The Mitochondrial Ca(2+) uniporter is a central regulator of interorganellar Ca(2+) transfer and NFAT activation

Mitochondrial Ca(2+) uptake tailors the strength of stimulation of plasma membrane phospholipase C–coupled receptors to that of cellular bioenergetics. However, how Ca(2+) uptake by the mitochondrial Ca(2+) uniporter (MCU) shapes receptor-evoked interorganellar Ca(2+) signaling is unknown. Here, we used CRISPR/Cas9 gene knockout, subcellular Ca(2+) imaging, and mathematical modeling to show that MCU is a universal regulator of intracellular Ca(2+) signaling across mammalian cell types. MCU activity sustains cytosolic Ca(2+) signaling by preventing Ca(2+)-dependent inactivation of store-operated Ca(2+) release–activated Ca(2+) channels and by inhibiting Ca(2+) extrusion. Paradoxically, MCU knockout (MCU-KO) enhanced cytosolic Ca(2+) responses to store depletion. Physiological agonist stimulation in MCU-KO cells led to enhanced frequency of cytosolic Ca(2+) oscillations, endoplasmic reticulum Ca(2+) refilling, nuclear translocation of nuclear factor for activated T cells transcription factors, and cell proliferation, without altering inositol-1,4,5-trisphosphate receptor activity. Our data show that MCU has dual counterbalancing functions at the cytosol–mitochondria interface, whereby the cell-specific MCU-dependent cytosolic Ca(2+) clearance and buffering capacity of mitochondria reciprocally regulate interorganellar Ca(2+) transfer and nuclear factor for activated T cells nuclear translocation during receptor-evoked signaling. These findings highlight the critical dual function of the MCU not only in the acute Ca(2+) buffering by mitochondria but also in shaping endoplasmic reticulum and cytosolic Ca(2+) signals that regulate cellular transcription and function.