Mechanisms of axon guidance during development

我们试图回答两个问题：神经元是如何在发育过程中连接起来的，为什么它们在神经退行性疾病期间会断开连接？ 我们目前在轴突生长和引导方面的大部分工作集中在了解信号模块的作用，这些信号模块将生长锥体表面的受体信号转化为轴突尖端的引导运动。我们特别关注Abl酪氨酸激酶信号通路，它在许多最常见的、系统发育保守的指导受体下游发挥着核心作用。完全意想不到的是，在过去的一年里，我们发现Abl的指导功能部分是通过调节分泌装置来执行的。因此，我们取得了以下发现： 1.在发育中的果蝇光感受器神经元中，关键的abl效应器，即肌动蛋白调节蛋白，选择性地与顺式高尔基体相关联。我们已经用标准荧光显微镜、超分辨荧光和免疫电子显微镜证明了这一点。 2.Abl激酶功能丧失或其拮抗剂功能增强，导致高尔基体碎裂，高尔基池在光感受器神经元胞体内重新定位。活体成像显示，降低Abl活性会增加高尔基体池分裂的频率，并降低融合的频率。 3.遗传学和药理学实验表明，Abl对高尔基体结构和分布的影响是通过Abl依赖的肌动蛋白细胞骨架的调节而实现的。 4.两条证据表明，Abl通路突变对轴突引导的影响部分是通过它们对分泌的影响来介导的。首先，直接干扰高尔基体功能的突变会产生类似Abl突变体的轴突缺陷。其次，基因测试表明，真正的分泌型突变体在导致轴突形成的途径中作用于Abl激酶的下游。 综上所述，这些数据强烈表明，Abl突变体的轴突表型在一定程度上是由于Abl调节分泌装置，通过Abl依赖的肌动蛋白结构和动力学调节发挥作用。 ABL是脊椎动物和无脊椎动物轴突形态、细胞极性、上皮组织和完整性的中央调节因子。它是两种常见人类白血病的致癌基因，最近的证据表明，它在帕金森氏病的病因学中起着关键作用。三十年来，ABL的研究一直集中在其对皮质肌动蛋白细胞骨架的影响上。我们的结果表明，现在必须重新考虑大量关于Abl的工作，以确定其有多少影响实际上是通过其对蛋白质分类、运输和分泌的影响而部分或全部介导的。

We seek to answer two questions: how do neurons become connected during development, and why do they become disconnected during neurodegenerative disease? 

 Most of our effort on axon growth and guidance currently focuses on understanding the action of the signaling modules that translate receptor signals at the growth cone surface into guided motility of the axon tip. We focus in particular on the Abl tyrosine kinase signaling pathway, which plays a central role downstream of many of the most common, phylogenetically-conserved guidance receptors. In a completely unexpected twist, in the past year we discovered that the guidance function of Abl is executed, in part, through regulation of the secretory apparatus. Thus, we have made the following discoveries: 1. The key Abl effector, the actin-regulatory protein Enabled, is selectively associated with the cis-Golgi compartment in developing Drosophila photoreceptor neurons. We have shown this by standard fluorescence microscopy, super-resolution fluorescence and by immunoelectron microscopy. 2. Loss-of function of Abl kinase, or gain-of-function of its antagonist Enabled, cause fragmentation of the Golgi apparatus, and relocalization of Golgi cisternae within the cell bodies of photoreceptor neurons. Live imaging demonstrates that reducing Abl activity increases the frequency of splitting of Golgi cisternae, and decreases the frequency of fusions. 3. Genetic and pharmacological experiments demonstrate that the effect of Abl on the Golgi structure and distribution is mediated through Abl-dependent regulation of the actin cytoskeleton. 4. Two lines of evidence suggest that the effect of Abl pathway mutations on axon guidance are mediated, in part, by their effects on secretion. First, mutations that interfere directly with Golgi function produce axonal defects that mimic those of Abl mutants. Second, genetic tests show that bona fide secretory mutants act downstream of Abl kinase in the pathway leading to axon patterning. Together, these data strongly suggest that the axonal phenotypes of Abl mutants are due, in part, to modulation of the secretory apparatus by Abl, working through the Abl-dependent regulation of actin structure and dynamics. Abl is a central regulator of axon patterning, cell polarity and epithelial organization and integrity in both vertebrates and invertebrates. It is the causative oncogene for two common forms of human leukemia, and recent evidence suggest it has a critical role in the etiology of Parkinsons disease. For three decades, studies of Abl have focused on its effects on the cortical actin cytoskeleton. Our results suggest that a vast body of work on Abl must now be reconsidered to ascertain how many of its effects are actually mediated, in part or in whole, through its effects on protein sorting, trafficking and secretion.