Calcium-mediated calreticulin-IRE1α interaction drives dynamic fluctuation of IRE1α activity under chronic endoplasmic reticulum stress

The unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress can be both pro-survival or pro-apoptotic, depending on the duration and intensity of the stress. ER stress under (patho)physiological conditions can last for long time, yet the dynamic regulation of the UPR under prolong ER stress is largely unknown. Here, we characterized the UPR dynamics during pharmacologically induced long-term ER stress and revealed an "up-down-up" fluctuation pattern of the IRE1α signal in various types of cells. A fluctuation of the calreticulin-IRE1α interaction intensity orchestrates dynamic regulation of IRE1α activity, which negatively correlates with the intensity of the interaction between IRE1α and BIP, a known suppressor of IRE1α. The calreticulin-IRE1α interaction is negatively affected by Ca²⁺ concentration, which showed "down-up-down" pattern in the ER lumen over time. Furthermore, a circadian rhythmic fluctuation of calreticulin-IRE1α interaction intensity is observed in mouse liver, accompanied by oscillation of IRE1α phosphorylation level at regular physiological conditions. Our study suggests a calcium-mediated, calreticulin-driven IRE1α activity fluctuation, representing an intermediate status that the cell adopts to cope with chronic ER stress that may exist under both pathophysiological and physiological conditions.