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* Departments of Pediatrics, Utrophin is a large cytoskeletal protein that
is homologous to dystrophin, the protein mutated in
Duchenne and Becker muscular dystrophy. In skeletal
muscle, dystrophin is broadly distributed along the sarcolemma whereas utrophin is concentrated at the neuromuscular junction. This differential localization,
along with studies on cultured cells, led to the suggestion that utrophin is required for synaptic differentiation. In addition, utrophin is present in numerous nonmuscle cells, suggesting that it may have a more generalized role in the maintenance of cellular integrity. To test these hypotheses we generated and characterized utrophin-deficient mutant mice. These mutant
mice were normal in appearance and behavior and
showed no obvious defects in muscle or nonmuscle tissue. Detailed analysis, however, revealed that the density of acetylcholine receptors and the number of junctional folds were reduced at the neuromuscular
junctions in utrophin-deficient skeletal muscle. Despite
these subtle derangements, the overall structure of the mutant synapse was qualitatively normal, and the specialized characteristics of the dystrophin-associated
protein complex were preserved at the mutant neuromuscular junction. These results point to a predominant role for other molecules in the differentiation and
maintenance of the postsynaptic membrane.
Molecular Biology and Pharmacology, and § Anatomy and Neurobiology, Washington University
School of Medicine, St. Louis, Missouri
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