Previous work had suggested that microtubules mimic F-actin movements in migrating cells, but technological limitations have made this idea difficult to test. On page 31, Salmon et al., studying migrating epithelial cells from newt lung, used new fluorescence speckle microscopy (FSM) imaging techniques to observe the movement of F-actin and microtubules nearly simultaneously in living cells. F-actin moves differently in four zones of the cell. Microtubules oriented parallel to the leading edge precisely mimic F-actin movements in all four zones, whereas microtubules perpendicular to the leading edge are often uncoupled from F-actin movements in two of the zones. F-actin movement appears to drive much of the dynamic organization of microtubules, and the two proteins seem to interact.
On page 139, Schaefer et al. used FSM to measure actin and tubulin dynamics in neuronal growth cones, and correlated these measurements with motility. As in migrating cells, F-actin structures in the growth cone interact strongly with microtubules. Polarized F-actin bundles in the filopodia act as polymerization guides for a population of highly dynamic microtubules that continuously explores the peripheral P-domain. The filopodial actin also provides retrograde transport for the microtubules. Meanwhile, a separate, less dynamic population of microtubules associates with actin filament arcs in the T-zone, possibly promoting axon elongation. Schaefer et al. propose that the steady-state movement of F-actin and microtubules in the filopodia allows the system to adapt quickly: a slight decrease in retrograde F-actin flow, for example, would drive rapid microtubule advance along the filopodia. ▪