STIM1 long and STIM1 gate differently TRPC1 during store-operated calcium entry

During myogenesis, a long splice variant of STIM1, called STIM1L is getting expressed, while the level of STIM1 remains constant. Previous work demonstrated that STIM1L is more efficient in eliciting store-operated Ca²⁺ entry (SOCE), but no current analysis of the channel(s) activated by this new STIM1L isoform was performed until now. In this study, we investigate the ionic channel(s) activated by STIM1L and whether differences exist between the two STIM1 isoforms, using HEK-293 T cells as a model system. Our data show that STIM1 and STIM1L activate Orai1 channel but also the endogenously expressed TRPC1. The channel activation occurs in two steps, with first Orai1 activation followed, in a subset of cells, by TRPC1 opening. Remarkably, STIM1L more frequently activates TRPC1 and preferentially interacts with TRPC1. In low intracellular Ca²⁺ buffering condition, the frequency of TRPC1 opening increases significantly, strongly suggesting a Ca²⁺-dependent channel activation. The ability of STIM1L to open Orai1 appears decreased compared to STIM1, which might be explained by its stronger propensity towards TRPC1. Indeed, increasing the amount of STIM1L results in an enhanced Orai1 current. The role of endogenous TRPC1 in STIM1- and STIM1L-induced SOCE was confirmed by Ca²⁺ imaging experiments. Overall, our findings provide a detailed analysis of the channels activated by both STIM1 isoforms, revealing that STIM1L is more prone to open TRPC1, which might explain the larger SOCE elicited by this isoform.