Mitochondrial Sodium/Calcium Exchanger (NCLX) Regulates Basal and Starvation-Induced Autophagy Through Calcium Signaling

In addition to their central role in energy metabolism, mitochondria can shape intracellular calcium (Ca2+) signaling through the concerted activity of Ca2+ uptake to the matrix through the mitochondrial calcium uniporter, and efflux from the matrix mediated by the mitochondrial Na+/Ca2+ exchanger, NCLX. Ca2+ levels in the cytosol are known to regulate both metabolism and autophagic activity. Here, we describe a novel and direct relationship between NCLX, cytosolic Ca2+, and autophagic activity. We find that conditions that stimulate autophagy in vivo and in vitro such as caloric restriction and nutrient deprivation upregulate NCLX expression in hepatic tissue and cells. Conversely, siRNA-mediated knockdown of NCLX impairs basal and starvation-induced autophagy, by decreasing LC3 II flux. Similarly, acute inhibition of NCLX activity by CGP 37157 affects early steps of autophagy activation in hepatocyte-derived cells and mouse embryonic fibroblasts, resulting in reduced autophagic activity. Inhibition of NCLX caused decreased cytosolic Ca2+ levels, and pre-incubation with intracellular Ca2+ chelator BAPTA-AM promotes similar results to those of CGP 37157 on autophagy. BAPTA-AM also completely abolished the effects of NCLX inhibition on autophagy, demonstrating that mitochondrial Ca2+ efflux regulates autophagy through the modulation of cytosolic Ca2+ signaling. Collectively, our results demonstrate that the mitochondrial Ca2+ extrusion pathway NCLX is an important regulatory node linking nutrient restriction and autophagy regulation through the modulation of cytosolic Ca2+ levels.