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

TMF inhibits extracellular matrix degradation by regulating the C/EBPβ/ADAMTS5 signaling pathway in osteoarthritis

Osteoarthritis (OA) is a chronic joint disease, characterized by degenerative destruction of articular cartilage. Chondrocytes, the unique cell type in cartilage, mediate the metabolism of extracellular matrix (ECM), which is mainly constituted by aggrecan and type II collagen. A disintegrin and metalloproteinase with thrombospondin 5 (ADAMTS5) is an aggrecanase responsible for the degradation of aggrecan in OA cartilage. CCAAT/enhancer binding protein β (C/EBPβ), a transcription factor in the C/EBP family, has been reported to mediate the expression of ADAMTS5. Our previous study showed that 5,7,3',4'-tetramethoxyflavone (TMF) could activate the Sirt1/FOXO3a signaling in OA chondrocytes. However, whether TMF protected against ECM degradation by down-regulating C/EBPβ expression was unknown. In this study, we found that aggrecan expression was down-regulated, and ADAMTS5 expression was up-regulated. Knockdown of C/EBPβ could up-regulate aggrecan expression and down-regulate ADAMTS5 expression in IL-1β-treated C28/I2 cells. TMF could compromise the effects of C/EBPβ on OA chondrocytes by activating the Sirt1/FOXO3a signaling. Conclusively, TMF exhibited protective activity against ECM degradation by mediating the Sirt1/FOXO3a/C/EBPβ pathway in OA chondrocytes.

IL-37b alleviates inflammation in the temporomandibular joint cartilage via IL-1R8 pathway

Objectives

Interleukin (IL)‐37 is a natural suppressor of innate inflammation. This study was conducted to explore the anti‐inflammatory effects of IL‐37 in temporomandibular joint (TMJ) inflammation.

Materials and Methods

The expression of IL‐37 in the TMJ was measured using ELISA and IHC. Human TMJ chondrocytes were treated with IL‐37b and IL‐1β, and inflammation‐related factors were detected. siRNA‐IL‐1R8 was transfected into chondrocytes, and the affected pathways were detected. IL‐37b was used in disc‐perforation‐induced TMJ inflammation in SD rats. Micro‐CT, IHC, real‐time PCR and histological staining were used to quantify the therapeutic effect of IL‐37b.

Results

IL‐37 was expressed in the synovium and the disc of patients with osteoarthritis (OA) and in the articular cartilage of condylar fracture patients. IL‐37 was highly expressed in synovial fluid of patients with synovitis than in those with OA and disc displacement and was closely related to visual analogue scale (VAS) score. In vitro, IL‐37b suppressed the expression of pro‐inflammatory factors. In addition, IL‐37b exerted anti‐inflammatory effects via IL‐1R8 by inhibiting the p38, ERK, JNK and NF‐κB activation, while silencing IL‐1R8 led to inflammation and upregulation of these signals. In disc‐perforation‐induced TMJ inflammation in SD rats, IL‐37b suppressed inflammation and inhibited osteoclast formation to protect against TMJ.

Conclusions

IL‐37b may be a novel therapeutic agent for TMJ inflammation.

Chronic inflammation deteriorates structure and function of collagen fibril in rat temporomandibular joint disc

Collagen is the building component of temporomandibular joint (TMJ) discs and is often affected by inflammation in temporomandibular disorders. The macromechanical properties of collagen are deteriorated by chronic inflammation. However, the mechanism by which inflammation influences disc function remains unknown. The relationship between the ultrastructure and nanomechanical properties of collagen in inflamed discs should be clarified. Seven-week-old female Sprague-Dawley rats were randomly divided into two groups. Chronic TMJ inflammation was induced by intra-articular injection of complete Freund's adjuvant, and samples were harvested after 5 weeks. Picrosirius staining revealed multiple colours under polarized light, which represented alternative collagen bundles in inflamed discs. Using atomic force microscopy scanning, the magnitude of Young's modulus was reduced significantly accompanied with disordered collagen fibril arrangement with porous architecture of inflamed discs. Transmission electron microscopy scanning revealed a non-uniform distribution of collagen fibres, and oversized collagen fibrils were observed in inflamed discs. Fourier transform infrared microspectroscopy revealed a decrease in 1 338 cm^-1/amide II area ratio of collagen in different regions. The peak positions of amide I and amide II bands were altered in inflamed discs, indicating collagen unfolding. Our results suggest that sustained inflammation deteriorates collagen structures, resulting in the deterioration of the ultrastructure and nanomechanical properties of rat TMJ discs.

Keratan sulfate, a complex glycosaminoglycan with unique functional capability

From an evolutionary perspective keratan sulfate (KS) is the newest glycosaminoglycan (GAG) but the least understood. KS is a sophisticated molecule with a diverse structure, and unique functional roles continue to be uncovered for this GAG. The cornea is the richest tissue source of KS in the human body but the central and peripheral nervous systems also contain significant levels of KS and a diverse range of KS-proteoglycans with essential functional roles. KS also displays important cell regulatory properties in epithelial and mesenchymal tissues and in bone and in tumor development of diagnostic and prognostic utility. Corneal KS-I displays variable degrees of sulfation along the KS chain ranging from non-sulfated polylactosamine, mono-sulfated and disulfated disaccharide regions. Skeletal KS-II is almost completely sulfated consisting of disulfated disaccharides interrupted by occasional mono-sulfated N-acetyllactosamine residues. KS-III also contains highly sulfated KS disaccharides but differs from KS-I and KS-II through 2-O-mannose linkage to serine or threonine core protein residues on proteoglycans such as phosphacan and abakan in brain tissue. Historically, the major emphasis on the biology of KS has focused on its sulfated regions for good reason. The sulfation motifs on KS convey important molecular recognition information and direct cell behavior through a number of interactive proteins. Emerging evidence also suggest functional roles for the poly-N-acetyllactosamine regions of KS requiring further investigation. Thus further research is warranted to better understand the complexities of KS.

IL37 dampens the IL1β-induced catabolic status of human OA chondrocytes

Objective

A crucial feature of OA is cartilage degradation. This process is mediated by pro-inflammatory cytokines, among other factors, via induction of matrix-degrading enzymes. Interleukin 37 (IL37) is an anti-inflammatory cytokine and is efficient in blocking the production of pro-inflammatory cytokines during innate immune responses. We hypothesize that IL37 is therapeutic in treating the inflammatory cytokine cascade in human OA chondrocytes and can act as a counter-regulatory cytokine to reduce cartilage degradation in OA.

Methods

Human OA cartilage was obtained from patients undergoing total knee or hip arthroplasty. Immunohistochemistry was applied to study IL37 protein expression in cartilage biopsies from OA patients. Induction of IL37 expression by IL1β, OA synovium-conditioned medium and TNFα was investigated in human OA chondrocytes. Adenoviral overexpression of IL37 followed by IL1β stimulation was performed to investigate the anti-inflammatory potential of IL37.

Results

IL37 expression was detected in cartilage biopsies of OA patients and induced by IL1β. After IL1β stimulation, increased IL1β, IL6 and IL8 expression was observed in OA chondrocytes. Elevated IL37 levels diminished the IL1β-induced IL1β , IL6 and IL8 gene levels and IL1β and IL8 protein levels. In addition to the reduction in pro-inflammatory cytokine expression, IL37 reduced MMP1 , MMP3 , MMP13 and disintegrin and metalloproteinase with thrombospondin motifs 5 gene levels and MMP3 and MMP13 protein levels.

Conclusion

IL37 is induced by IL1β, and IL37 itself reduced IL1β, IL6 and IL8 production, indicating that IL37 is able to induce a counter-regulatory anti-inflammatory feedback loop in chondrocytes. In addition, IL37 dampens catabolic enzyme expression. This supports IL37 as a potential therapeutic target in OA.

The role of cytokines in posttraumatic arthritis

The development of arthritis after joint injury is commonly known as posttraumatic arthritis (PTA). The inciting traumatic event may range from cartilage contusion and bone bruise combined with meniscus or ligament tear, to intra-articular fracture. End-stage PTA is often indistinguishable from primary osteoarthritis. However, knowing the time of the inciting traumatic event in a patient with PTA provides an opportunity to understand the events following joint injury that lead to the progression of arthritis. Joint injury often leads to mechanical alterations in loading of the injured joint, and restoration of joint mechanics through surgical repair remains an important aspect of treatment. However, the accuracy of joint reduction by itself does not account for the variability in outcome following joint injury, as evidenced by the fact that PTA remains a significant clinical problem. Emerging research in animal models and human subjects indicates that several inflammatory cytokines and related inflammatory mediators are elevated following joint injury. Data from animal studies and early clinical trials suggest that early inhibition of the intra-articular inflammatory response may improve clinical outcomes.

Internalization of aggrecan G1 domain neoepitope ITEGE in chondrocytes requires CD44

Degradation of the cartilage proteoglycan aggrecan is one of the earliest events that occurs in association with osteoarthritis. Little is known concerning the fate of the residual N-terminal G1 domains of cleaved aggrecan; domains that remain bound to hyaluronan. In this study, 68-72-kDa bands representative of aggrecan G1 domains containing ITEGE(373) neoepitope were detected within a hyaluronidase-sensitive pool at the cell surface of bovine articular chondrocytes and within a hyaluronidase-insensitive, intracellular pool. To determine the mechanisms that contribute to this distribution, CD44 expression was knocked down by siRNA or function by CD44-DN. Both approaches prevented the retention and internalization of G1-ITEGE. Inhibition of CD44 transit into lipid rafts blocked the endocytosis of G1-ITEGE but not the retention at the cell surface. Chondrocytes derived from CD44 null mice also exhibited limited potential for retention and internalization of G1-VTEGE. The consequence of a lack of chondrocyte-mediated endocytosis of these domains in cartilage of the CD44 null mice was the accumulation of the degradation fragments within the tissue. Additionally, chondrocytes or fibroblasts derived from CD44 null mice exhibited little capacity for retention and internalization of exogenous G1-ITEGE derived from bovine cartilage explants. Bovine or wild type mouse fibroblasts were able to bind and internalize bovine-derived G1-ITEGE. Although several pathways are available for the clearance of these domains, CD44-mediated cellular internalization is the most prominent.

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.

Effective treatment of psoriasis with etanercept is linked to suppression of IL-17 signaling, not immediate response TNF genes

BACKGROUND

TNF inhibitors have revolutionized the treatment of psoriasis vulgaris as well as psoriatic and rheumatoid arthritis and Crohn disease. Despite our understanding that these agents block TNF, their complex mechanism of action in disease resolution is still unclear.

OBJECTIVE

To analyze globally the genomic effects of TNF inhibition in patients with psoriasis, and to compare genomic profiles of patients who responded or did not respond to treatment.

METHODS

In a clinical trial using etanercept TNF inhibitor to treat psoriasis vulgaris (n = 15), Affymetrix gene arrays were used to analyze gene profiles in lesional skin at multiple time points during drug treatment (baseline and weeks 1, 2, 4, and 12) compared with nonlesional skin. Patients were stratified as responders (n = 11) or nonresponders (n = 4) on the basis of histologic disease resolution. Cluster analysis was used to define gene sets that were modulated with similar magnitude and velocity over time.

RESULTS

In responders, 4 clusters of downregulated genes and 3 clusters of upregulated genes were identified. Genes downmodulated most rapidly reflected direct inhibition of myeloid lineage immune genes. Upregulated genes included the stable dendritic cell population genes CD1c and CD207 (langerin). Comparison of responders and nonresponders revealed rapid downmodulation of innate IL-1beta and IL-8 sepsis cascade cytokines in both groups, but only responders downregulated IL-17 pathway genes to baseline levels.

CONCLUSION

Although both responders and nonresponders to etanercept inactivated sepsis cascade cytokines, response to etanercept is dependent on inactivation of myeloid dendritic cell genes and inactivation of the T(H)17 immune response.

Do non-steroidal anti-inflammatory drugs influence chronic inflammation? The effects of piroxicam on chronic antigen-induced arthritis in rats

OBJECTIVE

The effects of non-steroidal anti-inflammatory drugs (NSAIDs) on acute inflammation have been thoroughly investigated. NSAIDs are, however, also prescribed for patients with chronic inflammation, such as rheumatoid arthritis (RA), and objective improvement suggestive of anti-inflammatory action from NSAIDs has not been convincingly shown in chronic RA. An antigen-induced arthritis (AIA) model was used to investigate the effects of piroxicam on chronic inflammation.

METHODS

AIA was induced by injecting methylated bovine serum albumin (mBSA) into the knee joints of previously immunized rats that were treated orally with the NSAID piroxicam or with saline. This treatment was started either before AIA was induced or after it had reached a chronic phase. The findings were recorded by clinical and histological assessment of the joints.

RESULTS

The piroxicam group developed significantly less acute and subsequent chronic knee joint inflammation but this was only evident if the drug was administered prior to the intra-articular mBSA injections. Piroxicam treatment that was initiated during the chronic inflammation did not have any clinical effect, whereas short-term corticosteroid treatment abolished the chronic inflammation. Moreover, histological analysis of the chronic inflammation revealed significantly more inflammatory changes in the piroxicam group compared with the control group.

CONCLUSIONS

Piroxicam treatment had no beneficial effects on the chronic stable inflammation in this model and might even delay histological resolution. As the anti-inflammatory effect of piroxicam is restricted to acute inflammation, the use of NSAIDs during periods of chronic stable arthritis in humans, such as in RA, may need to be investigated.

Induction of increased cAMP levels in articular chondrocytes blocks matrix metalloproteinase-mediated cartilage degradation, but not aggrecanase-mediated cartilage degradation

OBJECTIVE

Calcitonin has been suggested to have chondroprotective effects. One signaling pathway of calcitonin is via the second messenger cAMP. We undertook this study to investigate whether increased cAMP levels in chondrocytes would be chondroprotective.

METHODS

Cartilage degradation was induced in bovine articular cartilage explants by 10 ng/ml oncostatin M (OSM) and 20 ng/ml tumor necrosis factor (TNF). In these cultures, cAMP levels were augmented by treatment with either forskolin (4, 16, or 64 microM) or 3-isobutyl-1-methyl xanthine (IBMX; 4, 16, or 64 microM). Cartilage degradation was assessed by 1) quantification of C-terminal crosslinking telopeptide of type II collagen fragments (CTX-II), 2) matrix metalloproteinase (MMP)-mediated aggrecan degradation by (342)FFGV- G2 assay, 3) aggrecanase-mediated degradation by (374)ARGS-G2 assay, 4) release of sulfated glycosaminoglycans (sGAG) into culture medium, 5) immunohistochemistry with a monoclonal antibody recognizing the CTX-II epitope, and 6) toluidine blue staining of proteoglycans. MMP expression and activity were assessed by gelatin zymography.

RESULTS

OSM and TNF induced an 8,000% increase in CTX-II compared with control (P < 0.001). Both forskolin and IBMX dose-dependently inhibited release of CTX-II (P < 0.001). OSM and TNF induced a 6-fold increase in (342)FFGV-G2, which was abrogated by forskolin and IBMX (by >80%). OSM and TNF stimulated MMP expression as visualized by zymography, and MMP expression was dose-dependently inhibited by forskolin and IBMX. The highest concentration of IBMX lowered cytokine-induced release of sGAG by 72%.

CONCLUSION

Levels of cAMP in chondrocytes play a key role in controlling catabolic activity. Increased cAMP levels in chondrocytes inhibited MMP expression and activity and consequently strongly inhibited cartilage degradation. Specific cAMP modulators in chondrocytes may be potential treatments for cartilage degenerative diseases.

ADAMTS-5 Deficiency Does Not Block Aggrecanolysis at Preferred Cleavage Sites in the Chondroitin Sulfate-rich Region of Aggrecan

In the mouse, proteolysis in the aggrecan interglobular domain is driven by ADAMTS-5, and mice deficient in ADAMTS-5 catalytic activity are protected against aggrecan loss and cartilage damage in experimental models of arthritis. Here we show that despite ablation of ADAMTS-5 activity, aggrecanolysis can still occur at two preferred sites in the chondroitin sulfate-rich region. Retinoic acid was more effective than interleukin-1alpha (IL) in promoting cleavage at these sites in ADAMTS-5-deficient cartilage. These results suggest that cleavage at preferred sites in the chondroitin sulfate-rich region is mediated by ADAMTS-4 or an aggrecanase other than ADAMTS-5. Following retinoic acid or IL-1alpha stimulation of cartilage explants, aggrecan fragments in medium and extracts contained SELE(1279) or FREEE(1467) C-terminal sequences. Some SELE(1279) and FREEE(1467) fragments were retained in the cartilage, with intact G1 domains. Other SELE(1279) fragments were released into the medium and co-migrated with the (374)ALGS neoepitope, indicating they were aggrecanase-derived fragments. In contrast none of the FREEE(1467) fragments released into the medium co-migrated with the (374)ALGS neoepitope, suggesting that, despite their size, these fragments were not products of aggrecanase cleavage in the interglobular domain. ADAMTS-5, but not ADAMTS-1, -4, or -9, was up-regulated 8-fold by retinoic acid and 17-fold by IL-1alpha treatment. The data show that whereas ADAMTS-5 is entirely responsible for cleavage in the interglobular domain, cleavage in the chondroitin sulfate-rich region is driven either by ADAMTS-4, which compensates for loss of ADAMTS-5 in this experimental system, or possibly by another aggrecanase. The data show that there are differential aggrecanase activities with preferences for separate regions of the core protein.

The accumulation of intracellular ITEGE and DIPEN neoepitopes in bovine articular chondrocytes is mediated by CD44 internalization of hyaluronan

OBJECTIVE

A dramatic loss of aggrecan proteoglycan from cartilage is associated with osteoarthritis. The fate of residual G1 domains of aggrecan is unknown, but inefficient turnover of these domains may impede subsequent repair and retention of newly synthesized aggrecan. Thus, the objective of this study was to determine whether ITEGE- and DIPEN-containing G1 domains, generated in situ, are internalized by articular chondrocytes, and whether these events are dependent on hyaluronan (HA) and its receptor, CD44.

METHODS

ITEGE and DIPEN neoepitopes were detected by immunofluorescence staining of bovine articular cartilage chondrocytes treated with or without interleukin-1alpha (IL-1alpha). Additionally, purified ITEGE- or DIPEN-containing G1 domains were aggregated with HA and then added to articular chondrocytes, articular chondrocytes transfected with CD44delta67, or COS-7 cells transfected with or without full-length CD44. Internalized epitopes were distinguished by their resistance to extensive trypsinization of the cell surface.

RESULTS

Both ITEGE and DIPEN were visualized within the extracellular cell-associated matrix of chondrocytes as well as within intracellular vesicles. Following trypsinization, the intracellular accumulation of both epitopes was clearly visible. IL-1 treatment increased extracellular as well as intracellular ITEGE epitope accumulation. Once internalized, the ITEGE neoepitope became localized within the nucleus and displayed little colocalization with HA, DIPEN, or other G1 domain epitopes. The internalization of both ITEGE and DIPEN G1 domains was dependent on the presence of HA and CD44.

CONCLUSION

One important mechanism for the elimination of residual G1 domains following extracellular degradation of aggrecan is CD44-mediated co-internalization with HA.

Autologous chondrocyte implantation (ACI) for aged patients: development of the proper cell expansion conditions for possible therapeutic applications

OBJECTIVE

Proliferation and chondrogenic commitment of cultured articular chondrocytes are impaired when cells derive from aged donors. In those subjects the feasibility of cell-based therapies for articular surface repair is reduced. Moreover, the use of serum as medium supplement elicits non-physiological responses in cultured chondrocytes. This study was therefore undertaken to identify the expansion culture conditions needed to sustain growth and chondrogenic commitment of chondrocytes harvested from aged human subjects.

DESIGN

Articular cartilage was obtained from aged (69-75 years) and from young adult subjects (27-35 years). Chondrocytes were isolated and cultured in serum-free (SF) or in serum-supplemented [fetal calf serum (FCS)] conditions. Chondrocytes were expanded in monolayer for five duplications and processed for RNA extraction and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. The differentiation potential was assessed by micromass pellet cultures before and after expansion in either culture medium, or after a prolonged exposure to serum followed by a period in SF condition.

RESULTS

Only SF-cultured chondrocytes reached five duplications within 25-35 days, maintaining the expression of some chondrogenic markers and without altering the levels of active matrix metalloproteinase 3 (MMP-3). Only the pellets derived from SF-expanded cultures positively stained for cartilage matrix deposition. On the contrary, exposure to serum diminished the proliferation capacities, abolished the differentiation potential in the same cells and elicited transcription of the MMP-3 gene. Shifting culture conditions from FCS to SF resumed growth rates but proper extracellular matrix deposition was only partially restored.

CONCLUSIONS

The SF conditions have proven valuable to prime cell proliferation and to sustain proper commitment in chondrocytes from aged patients. This culturing approach may represent a therapeutic chance extendable to a range of patients normally excluded from clinical protocols based on autologous chondrocyte implantation (ACI).

N-linked keratan sulfate in the aggrecan interglobular domain potentiates aggrecanase activity

Keratan sulfate is thought to influence the cleavage of aggrecan by metalloenzymes. We have therefore produced a recombinant substrate, substituted with keratan sulfate, suitable for the study of aggrecanolysis in vitro. Recombinant human G1-G2 was produced in primary bovine keratocytes using a vaccinia virus expression system. Following purification and digestion with specific hydrolases, fluorophore-assisted carbohydrate electrophoresis was used to confirm the presence of the monosulfated Gal-GlcNAc6S and GlcNAc6s-Gal disaccharides and the disulfated Gal6S-GlcNAc6S disaccharides of keratan sulfate. Negligible amounts of fucose or sialic acid were detected, and the level of unsulfated disaccharides was minimal. Treatment with keratanases reduced the size of the recombinant G1-G2 by approximately 5 kDa on SDS-PAGE. Treatment with N-glycosidase F also reduced the size of G1-G2 by approximately 5 kDa and substantially reduced G1-G2 immunoreactivity with monoclonal antibody 5-D-4, indicating that keratan sulfate on the recombinant protein is N-linked. Cleavage of G1-G2 by aggrecanase was markedly reduced when keratan sulfate chains were removed by treatment with keratanase, keratanase II, endo-beta-galactosidase, or N-glycosidase F. These results indicate that modification of oligosaccharides in the aggrecan interglobular domain with keratan sulfate, most likely at asparagine residue 368, potentiates aggrecanase activity in this part of the core protein.

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.

ADAMTS5 is the major aggrecanase in mouse cartilage in vivo and in vitro

Aggrecan is the major proteoglycan in cartilage, endowing this tissue with the unique capacity to bear load and resist compression. In arthritic cartilage, aggrecan is degraded by one or more 'aggrecanases' from the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of proteinases. ADAMTS1, 8 and 9 have weak aggrecan-degrading activity. However, they are not thought to be the primary aggrecanases because ADAMTS1 null mice are not protected from experimental arthritis, and cleavage by ADAMTS8 and 9 is highly inefficient. Although ADAMTS4 and 5 are expressed in joint tissues, and are known to be efficient aggrecanases in vitro, the exact contribution of these two enzymes to cartilage pathology is unknown. Here we show that ADAMTS5 is the major aggrecanase in mouse cartilage, both in vitro and in a mouse model of inflammatory arthritis. Our data suggest that ADAMTS5 may be a suitable target for the development of new drugs designed to inhibit cartilage destruction in arthritis, although further work will be required to determine whether ADAMTS5 is also the major aggrecanase in human arthritis.

Local expression of matrix metalloproteinases, cathepsins, and their inhibitors during the development of murine antigen-induced arthritis

Cartilage and bone degradation, observed in human rheumatoid arthritis (RA), are caused by aberrant expression of proteinases, resulting in an imbalance of these degrading enzymes and their inhibitors. However, the role of the individual proteinases in the pathogenesis of degradation is not yet completely understood. Murine antigen-induced arthritis (AIA) is a well-established animal model of RA. We investigated the time profiles of expression of matrix metalloproteinase (MMP), cathepsins, tissue inhibitors of matrix metalloproteinases (TIMP) and cystatins in AIA. For primary screening, we revealed the expression profile with Affymetrix oligonucleotide chips. Real-time polymerase chain reaction (PCR) analyses were performed for the validation of array results, for tests of more RNA samples and for the completion of the time profile. For the analyses at the protein level, we used an MMP fluorescence activity assay and zymography. By a combination of oligonucleotide chips, real-time PCR and zymography, we showed differential expressions of several MMPs, cathepsins and proteinase inhibitors in the course of AIA. The strongest dysregulation was observed on days 1 and 3 in the acute phase. Proteoglycan loss analysed by safranin O staining was also strongest on days 1 and 3. Expression of most of the proteinases followed the expression of pro-inflammatory cytokines. TIMP-3 showed an expression profile similar to that of anti-inflammatory interleukin-4. The present study indicates that MMPs and cathepsins are important in AIA and contribute to the degradation of cartilage and bone.

Chronicity of pristane-induced arthritis in rats is controlled by genes on chromosome 14

To address the possibility that genes specifically control the chronic phase of arthritis we have isolated a congenic fragment from the resistant E3 rat on the susceptible DA rat background. The isolated fragment covers the Pia6 quantitative trait locus on chromosome 14, which previously has been identified to be linked to chronic pristane induced arthritis (PIA) in gene segregation experiments of an (E3 x DA)F(2)-cross. Heterozygous Pia6 congenic rats (DA.Pia6) were protected from chronic active arthritis specifically, as determined by macroscopic scoring, histopathology and release of cartilage oligomeric matrix protein (-reflecting ongoing cartilage destruction). The difference was seen only during the chronic active phase of arthritis starting approximately 5 weeks after onset of arthritis. Interestingly, the plasma concentration of the lipocalins alpha(1)-acid glycoprotein and alpha(1)-microglobulin was found significantly altered in nai;ve congenic rats compared to the DA rat. The concentration of alpha(1)-microglobulin was found to be significantly higher throughout the disease course, while alpha(1)-acid glycoprotein had a lower concentration in nai;ve rats, which was highly significant in the chronic phase. The altered concentrations of these proteins already before development of chronic arthritis may provide a clue to the pathogenic process controlled by the Pia6 genes. We conclude that the active relapsing chronic arthritis is under a unique genetic control that is different from the control of acute arthritis and postulate that the liver plays an important role in this regulation.

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.

Matrix turnover in human cartilage repair tissue in autologous chondrocyte implantation

OBJECTIVE

Autologous chondrocyte implantation (ACI) is a form of tissue engineering that is being used increasingly to treat damaged articular cartilage. What happens at the graft site subsequent to the transplantation of chondrocytes beneath a periosteal flap has largely remained a matter of conjecture. We examined biopsy samples from the graft site using a panel of specific antibodies to investigate the cellular mechanisms involved and to determine whether remodeling of the matrix occurs.

METHODS

Ten full-depth core biopsy samples were obtained from patients who had undergone ACI 9-30 months previously (ages 28-53 years), in addition to 6 "control" biopsy samples. Cryosections were evaluated by standard histologic examination using polarized light and immunohistochemistry. Antibodies specific for type II collagen (CIIC1) were used, as well as antibodies against the C-propeptide of type II collagen (R160) and its denaturation product (Col2-3/4m), as indicators of anabolism or catabolism. In addition, antibodies to the matrix proteinase-generated neoepitopes of the aggrecan core protein were used to demonstrate either aggrecanase (BC-3 and BC-13) or matrix metalloproteinase (MMP) (BC-4 and BC-14) activity.

RESULTS

All biopsy samples stained for type II collagen, even in areas of fibrocartilaginous morphology. There was evidence of newly synthesized type II collagen in addition to denatured collagen. MMP and aggrecanase activity on the proteoglycan population was evident, with aggrecanase being more active in fibrocartilaginous areas.

CONCLUSION

The findings of this study indicate that ACI is capable of not only cartilage repair but, in some cases, regeneration. This may be achieved by the turnover and remodeling of an initial fibrocartilaginous matrix via enzymatic degradation and synthesis of newly formed type II collagen.

Cysteine protease activity is up-regulated in inflamed ankle joints of rats with adjuvant-induced arthritis and decreases with in vivo administration of a vinyl sulfone cysteine protease inhibitor

OBJECTIVE

Cysteine proteases are postulated to play a role in tissue destruction in the joints of animals with arthritis. The purpose of the present study was to confirm the concept that cysteine proteases are enzymes involved in the pathology of rheumatoid arthritis (RA).

METHODS

Arthritis was induced in Lewis rats by adjuvant injection (adjuvant-induced arthritis [AIA] model) and scored for inflammation. At necropsy, the rear paws were either fixed in formalin and assigned a histologic score (based on synovial cell proliferation, cartilage erosion, bone erosion, and fibroproliferative pannus) or frozen, cryosectioned, and assayed for enzyme activity either by in situ cytochemical staining with a post-azo-coupling method using a chromogenic substrate (Z-arg-arg-MNA) or by a novel assay placing the tissue section directly in a cuvette using the fluorogenic substrate Z-arg-arg-AMC.

RESULTS

Enzymatic activity, measured either in frozen sections in situ or in the cuvette assay, was positively correlated with joint destruction (r = 0.7) and inflammation (r = 0.8). Activity was not inhibited significantly by Pefabloc (a serine protease inhibitor), EDTA (a metalloprotease inhibitor), or pepstatin A (an aspartyl protease inhibitor) but was inhibited by E-64 and vinyl sulfone irreversible inhibitors of cysteine proteases. The effect of one of the vinyl sulfone cysteine protease inhibitors, Mu-Leu-HomoPhe-vinylsulfone, was tested in vivo by dietary administration at 2.2 mg/kg/day in the AIA model; this resulted in a significant decrease in inflammation and in the amount of cysteine protease activity measured in the joint tissue.

CONCLUSION

Cysteine protease activity levels increase in the diseased state and may be an important target for designing small molecule inhibitors to reduce the inflammation and tissue destruction associated with RA.

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.

Mechanisms involved in cartilage proteoglycan catabolism

The increased catabolism of the cartilage proteoglycan aggrecan is a principal pathological process which leads to the degeneration of articular cartilage in arthritic joint diseases. The consequent loss of sulphated glycosaminoglycans, which are intrinsic components of the aggrecan molecule, compromises both the functional and structural integrity of the cartilage matrix and ultimately renders the tissue incapable of resisting the compressive loads applied during joint articulation. Over time, this process leads to irreversible cartilage erosion. In situ degradation of aggrecan is a proteolytic process involving cleavage at specific peptide bonds located within the core protein. The most well characterised enzymatic activities contributing to this process are engendered by zinc-dependent metalloproteinases. In vitro aggrecanolysis by matrix metalloproteinases (MMPs) has been widely studied; however, it is now well recognised that the principal proteinases responsible for aggrecan degradation in situ in articular cartilage are the aggrecanases, two recently identified isoforms of which are members of the 'A Disintegrin And Metalloproteinase with Thrombospondin motifs' (ADAMTS) gene family. In this review we have described: (i) the development of monoclonal antibody technologies to identify catabolic neoepitopes on aggrecan degradation products; (ii) the use of such neoepitope antibodies in studies designed to characterise and identify the enzymes responsible for cartilage aggrecan metabolism; (iii) the biochemical properties of soluble cartilage aggrecanase(s) and their differential expression in situ; and (iv) model culture systems for studying cartilage aggrecan catabolism. These studies have clearly established that 'aggrecanase(s)' is primarily responsible for the catabolism and loss of aggrecan from articular cartilage in the early stages of arthritic joint diseases that precede overt collagen catabolism and disruption of the tissue integrity. At later stages, when collagen catabolism is occurring, there is evidence for MMP-mediated degradation of the small proportion of aggrecan remaining in the tissue, but this occurs independently of continued aggrecanase activity. Furthermore, the catabolism of link proteins by MMPs is also initiated when overt collagen degradation is evident.