ABSTRACT
We have previously observed that Wnt signaling activates a fibrogenic program in adult muscle stem cells, called satellite cells, during aging. We genetically labeled satellite cells in a mouse model of Duchenne muscular dystrophy to follow their fate during the progression of the disease. We observed that a fraction of satellite cells had a reduced myogenic potential and showed enhanced expression of profibrotic genes compared to age-matched controls. By combining in vitro and in vivo results, we found that expression of transforming growth factor-ß2 (TGFß2) was induced in response to elevated canonical Wnt signaling in dystrophic muscles and that the resulting increase in TGFß activity affected the behavior of satellite cells in an autocrine or paracrine fashion. Indeed, pharmacological inhibition of the TGFß pathway in vivo reduced the fibrogenic characteristics of satellite cells. These studies shed new light on the cellular and molecular mechanisms responsible for stem cell dysfunction in dystrophic muscle and may contribute to the development of more effective and specific therapeutic approaches for the prevention of muscle fibrosis.
Subject(s)
Muscular Dystrophy, Duchenne/metabolism , Satellite Cells, Skeletal Muscle/metabolism , Transforming Growth Factor beta2/metabolism , Wnt Proteins/metabolism , Wnt Signaling Pathway , Animals , Autocrine Communication , Cell Differentiation , Cell Line , Cell Lineage , Disease Models, Animal , Fibrosis , Gene Expression Regulation , Male , Mice, Inbred mdx , Mice, Transgenic , Muscle Development , Muscular Dystrophy, Duchenne/genetics , Muscular Dystrophy, Duchenne/pathology , Paracrine Communication , Satellite Cells, Skeletal Muscle/pathology , Transfection , Up-Regulation , Wnt Proteins/geneticsABSTRACT
Myf5 is a member of the muscle-specific determination genes and plays a critical role in skeletal muscle development. Whereas the expression of Myf5 during embryonic and fetal myogenesis has been extensively studied, its expression in progenitors that will ultimately give rise to adult satellite cells, the stem cells responsible for muscle repair, is still largely unexplored. To investigate this aspect, we have generated a mouse strain carrying a CreER coding sequence in the Myf5 locus. In this strain, Tamoxifen-inducible Cre activity parallels endogenous Myf5 expression. Combining Myf5(CreER) and Cre reporter alleles, we were able to evaluate the contribution of cells expressing Myf5 at distinct developmental stages to the pool of satellite cells in adult hindlimb muscles. Although it was possible to trace back the origin of some rare satellite cells to a subpopulation of Myf5(+ve) progenitors in the limb buds at the late embryonic stage (â¼E12), a significant number of satellite cells arise from cells which expressed Myf5 for the first time at the fetal stage (â¼E15). These studies provide direct evidence that adult satellite cells derive from progenitors that first express the myogenic determination gene Myf5 during fetal stages of myogenesis.
