期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2014
卷号：111
期号：46
页码：16556-16561
DOI：10.1073/pnas.1408409111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe neuromuscular junction (NMJ) is a synapse between the motor nerve and myotube essential for controlling skeletal muscle contraction. Motor nerve-derived glycoprotein agrin is indispensable for the formation and maintenance of NMJs, and genetic defects in agrin underlie a congenital myasthenic syndrome (CMS). Agrin's role has been thought to be activation of the muscle-specific receptor kinase MuSK. Here, we demonstrate that forced activation of MuSK in agrin-deficient mice restored embryonic formation, but not postnatal maintenance, of NMJs, demonstrating that agrin plays an essential role distinct from MuSK activation in the postnatal maintenance of NMJs. Given that CMSs frequently show postnatal onset, this finding provides key insights not only into NMJ homeostasis but also into CMS pathology with unknown etiology. 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.
