352-OR: GABA Modulates Pancreatic Beta-Cell Function through Ca2+ Signaling

Introduction and Objective: Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter uniquely found outside the central nervous system at high concentrations in pancreatic beta cells. Understanding the functional role of GABA in the islet has been challenging due to conflicting results that often relied on supraphysiological concentrations of exogenous GABA. Surprisingly, a genetic model to delete GABA from the islet has not been developed. Here, we generated and characterized a beta-cell specific knockout of both GABA-synthesizing enzymes (GAD65 and GAD67) (β-Gad1,2-/-). Methods: Pancreatic beta cell function was analyzed in vivo by glucose tolerance test and in vitro using isolated islets. Results: We confirmed deletion of GAD65/GAD67 by western blot and showed that the knockout islets were devoid of GABA by HPLC. β-Gad1,2-/- mice exhibited hypersecretion of insulin, with normal islet size, number, and endocrine cell ratio. In vivo, male β-Gad1,2-/- mice showed impaired glucose tolerance, increased insulin and c-peptide secretion, and faster weight gain. Ca2+ imaging showed that β-Gad1,2-/- islets had a shorter delay to initiate Ca2+ mobilization in response to glucose, sustained longer maximal Ca2+ activation, and altered Ca2+ oscillations with a prolonged active phase of Ca2+ wave and reduced differences between maximal and minimal Ca2+ levels, suggesting impaired ability to generate distinct active and silent phases critical for pulsatile insulin secretion. Pharmacological inhibition of GABA signaling in control islets with a GABAAR antagonist mimicked the effect of the GAD knockout, indicating that GABA functions via the GABAAR. Conclusion: We conclude that GABA's principal role in beta cells is to regulate insulin secretion by inhibiting beta cell excitability. These findings identify the functional role for GABA in the islet is to ensure proper beta cell stimulus-secretion coupling by regulating the strength and speed of the initial Ca2+ response followed by enforcing symmetry and enhancing the amplitude of subsequent Ca2+ oscillations. Disclosure S. Ferreira: None. A.E. Stis: None. A. Widener: None. A.E. Cuaycal: None. E. Phelps: Research Support; Immunocore, Ltd, Meso Scale Diagnostics, LLC. Funding NIH (R01DK124267); DRC (AWD11953)