Cyclic tensile stretch on bovine articular chondrocytes inhibits protein kinase C activity

A high throughput cell stretch device for investigating mechanobiology in vitro

Mechanobiology is a rapidly advancing field, with growing evidence that mechanical signaling plays key roles in health and disease. To accelerate mechanobiology-based drug discovery, novel in vitro systems are needed that enable mechanical perturbation of cells in a format amenable to high throughput screening. Here, both a mechanical stretch device and 192-well silicone flexible linear stretch plate were designed and fabricated to meet high throughput technology needs for cell stretch-based applications. To demonstrate the utility of the stretch plate in automation and screening, cell dispensing, liquid handling, high content imaging, and high throughput sequencing platforms were employed. Using this system, an assay was developed as a biological validation and proof-of-concept readout for screening. A mechano-transcriptional stretch response was characterized using focused gene expression profiling measured by RNA-mediated oligonucleotide Annealing, Selection, and Ligation with Next-Gen sequencing. Using articular chondrocytes, a gene expression signature containing stretch responsive genes relevant to cartilage homeostasis and disease was identified. The possibility for integration of other stretch sensitive cell types (e.g., cardiovascular, airway, bladder, gut, and musculoskeletal), in combination with alternative phenotypic readouts (e.g., protein expression, proliferation, or spatial alignment), broadens the scope of high throughput stretch and allows for wider adoption by the research community. This high throughput mechanical stress device fills an unmet need in phenotypic screening technology to support drug discovery in mechanobiology-based disease areas.

Spectral characterization of cell surface motion for mechanistic investigations of cellular mechanobiology

Understanding the specific mechanisms responsible for anabolic and catabolic responses to static or dynamic force are largely poorly understood. Because of this, most research groups studying mechanotransduction due to dynamic forces employ an empirical approach in deciding what frequencies to apply during experiments. While this has been shown to elucidate valuable information regarding how cells respond under controlled provocation, it is often difficult or impossible to determine a true optimal frequency for force application, as many intracellular complexes are involved in receiving, propagating, and responding to a given stimulus. Here we present a novel adaptation of an analytical technique from the fields of civil and mechanical engineering that may open the door to direct measurement of mechanobiological cellular frequencies which could be used to target specific cell signaling pathways leveraging synergy between outside-in and inside-out mechanotransduction approaches. This information could be useful in identifying how specific proteins are involved in the homeostatic balance, or disruption thereof, of cells and tissue, furthering the understanding of the pathogenesis and progression of many diseases across a wide variety of cell types, which may one day lead to the development of novel mechanobiological therapies for clinical use.

Combining stretching and gallic acid to decrease inflammation indices and promote extracellular matrix production in osteoarthritic human articular chondrocytes

Osteoarthritis (OA) patients undergo cartilage degradation and experience painful joint swelling. OA symptoms are caused by inflammatory molecules and the upregulation of catabolic genes leading to the breakdown of cartilage extracellular matrix (ECM). Here, we investigate the effects of gallic acid (GA) and mechanical stretching on the expression of anabolic and catabolic genes and restoring ECM production by osteoarthritic human articular chondrocytes (hAChs) cultured in monolayers. hAChs were seeded onto conventional plates or silicone chambers with or without 100 μM GA. A 5% cyclic tensile strain (CTS) was applied to the silicone chambers and the deposition of collagen and glycosaminoglycan, and gene expressions of collagen types II (COL2A1), XI (COL11A2), I (COL1A1), and X (COL10A1), and matrix metalloproteinases (MMP-1 and MMP-13) as inflammation markers, were quantified. CTS and GA acted synergistically to promote the deposition of collagen and glycosaminoglycan in the ECM by 14- and 7-fold, respectively. Furthermore, the synergistic stimuli selectively upregulated the expression of cartilage-specific proteins, COL11A2 by 7-fold, and COL2A1 by 47-fold, and, in contrast, downregulated the expression of MMP-1 by 2.5-fold and MMP-13 by 125-fold. GA supplementation with CTS is a promising approach for restoring osteoarthritic hAChs ECM production ability making them suitable for complex tissue engineering applications.

Chondrocytes Proliferation of Patients with Cartilage Lesions in Their Own Body for Use in Cartilage Tissue Engineering: Hypotheses on a New Approach for the Proliferation of Autologous Chondrocytes

Osteoarthritis is one of the most common chronic diseases, which have involved 250 million people around the world. One of the challenges in the field of cartilage tissue engineering is to provide an adequate source of chondrocytes to prevent changes in gene expression profile as a result of multiple passages.We hypothesized that by creating a low invasive lesion by scalpel or shear laser in the outer ear cartilage and stimulation of wound healing process, hyperplasia occurs and will provide an appropriate number of autologous chondrocytes for extraction and use in articular cartilage tissue engineering. Also, due to the effect of platelet-rich plasma and biomechanical forces in stimulating and accelerating of the repair process, these two factors can be used to achieve more desirable results.We describe a new approach to proliferate chondrocytes in the body. To evaluate this idea, various techniques of gene expression at the level of RNA or protein and animal experiments for histological studies can be used. Also, flowcytometry technique can be used to determine the cell viability and counting them.The use of autologous cell sources with minimal changes in gene expression profile can be promising in tissue engineering products.

Importance of reference gene selection for articular cartilage mechanobiology studies

OBJECTIVE

Identification of genes differentially expressed in mechano-biological pathways in articular cartilage provides insight into the molecular mechanisms behind initiation and/or progression of osteoarthritis (OA). Quantitative PCR (qPCR) is commonly used to measure gene expression, and is reliant on the use of reference genes for normalisation. Appropriate validation of reference gene stability is imperative for accurate data analysis and interpretation. This study determined in vitro reference gene stability in articular cartilage explants and primary chondrocytes subjected to different compressive loads and tensile strain, respectively.

DESIGN

The expression of eight commonly used reference genes (18s, ACTB, GAPDH, HPRT1, PPIA, RPL4, SDHA and YWHAZ) was determined by qPCR and data compared using four software packages (comparative delta-Ct method, geNorm, NormFinder and BestKeeper). Calculation of geometric means of the ranked weightings was carried out using RefFinder.

RESULTS

Appropriate reference gene(s) for normalisation of mechanically-regulated transcript levels in articular cartilage tissue or isolated chondrocytes were dependent on experimental set-up. SDHA, YWHAZ and RPL4 were the most stable genes whilst glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and to a lesser extent Hypoxanthine-guanine phosphoribosyltransferase (HPRT), showed variable expression in response to load, demonstrating their unsuitability in such in vitro studies. The effect of using unstable reference genes to normalise the expression of aggrecan (ACAN) and matrix metalloproteinase 3 (MMP3) resulted in inaccurate quantification of these mechano-sensitive genes and erroneous interpretation/conclusions.

CONCLUSION

This study demonstrates that commonly used 'reference genes' may be unsuitable for in vitro cartilage chondrocyte mechanobiology studies, reinforcing the principle that careful validation of reference genes is essential prior to each experiment to obtain robust and reproducible qPCR data for analysis/interpretation.

Impact of mechanical stretch on the cell behaviors of bone and surrounding tissues

Mechanical loading is recognized to play an important role in regulating the behaviors of cells in bone and surrounding tissues in vivo. Many in vitro studies have been conducted to determine the effects of mechanical loading on individual cell types of the tissues. In this review, we focus specifically on the use of the Flexercell system as a tool for studying cellular responses to mechanical stretch. We assess the literature describing the impact of mechanical stretch on different cell types from bone, muscle, tendon, ligament, and cartilage, describing individual cell phenotype responses. In addition, we review evidence regarding the mechanotransduction pathways that are activated to potentiate these phenotype responses in different cell populations.

Effects of cyclic tensile strain on chondrocyte metabolism: a systematic review

Chondrocytes reorganize the extracellular matrix of articular cartilage in response to externally applied loads. Thereby, different loading characteristics lead to different biological responses. Despite of active research in this area, it is still unclear which parts of the extracellular matrix adapt in what ways, and how specific loading characteristics affect matrix changes. This review focuses on the influence of cyclic tensile strain on chondrocyte metabolism in vitro. It also aimed to identify anabolic or catabolic chondrocyte responses to different loading protocols. The key findings show that loading cells up to 3% strain, 0.17 Hz, and 2 h, resulted in weak or no biological responses. Loading between 3–10% strain, 0.17–0.5 Hz, and 2–12 h led to anabolic responses; and above 10% strain, 0.5 Hz, and 12 h catabolic events predominated. However, this review also discusses that various other factors are involved in the remodeling of the extracellular matrix in response to loading, and that parameters like an inflammatory environment might influence the biological response.

Regulation of chondrogenesis by protein kinase C: Emerging new roles in calcium signalling

During chondrogenesis, complex intracellular signalling pathways regulate an intricate series of events including condensation of chondroprogenitor cells and nodule formation followed by chondrogenic differentiation. Reversible phosphorylation of key target proteins is of particular importance during this process. Among protein kinases known to be involved in these pathways, protein kinase C (PKC) subtypes play pivotal roles. However, the precise function of PKC isoenzymes during chondrogenesis and in mature articular chondrocytes is still largely unclear. In this review, we provide a historical overview of how the concept of PKC-mediated chondrogenesis has evolved, starting from the first discoveries of PKC isoform expression and activity. Signalling components upstream and downstream of PKC, leading to the stimulation of chondrogenic differentiation, are also discussed. Although it is evident that we are only at the beginning to understand what roles are assigned to PKC subtypes during chondrogenesis and how they are regulated, there are many yet unexplored aspects in this area. There is evidence that calcium signalling is a central regulator in differentiating chondroprogenitors; still, clear links between intracellular calcium signalling and prototypical calcium-dependent PKC subtypes such as PKCalpha have not been established. Exploiting putative connections and shedding more light on how exactly PKC signalling pathways influence cartilage formation should open new perspectives for a better understanding of healthy as well as pathological differentiation processes of chondrocytes, and may also lead to the development of novel therapeutic approaches.

Interleukin-4 protects matrix synthesis in chondrocytes under excessive mechanical stress in vitro

Abstract We examined the effect of interleukin-4 (IL-4) on matrix synthesis in chondrocytes under excessive mechanical stress in vitro. Chondrocytes from 7-day-old rat articular cartilage were incubated in the presence of rat IL-4 (0, 1, and 10 ng/ml) under a 5% CO2 atmosphere for 36 h. Cyclic mechanical stress (0.5 Hz, 7% elongation) was loaded using a Flexercell strain unit for 12, 16, and 24 h. Levels of messenger RNA (mRNA) for aggrecan (AGG), type II collagen (CII), fibronectin (FN), and integrin-β1 (INTb1) were measured by real-time reverse transcriptase polymerase chain reaction (RT-PCR) using GAPDH as the internal control. Mechanical stress for 16 h significantly decreased levels of mRNA for both AGG and CII (P < 0.01), but with rat IL-4 at doses of 1 or 10 ng/ml these levels recovered (P < 0.05). In addition, mRNA levels of FN and INTb1 were increased by IL-4 in chondrocytes under mechanical stress (P < 0.05). IL-4 protects chondrocytes loaded with excessive mechanical stress against degradation.

CCN family member 2/connective tissue growth factor (CCN2/CTGF) has anti-aging effects that protect articular cartilage from age-related degenerative changes

To examine the role of connective tissue growth factor CCN2/CTGF (CCN2) in the maintenance of the articular cartilaginous phenotype, we analyzed knee joints from aging transgenic mice (TG) overexpressing CCN2 driven by the Col2a1 promoter. Knee joints from 3-, 14-, 40-, and 60-day-old and 5-, 12-, 18-, 21-, and 24-month-old littermates were analyzed. Ccn2-LacZ transgene expression in articular cartilage was followed by X-gal staining until 5 months of age. Overexpression of CCN2 protein was confirmed through all ages in TG articular cartilage and in growth plates. Radiographic analysis of knee joints showed a narrowing joint space and other features of osteoarthritis in 50% of WT, but not in any of the TG mice. Transgenic articular cartilage showed enhanced toluidine blue and safranin-O staining as well as chondrocyte proliferation but reduced staining for type X and I collagen and MMP-13 as compared with those parameters for WT cartilage. Staining for aggrecan neoepitope, a marker of aggrecan degradation in WT articular cartilage, increased at 5 and 12 months, but disappeared at 24 months due to loss of cartilage; whereas it was reduced in TG articular cartilage after 12 months. Expression of cartilage genes and MMPs under cyclic tension stress (CTS) was measured by using primary cultures of chondrocytes obtained from wild-type (WT) rib cartilage and TG or WT epiphyseal cartilage. CTS applied to primary cultures of mock-transfected rib chondrocytes from WT cartilage and WT epiphyseal cartilage induced expression of Col1a1, ColXa1, Mmp-13, and Mmp-9 mRNAs; however, their levels were not affected in CCN2-overexpressing chondrocytes and TG epiphyseal cartilage. In conclusion, cartilage-specific overexpression of CCN2 during the developmental and growth periods reduced age-related changes in articular cartilage. Thus CCN2 may play a role as an anti-aging factor by stabilizing articular cartilage.

The design and development of a high-throughput magneto-mechanostimulation device for cartilage tissue engineering

To recapitulate the in vivo environment and create neo-organoids that replace lost or damaged tissue requires the engineering of devices, which provide appropriate biophysical cues. To date, bioreactors for cartilage tissue engineering have focused primarily on biomechanical stimulation. There is a significant need for improved devices for articular cartilage tissue engineering capable of simultaneously applying multiple biophysical (electrokinetic and mechanical) stimuli. We have developed a novel high-throughput magneto-mechanostimulation bioreactor, capable of applying static and time-varying magnetic fields, as well as multiple and independently adjustable mechanical loading regimens. The device consists of an array of 18 individual stations, each of which uses contactless magnetic actuation and has an integrated Hall Effect sensing system, enabling the real-time measurements of applied field, force, and construct thickness, and hence, the indirect measurement of construct mechanical properties. Validation tests showed precise measurements of thickness, within 14 μm of gold standard calliper measurements; further, applied force was measured to be within 0.04 N of desired force over a half hour dynamic loading, which was repeatable over a 3-week test period. Finally, construct material properties measured using the bioreactor were not significantly different (p=0.97) from those measured using a standard materials testing machine. We present a new method for articular cartilage-specific bioreactor design, integrating combinatorial magneto-mechanostimulation, which is very attractive from functional and cost viewpoints.

Effects of Wnt3A and mechanical load on cartilage chondrocyte homeostasis

Introduction

Articular cartilage functions in withstanding mechanical loads and provides a lubricating surface for frictionless movement of joints. Osteoarthritis, characterised by cartilage degeneration, develops due to the progressive erosion of structural integrity and eventual loss of functional performance. Osteoarthritis is a multi-factorial disorder; two important risk factors are abnormal mechanical load and genetic predisposition. A single nucleotide polymorphism analysis demonstrated an association of hip osteoarthritis with an Arg324Gly substitution mutation in FrzB, a Wnt antagonist. The purpose of this study was two-fold: to assess whether mechanical stimulation modulates β-catenin signalling and catabolic gene expression in articular chondrocytes, and further to investigate whether there is an interplay of mechanical load and Wnt signalling in mediating a catabolic response.

Methods

Chondrocytes were pre-stimulated with recombinant Wnt3A for 24 hours prior to the application of tensile strain (7.5%, 1 Hz) for 30 minutes. Activation of Wnt signalling, via β-catenin nuclear translocation and downstream effects including the transcriptional activation of c-jun, c-fos and Lef1, markers of chondrocyte phenotype (type II collagen (col2a1), aggrecan (acan), SOX9) and catabolic genes (MMP3, MMP13, ADAMTS-4, ADAMTS-5) were assessed.

Results

Physiological tensile strain induced col2a1, acan and SOX9 transcription. Load-induced acan and SOX9 expression were repressed in the presence of Wnt3A. Load induced partial β-catenin nuclear translocation; there was an additive effect of load and Wnt3A on β-catenin distribution, with both extensive localisation in the nucleus and cytoplasm. Immediate early response (c-jun) and catabolic genes (MMP3, ADAMTS-4) were up-regulated in Wnt3A stimulated chondrocytes. With load and Wnt3A there was an additive up-regulation of c-fos, MMP3 and ADAMTS-4 transcription, whereas there was a synergistic interplay on c-jun, Lef1 and ADAMTS-5 transcription.

Conclusion

Our data suggest that load and Wnt, in combination, can repress transcription of chondrocyte matrix genes, whilst enhancing expression of catabolic mediators. Future studies will investigate the respective roles of abnormal loading and genetic predisposition in mediating cartilage degeneration.

CYCLIC COMPRESSION AND TENSION REGULATE DIFFERENTLY THE METABOLISM OF CHONDROCYTES

Articular cartilage is the primary structure in joints which is responsible for withstanding compressive loading; however, it is possible that cartilage also experiences localized tensile loading under various loading and boundary conditions. To date, little is known as to whether chondrocyte functions are dependent on the loading variation, especially when loading is switched from compression to tension. The purpose of this study is to examine the metabolism of chondrocytes under both compressive and tensile loadings in vitro. Our results suggest that synthesis of both collagen II and aggrecan is regulated differently by compression and tension at the mRNA level. Tensile loading significantly inhibited the mRNA expression of both collagen II and aggrecan. Since poor content of collagen II and proteoglycan has been considered a detrimental factor in the integrity of cartilage matrix, cartilage degradation and the possible formation of osteoarthritis may be the consequence of loading patterns switching from compression to tension.

Physical stimulation of chondrogenic cells in vitro: a review

BACKGROUND

Mechanical stimuli are of crucial importance for the development and maintenance of articular cartilage. For conditioning of cartilaginous tissues, various bioreactor systems have been developed that have mainly aimed to produce cartilaginous grafts for tissue engineering applications. Emphasis has been on in vitro preconditioning, whereas the same devices could be used to attempt to predict the response of the cells in vivo or as a prescreening method before animal studies. As a result of the complexity of the load and motion patterns within an articulating joint, no bioreactor can completely recreate the in vivo situation.

QUESTIONS/PURPOSES

This article aims to classify the various loading bioreactors into logical categories, highlight the response of mesenchymal stem cells and chondrocytes to the various stimuli applied, and determine which data could be used within a clinical setting.

METHODS

We performed a Medline search using specific search terms, then selectively reviewed relevant research relating to physical stimulation of chondrogenic cells in vitro, focusing on cellular responses to the specific load applied.

RESULTS

There is much data pertaining to increases in chondrogenic gene expression as a result of controlled loading protocols. Uniaxial loading leads to selective upregulation of genes normally associated with a chondrogenic phenotype, whereas multiaxial loading results in a broader pattern of chondrogenic gene upregulation. The potential for the body to be used as an in vivo bioreactor is being increasingly explored.

CONCLUSIONS

Bioreactors are important tools for understanding the potential response of chondrogenic cells within the joint environment. However, to replicate the natural in vivo situation, more complex motion patterns are required to induce more physiological chondrogenic gene upregulation.

Mechanical influences on morphogenesis of the knee joint revealed through morphological, molecular and computational analysis of immobilised embryos

Very little is known about the regulation of morphogenesis in synovial joints. Mechanical forces generated from muscle contractions are required for normal development of several aspects of normal skeletogenesis. Here we show that biophysical stimuli generated by muscle contractions impact multiple events during chick knee joint morphogenesis influencing differential growth of the skeletal rudiment epiphyses and patterning of the emerging tissues in the joint interzone. Immobilisation of chick embryos was achieved through treatment with the neuromuscular blocking agent Decamethonium Bromide. The effects on development of the knee joint were examined using a combination of computational modelling to predict alterations in biophysical stimuli, detailed morphometric analysis of 3D digital representations, cell proliferation assays and in situ hybridisation to examine the expression of a selected panel of genes known to regulate joint development. This work revealed the precise changes to shape, particularly in the distal femur, that occur in an altered mechanical environment, corresponding to predicted changes in the spatial and dynamic patterns of mechanical stimuli and region specific changes in cell proliferation rates. In addition, we show altered patterning of the emerging tissues of the joint interzone with the loss of clearly defined and organised cell territories revealed by loss of characteristic interzone gene expression and abnormal expression of cartilage markers. This work shows that local dynamic patterns of biophysical stimuli generated from muscle contractions in the embryo act as a source of positional information guiding patterning and morphogenesis of the developing knee joint.

Signalling cascades in mechanotransduction: cell-matrix interactions and mechanical loading

Mechanical loading of articular cartilage stimulates the metabolism of resident chondrocytes and induces the synthesis of molecules to maintain the integrity of the cartilage. Mechanical signals modulate biochemical activity and changes in cell behavior through mechanotransduction. Compression of cartilage results in complex changes within the tissue including matrix and cell deformation, hydrostatic and osmotic pressure, fluid flow, altered matrix water content, ion concentration and fixed charge density. These changes are detected by mechanoreceptors on the cell surface, which include mechanosensitive ion channels and integrins that on activation initiate intracellular signalling cascades leading to tissue remodelling. Excessive mechanical loading also influences chondrocyte metabolism but unlike physiological stimulation leads to a quantitative imbalance between anabolic and catabolic activity resulting in depletion of matrix components. In this article we focus on the role of mechanical signalling in the maintenance of articular cartilage, and discuss how alterations in normal signalling can lead to pathology.

Yeast as a model system for identification of metabolic targets of a 'glucosamine complex' used as a therapeutic agent of osteoarthritis

This manuscript describes the effect of a glucosamine complex and its different constituents on the metabolism of yeast cells. Indeed, the yeast model biosystem offers important advantages in the understanding of basic cellular and molecular processes. For example, the possibility to differentiate aerobic and anaerobic metabolism allows the measurement of glycolysis and mitochondria importance in the control of energetic metabolism and stress-responsive. Yeast growth and division can be controlled efficiently and effectively by adjusting environmental conditions that mimic some aspect of those experienced by chondrocytes in an osteoarthritic milieu, such as low oxygen and nutriment availabilities, high oxidative stress, etc. The glucosamine complex or some of its components (glucosamine sulphate, MSM, Ribes nigrum and silicon) enhanced cellular proliferation and CO(2) production of yeast cells cultured under severe conditions. In addition, it allows a larger output of protons from the cells into the medium. Glucosamine complex supplementation also boosted cellular resistance to stresses such as heat shock, H(2)O(2)-induced peroxidation and ethanol. The beneficial effects of the complex were primarily due to R. nigrum and to glucosamine sulphate components. The protective effect of the glucosamine complex can be explained by an increase of cellular energy level through intensification of mitochondrial functionality and intracellular machinery (anaerobic glycolysis). An additional effect on protein kinase activation is not unlikely.

Cartilage tissue engineering and bioreactor systems for the cultivation and stimulation of chondrocytes

Damage to and degeneration of articular cartilage is a major health issue in industrialized nations. Articular cartilage has a particularly limited capacity for auto regeneration. At present, there is no established therapy for a sufficiently reliable and durable replacement of damaged articular cartilage. In this, as well as in other areas of regenerative medicine, tissue engineering methods are considered to be a promising therapeutic component. Nevertheless, there remain obstacles to the establishment of tissue-engineered cartilage as a part of the routine therapy for cartilage defects. One necessary aspect of potential tissue engineering-based therapies for cartilage damage that requires both elucidation and progress toward practical solutions is the reliable, cost effective cultivation of suitable tissue. Bioreactors and associated methods and equipment are the tools with which it is hoped that such a supply of tissue-engineered cartilage can be provided. The fact that in vivo adaptive physical stimulation influences chondrocyte function by affecting mechanotransduction leads to the development of specifically designed bioreactor devices that transmit forces like shear, hydrostatic pressure, compression, and combinations thereof to articular and artificial cartilage in vitro. This review summarizes the basic knowledge of chondrocyte biology and cartilage dynamics together with the exploration of the various biophysical principles of cause and effect that have been integrated into bioreactor systems for the cultivation and stimulation of chondrocytes.

Cyclic tensile stretch loaded on bovine chondrocytes causes depolymerization of hyaluronan: involvement of reactive oxygen species

OBJECTIVE

We have previously demonstrated that reactive oxygen species (ROS) are involved in cartilage degradation. Decreased size of hyaluronan (HA), the major macromolecule in synovial fluid, to which it imparts viscosity, is reported in patients with arthritis. The purpose of this study was to determine the alteration in the molecular weight range of HA as a result of mechanical deformation loaded on the chondrocytes, as well as the involvement of ROS in this action.

METHODS

ROS were generated via the oxidation of hypoxanthine by xanthine oxidase. Cyclic tensile stretch was loaded using a vacuum-operated instrument. Levels of HA were measured using a sandwich enzyme-binding assay. Superoxide dismutase (SOD) activity and ROS were measured using water-soluble tetrazolium and a chemiluminescent probe, respectively.

RESULTS

ROS depolymerized HA molecules. Cyclic tensile stretch depolymerized HA and induced ROS. SOD inhibited not only ROS induction but also HA depolymerization caused by the mechanical stress.

CONCLUSION

ROS play an important role in mechanical stress-induced HA depolymerization.

Activation of Integrin-RACK1/PKCalpha signalling in human articular chondrocyte mechanotransduction

OBJECTIVE

The objective of this study was to examine PKC isozyme expression in human articular chondrocytes and assess roles for RACK1, a receptor for activated C kinase in the mechanotransduction process.

METHODS

Primary cultures of human articular chondrocytes and a human chondrocyte cell line were studied for expression of PKC isozymes and RACK1 by western blotting. Following mechanical stimulation of chondrocytes in vitro in the absence or presence of anti-integrin antibodies and RGD containing oligopeptides, subcellular localization of PKCalpha and association of RACK1 with PKCalpha and beta1 integrin was assessed.

RESULTS

Human articular chondrocytes express PKC isozymes alpha, gamma, delta, iota, and lambda. Following mechanical stimulation at 0.33Hz chondrocytes show a rapid, beta1 integrin dependent, translocation of PKCalpha to the cell membrane and increased association of RACK1 with PKCalpha and beta1 integrin.

CONCLUSIONS

RACK1 mediated translocation of activated PKCalpha to the cell membrane and modulation of integrin-associated signaling are likely to be important in regulation of downstream signaling cascades controlling chondrocyte responses to mechanical stimuli.

[Cartilage: from biomechanics to physical therapy]

OBJECTIVES

To review the current knowledge about the relationship between physical activities, cartilage biology, osteoarthritis and rehabilitation.

METHOD

PubMed, Ovid, Cochrane Data base were interrogated for the period 1966-2000. Key words were: chondrocyte, cartilage, osteoarthritis, mechanical stimulation, exercises, physical therapy, rehabilitation. Were reviewed: the mechanical biology of the chondrocytes and the cartilage, the mechanisms of transduction, the metabolic response of the chondrocytes to mechanical stresses; the effects of physical activity and immobilization on the cartilage in animal models, the main studies on the epidemiology of limbs osteoarthritis and clinical trials on rehabilitation.

RESULTS

In vitro studies have demonstrated that some molecules are involved in the transduction of mechanical stress into intracellular biological event. Chondrocytes and cartilage are sensitive to mechanical stress and cartilage extracellular matrix synthesis and degradation can be modulated by mechanical events. Applications of cyclic loads usually lead to an enhanced matrix synthesis while static loads usually decrease matrix production. In animal models, intensive physical activity or immobilization lead to cartilage alteration mimicking osteoarthritis. In human, intensive and prolonged physical activities are probably associated with hip and knee osteoarthritis. However, there is evidence that exercise therapy and continuous passive motion have beneficial effects on patients with knee or hip osteoarthritis. Fundamental and clinical studies are still needed to determine if exercise programs could have an effect on chondromodulation. Continuous passive motion could help, in the future, to better understand the relationship between mechanical stimulation and cartilage homeostasis.

CONCLUSION

Rehabilitation could be beneficial in the therapeutic management of limbs osteoarthritis. The protocols of rehabilitation should however be more evaluated in controlled trials.

Cyclic tensile stretch inhibition of nitric oxide release from osteoblast-like cells is both G protein and actin-dependent

Recent reports indicate the alteration of nitric oxide (NO) synthesis with mechanical stress loaded on the osteoblast and NO is considered to have a significant role in mechanotransduction. We found the involvement of guanine-nucleotide-binding regulatory proteins (G proteins), especially Gi, in stress-inhibited NO release of osteoblast-like cells (JOR:17;593-597, 1999). To determine further the mechanism involved in this process, we measured c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) activity under cyclic tensile stretch loaded on osteoblast-like cells. Cyclic stretch significantly enhanced JNK/SAPK activity and pertussis toxin clearly reversed stress-enhanced JNK/SAPK activity. Cytochalasin D, actin microfilament disrupting reagent, also abolished the stress activation of JNK/SAPK. We propose a model for signaling events induced by cyclic tensile stretch, namely a transmembrane mechanosensor which couples Gi-protein, actin cytoskeleton and finally activates JNK/SAPK activity of osteoblasts.

Monolayer anulus fibrosus cell cultures in a mechanically active environment: local culture condition adaptations and cell phenotype study

Intervertebral disc cells can be cultured in vitro. Several culturing systems in a mechanically active environment have been developed to study the relationship between mechanical stimulations and biochemical events. The aim of this study was to assess the phenotype of rabbit intervertebral disc cells from the anulus fibrosus (AF) region cultured on flexible substrate before and after application of cyclic tensile stretch (CTS) and to control culture conditions during application of CTS. CTS was applied with a pressure-operated instrument, inducing the deformation of flexible-bottomed culture plates (Flexercell) at 20% and 5% stretch, at a frequency of 1 Hz, during 30 minutes to 24 hours. A significant decrease in culture medium volume and temperature was observed (52% and 2.1 degrees C at 20% stretch and 24 hours' application of CTS). These phenomena were inhibited by adding culture medium around culture wells and by a culture medium temperature control system. Like AF cells cultured in plastic wells, AF cells cultured on flexible substrate expressed collagen type II, but collagen type I mRNA was not detected. In both culture conditions, neosynthesized proteoglycans had the same aggregating properties. CTS at 20% stretch during 12 hours did not induce cell detachment from the substrate and did not modify aggregating properties of neosynthesized proteoglycans; AF cells continued to express collagen type II but not collagen type I mRNA. In conclusion, the Flexercell system appears to be appropriate for studying, at the cellular level, the metabolic responses to CTS.

Mechanical compression modulates proliferation of transplanted chondrocytes

The presence of an appropriate number of reparative cells in an articular cartilage defect is probably necessary for consistent and successful repair. Following the transplantation of chondrocytes into a defect, cell proliferation may modulate local defect cellularity. Transplanted cells can be compressed during cartilage repair as a result of joint-loading or press-fitting a graft into a cartilage defect. The objective of this study was to characterize the proliferative response of chondrocytes after attachment to cartilage and application of static compressive stress between cartilaginous surfaces in an ex vivo model. The chondrocytes were isolated from adult bovine cartilage, cultured in high-density monolayer, resuspended, and then transplanted onto the surface of devitalized cartilage at a density of 250,000 cells/cm2. The total DNA content of transplanted cell layers increased steadily to a plateau by 5 days and represented a 4-fold increase in cell number during incubation in medium including serum and ascorbate. Over the culture period, the level of DNA synthesis ([3H]thymidine incorporation), on a per cell basis, decreased steadily (88% between days 0 and 6). The application of 24 hours of static compressive stress (0.06-0.4 MPa) to the adherent cells at 1 and 4 days after transplantation inhibited overall DNA synthesis by 70-approximately 87% compared with unloaded controls. After release from load, cell proliferation generally remained at low levels. The marked proliferation of chondrocytes when attached to cartilage without applied load and the inhibition of this proliferation by relatively low-amplitude static compressive stress may be relevant to the occasional overgrowth of tissue in some chondrocyte transplantation procedures. The dosimetry of these effects suggests that the in vivo mechanical environment may have a marked effect on proliferation of transplanted chondrocytes.

Cyclic Tensile Stress Exerts Antiinflammatory Actions on Chondrocytes by Inhibiting Inducible Nitric Oxide Synthase

Continuous passive motion manifests therapeutic effects on inflamed articular joints by an as-yet-unknown mechanism. Here, we show that application of cyclic tensile stress (CTS) in vitro abrogates the catabolic effects of IL-1β on chondrocytes. The effects of CTS are mediated by down-regulation of IL-1β-dependent inducible NO production, and are directly attributed to the inhibition of inducible NO synthase (iNOS) mRNA expression and protein synthesis. The inhibition of iNOS induction by CTS is paralleled by abrogation of IL-1β-induced down-regulation of proteoglycan synthesis. Furthermore, CTS inhibits iNOS expression and up-regulates proteoglycan synthesis at concentrations of IL-1β frequently observed in inflamed arthritic joints, suggesting that the actions of CTS may be clinically relevant in suppressing the sustained effects of pathological levels of IL-1β in vivo. These results are the first to demonstrate that mechanisms of the intracellular actions of CTS in IL-1β-activated chondrocytes are mediated through inhibition of a key molecule in the signal transduction pathway that leads to iNOS expression.

The effects of tensile load on the metabolism of cultured chondrocytes

The effect of cyclic tensile load on articular cartilage metabolism was investigated experimentally using 12 Japanese White rabbits. Chondrocytes obtained from the knee joints were cultured on plates with flexible silicone rubber bases. They were subjected to a cyclic (3 seconds on and 3 seconds off) tensile load for 24 hours with a maximum increase in area of 17%. Proteoglycan synthesis, collagen synthesis, and tissue inhibitors of metalloproteinases production by the chondrocytes under the load were quantified and compared with those produced by the control cells in an unloaded condition. The cultured chondrocytes under the cyclic tensile load perpendicularly aligned to the direction of the tensile load. Collagen synthesis and tissue inhibitors of metalloproteinases production increased significantly under the cyclic tensile load, although no significant change in proteoglycan synthesis was observed. These results suggested that the cyclic tensile load on the chondrocytes contribute to the regulation of articular cartilage metabolism in part.

Factors related to degradation of articular cartilage in osteoarthritis: a review

OBJECTIVES

Osteoarthritis (OA) is a common joint deterioration initiated by multiple factors. To better understand related factors in the development of this disease, we focused on the mechanical stress loaded on articular cartilage.

MATERIALS AND METHODS

The anterior cruciate ligaments of rabbit knee joints were transected, and expression of protein kinase C (PKC) examined immunohistochemically. The PKC activator 12-o-tetradecanoyl-phorbol-13-acetate (TPA) was then administered intraarticularly. To determine the involvement of gas mediators, a cartilage defect was made on the medical femoral condyle of rabbit knee joints. Hydrostatic pressure was loaded on the cartilage taken from the surrounding defects, and levels of superoxide anion and nitric oxide (NO) were measured. Bovine chondrocytes were subjected to cyclic mechanical stretch using a Flexercell Strain Instrument. Proteoglycan synthesis and PKC activity were measured. Expression of matrix metalloproteinase (MMP)-3 and tissue inhibitor of metalloproteinase (TIMP)-1 in articular cartilages obtained from OA patients were examined using Northern blots.

RESULTS

Chondrocytes from experimentally induced OA were stained positively with anti-alpha-PKC antibody. Intraarticular administration of TPA prevented the development of OA changes. Cyclic tensile stretch loaded on chondrocytes decreased proteoglycan synthesis and PKC activity. Thus, PKC is involved in the stress-mediated degradation of articular cartilage. Cartilage defects led to degradation of surrounding cartilage and to enhanced superoxide anion and NO synthesis. We also noted increased and decreased expressions of MMP-3 and TIMP-1 mRNA in human OA cartilage, respectively.

CONCLUSION

PKC, gas mediators (superoxide anion, NO), and proteinases are all involved in OA.

Mechanical strain inhibits repair of airway epithelium in vitro

The repair of airway epithelium after injury is crucial in restoring epithelial barrier integrity. Although the airway epithelium is stretched and compressed due to changes in both circumferential and longitudinal dimensions during respiration and may be overdistended during mechanical ventilation, the effect of cyclic strain on the repair of epithelial wounds is unknown. Human and cat airway epithelial cells were cultured on flexible membranes, wounded by scraping with a metal spatula, and subjected to cyclic strain using the Flexercell Strain Unit. Because the radial strain profile in the wells was nonuniform, we compared closure in regions of elongation and compression within the same well. Both cyclic elongation and cyclic compression significantly slowed repair, with compression having the greatest effect. This attenuation was dependent upon the time of relaxation (TR) during the cycle. When wells were stretched at 10 cycles/min (6 s/cycle) with TR = 5 s, wounds closed similarly to wounds in static wells, whereas in wells with TR = 1 s, significant inhibition was observed. As the TR during cycles increased (higher TR), wounds closed faster. We measured the effect of strain at various TRs on cell area and centroid-centroid distance (CD) as a measure of spreading and migration. While cell area and CD in static wells significantly increased over time, the area and CD of cells in the elongated regions did not change. Cells in compressed regions were significantly smaller, with significantly lower CD. Cell area and CD became progressively larger with increasing TR. These results suggest that mechanical strain inhibits epithelial repair.