GABRA5 - Too much, or too little in Autism and Down Syndrome?

 

It is easy to get things the wrong way round.

This applies to science and to some people getting dressed

 

Today’s post was prompted by a reader updating me about Roche’s autism drug RO7017773, which targets the alpha 5 sub-unit of GABA_A receptors, encoded by the gene GABRA5.

Enrollment opens in phase II study of RO-7017773 for autism spectrum disorder

A 12-Week Placebo-Controlled Study to Investigate the Efficacy, Safety, and Tolerability of RO7017773 in Participants Aged 15-45 Years With Autism Spectrum Disorder (ASD)

 

Some people with severe autism, or just plain old ID/MR, which has gone out of fashion as a diagnosis these days, struggle to dress themselves because they do not notice what is inside out, or back to front.  I recall reading a few years ago about one autism parent who started a clothing company to get round this problem.

 

ADAPTIVE CLOTHING with no front/back and reversible so cannot be inside out.

 

I must say that writing this blog I am often left wondering which way round things are.  Do we want an agonist or an antagonist, a positive allosteric modulator or a negative one.  Many times things do seem to work backwards.

If you follow the research you will see that researchers often get things mixed up, with one group trying one strategy and yet another group of Ivy league bright-sparks doing exactly the opposite. The Vasopressin research is a good example.

Are they dyslexic? Perhaps dyspraxic?

Today it is the turn of GABRA5: do we want to upregulate it, or downregulate it?

GABRA5 is the gene that encodes the alpha 5 sub-unit of GABAa receptors.

A few years ago, the drug firm Roche spent a lot of money developing a negative modulator of these receptors.  That did not work and Basmisanil  (developmental codes RG-1662 and RO5186582) was abandoned as a treatment to raise cognition in Down syndrome.

Roche are now trialing the opposite therapy, a positive allosteric modulator of alpha 5 sub-unit of GABAa receptors, this time to treat autism.

 

Targeting GABA to treat autism

GABA is an important neurotransmitter and it seems to be dysfunctional in many types of autism, as well as other neurological conditions.

Both the A-type and the B-type of GABA receptors can respond to treatment.

When it comes to the A-type, we can be very clever and target specific sub-units of the receptor to achieve different goals.

 

Each receptor is made up of two α subunits, two βs and one γ.

In humans, the possibilities are made up of :

·         six types of α subunits (GABRA1, GABRA2, GABRA3, GABRA4, GABRA5, GABRA6)

·         three βs (GABRB1, GABRB2, GABRB3)

·         three γs (GABRG1, GABRG2, GABRG3)

 

What is particularly interesting is that the make up these receptors is not fixed, it is changing all the time and you can influence it with therapy.

It looks like you might even be able to treat alcohol addiction by targeting one of the sub-units.

In the world of autism it is more anxiety and cognition that we are targeting, but some types of seizure may also be targeted.

In previous posts I identified alpha 3 (GABRA3)  and alpha 5 (GABRA5) as subunits that I felt were the interesting ones to improve cognition in autism.  Alpha 3 is the target of the low dose clonazepam therapy.

Alpha 5 also fits in with my experience of inflammation-induced reduction in cognitive function.

 

α5 GABAA Receptors Regulate Inflammation-Induced Impairment of Long-Term Potentiation 

these results show that α5GABA_A receptor activity increases during inflammation and that this increase is critical for inflammation-induced memory deficits.

 

We know that female hormones modulate subunit expression, today we see that oxytocin also does this. So, yet another possible effect of a little more oxytocin.

 

Oxytocin modulates GABA_AR subunits to confer neuroprotection in stroke in vitro

Before I forget, I should add that that the nootropic herb Bacopa affects GABRA5 (in rats):-

https://www.sciencedirect.com/science/article/pii/S0753332218383914

“BME (Bacopa monnieri) significantly reversed the down-regulated Gabra1, Gabra4, Gabra5 gene expression of GABAA receptors subunits”

 

The following paper has been published since I wrote my earlier posts on GABRA5 and is very thorough.

 

Neurobiology and Therapeutic Potential of α5-GABA Type A Receptors

α5 subunit containing GABA type A receptors (GABA_ARs) have long been an enigmatic receptor subtype of interest due to their specific brain distribution, unusual surface localization and key role in synaptic plasticity, cognition and memory. These receptors are uniquely positioned to sculpt both the developing and mature hippocampal circuitry due to high overall expression and a distinct peak within the critical synapse formation period during the second postnatal week. Unlike the majority of other GABA_ARs, they exhibit both receptor clustering at extrasynaptic sites via interactions with the radixin scaffold as well as synaptic sites via gephyrin, thus contributing respectively to tonic currents and synaptic GABAergic neurotransmission. α5 GABA_AR signaling can be altered in neurodevelopmental disorders including autism and mental retardation and by inflammation in CNS injury and disease. Due to the unique physiology and pharmacology of α5 GABA_ARs, drugs targeting these receptors are being developed and tested as treatments for neurodevelopmental disorders, depression, schizophrenia, and mild cognitive impairment. This review article focuses on advances in understanding how the α5 subunit contributes to GABA_AR neurobiology. In particular, I discuss both recent insights and remaining knowledge gaps for the functional role of these receptors, pathologies associated with α5 GABA_AR dysfunction, and the effects and potential therapeutic uses of α5 receptor subtype targeted drugs.

 

Genetic Disorders with Altered α5 GABA_AR Neurotransmission

While acute reduction in α5 GABA_ARs has shown potential for improving cognition and memory, further studies both in mouse models and human patients link long term reduction with significant pathologies. Reduced α5 GABA_AR levels, function or protein interactions have been observed in patients with neurodevelopmental disorders including intellectual disability, epilepsy and autism. Common conditions among these disorders include cognitive impairments, increased anxiety, autism-related behaviors, sleep disorders and epilepsy susceptibility

 

α5 GABA_AR Therapeutics

NAMs that selectively reduce α5 GABA_AR function have been heavily pursued for the potential development of cognitive enhancing or “smart” drugs. The following are a selection of α5 GABA_AR NAMs: L-655,708, α5IA, Ro15-4513, MRK-016, RO4938581, and RY-80 Importantly, α5 NAMs did not exhibit the convulsant or pro-convulsant activity of more general alpha subunit NAMs, had good oral bioavailability and easily crossed the blood brain barrierIn contrast to NAMs which act via the GABA_AR benzodiazepine binding site, S44819 was recently identified as a competitive antagonist of GABA at α5 GABA_AR and showed similar pro-cognitive effects as NAMs: blocking α5-GABA_AR tonic current, enhancing LTP, reversing scopolamine-induced impairment of spatial working memory and enhancing object recognition memory). Finally, recent evidence for beneficial effects of positive allosteric modulators (PAMs) in aged brain cognition, autism, depression and schizophrenia has bolstered α5 PAM drug development. A selection of α5 preferring PAMs includes SH-053-R-CH3-2′F, MP-III-022, and GL-II-73. Potential therapeutic applications for α5 preferring NAMs and PAMs are discussed below with a focus on CNS specific uses (Table 1).

 

NAM α5 GABA_AR Therapeutic Applications

Pro-cognition

Developmental Disorders

Although these pharmacological successes led to a Phase II clinical trial for a related compound RG1662 (Hoffman-La Roche) in Down syndrome patients, the trial did not meet the primary and secondary endpoints of improved cognition and function.

Inflammation Induced Mild Cognitive Impairment and Post Anesthesia Memory Blockade

 

 

PAM α5 GABA_AR Therapeutic Applications

Neurodevelopmental Disorders

Mouse models of neurodevelopmental disorders that present with insufficient inhibitory tone show improvement with positive modulators of GABA_AR signaling. In the Scn1a+/− mouse model of Dravet syndrome, a severe childhood epileptic encephalopathy syndrome with hyperactivity and autism behaviors, abnormal social behaviors and fear memory deficits were rescued following treatment with a benzodiazepine, clonazepam. In an ASD mouse model with reduced GABA_AR-mediated inhibition, the BTBR T+tf/J mouse, the α2,3 and 5 PAM L-838,417, improved deficits in social interaction, repetitive behaviors, and spatial learning.

 

Mild Cognitive Impairment in Aging

Although α5 GABA_AR NAMs enhance memory in young rodents, it appears positive modulation may be more therapeutic in aging brains impaired by excess activity. Particularly in disorders such as Alzheimer’s which are hallmarked by overexcitation 

 

Depression and Schizophrenia

Another important unmet need where α5 GABA_ARs PAM pharmacotherapy may be applicable is in the development of new fast-acting anti-depressant drugs

  

 

Roche

Roche did develop a (NAM) drug to target the alpha 5 sub-unit in order to improve cognition in Down Syndrome. 

 

The GABA _A α5-selective Modulator, RO4938581, Rescues Protein Anomalies in the Ts65Dn Mouse Model of Down Syndrome

 

RG1662, a Selective GABAA α5 Receptor Negative Allosteric Modulator, Increases Gamma Power in Young Adults with Down Syndrome.

 

Basmisanil ( RG-1662 and RO5186582) is a highly selective inverse agonist/negative allosteric modulator of α₅ subunit-containing GABA_A receptors which is under development by Roche for the treatment of cognitive impairment associated with Down syndrome. As of June 2016, it is no longer studied.

 

Then came the opposite strategy, a PAM (positive allosteric modulator):-

 

RG 7816

Alternative Names: RG-7816; RO-7017773

 

Mechanism of Action  GABA A alpha 5 receptor modulators

Orphan Drug Status  No

New Molecular Entity  Yes

Highest Development Phases

Phase II  Pervasive child development disorders

Most Recent Events

·         23 Feb 2021Phase-II clinical trials in Pervasive child development disorders (In adolescents, In adults) in Canada (PO) (NCT04299464)

·         12 Mar 2020Hoffmann-La Roche plans a phase II trial for Pervasive child development disorders (Autism Spectrum Disorder) in USA (PO) (NCT04299464) (EudraCT2019-003524-20)

·         22 Apr 2019Roche completes a phase I trial in Pervasive child development disorders (In volunteers) in USA (PO, Capsule, Tablet) (NCT03847987)

 

RG7816 GABA-Aa5 PAM

autism spectrum disorder

4. Phase 1

Description/Summary:

RG7816 is a small molecule highly selective positive allosteric modulator of the GABAA α5 receptor, which is expressed in key brain regions for autism spectrum disorder. Two phase I clinical trial is evaluating RG7816 for the treatment of patients with autism spectrum disorder.

 

Conclusion

Modifying the response specific to sub-units of GABA_A receptors is a really nuanced therapy.

In a way I am not surprised that there is, as yet, no one size fits all therapy.

Will Roche’s trial of a drug to increase the effect of GABRA5 (a PAM) be more successful than their drug to reduce the effect of GABRA5 a (NAM)?

I do not know, but in the perfect world you would have both drugs and then see if fine-tuning GABRA5 ( + or -), on a case by case basis, was therapeutic. That would be personalized medicine.

At least we can modify GABRA3 extremely cheaply with Professor Catterall’s low dose clonazepam.

Note that we saw in my original posts that the Japanese attribute the benefit of low dose clonazepam to the γ2 subunit of GABAa receptors, which is encoded by GABRG2, for those who don’t speak Greek.

 

PX-RICS-deficient mice mimic autism spectrum disorder in Jacobsen syndrome through impaired GABAA receptor trafficking  

A curative effect of clonazepam on autistic-like behaviour

 

These results demonstrate that ASD-like behaviour in PX-RICS^−/− mice is caused by impaired postsynaptic GABA signalling and that GABA_AR agonists have the potential to treat ASD-like behaviour in JBS patients and possibly non-syndromic ASD individuals.

 

Jacobsen syndrome is a condition mainly found in girls and it is one of those more rare small-headed conditions (microcephaly). It features MR/ID and often an autism diagnosis.  It is caused by missing part of chromosome 11, apparently one of the most disease-rich chromosomes.

The fact that low-dose (sub-anxiolytic) clonazepam rescued the autistic behavior in mice does not mean that anyone has tried it in little girls with Jacobsen syndrome; that would require too much common sense.

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 GABA_A 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 GABA_A receptors. These combinations are not fixed, or “hard-wired”; they vary all the time.

The composition of the GABA_A 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. 

The brain GABA-benzodiazepine receptor alpha-5 subtype in autism spectrum disorder: A pilot [11C]Ro15-4513 positron emission tomography study

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 GABA_A 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 GABA_A signaling, especially at GABA_A α5-subunit containing receptors, might potentially be a novel therapeutic target for ASD. Unselective GABA_A 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 GABA_A 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 GABA_A α5 expression in adult males with ASD, consistent with the hypothesis that ASD is characterised by a defect in GABA signaling. 

α5GABAA receptor deficiency causes autism-like behaviors

The prevalence of autism spectrum disorders (ASDs), which affect over 1% of the population, has increased twofold in recent years. Reduced expression of GABA_A 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 GABA_A 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 α5GABA_A receptor-anchoring protein radixin, further supporting a α5GABA_A receptor deficiency in ASDs.

The results from the current study suggest that drugs that act as positive allosteric modulators of α5GABA_A receptors may ameliorate autism-like behaviors 

  

Too many or too few the α5GABA_A 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 α5GABA_A, 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 α5GABA_A are behaving, because even if you have an appropriate number of these receptors, you may not have optimal activity from them. Over activity from α5GABA_A 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 α5GABA_A. It has been shown that increased α5GABA_A 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 (α5GABA_A) 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 α5GABA_A receptor function. A tonic inhibitory current generated by α5GABA_A 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 α5GABA_A receptors in the hippocampus. Collectively, these results show that α5GABA_A 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 GABA_A 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 GABA_A 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 α5GABA_A 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 α5GABA_A 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

https://epiphanyasd.blogspot.com/2017/10/sodium-benzoate-and-gabra5.html

to use cinnamon/NaB to reduce GABRA5 expression, which has got to consequently reduce α5GABA_A activity.

All of these strategies are crude, because what matters is α5GABA_A 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 α5GABA_A 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 α5GABA_A 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 α5GABA_A 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:-

The α-5 subunit-containing GABA-A receptor: a target for the treatment of cognitive defects

The role of α5 GABAA receptor agonists in the treatment of cognitive deficits in schizophrenia