Analysis of aggrecan in human knee cartilage and synovial fluid indicates that aggrecanase (ADAMTS) activity is responsible for the catabolic turnover and loss of whole aggrecan whereas other protease activity is required for C-terminal processing in vivo

Neoepitope antibodies against MMP-cleaved and aggrecanase-cleaved aggrecan

Neoepitope antibodies recognize the newly created N or C terminus of protein degradation products but fail to recognize the same sequence of amino acids present in intact or undigested protein. Aggrecan neoepitope antibodies have been pivotal in studies determining the contribution of matrix metalloproteinases (MMPs) and aggrecanases to aggrecanolysis. In particular, an antibody to the A(374)RGSV N terminus was instrumental in the landmark discovery of the aggrecanases, ADAMTS-4 and ADAMTS-5. Antibodies to neoepitopes at the major MMP cleavage site DIPEN(341)/(342)FFGVG helped to distinguish MMP-driven aggrecan loss from aggrecanase-driven aggrecan loss and identified a role for MMPs in late-stage disease. More recently, neoepitope antibodies that recognize cleavage sites in the chondroitin sulphate-rich region of aggrecan have been used to show that aggrecanase cleavage proceeds in a defined manner, beginning at the C terminus and proceeding to the signature cleavage at NITEGE(373)/(374)ARGSV in the interglobular domain. Work with the C-terminal neoepitope antibodies has underscored the need to use a suite of neoepitope antibodies to fully describe aggrecanolysis in vitro. In this chapter, we describe the production of two aggrecan neoepitope antibodies as examples: the monoclonal anti-FFGVG antibody (AF-28) and the polyclonal anti-DIPEN antisera.

The role of ADAMTSs in arthritis

The ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family consists of 19 proteases. These enzymes are known to play important roles in development, angiogenesis and coagulation; dysregulation and mutation of these enzymes have been implicated in many disease processes, such as inflammation, cancer, arthritis and atherosclerosis. This review briefly summarizes the structural organization and functional roles of ADAMTSs in normal and pathological conditions, focusing on members that are known to be involved in the degradation of extracellular matrix and loss of cartilage in arthritis, including the aggrecanases (ADAMTS-4 and ADAMTS-5), ADAMTS-7 and ADAMTS-12, the latter two are associated with cartilage oligomeric matrix protein (COMP), a component of the cartilage extracellular matrix (ECM). We will discuss the expression pattern and the regulation of these metalloproteinases at multiple levels, including their interaction with substrates, induction by pro-inflammatory cytokines, protein processing, inhibition (e.g., TIMP-3, alpha-2-macroglobulin, GEP), and activation (e.g., syndecan-4, PACE-4).

Aggrecanolysis and in vitro matrix degradation in the immature bovine meniscus: mechanisms and functional implications

Introduction

Little is known about endogenous or cytokine-stimulated aggrecan catabolism in the meniscal fibrocartilage of the knee. The objectives of this study were to characterize the structure, distribution, and processing of aggrecan in menisci from immature bovines, and to identify mechanisms of extracellular matrix degradation that lead to changes in the mechanical properties of meniscal fibrocartilage.

Methods

Aggrecanase activity in the native immature bovine meniscus was examined by immunolocalization of the aggrecan NITEGE neoepitope. To investigate mechanisms of cytokine-induced aggrecan catabolism in this tissue, explants were treated with interleukin-1α (IL-1) in the absence or presence of selective or broad spectrum metalloproteinase inhibitors. The sulfated glycosaminoglycan (sGAG) and collagen contents of explants and culture media were quantified by biochemical methods, and aggrecan catabolism was examined by Western analysis of aggrecan fragments. The mechanical properties of explants were determined by dynamic compression and shear tests.

Results

The aggrecanase-generated NITEGE neoepitope was preferentially localized in the middle and outer regions of freshly isolated immature bovine menisci, where sGAG density was lowest and blood vessels were present. In vitro treatment of explants with IL-1 triggered the accumulation of NITEGE in the inner and middle regions. Middle region explants stimulated with IL-1 exhibited substantial decreases in sGAG content, collagen content, and mechanical properties. A broad spectrum metalloproteinase inhibitor significantly reduced sGAG loss, abrogated collagen degradation, and preserved tissue mechanical properties. In contrast, an inhibitor selective for ADAMTS-4 and ADAMTS-5 was least effective at blocking IL-1-induced matrix catabolism and loss of mechanical properties.

Conclusions

Aggrecanase-mediated aggrecanolysis, typical of degenerative articular cartilage, may play a physiologic role in the development of the immature bovine meniscus. IL-1-induced release of sGAG and loss of mechanical properties can be ascribed primarily to the activity of MMPs or aggrecanases other than ADAMTS-4 and ADAMTS-5. These results may have implications for the clinical management of osteoarthritis.

The emerging roles of ADAMTS-7 and ADAMTS-12 matrix metalloproteinases

The a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) comprise a family of secreted zinc metalloproteinases with a precisely ordered modular organization. These enzymes play an important role in the turnover of extracellular matrix proteins in various tissues and their dysregulation has been implicated in disease-related processes such as arthritis, atherosclerosis, cancer, and inflammation. ADAMTS-7 and ADAMTS-12 share a similar domain organization to each other and form a subgroup within the ADAMTS family. Emerging evidence suggests that ADAMTS-7 and ADAMTS-12 may play an important role in the development and pathogenesis of various kinds of diseases. In this review, we summarize what is currently known about the roles of these two metalloproteinases, with a special focus on their involvement in chondrogenesis, endochondral ossification, and the pathogenesis of arthritis, atherosclerosis, and cancer. The future study of ADAMTS-7 and ADAMTS-12, as well as the molecules with which they interact, will help us to better understand a variety of human diseases from both a biological and therapeutic standpoint.

Synovial fluid level of aggrecan ARGS fragments is a more sensitive marker of joint disease than glycosaminoglycan or aggrecan levels: a cross-sectional study

INTRODUCTION

Aggrecanase cleavage at the 392Glu-393Ala bond in the interglobular domain (IGD) of aggrecan, releasing N-terminal 393ARGS fragments, is an early key event in arthritis and joint injuries. Here, we use a quantitative immunoassay of aggrecan ARGS neoepitope fragments in human synovial fluid to determine if this cleavage-site specific method better identifies joint pathology than previously available less specific aggrecan assays.

METHODS

Synovial fluid (SF) from 26 people with healthy knees (reference) and 269 patients were analyzed in a cross-sectional study. Patient groups were acute inflammatory arthritis, acute knee injury, chronic knee injury and knee osteoarthritis (OA). Aggrecan ARGS fragments were assayed by ELISA using the monoclonal antibody OA-1. Total aggrecan content was analyzed by an ELISA using the monoclonal antibody 1-F21, and sulfated glycosaminoglycan by Alcian blue precipitation.

RESULTS

Aggrecan ARGS fragment concentrations in all groups differed from the reference group (P < 0.001). The acute inflammatory arthritis group had the highest median level, 177-fold greater than that of the reference group. Median levels (in pmol ARGS/ml SF) were: reference 0.5, acute inflammatory arthritis 88.5, acute knee injury 53.9, chronic knee injury 0.5 and OA 4.6. In contrast, aggrecan and sulfated glycosaminoglycan concentrations varied much less between groups, and only acute inflammatory arthritis and acute knee injury were found to have a two-fold increase in median levels compared to the reference.

CONCLUSIONS

Levels of aggrecan ARGS fragments in human synovial fluid are increased in human arthritis, OA and after knee injury, likely reflecting an enhanced cleavage at the 392Glu-393Ala bond in the IGD by aggrecanase. An assay that specifically quantified these fragments better distinguished samples from joints with pathology than assays monitoring aggrecan or glycosaminoglycan concentrations. The newly developed ARGS fragment assay can be used to monitor aggrecanase activity in human joint disease and experimental models.

Removal of O-linked and N-linked oligosaccharides is required for optimum detection of NITEGE neoepitope on ADAMTS4-digested fetal aggrecans: implications for specific N-linked glycan-dependent aggrecanolysis at Glu373-Ala374

OBJECTIVES

We have observed that Western blot analysis with an anti-G1 antibody detects G1-NITEGE product in a disintegrin and metalloprotease with thrombospondin motifs-4 (ADAMTS4)-digested fetal and mature human and bovine aggrecan, but the neoepitope-specific anti-NITEGE antibody only detects this product in digests of mature aggrecan. Our objective was to determine whether enzymatic removal of O- and/or N-linked oligosaccharides from the fetal products would enable detection of the NITEGE neoepitope with anti-NITEGE antibody.

METHODS

Aggrecan was purified from fetal and mature human and bovine cartilage and digested with: (1) ADAMTS4, (2) ADAMTS4, sialidase II, and N-glycanase, (3) ADAMTS4, sialidase II, and O-glycanase, or (4) ADAMTS4, sialidase II, and both N- and O-glycanases. Western blot analysis was performed using anti-G1 and anti-NITEGE antibodies.

RESULTS

When fetal G1-NITEGE products were treated with a combination of ADAMTS4, sialidase II, O-glycanase and N-glycanase, the resultant products migrated faster on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and the NITEGE neoepitope was rendered detectable.

CONCLUSIONS

It appears that the NITEGE neoepitope is blocked on Western blots by oligosaccharide structures present on Asn368 and Thr370 of fetal human and bovine aggrecans. Such masking structures do not appear to be present on mature aggrecans from these species. We suggest that when anti-NITEGE antibody is used in Western analysis, enzyme-linked immunosorbent assay (ELISA), fluorescence-activated cell sorting (FACS), and immunohistochemistry (IHC), removal of oligosaccharides with appropriate glycosidases may unmask reactivity that would otherwise go undetected. The implications of these findings for the much-studied effect of Asn368-linked keratan sulfate (KS)-based structures on ADAMTS4 and ADAMTS5 activity are discussed.

ADAMTS5-/- mice have less subchondral bone changes after induction of osteoarthritis through surgical instability: implications for a link between cartilage and subchondral bone changes

OBJECTIVE

Osteoarthritis (OA) is characterized by damaged articular cartilage and changes in subchondral bone. Previous work demonstrated aggrecanase-2 deficient (ADAMTS5-/-) mice to be protected from cartilage damage induced by joint instability. This study analyzed whether this protective effect on cartilage is also reflected in the subchondral bone structure.

METHODS

Right knee joints from 10-week old male wild type (WT) and ADAMTS5-/- mice received transection of the medial meniscotibial ligament to induce OA, whereas left knees were left unoperated. After 8 weeks knee joints were scanned by micro-CT. The proximal tibia was selected for further analysis. Histology was performed to evaluate cartilage damage and osteoclast presence.

RESULTS

ADAMTS5-/- joints had a significantly thinner subchondral plate and less epiphyseal trabecular bone compared to WT joints. Histology confirmed previous findings that ADAMTS5-/- mice have significantly less cartilage damage than WT in the instability-induced OA model. Although the subchondral bone plate became significantly thicker at the medial tibial plateau in operated joints of both genotypes, the percentage increase was significantly smaller in ADAMTS5-/- mice (WT: 20.7+/-4.7%, ADAMTS5-/-: 8.3+/-1.2% compared to the left unoperated control joint). In ADAMTS5-/- animals a significant decrease was found in both Oc.N./BS and Oc.S./BS. Finally, in WT but not in ADAMTS5-/- mice a significant correlation was found between medial subchondral bone plate thickness and cartilage damage at the medial tibial plateau.

CONCLUSION

ADAMTS5-/- joints that were protected from cartilage damage showed minor changes in the subchondral bone structure, in contrast to WT mice where substantial changes were found. This finding suggests links between the process of cartilage damage and subchondral bone changes in instability-induced OA.

Western blot quantification of aggrecan fragments in human synovial fluid indicates differences in fragment patterns between joint diseases

OBJECTIVE

To develop a Western blot method for quantification of multiple aggrecan fragments in human synovial fluids (SFs).

METHOD

SF aggrecan fragments were prepared from knee healthy (reference), knee injury and arthritis subjects by CsCl gradient centrifugations collecting D1 fractions. Samples were analyzed by Western blot, using antibodies against the N-terminal epitope ARGS and the G3 domain, and fragments were quantified using a digital luminescence image analyzer.

RESULTS

The method had a coefficients of variation of 10-30%, and a high correlation (r(S)=0.86) with a corresponding enzyme-linked immunosorbent assay (ELISA). The SFs from reference, knee injured and arthritic subjects contained two major ARGS fragments, ARGS-SELE and ARGS-CS1, and three major G3 fragments (GRGT-G3, GLGS-G3 and AGEG-G3). Compared to the reference, the acute arthritis and acute joint injury groups had a 30-fold elevated concentration of ARGS fragments, and both groups had a higher proportion of the aggrecan in joint fluid as ARGS fragments compared to the other groups. The reference and chronic injury groups had an excess of ARGS-CS1 fragments over ARGS-SELE fragments, while subjects with acute arthritis or osteoarthritis had a more even distribution between these fragments.

CONCLUSIONS

We have developed a novel Western blot quantification method for quantification of SF aggrecan fragments which can differentiate fragments of different sizes sharing the same epitope. The anti-ARGS and anti-G3 quantitative Western blots provided information important for a better understanding of the proteolytic pathways in aggrecan breakdown, information that discriminates between different joint diseases, and may aid in identification of new biomarkers.

The role of ADAMTS-7 and ADAMTS-12 in the pathogenesis of arthritis

Loss of articular cartilage caused by extracellular matrix breakdown is the hallmark of arthritis. Degradative fragments of cartilage oligomeric matrix protein (COMP) have been observed in arthritic patients. ADAMTS-7 and ADAMTS-12, two members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family, have been associated with COMP degradation in vitro, and are significantly overexpressed in the cartilage and synovium of patients with rheumatoid arthritis. Recent studies have demonstrated the importance of COMP degradation by ADAMTS-7 and ADAMTS-12. Specifically, the size of COMP fragments generated by ADAMTS-7 or ADAMTS-12 is similar to that of COMP-degradative fragments seen in arthritic patients. In addition, antibodies against ADAMTS-7 or ADAMTS-12 dramatically inhibit tumor necrosis factor-induced and interleukin-1beta-induced COMP degradation in cartilage explants. Furthermore, suppression of ADAMTS-7 or ADAMTS-12 expression using the small interfering RNA silencing approach in human chondrocytes markedly prevents COMP degradation. COMP degradation mediated by ADAMTS-7 and ADAMTS-12 is inhibited by alpha(2)-macroglobulin. More significantly, granulin-epithelin precursor, a newly characterized chondrogenic growth factor, disturbs the interaction between COMP and ADAMTS-7 and ADAMTS-12, preventing COMP degradation by these enzymes. This Review summarizes the evidence demonstrating that ADAMTS-7 and ADAMTS-12 are newly identified enzymes responsible for COMP degradation in arthritis, and that alpha(2)-macroglobulin and granulin-epithelin precursor represent their endogenous inhibitors.

Expression of ADAMTS-4 by chondrocytes in the surface zone of human osteoarthritic cartilage is regulated by epigenetic DNA de-methylation

The two major aggrecanases involved in osteoarthritis (OA) are ADAMTS-4 and ADAMTS-5. Knock-out studies suggested that ADAMTS-5, but not ADAMTS-4, is the major aggrecanase in murine OA. However, studies of human articular cartilage suggest that ADAMTS-4 also contributes to aggrecan degradation in human OA. This study investigated ADAMTS-4 in human OA. While ADAMTS-4 was virtually absent in control cartilage, numerous ADAMTS-4 immuno-positive chondrocytes were present in OA cartilage and their numbers increased with disease severity. RT-PCR confirmed expression, especially in the surface zone. DNA methylation was lost at specific CpG sites in the ADAMTS-4 promoter in OA chondrocytes, suggesting that the increased gene expression was more than a simple up-regulation, but involved loss of DNA methylation at specific CpG sites, resulting in a heritable and permanent expression of ADAMTS-4 in OA chondrocytes. These results suggest that ADAMTS-4 is epigenetically regulated and plays a role in aggrecan degradation in human OA.

Characterization of an ADAMTS-5-mediated cleavage site in aggrecan in OSM-stimulated bovine cartilage

OBJECTIVE

In a previous study, we identified a 50-kDa G3-containing aggrecan degradation product in bovine cartilage, released from the tissue after interleukin-1 (IL-1) stimulation in the presence of oncostatin M (OSM). Our objective was to purify, determine the N-terminal sequence of this fragment and verify whether this cleavage could be attributed to a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and ADAMTS-5 action in vitro.

METHODS

Collected media from bovine cartilage explant cultures stimulated with IL-1+OSM were subjected to anion-exchange chromatography. The N-terminal sequence of the fragment of interest in the purified fractions was determined by automated Edman sequencing. Fetal bovine aggrecan was digested with full-length recombinant ADAMTS-4 and ADAMTS-5 and resulting degradation products were analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE) and immunoblotting using an anti-G3 antiserum and an anti-neoepitope antibody that had been generated to the new N-terminus of the G3 fragment.

RESULTS

Characterization of the 50-kDa fragment showed that it possesses chondroitin sulfate (CS) and is the result of a cleavage within the C-terminal portion of the CS-2 domain, adjacent to the G3 region. Sequence analysis identified the cleavage region as TQRPAE(2047)-(2048)ARLEIE, suggesting an aggrecanase-derived product. Using an anti-neoepitope antibody specific for the additional cleavage site, it was shown that the product is generated in vitro upon digestion of aggrecan by ADAMTS-5 and, to a much lesser extent, by ADAMTS-4.

CONCLUSIONS

The abundance and rapid rate of release of this degradation product in organ cultures in the presence of OSM suggest that it could result from a unique aggrecan proteolysis mediated by aggrecanases.

Articular chondrocytes derived from distinct tissue zones differentially respond to in vitro oscillatory tensile loading

OBJECTIVE

The cell morphology, gene expression, and matrix synthesis of articular chondrocytes are known to vary with depth from the tissue surface. The objective of this study was to investigate if chondrocytes from different zones respond to in vitro oscillatory tensile loading in distinct ways and whether tensile strain, which is most prevalent near the articular surface, would preferentially stimulate superficial zone chondrocytes.

DESIGN

Chondrocytes were separately isolated from the superficial, middle, and deep zones of articular cartilage and seeded into three-dimensional fibrin hydrogel constructs. An intermittent protocol of oscillatory tensile loading was applied for 3 days, and the effects on extracellular matrix (ECM) synthesis were assessed by measuring the incorporation of radiolabed precursors, size exclusion gel chromatography, and western blotting.

RESULTS

Tensile loading was found to be a potent stimulus for proteoglycan synthesis only in superficial zone chondrocytes. Although overall biosynthesis rates by deep zone chondrocytes were unaffected by tensile loading, the molecular characteristics of proteins and proteoglycans released to the culture medium were significantly altered so as to resemble those of superficial zone chondrocytes.

CONCLUSIONS

Oscillatory tensile loading differentially affected subpopulations of articular chondrocytes in three-dimensional fibrin hydrogel constructs. Cells isolated from deeper regions of the tissue developed some characteristics of superficial zone chondrocytes after exposure to tensile loading, which may indicate an adaptive response to the new mechanical environment. Understanding how exogenous mechanical stimuli can differentially influence chondrocytes from distinct tissue zones will yield important insights into mechanobiological processes involved in cartilage tissue development, maintenance, disease, and repair.

Aggrecanolysis in human osteoarthritis: confocal localization and biochemical characterization of ADAMTS5-hyaluronan complexes in articular cartilages

OBJECTIVE

Human osteoarthritis (OA) is characterized by aggrecanase-mediated depletion of cartilage aggrecan. We have examined the abundance, location and some biochemical properties of the six known aggrecanases (A disintegrin and metalloproteinase with thrombospondin-like motifs 1 (ADAMTS1) 4, 5, 8, 9 and 15) in normal and OA human cartilages.

METHODS

Formalin-fixed, ethylenediamine tetraacetic acid (EDTA)-decalcified sections of full-depth cartilage from human OA tibial plateaus and normal control samples were studied by confocal imaging. Probes included specific antibodies to aggrecanases and two aggrecan epitopes, as well as biotinylated hyaluronan binding protein (HABP) for hyaluronan (HA) visualization. Cartilage extracts were analyzed by Western blot for the individual proteinases and aggrecan fragments.

RESULTS

ADAMTS5 was present in association with cells throughout normal cartilage and was markedly increased in OA, particularly in clonal groups in the superficial and transitional zones, where it was predominantly co-localized with HA. Consistent with the confocal analysis, a high molecular weight complex of ADAMTS5 and HA was isolated from human OA cartilage by isotonic salt extraction and chromatography on Superose 6. The complex eluted with an apparent molecular size of about 2x10(6) and contained major ADAMTS5 forms of 150, 60, 40 and 30kDa. The yield of most forms on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was markedly enhanced by prior digestion of the complex with either Streptomyces hyaluronidase or chondroitinase ABC.

CONCLUSION

ADAMTS5 abundance and distribution in human OA cartilages is consistent with a central role for this enzyme in destructive aggrecanolysis. HA-dependent sequestration of ADAMTS5 in the pericellular matrix may be a mechanism for regulating the activity of this proteinase in human OA cartilage.

Signaling pathways implicated in oncostatin M-induced aggrecanase-1 and matrix metalloproteinase-13 expression in human articular chondrocytes

Molecular mechanisms of oncostatin M (OSM)-stimulated cartilage extracellular matrix catabolism and signaling pathways were investigated in human arthritic chondrocytes. OSM, alone or with Interleukin-1 (IL-1beta), increased glycosaminoglycan release and induced ADAMTS-4 and MMP-13 protein expression in human cartilage explants. OSM dose- and time-dependently increased ADAMTS-4 mRNA and MMP-13 protein expression in human femoral head chondrocytes. Extracellular signal-regulated kinases (ERK1/2)-MAPK pathway inhibitor, U0126, down-regulated ADAMTS-4 and MMP-13 induction by OSM. Janus kinase 2 (JAK2) inhibitor, AG490, suppressed OSM-induced ADAMTS-4 mRNA expression but did not affect MMP-13 levels while JAK3 pharmacological inhibitor and siRNA transfection suppressed both. Parthenolide, a signal transducer and activator of transcription (STAT1 and STAT3) phosphorylation inhibitor, reduced OSM-induced ADAMTS-4 and MMP-13 gene expression and prevented STAT1/3 DNA binding activity. Additionally, OSM-enhanced ADAMTS-4 mRNA and MMP-13 expression was down-regulated by phosphatidylinositol 3-kinase (PI3K) and Akt/PKB inhibitors, LY294002 and NL-71-101. Furthermore, JAK3 inhibition time-dependently down-regulated Akt but not ERK1/2 phosphorylation suggesting that Akt is a downstream target of JAK3. These results suggest that OSM-stimulated ADAMTS-4 and MMP-13 expression is mediated by ERK1/2, JAK3/STAT1/3 and PI3K/Akt and by cross talk between these pathways. The inhibitors of these cascades could block OSM-evoked degeneration of cartilage by ADAMTS-4 and MMP-13.

Analysis of cartilage matrix fixed charge density and three-dimensional morphology via contrast-enhanced microcomputed tomography

Small animal models of osteoarthritis are often used for evaluating the efficacy of pharmacologic treatments and cartilage repair strategies, but noninvasive techniques capable of monitoring matrix-level changes are limited by the joint size and the low radiopacity of soft tissues. Here we present a technique for the noninvasive imaging of cartilage at micrometer-level resolution based on detecting the equilibrium partitioning of an ionic contrast agent via microcomputed tomography. The approach exploits electrochemical interactions between the molecular charges present in the cartilage matrix and an ionic contrast agent, resulting in a nonuniform equilibrium partitioning of the ionic contrast agent reflecting the proteoglycan distribution. In an in vitro model of cartilage degeneration we observed changes in x-ray attenuation magnitude and distribution consistent with biochemical and histological analyses of sulfated glycosaminoglycans, and x-ray attenuation was found to be a strong predictor of sulfated glycosaminoglycan density. Equilibration with the contrast agent also permits direct in situ visualization and quantification of cartilage surface morphology. Equilibrium partitioning of an ionic contrast agent via microcomputed tomography thus provides a powerful approach to quantitatively assess 3D cartilage composition and morphology for studies of cartilage degradation and repair.

Selective and non-selective metalloproteinase inhibitors reduce IL-1-induced cartilage degradation and loss of mechanical properties

Articular cartilage undergoes matrix degradation and loss of mechanical properties when stimulated with proinflammatory cytokines such as interleukin-1 (IL-1). Aggrecanases and matrix metalloproteinases (MMPs) are thought to be principal downstream effectors of cytokine-induced matrix catabolism, and aggrecanase- or MMP-selective inhibitors reduce or block matrix destruction in several model systems. The objective of this study was to use metalloproteinase inhibitors to perturb IL-1-induced matrix catabolism in bovine cartilage explants and examine their effects on changes in tissue compression and shear properties. Explanted tissue was stimulated with IL-1 for up to 24 days in the absence or presence of inhibitors that were aggrecanase-selective, MMP-selective, or non-selective. Analysis of conditioned media and explant digests revealed that aggrecanase-mediated aggrecanolysis was delayed to varying extents with all inhibitor treatments, but that aggrecan release persisted. Collagen degradation was abrogated by MMP- and non-selective inhibitors and reduced by the aggrecanase inhibitor. The inhibitors delayed but did not reduce loss of the equilibrium compression modulus, whereas the losses of dynamic compression and shear moduli were delayed and reduced. The data suggest that non-metalloproteinase mechanisms participate in IL-1-induced matrix degradation and loss of tissue material properties.

Estimation of the identity of proteolytic aggrecan fragments using PAGE migration and Western immunoblot

OBJECTIVE

To develop calculation models, using Western immunoblot, as a tool for the estimation of proteolytic human aggrecan fragment identity.

METHOD

Seven human aggrecan fragments (calibrators), purified by CsCl gradient centrifugation and identified by Western immunoblot of N- and C-terminals, were used to develop calculation models. The models were used for identification of unknown aggrecan fragments each having one of their N- or C-terminals identified.

RESULTS

The calibrator molecular weights (Mw) from sodium dodecyl sulfate (SDS)-gels (m), the Mw of amino acids (a) and the Mw of their carbohydrate substitution (g) were expressed as K = m/(a+g), or as K = 1.085m/(a+g) when compensation for the G1 domain was required. Using these models together with average K-values, 12 out of the 17 immuno-detected aggrecan fragments were calculated to a known protease cleavage site, while five were identified to domain levels.

CONCLUSIONS

With six neoepitope antibodies together with antibodies against the G1- and G3-domain it was possible to predict the identity of several proteolytic fragments from different regions within the aggrecan monomer.

Effects of covalently attached chondroitin sulfate on aggrecan cleavage by ADAMTS-4 and MMP-13

Aggrecan is degraded by several aggrecanase-1 (ADAMTS-4) isoforms differing in the number of sulfated glycosaminoglycan (sGAG)-binding motifs. ADAMTS-4 and MMPs cleave aggrecan more efficiently within the chondroitin sulfate (CS)-rich region than the interglobular domain (IGD). We investigated the influence of CS on aggrecan core protein cleavage by ADAMTS-4 (p68) and (p40) as well as MMP-13, which has no recognizable GAG-binding sites. Chondroitinase ABC-treated cartilage aggrecan was cleaved with ADAMTS-4 (p68) less efficiently than CS-substituted aggrecan within the CS-2 domain. Keratanase-treated aggrecan exhibited reduced IGD cleavage, but when both CS and KS were removed, the IGD cleavage was restored. This result suggests that KS in the IGD may compete with CS for ADAMTS-4 (p68) binding. In the absence of KS, however, p68 binding was shifted to the CS-2 domain. CS-deficient full-length recombinant aggrecan (rbAgg) was produced by chondroitinase ABC treatment, or by expression in the xylosyltransferase-deficient CHO-pgsA745 cell line. When digested with the ADAMTS-4 (p68), each of these preparations exhibited reduced CS-2 domain cleavage compared to CS-substituted CHO-K1 cell-derived aggrecan. Additionally, CS-deficient rbAgg showed increased IGD scission prior to cleavage within the CS-2 domain. ADAMTS-4 (p40) readily cleaved both rbAggs within the IGD, but cleaved poorly within the CS-2 domain, indicating little CS dependence. MMP-13, in contrast, cleaved the CS region and the IGD of both CS-substituted and CS-deficient rbAgg equally well. These data indicate that covalently bound CS enhances ADAMTS-4-mediated cleavage within the CS-rich region. MMP-13 also cleaves preferentially within the CS-region, but by an apparently CS-independent mechanism.

Development and characterization of a highly specific and sensitive sandwich ELISA for detection of aggrecanase-generated aggrecan fragments

OBJECTIVE

To develop an enzyme linked immunosorbent assay (ELISA) to quantify the levels of specific aggrecan fragments generated by aggrecanase-mediated cleavage at the 373Glu-374 Ala bond within the aggrecan interglobular domain.

METHODS

The ELISA employs a commercially available monoclonal antibody to capture aggrecan fragments containing keratan sulfate (KS). Aggrecan fragments generated by cleavage at the Glu-Ala bond were then detected using a monoclonal neoepitope antibody (mAb OA-1) that specifically recognizes the N-terminal sequence 'ARGSVIL'.

RESULTS

The mAb OA-1 antibody was highly specific for the immunizing neoepitope peptide since neither peptides spanning the cleavage site nor mutated peptides were detected. Aggrecan fragments generated by ADAMTS-4 digested human aggrecan monomers and from IL-1-stimulated human cartilage explants were quantified by the ELISA, and we observed increased sensitivity of the ELISA compared to mAb OA-1 Western analysis. We also observed that the basal, as well as IL-1-stimulated production of ARGS aggrecan fragments from human articular cartilage explants was blocked by a selective aggrecanase inhibitor, consistent with generation of the ARGS neoepitope in human articular cartilage being mediated by aggrecanase. Using purified human aggrecan digested by ADAMTS-4 as standard to quantify ARGS aggrecan fragments in human synovial fluids, we determined that the calculated amount of ARGSVIL-aggrecan fragments by ELISA measurement is in agreement with the published levels of these fragments, supporting its potential utility as a biomarker assay for osteoarthritis.

CONCLUSION

We have developed an assay that detects and quantifies specific aggrecan fragments generated by aggrecanase-mediated cleavage. Because aggrecanase mediates degradation of human articular aggrecan in joint disease, the KS/mAb OA-1 ELISA may serve as a biomarker assay for evaluation of preclinical and clinical samples.

Assorted effects of TGFbeta and chondroitinsulfate on p38 and ERK1/2 activation levels in human articular chondrocytes stimulated with LPS

OBJECTIVES

Inadequate cellular response of chondrocytes to stress frequently terminates in osteoarthritis (OA). Adequate response is fundamentally modulated by concerted cytokine signaling events, directing degradation and synthesis of cartilage on articular surfaces where and whenever necessary. Transforming growth factor (TGF)beta is a prominent mediator in cartilage anabolism, although particular catabolic activities are occasionally reported. Clearly, before the TGFbeta signal gets through to the gene regulatory machinery, cross talk with modulators occurs.

METHOD

We tested the hypothesis whether chondroitinsulfate (CS) modulates cell signaling. TGFbeta and/or soluble CS was added to human articular chondrocytes (HACs) and activation of p38 and extracellular signal related kinase (ERK)1/2 was determined by immunoblot analysis. Expression levels of mRNA of matrix metalloproteinase (MMP)-2, -3 and -13 were determined by real-time polymerase chain reaction (PCR).

RESULTS

No significant effects were observed unless cells were stimulated with lipopolysaccharide (LPS), invigorating catabolic metabolism in chondrocytes. LPS effects, however, were profoundly modulated by TGFbeta, CS and both applied in combination. Most prominent, the silencing of p38 stress signal by CS was superimposable to that of TGFbeta. Phospho-ERK1/2 levels were raised by TGFbeta three-fold over LPS induced levels. In contrast, CS treatment, alone or combined with TGFbeta, reduced phosphorylation significantly below LPS induced levels. Finally, suppression of LPS induced MMP-13 mRNA levels resulted with CS.

CONCLUSION

Soluble CS modulates signaling events in chondrocytes concurrent with MMP-13 down regulation. The effects observed suggest a feedback signaling mechanism cross talking with TGFbeta-signal pathways and may serve an explanation, on the cellular level, for the beneficial effects found in clinical studies with pharmacologic application of CS.

Products resulting from cleavage of the interglobular domain of aggrecan in samples of synovial fluid collected from dogs with early- and late-stage osteoarthritis

OBJECTIVE

To investigate interglobular domain (IGD) cleavage of aggrecan in dogs with naturally developing osteoarthritis (OA).

SAMPLE POPULATION

Samples of synovial fluid (SF) obtained from 3 cubital (elbow) joints and 3 stifle joints of 4 clinically normal dogs, 24 elbow joints of 12 dogs with early-stage OA, 8 stifle joints of 5 dogs with early-stage OA, and 10 stifle joints of 9 dogs with late-stage OA.

PROCEDURE

Fractions of SF were assayed for total glycosaminoglycan (GAG) content and also subjected to Western blot analysis by use of monoclonal antibodies against neoepitopes generated by cleavage of the IGD of the aggrecan protein core by matrix metalloproteinase (MMP; BC-14) and aggrecanase (BC-3).

RESULTS

Total GAG content of SF from joints of clinically normal dogs did not differ from that of dogs with early-stage OA. The GAG content of SF from joints of dogs with late-stage OA was significantly lower, compared with GAG content for other SF samples. Aggrecanase-generated fragments were detected in SF from all groups but not in all samples. Matrix metalloproteinase-generated fragments were not detected in any SF samples. In early-stage OA, high-molecular-weight aggrecanase-generated aggrecan catabolites were evident.

CONCLUSIONS AND CLINICAL RELEVANCE

GAG content of SF obtained from dogs with late-stage OA is significantly decreased, suggesting proteoglycan depletion of cartilage. Aggrecanases, but not MMPs, are the major proteolytic enzymes responsible for IGD cleavage of aggrecan in canine joints. Analyses of SF samples to detect aggrecanase-generated catabolites may provide an early biomarker for discriminating early- and late-stage OA in dogs.

A microplate assay for the screening of ADAMTS-4 inhibitors

Aggrecanase plays a major role in cartilage proteoglycan degradation in rheumatic diseases such as osteoarthritis and rheumatoid arthritis. The search of new inhibitors of aggrecanase activity necessitates a robust assays in order to be able to screen large numbers of compounds. We present in this paper an assay based on the cleavage of His-tagged aggrecan interglobular domain by N- and C- terminus truncated, active aggrecanase-1/ADAMTS-4, with formation of the aggrecanase-specific ARGSV neoepitope. This is detected by anti-ARGSV antibody, in turn recognized by a fluorescent anti-IgG. Furthermore, the formation of the reaction products was confirmed by high-pressure capillary electrophoresis. This assay allows the rapid screening of aggrecanase inhibitors in a 96-well plate format, allowing an immediate transposition to high-throughput scale up.

Comparison of age-dependent expression of aggrecan and ADAMTSs in mandibular condylar cartilage, tibial growth plate, and articular cartilage in rats

A disintegrin and metalloproteinase with thrombospondin motif (adamalysin-thrombospondins, ADAMTS) degrades aggrecan, one of the major extracellular matrix (ECM) components in cartilage. Mandibular condylar cartilage differs from primary cartilage, such as articular and growth plate cartilage, in its metabolism of ECM, proliferation, and differentiation. Mandibular condylar cartilage acts as both articular and growth plate cartilage in the growing period, while it remains as articular cartilage after growth. We hypothesized that functional and ECM differences between condylar and primary cartilages give rise to differences in gene expression patterns and levels of aggrecan and ADAMTS-1, -4, and -5 during growth and aging. We employed in situ hybridization and semiquantitative RT-PCR to identify mRNA expression for these molecules in condylar cartilage and primary cartilages during growth and aging. All of the ADAMTSs presented characteristic, age-dependent expression patterns and levels among the cartilages tested in this study. ADAMTS-5 mainly contributed to ECM metabolism in growth plate and condylar cartilage during growth. ADAMTS-1 and ADAMTS-4 may be involved in ECM turn over in articular cartilage. The results of the present study reveal that ECM metabolism and expression of related proteolytic enzymes in primary and secondary cartilages may be differentially regulated during growth and aging.

ADAMTS5-mediated aggrecanolysis in murine epiphyseal chondrocyte cultures

OBJECTIVE

Aggrecan degradation by aggrecanases [a disintegrin and metalloproteinase with thrombospondin-like motifs (ADAMTS) 1, 4, 5, 8, 9, 15] is considered to initiate much of the cartilage pathology seen in human arthritis, however, the proteinase responsible and its mode of control is unclear. The present work was done to examine mechanisms of aggrecanase control in a novel murine epiphyseal cell system and to determine whether ADAMTS5 alone is responsible for aggrecanolysis by these cells.

METHODS

Epiphyseal cells from 4-day-old mice (wild type, TS-5 (-/-), CD44(-/-), syndecan-1(-/-), membrane type-4 matrix metalloproteinase [MT4MMP(-/-)]) were maintained in non-adherent aggregate cultures and aggrecanolysis studied by biochemical and histochemical methods. Confocal immunolocalization analyses were done with specific probes for ADAMTS5, hyaluronan (HA) and aggrecanase-generated fragments of aggrecan.

RESULTS

Aggrecanolysis by these cells was specifically aggrecanase-mediated and it occurred spontaneously without the need for addition of catabolic stimulators. Chondrocytes from ADAMTS5-null mice were aggrecanase-inactive whereas all other mutant cells behaved as wild type in this regard suggesting that ADAMTS5 activity is not controlled by CD44, syndecan-1 or MT4MMP in this system. Immunohistochemical analysis supported the central role for ADAMTS5 in the degradative pathway and indicated that aggrecanolysis occurs primarily in the HA-poor pericellular region in these cultures.

CONCLUSION

These findings are consistent with published in vivo studies showing that single-gene ADAMTS5 ablation confers significant protection on cartilage in murine arthritis. We propose that this culture system and the analytical approaches described provide a valuable framework to further delineate the expression, activity and control of ADAMTS-mediated aggrecanolysis in human arthritis.

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.

A contentious issue finds some clarity: on the independent and complementary roles of aggrecanase activity and MMP activity in human joint aggrecanolysis

Our understanding of aggrecanolysis in the human joint has recently been clarified by detailed analysis of naturally occurring intermediates in cartilage and synovial fluids. The most studied aspect has been the proteolysis of the interglobular domain (IGD) of aggrecan with release of the glycosaminoglycan (GAG)-attachment regions, because this appears to be most destructive to tissue function. In this Editorial review, a working model is presented which supports the view that one or more aggrecanases (ADAMTS 1, 4, 5, 8, 9, 15) are responsible for cleavage of the IGD with destructive loss of tissue GAG. In contrast, one or more metalloproteinases (MMPs) (MMP 1, 2, 3, 7, 8, 9, 10, 13, 14, 19, 20) are responsible for cleavage of the IGD (at Asn360-Phe361) within a separate pool of aggrecan, which does not bear GAG, because it has previously been C-terminally truncated in a separate slow turnover process. These findings, along with recent gene deletion studies in mice, suggest that ADAMTS-mediated aggrecanolysis is destructive to cartilage function whereas MMP-mediated aggrecanolysis may actually be beneficial.

Mammalian expression of full-length bovine aggrecan and link protein: formation of recombinant proteoglycan aggregates and analysis of proteolytic cleavage by ADAMTS-4 and MMP-13

Aggrecan, a large chondroitin sulfate (CS) and keratan sulfate (KS) proteoglycan, has not previously been expressed as a full-length recombinant molecule. To facilitate structure/function analysis, we have characterized recombinant bovine aggrecan (rbAgg) and link protein expressed in COS-7 cells. We demonstrate that C-terminally truncated rbAgg was not secreted. Gel filtration chromatography of rbAgg and isolated glycosaminoglycan (GAG) chains, and their susceptibility to chondroitinase ABC digestion indicate that the GAG chains are predominantly CS, which likely occupy fewer serine residues than native aggrecan. To confirm functionality, we determined that rbAgg bound hyaluronan and recombinant link protein to form proteoglycan aggregates. In addition, cleavage of rbAgg by ADAMTS-4 revealed that the p68 form of ADAMTS-4 preferentially cleaves within the CS-2 domain, whereas the p40 form only effectively cleaves within the interglobular domain (IGD). MMP-13 cleaved rbAgg within the IGD, but cleaved more rapidly at a site within the CS domains, suggesting a role in C-terminal processing of aggrecan. Our results demonstrate that recombinant aggrecan can be used for in vitro analyses of matrix protease-dependent degradation of aggrecan in the IGD and CS domains, and both recombinant aggrecan and link protein can be used to study the assembly of proteoglycan aggregates with hyaluronan.

Effect of glucosamine and chondroitin sulfate on regulation of gene expression of proteolytic enzymes and their inhibitors in interleukin-1-challenged bovine articular cartilage explants

OBJECTIVE

To determine the effects of glucosamine (GLN) and chondroitin sulfate (CS), at concentrations attainable in vivo, on expression of genes encoding proteolytic enzymes, enzyme inhibitors, and macromolecules of articular cartilage in interleukin-1(IL-1)-challenged bovine cartilage explants.

SAMPLE POPULATION

Articular cartilage explants harvested from 9 steers.

PROCEDURES

Cartilage explants were exposed to media containing 10% fetal bovine serum (FBS) only, IL-1 (50 ng/mL), IL-1 with GLN (5 microg/mL), IL-1 with CS (20 microg/mL), or IL-1 with GLN and CS for 24 and 48 hours. Cartilage was frozen, and RNA was extracted. Gene expression of matrix metalloproteinases (MMPs)-2, -3, -9, -13, and -14; aggrecanases (Aggs)-1 and -2; tissue inhibitors of metalloproteinases (TIMPs)-1, -2, and -3; and type II collagen and aggrecan were assessed with quantitative real-time polymerase chain reaction.

RESULTS

Upregulated MMP-3, MMP-13, and Agg-1 transcripts at 24 hours were repressed by the GLN and CS combination by at least approximately 6-fold. Glucosamine was effective in suppressing IL-1-induced mRNA expression of MMP-13, Agg-1, and Agg-2, whereas CS was effective in decreasing IL-1-induced MMP-13 transcript at 24 hours. At 48 hours, GLN and CS added separately and in combination significantly abrogated Agg-1 and Agg-2 gene induction. The combination also decreased IL-1-stimulated MMP-13 transcript.

CONCLUSIONS AND CLINICAL RELEVANCE

GLN and CS, at concentrations that are within the range measured in synovial fluid and blood after oral administration, may regulate expression of matrix degrading enzymes and their inhibitors at the transcriptional level, providing a plausible mechanism for their purported chondroprotective properties.

ADAMTS proteinases: a multi-domain, multi-functional family with roles in extracellular matrix turnover and arthritis

Members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family are known to influence development, angiogenesis, coagulation and progression of arthritis. As proteinases their substrates include the von Willebrand factor precursor and extracellular matrix components such as procollagen, hyalectans (hyaluronan-binding proteoglycans including aggrecan), decorin, fibromodulin and cartilage oligomeric matrix protein. ADAMTS levels and activities are regulated at multiple levels through the control of gene expression, mRNA splicing, protein processing and inhibition by TIMP (tissue inhibitor of metalloproteinases). A recent screen of human cartilage has shown that multiple members of the ADAMTS family may be important in connective tissue homeostasis and pathology.

Matrix metalloproteinases are not essential for aggrecan turnover during normal skeletal growth and development

The growth plate is a transitional region of cartilage and highly diversified chondrocytes that controls long bone formation. The composition of growth plate cartilage changes markedly from the epiphysis to the metaphysis, notably with the loss of type II collagen, concomitant with an increase in MMP-13; type X collagen; and the C-propeptide of type II collagen. In contrast, the fate of aggrecan in the growth plate is not clear: there is biosynthesis and loss of aggrecan from hypertrophic cartilage, but the mechanism of loss is unknown. All matrix metalloproteinases (MMPs) cleave aggrecan between amino acids N341 and F342 in the proteinase-sensitive interglobular domain (IGD), and MMPs in the growth plate are thought to have a role in aggrecanolysis. We have generated mice with aggrecan resistant to proteolysis by MMPs in the IGD and found that the mice develop normally with no skeletal deformities. The mutant mice do not accumulate aggrecan, and there is no significant compensatory proteolysis occurring at alternate sites in the IGD. Our studies reveal that MMP cleavage in this key region is not a predominant mechanism for removing aggrecan from growth plate cartilage.

Analysis of ADAMTS4 and MT4-MMP indicates that both are involved in aggrecanolysis in interleukin-1-treated bovine cartilage

OBJECTIVE

To investigate the mechanism of aggrecanolysis in interleukin-1 (IL-1)-treated cartilage tissue by examining the time course of aggrecan cleavages and the tissue and medium content of membrane type 4-matrix metalloproteinases (MT4-MMP) and a disintegrin and metalloproteinase with thrombospondin type I motifs (ADAMTS)4.

METHODS

Articular cartilage explants were harvested from newborn bovine femoropatellar groove. The effects of IL-1 treatment with or without aggrecanase blockade were investigated by Western analysis of aggrecan fragment generation, ADAMTS4 species (p68 and p53), and MT4-MMP, as well as by realtime PCR (polymerase chain reaction) for ADAMTS4 and 5. Aggrecanase was blocked with mannosamine (ManN), an inhibitor of glycosylphosphatidylinositol anchor synthesis, and esculetin (EST), an inhibitor of MMP-1, MMP-3, and MMP-13 gene expression.

RESULTS

IL-1 treatment caused a major increase in MT4-MMP abundance in the tissue and medium. ADAMTS4 (p68) was abundant in fresh cartilage and this was retained in the tissue in untreated cartilage. IL-1 treatment for 6 days caused a marked loss of p68 from the cartilage and the appearance of p53 in the medium. Addition of either 1.35 mM ManN or 31-500 microM EST blocked IL-1-mediated aggrecanolysis and this was accompanied by nearly complete inhibition of the MT4-MMP increase, the p68 loss and the formation of p53. IL-1 treatment increased mRNA abundance for ADAMTS4 ( approximately 3-fold) and ADAMTS5 ( approximately 10-fold) but this was not accompanied by a marked change in enzyme protein abundance.

CONCLUSION

These studies support a central role for MT4-MMP in IL-1-induced cartilage aggrecanolysis and are consistent with the identification of p68 as the aggrecanase that cleaves within the CS2 domain, and of p53 as the aggrecanase that generates G1-NITEGE. Since the induction by IL-1 was not accompanied by marked changes in total ADAMTS4 protein, but rather in partial conversion of p68 to p53 and release of both from the tissue, we conclude that aggrecanolysis in this model system results from MT4-MMP-mediated processing of a resident pool of ADAMTS4 and release of the p68 and p53 from their normal association with the cell surface.

Mature bovine articular cartilage contains abundant aggrecan that is C-terminally truncated at Ala719-Ala720, a site which is readily cleaved by m-calpain

Extracts of normal mature articular cartilage contain aggrecan molecules which bear the G1 domain (the N-terminal globular domain of aggrecan) and are C-terminally truncated by proteolysis at a number of sites. A proportion of these molecules are generated by an aggrecanase and/or matrix-metalloproteinase-mediated cleavage in the IGD (interglobular domain between the G1 and G2 domains of aggrecan). However, the proteinase(s) responsible for formation of the majority of the larger G1-G2 and glycosaminoglycan-bearing truncated species is (are) unknown. N-terminal sequencing of aggrecan core fragments generated by m-calpain digestion of bovine aggrecan has identified four novel cleavage sites: one within the CS (chondroitin sulphate)-1 domain (at one or more of the bonds Ser1229-Val1230, Ser1249-Val1250, Ser1287-Val1288, Gly1307-Val1308 and Ser1346-Val1347), two within the IGD (at bonds Ala474-Ala475 and Gly365-Gly366) and one within the KS (keratan sulphate) domain (at Ala719-Ala720). A new monoclonal antibody (SK-28) to the C-terminal neoepitope at M710VTQVGPGVA719 showed that aggrecan products generated by this cleavage are present in high abundance in mature bovine articular cartilage extracts. We conclude that m-calpain, or an unidentified proteinase with the capacity to cleave at the same site, is active during aggrecan biosynthesis/secretion by mature chondrocytes or in the matrix of mature bovine articular cartilage in vivo.

ADAMTS10 mutations in autosomal recessive Weill-Marchesani syndrome

Weill-Marchesani syndrome (WMS) is characterized by the association of short stature; brachydactyly; joint stiffness; eye anomalies, including microspherophakia and ectopia of the lenses; and, occasionally, heart defects. We have recently mapped a gene for the autosomal recessive form of WMS to chromosome 19p13.3-p13.2, in a 12.4-cM interval. Here, we report null mutations in a member of the extracellular matrix protease family, the gene encoding ADAMTS10, a disintegrin and metalloprotease with thrombospondin motifs. A total of three distinct mutations were identified in two consanguineous families and in one sporadic WMS case, including one nonsense mutation (R237X) and two splice mutations (1190+1G-->A and 810+1G-->A). ADAMTS10 expression studies using reverse-transcriptase polymerase chain reaction, northern blot, and dot-blot analyses showed that ADAMTS10 is expressed in skin, fetal chondrocytes, and fetal and adult heart. Moreover, electron microscopy and immunological studies of the skin fibroblasts from the patients confirmed impairment of the extracellular matrix. We conclude, therefore, that ADAMTS10 plays a major role in growth and in skin, lens, and heart development in humans.

Extracellular matrix metabolites as potential biomarkers of disease activity in wound fluid: lessons learned from other inflammatory diseases?

The new era of pharmacogenetics has identified a potential for individuals to receive customized treatments for a variety of disease states. For such individualized treatments to fulfil their potential, it will be essential for clinicians to be able to monitor disease activity, ideally in a rapid, noninvasive fashion. The accessibility of the skin offers much potential to develop noninvasive tests of metabolic and disease activity for clinical use. Impaired human wound healing in the skin is a chronic inflammatory disorder in which the development of such tests has considerable potential, aiding clinical decision making and monitoring responses to treatment. This review article discusses how studies in other human diseases have highlighted potential biochemical markers (biomarkers) of disease activity in secreted biofluids, as aids to determining disease and metabolic activity within tissues. Using, as examples, lessons learned in the study of disease activity and prognosis of other chronic inflammatory conditions, such as osteoarthritis and periodontal disease, this review highlights the potential of dermal extracellular matrix (ECM) components (collagens, proteoglycans, hyaluronan and glycoproteins) for such uses. The limitations of currently utilized techniques and the concept that analysis of ECM components in wound fluid may represent useful biomarkers of disease activity are also discussed.

ADAMTS4 (aggrecanase-1) cleaves human brain versican V2 at Glu405-Gln406 to generate glial hyaluronate binding protein

Human brain tissue from cerebellum and hippocampus was obtained between 2 h and 24 h post mortem and, after extraction in the presence of proteinase inhibitors, proteoglycans were purified by anion-exchange chromatography. The versican component was characterized by Western analysis with antibodies to the N-terminal peptide (LF99), the N-terminal globular domain (12C5) and the two GAG (glycosaminoglycan) attachment regions (anti-GAG-alpha and anti-GAG-beta). The results indicated that versican V2 is the major variant in all brain samples, and that it exists as the full-length form and also as at least six C-terminally truncated forms. The major immunoreactive species present is a 64 kDa product, which we identified by biochemical and immunological analysis as the brain protein previously termed GHAP (glial hyaluronate binding protein) [Perides, Lane, Andrews, Dahl and Bignami (1989) J. Biol. Chem. 264, 5981-5987]. Immunological analysis of purified human GHAP using a new anti-neoepitope antiserum (JSCNIV) showed that its C-terminal sequence is NIVSFE(405), and digestion of human cerebellum proteoglycans with ADAMTS4 (aggrecanase-1, where ADAMTS, a disintegrin and metalloproteinase with thrombospondin-1-like motifs) indicated that GHAP is a product of cleavage of versican V0 or V2 at the Glu(405)-Gln(406) bond. Since human cerebellum extracts contained multiple forms of ADAMTS4 protein on Western analysis, these data suggest that one or more members of the 'aggrecanase' group of the ADAMTS family (ADAMTS 1, 4, 5 and 9) are responsible for turnover of versican V2 in the adult human brain.

Variations in aggrecan structure modulate its susceptibility to aggrecanases

Proteoglycan aggregates and purified aggrecan from adult and fetal bovine cartilage and adult and neonatal human cartilage were subjected to in vitro degradation by recombinant aggrecanase-1 and aggrecanase-2. The ability of the aggrecanases to cleave within the aggrecan IGD (interglobular domain) and CS2 domain (chondroitin sulphate-rich domain 2) was monitored by SDS/PAGE and immunoblotting. Aggrecanase-2 showed a similar ability to cleave within the IGD of adult and immature aggrecan, whereas aggrecanase-1 was less efficient in cleavage in the IGD of immature aggrecan, for both the bovine and the human substrates. Both aggrecanases showed a similar ability to cleave within the CS2 domain of bovine aggrecan irrespective of age, but showed a much lower ability to cleave within the CS2 domain of human aggrecan. Equivalent results were obtained whether aggrecan was present in isolation or as part of proteoglycan aggregates. When proteoglycan aggregates were used, neither aggrecanase was able to cleave link protein. Thus, for aggrecan cleavage by aggrecanases, variations in cleavage efficiency exist with respect to the species and age of the animal from which the aggrecan is derived and the type of aggrecanase being used.

Individual cartilage aggrecan macromolecules and their constituent glycosaminoglycans visualized via atomic force microscopy

Atomic force microscopy was used in ambient conditions to directly image dense and sparse monolayers of bovine fetal epiphyseal and mature nasal cartilage aggrecan macromolecules adsorbed on mica substrates. Distinct resolution of the non-glycosylated N-terminal region from the glycosaminoglycan (GAG) brush of individual aggrecan monomers was achieved, as well as nanometer-scale resolution of individual GAG chain conformation and spacing. Fetal aggrecan core protein trace length (398+/-57 nm) and end-to-end length (257+/-87 nm) were both larger than that of mature aggrecan (352+/-88 and 226+/-81 nm, respectively). Similarly, fetal aggrecan GAG chain trace length (41+/-7 nm) and end-to-end (32+/-8 nm) length were both larger than that of mature aggrecan GAG (32+/-5 and 26+/-7 nm, respectively). GAG-GAG spacing along the core protein was significantly smaller in fetal compared to mature aggrecan (3.2+/-0.8 and 4.4+/-1.2nm, respectively). Together, these differences between the two aggrecan types were likely responsible for the greater persistence length of the fetal aggrecan (110 nm) compared to mature aggrecan (82 nm) calculated using the worm-like chain model. Measured dimensions and polymer statistical analyses were used in conjunction with the results of Western analyses, chromatographic, and carbohydrate electrophoresis measurements to better understand the dependence of aggrecan structure and properties on its constituent GAG chains.

Association between protease-specific proteolytic cleavage of brevican and synaptic loss in the dentate gyrus of kainate-treated rats

Proteolytic fragments generated by ADAMTS (a disintegrin and metalloprotease with thrombospondin motifs)-mediated cleavage of the aggregating chondroitin sulfate proteoglycan, brevican, have been identified, but not localized in the CNS. The purpose of this study, using kainate-induced CNS lesion, was to examine the spatial and quantitative relationship between ADAMTS1 and 4 mRNA expression and ADAMTS-mediated cleavage of brevican (as determined by the abundance of the neo-epitope QEAVESE at the C-terminal of the cleaved brevican G1 domain). In untreated rats, in situ hybridization and reverse transcriptase polymerase chain reaction indicated that ADAMTS4 expression was higher than ADAMTS1 and was localized to hippocampus, temporal lobe and other areas of cortex, striatum and hypothalamus. ADAMTS4 mRNA expression in these regions correlated with the presence of the QEAVESE neo-epitope, which was concentrated in perineuronal nets and in neuropil. In rats that seized after kainate, there was a dramatic elevation in ADAMTS1 and ADAMTS4 transcript that correlated and co-localized with a robust elevation in an extractable, 55-kDa fragment of brevican in temporal lobe and hippocampus. This fragment consisted, at least in part, of the ADAMTS-cleaved epitope G1-QEAVESE. The kainate-induced elevation in this ADAMTS-cleaved fragment was localized to amygdaloid and thalamic nuclei, hippocampus, caudate-putamen, cingulate cortex, and the outer molecular layer of the dentate gyrus where it was accompanied by a robust elevation in ADAMTS1 and 4 mRNA and a 28% decline in synaptic density 5 days after kainate.Thus, complexes of extracellular matrix proteins that exist in perineuronal nets and in the neuropil are cleaved by specific matrix-degrading proteases at early time points during excitotoxic neurodegeneration. The observed ADAMTS-induced cleavage of brevican in the dentate outer molecular layer is closely associated with diminished synaptic density, and may, therefore, contribute to synaptic loss and/or reorganization in this region.

Aggrecanases and cartilage matrix degradation

The loss of extracellular matrix macromolecules from the cartilage results in serious impairment of joint function. Metalloproteinases called 'aggrecanases' that cleave the Glu373-Ala374 bond of the aggrecan core protein play a key role in the early stages of cartilage destruction in rheumatoid arthritis and in osteoarthritis. Three members of the ADAMTS family of proteinases, ADAMTS-1, ADAMTS-4 and ADAMTS-5, have been identified as aggrecanases. Matrix metalloproteinases, which are also found in arthritic joints, cleave aggrecans, but at a distinct site from the aggrecanases (i.e. Asn341-Phe342). The present review discuss the enzymatic properties of the three known aggrecanases, the regulation of their activities, and their role in cartilage matrix breakdown during the development of arthritis in relation to the action of matrix metalloproteinases.

Matrix metalloproteinases are involved in C-terminal and interglobular domain processing of cartilage aggrecan in late stage cartilage degradation

Monoclonal antibody (MAb) technology was used to examine aggrecan metabolites and the role of aggrecanases and matrix metalloproteinases (MMPs) in proteolysis of the interglobular domain (IGD) and C-terminus of aggrecan. An in vitro model of progressive cartilage degradation characterized by early proteoglycan loss and late stage collagen catabolism was evaluated in conjunction with a broad-spectrum inhibitor of MMPs. We have for the first time demonstrated that IGD cleavage by MMPs occurs during this late stage cartilage degeneration, both as a primary event in association with glycosaminoglycan (GAG) release from the tissue and secondarily in trimming of aggrecanase-generated G1 metabolites. Additionally, we have shown that MMPs were responsible for C-terminal catabolism of aggrecan and generation of chondroitin sulfate (CS) deficient aggrecan monomers and that this aggrecan truncation occurred prior to detectable IGD cleavage by MMPs. The onset of this later stage MMP activity was also evident by the generation of MMP-specific link protein catabolites in this model culture system. Recombinant MMP-1, -3 and -13 were all capable of C-terminally truncating aggrecan with at least two cleavage sites N-terminal to the CS attachment domains of aggrecan. Through analysis of aggrecan metabolites in pathological synovial fluids from human, canine and equine sources, we have demonstrated the presence of aggrecan catabolites that appear to have resulted from similar C-terminal processing of aggrecan as that induced in our in vitro culture systems. Finally, by developing a new MAb recognizing a linear epitope in the IGD of aggrecan, we have identified two novel aggrecan metabolites generated by an as yet unidentified proteolytic event. Collectively, these results suggest that C-terminal processing of aggrecan by MMPs may contribute to the depletion of cartilage GAG that leads to loss of tissue function in aging and disease. Furthermore, analysis of aggrecan metabolites resulting from both C-terminal and IGD cleavage by MMPs may prove useful in monitoring different stages in the progression of cartilage degeneration.

Cell biology of osteoarthritis: the chondrocyte's response to injury

Cartilage is comprised of a large amount of functional extracellular matrix that is made and maintained by a small number of chondrocytes, the sole resident cell type. Normal cartilage exists in a relatively steady state: that is, the anabolic processes (those that result in the synthesis of cartilage matrix components) are in equilibrium with the catabolic processes (those that result in the normal turnover of matrix molecules). If the functional extracellular matrix is disturbed by physical or molecular means, the cells respond in an attempt to repair the matrix. This stimulated activity does not result in repair due to the extent and complexity of the extracellular matrix. Eventually, the newly synthesized and activated catabolic enzymes degrade the matrix components. This review presents the cellular and molecular mechanisms that account for this activity and provides some possible solutions.

Intact aggrecan and fragments generated by both aggrecanse and metalloproteinase-like activities are present in the developing and adult rat spinal cord and their relative abundance is altered by injury

Aggrecan is a large proteoglycan (PG) that has been grouped with different PG families on the basis of its physical characteristics. These families include the chondroitin sulfate PGs, which appear to inhibit the migration of cells and axons during development. Although aggrecan has been studied primarily in cartilage, in the present study, tissue samples from developing, mature, and injured-adult rat spinal cords were used to determine whether aggrecan is present in the mammalian spinal cord. By the use of Western blot analysis, tissues were probed with aggrecan-specific antibodies (ATEGQV, TYKHRL, and LEC-7) and aggrecan-specific neoepitope antibodies (NITEGE, FVDIPEN, and TFKEEE) to identify full-length aggrecan and several fragments. Unlike many other aggrecan gene family members, aggrecan species were similar in embryonic day 14, postnatal day 1, and adult spinal cords. Spinal cord injury caused significant decreases in aggrecan. Partial recovery in some aggrecan species was evident by 2 weeks after injury. The presence of specific aggrecan neoepitopes suggested that aggrecan is cleaved in the spinal cord by both a disintegrin and metalloproteinase thrombospondin (also known as aggrecanase) and metalloproteinase-like activities. Many aggrecan species found in the spinal cord were similar to species in cartilage. Additional antibodies were used to identify two other aggrecan gene family members, neurocan and brevican, in the adult spinal cord. These studies present novel information on the aggrecan core protein species and enzymes involved in aggrecan cleavage in vivo in the rat spinal cord throughout development and after injury. They also provide the basis for investigating the function of aggrecan in the spinal cord.

Intact aggrecan and fragments generated by both aggrecanse and metalloproteinase-like activities are present in the developing and adult rat spinal cord and their relative abundance is altered by injury

Aggrecan is a large proteoglycan (PG) that has been grouped with different PG families on the basis of its physical characteristics. These families include the chondroitin sulfate PGs, which appear to inhibit the migration of cells and axons during development. Although aggrecan has been studied primarily in cartilage, in the present study, tissue samples from developing, mature, and injured-adult rat spinal cords were used to determine whether aggrecan is present in the mammalian spinal cord. By the use of Western blot analysis, tissues were probed with aggrecan-specific antibodies (ATEGQV, TYKHRL, and LEC-7) and aggrecan-specific neoepitope antibodies (NITEGE, FVDIPEN, and TFKEEE) to identify full-length aggrecan and several fragments. Unlike many other aggrecan gene family members, aggrecan species were similar in embryonic day 14, postnatal day 1, and adult spinal cords. Spinal cord injury caused significant decreases in aggrecan. Partial recovery in some aggrecan species was evident by 2 weeks after injury. The presence of specific aggrecan neoepitopes suggested that aggrecan is cleaved in the spinal cord by both a disintegrin and metalloproteinase thrombospondin (also known as aggrecanase) and metalloproteinase-like activities. Many aggrecan species found in the spinal cord were similar to species in cartilage. Additional antibodies were used to identify two other aggrecan gene family members, neurocan and brevican, in the adult spinal cord. These studies present novel information on the aggrecan core protein species and enzymes involved in aggrecan cleavage in vivo in the rat spinal cord throughout development and after injury. They also provide the basis for investigating the function of aggrecan in the spinal cord.