Sarcalumenin regulates muscle regeneration via modulating calcium homeostasis between ER and cytosol

Myoblast fusion is essential for muscle formation during embryogenesis and for muscle regeneration. We have recently demonstrated that cytosolic calcium (Ca2+) and calcium-responsive-genes (CRGs) that were previously implicated in muscle contraction also regulate myoblast-to-myotube fusion via Myomaker (MYMK), which is essential for fusion. Based on the hypothesis that genes in the same pathway are likely co-regulated, we asked if genes involved in myoblast fusion could be identified using features of the MYMK promoter. We generated promoter models for MYMK using the evolutionarily conserved regions and identified genes with homologous promoters across mammalian genomes, including several CRGs not previously implicated in fusion. We confirmed that these genes express differentially between myoblasts and myotubes using RNA-seq, and subsequently focused on those CRGs that are enriched in myotubes during fusion and share promoter homology with MYMK. Of these, we characterized Sarcalumenin (SRL) that was the only ER-Ca2+ sensor protein identified amongst shortlisted genes. SRL has been speculated for sarcoplasmic reticulum (SR) formation in myofibers and for muscle contraction via modulation of ER-to-cytosolic Ca2+ homeostasis. SRLKO(+/-) C2C12 cells and myoblasts extracted from SRLKO(+/-) mice exhibit earlier differentiation and fusion, relative to wild-type (WT) in vitro. This phenotype can be recapitulated in vitro upon treatment of WT myoblasts by Ionomycin. Consistently, upon injury, muscles of SRLKO(+/-) mice regenerate faster than WT. Taken together, we demonstrate the role of SRL in temporal regulation of myogenic differentiation in vitro muscle regeneration in vivo via modulation of ER-to-cytosolic Ca2+ homeostasis that operates via Ca2+-driven IGF1/MyoD/RyR1/p-CamKII/p-CREB axis.