Erratum Increased ER–mitochondrial coupling promotes mitochondrial respiration and bioenergetics during early phases of ER stress

Increased mitochondrial Ca2+ drives the adaptive metabolic boost observed during early phases of ER stress Increases in mitochondrial respiration and ATP production are often consequences of increases in mitochondrial Ca2+ (Green and Wang, 2010). In order to determine whether early phases of ER stress induced by tunicamycin increased mitochondrial Ca2+ , we treated cells expressing cytosolic or mitochondrial aequorins with histamine [which evokes Ins(1,4,5)P3-dependent Ca2+ release] and compared their mitochondrial Ca2+ uptake. We observed that histamine led to a mitochondrial Ca2+ uptake that was significantly higher in tunicamycinpretreated cells (P<0.05; 4 hours) than in untreated cells (Fig. 6A). Cytosolic Ca2+ increased similarly in tunicamycin-treated and untreated cells (Fig. 6B). These results indicate that the differences in mitochondrial Ca2+ levels are not due to altered Ca2+ release mediated by the Ins(1,4,5)P3 receptor but to an enhanced mitochondrial Ca2+ uptake, presumably due to the increased apposition of ER and mitochondrial Ca2+ channels. By using a different dye, Fura-2, we monitored the peak cytosolic Ca2+ levels after thapsigargin addition, reflecting the kinetics of Ca2+ release after sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) inhibition. After 4 hours of tunicamycin treatment, the thapsigargin-induced Ca2+ peak was increased, and it was further elevated by inhibition of mitochondrial Ca2+ uptake using Ru360 (Fig. 6C). These results suggest that, besides the Ins(1,4,5)P3-receptor-mediated direct Ca2+ transfer from the ER to neighboring mitochondria, an additional phenomenon associated with the early phases of ER stress involves Ca2+ leak from the ER, also resulting in mitochondrial Ca2+ uptake. Indeed, no mitochondrial Ca2+ uptake following the thapsigargin-induced Ca2+ leak was observed in Mfn2knockout cells (Fig. 6D), which is reflected by the lack of effect of Ru360. This result further indicates that juxtaposition of mitochondria with the ER is necessary for the mitochondrial Ca2+ uptake evoked by Ca2+ leak during early phases of ER stress.