Gene expression analysis of murine and human osteoarthritis synovium reveals elevation of transforming growth factor β-responsive genes in osteoarthritis-related fibrosis

OBJECTIVE

Synovial fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). Transforming growth factor β (TGFβ), which is elevated in OA, plays a key role in the onset and persistence of synovial fibrosis. However, blocking of TGFβ in OA as a therapeutic intervention for fibrosis is not an option since TGFβ is crucial for cartilage maintenance and repair. Therefore, we undertook the present study to seek targets downstream of TGFβ for preventing OA-related fibrosis without interfering with joint homeostasis.

METHODS

Experiments were performed to determine whether genes involved in extracellular matrix turnover were responsive to TGFβ and were elevated in OA-related fibrosis. We analyzed gene expression in TGFβ-stimulated human OA synovial fibroblasts and in the synovium of mice with TGFβ-induced fibrosis, mice with experimental OA, and humans with end-stage OA. Gene expression was determined by microarray, low-density array, or quantitative polymerase chain reaction analysis.

RESULTS

We observed an increase in expression of procollagen genes and genes encoding collagen crosslinking enzymes under all of the OA-related fibrotic conditions investigated. Comparison of gene expression in TGFβ-stimulated human OA synovial fibroblasts, synovium from mice with experimental OA, and synovium from humans with end-stage OA revealed that the genes PLOD2, LOX, COL1A1, COL5A1, and TIMP1 were up-regulated in all of these conditions. Additionally, we confirmed that these genes were up-regulated by TGFβ in vivo in mice with TGFβ-induced synovial fibrosis.

CONCLUSION

Most of the up-regulated genes identified in this study would be poor targets for therapy development, due to their crucial functions in the joint. However, the highly up-regulated gene PLOD2, responsible for the formation of collagen crosslinks that make collagen less susceptible to enzymatic degradation, is an attractive and promising target for interference in OA-related synovial fibrosis.

Inducible chondrocyte-specific overexpression of BMP2 in young mice results in severe aggravation of osteophyte formation in experimental OA without altering cartilage damage

OBJECTIVES

In osteoarthritis (OA) chondrocytes surrounding lesions express elevated bone morphogenetic protein 2 (BMP2) levels. To investigate the functional consequence of chondrocyte-specific BMP2 expression, we made a collagen type II dependent, doxycycline (dox)-inducible BMP2 transgenic mouse and studied the effect of elevated BMP2 expression on healthy joints and joints with experimental OA.

METHODS

We cloned a lentivirus with BMP2 controlled by a tet-responsive element and transfected embryos of mice containing a collagen type II driven cre-recombinase and floxed rtTA to gain a mouse expressing BMP2 solely in chondrocytes and only upon dox exposure (Col2-rtTA-TRE-BMP2). Mice were treated with dox to induce elevated BMP2 expression. In addition, experimental OA was induced (destabilisation of the medial meniscus model) with or without dox supplementation and knee joints were isolated for histology.

RESULTS

Dox treatment resulted in chondrocyte-specific upregulation of BMP2 and severely aggravated formation of osteophytes in experimental OA but not in control mice. Moreover, elevated BMP2 levels did not result in alterations in articular cartilage of young healthy mice, although BMP2-exposure did increase VDIPEN expression in the articular cartilage. Strikingly, despite apparent changes in knee joint morphology due to formation of large osteophytes there were no detectible differences in articular cartilage: none with regard to structural damage nor in Safranin O staining intensity when comparing destabilisation of the medial meniscus with or without dox exposure.

CONCLUSIONS

Our data show that chondrocyte-specific elevation of BMP2 levels does not alter the course of cartilage damage in an OA model in young mice but results in severe aggravation of osteophyte formation.

Osteoarthritis-related fibrosis is associated with both elevated pyridinoline cross-link formation and lysyl hydroxylase 2b expression

OBJECTIVE

Fibrosis is a major contributor to joint stiffness in osteoarthritis (OA). We investigated several factors associated with the persistence of transforming growth factor beta (TGF-β)-induced fibrosis and whether these factors also play a role in OA-related fibrosis.

DESIGN

Mice were injected intra-articularly (i.a.) with an adenovirus encoding either TGF-β or connective tissue growth factor (CTGF). In addition, we induced OA by i.a. injection of bacterial collagenase into the right knee joint of C57BL/6 mice. mRNA was isolated from the synovium for Q-PCR analysis of the gene expression of various extracellular matrix (ECM) components, ECM degraders, growth factors and collagen cross-linking-related enzymes. Sections of murine knee joints injected with Ad-TGF-β or Ad-CTGF or from experimental OA were stained for lysyl hydroxylase 2 (LH2). The number of pyridinoline cross-links per triple helix collagen in synovium biopsies was determined with high-performance liquid chromatography (HPLC).

RESULTS

Expression of collagen alpha-1(I) chain precursor (Col1a1), tissue inhibitor of metalloproteinases 1 (TIMP1) and especially procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2b (Plod2b) were highly upregulated by TGF-β but not by CTGF. Elevated expression of Plod2b mRNA was associated with high lysyl hydroxylase 2 (LH2) protein staining after TGF-β overexpression and in experimental OA. Furthermore, in experimental OA the number of hydroxypyridinoline cross-links was significant increased compared to control knee joints.

CONCLUSIONS

Our data show that elevated LH2b expression is associated with the persistent nature of TGF-β-induced fibrosis. Also in experimental OA, LH2b expression as well as the number of hydroxypyridinoline cross-link were significantly upregulated. We propose that LH2b, and the subsequent increase in pyridinoline cross-links, is responsible for the persistent fibrosis in experimental OA.

Resemblance of osteophytes in experimental osteoarthritis to transforming growth factor beta-induced osteophytes: limited role of bone morphogenetic protein in early osteoarthritic osteophyte formation

OBJECTIVE

Osteoarthritis (OA) is characterized by cartilage damage, synovial fibrosis, and osteophyte formation. Both transforming growth factor beta (TGFbeta) and bone morphogenetic protein 2 (BMP-2) can induce the formation of osteophytes during OA, but their specific role in this process is unclear. The purpose of this study was to investigate the respective contributions of TGFbeta and BMP-2 to OA.

METHODS

Mouse knee joints injected with adenovirus (Ad-TGFbeta or Ad-BMP-2) were compared histologically with knee joints from murine models of OA (joints injected with collagenase and joints from STR/Ort mice with spontaneous OA). To further investigate the role of BMP during osteophyte formation, adenovirus Ad-Gremlin was injected into knee joints that had previously been injected with Ad-TGFbeta or collagenase.

RESULTS

BMP-2 induced early osteophytes, which bulged from the growth plates on the femur and grew on top of the patella, whereas TGFbeta induced early osteophyte formation on the bone shaft beneath the collateral ligament on the femur as well as on top of the patella. The pattern of osteophyte formation during experimental OA closely resembled that of TGFbeta-induced osteophyte formation, but differed from the pattern induced by BMP-2. Ad-Gremlin proved to be able to totally block BMP-2-induced osteophyte formation. However, blocking BMP activity inhibited neither TGFbeta-induced nor experimental OA-associated osteophyte formation.

CONCLUSION

Our findings demonstrate that the role of BMP during the onset of TGFbeta-induced and experimental OA-induced osteophyte formation is limited. The latter finding does not rule out a role of BMP during osteophyte maturation.

Connective tissue growth factor/CCN2 overexpression in mouse synovial lining results in transient fibrosis and cartilage damage

OBJECTIVE

Characteristics of osteoarthritis (OA) include cartilage damage, fibrosis, and osteophyte formation. Connective tissue growth factor (CTGF; also known as CCN2), is found in high levels in OA chondrocytes and is frequently involved in fibrosis, bone formation, and cartilage repair. The present study was therefore undertaken to investigate the potential role of CTGF in OA pathophysiology.

METHODS

We transfected the synovial lining of mouse knee joints with a recombinant adenovirus expressing human CTGF and measured synovial fibrosis and proteoglycan content in cartilage on days 1, 3, 7, 14, and 28. Messenger RNA (mRNA) expression in synovium and cartilage was measured on days 3, 7, and 21.

RESULTS

CTGF induced synovial fibrosis, as indicated by accumulation of extracellular matrix and an increase in procollagen type I-positive cells. The fibrosis reached a maximum on day 7 and had reversed by day 28. Levels of mRNA for matrix metalloproteinase 3 (MMP-3), MMP-13, ADAMTS-4, ADAMTS-5, tissue inhibitor of metalloproteinases 1 (TIMP-1), and transforming growth factor beta were elevated in the fibrotic tissue. TIMP-1 expression was elevated on day 3, while expression of other genes did not increase until day 7 or later. CTGF induced proteoglycan depletion in cartilage as early as day 1. Maximal depletion was observed on days 3-7. Cartilage damage was reduced by day 28. A high level of MMP-3 mRNA expression was found in cartilage. CTGF overexpression did not induce osteophyte formation.

CONCLUSION

CTGF induces transient fibrosis that is reversible within 28 days. Overexpression of CTGF in knee joints results in reversible cartilage damage, induced either by the high CTGF levels or via factors produced by the CTGF-induced fibrotic tissue.

Expression of transforming growth factor-beta (TGFbeta) and the TGFbeta signalling molecule SMAD-2P in spontaneous and instability-induced osteoarthritis: role in cartilage degradation, chondrogenesis and osteophyte formation

BACKGROUND

The primary feature of osteoarthritis is cartilage loss. In addition, osteophytes can frequently be observed. Transforming growth factor-beta (TGFbeta) has been suggested to be associated with protection against cartilage damage and new cartilage formation as seen in osteophytes.

OBJECTIVE

To study TGFbeta and TGFbeta signalling in experimental osteoarthritis to gain insight into the role of TGFbeta in cartilage degradation and osteophyte formation during osteoarthritis progression.

METHODS

Histological sections of murine knee joints were stained immunohistochemically for TGFbeta3 and phosphorylated SMAD-2 (SMAD-2P). Expression patterns were studied in two murine osteoarthritis models, representing spontaneous (STR/ort model) and instability-associated osteoarthritis (collagenase-induced instability model).

RESULTS

TGFbeta3 and SMAD-2P staining was increasingly reduced in cartilage during osteoarthritis progression in both models. Severely damaged cartilage was negative for TGFbeta3. In contrast, bone morphogenetic protein-2 (BMP-2) expression was increased. In chondrocyte clusters, preceding osteophyte formation, TGFbeta3 and SMAD-2P were strongly expressed. In early osteophytes, TGFbeta3 was found in the outer fibrous layer, in the peripheral chondroblasts and in the core. Late osteophytes expressed TGFbeta3 only in the fibrous layer. SMAD-2P was found throughout the osteophyte at all stages. In the late-stage osteophytes, BMP-2 was strongly expressed.

CONCLUSION

Data show that lack of TGFbeta3 is associated with cartilage damage, suggesting loss of the protective effect of TGFbeta3 during osteoarthritis progression. Additionally, our results indicate that TGFbeta3 is involved in early osteophyte development, whereas BMP might be involved in late osteophyte development.

Reduced transforming growth factor-beta signaling in cartilage of old mice: role in impaired repair capacity

Osteoarthritis (OA) is a common joint disease, mainly effecting the elderly population. The cause of OA seems to be an imbalance in catabolic and anabolic factors that develops with age. IL-1 is a catabolic factor known to induce cartilage damage, and transforming growth factor (TGF)-beta is an anabolic factor that can counteract many IL-1-induced effects. In old mice, we observed reduced responsiveness to TGF-beta-induced IL-1 counteraction. We investigated whether expression of TGF-beta and its signaling molecules altered with age. To mimic the TGF-beta deprived conditions in aged mice, we assessed the functional consequence of TGF-beta blocking. We isolated knee joints of mice aged 5 months or 2 years, half of which were exposed to IL-1 by intra-articular injection 24 h prior to knee joint isolation. Immunohistochemistry was performed, staining for TGF-beta1, -2 or -3, TGF-betaRI or -RII, Smad2, -3, -4, -6 and -7 and Smad-2P. The percentage of cells staining positive was determined in tibial cartilage. To mimic the lack of TGF-beta signaling in old mice, young mice were injected with IL-1 and after 2 days Ad-LAP (TGF-beta inhibitor) or a control virus were injected. Proteoglycan (PG) synthesis (³⁵S-sulfate incorporation) and PG content of the cartilage were determined. Our experiments revealed that TGF-beta2 and -3 expression decreased with age, as did the TGF-beta receptors. Although the number of cells positive for the Smad proteins was not altered, the number of cells expressing Smad2P strongly dropped in old mice. IL-1 did not alter the expression patterns. We mimicked the lack of TGF-beta signaling in old mice by TGF-beta inhibition with LAP. This resulted in a reduced level of PG synthesis and aggravation of PG depletion. The limited response of old mice to TGF-beta induced-IL-1 counteraction is not due to a diminished level of intracellular signaling molecules or an upregulation of intracellular inhibitors, but is likely due to an intrinsic absence of sufficient TGF-beta receptor expression. Blocking TGF-beta distorted the natural repair response after IL-1 injection. In conclusion, TGF-beta appears to play an important role in repair of cartilage and a lack of TGF-beta responsiveness in old mice might be at the root of OA development.

Elucidation of IL-1/TGF-beta interactions in mouse chondrocyte cell line by genome-wide gene expression

OBJECTIVE

To elucidate the antagonism between interleukin-1 (IL-1) and transforming growth factor-beta (TGF-beta) at the gene expression level, as IL-1 and TGF-beta are postulated to be critical mediators of cartilage degeneration/protection in rheumatic diseases.

METHODS

The H4 chondrocyte cell line was validated by comparing metalloproteinase expression profile with intact murine cartilage by reverse transcription polymerase chain reaction. Genome-wide gene expression in the H4 cells in response to IL-1 and TGF-beta, alone and in combination, was analyzed by using oligonucleotide arrays negotiating approximately 12,000 genes.

RESULTS

The response of cartilage and the H4 cell line to IL-1 and TGF-beta was comparable. Oligonucleotide array analysis demonstrated a mutual but asymmetrical antagonism as the dominant mode of interaction between IL-1 and TGF-beta. Cluster analysis revealed a remarkable selectivity in the mode of action exerted by TGF-beta on IL-1 regulated genes: antagonistic on pro-inflammatory genes whereas additive on growth regulators such as vascular endothelial growth factor (VEGF) and connective tissue growth factor (CTGF). While the former cluster underlined the protective effect of TGF-beta, the latter underscored the adverse effect of TGF-beta. We further identified potentially novel classes of target genes under control of TGF-beta such as ras family, histones, proteasome components, and ubiquitin family, highlighting the importance of such genes in TGF signaling besides the well-characterized SMAD pathway.

CONCLUSIONS

We identified a cluster of genes as potential targets mediating the adverse effect of TGF-beta such as fibrosis. Transcriptional regulation of ras GTPase and ubiquitin/proteasome pathways is likely to be a novel mechanism mediating the effect of TGF-beta and its interaction with IL-1. These down-stream genes and pathways can be targets in future therapy.

Loss of transforming growth factor counteraction on interleukin 1 mediated effects in cartilage of old mice

OBJECTIVE

To investigate if a difference exists between young and old mice in the response of articular cartilage to interleukin 1 (IL1) and transforming growth factor beta (TGFbeta) alone or in combination.

METHODS

The interaction of IL1 and TGFbeta was studied in cartilage of young (three months) and old mice (18 months) both in vivo and in vitro. Therefore, IL1, TGFbeta, or IL1 together with TGFbeta was injected into the knee joints of mice on days 1, 3, and 5 before harvest of the patellae on day 6. Alternatively, isolated patellae were stimulated with IL1, TGFbeta, or IL1 together with TGFbeta in culture for 48 hours. Proteoglycan (PG) synthesis and nitric oxide (NO) production were measured.

RESULTS

IL1 inhibited PG synthesis and increased NO production in cartilage of both young and old mice. On the other hand, TGFbeta stimulated PG synthesis and reduced NO production in both age groups. Importantly, TGFbeta was able to counteract IL1 mediated effects on PG synthesis and NO production in young but not in old mice.

CONCLUSIONS

Contrary to the findings in young mice, the cartilage of old animals does not antagonise IL1 effects via TGFbeta. This loss of responsiveness to the pivotal cytokine TGFbeta on effects of IL1 can be important in the initiation and progression of osteoarthritis (OA).

Role of nitric oxide in the inhibition of BMP-2-mediated stimulation of proteoglycan synthesis in articular cartilage

OBJECTIVE

Bone morphogenetic protein-2 (BMP-2)-mediated stimulation of articular cartilage proteoglycan (PG) synthesis is suppressed in arthritic murine knee joints and by interleukin-1 (IL-1). The goal of this study was to investigate whether the gaseous mediator nitric oxide (NO) plays a crucial role in the inhibition of BMP-2 effects by IL-1.

METHODS

Bone morphogenetic protein-2 alone or in combination with IL-1 was injected into the right knee joint of wild-type and NOS2 deficient C57BI/6x129/Sv mice. Proteoglycan synthesis was measured ex vivo by incorporation of 35S-sulfate on day 1, 2 and 3 after injection. To study the role of NO in the inhibition BMP-2-mediated stimulation of PG synthesis in arthritic joints, BMP-2 was injected intra-articularly in the joints of wild-type and NOS2 deficient mice with zymosan-induced arthritis. To check for NOS2 deficiency, NO production was measured in conditioned medium after challenge of patellae with surrounding tissue with IL-1.

RESULTS

BMP-2 potently stimulated proteoglycan synthesis in articular cartilage of normal knees (up to 4-fold) but not in arthritic knees. Co-injection of BMP-2 with tumor necrosis factor alpha had no effect on BMP-2-mediated stimulation of PG synthesis but co-injection with IL-1 alpha resulted in a nearly total inhibition of BMP-2-mediated stimulation. In contrast, in NOS2 deficient mice IL-1 had no effect on BMP-2-mediated stimulation of PG synthesis. However, injection of BMP-2 into arthritic knee joints of NOS2 knock out mice did not result in significant stimulation of PG synthesis.

CONCLUSIONS

In this study we show that NO plays a role in the inhibition of BMP-2-mediated stimulation of PG synthesis by IL-1. However, NO, or at least NOS2, plays no dominant role in the inhibition of BMP-2 effects in arthritic knee joints.

Collagen type I antisense and collagen type IIA messenger RNA is expressed in adult murine articular cartilage

OBJECTIVE

Articular cartilage has only limited capacities for repair and it is not known what is the exact mechanism of matrix restoration. It was investigated whether the reparative process in murine articular cartilage after moderate proteoglycan depletion is accompanied by a change in the chondrocyte phenotype either to hypertrophy or to a less differentiated phenotype as assayed by the expression of specific collagen subtypes.

DESIGN

Moderate proteoglycan depletion was induced by injection of papain whereafter the expression of collagen type I mRNA, collagen IIA and IIB mRNA and type X collagen mRNA in patellar cartilage, as markers for chondrocyte phenotype, was investigated by RT-PCR during normal cartilage physiology and matrix restoration. In addition, in-situ expression of collagen subtypes was assayed by immunolocalisation.

RESULTS

In normal articular cartilage collagen I, collagen IIB and collagen type X transcripts were easily detected. Surprisingly, collagen type I sense as well as antisense mRNA was detected and in addition to IIB transcripts collagen IIA transcripts were detected in a number of samples. During cartilage matrix restoration no change in the expression of collagen I, collagen IIA or IIB or collagen type X mRNA transcripts could be detected. Immunolocalization demonstrated the presence of type I (pericellular) and type II collagen in the extracellular matrix. The pericellular matrix of hypertrophic chondrocytes showed collagen type X staining in the calcified cartilage in normal and papain-injected knee joints. Increased staining for collagen type X was found in the upper cartilage layer in the interterritorial matrix from day 7 after papain injection.

CONCLUSION

The absence of changes in collagen mRNA expression indicates that alteration of chondrocyte phenotype does not occur during the successful repair process after moderate proteoglycan depletion. Collagen type X appears to be deposited in the upper cartilage layer during this process.

Expression of recombinant human soluble type II transforming growth factor-beta receptor in Pichia pastoris and Escherichia coli: two powerful systems to express a potent inhibitor of transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) is a potent regulator of cell metabolism, proliferation, and differentiation. To study the role of endogenous TGF-beta in processes such as tissue repair and inflammation, potent and specific inhibitors are required. Because the type II TGF-beta receptor (TGF beta RII) has a high affinity for TGF-beta, the extracellular domain of TGF beta RII (TGF-beta sRII) was expressed in Pichia pastoris and Escherichia coli. Expression of the soluble TGF beta sRII using P. pastoris resulted in a soluble, heterogeneously glycosylated protein which was secreted into the medium. Although expression of TGF beta sRII in E. coli resulted in the formation of insoluble inclusion bodies, solubilization and refolding resulted in a biologically active protein. Because in both systems a C-terminal 6x His coding sequence was inserted behind the coding sequence for the extracellular domain of TGF beta RII the recombinant proteins could be purified by a powerful, single-step procedure using a Ni-NTA agarose. The purified proteins appeared to be potent inhibitors of TGF-beta 1 and TGF-beta 3. In contrast, TGF beta sRII was less effective in neutralization of TGF-beta 2. In conclusion, biologically active TGF beta sRII can be produced using P. pastoris and E. coli expression systems. The ease of these expression systems, the powerful single step purification and low costs makes it possible to produce TGF beta s RII in large amounts to further elucidate the role of TGF-beta 1 and TGF-beta 3 in physiological processes like tissue repair and inflammation.

Stimulation of articular cartilage repair in established arthritis by local administration of transforming growth factor-beta into murine knee joints

A severe consequence of rheumatoid arthritis is depletion of proteoglycans (PGs) from articular cartilage leading to functional impairment of this tissue. We investigated whether local administration of anabolic factors (transforming growth factors-beta1 and -beta2 [TGF-beta1 and -beta2, respectively] and bone morphogenetic protein-2 (BMP-2) into joints could stimulate cartilage repair during arthritis. A unilateral arthritis was induced in mice by intra-articular injection of zymosan. Starting on Day 4 after the induction of arthritis, three injections of TGF-beta1 (200 ng) were given (Days 4, 6, and 8). On Day 11, articular cartilage PG synthesis was measured by 35S-sulfate incorporation, and histologic knee joint sections were prepared, which were used to analyze cartilage PG content by quantification of safranin O staining. Additionally, histologic sections were used to analyze inflammation and chondrophyte-formation. Local administration of TGF-beta1 did not modify inflammation but clearly stimulated PG synthesis and restored PG content of depleted cartilage. TGF-beta2 appeared to be as potent as TGF-beta1 in the stimulation of cartilage repair, and both TGF-beta isoforms also stimulated the formation of chondrophytes in this rodent model. In contrast to TGF-beta, three intra-articular injections with 200 ng BMP-2 did not stimulate the repair process. In summary, this study demonstrates for the first time that local administration of TGF-beta into arthritic joints stimulates the replenishment of PGs in depleted cartilage.

Bone morphogenetic protein 2 stimulates articular cartilage proteoglycan synthesis in vivo but does not counteract interleukin-1alpha effects on proteoglycan synthesis and content

OBJECTIVE

To study the effect of bone morphogenetic protein 2 (BMP-2) on articular cartilage proteoglycan (PG) synthesis in vivo and to investigate whether BMP-2 is able to counteract the effects of interleukin-1 (IL-1) on articular cartilage PG synthesis and content.

METHODS

BMP-2 alone or in combination with IL-1alpha was injected into murine knee joints. PG synthesis was measured by 35S-sulfate incorporation using an ex vivo method or autoradiography. Cartilage PG content was analyzed by measuring Safranin O staining intensity on histologic sections.

RESULTS

BMP-2 appeared to be a potent stimulator of articular cartilage PG synthesis in vivo. However, BMP-2 was not able to counteract the deleterious effects of IL-1alpha on articular cartilage PG synthesis and content. In addition, intraarticular injections of BMP-2 induced chondrophytes.

CONCLUSION

Although BMP-2 is a very potent stimulator of cartilage PG synthesis in vivo, the therapeutic applications of BMP-2 are limited due to the inability of BMP-2 to counteract the effects of IL-1 and the induction of chondrophytes.

Species-specific expression of type II TGF-beta receptor isoforms by articular chondrocytes: effect of proteoglycan depletion and aging

Recently, a new isoform of the type II transforming growth factor beta receptor (TGF-beta RII) was identified. This isoform (TGF-beta RII2) contains an insertion of 25 amino acids in the extracellular domain of the receptor. Using RT-PCR the authors demonstrated that both TGF-beta RII1 and TGF-beta RII2 are expressed by chondrocytes in murine and human articular cartilage. Bovine articular chondrocytes expressed TGF-beta RII1 mRNA but did not express detectable levels of TGF-beta RII2 mRNA, suggesting that the new isoform does not play an important role in normal bovine cartilage physiology. Because TGF-beta responses seem to be age related and differential TGF-beta responses have been described between normal cartilage and cartilage undergoing repair the authors studied if the relative mRNA expression between these isoforms is altered during cartilage repair and aging. No differences in the relative mRNA expression of the two isoforms of the type II TGF-beta receptor could be demonstrated in murine cartilage during aging or during the repair phase after mild PG depletion indicating that it is unlikely that age-related TGF-beta responses and differential TGF-beta responses between normal cartilage and cartilage undergoing repair are the result of differences in the relative expression of the two TGF-beta RII isoforms.

Early elevation of transforming growth factor-beta, decorin, and biglycan mRNA levels during cartilage matrix restoration after mild proteoglycan depletion

OBJECTIVE

To elucidate the role of transforming growth factor-beta (TGF-beta) and the small proteoglycans biglycan and decorin in the repair of articular cartilage after proteoglycan depletion.

METHODS

Limited and reversible proteoglycan depletion was induced by injection of murine knee joints with 0.5% papain. Proteoglycan content of patellar cartilage was examined by safranin O staining on histological sections and overall proteoglycan synthesis was measured by incorporation of 35S sulfate. Changes in mRNA expression of TGF-beta, aggrecan, decorin, and biglycan were determined by semiquantitative reverse transcription polymerase chain reaction.

RESULTS

Papain injection led to rapid depletion of proteoglycans, which was partly overcome 7 days after injection, while total replenishment of the cartilage matrix with proteoglycans was observed on Day 24. The incorporation of radiolabeled sulfate in patellar proteoglycans was initially decreased (up to Day 3), but significantly enhanced on Days 4 and 7 after papain injection. Upregulation of TGF-beta, decorin, and biglycan mRNA in patellar cartilage was observed on Day 2, markedly before elevation of overall proteoglycan synthesis. mRNA levels were less augmented on Day 7, and on Day 24 all messenger RNA levels had returned to control values. As well, in the soft tissue adjoining the patella swift upregulation of TGF-beta mRNA was observed.

CONCLUSION

mRNA of both TGF-beta and the small proteoglycans decorin and biglycan are elevated at an early phase during cartilage repair after moderate proteoglycan depletion, implying a functional role for these molecules in this repair process.

Analysis of changes in proteoglycan content in murine articular cartilage using image analysis

The extracellular matrix of articular cartilage consists mainly of type II collagen and large aggregating proteoglycan (aggrecan). During arthritis and other joint diseases, the proteoglycan (PG) level of cartilage matrix is diminished, leading to impairment of normal joint function. A new method is described for measuring the changes in PG content of murine articular cartilage. The method is based on the automated densitometric analysis of patellar cartilage of standard, safranin O-stained sections of whole murine knee joints. It appeared to be possible to measure optical density in parallel layers of articular cartilage with high reproducibility. Approximately 25 sections can be evaluated within 1 h. Measuring a single section 10 times resulted in a coefficient of variation (CV) of 0.1-1.4%. A mean CV of 5-14% was calculated when a group of 18 sections was analyzed in quintuplicate. To validate the procedure, changes in PG content induced by arthritis or by intra-articular injection of TGFbeta-1 were analyzed by the image analysis method, the dimethylmethylene blue (DMB) assay and by visual grading. Although not a quantitive method, the newly developed image analysis method appeared to be more sensitive in detecting significant change in PG content of murine articular cartilage than the DMB method or visual grading. The image analysis method makes it possible to measure changes in PG content of specific areas of articular cartilage with higher sensitivity than the DMB method and eliminating the bias inherent to visual grading by human observers.

Stimulation of proteoglycan synthesis by triamcinolone acetonide and insulin-like growth factor 1 in normal and arthritic murine articular cartilage

OBJECTIVE

During experimentally induced arthritis, inhibition of proteoglycan synthesis is one of the mechanisms leading to cartilage destruction. Disturbed anabolic signalling might contribute to this impaired chondrocyte proteoglycan synthesis. We investigated the effects of insulin-like growth factor 1 (IGF-1) and the glucocorticoid, triamcinolone acetonide, on in vitro chondrocyte proteoglycan synthesis of articular cartilage obtained from normal and arthritic mouse knee joints.

METHODS

Proteoglycan synthesis was measured by 35S sulfate incorporation and the hydrodynamic volume of newly synthesized proteoglycans was analyzed with gel chromatography.

RESULTS

Culturing normal cartilage with IGF-1 resulted in significant enhancement of chondrocyte proteoglycan synthesis. Concerning the hydrodynamic volume of newly synthesized proteoglycans after culture with IGF-1, proteoglycan monomers with large hydrodynamic size, similar to those synthesized immediately after dissection were observed. In arthritic cartilage, IGF-1 failed to stimulate proteoglycan synthesis and only proteoglycans with relatively small dimensions were produced. However, in the presence of the steroid triamcinolone acetonide, synthesis of hydrodynamically large proteoglycans were found in arthritic as well as normal cartilage.

CONCLUSION

Our observations indicate that steroids may play a critical role in maintaining cartilage integrity in both normal and arthritic cartilage.

Correlation of the size of type II transforming growth factor beta (TGF-beta) receptor with TGF-beta responses of isolated bovine articular chondrocytes

OBJECTIVES

Transforming growth factor beta (TGF-beta) is a multipotent regulator of cell proliferation and extracellular matrix production. The effect of TGF-beta on chondrocyte matrix production was studied in relation to the expression of TGF-beta binding proteins. The effect of TGF-beta on proteoglycan synthesis of isolated articular chondrocytes depended on the culture period. Proteoglycan synthesis of chondrocytes which were cultured for one day was inhibited by TGF-beta whereas proteoglycan synthesis of chondrocytes cultured in monolayer for seven days or longer was stimulated by TGF-beta. To investigate if this differential response is related to a distinct expression of TGF-beta receptors, this parameter was studied by affinity labelling.

METHODS

Chondrocytes were incubated with 100 pM TGF-beta labelled with iodine-125. Crosslinking was performed using 0.25 mM disuccinimidyl suberate. Membrane proteins were extracted and analysed by denaturating sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography.

RESULTS

Freshly isolated and cultured chondrocytes expressed types I, II, and III TGF-beta receptors. The type II TGF-beta receptor of cultured chondrocytes appeared to be about 15 kilodaltons smaller than the type II TGF-beta receptor expressed on freshly isolated chondrocytes, however.

CONCLUSIONS

As the type II TGF-beta receptors appears to be involved in signal transduction, this change in size of the type II TGF-beta receptor might be related to the differential effect of TGF-beta on proteoglycan synthesis of freshly isolated and cultured bovine articular chondrocytes.

Maintenance of the synthesis of large proteoglycans in anatomically intact murine articular cartilage by steroids and insulin-like growth factor I

OBJECTIVES

The exact regulation of the synthesis of cartilage specific molecules, such as collagen type II and aggrecan, by articular chondrocytes is unknown, but growth factors and hormones probably play an important part. The effects of glucocorticosteroids (prednisolone and triamcinolone), in combination with insulin-like growth factor I (IGF-I), on the synthesis and hydrodynamic volume of proteoglycans from murine patellar cartilage were investigated.

METHODS

The in vitro effect of IGF-I and steroids on proteoglycan synthesis in murine patellar cartilage was evaluated by [35S]sulphate incorporation in combination with dissociative gel chromatography using a Sephacryl S-1000 column. The impact of in vivo prednisolone (0-5 mg/kg) on proteoglycan synthesis in murine patellar cartilage was analysed by [35S]sulphate incorporation immediately after dissection from the knee joint.

RESULTS

Prednisolone stimulated proteoglycan synthesis in murine patellar cartilage from normal knees and in cartilage from knees injected with papain in vitro in the absence and presence of IGF-I. Moreover, oral administration of prednisolone for seven days to C57Bl10 mice resulted in enhanced proteoglycan synthesis in patellar cartilage. The incubation of patellar cartilage for 48 hours without serum or growth factors led to the synthesis of proteoglycans with a smaller hydrodynamic volume than those synthesised immediately after dissection of the patellae. This could either be circumvented by the addition of IGF-I or by the addition of glucocorticosteroids (prednisolone or triamcinolone) to the culture medium.

CONCLUSIONS

These results show that in a dose range of 0.0003-0.3 mmol/l, glucocorticosteroids, like IGF-I, stimulate proteoglycan synthesis and maintain the synthesis of hydrodynamically large proteoglycans by chondrocytes from murine articular cartilage. This indicates that glucocorticosteroids might play a part in the preservation of matrix integrity in articular cartilage.

Induction of osteoarthritis by intra-articular injection of collagenase in mice. Strain and sex related differences

To study the effects of strain and sex on the development of injury-induced osteoarthritis (OA) in murine knee joints, two doses of highly purified bacterial collagenase (10 units and 30 units) were injected into male and female mice of two closely related strains, C57BL6 and C57BL10. Frontal histological sections of whole knee joints were made late in the disease process and examined for osteoarthritic lesions. Differences in prevalence of cartilage damage between strains and sexes were observed. Prevalence was higher in C57BL10 (male: almost 100%) than in C57BL6 (male: about 25%), and the prevalence was twice as high in males as in females in both strains. The amount of collagenase (10 or 30 units) did not affect the prevalence of lesions, however, it did influence the severity of the damage. The site of the damage appeared to be dose and strain dependent. Male C57BL6 always showed damage on the medial tibial plateau, independent of dose. In male C57BL10 damage almost always appeared on the lateral tibial plateau with 10 units, while with 30 units the medial plateau also became strongly involved. Since it is known that male mice are more prone to spontaneous OA than female mice and C57BL10 are more prone han C57BL6 mice, it can be concluded that predisposition to spontaneous osteoarthritis increases the risk of developing injury-induced osteoarthritis. Location and severity of the changes will probably be related to joint loading.

Variable TGF-beta receptor expression regulates TGF-beta responses of articular chondrocytes

TGF-beta inhibits the proteoglycan synthesis of freshly isolated articular chondrocytes while TGF-beta stimulates the proteoglycan synthesis of articular chondrocytes cultured for 14 days. To investigate if this differential effect was the result of differences in TGF-beta receptor expression this parameter was studied by affinity labeling in combination with SDS-PAGE. Using this method we demonstrated a difference in size of the TGF-beta type II receptor between freshly isolated and cultured chondrocytes. This difference might explain the differential effect of TGF-beta on the proteoglycan synthesis of chondrocytes cultured for 1 or 14 days.

Proteoglycan synthesis and osteophyte formation in 'metabolically' and 'mechanically' induced murine degenerative joint disease: an in-vivo autoradiographic study

We investigated the in-vivo proteoglycan synthesis in specific areas of murine knee joint articular cartilage after the induction of degenerative joint disease by means of 35S-sulphate autoradiography. Degenerative joint disease was induced either by direct interference with cartilage metabolism (papain and iodoacetate), or by the induction of joint instability (collagenase). Injection of iodoacetate and papain led to inhibition of proteoglycan synthesis mainly in the central parts of the patellae, patellaris femoris and the central part of the medial tibial plateau. Articular cartilage adjacent to the strongly inhibited areas frequently showed a significantly enhanced synthesis of proteoglycans. A strong inhibition of proteoglycan synthesis was observed in the central part of the medial plateau after collagenase injection while other cartilage sites and joint structures such as the capsule and ligaments were stimulated in their proteoglycan synthesis. This study shows that the localization of changes in cartilage metabolism in degenerative joint disease of the knee might be related to differences in the pathogenetic mechanism in different variants of this common joint disorder.

Local changes in proteoglycan synthesis during culture are different for normal and osteoarthritic cartilage

Proteoglycan synthesis of mild-to-moderate osteoarthritic human knee cartilage was compared with that of normal cartilage of the same donor. Immediately after cartilage was obtained, the synthesis rate of proteoglycans was higher for osteoarthritic cartilage than for normal cartilage. Proteoglycan synthesis was then located, for both normal and osteoarthritic cartilage, in the middle and deep zone. However, after 4 days of culture, proteoglycan synthesis rate was higher for normal cartilage than for osteoarthritic cartilage. The reason for this transition from a lower to a higher proteoglycan synthesis rate was a strong increase in the proteoglycan synthesis in the superficial zone of normal cartilage. This was not observed for the osteoarthritic cartilage. The chondrocytes in the superficial zone of osteoarthritic cartilage, in contrast to normal cartilage, were mainly joined in cell clusters and proliferating. This may explain their inability to contribute to proteoglycan synthesis.

Inhibition of proteoglycan synthesis by transforming growth factor beta in anatomically intact articular cartilage of murine patellae

The effect of transforming growth factor beta (TGF beta) on proteoglycan synthesis and degradation in anatomically intact articular cartilage of murine patellae was studied. Exogenously added TGF beta up to a concentration of 200 pmol/l had no effect on proteoglycan synthesis in intact articular cartilage. Neutralisation of endogenously produced TGF beta with a specific monoclonal antibody to TGF beta, however, led to stimulation of proteoglycan synthesis, indicating that TGF beta itself inhibits proteoglycan synthesis in anatomically intact cartilage. Transforming growth factor beta decreased the degradation of proteoglycans in intact cartilage in the absence of fetal calf serum or insulin-like growth factor 1. In the presence of fetal calf serum or insulin-like growth factor 1, TGF beta had no additional effect on proteoglycan breakdown.

High susceptibility of human articular cartilage glycosaminoglycan synthesis to changes in inorganic sulfate availability

The effect of sulfate concentration in the medium on glycosaminoglycan synthesis in articular cartilage of five different species was examined in relation to the physiological serum sulfate concentration in these species. Only the rate of sulfated glycosaminoglycan synthesis in human articular cartilage was sensitive to small deviations from the physiological sulfate concentration. A reduction in the sulfate concentration from 0.3 mM (physiological) to 0.2 mM resulted in a 33% reduction in glycosaminoglycan synthesis. In addition, we studied the effect of arthritic and "osteoarthritic" alterations in murine cartilage on the dependence of glycosaminoglycan synthesis on low sulfate concentrations. Arthritic and "osteoarthritic" cartilage had a similar dependence on the sulfate concentration in the medium as normal cartilage. Glycosaminoglycan synthesis in human articular cartilage appears to be very sensitive to the potential sulfate-depleting effects of drugs used in the treatment of rheumatoid arthritis and osteoarthritis.

The effect of chronic paracetamol administration to rats on the glycosaminoglycan content of patellar cartilage

Male Wistar rats were treated with paracetamol (200 mg/kg twice a day) for 2, 3, 4 and 9 weeks. During the first four weeks of paracetamol administration the serum sulfate concentration was significantly decreased. However, during the fourth until the ninth week, the serum sulfate concentration was only diminished to a small and insignificant extent. The paracetamol administration did not lead to serious liver or renal toxicity, as determined by alanine aminotransferase and creatinine levels in the serum of the rats. The paracetamol-induced serum sulfate depletion, observed during the first four weeks of the experiment, led to a significantly lower glycosaminoglycan content of the patellar cartilage of the rats after three and four weeks paracetamol treatment. When after the fourth week the serum sulfate concentration rose to nearly normal levels also the glycosaminoglycan content in the rat patellar cartilage reached control levels. These data indicate that the serum sulfate depletion might be the causative factor for the observed reduction in glycosaminoglycan content of rat patellar cartilage.

Degenerative knee joint lesions in mice after a single intra-articular collagenase injection. A new model of osteoarthritis

A single intra-articular injection with bacterial collagenase in the right knee joints of 10-week-old male C57bl10 mice led to osteoarthritic lesions within a few weeks in these joints. The collagenase-induced osteoarthritis was characterized by severe degenerative cartilage lesions on the medial side of the femorotibial joint associated with patellar dislocation to the medial side of the joint, sclerosis of subchondral bone below the cartilage erosions, osteophyte formation, and consequent deformity of the knee joints. The osteoarthritic alterations in the collagenase model closely resembled the changes observed in spontaneous osteoarthritis in aged mice. The intra-articular injection with collagenase probably results in damage to collagen type I-containing joint structures, such as tendons, ligaments and menisci, leading to an instable knee joint that results in the osteoarthritic joint lesions observed in this model. The collagenase-induced osteoarthritis model offers the possibility of studying experimental osteoarthritis in large animal groups of inbred strains within a restricted time span at low costs.

Development of osteoarthritic lesions in mice by "metabolic" and "mechanical" alterations in the knee joints

Male, 10-week-old C57B1 10 mice received a single intraarticular injection in the knee joints with papain, iodoacetate, or collagenase. This led to osteoarthritic lesions, such as matrix depletion, chondrocyte proliferation, and osteophyte formation, in the injected knee joints within several weeks. After injection of iodoacetate and papain, the main osteoarthritic alterations were localized in the femoropatellar joint, whereas injection of collagenase led to marked osteoarthritic lesions in the femorotibial joint. The mechanism of induction of these alterations appears to differ for iodoacetate and papain on one site and collagenase on the other site. Data are presented that collagenase injection, by way of damaging ligaments and tendons, destabilizes the knee joint eventually leading to osteoarthritic alterations. In contrast, injection of papain or iodoacetate directly interferes with cartilage metabolism resulting in osteoarthritic changes.

The effect of low sulfate concentrations on the glycosaminoglycan synthesis in anatomically intact articular cartilage of the mouse

We have studied the effect of environmental sulfate concentration on the glycosaminoglycan synthesis of anatomically intact patellar cartilage of the mouse in vitro. Incubation of mouse patellae in medium with sulfate concentrations below 0.5 mM resulted in a diminished incorporation of sulfate but in unaltered incorporation of glucosamine. This suggested the synthesis of undersulfated glycosaminoglycans under these conditions. We characterized glycosaminoglycans synthesized at three different sulfate concentrations: a sulfate concentration physiological for the mouse (1.0 mM), a sulfate concentration in the range where sulfate incorporation was strongly diminished (0.1 mM), and an extremely low sulfate concentration (10 nM). Analysis of glycosaminoglycan disaccharides and DEAE anion chromatography of the glycosaminoglycans could not confirm the synthesis of undersulfated glycosaminoglycans at 0.1 mM. The chromatogram of glycosaminoglycans synthesized in medium containing 10 nM showed the presence of a very low sulfated glycosaminoglycan pool not observed at higher medium sulfate concentrations. Intermediately sulfated glycosaminoglycans were also synthesized during incubation with 10 nM sulfate. So, our data indicate that only very low sulfate concentrations in the medium lead to the synthesis of undersulfated glycosaminoglycans and that the sulfation mechanism of murine patellar cartilage chondrocytes does not seem to fit completely in an "all-or-nothing" pattern.

Inhibition of glycosaminoglycan synthesis in anatomically intact rat patellar cartilage by paracetamol-induced serum sulfate depletion

We have studied the effect of low sulfate concentrations on the glycosaminoglycan synthesis in rat patellar cartilage in vivo as well as in vitro. The oral administration of 200 mg/kg paracetamol to male Wistar rats resulted in a significant reduction of the serum sulfate concentration. Reduced serum sulfate availability resulted in a 34% decrease of glycosaminoglycan synthesis in patellar cartilage. This is due to sulfate depletion since paracetamol had no direct effects on glycosaminoglycan synthesis and a slight but significant inhibitory effect on the catabolism of radiolabeled glycosaminoglycans in vitro. The glycosaminoglycans synthesized at low sulfate concentrations in vivo were similar to the glycosaminoglycans synthesized at physiological sulfate concentrations. Studying the effect of sulfate availability in vitro on glycosaminoglycan synthesis in patellar cartilage we found that incubation of rat patellae in medium containing less than 0.5 mM inorganic sulfate led to a decreased sulfate incorporation. The use of potential sulfate decreasing drugs can lead to inhibition of glycosaminoglycan synthesis. This argues for a reconsideration of the use of these drugs in patients with already dysfunctioning cartilage metabolism as in rheumatoid arthritis and osteoarthrosis.

Synthesis of aberrant glycosaminoglycans during cartilage culture in 'sulfate free' medium

Incorporation of radiolabeled sulfate into glycosaminoglycans is a widely accepted assay to measure the rate of proteoglycan synthesis. Although glycosaminoglycan synthesis is dependent on the quantity of inorganic sulfate available to proteoglycan synthesizing cells, 'sulfate free' medium is regularly used in studies regarding proteoglycan synthesis. In this study murine patellar cartilage glycosaminoglycans synthesized under 'sulfate free' conditions were compared with those synthesized at physiological sulfate concentration. Under 'sulfate free' conditions synthesis was not only decreased but low sulfated glycosaminoglycans were made that were not synthesized during incubation at physiological sulfate concentration. The use of 'sulfate free' medium should be avoided in proteoglycan synthesis studies.