Dynamic mechanical compression influences nitric oxide production by articular chondrocytes seeded in agarose

Keratoconus: a potential risk factor for osteoarthritis

PURPOSE

This study was undertaken to compare the distal femoral cartilage thickness in patients with keratoconus (KC) with that of age- and sex-matched healthy controls, in order to identify a potential risk factor for early osteoarthritis in patients with KC and to allow initiation of early rehabilitation.

METHODS

Thirty-six KC patients between 18 and 35 years of age and 36 healthy controls were included in this study. Keratometry readings (K1, K2), central corneal thickness (CCT), anterior chamber depth (ACD), iridocorneal angle (ICA), and corneal volumes (CV) were measured using a Sirius imaging system (Costruzioni Strumenti Oftalmici, Italy). Also, the distal femoral cartilage thickness (DFCT) was assessed bilaterally using ultrasound by the same physiatrist. Lateral femoral condyle (LFC), intercondylar area (ICA), medial femoral condyle (MFC), and body mass index (BMI) values were recorded.

RESULTS

Patient and control groups were comparable in terms of age, gender, and BMI. On the other hand, patients with KC had a significant reduction in right LFC, MFC thickness and left ICA, MFC as compared to controls (p < 0.05). In the corneal topographic evaluation of the groups, it was observed that K₁, K₂, CCT, and ACD values differed significantly.

CONCLUSIONS

Detection of thinner DFCT in KC patients suggests that these patients may be future candidates of osteoarthritis.

Effects of hydrostatic pressure and deviatoric stress on human articular chondrocytes for designing neo-cartilage construct

Autologous chondrocyte implantation is a promising therapy for the treatment of the articular cartilage defects. Recently, we have developed a three-dimensional chondrocyte construct manufactured with a collagen gel/sponge scaffold and cyclic hydrostatic pressure. However, the roles of various mechanical stresses, specifically hydrostatic pressure and deviatoric stress, as well as poststress loading, were unclear on metabolic function in chondrocytes. We hypothesized that hydrostatic pressure and deviatoric stresses each alter individual metabolic characteristics of chondrocytes. We embedded human articular chondrocytes within an agarose hydrogel and applied hydrostatic pressure and/or deviatoric stress individually or simultaneously for 4 days. Subsequently, we kept the cell constructs without stress for an additional 3 days. With hydrostatic pressure and/or deviatoric stress, more cells proliferated significantly than no stress (p < .05) and more cells proliferated near the inner side of the construct than the outer (p < .05). Cartilage specific aggrecan core protein and collagen type II were upregulated significantly after off-loading hydrostatic pressure alone at Day 7 (p < .05). On the other hand, these molecules were upregulated significantly immediately after deviatoric stress alone and combined with hydrostatic pressure at Day 4 (p < .05). Tissue inhibitor of metalloproteinase-2 was upregulated significantly after off-loading hydrostatic pressure alone and combined deviatoric stress at Day 7 (p < .05). Metalloproteinnase-13 was upregulated significantly with deviatoric stress at Day 4 (p < .05) and combined with hydrostatic pressure at Day 4. These results suggest that metabolic functions are regulated by the combination of hydrostatic pressure and deviatoric stress and by the timing of stress loading.

Osteochondral Graft Size Is Significantly Associated With Increased Force and Decreased Chondrocyte Viability

BACKGROUND

Insertion force has been shown to significantly reduce chondrocyte viability during osteochondral allograft transplantation. How graft size influences the required insertion force and chondrocyte viability has yet to be determined. Hypothesis/Purpose: The purpose was to characterize how graft size influences insertion force requirements and chondrocyte viability during osteochondral transplantation. The hypothesis was that larger grafts would require greater force and reduce chondrocyte viability.

STUDY DESIGN

Controlled laboratory study.

METHODS

Four graft sizes-15 × 5 mm, 15 × 10 mm, 25 × 5 mm, and 25 × 10 mm (diameter × depth)-were harvested from 13 thawed fresh-frozen human cadaveric distal femurs. Average, maximum, and cumulative force and number of impacts were recorded for 44 grafts by a surgical mallet embedded with a calibrated force sensor. In a separate experiment, fresh osteochondral tissues were subjected to mechanical loading. To capture a range of clinically important forces, categories were selected to correspond to impaction force data. Chondrocyte viability was assessed with confocal laser microscopy and live/dead staining.

RESULTS

Total force for all grafts averaged 4576 N. Median number of impacts for all grafts was 20 (range, 7-116). The mean number of impacts for 5-mm-deep grafts was 14.2 (95% CI, 10.8-18.6), as compared with 26.3 (95% CI, 19.9-34.4) for 10-mm-deep grafts ( P < .001). The mean cumulative force for 5-mm-deep grafts was 2128 N (95% CI, 1467-3087), as opposed to 4689 N (95% CI, 3232-6803) for 10-mm-deep grafts ( P = .001). For every 1 mm in graft depth, an average of 13.1% (95% CI, 6.2%-20.3%) more impacts are required when controlling for diameter and density ( P < .001). For every 1 mm in graft depth, the force required increases on average by 17.1% (95% CI, 7.7%-27.4%) when controlling for diameter and density ( P = .001). There was a significant reduction in chondrocyte viability for the forces required for graft thickness values >10 mm. Only forces associated with graft thickness <10 mm had chondrocyte viabilities consistently >70%.

CONCLUSION

Insertion force increases significantly with increasing graft depth. Controlling for diameter and bone density, a 1-mm increase in graft depth is associated with 13.1% more impacts and 17.1% more force. Chondrocyte viability was significantly reduced to <70% at average forces associated with grafts thicker than 10 mm.

CLINICAL RELEVANCE

Based on the current data, graft depth is an important consideration for surgeons when sizing osteochondral allograft transplant for chondral lesions of the knee.

Biomechanics, obesity, and osteoarthritis. The role of adipokines: When the levee breaks

Osteoarthritis is a high-incidence painful and debilitating disease characterized by progressive degeneration of articular joints, which indicates a breakdown in joint homeostasis favoring catabolic processes. Biomechanical loading, associated with inflammatory and metabolic imbalances of joint, strongly contributes to the initiation and progression of the disease. Obesity is a primary risk factor for disease onset, and mechanical factors increased the risk for disease progression. Moreover, inflammatory mediators, in particular, adipose tissue-derived cytokines (better known as adipokines) play a critical role linking obesity and osteoarthritis. The present article summarizes the knowledge about the role of adipokines in cartilage and bone function, highlighting their contribution to the imbalance of joint homeostasis and, consequently, pathogenesis of osteoarthritis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:594-604, 2018.

Effect of Dynamic Compression on in vitro Chondrocyte Metabolism

Background

Chondrocytes can detect and respond to the mechanical environment by altering their metabolism. This study was designed to explore the effects of dynamic compression on chondrocyte metabolism.

Methods

Chondrocytes were harvested from newborn Wistar rats. After 7 days of expansion, chondrocytes embedded in agarose discs underwent uniaxial unconfined dynamic compression loads at different amplitudes (5%, 10%, and 15%) and frequencies (0.5 Hz, 1.0 Hz, 2.0 Hz, and 3.0 Hz) with a duration of 24 hours. The delayed effects on the chondrocytes were studied at 1, 3, and 7 days after the experiment.

Results

The results showed that at 10% strain, higher-frequency compression pressure can enhance the proliferation of chondrocytes. The synthesis of glycosaminoglycan (GAG) increased at 10%-15% strain and a 1-Hz load. The synthesis of nitric oxide (NO) increased at the 0.5-Hz load; while decreasing at the 15% strain. With 10% strain, 1 Hz dynamic compression, the proliferation of chondrocytes and GAG synthesis increased and persisted for 7 days, and NO synthesis decreased at the third and seventh days of culture.

Conclusions

This study showed that chondrocytes respond metabolically to compressive loading, which is expected to modulate the growth and the resultant biomechanical properties of these tissue-engineered constructs during culture.

Lithium chloride prevents interleukin-1β induced cartilage degradation and loss of mechanical properties

Osteoarthritis is a chronic degenerative disease that affects the articular cartilage. Recent studies have demonstrated that lithium chloride exhibits significant efficacy as a chondroprotective agent, blocking cartilage degradation in response to inflammatory cytokines. However, conflicting literature suggests lithium may affect the physicochemical properties of articular cartilage and thus long-term exposure may negatively affect the mechanical functionality of this tissue. This study aims to investigate the effect of lithium chloride on the biomechanical properties of healthy and interleukin-1β treated cartilage in vitro and examines the consequences of long-term exposure to lithium on cartilage health in vivo. Bovine cartilage explants were treated with lithium chloride for 12 days. Chondrocyte viability, matrix catabolism and the biomechanical properties of bovine cartilage explants were not significantly altered following treatment. Consistent with these findings, long term-exposure (9 months) to dietary lithium did not induce osteoarthritis in rats, as determined by histological staining. Moreover, lithium chloride did not induce the expression of catabolic enzymes in human articular chondrocytes. In an inflammatory model of cartilage destruction, lithium chloride blocked interleukin-1β signaling in the form of nitric oxide and prostaglandin E2 release and prevented matrix catabolism such that the loss of mechanical integrity observed with interleukin-1β alone was inhibited. This study provides further support for lithium chloride as a novel compound for the treatment of osteoarthritis.

Biomechanical factors in osteoarthritis

Biomechanical factors play an important role in the health of diarthrodial joints. Altered joint loading - associated to obesity, malalignment, trauma or joint instability - is a critical risk factor for joint degeneration, whereas exercise and weight loss have generally been shown to promote beneficial effects for osteoarthritic joints. The mechanisms by which mechanical stress alters the physiology or pathophysiology of articular cartilage or other joint tissues likely involve complex interactions with genetic and molecular influences, particularly local or systemic inflammation secondary to injury or obesity. Chondrocytes perceive physical signals from their environment using a variety of mechanisms, including ion channels, integrin-mediated connections to the extracellular matrix that involve membrane, cytoskeletal and intracellular deformation. An improved understanding of the biophysical and molecular pathways involved in chondrocyte mechanotransduction can provide insight into the development of novel therapeutic approaches for osteoarthritis.

Hydromechanical stimulator for chondrocyte-seeded constructs in articular cartilage tissue engineering applications

Mechanical stimulation is a key technique used for controlling the mechanical properties of tissue engineered articular cartilage constructs proposed for defect repair. The present study introduces a new technical method and device for ‘hydromechanical’ stimulation of tissue engineered articular cartilage constructs. The stimulation consists of simultaneous cyclic compression, frictional shear from a sliding indenter contact and direct pressurized fluid perfusion. Each of these modes of mechanical loading has been shown by other research groups to effectively stimulate tissue engineered constructs. A device for applying these conditions was designed, developed and tested. Two sets (high and low perfusion flow rates) of three experiments were performed, each with two samples subjected to hydromechanical stimulation conditions (compression and friction forces along with perfusion). Two other samples from each set were subjected to just compression and dynamic frictional shear forces, and two more were used as controls (not stimulated). The average amount of glycosaminoglycan retained in the constructs after 3 weeks ranked from low to high as follows: controls, hydromechanical conditions with the low-flow rate, hydromechanical conditions with the high-flow rate and just compression plus dynamic frictional shear. Statistically significant differences were not detected. However, future studies would focus on glycosaminoglycan production in the superficial zone, measuring the glycosaminoglycan released to the nutrient media, and address altering the hydromechanical stimulation parameters using the results of the present study as guidance, in attempts to achieve statistically significant increases in glycosaminoglycan production compared with the controls.

Scaffold architecture determines chondrocyte response to externally applied dynamic compression

It remains unclear how specific mechanical signals generated by applied dynamic compression (DC) regulate chondrocyte biosynthetic activity. It has previously been suggested that DC-induced interstitial fluid flow positively impacts cartilage-specific matrix production. Modifying fluid flow within dynamically compressed hydrogels therefore represents a promising approach to controlling chondrocyte behavior, which could potentially be achieved by changing the construct architecture. The objective of this study was to first determine the influence of construct architecture on the mechanical environment within dynamically compressed agarose hydrogels using finite element (FE) modeling and to then investigate how chondrocytes would respond to this altered environment. To modify construct architecture, an array of channels was introduced into the hydrogels. Increased magnitudes of fluid flow were predicted in the periphery of dynamically compressed solid hydrogels and also around the channels in the dynamically compressed channeled hydrogels. DC was found to significantly increase sGAG synthesis in solid constructs, which could be attributed at least in part to an increase in DNA. DC was also found to preferentially increase collagen accumulation in regions of solid and channeled constructs where FE modeling predicted higher levels of fluid flow, suggesting that this stimulus is important for promoting collagen production by chondrocytes embedded in agarose gels. In conclusion, this study demonstrates how the architecture of cell-seeded scaffolds or hydrogels can be modified to alter the spatial levels of biophysical cues throughout the construct, leading to greater collagen accumulation throughout the engineered tissue rather than preferentially in the construct periphery. This system also provides a novel approach to investigate how chondrocytes respond to altered levels of biophysical stimulation.

Primary cilia elongation in response to interleukin-1 mediates the inflammatory response

Primary cilia are singular, cytoskeletal organelles present in the majority of mammalian cell types where they function as coordinating centres for mechanotransduction, Wnt and hedgehog signalling. The length of the primary cilium is proposed to modulate cilia function, governed in part by the activity of intraflagellar transport (IFT). In articular cartilage, primary cilia length is increased and hedgehog signaling activated in osteoarthritis (OA). Here, we examine primary cilia length with exposure to the quintessential inflammatory cytokine interleukin-1 (IL-1), which is up-regulated in OA. We then test the hypothesis that the cilium is involved in mediating the downstream inflammatory response. Primary chondrocytes treated with IL-1 exhibited a 50 % increase in cilia length after 3 h exposure. IL-1-induced cilia elongation was also observed in human fibroblasts. In chondrocytes, this elongation occurred via a protein kinase A (PKA)-dependent mechanism. G-protein coupled adenylate cyclase also regulated the length of chondrocyte primary cilia but not downstream of IL-1. Chondrocytes treated with IL-1 exhibit a characteristic increase in the release of the inflammatory chemokines, nitric oxide and prostaglandin E2. However, in cells with a mutation in IFT88 whereby the cilia structure is lost, this response to IL-1 was significantly attenuated and, in the case of nitric oxide, completely abolished. Inhibition of IL-1-induced cilia elongation by PKA inhibition also attenuated the chemokine response. These results suggest that cilia assembly regulates the response to inflammatory cytokines. Therefore, the cilia proteome may provide a novel therapeutic target for the treatment of inflammatory pathologies, including OA.

Biomechanical influence of cartilage homeostasis in health and disease

There is an urgent demand for long term solutions to improve osteoarthritis treatments in the ageing population. There are drugs that control the pain but none that stop the progression of the disease in a safe and efficient way. Increased intervention efforts, augmented by early diagnosis and integrated biophysical therapies are therefore needed. Unfortunately, progress has been hampered due to the wide variety of experimental models which examine the effect of mechanical stimuli and inflammatory mediators on signal transduction pathways. Our understanding of the early mechanopathophysiology is poor, particularly the way in which mechanical stimuli influences cell function and regulates matrix synthesis. This makes it difficult to identify reliable targets and design new therapies. In addition, the effect of mechanical loading on matrix turnover is dependent on the nature of the mechanical stimulus. Accumulating evidence suggests that moderate mechanical loading helps to maintain cartilage integrity with a low turnover of matrix constituents. In contrast, nonphysiological mechanical signals are associated with increased cartilage damage and degenerative changes. This review will discuss the pathways regulated by compressive loading regimes and inflammatory signals in animal and in vitro 3D models. Identification of the chondroprotective pathways will reveal novel targets for osteoarthritis treatments.

Numerical assessment on the effective mechanical stimuli for matrix-associated metabolism in chondrocyte-seeded constructs

The self-regeneration capacity of articular cartilage is limited, due to its avascular and aneural nature. Loaded explants and cell cultures demonstrated that chondrocyte metabolism can be regulated via physiologic loading. However, the explicit ranges of mechanical stimuli that correspond to favourable metabolic response associated with extracellular matrix (ECM) synthesis are elusive.Unsystematic protocols lacking this knowledge produce inconsistent results. This study aims to determine the intrinsic ranges of physical stimuli that increase ECM synthesis and simultaneously inhibit nitric oxide (NO) production in chondrocyte–agarose constructs, by numerically reevaluating the experiments performed by Tsuang et al. (2008). Twelve loading patterns were simulated with poro-elastic finite element models in ABAQUS. Pressure on solid matrix, von Mises stress, maximum principle stress and pore pressure were selected as intrinsic mechanical stimuli.Their development rates and magnitudes at the steady state of cyclic loading were calculated with MATLAB at the construct level. Concurrent increase in glycosaminoglycan and collagen was observed at 2300 Pa pressure and 40 Pa/s pressure rate. Between 0–1500 Pa and 0–40 Pa/s, NO production was consistently positive with respect to controls, whereas ECM synthesis was negative in the same range. A linear correlation was found between pressure rate and NO production (R =0.77). Stress states identified in this study are generic and could be used to develop predictive algorithms for matrix production in agarose–chondrocyte constructs of arbitrary shape, size and agarose concentration. They could also be helpful to increase the efficacy of loading protocols for avascular tissue engineering.

Static and dynamic compressive strains influence nitric oxide production and chondrocyte bioactivity when encapsulated in PEG hydrogels of different crosslinking densities

OBJECTIVE

Mechanical loading is an important regulator of chondrocytes; however, many of the mechanisms involved in chondrocyte mechanotransduction still remain unclear. Here, poly(ethylene glycol) (PEG) hydrogels are proposed as a model system to elucidate chondrocyte response due to cell deformation, which is controlled by gel crosslinking (rho(x)).

METHODS

Bovine articular chondrocytes (50 x 10(6)cells/mL) were encapsulated in gels with three rho(x)s and subjected to static (15% strain) or dynamic (0.3 Hz or 1 Hz, 15% amplitude strain) loading for 48 h. Cell deformation was examined by confocal microscopy. Cell response was assessed by total nitric oxide (NO) production, proteoglycan (PG) synthesis ((35)SO(4)(2-)-incorporation) and cell proliferation (CP) ([(3)H]-thymidine incorporation). Oxygen consumption was assessed using an oxygen biosensor.

RESULTS

An increase in rho(x) led to lower water contents, higher compressive moduli, and higher cell deformations. Chondrocyte response was dependent on both loading regime and rho(x). For example, under a static strain, NO was not affected, while CP and PG synthesis were inhibited in low rho(x) and stimulated in high rho(x). Dynamic loading resulted in either no effect or an inhibitory effect on NO, CP, and PG synthesis. Overall, our results showed correlations between NO and CP and/or PG synthesis under static and dynamic (0.3 Hz) loading. This finding was attributed to the hypoxic environment that resulted from the high cell-seeding density.

CONCLUSION

This study demonstrates gel rho(x) and loading condition influence NO, CP, and PG synthesis. Under a hypoxic environment and certain loading conditions, NO appears to have a positive effect on chondrocyte bioactivity.

Dynamic compression counteracts IL-1beta induced inducible nitric oxide synthase and cyclo-oxygenase-2 expression in chondrocyte/agarose constructs

Background

Nitric oxide and prostaglandin E₂ (PGE₂play pivotal roles in both the pathogenesis of osteoarthritis and catabolic processes in articular cartilage. These mediators are influenced by both IL-1β and mechanical loading, and involve alterations in the inducible nitric oxide synthase (iNOS) and cyclo-oxygenase (COX)-2 enzymes. To identify the specific interactions that are activated by both types of stimuli, we examined the effects of dynamic compression on levels of expression of iNOS and COX-2 and involvement of the p38 mitogen-activated protein kinase (MAPK) pathway.

Methods

Chondrocyte/agarose constructs were cultured under free-swelling conditions with or without IL-1β and/or SB203580 (inhibitor of p38 MAPK) for up to 48 hours. Using a fully characterized bioreactor system, constructs were subjected to dynamic compression for 6, 12 and 48 hours under similar treatments. The activation or inhibition of p38 MAPK by IL-1β and/or SB203580 was analyzed by western blotting. iNOS, COX-2, aggrecan and collagen type II signals were assessed utilizing real-time quantitative PCR coupled with molecular beacons. Release of nitrite and PGE₂ was quantified using biochemical assays. Two-way analysis of variance and the post hoc Bonferroni-corrected t-test were used to examine data.

Results

IL-1β activated the phosphorylation of p38 MAPK and this effect was abolished by SB203580. IL-1β induced a transient increase in iNOS expression and stimulated the production of nitrite release. Stimulation by either dynamic compression or SB203580 in isolation reduced the IL-1β induced iNOS expression and nitrite production. However, co-stimulation with both dynamic compression and SB203580 inhibited the expression levels of iNOS and production of nitrite induced by the cytokine. IL-1β induced a transient increase in COX-2 expression and stimulated the cumulative production of PGE₂ release. These effects were inhibited by dynamic compression or SB203580. Co-stimulation with both dynamic compression and SB203580 restored cytokine-induced inhibition of aggrecan expression. This is in contrast to collagen type II, in which we observed no response with the cytokine and/or SB203580.

Conclusion

These data suggest that dynamic compression directly influences the expression levels of iNOS and COX-2. These molecules are current targets for pharmacological intervention, raising the possibility for integrated pharmacological and biophysical therapies for the treatment of cartilage joint disorders.

Differential response of adult and embryonic mesenchymal progenitor cells to mechanical compression in hydrogels

Cells in the musculoskeletal system can respond to mechanical stimuli, supporting tissue homeostasis and remodeling. Recent studies have suggested that mechanical stimulation also influences the differentiation of MSCs, whereas the effect on embryonic cells is still largely unknown. In this study, we evaluated the influence of dynamic mechanical compression on chondrogenesis of bone marrow-derived MSCs and embryonic stem cell-derived (human embryoid body-derived [hEBd]) cells encapsulated in hydrogels and cultured with or without transforming growth factor beta-1 (TGF-beta1). Cells were cultured in hydrogels for up to 3 weeks and exposed daily to compression for 1, 2, 2.5, and 4 hours in a bioreactor. When MSCs were cultured, mechanical stimulation quantitatively increased gene expression of cartilage-related markers, Sox-9, type II collagen, and aggrecan independently from the presence of TGF-beta1. Extracellular matrix secretion into the hydrogels was also enhanced. When hEBd cells were cultured without TGF-beta1, mechanical compression inhibited their differentiation as determined by significant downregulation of cartilage-specific genes. However, after initiation of chondrogenic differentiation by administration of TGF-beta1, the hEBd cells quantitatively increased expression of cartilage-specific genes when exposed to mechanical compression, similar to the bone marrow-derived MSCs. Therefore, when appropriately directed into the chondrogenic lineage, mechanical stimulation is beneficial for further differentiation of stem cell tissue engineered constructs.

Purinergic pathway suppresses the release of .NO and stimulates proteoglycan synthesis in chondrocyte/agarose constructs subjected to dynamic compression

Mechanical loading plays a fundamental role in the physiological and pathological processes of articular cartilage. The application of dynamic compression to chondrocytes cultured in agarose, downregulates the release of nitric oxide (NO) and enhances cell proliferation and proteoglycan synthesis. We hypothesize that the observed metabolic changes in response to dynamic compression involve a purinergic signaling pathway. Chondrocyte/agarose constructs were subjected to dynamic compression (15%, 1 Hz, 48 h) in the presence of antagonists for the purinergic pathway. Gadolinium was used as a putative inhibitor of stretch-activated calcium ion channels including adenosine 5'-triphosphate (ATP) release channels; suramin was employed as a P2 receptor antagonist and apyrase was used to catalyze the hydrolysis of extracellular ATP. The data presented demonstrate that in the absence of the inhibitor, dynamic compression suppressed .NO release. Treatment with gadolinium and suramin caused a compression-induced upregulation of .NO release, a response abolished with apyrase. Compression-induced stimulation of cell proliferation was reversed with gadolinium, suramin, or apyrase. By contrast, compression-induced stimulation of proteoglycan synthesis was abolished under all treatment conditions. Thus, the purinergic pathway is important in suppressing the release of .NO and stimulation of proteoglycan synthesis. Indeed, high levels of .NO could trigger a downstream catabolic response and mediate the compression-induced inhibition of cell proliferation. The current study demonstrates for the first time the importance of a purinergic pathway in mediating the metabolic response to dynamic compression and suppressing an inflammatory effect.

Study on the effects of mechanical pressure to the ultrastructure and secretion ability of mandibular condylar chondrocytes

During mandibular movement, condyle is subjected to repetitive compression and the mandibular condylar chondrocytes (MCCs) can detect and respond to this biomechanical environment by altering their metabolism. The present study was undertaken to investigate the effects of pressure to the ultrastructure, aggrecan synthesis, nitric oxide (NO) and prostaglandin F(1)alpha(PGF(1)alpha) secretion in MCCs. In vitro cultured rabbit MCCs were incubated and pressed under continuous pressure of 90kPa for 60min and 360min by hydraulic pressure controlled cellular strain unit. The ultrastructure, aggrecan mRNA expression, activity of nitric oxide synthase (NOS) and PGF(1)alpha secretion were investigated. Besides, nitric oxide inhibitor was used together with pressure to investigate the role of NO in mechanical effects. The appearance of MCC on TEM showed that after been pressed under 90kPa for 60min, the cellular processes became elongated and voluminous, together with aggrecan mRNA increasing. Under 90kPa for 360min, some of the cells showed distinct sign of apotosis and the aggrecan mRNA decreased. Pressure of 90kPa could cause increase of NOS activity and decrease of PGF(1)alpha composition. Inhibitor experiments indicated that pressure-induced upregulation of aggrecan mRNA and inhibition of PGF(1)alpha synthesis was partly mediated by NO. Continuous pressure could cause changes on the ultrastructure and function of MCC, as well as up-regulation of aggrecan synthesis, increase of NO secretion and decrease of PGF(1)alpha composition. NO was the upstream molecule, which mediated the response of aggrecan and PGF(1)alpha to mechanical pressure.

Mechanical stimulation of TMJ condylar chondrocytes encapsulated in PEG hydrogels

Temporomandibular joint (TMJ) disorders are most commonly associated with TMJ disc dislocation and osteoarthritis, which can cause erosion of the articular cartilage on the head of the mandibular condyle. There has been little attention focused on treating the damaged condylar cartilage. Therefore, the overall goal of this research is to create a tissue engineering therapy for resurfacing the damaged cartilage of the condylar process with healthy living tissue. Initially, bovine condylar cartilage explants were studied to understand the tissue structure, composition, and gene expression of the native tissue. The cell response of isolated condylar chondrocytes encapsulated in photopolymerized poly(ethylene glycol) hydrogels as a tissue engineering scaffold was examined in the presence and absence of dynamic loading for up to three days of culture. Condylar chondrocyte viability was maintained within the PEG hydrogel constructs over the culture period and loading conditions. Cell response was examined through real-time RTPCR for collagen types I and II and aggrecan, nitric oxide production, cell proliferation, proteoglycan (PG) synthesis, and spatial distribution of extracellular matrix through histology. This study demonstrates that PEG hydrogel constructs are suitable for condylar chondrocyte encapsulation in the absence of loading. However, dynamic compressive strains resulted in inhibition of gene expression, cell proliferation, and PG synthesis.

Integrin-mediated mechanotransduction in IL-1 beta stimulated chondrocytes

Mechanical loading and interleukin-1 beta (IL-1 beta) influence the release of nitric oxide (*NO) and prostaglandin E2 (PGE2) from articular chondrocytes via distinct signalling mechanisms. The exact nature of the interplay between the respective signalling pathways remains unclear. Recent studies have shown that integrins act as mechanoreceptors and may transduce extracellular stimuli into intracellular signals, thereby influencing cellular response. The current study demonstrates that the application of dynamic compression induced an inhibition of *NO and an upregulation of cell proliferation and proteoglycan synthesis in the presence and absence of IL-1 beta. PGE2 release was not affected by dynamic compression in the absence of IL-1 beta but was inhibited in the presence of the cytokine. The integrin binding peptide, GRGDSP, abolished or reversed the compression-induced alterations in all four parameters assessed in the presence and absence of IL-1 beta. The non-binding control peptide, GRADSP, had no effect. These data clearly demonstrate that the metabolic response of the chondrocytes to dynamic compression in the presence and absence of IL-1 beta, are integrin mediated.

Dynamic biophysical strain modulates proinflammatory gene induction in meniscal fibrochondrocytes

Fibrochondrocytes of meniscus adapt to changes in their biomechanical environment by mechanisms that are yet to be elucidated. In this study, the mechanoresponsiveness of fibrochondrocytes under normal and inflammatory conditions was investigated. Fibrochondrocytes from rat meniscus were exposed to dynamic tensile forces (DTF) at various magnitudes and frequencies. The mechanoresponsiveness was assessed by examining the expression of inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha (TNF-alpha), and matrix metalloproteinase-13 mRNA expression. The mRNA and protein analyses revealed that DTF at magnitudes of 5% to 20% did not induce proinflammatory gene expression. IL-1beta induced a rapid increase in the iNOS mRNA. DTF strongly repressed IL-1beta-dependent iNOS induction in a magnitude-dependent manner. Exposure to 15% DTF resulted in >90% suppression of IL-1beta-induced mRNA within 4 h and this suppression was sustained for the ensuing 20 h. The mechanosensitivity of fibrochondrocytes was also frequency dependent and maximal suppression of iNOS mRNA expression was observed at rapid frequencies of DTF compared with lower frequencies. Like iNOS, DTF also inhibited IL-1beta-induced expression of proinflammatory mediators involved in joint inflammation. The examination of temporal effects of DTF revealed that 4- or 8-h exposure of DTF was sufficient for its sustained anti-inflammatory effects during the next 20 or 16 h, respectively. Our findings indicate that mechanical signals act as potent anti-inflammatory signals, where their magnitude and frequency are critical determinants of their actions. Furthermore, mechanical signals continue attenuating proinflammatory gene transcription for prolonged periods of time after their removal.

Overexpression of nitric oxide synthases in tendon overuse

Tendon disorders with a chronic nature, including the rotator cuff, are extremely common, and represent a major clinical problem. Mechanical overload has been proposed as an important etiologic factor in tendinopathies. Nitric oxide (NO), a free radical produced by nitric oxide synthases (NOSs), is a potent regulator and stimulator of biological processes including tendon degeneration and healing. It is also involved in response to mechanical stimuli in different tissues. In an animal model of acutely injured tendon healing temporal and differential expression of NOS isoforms has been demonstrated, suggesting that different patterns of NOSs expression may have different biological functions. Therefore, we hypothesized that tendon overuse may result in a differential upregulation of NOSs, particularly iNOS. An animal model of supraspinatus tendon overuse was utilized, which consisted of treadmill running. A group of animals of the same strain and age subjected to normal cage activity were used as controls. Following a 4-week exercise protocol supraspinatus tendons were harvested, RNA was extracted, and subjected to competitive reverse transcription and polymerase chain reaction (RT-PCR) to determine the expression levels of inducible-, endothelial-, and neuronal-NOS isoforms (i-, e-, and nNOS). The mRNA expression of all three NOS isoforms increased in the supraspinatus tendons as a result of overuse exercise. iNOS and eNOS mRNA expression increased fourfold (p < 0.01), and there was an increase, but statistically not significant, in nNOS mRNA expression in the overused tendons when compared with the controls. This study is the first to show that NOS isoforms are upregulated in rotator cuff tendon as a result of chronic overuse, and suggests the involvement of nitric oxide in the response of tendon tissue to increased mechanical stress.

Anti-inflammatory effects of IL-4 and dynamic compression in IL-1β stimulated chondrocytes

Mechanical loading can counteract inflammatory pathways induced by IL-1beta by inhibiting *NO and PGE2, catabolic mediators known to be involved in cartilage degradation. The current study investigates the potential of dynamic compression, in combination with the anti-inflammatory cytokine, IL-4, to further abrogate the IL-1beta induced effects. The data presented demonstrate that IL-4 alone can inhibit nitrite release in the presence and absence of IL-1beta and partially reverse the IL-1beta induced PGE2 release. When provided in combination, IL-4 and dynamic compression could further abrogate the IL-1beta induced nitrite and PGE2 release. IL-1beta inhibited [3H]thymidine incorporation and this effect could be reversed by IL-4 or dynamic strain alone or both in combination. By contrast, 35SO4 incorporation was not influenced by IL-4 and/or dynamic strain in IL-1beta stimulated constructs. IL-4 and mechanical loading may therefore provide a potential protective mechanism for cartilage destruction as observed in OA.

Extracellular superoxide dismutase and oxidant damage in osteoarthritis

OBJECTIVE

To use human cartilage samples and a mouse model of osteoarthritis (OA) to determine whether extracellular superoxide dismutase (EC-SOD) is a constituent of cartilage and to evaluate whether there is a relationship between EC-SOD deficiency and OA.

METHODS

Samples of human cartilage were obtained from femoral heads at the time of joint replacement surgery for OA or femoral neck fracture. Samples of mouse tibial cartilage obtained from STR/ort mice and CBA control mice were compared at 5, 15, and 35 weeks of age. EC-SOD was measured by enzyme-linked immunosorbent assay, Western blotting, and immunohistochemistry techniques. Real-time quantitative reverse transcription-polymerase chain reaction was used to measure messenger RNA for EC-SOD and for endothelial cell, neuronal, and inducible nitric oxide synthases. Nitrotyrosine formation was assayed by Western blotting in mouse cartilage and by fluorescence immunohistochemistry in human cartilage.

RESULTS

Human articular cartilage contained large amounts of EC-SOD (mean +/- SEM 18.8 +/- 3.8 ng/gm wet weight of cartilage). Cartilage from patients with OA had an approximately 4-fold lower level of EC-SOD compared with cartilage from patients with hip fracture. Young STR/ort mice had decreased levels of EC-SOD in tibial cartilage before histologic evidence of disease occurred, as well as significantly more nitrotyrosine formation at all ages studied.

CONCLUSION

EC-SOD, the major scavenger of reactive oxygen species in extracellular spaces, is decreased in humans with OA and in an animal model of OA. Our findings suggest that inadequate control of reactive oxygen species plays a role in the pathophysiology of OA.

The influence of mechanical compression on the induction of osteoarthritis-related biomarkers in articular cartilage explants

OBJECTIVE

Macromolecules of the articular cartilage extracellular matrix released into synovial fluid, blood, or urine can serve as potentially useful biomarkers of the severity of osteoarthritis (OA). Biomechanical factors play an important role in OA pathogenesis, yet their influence on biomarker production is not well understood. The goal of this study was to examine the hypothesis that dynamic mechanical stress influences the release of these biomarkers from articular cartilage.

METHODS

Explants of porcine cartilage were subjected to dynamic compression at 0.5 Hz for 24h at stresses ranging from 0.006 to 0.1 MPa. The concentrations of cartilage oligomeric matrix protein (COMP), keratan sulfate (KS measured as the 5 D 4 epitope), total sulfated glycosaminoglycan (S-GAG), and the KS (keratanase-digestible) and chondroitin sulfate (CS) (chondroitinase-digestible) fractions of S-GAG were measured. Radiolabel incorporation was used to determine the rates of proteoglycan and protein synthesis.

RESULTS

The magnitudes of mechanical stress applied in this study induced nominal tissue strains of 4-23%, consistent with a range of physiological to hyperphysiologic strains measured in situ. COMP release increased in proportion to the magnitude of dynamic mechanical stress, while KS, CS and total S-GAG release increased in a bimodal pattern with increasing stress. Protein and proteoglycan synthesis were significantly decreased at the highest level of stress.

CONCLUSION

Mechanical stress differentially regulates the turnover of distinct pools of cartilage macromolecules. These findings indicate that mechanical factors, independent of exogenous cytokines or other stimulatory factors, can influence the production and release of OA-related biomarkers from articular cartilage.

Menisci of the rabbit knee require mechanical loading to maintain homeostasis: cyclic hydrostatic compression in vitro prevents derepression of catabolic genes

BACKGROUND

The purpose of this study was to examine the influence of removing menisci from their in vivo loading environment on gene expression patterns and to determine whether in vitro loading can maintain the tissues in their in vivo phenotype.

METHODS

Lateral and medial rabbit meniscal explants from one leg were cultured in vitro and subjected to intermittent cyclic hydrostatic pressure (CHP) of 1 MPa at 0.5 Hz for 1 min and a rest period of 14 min (4 h of culture). The contralateral menisci were incubated at atmospheric pressure for 4 h. Menisci from both legs of another set of rabbits were frozen immediately to yield time zero values reflective of in vivo mRNA levels. Total RNA was isolated from all groups and processed for reverse transcription-polymerase chain reaction analysis for a subset of relevant genes (matrix molecules, cytokines, proteinases and inhibitors, enzymes).

RESULTS

It was found that mRNA levels for MMP-1, MMP-3, TIMPs, iNOS, COX-2, interleukin-1beta in both menisci, and interleukin-6 in medial menisci were significantly elevated in tissues cultured under nonloading conditions compared to the time zero controls. Subjecting menisci to CHP significantly prevented these increases in mRNA levels for nearly all of the indicated molecules. In contrast, there were no significant differences in mRNA levels for collagens, biglycan, MMP-13, or TIMP-4 between the time zero values and those cultured under either nonloading or loading conditions.

CONCLUSIONS

These studies demonstrate that removing rabbit menisci from their normal in vivo mechanical environment leads to an apparent up-regulation of a subset of potent effector molecules that could mediate catabolic activities, and that in vitro CHP can largely prevent this apparent up-regulation.

Microarray analysis of gene expression in chondrosarcoma cells treated with bee venom

Bee venom (BV) has a broad array of clinical applications in Korean medicine, including treatment of inflammatory conditions such as arthritis. The final common pathway of many arthropathies is the destruction of articular cartilage and consequent loss of articular function. Chondrocyte dysfunction plays a key role in the pathogenesis of such disorders. To explore the global gene expression profiles in a human chondrocyte-like cell line treated with BV, microarray analysis was performed. The HTB-94 human chondrosarcoma cells were treated with BV, lipopolysaccharide (LPS), or both. Of the 344 genes profiled in this study, with a cut-off level of 4-fold change in the expression, (1) 35 were downregulated following BV treatment, (2) 16 were upregulated and 7 downregulated following LPS treatment, and (3) 32 were downregulated following co-stimulation of BV and LPS. The results of the present study shows that treatment of BV reversed the LPS-induced upregulation of such genes as interleukin-6 (IL-6) receptor, matrix metalloproteinase 15 (MMP-15), tumor necrosis factor (ligand) superfamily-10, caspase-6 and tissue inhibitor of metalloproteinase-1 (TIMP-1). It is thought that microarrays will play an ever-growing role in the advance of our understanding of the pharmacologic actions of BV in the treatment of arthritis.

Do changes in the mechanical properties of articular cartilage promote catabolic destruction of cartilage and osteoarthritis?

Osteoarthritis (OA) is a joint disease characterized by cartilage degeneration, a thickening of subchondral bone, and formation of marginal osteophytes. Previous mechanical characterization of cartilage in our laboratory suggests that energy storage and dissipation is reduced in osteoarthritis as the extent of fibrillation and fissure formation increases. It is not clear whether the loss of energy storage and dissipation characteristics is a result of biochemical and/or biophysical changes that occur to hyaline cartilage in joints. The purpose of this study is to present data, on the strain rate dependence of the elastic and viscous behaviors of cartilage, in order to further characterize changes that occur in the mechanical properties that are associated with OA. We have previously hypothesized that the changes seen in the mechanical properties of cartilage may be due to altered mechanochemical transduction by chondrocytes. Results of incremental tensile stress-strain tests at strain rates between 100%/min and 10,000%/min conducted on OA cartilage indicate that the slope of the elastic stress-strain curve increases with increasing strain rate, unlike the reported behavior of skin and self-assembled collagen fibers. It is suggested that the strain-rate dependence of the elastic stress-strain curve is due to the presence of large quantities of proteoglycans (PGs), which protect articular cartilage by increasing the apparent stiffness. The increased apparent stiffness of articular cartilage at high strain rates may limit the stresses borne and prolong the onset of OA. It is further hypothesized that increased compressive loading of chondrocytes in the intermediate zone of articular cartilage occurs as a result of normal wear to the superficial zone or from excessive impact loading. Once the superficial zone of articular cartilage is worn away, the tension is decreased throughout all cartilage zones leading to increased chondrocyte compressive loading and up-regulation of mechanochemical transduction processes that elaborate catabolic enzymes.

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

OBJECTIVE

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

Integrin-mediated mechanotransduction processes in TGFbeta-stimulated monolayer-expanded chondrocytes

Previous studies have demonstrated that passage in monolayer detrimentally affects the response of articular chondrocytes to the application of dynamic compression. Transforming growth factor beta (TGFbeta) is known to regulate metabolic processes in articular cartilage and can enhance the re-expression of a chondrocytic phenotype following monolayer expansion. The current study tests the hypothesis that TGFbeta also modulates the response of monolayer-expanded human chondrocytes to the application of dynamic compression, via an integrin-mediated mechanotransduction process. The data presented demonstrate that TGFbeta3 enhanced 35SO4 and [3H]thymidine incorporation and inhibited nitrite release after 48 h of culture when compared to unsupplemented constructs. Dynamic compression also enhanced 35SO4 and [3H]thymidine incorporation and inhibited nitrite release in the presence of TGFbeta3. By contrast, dynamic compression did not alter these parameters in the absence of the growth factor. The addition of the peptide, GRGDSP, which acts as a competitive ligand for the alpha5beta1 integrin, reversed the compression-induced stimulation of 35SO4 incorporation, [3H]thymidine incorporation, and suppression of nitrite release. No effect was observed when the control peptide, GRADSP, was used. The current data clearly demonstrate that the dynamic compression-induced changes observed in cell metabolism for human monolayer-expanded chondrocytes were dependent on the presence of TGFbeta3 and are integrin-mediated.

The role of biomechanics and inflammation in cartilage injury and repair

Osteoarthritis is a painful and debilitating disease characterized by progressive degenerative changes in the articular cartilage and other joint tissues. Biomechanical factors play a critical role in the initiation and progression of this disease, as evidenced by clinical and animal studies of alterations in the mechanical environment of the joint caused by trauma, joint instability, disuse, or obesity. The onset of these changes after joint injury generally has been termed posttraumatic arthritis and can be accelerated by factors such as a displaced articular fracture. Within this context, there is considerable evidence that interactions between biomechanical factors and proinflammatory mediators are involved in the progression of cartilage degeneration in posttraumatic arthritis. In vivo studies have shown increased concentrations of inflammatory cytokines and mediators in the joint in mechanically induced models of osteoarthritis. In vitro explant studies confirm that mechanical load is a potent regulator of matrix metabolism, cell viability, and the production of proinflammatory mediators such as nitric oxide and prostaglandin E2. Knowledge of the interaction of inflammatory and biomechanical factors in regulating cartilage metabolism would be beneficial to an understanding of the etiopathogenesis of posttraumatic osteoarthritis and in the improvement of therapies for joint injury.

Dynamic compression of chondrocyte-seeded fibrin gels: effects on matrix accumulation and mechanical stiffness

OBJECTIVE

Various strategies have been tested to direct and control matrix synthesis in tissue engineered cartilage, including mechanical stimulation of the construct both before and after implantation. This study examined the effects of oscillatory compression on chondrocytes in a fibrin-based tissue engineered cartilage.

DESIGN

Chondrocyte-seeded fibrin gels were cultured under unconfined mechanical compression for 10 or 20 days (free-swelling, 10% static, or 10+/-4% at 0.1 or 1Hz). During the culture period, accumulation of nitrite, sGAG, and proteolytic enzymes in the culture media were monitored. Following culture, the mechanical stiffness and biochemical content of the gels (DNA, sGAG, and hydroxyproline content and GAG Delta-disaccharide composition) were assessed.

RESULTS

Compared to free-swelling conditions, static compression had little effect on the mechanical stiffness or biochemical content of the gels. Compared to static compression, oscillatory compression produced softer gels, inhibited sGAG and hydroxyproline accumulation in the gels, and stimulated accumulation of nitrite and sGAG in the culture media. Minimal differences were observed in DNA content and Delta-disaccharide composition across treatment conditions.

CONCLUSIONS

In this study, oscillatory compression inhibited formation of cartilage-like tissues by chondrocytes in fibrin gels. These results suggest that the effects of mechanical stimuli on tissue engineered cartilage may vary substantially between different scaffold systems.

Mechanical regulation of mitogen-activated protein kinase signaling in articular cartilage

Articular chondrocytes respond to mechanical forces by alterations in gene expression, proliferative status, and metabolic functions. Little is known concerning the cell signaling systems that receive, transduce, and convey mechanical information to the chondrocyte interior. Here, we show that ex vivo cartilage compression stimulates the phosphorylation of ERK1/2, p38 MAPK, and SAPK/ERK kinase-1 (SEK1) of the JNK pathway. Mechanical compression induced a phased phosphorylation of ERK consisting of a rapid induction of ERK1/2 phosphorylation at 10 min, a rapid decay, and a sustained level of ERK2 phosphorylation that persisted for at least 24 h. Mechanical compression also induced the phosphorylation of p38 MAPK in strictly a transient fashion, with maximal phosphorylation occurring at 10 min. Mechanical compression stimulated SEK1 phosphorylation, with a maximum at the relatively delayed time point of 1 h and with a higher amplitude than ERK1/2 and p38 MAPK phosphorylation. These data demonstrate that mechanical compression alone activates MAPK signaling in intact cartilage. In addition, these data demonstrate distinct temporal patterns of MAPK signaling in response to mechanical loading and to the anabolic insulin-like growth factor-I. Finally, the data indicate that compression coactivates distinct signaling pathways that may help define the nature of mechanotransduction in cartilage.

Heelstrike and the pathomechanics of osteoarthrosis: a pilot gait study

Involvement of mechanical factors in osteoarthrosis (OA) has been well documented. For OA of the human lower limb, the impulse imparted at heelstrike has been suggested as a pathogenic factor. It has also been reported that there is a large amount of variation in the level of impulse experienced by different individuals, and it is suggested that those who experience large impulses are at a greater risk of developing OA. The current study investigated gait patterns of 12 normal subjects to establish the gait determinants responsible for producing large impulses at heelstrike. The results suggest that subtle variations in the early part of the swing phase pattern are responsible for large differences in the impulse experienced at heelstrike; the usually reported gait variables mask these variations.

Temporal regulation of chondrocyte metabolism in agarose constructs subjected to dynamic compression

The temporal response of chondrocyte metabolism in agarose constructs subjected to different dynamic compression regimes was investigated. The current study explored the effects of continuous or intermittent compression using various duty cycles of dynamic compressive loading, over a 48 h culture period. For the continuous compression experiments, duty cycles ranged from 5400 to 172,800 and intermittent compression delivered a total of 86,400 cycles. Large numbers of duty cycles significantly stimulated proteoglycan synthesis with maximal levels obtained for constructs subjected to 12h of intermittent compression. The shortest duration of intermittent compression suggested that further cycles are inhibitory for cell proliferation. Nitrite release was independent of the length or type of compressive regime applied. The uncoupled nature of the metabolic response determined in this study suggests that mechanical conditioning regimes may be fine tuned to selectively stimulate key metabolic parameters of relevance to cartilage tissue engineering.

Dynamic compressive strain inhibits nitric oxide synthesis by equine chondrocytes isolated from different areas of the cartilage surface

REASONS FOR PERFORMING STUDY

Chondrocytes within articular cartilage respond to the mechanical stresses associated with normal joint loading via a series of signalling pathways. Specific biomolecules, such as nitric oxide (NO), have been implicated in these mechanotransduction processes. It has been shown that the synthesis of NO can be inhibited by dynamic compressive strain of chondrocytes in vitro which, in turn, leads to an up-regulation of specific metabolic parameters.

HYPOTHESIS

Chondrocytes isolated from different joint locations and seeded in agarose constructs respond in a distinct manner to the application of dynamic compression.

METHODS

Chondrocytes were isolated separately from the equine patella groove and the femoral condyle, representing high loaded areas (HLA) and low loaded areas (LLA), respectively, of 6 specimens of different ages. The cells were seeded in agarose constructs and cultured either in an unstrained state or strained under dynamic loading at 1 Hz for 48 h. The synthesis of nitric oxide (NO), proteoglycan synthesis and chondrocyte proliferation were assessed.

RESULTS

Equine chondrocytes were found to synthesise significant basal levels of NO, regardless of topographical origin or age of tissue. Marked differences in both proteoglycan synthesis and cell proliferation were, however, revealed between the 2 chondrocyte subpopulations. Dynamic compression inhibited NO synthesis but significant alterations in proteoglycan synthesis and cell proliferation were apparent in a minority of cases.

CONCLUSIONS AND POTENTIAL RELEVANCE

The differential response of the subpopulations of chondrocytes derived from the HLA and LLA provides a potential mechanism which enables the biomechanical demands of differing joint regions to be maintained.

Regulation of matrix turnover in meniscal explants: role of mechanical stress, interleukin-1, and nitric oxide

The meniscus is an intra-articular fibrocartilaginous structure that serves essential biomechanical roles in the knee. With injury or arthritis, the meniscus may be exposed to significant changes in its biochemical and biomechanical environments that likely contribute to the progression of joint disease. The goal of this study was to examine the influence of mechanical stress on matrix turnover in the meniscus in the presence of interleukin-1 (IL-1) and to determine the role of nitric oxide (NO) in these processes. Explants of porcine menisci were subjected to dynamic compressive stresses at 0.1 MPa for 24 h at 0.5 Hz with 1 ng/ml IL-1, and the synthesis of total protein, proteoglycan, and NO was measured. The effects of a nitric oxide synthase 2 (NOS2) inhibitor were determined. Dynamic compression significantly increased protein and proteoglycan synthesis by 68 and 58%, respectively, compared with uncompressed explants. This stimulatory effect of mechanical stress was prevented by the presence of IL-1 but was restored by specifically inhibiting NOS2. Release of proteoglycans into the medium was increased by IL-1 or mechanical compression and further enhanced by IL-1 and compression together. Stimulation of proteoglycan release in response to compression was dependent on NOS2 regardless of the presence of IL-1. These finding suggest that IL-1 may modulate the effects of mechanical stress on extracellular matrix turnover through a pathway that is dependent on NO.

Signal transduction by mechanical strain in chondrocytes

PURPOSE OF REVIEW

Exercise and passive motion exert reparative effects on inflamed joints, whereas excessive mechanical forces initiate cartilage destruction as observed in osteoarthritis. However, the intracellular mechanisms that convert mechanical signals into biochemical events responsible for cartilage destruction and repair remain paradoxical. This review summarizes how signals generated by mechanical stress may initiate repair or destruction of cartilage.

RECENT FINDINGS

Mechanical strain of low magnitude inhibits inflammation by suppressing IL-1beta and TNF-alpha-induced transcription of multiple proinflammatory mediators involved in cartilage degradation. This also results in the upregulation of proteoglycan and collagen synthesis that is drastically inhibited in inflamed joints. On the contrary, mechanical strain of high magnitude is proinflammatory and initiates cartilage destruction while inhibiting matrix synthesis. Investigations reveal that mechanical signals exploit nuclear factor-kappa B as a common pathway for transcriptional inhibition/activation of proinflammatory genes to control catabolic processes in chondrocytes. Mechanical strain of low magnitude prevents nuclear translocation of nuclear factor kappa B, resulting in the suppression of proinflammatory gene expression, whereas mechanical strain of high magnitude induces transactivation of nuclear factor kappa B, and thus proinflammatory gene induction.

SUMMARY

The beneficial effects of physiological levels of mechanical signals or exercise may be explained by their ability to suppress the signal transduction pathways of proinflammatory/catabolic mediators, while stimulating anabolic pathways. Whether these anabolic signals are a consequence of the inhibition of nuclear factor kappa B or are mediated via distinct anabolic pathways is yet to be elucidated.

The use of specific chondrocyte populations to modulate the properties of tissue-engineered cartilage

Tissue engineering of articular cartilage is a promising alternative to the conventional approaches for cartilage repair. However, recent attempts to develop articular cartilage in vitro have proven to be difficult. The tissue formed in vitro may not accumulate enough extracellular matrix, and the resulting mechanical properties are only a fraction of the native tissue. We investigated whether using specific populations of chondrocytes would improve the properties of the cartilaginous tissue that was generated in vitro. Full-thickness (FT), mid-and-deep zone (MD), and deep-zone (DEEP) chondrocytes were isolated, placed on the surface of porous ceramic substrates and maintained in culture for eight weeks. Tissue developed from DEEP chondrocytes was thicker (FT: 0.94+/-0.03, MD: 0.88+/-0.04, DEEP: 2.4+/-0.1 mm) and had accumulated larger amounts of extracellular matrix (FT: 1.61+/-0.05, MD: 1.5+/-0.1, DEEP: 3.8+/-0.2 mg dry weight) than the tissues formed by the FT and MD chondrocytes. The tissue formed by the FT chondrocytes accumulated the greatest amount of collagen (FT: 211+/-14, MD: 185+/-8, DEEP: 178+/-5 microg/mg dry weight) whereas the tissue formed by the MD chondrocytes accumulated significantly more proteoglycans (FT: 198+/-10, MD: 265+/-10, DEEP: 215+/-5 microg/mg dry weight). Interestingly, MD chondrocytes produced tissue that had compressive mechanical properties up to four times greater than the cartilaginous tissues formed by cells from either the FT or DEEP of cartilage. Thus, a combined population of intermediate and DEEP chondrocytes might be more suitable for the tissue engineering of articular cartilage.

Protective effects of intermittent hydrostatic pressure on osteoarthritic chondrocytes activated by bacterial endotoxin in vitro

The role of continuous passive motion (CPM) in the management of septic arthritis and inflammatory arthritis remains of interest. CPM produces cyclic variations in intraarticular pressure that facilitates transport of fluid, nutrients, and solutes within and/or across the joint and stimulates chondrocyte metabolism. However, the precise mechanisms mediating the responses of chondrocytes to joint motion remain unclear. This study tested the hypothesis that dynamic mechanical loading counteracts effects of bacterial lipopolysaccharide (LPS), an inflammatory mediator, on chondrocyte metabolism. Intermittent hydrostatic pressure (IHP) (10 MPa for 4 h) was applied to human chondrocytes pretreated with LPS (1 microg/ml for 18 h). LPS activation of chondrocytes decreased mRNA signal levels of type II collagen by 67% and aggrecan by 56% and increased nitric oxide by 3.1-fold, monocyte chemotactic protein-1 mRNA signal levels by 6.5-fold, and matrix metalloproteinase-2 mRNA signal levels by 1.3-fold. Application of IHP to LPS-activated chondrocytes decreased nitric oxide synthase mRNA signal levels and nitric oxide levels in the culture medium. Exposure of LPS-activated chondrocytes to IHP upregulated type II collagen and aggrecan mRNA signal levels by 1.7-fold, relative to chondrocytes activated by LPS and maintained without loading. In addition, application of IHP decreased the upregulation in signal levels of monocyte chemotactic factor-1 and matrix metalloproteinase-2 following LPS activation by 45% and 15%, respectively. These data show that mechanical loading counteract effects of inflammatory agents, such as bacterial LPS, and suggest that postinfection sequelae are influenced by the presence or absence of joint 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.

Tumor necrosis factor alpha-dependent proinflammatory gene induction is inhibited by cyclic tensile strain in articular chondrocytes in vitro

OBJECTIVE

To understand the intracellular mechanisms of the action of mechanical strain on articular chondrocytes during inflammation.

METHODS

One of the major mediators responsible for cartilage destruction in inflamed articular joints is tumor necrosis factor alpha (TNFalpha). Therefore, in this study we examined the intracellular mechanisms of actions of cyclic tensile strain (CTS) on the recombinant human TNFalpha (rHuTNFalpha)-induced proinflammatory pathways in primary cultures of chondrocytes. The expression of messenger RNA (mRNA) for TNFalpha-dependent proinflammatory proteins was examined by semiquantitative reverse transcriptase-polymerase chain reaction. The synthesis of proinflammatory proteins was examined by Western blot analysis in cell extracts, followed by semiquantitative measurement of bands using densitometric analysis. Nitric oxide production was measured by Griess reaction, and prostaglandin E2 production was assessed by radioimmunoassays. The proteoglycan synthesis in chondrocytes was assessed by incorporation of Na2(35)SO4 in chondroitin sulfate proteoglycans.

RESULTS

By exposing chondrocytes to CTS in the presence of TNFalpha in vitro, we showed that CTS is an effective antagonist of TNFalpha actions and acts as both an antiinflammatory signal and a reparative signal. CTS of low magnitude suppresses TNFalpha-induced mRNA expression of multiple proinflammatory proteins involved in catabolic responses, such as inducible nitric oxide synthase, cyclooxygenase 2, and collagenase. CTS also counteracts cartilage degradation by augmenting induction of tissue inhibitor of metalloproteinase 2. Additionally, CTS augments the reparative process via abrogation of TNFalpha-induced suppression of proteoglycan synthesis. Nonetheless, CTS acts on chondrocytes in a TNFalpha-dependent manner, since exposure of chondrocytes to CTS alone had no effect on these parameters.

CONCLUSION

CTS of low magnitude acts as an effective antagonist of TNFalpha not only by inhibiting the TNFalpha-dependent induction of proinflammatory proteins upstream of mRNA transcription, but also by augmenting the proteoglycan synthesis that is inhibited by TNFalpha.

Dynamic compression inhibits the synthesis of nitric oxide and PGE(2) by IL-1beta-stimulated chondrocytes cultured in agarose constructs

Both mechanical loading and interleukin-1beta (IL-1beta) are known to regulate metabolic processes in articular cartilage through pathways mediated by nitric oxide ((*)NO) and PGE(2). This study uses a well-characterized model system involving isolated chondrocytes cultured in agarose constructs to test the hypothesis that dynamic compression alters the synthesis of (*)NO and PGE(2) by IL-1beta-stimulated articular chondrocytes. The data presented demonstrate for the first time that dynamic compression counteracts the effects of IL-1beta on articular chondrocytes by suppressing both (*)NO and PGE(2) synthesis. Inhibitor experiments indicated that the dynamic compression-induced inhibition of PGE(2) synthesis and stimulation of proteoglycan synthesis were (*)NO mediated, while compression-induced stimulation of cell proliferation was (*)NO independent. The inhibition of (*)NO and PGE(2) by dynamic compression is a finding of major significance that could contribute to the development of novel strategies for the treatment of cartilage-degenerative disorders.