Molecular Mechanisms of L1-dependnet axon growth (intracellular L2-trafficking and signaling pathways)

在发育中的神经系统中，中枢和外周轴突束是由从神经元体延伸并到达适当目标的轴突形成的。生长轴突及其尖端（生长锥）上表达的各种细胞粘附分子（CAMs）与环境信号相互作用，调节生长锥的运动，并引导轴突沿着正确的路径运动。在本研究项目中，我们对免疫球蛋白超家族CAM Li及其相互作用蛋白ankyrin_B的功能进行了分析，得到以下结果：Ankyrin_B作为一个分子离合器，将逆行的f -肌动蛋白流与L1细胞质结构域（L1CD）连接起来，从而传递驱动神经突起始的牵引力。2.L1与逆行的f -肌动蛋白流偶联进入生长锥的中心区域后，通过AP-2和网格蛋白介导的途径被内吞，然后再循环到生长锥前缘的质膜。这种类型的细胞内L1运输是在生长锥中产生和维持L1介导的粘附梯度（前部强粘附和后部弱粘附）所必需的。它也是生长球果的顺行迁移所必需的。3.L1与AP-2的相互作用受L1CD中Tyr^<1176>的磷酸化/去磷酸化调控。网格蛋白介导的L1内吞作用是由酪氨酸去磷酸化触发的。4.这种酪氨酸可以被非受体酪氨酸激酶Src磷酸化，该激酶定位于脂筏，富含胆固醇和鞘脂的特殊膜微结构域。5.生长锥外周区域的脂筏对基于l1的轴突生长至关重要。这些结果有助于理解轴突生长的分子机制。

In the developing nervous system, central and peripheral axon tracts are formed by axons that have elongated from the neuronal soma and reached their appropriate targets. Various cell adhesion molecules (CAMs) expressed on the growing axon and its tip (the growth Cone) interact with environmental cues, regulate growth cone motility, and navigate the axon along the correct path. In this research project, we have analyzed the functions of the immunoglobulin superfamily CAM Li and its interacting protein, ankyrin_B, and obtained the following results : 1.Ankyrin_B functions as a molecular clutch that connects retrograde F-actin flow with the L1 cytoplasmic domain (L1CD), thereby transmitting traction force that drives neurite initiation. 2.After moving into the central domain of growth cones by coupling with retrograde F-actin flow, L1 is endocytosed via the AP-2 and clathrin-mediated pathway followed by recycling to the plasma membrane of the growth cone leading edge. This type of intracellular L1 trafficking is required to create and maintain a gradient of L1-mediated adhesion (strong adhesion at the front and weak adhesion at the rear) in growth cones. It is also required for anterograde migration of the growth cones. 3.L1 interaction with AP-2 is regulated by phosphorylation/dephosphorylation of Tyr^<1176> in the L1CD. Clathrin-mediated endocytosis of L1 is triggered by the tyrosine dephosphorylation. 4.This tyrosine can be phosphorylated by the non-receptor tyrosine kinase Src that is localized to lipid rafts, specialized membrane microdomains enriched in cholesterol and sphingolipids. 5.Lipid rafts in the growth cone peripheral domain are critical for L1-based axon growth. These results help to understand the molecular mechanisms of axon growth.