Effect of NGF on in vitro preformed microtubules. Evidence for a protective action against vinblastine.

The nerve growth factor protein (NGF) induces organization of in vitro preformed microtubules (MTs) to form multiple arrays. These complexes, in cross-section, appear to be formed by clusters of single tubules in parallel in close contact with center distance similar to the diameter of a microtubule. In the absence of NGF, on the contrary, microtubules are far apart from each other. Addition of vinblastine to NGF-MT complexes does not result in the usual reorganization of microtubules into paracrystalline arrays, noticeable when the growth factor is not present. The vinblastine treated NGF-MT complexes have several round-shaped elements along the tubules and at their endings which, at higher magnification, appear to be formed by closely packed, ring-like structures. These 'boutons' are postulated to represent the point of resistance to the depolymerizing action of the vinca alkaloid. These findings are discussed in relation to a similar antagonistic effect observed in vivo between NGF and vinblastine and to the possible involvement of microtubules in the mechanism of action of the growth factor.