Forces Controlling Motion of Topological Defects in Epithelial Cell Layers*

In confluent cell layers, there is a tendency for each cell to align its long axis with that of its neighbors. At flaws in the alignment, called topological defects, the local cell velocities are perturbed, which in turn affects, for example, morphogenesis and cancer cell invasion. Details on how forces are altered near topological defects remain unclear. Current understanding comes from theoretical predictions that motion of +1/2 topological defects can be used as an indicator of whether the active stresses in the cell layer are extensile (pushing) or contractile (pulling). Recent studies have observed topological defect motion consistent with extensile behavior, which is surprising considering that the stresses produced by each cell are contractile. Although multiple explanations for apparent extensile behavior have been proposed, detailed experimental investigation remains lacking. Here, we studied motion of cells and topological defects in monolayers of MDCK cells with and without perturbations of actomyosin contraction. In our experiments, we measured the cell-substrate tractions using traction force microscopy and the cell-cell stresses using monolayer stress microscopy. Our data show that the topological defects moved against gradients in intercellular stress but with the direction of cell-substrate traction. These findings indicate that traction controls defect motion, implying that defect motion is not a useful indicator of the stress state and that traction is a major source of activity in adherent epithelial cell layers.