Mitochondrial Ca²⁺efflux is linked to numerous cellular activities and pathophysiological processes. Although it is established that an Na⁺-dependent mechanism mediates mitochondrial Ca²⁺efflux, the molecular identity of this transporter has remained elusive. Here we show that the Na⁺/Ca²⁺exchanger NCLX is enriched in mitochondria, where it is localized to the cristae. Employing Ca²⁺and Na⁺fluorescent imaging, we demonstrate that mitochondrial Na⁺-dependent Ca²⁺efflux is enhanced upon overexpression of NCLX, is reduced by silencing of NCLX expression by siRNA, and is fully rescued by the concomitant expression of heterologous NCLX. NCLX-mediated mitochondrial Ca²⁺transport was inhibited, moreover, by CGP-37157 and exhibited Li⁺dependence, both hallmarks of mitochondrial Na⁺-dependent Ca²⁺efflux. Finally, NCLX-mediated mitochondrial Ca²⁺exchange is blocked in cells expressing a catalytically inactive NCLX mutant. Taken together, our results converge to the conclusion that NCLX is the long-sought mitochondrial Na⁺/Ca²⁺exchanger.

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.

New Findings: What is the topic of this review? This paper overviews the links between Ca²⁺ and Na⁺ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na⁺ and Ca²⁺ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na⁺–Ca²⁺ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca²⁺ and Na⁺ signalling for co‐ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed. Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca²⁺ and Na⁺ is tightly linked through several molecular pathways that generate Ca²⁺ and Na⁺ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca²⁺ release with generation of Na⁺ and Ca²⁺ currents. The plasmalemmal Na⁺–Ca²⁺ exchanger (NCX) flickers between forward and reverse mode to co‐ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na⁺–Ca²⁺ exchanger (NCLX) mediate Ca²⁺ entry into and release from this organelle and couple cytosolic Ca²⁺ and Na⁺ fluctuations with cellular energetics. Cellular Ca²⁺ and Na⁺ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.