Catabolism of aggrecan by explant cultures of human articular cartilage in the presence of retinoic acid

Distinguishing aggrecan loss from aggrecan proteolysis in ADAMTS-4 and ADAMTS-5 single and double deficient mice

Aggrecan loss from mouse cartilage is predominantly because of ADAMTS-5 activity; however, the relative contribution of other proteolytic and nonproteolytic processes to this loss is not clear. This is the first study to compare aggrecan loss with aggrecan processing in mice with single and double deletions of ADAMTS-4 and -5 activity (Deltacat). Cartilage explants harvested from single and double ADAMTS-4 and -5 Deltacat mice were cultured with or without interleukin (IL)-1alpha or retinoic acid and analyzed for (i) the kinetics of (35)S-labeled aggrecan loss, (ii) the pattern of (35)S-labeled aggrecan fragments released into the media and retained in the matrix, (iii) the pattern of total aggrecan fragments released into the media and retained in the matrix, and (iv) specific cleavage sites within the interglobular and chondroitin sulfate-2 domains. The loss of radiolabeled aggrecan from ADAMTS-4/-5 Deltacat cartilage was less than that from ADAMTS-4, ADAMTS-5, or wild-type cartilage under nonstimulated conditions. IL-1alpha and retinoic acid stimulated radiolabeled aggrecan loss from wild-type and ADAMTS-4 Deltacat cartilage, but there was little effect on ADAMTS-5 cartilage. Proteolysis of aggrecan contributed most to its loss in wild-type, ADAMTS-4, and ADAMTS-5 Deltacat cartilage explants. The pattern of proteolytic processing of aggrecan in these cultures was consistent with that occurring in cartilage pathologies. Retinoic acid, but not IL-1alpha, stimulated radiolabeled aggrecan loss from ADAMTS-4/-5 Deltacat cartilage explants. Even though there was a 300% increase in aggrecan loss from ADAMTS-4/-5 Deltacat cartilage stimulated with retinoic acid, the loss was not associated with aggrecanase cleavage but with the release of predominantly intact aggrecan consistent with the phenotype of the ADAMTS-4/-5 Deltacat mouse. Our results show that chondrocytes have additional mechanism for the turnover of aggrecan and that when proteolytic mechanisms are blocked by ablation of aggrecanase activity, nonproteolytic mechanisms compensate to maintain cartilage homeostasis.

Human osteoarthritis synovial fluid and joint cartilage contain both aggrecanase- and matrix metalloproteinase-generated aggrecan fragments

OBJECTIVE

To identify the major aggrecanase- and matrix metalloproteinase (MMP)-generated aggrecan fragments in human osteoarthritis (OA) synovial fluid and in human OA joint cartilage.

METHOD

Aggrecan fragments were prepared by CsCl gradient centrifugation. Fragment distributions were compared with aggrecanase-1 (ADAMTS-4) and MMP-3 digested human aggrecan by analysis with neoepitope antibodies and an anti-G1 domain antibody, using Western immuno-blots.

RESULTS

The overall fragment pattern of OA synovial fluid aggrecan was similar to the fragment pattern of cartilage aggrecan cleaved in vitro by ADAMTS-4. However, multiple glycosaminoglycan (GAG) containing aggrecanase and MMP-generated aggrecan fragments were identified in OA synovial fluid and some of these fragments were produced by the action of both types of proteinases. The synovial fluid content of large size aggrecan fragments with (374)ARGS- and (342)FFGV- N-terminals was about 107 and 40 pmoles per ml, respectively, out of a total concentration of aggrecan fragments of about 185 pmoles per ml. OA synovial fluid contained insignificant amounts of the G1-IPEN(341) fragment as compared to the G1-TEGE(373) fragment, while OA cartilage contained significant amounts of both fragments. OA cartilage contained several GAG-containing aggrecan fragments with N-terminals of G1- or (342)FFGV- but no fragments with an N-terminal of (374)ARGS-.

CONCLUSIONS

The overall pattern of aggrecan fragments in human OA synovial fluid and cartilage supports an important role for aggrecanase in aggrecan degradation. However, the fragment patterns and their differential distribution between cartilage and synovial fluid are consistent with the existence of at least two proteolytic pathways for aggrecan degradation in human OA, generating both (342)FFGV- and (374)ARGS-fragments.

Effect of matrix depleting agents on the expression of chondrocyte metabolism by equine chondrocytes

This study was carried out to investigate the effect of two enzymes (collagenase and chondroitinase) and two cytokines/metabolites (interleukin-1beta and retinoic acid) of known catabolic activity on the expression of cartilage metabolism/phenotype in equine articular cartilage. Articular cartilage explants from 11 horses (5-13 years old) were treated for 48 h and assayed for total sulphated glycosaminoglycan (GAG), the incorporation of 35S-sulphate, collagen degradation and mRNA expression of the proteoglycans collagen II, collagen IIA, collagen III, collagen IX, collagen X, collagen XI and glyceraldehyde-3-phosphate (GAPDH). Purified collagenase and retinoic acid were responsible for increased GAG loss from the tissues. Chondroitinase, responsible for catalysing the elimination of glucuronate residues from chondroitin A, B and C (Chondroitinase ABC) and retinoic acid treatment induced an inhibition of proteoglycan synthesis, whereas collagenase treatment did not. Collagenase activity was correlated with increased appearance of the CB11B epitope and type II collagen denaturation. By RT-PCR there was evidence of expression of altered collagen type IIA in purified collagenase treated tissues.

Cathepsin D cleaves aggrecan at unique sites within the interglobular domain and chondroitin sulfate attachment regions that are also cleaved when cartilage is maintained at acid pH

Bovine aggrecan was digested with bovine cathepsin D at pH 5.2 under conditions of partial digestion and the resulting aggrecan core protein fragments were separated by electrophoresis on gradient polyacrylamide gels. The fragments were characterized by their reactivity to specific antibodies and by N-terminal amino acid sequencing. It was also demonstrated that cathepsin D cleaved bovine aggrecan at five sites within the core protein, between residues Phe(342)-Phe(343) in the interglobular domain, Leu(1462)-Val(1463) between the chondroitin sulfate attachment regions 1 and 2 and Leu(1654)-Val(1655), Phe(1754)-Val(1755) and Leu(1854)-Ile(1855) that are located within the chondroitin sulfate attachment region 2 of the core protein. The time course of digestion showed that there was a continued degradation of aggrecan and there was no preferential cleavage of the core protein at any one site. It was shown that cathepsin D digested aggrecan over the pH range 5.2-6.5 resulting in the same products. When bovine cartilage was maintained in explant culture at pH 5.2 there was a rapid loss of both radiolabeled and chemical pools of sulfated glycosaminoglycans into the culture medium and this loss was inhibited by the inclusion in the medium of the aspartic proteinase inhibitor, pepstatin A. The aggrecan core protein fragments appearing in the medium of cultures maintained at pH 5.2 were characterized and it was shown that the fragments had N-terminal sequences starting at Phe(343), Ile(1855), and Val(1755) or Val(1463). This work demonstrates that cathepsin D present within the extracellular matrix of articular cartilage has the potential to contribute to the proteolytic processing of the core protein of aggrecan in this tissue.

Generation and novel distribution of matrix metalloproteinase-derived aggrecan fragments in porcine cartilage explants

We have studied aggrecan catabolism mediated by matrix metalloproteinases (MMPs) in a porcine cartilage culture system. Using antibodies specific for DIPEN(341) and (342)FFGVG neoepitopes, we have detected MMP-derived fragments in conditioned medium and cultured cartilage, by radioimmunoassay, Western blotting, and immunolocalization. Radioimmunoassay revealed that the amount (pmol of epitope/mg of total glycosaminoglycan) of (342)FFGVG epitope released from cartilage remained constant over a 5-day culture period and was not increased by IL-1alpha or retinoate. However, the proportion (pmol of epitope/mg of released glycosaminoglycan) of (342)FFGVG epitope released was decreased upon stimulation, consistent with the involvement of a non-MMP proteinase, such as aggrecanase. The data suggest that in vitro MMPs may be involved in the base-line catabolism of aggrecan. Immunolocalization experiments showed that DIPEN(341) and ITEGE(373) epitopes were increased by treatment with IL-1alpha and retinoate. Confocal microscopy revealed that ITEGE(373) epitope was largely intracellular but with matrix staining in the superficial zone, whereas DIPEN(341) epitope was cell-associated and widely distributed in the matrix. Surprisingly, the majority of (342)FFGVG epitope, determined by radioimmunoassay and Western blotting, was retained in the tissue despite the absence of a G1 domain anchor. Interleukin-1alpha stimulation caused a marked increase in tissue DIPEN(341) and (342)FFGVG epitope, and the (342)FFGVG fragments retained in the tissue were larger than those released into the medium. Active porcine aggrecanase was unable to cleave (342)FFGVG fragments at the downward arrowGlu(373) downward arrowAla(374) bond but cleaved intact aggrecan at this site, suggesting that (342)FFGVG fragments are not substrates for aggrecanase. The apparent retention of large (342)FFGVG fragments within cartilage, and their resistance to N-terminal cleavage by aggrecanase suggests that (342)FF6V6 fragments may have a role in cartilage homeostasis.

Mutations in the interglobular domain of aggrecan alter matrix metalloproteinase and aggrecanase cleavage patterns. Evidence that matrix metalloproteinase cleavage interferes with aggrecanase activity

We have expressed G1-G2 mutants with amino acid changes at the DIPEN(341) downward arrow(342)FFGVG and ITEGE(373) downward arrow(374)ARGSV cleavage sites, in order to investigate the relationship between matrix metalloproteinase (MMP) and aggrecanase activities in the interglobular domain (IGD) of aggrecan. The mutation DIPEN(341) to DIGSA(341) partially blocked cleavage by MMP-13 and MMP-8 at the MMP site, while the mutation (342)FFGVG to (342)GTRVG completely blocked cleavage at this site by MMP-1, -2, -3, -7, -8, -9, -13, -14. Each of the MMP cleavage site mutants, including a four-amino acid deletion mutant lacking residues ENFF(343), were efficiently cleaved by aggrecanase, suggesting that the primary sequence at the MMP site had no effect on aggrecanase activity in the IGD. The mutation (374)ARGSV to (374)NVYSV completely blocked cleavage at the aggrecanase site by aggrecanase, MMP-8 and atrolysin C but had no effect on the ability of MMP-8 and MMP-13 to cleave at the Asn(341) downward arrowPhe bond. Susceptibility to atrolysin C cleavage at the MMP site was conferred in the DIGSA(341) mutant but absent in the wild-type, (342)GTRVG, (374)NVYSV, and deletion mutants. To further explore the relationship between MMP and aggrecanase activities, sequential digest experiments were done in which MMP degradation products were subsequently digested with aggrecanase and vice versa. Aggrecanase-derived G1 domains with ITEGE(373) C termini were viable substrates for MMPs; however, MMP-derived G2 fragments were resistant to cleavage by aggrecanase. A 10-mer peptide FVDIPENFFG, which is a substrate analogue for the MMP cleavage site, inhibited aggrecanase cleavage at the Glu(373) downward arrowAla bond. This study demonstrates that MMPs and aggrecanase have unique substrate recognition in the IGD of aggrecan and suggests that sequences at the C terminus of the DIPEN(341) G1 domain may be important for regulating aggrecanase cleavage.

Calcium pentosan polysulfate inhibits the catabolism of aggrecan in articular cartilage explant cultures

OBJECTIVE

The catabolism of aggrecan and loss of aggrecan fragments from articular cartilage is a key event in the pathogenesis of arthritic diseases such as osteoarthritis. The catabolism of aggrecan is mediated by the specific proteolytic activity termed aggrecanase. The aim of this study was to investigate the effect of the chondroprotective agent calcium pentosan polysulfate (CaPPS) on the aggrecanase-mediated catabolism of aggrecan.

METHODS

The catabolism of 35S-labeled aggrecan and loss of tissue glycosaminoglycans (GAGs) were investigated using bovine articular cartilage explant cultures maintained in medium containing varying concentrations of CaPPS (1-100 microg/ml) in the presence or absence of 10(-6)M retinoic acid or 7 ng/ml recombinant human interleukin-1alpha (rHuIL-1alpha). In addition, the effect of CaPPS on the degradation of aggrecan monomers by aggrecanase activity present in conditioned medium from joint capsule explant cultures was investigated.

RESULTS

CaPPS inhibited the catabolism of 35S-labeled aggrecan in a dose-dependent manner, particularly when retinoic acid or rHuIL-1alpha was used to stimulate aggrecan catabolism. These effects were reflected in the tissue levels of GAG remaining in these cultures at the end of the experiment. CaPPS inhibited the degradation of aggrecan monomers by soluble aggrecanase activity.

CONCLUSION

CaPPS inhibits the catabolism of aggrecan by articular cartilage in a dose-dependent manner, particularly when the processes responsible for aggrecan loss are stimulated. This effect occurs, at least in part, through direct inhibition of aggrecanase activity. CaPPS did not adversely affect overall chondrocyte metabolism, as shown by the incorporation of 35S-sulfate and 3H-leucine into macromolecules and by lactate production in cartilage explant cultures.

Characterization of 5'-flanking region of human aggrecanase-1 (ADAMTS4) gene

Aggrecanase-1, also known as ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin motifs 4), cleaves at the Glu373-Ala374 site of aggrecan, thereby indicating aggrecan degradation. It is thought that ADAMTS4 plays a pivotal role in inflammatory joint diseases and cartilage degradation. To elucidate the mechanisms of regulation of ADAMTS4 gene expression, we cloned the 5'-flanking region of the human ADAMTS4 gene and characterized its promoter activity by means of reporter assay using porcine chondrocytes and NIH3T3 cells. Reporter gene analysis using deletion variants suggested that the region between -383 and +10 relative to the tentative transcription start site is necessary for full promoter activity; this region contains one Sp1 and three AP2 sites. In addition, the segment between -726 and -384 appears to contain silencer element(s). A complete deletion mutant of the nuclear factor I (NFI) binding site at -441 to -429 resulted in recovery of the promoter activity in chondrocytes, but not in NIH3T3 cells. Thus, the NFI site is involved in negative regulation of the human ADAMTS4 promoter activity in chondrocytes.

ADAMTS-1 cleaves a cartilage proteoglycan, aggrecan

A disintegrin-like and metalloproteinase with thrombospondin type I motifs-1 (ADAMTS-1) is an extracellular matrix-anchored metalloproteinase. In this study we have demonstrated that ADAMTS-1 is able to cleave a major cartilage proteoglycan, aggrecan. N-terminal sequencing analysis of the cleavage product revealed that ADAMTS-1 cleaves the Glu(1871)-Leu(1872) bond within the chondroitin sulfate attachment domain of aggrecan. In addition, deletional analysis demonstrated that the C-terminal spacer region of ADAMTS-1 is necessary to degrade aggrecan. These results suggest that ADAMTS-1 may be involved in the turnover of aggrecan in vivo.

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.

Cloning and characterization of ADAMTS11, an aggrecanase from the ADAMTS family

Aggrecan is responsible for the mechanical properties of cartilage. One of the earliest changes observed in arthritis is the depletion of cartilage aggrecan due to increased proteolytic cleavage within the interglobular domain. Two major sites of cleavage have been identified in this region at Asn(341)-Phe(342) and Glu(373)-Ala(374). While several matrix metalloproteinases have been shown to cleave at Asn(341)-Phe(342), an as yet unidentified protein termed "aggrecanase" is responsible for cleavage at Glu(373)-Ala(374) and is hypothesized to play a pivotal role in cartilage damage. We have identified and cloned a novel disintegrin metalloproteinase with thrombospondin motifs that possesses aggrecanase activity, ADAMTS11 (aggrecanase-2), which has extensive homology to ADAMTS4 (aggrecanase-1) and the inflammation-associated gene ADAMTS1. ADAMTS11 possesses a number of conserved domains that have been shown to play a role in integrin binding, cell-cell interactions, and extracellular matrix binding. We have expressed recombinant human ADAMTS11 in insect cells and shown that it cleaves aggrecan at the Glu(373)-Ala(374) site, with the cleavage pattern and inhibitor profile being indistinguishable from that observed with native aggrecanase. A comparison of the structure and expression patterns of ADAMTS11, ADAMTS4, and ADAMTS1 is also described. Our findings will facilitate the study of the mechanisms of cartilage degradation and provide targets to search for effective inhibitors of cartilage depletion in arthritic disease.

Increased vulnerability of postarthritic cartilage to a second arthritic insult: accelerated MMP activity in a flare up of arthritis

OBJECTIVE

Murine antigen induced arthritis (AIA) is a chronic, smouldering inflammation. Flares of arthritis can be induced by antigen rechallenge or exposure to inflammatory mediators like interleukin 1 (IL1). These flares are characterised by a fast and marked proteoglycan (PG) depletion if compared with the initial arthritis. This study investigated the involvement of metalloproteinases in both the initial and the flare phase of arthritis.

METHODS

Murine AIA was induced and a flare up of arthritis was induced by injection of 10 ng of IL1beta. Messenger RNA levels of MMP-1 and -3 were studied by RT-PCR. MMP activity in cartilage, during both primary AIA as well as the flare up of arthritis, was studied by immunodetection of MMP specific neoepitopes in aggrecan (VDIPEN). Cartilage just before flare induction was analysed for presence of MMPs at the mRNA level as well as at the protein level by zymography.

RESULTS

At the onset of AIA, a fast upregulation of mRNA for stromelysin and collagenase was noted. However, no VDIPEN epitopes were detected during this early phase of arthritis. They appeared when PG depletion was severe at day 7 of arthritis and disappeared when cartilage was repaired. IL1 injection into a knee joint at week 4 of AIA caused a flare up of arthritis, coinciding with a fast and marked PG degradation. This degradation was characterised by accelerated expression of VDIPEN epitopes if compared with the expression in primary AIA. Analysis of cartilage at week 4 of AIA showed still increased mRNA levels of MMP-1 and -3. Moreover, increased levels of latent MMPs were present as well, as APMA activation induced profound VDIPEN epitope. In vitro exposure to IL1 did show increased PG breakdown but no VDIPEN expression, suggesting that factors in addition to IL1 are needed to cause the in vivo VDIPEN expression.

CONCLUSIONS

The fast and marked PG depletion seen in a flare up of AIA coincides with accelarated expression of MMP induced neoepitopes compared with expression during primary AIA. This accelerated expression is probably linked to increased levels of latent enzyme, which were found to be present in the cartilage before induction of a flare up.

Kinetics of aggrecanase- and metalloproteinase-induced neoepitopes in various stages of cartilage destruction in murine arthritis

OBJECTIVE

Two major cleavage sites, one mediated by metalloproteinases (MMPs) and the other by an as-yet unidentified enzyme termed aggrecanase, have been observed in aggrecan. To learn more about the relative contribution of these enzymes during cartilage degradation, this study assessed the occurrence of both specific neoepitopes in cartilage during murine arthritis and examined the correlation between neoepitope formation and different aspects of cartilage damage.

METHODS

Reversible cartilage damage was induced in mice in the zymosan-induced arthritis (ZIA) model, partly irreversible cartilage damage in the antigen-induced arthritis (AIA) model, and irreversible, destructive cartilage damage in the collagen-induced arthritis (CIA) model. Immunolocalization techniques were used to detect the specific C-terminal neoepitopes VDIPEN (MMPS) and NITEGE (aggrecanase).

RESULTS

In normal cartilage from young adult mice, no VDIPEN epitopes were detected, but a limited amount of NITEGE epitopes were already present. During the early phase of proteoglycan (PG) depletion, NITEGE expression was raised substantially in all arthritis models. VDIPEN epitopes were not detected in this early phase of cartilage destruction. When PG depletion progressed toward advanced cartilage damage, VDIPEN epitopes were induced. During ZIA, minimal induction of VDIPEN was observed, whereas in AIA, strong, but partly reversible, VDIPEN staining was evident, and in CIA, an extensive presence and persistence of the MMP-induced neoepitope was seen. When VDIPEN epitopes were intensely present, NITEGE epitopes were greatly reduced at that site in the cartilage.

CONCLUSION

Presence of VDIPEN epitopes in cartilage correlated with severe cartilage damage, but these epitopes were not detected during early PG degradation. This suggests a limited role for VDIPEN-inducing MMPs in early PG degradation during murine arthritis. In contrast, aggrecanase epitopes were induced before the appearance of VDIPEN epitopes, but they disappeared with progression of cartilage damage.

Generation and characterization of aggrecanase. A soluble, cartilage-derived aggrecan-degrading activity

A method was developed for generating soluble, active "aggrecanase" in conditioned media from interleukin-1-stimulated bovine nasal cartilage cultures. Using bovine nasal cartilage conditioned media as a source of the aggrecanase enzyme, an enzymatic assay was established employing purified aggrecan monomers as a substrate and monitoring specific aggrecanase-mediated cleavage products by Western analysis using the monoclonal antibody, BC-3 (which recognizes the new N terminus, ARGS, on fragments produced by cleavage between amino acid residues Glu373 and Ala374). Using this assay we have characterized cartilage aggrecanase with respect to assay kinetics, pH and salt optima, heat sensitivity, and stability upon storage. Aggrecanase activity was inhibited by the metalloprotease inhibitor, EDTA, while a panel of inhibitors of serine, cysteine, and aspartic proteinases had no effect, suggesting that aggrecanase is a metalloproteinase. Sensitivity to known matrix metalloproteinase inhibitors as well as to the endogenous tissue inhibitor of metalloproteinases, TIMP-1, further support the notion that aggrecanase is a metalloproteinase potentially related to the ADAM family or MMP family of proteases previously implicated in the catabolism of the extracellular matrix.

Identification of distinct metabolic pools of aggrecan and their relationship to type VI collagen in the chondrons of mature bovine articular cartilage explants

The metabolism and distribution of newly synthesized aggrecan present in the extracellular matrix of intact explant cultures of mature articular cartilage was investigated with respect to type VI collagen-stained chondrons. Using biochemical, autoradiographical and novel confocal immunohistochemical techniques it was shown that aggrecan exists as a number of distinct pools that are located within the extracellular matrix of the tissue. The first was identified as a pool of high specific radioactivity, much of which appeared in the medium one day after incubation with radiolabeled sulfate. Of the radiolabeled aggrecan remaining within the extracellular matrix, three pools were differentiated on the basis of time and location within the extracellular matrix. One pool was resident within the pericellular microenvironment associated with the chondron, one migrated into the territorial matrix adjacent to the chondron and one was sequestered long term in the interterritorial matrix. Analysis of the kinetics of loss of radiolabeled aggrecan macromolecules present in the region of matrix defined by the chondron suggests that this pool rapidly turns over and is a precursor to the pools of aggrecan present in the territorial and interterritorial matrix. There were marked differences in the distribution of newly synthesized aggrecan present in these regions of the extracellular matrix in explant cultures maintained with or without fetal calf serum. In the absence of serum, more of the newly synthesized aggrecan moved into the territorial and interterritorial matrix indicating that the presence of serum in the culture medium influenced the tissue distribution of aggrecan. This work indicates that the pericellular microenvironment of the chondron plays an important role in the retention and maturation of aggrecan prior to the sequestration of aggrecan complexes into the functional load bearing matrices of adult articular cartilage.

Differential expression of aggrecanase and matrix metalloproteinase activity in chondrocytes isolated from bovine and porcine articular cartilage

The release of aggrecan catabolites from cartilage is an early event in the pathogenesis of degenerative joint diseases. The enzymes involved in this process are unknown, controversial, and the subject of intense investigation. In this paper we have utilized a recombinant substrate containing the interglobular domain (IGD) of aggrecan to study specifically aggrecanase versus matrix metalloproteinase (MMP) catabolism in this domain of aggrecan. Our studies have shown that (i) there are species differences in the expression of latent versus active MMP activity on the aggrecan IGD; (ii) interleukin-1alpha exposure induces both aggrecanase and MMP activities, whereas retinoic acid induces only aggrecanase activity and inhibits the MMP activity on the aggrecan IGD; (iii) activators of latent MMP activity (p-aminophenylmercuric acetate and trypsin) significantly reduce aggrecanase activity; (iv) the time course of the appearance of aggrecanase versus the MMP catabolism of aggrecan IGD differs; (v) aggrecanase is a protease with metalloprotease characteristics; however (vi) the physiological (tissue) inhibitors of MMPs show weak inhibition (TIMP-1) or no inhibition (TIMP-2) of aggrecanase activity. Collectively, these studies show that aggrecanase and MMP catabolism of the aggrecan IGD are independent and uncoupled.

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.

Membrane type 1 matrix metalloproteinase (MT1-MMP) cleaves the recombinant aggrecan substrate rAgg1mut at the 'aggrecanase' and the MMP sites. Characterization of MT1-MMP catabolic activities on the interglobular domain of aggrecan

The recent detection of membrane type 1 matrix metalloproteinase (MT1-MMP) expression in human articular cartilage [Büttner, Chubinskaya, Margerie, Huch, Flechtenmacher, Cole, Kuettner, and Bartnik (1997) Arthritis Rheum. 40, 704-709] prompted our investigation of MT1-MMP's catabolic activity within the interglobular domain of aggrecan. For these studies we used rAgg1mut, a mutated form of the recombinant fusion protein (rAgg1) that has been used as a substrate to monitor 'aggrecanase' catabolism in vitro [Hughes, Büttner, Eidenmüller, Caterson and Bartnik (1997) J. Biol. Chem. 272, 20269-20274]. The rAgg1 was mutated (G332 to A) to avoid the generation of a splice variant seen with the original genetic construct, which gave rise to heterogeneous glycoprotein products. This mutation yielded a homogeneous recombinant product. Studies in vitro with retinoic acid-stimulated rat chondrosarcoma cells indicated that the rAgg1mut substrate was cleaved at the 'aggrecanase' site equivalent to Glu373-Ala374 (human aggrecan sequence enumeration) in its interglobular domain sequence segment. The differential catabolic activities of the recombinant catalytic domain (cd) of MT1-MMP and matrix metalloproteinases (MMPs) 3 and 8 were then compared by using this rAgg1mut as a substrate. Coomassie staining of rAgg1mut catabolites separated by SDS/PAGE showed similar patterns of degradation with all three recombinant enzymes. However, comparative immunodetection analysis, with neoepitope antibodies BC-3 (anti-ARGS...) and BC-14 (anti-FFGV...) to distinguish between 'aggrecanase' and MMP-generated catabolites, indicated that the catalytic domain of MT1-MMP exhibited strong 'aggrecanase' activity, cdMMP-8 weak activity and cdMMP-3 no activity. In contrast, cdMMP-3 and cdMMP-8 led to strongly BC-14-reactive catabolic fragments, whereas cdMT1-MMP resulted in weak BC-14 reactivity. N-terminal sequence analyses of the catabolites confirmed these results and also identified other potential minor cleavage sites within the interglobular domain of aggrecan. These results indicate that MT1-MMP is yet another candidate for 'aggrecanase' activity in vivo.

Induction of matrix metalloproteinase activation cascades based on membrane-type 1 matrix metalloproteinase: associated activation of gelatinase A, gelatinase B and collagenase 3

SW1353 chondrosarcoma cells cultured in the presence of interleukin-1, concanavalin A or PMA secreted procollagenase 3 (matrix metalloproteinase-13). The enzyme was detected in the culture medium by Western blotting using a specific polyclonal antibody raised against recombinant human procollagenase 3. Oncostatin M enhanced the interleukin-1-induced production of procollagenase 3, whereas interleukin-4 decreased procollagenase 3 synthesis. The enzyme was latent except when the cells had been treated with concanavalin A, when a processed form of 48 kDa, which corresponds to the active form, was found in the culture medium and collagenolytic activity was detected by degradation of 14C-labelled type I collagen. The concanavalin A-induced activation of procollagenase 3 coincided with the processing of progelatinase A (matrix metalloproteinase-2) by the cells, as measured by gelatin zymography. In addition, progelatinase B (matrix metalloproteinase-9) was activated when gelatinase A and collagenase 3 were in their active forms. Concanavalin A treatment of SW1353 cells increased the amount of membrane-type-1 matrix metalloproteinase protein in the cell membranes, suggesting that this membrane-bound enzyme participates in an activation cascade involving collagenase 3 and the gelatinases. This cascade was effectively inhibited by tissue inhibitors of metalloproteinases-2 and -3. Tissue inhibitor of metalloproteinases-1, which is a much weaker inhibitor of membrane-type 1 matrix metalloproteinase than tissue inhibitors of metalloproteinases-2 and -3 [Will, Atkinson, Butler, Smith and Murphy (1996) J. Biol. Chem. 271, 17119-17123], was a weaker inhibitor of the activation cascade.

Presence of antibodies to native G1 domain of aggrecan core protein in synovial fluids from patients with various joint diseases

OBJECTIVE

To investigate the occurrence of IgG antibodies to aggrecan in synovial fluids (SF) from patients with arthritis and various articular diseases, and to determine the nature of epitopes present within aggrecan that react with these antibodies.

METHODS

SF samples were reacted with native aggrecan, reduced and alkylated aggrecan, chondroitin sulfate, and keratan sulfate, using dot-blots and a novel enzyme-linked immunosorbent assay (ELISA). The nature of the epitopes present on aggrecan was elucidated using Western blots and a competitive inhibition ELISA.

RESULTS

IgG antibodies to aggrecan were found in > 50% of the SF samples tested. No IgG antibody reactivity was observed in serum from the same patients. The antibodies appeared to react predominantly with native aggrecan, and there was no disease specificity. It was shown that the epitopes to these antibodies were located within the N-terminal region of the core protein.

CONCLUSION

This study demonstrates the frequent occurrence of IgG antibodies to aggrecan in human SF. The major epitope is located in the G1 domain of the aggrecan core protein. These IgG antibodies appear to be produced locally within the synovial cavity, probably in response to various articular diseases, resulting in the loss of native aggrecan from articular cartilage.

The effect of compressive loading on proteoglycan turnover in cultured fetal tendon

A fibrocartilaginous tissue develops in tendon at the point where the tendon wraps under bone and is subjected to transverse compressive loading in addition to tension. This tissue is characterized by a high level of large proteoglycan (aggrecan), which could accumulate because of increased synthesis, diminished turnover, or both. To examine the effect of loading on proteoglycan turnover segments of fetal tendon in sterile culture were subjected to cyclic, uniaxial compression loading to 30% of initial thickness once every 6 sec. for 72 h, and then allowed to incorporate 35S-sulfate for 12 h. The rate of loss of newly-synthesized 35S-proteoglycans from tissue was determined during subsequent culture for up to 12 days, with or without continued loading. Proteoglycan was lost from fetal tendon segments rapidly during the first 3 days of culture and slowly thereafter. Loss of newly-synthesized proteoglycan from adult tendon fibrocartilage was linear, with a half life of 12 d. Segments of fetal tendon subjected to cyclic compression before labeling synthesized more proteoglycan. These segments lost a greater percent of labeled proteoglycan to medium during a subsequent 12-day culture period than matched segments that had not experienced loading. Analysis of medium and tissue proteoglycans by SDS polyacrylamide gel electrophoresis and sieve chromatography indicated that small proteoglycans (decorin and biglycan) were retained in both loaded and non-loaded tissue whereas large proteoglycans (migrating in the Vo of a Sepharose CL-4B column) were readily lost. It is concluded that the 3-day loading regimen did not diminish turnover of large proteoglycan. To the contrary, although synthesis of large proteoglycan was enhanced by the loading regimen, these proteoglycans were still rapidly lost from the fetal tissue.