Mechanisms of axon guidance during development

在过去的一年里，我们在轴突生长和引导的分析方面取得了两项重大的学术进步。这个项目的中心工作是了解神经是如何生长的，以及为什么它们可能无法生长。该实验室之前的工作结合了对单个生长轴突的实时成像，以及对促进动物发育过程中正常神经生长的基因和蛋白质的遗传和生化分析。问题在于，神经生长的详细机制涉及到任何显微镜都无法分辨的过程。因此，在过去的一年里，我们求助于对正在生长的神经的运动机械--肌动蛋白细胞骨架--的计算机模拟，试图通过产生可测试的预测来将我们的成像与我们的遗传学和生物化学联系起来，从而产生可测试的预测，来预测神经的生化机械如何产生使神经生长的力量，并处理告诉神经在哪里生长的信息。这已经取得了惊人的成功；我们通过计算从生物物理第一原理生成的图片看起来与我们在显微镜中看到的蛋白质分布极其相似，这种相似性已经通过严格的定量分析得到了验证。因此，我们可以非常自信地说，我们提出的关于神经如何生长以及它们如何知道在哪里生长的详细分子模型，确实捕捉到了真正的神经在发育中的动物中所发生的事情的本质。这些计算论文目前正在为出版做准备。与此同时，我们还将注意力转向了为什么神经在疾病中不能生长或维持的问题。在这里，我们发现亨廷顿病中突变的基因HTT，是我们在早期开发中研究的同一神经生长机制的一部分。事实上，HTT是Abl激酶的抑制者，而Abl激酶是神经生长过程中肌动球蛋白的关键调节因子，而HTT突变体中出现的缺陷是由于Abl的过度活性造成的。因此，我们对发育早期的初始神经生长所做的分析，最终解释了导致晚年神经退化的基因突变的后果。

In the past year, we have made two significant intellectual advances in our analysis of axon growth and guidance. The central effort in this project is to understand how nerves grow, and why they may fail to grow. Previous work from the lab combined live imaging of single growing axons with genetic and biochemical analysis of genes and proteins that promote proper nerve growth during the development of the animal. The problem has been that the detailed mechanism of nerve growth involves processes that lie beyond the resolving power of any microscope. In the past year, therefore, we have turned to computational simulations of the motor machinery of the growing nerve, the actomyosin cytoskeleton, to try to connect our imaging with our genetics and biochemistry by generating testable predictions for how the biochemical machinery of the nerve generates the force to make the nerve grow, and processes the information that tells the nerve where to grow. This has been astonishingly successful; the pictures we generate computationally from biophysical first principles look extremely similar to the protein distributions we see in the microscope, and that similarity has been validated by rigorous quantitative analysis. We can therefore say with great confidence that the detailed molecular model we have proposed for how nerves grow, and how they know where to grow, indeed captures the essence of what goes on in a real nerve as it finds its way through the developing animal. These computational papers are currently in preparation for publication. In parallel, we also turned our attention to the problem of why nerves fail to grow, or to be maintained, in disease. Here we found that the gene that is mutated in Huntingtons Disease, HTT, is a piece of the same nerve growth machinery we have been studying in early development. Indeed, HTT is a repressor of the Abl kinase that is the key regulator of actomyosin during nerve growth, and the defects that occur in an htt mutant are due to overactivity of Abl. Thus, the analysis we have done of initial nerve growth early in development turns out to explain the consequences of mutating a gene that causes neurodegeneration late in life.