Standardization of nutrient media for isolated human articular chondrocytes in gelified agarose suspension culture

Collagen microsphere based 3D culture system for human osteoarthritis chondrocytes (hOACs)

The current study aims to evaluate collagen microencapsulation as an in vitro 3D culture platform for human osteoarthritic chondrocytes (hOACs), and to exemplify its feasibility in screening potential disease modifying factors. We first isolated and expanded hOACs from osteoarthritis (OA) cartilage samples harvested from multiple patients during total knee replacement (TKR) surgery. These cells were microencapsulated into collagen microspheres for subsequent 3D cultures. The change in chondrocyte phenotypes and OA phenotype was evaluated over time, using 2D monolayer culture and traditional 3D pellet culture as controls. The hOACs in the 3D collagen microsphere model resumed their in vivo phenotypes when compared to 2D monolayer. When compared with the 3D pellet model, the 3D hOAC-collagen microsphere model better recapitulated the OA phenotypes. We further demonstrated the responsiveness of the microencapsulated hOACs towards a number of external factors altering the chondrogenic phenotype, corroborating with previous studies. The hOAC encapsulated collagen microspheres better maintained the hOAC phenotype than the traditional 2D monolayer and 3D pellet cultures. The feasibility to use this hOAC-collagen microsphere in vitro model as a screening platform for disease-modifying agents has been demonstrated, contributing to future development of OA therapeutics.

Creating a spectrum of fibrocartilages through different cell sources and biochemical stimuli

In this study a scaffoldless approach was employed with two different cell sources and biochemical stimuli to engineer a spectrum of fibrocartilages representative of the different regions of the knee meniscus. Constructs composed of 100% fibrochondrocytes (FC) or a 50:50 co-culture of fibrochondrocytes and chondrocytes (CC) were cultured in 10% fetal bovine serum medium or serum-free "chondrogenic" medium, each +/-10 ng/mL TGF-beta1 (+T). Constructs from these two cell groups and four culture conditions were cultured for 6 weeks. By varying the cell type and presence of the growth factor, GAG per dry weight of the constructs spanned that of native tissue, ranging 16-45% and 1-7% in the CC and FC groups, respectively. Collagen density was most dependent on cell type and was significantly lower than tissue values. The collagen I/II ratio could be manipulated by cell type and serum presence to span the native range, from 3.5 in the serum-free CC group to over 1,000 in the FC groups treated with serum-containing medium. Using the CC cell group in the presence of serum-free medium dramatically increased the compressive stiffness to 128 +/- 34 kPa, similar to native tissue. Similarly, serum-free medium or TGF-beta1 treatment enhanced the tensile modulus by an order of magnitude, up to 3,000 kPa. Using two cell sources and manipulating biochemical stimuli, a range of fibrocartilaginous neotissues was engineered. Fibrocartilages such as the knee meniscus are characterized by heterogeneity in matrix and functional properties, and this work demonstrates a strategy for recreating these heterogeneous tissues.

Osteochondral repair in synovial joints

PURPOSE OF REVIEW

One of the major challenges in rheumatology remains the induction of osteochondral repair in synovial joints. Remarkable progress has been made in controlling the inflammatory pathways of chronic synovitis and tissue damage in rheumatoid arthritis and spondyloarthropathy. Here, we provide an overview of the current knowledge on the mechanisms involved in osteochondral repair in degenerative joint diseases, as well as in immune mediated inflammatory arthritides, with special emphasis on tumor necrosis factor alpha and IL-1.

RECENT FINDINGS

Homeostasis of articular cartilage and subchondral bone are essential for maintaining the integrity of osteochondral structures within synovial joints. This is achieved by the regulation of a delicate balance between anabolic and catabolic signals. In articular cartilage one cell type, the chondrocyte, is responsible for regulation of homeostasis. In bone, however, two distinct cell types, osteoblasts and osteoclasts, are responsible for anabolic and catabolic pathways, respectively. In inflammatory joint disorders, this tight regulation is profoundly dysregulated, with tumor necrosis factor alpha acting as an important catalyst of a disturbed homeostasis, together with IL-1. Targeting these cytokines may restore the intrinsic repair capacity of osteochondral structures.

SUMMARY

To restore catabolic cytokine balances appears to be a suitable strategy to promote osteochondral repair.

Collagen synthesis of articular cartilage explants in response to frequency of cyclic mechanical loading

Articular cartilage in vivo experiences the effects of both cell-regulatory proteins and mechanical forces. This study has addressed the hypothesis that the frequency of intermittently or continuously applied mechanical loads is a critical parameter in the regulation of chondrocyte collagen biosynthesis. Cyclic compressive pressure was applied intermittently to bovine articular cartilage explants by using a sinusoidal waveform of 0.1-1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5-20 s followed by a load-free period of 10-1,000 s. These loading protocols were repeated for a total duration of 6 days. In separate experiments, cyclic loading was continuously applied by using a sinusoidal waveform of 0.001-0.5 Hz frequency and a peak stress of 1.0 MPa for a period of 3 days. Unloaded cartilage discs of the same condyle were cultured in identically constructed loading chambers and served as controls. We report quantitative data showing that (1) no correlation exists between the relative rate of collagen synthesis expressed as the proportion of newly synthesized collagen among newly made proteins and either the frequency of intermittently or continuously applied loads or the overall time cartilage is actively loaded, and (2) individual protocols of intermittently applied loads can reduce the relative rate of collagen synthesis and increase the water content, whereas (3) continuously applied cyclic loads always suppress the relative rate of collagen synthesis compared with that of unloaded control specimens. The results provide further experimental evidence that collagen metabolism is difficult to manipulate by mechanical stimuli. This is physiologically important for the maintainance of the material properties of collagen in view of the heavy mechanical demands made upon it. Moreover, the unaltered or reduced collagen synthesis of cartilage explants might reflect more closely the metabolism of normal or early human osteoarthritic cartilage.

Chondroprotective drugs in degenerative joint diseases

Catabolic cytokine and anabolic growth factor pathways control destruction and repair in osteoarthritis (OA). A unidirectional TNF-alpha/IL-1-driven cytokine cascade disturbs the homeostasis of the extracellular matrix of articular cartilage in OA. Although chondrocytes in OA cartilage overexpress anabolic insulin-like growth factor (IGF) and its specific receptor (IGFRI) autocrine TNF-alpha released by apoptotic articular cartilage cells sets off an auto/paracrine IL-1-driven cascade that overrules the growth factor activities that sustain repair in degenerative joint disease. Chondroprotection with reappearance of a joint space that had disappeared has been documented unmistakably in peripheral joints of patients suffering from spondyloarthropathy when treated with TNF-alpha-blocking agents that repressed the unidirectional TNF-alpha/IL-1-driven cytokine cascade. A series of connective tissue structure-modifying agents (CTSMAs) that directly affect IL-1 synthesis and release in vitro and down-modulate downstream IL-1 features, e.g. collagenase, proteoglycanase and matrix metalloproteinase activities, the expression of inducible nitric oxide synthase, the increased release of nitric oxide, and the secretion of prostaglandin E(2), IL-6 and IL-8, have been shown to possess disease-modifying OA drug (DMOAD) activities in experimental models of OA and in human subjects with finger joint and knee OA. Examples are corticosteroids, some sulphated polysaccharides, chemically modified tetracyclines, diacetylrhein/rhein, glucosamine and avocado/soybean unsaponifiables.

Collagen biosynthesis of mechanically loaded articular cartilage explants

OBJECTIVE

The purpose of this study was to investigate systematically the effect of load amplitudes, frequencies and load durations of intermittently applied mechanical pressure on the biosynthesis of collagen and non-collagenous proteins (NCP) as well as on the water content of cultured bovine articular cartilage explants.

METHODS

Cyclic compressive pressure was applied using a sinusoidal waveform of 0.5 Hz frequency with a peak stress of 0.1, 0.5 or 1.0 MPa for a period of 10s followed by a load-free period of 10, 100 or 1000s. These intermittent loading protocols were repeated for a total duration of 1, 3 or 6 days. During the final 18 h of experiments, the incorporation of [(3)H]-proline into collagen and NCP, the content of water as well as the deformation of loaded explants were determined.

RESULTS

Intermittently applied, cyclic mechanical loading of articular cartilage explants consistently reduced the relative rate of collagen synthesis compared to load-free conditions. This reduced proportion of newly synthesized collagen among newly made proteins was independent of the mechanical stimuli applied. The release of newly synthesized collagen and NCP from loaded explants into the nutrient media was unaffected by any of the loading protocols applied. In addition, quantitative data are provided showing that only high amplitudes of loads and frequencies enhanced the water content of the explants.

CONCLUSIONS

Previous studies reporting that osteoarthritic cartilage in vivo can synthesize elevated amounts of collagen imply that the loading protocols chosen were inadequate for simulating in vitro osteoarthritic-like alterations of collagen synthesis. In our experiments the collagen biosynthesis of chondrocytes was only minor responsive to alterations in mechanical stimuli, applied over a wide range. Thus, our results imply that the synthesis of these structural macromolecules is under the strict control of normal chondrocytes enabling them to maintain the shape of this physical demanded tissue.

Influence of medium composition, static and stirred conditions on the proliferation of and matrix protein expression of bovine articular chondrocytes cultured in a 3-D collagen scaffold

Interest in chemical and physical modifications of culture conditions and composition, as a way to improve engineered cartilage, has grown over the last decade. To address some of these aspects, articular bovine chondrocytes seeded in collagen sponges (2.3x10(6) cells/cm(3), whose growth and metabolism have been previously reported) were grown under static or stirred conditions (orbital shaker at 30 rpm), in either 10% FCS-supplemented or serum-free media (1% ITS+1mM cysteine). Under stirred conditions, we observed a 2-fold increase in both cell proliferation and sulphated glycosaminoglycan deposition after 1 month of culture, compared to static conditions, and after 3 months, a more homogeneous distribution of both cells and neomatrix in the constructs. During the first month of culture, the substitution of FCS by ITS led to low cell proliferation and poor neomatrix deposition but, after 2 months a steep increase was observed with ITS for these two parameters that reached, after 3 months the levels observed with FCS. Aggrecan was the more abundant component at both gene and protein levels, whereas the collagenous network formed was looser than with FCS. In conclusion, the use of these simple culture conditions should improve, in long-term culture, the quality of the cartilage construct.

Fibronectin metabolism of cartilage explants in response to the frequency of intermittent loading

Chondrocytes within articular cartilage experience complete unloading between loading cycles and in so doing utilize mechanical signals to regulate their own metabolic activities. A strongly elevated fibronectin content is an early feature in osteoarthritis and appears to be related to increases in both the synthesis and retention of this glycoprotein. The objectives of this study were to investigate systematically whether the frequency of intermittently applied cyclic mechanical loading of cartilage explants alters the biosynthesis and retention of fibronectin, and to assess whether it is possible to induce in vitro osteoarthritic-like changes of this metabolic parameter by mechanical means over a period of 6 days. Cartilage plugs consisting of viability-checked chondrocytes were exposed to sinusoidal cyclic compressive pressure alterations of 0.1, 0.5 or 1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5, 10 or 20 s, followed by an unloading period of 10, 100 or 1000 s, and compared to unloaded reference plugs from the same joint and topographic origin. The incorporation of radioactive precursor into fibronectin during the last 18 h, the content of fibronectin, and the viability of chondrocytes were determined. Our data revealed that (a) the fibronectin synthesis was selectively, but non-linearly affected by the frequency of intermittent loads applied (as defined by the frequency of the applied force, the duration of the loading cycle and the duration of the force-free period between each loading cycle), and that (b) the retention of endogenous fibronectin and proteins within loaded cartilage explants is strongly elevated. These data support our hypothesis that the mechanical factor "frequency of intermittent loading" seems to be the crucial mechanical parameter controlling the metabolism of chondrocytes. The effect of the frequency of intermittent loading cannot be described by a simple statistical correlation, so that no specific predictions are possible. However, our results imply that distinct loading protocols have been established that can induce alterations of the fibronectin metabolism similar to those observed in human and animal osteoarthritis.

Homeostasis of the extracellular matrix of normal and osteoarthritic human articular cartilage chondrocytes in vitro

OBJECTIVE

In normal articular cartilage cells, the IGFRI/insulin-like growth factor 1 (IGF-1) autocrine pathway was shown to overrule the catabolic effects of the IL-1/IL-1RI pathway by up-regulation of the IL-1RII decoy receptor. The activity of the IGF-1/IGFR1 and IL-1/IL-1R pathways, and of the IL-1RII control mechanism in the synthesis and turnover of the extracellular matrix (ECM) by chondrocytes from normal and osteoarthritic (OA) articular cartilage was compared in order to identify possible therapeutic targets of this disease.

METHODS

Phenotypically stable human articular cartilage cells were obtained from normal and OA cartilage of the same knee showing focal OA. The cells were cultured in alginate beads over 1 week to re-establish the intracellular cytokine and growth factors, to reexpress the respective plasma membrane receptors and to reach equilibrium in accumulated cell-associated matrix (CAM) compounds. Following liberation of the cells from the alginate beads, the levels of cell-associated matrix (CAM) aggrecan, type II collagen and fibronectin, of intracellular IGF-1, IL-1alpha and beta and of their respective plasma membrane-bound receptors, IGFR1, IL-1RI and the decoy receptor IL-1RII, were assayed using flow cytometry.

RESULTS

Coordinated production and accumulation of CAM aggrecan and type II collagen under the effect of the IGFR1/IGF-1 autocrine pathway-as documented for chondrocytes from healthy controls-was absent when the chondrocytes had been obtained from OA joints. When compared with cells obtained from normal tissues, chondrocytes from fibrillated OA cartilage expressed significantly higher intracellular IGF-1 levels and plasma membrane-bound IGFR1. At the same time, significantly higher intracellular IL-1alpha and beta levels and upregulated plasma membrane-bound IL-1RI were observed. Plasma membrane-bound IL-1RII decoy receptor was downregulated in OA chondrocytes. The levels of CAM aggrecan, type II collagen and fibronectin were significantly reduced in the chondrocytes obtained from pathological tissue.

CONCLUSION

Paired analysis of normal and OA chondrocytes from the same knee joint has shown an enhanced capacity of chondrocytes from OA cartilage to produce ECM macromolecules. However, the same cells have increased catabolic signalling pathways. As a consequence of this increased IL-1 activity and the reduced amounts of IL-1RII decoy receptor, less of the produced ECM macromolecules may persist in the CAM of the OA chondrocytes.

Insulin-like growth factor 1-induced interleukin-1 receptor II overrides the activity of interleukin-1 and controls the homeostasis of the extracellular matrix of cartilage

OBJECTIVE

We examined the effect of the insulin-like growth factor 1 (IGF-1)/IGF receptor I (IGFRI) autocrine/paracrine anabolic pathway on the extracellular matrix (ECM) of human chondrocytes and the mechanism by which IGF-1 reverses the catabolic effects of interleukin-1 (IL-1).

METHODS

Phenotypically stable human articular cartilage cells were obtained from normal cartilage and maintained in culture in alginate beads for 1 week to reach equilibrium of accumulated cell-associated matrix (CAM) compounds. Levels of CAM components aggrecan and type II collagen (CII) and levels of intracellular IGF-1, IL-1alpha, and IL-1beta and their respective plasma membrane-bound receptors IGFRI, IL-1 receptor I (IL-1RI), and the decoy receptor IL-1RII were assayed using flow cytometry to investigate the relationship between the autocrine/paracrine pathways and the homeostasis of ECM molecules in the CAM. The effects of IGF-1 on the expression of IGF-1, IL-1alpha, and IL-1beta and their respective receptor systems, the aggrecan core protein, and CII were determined by flow cytometry.

RESULTS

Cause-effect relationship experiments showed that IGF-1 up-regulates the levels of IGF-1, IGFRI, aggrecan, and CII in the CAM. No effects on the expression of IL-1alpha and IL-1beta and their signaling receptor IL-1RI were observed. However, IGF-1 was able to reverse IL-1beta-mediated degradation of aggrecan and the repression of the aggrecan synthesis rate. Interestingly, levels of aggrecan and CII in the CAM strongly correlated not only with IGF-1, but also with IL-1RII, which acts as a decoy receptor for IL-1alpha and IL-1beta. This suggests that IGF-1 and IL-1RII may cooperate in regulating ECM homeostasis. Additional experiments demonstrated that IGF-1 up-regulated IL-1RII, thereby overriding the catabolic effects of IL-1.

CONCLUSION

These findings reveal a new paradigm by which IGF-1 influences chondrocyte metabolism, by reversing the IL-1-mediated catabolic pathway through up-regulation of its decoy receptor.

Control of extracellular matrix homeostasis of normal cartilage by a TGFbeta autocrine pathway. Validation of flow cytometry as a tool to study chondrocyte metabolism in vitro

OBJECTIVE

To validate flow cytometry as an experimental technique for the study of the homeostasis of the extracellular matrix (ECM) of human articular cartilage.

METHODS

Given the established insights in the relation between the transforming growth factor (TGF)-beta type II Receptor (TGF-betaRII)/TGF-beta auto/paracrine pathway, the intracellular levels of matrix metalloproteinases (MMPs) and their natural inhibitors (TIMPs), and the accumulation of ECM molecules in the ECM of articular cartilage, this metabolic pathway was used as a reference model to fulfill the objective. Chondrocytes were liberated from visually intact femoral condyle cartilage and cultured in gelled agarose to maintain their differentiated phenotype. After 2 weeks of culture, the chondrocytes were isolated from the agarose and flow cytometry was used to analyse the expression of TGF-betaRII on the plasmamembrane, the expression of TGFbeta1, MMP-1, MMP-3, TIMP-1 and TIMP-3 inside the cells, as well as the amounts of aggrecan, type II collagen and hyaluronan in the cell-associated matrix (CAM). The expression of the different substances was analysed with flow cytometry and reported as mean fluorescence intensity (MFI), which is due to the binding of FITC-labeled antibodies to their specific antigens. In addition, the effects of exogenous TGFbeta1 on the expression of these proteins was investigated on chondrocytes cultured in serum-free media. Enzyme Linked Immunosorbent Assay (ELISA) was performed to evaluate the MMP-1, MMP-3, TIMP-1 and MMP-1/TIMP-1 complex in the culture medium collected after the last 3 days of the culture period. The correlations between the data were analysed with the Spearman's test.

RESULTS

Exogenous TGF-beta1 increased the accumulation of aggrecan and hyaluronan in the CAM of chondrocytes and down-regulated the intracellular levels of MMP-1 and -3. TIMP-1 and -3 were increased after exposure to TGF-beta1. Baseline expression of TGF-betaRII on the plasmamembrane of normal human articular chondrocytes significantly correlated with the intracellular levels of TGFbeta1, TIMP-1 and TIMP-3. TGFbeta1 was correlated with TIMP-1, TIMP-3 and MMP-1. Aggrecan in the CAM was inversely correlated with the ratio of MMP-1 to TIMPs. In addition, there were correlations between TIMP-1 and TIMP-3, aggrecan and hyaluronan. ELISA also revealed the correlation between MMP-1 and TIMP-1 secreted by the chondrocytes into the nutrient medium. MMP-1/TIMP-1 complex was hardly found in the medium.

CONCLUSIONS

Some aspects of ECM metabolism of normal cartilage were evaluated by flow cytometry. Chondrocytes from normal human cartilage, when cultured in gelled agarose, showed correlations between the expression of TGF-betaRII/TGF-beta1 and the intracellular levels of TIMPs, indicating that TGF-beta autocrine pathway may contribute to homeostasis of the ECM in the normal cartilage. The relations between MMPs, TIMPs and the ECM molecules support that a physiological balance between MMPs and TIMPs results in a well-controlled matrix turnover in normal cartilage.

Biological freezing of human articular chondrocytes

AIM

To preserve viable, metabolically active chondrocytes cultured in alginate beads at -196 degrees C for further use in in vitro and in vivo studies.

METHODS

Human articular chondrocytes were isolated from femoral condyles within 24 h post mortem. To optimize the biological freezing procedure, the chondrocytes were control-rate frozen in different concentrations of dimethyl sulfoxide (DMSO) in Dulbecco's MEM supplemented with 10% FCS before being thawed and the cell viability was determined by Trypan Blue exclusion test. To investigate the effect of control-rate freezing on chondrocyte metabolism, control-rate frozen chondrocytes in 5% DMSO were thawed and cultured in gelled agarose for 2 weeks. Non-frozen chondrocytes cultured in agarose served as controls. Furthermore, human articular chondrocytes were cultured in 2% alginate beads for 2 weeks after which the beads were incubated with 5% DMSO for 0 h, 2.5 h, 5 h and 10 h and frozen at -196 degrees C. Non-frozen alginate beads containing chondrocytes and incubated with 5% DMSO served as a control. After 2 weeks in culture, chondrocytes in agarose or in alginate were sulfated with 10 microCi(35)SO(4)/ml for 48 h. The total production of aggrecans, and the aggrecan subtypes, were subsequently determined.

RESULTS

Five percent DMSO in the culture medium was the optimal condition to control-rate freeze and recover viable and functional isolated chondrocytes. Total aggrecan synthesis of control-rate frozen chondrocytes cultured in gelled agarose was not significantly reduced when compared with control cells. The proportion of aggrecan in the aggregate form of control-rate frozen chondrocytes kept in agarose remained unaltered. Chondrocytes, control-rate frozen in the alginate matrix, showed a 0-30% decrease in total aggrecan synthesis rates in culture when compared with the non-frozen chondrocytes. The optimal pre-incubation time of the alginate beads with 5% DMSO was 5 h, without any change in aggrecan synthesis rates when compared with the control situation. Shorter pre-incubation times resulted in an insufficient diffusion of DMSO into the beads and in cell death. There was no difference in the synthesis of the different aggrecan subtypes between frozen and non-frozen chondrocytes in alginate.

CONCLUSION

Human articular chondrocytes can be stored at -196 degrees C for 24 h without important decreases in their aggrecan synthesis rates when control-rate frozen as a cell suspension in 5% DMSO. Proportions of the aggrecan subtypes (monomers, aggregates) synthesized by chondrocytes cultured in agarose remained unchanged. The control-rate freezing procedure in the alginate beads pre-incubated with 5% DMSO for 5 h produced no decrease in total aggrecan synthesis rates and no change in the synthesized aggrecan subtypes. Further experiments have to confirm the suitability of this freezing method for long-term storage of chondrocytes allowing us to set up a 'chondrocyte' bank for further use in in vitro and in vivo manipulations.

Flow cytometric analysis of the human articular chondrocyte phenotype in vitro

OBJECTIVE

To develop flow cytometry for the study of human articular cartilage cell phenotype and to validate the method on chondrocytes cultured in different in-vitro systems.

METHODS

Chondrocyte phenotype was modulated by culturing the cells under different in-vitro conditions: i.e. in monolayer and in suspension culture in gelled agarose. Monolayer cultured chondrocyte phenotype was assayed by immunohistochemical staining with monoclonal antibodies against chondrocyte-specific aggrecan, type II and I collagen. Flow cytometry was used to quantify the proportions of chondrocytes expressing these extracellular matrix molecules in both culture conditions. To exclude the effects of cell-harvesting methods on the presence of cell-bound ECM molecules, non-proteolytic isolation procedures were used to obtain the chondrocytes for flow cytometry. Subconfluent cells from monolayer cultures were detached with EDTA. Chondrocytes cultured in gelled agarose were obtained after the agarose was enzymatically digested with agarase.

RESULTS

Immunohistochemical staining showed that monolayer-cultured chondrocytes, in the presence of serum, gradually lost the expression of chondrocyte-specific aggrecan and type II collagen, while type I collagen was increasingly expressed. Flow cytometry allowed monolayer cultured chondrocyte phenotype to be assessed reproducibly. Chondrocyte phenotype was characterized through the cell membrane-associated extracellular matrix antigens. EDTA, used to obtain single cells from monolayer cultures, did not affect the cell-associated matrix. Where the chondrocytes had been cultured in gelled agarose, flow cytometry allowed quantification of the percentages of chondrocytes maintaining or reexpressing their original phenotype. The agarase digestion procedure used to isolate the cells from the agarose gel did not affect the plasma membrane-associated extracellular matrix antigens.

CONCLUSION

Flow cytometry allows quantification of cells expressing aggrecan, type II and I collagen in their cell-associated extracellular matrix. A continuously increasing number of specific monoclonal antibodies will broaden the range of applications offered by this method.

Influence of aging on the synthesis and morphology of the aggrecans synthesized by differentiated human articular chondrocytes

OBJECTIVE

Synthesis rates of aggrecans by phenotypically stable human articular chondrocytes and the immobilization of these aggrecans in large aggregates were used as variables reflecting the capability of these cells of restoring the extracellular matrix of articular cartilage in vivo in an aging population.

DESIGN

Human articular chondrocytes were isolated from articular cartilage obtained from 33 different donors at autopsy. The chondrocytes were cultured in gelled agarose. Synthesis of aggrecans was investigated using Na(2)(35)SO(4)as a radioactive precursor after a 2-week culture period. Electron microscopic study of aggrecan aggregates was done on the macromolecules accumulated over 3 weeks in culture by the chondrocytes obtained from eight other donors with increasing ages.

RESULTS

Sulfate incorporation rates into aggrecans correlated inversely with the age of the donor. The value of sulfate incorporation in aggrecans for chondrocytes obtained from mature cartilage of a 20-year-old individual in this system drops to 50% and 25% for chondrocytes obtained from 45- and 69-year-old individuals respectively. Electron microscopic study of aggrecan aggregates showed that the 'de novo' synthesized hyaluronan molecules were fully loaded with aggrecans. Mature human articular cartilage cells were found to synthesize an aggrecan aggregate which carried an average number of 11.7 to 13.1 aggrecans. Cells obtained from immature donors synthesized aggrecan aggregates of which the hyaluronan chain carried twice the amount of aggrecans. These immature human articular cartilage cells were also found to synthesize significant proportions of large aggrecan aggregates with 20 to over 100 aggrecans immobilized on a single hyaluronan chain. The proportions of these large aggrecan aggregates decreased with increasing age of the donors of the chondrocytes.

CONCLUSION

The declining aggrecan synthesis rates and the decreased capability of assembling large molecular size aggregates with increasing age in humans illustrates a progressive failure of the repair function of articular cartilage cells in humans.

Synergistic action of transforming growth factor-beta and insulin-like growth factor-I induces expression of type II collagen and aggrecan genes in adult human articular chondrocytes

Reexpression of aggrecan and type II collagen genes in dedifferentiated adult human articular chondrocytes (AHAC) in suspension culture varied widely depending on the specific lot of bovine serum used to supplement the culture medium. Some lots of serum provided strong induction of aggrecan and type II collagen expression by AHAC while others did not stimulate significant production of these hyaline cartilage extracellular matrix molecules even following several weeks in culture. Addition of 50 ng/ml insulin-like growth factor-I (IGF-I) to a deficient serum lot significantly enhanced its ability to induce aggrecan and type II collagen mRNA. Given this observation, IGF-I and other growth factors were tested in defined serum-free media for their effects on the expression of these genes. Neither IGF-I nor insulin nor transforming growth factor beta (TGF-beta) alone stimulated induction of aggrecan or type II collagen production by dedifferentiated AHAC. However, TGF-beta 1 or TGF-beta 2 combined with IGF-I or insulin provided a strong induction as demonstrated by RNase protection and immunohistochemical assays. Interestingly, type I collagen, previously shown to be downregulated in serum supplemented suspension cultures of articular chondrocytes, persisted for up to 12 weeks in AHAC cultured in defined medium supplemented with TGF-beta and IGF-I.

Intermittent cyclic loading of cartilage explants modulates fibronectin metabolism

OBJECTIVE

The aim of this study was to evaluate systematically the effect of tissue load, its amplitude, time of intermittence and duration of loading on the biosynthesis and release of fibronectin by intermittently loaded mature bovine articular cartilage explants.

METHODS

Cyclic compressive pressure was introduced using a sinusoidal waveform of 0.5 Hz-frequency with a peak stress of 0.1, 0.5 or 1.0 MPa for a period of 10 s followed by an unloaded period lasting 10, 100 or 1000 s. Fibronectin and total proteins were radiolabeled with 10 microCi/ml [3H]-phenylalanine during the final 18 h of the 1, 3 or 6 day experiments. The content of endogenous fibronectin was determined using enzyme-linked immunosorbant assay (ELISA), whereas the viability of explants was measured using sections of cartilage explants stained with fluorescein diacetate and propidium iodide. The deformation of loaded explants was determined using a load-displacement transducer system.

RESULTS

The mechanical factor time of intermittence significantly altered the synthesis and release of fibronectin by cartilage explants, whereas the tested range of load magnitudes, as well as the duration of loading, seemed to be of subordinate importance. Loading affected the viability of the superficial zone in the cartilage, whereas the chondrocytes of the intermediate and deep zone remained viable. The compression of loaded explants was dependent on the magnitude of stress, as well as on the duration of unloading between each loading cycle. Synthesis of fibronectin, the retention of newly synthesized fibronectin within the extracellular matrix, and the portion of newly synthesized proteins that were fibronectin was significantly increased in cartilage explants which were cyclically compressed with 0.5 MPa for 10 s followed by a period of unloading lasting 100 s.

CONCLUSIONS

Previous studies reporting that cartilage explants of human and animal osteoarthritic joints synthesize and retain elevated amounts of fibronectin imply that in our experiments mechanical stimuli can induce a fibronectin metabolism in vitro which mimics some of the osteoarthritic characteristics.