AIMS/HYPOTHESIS: Unimolecular peptides targeting the receptors for glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon (GCG) have been shown to improve glycaemic management in both mice and humans. Yet the identity of the downstream signalling events mediated by these peptides remain to be elucidated. Here, we aimed to assess the mechanisms by which a validated peptide triagonist for GLP-1/GIP/GCG receptors (IUB447) stimulates insulin secretion in murine pancreatic islets. METHODS: Islets were isolated from wild-type (WT), Gipr-knockout (Gipr(-/-)), Gcgr-knockout (Gcgr(-/-)), Glp-1r (also known as Glp1r)/Gipr double-knockout and Trpm5-knockout (Trpm5(-/-)) mice, followed by assessment of beta cell function and insulin secretion in response to mono- and multi-agonist administration. Metabolic phenotypes of WT and Trpm5(-/-) mice under chow and high-fat diets were investigated following triagonist application. RESULTS: The triagonist promoted glucose-stimulated insulin secretion (GSIS) to a greater degree than co-administration of conventional mono-agonists in WT mouse islets. The triagonist-induced increase in GSIS was unchanged in the absence of either Gipr or Gcgr. However, the triagonist failed to enhance insulin secretion in islets lacking both Glp-1r and Gipr and upon treatment with the GLP-1 receptor-specific antagonist exendin-3 (9-39). Similarly, the specific blocking of G
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