Formation of axon by vesicle transport and cytoskeltal rearrangement

轴突和树突的形成和延伸，即所谓的神经突生长，是神经系统发育过程中神经元分化和成熟的关键事件。这些形态变化需要细胞骨架网络的重组和伴随的膜的扩张和收缩。Rap1小G蛋白已被证明通过调控MAPK级联和细胞骨架网络参与神经突生长和神经元极化。Rho小G蛋白通过调节肌动蛋白动力学参与神经元的形态发育。我们之前的研究表明，Rho的失活是诱导诱导的神经突生长所必需的。Rho gtpase激活蛋白，RA-RhoGAP，将Rap1信号转导到Rho并使Rho失活。Rap1的精确的暂时和空间激活对于调节RA-RhoGAP的活性以产生神经突是重要的。然而，Rap1如何以及在何处被激活的机制尚不清楚。在本研究项目中，我们发现Rap1的GDP/GTP交换因子（GEF） PDZ-GEF1与这些事件有关。1) PDZ-GEF1被GTP-Rap1以反馈机制激活。2)在PC12细胞中，神经生长因子（NGF）结合的TrkA受体转运到晚期核内体是NGF诱导rap1持续激活的必要条件。1.3)在PC12细胞中，ARMS结合S-SCAM与PDZ-GEF1形成复合物，并将复合物募集到晚期核内体。这种位于内体晚期的四聚体复合物诱导Rap1和ERK的持续激活，导致神经突的生长。4)在培养的大鼠海马神经元中，尾核体上的PDZ-GEF1四聚体复合物参与轴突的特异和伸长。

Formation and extension of axons and dendrites, so-called neurite outgrowth, is a crucial event in neuronal differentiation and maturation during development of the nervous system. These morphological changes require reorganization of the cytoskeletal networks and its accompanying membrane expansion and contraction. Rap1 small G protein has been shown to be involved in neurite outgrowth and neuronal polarization through regulating the MAP kinase (MAPK) cascade and the cytoskeletal network. Rho small G protein has been shown to be involved in the morphological development of neurons through regulating actin dynamics. We previously showed that inactivation of Rho is required for the Induced-induced neurite outgrowth. Activated-activated Rho GTPase-activating protein, RA-RhoGAP, transduces a signal from Rap1 to Rho and inactivates Rho. The precise temporary and spatially activation of Rap1 is important for regulating the activity of RA-RhoGAP to produce the neurite. However, the mechanism underlying how and where Rap1 is activated remains unclear. In the present research project, we identified that a GDP/GTP exchange factor (GEF) for Rap1,PDZ-GEF1,was implicated in these events.1) PDZ-GEF1 was activated by GTP-Rap1 in a feedback mechanism.2) In PC12 cells, transport of nerve growth factor (NGF)-bound TrkA receptor to late endosomes is required for the NGF-induced sustained activation of Rap1.3) In PC12 cells, ARMS bound S-SCAM, which formed a complex with PDZ-GEF1, and recruited the complex to late endosomes. This tetramer complex at late endosomes induced sustained activation of Rap1 and ERK, resulting in neurite outgrowth.4) In cultured rat hippocampal neurons, the PDZ-GEF1 tetramer complex on late endosomes is involved in the axon specification and elongation.