Live imaging of an axon in its native tissue shows that its growth is protrusive and occurs by stabilization of selected filopodia. Guidance signaling, however, for example, via Abl tyrosine kinase, does not control these morphological properties directly but rather controls actin distribution to determine where filopodial dynamics can occur.

The Abl tyrosine kinase signaling network controls cell migration, epithelial organization, axon patterning and other aspects of development. Although individual components are known, the relationships among them remain unresolved. We now use FRET measurements of pathway activity, analysis of protein localization and genetic epistasis to dissect the structure of this network in Drosophila. We find that the adaptor protein Disabled stimulates Abl kinase activity. Abl suppresses the actin-regulatory factor Enabled, and we find that Abl also acts through the GEF Trio to stimulate the signaling activity of Rac GTPase: Abl gates the activity of the spectrin repeats of Trio, allowing them to relieve intramolecular repression of Trio GEF activity by the Trio N-terminal domain. Finally, we show that a key target of Abl signaling in axons is the WAVE complex that promotes the formation of branched actin networks. Thus, we show that Abl constitutes a bifurcating network, suppressing Ena activity in parallel with stimulation of WAVE. We suggest that the balancing of linear and branched actin networks by Abl is likely to be central to its regulation of axon patterning.

Live imaging reveals that a forward bias of the stochastic actin fluctuations in the distal part of an axon drives axon extension by advancing the protrusive, filopodial domain of the growth cone. The actin mass is itself shaped by guidance signaling via Abl tyrosine kinase, which regulates growth cone length and minimizes its disorder.

How do single-molecule dynamics produce multimicron-scale changes in actin organization in an extending axon? Comparison of computational simulations to in vivo data suggests that Abl kinase and Arp2/3 expand actomyosin networks by fragmenting them into multiple domains, thus toggling the axon between states of local versus global internal connectivity.

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.

Extensive studies of growing axons have revealed many individual components and protein interactions that guide neuronal morphogenesis. Despite this, however, we lack any clear picture of the emergent mechanism by which this nanometer-scale biochemistry generates the multi-micron scale morphology and cell biology of axon growth and guidance in vivo. To address this, we studied the downstream effects of the Abl signaling pathway using a computer simulation software (MEDYAN) that accounts for mechanochemical dynamics of active polymers. Previous studies implicate two Abl effectors, Arp2/3 and Enabled, in Abl-dependent axon guidance decisions. We now find that Abl alters actin architecture primarily by activating Arp2/3, while Enabled plays a more limited role. Our simulations show that simulations mimicking modest levels of Abl activity bear striking similarity to actin profiles obtained experimentally from live-imaging of actin in wild type axons in vivo. Using a graph-theoretical filament-filament contact analysis, moreover, we find that networks mimicking hyperactivity of Abl (enhanced Arp2/3) are fragmented into smaller domains of actin that interact weakly with each other, consistent with the pattern of actin fragmentation observed upon Abl overexpression in vivo. Two perturbative simulations further confirm that high Arp2/3 actin networks are mechanically disconnected and fail to mount a cohesive response to perturbation. Taken together, these data provide a molecular-level picture of how the large-scale organization of the axonal cytoskeleton arises from the biophysics of actin networks.Highlight summaryHow do single-molecule dynamics produce multi-micron scale changes in actin organization in an extending axon? Comparison of computational simulations to in vivo data suggests that Abl kinase and Arp2/3 expand actomyosin networks by fragmenting into multiple domains, thus toggling the axon between states of local vs global internal connectivity.

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 morphogenetic processes of the plasma membrane, rather than for regulating 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.

33The fundamental problem in axon growth and guidance is to understand how cytoplasmic signaling 34 modulates the cytoskeleton to produce directed growth cone motility. We show here that the TSM1 35 pioneer axon of Drosophila extends by using Abl tyrosine kinase to shape the intrinsic fluctuations 36 of a mass of accumulated actin in the distal axon. The actin mass fluctuates stochastically in length, 37 but with a small, forward bias that drives the axon along its trajectory by promoting emergence of 38 protrusions in leading intervals where actin accumulates, and collapse of protrusions in lagging 39 intervals that actin has vacated. The actin mass is sculpted by Abl signaling, which 40 probabilistically modulates its key parameters -its width and internal disorder -to drive its 41 advance, while maintaining internal coherence. Comparison of TSM1 to other systems suggests 42 that the mechanism we demonstrate here is apt to be common among pioneer axons in many 43 organisms. 44 45 46