Myelin and collapsin-1 induce motor neuron growth cone collapse through different pathways: Inhibition of collapse by opposing mutants of Rac1

Precise growth cone guidance is the consequence of a continuous reorganizat ion of actin filament structures within filopodia and lamellipodia in respo nse to inhibitory and promoting cues. The small GTPases rad, cdc42, and rho A are critical for regulating distinct actin structures in non-neuronal cel ls and presumably in growth cones. Collapse, a retraction of filopodia and lamellipodia, is a typical growth cone behavior on contact with inhibitory cues and is associated with depolymerization and redistribution of actin fi laments. We examined whether small GTPases mediate the inhibitory propertie s of CNS myelin or collapsin-1, a soluble semaphorin, in chick embryonic mo tor neuron cultures. As demonstrated for collapsin-1, CNS myelin-evoked gro wth cone collapse was accompanied by a reduction of rhodamine-phalloidin st aining most prominent in the growth cone periphery, suggesting actin filame nt disassembly. Specific mutants of small GTPases were capable of desensiti zing growth cones to CNS myelin or collapsin-1. Adenoviral-mediated express ion of constitutively active rad or rhoA abolished CNS myelin-induced colla pse and allowed remarkable neurite extension on a CNS myelin substrate. In contrast, expression of dominant negative rad or cdc42 negated collapsin-1- induced growth cone collapse and promoted neurite outgrowth on a collapsin- 1 substrate. These findings suggest that small GTPases can modulate the sig naling pathways of inhibitory stimuli and, consequently, allow the manipula tion of growth cone behavior. However, the fact that opposite mutants of ra d were effective against different inhibitory stimuli speaks against a univ ersal signaling pathway underlying growth cone collapse.