Defects in dystrophin are thought to disrupt a mechanical link between the cytoskeleton, the membrane and the extracellular matrix of muscle cells resulting in muscular dystrophy and cardiomyopathy. Dystrophin binds a complex of proteins that anchor its attachment to the muscle membrane and in turn, bind the extracellular matrix protein, laminin. Laminin mutations also result in myopathy, but the membrane in laminin-deficient muscle does not seem to be as physically disrupted as dystrophin-deficient membranes implying a distinct molecular mechanism. Within the dystrophin-glycoprotein complex is a multisubunit protein, sarcoglycan, that is secondarily decreased when dystrophin is mutated. Recently, it was discovered that mutations in sarcoglycan genes are a primary cause of human myopathy. The function of sarcoglycan is unknown and its predicted structure suggests a cell surface receptor. To investigate membrane defects in muscle lacking sarcoglycan, we are using homologous recombination in embryonic stem cells to generate mice lacking different subunits of sarcoglycan. gamma-sarcoglycan and delta-sarcoglycan are related 35 kDa glycosylated, transmembrane proteins normally expressed in cardiac and skeletal muscle. We have generated mice lacking gamma-sarcoglycan and embryonic stem cells heterozygously lacking delta-sarcoglycan. Preliminary data indicate mice lacking gamma-sarcoglycan show a severe dystrophic pattern that resembles human myopathy arising from sarcoglycan mutations. Sarcoglycan-deficient mice will be characterized using a variety of immunocytochemical approaches. Changes in gene expression that result from sarcoglycan deficiency will be investigated. Lastly, we will perform genetic experiments aimed at disrupting multiple membrane-extracellular matrix connections to assess the relative contribution of these different attachments and their role in normal and abnormal heart and muscle function.
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