Introduction

Integrins are heterodimeric transmembrane receptors that mediate cell-adhesion.¹ With their extracellular head region, most integrins bind extracellular matrix (ECM) glycoproteins such as laminins and collagens in basement membranes or connective tissue components like fibronectin. Others bind counterreceptors on neighboring cells, bacterial polysaccharides, or viral coat proteins. Through all these interactions integrins mediate stable adhesion to basement membranes, the formation of extracellular matrices and migration on such matrices, the formation of platelet aggregates, the establishment of intercellular junctions in the immune system, and bacterial and viral entry during infectious diseases. Furthermore, integrin-mediated adhesion modulates signaling cascades in control of cell motility, survival, proliferation, and differentiation. Here, a brief overview of basic structure/function aspects of integrins is given with the aim of providing background information for the following chapters in this book.

The History of Integrins

Intercellular cohesion is critical for the structural organization and intercellular communication within multicellular organisms. The origin of cell adhesion precedes multicellularity: genes encoding cadherin and C-type lectin domains were discovered in the genome of choanoflagellates, a group of metazoa-related protozoa.² The integrin family emerged in metazoa, which appeared during evolution over 600 million years ago. In the most ancient living animals - the sponges, no cadherins have been detected and sponge cell aggregation is mediated by an extracellular proteoglycan matrix. The receptor that binds to this matrix also contains the typical integrin-binding motif, RGD (see below). Indeed, α and β integrin subunits have been cloned from sponges and RGD peptides block sponge cell aggregation.³ Some early metazoan must have evolved a laminin-binding integrin and an RGD-binding integrin, both of which have been preserved throughout metazoan evolution.¹ In the nematode Caenorhabditis elegans, one β subunit (termed βpat3) and two a subunits (termed αina1 and αpat2) form two integrins (Table 1). In the fruit fly Drosophila melanogaster, five integrins are formed through combination of one β subunit (termed βPS) with five α subunits (termed αPS1 through 5). With vertebrate evolution, the integrin family has expanded considerably. Many different laminin and RGD-binding integrins evolved and a new group of integrins appeared that have an extra inserted I-domain (see below) in the extracellular region and that contain the receptors for fibrillar collagens and the leukocyte-specific β2 integrins. Ultimately, sequencing of the human genome has identified as many as 18 α and 8 β subunits, from which 24 different functional integrins are currently known to be formed in humans.^4,5

Table 1

Integrins and their ligands.

Structural Aspects of Integrins

Integrins are formed through noncovalent association of two type I transmembrane glycoproteins, the α- and the β subunit. The extracellular parts are approximately 700 amino acids for α - and 1000 amino acids for β subunits and form elongated stalks and a globular ligand-binding head region⁶ (Fig. 1). The part of the a subunit that forms the globular head region and participates in ligand binding consists of seven repeats of about 60 amino acids that fold into a seven-bladed β propeller structure similar to the trimeric G protein β subunit.⁷ About half of the integrin a subunits has an insertion termed the I-domain within the β-propeller, which directly participates in ligand binding and appears closely related to the trimeric G protein α subunit and small G proteins.⁸ All integrin β subunits contain an “I-like domain” in the globular head region that also participates in ligand binding, perhaps most importantly in integrins lacking α I-domains. I domains contain a “metal ion-dependent adhesion site” (MIDAS) and I-like domains contain a structurally similar metal-binding motif. In analogy to small G proteins, where GTP hydrolysis leads to altered coordination of a Mg²⁺ ion causing conformational changes and loss of effector protein interaction, integrin ligand binding alters the coordination of the metal ion and shifts the I-domains from a closed to an open conformation. This process is thought to increase the affinity for ligand and to cause adhesion strengthening. ⁸

Figure 1

Structural aspects of integrins. A) Schematic drawing of integrin α and β subunits indicating basic structural units discussed in the text. B) Cartoon of inactive (i), activated (ii), and clustered integrins (iii). The exact nature of (more...)

The integrin cytoplasmic tails are less than 75 amino acids in length (the β4 tail forms an exception with its length of approximately 1000 amino acids containing 4 fibronectin type III repeats). There is a striking homology between the β cytoplasmic tails but the α cytoplasmic tails are highly divergent, except for a conserved GFFKR motif next to the transmembrane region that is important for association with the β tail.⁹ A large number of cytoskeletal and signaling proteins has been reported to bind to β cytoplasmic tails and some have been found to interact with specific α tails. Most β tails contain an NPxY motif, which can potentially bind a wide variety of cytoskeletal and signaling proteins that contain a PTB-domain.¹⁰ Furthermore, tyrosine phosphorylation in the NPxY motif may represent a mode of regulation of integrin interactions at the cytoplasmic face. By recruiting several proteins that bind actin filaments, the cytoplasmic tails mediate the integrin-cytoskeletal connection, which is essential for most, if not all, integrin mediated functions (see below for further details).

Finally, alternative mRNA splicing in the extracellular as well as in the intracellular regions gives rise to variant forms of several α and β subunits.¹¹ There is evidence that these variants can differ in ligand specificity or in their effect on signal transduction pathways. The selection for certain splice variants in specific cell types at specific stages of differentiation points to important functional differences. For instance, the switch to α7A and β1D variants that accompanies terminal differentiation of skeletal muscle cells probably reflects the need for a stronger interaction with laminin and a stronger connection to the cytoskeleton to be able to support muscle contraction.¹²

Extracellular Ligands and Lateral Associations

Most integrins bind to components of the ECM such as laminins, collagens, and fibronectin (Table 1). Many of the ECM proteins share a common integrin-binding motif, Arg-Gly-Asp (RGD), which is present in fibronectin, vitronectin, fibrinogen, and many others. Integrin-binding to laminins and collagens occurs at other recognition motifs, though cryptic RGD sites may become accessible after proteolytic cleavage of those proteins. Some integrins bind to counter receptors present on other cells, such as members of the ADAM (a disintegrin and metalloprotease) family or immunoglobulin-type receptors such as ICAM (intercellular adhesion molecule) and VCAM (vascular cell adhesion molecule) that are expressed on leukocytes and endothelial cells. Finally, bacterial polysaccharides and an increasing number of viruses are known to bind to integrins.

Various physical and functional interactions of integrins with other transmembrane receptors have been described. Some 11 integrins, in particular the laminin-binding integrins, associate with members of the tetraspanin family. It appears that such interactions do not modulate integrin-mediated adhesion but rather, that they affect the ability of cells to spread, migrate, and undergo morphological changes.¹³ In addition, integrins can functionally interact with cell surface proteoglycans termed syndecans and they can transactivate receptor tyrosine kinases (see below for details).

Regulation of Integrin-Mediated Adhesion

For many biological processes, most notably hemostasis and immunity, it is important that integrin-mediated adhesion can be regulated. The number of integrin-ligand bonds can be regulated through changes in cellular shape, lateral diffusion of integrins in the membrane, and integrin clustering; aspects that can be controlled through cytoskeletal organization. Additionally, the intrinsic affinity of individual integrins for their ligands can be regulated from within the cell, a process referred to as “inside-out signaling”.⁵ Early studies on platelets had shown that thrombin binding to its receptor leads to a very rapid increase in fibrinogen binding by aIIbβ3. Since then, it has become clear that the α and β cytoplasmic tails are required to maintain integrins in the inactive state, that they associate with each other, and that this association is disrupted upon treatment with agonists known to cause integrin activation.¹⁴

One key player in regulation of integrin affinity appears to be the cytoskeletal adaptor talin. Binding of the talin head region to the integrin β cytoplasmic tail causes dissociation of the cytoplasmic tails and induces a conformational change in the extracellular region that increases integrin affinity.^15-17 How dissociation of the cytoplasmic tails leads to conformational changes all the way to the ligand-binding head domain is a subject of intense study. Based on electron microscopy, mutational analysis, structural analysis, and functional studies two models have been proposed. In the “switchblade model” inactive integrins are in a “bent” conformation with the head region facing the membrane. Dissociation of the cytoplasmic and transmembrane regions (e.g., upon talin binding) leads to dislocation of an EGF-like repeat in the β stalk, which causes the head region to extend upward in a switchblade-like motion.^7,18 In the “deadbolt model” the bent conformation is maintained in activated integrins but piston-like movements of the transmembrane regions (e.g., in response to talin binding) cause sliding of the stalks, which disrupts an interaction between the head region and the β stalk just above the membrane (the “deadbolt”).^19,20 In both models, the described events during integrin activation ultimately cause conformational changes in the ligand-binding pocket of the head region that increase affinity for ligand.

The relative importance of conformational changes versus clustering in priming cells for adhesion versus post-ligand binding adhesion strengthening remains controversial.^21,22 In this respect, mutations in the β3 transmembrane region that cause constitutive αIIbβ3 activation have been reported to enhance the tendency of the β3 subunit to form homo-oligomers in one study but not in another, leaving open the question if clustering may in fact be a critical component of integrin activation.^23,24

Integrin-Containing Adhesions

Integrin-engagement triggers the formation of membrane extensions that are required for cell spreading on ECM surfaces, for migration of cells into sheets of other cells, or for engulfment of particles or pathogens by phagocytic cells. Activation of Rho GTPases, the enzymes controlling the actin cytoskeleton, is involved in this process.²⁵ Ultimately, ligands, integrins, cytoskeletal proteins, and signaling molecules assemble in high local concentrations as aggregates on each side of the plasma membrane, forming “cell-matrix adhesions” in the case of integrins binding to ECM proteins²⁶ or forming complex intercellular junctions such as the “immunological synapse” in the case of leukocyte β2 integrins binding to counter receptors on other cells.²⁷ In contrast to the initial ligand-driven adhesion complexes, the formation of these more elaborate structures requires integrin-cytoskeletal linkage and is driven by cytoskeletal remodeling. Indeed, Rho family GTPases control the formation of these integrin-containing adhesions.²⁸

The vast majority of integrins connect with filamentous (F)-actin in cell-matrix adhesions via cytoskeletal adaptor proteins such as talin, vinculin, paxillin, and others^26,29 (see below for details). In 2-dimensional culture systems these come in various flavors: cells can form “focal complexes” (small adhesions found in membrane protrusions of spreading and migrating cells), “focal adhesions” (larger adhesions connected by F-actin stress fibers that are derived from focal complexes in response to tension), and “fibrillar adhesions” (elongated adhesions associated with fibronectin matrix assembly)³⁰ (Fig. 2). In 3-dimensional cultures and in vivo, components of these structures coorganize in a mixed type of cell-matrix adhesions.³¹ In osteoclasts, macrophages, some epithelial cells, and Rous sarcoma-transformed cells exceptional cell-matrix adhesions are found, termed “podosomes”. These adhesions have been described mainly in vitro and consist of highly dynamic, dot-shaped F-actin-rich contacts whose function is not completely understood.³² Finally, a unique type of cell-matrix adhesions, termed “hemidesmosomes”, is formed by basal keratinocytes in the skin. Here, the unusually long cytoplasmic tail of the β4 subunit connects α6β4 not to the actin cytoskeleton but, instead, to the keratin cytoskeletal system. This association is not mediated by the proteins found in other cell-matrix adhesions (e.g., talin) but by a specialized protein complex including plectin, BP180, and BP230.³³

Figure 2

Cell-matrix adhesions. Immunofluorescence images showing four different types of cell-matrix adhesions. Arrows indicate clustering of paxillin in focal adhesions (upper left); tensin in fibrillar adhesions (upper right); activated Src in podosomes (lower (more...)

The Integrin-Cytoskeleton Connection

Integrin function largely depends on the connection of integrins to the cytoskeleton.³⁴ The integrin cytoplasmic tails connect to the F-actin filaments through an exquisitely regulated multiprotein complex (Fig. 3). Some important players in this complex are discussed here:

Figure 3

Integrin-F-actin connections. Schematic drawing of the various connections from integrin cytoplasmic tails to the actin cytoskeleton. Proteins involved in F-actin binding are underlined; those playing a regulatory role are indicated in small fonts.

Biological Processes Involving Integrins

Matrix Assembly

Integrins participate in the assembly of various types of ECM. In the case of basement membranes, β1 integrins cooperate with the dystroglycan receptor to promote synthesis and polymerization of laminin chains into a multivalent network in which subsequently, collagen and other components are incorporated⁷⁸ (Fig. 4). Fibronectin-rich matrices are essential structures for cell migration during embryogenesis and wound healing.⁷⁹ The functional interaction of integrins with syndecans is important for fibronectin matrix assembly,⁸⁰ with α5β1 being the typical integrin involved although other integrins can compensate to some extent for its absence.^81,82 The high efficiency with which α5β1 promotes fibronectin matrix assembly may be related to the fact that binding of fibronectin through α5β1 efficiently stimulates the activity of RhoA,⁸³ a Rho GTPase that stimulates actomyosin-driven contractility, which is required for fibronectin fibrillogenesis.⁸⁴

Figure 4

Functional implications of integrin-mediated adhesion. Schematic drawing of roles for integrins in ECM assembly, intracellular signaling pathways, and cell migration. See text for details.

Concluding Remarks

Here, some general features of the structure and function of integrins has been discussed. In the next three chapters we will learn in more detail how integrins are critically involved in extracellular matrix assembly, cell migration, and how integrins modulate many intracellular signaling cascades. Subsequently, the rest of this book will discuss what we can learn from genetic studies and from human diseases about the roles of integrins and associated proteins in animal development and tissue function.

Acknowledgements

I thank Stephan Huveneers and Sandy Litjens for providing images of podosomes and hemidesmosomes, and Arnoud Sonnenberg for critical reading of the manuscript. Funded by the Dutch Cancer Society (grant NKI 2003-2858).

References

1.

Hynes RO, Zhao Q. The evolution of cell adhesion. J Cell Biol. 2000;150:F89–F96. [PubMed: 10908592]

2.

King N, Hittinger CT, Carroll SB. Evolution of key cell signaling and adhesion protein families predates animal origins. Science. 2003;301:361–363. [PubMed: 12869759]

3.

Harwood A, Coates JC. A prehistory of cell adhesion. Curr Opin Cell Biol. 2004;16:470–476. [PubMed: 15363795]

4.

Bokel C, Brown N. Integrins in development. Moving on, responding to, and sticking to the extracellular matrix. Dev Cell. 2002;3:311. [PubMed: 12361595]

5.

Hynes RO. Integrins: Bidirectional, allosteric signaling machines. Cell. 2002;110:673–687. [PubMed: 12297042]

6.

Xiong JP, Stehle T, Diefenbach B. et al. Crystal structure of the extracellular segment of integrin alpha Vbeta3. Science. 2001;294:339–345. [PMC free article: PMC2885948] [PubMed: 11546839]

7.

Shimaoka M, Takagi J, Springer TA. Conformational regulation of integrin structure and function. Annu Rev Biophys Biomol Struct. 2002;31:485–516. [PubMed: 11988479]

8.

Liddington RC, Ginsberg MH. Integrin activation takes shape. J Cell Biol. 2002;158:833–839. [PMC free article: PMC2173150] [PubMed: 12213832]

9.

Hughes PE, Diaz-Gonzalez F, Leong L. et al. Breaking the integrin hinge. A defined structural constraint regulates integrin signaling. J Biol Chem. 1996;271:6571–6574. [PubMed: 8636068]

10.

Calderwood DA, Fujioka Y, de Pereda JM. et al. Integrin beta cytoplasmic domain interactions with phosphotyrosine-binding domains: A structural prototype for diversity in integrin signaling. Proc Natl Acad Sci USA. 2003;100:2272–2277. [PMC free article: PMC151330] [PubMed: 12606711]

11.

de Melker AA, Sonnenberg A. Integrins: Alternative splicing as a mechanism to regulate ligand binding and integrin signaling events. Bioessays. 1999;21:499–509. [PubMed: 10402956]

12.

Mayer U. Integrins: Redundant or important players in skeletal muscle? J Biol Chem. 2003;278:14587–14590. [PubMed: 12556453]

13.

Hemler ME. Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain. Annu Rev Cell Dev Biol. 2003;19:397–422. [PubMed: 14570575]

14.

Calderwood DA. Integrin activation. J Cell Sci. 2004;117:657–666. [PubMed: 14754902]

15.

Vinogradova O, Velyvis A, Velyviene A. et al. A structural mechanism of integrin alpha(IIb)beta(3) “inside-out” activation as regulated by its cytoplasmic face. Cell. 2002;110:587–597. [PubMed: 12230976]

16.

Garcia-Alvarez B, de Pereda JM, Calderwood DA. et al. Structural determinants of integrin recognition by talin. Mol Cell. 2003;11:49–58. [PubMed: 12535520]

17.

Tadokoro S, Shattil SJ, Eto K. et al. Talin binding to integrin beta tails: A final common step in integrin activation. Science. 2003;302:103–106. [PubMed: 14526080]

18.

Takagi J, Petre BM, Walz T. et al. Global conformational rearrangements in integrin extracellular domains in outside-in and inside-out signaling. Cell. 2002;110:599–11. [PubMed: 12230977]

19.

Xiong JP, Stehle T, Goodman SL. et al. New insights into the structural basis of integrin activation. Blood. 2003;102:1155–1159. [PubMed: 12714499]

20.

Adair BD, Xiong JP, Maddock C. et al. Three-dimensional EM structure of the ectodomain of integrin {alpha}V{beta}3 in a complex with fibronectin. J Cell Biol. 2005;168:1109–1118. [PMC free article: PMC2171847] [PubMed: 15795319]

21.

Bazzoni G, Hemler ME. Are changes in integrin affinity and conformation overemphasized? Trends Biochem Sci. 1998;23:30–34. [PubMed: 9478133]

22.

Carman CV, Springer TA. Integrin avidity regulation: Are changes in affinity and conformation underemphasized? Curr Opin Cell Biol. 2003;15:547–556. [PubMed: 14519389]

23.

Li R, Mitra N, Gratkowski H. et al. Activation of integrin alphaIIbbeta3 by modulation of transmembrane helix associations. Science. 2003;300:795–798. [PubMed: 12730600]

24.

Luo BH, Carman CV, Takagi J. et al. Disrupting integrin transmembrane domain heterodimerization increases ligand binding affinity, not valency or clustering. Proc Natl Acad Sci USA. 2005;102:3679–3684. [PMC free article: PMC553322] [PubMed: 15738420]

25.

DeMali KA, Burridge K. Coupling membrane protrusion and cell adhesion. J Cell Sci. 2003;116:2389–2397. [PubMed: 12766185]

26.

Geiger B, Bershadsky A, Pankov R. et al. Transmembrane crosstalk between the extracellular matrix —cytoskeleton crosstalk. Nat Rev Mol Cell Biol. 2001;2:793–805. [PubMed: 11715046]

27.

Sims TN, Dustin ML. The immunological synapse: Integrins take the stage. Immunol Rev. 2002;186:100–117. [PubMed: 12234366]

28.

Nobes CD, Hall A. Rho, rac, and cdc42 GTPases regulate the assembly of multimolecular focal complexes associated with actin stress fibers, lamellipodia, and filopodia. Cell. 1995;81:53–62. [PubMed: 7536630]

29.

Burridge K, Chrzanowska-Wodnicka M. Focal adhesions, contractility, and signaling. Annu Rev Cell Dev Biol. 1996;12:463–518. [PubMed: 8970735]

30.

Zamir E, Geiger B. Molecular complexity and dynamics of cell-matrix adhesions. J Cell Sci. 2001;114:3583–3590. [PubMed: 11707510]

31.

Cukierman E, Pankov R, Stevens DR. et al. Taking cell-matrix adhesions to the third dimension. Science. 2001;294:1708–1712. [PubMed: 11721053]

32.

Linder S, Aepfelbacher M. Podosomes: Adhesion hot-spots of invasive cells. Trends Cell Biol. 2003;13:376–385. [PubMed: 12837608]

33.

Nievers MG, Schaapveld RQ, Sonnenberg A. Biology and function of hemidesmosomes. Matrix Biol. 1999;18:5–17. [PubMed: 10367727]

34.

Brakebusch C, Fassler R. The integrin-actin connection, an eternal love affair. EMBO J. 2003;22:2324–2333. [PMC free article: PMC156003] [PubMed: 12743027]

35.

Critchley DR. Cytoskeletal proteins talin and vinculin in integrin-mediated adhesion. Biochem Soc Trans. 2004;32:831–836. [PubMed: 15494027]

36.

Priddle H, Hemmings L, Monkley S. et al. Disruption of the talin gene compromises focal adhesion assembly in undifferentiated but not differentiated embryonic stem cells. J Cell Biol. 1998;142:1121–1133. [PMC free article: PMC2132864] [PubMed: 9722622]

37.

Monkley SJ, Zhou XH, Kinston SJ. et al. Disruption of the talin gene arrests mouse development at the gastrulation stage. Dev Dyn. 2000;219:560–574. [PubMed: 11084655]

38.

Jiang G, Giannone G, Critchley DR. et al. Two-piconewton slip bond between fibronectin and the cytoskeleton depends on talin. Nature. 2003;424:334–337. [PubMed: 12867986]

39.

Di Paolo G, Pellegrini L, Letinic K. et al. Recruitment and regulation of phosphatidylinositol phosphate kinase type 1 gamma by the FERM domain of talin. Nature. 2002;420:85–89. [PubMed: 12422219]

40.

Ling K, Doughman RL, Firestone AJ. et al. Type I gamma phosphatidylinositol phosphate kinase targets and regulates focal adhesions. Nature. 2002;420:89–93. [PubMed: 12422220]

41.

Franco SJ, Rodgers MA, Perrin BJ. et al. Calpain-mediated proteolysis of talin regulates adhesion dynamics. Nat Cell Biol. 2004;6:977–983. [PubMed: 15448700]

42.

Otey CA, Carpen O. Alpha-actinin revisited: A fresh look at an old player. Cell Motil Cytoskeleton. 2004;58:104–111. [PubMed: 15083532]

43.

Cuevas BD, Abell AN, Witowsky JA. et al. MEKK1 regulates calpain-dependent proteolysis of focal adhesion proteins for rear-end detachment of migrating fibroblasts. EMBO J. 2003;22:3346–3355. [PMC free article: PMC165646] [PubMed: 12839996]

44.

Stossel TP, Condeelis J, Cooley L. et al. Filamins as integrators of cell mechanics and signalling. Nat Rev Mol Cell Biol. 2001;2:138–145. [PubMed: 11252955]

45.

van der Flier A, Sonnenberg A. Structural and functional aspects of filamins. Biochim Biophys Acta. 2001;1538:99–117. [PubMed: 11336782]

46.

Calderwood DA, Huttenlocher A, Kiosses WB. et al. Increased filamin binding to beta-integrin cytoplasmic domains inhibits cell migration. Nat Cell Biol. 2001;3:1060–1068. [PubMed: 11781567]

47.

Bailly M. Connecting cell adhesion to the actin polymerization machinery: Vinculin as the missing link? Trends Cell Biol. 2003;13:163–165. [PubMed: 12667752]

48.

Bakolitsa C, Cohen DM, Bankston LA. et al. Structural basis for vinculin activation at sites of cell adhesion. Nature. 2004;430:583–586. [PubMed: 15195105]

49.

Izard T, Evans G, Borgon RA. et al. Vinculin activation by talin through helical bundle conversion. Nature. 2004;427:171–175. [PubMed: 14702644]

50.

Xu W, Baribault H, Adamson ED. Vinculin knockout results in heart and brain defects during embryonic development. Development. 1998;125:327–337. [PubMed: 9486805]

51.

Volberg T, Geiger B, Kam Z. et al. Focal adhesion formation by F9 embryonal carcinoma cells after vinculin gene disruption. J Cell Sci. 1995;108(Pt 6):2253–2260. [PubMed: 7673345]

52.

Subauste MC, Pertz O, Adamson ED. et al. Vinculin modulation of paxillin-FAK interactions regulates ERK to control survival and motility. J Cell Biol. 2004;165:371–381. [PMC free article: PMC2172187] [PubMed: 15138291]

53.

Hannigan GE, Leung-Hagesteijn C, Fitz-Gibbon L. et al. Regulation of cell adhesion and anchorage-dependent growth by a new beta 1-integrin-linked protein kinase. Nature. 1996;379:91–96. [PubMed: 8538749]

54.

Zervas CG, Gregory SL, Brown NH. Drosophila integrin-linked kinase is required at sites of integrin adhesion to link the cytoskeleton to the plasma membrane. J Cell Biol. 2001;152:1007–1018. [PMC free article: PMC2198807] [PubMed: 11238456]

55.

Mackinnon AC, Qadota H, Norman KR. et al. C. elegans PAT-4/ILK functions as an adaptor protein within integrin adhesion complexes. Curr Biol. 2002;12:787–797. [PubMed: 12015115]

56.

Sakai T, Li S, Docheva D. et al. Integrin-linked kinase (ILK) is required for polarizing the epiblast, cell adhesion, and controlling actin accumulation. Genes Dev. 2003;17:926–940. [PMC free article: PMC196029] [PubMed: 12670870]

57.

Wu C. The PINCH-ILK-parvin complexes: Assembly, functions and regulation. Biochim Biophys Acta. 2004;1692:55–62. [PubMed: 15246679]

58.

Turner CE. Paxillin and focal adhesion signalling. Nat Cell Biol. 2000;2:E231–E236. [PubMed: 11146675]

59.

Liu S, Thomas SM, Woodside DG. et al. Binding of paxillin to alpha4 integrins modifies integrin-dependent biological responses. Nature. 1999;402:676–681. [PubMed: 10604475]

60.

Nikolopoulos SN, Turner CE. Molecular dissection of actopaxin-integrin-linked kinase-Paxillin interactions and their role in subcellular localization. J Biol Chem. 2002;277:1568–1575. [PubMed: 11694518]

61.

Brown MC, Turner CE. Paxillin: Adapting to change. Physiol Rev. 2004;84:1315–1339. [PubMed: 15383653]

62.

Kiyokawa E, Hashimoto Y, Kobayashi S. et al. Activation of Rac1 by a Crk SH3-binding protein, DOCK180. Genes Dev. 1998;12:3331–3336. [PMC free article: PMC317231] [PubMed: 9808620]

63.

Valles AM, Beuvin M, Boyer B. Activation of Rac1 by paxillin-Crk-DOCK180 signaling complex is antagonized by Rap1 in migrating NBT-II cells. J Biol Chem. 2004;279:44490–44496. [PubMed: 15308668]

64.

Richardson A, Malik RK, Hildebrand JD. et al. Inhibition of cell spreading by expression of the C-terminal domain of focal adhesion kinase (FAK) is rescued by coexpression of Src or catalytically inactive FAK: A role for paxillin tyrosine phosphorylation. Mol Cell Biol. 1997;17:6906–6914. [PMC free article: PMC232547] [PubMed: 9372922]

65.

Webb DJ, Donais K, Whitmore LA. et al. FAK-Src signalling through paxillin, ERK and MLCK regulates adhesion disassembly. Nat Cell Biol. 2004;6:154–161. [PubMed: 14743221]

66.

Lo SH. Tensin. Int J Biochem Cell Biol. 2004;36:31–34. [PubMed: 14592531]

67.

Lo SH, Yu QC, Degenstein L. et al. Progressive kidney degeneration in mice lacking tensin. J Cell Biol. 1997;136:1349–1361. [PMC free article: PMC2132507] [PubMed: 9087448]

68.

Chiang MK, Liao YC, Kuwabara Y. et al. Inactivation of tensin3 in mice results in growth retardation and postnatal lethality. Dev Biol. 2005;279:368–377. [PubMed: 15733665]

69.

Pankov R, Cukierman E, Katz BZ. et al. Integrin dynamics and matrix assembly: Tensin-dependent translocation of alpha(5)beta(1) integrins promotes early fibronectin fibrillogenesis. J Cell Biol. 2000;148:1075–1090. [PMC free article: PMC2174533] [PubMed: 10704455]

70.

Chen H, Lo SH. Regulation of tensin-promoted cell migration by its focal adhesion binding and Src homology domain 2. Biochem J. 2003;370:1039–1045. [PMC free article: PMC1223245] [PubMed: 12495434]

71.

Parsons JT. Focal adhesion kinase: The first ten years. J Cell Sci. 2003;116:1409–1416. [PubMed: 12640026]

72.

Cary LA, Guan JL. Focal adhesion kinase in integrin-mediated signaling. Front Biosci. 1999;4:D102–D113. [PubMed: 9889179]

73.

Vuori K, Hirai H, Aizawa S. et al. Introduction of p130cas signaling complex formation upon integrin-mediated cell adhesion: A role for Src family kinases. Mol Cell Biol. 1996;16:2606–2613. [PMC free article: PMC231251] [PubMed: 8649368]

74.

Sieg DJ, Hauck CR, Ilic D. et al. FAK integrates growth-factor and integrin signals to promote cell migration. Nat Cell Biol. 2000;2:249–256. [PubMed: 10806474]

75.

Chen R, Kim O, Li M. et al. Regulation of the PH-domain-containing tyrosine kinase Etk by focal adhesion kinase through the FERM domain. Nat Cell Biol. 2001;3:439–444. [PubMed: 11331870]

76.

Arthur WT, Petch LA, Burridge K. Integrin engagement suppresses RhoA activity via a c-Src-dependent mechanism. Curr Biol. 2000;10:719–722. [PubMed: 10873807]

77.

Ren XD, Kiosses WB, Sieg DJ. et al. Focal adhesion kinase suppresses Rho activity to promote focal adhesion turnover. J Cell Sci. 2000;113(Pt 20):3673–3678. [PubMed: 11017882]

78.

Li S, Edgar D, Fassler R. et al. The role of laminin in embryonic cell polarization and tissue organization. Dev Cell. 2003;4:613–624. [PubMed: 12737798]

79.

George EL, Georges-Labouesse EN, Patel-King RS. et al. Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin. Development. 1993;119:1079–1091. [PubMed: 8306876]

80.

Schwarzbauer JE, Sechler JL. Fibronectin fibrillogenesis: A paradigm for extracellular matrix assembly. Curr Opin Cell Biol. 1999;11:622–627. [PubMed: 10508649]

81.

Wu C, Bauer JS, Juliano RL. et al. The alpha 5 beta 1 integrin fibronectin receptor, but not the alpha 5 cytoplasmic domain, functions in an early and essential step in fibronectin matrix assembly. J Biol Chem. 1993;268:21883–21888. [PubMed: 7691819]

82.

Wennerberg K, Lohikangas L, Gullberg D. et al. Beta 1 integrin-dependent and -independent polymerization of fibronectin. J Cell Biol. 1996;132:227–238. [PMC free article: PMC2120698] [PubMed: 8567726]

83.

Danen EH, Sonneveld P, Brakebusch C. et al. The fibronectin-binding integrins {alpha}5{beta}1 and {alpha}v{beta}3 differentially modulate RhoA-GTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis. J Cell Biol. 2002;159:1071–1086. [PMC free article: PMC2173988] [PubMed: 12486108]

84.

Zhong C, Chrzanowska-Wodnicka M, Brown J. et al. Rho-mediated contractility exposes a cryptic site in fibronectin and induces fibronectin matrix assembly. J Cell Biol. 1998;141:539–551. [PMC free article: PMC2148448] [PubMed: 9548730]

85.

Ridley AJ, Schwartz MA, Burridge K. et al. Cell migration: Integrating signals from front to back. Science. 2003;302:1704–1709. [PubMed: 14657486]

86.

Ballestrem C, Hinz B, Imhof BA. et al. Marching at the front and dragging behind: Differential alphaVbeta3-integrin turnover regulates focal adhesion behavior. J Cell Biol. 2001;155:1319–1332. [PMC free article: PMC2199321] [PubMed: 11756480]

87.

Danen EH, van Rheenen J, Franken W. et al. Integrins control motile strategy through a Rho-cofilin pathway. J Cell Biol. 2005;169:515–526. [PMC free article: PMC2171933] [PubMed: 15866889]

88.

Chen Q, Lin TH, Der CJ. et al. Integrin-mediated activation of MEK and mitogen-activated protein kinase is independent of Ras [corrected] J Biol Chem. 1996;271:18122–18127. [PubMed: 8663436]

89.

Renshaw MW, Ren XD, Schwartz MA. Growth factor activation of MAP kinase requires cell adhesion. EMBO J. 1997;16:5592–5599. [PMC free article: PMC1170191] [PubMed: 9312018]

90.

Salgia R, Sattler M, Pisick E. et al. p210BCR/ABL induces formation of complexes containing focal adhesion proteins and the protooncogene product p120c-Cbl. Exp Hematol. 1996;24:310–313. [PubMed: 8641358]

91.

Lewis JM, Schwartz MA. Integrins regulate the association and phosphorylation of paxillin by c-Abl. J Biol Chem. 1998;273:14225–14230. [PubMed: 9603926]

92.

Chicurel ME, Singer RH, Meyer CJ. et al. Integrin binding and mechanical tension induce movement of mRNA and ribosomes to focal adhesions. Nature. 1998;392:730–733. [PubMed: 9565036]

93.

Miyamoto S, Teramoto H, Gutkind JS. et al. Integrins can collaborate with growth factors for phosphorylation of receptor tyrosine kinases and MAP kinase activation: Roles of integrin aggregation and occupancy of receptors. J Cell Biol. 1996;135:1633–1642. [PMC free article: PMC2133938] [PubMed: 8978828]

94.

Moro L, Venturino M, Bozzo C. et al. Integrins induce activation of EGF receptor: Role in MAP kinase induction and adhesion-dependent cell survival. EMBO J. 1998;17:6622–6632. [PMC free article: PMC1171008] [PubMed: 9822606]

95.

Wang JF, Zhang XF, Groopman JE. Stimulation of beta 1 integrin induces tyrosine phosphorylation of vascular endothelial growth factor receptor-3 and modulates cell migration. J Biol Chem. 2001;276:41950–41957. [PubMed: 11553610]

96.

Faham S, Linhardt RJ, Rees DC. Diversity does make a difference: Fibroblast growth factor-heparin interactions. Curr Opin Struct Biol. 1998;8:578–586. [PubMed: 9818261]

97.

Maniotis AJ, Chen CS, Ingber DE. Demonstration of mechanical connections between integrins, cytoskeletal filaments, and nucleoplasm that stabilize nuclear structure. Proc Natl Acad Sci USA. 1997;94:849–854. [PMC free article: PMC19602] [PubMed: 9023345]

98.

Lelievre SA, Weaver VM, Nickerson JA. et al. Tissue phenotype depends on reciprocal interactions between the extracellular matrix and the structural organization of the nucleus. Proc Natl Acad Sci USA. 1998;95:14711–14716. [PMC free article: PMC24514] [PubMed: 9843954]

99.

Assoian RK, Schwartz MA. Coordinate signaling by integrins and receptor tyrosine kinases in the regulation of G1 phase cell-cycle progression. Curr Opin Genet Dev. 2001;11:48–53. [PubMed: 11163150]

100.

Danen EH, Yamada KM. Fibronectin, integrins, and growth control. J Cell Physiol. 2001;189:1–13. [PubMed: 11573199]

101.

Khwaja A, Rodriguez-Viciana P, Wennstrom S. et al. Matrix adhesion and Ras transformation both activate a phosphoinositide 3-OH kinase and protein kinase B/Akt cellular survival pathway. EMBO J. 1997;16:2783–2793. [PMC free article: PMC1169887] [PubMed: 9184223]

102.

King WG, Mattaliano MD, Chan TO. et al. Phosphatidylinositol 3-kinase is required for integrin-stimulated AKT and Raf-1/mitogen-activated protein kinase pathway activation. Mol Cell Biol. 1997;17:4406–4418. [PMC free article: PMC232295] [PubMed: 9234699]

103.

Weaver VM, Lelievre S, Lakins JN. et al. beta4 integrin-dependent formation of polarized three-dimensional architecture confers resistance to apoptosis in normal and malignant mammary epithelium. Cancer Cell. 2002;2:205–216. [PMC free article: PMC2788997] [PubMed: 12242153]

104.

Varner JA, Emerson DA, Juliano RL. Integrin alpha 5 beta 1 expression negatively regulates cell growth: Reversal by attachment to fibronectin. Mol Biol Cell. 1995;6:725–740. [PMC free article: PMC301232] [PubMed: 7579691]

105.

Stupack DG, Puente XS, Boutsaboualoy S. et al. Apoptosis of adherent cells by recruitment of caspase-8 to unligated integrins. J Cell Biol. 2001;155:459–470. [PMC free article: PMC2150834] [PubMed: 11684710]

106.

Li ML, Aggeler J, Farson DA. et al. Influence of a reconstituted basement membrane and its components on casein gene expression and secretion in mouse mammary epithelial cells. Proc Natl Acad Sci USA. 1987;84:136–140. [PMC free article: PMC304157] [PubMed: 3467345]

107.

Edwards GM, Wilford FH, Liu X. et al. Regulation of mammary differentiation by extracellular matrix involves protein-tyrosine phosphatases. J Biol Chem. 1998;273:9495–9500. [PubMed: 9545277]

108.

Menko AS, Boettiger D. Occupation of the extracellular matrix receptor, integrin, is a control point for myogenic differentiation. Cell. 1987;51:51–57. [PubMed: 3115595]

109.

Haskill S, Johnson C, Eierman D. et al. Adherence induces selective mRNA expression of monocyte mediators and proto-oncogenes. J Immunol. 1988;140:1690–1694. [PubMed: 3257980]

110.

Adams JC, Watt FM. Fibronectin inhibits the terminal differentiation of human keratinocytes. Nature. 1989;340:307–309. [PubMed: 2473404]