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 Ca²⁺-dependent gene expression, is determined by the temporal and spatial aspects of agonist-induced Ca²⁺ signals. Stimulation of cells with physiological concentrations of agonists trigger increases Ca²⁺]_i due to intracellular Ca²⁺ release and Ca²⁺ influx. While Orai1-STIM1 channels account for agonist-stimulated Ca²⁺]_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 Ca²⁺]_i increases in human submandibular gland (HSG) cells. However, only Orai1-mediated Ca²⁺ entry regulates the activation of NFAT in HSG cells. Since both TRPC1 and Orai1 are activated following internal Ca²⁺ store depletion in these cells, it is not clear how the cells decode individual Ca²⁺ 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 Ca²⁺]_i signals and activation of NFAT in single cells. We report that Orai1-mediated Ca²⁺ entry generates Ca²⁺]_i oscillations at different CCh], ranging from very low to high. In contrast, TRPC1-mediated Ca²⁺ entry generates sustained Ca²⁺]_i elevation at high CCh] and contributes to frequency of Ca²⁺]_i oscillations at lower agonist]. More importantly, the two channels are coupled to activation of distinct Ca²⁺ dependent gene expression pathways, consistent with the different patterns of Ca²⁺]_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 Ca²⁺]_i generated by Orai1, independent of global Ca²⁺]_i changes or TRPC1-mediated Ca²⁺ entry. In contrast, Ca²⁺ entry via TRPC1 primarily regulates NFκB-mediated gene expression. Together, these findings reveal that Orai1 and TRPC1 mediate distinct local and global Ca²⁺ signals following agonist stimulation of cells, which determine the functional specificity of the channels in activating different Ca²⁺-dependent gene expression pathways.