Myosin II contributes to the posterior contraction and the anterior extension during the retraction phase in migrating Dictyostelium cells

Cells must exert force against the substrate to migrate. We examined the vectors (both the direction and the magnitude) of the traction force generated by Dictyostelium cells using an improved non-wrinkling silicone substrate. During migration, the cells showed two 'alternate' phases of locomotory behavior, an extension phase and a retraction phase. In accordance with these phases, two alternate patterns were identified in the traction force. During the extension phase, the cell exerted a 'pulling force' toward the cell body in the anterior and the posterior regions and a 'pushing force' in the side of the cell (pattern 1). During the retraction phase, the cell exerted a 'pushing force' in the anterior region, although the force disappeared in the side and the posterior regions of the cell (pattern 2). Myosin II heavy chain null cells showed a single pattern in their traction force comparable to 'pattern 1', although they still had the alternate biphasic locomotory behavior similar to the wild-type cells. Therefore, the generation of 'pushing force' in the anterior and the cancellation of the traction force in the side and the posterior during the retraction phase were deficient in myosin knock-out mutant cells, suggesting that these activities depend on myosin II via the posterior contraction. Considering all these results, we hypothesized that there is a highly coordinated, biphasic mechanism of cell migration in Dictyostelium.