The mechanisms that regulate the formation of multinucleated muscle fibers from mononucleated myoblasts are not well understood. We show here that extracellular matrix (ECM) receptors of the beta1 integrin family regulate myoblast fusion. beta1-deficient myoblasts adhere to each other, but plasma membrane breakdown is defective. The integrin-associated tetraspanin CD9 that regulates cell fusion is no longer expressed at the cell surface of beta1-deficient myoblasts, suggesting that beta1 integrins regulate the formation of a protein complex important for fusion. Subsequent to fusion, beta1 integrins are required for the assembly of sarcomeres. Other ECM receptors such as the dystrophin glycoprotein complex are still expressed but cannot compensate for the loss of beta1 integrins, providing evidence that different ECM receptors have nonredundant functions in skeletal muscle fibers.

Duchenne muscular dystrophy is a frequent muscular disorder caused by mutations in the gene encoding dystrophin, a cytoskeletal protein that contributes to the stabilization of muscle fiber membrane during muscle activity. Affected individuals show progressive muscle wasting that generally causes death by age 30. In this study, the dystrophic mdx(5Cv) mouse model was used to investigate the effects of green tea extract, its major component (-)-epigallocatechin gallate, and pentoxifylline on dystrophic muscle quality and function. Three-week-old mdx(5Cv) mice were fed for either 1 or 5 wk a control chow or a chow containing the test substances. Histological examination showed a delay in necrosis of the extensor digitorum longus muscle in treated mice. Mechanical properties of triceps surae muscles were recorded while the mice were under deep anesthesia. Phasic and tetanic tensions of treated mice were increased, reaching values close to those of normal mice. The phasic-to-tetanic tension ratio was corrected. Finally, muscles from treated mice exhibited 30-50% more residual force in a fatigue assay. These results demonstrate that diet supplementation of dystrophic mdx(5Cv) mice with green tea extract or (-)-epigallocatechin gallate protected muscle against the first massive wave of necrosis and stimulated muscle adaptation toward a stronger and more resistant phenotype.

Duchenne muscular dystrophy is caused by deficiency of dystrophin and leads to progressive weakness. It has been proposed that the muscle degeneration occurring in this disease is caused by increased Ca2+ influx due to enhanced activity of cationic channels that are activated either by stretch of the plasma membrane (stretch-activated channels) or by Ca2+-store depletion (store-operated channels). Using both cytosolic Ca2+ measurements with Fura-2 and the manganese quench method, we show here that store-operated Ca2+ entry is greatly enhanced in dystrophic skeletal flexor digitorum brevis fibers isolated from mdx5cv mice, a mouse model of Duchenne muscular dystrophy. Moreover, we show for the first time that store-operated Ca2+ entry in these fibers is under the control of the Ca2+-independent phospholipase A2 and that the exaggerated Ca2+ influx can be completely attenuated by inhibitors of this enzyme. Enhanced store-operated Ca2+ entry in dystrophic fibers is likely to be due to a near twofold overexpression of Ca2+-independent phospholipase A2. The Ca2+-independent phospholipase A2 pathway therefore appears as an attractive target to reduce excessive Ca2+ influx and subsequent degeneration occurring in dystrophic fibers.

Duchenne muscular dystrophy (DMD) is a severe disorder characterized by progressive muscle wasting,respiratory and cardiac impairments, and premature death. No treatment exists so far, and the identification of active substances to fight DMD is urgently needed. We found that tamoxifen, a drug used to treat estrogen-dependent breast cancer, caused remarkable improvements of muscle force and of diaphragm and cardiac structure in the mdx(5Cv) mouse model of DMD. Oral tamoxifen treatment from 3 weeks of age for 15 months at a dose of 10 mg/kg/day stabilized myofiber membranes, normalized whole body force, and increased force production and resistance to repeated contractions of the triceps muscle above normal values. Tamoxifen improved the structure of leg muscles and diminished cardiac fibrosis by~ 50%. Tamoxifen also reduced fibrosis in the diaphragm, while increasing its thickness,myofiber count, and myofiber diameter, thereby augmenting by 72% the amount of contractile tissue available for respiratory function. Tamoxifen conferred a markedly slower phenotype to the muscles.Tamoxifen and its metabolites were present in nanomolar concentrations in plasma and muscles,suggesting signaling through high-affinity targets. Interestingly, the estrogen receptors ERa and ERb were several times more abundant in dystrophic than in normal muscles, and tamoxifen normalized the relative abundance of ERb isoforms. Our findings suggest that tamoxifen might be a useful therapy for DMD.