NCLX is an essential component of mitochondrial Na⁺/Ca²⁺ exchange

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.