Mechanisms of alteration of gastrointestinal physiology by gut microbes

PROJECT SUMMARY/ABSTRACT Irritable bowel syndrome (IBS) is a globally prevalent disorder (~11%) characterized by an alteration in stool form/frequency in association with abdominal discomfort or pain. IBS is categorized into constipation, diarrhea or mixed (IBS-C, IBD-D, IBS-M) based on the predominant stool form/frequency. The pathophysiology of IBS is complex and therapeutic options targeting the underlying pathophysiology in IBS are limited. Recent studies support a role for gut microbial metabolites in maintaining normal gastrointestinal (GI) function, but how changes in different microbial metabolites and interactions among these metabolites affect molecular pathways involved in IBS pathophysiology remains a critical knowledge gap. Hence, it is not surprising that the current empirically designed microbial therapies (probiotics) have largely proven ineffective in IBS. To address this gap, in the previous grant cycle we focused on the bacterial metabolite tryptamine and found tryptamine increases secretion and mucus release in a 5- HT4R dependent manner, accelerates transit, and protects against inflammation in rodent models. The observations were supported by our finding of elevated levels of tryptamine in IBS-D in our human study. In the same longitudinal multi-omics human study, the most consistent finding in IBS-C across multiple -omics platforms were significant decreases in stool hypoxanthine and butyrate. The overall objective of this proposal is to determine the physiologic relevance of these metabolites by identifying the molecular pathways affected by each of these metabolites that are relevant to IBS-C. Our central hypothesis based on prior research and our preliminary data is that hypoxanthine is an effector metabolite that accelerates GI transit by increasing enterochromaffin (EC) cell serotonin release while butyrate is a regulatory metabolite that augments the biologic activity of effector metabolites. This will be tested in two Aims: In Aim 1, we will determine the mechanism by which hypoxanthine increases EC cell serotonin release and accelerates GI transit and in Aim 2, we will determine the mechanism by which butyrate regulates EC cell responses to effector metabolites and the resultant effects on GI function. We will use Ca2+ imaging in organoids/primary EC cell culture from novel transgenic mice, heterologous receptor expression with site- directed mutagenesis, and epigenomic and transcriptomics data, combined with ex vivo colon preparations, gnotobiotic- and EC cell-depleted mouse models, isogenic bacterial mutants, and novel encapsulation methods to address the above aims. Our findings will uncover specific pathways by which these microbial metabolites affect GI transit and allow development of novel mechanism-based microbial therapies for IBS-C.