Gut barrier integrity meets blood-brain barrier integrity with autism in mind

"In the ASD [autism spectrum disorder] brain, there is an altered expression of genes associated with BBB [blood-brain barrier] integrity coupled with increased neuroinflammation and possibly impaired gut barrier integrity."

Although pretty enthused to see research linking names like Anna Sapone, Tim Buie and Alessio Fasano in the recent paper published by Maria Fiorentino and colleagues [1] (open-access), I was slightly less impressed with the use of the term 'the ASD brain' in their paper potentially joining two concepts that I've been quite interested in down my research years: gut barrier and blood-brain barrier function in the context of autism. Yes, I accept that those most precious of resources, donated brains from the deceased, represented some of the 'material' under scientific scrutiny, but if science has learned anything about autism down the years, it is that sweeping generalisations such as terms like 'the autism brain' don't reflect what the existing research tells us about the heterogeneity under the label. I might just as well use the term 'blogger brain' to denote some of my activities, but such a label tells you nothing about me aside from my pastime.

After that little rant, the paper from Fiorentino is an interesting one in that the goal was to "investigate whether an altered BBB and gut permeability is part of the pathophysiology of ASD." To do this, tissue from both brain and gastrointestinal (GI) tract donated by a small number of deceased and non-deceased participants who were diagnosed with autism, schizophrenia or nothing related (not-autism controls) were analysed "for gene and protein expression profiles." This work was undertaken on the basis of "the interconnectivity of the gut–brain axis, [that] suggests that inappropriate antigen trafficking through an impaired intestinal barrier, followed by passage of antigens or activated immune complexes through a permissive blood–brain barrier (BBB), can be part of the chain of events leading to neuroinflammation and thereby subsequent disease." I might add that the use of the word 'disease' in that sentence is, I think, aiming to describe the physiological effects of 'leaky barriers' not the diagnosis of autism. It is unfortunate however that 'disease' still continues to be banded around in the context of autism [2].

I think it's important to stress that the Fiorentino study was in effect two studies: one that looked at brain samples from one participant group who had died, and one that looked at GI samples from those who were still living (at the time of sample collection) and who presented with "GI symptoms undergoing esophagogastroduodenoscopy (EGD) for clinically indicated reasons." This was not a study where biological samples - brain and gut - came from the same person but rather a mash-up. Keep that in mind for now. The sorts of genes that were focused in on were those "associated with the formation, integrity, and function of the BBB and neuroinflammation" and included the claudins and something called MMP-9 and MMP-2 that have been discussed previously on this blog (see here) with leaky barriers in mind. The key words are 'barrier integrity' when it comes to the list of compounds that were under inspection.

Results: well it was good to see the authors list details of each of the participants from which tissue were used in their study. Brain tissue from the deceased with autism for example, is subject to quite a few factors that can influence the outcome of any results obtained; not least whether specific comorbidity accompanied their autism diagnosis and the nature of their death. Indeed, looking through the various case report numbers, I'm struck by how young many participants, particularly those diagnosed with autism, were at the time of their death. This ties into other discussions and debates (see here).

"Our molecular analysis of the BBB integrity and function shows an altered BBB in the ASD subjects evaluated." This was evidenced by elevations in the gene expression of MMP-9 and its proposed connection to disturbances of BBB integrity. Further: "Of the four claudins (i.e., CLDN-1, -3, -5 and -12) that to date are thought to be incorporated in the BBB... we found that two were significantly more expressed in the ASD brain as compared in HC [healthy controls]." Once again I might suggest the term 'healthy controls' is not an inappropriate one when it comes to determining not-autism or not-schizophrenia.

Then to analysis of those [independent] gut biopsy samples: "results, showing increased expression levels of pore-forming (66% of the ASD samples) and decreased levels of barrier-forming (75% of the ASD samples) TJ [tight junction] components in the duodenal samples, suggest an impaired gut barrier and serve as a proof of concept to support the hypothesis of a gut–brain axis dysfunction in a subgroup of ASD patients." So, those compounds linked to making the gut barrier more 'leaky' were seemingly increased in expression, and those linked to making the gut barrier less 'leaky' were reduced in quite a few of the samples from those diagnosed with autism. Mmm...

There is quite a bit more science included in the Fiorentino study but I think I've gone on long enough in this post. Suffice to say that the whole gut-brain axis thing with autism in mind gets a boost but more work is indicated, not least with larger sample groups and perhaps combining tissues from gut and brain from the same person. I would also like to see a little more done on this topic with some 'interventions' in mind, based on the other autism research that potentially links the authors (see here). Drawing for example, on a paper written by Prof Fasano titled: 'Zonulin, regulation of tight junctions, and autoimmune diseases' [3] suggesting that "gliadin, a storage protein present in wheat and that triggers celiac disease in genetically susceptible individuals, also affect the intestinal barrier function by releasing zonulin" one might see how far from being a set-in-stone state of affairs, dietary changes for some on the autism spectrum, might actually set in motion a host of biological changes pertinent to this area of work. And such changes might not be just confined to accepted gluten-related conditions either...

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[1] Fiorentino M. et al. Blood–brain barrier and intestinal epithelial barrier alterations in autism spectrum disorders. Molecular Autism. 2016; 7:49.

[2] Simms MD. When Autistic Behavior Suggests a Disease Other than Classic Autism. Pediatr Clin North Am. 2017 Feb;64(1):127-138.

[3] Fasano A. Zonulin, regulation of tight junctions, and autoimmune diseases. Annals of the New York Academy of Sciences. 2012;1258(1):25-33.

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Fiorentino, M., Sapone, A., Senger, S., Camhi, S., Kadzielski, S., Buie, T., Kelly, D., Cascella, N., & Fasano, A. (2016). Blood–brain barrier and intestinal epithelial barrier alterations in autism spectrum disorders Molecular Autism, 7 (1) DOI: 10.1186/s13229-016-0110-z