Modulation of gut and brain serotonin signaling by tryptophan producing bacteria

Project Summary Functional disorders of the gastrointestinal (GI) tract represent a significant burden to patients and to society. These disorders are difficult to treat, and recurrence of symptoms is common. As a result, there are ongoing efforts to develop more effective treatment options. The gut microbiome is recognized as a dynamic entity that can influence a wide variety of physiological processes, ranging from the integrity of the intestinal epithelial barrier to brain neurochemistry and behavior. Studies included in this grant application are designed to exploit specific biochemical properties of certain bacterial strains to test the hypothesis that treatment of mice with bacteria that generate tryptophan (TRP) can improve GI motility and affect anxiety and depression. In the epithelium of the intestine, and in serotonin (5-HT) neurons, TRP is converted to 5-HT. When 5-HT is released from these cells it triggers reflexes that increase motility and epithelial secretion in the gut, and influences emotional well-being in the brain. We have recently demonstrated that Bacillus (B.) subtilis, a TRP- producing bacteria, can enhance colonic motility in normal and constipated mice through the activation of the 5- HT4 receptor. This grant proposal aims to elucidate the mechanisms by which TRP-synthesizing bacteria enhance colonic motility, and to determine whether and how increased circulatory TRP synthesized by gut bacteria can alter 5-HT signaling in the brain and affect anxiety and depression. Experiments proposed in Aim 1 will elucidate the mechanisms by which TRP-producing bacteria enhance motility. We will determine whether increased levels of enteric TRP by various TRP-producing bacteria increase the 5-HT release and mucus secretion, and whether epithelial 5-HT4 receptors are mediating the prokinetic effects of TRP synthesizing bacteria. We will test whether administering a TRP decarboxylase-producing bacteria in combination with a TRP-producing strain will further enhance motility by converting TRP to tryptamine, which has been shown to activate epithelial 5-HT4 receptors. Finally, we will identify changes in colonic motility patterns and in myenteric neuroglial activity resulting from this treatment and how these changes enhance propulsive motility. Experiments proposed in Aim 2 will determine whether TRP-producing bacteria can affect 5-HT signaling in brain regions involved in anxiety and depression. Preliminary data have shown that B subtilis treatment increases serum TRP levels. We will test whether this increase in circulatory TRP by enteric bacteria modify the levels of TRP and TRP metabolites in the raphe nuclei, hippocampus, amygdala, and prefrontal cortex. In addition, we will assess whether this treatment results in increased neuronal activity in these regions and affects anxiety- and depression-like behavior. The results of these studies could provide a predictive translational bacterial treatment strategy, based on biochemical features of the bacteria, for the treatment of constipation and associated psychological disorders.