Relationship between microtubule dynamics and lamellipodium formation revealed by direct imaging of microtubules in cells treated with nocodazole or taxol

Microtubules (MTs) contribute to the directional locomotion of many cell ty pes through an unknown mechanism. Previously, we showed that low concentrat ions (<200 nM) of nocodazole or taxol reduced the rate of locomotion of NRK fibroblasts over 60% without altering MT polymer level [Liao et al., 1995: J. Cell Sci. 108:3473-3483]. In this paper we directly measured the dynami cs of MTs in migrating NRK cells injected with rhodamine tubulin and treate d with low concentrations of nocodazole or taxol. Both drug treatments caus ed statistically significant reductions (approx. twofold) in growth and sho rtening rates and less dramatic effects on rescue and catastrophe transitio n frequencies. The percent time MTs were inactive (i.e., paused) increased greater than twofold in nocodazole- and taxol-treated cells, while the perc ent time growing was substantially reduced. Three parameters of MT dynamics were linearly related to the rates of locomotion determined previously: ra te of shortening, percent time pausing and percent time growing. The number of MTs that came within 1 mu m of the leading edge was reduced in drug-tre ated cells, suggesting that reduced MT dynamics may affect actin arrays nec essary for cell locomotion. We examined two such structures, lamellipodium and adhesion plaques, and found that lamelljpodia area was coordinately red uced with MT dynamics. No effect was detected on adhesion plaque density or distribution. In time-lapse recordings, MTs did not penetrate into the lam ellipodium of untreated cells, suggesting that MTs affect lamellipodia eith er through their interaction with factors at the base of the lamellipodium or by releasing factors that diffuse into the lamellipodia. In support of t he latter hypothesis, when all MTs were rapidly depolymerized by 20 mu M no codazole, we detected the rapid formation of exaggerated protrusions from t he leading edge of the cell. Our results show for the first time a linear r elationship between MT dynamics and the formation of the lamellipodium and support the idea that MT dynamics may contribute to cell locomotion by regu lating the size of the lamellipodium, perhaps through diffusable factors. ( C) 1998 Wiley-Liss, Inc.