Mechanisms of axon guidance during development

National Institute Of Neurological Disorders And Stroke

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Abstract

In the past year, we have made two significant advances in our analysis of axon growth and guidance. The central effort in this project is to understand how nerves 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. Over the past two years, therefore, we turned to computational simulations of the motor machinery of the growing nerve, the actomyosin cytoskeleton, 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 have now been accepted for publication and are in press. In parallel, we also performed an additional experimental axon imaging study to test a surprising result from the simulations. The Enabled gene product has historically been thought to be a major regulator of actin structure in the growth cone, but the simulations suggested it should be playing a relatively subtle role. We therefore performed live-imaging of the axon in flies with gain- or loss-of-function of Enabled. The experimental observations were precisely in line with the predictions from simulation, and showed a minor effect on actin organization. Rather, they suggested that the key effect of Enabled is to link the actin network to the plasma membrane and thereby control the efficiency with which morphological features are induced downstream of actin. This both further validated our computational studies and clarified the relationship between intracellular actin organization and external growth cone morphology