OPA1 Downregulation in Skeletal Muscle Induces MERC formation in an ATF4-Dependent Manner

Abstract

SummaryMitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are protein- and lipid-enriched hubs that mediate intracellular communications, contributing to the dynamic transfer of Ca2+, lipid, and other metabolites between these organelles. Defective MERCs are associated with cellular oxidative stress, neurodegenerative disease, and cardiac and skeletal muscle pathology via mechanisms that are poorly understood. We previously demonstrated that skeletal muscle-specific knockdown (KD) of the mitochondrial fusion mediator optic atrophy 1 (OPA1) induced ER stress and upregulated Mitofusin-2, a known MERC protein. In the present study, we tested the hypothesis that Opa1 downregulation in skeletal muscle cells induces MERC formation using multiple experimental models, including mice, Drosophila, and primary myotubes. Our results revealed that OPA1 deficiency increased MERC tethering and activated the integrated stress response (ISR) pathway effector, activating transcription factor 4 (ATF4). Loss of OPA1, reduced mitochondria calcium uptake and caffeine-induced calcium release by the ER, which was associated with changes in expression of mediators of calcium exchange IP3R3, GRP75, VDAC, at MERCs. Reducing Atf4 expression in Opa1-deficient muscle cells altered MERC ultrastructure changes and reestablished cytosolic calcium homeostasis. These data identify a role for ATF4 in the regulation of MERCs distance and mitochondrial calcium exchange.