Transcriptional regulation of cardiac development: implications for congenital heart disease and DiGeorge syndrome

In recent years, impressive advances have occurred in our understanding of transcriptional regulation of cardiac development. These insights have begun to elucidate the mystery of congenital heart disease at the molecular level. In addition, the molecular pathways emerging from the study of cardiac development are being applied to the understanding of adult cardiac disease. Preliminary results support the contention that a thorough understanding of molecular programs governing cardiac morphogenesis will provide important insights into the pathogenesis of human cardiac diseases. This review will focus on examples of transcription factors that play critical roles at various phases of cardiac development and their relevance to cardiac disease. This is an exciting and burgeoning area of investigation. It is not possible to be all-inclusive, and the reader will note important efforts in the areas of cardiomyocyte determination, left-right asymmetry, cardiac muscular dystrophies, electrophysiology and vascular disease are not covered. For a more complete discussion, the reader is referred to recent reviews including the excellent compilation of observations assembled by Harvey and Rosenthal (1).

Rnf4, a RING protein expressed in the developing nervous and reproductive systems, interacts with Gscl, a gene within the DiGeorge critical region

A yeast 2-hybrid screen was performed to identify possible transcriptional modulators interactive with goosecoid-like (gscl), a transcription factor with suppressive activity, expressed during early brain and gonad development. The screen resulted in the identification of a RING protein known as rnf4 or snrf. Gscl/rnf4 interactions were confirmed by affinity chromatography and by immunoprecipitation. Northern analysis confirmed earlier reports of ubiquitous rnf4 expression in adult tissues. Immunohistochemical analysis of mouse embryos revealed expression primarily in the developing nervous system, with strong expression in the dorsal root ganglia and developing gonads. In contrast to previous reports, both cytoplasmic and nuclear expression of rnf4 was documented. The results reported here confirm and extend earlier reports of rnf4 expression. They suggest for the first time, that in addition to acting as a modulator of transcriptional activity, rnf4 may function, as do other RING proteins, to promote the formation of intracytoplasmic complexes involved in shuttling information between the cytoplasm and the nucleus.

p57Kip2 expression is enhanced during mid-cardiac murine development and is restricted to trabecular myocardium

During embryonic development the heart is required to grow in size and cell number, undergo complex morphologic alterations, and function to circulate the blood. Between embryonic d 10.5 (E10.5) and E11.5, cardiac myocytes undergo rapid cell division, resulting in doubling of cardiac mass, while metabolic requirements are increased and contraction force is enhanced. Accelerated cardiomyocyte differentiation is accompanied by a significant increase in trabeculation of ventricular myocardium. Many single gene mutations in the mouse result in a "thinned myocardium" and embryonic lethality between E10.5 and E13.5 secondary to heart failure. This is the case in the Splotch mouse in which a mutation of the Pax3 gene results in neural crest and cardiac defects. Nevertheless, the molecular events governing these important developmental steps remain largely unknown. Here, we describe the use of suppression subtractive hybridization to identify mRNA transcripts whose expression is enhanced during this critical period in normal hearts. These genes encode functions related to maturation of the contractile apparatus, cardiomyocyte differentiation, altered cellular metabolism, and transcriptional regulation. One of the genes that we identified, p57Kip2, encodes a cyclin-dependent kinase inhibitor of the p21 family. We show that p57Kip2 is normally expressed in the inner trabecular layer of the developing heart. In Splotch embryos, expression of p57Kip2 is expanded to encompass the entire thickness of the myocardium. This result and further structural analysis suggests that the myocardial defect of Splotch embryos is associated with precocious cardiomyocyte differentiation.

Adenoviral-mediated expression of antisense RNA to fibroblast growth factors disrupts murine vascular development

Fibroblast growth factors (FGFs) are expressed in the developing embryo and are postulated to regulate embryonic and vascular growth. The goal of this study was to elucidate the role of basic fibroblast growth factor (FGF-2) in early murine embryonic cardiovascular development in the mouse embryo. Gestation day 7.5 embryos were harvested and placed in culture, and 12 hr later replication-defective adenovirus (0.5 x 10(6) plaque forming units) encoding either beta-galactosidase or antisense FGF-2 RNA was injected into the sinus venosus of the cultured embryos. Embryos receiving only replication-defective adenovirus expressing the beta-galactosidase gene continued to develop normally over the next 12 hr. In contrast, those receiving adenovirus encoding antisense FGF-2 RNA displayed marked morphogenetic abnormalities, including cessation of growth and abnormal yolk sac vascular development, even though the embryonic hearts continued to beat. Abnormal development of the yolk sac vasculature was confirmed by microangiography and by histologic examination. Coinjection of virus carrying FGF-2 cDNA in the sense orientation reversed the effects of antisense FGF-2 RNA expression. These results confirm the efficacy of the replication-defective adenovirus for targeting gene expression to the developing vasculature and provide evidence for a critical role of FGF in the normal vascular assembly in the early embryo. Cessation of embryonic growth on expression of antisense FGF-2 RNA was most likely attributable to failure of efficient circulation of the early embryonic blood cells from the yolk sac into the embryo.

Adenoviral gene transfer of beta3 integrin subunit induces conversion from radial to vertical growth phase in primary human melanoma

Expression of the beta3 subunit of the alphavbeta3 vitronectin receptor on melanoma cells is associated with tumor thickness and the ability to invade and metastasize. To address the role of alphavbeta3 in the complex process of progression from the nontumorigenic radial to the tumorigenic vertical growth phase of primary melanoma, we examined the biological consequences of overexpressing alphavbeta3 in early-stage melanoma cells using an adenoviral vector for gene transfer. Overexpression of functional alphavbeta3 in radial growth phase primary melanoma cells 1) promotes both anchorage-dependent and -independent growth, 2) initiates invasive growth from the epidermis into the dermis in three-dimensional skin reconstructs, 3) prevents apoptosis of invading cells, and 4) increases tumor growth in vivo. Thus, alphavbeta3 serves diverse biological functions during the progression from the nontumorigenic radial growth phase to the tumorigenic and-invasive vertical growth phase primary melanoma.

Immunohistochemical analysis of the expression of two serine-threonine kinases in the maturing mouse testis

Previously we identified two intronless serine-threonine kinase genes (Tsk1 and Tsk2) located 3 kb apart on mouse chromosome 16 (Galili, N., Baldwin, H.S., Lund, J., Reeves, R., Gong, W., Wang, Z., Roe, B.A., Emanuel, B.S., Nayak, S., Mickanin, C., Budraf, M.L., Buck, C.A., 1997. A region of mouse chromosome 16 is syntenic to the DiGeorge, velocardiofacial syndrome minimal critical region. Gen. Res. 7, 17-26). Tsk1 was identical to a putative testicular kinase reported by Bielke et al. (Bielke, W., Blaschke, R.J., Miescher, G.C., Zurcher, G., Andres, A.C., Ziemiecki, A., 1994. Characterization of a novel murine testis-specific serine/threonine kinase. Gene 13, 235-239). Here we document the expression patterns of each Tsk throughout spermiogenesis showing an initial association of Tsk1 with cells in meiotic metaphase and a later association of Tsk2 with tail-like structures in the lumen of the seminiferous tubule.

Gscl, a gene within the minimal DiGeorge critical region, is expressed in primordial germ cells and the developing pons

Gscl, a paired-type homeobox gene, has been implicated in the pathology of DGS/VCFS by virtue of its genomic location and its structural similarity to the Gsc gene family. Immunohistochemical and in situ studies were performed to examine the expression pattern of this gene during embryonic development. A polyclonal antibody, generated to the full-length protein and shown to be specific for GSCL by both Western blotting and immunofluorescence, was used for immunohistochemical localization. Both in situ and antibody staining localized GSCL expression to a cluster of cells in the pons region of the developing brain. This GSCL expression pattern showed partial overlap with that of Pax6. More detailed immunohistochemistry revealed the GSCL in primordial germ cells during migration from the epithelium of the hindgut and later as they colonize the developing gonads. GSCL was not detected in tissues affected in DGS/VCSF.

Identification and characterization of alpha 3 beta 1 integrin on primary and transformed rat islet cells

Dispersed rat islet cells embedded in a matrix of collagen I are known to form aggregates in vitro reminiscent of native islets. Furthermore, it appears that islet function and survival are better maintained in vitro when cells are grown in the presence of extracellular matrix. These studies suggest an important role of cell--matrix interactions in the formation and maintenance of islet structure and function. The molecular basis of these interactions is mostly unknown. In the present study, we confirm the presence of beta 1 integrins on primary and transformed (RIN-2A line) rat islet cells. Perturbation studies in vitro show that beta 1 integrins play a role in islet cell attachment and spreading on bovine extracellular matrix and on the matrix produced by A-431 cells. The alpha 3 integrin subunit is coimmunoprecipitated with beta 1 from extracts of both primary and transformed islet cells, and immunodepletion studies suggest that alpha 3 beta 1 represents nearly half of the total beta 1 integrins expressed on primary islet cells. In situ, alpha 3 and beta 1 are expressed on the surface of all islet cell types, as shown by indirect immunocytochemistry on paraformaldehyde-fixed sections of rat pancreas. In conclusion, the study demonstrates the presence of alpha 3 beta 1 on primary and transformed rat islet cells, and an important role of beta 1 integrins in islet cell attachment and spreading in vitro.

Epitopes of adhesion-perturbing monoclonal antibodies map within a predicted alpha-helical domain of the integrin beta 1 subunit

Several recent studies have demonstrated the involvement of various domains of the beta 1 integrin subunit in ligand binding. Thus, specific amino acids have been shown to be important in divalent cation binding, and others have been implicated by peptide crosslinking to play an intimate role in integrin-ligand interactions. Added to these data are previous observations that a group of adhesion-blocking anti-chicken beta 1 antibodies mapped within the first 160 amino acid residues of the subunit. These observations suggested that this region plays a critical role in integrin ligand recognition. In order to further define the domain in which the epitopes for these antibodies are clustered, a series of mouse/chicken chimeric beta 1 constructs were examined for their reactivity with each of these antibodies. Most of the antibodies recognize a region between residues 124 to 160 of the chicken beta 1 subunit. Computer modeling predicted a possible amphipathic alpha-helical configuration for the region between residues 141 to 160. Consistent with this prediction, circular dichroism and NMR analysis revealed a tendency for a synthetic peptide containing these residues to form an alpha-helix. The significance of this structural characteristic was demonstrated by a mutation at residue 149 that disrupted the alpha-helix formation and resulted in a loss of the ability to form heterodimers with alpha subunits, localize to focal contacts, or be transported to the cell surface. The direct involvement of residues 141 to 160 in ligand binding was supported by the ability of a peptide with this sequence to elute integrins from a fibronectin affinity column. Thus, our data suggest that residues 141 to 160 of the integrin beta 1 subunit, when arranged in an alpha-helix configuration, participate in ligand binding.

A region of mouse chromosome 16 is syntenic to the DiGeorge, velocardiofacial syndrome minimal critical region

DGS and VCFS, haploinsufficiencies characterized by multiple craniofacial and cardiac abnormalities, are associated with a microdeletion of chromosome 22q11.2. Here we document synteny between a 150-kb region on mouse chromosome 16 and the most commonly deleted portion of 22q11.2. Seven genes, all of which are transcribed in the early mouse embryo, have been identified. Of particular interest are two serine/threonine kinase genes and a novel goosecoid-like homeobox gene (Gscl). Comparative sequence analysis of a 38-kb segment reveals similarities in gene content, order, exon composition, and transcriptional direction. Therefore, if deletion of these genes results in DGS/VCFS in humans, then haploinsufficiencies involving this region of chromosome 16 should recapitulate the developmental field defects characteristic of this syndrome.

Expression patterns of adhesion receptors in the developing mouse lung: functional implications

A detailed, immunohistological study of mouse lung development from the first appearance of primary lung buds off the laryngo tracheal groove through the formation of the mature, adult lung has been carried out using monoclonal antibodies specific for endothelial cells, smooth muscle cells, adhesion receptors and markers of mature endothelial cell function. These included mAbs specific for PECAM-1, alpha-smooth muscle actin, ICAM-1, ICAM-2, VCAM-1, alpha 4 and alpha 6 integrin subunits, thrombomodulin and factor VIII. The results document a dynamic pattern of receptor expression and indicate that the expansion of the pulmonary vascular system may take place by both angiogenic and vasculogenic processes. They further document differences in receptor expression by vascular and airway smooth muscle. ICAM-1 expression was primarily extravascular during development. The expression patterns of alpha 4 integrin and its counter receptor VCAM-1 lacked the complementarity that might be expected if they were functioning as a receptor/counter-receptor pair in lung development. Thrombomodulin expression patterns support a major role for the thrombin/ thrombomodulin system in lung development. The expression of thrombomodulin only at sites of airway branching suggests that the thrombin/thrombomodulin system could play a pivotal, regulatory role in branching morphogenesis.

Monoclonal antibody 9EG7 defines a novel beta 1 integrin epitope induced by soluble ligand and manganese, but inhibited by calcium

The monoclonal antibody 9EG7 has been previously found to recognize an epitope induced by manganese on the integrin beta 1 chain (Lenter, M., Uhlig, H., Hamann, A., Jeno, P., Imhof, B., and Vestweber, D. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 9051-9055). Here we show that treatment of beta 1 integrins with manganese or soluble integrin ligands (e.g. fibronectin and RGD peptide) induced the 9EG7 epitope. This epitope was also induced upon EGTA treatment to remove calcium, and the addition of calcium inhibited 9EG7 epitope induction by manganese or by ligand. Further emphasizing the importance of the 9EG7 epitope, the 9EG7 antibody itself stimulated adhesion mediated by multiple beta 1 integrins, and conversely, ligands for alpha 2 beta 1, alpha 3 beta 1, alpha 4 beta 1, and alpha 5 beta 1 all stimulated 9EG7 expression. Together these results support a model whereby (i) calcium inhibits beta 1 integrin function because it prevents the appearance of a conformation favorable to ligand binding and (ii) manganese enhances beta 1 integrin function because it induces the same favorable conformation that is induced by adding ligand, or removing calcium. Notably, other beta 1-stimulating agents (magnesium and mAb TS2/16) did not induce 9EG7 expression unless ligand was also present. Thus, although 9EG7 may reliable detect the ligand-bound conformation of beta 1 integrins, its expression does not always correlate with integrin "activation". Finally, mouse/chicken beta 1 chimeric molecules were used to map the 9EG7 epitope to beta 1 residues 495-602 within the cysteine-rich region, and antibody cross-blocking studies showed that the 9EG7 epitope is distinct from all previously defined human beta 1 epitopes.

Alternative splicing of a specific cytoplasmic exon alters the binding characteristics of murine platelet/endothelial cell adhesion molecule-1 (PECAM-1)

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a membrane glycoprotein expressed on endothelial cells, platelets, and leukocytes. Analysis of PECAM-1 expression in the developing mouse embryo has revealed the presence of multiple isoforms of murine PECAM-1 (muPECAM-1) that appeared to result from the alternative splicing of exons encoding cytoplasmic domain sequences (exons 10-16) (Baldwin, H. S., Shen, H. M., Yan, H., DeLisser, H. M., Chung, A., Mickanin, C., Trask, T., Kirschbaum, N. E. Newman, P. J., Albelda, S., and Buck, C. A. (1994) Development 120, 2539-2553). To investigate the functional consequences of alternatively spliced muPECAM-1 cytoplasmic domains, L-cells were transfected with cDNA for each variant and their ability to promote cell aggregation was compared. In this assay, full-length muPECAM-1 and all three isoforms containing exon 14 behaved like human PECAM-1 in that they mediated calcium- and heparin-dependent heterophilic aggregation. In contrast, three muPECAM-1 variants, all missing exon 14, mediated calcium- and heparin-independent homophilic aggregation. Exon 14 thus appears to modulate the ligand and adhesive interactions of the extracellular domain of PECAM-1. These findings suggest that alternative splicing may represent a mode of regulating the adhesive function of PECAM-1 in vivo and provides direct evidence that alternative splicing involving the cytoplasmic domain affects the ligand specificity and binding properties of a cell adhesion receptor.

Distribution of integrin cell adhesion molecules in endometrial cancer

Integrins are ubiquitous cell adhesion molecules that are involved in maintaining normal tissue morphology and have been implicated in the behavior of certain malignancies. We examined the expression of nine integrin subunits in 38 endometrial adenocarcinomas using immunohistochemistry. The pattern of integrin expression in the cancers was compared with that seen in the endometrium of 20 normal cycling women and 7 postmenopausal women. Integrin expression was correlated with grade, stage, nodal status, depth of invasion, steroid receptor status, and histological pattern. In endometrial cancers there was an inverse relationship between the number of integrins expressed and histological grade (P = 0.011). Of the normally expressed, constitutive endometrial epithelial integrin subunits (alpha 2, alpha 3, alpha 6, and beta 4), the least frequently seen in the cancers was the alpha 3 subunit (44.7%) and the most frequently found was alpha 6 (81.6%). The alpha 5 beta 1 integrin, a fibronectin receptor normally found only on endometrial stromal cells, was seen in 17.8% of cases of these epithelial cancers. In addition, a significant association was found between the loss of the alpha 2 beta 1 integrin and the presence of lymph node metastases (P < 0.001). These data suggest that a decline in integrin expression occurs more frequently in poorly differentiated endometrial cancers and that the loss of specific integrins may be associated with metastatic nodal spread.

Defective development of the embryonic and extraembryonic circulatory systems in vascular cell adhesion molecule (VCAM-1) deficient mice

VCAM-1 is a cytokine-inducible cell surface protein capable of mediating adhesion to leukocytes expressing alpha 4 integrins. Mice deficient in VCAM-1 expression were produced by targeted homologous recombination in ES cells. VCAM-1-deficient embryos were not viable and exhibited either of two distinct phenotypes. Approximately half of the embryos died before embryonic day 11.5 and exhibited a severe defect in placental development in which the allantois failed to fuse with the chorion. The remaining VCAM-1-deficient embryos survived to embryonic day 11.5-12.5 and displayed several abnormalities in their developing hearts including a reduction of the compact layer of the ventricular myocardium and intraventricular septum. The hearts also contained significant amounts of blood in the pericardial space and lacked an epicardium. alpha 4 and VCAM-1 were found to be expressed in wild-type embryos in a reciprocal fashion in the chorion and allantois and in the epicardium and the underlying myocardium, although VCAM-1 was expressed in the intraventricular septum in the absence of adjacent alpha 4-expressing cells. These data suggest important roles for VCAM-1 and alpha 4 in the development of the placenta and the heart.

Further characterization of endometrial integrins during the menstrual cycle and in pregnancy

OBJECTIVE

To profile the changes in integrin expression in cycling and pregnant endometrium.

DESIGN

Immunohistochemistry was performed on endometrium from proliferative, early, and midsecretory phase and early pregnancy using a blinded panel of monoclonal antibodies directed against integrins. Closer examination of the cycle-dependent integrins was performed in 112 patients throughout the cycle and from the first trimester of pregnancy.

SETTING

An academic teaching hospital.

PATIENTS

Ovulatory women or women undergoing pregnancy termination.

MAIN OUTCOME MEASURE

Staining intensity of each antibody in epithelial or stromal cells.

RESULTS

Certain integrins were present constitutively throughout the menstrual cycle whereas others were expressed only during the luteal phase. Four integrins increased in the decidua of pregnancy. The timing of expression of the two cycle-dependent integrins (alpha 4 beta 1 and alpha v beta 3) framed the putative window of implantation and suggests a role in establishment of uterine receptivity.

CONCLUSIONS

Our findings confirm the patterns of endometrial integrins and suggest important roles for integrins in the process of implantation and decidualization.

Aberrant integrin expression in the endometrium of women with endometriosis

Integrins are ubiquitous cell adhesion molecules that undergo dynamic alterations during the normal menstrual cycle in the human endometrium. The alpha v beta 3 vitronectin receptor integrin is expressed in endometrium at the time of implantation, but its presence is delayed in endometrium that is assessed to be out of phase using classical histological features. To investigate the expression of this integrin in women with endometriosis, we assessed the presence of the beta 3-subunit throughout the menstrual cycle in 268 "in-phase" endometrial biopsies, using immunohistochemistry. The beta 3-subunit was expressed on endometrial epithelium after days 19-20 of the menstrual cycle. In 241 women whose biopsies were obtained after day 19, a lack of beta 3 expression was found to be closely related to the diagnosis of endometriosis (by Wilcoxon test, P = 0.02). This defect in integrin expression was associated with nulliparity, inversely related to the stage of disease, and occurred despite the presence of in-phase histological features. In a prospective double blind assessment of this integrin, we found endometrial beta 3 analysis to have a high specificity and positive predictive value as a nonsurgical diagnostic test for minimal and mild endometriosis. In conclusion, aberrant integrin expression in the native endometrium is associated with the finding of endometriosis and may identify some women with decreased cycle fecundity due to defects in uterine receptivity.

Deletions in the cytoplasmic domain of platelet-endothelial cell adhesion molecule-1 (PECAM-1, CD31) result in changes in ligand binding properties

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a member of the immunoglobulin superfamily present on platelets, endothelial cells, and leukocytes that may function as a vascular cell adhesion molecule. The purpose of this study was to examine the role of the cytoplasmic domain in PECAM-1 function. To accomplish this, wild-type and mutated forms of PECAM-1 cDNA were transfected into murine fibroblasts and the functional characteristics of the cells analyzed. Wild-type PECAM-1 localized to the cell-cell borders of adjacently transfected cells and mediated heterophilic, calcium-dependent L-cell aggregation that was inhibitable by a polyclonal and two monoclonal anti-PECAM-1 antibodies. A mutant protein lacking the entire cytoplasmic domain did not support aggregation or move to cell-cell borders. In contrast, both forms of PECAM-1 with partially truncated cytoplasmic domains (missing either the COOH-terminal third or two thirds of the cytoplasmic domain) localized to cell-cell borders in 3T3 cells in a manner analogous to the distribution seen in cultured endothelial cells. L-cells expressing these mutants demonstrated homophilic, calcium-independent aggregation that was blocked by the polyclonal anti-PECAM-1 antibody, but not by the two bioactive monoclonal antibodies. Although changes in the cytoplasmic domain of other receptors have been shown to alter ligand-binding affinity, to our knowledge, PECAM-1 is the first example of a cell adhesion molecule where changes in the cytoplasmic domain result in a switch in the basic mechanism of adhesion leading to different ligand-binding specificity. Variations in the cytoplasmic domain could thus be a potential mechanism for regulating PECAM-1 activity in vivo.

Basic fibroblast growth factor modulates integrin expression in microvascular endothelial cells

During angiogenesis capillary endothelial cells undergo a coordinated set of modifications in their interactions with extracellular matrix components. In this study we have investigated the effect of the prototypical angiogenic factor basic fibroblast growth factor (bFGF) on the expression and function of several integrins in microvascular endothelial cells. Immunoprecipitation experiments with antibodies to individual subunits indicated that microvascular cells express at their surface several integrins. These include the alpha 1 beta 1, alpha 2 beta 1, and alpha 3 beta 1 laminin/collagen receptors; the alpha 6 beta 1 laminin receptor; the alpha 5 beta 1 and alpha v beta 1 fibronectin receptors; the alpha 6 beta 4 basement membrane receptor; and the alpha v beta 3 and alpha v beta 5 vitronectin receptors. Treatment with bFGF caused a significant increase in the surface expression of the alpha 2 beta 1, alpha 3 beta 1, alpha 5 beta 1, alpha 6 beta 1, alpha 6 beta 4, and alpha v beta 5 integrins. In contrast, the level of expression of the alpha 1 beta 1 and alpha v beta 3 integrins was decreased in bFGF-treated cells. Immunoprecipitation of metabolically labeled cells indicated that bFGF increases the biosynthesis of the alpha 3, alpha 5, alpha 6, beta 4, and beta 5 subunits and decreases the production of the alpha v and beta 3 subunits. These results suggest that bFGF modulates integrin expression by altering the biosynthesis of individual alpha or beta subunits. In accordance with the upregulation of several integrins observed in bFGF-treated cells, these cells adhered better to fibronectin, laminin, vitronectin, and type I collagen than did untreated cells. The largest differences in beta 1 integrin expression occurred approximately 72 h after exposure to bFGF, at a time when the expression of the endothelial cell-to-cell adhesion molecule endoCAM was also significantly upregulated. In contrast, a shorter exposure to bFGF (24-48 h) was required for the maximal induction of plasminogen activator production in the same cells. Taken together, these results show that bFGF causes significant changes in the level of expression and function of several integrins in microvascular endothelial cells.

Structure/function analysis of the integrin beta 1 subunit by epitope mapping

Monoclonal antibodies (mAbs) have been produced against the chicken beta 1 subunit that affect integrin functions, including ligand binding, alpha subunit association, and regulation of ligand specificity. Epitope mapping of these antibodies was used to identify regions of the subunit involved in these functions. To accomplish this, we produced mouse/chicken chimeric beta 1 subunits and expressed them in mouse 3T3 cells. These chimeric subunits were fully functional with respect to heterodimer formation, cell surface expression, and cell adhesion. They differed in their ability to react with a panel anti-chicken beta 1 mAbs. Epitopes were identified by a loss of antibody binding upon substitution of regions of the chicken beta 1 subunit by homologous regions of the mouse beta 1 subunit. The identification of the epitope was confirmed by a reciprocal exchange of chicken and mouse beta 1 domains that resulted in the gain of the ability of the mouse subunit to interact with a particular anti-chicken beta 1 mAb. Using this approach, we found that the epitopes for one set of antibodies that block ligand binding mapped toward the amino terminal region of the beta 1 subunit. This region is homologous to a portion of the ligand-binding domain of the beta 3 subunit. In addition, a second set of antibodies that either block ligand binding, alter ligand specificity, or induce alpha/beta subunit dissociation mapped to the cysteine rich repeats near the transmembrane domain of the molecule. These data are consistent with a model in which a portion of beta 1 ligand binding domain rests within the amino terminal 200 amino acids and a regulatory domain, that affects ligand binding through secondary changes in the structure of the molecule resides in a region of the subunit, possibly including the cysteine-rich repeats, nearer the transmembrane domain. The data also suggest the possibility that the alpha subunit may exert an influence on ligand specificity by interacting with this regulatory domain of the beta 1 subunit.

Cell adhesion receptors and early mammalian heart development: an overview

Cardiovascular development is the end result of a complex genetic program subject to regulation by signals transmitted between a cell and its extracellular environment. As cells encounter new extracellular matrices or establish new cell-cell interactions, new genes must be activated to accommodate the altered developmental situation within which the cell finds itself. This is likely reflected in a program of adhesion receptor and counter receptor expression on the surface of cells engaged in the morphogenesis. To understand the molecular basis of development, it is necessary to first determine if such a program exists and then to establish the role of various receptors and counter receptors in the particular morphogenetic process under investigation. To this end, we have initiated an investigation into expression of specific adhesion receptors during cardiovascular development in the mouse. Here, we demonstrate that platelet endothelial cell adhesion molecule (PECAM)-1 is an excellent marker for following vascular formation in the mammalian embryo. It is expressed during development in several alternatively spliced forms involving the cytoplasmic domain of the molecule. These forms differ in their ligand binding properties. Thus, a change in the cytoplasmic domain affects the folding of the molecule in such a way as to structurally alter the extracellular domain. Further, several receptors including the laminin receptor, the fibronectin receptor and a hyaluronic acid receptor, display specific expression patterns during heart development. These include differential expression in the endocardium and myocardium, down regulation during endocardial and myocardium, down regulation during endocardial cushion formation and cessation of expression in particular regions of the heart upon maturation. Interference with the function of one of these receptors (the fibronectin receptor) results in aberrant heart formation. These observations strongly support the concept that morphogenesis requires specific cell adhesion molecules that are expressed in precisely choreographed programs.

Platelet/endothelial cell adhesion molecule-1 (CD31)-mediated cellular aggregation involves cell surface glycosaminoglycans

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130-kDa integral membrane glycoprotein expressed on endothelial cells, platelets, and leukocytes. Experiments analyzing the aggregation of mouse L-cells stably transfected with full-length PECAM-1 cDNA have demonstrated that PECAM-1 is capable of mediating calcium-dependent heterophilic aggregation. In this report the ligand interactions involved in the aggregation process were studied. This aggregation was inhibited by heparin and chondroitin sulfate, but not by other glycosaminoglycans. Enzymatic removal of cell surface glycosaminoglycans confirmed a PECAM-1-glycosaminoglycan interaction and suggested that this interaction involved glycosaminoglycans on adjacent cells. PECAM-1 contains a glycosaminoglycan consensus binding sequence in the second immunoglobulin-like domain of the molecule's extracellular domain. A comparable region in the related adhesion protein N-CAM has been shown to mediate the adhesive properties of N-CAM. Cells expressing mutant PECAM-1 protein missing the second domain failed to aggregate. Synthetic peptides mimicking the consensus glycosaminoglycan binding sequence, L-K-R-E-K-N, inhibited aggregation. These results demonstrate that PECAM-1-mediated aggregation is dependent on the binding of PECAM-1 to specific glycosaminoglycans on adjacent cells via a glycosaminoglycan consensus binding sequence in the second immunoglobulin-like homology domain.

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.

CD31 expressed on distinctive T cell subsets is a preferential amplifier of beta 1 integrin-mediated adhesion

The CD31 (platelet endothelial cell adhesion molecule-1 [PECAM-1]/endothelial cell adhesion molecule [endoCAM]) molecule expressed on leukocytes, platelets, and endothelial cells is postulated to mediate adhesion to endothelial cells and thereby function in immunity, inflammation, and wound healing. We report the following novel features of CD31 which suggests a role for it in adhesion amplification of unique T cell subsets: (a) engagement of CD31 induces the adhesive function of beta 1 and beta 2 integrins; (b) adhesion induction by CD31 immunoglobulin G (IgG) monoclonal antibodies (mAbs) is sensitive, requiring only bivalent mAb; (c) CD31 mAb induces adhesion rapidly, but it is transient; (d) unique subsets of CD4+ and CD8+ T cells express CD31, including all naive (CD45RA+) CD8 T cells; and (e) CD31 induction is selective, inducing adhesive function of beta 1 integrins, particularly very late antigen-4, more efficiently than the beta 2 integrin lymphocyte function-associated antigen-1. Conversely, CD3 is more effective in inducing beta 2-mediated adhesion. Taken together, these findings indicate that unique T cell subsets express CD31, and CD31 has the capacity to induce integrin-mediated adhesion of T cells in a sensitive and selective fashion. We propose that, in collaboration with other receptors/ligands, CD31 functions in an "adhesion cascade" by amplifying integrin-mediated adhesion of CD31+ T cells to other cells, particularly endothelial cells.

Integrin adhesion molecules in the human endometrium. Correlation with the normal and abnormal menstrual cycle

Integrins are a class of cell adhesion molecules that participate in cell-cell and cell-substratum interactions and are present on essentially all human cells. The distribution of nine different alpha and beta integrin subunits in human endometrial tissue at different stages of the menstrual cycle was determined using immunoperoxidase staining. Glandular epithelial cells expressed primarily alpha 2, alpha 3, and alpha 6 (collagen/laminin receptors), while stromal cells expressed predominantly alpha 5 (fibronectin receptor). The presence of alpha 1 on glandular epithelial cells was cycle specific, found only during the secretory phase. Expression of both subunits of the vitronectin receptor, alpha v beta 3, also underwent cycle specific changes on endometrial epithelial cells. Immunostaining for alpha v increased throughout the menstrual cycle, while the beta 3 subunit appeared abruptly on cycle day 20 on luminal as well as glandular epithelial cells. Discordant luteal phase biopsies (greater than or equal to 3 d "out of phase") from infertility patients exhibited delayed epithelial beta 3 immunostaining. These results demonstrate similarities, as well as specific differences, between endometrium and other epithelial tissues. Certain integrin moieties appear to be regulated within the cycling endometrium and disruption of integrin expression may be associated with decreased uterine receptivity and infertility.

Immunoglobulin superfamily: structure, function and relationship to other receptor molecules

The immunoglobulin superfamily includes the most diverse group of receptors known. They are united by the possession of a common structural feature, the immunoglobulin homology domain. They are found in species from insects to man. They are central to both the humoral and cell mediated immune reactions; they serve as cell surface receptors responsible for positional ques during embryonic development, as well as viral and growth factor receptors. They function in conjunction with other receptor families including the integrins and selectins. Thus, they are frequently parts of adhesive cascades and as such activate signal transducing systems required for the expression or activation of other cell surface receptors. They continue to be the targets of intense investigation into the molecular basis of adhesive interactions.

Distribution of integrin cell adhesion receptors in normal and malignant lung tissue

The integrins are a family of transmembrane glycoproteins that serve as cell-cell and cell-substratum adhesion molecules and help regulate cellular morphology, differentiation, and proliferation. The integrin repertoire of a cell may therefore influence its behavior under resting conditions or following malignant transformation. For this reason, the distribution of integrins in normal lung tissues was determined using monoclonal antibodies against integrins of the beta 1 (VLA) and beta 3 (cytoadhesin) subfamilies and compared with the distribution in a limited number of lung carcinomas. The integrin subunits that bind to collagen and laminin (alpha 1, alpha 2, alpha 3, and alpha 6) and the alpha subunit, which can pair with beta 1, beta 3, or beta 5 and promote fibronectin, fibrinogen, or vitronectin binding, were the predominant integrins expressed on the major cell types of the lung, i.e., bronchial epithelium, vascular endothelium, and smooth muscle. Strong expression of the alpha 5 beta 1 fibronectin receptor and the beta 3 subunit was restricted to the endothelium of large vessels. Integrin expression by the lung carcinoma cells was somewhat heterogeneous; however, the tumors tended to express fewer integrins than did the normal bronchial epithelium.

Molecular and cellular properties of PECAM-1 (endoCAM/CD31): a novel vascular cell-cell adhesion molecule

PECAM-1 is a 130-120-kD integral membrane glycoprotein found on the surface of platelets, at endothelial intercellular junctions in culture, and on cells of myeloid lineage. Previous studies have shown that it is a member of the immunoglobulin gene superfamily and that antibodies against the bovine form of this protein (endoCAM) can inhibit endothelial cell-cell interactions. These data suggest that PECAM-1 may function as a vascular cell adhesion molecule. The function of this molecule has been further evaluated by transfecting cells with a full-length PECAM-1 cDNA. Transfected COS-7, mouse 3T3 and L cells expressed a 130-120-kD glycoprotein on their cell surface that reacted with anti-PECAM-1 polyclonal and monoclonal antibodies. COS-7 and 3T3 cell transfectants formed cell-cell junctions that were highly enriched in PECAM-1, reminiscent of its distribution at endothelial cell-cell borders. In contrast, this protein remained diffusely distributed within the plasma membrane of PECAM-1 transfected cells that were in contact with mock transfectants. Mouse L cells stably transfected with PECAM-1 demonstrated calcium-dependent aggregation that was inhibited by anti-PECAM antibodies. These results demonstrate that PECAM-1 mediates cell-cell adhesion and support the idea that it may be involved in some of the interactive events taking place during thrombosis, wound healing, and angiogenesis.

Cytoplasmic and transmembrane domains of integrin beta 1 and beta 3 subunits are functionally interchangeable

Integrin beta subunits combine with specific sets of alpha subunits to form functional adhesion receptors. The structure and binding properties of integrins suggest the presence of domains controlling at least three major functions: subunit association, ligand binding, and cytoskeletal interactions. To more carefully define structure/function relationships, a cDNA construct consisting of the extracellular domain of the avian beta 1 subunit and the cytoplasmic and transmembrane domains of the human beta 3 subunit was prepared and expressed in murine 3T3 cells. The resulting chimeric beta 1/3 subunit formed heterodimers with alpha subunits from the beta 1 subfamily, could not interact with alpha IIb from the beta 3 subfamily, was targeted to focal contacts, and formed functional complexes within the focal contacts. A second cDNA construct was prepared that coded for an avian beta 1 subunit without a transmembrane or cytoplasmic domain. This subunit was not found in association with an accompanying alpha subunit, nor was it found expressed on the cell surface. Instead, it accumulated in vesicles within the cytoplasm and was eventually shed from the cell. The results from studies of the behavior of these two cDNA constructs demonstrate that the transmembrane and cytoplasmic domains play no role in alpha subunit selection, that the cytoplasmic domain of beta 3 is capable of functioning in the context of alpha subunits with which it is not normally paired, and that both integrin subunits must be membrane associated for normal assembly and transport to cell surface adhesive structures.

Integrin distribution in malignant melanoma: association of the beta 3 subunit with tumor progression

Since tumor progression is dependent on the ability of malignant cells to interact with the extracellular matrix, molecules on the cell surface which mediate cell-substratum interactions are likely to be important regulators of tumor invasion and metastasis. The purpose of this study was to examine the distribution of one such group of cell adhesion receptors, the integrins, in benign and malignant lesions of human melanocytes. The distribution of integrin adhesion receptors was defined on cells in culture derived from normal and malignant melanocytes and in tissue sections from benign to increasingly malignant melanocytic lesions using a panel of monoclonal antibodies against specific integrin subunits. Cells in culture expressed a large variety of integrins, including all of the previously characterized members of the beta 1 subfamily plus the alpha v/beta 3 vitronectin receptor. The expression of integrins was similar in cells cultured from either benign or malignant lesions. In contrast, consistent differences were noted in integrin expression by cells within tissues containing metastatic and vertical growth phase melanomas when compared to radial growth phase melanoma cells and cells within nevi. Most notably, the expression of the beta 3 subunit was restricted exclusively to cells within vertical growth phase and metastatic melanomas. The presence of this integrin may be important in the development of tumor invasiveness and could be useful as a marker of melanoma cells entering the more aggressive phase of the malignant process.

Integrins and other cell adhesion molecules

Cell-cell and cell-substratum interactions are mediated through several different families of receptors. In addition to targeting cell adhesion to specific extracellular matrix proteins and ligands on adjacent cells, these receptors influence many diverse processes including cellular growth, differentiation, junction formation, and polarity. Several families of adhesion receptors have been identified. These include: 1) the integrins, heterodimeric molecules that function both as cell-substratum and cell-cell adhesion receptors; 2) the adhesion molecules of the immunoglobulin superfamily, which are involved in cell-cell adhesion and especially important during embryo-genesis, wound healing, and the inflammatory response; 3) the cadherins, developmentally regulated, calcium-dependent homophilic cell-cell adhesion proteins; 4) the LEC-CAMs, cell adhesion molecules with lectin-like domains that mediate white blood cell/endothelial cell adhesion; and 5) homing receptors that target lymphocytes to specific lymphoid tissue. In this review we summarize recent data describing the structure and function of some of these cell adhesion molecules (with special emphasis on the integrin family) and discuss the possible role of these molecules in development, inflammation, wound healing, coagulation, and tumor metastasis.

EndoCAM: a novel endothelial cell-cell adhesion molecule

Cell-cell adhesion is controlled by many molecules found on the cell surface. In addition to the constituents of well-defined junctional structures, there are the molecules that are thought to play a role in the initial interactions of cells and that appear at precise times during development. These include the cadherins and cell adhesion molecules (CAMs). Representatives of these families of adhesion molecules have been isolated from most of the major tissues. The notable exception is the vascular endothelium. Here we report the identification of a cell surface molecule designated "endoCAM" (endothelial Cell Adhesion Molecule), which may function as an endothelial cell-cell adhesion molecule. EndoCAM is a 130-kD glycoprotein expressed on the surface of endothelial cells both in culture and in situ. It is localized to the borders of contiguous endothelial cells. It is also present on platelets and white blood cells. Antibodies against endoCAM prevent the initial formation of endothelial cell-cell contacts. Despite similarities in size and intercellular location, endoCAM does not appear to be a member of the cadherin family of adhesion receptors. The serologic and protease susceptibility characteristics of endoCAM are different from those of the known cadherins, including an endogenous endothelial cadherin. Although the precise biologic function of endoCAM has not been determined, it appears to be one of the molecules responsible for regulating endothelial cell-cell adhesion processes and may be involved in platelet and white blood cell interactions with the endothelium.

Expression of normal and mutant avian integrin subunits in rodent cells

We describe the expression of the beta 1 subunit of avian integrin in rodent cells with the purpose of examining the structure-function relationships of various domains within this subunit. The exogenous subunit is efficiently and stably expressed in 3T3 cells, and it forms hybrid heterodimers with endogenous murine alpha subunits, including alpha 3 and alpha 5. These heterodimers are exported to the cell surface and localize in focal contacts where both extracellular matrix and cytoskeleton associate with the plasma membrane. Hybrid heterodimers consisting of exogenous beta 1 and endogenous alpha subunits bind effectively and specifically to columns of cell-binding fragments of fibronectin. The exogenous avian beta 1 subunit appears to function as well as its endogenous murine equivalent, consistent with the high degree of conservation noted previously for integrins. In contrast, expression of a mutant form of avian integrin beta 1 subunit lacking the cytoplasmic domain produces hybrid heterodimers which, while efficiently exported to the cell surface and still capable of binding fibronectin, do not localize efficiently in focal contacts. This further implicates the cytoplasmic domain of the beta 1 subunit in interactions required for cytoskeletal organization.

Identification and characterization of cell-substratum adhesion receptors on cultured human endothelial cells

A series of immunological approaches was utilized to identify the molecules involved in cell-substratum adhesion of human endothelial cells (EC) derived from adult large vessels, fat capillaries, and umbilical veins. A polyclonal antibody prepared against partially purified extracellular matrix receptors disrupted adhesion of EC to a wide variety of substrates and identified four groups of glycoproteins migrating with apparent Mr of 150, 125, 110, and 95 kD in immunoprecipitation experiments. Specific monoclonal antibodies identified these proteins as members of the Integrin family of extracellular matrix receptors and included the alpha and beta chains of the fibronectin receptor (alpha 5/beta 1), a collagen receptor (alpha 2 beta 1), a multifunctional receptor that binds to fibronectin, collagen, and laminin (alpha 3/beta 1), as well as a receptor related to platelet IIb/IIIa (alpha v/beta 3). To directly test the importance of these molecules in cell-substratum adhesion, these proteins were purified by a combination of ion exchange, lectin affinity, and immunoaffinity chromatography and used to block the biological activity of the adhesion-disrupting polyclonal antibody. Immunofluorescence experiments further supported the role of these glycoproteins in adhesion. The GPIIb/IIIa-like receptor localized to well-formed adhesion plaques on EC plated on fibrinogen, but not on fibronectin, laminin, or type IV collagen. Receptors containing the beta 1 subunit were visualized as discontinuous fibrils which colocalized with fibronectin fibrils and actin stress fibers.

Myoblast migration specifically inhibited in the chick embryo by grafted CSAT hybridoma cells secreting an anti-integrin antibody

We report a teratological method in which mouse hybridoma cells are grafted into a chick host. CSAT (Cell Substratum ATtachment) hybridoma was used. It produces an antibody directed against the avian integrin complex. The grafts were performed during the second and third days of incubation either at the level of the somites or in the coelom of the chick embryo. The anomalies were revealed by means of a monoclonal antibody that recognizes myogenic cells as soon as they become committed in the myotome. When embryos were grafted at the level of the somites, body wall muscles failed to develop on the side of the graft only. After coelomic grafting, total agenesis of abdominal muscles was induced. The anomalies were specific since the engraftment of three control hybridoma clones induced no change in muscle formation. These control hybridomas produce antibodies directed against the same molecular complex but not against the same epitope as CSAT. The injection of hybridoma cells in an embryo appears as a method of general interest to determine the long-term consequences of perturbing a specific developmental process.

Cell surface receptors for extracellular matrix molecules

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

Soluble 80-kd fragment of cell-CAM 120/80 disrupts cell-cell adhesion

Calcium-dependent cell adhesion molecules (CAMs) mediate intercellular adhesion in epithelial cells and in preimplantation mammalian embryos. One of these molecules, cell-CAM 120/80, is found on cells as a 120-kd membrane glycoprotein and as a soluble 80-kd species in conditioned culture medium [Damsky et al: Cell 34:455, 1983]. We have purified to homogeneity the soluble 80-kd fragment of cell-CAM 120/80 by using monoclonal antibody affinity chromatography. We have shown that the purified molecule can disrupt cell-cell adhesion in cultured epithelial cells, thus indicating that it is directly involved in the adhesive process. In addition, we have further characterized both the 120-kd cell-associated molecule and its 80-kd fragment, including N-terminal sequence analysis.

Vasculogenesis in the early quail blastodisc as studied with a monoclonal antibody recognizing endothelial cells

QH1, a monoclonal antibody that recognizes quail endothelial and haemopoietic cells, was applied to quail blastodiscs in toto, in order to analyse by immunofluorescence the emergence of the vascular tree. The first endothelial cells were detected in the area opaca at the headfold stage and in the area pellucida at the 1-somite stage. Single cells then interconnected progressively, especially in the anterior intestinal portal and along the somites building up the linings of the heart and dorsal aortas. This study demonstrates that endothelial cells differentiate as single entities 4 h earlier in development than hitherto detected and that the vascular network forms secondarily. The horseshoe shape of the extraembryonic area vasculosa is also a secondary acquisition. A nonvascularized area persists until later (at least the 14-somite stage) in the region of the regressing primitive streak.

Integrin, a transmembrane glycoprotein complex mediating cell-substratum adhesion

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

Integrin (the CSAT antigen): functionality requires oligomeric integrity

Integrin, the cell-substrate attachment (CSAT) antigen, is a complex of integral membrane glycoproteins whose apparent function is to mediate cell-substratum adhesion by serving as a transmembrane link between the extracellular matrix and elements of the cytoskeleton. Previous attempts to separate the members of this complex under nondenaturing conditions have been successful. We have now produced a monoclonal antibody "G" that is specific for the lower molecular mass cysteine-rich band 3 of the complex. Using an antibody affinity column containing this monoclonal antibody, it is possible to dissociate integrin into two fractions, one containing band 3, the other containing bands 1 plus 2. Neither fraction will by itself bind fibronectin, laminin, or talin. However, when the fractions are combined, the reconstituted integrin elutes from a gel filtration column in the same position as the native complex, and binding activity to these molecules returns. Further, it is shown by gel filtration that the recognition site for the adhesion-disrupting monoclonal antibodies CSAT and JG22 is on band 3, supporting the contention that integrin is an oligomer. The data presented here is consistent with integrin being either a mixture of heterodimers, each with a common subunit and reacting with a particular extracellular matrix molecule, or a single heterotrimer capable of binding to several different extracellular matrix molecules.

Distribution of the cell substratum attachment (CSAT) antigen on myogenic and fibroblastic cells in culture

Previous studies (Neff et al., 1982, J. Cell. Biol. 95:654-666; Decker et al., 1984. J. Cell. Biol. 99:1388-1404) have described a monoclonal antibody (CSAT Mab) directed against a complex of three integral membrane glycoproteins of 120,000-160,000 mol wt (CSAT antigen [ag]) involved in the cell matrix adhesion of myoblasts and fibroblasts. In localization studies on fibroblasts presented here, CSAT ag has a discrete, well-organized distribution pattern. It co-aligns with portions of stress fibers and is enriched at the periphery of, but not directly beneath vinculin-rich focal contacts. In this last location, it co-distributes with fibronectin, consistent with the suggestion that the CSAT ag participates in the mechanism by which fibroblasts attach to fibronectin. In prefusion myoblasts, which are rapidly detached by CSAT Mab, CSAT ag is distributed diffusely as are vinculin, laminin, and fibronectin. After fusion, myotubes become more difficult to detach with CSAT Mab. The CSAT ag and vinculin are organized in a much more discrete pattern on the myotube surface, becoming enriched at microfilament bundle termini and in lateral lamellae which appear to attach myotubes to the substratum. These results suggest that the organization of CSAT ag-adhesive complexes on the surface of myogenic cells can affect the stability of their adhesive contacts. We conclude from the sum of the studies presented that, in both myogenic and fibroblastic cells, the CSAT ag is localized in sites expected of a surface membrane mediator of cell adhesion to extracelluon of CSAT ag-adhesive complexes on the surface of myogenic cells can affect the stability of their adhesive contacts. We conclude from the sum of the studies presented that, in both myogenic and fibroblastic cells, the CSAT ag is localized in sites expected of a surface membrane mediator of cell adhesion to extracellular matrix. The results from studies that use fibroblasts in particular suggest the involvement of CSAT ag in the adhesion of these cells to fibronectin.

Cell surface glycoproteins mediate compaction, trophoblast attachment, and endoderm formation during early mouse development

Early mouse embryos undergo several morphogenetic processes, such as compaction, trophoblast attachment, and endoderm formation that can be studied in vitro. Several polyspecific and monospecific antisera have been used to perturb these processes in a nontoxic, reversible fashion. One of the antibody-defined molecules, cell CAM 120/80, promotes epithelial cell adhesion, embryo compaction, and endoderm formation. The results suggest the presence of another such molecule(s) involved in these same processes. Evidence is also presented that another set of antibody-defined molecules, GP 140, involved in attachment of somatic cells to the substrate, mediates trophoblast attachment of the mouse blastocyst. The possible role of these molecules in governing the processes leading to cell lineages in the mouse embryo is discussed.

A monoclonal antibody identifies a glycoprotein complex involved in cell-substratum adhesion

The monoclonal antibody CSAT has been reported to perturb the adhesion of chick embryo cells to their substratum (Neff et al. [19]). Evidence is presented here that the antigen recognized by this monoclonal antibody is comprised of three membrane glycoproteins. The antigen is released from cells with non-ionic detergent and purified by monoclonal antibody affinity chromatography. When analysed by SDS-PAGE under non-reducing conditions, the antigen resolves into three components of apparent molecular weights 160 000 (band 1), 135000 (band 2), and 110 000 (band 3). Following reduction of each component, bands 1 and 2 migrate at slightly lower apparent molecular weights, while band 3 migrates at a higher apparent molecular weight, suggesting that band 3 has an internal disulfide bond. All three bands differ from one another as determined by peptide mapping and by immunologic cross-reactivity. It is postulated that the three glycoproteins function as a complex that plays a central role in cell-substratum adhesion.

Identification and purification of a cell surface glycoprotein mediating intercellular adhesion in embryonic and adult tissue

An antiserum against material shed into serum-free medium by MCF-7 human mammary carcinoma cells (anti-SFM II) disrupts cell-cell interactions in murine mammary tumor epithelial cells (MMTE). We now report purification of an 80 kd glycoprotein (GP80) from SFM of MCF-7 mammary carcinoma cells that blocks the activity of anti-SFM II. Anti-SFM II also inhibits compaction of eight-cell mouse embryos, and purified GP80 blocks this reaction. An antiserum against purified GP80 (anti-GP80) has all adhesion-disrupting activities displayed by anti-SFM II. It recognizes one band at 80 kd in SFM and a 120 kd band in detergent extracts of epithelial but not fibroblastic cells. In immunofluorescence studies it is restricted to sites of cell-cell interaction in cultured epithelial cells. Thus a cell surface glycoprotein of 120 kd, the medium form of which is approximately 80 kd, which is neither species nor tissue specific, is expressed at early stages of mammalian development and is found on epithelia.

Integral membrane glycoproteins related to cell-substratum adhesion in mammalian cells

Broad spectrum antisera have been raised against surface membrane-derived material from baby hamster kidney cells and mouse mammary tumor epithelial cells. These antisera disrupt cell-substratum adhesion in their respective cell types. Using an antibody neutralization (blocking) assay, adhesion-related glycoproteins have been isolated from non-ionic detergent extracts of each cell type. The purified material in each case consisted of a restricted population of glycoproteins of approximately 120,000-160,000 Mr. Purified material from each system blocked the disruption of adhesion induced by the heterologous antiserum on either cell type. The antisera were capable of disrupting cell-substratum adhesion of a large number of cell types and species sources. In addition, antibody blocking activity could be detected from partially purified extracts of several adult hamster cell types and a variety of cultured cell types. Thus, in addition to having similar substratum-associated glycoproteins ((eg, fibronectin) and cytoskeleton-associated proteins (eg, alpha-actinin and vinculin) cells from different species and tissue sources appear to have a relatively conserved class of integral membrane glycoproteins involved in cell substratum-adhesion.

Expression of adhesion-related membrane components in adherent versus nonadherent hamster melanoma cells

The existence of integral membrane components that are involved in cell-substratum adhesion has been postulated. Using an immunochemical approach developed in this laboratory, we provide further evidence for the role in cell-substratum adhesion of integral membrane glycoproteins within a molecular weight region of 120,000-140,000. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of material enriched approximately 100-fold in adhesion-related components revealed the 120,000-140,000 Mr glycoproteins in an adherent hamster melanoma cell line. These glycoproteins are greatly reduced in a nonadherent variant. Induction of adhesion in these cells by exposure to BudR is accompanied by re-expression of the surface adhesion antigens.

Membrane glycoproteins involved in cell--substratum adhesion

A combination of immunological and biochemical methods were used to identify surface membrane components involved in cell-substratum adhesion. Broad-spectrum antiserum, prepared against surface membranes from hamster cells, induced reversible rounding and detachment of hamster fibroblasts from a substratum in vitro. This phenomenon was inhibited by Nonidet P-40 extracts of hamster cells. Therefore, an antibody neutralization assay was developed to detect the presence of antigen during the fractionation of Nonidet P-40 extracts of cells. After two differential precipitation steps, anion exchange chromatography, and sequential lectin affinity chromatography, a fraction greatly enriched in ability to block antiserum-induced changes in cell adhesion and appearance was isolated. Analysis of this fraction by NaDodSO4/polyacrylamide gel electrophoresis revealed a highly restricted group of glycoproteins with Mr approximately 140,000. A lectin-purified glycoprotein fraction was used to raise a higher titer antiserum that was able to induce reversible rounding and detachment of cells from a substratum and, when immobilized on an antibody affinity column, was able to bind and release material capable of blocking antiserum-induced cell rounding. These methods have allowed us to focus attention on a restricted group of glycoproteins that are integral constituents of the surface membrane and which play some as yet undetermined role in the process of cell--substratum adhesion.