Integrins: versatility, modulation, and signaling in cell adhesion

Ligand-dependent and -independent integrin focal contact localization: the role of the alpha chain cytoplasmic domain

Many integrin receptors localize to focal contact sites upon binding their ligand. However, unoccupied integrin receptors do not localize to focal contact sites. Because the integrin beta 1 cytoplasmic domain appears to have a focal contact localization signal, there must be a mechanism by which this domain is kept inactive in the unoccupied state and becomes exposed or activated in the occupied receptor. We considered that this mechanism involves the alpha subunit cytoplasmic domain. To test this hypothesis, we have established two NIH 3T3 cell lines that express either the human alpha 1 wild-type subunit (HA1 cells) or the cytoplasmic domain deleted alpha 1 subunit (CYT cells). Both cell lines express similar levels of the human alpha 1 subunit, and there is no significant effect of the deletion on the dimerization and surface expression of the receptor. Furthermore, the deletion had no effect on the binding or adhesion via alpha 1 beta 1 to its ligand collagen IV. However, when these two cell lines are plated on fibronectin (FN), which is a ligand for alpha 5 beta 1 but not for alpha 1 beta 1, there is a striking difference in the cellular localization of alpha 1 beta 1. The HA1 cells show only alpha 5 in focal contacts, without alpha 1, demonstrating that all of the integrin localization is ligand dependent. In contrast, when the CYT cells are plated on FN, the mutant alpha 1 appears in focal contacts along with the alpha 5/beta 1. Thus, there is both ligand-dependent (alpha 5/beta 1) and ligand-independent (alpha 1/beta 1) focal contact localization in these cells. The truncated alpha 1 also localized to focal contacts in a ligand-independent manner on vitronectin. We conclude that the mutant alpha 1 no longer requires ligand occupancy for focal contact localization. These data strongly suggest that the alpha cytoplasmic domain plays a role in the normal ligand-dependent integrin focal contact localization.

The Drosophila Stubble-stubbloid gene encodes an apparent transmembrane serine protease required for epithelial morphogenesis

The Stubble-stubbloid (Sb-sbd) gene is required for hormone-dependent epithelial morphogenesis of imaginal discs of Drosophila, including the formation of bristles, legs, and wings. The gene has been cloned by using Sb-sbd-associated DNA lesions in a 20-kilobase (kb) region of a 263-kb genomic walk. The region specifies an approximately 3.8-kb transcript that is induced by the steroid hormone 20-hydroxyecdysone in imaginal discs cultured in vitro. The conceptually translated protein is an apparent 786-residue type II transmembrane protein (N terminus in, C terminus out), including an intracellular N-terminal domain of at least 35 residues and an extracellular C-terminal trypsin-like serine protease domain of 244 residues. Sequence analyses indicate that the Sb-sbd-encoded protease could activate itself by proteolytic cleavage. Consistent with the cell-autonomous nature of the Sb-sbd bristle phenotype, a disulfide bond between cysteine residues in the noncatalytic N-terminal fragment and the C-terminal catalytic fragment could tether the protease to the membrane after activation. Both dominant Sb and recessive sbd mutations affect the organization of microfilament bundles during bristle morphogenesis. We propose that the Sb-sbd product has a dual function. (i) It acts through its proteolytic extracellular domain to detach imaginal disc cells from extracellular matrices, and (ii) it transmits an outside-to-inside signal to its intracellular domain to modify the cytoskeleton and facilitate cell shape changes underlying morphogenesis.

Dynamic aspects of adhesion receptor function--integrins both twist and shout

The recognition of extracellular molecules by cell surface receptors is the principal mechanism used by cells to sense their environment. Consequently, signals transduced as a result of these interactions make a major contribution to the regulation of cellular phenotype. Historically, particular emphasis has been placed on elucidating the intracellular consequences of growth factor and cytokine binding to cells. In addition to these interactions, however, cells are usually in intimate contact with a further source of complex structural and functional information, namely immobilised extracellular matrix and/or cell surface adhesion proteins. A key question in recent years has been whether cells use the myriad of adhesion protein-receptor interactions purely for structural and migratory function, or whether these interactions also make a more varied contribution to cell phenotype. Here we review dynamic aspects of the function of one major class of adhesion receptor, the integrins. In particular, we focus on the evidence for shape changes in integrin molecules, the mechanisms responsible for regulating ligand binding, and the signals transduced following integrin occupancy.

Outer membrane protein YadA of enteropathogenic yersiniae mediates specific binding to cellular but not plasma fibronectin

The binding of bacteria or bacterial products to host proteins of tissue extracellular matrix may be a mechanism of tissue adherence. We investigated interactions of the plasmid-encoded outer membrane protein YadA, which confers pathogenic functions on enteropathogenic yersiniae, with fibronectin. Attachment of YadA-positive and YadA-negative recombinant Yersinia enterocolitica strains to cartilage-derived human cellular fibronectin and human plasma fibronectin in the solid phase revealed that YadA mediates binding of yersiniae to cellular fibronectin in a saturable, concentration-dependent manner. The interaction could be inhibited by an anti-YadA-specific anti-serum. An anti-beta 1-integrin antibody and the synthetic peptide G-R-G-D-S-P, representing the binding site for alpha 5 beta 1-integrin on fibronectin, did not block attachment of YadA-positive yersiniae to cellular fibronectin, indicating a binding site for YadA on cellular fibronectin independent of the R-G-D-S-containing site. By contrast, YadA failed to mediate binding to plasma fibronectin immobilized on nitrocellulose or plastic surfaces. These observations provide evidence for the hypothesis that the binding region for YadA in cellular fibronectin is not present in plasma fibronectin. This study is the first report on differential binding of bacteria to splicing variants of fibronectin. Further experiments might answer the question whether binding of YadA to cellular fibronectin contributes to the pathogenesis of yersiniae, both to the initial adhesion of the bacteria to the matrices of the host and to the arthritogenic potential of enteropathogenic yersiniae.

Characterization of adhesive interactions between human endothelial cells and megakaryocytes

Cell-cell adhesion is essential for many immunological functions and is believed to be important in the regulation of hematopoiesis. Adhesive interactions between human endothelial cells and megakaryocytes were characterized in vitro using the CMK megakaryocytic cell line as well as marrow megakaryocytes. Although there was no adhesion between unactivated human umbilical vein endothelial cells (HUVEC) and megakaryocytes, treatment of HUVEC with inflammatory cytokines such as IL-1 beta, tumor necrosis factor alpha, INF-gamma, or the phorbol ester phorbol myristate acetate (PMA) resulted in a time- and dose-dependent increase in adhesion. Stimulation of marrow megakaryocytes or CMK cells with the cytokines IL-1 beta, GM-CSF, IL-6, IL-3, or PMA augmented their adhesion to endothelium. Monoclonal antibodies against the LFA-1 subunit of the leukocyte adherence complex CD18 inhibited the binding of marrow megakaryocytes or CMK cells to HUVEC. Adhesion blocking experiments also demonstrated that the VLA-4/VCAM-1 pathway was important for megakaryocyte attachment to HUVEC. Adhesion promoted maturation of megakaryocytic cells as measured by increased expression of glycoproteins GpIb and GpIIb/IIIa and by increased DNA content. These observations suggest that alterations in megakaryocyte adhesion may occur during inflammatory conditions, mediated by certain cytokines, resulting in augmented megakaryocyte maturation.

Adhesion molecules in cell interactions

During a successful immune response, several families of adhesion molecules participate in a cascade of binding events that lead to the binding of leukocytes, both to each other and to cell types such as the endothelium and epithelium. A central theme emerging from recent studies is that the function of an adhesion receptor cannot be inferred from its expression alone; rather, adhesion receptors are 'selected' to perform distinct effector functions based on their cell-background and factors present in the local microenvironment. Thus, adhesion receptors expressed on different cell-types may find themselves in different states of 'activation-readiness' and may be further selected by prevailing conditions in the microenvironment to bind tissue-specific ligands and mediate leukocyte effector functions such as homing or transendothelial migration.

Interaction of human malignant melanoma tumor spheroids with endothelium and reconstituted basement membrane: modulation by RGDS

Tumor-cell extravasation involves sequential adhesive interactions with vascular endothelium and the subendothelial basement membrane. We have established a 3-dimensional model in vitro to simulate these events and to elucidate targets of the anti-cell-adhesive synthetic peptide RGDS. Tumor spheroids of the melanoma cell line ST-ML-12 served as models of tumor-cell emboli and were transferred onto human umbilical vein endothelial cells. To imitate the vascular anatomy, the latter were grown on reconstituted basement membranes produced by dextran-stimulated bovine corneal endothelial cells. RGDS did not affect the homotypic aggregation of the tumor cells and only minimally inhibited the attachment of the spheroids to the reconstituted vessel. A short-term (20 min) inhibition of adhesion to denuded basement membranes was observed. The attachment was closely associated with damage to the endothelial cells by oxygen-derived free radicals. RGDS retarded endothelial injury for up to 3 hr. The most prominent effect was observed after penetration of the endothelium. RGDS suppressed the emigration of tumor cells from the attached tumor-cell cluster in a dose- and time-dependent fashion. After 12 hr, the inhibitory effect progressively declined. This was not due to loss of activity of the peptide, indicating a resistance mechanism in the melanoma cells. On purified components of the basement membrane, RGDS effectively inhibited the initial spheroid attachment to fibronectin and collagen IV but had no effect on attachment to laminin. By contrast, subsequent migration was significantly suppressed on all substrata. Our model permits the study of dynamic cell-cell and cell-extracellular-matrix interactions and indicates that RGDS might predominantly act on early tumor-cell locomotion after penetration of the endothelium.

On the crawling of animal cells

Cells crawl in response to external stimuli by extending and remodeling peripheral elastic lamellae in the direction of locomotion. The remodeling requires vectorial assembly of actin subunits into linear polymers at the lamella's leading edge and the crosslinking of the filaments by bifunctional gelation proteins. The disassembly of the crosslinked filaments into short fragments or monomeric subunits away from the leading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic signals elicited by sensory cues escort actin through this cycle in which filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory receptors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.

Regulation of integrin-mediated adhesion to laminin and collagen in human melanocytes and in non-metastatic and highly metastatic human melanoma cells

We compared integrin-mediated adhesion to extracellular matrix (ECM) components of cultured human melanocytes and 6 human melanoma cell lines with different metastatic capacities in nude mice. Cultured melanocytes and most melanoma cell lines adhered strongly to fibronectin (FN), whereas only highly metastatic cell lines adhered to laminin (LM), collagen type I (COI) and type IV (COIV). Adhesion to LM and CO could be blocked by anti-alpha 6 and anti-alpha 2 monoclonal antibodies (MAbs) respectively. This observation is consistent with the finding that expression of LM receptor alpha 6 beta 1 and LM/CO receptor alpha 2 beta 1 was low on melanocytes and non- or poorly metastatic cell lines, whereas these integrins were strongly expressed on highly metastatic cell lines. In addition, immunoprecipitation from [35S]-methionine-labeled cells demonstrated increased synthesis of alpha 6, alpha 2 and beta 1 in highly metastatic cell lines and immunohistochemistry showed expression of alpha 6 beta 1 and alpha 2 beta 1 only in xenograft lesions from highly metastatic cell lines. Furthermore, the observation that adhesion of melanocytes and non- or poorly metastatic cell lines could be stimulated with anti beta 1 MAbs demonstrates that these receptors, on these cells, are expressed in an inactive state. Our results suggest that alpha 2 beta 1 and alpha 6 beta 1 play a role in human melanoma metastasis in nude mice and demonstrate that interactions of these integrins with their ligands can be regulated at the level of surface expression and activation state of the receptor.

Recombinant entactin promotes mouse primary trophoblast cell adhesion and migration through the Arg-Gly-Asp (RGD) recognition sequence

In vitro culture of mouse blastocysts during the period coinciding with implantation has revealed that primary trophoblast cells can adhere and migrate in serum-free medium when provided with certain extracellular matrix components, including fibronectin and laminin. Tightly associated with laminin is the glycoprotein, entactin, that may play an important role in basement membrane assembly and cell attachment. Mouse blastocysts were studied using this in vitro model to determine whether entactin was capable of mediating trophoblast invasive activity. Although entactin has never been shown to promote cell migration, we report here that recombinant entactin supported blastocyst outgrowth in a dose-dependent manner, with a maximal effect at 20-50 micrograms/ml. The ability of trophoblast cells to adhere and migrate on entactin was specifically inhibited by anti-entactin antibody, but not by antibodies raised against laminin. The synthetic peptide, Gly-Arg-Gly-Asp-Ser-Pro, that contains the Arg-Gly-Asp (RGD) integrin recognition site, reversibly inhibited entactin-mediated blastocyst outgrowth in a dose-dependent manner, but had no effect on laminin-mediated outgrowth. The synthetic peptide, Gly-Phe-Arg-Gly-Asp-Gly-Gln, that comprises the actual RGD-containing sequence within entactin, promoted trophoblast outgrowth when immobilized on the substratum. Furthermore, a mutated recombinant entactin, altered to contain a Glu in place of Asp at the RGD site, provided no trophoblast cell adhesive activity. We conclude that entactin promotes trophoblast outgrowth through a mechanism mediated by the RGD recognition site, and that it may play an important role during invasion of the endometrial basement membrane at implantation.

Expression of chicken vinculin complements the adhesion-defective phenotype of a mutant mouse F9 embryonal carcinoma cell

A mutant cell line, derived from the mouse embryonal carcinoma cell line F9, is defective in cell-cell adhesion (compaction) and in cell-substrate adhesion. We have previously shown that neither uvomorulin (E-cadherin) nor integrins are responsible for the mutant phenotype (Calogero, A., M. Samuels, T. Darland, S. A. Edwards, R. Kemler, and E. D. Adamson. 1991. Dev. Biol. 146:499-508). Several cytoskeleton proteins were assayed and only vinculin was found to be absent in mutant (5.51) cells. A chicken vinculin expression vector was transfected into the 5.51 cells together with a neomycin-resistance vector. Clones that were adherent to the substrate were selected in medium containing G418. Two clones, 5.51Vin3 and Vin4, were analyzed by Nomarski differential interference contrast and laser confocal microscopy as well as by biochemical and molecular biological techniques. Both clones adhered well to substrates and both exhibited F-actin stress fibers with vinculin localized at stress fiber tips in focal contacts. This was in marked contrast to 5.51 parental cells, which had no stress fibers and no vinculin. The mutant and complemented F9 cell lines will be useful models for examining the complex interactions between cytoskeletal and cell adhesion proteins.

CD11b/CD18-dependent polymorphonuclear leucocyte interaction with matrix proteins in adhesion and migration

Adhesion of human polymorphonuclear leucocytes (PMN) stimulated with phorbol myristate acetate (PMA) to plastic dishes coated with the matrix proteins laminin (LM), fibronectin (FN), collagen type I (CI) or collagen type IV (CIV) was inhibited by the monoclonal antibody 60.3 (MoAb 60.3; anti-CD18). The highest inhibitory effect was seen on adhesion to CI. PMN adhesion to CI was also effectively inhibited by Mo1 (anti-CD11b) but this antibody had only a minor effect on attachment of PMN to the other matrix proteins. In other experiments MoAb 60.3 inhibited LTB4-induced migration of PMN through polycarbonate filters (3 microns pores) coated with LM, FN, CI or CIV, with the most pronounced effect on migration through those filters coated with CI. By contrast, the antibody Mo1 had no effect on migration through any of the protein-coated filters tested. The results in this study suggest that the CD18 epitope, recognized by 60.3, mediates both adhesion and migration of PMN while the epitope on CD11b recognized by the antibody Mo1 is restricted to adhesion. The results also indicate that CD11b/CD18 is the major receptor on human PMN for CI while interaction with LM, FN and CIV may in addition involve other mechanisms.

Integrin receptor-mediated mobilisation of intranuclear calcium in rat osteoclasts

Cell-matrix interactions have been shown to play an important role in regulating cell function and behaviour. In bone, where calcified matrix formation and resorption events are required to be in dynamic equilibrium, regulation of adhesive interactions between bone cells and their matrix is critical. The present study focuses on the osteoclast, the bone resorbing cell, as well as integrins, which are cell surface adhesion receptors that mediate osteoclast attachment to bone matrix. In osteoclasts, the most abundant integrin receptor is the vitronectin receptor (VNR, alpha v beta 3). The objective of the study was to investigate changes in intracellular calcium, a regulator of osteoclast function, following addition of peptides that bind integrins, in particular the alpha v beta 3 form of the vitronectin receptor (VNR), which is highly expressed in osteoclasts. The study demonstrated a unique spatial localisation of the calcium signal in response to cell membrane receptor occupancy by integrin ligands in rat osteoclasts. Addition of peptides with the Arg-Gly-Asp (RGD) sequence such as BSP-IIA, GRGDSP and GRGDS to rat osteoclasts evoked an immediate increase in free calcium ion concentration [Ca2+]i, localised to the nuclei and to the thin cytoplasmic skirt. These responses were inhibited by F11, a monoclonal antibody to the rat integrin beta 3 chain, as well as echistatin, a snake venom shown to colocalise with the alpha v chain in osteoclasts, suggesting that the calcium signal is mediated by the alpha v beta 3 form of VNR.(ABSTRACT TRUNCATED AT 250 WORDS)

Adhesion to fibronectin stimulates inositol lipid synthesis and enhances PDGF-induced inositol lipid breakdown

The aim of these experiments was to investigate whether inositol lipids might mediate some of the effects of extracellular matrix (ECM) on cellular form and functions. The lipid phosphatidylinositol bisphosphate (PIP2) plays a role in cytoskeletal regulation while its hydrolysis products, diacylglycerol and inositol triphosphate, serve as second messengers. We therefore measured the effect of adhesion to fibronectin (FN) on PIP2 and its hydrolysis products, in the presence and absence of the soluble mitogen PDGF. PDGF induced a threefold increase in release of water-soluble inositol phosphates in C3H 10T1/2 fibroblasts when cells were attached to FN, but had little effect in suspended cells. Suppression of inositol phosphate release in unattached cells was not due to dysfunction of the PDGF receptor or failure to activate phospholipase C-gamma; PDGF induced similar tyrosine phosphorylation of PLC-gamma under both conditions. By contrast, the total mass of phosphatidylinositol bisphosphate (PIP2), the substrate for PLC-gamma, was found to decrease by approximately 80% when cells were detached from their ECM attachments and placed in suspension in the absence of PDGF. PIP2 levels were restored when suspended cells were replated on FN, demonstrating that the effect was reversible. Furthermore, a dramatic increase in synthesis of PIP2 could be measured in cells within 2 min after reattachment to FN in the absence of PDGF. These results show that FN acts directly to stimulate PIP2 synthesis, and that it also enhances PIP2 hydrolysis in response to PDGF. The increase in PIP2 induced by adhesion may mediate some of the known effects of FN on cell shape and cytoskeletal organization, while regulation of inositol lipid hydrolysis may provide a means for integrating hormone- and ECM-dependent signaling pathways.

Regulation of E-cadherin-mediated adhesion by muscarinic acetylcholine receptors in small cell lung carcinoma

We present the first evidence that adhesion mediated by a member of the cadherin gene family can be regulated by a G protein-coupled receptor. We show that activating the M3 muscarinic acetylcholine receptor (mAChR) rapidly induces E-cadherin-mediated adhesion in a small cell lung carcinoma (SCLC) cell line. This response is inhibited by E-cadherin antibodies, and does not occur in another SCLC cell line which expresses functional mAChR but reduced levels of E-cadherin. Protein kinase C may be involved, since phorbol 12-myristate 13-acetate also induces E-cadherin-mediated aggregation. Immunofluorescence analyses indicate that mAChR activation does not grossly alter E-cadherin surface expression or localization at areas of cell-cell contact, suggesting mAChR activation may increase E-cadherin binding activity. Our findings suggest that G protein-coupled receptors may regulate processes involving cadherin-mediated adhesion, such as embryonic development, neurogenesis, and cancer metastasis.

A 30 kD sulfated extracellular matrix protein immunologically crossreactive with vitronectin

In this study we describe a human sulfated 30 kD protein (sp30) that is recognized by a monoclonal antibody raised against human vitronectin (mAb 8E6). Another monoclonal antibody raised against human vitronectin, mAb MaSp, and a polyclonal antiserum against vitronectin did not react detectably with sp30. Sp30, unlike vitronectin, is synthesized by a variety of non-hepatic human cell lines in culture, including cells of lymphoid origin. It is synthesized in sulfated form as indicated by metabolic labeling of MG-63 human osteosarcoma cells with 35SO4. Sp30 is an extracellular matrix protein as indicated by its association with the matrix of MG-63 cells after removal of the cells with EDTA and its fibrillar pattern by immunofluorescence of non-permeabilized confluent MG-63 cell monolayers detected with mAb 8E6. This antibody also stained short fibrils in human embryonic tissue. This pattern was distinct from the fainter diffuse staining obtained with mAb MaSp and the polyclonal antiserum to vitronectin, suggesting that the 8E6 staining in embryonic tissues was mostly due to sp30 rather than vitronectin. A polyclonal antiserum against bovine microfibril associated glycoprotein (MAGP) precipitated a [35SO4]-30 kD protein from [35SO4]-labeled MG-63 medium that co-migrated with a band precipitated by mAb 8E6. Double-labeling immunofluorescence studies of embryonic tissues showed an identical distribution of anti-bovine MAGP antiserum and mAb 8E6 staining. These data indicate that sp30 is the human homolog of bovine MAGP. Distinction between sp30 and vitronectin will be important in ascertaining the localization and function of both proteins. The findings that sp30 is sulfated and synthesized and secreted by a variety of cells in culture should aid in defining its role in microfibrillogenesis. That sp30 is secreted by cells of lymphoid origin suggests that it might also have a heretofore unsuspected role in immune responses.

CR3 (CD11b/CD18) expressed by cytotoxic T cells and natural killer cells is upregulated in a manner similar to neutrophil CR3 following stimulation with various activating agents

CR3 (CD11b/CD18) functions both as an iC3b-receptor and as an adhesion molecule for cellular ligands such as ICAM-1. Although CR3 has been well characterized on phagocytic cells, much less is known about CR3 on lymphocytes. In this study, the expression of CR3 was examined on resting and stimulated B, T, and natural killer (NK) cells by three-color flow cytometry. Biotinylated anti-CR3 mAb and streptavidin-FITC were used in combination with anti-CD3 mAb conjugated with peridinin chlorophyll-alpha protein (PerCP) and phycoerythrin-labeled mAbs to CD4, CD8, CD19, or CD56. Among resting lymphocytes, CR3 was expressed on nearly all NK cells (CD56+CD3-), 1% of CD4+CD3+ helper T cells, 7% of CD8+CD3+ cytotoxic T cells, and 20% of B cells (CD19+). Among the 5% of T cells (CD3+) expressing CR3, the majority was CD56+. Incubation of PBMC for 30 min with PMA induced a three- to fivefold increase in CR3 expression on NK cells and a twofold increase on T cells but did not change the expression of CR3 on B cells. This effect of PMA was not blocked by the presence of cycloheximide, suggesting the presence of cytoplasmic (granule) stores of CR3 in these lymphoid cells resembling those previously reported in neutrophils and monocytes. When PBMC were incubated with rIFN-alpha, rIL-2, beta-glucan, or high concentrations of LPS, expression of CR3 on NK cells increased significantly, but > or = 4 hr of stimulation was required. Other cytokines (rIFN-gamma, rIL-1, rIL-4, rIL-6, TNF-alpha) and rC5a had no significant effect on CR3 expression. Among NK cells, both the CD56bright and the CD56dim cells expressed CR3, and the expression of CR3 on both of these NK cell subsets was increased in a similar manner by PMA. However, rIL-2 stimulated a greater increase in CR3 expression on CD56bright cells than on CD56dim cells. These studies suggest that CR3 expressed by NK cells or cytotoxic T cells resembles phagocyte CR3 in that cellular activation stimulates increased surface expression of CR3 derived from cytoplasmic reserves of the receptor.

5-Bromo-2-deoxyuridine regulates invasiveness and expression of integrins and matrix-degrading proteinases in a differentiated hamster melanoma cell

Cell interactions with the extracellular matrix play a critical role in regulating complex processes such as terminal differentiation and tumor progression. In these studies we describe a melanoma cell system that should be useful in addressing the regulation of cell-matrix interactions and the roles they play in regulating differentiation and cell invasiveness. CS (suspension)-1 melanoma cells are relatively well differentiated: they are melanotic, responsive to melanocyte-stimulating hormone, and express TA99, a melanosome membrane differentiation marker. Their repertoire of integrin receptors for extracellular matrix ligands is limited; in particular, they lack receptors for vitronectin, accounting for the observation that they are nonadherent when cultured in the presence of serum. CS-1 cells are noninvasive as well, and express low levels of both metalloproteinases and activated plasminogen activators. Treatment of these cells with melanocyte-stimulating hormone causes them to increase melanin production and assume an arborized phenotype, suggesting that it promotes their further differentiation. In contrast, treatment of CS-1 with the thymidine analog 5-bromodeoxyuridine, converts them to a highly invasive cell population (termed BCS-1) that loses its differentiated properties and responsiveness to melanocyte-stimulating hormone, acquires a broad integrin repertoire (including vitronectin receptors), and expresses elevated levels of metalloproteinases and activated urokinase. From these observations and findings of others on BrdU treatment of other developmental lineages, we hypothesize that BrdU both suppresses differentiation and promotes invasiveness of CS-1 cells. The demonstrated manipulability of CS-1 cells should make them extremely useful for studying the regulation of both terminal differentiation and tumor progression in the melanocyte lineage.

Variants of the alpha 6 beta 1 laminin receptor in early murine development: distribution, molecular cloning and chromosomal localization of the mouse integrin alpha 6 subunit

Laminin (A:B1:B2) is a major component of the first basement membrane to appear in the developing mouse embryo. Its effects on morphogenesis and differentiation are mediated by interaction with cell surface receptors that are members of the integrin family. We have studied the expression of the alpha 6 subunit of murine alpha 6 beta 1 and its ligand, laminin, in preimplantation mouse embryos, embryo outgrowths and in embryonic stem (ES) cells and embryonal carcinoma (EC) cells. The alpha 6 subunit is present in the oocyte and throughout preimplantation development. Laminin A chain appears later than alpha 6 and has a more restricted distribution until the late blastocyst stage. alpha 6 beta 1 is strongly expressed in ES and EC cells; the levels of mRNA expression are not altered by differentiation. Molecular cloning of cDNA for the murine integrin alpha 6 subunit from a mammary gland lambda gt11 library showed, as in man, an open reading frame encoding two variants of alpha 6, alpha 6A and alpha 6B. The identity of the alpha 6 amino acid sequence to that in man and chicken is 93% and 73%, respectively. The gene for murine alpha 6 was mapped to chromosome 2. While undifferentiated ES and EC cells express only alpha 6B, alpha 6A is co-expressed in ES cells after differentiation is induced by retinoic acid. alpha 6B is also the only variant expressed in blastocyst stage embryos, but when blastocysts have grown out in culture both alpha 6A and alpha 6B are expressed reflecting the results in the cell lines. We suggest that the deposition of laminin in the embryo is a receptor-mediated process and that the shift in the expression of the variants, as the inner cell mass forms its first differentiated progeny, reflects a change in functional properties.

A study of the structure, function and distribution of beta 5 integrins using novel anti-beta 5 monoclonal antibodies

Here, we have utilized six new anti-human beta 5 monoclonal antibodies to perform a detailed investigation of the structure, function and distribution of beta 5 integrins. Monoclonal anti-beta 5 specificity was confirmed by reactivity with beta 5-transfected CHO cells, by direct binding to the beta 5 subunit (immunoblotting), and by immunodepletion experiments using polyclonal anti-beta 5 serum. The beta 5 subunit was predominantly associated with the alpha v subunit, although on some cell lines, the level of beta 5 exceeded that of alpha v for unknown reasons. Cell adhesion studies showed that the adhesive function of beta 5 could be stimulated, inhibited or unaltered by different anti-beta 5 monoclonal antibodies. The beta 5 subunit was involved in adhesion to both vitronectin and fibronectin and, at least for K562 cells, fibronectin appeared to be the preferred ligand. Flow-cytometry studies showed that the beta 5 subunit was expressed at moderate to high levels on all adherent cell lines examined, was absent from all lymphoid cell lines, and was only weakly expressed on myeloid cell lines. Staining of thymic sections showed the distribution of beta 5 on blood vessels, Hassal's corpuscles, cortical and medullary stromal cells, and basement membranes. Skin sections showed beta 5 on the basal layer of the epidermis and on some dermal blood vessel walls, and kidney sections showed staining of glomerular regions, juxta glomerular apparatus, proximal convoluted tubules and collecting tubules, and at least one anti-beta 5 antibody also stained epithelial cells of proximal tubules.

Tumor necrosis factor alpha (TNF-alpha)-induced cell adhesion to human endothelial cells is under dominant control of one TNF receptor type, TNF-R55

Tumor necrosis factor alpha (TNF-alpha) is a pleiotropic cytokine triggering cell responses through two distinct membrane receptors. Stimulation of leukocyte adhesion to the endothelium is one of the many TNF-alpha activities and is explained by the upregulation of adhesion molecules on the endothelial cell surface. Human umbilical vein endothelial cells (HUVEC) were isolated, cultured, and demonstrated to express both TNF receptor types, TNF-R55 and TNF-R75. Cell adhesion to HUVEC was studied using the HL60, U937, and MOLT-4 cell lines. HUVEC were activated by either TNF-alpha, binding to both TNF-R55 and TNF-R75, and by receptor type-specific agonists, binding exclusively to TNF-R55 or to TNF-R75. The TNF-alpha-induced cell adhesion to HUVEC was found to be controlled almost exclusively by TNF-R55. This finding correlated with the exclusive activity of TNF-R55 in the TNF-alpha-dependent regulation of the expression of the intercellular adhesion molecule type 1 (ICAM-1), E-selectin, and vascular cell adhesion molecule type 1 (VCAM-1). The CD44 adhesion molecule in HUVEC was also found to be upregulated through TNF-R55. However, both TNF-R55 and TNF-R75 upregulate alpha 2 integrin expression in HUVEC. The predominant role of TNF-R55 in TNF-alpha-induced adhesion in HUVEC may correlate with its specific control of NF-kappa B activation, since kappa B elements are known to be present in ICAM-1, E-selectin, and VCAM-1 gene regulatory sequences.

Integrin expression in human bone

Integrins are a family of heterodimeric transmembrane glycoproteins that are known to mediate cell-cell and cell-matrix interactions. Members of the VLA (very late activation) family, which consists of beta 1 integrin in association with the VLA alpha chains (alpha 1-6), mediate adhesion of a wide range of cells to matrix proteins, such as fibronectin, collagen, and laminin, and may therefore be important for cell-matrix interactions in bone. Integrin expression in human bone was studied immunohistochemically using cryostat sections of fracture callus, tumor-associated reactive bone, and neonatal costochondral junctions, with a panel of well-characterized antibodies against beta 1-4 integrins, alpha 1-6 and alpha V integrins, and the alpha V beta 3 dimer (the classic vitronectin receptor). All cell types present in bone expressed beta 1 and alpha 5 integrins; a subpopulation of osteoblastic cells expressed alpha 4. The alpha V was uniformly expressed by osteoblasts but was heterogeneously expressed by osteocytes. Osteoclasts also expressed alpha 2, alpha V, and alpha V beta 3. These results demonstrate differential expression of a restricted range of integrins in bone. This supports the possibility that integrins may mediate the differing interactions of cells of the osteoblast and osteoclast lineages with the matrix of bone.

Collagen II from articular cartilage and annulus fibrosus. Structural and functional implication of tissue specific posttranslational modifications of collagen molecules

Collagen II was isolated and characterized from hyaline cartilage (articular cartilage) and fibro-cartilage (annulus fibrosus). Collagen II from the latter tissue has a substantially higher degree of hydroxylation and glycosylation than that isolated from articular cartilage. The higher degree of posttranslational modification was associated with a slower electrophoretic mobility, a greater resistance to mammalian collagenase digestion and a higher thermal stability. An increase of glycosylation accelerates the initial steps in fibril formation of collagen molecules but slows down the following lateral growth. The newly formed aggregates of collagen II from annulus fibrosus consisted of fibrils with a smaller diameter.

Evidence against a major role for integrins in calcium-dependent intercellular adhesion of epidermal keratinocytes

It is well established that integrins mediate keratinocyte adhesion to extracellular matrix proteins, but, in addition, there is some evidence that they mediate intercellular adhesion. We have investigated the role of integrins in keratinocyte-keratinocyte adhesion by adding anti-integrin antibodies to cells in three assays that differ according to the calcium ion concentration of the medium, the presence or absence of an adhesive substrate (glass or tissue culture plastic) and the timing of antibody addition. As previously reported by Larjava et al., (J. Cell Biol. 110:803-815), a monoclonal antibody to the beta 1 subunit perturbed cell-cell adhesion when added to adherent monolayers in low calcium medium (0.1 mM calcium ions), but did not prevent cell-cell adhesion or stratification induced by raising the level of calcium ions to 1.8mM (the concentration in standard medium). Monoclonal antibodies to both the alpha 3 and beta 1 subunits inhibited the attachment, spreading and motility of keratinocytes in low or standard calcium medium when added at the time of plating; however, they had only a modest effect on the accumulation of cells in adherent clusters. Aggregation of keratinocytes in suspension required a calcium ion concentration of greater than 0.1mM and was not inhibited by any of a large panel of anti-integrin antibodies, including three new antibodies that recognise alpha 2 beta 1. We conclude that any inhibitory effects of individual anti-integrin antibodies on cell-cell adhesion are abrogated by a calcium ion concentration above 0.1mM and that in low calcium medium at least some of the inhibition of cell-cell adhesion is a consequence of the inhibition of cell-substrate adhesion and motility.

The effect of ebselen on polymorphonuclear leukocyte and lymphocyte migration to inflammatory reactions in rats

Ebselen, a selinyl organic compound with anti-inflammatory properties was found by us previously to inhibit in vitro human polymorphonuclear leukocyte (PMNL) adhesion to and migration through umbilical vein endothelium monolayers. Here we investigated in rats the effect of ebselen on PMNL and spleen T lymphocyte (SPLT) migration to inflamed joints induced by intra-articular (i.a) injection of recombinant murine tumor necrosis factor alpha (mTNF alpha) and to dermal inflammatory reactions. Inflammation was induced in the carpal and talar joints of rats by intra-articular (i.a.) injection (100 ng) of mTNF alpha once daily for 2 days. Corresponding joints in the opposite limb received diluent. Simultaneously, the rats were treated p.o. with either ebselen (100 mg/kg/day) or indomethacin (2 mg/kg/day) or vehicle for 2 days. Dermal inflammation was induced by intradermal injection (0.05 ml) of inflammatory stimuli. Accumulation of 51Cr-labelled rat blood PMNL, 111In-labeled SPLT, and extravasation of 125I-labelled human serum albumin (HSA) in the joints and in skin sites were measured. Treatment of rats with ebselen inhibited by 33-65% PMNL migration to the mTNF alpha inflamed joints, and to dermal inflammation induced by zymosan activated serum (ZAS; containing C5adesArg), endotoxin (LPS), mIL-1 beta and mTNF alpha. Migration of SPLT to dermal inflammation induced by interferon gamma (IFN gamma), poly-inosine-cytosine (poly I:C) and LPS was also significantly inhibited (22-33%), but SPLT migration into the inflamed joints was not effected by ebselen. Indomethacin treatment of rats also inhibited PMNL migration into the inflamed joints, but unlike ebselen, indomethacin inhibited only ZAS induced dermal PMNL accumulation. In contrast to ebselen, indomethacin inhibited SPLT migration into the inflamed joints as well as to the dermal inflammation induced by poly I:C and a delayed-type hypersensitivity reaction (DTH). In addition, treatment of rats with indomethacin significantly inhibited plasma protein (125I-HSA) extravasation in the inflamed joints and the dermal inflammatory reaction induced by ZAS, but ebselen had no such effect. In conclusion, ebselen appears to have a distinct antiinflammatory mechanism of action from indomethacin and the PMNL findings are consistent with a direct inhibitory action on PMNL activation and PMNL transendothelial migration as observed previously in vitro.

Expression of beta 1 integrins in non-neoplastic mammary epithelium, fibroadenoma and carcinoma of the breast

beta 1 Integrins were examined immunohistochemically in normal and mastopathic mammary glands, 12 benign tumours and 90 carcinomas of the breast using monoclonal antibodies against beta 1 and alpha 1 to alpha 6 subunits. When compared with epithelial cells of non-neoplastic mammary glands and of benign tumours, carcinoma cells showed considerable quantitative changes in the pattern of alpha 2, alpha 3 and alpha 6 subunit expression. In contrast, the distribution pattern of beta 1, alpha 1, alpha 4 and alpha 5 antigens corresponded to the situation observed in non-neoplastic mammary gland epithelium in most instances. An abnormal expression of alpha 2 was found in 71.0% of the carcinomas ranging from a remarkably low number of alpha 2-positive tumour cells in 27.5% of the cases to a complete absence of the alpha 2 molecule in 43.5% of the carcinomas. Of the carcinomas 39.9% exhibited quantitative changes in alpha 3 expression with an abnormally low content of alpha 3-positive neoplastic cells in 15.4% and a complete absence of this molecule in 24.5% of the cases. Expression of alpha 6 was abnormal in 73.2% of the carcinomas, consisting in a greater number of alpha 6-negative tumour cells in 31.9% and in a complete absence of alpha 6 in 41.3% of the tumours. The abnormally low expression/absence of alpha 2 and alpha 3 subunits correlated with oestrogen receptor negativity (P < 0.033 and P < 0.04, respectively). In addition, abnormally low expression/absence of alpha 2 correlated with poor differentiation of the tumours (P < 0.014). The quantitative changes in the expression pattern of beta 1-associated alpha subunits in breast carcinomas may cause a disturbed cell-cell and/or cell-matrix interaction that increases the invasive and migratory property of the tumour cells.

Distribution of integrin cell adhesion receptors on normal bronchial epithelial cells and lung cancer cells in vitro and in vivo

The interactions of bronchial epithelial cells with the basement membrane control cell morphology, differentiation, and proliferation in addition to having a major role in malignant transformation. Since these interactions are mediated by the integrin family of cell adhesion receptors, we characterized the integrin repertoire and adhesive properties of normal human bronchial epithelial cells in culture and cell lines derived from nine lung carcinomas using subunit-specific monoclonal antibodies. In addition, the integrin repertoire of three of the transformed cell lines was reexamined after the cells formed tumor nodules in immunodeficient mice. Bronchial epithelial cells in culture expressed multiple integrin subunits with the capability of binding to collagen and laminin (alpha 2, alpha 3, and alpha 6) and at least two subunits that are capable of mediating adhesion to fibronectin (alpha 3 and an alpha v-containing integrin). The alpha v beta 3 vitronectin receptor was not present. This distribution closely mimicked that seen by bronchial epithelial cells in situ. Cell lines derived from transformed pulmonary epithelial cells showed great heterogeneity with respect to integrin expression--some showing fewer, some greater, and some the same types of integrins as nontransformed epithelial cells. Only slight changes in integrin expression were seen in tumor cells propagated in immunodeficient mice. Although the adhesion characteristics of the transformed cells mirrored their adhesion receptor profile, no correlation between integrin profile and the ability to grow in SCID mice was observed. This study defines the integrin repertoire of human bronchial epithelial cells and sets the stage for future investigations exploring how the regulation and signal transduction mechanisms of these receptors might affect important pulmonary processes such as bronchial cell differentiation, wound healing, and malignant transformation.

Expression of integrin receptors and their role in adhesion, spreading and migration of normal human melanocytes

Integrin receptors of human melanocytes in vivo and of melanocytes isolated and cultured from in vitro reconstituted normal human epidermis were investigated. Melanocytes were studied by high-resolution immunocytochemistry of in situ epidermis and were found to expose only the integrin subunits alpha 3, alpha 6, alpha v and beta 1 on their plasma membrane surface. Instead, cultured normal melanocytes expressed alpha 3 beta 1, alpha 5 beta 1, alpha 6 beta 1 and alpha v beta 3, which were immunoprecipitated from both metabolically and surface-labeled cells. Beta 1 integrins were diffused on the adhesion surface, while alpha v beta 3 was clustered in focal contacts both in control cells and upon dendrite induction with phorbol 12-myristate 13-acetate (PMA). The functional roles of integrins were studied in vitro by cell adhesion, spreading and migration assays. The sum of the data indicated that, in normal human melanocytes: (i) adhesion to defined substrata is mainly mediated by specific beta 1 integrins; (ii) spreading is mainly modulated by alpha v beta 3; (iii) the beta 1 and beta 3 heterodimers cooperate in regulating migration. The in vitro expression of two integrins (alpha v beta 3 and alpha 5 beta 1) that are not exposed in situ, and their role in the spreading and migratory properties of melanocytes, strongly suggest that they are involved in regenerating a normally pigmented epidermis during wound healing by controlling melanocyte spreading and migration over a provisional matrix. Tumor promoters, such as PMA, selectively increased the expression of alpha 3 beta 1. We suggest that this integrin might be involved in melanocyte migration on the newly formed basement membrane during wound healing as well as in intercellular recognition of adjacent keratinocytes.

Integrins alpha v beta 3 and alpha v beta 5 promote adenovirus internalization but not virus attachment

Adenovirus contains a heterodimeric protein complex consisting of 186 kd fiber protein that mediates high affinity virus attachment to cells and a 400 kd pentavalent subunit (penton base) that contains five Arg-Gly-Asp sequences, implying a role for integrins in adenovirus infection. We demonstrate that the vitro-nectin-binding integrins alpha v beta 3 and alpha v beta 5 promote viral infection in a novel way since antibodies against these receptors or soluble penton base block virus internalization without affecting attachment. Moreover, adenovirus binds to cultured cells lacking alpha v integrins but fail to become internalized, thus restricting infection of these cells. Transfection of alpha v(-) cells with a cDNA encoding alpha v results in the expression of integrins alpha v beta 3 and alpha v beta 5 and allows virus internalization and infection. These data indicate that adenovirus attachment and uptake into cells are separate but cooperative events that result from the interaction of distinct viral coat proteins with a receptor for attachment and alpha v integrin receptors for internalization.

Preferential migration of activated CD4+ and CD8+ T cells in response to MIP-1 alpha and MIP-1 beta

Recombinant human macrophage inflammatory protein-1 alpha (rhMIP-1 alpha) and rhMIP-1 beta were potent chemoattractants of human T lymphocytes. These rhMIP-1 cytokines attracted only T cells activated by monoclonal antibody to CD3 and did not attract unstimulated lymphocytes. Phenotypic analysis revealed that CD4+ T cells were capable of migrating in response to rhMIP-1 beta, whereas rhMIP-1 alpha induced chemotaxis of predominantly CD8+ T lymphocytes. Activated naïve and memory T cells also migrated in response to rhMIP-1 cytokines. Furthermore, these cytokines enhanced the ability of T cells to bind to an endothelial cell monolayer. These results suggest that rhMIP-1 cytokines preferentially recruit specific T cell subsets during the evolution of the immune response.

Streptavidin blocks immune reactions mediated by fibronectin-VLA-5 recognition through an Arg-Gly-Asp mimicking site

Streptavidin is a biotin-binding analogue of egg-white avidin which is secreted by the bacterium Streptomyces avidinii. We have recently reported that streptavidin contains an Arg-Tyr-Asp-Ser (RYDS) sequence which exhibits structural homology to the Arg-Gly-Asp-Ser (RGDS) cell adhesion domain of fibronectin and other matrix-associated glycoproteins. Competition studies with RGD peptides indicated that streptavidin binds to cells via this site and that the binding is independent of biotin recognition. Since the RGD-containing peptide has been shown to play a key role in integrin-mediated cell adhesion, we assumed that streptavidin may utilize the RYDS site to bind to immune cells and thereby abrogate their adhesion-dependent functions. We now report that streptavidin modulates several matrix-dependent interactions of immune cells. In this context, immobilized streptavidin was found to support activated human CD4+ T cell adhesion in an RGD-specific, alpha 5 beta 1-dependent manner. In addition, soluble streptavidin (the commercially available or biotin-blocked forms) inhibited T cell adhesion to fibronectin and interfered with its co-stimulatory effect on tumor necrosis factor-alpha secretion by co-cultures of CD4+ T cells and macrophages. These results suggest that streptavidin is a novel example of a bacterial protein which utilizes RGD mimicry to interfere with integrin-mediated immune responses.

Bendless alters thoracic musculature in Drosophila

bendless- (ben-) is an X chromosome mutation in Drosophila melanogaster, known to alter patterns of connections in the CNS and thus modify behavior (Thomas and Wyman, 1984). We report that in addition to its CNS effects, ben- has pleiotropic phenotypes affecting thoracic muscle patterning, pupal mortality, and post-eclosional mobility. The tergal depressor of the trochanter (TDT) normally attaches ventrally to an apodeme on the trochanter and dorsally to the lateral scutum just posterior to the intrascutal suture. In ben- individuals, TDT may attach anywhere within the boundaries of the attachment areas for TDT and dorsoventral muscles I (DVM I) and II (DVM II). Furthermore, TDT may completely lack a dorsal attachment, although it still maintains a ventral attachment. DVMs may also attach abnormally to dorsal sites normally occupied by an adjacent DVM, or may be entirely eliminated. DVM loss occurs independently of the position or presence of TDT dorsal attachment. The cytology of ben- TDT is altered. Muscles may have fibers that are swollen and stain abnormally. Other fibers may have large, axially aligned holes. ben- flies have an increased likelihood of failing to eclose and, upon eclosion, show impaired mobility. We describe several possible mechanisms for the ben- developmental defects and discuss this mutation in light of its evolutionary significance.

A re-examination of the molecular basis of cell movement

A model for cell movement is presented. It is suggested that cells do not migrate on collagen using their VLA (very late antigen) integrins that bind this extracellular matrix protein. Rather, the cells utilize alpha v integrins to bind endogenously produced fibronectin, which binds to the underlying collagen. It is envisaged that cells proceed by a process of engagement and disengagement of alpha v integrins to the extracellular matrix, somewhat analogous to the motion of a monkey climbing a tree. Secretion of isoforms of the adhesion modulator, thrombospondin, regulates disengagement of the integrin from its ligand in migrating cells. The integrin disengagement signal is mediated by thrombospondin cross-linking the alpha v integrin to an integrin accessory molecule and thus activating protein kinases. The cross-linked receptor complex undergoes recycling back along actin stress fibres, guided by the integrin beta-subunit. After endocytosis and protein sorting the alpha v integrin is transported back to the leading edge off migrating cells in vesicles guided by the tubulin-binding capabilities of an integrin accessory molecule. Direct attachment to collagen required for processes, such as matrix contraction, is mediated by VLA integrins which displace alpha v integrins from points of attachment during integrin recycling, possibly through an alpha v beta 1 intermediary receptor.

A novel integrin specificity exemplified by binding of the alpha v beta 5 integrin to the basic domain of the HIV Tat protein and vitronectin

Several studies have addressed the interaction of the HIV Tat protein with the cell surface. Our analysis of the cell attachment-promoting activity of Tat and peptides derived from it revealed that the basic domain of Tat, not the arg-gly-asp (RGD) sequence, is required for cell attachment to Tat. Affinity chromatography with Tat peptides and immunoprecipitation with various anti-integrin antibodies suggest that the vitronectin-binding integrin, alpha v beta 5, is the cell surface protein that binds to the basic domain of Tat. The Tat basic domain contains the sequence RKKRRQRRR. A related sequence, KKQRFRHRNRKG, present in the heparin-binding domain of an alpha v beta 5 ligand, vitronectin, also bound alpha v beta 5 in affinity chromatography and, in combination with an RGD peptide, was an inhibitor of cell attachment to vitronectin. The alpha v beta 5 interaction with these peptides was not solely due to high content of basic amino acids in the ligand sequences; alpha v beta 5 did not bind substantially to peptides consisting entirely of arginine or lysine, whereas a beta 1 integrin did bind to these peptides. The interaction of alpha v beta 5 with Tat is atypical for integrins in that the binding to Tat is divalent cation independent, whereas the binding of the same integrin to an RGD-containing peptide or to vitronectin requires divalent cations. These data define an auxiliary integrin binding specificity for basic amino acid sequences. These basic domain binding sites may function synergistically with the binding sites that recognize RGD or equivalent sequences.

Biochemical characterization and tissue distribution of the A and B variants of the integrin alpha 6 subunit

Two cytoplasmic variants of the alpha 6 integrin, alpha 6A and alpha 6B, have been identified previously (Hogervorst, F., I. Kuikman, A. G. van Kessel, and A. Sonnenberg. 1991. Eur. J. Biochem. 199:425-433; Cooper, H. M., R. N. Tamura, and V. Quaranta. 1991. J. Cell Biol. 115:843-850). Using synthetic peptides, containing sequences of their cytoplasmic domains, we have produced mAbs specific for either of the variants. These antibodies reacted with a variety of different epithelial tissues. In some tissues (e.g., salivary gland) both variants could be detected while in others only one of the variants was found (e.g., alpha 6A in epidermis and alpha 6B in kidney). Among nonepithelial cells and tissues, perineural fibroblasts and Schwann cells in peripheral nerves and platelets reacted with anti-alpha 6A, while microvascular endothelia reacted with both anti-alpha 6A and anti-alpha 6B. From our immunohistochemical results there is not evidence that combination with beta 1 or beta 4 is restricted to one of the two variants of alpha 6. This was confirmed by immunoprecipitation studies which showed that both beta 1 and beta 4 were coprecipitated by both anti-alpha 6A or anti-alpha 6B antibodies from cells. Also, the distribution of alpha 6A and alpha 6B subunits associated with beta 1 on cells attached to laminin was similar: both were found in focal contacts colocalizing with vinculin. In contrast, the alpha 6A subunit, associated with beta 4 in cultures of a squamous cell carcinoma cell line, was found to codistribute with bullous pemphigoid antigen 230 in hemidesmosomal-like structures. The alpha 6A and alpha 6B variants, immunoprecipitated from various cell lines, exhibited slightly different electrophoretic mobilities. Analysis of the antigens under reducing conditions showed that the mobility of the light chains, but not of the heavy chains, is different. In addition, in some cells the light chains of alpha 6A and alpha 6B, each are of two different sizes. Treatment with N-glycanase showed that these two light chain variants of alpha 6A and alpha 6B are not due to differences in N-linked glycosylation, and may therefore represent alternative proteolytic products of the alpha 6 precursor. We further demonstrate that alpha 6A, but not alpha 6B, is a major target for PMA-induced phosphorylation. Phosphorylated alpha 6A contained phosphoserine and a small amount of phosphotyrosine. There are also two variants of the integrin alpha 3 subunit with different cytoplasmic domains, but in the cell lines examined only alpha 3A could be demonstrated by RT-PCR.(ABSTRACT TRUNCATED AT 400 WORDS)

Interference with endogenous ras function inhibits cellular responses to wounding

Wounding of tissue induces cellular responses that ultimately result in wound repair. Studies in tissue culture model systems indicate that these responses include induction of AP-1 regulated genes, cell migration and mitogenesis which are also characteristic of cellular responses to growth factors. Investigations have identified cellular ras proteins as critical components of growth factor-stimulated signal transduction pathways, however their role in the wounding response is less clear. Investigation of the potential involvement of c-Ras in this process utilized quiescent living bovine corneal endothelium cells (BCE) which were microinjected with ras dominant interfering mutant protein (N17) and subsequently stimulated by mechanical wounding. Analysis of these cells demonstrated that microinjection of dominant-interfering ras protein, but not control proteins, inhibited the wounding response as evidenced by diminished Fos expression, lack of cell migration and a block in DNA synthesis.

Extracellular matrix proteins in intrathymic T-cell migration and differentiation?

Intrathymic T-cell migration and differentiation is not completely understood. Here, Wilson Savino and colleagues argue that certain interactions between differentiating thymocytes and thymic epithelial cells are mediated by extracellular matrix proteins and that these interactions influence intrathymic migration events and thymocyte differentiation.

Expression of beta 1 integrin complexes on the surface of unfertilized mouse oocyte

Integrins are a family of cell surface receptors that mediate cell-cell and cell-matrix interactions in a variety of different cellular systems. Here we show that unfertilized mouse oocytes express beta 1 class integrins both at mRNA and protein levels. Using the reverse transcription polymerase chain reaction and oligonucleotide primers based on the DNA sequence of mouse integrins, the RNA transcripts for the beta 1, alpha 5 and alpha 6 subunits were detected in unfertilized oocytes. The expression of the mRNAs is paralleled by the expression of the corresponding proteins, in fact, the alpha 5/beta 1 and the alpha 6/beta 1 complexes can be immunoprecipitated with specific antibodies form 125I-surface-labeled oocytes. Using subunit-specific antibodies we also demonstrate the presence of the alpha 3/beta 1 at the oocyte surface but alpha 1, alpha 2, alpha 4 or alpha V subunits were not detectable. Since the mouse alpha 3 DNA sequence is not available, we have not tested for the corresponding transcript. Integrin subunits alpha 6 and beta 1 were differently distributed on the oocyte surface, as visualized by immunofluorescence staining and by immunoelectron microscopy. alpha 6 antigen was mainly confined to the microvillous area of the oocyte surface, while beta 1 was more homogeneously distributed over the whole oolemma. These data demonstrate for the first time the expression of three beta 1 integrin complexes in unfertilized mouse oocytes. Such proteins may have a role in sperm-egg interaction or during very early steps of embryogenesis.

Diverse mechanisms for cell attachment to platelet thrombospondin

Thrombospondin-1 is a component of the extracellular matrix which is thought to play important roles in cell migration and proliferation, during embryogenesis and wound repair. To understand the basis for these activities, we are mapping the regions of the molecule with cell adhesive activity. Here, we use antagonists of specific cell binding sites, adhesion-perturbing thrombospondin monoclonal antibodies and proteolytic fragments of platelet thrombospondin, to investigate the adhesive mechanisms used by G361 melanoma cells, human intestinal smooth muscle cells (HISM), epidermal keratinocytes and MG-63 osteosarcoma cells. When attached to the same preparations of platelet thrombospondin, HISM and MG-63 cells underwent spreading, whereas G361 cells and keratinocytes did not. Attachment of all four cell types involved the carboxyterminal domain. The type 1 repeats and the amino-terminal heparin binding domain were important for stable attachment of G361, HISM and MG-63 cells, but were not involved in keratinocyte attachment. GRGDSP peptide caused near complete inhibition of HISM and MG-63 cell attachment, partially inhibited G361 attachment, but did not inhibit keratinocyte attachment. Attachment of HISM and MG-63 cells involved the alpha v beta 3 integrin. The integrity of the thrombospondin molecule was important for its adhesivity towards G361, HISM, and MG-63 cells, whereas keratinocytes attached to the 140 kDa tryptic fragment as effectively as they did to the intact molecule. These results show that cell attachment to platelet thrombospondin typically involves multiple binding interactions, but the exact profile of interactions is cell type specific. Usage of particular cell-binding sites does not predict whether cells will undergo spreading or not. These data may, in part, explain some of the current controversies surrounding the mechanisms of cell attachment to thrombospondin.

Integrin alpha subunit mRNAs are differentially expressed in early Xenopus embryos

Adhesion of cells to extracellular matrix proteins is mediated, in large part, by transmembrane receptors of the integrin family. The identification of specific integrins expressed in early embryos is an important first step to understanding the roles of these receptors in developmental processes. We have used polymerase chain reaction methods and degenerate oligodeoxynucleotide primers to identify and clone Xenopus integrin alpha subunits from neurula-stage (stage 17) cDNA. Partial cDNAs encoding integrin subunits alpha 2, alpha 3, alpha 4, alpha 5, alpha 6 and an alpha IIb-related subunit were cloned and used to investigate integrin mRNA expression in early embryos by RNase protection assay and whole-mount in situ hybridization methods. Considerable integrin diversity is apparent early in development with integrins alpha 2, alpha 3, alpha 4, alpha 5 and alpha 6 each expressed by the end of gastrulation. Both alpha 3 and alpha 5 are expressed as maternal mRNAs. Zygotic expression of alpha 2, alpha 3, alpha 4 and alpha 6 transcripts begins during gastrulation. Integrin alpha 5 is expressed at relatively high levels during cleavage, blastula and gastrula stages suggesting that it may represent the major integrin expressed in the early embryo. We demonstrated previously that integrin beta 1 protein synthesis remains constant following induction of stage 8 animal cap cells with activin (Smith, J. C., Symes, K., Hynes, R. O. and DeSimone, D. W. (1990) Development 108, 289–298.). Here we report that integrin alpha 3, alpha 4 and alpha 6 mRNA levels increase following induction with 10 U/ml activin-A whereas alpha 5, beta 1 and beta 3 mRNA levels remain unchanged. Whole-mount in situ hybridization reveals that alpha 3 mRNAs are expressed by cells of the involuting mesoderm in the dorsal lip region of early gastrulae. As gastrulation proceeds, alpha 3 expression is localized to a stripe of presumptive notochordal cells along the dorsal midline. In neurulae, alpha 3 mRNA is highly expressed in the notochord but becomes progressively more restricted to the caudalmost portion of this tissue as development proceeds from tailbud to tadpole stages. In addition, alpha 3 is expressed in the forebrain region of later stage embryos. These data suggest that integrin-mediated adhesion may be involved in the process of mesoderm involution at gastrulation and the organization of tissues during embryogenesis.