RAD MEDIATES COLLAPSIN-1-INDUCED GROWTH CONE COLLAPSE

Collapsin-1 or semaphorin III(D) inhibits axonal outgrowth by collapsi ng the lamellipodial and filopodial structures of the neuronal growth cones. Because growth cone collapse is asociated with actin depolymeri zation, we considered whether GTP-binding proteins of the rho subfamil y might particpate in collapsin-1 signal transduction. Recombinant rho , rac1, and cdc42 proteins were triturated into embryonic chick (DRG) neurons. Constitutively active rad increases the proportion of collaps ed growth cones, and dominant negative rad inhibits collapsin-1-induce d collapse of growth cones and collapsin-1 inhibition neurite outgrowt h. DRG neurons treated with dominant negative rac1 remain sensitive to myelin-induced growth cone collapse. Similar mutants of cdc42 do not alter growth cone structure, neurite elongation, or collapsin-1 sensit ivity. Whereas the addition of activated rho has no effect, the inhibi tion of rho with Clostridium botulinum C3 transferase stimulates the o utgrowth of DRG neurites. C3 transferase-treated growth cones exhibit little or no lamellipodial spreading and are minimally responsive to c ollapsin-1 and myelin. These data demonstrate a prominent role for rho and rac1 in modulating growth cone motility and indicate that rac1 ma y mediate collapsin-1 action.