Specific responses of axons and dendrites to cytoskeleton perturbations: an in vitro study

Several factors can influence the development of axons and dendrites in vitro. Some of these factors modify the adhesion of neurons to their substratum. We have previously shown that the threshold of neuron-substratum adhesion necessary for initiation and elongation of dendrites is higher than that required for axonal growth. To explain this difference we propose that, in order to antagonize actin-driven surface tension, axons primarily rely on the compression forces of microtubules whereas dendrites rely on adhesion. This model was tested by seeding the cells in conditions allowing the development either of axons or of axons and dendrites, then adding cytochalasin B or nocodazole 1 hour or 24 hours after plating. The addition of cytochalasin B, which depolymerizes actin filaments and thus reduces actin-tensile forces, increases the length of both axons and dendrites, indicating that both axons and dendrites have to antagonize surface tension in order to elongate. The addition of nocodazole, which acts primarily on microtubules, slightly reduces dendrite elongation and totally abolishes axonal growth. Similar results are obtained when the drugs are added 1 or 24 hours after plating, suggesting that the same mechanisms are at work both in initiation and in elongation. Finally, we find that in the presence of cytochalasin B axons adopt a curly morphology, a fact that could be explained by the importance of tensile forces in antagonizing the asymmetry created by polarized microtubules presenting a uniform minus/plus orientation.