STIM1 as a key regulator for Ca(2+ )homeostasis in skeletal-muscle development and function

Stromal interaction molecules (STIM) were identified as the endoplasmic-reticulum (ER) Ca(2+ )sensor controlling store-operated Ca(2+ )entry (SOCE) and Ca(2+)-release-activated Ca(2+ )(CRAC) channels in non-excitable cells. STIM proteins target Orai1-3, tetrameric Ca(2+)-permeable channels in the plasma membrane. Structure-function analysis revealed the molecular determinants and the key steps in the activation process of Orai by STIM. Recently, STIM1 was found to be expressed at high levels in skeletal muscle controlling muscle function and properties. Novel STIM targets besides Orai channels are emerging. Here, we will focus on the role of STIM1 in skeletal-muscle structure, development and function. The molecular mechanism underpinning skeletal-muscle physiology points toward an essential role for STIM1-controlled SOCE to drive Ca(2+)/calcineurin/nuclear factor of activated T cells (NFAT)-dependent morphogenetic remodeling programs and to support adequate sarcoplasmic-reticulum (SR) Ca(2+)-store filling. Also in our hands, STIM1 is transiently up-regulated during the initial phase of in vitro myogenesis of C2C12 cells. The molecular targets of STIM1 in these cells likely involve Orai channels and canonical transient receptor potential (TRPC) channels TRPC1 and TRPC3. The fast kinetics of SOCE activation in skeletal muscle seem to depend on the triad-junction formation, favoring a pre-localization and/or pre-formation of STIM1-protein complexes with the plasma-membrane Ca(2+)-influx channels. Moreover, Orai1-mediated Ca(2+ )influx seems to be essential for controlling the resting Ca(2+ )concentration and for proper SR Ca(2+ )filling. Hence, Ca(2+ )influx through STIM1-dependent activation of SOCE from the T-tubule system may recycle extracellular Ca(2+ )losses during muscle stimulation, thereby maintaining proper filling of the SR Ca(2+ )stores and muscle function. Importantly, mouse models for dystrophic pathologies, like Duchenne muscular dystrophy, point towards an enhanced Ca(2+ )influx through Orai1 and/or TRPC channels, leading to Ca(2+)-dependent apoptosis and muscle degeneration. In addition, human myopathies have been associated with dysfunctional SOCE. Immunodeficient patients harboring loss-of-function Orai1 mutations develop myopathies, while patients suffering from Duchenne muscular dystrophy display alterations in their Ca(2+)-handling proteins, including STIM proteins. In any case, the molecular determinants responsible for SOCE in human skeletal muscle and for dysregulated SOCE in patients of muscular dystrophy require further examination.