Neural agrin, in the absence of a nerve terminal, can induce the activity-resistant expression of acetylcholine receptor (AChR) subunit genes and the clustering of synapse-specific adult-type AChR channels in nonsynaptic regions of adult skeletal muscle fibers. Here we show that, when expression plasmids for neural agrin are injected into the extrasynaptic region of innervated muscle fibers, the following components of the postsynaptic apparatus are aggregated and colocalized with ectopic agrininduced AChR clusters: laminin-␤2, MuSK, phosphotyrosinecontaining proteins, ␤-dystroglycan, utrophin, and rapsyn. These components have been implicated to play a role in the differentiation of neuromuscular junctions. Furthermore, ErbB2 and ErbB3, which are thought to be involved in the regulation of neurally induced AChR subunit gene expression, were colocalized with agrin-induced AChR aggregates at ectopic nerve-free sites. The postsynaptic muscle membrane also contained a high concentration of voltage-gated Na ϩ channels as well as deep, basal lamina-containing invaginations comparable to the secondary synaptic folds of normal endplates. The ability to induce AChR aggregation in vivo was not observed in experiments with a muscle-specific agrin isoform. Thus, a motor neuron-specific agrin isoform is sufficient to induce a full ectopic postsynaptic apparatus in muscle fibers kept electrically active at their original endplate sites.

The extracellular matrix molecule agrin is both necessary and sufficient for inducing the formation of postsynaptic specializations at the neuromuscular junction (NMJ). At the mature NMJ, agrin is stably incorporated in synaptic basal lamina. The postsynapse-inducing activity of chick agrin, as assayed by its capability of causing aggregation of acetylcholine receptors (AChRs) on cultured muscle cells, maps to a 21 kDa, C-terminal domain. Binding of chick agrin to muscle basal lamina is mediated by the laminins and maps to a 25 kDa, N-terminal fragment of agrin. Here we show that an expression construct encoding a 'mini'-agrin, in which the laminin-binding fragment was fused to the AChR-clustering domain, is sufficient to induce postsynaptic differentiation in vivo when injected into non-synaptic sites of rat soleus muscle. As shown for ectopic postsynaptic differentiation induced by full-length neural agrin, myonuclei underneath the ectopic sites expressed the gene for the AChR epsilon-subunit. Altogether, our data show that a 'mini'-agrin construct encoding only a small fraction of the entire agrin protein is sufficient to induce postsynapse-like structures that are reminiscent of those induced by full-length neural agrin or innervation by motor neurons.

Abstract. Upon arrival of a motor axon at the muscle fiber, signals released from its growth cone initiate the formation of a synapse. This process consists of two stages: arrest of axon growth at the target area and differentiation of pre-and postsynaptic cells at the site of nerve-muscle contact. Studies of regenerating neuromuscular junctions in vertebrates have revealed that important signals for the formation of this synapse are located in the synaptic basal lamina, and attempts to identify these signals have led to the isolation of agrin and other components. In this review, we discuss the evidence for the involvement of these molecules and their potential functional role in the formation and maintenance of the neuromuscular junction, with emphasis on agrin.

Abstract. Upon arrival of a motor axon at the muscle fiber, signals released from its growth cone initiate the formation of a synapse. This process consists of two stages: arrest of axon growth at the target area and differentiation of pre-and postsynaptic cells at the site of nerve-muscle contact. Studies of regenerating neuromuscular junctions in vertebrates have revealed that important signals for the formation of this synapse are located in the synaptic basal lamina, and attempts to identify these signals have led to the isolation of agrin and other components. In this review, we discuss the evidence for the involvement of these molecules and their potential functional role in the formation and maintenance of the neuromuscular junction, with emphasis on agrin.