This chapter reviews studies on agrin, which is likely to be similar to the extracellular synaptic organizing molecules (ESOMs) that cause acetylcholine receptors (AChR) and acetylcholinesterase (AChE) aggregation at the regenerating neuromuscular junction. Monoclonal antibodies against agrin recognize molecules highly concentrated in the neuromuscular junction's synaptic cleft. Agrin causes a 3- to 20-fold increase in the number of AChR aggregates on cultured chick myotubes without influencing myotube size. There is no discernible increase in total AChR number on the myotube surface, nor is there a change in the AChR degradation rate. The AChR aggregating effect is dose-dependent and is because, at least in part, of lateral migration of AChRs present in the muscle cell plasma membrane at the time agrin is applied. Extracts containing agrin cause the formation of AChE and butyrylcholinesterase (BuChE) aggregates on cultured chick myotubes. Monoclonal antibodies against at least five different agrin epitopes recognize molecules at the Torpedo neuromuscular junction. Three antibodies against agrin stain the neuromuscular junction in the muscles of other species. 1987, Elsevier B.V. All rights reserved.
