Primary Cilia Regulate GLP-1 Signaling in Pancreatic Islets

Project Summary Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are clinically proven drugs for weight loss and diabetes, yet their mechanisms of action are incompletely understood. Notably, in pancreatic β cells, how does the incretin hormone glucagon-like peptide-1 (GLP-1) amplify glucose-stimulated insulin secretion is still an open question. We know that GLP-1 signals through a class B GPCR, the GLP-1 receptor (GLP-1R), that is expressed abundantly in β cells. Little is known, however, about the subcellular compartmentalization of GLP-1R signaling in β cells, and how these organellar compartments cooperatively bring about GLP-1 signaling effects on insulin secretion. My preliminary data shows that the primary cilium, a sensory organelle of the β cell, contains a distinct pool of GLP-1R that may mediate early responses to GLP-1 and trigger downstream cytosolic second- messenger cascades that lead to insulin secretion. These findings build on prior work by my mentors’ labs and others that demonstrated a role for primary cilia in regulating glucose-stimulated Ca2+ signaling and insulin secretion in the β cell, while mice lacking cilia on β cells develop glucose intolerance and diet-induced diabetes. My project tests the hypothesis that β-cell primary cilium acts as a unique GLP-1-sensing domain that determines whole-cell responses to GLP-1. To dissect the subcellular mechanisms of GLP-1 signaling, I will utilize recently developed imaging tools and genetic models to quantitatively measure GLP-1-dependent Ca2+ and cAMP signaling in the β-cell primary cilium and cell body (Aim 1). In parallel, I will examine mechanisms of GLP-1R trafficking to the primary cilium and test the effect of ciliary de-localization of GLP-1R using genetic deletion of Tulp3, a ciliary GPCR adaptor protein (Aim 2). Completion of these studies will provide a detailed molecular understanding of how primary cilia regulate GLP-1 signaling in β cells and may identify new cilia signaling pathways that have the potential to improve diabetes and human metabolic health. Through this fellowship application, I will develop 1) a novel understanding of the regulation and compartmentalization of GLP- 1 signaling and 2) my potential as an independent investigator focused on endocrine cell function and diabetes. These training goals will be facilitated by the detailed research plan, a team of remarkably qualified mentors with expertise in the proposed study design, and exceptional facilities and training environment through Washington University.