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.

The Notch pathway has recently been implicated in mesenchymal progenitor cell (MPC) differentiation from bone marrow-derived progenitors. However, whether Notch regulates MPC differentiation in an RBPjκ-dependent manner, specifies a particular MPC cell fate, regulates MPC proliferation and differentiation during early skeletal development or controls specific Notch target genes to regulate these processes remains unclear. To determine the exact role and mode of action for the Notch pathway in MPCs during skeletal development, we analyzed tissue-specific loss-of-function (Prx1Cre; Rbpjkf/f), gain-of-function (Prx1Cre; Rosa-NICDf/+) and RBPjκ-independent Notch gain-of-function (Prx1Cre; Rosa-NICDf/+; Rbpjkf/f) mice for defects in MPC proliferation and differentiation. These data demonstrate for the first time that the RBPjκ-dependent Notch signaling pathway is a crucial regulator of MPC proliferation and differentiation during skeletal development. Our study also implicates the Notch pathway as a general suppressor of MPC differentiation that does not bias lineage allocation. Finally, Hes1 was identified as an RBPjκ-dependent Notch target gene important for MPC maintenance and the suppression of in vitro chondrogenesis.

Objective. Osteoarthritis is a degenerative joint disease whose molecular mechanism is currently unknown. Wnt/␤-catenin signaling has been demonstrated to play a critical role in the development and function of articular chondrocytes. To determine the role of ␤-catenin signaling in articular chondrocyte function, we generated Col2a1-ICAT-transgenic mice to inhibit ␤-catenin signaling in chondrocytes.Methods. The expression of the ICAT transgene was determined by immunostaining and Western blot analysis. Histologic analyses were performed to determine changes in articular cartilage structure and morphology. Cell apoptosis was determined by TUNEL staining and the immunostaining of cleaved caspase 3 and poly(ADP-ribose) polymerase (PARP) proteins. Expression of Bcl-2, Bcl-x L , and Bax proteins and caspase 9 and caspase 3/7 activities were examined in primary sternal chondrocytes isolated from 3-day-old neonatal Col2a1-ICAT-transgenic mice and their wild-type littermates and in primary chicken and porcine articular chondrocytes.Results. Expression of the ICAT transgene was detected in articular chondrocytes of the transgenic mice. Associated with this, age-dependent articular cartilage destruction was observed in Col2a1-ICATtransgenic mice. A significant increase in cell apoptosis in articular chondrocytes was identified by TUNEL staining and the immunostaining of cleaved caspase 3 and PARP proteins in these transgenic mice. Consistent with this, Bcl-2 and Bcl-x L expression were decreased and caspase 9 and caspase 3/7 activity were increased, suggesting that increased cell apoptosis may contribute significantly to the articular cartilage destruction observed in Col2a1-ICAT-transgenic mice.Conclusion. Inhibition of ␤-catenin signaling in articular chondrocytes causes increased cell apoptosis and articular cartilage destruction in Col2a1-ICATtransgenic mice.

Obesity is a risk factor for osteoarthritis (OA), the greatest cause of disability in the US. The impact of obesity on OA is driven by systemic inflammation, and increased systemic inflammation is now understood to be caused by gut microbiome dysbiosis. Oligofructose, a nondigestible prebiotic fiber, can restore a lean gut microbial community profile in the context of obesity, suggesting a potentially novel approach to treat the OA of obesity. Here, we report that - compared with the lean murine gut - obesity is associated with loss of beneficial Bifidobacteria, while key proinflammatory species gain in abundance. A downstream systemic inflammatory signature culminates with macrophage migration to the synovium and accelerated knee OA. Oligofructose supplementation restores the lean gut microbiome in obese mice, in part, by supporting key commensal microflora, particularly Bifidobacterium pseudolongum. This is associated with reduced inflammation in the colon, circulation, and knee and protection from OA. This observation of a gut microbiome-OA connection sets the stage for discovery of potentially new OA therapeutics involving strategic manipulation of specific microbial species inhabiting the intestinal space.

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