PROJECT SUMMARY Worldwide, the combined syndromes of Type 1 and Type 2 diabetes mellitus affect nearly 387 million individuals, with a prevalence that is expected to increase by more than 50% over the next 10 years. Inadequate insulin secretion from the pancreatic beta cell plays a central role in the pathophysiology of both forms of diabetes, however the pathways that lead to beta cell failure remain poorly understood. This knowledge gap has led to a paucity of clinical therapies specifically focused on restoration of beta cell health in either disease. The long-term goal of our research is to define common and overlapping pathways of beta cell dysfunction in Type 1 and Type 2 diabetes and to delineate the role of altered beta cell calcium homeostasis in disease pathophysiology. Calcium plays a vital role in many processes that govern beta cell function, including the production, maturation, and regulated secretion of insulin. The fidelity of these processes depends on the maintenance of calcium subcompartments and their respective transmembrane gradients, which are organized at both the cellular and organelle level. The endoplasmic reticulum, Golgi apparatus, and secretory granules comprise a functional module referred to as the beta cell secretory pathway, through which insulin mRNA is translated and preproinsulin protein is processed to mature insulin and packaged for exocytosis. In aggregate, these organelles also represent the largest depots of intracellular calcium. Here, we will test the hypothesis that calcium dyshomeostasis arising within this integrated secretory unit is a key determinant of impaired beta cell health in both Type 1 and Type 2 diabetes. The specific goals of this work are to: (1) elucidate a role for the sarco-endoplasmic reticulum calcium ATPase 2b (SERCA2b) pump in the molecular regulation of calcium in the proximal secretory pathway; (2) define a complementary yet distinct role for the secretory pathway calcium ATPase 1 (SPCA1) pump within the distal secretory pathway; (3) identify how altered calcium levels within this functional module impact insulin translation, processing, and secretion; (4) dissect calcium-regulated communication between secretory pathway organelles; (5) establish a novel role for beta cell Golgi stress in diabetes pathophysiology; and (6) test genetic and pharmacologic strategies to restore secretory pathway calcium homeostasis and beta cell function.