Investigating the regulatory role of phenethylamine for serotonin production in the pathogenesis of diarrhea-predominant Irritable Bowel Syndrome

Irritable Bowel Syndrome (IBS) is one of the most prevalent functional bowel disorders characterized by symptoms including abdominal pain, bloating, abdominal distention and bowel habit abnormalities. Increased production of peripheral serotonin from intestinal enterochromaffin (EC) cells is found to be a major factor in the pathogenesis of diarrhoea- predominant IBS (IBS-D). The treatment with serotonin antagonists has been shown to be beneficial in relieving symptoms of IBS-D. Despite the close association between serotonin dysregulation and IBS-D, the mechanism underlying increased serotonin production in IBS-D patients remains largely unknown.

The conversion of dietary nutrients into metabolites mediated by gut microbes is a potential regulatory mechanism for the production of peripheral serotonin. In our preliminary studies, we found that deregulated metabolism of amino acids may contribute to serotonin dysregulation in IBS-D. Our findings revealed a significant increase in the level of phenethylamine, a metabolite produced by bacterial metabolism of phenylalanine, in faecal samples of IBS-D patients. The level of phenethylamine was positively associated with scores of diarrheal symptoms including Bristol stool scale and defecation frequency. Consistent with increased phenethylamine level, our preliminary studies along with our published findings (Journal of Clinical Investigation 2020) revealed that the relative abundance of phenethylamine-producing bacteria, such as Ruminoccocus genus and R. gnavus species, was enriched in IBS-D microbiota. We further found that the fecal microbiota isolated from IBS-D patients exhibited an increased capacity for converting phenylalanine to phenethylamine. Furthermore, the increased level of peripheral serotonin was detected in sera of IBS-D patients. These findings demonstrate alteration of microbiota-mediated phenylalanine metabolism in IBS-D patients, accompanied by excessive serotonin production and increased severity of diarrheal symptoms. In the follow-up study, we found that phenethylamine stimulates the production of serotonin in both mouse colon tissues cultured ex vivo, intestinal organoids and QGP-1 cells, a human pancreatic endocrine cell line that serves as a model of enterochromaffin cells for the production and secretion of serotonin. In contrast, phenylalanine, the precursor of phenethylamine, failed to alter the level of serotonin in QGP-1 cells, revealing the specific effect of phenethylamine on the regulation of serotonin production. Given that Trace amine- associated receptor 1 (TAAR1), a G protein-coupled receptor, can be activated by phenethylamine, we therefore investigated whether phenethylamine-mediated serotonin production is dependent on TAAR1 activation. Inhibition of TAAR1 by specific TAAR1 antagonist indeed suppressed phenethylamine-induced increase in serotonin synthesis in QGP- 1 cells.

We hypothesize that gut microbiota-produced phenethylamine mediated via TAAR1 promotes gut serotonin production, thereby increasing colonic secretion. To address this hypothesis, we will investigate (1) the role of microbiota-producing phenethylamine in the regulation of serotonin production and gastrointestinal motility; (2) the in vivo function of TAAR1 in phenethylamine-mediated regulation of serotonin synthesis; (3) whether phenethylamine regulates serotonin production via controlling enterochromaffin cell development. This study will not only for the first time reveal the molecular mechanism underlying the regulation of serotonin synthesis and secretion by gut microbiota, but also lay a foundation for developing therapeutics for the treatment of IBS-D.