Integrins: versatility, modulation, and signaling in cell adhesion

Tyrosine phosphorylation of paxillin and pp125FAK accompanies cell adhesion to extracellular matrix: a role in cytoskeletal assembly

Cells in culture reveal high levels of protein tyrosine phosphorylation in their focal adhesions, the regions where cells adhere to the underlying substratum. We have examined the tyrosine phosphorylation of proteins in response to plating cells on extracellular matrix substrata. Rat embryo fibroblasts, mouse Balb/c 3T3, and NIH 3T3 cells plated on fibronectin-coated surfaces revealed elevated phosphotyrosine levels in a cluster of proteins between 115 and 130 kD. This increase in tyrosine phosphorylation was also seen when rat embryo fibroblasts were plated on laminin or vitronectin, but not on polylysine or on uncoated plastic. Integrin mediation of this effect was suggested by finding the same pattern of elevated tyrosine phosphorylation in cells plated on the cell-binding fragment of fibronectin and in cells plated on a synthetic polymer containing multiple RGD sequences. We have identified one of the proteins of the 115-130-kD cluster as pp125FAK, a tyrosine kinase recently localized in focal adhesions (Schaller, M. D., C. A. Borgman, B. S. Cobb, R. R. Vines, A. B. Reynolds, and J. T. Parsons. 1992. Proc. Natl. Acad. Sci. USA. 89:5192). A second protein that becomes tyrosine phosphorylated in response to extracellular matrix adhesion is identified as paxillin, a 70-kD protein previously localized to focal adhesions. Treatment of cells with the tyrosine kinase inhibitor herbimycin A diminished the adhesion-induced tyrosine phosphorylation of these proteins and inhibited the formation of focal adhesions and stress fibers. These results suggest a role for integrin-mediated tyrosine phosphorylation in the organization of the cytoskeleton as cells adhere to the extracellular matrix.

Integrin-dependent phosphorylation and activation of the protein tyrosine kinase pp125FAK in platelets

We have investigated mechanisms involved in integrin-mediated signal transduction in platelets by examining integrin-dependent phosphorylation and activation of a newly identified protein tyrosine kinase, pp125FAK (FAK, focal adhesion kinase). This kinase was previously shown to be localized in focal adhesions in fibroblasts, and to be phosphorylated on tyrosine in normal and Src-transformed fibroblasts. We show that thrombin and collagen activation of platelets causes an induction of tyrosine phosphorylation of pp125FAK and that pp125FAK molecules isolated from activated platelets display enhanced levels of phosphorylation in immune-complex kinase assays. pp125FAK was not phosphorylated on tyrosine after thrombin or collagen treatment of Glanzmann's thrombasthenic platelets deficient in the fibrinogen receptor GPIIb-IIIa, or of platelets pretreated with an inhibitory monoclonal antibody to GP IIb-IIIa. Fibrinogen binding to GP IIb-IIIa was not sufficient to induce pp125FAK phosphorylation because pp125FAK was not phosphorylated on tyrosine in thrombin-treated platelets that were not allowed to aggregate. These results indicate that tyrosine phosphorylation of pp125FAK is dependent on platelet aggregation mediated by fibrinogen binding to the integrin receptor GP IIb-IIIa. The induction of tyrosine phosphorylation of pp125FAK was inhibited in thrombin- and collagen-treated platelets preincubated with cytochalasin D, which prevents actin polymerization following activation. Under all of these conditions, there was a strong correlation between the induction of tyrosine phosphorylation of pp125FAK in vivo and stimulation of the phosphorylation of pp125FAK in vitro in immune-complex kinase assays. This study provides the first genetic evidence that tyrosine phosphorylation of pp125FAK is dependent on integrin-mediated events, and demonstrates that there is a strong correlation between tyrosine phosphorylation of pp125FAK in platelets, and the activation of pp125FAK-associated phosphorylating activity in vitro.

Enhancement of CD3-induced activation of human intestinal intraepithelial lymphocytes by stimulation of the beta 7-containing integrin defined by HML-1 monoclonal antibody

Intraepithelial lymphocytes (IEL) form a large population of T cells in close contact with the intestinal lumen and differ from lymphocytes in other lymphoid compartments by their predominant CD8+ phenotype and the strong expression of the recently characterized beta 7-containing integrin defined by the monoclonal antibody (mAb) HML-1. The aim of the present in vitro study was to investigate the possible role of the integrin defined by HML-1 in the activation of human IEL via the CD3-T cell receptor (TcR) pathway. The proliferative response of IEL to optimal concentrations of immobilized OKT3 was found to be similar to that of peripheral blood lymphocytes enriched in CD8+ cells. When co-immobilized with suboptimal concentrations of OKT3, antibodies directed against CD11a, CD29 and the beta 7-containing integrin defined by HML-1 exerted a strong synergistic effect on the proliferative response and on the expression of CD25 and CD71 antigens by human IEL. These data indicate that the CD3-TcR pathway is functional in human IEL and contrast with previous observations suggesting that the CD3-TcR pathway was difficult to elicit in human IEL. Furthermore, the present data show that the immune response of human IEL can be modulated via interactions between integrins expressed by IEL and their respective ligands in the mucosa and suggest that IEL's activation may depend on the level of expression of integrin ligands in the epithelium, particularly of the expression of the as yet unknown ligand for the IEL-specific integrin defined by HML-1.

Patching and capping of LFA-1 molecules on human lymphocytes

The distribution and dynamics of LFA-1 molecules over the surface of human lymphocytes were analysed using immunogold label-fracture and fracture-flip methods. Patching and capping were induced by incubation at 37 degrees C with antibodies directed against the alpha and beta chains respectively of the heterodimeric LFA-1 molecule, and were followed by immunofluorescence. Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) to link LFA-1 molecules to the cytoskeleton increased the percentage of capped cells, implying a faster and more efficient process of capping. At all times of clustering or upon phorbol ester treatment, the concentration of LFA-1 in patches and then in caps was not accompanied by a parallel concentration of membrane particles on the freeze-fractured plasma membranes. Our results support the role of the cytoskeleton in regulating the capping phenomenon and in controlling the structural organization of the plasma membranes.

Expression of alpha 1 integrin, a laminin-collagen receptor, during myogenesis and neurogenesis in the avian embryo

In this study, we have examined the spatiotemporal distribution of the alpha 1 integrin subunit, a putative laminin and collagen receptor, in avian embryos, using immunofluorescence microscopy and immunoblotting techniques. We used an antibody raised against a gizzard 175 × 10(3) M(r) membrane protein which was described previously and which we found to be immunologically identical to the chicken alpha 1 integrin subunit. In adult avian tissues, alpha 1 integrin exhibited a very restricted pattern of expression; it was detected only in smooth muscle and in capillary endothelial cells. In the developing embryo, alpha 1 integrin subunit expression was discovered in addition to smooth muscle and capillary endothelial cells, transiently, in both central and peripheral nervous systems and in striated muscles, in association with laminin and collagen IV. alpha 1 integrin was practically absent from most epithelial tissues, including the liver, pancreas and kidney tubules, and was weakly expressed by tissues that were not associated with laminin and collagen IV. In the nervous system, alpha 1 integrin subunit expression occurred predominantly at the time of early neuronal differentiation. During skeletal muscle development, alpha 1 integrin was expressed on myogenic precursors, during myoblast migration, and in differentiating myotubes. alpha 1 integrin disappeared from skeletal muscle cells as they became contractile. In visceral and vascular smooth muscles, alpha 1 integrin appeared specifically during early smooth muscle cell differentiation and, later, was permanently expressed after cell maturation. These results indicate that (i) the expression pattern of alpha 1 integrin is consistent with a function as a laminin/collagen IV receptor; (ii) during avian development, expression of the alpha 1 integrin subunit is spatially and temporally regulated; (iii) during myogenesis and neurogenesis, expression of alpha 1 integrin is transient and correlates with cell migration and differentiation.

Regulation of expression of fibronectin and its receptor, alpha 5 beta 1, during development and regeneration of peripheral nerve

The extracellular matrix glycoprotein, fibronectin, is a potent promoter of peripheral neurite outgrowth. Interactions of peripheral neurons with fibronectin have been shown to be primarily mediated by the beta 1 class of integrin heterodimers. In the present study, we have examined the expression and regulation of fibronectin and its integrin receptor, alpha 5 beta 1, in developing and regenerating chick peripheral nerve. We show that fibronectin and alpha 5 beta 1 are expressed at comparatively high levels in developing nerve with alpha 5 beta 1 expression on axons and non-neuronal cells. With nerve maturation, both proteins are less prominently expressed and the cellular pattern of alpha 5 beta 1 expression becomes more restricted. Following lesion of mature nerve, both fibronectin and alpha 5 beta 1 are strongly induced with prominent expression of alpha 5 beta 1 on regenerating neurites and Schwann cells. The elevation in fibronectin levels in the regenerating nerve is highest in the vicinity of the lesion, an area undergoing extensive cellular remodeling including Schwann cell migration and growth cone extension. Our results suggest that fibronectin and its receptor, alpha 5 beta 1, may mediate functionally important interactions in the development and regeneration of peripheral nerve.

Triggering through CD16 or phorbol esters enhances adhesion of NK cells to laminin via very late antigen 6

Very late antigens VLA-1, VLA-2, VLA-3, and VLA-6, belonging to the beta 1 subfamily of integrins, have been identified as receptors for different binding domains of laminin (LM). We have detected VLA-6, but not VLA-1 and VLA-2 on a subset (50-70%) of fresh peripheral blood CD3-, CD16+, CD56+ human natural killer (NK) cells by immunofluorimetric and biochemical analysis. Binding assays performed on LM-coated plates showed that 10-15% of NK cells spontaneously adhere to LM, and this adhesion is mediated by VLA-6. Activation of NK cells through CD16 triggering or by phorbol ester results in a rapid increase of adhesion to LM, which is still mediated by VLA-6. The enhanced adhesiveness is not associated with changes in beta 1 LM receptor expression, while it correlates with changes in the phosphorylation status of alpha 6 subunit. The expression of VLA-6 on NK cells and the modulation of its avidity by activating stimuli may be relevant for NK cell migration and tissue location during inflammation or immune response.

Further characterisation of the talin-binding site in the cytoskeletal protein vinculin

The cytoskeletal protein vinculin is a component of adherens-type junctions where it is one of a number of interacting proteins thought to link the cytoplasmic domain of adhesion receptors to F-actin. Vinculin has been shown to bind to at least three other cytoskeletal proteins, talin, paxillin and alpha-actinin. In this study, we further characterise the talin-binding domain in vinculin using a series of chick vinculin polypeptides expressed as glutathione-S-transferase fusion proteins in Escherichia coli. Thus 125I-talin bound to a fusion protein spanning residues 1–398, but not to those spanning residues 399–881 or 881–1066 in an SDS-PAGE gel-blot assay. We have previously characterised two chick vinculin cDNAs (2.89 kb cDNA and cVin5) which are identical in the region of overlap except that cVin5 lacks coding sequence for residues 167–207. Interestingly, a fusion protein spanning residues 1–398, but lacking residues 167–207, was unable to bind talin. However, further analysis showed that residues 167–207 are insufficient to support binding, and deletion of as few as 31 N-terminal residues abolished binding activity. The results of the gel-blot assay were essentially confirmed using purified fusion proteins adsorbed to glutathione-agarose beads. The smallest vinculin fusion protein able to bind talin contained residues 1–258. This fusion protein was as effective as whole vinculin in inhibiting the binding of 125I-vinculin to talin-coated microtitre wells. Interestingly, mutations which altered the charge characteristics of the highly conserved residues 178 and 181 abolished binding, whereas conservative substitutions were without effect. However, such mutations did not abolish the ability of mutant polypeptides spanning residues 1–398 to target to cell-matrix junctions in Cos cells. We have investigated the possible origin of the cDNA clone cVin5 by defining the structure of a 5′ portion of the chicken vinculin gene, and by analysing vinculin transcripts in a variety of adult tissues and embryonic fibroblasts using reverse transcriptase and polymerase chain reaction. Although residues 167–207 are encoded on a separate exon, we have been unable to identify a tissue where this exon is alternatively spliced.

Integrins and other adhesion molecules on lymphocytes from synovial fluid and peripheral blood of rheumatoid arthritis patients

Cell and matrix adhesion of lymphocytes participates in homing, migration and accumulation of these cells in inflamed tissues as well as in the generation of immune and inflammatory responses. In inflamed joints of rheumatoid arthritis (RA) patients, lymphocytes accumulate in the synovial membrane and the synovial fluid. In the present study we have analyzed the expression of integrins and other adhesion molecules in synovial fluid lymphocytes (RA-SFL) and paired peripheral blood lymphocytes (RA-PBL) from 21 RA patients by immunofluorescence flow cytometry. We have also investigated the expression of these adhesion molecules on peripheral blood lymphocytes obtained from 13 sex- and age-matched healthy controls (CO-PBL). RA-SFL, which consisted mostly of T cells, showed higher expression of the integrin subunits beta 1 (CD29), VLA-1 alpha, -3 alpha, -4 alpha, -5 alpha and -6 alpha when compared to RA-PBL. In turn, RA-PBL showed lower expression of these molecules than CO-PBL. The expression of the immunoglobulin-related molecules CD2, CD54 (ICAM-1) and CD58 (LFA-3) was higher on RA-SFL when compared to RA-PBL or CO-PBL, and similar results were obtained with the beta 2 integrin subunits CD11a and CD18. In contrast, L-selectin (LECAM-1) and ICAM-2 were expressed at much lower levels on RA-SFL than on RA-PBL or CO-PBL. CD44, a receptor for hyaluronic acid and collagen, was expressed by most RA-SFL, RA-PBL and CO-PBL cells but at higher density on RA-SFL. The results indicate that RA-SFL express a distinct array of adhesion molecules, similar to the one of memory T lymphocytes. This characteristic phenotype may contribute to the lymphocytic infiltration of the synovium and to the pathogenesis of RA.

The lipoxygenase metabolite 12(S)-HETE promotes alpha IIb beta 3 integrin-mediated tumor-cell spreading on fibronectin

Tumor-cell interaction with the vessel wall during metastasis involves adhesion, induction of endothelial-cell retraction and spreading on the exposed sub-endothelial matrix. The signals for initiation of tumor-cell spreading and the receptors involved are unknown. A protocol was developed to distinguish between initial tumor-cell (B16 amelanotic melanoma; B16a) adhesion to and spreading on fibronectin. The time for maximum spreading was 50 min. Treatment with a lipoxygenase metabolite of arachidonic acid [12(S)-HETE] resulted in maximum spreading in 15 min (max. effect approx. 0.1 microM). Other lipoxygenase metabolites were ineffective. 12(S)-HETE treatment induced a rearrangement of F-actin, vinculin, vimentin intermediate filaments and integrin alpha IIb beta 3, but not integrin alpha 5 beta 1. Antibodies to alpha IIb beta 3 but not alpha 5 beta 1 blocked the 12(S)-HETE effect on B16a spreading. B16a-cell attachment to fibronectin resulted in increased metabolism of arachidonic acid to 12(S)-HETE, which was inhibited by lipoxygenase but not by cyclo-oxygenase inhibitors. Accordingly, lipoxygenase inhibitors but not cyclo-oxygenase inhibitors blocked spontaneous B16a-cell spreading. The protein-kinase-C inhibitors calphostin C, H7 and staurosporine also inhibited spreading, while the protein-kinase-A inhibitor H8 was ineffective. These data suggest that B16a-cell spreading on fibronectin is initiated by a lipoxygenase metabolite [12(S)-HETE] of arachidonic acid and is mediated by protein kinase C.

Signal transduction by integrin receptors for extracellular matrix: cooperative processing of extracellular information

Adhesion receptors allow cells to interact with a dynamic and information-rich environment of extracellular matrix molecules. The integrin family of adhesion receptors transduces signals from the extracellular matrix that regulate growth, gene expression and differentiation, as well as cell shape, motility and cytoskeletal architecture. Recent data support the hypothesis that integrins transduce signals cooperatively with other classes of adhesion receptors or with growth factor receptors. Furthermore, the ability of integrins to interact with the cytoskeleton appears to be fundamental to their mechanism for signal transduction.

[Communication between stem cells and the hematopoietic microenvironment. Experimental data and models of interaction]

The aim of this review is to analyze the different ways by which stem cells and microenvironmental cells may interact. Stem cells are defined as immature cells that ensure the continuous renewal of blood cells. This small set of marrow cells comprises different kinds of cells, differing by their degree of maturity, their commitment, self-renewal ability and repopulating capacity. Microenvironmental cells are fixed marrow cells involved in stem cell survival, proliferation and differentiation. In vivo studies on the distribution within spleen or marrow, of stem cells injected to lethally irradiated mice, have suggested that cells of the microenvironment play a significant role in stem cell proliferation and differentiation. This role has been demonstrated using an in vitro model, i.e. the long-term marrow cultures as described in 1976 by M. Dexter. Analysis of stem cell maintenance in this culture system has made it possible to define the different means by which stromal cells and macrophages (the microenvironmental cells) may control stem cell behavior. Different molecules play a critical role: cytokines (growth factors and inhibitors), adhesion molecules (cell adhesion molecules and molecules belonging to the extracellular matrix) and eventually small peptides. It appears nowadays possible to materially represent the hemopoietic niche, whose existence was postulated by R. Schofield 10 years ago for theoretical reasons related to the the physiology of stem cells.

Expression and distribution of osteopontin in human tissues: widespread association with luminal epithelial surfaces

Osteopontin, a glycoprotein with a glycine-arginine-glycine-aspartate-serine (GRGDS) cell-binding domain, has been described in bone and is also known to be expressed in other organs, particularly kidney. The goal of the present work was to define the distribution of osteopontin synthesis and deposition in a wide variety of normal adult human tissues using a multifaceted approach that included immunohistochemistry, in situ hybridization, and Northern analysis. Immunohistochemical studies have revealed the unexpected finding that osteopontin is deposited as a prominent layer at the luminal surfaces of specific populations of epithelial cells of the gastrointestinal tract, gall bladder, pancreas, urinary and reproductive tracts, lung, breast, salivary glands, and sweat glands. Northern analyses identified gallbladder as a major site of osteopontin gene transcription comparable in magnitude with that of kidney, and immunoblotting identified osteopontin in bile. In situ hybridization localized osteopontin gene transcripts predominantly to the epithelium of a variety of organs as well as to ganglion cells of bowel wall. Osteopontin of epithelial cell origin, like bone-derived osteopontin, promoted GRGDS-dependent cell spreading in attachment assays. We postulate that osteopontin secreted by epithelium binds integrins on luminal surfaces. Collectively, these findings suggest an important role for osteopontin on many luminal epithelial surfaces communicating with the external environment.

Variant choriocarcinoma (BeWo) cells that differ in adhesion and migration on fibronectin display conserved patterns of integrin expression

Starting from the BeWo choriocarcinoma cell line, two stable variant cell lines (epi and lc) were isolated. Epi cells displayed an epithelioid colony morphology while lc were fibroblastoid. lc cells attached and spread on fibronectin-coated surfaces at significantly lower density of fibronectin than epi or the parent cell line. lc also migrated more efficiently to fibronectin in a trans-filter assay than either epi or parent cells. Integrin expression by the cell lines was investigated by flow cytometry and immunoprecipitation from surface-labelled cells with a panel of subunit-specific antibodies. Integrins alpha 2 beta 1, alpha 5 beta 1, alpha v beta 1 and alpha 6 beta 4 were detected in each case, and levels of expression were identical in the two variant lines. Anti-functional antibodies were used to probe the role of integrins in fibronectin- and vitronectin-mediated adhesion. Complete inhibition of adhesion to fibronectin was observed with anti-beta 1 antibody, and partial inhibition with anti-alpha 5, suggesting that integrin alpha 5 beta 1 is mainly responsible for the interaction. Adhesion to vitronectin was inhibitable using anti-alpha v and anti-beta 1 antibodies, suggesting that integrin alpha v beta 1 is active in these cells as a vitronectin receptor. There was a correlation between the altered morphology of the variant cells and alterations in the distribution of integrin alpha 6 beta 4 and laminin in monolayer cultures. The results support the idea that fibronectin may mediate the migratory behaviour of extravillous trophoblast in vivo. Switch to a more migratory phenotype may be mediated by the selective activation of integrins and altered interaction with basement membrane.

Modulators of neuronal migration and neurite growth

A multitude of molecules have been identified over the past few years that promote neurite outgrowth in vitro. The concept that these molecules work mainly by providing an adhesive surface for neuronal growth cones has been challenged by evidence from recent experiments. Some of the substrate molecules have diverse actions on cell migration and neurite growth. In addition, there is now evidence that there are molecules that specifically inhibit growth cone locomotion. This has given rise to the hypothesis that growth cones integrate a variety of growth-promoting and inhibitory signals and translate them into directed locomotion.

The junction between cytokines and cell adhesion

Several aspects of the interactions between growth factors and cell adhesion are described. Recent advances in the field come from the identification of molecules resembling growth factors or growth factor receptors, which bear cell adhesion motifs as well as molecules participating in both cell growth control and adhesion.

Cellular mechanisms of rejection

Studies by molecular biologists, protein chemists and cell biologists are rapidly providing new tools and information for those interested in the cellular mechanisms of graft rejection. Despite these contributions, a clear picture of the mechanisms involved in rejection has not yet evolved. However, whole new areas for research have developed, providing opportunities for new insights as well as therapeutic interventions.

Morphoregulatory activities of E-cadherin and beta-1 integrins in colorectal tumour cells

The cadherin family of adhesion molecules are prime mediators of cell-cell interactions while the integrins predominantly mediate cell-matrix and to a lesser extent cell-cell binding specificity. We have recently shown that a human colon carcinoma cell line (SW1222) organizes into glandular structures, with well defined polarity when cultured in three-dimensional type I collagen gel. The current study indicates that SW1222 cells display high levels of E-cadherin (E-cd, epithelial cadherin) by western blotting and immunohistochemical staining. A monoclonal antibody (HECD-1) specific for human E-cd blocks cell-cell adhesion (100%) and inhibits (up to 75%) the glandular differentiation of SW1222 cells growing in collagen gel. Furthermore the anti-beta 1 integrin monoclonal antibody (mAb13) inhibits the glandular differentiation of SW1222 cells (61%) and their cellular binding to type I collagen (60%). However, no significant inhibition of cell-cell adhesion was demonstrated using mAb13 nor the anti-carcinoembryonic antigen monoclonal antibody (PR3B10). These results are consistent with E-cd being a cell-cell adhesion molecule expressed by SW1222 cells. These data indicate that E-cd and beta 1 integrins mediate cell-cell and cell-collagen interactions required for the induction and maintenance of the glandular differentiation of colorectal tumour cells. Thus the down-regulation or loss of E-cd and beta 1 integrins seen in poorly differentiated colorectal tumours may represent one of the abnormalities underlying their progression towards an undifferentiated phenotype in vivo.

Adhesion cascades: diversity through combinatorial strategies

Cell adhesion occurs via a highly regulated set of sequential interactions. Prototypic components of a variety of adhesion cascades are discussed, including integrins, triggering molecules and lectin-carbohydrate interactions. Selectivity and efficiency are achieved by utilizing the right combination of multiple adhesion molecules and by their coordinated biochemical regulation.

Inside-out integrin signalling

Integrins are expressed by virtually all cells and play key roles in a range of cellular processes. Changes in the integrin surface repertoire provide a means of altering the strength and ligand preferences of cell adhesion. Recent research has examined the affinity modulation of integrins, a rapid and versatile mechanism of cell adhesion regulation. Studies with a prototype, alpha IIb beta 3, indicate that intracellular events influence the conformation and ligand-binding affinity of the extracellular domain of integrins. This 'inside-out' signal transduction appears to be mediated through the integrin cytoplasmic domains. In addition, in some cases affinity modulation of integrins may be cell-type specific. The clarification of the mechanisms of integrin affinity modulation should help explain rapid changes in cell adhesion that occur during cell migration, aggregation and the cell cycle.

Structure, function and evolutionary relationship of proteins containing a disintegrin domain

Disintegrins are soluble integrin ligands from snake venoms that disrupt cell-matrix interactions. Recently, the nuclear magnetic resonance structures of two disintegrins were determined, providing provocative molecular insight into how a disintegrin may engage an integrin. In addition, it has recently been realized that disintegrins are derived from larger multifunctional proteins, and that there is a family of membrane-anchored, disintegrin domain-containing proteins that may promote important cell-cell interactions.

Molecular cloning and functional expression of the second mouse nm23/NDP kinase gene, nm23-M2

A new murine cDNA of nm23/NDP kinase was isolated. A RT-PCR product was obtained from the normal mouse liver mRNA with primers designed for the human nm23-H2 gene. The product was used as a probe to screen a cDNA library from the murine melanoma cell line, B16, and two clones containing the entire open reading frame were obtained. It was predicted that the DNA sequence encoded 152 amino acids which was 98% identical to the nm23-H2 protein. The entire nm23-M1 and -M2 gene-coding regions were translated as fusion proteins with a glutathione S-transferase. These fusion proteins displayed NDP kinase activities.

A GTPase controls cell-substrate adhesion in Xenopus XTC fibroblasts

Cell-substrate adhesion is crucial at various stages of development and for the maintenance of normal tissues. Little is known about the regulation of these adhesive interactions. To investigate the role of GTPases in the control of cell morphology and cell-substrate adhesion we have injected guanine nucleotide analogs into Xenopus XTC fibroblasts. Injection of GTP gamma S inhibited ruffling and increased spreading, suggesting an increase in adhesion. To further investigate this, we made use of GRGDSP, a peptide which inhibits binding of integrins to vitronectin and fibronectin. XTC fibroblasts injected with non-hydrolyzable analogs of GTP took much more time to round up than mock-injected cells in response to treatment with GRGDSP, while GDP beta S-injected cells rounded up in less time than controls. Injection with GTP gamma S did not inhibit cell rounding induced by trypsin however, showing that cell contractility is not significantly affected by the activation of GTPases. These data provide evidence for the existence of a GTPase which can control cell-substrate adhesion from the cytoplasm. Treatment of XTC fibroblasts with the phorbol ester 12-o-tetradecanoylphorbol-13-acetate reduced cell spreading and accelerated cell rounding in response to GRGDSP, which is essentially opposite to the effect exerted by non-hydrolyzable GTP analogs. These results suggest the existence of at least two distinct pathways controlling cell-substrate adhesion in XTC fibroblasts, one depending on a GTPase and another one involving protein kinase C.

Signal transduction by the platelet integrin alpha IIb beta 3: induction of calcium oscillations required for protein-tyrosine phosphorylation and ligand-induced spreading of stably transfected cells

We demonstrate an example of signal transduction by an integrin and have begun to define the pathway through which this signaling is achieved. We constructed a stably transfected derivative of 293 cells (ATCC 1573) that expresses the platelet integrin GPIIbIIIa (alpha IIb beta 3). This cell line, clone B, adheres to and spreads on fibrinogen, a ligand for alpha IIb beta 3, while the parent cell line does not. Stimulation of these cells either by adhesion to fibrinogen or with antiserum directed against alpha IIb beta 3 results in induction of calcium oscillations, followed by tyrosine phosphorylation of at least one protein of molecular weight approximately 125 kDa. We establish that this phosphorylation, as well as the morphological rearrangements, requires the mobilization of calcium.

Transforming growth factor-beta control of cell-substratum adhesion during avian neural crest cell emigration in vitro

It has been proposed that, in higher vertebrates, the onset of neural crest cell migration from the neural tube involves spatially and temporally coordinated changes in cellular adhesiveness that are under the control of external signals released in the extracellular milieu by neighboring tissues. In the present study, we have analyzed the dynamics of changes in cell-substratum adhesiveness during crest cell emigration and searched for regulatory cues using an in vitro model system. This model is based on the fact that, in vivo, crest cell dispersion occurs gradually along a rostrocaudal wave, allowing us to explant portions of the neural axis, termed migratory and premigratory levels, that differ in the time in culture at which neural crest cells initiate migration and in the locomotory behavior of the cells. We found that neural crest cell emigration is not triggered by the main extracellular matrix molecules present in the migratory pathways, as none of these molecules could abolish the intrinsic difference in the timing of emigration between the different axial levels. Using an in vitro adhesion assay, we found that presumptive neural crest cells from premigratory level explants gradually acquired the ability to respond to extracellular matrix material with time in culture, suggesting that acquisition of appropriate, functional integrin receptors was a necessary step for migration. Finally, we showed that members of the transforming growth factor-beta family reduced in a dose-dependent manner the delay of neural crest cell emigration from premigratory level explants and were able to increase significantly the substratum-adhesion properties of crest cells. Our results suggest that acquisition of substratum adhesion by presumptive neural crest cells is a key event during their dispersion from the neural tube in vitro, and that members of the transforming growth factor-beta family may act as potent inducers of crest cell emigration, possibly by increasing the substratum adhesion of the cells.

Cloning and expression of mouse integrin beta p(beta 7): a functional role in Peyer's patch-specific lymphocyte homing

Lymphocytes express integrin receptors, termed lymphocyte Peyer's patch high endothelial venule (HEV) adhesion molecules (LPAMs), that mediate their organ-specific adhesion to specialized HEVs found in mucosal lymphoid organs (Peyer's patches). LPAM-1 consists of a murine integrin alpha 4 noncovalently associated with integrin beta p. Here, we describe the cloning and expression of a mouse cDNA encoding beta p, which is an 806-amino acid transmembrane glycoprotein. The genomic Southern blot analysis indicates that beta p is the murine homologue of human beta 7. The function of alpha 4 beta 7 as a Peyer's patch-specific adhesion molecule was tested directly by expression of the murine beta 7 cDNA in an alpha 4+ beta 7-B-cell line or coexpression of the alpha 4 and beta 7 cDNAs in an alpha 4-beta 7-T-cell line. The transfected cells exhibited a new Peyer's patch-specific adhesive phenotype that could be specifically blocked by monoclonal antibodies against alpha 4 and beta 7. Moreover, an anti-beta 7 monoclonal antibody specifically blocked binding of normal lymphocytes to Peyer's patch HEV but did not inhibit their binding to peripheral lymph node HEVs, indicating that beta 7 is a unique component of the Peyer's patch-specific homing receptor.

Regulation of focal adhesion-associated protein tyrosine kinase by both cellular adhesion and oncogenic transformation

Increasing evidence indicates that the integrin family of cell adhesion receptors can transduce biochemical signals from the extracellular matrix to the cell interior to modulate cell growth and differentiation. We have shown that integrin/ligand interactions can trigger tyrosine phosphorylation of a protein of M(r) 120,000 (pp120), so it is possible that signal transduction by integrins might involve activation of intracellular protein tyrosine kinases as an early event in cell binding to the extracellular matrix. Here we report that pp120 is identical to the focal adhesion-associated protein tyrosine kinase pp125FAK (refs 3, 4). We show that tyrosine phosphorylation of this protein is modulated both by cell adhesion and transformation by pp60v-src, and that these changes in phosphorylation are correlated with increased pp125FAK tyrosine kinase activity. A model is proposed to relate these findings to the molecular basis of anchorage-independent growth of transformed cells.

Dichotomy in the laminin-binding properties of soluble and membrane-bound human galactoside-binding protein

Recent studies indicate that galactoside-binding proteins may bind the poly-N-acetyllactosamine sequences of laminin. We questioned whether human galactoside-binding protein (hL-31) binds to laminin and whether cells that express hL-31 on their surface use it as a laminin receptor to promote cellular attachment. The data show that both lectin and cells bind to immobilized laminin. The binding of soluble lectin to laminin is inhibited by lactose, while cell adhesion to it is not. The results indicate that laminin may be a ligand for soluble galactoside-binding proteins.

The Walter Herbert Lecture. Control of cell motility and tumour invasion by extracellular matrix interactions

Integrins are heterodimeric transmembrane proteins with large ectodomains and a short cytoplasmic tail inside the cell. They mediate cell adhesion to extracellular matrix proteins and to the surfaces of other cells. In many cases the sequence recognised by the integrins in the extracellular matrix proteins is the tripeptide Arg-Gly-Asp (RGD). Short synthetic peptides containing this sequence can inhibit invasion in vitro and tumour dissemination in vivo. Thus, the alpha 5 beta 1 fibronectin binding integrin appears to be the key integrin in the invasion of at least melanoma, osteosarcoma and glioblastoma cells. Modulation of the level and activities of this integrin can suppress invasion, whereas the alpha v beta 3 vitronectin binding integrin appears to be associated with increased invasiveness. There is increasing evidence that some of these effects are mediated through signals elicited by the binding of integrins to their target proteins. This possibility has generated a great deal of interest in the cytoplasmic molecules that might mediate the integrin-associated signalling.

Specificity of cell adhesion in development: the cadherin superfamily

Cadherins are major cell-surface receptors involved in specific cell adhesion during development. Recent results reveal the existence of a growing array of related molecules involved in various forms of cell-cell adhesion, including that mediated by desmosomes. Comparisons with other families of adhesion receptors suggest testable models for functions of the emerging cadherin superfamily in development and disease.

Differential expression of cell surface integrins on human mast cells and human basophils

Integrins are multifunctional recognition molecules and are expressed on various hematopoietic cells. In the present study expression of integrins on the cell surface of human mast cells and human basophils was investigated by using monoclonal antibodies (mAbs) and indirect immunofluorescence. Human mast cells were obtained from lung (n = 5), uterus (n = 5) and skin (n = 4). Human blood basophils were obtained from normal donors (n = 2). In addition, HMC-1 cells (human mast cell line) and KU-812 cells (a basophil cell line) were analyzed. Primary mast cells were found to react with mAbs directed against the common beta chain of beta 1 integrins (CD 29), the alpha chain of VLA-4 (CD 49 d) and VLA-5 (CD 49 e), the beta chain of beta 3 integrins (CD 61), and the alpha chain of the vitronectin receptor (VNR) (CD 51). Mast cells were not recognized by mAbs to beta 2 integrins (CD 18, CD 11 a, CD 11 b, CD 11 c), the alpha chain of VLA-2 (CD 49 b), and VLA-6 (CD 49 f). No differences in expression of integrins on human mast cells obtained from different organs were found. HMC-1 cells and primary mast cells expressed an almost identical pattern of integrins. Human basophils and KU-812 cells were found to react with mAbs directed against beta 1-integrins (CD 29, CD 49 b, CD 49 d, CD 49 e) and beta 2-integrins (CD 18, CD 11 a, CD 11 b, CD 11 c). Together, mast cells and blood basophils express a unique pattern of integrins. These cell surface structures may be involved in the distribution of basophils and tissue mast cells and their accumulation and function in inflammed tissues.

A reticulocyte-binding protein complex of Plasmodium vivax merozoites

Plasmodium vivax merozoites primarily invade reticulocytes. The basis of this restricted host cell preference has been debated. Here we introduce two novel P. vivax proteins that comigrate on reducing SDS-polyacrylamide gels, colocalize at the apical pole of merozoites, and adhere specifically to reticulocytes. The genes encoding these proteins, P. vivax reticulocyte-binding proteins 1 and 2 (PvRBP-1 and PvRBP-2), have been cloned and analyzed. Homologous genes are evident in the closely related simian malaria parasite, P. cynomolgi, which also prefers to invade reticulocytes, but are not evident in the genome of another related simian malaria parasite, P. knowlesi, which invades all red blood cell subpopulations. Native PvRBP-1 is likely a transmembrane-anchored disulfide-linked protein, and along with PvRBP-2 may function as an adhesive protein complex. We propose that the RBPs of P. vivax, and homologous proteins of P. cynomolgi, function to target the reticulocyte subpopulation of red blood cells for invasion.

Morphology of cell-substratum adhesion. Influence of receptor heterogeneity and nonspecific forces

Many cell types modulate growth, differentiation, and motility through changes in cell substrate adhesion, including regulation of focal contact formation. Clustering of cell surface adhesion receptors is an essential early step in the development of focal contacts, and thus may influence cell physiology. In this paper, we present a theoretical framework to examine how cell surface chemistry affects receptor clustering. Our one-dimensional tape-peeling model couples the equations of mechanical equilibrium for a cell membrane with kinetic receptor-ligand binding relations. We considered two distinct model scenarios: Adhesion mediated by multiple receptor-ligand interactions of different length and specific binding of a single receptor type occurs in the presence of van der Waals attraction and nonspecific repulsion. In each case, nonuniform (wave-like) membrane morphologies are observed in certain parameter ranges that support the clustering of adhesion receptors. The formation of these morphologies is described in terms of a balance of membrane stresses; when cell-surface potential as a function of separation distance is symmetric between two potential energy minima, nonuniform morphologies are obtained. Increases in the chemical binding energy between receptor and ligand (e.g., increases in ligand density) or decreases in the membrane rigidity result in smaller wavelengths for nonuniform interfaces. Additionally, we show wave-like geometries appear only when the mechanical compliance of receptor-ligand bonds is within an intermediate range, and examine how the mobility of "repellers"--glycocalyx molecules that exert a nonspecific repulsive force--influences membrane morphology. We find fully mobile repellers always redistribute to prevent nonuniform morphologies.

Colocalization of thrombospondin and syndecan during murine development

Thrombospondin is an adhesive glycoprotein that is thought to play a role in tissue genesis and repair. We have used a monoclonal anti-thrombospondin antibody, designated 5G11, to localize thrombospondin in paraformaldehyde fixed, paraffin-embedded sections of developing mouse embryos. Thrombospondin expression is observed in uterine smooth muscle, endometrial glands, the decidua, and trophoblastic giant cells during the initial phase of post-implantation development in the embryo. Cardiac myocytes and neuroepithelial cells show positive staining for thrombospondin at day 8.5 of gestation, and this expression continues throughout the development of the myocardium and central nervous system. Strong staining for thrombospondin is seen in developing bone and in the liver. Thrombospondin is also observed in developing smooth muscle and skeletal muscle, as well as in a variety of epithelia, including the epidermis, small intestinal epithelium, lens epithelium, renal tubular epithelium, and the epithelium of the developing tooth. Comparison of thrombospondin staining with that of two known cell surface receptors for thrombospondin, syndecan and the vitronectin receptor, reveals remarkable colocalization of thrombospondin and syndecan in all tissues, but almost no coexpression with the vitronectin receptor. Coexpression of thrombospondin and syndecan may play an important role in cell-cell or cell-matrix interactions during development.