654B Poster - 10. Cell biology: Cytoskeleton, organelles and trafficking
Friday April 08, 2:00 PM - 4:00 PM

Authors:
Aravind Chandrasekaran ^{1, 2}; Akanni Clarke ¹; Philip McQueen ¹; Hsiao-Yu Fa1ng ¹; Garegin Papoian ²; Edward Giniger ¹

Affiliations:
1) NINDS, NIH, Bethesda, MD; 2) University of Maryland, College Park, MD

Keywords:
a. cytoskeleton; a. axon guidance

Cytoplasmic signaling pathways regulate cellular morphogenesis, but how do the nanoscale dynamics of a signaling protein create structures and direct forces at the multi-micron spatial scales of a cell? We have now compared experimental measurements of actin distribution and dynamics in single axonal growth cones in vivo in the fly wing with the results of single-molecule computational simulations of actomyosin dynamics. The results suggest a simple framework for understanding multiscale regulation of structure and force in a living cell.

We have performed live imaging of the tip of an axon – its growth cone – in vivo in the developing fly wing. This reveals that the core of the growth cone is a mass of non-polarized actin that is undergoing constant, stochastic fluctuations. Those fluctuations are the engine of growth cone motility, as we have shown that net axon advance comes from a spatial bias in the actin fluctuations that causes the network to take more and longer steps forward than steps back. The bias comes from the cytoplasmic signaling pathways that control actin polymerization and branching. Our data show that Abl tyrosine kinase, a key downstream effector of axon guidance receptors, controls the spatial spread of the actin network: increasing Abl activity causes the growth cone actin core to expand, while decreasing Abl causes the network to contract. Computational simulations now reveal that the mechanism by which Abl controls network size is mediated through modulation of the lengths of actin filaments. Activating Abl causes a net decrease in the lengths of actin filaments, reducing their ability to link the actin network together. Consequently, the network as a whole fragments and spreads out in the growth cone, just as we observe experimentally. Conversely, reduction of Abl promotes extension of actin filaments, allowing myosin-dependent contractility to act across long length scales to condense the entire actin network, again, just as we observe experimentally.

These data provide a framework for interpreting the effects not only of Abl, but of many regulators of growth cone function. Moreover, the principles revealed here should apply to many other developmental contexts that rely on cytoplasmic signaling mechanisms to control the spatial distribution of actin-dependent structures and forces.