Showing posts with label GABRA5. Show all posts
Showing posts with label GABRA5. Show all posts

Thursday, 9 November 2017

Variable Expression of GABRA5 and Activation of α5 -  a Modifier of Cognitive Function in Autism?

Today’s post sounds complicated. We actually already know that the gene GABRA5, and hence the alpha 5 sub-unit of GABAA receptors, can affect cognition, but we do not know for sure in whom it is relevant.
Most readers of this blog are lay people, as such we tend to be predisposed to the idea that autism is somehow “hardwired”, something that just happened and cannot be reversed. Some of autism is indeed “hardwired”, you cannot take an adult with autism and “re-prune” his synapses, to produce a more elegant robust network in his brain. But much can be done, because many things in the brain are changing all the time, they are not fixed at all. Today’s post is good example.
GABA is the most important inhibitory neurotransmitter in the brain. There are two types of GABA receptor, A and B. These receptors are made up of sub-units. There are many different possible combinations of sub-units to make GABAA receptors. These combinations are not fixed, or “hard-wired”; they vary all the time.
The composition of the GABAA receptor changes its effect. It can change how you feel (anxiety) and it can change you think/learn.
You can actually measure GABRA5 expression in different regions of the brain in a test subject using a PET-CT (Positron Emission Tomography–Computed Tomography) scan and it has been done in some adults with high functioning autism. This machine looks like a big front-leading washing machine, just a bit cleverer. 

our primary hypothesis was that, compared to controls, individuals with ASD have a significant reduction in α5 GABA receptor availability in these areas.
Due to the small sample size, we could not examine possible correlations between GABAA binding and particular symptoms of ASD, age, IQ, or symptoms of comorbidities frequently associated with ASD, such as anxiety disorders, OCD and depression. We were also unable to address the effects of possible neuroanatomical differences between people with ASD and controls, which might lead to partial volume effects in PET studies. However, the modest magnitude of the volumetric differences seen in most studies of high-functioning ASD suggests that it is unlikely that these could fully explain the present findings.

These preliminary results suggest that potentiation of GABAA signaling, especially at GABAA α5-subunit containing receptors, might potentially be a novel therapeutic target for ASD. Unselective GABAA agonists and positive allosteric modulators, such as benzodiazepines, have undesirable features such as abuse potential and tolerance, but more selective modulators might avoid such limitations. Further research should extend this work in a larger sample of ASD individuals. It would also be interesting to use PET with the ligand [11C]Ro15-4513 to measure GABAA in disorders of known etiology characterised by ASD symptoms, such as Fragile X and 15q11-13 duplication
In summary, we present preliminary evidence of reduced GABAA α5 expression in adult males with ASD, consistent with the hypothesis that ASD is characterised by a defect in GABA signaling. 

The prevalence of autism spectrum disorders (ASDs), which affect over 1% of the population, has increased twofold in recent years. Reduced expression of GABAA receptors has been observed in postmortem brain tissue and neuroimaging of individuals with ASDs. We found that deletion of the gene for the α5 subunit of the GABAA receptor caused robust autism-like behaviors in mice, including reduced social contacts and vocalizations. Screening of human exome sequencing data from 396 ASD subjects revealed potential missense mutations in GABRA5 and in RDX, the gene for the α5GABAA receptor-anchoring protein radixin, further supporting a α5GABAA receptor deficiency in ASDs.

The results from the current study suggest that drugs that act as positive allosteric modulators of α5GABAA receptors may ameliorate autism-like behaviors 

Too many or too few the α5GABAA receptors or too much/little activity?

Regular readers will know that autism is all about extremes hypo/hyper, macro/micro etc. The same is true with α5GABAA, too few can cause autistic behaviors, but too many can impede learning. You need just the right amount.
The next variable is how well your α5GABAA are behaving, because even if you have an appropriate number of these receptors, you may not have optimal activity from them. Over activity from α5GABAA is likely to have the same effect as having too many of them.
Here it becomes very relevant to many with autism and inflammatory comorbidities, because systemic inflammation has been shown to activate α5GABAA. It has been shown that increased α5GABAA receptor activity contributes to inflammation-induced memory deficits and, by my extension, to inflammation-induced cognitive decline.

α5GABAA Receptors Regulate Inflammation-Induced Impairment of Long-Term Potentiation

Systemic inflammation causes learning and memory deficits through mechanisms that remain poorly understood. Here, we studied the pathogenesis of memory loss associated with inflammation and found that we could reverse memory deficits by pharmacologically inhibiting α5-subunit-containing γ-aminobutyric acid type A (α5GABAA) receptors and deleting the gene associated with the α5 subunit. Acute inflammation reduces long-term potentiation, a synaptic correlate of memory, in hippocampal slices from wild-type mice, and this reduction was reversed by inhibition of α5GABAA receptor function. A tonic inhibitory current generated by α5GABAA receptors in hippocampal neurons was increased by the key proinflammatory cytokine interleukin-1β through a p38 mitogen-activated protein kinase signaling pathway. Interleukin-1β also increased the surface expression of α5GABAA receptors in the hippocampus. Collectively, these results show that α5GABAA receptor activity increases during inflammation and that this increase is critical for inflammation-induced memory deficits.

We saw in an earlier post that overexpression of GABRA5 is found in slow learners and we know that this is a key target of Down Syndrome research, aimed at raising cognitive function.

What can be modified?
It appears that you can modify the expression of GABRA5, which means you can increase/decrease the number of GABAA receptors that contain an α5 subunit.
You can also tune the response of those α5 subunits. You can increase it or decrease it.
Activation of the α5 subunit is thought to be the reason why benzodiazepine drugs  have cognitive (reducing) side effects. By extension, inverse agonists of α5 are seen as likely to be nootropic.
One such drug is LS-193,268  is a nootropic drug invented in 2004 by a team working for Merck, Sharp and Dohme.
A complication is that you do not want to affect the α2 subunit, or you will cause anxiety. So you need a highly selective inverse agonist.
The new Down Syndrome drug, Basmisanil, is just such a selective inverse agonist of α5.
Basmisanil (developmental code names RG-1662, RO5186582) is a highly selective inverse agonist/negative allosteric modulator of α5 subunit-containing GABAA receptors which is under development by Roche for the treatment of cognitive impairment associated with Down syndrome.  As of August 2015, it is in phase II clinical trials for this indication.

A contradiction
As is often the case, there is an apparent contradiction, because on the one hand a negative allosteric modulator should be nootropic in NT people and appears to raise cognition in models of Down Syndrome; but on the other hand results from a recent study suggests that drugs that act as positive allosteric modulators of α5GABAA receptors may ameliorate autism-like behaviors.
So which is it?
Quite likely both are right.
It is exactly as we saw a long while back with NMDAR activity, some people have too much and some have too little. Some respond to an agonist, some to an antagonist and some to neither.
What we can say is that fine-tuning α5GABAA in man and mouse seems a viable option to enhance cognition in those with learning difficulties.
The clever option is probably the positive/negative allosteric modulator route, the one being pursued by big Pharma for Down Syndrome.
I like Dr Pahan’s strategy from this previous post, for poor learners and those with early dementia

to use cinnamon/NaB to reduce GABRA5 expression, which has got to consequently reduce α5GABAA activity.
All of these strategies are crude, because what matters is α5GABAA activity in each part of the brain. This is why changing GABRA5 expression will inevitably have good effects in one area and negative effects in another area. What matter is the net effect, is it good, bad or negligible?
The fact that systemic inflammation increases α5GABAA activity may contribute to the cognitive decline some people with autism experience.
We previously saw how inflammation changes KCC2 expression and hence potentially increases intra cellular chloride, shifting GABA towards excitatory.
Ideally you would avoid systemic inflammation, but in fact all you can do is treat it.
Increasing α5GABAA activity I would see as possible strategy for people with high IQ, but some autistic features.
I think those with learning problems are likely to be the ones wanting less α5GABAA activity.
The people for whom “bumetanide has stopped working” or “NAC has stopped working” are perhaps the ones who have developed systemic inflammation for some reason.  You might only have to measure C-reactive protein (CRP) to prove this.

More reading for those interested:-