The neural cell adhesion molecule (NCAM) is expressed both presynaptically and postsynaptically during neuromuscular junction formation. Genetic deletion in mice of all three isoforms (180, 140, and 120 kDa), or just the 180 isoform, suggested that different isoforms played distinct roles in synaptic maturation. Here we characterized in mice of either sex the earliest adhesive contacts between the growth cones of motoneurons and myotubes and their subsequent maturation into functional synapses in cocultures of motoneurons and myotubes, which expressed their normal complement of NCAM isoforms, or were lacking all isoforms either presynaptically or postsynaptically. Growth cone contact with +/+ mouse myotubes resulted in immediate adhesive contacts and the rapid downregulation of growth cone motility. When contacting NCAM −/− myotubes, growth cones touched and retracted/collapsed multiple times and failed to form stable contacts, even after 10 h. Exogenous expression in myotubes of either the 180 or 140 isoform, but not the 120 kDa isoform, rescued the rapid formation of stable contacts, the accumulation of presynaptic and postsynaptic molecules, and functional transmission. When NCAM was absent only in motoneurons, growth cones did not retract upon myotube contact, but, since their motility was not downregulated, they grew off the ends of the myotubes, failing to form synapses. The agrin receptor Lrp4 was strongly downregulated in NCAM-negative myotubes, and motoneuron growth cones did not make stable contacts with Lrp4-negative myotubes. These studies have identified novel roles for presynaptic and postsynaptic NCAM in mediating early cell–cell interactions required for synapse formation. SIGNIFICANCE STATEMENT Although many molecular signals needed to form the functionally effective neuromuscular synapses required for normal movement have been described, the earliest signals that let motoneuron growth cones make stable adhesive contacts with myotubes and cease motility are not well understood. Using dynamic imaging of motoneuron–myotube cocultures, we show that NCAM is required on both the growth cone and myotube and that different NCAM isoforms mediate initial adhesion and the downregulation of growth cone motility. The agrin receptor Lrp4 was also essential for initial adhesive contacts and was downregulated on NCAM −/− myotubes. Our identification of novel roles for NCAM and Lrp4 and possible interactions between them in transforming motile growth cones into stable contacts opens interesting new avenues for exploration.

Muscle-specific kinase (MuSK) is essential for each step in neuromuscular synapse formation. Before innervation, MuSK initiates postsynaptic differentiation, priming the muscle for synapse formation. Approaching motor axons recognize the primed, or prepatterned, region of muscle, causing motor axons to stop growing and differentiate into specialized nerve terminals. MuSK controls presynaptic differentiation by causing the clustering of Lrp4, which functions as a direct retrograde signal for presynaptic differentiation. Developing synapses are stabilized by neuronal Agrin, which is released by motor nerve terminals and binds to Lrp4, a member of the low-density lipoprotein receptor family, stimulating further association between Lrp4 and MuSK and increasing MuSK kinase activity. In addition, MuSK phosphorylation is stimulated by an inside-out ligand, docking protein-7 (Dok-7), which is recruited to tyrosine-phosphorylated MuSK and increases MuSK kinase activity. Mutations in MuSK and in genes that function in the MuSK signaling pathway, including Dok-7, cause congenital myasthenia, and autoantibodies to MuSK, Lrp4, and acetylcholine receptors are responsible for myasthenia gravis.

Motor axons receive retrograde signals from skeletal muscle that are essential for the differentiation and stabilization of motor nerve terminals. Identification of these retrograde signals has proved elusive, but their production by muscle depends on the receptor tyrosine kinase, MuSK (muscle, skeletal receptor tyrosine-protein kinase), and Lrp4 (low-density lipoprotein receptor (LDLR)-related protein 4), an LDLR family member that forms a complex with MuSK, binds neural agrin and stimulates MuSK kinase activity. Here we show that Lrp4 also functions as a direct muscle-derived retrograde signal for early steps in presynaptic differentiation. We demonstrate that Lrp4 is necessary, independent of MuSK activation, for presynaptic differentiation in vivo, and we show that Lrp4 binds to motor axons and induces clustering of synaptic-vesicle and active-zone proteins. Thus, Lrp4 acts bidirectionally and coordinates synapse formation by binding agrin, activating MuSK and stimulating postsynaptic differentiation, and functioning in turn as a muscle-derived retrograde signal that is necessary and sufficient for presynaptic differentiation.

Background: Development of the neuromuscular junction (NMJ) is initiated by the formation of postsynaptic specializations in the central zones of muscles, followed by the arrival of motor nerve terminals opposite the postsynaptic regions. The post- and presynaptic components are then stabilized and modified to form mature synapses. Roles of ADAM (A Disintegrin And Metalloprotease) family proteins in the formation of the NMJ have not been reported previously.Principal Findings: We report here that Meltrin beta, ADAM19, participates in the formation of the NMJ. The zone of acetylcholine receptor alpha mRNA distribution was broader and excess sprouting of motor nerve terminals was more prominent in meltrin beta-deficient than in wild-type embryonic diaphragms. A microarray analysis revealed that the preferential distribution of ephrin-A5 mRNA in the synaptic region of muscles was aberrant in the meltrin beta-deficient muscles. Excess sprouting of motor nerve terminals was also found in ephrin-A5 knockout mice, which lead us to investigate a possible link between Meltrin beta and ephrin-A5-Eph signaling in the development of the NMJ. Meltrin beta and EphA4 interacted with each other in developing motor neurons, and both of these proteins localized in the NMJ. Coexpression of Meltrin beta and EphA4 strongly blocked vesicular internalization of ephrin-A5-EphA4 complexes without requiring the protease activity of Meltrin beta, suggesting a regulatory role of Meltrin beta in ephrin-A5-Eph signaling.Conclusion: Meltrin beta plays a regulatory role in formation of the NMJ. The endocytosis of ephrin-Eph complexes is required for efficient contact-dependent repulsion between ephrin and Eph. We propose that Meltrin beta stabilizes the interaction between ephrin-A5 and EphA4 by regulating endocytosis of the ephrinA5-EphA complex negatively, which would contribute to the fine-tuning of the NMJ during development.

Remyelination is an important aspect of nerve regeneration after nerve injury, but the underlying mechanisms are not fully understood. Here, we show that meltrin-β (ADAM19), a member of the ADAM (a disintegrin and metalloprotease) family, plays crucial roles in nerve regeneration after a crush injury to the sciatic nerves. The expression of meltrin-β was up-regulated in neurons after the crush injury. Morphometrical analysis revealed a delay in remyelination in meltrin-β-deficient nerves, whereas no significant defects were observed in their axon elongation. The activation of Krox-20, an indispensable transcription factor for myelination, was delayed in meltrin-β-deficient nerves and was accompanied by the retarded expression of myelin-related proteins. Expression of Krox-20 in Schwann cells was mediated by Akt. Phosphorylation of Akt but not that of Erks was reduced in regenerating nerves of meltrin-β-deficient mice. The cell membrane fraction prepared from meltrin-β-deficient nerves showed a defective activation of Akt in the membrane-loaded Schwann cells. Meltrin-β-deficient mice exhibited delayed sciatic functional recovery after the nerve crush. Altogether, these results reveal a role of meltrin-β in Schwann cell differentiation and re-myelination in nerve regeneration. Moreover, this study suggests that meltrin-β functions as a modulator of juxtacrine signaling from axons that activate the Akt pathway and the Krox-20 expression, which is the prerequisite for Schwann cell differentiation.

Maturation of the neuromuscular junction is accompanied by molecular switching of acetylcholine receptor (AChR) channels from embryonic types with gamma-subunits to adult ones with epsilon-subunits after birth. As a step toward understanding the molecular mechanisms of the gamma-to-epsilon switch, we addressed the question of whether embryonic- and adult-type AChRs constitute different endplates during the transitional period. From analyses with double- or triple-staining with anti-gamma- and/or anti-epsilon-antibodies together with alpha-bungarotoxin, which binds to alpha-subunits, we demonstrated that during neonatal stages in mice, adult-type AChRs are incorporated into individual endplates expressing embryonic-AChRs and replace these embryonic-AChRs gradually. The main period of AChR transition in the mouse diaphragm was between postnatal days 5 (P5) and P7, similar to the period described previously in which endplates shift from multi-axon to single-axon innervation. This finding will help our understanding of the mechanisms of the gamma-to-epsilon switch during establishment of the neuromuscular junction.