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

A murine segmental femoral bone graft model was used to show the essential role of donor periosteal progenitor cells in bone graft healing. Transplantation of live bone graft harvested from Rosa 26A mice showed that ∼70% of osteogenesis on the graft was attributed to the expansion and differentiation of donor periosteal progenitor cells. Furthermore, engraftment of BMP-2-producing bone marrow stromal cells on nonvital allografts showed marked increases in cortical graft incorporation and neovascularization, suggesting that gene-enhanced, tissue engineered functional periosteum may improve allograft incorporation and repair. Introduction: The loss of cellular activity in a structural bone allograft markedly reduces its healing potential compared with a live autograft. To further understand the cellular mechanisms for structural bone graft healing and repair and to devise a therapeutic strategy aimed at enhancing the performance of allograft, we established a segmental femoral structural bone graft model in mice that permits qualitative and quantitative analyses of graft healing and neovascularization. Materials and Methods: Using this segmental femoral bone graft model, we transplanted live isografts harvested from Rosa 26A mice that constitutively express β-galactosidase into their wildtype control mice. In an attempt to emulate the osteogenic and angiogenic properties of periosteum, we applied a cell-based, adenovirus-mediated gene therapy approach to engraft BMP-2-producing bone marrow stromal cells onto devitalized allografts. Results: X-gal staining for donor cells allowed monitoring the progression of periosteal progenitor cell fate and showed that 70% of osteogenesis was attributed to cellular proliferation and differentiation of donor progenitor cells on the surface of the live bone graft. Quantitative μCT analyses showed a 3-fold increase in new bone callus formation and a 6.8-fold increase in neovascularization for BMP-2/stromal cell-treated allograft compared with control acellular allografts. Histologic analyses showed the key features of autograft healing in the BMP-2/stromal cell-treated allografts, including the formation of a mineralized bone callus completely bridging the segmental defects, abundant neovascularization, and extensive resorption of bone graft. Conclusions: The marked improvement of healing in these cellularized allografts suggests a clinical strategy for engineering a functional periosteum to improve the osteogenic and angiogenic properties of processed allografts.

The roles of the androgen receptor (AR) in female fertility and ovarian function remain largely unknown. Here we report on the generation of female mice lacking AR ( AR ^–/– ) and the resulting influences on the reproductive system. Female AR ^–/– mice appear normal but show longer estrous cycles and reduced fertility. The ovaries from sexually mature AR ^–/– females exhibited a marked reduction in the number of corpora lutea. After superovulation treatment, the AR ^–/– ovaries produced fewer oocytes and also showed fewer corpora lutea. During the periovulatory period, an intensive granulosa apoptosis event occurs in the AR ^–/– preovulatory follicles, concurrent with the down-regulation of p21 and progesterone receptor expression. Furthermore, the defective conformation of the cumulus cell–oocyte complex from the AR ^–/– females implies a lower fertilization capability of the AR ^–/– oocytes. In addition to insufficient progesterone production, the diminished endometrial growth in uteri in response to exogenous gonadotropins indicates that AR ^–/– females exhibit a luteal phase defect. Taken together, these data provide in vivo evidence showing that AR plays an important role in female reproduction.

Phenotype analysis of female mice lacking androgen receptor (AR) deficient (AR^−/−) indicates that the development of mammary glands is retarded with reduced ductal branching in the prepubertal stages, and fewer Cap cells in the terminal end buds, as well as decreased lobuloalveolar development in adult females, and fewer milk-producing alveoli in the lactating glands. The defective development of AR^−/− mammary glands involves the defects of insulin-like growth factor I–insulin-like growth factor I receptor and mitogen-activated protein kinase (MAPK) signals as well as estrogen receptor (ER) activity. Similar growth retardation and defects in growth factor–mediated Ras/Raf/MAPK cascade and ER signaling are also found in AR^−/− MCF7 breast cancer cells. The restoration assays show that AR NH₂-terminal/DNA-binding domain, but not the ligand-binding domain, is essential for normal MAPK function in MCF7 cells, and an AR mutant (R608K), found in male breast cancer, is associated with the excessive activation of MAPK. Together, our data provide the first in vivo evidence showing that AR-mediated MAPK and ER activation may play important roles for mammary gland development and MCF7 breast cancer cell proliferation.

The cellular and molecular events responsible for reduced fracture healing with aging are unknown. Cyclooxygenase 2 (COX-2), the inducible regulator of prostaglandin E2 (PGE2) synthesis, is critical for normal bone repair. A femoral fracture repair model was used in mice at either 7–9 or 52–56 wk of age, and healing was evaluated by imaging, histology, and gene expression studies. Aging was associated with a decreased rate of chondrogenesis, decreased bone formation, reduced callus vascularization, delayed remodeling, and altered expression of genes involved in repair and remodeling. COX-2 expression in young mice peaked at 5 days, coinciding with the transition of mesenchymal progenitors to cartilage and the onset of expression of early cartilage markers. In situ hybridization and immunohistochemistry showed that COX-2 is expressed primarily in early cartilage precursors that co-express col-2.COX-2 expression was reduced by 75% and 65% in fractures from aged mice compared with young mice on days 5 and 7, respectively. Local administration of an EP4 agonist to the fracture repair site in aged mice enhanced the rate of chondrogenesis and bone formation to levels observed in young mice, suggesting that the expression of COX-2 during the early inflammatory phase of repair regulates critical subsequent events including chondrogenesis, bone formation, and remodeling. The findings suggest that COX-2/EP4 agonists may compensate for deficient molecular signals that result in the reduced fracture healing associated with aging.