title: The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses.
abstract: The motoneural control of skeletal muscle contraction requires the neuromuscular junction (NMJ), a midmuscle synapse between the motor nerve and myotube. The formation and maintenance of NMJs are orchestrated by the muscle-specific receptor tyrosine kinase (MuSK). Motor neuron-derived agrin activates MuSK via binding to MuSK's coreceptor Lrp4, and genetic defects in agrin underlie a congenital myasthenic syndrome (an NMJ disorder). However, MuSK-dependent postsynaptic differentiation of NMJs occurs in the absence of a motor neuron, indicating a need for nerve/agrin-independent MuSK activation. We previously identified the muscle protein Dok-7 as an essential activator of MuSK. Although NMJ formation requires agrin under physiological conditions, it is dispensable for NMJ formation experimentally in the absence of the neurotransmitter acetylcholine, which inhibits postsynaptic specialization. Thus, it was hypothesized that MuSK needs agrin together with Lrp4 and Dok-7 to achieve sufficient activation to surmount inhibition by acetylcholine. Here, we show that forced expression of Dok-7 in muscle enhanced MuSK activation in mice lacking agrin or Lrp4 and restored midmuscle NMJ formation in agrin-deficient mice, but not in Lrp4-deficient mice, probably due to the loss of Lrp4-dependent presynaptic differentiation. However, these NMJs in agrin-deficient mice rapidly disappeared after birth, and postsynaptic specializations emerged ectopically throughout myotubes whereas exogenous Dok-7-mediated MuSK activation was maintained. These findings demonstrate that the MuSK activator agrin plays another role essential for the postnatal maintenance, but not for embryonic formation, of NMJs and also for the postnatal, but not prenatal, midmuscle localization of postsynaptic specializations, providing physiological and pathophysiological insight into NMJ homeostasis.
authors.affiliation: Division of Genetics, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan;
authors.forename: Tohru
authors.initials: T
authors.lastname: Tezuka
authors.affiliation: Division of Genetics, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan;
authors.forename: Akane
authors.initials: A
authors.lastname: Inoue
authors.affiliation: Division of Genetics, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan;
authors.forename: Taisuke
authors.initials: T
authors.lastname: Hoshi
authors.affiliation: Department of Genetics, Yale University, New Haven, CT 06520; and.
authors.forename: Scott D
authors.initials: SD
authors.lastname: Weatherbee
authors.affiliation: The Jackson Laboratory, Bar Harbor, ME 04609.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Burgess
authors.affiliation: Division of Genetics, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan;
authors.forename: Ryo
authors.initials: R
authors.lastname: Ueta
authors.affiliation: Division of Genetics, Department of Cancer Biology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan; yyamanas@ims.u-tokyo.ac.jp.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yamanashi
mesh_terms: D018171:Agrin / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D017398:Alternative Splicing; D000818:Animals; D003964:Diaphragm / Q000196:embryology / Q000254:growth & development; D004789:Enzyme Activation; D005260:Female; D026502:LDL-Receptor Related Proteins; D008136:Longevity / Q000235:genetics; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000502:physiology; D009124:Muscle Proteins / Q000172:deficiency / Q000502:physiology; D009469:Neuromuscular Junction / Q000196:embryology / Q000201:enzymology* / Q000254:growth & development; D020511:Neuromuscular Junction Diseases / Q000201:enzymology / Q000235:genetics / Q000503:physiopathology; D010766:Phosphorylation; D057907:Post-Synaptic Density / Q000502:physiology; D011499:Protein Processing, Post-Translational; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000502:physiology; D011973:Receptors, LDL / Q000172:deficiency / Q000502:physiology; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D045442:Rotarod Performance Test
chemical_list: D018171:Agrin; C511694:Dok-7 protein, mouse; D026502:LDL-Receptor Related Proteins; C116415:Lrp4 protein, mouse; D009124:Muscle Proteins; D011950:Receptors, Cholinergic; D011973:Receptors, LDL; D011993:Recombinant Fusion Proteins; C511695:MuSK protein, mouse; D020794:Receptor Protein-Tyrosine Kinases
keywords: Dok-7; Lrp4; neuromuscular junction; postsynaptic specialization