NCLX controls hepatic mitochondrial Ca²⁺extrusion and couples hormone-mediated mitochondrial Ca²⁺oscillations with gluconeogenesis

Abstract

Hepatic Ca2+ signaling is emerging as a key factor in mediating gluconeogenesis. However, the identity of the hepatic mitochondrial Ca2+ transporter is controversial and the role of mitochondria in controlling hormonal Ca2+ signaling and linking them to metabolic activity is poorly understood. We first interrogated the role of the mitochondrial Na+/Ca2+ exchanger NCLX by triggering cytosolic Ca2+ purinergic signaling in primary hepatocytes, and Ca2+ responses in isolated mitochondria from WT, global NCLX KO, and conditional hepatic NCLX KO mice models. We monitored a higher rate of Na+-dependent mitochondrial Ca2+ efflux in NCLX-expressing hepatocytes, indicating that it constitutes the major Ca2+ efflux pathway. We then asked if NCLX is controlling the hormone-dependent mitochondrial Ca2+ oscillations by employing physiological concentrations of glucagon and vasopressin. Consistent with previous studies, hormone applications triggered mitochondrial Ca2+ oscillations in WT hepatocytes. In NCLX KO hepatocytes the cytosolic oscillations persisted, however, the mitochondrial Ca2+ oscillations were suppressed. To further understand the metabolic role of NCLX in the hepatic system, we examined gluconeogenic function in vivo and ex vitro by monitoring hepatic glucose production. We found that blood glucose dropped faster in the conditional KO mice and their hepatic glucagon-dependent glucose production was reduced, indicating that gluconeogenesis was impaired in hepatic conditional NCLX KO mice. Taken together, our results indicate that NCLX is the primary Ca2+ extruder in hepatocytes and is required for mediating the hormone-dependent mitochondrial Ca2+ oscillations and gluconeogenesis.