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The Journal of Neuroscience, May 15, 1999, 19(10):3860-3873
Different Contributions of Microtubule Dynamics and Transport to
the Growth of Axons and Collateral Sprouts
Gianluca
Gallo and
Paul C.
Letourneau
University of Minnesota, Department of Cell Biology and
Neuroanatomy, Minneapolis, Minnesota 55455
Axonal growth is believed to depend on microtubule transport and
microtubule dynamic instability. We now report that the growth of axon
collateral branches can occur independent of microtubule dynamic
instability and can rely mostly on the transport of preassembled polymer. Raising embryonic sensory neurons in concentrations of either
taxol or nocodazole (NOC) that largely inhibit microtubule dynamics
significantly inhibited growth of main axonal shafts but had only minor
effects on collateral branch growth. The collaterals of axons raised in
taxol or nocodazole often contained single microtubules with both ends
clearly visible within the collateral branch ("floating"
microtubules), which we interpret as microtubules undergoing transport.
Furthermore, in these collaterals there was a distoproximal gradient in
microtubule mass, indicating the distal accumulation of transported
polymer. Treatment of cultures with a high dose of nocodazole to
deplete microtubules from collaterals, followed by treatment with 4-20
nM vinblastine to inhibit microtubule repolymerization,
resulted in the time-dependent reappearance and subsequent distal
accumulation of floating microtubules in collaterals, providing further
evidence for microtubule transport into collateral branches. Our data
show that, surprisingly, the contribution of microtubule dynamics to
collateral branch growth is minor compared with the important role of
microtubule dynamics in growth cone migration, and they indicate that
the transport of microtubules may provide sufficient cytoskeletal
material for the initial growth of collateral branches.
Key words:
microtubule; filopodium; axon; collateral; dynamic
instability; transport
Copyright © 1999 Society for Neuroscience 0270-6474/99/19103860-14$05.00/0
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