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

Abstract

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 live imaging of morphology and actin organization in the TSM1 axon of the Drosophila wing to dissect the mechanism of Ena action. We find that altering Ena activity has a substantial impact on filopodial morphology in this growth cone, but exerts only modest effects on actin organization. This is in contrast to the main regulator of Ena, Abl tyrosine kinase, which has profound effects on actin and only mild effects on TSM1 growth cone morphology. These data suggest that the primary role of Ena in this axon may be to link actin to morphogenetic processes of the plasma membrane, rather than regulating actin organization itself. These data also suggest that a key role of Ena, acting downstream of Abl, may be to maintain a constant filopodial organization of the growth cone, even as Abl activity varies in response to guidance cues in the environment.Summary statementWe dissect the function of the actin polymerase, Enabled, in axon growth by live-imaging of actin dynamics and axon morphology of the TSM1 neuron in its native environment in vivo.

A new view of axon growth and guidance grounded in the stochastic dynamics of actin networks

“…Enhanced ability to watch axons as they grow in their native tissues, and extension of in vitro conditions to the more physiological conditions of a threedimensional growth matrix and a mechanically compliant environment, has revealed that there is another mode of growth, at least as common and perhaps even more common than the adhesive modes. These are growth cones that seem to move through non-adhesive mechanisms [37][38][39]. They tend to be characterized by a local profusion of filopodia in the distal part of the axon and have few, if any, lamellipodia (figure 1b).…”

“…Instead, the key effect of Ena is evidently on the linkage of that actin to the plasma membrane, that is, on the efficiency with which filopodia are produced in response to accumulation of actin. It seems that the complementarity of Abl effects on Ena versus Arp2/3 is likely a mechanism to ensure that growth cone morphology remains stable even as Abl activity is altered to vary the properties of the growth cone actin network [39]. It is interesting that in other biological contexts, Ena, as well as its vertebrate orthologues, is evidently capable of having more pronounced effects on actin organization than is seen in the TSM1 growth cone [96][97][98].…”

Kinetic trapping organizes actin filaments within liquid-like protein droplets

“…The interactions of actin with VASP have been studied previously both in cellular (12,13) and reconstituted systems (14,15). VASP was originally isolated from platelets (16) where it is known to inhibit platelet activation/aggregation (17,18).…”