Distinct Contributions of Orai1 and TRPC1 to Agonist-Induced [Ca2+]i Signals Determine Specificity of Ca2+-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.