Induction by agrin of ectopic and functional postsynaptic-like membrane in innervated muscle (endplateyinnervation)

Two factors secreted from the nerve termi- nal, agrin and neuregulin, have been postulated to induce localization of the acetylcholine receptors (AChRs) to the subsynaptic membrane in skeletal muscle fibers. The princi- pal function ascribed to neuregulin is induction of AChR subunit gene expression and to agrin is the aggregation of AChRs. Here we report that when myoblasts engineered to secrete an agrin fragment were placed into the nerve-free region of denervated rodent muscle, the host muscle fibers expressed AChR «-subunit gene transcripts, characteristic of theneuromuscularsynapseinadultmuscle.Transcriptswere colocalized with agrin deposits and AChR clusters that were resistant to electrical muscle activity. More directly, single innervated muscle fibers injected intracellularly with agrin expressionplasmidsintheirextrasynapticregiondevelopeda functional ectopic postsynaptic membrane with clusters of adult-type AChR channels and acetylcholinesterase and ac- cumulation of myonuclei. The results demonstrate that agrin istheprincipalneuralsignalthatinducestheformationofthe subsynaptic apparatus in the muscle fiber and controls lo- cally, either indirectly or directly, the transcription of AChR subunit genes and the aggregation of AChRs.