Na⁺ Dysregulation Coupled with Ca²⁺ Entry through NCX1 Promotes Muscular Dystrophy in Mice

Abstract

f Unregulated Ca 2؉ entry is thought to underlie muscular dystrophy. Here, we generated skeletal-muscle-specific transgenic (TG) mice expressing the Na ؉ -Ca 2؉ exchanger 1 (NCX1) to model its identified augmentation during muscular dystrophy. The NCX1 transgene induced dystrophy-like disease in all hind-limb musculature, as well as exacerbated the muscle disease phenotypes in ␦-sarcoglycan (Sgcd ؊/؊ ), Dysf ؊/؊ , and mdx mouse models of muscular dystrophy. Antithetically, muscle-specific deletion of the Slc8a1 (NCX1) gene diminished hind-limb pathology in Sgcd ؊/؊ mice. Measured increases in baseline Na ؉ and Ca 2؉ in dystrophic muscle fibers of the hind-limb musculature predicts a net Ca 2؉ influx state due to reverse-mode operation of NCX1, which mediates disease. However, the opposite effect is observed in the diaphragm, where NCX1 overexpression mildly protects from dystrophic disease through a predicted enhancement in forward-mode NCX1 operation that reduces Ca 2؉ levels. Indeed, Atp1a2 ؉/؊ (encoding Na ؉ -K ؉ ATPase ␣2) mice, which have reduced Na ؉ clearance rates that would favor NCX1 reverse-mode operation, showed exacerbated disease in the hind limbs of NCX1 TG mice, similar to treatment with the Na ؉ -K ؉ ATPase inhibitor digoxin. Treatment of Sgcd ؊/؊ mice with ranolazine, a broadly acting Na ؉ channel inhibitor that should increase NCX1 forward-mode operation, reduced muscular pathology. M uscular dystrophy (MD) is characterized by myofiber degeneration that results in muscle loss, functional impairment, and eventually death. MD is generally caused by genetic mutations in genes encoding proteins that are either part of the membrane-stabilizing dystrophin-glycoprotein complex (DGC) or otherwise impact some aspect of sarcolemmal integrity and membrane channel activity (1). Such alterations cause enhanced Ca 2ϩ entry through microtears or Ca 2ϩ channels/exchangers (2). Downstream consequences of increased Ca 2ϩ entry include altered signaling, calpain activation leading to unregulated intracellular protein degradation, and induction of necrosis through opening of the mitochondrial permeability transition pore with mitochondrial rupture (3, 4). However, the hypothesis that Ca 2ϩ elevations directly induce myofiber necrosis and lead to MD is controversial (2). While some groups have indeed reported global or even subsarcolemmal increases in Ca 2ϩ in dystrophic myofibers (5-10), such measurements are often technically difficult, which may be the reason why other studies have not observed a significant increase (11-13). Recent studies in transgenic (TG) mice have supported the Ca 2ϩ hypothesis of disease. For example, overexpression of dominant-negative transient receptor potential canonical 6 (dnTRPC6) or dnTRPV2 was sufficient to abrogate the dystrophic phenotype in mice by inhibiting a type of storeoperated Ca 2ϩ entry that characterizes these channels (14, 15). TRPC3 overexpression in skeletal muscle, which dramatically enhanced Ca 2ϩ entry, was sufficient to induce MD in mice (14). Finally, overexpre...

Age-dependent changes in diastolic Ca2+ and Na+ concentrations in dystrophic cardiomyopathy: Role of Ca2+ entry and IP3

“…These dysfunctional ion concentrations are considered the hallmark of the muscle pathology and a triggering agent for muscle destruction [2]. Excessive elevation of intracellular Ca 2+ and Na + , a status known as Ca 2+ and Na + overload respectively, have been shown to be deleterious to skeletal and cardiac cells, and associated with either necrotic or apoptotic cell death [14,15]. In addition, in heart this abnormal handling of intracellular Ca 2+ and Na + may induce severe arrhythmias and ventricular fibrillation [16].…”

Functional in situ assessment of muscle contraction in wild-type and mdx mice

“…The increase in force generation rate in young mdx muscles has been attributed to an increase in myosin light‐chain kinase phosphorylation due to an increase in intracellular calcium, a common defect in mdx muscle . In addition, persistent rounds of regeneration in mdx muscle favor emergence of the slow‐twitch fiber type, which has low myosin light‐chain kinase activity.…”

Whole Body Periodic Acceleration Is an Effective Therapy to Ameliorate Muscular Dystrophy in mdx Mice

“…Ca 2+ and Na + overload have been shown to be deleterious to skeletal muscle fibers, and are associated with either necrotic or apoptotic cell death [29], [45]. These results suggest that the benefits of pGz would not only arise from paracrine effects of the surrounding endothelium but also from direct effects on the muscle cells, possible through activation of mechanoreceptors.…”