Timothy Gomez

• 
  Mechanochemical signals that guide axon development.
  Timothy Gomez

  Growth cones interact with the extracellular matrix (ECM) through integrin receptors at adhesion sites termed point contacts. Point contact adhesions link ECM proteins to the actin cytoskeleton through numerous adaptor and signaling proteins. One presumed function of growth cone point contacts is to restrain or “clutch� myosin II-based F-actin retrograde flow (RF) to promote leading edge membrane protrusion. In motile non-neuronal cells, myosin II binds and exerts force upon actin filaments at the leading edge where clutching forces occur. However, in growth cones it is unclear whether similar F-actin clutching forces affect axon outgrowth and guidance. I will show that RF is reduced in rapidly migrating growth cones on laminin (LN) compared to non-integrin binding poly-d-lysine (PDL). Moreover, acute stimulation with LN leads to accelerated axon outgrowth over a time course that correlates with point contact formation and reduced RF. These results suggest that actin RF is restricted by the assembly of point contacts, which we show directly by two color imaging of actin RF and paxillin. Further, using micro-patterns of PDL and LN, we demonstrate that individual growth cones have differential actin RF rates while interacting with two distinct substrata. Opposing effects on actin RF rates were also observed in growth cones treated with chemoattractive and chemorepulsive axon guidance cues known to influence point contact adhesions. Finally, using GFP-actin, we show that actin RF within growth cones in the spinal cord is slow, suggesting RF is being restrained by molecular clutching forces in vivo.