Modulation of aggrecan and link-protein synthesis in articular cartilage

Conversion of senescent cartilage into a pro-chondrogenic microenvironment with antibody-functionalized copper sulfate nanoparticles for efficient osteoarthritis therapy

The development of osteoarthritis (OA) correlates with the expansion of senescent cells in cartilage, which contributes to an inflammatory microenvironment that accelerates matrix degradation and hampers cartilage generation. To address OA, we synthesized small copper sulfide nanoparticles functionalized with anti-beta-2-microglobulin antibodies (B2M-CuS NPs) that catalyze the formation of toxic •OH from H2O2 via peroxidase-like activity. These B2M-CuS NPs are specifically targeted to induce apoptosis in senescent chondrocytes while showing no toxicity toward normal chondrocytes. Furthermore, B2M-CuS NPs enhance the chondrogenesis of normal chondrocytes. Thus, B2M-CuS NPs can effectively treat OA by clearing senescent chondrocytes and promoting cartilage regeneration after intra-articular injection into the knee joints of surgery-induced OA mice. This study uses smart nanomaterials to treat OA with a synergistic strategy that both remodels senescent cartilage and creates a pro-chondrogenic microenvironment.

Intraarticular injection of heparin-binding insulin-like growth factor 1 sustains delivery of insulin-like growth factor 1 to cartilage through binding to chondroitin sulfate

OBJECTIVE

Insulin-like growth factor 1 (IGF-1) stimulates cartilage repair but is not a practical therapy due to its short half-life. We have previously modified IGF-1 by adding a heparin-binding domain and have shown that this fusion protein (HB-IGF-1) stimulates sustained proteoglycan synthesis in cartilage. This study was undertaken to examine the mechanism by which HB-IGF-1 is retained in cartilage and to test whether HB-IGF-1 provides sustained growth factor delivery to cartilage in vivo and to human cartilage explants.

METHODS

Retention of HB-IGF-1 and IGF-1 was analyzed by Western blotting. The necessity of heparan sulfate (HS) or chondroitin sulfate (CS) glycosaminoglycans (GAGs) for binding was tested using enzymatic removal and cells with genetic deficiency of HS. Binding affinities of HB-IGF-1 and IGF-1 proteins for isolated GAGs were examined by surface plasmon resonance and enzyme-linked immunosorbent assay.

RESULTS

In cartilage explants, chondroitinase treatment decreased binding of HB-IGF-1, whereas heparitinase had no effect. Furthermore, HS was not necessary for HB-IGF-1 retention on cell monolayers. Binding assays showed that HB-IGF-1 bound both CS and HS, whereas IGF-1 did not bind either. After intraarticular injection in rat knees, HB-IGF-1 was retained in articular and meniscal cartilage, but not in tendon, consistent with enhanced delivery to CS-rich cartilage. Finally, HB-IGF-1 was retained in human cartilage explants but IGF-1 was not.

CONCLUSION

Our findings indicate that after intraarticular injection in rats, HB-IGF-1 is specifically retained in cartilage through its high abundance of CS. Modification of growth factors with heparin-binding domains may be a new strategy for sustained and specific local delivery to cartilage.

Cyclic AMP regulates extracellular matrix gene expression and metabolism in cultured primary rat chondrocytes

In osteo- and rheumatoid arthritis, the synovial fluid surrounding chondrocytes contains increased levels of prostaglandin E(2) (PGE(2)), an agent known to elevate intracellular cyclic AMP (cAMP). However, the effect of PGE(2)/cAMP on mRNA expression in chondrocytes is largely unknown. In this report, we assess the effect of the cell-permeable cAMP analog adenosine 8-(4-chloro-phenylthio)-3',5'-cyclic monophosphate (CPT-cAMP) and PGE(2) on mRNA expression in primary neonatal rat chondrocytes. CPT-cAMP decreased type II collagen, link protein, parathyroid hormone/parathyroid hormone-related peptide receptor and alkaline phosphatase, increased glyceraldehyde-3-phosphate dehydrogenase mRNA and lactate efflux, but did not alter type X collagen or aggrecan mRNA. The effect of CPT-cAMP on type II collagen and link protein mRNAs and chondrocyte metabolism were attenuated by the transcriptional inhibitor actinomycin D, indicating that the ability of CPT-cAMP to suppress mRNA expression was not due to alterations in mRNA stability, but were instead likely due to transcriptional mechanisms. CPT-cAMP-treatment induced GSK3 beta phosphorylation and beta-catenin-mediated transcriptional activity. Pharmacological inhibition of GSK3 beta paralleled the effects of CPT-cAMP on type II collagen, link protein and chondrocyte metabolism, suggesting that the effect of CPT-cAMP on chondrocytes may be GSK3 beta/beta-catenin-dependent. The effects of CPT-cAMP on beta-catenin-mediated transcription, cell metabolism and mRNA expression were mimicked by the cAMP-elevating agent PGE(2), providing a physiologically relevant context for our studies. Collectively, these results suggest that agents that elevate cAMP signaling may impair chondrocyte function in conditions such as arthritis.

Egr-1 inhibits the expression of extracellular matrix genes in chondrocytes by TNFalpha-induced MEK/ERK signalling

Introduction

TNFα is increased in the synovial fluid of patients with rheumatoid arthritis and osteoarthritis. TNFα activates mitogen-activated kinase kinase (MEK)/extracellular regulated kinase (ERK) in chondrocytes; however, the overall functional relevance of MEK/ERK to TNFα-regulated gene expression in chondrocytes is unknown.

Methods

Chondrocytes were treated with TNFα with or without the MEK1/2 inhibitor U0126 for 24 hours. Microarray analysis and real-time PCR analyses were used to identify genes regulated by TNFα in a MEK1/2-dependent fashion. Promoter/reporter, immunoblot, and electrophoretic mobility shift assays were used to identify transcription factors whose activity in response to TNFα was MEK1/2 dependent. Decoy oligodeoxynucleotides bearing consensus transcription factor binding sites were introduced into chondrocytes to determine the functionality of our results.

Results

Approximately 20% of the genes regulated by TNFα in chondrocytes were sensitive to U0126. Transcript regulation of the cartilage-selective matrix genes Col2a1, Agc1 and Hapln1, and of the matrix metalloproteinase genes Mmp-12 and Mmp-9, were U0126 sensitive – whereas regulation of the inflammatory gene macrophage Csf-1 was U0126 insensitive. TNFα-induced regulation of Sox9 and NFκB activity was also U0126 insensitive. Conversely, TNFα-increased early growth response 1 (Egr-1) DNA binding was U0126 sensitive. Transfection of chondrocytes with cognate Egr-1 oligodeoxynucleotides attenuated the ability of TNFα to suppress Col2a1, Agc1 or Hapln1 mRNA expression.

Conclusions

Our results suggest that MEK/ERK and Egr1 are required for TNFα-regulated catabolic and anabolic genes of the cartilage extracellular matrix, and hence may represent potential targets for drug intervention in osteoarthritis or rheumatoid arthritis.

Proteomic characterization of mouse cartilage degradation in vitro

OBJECTIVE

To develop proteomics to analyze mouse cartilage degradation and correlate transcriptional and translational responses to catabolic stimuli.

METHODS

Proteomic techniques were used to analyze catabolism in mouse femoral head cartilage. Using specific methods to prepare cartilage extracts and conditioned media for 2-dimensional polyacrylamide gel electrophoresis and subsequent tandem mass spectrometry, we identified novel proteins and fragments released into the media of control, interleukin-1alpha (IL-1alpha)-treated, and all-trans-retinoic acid (RetA)-treated explants. Fluorescence 2-dimensional difference gel electrophoresis was used to quantify protein expression changes. We also measured changes in messenger RNA (mRNA) expression to distinguish transcriptional and posttranslational regulation of released proteins.

RESULTS

Differentially abundant proteins in the media of control and treated explants included fragments of thrombospondin 1 and connective tissue growth factor. IL-1alpha stimulated release of the cartilage degeneration marker matrix metalloproteinase 3, as well as proteins with uncharacterized roles in cartilage pathology, such as neutrophil gelatinase-associated lipocalin. RetA stimulated release of the extracellular matrix proteins cartilage oligomeric matrix protein, link protein, and matrilin-3 into the media, which was accompanied by a dramatic reduction in the corresponding mRNA transcript levels. Gelsolin, which has been implicated in cytoskeletal reorganization in arthritis synovial fibroblasts but has not been previously associated with cartilage pathology, was regulated by IL-1alpha and RetA.

CONCLUSION

In this first analysis of mouse cartilage degradation and protein release using proteomics, we identified proteins and fragments, some of which represent novel candidate biomarkers for cartilage degradation. Applying these proteomic techniques to wild-type and genetically modified mouse cartilage will provide insights into the mechanisms of cartilage degeneration.

Cartilage degradation is fully reversible in the presence of aggrecanase but not matrix metalloproteinase activity

Introduction

Physiological and pathophysiological cartilage turnover may coexist in articular cartilage. The distinct enzymatic processes leading to irreversible cartilage damage, compared with those needed for continuous self-repair and regeneration, remain to be identified. We investigated the capacity of repair of chondrocytes by analyzing their ability to initiate an anabolic response subsequent to three different levels of catabolic stimulation.

Methods

Cartilage degradation was induced by oncostatin M and tumour necrosis factor in articular cartilage explants for 7, 11, or 17 days. The catabolic period was followed by 2 weeks of anabolic stimulation (insulin growth factor-I). Cartilage formation was assessed by collagen type II formation (PIINP). Cartilage degradation was measured by matrix metalloproteinase (MMP) mediated type II collagen degradation (CTX-II), and MMP and aggrecanase mediated aggrecan degradation by detecting the ³⁴²FFGVG and ³⁷⁴ARGSV neoepitopes. Proteoglycan turnover, content, and localization were assessed by Alcian blue.

Results

Catabolic stimulation resulted in increased levels of cartilage degradation, with maximal levels of ³⁷⁴ARGSV (20-fold induction), CTX-II (150-fold induction), and ³⁴²FFGVG (30-fold induction) (P < 0.01). Highly distinct protease activities were found with aggrecanase-mediated aggrecan degradation at early stages, whereas MMP-mediated aggrecan and collagen degradation occurred during later stages. Anabolic treatment increased proteoglycan content at all time points (maximally, 250%; P < 0.001). By histology, we found a complete replenishment of glycosaminoglycan at early time points and pericellular localization at an intermediate time point. In contrast, only significantly increased collagen type II formation (200%; P < 0.01) was observed at early time points.

Conclusion

Cartilage degradation was completely reversible in the presence of high levels of aggrecanase-mediated aggrecan degradation. After induction of MMP-mediated aggrecan and collagen type II degradation, the chondrocytes had impaired repair capacity.

In vivo intervertebral disc remodeling: kinetics of mRNA expression in response to a single loading event

Kinetics of mRNA expression following a single loading event was measured using an in vivo rat tail model. Animals were instrumented and loaded in compression for 1.5 h at 1 MPa and 1 Hz. Real-time RT-PCR was used to measure mRNA levels 0, 8, 24 and 72 h after mechanical stimulation for genes associated with matrix proteins (aggrecan, collagen-I, collagen-II), proteases (MMP-2, MMP-3, MMP-13, ADAMTS-4), and their inhibitors (TIMP-1, TIMP-3) in anulus fibrosus and nucleus pulposus regions. Baseline mRNA levels were of greatest abundance for matrix proteins and lowest for proteases. The mRNA levels reached maximum levels 24 h following mechanical stimulation for the majority of genes evaluated, but some had maximum levels 8 and 72 h following loading. The mRNA levels returned to baseline levels for all genes in the nucleus 72 h following loading, but the majority of genes in the anulus remained upregulated. Results support a coordinated strategy of relative mRNA expression that varied over time beginning with inhibition of tissue breakdown, followed by synthesis of aggrecan and matrix degrading enzymes, and eventually collagen metabolism days following loading. Consequently, optimal time for tissue harvest for mRNA measurements depends on genes of interest. Results suggest attempts at anabolic remodeling must be given adequate time for metabolic processes and protein synthesis to occur, and that changes in TIMP and MMP levels may have greater potency in affecting structural protein abundance than direct changes in the structural protein messages. Results have important implications for disc remodeling and tissue engineering.

In vitro chondrocyte differentiation using costochondral chondrocytes as a source of primary rat chondrocyte cultures: an improved isolation and cryopreservation method

INTRODUCTION

Isolating and culturing primary chondrocytes such that they retain their cell type and differentiate to a hypertrophic state is central to many investigations of skeletal growth and its regulation. The ability to store frozen chondrocytes has additional scientific and tissue engineering interest. Previous work has produced approaches of varying yield and complexity but does not permit frozen storage of cells for subsequent differentiation in culture. Investigations of growth plate dysplasias secondary to defective osteoclastogenesis in rodent models of osteopetrosis led us to adapt and modify a culture method and to cryopreserve neonatal rat costochondral chondrocytes.

METHODS

Chondrocytes were isolated from dissected ribs of 3-day-old rat pups by collagenase, hyaluronidase, and trypsin serial digestions. This was done either immediately or after the isolation was interrupted following an initial protease treatment to allow the chondrocytes, still in partially digested rib rudiments, to be frozen and later thawed for culture. Cells were plated in flat-bottom wells and allowed to adhere and grow under different conditions. Choice of media permitted cells to be maintained or induced to differentiate. Cell growth was monitored, as was expression of several relevant genes: collagen types II and X; osteocalcin, Sox9, adipocyte FABP, MyoD, aggrecan, and others. Mineralization was measured by alizarin red binding, and cultures were examined by light, fluorescence, and electron microscopy.

RESULTS

Cells retained their chondrocyte phenotype and ability to differentiate and mineralize the collagen-rich extracellular matrix even after freezing-thawing. RT-PCR showed retention of chondrocyte-specific gene expression, including aggrecan and collagen II. The cells had a flattened, "proliferating zone" appearance initially, and by 2 weeks post-confluence, exhibited swelling and other salient features of hypertrophic cells seen in vivo. Collagen fibrils were abundant in the extracellular matrix, along with matrix vesicles. The switch to collagen type X as marker for hypertrophy was not rigidly temporally regulated as happens in vivo, but its expression increased during hypertrophic differentiation.

CONCLUSIONS

This method should prove valuable as a means of studying chondrocyte regulation and has the advantages of simpler initial dissection, yields of a purer chondrocyte population, and the ability to stockpile frozen raw material for subsequent studies.

Proteoglycans and catabolic products of proteoglycans present in ligament

The aim of the present study was to characterize the proteoglycans and catabolic products of proteoglycans present in the tensile region of ligament and explant cultures of this tissue, and to compare these with those observed in the tensile region of tendon. Approx. 90% of the total proteoglycans in fresh ligament was decorin, as estimated by N-terminal amino acid sequence analysis. Other species that were detected were biglycan and the large proteoglycans versican (splice variants V(0) and/or V1 and/or V2) and aggrecan. Approx. 23% of decorin detected in the matrix was degraded. Intact decorin and decorin fragments similar to those observed in the matrix that retained the N-terminus were also observed in the medium of ligament cultures. Intact biglycan core protein was detected in the matrix and medium of ligament cultures, and two fragments originating from the N-terminal region of biglycan were observed in the matrix of cultured ligament. Versican and versican fragments that retained the N-terminus of versican core protein were detected in fresh matrix and medium of tendon cultures. Approx. 42% of versican present in the fresh ligament was degraded. Aggrecan catabolites appearing in the culture medium were derived from aggrecanase cleavage of the core protein. An intact link protein and a degradation product from the N-terminal region of type XII collagen were also detected in the medium of the ligament explant.

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.

Hydrostatic pressure modulates proteoglycan metabolism in chondrocytes seeded in agarose

OBJECTIVE

To investigate the effect of isolated hydrostatic pressure on proteoglycan metabolism in chondrocytes.

METHODS

Bovine articular chondrocytes cultured in agarose gels were subjected to 5 MPa hydrostatic pressure for 4 hours in either a static or a pulsatile (1 Hz) mode, and changes in glycosaminoglycan (GAG) synthesis, hydrodynamic size, and aggregation properties of proteoglycans and aggrecan messenger RNA (mRNA) levels were determined.

RESULTS

The application of 5 MPa static pressure caused a significant increase in GAG synthesis of 11% (P < 0.05). Column chromatography showed that this increase in GAG synthesis was associated with large proteoglycans. In addition, semiquantitative reverse transcriptase-polymerase chain reaction showed a 4-fold increase in levels of aggrecan mRNA (P < 0.01).

CONCLUSION

Hydrostatic pressure in isolation, which does not cause cell deformation, can affect proteoglycan metabolism in chondrocytes cultured in agarose gels, indicating an important role of hydrostatic pressure in the regulation of extracellular matrix turnover in articular cartilage.

Link protein as an enhancer of cumulus cell-oocyte complex expansion

To investigate the specific components involved in regulating cumulus cell-oocyte complex (COC) expansion in an in vitro mouse experiment, freshly-isolated COC were cultured in the presence of various combinations of FSH (1.0 microg/ml), proteins of the inter-alpha-inhibitor (I alpha I) family (a light chain, also known as bikunin, heavy chains [HC1 + HC2] and I alpha I [0.01-2.0 microg/ml]) and link protein (LP) (0.016-10 microg/ml) for 24 h and were observed for expansion of their cumulus cells (percent of COC with + 3 and + 4 expansion and average projected area). The COC were videotaped in real time at the initiation of culture and after 24 h of culture. FSH alone did not stimulate cumulus expansion under serum-free conditions; however, treatment with I alpha I (0.1-2.0 microg/ml) or heavy chains (10 microg/ml), but not bikunin (10 micro g/ml), in the presence of FSH significantly increased COC expansion, with maximal promotion occurring at 1.0 microg/ ml of I alpha I. Addition of LP (2.0 micro g/ml) to the medium containing I alpha I (1.0 microg/ml) and FSH resulted in significantly higher expansion levels than were observed in response to I alpha I alone, although LP alone (10 microg/ml) had no or very little effect by itself. Anti-I alpha I or anti-LP polyclonal antibody, which inhibits binding of I alpha I and LP, respectively, to hyaluronic acid (HA), markedly reduced expansion of the surrounding cumulus cell extracellular matrices. Therefore, in vitro, LP might serve, in part, to enhance the COC expansion possibly by stabilizing HA-I alpha I (or heavy chains) complex on the surrounding cumulus cell matrices.

Growth factors for sequential cellular de- and re-differentiation in tissue engineering

A model system for the in vitro generation of cartilaginous constructs was used to study a tissue engineering paradigm whereby sequentially applied growth factors promoted chondrocytes to first de-differentiate into a proliferative state and then re-differentiate and undergo chondrogenesis. Early cultivation in medium with supplemental TGF-beta1/FGF-2 doubled cell fractions in 2-week constructs compared to unsupplemented controls. Subsequent culture with supplemental IGF-I yielded large 4-week constructs with high fractions of cartilaginous extracellular matrix (ECM) and high compressive moduli, whereas prolonged culture with supplemental FGF-2 yielded small 4-week constructs with low ECM fractions and moduli. Sequential supplementation with TGF-beta1/FGF-2 and then IGF-I yielded 4-week constructs with type-specific mRNA expression and protein levels that were high for type II and negligible for type I collagen, in contrast to other growth factor regimens studied. The data demonstrate that structural, functional, and molecular properties of engineered cartilage can be modulated by sequential application of growth factors.

Time-dependent aggrecan gene expression of articular chondrocytes in response to hyperosmotic loading

OBJECTIVE

To investigate the effects of increasing extracellular osmolality on aggrecan gene expression and cell size in cultured chondrocytes.

DESIGN

Aggrecan promoter activity and mRNA levels were measured in bovine monolayer chondrocytes subjected to hyperosmotic loading for different time periods, using transient transfection assays or RT-PCR. Cell size changes were also determined using an epifluorescence microscopy system.

RESULTS

Hyperosmotic loading for 24 h suppressed aggrecan promoter activity and mRNA levels approximately two-fold. However no suppression of promoter activity was observed when exon 1 was deleted from the human aggrecan promoter construct. Osmotic regulation of aggrecan gene expression was time-dependent and found to correlate with cell shrinking and swelling. No suppression in promoter activity was observed when the hyperosmotic stimulus was applied in a cyclic manner, or when serum was present in the culture medium.

CONCLUSION

Hyperosmotic loading regulates aggrecan gene expression and cell size in isolated chondrocytes. Osmotic regulation of gene expression is also affected by the time-varying nature of loading and the presence of serum.

Coordinate upregulation of cartilage matrix synthesis in fibrin cultures supplemented with exogenous insulin-like growth factor-I

The addition of insulin-like growth factor-I to cartilage cultures is known to stimulate the synthesis of proteoglycan and type-II collagen in explant and monolayer studies. The purpose of this study was to determine the effects of long-term supplementation with insulin-like growth factor-I in chondrocytes cultured in fibrin discs as a preliminary investigation to in vivo application of chondrocyte/insulin-like growth factor-I/fibrin grafts to articular-cartilage repair procedures. Chondrocyte-fibrin cultures were maintained for 14 days, with insulin-like growth factor-I added at varying concentrations of 0, 10, 50, or 100 ng/ml medium. Cultures supplemented with 50 or 100 ng of growth factor/ml had increased levels of aggrecan and type-IIB procollagen mRNA, and translation to aggrecan and type-IIB collagen was confirmed by dye-binding assay of total proteoglycan, type-II collagen immunohistochemistry, and determination of collagen content by high-performance liquid chromatography. Maintenance of the chondrocyte phenotype during the 14 days of culture was confirmed by round cell morphology on routine staining, expression of type-II procollagen mRNA on in situ hybridization, evidence of production of pericellular type-II collagen on immunocytochemistry, synthesis of large-molecular-size aggrecan monomer on CL-2B column chromatography, and lack of appreciable message expression for type I or IIA collagen on Northern blot hybridization. Dose-response effects of insulin-like growth factor-I on the expression of chondrocyte matrix constituents were most pronounced at 50 and 100 ng of growth factor per milliliter of medium. These data confirm that (a) culture of chondrocytes for extended periods in three-dimensional cultures of fibrin maintains the chondrocyte phenotype and (b) supplementation with increasing concentrations of insulin-like growth factor-I enhances chondrocyte matrix synthesis and may provide a means to enhance chondrocyte phenotypic stability and function during transplantation grafting procedures.

Growth factor modulation of mitogenic responses and proteoglycan synthesis by human periodontal fibroblasts

In order to understand the relationship between specific growth factors and matrix synthesis by periodontal cells, we have investigated the effects of platelet-derived growth factor BB (PDGF-BB), insulin-like growth factor-I (IGF-1), and growth hormone on DNA and proteoglycan synthesis by cultured human gingival and periodontal ligament fibroblasts in vitro. PDGF-BB and IGF-1, but not growth hormone, were mitogenic for both periodontal ligament fibroblasts and gingival fibroblasts, although the periodontal ligament cells responded more strongly. The mitogenic response was accompanied by alterations in expression of matrix proteoglycan mRNA. For both the gingival and periodontal ligament cells, there was a decrease in mRNA for decorin and an increase in mRNA for versican following exposure to IGF-1 and PDGF-BB. Although no change was seen in response to PDGF, biglycan mRNA level was increased by IGF-1 in periodontal ligament fibroblasts. With the gingival fibroblasts, biglycan mRNA levels were unaffected by IGF-1, PDGF-BB, or growth hormone. These findings suggest variable responses of fibroblasts to growth factors depending upon anatomical site within the periodontium. Moreover, there appears to be a correlation between cell proliferation and the types of proteoglycan synthesised with decorin expression being suppressed, and versican being increased during fibroblast proliferation.

Elevated aggrecan mRNA in early murine osteoarthritis

Male STR/ort mice develop osteoarthritis in the tibial articular cartilage. Low grade histological lesions first appear between 10-20 weeks of age. A previous study showed that the level of aggrecan, the major cartilage proteoglycan, is approximately twofold greater in the tibial cartilage of 16-19-week-old STR/ort mice compared with that in normal cartilage in control CBA mice. In the present investigation aggrecan gene transcription was investigated in 20-week-old STR/ort and CBA tibial cartilage using a quantitative reverse transcription-polymerase chain reaction (RT-PCR). The amount of aggrecan cDNA obtained from the STR/ort medial and lateral plateau was 2.8- and 4.6-fold greater per milligram of wet cartilage than that from the CBA tibial plateau. The difference was not due to differences in cellularity of tibial cartilage in the two strains and indicates that aggrecan gene transcription is elevated in early osteoarthritis.

Recombinant human bone morphogenetic protein-2 maintains the articular chondrocyte phenotype in long-term culture

Bone morphogenetic proteins have been shown to increase matrix synthesis by articular chondrocytes in short-term cultures. Members of this family of proteins have also been shown to induce endochondral ossification in vivo. The present study was performed to determine if the addition of human recombinant bone morphogenetic protein-2 to a long-term monolayer articular chondrocyte cell culture system affected the ability of the chondrocytes to divide in vitro, whether the cytokine altered expression of the articular chondrocyte phenotype and synthesis of matrix proteoglycans, and whether the cytokine was capable of inducing differentiation to a hypertrophic chondrocyte. Human recombinant bone morphogenetic protein-2 did not alter cell proliferation. It caused 3.5-6.2 times more proteoglycan synthesis by articular chondrocytes during each of the time points tested after 4 days in culture. Total proteoglycan accumulation in the extracellular matrix after 28 days in culture was 6.7 times as great in the treated cultures as in the control. Treatment with human recombinant bone morphogenetic protein-2 maintained the articular chondrocyte phenotype of cells in culture as demonstrated by Northern blot analysis: the expression of type-I collagen genes was increased and that of type-II collagen and aggrecan mRNA was lost in untreated chondrocyte cultures after 14-21 days in culture. In contrast, exposure to 100 ng/ml human recombinant bone morphogenetic protein-2 maintained expression of type-II collagen and increased expression of aggrecan compared with controls during the 28-day culture period. Northern blot analysis of the expression of type-X collagen and osteocalcin by chondrocytes treated with human recombinant bone morphogenetic protein-2 showed a lack of expression of these genes, indicating no alteration in phenotype. These experiments demonstrated the ability of human recombinant bone morphogenetic protein-2 to promote the articular chondrocyte phenotype and matrix synthesis in long-term culture. Characteristics of cell growth were not affected, and the cytokine did not induce differentiation to a hypertrophic chondrocyte.

Quantification of aggrecan and link-protein mRNA in human articular cartilage of different ages by competitive reverse transcriptase-PCR

A competitive reverse transcriptase-PCR (RT-PCR) assay has been developed for the quantification of particular mRNA species in human articular cartilage. Competitor RNA species were synthesized that differed from the amplified target sequence only by the central insertion of an EcoRI restriction site. By using known amounts of synthetic target and competitor RNA, it was shown that competitor RNA molecules designed in this way are reverse-transcribed and amplified with equal efficiency to the target of interest. Furthermore quantification could be performed during the plateau phase of the PCR, which was necessary when using ethidium bromide fluorescence as a detection system. The inhibition of aggrecan and link-protein mRNA expression by interleukin 1 or tumour necrosis factor in monolayers of human articular chondrocytes quantified by this competitive RT-PCR method compared favourably with Northern hybridization studies. The main advantage of this technique is that it can be used to quantify levels of mRNA with RNA extracted directly from 100 mg wet weight of human articular cartilage. Age-related changes in aggrecan and link-protein mRNA were therefore quantified in human articular cartilage directly after dissection from the joint. The concentration of link-protein mRNA was higher in immature cartilage than in mature cartilage when expressed relative to the amount of glyceraldehyde-3-phosphate dehydrogenase mRNA, but no age-related changes were observed in aggrecan mRNA expression. The ratio of aggrecan to link-protein mRNA was higher in mature cartilage than in immature tissue. These age-related differences in the molecular stoichiometry of aggrecan and link-protein mRNA might have implications with respect to the regulation of the formation and the stability of the proteoglycan aggregates in cartilage.

Surface adhesion-mediated regulation of chondrocyte-specific gene expression in the nontransformed RCJ 3.1C5.18 rat chondrocyte cell line

Recent evidence suggests that decreased chondrocyte function in osteoarthritis and other articular disorders may be due to chondrocyte dedifferentiation produced by altered regulatory signals from the cartilage extracellular matrix (ECM). However, there are currently no mammalian chondrocytic cell line systems adapted to the study of this process. We therefore examined the effects of ECM growth conditions on markers of differentiated chondrocytic phenotype expression in the nontransformed rat RCJ 3.1C5.18 (RCJ) chondrocyte cell line, including type II collagen expression, aggrecan production, link protein gene expression, and parathyroid hormone (PTH) receptor number. RCJ cells grown in monolayer on plastic exhibited a dedifferentiated phenotype characterized by flattened cell morphology, with > 80% type I collagen and < 5% type II collagen production, as determined by two-dimensional gel mapping electrophoresis of collagen cyanogen bromide peptides. In addition, aggrecan production was low, and link protein mRNA was not expressed at detectable levels. After transfer to growth under minimal attachment conditions on the surface of a composite type I collagen/agarose (0.15%-0.8%) gel (CAG) for 7 days, RCJ cells developed a rounded, chondrocytic morphology and a pattern of differentiated, chondrocytic gene expression, with 79% type II and 8% type I collagen production. Steady-state type I and type II procollagen mRNA levels were altered in parallel with collagen protein expression. In cells grown on CAG, aggrecan production increased 6-fold, and there was a marked increase in both aggrecan core protein and link protein mRNA levels. In addition, maximal PTH-stimulated cAMP generation increased 15-fold in association with an increased PTH receptor number. Therefore, the RCJ chondrocyte cell line is highly sensitive to ECM regulation of chondrocyte-specific gene expression.

Expression of reduced amounts of structurally altered aggrecan in articular cartilage chondrocytes exposed to high hydrostatic pressure

The effect of hydrostatic pressure on proteoglycan (PG) metabolism of chondrocyte cultures was examined using a specially designed test chamber. Primary cultures of bovine articular chondrocytes at confluence were exposed for 20 h to 5 and 30 MPa continuous hydrostatic pressures and 5 MPa hydrostatic pulses (0.017, 0.25 and 0.5 Hz) in the presence of [35S]sulphate. Northern blot analyses showed that chondrocyte cultures used in this study expressed abundant mRNA transcripts of aggrecan, typical of chondrocytes, but not versican. The cultures also expressed biglycan and decorin. Enzymic digestions with keratanase and chondroitinases AC, ABC and B and subsequent SDS/agarose gel electrophoresis confirmed the synthesis of aggrecans and small dermatan sulphate PGs. The continuous 30 MPa pressure reduced total PG synthesis by 37% as measured by [35S]sulphate incorporation, in contrast to the 5 MPa continuous pressure which had no effect. The high static pressure also reduced total [3H]glucosamine incorporation by 63% and total [14C]leucine incorporation by 57%. The cyclic pressures showed a frequency-dependent stimulation (0.5 Hz, 11%) or inhibition (0.017 Hz, -17%) of [35S]sulphate incorporation. Aggrecans secreted under continuous 30 MPa pressure showed a retarded migration in 0.75% SDS/agarose gel electrophoresis and they also eluted earlier on Sephacryl S-1000 gel filtration, indicative of a larger molecular size. The increased size was consistent with an increase of average glycosaminoglycan chain length as determined by Sephacryl S-300 gel filtration. No change in aggrecan size was observed with the lower (5 MPa) static or cyclic pressures. Continuous 30 MPa hydrostatic pressure slightly reduced the steady-state mRNA level of aggrecan, in parallel with the decline in PG synthesis measured by [35S]sulphate incorporation. The results demonstrated that high hydrostatic pressure could influence the synthesis of PGs, especially of aggrecans, in chondrocytes both at the transcriptional and translational/post-translational levels.