Wednesday, 15 February 2012

The glutamate:glutamine ratio and autism

There they go again.

An Athenian butterfly @ Paul Whiteley
That Saudi Arabian autism research group who figured so heavily on this blog in 2011 have yet another publication out in this one by Abu Shmais and colleagues* (full-text) examining nitrogen-related parameters and autism. I'll admit that I'm a fan of this group; not only for the quantity of research that they are producing but the fact that there is clear evidence of 'blue-sky' thinking in their research material. These chaps are quietly contributing to some really interesting avenues in some cases of autism spectrum conditions, albeit at the moment based on quite small participant numbers.

Their latest offering asks a few interesting questions but before summarising some of them, I just want to run a sentence by you from the paper's introduction: "A recent epidemiological study in Saudi Arabia established the autism prevalence at 6:1000 (Talat; unpublished data, personal communication)". Bearing in mind that there are some important differences between personal communications about unpublished data and the public communication of the peer-reviewed variety, lots and lots of questions immediately start to appear in my mind about how this figure was arrived at, what criteria for autism used and if accurate, why the slight discrepancy with the [less than] 1 in 100 figure usually cited for the UK, US, Canada, etc. (or 1 in 38 if you believe the SK study last year). Answers on a postcard please addressed to...

Back to the paper:

  • The analysis of nitrogen containing compounds and their metabolism makes up the main reason for this paper so inevitably compounds such as ammonia (NH3) get a mention. More specifically how the the body harvests nitrogen from lots of different sources (including those funny amino acids which I seem to been talking a lot about recently) and consequently disposes of it. 
  • In terms of nitrogen metabolism, and concepts like the nitrogen balance, some familiar names crop up noted previously in autism research including glutamate and glutamine and how these compounds are involved in the removal of ammonia from the brain in the absence of important urea cycle enzymes.
  • Fasting blood samples were collected from 20 participants with autism and compared with 20 controls. There is a bit of typo in the methods section which states that controls were all male and in the next breath, 16 males and 4 females(!) 
  • Based on analyses looking at enzyme activity (5'-nucleotidase, adenosine deaminase and glutamate dehydrogenase), ammonia, urea, creatine, nitric oxide (NO), GABA, glutamate and glutamine - catch my breath - there were a few interesting results.
  • Mean plasma levels of creatine, glutamate and GABA were elevated in autism compared to control means. By contrast plasma glutamine levels were lower. Looking at the values and standard deviations, these differences seemed pretty pronounced particularly across the glutamate:glutamine ratio. The results coincided with significant increases in 5'-nucleotidase and a marginal mean decrease in adenosine deaminase activity (p=0.048).
  • A little bit of statistical wizardry (ROC) applied to the results suggested that the glutamate:glutamine ratio was the best measure of determining autism samples from controls based on a ratio cut-off value of 0.906. 

So what does all this tell us? Well, lots of things; some perhaps more important than others bearing in mind the small participant numbers. I'm going to focus specifically on the glutamate - glutamine findings as per the title of the post because the results are cumulatively most significant, as once again amino acid chemistry red flags are raised. I should also point out that plasma levels of glutamate and glutamine might not necessarily be the same as 'brain levels' as per the blood-brain barrier (BBB) and its amino acids transporters [note: I have a post scheduled soon on P-glycoprotein and BBB transport].

Not for the first time have elevated levels of glutamate and lower levels of glutamine been reported in autism research. The paper by Shimmura and colleagues** (open-access) found similar results in their analysis and once again pointed to the potential discriminatory power of this finding in cases of autism independent of learning disability from asymptomatic controls. Even Jim Adams and colleagues have reported similar trends (at least with regards to glutamate) on top of other group findings. As for searching the brain, well there is some evidence of issues there too at least in certain areas of the brain.

Fine, the result is pretty consistent but what does it mean?

Most commentary on glutamate reverts back to it being an excitatory neurotransmitter with reference to the hyperglutaminergic hypothesis of autism. I don't really want to go to heavily into the details of the hypothesis in this post but alongside elevated levels of glutamate, there is a corresponding decrease in activity of enzymes linked to glutamate metabolism (GAD65 & 67) and increased gliosis. Only a few days ago, a new paper*** was discussing the presence of autoantibodies to GAD65 in a subgroup of children with autism (and ADHD) which is probably not good news either.

The glutamine findings are also quite interesting. Abu Shmais and colleagues talk about how glutamine is an essential part of the process removing ammonia from the brain via processes such as glutamine synthetase. Low levels of glutamine may potentially reflect issues with this process. Outside of this however, other potential roles for glutamine need to be highlighted seemingly independent of the brain, in the liver and gut for example. The evidence for a connection between glutamine and glutathione (GSH) is also at the back of my mind bearing in mind the evidence so far on GSH in relation to autism.

A post on a sister blog discussed the issue of glutamine and its relationship to some gastrointestinal findings with its simplified role as the 'food of the gut' in terms of things like its effect on gut barrier function. It is therefore conceivable that low plasma levels of glutamine are probably not going to be particularly good for maintaining a healthy gut barrier; even outside of other confounding issues such as sulphation, etc. It might seem like a peripheral finding but more and more there is a realisation that physiology and psychology are linked as per the recent constipation - language impairment findings in autism.

So, another finding which requires large scale replication and another possible 'marker' for autism research is discussed. As time goes on, I am really starting to appreciate just how important amino acid chemistry is to lots of different functions and states outside of the classsic phenylalanine and PKU direction.

To finish something a little bit different as I ask the question: are you a man or a muppet (or perhaps neither)?

* Abu Shmais GA. et al. Mechanism of nitrogen metabolism-related parameters and enzyme activities in the pathophysiology of autism. Journal of Neurodevelopmental disorders. February 2012.
DOI: 10.1186/1866-1955-4-4

** Shimmura C. et al. Alteration of plasma glutamate and glutamine levels in children with high-functioning autism. PLoS ONE. October 2011.
DOI: 10.1371/journal.pone.0025340

*** Rout UK. et al. Presence of GAD65 autoantibodies in the serum of children with autism or ADHD. European Child & Adolescent Psychiatry. February 2012