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The Biologically Plausible Model Of Perturbed Microglial Colonization And Developmental Trajectories Participating In (Some Cases Of) Autism And The Ribs Hypothesis Of Maternal Inflammation And Causation, Or, Venturing Into Uncharted Realms Of Guessing
Posted February 3, 2013on:
- In: Autism | Beautiful Complexity | Bilbo | Developmentall Programming | Early Life Immune Activation | Glial Priming | Humbling Complexity | Immunology | Inflammation | LPS | Microglia | microglia colonization | microglial proliferation | Mr. Rat | Phenotypes | Primed Phenotype | Proliferation | Pruning | Synapse | Uncategorized
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Hello Friends – There are (at least) two big
classifications of microglia findings in autism, an altered
morphology (i.e., shape and function, or ‘activated’ versus
‘quiescent’), and an increased number (i.e., more), with both
parameters varying with each other and spatially. In other
words, disparate parts of the brain have different numbers of
microglia in them, and the functional profile of those microglia
also varies from one area to another.
[Note: There is ongoing
discussion regarding the appropriate definition of
‘activation’ of microglia, with evidence of (at least) four states
of microglial morphology.] Recently I saw a discussion on the SFARI
site about the fancy in
vivo study of microglia numbers in high functioning males with
autism. (I believe I am growing
increasingly jaded, as it occurs to me that radio tracing against
show microglial activation is a fancy trick, but one leaving us
open to detecting other stuff too.) In any case, the findings
are not especially unexpected by now, well not to me anyways, but a
comment at the SFARI site really got me thinking about the chain of
events that could lead to different spatial and
morphological characteristics of microglia. Perhaps we could
gain insight into the question of what the microglia are doing by
trying to understanding how they got there. Do we have any
biologically plausible models that might educate us on how a
different morphology and distribution of microglia could be
achieved? A while ago I got a copy of a few
articles that don’t have autism in them per se, but they kept
coming to the forefront of my mind when I thought about that
question. The first is Distribution
of microglia in the postnatal murine nigrostriatal
system, which had a disease focus on
Parkinson’s, but what really grabbed my attention is what they
learned about the developmental pathway microglia took to populate,
and then depopulate the substantia nigra (SN), a little wedge of
brain involved with motor skills, reward seeking, and addiction.
Interestingly, the SN
has been shown to contain more microglia than
adjacent structures. We have analysed
changes in microglia numbers and in microglial morphology in the
postnatal murine nigrostriatal system at various stages ranging
from postnatal day 0 (P0) up to 24 months of age. We
clearly show that the microglia numbers in the SN and in the
striatum dramatically increase from P0 to P15 and
significantly decrease in both areas in 18-month-old and
[Note: There seems to be
some variance in the appropriate ‘rat-to-human-age’ approximations;
especially when trying to do something as
expeditionary as comparing brain development. We should
extrapolate only with caution.] The part that makes me grin is that
it illustrates our nascent understanding of the process of
microglial colonization into the CNS, the hows,
whens, wheres, and whys are still
shrouded in mystery. The broadest outlines tell us that microglial
penetration into the brain is a long running, dynamic process; the
microglia are slow infiltrators, gaining access into parts of the
brain in concert with a swath of proliferating and inhibitory
factors, all at a time of once in a lifetime neurodevelopmental
of postnatal forebrain amoeboid microglial cell proliferation and
development by the transcription factor Runx1 paints a
beautiful portrait of functionality. Runx1 is a chemical
messenger that participates in phenotyopic determination of blood
cell progenitors into mature cells. The researchers observed
spatial, time dependent expression of Runx1 in the developing
forebrain, and differential levels following injury.
Here we show that the mouse
transcription factor Runx1, a key regulator of myeloid
cell proliferation and differentiation, is expressed in forebrain
amoeboid microglia during the first two postnatal weeks.
Runx1 expression is then downregulated in ramified microglia. Runx1
inhibits mouse amoeboid microglia proliferation and promotes
progression to the ramified state. We show further that
Runx1 expression is upregulated in microglia following nerve injury
in the adult mouse nervous system. These findings provide
insight into the regulation of postnatal microglia activation and
maturation to the ramified state and have implications for
microglia biology in the developing and injured brain.
It doesn’t really tell us much about a
persistent change in microglia per se, but it does render a picture
of proliferation and differentiation as an easily
disrupted symphony. When we think about the
developing brain, I won’t pretend to have more than a lightyear
close guess at what microglia might be doing
differently between amoeboid and ramified
morphologies in this locale, at this time, but I
very highly doubt there isn’t
a functional impact on microenvironment neurodevelopment; our
developing brains are using opportunities like the Indians used the
buffalo, no waste, no excess, and because balance is important,
everything is important. Moving back to the
question of the plausibility of a pathway to the autism state,
luckily (or unluckily?) the literature is veritably littered with
insults that perturb microglial development, leading to
persistent changes to microglial morphology, ultimately
percolating up to behavioral changes. Prenatal stress is a bad, bad
thing, and here is a study that finds that extreme mice stress can
persistently alter the mice activation profile of mice microglia.
stress increases the expression of proinflammatory cytokines and
exacerbates the inflammatory response to LPS in the hippocampal
formation of adult male mice, was just published, and
comes wrapped up with a double hit, and
different resting and stimulated neuroimmune environments.
Under basal conditions,
prenatally stressed animals showed increased expression of
interleukin 1ß and tumor necrosis factor-a (TNF-a) in the
hippocampus and an increased percentage of microglia cells with
reactive morphology in CA1 compared to non-stressed males.
Furthermore, prenatally stressed mice showed increased TNF-a
immunoreactivity in CA1 and increased number of Iba-1
immunoreactive microglia and GFAP-immunoreactive astrocytes in the
dentate gyrus after LPS administration. In contrast, LPS did not
induce such changes in non-stressed animals. These
findings indicate that prenatal stress induces a basal
proinflammatory status in the hippocampal formation during
adulthood that results in an enhanced activation of microglia and
astrocytes in response to a proinflammatory
Note: I have not read this
paper so I do not know if a qualitative number of microglia, or
just more immune-targeted microglia were found, but likely the
latter. A similar, full free paper, Prenatal
stress causes alterations in the morphology of microglia and the
inflammatory response of the hippocampus of adult female
mice, found broadly similar results; perturbed resting
and stimulated states in the treatment group.
Prenatal stress, per se,
increased IL1ß mRNA levels in the hippocampus, increased the total
number of Iba1-immunoreactive microglial cells and increased the
proportion of microglial cells with large somas and retracted
cellular processes. In addition, prenatally stressed and
non-stressed animals showed different responses to peripheral
inflammation induced by systemic administration of LPS.
LPS induced a significant increase in mRNA levels of IL-6,
TNF-a and IP10 in the hippocampus of prenatally stressed mice but
not of non-stressed animals.
Going back to my
ramblings on glial priming, it seems that here we have an
example of a type of cross system priming (sweet!), where
disturbing the stress response system changed the immune system;
such is the way of the polyamorous chemical families interacting in
our brain. It also occurs to me that given the
delicate nature of the developing brain, and the
tasks going on in there, we might want to think very
carefully before we ‘induced a significant increase in
mRNA levels of IL-6, TNF-a and IP10 in the hippocampus‘
on subgroups who might be environmentally predisposed to react with
exaggerated vigor. But what do I know? Of course, the
prenatal immune challenge arena holds a ton of studies on
persistent microglial function, and ‘consequences’. There are
way too many to list, but a quick overview of some very recent ones
would include: Enduring
consequences of early-life infection on glial and neural cell
genesis within cognitive regions of the brain, an early
life real infection model with e coli that
concludes, “Taken together, we have provided evidence that
systemic infection with E. coli early in life has significant,
enduring consequences for brain development and subsequent adult
function.” (Staci Bilbo!) This paper was sort
of a quinella, as it showed both changes in immune responsiveness
into adulthood; it also demonstrated the ability
of an immune insult to alter
the developmental trajectory of the
microglia, i.e., E. coli increased the number of newborn
microglia within the hippocampus and PAR compared to controls. The
total number of microglia was also significantly increased in E.
coli-treated pups, with a concomitant decrease in total
lipopolysaccharide exposure induces long-lasting learning
impairment, less anxiety-like response and hippocampal injury in
adult rats very directly blasted rats with some LPS
immune activation action, and includes, ”Neonatal LPS
exposure also resulted in sustained inflammatory responses in the
P71 rat hippocampus, as indicated by an increased number of
activated microglia and elevation of interleukin-1ß content in the
rat hippocampus.” (Sound familiar?) Interleukin-1
receptor antagonist ameliorates neonatal lipopolysaccharide-induced
long-lasting hyperalgesia in the adult rats took the
extra step of adding a set of animals that got inhibited
inflammatory responses. Results are increasingly
administration of an IL-1 receptor antagonist (0.1mg/kg)
significantly attenuated long-lasting hyperalgesia induced by LPS
and reduced the number of activated microglia in the adult rat
brain. These data reveal that neonatal intracerebral LPS
exposure results in long-lasting hyperalgesia and an elevated
number of activated microglia in later life. This effect is similar
to that induced by IL-1ß and can be prevented by an IL-1 receptor
I love how (once again) we
can see how interrupting the immune response can have an effect.
Environmental impacts outside of the immune
activation realm may also find a place within the ‘big tent’ of
microglial agitation with consequent developmental impacts.
The people who made the first big neuroimmune / autism splash at
Johns Hopkins later came out with Neuroinflammation
and behavioral abnormalities after neonatal terbutaline treatment
in rats: implications for autism, which found that an
agent used to prevent labor in some situations could
“produced a robust increase in microglial activation on PN
30 in the cerebral cortex” in treatment animals.
The drug in question, terbutaline, has been weakly associated with
increased incidence of autism, i.e., Prenatal
exposure to ß2-adrenergic receptor agonists and risk of autism
spectrum disorders, and beta2-adrenergic
receptor activation and genetic polymorphisms in autism: data from
dizygotic twins. And now, in 2013, Beta-adrenergic
receptor activation primes microglia cytokine production,
displays another example of cross system
if ß-AR stimulation is sufficient to prime microglia, rats were
intra-cerebroventricularly administered isoproterenol (ß-AR
agonist) or vehicle and 24h later hippocampal microglia were placed
in culture with media or LPS. Prior isoproterenol treatment
significantly enhanced IL-1ß and IL-6, but not TNF-a production
following LPS stimulation. These data suggest that central
ß-AR stimulation is sufficient to prime microglia cytokine
In other words, they gave
the rats a drug in the class of terbutline, and subsequently
observed an increased microglia responsiveness in cultured
cells. What a crazy coincidence. Detecting total
populations of microglia in adulthood, either regionally
or in the brain as a whole is a little more difficult, the little
buggers are a lot easier to detect when we light them up with neon
green tracers that stick to proteins expressed at ‘activation’
time, and it just doesn’t look like the question has been asked too
many times. I did, however, find something that has a sort of
chip shot on this analysis, Prenatal
stress alters microglial development and distribution in postnatal
rat brain, which looked at regional microglia populations
and phenotypes at two time periods following prenatal stress
stress consisting of 20 min of forced swimming occurred on
embryonic days 10–20. On postnatal days 1 and 10, stressed and
control pups were killed. Microglia were identified using Griffonia
simplicifolia lectin and quantified in the whole encephalon.
In addition, plasma corticosterone was measured in dams at
embryonic day 20, and in pups on postnatal days 1 and 10.
At postnatal day 1, there was an increase in number of
ramified microglia in the parietal, entorhinal and frontal
cortices, septum, basal ganglia, thalamus, medulla oblongata and
internal capsule in the stressed pups as compared to controls, but
also there was a reduction of amoeboid microglia and the total
number of microglia in the corpus callosum. By postnatal
day 10, there were no differences in the morphologic type or the
distribution of microglia between the prenatal stress and control
groups, except in the corpus callosum; where prenatal stress
decreased the number of ramified microglia. The stress procedure
was effective in producing plasma rise in corticosterone levels of
pregnant rats at embryonic day 20 when compared to same age
controls. Prenatal stress reduced the number of immature
microglia and promoted an accelerated microglial differentiation
into a ramified form.
They did a lot
of clever stuff at analysis time, taking samples from several
locations after birth and ten days later, and
also did some fine grained classification of the
shape of the microglia. They include spatial and temporal
mappings of four microglial developmental profiles. It looks
as if prental stress was able to alter the developmental speed of
microglia from one morphology to another in different parts of the
brain. There was as small section in the discussion that
speculated on what such changes might mean for neurodevelopment.
Given that during the
early postnatal period occur numerous brain developmental processes
(e.g. neurogenesis, myelination, synaptogenesis, astrogliogenesis,
neuronal cell death and blood–brain barrier maturation) [6, 19, 22,
25, 36, 52] it is possible that altered microglial
development induced by in utero stress may affect other
developmental processes either changing microenvironment molecular
constitution or triggering earlier inflammatory changes secondary
to the blood–brain barrier opening induced by prenatal
stress . Although punctual, the altered microglial
development might alter extensively the other
neurodevelopmental processes ensuing perdurable
structural changes; for example it is possible that the
change in the distribution pattern of microglia in the prenatal
stress group may render vulnerable some neuroanatomic
regions due to the reduction of neurotrophic factors,
such as the corpus callosum where there is a continuous axonal
No kidding! [There is also some very
interesting notes regarding microglial participation in purkinje
cell death that deserves and entire post. . .] This should be the
point that any rational observer must accept that we several lines
of evidence that early life experiences can persistently alter
microglial function with plausible mechanisms that could affect
neurodevelopment. Our data concerning total population
numbers in adulthood is a lot more difficult to come by, but I
think this will probably be getting looked at soon enough. Of
course, in any particular individual it is difficult (or
impossible?) to know how they may have arrived at a state of
increased microglial activation, but at the same time, it is not as
if we have no clue on possible pathways to this destination; our
short list of environmental factors includes immune insult, stress,
and chemical agents. If the question is, ‘what are the microglia
doing in the autism population?’, one plausible answer is ‘their
phenotype was persistently altered by an early life event through a
developmental programming model’. As I was mulling all of this
over, two things happened. First, a maternal CRP
study came out, and found a pretty strong relationship
between direct measurements of mommy inflammation with increased
risk of baby autism. The nice part is that they had a
gigantic data set (1.2M births!) to work with thanks to a few
decades of single payer medicine. (Very
For maternal CRP
levels in the highest quintile, compared with the lowest quintile,
there was a significant, 43% elevated
risk. This finding suggests that
maternal inflammation may have a significant role in autism, with
possible implications for identifying preventive strategies and
pathogenic mechanisms in autism and other neurodevelopmental
Just after that paper came out, I
made some Fred Flintstone style beef ribs. I ‘primed’ the
meat with a Moroccan inspired spice rub overnight, then
slow, slow, slow cooked them with a
low, low, low heat all day
long, and blasted away with a date glaze under the
broiler just before go time and they were caveman style
primal fucking awesome. The key to arriving there
was the slow cooking. The rib preparation
process got me thinking about our population wide experiment
in replacing infection with inflammation where we have
traded in death by pathogens or other once fatal ailments in
exchange for a longer life frequently plagued by conditions
associated with higher inflammation. Our analysis on long
term alterations to microglial proliferation and morphology is
largely comprised of studying acute insults
(sound familiar?), i.e., injection of purified bacterial cell
components known to trigger a robust immune response, ten sessions
of mouse based pregnant forced swimming, or exposure to chemicals
with rare and particular exposure routes in humans. Mostly I
think this is due to the black swan nature of the developmental programming
model alongside the very new idea that microglia are
doing jobs other than responding to infections; our models are
crude because of our relative ignorance. What will we find
when our filters are appropriately powered to detect for chronic,
but subtle insults? It occurs to me that there may be a ribs model
of altered microglial colonization of the fetal brain; it seems
clear that proliferation and differentiation of microglia can
clearly be changed by powerful inputs, but the
chemical messengers that impact that change are closely related (or
the same) as the measurement points in the maternal CRP study.
Could a slow cooking of slightly higher but not acutely
increased maternal inflammation be participating in the
genesis of autism (in some children) through altering the migration
and proliferation of microglia into the neonatal brain? Could
the same chemical messengers of inflammation be subtly
priming the microglia to respond with increased vigor to
insults later in life? Has our replacement of infection with
inflammation included an unanticipated effect that alters the
developmental pathway of the very cells that help shape our
children’s brains? I don’t think we are (quite) clever enough to
answer these types of questions yet, but we are at least starting
to generate the right kind of data to inform us on how to get
started. I don’t know what we will find, but the initial data
doesn’t look very good. In the meantime, I am recommending
you go get some ribs and let them cook all day long. –
A Sense Of Relief After (Some Of) Your Phantoms Are Observed By Others, A Distillation of Humbling Complex Early Life Neuroimmune Literature: “Microglia in the developing brain: a potential target with lifetime effects”, and The Need For Dispassionate Analysis
Posted June 15, 2012on:
Hello friends –
I have a confession to make. The fact that a lot of very smart people have ignored or flat out laughed at my arguments bothers me sometimes. I have applied non-trivial, not to be rebated time and effort to put forth what I considered to be logical views, scientifically defendable and important ideas; and yet people I knew were otherwise rational, and in some cases, very intelligent, just hadn’t seemed to get what I was saying. Often this was within the context of a discussion argument of vaccination, but my larger concern, that of a non-imaginary, non-trivial increase in children with autism in the past decades, also usually falls on deaf ears. If “environmental changes” incorporate the chemical milieu of our mother’s wombs, the microbial world our infants are born into, or the ocean of synthetic chemicals we all swim through every day, we have no rational conclusion but that our environment has changed a lot in the past few decades. Considered within the context of the reality based model where the events of early life can be disproportionally amplified through the lifetime of an organism, clinging to the idea that there has been a stable incidence of autism seems dangerously naïve, at most charitable.
And yet, for the most part, many or most of the people who are alarmed are crackpots. There were times I questioned myself. Am I missing something? Am I chasing phantoms? Why aren’t any of these other smart people as worried as I am?
A while ago I got a copy of Microglia in the developing brain: A potential target with lifetime effects (Harry et all), a paper that tells me that if nothing else, I have some good company in pondering the potential for disturbances in early life to uniquely affect developmental outcome, in this instance through alterations to the neuroimmune system. If I am incorrect about the validity of a developmental programming model with lifetime effects, lots of prolific researchers are wrong about the same thing in the same way. Harry is a very thorough (and terrifying) review of the relevant literature. Here is the abstract:
Microglia are a heterogenous group of monocyte-derived cells serving multiple roles within the brain, many of which are associated with immune and macrophage like properties. These cells are known to serve a critical role during brain injury and to maintain homeostasis; yet, their defined roles during development have yet to be elucidated. Microglial actions appear to influence events associated with neuronal proliferation and differentiation during development, as well as, contribute to processes associated with the removal of dying neurons or cellular debris and management of synaptic connections. These long-lived cells display changes during injury and with aging that are critical to the maintenance of the neuronal environment over the lifespan of the organism. These processes may be altered by changes in the colonization of the brain or by inflammatory events during development. This review addresses the role of microglia during brain development, both structurally and functionally, as well as the inherent vulnerability of the developing nervous system. A framework is presented considering microglia as a critical nervous system-specific cell that can influence multiple aspects of brain development (e.g., vascularization, synaptogenesis, and myelination) and have a long term impact on the functional vulnerability of the nervous system to a subsequent insult, whether environmental, physical, age-related, or disease-related.
The body of Microglia in the developing brain: A potential target with lifetime effects has tons of great stuff. From the Introduction
The evidence of microglia activation in the developing brain of patients with neurodevelopmental disorders(e.g., autism) and linkage to human disease processes that have a developmental basis (schizophrenia) have raised questions as to whether developmental neuroinflammation actively contributes to the disease process. While much of the available data represent associative rather than causative factors, it raises interesting questions regarding the role of these ‘‘immune-type’’ cells during normal brain development and changes that may occur with developmental disorders. Within the area of developmental neurotoxicology, the potential for environmental factors or pharmacological agents to directly alter microglia function presents a new set of questions regarding the impact on brain development.
There is a short section on what is known about the colonization of the brain by microglia, it is a busy, busy environment, and while we are just scratching the surface, microglia seem to be involved in scads of uber-critical operations, many of which pop up in the autism literature. It is just being confirmed that microglia constitute a distinct developmental path that diverges as an embryo, two papers from 2007 and 2010 are referenced as reasons we now believe microglia are a population of cells that migrate into the CNS before birth and are not replaced from the periphery in adulthood. From there, the beautiful complexity is in full effect; as the microglia develop and populate the brain there are specific spatial and morphological conditions, microglia are first evident at thirteen weeks after conception, and do not reach a stable pattern until after birth. In fact, it appears that microglia aren’t done finishing their distribution in the CNS until the postnatal period, “With birth, and during the first few postpartum weeks, microglia disseminate throughout all parts of the brain, occupying defined spatial territories without significant overlap (Rezaie and Male, 2003) suggesting a defined area of surveillance for each cell.”
It occurred to me to wonder if there are differences in microglia settlement patterns in males and females in human infants, as has been observed in other models? Could a spatially or temporally different number of micoglia, or different developmental profiles of microglia based on sex be a participant in the most consistent finding in the autism world, a rigid 4:1 male/female ratio?
Speaking towards the extremely low replacement rates for microglia in adulthood, the authors wonder aloud on the possible effects of perturbations of the process of microglial colonization.
The slow turn-over rate for mature microglia raises an issue related to changes that may occur in this critical neural cell population. While this has not been a primary issue of investigation there is limited data suggesting that microglia maintain a history of previous events. Thus, if this history alters the appropriate functioning of microglia then the effects could be long lasting. Additionally, a simple change in the number of microglia colonizing the brain during development, either too many or too few, could have a significant impact not on only the establishment of the nervous system network but also on critical cell specific processes later in life.
Perhaps coincidentally (*cough*), we have abundant evidence of an altered microglial state and population in the autism population; while we do not know that these findings are the result of a disturbance during development, it is an increasingly biologically plausible mechanism, and thus far, I’ve yet to see other mechanisms given much thought, excepting the chance of an ongoing, undetected infection.
There is a brief section concerning the changes found in adult microglial populations in terms of density, form, and gene expression in different areas of the brain, “With further investigation into the heterogeneity of microglia one would assume that a significant number of factors, both cell membrane and secreted, will be found to be differentially expressed across the various subpopulations.” Nice.
There is a section of the paper on microglial phenotypes, there are a lot of unknowns and the transformation microglia undergo between functional states is even more nebulously understood during brain development. “It is now becoming evident that in the developing brain, many of the standards for microglia morphology/activation may require readdressing.” We haven’t even figured out what they’re doing in the adult brain!
There is a really cool reference for a study that shows altered microglial function dependent on the age of the organism.
In the adult rodent, ischemia can induce microglia to display either a more ramified and bushy appearance or an amoeboid morphology depending on the level of damage and distance from the infarct site(s). In the immature rodent, ischemia-induced changes in capillary flow or, presumably, altered CNS vascularization can retain the microglia in an amoeboid phenotype for longer and delay the normal ramification process (Masuda et al., 2011).
One way of looking at this would be to say that we should exercise extreme caution in trying to translate our nascent understanding of how mature microglia react when speculating on how immature microglia will act. To follow up on just how little we know, there is a long discussion about the shortcomings of a the term ‘activated’ microglia with some details on chemical profiles of broadly generalized ‘classically inflammatory, ‘alternatively activated’, ‘anti-inflammatory’, and ‘tissue repair’ phenotypes.
Next up is a dizzyingly list of brain development functions that microglia are known, or suspected to participate in. Without getting too deep in the weeds, of particular interest to the autism realm, that list includes neurogenesis and differentiation in the cortex [related: Courchesne, me], cell maturation via cytokine generation, axon survival and proliferation [related: Wolff, me], programmed cell death of Purkinje cells, clearance of ‘early postnatal hippocampul neurons’, and the ‘significant contribution to synaptic stripping or remodeling events’, i.e., pruning (Paolicelli / fractaltine), and even experience dependent microglia / neuron interactions. Taking all of this (and more) into consideration, the authors conclude “Thus, one can propose that alterations in microglia functioning during synapse formation and maturation of the brain can have significant long-term effects on the final established neural circuitry. “ Ouch.
Next up is a summary of many of the animal studies on microglial participation in brain formation, there is a lot there. Interestingly (and particularly inconvenient) is the finding that a lot of the functional actions of microglia during development appear to operate after birth. “Overall, the data suggest that microglial actions may be most critical during postnatal brain maturation rather than during embryonic stages of development.” Doh!
Early life STRESS gets some attention, and for once there is some good news if you look at it the right way. There is something about a very cool study from Schwarz (et all / Staci Bilbo!) involving drug challenge that peered deep into the underlying mechanisms of an environmental enrichment model; animals given a preferential handling treatment were found by two metrics to have differential microglia response in adulthood with (biologically plausible) observations, increased mRNA levels for IL-10 production, and decreased DNA methylation; i.e., less restriction on the gene that produces IL-10, and more messenger RNA around to pass off the production orders [totally beautiful!]. There is more including thyroid disruption (though in a way that I found surprising), and the observations of time dependent effects on immue disturbances. (super inconvenient)
There is so much data that keeps piling on that the authors end up with “Overall, the existing data suggest a critical regulatory role for microglia in brain development that is much expanded from initial considerations of microglia in the context of their standard, immune mediated responses.”
A terrifying concept that I haven’t found time to dedicate a post towards is microglia priming, which gets some attention in Harry.
There is a significant amount of evidence regarding what is often termed ‘‘priming’’ and ‘‘preconditioning’’ events that serve to either exacerbate or provide neuroprotection from a secondary insult, respectively. In these states, the constitutive level of proinflammatory mediators would not be altered; however, upon subsequent challenge, an exaggerated response would be induced. The phenomena of priming represent a phenotypic shift of the cells toward a more sensitized state. . . Exactly how long this primed state will last has not been determined; however, data from microglia suggest that it can extend over an expanded period of time. Preconditioning can also represent changes that would occur not only over the short term but may be long lasting.”
I happen to think that microglia priming is going to be a very important cog in the machinery for this journey when all is said and done; the evidence to support a preconditioning system is strong, and in parallel, the things we see different in autism (and elsewhere) is consistent with a different set of operations of microglia, AND we also have evidence the disturbances that would invoke microglial change are subtle but real risk factors for autism.
What comes next is a type of greatest hits mashup of very cool papers on developmental programming in the CNS.
Galic et al.(2008) examined age related vulnerabilities to LPS in rats to determine critical age periods. Postnatal injection of LPS did not induce permanent changes in microglia or hippocampal levels of IL-1b or TNFa; however, when LPS was given during the critical postnatal periods, PND 7 and 14, an increased sensitivity to drug induced seizures was observed in 8-week-old rats. This was accompanied by elevated cytokine release and enhanced neuronal degeneration within the hippocampus after limbic seizures. This persistent increase in seizure susceptibility occurred only with LPS injection at postnatal day 7 or 14 and not with injections during the first day of life or at PND 20. Similar long-lasting effects were observed for pentylenetetrazol-induced seizures when PND 11 or 16 rat pups were subjected to LPS and hyperthermic seizures (Auvin et al., 2009). These results again highlight this early postnatal period as a ‘‘critical window’’ of development vulnerable to long-lasting modification of microglia function by specific stimuli. Work by Bilbo and co-workers demonstrated LPS-induced deficits in fear conditioning and a water maze task following infection of PND 4 rats with Escherichia coli. In the young adult, an injection of LPS induced an exaggerated IL-1b response and memory deficits in rats neonatally exposed to infection (Bilbo et al., 2005). Consistent with the earlier work by Galic et al. (2008), an age dependency for vulnerability was detected with E. coli-induced infection at PND 30 not showing an increased sensitivity to LPS in later life (Bilbo et al., 2006).
In particular, Galic 2008, or Postnatal Inflammation Increases Seizure Susceptibility in Adult Rats (full paper) was a very formative paper for me; it was elegant in design and showed alarming differences in outcome from a single immune challenge experience, if it occurred during a critical developmental timeframe. If you haven’t read it, you should.
This paper has a nice way of distilling the complexity of the literature in a readable way.
One hypothesis for developmental sensitivity is the heterogeneous roles for inflammatory factors and pro-inflammatory cytokines during development, including their timing-, region and situation-specific neurotrophic properties. Many of the proinflammatory cytokines are lower at birth with a subsequent rapid elevation occurring during the first few weeks of life. In an examination of the developing mouse cortex between PND 5 and 11, mRNA levels for TNFa, IL-1b, and TNFp75 receptor remained relatively constant while a significant increase in mRNA levels of CR3, macrophage-1 antigen (MAC-1), IL-1a, IL-1 receptor 1 (IL- )R1, TNFp55 receptor (TNFp55R), IL-6, and gp130 occurred (Fig. 2). This data suggests that an upregulation of interleukins and cytokine receptors may contribute to enhanced cytokine signaling during normal cortical development.
One hypothesis put forward using a model reliant on postnatal exposure to LPS suggests that these types of exposure may ‘‘reprogram’’ neuroimmune responses such that adult stress results in hyperactivation of the hypothalamic pituitary adrenal (HPA) axis (Mouihate et al., 2010) and corticosterone changes (Bilbo and Schwarz, 2009).While limited, the available data suggest that events occurring during development, especially postnatal development, have the potential to cause long term alterations in the phenotype of microglia and that this can be done in a region specific manner.
In what could, conceivably, be a coincidence, our available information on the autism brain also shows region specific changes in microglia populations, microglial activation profiles, and oxidative stress. I do not believe the findings reviewed in Microglia in the developing brain: A potential target with lifetime effects will be meaningless artifacts; the likelihood that our observations of an altered neuroimmune state in autism are not, at least, participatory has become vanishingly small.
Can these findings inform us on the incidence question? I was lurking on a thread on Respectful Insolence a while ago, and someone gave what I thought was a very succinct way of thinking about the changes that our species has encountered the past few decades; it went something like “we have replaced infection with inflammation”. That’s a pretty neat way of looking at how things have gotten different for humanity, at least lots of us, and especially those of us in the first world. We used to get sick and die early; now we live longer, but oftentimes alongside chronic disorders that share a common underlying biological tether point, inflammation.
Any dispassionate analysis of the available data can tell us that we have, indeed, replaced infection with inflammation; we suffer from less death and misery from infection, but more metabolic disorder, more diabetes, more hypertension, more asthma and autoimmune conditions than previous generations. We have largely replaced good fatty acids with poor ones in our diet. All of these conditions are characterized by altered immune biomarkers, including an increase in proinflammatory cytokines. Those are the facts that no one can deny; we have replaced infection with inflammation.
But when we look to the findings of Microglia in the developing brain: A potential target with lifetime effects, it becomes clear that our newfound knowledge of microglial function and crosstalk with the immune system raises some very troubling possibilities.
Lately it has been quite in vogue among a lot of the online posting about autism to at least mention environmental factors which could participate in developmental trajectory leading to autism; that’s a big step, an important and long overdue acknowledgement. If you pay close attention, you will notice that 99% of these admissions are handcuffed to the word “prenatal”. This is likely an attempt to deflect precise questions about the robustness of our evaluation of the vaccine schedule, but the big question, the incidence question, still hinges on fulcrum of the genetic versus environmental ratio ; that is a problem for the purveyors of the fairytale because the prenatal environment of our fetuses, the chemical milieu of their development, is qualitatively different compared to generations past. That chemical soup is their environment; and that environment has unquestionably changed in the past decades as we have replaced infection with inflammation.
Our previous analysis tells us that invoking inflammation outside the brain modifies microglial function inside the wall of the blood brain barrier; good or bad, no honest evaluation of the literature can argue against a lack of effect. What happens outside the brain affects what happens inside the brain. If, however, microglia are active participants in brain formation, as a swath of recent research indicates, can this fact give us insight into the incidence question?
Is a state of increased inflammation the pathway between maternal asthma, depression, stress, and obesity being associated with increased risk of autistic offspring? Have we replaced infection with inflammation plus?
What could be more lethal to the fairytale of a static tale of autism than a positive relationship between a lifestyle characterized by increased inflammation and the chances of having a baby with autism?
Are we totally fucked?
We cannot know the answers unless we have the courage to ask the difficult questions with methods powerful enough to provide good data, and it won’t be easy. The static rate of autism fairytale is a comforting notion; it expunges responsibility for the coronal mass ejection sized change to our fetuses developing environment, and while hiding behind the utterly frail findings of social soft scientists, we can happily place tin foil hats and accusations of scientific illiteracy on anyone who might be worried that our abilities have outstripped our wisdom. That is a terrible, cowardly way to approach the incidence question, what we should be doing is exactly the opposite, ridiculing the epidemic sized error bars in prevalence studies and demanding more answers from the hard scientists. Eventually we will get there and it will be a critical mass of information from studies like Harry that will propel decision makers to abandon the fairytale for a course regulated by dispassionate analysis.
Extremely Long Overdue Clinical Findings “Impaired Carbohydrate Digestion and Transport and Mucosal Dysbiosis in the Intestines of Children with Autism and Gastrointestinal Disturbances”, and The Swan Song Of A Tragically Overused Autism Canard
Posted January 9, 2012on:
Hello friends –
My son was a ‘gut kid’. The irony is, for a while, because we were first time parents, we didn’t even know. My son was flagged for evaluation for autism around a year of age and we met with the autism center people several times between his first and third birthdays, with his official diagnosis coming just after he turned three. My wife came home from one of the early meetings convinced that his evaluators didn’t know the first thing about our son, autism, or anything else, and that in fact, they might be insane.
‘Do you know what those idiots asked me today?’
‘What his shits look like. My kid can’t talk and they want to ask me about his diapers!’
‘Who fucking cares?‘
We wound up caring, a lot. It turns out, this wasn’t a stupid question, it just seemed like one to us. The answer to their question was that our son was having at least four or six very messy diapers a day, his stools were never firm logs that look like they came from an spherical filter, but always, always more liquid than solid, and frequently contained chunks of identifiable food. But from our viewpoint, within the context of a child who was not speaking, hurting himself, and never looked at anyone, the idea that we should be worrying about his shit was the stupidest thing we’d ever heard.
But. When we started paying attention, starting reading, and started meeting more people with children with autism, our incredulity waned. We tried GF/CF and probiotics. We paid for lab tests to analyze the bacterial populations in his intestines. We experienced a life saving miracle with anti-fungal agents wherein my son essentially stopped hurting himself over the course of weeks after persistently banging his head dozens of times a day for six months. For nearly a year we removed all complex carbohydrates from our son’s diet, an intervention that makes GFCF feel like a Sunday afternoon after college but before kids and autism. We saw changes in our son based on how his GI tract was performing. For our son, for us, we knew that by some mechanism, what got put in his mouth, and what happened along the way was tightly coupled with how our son felt and behaved.
This is why my vision with spots of rage when I see the ideas of GI and dietary involvement with autism mocked by pseudo-skeptics so rampantly on the Internet. I cannot stand the thought of a single child continuing to suffer the way I watched my son suffer because they were told that there was no basis of GI interaction in autism. That thought hurts.
Those biases stated, we are now, finally, starting to see research indicating that in some cases of autism, there are very real, non imaginary differences in GI function.
A few months ago, Impaired Carbohydrate Digestion and Transport and Mucosal Dysbiosis in the Intestines of Children with Autism and Gastrointestinal Disturbances was published [full, dense, but very cool paper available online]. Here is the abstract.
Gastrointestinal disturbances are commonly reported in children with autism, complicate clinical management, and may contribute to behavioral impairment. Reports of deficiencies in disaccharidase enzymatic activity and of beneficial responses to probiotic and dietary therapies led us to survey gene expression and the mucoepithelial microbiota in intestinal biopsies from children with autism and gastrointestinal disease and children with gastrointestinal disease alone. Ileal transcripts encoding disaccharidases and hexose transporters were deficient in children with autism, indicating impairment of the primary pathway for carbohydrate digestion and transport in enterocytes. Deficient expression of these enzymes and transporters was associated with expression of the intestinal transcription factor, CDX2. Metagenomic analysis of intestinal bacteria revealed compositional dysbiosis manifest as decreases in Bacteroidetes, increases in the ratio of Firmicutes to Bacteroidetes, and increases in Betaproteobacteria. Expression levels of disaccharidases and transporters were associated with the abundance of affected bacterial phylotypes. These results indicate a relationship between human intestinal gene expression and bacterial community structure and may provide insights into the pathophysiology of gastrointestinal disturbances in children with autism.
I’ll admit it. From the outside, from the don’t-have-a-kid-with-autism-and-GI-problems perspective, that is some dense and seemingly bland stuff. Essentially children with GI distress and children with GI distress and autism were compared and it was found that there were distinctly qualitative differences regarding the GI function in the groups. This is validation of what a lot of us have been saying for a long time, that the GI problems our kids were experiencing weren’t coincidental to the autism, but somehow related.
For anyone who has been paying attention to the details of the autism-gut debate, these are dynamite findings. These observations are the death knell for the overused, oversimplified notion that the GI connection to autism was a function of some kids having autism, some kids having GI distress, and that therefore, some kids with autism also have GI distress. This research tells us that the reality is not so simple.
This study is the view from the microscope as opposed to the telescope, and took care not to study just anyone with an autism diagnosis, but those with an autism diagnosis and GI distress, problems so severe that invasive procedures to obtain tissue samples from the GI tract was warranted. This is a critically important facet of the study design in my opinion, a lot of the negative findings in this arena have been epidemiological, and cast the widest possible net, capturing everyone with autism and comparing them with a sample of everyone else. This is a great strength of the paper; for too long everyone has acknowledged the heterogeneous nature of autism, but few studies have tried to understand differences at a phenotype level. This paper is different.
As evidence of the non-random population, the autism patient group had a regression incidence of over eighty percent, and nearly as many of the children in both groups were reported to have food allergies.
The details of the findings in the paper get deep pretty fast, but at a high level there were differences found in proteins involved with the digestion of carbohydrates and changes in bacterial populations between the groups, with some differences found with regard to specific locations in the intestine. Based on these findings, the authors speculate that alterations in carbohydrate processing could result in abnormal bacterial populations by way of altered microbial food availability in parts of the gut.
Based on these findings, we propose a model whereby deficiencies in disaccharidases and hexose transporters alter the milieu of carbohydrates in the distal small intestine (ileum) and proximal large intestine (cecum), resulting in the supply of additional growth substrates for bacteria. These changes manifest in significant and specific compositional changes in the microbiota of AUT-GI children (see Figure 7A–C).
The authors discuss a potential feedback loop of effects of intestinal microbes and nutritional processing, and of the known and potential effects of altered bacterial populations.
Additionally, intestinal microbes can influence the expression of disaccharidases and transporters  through the influence of pathogen-associated molecular patterns (PAMPs) and butyrate (a byproduct of bacterial fermentation) on CDX2 expression and activity , , , . In this regard, the observation that CDX2 was decreased in AUT-GI children with increased levels of Betaproteobacteria may be important.
Whatever the underlying mechanisms, reduced capacity for digestion and transport of carbohydrates can have profound effects. Within the intestine, malabsorbed carbohydrates can lead to osmotic diarrhea ; non-absorbed sugars may also serve as substrates for intestinal microflora that produce fatty acids and gases (methane, hydrogen, and carbon dioxide), promoting additional GI symptoms such as bloating and flatulence .
This is very similar to the underlying theory of the Specific Carbohydrate Diet, impaired carbohydrate digestion promotes bacterial imbalances in the intestine by altered food availability, leading to gastrointestinal distress.
Because of the varied nature of the protein imbalances found and absence of the common alleles associated with such conditions, the authors report that it is unlikely that the underlying cause of the imbalances is genetically based.
In our study, 93.3% of AUT-GI children had decreased mRNA levels for at least one of the three disaccharidases (SI, MGAM, or LCT). In addition, we found decreased levels of mRNA for two important hexose transporters, SGLT1 and GLUT2. Congenital defects in these enzymes and transporters are extremely rare , , and even the common variant for adult-type hypolactasia was not responsible for reduced LCT expression in AUT-GI children in this cohort. Therefore, it is unlikely that the combined deficiency of disaccharidases (maldigestion) and transporters (malabsorption) are indicative of a primary malabsorption resulting from multiple congenital or acquired defects in each of these genes.
There are a couple of ideas presented on what might be causing the altered disaccharide transporter levels, with food composition intake, immune or hormonal irregularities, and bacterial populations and their associated fermentation byproducts listed as possible candidates. This study did not attempt to determine if any of these things were actually responsible, but an upcoming paper will also detail qualitative differences in expression of genes involved with inflammation in the autism group.
Regarding bacterial populations found, there were several differences identified by bacterial classification and location as well as some associations with onset of autistic behaviors and GI distress.
Pyrosequencing analysis of mucoepithelial bacteria revealed significant multicomponent dysbiosis in AUT-GI children, including decreased levels of Bacteroidetes, an increase in the Firmicute/Bacteroidete ratio, increased cumulative levels of Firmicutes and Proteobacteria, and increased levels of bacteria in the class Betaproteobacteria.
Stratification of AUT-GI children based on the timing of GI symptom development relative to autism onset revealed that the levels of Clostridiales and cumulative levels of Lachnospiraceae and Ruminococcaceae were significantly higher in AUT-GI children for whom GI symptoms developed before or at the same time as the onset of autism symptoms compared to AUT-GI children for whom GI symptoms developed after the onset of autism and compared to Control-GI children. However, we cannot discern whether changes in Clostridiales occurred before the onset of autism in this subgroup. We can only conclude that increased levels of Clostridiales members in biopsies taken after the development of both GI symptoms and autism are associated with the timing of GI onset relative to autism onset in this cohort. Although the reason for this association remains unclear, this finding may suggest that the timing of GI onset relative to autism is an important variable to consider in the design of future prospective studies investigating the microbiota of children with autism.
I am in love with the appreciation of the subtlety on display at the end, it may not be sufficient to simply categorize by GI and non GI autism, but also by the temporal relationship to onset of behavioral symptoms. It makes for a messy outlook going forward, but one based on pragmatism as far as coming to valid conclusions.
As is appropriate, the authors end with an admission that we are still largely groping in the dusk about how the microbiome interacts with our tightly coupled systems, but give a variety of reasons to believe that what we do know makes system wide effects reasonable and a relationship with autism plausible.
Metabolic interactions between intestinal microflora and their hosts are only beginning to be understood. Nonetheless, there is already abundant evidence that microflora can have system-wide effects , , , , , , ,  and influence immune responses, brain development and behavior , , , , .
It should be noted that this paper is a child of a 2010 IMFAR abstract, Intestinal Inflammation, Impaired Carbohydrate Metabolism and Transport, and Microbial Dysbiosis in Autism. If I understand correctly, another paper is being prepared regarding the findings of intestinal inflammation that will be complimentary to Impaired Carbohydrate Digestion and Transport and Mucosal Dysbiosis in the Intestines of Children with Autism and Gastrointestinal Disturbances. I’ll try to post something when it is published.
This study was small, with only twenty two participants, largely as a result of the difficulty in obtaining tissue specimens. While this does give us cause for caution, it is important to note that this research does not exist in a vacuum, but rather, as a much larger set of research that tell us that the relationship between GI complaints and autism is more than the inceptions of DAN doctors. Previously, Gastrointestinal abnormalities in children with autistic disorder, performed similar biochemistry and reported broadly consistent carbohydrate digestion problems, ‘Low intestinal carbohydrate digestive enzyme activity was reported in 21 children (58.3%), although there was no abnormality found in pancreatic function.’ Several other papers analyzing fecal samples have reported altered bacterial populations, including Low relative abundances of the mucolytic bacterium Akkermansia muciniphila and Bifidobacterium spp. in feces of children with autism, Gastrointestinal flora and gastrointestinal status in children with autism–comparisons to typical children and correlation with autism severity, Fecal lactoferrin and Clostridium spp. in stools of autistic children, and Pyrosequencing study of fecal microflora of autistic and control children, among others.
If the findings from this latest paper are spurious finding based on sample size problems, a lot of other studies are coincidentally finding the same type of thing in the wrong way. Does anyone think that is likely?
I entered the autism world and online autism debate from a place of seeing with my own eyes the failures of a toddlers GI function and the difficult to overstate changes in that toddler alongside improvements in his GI health. On one of the first autism blogs on which I participated I got into a discussion (argument?) with a blogger who I came to respect very much, but has since moved on. I described the fact that my son had six or more diarrhea stools, a day, every day, for months on end, and that when we added dietary changes, probiotics, and later antifungal agents, the changes to his GI function were profound and impossible to misinterpret. He told me something along the lines that humans were susceptible to illusions and sleight of hand, and I thought, ‘as if not knowing the difference between diarrhea and a log was along the lines of figuring out where the jack of spades went!’. I couldn’t believe, could not fucking believe, someone would try to convince me that I had imagined my sons problems, and associated recovery. This wasn’t a sugar pill study where I was asked if my child acted more or less hyperactive, this was a matter of asking myself, ‘How many diarrhea diapers did I change today? Six? Or Zero?’ [Repeat once a day for 180 days.]
I doubt this is necessary, but just in case, I will go on the record to state that it is easy, very easy, to tell the difference between a condition of chronic diarrhea and normal GI function. There might not be a more simpleminded determination to make on Planet Earth or indeed, our perceptible universe. This is a situation that is susceptible to placebo effects only in the most elaborate imaginations of people who have never experienced chronic GI problems.
From that time on, with nearly zero exceptions, I have become a little less shocked, but a little more saddened by the doublethink style skepticism applied to GI distress and autism in nearly every single conversation I have ever seen on the Internet. I’ve put some time thinking toward this, why so many otherwise intelligent people house such extreme hostility on a relationship between GI function and autism. I believe that the Wakefield / MMR autism debacle is at the heart of this disconnect; his ill fated and now retracted paper that launched a thousand Internet scribbles has seemingly forever tied GI complaints and autism to bad science.
It doesn’t have to be this way. As a community, the vaccine wars and kissing cousin prevalence question has done a lot to fracture us, and very little to unite us. That is a sad statement, and nothing makes it more unfortunate than the fact that it does not have to be this way. Wakefield can be wrong about the MMR and there can still be very real differences in GI function in some cases of autism. We can respectfully disagree about how well our existing prevalence studies inform us on the incidence of autism without also needing to accept a world view where every child with autism has raging bowel problems.
We should have the intellectual honesty to admit that there is nothing inherently dangerous about acknowledging what the data tells us; GI function seems to be abnormal in a subset of children with autism, and the underlying features of that GI distress are qualitatively different than what is found in ‘normal’ children.
The Interconnectedness of the Brain, Behavior, and Immunology and the Difficult to Overstate Flaccidity of The Correlation Is Not Causation Argument
Posted May 12, 2011on:
Hello friends –
I’ve gotten into a lot of discussions online about the vaccines and autism; generally with very poor, if not nonexistent, evidence of having changed any opinions, but relatively strong evidence ( p > .001) that persisting in making my arguments can get you called ‘an antivaccine loon’, ‘idiot’, someone who engages in ‘Gish Gallop’, or the worst insult I’ve received so far, ‘anti-science’. While I am really torn on the vaccine issue, I am certain that both peripheries of this debate are at least somewhat wrong in the conclusions that they’ve drawn from the available evidence. I do believe that lots of parents have witnessed a very real change in their children post vaccination, and I also don’t believe for a single second that vaccines are the cause of an epidemic of autism. It’s a mess and I’ve been poking around the Internet almost five years into journey autism and from my eyes, it hasn’t improved any in the past half decade. This is very sad.
That being said, while I do think we need to have a rational and dispassionate discussion about what our existing vaccine studies can and cannot tell us about autism, I’m really concerned about the fact that the vaccine wars seem to have inoculated otherwise intelligent people from any semblance of intellectual curiosity regarding the immunological findings in the autism realm. That’s a problem, because there are lots of things other than vaccines that can modify the immune response, various environmental agents and cultural changes that are relatively new, and ignoring immunological findings in autism because they happen to intersect with the function of vaccination is a huge, massive, supernova sized disservice to what history will view us poorly on, refusing to perform honest evaluation due to fear and the comfort of willful ignorance.
Here, in this post, I will make the case that this lack of curiosity on immunological findings in autism is either born of a lack of understanding on how much we know about the ties between the immune system and the brain, or alternatively, originates from a deep seated desire to avoid honest interactions. This isn’t to make the case that vaccines can cause autism, or even that the immunological disturbances observed in autism are causative, but rather that an obstinate refusal to consider these as possibilities is the sign of someone who cannot, or will not accept, the biological plausibility of immunologically driven behaviors despite a constellation of evidence.
One of the things that jumps out to me why the autism population might be a subgroup of the population susceptible to changes as a result of immune dysfunction (and thus, potentially adversely affected as a result of vaccination), is the sheer volume of evidence we now have available to us indicating an altered immune response, and indeed, an ongoing state of inflammation within the brain in the autism population, and most strikingly, repeated observations of a correlation between the degree of immune dysregulation as a propensity of an inflammatory state, and the severity of autism behaviors. Again and again we’ve seen that as markers indicative of an inflammatory state increase, so too, do severity of autism behaviors. Not only that, but there are instances wherein the decrease of components known to regulate the immune response decrease, autistic behaviors are more severe. Subtle shifts in either the start or the resolution of the immune response seems to affect autistic behavior severity in the same way. I know coincidences happen all the time, but that doesn’t mean that everything is a coincidence.
We also have a large number of studies that tell us that in vitro, similar levels of stimulation with a variety of agents cause exaggerated or dysregulated production of immune markers in the autism population.
A large percentage of the time that I mention these findings, usually within discussions with an origin in vaccination, someone decides to educate me on one of the most rudimentary scientific axioms:
Correlation does not equal causation.
It must be stated, the above statement is absolutely true. Unfortunately for the people for whom this accurate, but simplistic catchphrase comprises the entirety of their argument, it completely ignores a wealth of research that tells us in very unambiguous terms that there is incontrovertible evidence that crosstalk between the immune system and central nervous system can modify behavior. The research indicating a relationship between immune dysregulation and autism does not exist in a vacuum, but rather, is only a tiny fragment of evidence, mostly accumulated within the last few years, that tells us that the paradigm of the past decades, that of the brain as a immune privileged organ without communication to the immune system, is as antiquated as refrigerator moms and a one in ten thousand prevalence.
From a common sense, why didn’t I think of that standpoint, the best example of the interaction between the brain and the immune response is the old standard, just plain old getting sick. You live in the dirty world, you pick up a pathogen, you get sick, and suddenly you get lethargic and you start to run a fever. But is it the pathogen itself that is actually making you feel like staying in bed all day?
What is being learned is that it is not necessarily the microbial invader that is causing you to get tired and feel sore, but rather, that your decreased energy levels are centrally mediated through your brain, and the triggers for your brain to start a fever include molecules our bodies use for a wide range of communications, including immune based messaging, cytokines. Some of the most common cytokines in the research to follow include IL-6, IL-1B, and TNF-Alpha; so called ‘pro-inflammatory’ cytokines. Researchers have been plugging away at just how the immune response is capable of modifying behaviors, i.e., inducing, sickness behavior for a while now, at least in terms of autism research. From 1998, we have Molecular basis of sickness behavior:
Peripheral and central injections of lipopolysaccharide (LPS), a cytokine inducer, and recombinant proinflammatory cytokines such as interleukin-1 beta (IL-1 beta) induce sickness behavior in the form of reduced food intake and decreased social activities. Mechanisms of the behavioral effects of cytokines have been the subject of much investigation during the last 3 years. At the behavioral level, the profound depressing effects of cytokines on behavior are the expression of a highly organized motivational state. At the molecular level, sickness behavior is mediated by an inducible brain cytokine compartment that is activated by peripheral cytokines via neural afferent pathways. Centrally produced cytokines act on brain cytokine receptors that are similar to those characterized on peripheral immune and nonimmune cells, as demonstrated by pharmacologic experiments using cytokine receptor antagonists, neutralizing antibodies to specific subtypes of cytokine receptors, and gene targeting techniques. Evidence exists that different components of sickness behavior are mediated by different cytokines and that the relative importance of these cytokines is not the same in the peripheral and central cytokine compartments.
The first sentence in this abstract references a practice that is extremely common in studying the immune system, intentionally invoking a robust immune response by exposing either animals, or cells in vitro, to the components that comprise the cell wall of certain types of bacteria; lipopolysaccharide, or LPS. LPS could be considered a sort of bacterial fingerprint, a pattern that our immune systems, and the immune system of almost everything, has evolved to recognize, and correspondingly initiates an immune response.
Because this is a conversation that frequently has an origin in vaccination, essentially the act of faking an infection, it is salient to remember that the animals or cell cultures aren’t really getting sick when exposed to LPS; there is no pathology associated with whatever type of bacteria might be housed within a cell membrane containing LPS. Usually, when the body is exposed to a gram negative bacteria, and the consequent LPS exposure, there are also effects of the bacteria that interact with the organism, but by only incorporating the alert signal for a bacterial invader, we can gain insight into the effect of the immune response itself; there isn’t anything else to cause any changes. This means that similarly to LPS administration, straight administration of these pro-inflammatory cytokines are similar to the result of getting sick with a pathogen, at least as far as the immune response is concerned.
In the above instance, administration of LPS, or simply cytokines, had been shown to be capable of causing reduced food intake and ‘decreased social activities’.
Later in 1998, Central administration of rat IL-6 induces HPA activation and fever but not sickness behavior in rats (full version), was published wherein the authors report that central administration (i.e., directly into the CNS), of cytokines in isolation (IL-6) or in combination (IL-6 + IL-1B) were capable of inducing altered HPA activation, fevers, and sickness behaviors. Effects of peripheral administration of recombinant human interleukin-1 beta on feeding behavior of the rat was published a few years later, and observed that peripheral administration (i.e., not the CNS) of IL-1B could affect how much a rat ate, with sucrose ingestion being consistently altered during periods of sickness.
Jumping ahead a few years, a review paper Expression and regulation of interleukin-1 receptors in the brain. Role in cytokines-induced sickness behavior reviewed how cytokines participate in sickness behavior, Interleukin-6 and leptin mediate lipopolysaccharide-induced fever and sickness behavior examined the interactions of IL-6 and leptin in sickness behavior, and Behavioral and physiological effects of a single injection of rat interferon-alpha on male Sprague-Dawley rats: a long-term evaluation reported “these data suggest that a single IFN-alpha exposure may elicit long-term behavioral disruptions”.
Much more recently, Sickness-related odor communication signals as determinants of social behavior in rat: a role for inflammatory processes more elegantly found that behavior was modified by LPS exposure, and that this effect was neutralized by concurrent administration of the anti-inflammatory cytokine, IL-10. Similarly, Inhibition of peripheral TNF can block the malaise associated with CNS inflammatory diseases observed another distinct means by which interfering with the immune response could attenuate the effect of faux sickness, in part, concluding, “Thus behavioral changes induced by CNS lesions may result from peripheral expression of cytokines that can be targeted with drugs which do not need to cross the blood-brain barrier to be efficacious.” In other words, what is happening in the periphery, outside of the protective boundaries of the blood brain barrier, can none the less manipulate behaviors that are controlled by the brain.
There are tons, tons more studies like this, but the point should be clear by now; it is accepted that you can achieve some of the same behaviors the come alongside illness, such as fever and lethargy, without the presence of an actual bacteria or virus; all you need is for your brain to think that you are sick.
While it must be acknowledged that the behavioral disturbances observed in autism are a lot different than feeling the need to watch TV all day, these types of studies were among the first clues that the traditional view of the CNS as a separate entity within the gated community of the blood brain barrier needed revision.
Measuring how much sugar water a rat drank is great stuff, but the reality is that we have conservatively a gazillion studies telling us that disorders that manifest behaviorally have strong, strong ties to the immune system; and once we begin to understand the vast scope of these findings, the utter frailty of “correlation does not equal causation” becomes painfully clear to the intellectually honest observer.
The big problem I found myself with in crafting this posting was that the sheer volume of studies available really makes a complete illustration of the literature impossible; I started looking and pubmed nearly puked trying to return to me a listing of all of the things I wanted to summarize. So here is some of the best of the best; to keep things interesting, I thought I’d only include findings from 2007 or later as a mechanism to show just how nascent our understanding of the connections between the brain and the immune system really are.
Initially, we can start with a condition that nearly everyone agrees is diagnosed based on behavior, depression. It turns out, the number of findings establishing a link between immune system markers and depression is wide and deep.
Here’s a great one, Elevated macrophage migration inhibitory factor (MIF) is associated with depressive symptoms, blunted cortisol reactivity to acute stress, and lowered morning cortisol, which reports, that MIF can modify HPA axis function and is tied to depression; a particularly compelling finding considering well documented alterations in HPA axis metabolites in autism, and the fact that increased MIF has also been found in the autism population, and as levels increased, so too did autism severity.
Here is part of the abstract for Inflammation and Its Discontents: The Role of Cytokines in the Pathophysiology of Major Depression (full paper)
Patients with major depression have been found to exhibit increased peripheral blood inflammatory biomarkers, including inflammatory cytokines, which have been shown to access the brain and interact with virtually every pathophysiologic domain known to be involved in depression, including neurotransmitter metabolism, neuroendocrine function, and neural plasticity. Indeed, activation of inflammatory pathways within the brain is believed to contribute to a confluence of decreased neurotrophic support and altered glutamate release/reuptake, as well as oxidative stress, leading to excitotoxicity and loss of glial elements, consistent with neuropathologic findings that characterize depressive disorders.
Somewhere along the way, researchers discovered that some anti-depressants can exert anti-inflammatory effects, for examples of these findings we could look to Fluoxetine and citalopram exhibit potent antiinflammatory activity in human and murine models of rheumatoid arthritis and inhibit toll-like receptors, or Plasma cytokine profiles in depressed patients who fail to respond to selective serotonin reuptake inhibitor therapy, which concludes in part, “Suppression of proinflammatory cytokines does not occur in depressed patients who fail to respond to SSRIs and is necessary for clinical recovery”.
In Investigating the inflammatory phenotype of major depression: focus on cytokines and polyunsaturated fatty acids, the authors report that, “The findings of this study provide further support for the view that major depression is associated with a pro-inflammatory phenotype which at least partially persists when patients become normothymic.” A nice review of the evidence of immunological participation in depression can be found in The concept of depression as a dysfunction of the immune system (full paper).
Moving forward, we can look to schizophrenia, we have similar findings, including Serum levels of IL-6, IL-10 and TNF-a in patients with bipolar disorder and schizophrenia: differences in pro- and anti-inflammatory balance, which observed an imbalanced baseline cytokine profile in the schizophrenic group; findings very similar in form with An activated set point of T-cell and monocyte inflammatory networks in recent-onset schizophrenia patients involves both pro- and anti-inflammatory forces. Similarly, the findings from Dysregulation of chemo-cytokine production in schizophrenic patients versus healthy controls, (full paper) which states, in part:
Growing evidence suggests that specific cytokines and chemokines play a role in signalling the brain to produce neurochemical, neuroendocrine, neuroimmune and behavioural changes. A relationship between inflammation and schizophrenia was supported by abnormal cytokines production, abnormal concentrations of cytokines and cytokine receptors in the blood and cerebrospinal fluid in schizophrenia
Their findings include differentially increased and decreased production of chemokines and cytokines as a result of LPS stimulations in the case group. Of particular note, a similarly dysregulated immune profile of cytokine and chemokine generation has been found in the autism population in several studies.
We also have several trials of immunomodulatory drugs in the schizophrenic arena that further implicate the immune system in pathology, including Adjuvant aspirin therapy reduces symptoms of schizophrenia spectrum disorders: results from a randomized, double-blind, placebo-controlled trial, a ‘gold standard’ trial which found that, “Aspirin given as adjuvant therapy to regular antipsychotic treatment reduces the symptoms of schizophrenia spectrum disorders. The reduction is more pronounced in those with the more altered immune function. Inflammation may constitute a potential new target for antipsychotic drug development”. A similar clinical trial, Celecoxib as adjunctive therapy in schizophrenia: a double-blind, randomized and placebo-controlled trial , another gold standard trial, which also had findings in the same vein, “Although both protocols significantly decreased the score of the positive, negative and general psychopathological symptoms over the trial period, the combination of risperidone and celecoxib showed a significant superiority over risperidone alone in the treatment of positive symptoms, general psychopathology symptoms as well as PANSS total scores.” [Celecoxib is a cox-2 inhibitor; i.e., anti-inflammatory, i.e., immunomodulatory]
What about bi-polar disorder? More of the same, including, The activation of monocyte and T cell networks in patients with bipolar disorder, or Elevation of cerebrospinal fluid interleukin-1ß in bipolar disorder, which reports, in part, “Our findings show an altered brain cytokine profile associated with the manifestation of recent manic/hypomanic episodes in patients with bipolar disorder. Although the causality remains to be established, these findings may suggest a pathophysiological role for IL-1ß in bipolar disorder.”. These studies were published in April and March, 2011, respectively.
Brain tissue from persons with bi-polar disorder also showed increased levels of excitotoxicity and neuroinflammation in Increased excitotoxicity and neuroinflammatory markers in postmortem frontal cortex from bipolar disorder patients (full version), and authors report differential cytokine profiles depending on state of mania, depression, or remission in Comparison of cytokine levels in depressed, manic and euthymic patients with bipolar disorder.
Another disorder based solely around behavior, Tourette syndrome, has increasingly unsurprising findings. Polymorphisms of interleukin 1 gene IL1RN are associated with Tourette syndrome reports “The odds ratio for developing Tourette syndrome in individuals with the IL1RN( *)1 allele, compared with IL1RN( *)2, was 7.65.” (!!!) , and Elevated expression of MCP-1, IL-2 and PTPR-N in basal ganglia of Tourette syndrome cases is yet another example of observations of CNS based immune participation in a disorder that is diagnosed by behavior.
There are also some reviews that perform a cross talk of sorts between disorders; i.e., The mononuclear phagocyte system and its cytokine inflammatory networks in schizophrenia and bipolar disorder, or Immune system to brain signaling: Neuropsychopharmacological implications, published in May 2011, which has this abstract:
There has been an explosion in our knowledge of the pathways and mechanisms by which the immune system can influence the brain and behavior. In the context of inflammation, pro-inflammatory cytokines can access the central nervous system and interact with a cytokine network in the brain to influence virtually every aspect of brain function relevant to behavior including neurotransmitter metabolism, neuroendocrine function, synaptic plasticity, and neurocircuits that regulate mood, motor activity, motivation, anxiety and alarm. Behavioral consequences of these effects of the immune system on the brain include depression, anxiety, fatigue, psychomotor slowing, anorexia, cognitive dysfunction and sleep impairment; symptoms that overlap with those which characterize neuropsychiatric disorders, especially depression. Pathways that appear to be especially important in immune system effects on the brain include the cytokine signaling molecules, p38 mitogen-activated protein kinase and nuclear factor kappa B; indoleamine 2,3 dioxygenase and its downstream metabolites, kynurenine, quinolinic acid and kynurenic acid; the neurotransmitters, serotonin, dopamine and glutamate; and neurocircuits involving the basal ganglia and anterior cingulate cortex. A series of vulnerability factors including aging and obesity as well as chronic stress also appears to interact with immune to brain signaling to exacerbate immunologic contributions to neuropsychiatric disease. The elucidation of the mechanisms by which the immune system influences behavior yields a host of targets for potential therapeutic development as well as informing strategies for the prevention of neuropsychiatric disease in at risk populations.
All of the conditions above, depression, schizophrenia, bi-polar, and tourettes are diagnosed behaviorally; it is only in the last few years that the medical dimension of these disorders were even understood to exist. None of the studies that I referenced above are more than five years old; the idea that behavioral disorders were so closely entangled with the immune system is very, very new. It should be noted that I intentionally left out disorders that also have reams of evidence of immune participation, but which are more degenerative in nature; i.e., Alzheimer’s, ALS, Parkinson’s. When discussing autism, I also left out studies involving aberrant presence of auto-antibodies, of which there are many.
One of the things that I have learned in trying to refine my thought processes during my time on the Internet is that rarely does a single study tell us much about a condition; but the converse also holds true, if we have many studies with different methodologies or measurement end points, but they all reach similar conclusions, then the likely-hood that the findings are accurate is much, much greater. All of the studies I have listed above tell us something similar; that the immune system is clearly, unmistakably playing a part in a lot of conditions classically considered neurological and diagnosed behaviorally. It isn’t enough to nitpick flaws in a single one of the studies in order for ‘correlation does not equal causation’ to make meaningful headway into the implications of these studies; instead, all of the studies above, and lots more, have to be wrong in the same way if we would like to return to a place where we can keep our heads in the sand, hoping for coincidences and bleating out catchphrases in the face of clinical findings. That isn’t going to happen. Given this reality, we should not and cannot ignore the growing evidence of immune abnormalities in the autism population, no matter how inconvenient following that trail of evidence might become.
Hello friends –
I have decidedly mixed feelings on the genetic side of autism research; clearly genetics plays a part, but it does appear that autism has largely mirrored other complicated conditions in that what we thought we were getting when we cracked the genetic code has, for all practical purposes, failed to materialize. To what extent our genetic makeup really plays a part in autism more than any other condition that is currently mystifying us, I don’t think we can say with much certainty; unless you want to count some.
To my mind, one particularly bright spot in the gene realm is the associations of the MET-C allele and an increased risk of an autism diagnosis. At first glance, MET doesn’t seem like a big deal; lots of people have the MET-C mutation, in fact, nearly half of everyone has it. But people with autism have it just a little more frequently, an observation that has been replicated many times. But what is exciting is not only that the MET-C findings are robust, but they can also affect a lot of implicated systems in autism in biologically relevant ways. From an ideological standpoint, the fissure in the autism community about research priorities regarding genetics versus environment, the MET-C studies are a superb example of just how much useful knowledge there is by starting at the genome and working upwards, and finding once we get there that the reality involves lots more than just genes. There is something for everyone!
Getting to the big picture where we can appreciate the beautiful complexity takes a little bit of digging, but it’s worth the effort.
Every now and again you’ll see a period piece about the forties, fifties or sixties, and you’ll get a glimpse of the female operator, someone who would take a call and literally connect two parties together; the gatekeeper. The operator’s actions were binary; either she connected the lines and the call went through, or she didn’t, and nothing happened. Of course, one operator couldn’t connect you to any other phone, but participated in groupings of phones with some logical or functional structure. Ultimately, the operators were the enabler of communication, physically putting two entities into contact to perform whatever business they had with each other.
Within our bodies, tyrosine kinases are enzymes responsible transferring phosphate to proteins; a chemical exchange critical towards a great number of cellular functions, and in a sense, the tyrosine kinases act as cellular operators, helping implement a physical swap of chemicals that ultimately set in motion a great number of processes. Some very rudimentary cellular functions are initiated by the tyrosine kinases; for example, cell division, which is why mutated kinases can lead to the generation of tumors; i.e., the signaling for cell division gets turned on, and never gets turned off. Inhibiting tyrosine kinases is the mechanism of action for some drugs that target cancer.
The MET gene is responsible for creating the MET receptor tyrosine kinase. This particular receptor is involved in lots of processes that are of great interest to autism; the MET receptor is expressed heavily during embryogenesis in the brain, has immune modulating capacities, and is associated with wound healing, and is particularly implicated in repair of the gastro-intestinal track.
Kinases don’t just fire away, shuttling phosphates around any old time, they must be activated by a triggering molecule, or a ligand. There is only one known ligand for the MET receptor; hepatocyte growth factor, or HGF (also sometimes referred to as HGF/SF, or hepatocyte growth factor/scatter factor). We’ll get to why we bother worrying about HGF a little later on, but it is important to keep in mind that without HGF, the functions affected by the MET-C receptor, early brain development, immune modulating, and wound repair cannot be achieved.
So what about autism, and why is it a beautiful illustration of complexity? Walking our way through the MET findings in autism is a rewarding task; it is one of the few instances I’ve seen where the glimpses of relevance gleaned from straight genetic studies have been incrementally built upon to achieve a much grander understanding of autism. This is the kind of thing that I think a lot of people who dismiss the utility of genetic studies are missing; genetics are only the first piece of the puzzle, it doesn’t only implicate genes, it tells us about the processes and the proteins disturbed in autism; and with that knowledge, we can perform targeted analysis for environmental participants.
The first clues about MET involvement with autism came in 2006, when A genetic variant that disrupts MET transcription is associated with autism (full paper) was published. The abstract is longish, but here is a snipet:
MET signaling participates in neocortical and cerebellar growth and maturation, immune function, and gastrointestinal repair, consistent with reported medical complications in some children with autism. Here, we show genetic association (P = 0.0005) of a common C allele in the promoter region of the MET gene in 204 autism families. The allelic association at this MET variant was confirmed in a replication sample of 539 autism families (P = 0.001) and in the combined sample (P = 0.000005). Multiplex families, in which more than one child has autism, exhibited the strongest allelic association (P = 0.000007).
I appreciate the pleiotropic nature of what we are seeing here, a gene that is involved with brain growth and maturation, immune function, and GI repair. The association in ‘multiplex’ (i.e., families with more than one child with autism) was very, very strong. Even still, this was a pretty short paper, and it was all genetics. Coolness factor: 3.
Neater studies were on the horizon shortly thereafter, a year later, some of the same group looked for expression of MET in post mortem brain tissue and found significantly decreased levels of MET protein in Disruption of cerebral cortex MET signaling in autism spectrum disorder.
MET protein levels were significantly decreased in ASD cases compared with control subjects. This was accompanied in ASD brains by increased messenger RNA expression for proteins involved in regulating MET signaling activity. Analyses of coexpression of MET and HGF demonstrated a positive correlation in control subjects that was disrupted in ASD cases.
This is a nice follow up; lots of times a genetic study might suggest a hit, but we really don’t even know how such a genetic change might manifest physiologically, like having a jigsaw puzzle of solid black and finding two pieces that fit together. In those instances, we can’t really go looking for different levels of the protein, so there you are. In this case, the authors found an allele worth investigating, and then went looking to see if relevant proteins were altered in the population, and in the CNS no less! Not only that, but they also looked at the initiating end of the process, the ligand, HGF, and found abnormalities. Good stuff. Unfortunately, I haven’t found myself a copy of this paper yet, but the fact that other proteins in the pathway were altered is another line of evidence that something is amiss. I’ve begun to appreciate the fact that I have spent a long time under appreciating the interconnectedness of biological systems; you aren’t going to have a disturbance in one system without altering the way upstream, and downstream processes are working; so the fact that we see other proteins, those related to MET functions, modified, makes beautiful sense. Coolness factor: 5.
Likely because of the mixed findings of skewed proteins in the MET pathway (?), the next study in line is, Genetic Evidence Implicating Multiple Genes in the MET Receptor Tyrosine Kinase Pathway in Autism Spectrum Disorder (full paper available). Here’s the abstract:
A functional promoter variant of the gene encoding the MET receptor tyrosine kinase alters SP1 and SUB1 transcription factor binding, and is associated with autism spectrum disorder (ASD). Recent analyses of postmortem cerebral cortex from ASD patients revealed altered expression of MET protein and three transcripts encoding proteins that regulate MET signaling, hepatocyte growth factor (HGF), urokinase plasminogen activator receptor (PLAUR) and plasminogen activator inhibitor-1 (SERPINE1). To address potential risk conferred by multiple genes in the MET signaling pathway, we screened all exons and 5′ promoter regions for variants in the five genes encoding proteins that regulate MET expression and activity. Identified variants were genotyped in 664 families (2,712 individuals including 1,228 with ASD) and 312 unrelated controls. Replicating our initial findings, family-based association test (FBAT) analyses demonstrated that the MET promoter variant rs1858830 C allele was associated with ASD in 101 new families (P=0.033). Two other genes in the MET signaling pathway also may confer risk. A haplotype of the SERPINE1 gene exhibited significant association. In addition, the PLAUR promoter variant rs344781 T allele was associated with ASD by both FBAT (P=0.006) and case-control analyses (P=0.007). The PLAUR promoter rs344781 relative risk was 1.93 (95% Confidence Interval [CI]: 1.12−3.31) for genotype TT and 2.42 (95% CI: 1.38−4.25) for genotype CT compared to genotype CC. Gene-gene interaction analyses suggested a significant interaction between MET and PLAUR. These data further support our hypothesis that genetic susceptibility impacting multiple components of the MET signaling pathway contributes to ASD risk.
We’ve got two new genes added to the mix, PLAUR and SERPINE. The juicy part here is that the authors didn’t look for these variants at random, but performed a targeted search; they knew that the proteins encoded by these genes interact with either MET receptor function or HGF, and they also had found altered expression of these genes in the CNS study. From the Introduction:
The hepatocyte growth factor (HGF) gene encodes the activating ligand for the MET receptor. HGF is translated as an inactive precursor protein that requires cleavage for efficient binding to the MET receptor [Lokker et al 1992]. The activating cleavage of HGF is achieved most efficiently by the enzyme plasminogen activator (urokinase-type; uPA; gene symbol: PLAU) under conditions in which uPA binds to its receptor, the urokinase plasminogen activator receptor (uPAR; gene symbol: PLAUR). Activating cleavage of HGF can be suppressed by the plasminogen activator inhibitor-1 (PAI-1; gene symbol: SERPINE1). Together, these proteins regulate the activity of MET receptor tyrosine kinase signaling, and our recent microarray analyses of postmortem temporal lobe of individuals with ASD indicate that disrupted MET signaling may be common to ASD pathophysiology [Campbell et al 2007]. For example, we found that there is increased expression of the HGF, PLAUR and SERPINE1 transcripts in ASD in postmortem cerebral cortex. The observation of disrupted expression suggests a general dysfunction of MET signaling in the cerebral cortex of individuals with ASD.
The proteins encoded by PLAUR and SERPINE were also found increased in the expression study; a finding further supported by the genetic study here. The really grand slice here is that the SERPINE protein suppresses cleavage of HGF; essentially another way MET function can be affected, from a disturbance upstream of HGF binding. In other words, more SERPINE (possibly as a result of a ‘promoter allele’) would result in less MET receptor activation because the SERPINE interferes with the cleavage of HGF, and thus, another pathway to reduced MET activation. In a finding that seems 20/20 with hindsight, a functional promoter of the SERPINE gene was found to increase autism risk; i.e., if you have more SERPINE, you get less functional HGF, and therefore less triggering of the MET receptor. This is cool and begins a portrait of the complexity; it shows how the effect of reduced MET functionality can come from multiple drivers; the reduced MET allele, or, the promoter SERPINE allele, and what’s more, having both is an even bigger risk; the authors are describing a synergy of low penetrance genes.
From the discussion section of the paper:
Beyond genetic susceptibility, the functional integrity of the MET signaling system also is sensitive to environmental factors. This concept is supported by bioinformatics analyses that identified PLAUR, SERPINE1 and HGF as genes active in immune response regulation, sensitive to environmental exposures, and within chromosomal regions previously implicated in ASD linkage studies [Herbert et al 2006]. Moreover, a recent cell biological study shows that chemically diverse toxicants reduce the expression of MET in oligodendrocyte progenitor cells, a result that is interpreted as the convergence of toxicant effects on oxidative status and the MET-regulating Fyn/c-Cbl pathway
Here are links to the Hebert paper, Autism and environmental genomics, and the Li paper, Chemically Diverse Toxicants Converge on Fyn and c-Cbl to Disrupt Precursor Cell Function. What is neat here is that we are starting to be able to see a pathway of genes, and resultant proteins, that can effect disparate systems. I believe that there is a subset of acupuncture, acupressure that relies on more knuckles than needles, and while the science on accu* based therapies isn’t very good, it does occur to me that in a sense, our lattice work of HGF-PLAUR-SERPINE proteins that participate in the MET-C process are pressure points in a delicate system, push a little bit and things will bend down the line accordingly. It also exemplifies why I am offended by highly negative attitudes on genetic studies held by people who believe in a non trivial, environmentally mediated increase in the rates of autism; we are approaching a nearly impossible to overturn reality that genes we know to be associated with autism are particularly sensitive to interference from environmental agents, and participate in immune function. That is important information. Coolness factor 8. First glimpse of beauty factor: 10.
The establishment of appropriate neural circuitry depends upon the coordination of multiple developmental events across space and time. These events include proliferation, migration, differentiation, and survival – all of which can be mediated by hepatocyte growth factor (HGF) signaling through the Met receptor tyrosine kinase. We previously found a functional promoter variant of the MET gene to be associated with autism spectrum disorder, suggesting that forebrain circuits governing social and emotional function may be especially vulnerable to developmental disruptions in HGF/Met signaling. However, little is known about the spatiotemporal distribution of Met expression in the forebrain during the development of such circuits. To advance our understanding of the neurodevelopmental influences of Met activation, we employed complementary Western blotting, in situ hybridization and immunohistochemistry to comprehensively map Met transcript and protein expression throughout perinatal and postnatal development of the mouse forebrain. Our studies reveal complex and dynamic spatiotemporal patterns of expression during this period. Spatially, Met transcript is localized primarily to specific populations of projection neurons within the neocortex and in structures of the limbic system, including the amygdala, hippocampus and septum. Met protein appears to be principally located in axon tracts. Temporally, peak expression of transcript and protein occurs during the second postnatal week. This period is characterized by extensive neurite outgrowth and synaptogenesis, supporting a role for the receptor in these processes. Collectively, these data suggest that Met signaling may be necessary for the appropriate wiring of forebrain circuits with particular relevance to social and emotional dimensions of behavior.
Coooooool. Here we touch on the complexity of brain formation, all the little things that need to go exactly right, and how MET might play a role in that incredibly complicated dance. Even better, a mouse model is used to gain an understanding of where and when peak expression of MET proteins occur, a period of significant changes to neural structures and the formation of synapses, the physical structures that enable thought. This is a dense paper, too dense to get deeply into blockquoting for this posting, but there are some parts that deserve notice, namely, documentation of spatially localized MET expression in brain areas associated with social behaviors and some fine grained information on the specific parts of synapse formation that utilize MET. Coolness factor: 8. Complexity Factor: 12.
Here is a paper that a lot of people that play skeptics on the Internet ought to hate, Distinct genetic risk based on association of MET in families with co-occurring autism and gastrointestinal conditions. (full paper)
In the entire 214-family sample, the MET rs1858830 C allele was associated with both autism spectrum disorder and gastrointestinal conditions. Stratification by the presence of gastrointestinal conditions revealed that the MET C allele was associated with both autism spectrum disorder and gastrointestinal conditions in 118 families containing at least 1 child with co-occurring autism spectrum disorder and gastrointestinal conditions. In contrast, there was no association of the MET polymorphism with autism spectrum disorder in the 96 families lacking a child with co-occurring autism spectrum disorder and gastrointestinal conditions. chi(2) analyses of MET rs1858830 genotypes indicated over-representation of the C allele in individuals with co-occurring autism spectrum disorder and gastrointestinal conditions compared with non-autism spectrum disorder siblings, parents, and unrelated controls.
There is a lot of caution in this paper, but the nice part is that there are biologically plausible mechanisms by which a reduction in MET could snowball into problems in the gastro-intestinal track.
In the gastrointestinal system, MET signaling modulates intestinal epithelial cell proliferation, and thus acts as a critical factor in intestinal wound healing. For example, activation of MET signaling via application of exogenous hepatocyte growth factor has been shown to reduce the effects of experimentally induced colitis, inflammatory bowel disease, and diarrhea.
Pushing on the other end of the balloon, increasing MET signaling, has been shown to help GI problems; no less than evidence that a genetic change associated with autism has biologically plausible mechanisms by which GI problems would be more prevalent. In fact, unless our findings of MET alleles are in error, or our clinical findings of the effects of HGF are spurious, it is absolutely expected. There is also a section with the startlingly simple, and simultaneously great idea of why findings like these might be useful markers for phenotypic categorization in studies in the future; i.e., to discern the prevalence of GI problems in autism, it might, for example, make sense to design that study to take presence or absence of MET alleles into consideration. Nice. Coolness Factor: 7. Insidiousness factor: 9.
Here’s another one that found associations with MET and social behavior, and GI disturbances again. Association of MET with social and communication phenotypes in individuals with autism spectrum disorder
Autism is a complex neurodevelopmental disorder diagnosed by impairments in social interaction, communication, and behavioral flexibility. Autism is highly heritable, but it is not known whether a genetic risk factor contributes to all three core domains of the disorder or autism results from the confluence of multiple genetic risk factors for each domain. We and others reported previously association of variants in the gene encoding the MET receptor tyrosine kinase in five independent samples. We further described enriched association of the MET promoter variant rs1858830 C allele in families with co-occurring autism and gastrointestinal conditions. To test the contribution of this functional MET promoter variant to the domains of autism, we analyzed its association with quantitative scores derived from three instruments used to diagnose and describe autism phenotypes: the Autism Diagnostic Interview-Revised (ADI-R), the Autism Diagnostic Observation Schedule (ADOS), and both the parent and the teacher report forms of the Social Responsiveness Scale (SRS). In 748 individuals from 367 families, the transmission of the MET C allele from parent to child was consistently associated with both social and communication phenotypes of autism. Stratification by gastrointestinal conditions revealed a similar pattern of association with both social and communication phenotypes in 242 individuals with autism from 118 families with co-occurring gastrointestinal conditions, but a lack of association with any domain in 181 individuals from 96 families with ASD and no co-occurring gastrointestinal condition. These data indicate that the MET C allele influences at least two of the three domains of the autism triad.
Really sort of plain, but very nice to see the GI component validated in another data set. Coolness factor 5.
Then a few months ago, Prenatal polycyclic aromatic hydrocarbon exposure leads to behavioral deficits and downregulation of receptor tyrosine kinase, MET was released, an uber cool showcase of the autism bigfoot, the often regaled, only very rarely documented, gene/environment interaction.
Gene by environment interactions (G × E) are thought to underlie neurodevelopmental disorder, etiology, neurodegenerative disorders, including the multiple forms of autism spectrum disorder. However, there is limited biological information, indicating an interaction between specific genes and environmental components. The present study focuses on a major component of airborne pollutants, polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene [B(a)P], which negatively impacts cognitive development in children who have been exposed in utero. In our study, prenatal exposure of Cpr(lox/lox) timed-pregnant dams to B(a)P (0, 150, 300, and 600 μg/kg body weight via oral gavage) on embryonic day (E14-E17) consistent with our susceptibility-exposure paradigm was combined with the analysis of a replicated autism risk gene, the receptor tyrosine kinase, Met. The results demonstrate a dose-dependent increase in B(a)P metabolite generation in B(a)P-exposed Cpr(lox/lox) offspring. Additionally, a sustained persistence of hydroxy metabolites during the onset of synapse formation was noted, corresponding to the peak of Met expression. Prenatal B(a)P exposure also downregulated Met RNA and protein levels and dysregulated normal temporal patterns of expression during synaptogenesis. Consistent with these data, transcriptional cell-based assays demonstrated that B(a)P exposure directly reduces human MET promoter activity. Furthermore, a functional readout of in utero B(a)P exposure showed a robust reduction in novel object discrimination in B(a)P-exposed Cpr(lox/lox) offspring. These results confirm the notion that common pollutants, such as the PAH B(a)P, can have a direct negative impact on the regulated developmental expression of an autism risk gene with associated negative behavioral learning and memory outcomes.
Oh snap. A common pollutant (well, common in the last few decades anyways), is shown to interact with MET in a dose dependent fashion to reduce protein expression in the brain during embryonic development and cause ‘a robust reduction in novel object discrimination’. (Ouch) This is an example of just what we mentioned above, referenced Herbert, concerning the possibility of MET as a gene sensitive to ‘environmental exposures’. Indeed. From the discussions section:
The results from the present study demonstrate that the transcription and developmental expression patterns of a replicated ASD risk gene, MET, are highly sensitive to a common PAH pollutant. In utero exposure to B(a)P produces an oxidative milieu of B(a)P metabolites in offspring during a key postnatal period of synapse development, providing evidence that environmental exposure creates a sustained cerebral cortical burden that likely contributes to an increased oxidative load. Oxidative stressors in the form of metabolites would be expected to negatively impact gene expression (Kerzee and Ramos 2000) and, more specifically, receptor tyrosine kinase function, including Met (Li et al. 2007). These data suggest that B(a)P-induced exposure would impact the expression of key neurodevelopmental genes, including Met. Additionally, the predominance of the 3-OH and 9-OH metabolites places a sustained burden in the brain because of the potential for further oxidization to form B(a)P quinones (McCallister et al. 2008, Hood et al. 2000, Brown et al. 2007) which undergo redox cycling to generate reactive oxygen species (Kerzee and Ramos 2000, Bolton et al., 2000).
In conclusion, specific developmental events such as glutamatergic excitatory synapse formation and maturation may be particularly vulnerable to G x E effects that impact regulatory and signaling proteins involved in this process. While we do not suggest that the current study reflects specific defects related to a complex clinical condition such as the ASDs, current molecular, behavioral and functional imaging data are converging on the concept that the ASDs are a manifestation of altered local and long-distance cortical connectivity (Geschwind et al. 2007, Bill and Geschwind 2009, Geschwind and Levitt, 2007, Levitt and Campbell 2009). Also, Met and other related signaling components of this receptor tyrosine kinase pathway have been implicated in both syndromic and idiopathic disorders where the ASDs are diagnosed at a high rate. In combination with risk alleles in key genes, the in utero exposure to PAHs such as B(a)P, which results in both a reduction in absolute levels and the mistiming of peak Met expression, could drive the system toward a pathophysiological threshold that neither genetic risk nor environmental factors could produce individually. The present study focused on the neocortex, but given the highly restricted spatial and temporal expression of Met in mouse limbic circuits associated with social-emotional development and cognition (Judson et al. 2009), it is likely that perturbations occur throughout these key circuits, including in the hippocampus.
Really cool stuff; particularly the finding that developmental, in utero exposure was capable of driving abnormal protein expression well after birth. This is the best of both sides of the genetics versus environment conundrum; the kind of finding that sheds light on how environmental pollutants could be participating in increasing the number of children with autism by interacting with genetically susceptible children. But what I love about this is that it is the death knell of the fairytale of a static rate, or near static rate of autism, just having the genes or the exposure isn’t enough; instead, the interaction of alleles and timed exposure ‘could drive the system toward a pathophysiolical threshold that neither genetic risk nor environmental factors could produce individually’. I think there are some more findings coming from this group soon that might be exciting, or terrifying, depending on how you see it. (or both). Coolness factor: 99.
So what have we learned and just how cool is it?
1) The MET receptor enables some types of cellular signaling that have relevance to the autism community including synapse formation, immune modulation, and gastro intestinal function. The ligand, or trigger of the MET receptor is HGF.
2) Certain alleles of the MET gene that result in decreased expression are more common in children with autism than people without autism.
3) Consistent with findings of increased prevalence of MET alleles, MET protein expression was found to be decreased in brain tissue from people with autism. Other, related proteins, HGF, PLAUR, and SERPINE were also found to be disturbed.
4) Following up on the differential findings of SERPINE and PLAUR, genetic studies found gene to gene interactions between the MET allele and alleles involved with production of SERPINE and PLAUR. Some of the proteins in question are known to be particularly vulnerable to environmental interference.
5) Animal models tell us that MET is heavily expressed in many areas of the mammalian brain during prenatal and postnatal development, and we gain insight into the spatial and temporal expression of MET during the intricate dance of brain formation.
6) Two studies add evidence that the one function of decreased MET expression, GI disturbances, are indeed found with greater consistency within children with autism and the MET allele. This should be a relatively unsurprising finding considering what we know about MET and children with autism.
7) Finally, a portrait of genetic / environmental interactions capable of disturbing physiology and behavior in ways consistent with findings in autism is rendered using an agent that is the product of the automobile age and already associated with decreased cognitive skills for groups with the highest gestational exposure.
It should be noted that this is just a slice of the MET papers out there in the autism realm; they all shared one or more authors, I picked them because they seem to show a nice progression of knowledge, and incremental approach towards learning more. There is a lof more to learn, in particular, I think that the immune modulating effects of reduced expression would be an interesting subject, but one that will have to wait for another posting.
Increasingly Unsurprising Findings – Microglial Activation and Increased Microglial Density Observed in the Dorsolateral Prefrontal Cortex in Autism – With Bonus Theoretical Pontifications
Posted August 22, 2010on:
Hello friends –
A new paper looking for evidence of an ongoing immune reaction in the brain of people with autism landed the other day, Microglial Activation and Increased Microglial Density Observed in the Dorsolateral Prefrontal Cortex in Autism
BACKGROUND: In the neurodevelopmental disorder autism, several neuroimmune abnormalities have been reported. However, it is unknown whether microglial somal volume or density are altered in the cortex and whether any alteration is associated with age or other potential covariates. METHODS: Microglia in sections from the dorsolateral prefrontal cortex of nonmacrencephalic male cases with autism (n = 13) and control cases (n = 9) were visualized via ionized calcium binding adapter molecule 1 immunohistochemistry. In addition to a neuropathological assessment, microglial cell density was stereologically estimated via optical fractionator and average somal volume was quantified via isotropic nucleator. RESULTS: Microglia appeared markedly activated in 5 of 13 cases with autism, including 2 of 3 under age 6, and marginally activated in an additional 4 of 13 cases. Morphological alterations included somal enlargement, process retraction and thickening, and extension of filopodia from processes. Average microglial somal volume was significantly increased in white matter (p = .013), with a trend in gray matter (p = .098). Microglial cell density was increased in gray matter (p = .002). Seizure history did not influence any activation measure. CONCLUSIONS: The activation profile described represents a neuropathological alteration in a sizeable fraction of cases with autism. Given its early presence, microglial activation may play a central role in the pathogenesis of autism in a substantial proportion of patients. Alternatively, activation may represent a response of the innate neuroimmune system to synaptic, neuronal, or neuronal network disturbances, or reflect genetic and/or environmental abnormalities impacting multiple cellular populations.
This is a neat paper, to my eye not as comprehensive as the landmark paper on microglial activation, Neuroglial Activation and Neuroinflammation in the Brain of Patients with Autism Neuroglial Activation andNeuroinflammation in the Brain of Patientswith Autism, but still a very interesting read. Here are the some areas that caught my eye. From the introduction:
These results provide evidence for microglial activation in autism but stop short of demonstrating quantifiable microglial abnormalities in the cortex, as well as determining the nature of these abnormalities. Somal volume increases are often observed during microglial activation, reflecting a shift toward an amoeboid morphology that is accompanied by retraction and thickening of processes (13). Microglial density may also increase, reflecting either proliferation of resident microglia or increased trafficking of macrophages across a blood-brain barrier opened in response to signaling by cytokines, chemokines, and other immune mediators (13–16). These results provide evidence for microglial activation in autism but stop short of demonstrating quantifiable microglial abnormalitiesin the cortex, as well as determining the nature of these abnormalities. Somal volume increases are often observed during microglial activation, reflecting a shift toward an amoeboid morphology that is accompanied by retraction and thickening ofprocesses (13). Microglial density may also increase, reflecting either proliferation of resident microglia or increased trafficking of macrophages across a blood-brain barrier opened in response to signaling by cytokines, chemokines, and other immune mediators(13–16).
Tragically, my ongoing google based degree in neurology has yet to cover the chapters on specific brain geography, so the finer points, such as the difference between the middle frontal gyri and the neocortex are lost on me. None the less, several things jump out at me from what I have managed to understand so far. The shift to an ‘ameboid morphology’ is one that I’ve run into previously, notably in Early-life programming of later-life brain and behavior: a critical role for the immune system, which is a paper I really need to dedicate an entire post towards, but as applicable here, the general idea is that the microglia undergo structural and functional changes during times of immune response; the ‘ameboid’ morphology is associated with an active immune response. Regarding increased trafficking of macrophages across the BBB, Vargas 2005 noted chemokines (MCP-1) increases in the CNS, so we do have reason to believe such signalling molecules are present.
The authors went on to look for structural changes in microglia, differences in concentration of microglia, and evaluated for markers indicative of an acute inflammatory response. Measurements such as grey and white matter volumes and relationships to microglia structural differences, and correlations with seizure activity were also performed. There were three specimens from children under the age of six that were analyzed as a subgroup to determine if immune activation was present at early ages. From the discussion section:
Moderate to strong alterations in Iba-1 positive microglial morphology indicative of activation (13,29) are present in 5 of 13 postmortem cases with autism, and mild alterations are present in an additional 4 of 13 cases. These alterations are reflected in a significant increase in average microglial somal volume in white matter and microglial density in gray matter, as well as a trend in microglial somal volume in gray matter. These observations appear to reflect a relatively frequent occurrence of cortical microglial activation in autism.
Of particular interest are the alterations present in two thirds of our youngest cases, during a period of early brain overgrowth in the disorder. Indeed, neither microglial somal volume nor density showed significant correlation with age in autism, suggesting long running alteration that is in striking contrast with neuronal features examined in the same cases (Morgan et al., unpublished data, 2009). The early presence of microglial activation indicates it may play a central pathogenic role in some patients with autism.
The authors evaluated for IL-1R1 receptor presence, essentially a marker for an inflammatory response, and found that the values did not differ between the autism population and controls, and that in fact the controls trended towards expressing more IL-1R1 than the autism group. I think this was the opposite of what the authors expected to find.
While Iba-1 staining intensity increases modestly in activated microglia (30), strong staining and fine detail were apparent in Iba-1 positive resting microglia in our samples. Second, there is no increase in microglial colocalization with a receptor, IL-1R1, typically upregulated in acute inflammatory reactions (28). The trend toward an increase in colocalization in control cases may also hint at downregulation of inflammatory signal receptors in a chronically activated system.
I don’t think I’ve seen this type of detail in qualitative measures of the neuroimmune response in autism measured previously, so I definitely appreciate the detail. Furthermore, from a more speculative standpoint, we may have some thoughts on why we might see this in the autism population specifically that I’ll go into detail below a little bit.
The authors failed to find a relationship between seizure activity and microglial activation, which came as a surprise to me, to tell the truth. Also discussed was the large degree of heterogeneity in the findings in so far as the type and severity of microglial morphological differences observed. The potential confounds in the study included an inability to control for medication history, and the cause of death, eight of which were drowning in the autism cases. There was some discussion of potential causes, including, of course, gene-environment interactions, maternal immune activation, neural antibodies, and the idea that “chronic innate immune system activation might gradually produce autoimmune antibodies via the occasional presentation of brain proteins as antigens” (!) There was also this snipet:
Microglial activation might also represent an aberrant event during embryonic monocyte infiltration that may or may not also be reflected in astroglial and neuronal populations (17), given the largely or entirely separate developmental lineage of microglia (13). Alternatively, alterations might reflect an innate neuroimmune response to events in the brain such as excessive early neuron generation or aberrant development of neuronal connectivity.
There is a short discussion of the possible effects of an ongoing microglial immune response, including damage to neural cells, reductions in cells such as Purkinjes, and increases in neurotrophic factors such as BDNF.
This is another illustration of an ongoing immune response in the CNS of the autism population, though in this instance, only some of the treatment group appeared to be affected. It would have been nice to see if there were correlations between behavioral severity and/or specific behavior types, but it would seem that this information is was not available in sufficient quality for this type of analysis, which is likely going to be an ongoing problem with post mortem studies for some time to come. I believe that an effort to develop an autism tissue bank is underway, perhaps eventually some of these logistical problems will be easier to address. The fact that some of the samples were from very young children provides evidence that when present, the neuroinflammatory response is chronic, and indeed, likely lifelong.
Stepping away from the paper proper, I had some thoughts about some of these findings that are difficult to defend with more than a skeletal framework, but have been rattling around my head for a little while. Before we move forward, let’s be clear on a couple of things:
1) The jump from rodent to human is fraught with complications, most of which I doubt we even understand.
2) We can’t be positive that an activated neuroimmune system is the cause of autistic behaviors, as opposed to a result of having autism. I still think a very strong argument can be made that an ongoing immune response is ultimately detrimental, even if it cannot be proven to be completely responsible for the behavioral manifestation of autism.
3) At the end of the day, I’m just Some Jerk On The Internet.
Those caveats made, Morgan et all spend a little time on the potential cause of a persistent neuroinflammatory state as referenced above. One of the ideas, “an aberrant event during embyronic monocyte infiltration that may or may not also be reflected in astroglial and neuronal populations given the largely or entirely separate developmental lineage of microglia” struck me as particularly salient when considered alongside the multitude of data we have concerning the difficult to predict findings regarding an immune insult during critical developmental timeframes.
We now have several papers that dig deeper into the mechanism by which immune interaction during development seem to have physiological effects with some parallels to autism; specifically, Enduring consequences of early-life infection on glial and neural cell genesis within cognitive regions of the brain (Bland et all), and Early-Life Programming of Later-Life Brain and Behavior: A Critical Role for the Immune System (Bilbo et all) ; both of which share Staci Bilbo as an author and I think she is seriously onto something. Here is the abstract for Bland et all:
Systemic infection with Escherichia coli on postnatal day (P) 4 in rats results in significantly altered brain cytokine responses and behavioral changes in adulthood, but only in response to a subsequent immune challenge with lipopolysaccharide [LPS]. The basis for these changes may be long-term changes in glial cell function. We assessed glial and neural cell genesis in the hippocampus, parietal cortex (PAR), and pre-frontal cortex (PFC), in neonates just after the infection, as well as in adulthood in response to LPS. E. coli increased the number of newborn microglia within the hippocampus and PAR compared to controls. The total number of microglia was also significantly increased in E. coli-treated pups, with a concomitant decrease in total proliferation. On P33, there were large decreases in numbers of cells coexpressing BrdU and NeuN in all brain regions of E. coli rats compared to controls. In adulthood, basal neurogenesis within the dentate gyrus (DG) did not differ between groups; however, in response to LPS, there was a decrease in neurogenesis in early-infected rats, but an increase in controls to the same challenge. There were also significantly more microglia in the adult DG of early-infected rats, although microglial proliferation in response to LPS was increased in controls. Taken together, we have provided evidence that systemic infection with E. coli early in life has significant, enduring consequences for brain development and subsequent adult function. These changes include marked alterations in glia, as well as influences on neurogenesis in brain regions important for cognition.
Bland et all went on to theorize on the mechanism by which an infection in early life can have such long lasting effects.
We have hypothesized that the basis for this vulnerability may be long-term changes in glial cell function. Microglia are the primary cytokine producers within the brain, and are an excellent candidate for long-term changes, because they are long-lived and can become and remain activated chronically (Town et al., 2005). There is increasing support for the concept of ‘‘glial priming”, in which cells can become sensitized by an insult, challenge, or injury, such that subsequent responses to a challenge are exaggerated (Perry et al., 2003).
The authors infected some rodents with e-coli on postnatal day four, and then evaluated for microglial function in adulthood.
We have hypothesized that the basis for early-life infection-induced vulnerability to altered cytokine expression and cognitive deficits in adulthood may be due to long-term changes in glial cell function and/or influences on subsequent neural development. E. coli infection on P4 markedly increased microglial proliferation in the CA regions of the hippocampus and PAR of newborn pups, compared to a PBS injection (Figs. 3 and 4). The total number of microglia, and specifically microglia with an ‘‘active” morphology (amoeboid, with thick processes), were also increased as a consence of infection. There was a concomitant decrease in non microglial newborn cells (BrdU + only) in the early-infected rats, in the same regions.
Check that shit out! Rodents infected with E-coli during the neonatal period had an increased number of active microglia when compared to rodents that got saline as neonates. Keep in mind that the backbone of these studies, and studies from other groups indicate that this persistence of effects are not specific to an e-coli infection, but rather, can be triggered by any immune response during critical timeframes. In fact, at least two studies have employed anti-inflammatory agents, and observed an attenuation of effect regarding seizure susceptibility.
A final snipet from Bland et all Discussion section:
Although the mechanisms remain largely unknown, the ‘‘glial cell priming” hypothesis posits that these cells have the capacity to become chronically sensitized by an inflammatory event within the brain (Perry et al., 2003). We assessed whether glial priming may be a likely factor in the current study by measuring the volume of each counted microglial cell within our stereological analysis. The morphology of primed glial cells is similar to that of ‘‘activated” cells (e.g., amoeboid, phagocytic), but primed glial cells do not chronically produce cytokines and other pro-inflammatory mediators typical of cells in an activated state. There was a striking increase in cell volume within the CA1 region of adult rats infected as neonates (Figs. 2 and 8), the same region in which a marked increase in newborn glia was observed at P6. These data are consistent with the hypothesis that an inflammatory environment early in life may prime the surviving cells long-term, such that they over-respond to a second challenge, which we have demonstrated at the mRNA level in previous studies (Bilbo et al., 2005a, 2007; Bilbo and Schwarz, in press).
The concept of glial priming, close friends with the ‘two hit’ hypothesis (or soon to be, the multi-hit hypothesis?), has some other very neat studies behind it, the coolest ones I’ve found so far are from a group at Northwestern, and include “hits” such as Glial activation links early-life seizures and long-term neurologic dysfunction: evidence using a small molecule inhibitor of proinflammatory cytokine upregulation, Enhanced microglial activation and proinflammatory cytokine upregulation are linked to increased susceptibility to seizures and neurologic injury in a ‘two-hit’ seizure model and Minozac treatment prevents increased seizure susceptibility in a mouse “two-hit” model of closed skull traumatic brain injury and electroconvulsive shock-induced seizures. Also the tragically, hilariously titled, Neonatal lipopolysaccharide and adult stress exposure predisposes rats to anxiety-like behaviour and blunted corticosterone responses: implications for the double-hit hypothesis. (!) These are potentially very inconvenient findings, the details for which I’ll save for another post.
Moving on to Bilbo et all, though a pure review paper than an experiment, it provides additional detailed theories on the mechanisms behind persistent effects of early life immune challenge. Here’s the abstract:
The immune system is well characterized for its critical role in host defense. Far beyond this limited role however, there is mounting evidence for the vital role the immune system plays within the brain, in both normal, “homeostatic” processes (e.g., sleep, metabolism, memory), as well as in pathology, when the dysregulation of immune molecules may occur. This recognition is especially critical in the area of brain development. Microglia and astrocytes, the primary immunocompetent cells of the CNS, are involved in every major aspect of brain development and function, including synaptogenesis, apoptosis, and angiogenesis. Cytokines such as tumor necrosis factor (TNF)α, interleukin [IL]-1β, and IL-6 are produced by glia within the CNS, and are implicated in synaptic formation and scaling, long-term potentiation, and neurogenesis. Importantly, cytokines are involved in both injury and repair, and the conditions underlying these distinct outcomes are under intense investigation and debate. Evidence from both animal and human studies implicates the immune system in a number of disorders with known or suspected developmental origins, including schizophrenia, anxiety/depression, and cognitive dysfunction. We review the evidence that infection during the perinatal period of life acts as a vulnerability factor for later-life alterations in cytokine production, and marked changes in cognitive and affective behaviors throughout the remainder of the lifespan. We also discuss the hypothesis that long-term changes in brain glial cell function underlie this vulnerability.
Bilbo et all go on to discuss the potential for time sensitive insults that could result in an altered microglial function. Anyone that has been paying attention should know that the concept of time dependent effects is, to my mind, the biggest blind spot in our existing research concerning autism and everyones favorite environmental agent.
Is there a sensitive period? Does an immune challenge early in life influence brain and behavior in a way that depends on developmental processes? Since 2000 alone, there have been numerous reports in the animal literature of perinatal immune challenges ranging from early gestation to the juvenile period, and their consequences for adult offspring phenotypes (see Table 1). It is clear that the timing of a challenge is likely a critical factor for later outcomes, impacting the distinct developmental time courses of different brain regions and their underlying mechanisms (e.g., neurotransmitter system development, synapse formation, glial and neural cell genesis, etc; Herlenius and Lagercrantz, 2004; Stead et al., 2006). However, the original question of whether these changes depend on development has been surprisingly little addressed. We have demonstrated that infection on P30 does not result in memory impairments later in life (Bilbo et al., 2006), nor does it induce the long-term changes in glial activation and cytokine expression observed with a P4 infection (Bilbo et al., unpublished data). The factors defining this “sensitive period” are undoubtedly many, as suggested above. However, our working hypothesis is that one primary reason the early postnatal period in rats is a sensitive or critical period for later-life vulnerabilities to immune stimuli, is because the glia themselves are functionally different at this time. Several studies have demonstrated that amoeboid, “macrophage-like”, microglia first appear in the rat brain no earlier than E14, and steadily increase in density until about P7. By P15 they have largely transitioned to a ramified, adult morphology. Thus, the peak in density and amoeboid morphology (and function) occurs within the first postnatal week, with slight variability depending on brain region (Giulian et al., 1988; Wu et al., 1992). [emphasis theirs]
[Note: The authors go on to state that this time period is likely developmentally equivalent to the late second, to early third trimester of human fetal development.]
We seem to have a growing abundance of evidence that immune stimulation in utero can have neurological impacts on the fetus that include schizophrenia, and autism. In some instances, we have specific viral triggers; i.e., the flu or rubella, but I’d further posit that we have increasing reason to believe that any immune response can have a similar effect. The Patterson studies involving IL-6 in a rodent model of maternal activation seem to make this point with particular grace, as the use of IL-6 knockout mice attenuated the effect, as did IL-6 antibodies; and direct injection of IL-6 in the absence of actual infection produced similar outcomes. In animal models designed to study a variety of effects, we have a veritable spectrum of studies that tell us that immune insults during critical developmental timeframes can have lifelong effects on neuroimmune activity, HPA-axis reactions, seizure susceptibility, and ultimately, altered behaviors. I believe that we are rapidly approaching a point where there will be little question as towards if a robust immune response during development can lead to a developmental trajectory that includes autism, and will instead be faced with attempting to detangle the more subtle, and inconvenient, mechanisms of action, temporal windows of vulnerability, and indeed if there are subgroups of individuals that are predisposed to be more likely to suffer from such an insult.
Another thing that struck me about Morgan was the speculation that an increased presence of IL-1R in controls may have been suggestive of an attempt to muzzle the immune response in the case group; repeated from Morgan “The trend toward an increase in colocalization in control cases may also hint at downregulation of inflammatory signal receptors in a chronically activated system.” In other words, for controls it wasn’t a big deal to be expressing IL-1R in a ‘normal’ fashion, because the immune system is in a state of balance. Another way of looking at our observations would be to ask the question as towards what has caused the normally self regulating immune system to fail to return to a state of homeostasis? Ramping up an immune response to fight off pathogens and ratcheting back down to avoid unnecessary problems is something most peoples immune systems do with regularity. Is the immune system in autism trying to shut down unsuccessfully?
There are clues that the homeostatic mechanisms are trying to restore a balanced system. For example, in Immune transcriptome alterations in the temporal cortex of subjects with autism, researchers reported that the genetic pathway analysis reveals a pattern that could be consistent with “an inability to attenuate a cytokine activation signal.” Another paper that I need to spend some read in full, Involvement of the PRKCB1 gene in autistic disorder: significant genetic association and reduced neocortical gene expression describes a genetic and expression based study that concludes, in part, that downregulation of PRKCB1 “could represent a compensatory adjustment aimed at limiting an ongoing dysreactive immune process“.
If we look to clinical evidence for a decreased capacity to regulate an immune response, one paper that might help is Decreased transforming growth factor beta1 in autism: a potential link between immune dysregulation and impairment in clinical behavioral outcomes, the authors report an inverse dose relationship between peripheral levels of an important immune regulator, TGF-Beta1, and autism severity; i.e., the less TGF-Beta1 in a subject, the worse the autism behaviors [the autism group also, as a whole, had less TGF-Beta1 than the controls].
And then, in between the time that Morgan came out, and I completed this posting, another paper hit my inbox that might provide some clues,a title that is filled to the brim with autism soundbytes, “Effects of mitochondrial dysfunction on the immunological properties of microglia“. The whole Hannah Poling thing seemed so contrived to me, basically two sets of people trying to argue past each other to reach a predetermined conclusions, and as a result, I’ve largely shied away from digging too deeply into the mitochondrial angle. This may not be a luxury I have anymore after reading Ferger et all. For our purposes, lets forget about classically diagnosed and acute mitochonrdrial disease, as Hannah Poling supposedly has, and just acknowledge that we have several studies that show that children with autism seem to have signs of mitochondrial dysfunction, as I understand it, sort of a halfway between normal mitochondrial processing and full blown mitochondrial disorder. Given that, what does Ferger tell us? Essentially an in vitro study, the group took microglial cells from mice, exposed some of them to toxins known to interferre with the electron transport chain, and exposed the same cells to either LPS or IL-4 to measure the subsequent immunological response. What they observed was that the response to LPS was unchanged, but the response to IL-4, was blunted; and pertinently for our case, the IL-4 response is a so called ‘alternative’ immune response, that participates in shutting down the immune response. From the conclusion of Ferger:
In summary, we have shown that mitochondrial dysfunction in mouse microglial cells inhibit some aspects of alternative activation, whereas classic activation seems to remain unchanged. If, in neurological diseases, microglial cells are also affected by mitochondrial dysfunction, they might not be able to induce a full anti-inflammatory alternative response and thereby exacerbate neuroinflammation. This would be associated with detrimental effects for the CNS since wound healing and attenuation of inflammation would be impaired.
If our model of interest is autism, our findings can begin to fit together with remarkable elegance. And we haven’t even gone over our numerous studies that show the flip side of the immunological coin; that children with autism have been shown time and time again to have a tendency towards an exaggerated immune response, and increased baseline pro-inflammatory cytokines when compared with their non diagnosed peers!
Anyways, those are my bonus theoretical pontifications regarding Morgan.
Hello friends –
I ran into a few abstracts, read a few papers, and tried to get my way through one really dense paper in the past few weeks that got me thinking about this post. It’s all shook up, like pasta primavera in my head, but hopefully something cogent will come out the other end. (?)
Of the metabolic conditions known to be associated with having a child with autism, hypothyroidism is one that I keep on running into by way of the pubmed alert grapevine. By way of example, we have two studies that looked for autoimmune conditions in family members which found hypothyroidism to be one of many autoimmune diseases as a risk factor for autism, including, Familial clustering of autoimmune disorders and evaluation of medical risk factors in autism, and Increased prevalence of familial autoimmunity in probands with pervasive developmental disorders. This shouldn’t be too surprising, we know that, for example, perinatal hypothyroidism is a leading cause of mental retardation, with similar findings for the condition during pregnancy. It turns out, it appears that rates of hypothyroidism are slightly increasing, though at this time, the increases are of relatively small proportions, and as such, may be artifacts unrelated to an actual increase in classically recognized hypothyroidism. In any case, I think it is safe to say that interference with thyroid metabolism is something to be avoided at all costs when possible.
So after having read about that, this paper showed up in my inbox a while ago:
Thyroid hormones have long been known to play important roles in the development and functions of the central nervous system, however, the precise molecular mechanisms that regulate thyroid hormone-responsive gene expression are not well understood. The present study investigated the role of DNA methylaion and histone acetylation in the effects of perinatal hypothyroidism on regulation of reelin and brain-derived neurotrophic factor (BDNF) gene expression in rat hippocampus. The findings indicated that the activities of DNA methyltransferase (DNMT), methylated reelin and BDNF genes were up-regulated, whereas, the activities of histone acetylases (HAT), the levels of global acetylated histone 3 (H3) and global acetylated histone 4 (H4), and acetylated H3, acetylated H4 at reelin promoter and at BDNF gene promoter for exon II were down-regulated in the hippocampus at the developmental stage of the hypothyroid animals. These results suggest that epigenetic modification of chromatin might underlie the mechanisms of hypothyroidism-induced down-regulation of reelin and BDNF gene expression in developmental rat hippocampus
This gets interesting for autism because reelin, and bdnf levels have been found to be decreased in several studies in the autism population, with direct measurements, genetic expression, mouse knockout based models of autism , and genomic alterations all being implicated. There have been some negative genetic studies, but considering that it isn’t always the genes you have, but the genes you use, our other available evidence certainly points to BDNF and reelin involvement with some percentage of children with autism, and the association is such that a reduction in reelin or BDNF is a risk factor for developing autism. It would seem that the paper above might give some insight into the lower level details of the effects of hypothyroidism and subsequent developmental trajectories; modifications of reelin expression; through epigentic mechanisms, no less!. That’s pretty cool!
Then, I got my hands on a review paper that tries to go into detail as to the functional mechanism by which reelin deficiency could contribute to ASD, Neuroendocrine pathways altered in autism. Special role of reelin. It is a review that touches on a variety of ways that reelin contributes to neurodevelopment that have findings in the autism realm, including neuronal targeting and migration during brain formation, interactions with the serotonin and GABA systems, testosterone, and oxytocin. In short, there are plenty of ways that decreased reelin expression can impact development in ways that mirror our some of our observations in autism.
Of the many things that convince me that we are doomed, the proliferation of chemical compounds whose interactions within our bodies we scarcely understand is among them. In my readings on endocrine disruptors, one thing I found that seemed to be worrying lots of researchers was that some classes of these chemicals are capable of interfering with thyroid metabolism, and in some cases interfering with development of cells known to be associated with autism. Terrifyingly enough, since I read those papers, several others have come out, including Polybrominated Diphenylether (PBDE) Flame Retardants and Thyroid Hormone during Pregnancy and Mini-review: polybrominated diphenyl ether (PBDE) flame retardants as potential autism risk factors. At this point, it is important to point out that, as far as I know, there have not been any studies showing that non occupational exposure to PDBEs or other environmental pollutants can lead to classically defined hypothyroidism, at least none that I know of. (?) Be that as it may, I think it is realistic to assume any interference in thyroid metabolism is a bad thing, and while finding people in the outlier regions of hypo (or hyper) thyroidism gives us information on extreme environments, it would take someone with a lot of misplaced faith to assume that we can safely disturb thyroid metabolism just a little bit, and everything will come out in the wash.
I’ve had the argument made to me in the past that environmental pollutant driven increases in autism lacked biological plausible mechanisms; this argument is almost always made within a context of trying to defend the concept of a static rate of autism. While the papers I’ve linked to above do not provide conclusive proof that our changing environment is causing more children to be born with autism, they do provide increasing evidence of a pathway from pollutants to ASD, and indeed, the lack of biological plausibility becomes an increasingly flacid foundation on which to assume that our observations of an increased rate of autism are illusory. Unfortunately, in my opinion, the focus on vaccines has contributed to the mindset that a static rate of autism (or nowadays, maybe a tiny increase), must be protected at all costs, including some ideas on the application of a precautionary principle that seem outright insane to me (or at least, the exact opposite of what I would consider to be a precautionary path).
One thing is for certain, the number of child bearing women in developing countries with measurable concentrations of chemicals known to interferre with thyroid metabolism nears 100% in the industrialized nation as we eat , drink, breathe and bathe in the microscopic remnants of packaging materials, deteriorating carpet fibers, and baby clothes that are made to be fire resistant. This is an environment unambiguously different than that encountered by any other generation of infants in the history of mankind. To believe that we can modify our environment so drastically without having an impact seems incredibly naive to me, or on some days, just plain old stupid.