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

Cytoskeletal prestress: The cellular hallmark in mechanobiology and mechanomedicine

Increasing evidence demonstrates that mechanical forces, in addition to soluble molecules, impact cell and tissue functions in physiology and diseases. How living cells integrate mechanical signals to perform appropriate biological functions is an area of intense investigation. Here, we review the evidence of the central role of cytoskeletal prestress in mechanotransduction and mechanobiology. Elevating cytoskeletal prestress increases cell stiffness and reinforces cell stiffening, facilitates long-range cytoplasmic mechanotransduction via integrins, enables direct chromatin stretching and rapid gene expression, spurs embryonic development and stem cell differentiation, and boosts immune cell activation and killing of tumor cells whereas lowering cytoskeletal prestress maintains embryonic stem cell pluripotency, promotes tumorigenesis and metastasis of stem cell-like malignant tumor-repopulating cells, and elevates drug delivery efficiency of soft-tumor-cell-derived microparticles. The overwhelming evidence suggests that the cytoskeletal prestress is the governing principle and the cellular hallmark in mechanobiology. The application of mechanobiology to medicine (mechanomedicine) is rapidly emerging and may help advance human health and improve diagnostics, treatment, and therapeutics of diseases.

Distortion of the normal function of synaptic cell adhesion molecules by genetic variants as a risk for autism spectrum disorders

Synaptic cell adhesion molecules (SCAMs) are a functional category of cell adhesion molecules that connect pre- and postsynapses by the protein-protein interaction via their extracellular cell adhesion domains. Countless numbers of common genetic variants and rare mutations in SCAMs have been identified in the patients with autism spectrum disorders (ASDs). Among these, NRXN and NLGN family proteins cooperatively function at synaptic terminals both of which genes are strongly implicated as risk genes for ASDs. Knock-in mice carrying a single rare point mutation of NLGN3 (NLGN3 R451C) discovered in the patients with ASDs display a deficit in social interaction and an enhancement of spatial learning and memory ability reminiscent of the clinical phenotype of ASDs. NLGN4 knockout (KO) and NRXN2α KO mice also show a deficit in sociability as well as some specific neuropsychiatric behaviors. In this review, we selected NRXNs/NLGNs, CNTNAP2/CNTNAP4, CNTN4, ITGB3, and KIRREL3 as strong ASD risk genes based on SFARI score and summarize the protein structures, functions at synapses, representative discoveries in human genetic studies, and phenotypes of the mutant model mice in light of the pathophysiology of ASDs.

Recent Innovations in Peptide Based Targeted Drug Delivery to Cancer Cells

Targeted delivery of chemotherapeutics and diagnostic agents conjugated to carrier ligands has made significant progress in recent years, both in regards to the structural design of the conjugates and their biological effectiveness. The goal of targeting specific cell surface receptors through structural compatibility has encouraged the use of peptides as highly specific carriers as short peptides are usually non-antigenic, are structurally simple and synthetically diverse. Recent years have seen many developments in the field of peptide based drug conjugates (PDCs), particularly for cancer therapy, as their use aims to bypass off-target side-effects, reducing the morbidity common to conventional chemotherapy. However, no PDCs have as yet obtained regulatory approval. In this review, we describe the evolution of the peptide-based strategy for targeted delivery of chemotherapeutics and discuss recent innovations in the arena that should lead in the near future to their clinical application.

Integrins

Integrins are cell adhesion receptors that are evolutionary old and that play important roles during developmental and pathological processes. The integrin family is composed of 24 αβ heterodimeric members that mediate the attachment of cells to the extracellular matrix (ECM) but that also take part in specialized cell-cell interactions. Only a subset of integrins (8 out of 24) recognizes the RGD sequence in the native ligands. In some ECM molecules, such as collagen and certain laminin isoforms, the RGD sequences are exposed upon denaturation or proteolytic cleavage, allowing cells to bind these ligands by using RGD-binding receptors. Proteolytic cleavage of ECM proteins might also generate fragments with novel biological activity such as endostatin, tumstatin, and endorepellin. Nine integrin chains contain an αI domain, including the collagen-binding integrins α1β1, α2β1, α10β1, and α11β1. The collagen-binding integrins recognize the triple-helical GFOGER sequence in the major collagens, but their ability to recognize these sequences in vivo is dependent on the fibrillar status and accessibility of the interactive domains in the fibrillar collagens. The current review summarizes some basic facts about the integrin family including a historical perspective, their structure, and their ligand-binding properties.

Immunopurification of a sarcomeric junctional protein complex containing GAPDH

We have isolated a monoclonal antibody, P4B2, which localizes to multiple anchorage junctions, namely, a subset of focal adhesions, the Z-disk of muscle, and neuromuscular junctions. Immunopurification of the antigen to this antibody from chicken brain tissue yielded a complex of three prominent proteins with mobilities of 36, 30, and 18 kDa. Amino acid sequencing of the purified proteins identified the 36-kDa protein as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The other two protein bands were heterogeneous, containing proteins found in the synaptic vesicle fusion core complex. Immunolocalization of P4B2 antigen in developing cultured muscle cells showed that the antigen is incorporated into Z-lines soon after the sarcomeric architecture was positive for alpha-actinin. Together, the data indicate the P4B2 antigen is part of a unique GAPDH-containing protein complex that may be involved in reinforcement of established cytoskeletal structures.

Epithelial cell-substrate adhesion in the cornea: localization of actin, talin, integrin, and fibronection [corrected]

During corneal wound healing, epithelial cells migrate and spread over a basement membrane to cover the denuded area. We have initiated studies on the proteins involved in this cell-substrate interaction. In the present report, we show the distribution of actin, talin, integrin and fibronectin in the unwounded chicken corneal epithelium. Rhodamine-phalloidin and antibodies against talin, the beta-subunit of integrin, and fibronectin were used to fluorescently label semi-thin cryosections of chicken cornea. Phalloidin labeling indicated the presence of f-actin around the boundaries of all epithelial cells. Antitalin labeled the most basal aspect of the basal cells. Diffuse cytoplasmic labeling of the basal cells was also seen. Integrin was detected by the polyclonal anti-CSAT and monoclonal 30B6 antibodies. With both antibodies, the basal cells were outlined by label. These cells were similarly labeled by antifibronectin. Less distinct labeling of fibronectin was present around the boundaries of the outer epithelial cells. Our results indicate that many of the proteins associated with cell-substrate adherens junctions in other systems are also present in cell-substrate adhesion of the corneal epithelium. Details of the distribution of some of the proteins appear to be somewhat unusual, however.

N-cadherin-associated proteins in chicken muscle

The development and functional activity of the heart depends on the regulated interaction of cardiac cells. This is in part mediated by cell-cell adhesion molecules such as N-cadherin. N-cadherin belongs to a family of Ca+(+)-dependent, transmembrane, adhesion glycoproteins that promote cell-cell adhesion by molecular self-association extracellularly, and interact intracellularly with the cytoskeleton through highly conserved carboxy-terminal domains. In this paper we show that embryonic chicken cardiac myocytes grown in vitro display Ca+(+)-dependent adhesion and express N-cadherin. When immunoprecipitated from detergent extracts of embryonic chicken cardiac and skeletal muscle cultures, N-cadherin associates with proteins immunologically unrelated to itself. The associated proteins are similar in molecular weight to proteins that coimmunoprecipatate with E-cadherin from human epithelial cells. We postulate that the coimmunoprecipitating proteins are involved in linking the cadherins to the cytoskeleton.

Rapid screening for cell surface binding monoclonal antibodies

A procedure is described which allows for rapid detection of cell surface binding or cytoskeleton binding monoclonal antibodies. At the same time this procedure ensures that selected antibodies will be useful in Western blot analysis. This procedure including the cytochemistry and Western blot analysis requires only 100 microliters of supernatant and can be done directly from the original 96 well plates into which the fusion was plated. One person can easily assay several hundred supernatants in one day for the ability to stain both cells and selected proteins in Western blot analysis.

A monoclonal antibody (8H3) that binds to rat T lineage cells and augments in vitro proliferative responses

A murine monoclonal antibody, designated 8H3, recognizes a cell surface antigen expressed exclusively on rat T lineage cells. 8H3 antibody immunoprecipitated 180-, 120-, and 90-kD components from rat thymocytes as well as splenic T cells under nonreducing conditions. 8H3 antibody specifically inhibited the binding of thymocytes to fibronectin. Furthermore, binding of rat thymocytes to immobilized synthetic peptide Gly-Arg-Gly-Asp-Ser-Pro-Cys-BSA was inhibited by 8H3 antibody as was Gly-Arg-Gly-Asp-Ser-Pro-Cys, but not by Gly-Arg-Ala-Asp-Ser-Pro-Lys or Gly-Arg-Gly-Glu-Ser-Pro. Crosslinking of 8H3 antigen on double-negative thymocytes and adult thymocytes, as well as splenic T lymphocytes by 8H3 antibody and F(ab')2 fragments of goat antibodies to mouse immunoglobulin, led to an increase in the concentration of cytoplasmic free Ca2+ due to the release of Ca2+ from intracellular stores as well as the influx of Ca2+ from extracellular sources. Expression of interleukin 2 receptor and subsequently cell proliferation was observed upon incubation of thymocytes and splenic T cells with 8H3 antibody. Furthermore, 8H3 antibody induced the proliferation of double-negative thymocytes. These data collectively indicated that a cell surface antigen, 8H3, is involved in not only cell adhesion but also involved in the expression of immature as well as mature thymocytes.

A role for the Ca2(+)-dependent adhesion molecule, N-cadherin, in myoblast interaction during myogenesis

The formation of multinucleate skeletal muscle cells (myotubes) is a Ca2(+)-dependent process involving the interaction and fusion of mononucleate muscle cells (myoblasts). Specific cell-cell adhesion precedes lipid bilayer union during myoblast fusion and has been shown to involve both Ca2(+)-independent (CI)2 and Ca2(+)-dependent (CD) mechanisms. In this paper we present evidence that CD myoblast adhesion involves a molecule similar or identical to two known CD adhesion glycoproteins, N-cadherin and A-CAM. These molecules were previously identified by other laboratories in brain and cardiac muscle, respectively, and are postulated to be the same molecule. Antibodies to N-cadherin and A-CAM immunoblotted a similar band with a molecular weight of approximately 125,000 in extracts of brain, heart, and pectoral muscle isolated from chick embryos and in extracts of muscle cells grown in vitro at Ca2+ concentrations that either promoted or inhibited myotube formation. In assays designed to measure the interaction of fusion-competent myoblasts in suspension, both polyclonal and monoclonal anti-N-cadherin antibodies inhibited CD myoblast aggregation, suggesting that N-cadherin mediates the CD aspect of myoblast adhesion. Anti-N-cadherin also had a partial inhibitory effect on myotube formation likely due to the effect on myoblast-myoblast adhesion. The results indicate that N-cadherin/A-CAM plays a role in myoblast recognition and adhesion during skeletal myogenesis.

The alpha 1/beta 1 and alpha 6/beta 1 integrin heterodimers mediate cell attachment to distinct sites on laminin

This study was undertaken to determine the roles of individual alpha/beta 1 integrin heterodimers in promoting cellular interactions with the different attachment-promoting domains of laminin (LN). To do this, antibodies to the integrin beta 1 subunit or to specific integrin alpha subunits were tested for effects on cell attachment to LN, to elastase fragments E1-4 and E1, derived from the short arms and core of LN's cruciform structure, and to fragment E8 derived from the long arm of this structure. The human JAR choriocarcinoma cells used in this study attached to LN and to fragments E1 and E8. Attachment to E1-4 required a much higher substrate coating concentration, suggesting that it is a poor substrate for JAR cell attachment. The ability of cells to attach to different LN domains suggested the presence of more than one LN receptor. These multiple LN receptors were shown to be beta 1 integrin heterodimers because antibodies to the integrin beta 1 subunit inhibited attachment of JAR cells to LN and its three fragments. To identify the individual integrin alpha/beta 1 heterodimers that mediate interactions with these LN domains, mAbs specific for individual beta 1 heterodimers in human cells were used to study JAR cell interactions with LN and its fragments. An anti-alpha 6/beta 1-specific mAb, GoH3, virtually eliminated cell attachment to E8 and partially inhibited attachment to E1 and intact LN. Thus the major alpha 6/beta 1 attachment domain is present in fragment E8. An alpha 1/beta 1-specific mAb (S2G3) strongly inhibited cell attachment to collagen IV and partially inhibited JAR attachment to LN fragment E1. Thus, the alpha 1/beta 1 heterodimer is a dual receptor for collagen IV and LN, interacting with LN at a site in fragment E1. In combination, the anti-alpha 1- and anti-alpha 6-specific antibodies completely inhibited JAR cell attachment to LN and fragment E1. Thus, the alpha 1/beta 1 and alpha 6/beta 1 integrin heterodimers each function as LN receptors and act together to mediate the interactions of human JAR choriocarcinoma cells with LN.

A role for the neural cell adhesion molecule, NCAM, in myoblast interaction during myogenesis

The Ca2(+)-independent neural cell adhesion molecule, NCAM, is expressed by both nerve and muscle cells and has been shown to mediate both nerve-nerve and nerve-muscle cell interaction. A role for NCAM in muscle-muscle cell interaction has been proposed but not demonstrated. Here we report evidence that NCAM is expressed by embryonic chick muscle cells during in vitro development and functions together with Ca2(+)-dependent adhesion molecules in mediating myoblast interaction during the formation of multinucleate cells.

Distribution of integrins and their ligands in the trunk of Xenopus laevis during neural crest cell migration

We have examined the distribution in Xenopus embryos of beta 1 subunits of integrin, as recognized by cross-reactive antibodies against the avian integrin beta 1 subunit. These antibodies recognize a doublet of bands of approximately 120 kD in Xenopus embryos. The distribution pattern of these integrin cell surface receptors was compared with that of two possible ligands, fibronectin and laminin, in the extracellular matrix during the time of neural crest cell migration. Integrin immunoreactivity in the early neurula was observed lightly outlining somite and epidermal cells and the notochord. The integrin immunostaining increased with developmental age and was observed on most cell types in the embryo but was particularly notable in the intersomitic clefts through which motoraxons grow. The immunoreactivity in this region was not, however, wholly on the axon surfaces, since intersomitic integrin remained detectable in embryos in which the neural tube had been ablated. Fibronectin and laminin were more extensively distributed than integrin at all stages examined. Immunoreactivity for both was observed around the neural tube, notochord, somites, epidermis, dorsal mesentery, and lateral plate mesoderm. The distribution of laminin and fibronectin around the somites was particularly interesting since it was non-uniform and similar to that of integrin. Strongest staining was observed in the intersomitic clefts, and weakest staining was observed on the medial surface of the somites, which faces the neural tube and notochord. The major differences in distribution pattern between the fibronectin and laminin immunoreactivities were that only fibronectin was detected in the mesenchyme of the dorsal fin. Our results demonstrate that a molecule homologous to avian integrin is present in Xenopus embryos during neural crest cell migration and motoraxon outgrowth. Its presence in the intersomitic clefts and on the surface of many embryonic cell types together with the abundant distribution of its ligands are consistent with a potentially important developmental function in neurite outgrowth and/or muscle development.

Expression and function of chicken integrin beta 1 subunit and its cytoplasmic domain mutants in mouse NIH 3T3 cells

Chicken integrin beta 1 cDNA and its site-directed mutants were cloned into a mammalian expression vector and introduced into mouse NIH 3T3 cells. Stable transfectants expressing the chicken beta 1 subunit or its site-directed mutants were identified by immunostaining with antibodies specific for the chicken integrin beta 1 subunit. The chicken beta 1 proteins were expressed predominately in the endoplasmic reticulum of transfectants and to a lesser degree in the plasma membrane. Immunoblots and immunoprecipitations, using anti-chicken integrin antibodies, revealed three different sizes of the chicken subunit (90, 95, and 120 kD) and a mouse 140-kD alpha subunit. Immunoprecipitations of the cell surface receptors showed only two peptides, an 120-kD beta 1 and an 140-kD alpha subunit. Antibodies perturbing mouse and chicken integrin-specific cell adhesions were used to demonstrate that the chimeric receptors functioned in adhesion to both laminin and fibronectin. Immunofluorescent staining with antibodies specific for either the chicken or mouse receptors showed that both the wild type and the chimeric receptors localized in focal contacts. Several mutations in the cytoplasmic domain were synthesized and used in the transfection experiments. In one mutant the tyrosine (Tyr 788) in the consensus sequence for phosphorylation was replaced by a phenylalanine. In another the lysine (Lys 757) at the end of the membrane spanning region was replaced by a leucine. Both of these mutants formed dimers with mouse alpha subunits, participated in adhesion, localized in focal contacts, and displayed biological properties indistinguishable from the wild-type transfection. In contrast, mutants containing deletions greater than 5-15 amino acids nearest the carboxyl end in the cytoplasmic domain neither promoted adhesion nor localized in focal contacts. They did, however, form heterodimers that were expressed on the cell surface.

Dynamic cytoskeleton-integrin associations induced by cell binding to immobilized fibronectin

We have examined the early events of cellular attachment and spreading (10-30 min) by allowing chick embryonic fibroblasts transformed by Rous sarcoma virus to interact with fibronectin immobilized on matrix beads. The binding activity of cells to fibronectin beads was sensitive to both the mAb JG22E and the GRGDS peptide, which inhibit the interaction between integrin and fibronectin. The precise distribution of cytoskeleton components and integrin was determined by immunocytochemistry of frozen thin sections. In suspended cells, the distribution of talin was diffuse in the cytoplasm and integrin was localized at the cell surface. Within 10 min after binding of cells and fibronectin beads at 22 degrees C or 37 degrees C, integrin and talin aggregated at the membrane adjacent to the site of bead attachment. In addition, an internal pool of integrin-positive vesicles accumulated. The mAb ES238 directed against the extracellular domain of the avian beta 1 integrin subunit, when coupled to beads, also induced the aggregation of talin at the membrane, whereas ES186 directed against the intracellular domain of the beta 1 integrin subunit did not. Cells attached and spread on Con A beads, but neither integrin nor talin aggregated at the membrane. After 30 min, when many of the cells were at a more advanced stage of spreading around beads or phagocytosing beads, alpha-actinin and actin, but not vinculin, form distinctive aggregates at sites along membranes associated with either fibronectin or Con A beads. Normal cells also rapidly formed aggregates of integrin and talin after binding to immobilized fibronectin in a manner that was similar to the transformed cells, suggesting that the aggregation process is not dependent upon activity of the pp60v-src tyrosine kinase. Thus, the binding of cells to immobilized fibronectin caused integrin-talin coaggregation at the sites of membrane-ECM contact, which can initiate the cytoskeletal events necessary for cell adhesion and spreading.

Involvement of cell surface phosphatidylinositol-anchored glycoproteins in cell-cell adhesion of chick embryo myoblasts

During myogenesis myoblasts fuse to form multinucleate cells that express muscle-specific proteins. A specific cell-cell adhesion process precedes lipid bilayer union during myoblast fusion (Knudsen, K. A., and A. F. Horwitz. 1977. Dev. Biol. 58:328-338) and is mediated by cell surface glycoproteins (Knudsen, K. A., 1985. J. Cell Biol. 101:891-897). In this paper we show that myoblast adhesion and myotube formation are inhibited by treating fusion-competent myoblasts with phosphatidylinositol-specific phospholipase C (PI-PLC). The effect of PI-PLC on myoblast adhesion is dose dependent and inhibited by D-myo-inositol 1-monophosphate and the effect on myotube formation is reversible, suggesting a specific, nontoxic effect on myogenesis by the enzyme. A soluble form of adhesion-related glycoproteins is released from fusion-competent myoblasts by treatment with PI-PLC as evidenced by (a) the ability of phospholipase C (PLC)-released material to block the adhesion-perturbing activity of a polyclonal antiserum to intact myoblasts; and (b) the ability of PLC-released glycoprotein to stimulate adhesion-perturbing antisera when injected into mice. PI-PLC treatment of fusion-competent myoblasts releases an isoform of N-CAM into the supernate, suggesting that N-CAM may participate in mediating myoblast interaction during myogenesis.

Evidence for the direct interaction of chicken gizzard 5'-nucleotidase with laminin and fibronectin

The ectoenzyme 5'-nucleotidase purified from chicken gizzard is shown to specifically interact with laminin and fibronectin, components of the extracellular matrix, by a number of different techniques: (i) cosedimentation with laminin by sucrose gradient centrifugation; (ii) affinity adsorption to both laminin- and fibronectin-Sepharose 4-B; (iii) specific binding to both laminin and fibronectin dotted onto cellulose filters; and (iv) monoclonal antibodies against 5'-nucleotidase are shown to interfere with the interaction of 5'-nucleotidase with laminin and fibronectin. For all the techniques employed, the interactions were found to be specific, since 5'-nucleotidase did not bind to unrelated proteins such as bovine serum albumin or to monomeric actin. The interaction of purified chicken gizzard 5'-nucleotidase could be demonstrated for the hydrophobic enzyme solubilized in detergent and after its reconstitution into artificial phospholipid vesicles. The affinity adsorption experiments indicate that reconstituted enzyme binds more strongly to both laminin and fibronectin. The 5'-nucleotidase employed in this study is anchored to the plasma membrane by a glycan-phosphatidylinositol linker. After treatment with phosphatidylinositol-specific phospholipase C, the enzyme is transformed into a hydrophilic form, for which interactions with laminin and fibronectin could also be demonstrated by the dot-blot technique. Thus controlled cleavage of the phosphatidylinositol linker of 5'-nucleotidase could enable cells to rapidly alter their adhesiveness to certain components of the extracellular matrix.

Integrin heterodimer and receptor complexity in avian and mammalian cells

We report data showing that the integrin receptor complex in chickens contains several discrete heterodimers all sharing the beta 1-integrin subunit combined separately with different alpha-subunits. Using antisera to synthetic peptides based on cDNA sequences of chicken and human alpha-integrin subunits to analyze the integrin complement of avian and mammalian cells, we show that band 2 of the chicken integrin complex contains alpha-subunits related to both alpha 3- and alpha 5-subunits of human integrins. alpha 3 beta 1 and alpha 5 beta 1 have both previously been shown in human cells to be fibronectin receptors and alpha 3 beta 1 can also act as a receptor for laminin and collagen. We also provide evidence for the presence, in band 1 of the chicken integrin complex, of a third integrin alpha-subunit which is also alpha 5 related. This integrin subunit exists in a separate heterodimer complex with beta 1 and binds to fibronectin-affinity columns. These results provide explanations for published data showing that the avian integrin complex contains receptor activity for a variety of extracellular matrix proteins. We conclude that the chicken integrin complex comprises a set of beta 1-integrin heterodimers equivalent to the human VLA antigens and includes at least two fibronectin receptors. Finally, we show that chicken embryo fibroblasts also contain a beta 3-class integrin related to the RGD receptors defined in various human cells.

Integrin on developing and adult skeletal muscle

Avian integrin is a complex of integral membrane glycoproteins that appears to function as a dual receptors for both intracellular cytoskeletal and extracellular matrix components. Antibodies were raised against this complex and used to (1) immunolocalize integrin on cryosections of developing and adult muscle tissue and on developing myotube cultures in vitro and (2) immunoaffinity purify integrin from various fiber-type specific muscles. Integrin localization was compared with that of its putative cytoskeletal-associated and extracellular matrix ligands, talin and vinculin and fibronectin and laminin, respectively. The goal was to identify putative sites of interaction between the muscle sarcolemma and the cytoskeleton and the extracellular matrix and to reveal any differences in the molecular composition at these sites. Integrin's distribution on the sarcolemma of early (Day 12) embryonic limb muscle was random and punctate. On late embryonic (Days 17-19) limb muscle tissue its distribution was generally uniform but with occasional increased densities at specific sites along the sarcolemma. Posthatch (greater than 3 weeks) fast twitch muscle showed a highly regionalized distribution. These regions of integrin concentration coincided with densities of acetylcholine receptors, revealed by TRITC alpha-bungarotoxin labeling, and regions of muscle-tendon interaction, identified by morphological criteria. Tissue culture studies also demonstrated integrin densities at analogous sites in vitro, e.g., acetylcholine receptor clusters and sites at which myofibrils terminate at the sarcolemma. These integrin-rich sites were also shown to be Triton X-100 insoluble and therefore presumably are linked to the cytoskeleton or extracellular matrix. The localization of integrin on developing and adult muscle tissue was compared with that of fibronectin, laminin, vinculin, and talin using double, immunofluorescently labeled cryosections. In general, integrin did not colocalize exclusively with any one of its putative ligands. In the embryo, discrete densities of both talin and vinculin were observed at the myotendinous junction, whereas integrin immunoreactivity was widely distributed on muscle, vasculature, nerve, and connective tissue with no discernible sites of increased density. Laminin was primarily associated with muscle and nerve whereas fibronectin was prominent on connective tissue. On posthatch tissue, the distributions of talin, vinculin, laminin, and fibronectin were similar to those in the embryo, whereas the distribution of integrin was restricted to specific sites. The distribution of integrin was also examined for fiber-type specific differences on adu

A substrate of the cell-attachment sequence of fibronectin (Arg-Gly-Asp-Ser) is sufficient to promote transition of arterial smooth muscle cells from a contractile to a synthetic phenotype

Extracellular matrix components strongly influence the differentiated properties of isolated rat arterial smooth muscle cells during in vitro cultivation. The attachment and spreading of the cells on a substrate of fibronectin or a 105-kDa cell-binding fragment of fibronectin are accompanied by a structural and functional transformation, referred to as a transition or modulation from a contractile to a synthetic phenotype. Here, the ability of the cell-attachment sequence of fibronectin, Arg-Gly-Asp-Ser (RGDS), to promote this process was studied. The results demonstrate that freshly isolated smooth muscle cells attached to a substrate of the synthetic peptide Gly-Arg-Gly-Asp-Ser-Cys (GRGDSC) in a specific manner and as well as to substrates of fibronectin and the 105-kDa fragment. Subsequent spreading of the cells on the peptide substrate followed the same kinetics and was as extensive as on fibronectin, even if protein synthesis was blocked by treatment of the cultures with cycloheximide. Like fibronectin, the peptide substrate induced formation of actin filament bundles, again without ongoing protein synthesis. Moreover, it was as efficient as fibronectin in supporting the transition of the cells from a contractile to a synthetic phenotype as analyzed by electron microscopy. Antibodies against the beta subunit of the fibronectin receptor interfered with the attachment, spreading, and fine structural reorganization of the cells in a similar manner on substrates of fibronectin, the 105-kDa fragment, and GRGDSC. Taken together, the findings indicate that the cell-attachment sequence (RGDS) mimics intact fibronectin in promoting a change in the differentiated properties of arterial smooth muscle cells and does so by interacting with a cell surface receptor for fibronectin.

Modulation of growth factor induced fiber outgrowth in rat pheochromocytoma (PC12) cells by a fibronectin receptor antibody

Rat pheochromocytoma PC12 cells respond to the binding of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) by extending neurites in a manner resembling sympathetic neurons. This response requires cell attachment to an appropriate substratum (Fujii et al., J. Neurosci., 2:1157, 1982); attachment factors which function in this capacity include the adhesive proteins fibronectin and laminin. Incubating PC12 cells with a polyclonal antiserum directed against a putative 140-kDa fibroblast cell surface fibronectin receptor (anti-gp140) perturbed spreading but not attachment of the cells to fibronectin and laminin substrates. However, in the presence of anti-gp 140 or its Fab fragments, NGF-stimulated neurite outgrowth was dramatically reduced. The antibody also caused a retraction of previously extended neurites. SDS-PAGE analysis of immunoprecipitates of PC12 cells surface labeled with 125I identified a prominent 120-140-kDa band, suggesting that the site of anti-gp140 action in PC12 cells is also through a fibronectin receptor.

Transforming growth factor beta increases cell surface binding and assembly of exogenous (plasma) fibronectin by normal human fibroblasts

Transforming growth factor beta (TGF-beta) enhances the cell surface binding of 125I-fibronectin by cultured human fibroblasts. The effect of TGF-beta on cell surface binding was maximal after 2 h of exposure to TFG-beta and did not require epidermal growth factor or protein synthesis. The enhancement was dose dependent and was found with the 125I-labeled 70-kilodalton amino-terminal fragment of fibronectin as well as with 125I-fibronectin. Treatment of cultures with TGF-beta for 6 h resulted in a threefold increase in the estimated number of fibronectin binding sites. The increase in number of binding sites was accompanied by an increased accumulation of labeled fibronectin in detergent-insoluble extracellular matrix. The effect of TGF-beta was biphasic; after 6 h of exposure, less labeled fibronectin bound to treated cultures than to control cultures. Exposure of cells to TGF-beta for greater than 6 h caused a two- to threefold increase in the accumulation of cellular fibronectin in culture medium as detected by a quantitative enzyme-linked immunosorbent assay. The second phase of the biphasic effect and the increase in soluble cellular fibronectin were blocked by cycloheximide. Immunofluorescence staining of fibroblast cultures with antifibronectin revealed that TGF-beta caused a striking increase in fibronectin fibrils. The 70-kilodalton amino-terminal fragment of fibronectin, which blocks incorporation of fibronectin into extracellular matrix, blocked anchorage-independent growth of NRK-49F cells in the presence of epidermal growth factor. Our results show that an increase in the binding and rate of assembly of exogenous fibronectin is an early event preceding the increase in expression of extracellular matrix proteins. Such an early increase in cell surface binding of exogenous fibronectin may be a mechanism whereby TGF-beta can modify extracellular matrix characteristics rapidly after tissue injury or during embryonic morphogenesis.

Fibronectin receptor exhibits high lateral mobility in embryonic locomoting cells but is immobile in focal contacts and fibrillar streaks in stationary cells

The dynamic process of embryonic cell motility was investigated by analyzing the lateral mobility of the fibronectin receptor in various locomotory or stationary avian embryonic cells, using the technique of fluorescence recovery after photobleaching. The lateral mobility of fibronectin receptors, labeled by a monoclonal antibody, was defined by the diffusion coefficient and mobile fraction of these receptors. Even though the lateral diffusion coefficient did not vary appreciably (2 X 10(-10) cm2/S less than or equal to D less than or equal to 4 X 10(-10) cm2/S) with the locomotory state and the cell type, the mobile fraction was highly dependent on the degree of cell motility. In locomoting cells, the population of fibronectin receptors, which was uniformly distributed on the cell surface, displayed a high mobile fraction of 66 +/- 19% at 25 degrees C (82 +/- 14% at 37 degrees C). In contrast, in nonmotile cells, the population of receptors was concentrated in focal contacts and fibrillar streaks associated with microfilament bundles and, in these sites, the mobile fraction was small (16 +/- 8%). When cells were in a stage intermediate between highly motile and stationary, the population of fibronectin receptors was distributed both in focal contacts with a small mobile fraction and in a diffuse pattern with a reduced mobile fraction (33 +/- 9%) relative to the diffuse population in highly locomotory cells. The mobile fraction of the fibronectin receptor was found to be temperature dependent in locomoting but not in stationary cells. The mobile fraction could be modulated by affecting the interaction between the receptor and the substratum. The strength of this interaction could be increased by growing cells on a substratum coated with polyclonal antibodies to the receptor. This caused the mobile fraction to decrease. The interaction could be decreased by using a probe, monoclonal antibodies to the receptor known to perturb the adhesion of certain cell types which caused the mobile fraction to increase. From these results, we conclude that in locomoting embryonic cells, most fibronectin receptors can readily diffuse in the plane of the membrane. This degree of lateral mobility may be correlated to the labile adhesions to the substratum presumably required for high motility. In contrast, fibronectin receptors in stationary cells are immobilized in focal contacts and fibrillar streaks which are in close association with both extracellular and cytoskeletal structures; these stable complexes appear to provide firm anchorage to the substratum.

Transforming growth factor beta increases cell surface binding and assembly of exogenous (plasma) fibronectin by normal human fibroblasts

Transforming growth factor beta (TGF-beta) enhances the cell surface binding of 125I-fibronectin by cultured human fibroblasts. The effect of TGF-beta on cell surface binding was maximal after 2 h of exposure to TFG-beta and did not require epidermal growth factor or protein synthesis. The enhancement was dose dependent and was found with the 125I-labeled 70-kilodalton amino-terminal fragment of fibronectin as well as with 125I-fibronectin. Treatment of cultures with TGF-beta for 6 h resulted in a threefold increase in the estimated number of fibronectin binding sites. The increase in number of binding sites was accompanied by an increased accumulation of labeled fibronectin in detergent-insoluble extracellular matrix. The effect of TGF-beta was biphasic; after 6 h of exposure, less labeled fibronectin bound to treated cultures than to control cultures. Exposure of cells to TGF-beta for greater than 6 h caused a two- to threefold increase in the accumulation of cellular fibronectin in culture medium as detected by a quantitative enzyme-linked immunosorbent assay. The second phase of the biphasic effect and the increase in soluble cellular fibronectin were blocked by cycloheximide. Immunofluorescence staining of fibroblast cultures with antifibronectin revealed that TGF-beta caused a striking increase in fibronectin fibrils. The 70-kilodalton amino-terminal fragment of fibronectin, which blocks incorporation of fibronectin into extracellular matrix, blocked anchorage-independent growth of NRK-49F cells in the presence of epidermal growth factor. Our results show that an increase in the binding and rate of assembly of exogenous fibronectin is an early event preceding the increase in expression of extracellular matrix proteins. Such an early increase in cell surface binding of exogenous fibronectin may be a mechanism whereby TGF-beta can modify extracellular matrix characteristics rapidly after tissue injury or during embryonic morphogenesis.

Role of IIb-IIIa-like glycoproteins in cell-substratum adhesion of human melanoma cells

The platelet fibrinogen receptor, glycoprotein complex IIb-IIIa, was isolated from human platelets by lectin and monoclonal antibody affinity chromatography and a polyclonal antiserum (anti-IIb-IIIa) was generated and used to probe for the presence and function of IIb-IIIa-like molecules in two adherent human cell lines. Both C32 melanoma cells and WI38 fibroblasts expressed a IIb-IIIa-like complex on their surface as indicated by immunoprecipitation of detergent extracts of surface radiolabeled cells. When added to cells plated in medium containing 10% serum, the anti-IIb-IIIa antiserum perturbed the adhesion of C32 melanoma cells, but not of WI38 fibroblasts. In a serum-free system, anti-IIb-IIIa antibodies inhibited attachment and spreading of C32 cells to fibrinogen, vitronectin, and fibronectin adsorbed to glass. Anti-IIb-IIIa had no effect on the attachment and spreading of WI38 cells to the extracellular matrix proteins, however. Thus, the IIb-IIIa-like complex appears to play a predominant role in cell-substratum adhesion of C32 cells, but not WI38 cells, and may result from the fact that, on a protein basis, the C32 melanoma cells express approximately 3 times more complex on their surface than do WI38 fibroblasts. The results suggest that the relative abundance of a particular adhesion receptor on the cell surface may govern its importance to cell-substratum adhesion.

Monoclonal antibodies to distinctive epitopes on the alpha and beta subunits of the fibronectin receptor

Monoclonal antibodies (MAbs) have been developed that can recognize epitopes that are unique to either the alpha or beta subunit of the fibronectin receptor (FnR). MAbs 11B4 and 7A8 immunoblot the alpha subunit of FnR either in purified form from Chinese hamster ovary (CHO) cells or in nonionic detergent extracts of cells of human and rodent origin electrophoresed under reducing or nonreducing conditions. The MAbs seem to be more reactive to the subunit when it has been electrophoresed under reducing conditions, suggesting that the epitope may be partially masked by the conformation conferred by disulfide bonding. A second set of MAbs, 7E2 and 7F9, is directed to an epitope on the beta subunit that is conformationally dependent upon disulfide bonding, as reduction of the subunit leads to loss of reactivity with both MAbs. Further, 7E2/7F9 immunoblots of nonionic detergent extracts of CHO cells, run under nonreducing conditions, reveal the presence of a third band (90-kDa), immunologically related to the beta subunit, which is not surface-labeled with 125I in intact cells and which does not copurify with the alpha and beta subunits isolated by immunoaffinity purification of FnR using the MAb PB1. The 90-kDa component is not found associated with a plasma membrane fraction prepared by crude cell fractionation, but is abundant in a low-speed pellet containing nuclei and intracellular membranes. This finding suggests that the 90-kDa component is a precursor to the beta subunit. Finally, the epitope of 7E2/7F9 is unique to CHO cells, as cross-reactivity to other cell types cannot be demonstrated by either immunoblotting or immunoprecipitation.

Laminin: molecular organization and biological function

Laminin, the most abundant glycoprotein molecule found in basement membrane, has multiple functions in eukaryotic tissues. It serves to attach epithelial cells to basement membrane, aids development and migration of specific cell types in growth and maturation, and has been implicated in tumor metastasis and some types of infection. Current concepts of the molecular organization and myriad functions of the laminin molecule are reviewed.

Expression and localization of the fibronectin receptor in the mouse nervous system

Cell surface receptors for extracellular matrix components have recently been characterized as integral membrane complexes with common features in their structural and functional properties. We have investigated the expression of the mammalian fibronectin receptor in the mouse nervous system using immunocytological and immunochemical methods. The fibronectin receptor was detectable on immature oligodendrocytes and immature and mature astrocytes in culture, while central nervous system neurons did not reveal detectable levels of fibronectin receptor at the developmental stages studied. In the peripheral nervous system both glia and neurons were found to express the fibronectin receptor. The receptor complex in both peripheral and central nervous system has an apparent molecular weight of approximately 140 kD under reducing conditions and resolves into two or three distinct protein bands under nonreducing conditions. The fibronectin receptor expresses the L2/HNK-1 epitope that is characteristic of several adhesion molecules, including L1, N-CAM, the myelin-associated glycoprotein, and J1 and thus is another member of the L2/HNK-1 family of adhesion molecules. The L2/HNK-1 carbohydrate epitope is expressed differently and independently of the fibronectin receptor protein backbone in that it is detectable in neonatal brain but not in adult brain. Our observations attribute a functional role to the fibronectin receptor and its L2/HNK-1 carbohydrate epitope during development and maintenance of cell interactions in the central and peripheral nervous systems.

Regulation of proteins in the VLA cell substrate adhesion family: influence of cell growth conditions on VLA-1, VLA-2, and VLA-3 expression

Cell quiescence resulting from culture of normal human fibroblasts in low serum (0.5%) was associated with a subsequent gradual increase in the expression of the cell-surface glycoprotein VLA-1, and a corresponding decrease in the expression of extracellular matrix adhesion receptors VLA-2 and VLA-3. Quantitation using either flow cytometry or immunoprecipitation showed that both the VLA-1/VLA-2 and VLA-1/VLA-3 ratios increased 10- to 28-fold and were still rising when cells remained quiescent for 20-30 days. Although induced by cell quiescence, changes in the levels of VLA-1, VLA-2, and VLA-3 continued to occur well after cell proliferation had stopped and thus do not directly correlate with cell cycle transition events. Despite prolonged serum deprivation resulting in elevated VLA-1/VLA-2 and VLA-1/VLA-3 ratios, growth-arrested cells remained viable and were fully capable of proliferating when restimulated. The increases in VLA-1/VLA-2 and VLA-1/VLA-3 ratios observed on quiescent cells were readily reversible, since after restimulation with 10% serum, these ratios quickly returned within 1-2 days to a level near that found on normal exponentially grown cells. Elevation of VLA-1/VLA-2 and VLA-1/VLA-3 ratios is generally associated with quiescence and is not due just to serum deprivation since density arrest of cells at confluence had similar effects on these ratios.

An integral membrane protein antigen associated with the membrane attachment sites of actin microfilaments is identified as an integrin beta-chain

A monoclonal antibody (MAb 30B6) was recently described by Rogalski and Singer (J. Cell Biol. 101:785-801, 1985) which identified an integral membrane glycoprotein of chicken cells that was associated with a wide variety of sites of actin microfilament attachments to membranes. In this report, we present a further characterization of this integral protein. An immunochemical comparison was made of MAb 30B6 binding properties with those of two other MAbs, JG9 and JG22, which identify a component of a membrane protein complex that interacts with extracellular matrix proteins including fibronectin. We showed that the 110-kilodalton protein recognized by MAb 30B6 in extracts of chicken gizzard smooth muscle is identical, or closely related, to the protein that reacts with MAbs JG9 and JG22. These 110-kilodalton proteins are also structurally closely similar, if not identical, to one another as demonstrated by 125I-tryptic peptide maps. However, competition experiments showed that MAb 30B6 recognizes a different epitope from those recognized by MAbs JG9 and JG22. In addition, the 30B6 antigen is part of a complex that can be isolated on fibronectin columns. These results together establish that the 30B6 antigen is the same as, or closely similar to, the beta-chain of the protein complex named integrin, which is the complex on chicken fibroblast membranes that binds fibronectin. Although the 30B6 antigen is present in a wide range of tissues, its apparent molecular weight on gels varies in different tissues. These differences in apparent molecular weight are due, in large part, to differences in glycosylation.

Antibodies to the conserved cytoplasmic domain of the integrin beta 1 subunit react with proteins in vertebrates, invertebrates, and fungi

The integrin family of cell surface receptors can be divided into three groups on the basis of their homologous beta subunits: beta 1, beta 2, and beta 3. We have raised an antibody against a synthetic peptide corresponding to the COOH-terminal domain of the chicken integrin beta 1 subunit that reacts with beta subunits from a variety of vertebrates, invertebrates, and fungi, demonstrating strong evolutionary conservation of sequences in this domain. In Drosophila cells, the antibody recognizes integrin alpha beta complexes that appear to be identical with position-specific antigens. Cross-reactive proteins are also detected in Caenorhabditis elegans and Candida albicans. The antiserum is specific for beta 1 subunits and does not recognize other integrin beta subunits in humans. In immunofluorescence analyses of cultured cells, the antibody reacts only with permeabilized cells confirming that this highly conserved COOH-terminal segment is a cytoplasmic domain.

Differential effects of laminin, intact type IV collagen, and specific domains of type IV collagen on endothelial cell adhesion and migration

Laminin and type IV collagen were compared for the ability to promote aortic endothelial cell adhesion and directed migration in vitro. Substratum-adsorbed IV promoted aortic endothelial cell adhesion in a concentration dependent fashion attaining a maximum level 141-fold greater than controls within 30 min. Aortic endothelial cell adhesion to type IV collagen was not inhibited by high levels (10(-3) M) of arginyl-glycyl-aspartyl-serine. In contrast, adhesion of aortic endothelial cells on laminin was slower, attaining only 53% of the adhesion observed on type IV collagen by 90 min. Type IV collagen when added to the lower well of a Boyden chamber stimulated the directional migration of aortic endothelial cells in a concentration dependent manner with a maximal response 6.9-fold over control levels, whereas aortic endothelial cells did not migrate in response to laminin at any concentration (.01-2.0 X 10(-7) M). Triple helix-rich fragments of type IV collagen were nearly as active as intact type IV collagen in stimulating both adhesion and migration whereas the carboxy terminal globular domain was less active at promoting adhesion (36% of the adhesion promoted by intact type IV collagen) or migration. Importantly, aortic endothelial cells also migrate to substratum adsorbed gradients of type IV collagen suggesting that the mechanism of migration is haptotactic in nature. These results demonstrate that the aortic endothelial cell adhesion and migration is preferentially promoted by type IV collagen compared with laminin, and has a complex molecular basis which may be important in angiogenesis and large vessel repair.

Alternative splicing of chicken fibronectin in embryos and in normal and transformed cells

To study the alternative splicing of fibronectin during embryogenesis and oncogenic transformation, we isolated cDNA clones of chicken fibronectin. The partial amino acid sequence deduced from sequencing of these clones showed that, overall, chicken fibronectin is approximately 80% identical with mammalian fibronectins. However, two of the three known regions of alternative splicing differed from this average. The V region was significantly more divergent, and RNA from embryonic chicken liver showed a pattern of V exon splicing which was distinct from that seen in human or rat fibronectins. In contrast, the EIIIB segment was very highly conserved (96%). As in mammals, this segment and another (EIIIA) were alternatively spliced in a cell-type-specific fashion. EIIIA+ and EIIIB+ species were almost absent in liver but predominated in total embryo RNA at all times from 2.5 to 11 days postfertilization. We also examined the possible contributions of fibronectin splicing and integrin receptor expression to the loss of fibronectin on oncogenic transformation. We detected little change in fibronectin splicing, other than a slight increase in representation of EIIIB+ species in fibroblasts after transformation by Rous sarcoma virus. It was also established that the overall reduction in fibronectin mRNA level observed after transformation was not accompanied by a decrease in integrin mRNA levels, indicating that fibronectin and integrin receptors are not coordinately regulated at the transcriptional level.

Expression and function of cell surface extracellular matrix receptors in mouse blastocyst attachment and outgrowth

Mouse-hatched blastocysts cultured in vitro will attach and form outgrowths of trophoblast cells on appropriate substrates, providing a model for implantation. Immediately after hatching, the surfaces of blastocysts are quiescent and are not adhesive. Over the period 24-36 h post-hatching, blastocysts cultured in serum-free medium become adhesive and attach and spread on the extracellular matrix components fibronectin, laminin, and collagen type IV in a ligand specific manner. Attachment and trophoblast outgrowth on these substrates can be inhibited by addition to the culture medium of an antibody, anti-ECMr (anti-extracellular matrix receptor), that recognizes a group of 140-kD glycoproteins similar to those of the 140-kD extracellular matrix receptor complex (integrin) recognized in avian cells by CSAT and JG22 monoclonal antibodies. Addition to the culture medium of a synthetic peptide containing the Arg-Gly-Asp tripeptide cell recognition sequence of fibronectin inhibits trophoblast outgrowth on both laminin and fibronectin. However, the presence of the peptide does not affect attachment of the blastocysts to either ligand. Immunoprecipitation of 125I surface-labeled embryos using anti-ECMr reveals that antigens recognized by this antibody are exposed on the surfaces of embryos at a time when they are spreading on the substrate, but are not detectable immediately after hatching. Immunofluorescence experiments show that both the ECMr antigens and the cytoskeletal proteins vinculin and talin are enriched on the cell processes and ventral surfaces of trophectoderm cells in embryo outgrowths, in patterns similar to those seen in fibroblasts, and consistent with their role in adhesion of the trophoblast cells to the substratum.

The migratory behavior of avian embryonic cells does not require phosphorylation of the fibronectin-receptor complex

When locomotory embryonic cells become stationary, they acquire new substratum-adhesion properties. In particular, the distribution of fibronectin receptors shifts from diffuse and highly mobile on the cell membrane to immobilized in close association with fibronectin molecules and cytoskeletal elements in focal contacts. Receptor phosphorylation has been proposed as a possible regulator of the interaction between the receptor and its intracellular and extracellular ligands. In the present study, we have compared the phosphorylation state of the fibronectin receptor in motile neural crest and somitic cells, in stationary somitic cells, and in Rous-sarcoma virus transformed-chick embryo fibroblasts, using immunoprecipitation following metabolic labeling. While no receptor phosphorylation was detected in motile embryonic cells, the beta subunit of the receptor was phosphorylated in stationary cells. This subunit was also highly phosphorylated in Rous-sarcoma virus-transformed chicken cells. These results suggest that phosphorylation of the fibronectin receptor cannot account for its distribution in the cell membrane and for the nature of the interactions between this receptor and its ligands in embryonic cells.

Purification of the beta subunit of the fibronectin receptor

In this work we describe a method for purification of the beta subunit of the mouse fibronectin receptor (GP135). Cellular glycoproteins were isolated from a detergent extract of SR-Balb tumor cell membranes by two steps of affinity chromatography on lentil lectin-Sepharose and wheat-germ-agglutinin--agarose. This material was subsequently bound to an Affi gel 102 column and eluted by increasing salt concentration. Most of the GP135 was eluted at 80 mM sodium chloride together with a few other components. A final step of hydroxyapatite chromatography in sodium dodecyl sulphate allowed elution of GP135 as a single chromatographic peak. Fractions containing GP135 were identified at each chromatographic step by immunoblotting with a specific antiserum. By this procedure GP135 was purified to homogeneity as judged by SDS-PAGE analysis of 125I-labelled material.

The 140-kDa fibronectin receptor complex is required for mesodermal cell adhesion during gastrulation in the amphibian Pleurodeles waltlii

We have studied the localization and function of a 140-kDa glycoprotein complex implicated in cell adhesion to fibronectin- and laminin-rich extracellular matrices in Pleurodeles waltlii gastrulae. In particular, we have shown that antibodies directed against highly purified avian fibronectin (FN) receptor complex cross-react with two major polypeptides of apparent molecular weights of 140,000 and 100,000 and a third minor component of 90,000. Using sections of embryos or whole mounts, we have also discovered that the putative FN receptor is widely distributed on the early embryonic cell surface. We have also found that the basal surface of the roof of the blastocoel, a region particularly enriched in an extracellular matrix consisting of fibronectin- and laminin-rich fibrils, is rich in receptor complex. We have prepared monovalent Fab' fragments of this antibody and have found that they cause detachment of cells previously attached to substrata coated with fibronectin, and they also arrest gastrulation when injected into the blastocoel of early gastrulae. Thus, it appears that the fibronectin receptor complex plays a significant functional role in cell attachment of gastrula-stage cells in vitro and in cell migration in vivo during gastrulation.

An integral membrane protein antigen associated with the membrane attachment sites of actin microfilaments is identified as an integrin beta-chain

A monoclonal antibody (MAb 30B6) was recently described by Rogalski and Singer (J. Cell Biol. 101:785-801, 1985) which identified an integral membrane glycoprotein of chicken cells that was associated with a wide variety of sites of actin microfilament attachments to membranes. In this report, we present a further characterization of this integral protein. An immunochemical comparison was made of MAb 30B6 binding properties with those of two other MAbs, JG9 and JG22, which identify a component of a membrane protein complex that interacts with extracellular matrix proteins including fibronectin. We showed that the 110-kilodalton protein recognized by MAb 30B6 in extracts of chicken gizzard smooth muscle is identical, or closely related, to the protein that reacts with MAbs JG9 and JG22. These 110-kilodalton proteins are also structurally closely similar, if not identical, to one another as demonstrated by 125I-tryptic peptide maps. However, competition experiments showed that MAb 30B6 recognizes a different epitope from those recognized by MAbs JG9 and JG22. In addition, the 30B6 antigen is part of a complex that can be isolated on fibronectin columns. These results together establish that the 30B6 antigen is the same as, or closely similar to, the beta-chain of the protein complex named integrin, which is the complex on chicken fibroblast membranes that binds fibronectin. Although the 30B6 antigen is present in a wide range of tissues, its apparent molecular weight on gels varies in different tissues. These differences in apparent molecular weight are due, in large part, to differences in glycosylation.

Integrin, the cell surface receptor for fibronectin and laminin, expresses the L2/HNK-1 and L3 carbohydrate structures shared by adhesion molecules

On the basis of recent evidence that the carbohydrate structures designated L2/HNK-1 and L3 are shared by several neural adhesion molecules including L1, N-CAM, the myelin-associated glycoprotein and J1, we have suggested that other members of the L2/HNK-1 and L3 families are adhesion molecules. Here we show by Western blot analysis that integrin, the cell surface receptor for the extracellular matrix constituents fibronectin and laminin in chicken, also expresses the L2/HNK-1 and L3 epitopes and thus is another family member proven to be involved in adhesion.