SEC24A facilitates colocalization and Ca2+ flux between the endoplasmic reticulum and mitochondria

A genome-wide screen recently identified SEC24A as a novel mediator of thapsigargin-induced cell death in HAP1 cells. Here, we determined the cellular mechanism and specificity of SEC24A-mediated cytotoxicity. Measurement of Ca2+ levels using organelle-specific fluorescent indicator dyes showed that Ca2+ efflux from endoplasmic reticulum (ER) and influx into mitochondria were significantly impaired in SEC24A-knockout cells. Furthermore, SEC24A-knockout cells also showed similar to 44% less colocalization of mitochondria and peripheral tubular ER. Knockout of SEC24A, but not its paralogs SEC24B, SEC24C or SEC24D, rescued HAP1 cells from cell death induced by three different inhibitors of sarcoplasmic/endoplasmic reticulum Ca2+ ATPases (SERCA) but not from cell death induced by a topoisomerase inhibitor. Thapsigargin-treated SEC24A-knockout cells showed a similar to 2.5-fold increase in autophagic flux and similar to 10-fold reduction in apoptosis compared to wild-type cells. Taken together, our findings indicate that SEC24A plays a previously unrecognized role in regulating association and Ca2+ flux between the ER and mitochondria, thereby impacting processes dependent on mitochondrial Ca2+ levels, including autophagy and apoptosis.Y