Luminal Ca²⁺ depletion during the unfolded protein response in Xenopus oocytes: Cause and consequence

The endoplasmic reticulum (ER) is a Ca²⁺ storing organelle that plays a critical role in the synthesis, folding and post-translational modifications of many proteins. The ER enters into a condition of stress when the load of newly synthesized proteins exceeds its folding and processing capacity. This activates a signal transduction pathway called the unfolded protein response (UPR) that attempts to restore homeostasis. The precise role of ER Ca²⁺ in the initiation of the UPR has not been defined. Specifically, it has not been established whether ER Ca²⁺ dysregulation is a cause or consequence of ER stress. Here, we report that partial depletion of ER Ca²⁺ stores induces a significant induction of the UPR, and leads to the retention of a normally secreted protein Carboxypeptidase Y. Moreover, inhibition of protein glycosylation by tunicamycin rapidly induced an ER Ca²⁺ leak into the cytosol. However, blockade of the translocon with emetine inhibited the tunicamycin-induced Ca²⁺ release. Furthermore, emetine treatment blocked elF2α phosphorylation and reduced expression of the chaperone BiP. These findings suggest that Ca²⁺ may be both a cause and a consequence of ER protein misfolding. Thus, it appears that ER Ca²⁺ leak is a significant co-factor for the initiation of the UPR.