Cytoskeletal dynamics in growth-cone steering.

Interactions between dynamic microtubules and actin filaments are essential to a wide range of cell biological processes including cell division, motility and morphogenesis. In neuronal growth cones, interactions between microtubules and actin filaments in filopodia are necessary for growth cones to make a turn. Growth-cone turning is a fundamental behaviour during axon guidance, as correct navigation of the growth cone through the embryo is required for it to locate an appropriate synaptic partner. Microtubule-actin filament interactions also occur in the transition zone and central domain of the growth cone, where actin arcs exert compressive forces to corral microtubules into the core of the growth cone and thereby facilitate microtubule bundling, a requirement for axon formation. We now have a fairly comprehensive understanding of the dynamic behaviour of the cytoskeleton in growth cones, and the stage is set for discovering the molecular machinery that enables microtubule-actin filament coupling in growth cones, as well as the intracellular signalling pathways that regulate these interactions. Furthermore, recent experiments suggest that microtubule-actin filament interactions might also be important for the formation of dendritic spines from filopodia in mature neurons. Therefore, the mechanisms coupling microtubules to actin filaments in growth-cone turning and dendritic-spine maturation might be conserved.