Actin-dependent lamellipodia formation and microtubule-dependent tail retraction control-directed cell migration

Migrating cells are polarized with a protrusive lamella at the cell front f ollowed by the main cell body and a retractable tail at the rear of the cel l. The lamella terminates in ruffling lamellipodia that face the direction of migration. Although the role of actin in the formation of lamellipodia i s well established, it remains unclear to what degree microtubules contribu te to this process. Herein, we have studied the contribution of microtubule s to cell motility by time-lapse video microscopy on green flourescence pro tein-actin- and tubulin-green fluorescence protein-transfected melanoma cel ls. Treatment of cells with either the microtubule-disrupting agent nocodaz ole or with the stabilizing agent taxol showed decreased ruffling and lamel lipodium formation. However, this was not due to an intrinsic inability to form ruffles and lamellipodia because both were restored by stimulation of cells with phorbol 12-myristate 13-acetate in a Rac-dependent manner, and b y stem cell factor in melanoblasts expressing the receptor tyrosine kinase c-kit. Although ruffling and lamellipodia were formed without microtubules, the microtubular network was needed fur advancement of the cell body and t he subsequent retraction of the tail. In conclusion, we demonstrate that th e formation of lamellipodia can occur via actin polymerization independentl y of microtubules, but that microtubules are required for cell migration, t ail retraction, and modulation of cell adhesion.