Biosynthesis of basement membrane collagen by rabbit corneal endothelium in vitro

Basement membranes in the cornea and other organs that commonly develop fibrosis

Basement membranes are thin connective tissue structures composed of organ-specific assemblages of collagens, laminins, proteoglycan-like perlecan, nidogens, and other components. Traditionally, basement membranes are thought of as structures which primarily function to anchor epithelial, endothelial, or parenchymal cells to underlying connective tissues. While this role is important, other functions such as the modulation of growth factors and cytokines that regulate cell proliferation, migration, differentiation, and fibrosis are equally important. An example of this is the critical role of both the epithelial basement membrane and Descemet's basement membrane in the cornea in modulating myofibroblast development and fibrosis, as well as myofibroblast apoptosis and the resolution of fibrosis. This article compares the ultrastructure and functions of key basement membranes in several organs to illustrate the variability and importance of these structures in organs that commonly develop fibrosis.

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.

Posterior polymorphous dystrophy and Alport syndrome

Seventeen Thai patients from nine families with Alport syndrome underwent complete ocular examination and specular microscopy. Fourteen (82.3%) patients had ocular changes. Eleven (64.7%) had endothelial vesicles compatible with posterior polymorphous dystrophy. Four of these also had subepithelial opacities, a previously undescribed phenomenon. Other ocular changes included lenticonus and macular and midperipheral retinal flecks. A second group of 18 consecutive patients from 14 families with posterior polymorphous dystrophy detected during routine ocular examination underwent renal evaluation. Five had hematuria, four of whom had sensorineural hearing loss. Two of the four patients also had characteristic renal biopsy findings. Another had sensorineural hearing loss without hematuria, and renal biopsy showed a thin glomerular basement membrane. Posterior polymorphous dystrophy is a common but frequently overlooked finding in Alport syndrome. The frequent association of these two hereditary conditions suggests a common defect in basement membrane formation. Patients with posterior polymorphous dystrophy should be examined for renal abnormalities and hearing loss.

Myelofibrosis: a review of clinical and pathologic features and treatment

The purpose of this review is to discuss and clarify the current understanding of the pathogenesis, clinical manifestations, and treatment of MF. MF may be either a primary or secondary disorder. It is characterized by an increased deposition of bone marrow collagen, fibronectin, and laminin. Present evidence indicates that MF may be mediated by platelet or megakaryocyte growth factors, decreased prostaglandin mediated stem cell inhibition, immune complex deposition, and both fibroblast and endothelial cell proliferation. Recently acute MF has been recognized to be identical to acute megakaryocytic leukemia. Secondary MF usually responds to appropriate treatment of the underlying disease. Primary MF is usually treated by blood product support, but may be responsive to androgens, splenectomy, splenic irradiation, chemotherapy, or bone marrow ablation with marrow reconstitution.

Myelofibrosis: a concise review of clinical and pathologic features and treatment

This paper reviews and clarifies the current understanding of the clinical and pathologic features and treatment of MF. Recent investigations indicate that MF may be mediated by platelet- and megakaryocyte-derived growth factors, impaired prostaglandin-mediated stem cell growth inhibition, or excessive endothelial cell and fibroblast proliferation. Immunologic disorders have been associated with MF. MF may be either a primary or a secondary phenomenon. Secondary MF often regresses with appropriate treatment of this underlying disorder. Primary MF may require androgen therapy, splenectomy, splenic irradiation, bone curettage, chemotherapy, or bone marrow transplantation.

Morphogenesis of rabbit corneal endothelium

We studied ultrastructurally the development of rabbit corneal endothelium from the 13th day of gestation to 3 days after birth. Precursor corneal endothelial cells, stromal cells, and a vascular network migrate in close association with each other between the developing corneal and lens epithelia. During development, newly deposited extracellular fibrous matrices separate the prospective endothelium from the capillaries and corneal stroma. The extracellular matrix between the apical endothelial surface and the vascular network loses its fibrous appearance early in development. Simultaneously, randomly organized fibrils are deposited on the basal endothelial surface facing the stroma. These fibrils, gradually obscured by the deposition of a nonfibrous component, eventually become part of Descemet's membrane. Early in development, prospective endothelial cells cannot be distinguished morphologically from the overlying corneal stromal cells. Morphologic differentiation of the endothelial cell is characterized by the formation of sinuous lateral borders that interdigitate with those of adjacent cells to form a continuous single-cell layer of tissue. The basal endothelial membrane forms a pitted surface, distinguishing it from the apical cell membrane. Intercellular junctions between lateral membranes, a cilium projecting into the anterior chamber, and deposition of Descemet's membrane on the basal endothelial surface contribute to the polarization of the endothelium. Throughout most of corneal development the vascular pupillary membrane maintains a close association with the apical surface of the differentiating endothelium. We conclude that fetal corneal endothelium develops within a complex extracellular matrix environment and in proximity to the underlying vascular network. These structures play an important role in the morphogenesis of corneal endothelium.

Endothelial cell proliferation in myelofibrosis

The distribution of laminin and collagen type IV in normal human bone marrow appeared to be limited to a small number of capillary and sinusoidal basement membranes. Cellular elements including megakaryocytes showed a negative staining reaction. In contrast, myelofibrotic bone marrow showed markedly increased quantities of both laminin and collagen type IV. This was consistent with a proliferation of endothelial cells within the marrow as confirmed by factor VIII RAG staining. It is suggested that in myelofibrosis in addition to a marked fibrous reaction proliferation of vascular elements is also prominent.

A method for the selective release and, hence, assay of the carbohydrate moieties of collagen

The carbohydrate residues of collagen were selectively released in high yield by nitrosation of the hydroxylysines of the intact collagen or peptides derived from collagen. The carbohydrate residues (Glc-Gal and Gal) released were separated from the modified protein or peptide by gel chromatography and were assayed by gas-liquid chromatography of their trimethylsilyl derivatives. The results agreed closely with those obtained from methanolysis-gas chromatography or from alkaline hydrolysis followed by amino acid analysis of the hydroxylysyl glycosides. With a more sensitive perbenzoylation-high-performance liquid chromatography method and uv detection at 230 nm, the carbohydrates released by nitrosation of submilligram quantities of collagen or peptide could be assayed accurately.

Characterization of the Descemet's membrane/posterior collagenous layer isolated from Fuchs' endothelial dystrophy corneas

The combined Descemet's membrane (DM) and posterior collagenous layer (PCL) of Fuchs' endothelial dystrophy corneas were isolated and characterized by biochemical and immunofluorescence methods. The amino acid composition of the Fuchs' DM-PCL was similar to age-matched normal Descemet's membranes (DM). As determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and 125I two-dimensional peptide mapping, normal DM and Fuchs' DM-PCL contained the same collagen types [type IV and endothelial cell (EC) collagen], but a slight discrepancy was seen in the electrophoretic mobility of some collagen chains. Immunofluorescence staining localized fibrinogen/fibrin to Fuchs' DM-PCL but not to normal DM. These data suggest that the appearance of 110 nm banded material in sheets and fusiform bundles characteristic of Fuchs' PCL is not due to the presence of a new (abnormal) collagen type but may represent altered assembly of collagen molecules, and that the fibrinolytic system may play a role in the degenerative process of Fuchs' endothelial dystrophy.

Identification of collagens isolated from bovine Descemet's membrane

In this study collagens were isolated and identified from a morphologically pure preparation of bovine Descemet's membrane (DM) which was obtained by gentle scraping of the cornea, sieving and subsequent treatment with detergents. An alternative procedure of DM isolation with forceps resulted in stromal contamination of the preparation as verified by light and transmission electron microscopy, and gel electrophoresis. The amino acid profile of collagens isolated by pepsinization and salt precipitation from the pure sample was similar to the analysis of the intact bovine DM. Polyacrylamide gel electrophoresis of this collagen under non-reducing conditions resulted in five major bands: 300 000 daltons (300 K), 200 000 daltons (200 K), 100 000 daltons (100 K) and lower molecular weight fractions (50 K1 and 50 K2). Individual collagen chains were isolated from preparative polyacrylamide gels and characterized by 125I two dimensional peptide mapping patterns. This data suggests that (1) the majority of collagen fragments seen in bovine DM pepsin supernatant are derived from a single genetically distinct collagen molecule, and (2) that type I and V collagens are not major components of bovine DM.

Production of collagen by cells isolated from a retrocorneal fibrous membrane rabbit model

The morphological and biosynthetic characteristics of cells from an experimentally induced rabbit retrocorneal fibrous membrane (RCFM) model were investigated. By transmission electron microscopy the cells within the RCFM demonstrated overlapping cytoplasmic processes and intercellular junctions, neither of which are fibroblast-like characteristics. The extracellular matrix within the RCFM had a fibrillar and amorphous component. Collagenous biosynthetic products of primary cultures of RCFM cells were compared to normal corneal endothelial cells, which produce mainly type IV collagen and a small amount of type V collagen, and fibroblasts, which produce types I, III and V collagens. The collagenous components produced by the RCFM cells were a combination of types I, V and low molecular weight fragments of type IV collagen. Therefore, these morphological and biochemical data suggest that RCFM cells are a type of modified corneal endothelial cell that produce collagens distinctly different from normal corneal endothelial cells.

Secretory phenotypes of endothelial cells in culture: comparison of aortic, venous, capillary, and corneal endothelium

Endothelial cells from different tissues display variations in morphology, intercellular junctions, cell surface and growth properties, and in production of basal lamina components, both in vivo and in vitro. We have investigated the spectra of extracellular proteins secreted by bovine endothelial cells cultured from large vessels, cornea, and capillaries. Aortic, venous, and corneal endothelial cells displayed highly similar patterns of protein synthesis as judged by analysis of the culture medium; the major products were fibronectin, a glycoprotein similar or identical to platelet thrombospondin, and Type III procollagen. Ion-exchange chromatography, followed by peptide mapping, confirmed the presence of EC, a novel endothelial collagen previously described in bovine aortic endothelial cell cultures. Minor variations were found in the collagens of the cell layers: Type III, the predominant interstitial collagen, was associated with the basement membrane Types IV and V and, in the case of corneal endothelium, with Type I. In contrast, capillary endothelial cells secreted significantly more collagen than did the aortic, venous, and corneal cells. Approximately 50% of the protein in the culture medium was collagenous and consisted of Types I and III collagen in a ratio of 2:3. These interstitial collagens were the only types detected in capillary cell layers as well. The pattern and overall rate of collagen synthesis by capillary endothelial cells in vitro contrasted significantly with that of the other endothelial cell types and closely resembled that described for cultures of sprouting endothelium. These alterations in secretory phenotype may reflect: 1) a true difference in cell type between capillary and other types of endothelium, 2) differences resulting from cell isolation and initial culture conditions, or 3) a correlation between growth regulation and protein synthesis.

Biosynthesis of two subunits of type IV procollagen and of other basement membrane proteins by a human tumor cell line

The major collagenous component secreted into the medium of cultured HT-1080 tumor cells was identified as type IV procollagen by specific antibodies and characteristic ratios of incorporated labeled 3-hydroxyproline and 4-hydroxyproline. The disulfide-bonded molecules consisted of two subunits, pro-alpha 1(IV) and pro-alpha 2(IV) chains with apparent molecular weights of 180 000 and 165 000. No conversion of the procollagen to collagen or to procollagen intermediates was detected in the cell cultures. The two subunits apparently represent different gene products, since enzymatic digestion of the separated chains produced quite different peptide maps. Pepsin degraded native type IV procollagen successively into several fragments, some still disulfide-linked, giving rise to a complex set of polypeptide chains (Mr = 30 000-140 000). This agrees with similar diverse patterns produced by pepsin from authentic type IV collagens. The ratio between the pro-alpha 1(IV) and pro-alpha 2(IV) chains varied in several experiments between 1.3 and 1.8, suggesting that the two chains belong to different triple-helical molecules. The cells also produced distinct amounts of fibronectin (subunit Mr = 230 000) and of the basement membrane glycoprotein laminin. The latter showed three subunits with Mr = 220 000, 210 000, and 400 000. A further disulfide-bonded, non-collagenous polypeptide (Mr = 160 000) was detected but not yet identified. Immunofluorescence demonstrated these proteins within the cells but not in a pericellular matrix. The production of basement membrane components by HT-1080 cells and lack of interstitial collagens disagree with the original classification of the cell line as a fibrosarcoma.

Isolation and characterization of hydroxyproline-containing proteins secreted by a murine carcinoma cell culture

A collagenous protein was isolated from a murine carcinoma cell culture, which has been shown to synthesize basement membrane. The molecular weight of this protein was estimated to be 155 000. It eluted from carboxymethyl-cellulose in the region near the alpha 1 and beta 11 components of calf skin collagen. 63--69% of the peptide-bound prolines were hydroxylated, and the 4-/3-hydroxyproline ratios ranged from 12 : 1 to 14 : 1. About 95% of the hydroxylysines in the peptide were glycosylated, and almost all of them were in the glucosylgalactosyl dissacharide form. Judging from the posttranslational characteristics, this collagenous protein is probably of basement membrane type.

Isolation and characterization of a native placental basement-membrane collagen and its component alpha chains

Native type IV collagen was isolated from human placenta using pepsin solubilisation followed by fractional salt precipitation and chromatogarphic purification. The native preparation was characterised using amino acid analyses, disc gel electrophoresis, segment-long-spacing crystallites and immunological methods. Two component alpha chains were isolated with molecular weights of approximately 95000 and 70000. Cyanogen bromide digests of these chains indicated that they are not related to any of the known alpha chains of interstitial collagens or to the recently described collagen containing alphaA and alphaB chains. They are also not related to one another and are therefore probably fragments of two genetically distinct type IV collagen alpha chains.

Biosynthesis of basement membrane matrix by isolated rat renal glomeruli

The incorporation of radioactive precursors into the extracellular basement membrane matrix has been investigated in a purified preparation of isolated rat kidney glomeruli. Using deoxycholate extraction of isolated glomeruli which were incubated with radioactively labeled amino acids and carbohydrates, as an assay system for the measurement of incorporation of basement membrane precursors into intact deoxycholate insoluble basement membrane material, we demonstrated the in vitro biosynthesis of this structure. The assay system minimized the possibility of contamination of the isolated labeled basement membrane with labeled cell membrane fragments which may occur with the standard isolation procedures involving sonication and centrifugation. A linear incorporation of labeled proline, lysine, glycine, glucosamine, and galactose into glomerular basement membrane was shown. Basement membrane synthesis was inhibited by metabolic poisons and protein synthesis inhibitors as well as by inhibitors of collagen synthesis, but not by colchicine, an inhibitor of collagen secretion. Rhe appearance of 14C-hydroxyproline in the basement membrane matrix was negligible during the first 4 hours of incubation and rose only to 1% of the total proline counts thereafter. The results are consistent with the characterization of glomerular basement membrane synthesis and deposition as a two component system comprising a rapidly synthesized and deposited noncollagenous glycoprotein component(s) and a collagenous component which is only deposited after a delay of 4 to 6 hours.

Collagen synthesis by human glomerular cells in culture

Collagen synthesis was studied in three subcultured human glomerular cell types, by radiolabeled incorporation of [14C]proline and [3H]lysine. The epithelioid circular glomerular cells secrete a collagen with a single size of chain (possibly type IV) with a high ratio of hydroxyproline to proline, hydroxylysine to lysine, and 11--17% of hydroxyproline as the 3-isomer. The smooth muscle-like rhomboid glomerular cells secrete collagen with a chain pattern suggesting types III and I collagen, distinct from that found in the media of fibroblasts. Small ovoid glomerular cells are morphologically and biochemically intermediate between circular glomerular cells and rhomboid glomerular cells, and may represent an in vitro modification of either circular glomerular cells or rhomboid glomerular cells.

Glomerular basement membrane collagen and activities of the intracellular enzymes of collagen biosynthesis in congenital nephrotic syndrome of the Finnish type

The composition of pepsin-solubilized glomerular basement membrane (GBM) collagen was studied in kidneys from patients with congenital nephrotic syndrome of the Finnish type (CNF). The 3-hydroxyproline content in the GBM collagen in CNF was only about one half of that noted in the controls. The alanine content was slightly higher in CNF, but no differences were found in the contents of the other amino acids or carbohydrates. Analysis of the GBM collagen by sodium dodecyl sulphate polyacrylamide gel electrophoresis after reduction indicated a lower proportion of one of the major polypeptide chains in CNF than in the controls, while a higher proportion of one low molecular weight minor component was noted. The activities of the five intracellular enzymes of collagen biosynthesis, including prolyl 3-hydroxylase, were not significancy altered in the CNF kidney cortex samples when compared with the controls.

Diabetic microangiopathy. I. Current status of the chemistry and metabolism of the glomerular basement membrane

In recent years, the nature of the renal glomerular basement membrane has been the subject of numerous investigations. In diabetes mellitus, the renal glomerulus is characterized primarily by thickening of the basement membrane and excessive accumulation of basement membrane-like material in the mesangial region. Compositional analyses have shown that basement membranes are glycoprotein in nature. Studies of the glomerular basement membranes in diabetes have indicated a change from normal chemical composition. Furthermore, studies of the metabolism of diabetic kidneys in experimental animals, using cortical homogenates and isolated glomeruli, have demonstrated higher anabolic and lower catabolic enzyme activities. However, contradictory data have been reported with regard to both the chemical composition and metabolism of the kidney in human and experimental diabetes. This review attempts to examine these reports in detail and discuss the possible causes for these discrepancies.

Differentiation in human amniotic fluid cell cultures: I: Collagen production

The collagen produced by differentiated cells cultured from human amniotic fluid was characterized in two ways. By chain composition and by 4-hydroxyproline:3-hydroxyproline isomer ratio, the collagen synthesized by F-type (fibroblast) cells was indistinguishable from that made by cultured fetal dermal fibroblasts. The predominant cells in young amniotic fluid cultures, termed AF-type, produced collagen with a lower isomer ratio, resembling that of basement membrane collage. The chain composition, as determined by chromatography on carboxymethyl cellulose, varied for different cultures of the AF-type, but the major pattern was consistent with that of basement membrane collagen. On the basis of these characteristics, F cells are of fibroblast origin, whereas most AF cells are of a different origin either endothelial or epithelial. Other evidence (Megaw et al., 1977) suggests an epithelial origin for AF cells.