Effects of compression on the loss of newly synthesized proteoglycans and proteins from cartilage explants

Effects of dynamic compressive loading on chondrocyte biosynthesis in self-assembling peptide scaffolds

Dynamic mechanical loading has been reported to affect chondrocyte biosynthesis in both cartilage explant and chondrocyte-seeded constructs. In this study, the effects of dynamic compression on chondrocyte-seeded peptide hydrogels were analyzed for extracellular matrix synthesis and retention over long-term culture. Initial studies were conducted with chondrocyte-seeded agarose hydrogels to explore the effects of various non-continuous loading protocols on chondrocyte biosynthesis. An optimized alternate day loading protocol was identified that increased proteoglycan (PG) synthesis over control cultures maintained in free-swelling conditions. When applied to chondrocyte-seeded peptide hydrogels, alternate day loading stimulated PG synthesis up to two-fold higher than that in free-swelling cultures. While dynamic compression also increased PG loss to the medium throughout the 39-day time course, total PG accumulation in the scaffold was significantly higher than in controls after 16 and 39 days of loading, resulting in an increase in the equilibrium and dynamic compressive stiffness of the constructs. Viable cell densities of dynamically compressed cultures differed from free-swelling controls by less than 20%, demonstrating that changes in PG synthesis were due to an increase in the average biosynthesis per viable cell. Protein synthesis was not greatly affected by loading, demonstrating that dynamic compression differentially regulated the synthesis of PGs. Taken together, these results demonstrate the potential of dynamic compression for stimulating PG synthesis and accumulation for applications to in vitro culture of tissue engineered constructs prior to implantation.

The spread of cell death from impact damaged cartilage: lack of evidence for the role of nitric oxide and caspases

Over 21 days in culture, cell death spreads, both radially and transversely, from loaded to surrounding cartilage. This spread was prevented by physical separation and separate culture post-impact.

OBJECTIVE

One aim was to determine if nitric oxide (NO) is the intercellular signal mediating cell death. Another aim was to clarify the nature of the cell death, whether caspase mediated apoptosis or necrosis.

DESIGN

Cyclic impacts were applied to the central 2 mm core of 4 mm canine articular cartilage discs. Post-impact culturing was for 21 days in the presence or absence of the iNOS inhibitor, L-NAME, or the broad-spectrum caspase inhibitor, Z-VAD FMK. Cell death was quantified using the TUNEL assay. Culture media were collected every 2 days for measurements of glycosaminoglycan (GAG) and NO release.

RESULTS

Cell death spread from the loaded core into the surrounding ring over 21 days in culture. Although L-NAME significantly reduced nitrite release into the culture media of both loaded and control cartilage, the spread of cell death was not prevented. Neither was the spread of cell death prevented by Z-VAD FMK.

CONCLUSIONS

These data indicate that NO is not acting as an intercellular signalling factor in this in vitro system and that the cell death post-impact is not caspase mediated.

Growth of immature articular cartilage in vitro: correlated variation in tensile biomechanical and collagen network properties

Articular cartilage biochemical composition and mechanical properties evolve during in utero and in vivo growth, with marked differences between fetus, newborn, and young adult. The objectives of this study were to test whether in vitro growth of bovine fetal and newborn calf articular cartilage explants resulted in changes in biochemical and tensile properties during up to 6 weeks of free-swelling culture in serum-supplemented medium. During this culture period, both fetal and calf cartilage grew markedly in size, increasing in dry and wet mass by 150-270%. This was due in part to increases in sulfated glycosaminoglycan (+248%), collagen (+96%), and pyridinoline cross-link (+133%). This was accompanied by an increase in water content so that the concentration of matrix components decreased, despite the overall net increase in mass. The ratio of pyridinoline cross-link to collagen remained low and characteristic of immature tissue. The equilibrium and dynamic tensile moduli and strength of both fetal and calf cartilage decreased during the culture period. The biochemical and biomechanical properties of the cartilage explants were correlated, such that the low values of modulus and strength were associated with low concentrations of collagen and pyridinoline. Thus, the tested culture conditions supported growth and maintenance cartilage in an immature state, but did not induce biomechanical or collagen network maturation.

Regulation of gelatinases expression by cytokines, endotoxin, and pharmacological agents in the human osteoarthritic knee

We examined the amount of gelatinases (matrix metalloproteinase-2 and -9 [MMP-2 and MMP-9] in a series of chondral, meniscal, and synovial cultures of early osteoarthritis (OA) after treatment with or without catabolic cytokines. These included interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS), and pharmacological agents, including plasmin/serine proteinase antagonist aprotinin, protein synthesis inhibitor cycloheximide, and protein kinase C (PKC) inhibitors staurosporine, H7, and Gö6976 for investigation of their effects on MMP-2 and -9 production in OA. Gelatin zymography revealed that IL-alpha, TNF-alpha, and LPS could elevate MMP-2 secretion in all tissue cultures and also increase MMP-9 production in all synovial and some meniscal cultures. In contrast, aprotinin, cycloheximide, staurosporine, H7, and Gö6976 could suppress MMP-2 secretion in all tissue cultures and also decrease MMP-9 production in all synovial and some meniscal cultures. Our data indicate that catabolic cytokines and LPS may promote tissue destruction and disintegration of extracellular matrix in early OA. Agents that target on the PKC pathway, plasmin/serine proteinase or protein synthesis for MMP-2 and -9 in early OA may inhibit the production of MMPs. These findings might contribute to the design of more efficacious therapies.

Time, stress, and location dependent chondrocyte death and collagen damage in cyclically loaded articular cartilage

We investigated the effect of light (0.1 MPa), moderate (1 MPa) or heavy (5 MPa) cyclical stresses applied continuously or intermittently for 0 to 72 h on cell death and collagen damage in adult bovine cartilage explants. No increase in cell death was observed in the cartilage loaded with a continuous cyclic stress at 0.1 MPa for up to 72 h. Cell death occurred in the uppermost superficial tangential zone (STZ) of explants after loading for 1 h at 1 MPa, and reached a maximum depth of 61+/-23 micro m by 6 h (at the rate of 9+/-6 micro m/h). At 5 MPa, cell death occurred in the STZ after as little as 1 min (30 cycles) of loading, and reached a maximum depth of 70+/-2 micro m by 60 min (47+/-8 micro m/h). When an intermittent (with 2 s on, 2 s off) stress of 5 MPa was applied, cell death appeared in the STZ after 2 min (30 cycles) and increased to a depth of 63+/-2 micro m at 60 min (45+/-11 micro m/h). No significant differences were observed between the continuous and intermittent loading conditions. Both collagenase-cleaved and denatured collagen fibers were found in the STZ of explants loaded at 1 and 5 MPa. We concluded that load-induced cell death depends on load duration and magnitude, and that the chondrocytes in the STZ are more vulnerable to load-induced injury than those in the middle and deep zones.

Combined effects of dynamic tissue shear deformation and insulin-like growth factor I on chondrocyte biosynthesis in cartilage explants

Biophysical forces and biochemical factors play crucial roles in the maintenance of the integrity of articular cartilage. In this study, we explored the effect of dynamic tissue shear deformation and insulin-like growth factor I (IGF-I) on matrix synthesis by chondrocytes within native cartilage explants. Dynamic tissue shear in the range of 0.5-6% strain amplitude at 0.1 Hz was applied to cartilage explants cultured in serum-free medium. Dynamic tissue shear above 1.5% strain amplitude significantly stimulated protein and proteoglycan synthesis, by maximum values of 35 and 25%, respectively, over statically held control specimens. In the absence of tissue shear, IGF-I augmented protein and proteoglycan synthesis up to twofold at IGF-I concentrations in the range of 100-300 ng/ml. When tissue shear and IGF-I stimuli were combined, matrix biosynthesis levels were significantly higher than the maximal effect caused by either stimulus alone. However, there was no significant interaction between tissue shear and IGF-I as determined by two-way ANOVA. We then quantified the effect of dynamic tissue shear on the transport of IGF-I into and within cartilage explants. [125I]IGF-I was added to the medium, and the levels of intratissue [125I]IGF-I were directly measured as a function of time over 48 h in the presence and absence of continuous dynamic shear strain. Dynamic shear did not alter the rate of uptake of [125I]IGF-I into the explants, suggesting that convective diffusion of [125I]IGF-I is negligible under the shear strain conditions used. This is in marked contrast to the enhancement of transport reported in response to uniaxial dynamic compression. Taken together, these data suggest that (1) the stimulatory effect of tissue shear is via mechanotransduction pathways and not by facilitated transport of biochemical factors and (2) chondrocytes may possess complementary signal transduction pathways for biophysical and biochemical factors leading to changes in metabolic activity.

Increasing strain and strain rate strengthen transient stiffness but weaken the response to subsequent compression for articular cartilage in unconfined compression

Strain amplitude and strain rate dependent nonlinear behavior and load-induced mechanical property alterations of full-thickness bovine articular cartilage attached to bone were investigated in unconfined compression. A sequence of test compressions of finite deformation (ranging from 0.9% to 34.5% nominal strain) was performed at strain rates ranging from approximately 0.053%/s to 5.8%/s. Peak and equilibrium loads were analyzed to determine strain amplitude and strain rate dependence of linear versus nonlinear responses. The test protocol was designed to reveal changes in mechanical properties due to these finite deformations by interspersing small-amplitude witness ramps of approximately 1.1% deformation and approximately 0.44%/s strain rate between the test ramps ("witness" meaning to assess any mechanical property changes). We found that peak loads displayed high nonlinearity, stiffening with both increasing compression amplitude and more so with increasing strain rate. The response to witness ramps suggested that mechanical weakening occurred when compression amplitude reached 1.9-2.9% strain and beyond, and that weakening was much more significant at higher strain rate. These findings delineate regimes of linear versus nonlinear behavior of cartilage, and indicate the types of loads which can cause mechanical property alterations. Biological implications of this study are that strain amplitude and strain rate dependent stiffening may be essential to bear physiological loads and to protect cells and matrix from mechanical damage. Structural changes reflected by mechanical weakening at small compression could also initiate remodeling or disease processes.

Proteoglycan metabolism and viability of articular cartilage explants as modulated by the frequency of intermittent loading

OBJECTIVE

This study was designed to systematically determine whether and to what extent the frequency of intermittent loading modulates the biosynthesis and release of proteoglycans (PGs), and to assess chondrocyte viability within mature bovine articular cartilage explants exposed to different loading patterns.

METHODS

Cultured full-thickness cartilage explants from the weight-bearing area of healthy bovine fetlock joints were exposed to intermittently applied, uniaxial cyclic loads by introducing a sinusoidal waveform of 0.1, 0.5 or 1.0Hz, frequency and a peak stress of 0.5MPa for a period of 6 days. The cyclic loads were applied for 5, 10 or 20s followed by a period of unloading lasting 10, 100 or 1000s. The incorporation of radiolabeled sulfate into glycosaminoglycans (GAGs) during the final 18h, the content of GAGs and DNA, the deformation of loaded explants as well as the viability of chondrocytes within the different zones of explants were determined.

RESULTS

PG synthesis and loss of endogenous PGs were non-linearly and independently regulated by the frequency of the chosen intermittent load, whereas the release of newly synthesized PGs remained unaffected. The viability of chondrocytes within the superficial zone decreased drastically under intermittent loading in a manner independent of the frequency applied.

CONCLUSIONS

Our results confirm the hypothesis that the frequency of intermittent loading is an important mechanical factor controlling the metabolic activities of chondrocytes. They also implicate that an initially healthy cartilage explant can be mechanically manipulated to generate an in vitro model of degenerative, osteoarthritic-like cartilage.

Proteoglycan degradation after injurious compression of bovine and human articular cartilage in vitro: interaction with exogenous cytokines

OBJECTIVE

Traumatic joint injury leads to an increased risk of osteoarthritis (OA), but the progression to OA is not well understood. We undertook this study to measure aspects of proteoglycan (PG) degradation after in vitro injurious mechanical compression, including up-regulation of enzymatic degradative expression and cytokine-stimulated degradation.

METHODS

Articular cartilage tissue explants were obtained from newborn bovine femoropatellar groove and from adult normal human donor knee and ankle tissue. Following injurious compression of the cartilage, matrix metalloproteinase 3 (MMP-3) and MMP-13 messenger RNA (mRNA) expression levels were measured by Northern analysis, and PG loss to the medium after cartilage injury was measured in the presence and absence of added exogenous cytokine (interleukin-1alpha [IL-1alpha] or tumor necrosis factor alpha [TNFalpha]).

RESULTS

During the first 24 hours after injury in bovine cartilage, MMP-3 mRNA levels increased 10-fold over the levels in control cartilage (n = 3 experiments), whereas MMP-13 mRNA levels were unchanged. PG loss was significantly increased after injury, but only by 2% of the total PG content and only for the first 3 days following injury. However, compared with injury alone or cytokine treatment alone, treatment of injured tissue with either 1 ng/ml IL-1alpha or 100 ng/ml TNFalpha caused marked increases in PG loss (35% and 54%, respectively, of the total cartilage PG content). These interactions between cytokine treatment and injury were statistically significant. In human knee cartilage, the interaction was also significant for both IL-1alpha and TNFalpha, although the magnitude of increase in PG loss was lower than that in bovine cartilage. In contrast, in human ankle cartilage, there was no significant interaction between injury and IL-1alpha.

CONCLUSION

The cytokines IL-1alpha and TNFalpha can cause a synergistic loss of PG from mechanically injured bovine and human cartilage. By attempting to incorporate interactions with other joint tissues that may be sources of cytokines, in vitro models of mechanical cartilage injury may explain aspects of the interactions between mechanical forces and degradative pathways which lead to OA progression.

Biosynthetic response of passaged chondrocytes in a type II collagen scaffold to mechanical compression

To investigate the potential utility of mechanical loading in articular cartilage tissue engineering, porous type II collagen scaffolds seeded with adult canine passaged chondrocytes were subjected to static and dynamic compressions of varying magnitudes (0-50% static strain) and durations (1-24 h), and at different times during culture (2-30 days postseeding). The effects of mechanical compression on the biosynthetic activity of the chondrocytes were evaluated by measuring the amount of (3)H-proline-labeled proteins and (35)S-sulfate-labeled proteoglycans that accumulated in the cell-scaffold construct and was released to the medium during the loading period. Similar to published results on loading of articular cartilage explants, static compression decreased protein and proteoglycan biosynthesis in a time- and dose-dependent manner (each p < 0.005), and selected dynamic compression protocols were able to increase rates of biosynthesis (p < 0.05). The main difference between the results seen for this tissue engineering system and cartilage explants was in the amount of newly synthesized matrix molecules that accumulated within the construct under dynamic loading, with less accumulating in the type II collagen scaffold. In summary, the general biosynthetic response of passaged chondrocytes in the porous type II collagen scaffolds is similar to that seen for chondrocytes in their native environment. Future work needs to be directed to modifications of the cell-seeded construct to allow for the capture of the newly synthesized matrix molecules by the scaffold.

Cyclic compression of cartilage/bone explants in vitro leads to physical weakening, mechanical breakdown of collagen and release of matrix fragments

Mechanical loading of articular cartilage can produce catabolic and anabolic changes in tissue metabolism. Most previous studies in this area have focussed on aggrecan. Little information concerning load-induced collagen modifications has been obtained. We have therefore conducted studies where mechanical loads are applied in vitro to full thickness cartilage explants retaining a thin layer of bone, in order to investigate mechanically induced collagen breakdown and consequent turnover, in addition to aggrecan changes and mechanical property alterations. Tissue explant disks were subjected to unconfined compression and either immediately frozen or kept in static culture for 10 days. Mechanical tests of the disks immediately prior to and just after the cyclic loading period were also performed. They showed a weakening of the collagen network and an increased hydraulic permeability due to the cyclic loading. Load-induced alterations of the extracellular matrix was then clearly evidenced by an increase in denatured collagen in the disks frozen immediately after loading compared to unloaded controls. Loaded disks maintained in culture for 10 additional days following cyclic loading no longer expressed this increase in denatured collagen suggesting that mechanically denatured collagen II had undergone a removal process which could represent turnover or repair, or the beginning of progressive degradation. Indeed matrix fragments of collagen II and glycosaminoglycans were found to be released to post-loading culture medium in increased quantities compared to unloaded controls. Our data further demonstrates the ability of mechanical load of articular cartilage to modulate turnover and metabolism of collagen and proteoglycan in a complex and multifactorial manner that may be of particular significance in the pathogenesis of osteoarthritis and in the development of pharmacological agents to modulate its progression.

Perfusion increases cell content and matrix synthesis in chondrocyte three-dimensional cultures

This work examines the effect of perfusion on the cell content and sulfated glycosaminoglycan synthesis of ovine articular chondrocytes cultured on polyglycolic acid (PGA) scaffolds. Ovine chondrocytes were seeded onto the scaffolds and cultured for up to 9 days. During this time the cells were subjected to perfusion at velocities of up to 170 microm/s. The samples were radiolabeled with (35)SO(4) to quantify the overall synthesis of sulfated glycosaminoglycans (S-GAGs) and the retention of S-GAGs in the construct. The constructs were also analyzed for DNA as a measure of cellular content. Constructs subjected to perfusion during culture had significantly higher DNA contents than those cultured statically. Matrix metabolism was also modulated by perfusion, with this modulation depending on culture duration. Nine days of continuous perfusion increased S-GAG synthesis and deposition by approximately 40% when compared with static controls. However, perfusion at early time points (during the initial 3-day culture period) suppressed the synthesis and retention of S-GAGs when compared with controls. This work demonstrates the effects of perfusion on cartilage growth in vitro, illustrating the use of perfusion to modulate the growth of tissue-engineered cartilage constructs, and potentially enhance tissue growth in vitro.

Self-assembling peptide hydrogel fosters chondrocyte extracellular matrix production and cell division: implications for cartilage tissue repair

Emerging medical technologies for effective and lasting repair of articular cartilage include delivery of cells or cell-seeded scaffolds to a defect site to initiate de novo tissue regeneration. Biocompatible scaffolds assist in providing a template for cell distribution and extracellular matrix (ECM) accumulation in a three-dimensional geometry. A major challenge in choosing an appropriate scaffold for cartilage repair is the identification of a material that can simultaneously stimulate high rates of cell division and high rates of cell synthesis of phenotypically specific ECM macromolecules until repair evolves into steady-state tissue maintenance. We have devised a self-assembling peptide hydrogel scaffold for cartilage repair and developed a method to encapsulate chondrocytes within the peptide hydrogel. During 4 weeks of culture in vitro, chondrocytes seeded within the peptide hydrogel retained their morphology and developed a cartilage-like ECM rich in proteoglycans and type II collagen, indicative of a stable chondrocyte phenotype. Time-dependent accumulation of this ECM was paralleled by increases in material stiffness, indicative of deposition of mechanically functional neo-tissue. Taken together, these results demonstrate the potential of a self-assembling peptide hydrogel as a scaffold for the synthesis and accumulation of a true cartilage-like ECM within a three-dimensional cell culture for cartilage tissue repair.

Age-related changes in the composition and mechanical properties of human nasal cartilage

Nasal cartilage is widely used in reconstructive surgery for the replacement of soft tissue defects and nasal reconstruction procedures. The ability to shape harvested tissue and the performance in the transplant site are related to the mechanical properties of nasal cartilage. Several studies have documented changes in composition and mechanical properties of other cartilages with age, but little is known about these processes in nasal cartilage. In this study, 45 human nasal septum specimens were gathered from patients 15-60 years of age after reconstructive surgery. Samples were cut to 6 mm in diameter and tested in confined compression to determine equilibrium modulus and hydraulic permeability and analyzed for glycosaminoglycan and hydroxyproline content. Equilibrium modulus decreased significantly with increasing donor age (P<0.01) while hydraulic permeability increased significantly (P<0.02). Glycosaminoglycan (GAG) content decreased significantly with age (P<0.05), while hydroxyproline content showed a slight, but not significant, increase with age (P>0.2). These trends are qualitatively similar to those observed in articular cartilage, suggesting the existence of a systemic process of cartilage degradation that is independent of mechanical loading. Further, the relationships between biochemical composition and mechanical properties were age-dependent, with cartilage from patients less than 30 years of age showing greater dependence of equilibrium modulus and hydraulic permeability on GAG and hydroxyproline content. This suggests that changes in matrix organization may accompany changes in tissue composition.

Static and dynamic compression modulate matrix metabolism in tissue engineered cartilage

Static and dynamic compression are known to modulate the metabolism of articular cartilage. The present study focused on determining the effects of compressive loading on the metabolism of sulfated glycosaminoglycans (S-GAG) and protein in tissue engineered cartilage constructs. Cartilage constructs were subjected to static or dynamic compression for 24 h and radiolabeled with 35SO4 and 3H-proline to assess the total synthesis and percentage retention of S-GAG and total protein, respectively. Static compression at an amplitude of 50% suppressed the synthesis of both S-GAG and protein by 35% and 57%, respectively. Dynamic compression at an amplitude of 5% had stimulatory effects on synthesis that were dependent on the static offset compression amplitude (10% or 50%) and dynamic compression frequency (0.001 or 0.1 Hz). Thus, tissue engineered cartilage demonstrated the ability to respond to mechanical loading in a manner similar to that observed with articular cartilage. Mechanical loading may therefore potentially be used to modulate the growth of cartilaginous tissues in vitrd, potentially facilitating the culture of functional cartilage tissues suitable for implantation.

Integrative articular cartilage repair: dependence on developmental stage and collagen metabolism

OBJECTIVES

The objectives of this research were to determine whether the integrative repair of bovine cartilage explants was dependent on developmental stage, and whether observed differences in integration with developmental stage were related to deposition of newly synthesized collagen and lysyl oxidase-mediated collagen cross-linking.

METHODS

Pairs of fetal, newborn calf, and adult bovine cartilage blocks were cultured in partial apposition for 2 weeks in medium supplemented with serum, ascorbate, and [3H]proline. Following culture, mechanical integration between apposed cartilage blocks was assessed by measuring adhesive strength in a single-lap shear configuration. Formation and stabilization of newly synthesized protein and collagen was investigated by determination of [3H]proline and [3H]hydroxyproline in tissue digests and guanidine extracts.

RESULTS

Calf cartilage exhibited a relatively high integrative repair phenotype, achieving an adhesive strength that was three--four-fold that of adult or fetal specimens. The low and high integrative repair phenotypes appeared related in part to different levels of collagen biosynthesis, which was approximately four--five-fold higher in calf cartilage samples than in the adult. However, fetal cartilage also exhibited a high level of biosynthesis. The different integrative repair phenotypes were not associated with marked differences in the kinetics of chemical stabilization of newly synthesized collagen, as the proportion of incorporated [3H]proline and newly-formed [3H]hydroxyproline that was resistant to extraction by 4M guanidine-HCl following culture was similar for cartilage from all developmental stages. Integration of calf cartilage appeared to depend on lysyl oxidase-mediated collagen cross-link formation, since inclusion of beta-aminopropionitrile (BAPN) in the culture medium completely eliminated development of adhesive strength. BAPN treatment also increased the percentage of newly synthesized protein in the guanidine extracts from 10% to 36% of the total, and that of newly synthesized collagen from 2% to 20%, while having only slight inhibitory effects on overall protein and collagen biosynthesis.

CONCLUSION

The finding that cartilage exhibits enhanced integrative repair at a certain developmental stage suggests that it may ultimately be possible to enhance repair when needed in clinical situations.

Streaming potentials maps are spatially resolved indicators of amplitude, frequency and ionic strength dependant responses of articular cartilage to load

Streaming potential distributions were measured on the surface of articular cartilage in uniaxial unconfined compression using a linear array of microelectrodes. Potential profiles were obtained for sinusoidal and ramp/stress-relaxation displacements and exhibited dependencies on radial position, sinusoidal amplitude and frequency, time during stress relaxation, and on ionic strength. The measurements agreed with trends predicted by biphasic and related models. In particular, the absolute potential amplitude was maximal at the disk center, as was the predicted fluid pressure and the potential gradient (the electric field) was seen to be maximal at the disk periphery, as was the predicted fluid velocity. We also observed a similarity between non-linear behavior of streaming potential amplitude and load amplitude with respect to sinusoidal displacement amplitude. Taken together, these results support many of the phenomena concerning relative fluid-solid movement and fluid pressurization predicted by biphasic and related models, and they indicate the general utility of spatially resolved measurements of streaming potentials for the investigation of electromechanical phenomena in tissues. For example, these streaming potential maps could be used to non-destructively diagnose cartilage extracellular matrix composition and function, as well as to quantify spatially and temporally varying physical signals in cartilage that can induce cellular and extracellular biological responses to load.

Cartilage tissue remodeling in response to mechanical forces

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

Cartilage viability after repetitive loading: a preliminary report

OBJECTIVE

To assess matrix changes and chondrocyte viability during static and continuous repetitive mechanical loading in mature bovine articular cartilage explants.

METHODS

Cartilage explants were continuously loaded either statically or cyclically (0.5 Hz) for 1-72 h (max. stress 1 megapascal). Cell death was assessed using fluorescent probes and detection of DNA strand breakage characteristic of apoptosis. Cell morphology and matrix integrity were evaluated using histology and transmission electron microscopy.

RESULTS

Repetitive loading of articular cartilage at physiological levels of stress (1 megapascal) was found to be harmful to only the chondrocytes in the superficial tangential zone (STZ) and depended on the characteristics (static vs cyclic) and duration (1-72 h) of the applied load. The chondrocytes in the middle and deep zone remained viable at all times. Static loads caused cell death at an early time (3 h) as compared with cyclic loads (sinusoidal, 0.5 cycles per s for 6 h). The amount and extent of cell death peaked at 6 h of cyclic loading, and did not change in subsequent experiments run for longer periods of time (up to 72 h). There was no indication of fragmented nuclear DNA but there was evidence of injurious cell death (necrosis) by electron microscopy. Morphological analysis of cartilage repetitively loaded for 24 h showed matrix damage only in the uppermost superficial layer at the articular surface, reminiscent of the early stages of osteoarthritis.

CONCLUSIONS

Cell death in mature cartilage explants occurred after 6 hours of continuous repetitive load or 3 h of static load. Cell death was directly related to the mechanical load, as control (free-swelling) explants remained viable at all times. The excessive, repetitive loading conditions imposed are not physiological, and demonstrate the deleterious effects of mechanical overload resulting in morphological and cellular damage similar to that seen in degenerative joint disease.

Up-regulation of matrix metalloproteinase expression and activation following cyclical compressive loading of articular cartilage in vitro

Osteoarthritis (OA) results in articular cartilage degeneration and subchondral bone remodeling. Excessive or abnormal loading of the joint may contribute to matrix destruction by creating an imbalance between proteinases and their inhibitors. This study investigates whether cyclical loading regulates expression and/or activation of metalloproteinases 2 and 9 (MMPs) in articular cartilage explants. Gelatin zymography, reverse zymography, and MMP activity assays of mechanically loaded bovine cartilage explants (0.5 MPa, 1 Hz, 3 h) showed increased expression and activation of MMPs 2 and 9, whereas expression of the tissue inhibitors of MMPs was unaffected. This shows, for the first time that mechanical loading can influence tissue homeostasis generating an imbalance of proteinases and their inhibitors inducing turnover and/or catabolic events in cartilage.

Biosynthetic response and mechanical properties of articular cartilage after injurious compression

Traumatic joint injury is known to produce osteoarthritic degeneration of articular cartilage. To study the effects of injurious compression on the degradation and repair of cartilage in vitro, we developed a model that allows strain and strain rate-controlled loading of cartilage explants. The influence of strain rate on both cartilage matrix biosynthesis and mechanical properties was assessed after single injurious compressions. Loading with a strain rate of 0.01 s(-1) to a final strain of 50% resulted in no measured effect on the cells or on the extracellular matrix, although peak stresses reached levels of about 12 MPa. However, compression with strain rates of 0.1 and 1 s(-1) caused peak stresses of approximately 18 and 24 MPa, respectively, and resulted in significant decreases in both proteoglycan and total protein biosynthesis. The mechanical properties of the explants (compressive and shear stiffness) were also reduced with increasing strain rate. Additionally, cell viability decreased with increasing strain rate, and the remaining viable cells lost their ability to exhibit an increase in biosynthesis in response to low-amplitude dynamic mechanical stimulation. This latter decrease in reparative response was most dramatic in the tissue compressed at the highest strain rates. We conclude that strain rate (like peak stress or strain) is an important parameter in defining mechanical injury, and that cartilage injuriously compressed at high strain rates can lose its characteristic anabolic response to low-amplitude cyclic mechanical loading.

Human chondrocyte apoptosis in response to mechanical injury

OBJECTIVE

The effect of mechanical injury on chondrocyte viability and matrix degradation was studied. It was proposed that mechanical injury to human cartilage explants results in chondrocyte apoptosis with associated loss of glycosaminoglycans.

DESIGN

Full thickness human cartilage explants, 5 mm in diameter were subjected to a single static mechanical stress of 14 MPa for 500 ms under radially unconfined compression. Glycosaminoglycan (GAG) release and percentage of cells undergoing apoptosis were measured at 96 h after injury. To establish the time course of apoptosis, explants were subjected to 30% strain and cultured for varying intervals up to 7 days after injury. A group of loaded explants were also treated with the broad spectrum caspase inhibitor z-Vad.fmk after injury.

RESULTS

Internucleosomal DNA fragmentation as one indicator of apoptosis was observed in 34% (S.D.+/-11) of chondrocytes at 96 h in response to mechanical loading at 14 MPa, compared to 4% (S.D.+/-2) in the non-loaded explants. Evidence for cell death induction via apoptosis was also obtained by electron microscopy and caspase cleavage of cytokeratin. GAG release was also higher for the loaded explants, mean 1.9% (S.D.+/-0.14) of total GAG content, compared to control explants, mean 0.8% (S.D.+/-0.28). The percentage of apoptotic cells also correlated with the level of GAG release into the culture media. The percentage of apoptotic chondrocytes demonstrated a progressive increase from 6 h to 7 days post-injury. When loaded explants were cultured in z-Vad.fmk after injury, a 50% reduction in apoptosis rates was seen.

CONCLUSIONS

These results demonstrate that mechanical injury induces chondrocyte apoptosis and release of GAG from the matrix. The time course suggests that a therapeutic window may exist where apoptosis could be inhibited. This potentially identifies a new approach to chondroprotection.

Tissue shear deformation stimulates proteoglycan and protein biosynthesis in bovine cartilage explants

Chondrocytes are known to sense and respond to mechanical and physicochemical stimuli by multiple regulatory pathways, including upstream signaling, transcription, translation, posttranslational modifications, and vesicular transport. Due to the complexity of identifying the biophysical phenomena that occur during cartilage loading in vivo, the regulatory mechanisms that govern chondrocyte mechanotransduction are not fully understood. Recent studies have shown that fluid flow during dynamic compression of cartilage explants can stimulate proteoglycan and protein synthesis. In this study, we examined the effect of deformations of cell and extracellular matrix on chondrocyte biosynthesis. We used tissue shear loading, since tissue shear causes little volumetric deformation and can thereby decouple fluid flow from cell and matrix deformation. Shear loading was applied over a wide range of frequencies, 0.01-1.0 Hz, using 1-3% sinusoidal shear strain amplitudes, and the resulting proteoglycan and protein syntheses were measured using radiolabel incorporation. In addition, quantitative autoradiography was used to investigate spatial variations in matrix biosynthesis and to correlate these variations with the spatial profiles of biophysical stimuli. Our data show that tissue shear loading at 1-3% strain amplitude stimulated the synthesis of protein by approximately 50% and proteoglycans by approximately 25% at frequencies between 0.01 and 1.0 Hz. The relatively uniform patterns of biosynthesis in the radial and vertical directions within cylindrical explants revealed by autoradiography suggest that the stimulatory effect was associated with the relatively uniform deformation caused by simple shear loading. These results suggest that chondrocytes can respond to tissue shear stress-initiated pathways for the production of collagen and proteoglycan, which include deformation of cells and pericellular matrix, even in the absence of macroscopic tissue-level fluid flow.

Integrative cartilage repair: adhesive strength is correlated with collagen deposition

Procedures to repair focal articular cartilage defects often result in poor integration between the host cartilage and the graft tissue, and this may be related to the lack of matrix deposition and the death of chondrocytes near a cut cartilage surface. The objective of this study was to determine if cartilage repair was related to deposition of newly synthesized collagen. The mechanical integration that occurred between two live adult bovine cartilage blocks cultured in partial apposition for two weeks was correlated with [3H]proline incorporation, a measure of protein synthesis, of which more than 66% was accounted for by collagen. A similar level of mechanical integration occurred in sample pairs consisting of a live and killed cartilage block, and this adhesive strength was also correlated with [3H]proline deposition into both the live and the killed blocks. In these samples, the [3H]proline deposited into the killed cartilage appeared to originate from chondrocytes in the live cartilage, since live cells were not detected in the killed cartilage block by either viability staining or [35S]sulfate incorporation. These results suggest a mechanism of integrative cartilage repair in which live chondrocytes within cartilage secrete matrix molecules that are components of a collagen network, and subsequent deposition of these molecules near the repair interface contributes to functional integration.

Impact of mechanical trauma on matrix and cells

Posttraumatic arthritis is one of the most common causes of secondary osteoarthritis. The contribution of cell death to matrix degradation has not been characterized fully. The current study was designed to determine the effect of mechanical injury on chondrocyte viability and matrix degradation. Full-thickness bovine and human cartilage explants, 5 mm in diameter were subjected to mechanical loads representative of traumatic joint injury. Glycosaminoglycan release and percent apoptotic cells were measured. Unilateral patellas in eight anesthetized rabbits were subjected to an impact load. Rabbits were euthanized at 96 hours after injury and patellar cartilage was harvested for analysis. The effect of a pan-caspase inhibitor, z-VAD.fmk [benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethylketone] in preventing chondrocyte apoptosis in human articular cartilage explants was determined. A significant increase in the number of apoptotic cells was observed in response to mechanical loading. The mean in vivo apoptotic rates were 1% in control rabbits and 15% in impacted patellas. Caspase inhibition reduced chondrocyte apoptosis from 34% to 25% after mechanical injury and was associated with reduction in glycosaminoglycan release. Mechanical injury induces chondrocyte apoptosis that is sensitive to pharmacologic inhibition. This identifies a new approach to limit traumatic cartilage injury and the subsequent development of secondary osteoarthritis.

Chondrocyte necrosis and apoptosis in impact damaged articular cartilage

A decrease in chondrocyte numbers is one characteristic of osteoarthritic cartilage. This decrease may be the result of apoptosis or other forms of cell death induced by mechanical damage. Furthermore, cell death may contribute to the structural and metabolic changes found in osteoarthritic cartilage. Therefore, we investigated cell viability and the mode of cell death in cartilage subjected to an increasing severity of impact loads expected to cause compositional damage and osteoarthritic-like metabolic alterations. Canine cartilage explants were subjected to cyclic indentation impacts of 5 megapascals at 0.3 Hz for 0, 2, 20, and 120 min and then kept in culture for 2, 4, 48, and 144 h. Cell death was assessed by the TUNEL assay and by uptake of propidium iodide. Viable cells were detected by the ability to metabolize fluorescein diacetate. Nuclear morphology and ultrastructure of the cell were examined using Hoechst 33342 fluorescent staining and transmission electron microscopy (TEM). As controls for necrosis and apoptosis, cartilage was, respectively, frozen and thawed or incubated with mitomycin-C, an apoptosis inducer. In cartilage that had been loaded for 2 h, 32% of the chondrocytes in the loaded core took up propidium iodide within 2 h after loading. Most of these were in the middle to superficial zones and reflected leaky cell membranes usually characteristic of necrosis. Less than 1% of these chondrocytes were positive in the TUNEL assay after 4 h. After additional culture for 2 days, however, the proportion of chondrocytes which were positive in the TUNEL assay reached 73%. A dose dependent response to duration of loading was detected with the TUNEL assay at this time. The TUNEL assay was not specific for apoptosis since 92% of chondrocytes in freeze/thawed cartilage were TUNEL positive. However, some cells with apoptotic bodies and chromatin condensation characteristic of apoptosis were found in the transition zone between necrotic and normal chondrocytes, but not in the superficial and upper zones, in impact damaged cartilage. We concluded that in this study, necrosis occurred first, followed by apoptosis.

Matrix and cell injury due to sub-impact loading of adult bovine articular cartilage explants: effects of strain rate and peak stress

Mechanical overloading of cartilage has been implicated in the initiation and progression of osteoarthrosis. Our objectives were to identify threshold levels of strain rate and peak stress at which sub-impact loads could induce cartilage matrix damage and chondrocyte injury in bovine osteochondral explants and to explore relationships between matrix damage, spatial patterns of cell injury, and applied loads. Single sub-impact loads characterized by a constant strain rate between 3 x 10(-5) and 0.7 s(-1) to a peak stress between 3.5 and 14 MPa were applied, after which explants were maintained in culture for four days. At the higher strain rates, matrix mechanical failure (tissue cracks) and cell deactivation were most severe near the cartilage superficial zone and were associated with sustained increased release of proteoglycan from explants. In contrast, low strain rate loading was associated with cell deactivation in the absence of visible matrix damage. Furthermore, cell activity and proteoglycan synthesis were suppressed throughout the cartilage depth, but in a radially dependent manner with the most severe effects at the center of cylindrical explants. Results highlight spatial patterns of matrix damage and cell injury which depend upon the nature of injurious loading applied. These patterns of injury may also differ in terms of their long-term implications for progression of degradative disease and possibilities for cartilage repair.

An analysis of the effects of depth-dependent aggregate modulus on articular cartilage stress-relaxation behavior in compression

An accurate description of the mechanical environment around chondrocytes embedded within their dense extracellular matrix (ECM) is essential for the study of mechano-signal transduction mechanism(s) in explant experiments. New methods have been developed to determine the inhomogeneous strain distribution throughout the depth of the ECM during compression (Schinagl et al., 1996, Annals of Biomedical Engineering 24, 500-512; Schinagl et al 1997. Journal of Orthopaedics Research 15, 499-506) and the corresponding depth-dependent aggregate modulus distribution (Wang and Mow, 1998. Transactions of the Orthopaedics Research Society 23, 484; Chen and Sah, 1999. Transactions of the Orthopaedics Research Society 24, 635). These results provide the motivation for the current investigation to assess the influence of tissue inhomogeneity on the chondrocyte milieu in situ, e.g. stress, strain, fluid velocity and pressure fields within articular cartilage. To describe this inhomogeneity, we adopted the finite deformation biphasic constitutive law developed by Holmes and Mow (1990 Journal of Biomechanics 23, 1145-1156). Our calculations show that the mechanical environment inside an inhomogeneous tissue differs significantly from that inside a homogeneous tissue. Furthermore, our results indicate that the need to incorporate an inhomogeneous aggregate modulus. or an anisotropy, into the biphasic theory to describe articular cartilage depends largely on the motivation for the study.

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

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

Diffusion of paramagnetically labeled proteins in cartilage: enhancement of the 1-D NMR imaging technique

Quantifying the diffusive transport of large molecules in avascular cartilage tissue is important both for planning potential pharamacological treatments and for gaining insight into the molecular-scale structure of cartilage. In this work, the diffusion coefficients of gadolinium-DTPA and Gd-labeled versions of four proteins-lysozyme, trypsinogen, ovalbumin, and bovine serum albumin (BSA) with molecular weights of 14,300, 24,000, 45,000, and 67,000, respectively-have been measured in healthy and degraded calf cartilage. The experimental technique relies on the effect of the paramagnetic on the relaxation properties of the surrounding water, combined with the time course of a 1-dimensional spatial profile of the water signal in the cartilage sample. The enhanced technique presented here does not require a prior measurement of the relaxivity of the paramagnetic compound in the sample of interest. The data are expressed as the ratio of the diffusion coefficient of a compound in cartilage to its diffusion coefficient in water. For healthy cartilage, this ratio was 0.34 +/- 0.07 for Gd-DTPA, the smallest compound, and fell to 0.3 +/- 0.1 for Gd-lysozyme, 0.08 +/- 0.04 for Gd-trypsinogen, and 0.07 +/- 0.04 for Gd-ovalbumin. Gd-BSA did not appear to enter healthy cartilage tissue beyond a surface layer. After the cartilage had been degraded by 24-h trypsinization, these ratios were 0.60 +/- 0.03 for Gd-DTPA, 0.40 +/- 0.08 for Gd-lysozyme, 0.42 +/- 0.09 for Gd-trypsinogen, 0.16 +/- 0.14 for Gd-ovalbumin, and 0.11 +/- 0.05 for Gd-BSA. Thus, degradation of the cartilage led to increases in the diffusion coefficient of up to fivefold for the Gd-labeled proteins. These basic transport parameters yield insights on the nature of pore sizes and chemical-matrix interactions in the cartilage tissue and may prove diagnostically useful for identifying the degree and nature of damage to cartilage.

Chondrocyte extracellular matrix synthesis and turnover are influenced by static compression in a new alginate disk culture system

The goal of this study was to examine the effects of mechanical compression on chondrocyte biosynthesis of extracellular matrix (ECM) components during culture in a new alginate disk culture system. Specifically, we have examined chondrocyte biosynthesis rates, and the structure of aggrecan core protein species present in the cell-associated matrix (CM), in the further removed matrix (FRM) and in the surrounding culture medium. In this alginate disk culture system, chondrocytes can be subjected to mechanical deformations similar to those experienced in vivo. Our results show that over an 8-week culture period, chondrocytes synthesize a functional ECM and can respond to mechanical forces similarly to chondrocytes maintained in native cartilage. In the alginate disk system, static compression was shown to decrease and dynamic compression to increase synthesis of aggrecan of bovine chondrocytes. Western blot analysis of the core proteins of aggrecan molecules identified a number of different species that were present in different relative amounts in the CM, FRM, and medium. Over 21 days of culture, the predominant form of aggrecan found in the ECM was a full-length link-stabilized species. In addition, our data show that the application of 40 h of static compression caused an increase in the proportion of newly synthesized aggrecan molecules released into the medium. However, this was not accompanied by a significant change in the size and composition of aggrecan and aggrecan fragments in the different compartments, suggesting that mechanical compression did not alter the catabolic pathways. Together, these data show that chondrocyte function is maintained in an alginate disk culture system and that this culture system is a useful model to examine chondrocyte ECM assembly and some aspects of catabolism normally found in vivo.

The influence of strenuous exercise on collagen characteristics of articular cartilage in Thoroughbreds age 2 years

In order to assess the influence of strenuous exercise on collagen characteristics of articular cartilage, the response of the collagen network was studied in seven 2-year-old Thoroughbreds subjected to strenuous exercise compared to 7 nontrained individuals. After 13 weeks, the animals were subjected to euthanasia, fetlock joints of the forelimbs were scored macroscopically after Indian Ink staining, and articular cartilage from different locations of the articular surface of the proximal first phalanx was sampled and analysed for water content, collagen content, hydroxylysine content and amount of hydroxylysylpyridinoline (HP) crosslinks. Gross lesions were significantly more severe in the exercised than in the nonexercised group. In the control animals, the characteristic site-specific differences in collagen parameters were found as described earlier, but in the strenuously exercised animals this physiological biochemical heterogeneity had disappeared. In the exercised animals, an increase in water content and a sharp decrease in HP crosslinking was found that was correlated with the presence of wear lines. It is concluded that the strenuous exercise provoked significant alterations in the characteristics of the collagen network of the articular cartilage of the fetlock joint which were suggestive of microdamage and loosening of the collagen network. The collagen component of cartilage, in contrast to the proteoglycan component, is known to have a very limited capacity for repair and remodelling due to an extremely low turnover rate. Therefore, alterations within the articular collagen network might be expected to play an important role in the pathophysiology of degenerative joint disorders.

Injurious mechanical compression of bovine articular cartilage induces chondrocyte apoptosis

A bovine cartilage explant system was used to evaluate the effects of injurious compression on chondrocyte apoptosis and matrix biochemical and biomechanical properties within intact cartilage. Disks of newborn bovine articular cartilage were compressed in vitro to various peak stress levels and chondrocyte apoptotic cell death, tissue biomechanical properties, tissue swelling, glycosaminoglycan loss, and nitrite levels were quantified. Chondrocyte apoptosis occurred at peak stresses as low as 4.5 MPa and increased with peak stress in a dose-dependent manner. This increase in apoptosis was maximal by 24 h after the termination of the loading protocol. At high peak stresses (>20 MPa), greater than 50% of cells apoptosed. When measured in uniaxial confined compression, the equilibrium and dynamic stiffness of explants decreased with the severity of injurious load, although this trend was not significant until 24-MPa peak stress. In contrast, the equilibrium and dynamic stiffness measured in radially unconfined compression decreased significantly after injurious stresses of 12 and 7 MPa, respectively. Together, these results suggested that injurious compression caused a degradation of the collagen fibril network in the 7- to 12-MPa range. Consistent with this hypothesis, injurious compression caused a dose-dependent increase in tissue swelling, significant by 13-MPa peak stress. Glycosaminoglycans were also released from the cartilage in a dose-dependent manner, significant by 6- to 13-MPa peak stress. Nitrite levels were significantly increased above controls at 20-MPa peak stress. Together, these data suggest that injurious compression can stimulate cell death as well as a range of biomechanical and biochemical alterations to the matrix and, possibly, chondrocyte nitric oxide expression. Interestingly, chondrocyte programmed cell death appears to take place at stresses lower than those required to stimulate cartilage matrix degradation and biomechanical changes. While chondrocyte apoptosis may therefore be one of the earliest responses to tissue injury, it is currently unclear whether this initial cellular response subsequently drives cartilage matrix degradation and changes in the biomechanical properties of the tissue.

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

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

Functional tissue engineering of articular cartilage through dynamic loading of chondrocyte-seeded agarose gels

Due to its avascular nature, articular cartilage exhibits a very limited capacity to regenerate and to repair. Although much of the tissue-engineered cartilage in existence has been successful in mimicking the morphological and biochemical appearance of hyaline cartilage, it is generally mechanically inferior to the natural tissue. In this study, we tested the hypothesis that the application of dynamic deformational loading at physiological strain levels enhances chondrocyte matrix elaboration in cell-seeded agarose scaffolds to produce a more functional engineered tissue construct than in free swelling controls. A custom-designed bioreactor was used to load cell-seeded agarose disks dynamically in unconfined compression with a peak-to-peak compressive strain amplitude of 10 percent, at a frequency of 1 Hz, 3 x (1 hour on, 1 hour off)/day, 5 days/week for 4 weeks. Results demonstrated that dynamically loaded disks yielded a sixfold increase in the equilibrium aggregate modulus over free swelling controls after 28 days of loading (100 +/- 16 kPa versus 15 +/- 8 kPa, p < 0.0001). This represented a 21-fold increase over the equilibrium modulus of day 0 (4.8 +/- 2.3 kPa). Sulfated glycosaminoglycan content and hydroxyproline content was also found to be greater in dynamically loaded disks compared to free swelling controls at day 21 (p < 0.0001 and p = 0.002, respectively).

Proteoglycan synthesis in porcine nasal cartilage grafts following Nd:YAG (lambda = 1.32 microns) laser-mediated reshaping

Mechanically deformed morphologic cartilage grafts undergo temperature-dependent stress relaxation during sustained laser irradiation resulting in stable shape changes. In this study, porcine nasal septal cartilage specimens were evaluated for viability by measuring the incorporation of Na2(35)SO4 into proteoglycan (PTG) macromolecules in whole tissue culture following laser-mediated reshaping. Synthesis rates of PTG were determined by scintillation counting lyophilized specimens and normalizing these values by total protein content. Positive controls were established by inducing chondrocyte apoptosis using prolonged exposure to nitric oxide (NO). In chondrocytes, apoptosis induced using NO resulted in significantly lower PTG synthesis rates compared to untreated native specimens. Cartilage specimens were irradiated with light emitted from a Nd:YAG laser (25 W/cm2, lambda = 1.32 microns) while recording simultaneously radiometric surface temperature, internal stress and back-scattered light intensity from a probe laser. Each specimen received one, two or three sequential laser exposures. The duration of each exposure was determined from real-time measurements of characteristic changes in back-scattered light intensity that correlate with accelerated stress relaxation. A 5 min time interval between each laser exposures allowed the cartilage specimen to return to thermal equilibrium. Average PTG synthesis rates decreased with successive laser exposures, though these were always higher than baseline rates established for NO-treated tissues, suggesting that laser-mediated cartilage reshaping acutely does not eliminate the entire population of viable chondrocytes. The reduction in PTG synthesis is correlated with the time-temperature-dependent heating profile created during laser irradiation, supporting our hypothesis that careful monitoring of laser dosimetry is required to ensure chondrocyte viability.

Chondrocyte deformation within compressed agarose constructs at the cellular and sub-cellular levels

Mechanotransduction events in articular cartilage may be resolved into extracellular components followed by intracellular signalling events, which finally lead to altered cell response. Cell deformation is one of the former components, which has been examined using a model involving bovine chondrocytes seeded in agarose constructs. Viable fluorescent labels and confocal laser scanning microscopy were used to examine cellular and sub-cellular morphology. It was observed that cell size increased up to day 6 in culture, associated with an increase in the contents of proteoglycan and collagen. In addition, the organisation of the cytoskeleton components, described using a simple scoring scale, revealed temporal changes for actin fibres, microtubules and vimentin intermediate filaments. The constructs on day 1 were also subjected to unconfined compressive strains. A series of confocal scans through the centre of individual cells revealed a change from a spherical to an elliptical morphology. This was demonstrated by a change in diameter ratio, from a mean value of 1.00 at 0% strain to 0.60 at 25% strain. Using simple equations, the volume and surface areas were also estimated from the scans. Although the former revealed little change with increasing construct strain, surface area appeared to increase significantly. However further examination, using transmission electron microscopy to reveal fine ultrastructural detail at the cell periphery, suggest that this increase may be due to an unravelling of folds at the cell membrane. Cell deformation was associated with a decrease in the nuclear diameter, in the direction of the applied strain. The resulting nuclear strain in one direction increased in constructs compressed at later time points, although its values at all three assessment times were less than the corresponding values for cell strain. It is suggested that the nuclear behaviour may be a direct result of temporal changes observed in the organisation of the cytoskeleton. The study demonstrated that the chondrocyte-agarose model provides a useful system for the examination of compression events at both cellular and sub-cellular levels.

Influence of different exercise levels and age on the biochemical characteristics of immature equine articular cartilage

This study aimed to examine whether biochemical characteristics of juvenile articular cartilage are changing during the first year post partum and whether they can be influenced by exercise at young age. Water, glycosaminoglycan (GAG), DNA, total collagen, hydroxylysine and hydroxylysylpyridinoline (HP) content were measured in articular cartilage of 43 foals that were subdivided into 3 groups (n = 15, 14 and 14) which were subjected to different exercise regimens from one week after birth to age 5 months. At the age of 5 months all foals were weaned and 8 foals were selected randomly from each exercise group and subjected to euthanasia. The remaining foals (n = 19) were grouped and subjected to a similar exercise regimen for an additional 6 months. Differences were tested by student's t test (P<0.01). No effect of exercise on the water or DNA content was found. GAG content increased with increasing exercise in the 5 months group. These differences had disappeared after 6 months of similar exercise. No influence of exercise could be demonstrated on any of the collagen parameters. When comparing 5 months with 11 months group, all parameters except hydroxylysine changed significantly during these 6 months. Water, DNA and GAG content decreased during maturation. Collagen and HP content increased. It is hypothesised that juvenile equine articular cartilage may be seen as a dynamic, continuously remodelling tissue that is gradually taking on the biochemical characteristics it will have during the rest of the life of the animal. Moderate exercise does not influence the collagen component of the extracellular matrix. It has a beneficial, but reversible, effect on the glycosaminoglycan component.

Down-regulation of chondrocyte aggrecan and type-II collagen gene expression correlates with increases in static compression magnitude and duration

The goal of this study was to examine the simultaneous effects of mechanical compression of chondrocytes on mRNA expression and macromolecular synthesis of aggrecan and type-II collagen. Bovine cartilage explants were exposed to different magnitudes and durations of applied mechanical compression, and levels of aggrecan and type-IIa collagen mRNA normalized to glyceraldehyde-3-phosphate dehydrogenase were measured and quantified by Northern blot analysis. Synthesis of aggrecan and type-II collagen protein was measured by radiolabel incorporation of [35S]sulfate and [3H]proline into macromolecules. The results showed a dose-dependent decrease in mRNA levels for aggrecan and type-II collagen, with increasing compression relative to physiological cut thickness applied for 24 hours. Radiolabel incorporation into glycosaminoglycans and collagen also decreased with increasing compression in a dose-related manner similar to the changes seen in mRNA expression. The modulation of aggrecan and type-II collagen mRNA and protein synthesis were dependent on the duration of the compression. Aggrecan and type-II collagen mRNA expression increased during the initial 0.5 hours of static compression; however, 4-24 hours after compression was applied total mRNA levels had significantly decreased. The synthesis of aggrecan and collagen protein decreased more rapidly than did mRNA levels after the application of a step compression. Together, these results suggest that mechanical compression rapidly alters chondrocyte aggrecan and type-II collagen gene expression on application of load. However, our results indicate that the observed decreases in biosynthesis may not be related solely to changes in mRNA expression. The mechanisms by which mechanical forces affect different segments of the biosynthetic pathways remain to be determined.

Integrative cartilage repair: inhibition by beta-aminopropionitrile

The effects of beta-aminopropionitrile, a known inhibitor of lysyl oxidase, on the extractability of newly synthesized collagen and integrative cartilage repair were determined in explant cultures of adult bovine articular cartilage. Dose-escalation studies indicated that treatment of cartilage explants for 6 days with beta-aminopropionitrile caused a dose-dependent inhibition of proteoglycan synthesis ([35S]sulfate incorporation) with a 50% inhibition at 2.2 mM. However, 0.25 mM beta-aminopropionitrile had no detectable effect on proteoglycan synthesis and was thus used for subsequent experiments. Treatment of cartilage with beta-aminopropionitrile for 14 days increased the extractability of newly synthesized collagen with 4 M guanidine-HCl while having little effect on proteoglycan synthesis, proteoglycan deposition, collagen synthesis (formation of [3H]hydroxyproline after labeling with [3H]proline), collagen deposition, or cartilage cellularity (DNA content). In untreated cultures, the percentage of radiolabeled collagen ([3H]hydroxyproline) that was extractable after 1 day of radiolabeling, 6 days of radiolabeling, or 6 days of label and 6 days of chase decreased from 81 to 25 and 9%, respectively. In beta-aminopropionitrile-treated cultures, the extractability was relatively higher (96, 62, and 47%, respectively). Treatment with beta-aminopropionitrile after radiolabeling with [14C]lysine also significantly inhibited the formation of the reducible crosslink [14C]dihydroxylysinonorleucine without affecting the overall deposition in cartilage of [14C]lysine and [14C]hydroxylysine. In functional repair studies, treatment with beta-aminopropionitrile caused an almost complete inhibition of integration between pairs of cartilage explants maintained in apposition for 2 weeks. These results indicate that beta-aminopropionitrile blocks the formation of collagen crosslinks in cartilage explants and suggest that such crosslinks are critical to integrative cartilage repair.

Some bioengineering considerations for tissue engineering of articular cartilage

The mechanism(s) by which chondrocytes convert physical stimuli to intracellular signals, which in turn direct cell activities, represents an area of intense current orthopaedic tissue engineering research. This report is aimed at providing an overview of some biomechanical engineering factors that are required for pursuing this type of research. Two specific aspects of cartilage are addressed: (1) how does the tissue function biomechanically; and (2) what is the nature of physical stimuli inside articular cartilage. By focusing on the effects of inhomogeneities of material properties, a description of some of the mechanical and electrochemical events (the physical stimuli) that would occur in cartilage during loading is presented. Two simple and common tests are considered: permeation and confined compression. Theoretical analyses using appropriate constitutive laws (the biphasic and triphasic theories) reveal the details of how surface loadings are converted to mechanical and electrochemical signals by the extracellular matrix to hydraulic and osmotic pressures, fluid, solute and ion flows, matrix deformations, and electrical fields. The material inhomogeneities are shown to be able to significantly change the mechanical and electrochemical events within the extracellular matrix, and thus the environments around chondrocytes. Material inhomogeneities arising from the flow of interstitial fluid through the porous and permeable extracellular matrix also are discussed. In the authors' view, the charged extracellular matrix, together with the associated interstitial fluid and ions, collectively can be thought of as a signal transducer. Knowledge of the nature of the mechanical and electrochemical events in the extracellular matrix, and their variations with time and location during and after loading, is essential in the understanding of the mechanical signal transduction mechanism(s) in chondrocytes and articular cartilage.

Physical and biological regulation of proteoglycan turnover around chondrocytes in cartilage explants. Implications for tissue degradation and repair

The development of clinical strategies for cartilage repair and inhibition of matrix degradation may be facilitated by a better understanding of (1) the chondrocyte phenotype in the context of a damaged extracellular matrix, and (2) the roles of biochemical and biomechanical pathways by which matrix metabolism is mediated. Using methods of quantitative autoradiography, we examined the cell-length scale patterns of proteoglycan deposition and turnover in the cell-associated matrices of chondrocytes in adult bovine and calf cartilage explants. Results highlight a rapid turnover in the pericellular matrix, which may indicate spatial organization of PG metabolic pools, and specific biomechanical roles for different matrix regions. Subsequent to injurious compression of calf explants, which resulted in grossly visible tissue cracks and caused a decrease in the number of viable chondrocytes within explants, cell-mediated matrix catabolic processes appeared to increase, resulting in apparently increased rates of proteoglycan turnover around active cells. Furthermore, the influences of cell-stimulatory factors such as IL-1 beta appeared to be delayed in their effects subsequent to injurious compression, suggesting interactions between biomechanical and biochemical pathways of PG degradation. These results may provide a useful reference point in the development of in vitro models for cartilage injury and disease, and hint at possible new approaches in the development of cartilage repair strategies.

Gas exchange is essential for bioreactor cultivation of tissue engineered cartilage

Tissue engineered cartilage can be grown in vitro if the necessary physical and biochemical factors are present in the tissue culture environment. Cell metabolism and tissue composition were studied for engineered cartilage cultured for 5 weeks using bovine articular chondrocytes, polymer scaffolds (5 mm diameter x 2 mm thick fibrous discs), and rotating bioreactors. Medium pH and concentrations of oxygen, carbon dioxide, glucose, lactate, ammonia, and glycosoaminoglycan (GAG) were varied by altering the exchange rates of gas and medium in the bioreactors. Cell-polymer constructs were assessed with respect to histomorphology, biochemical composition and metabolic activity. Low oxygen tension ( approximately 40 mmHg) and low pH ( approximately 6.7) were associated with anaerobic cell metabolism (yield of lactate on glucose, YL/G, of 2.2 mol/mol) while higher oxygen tension ( approximately 80 mmHg) and higher pH ( approximately 7.0) were associated with more aerobic cell metabolism (YL/G of 1.65-1.79 mol/mol). Under conditions of infrequent medium replacement (50% once per week), cells utilized more economical pathways such that glucose consumption and lactate production both decreased, cell metabolism remained relatively aerobic (YL/G of 1.67 mol/mol) and the resulting constructs were cartilaginous. More aerobic conditions generally resulted in larger constructs containing higher amounts of cartilaginous tissue components, while anaerobic conditions suppressed chondrogenesis in 3D tissue constructs.

Organisation of the chondrocyte cytoskeleton and its response to changing mechanical conditions in organ culture

Articular cartilage undergoes cycles of compressive loading during joint movement, leading to its cyclical deformation and recovery. This loading is essential for chondrocytes to perform their normal function of maintenance of the extracellular matrix. Various lines of evidence suggest the involvement of the cytoskeleton in load sensing and response. The purpose of the present study is to describe the 3-dimensional (3D) architecture of the cytoskeleton of chondrocytes within their extracellular matrix, and to examine cytoskeletal responses to experimentally varied mechanical conditions. Uniformly sized explants of articular cartilage were dissected from adult rat femoral heads. Some were immediately frozen, cryosectioned and labelled for filamentous actin using phalloidin, and for the focal contact component vinculin or for vimentin by indirect immunofluorescence. Sections were examined by confocal microscopy and 3D modelling. Actin occurred in all chondrocytes, appearing as bright foci at the cell surface linked to an irregular network beneath the surface. Cell surface foci colocalised with vinculin, suggesting the presence of focal contacts between the chondrocyte and its pericellular matrix. Vimentin label occurred mainly in cells of the deep zone. It had a complex intracellular distribution, with linked networks of fibres surrounding the nucleus and beneath the plasma membrane. When cartilage explants were placed into organ culture, where in the absence of further treatments cartilage imbibes fluid from the culture medium and swells, cytoskeletal changes were observed. After 1 h in culture the vimentin cytoskeleton was disassembled, leading to diffuse labelling of cells. After a further hour in culture filamentous vimentin label reappeared in deep zone chondrocytes, and then over the next 48 h became more widespread in cells of the explants. Actin distribution was unaffected by culture. Further experiments were performed to test the effects of load on the cytoskeleton. Explants were placed in culture and immediately subjected to static uniaxial radially unconfined compressive loads of 0.5, 1, 2 or 4 MPa for 1 h using a pneumatic loading device. Loads greater than 0.5 MPa maintained the vimentin organisation over the culture period. At 0.5 MPa, the chondrocytes within the explant behaved as in free-swelling culture. The rapid change in vimentin organisation probably relates to rapid swelling of the explants--under free-swelling conditions, these reached their maximum swollen size in just 15 min of culture. The chondrocytes' response to change in tissue dimensions, and thus to their relationship to their immediate environment, was to disassemble their vimentin networks. Loading probably counteracts the swelling pressure of the tissue. Overall, this work suggests that chondrocytes maintain their actin cytoskeleton and modify their vimentin cytoskeleton in response to changing mechanical conditions.

Semi-continuous perfusion system for delivering intermittent physiological pressure to regenerating cartilage

A semi-continuous compression/perfusion system has been custom made to allow the application of intermittent hydrostatic pressure, at physiological levels, to regenerating tissues over the long term. To test the system, isolated foal chondrocytes were seeded in resorbable polyglycolic acid meshes and cultured in the system for 5 weeks. The cell/polymer constructs were subjected to an intermittent hydrostatic pressure of 500 psi and were fed semi-continuously. Assays of the resulting tissue constructs indicate that the reactor supports cartilage development and that physiological intermittent compression enhances the production of extracellular matrix by the chondrocytes. The concentrations of sulfated glycosaminoglycan were found to be at least twice as high as those in control (unpressurized) samples. A correlation between the sulfated glycosaminoglycan content and the compressive modulus in pressurized, but not control, samples suggests that physiological intermittent pressurization not only enhances the production of extracellular matrix but may also influence matrix organization resulting in a stronger construct.

Studies on growth cartilage in the horse and their application to aetiopathogenesis of dyschondroplasia (osteochondrosis)

The importance of osteochondrosis (dyschondroplasia) to the horse industry has been well documented since it was first recorded 50 years ago. The condition is known to be multifactorial in origin, arising from focal failure of endochondral ossification at predilection sites in articular/epiphyseal growth cartilage, but specific information on its aetiopathogenesis is sparse. This paper reviews the current knowledge of growth cartilage metabolism and the process of normal endochondral ossification in the horse. It highlights the localization of various protein products of chondrocytes and the differences in the zones of articular cartilage. In the early focal lesions (referred to as dyschondroplasia) there are alterations in the chondrocytes, extracellular matrix and some of the local protein products. The most obvious feature is an alteration in matrix metabolism which may be responsible for triggering a range of other factors leading to the development of a retained core of cartilage and a primary lesion of dyschondroplasia. Based on available evidence, a preliminary hypothesis for pathogenesis is presented. This suggests that there are a number of factors capable of initiating the condition. One of these involves high circulating insulin levels from high energy feeding which may affect chondrocyte maturation leading to altered matrix metabolism and faulty mineralization resulting in the formation of cartilage cores which characterize the condition. Further research to test this hypothesis is needed before there can be a rational basis for prophylaxis.