THE ORGANIZATION OF F-ACTIN AND MICROTUBULES IN GROWTH CONES EXPOSED TO A BRAIN-DERIVED COLLAPSING FACTOR

In previous work we characterized a brain derived collapsing factor th at induces the collapse of dorsal root ganglion growth cones in cultur e (Raper and Kapfhammer, 1990). To determine how the growth cone cytos keleton is rearranged during collapse, we have compared the distributi ons of F-actin and microtubules in normal and partially collapsed grow th cones. The relative concentration of F-actin as compared to all pro teins can be measured in growth cones by ratioing the intensity of rho damine-phalloidin staining of F-actin to the intensity of a general pr otein stain. The relative concentration of F-actin is decreased by abo ut one half in growth cones exposed to collapsing factor for five minu tes, a time at which they are just beginning to collapse. During this period the relative concentration of F-actin in the leading edges of g rowth cones decreases dramatically while the concentration of F-actin in the centers decreases little. These results suggest that collapse i s associated with a net loss of F-actin at the leading edge. The distr ibutions of microtubules in normal and collapsing factor treated growt h cones were examined with antibodies to tyrosinated and detyrosinated isoforms of alpha-tubulin. The tyrosinated form is found in newly pol ymerized microtubules while the detyrosinated form is not. The relativ e proximal-distal distributions of these isoforms are not altered duri ng collapse, suggesting that rates of microtubule polymerization and d epolymerization are not greatly affected by the presence of collapsing factor. An analysis of the distributions of microtubules before and a fter collapse suggests that microtubules are rearranged, but their pol ymerization state is unaffected during collapse. These results are con sistent with the hypothesis that the brain derived collapsing factor h as little effect on microtubule polymerization or depolymerization. In stead it appears to induce a net loss of F-actin at the leading edge o f the growth cone.