The Role of Pancreatic ÃÂò-cell Store-Operated Calcium Entry in Fatty Acid Metabolism

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Abstract The prevalence of Type 2 Diabetes Mellitus (T2DM) is increasing at an alarming rate, highlighting the need to better understand the pathophysiology this syndrome. T2DM develops as a result of increased peripheral insulin resistance that develops in the setting of obesity, coupled with inadequate insulin secretion from the pancreatic β cells. A number of factors are thought to contribute to β cell death and dysfunction in T2DM including endoplasmic reticulum (ER) stress, β cell calcium dyshomeostasis and lipotoxicity resulting from increased levels of non-esterified free fatty acids (FFA). The goal of this research project is to define how FFAs alter β cell calcium (Ca2+) homeostasis and triggers the transition to a state of hyperglycemia in T2DM. The maintenance of robust levels of insulin secretion and insulin production depends on intact Ca2+ signaling in the β cell, in part patterned by the sequestration of high levels of Ca2+ within the ER lumen. Store-operated Ca2+ entry (SOCE) is a mechanism that functions to refill ER Ca2+ stores in response to ER Ca2+ depletion. SOCE is activated by the ER Ca2+ sensor STIM1, which forms complexes with plasmalemmal Orai or TRP cation channels. The formation of these complexes allows SOCE-induced Ca2+ influx from the extracellular space to refill depleted ER Ca2+ stores. Previous work in our lab has shown that β cell SOCE is impaired in T2DM due to a reduction in the expression of STIM1, leading to ER stress and reduced insulin secretion. Preliminary data have shown that lipotoxic stress induced by chronic palmitate treatment leads to dysfunctional SOCE, reduced STIM1 expression, calpain activation, and β cell death. Lastly, STIM1 ablation in INS-1 β cells led to an intracellular accumulation of lipid droplets following FFA treatment. Based on this scientific premise, this project will test the hypothesis that STIM1-induced SOCE plays a critical role in normal lipid handling in the pancreatic β cell, while dysfunctional SOCE leads to increased susceptibility to lipotoxic stress. To address this hypothesis, β-cell specific knockout mouse model of STIM1 (STIM1Δβ) and STIM1 knockout (STIM1KO) INS-1 β cells will be utilized. The goal of Aim 1 is to explore the cellular mechanisms by which loss of STIM1 leads to susceptibility to lipotoxic stress through lipid droplet accumulation, defective lipolysis, cAMP signaling and ER stress leading to calpain-induced cell death. In Aim 2, STIM1Δβ mice will be utilized to define the physiological impact of the loss of β cell STIM1 on whole body glucose homeostasis in response to a high-fat diet. Taken together, this study will provide a novel mechanistic insight into SOCE as a potential therapeutic target in the progressive development of T2DM. This fellowship award will support two years of graduate research training and two years of clinical training in the Medical Scientist Training Program (MSTP) of Indiana University (IU) School of Medicine, under the mentorship of Dr. Carmella Evans-Molina. The active, collaborative environment of IU Center for Diabetes and Metabolic Diseases (CDMD) will ensure a successful training program.