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      Cartilage homeostasis in health and rheumatic diseases

      review-article
      1 , , 2 , 3
      Arthritis Research & Therapy
      BioMed Central

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          Abstract

          As the cellular component of articular cartilage, chondrocytes are responsible for maintaining in a low-turnover state the unique composition and organization of the matrix that was determined during embryonic and postnatal development. In joint diseases, cartilage homeostasis is disrupted by mechanisms that are driven by combinations of biological mediators that vary according to the disease process, including contributions from other joint tissues. In osteoarthritis (OA), biomechanical stimuli predominate with up-regulation of both catabolic and anabolic cytokines and recapitulation of developmental phenotypes, whereas in rheumatoid arthritis (RA), inflammation and catabolism drive cartilage loss. In vitro studies in chondrocytes have elucidated signaling pathways and transcription factors that orchestrate specific functions that promote cartilage damage in both OA and RA. Thus, understanding how the adult articular chondrocyte functions within its unique environment will aid in the development of rational strategies to protect cartilage from damage resulting from joint disease. This review will cover current knowledge about the specific cellular and biochemical mechanisms that regulate cartilage homeostasis and pathology.

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          Osteoarthritis.

          Osteoarthritis (OA) is characterized by degeneration of articular cartilage, limited intraarticular inflammation with synovitis, and changes in peri-articular and subchondral bone. Multiple factors are involved in the pathogenesis of OA, including mechanical influences, the effects of aging on cartilage matrix composition and structure, and genetic factors. Since the initial stages of OA involve increased cell proliferation and synthesis of matrix proteins, proteinases, growth factors, cytokines, and other inflammatory mediators by chondrocytes, research has focused on the chondrocyte as the cellular mediator of OA pathogenesis. The other cells and tissues of the joint, including the synovium and subchondral bone, also contribute to pathogenesis. The adult articular chondrocyte, which normally maintains the cartilage with a low turnover of matrix constituents, has limited capacity to regenerate the original cartilage matrix architecture. It may attempt to recapitulate phenotypes of early stages of cartilage development, but the precise zonal variations of the original cartilage cannot be replicated. Current pharmacological interventions that address chronic pain are insufficient, and no proven structure-modifying therapy is available. Cartilage tissue engineering with or without gene therapy is the subject of intense investigation. There are multiple animal models of OA, but there is no single model that faithfully replicates the human disease. This review will focus on questions currently under study that may lead to better understanding of mechanisms of OA pathogenesis and elucidation of effective strategies for therapy, with emphasis on mechanisms that affect the function of chondrocytes and interactions with surrounding tissues. 2007 Wiley-Liss, Inc.
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            The control of chondrogenesis.

            Chondrogenesis is the earliest phase of skeletal development, involving mesenchymal cell recruitment and migration, condensation of progenitors, and chondrocyte differentiation, and maturation and resulting in the formation of cartilage and bone during endochondral ossification. This process is controlled exquisitely by cellular interactions with the surrounding matrix, growth and differentiation factors, and other environmental factors that initiate or suppress cellular signaling pathways and transcription of specific genes in a temporal-spatial manner. Vertebrate limb development is controlled by interacting patterning systems involving prominently the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), and hedgehog pathways. Both positive and negative signaling kinases and transcription factors, such as Sox9 and Runx2, and interactions among them determine whether the differentiated chondrocytes remain within cartilage elements in articular joints or undergo hypertrophic maturation prior to ossification. The latter process requires extracellular matrix remodeling and vascularization controlled by mechanisms that are not understood completely. Recent work has revealed novel roles for mediators such as GADD45beta, transcription factors of the Dlx, bHLH, leucine zipper, and AP-1 families, and the Wnt/beta-catenin pathway that interact at different stages during chondrogenesis. (c) 2005 Wiley-Liss, Inc.
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              Activation of beta-catenin signaling in articular chondrocytes leads to osteoarthritis-like phenotype in adult beta-catenin conditional activation mice.

              Osteoarthritis (OA) is a degenerative joint disease, and the mechanism of its pathogenesis is poorly understood. Recent human genetic association studies showed that mutations in the Frzb gene predispose patients to OA, suggesting that the Wnt/beta-catenin signaling may be the key pathway to the development of OA. However, direct genetic evidence for beta-catenin in this disease has not been reported. Because tissue-specific activation of the beta-catenin gene (targeted by Col2a1-Cre) is embryonic lethal, we specifically activated the beta-catenin gene in articular chondrocytes in adult mice by generating beta-catenin conditional activation (cAct) mice through breeding of beta-catenin(fx(Ex3)/fx(Ex3)) mice with Col2a1-CreER(T2) transgenic mice. Deletion of exon 3 of the beta-catenin gene results in the production of a stabilized fusion beta-catenin protein that is resistant to phosphorylation by GSK-3beta. In this study, tamoxifen was administered to the 3- and 6-mo-old Col2a1-CreER(T2);beta-catenin(fx(Ex3)/wt) mice, and tissues were harvested for histologic analysis 2 mo after tamoxifen induction. Overexpression of beta-catenin protein was detected by immunostaining in articular cartilage tissues of beta-catenin cAct mice. In 5-mo-old beta-catenin cAct mice, reduction of Safranin O and Alcian blue staining in articular cartilage tissue and reduced articular cartilage area were observed. In 8-mo-old beta-catenin cAct mice, cell cloning, surface fibrillation, vertical clefting, and chondrophyte/osteophyte formation were observed. Complete loss of articular cartilage layers and the formation of new woven bone in the subchondral bone area were also found in beta-catenin cAct mice. Expression of chondrocyte marker genes, such as aggrecan, Mmp-9, Mmp-13, Alp, Oc, and colX, was significantly increased (3- to 6-fold) in articular chondrocytes derived from beta-catenin cAct mice. Bmp2 but not Bmp4 expression was also significantly upregulated (6-fold increase) in these cells. In addition, we also observed overexpression of beta-catenin protein in the knee joint samples from patients with OA. These findings indicate that activation of beta-catenin signaling in articular chondrocytes in adult mice leads to the premature chondrocyte differentiation and the development of an OA-like phenotype. This study provides direct and definitive evidence about the role of beta-catenin in the development of OA.
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                Author and article information

                Journal
                Arthritis Res Ther
                Arthritis Research & Therapy
                BioMed Central
                1478-6354
                1478-6362
                2009
                19 May 2009
                : 11
                : 3
                : 224
                Affiliations
                [1 ]Research Division, Hospital for Special Surgery, affiliated with Weill College of Medicine of Cornell University, Caspary Research Building, 535 E. 70th Street, New York, NY 10021, USA
                [2 ]Biochemistry and Cell Biology Department, Stony Brook University, Life Sciences Rm #330, Stony Brook, NY 11794, USA
                [3 ]Centro Ricerca Biomedica Applicata, S. Orsola-Malpighi University Hospital, University of Bologna, Via Massarenti 9, 40138 Bologna, Italy
                Article
                ar2592
                10.1186/ar2592
                2714092
                19519926
                c3a6996a-5737-4071-861c-2bda71c88bcc
                Copyright © 2009 BioMed Central Ltd
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                Orthopedics
                Orthopedics

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