Regulation of store-operated channels by endoplasmic reticulum chaperons

Abstract Calreticulin is a conserved Ca2+ binding chaperone protein that is localized to the lumen of the endoplasmic reticulum (ER). The protein is implicated in many cellular functions such as the regulation of intracellular Ca2+ homeostasis, the regulation of gene expression, the folding of the newly synthesized proteins, cell adhesion, cancer and auto-immunity. The role of calreticulin in Ca2+ homeostasis regulation through Ca2+ storage and signaling might be the key to explaining the involvement of the protein in many biological functions inside and outside the ER. In this study, we examined whether calreticulin is responsible for regulating store-operated channels (SOC) on the plasma membrane by assessing changes in Ca2+ levels and SOC activity in mouse embryonic fibroblasts that are deficient in calreticulin. Wild type and calreticulin deficient fibroblasts were loaded with fura2-AM and the release of Ca2+ from the ER stores was induced using Thapsigargin or Ionomycin, and Ca2+ levels were measured using InC lm2 computer program. Calreticulin-defiecint fibroblasts showed a marked decrease in Ca2+ influx through SOC compared to wild type. To investigate the mechanism that underlines this decrease in SOC activity we performed western blots using antibodies for ORAI (Calcium release-activated calcium channel protein) and STIM (stromal interacting molecule). Our data showed a significant decrease in the expression of ORAI in the calreticulin-deficient fibroblasts compared to wild type while STIM expression in calreticulin-defiecint fibroblasts did not significantly differ from wild type. We concluded from our data that calreticulin controls SOC activity and this seems to be through regulating the expression of ORAI on the plasma membrane.