Turnover of proteoglycans in articular-cartilage cultures. Characterization of proteoglycans released into the medium

IL-1 in osteoarthritis: time for a critical review of the literature

The concept of interleukin-1 (IL-1) as a target in osteoarthritis (OA) has been an attractive one for many years. It is a highly potent inducer of cartilage degradation, causing the induction of mRNA and controlling the bioavailability of disease-relevant proteases such as ADAMTS5 and MMP13. It drives synovitis and can induce other disease-relevant genes such as nerve growth factor, a key pain sensitiser in OA. However, the quality of evidence for its involvement in disease is modest. Descriptive studies have demonstrated expression of IL-1α and β in OA cartilage and elevated levels in the synovial fluid of some patients. Agnostic transcriptomic and genomic analyses do not identify IL-1 as a key pathway. In vivo models show a conflicting role for this molecule; early studies using therapeutic approaches in large animal models show a benefit, but most murine studies fail to demonstrate protection where the ligands (IL-1α/β), the cytokine activator (IL-1–converting enzyme), or the receptor (IL-1R) have been knocked out. Recently, a number of large double-blind randomised controlled clinical studies targeting IL-1 have failed. Enthusiasm for IL-1 as a target in OA is rapidly dwindling.

A novel organ culture model of a joint for the evaluation of static and dynamic load on articular cartilage

OBJECTIVES

The purpose of this study was to create a novel ex vivo organ culture model for evaluating the effects of static and dynamic load on cartilage.

METHODS

The metatarsophalangeal joints of 12 fresh cadaveric bovine feet were skinned and dissected aseptically, and cultured for up to four weeks. Dynamic movement was applied using a custom-made machine on six joints, with the others cultured under static conditions. Chondrocyte viability and matrix glycosaminoglycan (GAG) content were evaluated by the cell viability probes, 5-chloromethylfluorescein diacetate (CMFDA) and propidium iodide (PI), and dimethylmethylene blue (DMMB) assay, respectively.

RESULTS

Chondrocyte viability in the static model decreased significantly from 89.9% (sd 2.5%) (Day 0) to 66.5% (sd 13.1%) (Day 28), 94.7% (sd 1.1%) to 80. 9% (sd 5.8%) and 80.1% (sd 3.0%) to 46.9% (sd 8.5%) in the superficial quarter, central half and deep quarter of cartilage, respectively (p < 0.001 in each zone; one-way analysis of variance). The GAG content decreased significantly from 6.01 μg/mg (sd 0.06) (Day 0) to 4.71 μg/mg (sd 0.06) (Day 28) (p < 0.001; one-way analysis of variance). However, with dynamic movement, chondrocyte viability and GAG content were maintained at the Day 0 level over the four-week period without a significant change (chondrocyte viability: 92.0% (sd 4.0%) (Day 0) to 89.9% (sd 0.2%) (Day 28), 93.1% (sd 1.5%) to 93.8% (sd 0.9%) and 85.6% (sd 0.8%) to 84.0% (sd 2.9%) in the three corresponding zones; GAG content: 6.18 μg/mg (sd 0.15) (Day 0) to 6.06 μg/mg (sd 0.09) (Day 28)).

CONCLUSION

Dynamic joint movement maintained chondrocyte viability and cartilage GAG content. This long-term whole joint culture model could be of value in providing a more natural and controlled platform for investigating the influence of joint movement on articular cartilage, and for evaluating novel therapies for cartilage repair.Cite this article: Y-C. Lin, A. C. Hall, A. H. R. W. Simpson. A novel organ culture model of a joint for the evaluation of static and dynamic load on articular cartilage. Bone Joint Res 2018;7:205-212. DOI: 10.1302/2046-3758.73.BJR-2017-0320.

An enzyme-sensitive PEG hydrogel based on aggrecan catabolism for cartilage tissue engineering

A new cartilage-specific degradable hydrogel based on photoclickable thiol-ene poly(ethylene glycol) (PEG) hydrogels is presented. The hydrogel crosslinks are composed of the peptide, CRDTEGE-ARGSVIDRC, derived from the aggrecanase-cleavable site in aggrecan. This new hydrogel is evaluated for use in cartilage tissue engineering by encapsulating bovine chondrocytes from different cell sources (skeletally immature (juvenile) and mature (adult) donors and adult cells stimulated with proinflammatory lipopolysaccharide (LPS)) and culturing for 12 weeks. Regardless of cell source, a twofold decrease in compressive modulus is observed by 12 weeks, but without significant hydrogel swelling indicating limited bulk degradation. For juvenile cells, a connected matrix rich in aggrecan and collagen II, but minimal collagens I and X is observed. For adult cells, less matrix, but similar quality, is deposited. Aggrecanase activity is elevated, although without accelerating bulk hydrogel degradation. LPS further decreases matrix production, but does not affect aggrecanase activity. In contrast, matrix deposition in the nondegradable hydrogels consists of aggrecan and collagens I, II, and X, indicative of hypertrophic cartilage. Lastly, no inflammatory response in chondrocytes is observed by the aggrecanase-sensitive hydrogels. Overall, it is demonstrated that this new aggrecanase-sensitive hydrogel, which is degradable by chondrocytes and promotes a hyaline-like engineered cartilage, is promising for cartilage regeneration.

Chemical changes demonstrated in cartilage by synchrotron infrared microspectroscopy in an antibody-induced murine model of rheumatoid arthritis

Collagen antibody-induced arthritis develops in mice following passive transfer of monoclonal antibodies (mAbs) to type II collagen (CII) and is attributed to effects of proinflammatory immune complexes, but transferred mAbs may react directly and damagingly with CII. To determine whether such mAbs cause cartilage damage in vivo in the absence of inflammation, mice lacking complement factor 5 that do not develop joint inflammation were injected intravenously with two arthritogenic mAbs to CII, M2139 and CIIC1. Paws were collected at day 3, decalcified, paraffin embedded, and 5-μm sections were examined using standard histology and synchrotron Fourier-transform infrared microspectroscopy (FTIRM). None of the mice injected with mAb showed visual or histological evidence of inflammation but there were histological changes in the articular cartilage including loss of proteoglycan and altered chondrocyte morphology. Findings using FTIRM at high lateral resolution revealed loss of collagen and the appearance of a new peak at 1635 cm(-1) at the surface of the cartilage interpreted as cellular activation. Thus, we demonstrate the utility of synchrotron FTIRM for examining chemical changes in diseased cartilage at the microscopic level and establish that arthritogenic mAbs to CII do cause cartilage damage in vivo in the absence of inflammation.

Tumor necrosis factor alpha-dependent proinflammatory gene induction is inhibited by cyclic tensile strain in articular chondrocytes in vitro

OBJECTIVE

To understand the intracellular mechanisms of the action of mechanical strain on articular chondrocytes during inflammation.

METHODS

One of the major mediators responsible for cartilage destruction in inflamed articular joints is tumor necrosis factor alpha (TNFalpha). Therefore, in this study we examined the intracellular mechanisms of actions of cyclic tensile strain (CTS) on the recombinant human TNFalpha (rHuTNFalpha)-induced proinflammatory pathways in primary cultures of chondrocytes. The expression of messenger RNA (mRNA) for TNFalpha-dependent proinflammatory proteins was examined by semiquantitative reverse transcriptase-polymerase chain reaction. The synthesis of proinflammatory proteins was examined by Western blot analysis in cell extracts, followed by semiquantitative measurement of bands using densitometric analysis. Nitric oxide production was measured by Griess reaction, and prostaglandin E2 production was assessed by radioimmunoassays. The proteoglycan synthesis in chondrocytes was assessed by incorporation of Na2(35)SO4 in chondroitin sulfate proteoglycans.

RESULTS

By exposing chondrocytes to CTS in the presence of TNFalpha in vitro, we showed that CTS is an effective antagonist of TNFalpha actions and acts as both an antiinflammatory signal and a reparative signal. CTS of low magnitude suppresses TNFalpha-induced mRNA expression of multiple proinflammatory proteins involved in catabolic responses, such as inducible nitric oxide synthase, cyclooxygenase 2, and collagenase. CTS also counteracts cartilage degradation by augmenting induction of tissue inhibitor of metalloproteinase 2. Additionally, CTS augments the reparative process via abrogation of TNFalpha-induced suppression of proteoglycan synthesis. Nonetheless, CTS acts on chondrocytes in a TNFalpha-dependent manner, since exposure of chondrocytes to CTS alone had no effect on these parameters.

CONCLUSION

CTS of low magnitude acts as an effective antagonist of TNFalpha not only by inhibiting the TNFalpha-dependent induction of proinflammatory proteins upstream of mRNA transcription, but also by augmenting the proteoglycan synthesis that is inhibited by TNFalpha.

Recombinant human aggrecan G1-G2 exhibits native binding properties and substrate specificity for matrix metalloproteinases and aggrecanase

A recombinant human aggrecan G1-G2 fragment comprising amino acids Val(1)-Arg(656) has been expressed in Sf21 cells using a baculovirus expression system. The recombinant G1-G2 (rG1-G2) was purified to homogeneity by hyaluronan-Sepharose affinity chromatography followed by high performance liquid chromatography gel filtration, and gave a single band of M(r) 90,000-95,000 by silver stain or immunoblotting with monoclonal antibody 1-C-6. The expressed G1-G2 bound to both hyaluronan and link protein indicating that the immunoglobulin-fold motif and proteoglycan tandem repeat loops of the G1 domain were correctly folded. Further analysis of secondary structure by rotary shadowing electron microscopy confirmed a double globe appearance, but revealed that the rG1-G2 was more compact than its native counterpart. The size of rG1-G2 by SDS-polyacrylamide gel electorphoresis was unchanged following digestion with keratanase and keratanase II and reduced by only 2-5 kDa following digestion with either O-glycosidase or N-glycosidase F. Recombinant G1-G2 was digested with purified matrix metalloproteinases (MMP), isolated aggrecanase, purified atrolysin C, or proteinases present in conditioned medium from cartilage explant cultures, and the products analyzed on SDS gels by silver stain and immunoblotting. Neoepitope antibodies recognizing the N-terminal F(342)FGVG or C-terminal DIPEN(341) sequences were used to confirm MMP cleavage at the Asn(341) downward arrow Phe bond, while neoepitope antibodies recognizing the N-terminal A(374)RGSV or C-terminal ITEGE(373) sequences were used to confirm aggrecanase cleavage at the Glu(373) downward arrow Ala bond. Cleavage at the authentic MMP and aggrecanase sites revealed that these proteinases have the same specificity for rG1-G2 as for native aggrecan. Incubation of rG1-G2 with conditioned medium from porcine cartilage cultures revealed that active soluble aggrecanase but no active MMPs, was released following stimulation with interleukin-1alpha or retinoic acid. Atrolysin C, which cleaves native bovine aggrecan at both the aggrecanase and MMP sites, efficiently cleaved rG1-G2 at the aggrecanase site but failed to cleave at the MMP site. In contrast, native glycosylated G1-G2 with or without keratanase treatment was cleaved by atrolysin C at both the aggrecanase and MMP sites. The results suggest that the presence or absence per se of keratan sulfate on native G1-G2 does not affect the activity of atrolysin C toward the two sites.

Characterization of aggrecan retained and lost from the extracellular matrix of articular cartilage. Involvement of carboxyl-terminal processing in the catabolism of aggrecan

The catabolism of aggrecan in bovine articular cartilage explants is characterized by the release into the culture medium of high molecular weight aggrecan fragments, generated by the proteolytic cleavage of the core protein between residues Glu373 and Ala374 within the interglobular domain. In this study, the position of the carboxyl-terminus of these aggrecan fragments, as well as a major proteolytically shortened aggrecan core protein present in cartilage matrix, have been deduced by characterizing the peptides generated by the reaction of aggrecan core protein peptides with cyanogen bromide. It was shown that two out of three such peptide fragments having an amino terminus starting at Ala374 have their carboxyl terminus located within the chondroitin sulfate 1 domain. The third and largest aggrecan core protein peptide, with an amino terminus starting at Ala374, has a carboxyl terminus in a region of core protein between the chondroitin sulfate 1 domain and the chondroitin sulfate 2 domain. The carboxyl terminus of this peptide appeared to be the same as that of the proteolytically degraded aggrecan core protein, which is retained within the extracellular matrix of the tissue. Another two aggrecan fragments recovered from the medium of explant cultures with amino-terminal sequences in the chondroitin sulfate 2 domain at Ala1772 and Leu1872 were shown to have their carboxyl termini within the G3 globular domain. These results suggest that the catabolism of aggrecan between residues Glu373 and Ala374 in the interglobular domain by the putative proteinase, aggrecanase, may be dependent on prior proteolytic processing within the carboxyl-terminal region of the core protein.

Pharmacological effect of tetracyclines on proteoglycanases from interleukin-1-treated articular cartilage

Based on previous in vivo and in situ studies showing that tetracyclines possess antidegenerative effects on cartilage in conjunction with a reduced proteoglycan (PG) loss from the extracellular matrix, we investigated the effects of doxycycline, minocycline and tetracycline on the degradation and biosynthesis of PGs by bovine articular cartilage explants, both in vitro and in situ. Doxycycline, minocycline and tetracycline dose dependently, although weakly, inhibited PG degrading matrix metalloproteinases (MMPs) in vitro, when tested at concentrations ranging from 1 to 100 microM. Ro 31-4724 proved to be a potent inhibitor of MMP proteoglycanases (IC50 value 1.5 nM). Only at a concentration of 100 microM did doxycycline and minocycline significantly inhibit the interleukin-1 (IL-1)-induced augmentation of PG loss from cartilage explants into the nutrient media. The tetracyclines did not modulate the IL-1-mediated reduced aggregability of PGs, whereas 10 microM Ro 31-4724 partially restored the aggregability of PGs ex vivo. Tetracycline even at this high concentration was ineffective. Compared to the effects of the MMP inhibitor Ro 31-4724, treatment with tetracyclines at therapeutic serum levels of 1 or 10 microM was minimal, with little or no effect on cartilage proteoglycanases and PG biosynthesis. In our experiments, tetracyclines and Ro 31-4724 at doses evaluated had no cytotoxic effects on chondrocytes.

The proteoglycan metabolism of mature bovine articular cartilage explants superimposed to continuously applied cyclic mechanical loading

This study describes the effect of load magnitude, frequency and duration on proteoglycan (PG) biosynthesis and loss in mature bovine articular cartilage explants. Cultured full thickness cartilage discs were subjected to a continuously applied, uniaxial compressive cyclic load. The loads were applied using a sinusoidal waveform of 0.001, 0.01, 0.1 or 0.5 Hz-frequency and a peak stress of 0.1, 1.0, 2.5, or 5.0 MPa for a period of 1, 3 or 6 days. Increasing the load magnitude, as well as the duration of loading, reduced the PG biosynthesis. Reducing the load frequency abolished the inhibitory effect of a given load magnitude on PG biosynthesis, even though explants were more compressed. Increasing the load magnitude stimulated the release of newly synthesized PGs from explants, whereas an elevated duration of loading significantly decreased the release of endogenous PGs. Explants loaded for 1 or 3 days were viable as determined biochemically, whereas 6 days of loading resulted in a slightly diminished viability of explants. This study demonstrates that the duration and intensity of loading influences the inhibition of PG biosynthesis, while PG loss is only modulated by the magnitude and duration of loading.

The effect of strenuous versus moderate exercise on the metabolism of proteoglycans in articular cartilage from different weight-bearing regions of the equine third carpal bone

Articular cartilage degeneration in the middle carpal joint is a common problem in racing horses. This study evaluated the effect of exercise on the in-vitro synthesis of the large aggregating proteoglycans (aggrecan) and two small proteoglycans, biglycan and decorin, in articular cartilage taken from three weight bearing regions of the third carpal bone of horses which were subjected to moderate or strenuous exercise. Twelve Standardbred horses free from clinical and radiographic disease of the middle carpal joint were subjected to an 8 week moderate exercise program. The horses were then randomly assigned to two groups: group A--continued moderate exercise and group B--strenuous exercise for 17 weeks. Horses were then rested for 16 weeks. Full-depth articular cartilage explants from the dorsal radial facet (DRF), dorsal intermediate facet (DIF) and palmar condyle (PC) of the third carpal bone were collected and cultured. Cartilage proteoglycan content and release into culture media were measured. Newly synthesized proteoglycans were labeled with 35SO4(2-) for 48 h and analyzed by size exclusion and hydrophobic chromatography, sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) and autoradiography. Histologic sections of adjacent osteochondral regions were evaluated for evidence of arthritic change. No histologic abnormalities or differences in proteoglycan content were detected in any of the articular cartilage regions examined. There was however, a significant reduction (P < 0.05) in aggrecan synthesis and a concomitant increase in decorin synthesis (P < 0.05) in articular cartilage from the DRF of group B animals. There was no change in biglycan synthesis, aggrecan hydrodynamic size or ability to aggregate in any articular cartilage region. This study has demonstrated that strenuous exercise in horses can lead to a disturbance in the biosynthesis of proteoglycans in articular cartilage regions subjected to high contact stresses (DRF). These metabolic abnormalities, which persisted for 16 weeks after cessation of exercise, could have deleterious effects on the biomechanical properties of the tissue. We suggest that the observed alteration in articular cartilage metabolism in CRF cartilage of strenuously exercised horses could represent a predisposing factor for cartilage degeneration and osteoarthritis at a later stage.

The chondrocyte, architect of cartilage. Biomechanics, structure, function and molecular biology of cartilage matrix macromolecules

Chondrocytes are specialised cells which produce and maintain the extracellular matrix of cartilage, a tissue that is resilient and pliant. In vivo, it has to withstand very high compressive loads, and that is explicable in terms of the physico-chemical properties of cartilage-specific macromolecules and with the movement of water and ions within the matrix. The functions of the cartilage-specific collagens, aggrecan (a hydrophilic proteoglycan) and hyaluronan are discussed within this context. The structures of cartilage collagens and proteoglycans and their genes are known and a number of informative mutations have been identified. In particular, collagen fibrillogenesis is a complex process which can be altered by mutations whose effects fit what is known about collagen molecular structural functions. In other instances, mutations have indicated new functions for particular molecular domains. As cartilage provides the template for the developing skeleton, mutations in genes for cartilage-specific proteins often produce developmental abnormalities. The search for mutations amongst such genes in heritable disorders is being actively pursued by many groups, although mutation and phenotype are not always well correlated, probably because of compensatory mechanisms. The special nature of the chondrocyte is stressed in connection with its cell involvement in osteoarthritis, the most widespread disease of diarthrodial joints.

Characterization of proteoglycans isolated from associative extracts of human articular cartilage

Approx. 10% of the total proteoglycan content of normal young human articular cartilage was extracted under associative conditions with Dulbecco's PBS. Proteoglycans isolated from the extract by Q-Sepharose chromatography were separated by gel chromatography and characterized by gradient gel SDS/PAGE and immunoblotting. Three species of small proteoglycans, two main populations of aggrecan and a population of its smaller fragments were identified. The major populations of aggrecan contained chondroitin sulphate chains, all or part of the N-terminal G1 and G2 domains and, therefore, intact keratan sulphate domains. The larger population was estimated by gradient SDS/PAGE to have a molecular mass of approx. 600 kDa or greater. The second population had an apparent molecular mass of approx. 300-600 kDa. Core proteins derived from these populations of proteoglycans separated on SDS/PAGE into several clusters of bands in the range from 120 to approx. 360 kDa. The extract further contained smaller fragments which lacked chondroitin sulphate but reacted with antibodies against keratan sulphate, and against epitopes present in the G2 domain of aggrecan. The presence of the G2 domain in a broad range of populations of decreasing size indicated extensive cleavage of the aggrecan core protein within its chondroitin sulphate domain. These findings suggest that fragmentation of aggrecan probably occurs in vivo in normal articular cartilage of young individuals. Associative extracts also contained decorin, biglycan and fibromodulin. These were resolved from aggrecan by gel chromatography and identified by immunodetection.

Cell vitality in cartilage tissue culture following excimer laser radiation: an in vitro examination

Excimer laser is used for cartilage debridement, although the resulting cell damage is yet unclear. For examination of cartilage survival after treatment, we used short-term tissue cultures of human joint cartilage. Specimens were treated with a XeCl-Excimer laser using different laser parameters, pulse energies, and repetition rates. Following treatment, discs were cultured for 8 days prior to examination. In contrast to the 20 microns damage zone as instant visible effect in histomorphologic examinations, we found a 0.3 mm zone in which approximately 50% of cartilage cells had morphological signs of damage on light microscopic examinations. Autoradiography revealed that cartilage cells in an 0.5-0.7 mm area surrounding the laser craters had no collagen synthesis. This examination indicates that cell damage of excimer laser is higher than expected from prior studies.

N-terminal sequence of proteoglycan fragments isolated from medium of interleukin-1-treated articular-cartilage cultures. Putative site(s) of enzymic cleavage

Bovine articular cartilage was cultured both in the presence and in the absence of human recombinant interleukin-1 alpha (IL-1) (100 units/ml). Addition of this cytokine stimulated matrix degradation approx. 3-fold. This increased degradation permitted characterization of the large chondroitin sulphate proteoglycan (aggrecan) fragments accumulating in the media. When compared with controls, the proteoglycans isolated from the medium of cultures treated with IL-1 exhibited a decrease in the Kav. (control 0.25; IL-1-treated 0.37), determined by Sepharose CL-2B chromatography. This decrease in proteoglycan size was accompanied by a decreased ability of these monomers to associate with hyaluronic acid. Thus only 20% of the proteoglycans isolated from the medium of IL-1-treated cultures, compared with 39% for control cultures, had the capacity to form high-M(r) aggregates with hyaluronic acid. SDS/PAGE analysis of the proteoglycans from the media of IL-1-treated cultures demonstrated several large proteoglycan protein-core bands (M(r) 144,000-380,000). The protein-core bands with M(r) 144,000-266,000 exhibited a significantly decreased reactivity with monoclonal antibody 1-C-6 (specific for domains G1 and G2). The N-terminal amino acid sequence of four of these protein-core bands (M(r) 144,000, 173,000, 214,000 and 266,000) yielded sequences LGQRPPV-Y-PQLF(E), AGEGP(S)GILEL-GAP(S)-AP(D)M, GLG-VEL-LPGE and (A)RGSVIL-AKPDFEV-P-A. A comparison of these N-terminal amino acid sequences with the published proteoglycan sequence for bovine nasal cartilage [Oldberg, Antonsson & Heinegård (1987) Biochem. J. 243, 255-259], rat chondrosarcoma [Doege, Sasaki, Horigan, Hassell & Yamada (1987) J. Biol. Chem. 262, 17757-17769] and human articular cartilage [Doege, Sasaki, Kimura & Yamada (1991) J. Biol. Chem. 266, 894-902] permitted assignment of their relative positions on the core protein. Furthermore, on the basis of this similarity to published sequence, putative sites of enzymic cleavage were constructed. These theoretical cleavage sites revealed a glutamic acid residue in the P1 position and an uncharged polar or non-polar residue in the P1' position.

Mechanism of catabolism of aggrecan by articular cartilage

Characterization of aggrecan core protein peptides appearing in the medium of adult articular cartilage maintained in tissue culture showed that eight major peptides could be detected. The two largest peptides had the same N-terminal sequence as bovine aggrecan core protein and probably represent partly degraded aggrecan lost to the medium in the form of the proteoglycan aggregate. The three next smallest peptides were all shown to have another N-terminal sequence which corresponded to a sequence in the interglobular domain starting at alanine residue 393 of the human aggrecan core protein (K. Doege et al., 1991, J. Biol. Chem. 266, 894-902). Two other peptides were isolated and shown to have two different N-terminal amino sequences corresponding to sequences in the chondroitin sulfate attachment domain 2 of the core protein starting at alanine residue 1839 and leucine residue 1939 of human aggrecan. This suggests that the catabolism of aggrecan by adult articular cartilage occurs by the proteolytic cleavage of the core protein of this proteoglycan at three separate sites. Examination of the amino acid sequences around each of these cleavage sites showed a similar pattern TEGE decreases ARGS, TAQE decreases AGEG, and VSQE decreases LGQR, suggesting that a single proteinase may be involved in the catabolism of aggrecan. Analysis of synovial fluids and serum of age-matched animals revealed the presence of aggrecan core protein peptides corresponding in size to those detected in vitro, thus indicating the cleavage observed in explant culture is the same as that which occurs in vivo.

Cleavage of proteoglycan aggregate by leucocyte elastase

The partial degradation of proteoglycan aggregate by human leucocyte elastase yielded products that banded with Mr 190,000, 140,000, 88,000, and 71,000 when analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide electrophoresis. Analysis of these bands revealed that the 190,000- and 140,000-Da bands contained chondroitin and keratan sulfate stubs and had N-terminal amino acid sequences corresponding to a sequence starting at residue 398 of the core protein of rat or human aggrecan. With increased time of digestion, the staining intensities of the 190,000-, 140,000-, and 88,000-Da bands decreased relative to the 71,000-Da band. Analysis of the 88,000- and 71,000-Da bands showed that they contained peptides substituted only with keratan sulfate stubs and that each band contained two peptides with different N-terminal sequences. One of these corresponded to a sequence that started at residue 398 of rat or human aggrecan and the other to the N-terminal sequence of bovine aggrecan. Under conditions of complete digestion, bands of 71,000 and 56,000 Da which contained only keratan sulfate stubs were observed on SDS-polyacrylamide electrophoresis. The 71,000-Da band was shown to have a single sequence similar to that starting at residue 398 of human and rat aggrecan and thus represents the globular domain 2 (G2) of the core protein of aggrecan. The 56,000-Da band was shown to have a sequence similar to that of the N-terminal sequence of bovine aggrecan indicating that this peptide corresponds to the globular domain 1 (G1) of the molecule. These results suggest that leucocyte elastase cleaves the core protein of aggrecan between valine 397 and isoleucine 398, which are located in the interglobular domain linking the G1 and G2 domains of the core protein of aggrecan. Further digestion of the proteoglycan aggregate with elastase resulted in the cleavage of the core protein within the chondroitin sulfate attachment domains.

Methylprednisolone acetate induced release of cartilage proteoglycans: determination by high performance liquid chromatography

A high performance liquid chromatography (HPLC) procedure suitable for the simultaneous determination of the molecular size and concentration of macromolecular hyaluronate and proteoglycans in synovial fluid has been developed. Irrigation of the equine tarsocrural joint with 20 ml physiological saline (PSS) caused a mild inflammation with an increase of proteoglycans in the synovial fluid over the baseline arthrocentesis control sample. Proteoglycan and hyaluronate in the synovial fluid did not interact to form hyaluronate-proteoglycan aggregates, but separated as distinct chromatographic peaks. This suggests that the cartilage derived proteoglycans in synovial fluid in the inflamed joint have been proteolytically cleaved from the non-covalent aggregates containing link protein and hyaluronate. Hyaluronidase digestion completely abolished the hyaluronate peak without affecting the proteoglycans. This seems to indicate that proteoglycan in synovial fluid is unable to interact with hyaluronate in synovial fluid to form cartilage type aggregates. Proteolytic degradation and the time dependent release into the synovial fluid of such digested proteoglycan also resulted from the intra-articular injection of methylprednisolone acetate into normal tarsocrural joints and joints irrigated with PSS. These proteoglycans were insensitive to hyaluronidase but may consist of a protein moiety with attached glycosaminoglycans, as suggested by their sensitivity to proteinase and keratanase/chondroitinase digestion. These observations with cartilage treated with methylprednisolone acetate and mildly stimulated articular cartilage are inconsistent with earlier work on osteoarthritic and rheumatoid articular cartilage and have interesting implications for the pathogenesis and for the therapeutic action of intraarticular corticosteroids. A rapid HPLC procedure applicable to unprocessed small volume samples of synovial fluid gives information simultaneously on hyaluronate and proteoglycan in synovial fluid which is not attainable with immunoradiometric or isotope tracer techniques. It therefore appears to be useful for the analysis of cartilage turnover and destruction in health and disease.

Effects of compression on the loss of newly synthesized proteoglycans and proteins from cartilage explants

The effects of mechanical compression of calf cartilage explants on the catabolism and loss into the medium of proteoglycans and proteins radiolabeled with [35S]sulfate and [3H]proline were examined. A single 2- or 12-h compression of 3-mm diameter cartilage disks from a thickness of 1.25 to 0.50 mm, or slow cyclic compression (2 h on/2 h off) from 1.25 mm to 1.00, 0.75, or 0.50 mm for 24 h led to transient alterations and/or sustained increases in loss of radiolabeled macromolecules. The effects of imposing or removing loads were consistent with several compression-induced physical mediators including fluid flow, diffusion, and matrix disruption. Cyclic compression induced convective fluid flow and enhanced the loss of 35S- and 3H-labeled macromolecules from tissue into medium. In contrast, prolonged static compression induced matrix consolidation and appeared to hinder the diffusional transport and loss of 35S- and 3H-labeled macromolecules. Since high amplitude cyclic compression led to a sustained increase in the rate of loss of 3H- and 35S-labeled macromolecules that was accompanied by an increase in the rate of loss of [3H]hydroxyproline residues and an increase in tissue hydration, such compression may have caused disruption of the collagen meshwork. The 35S-labeled proteoglycans lost during such cyclic compression were of smaller average size than those from controls, but contained a similarly low proportion (approximately 15%) that could form aggregates with excess hyaluronate and link protein. The size distribution and aggregability of the remaining tissue proteoglycans and 35S-labeled proteoglycans were not markedly affected. The loss of tissue proteoglycan paralleled the loss of 35S-labeled macromolecules. This study provides a framework for elucidating the biophysical mechanisms involved in the redistribution, catabolism, and loss of macromolecules during cartilage compression.

1-C-6 epitope in cartilage proteoglycan G2 domain is masked by keratan sulphate

The presence of the protein epitope recognized by monoclonal antibody 1-C-6 was investigated on the globular G1 and G2 domains of pig cartilage proteoglycan core protein. After reduction of disulphide bonds and removal of keratan sulphate chains with keratanase, both G1 and G2 domains were shown to contain the epitope. However, without keratanase digestion the epitope on the G2 domain was poorly detected. The results suggest that a keratan sulphate chain substituted close to the epitope sequence in the G2 domain prevents antibody access to the epitope and thus masks its detection. This shows the 1-C-6 epitope to be a conserved protein sequence in the G2 domain of proteoglycans from different species, but its detection may be masked by glycosylation.

The oxidant hypochlorite (OCl-), a product of the myeloperoxidase system, degrades articular cartilage proteoglycan aggregate

The myeloperoxidase-derived oxidant, hypochlorite (OCl-) was shown to be able to degrade proteoglycan aggregate prepared from bovine articular cartilage. Exposure of proteoglycan aggregate to OCl- concentrations less than 10(-4) M resulted in a decrease in the size of the constituent proteoglycan monomers, which were unable to reaggregate with hyaluronate due to the loss of the hyaluronic acid binding region as indicated by immunoblotting using the monoclonal 1-C-6 antibody. Analysis of the [35S]-labeled core proteins by SDS/polyacrylamide electrophoresis and fluorography indicated a decrease in the size of the core protein. These data suggest that concentrations of OCl- below 10(-3) M results in the cleavage of the proteoglycan core protein in or near the hyaluronic acid binding region. The physiological consequences of these data are discussed. Exposure to higher concentrations (greater than 10(-3)) of OCl- caused more extensive degradation of the core protein; however, there was no evidence to suggest that OCl- cleaves glycosaminoglycan (GAG) chains.

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.

Characterization and synthesis of macromolecules by adult collateral ligament

Bovine collateral ligament was found to have a water content of 67.5 +/- 2.5%, the tissue was highly collagenous containing 100.3 +/- 15.1 micrograms hydroxyproline/mg dry weight. Type I collagen was the major collagen present with small amounts of Type III and V. The hexuronate content of the tissue was found to be 2.62 +/- 0.40 micrograms hexuronate/mg dry weight of tissue. On incubation in vitro collateral ligament incorporated [35S]sulfate and [3H]acetate into proteoglycans and [3H]acetate into hyaluronate and glycoproteins. The rate of synthesis of proteoglycans by collateral ligament was shown on a weight basis to be greater than that of tendon but lower than that of articular cartilage. Analysis of the proteoglycans present in collateral ligament showed two populations of proteoglycans to be present. Approx. 20% of the total proteoglycans present were large chondroitin- and keratan sulfate-containing proteoglycans capable of forming aggregates with hyaluronate. The major species of proteoglycan present were small dermatan sulfate proteoglycans made up of a core protein with a molecular mass of 45,000 daltons with one dermatan/chondroitin sulfate glycosaminoglycan chain of 30,000 daltons attached. The N-terminal amino acid sequence of the core protein of this proteoglycan showed it to be analogous to the core protein of dermatan sulfate proteoglycan II.

Passive loss of proteoglycan from articular cartilage explants

The addition of proteinase inhibitors (1 mM phenylmethylsulfonyl fluoride, 10 mM N-ethylmaleimide, 0.25 mM benzamidine hydrochloride, 6.25 mM EDTA, 12.5 mM 6-aminohexanoic acid and 2 mM iodoacetic acid) to explant cultures of adult bovine articular cartilage inhibits proteoglycan synthesis as well as the loss of the macromolecule from the tissue. Those proteoglycans lost to the medium of explant cultures treated with proteinase inhibitors were either aggregates or monomers with functional hyaluronic acid-binding regions, whereas proteoglycans lost from metabolically active tissue also included a population of monomers that were unable to aggregate with hyaluronate. Analysis of the core protein from proteoglycans lost into the medium of inhibitor-treated cultures showed the same size distribution as the core proteins of proteoglycans present in the extracellular matrix of metabolically active cultures. The core proteins of proteoglycans appearing in the medium of metabolically active cultures showed that proteolytic cleavage of these macromolecules occurred as a result of their loss from the tissue. Explant cultures of articular cartilage maintained in medium with proteinase inhibitors were used to investigate the passive loss of proteoglycan from the tissue. The rate of passive loss of proteoglycan from the tissue was dependent on surface area, but no difference in the proportion of proteoglycan aggregate to monomer appearing in the medium was observed. Furthermore, proteoglycans were lost at the same rate from the articular and cut surfaces of cartilage. Proteoglycan aggregates and monomer were lost from articular cartilage over a period of time, which indicates that proteoglycans are free to move through the extracellular matrix of cartilage. The movement of proteoglycans out of the tissue was shown to be temperature dependent, but was different from the change of the viscosity of water with temperature, which indicates that the loss of proteoglycan was not solely due to diffusion. The activation energy for the loss of proteoglycans from articular cartilage was found to be similar to the binding energies for electrostatic and hydrogen bonds.