Calreticulin as a stress-responsive integrator of ER proteostasis, calcium homeostasis, and immune clearance

Calreticulin (CALR) is a multifunctional endoplasmic reticulum (ER) protein that couples lectin-like chaperone activity in the calnexin/calreticulin cycle with high-capacity Ca²⁺-binding, thereby linking ER proteostasis to luminal calcium homeostasis. Although classically viewed as an ER-resident chaperone, CALR is increasingly recognized as a stress-responsive regulator whose functions extend beyond the ER lumen. Under stress, CALR can relocalize to the cell surface or extracellular space, where it functions as an immune-recognition and pro-clearance signal for damaged or dying cells. Across aging and chronic degenerative conditions, persistent ER stress, altered calcium handling, and defective clearance of stressed or senescent cells may reshape CALR expression, localization, and stress-responsive functions. However, CALR has not been widely conceptualized as an integrative regulator linking ER proteostatic stress, calcium dysregulation, senescence-associated remodeling, and immune surveillance. In this review, we examine CALR as a stress-responsive integrator of ER proteostasis, calcium homeostasis, senescence-associated stress adaptation, and immune-mediated clearance, and discuss how this framework may inform mechanistic studies and therapeutic strategies in chronic degenerative diseases.