Damage to skeletal muscle can occur following acute trauma (e.g., blunt force trauma, invasive surgery, eccentric exercise) or during chronic disease progression (e.g., muscular dystrophy, inflammatory myopathies). In healthy individuals, the regenerative capacity of skeletal muscle is robust, and full recovery from muscle damage can be achieved even when myofiber necrosis occurs. However, the regenerative capacity of skeletal muscle is impaired during aging and disease, leading to incomplete muscle regeneration following injury and consequent impairments in muscle function, mobility, and quality of life. While the mechanisms regulating impaired regenerative capacity in aged/diseased individuals remains to be fully elucidated, reduced function of muscle resident stem cells (i.e. satellite cells) has been shown to inhibit tissue repair in these conditions. Satellite cells are required for a normal muscle regenerative response; and thus, it is important to develop interventions to enhance satellite cell function in an effort to restore regenerative capacity. Our preliminary findings show that the E3 ligase, Fbxw7, is a novel regulator of satellite cell self-renewal and muscle regeneration, and that Fbxw7 expression is increased in myogenic cells isolated from dystrophic patients. Thus, we hypothesize that Fbxw7 regulates satellite cell function via its proteasomal-mediated degradation of c-Jun, and that increased Fbxw7-mediated degradation of c-Jun is a contributing cause of the reduced regenerative capacity observed in muscular dystrophy (via impaired satellite cell proliferation). This hypothesis will be tested in a series of experiments utilizing wild type and dystrophic (mdx) mice with conditional satellite cell-specific deletions in the Fbxw7 gene.
First (Aim 1 a), I will validate that Fbxw7-null satellite cells have less ubiquitinated, and thus, a greater accumulation of total c-Jun than wild type controls. I will then transfect satellite cells with plasmids encoding either wild type c-Jun or mutant c-Jun that cannot be recognized by Fbxw7 to determine if Fbxw7-mediated c-Jun degradation is required for cell cycle exit and myogenic differentiation (Aim 1b). Next, I will examine the impact of increased Fbxw7 levels in dystrophic satellite cells by assessing total and ubiquitinated c-Jun levels. I will also elucidate which upstream regulators and/or miRNAs are the cause of increased Fbxw7 expression in dystrophic satellite cells, and whether manipulating these molecules can reduce Fbxw7 levels and improve proliferative capacity (Aim 2a). I will then determine whether reducing Fbxw7 levels in dystrophic mice can improve their regenerative capacity by histologically evaluating the regenerative potential of mdx mice that have satellite cell-specific deletions in the Fbxw7 gene (Aim 2b). Finally, I will determine whether depletion of Fbxw7 can rescue the phenotype of myoblasts isolated from DMD patients (Aim 2c). The proposed experiments will improve our understanding of satellite cell-mediated muscle regeneration, thus helping to advance the development of therapies to improve muscle regeneration in aged/diseased conditions.
Skeletal muscle has a remarkable capacity to regenerate following damage due to the activation of muscle resident stem cells (i.e. satellite cells). However, the function of these cells is compromised during aging and disease, leading to impaired regeneration. The experiments proposed in this research plan will further our understanding of the mechanisms controlling the satellite cell response to muscle damage, thus aiding in the development of novel therapies for individuals with impaired regenerative capacity.
