The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses
The MuSK activator agrin has a separate role essential for postnatal maintenance of neuromuscular synapses
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DOI：
10.1073/pnas.1408409111
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发表时间：
2014-11-18
影响因子：
11.1
通讯作者：
Yamanashi, Yuji
中科院分区：
文献类型：
--
作者：
Tezuka, Tohru;Inoue, Akane;Yamanashi, Yuji
运动神经对骨骼肌收缩的控制需要神经肌肉接头（NMJ），它是运动神经和肌管之间的中间肌突触。NMJ的形成和维持由肌肉特异性受体酪氨酸激酶（MuSK）协调。运动神经元衍生的聚集蛋白通过与MuSK的辅助受体Lrp 4结合来激活MuSK，聚集蛋白的遗传缺陷是先天性肌无力综合征（一种NMJ疾病）的基础。然而，在运动神经元不存在的情况下，NMJ的MuSK依赖性突触后分化发生，表明需要神经/聚集蛋白独立的MuSK激活。我们以前确定肌肉蛋白Dok-7是MuSK的重要激活剂。虽然在生理条件下NMJ的形成需要聚集蛋白，但在缺乏抑制突触后特化的神经递质乙酰胆碱的情况下，NMJ的形成在实验上是不可能的。因此，假设MuSK需要聚集蛋白与Lrp 4和Dok-7一起实现足够的活化以克服乙酰胆碱的抑制。在这里，我们发现，Dok-7在肌肉中的强制表达增强了缺乏聚集蛋白或Lrp 4的小鼠的MuSK激活，并恢复了聚集蛋白缺陷小鼠的中肌NMJ形成，但在Lrp 4缺陷小鼠中没有，这可能是由于Lrp 4依赖的突触前分化的丧失。然而，这些NMJ在聚集蛋白缺陷小鼠出生后迅速消失，突触后特化出现异位整个肌管，而外源性Dok-7介导的MuSK激活保持。这些研究结果表明，MuSK激活剂聚集蛋白起着另一个重要的作用，出生后的维护，但不是胚胎形成，NMJ，也为出生后，但不是产前，midmuscle定位的突触后专业化，提供生理和病理生理学洞察NMJ稳态。
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.