The effect of continuously applied cyclic mechanical loading on the fibronectin metabolism of articular cartilage explants

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Subphysiological compressive loading reduces apoptosis following acute impact injury in a porcine cartilage model

BACKGROUND

Acute cartilage injuries induce cell death and are associated with an increased incidence of osteoarthritis development later in life. The objective of this study was to investigate the effect of posttraumatic cyclic compressive loading on chondrocyte viability and apoptosis in porcine articular cartilage plugs.

HYPOTHESIS

Compressive loading of acutely injured cartilage can maintain chondrocyte viability by reducing apoptosis after a traumatic impact injury.

STUDY DESIGN

In vitro controlled laboratory study.

LEVEL OF EVIDENCE

Level 5.

METHODS

Each experiment compared 4 test groups: control, impact, impact with compressive loading (either 0.5 or 0.8 MPa), and no impact but compressive loading (n = 15 per group). Flat, full-thickness articular cartilage plugs were harvested from the trochlear region of porcine knees. A drop tower was utilized to introduce an impact injury. The articular plugs were subjected to two 30-minute cycles of either 0.5 or 0.8 MPa of dynamic loading. Cell viability, apoptosis, and gene expression of samples were evaluated 24 hours postimpaction.

RESULTS

Cell viability staining showed that 0.5 MPa of dynamic compressive loading increased cell viability compared with the impact group. Apoptotic analysis revealed a decrease in apoptotic expression in the group with 0.5 MPa of dynamic compressive loading compared with the impact group. Significantly higher caspase 3 and lower collagen II expressions were observed in impacted samples without compressive loading, compared with those with. Compressive loading of nonimpacted samples significantly increased collagen II and decreased caspase 3 expressions.

CONCLUSION

In this porcine in vitro model, dynamic compressive loading at subphysiological levels immediately following impact injury decreases apoptotic expression, thereby maintaining chondrocyte viability.

CLINICAL RELEVANCE

Therapeutic exercises could be designed to deliver subphysiological loading to the injured cartilage, thereby minimizing injury.

Low oxygen tension increased fibronectin fragment induced catabolic activities--response prevented with biomechanical signals

Introduction

The inherent low oxygen tension in normal cartilage has implications on inflammatory conditions associated with osteoarthritis (OA). Biomechanical signals will additionally contribute to changes in tissue remodelling and influence the inflammatory response. In this study, we investigated the combined effects of oxygen tension and fibronectin fragment (FN-f) on the inflammatory response of chondrocytes subjected to biomechanical signals.

Methods

Chondrocytes were cultured under free-swelling conditions at 1%, 5% and 21% oxygen tension or subjected to dynamic compression in an ex vivo 3D/bioreactor model with 29 kDa FN-f, interleukin-1beta (IL-1β) and/or the nitric oxide synthase (NOS) inhibitor for 6 and 48 hours. Markers for catabolic activity (NO, PGE₂), tissue remodelling (GAG, MMPs) and cytokines (IL-1β, IL-6 and TNFα) were quantified by biochemical assay. Aggrecan, collagen type II, iNOS and COX-2 gene expression were examined by real-time quantitative PCR. Two-way ANOVA and a post hoc Bonferroni-corrected t-test were used to analyse data.

Results

Both FN-fs and IL-1β increased NO, PGE₂ and MMP production (all P < 0.001). FN-f was more active than IL-1β with greater levels of NO observed at 5% than 1% or 21% oxygen tension (P < 0.001). Whilst FN-f reduced GAG synthesis at all oxygen tension, the effect of IL-1β was significant at 1% oxygen tension. In unstrained constructs, treatment with FN-f or IL-1β increased iNOS and COX-2 expression and reduced aggrecan and collagen type II (all P < 0.001). In unstrained constructs, FN-f was more effective than IL-1β at 5% oxygen tension and increased production of NO, PGE₂, MMP, IL-1β, IL-6 and TNFα. At 5% and 21% oxygen tension, co-stimulation with compression and the NOS inhibitor abolished fragment or cytokine-induced catabolic activities and restored anabolic response.

Conclusions

The present findings revealed that FN-fs are more potent than IL-1β in exerting catabolic effects dependent on oxygen tension via iNOS and COX-2 upregulation. Stimulation with biomechanical signals abolished catabolic activities in an oxygen-independent manner and NOS inhibitors supported loading-induced recovery resulting in reparative activities. Future investigations will utilize the ex vivo model as a tool to identify key targets and therapeutics for OA treatments.

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.

Biomechanical modulation of collagen fragment-induced anabolic and catabolic activities in chondrocyte/agarose constructs

INTRODUCTION

The present study examined the effect of collagen fragments on anabolic and catabolic activities by chondrocyte/agarose constructs subjected to dynamic compression.

METHODS

Constructs were cultured under free-swelling conditions or subjected to continuous and intermittent compression regimes, in the presence of the N-terminal (NT) and C-terminal (CT) telopeptides derived from collagen type II and/or 1400 W (inhibits inducible nitric oxide synthase (iNOS)). The anabolic and catabolic activities were compared to the amino-terminal fibronectin fragment (NH2-FN-f) and assessed as follows: nitric oxide (NO) release and sulphated glycosaminoglycan (sGAG) content were quantified using biochemical assays. Tumour necrosis factor-alpha (TNFalpha) and interleukin-1beta (IL-1beta) release were measured by ELISA. Gene expression of matrix metalloproteinase-3 (MMP-3), matrix metalloproteinase-13 (MMP-13), collagen type II and fibronectin were assessed by real-time quantitative polymerase chain reaction (qPCR). Two-way ANOVA and the post hoc Bonferroni-corrected t-test was used to examine data.

RESULTS

The presence of the NT or CT peptides caused a moderate to strong dose-dependent stimulation of NO, TNFalpha and IL-1beta production and inhibition of sGAG content. In some instances, high concentrations of telopeptides were just as potent in stimulating catabolic activities when compared to NH2-FN-f. Depending on the concentration and type of fragment, the increased levels of NO and cytokines were inhibited with 1400 W, resulting in the restoration of sGAG content. Depending on the duration and type of compression regime employed, stimulation with compression or incubation with 1400 W or a combination of both, inhibited telopeptide or NH2-FN-f induced NO release and cytokine production and enhanced sGAG content. All fragments induced MMP-3 and MMP-13 expression in a time-dependent manner. This effect was reversed with compression and/or 1400 W resulting in the restoration of sGAG content and induction of collagen type II and fibronectin expression.

CONCLUSIONS

Collagen fragments containing the N- and C-terminal telopeptides have dose-dependent catabolic activities similar to fibronectin fragments and increase the production of NO, cytokines and MMPs. Catabolic activities were downregulated by dynamic compression or by the presence of the iNOS inhibitor, linking reparative activities by both types of stimuli. Future investigations which examine the signalling cascades of chondrocytes in response to matrix fragments with mechanical influences may provide useful information for early osteoarthritis treatments.

Passage and reversal effects on gene expression of bovine meniscal fibrochondrocytes

The knee meniscus contains a mixed population of cells that exhibit fibroblastic as well as chondrocytic characteristics. Tissue engineering studies and future therapies for the meniscus require a large population of cells that are seeded on scaffolds. To achieve this, monolayer expansion is often used as a technique to increase cell number. However, the phenotype of these cells may be significantly different from that of the primary population. The objective of this study was to investigate changes in meniscal fibrochondrocytes at the gene expression level over four passages using quantitative real-time reverse transcriptase polymerase chain reaction. Cells from the inner two-thirds of bovine medial menisci were used. Four extracellular matrix (ECM) molecules, commonly found in the meniscus, were investigated, namely collagen I, collagen II, aggrecan and cartilage oligomeric matrix protein (COMP). In addition, primary and passaged meniscus fibrochondrocytes were placed on surfaces coated with collagen I or aggrecan protein to investigate whether any gene expression changes resulting from passage could be reversed. Collagen I expression was found to increase with the number of passages, whereas collagen II and COMP expression decreased. Collagen I and aggrecan surface coatings were shown to downregulate and upregulate collagen I and COMP expression levels, respectively, in passaged cells. However, decreases in collagen II expression could not be reversed by either protein coating. These results indicate that although monolayer expansion results in significant changes in gene expression in meniscal fibrochondrocytes, protein coatings may be used to regain the primary cell expression of several ECM molecules.

Mandibular appliance modulates condylar growth through integrins

Functional orthopedic therapy corrects growth discrepancies between the maxilla and mandible, possibly through postural changes in the musculature and modulation of the mandibular condylar cartilage growth. Using Wistar rats, we tested the hypothesis that chondrocytes respond to forces generated by a mandibular propulsor appliance by changes in gene expression, and that integrins are important mediators in this response. Immunohistochemical analyses demonstrated that the use of the appliance for different periods of time modulated the expression of fibronectin, alpha5 and alphav integrin subunits, as well as cell proliferation in the cartilage. In vitro, cyclic distension of condylar cartilage-derived cells increased fibronectin mRNA, as well as Insulin-like Growth Factor-I and II mRNA and cell proliferation. A peptide containing the Arginine-Glycine-Asparagine sequence (RGD), the main cell-binding sequence in fibronectin, blocked almost all these effects, confirming that force itself modulates the growth of the rat condylar cartilage, and that RGD-binding integrins participate in mechanotransduction.

Intermittent mechanical loading of articular cartilage explants modulates chondroitin sulfate fine structure

OBJECTIVE

Alterations in the sulfation pattern of chondroitin sulfate (CS) chains of proteoglycans have been associated with aging and degeneration of articular cartilage. The purpose of the present study was to investigate systematically the effect of load amplitudes, frequencies and load durations of intermittently applied mechanical pressure on the sulfation of CS chains of cultured bovine articular cartilage explants.

METHODS

Using a sinusoidal waveform of 0.5 Hz frequency, cyclic compressive pressure of 0.1-1.0 MPa was applied for 10s followed by a period of unloading lasting 10-1000 s. These intermittent loading protocols were repeated for a total duration of 1-6 days. Newly synthesized as well as endogenous CS chains were isolated, depolymerized and subsequently quantitated after fractionation by high-performance anion-exchange chromatography.

RESULTS

Increasing the mechanical demands on cartilage explants by elevating either the duration or the frequency of loading can significantly alter the fine structure of newly synthesized CS in that less chains terminate on galNAc4,6S and, in that simultaneously the ratio of the internal disaccharides DeltaDi6S to DeltaDi4S is increased. Similar results were obtained with explants being slightly mechanically challenged by low magnitudes of loads.

CONCLUSION

Our data show for the first time that intermittent loading of articular cartilage explants can significantly alter the sulfation pattern of the terminal CS residues as well as of the internal disaccharides. Furthermore, our results indicate that explants possess a physiological window of stress in which they are able to produce also a normal extracellular matrix.

A Direct Compression Stimulator for Articular Cartilage and Meniscal Explants

This paper describes the development and use of a direct compression stimulator for culturing explants from the meniscus of the knee and articular cartilage. Following design and fabrication of the instrument along with its data acquisition system, the function of the machine was verified by both mechanical means and tissue effect. The loading chamber can hold up to 45 5 mm diameter samples. While designed to stimulate samples up to 4 mm thick, axial displacements as little as 0.127 microm are within the theoretical capacity of the stimulator. In gene expression studies, collagen II and aggrecan expression were examined in explants from articular cartilage as well as medial and lateral menisci subjected to dynamic stimulation and static compression. These results were then compared to free swelling samples. It was found that static compression to cut thickness down-regulated aggrecan and collagen II expression in articular cartilage explants compared to free swelling controls by 94% and 90%, respectively. The application of a dynamic, intermittent, 2% oscillation around the cut thickness returned expression levels to those of free swelling controls at 4 h but not at 76 h. In medial meniscus samples, dynamic compression up-regulated aggrecan expression by 108%, but not collagen II expression, at 4 and 76 h compared to static controls. No difference in gene expression was observed for lateral meniscal explants. Thus, effects of direct compression seen in articular cartilage may not necessarily translate to the knee meniscus. The design of this stimulator will allow a variety of tissues and loading regimens to be examined. It is hoped that regimens can be found that not only return samples to the production levels of free swelling controls, but also surpass them in terms of gene expression, protein synthesis, and functional properties.

Pathomechanisms of cartilage destruction by mechanical injury

Mechanical injury is considered to be a major inductor of articular cartilage destruction and therefore a risk factor for the development of secondary osteoarthritis. Mechanical injury induces damage to the tissue matrix directly or mediated by chondrocytes via expression of matrix-degrading enzymes and reduction of biosynthetic activity. As a consequence the mechanical properties of cartilage change. Some of the pathomechanisms of mechanical injury have already been uncovered by the use of a broad range of in vitro-models. They demonstrate that mechanical injury induces tissue swelling and decrease in both the compressive and shear stiffness of articular cartilage, probably due to disruption of the collagen network. Injurious compression induces chondrocyte death by necrosis and apoptosis and the remaining cells decrease their biosynthetic activity. The tissue content of proteoglycans also decreases with time in injured cartilage, and the tissue loses its ability to respond to physiological levels of mechanical stimulation with an increase in biosynthesis. Immature cartilage seems to be more vulnerable to injurious compression than more mature tissue. The expression of several matrix-degrading enzymes like ADAM-TS5 and matrix-metalloproteinases (MMP-1, MMP-2, MMP-3, MMP-9, MMP-13) is increased after injury and may in part be regulated by an autocrine vascular endothelial growth factor (VEGF)-dependent signalling pathway. Apoptosis seems to be mediated by caspase activity and reactive oxygen species. For that reason activation of antioxidative defense mechanisms as well as the inhibition of angiogenetic factors and MMPs might be key regulators in the mechanically induced destruction of cartilage and might be suggested as potential therapeutic interventions. This review summarizes some of the most important data from in vitro injury studies dealing with the pathomechanisms of cartilage destruction.

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

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

The sulfation pattern of chondroitin sulfate from articular cartilage explants in response to mechanical loading

Chondrocytes within articular cartilage experience complete unloading between loading cycles thereby utilizing mechanical signals to regulate their own anabolic and catabolic activities. Structural alterations of proteoglycans (PGs) during aging and the development of osteoarthritis (OA) have been reported; whether these can be attributed to altered load or compression is largely unknown. We report here on experiments in which the effect of intermittent loading on the fine structure of newly synthesized chondroitin sulfate (CS) in bovine articular cartilage explants was examined. Tissues were subjected for 6 days to cyclic compressive pressure using a sinusoidal waveform of 0.1, 0.5 or 1.0 Hz frequency with a peak stress of 0.5 MPa for a period of 5, 10 or 20 s, followed by an unloading period lasting 10, 100 or 1000 s. During the final 18 h of the culture, cartilage explants were radiolabeled with 50 microCi/ml D-6-[3H]glucosamine, and newly synthesized as well as endogenous CS chains were isolated after proteinase solubilization of the tissue. CS chains were depolymerized with chondroitinase ABC and ACII, and the 3H-digestion products were quantified after fractionation by high-performance anion-exchange chromatography using a CarboPac PA1 column. Intermittently applied cyclic mechanical loading did not affect the proportion of 4- and 6-sulfated disaccharide repeats, but caused a significant decrease in the abundance of the 4,6-disulfated nonreducing terminal galNAc residues. In addition, loading induced elongation of CS chains. Taken together, these data provide evidence for the first time that long-term in vitro loading results in marked and reproducible changes in the fine structure of newly synthesized CS, and that accumulation of such chains may in turn modify the physicochemical and biological response of articular cartilage. Moreover, data presented here suggest that in vitro dynamic compression of cartilage tissue can induce some of the same alterations in CS sulfation that have previously been shown to occur during the development of degenerative joint diseases such as OA.

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.