title: Filopodial retraction force is generated by cortical actin dynamics and controlled by reversible tethering at the tip.
abstract: Filopodia are dynamic, finger-like plasma membrane protrusions that sense the mechanical 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 flow at 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 the membrane at the tip. Upon mechanical rupture of the tip connection, filopodia exert a passive retraction force of 15 pN via their plasma membrane. Transient reconnection at the tip allows filopodia to continuously probe their surroundings in a load-and-fail manner within a well-defined force range.
authors.affiliation: Institut Curie, Centre de Recherche, 75248 Paris, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Bornschlögl
authors.forename: Stéphane
authors.initials: S
authors.lastname: Romero
authors.forename: Christian L
authors.initials: CL
authors.lastname: Vestergaard
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Joanny
authors.forename: Guy Tran
authors.initials: GT
authors.lastname: Van Nhieu
authors.forename: Patricia
authors.initials: P
authors.lastname: Bassereau
mesh_terms: D000199:Actins / Q000378:metabolism*; D001696:Biomechanical Phenomena / Q000502:physiology; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D018613:Microscopy, Confocal; D008863:Microspheres; D052898:Optical Tweezers; D038761:Photobleaching; D058105:Polymerization; D011554:Pseudopodia / Q000502:physiology*
chemical_list: D000199:Actins; D049452:Green Fluorescent Proteins
keywords: cytoskeleton; lamellipodium; mechanics; membrane-tension; pulling