  Collective cell migration is key during development, wound healing and
metastasis and relies on coordinated cell behaviors at the group level. Src
kinase is a signalling enzyme regulating many cellular processes including
adhesion and motility and its deregulated activation has been associated to
aggressiveness of different cancers. Here, we take advantage of optogenetics to
precisely control Src activation in time to study the effect of its over
activation on collective rotation of confined monolayers. We show that Src
activation slows down collective rotation of epithelial cells confined into
circular adhesive patches. We interpret velocity, force and stress data during
period of non-activation and period of activation of Src thanks to an
hydrodynamic description of the cell assembly as a polar active fluid. Src
activation leads to a 2-fold decrease in the ratio of polar angle to friction,
which could result from increased adhesive bonds at the cell-substrate
interface. Our work reveals the importance of fine-tuning the level of Src
activity for coordinated collective behaviors.
