Identification of a high-molecular-weight (greater than 400 000) protein in hyaline cartilage

A high-molecular-weight (greater than 400 000) non-collagenous protein has been identified in normal articular cartilage from several mammalian species and in bovine tracheal cartilage. This protein is reduced by 2-mercaptoethanol to subunits with a molecular weight of 116 000, which appear to constitute approx. 2-4% of the total protein detectable by the Lowry assay in 4 M guanidinium chloride extracts of normal bovine and canine articular cartilage. Antiserum to the 116 kDa subunit protein from bovine articular cartilage cross-reacts with the intact and subunit proteins from bovine trachea and from normal canine, porcine and human articular cartilage. This protein is not found in non-cartilagenous tissues, suggesting that it is a cartilage-specific protein. We conclude that the greater than 400 kDa protein and its subunit are ubiquitous and quantitatively significant proteins in hyaline cartilage.     

Topographic localization of a 116,000-dalton protein in cartilage

A disulfide-bonded greater than 400,000-dalton (greater than 400-kD) protein with 116-kD subunits in hyaline cartilage from several species has recently been described. It constitutes 2-4% of the total noncollagenous protein in 4 M guanidinium chloride extracts of normal articular cartilage and accounts for most of the total noncollagen, nonproteoglycan protein synthesized in short-term organ cultures of canine articular cartilage. In the present study, immunofluorescence techniques were used to examine the topographic distribution of the 116-kD subunit protein in normal cartilage. In specimens of normal adult articular cartilage from several species, the protein was located throughout the matrix. More intense staining was observed at the articular surface than in the remainder of the uncalcified cartilage. In contrast, in fetal cartilage, the protein was uniformly distributed throughout the matrix without a marked increase in surface staining. Normal canine menisci and annulus fibrosus also demonstrated moderate fluorescence after incubation with the antiserum to the 116-kD subunit protein. Normal canine nucleus pulposus, synovium, aorta, and monolayer cultures of canine synovial cells exhibited only weak immunofluorescence after incubation with the antiserum. Therefore, the 116-kD subunit protein appears to be a ubiquitous matrix protein in cartilage.     

Identification of link proteins and a 116,000-Dalton matrix protein in canine meniscus

The menisci are collagen-rich, fibrocartilagenous structures which are important in protecting the articular cartilage of the knee from some of the impact of weight-bearing. Meniscal proteoglycans have been studied in several mammalian species, including the dog, but very little is known about the noncollagenous proteins of the menisci. In the present study, 4 M guanidinium chloride extracts of meniscal cartilage from normal adult mongrel dogs were studied, and several noncollagenous proteins, including the link proteins and a 116,000-Da subunit protein, which we have recently described in articular cartilage, were found in meniscal cartilage. The 116,000-Da subunit protein represents 3.8% of the total protein extracted from meniscal cartilage. The link proteins sedimented in the bottom of an associative cesium chloride density gradient, where high-buoyant-density proteoglycans sediment.     

Effects of Hyaluronic Acid and γ–Globulin Concentrations on the Frictional Response of Human Osteoarthritic Articular Cartilage

Synovial fluid plays an important role in lubricating synovial joints. Its main constituents are hyaluronic acid (HA) and γ–globulin, acting as boundary lubricants for articular cartilage. The aim of the study was to demonstrate the concentration-dependent effect of HA and γ–globulin on the boundary-lubricating ability of human osteoarthritis (OA) cartilage. Normal, early and advance stage articular cartilage samples were obtained from human femoral heads and in presence of either HA or γ–globulin, cartilage frictional coefficient (µ) was measured by atomic force microscopy (AFM). In advanced stage OA, the cartilage superficial layer was observed to be completely removed and the damaged cartilage surface showed a higher µ value (∼0.409) than the normal cartilage surface (∼0.119) in PBS. Adsorbed HA and γ–globulin molecules significantly improved the frictional behavior of advanced OA cartilage, while they were ineffective for normal and early OA cartilage. In advanced-stage OA, the concentration-dependent frictional response of articular cartilage was observed with γ–globulin, but not with HA. Our result suggested that HA and γ–globulin may play a significant role in improving frictional behavior of advanced OA cartilage. During early-stage OA, though HA and γ–globulin had no effect on improving frictional behavior of cartilage, however, they might contribute to disease modifying effects of synovial fluid as observed in clinical settings.     

Articular cartilage chondrocytes express aromatase and use enzymes involved in estrogen metabolism

Introduction

Sex hormones, especially estrogens, have been implicated in articular cartilage metabolism and the pathogenesis of postmenopausal osteoarthritis. The conversion by aromatase (CYP19A1) of androstenedione into estrone (E1) and of testosterone into 17β-estradiol (E2) plays a key role in the endogenous synthesis of estrogens in tissue.

Methods

We analyzed the expression of aromatase (CYP19A1) in immortalized C-28/I2 and T/C-28a2 chondrocytes, as well as in cultured primary human articular chondrocytes and human articular cartilage tissue, by means of RT-PCR, Western blotting and immunohistochemistry. By means of quantitative RT-PCR and enzyme-linked immunosorbent assay, we also determined whether the aromatase inhibitor letrozole influences estrogen metabolism of cultured chondrocytes in immortalized C-28/I2 chondrocytes.

Results

Aromatase mRNA was detected in both immortalized chondrocyte cell lines, in cultured primary human chondrocytes, and in human articular cartilage tissue. By means of Western blot analysis, aromatase was detected at the protein level in articular cartilage taken from various patients of both sexes and different ages. Cultured primary human articular chondrocytes, C-28/I2 and T/C-28a2, and human articular cartilage tissue reacted with antibodies for aromatase. Incubation of C-28/I2 chondrocytes with 10⁻¹¹ M to 10⁻⁷ M letrozole as an aromatase inhibitor revealed significantly increased amounts of the mRNAs of the enzyme cytochrome P4501A1 (CYP1A1), which is involved in the catagen estrogen metabolism, and of the estrogen receptors ER-α and ER-β. Concomitantly, synthesis of estrone (E1) was significantly downregulated after incubation with letrozole.

Conclusions

We demonstrate that human articular cartilage expresses aromatase at the mRNA and protein levels. Blocking of estrone synthesis by the aromatase inhibitor letrozole is counteracted by an increase in ER-α and ER-β. In addition, CYP1A1, an enzyme involved in catabolic estrogen metabolism, is upregulated. This suggests that articular chondrocytes use ERs functionally. The role of endogenous synthesized estrogens in articular cartilage health remains to be elucidated.     

Carprofen inhibits the release of matrix metalloproteinases 1, 3, and 13 in the secretome of an explant model of articular cartilage stimulated with interleukin 1β

Introduction

Arthritic diseases are characterized by the degradation of collagenous and noncollagenous extracellular matrix (ECM) components in articular cartilage. The increased expression and activity of matrix metalloproteinases (MMPs) is partly responsible for cartilage degradation. This study used proteomics to identify inflammatory proteins and catabolic enzymes released in a serum-free explant model of articular cartilage stimulated with the pro-inflammatory cytokine interleukin 1β (IL-1β). Western blotting was used to quantify the release of selected proteins in the presence or absence of the cyclooxygenase-2 specific nonsteroidal pro-inflammatory drug carprofen.

Methods

Cartilage explant cultures were established by using metacarpophalangeal joints from horses euthanized for purposes other than research. Samples were treated as follows: no treatment (control), IL-1β (10 ng/ml), carprofen (100 μg/ml), and carprofen (100 μg/ml) + IL-1β (10 ng/ml). Explants were incubated (37°C, 5% CO₂) over twelve day time courses. High-throughput nano liquid chromatography/mass spectrometry/mass spectrometry uncovered candidate proteins for quantitative western blot analysis. Proteoglycan loss was assessed by using the dimethylmethylene blue (DMMB) assay, which measures the release of sulfated glycosaminoglycans (GAGs).

Results

Mass spectrometry identified MMP-1, -3, -13, and the ECM constituents thrombospondin-1 (TSP-1) and fibronectin-1 (FN1). IL-1β stimulation increased the release of all three MMPs. IL-1β also stimulated the fragmentation of FN1 and increased chondrocyte cell death (as assessed by β-actin release). Addition of carprofen significantly decreased MMP release and the appearance of a 60 kDa fragment of FN1 without causing any detectable cytotoxicity to chondrocytes. DMMB assays suggested that carprofen initially inhibited IL-1β-induced GAG release, but this effect was transient. Overall, during the two time courses, GAG release was 58.67% ± 10.91% (SD) for IL-1β versus 52.91% ± 9.35% (SD) with carprofen + IL-1β.

Conclusions

Carprofen exhibits beneficial anti-inflammatory and anti-catabolic effects in vitro without causing any detectable cytotoxicity. Combining proteomics with this explant model provides a sensitive screening system for anti-inflammatory compounds.     

Identification of latexin by a proteomic analysis in rat normal articular cartilage

Background  

Osteoarthritis (OA) is characterized by degeneration of articular cartilage. Animal models of OA induced are a widely used tool in the study of the pathogenesis of disease. Several proteomic techniques for selective extraction of proteins have provided protein profiles of chondrocytes and secretory patterns in normal and osteoarthritic cartilage, including the discovery of new and promising biomarkers. In this proteomic analysis to study several proteins from rat normal articular cartilage, two-dimensional electrophoresis and mass spectrometry (MS) were used. Interestingly, latexin (LXN) was found. Using an immunohistochemical technique, it was possible to determine its localization within the chondrocytes from normal and osteoarthritic articular cartilage.     

Skeletal Characterization of Smurf2-Deficient Mice and In Vitro Analysis of Smurf2-Deficient Chondrocytes

Overexpression of Smad ubiquitin regulatory factor 2 (Smurf2) in chondrocytes was reported to cause spontaneous osteoarthritis (OA) in mice. However, it is unclear whether Smurf2 is involved in bone and cartilage homeostasis and if it is required for OA pathogenesis. Here we characterized age-related changes in the bone and articular cartilage of Smurf2-deficient (MT) mice by microCT and histology, and examined whether reduced Smurf2 expression affected the severity of OA upon surgical destabilization of the medial meniscus (DMM). Using immature articular chondrocytes (iMAC) from MT and wild-type (WT) mice, we also examined how Smurf2 deficiency affects chondrogenic and catabolic gene expressions and Smurf2 and Smurf1 proteins upon TGF-β3 or IL-1β treatment in culture. We found no differences in cortical, subchondral and trabecular bone between WT and MT in young (4 months) and old mice (16–24 months). The articular cartilage and age-related alterations between WT and MT were also similar. However, 2 months following DMM, young MT showed milder OA compared to WT (~70% vs ~30% normal or exhibiting only mild OA cartilage phenotype). The majority of the older WT and MT mice developed moderate/severe OA 2 months after DMM, but a higher subset of aged MT cartilage (27% vs. 9% WT) remained largely normal. Chondrogenic gene expression (Sox9, Col2, Acan) trended higher in MT iMACs than WT with/without TGF-β3 treatment. IL-1β treatment suppressed chondrgenic gene expression, but Sox9 expression in MT remained significantly higher than WT. Smurf2 protein in WT iMACs increased upon TGF-β3 treatment and decreased upon IL-1β treatment in a dose-dependent manner. Smurf1 protein elevated more in MT than WT upon TGF-β3 treatment, suggesting a potential, but very mild compensatory effect. Overall, our data support a role of Smurf2 in regulating OA development but suggest that inhibiting Smurf2 alone may not be sufficient to prevent or consistently mitigate post-traumatic OA across a broad age range.     

A comparison of isometallothionein synthesis in rat liver after partial hepatectomy and parenteral zinc injection.

Proteoglycan aggregates free of non-aggregating proteoglycan have been prepared from the annuli fibrosi and nuclei pulposi of intervertebral discs of three human lumbar spines by extraction with 4M-guanidinium chloride, associative density gradient centrifugation, and chromatography on Sepharose CL-2B. The aggregate (A1-2B.V0) was subjected to dissociative density-gradient ultracentrifugation. Three proteins of Mr 38 900, 44 200 and 50 100 found in the fraction of low buoyant density (A1-2B.V0-D4) reacted with antibodies to link protein from newborn human articular cartilage. After reduction with mercaptoethanol, two proteins of Mr 43 000 and two of Mr 20 000 and 14 000 were seen. The A1-2B.V0-D4 fraction, labelled with 125I, coeluted with both hyaluronate and a hyaluronate oligosaccharide (HA14) on a Sepharose CL-2B column. HA10 and HA14 reduced the viscosity of A1 fractions; HA4, HA6 and HA8 did not. HA14 decreased the viscosity of disc proteoglycans less than it did that of bovine cartilage proteoglycans. Thus, although a link protein was present in human intervertebral disc, it stabilized proteoglycan aggregates less well than did the link protein from bovine nasal cartilage.     

Integrin α2β1 Is a Receptor for the Cartilage Matrix Protein Chondroadherin

Chondroadherin (the 36-kD protein) is a leucine-rich, cartilage matrix protein known to mediate adhesion of isolated chondrocytes. In the present study we investigated cell surface proteins involved in the interaction of cells with chondroadherin in cell adhesion and by affinity purification. Adhesion of bovine articular chondrocytes to chondroadherin-coated dishes was dependent on Mg²⁺ or Mn²⁺ but not Ca²⁺. Adhesion was partially inhibited by an antibody recognizing β1 integrin subunit. Chondroadherin-binding proteins from chondrocyte lysates were affinity purified on chondroadherin-Sepharose. The β1 integrin antibody immunoprecipitated two proteins with molecular mass ~110 and 140 kD (nonreduced) from the EDTA-eluted material. These results indicate that a β1 integrin on chondrocytes interacts with chondroadherin. To identify the α integrin subunit(s) involved in interaction of cells with the protein, we affinity purified chondroadherin-binding membrane proteins from human fibroblasts. Immunoprecipitation of the EDTA-eluted material from the affinity column identified α2β1 as a chondroadherin-binding integrin. These results are in agreement with cell adhesion experiments where antibodies against the integrin subunit α2 partially inhibited adhesion of human fibroblast and human chondrocytes to chondroadherin. Since α2β1 also is a receptor for collagen type II, we tested the ability of different antibodies against the α2 subunit to inhibit adhesion of T47D cells to collagen type II and chondroadherin. The results suggested that adhesion to collagen type II and chondroadherin involves similar or nearby sites on the α2β1 integrin. Although α2β1 is a receptor for both collagen type II and chondroadherin, only adhesion of cells to collagen type II was found to mediate spreading.     

Ultrastructural histochemistry of the surface lamina of normal articular cartilage

Summary Two collagen-poor, ultramicroscopic layers are described at the surface of canine articular cartilage. They are distinguished by staining with an electron-dense cationic dye, Cupromeronic Blue, in a critical electrolyte concentration technique and by digestion with testicular hyaluronidase. The superficial layer, approximately 50 nm thick, stained at low electrolyte concentrations but failed to stain in conditions specific for sulphated glycosaminoglycans. It was hyaluronidase-resistant and may be either glycoprotein or protein in nature. The deeper layer, 100–400 nm thick, stained positively at electrolyte concentrations specific for sulphated glycosaminoglycans but not in conditions specific for keratan sulphate. It was removed by hyaluronidase digestion. This layer probably represents a chondroitin sulphate-rich proteoglycan.These surface layers may be important in the lubrication of the articular surface and in the permeability and compression resistance of the superficial cartilage zone.     

Transient anabolic effects accompany epidermal growth factor receptor signal activation in articular cartilage in vivo

Introduction

Signals from the epidermal growth factor receptor (EGFR) have typically been considered to provide catabolic activities in articular cartilage, and accordingly have been suggested to have a causal role in osteoarthritis progression. The aim of this study was to determine in vivo roles for endogenous EGFR signal activation in articular cartilage.

Methods

Transgenic mice with conditional, limb-targeted deletion of the endogenous intracellular EGFR inhibitor Mig-6 were generated using CreLoxP (Mig-6-flox; Prx1Cre) recombination. Histology, histochemical staining and immunohistochemistry were used to confirm activation of EGFR signaling in the articular cartilage and joints, and to analyze phenotypic consequences of Mig-6 loss on articular cartilage morphology, proliferation, expression of progenitor cell markers, presence of chondrocyte hypertrophy and degradation of articular cartilage matrix.

Results

The articular cartilage of Mig-6-conditional knockout (Mig-6-cko) mice was dramatically and significantly thicker than normal articular cartilage at 6 and 12 weeks of age. Mig-6-cko articular cartilage contained a population of chondrocytes in which EGFR signaling was activated, and which were three to four times more proliferative than normal Mig-6-flox articular chondrocytes. These cells expressed high levels of the master chondrogenic regulatory factor Sox9, as well as high levels of putative progenitor cell markers including superficial zone protein (SZP), growth and differentiation factor-5 (GDF-5) and Notch1. Expression levels were also high for activated β-catenin and the transforming growth factor beta (TGF-β) mediators phospho-Smad2/3 (pSmad2/3). Anabolic effects of EGFR activation in articular cartilage were followed by catabolic events, including matrix degradation, as determined by accumulation of aggrecan cleavage fragments, and onset of hypertrophy as determined by type × collagen expression. By 16 weeks of age, the articular cartilage of Mig-6-cko knees was no longer thickened and was degenerating.

Conclusions

These results demonstrate unexpected anabolic effects of EGFR signal activation in articular cartilage, and suggest the hypothesis that these effects may promote the expansion and/or activity of an endogenous EGFR-responsive cell population within the articular cartilage.     

Identification of link proteins in canine synovial cell cultures and canine articular cartilage

Link proteins are glycoproteins in cartilage that are involved in the stabilization of aggregates of proteoglycans and hyaluronic acid. We have identified link proteins in synovial cell cultures form normal canine synovium using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunofluorescence, and immunolocation with specific antibodies by electrophoretic transfer. We have also found evidence for the synthesis of link proteins in these cultures by fluorography of radiolabeled synovial cell extracts. We have identified a 70,000 mol-wt protein in canine synovial cell culture extracts that has antigenic cross-reactivity with the 48,000-mol-wt link protein. Three link proteins were identified in normal canine articular cartilage. These results indicate that link proteins are more widely distributed in connective tissues than previously recognized and may have biological functions other than aggregate stabilization.     

The phosphorylation of human link proteins

Three link proteins of 48,44 and 40 kDa were purified from human articular cartilage and identified with monoclonal anti-link protein antibody 8-A-4. Two sets of lower molecular weight proteins of 30-31 kDa and 24-26 kDa also contained link protein epitopes recognized by the monoclonal antibody and were most likely degradative products of the intact link proteins. The link proteins of 48 and 40 kDa were identified as phosphoproteins while the 44 kDa link protein did not contain 32P. The phosphorylated 48 and 40 kDa link proteins contained approximately 2 moles PO4/mole link protein.     

Specific Changes of Serum Proteins in Parkinson's Disease Patients

The aim of this study is to identify and validate protein change in the serum from PD patients. We used serum samples from 21 PD patients and 20 age-matched normal people as control to conduct a comparative proteomic study. We performed 2-DE and analyzed the differentially expressed protein spots by LC-MS/MS. In PD group 13 spots were shown to be differentially expressed compared to control group. They were identified as 6 proteins. Among these, 3 proteins were confirmed by Western blot analysis. It showed that the frequency of fibrinogen γ-chain (FGG) appeared 70% in PD, which could not be detected in control group. The protein of inter-alpha-trypsin inhibitor heavy chain H4 (ITI-H4) was found to exist two forms in serum. The full size (120 kDa) of the protein was increased and the fragmented ITI-H4 (35 kDa) was decreased in PD group. The ratio of full size ITI-H4 to fragmented ITI-H4 in PD patients was 3.85±0.29-fold higher than in control group. Furthermore, fragmented Apo A-IV (∼26 kDa) was mainly detected in control group, while it was rare to be found in PD group. Above findings might be useful for diagnosis of PD. When the expressions of FGG and 120 kDa ITI-H4 are increase, as well as ∼26 kDa Apo A-IV disappear would provide strong evidence for PD.     

Distribution,Molecular Structure and Functional Analysis of Carnitine Transporter (SLC22A5) in Canine Lens Epithelial Cells

While carnitine has been reported to have an anti-oxidative role on the ocular surface, there has been no report on the existence of a carnitine transporter (SLC22A5) in the lens. Therefore, we investigated the carnitine transport activity of canine lens epithelial cells (LEC) and determined the molecular structure of canine SLC22A5. The carnitine transport activity was 7.16 ± 0.48 pmol/mg protein/30 min. Butyrobetaine, the analogue of carnitine, reduced 30% of the activity at 50 µM. A coding sequence of canine carnitine transporter was 1694 bp long and was predicted to encode 557 amino acid polypeptides. The deduced amino acid sequence of canine carnitine transporter showed >80% similarity to that of mouse and human. Western blot analysis detected the band at 60 kDa in the membrane of lens epithelial cells. The high content of carnitine in the lens is possibly transported from aqueous humor by SLC22A5.     

Temperature affects transport of polysaccharides and proteins in articular cartilage explants

Solute transport phenomena mediate many aspects of the physiology and contrast agent-based clinical imaging of articular cartilage. Temperatures up to 10°C below standard body temperature (37°C) are common in articulating joints during normal activities and clinically (e.g. cold treatment of injuries). Therefore it is of interest to characterize the effects of temperature changes on solute transport parameters in cartilage. A range of fluorescent solutes including fluorescein isothiocyanate, 4 and 40kDa dextrans, myoglobin, insulin and chondroitin sulfate were prepared and used in assays of solute effective partition coefficient and effective diffusivity in bovine intermediate zone articular cartilage explants maintained at 10, 22 or 37°C. Trends for increasing partition coefficient with increasing temperature were evident for all solutes except chondroitin sulfate, with significant changes between 22 and 37°C for 4kDa dextran, insulin and myoglobin. Diffusivities of most solutes tested also tended to increase with increasing temperature, with significant changes between 10 and 22°C for FITC, 40kDa dextran and myoglobin. Oddly, insulin diffusivity decreased significantly as temperature increased from 22 to 37°C while chondroitin sulfate diffusivity exhibited no clear temperature dependence. These results highlight solute-specific temperature dependences of transport phenomena which may depend upon molecular weight, chemical structure, molecular conformation, and solute-matrix and solute-solute interactions. The articular cartilage explants themselves exhibited small but significant changes in water and glycosaminoglycan contents during experiments, underscoring the importance of solute-matrix interactions. Solute transport parameters in cartilage and their temperature dependences are therefore not easily predicted, and case-by-case experimental determination may be essential.     

Characterization of the dermatan sulfate proteoglycans, DS-PGI and DS-PGII, from bovine articular cartilage and skin isolated by octyl-sepharose chromatography

Two forms of dermatan sulfate proteoglycans, called DS-PGI and DS-PGII, have been isolated from both bovine fetal skin and calf articular cartilage and characterized. The proteoglycans were isolated using either (a) molecular sieve chromatography under conditions where DS-PGI selectively self-associates or (b) chromatography on octyl-Sepharose, which separates DS-PGI from DS-PGII based on differences in the hydrophobic properties of their core proteins. The NH2-terminal amino acid sequence of DS-PGI from skin and cartilage is identical. The NH2-terminal amino acid sequence of DS-PGII from skin and cartilage is identical. However, the amino acid sequence data and tryptic peptide maps demonstrate that the core proteins of DS-PGI and DS-PGII differ in primary structure. In DS-PGI from bovine fetal skin, 81-84% of the glycosaminoglycan was composed of IdoA-GalNAc(SO4) disaccharide repeating units. In DS-PGI from calf articular cartilage, only 25-29% of the glycosaminoglycan was composed of IdoA-GalNAc(SO4). In DS-PGII from bovine fetal skin, 85-93% of the glycosaminoglycan was IdoA-GalNAc(SO4), whereas in DS-PGII from calf articular cartilage, only 40-44% of the glycosaminoglycan was IdoA-GalNAc(SO4). Thus, analogous proteoglycans from two different tissues, such as DS-PGI from skin and cartilage, possess a core protein with the same primary structure, yet contain glycosaminoglycan chains which differ greatly in iduronic acid content. These differences in the composition of the glycosaminoglycan chains must be determined by tissue-specific mechanisms which regulate the degree of epimerization of GlcA-GalNAc(SO4) into IdoA-GalNAc(SO4) and not by the primary structure of the core protein.     

Species-common antigen of connective tissues.

Collagen-free extracts were prepared from bovine, porcine and canine hyaline, elastic and fibrous cartilages, articular capsule, tendon, aorta, cortical bone and regenerating articular surfaces. The extracts were investigated with antisera to bovine nasal septal cartilage, dog articular cartilage and non-collagenous protein fraction of bovine cortical bone. Immunodiffusion, immunoelectrophoresis, and immunohistochemical methods were used. In the different supporting tissues of the three animal species a common antigen, probably of proteoglycan origin, was demonstrated. The finer differences in antigenicity between the different tissues are probably due to the variations in proteoglycan composition of the given supporting tissues. Owing to the wide-spread occurrence of the antigen, the authors suggest the term "species-common connective tissue antigen" instead of the "species-common cartilage antigen" used so far.     

Identification of a hyaluronic acid-binding protein that interferes with the preparation of high-buoyant-density proteoglycan aggregates from adult human articular cartilage.

Adult human articular cartilage contains a hyaluronic acid-binding protein of Mr 60 000-75 000, which contains disulphide bonds essential for this interaction. The molecule can compete with proteoglycan subunits for binding sites on hyaluronic acid, and can also displace proteoglycan subunits from hyaluronic acid if their interaction is not stabilized by the presence of link proteins. The abundance of this protein in the adult accounts for the reported inability to prepare high-buoyant-density proteoglycan aggregates from extracts of adult human cartilage [Roughley, White, Poole & Mort (1984) Biochem. J. 221, 637-644], whereas the deficiency of the protein in newborn human cartilage allows the normal recovery of proteoglycan aggregates from this tissue. The protein shares many common features with a hyaluronic acid-binding region derived by proteolytic treatment of a proteoglycan aggregate preparation, and this may also represent its origin in the cartilage, with its production increasing during tissue maturation.