Loss of dystrophin protein due to mutations in the DMD gene causes Duchenne muscular dystrophy. Dystrophin loss also leads to the loss of the dystrophin glycoprotein complex (DGC) from the sarcolemma which contributes to the dystrophic phenotype. Tyrosine phosphorylation of dystroglycan has been identified as a possible signal to promote the proteasomal degradation of the DGC. In order to test the role of tyrosine phosphorylation of dystroglycan in the aetiology of DMD, we generated a knock-in mouse with a phenylalanine substitution at a key tyrosine phosphorylation site in dystroglycan, Y890. Dystroglycan knock-in mice (Dag1(Y890F/Y890F)) had no overt phenotype. In order to examine the consequence of blocking dystroglycan phosphorylation on the aetiology of dystrophin-deficient muscular dystrophy, the Y890F mice were crossed with mdx mice an established model of muscular dystrophy. Dag1(Y890F/Y890F)/mdx mice showed a significant improvement in several parameters of muscle pathophysiology associated with muscular dystrophy, including a reduction in centrally nucleated fibres, less Evans blue dye infiltration and lower serum creatine kinase levels. With the exception of dystrophin, other DGC components were restored to the sarcolemma including α-sarcoglycan, α-/β-dystroglycan and sarcospan. Furthermore, Dag1(Y890F/Y890F)/mdx showed a significant resistance to muscle damage and force loss following repeated eccentric contractions when compared with mdx mice. While the Y890F substitution may prevent dystroglycan from proteasomal degradation, an increase in sarcolemmal plectin appeared to confer protection on Dag1(Y890F/Y890F)/mdx mouse muscle. This new model confirms dystroglycan phosphorylation as an important pathway in the aetiology of DMD and provides novel targets for therapeutic intervention.

Identification of a systemically acting and universal small molecule therapy for Duchenne muscular dystrophy would be an enormous advance for this condition. Based on evidence gained from studies on mouse genetic models, we have identified tyrosine phosphorylation and degradation of β-dystroglycan as a key event in the aetiology of Duchenne muscular dystrophy. Thus, preventing tyrosine phosphorylation and degradation of β-dystroglycan presents itself as a potential therapeutic strategy. Using the dystrophic sapje zebrafish, we have investigated the use of tyrosine kinase and other inhibitors to treat the dystrophic symptoms in this model of Duchenne muscular dystrophy. Dasatinib, a potent and specific Src tyrosine kinase inhibitor, was found to decrease the levels of β-dystroglycan phosphorylation on tyrosine and to increase the relative levels of non-phosphorylated β-dystroglycan in sapje zebrafish. Furthermore, dasatinib treatment resulted in the improved physical appearance of the sapje zebrafish musculature and increased swimming ability as measured by both duration and distance of swimming of dasatinib-treated fish compared with control animals. These data suggest great promise for pharmacological agents that prevent the phosphorylation of β-dystroglycan on tyrosine and subsequent steps in the degradation pathway as therapeutic targets for the treatment of Duchenne muscular dystrophy.

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