ER proteostasis meets mitochondrial function: contact sites as hubs of communication and therapeutic targets.

Proteostasis maintains the balance between protein synthesis, folding, and degradation within the endoplasmic reticulum (ER). This quality-control system ensures that proteins undergo proper post-translational modifications-such as PDI-ERO1-mediated oxidative folding and STT3-dependent N-glycosylation-so that only correctly folded proteins proceed through the secretory pathway. Impairment of protein load, folding capacity, or degradation via the ER-associated degradation (ERAD) pathway leads to the accumulation of unfolded proteins, triggering ER stress and activating the unfolded protein response (UPR), which, in the first instance, is an adaptive signaling network designed to restore homeostasis by adjusting protein synthesis, enhancing folding capacity, and promoting the clearance of misfolded proteins. During ER stress, the ER undergoes morphological and functional remodeling to manage the increased folding burden, including an increase of ER-mitochondria contact sites (ERMCs). These nanometric junctions (~10-100 nm) facilitate lipid and metabolite exchange and mediate calcium and reactive oxygen species signaling to support cellular metabolism. However, chronic ER stress can further tighten ERMCs, leading to calcium overload, mitochondrial dysfunction, and apoptosis. This review examines the core mechanisms underlying ER proteostasis in the context of ER stress and explores how ER stress first boosts mitochondrial activity and later impairs it through ERMCs, contributing to cell death and disease. Finally, emerging therapeutic strategies aimed at restoring proteostasis and modulating the dynamics of ERMCs are highlighted as promising interventions for conditions, such as cancer and congenital myopathies, where ER and mitochondrial dysfunction play central roles in pathogenesis.