Control of Actin Dynamics in Cell Motility

Abstract

actin-depolymerizing factor A large number of cellular processes, including cytokinesis, endocytosis, chemotaxis, or neurite outgrowth, is mediated by polymerization of actin filaments. In response to extracellular stimuli, motile protrusions of the plasma membrane, in the form of lamellipodia or filopodia, are driven by the continuous initiation, polarized growth, and turnover of actin filaments at the leading edge of the cell. Recent progress has been made in identifying the key players responsible for the spatio-temporal control of actin-based motility and in understanding the molecular mechanism supporting their function. The linkage of the actin cytoskeleton to the signaling pathway is generally mediated by the interaction of a small G-protein (Rac, Cdc42) in its active, GTP-bound form, with a multipartner “connector” at the plasma membrane. The activation of the connector allows it to recruit the Arp2/3 complex, which initiates branched barbed end growth of actin filaments (see the first minireview in this series (60Higgs H.N. Pollard T.D. J. Biol. Chem. 1999; 274: 32531-32534Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar)) forming the dense reticulated actin network seen in high resolution electron microscopy images of lamellipodia (1Svitkina T.M. Verkhovsky A.B. McQuade K.M. Borisy G.G. J. Cell Biol. 1997; 139: 397-415Crossref PubMed Scopus (561) Google Scholar, 2Bailly M. Macaluso F. Cammer M. Chang A. Segall J.E. Condeelis J.S. J. Cell Biol. 1999; 145: 331-345Crossref PubMed Scopus (176) Google Scholar, 3Svitkina T.M. Borisy G.G. J. Cell Biol. 1999; 145: 1009-1026Crossref PubMed Scopus (906) Google Scholar). Pathogens like Listeria or Shigella or the vaccinia virus, which propel themselves through the cytoplasm by polymerizing actin at their surface (4Higley S. Way M. Curr. Opin. Cell Biol. 1997; 9: 62-69Crossref PubMed Scopus (44) Google Scholar), harness the cytoskeletal machinery downstream of the signaling pathway in a constitutive fashion and provide a biochemical approach of the mechanism of actin-based motility. Once actin polymerization is initiated, continuous filament growth causes membrane protrusion (or bacterium propulsion) at a rate of 1–25 μm/min. Growing filaments remain stationary with respect to the substratum (5Theriot J.A. Mitchison T.J. Nature. 1991; 352: 126-131Crossref PubMed Scopus (650) Google Scholar), demonstrating that actin polymerization is linked to the movement. Barbed end growth of filaments initiated at the leading edge (or at the surface of Listeria or Shigella) is fed by subunits provided by continuous depolymerization of filaments from their pointed ends at the rear of the lamellipodial extension, which is thus maintained at a constant width (6Small J.V. Rottner K. Kaverina I. Anderson K.I. Biochim. Biophys. Acta. 1998; 1404: 271-281Crossref PubMed Scopus (211) Google Scholar). Conversely, the actin tail formed at the rear of Listeria or Shigellaremains at a constant length in a stationary regime of propulsion (7Theriot J.A. Mitchison T.J. Tilney L.G. Portnoy D.A. Nature. 1992; 357: 257-260Crossref PubMed Scopus (412) Google Scholar). The steady polymerization reflects actual turnover of actin filaments according to a treadmilling process (8Wang Y.L. J. Cell Biol. 1985; 101: 597-602Crossref PubMed Scopus (453) Google Scholar). One of the fascinating aspects of actin-based motility is the rapid rate of filament turnover, which supports the movement. Typically, to push the membrane forward at 10 μm/min, individual barbed ends must readily incorporate 100 subunits/s. This rate is 200-fold higher than the treadmilling rate measured in vitro for pure F-actin turnover at steady state. From a thermodynamic point of view, both the activated nucleation and the rapid barbed end growth of filaments imply that upon cell stimulation, the concentration of monomeric ATP-actin is increased above its level in quiescent cells, so as to promote nucleation, and remains high, well above the critical concentration for barbed end assembly during movement, so as to sustain rapid steady filament growth. Recent works have shown that actin-binding proteins of the actin-depolymerizing factor (ADF)1/cofilin family are the regulatory factors that elicit the rapid turnover-barbed end growth of actin filaments driving the forward movement of the leading edge. ADF/cofilins have been recognized early as a family of essential, conserved, widespread, small (15–18 kDa) actin-binding proteins (see Ref. 9Moon A. Drubin D.G. Mol. Biol. Cell. 1995; 6: 1423-1431Crossref PubMed Scopus (226) Google Scholar for review) playing an important role in cytokinesis (10Abe H. Obinata T. Minamide L.S. Bamburg J.R. J. Cell Biol. 1996; 132: 871-885Crossref PubMed Scopus (165) Google Scholar), endocytosis, and in the development of all embryonic tissues (11Bamburg J.R. Bray D. J. Cell Biol. 1987; 105: 2817-2825Crossref PubMed Scopus (158) Google Scholar), as well as in pathological situations su