Figure 22-5 Diagram of agrin- and ARIA/neuregulin-dependent events during neuromuscular junction maturation. After establishment of a nerve on the muscle, growth factors, including agrin and ARIA (acetylcholine receptor-inducing activity), are released. Agrin interacting with its receptor MuSK (muscle-specific kinase) enhances the clustering of the synaptic proteins, including acetylcholine receptors (AChRs), rapsyn, and ERBB receptors. ARIA is the best candidate for involvement in the conversion of γ-subunit-containing immature receptor to epsilon-subunit-containing mature (innervated) receptor, which is synapse specific and therefore not inserted in the extrajunctional area.