Inhibition of nerve- and agrin-induced acetylcholine receptor clustering on Xenopus muscle cells in culture.

During an early stage of neuromuscular junction formation the nerve induces acetylcholine (ACh) receptors to accumulate at the contact area. To elucidate the induction process we tested various glycosaminoglycans for their ability to inhibit nerve-induced receptor accumulation. The potency sequence was found as follows: fucyoidin > dextran sulfate > heparin = heparan sulfate > chondroitin sulfate type A and B. This sequence is similar to that for agrin-induced receptor clustering in chick myotubes [J. Neurosci., 10 (1990) 3576-3582], suggesting that agrin-like molecules are involved in nerve-induced receptor accumulation in the Xenopus system. We further tested whether agrin in the culture medium competes with the endogenous inducing substance and found that agrin partially inhibited nerve-induced receptor accumulation. We compared nerve- and agrin-induced receptor accumulation under various experimental conditions. Generally, they behaved similarly except in the presence of heparin. Heparin in the culture medium partially blocked nerve-induced receptor accumulation, whereas it totally inhibited agrin-induced receptor clustering. Our observations are consistent with the hypothesis that an agrin-like molecule released by the nerve is the induction signal for receptor accumulation in the Xenopus system.