This blog aims to synthesize the relevant parts of the research
and make connections that point towards some potential therapeutic avenues. Most researchers work in splendid isolation
and concentrate on one extremely narrow area of interest.
The GABAA reset, not functional in some autism
On the one hand things are very simple, if the GABAA
receptors function correctly and are inhibitory and the glutamate receptors
(particularly NMDA and mGluRx) function correctly, there is harmony and a perfect excitatory/inhibitory balance.
Unfortunately numerous different things can go wrong and you could
write a book about each one.
As you dig deeper you see that the sub-unit make-up of GABAA
receptors is not only critical but changes.
The plus side is that you can influence this.
Today we see that the receptors themselves are physically movable
and sometimes get stuck in the “wrong place”. When the receptors cluster close
together they produce a strong inhibitory effect, but continual activation of NMDA receptors by the
neurotransmitter glutamate - as occurs naturally during learning and memory, or
in epilepsy - leads to an excess of incoming calcium, which ultimately causes
the receptors to become more spread out, reducing how much the neuron can be
inhibited by GABA. There needs to be a mechanism to move the GABAA receptors
back into their original clusters.
Very clever Japanese researchers have figured
out the mechanism and to my surprise it involves one of those hubs, where
strange things in autism seem to connect to, this time
IP3R.
I guess the
Japanese answer to my question above is simple. YES,
A very
recent science-light article by Gargus on IP3:-
Now to the Japanese.
I wonder if Gargus has
read the Japanese research, because both the cause and cure for the GABAA
receptors dispersing and clustering is an increase in calcium and both mediated
by glutamate.
The excitatory
neurotransmitter glutamate binds to the mGluR receptor and activates IP3
receptor-dependent calcium release and protein kinase C to promote clustering
of GABAA receptors at the postsynaptic membrane - the place on a
neuron that receives incoming neurotransmitters from connecting neurons.
If Professor Gargus is
correct, and IPR3 does not work properly in autism, the GABAA
receptors are likely not sitting there in nice neat clusters. As a result their
inhibitory effect is reduced and neurons fire when they should not.
Gargus has found that in the types of autism he has
investigated IP3
receptor open as they should, but close too fast and so do not release enough
calcium from the cell’s internal calcium store (the endoplasmic
reticulum).
In particular the Japanese researchers found that:-
“Stabilization of GABA
synapses by mGluR-dependent Ca2+ release from IP3R via
PKC”
If the IP3
receptor does not stay open as long as it should, not enough Ca2+ will
be released and GABA synapses will not be stabilized. Then GABAA
receptors will be diffused rather than being in neat clusters.
The science-light version of the Japanese study:-
Just as a thermostat is used to maintain a
balanced temperature in a home, different biological processes maintain the
balance of almost everything in our bodies, from temperature and oxygen to
hormone and blood sugar levels. In our brains, maintaining the balance -- or
homeostasis -- between excitation and inhibition within neural circuits is
important throughout our lives, and now, researchers at the RIKEN Brain Science
Institute and Nagoya University in Japan, and École Normale Supérieure in
France have discovered how disturbed inhibitory connections are restored.
Published in Cell Reports, the work shows how inhibitory synapses are
stabilized when the neurotransmitter glutamate triggers stored calcium to be
released from the endoplasmic reticulum in neurons.
"Imbalances in excitation and inhibition
in the brain has been linked to several disorders," explains lead author
Hiroko Bannai. "In particular, forms of epilepsy and even autism appear to
be related to dysfunction in inhibitory connections."
One of the key molecules that regulates
excitation/inhibition balance in the brain is the inhibitory neurotransmitter
GABA. When GABA binds to GABAA receptors on the outside of a neuron, it
prevents that neuron from sending signals to other neurons. The strength of the
inhibition can change depending on how these receptors are spaced in the
neuron's membrane.
While GABAA receptors are normally clustered
together, continual neural activation of NMDA receptors by the neurotransmitter
glutamate -- as occurs naturally during learning and memory, or in epilepsy --
leads to an excess of incoming calcium, which ultimately causes the receptors
to become more spread out, reducing how much the neuron can be inhibited by
GABA.
To combat this effect, the receptors are
somehow continually re-clustered, which maintains the proper
excitatory/inhibitory balance in the brain. To understand how this is
accomplished, the team focused on another signaling pathway that also begins
with glutamate, and is known to be important for brain development and the
control of neuronal growth.
In this pathway glutamate binds to the mGluR
receptor and leads to the release of calcium from internal storage into the
neuron's internal environment. Using quantum dot-single particle tracking, the
team was able to show that after release, this calcium interacts with protein
kinase C to promote clustering of GABAA receptors at the postsynaptic
membrane--the place on a neuron that receives incoming neurotransmitters from
connecting neurons.
These findings show that glutamate activates
distinct receptors and patterns of calcium signaling for opposing control of
inhibitory GABA synapses.
Notes Bannai, "it was surprising that
the same neurotransmitter that triggers GABAA receptor dispersion from the
synapse, also plays a completely opposite role in stabilizing GABAA receptors,
and that the processes use different calcium signaling pathways. This shows how
complex our bodies are, achieving multiple functions by maximizing a limited
number of biological molecules.
Pre-activation of the cluster-forming pathway
completely prevented the dispersion of GABAA receptors that normally results
from massive excitatory input, as occurs in status epilepticus -- a condition
in which epileptic seizures follow one another without recover of
consciousness. Bannai explains, "further study of the molecular mechanisms
underlying the process we have uncovered could help develop treatments or
preventative medication for pathological excitation-inhibition imbalances in
the brain.
"The next step in understanding how
balance is maintained in the brain is to investigate what controls which
pathway is activated by glutamate. Most types of cells use calcium signals to
achieve biological functions. On a more basic level, we believe that decoding
these signals will help us understand a fundamental biological question: why
and how are calcium signals involved in such a variety of biological
phenomena?"
The full Japanese study:-
·
Bidirectional synaptic control system by glutamate and Ca2+
signaling
·
Stabilization of GABA synapses by mGluR-dependent Ca2+
release from IP3R via PKC
·
Synaptic GABAAR clusters stabilized through
regulation of GABAAR lateral diffusion
·
Competition with an NMDAR-dependent Ca2+
pathway driving synaptic destabilization
GABAergic synaptic transmission
regulates brain function by establishing the appropriate excitation-inhibition
(E/I) balance in neural circuits. The structure and function of GABAergic
synapses are sensitive to destabilization by impinging neurotransmitters.
However, signaling mechanisms that promote the restorative homeostatic
stabilization of GABAergic synapses remain unknown. Here, by quantum dot
single-particle tracking, we characterize a signaling pathway that promotes the
stability of GABAA receptor (GABAAR) postsynaptic
organization. Slow metabotropic glutamate receptor signaling activates IP3
receptor-dependent calcium release and protein kinase C to promote GABAAR
clustering and GABAergic transmission. This GABAAR stabilization
pathway counteracts the rapid cluster dispersion caused by glutamate-driven
NMDA receptor-dependent calcium influx and calcineurin dephosphorylation,
including in conditions of pathological glutamate toxicity. These findings show
that glutamate activates distinct receptors and spatiotemporal patterns of
calcium signaling for opposing control of GABAergic synapses.
In this study, we demonstrate that the mGluR/IICR/PKC
pathway stabilizes GABAergic synapses by constraining lateral diffusion and
increasing clustering of GABAARs, without affecting the total number
of GABAAR on the cell surface. This pathway defines a unique form of
homeostatic regulation of GABAergic transmission under conditions of basal
synaptic activity and during recovery from E/I imbalances. The study also
highlights the ability of neurons to convert a single neurotransmitter
(glutamate) into an asymmetric control system for synaptic efficacy using
different calcium-signaling pathways.
The most striking
conceptual finding in this study is that two distinct intracellular signaling
pathways, NMDAR-driven Ca2+ influx and mGluR-driven Ca2+
release from the ER, effectively driven by the same neurotransmitter,
glutamate, have an opposing impact on the stability and function of GABAergic
synapses. Sustained Ca2+
influx through ionotropic glutamate receptor-dependent calcium signaling
increases GABAAR lateral diffusion, thereby causing the dispersal of
synaptic GABAAR, while tonic mGluR-mediated IICR restrains the
diffusion of GABAAR, thus increasing its synaptic density. How can
Ca2+ influx and IICR exert opposing effects on GABA synaptic
structure? Our research indicates that each Ca2+ source activates a
different Ca2+-dependent phosphatase/kinase: NMDAR-dependent Ca2+
influx activates calcineurin, while ER Ca2+ release activates PKC.
Taken together, these results lead us to
propose the following model for bidirectional competitive regulation of
GABAergic synapses by glutamate signaling. Phasic Ca2+ influx
through NMDARs following sustained neuronal excitation or injury leads to the
activation of calcineurin, overcoming PKC activity and relieving GABAAR
diffusion constraints. In contrast, during the maintenance of GABAergic
synaptic structures or the recovery from GABAAR dispersal, the
ambient tonic mGluR/IICR pathway constrains GABAAR diffusion by PKC
activity, overcoming basal calcineurin activity. One possible mechanism of dual
regulation of GABAAR by Ca2+ is that each Ca2+-dependent
enzyme has a unique sensitivity to the frequency and number of external
glutamate release events and can act to decode neuronal inputs (Fujii et al., 2013, Li et al., 2012, Stefan et al., 2008) in inhibitory synapses.
Tight
control of E/I balance, the loss of which results in epilepsy and other brain
and nervous system diseases/disorders, is dependent on GABAergic synaptic
transmission (Mann and
Paulsen, 2007). A recent study showed that
the excitation-induced acceleration of GABAAR diffusion and
subsequent dispersal of GABAARs from synapses is the cause of
generalized epilepsy febrile seizure plus (GEFS+) syndrome (Bouthour et al., 2012). Our results indicate that pre-activation of the mGluR/IICR pathway by DHPG
could completely prevent the dispersion of synaptic GABAARs induced
by massive excitatory input similar to status epilepticus. Thus,
further study of the molecular mechanisms underlying the mGluR/IICR-dependent
stabilization of GABAergic synapses via regulation of GABAAR lateral
diffusion and synaptic transmission could be helpful in the prevention or
treatment of pathological E/I imbalances, for example, in the recovery of
GABAergic synapses from epileptic states
DHPG = group I mGluR agonist
dihydroxyphenylglycine.
On a practical level you want to inhibit GABAA
dispersion and promote GABAA stabilization.
How you might do this would depend on exactly what was the underlying problem.
If the problem is IP3R not releasing
enough calcium, you might activate PKC in a different way or just increase the
signal from Group 1 mGluR. If the problem is too much calcium influx through
NMDA receptors due to excess glutamate, you could increase the re-uptake of
glutamate, via GLT-1, using Riluzole. You could block the flow of Ca2+ through
NMDA receptors using an antagonist.
The Japanese used dihydroxyphenylglycine
(DHPG) as their Group 1 mGluR agonist.
DHPG is an agonist of mGluR1 and mGluR5. We have come across mGluR5 many times before in this blog. Mavoglurant
is an experimental drug candidate for the treatment of fragile X syndrome. It is an antagonist of mGluR5.
We have seen many times before that there is
both hypo and hyper function possible and indeed that fragile X is not
necessarily a good model for autism.
The selective mGluR5 agonist CHPG protects against traumatic brain
injury, which would indeed make sense. Although, that research suggests an
entirely different mechanism.
The selective mGluR5 agonist CHPG protects
against traumatic brain injury in vitro and in vivo via ERK and Akt pathway
Ideally you would have enough calcium released
from IP3, but you could also increase DAG. It depends which part of the process
is rate-limiting.
Diacylglycerol
(DAG) has been investigated
extensively as a fat substitute due to its ability to suppress the accumulation
of body fat. Diglycerides, generally in
a mix with monoglycerides are common food additives largely used as emulsifiers.
In Europe, when used in food the mix is called E471.
Conclusion
On the one hand things are getting very complicated, but on the
other we keep coming back to the same variables (IP3R, mGlur5, GABAA
etc.).
It is pretty clear that some very personalized therapy will be
needed. Is it an mGlur5 agonist or
antagonist? Or quite possibly neither, because in different parts of the brain
it will have a good/bad effect.
It does look like Riluzole should work well in some people.
A safe IP3R agonist looks a possibility. As shown in the diagram
earlier in this post,IP3 is usually made in situ, but agonists exist.
In effect autism could be the opposite of
Huntington’s disease. In Huntington’s,
type 1 IP3 receptors are
more sensitive to IP3, which leads to the release of too much
Ca2+ from the ER. The release of Ca2+ from the ER causes
an increase in concentrations of Ca2+inside cells and in
mitochondria.
According to Gargus we should have reduced
concentrations of Ca2+inside cells in autism.
I suspect it is much more complicated in
reality, because it is not just the absolute
level of Ca2+ but rather the flow of Ca2+; so it
matters where it is coming from. I think we likely have impaired calcium
channel activity of multiple types in autism and the actual level of
intracellular calcium will not tell you much at all.
As the Japanese commented, it is surprising that glutamate is the
neurotransmitter that controls the clustering, or not, of GABAA
receptors. This suggests that you cannot
ignore glutamate and just “fix” GABA.
