Amino acid sequence of the human fibronectin receptor

Regulation of alpha 2 beta 1-mediated fibroblast migration on type I collagen by shifts in the concentrations of extracellular Mg2+ and Ca2+

Extracellular Ca2+ can reverse the Mg(2+)-dependent, alpha 2 beta 1-mediated adhesion of WI38 human fibroblasts to type I collagen substrates. Affinity chromatography data also demonstrate that Ca2+ can specifically elute the fibroblast alpha 2 beta 1 integrin bound to type I collagen-Sepharose in Mg2+. In modified Boyden chamber migration assays, Mg2+ alone supports the alpha 2 beta 1-mediated migration of fibroblasts on type I collagen substrates, while Ca2+ does not. However, a twofold enhancement in migration was observed when combinations of the two cations were used, with optimal migration observed when the Mg2+/Ca2+ ratio was higher than one. Inhibitory mAbs directed against various integrin subunits demonstrate that these observed cation effects appear to be mediated primarily by alpha 2 beta 1. These data, together with reports that under certain physiological conditions significant fluctuations in the concentrations of extracellular Ca2+ and Mg2+ can take place in vivo, suggest that the ratio between these two cations is involved in the up- and downregulation of integrin function, and thus, may influence cell migratory behavior.

A new variant of Glanzmann's thrombasthenia (Strasbourg I). Platelets with functionally defective glycoprotein IIb-IIIa complexes and a glycoprotein IIIa 214Arg----214Trp mutation

We describe a new variant of Glanzmann's thrombasthenia (variant Strasbourg I). The patient (M.S.) showed an absence of platelet aggregation to ADP, thrombin, and collagen, and a decreased clot retraction. Platelet fibrinogen was approximately 20% of normal levels. ADP-stimulated platelets bound markedly reduced amounts of soluble fibrinogen and platelet adhesion to surface-bound fibrinogen was defective. Normal to subnormal amounts of glycoprotein (GP) IIb-IIIa (alpha IIb beta 3) complexes, the platelet fibrinogen receptor, were revealed by SDS-PAGE, crossed immunoelectrophoresis, and antibody binding. However, the complexes were unusually sensitive to dissociation with EDTA at room temperature. Furthermore, flow cytometry showed that the platelets failed to bind the activation-dependent monoclonal antibody, PAC-1, after stimulation. In contrast, an RGDS-containing peptide induced significant binding of the anti-ligand-induced binding site antibody, D3GP3, suggesting the presence of a functional RGD binding domain on the patient's GPIIb-IIIa complex. Sequence analysis was performed after polymerase chain reaction amplification of selected patient's GPIIIa exons, and of the patient's platelet GPIIb and GPIIIa mRNAs. A point mutation (C to T) was localized in exon D (iv) of GPIIIa that resulted in an 214Arg to 214Trp amino acid substitution. The defect has been inherited from the parents who are heterozygous for the same mutation. This substitution points to an essential amino acid in a region of GPIIIa involved in the binding of fibrinogen and influencing the Ca(2+)-dependent stability of the GPIIb-IIIa complex.

Leukocyte adhesion deficiency: identification of novel mutations in two Japanese patients with a severe form

Leukocyte adhesion deficiency is a disorder with mutations of the gene for the beta subunit, a component common to three adhesion molecules; LFA-1, Mac-1 and p150,95. The molecular basis of the disorder was studied in two patients with its severe form. In the first patient, the mutant gene expressed an aberrant mRNA, 1.2 kb longer than usual, resulting from a G to A substitution at the splice donor site of a 1.2 kb intron. Several aberrantly spliced messages, arising from splicing at cryptic donor sites, were also identified. The beta subunit proteins deduced from the mRNA sequences lacked half the carboxyl terminal portion. In the second patient, the mutation was a G to A transition at nucleotide 454, which resulted in an Asp128 to Asn substitution of the beta subunit. The 128th Asp residue is located in a region crucial for the association with alpha subunits and strictly conserved among the integrin beta subunits so far analyzed.

Regulation of fibronectin receptor distribution

To determine the role of each intracellular domain of the fibronectin receptor in receptor distribution, chimeric receptors were constructed containing the human interleukin-2 receptor (gp55 subunit) as the extracellular and transmembrane domains, in combination with either the alpha 5 or beta 1 intracellular domain of the fibronectin receptor as the cytoplasmic domain. These chimeric receptors were transiently expressed in normal fibroblasts, and their localization on the cell surface was determined by immunofluorescence using antibodies to the human interleukin-2 receptor. The alpha 5 chimera was expressed diffusely on the plasma membrane. The beta 1 chimera, however, colocalized with the endogenous fibronectin receptor at focal contacts of cells spread on fibronectin. On cells spread in the presence of serum, the beta 1 chimera colocalized both with the fibronectin receptor at sites of extracellular fibronectin fibrils and with the vitronectin receptor at focal contacts. The beta 1 intracellular domain alone, therefore, contains sufficient information to target the chimeric receptor to regions of the cell where ligand-occupied integrin receptors are concentrated. The finding that the beta 1 chimeric protein behaves like a ligand-occupied receptor, even though the beta 1 chimera cannot itself bind extracellular ligand, suggests an intracellular difference between occupied and unoccupied receptors, and predicts that the distribution of integrin receptors can be regulated by ligand occupancy. We tested this prediction by providing a soluble cell-binding fragment of fibronectin to cells spread on laminin. Under conditions preventing further ligand adsorption to the substrate, this treatment nevertheless resulted in the relocation of diffuse fibronectin receptors to focal contacts. Similarly, a redistribution of diffuse vitronectin receptors to focal contacts occurred on cells spread on laminin after the addition of the small soluble peptide GRGDS. We conclude that the propensity for receptor redistribution to focal contacts driven by the beta 1 cytoplasmic domain alone is suppressed in heterodimeric unoccupied fibronectin receptors, and that ligand occupancy can release this constraint. This redistribution of integrin receptors after the binding of a soluble substrate molecule may provide a direct means of assembling adhesion sites.

A new extracellular matrix protein of the sea urchin embryo with properties of a substrate adhesion molecule

A new embryonic extracellular matrix protein has been purified from eggs of the sea urchin Paracentrotus lividus. The molecule is a 210 kD dimer consisting of two 105 kD subunits that are held together by S-S bridges. In the unfertilized egg, the protein is found within granules uniformly distributed throughout the cytoplasm. After the egg is fertilized, the antigen is polarized to the apical surface of ectodermal and endodermal cells during all of the developmental stages examined, until the pluteus larva is formed. The protein promotes the adhesion of blastula cells to the substrate and is antigenically distinct from echinonectin, a well characterized substrate adhesion molecule. This report adds a new candidate to the list of known extracellular matrix molecules for the regulation of differentiation and morphogenesis in the sea urchin embryo.

H36-alpha 7 is a novel integrin alpha chain that is developmentally regulated during skeletal myogenesis

H36 is a 120,000-D membrane glycoprotein that is expressed during the differentiation of skeletal muscle. H36 cDNA clones were isolated from a lambda UniZapXR rat myotube cDNA library and sequenced. The deduced amino acid sequence demonstrates that H36 is a novel integrin alpha chain that shares extensive homology with other alpha integrins that includes: (a) the GFFKR sequence found in all alpha integrins; (b) a single membrane spanning region; (c) conservation of 18 of 22 cysteines; and (d) a protease cleavage site found in the non-I region integrin alpha chains. The cytoplasmic domain of H36 is unique and additional regions of nonhomology further indicate H36 is distinct from all other alpha chains. In keeping with current nomenclature we designate this alpha chain alpha 7. Northern blots demonstrate that expression of H36-alpha 7 mRNA is regulated both early in the development of the myogenic lineage and later, during terminal differentiation. Detection of H36-alpha 7 mRNA coincides with conversion of H36- myogenic precursor cells to H36+ cells. H36-alpha 7 mRNA is present in replicating myoblasts: expression increases upon terminal differentiation and is markedly reduced in developmentally defective myoblasts. In addition, H36-alpha 7 mRNA is not detected in C3H10T1/2 cells. It is in myotubes derived from myoblasts obtained by treatment of 10T1/2 cells with azacytidine or transfection with MRF4. Immunoblots and immunofluorescence demonstrate that the H36-alpha 7 chain is associated with integrin beta 1. Affinity chromatography demonstrates that H36-alpha 7 beta 1 selectively binds to laminin. The expression of H36-alpha 7 on secondary myoblasts during the development of the limb in vivo corresponds with the appearance of laminin in the limb, with the responsiveness of secondary myoblast proliferation to laminin, and with the onset of increased muscle mass, suggesting that H36-alpha 7 modulates this stage in limb development. We conclude that H36-alpha 7 is a novel alpha integrin laminin binding protein whose expression is developmentally regulated during skeletal myogenesis.

Identification of the integrin VLA-2 as a receptor for echovirus 1

Cell surface receptors for echovirus, a common human pathogen, were identified with monoclonal antibodies that protected susceptible cells from infection with echovirus 1. These monoclonal antibodies, which prevented virus attachment to specific receptor sites, recognized the alpha and beta subunits of the integrin VLA-2 (alpha 2 beta 1), a receptor for collagen and laminin. RD rhabdomyosarcoma cells expressed little VLA-2, did not bind to 35S-labeled virus, and resisted infection until transfected with complementary DNA encoding the alpha 2 subunit of VLA-2. Thus, integrins, adhesion receptors important in interactions between cells and with the extracellular matrix, can mediate virus attachment and infection.

Distinct cellular functions mediated by different VLA integrin alpha subunit cytoplasmic domains

To characterize VLA alpha subunit cytoplasmic domain functions, unaltered alpha 2 cDNA (called X2C2) and two chimeric cDNAs (called X2C5 and X2C4) were constructed with extracellular alpha 2 domains and cytoplasmic alpha 2, alpha 5, and alpha 4 domains respectively. Upon transfection into rhabdomyosarcoma (RD) cells, each construct yielded comparable expression levels, immunoprecipitation profiles, and avidity for collagen and laminin. However, while RDX2C2 and RDX2C5 transfectants mediated collagen gel contraction, RDX2C4 and a mock transfectant (RDpF) did not. Conversely, only RDX2C4 cells (but not RDX2C2 or RDX2C5) showed enhanced cell migration on collagen and laminin compared with RDpF cells. This indicates markedly differing roles for integrin alpha subunit cytoplasmic domains in post-ligand binding events. Furthermore, stable exertion of physical force (collagen gel contraction) may involve fundamentally different cellular machinery than the transient adhesion occurring during cell migration. Finally, these findings provide insight into a functional flexibility perhaps resulting from multiple integrins binding to identical ligands.

Integrin expression in human melanoma cells with differing invasive and metastatic properties

During the process of tumor cell invasion and metastasis, tumor cells are known to interact with extracellular matrix proteins, endothelial cells, platelets and other organ-specific structures. Integrins are cell surface molecules which mediate cell-matrix and cell-cell interactions and are likely to be important for tumor cell survival and dissemination. The purpose of this study was to characterize the integrin and proteolytic enzyme repertoire from low (A375P), medium (A375M) and high metastatic (A375SM) human melanoma cell lines. These cell lines are also invasive through human amniotic membranes in vitro and their invasiveness parallels the reported metastatic phenotype. The types and levels of expression of the various integrin receptors were analysed by quantitative immunoprecipitation using a panel of monoclonal antibodies directed to known integrin subunits. In addition, cDNA probes to the integrin subunits were used in quantitative northern blot analysis. These data show that the integrin alpha v beta 3 increases 50- to 100-fold as these cells progress to a more metastatic phenotype. alpha 4 beta 1 levels also appeared to increase several fold, while other beta 1 integrins did not differ in their expression levels. The increased alpha v beta 3 expression in the more metastatic cells resulted in an increased adhesion to vitronectin and fibrinogen substrates in cell attachment assays. However, alpha v- and beta 3-specific antibodies did not inhibit A375 cell invasion through the amnion. Each cell line was found to release similar quantities of a 72-kDa gelatinase/type IV collagenase and tissue type plasminogen activator. These results suggest that during the progression of these tumor cells from a low to high metastatic phenotype, marked changes in integrin expression occurred which may facilitate interactions with platelets, endothelial cells and specific extracellular matrix proteins to promote metastasis.

GP-2/THP gene family encodes self-binding glycosylphosphatidylinositol-anchored proteins in apical secretory compartments of pancreas and kidney

A family of homologous genes is shown to encode GP-2, the major glycosylphosphatidylinositol (GPI)-linked glycoprotein of pancreatic zymogen granule membranes, and Tamm-Horsfall protein (THP), a GPI-linked glycoprotein associated with apical vesicles in kidney thick ascending limb of Henle (TALH) cells. The C-terminal regions of GP-2 (Asp54-Phe530) and THP (Asp175-His644) from rat show 53% identity, 86% similarity, and 26 conserved cysteine residues including one epidermal growth factor motif. The unique N-terminal domain of rat THP (unique-THP, Pro29-Gln174) shows four conserved epidermal growth factor motifs, three in tandem and one in reverse orientation. GP-2 homologues are observed in a wide variety of epithelial cells, several of which contain highly regulated secretory processes. GP-2 released from zymogen granule membranes with phosphatidylinositol phospholipase C reacts with anti-cross-reactive determinant antibody (anti-CRD), confirming the GPI nature of the pancreatic homologue. In contrast, GP-2 and THP, released endogenously from pancreas and kidney, respectively, do not react with anti-cross-reactive determinant antibody, suggesting alternative enzymatic mechanisms for their physiological release. Globular domains of GP-2 and THP, but not albumin, show pH- and ion-dependent self-association in vitro. The GP-2/THP family appears to represent a newly discovered class of GPI-anchored proteins, which may utilize pH- and ion-dependent self-association mechanisms for establishing membrane (micro)domains targeted to intracellular secretory compartments.

The gene organisation of the human beta 2 integrin subunit (CD18)

We have studied the gene of the human beta 2 integrin subunit (CD18) and found it to be organised into 16 exons spanning a region of about 40 kb. All exon/intron boundaries conform to the GT/AG splicing consensus. The exons coding for the cysteine-rich region, which has been postulated to consist of 3 or 4 repeating elements, are not organised correspondingly. Transcription of the gene initiates from multiple sites which may be due to the absence of an upstream TATA box. The polyadenylation site is also heterogeneous. Five different sites were identified over a stretch of 10 bases.

Receptor sequence in the terminal protein of bacteriophage M2 that interacts with an RGD (Arg-Gly-Asp) sequence of the primer protein

At the initiation of protein-primed DNA replication of bacteriophages M2 and phi 29, the Arg-Gly-Asp (RGD) sequence of primer protein participates in the recognition of terminal protein (TP), where the initiation site for protein-primed DNA replication of template DNA is located. We compared the sequences of M2 and phi 29 TP with those of the members of the integrin superfamily and found the highly homologous sequences Lys-Lys-Ile-Pro-Pro-Asp-Asp (KKIPPDD) in M2 and phi 29 TP and Lys-Lys-Gly-Cys-Pro-Pro-Asp-Asp (KKGCPPDD) in the beta-subunit of fibronectin receptor protein. A synthetic 20mer peptide that contained the KKIPPDD sequence interfered with the inhibitory effect of the RGD peptide on both transfection and the protein-priming reaction in vitro. We propose that the sequence KKIPPDD of M2 TP is the receptor sequence for RGD.

Integrin alpha 2 beta 1 is upregulated in fibroblasts and highly aggressive melanoma cells in three-dimensional collagen lattices and mediates the reorganization of collagen I fibrils

The ability of cultured human fibroblasts to reorganize and contract three dimensional collagen I gels is regarded as an in vitro model for the reorganization of connective tissue during wound healing. We investigated whether adhesion receptors of the integrin family are involved. It was found that synthesis and transcription of the alpha 2 beta 1 integrin (but not of alpha 1 beta 1 or alpha 3 beta 1) is selectively upregulated when fibroblasts are seeded into type I collagen gels. Time course experiments revealed that high synthetic levels of alpha 2 beta 1 parallel the gel contraction process and return to "baseline" levels after the contraction has subsided. Furthermore, function-blocking mAbs directed to the alpha 2 and beta 1 chain of integrins inhibited gel contraction. Remodelling of connective tissue can be important for tumor cells during invasion and formation of metastases. Therefore, we tested human melanoma cell lines for this function. Five out of nine melanoma lines contracted collagen gels in vitro. Among these, two highly aggressive melanoma cell lines (MV3 and BLM) most efficiently contracted gels almost reaching the rate of normal adult fibroblasts. In these cells, synthesis of alpha 2 beta 1 was also significantly upregulated when seeded into collagen I gels. Moreover, function blocking anti-alpha 2 in conjunction with anti-beta 1 chain mAbs completely inhibited gel contraction for several days. Other melanoma cells (530) with lower metastatic potential which were not able to contract gels, showed no induction of alpha 2 beta 1 synthesis in gel culture. Our results suggest an important role of integrin alpha 2 beta 1 in the contraction of collagen I by normal diploid fibroblasts during wound healing and in the reorganization of collagen matrices by highly aggressive human melanoma cells.

B lymphocyte fibronectin receptors: expression and utilization

Lymphocytes adhere to fibronectin (FN) via multiple receptors of the VLA (beta 1, CD29 integrin) family. The cellular requirement for the variety of FN receptors (FNR) which have been described is unclear, but they may be associated with differential signalling processes, cooperative effects which may stabilize cellular attachment, or cell homing and retention processes. The present study was undertaken to examine the FN adherence properties and receptor utilization patterns of human B cells. Of ten B-cell culture lines which were studied, six demonstrated a significant adherence to FN. Among these, four employed alpha 4 beta 1, (CD49d/29) and two employed alpha 4 beta 1/alpha 5 beta 1, (CD49d/29, CD49e/29). There was no apparent correlation between the differentiation status of the lines and their FNR utilization patterns. Furthermore, expression of FNR alone was not sufficient to confer FN binding potential. Freshly isolated tonsil B cells did not display significant adherence to FN. Following stimulation, a marked increase in VLA antigens was observed, and the capacity to attach to FN-coated surfaces was co-acquired. Analysis of the induced bulk B-cell population demonstrated that both alpha 4 beta 1 and alpha 5 beta 1 were used for adherence. These results clearly indicate that activated B cells, similar to T cells, may express and utilize alpha 5 beta 1 as a FNR.

Cloning and expression of cDNAs for the alpha subunit of the murine lymphocyte-Peyer's patch adhesion molecule

cDNA clones encoding the alpha chain of the murine lymphocyte-Peyer's patch adhesion molecule (LPAM), which is associated with lymphocyte homing, have been isolated by screening with the human VLA-4 (alpha 4h) probe. Several alpha 4 antigenic determinants were identified on COS-7 cells after transfection. From overlapping clones, approximately 5 kb of contiguous nucleotide sequence have been determined, encoding a protein sequence of 1039 amino acids for the LPAM alpha chain (alpha 4m). LPAM is a member of the integrin family of cell-surface heterodimers, and alpha 4m is the murine homologue of the human alpha 4 h chain. The two proteins have a total sequence similarity of 84%, with an almost perfect conservation (31/32 amino acids) in the cytoplasmic domain. Like alpha 4h, alpha 4m is distinct from other integrin alpha chains because it has neither an I-domain nor a COOH-terminal cleavage site. The positions of the characteristic Cysteine residues are conserved, and a putative protease cleavage site is located near the middle of the protein sequence. The NH2-terminal part of the protein contains seven homologous repeats, and three of them include putative divalent cation-binding sites. These sites are among the most conserved between the alpha 4m sequence and other alpha chains, and may therefore be involved in the binding of integrin alpha and beta chains. An additional cDNA clone was isolated which shares a sequence of perfect homology with the alpha 4m encoding cDNAs, but has a unique 3' poly-A end. This observation correlates with the fact that three discrete murine RNA bands are observed in Northern blot experiments using alpha 4m as a probe, whereas only two human RNA species are described for alpha 4h, indicating a higher complexity for murine than for human sequences.

IL-1 beta and prostaglandins regulate integrin mRNA expression

The purpose of this study was to examine the effects of IL-1 beta on integrin expression in MG-63 human osteosarcoma cells. Human recombinant IL-1 beta (rIL-1 beta) produced significant increases in both alpha 2- and alpha 5-subunit mRNA levels, as well as a smaller increase in alpha v-subunit mRNA. In contrast, IL-1 beta decreased alpha 4-subunit mRNA levels by approximately 30% relative to untreated controls. These findings suggest that human IL-1 beta differentially regulates expression of integrins. When cultures were treated with both IL-1 beta and the cyclooxygenase inhibitor, indomethacin, the expression of alpha 2-, alpha 5-, and alpha v-subunit mRNA levels were dramatically increased relative to untreated controls; co-treatment with 0.5 mM prostaglandin E2 (PGE2) partially reversed this effect. Indomethacin alone did not affect integrin mRNA levels. Treatment with IL-1 beta or IL-1 beta + indomethacin also induced significant changes in MG-63 morphology (i.e., increased cell elongation) and increased the ability of cells to contract collagen gels. PGE2 reversed the above effects on cell morphology and gel contraction. These findings indicate that (a) IL-1 beta differentially regulates the expression of integrins and (b) that PGE2, which is induced by IL-1 beta, may provide a negative feedback loop which counteracts the stimulatory effect of IL-1 beta on integrin gene expression. It is suggested that products of inflammation may affect cell behavior by differentially regulating the expression of various integrins.

The cytoplasmic domain of the integrin lymphocyte function-associated antigen 1 beta subunit: sites required for binding to intercellular adhesion molecule 1 and the phorbol ester-stimulated phosphorylation site

We have defined the regions of the cytoplasmic domain of the leukocyte integrin lymphocyte function-associated antigen 1 (LFA-1) that are required for active binding of its extracellular domain to intercellular adhesion molecule 1 (ICAM-1). The NH2-terminal 28 amino acids in the cytoplasmic domain are dispensable, but a segment of 5 amino acids including three contiguous threonines (758-760) and Phe 766 in the COOH-terminal third of the cytoplasmic domain are required for binding to ICAM-1. Mutation and phosphoamino acid analysis show that Ser 756 is the major residue phosphorylated in response to phorbol ester. Furthermore, multiple mutations demonstrate that serine phosphorylation can be dissociated from phorbol ester-stimulated binding of LFA-1 to ICAM-1. The sites we have defined are previously unremarked, are well conserved in the beta 1, beta 3, and beta 7 integrin subunits, and may be of broad importance in regulating adhesiveness of integrins.

Separation of important new platelet glycoproteins (GPIa, GPIc, GPIc*, GPIIa and GMP-140) by f.p.l.c. Characterization by monoclonal antibodies and gas-phase sequencing

A large number of membrane glycoproteins (around 40) are present on the surface of human blood platelets. Some of these glycoproteins are expressed in relatively small amounts, and their functions, as well as their structure, remain to be elucidated. The aim of the present study was to separate rapidly, under non-denaturing conditions, and characterize minor glycoproteins such as Very Late Antigens (VLA) (GPIa, GPIc, GPIc* and GPIIa) and GMP-140 (also known as PADGEM). VLAs and GMP-140 are respectively members of the integrin and selectin families. Platelet membrane glycoproteins were separated by wheat-germ agglutinin lectin affinity and Mono Q anion-exchange f.p.l.c. Peaks bearing isolated glycoproteins were electrophoresed on one- or two-dimensional SDS/polyacrylamide gels, Western blotted on to Immobilon poly(vinylidene difluoride) membranes and gas-phase-sequenced. The identity of isolated glycoproteins was also obtained by the use of monoclonal or polyclonal antibodies and tryptic peptide maps. Five minor [GPIa, GPIc, GPIc*, GPIIa and GMP 140 (PADGEM)], as well as a major (GPIIIb) glycoprotein, were eluted at low salt concentrations. GPIIb-IIIa and GPIb were eluted at high salt concentrations. The N-terminal sequence of platelet GPIa was identical with that obtained by Takada & Hemler [(1989) J. Cell Biol. 109, 397-407]. However, the N-terminal sequence of platelet GPIc + Ic* and GPIIa were found to differ from those deduced from cDNA sequences isolated from human placenta or umbilical-vein endothelial-cell cDNA libraries. The combined use of f.p.l.c. and gas-phase sequencing techniques provides a very powerful tool to separate and characterize rapidly platelet or other cellular proteins for structural, immunological and functional studies.

Laminin-binding integrin alpha 7 beta 1: functional characterization and expression in normal and malignant melanocytes

A novel integrin, alpha 7 beta 1, that specifically binds with high affinity to laminin has been identified on melanoma cells. This complex was purified from both human and murine melanoma cells by laminin-affinity chromatography, and the alpha 7 subunit was recovered after gel electrophoresis. N-terminal amino acid sequence analysis of the alpha 7 subunit from both human and mouse cells verifies that this integrin is distinct from other alpha chains in the beta 1 family, although strikingly similar to the alpha 6 subunit. By using specific proteolytically derived fragments of laminin, it was determined that the alpha 7 beta 1 complex binds selectively to the E8 region, which represents part of the long arm of laminin. In contrast, the receptor failed to bind to the P1 fragment, which contains the intersection of the short arms of laminin. Although the alpha 7 beta 1 complex was commonly expressed in melanoma cells, this integrin was not detected in normal melanocytes, suggesting that alpha 7 expression may be associated with malignant transformation. These results establish the existence of a novel integrin that binds to the E8 domain of laminin and appears to mediate cell adhesion to this ligand.

Molecular cloning and expression of the cDNA for alpha 3 subunit of human alpha 3 beta 1 (VLA-3), an integrin receptor for fibronectin, laminin, and collagen

alpha 3 beta 1 (VLA-3), a member of the integrin family of cell adhesion receptors, may function as a receptor for fibronectin, laminin, and collagen. A partial cDNA clone (2.4 kb) for the human alpha 3 subunit was selected from an endothelial cell lambda gt11 cDNA library by specific antibody screening. Several overlapping cDNA clones were subsequently obtained, of a total length of 4.6 kb from various cDNA libraries. The reconstructed alpha 3 cDNA was expressed on the surface of chinese hamster ovary cells as detected by an alpha 3-specific mAb after transfection, suggesting that the cDNA is authentic. Within this sequence was an open reading frame, encoding for 1,051 amino acids, including a signal peptide of 32 residues, a long extracellular domain (959 residues), a transmembrane domain (28 residues), and a short cytoplasmic segment (32 residues). Overall, the alpha 3 amino acid sequence was 25-37% similar to the other integrin alpha subunits that are cleaved, with most similarity to the alpha 6 sequence (37%), and less similarity to those alpha subunits that have I domains (15-20%, excluding the I domain sequence itself). Features most like those in other alpha subunits are (a) the positions of 18/19 cysteine residues, (b) three potential metal binding domains of the general structure DX(D/N)X(D/N)GXXD, and (c) the predicted transmembrane domain. The mass of alpha 3 calculated from its amino acid sequence is 113,505. The human alpha 3 sequence was 89% identical to hamster galactoprotein b3, and 70% similar to the chicken CSAT antigen band 2 protein partial sequence, suggesting that these two polypeptides are homologues of human alpha 3.

Detection of fibronectin receptor in sera: its clinical significance as a parameter of hepatic fibrosis

Pooled sera collected from cirrhotic patients was fractionated by affinity chromatography with a fibronectin receptor monoclonal antibody against the beta-subunit of fibronectin receptor. Eluates were assayed using Western immunoblotting. The relative mobility of the protein reactive with fibronectin receptor antibody was nearly identical to that of the beta-subunit of fibronectin receptor, confirming that fibronectin receptor is present in human serum. Serum levels of the beta-subunit of fibronectin receptor were analyzed by sandwich enzyme-linked immunosorbent assay in patients with various liver diseases. The serum level of fibronectin receptor (micrograms/ml) was significantly higher in patients with chronic hepatitis (inactive, 2.59 +/- 0.04; active, 3.45 +/- 0.13), cirrhosis (4.77 +/- 0.30), alcoholic liver disease (2.96 +/- 0.16) and hepatocellular carcinoma (4.71 +/- 0.49) than in normal subjects (2.11 +/- 0.08). Strong positive correlation was observed between serum levels of fibronectin receptor and histological findings, particularly in the degree of hepatic fibrosis. Immunohistochemical studies with fibronectin receptor antibody revealed that the beta-subunit of fibronectin receptor was present on the plasma membrane of hepatocytes and sinusoidal lining cells in the normal liver and was increased in fibrotic areas and on the plasma membrane of hepatocytes and sinusoidal lining cells of fibrotic liver. The serum level of fibronectin receptor in patients with chronic liver diseases may therefore be a useful marker of hepatic fibrosis.

Ependymin, a brain extracellular glycoprotein, and CNS plasticity

Ependymin, a glycoprotein of the brain ECF, has been implicated in the neurochemistry of memory and neuronal regeneration. Three behavioral experiments (swimming with a float, avoidance conditioning, and classical conditioning) in the goldfish and one in the mouse (T-maze learning) indicate that ependymin has a role in the synaptic changes that take place in the consolidation step of memory formation and the activity-dependent phase of sharpening of goldfish retinotectal connections during neuronal regeneration. The ECF concentration of the protein was found to decrease after the goldfish learned to associate a light stimulus (CS) with the subsequent arrival of a shock (US): paired CS-US gave changes whereas an unpaired presentation of CS-US gave no changes relative to the unstimulated controls. Ependymin is present in ECF as a mixture of three disulfide-linked dimers of two acidic (alpha and beta) polypeptide chains (37 kDa and 31 kDa). Upon removal of its N-linked glycan fragment by N-glycosidase F, the beta chain yields gamma-ependymin (26 kDa). Determinations of the amino acid sequence of gamma-ependymin indicate that it is a unique protein with no long sequence homologies to any known polypeptide. There are, however, small segments (5-7 amino acids long) with homologies to fibronectin, laminin, and tubulin. Ependymin has the capacity to polymerize into FIP (after activation by phosphorylation) in response to events that deplete ECF calcium. FIP is insoluble in 2% SDS in 6 M urea, 10 mM Ca2+Ac2, 100% acetic acid, chloroform/methanol (2/1), saturated KCNS, and even 100% trifluoroacetic acid. FIP was found to be present in goldfish brain and to be formed as a labeled product in vivo. Ependymin's FIP-forming property was used to propose a molecular hypothesis for generating synaptic changes in response to local extracellular depletions of calcium at sites of "associating inputs." The model assumes that, following NMDA receptor stimulation, the translocated PKC that is generated activates extracellular ependymin by converting it to its phosphorylated form using presynaptically released ATP. The hypothesis was tested in studies of LTP of rat hippocampal slices at CA1. After LTP, new sites that stained with antisera to ependymin, visible at 100x, were obtained in its potentiated radiatum in the CA1 region but not in the unpotentiated CA3. Electron microscopic studies showed that the horseradish peroxidase reaction product obtained was localized at synaptic clefts and postsynaptic regions. The results suggest that FIP may be formed at extracellular and postsynaptic loci where multiple associating inputs interact at CA1.

Novel integrin alpha and beta subunit cDNAs identified in airway epithelial cells and lung leukocytes using the polymerase chain reaction

The integrins are a large group of cell surface glycoproteins that mediate cell-matrix and cell-cell adhesive interactions. Integrins play a role in normal lung development, in host defense against pulmonary infection, and in the pathogenesis of the adult respiratory distress syndrome. Integrins are heterodimers consisting of one alpha subunit and one beta subunit. We identified consensus sequences within integrin subunits and used oligonucleotide primers based on these sequences to amplify cDNA by the polymerase chain reaction (PCR). We previously reported the use of this homology PCR technique for the identification of one novel integrin beta subunit, beta 6, from guinea pig airway epithelial cells. Here we demonstrate that primers based on alpha subunit consensus sequences can also be used for homology PCR. We have used the alpha and beta subunit primers to amplify and clone a large variety of integrin partial cDNAs from several cell types and species. Comparison of the deduced amino acid sequences reveals a high degree of cross-species conservation (86 to 98% identity). One alpha subunit (identified in guinea pig airway epithelial cells) and one beta subunit (identified in rabbit leukocytes obtained by bronchoalveolar lavage and in human and mouse leukocyte cell lines) have novel sequences that are related to but clearly distinct from all previously reported integrin sequences (24 to 61% identity). These novel cDNAs are very likely to encode previously unsequenced integrin subunit proteins that are expressed in the lung. Homology PCR is a powerful technique for the identification of known and novel integrin alpha and beta subunit cDNAs in cells from the lung and other organs.

Molecular cloning of the human alpha 6 integrin subunit. Alternative splicing of alpha 6 mRNA and chromosomal localization of the alpha 6 and beta 4 genes

We have isolated cDNAs encoding the alpha 6 subunit from a lambda gt11 expression library from human keratinocytes by combined screening with a rabbit polyclonal anti-alpha 6 antibody and the polymerase chain reaction. The alpha 6 subunit encoded by this cDNA consists of 1050 amino acids with a 991-amino-acid extracellular, a 23-amino-acid transmembrane and a 36-amino-acid cytoplasmic domain. The extracellular domain contains three putative divalent cation-binding sites and nine potential N-linked glycosylation sites. From a cDNA library from normal human mammary gland cells two different cDNAs for alpha 6 were isolated, one of which is identical to the above cDNA. The two alpha 6 subunits, called alpha 6A and alpha 6B, encoded by the two cDNAs each have a unique cytoplasmic domain, that of alpha 6B being 18 amino acids longer than that of alpha 6A. Different carcinoma cell lines contain transcripts for both alpha 6 subunits. K562 leukemic cells have little alpha 6A or alpha 6B mRNAs. The overall level of expression varies in the carcinoma cell lines, but reflects alpha 6 cell surface expression. In A375 melanoma cells, however, cell surface expression of alpha 6 was low in spite of a high level of mRNA. This suggest that other mechanisms may be involved in regulating the expression of alpha 6 on the surface of these cells. The mRNA for both alpha 6 subunits is around 6 kb. The alpha 6 subunits are similar to other alpha subunits (26-31% identity with cleaved alpha subunits) of the integrin family but they are more similar to the alpha 3 subunit (40% identity). This high degree of similarity may be the basis for their functional resemblance since both alpha 3 and alpha 6 subunits, when associated with beta 1, function as laminin receptors and bind to the long arm of laminin. The genes for alpha 6 and beta 4, the alternative beta subunit with which alpha 6 combines on certain epithelial cells, were mapped to chromosome 2 and 17q11-qter, respectively.

Participation of D-galactose-specific receptors of liver macrophages in recognition of fibronectin-opsonized particles

The interaction of immobilized human or rat plasma fibronectin with isolated rat liver macrophages was studied in a model system using colloidal gold of 17-nm diameter (Au-17) as test particles. Plasma fibronectin (pFn)-coated gold particles were rapidly bound and endocytosed via the coated pit-coated vesicle pathway as demonstrated by photometry, and light and electron microscopy. The isolated macrophages bind 2.5 +/- 2 particles/10 microns of plasma membrane (incubation at 4 degrees), equalling a binding capacity of approximately 3.5 x 10(4) pFn-Au-17 particles per cell. Binding and uptake (at 37 degrees) was specifically inhibited by D-galactose-related carbohydrates, but not by D-mannose, N-acetyl-D-glucosamine, nor by excess soluble pFn. Uptake was also inhibited by lactosylated bovine serum albumin at a concentration of 10(-6) M but not by bovine serum albumin. India ink uptake by the liver macrophages in the presence of fibronectin was also inhibited by D-galactose-related monosaccharides. The presence of terminal, nonreducing D-galactosyl groups on pFn could be demonstrated by agglutination experiments with the D-galactose-specific plant lectin, Ricinus communis agglutinin (RCA), which could also be used for isolation of pFn from rat plasma. The 29-kDa molecular mass D-galactose-specific receptor, known to be expressed on the liver macrophage membrane and recently shown to be a membrane-bound form of C-reactive protein, was found to bind the pFn-coated gold particles in dot blotting experiments. It was concluded that the D-galactose-specific macrophage receptor binds to terminal D-galactose-related units of immobilized pFn and participates in recognition of fibronectin-opsonized particles.

Fibronectin augments anti-CD3-mediated IL-2 receptor (CD25) expression on human peripheral blood lymphocytes

The extracellular matrix (ECM) is composed of a number of macromolecules that promote cell adhesion, cell migration, and differentiation. Receptors for these molecules have been identified and belong to a superfamily of cell surface proteins, collectively known as the integrins. In this study, we show that the matrix protein fibronectin (FN) acts synergistically with immobilized anti-CD3 antibody to promote proliferation of total human peripheral blood lymphocytes (HPBL) in the absence of exogenous IL-2. Proliferation was inhibited by both the alpha 5 beta 1 and alpha 4 beta 1 recognition peptides. ARG-GLY-ASP (RGD), and GLU-ILE-LEU-ASP-VAL-PRO-SER-THR (EILDVPST), respectively. Expression of CD25 (IL-2 receptor) was significantly higher on cells cultured on anti-CD3 and FN, indicative of T-cell activation. Additionally, cells cultured on immobilized anti-CD3 and FN for 3 days showed increased adhesion to FN and increased forward light scatter/side scatter profile. Synthesis of both IL-1 and to a lesser extent IL-2 was elevated in supernatants from cultures containing both anti-CD3 and FN. These data are consistent with published reports which demonstrate that ECM proteins can act as costimulants of lymphocyte proliferation. Finally, our results show that cells cultured on anti-CD3 antibody and FN have an activated phenotype and that cytokines may be involved in this process.

Human monoclonal autoantibody 2E7 is specific for a peptide sequence of platelet glycoprotein IIb. Localization of the epitope to IIb231-238 with an immunodominant Trp235

2E7 is a human monoclonal IgM autoantibody that binds to a site on the heavy chain of the human platelet integrin alpha subunit glycoprotein IIb. The epitope recognized by 2E7 is stable to denaturation with sodium dodecyl sulfate and reduction of disulfide bonds but is destroyed by proteolysis with papain, chymotrypsin or elastase. By evaluating the reaction of 2E7 with a number of protein sequences from the IIb heavy chain, we have determined that the epitope is located in the octapeptide Phe-Asp-Gly-Tyr-Trp-Gly-Tyr-Ser (FDGYWGYS), corresponding to residues 231-238, and that substitution of the Trp at position 235 completely destroys the epitope. This represents the first precise localization of an epitope on the human platelet integrin IIb-IIIa or on any platelet membrane glycoprotein that is recognized by a human autoantibody.

Changes in the gene expression of collagens, fibronectin, integrin and proteoglycans during matrix-induced bone morphogenesis

Subcutaneous implantation of demineralized bone matrix in rat results in the local cartilage and bone development. This in vivo model of bone formation was used to examine the expression patterns of cartilage and bone specific extracellular matrix genes. The steady state levels of mRNA in implants for cartilage specific type II collagen, type IX collagen, proteoglycan link protein and cartilage proteoglycan core protein (aggrecan) were increased during chondrogenesis and cartilage hypertrophy. Fibronectin mRNA levels were high during mesenchymal cell migration, attachment and chondrogenesis. Integrin (beta 1 chain) mRNA was expressed throughout the endochondral bone development. Type I collagen mRNA levels in implants increased as early as day 3, reached its peak during osteogenesis. These gene markers will be useful in the study of the mechanism of action of bone morphogenetic proteins present in the demineralized bone matrix.

Characterization of the alpha 4 integrin gene promoter

A cDNA for the alpha 4 chain of the alpha 4 beta 1 integrin was described previously [Takada, Y., Elices, M. J., Crouse, C. & Hemler, M. E. (1989) EMBO J. 8, 1361-1368]. Primer extension analysis indicated that alpha 4 mRNA extended well beyond the 5' end of this cDNA. To clone this 5' sequence, a primer extension cDNA library was constructed, and a cDNA extending an additional 660 base pairs was isolated. This cDNA hybridized to multiple mRNAs in both T and B lymphocytes, but no alpha 4 mRNA was found in different tissues or in adherent cell lines. A single alpha 4 gene was detected in a genomic Southern blot when hybridization was done at high stringency; however, additional bands were observed at lower stringency, indicating the presence of alpha 4-related genes. Some of the different mRNAs that hybridize to the alpha 4 cDNA may then be the products of these related genes. Analysis of the alpha 4 genomic sequence revealed a large first exon of 958 base pairs. Interestingly, translation of alpha 4 initiates from the second ATG in this exon (nucleotide + 744). The first ATG (nucleotide +21) is followed by a termination codon 21 amino acids downstream. Such upstream ATG codons have been implicated in translational control of protooncogenes. One major transcriptional start site was identified by using S1 nuclease and primer extension mapping. Consensus sequences for DNA regulatory elements were found upstream of the gene and in exon 1 and intron 1. The alpha 4 gene 5' flanking region acted as a promoter in transfection assays. Detailed characterization of the promoter should provide insight into molecular events regulating expression and tissue specificity of alpha 4.

Expression of the fibronectin receptor VLA-5 is regulated during human B cell differentiation and activation

We examined the expression of VLA-5, a fibronectin receptor, during human B cell development and activation. VLA-5 is a member of the integrin supergene family; VLAs are heterodimers of at least six unique alpha chains sharing a common beta chain; most are involved in cell attachment to extracellular matrix (ECM). A hypothesis of haematopoietic development is that maturing cells leave the bone marrow because of the loss of VLA-5 during differentiation. However, mature B cells are not primarily circulating cells, and the role of ECM receptors in homing to peripheral lymphoid tissue and inflammatory sites is unknown. To examine the expression of VLA-5 during B cell development, cell lines blocked at specific stages of differentiation were evaluated for their synthesis and surface expression of VLA-5 using VLA-5-specific antibody and cDNA probes. VLA-5 mRNA and surface expression were found in the pre-B cell lines, REH and Nall 1, but not in more differentiated Raji cells or in several EBV-transformed peripheral B cell lines. Circulating peripheral B lymphocytes and resting tonsillar and splenic B lymphocytes expressed no VLA-5 by FACS analysis. Interestingly, mRNA and surface expression of VLA-5 were found in SKW, a highly differentiated, IgM-secreting line. In addition, low levels of staining for VLA-5 expression could be demonstrated when tonsillar or peripheral blood B lymphocytes were stimulated by Staphylococcus aureus Cowan (SAC). All cell lines expressed VLA-3 and VLA-4, two other receptors reported to mediate fibronectin binding in some cell types. Thus, our studies provided no evidence for developmental or inflammatory regulation of these receptors. Binding studies, however, demonstrated that adherence of both pre-B REH cells and SKW cells to fibronectin was almost completely inhibited by a monoclonal antibody to VLA-5 alpha. In addition, Raji cells, which lack VLA-5 but express VLA-3 and VLA-4, showed very low level binding to fibronectin. This demonstrates that for some B lymphocytes VLA-5, rather than other possible fibronectin receptors, primarily mediates attachment to fibronectin. These data also suggest that human VLA-5 expression is regulated during B cell development, with expression at a very early stage and then again after activation. This pattern of loss and reacquisition of an ECM receptor may be relevant to normal B cell maturation and to function during immunologic injury.

Primary structure and functional activity of a phosphatidylinositol-glycan-specific phospholipase D

A phosphatidylinositol-glycan-specific phospholipase D (PI-G PLD) that specifically hydrolyzes the inositol phosphate linkage in proteins anchored by phosphatidylinositol-glycans (PI-Gs) has recently been purified from human and bovine sera. The primary structure of bovine PI-G PLD has now been determined and the functional activity of the enzyme has been studied. Expression of PI-G PLD complementary DNA in COS cells produced a protein that specifically hydrolyzed the inositol phosphate linkage of the PI-G anchor. Cotransfection of PI-G PLD with a PI-G-anchored protein resulted in the secretion of the PI-G-anchored protein. The results suggest that the expression of PI-G PLD may influence the expression and location of PI-G-anchored proteins.

Isolation and sequence of partial cDNA clones of human L1: homology of human and rodent L1 in the cytoplasmic region

We have isolated cDNA clones coding for the human homologue of the neuronal cell adhesion molecule L1. The nucleotide sequence of the cDNA clones and the deduced primary amino acid sequence of the carboxy terminal portion of the human L1 are homologous to the corresponding sequences of mouse L1 and rat NILE glycoprotein, with an especially high sequences identity in the cytoplasmic regions of the proteins. There is also protein sequence homology with the cytoplasmic region of the Drosophila cell adhesion molecule, neuroglian. The conservation of the cytoplasmic domain argues for an important functional role for this portion of the molecule.

PCR primers for human chromosomes: reagents for the rapid analysis of somatic cell hybrids

Rapid analysis of somatic cell hybrids can be facilitated by using the polymerase chain reaction (PCR) to assay for genes assigned to specific human chromosomes. We describe PCR primer pairs for genes on the short and long arms of the 22 autosomes and the X chromosome. Some of the primers were designed from the 3' untranslated region of cDNA sequences, whereas others were derived from genomic sequence. Each primer set was tested for its specificity and mapped to a chromosome by screening a somatic cell hybrid panel. Two of the primer pairs (APOC2 and G6PD) detect CA dinucleotide repeat polymorphisms.

Expression and role of integrins in adhesion of human colonic carcinoma cells to extracellular matrix components

We have examined integrin expression and function in the human colon carcinoma cell line HT29, and in clonal sublines derived from the HT29 line. These cells express several different integrin subunits including beta 1, alpha 2, 3, 6 and alpha v, but do not express the classic alpha 5/beta 1 fibronectin receptor. Clonal variation in the pattern of integrin expression was quite limited. The profile of integrin expression correlates well with the adhesive behavior of HT29 cells. Thus the cells adhere well to vitronectin, laminin and type IV collagen, but not at all to fibronectin. Adhesion to collagen was completely blocked by an anti-beta 1 monoclonal antibody, indicating that beta 1 integrins mediate this process. Adhesion to laminin was strongly blocked by anti-beta 1 monoclonal or anti-beta 6 monoclonal, suggesting that the alpha 6/beta 1 complex functions in attachment to laminin; this was somewhat surprising since immunoprecipitation experiments indicate that most of the alpha 6 subunit seems to be associated with the beta 4 subunit. Despite their strong adherence to laminin, collagen and vitronectin, HT29 cells are not very motile and, in response to gradients of these proteins, do not migrate nearly as well as CHO cells tested under similar conditions. Since HT29 cells can undergo an enterocyte-like differentiation in glucose-free medium, we compared integrin expression in HT29 and its subclones during the process of differentiation. There was no correlation between the state of differentiation, as assessed by expression of brush-border hydrolases, and the level of expression of any of the integrin subunits measured. Thus the pattern of integrin expression in these colonic tumor cells seems to be a characteristic of the cell line, and is not readily modified by changes in cell growth or differentiation.

Vitronectin and proliferative intraocular disorders. II. Expression of cell surface receptors for fibronectin and vitronectin in periretinal membranes

Several cell types participate in the formation of vitreoretinal traction membranes in proliferative intraocular disorders. The communication between these cells involves hormones, growth factors, and the interaction with extracellular matrix molecules. We have previously demonstrated a partial colocalisation of two potent mediators of cell attachment, fibronectin and vitronectin, in periretinal membranes from patients with proliferative vitreoretinopathy (PVR). We found a similar pattern of vitronectin and fibronectin deposition in proliferative diabetic retinopathy (PDR) (n = 6). Now we show the expression of the corresponding cell surface receptors, integrins, for fibronectin and vitronectin by proliferating cells in 22 periretinal membranes, including traumatic (n = 8) and idiopathic (n = 8) PVR as well as PDR membranes (n = 6). Integrins are membrane receptors for extracellular matrix macromolecules which are involved in such basic biological phenomena as embryogenesis and metastasis. Future studies on the pathogenesis of vitreoretinal proliferation will have to focus on the initiation, maintenance, and regulation of this intercellular communication network involving attachment proteins and integrins.