Filopodial retraction force is generated by cortical actin dynamics and controlled by reversible tethering at the tip

Technical University of Denmark, Kongens Lyngby, 2450, Denmark Filopodia are dynamic, finger-like plasmamembrane protrusions that sense themechanical and chemical surroundings of the cell. Here,we show in epithelial cells that the dynamics of filopodial extension and retraction are determined by the difference between the actin polymerization rate at the tip and the retrograde flowat the base of the filopodium. Adhesion of a bead to the filopodial tip locally reduces actin polymerization and leads to retraction via retrograde flow, reminiscent of a process used by pathogens to invade cells. Using optical tweezers, we show that filopodial retraction occurs at a constant speed against counteracting forces up to 50 pN. Our measurements point toward retrograde flow in the cortex together with frictional coupling between the filopodial and cortical actin networks as the main retraction-force generator for filopodia. The force exerted by filopodial retraction, however, is limited by the connection between filopodial actin filaments and themembrane at the tip. Upon mechanical rupture of the tip connection, filopodia exert a passive retraction force of 15 pN via their plasmamembrane. Transient reconnection at the tip allows filopodia to continuously probe their surroundings in a load-and-fail manner within a welldefined force range.