STIM1 Negatively Regulates the Ca2+ Release from the Sarcoplasmic Reticulum in Skeletal Myotubes

Abstract

Stromal interaction molecule 1 (STIM1) mediates store-operated Ca2+ entry (SOCE) in skeletal muscle. However, the direct role(s) of STIM1 in the innate skeletal muscle event such as the Ca2+ release from the sarcoplasmic reticulum (SR) for muscle contraction have not been identified. In the present study, wild-type STIM1 and two STIM1 mutants (the Triple mutant, missing Ca2+-sensing residues, and E136X, missing the C-terminus) were over-expressed in mouse primary skeletal myotubes. The wild-type STIM1 increased SOCE, while neither mutant had an effect on SOCE. Interestingly, the development of puncta by endogenous STIM1 and Orai1 was detected without any stimulus during the differentiation of myoblasts to myotubes, and increased puncta formation was observed in the triple mutant as well as the wild-type STIM1, suggesting that, in skeletal muscle, the formation of puncta is part of the differentiation process and not the necessary and sufficient condition for SOCE. On the other hand, the Triple mutant, but not E136X, decreased the Ca2+ release from the SR in response to KCl in a dominant-negative manner without affecting the SR Ca2+ amount or resting Ca2+ level. STIM1 was co-immunoprecipitated with the dihydropyridine receptor (DHRP). These results suggest that STIM1 could negatively regulate the Ca2+ release from the SR, possibly via its C-terminal interaction with DHPR.