The Mitochondrial Na+/Ca2+ Exchanger Upregulates Glucose Dependent Ca2+ Signalling Linked to Insulin Secretion

Abstract

Mitochondria mediate dual metabolic and Ca2+ shuttling activities. While the former is required for Ca2+ signalling linked to insulin secretion, the role of the latter in β cell function has not been well understood, primarily because the molecular identity of the mitochondrial Ca2+ transporters were elusive and the selectivity of their inhibitors was questionable. This study focuses on NCLX, the recently discovered mitochondrial Na+/Ca2+ exchanger that is linked to Ca2+ signalling in MIN6 and primary β cells. Suppression either of NCLX expression, using a siRNA construct (siNCLX) or of its activity, by a dominant negative construct (dnNCLX), enhanced mitochondrial Ca2+ influx and blocked efflux induced by glucose or by cell depolarization. In addition, NCLX regulated basal, but not glucose-dependent changes, in metabolic rate, mitochondrial membrane potential and mitochondrial resting Ca2+. Importantly, NCLX controlled the rate and amplitude of cytosolic Ca2+ changes induced by depolarization or high glucose, indicating that NCLX is a critical and rate limiting component in the cross talk between mitochondrial and plasma membrane Ca2+ signalling. Finally, knockdown of NCLX expression was followed by a delay in glucose-dependent insulin secretion. These findings suggest that the mitochondrial Na+/Ca2+ exchanger, NCLX, shapes glucose-dependent mitochondrial and cytosolic Ca2+ signals thereby regulating the temporal pattern of insulin secretion in β cells.

Beta-cell mitochondrial carriers and the diabetogenic stress response

“…m evoked by cell depolarization and accelerates the rise in ATP/ ADP ratio in response to glucose stimulation [46]. Consistently, the rise in [Ca 2+ ] m evoked by glucose is enhanced in β-cells when NCLX is expressed in a dominant negative form [50], in agreement with CGP37157-mediated inhibition of the Na 2+ /Ca 2+ exchanger [51].…”

Calcium signaling in pancreatic β-cells in health and in Type 2 diabetes

“…As for the MCU, the study of the mitochondrial Na 2+ /Ca 2+ exchanger was rendered possible only a few years ago thanks to the cloning of the exchanger, since previous studies used pharmacological inhibitors that displayed side effects [265][266][267]. The silencing of NCLX in mouse ␤-cells strongly reduces the efflux of Ca 2+ and regulates the rate of Ca 2+ influx in mitochondria [9,268]. Additionally it reduces [Ca 2+ ] c changes and the first phase of insulin secretion [268].…”

Essential Role of Mitochondrial Ca2+ Uniporter in the Generation of Mitochondrial pH Gradient and Metabolism-Secretion Coupling in Insulin-releasing Cells

“…Especially MICU1/2 negatively regulate MCU activity under resting cytosolic Ca 2ϩ ([Ca 2ϩ ] i ). At stimulating [Ca 2ϩ ] i (Ͼ10 M), however, MICU1 activates MCU activity, implying that the regulatory subunits of the MCU complex modulate mitochondrial Ca 2ϩ loads of ⌬⌿ mito -driven Ca 2ϩ uptake without perturbing the important signal propagation from ER to mitochondria (13,18,19 ] mito , accelerates mitochondrial oxidative metabolism and GSIS (25)(26)(27)(28).…”