Defects in skeletal myotubes caused by STIM1 I115F that lead to tubular aggregate myopathy and Stormorken syndrome and their restoration at the cellular level

A genetic mutation in stromal interaction molecule 1 (STIM1) at I115 (I115F) causes tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK), which are multisystemic disorders characterized by miosis, thrombocytopenia, asplenia and congenital skeletal muscle weakness. The main cause of this skeletal muscle weakness is excess store-operated Ca²⁺ entry (SOCE) resulting from the constitutively active I115F STIM1 mutant. This study investigated the detailed mechanisms underlying I115F-induced pathological defects and the possible mechanisms by which these defects can be restored at the cellular level. I115F was overexpressed in mouse primary skeletal myotubes, which were subsequently examined using live single-cell Ca²⁺ imaging, transmission electron microscopy, and biochemical approaches. Additionally, the restoration of I115F-induced pathological defects was examined using I115F-overexpressing myotubes codifferentiated with normal immature myotubes on day 2 of differentiation. Constitutively active I115F induced cytosolic Ca²⁺ overload in I115F-overexpressing myotubes by increasing SOCE and the expression of canonical transient receptor potential cation channel 6 (TRPC6), resulting in an imbalanced Ca²⁺ distribution between the cytosol and sarcoplasmic reticulum, abnormal mitochondria, low ATP production, and aberrant Ca²⁺ release for skeletal muscle contraction. Codifferentiation reversed the I115F-induced defects, normalizing cytosolic Ca²⁺ levels by increasing myogenin expression and myotube width and decreasing ORAI1 expression while maintaining TRPC6 expression. Moreover, codifferentiation reset the intracellular Ca²⁺ distribution by increasing SERCA1a expression and providing sufficient ATP production. Therefore, this study suggests that I115F-induced cellular Ca²⁺ dysregulation, which may contribute to skeletal muscle weakness in TAM and STRMK, may be attenuated by modulating myogenin, TRPC6, ORAI1, or SERCA1a expression or activity.