Mitochondrial Ca2+ uptake 1 (MICU1) and mitochondrial ca2+ uniporter (MCU) contribute to metabolism-secretion coupling in clonal pancreatic β-cells

In pancreatic β-cells, uptake of CA(2+) into mitochondria facilitates metabolism-secretion coupling by activation of various matrix Enzymes, thus facilitating ATP generation by Oxidative Phosphorylation and, in turn, augmenting Insulin release. We employed an siRNA-based approach to evaluate the individual contribution of four proteins that were recently described to be engaged in mitochondrial CA(2+) sequestration in clonal INS-1 832/13 pancreatic β-cells: the mitochondrial CA(2+) uptake 1 (MICU1), mitochondrial CA(2+) uniporter (MCU), uncoupling protein 2 (UCP2), and leucine zipper EF-hand-containing transmembrane protein 1 (LETM1). Using a FRET-based genetically encoded CA(2+) sensor targeted to mitochondria, we show that a transient knockdown of MICU1 or MCU diminished mitochondrial CA(2+) uptake upon both intracellular CA(2+) release and CA(2+) entry via L-type channels. In contrast, knockdown of UCP2 and LETM1 exclusively reduced mitochondrial CA(2+) uptake in response to either intracellular CA(2+) release or CA(2+) entry, respectively. Therefore, we further investigated the role of MICU1 and MCU in metabolism-secretion coupling. Diminution of MICU1 or MCU reduced mitochondrial CA(2+) uptake in response to d-glucose, whereas d-glucose-triggered cytosolic CA(2+) oscillations remained unaffected. Moreover, d-glucose-evoked increases in cytosolic ATP and d-glucose-stimulated Insulin secretion were diminished in MICU1- or MCU-silenced cells. Our data highlight the crucial role of MICU1 and MCU in mitochondrial CA(2+) uptake in pancreatic β-cells and their involvement in the positive feedback required for sustained Insulin secretion.