Cell Motility and the Cytoskeleton 8:193-209 (1987) “Pull” and “Push” in Neurite Elongation: Observations on the Effects of Different Concentrations of Cytochalasin B and Tax01 Paul C. Letourneau, Terri A. Shattuck, and Alice H. Ressler Department of Cell Biology and Neuroanatomy, University of Minnesota, Minneapolis 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 compo- nents, microtubules and actin filaments, respectively. When neurons are treated with concentrations of CB below 0.2 pg/ml, neurite elongation, growth cone protrusion, and neurite tension are all inhibited in a similar concentration depen- dent manner. Protrusive activity and tensions are absent at CB concentrations above 0.3 pg/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. Key words: growth cone, microtubules, actin IN TROD U CT I0 N Neurons are distinguished from other cells by their extremely elongated and branched axons and dendrites. Cytoskeletal components have a major role in the for- mation of dendrites and axons, and neuron-specific vari- ations exist in actin filaments, microtubules, and intermediate filaments [Bernhardt and Matus, 1984; Binder et al, 1985; Ciment et al, 1986; Drake and Lasek, 1984; Izant and McIntosh, 1980; Jacobs et al, 1982; Letourneau, 1983; Ludueiia and Wessells, 1973; Mar- otta, 1983; Pachter et al, 1984; Tapscott et al, 19811. The significance of these characteristics to neuronal morpho- genesis is largely unknown. Bunge et a1 [1983] asked whether neurites elongate primarily because of “pull” exerted by the growth cone or because of “push” provided by anterograde transport and assembly of materials at the neurite tip. ‘‘Pull” is generated by tensions exerted on the adherent contacts of growth cones and their filipodial and lamellipodial pro- trusions. The need for a substratum against which growth cones pull was first demonstrated by Harrison [ 19141 and was further explored by Letourneau [1975] and Bray [1982]. Bray [1984] attached neurite tips to a needle, pulled the needle, and found that the extrinsic tension induced neurite elongation up to the maximal rates of Received November 3, 1986; accepted June 4, 1987. Address reprint requests to Dr. Paul C. Letourneau, Department of Cell Biology and Neuroanatomy, 4-135 Jackson Hall, University of Minnesota, Minneapolis, MN 55455. 0 1987 Alan R. Liss, Inc.