Enabled primarily controls filopodial morphology, not actin organization, in the TSM1 growth cone in Drosophila

Ena/VASP proteins are processive actin polymerases that are required throughout animal phylogeny for many morphogenetic processes, including axon growth and guidance. Here we use in vivo live imaging of morphology and actin distribution to determine the role of Ena in promoting the growth of the TSM1 axon of the Drosophila wing. Altering Ena activity causes stalling and misrouting of TSM1. Our data show that Ena has a substantial impact on filopodial morphology in this growth cone but exerts only modest effects on actin distribution. This is in contrast to the main regulator of Ena, Abl tyrosine kinase, which was shown previously to have profound effects on actin and only mild effects on TSM1 growth cone morphology. We interpret these data as suggesting that the primary role of Ena in this axon may be to link actin to the morphogenetic processes of the plasma membrane, rather than to regulate actin organization itself. These data also suggest that a key role of Ena, acting downstream of Abl, may be to maintain consistent organization and reliable evolution of growth cone structure, even as Abl activity varies in response to guidance cues in the environment.