Mechanisms of neuromuscular junction formation

神经肌肉接头(NMJ)是运动神经元和肌纤维之间形成的突触， 对理解突触发生的一般原理以及 神经肌肉疾病。NMJ的形成需要运动神经元之间的密切相互作用 和肌肉纤维。例如，在顺行信号中，运动神经元释放结合的集聚蛋白。 LRP4，低密度脂蛋白受体家族的成员，在肌肉细胞中激活受体酪氨酸 激酶麝香，两者都是形成NMJ所必需的。马斯克下游的情况不太好 了解，除非AChR的浓度绝对需要接头蛋白Rapsyn。 然而，信号是如何从麝香激活传递到AChR浓度的，目前还不清楚 很好理解。另一方面，骨骼肌被认为是发育的关键。 运动神经元的轴突终末。与顺行监管相比，人们理解的要少得多 肌肉逆行调节突触前分化的分子机制 纤维。在初步研究中，我们发现经典的接头蛋白Rapsyn是一个E3 连接酶。Knockin小鼠携带酶所必需的单一残基突变 活动无法形成NMJ。我们的结果表明Rapsyn在AChR中起作用 通过促进新陈代谢来实现集群。这些观察发现了一种以前没有被认识到的 Rapsyn的酶活性及其在突触形成中的作用。我们对LRP4的研究 这表明它可以通过独立于马斯克的机制来调节发育中的轴突。这个 该提案将检验两个假设。首先，麝香增加Rapsyn酶的活性以促进 AChR聚集和NMJ形成的关系。第二，肌肉调节突触前 通过LRP4进行分化。这项提案的结果将使您更好地理解蜂窝AS 以及哺乳动物NMJ形成的分子机制，并有助于更好地 了解神经肌肉疾病的发病机制。

The neuromuscular junction (NMJ), a synapse formed between motoneurons and muscle fibers, has contributed greatly to understanding of the general principles of synaptogenesis, as well as of neuromuscular disorders. NMJ formation requires intimate interaction between motoneurons and muscle fibers. For example, in antegrade signaling, motoneurons release agrin that binds LRP4, a member of the LDL receptor family, in muscle cells to activate the receptor tyrosine kinase MuSK, both of which are required for NMJ formation. Downstream of MuSK was not well understood, except that AChR concentration absolutely requires the adapter protein rapsyn. However, exactly how signals are transduced from MuSK activation to AChR concentration is not well understood. On the other hand, skeletal muscles are known to be critical to the development of axon terminals of motoneurons. In contrast to antegrade regulation, much less is understood about molecular mechanisms of retrograde regulation of presynaptic differentiation by muscle fibers. In preliminary studies, we discovered that the classic adaptor protein rapsyn is an E3 ligase. Knockin mice carrying the mutation of a single residue necessary for the enzymatic activity are unable to form the NMJ. Our results suggest that rapsyn contributes to AChR clustering by promoting neddylation. These observations identify a previously unappreciated enzymatic activity of rapsyn and a role of neddylation in synapse formation. Our studies of LRP4 suggest that it could regulate developing axons by mechanisms independent of MuSK. The proposal will test two hypotheses. First, MuSK increases rapsyn enzyme activity to promote neddylation for AChR clustering and NMJ formation. Second, muscle regulates presynaptic differentiation via LRP4. Results of this proposal will provide a better understanding of cellular as well as molecular mechanisms of mammalian NMJ formation and contribute to a better understanding of pathogenic mechanisms of neuromuscular disorders.