Duchenne muscular dystrophy (DMD), the most common lethal genetic disorder in children, is a severe muscle-wasting disease caused by mutations in the dystrophin gene. Dystrophin is a core component of the dystrophin-glycoprotein complex (DGC), which is thought to confer mechanical stability to the muscle fiber by providing a strong, flexible connection between the internal actin cytoskeleton and the extracellular matrix. Numerous studies have examined the function of dystrophin and the core components of the DGC, however, less is known about the function of other members of the DGC that bind the C-terminal portion of dystrophin. Some of these proteins, including syntrophin and neuronal nitric oxide synthase (nNOS), have documented signaling properties and may play important signaling roles in muscle tissue. We previously generated transgenic mice with skeletal muscle expression of Dpi 16 and have introduced this transgene into dystrophin-deficient mdx mice, which serve as a model for DMD. Dpi 16 is a non-muscle isoform of dystrophin that lacks the actin-binding domain and the majority of the rod domain found in the full length dystrophin isoform. It is postulated that Dpi 16 has no direct, mechanical link to the actin cytoskeleton, but can still assemble and stabilize the DGC at the sarcolemma. Thus, Dpi 16 should permit functional and structural studies of both the C-terminal portion of dystrophin and its binding partners, independent of the mechanical function of the dystrophin N-terminus and rod domain. We have shown that Dpi 16 has minimal effect in muscles of mdx mice, in which utrophin is upregulated to partially compensate for the lack of dystrophin. However, when expressed in the severely dystrophic dystrophin:utrophin double knockout (mdx:utrn-/-) mouse. Dpi 16 expression imparts a dramatic increase in longevity and a delay in the formation and progression of joint contractures and kyphosis. Thus, we propose to study the structural and functional mechanism through which Dp116-mediate(j DGC assembly leads to phenotypic rescue in mdx:utrn-/- mice and explore possible signaling roles of the DGC in skeletal muscle. We will generate deletion constructs to identify the domains of Dpi 16 that are critical for rescue of lethality. We will also evaluate aspects of skeletal muscle biology, including regeneration, satellite cell activation, extracellular matrix organization, and mitochondrial function that may be influenced by the DGC.
This proposal addresses questions regarding the function of dystrophin. Understanding of the function of Dpi 16 and the dystrophin-glycoprotein complex in severely dystrophic mice will provide insight into the mechanism of muscle degeneration, which may suggest novel strategies for therapy of muscular dystrophy. NOTE: The critiques of individual reviewers are provided below in an essentially unedited form. These critiques were prepared prior to the review meeting and may not have been updated or revised subsequent to the discussion at the meeting. Therefore, they may not fully reflect the final opinions of the individual reviewers at the close of group discussion or the final majority opinion of the group. The Resume and Summary of Discussion above summarizes the final outcome of the group discussion.
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