title: Reduced IRE1α mediates apoptotic cell death by disrupting calcium homeostasis via the InsP3 receptor.
abstract: The endoplasmic reticulum (ER) is not only a home for folding and posttranslational modifications of secretory proteins but also a reservoir for intracellular Ca(2+). 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 Ca(2+) homeostasis during ER stress responses is inositol-requiring enzyme 1α (IRE1α). 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 Ca(2+) dysregulation via the IRE1α-dependent signaling pathway. In this study, we show that inactivation of IRE1α by RNA interference increases cytosolic Ca(2+) concentration in SH-SY5Y cells, leading to cell death. This dysregulation is caused by an accelerated ER-to-cytosolic efflux of Ca(2+) through the InsP3 receptor (InsP3R). The Ca(2+) efflux in IRE1α-deficient cells correlates with dissociation of the Ca(2+)-binding InsP3R inhibitor CIB1 and increased complex formation of CIB1 with the pro-apoptotic kinase ASK1, which otherwise remains inactivated in the IRE1α-TRAF2-ASK1 complex. The increased cytosolic concentration of Ca(2+) 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 Ca(2+) dysregulation-induced mitochondrial abnormalities and cell death in IRE1α-deficient cells can be blocked by depleting ROS or inhibiting Ca(2+) influx into the mitochondria. These results demonstrate the importance of IRE1α in Ca(2+) homeostasis and cell survival during ER stress and reveal a previously unknown Ca(2+)-mediated cell death signaling between the IRE1α-InsP3R pathway in the ER and the redox-dependent apoptotic pathway in the mitochondrion.
authors.affiliation: Department of Biochemistry and Biomedical Sciences, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: S M
authors.initials: SM
authors.lastname: Son
authors.affiliation: Department of Biochemistry and Biomedical Sciences, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: J
authors.initials: J
authors.lastname: Byun
authors.affiliation: Department of Physiology, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: S-E
authors.initials: SE
authors.lastname: Roh
authors.affiliation: Department of Physiology, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: S J
authors.initials: SJ
authors.lastname: Kim
authors.affiliation: Department of Biochemistry and Biomedical Sciences, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: I
authors.initials: I
authors.lastname: Mook-Jung
mesh_terms: D017209:Apoptosis; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism / Q000648:ultrastructure; D059865:Endoplasmic Reticulum Stress; D004722:Endoribonucleases / Q000378:metabolism*; D055785:Gene Knockdown Techniques; D006706:Homeostasis; D006801:Humans; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000378:metabolism*; D042541:Intracellular Space / Q000378:metabolism; D048848:MAP Kinase Kinase Kinase 5 / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism; D008954:Models, Biological; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D017382:Reactive Oxygen Species / Q000378:metabolism; D015854:Up-Regulation
chemical_list: C109198:CIB1 protein, human; D002135:Calcium-Binding Proteins; D053496:Inositol 1,4,5-Trisphosphate Receptors; D017382:Reactive Oxygen Species; C528229:ERN1 protein, human; D017346:Protein Serine-Threonine Kinases; D048848:MAP Kinase Kinase Kinase 5; D004722:Endoribonucleases; D002118:Calcium
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