Differential Spatial cAMP Signaling in the Pancreatic Beta cell - PROJECT SUMMARY/ABSTRACT Cyclic adenosine monophosphate (cAMP) is a key signaling molecule that is essential for pancreatic beta- cell gene expression, function, and survival. Recent studies indicate that the cellular response to cAMP signaling varies based on the location within the cell where cAMP is generated. Important targets of diabetes therapy are G-protein coupled incretin hormone receptors located on beta-cells. Upon ligand binding on the cell surface, the incretin hormone receptors glucagon like peptide-1 (GLP-1R), glucagon (GCGR), and glucose dependent insulinotropic peptide (GIPR) activate the G-protein Gas-to stimulate cAMP synthesis at the inner surface of the cytoplasmic membrane. Furthermore, these ligand-bound receptors are internalized into early endosomes within the cell body, from where they continue to generate cAMP before recycling to the cell surface or before being degraded through the ubiquitination pathway. Although incretin hormone receptor agonists are central to diabetes treatment and generate cAMP from different locations within the beta-cell, very little is understood about the differential role of cAMP signaling events that originate from different intracellular compartments. Furthermore, therapeutic incretin receptor agonists are designed to potentiate insulin secretion, while their effects on beta-cell gene expression remain poorly defined. Based on our preliminary studies, we hypothesize that the beta-cell differentially interprets cAMP signaling based on the spatial location of cAMP synthesis. We hypothesize that incretin hormone GPCR-stimulated cAMP signaling from the cell surface predominantly potentiates glucose-stimulated insulin secretion (GSIS) with little effects on gene expression in the nucleus. Conversely, cAMP generated at endosomes has minimal effects on GSIS potentiation, but potently regulates cAMP-dependent gene expression in the nucleus. We also find that in mice that are fed a diabetogenic diet, incretin hormones retain their ability to potentiate GSIS (cAMP from the plasma membrane) but lose their ability to regulate beta-cell gene expression (cAMP from the endosome). Thus, beta-cell endosomal cAMP signaling is defective early in diabetes pathogenesis, thereby leading to altered gene expression. We now seek to expand our novel and exciting findings specifically a) to understand the differential roles of cAMP generated from different cellular locations in regulating beta-cell function and gene expression; b) to understand the consequences of selective disruption of endosome-derived cAMP signaling on beta-cell function and gene expression Our studies will yield important and fundamental insights into the role of cAMP signaling in beta-cell biology. Furthermore, our studies have the potential to identify new molecular mechanisms that can be targeted in order to prevent and/or reverse the relentlessly progressive beta-cell deterioration in diabetes mellitus.
