Glycosaminoglycans modulate fibronectin matrix assembly and are essential for matrix incorporation of tenascin-C

Requirements for sulfate transport and the diastrophic dysplasia sulfate transporter in fibronectin matrix assembly

Diastrophic dysplasia sulfate transporter (DTDST) is a sulfate/chloride antiporter whose function is impaired in several human chondrodysplasias. We show that DTDST is upregulated by dexamethasone stimulation of HT1080 fibrosarcoma cells and is required for fibronectin (FN) extracellular matrix deposition by these cells. DTDST imports sulfate for the modification of glycosaminoglycans. We find that N-sulfation of these chains is important for FN matrix assembly and that sulfation of cell surface proteoglycans is reduced in the absence of DTDST. Of the candidate HT1080 cell surface proteoglycans, only loss of syndecan-2 compromises FN assembly, as shown by syndecan-2 small interfering RNA knockdown. DTDST is both necessary and sufficient to induce FN matrix assembly in HT1080 cells. Knockdown of DTDST ablates FN matrix, whereas its overexpression increases assembly without dexamethasone stimulation. These results identify a previously unrecognized regulatory pathway for matrix assembly via modulation of a sulfate transporter and proteoglycan sulfation. These data raise the possibility that FN assembly defects contribute to chondrodysplasias.

Switch to an invasive growth phase in melanoma is associated with tenascin-C, fibronectin, and procollagen-I forming specific channel structures for invasion

Malignant melanomas are characterized by their high propensity to invade and metastasize, but the molecular mechanisms of these traits have remained elusive. Our DNA microarray analyses of benign nevi and melanoma tissue specimens revealed that the genes encoding extracellular matrix proteins tenascin-C (TN-C), fibronectin (FN), and procollagen-I (PCOL-I) are highly upregulated in invasive and metastatic melanomas. The expression and distribution of these proteins were further studied by immunohistochemistry in benign nevi, radially and vertically growing melanomas, sentinel node micrometastases, and macrometastases. TN-C was increased in all invasive tumours and metastases, especially at invasion fronts, but not in benign nevi or non-invasive melanomas. Significantly, the intensity of TN-C staining correlated with metastasis to sentinel lymph nodes, better than tumour thickness (Breslow). Moreover, TN-C, FN, and PCOL-I appeared to co-localize in the tumours and form tubular meshworks and channels ensheathing the melanoma cells. Our data suggest that melanoma invasion is associated with the formation of special channel-like structures, providing a new concept, structured tumour cell spreading. Altogether, these data provide potential new prognostic markers and therapeutic targets/strategies for preventing melanoma dissemination.

Engineered Cell-Adhesive Nanoparticles Nucleate Extracellular Matrix Assembly

Tissue engineering aims to regenerate new biological tissue for replacing diseased or injured tissues. We propose a new approach to accelerate the deposition of cell-secreted matrix proteins into extracellular matrix fibrils. We examined whether dynamic substrates with nanoscale ligand features allowing for alpha5beta1 integrin recruiting, cellular tension generation, and alpha5beta1 integrin mobility would enhance fibronectin matrix assembly in a ligand model system that is routinely not sufficient for its induction. To this end, we developed biodynamic substrates consisting of cell adhesive fragment from the 9th and 10th type repeats of fibronectin (FNf ) functionalized to 100 nm prefabricated albumin nanoparticles (ANPs). FNf-ANPs modulated cellular spreading processes, promoting the development of stellate or dendritic morphologies. Concomitant with the spreading, FNf-ANPs rapidly recruited beta1 integrins to focal contacts and promoted the migration of beta1 integrins centripetally from the cell periphery toward the center. FNf-ANPs stimulated the deposition of secreted fibronectin into matrix fibrils; FNf, the key ligand alone, was not sufficient for fibronectin fibrillogenesis. When FNf-ANPs were displayed from "immobilized" substrates, abolishing any mobility of ligated beta1 integrins, fibronectin matrix assembly was abrogated, implicating the role of dynamic matrix display on matrix assembly. Receptor ligation of FNf-ANPs via noncontractile adhesions was not sufficient to stimulate fibrillogenesis, and Rho-kinase inhibitors abolished fibronectin matrix deposition. Our approach highlights the possibility of engineering integrin-based extracellular matrix assembly using nanotechnology, which may have implications for improved biomaterials for wound repair and basic understanding of matrix remodeling within pathogenesis and biomedicine.

Heparin-mediated conformational changes in fibronectin expose vascular endothelial growth factor binding sites

Regulation of angiogenesis involves interactions between vascular endothelial growth factor (VEGF) and components of the extracellular matrix, including fibronectin and heparan sulfate. In the present study, we identified two classes of VEGF binding sites on fibronectin. One was constitutively available whereas the availability of the other was modulated by the conformational state of fibronectin. Atomic force microscopy studies revealed that heparin and hydrophilic substrates promoted the extended conformation of fibronectin, leading to increased VEGF binding. The ability of heparin to enhance VEGF binding to fibronectin was dependent on the chemical composition and chain length of heparin, since long (>22 saccharides) heparin chains with sulfation on the 6-O and N positions of glucosamine units were required for full activity. Treatment of the complex endothelial extracellular matrix with heparin also increased VEGF binding, suggesting that heparin/heparan sulfate might regulate VEGF interactions within the extracellular matrix by controlling the structure and organization of fibronectin matrices.

Circulating level of large splice variants of tenascin-C is a marker of piecemeal necrosis activity in patients with chronic hepatitis C

BACKGROUND

It has been reported that histological activity index and piecemeal necrosis are good factors to evaluate the prognosis in patients with chronic hepatitis C (CHC). Thus, there is a need for simple and noninvasive means to assess disease activity and piecemeal necrosis in patients with CHC. In this study, we measured the serum concentrations of large splice variants of tenascin-C (cTN-C) in patients with CHC, and examined their correlation with the degree of inflammatory activity and fibrosis as evaluated in liver biopsy specimens.

METHODS

The serum levels of cTN-C in 150 patients with CHC and 50 healthy volunteers were measured by enzyme-linked immunosorbent. The histology of liver biopsy specimens was also evaluated following the Desmet's grading/staging system and the Ishak's classification. Liver specimens obtained by biopsy were also immunohistochemically evaluated with anti-human tenascin-C antibodies.

RESULTS

Serum cTN-C concentrations were significantly higher in CHC patients than in healthy volunteers (P < 0.0001). The levels of cTN-C showed no significant difference among the fibrosis stages as assessed by the Desmet's grading/staging system and Ishak's classification scores. However, the concentration of cTN-C was significantly correlated with the grade of activity. According to the Ishak's classification, the concentration of cTN-C was increased in proportion to the degree of piecemeal necrosis. Specific immunostaining of cTN-C was observed in limited areas of piecemeal necrosis but not in fibrotic areas.

CONCLUSION

The measurement of serum levels of cTN-C is a useful marker of the activity of CHC, in particular of the degree of piecemeal necrosis.

The shed ectodomain of type XIII collagen associates with the fibrillar fibronectin matrix and may interfere with its assembly in vitro

Type XIII collagen is a transmembrane collagen, which is known to exist also as a soluble variant due to ectodomain shedding. Earlier studies with the recombinant ectodomain have shown it to interact in vitro with a number of extracellular matrix proteins, e.g. Fn (fibronectin). In view of its strong binding to Fn, we examined in the present study whether the released soluble ectodomain can bind to the fibrillar Fn matrix under cell-culture conditions and, if so, influence its assembly. In this study, we demonstrate that the type XIII collagen ectodomain of mammalian cells can associate with Fn fibres and may eventually hamper incorporation of the fibrillar Fn meshwork. The association between type XIII collagen and Fn was implicated to be mediated by the C-terminal end of type XIII collagen and the N-terminal end of Fn. The results presented here imply that the shedding of the type XIII collagen ectodomain results in a biologically active molecule capable of remodelling the structure of the pericellular matrix.

Immunohistochemical assessment of fibronectin and tenascin and their integrin receptors alpha5beta1 and alpha9beta1 in gastric and colorectal cancers with lymph node and liver metastases

The aim of the current study was to assess immunohistochemically and compare the level of expression of tenascin (TN) and fibronectin (FN) and their integrin receptors alpha9beta1 and alpha5beta1 in the primary colorectal and gastric tumors, and in corresponding lymph node and liver metastases from 53 patients. We detected similar high deposition of the studied ECM proteins and their receptors in the stroma of primary tumors and in liver metastases and a lower deposition in lymph node metastases. Cytoplasmic immune reaction for FN and TN was also seen in the tumor cells. A pronounced co-localization of immune deposits for FN and TN and their receptors was found in the stroma of the center and the invasion front (IF) (p<0.0001). A significant decrease of FN immune signal was observed in the IF in primary tumors and liver metastases (p<0.0001). The levels of immunolabeling of FN and TN correlated with the differentiation grade of primary tumors (p<0.0001). In conclusion, we may say that there is heterogeneous deposition of TN, FN and their integrin receptors in the different areas of primary colorectal and gastric tumors and of their metastases. These findings imply that the studied proteins may be involved in cell processes such as growth, adhesion, migration and apoptosis.

Dynamics of Assembly and Reorganization of Extracellular Matrix Proteins

This chapter will review advances in our understanding of the dynamics of assembly and reorganization of extracellular matrix (ECM) proteins and will highlight the role of fibronectin as a key orchestrator for the assembly of multiple ECM proteins. The dynamic rather than static nature of the ECM will be emphasized by reviewing time-lapse imaging studies in living cell and embryo systems, with a particular focus on fibronectin and members of the fibrillin superfamily. These studies have provided new insights into the assembly and reorganization of ECM fibrillar networks, suggesting that fibril assembly is a hierarchical process, with increasingly larger fibrillar structures formed by the progressive aggregation of smaller units. These studies have also revealed that motile cells appear to be actively involved in the assembly and reorganization of ECM fibrillar networks by shunting fibrillar material from one location to another, adding fibrillar material to the ends of growing fibrils, and exchanging material between fibrils. A common theme emerging from these studies is that cell- and tissue-generated mechanical forces are critical in the assembly and remodeling of the ECM.

Fucoidan a sulfated polysaccharide from brown algae is a potent modulator of connective tissue proteolysis

Fucoidans are sulfated fucosylated polymers from brown algae cell wall that exhibit some heparin/heparan sulfate properties. We previously demonstrated that these polysaccharides were able in vitro to stimulate dermal fibroblast proliferation and extracellular matrix deposition. Here, we investigated the action of a 16kDa fucoidan fraction on parameters involved in connective tissue breakdown. This fucoidan is able to inhibit gelatinase A secretion and stromelysin 1 induction by interleukin-1beta on dermal fibroblasts in culture. Furthermore, we observed that fucoidan increases the rate of association of MMPs with their specific inhibitors namely TIMPs. Using tissue sections of human skin in ex vivo experiments, we evidenced that this polysaccharide was able to minimize human leukocyte elastase activity resulting in the protection of human skin elastic fiber network against the enzymatic proteolysis due to this serine proteinase. These results suggested that fucoidan could be used for treating some inflammatory pathologies in which uncontrolled extracellular matrix degradation takes place.

Ultrastructural Dynamics of Human Reproduction, from Ovulation to Fertilization and Early Embryo Development1

This study describes the updated, fine structure of human gametes, the human fertilization process, and human embryos, mainly derived from assisted reproductive technology (ART). As clearly shown, the ultrastructure of human reproduction is a peculiar multistep process, which differs in part from that of other mammalian models, having some unique features. Particular attention has been devoted to the (1) sperm ultrastructure, likely "Tygerberg (Kruger) strict morphology criteria"; (2) mature oocyte, in which the MII spindle is barrel shaped, anastral, and lacking centrioles; (3) three-dimensional microarchitecture of the zona pellucida with its unique supramolecular filamentous organization; (4) sperm-egg interactions with the peculiarity of the sperm centrosome that activates the egg and organizes the sperm aster and mitotic spindles of the embryo; and (5) presence of viable cumulus cells whose metabolic activity is closely related to egg and embryo behavior in in vitro as well as in vivo conditions, in a sort of extraovarian "microfollicular unit." Even if the ultrastructural morphodynamic features of human fertilization are well understood, our knowledge about in vivo fertilization is still very limited and the complex sequence of in vivo biological steps involved in human reproduction is only partially reproduced in current ART procedures.

Potential oncogenic action of tenascin-C in tumorigenesis

The prominent expression of tenascin-C in the stroma of most solid tumors, first observed in the mid 1980s, implicates tenascin-C in tumorigenesis. This is also supported by in vitro experiments that demonstrate the capacity of tenascin-C to stimulate tumor growth by various mechanisms including promotion of proliferation, escaping immuno-surveillance and positively influencing angiogenesis. However, tumorigenesis in tenascin-C knock-out mice is not significantly different from that observed in control animals. Perhaps this is not unexpected if one considers that tenascin-C may act as an oncogene. The potential role of tenascin-C in tumorigenesis through its oncogenic action on cellular signaling will be discussed in this review, including how tenascin-C mediated tumor cell detachment might affect genome stability.

Fibronectin matrix turnover occurs through a caveolin-1-dependent process

Extracellular matrix remodeling occurs during development, tissue repair, and in a number of pathologies, including fibrotic disorders, hypertension, and atherosclerosis. Extracellular matrix remodeling involves the complex interplay between extracellular matrix synthesis, deposition, and degradation. Factors that control these processes are likely to play key roles in regulating physiological and pathological extracellular matrix remodeling. Our data show that fibronectin polymerization into the extracellular matrix regulates the deposition and stability of other extracellular matrix proteins, including collagen I and thrombospondin-1 (Sottile and Hocking, 2002. Mol. Biol. Cell 13, 3546). In the absence of continual fibronectin polymerization, there is a loss of fibronectin matrix fibrils, and increased levels of fibronectin degradation. Fibronectin degradation occurs intracellularly after endocytosis and can be inhibited by chloroquine, an inhibitor of lysosomal degradation, and by caveolae-disrupting agents. Down-regulation of caveolin-1 by RNAi inhibits loss of fibronectin matrix fibrils, fibronectin internalization, and fibronectin degradation; these processes can be restored by reexpression of caveolin-1. These data show that fibronectin matrix turnover occurs through a caveolin-1-dependent process. Caveolin-1 regulation of fibronectin matrix turnover is a novel mechanism regulating extracellular matrix remodeling.

Enhanced tenascin-C expression and matrix deposition during Ras/TGF-beta-induced progression of mammary tumor cells

Overexpression of tenascin-C (TN-C) in breast carcinomas has been associated with a migratory or even invasive tumor cell phenotype. The mechanisms regulating expression and matrix deposition of TN-C in normal and cancerous breast tissues are, however, little understood. Here, we demonstrate that mouse mammary epithelial cells (EpH4) transformed by oncogenic Ha-Ras (EpRas) overexpress TN-C, which accumulates in the cytoplasm. When EpRas cells undergo epithelial-mesenchymal transition (EMT) in response to TGFbeta1, they secrete TN-C into the culture medium. In EpRas cells undergoing TGFbeta1-induced EMT in three-dimensional (3D)-collagen gel cultures, TN-C was deposited into an extracellular matrix (ECM) already containing fibronectin and perlecan. Under less physiological 2D plastic cultures, EpRas cells undergoing EMT failed to deposit TN-C into an (apparently incomplete) ECM. Ras-downstream signaling was dissected by pharmacological inhibitors and effector-specific Ras mutants (V12S35, V12C40), specifically inhibiting or activating ERK/MAPK or PI3K signaling, respectively. We showed that TN-C overexpression required a hyperactive ERK/MAPK-signaling pathway, while elevated PI3K signaling did not enhance TN-C expression. Similarly, tumors induced by cells exhibiting hyperactive ERK/MAPK signaling showed expression of TN-C in the tumor cells themselves, while only endothelial cells expressed TN-C in tumors caused by the V12C40 mutant (incapable of EMT in vivo). Taken together, our data indicate that hyperactive ERK/MAPK signaling causes enhanced expression of TN-C, while its secretion is induced by TGFbeta1 and both signals cooperate in TN-C matrix deposition. Importantly, both signals also cooperate to induce EMT in vitro and tumor progression/metastasis in vivo.

Localization of a cryptic binding site for tenascin on fibronectin

Fibronectin and tenascin are large extracellular matrix proteins that interact with each other and with integrin receptors to regulate cell growth and movement. They are both modular proteins composed of independently folded domains (modules) that are arranged in linear fashion. Fibronectin is a covalent dimer and tenascin is a hexamer. The site on tenascin to which fibronectin binds has been localized to type III modules 3-5. In this study we use surface plasmon resonance to examine the interaction between various fragments of fibronectin and tenascin to further characterize and localize the binding sites. We found that tenascin fragments that contain type III modules 3-5 bind primarily to the N-terminal 29-kDa hep-1/fib-1 domain, which contains the first five type I modules of fibronectin. The dissociation constant, K(d), is approximately 1 microm. The binding site on fibronectin appears to be cryptic in the whole molecule in solution but is exposed on the proteolytic fragments and probably when fibronectin is in the extended conformation.

Solid phase assays in glycoconjugate research: applications to the analysis of proteoglycans, glycosaminoglycans and metalloproteinases

Glycoconjugates are a class of macromolecules consisting of different constituents, one of which is sugar moieties. Glycoconjugates comprise the majority of tissue constituents, both intracellular and extracellular. Extracellular glycoconjugates (glycoproteins and proteoglycans) participate in a wide variety of interactions, through which they maintain tissue integrity. Therefore, their analysis or the study of their possible interactions would give evidence for the state of tissues. Since the amounts of some of the extracellular glycoconjugates are usually low or the amounts of tissue to be examined come from biopsies, specific analytical systems are developed for their study, the most familiar being solid phase assays, which have the advantages of analysis of multiple samples on the same time, cheap instrumentation and high specificity.

Protease nexin-1 inhibits plasminogen activation-induced apoptosis of adherent cells

Degradation of adhesive glycoproteins by plasmin is implicated in cell migration. In this study, we further explored the role of plasminogen activation in cell adhesion and survival and show that uncontrolled plasminogen activation at the cell surface may induce cell detachment and apoptosis. We hypothesized that this process could be prevented in adherent cells by expression of protease nexin-1, a potent serpin able to inhibit thrombin, plasmin, and plasminogen activators. Using two- and three-dimensional culture systems, we demonstrate that Chinese hamster ovary fibroblasts constitutively express tissue-type plasminogen activator and efficiently activate exogenously added plasminogen in a specific and saturable manner (K(m) = 46 nm). The formation of plasmin results in proteolysis of fibronectin and laminin, which is followed by cell detachment and apoptosis. Protease nexin-1 expressed by transfected cells significantly inhibited the activity of plasmin and tissue-type plasminogen activator via the formation of inhibitory complexes and prevented cell detachment and apoptosis. In conclusion, protease nexin-1 may be an important anti-apoptotic factor for adherent cells. This cell model could be a useful tool to evaluate therapeutic agents such as serpins in vascular pathologies involving pericellular protease-protease inhibitor imbalance.

The ins and outs of fibronectin matrix assembly

Cell phenotype is specified by environmental cues embedded in the architecture and composition of the extracellular matrix (ECM). Much has been learned about matrix organization and assembly through analyses of the ECM protein fibronectin (FN). FN matrix assembly is a cell-mediated process in which soluble dimeric FN is converted into a fibrillar network. Binding of cell surface integrin receptors to FN converts it to an active form, which promotes fibril formation through interactions with other cell-associated FN dimers. As FN fibrils form on the outside of the cell, cytoplasmic domains of integrin receptors organize cytoplasmic proteins into functional complexes inside. Intracellular connections to the actin cytoskeletal network and stimulation of certain key intracellular signaling pathways are essential for FN-integrin interactions and propagation of FN fibril formation. Thus, assembly of native functional ECM depends on exquisite coordination between extracellular events and intracellular pathways.

Involvement of large tenascin-C splice variants in breast cancer progression

Alternative splicing of fibronectin-like type III (FNIII) repeats of tenascin-C (Tn-C) generates a number of splice variants. The distribution of large variants, typical components of provisional extracellular matrices that are up-regulated during tumor stroma remodeling, was here studied by immunoblotting and immunohistochemistry using a monoclonal antibody against the FNIII B domain (named 4C8MS) in a series of human breast cancers. Large Tn-C variants were found at only low levels in normal breast tissues, but were highly expressed at invading sites of intraductal cancers and in the stroma of invasive ductal cancers, especially at invasion fronts. There was a positive correlation between the expression of large Tn-C variants and the cell proliferation rate determined by immunolabeling of the Ki-67 antigen. Of the Tn-C recombinant fragments (all FNIII repeats or mFNIII FL, the conserved FNIII domain only, the epidermal growth factor-like domain, and the fibrinogen-like domain) which were expressed by CHO-K1 cells transfected with mouse Tn-C cDNAs, only the mFNIII FL enhanced in vitro migration and mitotic activity of mammary cancer cells derived from a Tn-C-null mouse. Addition of 4C8MS blocked the function of mFNIII FL. These findings provide strong evidence that the FNIII alternatively spliced region has important roles in tumor progression of breast cancer.

Markers for the development of early prostate cancer

Biochemical and genetic changes precede histologically identifiable changes accompanying cell transformation often by months or years. De-expression of the extracellular matrix adhesive glycoprotein tenascin and the cell-to-cell adherent protein E-cadherin have been suggested as markers of early neoplastic change in prostate epithelial cells. Previous studies have been inconclusive, probably due to epitope masking. This study examined 2,378 biopsy cores from 289 prostates using a heat antigen retrieval protocol at low pH to improve the accuracy of detection. Tenascin and E-cadherin de-expression was correlated with purinergic receptor and telomerase-associated protein labelling, as well as prostate-specific antigen (PSA) levels and Gleason scores. E-cadherin was a poor marker, as it was expressed in all lesions except carcinomas of the highest Gleason score. Tenascin was maximally expressed in the extracellular matrix and acinar basement membrane in normal and prostatic intraepithelial neoplasia tissue. In prostate cancer tissue, tenascin expression did not correlate with Gleason score but was significantly de-expressed as purinergic receptor and telomerase-associated protein expression increased. Marked changes in tenascin, telomerase-associated protein, and purinergic receptor expression were apparent before any histological abnormalities were visible by haematoxylin and eosin (H&E) stain, making these potential markers for early and developing prostate cancer. Moreover, the potential increased accuracy of diagnosis of underlying prostate cancer using purinergic receptor translocation (PRT) assessment suggests that PSA levels may be more accurate than has generally been supposed when apparent false negatives arising from H&E-based diagnoses are correctly categorized.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.

The incorporation of fibrinogen into extracellular matrix is dependent on active assembly of a fibronectin matrix

Fibrinogen is a soluble protein produced by hepatocytes and secreted into plasma, where it functions in hemostasis. During inflammation, the hepatic synthesis of fibrinogen is induced 2-10 fold. Recent studies demonstrate that after an inflammatory stimulus, fibrinogen gene expression and protein production is upregulated in lung epithelial cells, where it is secreted basolaterally and consequently deposited into the extracellular matrix in fibrils that extensively colocalize with fibronectin fibrils. In this study, we show that the deposition of fibrinogen into the matrix of fibroblasts occurred rapidly and in a Rho-dependent manner in response to serum or lysophosphatidic acid; RhoA GTPase signaling is also required for fibronectin matrix assembly. Using mouse embryonic fibronectin-null cells, we show that incorporation of exogenous fibrinogen into matrix fibrils occurred only in the presence of exogenous fibronectin, which is also assembled into matrix fibrils. Furthermore, treatment of fibroblasts and fibronectin-null cells with an antibody that inhibits fibronectin matrix assembly impaired incorporation of fibrinogen into matrix fibrils. Collectively, these data suggest that incorporation of fibrinogen into the extracellular matrix requires active fibronectin polymer elongation into matrix fibrils. From these data, we hypothesize that fibrinogen deposition rapidly changes the topology of the extracellular matrix to provide a surface for cell migration and matrix remodeling during tissue repair.

Identification and characterization of a conformational heparin-binding site involving two fibronectin type III modules of bovine tenascin-X

Tenascin-X is known as a heparin-binding molecule, but the localization of the heparin-binding site has not been investigated until now. We show here that, unlike tenascin-C, the recombinant fibrinogen-like domain of tenascin-X is not involved in heparin binding. On the other hand, the two contiguous fibronectin type III repeats b10 and b11 have a predicted positive charge at physiological pH, hence a set of recombinant proteins comprising these domains was tested for interaction with heparin. Using solid phase assays and affinity chromatography, we found that interaction with heparin was conformational and involved both domains 10 and 11. Construction of a three-dimensional model of domains 10 and 11 led us to predict exposed residues that were then submitted to site-directed mutagenesis. In this way, we identified the basic residues within each domain that are crucial for this interaction. Blocking experiments using antibodies against domain 10 were performed to test the efficiency of this site within intact tenascin-X. Binding was significantly reduced, arguing for the activity of a heparin-binding site involving domains 10 and 11 in the whole molecule. Finally, the biological significance of this site was tested by cell adhesion studies. Heparan sulfate cell surface receptors are able to interact with proteins bearing domains 10 and 11, suggesting that tenascin-X may activate different signals to regulate cell behavior.

Binding of tenascin-X to decorin

Tenascin-X (TN-X) is an extracellular matrix protein whose absence results in an alteration of the mechanical properties of connective tissue. To understand the mechanisms of integration of TN-X in the extracellular matrix, overlay blot assays were performed on skin extracts. A 100 kDa molecule interacting with TN-X was identified by this method and this interaction was abolished when the extract was digested by chondroitinase. By solid-phase assays, we showed that dermatan sulfate chains of decorin bind to the heparin-binding site included within the fibronectin-type III domains 10 and 11 of TN-X. We thus postulate that the association of TN-X with collagen fibrils is mediated by decorin and contributes to the integrity of the extracellular network.

Tenascin-C in development and disease: gene regulation and cell function

Tenascin-C (TN-C) is a modular and multifunctional extracellular matrix (ECM) glycoprotein that is exquisitely regulated during embryonic development and in adult tissue remodeling. TN-C gene transcription is controlled by intracellular signals that are generated by multiple soluble factors, integrins and mechanical forces. These external cues are interpreted by particular DNA control elements that interact with different classes of transcription factors to activate or repress TN-C expression in a cell type- and differentiation-dependent fashion. Among the transcriptional regulators of the TN-C gene that have been identified, the homeobox family of proteins has emerged as a major player. Downstream from TN-C, intracellular signals that are relayed via specific cell surface receptors often impart contrary cellular functions, even within the same cell type. A key to understanding this behavior may lie in the dual ability of TN-C-enriched extracellular matrices to generate intracellular signals, and to define unique cellular morphologies that modulate these signal transduction pathways. Thus, despite the contention that TN-C null mice appear to develop and act normally, TN-C biology continues to provide a wealth of information regarding the complex nature of the ECM in development and disease.

Modulation of extracellular matrix adhesiveness by neurocan and identification of its molecular basis

Neurocan is one of the major chondroitin sulfate proteoglycans of perinatal rodent brain. HEK-293 cells producing neurocan recombinantly show changes in their behavior. The expression of full-length neurocan led to a detachment of the secreting cells and the formation of floating spheroids. This occurred in the continuous presence of 10% fetal bovine serum in the culture medium. Cells secreting fragments of neurocan-containing chondroitin sulfate chains and the C-terminal domain of the molecule showed a similar behavior, whereas cells expressing fragments of neurocan-containing chondroitin sulfate chains but lacking parts of the C-terminal domain did not show spheroid formation. Cells secreting the hyaluronan-binding N-terminal domain of neurocan showed an enhanced adhesiveness. When untransfected HEK-293 cells were plated on a surface conditioned by spheroid-forming cells, they also formed spheroids. This effect could be abolished by chondroitinase treatment of the conditioned surface. The observations indicate that the ability of the chondroitin sulfate proteoglycan neurocan to modulate the adhesive character of extracellular matrices is dependent on the structural integrity of the C-terminal domain of the core protein.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Chondroitin sulfates affect the formation of the segmental motor nerves in zebrafish embryos

Chondroitin sulfates have been implicated in the promotion and in the inhibition of axon growth. In the zebrafish embryo, chondroitin sulfates are present at the interface of the somites and the notochord where spinal motor axons extend ventrally to establish the midsegmental ventral motor nerves. Injection of chondroitinase ABC prior to motor axon outgrowth effectively removed all chondroitin sulfate immunoreactivity and induced abnormal axonal outgrowth in many (39%) of the ventral motor nerves. The most common abnormality was the formation of side branches, approximately half of which extended posteriorly, the others anteriorly. The effect was specific to the removal of chondroitin sulfates, since injections of vehicle solution or of heparinase III did not affect the ventral motor nerves. Electron microscopic examination demonstrated that the injections caused no damage to spinal cord, somite, and notochord. This suggests that chondroitin sulfates normally constrain the outgrowth of the ventral motor nerves. Consistent with this hypothesis, injections of soluble chondroitin sulfates, either as a mixture or individually, led to truncated or missing ventral motor nerves. Truncations were most frequent after injection of chondroitin sulfate-B (up to 23%) while chondroitin sulfate-A had a lesser, and chondroitin sulfate-C no apparent, effect.

Involvement of chondroitin sulfates on brain-derived tenascin-R in carbohydrate-dependent interactions with fibronectin and tenascin-C

Tenascin-R (TN-R), a matrix glycoprotein of the central nervous system (CNS), has been implicated in a variety of cell-matrix interactions involved in the control of axon growth, myelination and cell adhesion to fibronectin during development and regeneration. While most of the functional analyses have concentrated exclusively on the role of the core protein, the contribution of TN-R glycoconjugates present on many potential sites for N- and O-glycosylation is presently unknown. Here we provide evidence that TN-R derived from adult mouse brain expresses chondroitin sulfate (CS) glycosaminoglycans (GAGs), i.e. C-6S and C-4S, that are recognized by the CS/dermatan sulfate-specific monoclonal antibodies 473 HD and CS-56. Using ligand-binding, cell adhesion and neurite outgrowth assays, we show that TN-R-linked CS GAGs (i) are involved in the interaction with the heparin-binding sites of fibronectin and are responsible for TN-R-mediated inhibition of cell adhesion to a 33/66-kD heparin-binding fibronectin fragment or to FN-C/H I and FN-C/H II peptides, known to participate in fibronectin binding to cell surface proteoglycans; and (ii) partially contribute to the interaction between TN-R and TN-C which, however, does not lead to an interference with TN-R- and TN-C-mediated inhibition of neurite outgrowth when the two molecules are offered as a mixed substrate in culture. Our findings suggest the functional implication of TN-R-linked CS GAGs in matrix interactions with fibronectin and TN-C that are likely to contribute to a modulation of cellular behavior and the macromolecular organization of matrix components in the developing or injured adult CNS.

Control of extracellular matrix assembly by syndecan-2 proteoglycan

Extracellular matrix (ECM) deposition and organization is maintained by transmembrane signaling and integrins play major roles. We now show that a second transmembrane component, syndecan-2 heparan sulfate proteoglycan, is pivotal in matrix assembly. Chinese Hamster Ovary (CHO) cells were stably transfected with full length (S2) or truncated syndecan-2 lacking the C-terminal 14 amino acids of the cytoplasmic domain (S2deltaS). No differences in the amount of matrix assembly were noted with S2 cells, but those expressing S2deltaS could not assemble laminin or fibronectin into a fibrillar matrix. The loss of matrix formation was not caused by a failure to synthesize or externalize ECM components as determined by metabolic labeling or due to differences in surface expression of alpha5 or beta1 integrin. The matrix assembly defect was at the cell surface, since S2deltaS cells also lost the ability to rearrange laminin or fibronectin substrates into fibrils and to bind exogenous fibronectin. Transfection of activated alphaIIbalphaLdeltabeta3 integrin into alpha(5)-deficient CHO B2 cells resulted in reestablishment of the previously lost fibronectin matrix. However, cotransfection of this cell line with S2deltaS could override the presence of activated integrins. These results suggest a regulatory role for syndecan-2 in matrix assembly, along with previously suggested roles for activated integrins.

Chondroitin sulphate and heparan sulfate proteoglycan are sequentially expressed in the uterine extracellular matrix during early pregnancy in the rat

Chondroitin sulfate proteoglycan (CSPG) and heparan sulfate proteoglycan (HSPG) are extracellular matrix proteins that regulate cell adhesion, growth, migration, differentiation and gene expression in many systems. In this study, stromal CSPG label was intense within 10 microm of the uterine lumen. From that distance to the myometrium, CSPG was de-expressed. From the time of implantation on Day 6, this pattern was reversed. CSPG was de-expressed from the uterine epithelium to a distance of approximately 10 microm from the uterine lumen. From that region to the myometrium, labeling was homogeneously intense. This finding suggests that CSPG may inhibit attachment and implantation. Heparan sulfate core proteoglycan (perlecan) was increasingly expressed in the uterine epithelium from the time of implantation, commencing in the basement membrane on Day 6 and extending to the apical epithelium and lateral plasma membranes by Day 7. Perlecan thus appears to facilitate trophoblast attachment and implantation. We propose that attachment and implantation is regulated, at least in part, by the selective and sequential expression of CSPG and perlecan.

Dynamics and elasticity of the fibronectin matrix in living cell culture visualized by fibronectin-green fluorescent protein

Fibronectin (FN) forms the primitive fibrillar matrix in both embryos and healing wounds. To study the matrix in living cell cultures, we have constructed a cell line that secretes FN molecules chimeric with green fluorescent protein. These FN-green fluorescent protein molecules were assembled into a typical matrix that was easily visualized by fluorescence over periods of several hours. FN fibrils remained mostly straight, and they were seen to extend and contract to accommodate movements of the cells, indicating that they are elastic. When fibrils were broken or detached from cells, they contracted to less than one-fourth of their extended length, demonstrating that they are highly stretched in the living culture. Previous work from other laboratories has suggested that cryptic sites for FN assembly may be exposed by tension on FN. Our results show directly that FN matrix fibrils are not only under tension but are also highly stretched. This stretched state of FN is an obvious candidate for exposing the cryptic assembly sites.