AGRIN BINDS TO THE NERVE-MUSCLE BASAL LAMINA VIA LAMININ

Agrin is a heparan sulfate proteoglycan that is required for the forma tion and maintenance of neuromuscular junctions. During development, a grin is secreted from motor neurons to trigger the local aggregation o f acetylcholine receptors (AChRs) and other proteins in the muscle fib er, which together compose the postsynaptic apparatus. After release f rom the motor neuron, agrin binds to the developing muscle basal lamin a and remains associated with the synaptic portion throughout adulthoo d. We have recently shown that full-length chick agrin binds to a base ment membrane-like preparation called Matrigel(TM). The first 130 amin o acids from the NH2 terminus are necessary for the binding, and they are the reason why, on cultured chick myotubes, AChR clusters induced by full-length agrin are small. In the current report we show that an NH2-terminal fragment of agrin containing these 130 amino acids is suf ficient to bind to Matrigel(TM) and that the binding to this preparati on is mediated by laminin-1. The fragment also binds to laminin-2 and -4, the predominant laminin isoforms of the muscle fiber basal lamina. On cultured myotubes, it colocalizes with laminin and is enriched in AChR aggregates. In addition, we show that the effect of full-length a grin on the size of AChR clusters is reversed in the presence of the N H2-terminal agrin fragment. These data strongly suggest that binding o f agrin to laminin provides the basis of its localization to synaptic basal lamina and other basement membranes.