Integrins and Actin Filaments: Reciprocal Regulation of Cell Adhesion and Signaling

Abstract

extracellular matrix focal adhesions ezrin, radixin, and moesin vasodilator-stimulated phosphoprotein Wiskott-Aldrich syndrome protein integrin-linked kinase Integrin adhesion receptors link the extracellular matrix (ECM)1to the actin cytoskeleton and transmit biochemical signals and mechanical force across the plasma membrane. This enables cells to generate traction during migration and exert tension during matrix remodeling (1Choquet D. Felsenfeld D.P. Sheetz M.P. Cell. 1997; 88: 39-48Abstract Full Text Full Text PDF PubMed Scopus (1090) Google Scholar). Cytoskeletal linkages also enable integrins to mediate cell adhesion and regulate cell shape and gene expression (1Choquet D. Felsenfeld D.P. Sheetz M.P. Cell. 1997; 88: 39-48Abstract Full Text Full Text PDF PubMed Scopus (1090) Google Scholar). Here we will summarize the evidence for direct interactions between integrin cytoplasmic tails and specific actin-binding proteins and discuss how these interactions influence cell adhesion, cell spreading, and migration. Integrin α and β subunits are type I transmembrane proteins expressed in surface membranes as heterodimers. Each consists of a large extracellular domain, a single transmembrane segment, and a relatively short cytoplasmic tail. The latter contains anywhere from 20 to 70 amino acid residues, with the notable exception of the much larger β4 tail, which is linked primarily to intermediate filaments instead of actin filaments (2Burridge K. Chrzanowska-Wodnicka M. Annu. Rev. Cell Dev. Biol. 1996; 12: 463-518Crossref PubMed Scopus (1674) Google Scholar). β-Cytoplasmic tails are necessary and sufficient to link integrins to the actin cytoskeleton (2Burridge K. Chrzanowska-Wodnicka M. Annu. Rev. Cell Dev. Biol. 1996; 12: 463-518Crossref PubMed Scopus (1674) Google Scholar). In contrast, there is less evidence to date that α tails are directly linked to the cytoskeleton; indeed the removal of the α1, α4, or αIIb cytoplasmic tail appears to increase β tail-mediated interactions with the cytoskeleton (2Burridge K. Chrzanowska-Wodnicka M. Annu. Rev. Cell Dev. Biol. 1996; 12: 463-518Crossref PubMed Scopus (1674) Google Scholar). Direct binding of the signaling adapter protein paxillin to α4 cytoplasmic tails has recently been demonstrated, and this binding regulates α4β1-mediated cell spreading, migration, and stress fiber formation (3Liu S. Thomas S.M. Woodside D.G. Rose D.M. Kiosses W.B. Pfaff M. Ginsberg M.H. Nature. 1999; 402: 676-681Crossref PubMed Scopus (294) Google Scholar). There is direct biochemical support for the interaction of α and β tails with each other (4Muir T.W. Williams M.J. Ginsberg M.H. Kent S.B.H. Biochemistry. 1994; 33: 7701-7708Crossref PubMed Scopus (84) Google Scholar, 5Haas T.A. Plow E.F. J. Biol. Chem. 1996; 271: 6017-6026Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar) and for the modulation of this interaction by the binding of ligands to the extracellular domain (6Leisner T.M. Wencel-Drake J.D. Wang W. Lam S.C. J. Biol. Chem. 1999; 274: 12945-12949Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). Consequently, regulated changes in the interactions between the α and β tails may affect integrin-cytoskeleton linkages. In addition to mediating integrin linkages with the actin cytoskeleton, β cytoplasmic tails are important for adhesion, spreading, and migration of cells on ECM, processes dependent on an intact actin cytoskeleton. Integrins typically cluster within “matrix adhesions,” sites of close apposition of the cell membrane to the ECM. Matrix adhesions are extremely dynamic and heterogeneous structures with respect to size, composition, and orientation to actin filaments (7Zamir E. Katz B.Z. Aota S. Yamada K.M. Geiger B. Kam Z. J. Cell Sci. 1999; 112: 1655-1669Crossref PubMed Google Scholar, 9Hall A. Science. 1998; 23: 509-514Crossref Scopus (5278) Google Scholar). The largest ones are usually referred to as focal adhesions (FA), which are aligned at the ends of actin stress fibers (2Burridge K. Chrzanowska-Wodnicka M. Annu. Rev. Cell Dev. Biol. 1996; 12: 463-518Crossref PubMed Scopus (1674) Google Scholar, 8Jockusch B.M. Bubeck P. Giehl K. Kroemker M. Moschner J. Rothkegel M. Rudiger M. Schluter K. Stanke G. Winkler J. Annu. Rev. Cell Dev. Biol. 1995; 11: 379-416Crossref PubMed Scopus (432) Google Scholar, 9Hall A. Science. 1998; 23: 509-514Crossref Scopus (5278) Google Scholar). As such, FA represent a morphologically prominent association between integrins and the cytoskeleton, and investigation of integrin targeting to FA has shed light on the mechanisms of integrin-cytoskeleton association. Mutational analysis of the 47-amino acid β1 cytoplasmic tail has identified three clusters of amino acids important for integrin localization to FA, a membrane-proximal region and two conserved NPXY (single-letter amino acid code) motifs (10Reszka A.A. Hayashi Y. Horwitz A.F. J. Cell Biol. 1992; 117: 1321-1330Crossref PubMed Scopus (242) Google Scholar). Similar motifs in the β3 cytoplasmic tail are important for localization of β3 integrins to FA (11Ylanne J. Huuskonen J. O'Toole T.E. Ginsberg M.H. Virtanen I. Gahmberg C.G. J. Biol. Chem. 1995; 270: 9550-9557Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar). An additional Thr-containing motif between the two NPXY sites has also been implicated in β2 integrin-cytoskeleton linkages (12Peter K. O'Toole T.E. J. Exp. Med. 1995; 181: 315-326Crossref PubMed Scopus (145) Google Scholar). Integrins are linked to actin filaments by specific actin-binding proteins, and there is now an emerging consensus concerning which proteins are involved (Fig. 1). Talin is composed of two ∼270-kDa subunits arranged as an anti-parallel homodimer (8Jockusch B.M. Bubeck P. Giehl K. Kroemker M. Moschner J. Rothkegel M. Rudiger M. Schluter K. Stanke G. Winkler J. Annu. Rev. Cell Dev. Biol. 1995; 11: 379-416Crossref PubMed Scopus (432) Google Scholar), and it co-localizes with integrins at certain sites of cell-substratum contact. Talin is a major structural component of FA along with actin and vinculin. It consists of an N-terminal ∼50-kDa globular head domain, which includes an ∼200-amino acid region with homology to the ezrin, radixin, and moesin (ERM) family of proteins, and an ∼220-kDa, C-terminal rod domain containing a conserved ILWEQ actin-binding domain (8Jockusch B.M. Bubeck P. Giehl K. Kroemker M. Moschner J. Rothkegel M. Rudiger M. Schluter K. Stanke G. Winkler J. Annu. Rev. Cell Dev. Biol. 1995; 11: 379-416Crossref PubMed Scopus (432) Google Scholar, 13Schultz J. Milpetz F. Bork P. Ponting C.P. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 5857-5864Crossref PubMed Scopus (3058) Google Scholar). Talin contains binding sites for actin, vinculin, focal adhesion kinase, phospholipids, and the transmembrane protein laylin (8Jockusch B.M. Bubeck P. Giehl K. Kroemker M. Moschner J. Rothkegel M. Rudiger M. Schluter K. Stanke G. Winkler J. Annu. Rev. Cell Dev. Biol. 1995; 11: 379-416Crossref PubMed Scopus (432) Google Scholar, 14Borowsky M.L. Hynes R.O. J. Cell Biol. 1998; 143: 429-442Crossref PubMed Scopus (116) Google Scholar). Talin was the first actin-binding protein shown to directly bind integrins and was proposed to mediate the link to the actin cytoskeleton (2Burridge K. Chrzanowska-Wodnicka M. Annu. Rev. Cell Dev. Biol. 1996; 12: 463-518Crossref PubMed Scopus (1674) Google Scholar). Talin binds to β1, β2, and β3 and more weakly to β7 integrin cytoplasmic tails (15Pfaff M. Liu S. Erle D.J. Ginsberg M.H. J. Biol. Chem. 1998; 273: 6104-6109Abstract Full Text Full Text PDF PubMed Scopus (243) Google Scholar, 16Sampath R. Gallagher P.J. Pavalko F.M. J. Biol. Chem. 1998; 273: 33588-33594Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar, 17Calderwood D.A. Zent R. Grant R. Rees D.J.G. Hynes R.O. Ginsberg M.H. J. Biol. Chem. 1999; 274: 28071-28074Abstract Full Text Full Text PDF PubMed Scopus (574) Google Scholar, 18Patil S. Jedsadayanmata A. Wencel-Drake J.J. Wang W. Knezevic I. Lam S.C.T. J. Biol. Chem. 1999; 274: 28575-28583Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). Talin accumulation is an early step in FA formation and requires integrins but not vinculin (19Moulder G.L. Huang M.M. Waterston R.H. Barstead R.J. Mol. Biol. Cell. 1996; 7: 1181-1193Crossref PubMed Scopus (70) Google Scholar). Microinjection of antibodies to talin or talin antisense RNA disrupts stress fibers and inhibits adhesion, spreading, and migration of fibroblasts and HeLa cells (8Jockusch B.M. Bubeck P. Giehl K. Kroemker M. Moschner J. Rothkegel M. Rudiger M. Schluter K. Stanke G. Winkler J. Annu. Rev. Cell Dev. Biol. 1995; 11: 379-416Crossref PubMed Scopus (432) Google Scholar, 20Albiges-Rizo C. Frachet P. Block M.R. J. Cell Sci. 1995; 108: 3317-3329Crossref PubMed Google Scholar). The significance of talin for integrin function has been underscored by studies of talin-null ES cells, which exhibit extensive membrane blebbing, defects in cell adhesion and spreading, and a failure to assemble FA or stress fibers (21Priddle H. Hemmings L. Monkley S. Woods A. Patel B. Sutton D. Dunn G.A. Zicha D. Critchley D.R. J. Cell Biol. 1998; 142: 1121-1133Crossref PubMed Scopus (153) Google Scholar). These results suggest that talin is required for the integrin-cytoskeleton associations needed for FA and stress fiber formation. However, undifferentiated talin-null ES cells also express reduced levels of β1 integrin, vinculin, and α-actinin, which may contribute to the phenotype. Following differentiation of talin-null ES cells, only two morphologically distinct cell types emerged, and no organized tissues were formed (21Priddle H. Hemmings L. Monkley S. Woods A. Patel B. Sutton D. Dunn G.A. Zicha D. Critchley D.R. J. Cell Biol. 1998; 142: 1121-1133Crossref PubMed Scopus (153) Google Scholar). The differentiated cells expressed normal levels of β1integrin and vinculin and were capable of spreading and form

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• Scripps Research Institute US

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