Combined Therapies for Duchenne Muscular Dystrophy to Optimize Treatment Efficacy

Modified muscle use or injury can produce a stereotypic inflammatory response in which neutrophils rapidly invade, followed by macrophages. This inflammatory response coincides with muscle repair, regeneration, and growth, which involve activation and proliferation of satellite cells, followed by their terminal differentiation. Recent investigations have begun to explore the relationship between inflammatory cell functions and skeletal muscle injury and repair by using genetically modified animal models, antibody depletions of specific inflammatory cell populations, or expression profiling of inflamed muscle after injury. These studies have contributed to a complex picture in which inflammatory cells promote both injury and repair, through the combined actions of free radicals, growth factors, and chemokines. In this review, recent discoveries concerning the interactions between skeletal muscle and inflammatory cells are presented. New findings clearly show a role for neutrophils in promoting muscle damage soon after muscle injury or modified use. No direct evidence is yet available to show that neutrophils play a beneficial role in muscle repair or regeneration. Macrophages have also been shown capable of promoting muscle damage in vivo and in vitro through the release of free radicals, although other findings indicate that they may also play a role in muscle repair and regeneration through growth factors and cytokine-mediated signaling. However, this role for macrophages in muscle regeneration is still not definitive; other cells present in muscle can also produce the potentially regenerative factors, and it remains to be proven whether macrophage-derived factors are essential for muscle repair or regeneration in vivo. New evidence also shows that muscle cells can release positive and negative regulators of inflammatory cell invasion, and thereby play an active role in modulating the inflammatory process. In particular, muscle-derived nitric oxide can inhibit inflammatory cell invasion of healthy muscle and protect muscle from lysis by inflammatory cells in vivo and in vitro. On the other hand, muscle-derived cytokines can signal for inflammatory cell invasion, at least in vitro. The immediate challenge for advancing our current understanding of the relationships between muscle and inflammatory cells during muscle injury and repair is to place what has been learned in vitro into the complex and dynamic in vivo environment.

M1 macrophages play a major role in worsening muscle injury in the mdx mouse model of Duchenne muscular dystrophy. However, mdx muscle also contains M2c macrophages that can promote tissue repair, indicating that factors regulating the balance between M1 and M2c phenotypes could influence the severity of the disease. Because interleukin-10 (IL-10) modulates macrophage activation in vitro and its expression is elevated in mdx muscles, we tested whether IL-10 influenced the macrophage phenotype in mdx muscle and whether changes in IL-10 expression affected the pathology of muscular dystrophy. Ablation of IL-10 expression in mdx mice increased muscle damage in vivo and reduced mouse strength. Treating mdx muscle macrophages with IL-10 reduced activation of the M1 phenotype, assessed by iNOS expression, and macrophages from IL-10 null mutant mice were more cytolytic than macrophages isolated from wild-type mice. Our data also showed that muscle cells in mdx muscle expressed the IL-10 receptor, suggesting that IL-10 could have direct effects on muscle cells. We assayed whether ablation of IL-10 in mdx mice affected satellite cell numbers, using Pax7 expression as an index, but found no effect. However, IL-10 mutation significantly increased myogenin expression in vivo during the acute and the regenerative phase of mdx pathology. Together, the results show that IL-10 plays a significant regulatory role in muscular dystrophy that may be caused by reducing M1 macrophage activation and cytotoxicity, increasing M2c macrophage activation and modulating muscle differentiation.

Connective tissue growth factor (CTGF) is a cysteine-rich mitogenic peptide that binds heparin and is secreted by fibroblasts after activation with transforming growth factor beta (TGF-beta). CTGF is a member of a highly conserved family of peptides that include immediate early gene products cef10, cyr61, fisp12; a putative avian proto-oncogene, nov; and a drosophila gene, twisted gastrulation, tsg, that controls medial mesoderm induction during dorsal-ventral axis pattern formation, a process also controlled by TGF-beta related peptides (dpp, scw). In the adult mammal, CTGF functions as a downstream mediator of TGF-beta action on connective tissue cells, where it stimulates cell proliferation and extracellular matrix synthesis. CTGF does not appear to act on epithelial cells or immune cells. Because the biological actions of TGF-beta are complex and affect many different cell types, CTGF may serve as a more specific target for selective intervention in processes involving connective tissue formation during wound repair or fibrotic disorders. Northern blot and in situ hybridization studies have demonstrated that CTGF is coordinately expressed with TGF-beta in every fibrotic disorder examined to date. Agents that inhibit CTGF production or action could lead to the development of new therapeutic approaches for the control of fibrotic disorders in humans.