Reduced IRE1α mediates apoptotic cell death by disrupting calcium homeostasis via the InsP3 receptor

The endoplasmic reticulum (ER) is not only a home for folding and posttranslational modifications of secretory proteins but also a reservoir for intracellular Ca2+. Perturbation of ER homeostasis contributes to the pathogenesis of various neurodegenerative diseases, such as Alzheimer's and Parkinson diseases. One key regulator that underlies cell survival and Ca2+ homeostasis during ER stress responses is inositol-requiring enzyme 1 alpha(IRE1 alpha). Despite extensive studies on this ER membrane-associated protein, little is known about the molecular mechanisms by which excessive ER stress triggers cell death and Ca2+ dysregulation via the IRE1 alpha-dependent signaling pathway. In this study, we show that inactivation of IRE1 alpha by RNA interference increases cytosolic Ca2+ concentration in SH-SY5Y cells, leading to cell death. This dysregulation is caused by an accelerated ER-to-cytosolic efflux of Ca2+ through the InsP3 receptor (InsP3R). The Ca2+ efflux in IRE1 alpha-deficient cells correlates with dissociation of the Ca2+ -binding InsP3R inhibitor CIB1 and increased complex formation of CIB1 with the pro-apoptotic kinase ASK1, which otherwise remains inactivated in the IRE1 alpha-TRAF2-ASK1 complex. The increased cytosolic concentration of Ca2+ induces mitochondrial production of reactive oxygen species (ROS), in particular superoxide, resulting in severe mitochondrial abnormalities, such as fragmentation and depolarization of membrane potential. These Ca2+ dysregulation-induced mitochondrial abnormalities and cell death in IRE1 alpha-deficient cells can be blocked by depleting ROS or inhibiting Ca2+ influx into the mitochondria. These results demonstrate the importance of IRE1 alpha in Ca2+ homeostasis and cell survival during ER stress and reveal a previously unknown Ca2+ -mediated cell death signaling between the IRE1 alpha-InsP3R pathway in the ER and the redox-dependent apoptotic pathway in the mitochondrion.