Pathways of aggrecan processing in joint tissues Implications for disease mechanism and monitoring

Degeneration of the articular disc in the human triangular fibrocartilage complex

INTRODUCTION

Traumatic injuries of the triangular fibrocartilage complex (TFCC) are frequent reasons for ulnar wrist pain. The assessment of the extent of articular disc (AD) degeneration is important for the differentiation of acute injuries versus chronic lesions.

MATERIALS AND METHODS

The AD of the TFCC of eleven human cadaver wrists was dissected. Degeneration was analyzed according to the grading of Krenn et al. Hematoxylin-eosin was used to determine the tissue morphology. Degeneration was evaluated using the staining intensity of alcian blue, the immunohistochemistry of the proteoglycan versican and the immunoreactivity of NITEGE, an aggrecan fragment.

RESULTS

The staining homogeneity of HE decreased with higher degeneration of the AD and basophilic tissue areas were more frequently seen. Two specimens were characterized as degeneration grade 1, five specimens as grade 2, and four specimens as grade 3, respectively. Staining intensity of alcian blue increased with higher degeneration grade of the specimens. Immunoreactivity for NITEGE was detected around tissue fissures and perforations as well as matrix splits. Immunoreactivity for versican was found concentrated in the tissue around matrix fissures and lesions as well as loose connective tissue at the ulnar border of the AD. Specimens with degeneration grade 2 had the strongest immunoreactivity of NITEGE and versican. Cell clusters were observed in specimens with degeneration grade 2 and 3, which were stained by alcian blue and immunoreactive for NITEGE and versican. Increasing age of the cadaver wrists correlated with a higher degree of degeneration (p < 0.0001, r = 0.68).

CONCLUSIONS

The fibrocartilage of degenerated ADs contains NITEGE and versican. The amount of the immunoreactivity of these markers allows the differentiation of degenerative changes into three grades. The degeneration of the AD increases with age and emphasizes its important mechanical function.

Adipokines induce catabolism of newly synthesized matrix in cartilage and meniscus tissues

PURPOSE

Altered synovial levels of various adipokines (factors secreted by fat as well as other tissues) have been associated with osteoarthritis (OA) onset and progression. However, the metabolic effects of adipokines on joint tissues, in particular the fibrocartilaginous menisci, are not well understood. This study investigated effects of several adipokines on release of recently synthesized extracellular matrix in bovine cartilage and meniscus tissue explants.

MATERIALS AND METHODS

After labeling newly synthesized proteins and sulfated glycosaminoglycans (sGAGs) with ³H-proline and ³⁵S-sulfate, respectively; bovine cartilage and meniscus tissue explants were cultured for 6 days in basal medium (control) or media supplemented with adipokines (1 µg/ml of leptin, visfatin, adiponectin, or resistin) or 20 ng/ml interleukin-1 (IL-1). Release of radiolabel and sGAG to the media during culture and the final explant water, DNA, sGAG, and retained radiolabel were measured. Matrix metalloproteinase (MMP-2) and MMP-3 activities were assessed using gelatin and casein zymography, respectively.

RESULTS

Water and DNA contents were not significantly altered by any treatment. Visfatin, adiponectin, resistin, and IL-1 stimulated sGAG release from meniscus, whereas only IL-1 stimulated sGAG release from cartilage. Release of ³H and ³⁵S was stimulated not only by resistin and IL-1 in meniscus but also by IL-1 in cartilage. Retained ³H was unaltered by any treatment, while retained ³⁵S was reduced by visfatin, resistin, and IL-1 in meniscus and by only IL-1 in cartilage. Resistin and IL-1 elevated active MMP-2 and total MMP-3 in meniscus, whereas cartilage MMP-3 activity was elevated by only IL-1.

CONCLUSIONS

Resistin stimulated rapid and extensive catabolism of meniscus tissue, similar to IL-1, whereas adipokines minimally affected cartilage. Release of newly synthesized matrix was similar to overall release in both tissues. These observations provide further indications that meniscal tissue is more sensitive to pro-inflammatory factors than cartilage and also suggest further study of resistin's role in OA.

Bovine meniscal tissue exhibits age- and interleukin-1 dose-dependent degradation patterns and composition-function relationships

Despite increasing evidence that meniscal degeneration is an early event in the development of knee osteoarthritis, relatively little is known regarding the sequence or functional implications of cytokine-induced meniscal degradation or how degradation varies with age. This study examined dose-dependent patterns of interleukin-1 (IL-1)-induced matrix degradation in explants from the radially middle regions of juvenile and adult bovine menisci. Tissue explants were cultured for 10 days in the presence of 0, 1.25, 5, or 20 ng/ml recombinant human IL-1α. Juvenile explants exhibited immediate and extensive sulfated glycosaminoglycan (sGAG) loss and subsequent collagen release beginning after 4-6 days, with relatively little IL-1 dose-dependence. Adult explants exhibited a more graded response to IL-1, with dose-dependent sGAG release and a lower fraction of sGAG released (but greater absolute release) than juvenile explants. In contrast to juvenile explants, adult explants exhibited minimal collagen release over the 10-day culture. Compressive and shear moduli reflected the changes in explant composition, with substantial decreases for both ages but a greater relative decrease in juvenile tissue. Dynamic moduli exhibited stronger dependence on explant sGAG content for juvenile tissue, likely reflecting concomitant changes to both proteoglycan and collagen tissue components. The patterns of tissue degradation suggest that, like in articular cartilage, meniscal proteoglycans may partially protect collagen from cell-mediated degeneration. A more detailed view of functional changes in meniscal tissue mechanics with degeneration will help to establish the relevance of in vitro culture models and will advance understanding of how meniscal degeneration contributes to overall joint changes in early stage osteoarthritis. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:801-811, 2016.

[Histopathological degeneration score of fibrous cartilage. Low- and high-grade meniscal degeneration]

Although histopathology of meniscal degeneration plays an important role, no criteria to assess severity of the degeneration are available to date. Our aim was to create a histopathological scoring system for meniscal degeneration with good interobserver variability, taking matrix degradation and cellularity in meniscal tissue into consideration. Degeneration is classified as follows: grade 1 (low), grade 2 (intermediate), grade 3 (high). The pattern of NITEGE deposits (G1 fragment of aggrecan) was assessed immunohistochemically (n=38) and compared with the grades of degeneration. In 48% of the patients with grade 2 or 3 degeneration extracellular NITEGE deposits (specificity 100%) were found, whereas grade 1 patients showed no deposits. Extracellular NITEGE deposits correlated positively with the grade of degeneration. In all, 30 cases (10 per grade) were assessed by three pathologists (A, B, C). Grading conformity was 70% for grade 1, 66% for grade 2 and 100% for grade 3. Cohen's Kappa coefficient was 0.6--0.7 between pairs of observers. Combining grade 1 and 2 to low-grade degeneration, compared to a grade-3 high-grade degeneration achieved Kappa coefficients of between 0.93 and 1.0. This reproducible degeneration score for fibrous cartilage could form the basis for the standardized assessment of meniscal degeneration.

[Meniscal degeneration score and NITEGE expression : immunohistochemical detection of NITEGE in advanced meniscal degeneration]

BACKGROUND

Meniscal degeneration (MD) is a structural change of fibrous cartilage that is common in orthopaedic diagnostics and relevant for health insurance matters. So far, there has been neither a standardised scoring system nor an immunohistochemical marker for MD.

MATERIAL AND METHOD

In this retrospective trial, the meniscal tissue of 60 patients was assessed immunohistochemically for NITEGE (G1 fragment of the proteoglycan aggrecan) expression. NITEGE expression was correlated with defined grades of MD: little (grade 0/1), medium (grade 2), or severe (grade 3).

RESULTS

Detection of extracellular NITEGE deposits in grade 2 or 3 MD had a positive predictive value and specificity of 100%, whereas no deposits were found in grade 0/1 MD. Sensitivity in advanced MD was 55%. Detection of extracellular NITEGE correlated positively with the grade of degeneration, as did patient age and the grade of degeneration. The patient age of those with grade 0/1 MD was significantly lower than for grade 3 (p<0.0001).

CONCLUSION

The thoroughly defined degeneration score (grade 1 - grade 3 MD) is suitable to assess the severity of degeneration. Extracellular NITEGE deposits can be regarded as an immunohistochemical marker for advanced (grades 2 and 3) MD.

Interleukin-1alpha treatment of meniscal explants stimulates the production and release of aggrecanase-generated, GAG-substituted aggrecan products and also the release of pre-formed, aggrecanase-generated G1 and m-calpain-generated G1-G2

Pro-inflammatory cytokines induce meniscal matrix degradation and inhibition of endogenous repair mechanisms, but the pathogenic mechanisms behind this are mostly unknown. Therefore, we investigated details of interleukin-1 (IL-1alpha)-induced aggrecan turnover in mature meniscal tissue explants. Fibro-cartilagenous disks (3 mm diameter x 1 mm thickness) were isolated from the central, weight-bearing region of menisci from 2-year-old cattle. After 3 or 6 days of IL-1alpha-treatment, GAG loss (DMMB assay), biosynthetic activity ([(35)SO(4)]-sulfate and [(3)H]-proline incorporation), gene expression (quantitative RT-PCR) and the abundance (zymography, Western blot) of matrix-degrading enzymes and specific aggrecan products were determined. Meniscal fibrocartilage had a 4-fold lower GAG content (per wet weight) than adjacent articular cartilage, and expressed MMPs-1, -2, -3 and ADAMTS4 constitutively, whereas ADAMTS5 m-RNA was essentially undetectable. Significant IL-1 effects were a decrease in biosynthetic activity, an increase in GAG release and in the expression/abundance of MMP-2, MMP-3 and ADAMTS4. Fresh tissue contained aggrecan core protein products similar to those previously described for bovine articular cartilage of this age. IL-1 induced the release of aggrecanase-generated CS-substituted products including both high (>250 kDa) and low molecular weight (about 75 kDa) species. TIMP-3 (but not TIMP-1 and -2 or a broad spectrum MMP inhibitor) inhibited IL-1-dependent GAG loss. In addition, IL-1 induced the release of preformed pools of three known G1-bearing products. We conclude that aggrecanases are responsible for IL-1-stimulated GAG release from meniscal explants, and that IL-1 also stimulates release of G1-bearing products, by a process possibly involving hyaluronan fragmentation.

Chondrocytes and meniscal fibrochondrocytes differentially process aggrecan during de novo extracellular matrix assembly

Aggrecan is an extracellular matrix molecule that contributes to the mechanical properties of articular cartilage and meniscal fibrocartilage, but the abundance and processing of aggrecan in these tissues are different. The objective of this study was to compare patterns of aggrecan processing by chondrocytes and meniscal fibrochondrocytes in tissue explants and cell-agarose constructs. The effects of transforming growth factor-beta 1 (TGF-beta1) stimulation on aggrecan deposition and processing were examined, and construct mechanical properties were measured. Fibrochondrocytes synthesized and retained less proteoglycans than did chondrocytes in tissue explants and agarose constructs. In chondrocyte constructs, TGF-beta1 induced the accumulation of a 120-kDa aggrecan species previously detected in mature bovine cartilage. Fibrochondrocyte-seeded constructs contained high-molecular-weight aggrecan but lacked aggrecanase-generated fragments found in native, immature meniscus. In addition, reflecting the lesser matrix accumulation, fibrochondrocyte constructs had significantly lower compression moduli than did chondrocyte constructs. These cell type-specific differences in aggrecan synthesis, retention, and processing may have implications for the development of functional engineered tissue grafts.

Aggrecan expression is substantially and abnormally upregulated in Hutchinson–Gilford Progeria Syndrome dermal fibroblasts

Hutchinson-Gilford Progeria syndrome (HGPS) is a rare genetic disorder that displays features of segmental aging. It is manifested predominantly in connective tissue, with most prominent histological changes occurring in the skin, cartilage, bone and cardiovascular tissues. Detailed quantitative real time reverse-transcription polymerase chain reaction studies confirmed the previous observation that platelet-derived growth factor A-chain transcripts are consistently elevated 11+/-2- to 13+/-2-fold in two HGPS dermal fibroblast lines compared with age-matched controls. Furthermore, we identified two additional genes with substantially altered transcript levels. Nucleotide pyrophosphatase transcription was virtually shut down with decreased expression of 13+/-3- to 59+/-3-fold in HGPS, whereas aggrecan mRNA was elevated to 24+/-5 times to 41+/-4 times that of chronologically age-matched controls. Aggrecan, normally a component of cartilage and not always detectable in normal fibroblasts cultures, was secreted by HGPS fibroblast lines and was produced as a proteoglycan. This demonstrates that elevated aggrecan expression and its secretion are aberrant features of HGPS. We conclude that HGPS cells can display massively altered transcript levels leading to the secretion of inappropriate protein species.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

OBJECTIVE

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

METHOD

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

RESULTS

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

CONCLUSIONS

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

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

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

Characterisation of proteoglycans and their catabolic products in tendon and explant cultures of tendon

Tendons are collagenous tissues made of mainly Type I collagen and it has been shown that the major proteoglycans of tendons are decorin and versican. Little is still known about the catabolism of these proteoglycans in tendon. Therefore, the aim of the study was to characterise the proteoglycans including their catabolic products present in uncultured bovine tendon and in the explant cultures of tendon. In this study, the proteoglycans were extracted from the tensile region of deep flexor tendon and isolated by ion-exchange chromatography and after deglycosylation analysed by SDS-polyacrylamide electrophoresis, Western blotting and amino-terminal amino acid sequence analysis. Based on amino acid sequence analysis, approximately 80% of the total proteoglycan core proteins in fresh tendon was decorin. Other species that were detected were biglycan and the large proteoglycans versican (splice variants V(0) and/or V(1)) and aggrecan. Approximately 35% of decorin present in the matrix showed carboxyl-terminal proteolytic processing at a number of specific sites. The analysis of small proteoglycans lost to the medium of tendon explants showed the presence of biglycan and decorin with the intact core protein as well as decorin fragments that contained the amino terminus of the core protein. In addition, two core protein peptides of decorin starting at residues K(171) and D(180) were observed in the matrix and one core protein with an amino-terminal sequence commencing at G(189) was isolated from the culture medium. The majority of the large proteoglycans present in the matrix of tendon were degraded and did not contain the G1 globular domain. Furthermore the aggrecan catabolites present in fresh tendon and lost to the medium of explants were derived from aggrecanase cleavage of the core protein at residues E(373)-A(374), E(1480)-G(1481) and E(1771)-A(1772). The analysis of versican catabolites (splice variants V(0) and/or V(1)) also showed evidence of degradation of the core protein by aggrecanase within the GAG-beta subdomain, as well as cleavage by other proteinase(s) within the GAG-alpha and GAG-beta subdomains of versican (variants V(0) and/or V(2)). Degradation products from the amino terminal region of type XII collagen were also detected in the matrix and medium of tendon explants. This work suggests a prominent role for aggrecanase enzymes in the degradation of aggrecan and to a lesser extent versican. Other unidentified proteinases are also involved in the degradation of versican and small leucine-rich proteoglycans.

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

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

Cartilage degradation independent of MMP/aggrecanases

OBJECTIVE

To identify and characterize a cartilage degradation mechanism that is independent of the proteolytic cleavages by matrix metalloproteinases (MMPs) and aggrecanases.

METHODS

The sensitivity of glycosaminoglycan (GAG) release and collagen release to an MMP/aggrecanase inhibitor, AG3340, was compared using a bovine nasal cartilage explant culture. The release of matrix proteins and hyaluronan (HA) from the culture was analyzed by immunoblotting and radioimmunoassay, respectively. Induction of HA-degrading activity by retinoic acid was examined using the cartilage explant culture and a primary culture of chondrocytes. Degradation of the matrix components of cartilage was also characterized in vivo using an acute arthritis model induced by an intra-articular injection of interleukin 1alpha (IL-1alpha).

RESULTS

AG3340 did not effectively inhibit GAG release at a concentration of more than 10muM, while 10nM of the inhibitor completely suppressed collagen degradation. Retinoic acid induced the release of the aggrecan G1 domain, link protein and HA into the culture medium, and the release of these molecules was not completely inhibited by 10muM of AG3340. The molecules were released as ternary complexes. Retinoic acid induced HA degradation in the explant culture and hyaluronidase activity in the primary culture of chondrocytes. The release of the G1 domain of aggrecan and link protein into the synovial fluid was also observed in the IL-1alpha-induced acute arthritis model.

CONCLUSION

A novel mechanism by chondrocyte-derived hyaluronidase(s) is involved in the release of the matrix components from cartilage, and the hyaluronidase(s) and MMPs/aggrecanases act in a coordinated manner in cartilage degradation.

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

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

Human knee and ankle cartilage explants: catabolic differences

The prevalence of osteoarthritis (OA) is lower in some joints, i.e., the ankle, than in the knee. We have compared the cartilages from these two joints of the same limb in adult donors (matched pairs). Our data to date suggest that there are metabolic, biochemical and biomechanical differences between the cartilages of the two joints. The current study has focused on extending the metabolic studies comparing the response of chondrocytes to Interleukin-1beta (IL-1beta) and osteogenic protein 1 (OP-1) by analyzing changes in sulfate incorporation into glycosaminoglycans (GAGs) as a measure of proteoglycan (PG) synthesis. Human adult chondrocytes from normal knees (tibiofemoral) and ankles (talocrural) joints cultured as explants both responded to IL-1beta after 72 h by decreasing PG synthesis; however, the IC50 for the knee chondrocytes was 6.2 pg/ml, while that for the ankle was 35 pg/ml. When the explants were incubated for 72 h with IL-1beta and allowed to rebound without IL-1beta, synthesis of PG was significantly elevated by ankle chondrocytes within five days; knee chondrocytes were unable to significantly increase synthesis even after eight days. However, in both knee and ankle, application of OP-I enhanced PG synthesis in the rebound phase. In response to IL-1, an upregulation of proteinase activity was detectable by an increase in the neoepitopes proteolytically-generated by both aggrecanase and matrix metalloproteinases (MMPs), in the deep zone of the knee cartilage. Stromelysin and collagenase were upregulated as well. The data emerging from these studies confirm that the ankle is less responsive to catabolic stimulation and more responsive to anabolic stimulation following IL-1 removal. These differences in metabolic activity between the cartilages of the two joints could in part help to explain their differences in susceptibility to OA.

Expression and activity of ADAMTS-5 in synovium

ADAMTS proteinases, belonging to the adamalysin subfamily of metalloproteinases, have been implicated in a variety of cellular events such as morphogenesis, cell migration, angiogenesis, ovulation and extracellular matrix breakdown. Aggrecanase-1 (ADAMTS-4) and aggrecanase-2 (ADAMTS-5) have been identified in cartilage and are largely responsible for cartilage aggrecan breakdown. We have shown previously that synovium, the membrane lining diarthrodial joints, generates soluble aggrecanase activity. We report here the expression, localization and activity of ADAMTS-5 from human arthritic and bovine synovium. ADAMTS-5 was expressed constitutively in synovium with little or no transcriptional regulation by recombinant human interleukin-1 alpha or all-trans-retinoate, factors previously shown to upregulate aggrecanase activity in cartilage. Aggrecanase activity generated by synovium in vitro and recombinant ADAMTS-5 cleaved aggrecan extensively, resulting in aggrecan fragments similar to those generated by chondrocyte-derived aggrecanases, and the activity was inhibited by heparin. ADAMTS-5 was immunolocalized in human arthritic synovium, where staining was mostly pericellular, particularly in the synovial lining and around blood vessels; some matrix staining was also seen. The possibility that synovium-derived ADAMTS-5 may play a role in cartilage aggrecan breakdown is discussed.

Age-related changes in aggrecan glycosylation affect cleavage by aggrecanase

Aggrecan degradation involves proteolytic cleavage of the core protein within the interglobular domain. Because aggrecan is highly glycosylated with chondroitin sulfate (CS) and keratan sulfate (KS), we investigated whether glycosylation affects digestion by aggrecanase at the Glu(373)-Ala(374) bond. Treatment of bovine aggrecan monomers to remove CS and KS resulted in loss of cleavage at this site, suggesting that glycosaminoglycans (GAGs) play a role in cleavage at the Glu(373)-Ala(374) bond. In contrast, MMP-3 cleavage at the Ser(341)-Phe(342) bond was not affected by glycosidase treatment of aggrecan. Removal of KS, but not CS, prevented cleavage at the Glu(373)-Ala(374) bond. Thus, KS residues may be important for recognition of this cleavage site by aggrecanase. KS glycosylation has been observed at sites adjacent to the Glu(373)-Ala(374) bond in steer aggrecan, but not in calf aggrecan (Barry, F. P., Rosenberg, L. C., Gaw, J. U., Gaw, J. U., Koob, T. J., and Neame, P. J. (1995) J. Biol. Chem. 270, 20516-20524). Interestingly, although we found that aggrecanase degraded both calf and steer cartilage aggrecan, the proportion of fragments generated by cleavage at the Glu(373)-Ala(374) bond was higher in steer than in calf, consistent with our observations using aggrecan treated to remove KS. We conclude that the GAG content of aggrecan influences the specificity of aggrecanase for cleavage at the Glu(373)-Ala(374) bond and suggest that age may be a factor in aggrecanase degradation of cartilage.

Chondrocyte extracellular matrix synthesis and turnover are influenced by static compression in a new alginate disk culture system

The goal of this study was to examine the effects of mechanical compression on chondrocyte biosynthesis of extracellular matrix (ECM) components during culture in a new alginate disk culture system. Specifically, we have examined chondrocyte biosynthesis rates, and the structure of aggrecan core protein species present in the cell-associated matrix (CM), in the further removed matrix (FRM) and in the surrounding culture medium. In this alginate disk culture system, chondrocytes can be subjected to mechanical deformations similar to those experienced in vivo. Our results show that over an 8-week culture period, chondrocytes synthesize a functional ECM and can respond to mechanical forces similarly to chondrocytes maintained in native cartilage. In the alginate disk system, static compression was shown to decrease and dynamic compression to increase synthesis of aggrecan of bovine chondrocytes. Western blot analysis of the core proteins of aggrecan molecules identified a number of different species that were present in different relative amounts in the CM, FRM, and medium. Over 21 days of culture, the predominant form of aggrecan found in the ECM was a full-length link-stabilized species. In addition, our data show that the application of 40 h of static compression caused an increase in the proportion of newly synthesized aggrecan molecules released into the medium. However, this was not accompanied by a significant change in the size and composition of aggrecan and aggrecan fragments in the different compartments, suggesting that mechanical compression did not alter the catabolic pathways. Together, these data show that chondrocyte function is maintained in an alginate disk culture system and that this culture system is a useful model to examine chondrocyte ECM assembly and some aspects of catabolism normally found in vivo.

Truncation of the amino-terminus of the recombinant aggrecan rAgg1mut leads to reduced cleavage at the aggrecanase site. Efficient aggrecanase catabolism may depend on multiple substrate interactions

Aggrecanase cleavage at the Glu(373)-Ala(374) site in the interglobular domain of the cartilage proteoglycan aggrecan is a key event in arthritic diseases. The observation that substrates representing only the aggrecanase cleavage site are not catabolized efficiently by aggrecanase prompted us to investigate the requirement of aggrecanase for additional structural elements of its substrate other than the actual cleavage site. Based on the recombinant substrate rAgg1mut we constructed deletion mutants with successively truncated N- or C-termini of the interglobular domain. Catabolism by aggrecanase activities induced in rat chondrosarcoma cells, porcine chondrocytes, and by human recombinant ADAMTS4 showed a gradually decreasing catabolism of progressively shortened, N-terminal deletion mutants of the substrate rAgg1mut. A reduction to 32 amino acids N-terminal to the aggrecanase site resulted in a decrease of at least 42% of aggrecanase cleavage products as compared with the wild-type substrate. When only 16 amino acids preceded the Glu(373)-Ala(374) site, aggrecanase cleavage was completely inhibited. In contrast, C-terminal deletions did not negatively affect aggrecanase cleavage up to the reduction to 13 amino acids C-terminal to the cleavage site. Unlike aggrecanase(s), membrane type 1-matrix metalloprotease (MT1-MMP), able to cleave rAgg1mut both at the aggrecanase and the MMP site, was insensitive to N-terminal deletions regarding aggrecanase cleavage, indicating that the importance of the N-terminus is characteristic for aggrecanase(s). Taken together, the results demonstrate that the amino-terminus of rAgg1mut, containing the MMP site, plays an important role for efficient cleavage by aggrecanase(s), possibly by serving as a further site of interaction between the enzyme and its substrate.

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.

ADAMTS-1 cleaves a cartilage proteoglycan, aggrecan

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

Bovine joint capsule and fibroblasts derived from joint capsule express aggrecanase activity

Bovine joint capsule was maintained in explant culture in the presence of bovine aggrecan monomer and it was shown that the aggrecan monomer was degraded. Amino-terminal sequence analysis of the resulting aggrecan core protein fragments revealed that the core protein was cleaved at five specific sites attributed to glutamyl endopeptidases referred to as aggrecanase activity. Fibroblast cultures were established from explant cultures of joint capsule and when these cells were exposed to aggrecan, cleavage of the core protein of aggrecan at the aggrecanase sites was observed. Inclusion of either retinoic acid or interleukin-1alpha in medium of either joint capsule explant cultures or fibroblast cultures did not increase the rate of cleavage of exogenous aggrecan present in the culture medium. When aggrecan monomer was incubated with conditioned medium from explant cultures of joint capsule maintained in medium, degradation could be detected after 10 min. After a 6-h incubation period the same fragments of aggrecan core protein were observed as those for tissue or cells incubated directly with aggrecan monomer. RT-PCR analysis of mRNA extracted from joint capsule fibroblasts showed that these cells express both aggrecanase-1 and -2 [ADAMTS-2 (Tang) and ADAMTS-5].

Sites of aggrecan cleavage by recombinant human aggrecanase-1 (ADAMTS-4)

Aggrecan, the major proteoglycan of cartilage that provides its mechanical properties of compressibility and elasticity, is one of the first matrix components to undergo measurable loss in arthritic diseases. Two major sites of proteolytic cleavage have been identified within the interglobular domain (IGD) of the aggrecan core protein, one between amino acids Asn(341)-Phe(342) which is cleaved by matrix metalloproteinases and the other between Glu(373)-Ala(374) that is attributed to aggrecanase. Although several potential aggrecanase-sensitive sites had been identified within the COOH terminus of aggrecan, demonstration that aggrecanase cleaved at these sites awaited isolation and purification of this protease. We have recently cloned human aggrecanase-1 (ADAMTS-4) (Tortorella, M. D., Burn, T. C., Pratta, M. A., Abbaszade, I., Hollis, J. M., Liu, R., Rosenfeld, S. A., Copeland, R. A., Decicco, C. P., Wynn, R., Rockwell, A., Yang, F., Duke, J. L., Solomon, K., George, H., Bruckner, R., Nagase, H., Itoh, Y., Ellis, D. M., Ross, H., Wiswall, B. H., Murphy, K., Hillman, M. C., Jr., Hollis, G. F., Newton, R. C., Magolda, R. L., Trzaskos, J. M., and Arner, E. C. (1999) Science 284, 1664-1666) and herein demonstrate that in addition to cleavage at the Glu(373)-Ala(374) bond, this protease cleaves at four sites within the chondroitin-sulfate rich region of the aggrecan core protein, between G2 and G3 globular domains. Importantly, we show that this cleavage occurs more efficiently than cleavage within the IGD at the Glu(373)-Ala(374) bond. Cleavage occurred preferentially at the KEEE(1667-1668)GLGS bond to produce both a 140-kDa COOH-terminal fragment and a 375-kDa fragment that retains an intact G1. Cleavage also occurred at the GELE(1480-1481)GRGT bond to produce a 55-kDa COOH-terminal fragment and a G1-containing fragment of 320 kDa. Cleavage of this 320-kDa fragment within the IGD at the Glu(373)-Ala(374) bond then occurred to release the 250-kDa BC-3-reactive fragment from the G1 domain. The 140-kDa GLGS-reactive fragment resulting from the preferential cleavage was further processed at two additional cleavage sites, at TAQE(1771)-(1772)AGEG and at VSQE(1871-1872)LGQR resulting in the formation of a 98-kDa fragment with an intact G3 domain and two small fragments of approximately 20 kDa. These data elucidate the sites and efficiency of cleavage during aggrecan degradation by aggrecanase and suggest potential tools for monitoring aggrecan cleavage in arthritis.

Mannosamine inhibits aggrecanase-mediated changes in the physical properties and biochemical composition of articular cartilage

The enzymatic processes underlying the degradation of aggrecan in cartilage and the corresponding changes in the biomechanical properties of the tissue are an important part of the pathophysiology of osteoarthritis. Recent studies have demonstrated that the hexosamines glucosamine (GlcN) and mannosamine (ManN) can inhibit aggrecanase-mediated cleavage of aggrecan in IL-1-treated cartilage cultures. The term aggrecanase describes two or more members of the ADAMTS family of metalloproteinases whose glutamyl endopeptidase activity is known to be responsible for much of the aggrecan degradation seen in human arthritides. In this study we examined the effect of ManN and GlcN on aggrecanase-mediated degradation of aggrecan induced by IL-1alpha and the corresponding tissue mechanical properties in newborn bovine articular cartilage. After 6 days of culture in 10 ng/ml IL-1 plus ManN, mechanical testing of explants in confined compression demonstrated that ManN inhibited the IL-1alpha-induced degradation in tissue equilibrium modulus, dynamic stiffness, streaming potential, and hydraulic permeability, in a dose-dependent fashion, with peak inhibition ( approximately 75-100% inhibition) reached by a concentration of 1.35 mM. Aggrecan from explants cultured in IL-1 was found by Western analysis to be almost entirely processed down to the G1-NITEGE(373) end product. Addition of ManN or GlcN was found to produce 75-90% inhibition of this cleavage, but the proportion of aggrecan remaining in the tissue which was cleaved at aggrecanase sites in the chondroitin sulfate (CS)-rich region (Glu(1501) and Glu(1687)) was higher than with IL-1 alone. This result suggests that the preservation of mechanical properties by hexosamines in explants is primarily due to inhibition of cleavage at the Glu(373) site in the interglobular domain. While the precise mechanism by which hexosamines function in this system is unclear, the present analysis suggests that the mechanical properties examined may be predominantly a function of electrostatic repulsion due to the charged CS chains in the tightly packed repetitive sequences of the CS-1 region.

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

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

Chondrocyte-mediated catabolism of aggrecan: evidence for a glycosylphosphatidylinositol-linked protein in the aggrecanase response to interleukin-1 or retinoic acid

The control of chondrocyte-mediated degradation of aggrecan has been studied in rat chondrosarcoma cells and bovine cartilage explants treated with either IL-1 or retinoic acid. The capacity of glucosamine to inhibit the aggrecanase-mediated response (J. D. Sandy, D. Gamett, V. Thompson, and C. Verscharen (1998) Biochem. J. 335, 59-66) has been extended to an investigation of the effect of other hexosamines. Mannosamine inhibits the aggrecanase response to both IL-1 and RA at about one-tenth the concentration of glucosamine in both rat cell and bovine explant systems. This effect of mannosamine appears to be due to its capacity to inhibit the synthesis of glycosylphosphatidylinositol (GPI)-linked proteins by chondrocytes since the GPI synthesis inhibitor 2-deoxyfluoroglucose (2-DFG) also inhibited the aggrecanase response to IL-1b and RA in rat cells. Moreover, phosphatidylinositol-specific phospholipase C (PIPLC) treatment of rat cells markedly inhibited the aggrecanase response to IL-1b and RA. These inhibitory effects of mannosamine, 2-DFG, and PIPLC in rat cells did not appear to be due to an interference with general biosynthetic activity of the cells as measured by [3H]proline incorporation into secreted proteins. We suggest that the aggrecanase response by chondrocytes to IL-1 and RA is dependent on the activity of a GPI-anchored protein on the chondrocyte cell surface.

Physical and biological regulation of proteoglycan turnover around chondrocytes in cartilage explants. Implications for tissue degradation and repair

The development of clinical strategies for cartilage repair and inhibition of matrix degradation may be facilitated by a better understanding of (1) the chondrocyte phenotype in the context of a damaged extracellular matrix, and (2) the roles of biochemical and biomechanical pathways by which matrix metabolism is mediated. Using methods of quantitative autoradiography, we examined the cell-length scale patterns of proteoglycan deposition and turnover in the cell-associated matrices of chondrocytes in adult bovine and calf cartilage explants. Results highlight a rapid turnover in the pericellular matrix, which may indicate spatial organization of PG metabolic pools, and specific biomechanical roles for different matrix regions. Subsequent to injurious compression of calf explants, which resulted in grossly visible tissue cracks and caused a decrease in the number of viable chondrocytes within explants, cell-mediated matrix catabolic processes appeared to increase, resulting in apparently increased rates of proteoglycan turnover around active cells. Furthermore, the influences of cell-stimulatory factors such as IL-1 beta appeared to be delayed in their effects subsequent to injurious compression, suggesting interactions between biomechanical and biochemical pathways of PG degradation. These results may provide a useful reference point in the development of in vitro models for cartilage injury and disease, and hint at possible new approaches in the development of cartilage repair strategies.

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

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

Coincubation of bovine synovial or capsular tissue with cartilage generates a soluble "Aggrecanase" activity

The culture of bovine synovial or capsular tissue generated proteoglycan-degrading activity. When these tissues were incubated with living or dead bovine articular cartilage significantly more proteoglycan-degrading activity was revealed. The activity was present in a soluble form and required protein synthesis for its generation. The conditioned medium did not contain matrixin activity, although experiments with proteinase inhibitors suggested that the activity was due to a metalloproteinase. Western blotting of the aggrecan fragments suggested cleavage of aggrecan within the interglobular domain at the "aggrecanase" site, but not at the major matrixin site. N-terminal sequencing confirmed cleavage of aggrecan at a number of glutamyl bonds, including the aggrecanase site in the interglobular domain. We conclude that cultured synovial or capsular tissue produces soluble aggrecanase and an enzyme which releases aggrecanase from cartilage, possibly by cleavage of a chondrocyte membrane-bound form of aggrecanase.

Effects of injurious compression on matrix turnover around individual cells in calf articular cartilage explants

The effects of mechanical injury on the metabolism of cartilage matrix are of interest for understanding the pathogenesis of osteoarthrosis and the development of strategies for cartilage repair. The purpose of the present study was to examine the effects of injury on matrix turnover in a calf articular cartilage explant system for which the effects of mechanical loading on cell activity and the cell-mediated pathways of matrix metabolism are already well characterized. New methods of quantitative autoradiography were used in combination with established biochemical and biomechanical techniques for the analysis of cell and matrix responses to acute mechanical injury, with particular attention to the processes of localized matrix turnover in the cell-associated matrices of individual chondrocytes. Matrix deposition and turnover around cells in control explants was spatially dependent, with the highest rates of proteoglycan deposition and turnover and the lowest rates of collagen deposition (as indicated by [3H]proline autoradiography) occurring in the pericellular matrix. Injurious compression was associated with (a) an abrupt decrease in the tensile load-carrying capacity of the collagen matrix, apparently associated with mechanical failure of the tissue, (b) a considerable but subtotal decrease in cell viability, marked by the emergence of an apparently inactive cell population interspersed within catabolically active but abnormally large cells, and (c) sustained, elevated rates of proteoglycan turnover, particularly in the cell-associated matrices of apparently viable cells, which involved the increased release of aggregating species in addition to a spectrum of degradation fragments that were also in controls. These results may represent an in vitro model for the responses of chondrocytes and the cartilage extracellular matrix to mechanical injury.

Complete coding sequence of bovine aggrecan: comparative structural analysis

The previously available sequence for bovine aggrecan included only the KS domain, the C-terminal portion of the CS-2 domain, and the entire CS-3 and G3 domains. We have isolated cDNA clones for previously uncharacterized portions of the bovine aggrecan sequence, and, when we combined them with previously published incomplete sequences, have obtained a complete sequence for the entire core protein. The bovine aggrecan sequence, which is a composite of new sequence data and previously published incomplete sequences, is 2327 residues in length. Although there is significant conservation of G1, G2, and G3 globular domains between species, there are differences in the length of the interglobular domain, in the number of KS domain hexapeptide repeats and CS domain repeats, and in alternative splicing within the G3 domain. The bovine aggrecan KS domain contains 24 repeats of a hexapeptide motif. The largely uncharacterized CS-1 domain of bovine aggrecan was found to contain 27 variable repeats of a 21-residue consensus sequence. A notable feature of the bovine CS-1 domain is in the distribution of single Ser-Gly dipeptides, the majority of which are separated by 7 or 8 amino acids, compared to the human, where discrete pairs of Ser-Gly dipeptides are separated by 13 amino acids. The CS-2 domain contains a total of six "homology domains" with 4 complete and 2 partial approximately 100-residue repeats. Each "homology domain" contains a "nodal" region with few sites for CS chain addition that is highly conserved between species, suggesting a possible role in aggrecan biosynthesis or catabolism.

Inhibition of cartilage degradation and changes in physical properties induced by IL-1beta and retinoic acid using matrix metalloproteinase inhibitors

Bovine cartilage explants were treated with 100 ng/ml recombinant human interleukin-1beta (IL-1beta) or 1 microM all-trans retinoic acid (RA) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, samples cultured with IL-1beta or RA were treated with 4 microM recombinant human tissue inhibitor of metalloproteinases-1 (TIMP-1) or a synthetic metalloproteinase inhibitor (L-758,354) to inhibit this degradation. Treatment with IL-1beta or RA each resulted in >90% GAG loss after 8 days in culture. Addition of TIMP or L-758,354 to the culture media inhibited IL-1beta-induced loss of tissue GAG by 40 and 65%, respectively, and inhibited RA-induced GAG loss by 35 and 65%, respectively. Analysis of degradation products in the culture media using a G1 antibody indicated that IL-1beta- and RA-treated plugs released 68-kDa fragments of aggrecan, corresponding to a segment of the aggrecan core protein from the G1 domain to the C-terminus NITEGE, consistent with "aggrecanase" activity. Release of the G1 fragment was inhibited by treatment with L-758,354. Both IL-1beta and RA induced significant loss of hyaluronan from cartilage explants after 8 days of exposure and HA loss was also inhibited by addition of L-756,354 to the culture media. IL-1beta, but not RA, induced a significant increase in swelling ratio (wet weight in 0.01 M NaCl normalized to wet weight in DMEM) after 8 days in culture, consistent with degradation of the collagen network, and the increase in tissue swelling was inhibited by treatment with TIMP-1 or L-758,354. Exposure to IL-1beta or RA resulted in significant changes in cartilage physical properties including streaming potential, equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient after 8 days in culture, and these changes were inhibited by 40-90% by exposure to TIMP and by 50-90% by exposure to L-758,354. Measurement of dynamic streaming potential showed that changes due to treatment with IL-1beta alone were highly dependent in compression frequency, with dramatic changes seen at high frequency prior to changes in mechanical properties, and little initial change seen at low frequency. Streaming potential and equilibrium modulus of explants treated with RA decreased to 10% of their initial values after 8 days in culture, but decreased to only 40 and 90%, respectively, when treated with RA plus TIMP-1.

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

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