ESSENTIAL ROLE OF FILOPODIA IN CHEMOTROPIC TURNING OF NERVE GROWTH CONE INDUCED BY A GLUTAMATE GRADIENT

Pathfinding of growing neurites depends on turning of the growth cone in response to extracellular cues. Motile filopodia of the growth cone are known to be critical for mediating contact-dependent guidance of the growth cone. However, whether filopodia also play an essential rol e in growth cone turning response induced by a diffusible chemotropic substance is unclear. Growth cones of cultured Xenopus spinal neurons exhibited chemotropic turning responses in a gradient of glutamate wit hin a limited range of concentrations. This turning response depends o n the activation of the NMDA subtype of glutamate receptors and requir es the presence of extracellular Ca2+. Time-lapse differential interfe rence contrast microscopy with quantitative analysis of filopodia dyna mics showed a close correlation between an increased number of filopod ia on the side of the growth cone facing the glutamate source and the turning. Such filopodia asymmetry was observed within minutes after th e onset of the glutamate gradient, before any detectable turning of th e growth cone. In Ca2+-free medium, no filopodia asymmetry was induced by the glutamate gradient, and no growth cone turning was observed. F urthermore, elimination of filopodia with a low concentration of cytoc halasin B completely abolished the turning response without substantia lly affecting neurite extension. Thus, filopodia may be required for c hemotropic guidance of the growth cone, and an asymmetry in filopodia distribution may be an early cellular event responsible for determinin g the direction the growth cone advances.