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

Diarrhea-predominant irritable bowel syndrome (IBS-D) is a prevalent functional bowel disorder characterized by symptoms including diarrhea, abdominal pain and bloating. Our previous findings (Nature Communications, 2019) and other clinical studies reveal increased plasma serotonin (5-HT) levels in patients with IBS-D. Effective symptom relief from treatment with 5-HT antagonists further suggests that dysregulated intestinal 5-HT activity contributes to IBS-D pathogenesis. The peripheral 5-HT, majorly synthesized by intestinal enterochromaffin cells, is inactivated by the selective serotonin reuptake transporter (SERT) that is found to be downregulated in the colon of IBS-D patients. As a result, dysregulated SERT increases mucosal 5-HT bioavailability, driving the development of IBS-D. Currently, the regulation of SERT expression in IBS-D is not fully understood. Growing evidence suggests that gut microbiota may have a role in the regulation of SERT levels in IBS-D since fecal transplants from IBS-D patients induce SERT downregulation and hence elevated peripheral serotonin levels in recipient germ-free mice. Despite the importance of gut microbiota in modulating SERT expression and gut 5-HT levels, the identity of the bacterial species responsible for the regulation of 5-HT homeostasis, especially for 5-HT reuptake, remains unknown, and the molecular mechanisms by which the gut microbiota modulates SERT expression are largely unclear.

By metagenomic analyses of our IBS-D cohorts, we found that the human bacterium Ruminococcus gnavus strain may play a key pathogenic role in regulating serotonin levels in IBS-D (Cell Host & Microbe, accepted in principle). Our clinical studies revealed that R. gnavus was positively correlated with the severity of diarrheal symptoms and serum serotonin levels in IBS-D patients. In germ-free mice, monocolonization of R. gnavus induced IBS-Dlike diarrheal symptoms by elevating peripheral serotonin levels. From a metabolic perspective, we identified that phenethylamine derived from R. gnavus-mediated catabolism of dietary amino acid phenylalanine directly stimulated serotonin biosynthesis in intestinal enterochromaffin cells and this effect was mediated through the activation of TAAR1, a Gprotein coupled receptor for aromatic trace amines (e.g. phenethylamine). The activation of phenethylamine/TAAR1/serotonin signaling axis as a result of aberrant R. gnavus enrichment thereby contributed to elevated gastrointestinal transit and colonic secretion in IBS-D. In our follow-up study, we further demonstrated that the mRNA expression of colonic Sert was significantly downregulated in germ-free mice monocolonized with R. gnavus and normal mice treated with phenethylamine. In addition, we showed that diet low in phenylalanine, a precursor of phenethylamine, led to reduced gastrointestinal transit along with decreased levels of fecal phenethylamine and serum serotonin in normal mice.

Based on these observations, we hypothesize that, in addition to its stimulatory role in serotonin biosynthesis, R. gnavus may contribute to the impaired serotonin reuptake in IBS-D by phenethylamine-mediated downregulation of SERT. To address this hypothesis, we will investigate the regulatory role of phenethylamine for serotonin reuptake in the pathogenesis and treatment of IBS-D with following objectives: (1) To characterize the regulatory role of R. gnavus and its metabolite phenethylamine in the inhibition of serotonin reuptake and gastrointestinal motility; (2) To investigate the mechanism by which phenethylamine derived from R. gnavus regulates SERT expression and hence serotonin uptake; (3) To investigate the therapeutic potential of dietary intervention with a diet low in phenylalanine in the treatment of IBS-D. This study will not only provide new insights into the regulation of serotonin reuptake by host-microbe interactions, but also lay a foundation for developing new therapeutics for the treatment of IBS-D.