Integrins: versatility, modulation, and signaling in cell adhesion.

The adhesive interactions of cells with other cells and with the extracellular matrix are crucial to all developmental processes, but have a central role in the functions of the immune system throughout life Three families of cell-surface molecules regulate the migration of lymphocytes and the interactions of activated cells during immune responses

Adhesion receptors of the immune system.

Rapid progress has been made in the understanding of the molecular interactions that result in cell adhesion. Many adhesive proteins present in extracellular matrices and in the blood contain the tripeptide arginine-glycine-aspartic acid (RGD) as their cell recognition site. These proteins include fibronectin, vitronectin, osteopontin, collagens, thrombospondin, fibrinogen, and von Willebrand factor. The RGD sequences of each of the adhesive proteins are recognized by at least one member of a family of structurally related receptors, integrins, which are heterodimeric proteins with two membrane-spanning subunits. Some of these receptors bind to the RGD sequence of a single adhesion protein only, whereas others recognize groups of them. The conformation of the RGD sequence in the individual proteins may be critical to this recognition specificity. On the cytoplasmic side of the plasma membrane, the receptors connect the extracellular matrix to the cytoskeleton. More than ten proved or suspected RGD-containing adhesion-promoting proteins have already been identified, and the integrin family includes at least as many receptors recognizing these proteins. Together, the adhesion proteins and their receptors constitute a versatile recognition system providing cells with anchorage, traction for migration, and signals for polarity, position, differentiation, and possibly growth.

https://science.sciencemag.org/content/sci/238/4826/491.full.pdf

New perspectives in cell adhesion : RGD and integrins

Integrins: a family of cell surface receptors.

Mechanical forces associated with blood flow play important roles in the acute control of vascular tone, the regulation of arterial structure and remodeling, and the localization of atherosclerotic lesions. Major regulation of the blood vessel responses occurs by the action of hemodynamic shear stresses on the endothelium. The transmission of hemodynamic forces throughout the endothelium and the mechanotransduction mechanisms that lead to biophysical, biochemical, and gene regulatory responses of endothelial cells to hemodynamic shear stresses are reviewed.

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Flow-mediated endothelial mechanotransduction

Many observations suggest the presence of transmembrane linkages between the cytoskeleton and the extracellular matrix. In fibroblasts both light and electron microscopic observations reveal a co-alignment between actin filaments at the cell surface and extracellular fibronectin1–3. These associations are seen at sites of cell matrix interaction, frequently along stress fibres and sometimes where these bundles of microfilaments terminate at adhesion plaques (focal contacts). Non-morphological evidence also indicates a functional linkage between the cytoskeleton and extracellular matrix. Addition of fibronectin to transformed cells induces flattening of the cells and a reorganization of the actin cytoskeleton, with the concomitant appearance of arrays of stress fibres4–6. Conversely, disruption of the actin cytoskeleton by treatment with cytochalasin B leads to release of fibronectin from the cell surface7. As yet, there is no detailed knowledge of the molecules involved in this transmembrane linkage, although several proteins have been suggested as candidates in the chain of attachment between bundles of actin filaments and the cytoplasmic face of the plasma membrane: these include vinculin8, α-actinin9 and talin10, each one having been identified at regions where bundles of actin filaments interact with the plasma membrane and underlying cell-surface fibronectin10–13. Recently, the cell-substrate attachment (CSAT) antigen14 has been identified as a plasma membrane receptor for fibronectin15, raising the possibility that this glycoprotein complex may serve as a bridge between fibronectin and one or more of the underlying cytoskeletal components mentioned. Here we have investigated the interaction of the purified CSAT antigen with these cytoskeletal components, and we demonstrate an interaction specifically between the CSAT antigen and talin.

/pdf/interaction-of-plasma-membrane-fibronectin-receptor-with-53gbi6k470.pdf

Interaction of plasma membrane fibronectin receptor with talin—a transmembrane linkage

https://www.cell.com/cell/pdf/0092-8674(86)90744-0.pdf

Structure of Integrin, a Glycoprotein Involved in the Transmembrane Linkage between Fibronectin and Actin

Table 2 lists most of the extracellular matrix and related receptors identified to date. The wide range of binding affinities of these receptors for their ligands may be important to their function. The affinity of integrins for fibronectin is moderate, with a dissociation constant in the micromolar range. This affinity level leads to relatively rapid dissociation and reformation of receptor-ligand complexes. Thus changes in component concentration can shift binding equilibria within minutes (the time scale of many biologic phenomena) and change the number or organization of adhesive complexes. This type of interaction would be useful in motile cells, in which adhesions must form and dissociate rapidly. In contrast, the affinity of the 68-kDa laminin receptor for its ligand is three orders of magnitude higher. Such levels of affinity would be useful in stabilizing tissue. Members of the integrin family appear to recognize an RGD sequence on the ligands to which they bind. Since there are many ligands containing the RGD sequence, the question of specificity arises. Avian integrin shows little specificity and appears to behave as a multifunctional, promiscuous receptor for extracellular matrix molecules. Figure 1 summarizes our current view of the structural and functional features of avian integrin. In contrast, the mammalian receptors for vitronectin and fibronectin are specific for their respective ligands. More than one of these receptors may be present simultaneously on a cell surface, e.g. fibroblasts express receptors for fibronectin, laminin, and vitronectin at the same time. This multiplicity of receptors provides potential mechanisms for generating the adhesive differences among cells believed to play a prominent role in morphogenesis. Further adhesive differences may stem from the formation of different combinations of various alpha- and beta-subunits expressed in the cell. The mechanism of regulation of adhesive interactions with the extracellular matrix is only beginning to be explored. There are several levels at which this regulation might occur. Integrin appears to be more regionalized in more developed cells that are integral parts of tissue structures. Changes in receptor distribution could alter the relative strength of adhesive interactions. In certain instances, avian integrin disappears, or its concentration is reduced, e.g. during the development of embryonic lung (Chen et al 1986) and erythroid cells (Patel & Lodish 1985). Post-translational modifications provide yet another mechanism for regulating integrin-ligand binding.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell surface receptors for extracellular matrix molecules

The fibronectin (FN) receptor in avian cells has been characterized previously as a complex of three membrane glycoproteins of about Mr 160,000, Mr 140,000, and Mr 120,000 (simply termed protein band 1, band 2, and band 3, respectively). Monoclonal antibodies to the band 3 protein of the complex prevent FN and laminin binding both in vivo and in vitro and enable the detection of the receptor proteins in the plasma membrane and in adhesion plaques. Association of the FN receptor proteins with the adhesion-plaque protein talin also has been reported. We now find that the band 2 and band 3 proteins in the complex are phosphorylated in Rous sarcoma virus-transformed chicken cells but not in normal chicken cells. Phosphorylation occurs predominantly on tyrosine and is accompanied by a reorganization of the receptor complex in the membrane of the transformed cells. Whereas normal cells contain the FN receptor in focal contacts and cellular processes between cells, v-src-transformed cells exhibit a more diffuse distribution of this receptor. In addition to the viral v-src oncogene, cells transformed by other avian oncogenes that also encode tyrosine kinases (v-fps, v-erbB, and v-yes) also express the receptor complex proteins in the phosphorylated state regardless of whether the transforming protein is detectable in adhesion plaques. These results suggest that the altered FN and laminin receptor proteins may contribute to the transformed phenotype, but their significance and role in the transformed state remain to be established.

Phosphorylation of the fibronectin receptor complex in cells transformed by oncogenes that encode tyrosine kinases.

A monoclonal antibody, CSAT, which inhibits the adhesion of chick cells to substrata coated with fibronectin, laminin and vitronectin, has been used to identify a cell surface receptor required for cell-substratum adhesion. This receptor, termed integrin, is found on the ventral surface of cells in close contact adhesion sites, at the periphery of adhesion plaques and beneath stress fibres. It is a heterodimer consisting of non-covalently linked alpha and beta subunits. Integrin binds directly to laminin, fibronectin and vitronectin with dissociation constants in the micromolar range. The binding of integrin to matrix molecules is sensitive to peptides carrying the cell-binding sequence Arg-Gly-Asp and requires heteromeric integrity. Integrin also binds directly to the cytoskeleton-associated protein talin. Thus, integrin has the properties of a transmembrane molecule capable of bringing extracellular matrix and cytoskeleton-associated molecules in proper juxtaposition to form adhesion structures. The integrin beta subunit is phosphorylated following Rous sarcoma virus transformation. Phosphorylation alters the ability of the receptor to bind extracellular matrix molecules as well as talin, suggesting a mechanism for the alteration of cellular adhesive and morphological properties following malignant transformation. A major phosphorylation site is on the cytoplasmic domain of the beta subunit. Synthetic peptides homologous with this region of integrin inhibit integrin-talin binding. The gene for the beta subunit of integrin has been sequenced. Its structure is consistent with the membrane-spanning properties of the receptor. Integrin is structurally and serologically related to adhesion receptors from mammalian tumour cells, fibroblasts, platelets and lymphocytes. It appears to be a member of a supergene family of receptors involved in cellular adhesive interactions. Antibody and peptide inhibition experiments have suggested a role for integrin and integrin-like molecules in cell migration, neurite extension, neural differentiation, histogenesis and embryonic development in Drosophila. Thus, integrin appears representative of a set of evolutionarily conserved, biologically important adhesive molecules.

Alan F. Horwitz

Papers

Interaction of plasma membrane fibronectin receptor with talin—a transmembrane linkage

Structure of Integrin, a Glycoprotein Involved in the Transmembrane Linkage between Fibronectin and Actin

Cell surface receptors for extracellular matrix molecules

Phosphorylation of the fibronectin receptor complex in cells transformed by oncogenes that encode tyrosine kinases.

Integrin, a transmembrane glycoprotein complex mediating cell-substratum adhesion.