Signaling by MuSK a Component of the Agrin Receptor

描述（由申请人提供）：神经肌肉突触是在运动神经元、肌肉纤维和雪旺细胞之间一系列复杂的相互作用后形成的。运动神经元提供的信号在指导突触部位肌纤维的分化中起着关键作用，而肌纤维提供的信号在调节运动轴突的生长和分化中起着重要作用。Agrin是一种由运动神经元合成的200 kD蛋白，是一种重要的突触信号分子，通过刺激肌肉特异性受体酪氨酸激酶（MuSK）来组织突触后分化。缺乏麝香的小鼠无法形成神经肌肉突触，因此由于无法移动或呼吸而在出生时死亡。突触前和突触后分化的多个方面都需要MuSK， MuSK突变小鼠无法聚集包括乙酰胆碱受体在内的骨骼肌源性蛋白，无法在突触核中优先转录乙酰胆碱受体基因，也无法为神经末梢分化提供逆行信号。MuSK激活后的步骤以及导致神经肌肉突触形成的步骤，人们知之甚少。我们的研究表明，MuSK的近膜结构域在调节MuSK信号传导和指导突触分化中具有关键作用。本文描述的实验旨在通过识别和研究与MuSK关键结构域相互作用的蛋白质，揭示MuSK近膜区域如何调节突触分化。MuSK激活导致乙酰胆碱受体β亚基和δ亚基酪氨酸磷酸化，但乙酰胆碱受体酪氨酸磷酸化的作用尚不清楚。这里描述的分子遗传学实验旨在揭示乙酰胆碱受体酪氨酸磷酸化可能在突触分化中的作用。

DESCRIPTION (provided by applicant): Neuromuscular synapses form following a series of complex interactions between motor neurons, muscle fibers and Schwann cells. Signals provided by motor neurons have a key role in directing the differentiation of the muscle fiber at synaptic sites, and signals provided by muscle fibers have an important role in regulating motor axon growth and differentiation. Agrin, a 200 kD protein synthesized by motor neurons, is a critical synaptic signaling molecule that organizes postsynaptic differentiation by stimulating MuSK, a muscle-specific receptor tyrosine kinase. Mice lacking MuSK fail to form neuromuscular synapses and consequently die at birth due to their failure to move or breathe. MuSK is required for multiple aspects of presynaptic and postsynaptic differentiation, as MuSK mutant mice fail to cluster skeletal muscle-derived proteins, including acetylcholine receptors, to preferentially transcribe acetylcholine receptor genes in synaptic nuclei, and to provide retrograde signals for nerve terminal differentiation. The steps that follow MuSK activation and that lead to neuromuscular synapse formation are poorly understood. Our studies have demonstrated that the juxtamembrane domain of MuSK has a critical role in regulating MuSK signaling and directing synaptic differentiation. Experiments described here are designed to reveal how the MuSK juxtamembrane region regulates synaptic differentiation by identifying and studying the proteins that interact with this critical domain of MuSK. MuSK activation leads to tyrosine phosphorylation of the acetylcholine receptor beta and delta subunits, but the role of acetylcholine receptor tyrosine phosphorylation is not known. Molecular genetic experiments described here are designed to reveal the role that acetylcholine receptor tyrosine phosphorylation may have in synaptic differentiation.