TL;DR: The hypothesis that the dramatic reduction in the 156K DAG in Duchenne muscular dystrophy leads to a loss of a linkage between the sarcolemma and extra-cellular matrix and that this may render muscle fibres more susceptible to necrosis is supported.
Abstract: The primary sequence of two components of the dystrophin–glycoprotein complex has been established by complementary DNA cloning. The transmembrane 43K and extracellular 156K dystrophin-associated glycoproteins (DAGs) are encoded by a single messenger RNA and the extracellular 156K DAG binds laminin. Thus, the 156K DAG is a new laminin-binding glycoprotein which may provide a linkage between the sarcolemma and extracellular matrix. These results support the hypothesis that the dramatic reduction in the 156K DAG in Duchenne muscular dystrophy leads to a loss of a linkage between the sarcolemma and extra-cellular matrix and that this may render muscle fibres more susceptible to necrosis.
TL;DR: The results support a role for the striated muscle dystrophin-glycoprotein complex in linking the actin- based cytoskeleton with the extracellular matrix and suggest that dystophin and dystroglycan may play substantially different functional roles in nonmuscle tissues.
Abstract: The dystrophin-glycoprotein complex was tested for interaction with several components of the extracellular matrix as well as actin. The 156-kD dystrophin-associated glycoprotein (156-kD dystroglycan) specifically bound laminin in a calcium-dependent manner and was inhibited by NaCl (IC50 = 250 mM) but was not affected by 1,000-fold (wt/wt) excesses of lactose, IKVAV, or YIGSR peptides. Laminin binding was inhibited by heparin (IC50 = 100 micrograms/ml), suggesting that one of the heparin-binding domains of laminin is involved in binding dystroglycan while negatively charged oligosaccharide moieties on dystroglycan were found to be necessary for its laminin-binding activity. No interaction between any component of the dystrophin-glycoprotein complex and fibronectin, collagen I, collagen IV, entactin, or heparan sulfate proteoglycan was detected by 125I-protein overlay and/or extracellular matrix protein-Sepharose precipitation. In addition, laminin-Sepharose quantitatively precipitated purified dystrophin-glycoprotein complex, demonstrating that the laminin-binding site is accessible when dystroglycan is associated with the complex. Dystroglycan of nonmuscle tissues also bound laminin. However, the other proteins of the striated muscle dystrophin-glycoprotein complex appear to be absent, antigenically dissimilar or less tightly associated with dystroglycan in nonmuscle tissues. Finally, we show that the dystrophin-glycoprotein complex cosediments with F-actin but does not bind calcium or calmodulin. Our results support a role for the striated muscle dystrophin-glycoprotein complex in linking the actin-based cytoskeleton with the extracellular matrix. Furthermore, our results suggest that dystrophin and dystroglycan may play substantially different functional roles in nonmuscle tissues.
TL;DR: It is demonstrated that dystrophin and its 59 kd associated protein are cytoskeletal elements that are tightly linked to a 156 kd extracellular glycoprotein by way of a complex of transmembrane proteins.
TL;DR: It is shown that four glycoproteins are integral components of the dystrophin complex and that the concentration of one of these is greatly reduced in DMD patients, suggesting the reduction in this glycoprotein may be one of the first stages of the molecular pathogenesis of muscular dystrophy.
Abstract: Dystrophin, the protein encoded by the Duchenne muscular dystrophy (DMD) gene, exists in a large oligomeric complex. We show here that four glycoproteins are integral components of the dystrophin complex and that the concentration of one of these is greatly reduced in DMD patients. Thus, the absence of dystrophin may lead to the loss of a dystrophin-associated glycoprotein, and the reduction in this glycoprotein may be one of the first stages of the molecular pathogenesis of muscular dystrophy.
TL;DR: The present review focuses on the molecular basis of three muscular dystrophic diseases that may be caused by disruptions in the dystrophin-glycoprotein complex, which normally links the subsarcolemmal cytoskeleton to the extracellular matrix in skeletal muscle.