Laminin α4 and integrin α6 are upregulated in regenerating dy/dy skeletal muscle:: Comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha 2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions, Laminin alpha 1 was not detectable in skeletal muscle. Laminin alpha 2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha 2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha 4 and alpha 5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin pi was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha 3 was not expressed in uninjured or regenerating muscle, while integrin alpha 6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha 6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha 6 occurred on some newly formed myotubes, Integrin alpha 7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha 7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha 7 negative. No marked difference was observed in integrin alpha 7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha 4 and integrin alpha 6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes, In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha 4 and integrin alpha 6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha 2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation. (C) 2000 Academic Press.