ER stress disrupts Ca²⁺-signaling complexes and Ca²⁺ regulation in secretory and muscle cells from PERK-knockout mice

Disruption of protein synthesis and folding results in ER stress, which is associated with the pathophysiology of diverse diseases affecting secretory and muscle cells. Cells are protected against ER stress by activation of the unfolded protein response (UPR) that is regulated by the protein kinase PERK, which phosphorylates the translation initiation factor 2 eIF2α to attenuate protein synthesis. PERK^-/- cells are unable to modulate ER protein load and experience high levels of ER stress. In addition to its role in protein synthesis, the ER also orchestrates many signaling events essential for cell survival, prominent among which is Ca²⁺ signaling. It is not known, however, whether there is a relationship between ER stress and the function of the Ca²⁺ signaling pathway in muscle and non-muscle cells. To directly address this question we characterized Ca²⁺ signaling in the secretory pancreatic and parotid acinar cells and in urinary bladder smooth muscle (UBSM) cells obtained from PERK^-/- and wild-type mice. Deletion of PERK that results in high levels of ER stress, and distention and fragmentation of the ER slowed the rate of agonist-mediated Ca²⁺ release from the ER and reduced Ca²⁺-induced Ca²⁺ release, although IP₃ production, localization of the IP₃ receptors, IP₃-mediated Ca²⁺ release, Ca_v1.2 current and RyRs activity remained unaltered. On the other hand, ER stress disrupted the integrity of the Ca²⁺-signaling complexes in both secretory and UBSM cells, as revealed by markedly reduced co-immunoprecipitation of plasma membrane- and ER-resident Ca²⁺-signaling proteins. These findings establish a relationship between the unfolding protein response, ER stress and Ca²⁺ signaling and highlight the importance of communication within the terminal ER-plasma membrane microdomain for propagation of the Ca²⁺ signal from the plasma membrane into the cell.