Interactions, functions, and independence of plasma membrane STIM1 and TRPC1 in vascular smooth muscle cells

Stromal interaction molecule 1 (STIM1) is a predicted single membrane-spanning protein involved in store-operated calcium entry and interacting with ion channels including TRPC1. Here, we focus on endogenous STIM1 of modulated vascular smooth muscle cells, which exhibited a nonselective cationic current in response to store depletion despite strong buffering of intracellular calcium at the physiological concentration. STIM1 mRNA and protein were detected and suppressed by specific short interfering RNA. Calcium entry evoked by store depletion was partially inhibited by STIM1 short interfering RNA, whereas calcium release was unaffected. STIM1 short interfering RNA suppressed cell migration but not proliferation. Antibody that specifically bound STIM1 revealed constitutive extracellular N terminus of STIM1 and extracellular application of the antibody caused fast inhibition of the current evoked by store depletion. The antibody also inhibited calcium entry and cell migration but not proliferation. STIM1 interacted with TRPC1, and TRPC1 contributed partially to calcium entry and cationic current. However, the underlying processes could not be explained only by a STIM1-TRPC1 partnership because extracellular TRPC1 antibody suppressed cationic current only in a fraction of cells, TRPC1-containing channels were important for cell proliferation as well as migration, and cell surface localization studies revealed TRPC1 alone, as well as with STIM1. The data suggest a complex situation in which there is not only plasma membrane-spanning STIM1 that is important for cell migration and TRPC1-independent store-operated cationic current but also TRPC1-STIM1 interaction, a TRPC1-dependent component of store-operated current, and STIM1-independent TRPC1 linked to cell proliferation.