Mitochondrial clearance of calcium facilitated by MICU2 controls insulin secretion

Objective: Transport of Ca²⁺ into pancreatic β cell mitochondria facilitates nutrient-mediated insulin secretion. However, the underlying mechanism 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 the INS-1 832/13 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 the INS-1 832/13 and EndoC-βH1 cells. The Micu2^−/− mice also displayed attenuated GSIS. Mitochondrial Ca²⁺ uptake declined in MICU2-deficient INS-1 832/13 and EndoC-βH1 cells in response to high glucose and high K⁺. MICU2 silencing in INS-1 832/13 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 elevated glucose. Despite the loss of mitochondrial Ca²⁺ uptake, cytosolic Ca²⁺ was lower in siMICU2-treated INS-1 832/13 cells in response to high K⁺. It was hypothesized that Ca²⁺ accumulated in the submembrane compartment in MICU2-deficient cells, resulting in desensitization of voltage-dependent Ca²⁺ channels, lowering total cytosolic Ca²⁺. Upon high K⁺ stimulation, MICU2-silenced cells showed higher and prolonged increases in submembrane Ca²⁺ levels. Conclusions: MICU2 plays a critical role in β cell mitochondrial Ca²⁺ uptake. β cell mitochondria sequestered Ca²⁺ from the submembrane compartment, preventing desensitization of voltage-dependent Ca²⁺ channels and facilitating GSIS.