Glucagon-like peptide-2 promotes gallbladder refilling via a TGR5-independent, GLP-2R-dependent pathway

Abstract

Objective: Glucagon-like peptides (GLPs) are secreted from enteroendocrine cells in response to nutrients and bile acids and control metabolism via actions on structurally-related yet distinct G protein coupled receptors. GLP-1 regulates gut motility, appetite, islet function, and glucose homeostasis, whereas GLP-2 enhances intestinal nutrient absorption. GLP-1R agonists are used to treat diabetes and obesity, and a GLP-2R agonist is approved to treat short bowel syndrome. Unexpectedly, reports of gallbladder disease have been associated with the use of both GLP-1R and GLP-2R agonists and after bariatric surgery, although the mechanisms remain unknown. Methods: We investigated whether GLP-1 or GLP-2 acutely controls gallbladder (GB) volume and whether GLP-2 regulates GB muscle activity in mice. The expression of Tgr5, Glp2r, and Glp1r was assessed in mouse GB, and the effects of GLP-2 on hepatic bile acid (BA) flow, intestinal and liver BA uptake, and GB gene expression were determined. GLP-2 regulation of GB volume was assessed in wildtype, Glp2r−/− and Tgr5−/− mice. The effect of GLP-2 on GB smooth muscle (GBSM) calcium transients was characterized ex vivo. Results: Acute administration of the GLP-1R agonist exendin-4 lowered glucose but had no effect on GB volume in mice. In contrast, GLP-2 rapidly enhanced GB filling in a dose-dependent manner, actions maintained in the presence of cholecystokinin, and mediated through the canonical GLP-2R. GLP-2 also rapidly induced immediate early gene expression in GB, consistent with detection of the endogenous Glp2r in GB RNA. The ability of GLP-2 to increase GB volume was not abrogated by systemic administration of hexamethonium, propranolol, a vasoactive peptide receptor antagonist or N-Nitroarginine methyl ester, and was maintained in Tgr5−/− mice. In contrast, lithocholic acid, a Tgr5 agonist, increased GB filling in Glp2r−/− but not in Tgr5−/− mice. GLP-2 had no effect on ileal uptake or hepatic clearance of taurocholic acid or on hepatic bile flow, yet reduced the frequency of spontaneous calcium transients in murine GBSM ex vivo, in a tetrodotoxin-sensitive manner. Conclusions: Our data extend endocrine concepts of regulation of GB filling beyond FXR-FGF15/19 and the direct effects of BA via Tgr5, to encompass a novel BA-Tgr5-L cell GLP-2 axis providing nutrient-mediated feedback from BA to terminate meal-related GB contraction. These findings have implications for conditions characterized by elevated circulating levels of GLP-2 such as after bariatric surgery and the development and use of agents that promote Tgr5 activation, L cell secretion, or GLP-2R agonism for the treatment of metabolic disease. Keywords: Bile acids, Gallbladder, Enteroendocrine, Glucagon-like peptide, TGR5, GLP-1, GLP-2