NEURAL AGRIN INDUCES ECTOPIC POSTSYNAPTIC SPECIALIZATIONS IN INNERVATED MUSCLE-FIBERS

Neural agrin, in the absence of a nerve terminal, can induce the activ ity-resistant expression of acetylcholine receptor (AChR) subunit gene s and the clustering of synapse-specific adult-type AChR channels in n onsynaptic regions of adult skeletal muscle fibers. Here we show that, when expression plasmids for neural agrin are injected into the extra synaptic region of innervated muscle fibers, the following components of the postsynaptic apparatus are aggregated and colocalized with ecto pic agrin-induced AChR clusters: laminin-beta 2, MuSK, phosphotyrosine -containing proteins, beta-dystroglycan, utrophin, and rapsyn. These c omponents have been implicated to play a role in the differentiation o f neuromuscular junctions. Furthermore, ErbE2 and ErbB3, which are tho ught to be involved in the regulation of neurally induced AChR subunit gene expression, were colacalized with agrin-induced AChR aggregates at ectopic nerve-free sites. The postsynaptic muscle membrane also con tained a high concentration of voltage-gated Na+ channels as well as d eep, basal lamina-containing invaginations comparable to the secondary synaptic folds of normal endplates. The ability to induce AChR aggreg ation in vivo was not observed in experiments with a muscle-specific a grin isoform. Thus, a motor neuron-specific agrin isoform is sufficien t to induce a full ectopic postsynaptic apparatus in muscle fibers kep t electrically active at their original endplate sites.