Mechanisms of axon guidance during development

我们试图回答两个问题：神经元在发育过程中是如何连接的，为什么在神经退行性疾病期间它们会断开连接？ 在脊椎动物和无脊椎动物中，轴突沿着中枢神经系统(CNS)纵轴生长的机制一直是个谜。这是后生动物神经发育的关键问题，也是修复灾难性脊髓损伤的必要过程。根据我们在过去一年中取得的结果，我们现在可以在很大程度上解释这一过程是如何在果蝇的中枢神经系统中发生的。首先，我们解开了四种不同中枢神经系统细胞类型的协调发展机制，以产生一个轨迹，在连续的节段之间，纵向轴突可以沿着这个轨迹生长。这涉及到沿着专门的神经胶质细胞表面铺开一层薄薄的神经元组织，并将这一层塑造成连接节段之间的连续粘连带。其次，我们已经确定了是什么驱使生长的轴突沿着这条轨道延伸。生长轴突表面的受体Notch抑制主要的细胞质信号蛋白Abl酪氨酸激酶的活性。这促进了生长中的轴突形成长丝足，并抑制了这些轴突与其底物的粘连。总而言之，这些影响在有利于生长的细胞骨架动态循环中的各个步骤之间建立了平衡。轴突引导分子通过在细胞骨架组织的动态中产生特定的平衡来实现其效果的观点，而不是通过显性的促进和抑制生长的活动，是一个新的假设，它使人们关注到关于神经连接的大量数据，这些数据到目前为止一直是相互矛盾和令人困惑的。我们认为，这种不同的视角将极大地促进我们对神经连接机制的理解，以及对细胞运动的理解。 在这项工作的过程中，我们被迫剖析了由Abl蛋白酪氨酸激酶定义的信号网络。ABL是第一个被发现与人类重大癌症有关的细胞原癌基因。它的信号通路的中心组成部分多年来一直是已知的，但它们如何结合形成一条通路一直拒绝分析。我们发现，通过分析Abl在上皮细胞中的功能，我们可以对这一信号通路中的步骤进行排序。我们发现，首先，长期以来被认为是Abl效应蛋白的失活蛋白，实际上是定位该激酶的上游调节因子。此外，初步结果表明，Abl信号系统不是一条单一的线性途径，而是有两个基本上分开的分支，尽管它们是协调的。这可能解释了以前对这一途径的研究中的许多模棱两可和复杂的问题。了解Abl信号转导机制对于了解神经生长的调控和机制，以及治疗慢性粒细胞白血病等血液系统恶性肿瘤具有重要意义。

We seek to answer two questions: how do neurons become connected during development, and why do they become disconnected during neurodegenerative disease? The mechanisms responsible for growth of axons along the longitudinal axis of the central nervous system (CNS) have been mysterious in both vertebrates and invertebrates. This is a crucial question for metazoan neural development, and is also the essential process for repair of catastrophic spinal cord injuries. In light of results we have obtained over the past year, we can now largely account for how this process occurs in the Drosophila central nervous system. First, we have disentangled the mechanism by which the development of four different CNS cell types are coordinated to produce a track along which longitudinal axons can grow between successive segments. This involves spreading a thin layer of neuronal tissue along the surfaces of specialized glial cells, and shaping that layer into a continuous adhesive band that bridges between segments. Second, we have determined what drives growing axons to extend along that track. A receptor on the surface of the growing axon, Notch, suppresses the activity of a major cytoplasmic signaling protein, the Abl tyrosine kinase. This promotes the formation of long filopodia by the growing axons, and suppresses the adhesion of those axons to their substratum. Together, these effects establish a balance among the steps in the cycle of dynamics of the cytoskeleton that is conducive to growth. The view that axon guidance molecules achieve their effects by producing a particular balance in the dynamics of cytoskeleton organization, and not by explicit growth-promoting and growth-retarding activities, is a novel hypothesis that brings into focus a large body of data on neural wiring that has until now been contradictory and confusing. We argue that this alternate perspective will greatly advance our understanding of the mechanisms of neural wiring, and also of cell motility. In the course of this work we have been forced to dissect the signaling network defined by the Abl protein tyrosine kinase. Abl was the first cellular proto-oncogene found to be responsible for a major human cancer. Central components of its signaling pathway have been known for many years, but how they combine to form a pathway has resisted analysis. We found that by analyzing Abl function in epithelia we could order the steps in this signaling pathway. We found, first, that the Disabled protein, long thought to be an effector of Abl, is actually an upstream regulator that localizes the kinase. Moreover, preliminary results suggest that rather than being a single, linear pathway, the Abl signaling system has two branches that are largely separate, though coordinated. This likely accounts for many of the ambiguities and complexities in prior studies of this pathway. Unraveling the mechanism of Abl signaling is central to understanding the regulation and machinery of nerve growth, and also for treatment of hematopoietic malignancies such as chronic myelogenous leukemia.