TUBULIN TRANSPORT IN NEURONS

A question of broad importance in cellular neurobiology has been, how is microtubule cytoskeleton of the axon organized? It is of particular interest because of the history of conflicting-results concerning the form in which tubulin is transported in the axon. While many studies indicate a stationary nature of axonal microtubules, a recent series o f experiments reports that microtubules are recruited into axons of ne urons grown in the presence of a microtubule-inhibitor, vinblastine (B aas, P.W., and F.J. Ahmad. 1993. J. Cell Biol. 120:1427-1437; Ahmad F. J., and P.W. Baas. 1995. J. Cell Sci. 108:2761-2769; Sharp, D.J., W. Y u, and P.W. Baas. 1995. J. Cell Biol. 130:93-103; Yu, W., and P.W. Baa s, 1995. J. Neurosci. 15:6827-6833.). Since vinblastine stabilizes bul k microtubule-dynamics in vitro, it was concluded that preformed micro tubules moved into newly grown axons. By visualizing the polymerizatio n of injected fluorescent tubulin, we show that substantial microtubul e polymerization occurs in neurons grown at reported vinblastine conce ntrations. Vinblastine inhibits, in a concentration-dependent manner, both neurite outgrowth and microtubule assembly. More importantly, the neuron growth conditions of low vinblastine concentration allowed us to visualize the footprints of the tubulin wave as it polymerized and depolymerized during its slow axonal transport. In contrast, depolymer ization resistant fluorescent microtubules did not move when injected in neurons, We show that tubulin subunits, not microtubules, are the p rimary form of tubulin transport in neurons.