2078-P: APPL2 Deficiency Diminishes Glucose-Stimulated Insulin Secretion in Pancreatic Beta Cells

In response to elevated postprandial blood glucose, remodeling of F-actin enables exocytosis of insulin granules from pancreatic β cells. Dysregulation of F-actin remodeling contributes to defective glucose-stimulated insulin secretion (GSIS) in β cells and subsequent type 2 diabetes. The adaptor protein APPL1 potentiates GSIS and prevents β cell loss in diabetes, but the role of its close homolog APPL2 in β cell function remains unknown. Here we demonstrated that APPL2 controls GSIS by Rac1-mediated F-actin depolymerization via interaction with Rac GTPase activating protein 1 (RacGAP1). β-cell specific deletion of APPL2 induced defective GSIS and glucose intolerance in mice. Ex vivo study revealed a dramatic reduction in both first- and second-phase GSIS in islets, accompanied by a dysregulation of F-actin remodeling. Consistently, knockdown of APPL2 caused impaired glucose-induced F-actin remodeling and insulin secretion in INS-1E β cells. The defects were mainly due to the inactivation of Rac1- the key player controlling insulin granule secretion via F-actin remodeling. Ectopic expression of constitutively active Rac1 mutant rescued the defective GSIS and F-actin remodeling in APPL2 deficient β cells. Furthermore, we identified RacGAP1 as an interacting partner of APPL2. RacGAP1 interacted with and regulate activity of Rac1, and overexpression of RacGAP1 significantly suppressed glucose-induced F-actin depolymerization, whereas co-expression with APPL2 antagonized this suppressive effect. In addition, the defects in APPL2 knockdown β cells were largely reversed by simultaneous downregulation of RacGAP1 or co-expression of constitutively active Rac1 mutant.

In summary, our study suggest that APPL2 potentiates GSIS by antagonizing the inhibitory effect of RacGAP1 on Rac1 activation and F-actin remodeling and insulin secretion in β cells, highlighting the importance of APPL2 in the regulation of GSIS in pancreatic β cells and glucose homeostasis.