NCX3 regulates mitochondrial calcium handling through AKAP121-anchored signaling complex and prevents hypoxia-induced cell death.

The mitochondrial influx and efflux calcium pathways play a relevant role in cytosolic and mitochondrial calcium homeostasis and contribute to the regulation of mitochondrial functions in neurons. The mitochondrial Na+/Ca2+ exchanger, although hypothesized 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 last three decades. Recently, a lithium-dependent sodium/calcium exchanger extruding calcium 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 calcium efflux into the cytoplasm. In this study, we have demonstrated for the first time that the nuclear encoded NCX3 isoform (a) is localized on the outer mitochondrial membrane (OMM) of neurons, (b) co-localizes and immunoprecipitates with AKAP121, a member of the protein kinase A anchoring proteins (AKAPs) present on the outer membrane, (c) extrudes calcium from mitochondria through AKAP121 interaction in a PKA-mediated manner, both under normoxia and hypoxia, and (d) 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 calcium handling from the OMM through an AKAP121-anchored signalling complex, thus promoting cell survival during hypoxia.