Echoviruses 1 and 8 are closely related genetically, and bind to similar determinants within the VLA-2 I domain

Echoviruses (EV) 1 and 8 were originally considered to be distinct serotypes, but more recently have been considered strains of the same virus. In experiments with chimeric recombinant fusion proteins, both viruses bound to the I domain of the integrin VLA-2, and both required the same receptor residues for attachment. A full-length, infectious cDNA clone encoding EV1 was obtained; its nucleotide sequence was determined, as were the sequences encoding the EV8 capsid. EV1 and 8 show 94% amino acid identity within the capsid region and are more similar to each other than to any other human picornavirus.

Novel inhibitors of alpha 4 beta 1 integrin receptor interactions through library synthesis and screening

A library of 2302 small molecule beta-turn mimetics was screened for inhibition of the alpha 4 beta 1 integrin-CS1 splice variant binding interaction. Preliminary data revealed several active ligands, and validation with purified material culminated in the identification of some of the first small molecule ligands (1, IC50 = 5 microM, and 2, IC50 = 8 microM) to be reported for this class of integrins.

CD9, CD63, CD81, and CD82 are components of a surface tetraspan network connected to HLA-DR and VLA integrins

CD9, CD63, CD81, and CD82 are glycoproteins of unknown function which belong to the tetraspan superfamily. These molecules have short cytoplasmic sequences, four transmembrane domains and two unequal extracellular regions. Here, we show that these molecules are associated with each other on cell surface and with other glycoproteins such as very late antigen (VLA) integrins and HLA-DR antigens. Moreover, the VLA integrins and HLA-DR antigens were also found to be associated. The interactions of these molecules were analyzed by transfection experiments. It is demonstrated that overexpression of CD9 antigen in Raji cells leads to a lower efficiency of precipitation of CD81 and CD82, suggesting a direct interaction between these molecules. In these cells, the co-precipitation of CD81 and CD82 was not modified, suggesting that these tetraspans did not compete for association. However, in COS-7 cells, transfection of both CD81 and CD82 led to a marked reduction of the number of CD9/CD81 or CD9/CD82 complexes compared to single-transfected cells, and this was associated with the appearance of CD81/CD82 complexes. Therefore, in this cellular system, CD9 competes with CD81 and CD82 for association with the other tetraspan proteins. Finally, the tetraspans do not compete for the association with integrins or HLA-DR. Indeed, when CD9 was expressed in Raji cells, it was incorporated into the pre-existing complexes of these molecules with CD81 and CD82. These data suggest the existence of a tetraspan network which, by connecting several molecules, may organize the positioning of cell surface proteins and play a role in signal transduction, cell adhesion, and motility.

A region of the integrin VLA alpha 4 subunit involved in homotypic cell aggregation and in fibronectin but not vascular cell adhesion molecule-1 binding

The VLA-4 (alpha 4 beta 1) integrin is involved in the adhesion of cells to fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In order to study alpha 4 structure-function relationships, we have expressed mutated alpha 4 subunit by transfection into VLA-4-negative K562 cells. Substitutions at alpha 4 residues Arg89-Asp90, which show the highest surface probability indexes inside the N-terminal alpha 4/80 fragment, resulted in a reduction in the reactivity of all anti-alpha 4 epitope A monoclonal antibodies (mAbs) tested, compared with the reactivity with anti-alpha 4 epitopes B1, B2, and C mAb, both by transfectant flow cytometry, and by immunoprecipitation and SDS-polyacrylamide gel electrophoresis analysis of transfectant surface-iodinated proteins. In contrast, substitutions at nearby residues, Gln101, Pro102, and Ile105 did not affect the reactivity of any anti-alpha 4 mAb representing the known alpha 4 epitopes. Homotypic cell aggregation triggered by anti-alpha 4 epitope A mAb was prevented in the transfectants expressing mutated alpha 4 Arg89-Asp90Asp residues, while cell aggregation was fully achieved with either anti-alpha 4 epitope B2 or anti-beta 1 mAb. Mutations at alpha 4 residues Gln101, Pro102, and Ile108 did not affect the homotypic cell aggregation of the transfectants expressing these mutations. In addition, the adhesion of mutant Arg89-Asp90 alpha 4 transfectants to the connecting segment-1-containing fibronectin-40 (FN-40) fragment of fibronectin was diminished compared to wild type alpha 4 transfectants, as well as to other mutant alpha 4 transfectants. This adhesion to FN-40 was restored when the activating anti-beta 1 TS2/16 mAb was present in the adhesion assays. In contrast, adhesion to VCAM-1 was not affected by mutations at Arg89-Asp90, nor at Gln101, Pro102, and Ile108 alpha 4 residues. Altogether, these results indicate that alpha 4 residues Arg89 and Asp90 are included in a region involved in homotypic cell aggregation, as well as in adhesion to FN-40, but not to VCAM-1.

Alpha 4 beta 1 integrin-dependent cell adhesion is regulated by a low affinity receptor pool that is conformationally responsive to ligand

alpha 4 beta 1 integrin (VLA-4) appears to be unique among the leukocyte integrins in that it can initiate the adhesion of circulating lymphocytes without cellular activation. It is not known how lymphocytes or other cell types maintain constitutive levels of alpha 4 beta 1 integrin activity. The current report describes a monoclonal antibody, 15/7, that recognizes a high affinity or ligand-occupied conformation of beta 1 integrin. Studies with 15/7 revealed that alpha 4 beta 1 integrin-dependent adhesion of leukocytic cell lines is mediated by a population of low affinity receptors that is conformationally responsive to ligand; the 15/7 epitope could be induced by nanomolar concentrations of soluble VCAM-1 or by micromolar concentrations of a peptide derived from the type III connecting segment domain of fibronectin (as ligands for alpha 4 beta 1 integrin). The same receptors were also responsive to adhesion activating reagents, such as Mn2+, activating anti-beta 1 integrin antibodies, and phorbol myristate acetate, which induced the 15/7 epitope directly and/or decreased the concentration of ligand required for epitope induction. In addition to the responsive receptor pool, cells expressed a second population of alpha 4 beta 1 integrin that was conformationally restrained, failing to respond to ligand or to any of the activating reagents. The relative size of the responsive and inactive receptor pools, as well as the affinity of the responsive receptors, represented a stable phenotype of different cell types and played important roles in defining the cells' adhesive capacity and ligand specificity. Similar receptor populations were measured on lymphocyte subsets in whole blood. These studies provide insight into how cells maintain different constitutive levels of alpha 4 beta 1 integrin activity, and how the activity of beta 1 integrin can be modulated by activators of cell adhesion.

Vascular cell adhesion molecule (VCAM)-Ig fusion protein defines distinct affinity states of the very late antigen-4 (VLA-4) receptor

The Very Late Antigen-4 receptor (VLA-4) (alpha 4 beta 1) is constitutively expressed on leukocytes and plays a role in cell trafficking, activation and development through its interaction with two alternative ligands, Vascular Cell Adhesion Molecule (VCAM-1) and fibronectin (FN). VLA-4-dependent cell adhesion is augmented by various stimuli, such as divalent cations, certain beta 1-specific monoclonal antibodies (mAbs) and cell activation. However, the steps of the adhesive process which they affect are currently undefined. In order to investigate whether or not these stimuli affect the primary step, VLA-4/ligand binding, we employed a recombinant VCAM-IgG fusion protein (VCAM-Ig) as a soluble ligand for VLA-4. Using this soluble ligand, we have directly demonstrated that the VLA-4 receptor can exist in at least three different affinity states on the cell surface. Two distinct high affinity states are induced on normal peripheral blood T cells, one by the anti-beta 1 mAb TS2/16, and one of 15-20 fold higher affinity by the divalent cation Mn2+. Interestingly, activation through the T cell receptor (TcR), through CD31 or by the Macrophage Inflammatory Protein-1 beta chemokine (MIP-1 beta) do not detectably increase VLA-4 affinity although they do augment VLA-4 dependent cell adhesion in vitro. Thus, VCAM-Ig binding defines high affinity VLA-4 receptors, revealing unique effects of the TS2/16 mAb and Mn2+ cations in vitro, and distinguishes VLA-4/VCAM interactions from subsequent steps in cell adhesion.

cDNA cloning of mouse VLA-3 alpha subunit

cDNA clones for mouse VLA (very late antigen)-3 alpha subunit (alpha 3 integrin) were isolated and sequenced. The encoded mouse alpha 3 integrin subunit was composed of 1,053 amino acid residues. The results of sequence analysis revealed similar structural characteristics of other VLA alpha subunits. For example, the presence of a large extracellular domain including three putative metal binding sequences, a transmembrane domain, and a short cytoplasmic domain. A higher level of its message was detected in thymus than in kidney, stomach, spleen, liver, brain, or lung by Northern blotting analysis.

The VLA-2 (alpha 2 beta 1) I domain functions as a ligand-specific recognition sequence for endothelial cell attachment and spreading: molecular and functional characterization

The integrin VLA-2 (alpha 2 beta 1), generally considered to represent the specific collagen receptor on human endothelial cells, contains an alpha 2-subunit inserted I domain with structural similarity to the type A domains found within the recently described superfamily of receptor-ligand recognition proteins. This region of the cDNA has now been isolated and used for molecular and functional characterization of this heterodimeric receptor complex. Comparative sequence analysis with the porcine homologue revealed 93% amino acid sequence identity, suggestive of a developmentally conserved function. To complete structure/function studies, this region of the human cDNA was expressed as a chimeric protein in Escherichia coli, and a rabbit polyclonal antibody (anti-I domain) was used to study determinants of endothelial cell attachment and spreading in vitro. Quantifiable and visual disruption of endothelial cell attachment to gelatin, type I collagen, and laminin was evident using the specific anti-I domain antibody, with minimal inhibitory effects demonstrable using fibronectin or fibrinogen matrices. Therefore, these data would suggest that the alpha 2 beta 1 I domain confers ligand-binding specificity for both known alpha 2 beta 1 substrates (laminin and collagen), and that this region subserves a regulatory function in the molecular processes controlling endothelial cell attachment and spreading in vitro.

Cloning and characterization of the promoter region of the murine alpha-4 integrin subunit

To study the differential expression of the murine VLA-4 (alpha 4 beta 1) integrin, the 5'-flanking region of the gene for the alpha subunit (alpha 4m) was isolated and a cDNA for alpha 4m was obtained with reverse transcriptase polymerase chain reaction (RT-PCR). The cDNA sequence contained a difference in the signal peptide region compared to the previously described cDNA (Neuhaus et al., 1991). As a consequence, another start codon is predicted, resulting in a decrease in size of the signal peptide. This was confirmed by genomic sequencing. The promoter region was delimited by ribonuclease protection assay (RPA) and transfection experiments fusing 5'-upstream fragments to the luciferase gene. A fragment extending from -936 to +221 was capable of controlling the expected cell-type-specific expression. Sequence comparison of the mouse alpha 4m promoter region with the human alpha 4h promoter revealed little homology. Like most integrin subunits, alpha 4m lacks TATA anc CCAAT boxes. Putative recognition sites for DNA-binding nuclear factors (AP1, AP2, Sp1, and PU1) were identified. The characterization of the promoter region and further identification of the transcription regulatory elements should provide insight in the regulation of alpha 4m integrin gene expression.

Identification of putative ligand binding sites within I domain of integrin alpha 2 beta 1 (VLA-2, CD49b/CD29)

Integrin alpha 2 beta 1 is a cell surface adhesion receptor for collagen and echovirus 1. Here we localized the epitopes for anti-alpha 2 monoclonal antibodies using interspecies (human/bovine) alpha 2 chimeras with different lengths of human alpha 2 sequence on the amino-terminal side and site-directed mutagenesis. The antibodies that block the collagen and/or echovirus 1 binding to human alpha 2 beta 1 (6F1, RMAC11, 12F1, and AA10) recognizes a small region (residues 173-259) within the I domain. Asp-160 and Arg-242 are critical for binding of the two other function-inhibiting antibodies, P1H5 and 5E8, respectively. Notably, mutations of Asp-151 and Asp-254 block the binding of alpha 2 beta 1 to collagen. These data suggest that the I domain (residues 140-359) is critically involved in the ligand/receptor interactions, and collagen and echovirus 1 binding sites are adjacent or overlapping within the I domain. The sequence of the residues 173-259 of alpha 2 overlap with the peptide sequences (M11 and M20) that derive from von Willebrand factor A1 and A3 domains (homologous to the alpha 2 I domain) and block von Willebrand factor/collagen interaction, suggesting that the epitope region of alpha 2 (residues 173-259) may really be involved in ligand recognition.

Interchangeable alpha chain cytoplasmic domains play a positive role in control of cell adhesion mediated by VLA-4, a beta 1 integrin

Integrins can exist in a range of functional states, depending on the cell type and its state of activation. Although the mechanism that controls activity is unknown, it has been suggested that for some integrins, alpha chain cytoplasmic domains may exert either a negative effect or no effect on adhesion function. To address this issue for VLA-4 (an alpha 4 beta 1 heterodimer), we constructed an alpha 4 cytoplasmic deletion mutant and chimeric alpha chains composed of the extracellular domains of alpha 4 and the cytoplasmic domains of alpha 2, alpha 4, or alpha 5. Upon stable transfection of wild-type alpha 4, VLA-4 heterodimer was obtained that mediated (a) poor adhesion to CS1 peptide, fibronectin, or vascular cell adhesion molecule 1 (VCAM-1) (in K562 cells); (b) poor adhesion to CS1 peptide but moderate adhesion to VCAM-1 (in MIP101 cells); and (c) moderate adhesion to both CS1 peptide and VCAM-1 (in PMWK cells). Chimeric alpha 4 constructs and wild-type alpha 4 yielded similar results in these cell lines. In contrast, truncation of the alpha 4 cytoplasmic domain (after the conserved GFFKR motif) caused an almost complete loss of adhesive activity in all three cell lines. Thus, several interchangeable alpha chain cytoplasmic domains play a fundamentally positive role in determining the state of constitutive activity for VLA-4. The alpha chain cytoplasmic domain is also required for agonist-stimulated adhesion, since phorbol ester stimulated the cell adhesion mediated by wild-type and chimeric alpha chains, but not by the cytoplasmic deletion mutant. The inactivity of both wild-type VLA-4 (in K562 cells), and truncated VLA-4 (in all three cell lines) was overcome by the addition of a stimulatory anti-beta 1 monoclonal antibody. Thus, the alpha cytoplasmic domain-dependent cellular mechanism controlling both constitutive and agonist-stimulated VLA-4 activity could be bypassed by external manipulation of the integrin.

A high affinity conformational state on VLA integrin heterodimers induced by an anti-beta 1 chain monoclonal antibody

The VLA integrin subfamily includes receptors for extracellular matrix proteins as well as receptors involved in cell-cell adhesive interactions. We have previously described the up-regulation of VLA integrin-mediated cell attachment to different ligands by the anti-beta 1 TS2/16 monoclonal antibody (mAb) (Arroyo, A. G., Sánchez-Mateos, P., Campanero, M. R., Martín-Padura, I., Dejana, E., and Sánchez-Madrid, F. (1992) J. Cell Biol. 117, 659-670). In this report, we have investigated the mechanism involved in this regulatory effect. The anti-beta 1-mediated regulatory effect on cell adhesion did not require "de novo" protein synthesis, since it was not affected by pretreatment with either cycloheximide or actinomycin D. To quantitate the effect of the regulatory anti-beta 1 TS2/16 mAb on the affinity of VLA-5 for its ligand, an RGD-containing fragment of fibronectin (FN80), we performed binding studies of radiolabeled soluble FN80 to U-937 cells. The affinity of VLA-5 for FN80 was enhanced about 4-fold in the presence of TS2/16 mAb (Kd = 0.98 +/- 0.07 microM) compared to the functionally irrelevant anti-beta 1 Alex 1/4 mAb (Kd = 4.23 +/- 0.92 microM), whereas no alteration in the number of binding sites was observed. Indeed, the anti-beta 1 TS2/16 mAb is inducing this high affinity state on VLA heterodimers by a direct change on the conformation of these receptors as demonstrated by affinity chromatography analysis using extracellular matrix proteins covalently bound to Sepharose. The yield of VLA-5 fibronectin receptor bound to FN80-Sepharose columns was strongly increased upon treatment of U-937 cell lysates with mAb TS2/16. Moreover, higher concentrations of EDTA were required for eluting the VLA-5 integrin from this matrix. This up-regulatory effect was also observed with F(ab')2 and Fab fragments of the anti-beta 1 TS2/16 mAb, and was also exerted on the purified VLA-5 receptor. Similarly, the yield of VLA-2 retained on a collagen I-Sepharose column was dramatically increased by pretreatment of A375 melanoma cell lysates with the mAb TS2/16. Altogether, these results indicate that the interaction of VLA beta 1 heterodimers with their ligands can be regulated by switching between differently active conformations inherent to the alpha beta 1 receptors.

Alpha 4/180, a novel form of the integrin alpha 4 subunit

The integrin alpha 4 beta 1 (VLA-4) is a versatile cell-cell and cell-extracellular matrix adhesion receptor. The alpha 4 subunit can be expressed on the cell surface in two forms: an intact form (alpha 4/150) and a cleaved form (alpha 4/80,70). Here we have characterized a third form of alpha 4, called alpha 4/180. Although alpha 4/180 (M(r) 180, nonreduced) is different in size than alpha 4/150 (M(r) 150, nonreduced), these two forms are clearly related, as they (i) shared the same amino-terminal sequence, (ii) were both recognized in Western blots by an anti-alpha 4 COOH-terminal antiserum, (iii) migrated with the same apparent size and charge when reduced, and (iv) were both immunoprecipitated using anti-VLA-4 reagents. In pulse-chase experiments, precursors to both forms appeared simultaneously and matured at the same rate, indicating that one is most likely not the biosynthetic precursor of the other. Although reduction of alpha 4/180 to yield alpha 4/150 suggested the release of a cysteine-linked 30-kDa fragment, seven different biochemical techniques failed to identify such a fragment. Also, alpha 4/180 was converted to alpha 4/150 by incubation at pH 11, by treatment with EDTA at 56 degrees C, or by heating in the presence of elevated SDS levels. Together our findings suggest that alpha 4/180 and alpha 4/150 represent different conformations of the same alpha 4 polypeptide, with the former having anomalous slower migration in SDS-polyacrylamide gel electrophoresis. This unusual biochemical feature of alpha 4 is not shared by other beta 1-associated integrin alpha subunits and suggests that VLA-4 has unique structural properties.

Specific cleavage of the alpha 4 integrin associated with activation of peripheral T lymphocytes

The heterodimeric VLA-4 integrin has been implicated in lymphocyte migration to inflamed peripheral tissues, lympho-haemopoiesis and autoimmune disease. To determine the structure and function of VLA-4 in physiological processes, molecular forms of the VLA-4 alpha-chain were analysed during T-cell activation. The results showed that prolonged activation of human peripheral T cells was associated with increased cleavage of the 150,000 MW alpha 4 chain into 80,000 and 66,000 MW fragments. Similar-sized alpha 4 fragments could also be generated from 150,000 MW alpha 4 on intact resting T cells by brief trypsinization, whereas trypsin treatment of isolated 150,000 MW alpha 4 resulted in nearly complete protein degradation. Native 80,000 and 66,000 MW alpha 4 chains on activated T cells could not be digested further by trypsin. These results indicated that specific cleavage of 150,000 MW alpha 4 was largely dependent on the tertiary structure of native alpha 4 chains. To determine the specific cleavage site in alpha 4 on peripheral T cells, VLA-4 was isolated and purified from in vitro-activated T cells and the N-terminus of the 66,000 MW fragment was partially sequenced. The sequence SKR/STE was identified as the specific alpha 4 cleavage site on T cells. These results indicate that T cells, upon activation, acquire an enhanced dipeptidase processing activity, which cleaves alpha 4 at a specific site.

A single mRNA encodes the alpha 150 and alpha 80/70 forms of the alpha subunit of VLA4

VLA4 is a cell surface heterodimer (alpha 4 beta 1, CD49d/CD29) which belongs to the integrin family and is involved in cell-extracellular matrix interactions, as well as in intercellular adhesion. Unlike other integrin alpha subunits, the alpha subunit of VLA4 (alpha 4) can appear as a 150-kDa polypeptide (alpha 150), or cleaved into two non-covalently associated polypeptides (alpha 70-80). The relative proportion of each form is highly variable among different cells and is dependent on the state of cellular activation. The alpha 4 cleavage site has recently been shown to occur between residues Arg558-Ser559. We report the isolation of genomic clones encoding the alpha 4 subunit, the location of the cleavage site-encoding nucleotides to a specific exon and the determination of the exon-intron organization around the cleavage site. Comparison with the gpIIb (CD41) and the alpha x (p150,95 alpha, CD11c) genes revealed a similar genomic structure in this region, with exons of similar length separated by introns of identical phase. The structure of the alpha 4 mRNA in cells expressing the alternative forms of alpha 4 has been analyzed by means of reverse transcription-polymerase chain reaction. Our results indicate that the exon encoding the cleavage site is present in alpha 4 mRNA molecules from cells expressing either the alpha 150 or the alpha 70-80 form on the cell surface. Moreover, the structure of the alpha 4 mRNA around the cleavage site does not change during the switch towards the alpha 70-80 form that takes place upon lymphocyte activation. Therefore, both forms of alpha 4 arise from a common mRNA, the alpha 150 form contains the cleavage sequence and the alpha 70-80 form must be generated by a post-translational proteolytic event.

VLA-2 is a collagen receptor on endothelial cells

In order to identify cell-substrate adhesion receptors on vascular endothelium, murine monoclonal antibodies (MoAb) were raised against human umbilical vein endothelial cells (HUVE). One anti-HUVE MoAb, RMAC11, identified the adhesion receptor VLA-2 as it immunoprecipitated a non-covalently linked heterodimer of 160 kD and 130 kD, which was identical to the heterodimer immunoprecipitated by the anti-VLA-2 MoAb, 12F1 and 5E8. Furthermore, proteolytic peptide maps of the VLA-2 alpha- and beta-chains were highly homologous with those of the RMAC11-recognized molecule. However, unlike other VLA-2 MoAb, RMAC11 also identified an 85 kD band which migrated to 90 kD under reducing conditions. This band was most likely a fragment of the 160 kD alpha-chain as a similar alpha-chain derived fragment has been demonstrated in the immunoprecipitates of some VLA-4 reactive monoclonals. However the possibility that this may be a novel molecule associated with VLA-2 has not been excluded. In vitro assays of HUVE adhesion to collagen types 1 and 4, laminin and fibronectin showed that RMAC11 blocked adhesion to collagen (types 1 and 4) and laminin, but had no effect on HUVE adhesion to fibronectin, confirming that VLA-2 is a collagen and laminin receptor for HUVE.

The alpha 6 beta 1 (VLA-6) and alpha 6 beta 4 protein complexes: tissue distribution and biochemical properties

A member of the integrin family, the alpha 6 beta 4 complex was previously identified on human and mouse carcinoma cell lines by using a rat monoclonal antibody to alpha 6. Here we describe two monoclonal antibodies that recognize epitopes on the beta 4 subunit of the human and mouse alpha 6 beta 4 complexes. The monoclonal antibodies against beta 4 were able to preclear alpha 6 beta 4, but not alpha 6 beta 1 from cell line extracts. A substantial fraction of the total beta 4 subunits present on the cell surface was not associated with alpha 6, as it could not be removed by anti-alpha 6 antibodies, but remained precipitable with anti-beta 4 antibodies. There was no evidence for novel alpha subunits associated with beta 4. The alpha 6 subunit consists of disulfide-linked heavy and light chains. The variability in size of these two chains from different cell types is largely due to differences in modifications of N-linked glycans. Additional heterogeneity may be caused by differential proteolytic cleavage of the alpha 6 precursor. Immunoperoxidase staining of tissue sections of neonatal and adult mice revealed that beta 4 expression is limited to epithelial tissues and peripheral nerves. The alpha 6 subunit has a wider distribution that includes all tissues and cells stained by antibodies against beta 4. Cells and tissue that are positive for alpha 6, but negative for beta 4, may express the alpha 6 beta 1 complex.