Madhavan Raghavan, Zhao Xiaotao T, Reynolds Albert B, Peng H Benjamin
Department of Biology, Hong Kong University of Science and Technology, Kowloon, Hong Kong.
J Neurobiol. 2006 Nov;66(13):1511-27. doi: 10.1002/neu.20320.
At developing neuromuscular junctions (NMJs), muscles initially contact motor axons by microprocesses, or myopodia, which are induced by nerves and nerve-secreted agrin, but it is unclear how myopodia are assembled and how they influence synaptic differentiation at the NMJ. Here, we report that treatment of cultured muscle cells with agrin transiently depleted p120 catenin (p120ctn) from cadherin junctions in situ, and increased the tyrosine phosphorylation and decreased the cadherin-association of p120ctn in cell extracts. Whereas ectopic expression of wild-type p120ctn in muscle generated myopodia in the absence of agrin, expression of a specific dominant-negative mutant form of p120ctn, which blocks filopodial assembly in nonmuscle cells, suppressed nerve- and agrin-induction of myopodia. Significantly, approaching neurites triggered reduced acetylcholine receptor (AChR) clustering along the edges of muscle cells expressing mutant p120ctn than of control cells, although the ability of the mutant cells to cluster AChRs was itself normal. Our results indicate a novel role of p120ctn in agrin-induced myopodial assembly and suggest that myopodia increase muscle-nerve contacts and muscle's access to neural agrin to promote NMJ formation.
在发育中的神经肌肉接头（NMJ）处，肌肉最初通过微突起（即肌足）与运动轴突接触，这些微突起由神经和神经分泌的聚集蛋白诱导产生，但目前尚不清楚肌足是如何组装的，以及它们如何影响NMJ处的突触分化。在此，我们报告，用聚集蛋白处理培养的肌肉细胞会使钙黏蛋白连接处的p120连环蛋白（p120ctn）在原位短暂耗尽，并增加细胞提取物中p120ctn的酪氨酸磷酸化，同时降低其与钙黏蛋白的结合。虽然在没有聚集蛋白的情况下，肌肉中野生型p120ctn的异位表达会产生肌足，但一种在非肌肉细胞中阻断丝状伪足组装的p120ctn特异性显性负突变体形式的表达，会抑制神经和聚集蛋白诱导的肌足形成。值得注意的是，接近的神经突触发表达突变型p120ctn的肌肉细胞边缘的乙酰胆碱受体（AChR）聚集减少，低于对照细胞，尽管突变细胞聚集AChR的能力本身是正常的。我们的结果表明p120ctn在聚集蛋白诱导的肌足组装中具有新作用，并表明肌足增加了肌肉与神经的接触以及肌肉获取神经聚集蛋白的机会，从而促进NMJ的形成。
