MICROTUBULES RELEASED FROM THE NEURONAL CENTROSOME ARE TRANSPORTED INTO THE AXON

There is controversy concerning the source of new microtubules require d for the development of neuronal axons. We have proposed that microtu bules are released from the centrosome within the cell body of the neu ron and are then translocated into the axon to support its growth. To investigate this possibility, we have developed an experimental regime that permits us to determine the fate of a small population of microt ubules nucleated at the neuronal centrosome. Microtubules within cultu red sympathetic neurons were depolymerized with the anti-microtubule d rug nocodazole, after which the drug was removed. Microtubules rapidly and specifically reassembled from the centrosome within three minutes of nocodazole removal. At this point, low levels of vinblastine, anot her anti-microtubule drug, were added to the culture to inhibit furthe r microtubule assembly while not substantially depolymerizing the smal l population of microtubules that had already assembled at the centros ome. Within minutes, released microtubules were apparent in the cytopl asm, and many of these had already translocated to the cell periphery by ten minutes, By one hour, virtually all of the microtubules had bee n released from the centrosome and were concentrated at the cell perip hery. With increasing time, these microtubules appeared within and pro gressively farther down developing axons, Nonneuronal cells within the culture also reassembled microtubules at the centrosome, but only a s mall portion of these microtubules were released. These observations i ndicate that microtubules are released from the neuronal centrosome an d transported into growing axons, and that microtubule release and rel ocation from the centrosome are especially active in neurons compared to nonneuronal cells.