Axon motility: Mechanics and Chemotaxis
In collaboration with neuroscientist Dr. Herb Geller at NIH,we have been investigating the biomechanics of growing axons and their response to the structure and mechanics of their environments. We performed careful measurements of neuronal outgrowth and growth cone traction force generation on substrates of different stiffnesses, comparing primary neurons from the peripheral and central nervous systems. We found that peripheral axons showed maximal outgrowth on substrates with a Young’s modulus of 1000 Pa, comparable to the stiffness of many tissues, whereas outgrowth from central nervous system axons, which normally extend in an extremely soft environment, is independent of substrate stiffness. We showed that these differences are correlated with dramatic differences in internally generated forces (although both types of neurons are substantially weaker than other motile cells) [1]. Our measurements of cytoskeletal dynamics and adhesion protein distributions showed that the difference in force generation arises from a differing degree of coupling between the cytoskeleton and the extra-cellular environment. We also performed the first measurements of dynamic tension fluctuations in extending axons, showing that they are consistent with the maturation and dissolution of stress-bearing adhesions in growth cones [2]. Finally, we have also shown that changes in structural confinement, but not confinement by itself, affect the rate of axon outgrowth [3]. These are part of the small but growing body of work that shows the similarities and differences in mechanosensing in neurons compared to other cell types.
Axon Chemotaxis
In collaboration with neuroscientist Dr. Geoff Goodhill, we developed the first theoretical models for calculating the limits of growth cone detection of molecular gradients [4]. The goal of this work was to determine the role of chemotactic signaling for long range guidance during nervous system development. However, it was clear that existing data on chemotactic response of growing axons did not meaningfully constrain our models, and that existing assays for measuring chemotaxis were not appropriate for studying cultured neurons. Thus we developed a new technology for producing controlled molecular gradients in three-dimensional gels, taking advantage of then emerging technologies in precision droplet printing and the nature of slow diffusion of large proteins [5]. We used this technology to demonstrate that axonal growth cones are among nature’s most sensitive gradient detectors [6], capable of responding to gradients as small as 0.1% across the width of the growth cone, the dynamic structure at the tip of the growing axon. In some regimes this represents an average difference in the number of molecules between the high and low concentration sides of the growth cone of about one molecule. We combined these results with the theoretical analysis to demonstrate that the concentration-dependent chemotactic response does not require an adaptive signaling network, but does take maximal advantage of both spatial and temporal averaging [7].
