Nox4 regulates InsP3 receptor‐dependent Ca²⁺ release into mitochondria to promote cell survival.

Cells subjected to environmental stresses undergo regulated cell death (RCD) when homeostatic programs fail to maintain viability. A major mechanism of RCD is the excessive calcium loading of mitochondria and consequent triggering of the mitochondrial permeability transition (mPT), which is especially important in post‐mitotic cells such as cardiomyocytes and neurons. Here, we show that stress‐induced upregulation of the ROS‐generating protein Nox4 at the ER‐mitochondria contact sites (MAMs) is a pro‐survival mechanism that inhibits calcium transfer through InsP3 receptors (InsP3R). Nox4 mediates redox signaling at the MAM of stressed cells to augment Akt‐dependent phosphorylation of InsP3R, thereby inhibiting calcium flux and mPT‐dependent necrosis. In hearts subjected to ischemia–reperfusion, Nox4 limits infarct size through this mechanism. These results uncover a hitherto unrecognized stress pathway, whereby a ROS‐generating protein mediates pro‐survival effects through spatially confined signaling at the MAM to regulate ER to mitochondria calcium flux and triggering of the mPT. Synopsis: Excessive calcium loading into mitochondria induced by environmental stresses triggers mitochondrial permeability transition (mPT) and regulated cell death (RCD) in mammalian cells. This study shows that NADPH oxidase 4 (Nox4) prevents mitochondrial calcium overload by inhibiting InsP3 receptor at ER‐mitochondria contact sites (MAMs) in cells and infarcted murine hearts. Nox4‐deficient cells show increased mPT‐dependent cell necrosis during serum starvation in cardiac myocytes.Serum starvation induces upregulation of Nox4, which in turn inhibits InsP3R‐mediated calcium transfer from the ER to mitochondria.Nox4 is located at MAMs in multiple cell types and tissues.Nox4‐dependent inhibition of calcium transfer is mediated by redox‐dependent Akt activation and phosphorylation of InsP3R.Nox4 limits infarct size in murine hearts subjected to ischemia‐reperfusion by inhibiting InsP3R. [ABSTRACT FROM AUTHOR]