Novel Targets for GI Hormones and Neuropeptides in Regulation of Intestinal Motility

Summary of the Project: Gastrointestinal (GI) motility is a complex group motor behaviors controlled by myogenic, neurogenic, hormonal and additional levels of regulation. Previous research has demonstrated that myogenic regulation is actually accomplished by smooth muscle cells (SMCs), interstitial cells of Cajal (ICC) and cells labeled by antibodies to PDGFRa (PDGFRa+ cells). These cells are electrically coupled, forming a network known as the SIP syncytium. Hormonal regulation has been thought to be due to direct regulation of the excitability of SMCs or to occur indirectly through actions on the enteric nervous system. However, RNAseq of SIP cells sorted to purity by FACS has shown that ICC and PDGFRa+ cells have significant and sometimes dominant expression of receptors for neural and hormonal peptides, especially secretin, GLP-2 and CGRP. This project will explore novel targets for these peptides first by determining the localization of receptors in specific subtypes of ICC and PDGFRa+ cells in muscles of the small intestine and colon and then characterizing the actions of secretin, GLP-2 and CGRP on these cells. Ca2+ signaling is the main mechanism responsible for the behaviors of ICC and PDGFRa+ cells, as these cells function by activation of signature Ca2+-dependent conductances (e.g. ANO1 in ICC and SK3 in PDGFRa+ cells). Preliminary experiments using confocal imaging of Ca2+ transients before and during exposure to tetrodotoxin show direct, non-neurogenic effects of secretin, GLP-2 and CGRP on ICC or PDGFRa+ cells. The effects of these peptides, mediated by ICC or PDGFRa+ cells, have profound effects on the electrophysiology and contractile behaviors of intact muscles and dramatically alter motility of the small intestine and colon. Cellular mechanisms utilized by secretin, GLP-2 and CGRP will be investigated in depth using cell-specific, transgenic manipulation and optogenetic, chemogenic and FRET techniques. Preliminary experiments suggest this project will demonstrate 3 important novel concepts of regulation of GI motility: i) imbalances in hormonal responses in the SIP syncytium can compromise or even nullify post-junctional neural responses; ii) cAMP-dependent mechanisms control Ca2+ signaling in ICC; iii) axon reflexes, mediated through PDGFRa+ cells, provide powerful regulation of colonic motility. These new concepts may provide new ideas for causes of and therapies for GI motor diseases.