The recovery of articular cartilage in explant culture from interleukin-1 alpha: effects on proteoglycan synthesis and degradation

Cellular Patterning Alone Using Bioprinting Regenerates Articular Cartilage Through Native-Like Cartilagenesis

Few studies have proved that bioprinting itself helps recapitulate native tissue functions mainly because the bioprinted macro shape can rarely, if ever, influence cell function. This can be more problematic in bioprinting cartilage, generally considered more challenging to engineer. Here a new method is shown to micro-pattern chondrocytes within bioprinted sub-millimeter micro tissues, denoted as patterned micro-articular-cartilages tissues (PA-MCTs). Under the sole influence of bioprinted cellular patterns. A pattern scoring system is developed after over 600 bioprinted cellular patterns are analyzed. The top-scored pattern mimics that of the isogenous group in native articular cartilage. Under the sole influence of this pattern during PA-MCTs bio-assembling into macro-cartilage and repairing cartilage defects, chondrogenic cell phenotype is preserved, and cartilagenesis is initiated and maintained. Neocartilage tissues from individual and assembled PA-MCTs are comparable to native articular cartilage and superior to cartilage bioprinted with homogeneously distributed cells in morphology, biochemical components, cartilage-specific protein and gene expression, mechanical properties, integration with host tissues, zonation forming and stem cell chondrogenesis. PA-MCTs can also be used as osteoarthritic and healthy cartilage models for therapeutic drug screening and cartilage development studies. This cellular patterning technique can pave a new way for bioprinting to recapitulate native tissue functions via tissue genesis.

Stratification of knee osteoarthritis: two major patient subgroups identified by genome-wide expression analysis of articular cartilage

Introduction

Osteoarthritis (OA) is a heterogeneous and complex disease. We have used a network biology approach based on genome-wide analysis of gene expression in OA knee cartilage to seek evidence for pathogenic mechanisms that may distinguish different patient subgroups.

Methods

Results from RNA-Sequencing (RNA-Seq) were collected from intact knee cartilage at total knee replacement from 44 patients with OA, from 16 additional patients with OA and 10 control patients with non-OA. Results were analysed to identify patient subsets and compare major active pathways.

Results

The RNA-Seq results showed 2692 differentially expressed genes between OA and non-OA. Analysis by unsupervised clustering identified two distinct OA groups: Group A with 24 patients (55%) and Group B with 18 patients (41%). A 10 gene subgroup classifier was validated by RT-qPCR in 16 further patients with OA. Pathway analysis showed increased protein expression in both groups. PhenomeExpress analysis revealed group differences in complement activation, innate immune responses and altered Wnt and TGFβ signalling, but no activation of inflammatory cytokine expression. Both groups showed suppressed circadian regulators and whereas matrix changes in Group A were chondrogenic, in Group B they were non-chondrogenic with changes in mechanoreceptors, calcium signalling, ion channels and in cytoskeletal organisers. The gene expression changes predicted 478 potential biomarkers for detection in synovial fluid to distinguish patients from the two groups.

Conclusions

Two subgroups of knee OA were identified by network analysis of RNA-Seq data with evidence for the presence of two major pathogenic pathways. This has potential importance as a new basis for the stratification of patients with OA for drug trials and for the development of new targeted treatments.

Xylosyltransferase-I regulates glycosaminoglycan synthesis during the pathogenic process of human osteoarthritis

Loss of glycosaminoglycan (GAG) chains of proteoglycans (PGs) is an early event of osteoarthritis (OA) resulting in cartilage degradation that has been previously demonstrated in both huma and experimental OA models. However, the mechanism of GAG loss and the role of xylosyltransferase-I (XT-I) that initiates GAG biosynthesis onto PG molecules in the pathogenic process of human OA are unknown. In this study, we have characterized XT-I expression and activity together with GAG synthesis in human OA cartilage obtained from different regions of the same joint, defined as "normal", "late-stage" or adjacent to "late-stage". The results showed that GAG synthesis and content increased in cartilage from areas flanking OA lesions compared to cartilage from macroscopically "normal" unaffected regions, while decreased in "late-stage" OA cartilage lesions. This increase in anabolic state was associated with a marked upregulation of XT-I expression and activity in cartilage "next to lesion" while a decrease in the "late-stage" OA cartilage. Importantly, XT-I inhibition by shRNA or forced-expression with a pCMV-XT-I construct correlated with the modulation of GAG anabolism in human cartilage explants. The observation that XT-I gene expression was down-regulated by IL-1β and up-regulated by TGF-β1 indicates that these cytokines may play a role in regulating GAG content in human OA. Noteworthy, expression of IL-1β receptor (IL-1R1) was down-regulated whereas that of TGF-β1 was up-regulated in early OA cartilage. Theses observations may account for upregulation of XT-I and sustained GAG synthesis prior to the development of cartilage lesions during the pathogenic process of OA.

Estrogen modulates iodoacetate-induced gene expression in bovine cartilage explants

Estrogen loss may be involved in onset or progression of osteoarthritis. Estrogen receptors are present in chondrocytes, thus estrogen may exert effects directly on cartilage. However, studies on direct estrogen effects on cartilage are limited. We investigated, in an in vitro cartilage explant model, whether estrogen prevents damage or stimulates repair after damage induced by addition of iodoacetate (IA), as an experimental model for osteoarthritis. We used healthy bovine cartilage explants. Prevention experiment: Explants precultured with/without estradiol (E) for 3 days were cultured with IA for 4 h on day 0, and subsequently cultured as in preculture: with/without E. Explants were harvested at day 2 for gene expression analysis. Repair experiment: At day 0, explants were cultured with IA for 4 h on day 0, and subsequently cultured without E or with E. Explants were harvested at days 2, 10, and 14 for gene expression analysis. IA transiently downregulated most genes tested, whereas vascular endothelial growth factor (VEGF) was upregulated on day 2. On day 14, transforming growth factor beta (TGFB)1 and TGFB3 were upregulated, and matrix metalloproteinase (MMP)13 and VEGF downregulated. Estradiol affected gene expression of aggrecan (AGC)1, MMP2, MMP14, tissue inhibitor of metalloproteinase (TIMP)2, TGFB2, and TGFB3. Prevention experiment: Estradiol did not significantly affect IA-induced changes in gene expression (no significant interaction). Repair experiment: Estradiol affected IA-induced changes in expression of collagen (COL)2, MMP2, MMP3, MMP13, MMP14, TIMP2, TGFB2, TGFB3, and VEGF. Estradiol affects expression of anabolic and catabolic genes in bovine cartilage explants and modulates the effects of IA. These effects of estradiol may be beneficial for cartilage maintenance and repair.

Composition-function relationships during IL-1-induced cartilage degradation and recovery

OBJECTIVE

To examine the relationships between biochemical composition and mechanical properties of articular cartilage explants during interleukin-1 (IL-1)-induced degradation and post-exposure recovery.

DESIGN

Bovine articular cartilage explants were cultured for up to 32 days with or without 20 ng/mL IL-1. The dynamic shear modulus |G*(dyn)| and equilibrium and dynamic unconfined compression moduli (E(equil) and |E*(dyn)|) were measured at intervals throughout the culture period. In a subsequent recovery study, explants were cultured for 4 days with or without 20ng/mL IL-1 and for an additional 16 days in control media. The dynamic moduli |E*(dyn)| and |G*(dyn)| were measured at intervals during degeneration and recovery. Conditioned media and explant digests were assayed for sulfated glycosaminoglycans (sGAG) and collagen content.

RESULTS

Continuous IL-1 stimulation triggered progressive decreases in E(equil), |E*(dyn)|, and |G*(dyn)| concomitant with the sequential release of sGAG and collagen from the explants. Brief IL-1 exposure resulted in a short release of sGAG but not collagen, followed by a gradual and incomplete repopulation of sGAG. The temporary sGAG depletion was associated with decreases in both |E*(dyn)| and |G*(dyn)| which also recovered after removal of IL-1. During IL-1-induced degradation and post-exposure recovery, explant mechanical properties correlated well with tissue sGAG concentration.

CONCLUSIONS

As previously shown for developing cartilages and engineered cartilage constructs, cytokine-induced changes in sGAG concentration (i.e., fixed charge density) are coincident with changes in compressive and shear properties of articular cartilage. Further, recovery of cartilage mechanical properties can be achieved by relief from proinflammatory stimuli and subsequent restoration of tissue sGAG concentration.

Regulation of immature cartilage growth by IGF-I, TGF-beta1, 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.

Perspectives on articular cartilage biology and osteoarthritis

Cartilage biochemistry and cell biology is presented in context with osteoarthritis and cartilage regeneration and repair. Success in current efforts towards cell-based orthopaedic treatment options in cases of cartilage trauma and early stages of osteoarthritic degeneration will strictly depend on strategies that rely on known mechanisms of a chondrocyte's regulation.

Growth and differentiation factors for cartilage healing and repair

Chondrocyte differentiation and the maintenance of function requires both transient and long-lasting control through humoral factors, particularly under stress, repair and regeneration in vivo or in vitro as in cell and tissue culture. To date, humoral factors from all major classes of molecules are known to contribute: ions (calcium), steroids (estrogens), terpenoids (retinoic acid), peptides (PTHRP, PTH, insulin, FGFs) and complex proteins (IGF-1, BMPs). They may act indirectly through membrane receptors and signal pathways or directly on transcriptional control elements. Those molecules may reach chondrocytes via free diffusion or may be bound to collagens or proteoglycans on extracellular matrix superstructures becoming available on metabolic processing of collagens and/or proteoglycans. Depending on their position in the metabolic cascade controlling chondrocyte development and homeostasis, they may be used in tissue engineering and regenerative approaches towards cartilage repair by direct application, carrier-mediated release or genetic delivery.

Analysis of cartilage matrix fixed charge density and three-dimensional morphology via contrast-enhanced microcomputed tomography

Small animal models of osteoarthritis are often used for evaluating the efficacy of pharmacologic treatments and cartilage repair strategies, but noninvasive techniques capable of monitoring matrix-level changes are limited by the joint size and the low radiopacity of soft tissues. Here we present a technique for the noninvasive imaging of cartilage at micrometer-level resolution based on detecting the equilibrium partitioning of an ionic contrast agent via microcomputed tomography. The approach exploits electrochemical interactions between the molecular charges present in the cartilage matrix and an ionic contrast agent, resulting in a nonuniform equilibrium partitioning of the ionic contrast agent reflecting the proteoglycan distribution. In an in vitro model of cartilage degeneration we observed changes in x-ray attenuation magnitude and distribution consistent with biochemical and histological analyses of sulfated glycosaminoglycans, and x-ray attenuation was found to be a strong predictor of sulfated glycosaminoglycan density. Equilibration with the contrast agent also permits direct in situ visualization and quantification of cartilage surface morphology. Equilibrium partitioning of an ionic contrast agent via microcomputed tomography thus provides a powerful approach to quantitatively assess 3D cartilage composition and morphology for studies of cartilage degradation and repair.

Adenoviral delivery of IL-1 receptor antagonist abrogates disease activity during the development of autoimmune arthritis in IL-1 receptor antagonist-deficient mice

Currently available treatments for rheumatoid arthritis (RA) are limited in terms of their long-term effects and their abilities to control disease progression. Interleukin-1 receptor antagonist (IL-1Ra) is a natural inhibitor of the biologic actions of IL-1, which is known to promote inflammation and degeneration of the joint. In this study, we investigated whether human IL-1Ra gene transfer is effective at treating an established experimental arthritis model. A recombinant adenovirus carrying the gene that encode human hIL-1Ra and GFP (Ad.hIL-1Ra/GFP) was administered by intra-articular injection into the ankle joints of the mice with established the IL-1Ra-deficient Balb/cA mice (IL-1Ra(-/-)), which develop spontaneously chronic inflammatory arthropathy. The effects of two injections of Ad.hIL-1Ra/GFP or control virus with no inserted target gene (Ad.GFP) were compared with the effects of PBS injection with respect to the clinical characteristics of arthritis, as determined by articular index scores, histopathological and immunological assays. We further divided the outcomes of Ad.hIL-1Ra/GFP gene therapy in IL-1Ra(-/-) mice according arthritis stage; early stage and chronic stage corresponding to 8 and 15 weeks of age, respectively. Intra-articular injections of Ad.hIL-1Ra/GFP reduced arthritis severity and footpad swelling compared with control groups treated with Ad.GFP or PBS in early stage IL-1Ra(-/-) mice. Moreover, the histopathology of the ankle joints of IL-1Ra(-/-) mice treated with Ad.hIL-1Ra/GFP showed a significant decrease in synovial proliferation and inflammatory cell infiltration, and preserved proteoglycan levels in the joints of early stage IL-1Ra(-/-) mice compared with the control mice. Moreover, Ad.hIL-1Ra/GFP treated mice showed reduced levels of inflammatory T helper type 1 (Th1) driven IgG2a antibodies to collagen type II but increased levels Th2 driven IgG1 antibody. These results suggest that adenovirus-mediated gene transfer of IL-1Ra may be a promising therapeutic option in the early stage of autoimmune arthritis.

Retroviral transduction with SOX9 enhances re-expression of the chondrocyte phenotype in passaged osteoarthritic human articular chondrocytes

OBJECTIVES

Articular chondrocytes proliferate in monolayer culture, but the expression of the transcription factor SOX9 falls and the ability of the cells to reform cartilage tissue declines. We have investigated whether retroviral SOX9 expression in extensively passaged human articular chondrocytes from osteoarthritic (OA) joints enables the cells to regain a cartilage matrix forming phenotype in pellet culture.

DESIGN

Chondrocytes from normal and OA joints were retrovirally transduced with SOX9 and grown to passages 7-10 before being cultured as pellets of 500,000 cells for 14 days. Pellets were analysed by real time polymerase chain reaction, histology, immunohistochemistry and 1,9-dimethylmethylene blue assay.

RESULTS

Chondrocytes from OA joints displayed higher expression of COL2A1 gene when transduced with SOX9 and cultured as pellets with 10% serum, but glycosaminoglycan (GAG) synthesis was low. Addition of transforming growth factor beta-3 and insulin like growth factor-1 increased collagen II expression and GAG synthesis in these SOX9 transduced cell pellets. The cells adopted a rounded morphology and there was increased deposition of collagen II protein compared to control green fluorescent protein transduced cell pellets. Similar results were seen with transduced chondrocytes from OA or healthy cartilage. SOX9 transduced human dermal fibroblasts did not show any chondrogenic response.

DISCUSSION

Transduction with SOX9 primed the passaged articular chondrocytes to regain a chondrocytic phenotype in pellet culture and to form a cartilaginous matrix, which was enhanced by growth factors. Following transduction, chondrocytes from OA joints showed a similar capacity for chondrogenic recovery as those from healthy joints, which suggested that OA does not permanently compromise the chondrocyte phenotype.

Effects of physical stimulation with electromagnetic field and insulin growth factor-I treatment on proteoglycan synthesis of bovine articular cartilage

OBJECTIVE

To investigate the single and combined effects of electromagnetic field (EMF) exposure and the insulin growth factor-I (IGF-I) on proteoglycan (PG) synthesis of bovine articular cartilage explants and chondrocytes cultured in monolayers.

DESIGN

Bovine articular cartilage explants and chondrocyte monolayers were exposed to EMF (75Hz; 1.5mT) for 24h in the absence and in the presence of both 10% fetal bovine serum (FBS) and IGF-I (1-100ng/ml). PG synthesis was determined by Na(2)-(35)SO(4) incorporation. PG release into culture medium was determined by the dimethylmethylene blue (DMMB) assay.

RESULTS

In cartilage explants, EMF significantly increased (35)S-sulfate incorporation both in the absence and in the presence of 10% FBS. Similarly, IGF-I increased (35)S-sulfate incorporation in a dose-dependent manner both in 0% and 10% FBS. At all doses of IGF-I, the combined effects of the two stimuli resulted additive. No effect was observed on medium PG release. Also in chondrocyte monolayers, IGF-I stimulated (35)S-sulfate incorporation in a dose-dependent manner, both in 0% and 10% FBS, however, this was not modified by EMF exposure.

CONCLUSIONS

The results of this study show that EMF can act in concert with IGF-I in stimulating PG synthesis in bovine articular cartilage explants. As this effect is not maintained in chondrocyte monolayers, the native cell-matrix interactions in the tissue may be fundamental in driving the EMF effects. These data suggest that in vivo the combination of both EMF and IGF may exert a more chondroprotective effect than either treatment alone on articular cartilage.

Modulation of endogenous osteogenic protein-1 (OP-1) by interleukin-1 in adult human articular cartilage

BACKGROUND

Osteogenic protein-1 (OP-1, BMP-7) induces bone formation and cartilage growth. Since OP-1 is an anabolic factor expressed by human articular chondrocytes, we examined the response of endogenous OP-1 to interleukin-1beta (IL-1beta) in human articular cartilage.

METHODS

Normal adult human articular cartilage explants were cultured for twenty-five days in the presence of medium only or were treated with a low dose (0.1 ng/mL) or high dose (1.0 ng/mL) of IL-1beta for forty-eight or ninety-six hours. Alternately, cartilage explants were cultured forty-eight hours with IL-1beta, followed by forty-eight hours in standard medium (recovery). Tissue was analyzed for OP-1 message (by means of the reverse transcriptase-polymerase chain reaction), protein (by means of enzyme-linked immunosorbent assay and Western blot analysis) and proteoglycan content. Medium was analyzed for released proteoglycans and OP-1.

RESULTS

In the presence of medium, OP-1 maintained its steady state of mRNA and protein expression for as long as twenty-five days in culture. A low dose of IL-1beta led to some upregulation in message and a twofold (p < 0.02) increase in OP-1 protein characterized by enhanced processing and activation of OP-1. Removal of IL-1beta (recovery experiments) did not reverse its effect on OP-1 synthesis. A high dose of IL-1beta caused stronger upregulation of message and a twofold decrease in OP-1 protein content (p < 0.007) in the cartilage matrix. However, this decrease in the matrix was primarily due to a release of active OP-1 into the medium. After removal of the 1.0-ng/mL IL-1beta, the levels of OP-1 protein did not recover.

CONCLUSION

The results of the present study indicate that human adult chondrocytes have an ability to respond anabolically to initial or early catabolic events through an upregulation of endogenous OP-1.

Cartilage tissue remodeling in response to mechanical forces

Recent studies suggest that there are multiple regulatory pathways by which chondrocytes in articular cartilage sense and respond to mechanical stimuli, including upstream signaling pathways and mechanisms that may lead to direct changes at the level of transcription, translation, post-translational modifications, and cell-mediated extracellular assembly and degradation of the tissue matrix. This review focuses on the effects of mechanical loading on cartilage and the resulting chondrocyte-mediated biosynthesis, remodeling, degradation, and repair of this tissue. The effects of compression and tissue shear deformation are compared, and approaches to the study of mechanical regulation of gene expression are described. Of particular interest regarding dense connective tissues, recent experiments have shown that mechanotransduction is critically important in vivo in the cell-mediated feedback between physical stimuli, the molecular structure of newly synthesized matrix molecules, and the resulting macroscopic biomechanical properties of the tissue.

The effect of dynamic compression on the response of articular cartilage to insulin-like growth factor-I

Articular cartilage is routinely subjected to mechanical forces and to cell-regulatory molecules. Previous studies have shown that mechanical stimuli can influence articular chondrocyte metabolic activity, and biochemical studies have shown that growth factors and cytokines control many of the same cell functions. Little is known, however, of the relationships or interplay, if any, between these two key components of the articular environment. This study investigated the comparative and interactive effects of low amplitude, sinusoidal, dynamic compression and insulin-like growth factor-I (IGF-I), a polypeptide in synovial fluid that is anabolic for cartilage. In bovine patellofemoral cartilage explants, IGF-I increased protein and proteoglycan synthesis 90% and 120%, respectively while dynamic compression increased protein and proteoglycan synthesis 40% and 90%, respectively. Stimulation by IGF-I was significantly greater than by dynamic compression for both protein and proteoglycan synthesis. When applied together, the two stimuli enhanced protein and proteoglycan synthesis by 180% and 290%, respectively, a degree greater than that achieved by either stimulus alone. IGF-I augmented protein synthesis with a time constant of 12.2 h. Dynamic compression increased protein synthesis with a time constant of 2.9 h, a rate significantly faster than that of IGF-I, suggesting that these signals act via distinct cell activation pathways. When used together, dynamic compression and IGF-I acted with a time constant of 5.6 h. Thus, dynamic compression accelerated the biosynthetic response to IGF-I and increased transport of IGF-I into the articular cartilage matrix, suggesting that, in addition to independently stimulating articular chondrocytes, cyclic compression may improve the access of soluble growth factors to these relatively isolated cells.

Mechanical and physicochemical regulation of the action of insulin-like growth factor-I on articular cartilage

The development and maintenance of healthy joints is a complex process involving many physical and biological stimuli. This study investigates the interaction between insulin-like growth factor-I (IGF-I) and static mechanical compression in the regulation of articular cartilage metabolism. Bovine cartilage explants were treated with concentrations of IGF-I from 0 to 300 ng/ml in the presence or absence of 0-50% static compression, and the transient and steady-state incorporation of [(3)H]proline and [(35)S]sulfate into matrix components were measured. In parallel studies, cartilage explants were treated with 0-300 ng/ml IGF-I at media pH ranging from 6.4 to 7.2 and the steady-state incorporation of [(3)H]proline and [(35)S]sulfate was measured. The effect of 50% static compression on IGF-I transport was determined by measuring the uptake of (125)I-labeled IGF-I into cartilage explants. Static compression decreased both [(3)H]proline and [(35)S]sulfate incorporation in a dose-dependent manner in the presence or absence of IGF-I. IGF-I increased [(3)H]proline and [(35)S]sulfate incorporation in a dose-dependent manner in the presence or absence of compression, but the anabolic effect of the growth factor was lessened when the tissue was compressed by 50%. The response of cartilage explants to IGF-I was similarly lessened in unstrained tissue cultured in media at pH 6.4, a condition which results in a similar intratissue pH to that when cartilage is compressed by 50%. The characteristic time constant (tau) for IGF-I stimulation of cartilage explants was approximately 24 h, while tau for inhibition of biosynthesis by static compression was approximately 2 h. Samples which were both compressed and treated with IGF-I demonstrated an initial decrease in biosynthetic activity at 2 h, followed by an increase at 24 h. Static compression did not alter tau for (125)I-labeled IGF-I transport into cartilage but decreased the concentration of (125)I-labeled IGF-I in the tissue at equilibrium.

Phenotypic stability of articular chondrocytes in vitro: the effects of culture models, bone morphogenetic protein 2, and serum supplementation

Numerous in vitro culture models have been developed for the investigation of chondrocyte and cartilage biology. In this study, we investigated the stability of the chondrocytic phenotype in monolayer, aggregate, pellet, and explant culture models and assessed the effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) and serum supplementation on the phenotype in each model. Phenotypic effects were assessed by analyses of procollagen type II, aggrecan, (V + C)- fibronectin, and procollagen type I messenger RNA expression. In monolayer cultures, we noted a characteristic loss of procollagen type II and induction of procollagen type I expression. The aggregate and pellet culture models supported matrix protein gene expression profiles more reflective of in vivo levels. In explant cultures, expression of matrix protein genes was consistently depressed. Treatment with rhBMP-2 significantly increased the expression of procollagen type II and aggrecan in monolayer cultures; however, other models showed comparatively little response. Similarly, serum supplementation significantly down-regulated procollagen type II and aggrecan expression in monolayer cultures but had less effect on gene expression in the other models. Serum supplementation increased procollagen type I expression in monolayer and aggregate cultures. These results suggest that the influence of exogenous BMP-2 and serum on expression of chondrocyte-specific matrix protein genes is influenced by aspects of substrate attachments, cellular morphology, and/or cytoskeletal organization. Finally, the analyses of fibronectin expression suggest that V and C region alternative splicing in chondrocytes is linked to the establishment of a three-dimensional multicellular complex.

The long-term effect of a short course of transforming growth factor-beta1 on rat articular cartilage

The long-term effect of 3 days' topical administration of TGF-beta1 on unloaded articular cartilage in growing rats was investigated. Three to five rats were sacrificed on days 15, 30, 45, 60 and 90 after the last injection and the patellas were subjected to light and electron microscopic evaluation. The patellas showed age-related changes when entering the maturation phase. Absence of a hypertrophic zone and formation of a subchondral bone plate were observed in both treated rats and controls from that time point. Reduction of total cell amount was observed between older and younger patellas in both treated rats and controls. TGF-beta1 treatment resulted in accelerated maturation and ageing. Furthermore, the administration of TGF-beta1 resulted in a higher unmineralized cartilage with an increased total amount of cells in the intermediate zone. Matrix areas showing ultrastructural features of disturbed matrix composition were seen in the deeper part of this zone at all time points, but in the treated patellas only. The present results indicate that TGF-beta1 treatment may induce changes in articular cartilage in some respects similar to those seen in the early stages of degenerative and inflammatory joint diseases.

Chondrocyte-mediated catabolism of aggrecan: aggrecanase-dependent cleavage induced by interleukin-1 or retinoic acid can be inhibited by glucosamine

A rat chondrosarcoma cell line and bovine cartilage explants have been used to study the control of aggrecan degradation by chondrocytes treated with interleukin-1 (IL-1) or retinoic acid (RA). Aggrecan fragment analysis with anti-neo-epitope antibodies suggests that aggrecanase (an as yet unidentified enzyme) is the only aggrecan-degrading proteinase active in these cultures. With rat cells, aggrecanase converts the aggrecan core protein into two major G1-domain-bearing products (60 kDa with a C-terminal Glu-373, and 220 kDa with a C-terminal Glu-1459). Both products were quantified on a standardized Western analysis system with a G1-specific antibody. Immunoblots were analysed by scanning densitometry and the sensitivity, linearity and reproducibility of the assay were established. With rat cells the aggrecanase response to IL-1 was optimal at about 2 mM glutamine, but was progressively inhibited at higher concentrations, with about 90% inhibition at 10 mM glutamine. Such inhibition by glutamine was not, however, observed with bovine explants. On the other hand, marked inhibition of aggrecanase-dependent cleavage was observed with both rat cells and bovine explants when d(+)-glucosamine was included at concentrations above 2 mM. Inhibition was apparently not due to cytotoxicity or interference with IL-1 signalling, since biosynthetic activity was not inhibited and inhibition of the aggrecanase response was also obtained when RA was used as the catabolic stimulator. Possible mechanisms for the inhibition of the aggrecanase response by glucosamine in chondrocytes treated with IL-1 or RA are discussed.

Suppression of cartilage matrix gene expression in upper zone chondrocytes of osteoarthritic cartilage

OBJECTIVE

To evaluate the anabolic activity of osteoarthritic chondrocytes in situ by investigating the messenger RNA (mRNA) expression of 3 major cartilage components, type II collagen, aggrecan, and link protein:

METHODS

In situ hybridization experiments and histochemical analysis for proteoglycan content were performed on parallel sections of normal and osteoarthritic (OA) cartilage specimens.

RESULTS

Most chondrocytes in the deeper.zones of OA cartilage showed an increase in mRNA expression, in particular, of type II collagen and to a lesser extent, aggrecan, compared with normal specimens. However, chondrocytes of the upper zone were largely negative for aggrecan or type II collagen mRNA. The expression of link protein mRNA was low in normal and OA specimens.

CONCLUSION

These observations suggest that suppression of the anabolic activity of chondrocytes in the upper zones contributes to the metabolic imbalance observed in OA cartilage. Stimulation of matrix anabolism in superficial chondrocytes might be a suitable target for therapeutic intervention.

Articular cartilage destruction in experimental inflammatory arthritis: insulin-like growth factor-1 regulation of proteoglycan metabolism in chondrocytes

Rheumatoid arthritis, a disease of unknown aetiology, is characterized by joint inflammation and, in its later stages, cartilage destruction. Inflammatory mediators may exert not only suppression of matrix synthesis but also cartilage degradation, which eventually leads to severe cartilage depletion. Systemically and locally produced growth factors and hormones regulate cartilage metabolism. Alterations in levels of these factors or in their activity can influence the pathogenesis of articular cartilage destruction in arthritic joints. The main topic of the present review is the role of the anabolic factor insulin-like growth factor-1 in the regulation of chondrocyte metabolic functions in normal and in diseased cartilage. This is the most important growth factor that balances chondrocytes proteoglycan synthesis and catabolism to maintain a functional cartilage matrix. A brief overview of how chondrocytes keep the cartilage matrix intact, and how catabolic and anabolic factors are thought to be involved in pathological cartilage destruction precedes the review of the role of this growth factor in proteoglycan metabolism in cartilage.

IL-1 has no direct role in the IGF-1 non-responsive state during experimentally induced arthritis in mouse knee joints

OBJECTIVE

To investigate the involvement of interleukin-1 (IL-1) in the induction or maintenance of the insulin-like growth factor 1 (IGF-1) non-responsive state of chondrocytes during experimental arthritis in mouse knee joints.

METHODS

To characterise IGF-1 nonresponsiveness during arthritis, we measured chondrocyte proteoglycan (PG) synthesis by assaying incorporation of 35S-sulphate into mouse patellar cartilage, obtained from knee joints with experimentally induced arthritis and normal knee joints, cultured with IGF-1. We investigated whether suppressive mediators produced by the arthritic synovium or chondrocytes abolished the IGF-1 stimulation of normal cartilage, and used IL-1 primed cartilage to mimic the arthritic in vivo state. Specific inflammatory mediators responsible for the maintenance of the suppressed IGF-1 response were sought. We measured IGF-1 responsiveness in normal and arthritic patellae cultured with antibodies against tumour necrosis factor (TNF) or IL-1 alpha/beta, with IL-1 receptor antagonist (IL-1ra), and with several inhibitors of proteolytic enzymes or reactive oxygen species, and analysed the role of IL-1 in the development of IGF-1 non-responsiveness by studying IGF-1 responses in cartilage treated with IL-1 antibodies in vivo, at the onset of arthritis.

RESULTS

Mediators from the surrounding tissue of both normal and arthritic cartilage suppressed chondrocyte IGF-1 responses. Priming the cartilage with IL-1 did not directly induce IGF-1 non-responsiveness, but enhanced the ability of suppressive mediators from synovium or chondrocytes to downregulate the IGF-1 responsive state. IL-1ra, IL-1 alpha/beta antibody, TNF antibody, or the inhibitors tested did not markedly improve the disturbed IGF-1 response, but treatment with anti-IL-1 at the onset of arthritis prevented the development of IGF-1 non-responsiveness.

CONCLUSION

IL-1 alone does not induce IGF-1 non-responsiveness and is not critical in the maintenance of this phenomenon. However, IL-1 does appear to be an important cofactor in the generation of the IGF-1 non-responsive state.

Increased proteoglycan synthesis in cartilage in experimental canine osteoarthritis does not reflect a permanent change in chondrocyte phenotype

OBJECTIVE

To determine whether chondrocytes in early experimental osteoarthritic (OA) cartilage continue to show increased synthesis and turnover of proteoglycans (PGs) during explant culture. A comparison was also made between the responsiveness of experimental OA and control cartilage to interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) after 1 day and 3 days in culture.

METHODS

OA was induced in mature animals by sectioning of the anterior cruciate ligament followed by 3 months of normal exercise. PG synthesis in the articular cartilage was determined by measuring 35S-sulfate incorporation during explant culture over 1-3 days. Inhibition of PG synthesis was also determined with various concentrations of IL-1 beta and TNF alpha after 1 and 3 days in culture. PGs extracted from the articular cartilage over 1-3 days in culture were examined by agarose-polyacrylamide gel electrophoresis.

RESULTS

Up to 24 hours after excision from the joint, PG synthesis was higher in experimental OA cartilage than in control cartilage. It was also less sensitive to inhibition by TNF alpha. These differences were no longer detected after 48-72 hours in culture. There were no changes in the relative proportions of aggrecan and decorin/biglycan extracted from and synthesized by control and experimental OA cartilage over the 3 days in culture.

CONCLUSION

Previous results indicated that PG synthesis and turnover in articular cartilage was increased for many months after induction of experimental OA. Our present results show that the enhanced rate of PG synthesis and turnover were evident in freshly explanted tissue, but the differences were lost over 3 days in culture. A decreased responsiveness to TNF alpha was also lost. The hypermetabolic activity of experimental OA chondrocytes was thus reversible and not a permanent change in chondrocyte phenotype.

Neutrophil collagenase (MMP-8) cleaves at the aggrecanase site E373-A374 in the interglobular domain of cartilage aggrecan

Native and recombinant neutrophil collagenase (MMP-8) was shown to cleave at the E373-A374 'aggrecanase' site in the interglobular domain of aggrecan. The time course of digestion in vitro showed that MMP-8 cleaved initially at N341-F342, the predominant metalloproteinase site, before cleaving at the E373-A374 site. A synthetic peptide, IPENFFG, inhibited cleavage at E373-A374 but not N341-F342 in vitro, indicating that the E373-A374 sequence was a less preferred site for MMP-8 cleavage than N341-F342. IPENFFG also inhibited release of A374 RGSVI fragments from cartilage in explant culture, suggesting that a metalloproteinase cleaved at the aggrecanase site in situ. The possibility remains that 'aggrecanase' may be a metalloproteinase in cartilage.