NCX3 regulates mitochondrial Ca2+ handling through the AKAP121-anchored signaling complex and prevents hypoxia-induced neuronal death

The mitochondrial influx and efflux of Ca2+ play a relevant role in cytosolic and mitochondrial Ca2+ homeostasis, and contribute to the regulation of mitochondrial functions in neurons. The mitochondrial Na+/Ca2+ exchanger, which was first postulated in 1974, has been primarily investigated only from a functional point of view, and its identity and localization in the mitochondria have been a matter of debate over the past three decades. Recently, a Li+-dependent Na+/Ca2+ exchanger extruding Ca2+ from the matrix has been found in the inner mitochondrial membrane of neuronal cells. However, evidence has been provided that the outer membrane is impermeable to Ca2+ efflux into the cytoplasm. In this study, we demonstrate for the first time that the nuclear-encoded NCX3 isoform (1) is located on the outer mitochondrial membrane (OMM) of neurons; (2) colocalizes and immunoprecipitates with AKAP121 (also known as AKAP1), a member of the protein kinase A anchoring proteins (AKAPs) present on the outer membrane; (3) extrudes Ca2+ from mitochondria through AKAP121 interaction in a PKA-mediated manner, both under normoxia and hypoxia; and (4) improves cell survival when it works in the Ca2+ efflux mode at the level of the OMM. Collectively, these results suggest that, in neurons, NCX3 regulates mitochondrial Ca2+ handling from the OMM through an AKAP121-anchored signaling complex, thus promoting cell survival during hypoxia.