Co-localization and molecular association of dystrophin with laminin at the surface of mouse and human myotubes

Abstract

In Duchenne muscular dystrophy (DMD), deficiency of the protein dystrophin results in necrosis of muscle myofibres, associated with lesions in the sarcolemma and surrounding basal lamina. Dystrophin has been proposed to be a major component of the sub-sarcolemmal cytoskeleton involved in maintaining the integrity of the myofibre plasma membrane, and is known to associate with a group of sarcolemmal glycoproteins, one of which exhibits high affinity binding to the basal lamina component laminin. However, a direct or indirect transmembrane association of dystrophin in muscle cells with the myofibre basal lamina has not been demonstrated. To address this question we have examined dystrophin immunostaining and immunoprecipitation patterns in cultured mouse and human myotubes in comparison with that of the basal lamina component, laminin. Dual-immunolabelling revealed virtually complete co-localization of dystrophin on the inside surface of the muscle cell sarcolemma with plaques and veined arrays of laminin accumulating on the extracellular face. This pattern of laminin and dystrophin distribution was distinct from that of other cell surface molecules expressed in myotubes such as the neural cell adhesion molecule, NCAM, and the beta 1 integrin receptor, and immunoprecipitation of dystrophin from solubilized myotube extracts resulted in co-purification of laminin B1 chain confirming an association between these two components. The results thus provide the first direct cellular evidence of a transmembrane linkage between dystrophin in the sarcolemmal cytoskeleton with laminin in the overlying basal lamina. While the immunocytochemical distribution of laminin was apparently normal in dystrophin-deficient muscle cells, elevated levels of soluble laminin were present in extracts of mdx compared with normal mouse skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)