Although functional gastrointestinal (GI) disorders (FGIDs) are the most common causes of bowel dysfunction worldwide, many patients are inadequately treated because current therapies are frequently ineffective. The inadequacy of therapy is largely due to an incomplete comprehension of underlying mechanisms that are critical for the design of new treatments. Up to half of individuals with FGIDs also suffer from mood disorders. Evidence supports the idea that the GI tract is vulnerable to genetic perturbations that can exert lasting effects on GI function and mood. It is thus conceivable that a co-morbid FGID and psychiatric condition result from abnormalities occurring as the result of a genetic variant. Discovery of the pathophysiology underlying FGIDs and psychiatric co-morbidities is likely to enhance understanding of their relationship and thus lead to novel therapies for both. Serotonin (5-HT), which is a major determinant of enteric and central nervous system (ENS and CNS) development and also modulates FGID-related symptoms (GI motility) as well as mood, may be an important developmental modulator of FGID pathogenesis. It is 5-HT stimulation of the 5-HT4 receptor, however, that has the most well-studied prokinetic, anti-nociceptive, anti- depressive, and anti-anxiety effects and has thus been targeted to treat both GI dysmotility and mood dysfunction. Curiously, however, >50% of patients with FGIDs are relatively unresponsive to them. The reason for 5-HT4 treatment failure is unknown, making this a critical treatment obstacle. Our preliminary data strongly suggest that failure to respond to a 5-HT4 agonist is due to a defect in 5-HT4 trafficking. Although defects in enteric 5-HT4 trafficking have never previously been explored, such abnormalities have been described in the CNS; p11 is a critical adaptor molecule involved in this transport of 5-HT4 receptors to cell surfaces, where the receptors become available to mediate 5-HT signaling. P11 also plays a role in depression (p11KO mice exhibit depressive behaviors and CNS p11 transcription is impaired in mouse models of depression and human suicide victims). We have found that in the ENS, as in the CNS, gut p11 is co- expressed with 5-HT4 receptors where they co-immunoprecipitate, suggesting that p11 interacts with 5-HT4 in the gut and, further, that p11 affects 5-HT4 receptor-mediated actions on ENS development and GI motility. Our hypotheses are thus that p11 facilitation of trafficking of 5-HT4 receptors to cell surfaces is essential for 5-HT4- modulation of ENS development and function and that p11 dysfunction thus underlies comorbid FGID and depression. In this application we will explore: (1) How critical the p11-5-HT4 interaction is for effective GI motility utilizing a comprehensive array of in vivo and ex vivo studies the selective 5-HT4 agonist, prucalopride, in WT and p11KO mice; (2) If effects of p11 on mood (depression / anxiety), ENS development, and GI motility depend on mucosal or enteric neuronal p11, using novel transgenic mice that selectively lack p11 in the enteric epithelium or ENS and; (3) If enteric 5-HT4 trafficking and function are p11-dependent.
Signaling molecules, such as serotonin (5-HT), which do not cross cell membranes, must activate receptors on cell surfaces to which receptors are transported from their intracellular site of biosynthesis. Evidence suggests that insertion of the 5-HT4 receptor subtype into cell surface membranes in the brain is critically dependent on an interaction with another molecule, p11, which functions as an adaptor. We plan to test three hypotheses strongly supported by our preliminary data, (i) that the p11-5-HT4 interaction is necessary for insertion of 5-HT4 receptors into cell surface membranes of intestinal nerve and epithelial cells, is thus (ii) essential for 5- HT4-stimulation of enteric neuronal development and function, and, (iii) when defective, underlies the frequent co-existence of 5-HT4-mediated brain and gut disorders, including constipation and depression.