The role of chondrocyte-matrix interactions in maintaining and repairing articular cartilage

Throughout life chondrocytes maintain the articular cartilage matrix by replacing degraded macromolecules and respond to focal cartilage injury or degeneration by increasing local synthesis activity. These observations suggest that mechanisms exist within articular cartilage that stimulate chondrocyte anabolic activity in response to matrix degradation or damage. An important cartilage anabolic factor, insulin-like growth factor I (IGF-I), appears to have a role in stimulating chondrocyte anabolic activity. Although IGF-I is ubiquitous, its bioavailability is controlled by a class of secreted proteins, IGF binding proteins (IGFPBs). Of the six known IGFPBs, IGFBP-3 is the most abundant in human articular cartilage. We recently found that with increasing age, articular chondrocytes increase their expression of IGFBP-3. This observation led us to investigate the potential role of IGFBP-3 in chondrocyte-matrix interactions. Using immunofluorescent staining and confocal microscopy we found that IGFBP-3 accumulates with increasing age in the chondrocyte territorial matrix where it co-localizes with fibronectin, but not with tenascin-C or type VI collagen. Using purified proteins we demonstrated that IGFBP-3 binds to fibronectin in a dose dependent manner, but not to tenascin-C. In vitro studies showed that IGFBP-3 alone inhibited chondrocyte synthetic activity while intact fibronectin alone significantly stimulated activity. When fibronectin and IGFBP-3 were combined we found that the inhibitory activity of low concentrations of IGFPB-3 was enhanced. These observations indicate that in mature articular cartilage IGF-I is stored in the chondrocyte territorial matrix through binding to a complex of IGFPB-3 and intact fibronectin. Storage of IGF-I of the territorial matrix may help maintain a relatively constant level of available IGF-I and the local increase in matrix synthesis following matrix damage may result from release of IGF-I. This mechanism may have an important role in maintaining and repairing articular cartilage and failure of this mechanism may lead to progressive articular cartilage degeneration.     

Effects of shear stress on articular chondrocyte metabolism

The articular cartilage of diarthrodial joints experiences a variety of stresses, strains and pressures that result from normal activities of daily living. In normal cartilage, the extracellular matrix exists as a highly organized composite of specialized macromolecules that distributes loads at the bony ends. The chondrocyte response to mechanical loading is recognized as an integral component in the maintenance of articular cartilage matrix homeostasis. With inappropriate mechanical loading of the joint, as occurs with traumatic injury, ligament instability, bony malalignment or excessive weight bearing, the cartilage exhibits manifestations characteristic of osteoarthritis. Breakdown of cartilage in osteoarthritis involves degradation of the extracellular matrix macromolecules and decreased expression of chondrocyte proteins necessary for normal joint function. Osteoarthritic cartilage often exhibits increased amounts of type I collagen and synthesis of proteoglycans characteristic of immature cartilage. The shift in cartilage phenotype in response to altered load yields a matrix that fails to support normal joint function. Mathematical modeling and experimental studies in animal models confirm an association between altered loading of diarthrotic joints and arthritic changes. Both types of studies implicate shear forces as a critical component in the destructive profile. The severity of cartilage destruction in response to altered loads appears linked to expression of biological factors influencing matrix integrity and cellular metabolism. Determining how shear stress alters chondrocyte metabolism is fundamental to understanding how to limit matrix destruction and stimulate cartilage repair and regeneration. At present, the precise biochemical and molecular mechanisms by which shear forces alter chondrocyte metabolism from a normal to a degenerative phenotype remain unclear. The results presented here address the hypothesis that articular chondrocyte metabolism is modulated by direct effects of shear forces that act on the cell through mechanotransduction processes. The purpose of this work is to develop critical knowledge regarding the basic mechanisms by which mechanical loading modulates cartilage metabolism in health and disease. This presentation will describe the effects of using fluid induced shear stress as a model system for stimulation of articular chondrocytes in vitro. The fluid induced shear stress was applied using a cone viscometer system to stimulate all the cells uniformly under conditions of minimal turbulence. The experiments were carried using high-density primary monolayer cultures of normal and osteoarthritic human and normal bovine articular chondrocytes. The analysis of the cellular response included quantification of cytokine release, matrix metalloproteinase expression and activation of intracellular signaling pathways. The data presented here show that articular chondrocytes exhibit a dose- and time-dependent response to shear stress that results in the release of soluble mediators and extracellular matrix macromolecules. The data suggest that the chondrocyte response to mechanical stimulation contributes to the maintenance of articular cartilage homeostasis in vivo.     

Chondrocyte response to growth factors is modulated by p38 mitogen-activated protein kinase inhibition

Inhibitors of p38 mitogen-activated protein kinase (MAPK) diminish inflammatory arthritis in experimental animals. This may be effected by diminishing the production of inflammatory mediators, but this kinase is also part of the IL-1 signal pathway in articular chondrocytes. We determined the effect of p38 MAPK inhibition on proliferative and synthetic responses of lapine chondrocytes, cartilage, and synovial fibroblasts under basal and IL-1-activated conditions.Basal and growth factor-stimulated proliferation and proteoglycan synthesis were determined in primary cultures of rabbit articular chondrocytes, first-passage synovial fibroblasts, and cartilage organ cultures. Studies were performed with or without p38 MAPK inhibitors, in IL-1-activated and control cultures. Media nitric oxide and prostaglandin E2 were assayed.p38 MAPK inhibitors blunt chondrocyte and cartilage proteoglycan synthesis in response to transforming growth factor beta; responses to insulin-like growth factor 1 (IGF-1) and fetal calf serum (FCS) are unaffected. p38 MAPK inhibitors significantly reverse inhibition of cartilage organ culture proteoglycan synthesis by IL-1. p38 MAPK inhibition potentiated basal, IGF-1-stimulated and FCS-stimulated chondrocyte proliferation, and reversed IL-1 inhibition of IGF-1-stimulated and FCS-stimulated DNA synthesis. Decreases in nitric oxide but not prostaglandin E2 synthesis in IL-1-activated chondrocytes treated with p38 MAPK inhibitors are partly responsible for this restoration of response. Synovial fibroblast proliferation is minimally affected by p38 MAPK inhibition.p38 MAPK activity modulates chondrocyte proliferation under basal and IL-1-activated conditions. Inhibition of p38 MAPK enhances the ability of growth factors to overcome the inhibitory actions of IL-1 on proliferation, and thus could facilitate restoration and repair of diseased and damaged cartilage.     

Effects of Cyclic Tensile Strain on Chondrocyte Metabolism: A Systematic Review

Chondrocytes reorganize the extracellular matrix of articular cartilage in response to externally applied loads. Thereby, different loading characteristics lead to different biological responses. Despite of active research in this area, it is still unclear which parts of the extracellular matrix adapt in what ways, and how specific loading characteristics affect matrix changes. This review focuses on the influence of cyclic tensile strain on chondrocyte metabolism in vitro. It also aimed to identify anabolic or catabolic chondrocyte responses to different loading protocols. The key findings show that loading cells up to 3% strain, 0.17 Hz, and 2 h, resulted in weak or no biological responses. Loading between 3–10% strain, 0.17–0.5 Hz, and 2–12 h led to anabolic responses; and above 10% strain, 0.5 Hz, and 12 h catabolic events predominated. However, this review also discusses that various other factors are involved in the remodeling of the extracellular matrix in response to loading, and that parameters like an inflammatory environment might influence the biological response.     

Effects of diacerein on biosynthesis activities of chondrocytes in culture

The maintenance of articular cartilage integrity requires a balance between anabolic and catabolic processes which are under the control of chondrocytes. These cells are living in an anaerobic environment and normally do not divide. They are responsible for the continuous maintenance of the cartilage extracellular matrix (ECM). Although multiple factors are involved in the dynamic homeostasis of cartilage, increases in cytokines such as interleukin-1 (IL-1) are associated with a decrease in synthesis and an increase in degradation of the proteoglycans and collagens. Conversely, growth factors such as transforming growth factor-beta (TGF-beta) stimulate chondrocyte synthesis of collagens and proteoglycans, and reduce the activity of IL-1 stimulated metalloproteases, thus opposing the inhibitory and catabolic effects of IL-1. By its capability to reduce IL-1 effects and to stimulate TGF-beta expression in cultured articular chondrocytes, diacerein could favour anabolic processes in the OA cartilage and, hence may contribute to delay the progression of the disease.     

The effect of TGF-beta1 and beta-estradiol on glycosaminoglycan and type II collagen distribution in articular chondrocyte cultures

Articular cartilage extracellular matrix (ECM) plays a crucial role in regulating chondrocyte functions via cell-matrix interaction, cytoskeletal organization and integrin-mediated signaling. Factors such as interleukins, basic fibroblast growth factor (bFGF), bone morphogenic proteins (BMPs) and insulin-like growth factor (IGF) have been shown to modulate the synthesis of extracellular matrix in vitro. However, the effects of TGF-beta1 and beta-estradiol in ECM regulation require further investigation, although there have been suggestions that these factors do play a positive role. To establish the role of these factors on chondrocytes derived from articular joints, a study was conducted to investigate the effects of TGF-beta1 and beta-estradiol on glycosaminoglycan secretion and type II collagen distribution (two major component of cartilage ECM in vivo). Thus, chondrocyte cultures initiated from rabbit articular cartilage were treated with 10ng/ml of TGF-beta1, 10nM of beta-estradiol or with a combination of both factors. Sulphated glycosaminoglycan (GAG) and type II collagen levels were then measured in both these culture systems. The results revealed that the synthesis of GAG and type II collagen was shown to be enhanced in the TGF-beta1 treated cultures. This increase was also noted when TGF-beta1 and beta-estradiol were both used as culture supplements. However, beta-estradiol alone did not appear to affect GAG or type II collagen deposition. There was also no difference between the amount of collagen type II and GAG being expressed when chondrocyte cultures were treated with TGF-beta1 when compared with cultures treated with combined factors. From this, we conclude that although TGF-beta1 appears to stimulate chondrocyte ECM synthesis, beta-estradiol fails to produce similar effects. The findings of this study confirm that contrary to previous claims, beta-estradiol has little or no effect on chondrocyte ECM synthesis. Furthermore, the use of TGF-beta1 may be useful in future studies looking into biological mechanisms by which ECM synthesis in chondrocyte cultures can be augmented, particularly for clinical application.     

Estradiol receptors in articular chondrocytes

Recent experimental studies suggest a role for estrogens in chondrocyte metabolism and the development and treatment of osteoarthritis. Type I cytoplasmic estradiol receptors were found in articular chondrocytes. The presence of these receptors supports a possible mechanism for previous observations on estrogen suppressive effects on cartilage proteoglycan synthesis and offers a basis for further study on the role of estrogens in the physiology of cartilage and in the pathogenesis and treatment of osteoarthritis.     

Post-traumatic osteoarthritis: the role of accelerated chondrocyte senescence

Joint injuries frequently lead to progressive joint degeneration that causes the clinical syndrome of post-traumatic osteoarthritis. The pathogenesis of osteoarthritis remains poorly understood, but patient age is a significant risk factor for progressive joint degeneration. We have found that articular cartilage chondrocytes show strong evidence of senescence with increasing age, including synthesis of smaller more irregular aggrecans; increased expression of lysosomal beta-galactosidase and telomere erosion; and decreased proteoglycan synthesis, response to the anabolic cytokine IGF-I, proliferative capacity, and mitochondrial function. These observations help explain the strong association between age and joint degeneration, but they do not explain how joint injury increases the risk of joint degeneration in younger individuals. We hypothesized that excessive loading of articular surfaces due to acute joint trauma or post-traumatic joint instability, incongruity or mal-alignment increases release of reactive oxygen species, and that the increased oxidative stress on chondrocytes accelerates chondrocyte senescence thereby decreasing the ability of the cells to maintain or restore the tissue. To test this hypothesis, we exposed human articular cartilage chondrocytes from young adults to mechanical and oxidative stress. We found that shear stress applied to cartilage explants in a triaxial pressure vessel increased release of reactive oxygen species and oxidative stress induced chondrocyte senescence (as measured by expression of lysosomal beta-galactosidase, nuclear and mitochondrial DNA damage and decreased mitochondrial function). These observations support the hypothesis that joint injury accelerates chondrocyte senescence and that this acceleration plays a role in the joint degeneration responsible for post-traumatic osteoarthritis.     

The influence of cyclic tension amplitude on chondrocyte matrix synthesis: experimental and finite element analyses

While not generally viewed as physiologically significant in articular cartilage, substantial tension can develop in fibrocartilage structures and in articular cartilage injuries. This study examined how different amplitudes of cyclic tension influence chondrocyte matrix synthesis. Bovine articular chondrocytes seeded in fibrin gels were loaded continuously for 48 hours at 1.0 Hz with displacements of 5%, 10%, or 20%. Protein and proteoglycan synthesis were measured by (3)H-proline and (35)S-sulfate incorporation, respectively. A poroelastic finite element model of the fibrin gel was developed to determine the strain distributions, hydrostatic pressures, and fluid velocities within the constructs at the various levels of displacement. Compared to unloaded controls, 10% and 20% displacements inhibited proteoglycan synthesis to the same extent, while 5% displacement had no effect. Tensile loading did not significantly affect protein synthesis. The finite element model predicted a wide range of strains and fluid velocities within the region of the gel analyzed for matrix synthesis, and the ranges overlapped for the different levels of displacement. These results indicate that the cyclic tension amplitude influences chondrocyte proteoglycan synthesis and that there may be a threshold in the response.     

Effects of interleukin-6 on proliferation and proteoglycan metabolism in articular chondrocyte cultures.

Interleukin-6 (IL-6) levels are markedly increased in the synovial fluid of patients with rheumatoid arthritis or osteoarthritis. However, the effects of IL-6 on proliferation and proteoglycan metabolism in articular cartilage are not known. We demonstrated here the effects of human recombinant (hr) IL-6 on proliferation and proteoglycan metabolism in rabbit articular chondrocyte cultures. In vitro, these cells proliferated and produced abundant extracellular matrices. We found that 1-10 ng/ml of hrIL-6 inhibited proliferation to approximately 65% of control levels and suppressed colony formation induced by bFGF in soft agarose. The same concentration of hrIL-6 depressed proteoglycan synthesis to approximately 60% of control levels. Moreover, hrIL-6 significantly enhanced proteoglycan degradation induced by hrIL-1beta, although hrIL-6 alone did not affect proteoglycan degradation. These findings suggest that IL-6 is a negative regulator for chondrocyte proliferation and articular cartilage metabolism.     

Scorbutic and fasted guinea pig sera contain an insulin-like growth factor I-reversible inhibitor of proteoglycan and collagen synthesis in chick embryo chondrocytes and adult human skin fibroblasts

Chick embryo chondrocytes cultured in sera from scorbutic and fasted guinea pigs exhibited decreases in collagen and proteoglycan production to about 30-50% of control values (I. Oyamada et al., 1988, Biochem. Biophys. Res. Commun. 152, 1490-1496). Here we show by pulse-chase labeling experiments that in the chondrocyte system, as in the cartilage of scorbutic and fasted guinea pigs, decreased incorporation of precursor into collagen was due to decreased synthesis rather than to increased degradation. There was a concomitant decrease in type II procollagen mRNA to about 32% of the control level. As in scorbutic cartilage, proteoglycan synthesis by chondrocytes in scorbutic serum was blocked at the stage of glycosaminoglycan chain initiation. Scorbutic and fasted guinea pig sera also caused a 50-60% decrease in the rates of collagen and proteoglycan synthesis in adult human skin fibroblasts, which synthesize mainly type I collagen. Decreased matrix synthesis in both cell types resulted from the presence of an inhibitor in scorbutic and fasted sera. Elevated cortisol levels in these sera were not responsible for inhibition, as determined by the addition of dexamethasone to chondrocytes cultured in normal serum. Insulin-like growth factor I (IGF-I, 300-350 ng/ml) reversed the inhibition of extracellular matrix synthesis by scorbutic and fasted guinea pig sera in both cell types and prevented the decrease in type II procollagen mRNA in chondrocytes. Therefore, in addition to its established role in proteoglycan metabolism, IGF-I also regulates the synthesis of several collagen types. An increase in the circulating inhibitor of IGF-I action thus could lead to the negative regulation of collagen and cartilage proteoglycan synthesis that occurs in ascorbate-deficient and fasted guinea pigs.     

Regional assessment of articular cartilage gene expression and small proteoglycan metabolism in an animal model of osteoarthritis

Osteoarthritis (OA), the commonest form of arthritis and a major cause of morbidity, is characterized by progressive degeneration of the articular cartilage. Along with increased production and activation of degradative enzymes, altered synthesis of cartilage matrix molecules and growth factors by resident chondrocytes is believed to play a central role in this pathological process. We used an ovine meniscectomy model of OA to evaluate changes in chondrocyte expression of types I, II and III collagen; aggrecan; the small leucine-rich proteoglycans (SLRPs) biglycan, decorin, lumican and fibromodulin; transforming growth factor-β; and connective tissue growth factor. Changes were evaluated separately in the medial and lateral tibial plateaux, and were confirmed for selected molecules using immunohistochemistry and Western blotting. Significant changes in mRNA levels were confined to the lateral compartment, where active cartilage degeneration was observed. In this region there was significant upregulation in expession of types I, II and III collagen, aggrecan, biglycan and lumican, concomitant with downregulation of decorin and connective tissue growth factor. The increases in type I and III collagen mRNA were accompanied by increased immunostaining for these proteins in cartilage. The upregulated lumican expression in degenerative cartilage was associated with increased lumican core protein deficient in keratan sulphate side-chains. Furthermore, there was evidence of significant fragmentation of SLRPs in both normal and arthritic tissue, with specific catabolites of biglycan and fibromodulin identified only in the cartilage from meniscectomized joints. This study highlights the focal nature of the degenerative changes that occur in OA cartilage and suggests that altered synthesis and proteolysis of SLRPs may play an important role in cartilage destruction in arthritis.     

Cyclic mechanical stress induces extracellular matrix degradation in cultured chondrocytes via gene expression of matrix metalloproteinases and interleukin-1

To clarify the mechanism of cartilage degradation induced by mechanical stress, we investigated the influence of cyclic tension force (CTF) on the metabolism of cultured chondrocytes. The chondrocytes were exposed to CTF using a Flexercell strain unit. Five or 15 kPa of high frequency CTF significantly inhibited the syntheses of DNA, proteoglycan, collagen, and protein. Fifteen kPa of high frequency CTF induced the expression of interleukin-1 (IL-1), matrix metalloproteinase (MMP)-2 and -9 mRNA, and increased the production of pro- and active-MMP-9. The degradation of proteoglycan was inhibited by and MMP inhibitor, indicating that MMPs are involved in the degradation of proteoglycans induced by high frequency CTF. Moreover, reducing the frequency of CTF from high to low decreased the inhibition of proteoglycan synthesis. These findings suggest that the CTF frequency is one of the key determinants of chondrocyte metabolism. Low magnitude CTF, whether high or low frequency, did not cause the gene expression of cartilage degradation factors, suggesting that this CTF magnitude causes only minor changes in the cartilage matrix. High magnitude and frequency CTF caused the gene expression of IL-1 and MMP-9, followed by increases in the production of MMP-2 and -9 proteins, suggesting that excessive and continuous cyclic mechanical stress induces the production of IL-1 and MMP-9, resulting in cartilage degradation.     

Transforming growth factor-beta in calf articular cartilage organ cultures: synthesis and distribution.

Transforming growth factor beta 1 (TGF-beta 1) has been shown to play a prominent role in controlling proteoglycan synthesis and breakdown as measured following addition to organ cultures of calf articular cartilage (Morales, T. I., and Roberts, A. B., J. Biol. Chem., 263, 12,828-12,831, 1988). In this study, we compare two closely related TGF-beta isoforms, TGF-beta 1 and TGF-beta 2, both by assessing the effects of exogenous peptide as well as by analyzing the biosynthesis and total amount of these two isoforms in cartilage explants. Added exogenously, TGF-beta 1 and TGF-beta 2 induce a comparable increase in proteoglycan synthesis over basal controls with saturation at approximately 5 ng/ml. Synthesis of TGF-beta by basal calf cartilage cultures is demonstrated by (i) immunolocalization of intracellular TGF-beta, (ii) Northern blot analysis of steady-state mRNA levels, and (iii) immunoprecipitation of metabolically labeled TGF-beta from tissue extracts and conditioned culture medium. The net amount of extractable TGF-beta 1 and TGF-beta 2 in the basal cartilage cultures was assessed by a functional assay involving inhibition of proliferation of CCL-64 mink lung epithelial cells and by sandwich enzyme-linked immunosorbent assay. The predominant isoform was TGF-beta 1 (60-85%) and the total TGF-beta 1 + TGF-beta 2 was in excess of the amount required for maximal activation of proteoglycan synthesis. The level of both isoforms was maintained relatively constant between Days 2 and 7 of culture despite a sharp (approximately two to fourfold) drop in proteoglycan synthesis. This suggests that cartilage contains a large pool of TGF-beta which is not readily accessible to the chondrocyte. We propose that much of the polypeptide is sequestered in the matrix awaiting release upon demand.     

Pharmacological actions of 17 beta-oestradiol on articular cartilage chondrocytes and chondrosarcoma chondrocytes in the absence of oestrogen receptors

(1) Pharmacological concentrations (greater than 10(-5) M) of 17 beta-oestradiol inhibited 35S-labelled proteoglycan synthesis in bovine articular cartilage explant cultures. They also inhibited 35S-labelled proteoglycan synthesis and 3H-labelled protein synthesis in cell cultures of chondrocytes from bovine articular cartilage and Swarm rat chondrosarcoma. Maximal inhibition was about 30-50%. Physiological concentrations (10(-9)-10(-8) M) of oestradiol had no effect on the synthesis of either protein or proteoglycan. (2) The inhibitory action of high concentrations of oestradiol on these biosynthetic pathways is not common to all steroids since 10(-4) M cortisol had no effect on articular chondrocyte cell cultures. 10(-4) M testosterone had a similar action to oestradiol. (3) Neither physiological nor pharmacological concentrations of 17 beta-oestradiol had any effect on 35S-labelled proteoglycan turnover in the cartilage explant system. (4) 10(-5) M oestradiol inhibited cell division in cultures of articular chondrocytes which had entered the log growth phase. 10(-7) M oestradiol had no effect on articular chondrocyte growth. (5) In male rats implanted with silastic capsules releasing 17 beta-oestradiol, increase in body weight was retarded by about 25% over a period of 6 weeks, compared to control rats. Rat chondrosarcoma grew to the same size in oestrogen-treated rats as it did in controls. (6) Oestrogen receptors could not be detected in freshly isolated bovine articular chondrocytes or in rat chondrosarcoma. (7) In conclusion, neither the mitotic rate of articular chondrocytes nor their proteoglycan metabolism is under the direct physiological control of oestradiol. Growth and biosynthetic activity of the rat chondrosarcoma chondrocytes are independent of either direct control by the hormone or control effected by oestradiol regulation of a second hormone or growth factor.     

Interleukin-1-induced alterations in proteoglycan metabolism and matrix assembly

In cartilage, the large chondroitin sulfate proteoglycan exists as aggregates by interacting with link protein and hyaluronic acid. In diseases associated with cartilage degeneration, the proteoglycan does not aggregate because of a defect in the hyaluronate-binding activity. Since interleukin-1 (IL-1) is a secretory product of activated macrophages and may influence the cartilage function in joints, we studied the effects of IL-1 on the synthesis and assembly of proteoglycan by rabbit articular chondrocytes in culture. IL-1-treated cells showed a modest increase in the total proteoglycan synthesis, but also showed a more pronounced decrease in the incorporation of extracellular matrix. Affinity chromatography of the conditioned media on hyaluronic acid-Sepharose revealed that all of the proteoglycan of control cells strongly bound to hyaluronate. The IL-1-treated medium contained two fractions: one that was strongly bound to the column and a second that did not bind. The results demonstrate that the IL-1-treated cells cannot incorporate proteoglycan into the matrix partly because of a defect in the proteoglycan molecules and partly due to other mechanisms regulating proteoglycan assembly.     

Regulation of immature cartilage growth by IGF-I,TGF-<Emphasis Type="Italic">β</Emphasis>1, BMP-7, and PDGF-AB: role of metabolic balance between fixed charge and collagen network

Cartilage growth may involve alterations in the balance between the swelling tendency of proteoglycans and the restraining function of the collagen network. Growth factors, including IGF-I, TGF-beta1, BMP-7, and PDGF-AB, regulate chondrocyte metabolism and, consequently, may regulate cartilage growth. Immature bovine articular cartilage explants from the superficial and middle zones were incubated for 13 days in basal medium or medium supplemented with serum, IGF-I, TGF-beta1, BMP-7, or PDGF-AB. Variations in tissue size, accumulation of proteoglycan and collagen, and tensile properties were assessed. The inclusion of serum, IGF-I, or BMP-7 resulted in expansive tissue growth, stimulation of proteoglycan deposition but not of collagen, and a diminution of tensile integrity. The regulation of cartilage metabolism by TGF-beta1 resulted in tissue homeostasis, with maintenance of size, composition, and function. Incubation in basal medium or with PDGF-AB resulted in small volumetric and compositional changes, but a marked decrease in tensile integrity. These results demonstrate that the phenotype of cartilage growth, and the associated balance between proteoglycan content and integrity of the collagen network, is regulated differentially by certain growth factors.     

Toll-like receptor 2 pathway drives streptococcal cell wall-induced joint inflammation: critical role of myeloid differentiation factor 88

The IL-1R/Toll-like receptor (TLR) superfamily of receptors has a key role in innate immunity and inflammation. In this study, we report that streptococcal cell wall (SCW)-induced joint inflammation is predominantly dependent on TLR-2 signaling, since TLR-2-deficient mice were unable to develop either joint swelling or inhibition of cartilage matrix synthesis. Myeloid differentiation factor 88 (MyD88) is a Toll/IL-1R domain containing adaptor molecule known to have a central role in both IL-1R/IL-18R and TLR signaling. Mice deficient for MyD88 did not develop SCW-induced arthritis; both joint swelling and disturbance of cartilage chondrocyte anabolic function was completely abolished. Local levels of proinflammatory cytokines and chemokines in synovial tissue washouts were strongly reduced in MyD88-deficient mice. Histology confirmed the pivotal role of MyD88 in acute joint inflammation. TLR-2-deficient mice still allow influx of inflammatory cells into the joint cavity, although the number of cells was markedly reduced. No influx of inflammatory cells was seen in joints of MyD88-deficient mice. In addition, cartilage matrix proteoglycan loss was completely absent in MyD88 knockout mice. These findings clearly demonstrated that MyD88 is a key component in SCW-induced joint inflammation. Since agonists of the Toll-like pathway are abundantly involved in both septic and rheumatoid arthritis, targeting of MyD88 may be a novel therapy in inflammatory joint diseases.