Integrin-mediated mechanotransduction in IL-1 beta stimulated chondrocytes

Revisiting prostaglandin E2: A promising therapeutic target for osteoarthritis

Osteoarthritis (OA) is a complex disease characterized by cartilage degeneration and persistent pain. Prostaglandin E2 (PGE2) plays a significant role in OA inflammation and pain. Recent studies have revealed the significant role of PGE2-mediated skeletal interoception in the progression of OA, providing new insights into the pathogenesis and treatment of OA. This aspect also deserves special attention in this review. Additionally, PGE2 is directly involved in pathologic processes including aberrant subchondral bone remodeling, cartilage degeneration, and synovial inflammation. Therefore, celecoxib, a commonly used drug to alleviate inflammatory pain through inhibiting PGE2, serves not only as an analgesic for OA but also as a potential disease-modifying drug. This review provides a comprehensive overview of the discovery history, synthesis and release pathways, and common physiological roles of PGE2. We discuss the roles of PGE2 and celecoxib in OA and pain from skeletal interoception and multiple perspectives. The purpose of this review is to highlight PGE2-mediated skeletal interoception and refresh our understanding of celecoxib in the pathogenesis and treatment of OA.

The interplay between biochemical mediators and mechanotransduction in chondrocytes: Unravelling the differential responses in primary knee osteoarthritis

In primary or idiopathic osteoarthritis (OA), it is unclear which factors trigger the shift of articular chondrocyte activity from pro-anabolic to pro-catabolic. In fact, there is a controversy about the aetiology of primary OA, either mechanical or inflammatory. Chondrocytes are mechanosensitive cells, that integrate mechanical stimuli into cellular responses in a process known as mechanotransduction. Mechanotransduction occurs thanks to the activation of mechanosensors, a set of specialized proteins that convert physical cues into intracellular signalling cascades. Moderate levels of mechanical loads maintain normal tissue function and have anti-inflammatory effects. In contrast, mechanical over- or under-loading might lead to cartilage destruction and increased expression of pro-inflammatory cytokines. Simultaneously, mechanotransduction processes can regulate and be regulated by pro- and anti-inflammatory soluble mediators, both local (cells of the same joint, i.e., the chondrocytes themselves, infiltrating macrophages, fibroblasts or osteoclasts) and systemic (from other tissues, e.g., adipokines). Thus, the complex process of mechanotransduction might be altered in OA, so that cartilage-preserving chondrocytes adopt a different sensitivity to mechanical signals, and mechanic stimuli positively transduced in the healthy cartilage may become deleterious under OA conditions. This review aims to provide an overview of how the biochemical exposome of chondrocytes can alter important mechanotransduction processes in these cells. Four principal mechanosensors, i.e., integrins, Ca2+ channels, primary cilium and Wnt signalling (canonical and non-canonical) were targeted. For each of these mechanosensors, a brief summary of the response to mechanical loads under healthy or OA conditions is followed by a concise overview of published works that focus on the further regulation of the mechanotransduction pathways by biochemical factors. In conclusion, this paper discusses and explores how biological mediators influence the differential behaviour of chondrocytes under mechanical loads in healthy and primary OA.

In vitro and in vivo efficacy of naturally derived scaffolds for cartilage repair and regeneration

Intrinsically present bioactive cues allow naturally derived materials to mimic important characteristics of cartilage while also facilitating cellular recruitment, infiltration, and differentiation. Such traits are often what tissue engineers desire when they fabricate scaffolds, and yet, literature from the past decade is replete with examples of how most natural constructs with native biomolecules have only offered sub-optimal results in the treatment of cartilage defects. This paper provides an in-depth investigation of the performance of such scaffolds through a review of a collection of natural materials that have been used so far in repairing/regenerating articular cartilage. Although in vivo and clinical studies are the best indicators of scaffold efficacy, it was, however, observed that a large number of natural constructs had very promising scaffold characteristics to begin with, and would often show good in vitro/in vivo results. Finally, an examination of the biochemistry and biomechanics of repair tissues in studies that reported positive outcomes showed that these attributes often approached target cartilage values. The paper concludes with an outline of current trends as well as future directions for the field. STATEMENT OF SIGNIFICANCE: This review offers an exclusive focus on natural scaffold materials for cartilage repair and regeneration and provides a quantitative and qualitative analysis of their performance under a variety of in vitro and in vivo conditions. Readers can learn about environments where natural scaffolds have had the most success and tailor strategies to optimize their own work. Furthermore, given how the glycosaminoglycan (GAG) to hydroxyproline (HYP) ratio and moduli are fundamental attributes of hyaline cartilage, this paper adds to the body of knowledge by exploring how these characteristics reflect in preclinical outcomes. Such perspectives can greatly aid researchers better utilize natural materials for Cartilage Tissue Engineering (CTE).

Transducing compressive forces into cellular outputs in cancer and beyond

In living organisms, cells sense mechanical forces (shearing, tensile, and compressive) and respond to those physical cues through a process called mechanotransduction. This process includes the simultaneous activation of biochemical signaling pathways. Recent studies mostly on human cells revealed that compressive forces selectively modulate a wide range of cell behavior, both in compressed and in neighboring less compressed cells. Besides participating in tissue homeostasis such as bone healing, compression is also involved in pathologies, including intervertebral disc degeneration or solid cancers. In this review, we will summarize the current scattered knowledge of compression-induced cell signaling pathways and their subsequent cellular outputs, both in physiological and pathological conditions, such as solid cancers.

Network-based modelling of mechano-inflammatory chondrocyte regulation in early osteoarthritis

Osteoarthritis (OA) is a debilitating joint disease characterized by articular cartilage degradation, inflammation and pain. An extensive range of in vivo and in vitro studies evidences that mechanical loads induce changes in chondrocyte gene expression, through a process known as mechanotransduction. It involves cascades of complex molecular interactions that convert physical signals into cellular response(s) that favor either chondroprotection or cartilage destruction. Systematic representations of those interactions can positively inform early strategies for OA management, and dynamic modelling allows semi-quantitative representations of the steady states of complex biological system according to imposed initial conditions. Yet, mechanotransduction is rarely integrated. Hence, a novel mechano-sensitive network-based model is proposed, in the form of a continuous dynamical system: an interactome of a set of 118 nodes, i.e., mechano-sensitive cellular receptors, second messengers, transcription factors and proteins, related among each other through a specific topology of 358 directed edges is developed. Results show that under physio-osmotic initial conditions, an anabolic state is reached, whereas initial perturbations caused by pro-inflammatory and injurious mechanical loads leads to a catabolic profile of node expression. More specifically, healthy chondrocyte markers (Sox9 and CITED2) are fully expressed under physio-osmotic conditions, and reduced under inflammation, or injurious loadings. In contrast, NF-κB and Runx₂, characteristic of an osteoarthritic chondrocyte, become activated under inflammation or excessive loading regimes. A literature-based evaluation shows that the model can replicate 94% of the experiments tested. Sensitivity analysis based on a factorial design of a treatment shows that inflammation has the strongest influence on chondrocyte metabolism, along with a significant deleterious effect of static compressive loads. At the same time, anti-inflammatory therapies appear as the most promising ones, though the restoration of structural protein production seems to remain a major challenge even in beneficial mechanical environments. The newly developed mechano-sensitive network model for chondrocyte activity reveals a unique potential to reflect load-induced chondroprotection or articular cartilage degradation in different mechano-chemical-environments.

Emulating the chondrocyte microenvironment using multi-directional mechanical stimulation in a cartilage-on-chip

The multi-directional mechanical stimulation experienced by articular cartilage during motion is transferred to the chondrocytes through a thin layer of pericellular matrix around each cell; chondrocytes in turn respond by releasing matrix proteins and/or matrix-degrading enzymes. In the present study we investigated how different types of mechanical stimulation can affect a chondrocyte's phenotype and extracellular matrix (ECM) production. To this end, we employed a cartilage-on-chip system which allows exerting well-defined compressive and multi-directional mechanical stimulation on a 3D chondrocyte-laden agarose hydrogel using a thin deformable membrane and three individually addressed actuation chambers. First, the 3D chondrocyte culture in agarose responded to exposure to mechanical stimulation by an initial increase in IL-6 production and little-to-no change in IL-1β and TNF-α secretion after one day of on-chip culture. Exposure to mechanical stimulation enhanced COL2A1 (hyaline cartilage marker) and decreased COL1A1 (fibrotic cartilage) expression, this being more marked for the multi-directional stimulation. Remarkably, the production of glycosaminoglycans (GAGs), one of the main components of native cartilage ECM, was significantly increased after 15 days of on-chip culture and 14 days of mechanical stimulation. Specifically, a thin pericellular matrix shell (1-5 μm) surrounding the chondrocytes as well as an interstitial matrix, both reminiscent of the in vivo situation, were deposited. Matrix deposition was highest in chips exposed to multi-directional mechanical stimulation. Finally, exposure to mechanical cues enhanced the production of essential cartilage ECM markers, such as aggrecan, collagen II and collagen VI, a marker for the pericellular matrix. Altogether our results highlight the importance of mechanical cues, and using the right type of stimulation, to emulate in vitro, the chondrocyte microenvironment.

Enhanced matrix production by cocultivated human stem cells and chondrocytes under concurrent mechanical strain

Conventional treatments of osteoarthritis have failed to re-build functional articular cartilage. Tissue engineering clinical treatments for osteoarthritis, including autologous chondrocyte implantation, provides an alternative approach by injecting a cell suspension to fill lesions within the cartilage in osteoarthritic knees. The success of chondrocyte implantation relies on the availability of chondrogenic cell lines, and their resilience to high mechanical loading. We hypothesize we can reduce the numbers of human articular chondrocytes necessary for a treatment by supplementing cultures with human adipose-derived stem cells, in which stem cells will have protective and stimulatory effects on mixed cultures when exposed to high mechanical loads, and in which coculture will enhance production of requisite extracellular matrix proteins over those produced by stretched chondrocytes alone. In this work, adipose-derived stem cells and articular chondrocytes were cultured separately or cocultivated at ratios of 3:1, 1:1, and 1:3 in static plates or under excessive cyclic tensile strain of 10% and results were compared to culturing of both cell types alone with and without cyclic strain. Results indicate 75% of chondrocytes in engineered articular cartilage can be replaced with stem cells with enhanced collagen over all culture conditions and glycosaminoglycan content over stretched cultures of chondrocytes. This can be done without observing adverse effects on cell viability. Collagen and glycosaminoglycan secretion, when compared to chondrocyte alone under 10% strain, was enhanced 6.1- and 2-fold, respectively, by chondrocytes cocultivated with stem cells at a ratio of 1:3.

Understanding collagen interactions and their targeted regulation by novel drugs

Introduction: Among protein and fibers in the extracellular matrix (ECM), collagen is the most copious and widely employed in cosmetic, food, pharmaceutical, and biomedical industries due to its extensive biocompatible and versatile properties. In the last years, the knowledge about functions of collagens increased and expanded dramatically. Once considered only crucial for the ECM scaffolding and mechanotransduction, additional functional roles have now been ascribed to the collagen superfamily which are defined by other recently discovered domains, supramolecular assembly and receptors.Areas covered: Given the importance of each step in the collagen biosynthesis, folding and signaling, medicinal chemists have explored small molecules, peptides, and monoclonal antibodies to modulate enzymes, receptors and interactions with the physiological ligands of collagen. These compounds were also explored toward diseases and pathological conditions. The authors discuss this providing their expert perspectives on the subject area.Expert opinion: Understanding collagen protein properties and its interactome is beneficial for therapeutic drug design. Nevertheless, compounds targeting collagen-based interactome suffered from the presence of different isoforms for each target and the lack of specific 3D crystal structures able to guide properly drug design.

Cationic solid lipid nanoparticles loaded by integrin β1 plasmid DNA attenuates IL-1β-induced apoptosis of chondrocyte

Aging-related inflammation is tightly linked with the development of osteoarthritis (OA). As the pro-inflammatory cytokine, IL-1β has been associated with physical dysfunction and frailty. It is still elusive whether and how IL-1β blockade improves the outcome of OA. Here we develop a cationic solid lipid nanoparticles (SLNs) system that effectively mediate non-viral delivery of plasmid DNA (pDNA) into cells. Compared with other DNA transfer technologies including lipofetamin 2000, SLNs-pDNA system is less toxic and exerts identical effectiveness on DNA transfer. Loaded with integrin β1 overexpression pDNA, the SLNs-pDNA mainly localized in cytoplasm and enforced expression of integrin β1 in rat chondrocytes. Moreover, upon exposure to IL-1β stimulation, SLNs-pDNA treatment attenuates the apoptosis rat chondrocytes and augments tissue repair. Our data thus demonstrate that SLNs-pDNA functions as a potential therapeutic nanomedicine in the treatment of osteoarthritis.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Effect of Cyclic Dynamic Compressive Loading on Chondrocytes and Adipose-Derived Stem Cells Co-Cultured in Highly Elastic Cryogel Scaffolds

In this study, we first used gelatin/chondroitin-6-sulfate/hyaluronan/chitosan highly elastic cryogels, which showed total recovery from large strains during repeated compression cycles, as 3D scaffolds to study the effects of cyclic dynamic compressive loading on chondrocyte gene expression and extracellular matrix (ECM) production. Dynamic culture of porcine chondrocytes was studied at 1 Hz, 10% to 40% strain and 1 to 9 h/day stimulation duration, in a mechanical-driven multi-chamber bioreactor for 14 days. From the experimental results, we could identify the optimum dynamic culture condition (20% and 3 h/day) to enhance the chondrocytic phenotype of chondrocytes from the expression of marker (Col I, Col II, Col X, TNF-α, TGF-β1 and IGF-1) genes by quantitative real-time polymerase chain reactions (qRT-PCR) and production of ECM (GAGs and Col II) by biochemical analysis and immunofluorescence staining. With up-regulated growth factor (TGF-β1 and IGF-1) genes, co-culture of chondrocytes with porcine adipose-derived stem cells (ASCs) was employed to facilitate chondrogenic differentiation of ASCs during dynamic culture in cryogel scaffolds. By replacing half of the chondrocytes with ASCs during co-culture, we could obtain similar production of ECM (GAGs and Col II) and expression of Col II, but reduced expression of Col I, Col X and TNF-α. Subcutaneous implantation of cells/scaffold constructs in nude mice after mono-culture (chondrocytes or ASCs) or co-culture (chondrocytes + ASCs) and subject to static or dynamic culture condition in vitro for 14 days was tested for tissue-engineering applications. The constructs were retrieved 8 weeks post-implantation for histological analysis by Alcian blue, Safranin O and Col II immunohistochemical staining. The most abundant ectopic cartilage tissue was found for the chondrocytes and chondrocytes + ASCs groups using dynamic culture, which showed similar neo-cartilage formation capability with half of the chondrocytes replaced by ASCs for co-culture. This combined co-culture/dynamic culture strategy is expected to cut down the amount of donor chondrocytes needed for cartilage-tissue engineering.

The mechanobiology of articular cartilage: bearing the burden of osteoarthritis

Articular cartilage injuries and degenerative joint diseases are responsible for progressive pain and disability in millions of people worldwide, yet there is currently no treatment available to restore full joint functionality. As the tissue functions under mechanical load, an understanding of the physiologic or pathologic effects of biomechanical factors on cartilage physiology is of particular interest. Here, we highlight studies that have measured cartilage deformation at scales ranging from the macroscale to the microscale, as well as the responses of the resident cartilage cells, chondrocytes, to mechanical loading using in vitro and in vivo approaches. From these studies, it is clear that there exists a complex interplay among mechanical, inflammatory, and biochemical factors that can either support or inhibit cartilage matrix homeostasis under normal or pathologic conditions. Understanding these interactions is an important step toward developing tissue engineering approaches and therapeutic interventions for cartilage pathologies, such as osteoarthritis.

The application of multiple biophysical cues to engineer functional neocartilage for treatment of osteoarthritis. Part II: signal transduction

The unique mechanoelectrochemical environment of cartilage has motivated researchers to investigate the effect of multiple biophysical cues, including mechanical, magnetic, and electrical stimulation, on chondrocyte biology. It is well established that biophysical stimuli promote chondrocyte proliferation, differentiation, and maturation within "biological windows" of defined dose parameters, including mode, frequency, magnitude, and duration of stimuli (see companion review Part I: Cellular Response). However, the underlying molecular mechanisms and signal transduction pathways activated in response to multiple biophysical stimuli remain to be elucidated. Understanding the mechanisms of biophysical signal transduction will deepen knowledge of tissue organogenesis, remodeling, and regeneration and aiding in the treatment of pathologies such as osteoarthritis. Further, this knowledge will provide the tissue engineer with a potent toolset to manipulate and control cell fate and subsequently develop functional replacement cartilage. The aim of this article is to review chondrocyte signal transduction pathways in response to mechanical, magnetic, and electrical cues. Signal transduction does not occur along a single pathway; rather a number of parallel pathways appear to be activated, with calcium signaling apparently common to all three types of stimuli, though there are different modes of activation. Current tissue engineering strategies, such as the development of "smart" functionalized biomaterials that enable the delivery of growth factors or integration of conjugated nanoparticles, may further benefit from targeting known signal transduction pathways in combination with external biophysical cues.

Role of integrins and their ligands in osteoarthritic cartilage

Osteoarthritis (OA) is a degenerative disease, which is characterized by articular cartilage destruction, and mainly affects the older people. The extracellular matrix (ECM) provides a vital cellular environment, and interactions between the cell and ECM are important in regulating many biological processes, including cell growth, differentiation, and survival. However, the pathogenesis of this disease is not fully elucidated, and it cannot be cured totally. Integrins are one of the major receptors in chondrocytes. A number of studies confirmed that the chondrocytes express several integrins including α5β1, αVβ3, αVβ5, α6β1, α1β1, α2β1, α10β1, and α3β1, and some integrins ligands might act as the OA progression biomarkers. This review focuses on the functional role of integrins and their extracellular ligands in OA progression, especially OA cartilage. Clear understanding of the role of integrins and their ligands in OA cartilage may have impact on future development of successful therapeutic approaches to OA.

Distinctive subcellular inhibition of cytokine-induced SRC by salubrinal and fluid flow

A non-receptor protein kinase Src plays a crucial role in fundamental cell functions such as proliferation, migration, and differentiation. While inhibition of Src is reported to contribute to chondrocyte homeostasis, its regulation at a subcellular level by chemical inhibitors and mechanical stimulation has not been fully understood. In response to inflammatory cytokines and stress to the endoplasmic reticulum (ER) that increase proteolytic activities in chondrocytes, we addressed two questions: Do cytokines such as interleukin 1 beta (IL1β) and tumor necrosis factor alpha (TNFα) induce location-dependent Src activation? Can cytokine-induced Src activation be suppressed by chemically alleviating ER stress or by applying fluid flow? Using live cell imaging with two Src biosensors (i.e., cytosolic, and plasma membrane-bound biosensors) for a fluorescence resonance energy transfer (FRET) technique, we determined cytosolic Src activity as well as membrane-bound Src activity in C28/I2 human chondrocytes. In response to TNFα and IL1β, both cytosolic and plasma membrane-bound Src proteins were activated, but activation in the cytosol occurred earlier than that in the plasma membrane. Treatment with salubrinal or guanabenz, two chemical agents that attenuate ER stress, significantly decreased cytokine-induced Src activities in the cytosol, but not in the plasma membrane. In contrast, fluid flow reduced Src activities in the plasma membrane, but not in the cytosol. Collectively, the results demonstrate that Src activity is differentially regulated by salubrinal/guanabenz and fluid flow in the cytosol and plasma membrane.

Cyclic hydrodynamic pressure induced proliferation of bladder smooth muscle cells via integrin alpha5 and FAK

According to previous studies, integrins play an important role in the mechanotransduction. The aim of this study was to examine the role of integrin subunits and its down-stream signaling molecules in the cyclic hydrodynamic pressure-induced proliferation of human bladder smooth muscle cells (HBSMCs) cultured in scaffolds. The HBSMCs cultured in scaffolds were subjected to four different levels of cyclic hydrodynamic pressure for 24 hours, which were controlled by a BOSE BioDynamic bioreactor. Flow cytometry was used to examine cell cycle distribution. Real-time RT-PCR and western blotting were used to examine the expression levels of integrin subunits and their downstream signaling molecules. Integrin alpha5 siRNA was applied to validate the role of integrin alpha5 in cell proliferation. Here, we showed that cyclic hydrodynamic pressure promoted proliferation of HBSMCs. The cyclic hydrodynamic pressure also increased expression of integrin alpha5 and phosphorylation of FAK, the key mediator of integrin alpha5 signaling, but not that of integrin alpha1, alpha3, alpha4, alphav, beta1 and beta3. Moreover, inhibition of integrin alpha5 decreased the level of p-FAK and abolished proliferation of HBSMCs stimulated by cyclic hydrodynamic pressure. Taken together, we demonstrate for the ?rst time that the integrin alpha5-FAK signaling pathway controls the proliferation of HBSMCs in response to cyclic hydrodynamic pressure.

Integrin - dependent mechanotransduction in mechanically stimulated human annulus fibrosus cells: evidence for an alternative mechanotransduction pathway operating with degeneration

Intervertebral disc (IVD) cells derived from degenerate tissue respond aberrantly to mechanical stimuli, potentially due to altered mechanotransduction pathways. Elucidation of the altered, or alternative, mechanotransduction pathways operating with degeneration could yield novel targets for the treatment of IVD disease. Our aim here was to investigate the involvement of RGD-recognising integrins and associated signalling molecules in the response to cyclic tensile strain (CTS) of human annulus fibrosus (AF) cells derived from non-degenerate and degenerate IVDs. AF cells from non-degenerate and degenerate human IVDs were cyclically strained with and without function blocking RGD – peptides with 10% strain, 1.0 Hz for 20 minutes using a Flexercell® strain device. QRT-PCR and Western blotting were performed to analyse gene expression of type I collagen and ADAMTS -4, and phosphorylation of focal adhesion kinase (FAK), respectively. The response to 1.0 Hz CTS differed between the two groups of AF cells, with decreased ADAMTS -4 gene expression and decreased type I collagen gene expression post load in AF cells derived from non-degenerate and degenerate IVDs, respectively. Pre-treatment of non-degenerate AF cells with RGD peptides prevented the CTS-induced decrease in ADAMTS -4 gene expression, but caused an increase in expression at 24 hours, a response not observed in degenerate AF cells where RGD pre-treatment failed to inhibit the mechano-response. In addition, FAK phosphorylation increased in CTS stimulated AF cells derived from non-degenerate, but not degenerate IVDs, with RGD pre-treatment inhibiting the CTS – dependent increase in phosphorylated FAK. Our findings suggest that RGD -integrins are involved in the 1.0 Hz CTS – induced mechano-response observed in AF cells derived from non-degenerate, but not degenerate IVDs. This data supports our previous work, suggesting an alternative mechanotransduction pathway may be operating in degenerate AF cells.

Mechanical regulation of chondrogenesis

Mechanical factors play a crucial role in the development of articular cartilage in vivo. In this regard, tissue engineers have sought to leverage native mechanotransduction pathways to enhance in vitro stem cell-based cartilage repair strategies. However, a thorough understanding of how individual mechanical factors influence stem cell fate is needed to predictably and effectively utilize this strategy of mechanically-induced chondrogenesis. This article summarizes some of the latest findings on mechanically stimulated chondrogenesis, highlighting several new areas of interest, such as the effects of mechanical stimulation on matrix maintenance and terminal differentiation, as well as the use of multifactorial bioreactors. Additionally, the roles of individual biophysical factors, such as hydrostatic or osmotic pressure, are examined in light of their potential to induce mesenchymal stem cell chondrogenesis. An improved understanding of biomechanically-driven tissue development and maturation of stem cell-based cartilage replacements will hopefully lead to the development of cell-based therapies for cartilage degeneration and disease.

Human articular chondrocytes express multiple gap junction proteins: differential expression of connexins in normal and osteoarthritic cartilage

Osteoarthritis (OA) is the most common joint disease and involves progressive degeneration of articular cartilage. The aim of this study was to investigate if chondrocytes from human articular cartilage express gap junction proteins called connexins (Cxs). We show that human chondrocytes in tissue express Cx43, Cx45, Cx32, and Cx46. We also find that primary chondrocytes from adults retain the capacity to form functional voltage-dependent gap junctions. Immunohistochemistry experiments in cartilage from OA patients revealed significantly elevated levels of Cx43 and Cx45 in the superficial zone and down through the next approximately 1000 μm of tissue. These zones corresponded with regions damaged in OA that also had high levels of proliferative cell nuclear antigen. An increased number of Cxs may help explain the increased proliferation of cells in clusters that finally lead to tissue homeostasis loss. Conversely, high levels of Cxs in OA cartilage reflect the increased number of adjacent cells in clusters that are able to interact directly by gap junctions as compared with hemichannels on single cells in normal cartilage. Our data provide strong evidence that OA patients have a loss of the usual ordered distribution of Cxs in the damaged zones and that the reductions in Cx43 levels are accompanied by the loss of correct Cx localization in the nondamaged areas.

Central role of SREBP-2 in the pathogenesis of osteoarthritis

Background

Recent studies have implied that osteoarthritis (OA) is a metabolic disease linked to deregulation of genes involved in lipid metabolism and cholesterol efflux. Sterol Regulatory Element Binding Proteins (SREBPs) are transcription factors regulating lipid metabolism with so far no association with OA. Our aim was to test the hypothesis that SREBP-2, a gene that plays a key role in cholesterol homeostasis, is crucially involved in OA pathogenesis and to identify possible mechanisms of action.

Methodology/Principal Findings

We performed a genetic association analysis using a cohort of 1,410 Greek OA patients and healthy controls and found significant association between single nucleotide polymorphism (SNP) 1784G>C in SREBP-2 gene and OA development. Moreover, the above SNP was functionally active, as normal chondrocytes’ transfection with SREBP-2-G/C plasmid resulted in interleukin-1β and metalloproteinase-13 (MMP-13) upregulation. We also evaluated SREBP-2, its target gene 3-hydroxy-3-methylglutaryl-coenzymeA reductase (HMGCR), phospho-phosphoinositide3-kinase (PI3K), phospho-Akt, integrin-alphaV (ITGAV) and transforming growth factor-β (TGF-β) mRNA and protein expression levels in osteoarthritic and normal chondrocytes and found that they were all significantly elevated in OA chondrocytes. To test whether TGF-β alone can induce SREBP-2, we treated normal chondrocytes with TGF-β and found significant upregulation of SREBP-2, HMGCR, phospho-PI3K and MMP-13. We also showed that TGF-β activated aggrecan (ACAN) in chondrocytes only through Smad3, which interacts with SREBP-2. Finally, we examined the effect of an integrin inhibitor, cyclo-RGDFV peptide, on osteoarthritic chondrocytes, and found that it resulted in significant upregulation of ACAN and downregulation of SREBP-2, HMGCR, phospho-PI3K and MMP-13 expression levels.

Conclusions/Significance

We demonstrated, for the first time, the association of SREBP-2 with OA pathogenesis and provided evidence on the molecular mechanism involved. We suggest that TGF-β induces SREBP-2 pathway activation through ITGAV and PI3K playing a key role in OA and that integrin blockage may be a potential molecular target for OA treatment.

Mechanical Loading of Articular Cartilage Reduces IL-1-Induced Enzyme Expression

OBJECTIVE: Exposure of articular cartilage to interleukin-1 (IL-1) results in increased synthesis of matrix degrading enzymes. Previously mechanical load applied together with IL-1 stimulation was found to reduce aggrecan cleavage by ADAMTS-4 and 5 and MMP-1, -3, -9, and -13 and reduce proteoglycan loss from the extracellular matrix. To further delineate the inhibition mechanism the gene expression of ADAMTS-4 and 5; MMP-1, -3, -9, and -13; and TIMP-1, -2, and -3 were measured. DESIGN: Mature bovine articular cartilage was stimulated with a 0.5 MPa compressive stress and 10 ng/ml of IL-1α for 3 days and then allowed to recover without stimulation for 1 additional day. The media was assayed for proteoglycan content on a daily basis, while chondrocyte gene expression (mRNA) was measured during stimulation and 1 day of recovery. RESULTS: Mechanical load alone did not change the gene expression for ADAMTS, MMP, or TIMP. IL-1 caused an increase in gene expression for all enzymes after 1 day of stimulation while not affecting the TIMP levels. Load applied together with IL-1 decreased the expression levels of ADAMTS-4 and -5 and MMP-1 and -3 and increased TIMP-3 expression. CONCLUSIONS: A mechanical load appears to modify cartilage degradation by IL-1 at the cellular level by reducing mRNA.

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.

Glatiramer acetate (GA), the immunomodulatory drug, inhibits inflammatory mediators and collagen degradation in osteoarthritis (OA) cartilage

OBJECTIVE

Glatiramer acetate (GA), the generic name for Copaxone, an immunomodulatory agent, has been shown to induce interleukin-1 receptor antagonist (IL-1Ra) production in macrophages. We therefore tested the effects of GA on the catabolic activities of osteoarthritis (OA) chondrocytes.

DESIGN

Primary human chondrocytes and OA cartilage explants were utilized in this study. IL-1Ra, pro-matrix metalloproteinase-13 (proMMP-13) and prostaglandin E(2) (PGE(2)) were estimated in the cell culture supernatants and in vitro MMP-13 activity was measured using fluorogenic substrate. TaqMan Real-Time quantitative polymerase chain reaction (RT-qPCR) was performed to estimate relative expression levels of genes.

RESULTS

GA treatment significantly increased transcription and production of sIL-1Ra (P=0.001) in both culture models. Furthermore, addition of GA (100 μg) inhibited: (1) spontaneous collagen degradation as assayed by CTX II enzyme-linked immunosorbent assay (ELISA) [mean CTX II (ng/g cartilage)] in control was 7.79 [95% confidence interval (CI) 2.57-13.02]-3.415 (95% CI 0.81-6.02) (P=0.0286); (2) spontaneous proMMP-13 secretion [mean MMP-13 (ng/g cartilage)] in control was 16.98 (95% CI 7.739-26.23)-6.973 (95% CI 1.632-12.31) (P=0.0286); (3) production of IL-1β-induced inflammatory mediators such as nitric oxide (NO) [mean NO (μM)] in IL-1 cultures was 11.47 (95% CI 7.10-15.83)-0.87 (95% CI 0.18-1.56) (P=0.0022); and (4) recombinant MMP-13 in vitro activity (15-25%; P=0.004).

CONCLUSIONS

These data suggest that GA effects may be due to upregulation of IL-1Ra as well as direct inhibition of MMP-13 activity. Based on these studies, we propose that GA has potential for disease modifying properties in OA and should be evaluated in vivo in animal studies.

The synergistic effects of 3-D porous silk fibroin matrix scaffold properties and hydrodynamic environment in cartilage tissue regeneration

Autologous cell-based tissue engineering using three-dimensional porous scaffolds has provided a good option for the repair of cartilage defects. Silk fibroin-based scaffolds are naturally degradable materials with excellent biocompatibility and robust mechanical properties, indicating potential applications in cartilage tissue engineering. In this study, silk fibroin scaffolds prepared by freeze-drying (FD) and salt-leaching (SL300 and SL500) were fully characterized and used to study the effects of silk fibroin scaffold properties on chondrocyte attachment, proliferation and differentiation. The synergistic effects of scaffold properties and hydrodynamic environment generated by in vitro rocking culture were also investigated using static cultures as control. FD scaffolds with small pore size and lower porosity increased cell attachment but inhibited cell penetration and limited cell proliferation and differentiation. In contrast, SL scaffolds displaying a bigger pore size, higher porosity and crystallinity resulted in homogenous cell distribution, increasing cell proliferation and advanced chondrocyte differentiation in terms of their spherical morphology, predominant chondrogenic gene expression and abundant cartilaginous extracellular matrix production. A hydrodynamic environment was beneficial to chondrocyte proliferation, differentiation, and integrin gene expression in a pore size dependent manner with superior cartilage matrix production but limited hypertrophic differentiation obtained using chondrocyte-seeded SL500 scaffolds. Integrin alpha5beta1 might mediate these effects. Chondrocyte/SL500 silk fibroin constructs obtained under in vitro rocking culture might serve as an excellent implant for in vivo cartilage defect reparation.

Mechanical load inhibits IL-1 induced matrix degradation in articular cartilage

OBJECTIVE

Osteoarthritis is a disease process of cellular degradation of articular cartilage caused by mechanical loads and inflammatory cytokines. We studied the cellular response in native cartilage subjected to a mechanical load administered simultaneously with an inflammatory cytokine interleukin-1 (IL-1), hypothesizing that the combination of load and cytokine would result in accelerated extracellular matrix (ECM) degradation.

METHODS

Mature bovine articular cartilage was loaded for 3 days (stimulation) with 0.2 and 0.5 MPa stresses, with and without IL-1 (IL-1alpha, 10 ng/ml), followed by 3 days of no stimulation (recovery). Aggrecan and collagen loss were measured as well as aggrecan cleavage using monoclonal antibodies AF-28 and BC-3 for cleavage by aggrecanases (ADAMTS) and matrix metalloproteinases (MMPs), respectively.

RESULTS

Incubation with IL-1 caused aggrecan cleavage by aggrecanases and MMPs during the 3 days of stimulation. A load of 0.5 MPa inhibited the IL-1-induced aggrecan loss while no inhibition was found for the 0.2 MPa stress. There was no collagen loss during the treatments but upon load and IL-1 removal proteoglycan and collagen loss increased. Load itself under these conditions was found to have no effect when compared to the unloaded controls.

CONCLUSIONS

A mechanical load of sufficient magnitude can inhibit ECM degradation by chondrocytes when stimulated by IL-1. The molecular mechanisms involved in this process are not clear but probably involve altered mechanochemical signal transduction between the ECM and chondrocyte.

Cell-matrix interactions and dynamic mechanical loading influence chondrocyte gene expression and bioactivity in PEG-RGD hydrogels

The pericellular matrix (PCM) surrounding chondrocytes is thought to play an important role in transmitting biochemical and biomechanical signals to the cells, which regulates many cellular functions including tissue homeostasis. To better understand chondrocytes interactions with their PCM, three-dimensional poly(ethylene glycol) (PEG) hydrogels containing Arg-Gly-Asp (RGD), the cell-adhesion sequence found in fibronectin and which is present in the PCM of cartilage, were employed. RGD was incorporated into PEG hydrogels via tethers at 0.1, 0.4 and 0.8 mM concentrations. Bovine chondrocytes were encapsulated in the hydrogels and subjected to dynamic compressive strains (0.3 Hz, 18% amplitude strain) for 48h, and their response assessed by cell morphology, ECM gene expression, cell proliferation and matrix synthesis. Incorporation of RGD did not influence cell morphology under free swelling conditions. However, the level of cell deformation upon an applied strain was greater in the presence of RGD. In the absence of dynamic loading, RGD appears to have a negative effect on chondrocyte phenotype, as seen by a 4.7-fold decrease in collagen II/collagen I expressions in 0.8mM RGD constructs. However, RGD had little effect on early responses of chondrocytes (i.e. cell proliferation and matrix synthesis/deposition). When isolating RGD as a biomechanical cue, cellular response was very different. Chondrocyte phenotype (collagen II/collagen I ratio) and proteoglycan synthesis were enhanced with higher concentrations of RGD. Overall, our findings demonstrate that RGD ligands enhance cartilage-specific gene expression and matrix synthesis, but only when mechanically stimulated, suggesting that cell-matrix interactions mediate chondrocyte response to mechanical stimulation.

Dynamic loading stimulates chondrocyte biosynthesis when encapsulated in charged hydrogels prepared from poly(ethylene glycol) and chondroitin sulfate

This study aimed to elucidate the role of charge in mediating chondrocyte response to loading by employing synthetic 3D hydrogels. Specifically, neutral poly(ethylene glycol) (PEG) hydrogels were employed where negatively charged chondroitin sulfate (ChS), one of the main extracellular matrix components of cartilage, was systematically incorporated into the PEG network at 0%, 20% or 40% to control the fixed charge density. PEG hydrogels were employed as a control environment for extracellular events which occur as a result of loading, but which are not associated with a charged matrix (e.g., cell deformation and fluid flow). Freshly isolated bovine articular chondrocytes were embedded in the hydrogels and subject to dynamic mechanical stimulation (0.3Hz, 15% amplitude strains, 6h) and assayed for nitric oxide production, cell proliferation, proteoglycan synthesis, and collagen deposition. In the absence of loading, incorporation of charge inhibited cell proliferation by approximately 75%, proteoglycan synthesis by approximately 22-50% depending on ChS content, but had no affect on collagen deposition. Dynamic loading had no effect on cellular responses in PEG hydrogels. However, dynamically loading 20% ChS gels inhibited nitrite production by 50%, cell proliferation by 40%, but stimulated proteoglycan and collagen deposition by 162% and 565%, respectively. Dynamic loading of 40% ChS hydrogels stimulated nitrite production by 62% and proteoglycan synthesis by 123%, but inhibited cell proliferation by 54% and collagen deposition by 52%. Upon removing the load and culturing under free-swelling conditions for 36h, the enhanced matrix synthesis observed in the 20% ChS gels was not maintained suggesting that loading is necessary to stimulate matrix production. In conclusion, extracellular events associated with a charged matrix have a dramatic affect on how chondrocytes respond to mechanical stimulation within these artificial 3D matrices suggesting that streaming potentials and/or dynamic changes in osmolarity may be important regulators of chondrocytes while cell deformation and fluid flow appear to have less of an effect.

Dynamic compression alters NFkappaB activation and IkappaB-alpha expression in IL-1beta-stimulated chondrocyte/agarose constructs

OBJECTIVE AND DESIGN

Determine the effect of IL-1beta and dynamic compression on NFkappaB activation and IkappaB-alpha gene expression in chondrocyte/agarose constructs.

METHODS

Constructs were cultured under free-swelling conditions or subjected to dynamic compression for up to 360 min with IL-1beta and/or PDTC (inhibits NFkappaB activation). Nuclear translocation of NFkappaB-p65 was analysed by immunofluoresence microscopy. Gene expression of IkappaB-alpha, iNOS, IL-1beta and IL-4 was assessed by real-time qPCR.

RESULTS

Nuclear translocation of NFkappaB-p65 was concomitant with an increase in nuclear fluorescence intensity which reached maximum values at 60 min with IL-1beta (p < 0.001). Dynamic compression or PDTC reduced nuclear fluorescence and NFkappaB nuclear translocation in cytokine-treated constructs (p < 0.001 and p < 0.01 respectively). IL-1beta increased IkappaB-alpha expression (p < 0.001) at 60 min and either induced iNOS (p < 0.001) and IL-1beta (p < 0.01) or inhibited IL-4 (p < 0.05) expression at 360 min. These time-dependent events were partially reversed by dynamic compression or PDTC (p < 0.01) with IL-1beta. Co-stimulation by dynamic compression and PDTC favoured suppression (IkappaB-alpha, iNOS, IL-1beta) or induction (IL-4) of gene expression.

CONCLUSIONS

NFkappaB is one of the key players in the mechanical and inflammatory pathways, and its inhibition by a biophysical/therapeutic approach could be a strategy for attenuating the catabolic response in osteoarthritis.

RGD-dependent integrins are mechanotransducers in dynamically compressed tissue-engineered cartilage constructs

Recent studies have shown that integrins act as mechanoreceptors in articular cartilage. In this study, we examined the effect of blocking RGD-dependent integrins on both ECM gene expression and ECM protein synthesis. Chondrocytes were isolated from full-depth porcine articular cartilage and seeded in 3% agarose constructs. These constructs were loaded in compression with 15% strain at 0.33 and 1 Hz for 12h, in the presence or absence of GRGDSP, which blocks RGD-dependent integrin receptors. The levels of mRNA for aggrecan, collagen II and MMP-3 were determined by semi-quantitative PCR at several time points up to 24h post-stimulation. DNA and sGAG content were determined at several time points up to 28 days post-stimulation. At 0.33 Hz, the mRNA levels for aggrecan and MMP-3 were increased after loading, but the mRNA levels for collagen II remained unchanged. Incubation with GRGDSP counteracted these effects. Loading at 1 Hz led to increased mRNA levels for all three molecules directly after loading and these effects were counteracted by incubation with GRGDSP. The constructs that were loaded at 0.33 Hz showed a lower amount of sGAG, compared to the unstrained control. In contrast, loading at 1 Hz caused an increase in sGAG deposition over the culture period. Blocking integrins had only a counteracting effect on the long-term biosynthetic response of constructs that were compressed at 1 Hz. The results confirmed the role of RGD-dependent integrins as mechanotransducers in the regulation of both ECM gene expression and matrix biosynthesis for chondrocytes seeded in agarose under the applied loading regime. Interestingly, this role seems to be dependent on the applied loading frequency.

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.

Novel electrospun scaffolds for the molecular analysis of chondrocytes under dynamic compression

Mechanical training of engineered tissue constructs is believed necessary to improve regeneration of cartilaginous grafts. Nevertheless, molecular mechanisms underlying mechanical activation are not clear. This is partly due to unavailability of appropriate scaffolds allowing exposure of cells to dynamic compressive strains (DCS) in vitro while permitting subsequent molecular analyses. We demonstrate that three-dimensional macroporous electrospun poly(epsilon-caprolactone) scaffolds can be fabricated that are suitable for the functional and molecular analysis of dynamically loaded chondrocytes. These scaffolds encourage chondrocytic proliferation promoting expression of collagen type II, aggrecan, and Sox9 while retaining mechanical strength after prolonged dynamic compression. Further, they exhibit superior infiltration of exogenous agents into the cells and permit easy retrieval of cellular components postcompression to allow exploration of molecular mechanisms of DCS. Using these scaffolds, we observed that chondrocytes responded to DCS in a magnitude-dependent manner exhibiting antiinflammatory and proanabolic responses at low physiological magnitudes. Proinflammatory responses and decreased cellular viability were observed at hyperphysiological magnitudes. These scaffolds provide a means of unraveling the mechanotransduction-induced transcriptional and posttranslational activities involved in cartilage regeneration and repair.

Integrative microRNA and proteomic approaches identify novel osteoarthritis genes and their collaborative metabolic and inflammatory networks

BACKGROUND

Osteoarthritis is a multifactorial disease characterized by destruction of the articular cartilage due to genetic, mechanical and environmental components affecting more than 100 million individuals all over the world. Despite the high prevalence of the disease, the absence of large-scale molecular studies limits our ability to understand the molecular pathobiology of osteoathritis and identify targets for drug development.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we integrated genetic, bioinformatic and proteomic approaches in order to identify new genes and their collaborative networks involved in osteoarthritis pathogenesis. MicroRNA profiling of patient-derived osteoarthritic cartilage in comparison to normal cartilage, revealed a 16 microRNA osteoarthritis gene signature. Using reverse-phase protein arrays in the same tissues we detected 76 differentially expressed proteins between osteoarthritic and normal chondrocytes. Proteins such as SOX11, FGF23, KLF6, WWOX and GDF15 not implicated previously in the genesis of osteoarthritis were identified. Integration of microRNA and proteomic data with microRNA gene-target prediction algorithms, generated a potential "interactome" network consisting of 11 microRNAs and 58 proteins linked by 414 potential functional associations. Comparison of the molecular and clinical data, revealed specific microRNAs (miR-22, miR-103) and proteins (PPARA, BMP7, IL1B) to be highly correlated with Body Mass Index (BMI). Experimental validation revealed that miR-22 regulated PPARA and BMP7 expression and its inhibition blocked inflammatory and catabolic changes in osteoarthritic chondrocytes.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that obesity and inflammation are related to osteoarthritis, a metabolic disease affected by microRNA deregulation. Gene network approaches provide new insights for elucidating the complexity of diseases such as osteoarthritis. The integration of microRNA, proteomic and clinical data provides a detailed picture of how a network state is correlated with disease and furthermore leads to the development of new treatments. This strategy will help to improve the understanding of the pathogenesis of multifactorial diseases such as osteoarthritis and provide possible novel therapeutic targets.

The mechanical transduction of physiological strength electric fields

In this article it is proposed that electric fields of physiological strength (approximately 100 V/m) are transduced by the mechanical torque they exert on glycoproteins. The resulting mechanical signal is then transmitted to the cytoskeleton and propagated throughout the cell interior. This mechanical coupling is analyzed for transmembrane glycoproteins, such as integrins and the glycocalyx, and for glycoproteins in the extracellular matrix of cartilage. The applied torque is opposed by viscous fluid drag and restoring forces exerted by adjacent molecules in the membrane or cartilage. The resulting system represents a damped, driven harmonic oscillator. The amplitude of oscillation is constant at low frequencies, but falls off rapidly in the range 1-1000 Hz. The transition frequency depends on parameters such as the viscosity of the surrounding fluid and the restoring force exerted by the surrounding structure. The amplitude increases as the fourth power of the length of the transmembrane glycoproteins and as the square of the applied field. This process may operate in concert with other transduction mechanisms, such as the opening of voltage-gated channels and electrodiffusion/osmosis for DC fields.

Expression of ion channels of the TRP family in articular chondrocytes from osteoarthritic patients: changes between native and in vitro propagated chondrocytes

The maintenance of a differentiated chondrocyte phenotype is influenced by several factors of which signal transduction of extracellular stimuli through the cell membrane is of major interest. One important group of membrane-bound proteins which are involved in transmembrane signal transduction are ion channels. Human articular chondrocytes were obtained from osteoarthritic femoral condyles. Cells were released from the surrounding matrix and cultivated under standard conditions. We investigated gene expression of 12 members of the TRP ion channel family of freshly prepared (passage 0; P0) and in vitro propagated human articular chondrocytes (passage 2; P2) using conventional and real-time PCR (RT-PCR). In addition, the protein appearance of four TRP channels was demonstrated by immunofluorescence and western blotting. Chondrocyte differentiation was monitored by quantification of collagen type-II, type-I, and aggrecan gene expression. By conventional PCR, 8 channels could be detected, of which some displayed a heterogeneous PCR pattern. RT-PCR quantification revealed that TRPC1 was expressed on the same level in P0 and P2 chondrocytes while gene expression of TRPC3 and TRPC6 was elevated in passage 2 cells. TRPM5, TRPM7, and TRPV1 displayed an enhanced gene expression in freshly isolated chondrocytes. Immunofluorescence signal intensity of all four investigated TRP proteins was consistent with the corresponding gene expression data. In the present study, a correlation between the appearance of some members of the TRP ion channel family and the state of de-differentiation of osteoarthritic articular chondrocytes was shown. A possible direct involvement in the process of chondrocyte de-differentiation has to be investigated in further studies.

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.

Crosstalk between integrin and G protein pathways involved in mechanotransduction in mandibular condylar chondrocytes under pressure

To investigate the role of integrin and G protein pathways in the mechanotransduction process within MCCs and explore the possible crosstalk between the two traditional signal pathways, in vitro-cultured rabbit MCCs were treated with pressure. The mRNA level of alpha5beta1 integrin was determined by in situ hybridization and the distributions of vinculin, Galphaq/11 protein, F-actin and intracellular calcium were studied with a laser scanning confocal microscope. Increased integrin alpha5beta1 expression, enhanced stress fiber assembly, elevated G protein and vinculin level and up-regulated IP(3) channel sensitivity were found in the mechanotransduction process of MCCs under pressure. Furthermore, the vinculin and the Galphaq/11 were observed co-localized with each other, and the F-actin reassembly and stress fibers formation could be inhibited by intracellular calcium channel blocking, which gave direct evidence that the traditional integrin-mediated or G protein-mediated signaling pathways coordinately regulate the function of MCCs under mechanical stimulation.

Regulation of chondrocytic gene expression by biomechanical signals

Cartilage is a mechanosensitive tissue, which means that it can perceive and respond to biomechanical signals. Despite the known importance of biomechanical signals in the etiopathogenesis of arthritic diseases and their effectiveness in joint restoration, little is understood about their actions at the cellular level. Recent molecular approaches have revealed that specific biomechanical stimuli and cell interactions generate intracellular signals that are powerful inducers or suppressors of proinflammatory and reparative genes in chondrocytes. Biomechanical signals are perceived by cartilage in magnitude-, frequency-, and time-dependent manners. Static and dynamic biomechanical forces of high magnitudes induce proinflammatory genes and inhibit matrix synthesis. Contrarily, dynamic biomechanical signals of low/physiologic magnitudes are potent antiinflammatory signals that inhibit interleukin-1beta (IL-1beta)-induced proinflammatory gene transcription and abrogate IL-1beta/tumor necrosis factor-alpha-induced inhibition of matrix synthesis. Recent studies have identified nuclear factor-kB (NF-kB) transcription factors as key regulators of biomechanical signal-mediated proinflammatory and antiinflammatory actions. These signals intercept multiple steps in the NF-kappaB signaling cascade to regulate cytokine gene expression. Taken together, these findings provide insight into how biomechanical signals regulate inflammatory and reparative gene transcription, underscoring their potential in enhancing the ability of chondrocytes to curb inflammation in diseased joints.

RGD-dependent mechanotransduction of suspension cultured Taxus cell in response to shear stress

Plant cells cultured in bioreactors are strongly influenced by mechanical forces. However, the molecular mechanism of plant cell mechanoreception has maintained unclear. In animal cells, the Arg-Gly-Asp (RGD) motif can be found in proteins of the extracellular matrix. Integrins link the intracellular cytoskeleton of cells with the extracellular matrix by recognizing this RGD motif. Integrin has been demonstrated to function as an apparatus not only for adhesion but also for mechanotransduction. In plant cells, the molecules that mediate the structural continuity between wall and membrane are unknown. Here, we found that synthetic RGD peptide could dramatically reduce the level of phosphorylation of MAPK-like cascades that are activated by shear stress and reduce the alkalinization response, production of reactive oxygen species (ROS) and accumulation of phenolics by Taxus cuspidata cells during shear stress. These results implicate that a RGD recognition system may exist in Taxus cells and play an important role in signal transduction of shear stress. Although the Arabidopsis genome database shows that the plant seems to lack a homologue of animal integrin, plant cells may use other RGD-binding proteins to recognize the RGD motif. The correlative mechanism is discussed.

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.