Fig. 2.

Coordinated action of positive and negative signals in NMJ assembly. At least three cellular mechanisms contribute to the high density of AChRs at the NMJ. First, AChR might redistribute from primitive clusters to the synaptic area, either by lateral movement, by diffusion in the plasma membrane or by endo- and exocytosis. Second, muscle fibers are multi-nucleated cells, and only the nuclei beneath the postsynaptic membrane (synaptic nuclei) are actively transcribing the AChR subunit genes, contributing to synapse-specific AChR expression. Third, AChR turnover rate is reduced at mature NMJ or when clustered (as shown by the half-life of AChRs at the NMJ at 8-14 days compared with 17-24 hours for non-clustered or embryonic AChR). Motor nerves activate muscle fibers by releasing ACh, a negative signal, which activates AChR. Muscle activation stimulates the serine/threonine kinases cyclin-dependent kinase 5 (Cdk5) and Ca²⁺/calmodulin-dependent kinase II (CaMKII) to inhibit AChR clustering, to suppress AChR expression and to destabilize AChR clusters in entire muscle fibers. At the same time, nerves also release positive signals, such as agrin, which counteract the effects of negative signals, resulting in a high AChR concentration at the NMJ.