The Ca2+ Sensor Stromal Interaction Molecule 1 (STIM1) Is Necessary and Sufficient for the Store-Operated Ca2+ Entry Function of Transient Receptor Potential Canonical (TRPC) 1 and 4 Channels in Endothelial Cells

We addressed the requirement for stromal interaction molecule 1 (STIM1), the endoplasmic reticulum (ER) Ca2+-sensor, and Orai1, a Ca2+ selective channel, in regulating Ca2+ entry through the store-operated channels mouse transient receptor potential canonical (TRPC) 4 or human TRPC1. Studies were made using murine and human lung endothelial cells (ECs) challenged with thrombin known to induce Ca2+ entry via TRPC1/4. Deletion or knockdown of TRPC4 abolished Ca2+ entry secondary to depletion of ER Ca2+ stores, preventing the disruption of the endothelial barrier. Knockdown of STIM1 (but not of Orai1or Orai3) or expression of the dominant-negative STIM1(K684E-K685E) mutant in ECs also suppressed Ca2+ entry secondary to store depletion. Ectopic expression of WT-STIM1 or WT-Orai1 in TRPC4(-/-)-ECs failed to rescue Ca2+ entry; however, WT-TRPC4 expression in TRPC4(-/-)-ECs restored Ca2+ entry indicating the requirement for TRPC4 in mediating store-operated Ca2+ entry. Moreover, expression of the dominant-negative Orai1(R91W) mutant or Orai3(E81W) mutant in WT-ECs failed to prevent thrombin-induced Ca2+ entry. In contrast, expression of the dominant-negative TRPC4(EE647-648KK) mutant in WT-ECs markedly reduced thrombin-induced Ca2+ entry. In ECs expressing YFP-STIM1, ER-store Ca2+ depletion induced formation of fluorescent membrane puncta in WT but not in TRPC4(-/-) cells, indicating that mobilization of STIM1 and engagement of its Ca2+ sensing function required TRPC4 expression. Coimmunoprecipitation studies showed coupling of TRPC1 and TRPC4 with STIM1 on depletion of ER Ca2+ stores. Thus, TRPC1 and TRPC4 can interact with STIM1 to form functional store-operated Ca2+-entry channels, which are essential for mediating Ca2+ entry-dependent disruption of the endothelial barrier.