Mechanism of autophagy-controlled axon branching

对科学家来说，最重要的未解决问题之一是神经系统是如何形成的。在胚胎发育过程中，神经细胞会发出长长的线状结构，即轴突，这些结构会长很长一段距离到达目的地，在那里它们会与其他神经细胞连接起来。为了与多个其他神经细胞形成连接，轴突必须在目标区域分支，这遵循一个复杂而特定的模式。这个过程对神经元回路的发育至关重要。我们的研究旨在理解视觉系统中的轴突分支，它连接着眼睛的视网膜和大脑。关于视觉系统积累的大量知识使其成为揭示发展基本原理的理想工具。我们之前的工作表明，当视网膜神经元被培养时，它们形成的分支模式与完整生物体中的分支模式非常相似。我们最近发现神经元的废物处理过程，称为自噬，参与了这个过程。神经元利用自噬来降解不需要的分子。我们已经证明，自噬的增加导致分支数量的增加，而自噬的减少导致分支数量的减少。我们认为神经元通过局部破坏（通过自噬）通常抑制轴突分支形成的分子来控制分支的数量和位置。因此，在发育过程中，轴突分支通常受到抑制，只有在自噬激活的区域才会发生分支。我们已经证明了一种叫做ar18b的小分子，它在轴突中运输溶酶体，可能控制自噬被激活的位置。ar18b表达水平的变化与在自噬过程中起关键作用的囊泡分布和密度的变化有关，这些囊泡被称为自噬体。这个令人兴奋的项目将测试自噬影响轴突分支和神经回路发育的假设。到目前为止，我们的数据显示了很好的相关性，我们现在的目标是显示一种因果关系。为此，我们将使用时间推移实验来揭示自噬体/溶酶体的运动与分支形成之间的时空相关性。然后我们要确定控制轴突自噬激活位置的细胞外分子，我们还要确定通常阻断分支的抑制分子。这项关于神经细胞培养的工作将扩展到完整的动物。我们想要证明单个轴突在基因上消除视网膜中的自噬会干扰它们在小鼠视觉系统中的连通性。这将表明，我们在文化中发现的机制在视觉系统的发展过程中发挥了作用。我们认为，这些相互交织的方法的整合将导致对自噬及其关键成分之一ar18b在神经系统回路发育中的作用有更深入的了解。到目前为止，我们对自噬在神经系统发育中的作用知之甚少，我们的研究将为这些问题提供大量的新信息。最近的出版物揭示了自噬解除管制与自闭症谱系障碍病因之间的密切联系。我们预测，我们对小G蛋白Arl8B的研究将把这些不同的方面整合到一个概念中，并代表了一种研究自噬和神经回路形成之间联系的新方法。我们预测这项研究将有助于更好地理解自闭症谱系障碍的病因学。

One of the most important unsolved questions for scientists is how the nervous system is formed. During embryonic development nerve cells send out long thread-like structures, or axons, which grow over long distance to reach their destinations, where they will wire up with other nerve cells. In order to form connections with multiple other nerve cells, axons must branch in the target area which follows an intricate and specific pattern. This process is crucial for the development of neuronal circuits. Our research seeks to understand axon branching in the visual system, which connects the retina of the eye with the brain. The large body of knowledge accrued about the visual system makes it ideal for uncovering fundamental principles of development. Our previous work has shown that when retinal neurons are cultured, they form branched patterns which are closely similar to those found in the intact organism. We have recently found that the neuron's waste disposal processes, termed autophagy, is involved in this process. Neurons deploy autophagy as a means of degrading unwanted molecules. We have shown that an increase in autophagy leads to an increase in the number of branches, while a reduction autophagy leads to a decrease in the number of branches. We believe that neurons control the number and location of branches by locally destroying (via autophagy) molecules which normally suppress the formation of axon branches. Thus during development, axon branching is normally suppressed, and only in regions where autophagy is activated, branching can occur. We have shown that a small molecule called Arl8B, which transports lysosomes in axons, might control where autophagy is activated. Changes in the expression level of Arl8B correlate with changes in the distribution and density of vesicles having a key role in executing autophagy, named autophagosomes. This exciting project will test the hypothesis that autophagy shapes axonal branching and neural circuit development. Our data so far show a good correlation, and we aim to show now a causal relationship. For this we will use time lapse experiments to unravel the spatio-temporal correlation between the movement of autophagosomes/lysosomes and the formation of branches. Then we would like to identify the extracellular molecules which control where along an axon autophagy is activated, and we would like to identify the suppressor molecules which normally block branching. This work on neuronal cell culture will be extended into intact animals. We would like to show for individual axons that genetically eliminating autophagy in the retina disturbs their connectivity in the visual system of mice. This will suggest that the mechanisms we have discovered in culture play a role during the development of the visual system. We think that an integration of these interwoven approaches will lead to a deeper understanding of the role of autophagy and one of its key components, Arl8B, in circuit development within the nervous system. Up to now, very little is known about the role of autophagy during nervous system development, and our research will provide substantial new information on these issues. Recent publications have uncovered a close link between deregulation of autophagy and the aetiology of autism spectrum disorders. We predict that our investigation of the small G protein Arl8B will integrate these different aspects into one concept, and represents a novel approach to investigate the connection between autophagy and neural circuit formation. We predict that this investigation will contribute to a better understanding of the aetiology of autism spectrum disorders.