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.

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that spans the muscle plasma membrane and forms a link between the F-actin cytoskeleton and the extracellular matrix. The proteins of the DGC are structurally organized into distinct subcomplexes, and genetic mutations in many individual components are manifested as muscular dystrophy. We recently identified a unique tetraspan-like dystrophin-associated protein, which we have named sarcospan (SPN) for its multiple sarcolemma spanning domains (Crosbie, R.H., J. Heighway, D.P. Venzke, J.C. Lee, and K.P. Campbell. 1997. J. Biol. Chem. 272:31221-31224). To probe molecular associations of SPN within the DGC, we investigated SPN expression in normal muscle as a baseline for comparison to SPN's expression in animal models of muscular dystrophy. We show that, in addition to its sarcolemma localization, SPN is enriched at the myotendinous junction (MTJ) and neuromuscular junction (NMJ), where it is a component of both the dystrophin- and utrophin-glycoprotein complexes. We demonstrate that SPN is preferentially associated with the sarcoglycan (SG) subcomplex, and this interaction is critical for stable localization of SPN to the sarcolemma, NMJ, and MTJ. Our experiments indicate that assembly of the SG subcomplex is a prerequisite for targeting SPN to the sarcolemma. In addition, the SG- SPN subcomplex functions to stabilize alpha-dystroglycan to the muscle plasma membrane. Taken together, our data provide important information about assembly and function of the SG-SPN subcomplex.