Structure and function of heparan sulphate proteoglycans

Mouse mammary epithelial cells produce basement membrane and cell surface heparan sulfate proteoglycans containing distinct core proteins

Cultured mouse mammary (NMuMG) cells produce heparan sulfate-rich proteoglycans that are found at the cell surface, in the culture medium, and beneath the monolayer. The cell surface proteoglycan consists of a lipophilic membrane-associated domain and an extracellular domain, or ectodomain, that contains both heparan and chondroitin sulfate chains. During culture, the cells release into the medium a soluble proteoglycan that is indistinguishable from the ectodomain released from the cells by trypsin treatment. This medium ectodomain was isolated, purified, and used as an antigen to prepare an affinity-purified serum antibody from rabbits. The antibody recognizes polypeptide determinants on the core protein of the ectodomain of the cell surface proteoglycan. The reactivity of this antibody was compared with that of a serum antibody (BM-1) directed against the low density basement membrane proteoglycan of the Englebarth-Holm-Swarm tumor (Hassell, J. R., W. C. Leyshon, S. R. Ledbetter, B. Tyree, S. Suzuki, M. Kato, K. Kimata, and H. Kleinman. 1985. J. Biol. Chem. 250:8098-8105). The BM-1 antibody recognized a large, low density heparan sulfate-rich proteoglycan in the cells and in the basal extracellular materials beneath the monolayer where it accumulated in patchy deposits. The affinity-purified anti-ectodomain antibody recognized the cell surface proteoglycan on the cells, where it is seen on apical cell surfaces in subconfluent cultures and in fine filamentous arrays at the basal cell surface in confluent cultures, but detected no proteoglycan in the basal extracellular materials beneath the monolayer. The amino acid composition of the purified medium ectodomain was substantially different from that reported for the basement membrane proteoglycan. Thus, NMuMG cells produce at least two heparan sulfate-rich proteoglycans that contain distinct core proteins, a cell surface proteoglycan, and a basement membrane proteoglycan. In newborn mouse skin, these proteoglycans localize to distinct sites; the basement membrane proteoglycan is seen solely at the dermal-epidermal boundary and the cell surface proteoglycan is seen solely at the surfaces of keratinocytes in the basal, spinous, and granular cell layers. These results suggest that although heparan sulfate-rich proteoglycans may have similar glycosaminoglycan chains, they are sorted by the epithelial cells to different sites on the basis of differences in their core proteins.

Glomerular basement membrane proteoglycans are derived from a large precursor

The basement membrane heparan sulfate proteoglycan produced by the Englebreth-Holm-Swarm (EHS) tumor and by glomeruli were compared by immunological methods. Antibodies to the EHS proteoglycan immunoprecipitated a single precursor protein (Mr = 400,000) from [35S]methionine-pulsed glomeruli, the same size produced by EHS cells. These antibodies detected both heparan sulfate proteoglycans and glycoproteins in extracts of unlabeled glomeruli and glomerular basement membrane. The proteoglycans contained core proteins of varying size (Mr = 150,000 to 400,000) with a Mr = 250,000 species being predominant. The glycoproteins are fragments of the core protein which lack heparan sulfate side chains. Antibodies to glomerular basement membrane proteoglycan immunoprecipitated the precursor protein (Mr = 400,000) synthesized by EHS cells and also reacted with most of the proteolytic fragments of the EHS proteoglycan. This antibody did not, however, react with the P44 fragment, a peptide situated at one end of the EHS proteoglycan core protein. These data suggest that the glomerular basement membrane proteoglycan is synthesized from a large precursor protein which undergoes specific proteolytic processing.

Identification of the glycosaminoglycan-attachment site of mouse invariant-chain proteoglycan core protein by site-directed mutagenesis

The invariant chain (Ii), a nonpolymorphic glycoprotein that associates with the immunoregulatory Ia proteins encoded by the major histocompatibility complex, has a proteoglycan form (Ii-CS) that bears a chondroitin sulfate glycosaminoglycan. In this proteoglycan form, Ii may remain associated with Ia at the cell surface. Inhibitors that prevent the addition of glycosaminoglycan to Ii have been found to depress antigen-presenting function. Ii does not have multiple candidate glycosaminoglycan-attachment sites, and we used site-directed mutagenesis to replace a candidate serine glycosaminoglycan-acceptor site with alanine at position 201 in the murine Ii protein. Transfection of the normal or altered gene into Ii-negative COS-7 cells showed that equivalent amounts of core Ii protein and its acidic, terminally glycosylated forms were synthesized, but the Ala-201 mutant Ii did not give rise to Ii-CS. The mutant protein had apparently normal transport through the Golgi compartment and associated stably with Ia molecules. Thus, this mutation directly identifies the site of glycosaminoglycan addition and shows that it can be eliminated without adversely affecting the overall biosynthesis of Ii.

Heparin uncouples alpha 2-adrenoceptors from the Gi-protein in membranes of human platelets

The influence of heparin was studied on the inhibitory regulation of adenylate cyclase in human platelet membranes. Heparin blocked the adrenaline-induced inhibition of adenylate cyclase and the stimulation of GTP hydrolysis with half-maximal and maximal efficiency at 0.3 and 1-3 micrograms/ml, respectively. The effect of heparin was reversed by washing the membranes. Heparin did not change the number of alpha-adrenoceptors. In contrast, the affinity of the alpha-adrenoceptor for adrenaline was decreased in the presence of heparin. The pertussis toxin-catalysed ADP-ribosylation of the inhibitory guanine nucleotide-binding Gi-protein was not altered by heparin. Heparin also abolished the inhibition of adenylate cyclase caused by GTP itself. The data indicate that heparin can impair the hormone-induced inhibition of adenylate cyclase and the stimulation of GTP hydrolysis and suggest that the effects of heparin are caused by an action at the Gi-protein of the adenylate cyclase system.

Characterization and macromolecular association of proteoglycans produced by pig arterial smooth muscle cells in culture

1. Medium and cell-layer proteoglycans from pig aorta smooth muscle cells in culture were compared. In both compartments, the main proteoglycans contained chondroitin sulfate-dermatan sulfate chains of 40 kDalton. 2. However, cell-layer proteoglycans differed from those of the medium by the presence of: (a) some small-size proteoglycans; (b) a greater amount of heparan sulfate; (c) chondroitin sulfate-dermatan sulfate enriched in iduronate and in 4 sulfate- (instead of 6 sulfate-) residues. 3. During dissociation-reassociation assays of arterial proteoglycans with exogenous hyaluronate or "aggregate" proteoglycans, the in vitro formation of complexes appeared to involve inter-associations between proteoglycans molecules, in addition to aggregation with hyaluronate.

Heparan sulfate proteoglycan from human and equine glomeruli and tubules

1. Proteoglycans were isolated from human and equine glomeruli or tubules by guanidine extraction and anion exchange chromatography. 2. These proteoglycan preparations contained about equal amounts of heparan sulfate and chondroitin sulfates. 3. During the preparation of glomerular or tubular basement membranes the main part of proteoglycans (greater than 50%) was extracted in the salt extract. Chondroitin sulfate proteoglycan was mainly found in the water and salt extracts of glomeruli and tubules, heparan sulfate proteoglycan in the deoxycholate extracts and the basement membranes. 4. The glomerular basement membrane (GBM) contains about 12% (human) or 20% (equine) of the proteoglycans of the total glomerulus. They consist of greater than 70% (equine) or 80% (human) of heparan sulfate. 5. Heparan sulfate proteoglycan was isolated from the proteoglycan preparations of human or equine glomeruli and tubules by additional treatment with nucleases and chondroitinase ABC followed by CsCl gradient centrifugation. 6. Protein accounts for about 40% (dry weight) of the heparan sulfate proteoglycans. Their amino acid composition is characterized by a high content of glycine, but 3-hydroxyproline, 4-hydroxyproline and hydroxylysine are lacking. 7. The biochemical characteristics of the heparan sulfate proteoglycan of human or equine glomeruli or tubules differ from that isolated from rat glomeruli by their higher protein content and their amino acid composition. The significance of these differences is discussed.

Glycosaminoglycans in human gingival crevicular fluid during orthodontic movement

Glycosaminoglycans (GAG) in gingival crevicular fluid (GCF) were investigated by cellulose acetate electrophoresis of simultaneously collected samples from the mesial and distal surfaces of teeth in 3 groups of young persons. In a control group, which had not undergone orthodontic treatment, a major band of hyaluronic acid (HA) and a minor band of chondroitin sulphate (CS) were present. No differences in the mean content of either GAG between the mesial and distal surfaces were detected. From teeth undergoing movement by fixed appliances (active group), a raised mean level of CS was present in GCF from the surface towards which movement was directed. Teeth held passively by an appliance following cessation of active movement (retention group) showed raised levels of CS at mesial and distal surfaces. A heparan sulphate-like GAG was commonly present in this group only. No significant increase in the levels of HA were detected at the mesial and distal surfaces of either the active or the retention groups, despite increased GCF flow rates unassociated with more severe gingival inflammation. The GAG composition of GCF, particularly CS, appears to reflect changes occurring in the deeper periodontal tissues of alveolar bone and periodontal ligament during orthodontic treatment.

Basement-membrane heparan sulphate with high affinity for antithrombin synthesized by normal and transformed mouse mammary epithelial cells

Basement-membrane proteoglycans, biosynthetically labelled with [35S]sulphate, were isolated from normal and transformed mouse mammary epithelial cells. Proteoglycans synthesized by normal cells contained mainly heparan sulphate and, in addition, small amounts of chondroitin sulphate chains, whereas transformed cells synthesized a relatively higher proportion of chondroitin sulphate. Polysaccharide chains from transformed cells were of lower average Mr and of lower anionic charge density compared with chains isolated from the untransformed counterparts, confirming results reported previously [David & Van den Berghe (1983) J. Biol. Chem. 258, 7338-7344]. A large proportion of the chains isolated from normal cells bound with high affinity to immobilized antithrombin, and the presence of 3-O-sulphated glucosamine residues, previously identified as unique markers for the antithrombin-binding region of heparin [Lindahl, Bäckström, Thunberg & Leder (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6551-6555], could be demonstrated. A significantly lower proportion of the chains derived from transformed cells bound with high affinity to antithrombin, and a corresponding decrease in the amount of incorporated 3-O-sulphate was observed.

Paper disk assay for glycosaminoglycan sulfotransferases

A method is described for the assay of sulfotransferases, which transfer sulfate from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to glycosaminoglycan acceptors. Following the sulfation reactions, the [35S]sulfate-labeled products are precipitated and then separated from a sulfate donor ([35S]PAPS) and its degradation products by a paper disk method, and then the radioactivity remaining on the paper disk is subsequently determined by liquid scintillation counting. The rapidity and simplicity of the method are advantageous for multiple assays and have allowed us to establish assay conditions for serum sulfotransferases which introduce sulfate at position 6 of the internal N-acetylgalactosamine units of chondroitin, position 2 (amino group) of the glucosamine units of heparan sulfate and sugar units of keratan sulfate, respectively. The assay method will be applicable with modification to the assay of other glycosaminoglycan sulfotransferases and glycoprotein sulfotransferases.

Identification of a 64 kDa heparan sulphate proteoglycan core protein from human lung fibroblast plasma membranes with a monoclonal antibody

Human lung fibroblasts produce heparan sulphate proteoglycans (HSPG) that are associated with the plasma membrane. A monoclonal-antibody (Mab)-secreting hybridoma, S1, was produced by fusion of SP 2/0-AG 14 mouse myeloma cells with spleen cells from mice immunized with partially purified cellular HSPG fractions. The HSPG character of the material carrying the epitope recognized by Mab S1 was demonstrated by: (i) the co-purification of the S1 epitope with the membrane HSPG of human lung fibroblasts; (ii) the decrease in size of the material carrying the S1 epitope upon treatment with heparinase or heparitinase, and the resistance of this material to heparinase treatment after N-desulphation. The S1 epitope appears to be part of the core protein, since it was destroyed by proteinase treatment and by disulphide-bond reduction, but not by treatments that depolymerize the glycosaminoglycan chains and N-linked oligosaccharide chains. Polyacrylamide-gel electrophoresis of non-reduced heparitinase-digested membrane HSPG followed by Western blotting and immunostaining with Mab S1 revealed a single band with apparent molecular mass of 64 kDa. Membrane proteoglycans isolated from detergent extracts or from 4 M-guanidinium chloride extracts of the cells yielded similar results. Additional digestion with N-glycanase lowered the apparent molecular mass of the immunoreactive material to 56 kDa, suggesting that the core protein also carries N-linked oligosaccharides. Fractionation of 125I-labelled membrane HSPG by immuno-affinity chromatography on immobilized Mab S1, followed by heparitinase digestion and polyacrylamide-gel electrophoresis of the bound material, yielded a single labelled band with apparent molecular mass 64 kDa. Treatment with dithiothreitol caused a slight increase in apparent molecular mass, suggesting that the core protein of this membrane proteoglycan of a single subunit containing (an) intrachain disulphide bond(s).

Heparin potentiates the action of plasma membrane-associated growth stimulatory activity

Plasma membranes prepared from mouse liver have been previously shown to contain growth stimulatory activity as determined with cultured mouse fibroblasts. This growth stimulatory activity, termed plasma membrane-associated growth stimulatory activity (PMGA), is highly mitogenic in the presence of platelet-poor plasma. We now demonstrate that the growth stimulatory action of PMGA is dramatically enhanced by the addition of heparin. The half-maximal effect of heparin was observed at 1-3 micrograms/ml. The synergistic effect was seen in two distinct assays; the stimulation of DNA synthesis in quiescent cells, and an increase of cell number over a 3-day culture period. Heparin, by itself, does not have any measurable influence on the growth of fibroblasts. The action of heparin is not unique to this glycosaminoglycan, as several other highly sulfated polysaccharides, including dextran sulfate, pentosan polysulfate, and fucoidan, also exhibited the highly synergistic effect. Among other glycosaminoglycans examined, chondroitin sulfate B and heparan sulfate had a small, but significant, effect on enhancing the growth stimulatory action of PMGA. Chondroitin sulfate A, chondroitin sulfate C, hyaluronic acid dextran, and poly-L-glutamic acid, however, had no detectable effect. Further experiments suggested that the effect of heparin is twofold, namely, both a potentiation of growth stimulatory activity and a protection of PMGA activity. The data presented here suggest that the association of various cell surface components, such as PMGA and specific proteoglycans, can modulate the growth potential of a cell.

Heterogeneous distribution of a basement membrane heparan sulfate proteoglycan in rat tissues

A heparan sulfate proteoglycan (HSPG) synthesized by murine parietal yolk sac (PYS-2) cells has been characterized and purified from culture supernatants. A monospecific polyclonal antiserum was raised against it which showed activity against the HSPG core protein and basement membrane specificity in immunohistochemical studies on frozen tissue sections from many rat organs. However, there was no reactivity with some basement membranes, notably those of several smooth muscle types and cardiac muscle. In addition, it was found that pancreatic acinar basement membranes also lacked the HSPG type recognized by this antiserum. Those basement membranes that lacked the HSPG strongly stained with antisera against laminin and type IV collagen. The striking distribution pattern is possibly indicative of multiple species of basement membrane HSPGs of which one type is recognized by this antiserum. Further evidence for multiple HSPGs was derived from the finding that skeletal neuromuscular junction and liver epithelia also did not contain this type of HSPG, though previous reports have indicated the presence of HSPGs at these sites. The PYS-2 HSPG was shown to be antigenically related to the large, low buoyant density HSPG from the murine Engelbreth-Holm swarm tumor. It was, however, confirmed that only a single population of antibodies was present in the serum. Despite the presence of similar epitopes on these two proteoglycans of different hydrodynamic properties, it was apparent that the PYS-2 HSPG represents a basement membrane proteoglycan of distinct properties reflected in its restricted distribution in vivo.

Structure of low density heparan sulfate proteoglycan isolated from a mouse tumor basement membrane

A large heparan sulfate proteoglycan of low buoyant density (p = 1.32 to 1.40 g/cm3 in 6 M-guanidine.HCl) was extracted from a tumor basement membrane with denaturing solvents and purified by chromatography and CsCl gradient centrifugation. Chemical, immunological, physical and electron microscopical analyses have demonstrated a high degree of purity and have allowed us to propose a structural model for this proteoglycan. It is composed of an 80 nm long protein core formed from a single polypeptide chain (Mr about 500,000) with intrachain disulfide bonds. This core is folded into a row of six globular domains of variable size as shown by electron microscopy after rotary shadowing and negative staining. A multidomain structure was confirmed by protease digestion experiments that allowed the isolation of a single heparan sulfate-containing peptide segment representing less than 5% of the total mass of the protein core. Electron microscopy has visualized generally three heparan sulfate chains in each molecule close to each other at one pole of the protein core. The molecular mass and length (100 to 170 nm) of the heparan sulfate chains were found to vary consistently between different preparations. The mass per length ratio (350 nm-1) indicated an extended conformation for the heparan sulfate side-chains. These structural features are distinctly different from those of the high density proteoglycan, suggesting that both forms of basement membrane heparan sulfate proteoglycan are genetically distinct and not derived from a common precursor.

Effects of nerve growth factor-induced differentiation on the heparan sulfate of PC12 pheochromocytoma cells and comparison with developing brain

We have examined the size, charge, and sulfation pattern of heparan sulfate in the cell-soluble fraction, membranes, and culture medium of PC12 pheochromocytoma cells cultured in the presence and absence of nerve growth factor (NGF) and compared the structural features of PC12 cell heparan sulfate to that of rat brain at several stages of early postnatal development. Nitrous acid degradation studies revealed significant differences in the distribution of N-sulfate and N-acetyl groups in heparan sulfate present in the PC12 cell-soluble fraction, membranes, and medium and demonstrated that NGF treatment led to an increased proportion of N-sulfated segments in the cell-associated heparan sulfate, although no such change was seen in that released into the culture medium. There was very little change in the N-sulfation of brain heparan sulfate during the first 30 days after birth. In brain, most of the heparan sulfate glucosamine residues are N-sulfated and yield predominantly di- and tetrasaccharide nitrous acid degradation products, whereas PC12 cell heparan sulfate contains large blocks of N-acetylglucosamine residues. There was very little difference in the overall charge or size (approximately 15,000 Da) of heparan sulfate chains between the different PC12 cell fractions or brain, although NGF treatment led to a decrease in the proportion of less-charged chains in the PC12 cell membranes and a small increase in molecular size. Our studies therefore demonstrate the presence in PC12 cells of several pools of heparan sulfate having different structural properties, and that significant alterations in the charge, size, and sulfation pattern of PC12 cell heparan sulfate accompany NGF-induced differentiation and neurite outgrowth.

Isolation of the heparan sulfate proteoglycans from the extracellular matrix of rat skeletal muscle

We have previously shown that asymmetric collagen-tailed acetylcholinesterase (AChE) is anchored to the extracellular matrix (ECM) by heparan sulfate proteoglycans (HSPGs). Here we present our studies on the characterization of such PGs from the ECM of rat skeletal muscles. After radiolabeling with 35SO4 for 24h, PGs were extracted from the muscle ECM with 4.0 M guanidine-HCl containing protease inhibitors. PGs were subsequently isolated using sequential DEAE-Sephacel chromatography, digestion with chondroitinase ABC, and Sepharose CL-4B. Two different hydrodynamic size species of HSPGs were found. One type had a Mr of 4-6 X 10(5) (Kav = 0.25) as estimated by gel chromatography in the presence of 1% SDS and accounted for 75% of the total HSPGs. The other HSPG had a Mr 1.5-2.5 X 10(5) (Kav = 0.41). The glycosaminoglycan (GAG) side chains (Mr 20,000 and 12,000) were found composed only of heparan sulfate as determined by nitrous acid oxidation and heparitinase treatment. The large-sized HSPG, which is concentrated in synaptic regions, contains only GAG chains of Mr 20,000, suggesting that each HSPG contains only one kind of heparan sulfate chain in its structure. Our results definitively establish by biochemical criteria that the basement membrane of mammalian skeletal muscle contains HSPGs, the likely matrix receptor for the immobilization of the asymmetric collagen-tailed AChE at the neuromuscular junction.

Hyaluronate synthetase inhibition by normal and transformed human fibroblasts during growth reduction

To establish the relation of glycosaminoglycan synthesis to cell proliferation, we investigated the synthesis of individual glycosaminoglycan species by intact cells and in a cell-free system, using normal and transformed human fibroblasts under differing culture conditions. Reducing serum concentration brought about a marked decline in the synthesis of hyaluronate (HA) as well as cell proliferation on both normal and transformed cells. Both HA synthesis and proliferation decreased with increasing cell densities markedly (in inverse proportion to cell density) in normal cells but gradually in transformed cells. This noticeable congruity of the changes in HA synthesis and proliferation indicates that the change in HA synthesis is related primarily to cell proliferation rather than to cell density or cellular transformation. Examination of HA synthesis in a cell-free system demonstrated that the activity of HA synthetase also fluctuated in conjunction with cell proliferation. Furthermore, growth-reduced cells (except crowded transformed cells) inhibited cell-free HA synthesis and this inhibition was induced coincidentally with a decrease in both HA synthetase activity and proliferation. These findings suggest that the change in HA synthesis is significant in the regulation of cell proliferation.

Conformational equilibria of alpha-L-iduronate residues in disaccharides derived from heparin

The disaccharides IdoA(2SO3)-anManOH(6SO3) and IdoA-anManOH (where IdoA represents alpha-L-iduronate, anManOH represents 2,5-anhydro-D-mannitol and SO3 represents sulphate ester) were prepared from bovine lung heparin using HNO2 depolymerization, borohydride reduction and desulphation, and were examined by 400 MHz 1H-n.m.r. spectroscopy. Three-bond proton-proton coupling constants around the IdoA ring were determined under a range of experimental conditions. For unsulphated IdoA all four proton-proton coupling constants varied markedly as a function of temperature, pH and solvent, providing clear evidence for a rapid conformational equilibrium. These data were analysed in terms of the three most energetically stable IdoA conformers: 1C4, 4C1, and 2S0. Predicted coupling constants for these conformers were determined using a modified Karplus-type relationship. For unsulphated IdoA in dimethyl sulphoxide the equilibrium was provoked strongly in favour of a slightly distorted 4C1 'chair' IdoA conformer for which coupling constants have not previously been reported. For sulphated IdoA in aqueous conditions and at low pH the equilibrium is strongly in favour of the alternative 1C4 chair conformer. Under many conditions, however, significant contributions from all three conformers occur for the non-reducing terminal IdoA in these disaccharides.

Compartmentalization of a haematopoietic growth factor (GM-CSF) by glycosaminoglycans in the bone marrow microenvironment

Haematopoietic progenitor cells proliferate and mature in semisolid media when stimulated by exogenous haematopoietic cell growth factors (HCGFs) such as granulocyte-macrophage colony-stimulating factor (GM-CSF). They also proliferate in association with marrow-derived stromal cells although biologically active amounts of HCGFs cannot be detected in stromal culture supernatants. It is possible that HCGFs are synthesized in small amounts by stromal cells but remain bound to the stromal cells and/or their extracellular matrix (ECM). This interpretation accords with haematopoietic progenitor cell proliferation in close association with stromal layers in long-term cultures. Glycosaminoglycans (GAGs) are found in the ECM produced by stromal cells. They are prime candidates for selectively retaining HCGFs in the stromal layer; they influence embryonic morphogenesis and cyto-differentiation and they may regulate haematopoiesis. We now report that granulocyte-macrophage colony-stimulating activity can be eluted from cultured stromal layers and that exogenous GM-CSF binds to GAGs from bone marrow stromal ECM. Selective compartmentalization of HCGFs in this manner may be an important function of the marrow microenvironment and may be involved in haematopoietic cell regulation.

Identification of an extended N-acetylated sequence adjacent to the protein-linkage region of fibroblast heparan sulphate

The distribution of N-sulphate groups within fibroblast heparan sulphate chains was investigated. The detergent-extractable heparan sulphate proteoglycan from adult human skin fibroblasts, radiolabelled with [3H]glucosamine and [35S]sulphate, was coupled to CNBr-activated Sepharose 4B. After partial depolymerization of the heparan sulphate with nitrous acid, the remaining Sepharose-bound fragments were removed by treatment with alkali. These fragments, of various sizes, but all containing an intact reducing xylose residue, were fractionated on Sephacryl S-300 and the distribution of the 3H and 35S radiolabels was analysed. A decreased degree of sulphation was observed towards the reducing termini of the chains. After complete nitrous acid hydrolysis of the Sepharose-bound proteoglycan, analysis of the proximity of N-sulphation to the reducing end revealed the existence of an extended N-acetylated sequence directly adjacent to the protein-linkage sequence. The size of this N-acetylated domain was estimated by gel filtration to be approximately eight disaccharide units. This domain appears to be highly conserved, being present in virtually all the chains derived from this proteoglycan, implying the existence of a mechanism capable of generating such a non-random sequence during the post-polymeric modification of heparan sulphate. Comparison with the corresponding situation in heparin suggests that different mechanisms regulate polymer N-sulphation in the vicinity of the protein-linkage region of these chemically related glycosaminoglycans.

Sulphation and fibronectin-binding properties of heparan sulphate glycosaminoglycans from transformed cultured human keratinocytes

Images

Glycosaminoglycan composition of electric organ basement membranes

The basement membranes of the innervated surface of the electric organ of Discopyge tschudii present a high concentration of mucopolysaccharides as revealed by intense ruthenium red-positive reaction. Glycosaminoglycans (GAGs) were isolated and characterized from these pure basement membranes by using a combination of agarose gel electrophoresis and enzymatic degradation with specific enzymes. The isolated basement membrane showed a high concentration of GAGs (130 mg/g of dry tissue); of this amount 49% was hyaluronic acid, 24% was chondroitin-6-sulfate, 12% was heparan sulfate, and 14% was dermatan sulfate. Controlled digestion with heparinase and heparitinases I and II was used to study the structural features of the heparan sulfate. Four unsaturated disaccharide units were found in the heparan sulfate: disulfated, N-sulfated, N-acetylated, and N-acetylated O-sulfated disaccharides. The disaccharide units of the cholinergic heparan sulfate present a high amount of disulfated disaccharides and a low amount of N-acetylated O-sulfated disaccharides. The N-sulfated disaccharides, in contrast to the N-acetylated ones, were found through all the structure of the cholinergic heparan sulfate. Finally our work shows for the first time the presence of dermatan sulfate in the basal lamina of the electric organ.

Expression of anionic sites at the dermoepibolic junction

The emergence of anionic sites during basement membrane zone formation was studied using migrating epidermis in organ culture as a model system. Ultrastructural investigations using a strongly cationized probe revealed that the heparitinase-sensitive, anionic sites were formed synchronously with the newly built basal lamina after 7 days in culture.

Internalization and metabolism of endogenous heparin by cultured endothelial cells

We have studied the ability of bovine adrenal capillary cells cultured in vitro to uptake and metabolize heparin. We have previously demonstrated that endogenous heparin can be extracted from human plasma (Vannucchi, S. et al., (1985) Biochem. J. 227, 57-65), and here we show that also endothelial cells contain heparin. However, experiments with (35S)sodium sulfate labeling indicate that these cells do not synthesize de novo heparin, but they uptake it from culture serum. Bovine adrenal capillary endothelial cells are able to bind and uptake (3H)heparin added to culture medium and they also release its low molecular weight degradation products, thus indicating a metabolism of heparin. We discuss about the role of endothelial cell-mediated uptake and metabolism of endogenous heparin in relationship with circulating heparin. We also discuss about these events as related to some of the antithrombogenic properties of the endothelium.