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

Signalling interaction between β-catenin and other signalling molecules during osteoarthritis development

Osteoarthritis (OA) is the most prevalent disorder of synovial joint affecting multiple joints. In the past decade, we have witnessed conceptual switch of OA pathogenesis from a 'wear and tear' disease to a disease affecting entire joint. Extensive studies have been conducted to understand the underlying mechanisms of OA using genetic mouse models and ex vivo joint tissues derived from individuals with OA. These studies revealed that multiple signalling pathways are involved in OA development, including the canonical Wnt/β-catenin signalling and its interaction with other signalling pathways, such as transforming growth factor β (TGF-β), bone morphogenic protein (BMP), Indian Hedgehog (Ihh), nuclear factor κB (NF-κB), fibroblast growth factor (FGF), and Notch. The identification of signalling interaction and underlying mechanisms are currently underway and the specific molecule(s) and key signalling pathway(s) playing a decisive role in OA development need to be evaluated. This review will focus on recent progresses in understanding of the critical role of Wnt/β-catenin signalling in OA pathogenesis and interaction of β-catenin with other pathways, such as TGF-β, BMP, Notch, Ihh, NF-κB, and FGF. Understanding of these novel insights into the interaction of β-catenin with other pathways and its integration into a complex gene regulatory network during OA development will help us identify the key signalling pathway of OA pathogenesis leading to the discovery of novel therapeutic strategies for OA intervention.

Elucidating the role of ubiquitination and deubiquitination in osteoarthritis progression

Osteoarthritis is non-inflammatory degenerative joint arthritis, which exacerbates disability in elder persons. The molecular mechanisms of osteoarthritis are elusive. Ubiquitination, one type of post-translational modifications, has been demonstrated to accelerate or ameliorate the development and progression of osteoarthritis via targeting specific proteins for ubiquitination and determining protein stability and localization. Ubiquitination process can be reversed by a class of deubiquitinases via deubiquitination. In this review, we summarize the current knowledge regarding the multifaceted role of E3 ubiquitin ligases in the pathogenesis of osteoarthritis. We also describe the molecular insight of deubiquitinases into osteoarthritis processes. Moreover, we highlight the multiple compounds that target E3 ubiquitin ligases or deubiquitinases to influence osteoarthritis progression. We discuss the challenge and future perspectives via modulation of E3 ubiquitin ligases and deubiquitinases expression for enhancement of the therapeutic efficacy in osteoarthritis patients. We conclude that modulating ubiquitination and deubiquitination could alleviate the osteoarthritis pathogenesis to achieve the better treatment outcomes in osteoarthritis patients.

E3 Ubiquitin Ligases: Potential Therapeutic Targets for Skeletal Pathology and Degeneration

The ubiquitination-proteasome system (UPS) is crucial in regulating a variety of cellular processes including proliferation, differentiation, and survival. Ubiquitin protein ligase E3 is the most critical molecule in the UPS system. Dysregulation of the UPS system is associated with many conditions. Over the past few decades, there have been an increasing number of studies focusing on the UPS system and how it affects bone metabolism. Multiple E3 ubiquitin ligases have been found to mediate osteogenesis or osteolysis through a variety of pathways. In this review, we describe the mechanisms of UPS, especially E3 ubiquitin ligases on bone metabolism. To date, many E3 ubiquitin ligases have been found to regulate osteogenesis or osteoclast differentiation. We review the classification of these E3 enzymes and the mechanisms that influence upstream and downstream molecules and transduction pathways. Finally, this paper reviews the discovery of the relevant UPS inhibitors, drug molecules, and noncoding RNAs so far and prospects the future research and treatment.

Cartilage degeneration is associated with activation of the PI3K/AKT signaling pathway in a growing rat experimental model of developmental trochlear dysplasia

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Established a new experimental rat model of the developmental trochlear dysplasia;

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Using the macroscopic morphological and micro-CT to assess trochlear dysplasia;

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Using Histological staining to investigate the cartilage degradation of the model;

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Investigated the relationship of the PI3K/AKT signaling pathway with trochlear dysplasia cartilage degeneration;

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Using immunohistochemistry and qPCR to investigate the PI3K/AKT and the marker of the cartilage degeneration.

Introduction

Trochlear dysplasia is a commonly encountered lower extremity deformity in humans. However, the molecular mechanism of cartilage degeneration in trochlear dysplasia is unclear thus far.

Objectives

The PI3K/AKT signaling pathway is known to be important for regulating the pathophysiology of cartilage degeneration. The aim of this study was to investigate the relationship of the PI3K/AKT signaling pathway with trochlear dysplasia cartilage degeneration.

Methods

In total, 120 female Sprague-Dawley rats (4 weeks of age) were randomly separated into control and experimental groups. Distal femurs were isolated from the experimental group at 4, 8, and 12 weeks after surgery; they were isolated from the control group at the same time points. Micro-computed tomography and histological examination were performed to investigate trochlear anatomy and changes in trochlear cartilage. Subsequently, expression patterns of PI3K/AKT, TGFβ1, and ADAMTS-4 in cartilage were investigated by immunohistochemistry and quantitative polymerase chain reaction.

Results

In the experimental group, the trochlear dysplasia model was successfully established at 8 weeks after surgery. Moreover, cartilage degeneration was observed beginning at 8 weeks after surgery, with higher protein and mRNA expression levels of PI3K/AKT, TGFβ1, and ADAMTS-4, relative to the control group.

Conclusion

Patellar instability might lead to trochlear dysplasia in growing rats. Moreover, trochlear dysplasia may cause patellofemoral osteoarthritis; cartilage degeneration in trochlear dysplasia might be associated with activation of the PI3K/AKT signaling pathway. These results provide insights regarding the high incidence of osteoarthritis in patients with trochlear dysplasia. However, more research is needed to clarify the underlying mechanisms.

SAG therapy restores bone growth and reduces enchondroma incidence in a model of skeletal chondrodysplasias caused by Ihh deficiency

Inactivation mutations in the Indian hedgehog (Ihh) gene in humans cause numerous skeletal chondrodysplasias, including acrocapitofemoral dysplasia, brachydactyly type A1, and human short stature. The lack of an appropriate human-relevant model to accurately represent these chondrodysplasias has hampered the identification of clinically effective treatments. Here, we established a mouse model of human skeletal dysplasia induced by Ihh gene mutations via ablation of Ihh in Aggrecan-positive (Acan+) cells using Aggrecan (Acan)-creERT transgenic mice. Smoothen agonist (SAG) promoted Hh activity and rescued chondrocyte proliferation and differentiation by stimulating smoothened trafficking to the cilium in Ihh-silenced cells. SAG treatment corrected mouse stature and significantly decreased mortality without evidence of toxicity. Moreover, Ihh ablation in Acan+ cells produced enchondroma-like tissues near the growth plates that were significantly reduced by SAG treatment. These results demonstrated that SAG effectively treats skeletal dysplasia caused by Ihh gene mutations in a mouse model, suggesting that SAG may represent a potential drug for the treatment of these diseases and/or enchondromas.

Changes in cartilage and subchondral bone in a growing rabbit experimental model of developmental trochlear dysplasia of the knee

Purpose: Trochlear dysplasia is one of the most frequent lower extremities deformities. Aim of this research was to investigate the changes in cartilage and subchondral bone of trochlea after patellar dislocation in growing rabbits. Materials and Methods: Ninety-six knees from 48 one-month-old rabbits were divided into two groups (experimental, control). Lateral patellar dislocation was established in the experimental group and distal femurs were collected at 4, 8, 12 and 24-week time points, respectively. General examination and histological observations were conducted to research the anatomical structure of the trochlear cartilage and subchondral bone. Structural parameters of trochlear subchondral bone were measured by MicroCT. Subsequently, the expression of TRPV4, collagen II and MMP-13 in cartilage were detected by western blot and RT-PCR analysis, respectively.Results: Subchondral bone loss was found in experimental group from 4 weeks after patellar dislocation, accompanied by increased TRAP-positive osteoclasts in subchondral bone. The trochlear dysplasia model was well established from 8 weeks after patellar dislocation. In addition, degeneration of cartilage was found from 8 weeks, accompanied by decreased expression of mechanically sensitive TRPV4 and collagen II, and increased expression of MMP-13.Conclusions: This study proved that trochlear dysplasia can be caused by patellar dislocation in growing rabbits, accompanied by significant subchondral bone loss. What is more, this study also shows that degenerative cartilage changes occur in the patellar dislocation model and become aggravated with time, accompanied by decreased TRPV4 and collagen II, but increased MMP-13.

Runx2 plays a central role in Osteoarthritis development

Osteoarthritis (OA) is the most common form of arthritis, is the leading cause of impaired mobility in the elderly, and accounts for more than a third of chronic moderate to severe pain. As a degenerative joint disorder, OA affects the whole joint and results in synovial hyperplasia, degradation of articular cartilage, subchondral sclerosis, osteophyte formation, and chronic pain. Currently, there is no effective drug to decelerate OA progression and molecular targets for drug development have been insufficiently investigated. Anti-OA drug development can benefit from more and precise knowledge of molecular targets for drug development. Runt-related transcription factor 2 (Runx2) is a key transcription factor controlling osteoblast and chondrocyte differentiation and is among the most promising potential therapeutic targets. Notably, Runx2 expression is upregulated in several murine OA models, suggesting a role in disease pathogenesis. In this review article, we summarized recent findings on Runx2 related to OA development and evaluated its potential as a therapeutic target.

The translational potential of this article

A better understanding of the role of Runx2 in osteoarthritis pathogenesis will contribute to the development of novel intervention of osteoarthritis disease.

Scaffold-Based microRNA Therapies in Regenerative Medicine and Cancer

microRNA-based therapies are an advantageous strategy with applications in both regenerative medicine (RM) and cancer treatments. microRNAs (miRNAs) are an evolutionary conserved class of small RNA molecules that modulate up to one third of the human nonprotein coding genome. Thus, synthetic miRNA activators and inhibitors hold immense potential to finely balance gene expression and reestablish tissue health. Ongoing industry-sponsored clinical trials inspire a new miRNA therapeutics era, but progress largely relies on the development of safe and efficient delivery systems. The emerging application of biomaterial scaffolds for this purpose offers spatiotemporal control and circumvents biological and mechanical barriers that impede successful miRNA delivery. The nascent research in scaffold-mediated miRNA therapies translates know-how learnt from studies in antitumoral and genetic disorders as well as work on plasmid (p)DNA/siRNA delivery to expand the miRNA therapies arena. In this progress report, the state of the art methods of regulating miRNAs are reviewed. Relevant miRNA delivery vectors and scaffold systems applied to-date for RM and cancer treatment applications are discussed, as well as the challenges involved in their design. Overall, this progress report demonstrates the opportunity that exists for the application of miRNA-activated scaffolds in the future of RM and cancer treatments.

Knock-in human GDF5 proregion L373R mutation as a mouse model for proximal symphalangism

Proximal symphalangism (SYM1) is an autosomal dominant disorder, mainly characterized by bony fusions of the proximal phalanges of the hands and feet. GDF5 and NOG were identified to be responsible for SYM1. We have previously reported on a p.Leu373Arg mutation in the GDF5 proregion present in a Chinese family with SYM1. Here, we investigated the effects of the GDF-L373R mutation. The variant caused proteolysis efficiency of GDF5 increased in ATDC5 cells. The variant also caused upregulation of SMAD1/5/8 phosphorylation and increased expression of target genes SMURF1, along with COL2A1 and SOX9 which are factors associated with chondrosis. Furthermore, we developed a human-relevant SYM1 mouse model by making a Gdf5^L367R (the orthologous position for L373R in humans) knock-in mouse. Gdf5^L367R/+ and Gdf5^L367R/L367R mice displayed stiffness and adhesions across the proximal phalanx joint which were in complete accord with SYM1. It was also confirmed the joint formation and development was abnormal in Gdf5^L367R/+ and Gdf5^L367R/L367R mice, including the failure to develop the primary ossification center and be hypertrophic chondrocytes during embryonic development. This knock-in mouse model offers a tool for assessing the pathogenesis of SYM1 and the function of the GDF5 proregion.

Ectopic expression of Smurf2 and acceleration of age-related intervertebral disc degeneration in a mouse model

OBJECTIVE

Lumbar intervertebral disc degeneration, an age-related process, is a major cause of low-back pain. Although low-back pain is a very common clinical problem in the aging population, no effective treatment is available, largely owing to lack of understanding of the molecular mechanisms underlying disc degeneration. The goal of this study was to characterize how ectopic expression of Smurf2 driven by the collagen Type II alpha 1 (Col2a1) promoter alters disc cell phenotype and associated cellular events, matrix synthesis, and gene expression during disc degeneration in mice.

METHODS

To characterize how ectopic expression of Smurf2 in Col2a1-promoter working cells affects the disc degeneration process, the authors performed histological and immunohistochemical analysis of lumbar spine specimens harvested from wild-type (WT) and Col2a1-Smurf2 transgenic mice at various ages (n ≥ 6 in each age group). To elucidate the molecular mechanism underlying Smurf2-mediated disc degeneration, the authors isolated cells from WT and Col2a1-Smurf2 transgenic lumbar intervertebral discs and performed Western blot and real-time RT-PCR (reverse transcription polymerase chain reaction) to examine the protein and mRNA levels of interesting targets.

RESULTS

The authors demonstrated that approximately 30% of WT mice at 10–12 months of age had started to show disc degeneration and that the disc degeneration process was accelerated by 3–6 months in Col2a1-Smurf2 transgenic mice. Chondrocyte-like cell proliferation, maturation, and fibrotic tissue formation in the inner annulus were often accompanied by fibroblast-to-chondrocyte differentiation in the outer annulus in transgenic discs. The chondrocyte-like cells in transgenic discs expressed higher levels of connective tissue growth factor (CTGF) than were expressed in WT counterparts.

CONCLUSIONS

The findings that ectopic expression of Smurf2 driven by the Col2a1 promoter accelerated disc degeneration in Col2a1-Smurf2 transgenic mice, and that higher levels of CTGF protein and mRNA were present in Col2a1-Smurf2 transgenic discs, indicate that Smurf2 accelerates disc degeneration via upregulation of CTGF.

Ubiquitin E3 ligase Itch negatively regulates osteoblast function by promoting proteasome degradation of osteogenic proteins

OBJECTIVES

Ubiquitin E3 ligase-mediated protein degradation regulates osteoblast function. Itch, an E3 ligase, affects numerous cell functions by regulating ubiquitination and proteasomal degradation of related proteins. However, the Itch-related cellular and molecular mechanisms by which osteoblast differentiation and function are elevated during bone fracture repair are as yet unknown.

METHODS

We examined the expression levels of E3 ligases and NF-κB members in callus samples during bone fracture repair by quantitative polymerase chain reaction (qPCR) and the total amount of ubiquitinated proteins by Western blot analysis in wild-type (WT) mice. The expression levels of osteoblast-associated genes in fracture callus from Itch knockout (KO) mice and their WT littermates were examined by qPCR. The effect of NF-κB on Itch expression in C2C12 osteoblast cells was determined by a chromatin immunoprecipitation (ChIP) assay.

RESULTS

The expression levels of WW Domain Containing E3 Ubiquitin Protein Ligase 1 (Wwp1), SMAD Specific E3 Ubiquitin Protein Ligase 1 (Smurf1), SMAD Specific E3 Ubiquitin Protein Ligase 2 (Smurf2) and Itch were all significantly increased in the fracture callus of WT mice, which was associated with elevated expression of NF-κB members and total ubiquitinated proteins. Callus tissue isolated from Itch KO mice expressed higher levels of osteoblast-associated genes, including Runx2, a positive regulator of osteoblast differentiation, but osteoclast-associated genes were not increased. Both NF-κB RelA and RelB proteins were found to bind to the NF-κB binding site in the mouse Itch promoter.

CONCLUSIONS

Our findings indicate that Itch depletion may have a strong positive effect on osteoblast differentiation in fracture callus. Thus, ubiquitin E3 ligase Itch could be a potential target for enhancing bone fracture healing.Cite this article: J. Liu, X. Li, H. Zhang, R. Gu, Z. Wang, Z. Gao, L. Xing. Ubiquitin E3 ligase Itch negatively regulates osteoblast function by promoting proteasome degradation of osteogenic proteins. Bone Joint Res 2017;6:154-161. DOI: 10.1302/2046-3758.63.BJR-2016-0237.R1.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Ubiquitin e3 ligase itch negatively regulates osteoblast differentiation from mesenchymal progenitor cells

Itch, a HECT family E3 ligase, affects numerous cell functions by regulating ubiquitination and proteasomal degradation of target proteins. However, the role of Itch in osteoblasts has not been investigated. We report that Itch(-/-) mice have significantly increased bone volume, osteoblast numbers, and bone formation rate. Using bone marrow stromal cells from Itch(-/-) mice and wild-type (WT) littermates as bone marrow mesenchymal precursor cells (BM-MPCs), we found that BM-MPCs from Itch(-/-) mice have compatible numbers of cells expressing mesenchymal stem cell markers. However, Itch(-/-) BM-MPCs grew faster in an in vitro culture, formed more CFU-F mesenchymal colonies, and exhibited increased osteoblast differentiation and decreased adipogenesis. Importantly, Itch(-/-) mesenchymal colony cells formed significantly more new bone in a tibial defect of recipient mice compared with WT cells. The expression levels of JunB, an AP-1 transcription factor that positively regulate osteoblast differentiation, were significantly increased in Itch(-/-) BM-MPCs when proteasome function is intact. In contrast, the amount of ubiquitinated JunB protein was markedly decreased in Itch(-/-) cells when proteasome function was blocked. Overexpression of WT Itch, but not an Itch ligase-inactive mutant, rescued differentiation defects of Itch(-/-) BM-MPCs. Itch(-/-) BM-MPCs had a similar role in immune modulation as WT cells. Thus, Itch negatively controls osteoblast differentiation from BM-MPCs through the regulation of proteasomal degradation of positive osteoblast regulator JunB protein. Itch is a potential new target for bone anabolic drug development to treat patients with bone loss.

TGF-β signaling and the development of osteoarthritis

Osteoarthritis (OA) is a common joint degenerative disease affecting the whole joint structure, including articular cartilage, subchondral bone and synovial tissue. Although extensive work has been done in recent years to explore the molecular mechanism underlying this disease, the pathogenesis of OA is still poorly understood and currently, there is no effective disease-modifying treatment for OA. Recently, both in vitro and in vivo studies suggest that confirmed (TGF-β)/SMAD pathway plays a critical role during OA development. This short review will focus on the function and signaling mechanisms of TGF-β/SMAD pathway in articular chondrocytes, mesenchymal progenitor cells of subchondral bone and synovial lining cells during OA development.

Mouse p63 variants and chondrogenesis

As a critical member of the p53 family of transcription factors, p63 has been implicated a role in development than in tumor formation, because p63 is seldom mutated in human cancers, while p63 null mice exhibit severe developmental abnormalities without increasing cancer susceptibility. Notably, besides the major epithelial and cardiac defect, p63 deficient mice show severe limb and craniofacial abnormalities. In addition, humans with p63 mutations also show severe limb and digit defects, suggesting a putative role of p63 in skeletal development. There are eight p63 variants which encode for the TAp63 and ΔNp63 isoforms by alternative promoters. How these isoforms function during skeletal development is currently largely unknown. Our recent transgenic studies suggest a role of TAP63α, but not ΔNP63α, during embryonic long bone development. However, the moderate skeletal phenotypes in the TAP63α transgenic mice suggest requirement of additional p63 isoform(s) for the limb defects in p63 null mice. Here, we report analysis of mouse p63 variants in MCT and ATDC5 cells, two cell models undergo hypertrophic differentiation and mimic the process of endochondral bone formation upon growth arrest or induction. We detected increased level of p63 variants in hypertrophic MCT cells by regular RT-PCR analysis. Further analysis by qRT-PCR, we detected significantly upregulated level of γ variant (p<0.05), but not α or β variant (p>0.05), in hypertrophic MCT cells than in proliferative MCT cells. Moreover, we detected upregulated TAP63γ in ATDC5 cells undergoing hypertrophic differentiation. Our results suggest that TAp63γ plays a positive role during endochondral bone formation.

Ubiquitin E3 ligase Wwp1 negatively regulates osteoblast function by inhibiting osteoblast differentiation and migration

Ubiquitin E3 ligase-mediated protein degradation promotes proteasomal degradation of key positive regulators of osteoblast functions. For example, the E3 ligases--SMAD-specific E3 ubiquitin protein ligase 1 (Smurf1), Itch, and WW domain-containing E3 ubiquitin protein ligase 1 (Wwp1)--promote degradation of Runt-related transcription factor 2 (Runx2), transcription factor jun-B (JunB), and chemokine (C-X-C) receptor type 4 (CXCR-4) proteins to inhibit their functions. However, the role of E3 ligases in age-associated bone loss is unknown. We found that the expression level of Wwp1, but not Smurf1 or Itch, was significantly increased in CD45-negative (CD45(-)) bone marrow-derived mesenchymal stem cells from 6-month-old and 12-month-old wild-type (WT) mice. Wwp1 knockout (Wwp1(-/-)) mice developed increased bone mass as they aged, associated with increased bone formation rates and normal bone resorption parameters. Bone marrow stromal cells (BMSCs) from Wwp1(-/-) mice formed increased numbers and areas of alkaline phosphatase(+) and Alizarin red(+) nodules and had increased migration potential toward chemokine (C-X-C motif) ligand 12 (CXCL12) gradients. Runx2, JunB, and CXCR-4 protein levels were significantly increased in Wwp1(-/-) BMSCs. Wwp1(-/-) BMSCs had increased amount of ubiquitinated JunB protein, but Runx2 ubiquitination was no change. Knocking down JunB in Wwp1(-/-) BMSCs returned Runx2 protein levels to that in WT cells. Thus, Wwp1 negatively regulates osteoblast functions by affecting both their migration and differentiation. Mechanisms designed to decrease Wwp1 levels in BMSCs may represent a new approach to prevent the decrease in osteoblastic bone formation associated with aging.

Agonism of Wnt-β-catenin signalling promotes mesenchymal stem cell (MSC) expansion

Promoting mesenchymal stem cell (MSC) proliferation has numerous applications in stem cell therapies, particularly in the area of regenerative medicine. In order for cell-based regenerative approaches to be realized, MSC proliferation must be achieved in a controlled manner without compromising stem cell differentiation capacities. Here we demonstrate that 6-bromoindirubin-3'-oxime (BIO) increases MSC β-catenin activity 106-fold and stem cell-associated gene expression ~33-fold, respectively, over untreated controls. Subsequently, BIO treatment increases MSC populations 1.8-fold in typical 2D culture conditions, as well as 1.3-fold when encapsulated within hydrogels compared to untreated cells. Furthermore, we demonstrate that BIO treatment does not reduce MSC multipotency where MSCs maintain their ability to differentiate into osteoblasts, chondrocytes and adipocytes using standard conditions. Taken together, our results demonstrate BIO's potential utility as a proliferative agent for cell transplantation and tissue regeneration.

Distinct function of P63 isoforms during embryonic skeletal development

P63 belongs to the P53 family of transcription factors. There are multiple P63 isoforms that play important functions both in cancer and development. The obvious limb defect in p63 null mice and in human skeletal syndromes with P63 mutations suggest its essential role in long bone development. However, how the different P63 isoforms function during long bone development is largely unknown. We have previously shown that TAP63α, the longest P63 isoform, plays a positive role in embryonic skeletal development, since targeting TAP63α expression in hypertrophic chondrocytes accelerates endochondral ossification at both E17.5 and P1 stages. Here, we report transgenic studies of ΔNP63α, another P63 isoform which lacks the N-terminal transactivation domain compared to TAP63α, using the same hypertrophic chondrocyte-specific Col10a1 control element. No skeletal abnormalities were detected in these Col10a1-ΔNP63α transgenic mice at both E17.5 and P1 stages, suggesting less importance of ΔNP63α during late embryonic skeletal development. To further investigate the function of P63 isoforms during early skeletal development, we have generated ΔNP63α and TAP63α transgenic mice using a chondrocyte-specific Col2a1 control element. Surprisingly, while no skeletal defect was shown in the Col2a1-ΔNP63α transgenic mice, reduced ossification was observed in the digit and tail bones of Col2a1-TAP63α transgenic mice at both E17.5 and P1 stages compared to their wild-type littermates. Expression profiling and immunohistochemical analysis detected upregulated expression of Sox9, a major negative regulator of endochondral ossification, in Col2a1-TAP63α transgenic mice. Taken together, our results suggest a distinct function of P63 isoforms, herein, ΔNP63α and TAP63α, during endochondral ossification.

Early cartilage degeneration in a rat experimental model of developmental dysplasia of the hip

Osteoarthritis (OA) is a common long-term complication of developmental dysplasia of the hip (DDH) that is associated with a higher incidence of OA. In addition, the age of onset of OA in DDH patients is significantly younger than in the general population. In order to investigate the early degeneration in DDH cartilage, we used a rat DDH model that was established by the straight-leg swaddling position. The hips were isolated from the DDH model rats and an untreated control group at postnatal weeks 2, 4, 6, and 8. Histology and proteoglycan levels were observed in articular cartilage using Safranin O staining. Biomarkers of cartilage degeneration, including type X collagen and matrix metalloproteinase (MMP)-13, were assessed using immunohistochemistry and quantitative real-time polymerase chain reaction. In addition, expressions of ADAMTS-4 and ADAMTS-5 were studied using quantitative real-time polymerase chain reaction at different ages. DDH rats showed decreased proteoglycans and derangement of chondrocytes when compared with the control group. Collagen X and MMP-13 expressions were higher in the superficial zone of DDH rats than in that of controls (p < 0.05), and the increase was age-dependent. mRNA expression of Collagen X and MMP-13 showed similar results (p < 0.05). A significant increase in mRNA expression of ADAMTS-5 was found in the DDH model cartilage at 8 weeks (p < 0.05). However, no change was observed in ADAMTS-4 expression. This study shows that degenerative cartilage changes occur at an early stage in the rat DDH model and become aggravated with age.

Regulation of early xenopus embryogenesis by Smad ubiquitination regulatory factor 2

BACKGROUND

Smad ubiquitination regulatory factor (Smurf) 1 and 2 are E3 ubiquitin ligases originally identified as inhibitors of transforming growth factor beta signaling and are shown to modulate multiple cellular activities. The roles of Smurfs in vertebrate embryogenesis, however, are not completely understood.

RESULTS

Here we investigate the function of Smurf2 during early Xenopus development. We show that distinctly from Smurf1, overexpression of Smurf2 in presumptive mesoderm interfered with mesoderm induction and caused axial defects, whereas knockdown of Smurf2 with antisense morpholino oligonucleotides resulted in expansion of the mesoderm. These results imply that Smurf2 may modulate nodal-mediated mesodermal induction. Consistently, ventral expression of Smurf2 induced a partial secondary axis with head structures. In the ectoderm, Smurf2 resembled Smurf1 in controlling neural and epidermal marker expression and influencing head formation. Smurf1, but not Smurf2, additionally affected neural tube closure. Interestingly, both Smurfs could enhance as well as repress neural crest markers, implying that they modulate their targets dynamically during neural plate border specification.

CONCLUSION

Our data demonstrate that Smurf1 and Smurf2 have overlapping and distinct functionalities during early frog embryogenesis; collectively, they regulate ectodermal and mesodermal induction and patterning to ensure normal development of Xenopus embryos.

Cyclic tensile strain facilitates the ossification of ligamentum flavum through β-catenin signaling pathway: in vitro analysis

STUDY DESIGN

Histological, immunohistochemical, and real-time reverse transcription-polymerase chain reaction analyses of the expression of cell signaling and transcriptional factors in human ossification of ligamentum flavum (OLF).

OBJECTIVE

To test the hypothesis that β-catenin plays a role in the ossification of OLF cells in response to cyclic tensile strain.

SUMMARY OF BACKGROUND DATA

Several studies have investigated the roles of biomechanical and metabolic factors in the development and progression of OLF, based on the importance of genetic and biological factors. The process of ossification includes enchondral ossification, although such pathology remains poorly defined.

METHODS

Using real-time reverse transcription-polymerase chain reaction, we analyzed the mRNA expression levels of signaling factors known to be involved in the ossification process (β-catenin, Runx2, Sox9, and osteopontin) in cultured OLF cells subjected to cyclic tensile strain. Cyclic tensile strain was produced by Flexercell FX-3000 (Flexercell International, Hillsborough, NC), applied for 0, 6, 12, or 24 hours. The localization of these factors was examined in decalcified paraffin OLF sections by immunohistochemistry. Controlled samples were harvested from nonossified ligamentum flavum of patients who underwent thoracic posterior surgical procedures.

RESULTS

Under resting conditions (no tensile strain), the mRNA levels of β-catenin, Runx2, Sox9, and osteopontin in cultured OLF cells were significantly higher than in the control non-OLF cells. Application of cyclic tensile strain to OLF cells resulted in significant increases in mRNA expression levels of β-catenin, Runx2, Sox9, and osteopontin at 24 hours. Hypertrophic chondrocytes present around the calcification front were immunopositive for Runx2 and osteopontin. Immunoreactivity of β-catenin and Sox9 was strongly present in premature chondrocytes in the fibrocartilage area.

CONCLUSION

Our results indicated that cyclic tensile strain applied to OLF cells activated their ossification through a process mediated by the β-catenin signaling pathway.

Smad signaling determines chondrogenic differentiation of bone-marrow-derived mesenchymal stem cells: inhibition of Smad1/5/8P prevents terminal differentiation and calcification

The aim of this study was to investigate the roles of Smad2/3 and Smad1/5/8 phosphorylation in transforming growth factor-beta-induced chondrogenic differentiation of bone-marrow-derived mesenchymal stem cells (BMSCs) to assess whether specific targeting of different Smad signaling pathways offers possibilities to prevent terminal differentiation and mineralization of chondrogenically differentiated BMSCs. Terminally differentiated chondrocytes produced in vitro by chondrogenic differentiation of BMSCs or studied ex vivo during murine embryonic limb formation stained positive for both Smad2/3P and Smad1/5/8P. Hyaline-like cartilage produced in vitro by articular chondrocytes or studied in ex vivo articular cartilage samples that lacked expression for matrix metalloproteinase 13 and collagen X only expressed Smad2/3P. When either Smad2/3 or Smad1/5/8 phosphorylation was blocked in BMSC culture by addition of SB-505124 or dorsomorphin throughout culture, no collagen II expression was observed, indicating that both pathways are involved in early chondrogenesis. Distinct functions for these pathways were demonstrated when Smad signaling was blocked after the onset of chondrogenesis. Blocking Smad2/3P after the onset of chondrogenesis resulted in a halt in collagen II production. On the other hand, blocking Smad1/5/8P during this time period resulted in decreased expression of matrix metalloproteinase 13, collagen X, and alkaline phosphatase while allowing collagen II production. Moreover, blocking Smad1/5/8P prevented mineralization. This indicates that while Smad2/3P is important for continuation of collagen II deposition, Smad1/5/8 phosphorylation is associated with terminal differentiation and mineralization.

Smurf control in bone cells

The homologous to the E6-associated protein carboxyl terminus (HECT) domain E3 ubiquitin ligase Smurf1 is the first E3 ligase to be implicated in regulating bone cell function. The involvement of Smurf1 in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review highlights recent works exploring Smurf-regulated biological processes in bone cells and highlights recent discoveries surrounding the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Moreover, we discuss the relevance of targeting the HECT E3s through the development of small-molecule inhibitors as an anticancer therapeutic strategy.

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.

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.

Smurf2 induces degradation of GSK-3beta and upregulates beta-catenin in chondrocytes: a potential mechanism for Smurf2-induced degeneration of articular cartilage

We have previously demonstrated that Smurf2 is highly expressed in human osteoarthritis (OA) tissue, and overexpression of Smurf2 under the control of the type II collagen promoter (Col2a1) induces an OA-like phenotype in aged Col2a1-Smurf2 transgenic mice, suggesting that Smurf2 is located upstream of a signal cascade which initiates OA development. However, the factors downstream of Smurf2 in this signal cascade and how Smurf2-induced OA is initiated are largely unknown. In this study, we further characterized the phenotypic changes in Col2a1-Smurf2 transgenic and WT articular cartilage from the postnatal stage to adulthood. We found that the articular cartilage degeneration occurring at the cartilage surface in 6 month-old Col2a1-Smurf2 transgenic mice progressed from an expanded hypertrophic domain in the basal layer of the deep articular cartilage at 2.5 weeks of age, which may lead to an accelerated calcification and ectopic ossification of this region at 1 month of age, and aggregation and maturation of articular chondrocytes in the middle and deep zones at 2 months and 4.5 months of age, respectively. Furthermore, we discovered that ectopically expressed Smurf2 interacted with GSK-3beta and induced its ubiquitination and subsequent proteasomal degradation, and hence upregulated beta-catenin in Col2a1-Smurf2 transgenic chondrocytes ex vivo. It is therefore likely that Smurf2-mediated upregulation of beta-catenin through induction of proteasomal degradation of GSK-beta in chondrocytes may activate articular chondrocyte maturation and associated alteration of gene expression, the early events of OA.