“Pull” and “push” in neurite elongation: Observations on the effects of different concentrations of cytochalasin B and taxol

Neurite elongation involves two distinct cytoskeletal functions the "push" of anterograde transport of the cytoskeleton and associated organelles to the neurite tip, and the "pull" exerted by protrusion and generation of tensions in the growth cone. We investigated the roles of these two activities in neurite elongation via the drugs taxol and cytochalasin B (CB), which act on the key cytoskeletal components, microtubules and actin filaments, respectively. When neurons are treated with concentrations of CB below 0.2 micrograms/ml, neurite elongation, growth cone protrusion, and neurite tension are all inhibited in a similar concentration dependent manner. Protrusive activity and tensions are absent at CB concentrations above 0.3 micrograms/ml, yet neurite elongation continues at a plateau level. Thus, "pull" does modulate, but it is not required for neurite elongation. Surprisingly, the inhibitory effects of taxol on neurite elongation are removed by the addition of CB at levels that substantially disrupt the actin filaments of neurites. The neurites extended by taxol-CB neurons are unbranched and curiously unattached to the substratum. When CB is added to taxol-treated neurons, neurite extension begins rapidly, even if protein synthesis is severely reduced. We propose that taxol inhibits microtubule transport in neurites, and this inhibition of "push" is reversed by the disruptive effects of CB on the cytoplasmic matrix, allowing taxol-induced microtubule bundles to be transported distally.