STIM1 but not STIM1L couples Ca²⁺ extrusion to SOCE, promoting NFATc1 activation and human myotube growth

Stromal Interaction Molecule 1 (STIM1) is a ubiquitous protein that triggers extracellular Ca²⁺ entry after endoplasmic reticulum store depletion, a mechanism called store-operated Ca²⁺ entry (SOCE). In skeletal muscle, a longer splicing variant of STIM1 accounts for half of the SOCE. In this study, we deciphered the impact of the canonical STIM1, or the long muscle variant (STIM1L) on human muscle regeneration. We showed that both protein knockdowns led to defects in excitation-contraction coupling. Furthermore, STIM1 but not STIM1L functionally interacts with PMCA1, enhancing Ca²⁺ extrusion. Additionally, STIM1L-dependent SOCE did not trigger NFATc1 translocation, while the SOCE induced by STIM1 led to NFATc1 translocation both at rest and under stimulated conditions. In parallel, the downregulation of PMCA1 decreased basal Ca²⁺ entry as well as NFATc1 translocation. Overall, we propose that STIM1 but not STIM1L increases PMCA1-dependent Ca²⁺ extrusion, facilitating NFATc1 activation, possibly through the attenuation of Ca²⁺-dependent inactivation of Orai1. This mechanism, in turn, favors skeletal muscle maturation by increasing myotube growth.