Mechanisms of axon guidance during development

点击翻译按钮获取中文摘要

Abl tyrosine kinase is a master regulator of axon growth and guidance throughout the animal kingdom. It is a key regulator of actin structure and dynamics, and a key point of integration of signals from many guidance cue receptors in axonal growth cones. As we reported in two back-to-back papers this year, we can now trace the mechanism by which Abl controls actin structure, and thereby growth cone motility, all the way back to its roots in actin biophysics. In brief, our previous genetics and biochemistry showed that Abl controls the ratio of linear actin polymerization to actin branching (through its antagonistic regulation of the actin polymerase, Enabled, and of the Rac GEF, Trio). Now, by performing high-resolution, quantitative, live imaging of actin in a growth cone advancing in vivo, we have revealed the significance of this regulation. The intermediate level of actin branching maintained by Abl allows a mass of actin in the core of the growth cone to oscillate in length, but only within narrow limits consistent with maintenance of order in the actin distribution. The role of guidance cue receptors is to locally favor actin extension or branching (respectively), which we show introduces a spatial bias to the oscillations of actin, and thus causes the actin mass to advance along a predictable trajectory. Motion of the actin, in turn, favors filopodial assembly in places where the axon is destined to extend, and disassembly of filopodia in places the growth cone is destined to vacate. Over time, this mechanism produces net motion of the dynamic core of the growth cone along the trajectory defined by guidance cues. This molecular model is entirely without precedent in the literature of axon growth and guidance, but in addition to explaining our own observations, it can account for the effects of a wide range of actin modulatory proteins, and also provides simple explanations for numerous longstanding mysteries in the mechanism of axon guidance. Ongoing experiments are performing further tests of our model, both by imaging the phenotypes observed upon genetic manipulation of other Abl pathway components, and by generating detailed computational simulations of the consequences of altering various parameters of actin dynamics. In addition, together with collaborators we are (1) developing novel methods of advanced microscopy that increase the spatial and temporal resolution with which we can monitor growth cone structure and dynamics in live samples, (2) developing novel methods of image analysis to automate tracing of axon images, and (3) investigating the implications that uncontrolled excursions of Abl activity have for the production and maintenance of neural circuits in the context of neurodegenerative disease.