Effect of link protein concentration on articular cartilage proteoglycan aggregation

Hyaluronic Acid: Incorporating the Bio into the Material

In the last few decades, hyaluronic acid (HA) has become increasingly employed as a biomaterial in both clinical and research applications. The abundance of HA in many tissues, together with its amenability to chemical modification, has made HA an attractive material platform for a wide range of applications including regenerative medicine, drug delivery, and scaffolds for cell culture. HA has traditionally been appreciated to modulate tissue mechanics and remodeling through its distinctive biophysical properties and ability to organize other matrix proteins. However, HA can influence cell behavior in much more direct and specific ways by engaging cellular HA receptors, which can trigger signals that influence cell survival, proliferation, adhesion, and migration. In turn, cells modify HA by regulating synthesis and degradation through a dedicated arsenal of enzymes. Optimal design of HA-based biomaterials demands full consideration of these diverse modes of regulation. This review summarizes how HA-based signaling regulates cell behavior and discusses how these signals can be leveraged to create cell-instructive biomaterials.

Non-destructive detection of matrix stabilization correlates with enhanced mechanical properties of self-assembled articular cartilage

Tissue engineers rely on expensive, time-consuming, and destructive techniques to monitor the composition, microstructure, and function of engineered tissue equivalents. A non-destructive solution to monitor tissue quality and maturation would greatly reduce costs and accelerate the development of tissue-engineered products. The objectives of this study were to (a) determine whether matrix stabilization with exogenous lysyl oxidase-like protein-2 (LOXL2) with recombinant hyaluronan and proteoglycan link protein-1 (LINK) would result in increased compressive and tensile properties in self-assembled articular cartilage constructs, (b) evaluate whether label-free, non-destructive fluorescence lifetime imaging (FLIm) could be used to infer changes in both biochemical composition and biomechanical properties, (c) form quantitative relationships between destructive and non-destructive measurements to determine whether the strength of these correlations is sufficient to replace destructive testing methods, and (d) determine whether support vector machine (SVM) learning can predict LOXL2-induced collagen crosslinking. The combination of exogenous LOXL2 and LINK proteins created a synergistic 4.9-fold increase in collagen crosslinking density and an 8.3-fold increase in tensile strength as compared with control (CTL). Compressive relaxation modulus was increased 5.9-fold with addition of LOXL2 and 3.4-fold with combined treatments over CTL. FLIm parameters had strong and significant correlations with tensile properties (R²  = 0.82; p < 0.001) and compressive properties (R²  = 0.59; p < 0.001). SVM learning based on FLIm-derived parameters was capable of automating tissue maturation assessment with a discriminant ability of 98.4%. These results showed marked improvements in mechanical properties with matrix stabilization and suggest that FLIm-based tools have great potential for the non-destructive assessment of tissue-engineered cartilage.

Hyaluronan: a simple polysaccharide with diverse biological functions

Hyaluronan (HA) is a linear polysaccharide with disaccharide repeats of d-glucuronic acid and N-acetyl-d-glucosamine. It is evolutionarily conserved and abundantly expressed in the extracellular matrix (ECM), on the cell surface and even inside cells. Being a simple polysaccharide, HA exhibits an astonishing array of biological functions. HA interacts with various proteins or proteoglycans to organize the ECM and to maintain tissue homeostasis. The unique physical and mechanical properties of HA contribute to the maintenance of tissue hydration, the mediation of solute diffusion through the extracellular space and the lubrication of certain tissues. The diverse biological functions of HA are manifested through its complex interactions with matrix components and resident cells. Binding of HA with cell surface receptors activates various signaling pathways, which regulate cell function, tissue development, inflammation, wound healing and tumor progression and metastasis. Taking advantage of the inherent biocompatibility and biodegradability of HA, as well as its susceptibility to chemical modification, researchers have developed various HA-based biomaterials and tissue constructs with promising and broad clinical potential. This paper illustrates the properties of HA from a matrix biology perspective by first introducing the principles underlying the biosynthesis and biodegradation of HA, as well as the interactions of HA with various proteins and proteoglycans. It next highlights the roles of HA in physiological and pathological states, including morphogenesis, wound healing and tumor metastasis. A deeper understanding of the mechanisms underlying the roles of HA in various physiological processes can provide new insights and tools for the engineering of complex tissues and tissue models.

Biochemical and atomic force microscopic characterization of salmon nasal cartilage proteoglycan

Biological activities of salmon nasal cartilage proteoglycan fractions are known, however, structural information is lacking. Recently, the major proteoglycan of this cartilage was identified as aggrecan. In this study, global molecular images and glycosaminoglycan structure of salmon nasal cartilage aggrecan purified from 4M guanidine hydrochloride extract were analyzed using HPLCs and atomic force microscopy with bovine tracheal cartilage aggrecan as a control. The estimated numbers of sulfates per disaccharide unit of chondroitin sulfate chains of salmon and bovine aggrecans were similar (approximately 0.85). However, the disaccharide composition showed a higher proportion of chondroitin 6-sulfate units in salmon aggrecan, 60%, compared to 40% in bovine. Gel filtration HPLC and monosaccharide analysis showed the salmon aggrecan had a lower number (approximately one-third), but 1.5-3.3 times longer chondroitin sulfate chains than the bovine aggrecan. Atomic force microscopic molecular images of aggrecan supported the images predicted by biochemical analyses.

Cocultures of adult and juvenile chondrocytes compared with adult and juvenile chondral fragments: in vitro matrix production

BACKGROUND

The use of allogenic juvenile chondrocytes or autologous chondral fragments has shown promising laboratory results for the repair of chondral lesions.

HYPOTHESIS

Juvenile chondrocytes would not affect matrix production when mixed with adult chondrocytes or cartilage fragments.

STUDY DESIGN

Controlled laboratory study.

METHODS

Cartilage sources consisted of 3 adult and 3 juvenile (human) donors. In part 1, per each donor, juvenile chondrocytes were mixed with adult chondrocytes in 5 different proportions: 100%, 50%, 25%, 12.5%, and 0%. Three-dimensional cultures in low-melt agarose were performed. At 6 weeks, biochemical and histologic analyses were performed. In part 2, isolated adult, isolated juvenile, and mixed 3-dimensional cultures (1:1) were performed with chondral fragments (<1 mm), both with low-melt agarose and a hyaluronic acid scaffold. At 2 and 6 weeks, cultures were evaluated with biochemical and histologic analyses.

RESULTS

Part 1: Biochemical and histologic analyses showed that isolated juvenile cultures performed significantly better than mixed and isolated adult cultures. No significant differences were noted between mixed cultures (1:1) and isolated adult cultures. Part 2: Biochemical and histologic results at 6 weeks showed that mixed cartilage fragment cultures performed better than isolated adult cultures in terms of proteoglycans/DNA ratio (P = .014), percentage of safranin O-positive cells (P = .012), Bern score (P = .001), and collagen type II. No statistically significant difference was noted between juvenile and mixed cultures.

CONCLUSION

Extracellular matrix production of juvenile chondrocytes is inhibited by adult chondrocytes. The addition of juvenile cartilage fragments to adult fragments improves matrix production, with a positive interaction between the 2 sources.

CLINICAL RELEVANCE

Even if the underlying mechanisms are still unknown, this study describes the behavior of juvenile/adult cocultures using both chondrocytes and cartilage fragments, with potential for new research and clinical applications.

Acute Osteoclast Activity following Subchondral Drilling Is Promoted by Chitosan and Associated with Improved Cartilage Repair Tissue Integration

OBJECTIVE

Cartilage-bone integration is an important functional end point of cartilage repair therapy, but little is known about how to promote integration. We tested the hypothesis that chitosan-stabilized blood clot implant elicits osteoclasts to drilled cartilage defects and promotes repair and cartilage-bone integration.

DESIGN

Bilateral trochlear defects in 15 skeletally mature rabbit knees were microdrilled and then treated with chitosan-glycerol phosphate (GP)/blood implant with fluorescent chitosan tracer and thrombin to accelerate in situ solidification or with thrombin alone. Chitosan clearance, osteoclast density, and osteochondral repair were evaluated at 1, 2, and 8 weeks at the outside, edge, and through the proximal microdrill holes.

RESULTS

Chitosan was retained at the top of the drill holes at 1 week as extracellular particles became internalized by granulation tissue cells at 2 weeks and was completely cleared by 8 weeks. Osteoclasts burst-accumulated at microdrill hole edges at 1 week, in new woven bone at the base of the drill holes at 2 weeks, and below endochondral cartilage repair at 8 weeks. Implants elicited 2-fold more osteoclasts relative to controls (P < 0.001), a more complete drill hole bone repair, and improved cartilage-bone integration and histological tissue quality. Treated and control 8-week cartilage repair tissues contained 85% collagen type II. After 8 weeks of repair, subchondral osteoclast density correlated positively with bone-cartilage repair tissue integration (P < 0.0005).

CONCLUSIONS

Chitosan-GP/blood implant amplified the acute influx of subchondral osteoclasts through indirect mechanisms, leading to significantly improved repair and cartilage-bone integration without inducing net bone resorption. Osteoclasts are cellular mediators of marrow-derived cartilage repair integration.

Tissue engineering by molecular disassembly and reassembly: biomimetic retention of mechanically functional aggrecan in hydrogel

In vitro assembly of key functional extracellular matrix constituents for tissue-engineered constructs may provide a tool to modulate the retention of proteoglycan (PG) aggregates, which are crucial to compressive biomechanical properties of connective tissues. This study tested the hypotheses that (1) biomimetic molecular reassembly of PG aggregates (native aggrecan [AGC] with hyaluronan [HA] ± link protein [LP]) affects AGC retention kinetics in hydrogel constructs, (2) the compressive properties of such hydrogel constructs are related to the content of retained AGC, and (3) the reassembly method is compatible with chondrocytes. Addition of HA to AGC in hydrogel constructs increased AGC retention in a dose-dependent manner, and the addition of LP to AGC + HA further enhanced AGC retention. The level of AGC retention, in turn, was associated with increased equilibrium compressive stress of the constructs. Chondrocytes could be included in the process, and maintained expression of the chondrogenic phenotype, secreting type II collagen but little type I collagen. Thus, by altering the assembly of PG aggregates with HA ± LP, which affects AGC retention, it may be possible to achieve the targeted levels of PG components to modulate the mechanical properties of the engineered construct for cartilage as well as other tissues containing PG and PG aggregates.

Inhibition of hyaluronan export reduces collagen degradation in interleukin-1 treated cartilage

Background

Osteoarthrosis is characterized by cartilage erosion, proteolysis of aggrecan and collagen, and disturbed rates of synthesis of aggrecan and hyaluronan by chondrocytes, with hyaluronan over-production being an early reaction. We considered that inhibition of hyaluronan export might prevent subsequent proteoglycan loss and collagen degradation.

Methods

To test this hypothesis, we studied a tissue culture model using bovine cartilages explants activated with IL-1α to induce osteoarthritic reactions using the phosphodiesterase-5 inhibitors tadalafil, zaprinast and vardenafil.

Results

These drugs inhibited hyaluronan export, but they did not inhibit hyaluronan synthase activity. Simultaneously, they inhibited proteoglycan loss and collagen degradation, but not their synthesis. They also reduced the release of gelatinases into the culture media and diffusion of the indicator protein horseradish peroxidase through the cartilage explants. The mechanism of action of these compounds may be through inhibition of hyaluronan exporter multidrug resistance-associated protein 5 (MRP5), because the effective drug concentrations were much higher than required for phosphodiesterase-5 inhibition and intracellular cGMP accumulation.

Conclusion

Inhibition of hyaluronan over-production may be an appropriate target to attenuate IL-1-induced reactions in osteoarthritic cartilage.

Age-related changes in muscles and joints

Aging muscle and joint changes can have a tremendous impact on the functionality of elderly people with and without disabilities. Studies in animal models have shown some potentially beneficial interventions (eg, gene therapy). Further research is needed to ascertain their benefits in humans. A better understanding of mechanisms by which skeletal muscle and joint changes take place in a geriatric population will be helpful to find reasonable ways to prevent age-related change and improve disability. Although some agents have been reported to have significant positive effects, further studies are needed to determine long-term side effects. More information is needed with respect to the changes in muscles and joints in various disabilities.

The structure of proteoglycan aggregate determined by atomic force microscopy

Proteoglycan aggregate is the major extracellular matrix component in cartilage, comprising about 18% of the dry weight of hyaline cartilage. The proteoglycan aggregate is the major substance in cartilage which resists compression in the joint. The purpose of this study was to utilize the newly developed imaging technique, Atomic force Microscopy (AFM), to visualize the ultrastructure of proteoglycan aggregates. The proteoglycan aggregate molecules were imaged in air using the tapping mode of the AFM. The images illustrated the ultrastructure of the aggregates, especially the individual proteoglycan and the core hyaluronic acid. In addition to the length and width of each molecule, the height of the proteoglycan aggregates and the individual proteoglycans could be directly measured. The images of the ultrastructures of proteoglycan aggregates visualized from the AFM are comparable with those using conventional electron microscopy approaches. Nevertheless, the sample preparation for AFM imaging does not involve fixation, staining, coating, and other routine procedures required for traditional electron microscopy imaging. Thus, this technique could be a simple alternative approach for future analysis of proteoglycan aggregate and its assembly.

Chondrocyte senescence, joint loading and osteoarthritis

cellular level is not completely understood, but both aging and loading-induced stresses have been shown to undermine cell functions related to the maintenance and restoration of the cartilage matrix. Based on precedents set by studies of other age-related degenerative diseases, we have focused our laboratory work on senescence as the cause of age-dependent decline in chondrocytes and on the impact of excessive mechanical stresses in promoting senescence. We hypothesized that senescent chondrocytes accumulate with age in articular cartilage and we propose that excessive mechanical stress plays a role in this process by promoting oxidative damage in chondrocytes that ultimately causes them to senesce. To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes, and determined the effects of chronic exposure to oxidative stress on chondrocyte growth and senescence. In addition, we measured the effects of abnormally high levels of mechanical shear stress on the release of oxidants in cartilage explants. We found that senescent chondrocytes accumulated with age in articular cartilage. In vitro studies showed that chronic oxidative stress caused by repeated exposure to peroxide, or by growth under superphysiologic oxygen tension caused chondrocyte populations to senesce prematurely, before extensive telomere erosion occurred. Mechanical shear stress applied to cartilage explants considerably increased the production of oxidants. These observations support the hypothesis that senescence accounts for age-related decline in chondrocyte function and indicate that mechanically induced oxidative damage plays a role in this process. This suggests that new efforts to prevent the development and progression of osteoarthritis should include strategies that slow the progression of chondrocyte senescence or replace senescent cells.

Direct quantification of the rupture force of single hyaluronan/hyaluronan binding protein bonds

The non-covalent bond between aggrecan and hyaluronan is critical for maintaining the normal structure and function of the extracellular matrix in articular cartilage. The failure of this bond can cause the loss of aggrecan and destruction of the extracellular matrix of articular cartilage. In this study, the rupture force of the single bond between hyaluronan and hyaluronan binding protein - the complex of the hyaluronan binding region of aggrecan and link protein - was directly measured with a nanomechanical testing system as 40+/-11 pN. The results were compared to a theoretical prediction based on a smart version of the Monte Carlo simulation.

Direct measurements of the compressive properties of single proteoglycan aggregates

Proteoglycan aggregate is a major component of the extracellular matrix in articular cartilage and is considered to be responsible for the resistance to compression of this tissue. The reduced stiffness of articular cartilage due to the loss of proteoglycan aggregate has been reported in osteoarthritis. In order to understand the mechanical properties of extracellular matrix in articular cartilage at molecular level, the compressive properties of 36 single molecules of proteoglycan aggregate were directly measured using a laser tweezers/interferometer system. The proteoglycan aggregates showed resistance when compressed to approximately 30% of their contour length. The stiffness of proteoglycan aggregates increased non-linearly from 2.6+/-3.8 pN/microm (compressed to 30-35% of their contour length) to 115.5+/-30.9 pN/microm (compressed to 2.5-5% of their contour length).

A method for testing compressive properties of single proteoglycan aggregates

This study presents a method for direct measurement of the compressive properties of single molecules of proteoglycan aggregate using a state-of-the-art laser tweezers/interferometer system previously developed to test the tensile properties of single molecules. A typical molecule of proteoglycan aggregate showed a highly non-linear resistance to compression after being compressed to about 25% of its original molecule length.

The role of chondrocyte senescence in the pathogenesis of osteoarthritis and in limiting cartilage repair

BACKGROUND

With increasing age, the prevalence of osteoarthritis increases and the efficacy of articular cartilage repair decreases. As chondrocytes age, they synthesize smaller, less uniform aggrecan molecules and less functional link proteins, their mitotic and synthetic activity decline, and their responsiveness to anabolic mechanical stimuli and growth factors decreases. These observations led us to hypothesize that progressive cell senescence decreases the ability of chondrocytes to maintain and to restore articular cartilage.

METHODS

To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes from twenty-seven donors ranging in age from one to eighty-seven years. We also assessed mitochondrial DNA, membrane potential, and numerical density. To determine if chondrocyte age changes are reversible, we transfected human articular cartilage chondrocytes with the human telomerase gene (hTERT) and human papilloma virus oncogenes (E6 and E7).

RESULTS

Beta-galactosidase expression increased with age (r = 0.84, p = 0.0001), while mitotic activity and telomere length declined (r = -0.77, p = 0.001 and r = -0.71, p = 0.0004, respectively). Decreasing telomere length was closely correlated with increasing expression of beta-galactosidase and decreasing mitotic activity. As the number of population doublings increased, mitochondrial DNA was degraded, mitochondrial membrane potential was lost, and the number of mitochondria per cell declined. Transfection of human articular cartilage chondrocytes from a forty-seven-year-old donor with hTERT and human papilloma virus proto-oncogenes E6 and E7 created a cell line that has completed more than 300 population doublings as compared with an upper limit of twenty-five population doublings for normal cells. Telomere length increased in cells transduced with hTERT.

CONCLUSIONS

These findings help to explain the previously reported age-related declines in chondrocyte synthetic activity, mitotic activity, and responsiveness to anabolic cytokines and mechanical stimuli. They also suggest that in vivo chondrocyte senescence contributes to the age-related increase in the prevalence of osteoarthritis and decrease in the efficacy of cartilage repair. The creation of immortal cells with increased telomere length suggests that the progression of human chondrocytes toward senescence is not inevitable.

Fibrocartilage in the transverse ligament of the human atlas

STUDY DESIGN

Immunohistochemical investigation.

OBJECTIVE

To determine whether molecules typical of articular cartilage are present in the transverse ligament and whether the ligament may be a target for an autoimmune response in rheumatoid arthritis.

SUMMARY OF BACKGROUND DATA

In chronic rheumatoid arthritis there is often a marked instability of the atlantoaxial complex, and the transverse ligament can show degenerative changes that compromise its mechanical function. In some rheumatoid patients there can be an autoimmune response to cartilage link protein, aggrecan, and Type II collagen.

METHODS

Transverse ligaments were removed from 13 cadavers and fixed in 90% methanol. Cryosections were immunolabeled with antibodies against proteoglycans (aggrecan, link protein, and versican), glycosaminoglycans (chondroitin-4-sulfate, chondroitin-6-sulfate, dermatan sulfate, and keratan sulfate), and collagens (Types I, II, III, and VI).

RESULTS

Labeling for aggrecan and link protein was characteristic of the fibrocartilages, but versican was only detected in the fibrous regions. Equally, Types I, III, and VI collagens and keratan, dermatan, and chondroitin-4-sulfates were found throughout the ligament, but labeling for Type II collagen and chondroitin-6-sulfate was restricted to the fibrocartilages.

CONCLUSION

The presence of molecules typical of articular cartilage (aggrecan, link protein, and Type II collagen) in the transverse ligament explains why it can be a target for destruction in rheumatoid arthritis and also suggests that it is subject to constant compression against the dens rather than only at the extremes of movement.

Effects of interleukin-1beta and tumor necrosis factor-alpha on expression of matrix-related genes by cultured equine articular chondrocytes

OBJECTIVE

To determine the effects of interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) on expression and regulation of several matrix-related genes by equine articular chondrocytes.

SAMPLE POPULATION

Articular cartilage harvested from grossly normal joints of 8 foals, 6 yearling horses, and 8 adult horses.

PROCEDURE

Chondrocytes maintained in suspension cultures were treated with various doses of human recombinant IL-1beta or TNF-alpha. Northern blots of total RNA from untreated and treated chondrocytes were probed with equine complementary DNA (cDNA) probes for cartilage matrix-related genes. Incorporation of 35S-sulfate, fluorography of 14C-proline labeled medium, zymography, and western blotting were used to confirm effects on protein synthesis.

RESULTS

IL-1beta and TNF-alpha increased steady-state amounts of mRNA of matrix metalloproteinases 1, 3, and 13 by up to 100-fold. Amount of mRNA of tissue inhibitor of metalloproteinase-1 also increased but to a lesser extent (1.5- to 2-fold). Amounts of mRNA of type-II collagen and link protein were consistently decreased in a dose-dependent manner. Amount of aggrecan mRNA was decreased slightly; amounts of biglycan and decorin mRNA were minimally affected.

CONCLUSIONS AND CLINICAL RELEVANCE

Treatment of cultured equine chondrocytes with IL-1beta or TNF-alpha resulted in marked alterations in expression of various matrix and matrix-related genes consistent with the implicated involvement of these genes in arthritis. Expression of matrix metalloproteinases was increased far more than expression of their putative endogenous inhibitor. Results support the suggestion that IL-1beta and TNF-alpha play a role in the degradation of articular cartilage in arthritis.

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.

Blood-induced joint damage: a human in vitro study

OBJECTIVE

To investigate mechanisms underlying cartilage damage caused by brief exposure of cartilage to blood, such as that occurring during intraarticular bleeding.

METHODS

Human articular cartilage was cultured for 4 days in the presence of blood (components; 7.5-50% volume/volume). The synthesis of cartilage matrix, as determined by proteoglycan synthesis (incorporation of 35SO4(2-)), was measured directly after exposure and after a recovery period of 20 days, during which the cartilage was cultured in the absence of blood or blood components. The production of the cytokines interleukin-1 (IL-1) and tumor necrosis factor a (TNFalpha), which have a destructive effect on cartilage, was determined by enzyme-linked immunosorbent assay, and the viability of chondrocytes was determined by measuring lactate dehydrogenase release and with electron microscopy. The involvement of oxygen metabolites was evaluated by using N-acetylcysteine.

RESULTS

Brief exposure to blood resulted in dose-dependent inhibition of proteoglycan synthesis. The combination of mononuclear cells and red blood cells was responsible for this effect. The effect was irreversible, independent of IL-1 and TNFalpha production, and was accompanied by chondrocyte death. These effects were partially prevented by N-acetylcysteine.

CONCLUSION

Brief exposure of cartilage to blood, as occurs after a single episode or a limited number of bleeding episodes, results in lasting cartilage damage in vitro, in which cytotoxic oxygen metabolites play a role.

Aggrecan and link protein affect cell adhesion to culture plates and to type II collagen

Cartilage is a hypocellular tissue in which a balance of matrix molecules, especially aggrecan and link protein, play a critical role in maintaining structural integrity. To study the role of aggrecan and link protein in mediating cell activities, we have stably expressed them in NIH/3T3 fibroblasts and observed the effect on cell-substratum interactions. Overexpression of either protein destabilized the cell-substratum interaction. However, when both were co-expressed, the interaction between cell and substratum was less impaired. Similar results were obtained on type II collagen-coated plates. The addition of exogenous gene products into fibroblast cell lines and chondrocyte culture had the same effect as expression of the genes. The addition of exogenous hyaluronan to the growth medium or treatment of cells with hyaluronidase also decreased cell adhesion, indicating that hyaluronan also plays a role in the cell-substratum adhesion. The presence of aggrecan seems to increase the amount of link protein on the cell surface. Chondrocytes expressing high concentrations of aggrecan and link protein were maintained within a matrix network and were able to survive in suspended culture. Imbalances in aggrecan or link protein concentrations, or degradation of hyaluronan, disrupted the network and caused the chondrocytes to aggregate or adhere to the plates.