The Interplay Between Ca²⁺ Homeostasis, Endoplasmic Reticulum Stress, and the Unfolded Protein Response in Human...

The maintenance of endoplasmic reticulum (ER) Ca²⁺ homeostasis is intrinsically linked to the fidelity of protein folding, forming a functional tether that, when disrupted, triggers the Unfolded Protein Response (UPR). This bidirectional axis serves as a critical rheostat for cellular viability, yet its chronic dysregulation underpins the molecular etiology of numerous pathologies, including neurodegeneration, heart failure, and malignant transformation. This review provides a comprehensive interrogation of the Ca²⁺-ER Stress-UPR network, delineating how primary stress sensors-PERK, IRE1alpha, and ATF6-engage in complex feedback loops that either reinstate equilibrium or commit the cell to apoptosis. We specifically examine the PERK-CHOP-SERCA2b inhibitory circuit as a central driver of persistent Ca²⁺ depletion and discuss the role of Mitochondria-Associated Membranes (MAMs) in governing lethal Ca²⁺ transfer. Notably, we move beyond the classical paradigm of CHOP as a terminal apoptotic executioner, incorporating emerging evidence of its context-dependent adaptive functions. By synthesizing mechanistic insights across diverse disease models, this work highlights the transition from adaptive to maladaptive UPR as a universal pathological checkpoint. Ultimately, we evaluate the therapeutic potential of 'axis-targeted' interventions, such as SERCA activators and selective UPR modulators, aimed at resolving the underlying Ca²⁺ signaling defects in ER stress-related disorders.