Different Contributions of Microtubule Dynamics and Transport to the Growth of Axons and Collateral Sprouts

Abstract

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 nmvinblastine 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.

A Novel Forward Genetic Screen for Identifying Mutations Affecting Larval Neuronal Dendrite Development in Drosophila melanogaster

“…As embryonic dendrite outgrowth proceeds, ''floating microtubules'' can be observed in these F-actin-rich growth-cone-like structures ( Figure 2C). Interestingly, ''floating microtubules'' were first identified in growing axon collateral branches and believed to be transported microtubule polymer (Gallo and Letourneau 1999). We reasoned that this genetic screen could also potentially identify mutations affecting the coupling of actin/ (A-D).…”

Developmental regulation of axon branching in the vertebrate nervous system

“…Both microtubule transport and microtubule assembly play important roles in branching (Dent et al, 1999;Gallo and Letourneau, 1999;Yu et al, 1994;Black, 1994;Kornack and Giger, 2005). Microtubule loops are present in regions prior to branch formation (Dent et al, 2004;Dent et al, 1999) and interact with actin at putative branching sites ).…”

Axon Branching Requires Interactions between Dynamic Microtubules and Actin Filaments

“…Furthermore, our data suggest that the ability of microtubules to bind certain proteins may depend on the degree of tubulin tyrosination. Later, once axon branches are formed, their further growth is dominated by transport of microtubules into the growing branch (Dent et al, 1999;Gallo and Letourneau, 1999).…”

“…Changes in the direction of axon outgrowth depend on reorganization of the microtubule and actin cytoskeleton (Lin and Forscher, 1993;Challacombe et al, 1996Challacombe et al, , 1997Williamson et al, 1996;Suter et al, 1998;Gallo and Letourneau, 1999). Microtubules in the central region of advancing growth cones are splayed apart but become bundled and form loops in slowly growing axons (Tsui et al, 1984;Sabry et al, 1991;Tanaka and Kirschner, 1991).…”