Presence of fibronectin in basement membranes and acidic structural glycoproteins from human placenta and lung

Dilated capillaries, disorganized collagen fibers and differential gene expression in periodontal ligaments of hypomorphic fibrillin-1 mice

The periodontal ligaments (PDLs) are soft connective tissue between the cementum covering the tooth root surface and alveolar bone. PDLs are composed of collagen and elastic system fibers, blood vessels, nerves, and various types of cells. Elastic system fibers are generally formed by elastin and microfibrils, but PDLs are mainly composed of the latter. Compared with the well-known function of collagen fibers to support teeth, little is known about the role of elastic system fibers in PDLs. To clarify their role, we examined PDLs of mice under-expressing fibrillin-1 (mgR mice), which is one of the major microfibrillar proteins. The PDLs of homozygous mgR mice showed one-quarter of the elastic system fibers of wild-type (WT) mice. A close association between the elastic system fibers and the capillaries was noted in WT, homozygous and heterozygous mgR mice. Interestingly, capillaries in PDLs of homozygous mice were dilated or enlarged compared with those of WT mice. A comparable level of type I collagen, which is the major collagen in PDLs, was expressed in PDL-cells of mice with three genotypes. However, multi-oriented collagen fiber bundles with a thinner appearance were noted in homozygous mice, whereas well-organized collagen fiber bundles were seen in WT mice. Moreover, there was a marked decrease in periostin expression, which is known to regulate the fibrillogenesis and crosslinking of collagen. These observations suggest that the microfibrillar protein, fibrillin-1, is indispensable for normal tissue architecture and gene expression of PDLs.

Inhibition of cryogelation by the novel synthetic peptide (Gly-Arg-Lys-Lys-Thr): recognition site of extra domain A containing fibronectin for heparin in cryogelation

Cryogel is a physical gel formed by the heterophilic aggregation of extra domain A (EDA) containing fibronectin [EDA(+)FN], plasma fibronectin (pFN), fibrinogen (Fbg) and heparin (Hep) in the blood of rheumatoid arthritis (RA) patients. In cryogelation EDA(+)FN cross-links to form an interaggregate of cryogel with Hep. In the present study, we determined the recognition structure of Hep for EDA(+)FN by using oligo- and desulfonated-Hep. The affinity constant (KA) (1.2 x 10(8) per M) of oligo-Hep for EDA(+)FN did not change with a decrease in number-average molecular weight (4.9 x 10(4)-->6.0 x 10(3)). The KA-value of desulfonated-Hep for EDA(+)FN decreased from 3.2 x 10(8) to 1.0 x 10(7) per M with a decrease in the sulfonation ratio (7.0-->4.3%). We also determined the recognition structure of EDA(+)FN for Hep by an inhibition experiment on the heparin binding domain II (HepII) in EDA(+)FN with the synthetic peptides, Arg-Arg-Ala-Arg (RRAR), Asp-Gln-Ala-Arg (DNAR), Ile-Lys-Tyr-Glu-Lys (IKYEK), and Gly-Arg-Lys-Lys-Try (GRKKT). The GRKKT sequence clearly inhibited bonding between EDA(+)FN and Heps containing oligo- and desulfonated-Hep. The amount of cryogel formed in the RA-patient model plasma corresponded to the EDA(+)FN concentration in cryogel (36.7%) normalized by the EDA(+)FN concentration in plasma. When GRKKT was added to plasma, the EDA(+)FN concentration fell to 10.5%. These results demonstrated that inhibition of cryogelation in plasma could progress to a novel treatment for RA.

Specific interactions between cryogel components: role of extra domain A containing fibronectin in cryogelation

Cryogel is a physical gel formed by heterophilic aggregation of extra domain A containing fibronectin [EDA(+)FN], plasma fibronectin (pFN), fibrinogen (Fbg) and heparin (Hep), which are found in high concentrations in the blood of patients suffering from rheumatoid arthritis. In this study, we clarify the specific interactions between cryogel components in terms of the affinity constant (K(A)), obtained by surface plasmon resonance (SPR). It is found that Fbg self-interactions occur at lower temperatures, and that K(A) of Fbg-Hep changes with temperature. Specifically, K(A) (2.0 x 10(8) [M(-1)]) of Fbg-Hep at 5 degrees C increases significantly from that (1.0x10(7) [M(-1)]) at 40 degrees C. K(A) of EDA(+)FN-Hep increases with temperature, by approximately 100-fold between 40 degrees C (K(A)=10(12) [M(-1)]) and 20 degrees C (K(A)=10(10) [M(-1)]). Although K(A) of the FN fragments of Hep-binding domain containing an EDA region [EDA(+)HBD(+)] and Hep increases with temperatures above 30 degrees C, K(A)s of HBD(+)-Hep and EDA(+)-Hep are not temperature-dependent. Therefore, EDA(+)HBD(+), formed as a special structure for high Hep affinity, exhibits temperature-dependent interaction with Hep. These results suggest that the main role of EDA(+)FN in cryogelation is to support the interaction with Hep.

The fibrinoids of the human placenta: origin, composition and functional relevance

Placental fibrinoids are extracellularly deposited materials which are histologically glossy and acid staining, and can be found in every normal and pathological placenta at all stages of pregnancy. The amount of fibrinoid is, in general, independent of pregnancy outcome and fetal wellbeing. According to new findings, the classical histological term "fibrinoid" covers two distinctive extracellular matrices which differ as regards structure, composition and function. Fibrin-type fibrinoid is mostly composed of fibrin together with other molecules derived from blood clotting or degenerative processes. It is mainly a maternal blood-clot product which is used (a) to adapt the intervillous space to optimized flow conditions and (b) to control growth of the villous trees by encasing new villous branches which caused intervillous stasis or turbulence of maternal blood. Moreover, fibrin-type fibrinoid replaces degenerative syncytiotrophoblast at the maternofetal exchange surfaces, thus acting as a kind of substitute barrier. Matrix-type fibrinoid is a secretory product of invasive extravillous trophoblast cells. It shares some similarities with basement membranes, however, it is secreted in an apolar fashion, embedding the secreting cells. Like basement membranes, it contains laminins, collagen IV, and heparan sulfate. In addition, oncofetal fibronectins, vitronectin, and i-glycosylated molecules but no collagens I, III, and VII can be found. Matrix-type fibrinoid is thought to regulate trophoblast invasion by specific interactions with cell surface integrins. As a kind of "glue", it anchors the placenta to the uterine wall and seems to play an important role in materno-fetal immune interactions at this particular site. Both types of fibrinoid are usually co-localized, thus indicating close morphogenetic and functional interrelations.

Novel hyperglycosylated weak gelatin-binding fibronectin from human fetal placenta. Fractionation of a high poly(N-acetyllactosamine) fragment by tomato lectin affinity chromatography

A novel hyperglycosylated fraction of human term fetal placental fibronectin was detected by long-term affinity binding to gelatin-Sepharose. An 18-h batch-wise gelatin-binding step was necessary to obtain a very low-affinity binding fraction, characterized by especially high N-acetylglucosamine and galactose content, and diffuse, poorly stained Coomassie bands on SDS/polyacrylamide electrophoretograms. The presence of a high proportion of long 7-10-kDa poly(N-acetyllactosamine)-containing N-linked carbohydrate chains was confirmed by their gel permeation behavior, susceptibility to endo-beta-galactosidase and by methylation analysis. Our previous results suggest that 4.5-7-kDa poly(N-acetyllactosamine) structures reduce the binding of fibronectin and its chymotryptic Ala260-Trp599 subdomain GB44 to gelatin [Zhu, B. C. R. & Laine, R. A. (1985) J. Biol. Chem. 260, 4041-4045]. Based on a gradient of urea used to dissociate gelatin-bound GB44, in the present study, fractions containing the novel 7-10-kDa carbohydrates showed significantly weaker binding to gelatin. Weak gelatin-binding characteristics of this novel hyperglycosylated fraction suggest that extended poly(N-acetyllactosamine) N-linked chains can significantly weaken heterotropic binding functions of fetal glycoproteins. The combined properties of weak Coomassie staining and weak gelatin binding have caused the novel hyperglycosylated fibronectin to be overlooked in previous investigations.

The fibronectin content of canine lungs is increased in bleomycin-induced fibrosis

Fibronectin (Fn), a high molecular weight glycoprotein, was found to constitute 0.43% of the normal adult beagle dog lung. The tissue Fn (TFn) was solubilized by sequential chemical extractions and quantified by ELISA (enzyme-linked immunoabsorbent assay). Subsequent plasmin digestion did not appear to solubilize significantly more Fn. Since 70% of the lung tissue was solubilized by the extractions and plasmin digestions, the TFn quantified represented the bulk of lung Fn. The TFn was identical to plasma fibronectin in the ELISA and one can infer that the Fn molecule is not significantly altered as it is incorporated into the lung connective tissue matrix. Lungs from beagles in which fibrosis had been induced with bleomycin contained 0.99% Fn, more than a twofold increase over normal. In the ELISA TFn from fibrotic lungs gave an inhibition curve of the same shape as did TFn from normal lungs. Thus, Fn from fibrotic lungs is not different qualitatively from Fn from normal lungs in any way detectable with this antiserum. The TFn content of plasmin digests of intact lung was less than that of extracts, which was the converse of results obtained on placenta (B. A. Bray (1985) Biochem. J. 226, 811-815). This difference between lung TFn and placental TFn may be due to differences in degree of glycosylation, which determines susceptibility to proteases.

Changes in the distribution of fibronectin in the placenta during normal human pregnancy

The distribution of fibronectin, a major extracellular glycoprotein with various activities that affect the proliferation and differentiation of cells, was studied by immunofluorescence staining in first-, second- and third-trimester placentas from normal human pregnancy. In early chorionic villi, fibronectin was localized mainly in the trophoblastic basement membranes; this fluorescence became weaker after 10 weeks' gestation. In term placenta, fibronectin was densely deposited around the fetal vessels but not in the trophoblastic basement membranes. Both syncytiotrophoblasts and cytotrophoblasts of the villous epithelium were virtually negative throughout pregnancy. However, the pericellular matrices of nonvillous trophoblasts in early chorionic villi were strongly stained. These findings suggest that fibronectin plays an important role in the proliferation of trophoblastic cells and the tissue organization of the placenta.

Quantification of tissue fibronectin from terminal villi of placenta

Tissue fibronectin (TFn) was solubilized from the terminal villi of perfused human placentas by sequential chemical extractions and plasmin digestion. Alternatively, plasmin digestion of intact tissue solubilized all the TFn, which amounted to 1.8-2.9% of the dry weight of the villi. Concomitantly, 69% of the tissue was solubilized. The non-equilibrium competitive e.l.i.s.a. (enzyme-linked immunoabsorbent assay), in which the TFn was immunologically identical with plasma fibronectin (PFn), was used for the quantification of TFn. This study demonstrates that the bulk of TFn can be obtained in a form that can be quantified by e.l.i.s.a. and that TFn is immunologically identical with PFn. Thus the fibronectin molecule is not significantly altered as it is incorporated into the connective-tissue matrix and could exchange with PFn.

Immunochemical analysis of fibronectin using monoclonal antibodies

Cloned hybrid cell lines secreting antibodies directed against human plasma fibronectin were prepared according to the methods of Kohler and Milstein (Kohler, G. and Milstein, C. (1975) Nature (London) 256, 495-497 and (1976) Eur. J. Immunol. 6, 511-519). The specificity of each monoclonal antibody for fibronectin was established from autoradiograms of radioimmunoprecipitates following SDS-polyacrylamide gel electrophoresis. The monoclonal antibodies were reactive with both native and SDS-denatured fibronectin. Ascites fluids obtained from infected isogenic mice precipitated 85-95% of the 125I-labelled fibronectin radioactivity in indirect radioimmunoprecipitation tests. Localization of specific epitopes to restricted regions of the fibronectin molecule was carried out by monitoring monoclonal antibody binding to proteolytic fragments. Of the five monoclonal antibodies analyzed in this study, three recognized determinants which resided in the terminal 35 kDa region of the fibronectin monomer. Furthermore, these epitopes were localized to fragments as small as 20 kDa. Competition studies carried out using plasma fibronectins isolated from different species revealed that three monoclonal antibodies recognized sites which were relatively conserved, while two monoclonal antibodies recognized epitopic sequences which were unique to the human protein. The corresponding anti-fibronectin serum also demonstrated discriminatory capabilities. Immunofluorescent analysis of human fibroblasts grown in vitro demonstrated that all the monoclonal antibodies tested were reactive with pericellular fibronectin.

Current concepts of basement-membrane structure and function

Conclusion

In this brief review we have attempted to describe the known components of basement membranes in relation to the morphology and function of these matrices. Further details of the molecular structures and biosynthesis of these components may be found in original papers and in various reviews (Kefalides, 1973; Spiro, 1976; Kefalides et al., 1979; Heathcote & Grant, 1981).

Although basement membranes appear to contain essentially similar protein and carbohydrate moieties, the proportions and organization of these may differ and, in the opinion of the authors, the key to an understanding of basement membranes lies in the recognition of this heterogeneity. At present, structural models of basement membrane are far from satisfactory and should be regarded with reservation.

Distribution of the LETS protein (fibronectin) in rat cerebellum. An in vitro and in vivo developmental study

The distribution of the large, external, transformation-sensitive (LETS; fibronectin) protein was investigated in rat cerebellum, both in vitro and in vivo, by biochemical and immunocytochemical methods. Biochemical analyses indicated that LETS protein is not demonstrable on the surfaces of cerebellar neurons from postnatal rats maintained in cell culture for varying periods of time, but is present on the surfaces of at least some fraction of the total nonneuronal cell population in vitro. Indirect immunofluorescence microscopy with an anti-LETS antiserum substantiated these observations and further indicated that LETS-bearing cells of cerebellum maintained in vitro are probably of endothelial and fibroblastic origin. The LETS protein is arranged in a reticular network of filaments spanning the surfaces of the cells, and the filaments are often extensively interdigitated with each other. At all stages of development investigated (two days postnatal to adult) LETS antigen was observed in vivo to be primarily localized in the meninges covering the surface of the cerebellum and between folia, and in the walls of blood vessels within the tissue. Neuroblasts and neurons of the external and internal granule layers of the cerebellum, respectively, were negative for the presence of LETS antigen.

Indirect immunofluorescent staining of fibronectin associated with the floor of the foregut during formation and rupture of the oral membrane in the chick embryo

The distribution of the glycoprotein, fibronectin, within the cranial region of stage 8--16 chick embryos was examined by indirect immunofluorescence using paraffin sections exposed to affinity-purified rabbit anti-human CIG and FITC-conjugated goat anti-rabbit immunoglobulins. Fluorescence was present within the matrix surrounding the cranial mesenchyme, along the basal surfaces of all epithelia, and surrounding the notochord at all stages. Fluorescence associated with the floor of the foregut was particularly intense. The fluorescent layers beneath the ectoderm and endoderm of the oral (oropharyngeal) membrane at stage 8 merged into a single, continuous, intensely fluorescent line as the extracellular space within the oral membrane narrowed during stages 9--12. This line of uniform fluorescence parallels the previously described histological reorganization of the extracellular compartment of the oral membrane, but the ultrastructural localization of this fluorescent material remains unknown. Fluorescence was also intense beneath the foregut endoderm in the presumptive cardiac region caudal to the oral membrane and was continuous with strands of fluorescent material extending into the matrix of the dorsal mesocardium and cardiac jelly of the developing tubular heart. These observations indicate that the extracellular matrix associated with the floor of the entire foregut contains fibronectin during stages encompassing the formation and rupture of the oral membrane. The presence of fibronectin within the oral membrane and dorsal mesocardium, as well as between Rathke's pouch and infundibulum and within the closing plates between ectodermal clefts and endodermal pouches, is consistent with the possibility that this glycoprotein may play a role in adhesion at these sites.

Fibronectin-collagen binding and requirement during cellular adhesion

Fibronectin isolated from human plasma and from the extracellular matrices of cell monolayers mediates the attachment in vitro and spreading of trypsin-treated cells on a collagen substratum. Fibronectin-dependent kinetics of cellular attachment to collagen were studied for several adherent cell types. It was shown that trypsin-treated human umbilical-cord cells, mouse sarcoma CMT81 cells, endothelial cells, and human fibroblasts from a patient with Glanzmann's disease were completely dependent on fibronectin for their attachment to collagen, whereas guinea-pig and monkey smooth-muscle cells and chick-embryo secondary fibroblasts displayed varying degrees of dependence on fibronectin for their attachment. Radiolabelled human plasma fibronectin possessed similar affinity for collagen types I, II and III from a variety of sources. The fibronectin bound equally well to the collagens with or without prior urea treatment. However, in the fibronectin-mediated adhesion assay using PyBHK fibroblasts, a greater number of cells adhered and more spreading was observed on urea-treated collagen. Fibronectin extracted from the extracellular matrix of chick-embryo fibroblasts and that purified from human plasma demonstrated very similar kinetics of complexing to collagencoated tissue-culture dishes. Fibronectin from both sources bound to collagen in the presence of 0.05-4.0m-NaCl and over the pH range 2.6-10.6. The binding was inhibited when fibronectin was incubated with 40-80% ethylene glycol, the ionic detergents sodium dodecyl sulphate and deoxycholate, and the non-ionic detergents Nonidet P-40, Tween 80 and Triton X-100, all at a concentration of 0.1%. From these results we proposed that fibronectin-collagen complexing is mainly attributable to hydrophobic interactions.

Fibronectin associated with the glial component of embryonic brain cell cultures

In the basic approach to investigations of neuronal--glial interactions during both normal brain development and its pathogenesis, embryonic brain cell populations were fractionated into purified neuronal and glial components. Using separation procedures based on differential adhesion and cytotoxicity, the isolated neuronal and glial phenotypes could be identified by distinct morphological and biochemical characteristics, including the visualization of glial fibrillary acid protein (GFA) within glial cells in immunohistochemical assays with monospecific anti-GFA serum. When unfractionated cerebrum cells dissociated from 10-day chick or 14-day mouse embryos were plated as monolayers and cultured for 1--14 days, monospecific antiserum against fibronectin (LETS glycoprotein) was found to react with many, but not all, of the cells as revealed by indirect immunofluorescence microscopy. The isolated neuronal and glial components of these populations were used to determine whether the appearance of membrane-associated fibronectin was characteristic of one cell type or the other, or both, and if neuronal--glial cell interaction was required for its expression. It was found that the surfaces of glial cells, completely isolated from neurons, showed an intense fluorescent reaction to the anti-fibronectin serum. In contrast, the purified neuronal cultures showed no fluorescence with either the anti-GFA or anti-fibronectin sera. These results demonstrate fibronectin as a cell surface protein associated primarily with glial cells and independent of neuronal--glial cell interaction for its expression. Furthermore, the results indicate that the fibronectin observed on glial cell surfaces in these cultures is produced endogenously and is not due to the preferential binding of fibronectin present in the culture medium. The role of fibronectin as an adhesive molecule in neuronal--glial interactions is discussed.