Cytoskeletal changes associated with cell motility.

This chapter reviews various aspects of changes in cytoskeletal organization which occur upon activating a highly motile cell phenotype. The first of these relates to the rapid formation of F-actin-rich ruffles on the apical cell surfaces following the addition of one of several motility factors. In a number of aspects these ruffles resemble leading edge lamellipodia. The ruffles form upon ligand binding and, at least for one cytokine, the receptors become associated with the ruffles. For several cytokines the ruffles are circular and in all cases they are associated with much increased pinocytosis. The possible significance is considered of these very early markers of a motile cell phenotype. A second topic covered is the role of microtubules (MTs) in maintaining cell polarity in some cell types but not others. The micro-injection of biotin-tubulin into cells has provided a valuable marker of MT turnover. Using this method it has been found that the microtubule network in secondary chick heart fibroblasts (2 degrees CHFs), where MTs are required to maintain a polarized motility, does not turn over significantly more slowly than in 1 degree CHFs which do not require an intact MT network for locomotion. In both cases the MT network turns over very quickly and no sub-population of longer-lived MTs has been found. In contrast, in motile epithelial (PtK2) cells, a sub-population of longer-lived microtubules has been identified and these appear to maintain the long cell processes.