PROJECT SUMMARY Insulin secretion deficiency is a hallmark of diabetes. In this process, the cortical actin of β cells has recently been recognized as a critical player contributing to diabetes pathogenesis. β cells from type-2 diabetes (T2D) patients show significant signaling alteration associated with cortical actin remodeling, and restoring such abnormality rescues insulin secretion to normal levels. Cortical actin is thought initially as a simple barrier to prevent insulin secretion, but recent studies challenge this view and imply an active role. Mounting evidence further indicates that cortical actin is a converging point and effector of multiple metabolic signals in β cells. However, the molecule and nanostructure basis of its remodeling remains unresolved. Addressing this question becomes imperative to understanding the regulation of insulin secretion and developing new therapeutics to restore β cell function in T2D. This proposal will investigate ArpC3, a gene encoding a subunit of the Arp2/3 complex that dictates actin filament branching and cortical remodeling. Although multiple signaling pathways converge on Arp2/3, the mechanism downstream Arp2/3 activity to regulate insulin secretion remains unknown. Based on recent progress in the field and our new data on β cell imaging and functional assays, we hypothesize that ArpC3-dependent actin polymerization regulates insulin exocytosis by driving the local cortical actin disassembly-reassembly cycles. Through this remodeling, cortical actin filaments may actively coordinate the critical steps of granule trafficking and insulin secretion. We have assembled a team of investigators with expertise in diabetes, β cell biology, vesicle trafficking, and super-resolution microscopy to test this hypothesis. We will focus on three specific aims. First, determine the in vivo role of ArpC3-mediated cortical actin remodeling in insulin secretion and glucose homeostasis. We have generated a new mouse model where ArpC3 is deleted selectively in adult β cells to ablate Arp2/3 activity and cortical actin branching. Second, define ArpC3 function in cortical actin nano-remodeling and its role in insulin granule recruitment, docking, and exocytosis. Third, evaluate the role of cortical actin branching in human β cells and its dysfunction in T2D patients. The results from this work will fill a long-standing knowledge gap between cortical actin remodeling and insulin secretion, providing mechanistic insights into T2D pathogenesis induced by cortical actin dysfunction.

PROJECT NARRATIVE Dysfunction of insulin secretion causes diabetes. This work investigates the molecular mechanism by which the cortical actin web of β cells regulates glucose-stimulated insulin granule secretion. The results from this work will inform new knowledge of the nanoscale remodeling of the actin cortex in insulin release, a question that is largely unknown yet critical to preserve and restore β cell function in diabetes.