Sequential forward and reverse transport of the Na(+) Ca(2+) exchanger generates Ca(2+) oscillations within mitochondria

Mitochondrial Ca(2+) homoeostasis regulates aerobic metabolism and cell survival. Ca(2+) flux into mitochondria is mediated by the mitochondrial calcium uniporter (MCU) channel whereas Ca(2+) export is often through an electrogenic Na(+)–Ca(2+) exchanger. Here, we report remarkable functional versatility in mitochondrial Na(+)–Ca(2+) exchange under conditions where mitochondria are depolarised. Following physiological stimulation of cell-surface receptors, mitochondrial Na(+)–Ca(2+) exchange initially operates in reverse mode, transporting cytosolic Ca(2+) into the matrix. As matrix Ca(2+) rises, the exchanger reverts to its forward mode state, extruding Ca(2+). Transitions between reverse and forward modes generate repetitive oscillations in matrix Ca(2+). We further show that reverse mode Na(+)–Ca(2+) activity is regulated by the mitochondrial fusion protein mitofusin 2. Our results demonstrate that reversible switching between transport modes of an ion exchanger molecule generates functionally relevant oscillations in the levels of the universal Ca(2+) messenger within an organelle.