Calreticulin differentially modulates calcium uptake and release in the endoplasmic reticulum and mitochondria

To study the role of calreticulin in Ca2+ homeostasis and apoptosis, we generated cells inducible for full-length or truncated calreticulin and measured Ca2+ signals within the cytosol, the endoplasmic reticulum (ER), and mitochondria with "cameleon" indicators. Induction of calreticulin increased the free Ca2+ concentration within the ER lumen, [Ca2+](ER), from 306 +/- 31 to 595 +/- 53 muM, and doubled the rate of ER refilling. [Ca2+](ER) remained elevated in the presence of thapsigargin, an inhibitor of SERCA-type Ca2+ ATPases. Under these conditions, store-operated Ca2+ influx appeared inhibited but could be reactivated by decreasing [Ca2+](ER) with the low affinity Ca2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. In contrast, [Ca2+](ER) decreased much faster during stimulation with carbachol. The larger ER release was associated with a larger cytosolic Ca2+ response and, surprisingly, with a shorter mitochondrial Ca2+ response. The reduced mitochondrial signal was not associated with visible morphological alterations of mitochondria or with disruption of the contacts between mitochondria and the ER but correlated with a reduced mitochondrial membrane potential. Altered ER and mitochondrial Ca2+ responses were also observed in cells expressing an N-truncated calreticulin but not in cells overexpressing calnexin, a P-domain containing chaperone, indicating that the effects were mediated by the unique C-domain of calreticulin. In conclusion, calreticulin overexpression increases Ca2+ fluxes across the ER but decreases mitochondrial Ca2+ and membrane potential. The increased Ca2+ turnover between the two organelles might damage mitochondria, accounting for the increased susceptibility of cells expressing high levels of calreticulin to apoptotic stimuli.