Microbial reactivation of sex steroids and visceral pain

PROJECT SUMMARY Abdominal pain is a common symptom of digestive disease that is poorly addressed by existing therapies. Probiotics are widely used to treat abdominal pain even though most studies that have examined their effects have had disappointing results. A more effective strategy might be to stimulate specific pathways of microbes already present in the gut that benefit the host. The overarching goal of this proposal is to determine if a microbial pathway that reactivates steroids in the gut lumen normally regulates the activity of sensory neurons that mediate visceral sensation, and whether this pathway can be manipulated to influence abdominal pain. Abdominal pain is mediated by visceral afferents, primary sensory neurons located outside the gut that communicate information from the gut to the central nervous system. Prior studies suggest that commensal microbiota normally limit visceral afferent sensitivity. Depletion of commensal microbes causes exaggerated responses to colorectal distention, evidence that visceral afferents become hypersensitive to non-noxious stimuli in the absence of microbes. The full extent of microbial effects on visceral pain and the signals that mediate them, however, are largely unclear. Androgens, steroid hormones that circulate at higher levels in males than females, are compelling candidates. Androgens are anti-nociceptive in somatic pain and emerging evidence suggests they have similar effects in visceral pain. In irritable bowel syndrome (IBS), a disorder defined by chronic abdominal pain, we found that low androgen levels were associated with both diagnosis and symptom severity in males and females. Furthermore, androgen homeostasis has clear links to gut microbiota. Like other steroids, androgens are inactivated by glucuronidation in the liver and excreted into bile. In the gut lumen, these inactive forms become substrates for microbial β-glucuronidase enzymes (GUS) that remove the glucuronide moieties, regenerating a large pool of active androgens. Thus, androgen reactivation could be a key mechanism by which commensal microbes limit visceral hypersensitivity, linking previous observations. The central hypothesis of this proposal is that androgens reactivated by microbial GUS signal directly to host visceral afferent neurons to limit peripheral sensitization and pain. First, we will establish the independent effects of commensal microbes, microbial GUS activity, and androgen signaling to visceral afferent neurons on abdominal pain. Then, we will test for mechanistic links between each component. Incorporating genetic and gnotobiotic mouse models as well novel inhibitors of microbial GUS enzymes developed by the co-I, a leader in GUS chemistry, this innovative project moves the PI's research program into new directions of visceral pain and host-microbe interactions. The impact of this work will be to advance the understanding of visceral sensation and generate key evidence for new rational therapeutic targets in abdominal pain.