Fibronectin and integrins in cell adhesion, signaling, and morphogenesis

Rapid stiffening of integrin receptor-actin linkages in endothelial cells stimulated with thrombin: a magnetic bead microrheology study

By using magnetic bead microrheology we study the effect of inflammatory agents and toxins on the viscoelastic moduli of endothelial cell plasma membranes in real time. Viscoelastic response curves were acquired by applying short force pulses of ~500 pN to fibronectin-coated magnetic beads attached to the surface membrane of endothelial cells. Upon addition of thrombin, a rapid stiffening of the membrane was observed within 5 s, followed by recovery of the initial deformability within 2 min. By using specific inhibitors, two known pathways by which thrombin induces actin reorganization in endothelial cells, namely activation of Ca2+-calmodulin-dependent myosin light chain kinase and stimulation of Rho/Rho-kinase, were excluded as possible causes of the stiffening effect. Interestingly, the cytotoxic necrotizing factor of Escherichia coli, a toxin which, in addition to Rho, activates the GTPases Rac and CDC42Hs, also induced a dramatic stiffening effect, suggesting that the stiffening may be mediated through a Rac- or Cdc42Hs-dependent pathway. This work demonstrates that magnetic bead microrheometry is not only a powerful tool to determine the absolute viscoelastic moduli of the composite cell plasma membrane, but also a valuable tool to study in real time the effect of drugs or toxins on the viscoelastic parameters of the plasma membrane.

Identification and characterization of a novel fibronectin in zebrafish

Sequence analysis of zebrafish fibronectin (FN) cDNAs indicates that at least two forms of the protein exist in fish. One form (FN1) is very similar to FNs identified in other vertebrates possessing 12 type I, 2 type II, and 17 type III repeats including two alternative splice sites (EIIIA and EIIIB) and a variable region (V). Zebrafish FN1 contains the RGD cell adhesion site in type III(10) and a second cell-binding site (LDV) in the V region. In addition to this conserved form of FN, a novel truncated form of zebrafish FN (FN2) was identified. The predicted structure of FN2 is identical to FN1 at the N-terminal region possessing 9 type I, 2 type II, and the first 3 type III repeats. Following III(3), FN2 contains a unique 20-amino-acid C-terminal tail that is different from the C-terminus of FN1, lacking the two cysteines that are usually involved in the formation of interchain disulfide bonds. Genomic sequence analysis has revealed that FN2 is generated by an alternative RNA splicing pattern that has not been described for FN in other organisms. Reverse transcription-polymerase chain reaction analysis and RNase protection assays reveal that FN2 mRNA is present in the zebrafish embryo throughout development as well as in cultures of an established liver cell line. Experiments conducted with recombinant FN2 synthesized in insect cells demonstrate that the protein promotes the attachment and spreading of fish embryo cells in culture.

Tumors and fibrinogen. The role of fibrinogen as an extracellular matrix protein

The progression of a tumor from benign and localized to invasive and metastatic growth is the major cause of poor clinical outcome in cancer patients. Much like in a healing wound, the deposition of fibrin(ogen), along with other adhesive glycoproteins, into the extracellular matrix (ECM) serves as a scaffold to support binding of growth factors and to promote the cellular responses of adhesion, proliferation, and migration during angiogenesis and tumor cell growth. Inappropriate synthesis and deposition of ECM constituents is linked to altered regulation of cell proliferation, leading to tumor cell growth and malignant transformation. Fibrin deposition occurs within the stroma of a majority of tumor types. In contrast, abundant FBG, not fibrin, is present within the stroma of breast cancers. It is thought to originate from exudation of plasma FBG and subsequent deposition into the tumor stroma and not endogenous synthesis and secretion of FBG by breast tumor cells. However, we show that MCF-7 human breast cancer cells synthesize and secrete FBG polypeptides, suggesting that the origin of FBG in the stroma of breast carcinoma may be due to endogenous synthesis and deposition. Moreover, FBG assembles into ECM as conformationally altered FBG, not as fibrin. Studies in our laboratory demonstrate that FBG alters the ability of breast cancer cells to migrate. Together, the results of studies from our laboratory, as well as the laboratories of others, indicate that the presence of fibrin(ogen) within the tumor stroma likely affects the progression of tumor cell growth and metastasis. This review focuses on FBG within tumors and its relationship with other tumor constituents, ultimately focusing on the role of FBG in breast cancer.

The putative collagen binding peptide hastens periodontal ligament cell attachment to bone replacement graft materials

BACKGROUND

Bone replacement graft (BRG) materials are often used to treat periodontal defects, to promote cellular invasion, and to encourage bone regrowth. Periodontal ligament fibroblasts (PDLF) incorporate these materials and form the basis of the renewed connection between the existing and newly formed alveolar bone and the tooth surface. A peptide (P-15) that mimics the putative cell-binding domain of collagen has been reported to promote dermal fibroblast attachment and proliferation.

METHODS

PDLF were quantitatively examined for their ability to adhere to a variety of BRG materials fluorometrically. In addition, scanning electron microscopy was used to examine the changes in morphology exhibited by these cells as they attached and spread on several BRG materials. Finally, BRG materials containing the P-15 peptide were quantitatively examined for their ability to promote PDLF attachment and proliferation.

RESULTS

Freeze-dried allograft bone supports greater PDLF attachment than does several xenograft and alloplastic anorganic bone replacement materials. An anorganic BRG material containing the P-15 peptide promoted more rapid cell attachment and spreading than a similar anorganic BRG material lacking this peptide. Finally, none of the BRG materials examined promoted PDLF proliferation.

CONCLUSIONS

Our data indicate that the addition of the P-15 peptide increases the rapidity of PDLF attachment to xenogeneic bone replacement materials. This increase in the rate of attachment may have clinical significance in the context of the dynamic regulation of cell attachment during periodontal regeneration. However, this peptide does not promote an increase in stable cell attachment or proliferation in vitro.

Galectin-8 Functions as a Matricellular Modulator of Cell Adhesion

The interaction of cells with the extracellular matrix regulates cell adhesion and motility. Here we demonstrate that different cell types adhere and spread when cultured in serum-free medium on immobilized galectin-8, a mammalian beta-galactoside-binding protein. At maximal doses, galectin-8 is equipotent to fibronectin in promoting cell adhesion and spreading. Cell adhesion to immobilized galectin-8 is mediated by sugar-protein interactions with integrins, and galectin-8 triggers integrin-mediated signaling cascades including Tyr phosphorylation of focal adhesion kinase and paxillin. Cell adhesion is potentiated in the presence of Mn(2+), whereas it is interrupted in the presence of soluble galectin-8, integrin beta(1) inhibitory antibodies, EDTA, or thiodigalactoside but not by RGD peptides. Furthermore, cells readily adhere onto immobilized monoclonal galectin-8 antibodies, which are equipotent to integrin antibodies in promoting cell adhesion. Cell adhesion to immobilized galectin-8 is partially inhibited by serum proteins, suggesting that complex formation between immobilized galectin-8 and serum components generates a matrix that is less supportive of cell adhesion. Accordingly, cell motility on immobilized galectin-8 readily takes place in the presence of serum. Truncation of the C-terminal half of galectin-8, including one of its two carbohydrate recognition domains, largely abolishes its ability to modulate cell adhesion, indicating that both carbohydrate recognition domains are required to maintain a functional form of galectin-8. Collectively, our findings implicate galectin-8 as a physiological modulator of cell adhesion. When immobilized, it functions as a matrix protein equipotent to fibronectin in promoting cell adhesion by ligation and clustering of cell surface integrin receptors. In contrast, when present in excess as a soluble ligand, galectin-8 (like fibronectin) forms a complex with integrins that negatively regulates cell adhesion. Because of its dual effects on the adhesive properties of the cells and its association with fibronectin, galectin-8 might be considered a novel type of matricellular protein.

Nischarin, a novel protein that interacts with the integrin alpha5 subunit and inhibits cell migration

Integrins have been implicated in key cellular functions, including cytoskeletal organization, motility, growth, survival, and control of gene expression. The plethora of integrin alpha and beta subunits suggests that individual integrins have unique biological roles, implying specific molecular connections between integrins and intracellular signaling or regulatory pathways. Here, we have used a yeast two-hybrid screen to identify a novel protein, termed Nischarin, that binds preferentially to the cytoplasmic domain of the integrin alpha5 subunit, inhibits cell motility, and alters actin filament organization. Nischarin is primarily a cytosolic protein, but clearly associates with alpha5beta1, as demonstrated by coimmunoprecipitation. Overexpression of Nischarin markedly reduces alpha5beta1-dependent cell migration in several cell types. Rat embryo fibroblasts transfected with Nischarin constructs have "basket-like" networks of peripheral actin filaments, rather than typical stress fibers. These observations suggest that Nischarin might affect signaling to the cytoskeleton regulated by Rho-family GTPases. In support of this, Nischarin expression reverses the effect of Rac on lamellipodia formation and selectively inhibits Rac-mediated activation of the c-fos promoter. Thus, Nischarin may play a negative role in cell migration by antagonizing the actions of Rac on cytoskeletal organization and cell movement.

Cellular invasion by Staphylococcus aureus involves a fibronectin bridge between the bacterial fibronectin-binding MSCRAMMs and host cell beta1 integrins

Although Staphylococcus aureus is primarily considered an extracellular pathogen, recent evidence suggests that this bacterium can invade a variety of nonprofessional phagocytic cells. Here we investigate the early stages of cellular invasion by S. aureus and determine the bacterial and host components that are required for this process. S. aureus expresses two cell surface-associated fibronectin (FN)-binding proteins (FnbpA and FnbpB) that mediate the interaction of the bacteria with both soluble and solid-phase FN in vitro. Using a mutant of S. aureus that lacks the expression of both Fnbps, we show that the expression of either protein is necessary for efficient uptake by the mouse fibroblast line GD25beta1A. Invasion could be inhibited by soluble recombinant proteins encompassing either the FN-binding D repeat region or the A region (and B repeats) of FnbpA, suggesting that the activities of both regions are important in this process. We demonstrate that FN is also required for invasion of this cell line. In the presence of FN-depleted fetal bovine serum, the invasion level was reduced by approximately 40% compared to in the presence of whole fetal bovine serum. Invasion could be further reduced by the addition of anti-mouse FN antibodies to the assay. Finally, we utilize a mutant mouse fibroblast line, which lacks beta1 integrin expression, to demonstrate that host cell beta1 integrins are necessary for efficient cellular invasion. The level of invasion of the mutant cell line GD25 was reduced by approximately 97% compared to the beta1-expressing complemented cell line GD25beta1A. In addition, invasion of the GD25beta1A cell line could be inhibited by an RGD-containing peptide, further implicating a role for integrins in this process. Based on these observations, we put forward a model of S. aureus invasion in which host FN forms a bridge between the bacterial Fnbps and host cell beta1 integrins, leading to bacterial uptake.

Conditional ablation of beta1 integrin in skin. Severe defects in epidermal proliferation, basement membrane formation, and hair follicle invagination

The major epidermal integrins are alpha3beta1 and hemidesmosome-specific alpha6beta4; both share laminin 5 as ligand. Keratinocyte culture studies implicate both integrins in adhesion, proliferation, and stem cell maintenance and suggest unique roles for alphabeta1 integrins in migration and terminal differentiation. In mice, however, whereas ablation of alpha6 or beta4 results in loss of hemidesmosomes, epidermal polarity, and basement membrane (BM) attachment, ablation of alpha3 only generates microblistering due to localized internal shearing of BM. Using conditional knockout technology to ablate beta1 in skin epithelium, we have uncovered biological roles for alphabeta1 integrins not predicted from either the alpha3 knockout or from in vitro studies. In contrast to alpha3 null mice, beta1 mutant mice exhibit severe skin blistering and hair defects, accompanied by massive failure of BM assembly/organization, hemidesmosome instability, and a failure of hair follicle keratinocytes to remodel BM and invaginate into the dermis. Although epidermal proliferation is impaired, a spatial and temporal program of terminal differentiation is executed. These results indicate that beta1's minor partners in skin are important, and together, alphabeta1 integrins are required not only for extracellular matrix assembly but also for BM formation. This, in turn, is required for hemidesmosome stability, epidermal proliferation, and hair follicle morphogenesis. However, beta1 downregulation does not provide the trigger to terminally differentiate.

Hyaluronan-based polymers in the treatment of osteochondral defects

Articular cartilage in adults has limited ability for self-repair. Some methods devised to augment the natural healing response stimulate some regeneration, but the repair is often incomplete and lacks durability. Hyaluronan-based polymers were tested for their ability to enhance the natural healing response. It is hypothesized that hyaluronan-based polymers recreate an embryonic-like milieu where host progenitor cells can regenerate the damaged articular surface and underlying bone. Osteochondral defects were made on the femoral condyles of 4-month-old rabbits and were left empty or filled with hyaluronan-based polymers. The polymers tested were ACP sponge, made of crosslinked hyaluronan, and HYAFF-11 sponge, made of benzylated hyaluronan. The rabbits were killed 4 and 12 weeks after surgery, and the condyles were processed for histology. All 12-week defects were scored with a 29-point scale, and the scores were compared with a Kruskall-Wallis analysis of variance on ranks. Untreated defects filled with bone tissue up to or beyond the tidemark, and the noncalcified surface layer varied from fibrous to hyaline-like tissue. Four weeks after surgery, defects treated with ACP exhibited bone filling to the level of the tidemark and the surface layer was composed of hyaline-like cartilage well integrated with the adjacent cartilage. At 12 weeks, the specimens had bone beyond the tidemark that was covered with a thin layer of hyaline cartilage. Four weeks after surgery, defects treated with HYAFF-11 contained a rim of chondrogenic cells at the interface of the implant and the host tissue. In general, the 12-week defects exhibited good bone fill and the surface was mainly hyaline cartilage. Treated defects received significantly higher scores than untreated defects (p < 0.05), and ACP-treated defects scored significantly higher than HYAFF-11-treated defects (p < 0.05). The introduction of these hyaluronan-based polymers into defects provides an appropriate scaffolding and favorable microenvironment for the reparative process. Further work is required to fully assess the long-term outcome of defects treated with these polymers.

Integrin alpha5/beta1 mediates fibronectin-dependent epithelial cell proliferation through epidermal growth factor receptor activation

Human integrin alpha5 was transfected into the integrin alpha5/beta1-negative intestinal epithelial cell line Caco-2 to study EGF receptor (EGFR) and integrin alpha5/beta1 signaling interactions involved in epithelial cell proliferation. On uncoated or fibronectin-coated plastic, the integrin alpha5 and control (vector only) transfectants grew at similar rates. In the presence of the EGFR antagonistic mAb 225, the integrin alpha5 transfectants and controls were significantly growth inhibited on plastic. However, when cultured on fibronectin, the integrin alpha5 transfectants were not growth inhibited by mAb 225. The reversal of mAb 225-mediated growth inhibition on fibronectin for the integrin alpha5 transfectants correlated with activation of the EGFR, activation of MAPK, and expression of proliferating cell nuclear antigen. EGFR kinase activity was necessary for both MAPK activation and integrin alpha5/beta1-mediated cell proliferation. Although EGFR activation occurred when either the integrin alpha5-transfected or control cells were cultured on fibronectin, coprecipitation of the EGFR with SHC could be demonstrated only in the integrin alpha5-transfected cells. These results suggest that integrin alpha5/beta1 mediates fibronectin-induced epithelial cell proliferation through activation of the EGFR.

Role of fibronectin deposition in branching morphogenesis of Madin-Darby canine kidney cells

BACKGROUND

Madin-Darby canine kidney (MDCK) epithelial cells grown in collagen gels in the presence of hepatocyte growth factor (HGF) form branching tubules. The tubule-lining epithelial cells are polarized with the basolateral surface in contact with the collagen gel and the apical surface facing the lumen. To delineate whether MDCK branching tubules construct the basal lamina, we characterized the composition of the extracellular matrix deposited by MDCK tubules. The tubule-lining cells produced an apparently incomplete basal lamina containing a discontinuous laminin substratum. In addition, a thick layer of fibronectin surrounded the basal cell surface of the branching tubule. In an attempt to delineate the role of fibronectin deposition in branching morphogenesis, we conducted this study.

METHODS

MDCK cells cultured in collagen gel were employed. We first used arginine-glycine-aspartate peptides containing disintegrin rhodostomin to disturb the interactions between fibronectin and cell surface integrins. Furthermore, we established several stable transfectants expressing fibronectin antisense RNA to examine the role of fibronectin in branching morphogenesis directly.

RESULTS

Rhodostomin inhibited the formation of branching tubules. The transfectants expressing fibronectin antisense RNA exhibited relatively lower levels of synthesized fibronectin and markedly diminished growth rates of branching tubules than the control clone. An inhibition of branching morphogenesis induced by the overexpression of fibronectin antisense RNA was manifested by the decrease in cell growth rates and cell migration.

CONCLUSION

These results indicate that the deposition of fibronectin underlying the tubule-lining epithelium serves to enhance cell proliferation and migration, and hence facilitates the branching tubulogenesis of MDCK cells.

Integrins regulate the linkage between upstream and downstream events in G protein-coupled receptor signaling to mitogen-activated protein kinase

Receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) can both activate mitogen-activated protein kinase (MAPK), a critical intermediate in the transduction of proliferative signals. Numerous observations have demonstrated that integrin-mediated cell anchorage can regulate the efficiency of signaling from RTKs to MAPK. Recently, a relationship between integrins and GPCR signaling has also emerged; however, little is understood concerning the mechanisms involved. Here, we investigate integrin regulation of GPCR signaling to MAPK, focusing on the P2Y class of GPCRs that function through activation of phospholipase Cbeta. P2Y receptor signaling to the downstream components mitogen-activated protein kinase kinase and MAPK is highly dependent on integrin-mediated cell anchorage. However, activation of upstream events, including inositol phosphate production and generation of calcium transients, is completely independent of cell anchorage. This indicates that integrins regulate the linkage between upstream and downstream events in this GPCR pathway, just as they do in some aspects of RTK signaling. However, the P2Y pathway does not involve cross-activation of a RTK, nor a role for Shc or c-Raf; thus, it is quite distinct from the classical RTK-Ras-Raf-MAPK cascade. Rather, integrin-modulated P2Y receptor stimulation of MAPK depends on calcium and on the activation of protein kinase C.

Fibronectin matrix composition and organization can regulate cell migration during amphibian development

Fibronectin (FN) is an adhesive extracellular matrix component that is essential for vertebrate development. It forms a fibrillar matrix at the cell surface which controls cell morphology, migration, proliferation, and other important cellular processes. To address specific functions of FN matrix structure during early vertebrate development, we introduced normal and mutant recombinant FNs (recFNs) into the blastocoel cavity of embryos of the amphibian Pleurodeles waltl. Here we show that a native recFN FN(A-B-) as well as recFNs with specific mutations in the cell-binding domain, FN(RGD-) and FN(syn-), or in a FN-binding region, FNDeltaIII(1), are assembled into fibrillar matrix. A recFN (FNDeltaIII(1-7)) that forms a structurally distinct matrix in cultured cells was assembled into aggregates at the cell periphery and was able to inhibit assembly of endogenous amphibian FN matrix in a dose-dependent manner. Cell adhesion, spreading, and migration were perturbed in vitro and in vivo on chimeric matrices containing FN(RGD-), FN(syn-), or FNDeltaIII(1-7) co-assembled with amphibian FN. Developmentally, this perturbation resulted in defects in mesoderm patterning and inhibition of gastrulation. These results indicate that FN matrix fibrillar structure and composition are important determinants of cell adhesion and migration during development.

Extracellular matrix cell adhesion peptides: functional applications in orthopedic materials

This review describes research on selected peptide sequences that affect cell adhesion as it applies in orthopedic applications. Of particular interest are the integrin-binding RGD peptides and heparin-binding peptides. The influence of these peptides on cell adhesion is described. Cell adhesion is defined as a sequence of four steps: cell attachment, cell spreading, organization of an actin cytoskeleton, and formation of focal adhesions. RGD sequences clearly influence cell attachment and spreading, whereas heparin-binding sequences appear to be less efficient. Collectively, these sequences appear to promote all steps of cell adhesion in certain cell types. This review also addresses issues related to peptide immobilization, as well as potential complexities that may develop as a result of using these versatile cell-binding sequences. Also described are future directions in the field concerning use of existing and more sophisticated peptide substrata.

Taste cell function. Structural and biochemical implications

Maintenance of constant relations between receptor cell types and branching from a single gustatory nerve fiber during normal cell turnover and regeneration requires cell-cell recognition likely mediated by timed expression of molecules at surfaces of taste bud cells, nerve endings, and in extracellular matrix. These processes assure stability of gustatory quality representation during intragemmal remodeling. Coincidentally, features of gemmal cell lifespan, including elongation, differentiation, and migration prior to apoptosis, must also be orchestrated by molecular signals. This article reviews the potential roles played by a variety of molecular markers for some relevant classes of proteins, peptides, and enzymes, which were presumed to be important for carrying out these gustatory cellular functions.

Single-cell analysis using capillary electrophoresis: influence of surface support properties on cell injection into the capillary

Capillary electrophoresis (CE) is an important tool of chemical cytometry. Whole-cell analysis using CE starts with cell injection into the capillary by either siphoning or electroosmosis. However, strong adherence of the cell to the support surface can prevent efficient cell injection and lead to irreproducible analysis. Here we evaluated several surfaces as potential cell supports for HT29 cells (human colon adenocarcinoma). These cells strongly adhered to the surface of untreated glass or polystyrene. Hydrophobic coating with dimethyldichlorosilane (DMS) or Sigmacote did not significantly reduce cell adhesion. In contrast, cell adhesion was reduced significantly when the surface was modified with hydrophilic polymers (hydrogels) such as poly(2-hydrohyethyl methacrylate) (PHEMA) and polyvinyl alcohol (PVA). In addition to their pronounced antiadhesive properties, PHEMA and PVA coatings were the most biocompatible (had highest survival of cells in contact with surface). Hydrogel-coated polystyrene plates were tested as a commercial alternative to hydrogel-coated glass slides. The cell adhesive properties of such plates were similar to those of PHEMA and PVA. However, the biocompatibility of the plates was lower than that of the other surfaces tested. Moreover, in contrast to PHEMA- and PVA-coated glass slides, the plates were sensitive to UV light and therefore should not be used when fluorescent image microscopy with UV excitation precedes CE. The analyses of the data obtained showed that PHEMA- and PVA-coated glass slides were the most suitable cell supports for cell injection into the capillary.