Distinct Contributions of Orai1 and TRPC1 to Agonist-Induced [Ca(2+)](i) Signals Determine Specificity of Ca(2+)-Dependent Gene Expression

Regulation of critical cellular functions, including Ca(2+)-dependent gene expression, is determined by the temporal and spatial aspects of agonist-induced Ca(2+) signals. Stimulation of cells with physiological concentrations of agonists trigger increases [Ca(2+)](i) due to intracellular Ca(2+) release and Ca(2+) influx. While Orai1-STIM1 channels account for agonist-stimulated [Ca(2+)](i) increase as well as activation of NFAT in cells such as lymphocytes, RBL and mast cells, both Orai1-STIM1 and TRPC1-STIM1 channels contribute to [Ca(2+)](i) increases in human submandibular gland (HSG) cells. However, only Orai1-mediated Ca(2+) entry regulates the activation of NFAT in HSG cells. Since both TRPC1 and Orai1 are activated following internal Ca(2+) store depletion in these cells, it is not clear how the cells decode individual Ca(2+) signals generated by the two channels for the regulation of specific cellular functions. Here we have examined the contributions of Orai1 and TRPC1 to carbachol (CCh)-induced [Ca(2+)](i) signals and activation of NFAT in single cells. We report that Orai1-mediated Ca(2+) entry generates [Ca(2+)](i) oscillations at different [CCh], ranging from very low to high. In contrast, TRPC1-mediated Ca(2+) entry generates sustained [Ca(2+)](i) elevation at high [CCh] and contributes to frequency of [Ca(2+)](i) oscillations at lower [agonist]. More importantly, the two channels are coupled to activation of distinct Ca(2+) dependent gene expression pathways, consistent with the different patterns of [Ca(2+)](i) signals mediated by them. Nuclear translocation of NFAT and NFAT-dependent gene expression display “all-or-none” activation that is exclusively driven by local [Ca(2+)](i) generated by Orai1, independent of global [Ca(2+)](i) changes or TRPC1-mediated Ca(2+) entry. In contrast, Ca(2+) entry via TRPC1 primarily regulates NFκB-mediated gene expression. Together, these findings reveal that Orai1 and TRPC1 mediate distinct local and global Ca(2+) signals following agonist stimulation of cells, which determine the functional specificity of the channels in activating different Ca(2+)-dependent gene expression pathways.