Optogenetic generation of leader cells reveals a force-velocity relation for collective cell migration

The front of migratory cellular clusters during development, wound healing and cancer invasion is typically populated with highly protrusive cells that are called leader cells. Leader cells are thought to physically pull and direct their cohort of followers, but how leaders and followers are mechanically organized to migrate collectively remains controversial. One possibility is that the autonomous local action of a leader cell is sufficient to drive migration of the group. Yet another possibility is that a global mechanical organization is required for the group to move cohesively. Here we show that the effectiveness of leader-follower organization is proportional to the asymmetry of traction and tension within the cellular cluster. By combining hydrogel micropatterning and optogenetic activation of Rac1, we locally generate highly protrusive leaders at the edge of minimal cell groups. We find that the induced leader can robustly drag one follower but is generally unable to direct larger groups. By measuring traction forces and tension propagation in groups of increasing size, we establish a quantitative relationship between group velocity and the asymmetry of the traction and tension profiles. We propose a model of the motile cluster as an active polar fluid that explains this force-velocity relationship in terms of asymmetries in the distribution of active tractions. Our results challenge the notion of autonomous leader cells by showing that collective cell migration requires a global mechanical organization within the cluster.