Adaptive ER stress promotes mitochondrial remodelling and longevity through PERK-dependent MERCS assembly

The transfer of information and metabolites between the mitochondria and the endoplasmic reticulum (ER) is mediated by mitochondria-ER contact sites (MERCS), allowing adaptations in response to changes in cellular homeostasis. MERCS are dynamic structures essential for maintaining cell homeostasis through the modulation of calcium transfer, redox signalling, lipid transfer, autophagy and mitochondrial dynamics. Under stress conditions such as ER protein misfolding, the Unfolded Protein Response (UPR^ER) mediates PERK and IRE1 activation, both of which localise at MERCS. Adaptive UPR^ER signalling enhances mitochondrial function and calcium import, whereas maladaptive responses lead to excessive calcium influx and apoptosis. In this study, induction of mild acute ER stress with tunicamycin (TM) in myoblasts promoted myogenesis that required PERK for increased MERCS assembly, mitochondrial turnover and function. Similarly, treatment of C. elegans embryos with an acute low concentration of TM, promoted an extension in lifespan and health-span. The adaptive ER stress response following a low dose of TM in both myoblasts and C. elegans, increased MERCS assembly and activated autophagy machinery, ultimately promoting an increase in mitochondrial remodelling. However, these beneficial adaptations were dependent on the developmental stage, as treatment of myotubes or adult C. elegans resulted in a maladaptive response. In both models the adaptations to UPR^ER activation were dependent on PERK signalling and its interaction with the UPR^mt. The results demonstrate PERK is required for the increased mitochondrial ER communication in response to adaptive UPR signalling, promoting mitochondrial remodelling and improved physiological function.