Involvement of the Wnt signaling pathway in experimental and human osteoarthritis: prominent role of Wnt-induced signaling protein 1

Osteochondral alterations in osteoarthritis

Osteoarthritis (OA) is a major cause of pain and disability in the aging population, but its pathogenesis remains incompletely understood. Alterations beneath the articular cartilage at the osteochondral junction are attracting interest as possible mediators of pain and structural progression in OA. Osteochondral changes occur early during the development of OA and may aggravate pathology elsewhere in the joint. Loss of osteochondral integrity removes the barrier between intra-articular and subchondral compartments, exposing subchondral bone and its nerves to abnormal chemical and biomechanical influence. Osteochondral plasticity results in a merging of tissue compartments across the junction. Loss of the clearly differentiated demarcation between bone and articular cartilage is associated with invasion of articular cartilage by blood vessels and sensory nerves, and advancing endochondral ossification. Increased subchondral bone turnover is intimately associated with these alterations at the osteochondral junction. Cells signal across the osteochondral junction, and this cross-talk may be both a consequence of, and contribute to these pathological changes. Bone turnover, angiogenesis and nerve growth are also features of other diseases such as osteoporosis and cancers, for which therapeutic interventions are already advanced in their development. Here we review pathological changes at the osteochondral junction and explore their potential therapeutic implications for OA. This article is part of a Special Issue entitled "Osteoarthritis".

Cartilage development and degeneration: a Wnt Wnt situation

The Wnt signaling pathway plays a crucial role in the development and homeostasis of a variety of adult tissues and, as such, is emerging as an important therapeutic target for numerous diseases. Factors involved in the Wnt pathway are expressed throughout limb development and chondrogenesis and have been shown to be critical in joint homeostasis and endochondral ossification. Therefore, in this review, we discuss Wnt regulation of chondrogenic differentiation, hypertrophy and cartilage function. Moreover, we detail the role of the Wnt signaling pathway in cartilage degeneration and its potential to act as a target for therapy in osteoarthritis.

Progressive polyepiphyseal dysplasia with arthropathy: a distinct disorder from idiopathic juvenile arthritis and pseudorheumatoid dysplasia?

Juvenile idiopathic arthritis is an inflammatory disease with various onset-forms which can sometimes be difficult to distinguish from genetic inflammatory/rheumatoid-like osteoarthropathies. In this report, we describe two boys with severe chronic arthralgia, stiffness and swelling of joints, motor weakness and joints contractures evolving despite immunosuppressive treatments and for whom all biological and molecular exams failed to identify a prompt diagnosis. Some findings also overlap with pseudorheumatoid dysplasia but WISP3 gene molecular analysis failed to identify any mutation for both patients. Therefore, we propose that these boys show a clinical entity distinct from the actually known genetic inflammatory/rheumatoid-like osteoarthropathies.

The interaction of canonical bone morphogenetic protein- and Wnt-signaling pathways may play an important role in regulating cartilage degradation in osteoarthritis

Bone morphogenetic proteins (BMPs) and Wnts are important signaling protein families with key roles in embryologic, patterning, development, and tissue remodeling in growth. BMP and Wnt-β-catenin are highly evolutionarily conserved pathways that, though often regulating similar cellular events, are independent signaling mechanisms that can have complementary or antagonistic effects depending on various factors, including cell type and developmental stage. Although BMP and Wnt-β-catenin have the ability to act entirely independently, there is a developing body of evidence for specific extra- and intra-cellular molecular interactions and crosstalk that occur between BMP and Wnt-β-catenin signaling and that again this may be cell type-specific. In the previous issue of Arthritis Research & Therapy, Papathanasiou and colleagues provide novel insights into the role and direct interaction of BMP2 and canonical Wnt-β-catenin signaling in regulating chondrocyte hypertrophy and matrix metalloproteinase/a disintegrin like and metalloproteinase with thrombospondin type I motif (MMP/ADAMTS) synthesis in osteoarthritis.

Recent progress in understanding molecular mechanisms of cartilage degeneration during osteoarthritis

Osteoarthritis (OA) is a highly prevalent disease affecting more than 20% of American adults. Predispositions include joint injury, heredity, obesity, and aging. Biomechanical alterations are commonly involved. However, the molecular mechanisms of this disease are complex, and there is currently no effective disease-modifying treatment. The initiation and progression of OA subtypes is a complex process that at the molecular level probably involves many cell types, signaling pathways, and changes in extracellular matrix. Ex vivo studies with tissue derived from OA patients and in vivo studies with mutant mice have suggested that pathways involving receptor ligands such as TGF-β1, WNT3a, and Indian hedgehog; signaling molecules such as Smads, β-catenin, and HIF-2a; and peptidases such as MMP13 and ADAMTS4/5 are probably involved to some degree. This review focuses on molecular mechanisms of OA development related to recent findings.

Taking aim at the extracellular matrix: CCN proteins as emerging therapeutic targets

Members of the CCN family of matricellular proteins are crucial for embryonic development and have important roles in inflammation, wound healing and injury repair in adulthood. Deregulation of CCN protein expression or activities contributes to the pathobiology of various diseases - many of which may arise when inflammation or tissue injury becomes chronic - including fibrosis, atherosclerosis, arthritis and cancer, as well as diabetic nephropathy and retinopathy. Emerging studies indicate that targeting CCN protein expression or signalling pathways holds promise in the development of diagnostics and therapeutics for such diseases. This Review summarizes the biology of CCN proteins, their roles in various pathologies and their potential as therapeutic targets.

Are we moving in the right direction with osteoarthritis drug discovery?

INTRODUCTION

The success of targeted biologic therapy against rheumatoid arthritis has meant that much research has been devoted to investigating the pathophysiology of osteoarthritis, in the hope of defining novel therapeutic targets. Osteoarthritis has long been thought of mainly as a degenerative disease of cartilage, with secondary bony damage and osteophytes. However, in recent years, the importance of the synovium, and in particular the synovial macrophages, has been highlighted in both in vitro and in vivo studies.

AREAS COVERED

The recent progress in osteoarthritis drug discovery, particularly with regard to the search for therapeutic targets for this disease and the development of disease-modifying anti-osteoarthritic drugs is critically assessed. Some important recent research with regard to possible therapeutic targets in osteoarthritis drug discovery is highlighted.

EXPERT OPINION

The concept that synovial macrophages and macrophage-produced cytokines, may play a role in driving inflammatory and destructive signalling pathways in osteoarthritis, is of importance for drug discovery in this disease, in spite of disappointing results from early studies of anti-cytokine strategies in osteoarthritis clinical trials. There is also an abundance of potential downstream therapeutic targets in osteoarthritis, including the matrix metalloproteinases, the aggrecanases, iNOS and elements of the Wnt pathway.

Interleukin-18/WNT1-inducible signaling pathway protein-1 signaling mediates human saphenous vein smooth muscle cell proliferation

We demonstrate for the first time that the pro-inflammatory cytokine interleukin (IL)-18 stimulates rapid and significant proliferation of SMC derived from human saphenous vein (VSMC), but not coronary artery. IL-18 also stimulates VSMC growth. Further investigations revealed that IL-18-induced VSMC proliferation was Wnt inducible secreted protein-1 (WISP1) dependent. In addition to inducing its own expression via phosphatidylinositol 3-kinase/Akt-dependent IKK/NF-κB activation, IL-18 stimulated glycogen synthase kinase 3β phosphorylation and degradation, β-catenin nuclear translocation and stabilization, T-cell factor-lymphoid enhancer binding factor (TCF-LEF) activation, and WISP1 induction. Moreover, WISP1 induced its own expression, and that of survivin and multiple matrix metalloproteinases via β-catenin/TCF-LEF interaction. WISP1 also activated AP-1, but not NF-κB, and induced matrix metalloproteinase (MMP)9 transcription in part via AP-1. Interestingly, WISP1 failed to regulate tissue inhibitors of matrix metalloproteinases (TIMP) expression. These novel findings indicate that IL-18 induces a series of signaling events that result in WISP1-mediated VSMC proliferation, survival and MMP induction that are key components of vein graft stenosis and this may be amplified by IL-18 and WISP1 autoregulation and cross-regulation.

Cartilage biology in osteoarthritis--lessons from developmental biology

The cellular and molecular mechanisms responsible for the initiation and progression of osteoarthritis (OA), and in particular cartilage degeneration in OA, are not completely understood. Increasing evidence implicates developmental processes in OA etiology and pathogenesis. Herein, we review this evidence. We first examine subtle changes in cartilage development and the specification and formation of joints, which predispose to OA development, and second, we review the switch from an articular to a hypertrophic chondrocyte phenotype that is thought to be part of the OA pathological process ultimately resulting in cartilage degeneration. The latest studies are summarized and we discuss the concepts emerging from these findings in cartilage biology, in the light of our understanding of the developmental processes involved.

Dual function of β-catenin in articular cartilage growth and degeneration at different stages of postnatal cartilage development

PURPOSE

The objective of this study was to determine the role of β-catenin in normal postnatal articular cartilage growth and degeneration.

METHODS

We investigated β-catenin gene and protein expression in hip cartilage cells of normal Wistar rats at two, four, six and eight weeks of age by using reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry. Primary articular chondrocytes from eight week old rats were cultured and treated with LiCl for activation of β-catenin. Collagen X and matrix metalloproteinase 13 (MMP-13) were detected by quantitative RT-PCR and immunofluorescence. A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4 and 5 were detected by quantitative RT-PCR, and terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) was used for detecting cell apoptosis.

RESULTS

The highest levels of β-catenin expressions were detected in two week old rats, after which a steady decline was observed over the remaining period of observation (p < 0.05). When primary articular chondrocytes from eight week old rats were treated with LiCl, β-catenin mRNA and protein were induced (p < 0.05). Moreover, LiCl-activated β-catenin in chondrocytes was associated with significant concomitant increases in mRNA expression of collagen X and the MMP-13 encoding collagenase 3. Significantly increased mRNA expression of ADAMTS-5 was also seen in primary chondrocytes from eight week old rats after LiCl treatment (p < 0.05). The effect was specific to ADAMTS-5 since ADAMTS-4, which has similar proteolytic activity but different aggrecanase activity, was unaffected. Finally, TUNEL staining revealed that LiCl-activated β-catenin signalling led to increased cell apoptotic events in chondrocytes (p < 0.05).

CONCLUSIONS

Our findings suggest that normal spatiotemporal patterns and degrees of Wnt/β-catenin signalling are needed to maintain postnatal articular cartilage growth and function. In the early stages of cartilage development, activation of β-catenin signalling is necessary for articular cartilage growth, while in adult cartilage it leads to degeneration and osteoarthritic-like chondrocytes.

Proteases involved in cartilage matrix degradation in osteoarthritis

Osteoarthritis is a common joint disease for which there are currently no disease-modifying drugs available. Degradation of the cartilage extracellular matrix is a central feature of the disease and is widely thought to be mediated by proteinases that degrade structural components of the matrix, primarily aggrecan and collagen. Studies on transgenic mice have confirmed the central role of Adamalysin with Thrombospondin Motifs 5 (ADAMTS-5) in aggrecan degradation, and the collagenolytic matrix metalloproteinase MMP-13 in collagen degradation. This review discusses recent advances in current understanding of the mechanisms regulating expression of these key enzymes, as well as reviewing the roles of other proteinases in cartilage destruction. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Increased chondrocyte sclerostin may protect against cartilage degradation in osteoarthritis

OBJECTIVES

To investigate the regulation of sclerostin (SOST) in osteoarthritis (OA) and its potential effects on articular cartilage degradation.

METHODS

SOST and other Wnt-β-catenin components were immuno-localised in osteochondral sections of surgically-induced OA in knees of sheep and mice, and human OA samples obtained at arthroplasty. Regulation of SOST mRNA and protein expression by ovine chondrocytes in response to interleukin-1α (IL-1α) or tumour necrosis factor-α (TNFα) was examined in explant cultures. The effect of 25 or 250 ng/ml recombinant SOST alone or in combination with IL-1α, on ovine articular cartilage explant aggrecan degradation, and chondrocyte gene expression of Wnt-β-catenin pathway proteins, metalloproteinases and their inhibitors, and cartilage matrix proteins was quantified.

RESULTS

Contrary to being an osteocyte-specific protein, SOST was expressed by articular chondrocytes, and mRNA levels were upregulated in vitro by IL-1α but not TNFα. Chondrocyte SOST staining was significantly increased only in the focal area of cartilage damage in surgically-induced OA in sheep and mice, as well as end-stage human OA. In contrast, osteocyte SOST was focally decreased in the subchondral bone in sheep OA in association with bone sclerosis. SOST was biologically active in chondrocytes, inhibiting Wnt-β-catenin signalling and catabolic metalloproteinase [matrix metalloproteinases (MMP) and distintegrin and metalloproteinase with thrombospndin repeats (ADAMTS)] expression, but also decreasing mRNA levels of aggrecan, collagen II and tissue inhibitors of metalloproteinaes (TIMPs). Despite this mixed effect, SOST dose-dependently inhibited IL-1α-stimulated cartilage aggrecanolysis in vitro.

CONCLUSIONS

These results implicate SOST in regulating the OA disease processes, but suggest opposing effects by promoting disease-associated subchondral bone sclerosis while inhibiting degradation of cartilage.

Age-dependent alteration of TGF-β signalling in osteoarthritis

Osteoarthritis (OA) is a disease of articular cartilage, with aging as the main risk factor. In OA, changes in chondrocytes lead to the autolytic destruction of cartilage. Transforming growth factor-β has recently been demonstrated to signal not only via activin receptor-like kinase 5 (ALK5)-induced Smad2/3 phosphorylation, but also via ALK1-induced Smad1/5/8 phosphorylation in articular cartilage. In aging cartilage and experimental OA, the ratio ALK1/ALK5 has been found to be increased, and the expression of ALK1 is correlated with matrix metalloproteinase-13 expression. The age-dependent shift towards Smad1/5/8 signalling might trigger the differentiation of articular chondrocytes with an autolytic phenotype.

Intervertebral disc development is regulated by Wnt/β-catenin signaling

STUDY DESIGN

Histologic analysis of intervertebral disc (IVD) in three types of transgenic mice.

OBJECTIVE

To investigate the role of Wnt/β-catenin signaling in regulation of IVD development and organization.

SUMMARY OF BACKGROUND DATA

β-catenin dependent Wnt signaling is one of the central regulators in cartilage development during limb skeletal formation. Little is known, however, about the physiologic relevance of this signaling pathway to IVD development and organization.

METHODS

Temporal-spatial distribution of Wnt/β-catenin signaling activity was examined in IVD using Wnt/β-catenin reporter (TOPGAL) mice. The structural changes in the mouse IVD components such as the nucleus pulposus (NP), endplate (EP), annulus fibrosus (AF), and the growth plate (GP) of the vertebral body were analyzed after transient activation of Wnt/β-catenin signaling or deletion of β-catenin in the mice.

RESULTS

Activity of Wnt/β-catenin signaling was high in EP, AF, and GP in the embryonic stages and decreased at the postnatal stage; it was undetectable in the embryonic NP but upregulated after birth. The transient activation of Wnt/β-catenin signaling caused severe deterioration of the GP and the AF, whereas deficiency of β-catenin accelerated bone formation in between EP and GP.

CONCLUSION

The findings in this study suggest that proper regulation of Wnt/β-catenin signaling is required for development and organization of IVD.

The Wnt/beta-catenin pathway is activated during advanced arterial aging in humans

Aging is the main risk factor for cardiovascular diseases, but the associated molecular mechanisms are poorly understood. The Wnt signaling pathway was shown to be induced during aging in muscle and in the skin, but the regulation and role of Wnt signaling in the aged vessel have not yet been addressed. While screening for age-related changes in gene expression in the intima/media of human mammary arteries, we observed that the expression of frizzled 4 (Fzd4), a Wnt receptor, and of several targets of the Wnt/β-catenin/TCF signaling pathway [Wnt-inducible secreted protein 1 (WISP1), versican, osteopontin (SPP1), insulin-like growth factor binding protein 2 (IGFBP-2), and p21] were modified with age, suggesting an activation of the Wnt/β-catenin pathway. In contrast, we did not observe any regulation of forkhead transcription factor (FoxO) target genes. Beta-catenin-activating phosphorylation at position Ser675 was increased in aging mammary arteries, confirming the activation of this pathway. We confirmed in vitro that Wnt3a or Wnt1 treatment of human vascular smooth muscle cells (VSMCs) induced β-catenin phosphorylation at Ser675 and WISP1, SPP1, and IGFBP-2 expression. In vitro, Wnt treatment induced proliferation and cyclin D1 expression in VSMC from young (6 weeks old) rats but not in cells from older rats (8 months old), even though low-density lipoprotein receptor-related protein 6 and β-catenin phosphorylation, and β-catenin nuclear translocation demonstrated β-catenin activation in both cell types. Beta-catenin silencing demonstrated that Wnt induction of cyclin D1 expression is β-catenin dependent. Altogether, our data show that the Wnt/β-catenin/TCF pathway is activated in aging human mammary artery cells, but fails to induce the proliferation of aging vascular cells.

WISP-1/CCN4 regulates osteogenesis by enhancing BMP-2 activity

Wnt-induced secreted protein 1 (WISP-1/CCN4) is a member of the CCN family that is highly expressed in skeletal tissue and in osteoprogenitor cells induced to differentiate in vitro. To determine the function of WISP-1 during osteogeneis, osteogenic bone marrow stromal cells (BMSCs) were transduced with WISP-1 adenovirus (adWISP-1) in the presence or absence of bone morphogenetic protein 2 (BMP-2) adenovirus (adBMP-2). WISP-1 overexpression enhanced the ability of BMP-2 to direct BMSCs toward osteogenic differentiation and appeared to work by stimulating Smad-1/5/8 phosphorylation and activation. The ability of WISP-1 to enhance BMP-2 activity also was shown in vivo using an ectopic osteogenesis assay with BMSCs transduced with WISP-1, BMP-2, or both. When BMSCs were infected with lentivirus containing human WISP1 shRNA, they formed less bone in vivo and were less responsive to BMP-2, confirming that WISP-1 and BMP-2 have a functional interaction. Immunoprecipitation (IP) and Western blot analysis showed that WISP-1 bound directly to BMP-2 and showed that WISP-1 increased BMP-2 binding to hBMSCs in a dose-dependent fashion. To understand how WISP-1 enhanced BMP-2 signaling, the influence of WISP-1 on integrin expression was analyzed. WISP-1 induced the mRNA and protein levels of α(5)-integrin and, further, was found to bind to it. Antibody-blocking experiments showed that the BMP-2 binding to BMSCs that was enhanced by WISP-1 was completely neutralized by treatment with anti-integrin α(5)β(1) antibody. Pilot studies and the use of transgenic mice that overexpressed human WISP-1 in preosteoblasts had increased bone mineral density (BMD), trabecular thickness, and bone volume (BV/TV) over wild-type controls, supporting observations using human osteoprogenitors that WISP-1 has a positive influence on osteogenesis in vivo. In conclusion, these studies show, for the first time, that WISP-1 has a positive influence on bone cell differentiation and function and may work by enhancing the effects of BMP-2 to increase osteogenesis through a mechanism potentially involving binding to integrin α(5)β(1).

2-Deoxy-D-glucose regulates dedifferentiation through beta-catenin pathway in rabbit articular chondrocytes

2-deoxy-D-glucose (2DG) is known as a synthetic inhibitor of glucose. 2DG regulates various cellular responses including proliferation, apoptosis and differentiation by regulation of glucose metabolism in cancer cells. However, the effects of 2DG in normal cells, including chondrocytes, are not clear yet. We examined the effects of 2DG on dedifferentiation with a focus on the beta-catenin pathway in rabbit articular chondrocytes. The rabbit articular chondrocytes were treated with 5 mM 2DG for the indicated time periods or with various concentrations of 2DG for 24 h, and the expression of type II collagen, c-jun and beta-catenin was determined by Western blot, RT-PCR, immunofluorescence staining and immunohistochemical staining and reduction of sulfated proteoglycan synthesis detected by Alcain blue staining. Luciferase assay using a TCF (T cell factor)/LEF (lymphoid enhancer factor) reporter construct was used to demonstrate the transcriptional activity of beta-catenin. We found that 2DG treatment caused a decrease of type II collagen expression. 2DG induced dedifferentiation was dependent on activation of beta-catenin, as the 2DG stimulated accumulation of beta-catenin, which is characterized by translocation of beta-catenin into the nucleus determined by immunofluorescence staining and luciferase assay. Inhibition of beta-catenin degradation by inhibition of glycogen synthase kinase 3-beta with lithium chloride (LiCl) or inhibition of proteasome with z-Leu-Leu-Leu-CHO (MG132) accelerated the decrease of type II collagen expression in the chondrocytes. 2DG regulated the post-translational level of beta-catenin whereas the transcriptional level of beta-catenin was not altered. These results collectively showed that 2DG regulates dedifferentiation via beta-catenin pathway in rabbit articular chondrocytes.

Will o' the wisp: CCN4 as a novel molecular target in osteoarthritis

Osteoarthritis (OA), or degenerative arthritis, is characterized by mechanical stress-induced changes in cartilage and bone. OA is a leading cause of chronic disability in North America and Europe. A recent study written by Blom and colleagues (Arthritis and Rheumatism 2009; 60:501-12) showed that elevated wnt signaling was observed in experimental OA as well as in patient samples. The authors found that the known wnt target WISP-1 (CCN4) was also overexpressed; CCN4 was sufficient to recapitulate an OA phenotype in vitro and in vivo, suggesting that CCN4 may be a novel target for drug intervention in OA. This commentary summarizes these exciting findings.

Role of proinflammatory cytokines in the pathophysiology of osteoarthritis

Osteoarthritis (OA) is associated with cartilage destruction, subchondral bone remodeling and inflammation of the synovial membrane, although the etiology and pathogenesis underlying this debilitating disease are poorly understood. Secreted inflammatory molecules, such as proinflammatory cytokines, are among the critical mediators of the disturbed processes implicated in OA pathophysiology. Interleukin (IL)-1β and tumor necrosis factor (TNF), in particular, control the degeneration of articular cartilage matrix, which makes them prime targets for therapeutic strategies. Animal studies provide support for this approach, although only a few clinical studies have investigated the efficacy of blocking these proinflammatory cytokines in the treatment of OA. Apart from IL-1β and TNF, several other cytokines including IL-6, IL-15, IL-17, IL-18, IL-21, leukemia inhibitory factor and IL-8 (a chemokine) have also been shown to be implicated in OA and could possibly be targeted therapeutically. This Review discusses the current knowledge regarding the role of proinflammatory cytokines in the pathophysiology of OA and addresses the potential of anticytokine therapy in the treatment of this disease.

WNT1 inducible signaling pathway protein 1 (WISP1): a novel mediator linking development and disease

WISP1 is a secreted, matricellular protein allocated to the CCN protein family. The CCN protein family consists of six, modular structured, secreted proteins. WISP1 is mainly expressed during organ development and under diseased conditions, such as fibrosis or cancer. Its expression is associated with proliferation, cytoprotection, as well as extracellular matrix production, thereby representing a highly attractive therapeutical target for future applications.

Hypertrophic differentiation of chondrocytes in osteoarthritis: the developmental aspect of degenerative joint disorders

Osteoarthritis is characterized by a progressive degradation of articular cartilage leading to loss of joint function. The molecular mechanisms regulating pathogenesis and progression of osteoarthritis are poorly understood. Remarkably, some characteristics of this joint disease resemble chondrocyte differentiation processes during skeletal development by endochondral ossification. In healthy articular cartilage, chondrocytes resist proliferation and terminal differentiation. By contrast, chondrocytes in diseased cartilage progressively proliferate and develop hypertrophy. Moreover, vascularization and focal calcification of joint cartilage are initiated. Signaling molecules that regulate chondrocyte activities in both growth cartilage and permanent articular cartilage during osteoarthritis are thus interesting targets for disease-modifying osteoarthritis therapies.

Beta-catenin, cartilage, and osteoarthritis

The early cellular events during the development of osteoarthritis (OA) are accelerated articular chondrocyte maturation and extracellular matrix degradation, which are usually seen in the weight-bearing region of articular cartilage. The results of our recent studies from transgenic OA mouse models indicate that upregulation of beta-catenin signaling in articular chondrocytes is most likely responsible for the conversion of normal articular chondrocytes into maturing (arthritic) chondrocytes, which is associated with activation of chondrocyte maturational genes and matrix degradation. Conditional activation of the beta-catenin gene in articular chondrocytes leads to an OA-like phenotype. Overexpression of Smurf2, an E3 ubiquitin ligase, also induces an OA-like phenotype through upregulation of beta-catenin signaling. In addition, beta-catenin upregulation was also found in articular cartilage tissues in patients with OA. These findings indicate that beta-catenin plays a central role in articular cartilage function and that activation of beta-catenin signaling may represent a pathologic mechanism for OA development.

New molecular targets for the treatment of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disorder characterized by destruction of the articular cartilage, subchondral bone alterations and synovitis. Current treatments are focused on symptomatic relief but they lack efficacy to control the progression of this disease which is a leading cause of disability. Therefore, the development of effective disease-modifying drugs is urgently needed. Different initiatives are in progress to define the molecular mechanisms involved in the initiation and progression of OA. These studies support the therapeutic potential of pathways relevant in joint metabolism such as Wnt/beta-catenin, discoidin domain receptor 2 or proteinase-activated receptor 2. The dysregulation in cartilage catabolism and subchondral bone remodeling could be improved by selective inhibitors of matrix metalloproteinases, aggrecanases and other proteases. Another approach would favor the activity of anabolic processes by using growth factors or regulatory molecules. Recent studies have also revealed the role of oxidative stress and synovitis in the progression of this disease, supporting the development of a number of inhibitory strategies. Novel targets in OA are represented by genes involved in OA pathophysiology discovered using gene network, epigenetic and microRNA approaches. Further insights into the molecular mechanisms involved in OA initiation and progression may lead to the development of new therapies able to control joint destruction and repair.

The anatomical basis for a novel classification of osteoarthritis and allied disorders

Osteoarthritis (OA) has historically been classified as 'primary' where no discernible cause was evident and 'secondary' where a triggering factor was apparent. Irrespective of the triggering events, late-stage OA is usually characterized by articular cartilage attrition and consequently the anatomical basis for disease has been viewed in terms of cartilage. However, the widespread application of magnetic resonance imaging in early OA has confirmed several different anatomical abnormalities within diseased joints. A key observation has been that several types of primary or idiopathic OA show ligament-related pathology at the time of clinical presentation, so these categories of disease are no longer idiopathic - at least from the anatomical perspective. There is also ample evidence for OA initiation in other structures including menisci and bones in addition to articular cartilage. Therefore, a new classification for OA is proposed, which is based on the anatomical sites of earliest discernible joint structural involvement. The major proposed subgroups within this classification are ligament-, cartilage-, bone-, meniscal- and synovial-related, in addition to disease that is mixed pattern or multifocal in origin. We show how such a structural classification for OA provides a useful reference framework for staging the magnitude of disease. For late-stage or end-stage/whole organ disease, the final common pathway of these different scenarios, joint replacement strategies are likely to remain the only viable option. However, for younger subjects in particular, near the time of clinical disease onset, this scheme has implications for therapy targeted to specific anatomical locations. Thus, in the same way that tumours can be classified and staged according to their tissue of origin and extent of involvement, OA can likewise be anatomically classified and staged. This has implications for therapeutic strategies including regenerative medicine therapy development.

Wnt pathway genes in osteoporosis and osteoarthritis: differential expression and genetic association study

In comparison with hip fractures, increased expression of genes in the Wnt pathway and increased Wnt activity were found in bone samples and osteoblast cultures from patients with osteoarthritis, suggesting the involvement of this pathway in subchondral bone changes. No consistent differences were found in the genetic association study.

INTRODUCTION

This study aims to explore the allelic variations and expression of Wnt pathway genes in patients with osteoporosis and osteoarthritis.

METHODS

The expression of 86 genes was studied in bone samples and osteoblast primary cultures from patients with hip fractures and hip or knee osteoarthritis. The Wnt-related activity was assessed by measuring AXIN2 and in transfection experiments. Fifty-five SNPs of the LRP5, LRP6, FRZB, and SOST genes were analyzed in 1,128 patients.

RESULTS

Several genes were differentially expressed in bone tissue, with the lowest values usually found in hip fracture and the highest in knee osteoarthritis. Overall, seven genes were consistently upregulated both in tissue samples and in cell cultures from patients with knee osteoarthritis (BCL9, FZD5, DVL2, EP300, FRZB, LRP5, and TCF7L1). The increased expression of AXIN2 and experiments of transient transfection of osteoblasts with the TOP-Flash construct confirmed the activation of Wnt signaling. Three SNPs of the LRP5 gene and one in the LRP6 gene showed marginally significant differences in allelic frequencies across the patient groups, but they did not resist multiple-test adjustment.

CONCLUSIONS

Genes in the Wnt pathway are upregulated in the osteoarthritic bone, suggesting their involvement not only in cartilage distortion but also in subchondral bone changes.

TGF-beta signaling in chondrocyte terminal differentiation and osteoarthritis: modulation and integration of signaling pathways through receptor-Smads

OBJECTIVE

Chondrocytes and alteration in chondrocyte differentiation play a central role in osteoarthritis. Chondrocyte differentiation is amongst others regulated by members of the transforming growth factor-beta (TGF-beta) superfamily. The major intracellular signaling routes of this family are via the receptor-Smads. This review is focused on the modulation of receptor-Smad signaling and how this modulation can affect chondrocyte differentiation and potentially osteoarthritis development.

METHODS

Peer reviewed publications published prior to April 2009 were searched in the Pubmed database. Articles that were relevant for the role of TGF-beta superfamily/Smad signaling in chondrocyte differentiation and for differential modulation of receptor-Smads were selected.

RESULTS

Chondrocyte terminal differentiation is stimulated by Smad1/5/8 activation and inhibited the by Smad2/3 pathway, most likely by modulation of Runx2 function. Several proteins and signaling pathways differentially affect Smad1/5/8 and Smad2/3 signaling. This will result in an altered Smad1/5/8 and Smad2/3 balance and subsequently have an effect on chondrocyte differentiation and osteoarthritis development.

CONCLUSION

Modulation of receptor-Smads signaling can be expect to play an essential role in both the regulation of chondrocyte differentiation and osteoarthritis development and progression.