|Not lecturing... @ Wikipedia|
The latest offering from Naviaux et al is not so dissimilar from their previous work on that mouse model of autism, this time testing "the hypothesis that the behavioral manifestations of the MIA [maternal immune activation] model are a consequence of pathological persistence of the evolutionarily conserved CDR [cell danger response]... and that the CDR is maintained by dysregulated purine metabolism and secondary abnormalities in purinergic signaling". The press release about the study can be viewed here.
What this all translates into is that a state of immune arousal above and beyond what would be typically expected during pregnancy is somehow impacting on offspring development pertinent to an elevated risk of conditions like autism or schizophrenia. There is quite a bit of literature on this topic from the autism perspective (see here) and based on animal models other than just rodents (see here). The cell danger response (CDR) described by Naviaux (Robert that is)  represents "the evolutionarily conserved metabolic response that protects cells and hosts from harm". Indeed that last reference  contains just about every biological link known to autism science at the current time... including mitochondria. The idea is that triggering immune activation activates "a conserved cellular response to stress" called the CDR and the proposed master regulator of the CDR is purinergic signalling  - "purine nucleotides and nucleosides as extracellular messengers". ATP (adenosine triphosphate) as well as being quite an important molecular fuel source, is suggested to be one of the nucleotides which "can bind to cell surface receptors and act as signaling molecules and neuromodulators that are important in inflammation.. neurotransmission.. and many other biological processes".
A few basic points about the Naviaux study:
- Mice were the lucky volunteers for this study, and again the 'good breeder' that is the C57BL/6J variety. As per their previous trial, pregnant female mice were given something to artificially stimulate their immune system and "initiate the MIA model" or a saline control and then their offspring were the study focus.
- Suramin or saline (as a control) was then administered to 6-month old offspring mice and thereafter "behaviors were evaluated". Suramin levels were also examined, as were a broad range of metabolites as part of some metabolomic analysis based on the use of triple quad mass spectrometry.
- Results: "MIA animals showed social deficits from an early age". Nothing too novel there bearing in mind previous observations in this area of research. But... "Single-dose APT [antipurinergic therapy] with suramin completely reversed the social abnormalities in 6.5-month-old adults". Social behaviour by the way, was quantified as "time spent interacting with a novel ("stranger") mouse".
- The benefit of suramin also lasted for quite a while: "a small residual benefit to social behavior was still detectable" even after 5 weeks following the intervention. There is some discussion by the authors about this effect; noted to be potentially "due to the development of metabolic memory and/or somatic epigenetic DNA changes that lasted longer than the physical presence of the drug".
- Then the biochemistry. Suramin seemed to by-pass that very important gateway, the blood-brain barrier (BBB) and end up in the brainstem following some analysis of sacrificed offspring mice: "consistent with the notion that nuclei in brainstem, or their projection targets in distant sites of the brain, may mediate the dramatic behavioral effects of acute and chronic APT in this model".
- And more: "Comprehensive metabolomic analysis revealed disturbances in several other metabolic pathways relevant to children with ASDs. These included disturbances in microbiome, phospholipid, cholesterol/sterol, sphingolipid, glycolytic and bile salt metabolism". This bearing in mind that only "male animals that had been behaviorally evaluated were tested". Purine metabolism and the gut microbiome seemed to be quite important to the author's results. At this point I'll refer you back to the work by Elaine Hsiao and colleagues...
- Moreover: "The top, non-microbiome-associated metabolite was quinolinic acid... which was decreased in the MIA model". Quinolinic acid implies the involvement of one of those aromatic amino acids, tryptophan, which is an autism research favourite . More than that is the literature on the intersecting kynurenine pathway and how that might relate to a condition like schizophrenia (see here).
- The authors caution that their results are (a) mouse based and (b) "suramin is a poor drug choice for chronic use because of potentially toxic side effects that can occur with prolonged treatment". That being said, they do suggest that "new drugs might be given only once, or intermittently, during sensitive windows to unblock metabolism, restore more normal neural network function, improve resilience and plasticity, and permit improved development in response to behavioral and interdisciplinary therapies, and to natural play". Just in case you'd like an alternative reading of this study, have a look at this write-up too.
I do apologise for all the quotes taken from the Naviaux paper and used in this post, but when the authors say it better than I could, why would I try and complicate things any further? Indeed, the more I read the paper by Naviaux and colleagues, the more I see what a potential gem it actually is. I say this based on the pretty comprehensive way that the authors went about looking at the MIA model and the effect of their intervention, crossing behavioural (mouse behavioural) and biochemical fields. Mention of the words 'microbiome' and 'metabolome' are also guaranteed to perk my interest. All that enthusiasm is however tempered dependent on the work being replicated and not just in a mouse model either.
A few closing comments are all that are required. Looking at the list of biochemical pathways altered in the MIA model of autism (see Table 1 here) we see lots of familiar names too numerous to mention. Outside of looking at the effect of suramin on these pathways, I'm minded to suggest that other interventions might also benefit from some similar inspection of their effects on said pathways. Even as one commentator has already noted: "[The findings] are valuable, but the main problem is that they rely on a model of immune infection, not a genuine model of autism... They should have tested one of the classical models of autism... meaning genetic models of autism — to see whether suramin indeed corrects autistic behaviors".
I've also not really gone into the detail when it comes to the cell danger response (CDR) in this post because it is a complicated area and seems to tie into various issues which are slightly beyond my level of expertise. I would like to bring in the letter by Theoharides  at this point however, and his discussion linking "extracellular mitochondrial material" to mast cell activation (see here) on the back of the previous study by this authorship group, as another area crying out for further investigation.
 Naviaux JC. et al. Reversal of autism-like behaviors and metabolism in adult mice with single-dose antipurinergic therapy. Translational Psychiatry. 2014; 4: e400; doi:10.1038/tp.2014.33
 Naviaux RK. et al. Antipurinergic therapy corrects the autism-like features in the poly(IC) mouse model. PLoS One. 2013;8(3):e57380.
 Naviaux RK. Metabolic features of the cell danger response. Mitochondrion. 2014 May;16:7-17.
 Burnstock G. Pathophysiology and therapeutic potential of purinergic signaling. Pharmacol Rev. 2006 Mar;58(1):58-86.
 Boccuto L. et al. Decreased tryptophan metabolism in patients with autism spectrum disorders. Mol Autism. 2013 Jun 3;4(1):16.
 Theoharides TC. Extracellular Mitochondrial ATP, Suramin, and Autism? Clinical Therapeutics. 2013; 35: 1454-1456.
Naviaux JC, Schuchbauer MA, Li K, Wang L, Risbrough VB, Powell SB, & Naviaux RK (2014). Reversal of autism-like behaviors and metabolism in adult mice with single-dose antipurinergic therapy. Translational psychiatry, 4 PMID: 24937094