Alpha-Smooth Muscle Actin Is Not A Marker Of Fibrogenic Cell Activity In Skeletal Muscle Fibrosis

alpha-Smooth muscle actin (alpha-SMA) is used as a marker for a subset of activated fibrogenic cells, myofibroblasts, which are regarded as important effector cells of tissue fibrogenesis. We address whether alpha-SMA-expressing myofibroblasts are detectable in fibrotic muscles of mdx(5cv) mice, a mouse model for Duchenne muscular dystrophy (DMD), and whether the alpha-SMA expression correlates with the fibrogenic function of intramuscular fibrogenic cells. alpha-SMA immunostaining signal was not detected in collagen I (GFP)-expressing cells in fibrotic muscles of ColI-GFP/mdx(5cv) mice, but it was readily detected in smooth muscle cells lining intramuscular blood vessel walls. alpha-SMA expression was detected by quantitative RT-PCR and Western blot in fibrogenic cells sorted from diaphragm and quadriceps muscles of the ColI-GFP/mdx(5cv) mice. Consistent with the more severe fibrosis in the ColI-GFP/mdx(5cv) diaphragm, the fibrogenic cells in the diaphragm exerted a stronger fibrogenic function than the fibrogenic cells in the quadriceps as gauged by their extracellular matrix gene expression. However, both gene and protein expression of alpha-SMA was lower in the diaphragm fibrogenic cells than in the quadriceps fibrogenic cells in the ColI-GFP/mdx(5cv) mice. We conclude that myofibroblasts are present in fibrotic skeletal muscles, but their expression of alpha-SMA is not detectable by immunostaining. The level of alpha-SMA expression by intramuscular fibrogenic cells does not correlate positively with the level of collagen gene expression or the severity of skeletal muscle fibrosis in the mdx(5cv) mice. alpha-SMA is not a functional marker of fibrogenic cells in skeletal muscle fibrosis associated with muscular dystrophy.