Runx2/Cbfa1 stimulation by retinoic acid is potentiated by BMP2 signaling through interaction with Smad1 on the collagen X promoter in chondrocytes

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

A naringin-derived bioink enhances shape fidelity of 3D bioprinting and efficiency of cartilage defects repair

3D bioprinting is a major area of interest in health sciences for customized manufacturing, but lacks specific bioinks to enhance shape fidelity of 3D bioprinting and efficiency of tissue repair...

Effect of N-Vinyl-2-Pyrrolidone (NVP), a Bromodomain-Binding Small Chemical, on Osteoblast and Osteoclast Differentiation and Its Potential Application for Bone Regeneration

The human skeleton is a dynamic and remarkably organized organ system that provides mechanical support and performs a variety of additional functions. Bone tissue undergoes constant remodeling; an essential process to adapt architecture/resistance to growth and mechanical needs, but also to repair fractures and micro-damages. Despite bone's ability to heal spontaneously, certain situations require an additional stimulation of bone regeneration, such as non-union fractures or after tumor resection. Among the growth factors used to increase bone regeneration, bone morphogenetic protein-2 (BMP2) is certainly the best described and studied. If clinically used in high quantities, BMP2 is associated with various adverse events, including fibrosis, overshooting bone formation, induction of inflammation and swelling. In previous studies, we have shown that it was possible to reduce BMP2 doses significantly, by increasing the response and sensitivity to it with small molecules called "BMP2 enhancers". In the present study, we investigated the effect of N-Vinyl-2-pyrrolidone (NVP) on osteoblast and osteoclast differentiation in vitro and guided bone regeneration in vivo. We showed that NVP increases BMP2-induced osteoblast differentiation and decreases RANKL-induced osteoclast differentiation in a dose-dependent manner. Moreover, in a rabbit calvarial defect model, the histomorphometric analysis revealed that bony bridging and bony regenerated area achieved with NVP-loaded poly (lactic-co-glycolic acid (PLGA) membranes were significantly higher compared to unloaded membranes. Taken together, our results suggest that NVP sensitizes BMP2-dependent pathways, enhances BMP2 effect, and inhibits osteoclast differentiation. Thus, NVP could prove useful as "osteopromotive substance" in situations where a high rate of bone regeneration is required, and in the management of bone diseases associated with excessive bone resorption, like osteoporosis.

Bone toxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the retinoid system: A causality analysis anchored in osteoblast gene expression and mouse data

Dioxin exposures impact on bone quality and osteoblast differentiation, as well as retinoic acid metabolism and signaling. In this study we analyzed associations between increased circulating retinol concentrations and altered bone mineral density in a mouse model following oral exposure to 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD). Additionally, effects of TCDD on differentiation marker genes and genes involved with retinoic acid metabolism were analysed in an osteoblast cell model followed by benchmark dose-response analyses of the gene expression data. Study results show that the increased trabecular and decreased cortical bone mineral density in the mouse model following TCDD exposure are associated with increased circulating retinol concentrations. Also, TCDD disrupted the expression of genes involved in osteoblast differentiation and retinoic acid synthesis, degradation, and nuclear translocation in directions compatible with increasing cellular retinoic acid levels. Further evaluation of the obtained results in relation to previously published data by the use of mode-of-action and weight-of-evidence inspired analytical approaches strengthened the evidence that TCDD-induced bone and retinoid system changes are causally related and compatible with an endocrine disruption mode of action.

Expression Profiling and Functional Analysis of Candidate Col10a1 Regulators Identified by the TRAP Program

Hypertrophic chondrocytes and their specific marker, the type X collagen gene (Col10a1), are critical components of endochondral bone formation during skeletal development. We previously found that Runx2 is an indispensable mouse Col10a1 gene regulator and identified many other transcription factors (TFs) that potentially interact with the 150-bp Col10a1 cis-enhancer. However, the roles of these candidate TFs in Col10a1 expression and chondrocyte hypertrophy have not been elucidated. Here, we focus on 32 candidate TFs recently identified by analyzing the 150-bp Col10a1 enhancer using the transcription factor affinity prediction (TRAP) program. We found that 12 TFs (Hoxa3, Lsx, Evx2, Dlx5, S8, Pax2, Egr2, Mef2a, Barhl2, GKlf, Sox17, and Crx) were significantly upregulated and four TFs (Lhx4, Tbx5, Mef2c, and Hb9) were significantly downregulated in hypertrophic MCT cells, which show upregulation of Col10a1 expression. Most of the differential expression pattern of these TFs conformed with the results obtained from ATDC5 cell model and primary mouse chondrocytes. Notably, Tbx5 was downregulated upon Col10a1 upregulation, overexpression of Tbx5 decreased Col10a1 expression, and knock-down of Tbx5 increased Col10a1 expression in hypertrophic chondrocytes, suggesting that Tbx5 is a negative regulator of Col10a1. We further generated a stable Tbx5-overexpressing ATDC5 cell line and ColX-Tbx5 transgenic mice driven by Col10a1-specific enhancers and promoters. Tbx5 overexpression decreased Col10a1 expression in ATDC5 cells cultured as early as day 7 and in limb tissue on post-natal day 1. Slightly weaker alkaline phosphatase staining was also observed in cell culture on day 7 and in limb digits on embryonic day 17.5, indicating mildly delayed ossification. Further characterization of these candidate Col10a1 transcriptional regulators could help identify novel therapeutic targets for skeletal diseases associated with abnormal chondrocyte hypertrophy.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype–phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

Transcriptional, epigenetic and microRNA regulation of growth plate

Endochondral ossification is a critical event in bone formation, particularly in long shaft bones. Many cellular differentiation processes work in concert to facilitate the generation of cartilage primordium to formation of trabecular structures, all of which occur within the growth plate. Previous studies have revealed that the growth plate is tightly regulated by various transcription factors, epigenetic systems, and microRNAs. Hence, understanding these mechanisms that regulate the growth plate is crucial to furthering the current understanding on skeletal diseases, and in formulating effective treatment strategies. In this review, we focus on describing the function and mechanisms of the transcription factors, epigenetic systems, and microRNAs known to regulate the growth plate.

Static magnetic fields accelerate osteogenesis by regulating FLRT/BMP pathway

OBJECTIVES

Static magnetic fields (SMF) have been proved to enhance osteogenic differentiation in mesenchymal stem cells (MSCs). However, the effect of SMF on mandibular condylar chondrocytes (MCCs) are less investigated, which contributes to the vertical formation of mandible. The purpose of the present study was to identify whether SMF accelerate the osteogenesis on mature condylar cartilage and explore the potential regulatory mechanism.

METHODS

In this study, we presented a 280 mT SMF stimulation set-up to investigate the genomic effects of SMF exposure on MCCs differentiation and osteoblast-related factor secretion in vitro. Induced by Oricell™ for osteogenesis, MCCs from primary SD Rat were stimulated with or without SMF for cell culture. Cell proliferation was determined by CCK-8. The enhanced osteogenetic capacity of the SMF stimulated MCCs was identified by Alizarin red staining (ARS). Additionally, the effects of SMF on the expression of transmembrane protein marker (FLRT3), terminal differentiation markers (BMP2), and transcription factors (Smad1/5/8) were quantified by Western blot and immunofluorescence analysis.

RESULTS

Compared with the control group, SMF decreased the proliferation of MCCs (p < 0.05) after 14 days osteogenesis-specific induction. The mineral synthesis of MCCs was upregulated by SMF (p < 0.0001). The expression of BMP2, Smad1/5/8 showed decrease trends while the protein level of FLRT3 acted in contrary manner (p < 0.05).

CONCLUSIONS

Our findings emphasized the ability of osteogenesis positively respond to SMF stimulation by exhibiting enhanced differentiation via FLRT/BMP signaling.

The wonders of BMP9: From mesenchymal stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism to regenerative medicine

Although bone morphogenetic proteins (BMPs) initially showed effective induction of ectopic bone growth in muscle, it has since been determined that these proteins, as members of the TGF-β superfamily, play a diverse and critical array of biological roles. These roles include regulating skeletal and bone formation, angiogenesis, and development and homeostasis of multiple organ systems. Disruptions of the members of the TGF-β/BMP superfamily result in severe skeletal and extra-skeletal irregularities, suggesting high therapeutic potential from understanding this family of BMP proteins. Although it was once one of the least characterized BMPs, BMP9 has revealed itself to have the highest osteogenic potential across numerous experiments both in vitro and in vivo, with recent studies suggesting that the exceptional potency of BMP9 may result from unique signaling pathways that differentiate it from other BMPs. The effectiveness of BMP9 in inducing bone formation was recently revealed in promising experiments that demonstrated efficacy in the repair of critical sized cranial defects as well as compatibility with bone-inducing bio-implants, revealing the great translational promise of BMP9. Furthermore, emerging evidence indicates that, besides its osteogenic activity, BMP9 exerts a broad range of biological functions, including stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism. This review aims to summarize our current understanding of BMP9 across biology and the body.

MicroRNA-29b regulates hypertrophy of murine mesenchymal stem cells induced toward chondrogenesis

OBJECTIVE

Chondrocyte hypertrophy, a terminal stage of chondrocyte differentiation, is essential to the endochondral bone formation and is one of the major pathological factors in osteoarthritis. This study investigated the role of microRNA-29b (miR-29b), which is involved in chondrogenesis, in the regulation of hypertrophy in chondrocytes.

METHODS

miR-29b expression was assessed during murine mesenchymal stem cells (mMSCs) chondrogenesis. To detect whether miR-29b affects chondrocyte hypertrophy, the mMSCs induced toward chondrogenesis were transfected with miR-29b or its antisense inhibitor (antagomiR-29b). Finally, the differential effects of antagomiR-29b on chondrocytes at different differentiation stages were evaluated by loss-of-function experiments.

RESULTS

miR-29b expression was low-level during the early chondrogenic differentiation, however, it was changed to high level during hypertrophy. Subsequently, the gain-of-function and loss-of-function experiments had confirmed that miR-29b promoted hypertrophy in mMSC-derived chondrocytes. In addition, we confirmed that on day 7, when cells were treated with antagomiR-29b, was the optimal intervention time for preventing hypertrophic phenotype of mMSCs in vitro.

CONCLUSION

miR-29b regulated chondrogenesis homeostasis and enhance hypertrophic phenotype. These data suggest that miR-29b is a key regulator of the chondrocyte phenotype derived from mMSCs and it might be a potential target for articular cartilage repair.

Poly(ADP-ribose) polymerase 1 accelerates vascular calcification by upregulating Runx2

Vascular calcification is highly prevalent in end-stage renal diseases and is predictive of cardiovascular events and mortality. Poly(ADP-ribose) polymerase 1 (PARP1) inhibition or deletion is vasoprotective in several disease models. Here we show that PARP activity is increased in radial artery samples from patients with chronic renal failure, in arteries from uraemic rats, and in calcified vascular smooth muscle cells (VSMCs) in vitro. PARP1 deficiency blocks, whereas PARP1 overexpression exacerbates, the transdifferentiation of VSMCs from a contractile to an osteogenic phenotype, the expression of mineralization-regulating proteins, and calcium deposition. PARP1 promotes Runx2 expression, and Runx2 deficiency offsets the pro-calcifying effects of PARP1. Activated PARP1 suppresses miRNA-204 expression via the IL-6/STAT3 pathway and thus relieves the repression of its target, Runx2, resulting in increased Runx2 protein. Together, these results suggest that PARP1 counteracts vascular calcification and that therapeutic agents that influence PARP1 activity may be of benefit to treat vascular calcification.

Vascular calcification is a hallmark of end stage renal disease. Here, Cheng et al. show that poly(ADP-ribose) polymerase (PARP) activity is increased in calcified arteries in patients and uremic rats, and that PARP1 promotes vascular calcification by suppressing miR-204 expression via IL-6/STAT3 signaling, thus relieving repression of the osteogenic regulator Runx2.

Loading of the Condylar Cartilage Can Rescue the Effects of Botox on TMJ

The purpose of this study is to evaluate whether the effects of botulinum neurotoxin (botox) injection into the masseter in the mandibular condylar cartilage (MCC) and subchondral bone could be rescued by compressive loading of the temporomandibular joint (TMJ). Twenty-four 6-week-old female mice (C57BL/6J) were used. Mice were divided in three groups: (1) Botox (n = 8); (2) Botox plus loading (n = 8); (3) Pure control (n = 8). Bone labels (3 and 1 day before sacrifice) and the proliferation marker EdU (2 and 1 day before sacrifice) were intraperitoneally injected into all groups of mice. Condyles were dissected and examined by micro-CT and histology. Sagittal sections of condyles were stained for TRAP, alkaline phosphatase, EdU, TUNEL, and toluidine blue. In addition, immunostaining for pSmad, VEGF, and Runx2 was performed. Bone volume fraction, tissue density, and trabecular thickness were significantly decreased on the subchondral bone of botox-injected side when compared to control side and control mice, 4 weeks after injection. Furthermore, histological analysis revealed decrease in mineralization, matrix deposition, TRAP activity, EdU, and TUNEL-positive cells in the MCC of the botox-injected side, 4 weeks after injection. However, compressive loading reversed the reduced bone volume and density and the cellular changes in the MCC caused by Botox injection. TMJ compressive loading rescues the negative effects of botox injection into the masseter in the MCC and subchondral bone.

Indomethacin induces differential effects on in vitro endochondral ossification depending on the chondrocyte's differentiation stage

Heterotopic ossification (HO) is the abnormal formation of bone in soft tissues and is a frequent complication of hip replacement surgery. Heterotopic ossifications are described to develop via endochondral ossification and standard treatment is administration of indomethacin. It is currently unknown how indomethacin influences heterotopic ossification on a molecular level; therefore, we aimed to determine whether indomethacin might influence heterotopic ossification via impairing the chondrogenic phase of endochondral ossification. Progenitor cell models differentiating in the chondrogenic lineage (ATDC5, primary human bone marrow stem cells and ex vivo periosteal agarose cultures) were treated with increasing concentrations of indomethacin and a decrease in gene- and protein expression of chondrogenic and hypertrophic markers (measured by RT-qPCR and immunoblotting) as well as decreased glycosamino-glycan content (by alcian blue histochemistry) was observed. Even when hypertrophic differentiation was provoked, the addition of indomethacin resulted in decreased hypertrophic marker expression. Interestingly, when mature chondrocytes were treated with indomethacin, a clear increase in collagen type 2 expression was observed. Similarly, when ATDC5 cells and bone marrow stem cells were pre-differentiated to obtain a chondrocyte phenotype and indomethacin was added from this time point onward, low concentrations of indomethacin also resulted in increased chondrogenic differentiation. Indomethacin induces differential effects on in vitro endochondral ossification, depending on the chondrocyte's differentiation stage, with complete inhibition of chondrogenic differentiation as the most pronounced action. This observation may provide a rational behind the elusive mode of action of indomethacin in the treatment of heterotopic ossifications. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:847-857, 2017.

Transcriptional Control by the SMADs

The transforming growth factor-β (TGF-β) family of ligands elicit their biological effects by initiating new programs of gene expression. The best understood signal transducers for these ligands are the SMADs, which essentially act as transcription factors that are activated in the cytoplasm and then accumulate in the nucleus in response to ligand induction where they bind to enhancer/promoter sequences in the regulatory regions of target genes to either activate or repress transcription. This review focuses on the mechanisms whereby the SMADs achieve this and the functional implications. The SMAD complexes have weak affinity for DNA and limited specificity and, thus, they cooperate with other site-specific transcription factors that act either to actively recruit the SMAD complexes or to stabilize their DNA binding. In some situations, these cooperating transcription factors function to integrate the signals from TGF-β family ligands with environmental cues or with information about cell lineage. Activated SMAD complexes regulate transcription via remodeling of the chromatin template. Consistent with this, they recruit a variety of coactivators and corepressors to the chromatin, which either directly or indirectly modify histones and/or modulate chromatin structure.

The normal and fractured physis: an anatomic and physiologic overview

The growth plate (physis) is responsible for enabling and regulating longitudinal growth of upper and lower limbs. This regulation occurs through interaction of the cells in the growth plate with systemic and locally produced factors. This complex interaction leads to precisely controlled changes in chondrocyte size, receptors, and matrix, which ultimately result in endochondral bone formation. With advances in cellular and molecular biology, our knowledge about these complex interactions has increased significantly over the past decade. Deficiency of any of the regulating factors or physeal injury during childhood can alter this well-orchestrated sequence of events and lead to abnormalities in growth. This review highlights the histology of the normal physis, including recent findings at the cellular and molecular levels, mechanics and mechanobiology of the growth plate, pathologies that can affect the physis, and treatment options, including interposition materials.

ATF6a, a Runx2-activable transcription factor, is a new regulator of chondrocyte hypertrophy

Our previous research has shown that the spliced isoform of XBP1 (XBP1s) is an important downstream mediator of BMP2 and is involved in BMP2-stimulated chondrocyte differentiation. Herein, we report that ATF6 and its cleaved N-terminal cytoplasmic domain (known as ATF6a) are expressed in growth plate chondrocytes. We find that these proteins are differentially induced during BMP2-triggered chondrocyte differentiation. This differential expression probably results from the activation of the ATF6 gene by Runx2 and its repression by the Sox6 transcription factor. Runx2 and Sox6 act through their respective binding elements on the ATF6 gene. When overexpressed, ATF6 and ATF6a intensify chondrogenesis; our studies demonstrate that under the stimulation of ATF6 and ATF6a, chondrocytes tend to be hypertrophied and mineralized, a process leading to bone formation. By contrast, lowering expression of ATF6a by use of its specific siRNA suppresses chondrocyte differentiation. Moreover, ATF6a interacts with Runx2 and augments the Runx2-mediated hypertrophication of chondrocytes. Importantly, overexpression and knockdown of ATF6a during the chondrocyte hypertrophy process also led to altered expressions of IHH and PTHrP (also known as PTHLH). Taken together, these findings indicate that ATF6a favorably controls chondrogenesis and bone formation (1) by acting as a co-factor of Runx2 and enhancing Runx2-incited hypertrophic chondrocyte differentiation, and (2) by affecting IHH and PTHrP signaling.

Transmission of ER stress response by ATF6 promotes endochondral bone growth

Background

We reported earlier that X-box binding protein1 spliced (XBP1S), a key regulator of the unfolded protein response (UPR), as a bone morphogenetic protein 2 (BMP2)-inducible transcription factor, positively regulates endochondral bone formation by activating granulin-epithelin precursor (GEP) chondrogenic growth factor. Under the stress of misfolded or unfolded proteins in the endoplasmic reticulum (ER), the cells can be protected by the mammalian UPR. However, the influence of activating transcription factor 6 (ATF6), another transcriptional arm of UPR, in BMP2-induced chondrocyte differentiation has not yet been elucidated. In the current study, we investigate and explore the role of ATF6 in endochondral bone formation, focus on associated molecules of hypertrophic chondrocyte differentiation, as well as the molecular events underlying this process.

Methods

High-cell-density micromass cultures were used to induce ATDC5 and C3H10T1/2 cell differentiation into chondrocytes. Quantitative real-time PCR, immunoblotting analysis, and immunohistochemistry were performed to examine (1) the expression of ATF6, ATF6α, collagen II, collagen X, and matrix metalloproteinase-13 (MMP13) and (2) whether ATF6 stimulates chondrogenesis and whether ATF6 enhances runt-related transcription factor 2 (Runx2)-mediated chondrocyte hypertrophy. Culture of fetal mouse bone explants was to detect whether ATF6 stimulates chondrocyte hypertrophy, mineralization, and endochondral bone growth. Coimmunoprecipitation was employed to determine whether ATF6 associates with Runx2 in chondrocyte differentiation.

Results

ATF6 is differentially expressed in the course of BMP2-triggered chondrocyte differentiation. Overexpression of ATF6 accelerates chondrocyte differentiation, and the ex vivo studies reveal that ATF6 is a potent stimulator of chondrocyte hypertrophy, mineralization, and endochondral bone growth. Knockdown of ATF6 via a siRNA approach inhibits chondrogenesis. Furthermore, ATF6 associates with Runx2 and enhances Runx2-induced chondrocyte hypertrophy. And, the stimulation effect of ATF6 is reduced during inhibition of Runx2 via a siRNA approach, suggesting that the promoting effect is required for Runx2.

Conclusions

Our observations demonstrate that ATF6 positively regulates chondrocyte hypertrophy and endochondral bone formation through activating Runx2-mediated hypertrophic chondrocyte differentiation.

A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

Decades of work have identified the signaling pathways that regulate the differentiation of chondrocytes during bone formation, from their initial induction from mesenchymal progenitor cells to their terminal maturation into hypertrophic chondrocytes. Here, we review how multiple signaling molecules, mechanical signals and morphological cell features are integrated to activate a set of key transcription factors that determine and regulate the genetic program that induces chondrogenesis and chondrocyte differentiation. Moreover, we describe recent findings regarding the roles of several signaling pathways in modulating the proliferation and maturation of chondrocytes in the growth plate, which is the 'engine' of bone elongation.

Optimized osteogenic differentiation protocol from R1 mouse embryonic stem cells in vitro

Embryonic stem cells (ESCs) are a unique model that allows the study of molecular pathways underlying commitment and differentiation. One such lineage is osteoblasts, which are responsible for forming bone tissue in the body. There are many osteogenic differentiation protocols in the literature utilizing different soluble factors. The goal of the present study was to increase the efficacy of our osteogenic differentiation protocol from R1 cells. We have studied the effects of the addition of the following factors: dexamethasone, retinoic acid, and peroxisome-proliferator-activated receptor-gamma inhibitor, which have been reported to enhance osteogenesis. We found that among the 6 different protocols that were tested, the addition of retinoic acid with later addition of dexamethasone gives the most enrichment of osteogenic lineage cells. Thus, our findings provide valuable guidelines for culture condition to differentiate mouse R1 ESCs to osteoblastic cells in vitro.

Mouse limb skeletal growth and synovial joint development are coordinately enhanced by Kartogenin

Limb development requires the coordinated growth of several tissues and structures including long bones, joints and tendons, but the underlying mechanisms are not wholly clear. Recently, we identified a small drug-like molecule - we named Kartogenin (KGN) - that greatly stimulates chondrogenesis in marrow-derived mesenchymal stem cells (MSCs) and enhances cartilage repair in mouse osteoarthritis (OA) models. To determine whether limb developmental processes are regulated by KGN, we tested its activity on committed preskeletal mesenchymal cells from mouse embryo limb buds and whole limb explants. KGN did stimulate cartilage nodule formation and more strikingly, boosted digit cartilaginous anlaga elongation, synovial joint formation and interzone compaction, tendon maturation as monitored by ScxGFP, and interdigit invagination. To identify mechanisms, we carried out gene expression analyses and found that several genes, including those encoding key signaling proteins, were up-regulated by KGN. Amongst highly up-regulated genes were those encoding hedgehog and TGFβ superfamily members, particularly TFGβ1. The former response was verified by increases in Gli1-LacZ activity and Gli1 mRNA expression. Exogenous TGFβ1 stimulated cartilage nodule formation to levels similar to KGN, and KGN and TGFβ1 both greatly enhanced expression of lubricin/Prg4 in articular superficial zone cells. KGN also strongly increased the cellular levels of phospho-Smads that mediate canonical TGFβ and BMP signaling. Thus, limb development is potently and harmoniously stimulated by KGN. The growth effects of KGN appear to result from its ability to boost several key signaling pathways and in particular TGFβ signaling, working in addition to and/or in concert with the filamin A/CBFβ/RUNX1 pathway we identified previously to orchestrate overall limb development. KGN may thus represent a very powerful tool not only for OA therapy, but also limb regeneration and tissue repair strategies.

Identification and characterization of the novel Col10a1 regulatory mechanism during chondrocyte hypertrophic differentiation

The majority of human skeleton develops through the endochondral pathway, in which cartilage-forming chondrocytes proliferate and enlarge into hypertrophic chondrocytes that eventually undergo apoptosis and are replaced by bone. Although at a terminal differentiation stage, hypertrophic chondrocytes have been implicated as the principal engine of bone growth. Abnormal chondrocyte hypertrophy has been seen in many skeletal dysplasia and osteoarthritis. Meanwhile, as a specific marker of hypertrophic chondrocytes, the type X collagen gene (COL10A1) is also critical for endochondral bone formation, as mutation and altered COL10A1 expression are often accompanied by abnormal chondrocyte hypertrophy in many skeletal diseases. However, how the type X collagen gene is regulated during chondrocyte hypertrophy has not been fully elucidated. We have recently demonstrated that Runx2 interaction with a 150-bp mouse Col10a1 cis-enhancer is required but not sufficient for its hypertrophic chondrocyte-specific reporter expression in transgenic mice, suggesting requirement of additional Col10a1 regulators. In this study, we report in silico sequence analysis of this 150-bp enhancer and identification of its multiple binding factors, including AP1, MEF2, NFAT, Runx1 and TBX5. Using this enhancer as bait, we performed yeast one-hybrid assay and identified multiple candidate Col10a1-interacting genes, including cyclooxygenase 1 (Cox-1) and Cox-2. We have also performed mass spectrometry analysis and detected EF1-alpha, Fus, GdF7 and Runx3 as components of the specific complex formed by the cis-enhancer and nuclear extracts from hypertrophic MCT (mouse chondrocytes immortalized with large T antigen) cells that express Col10a1 abundantly. Notably, some of the candidate genes are differentially expressed in hypertrophic MCT cells and have been associated with chondrocyte hypertrophy and Runx2, an indispensible Col10a1 regulator. Intriguingly, we detected high-level Cox-2 expression in hypertrophic MCT cells. Electrophoretic mobility shift assay and chromatin immunoprecipitation assays confirmed the interaction between Cox-2 and Col10a1 cis-enhancer, supporting its role as a candidate Col10a1 regulator. Together, our data support a Cox-2-containing, Runx2-centered Col10a1 regulatory mechanism, during chondrocyte hypertrophic differentiation.

Deletion of core-binding factor β (Cbfβ) in mesenchymal progenitor cells provides new insights into Cbfβ/Runxs complex function in cartilage and bone development

Core-binding factor β (Cbfβ) is a subunit of the Cbf family of heterodimeric transcription factors, which plays a critical role in skeletal development through its interaction with the Cbfα subunits, also known as Runt-related transcription factors (Runxs). However, the mechanism by which Cbfβ regulates cartilage and bone development remains unclear. Existing Cbfβ-deficient mouse models cannot specify the role of Cbfβ in skeletal cell lineage. Herein, we sought to specifically address the role of Cbfβ in cartilage and bone development by using a conditional knockout (CKO) approach. A mesenchymal-specific Cbfβ CKO mouse model was generated by using the Dermo1-Cre mouse line to specifically delete Cbfβ in mesenchymal stem cells, which give rise to osteoblasts and chondrocytes. Surprisingly, the mutant mice had under-developed larynx and tracheal cartilage, causing alveolus defects that led to death shortly after birth from suffocation. Also, the mutant mice exhibited severe skeletal deformities from defective intramembranous and endochondral ossification, owing to delayed chondrocyte maturation and impaired osteoblast differentiation. Almost all bones of the mutant mice, including the calvariae, vertebrae, tibiae, femurs, ribs, limbs and sternums were defective. Importantly, we showed that Cbfβ was expressed throughout the skeleton during both embryonic and postnatal development, which explains the multiple-skeletal defects observed in the mutant mice. Consistently, Cbfβ deficiency impaired both chondrocyte proliferation and hypertrophy zone hypertrophy during growth-plate development in the long bones of mutant mice. Notably, Cbfβ, Runx1 and Runx2 displayed different expression patterns in the growth plates of the wild-type mice, indicating that Cbfβ/Runx1 complex and Cbfβ/Runx2 complex may regulate chondrocyte proliferation and hypertrophy, respectively, in a spatial and temporal manner. Cbfβ deletion in the mesenchymal progenitors affected bone development by dramatically down-regulating Collagen X (Col X) and Osterix (Osx) but had a dispensable effect on osteoclast development. Collectively, the results demonstrate that Cbfβ mediates cartilage and bone development by interacting with Runx1 and Runx2 to regulate the expressions of Col X and Osx for chondrocyte and osteoblast development. These findings not only reveal a critical role for Cbfβ in cartilage and bone development but also facilitate the design of novel therapeutic approaches for skeletal diseases.

Chondrocyte-specific knockout of Cbfβ reveals the indispensable function of Cbfβ in chondrocyte maturation, growth plate development and trabecular bone formation in mice

Despite years of research into bone formation, the mechanisms by which transcription factors specify growth plate development and trabecular bone formation remain unclear and the role of hypertrophic chondrocytes in trabeculae morphogenesis is controversial. To study the role of Core binding factor beta (Cbfβ) in postnatal cartilage development and endochondral bone formation, we generated chondrocyte-specific Cbfβ-deficient mice (Cbfβf/fCol2α1-Cre mice) using floxed alleles of Cbfβ (Cbfβf/f) and Cre driven by the Collagen 2α1 promoter (Col2α1-Cre). Cbfβf/fCol2α1-Cre mice evaded developmental and newborn lethality to survive to adulthood and displayed severe skeletal malformation. Cbfβf/fCol2α1-Cre mice had dwarfism, hypoplastic skeletons, defective bone mineralization, shortened limbs, shortened sternum bodies, and un-calcified occipital bones and hyoid bones. In the long bone cartilage, the resting zone was elongated, and chondrocyte proliferation and hypertrophy were impaired in Cbfβf/fCol2α1-Cre mice, which led to deformation of the growth plates. Primary spongiosa formation was delayed, diaphysis was shortened and trabecular bone formation was almost absent in the mutant mice. In addition, lamellar bone formation in the secondary spongiosa was also impaired. However, osteoclast formation in the trabecular bone was not affected. Cbfβ deficiency led to down-regulation of chondrocyte-regulating genes [i.e, patched (Ptc1), Cyclin D1 and Indian hedgehog (Ihh)] in the cartilage. Interestingly, the expression of Runx2 and Runx3 was not changed in the cartilage of the mutants. Collectively, the results revealed that Cbfβ is crucial for postnatal skeletal development and endochondral bone formation through its function in growth plate development and chondrocyte proliferation and differentiation. This study also revealed that chondrocyte maturation, mediated by Cbfβ, was critical to trabecular bone morphogenesis. Significantly, these findings provide insight into the role of Cbfβ in postnatal skeletogenesis, which may assist in the development of new therapies for osteoporosis.

Indian hedgehog signaling regulates transcription and expression of collagen type X via Runx2/Smads interactions

Indian hedgehog (Ihh) is essential for chondrocyte differentiation and endochondral ossification and acts with parathyroid hormone-related peptide in a negative feedback loop to regulate early chondrocyte differentiation and entry to hypertrophic differentiation. Independent of this function, we and others recently reported independent Ihh functions to promote chondrocyte hypertrophy and matrix mineralization in vivo and in vitro. However, the molecular mechanisms for these actions and their functional significance are still unknown. We recently discovered that Ihh overexpression in chondrocytes stimulated the expression of late chondrocyte differentiation markers and induced matrix mineralization. Focusing on collagen type X (Col10α1) expression and transcription, we observed that hedgehog downstream transcription factors GLI-Krüppel family members (Gli) 1/2 increased COL10A1 promoter activity and identified a novel Gli1/2 response element in the 250-bp basic promoter. In addition, we found that Ihh induced Runx2 expression in chondrocytes without up-regulating other modulators of chondrocyte maturation such as Mef2c, Foxa2, and Foxa3. Runx2 promoted Col10α1 expression in cooperation with Ihh. Further analyses using promoter assays, immunofluorescence, and binding assays showed the interaction of Gli1/2 in a complex with Runx2/Smads induces chondrocyte differentiation. Finally, we could demonstrate that Ihh promotes in vitro matrix mineralization using similar molecular mechanisms. Our data provide an in vitro mechanism for Ihh signaling to positively regulate Col10α1 transcription. Thus, Ihh signaling could be an important player for not only early chondrocyte differentiation but maturation and calcification of chondrocytes.

Core binding factor beta (Cbfβ) controls the balance of chondrocyte proliferation and differentiation by upregulating Indian hedgehog (Ihh) expression and inhibiting parathyroid hormone-related protein receptor (PPR) expression in postnatal cartilage and bone formation

Core binding factor beta (Cbfβ) is essential for embryonic bone morphogenesis. Yet the mechanisms by which Cbfβ regulates chondrocyte proliferation and differentiation as well as postnatal cartilage and bone formation remain unclear. Hence, using paired-related homeobox transcription factor 1-Cre (Prx1-Cre) mice, mesenchymal stem cell-specific Cbfβ-deficient (Cbfβ(f/f) Prx1-Cre) mice were generated to study the role of Cbfβ in postnatal cartilage and bone development. These mutant mice survived to adulthood but exhibited severe sternum and limb malformations. Sternum ossification was largely delayed in the Cbfβ(f/f) Prx1-Cre mice and the xiphoid process was noncalcified and enlarged. In newborn and 7-day-old Cbfβ(f/f) Prx1-Cre mice, the resting zone was dramatically elongated, the proliferation zone and hypertrophic zone of the growth plates were drastically shortened and disorganized, and trabecular bone formation was reduced. Moreover, in 1-month-old Cbfβ(f/f) Prx1-Cre mice, the growth plates were severely deformed and trabecular bone was almost absent. In addition, Cbfβ deficiency impaired intramembranous bone formation both in vivo and in vitro. Interestingly, although the expression of Indian hedgehog (Ihh) was largely reduced, the expression of parathyroid hormone-related protein (PTHrP) receptor (PPR) was dramatically increased in the Cbfβ(f/f) Prx1-Cre growth plate, indicating that that Cbfβ deficiency disrupted the Ihh-PTHrP negative regulatory loop. Chromatin immunoprecipitation (ChIP) analysis and promoter luciferase assay demonstrated that the Runx/Cbfβ complex binds putative Runx-binding sites of the Ihh promoter regions, and also the Runx/Cbfβ complex directly upregulates Ihh expression at the transcriptional level. Consistently, the expressions of Ihh target genes, including CyclinD1, Ptc, and Pthlh, were downregulated in Cbfβ-deficient chondrocytes. Taken together, our study reveals not only that Cbfβ is essential for chondrocyte proliferation and differentiation for the growth and maintenance of the skeleton in postnatal mice, but also that it functions in upregulating Ihh expression to promoter chondrocyte proliferation and osteoblast differentiation, and inhibiting PPR expression to enhance chondrocyte differentiation.

The mineral dissolution function of osteoclasts is dispensable for hypertrophic cartilage degradation during long bone development and growth

During long bone development and post-natal growth, the cartilaginous model of the skeleton is progressively replaced by bone, a process known as endochondral ossification. In the primary spongiosa, osteoclasts degrade the mineralized cartilage produced by hypertrophic chondrocytes to generate cartilage trabeculae that osteoblasts embed in bone matrix. This leads to the formation of the trabecular bone network of the secondary spongiosa that will undergo continuous remodeling. Osteoclasts are specialized in mineralized tissue degradation, with the combined ability to solubilize hydroxyapatite and to degrade extracellular matrix proteins. We reported previously that osteoclasts lacking Dock5 could not degrade bone due to abnormal podosome organization and absence of sealing zone formation. Consequently, adult Dock5(-/-) mice have increased trabecular bone mass. We used Dock5(-/-) mice to further investigate the different functions of osteoclast during endochondral bone formation. We show that long bones are overall morphologically normal in developing and growing Dock5(-/-) mice. We demonstrate that Dock5(-/-) mice also have normal hypertrophic cartilage and cartilage trabecular network. Conversely, trabecular bone volume increased progressively in the secondary spongiosa of Dock5(-/-) growing mice as compared to Dock5(+/+) animals, even though their osteoclast numbers were the same. In vitro, we show that Dock5(-/-) osteoclasts do present acidic compartments at the ventral plasma membrane and produce normal amounts of active MMP9, TRAP and CtsK for matrix protein degradation but they are unable to solubilize minerals. These observations reveal that contrarily to bone resorption, the ability of osteoclasts to dissolve minerals is dispensable for the degradation of mineralized hypertrophic cartilage during endochondral bone formation.

Establishment of a bone-specific col10a1:GFP transgenic zebrafish

During skeletal development, both osteogenic and chondrogenic programs are initiated from multipotent mesenchymal cells, requiring a number of signaling molecules, transcription factors, and downstream effectors to orchestrate the sophisticated process. Col10a1, an important downstream effector gene, has been identified as a marker for maturing chondrocytes in higher vertebrates, such as mammals and birds. In zebrafish, this gene has been shown to be expressed in both osteoblasts and chondrocytes, but no study has reported its role in osteoblast development. To initially delineate the osteogenic program from chondrogenic lineage development, we used the zebrafish col10a1 promoter to establish a transgenic zebrafish expressing a GFP reporter specifically in osteoblast-specific bone structures that do not involve cartilaginous programs. A construct harboring a -2.2-kb promoter region was found to be sufficient to drive the reporter gene in osteoblast-specific bone structures within the endogenous col10a1 expression domain, confirming that separable cis-acting elements exist for distinct cell type-specific expression of col10a1 during zebrafish skeletal development. The -2.2-kb col10a1:GFP transgenic zebrafish marking only bone structures derived from osteoblasts will undoubtedly be an invaluable tool for identifying and characterizing molecular events driving osteoblast development in zebrafish, which may further provide a differential mechanism where col10a1 is involved in the development of chondrocytes undergoing maturation in other vertebrate systems.

SOX9 gene transfer via safe, stable, replication-defective recombinant adeno-associated virus vectors as a novel, powerful tool to enhance the chondrogenic potential of human mesenchymal stem cells

Introduction

Transplantation of genetically modified human bone marrow-derived mesenchymal stem cells (hMSCs) with an accurate potential for chondrogenic differentiation may be a powerful means to enhance the healing of articular cartilage lesions in patients. Here, we evaluated the benefits of delivering SOX9 (a key regulator of chondrocyte differentiation and cartilage formation) via safe, maintained, replication-defective recombinant adeno-associated virus (rAAV) vector on the capability of hMSCs to commit to an adequate chondrocyte phenotype compared with other mesenchymal lineages.

Methods

The rAAV-FLAG-hSOX9 vector was provided to both undifferentiated and lineage-induced MSCs freshly isolated from patients to determine the effects of the candidate construct on the viability, biosynthetic activities, and ability of the cells to enter chondrogenic, osteogenic, and adipogenic differentiation programs compared with control treatments (rAAV-lacZ or absence of vector administration).

Results

Marked, prolonged expression of the transcription factor was noted in undifferentiated and chondrogenically differentiated cells transduced with rAAV-FLAG-hSOX9, leading to increased synthesis of major extracellular matrix components compared with control treatments, but without effect on proliferative activities. Chondrogenic differentiation (SOX9, type II collagen, proteoglycan expression) was successfully achieved in all types of cells but strongly enhanced when the SOX9 vector was provided. Remarkably, rAAV-FLAG-hSOX9 delivery reduced the levels of markers of hypertrophy, terminal and osteogenic/adipogenic differentiation in hMSCs (type I and type X collagen, alkaline phosphatise (ALP), matrix metalloproteinase 13 (MMP13), and osteopontin (OP) with diminished expression of the osteoblast-related transcription factor runt-related transcription factor 2 (RUNX2); lipoprotein lipase (LPL), peroxisome proliferator-activated receptor gamma 2 (PPARG2)), as well as their ability to undergo proper osteo-/adipogenic differentiation. These effects were accompanied with decreased levels of β-catenin (a mediator of the Wnt signaling pathway for osteoblast lineage differentiation) and enhanced parathyroid hormone-related protein (PTHrP) expression (an inhibitor of hypertrophic maturation, calcification, and bone formation) via SOX9 treatment.

Conclusions

This study shows the potential benefits of rAAV-mediated SOX9 gene transfer to propagate hMSCs with an advantageous chondrocyte differentiation potential for future, indirect therapeutic approaches that aim at restoring articular cartilage defects in the human population.

TGFβ inhibition during expansion phase increases the chondrogenic re-differentiation capacity of human articular chondrocytes

OBJECTIVE

Autologous chondrocyte implantation is a cell-based treatment to repair articular cartilage defects, relying on the availability of expanded (de-differentiated) chondrocytes. Unfortunately, the expansion process causes several phenotypical changes, requiring re-establishment of the native chondrogenic phenotype to sustain proper repair. Among other proteins, transforming growth factor-β (TGFβ) is known to influence the chondrogenic re-differentiation of human articular chondrocytes (HACs) and their matrix deposition. Thus we investigated the effects of TGFβ-depletion during the expansion phase.

DESIGN

HACs were isolated from articular cartilage and expanded in the canonical serum-supplemented medium [fetal calf serum (FCS)] or in a chemically-defined (CD) medium, with or without anti-TGFβ antibody administration. The re-differentiation potential of the cells was assessed by pellet cultures, gene expression analysis and histology.

RESULTS

Cell proliferation proceeded more rapidly in CD-medium than in FCS-medium; it was not affected by the use of anti-TGFβ antibody but was further increased by addition of exogenous TGFβ1, via increased p-Smad1/5/8. Conversely, in FCS-medium, addition of anti-TGFβ antibody decreased both proliferation and p-Smad1/5/8 level. Challenging either FCS- or CD-medium with anti-TGFβ antibody during expansion enhanced chondrogenesis in the subsequent pellet cultures. Moreover, TGFβ-depletion during expansion in CD-medium inhibited mRNA expression of hypertrophic markers, collagen type-X (COL10) and matrix metalloproteinase-13 (MMP-13). Interestingly, the TGFβ1 level detected by enzyme-linked immunosorbent sandwich assay (ELISA) during cell expansion was correlated with COL10 mRNA expression after re-differentiation.

CONCLUSION

TGFβ-depletion during expansion improves the re-differentiation capacity of chondrocytes and inhibits hypertrophy. These results indicate the importance of the expansion medium composition to improve chondrogenic re-differentiation and to inhibit hypertrophy.

XBP1S associates with RUNX2 and regulates chondrocyte hypertrophy

BMP2 (bone morphogenetic protein 2) is known to activate unfolded protein response signaling molecules, including XBP1S and ATF6. However, the influence on XBP1S and ATF6 in BMP2-induced chondrocyte differentiation has not yet been elucidated. In this study, we demonstrate that BMP2 mediates mild endoplasmic reticulum stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. XBP1S is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. This expression is probably due to the activation of the XBP1 gene by ATF6 and splicing by IRE1a. ATF6 directly binds to the 5'-flanking regulatory region of the XBP1 gene at its consensus binding elements. Overexpression of XBP1S accelerates chondrocyte hypertrophy, as revealed by enhanced expression of type II collagen, type X collagen, and RUNX2; however, knockdown of XBP1S via the RNAi approach abolishes hypertrophic chondrocyte differentiation. In addition, XBP1S associates with RUNX2 and enhances RUNX2-induced chondrocyte hypertrophy. Altered expression of XBP1S in chondrocyte hypertrophy was accompanied by altered levels of IHH (Indian hedgehog) and PTHrP (parathyroid hormone-related peptide). Collectively, XBP1S may be a novel regulator of hypertrophic chondrocyte differentiation by 1) acting as a cofactor of RUNX2 and 2) affecting IHH/PTHrP signaling.

FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program

During endochondral ossification, small, immature chondrocytes enlarge to form hypertrophic chondrocytes, which express collagen X. In this work, we demonstrate that FoxA factors are induced during chondrogenesis, bind to conserved binding sites in the collagen X enhancer, and can promote the expression of a collagen X-luciferase reporter in both chondrocytes and fibroblasts. In addition, we demonstrate by both gain- and loss-of-function analyses that FoxA factors play a crucial role in driving the expression of both endogenous collagen X and other hypertrophic chondrocyte-specific genes. Mice engineered to lack expression of both FoxA2 and FoxA3 in their chondrocytes display defects in chondrocyte hypertrophy, alkaline phosphatase expression, and mineralization in their sternebrae and, in addition, exhibit postnatal dwarfism that is coupled to significantly decreased expression of both collagen X and MMP13 in their growth plates. Our findings indicate that FoxA family members are crucial regulators of the hypertrophic chondrocyte differentiation program.

Runx2 contributes to murine Col10a1 gene regulation through direct interaction with its cis-enhancer

We have recently shown that a 150-bp Col10a1 distal promoter (-4296 to -4147 bp) is sufficient to direct hypertrophic chondrocyte-specific reporter (LacZ) expression in vivo. More recently, through detailed sequence analysis we identified two putative tandem-repeat Runx2 binding sites within the 3'-end of this 150-bp region (TGTGGG-TGTGGC, -4187 to -4176 bp). Candidate electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation, and transfection studies demonstrate that these putative Runx2 sites bind Runx2 and mediate upregulated Col10a1/reporter activity in vitro. Transgenic studies using the 5'-sequence without Runx2 sites were not able to drive the cell-specific LacZ reporter activity, suggesting the in vivo requirement of the Runx2 sites located in the 3'-end in mediating Col10a1/reporter expression. Indeed, mutating the Runx2 sites in the context of the 150-bp promoter abolishes its capacity to drive hypertrophic chondrocyte-specific reporter expression in transgenic mice. We have also generated multiple transgenic mouse lines using only the 3'-sequence containing the Runx2 sites to drive the LacZ gene. Interestingly, no hypertrophic chondrocyte-specific blue staining was observed in these transgenic mice. Together, our data support that Runx2 directly interacts with murine Col10a1 cis-enhancer. This interaction is required but not sufficient for cell-specific Col10a1 promoter activity in vivo. Additional cooperative/repressive elements within the 5'- or 3'-sequences of this 150-bp promoter are needed to work with Runx2 together to mediate cell-specific Col10a1 expression. Further delineation of these elements/factors has the potential to identify novel therapeutic targets for multiple skeletal disorders, including osteoarthritis, that show abnormal Col10a1 expression and altered chondrocyte maturation.

Calcium hydroxide regulates bone sialoprotein gene transcription in human osteoblast-like Saos2 cells

Bone sialoprotein (BSP) is a mineralized tissue-specific protein expressed in differentiated osteoblasts that appears to function in the initial mineralization of bone. Calcium hydroxide (Ca(OH)(2)) is a basic salt that has been widely used for a variety of applications in dentistry, due to its antimicrobial effects and its capability of inducing hard tissue formation. However, details of the mechanism involved in the mineralization induced by Ca(OH)(2) are still unclear. In the present study, Ca(OH)(2) (0.4 mM) was found to increase the levels of BSP and Runx2 mRNA at 3 h in human osteoblast-like Saos2 cells. Transient transfection assays were performed using chimeric constructs of the human BSP gene promoter linked to a luciferase reporter gene. Treatment of Saos2 cells with Ca(OH)(2) (0.4 mM) increased the luciferase activities of the constructs between -60LUC and -927LUC at 12 h. Gel shift analysis showed that Ca(OH)(2) (0.4 mM) increased the binding of nuclear protein to CRE1, CRE2 and FRE. Antibodies against CREB1, c-Fos, c-Jun, JunD, Fra2 and P300 disrupted the formation of the CRE1- and CRE2-protein complexes, and antibodies against Dlx5, Msx2, Runx2 and Smad1 disrupted the formation of the FRE-protein complex. These findings demonstrate that Ca(OH)(2) stimulates BSP transcription by targeting the CRE1, CRE2 and FRE elements in the human BSP gene promoter.

Generation and characterization of Col10a1-mcherry reporter mice

We report here on the generation of a new fluorescent protein reporter transgenic mouse line, Col10a1-mCherry, which can be used as a tool to study chondrocyte biology and pathology. Collagen, Type X, alpha 1 (Col10a1) is highly expressed in hypertrophic chondrocytes and commonly used as a gene marker for this cell population. The Col10a1-mCherry reporter line was generated using a bacterial recombination strategy with the mouse BAC clone RP23-192A7. To aid in the characterization of this animal model, we intercrossed Col10a1-mCherry mice with Collagen, Type II, alpha 1 (Col2a1) enhanced cyan fluorescent protein (ECFP) reporter mice and characterized the expression of both chondrocyte reporters during embryonic skeletal development from days E10.5 to E17.5. Additionally, at postnatal day 0, Col10a1-mCherry reporter expression was compared to endogenous Col10a1 mRNA expression in long bones and revealed that mCherry fluorescence extended past the Col10a1 expression domain. However, in situ hybridization for mCherry was consistent with the zone of Col10a1 mRNA expression, indicating that the persistent detection of mCherry fluorescence was a result of the long protein half life of mCherry in conjunction with a very rapid phase of skeletal growth and not due to aberrant transcriptional regulation. Taking advantage of the continued fluorescence of hypertrophic chondrocytes at the chondro-osseus junction, we intercrossed Col10a1-mCherry mice with two different Collagen, Type 1, alpha 1, (Col1a1) osteoblast reporter mice, pOBCol3.6-Topaz and pOBCol2.3-Emerald to investigate the possibility that hypertrophic chondrocytes transdifferentiate into osteoblasts. Evaluation of long bones at birth suggests that residual hypertrophic chondrocytes and osteoblasts in the trabecular zone exist as two completely distinct cell populations. genesis 49:410-418, 2011.

cAMP and fibroblast growth factor 2 regulate bone sialoprotein gene expression in human prostate cancer cells

Bone sialoprotein (BSP) is a noncollagenous protein of the extracellular matrix in mineralized connective tissues that has been implicated in the nucleation of hydroxyapatite. Forskolin (FSK), an activator of adenylate cyclase, increased the intracellular cAMP level, which stimulates the proliferation and differentiation of osteoblasts. Fibroblast growth factor 2 (FGF2) is a potent mitogen in many cell types, including osteoblasts. In human prostate cancer DU145 cells, FSK (1 μM) and FGF2 (10 ng/ml) increased BSP and Runx2 mRNA and protein levels at 3 and 12h, respectively. Transient transfection analyses were performed using chimeric constructs of the human BSP gene promoter linked to a luciferase reporter gene. Treatment of DU145 cells with FSK (1 μM) and FGF2 (10 ng/ml) increased the luciferase activities of constructs between -60LUC to -927LUC and -108LUC to -927LUC, including the human BSP gene promoter. Effects of FSK and FGF2 abrogated in constructs included 2bp mutations in the two cAMP response elements (CRE1 and CRE2). Luciferase activities induced by FSK and FGF2 were blocked by protein kinase A and tyrosine kinase inhibitors. Gel mobility shift analyses showed that FSK and FGF2 increased the binding of CRE1 and CRE2. CRE1-protein complexes were supershifted by phospho-CREB1 and c-Fos antibodies, and disrupted by CREB1, c-Jun, JunD, Fra2, p300, Runx2, Dlx5 and Smad1 antibodies. CRE2-protein complexes were disrupted by CREB1, phospho-CREB1, c-Fos, c-Jun, JunD, Fra2, p300, Runx2, Dlx5 and Smad1 antibodies. These studies demonstrate that FSK and FGF2 stimulate BSP transcription in DU145 human prostate cancer cells by targeting the CRE1 and CRE2 elements in the human BSP gene promoter.

Retinoic acids potentiate BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Background

As one of the least studied bone morphogenetic proteins (BMPs), BMP9 is one of the most osteogenic BMPs. Retinoic acid (RA) signaling is known to play an important role in development, differentiation and bone metabolism. In this study, we investigate the effect of RA signaling on BMP9-induced osteogenic differentiation of mesenchymal progenitor cells (MPCs).

Methodology/Principal Findings

Both primary MPCs and MPC line are used for BMP9 and RA stimulation. Recombinant adenoviruses are used to deliver BMP9, RARα and RXRα into MPCs. The in vitro osteogenic differentiation is monitored by determining the early and late osteogenic markers and matrix mineralization. Mouse perinatal limb explants and in vivo MPC implantation experiments are carried out to assess bone formation. We find that both 9CRA and ATRA effectively induce early osteogenic marker, such as alkaline phosphatase (ALP), and late osteogenic markers, such as osteopontin (OPN) and osteocalcin (OC). BMP9-induced osteogenic differentiation and mineralization is synergistically enhanced by 9CRA and ATRA in vitro. 9CRA and ATRA are shown to induce BMP9 expression and activate BMPR Smad-mediated transcription activity. Using mouse perinatal limb explants, we find that BMP9 and RAs act together to promote the expansion of hypertrophic chondrocyte zone at growth plate. Progenitor cell implantation studies reveal that co-expression of BMP9 and RXRα or RARα significantly increases trabecular bone and osteoid matrix formation.

Conclusion/Significance

Our results strongly suggest that retinoid signaling may synergize with BMP9 activity in promoting osteogenic differentiation of MPCs. This knowledge should expand our understanding about how BMP9 cross-talks with other signaling pathways. Furthermore, a combination of BMP9 and retinoic acid (or its agonists) may be explored as effective bone regeneration therapeutics to treat large segmental bony defects, non-union fracture, and/or osteoporotic fracture.

Smad signaling in skeletal development and regeneration

Smad proteins are intracellular molecules that mediate the canonical signaling cascade of TGFbeta superfamily growth factors. The TGFbeta superfamily comprises two groups of growth factors, BMPs and TGFbetas. Both groups can be further divided into several sub-groups based on sequence homologies and functional similarities. Ligands of the TGFbeta superfamily bind to cell surface receptors to activate Smad proteins in the cytoplasm; then the activated Smad proteins translocate into the nucleus to activate or repress specific target gene transcription. Both groups of growth factors play important roles in skeletal development and regeneration. However, whether these effects reflect signaling through canonical Smad pathways, or other non-canonical signaling pathways in vivo remains a mystery. Moreover, the mechanisms utilized by Smad proteins to initiate nuclear events and their interactions with cytoplasmic proteins are still under intensive investigation. This review will discuss the most recent progress understanding Smad signaling in the context of skeletal development and regeneration.

A role for age-related changes in TGFbeta signaling in aberrant chondrocyte differentiation and osteoarthritis

Transforming growth factor beta (TGFβ) is a growth factor with many faces. In our osteoarthritis (OA) research we have found that TGFβ can be protective as well as deleterious for articular cartilage. We postulate that the dual effects of TGFβ on chondrocytes can be explained by the fact that TGFβ can signal via different receptors and related Smad signaling routes. On chondrocytes, TGFβ not only signals via the canonical type I receptor ALK5 but also via the ALK1 receptor. Notably, signaling via ALK5 (Smad2/3 route) results in markedly different chondrocyte responses than ALK1 signaling (Smad1/5/8), and we postulate that the balance between ALK5 and ALK1 expression on chondrocytes will determine the overall effect of TGFβ on these cells. Importantly, signaling via ALK1, but not ALK5, stimulates MMP-13 expression by chondrocytes. In cartilage of ageing mice and in experimental OA models we have found that the ALK1/ALK5 ratio is significantly increased, favoring TGFβ signaling via the Smad1/5/8 route, changes in chondrocyte differentiation and MMP-13 expression. Moreover, human OA cartilage showed a significant correlation between ALK1 and MMP-13 expression. In this paper we summarize concepts in OA, its link with ageing and disturbed growth factor responses, and a potential role of TGFβ signaling in OA development.

Retinoic acid enhances skeletal muscle progenitor formation and bypasses inhibition by bone morphogenetic protein 4 but not dominant negative beta-catenin

BACKGROUND

Understanding stem cell differentiation is essential for the future design of cell therapies. While retinoic acid (RA) is the most potent small molecule enhancer of skeletal myogenesis in stem cells, the stage and mechanism of its function has not yet been elucidated. Further, the intersection of RA with other signalling pathways that stimulate or inhibit myogenesis (such as Wnt and BMP4, respectively) is unknown. Thus, the purpose of this study is to examine the molecular mechanisms by which RA enhances skeletal myogenesis and interacts with Wnt and BMP4 signalling during P19 or mouse embryonic stem (ES) cell differentiation.

RESULTS

Treatment of P19 or mouse ES cells with low levels of RA led to an enhancement of skeletal myogenesis by upregulating the expression of the mesodermal marker, Wnt3a, the skeletal muscle progenitor factors Pax3 and Meox1, and the myogenic regulatory factors (MRFs) MyoD and myogenin. By chromatin immunoprecipitation, RA receptors (RARs) bound directly to regulatory regions in the Wnt3a, Pax3, and Meox1 genes and RA activated a beta-catenin-responsive promoter in aggregated P19 cells. In the presence of a dominant negative beta-catenin/engrailed repressor fusion protein, RA could not bypass the inhibition of skeletal myogenesis nor upregulate Meox1 or MyoD. Thus, RA functions both upstream and downstream of Wnt signalling. In contrast, it functions downstream of BMP4, as it abrogates BMP4 inhibition of myogenesis and Meox1, Pax3, and MyoD expression. Furthermore, RA downregulated BMP4 expression and upregulated the BMP4 inhibitor, Tob1. Finally, RA inhibited cardiomyogenesis but not in the presence of BMP4.

CONCLUSION

RA can enhance skeletal myogenesis in stem cells at the muscle specification/progenitor stage by activating RARs bound directly to mesoderm and skeletal muscle progenitor genes, activating beta-catenin function and inhibiting bone morphogenetic protein (BMP) signalling. Thus, a signalling pathway can function at multiple levels to positively regulate a developmental program and can function by abrogating inhibitory pathways. Finally, since RA enhances skeletal muscle progenitor formation, it will be a valuable tool for designing future stem cell therapies.

Finding partners: how BMPs select their targets

The bone morphogenetic protein (BMP) signaling pathway is a conserved and evolutionarily ancient regulatory module affecting a large variety of cellular behaviors. The evolutionary flexibility in using BMP responses presumably arose by co-option of a canonical BMP signaling cascade to regulate the transcription of diverse batteries of target genes. This begs the question of how seemingly interchangeable BMP signaling components elicit widely different outputs in different cell types, an important issue in the context of understanding how BMP signaling integrates with gene regulatory networks to control development. Because a molecular understanding of how BMP signaling activates different batteries of target genes is an essential prerequisite to comprehending the roles of BMPs in regulating cellular responses, here we review the current knowledge of how BMP-regulated target genes are selected by the signal transduction machinery. We highlight recent studies suggesting the evolutionary conservation of BMP target gene regulation signaling by Schnurri family zinc finger proteins. Developmental Dynamics 238:1321-1331, 2009. (c) 2009 Wiley-Liss, Inc.

The promotion of chondrogenesis in adipose-derived adult stem cells by an RGD-chimeric protein in 3D alginate culture

The dynamic regulation of integrin-binding peptides is crucial for chondrogenic differentiation. Here, we revealed the feasibility for flexible modification of RGD by embedding a large molecular weight and slightly charged (isoelectric point, 6-6.25) RGD-chimeric protein (CBD-RGD) with cellulose-binding domain (CBD) in three dimensional (3D) alginate beads to evaluate the chondrogenesis of adipose-derived adult stem cells (ADAS). The binding of CBD-RGD with cells and its diffusion from alginate beads were studied on fluorescein isothiocyanate (FITC)-conjugated CBD-RGD. The increases in gene expression (Sox9, Aggrecan, fibronectin and collagen II), accumulation of chondrogenic matrices and decrease of collagen X gene expression during TGF-beta3 induction were only observed for those beads containing 10mg/g CBD-RGD initially, with 20.18+/-0.73% of that released in a week. The contrary was observed for beads with CBD-RGD 20mg/g initially and having higher persistence (only 8.6+/-2.17% released in a week). The 10mg/g CBD-RGD-mediated enhancement was demonstrated via the activation of integrin alpha5 and beta1-dependent pathway, and especially related to the upregulation of Sox9 gene and the temporary block of fibronectin expression as well as sustained inhibition of RhoA activity in the early differentiation stage. Thus, we speculated that the dynamic mobility of CBD-RGD may account for the enhanced chondrogenesis. It was concluded that the CBD-RGD-alginate culture system promoted the chondrogenesis of mesenchymal stem cells coordinated with TGF-beta3 induction in an RGD dose-dependent manner.

Regulation of bone development and extracellular matrix protein genes by RUNX2

RUNX2 is a multifunctional transcription factor that controls skeletal development by regulating the differentiation of chondrocytes and osteoblasts and the expression of many extracellular matrix protein genes during chondrocyte and osteoblast differentiation. This transcription factor plays a major role at the late stage of chondrocyte differentiation: it is required for chondrocyte maturation and regulates Col10a1 expression in hypertrophic chondrocytes and the expression of Spp1, Ibsp, and Mmp13 in terminal hypertrophic chondrocytes. It is essential for the commitment of pluripotent mesenchymal cells to the osteoblast lineage. During osteoblast differentiation, RUNX2 upregulates the expression of bone matrix protein genes including Col1a1, Spp1, Ibsp, Bglap, and Fn1 in vitro and activates many promoters including those of Col1a1, Col1a2, Spp1, Bglap, and Mmp13. However, overexpression of Runx2 inhibits osteoblast maturation and reduces Col1a1 and Bglap expression. The inhibition of RUNX2 in mature osteoblasts does not reduce the expression of Col1a1 and Bglap in mice. Thus, RUNX2 directs pluripotent mesenchymal cells to the osteoblast lineage, triggers the expression of major bone matrix protein genes, and keeps the osteoblasts in an immature stage, but does not play a major role in the maintenance of the expression of Col1a1 or Bglap in mature osteoblasts. During bone development, RUNX2 induces osteoblast differentiation and increases the number of immature osteoblasts, which form immature bone, whereas Runx2 expression has to be downregulated for differentiation into mature osteoblasts, which form mature bone. During dentinogenesis, Runx2 expression is downregulated, and RUNX2 inhibits the terminal differentiation of odontoblasts.

Tracking gene expression during zebrafish osteoblast differentiation

The transcription factors RUNX2 and OSX have been shown to act sequentially to direct mammalian osteoblast differentiation. RUNX2 is required during the early stages of commitment and acts in part to activate Osx transcription. OSX and RUNX2 then act to direct transcription of bone matrix proteins. Here, we investigate the expression of these genes and others during zebrafish osteoblastogenesis. Using whole-mount in situ hybridization, we find that, during the formation of a given bone, the zebrafish homologues of mouse Runx2 (runx2a and runx2b) are typically expressed before the onset of osx. osx expression is usually followed by up-regulation of the bone matrix proteins, col1a2 and osteonectin. These results suggest that the mammalian pathway is conserved during development of the head and shoulder skeleton of zebrafish. We also analyze the expression of three atypical bone markers (tcf7, cvl2, and col10a1) in an effort to place them within this canonical hierarchy.

Thyroid hormone treatment of cultured chondrocytes mimics in vivo stimulation of collagen X mRNA by increasing BMP 4 expression

During endochondral bone formation, chondrocytes undergo terminal differentiation, during which the rate of proliferation decreases, cells become hypertrophic, and the extracellular matrix is altered by production of collagen X, as well as proteins required for matrix mineralization. This maturation process is responsible for most longitudinal bone growth, both during embryonic development and in postnatal long bone growth plates. Among the major signaling molecules implicated in regulation of this process are the positive regulators thyroid hormone (T3) and bone morphogenetic proteins (BMPs). Both T3 and BMPs are essential for endochondral bone formation and cannot compensate for each other, suggesting interaction of the two signaling pathways. We have analyzed the temporal and spatial expression patterns of numerous genes believed to play a role in chondrocyte maturation. Our results show that T3 stimulates collagen X gene expression in cultured chondrocytres with kinetics and magnitude similar to those observed in vivo. Stimulation of collagen X gene expression by T3 occurs only after a significant delay, implying that this hormone may act indirectly. We show further that T3 rapidly stimulates production of BMP 4, concomitant with a decrease in the BMP inhibitor Noggin, potentially resulting in a net increase in BMP signaling. Finally, inhibition of BMP signaling with exogenous Noggin prevents T3 stimulation of collagen X expression, indicating that BMP signaling is essential for this process. These data position thyroid hormone at the top of a T3/BMP cascade, potentially explaining why both pathways are essential for chondrocyte maturation. J. Cell. Physiol. 219: 595-605, 2009. (c) 2009 Wiley-Liss, Inc.

BMP canonical Smad signaling through Smad1 and Smad5 is required for endochondral bone formation

Bone morphogenetic protein (BMP) signaling is required for endochondral bone formation. However, whether or not the effects of BMPs are mediated via canonical Smad pathways or through noncanonical pathways is unknown. In this study we have determined the role of receptor Smads 1, 5 and 8 in chondrogenesis. Deletion of individual Smads results in viable and fertile mice. Combined loss of Smads 1, 5 and 8, however, results in severe chondrodysplasia. Smad1/5(CKO) (cartilage-specific knockout) mutant mice are nearly identical to Smad1/5(CKO);Smad8(-/-) mutants, indicating that Smads 1 and 5 have overlapping functions and are more important than Smad8 in cartilage. The Smad1/5(CKO) phenotype is more severe than that of Smad4(CKO) mice, challenging the dogma, at least in chondrocytes, that Smad4 is required to mediate Smad signaling through BMP pathways. The chondrodysplasia in Smad1/5(CKO) mice is accompanied by imbalances in cross-talk between the BMP, FGF and Ihh/PTHrP pathways. We show that Ihh is a direct target of BMP pathways in chondrocytes, and that FGF exerts antagonistic effects on Ihh expression. Finally, we tested whether FGF exerts its antagonistic effects directly through Smad linker phosphorylation. The results support the alternative conclusion that the effects of FGFs on BMP signaling are indirect in vivo.

A Runx2 threshold for the cleidocranial dysplasia phenotype

Cleidocranial dysplasia (CCD) in humans is an autosomal-dominant skeletal disease that results from mutations in the bone-specific transcription factor RUNX2 (CBFA1/AML3). However, distinct RUNX2 mutations in CCD do not correlate with the severity of the disease. Here we generated a new mouse model with a hypomorphic Runx2 mutant allele (Runx2(neo7)), in which only part of the transcript is processed to full-length (wild-type) Runx2 mRNA. Homozygous Runx2(neo7/neo7) mice express a reduced level of wild-type Runx2 mRNA (55-70%) and protein. This mouse model allowed us to establish the minimal requirement of functional Runx2 for normal bone development. Runx2(neo7/neo7) mice have grossly normal skeletons with no abnormalities observed in the growth plate, but do exhibit developmental defects in calvaria and clavicles that persist through post-natal growth. Clavicle defects are caused by disrupted endochondral bone formation during embryogenesis. These hypomorphic mice have altered calvarial bone volume, as observed by histology and microCT imaging, and decreased expression of osteoblast marker genes. The bone phenotype of the heterozygous mice, which have 79-84% of wild-type Runx2 mRNA, is normal. These results show there is a critical gene dosage requirement of functional Runx2 for the formation of intramembranous bone tissues during embryogenesis. A decrease to 70% of wild-type Runx2 levels results in the CCD syndrome, whereas levels >79% produce a normal skeleton. Our findings suggest that the range of bone phenotypes in CCD patients is attributable to quantitative reduction in the functional activity of RUNX2.