Prostaglandin E2 inhibits BMP signaling and delays chondrocyte maturation.

While cyclooxygenases are important in endochondral bone formation during fracture healing, mechanisms involved in prostaglandin E2 (PGE2) regulation of chondrocyte maturation are incompletely understood. The present study was undertaken to determine if PGE2 effects on chondrocyte differentiation are related to modulation of the bone morphogenetic protein (BMP) signaling pathway. In primary murine sternal chondrocytes, PGE2 differentially regulated genes involved in differentiation.

Reduced COX-2 expression in aged mice is associated with impaired fracture healing.

The cellular and molecular events responsible for reduced fracture healing with aging are unknown. Cyclooxygenase 2 (COX-2), the inducible regulator of prostaglandin E(2) (PGE(2)) 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.

Induction of an osteoarthritis-like phenotype and degradation of phosphorylated Smad3 by Smurf2 in transgenic mice.

Objective: To determine whether Smurf2, an E3 ubiquitin ligase known to inhibit transforming growth factor beta (TGFbeta) signaling, is expressed in human osteoarthritic (OA) cartilage and can initiate OA in mice.

Methods: Human OA cartilage was obtained from patients undergoing knee arthroplasty. Samples were graded histologically using the Mankin scale and were examined immunohistochemically for Smurf2 expression.

COX-2 from the injury milieu is critical for the initiation of periosteal progenitor cell mediated bone healing.

Although a critical role of COX-2 in bone repair has been established, the mechanism involved remains unclear. During early inflammatory phase of bone healing, COX-2 is produced by the surrounding inflammatory cells as well as bone/cartilage progenitors. Based on the temporal and spatial expression of COX-2 during the early phase of fracture healing, we hypothesize that COX-2 from both sources is critical for progenitor cell activation, proliferation and differentiation.

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.

What experimental approaches (eg, in vivo, in vitro, tissue retrieval) are effective in investigating the biologic effects of particles?

Understanding the complex cellular and tissue mechanisms and interactions resulting in periprosthetic osteolysis requires a number of experimental approaches, each of which has its own set of advantages and limitations. In vitro models allow for the isolation of individual cell populations and have furthered our understanding of particle-cell interactions; however, they are limited because they do not mimic the complex tissue environment in which multiple cell interactions occur. In vivo animal models investigate the tissue interactions associated with periprosthetic osteolysis, but the choice of species and whether the implant system is subjected to mechanical load or to unloaded conditions are critical in assessing whether these models can be extrapolated to the clinical condition.

Inhibition of beta-catenin signaling in articular chondrocytes results in articular cartilage destruction.

Objective: Osteoarthritis is a degenerative joint disease whose molecular mechanism is currently unknown. Wnt/beta-catenin signaling has been demonstrated to play a critical role in the development and function of articular chondrocytes. To determine the role of beta-catenin signaling in articular chondrocyte function, we generated Col2a1-ICAT-transgenic mice to inhibit beta-catenin signaling in chondrocytes.

Evaluating the effectiveness of air quality interventions.

Evaluating the extent to which air quality regulations improve public health--sometimes referred to as accountability--is part of an emerging effort to assess the effectiveness of environmental regulatory policies. Air quality has improved substantially in the United States and Western Europe in recent decades, with far less visible pollution and decreasing concentrations of several major pollutants. In large part, these gains were achieved through increasingly stringent air quality regulations.

A perspective: engineering periosteum for structural bone graft healing.

Autograft is superior to both allograft and synthetic bone graft in repair of large structural bone defect largely due to the presence of multipotent mesenchymal stem cells in periosteum. Recent studies have provided further evidence that activation, expansion and differentiation of the donor periosteal progenitor cells are essential for the initiation of osteogenesis and angiogenesis of donor bone graft healing. The formation of donor cell-derived periosteal callus enables efficient host-dependent graft repair and remodeling at the later stage of healing.

Teriparatide (1-34 human PTH) regulation of osterix during fracture repair.

Based on remarkable success of PTH as an anabolic drug for osteoporosis, case reports of off-label use of teriparatide (1-34 PTH) in patients with complicated fractures and non-unions are emerging. We investigated the mechanisms underlying PTH accelerated fracture repair. Bone marrow cells from 7 days 40 microg/kg of teriparatide treated or saline control mice were cultured and Osx and osteoblast phenotypic gene expression assessed by real-time RT-PCR in these cells.

The role of Snu114p during pre-mRNA splicing.

Pre-mRNA splicing is an essential step in gene expression where intron regions are removed and coding exon sequences are joined to form an mRNA for translation. Splicing is catalysed by an RNA-protein complex called the spliceosome. A number of spliceosome proteins are required for assembly and remodelling of the spliceosome with pre-mRNA to orient the splice sites correctly and catalyse the two steps of splicing.

Differential effects of biologic versus bisphosphonate inhibition of wear debris-induced osteolysis assessed by longitudinal micro-CT.

Aseptic loosening of total joint replacements is caused by wear debris-induced osteoclastic bone resorption, for which bisphosphonates (BPs) and RANK antagonists have been developed. Although BPs are effective in preventing metabolic bone loss, they are less effective for inflammatory bone loss. Because this difference has been attributed to the antiapoptotic inflammatory signals that protect osteoclasts from BP-induced apoptosis, but not RANK antagonists, we tested the hypothesis that osteoprotegerin (OPG) is more effective in preventing wear debris-induced osteolysis than zoledronic acid (ZA) or alendronate (Aln) in the murine calvaria model using in vivo micro-CT and traditional histology.

Inhibition of beta-catenin signaling causes defects in postnatal cartilage development.

The Wnt/beta-catenin signaling pathway is essential for normal skeletal development because conditional gain or loss of function of beta-catenin in cartilage results in embryonic or early postnatal death. To address the role of beta-catenin in postnatal skeletal growth and development, Col2a1-ICAT transgenic mice were generated. Mice were viable and had normal size at birth, but became progressively runted.

Tamoxifen-inducible CreER-mediated gene targeting in periosteum via bone-graft transplantation.

Periosteum plays a key role in bone repair through activation of residing stem and/or progenitor cells. The molecular signals regulating differentiation and expansion of periosteal stem cells during early repair are poorly understood. Understanding the molecular basis for initiation and completion of bone healing is vital for the success of bone-tissue engineering and regeneration therapy for impaired bone healing.

Regulation of chondrogenesis and chondrocyte differentiation by stress.

Chondrogenesis and endochondral ossification are the cartilage differentiation processes that lead to skeletal formation and growth in the developing vertebrate as well as skeletal repair in the adult. The exquisite regulation of these processes, both in normal development and in pathologic situations, is impacted by a number of different types of stress. These include normal stressors such as mechanical loading and hypoxia as well pathologic stressors such as injury and/or inflammation and environmental toxins.

Runx2-mediated activation of the Bax gene increases osteosarcoma cell sensitivity to apoptosis.

The Runx family of transcription factors regulate cell growth and differentiation, and control the expression of target genes involved in cell fate decisions. We examined the role of the bone-related member of this family, Runx2, in regulating apoptosis via modulation of the Bcl2 family of genes in the osteosarcoma cell line Saos2. Our data demonstrate that Runx2 directly binds to two Runx-specific regulatory elements on the human bax promoter thereby inducing Bax expression.

Adhesions in a murine flexor tendon graft model: autograft versus allograft reconstruction.

Reconstruction of flexor tendons often results in adhesions that compromise joint flexion. Little is known about the factors involved in the formation of flexor tendon graft adhesions. In this study, we developed and characterized a novel mouse model of flexor digitorum longus (FDL) tendon reconstruction with live autografts or reconstituted freeze-dried allografts.

Freeze-dried tendon allografts as tissue-engineering scaffolds for Gdf5 gene delivery.

Tendon reconstruction using grafts often results in adhesions that limit joint flexion. These adhesions are precipitated by inflammation, fibrosis, and the paucity of tendon differentiation signals during healing. In order to study this problem, we developed a mouse model in which the flexor digitorum longus (FDL) tendon is reconstructed using a live autograft or a freeze-dried allograft, and identified growth and differentiation factor 5 (Gdf5) as a therapeutic target.

Regulation of embryonic endochondral ossification by Smurf2.

Smurf2 is an E3 ubiquitin ligase that targets TGF-beta receptor activated Smad2 and Smad3 for the proteasome in primary articular chondrocytes, thus stimulating their hypertrophic differentiation. Comparatively, how Smurf2 functions in growth plate chondrocytes in a developing long bone is an open question. In this study, we measured the mRNA levels of endogenous Smurf2 and type X collagen in chick growth plate at different embryonic stages to monitor the correlation between the level of Smurf2 expression and chondrocyte maturational stage.

The U1, U2 and U5 snRNAs crosslink to the 5' exon during yeast pre-mRNA splicing.

Activation of pre-messenger RNA (pre-mRNA) splicing requires 5' splice site recognition by U1 small nuclear RNA (snRNA), which is replaced by U5 and U6 snRNA. Here we use crosslinking to investigate snRNA interactions with the 5' exon adjacent to the 5' splice site, prior to the first step of splicing. U1 snRNA was found to interact with four different 5' exon positions using one specific sequence adjacent to U1 snRNA helix 1.

Axin1 and Axin2 are regulated by TGF- and mediate cross-talk between TGF- and Wnt signaling pathways.

Chondrocyte maturation during endochondral bone formation is regulated by a number of signals that either promote or inhibit maturation. Among these, two well-studied signaling pathways play crucial roles in modulating chondrocyte maturation: transforming growth factor-beta (TGF-beta)/Smad3 signaling slows the rate of chondrocyte maturation, while Wingless/INT-1-related (Wnt)/beta-catenin signaling enhances the rate of chondrocyte maturation. Axin1 and Axin2 are functionally equivalent and have been shown to inhibit Wnt/beta-catenin signaling and stimulate TGF-beta signaling.

Overexpression of Smurf2 stimulates endochondral ossification through upregulation of beta-catenin.

Unlabelled: Ectopic expression of Smurf2 in chondrocytes and perichondrial cells accelerated endochondral ossification by stimulating chondrocyte maturation and osteoblast development through upregulation of beta-catenin in Col2a1-Smurf2 embryos. The mechanism underlying Smurf2-mediated morphological changes during embryonic development may provide new mechanistic insights and potential targets for prevention and treatment of human osteoarthritis.

Introduction: Our recent finding that adult Col2a1-Smurf2 mice have an osteoarthritis-like phenotype in knee joints prompted us to examine the role of Smurf2 in the regulation of chondrocyte maturation and osteoblast differentiation during embryonic endochondral ossification.

Murine and chicken chondrocytes regulate osteoclastogenesis by producing RANKL in response to BMP2.

Unlabelled: Chondrocytes express RANKL, but their role in osteoclastogenesis is not clear. We report that hypertrophic chondrocytes induce osteoclast formation through RANKL production stimulated by BMP2 and Runx2/Smad1 and thus they may regulate resorption of calcified matrix by osteoclasts at growth plates.

Introduction: Bone morphogenetic protein (BMP) signaling and Runx2 regulate chondrogenesis during bone development and fracture repair and RANKL expression by osteoblast/stromal cells.

Tamoxifen-inducible Cre-recombination in articular chondrocytes of adult Col2a1-CreER(T2) transgenic mice.

Objective: To determine the specificity and efficiency of the tamoxifen (TM)-induced Cre-recombination in articular chondrocytes of adult Col2a1-CreER(T2) transgenic mice.

Methods: Col2a1-CreER(T2) transgenic mice were bred with Rosa26 reporter mice. Two-week-old Col2a1-CreER(T2);R26R mice were administered TM for 5 days and were sacrificed 1 and 6 months after TM induction.

Lead induces chondrogenesis and alters transforming growth factor-beta and bone morphogenetic protein signaling in mesenchymal cell populations.

Background: It has been established that skeletal growth is stunted in lead-exposed children. Because chondrogenesis is a seminal step during skeletal development, elucidating the impact of Pb on this process is the first step toward understanding the mechanism of Pb toxicity in the skeleton.

Objectives: The aim of this study was to test the hypothesis that Pb alters chondrogenic commitment of mesenchymal cells and to assess the effects of Pb on various signaling pathways.