Orai1 Mediates Exacerbated Ca²⁺ Entry in Dystrophic Skeletal Muscle

There is substantial evidence indicating that disruption of Ca²⁺ homeostasis and activation of cytosolic proteases play a key role in the pathogenesis and progression of Duchenne Muscular Dystrophy (DMD). However, the exact nature of the Ca²⁺ deregulation and the Ca²⁺ signaling pathways that are altered in dystrophic muscles have not yet been resolved. Here we examined the contribution of the store-operated Ca²⁺ entry (SOCE) for the pathogenesis of DMD. RT-PCR and Western blot found that the expression level of Orai1, the pore-forming unit of SOCE, was significantly elevated in the dystrophic muscles, while parallel increases in SOCE activity and SR Ca²⁺ storage were detected in adult mdx muscles using Fura-2 fluorescence measurements. High-efficient shRNA probes against Orai1 were delivered into the flexor digitorum brevis muscle in live mice and knockdown of Orai1 eliminated the differences in SOCE activity and SR Ca²⁺ storage between the mdx and wild type muscle fibers. SOCE activity was repressed by intraperitoneal injection of BTP-2, an Orai1 inhibitor, and cytosolic calpain1 activity in single muscle fibers was measured by a membrane-permeable calpain substrate. We found that BTP-2 injection for 2 weeks significantly reduced the cytosolic calpain1 activity in mdx muscle fibers. Additionally, ultrastructural changes were observed by EM as an increase in the number of triad junctions was identified in dystrophic muscles. Compensatory changes in protein levels of SERCA1, TRP and NCX3 appeared in the mdx muscles, suggesting that comprehensive adaptations occur following altered Ca²⁺ homeostasis in mdx muscles. Our data indicates that upregulation of the Orai1-mediated SOCE pathway and an overloaded SR Ca²⁺ store contributes to the disrupted Ca²⁺ homeostasis in mdx muscles and is linked to elevated proteolytic activity, suggesting that targeting Orai1 activity may be a promising therapeutic approach for the prevention and treatment of muscular dystrophy.