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

Biochemical identification and immunolocalizaton of aggrecan, ADAMTS5 and inter-alpha-trypsin-inhibitor in equine degenerative suspensory ligament desmitis

We describe analysis of suspensory ligaments from horses with advanced degenerative suspensory ligament desmitis (DSLD) to identify the major proteoglycans (PGs), ADAMTS-aggrecanases and inter-alpha-trypsin inhibitor (IαI) components associated with ligament degeneration. Specific anatomical regions of suspensory ligaments from two normal horses and four diagnosed with DSLD were analyzed by Western blot and immunohistochemistry for the following: aggrecan, aggrecan fragments, decorin, ADAMTS4, ADAMTS5, and IαI components. When compared to normal, DSLD ligaments showed about a 15-fold increase (P < 0.0014) in aggrecan levels and markedly enhanced staining with Safranin O. The aggrecan was composed of two distinct high molecular weight core protein species. The largest species was found only in DSLD samples and it co-migrated with aggrecan synthesized by equine mesenchymal stem cells (MSC). Many of the DSLD samples also contained abnormally high concentrations of ADAMTS4, ADAMTS5, and IαI. Notably, the ADAMTS5 in DSLD samples, but not normals, was present largely as a high molecular weight complex. We conclude that ligament degeneration in DSLD is associated with matrix changes characteristic of an inflammatory nonhealing wound, specifically containing chondrogenic progenitor cells. Since aggrecan accumulation is a major feature of incomplete healing in tendon and skin of the ADAMTS5 knockout mouse, we propose that ligament failure in DSLD results from a process involving tissue inflammation and the complexation of ADAMTS5.

Adamts5 deletion blocks murine dermal repair through CD44-mediated aggrecan accumulation and modulation of transforming growth factor β1 (TGFβ1) signaling

ADAMTS5 has been implicated in the degradation of cartilage aggrecan in human osteoarthritis. Here, we describe a novel role for the enzyme in the regulation of TGFβ1 signaling in dermal fibroblasts both in vivo and in vitro. Adamts5(-/-) mice, generated by deletion of exon 2, exhibit impaired contraction and dermal collagen deposition in an excisional wound healing model. This was accompanied by accumulation in the dermal layer of cell aggregates and fibroblastic cells surrounded by a pericellular matrix enriched in full-length aggrecan. Adamts5(-/-) wounds exhibit low expression (relative to wild type) of collagen type I and type III but show a persistently elevated expression of tgfbRII and alk1. Aggrecan deposition and impaired dermal repair in Adamts5(-/-) mice are both dependent on CD44, and Cd44(-/-)/Adamts5(-/-) mice display robust activation of TGFβ receptor II and collagen type III expression and the dermal regeneration seen in WT mice. TGFβ1 treatment of newborn fibroblasts from wild type mice results in Smad2/3 phosphorylation, whereas cells from Adamts5(-/-) mice phosphorylate Smad1/5/8. The altered TGFβ1 response in the Adamts5(-/-) cells is dependent on the presence of aggrecan and expression of CD44, because Cd44(-/-)/Adamts5(-/-) cells respond like WT cells. We propose that ADAMTS5 deficiency in fibrous tissues results in a poor repair response due to the accumulation of aggrecan in the pericellular matrix of fibroblast progenitor cells, which prevents their transition to mature fibroblasts. Thus, the capacity of ADAMTS5 to modulate critical tissue repair signaling events suggests a unique role for this enzyme, which sets it apart from other members of the ADAMTS family of proteases.

Knockout of ADAMTS5 does not eliminate cartilage aggrecanase activity but abrogates joint fibrosis and promotes cartilage aggrecan deposition in murine osteoarthritis models

To investigate the role of ADAMTS5 in murine osteoarthritis (OA), resulting from destabilization of the medial meniscus (DMM model) or from TGFb1 injection and enforced uphill treadmill running (TTR model). Wild-type (WT) and ADAMTS5-/- mice were subjected to either DMM or TTR and joints were evaluated for meniscal damage, cartilage changes, and fibrotic ingrowths from the joint margins. Cartilage lesions were quantified on an 8-point scoring system. Cartilage chondroitin sulfate (CS) content was evaluated by SafraninO staining and by quantitative electrophoresis (FACE). The abundance of aggrecan, versican, and specific aggrecanase-generated products was determined by Western analysis. Joint changes were similar for WT mice taken through either the DMM or the TTR model. ADAMTS5 ablation essentially eliminated cartilage erosion and fibrous overgrowth in both models. In the TTR model, ADAMTS5 ablation did not eliminate aggrecanase activity from the articular cartilage but blocked fibrosis and resulted in the accumulation of aggrecan in the articular cartilage. The cartilage protection provided by ADAMTS5 ablation in the mouse does not result from prevention of aggrecanase activity per se, but it appears to be due to a blockade of joint tissue fibrosis and a concomitant increase in cartilage aggrecan content.

Intraarticular injection of hyaluronan prevents cartilage erosion, periarticular fibrosis and mechanical allodynia and normalizes stance time in murine knee osteoarthritis

Introduction

Intraarticular hyaluronan (HA) is used clinically for symptomatic relief in patients with knee osteoarthritis (OA); however, the mechanism of action is unclear. In this study, we examined the effects of a single injection of HA on joint tissue pathology, mechanical allodynia and gait changes (measured by stride times) in a murine model of OA.

Methods

OA was induced in the right knee joint (stifle) of 12-week-old male C57BL/6 mice by transforming growth factor β1 (TGFβ1) injection and treadmill running for 14 days. Gait parameters were quantified by using TreadScan, mechanical allodynia was evaluated with von Frey filaments, and joint pathology was evaluated by scoring of macroscopic images for both cartilage erosion and periarticular fibrosis. HA or saline control was injected 1 day after TGFβ1 injection but before the start of treadmill running.

Results

OA development in this model was accompanied by significant (P < 0.01) enhancement of the stance and propulsion times of affected legs. HA injection (but not saline injection) blocked all gait changes and also protected joints from femoral cartilage erosion as well as tibial and femoral tissue fibrosis. Both HA injection and saline injection attenuated acute allodynia, but the HA effect was more pronounced and prolonged than the saline injection.

Conclusions

We conclude that videographic gait analysis is an objective, sensitive and reproducible means of monitoring joint pathology in experimental murine OA, since stance time appears to correlate directly with OA severity. A single injection of HA prevents acute and prolonged gait changes and ameliorates the cartilage erosion and periarticular fibrosis normally seen in this model. We speculate that the capacity of HA to prevent cartilage erosion results from its normalization of joint biomechanics and its inhibitory effects on periarticular cells, which are involved in tissue hyperplasia and fibrosis. This effect of exogenous HA appears to mimic the protective effects of ablation of Adamts5 (a disintegrin and metalloproteinase with thrombospondin motifs 5) on experimental murine OA, and we speculate that a common mechanism is involved.

Abnormal human chondrocyte morphology is related to increased levels of cell-associated IL-1β and disruption to pericellular collagen type VI

Early osteoarthritis (OA) is poorly understood, but abnormal chondrocyte morphology might be important. We studied IL-1β and pericellular collagen type VI in morphologically normal and abnormal chondrocytes. In situ chondrocytes within explants from nondegenerate (grade 0/1) areas of human tibial plateaus (n = 21) were fluorescently labeled and visualized [2-photon laser scanning microscopy (2PLSM)]. Normal chondrocytes exhibited a "smooth" membrane surface, whereas abnormal cells were defined as demonstrating ≥1 cytoplasmic process. Abnormal chondrocytes were further classified by number and average length of cytoplasmic processes/cell. IL-1β or collagen type VI associated with single chondrocytes were visualized by fluorescence immuno-histochemistry and confocal laser scanning microscopy (CLSM). Fluorescence was quantified as the number of positive voxels (i.e., 3D pixels with fluorescence above baseline)/cell. IL-1β-associated fluorescence increased between normal and all abnormal cells in the superficial (99.7 ± 29.8 [11 (72)] vs. 784 ± 382 [15 (132)]; p = 0.04, positive voxels/cell) and deep zones (66.5 ± 29.4 [9 (64)] vs. 795 ± 224 [9 (56)]; p = 0.006). There was a correlation (r(2) = 0.988) between the number of processes/cell (0-5) and IL-1β, and an increase particularly with short processes (≤5 µm; p = 0.022). Collagen type VI coverage and thickness decreased (p < 0.001 and p = 0.005, respectively) with development of processes. Abnormal chondrocytes in macroscopically nondegenerate cartilage demonstrated a marked increase in IL-1β and loss of pericellular type VI collagen, changes that could lead to cartilage degeneration.

Identifying the human aggrecanase

It is clear that A Disintegrin And Metalloproteinase with ThromboSpondin motif (ADAMTS)-5 is the major aggrecanase in mouse cartilage, however it is not at all clear which enzyme is the major aggrecanase in human cartilage. Identifying the human aggrecanase is difficult because multiple, independent, molecular processes determine the final level of enzyme activity. As investigators, we have good methods for measuring changes in the expression of ADAMTS mRNA, and good methods for detecting aggrecanase activity, but no methods that distinguish the source of the activity. In between gene expression and enzyme action there are many processes that can potentially enhance or inhibit the final level of activity. In this editorial we discuss how each of these processes affects ADAMTS activity and argue that measuring any one process in isolation has little value in predicting overall ADAMTS activity in vivo.

Calpain is involved in C-terminal truncation of human aggrecan

Mature aggrecan is generally C-terminally truncated at several sites in the CS (chondroitin sulfate) region. Aggrecanases and MMPs (matrix metalloproteinases) have been suggested to be responsible for this digestion. To identify whether calpain, a common intracellular protease, has a specific role in the proteolysis of aggrecan we developed neoepitope antibodies (anti-PGVA, anti-GDLS and anti-EDLS) against calpain cleavage sites and used Western blot analysis to identify calpain-generated fragments in normal and OA (osteoarthritis) knee cartilage and SF (synovial fluid) samples. Our results showed that human aggrecan contains six calpain cleavage sites: one in the IGD (interglobular domain), one in the KS (keratan sulfate) region, two in the CS1 and two in the CS2 region. Kinetic studies of calpain proteolysis against aggrecan showed that the aggrecan molecule was cleaved in a specific order where cuts in CS1 was the most preferred and cuts in KS region was the second most preferred cleavage. OA and normal cartilage contained low amounts of a calpain-generated G1–PGVA fragment (0.5–2%) compared with aggrecanase-generated G1–TEGE (71–76%) and MMP-generated G1–IPEN (23–29%) fragments. Significant amounts of calpain-generated GDLS and EDLS fragments were found in OA and normal cartilage, and a ARGS–EDLS fragment was detected in arthritic SF samples. The results of the present study indicate that calpains are involved in the C-terminal truncation of aggrecan and might have a minor role in arthritic diseases.

10mM glucosamine prevents activation of proADAMTS5 (aggrecanase-2) in transfected cells by interference with post-translational modification of furin

OBJECTIVE

Glucosamine has been previously shown to suppress cartilage aggrecan catabolism in explant cultures. We determined the effect of glucosamine on ADAMTS5 (a disintegrin-like and metalloprotease domain (reprolysin type) with thrombospondin type-1 motifs 5), a major aggrecanase in osteoarthritis, and investigated a potential mechanism underlying the observed effects.

DESIGN

HEK293F and CHO-K1 cells transiently transfected with ADAMTS5 cDNA were treated with glucosamine or the related hexosamine mannosamine. Glucosamine effects on FURIN transcription were determined by quantitative RT-PCR. Effects on furin-mediated processing of ADAMTS5 zymogen, and aggrecan processing by glucosamine-treated cells, were determined by western blotting. Post-translational modification of furin and N-glycan deficient furin mutants generated by site-directed mutagenesis was analyzed by western blotting, and the mutants were evaluated for their ADAMTS5 processing ability in furin-deficient CHO-RPE.40 cells.

RESULTS

Ten mM glucosamine and 5-10mM mannosamine reduced excision of the ADAMTS5 propeptide, indicating interference with the propeptide excision mechanism, although mannosamine compromised cell viability at these doses. Although glucosamine had no effect on furin mRNA levels, western blot of furin from glucosamine-treated cells suggested altered post-translational modification. Glucosamine treatment led to decreased glycosylation of cellular furin, with reduced furin autoactivation as the consequence. Recombinant furin treated with peptide N-glycanase F had reduced activity against a synthetic peptide substrate. Indeed, site-directed mutagenesis of two furin N-glycosylation sites, Asn(387) and Asn(440), abrogated furin activation and this mutant was unable to rescue ADAMTS5 processing in furin-deficient cells.

CONCLUSIONS

Ten mM glucosamine reduces excision of the ADAMTS5 propeptide via interference with post-translational modification of furin and leads to reduced aggrecanase activity of ADAMTS5.

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).

Analysis of meniscal degeneration and meniscal gene expression

BACKGROUND

Menisci play a vital role in load transmission, shock absorption and joint stability. There is increasing evidence suggesting that OA menisci may not merely be bystanders in the disease process of OA. This study sought: 1) to determine the prevalence of meniscal degeneration in OA patients, and 2) to examine gene expression in OA meniscal cells compared to normal meniscal cells.

METHODS

Studies were approved by our human subjects Institutional Review Board. Menisci and articular cartilage were collected during joint replacement surgery for OA patients and lower limb amputation surgery for osteosarcoma patients (normal control specimens), and graded. Meniscal cells were prepared from these meniscal tissues and expanded in monolayer culture. Differential gene expression in OA meniscal cells and normal meniscal cells was examined using Affymetrix microarray and real time RT-PCR.

RESULTS

The grades of meniscal degeneration correlated with the grades of articular cartilage degeneration (r = 0.672; P < 0.0001). Many of the genes classified in the biological processes of immune response, inflammatory response, biomineral formation and cell proliferation, including major histocompatibility complex, class II, DP alpha 1 (HLA-DPA1), integrin, beta 2 (ITGB2), ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), ankylosis, progressive homolog (ANKH) and fibroblast growth factor 7 (FGF7), were expressed at significantly higher levels in OA meniscal cells compared to normal meniscal cells. Importantly, many of the genes that have been shown to be differentially expressed in other OA cell types/tissues, including ADAM metallopeptidase with thrombospondin type 1 motif 5 (ADAMTS5) and prostaglandin E synthase (PTGES), were found to be expressed at significantly higher levels in OA meniscal cells. This consistency suggests that many of the genes detected in our study are disease-specific.

CONCLUSION

Our findings suggest that OA is a whole joint disease. Meniscal cells may play an active role in the development of OA. Investigation of the gene expression profiles of OA meniscal cells may reveal new therapeutic targets for OA therapy and also may uncover novel disease markers for early diagnosis of OA.

Stigmasterol: a phytosterol with potential anti-osteoarthritic properties

OBJECTIVE

Although most studies have focused on the cholesterol-lowering activity of stigmasterol, other bioactivities have been ascribed to this plant sterol compound, one of which is a potential anti-inflammatory effect. To investigate the effects of stigmasterol, a plant sterol, on the inflammatory mediators and metalloproteinases produced by chondrocytes.

METHOD

We used a model of newborn mouse chondrocytes and human osteoarthritis (OA) chondrocytes in primary culture stimulated with or without IL-1beta (10 ng/ml), for 18 h. Cells were pre-incubated for 48 h with stigmasterol (20 microg/ml) compared to untreated cells. We initially investigated the presence of stigmasterol in chondrocyte, compared to other phytosterols. We then assessed the role of stigmasterol on the expression of various genes involved in inflammation (IL-6) and cartilage turn-over (MMP-3, -13, ADAMTS-4, -5, type II collagen, aggrecan) by quantitative Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR). Additional experiments were carried out to monitor the production of MMP-3 and prostaglandin E2 (PGE(2)) by specific immuno-enzymatic assays. We eventually looked at the role of stigmasterol on NF-kappaB activation by western blot, using an anti-IkappaBalpha antibody.

RESULTS

After 18 h of IL-1beta treatment, MMP-3, MMP-13, ADAMTS-4, but not ADAMTS-5 RNA expression were elevated, as well as MMP-3 and PGE(2) protein levels in mouse and human chondrocytes. Type II collagen and aggrecan mRNA levels were significatively reduced. Pre-incubation of stigmasterol to IL-1beta-treated cells significantly decreased these effects described above (significant reduction of MMP-3 mRNA in human and mouse, MMP-3 protein in mouse, MMP-13 mRNA in mouse and human, ADAMTS-4 mRNA in human, PGE(2) protein in human and mouse) Finally, stigmasterol was capable of counteracting the IL-1beta-induced NF-kappaB pathway.

CONCLUSION

This study shows that stigmasterol inhibits several pro-inflammatory and matrix degradation mediators typically involved in OA-induced cartilage degradation, at least in part through the inhibition of the NF-kappaB pathway. These promising results justify further ex vivo and in vivo investigations with stigmasterol.

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.

Cell death-associated ADAMTS4 and versican degradation in vascular tissue

High blood flow through baboon polytetrafluorethylene aorto-iliac grafts increases neointimal vascular smooth muscle cell (SMC) death, neointimal atrophy, and cleavage of versican to generate the DPEAAE neoepitope, a marker of ADAMTS-mediated proteolysis. In this study, we have determined the effect of high blood flow on transcript abundance in the neointima for ADAMTS1, -4, -5, -8, -9, -15, and -20. We found that after 24 hr of flow, the mRNA for ADAMTS4 was significantly increased, whereas that for the other family members was unchanged. Because vascular SMC death is markedly increased in the graft after 24 hr of high flow, we next examined the possibility that the ADAMTS4 induction and the cell death are causally related. The addition of Fas ligand to SMC cultures increased both ADAMTS4 mRNA and cell death approximately 5-fold, consistent with the idea that ADAMTS4-dependent cleavage of versican may be partly responsible for cell death and tissue atrophy under these conditions.

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.

Cartilage homeostasis in health and rheumatic diseases

As the cellular component of articular cartilage, chondrocytes are responsible for maintaining in a low-turnover state the unique composition and organization of the matrix that was determined during embryonic and postnatal development. In joint diseases, cartilage homeostasis is disrupted by mechanisms that are driven by combinations of biological mediators that vary according to the disease process, including contributions from other joint tissues. In osteoarthritis (OA), biomechanical stimuli predominate with up-regulation of both catabolic and anabolic cytokines and recapitulation of developmental phenotypes, whereas in rheumatoid arthritis (RA), inflammation and catabolism drive cartilage loss. In vitro studies in chondrocytes have elucidated signaling pathways and transcription factors that orchestrate specific functions that promote cartilage damage in both OA and RA. Thus, understanding how the adult articular chondrocyte functions within its unique environment will aid in the development of rational strategies to protect cartilage from damage resulting from joint disease. This review will cover current knowledge about the specific cellular and biochemical mechanisms that regulate cartilage homeostasis and pathology.

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.

Modulation of lubricin biosynthesis and tissue surface properties following cartilage mechanical injury

OBJECTIVE

To evaluate the effects of injurious compression on the biosynthesis of lubricin at different depths within articular cartilage and to examine alterations in structure and function of the articular surface following mechanical injury.

METHODS

Bovine cartilage explants were subdivided into level 1, with intact articular surface, and level 2, containing middle and deep zone cartilage. Following mechanical injury, lubricin messenger RNA (mRNA) levels were monitored by quantitative reverse transcriptase-polymerase chain reaction, and soluble or cartilage-associated lubricin protein was analyzed by Western blotting and immunohistochemistry. Cartilage morphology was assessed by histologic staining, and tissue functionality was assessed by friction testing.

RESULTS

Two days after injury, lubricin mRNA expression was up-regulated approximately 3-fold for level 1 explants and was down-regulated for level 2 explants. Lubricin expression in level 1 cartilage returned to control levels after 6 days in culture. Similarly, lubricin protein synthesis and secretion increased in response to injury for level 1 explants and decreased for level 2 cartilage. Histologic staining revealed changes in the articular surface of level 1 explants following injury, with respect to glycosaminoglycan and collagen content. Injured level 1 explants displayed an increased coefficient of friction relative to controls.

CONCLUSION

Our findings indicate that increased lubricin biosynthesis appears to be an early transient response of surface-layer cartilage to injurious compression. However, distinct morphologic changes occur with injury that appear to compromise the frictional properties of the tissue.

Adamts5, the gene encoding a proteoglycan-degrading metalloprotease, is expressed by specific cell lineages during mouse embryonic development and in adult tissues

The secreted metalloprotease ADAMTS5 is implicated in destruction of the cartilage proteoglycan aggrecan in arthritis, but its physiological functions are unknown. Its expression profile during embryogenesis and in adult tissues is therefore of considerable interest. beta-Galactosidase (beta-gal) histochemistry, enabled by a LacZ cassette inserted in the Adamts5 locus, and validated by in situ hybridization with an Adamts5 cRNA probe and ADAMTS5 immunohistochemistry, was used to profile Adamts5 expression during mouse embryogenesis and in adult mouse tissues. Embryonic expression was scarce prior to 11.5 days of gestation (E11.5) and noted only in the floor plate of the developing brain at E 9.5. After E11.5 there was continued expression in brain, especially in the choroid plexus, peripheral nerves, dorsal root ganglia, cranial nerve ganglia, spinal and cranial nerves, and neural plexuses of the gut. In addition to nerves, developing limbs have Adamts5 expression in skeletal muscle (from E13.5), tendons (from E16.5), and inter-digital mesenchyme of the developing autopod (E13.5-15.5). In adult tissues, there is constitutive Adamts5 expression in arterial smooth muscle cells, mesothelium lining the peritoneal, pericardial and pleural cavities, smooth muscle cells in bronchi and pancreatic ducts, glomerular mesangial cells in the kidney, dorsal root ganglia, and in Schwann cells of the peripheral and autonomic nervous system. Expression of Adamts5 during neuromuscular development and in smooth muscle cells coincides with the broadly distributed proteoglycan versican, an ADAMTS5 substrate. These observations suggest the major contexts in which developmental and physiological roles could be sought for this protease.

Effects of alendronate on a disintegrin and metalloproteinase with thrombospondin motifs expression in the developing epiphyseal cartilage in rats

A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) have been reported to play a role in the degradation of aggrecan, a major component of cartilage. This study was performed to examine the effects of alendronate on the expression of ADAMTS in developing femoral epiphyseal cartilage. Primary cultured chondrocytes from this cartilage were treated with alendronate in vitro and postnatal day 1 rats were injected subcutaneously with alendronate (1 mg/kg) every second day in vivo. The number of cultured chondrocytes and their aggrecan mRNA levels were unaffected by the alendronate treatment at 10(-6) to 10(-4) M concentrations. The mRNA levels of ADAMTS-1, -2 and -9 in chondrocytes were also unaffected. However, the levels of ADAMTS-5 and -4 were reduced significantly by the same treatment. The thickness of the proliferating chondrocyte layers and the aggrecan mRNA levels in the epiphysis were unaffected by the alendronate treatment in vivo. However, the hypertrophied chondrocyte layers became significantly thicker, and the size of the secondary ossification centre was reduced significantly by the same treatment (P < 0.05). Both ADAMTS-4 and -5 mRNA expressions were also reduced significantly in vivo. The immunoreactivity against ADAMTS-4 was seen in hypertrophied chondrocytes and reduced significantly by the alendronate treatment. These results suggested that alendronate can inhibit the degradation of aggrecan in the articular cartilage by downregulating the expression of matrix enzymes such as ADAMTS-4 and -5.

Role of bone in osteoarthritis pathogenesis

Much of the attention in developing diagnostic tools and therapeutic interventions for the management of osteoarthritis (OA) has focused on the preservation or repair of articular cartilage. It is clear that all of the joint components, including the ligaments, tendons, capsule, synovial lining, and periarticular bone, undergo structural and functional alterations during the course of OA progression. This article focuses on the specific skeletal features of OA and the putative mechanisms involved in their pathogenesis.

Superficial zone chondrocytes in normal and osteoarthritic human articular cartilages synthesize novel truncated forms of inter-alpha-trypsin inhibitor heavy chains which are attached to a chondroitin sulfate proteoglycan other than bikunin

OBJECTIVE

We have examined the occurrence of the inflammation-associated inter-alpha-trypsin inhibitor (IalphaI) components, bikunin, heavy chain (HC)1 and HC2 in normal cartilage and osteoarthritis (OA) cartilage and synovial fluids.

DESIGN/METHODS

Cartilage extracts from normal donors and late-stage OA patients, and synovial fluids from OA patients were studied by Western blot with multiple antibodies to bikunin, HC1 and HC2. Cell and matrix localization was determined by immunohistochemistry and mRNA by RT-PCR.

RESULTS

Bikunin.chondroitin sulfate (CS) and IalphaI were abundant in OA cartilages, but virtually undetectable in normal. In both OA and normal cartilages, HCs were largely present in a novel C-terminally truncated 50-kDa form, with most, if not all of these being attached to CS on a proteoglycan other than bikunin. Synovial fluids from OA patients contained bikunin.CS and full-length (approximately 90 kDa) HCs linked to hyaluronan (HA) as HC.HA (SHAP.HA). Immunohistochemistry showed intracellular and cell-associated staining for bikunin and HCs, consistent with their synthesis by superficial zone chondrocytes. PCR on multiple human normal and OA cartilage samples detected transcripts for HC1 and HC2 but not for bikunin. In OA cartilages, immunostaining was predominantly matrix-associated, being most intense in regions with a pannus-like fibrotic overgrowth.

CONCLUSION

The truncated structure of HCs, their attachment to a proteoglycan other than bikunin, PCR data and intracellular staining are all consistent with synthesis of HC1 and HC2 by human articular chondrocytes. The presence of bikunin.CS and IalphaI in OA cartilage, but not in normal, appears to be due to diffusional uptake and retention through fibrillated (but not deeply fissured) cartilage surfaces.

Characterization of proADAMTS5 processing by proprotein convertases

ADAMTS5 (aggrecanase-2), a key metalloprotease mediating cartilage destruction in arthritis, is synthesized as a zymogen, proADAMTS5. We report a detailed characterization of the propeptide excision mechanism and demonstrate that it is a major regulatory step with unusual characteristics. Using furin-deficient cells and a furin inhibitor, we found that proADAMTS5 was processed by proprotein convertases, specifically furin and PC7, but not PC6B. Mutagenesis of three sites containing basic residues within the ADAMTS5 propeptide (RRR(46), RRR(69) and RRRRR(261)) suggested that proADAMTS5 processing occurs after Arg(261). That furin processing was essential for ADAMTS5 activity was illustrated using the known ADAMTS5 substrate aggrecan, as well as a new substrate, versican, an important regulatory proteoglycan during mammalian development. When compared to other ADAMTS proteases, proADAMTS5 processing has several distinct features. In contrast to ADAMTS1, whose furin processing products were clearly present intracellularly, cleaved ADAMTS5 propeptide and mature ADAMTS5 were found exclusively in the conditioned medium. Despite attempts to enhance detection of intracellular proADAMTS5 processing, such as by immunoprecipitation of total ADAMTS5, overexpression of furin, and secretion blockade by monensin, neither processed ADAMTS5 propeptide nor the mature enzyme were found intracellularly, which was strongly suggestive of extracellular processing. Extracellular ADAMTS5 processing was further supported by activation of proADAMTS5 added exogenously to HEK293 cells stably expressing furin. Unlike proADAMTS9, which is processed by furin at the cell-surface, to which it is bound, ADAMTS5 does not bind the cell-surface. Thus, the propeptide processing mechanism of ADAMTS5 has several points of distinction from those of other ADAMTS proteases, which may have considerable significance in the context of osteoarthritis.

Innate immune system activation in osteoarthritis: is osteoarthritis a chronic wound?

PURPOSE OF REVIEW

Synovial inflammation is increasingly recognized as an important pathophysiologic process in osteoarthritis, but the stimuli and downstream pathways activated are not well defined. Innate immune system activation, best documented in responses to pathogens, likely plays a role in induction of inflammatory mediators and the specific cellular infiltrate seen in osteoarthritis. Thus, the Toll-like receptors (TLRs) and their signaling pathways are of particular interest. These innate pattern-recognition receptors are activated not only by pathogens but by endogenous 'danger signals'. In this report, we review evidence that certain extracellular matrix components of joint tissues (hyaluronan and fibronectin) may act as TLR stimuli, and summarize recent literature implicating TLR activation in osteoarthritis.

RECENT FINDINGS

Convincing evidence exists that hyaluronan/TLR interactions drive responses to tissue injury. Evidence of a similar role for fibronectin is growing. TLRs are expressed and functional in the joint, and many proteases and cytokines that promote cartilage catabolism are dependent on nuclear factor-kappaB, a TLR-activated transcription factor.

SUMMARY

Activation of TLR pathways seems likely in osteoarthritis and may play a central role in disease development and progression. A model of osteoarthritis as a chronic wound, in which the innate immune response is triggered by molecular signals of tissue damage, is presented as a framework for future study of inflammation in this prevalent joint disease.

Characterization of proteoglycan production and processing by chondrocytes and BMSCs in tissue engineered constructs

OBJECTIVE

The goal of this study was to characterize the proteoglycan (PG) production and processing by bone marrow stromal cells (BMSCs) within a tissue engineered construct.

METHODS

Bovine BMSCs and articular chondrocytes (ACs) were isolated from an immature calf, seeded into agarose gels, and cultured up to 32 days in the presence of TGF-beta1. The localization of various PGs was examined by immunofluorescence and histological staining. The role of proteolytic enzymes in construct development was further investigated by examining the effects of aggrecanase and MMP inhibitors on PG accumulation, aggrecan processing, and construct mechanics.

RESULTS

BMSCs developed a matrix rich in sulfated-glycosaminoglycans (sGAG) and full-length aggrecan, but had low levels of versican. The BMSC constructs had less collagen II and aggrecan compared to the AC constructs cultured under identical conditions. AC constructs also had high levels of pericellular collagen VI, while BMSCs had a pericellular matrix containing little collagen VI and greater levels of decorin, biglycan, and fibronectin. Treatment with the aggrecanase inhibitor did not affect sGAG accumulation or the dynamic moduli of the BMSC constructs. The MMP inhibitor slightly but significantly inhibited sGAG accumulation and lowered the dynamic moduli of BMSC constructs.

CONCLUSIONS

The results of this preliminary study indicate that long-term culture of BMSCs with TGF-beta1 promotes the development of an aggrecan-rich matrix characteristic of native articular cartilage; however, BMSCs accumulate significantly lower levels of sGAG and assemble distinct pericellular microenvironments compared to ACs. PG turnover does not appear to play a major role in the development of tissue engineered cartilage constructs by BMSCs.

TSG-6 regulates bone remodeling through inhibition of osteoblastogenesis and osteoclast activation

TSG-6 is an inflammation-induced protein that is produced at pathological sites, including arthritic joints. In animal models of arthritis, TSG-6 protects against joint damage; this has been attributed to its inhibitory effects on neutrophil migration and plasmin activity. Here we investigated whether TSG-6 can directly influence bone erosion. Our data reveal that TSG-6 inhibits RANKL-induced osteoclast differentiation/activation from human and murine precursor cells, where elevated dentine erosion by osteoclasts derived from TSG-6^-/- mice is consistent with the very severe arthritis seen in these animals. However, the long bones from unchallenged TSG-6^-/- mice were found to have higher trabecular mass than controls, suggesting that in the absence of inflammation TSG-6 has a role in bone homeostasis; we have detected expression of the TSG-6 protein in the bone marrow of unchallenged wild type mice. Furthermore, we have observed that TSG-6 can inhibit bone morphogenetic protein-2 (BMP-2)-mediated osteoblast differentiation. Interaction analysis revealed that TSG-6 binds directly to RANKL and to BMP-2 (as well as other osteogenic BMPs but not BMP-3) via composite surfaces involving its Link and CUB modules. Consistent with this, the full-length protein is required for maximal inhibition of osteoblast differentiation and osteoclast activation, although the isolated Link module retains significant activity in the latter case. We hypothesize that TSG-6 has dual roles in bone remodeling; one protective, where it inhibits RANKL-induced bone erosion in inflammatory diseases such as arthritis, and the other homeostatic, where its interactions with BMP-2 and RANKL help to balance mineralization by osteoblasts and bone resorption by osteoclasts.

Cells of the synovium in rheumatoid arthritis. Chondrocytes

Rheumatoid arthritis (RA) is one of the inflammatory joint diseases in a heterogeneous group of disorders that share features of destruction of the extracellular matrices of articular cartilage and bone. The underlying disturbance in immune regulation that is responsible for the localized joint pathology results in the release of inflammatory mediators in the synovial fluid and synovium that directly and indirectly influence cartilage homeostasis. Analysis of the breakdown products of the matrix components of joint cartilage in body fluids and quantitative imaging techniques have been used to assess the effects of the inflammatory joint disease on the local remodeling of joint structures. The role of the chondrocyte itself in cartilage destruction in the human rheumatoid joint has been difficult to address but has been inferred from studies in vitro and in animal models. This review covers current knowledge about the specific cellular and biochemical mechanisms that account for the disruption of the integrity of the cartilage matrix in RA.

Osteoarthritis

Osteoarthritis (OA) is characterized by degeneration of articular cartilage, limited intraarticular inflammation with synovitis, and changes in peri-articular and subchondral bone. Multiple factors are involved in the pathogenesis of OA, including mechanical influences, the effects of aging on cartilage matrix composition and structure, and genetic factors. Since the initial stages of OA involve increased cell proliferation and synthesis of matrix proteins, proteinases, growth factors, cytokines, and other inflammatory mediators by chondrocytes, research has focused on the chondrocyte as the cellular mediator of OA pathogenesis. The other cells and tissues of the joint, including the synovium and subchondral bone, also contribute to pathogenesis. The adult articular chondrocyte, which normally maintains the cartilage with a low turnover of matrix constituents, has limited capacity to regenerate the original cartilage matrix architecture. It may attempt to recapitulate phenotypes of early stages of cartilage development, but the precise zonal variations of the original cartilage cannot be replicated. Current pharmacological interventions that address chronic pain are insufficient, and no proven structure-modifying therapy is available. Cartilage tissue engineering with or without gene therapy is the subject of intense investigation. There are multiple animal models of OA, but there is no single model that faithfully replicates the human disease. This review will focus on questions currently under study that may lead to better understanding of mechanisms of OA pathogenesis and elucidation of effective strategies for therapy, with emphasis on mechanisms that affect the function of chondrocytes and interactions with surrounding tissues.