PDGF-receptor concentration is elevated in regenerative muscle fibers in dystrophin-deficient muscle

Dystrophin-deficient muscle undergoes sudden, postnatal onset of muscle necrosis that is either progressive, as in Duchenne muscular dystrophy, or successfully arrested and followed by regeneration, as in most muscles of mdx mice. The mechanisms regulating regeneration in mdx muscle are unknown, although the possibility that there is renewed expression of genes regulating embryonic muscle cell proliferation and differentiation may provide testable hypotheses. Here, we examine the possibility that necrotic and regenerating mdx muscles exhibit renewed or increased expression of PDGF-receptors. PDGF-binding to receptors on muscle has been shown previously to be associated with myogenic cell proliferation and delay of muscle differentiation. We find that PDGF-receptors are present in 4-week-old mdx mice in muscles that undergo brief, reversible necrosis (hindlimb muscles) or progressive necrosis (diaphragm), as well as in 4-week-old control mouse muscles. Immunoblots indicate that the concentrations of PDGF-receptors in 4-week-old dystrophic (necrotic) and control muscles are similar. Prenecrotic, dystrophic fibers and control fibers possess some cell surface labeling of fibers treated with anti-PDGF-receptor and viewed by indirect immunofluorescence. Necrotic fibers in dystrophic muscle show cytoplasmic labeling for PDGF-receptors and labeling of perinuclear regions at the muscle cell surface. Adult dystrophic muscle displays higher concentrations of PDGF-receptor in both regenerated muscle (hindlimb) and progressively necrotic muscle (diaphragm) than found in controls. Anti-PDGF-receptor labeling of regenerated, dystrophic muscle is observed primarily in granules surrounding central nuclei or surrounding nuclei located at the surface of regenerated fibers. No labeling of perinuclear regions of control muscle or prenecrotic fibers was observed. Myonuclei fractionated from adult mdx hindlimb muscles contained no PDGF-receptor, indicating that PDGF-receptor-positive structures are not tightly associated with nuclei or within nuclei. L6 myoblasts show PDGF-receptor distributed diffusely on the cell surface. Stimulation of L6 myoblasts with 10 ng/ml of PDGF-BB causes receptor internalization and concentration in granules at perinuclear regions. Thus, PDGF stimulation of myoblasts causes a redistribution of PDGF-receptors to resemble receptor localization observed during muscle regeneration. These findings implicate PDGF-mediated mechanisms in regeneration of dystrophic muscle.