Lamellipodial Actin Mechanically Links Myosin Activity with Adhesion-Site Formation

Abstract

Cell motility proceeds by cycles of edge protrusion, adhesion, and retraction. Whether these functions are coordinated by biochemical or biomechanical processes is unknown. We find that myosin II pulls the rear of the lamellipodial actin network, causing upward bending, edge retraction, and initiation of new adhesion sites. The network then separates from the edge and condenses over the myosin. Protrusion resumes as lamellipodial actin regenerates from the front and extends rearward until it reaches newly assembled myosin, initiating the next cycle. Upward bending, observed by evanescence and electron microscopy, results in ruffle formation when adhesion strength is low. Correlative fluorescence and electron microscopy shows that the regenerating lamellipodium forms a cohesive, separable layer of actin above the lamellum. Thus, actin polymerization periodically builds a mechanical link, the lamellipodium, connecting myosin motors with the initiation of adhesion sites, suggesting that the major functions driving motility are coordinated by a biomechanical process.

Podosome-type adhesions and focal adhesions, so alike yet so different

“…Extension of the leading edge is characterized Ponti , 2004 et al by a cyclic process: the protrusion of lamellipodium associated with a dense network of branching actin filaments is followed by the formation of focal adhesions at the rear of the lamellipodium with the assembly of stress fibers in the lamellum area (Giannone , 2007 et al ). Actin network undergoes a fast retrograde flow in the lamellipodium allowing actin polymerization and depolymerisation whereas a slower centripetal flow is observed in the actomyosin contraction associated lamellum.…”

Cell biology of embryonic migration

“…Focal adhesion formation and maturation is coupled to the assembly of prominent actin stress fibers and is believed to be mediated by actin interaction with the molecular motor myosin II (Galbraith et al, 2002;Geiger and Bershadsky, 2001;Giannone et al, 2007;Rottner et al, 1999;Schoenwaelder and Burridge, 1999). Myosin and its regulatory machinery play a major role in creating and maintaining tractional forces that enable the cell to attach and propel itself over the substrate (Even-Ram et al, 2007;Iwasaki and Wang, 2008).…”

Loss of Cell Adhesion Causes Hydrocephalus in Nonmuscle Myosin II-B–ablated and Mutated Mice

“…The defect may be accompanied by an obstructed aqueduct of Sylvius, the narrow channel connecting the third and fourth brain ventricles (noncommunicating hydrocephalus), or by a normal aqueduct (communicating hydrocephalus). The pathogenesis of most cases of communicating hydrocephalus is largely unknown (for reviews, see Perez-Figares et al, 2001;Crews et al, 2004).Nonmuscle myosin (NM) II, one of the major cytoskeletal motor proteins, plays an important role in cell migration (Svitkina et al, 1997;Ma et al, 2004;Even-Ram et al, 2007;Vicente-Manzanares et al, 2007), cell-cell adhesion Shewan et al, 2005;Giannone et al, 2007), and cell division (De Lozanne and Spudich, 1987;Takeda et al, 2003;Bao et al, 2005). The molecular structure of NM II is a hexamer consisting of a pair of myosin heavy chains (200 kDa) and two pairs of light chains (20 and 17 kDa).…”