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/␤-catenin signaling may be the key pathway to the development of OA. However, direct genetic evidence for ␤-catenin in this disease has not been reported. Because tissue-specific activation of the ␤-catenin gene (targeted by Col2a1-Cre) is embryonic lethal, we specifically activated the ␤-catenin gene in articular chondrocytes in adult mice by generating ␤-catenin conditional activation (cAct) mice through breeding of ␤-catenin fx(Ex3)/fx(Ex3) mice with Col2a1-CreER T2 transgenic mice. Deletion of exon 3 of the ␤-catenin gene results in the production of a stabilized fusion ␤-catenin protein that is resistant to phosphorylation by GSK-3␤. In this study, tamoxifen was administered to the 3-and 6-mo-old Col2a1-CreER T2 ;␤-catenin fx(Ex3)/wt mice, and tissues were harvested for histologic analysis 2 mo after tamoxifen induction. Overexpression of ␤-catenin protein was detected by immunostaining in articular cartilage tissues of ␤-catenin cAct mice. In 5-mo-old ␤-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 ␤-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 ␤-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 ␤-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 ␤-catenin protein in the knee joint samples from patients with OA. These findings indicate that activation of ␤-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 ␤-catenin in the development of OA.

SummaryThe BMP signaling pathway has a crucial role in chondrocyte proliferation and maturation during endochondral bone development. To investigate the specific function of the Bmp2 and Bmp4 genes in growth plate chondrocytes during cartilage development, we generated chondrocyte-specific Bmp2 and Bmp4 conditional knockout (cKO) mice and Bmp2,Bmp4 double knockout (dKO) mice. We found that deletion of Bmp2 and Bmp4 genes or the Bmp2 gene alone results in a severe chondrodysplasia phenotype, whereas deletion of the Bmp4 gene alone produces a minor cartilage phenotype. Both dKO and Bmp2 cKO mice exhibit severe disorganization of chondrocytes within the growth plate region and display profound defects in chondrocyte proliferation, differentiation and apoptosis. To understand the mechanism by which BMP2 regulates these processes, we explored the specific relationship between BMP2 and Runx2, a key regulator of chondrocyte differentiation. We found that BMP2 induces Runx2 expression at both the transcriptional and post-transcriptional levels. BMP2 enhances Runx2 protein levels through inhibition of CDK4 and subsequent prevention of Runx2 ubiquitylation and proteasomal degradation. Our studies provide novel insights into the genetic control and molecular mechanism of BMP signaling during cartilage development.Key words: Bmp2, Bmp4, Chondrocyte, Endochondral bone formation Introduction During skeletal development, the majority of the bones in the body are established by the endochondral bone formation process, which is initiated by mesenchymal cell condensation and subsequent mesenchymal cell differentiation into chondrocytes and surrounding perichondrial cells. The committed chondrocytes proliferate rapidly forming the cartilage growth plate where cells are arranged in columns of proliferating, differentiating and terminally hypertrophic chondrocytes. Chondrocytes near the center of the cartilage elements exit the cell cycle initiating the process of hypertrophic differentiation to generate a calcified cartilage matrix. Eventually, the local vasculature, perichondrial osteoblasts and various other types of cells invade the calcified cartilage, replacing the terminally mature chondrocytes with marrow components and trabecular bone matrix. Primary ossification occurs with osteoblast-mediated bone formation, which initially occurs on the calcified cartilage template. Chondrocyte maturation and the endochondral bone development process is tightly regulated by a series of growth factors and transcription factors, including bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), indian hedgehog (Ihh),

Objective The incidence of low back pain is extremely high and is often linked to intervertebral disc (IVD) degeneration. The mechanism of this disease is currently unknown. In this study, we have investigated the role of β-catenin signaling in IVD tissue function. Methods β-catenin protein levels were measured in disc samples derived from patients with disc degeneration and normal subjects by immunohistochemistry (IHC). To generate β-catenin conditional activation (cAct) mice, Col2a1-CreERT2 transgenic mice were bred with β-cateninfx(Ex3)/fx(Ex3) mice. Changes in disc tissue morphology and function were analyzed by micro-CT, histology and real-time PCR assays. Results We found that β-catenin protein was up-regulated in disc tissues from patients with disc degeneration. To assess the effects of increased β-catenin on disc tissue we generated β-catenin cAct mice. Overexpression of β-catenin in disc cells led to extensive osteophyte formation in 3- and 6-month-old β-catenin cAct mice which were associated with significant changes in the cells and extracellular matrix of disc tissues and growth plate. Gene expression analysis demonstrated that activation of β-catenin could enhance Runx2-dependent Mmp13 and Adamts5 expression. Moreover, genetic ablation of the Mmp13 or Adamts5 under β-catenin cAct background, or treatment of β-catenin cAct mice with a specific MMP13 inhibitor, ameliorated the mutant phenotype. Conclusions β-catenin signaling pathway plays a critical role in disc tissue function.