Proteoglycan-fibrillar collagen interactions

Developmental changes in the type I procollagen processing pathway in chick-embryo cornea

Type I procollagen processing in chick-embryo corneas was studied at days 12, 14 and 17 of development. Pulse-chase experiments and electrophoretic analysis of salt-soluble extracts showed developmental changes in the processing pathway. A kinetic model was fitted to the data to determine rate constants for processing of both N- and C-propeptides. Data for pro alpha 1(I)-chain processing and pro alpha 2(I)-chain processing were fitted separately (where pro means procollagen). Between days 12 and 17 the relative flux through the pC-collagen (procollagen chain lacking the N-propeptide) and pN-collagen (procollagen chain lacking the C-propeptide) pathways increased approx. 4-fold. Pro alpha 1(I) chains and pro alpha 2(I) chains were processed by slightly different routes. Variations in the rate constants were compared with electron-microscopic measurements of collagen fibril diameters at each stage of development. Diameters increased by less than 10% over the period from 12 to 17 days. It was concluded that fibril diameters are relatively insensitive to the pathway of procollagen processing in the salt-soluble pool.

Proteoglycan: collagen interactions in connective tissues. Ultrastructural, biochemical, functional and evolutionary aspects

Electron histochemical investigations of mammalian and echinoderm tissues, using cupromeronic blue to stain proteoglycans (PGs) specifically in critical electrolyte concentration methods, showed that collagen fibrils are associated with keratan sulphate and chondroitin (dermatan) sulphate ('tadpole') PGs at the a, c, d and e bands on the fibril surface, giving rise to the 'one proteoglycan: one binding site' hypothesis. Intra-fibrillar PGs have been observed, distributed in a regular way which suggests that collagen fibrils are aggregates of 'protofibrils', some of which carry PGs at their surfaces. A scheme for remodelling of collagen fibrils, based on recycling of these protofibrils, is outlined. The choice of which tadpole PG to use to carry out a given function is decided to a considerable extent by the availability of oxygen to the relevant tissue element.

Patterns of glycosaminoglycan/proteoglycan immunostaining in human skin during aging

Proteoglycans and their component glycosaminoglycans are involved in such cell-cell and cell-matrix interactions as cell adhesion and migration, processes that are essential for embryonic and fetal development. As definitive organs such as skin emerge, structurally different proteoglycans partition into highly defined compartments. In skin, these compartments correspond to morphologically and functionally distinct layers. However, during the normal aging process, the relative amounts of structurally distinct proteoglycans apparently varies independently in each of these layers. This was demonstrated, in an indirect immunocytochemical study, through the use of monoclonal antibodies that detect structurally distinct domains in glycosaminoglycan chains of proteoglycans. Using samples of normal human skin obtained from individuals ranging in age from 20 weeks of gestation to 98 years of age, we determined that a common distribution pattern existed in skin. The epidermis contained chondroitin 4- and keratan sulfates, the basal lamina was the only layer that contained chondroitin 6-sulfate, the papillary and reticular dermis contained principally dermatan sulfate. In addition, antibodies that recognize native domains in chondroitin sulfates identified proteoglycan subsets that partitioned into distinct layers. An important new finding was that the relative amounts of specific types of glycosaminoglycans varied in an age- and layer-dependent manner. In the epidermis there was a notable increase in keratan sulfate beginning at age 50. Chondroitin 6-sulfate, found principally in the basal lamina, decreased after age 60. In the papillary dermis, the amount of dermatan sulfate increased after age 50, whereas the amount of novel chondroitin sulfate epitope, detected by antibody 4C3, decreased with age. Thus, age-related changes in proteoglycan distribution exist and correlate with morphologic and functional changes that occur in the intrinsic process of aging in human skin.

Ultrastructural localisation of anionic sites at the dermo-epidermal junction in normal human skin

Glycosaminoglycans have been demonstrated throughout the cutaneous BMZ at the ultrastructural level. Colloidal iron and cationised ferritin proved of limited value, whilst staining with Alcian blue and application of the critical electrolyte concentration principle has provided evidence for the presence of sulphated GAGs at the lamina lucida and lamina reticularis. Digestions with chondroitin ABC lyase and heparin lyase have confirmed the existence of chondroitin and/or dermatan sulphates and heparan sulphates, although the results obtained with hyaluronate lyase have indicated that hyaluronates are also present.

Characterization of thrombospondin binding to collagen (type I) fibres: role of collagen telopeptides

We have shown that thrombospondin (tsp), like fibronectin (fn) and von Willebrand factor (vWf), exhibits a rapid, specific and saturable binding to collagen type I fibres (from bovine tendon). The level of binding at saturation is very similar to that of vWf. As with fn and vWf, the interaction is ionic in character and appears to occur by a polyvalent mechanism since there is little inhibition of interaction by monomeric collagen. The conformation of tsp, like that of fn and vWf, is important since denaturation causes reduced complexing. Furthermore, conformational changes in tsp due to the presence of Ca++ can modulate the amount of complex formed under physiological conditions. Tsp, like vWf but in contrast to fn, shows little affinity for denatured fibres emphasizing the importance of collagen conformation. Pepsin digestion suggests an important role for collagen telopeptides; vWf- and fn-binding sites are located more within the collagen triple helix. Comparison of the effect on binding after leucine aminopeptidase or carboxypeptidase digestion suggests involvement of the N- rather than C-terminal telopeptides. No evidence was found for a role for fn, the proteoglycan PG2 or collagen type V, which could be present in type I fibres, in mediating the interaction between tsp and the fibres. VWf did not inhibit the interaction of tsp, but fn did slightly when tested in large excess. This suggests separate binding sites in collagen for all three ligands since fn and vWf are also known to bind independently of each other.

Chondroitin sulphate and keratan sulphate are almost isosteric

Keratan sulphate and chondroitin sulphate (KS and CS) in the 2-fold helical configurations that are prevalent in solution are of very similar tacticity. The chiral centres, anionic sites and hydrophobic patches are in identical conformations. Only the position of the acetamido group varies from CS to KS, but part of its intramolecular H-bonding potential in CS is retained in KS. The formation of tertiary aggregates, observed in vitro and in tissues, is explicable on these bases. The proposal that KS may be a functional substitute for CS [Scott & Haigh (1988) J. Anat. 158, 95-108] under low-O2 conditions is relevant.

Secondary and tertiary structures of hyaluronan in aqueous solution, investigated by rotary shadowing-electron microscopy and computer simulation. Hyaluronan is a very efficient network-forming polymer

1. Hyaluronan from mesothelioma fluid, rooster comb and streptococci was examined by rotary shadowing and electron microscopy. All preparations showed extensive branched networks, but high-viscosity hyaluronan networks were essentially infinite, with no individual 'molecules' that were not integrated via multiple branched points into the meshwork. Low-viscosity hyaluronan, recovered after papain digestion of mesothelioma fluid, showed occasional single filaments that were independent of the main aggregates, some of which were themselves independent of other aggregates. 2. Hyaluronan is a polymer with a very marked capability to form meshworks at very low dilution (less than 1 microgram/ml). The longer the hyaluronan molecule, the more branching is potentially possible, and the more extensive and coherent is the network, with every hyaluronan molecule in contact with every other in the solution, via the network. This behaviour accounts for the mechanical properties of the soft tissues (e.g. vitreous humour) and fluids (e.g. synovial fluid) of which hyaluronan is a major component. 3. The hyaluronan twofold helix, previously demonstrated to be present in solution [Heatley & Scott (1988) Biochem. J. 254, 489-493] was shown by computer simulation and energy calculations to be sterically capable of extensive duplex formation, probably driven by interactions between the large hydrophobic patches on alternate sides of the tape-like polymer, forming stable aggregates at biological temperatures in water. This 'stickiness' is postulated to be the basis of the network-forming and laterally aggregating behaviour of hyaluronan. 4. The tertiary structures formed by hyaluronan may not be possible in the case of chondroitin 4-sulphate.

Purification and N-terminal amino acid sequence of a chondroitin sulphate/dermatan sulphate proteoglycan isolated from intima/media preparations of human aorta

A proteoglycan (PG) was purified to homogeneity from intima/media preparations of human aorta specimens by the following chromatographic steps: Sepharose Q anion exchange, Sepharose CL-4B size exclusion, hydroxyapatite, MonoQ anion exchange and TSK G 4000 SW size exclusion. The purity of the preparation was established by SDS/PAGE using direct staining by silver or Dimethylmethylene Blue, as well as by Western blots of biotin-labelled samples. The electrophoretic mobility of the native PG was less than that of a 200,000-Mr standard protein. After treatment with chondroitin sulphate lyase ABC, a core protein of Mr 15,000 was revealed. The Mr of the glycosaminoglycan (GAG) peptides was less than 24,000, by comparison with a keratan sulphate peptide. The composition of the GAG chains was determined by differential digestion of the PG by chondroitin sulphate lyases AC/ABC or chondroitin sulphate lyase AC alone followed by anion-exchange chromatography of the resulting disaccharides. The GAG chains are composed of approximately one-third of dermatan sulphate and two-thirds chondroitin sulphate disaccharide units. The sequence of the 20 N-terminal amino acids is identical with the sequence previously reported for PG I isolated from human developing bone [Fisher, Termine & Young (1989) J. Biol. Chem. 264, 4571-4576]. The assignment of glycosylation sites to the serine residues in positions 5 and 10 was confirmed. The findings indicate that the chondroitin sulphate/dermatan sulphate PG is a major PG in intima/media preparations of human aorta and represents a biglycan-type PG.

Stimulation by 1,25-dihydroxyvitamin D3 of in vitro mineralization induced by osteoblast-like MC3T3-E1 cells

Although vitamin D is essential for mineralization of bone, it is as yet unclear whether vitamin D has a direct stimulatory effect on the bone mineralization process. In the present study, the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on in vitro mineralization mediated by osteoblast-like MC3T3-E1 cells was examined. MC3T3-E1 cells continued to grow after they reached confluency, and DNA content and alkaline phosphatase activity increased linearly until about 16 days of culture, whereas 45Ca accumulation into cell and matrix layer remained low. After this period, DNA content plateaued, and 45Ca accumulation increased sharply. Histological examination by von Kossa staining revealed that calcium was accumulated into extracellular matrix. In addition, needle-shaped mineral crystals similar to hydroxyapatite crystals could be demonstrated in between collagen fibrils by electron microscopy. Thus, MC3T3-E1 cells differentiate in vitro into cells with osteoblastic phenotype and exhibit mineralization. When MC3T3-E1 cells were treated with 1,25(OH)2D3 at this stage of culture, there was a dose-dependent stimulation of 45Ca accumulation by 1,25(OH)2D3, and a significant stimulation of 45Ca accumulation was observed with 3 x 10(-10) M 1,25(OH)2D3. Although 1,25(OH)2D3 enhanced alkaline phosphatase activity and collagen synthesis at the early phase of culture, it did not affect any of these parameters at the late phase when 1,25(OH)2D3 stimulated mineralization. Neither 24,25-dihydroxyvitamin D3 nor human PTH(1-34) affected mineralization in the presence or absence of 1,25(OH)2D3. These results demonstrate that 1,25(OH)2D3 stimulates matrix mineralization induced by osteoblastic MC3T3-E1 cells, and are consistent with the possibility that 1,25(OH)2D3 has a direct stimulatory effect on bone mineralization process.(ABSTRACT TRUNCATED AT 250 WORDS)

Lectin binding pattern in the embryonal and early fetal human vertebral column

Paraffin sections from vertebral columns of ten human embryos and fetuses ranging from stage 16 to the 12th week were stained with the FITC-coupled lectins PNA, RCA I, Con A and WGA in order to investigate changes in carbohydrate-binding sites during vertebral development. PNA revealed a specific binding site in the vertebral body blastema in the precartilaginous stage of development. Beginning with the 25-mm CRL embryo, PNA-binding sites occurred in the developing fibrous annulus and the inner zone of the intervertebral discs. The first binding sites for RCA I were seen in the extracellular matrix of vertebral bodies during the cartilaginous stage of vertebral development. During early ossification of the vertebrae, staining for RCA I-binding sites in the cytoplasm of the chondrocytes and the area around the future cartilaginous end-plates was observed. Con A bound to the chondrocyte cytoplasm, and also very strongly to notochordal cells in all developmental stages examined. WGA-binding sites appeared simultaneously with cartilage formation. Connective tissue components, e.g. ligaments, were diffusely stained by WGA. Also this lectin showed an affinity for vertebral body chondrocytes. We discuss the biochemical aspects of these lectin-binding sites, and their possible roles in the differentiation process of the human vertebral column. The results of this first lectin histochemical study on human vertebral development are compared with related results in other species.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of cells responsible for synthesis of sulphated glycosaminoglycans in schistosome-induced hepatic granulomas

Sulphated glycosaminoglycans were isolated from schistosome-induced hepatic granuloma and from the pericellular, intracellular and extracellular compartments of two murine cell lines derived from granulomas: the primary cell line GR, and the permanent cell line GRX, established spontaneously from GR. The glycosaminoglycans composition in the whole granuloma was similar to that observed in the intracellular and extracellular compartments of GR cells. This result suggests that GR cells may be the major cell population involved in the synthesis and accumulation of glycosaminoglycans in the granulomas, and play an important role in the process of hepatic fibrosis. The conversion of the primary cell line GR into the established GRX cells did not modify the ratios that prevail among different glycosaminoglycans of the cell surface. However, it decreased the synthesis and secretion of glycosaminoglycans, reduced the proportion of iduronic acid units in the chondroitin sulphate, and increased the proportion of heparan sulphate in intracellular and extracellular pools. These characteristics of the GRX cells are similar to those observed in long-term cultures of smooth-muscle cells. In agreement with the general phenomenon of progressive de-differentiation during in-vitro culture of primary cell lines, these data indicate that the connective tissue cells of liver may belong to the myofibroblastic cell lineage.

Proteoglycans of articular cartilage: changes in aging and in joint disease

In human osteoarthritis and animal models of degenerative joint disease, damage to the structure of cartilage proteoglycan is a central event. Loss of proteoglycan from the matrix alters the physicochemical properties of the tissue, but the pathological process and biochemical mechanisms that lead to this loss are poorly understood. This review examines the present state of knowledge regarding proteoglycan structure and the changes that occur in aging and osteoarthritis. It also discusses how these studies will influence the development of new methods for measuring cartilage breakdown.

Interaction of the NG2 chondroitin sulfate proteoglycan with type VI collagen

The NG2 chondroitin sulfate proteoglycan is a membrane-associated molecule of approximately 500 kD with a core glycoprotein of 300 kD. Both the complete proteoglycan and a smaller quantity of the 300-kD core are immunoprecipitable with polyclonal and monoclonal antibodies against purified NG2. From some cell lines, the antibodies coprecipitate NG2 and type VI collagen, the latter appearing on SDS-PAGE as components of 140 and 250 kD under reducing conditions. The immunoprecipitation of type VI collagen does not seem to be due to recognition of the collagen by the antibodies, but rather to binding of the collagen to NG2. Studies on the NG2-type VI collagen complex suggest that binding between the two molecules is mediated by protein-protein interactions rather than by ionic interactions involving the glycosaminoglycans. Immunofluorescence double labeling in frozen sections of embryonic rat shows that NG2 and type VI collagen are colocalized in structures such as the intervertebral discs and arteries of the spinal column. In vitro the two molecules are highly colocalized on the surface of several cell lines. Treatment of these cells resulting in a change in the distribution of NG2 on the cell surface also causes a parallel change in type VI collagen distribution. Our results suggest that cell surface NG2 may mediate cellular interactions with the extracellular matrix by binding to type VI collagen.

The small proteoglycans of cartilage matrix

The small proteoglycans (PGs) of cartilage matrix represent a small fraction of the total mass of PGs, but with a small size they can be present in equivalent moles to the large PGs. Three types of PGs with a wide skeletal and extraskeletal distribution, biglycan (PGI), decorin (PGII) and fibromodulin have distinct but homologous core proteins containing leucin-rich sequences. Carbohydrate substituants (one or two chondroitin sulfate/dermatan sulfate chains for decorin and biglycan respectively, chains of keratan sulfate for fibromodulin and oligosaccharides) present variations from tissue to tissue and with age and other factors. Decorin and fibromodulin appear to interact with collagen and to participate in the regulation of collagen matrices. In vitro experiments indicate a role for small PGs in adhesion, multiplication, differentiation, and migration of cells. Recent data on molecular biology of the small PGs contribute to a better understanding of their functions and make the evaluation of their role in hereditary diseases.

A comparative biochemical and ultrastructural study of proteoglycan-collagen interactions in corneal stroma. Functional and metabolic implications

1. Corneas of mouse, rat, guinea pig, rabbit, sheep, cat, dog, pig and cow were quantitatively analysed for water, hydroxyproline, nucleic acid, total sulphated polyanion, chondroitin sulphate/dermatan sulphate and keratan sulphate, several samples or pools of tissue from each species being used. Ferret cornea was similarly analysed for water and hydroxyproline on one pool of eight corneas. Pooled frog (38) and ferret (eight) corneas and a single sample of human cornea were qualitatively examined for keratan sulphate and chondroitin sulphate/dermatan sulphate by electrophoresis on cellulose acetate membranes. Nine species (mouse, frog, rat, guinea pig, rabbit, sheep, cat, pig and cow) were examined by light microscopy and six (mouse, frog, rat, guinea pig, rabbit and cow) by electron microscopy, with the use of Alcian Blue or Cupromeronic Blue in critical-electrolyte-concentration (CEC) methods to stain proteoglycans. 2. Water (% of wet weight), hydroxyproline (mg/g dry wt.) and chondroitin sulphate (mg/g of hydroxyproline) contents were approximately constant across the species, except for mouse. 3. Keratan sulphate contents (mg/g of hydroxyproline) increased with corneal thickness, whereas dermatan sulphate contents decreased. The oversulphated domain of keratan sulphate was absent from mouse and frog corneas, increasing as percentage of total keratan sulphate with increasing corneal thickness. Sulphation of dermatan sulphate was essentially complete (i.e. one sulphate group per disaccharide unit). 4. Chondroitin sulphate/dermatan sulphate proteoglycans were present at the d bands of the collagen fibrils of all species examined, orthogonally arrayed, with high frequency, and occasionally at the e bands. Keratan sulphate proteoglycans were present at the a and c bands of all species examined, but with far higher frequency in the thicker corneas, where keratan sulphate contents were high. 5. Alcian Blue CEC staining showed much higher sulphation of keratan sulphate in thick corneas, e.g. that of cow, than in thin corneas, e.g. that of mouse, in keeping with biochemical analyses. 6. It is suggested that the constancy of interfibrillar volumes is regulated via the swelling and osmotic pressure of the interfibrillar polyanions, by adjustment of the extent of sulphation in two independent proteoglycan populations, to achieve an 'average sulphation' of the total polyanion similar to that of fully sulphated chondroitin sulphate/dermatan sulphate. 7. The balance of synthesis of the two kinds of proteoglycans may be determined by the O2 supply to the avascular cornea. O2 supply may also determine the conversion of chondroitin sulphate into dermatan sulphate.

Extracellular matrix interactions: sulfation of connective tissue polysaccharides creates macroion binding templates and conditions for dissipative structure formation

Evidence is now accumulating that the post-polymer modification process of sulfation of connective tissue polysaccharides is primarily to provide an interactive macroion for enthalpic interactions rather than influence thermodynamic non-ideality which primarily affects water distribution in biological systems. Metabolic energy considerations also distinguish these physicochemical classifications. Thermodynamic non-ideality is embodied in the carboxyl group and polysaccharide chain which are energetically favoured in biosynthesis, whereas considerable energy input is required for sulfation. The sulfation process gives rise to macroions, with a wide variety of negative charge patterns, that may participate in heterotypic macromolecular interactions. This partial informational specificity is discussed in terms of evolutionary flexibility of the extracellular matrix as rationalized on the qualitative aspects of dissipative structure formation. The concept of multiple binding interactions of varying specificity associated with connective tissue polysaccharides raises the awareness of a more random, less highly ordered, extracellular matrix as compared to the tight machine-like organization generally found for processes in the cell. This is discussed in terms of the physiological adaptation and development of multicellular-tissue systems.

Non-uniform influence of transforming growth factor-beta on the biosynthesis of different forms of small chondroitin sulphate/dermatan sulphate proteoglycan

The influence of transforming growth factor-beta (TGF-beta) on the expression of different forms of small proteoglycans was investigated in human skin fibroblasts and in a human osteosarcoma cell line. TGF-beta was not found to act as a general stimulator of small proteoglycan biosynthesis. In both cell types, an increased expression of the core protein of proteoglycan I was found. However, there was a profound decrease in the expression of a 106 kDa core protein, and either no alteration or a small decrease in the biosynthesis of the collagen-binding small proteoglycan II core protein. These results show that the production of individual members of the small proteoglycan family is differentially regulated.

Ultrastructure of proteoglycans in the tectorial membrane

The ultrastructure of proteoglycans (PGs) in the tectorial membrane (TM) of the mature chinchilla cochlea was investigated using the cationic dye Cuprolinic blue. When used at a high critical electrolyte concentration, Cuprolinic blue has been shown specifically to bind to the glycosaminoglycan residues of sulfated PGs. After Cuprolinic blue treatment, PGs were observed in the TM which were represented as rod-shaped, electron-dense structures. A perifibrillar, primarily orthogonal, array of PGs was associated with the type A protofibrils. These PGs were distributed in 50 nm intervals along the length of the type A protofibrils. A less common orientation was parallel to the axis of the type A protofibrils. PGs did not appear to be associated with the type B protofibrils. Based upon previous results by other investigators, the TM contains types II and IX collagen, and it appears likely that the type A protofibrils are composed of collagen type II. PGs visualized in the TM in this study thus may represent the glycosaminoglycan residue of type IX collagen which is associated with the type II collagen fibrils. Alternatively, the TM PGs may be small dermatan or chondroitin sulfate PGs.

Examination of corneal proteoglycans and glycosaminoglycans by rotary shadowing and electron microscopy

Proteoglycans (PGs) from cornea and their relevant glycosaminoglycan (GAG) chains, dermatan sulphate (DS) and keratin sulphate (KS), were examined by electron microscopy following rotary shadowing, and compared with hyaluronan (HA), chondroitin sulphate (CS), alginate, heparin, heparan sulphate (HS) and methyl cellulose. Corneal DS PG had the tadpole shape previously seen in scleral DS FG, and the images from corneal KS PG could be interpreted similarly, although the GAG (KS) chains were very much fainter than those of DS PG GAG. Isolated GAG (KS, DS, CS, HA, etc.) examined in the same way showed images that decreased very significantly in clarity and contrast, in the sequence HA greater than DS greater than CS greater than KS. The presence of secondary and tertiary structures in the GAGs may be at least partly responsible for these variations. HA appeared to be double stranded, and DS frequently self-aggregated, KS and HS showed tendencies to coil into globular shapes. It is concluded that it is unsafe to assume the absence of GAGs, based on these techniques, and quantitative measurements of length may be subject to error. The results on corneal DS PG confirm and extend the hypothesis that PGs specifically associated with collagen fibrils are tadpole shaped.

Proteoglycan:collagen interactions and subfibrillar structure in collagen fibrils. Implications in the development and ageing of connective tissues

Evidence is presented confirming that collagen fibrils are aggregates of subfibrils. Rat tail tendon fibrils swollen in acetate buffer, pH 5.8, (with or without MgCl2 or Cupromeronic blue, a proteoglycan (PG) precipitant used to locate PGs by electron histochemistry) showed two characteristic populations of subfibrils, of 10-15 nm, and approximately 25 nm thickness respectively. The smaller protofibrils, helically orientated, were derived from, or coalesced into, the larger subfibrils. The results prove that cross-links are not regularly distributed throughout the tissue but are limited to intra-protofibrillar collagen. Cupromeronic blue-stained PGs were seen in thick fibrils from cartilage and tendons, axially orientated in bands across the fibrils in longitudinal sections, or chordally in fibril cross-sections. Based on the intrafibrillar location of PGs vis-à-vis the fibril a-e banding pattern, the relative orientation of PGs and protofibrils and PG distribution in very young, as compared with mature or old, tissue, it is suggested that intrafibrillar PG is originally associated with protofibrils, which coalesce to larger fibrils, e.g. during development. The function of the large amount of PG and hyaluronan in young tissues is seen to keep the collagen protofibrils and subfibrils from coalescing. Disaggregation to protofibrils was demonstrated in conditions not very different from certain in vivo situations. It is suggested that this phenomenon is potentially important during tissue modelling and remodelling, allowing rapid access of reactants to the interior of thick fibrils and recycling of component protofibrils.

Proteoglycans of bovine cementum: isolation and characterization

The proteoglycans associated with the mineralized matrix of bovine cementum have been studied biochemically and their distribution within this tissue localized immunohistochemically. Both hyaluronate and proteoglycans were fractionated by DEAE-Sephacel ion-exchange chromatography. The proteoglycans eluted in three separate peaks of which two contained alkali labile protein associated with glycosaminoglycans, and one appeared as free glycosaminoglycan chains. Analysis of the glycosaminoglycans identified chondroitin sulfate as the predominant species, although minor quantities of dermatan sulfate and heparan sulfate were also identified. Agarose-acrylamide gel electrophoresis and Sepharose CL-6B molecular sieve chromatography of the proteoglycans indicated them to be smaller in size with respect to periodontal ligament and gingival proteoglycans, but similar to bone and dentine proteoglycans. Amino acid analyses indicated subtle differences between cementum and bone proteoglycans. Using a monoclonal antibody (9-A-2) which recognizes the unsaturated disaccharide of chondroitinase ACII-digested glycosaminoglycans, chondroitin sulfate was identified in the pericellular environment within the lacunae housing the cementoblasts as well as in the extracellular matrix of cementum.

Characteristics of the in vitro interaction of a small proteoglycan (PG II) of bovine tendon with type I collagen

Binding of the small dermatan sulfate proteoglycan of bovine tendon (PG II type/decorin-like) to type I collagen was characterized in an in vitro fibril-forming assay, using native collagen prepared from bovine tendon by acid extraction and radiolabeled proteoglycans synthesized by bovine tendon fibroblasts in culture. Substantial binding to collagen was noted for both intact small proteoglycan and core protein from which the glycosaminoglycan chain was removed. However, binding to collagen was minimal for free glycosaminoglycan chains or large proteoglycans. Binding of the small proteoglycan was optimal at approximately physiological conditions of salt concentration and pH. Scatchard analysis showed a binding affinity constant of 3.3 x 10(7) M-1 with 0.054 proteoglycan binding sites/collagen molecule, when about 0.25-6 micrograms proteoglycan was combined with 100 micrograms collagen. Binding to preformed fibrils of native tendon collagen and to pepsin-treated bovine skin collagen was similar to binding to native tendon collagen. Binding occurred in non-ionic detergents at concentrations up to 1% and once bound, the proteoglycan was not released by washing with up to 2 M NaCl. When both PG I and PG II small proteoglycans were added to collagen, only PG II was bound. This difference is not readily explained by differences in disulfide bond position. These studies indicate a strong, specific interaction between type I collagen fibrils and the core protein of the small (PG II) proteoglycan of tendon.

Self-aggregation of bovine skin proteodermatan sulphate promoted by removal of the three N-linked oligosaccharides

Progressive digestion of native bovine skin proteodermatan sulphate with glycopeptidase F (EC. 3.2.2.18), followed by electrophoresis and affinity-blotting with concanavalin A, demonstrated the presence of three N-linked oligosaccharide chains on the protein core. These oligosaccharides were localized to the C-terminal portion of the protein core. Proteodermatan sulphate purified after removal of the oligosaccharides exhibited an altered circular dichroism spectrum and apparently enhanced thermal stability which were explained by the finding that it had aggregated. The aggregates could be partially dissociated by urea. Aggregation could also be demonstrated without intervening preparative steps between digestion with glycopeptidase-F and electrophoresis. Oligosaccharide-free proteodermatan sulphate retained its ability to inhibit fibril formation from monomeric collagen but showed a tendency to self-aggregate in solution. These results suggest a role for the oligosaccharides of proteodermatan sulphate in maintaining the molecule in a predominantly monomeric form in the tissue, thus indirectly promoting its interaction with collagen.

The collagen fibril--a model system for studying the staining and fixation of a protein

A collagen fibril is made up of long rod-like molecules regularly D-staggered with respect to one another. This means that (i) its axially projected fine structure, resolvable to approximately 2 nm in electron micrographs, repeats D-periodically (D = 67 nm), and (ii) the amino acid residues contributing to each element of the fine structure can be inferred from sequence data. Electron-optical data from a fibril D-period can can therefore be correlated directly with chemical data. Such correlations confirm the electrostatic nature of the staining reaction when a fibril is positively stained. After negative staining, the principal factor determining the small-scale distribution of stain is local exclusion by 'bulky' amino acid side-chains. ('Bulkiness' is the average cross-sectional area, or 'plumpness', of a side-chain.) A small superimposed positive staining contribution can also be detected. Fixation of collagen by aldehydes and diimidoesters occurs via an initial reaction with lysyl (and hydroxylsyl) side-chains and alpha-amino groups, followed by secondary cross-linking reactions that differ from fixative to fixative. These secondary reactions determine the nature and abundance of the cross-links and the extent to which they influence subsequent staining behaviour.

Proteoglycan: collagen interactions in dermatosparactic skin and tendon. An electron histochemical study using cupromeronic blue in a critical electrolyte concentration method

Proteoglycans (PGs), stained for electron microscopy with Cupromeronic blue, were observed in skin and tendon from normal and dermatosparactic calves. Very frequently they (i.e. dermatan sulphate (DS) PGs) were seen arrayed orthogonally to the collagen fibrils, in the gap zone, usually at the d band, in both diseased and normal tissues. Where UO2(2+) staining showed regular and normal packing of collagen molecules, orthogonally located DS PGs were seen. No qualitative differences between controls and pathological tissues were identified, but quantitatively it appears likely that considerable areas of the surface of dermatosparactic skin collagen fibrils may be without associated PGs.

Biosynthesis of small proteoglycans by hepatic lipocytes in primary culture

Chondroitin sulfate/dermatan sulfate proteoglycans were obtained from the secretions of cultured rat hepatic lipocytes. The collagen-binding small proteoglycan II represented only a minor species (less than 10%), whereas similar amounts of small proteoglycan I and of a novel collagen-binding proteoglycan with a core protein of 101 kDa were found. These results support the concept of a special role of lipocytes in the pathogenesis of liver fibrosis.

Collagen and proteoglycan in a sea urchin ligament with mutable mechanical properties

The "problematic ligament" of sea urchins is a connective tissue which crosses the ball-and-socket joint between spine and body wall. The problem of this ligament is that it is composed of parallel collagen fibrils, yet normally undergoes rapid and dramatic alterations in mechanical properties and in length. Previous work has suggested that the collagen fibrils of the ligament are able to slide past one another during length changes but are inhibited from sliding when the ligament is in "catch". In this model of the ligament both the collagen fibrils and the interfibrillar matrix are mechanically important. We have found that the collagen fibrils of the spine ligament of the pencil urchin Eucidaris tribuloides are discontinuous and end by tapering within the body of the ligament. Intact fibrils that have been isolated from the ligament vary by more than an order of magnitude in length and in radius but have a constant length/radius (aspect) ratio of about 5,300. This is the first determination of the aspect ratio of collagen fibrils from any source. The constant aspect ratio of the fibrils is consistent with their functioning as the discontinuous fiber phase in a fiber-reinforced composite material, while the high value of the aspect ratio indicates that the nonfibrillar matrix, which must act to transfer stress between fibrils, can produce a stiff and strong ligament even if it is several orders of magnitude weaker and more compliant than the fibrils. Moreover, the tensile properties of the ligament may be determined by the properties of the matrix. A prominent component of the interfibrillar matrix is a proteoglycan which associates with specific bands at the surface of the collagen fibrils through noncovalent binding of its core protein. The glycosaminoglycan moiety of this proteoglycan is partly comprised of chondroitin sulfate/dermatan sulfate polymers. These results are consistent with the "sliding fibril" hypothesis and suggest that the proteoglycan may be an important component of the stress-transfer matrix.

Endocytosis of a small dermatan sulphate proteoglycan. Identification of binding proteins

Endosomal preparations from human osteosarcoma cells and from fibroblasts contain 51,000- and 26,000-Mr proteins which bind a small dermatan sulphate proteoglycan after SDS/polyacrylamide-gel electrophoresis and Western blotting. Binding can be inhibited by unlabelled proteoglycan core protein. The proteins co-precipitate with a proteoglycan core protein-antibody complex. Scatchard analysis of immobilized endosomal proteins yielded a KD of about 37 nM for the proteoglycan. In intact cells proteins of the same size can be found. They are sensitive to trypsinization. A 51,000-Mr protein is the predominant membrane protein with strong binding to immobilized dermatan sulphate proteoglycan. There are additional proteoglycan-binding proteins with Mr values of around 30,000 and 14,000 which are insensitive to trypsin treatment. In contrast with the 51,000- and 26,000-Mr proteins, they resist deoxycholate/Triton X-100 extraction several days after subcultivation.

Proteoglycans of the articular disc of the bovine temporomandibular joint. II. Low molecular weight dermatan sulphate proteoglycan

The low molecular weight proteoglycan fraction extracted from articular discs with 4 M guanidinium chloride was found to consist predominantly of an iduronate-rich dermatan sulphate proteoglycan, together with chondroitin sulphate-containing material. The dermatan sulphate proteoglycan was purified by ion-exchange and gel-filtration chromatography and its core protein isolated after digestion with chondroitinase ABC. The amino acid composition and pattern of cyanogen bromide peptides obtained from this core were closely similar to those of the protein core of bovine skin proteodermatan sulphate. Four monoclonal antibodies raised against bovine skin proteodermatan sulphate also reacted with the disc protein core and its cyanogen bromide peptides. Results of digestion with glycopeptidase F demonstrated the presence of three N-linked oligosaccharides. The combined size of these oligosaccharides appeared to be somewhat less than the size of those on skin proteodermatan sulphate. The glycosaminoglycan chain released by digestion with cathepsin C had a higher molecular weight than that from skin. These differences in glycosylated structures may be responsible for the different effects on collagen fibrillogenesis in vitro; whereas skin proteodermatan sulphate only reduced the rate of fibril growth, disc dermatan sulphate proteoglycan also increased the length of the lag-phase and the final opacity.

The regulation of size and form in the assembly of collagen fibrils in vivo

A possible mechanism for regulating the lateral growth of collagen fibrils in vivo is considered. A growth inhibitor associated with a particular part of the long semiflexible collagen molecule restricts that part of the molecule to the surface of the growing assembly. Lateral accretion ceases when these inhibitors form a complete circumferential layer around the fibril surface. Cell-mediated removal of the inhibitors allows lateral growth to proceed to a second limiting layer, and so on to subsequent limiting layers. In this way, cycles of inhibitor removal and limited lateral accretion permit growth to be synchronized over large populations of fibrils. Observed diameter distributions in bundles of embryonic and neonatal fibrils are those expected from a mechanism of this kind. The mechanism depends on the existence of axial order (D-periodicity) in fibrils, but not on any specific lateral packing of molecules. Rather, contacts between newly assembled molecules are presumed to be partly fluid-like in lateral directions (except where covalent cross-links have formed). Some initial fluidity in lateral packing prior to cross-linking does not preclude the subsequent emergence of quasi-crystalline packing as cross-links form. The cylindrical shape of fibrils in vivo may also be attributable in part to fluidity of intermolecular contacts at the growing surface.

Identification of the core proteins in proteoglycans synthesized by vascular endothelial cells

Proteoglycans, metabolically labelled with [3H]leucine and 35SO4(2-), were isolated from the spent media and from guanidinium chloride extracts of cultured human umbilical-vein endothelial cells by using isopycnic density-gradient centrifugation, gel filtration and ion-exchange h.p.l.c. The major proteoglycan species were subjected to SDS/polyacrylamide-gel electrophoresis before and after enzymic degradation of the polysaccharide chains. The cell extract contained mainly a heparan sulphate proteoglycan that has a buoyant density of 1.31 g/ml and a protein core with apparent molecular mass 300 kDa. The latter was heterogeneous and migrated as one major and one minor band. After reduction, the apparent molecular mass of the major band increased to approx. 350 kDa, indicating the presence of intrachain disulphide bonds. The proteoglycan binds to octyl-Sepharose and its polysaccharide chains are extensively degraded by heparan sulphate lyase. The proteoglycans of the medium contained 90% of all the incorporated 35SO4(2-). Here the predominant heparan sulphate proteoglycan was similar to that of the cell extract, but was more heterogeneous and contained an additional core protein with apparent molecular mass 210 kDa. Furthermore, two different chondroitin sulphate proteoglycans were found: one 200 kDa species with a high buoyant density (approx. 1.45 g/ml) and one 100 kDa species with low buoyant density (approx. 1.3 g/ml). Both these proteoglycans have a core protein of molecular mass approx. 47 kDa.

Collagen fibrillogenesis in situ: fibril segments are intermediates in matrix assembly

The assembly of discontinuous fibril segments and bundles was studied in 14-day chicken embryo tendons by using serial sections, transmission electron microscopy, and computer-assisted image reconstruction. Fibril segments were first found in extracytoplasmic channels, the sites of their polymerization; they also were found within fibril bundles. Single fibril segments were followed over their entire length in consecutive sections, and their lengths ranged from 7 to 15 microns. Structural differences in the ends of the fibril segments were identified, suggesting that the amino/carboxyl polarity of the fibril segment is reflected in its architecture. Our data indicate that fibril segments are precursors in collagen fibril formation, and we suggest that postdepositional fusion of fibril segments may be an important process in tendon development and growth.

Fibronectin and glycosaminoglycan synthesis by fibrotic pig fibroblasts in primary culture

Synthesis of fibronectin and glycosamingoglycans (GAGs) was studied in fibroblasts from pigs with post-irradiation subcutaneous fibrosis. Fibrosis was developed in the femoral muscle by local gamma irradiation with a dose of 60 Gy. Normal fibroblasts were obtained from the healthy skin of the same animal. To measure GAG and fibronectin synthesis fibrotic and normal fibroblasts were labeled with 3H-glucosamine, 35S-sulfate and 35S-methionine. Fibrotic fibroblasts synthesized 2.5 times as much fibronectin as normal skin fibroblasts but total protein synthesis did not change. Parallel enhanced secretion of hyaluronic acid and dermatan sulfate into the cell culture medium were also observed. GAGs from the pericellular layer of trypsin-digested fibrotic fibroblasts exhibited increased 3H incorporation, but reduced 35S-sulfate incorporation. The largest reduction in the latter was observed for heparan sulfate. These results indicate that the fibroblasts from the well developed fibrotic tissue maintain enhanced synthesis of matrix macromolecules in primary cultures. Structural and/or metabolic changes in secreted GAGs, combined with the stimulation of tissue repair by growth factors may be responsible for the excessive deposition of collagen in post-irradiation fibrosis.