Preventing phosphorylation of dystroglycan ameliorates the dystrophic phenotype in mdx mouse

Abstract

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 New Dystroglycan Complexes in Skeletal Muscle

“…Alternative proteins that may anchor these dystroglycan complexes include ankyrins and plectins that can bind directly to β-dystroglycan [67,68]. We have previously detected ankyrin in dystroglycan immunoprecipitations from mdx muscles [31] and association of dystroglycan with plectins can be modulated by tyrosine phosphorylation of the β-dystroglycan intracellular domain leading to amelioration of the mdx phenotype [15]. Although intracellular binding partners of dystroglycan that are distinct from dystrophin and utrophin likely exist in muscle, our data suggest that these interactions may be affected by loss of dystrophin in mdx muscle.…”

“…The experimental evidence stems from the observation that loss of dystrophin in mice ( mdx ) and humans (DMD) leads to a dramatic decrease in dystroglycan expression at the myofiber membrane [1,15–17]. The residual dystroglycan is believed to interact with utrophin, a homolog of dystrophin whose expression is normally restricted to the neuromuscular junction (NMJ) in adult muscles [18–20].…”

Rescue of dystrophic skeletal muscle by PGC-1α involves restored expression of dystrophin-associated protein complex components and satellite cell signaling

“…Of interest, accumulation of dystroglyan, independent of other treatments, has also been shown to decrease disease severity in dystrophic skeletal muscle (34). Improved resistance to damage caused by lengthening contractions indicates that the DAPC components are not only increased but also are functioning.…”

Dystrophin–Glycoprotein Complex in Blood Cells

“…Previous studies demonstrated that preventing tyrosine phosphorylation of β-Dg in mdx mouse alleviated the dystrophic phenotype in a genetic mouse model, ameliorating many of the main pathological symptoms associated with dystrophin deiciency [78]. The use of dasatinib was found to decrease β-Dg phosphorylation levels in tyrosine and to increase the relative levels of nonphosphorylated β-Dg in the sapje zebraish, improving its physical condition [79].…”

“…Additional modiications to β-Dg, however, include phosphorylation on tyrosine [75,76] and speciic proteolytic cleavage events. Tyrosine phosphorylation of β-Dg serves as a molecular switch to regulate the binding of diferent cellular-binding partners [77], but it is also a signal of the internalization of Dg from the plasma membrane [78,79] and may mediate some proteolytic events and nuclear translocation [80,81].…”