Non-monotonic relationships between cell adhesion and protrusions

Cells probe their environments by extending protrusions: this process is mediated by the polymerization of actin gels at the edge of cells. Although their molecular components have been widely studied, their mesoscopic properties remain to be characterized. In this paper, we show that cell adhesion modulates actin gel dynamics. By changing the grafting density of fibronectin on a surface, we changed the adhesion strength of a cell on this surface. We found that the length of filopodia, the speeds of their growth and the speeds of retrograde flows were non-monotonic functions of the grafting density of fibronectin. The minima of the length and speeds of filopodia and the maximum of the speeds of retrograde flows are found at the same fibronectin density; this implies that there are strong correlations between these parameters. We used a simple model to predict that retrograde flows show non-monotonic behaviors because integrin-fibronectin binding mediates actomyosin and friction forces applied to actin gels. This model also predicts that connectivity of actin gels is responsible for the strong correlations between retrograde flows and filopodial growth. Altogether, our study investigates how actomyosin forces and friction with the substrate influence actin gel dynamics in living cells.