Mitochondrial Clearance of Calcium Facilitated by MICU2 Controls Insulin Secretion

Abstract Objective Transport of Ca2+ into pancreatic β-cell mitochondria facilitates nutrient-mediated insulin secretion. The underlying mechanism, however, is unclear. Recent establishment of the molecular identity of the mitochondrial Ca2+ uniporter (MCU) and associated proteins allows modification of mitochondrial Ca2+ transport in intact cells. We examined the consequences of deficiency of the accessory protein, MICU2, in rat and human insulin-secreting cells and mouse islets. Methods siRNA-silencing of Micu2 in INS1-832/13 and EndoC-βH1 cell lines was performed; Micu2-/- mice were also studied. Insulin secretion and mechanistic analyses, utilizing live confocal imaging to assess mitochondrial function and intracellular Ca2+ dynamics, were performed. Results Silencing of Micu2 abrogated GSIS in INS1 832/13 and EndoC-βH1 cells. Micu2-/- mice also displayed attenuated GSIS. Mitochondrial Ca2+ uptake declined in MICU2-deficient INS1 832/13 and EndoC-βH1 cells in response to high glucose and high K+. Furthermore, MICU2 silencing in INS1 832/13 cells, presumably through its effects on mitochondrial Ca2+ uptake, perturbed mitochondrial function illustrated by absent mitochondrial membrane hyperpolarization and lowering of the ATP/ADP ratio in response to an elevation of glucose. Despite the loss of mitochondrial Ca2+ uptake, cytosolic Ca2+ was lower in siMICU2-treated INS1 832/13 cells in response to high K+. It was hypothesized that Ca2+ was accumulating in the submembrane compartment in MICU2-deficient cells, resulting in desensitization of voltage-dependent Ca2+ channels, thereby lowering total cytosolic Ca2+. Indeed, upon high K+ stimulation, MICU2-silenced cells showed higher and prolongated rises in submembrane Ca2+ levels. Conclusions MICU2 plays a critical role in β-cell mitochondrial Ca2+ uptake. β-cell mitochondria sequester Ca2+ from the submembrane compartment preventing desensitization of voltage-dependent Ca2+ channels, thereby facilitating GSIS.