Computational simulations reveal that Abl activity controls cohesiveness of actin networks in growth cones

Abstract

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.

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

“…The experimental observations made here showing that Ena has relatively modest effects on actin organization agree well with our recent results from computational simulations of actin networks (Chandrasekaran et al, 2021). There, we found that the effects on actin from changing Ena activity were manifested most strongly on fine details of network organization at very short range (sub-micron) length scales, beyond the effective resolution of our microscopy.…”

“…Moreover, it has been shown that Abl regulates the activity of Myosin 2 (Dudek et al, 2010), another key regulator of the mesoscopic organization of non-polarized actin assemblies in our simulations. Our computational analysis also suggested a simple mechanistic explanation for how nanometer-scale changes to actin filament length produce multi-micron scale changes in the overall distribution of actin density by modifying the connectivity (percolation) of the actomyosin network (Chandrasekaran et al, 2021).…”