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The role for proteoglycans in acetylcholine receptor clustering on cultured muscle.

The present dissertation investigated the possible role of proteoglycans (PGs) in acetycholine receptor (AChR) clusteling on cultured C2 myotubes. This system serves as a model of the molecular interactions that underlie synaptogenesis. PGs were manipulated in mutagenized cells or by chemical intervention in order to assess their roles in the clustering process. In addition, the relationship between PGs and the clusteling factor agrin was investigated. Analysis of valiant muscle cell lines and their hyblid products supported the hypothesis that PGs are required for the clustering of AChRs. Three PG-defective genetic valiants derived from the C2 cell line, S27, S 11 and S26, all form myotubes but fail to spontaneously cluster AChRs. The three variants show different broad-spectrum defects in glycosaminoglycan (GAG) biosynthesis and are especially deficient in the synthesis of chondroitin sulfate (CS) chains that elute at high salt in ion-exchange chromatography. Formation of heterokaryon myotubes containing nuclei from two different variants spontaneously clustered AChRs and recovered synthesis of GAGs, especially of CS eluting at high salt in ion-exchange chromatography. As it is highly unlikely that defects in AChR clustering would have alisen through a chance association with three different defects in GAG biosynthesis, it strongly suggests that there is a requirement for proper GAG biosynthesis in AChR clustering. Three lines of expelimentation suggested that CS is required for the clustering of AChRs. Chlorate was found to inhibit both GAG synthesis and the clusteling of AChRs in a dose-dependent manner. When extracellular calcium was raised from 1.8 to 6.8 mM in cultures of wild type C2 myotubes, both the frequency of spontaneous AChR clusters and the level of cell layer-associated CS were increased. Culture of wild type C2 myotubes in