Interactions between Sox9 and beta-catenin control chondrocyte differentiation

WNTers in La Jolla

A 'traditional' Wnt meeting, the first of which occurred over two decades ago as a meeting of the laboratories of Harold Varmus and Roel Nusse, was held at the University of California, San Diego, in June 2007. Organized by Karl Willert, Anthony Wynshaw-Boris and Katherine Jones, the meeting was attended by nearly 400 scientists interested in ;all things Wnt', including Wnt signal transduction mechanisms, and Wnt signaling in evolutionary and developmental biology, stem cell biology, regeneration and disease. Themes that dominated the meeting included the need for precise control over each step of the signal transduction mechanism and developing therapeutics for diseases caused by altered Wnt-signaling.

Thyroid hormone interacts with the Wnt/beta-catenin signaling pathway in the terminal differentiation of growth plate chondrocytes

Thyroid hormone activates Wnt-4 expression and Wnt/beta-catenin signaling in rat growth plate chondrocytes. Wnt antagonists Frzb/sFRP3 and Dkk1 inhibit T3-induced Wnt/beta-catenin activation and inhibit the maturation-promoting effects of T3 in growth plate cells. This study indicates that thyroid hormone regulates terminal differentiation of growth plate chondrocytes in part through modulating Wnt/beta-catenin signaling.

INTRODUCTION

Thyroid hormone is a potent regulator of skeletal maturation in the growth plate, yet the molecular mechanisms underlying this profound effect remain unknown. Wnt signaling has recently been recognized as an important signal transduction pathway in regulating chondrogenesis and terminal differentiation of growth plate chondrocytes. The objective of this study was to explore the interaction between the thyroid hormone and Wnt signaling pathways in the growth plate.

MATERIALS AND METHODS

Rat epiphyseal chondrocytes were maintained in 3D pellet culture and treated with triiodothyronine (T3). Activation of Wnt/beta-catenin signaling pathway in response to T3 was detected by measurement of the expression of Wnt-4 mRNA, the cellular accumulation of beta-catenin, the transcriptional activity of TCF/LEF, and the expression of the Wnt/beta-catenin responsive gene Runx2/cbfa1. Terminal differentiation of the chondrocytes was assessed by measurement of alkaline phosphatase enzymatic activity and Col10a1 gene expression.

RESULTS

Thyroid hormone treatment of growth plate chondrocytes upregulated both Wnt-4 mRNA and protein expression, increased cellular accumulation of stabilized beta-catenin, increased TCF/LEF transcriptional activity, and stimulated the expression of the Runx2/cbfa1 gene. Overexpression of either Wnt-4 or a stabilized form of beta-catenin promoted growth plate chondrocyte terminal differentiation. Blocking Wnt ligand/receptor interactions with the secreted Wnt antagonists Frzb/sFRP3 or Dkk1 inhibited these T3-induced increases in beta-catenin accumulation and Runx2 gene expression and inhibited the maturation-promoting effects of T3 in growth plate cells.

CONCLUSIONS

These data suggest that thyroid hormone regulates terminal differentiation of growth plate chondrocytes in part through modulating canonical Wnt/beta-catenin signaling.

Bone regeneration is regulated by wnt signaling

Tissue regeneration is increasingly viewed as reactivation of a developmental process that, when misappropriated, can lead to malignant growth. Therefore, understanding the molecular and cellular pathways that govern tissue regeneration provides a glimpse into normal development as well as insights into pathological conditions such as cancer. Herein, we studied the role of Wnt signaling in skeletal tissue regeneration.

INTRODUCTION

Some adult tissues have the ability to regenerate, and among these, bone is one of the most remarkable. Bone exhibits a persistent, lifelong capacity to reform after injury, and continual bone regeneration is a prerequisite to maintaining bone mass and density. Even slight perturbations in bone regeneration can have profound consequences, as exemplified by conditions such as osteoporosis and delayed skeletal repair. Here, our goal was to determine the role of Wnts in adult bone regeneration.

MATERIALS AND METHODS

Using TOPgal reporter mice, we found that damage to the skeleton instigated Wnt reporter activity, specifically at the site of injury. We used a skeletal injury model to show that Wnt inhibition, achieved through adenoviral expression of Dkk1 in the adult skeleton, prevented the differentiation of osteoprogenitor cells.

RESULTS

As a result, injury-induced bone regeneration was reduced by 84% compared with controls. Constitutive activation of the Wnt pathway resulting from a mutation in the Lrp5 Wnt co-receptor results in high bone mass, but our experiments showed that this same point mutation caused a delay in bone regeneration. In these transgenic mice, osteoprogenitor cells in the injury site were maintained in a proliferative state and differentiation into osteoblasts was delayed.

CONCLUSIONS

When considered together, these data provide a framework for understanding the roles of Wnt signaling in adult bone regeneration and suggest a feasible approach to treating clinical conditions where enhanced bone formation is desired.

Back to basics: Sox genes

Sox genes are indispensable for multiple aspects of development. This primer briefly describes shared properties of the Sox gene family, and five well-characterized examples of vertebrate developmental mechanisms governed by Sox gene subgroups: testis development, central nervous system neurogenesis, oligodendrocyte development, chondrogenesis, and neural crest cell development. Also featured is an interview about current issues in the field with experts Jonas Muhr, Ph.D. and Robert Kelsh, Ph.D.

Wnt signaling and skeletal development

Wnt proteins are a family of secreted proteins that regulate many aspects of cellular functions. The discovery that mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, could positively and negatively affect bone mass in humans generated an enormous amount of interest in the possible role of the Wnt signaling pathway in skeletal biology. Over the last decade, considerable progress has been made in determining the role of the canonical Wnt signaling pathway in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of non-canonical Wnt signaling in skeletal development. In this review we discuss the current understanding of the role of Wnt signaling in chondrogenesis, osteoblastogenesis, and osteoclastogenesis.

Small molecules in stem cell self-renewal and differentiation

The use of stem cells in regenerative medicine is a promising approach to the treatment of disease and injury. Natural and synthetic small molecules have been shown to be useful chemical tools for controlling and manipulating the fates of cells. Small molecules can target signaling transduction pathways (for example, tyrosine kinase receptors) and affect DNA replication, cell differentiation, tumor metastasis and apoptosis. Stem cells share many properties with cancer cells and these similarities can provide insights to control and direct cell behavior; small molecules are already standard chemotherapeutics in the treatment of cancer. Libraries of small molecules have been examined for anticancer behavior (especially apoptosis), and, more recently, for stem cell self-renewal and differentiation capabilities in potential approaches to regenerative medicine. Differentiation therapy for cancer is based on the idea that cancer cells are undifferentiated embryonic-like cells and proposes to promote the differentiation and hence block cell proliferation. For example, retinoids have a role in stem cell differentiation to several lineages and have also been used to promote differentiation of acute promyeloic leukemic cells. Small molecules are also important tools for understanding mechanistic and developmental processes. Strategies for generating functional small molecule libraries have been outlined previously. In this review, we will look at several small molecules that have been described in the recent literature as effectors of stem cell self-renewal or differentiation as associated with the Wnt, Hedgehog or NF-kappaB pathways.

A new approach to control condylar growth by regulating angiogenesis

OBJECTIVE

To provide a comprehensive review of the mechanisms of growth of mandibular condyle, the roles of angiogenesis enhancers and inhibitors during endochondral ossification in mandibular condyle and newly developed delivery methods for local gene delivery that may represent strategies to regulate condylar growth.

DESIGN

Narrative review.

RESULTS

Angiogenesis is the crucial step in mandibular condylar growth for it regulates the transformation from cartilage to bone. Angiognesis enhancers, especially VEGF and FGF, play important roles in the process of new blood lumen formation and invasion. On the other hand, angiostatin and endostatin inhibit angiogenesis by targeting endothelial cells and several signal cascades. Delivery methods such as liposomes, stem cells and virus vectors have been studied. Recombinant AAV-mediated gene therapy is considered as one of the most promising strategies of condylar growth management.

CONCLUSION

AAV-mediated gene therapy using VEGF or angiogenesis inhibitor will be a promising way to regulate condylar growth at an early stage.

Differential control of Wnt target genes involves epigenetic mechanisms and selective promoter occupancy by T-cell factors

Canonical Wnt signaling and its nuclear effectors, beta-catenin and the family of T-cell factor (TCF) DNA-binding proteins, belong to the small number of regulatory systems which are repeatedly used for context-dependent control of distinct genetic programs. The apparent ability to elicit a large variety of transcriptional responses necessitates that beta-catenin and TCFs distinguish precisely between genes to be activated and genes to remain silent in a specific context. How this is achieved is unclear. Here, we examined patterns of Wnt target gene activation and promoter occupancy by TCFs in different mouse cell culture models. Remarkably, within a given cell type only Wnt-responsive promoters are bound by specific subsets of TCFs, whereas nonresponsive Wnt target promoters remain unoccupied. Wnt-responsive, TCF-bound states correlate with DNA hypomethylation, histone H3 hyperacetylation, and H3K4 trimethylation. Inactive, nonresponsive promoter chromatin shows DNA hypermethylation, is devoid of active histone marks, and additionally can show repressive H3K27 trimethylation. Furthermore, chromatin structural states appear to be independent of Wnt pathway activity. Apparently, cell-type-specific regulation of Wnt target genes comprises multilayered control systems. These involve epigenetic modifications of promoter chromatin and differential promoter occupancy by functionally distinct TCF proteins, which together determine susceptibility to Wnt signaling.

Sox9 regulates cell proliferation and is required for Paneth cell differentiation in the intestinal epithelium

The HMG-box transcription factor Sox9 is expressed in the intestinal epithelium, specifically, in stem/progenitor cells and in Paneth cells. Sox9 expression requires an active beta-catenin-Tcf complex, the transcriptional effector of the Wnt pathway. This pathway is critical for numerous aspects of the intestinal epithelium physiopathology, but processes that specify the cell response to such multipotential signals still remain to be identified. We inactivated the Sox9 gene in the intestinal epithelium to analyze its physiological function. Sox9 inactivation affected differentiation throughout the intestinal epithelium, with a disappearance of Paneth cells and a decrease of the goblet cell lineage. Additionally, the morphology of the colon epithelium was severely altered. We detected general hyperplasia and local crypt dysplasia in the intestine, and Wnt pathway target genes were up-regulated. These results highlight the central position of Sox9 as both a transcriptional target and a regulator of the Wnt pathway in the regulation of intestinal epithelium homeostasis.

Growth and differentiation of the developing limb bud from the perspective of chondrogenesis

Limb skeletal elements develop from a cartilage template, which is formed by the process termed chondrogenesis. This process is crucial in determining the shape and size of definitive bones in vertebrates. During chondrogenesis, aggregated mesenchymal cells undergo a highly organized process of proliferation and maturation along with secretion of extracellular matrix followed by programmed cell death and replacement by bone. The molecular mechanisms underlying this sophisticated process have been extensively studied. It has been demonstrated that several transcription factors such as Sox genes and Runx genes are indispensable for the major steps in chondrogenesis. Additionally, a number of signaling molecules including Bmps, Fgfs and Ihh/PTHrP are known to regulate chondrogenesis through highly coordinated interactions. This review is meant to provide an overview of the current knowledge of chondrogenesis with particular emphasis on the cellular and molecular aspects.

Sox17 and Sox4 differentially regulate beta-catenin/T-cell factor activity and proliferation of colon carcinoma cells

The canonical Wnt pathway is necessary for gut epithelial cell proliferation, and aberrant activation of this pathway causes intestinal neoplasia. We report a novel mechanism by which the Sox family of transcription factors regulate the canonical Wnt signaling pathway. We found that some Sox proteins antagonize while others enhance beta-catenin/T-cell factor (TCF) activity. Sox17, which is expressed in the normal gut epithelium but exhibits reduced expression in intestinal neoplasia, is antagonistic to Wnt signaling. When overexpressed in SW480 colon carcinoma cells, Sox17 represses beta-catenin/TCF activity in a dose-dependent manner and inhibits proliferation. Sox17 and Sox4 are expressed in mutually exclusive domains in normal and neoplastic gut tissues, and gain- and loss-of-function studies demonstrate that Sox4 enhances beta-catenin/TCF activity and the proliferation of SW480 cells. In addition to binding beta-catenin, both Sox17 and Sox4 physically interact with TCF/lymphoid enhancer factor (LEF) family members via their respective high-mobility-group box domains. Results from gain- and loss-of-function experiments suggest that the interaction of Sox proteins with beta-catenin and TCF/LEF proteins regulates the stability of beta-catenin and TCF/LEF. In particular, Sox17 promotes the degradation of both beta-catenin and TCF proteins via a noncanonical, glycogen synthase kinase 3beta-independent mechanism that can be blocked by proteasome inhibitors. In contrast, Sox4 may function to stabilize beta-catenin protein. These findings indicate that Sox proteins can act as both antagonists and agonists of beta-catenin/TCF activity, and this mechanism may regulate Wnt signaling responses in many developmental and disease contexts.

Functional gene screening system identified TRPV4 as a regulator of chondrogenic differentiation

Sox9 is a transcription factor that is essential for chondrocyte differentiation and chondrocyte-specific gene expression. However, the precise mechanism of Sox9 activation during chondrogenesis is not fully understood. To investigate this mechanism, we performed functional gene screening to identify genes that activate SOX9-dependent transcription, using full-length cDNA libraries generated from a murine chondrogenic cell line, ATDC5. Screening revealed that TRPV4 (transient receptor potential vanilloid 4), a cation channel molecule, significantly elevates SOX9-dependent reporter activity. Microarray and quantitative real time PCR analyses demonstrated that during chondrogenesis in ATDC5 and C3H10T1/2 (a murine mesenchymal stem cell line), the expression pattern of TRPV4 was similar to the expression patterns of chondrogenic marker genes, such as type II collagen and aggrecan. Activation of TRPV4 by a pharmacological activator induced SOX9-dependent reporter activity, and this effect was abolished by the addition of the TRPV antagonist ruthenium red or by using a small interfering RNA for TRPV4. The SOX9-dependent reporter activity due to TRPV4 activation was abrogated by both EGTA and a calmodulin inhibitor, suggesting that the Ca2+/calmodulin signal is essential in this process. Furthermore, activation of TRPV4 in concert with insulin activity in ATDC5 cells or in concert with bone morphogenetic protein-2 in C3H10T1/2 cells promoted synthesis of sulfated glycosaminoglycan, but activation of TRPV4 had no effect alone. We showed that activation of TRPV4 increased the steady-state levels of SOX9 mRNA and protein and SOX6 mRNA. Taken together, our results suggest that TRPV4 regulates the SOX9 pathway and contributes to the process of chondrogenesis.

SOX9 is required for the differentiation of paneth cells in the intestinal epithelium

BACKGROUND AND AIMS

The transcription factor SOX9 has been shown previously to have an essential role in the differentiation of a small number of discrete cell lineages. In the intestine, Sox9 is expressed in the epithelial cells of the crypts and is a target of Wnt signaling.

METHODS

To examine the function of SOX9 in the intestine, we inactivated the Sox9 gene in intestinal epithelial cells by generating mice that harbored a conditional Sox9 gene and a Villin-Cre transgene.

RESULTS

In the absence of SOX9, Paneth cells were not formed, but the differentiation of other intestinal epithelial cell types was unaffected. The lack of SOX9 also lead to crypt enlargement, to a marked increase in cell proliferation throughout the crypts, and to replacement of the Paneth cells by proliferating epithelial cells.

CONCLUSIONS

We conclude that SOX9 is required for the differentiation of Paneth cells. Our results elucidate an essential step in the differentiation of gut epithelium.

SRY and the standoff in sex determination

SRY was identified as the mammalian sex-determining gene more than 15 yr ago and has been extensively studied since. Although many of the pathways regulating sexual differentiation have been elucidated, direct downstream targets of SRY are still unclear, making a top down approach difficult. However, recent work has demonstrated that the fate of the gonad is actively contested by both male-promoting and female-promoting signals. Sox9 and Fgf9 push gonads towards testis differentiation. These two genes are opposed by Wnt4, and possibly RSPO1, which push gonads toward ovary differentiation. In this review, we will discuss the history of the field, current findings, and exciting new directions in vertebrate sex determination.

The Drosophila HMG-domain proteins SoxNeuro and Dichaete direct trichome formation via the activation of shavenbaby and the restriction of Wingless pathway activity

Trichomes are cytoplasmic extrusions of epidermal cells. The molecular mechanisms that govern the differentiation of trichome-producing cells are conserved across species as distantly related as mice and flies. Several signaling pathways converge onto the regulation of a conserved target gene, shavenbaby (svb, ovo), which, in turn, stimulates trichome formation. The Drosophila ventral epidermis consists of the segmental alternation of two cell types that produce either naked cuticle or trichomes called denticles. The binary choice to produce naked cuticle or denticles is affected by the transcriptional regulation of svb, which is sufficient to cell-autonomously direct denticle formation. The expression of svb is regulated by the opposing gradients of two signaling molecules--the epidermal growth factor receptor (Egfr) ligand Spitz (Spi), which activates svb expression, and Wingless (Wg), which represses it. It has remained unclear how these opposing signals are integrated to establish a distinct domain of svb expression. We show that the expression of the high mobility group (HMG)-domain protein SoxNeuro (SoxN) is activated by Spi, and repressed by Wg, signaling. SoxN is necessary and sufficient to cell-autonomously direct the expression of svb. The closely related protein Dichaete is co-regulated with SoxN and has a partially redundant function in the activation of svb expression. In addition, we show that SoxN and Dichaete function upstream of Wg and antagonize Wg pathway activity. This suggests that the expression of svb in a discreet domain is resolved at the level of SoxN and Dichaete.

SOX6 Suppresses Cyclin D1 Promoter Activity by Interacting with β-Catenin and Histone Deacetylase 1, and Its Down-regulation Induces Pancreatic β-Cell Proliferation

Sex-determining region Y-box (SOX) 6 negatively regulates glucose-stimulated insulin secretion from beta-cells and is a down-regulated transcription factor in the pancreatic islet cells of hyperinsulinemic obese mice. To determine the contribution of SOX6 to insulin resistance, we analyzed the effects of SOX6 on cell proliferation. Small interfering RNA-mediated attenuation of SOX6 expression stimulated the proliferation of insulinoma INS-1E and NIH-3T3 cells, whereas retroviral overexpression resulted in inhibition of cell growth. Quantitative real time-PCR analysis revealed that the levels of cyclin D1 transcripts were markedly decreased by SOX6 overexpression. Luciferase-reporter assay with beta-catenin showed that SOX6 suppresses cyclin D1 promoter activities. In vitro binding experiments showed that the LZ/Q domain of SOX6 physically interacts with armadillo repeats 1-4 of beta-catenin. Furthermore, chromatin immunoprecipitation assay revealed that increased SOX6 expression significantly reduced the levels of acetylated histones H3 and H4 at the cyclin D1 promoter. By using a histone deacetylase (HDAC) inhibitor and co-immunoprecipitation analysis, we showed that SOX6 suppressed cyclin D1 activities by interacting withbeta-catenin and HDAC1. The data presented suggest that SOX6 may be an important factor in obesity-related insulin resistance.

Wnt gene expression in the post-natal growth plate: regulation with chondrocyte differentiation

Longitudinal growth of long bones occurs at the growth plate by endochondral ossification. In the embryonic mouse, this process is regulated by Wnt signaling. Little is known about which members of the Wnt family of secreted signaling proteins might be involved in the regulation of the postnatal growth plate. We used microdissection and real-time PCR to study mRNA expression of Wnt genes in the mouse growth plate. Of the 19 known members of the Wnt family, only six were expressed in postnatal growth plate. Of these, Wnts -2b, -4, and -10b signal through the canonical beta-catenin pathway and Wnts -5a, -5b, and -11 signal through the noncanonical calcium pathway. The spatial expression for these six Wnts was remarkably similar, showing low mRNA expression in the resting zone, increasing expression as the chondrocytes differentiated into the proliferative and prehypertrophic state and then (except Wnt-2b) decreasing expression as the chondrocytes underwent hypertrophic differentiation. This overall pattern is broadly consistent with previous studies of embryonic mouse growth cartilage suggesting that Wnt signaling modulates chondrocyte proliferation and hypertrophic differentiation. We also found that mRNA expression of these Wnt genes persisted at similar levels at 4 weeks, when longitudinal bone growth is waning. In conclusion, we have identified for the first time the specific Wnt genes that are expressed in the postnatal mammalian growth plate. The six identified Wnt genes showed a similar pattern of expression during chondrocyte differentiation, suggesting overlapping or interacting roles in postnatal endochondral bone formation.

Immunohistochemical localization of cadherin and catenin adhesion molecules in the murine growth plate

Mouse tibial growth plates were examined for the presence of adhesion molecules using immunohistochemistry and RT-PCR. All of the components of the classical cadherin/catenin complex (cadherin, alpha-, beta-, and gamma-catenin), as well as a heavy presence of p120, were identified in the murine growth plate. All of the major cadherins (1-5, 11, 13, and 15) were, for the first time, identified and localized in the murine growth plate. We have demonstrated that most of the cadherins and catenins reside in the zone of hypertrophy. Only alpha-catenin and E-, P-, R-, and VE-cadherin were found in all regions of the growth plate. The results for T-cadherin were inconclusive.

Shox2 is required for chondrocyte proliferation and maturation in proximal limb skeleton

Mutations in the short stature homeobox gene SHOX lead to growth retardation associated with Turner, Leri-Weill dyschondrosteosis, and Langer mesomelic dysplasia syndromes, which marked the shortening of the forearms and lower legs. We report here that in contrast to the SHOX mutations in humans, Shox2 deficiency in mice leads to a virtual elimination of the stylopod in the developing limbs, while the zeugopod and autopod appear relatively normal. This phenotype is consistent with the restriction of the Shox2 expression to the proximal mesenchyme in the limb bud and later to chondrocytes associated with the forming stylopod. In the Shox2(-/-) embryo, the mesenchymal condensation for the stylopod initiates normally but the cartilaginous element subsequently fails in growth, chondrogenesis and endochondral ossification. A dramatic down-regulation of Runx2 and Runx3 could account for the lack of chondrocyte hypertrophy, while a down-regulation of Ihh expression may be responsible for a significant reduction in chondrocyte proliferation in the mutant stylopod. We further demonstrate that an enhanced and ectopic Bmp4 expression in the proximal limb of the Shox2 embryo may underlie the down-regulation of Runx2, as ectopically applied exogenous BMP4 represses Runx2 expression in the early limb bud. Moreover, we show that mouse Shox2, similar to human SHOX, can perform opposite roles on gene expression: either as a transcription activator or a repressor in different cell types. Our results establish a key role for Shox2 in regulating the growth of stylopod by controlling chondrocyte maturation via Runx2 and Runx3.

Generation of a transgenic mouse model with chondrocyte-specific and tamoxifen-inducible expression of Cre recombinase

Postnatal cartilage development and growth are regulated by key growth factors and signaling molecules. To fully understand the function of these regulators, an inducible and chondrocyte-specific gene deletion system needs to be established to circumvent the perinatal lethality. In this report, we have generated a transgenic mouse model (Col2a1-CreER(T2)) in which expression of the Cre recombinase is driven by the chondrocyte-specific col2a1 promoter in a tamoxifen-inducible manner. To determine the specificity and efficiency of the Cre recombination, we have bred Col2a1-CreER(T2) mice with Rosa26R reporter mice. The X-Gal staining showed that the Cre recombination is specifically achieved in cartilage tissues with tamoxifen-induction. In vitro experiments of chondrocyte cell culture also demonstrate the 4-hydroxy tamoxifen-induced Cre recombination. These results demonstrate that Col2a1-CreER(T2) transgenic mice can be used as a valuable tool for an inducible and chondrocyte-specific gene deletion approach.

Sox17 is essential for the specification of cardiac mesoderm in embryonic stem cells

Early steps for cardiac specification are problematic for the study of mammalian embryos, which has favored using pluripotent cells that recapitulate cardiac myogenesis. Furthermore, circuits governing cardiac specification have relevance to the application of ES cells and other cells for heart repair. In mouse teratocarcinoma cells, canonical Wnts that inhibit heart formation in avian or amphibian embryos and explants activate cardiogenesis, paradoxically. Here, we show that the Wnt/beta-catenin pathway also is essential for cardiac myogenesis to occur in ES cells, acting at a gastrulation-like stage, mediating mesoderm formation and patterning (two prerequisites for cardiac myogenesis itself). Among genes associated temporally with this step was Sox17, encoding an endodermal HMG-box transcription factor. Using lentiviral vectors for RNA interference in differentiating ES cells, an essential role for Sox17 was proven in cardiac muscle cell formation. Sox17 short-hairpin RNA suppresses cardiac myogenesis selectively, acting subsequent to mesoderm formation yet before induction of Mesp1 and Mesp2, a pair of related basic helix-loop-helix transcription factors that together are indispensable for creating heart mesoderm. Sox17 short-hairpin RNA blocks cardiac myogenesis non-cell autonomously and impairs the induction of Hex, a homeodomain transcription factor that is known to be required for the production of endoderm-derived heart-inducing factors.

Multiple roles for neurofibromin in skeletal development and growth

Neurofibromatosis type 1 (NF1) is a prevalent genetic disorder primarily characterized by the formation of neurofibromas, café-au-lait spots and freckling. Skeletal abnormalities such as short stature or bowing/pseudarthrosis of the tibia are relatively common. To investigate the role of the neurofibromin in skeletal development, we crossed Nf1flox mice with Prx1Cre mice to inactivate Nf1 in undifferentiated mesenchymal cells of the developing limbs. Similar to NF1 affected individuals, Nf1(Prx1) mice show bowing of the tibia and diminished growth. Tibial bowing is caused by decreased stability of the cortical bone due to a high degree of porosity, decreased stiffness and reduction in the mineral content as well as hyperosteoidosis. Accordingly, osteoblasts show an increase in proliferation and a decreased ability to differentiate and mineralize in vitro. The reduction in growth is due to lower proliferation rates and a differentiation defect of chondrocytes. Abnormal vascularization of skeletal tissues is likely to contribute to this pathology as it exerts a negative effect on cortical bone stability. Furthermore, Nf1 has an important role in the development of joints, as shown by fusion of the hip joints and other joint abnormalities, which are not observed in neurofibromatosis type I. Thus, neurofibromin has multiple essential roles in skeletal development and growth.

Activation of WNT and BMP signaling in adult human articular cartilage following mechanical injury

Acute full thickness joint surface defects can undergo repair, which involves tissue patterning and endochondral bone formation. Molecular signals regulating this process may contribute to the repair outcome, chronic evolution and, eventually, the onset of osteoarthritis. We tested the hypothesis that mechanical injury modulates morphogenetic pathways in adult human articular cartilage explants. Adjacent articular cartilage explants were obtained from preserved areas of the femoral condyles of patients undergoing arthroplasty for osteoarthritis, or from a normal joint of a patient undergoing lower limb amputation. Paired explants were individually maintained in explant culture. From each pair, one explant was mechanically injured and the other left uninjured as a control. Cultures were terminated at different time points for histochemistry, immunohistochemistry and gene expression analysis by reverse transcription real time PCR. Bone morphogenetic protein 2 (BMP-2) mRNA was upregulated in the injured explants. We detected phosphorylation of SMAD-1 and SMAD-5, consistent with activation of the bone morphogenetic protein (BMP) pathway. FRZB-1 mRNA was downregulated in the injured explants, suggesting de-repression of WNT signaling. Accordingly, expression of the canonical WNT target genes Axin-2 and c-JUN was upregulated in the injured explants. Activation of the canonical WNT signaling pathway by LiCl treatment induced upregulation of COL2A1 and Aggrecan mRNA, suggesting an anabolic effect. Phosphorylation of SMAD-1/-5 and downregulation of FRZB were confirmed in vivo in a mouse model of joint surface injury. Taken together, these data show modulation of the BMP and WNT pathways following mechanical injury in vitro and in vivo, which may play a role in the reparative response of the joint surface. These pathways may, therefore, represent potential targets in protocols of biological joint surface defect repair.

Sry and the hesitant beginnings of male development

In mammals, Sry (sex-determining region Y gene) is the master regulator of male sex determination. The discovery of Sry in 1990 was expected to provide the key to unravelling the network of gene regulation underlying testis development. Intriguingly, no target gene of SRY protein has yet been discovered, and the mechanisms by which it mediates its developmental functions are still elusive. What is clear is that instead of the robust gene one might expect as the pillar of male sexual development, Sry function hangs by a thin thread, a situation that has profound biological, medical and evolutionary implications.

Alteration of matrix glycosaminoglycans diminishes articular chondrocytes' response to a canonical Wnt signal

OBJECTIVE

Although Wnt signaling is a key regulator of the chondrocyte life cycle during embryonic development, little is known about Wnt activity in articular cartilage. Recent studies have suggested an association between excess signaling through the canonical Wnt pathway and osteoarthritis (OA). Genetic and in vitro studies with Drosophila have shown that signaling by the orthologous protein, Wingless (Wg), is regulated by glycosaminoglycans (GAGs) found at the cell surface. The objective of this study was to determine whether alteration in GAG sulfation or matrix content, such as that occurs in OA cartilage, would affect articular chondrocytes' response to a canonical Wnt stimulus.

METHODS

Cells were isolated from shoulder joints of young calves (bovine articular chondrocytes, bACs) and from human cartilage (human articular chondrocytes, hACs) discarded during total knee replacement for OA. Conditioned media from a cell line that is stably transfected with Wnt3a was used as a source of Wnt protein that activates the canonical signaling pathway. Conditioned media from the parental cell line was used as a control. beta-catenin levels were measured by immunoblot. In some experiments, chondrocyte cultures were treated with sodium chlorate (NaClO3) to inhibit GAG sulfation, or with chondroitinase ABC (ChABC) to digest chondroitin sulfate (CS) in the matrix.

RESULTS

Cultured bACs showed low steady-state levels of beta-catenin that increased upon stimulation with Wnt3a. A decrease in either GAG sulfation or CS content diminished bACs' response to Wnt3a (approximately 40% and 37% of control, respectively). Similar effects on the response to Wnt3a via beta-catenin were observed for cultured hACs with undersulfation of GAGs (16% of control) and decreased CS content (20% of control).

CONCLUSION

This study demonstrates that articular chondrocytes respond to canonical Wnt stimulation, and that reduced sulfation or CS content diminishes that response.

The HMG-box transcription factor SoxNeuro acts with Tcf to control Wg/Wnt signaling activity

Wnt signaling specifies cell fates in many tissues during vertebrate and invertebrate embryogenesis. To understand better how Wnt signaling is regulated during development, we have performed genetic screens to isolate mutations that suppress or enhance mutations in the fly Wnt homolog, wingless (wg). We find that loss-of-function mutations in the neural determinant SoxNeuro (also known as Sox-neuro, SoxN) partially suppress wg mutant pattern defects. SoxN encodes a HMG-box-containing protein related to the vertebrate Sox1, Sox2 and Sox3 proteins, which have been implicated in patterning events in the early mouse embryo. In Drosophila, SoxN has previously been shown to specify neural progenitors in the embryonic central nervous system. Here, we show that SoxN negatively regulates Wg pathway activity in the embryonic epidermis. Loss of SoxN function hyperactivates the Wg pathway, whereas its overexpression represses pathway activity. Epistasis analysis with other components of the Wg pathway places SoxN at the level of the transcription factor Pan (also known as Lef, Tcf) in regulating target gene expression. In human cell culture assays, SoxN represses Tcf-responsive reporter expression, indicating that the fly gene product can interact with mammalian Wnt pathway components. In both flies and in human cells, SoxN repression is potentiated by adding ectopic Tcf, suggesting that SoxN interacts with the repressor form of Tcf to influence Wg/Wnt target gene transcription.

Wnt/beta-catenin signaling interacts differentially with Ihh signaling in controlling endochondral bone and synovial joint formation

Both the Wnt/beta-catenin and Ihh signaling pathways play essential roles in crucial aspects of endochondral ossification: osteoblast differentiation, chondrocyte proliferation and hypertrophy. To understand the genetic interaction between these two signaling pathways, we have inactivated the beta-catenin gene and upregulated Ihh signaling simultaneously in the same cells during endochondral skeletal development using beta-catenin and patched 1 floxed alleles. We uncovered previously unexpected roles of Ihh signaling in synovial joint formation and the essential function of Wnt/beta-catenin signaling in regulating chondrocyte survival. More importantly, we found that Wnt and Ihh signaling interact with each other in distinct ways to control osteoblast differentiation, chondrocyte proliferation, hypertrophy, survival and synovial joint formation in the developing endochondral bone. Beta-catenin is required downstream of Ihh signaling and osterix expression for osteoblast differentiation. But in chondrocyte survival, beta-catenin is required upstream of Ihh signaling to inhibit chondrocyte apoptosis. In addition, Ihh signaling can inhibit chondrocyte hypertrophy and synovial joint formation independently of beta-catenin. However, there is a strong synergistic interaction between Wnt/beta-catenin and Ihh signaling in regulating synovial joint formation.

Q89R polymorphism in the LDL receptor-related protein 5 gene is associated with spinal osteoarthritis in postmenopausal Japanese women

STUDY DESIGN

An association study investigating the genetic etiology for spinal osteoarthritis.

OBJECTIVE

To determine the association of single-nucleotide polymorphism (SNP) causing an amino-acid change (Q89R) in the low-density lipoprotein receptor-related protein 5 (LRP5) coding region with spinal osteoarthritis.

SUMMARY OF BACKGROUND DATA

Wnt/beta-catenin signaling pathway regulates bone density through a Wnt coreceptor LRP5. This pathway is also involved in cartilage development and homeostasis, suggesting that genetic variation in LRP5 gene may affect the pathogenesis of cartilage-related diseases, such as osteoarthritis.

METHODS

We evaluated the presence of osteophytes, endplate sclerosis, and narrowing of disc spaces in 357 Japanese postmenopausal women. Missense coding SNP for Q89R of LRP5 gene was determined using TaqMan polymerase chain reaction (PCR) method.

RESULTS

We found that subjects without the R allele (QQ; n = 321) had a significantly lower osteophyte formation score than did subjects bearing at least one R allele (QR + RR; n = 36) (7.80 vs. 10.89, P = 0.0019 by analysis of covariance).

CONCLUSIONS

We suggest that a genetic variation at the LRP5 gene locus is associated with spinal osteoarthritis, in line with the involvement of the LRP5 gene in the bone and cartilage metabolism.

Wnt4 action in gonadal development and sex determination

Wnt4 is a growth factor involved in multiple developmental processes such as the formation of the kidney, adrenal, mammary gland, pituitary and the female reproductive system. During mammalian embryogenesis, Wnt4 is expressed in the gonads of both sexes before sex determination events take place and is subsequently down-regulated in the male gonad. Inactivation of the Wnt4 gene in mice has revealed that it is involved at several steps of female reproductive development. Wnt4 is implicated in Müllerian duct regression, the formation of sex-specific vasculature, the inhibition of steroidogenesis and in sex-specific cell migration events. A mouse model of sex-reversal has partially unravelled the molecular pathways in which Wnt4 operates during the development of the female reproductive system. However, the specific molecular mechanism of action of Wnt4 during gonadal development remains unknown. This and downstream signaling pathways involved in Wnt4 action during female gonad development are reviewed and models of Wnt4 action are proposed for Müllerian duct formation, sex-specific vasculature development, and sex determination events. Further identification of critical downstream effectors of the Wnt4 signaling pathway in mouse models and in patients with sex-reversal conditions could help in understanding sex-reversal pathologies in humans.

Frontal nasal prominence expression driven by Tcfap2a relies on a conserved binding site for STAT proteins

The AP-2 transcription factor family is linked with development of the head and limbs in both vertebrate and invertebrate species. Recent evidence has also implicated this gene family in the evolution of the neural crest in chordates, a critical step that allowed the development and elaboration of the vertebrate craniofacial skeleton. In mice, the inappropriate embryonic expression of one particular AP-2 gene, Tcfap2a, encoding AP-2alpha, results in multiple developmental abnormalities, including craniofacial and limb defects. Thus, Tcfap2a provides a valuable genetic resource to analyze the regulatory hierarchy responsible for the evolution and development of the face and limbs. Previous studies have identified a 2-kilobase intronic region of both the mouse and human AP-2alpha locus that directs expression of a linked LacZ transgene to the facial processes and the distal mesenchyme of the limb bud in transgenic mice. Further analysis identified two highly conserved regions of approximately 200-400 bp within this tissue-specific enhancer. We have now initiated a transgenic and biochemical analysis of the most important of these highly conserved regions. Our analysis indicates that although the sequences regulating face and limb expression have been integrated into a single enhancer, different cis-acting sequences ultimately control these two expression domains. Moreover, these studies demonstrate that a conserved STAT binding site provides a major contribution to the expression of Tcfap2a in the facial prominences.

Misexpression of Sox9 in mouse limb bud mesenchyme induces polydactyly and rescues hypodactyly mice

Our previous studies have demonstrated the essential roles of the transcription factor Sox9 in the commitment of mesenchymal cells to a chondrogenic cell lineage and in overt chondrogenesis during limb bud development. However, it remains unknown if Sox9 induces chondrogenesis in mesenchyme ectopically in vivo as a master regulator of chondrogenesis. In this study, we first generated mutant mice in which Sox9 was misexpressed in the limb bud mesenchyme. The mutant mouse embryos exhibited polydactyly in limb buds in association with ectopic expression of Sox5 and Sox6 although markers for the different axes of limb bud development showed a normal pattern of expression. Misexpression of Sox9 stimulated cell proliferation in limb bud mesenchyme, suggesting that Sox9 has a role in recruiting mesenchymal cells to mesenchymal condensation. Second, despite the facts that misexpression of Sonic hedgehog (Shh) induces polydactyly in a number of mutant mice and Shh-null mutants have severely defective cartilage elements in limb buds, misexpression of Sox9 did not restore limb bud phenotypes in Shh-null mutants. Rather, there was no expression of Sox9 in digit I of Hoxa13Hd mutant embryos, and Sox9 partially rescued hypodactyly in Hoxa13Hd mutant embryos. These results provide evidence that Sox9 induces ectopic chondrogenesis in mesenchymal cells and strongly suggest that its expression may be regulated by Hox genes during limb bud development.

Dominance of SOX9 function over RUNX2 during skeletogenesis

Mesenchymal stem cell-derived osteochondroprogenitors express two master transcription factors, SOX9 and RUNX2, during condensation of the skeletal anlagen. They are essential for chondrogenesis and osteogenesis, respectively, and their haploinsufficiency causes human skeletal dysplasias. We show that SOX9 directly interacts with RUNX2 and represses its activity via their evolutionarily conserved high-mobility-group and runt domains. Ectopic expression of full-length SOX9 or its RUNX2-interacting domain in mouse osteoblasts results in an osteodysplasia characterized by severe osteopenia and down-regulation of osteoblast differentiation markers. Thus, SOX9 can inhibit RUNX2 function in vivo even in established osteoblastic lineage. Finally, we demonstrate that this dominant inhibitory function of SOX9 is physiologically relevant in human campomelic dysplasia. In campomelic dysplasia, haploinsufficiency of SOX9 results in up-regulation of the RUNX2 transcriptional target COL10A1 as well as all three members of RUNX gene family. In summary, SOX9 is dominant over RUNX2 function in mesenchymal precursors that are destined for a chondrogenic lineage during endochondral ossification.

Balancing the bipotential gonad between alternative organ fates: a new perspective on an old problem

The embryonic gonads give rise to one of two morphologically and functionally different organs, a testis or an ovary. Sex determination is the embryonic process that determines the developmental fate of the gonad. In mammals, sex determination is regulated by a DNA binding protein encoded on the Y chromosome, Sry, and it's downstream mediator, Sox9, which trigger testis determination in the bipotential gonad. However, evidence suggests that the extracellular signals. Fgf9 and Wnt4, are also required to establish divergent organogenesis of the gonad. In this review, we discuss how these extracellular signals interface with cell-autonomous factors to determine the fate of the mammalian gonad, and we derive a model that could provide a molecular explanation for testis determination in vertebrates where Sry is absent.

Igf-I extends the chondrogenic potential of human articular chondrocytes in vitro: Molecular association between Sox9 and Erk1/2

Expansion of articular chondrocytes in monolayer culture leads to loss of the unique chondrocyte phenotype and the cells' redifferentiation capacity. Dedifferentiation of chondrocytes in monolayer culture is a challenging problem for autologous chondrocyte transplantation (ACT). It is well established that Igf-I exerts positive anabolic effects on chondrocytes in vivo and in vitro. Accordingly, in this study, we examined whether the anabolic insulin-like growth factor-I (Igf-I) is capable of extending the chondrogenic potential of dedifferentiated chondrocytes in vitro. Chondrocyte monolayers were cultured up to 10 passages. At each passage chondrocytes were stimulated with Igf-I (10ng/ml) and introduced to high-density cultures for up to 7 days. Expression of collagen type II, cartilage-specific proteoglycans, activated caspase-3, integrin beta1, extracellular signal-regulated kinase (Erk) and Sox9 was examined by Western blotting, immunoprecipitation and immunomorphological techniques. Monolayer chondrocytes rapidly lost their differentiated phenotype. When introduced to high-density cultures, only chondrocytes from P1-P4 redifferentiated. In contrast, Igf-I treated cells from P1 up to P7 redifferentiated and formed cartilage-like tissue in high-density culture. P8-P10 cells exhibited apoptotic alterations and produced significantly less matrix. Igf-I markedly increased expression of integrin beta1, Erk and Sox9. Immunoprecipitation revealed that phosphorylated Erk1/2 physically interacts with Sox9 in chondrocyte nuclei, suggesting a previously unreported functional association which was markedly enhanced by Igf-I. Treatment of chondrocyte cultures with Igf-I stabilizes chondrogenic potential, stimulates Sox9 and promotes molecular interactions between Erk and Sox9. These effects appear to be regulated by the integrin/MAPK signaling pathways.

Craniosynostosis caused by Axin2 deficiency is mediated through distinct functions of beta-catenin in proliferation and differentiation

Targeted disruption of Axin2 in mice induces skeletal defects, a phenotype resembling craniosynostosis in humans. Premature fusion of cranial sutures, caused by deficiency in intramembranous ossification, occurs at early postnatal stages. Axin2 negatively regulates both expansion of osteoprogenitors and maturation of osteoblasts through its modulation on Wnt/beta-catenin signaling. We investigate the dual role of beta-catenin to gain further insights into the skull morphogenetic circuitry. We show that as a transcriptional co-activator, beta-catenin promotes cell division by stimulating its target cyclin D1 in osteoprogenitors. Upon differentiation of osteoprogenitors, BMP signaling is elevated to accelerate the process in a positive feedback mechanism. This Wnt-dependent BMP signal dictates cellular distribution of beta-catenin. As an adhesion molecule, beta-catenin promotes cell-cell interaction mediated by adherens junctions in mature osteoblasts. Finally, haploid deficiency of beta-catenin alleviates the Axin2-null skeletal phenotypes. These findings support a model for disparate roles of beta-catenin in osteoblast proliferation and differentiation.

Regulation of SOX9 mRNA in human articular chondrocytes involving p38 MAPK activation and mRNA stabilization

Human articular chondrocytes rapidly lose their phenotype in monolayer culture. Recently we have shown that overexpression of the transcription factor SOX9 greatly enhanced re-expression of the phenotype in three-dimensional aggregate cultures. Here we show that endogenous SOX9 mRNA can be rapidly up-regulated in subcultured human articular chondrocytes if grown in alginate, in monolayer with cytochalasin D, or with specific inhibition of the RhoA effector kinases ROCK1 and -2, which all prevent actin stress fiber formation. Disruption of actin stress fibers using any of these redifferentiation stimuli also supported the superinduction of SOX9 by cycloheximide. The superinduction was blocked by inhibitors of the p38 MAPK signaling pathway and involved the stabilization of SOX9 mRNA. Furthermore stimulation of chondrocyte p38 MAPK activity with interleukin-1beta resulted in increased levels of SOX9 mRNA, and this was again dependent on the absence of actin stress fibers in the cells. In this study of chondrocyte redifferentiation we have provided further evidence of the early involvement of SOX9 and have discovered a novel post-transcriptional regulatory mechanism activated by p38 MAPK, which stabilized SOX9 mRNA.

The role of TGFbetas and Sox9 during limb chondrogenesis

The majority of the skeletal elements, except the flat bones of the skull, are formed by endochondral ossification, in which cartilage is replaced by bone. The formation of cartilage is a multi-step process termed chondrogenesis, during which undifferentiated mesenchymal cells condense and undergo differentiation towards chondrocytes. Notwithstanding recent advances, our knowledge of the detailed mechanisms implicated in cartilage and bone formation is still scarce. Recent genetic, cellular and biochemical studies have highlighted the importance of TGFbeta signaling and the activity of the transcription factor Sox9 during the early stages of vertebrate limb chondrogenesis.

R-spondin1 is essential in sex determination, skin differentiation and malignancy

R-spondins are a recently characterized small family of growth factors. Here we show that human R-spondin1 (RSPO1) is the gene disrupted in a recessive syndrome characterized by XX sex reversal, palmoplantar hyperkeratosis and predisposition to squamous cell carcinoma of the skin. Our data show, for the first time, that disruption of a single gene can lead to complete female-to-male sex reversal in the absence of the testis-determining gene, SRY.

Involvement of cyclic guanosine monophosphate-dependent protein kinase II in chondrocyte hypertrophy during endochondral ossification

During vertebrate skeletal development, the appendicular skeleton forms through endochondral ossification, which involves the intricately regulated multistep differentiation of mesenchymal cells. During this process, mesenchymal condensations initially differentiate into chondrocytes. Then chondrocytes in the center further differentiate into hypertrophic chondrocytes. Hypertrophic chondrocytes express a number of osteogenic factors and induce bone formation. Although numerous studies have provided novel insights into the regulation and function of cartilage development, little is known about the intracellular signaling pathways regulating chondrocyte hypertrophy. Recent study revealed that cyclic guanosine monophosphate (cGMP)-dependent protein kinase II (cGKII) coupled the stop of proliferation and the start of hypertrophic differentiation of chondrocytes. Herein, we review the molecular mechanism of regulation of chondrocyte hypertrophy by cGKII and the interaction between cGKII and other signaling pathways.

The transcription factor ATF3 is upregulated during chondrocyte differentiation and represses cyclin D1 and A gene transcription

Background

Coordinated chondrocyte proliferation and differentiation are required for normal endochondral bone growth. Transcription factors binding to the cyclicAMP response element (CRE) are known to regulate these processes. One member of this family, Activating Tanscription Factor 3 (ATF3), is expressed during skeletogenesis and acts as a transcriptional repressor, but the function of this protein in chondrogenesis is unknown.

Results

Here we demonstrate that Atf3 mRNA levels increase during mouse chondrocyte differentiation in vitro and in vivo. In addition, Atf3 mRNA levels are increased in response to cytochalasin D treatment, an inducer of chondrocyte maturation. This is accompanied by increased Atf3 promoter activity in cytochalasin D-treated chondrocytes. We had shown earlier that transcription of the cell cycle genes cyclin D1 and cyclin A in chondrocytes is dependent on CREs. Here we demonstrate that overexpression of ATF3 in primary mouse chondrocytes results in reduced transcription of both genes, as well as decreased activity of a CRE reporter plasmid. Repression of cyclin A transcription by ATF3 required the CRE in the cyclin A promoter. In parallel, ATF3 overexpression reduces the activity of a SOX9-dependent promoter and increases the activity of a RUNX2-dependent promoter.

Conclusion

Our data suggest that transcriptional induction of the Atf3 gene in maturing chondrocytes results in down-regulation of cyclin D1 and cyclin A expression as well as activation of RUNX2-dependent transcription. Therefore, ATF3 induction appears to facilitate cell cycle exit and terminal differentiation of chondrocytes.

Beta-catenin is essential for lamination but not neurogenesis in mouse retinal development

During vertebrate retinal development, the seven retinal cell types differentiate sequentially from a single population of retinal progenitor cells (RPCs) and organize themselves into a distinct laminar structure. The purpose of this study was to determine whether beta-catenin, which functions both as a nuclear effector for the canonical Wnt signaling pathway and as a regulator of cell adhesion, is required for retinal neurogenesis or lamination. We used the Cre-loxP system to either eliminate beta-catenin or to express a constitutively active form during retinal neurogenesis. Eliminating beta-catenin did not affect cell differentiation, but did result in the loss of the radial arrangement of RPCs and caused abnormal migration of differentiated neurons. As a result, the laminar structure was massively disrupted in beta-catenin-null retinas, although all retinal cell types still formed. In contrast to other neural tissues, eliminating beta-catenin did not significantly reduce the proliferation rate of RPCs; likewise, activating beta-catenin ectopically in RPCs did not result in overproliferation, but loss of neural retinal identity. These results indicate that beta-catenin is essential during retinal neurogenesis as a regulator of cell adhesion but not as a nuclear effector of the canonical Wnt signaling pathway. The results further imply that retinal lamination and retinal cell differentiation are genetically separable processes.

Zebrafish Trap230/Med12 is required as a coactivator for Sox9-dependent neural crest, cartilage and ear development

The vertebrate Sox9 transcription factor directs the development of neural crest, otic placodes, cartilage and bone. In zebrafish, there are two Sox9 orthologs, Sox9a and Sox9b, which together perform the functions of the single-copy tetrapod Sox9. In a large-scale genetic screen, we have identified a novel zebrafish mutant that strongly resembles the Sox9a/Sox9b double mutant phenotype. We show that this mutation disrupts the zebrafish Trap230/Med12 ortholog, a member of the Mediator complex. Mediator is a coactivator complex transducing the interaction of DNA-binding transcription factors with RNA polymerase II, and our results reveal a critical function of the Trap230 subunit as a coactivator for Sox9.

Distinct roles for Hedgehog and canonical Wnt signaling in specification, differentiation and maintenance of osteoblast progenitors

Hedgehog and canonical Wnt/beta-catenin signaling are implicated in development of the osteoblast, the bone matrix-secreting cell of the vertebrate skeleton. We have used genetic approaches to dissect the roles of these pathways in specification of the osteoblast lineage. Previous studies indicate that Ihh signaling in the long bones is essential for initial specification of an osteoblast progenitor to a Runx2+ osteoblast precursor. We show here that this is a transient requirement, as removal of Hh responsiveness in later Runx2+, Osx1+ osteoblast precursors does not disrupt the formation of mature osteoblasts. By contrast, the removal of canonical Wnt signaling by conditional removal of the beta-catenin gene in early osteoblast progenitors or in Runx2+, Osx1+ osteoblast precursors results in a similar phenotype: osteoblasts fail to progress to a terminal osteocalcin+ fate and instead convert to a chondrocyte fate. By contrast, stabilization of beta-catenin signaling in Runx2+, Osx1+ osteoblast precursors leads to the premature differentiation of bone matrix secreting osteoblasts. These data demonstrate that commitment within the osteoblast lineage requires sequential, stage-specific, Ihh and canonical Wnt/beta-catenin signaling to promote osteogenic, and block chondrogenic, programs of cell fate specification.

Wnt9a signaling is required for joint integrity and regulation of Ihh during chondrogenesis

Joints, which separate skeleton elements, serve as important signaling centers that regulate the growth of adjacent cartilage elements by controlling proliferation and maturation of chondrocytes. Accurate chondrocyte maturation is crucial for endochondral ossification and for the ultimate size of skeletal elements, as premature or delayed maturation results predominantly in shortened elements. Wnt9a has previously been implicated as being a player in joint induction, based on gain-of function experiments in chicken and mouse. We show that loss of Wnt9a does not affect joint induction, but results to synovial chondroid metaplasia in some joints. This phenotype can be enhanced by removal of an additional Wnt gene, Wnt4, suggesting that Wnts are playing a crucial role in directing bi-potential chondro-synovioprogenitors to become synovial connective tissue, by actively suppressing their chondrogenic potential. Furthermore, we show that Wnt9a is a temporal and spatial regulator of Indian hedgehog (Ihh), a central player of skeletogenesis. Loss of Wnt9a activity results in transient downregulation of Ihh and reduced Ihh-signaling activity at E12.5-E13.5. The canonical Wnt/beta-catenin pathway probably mediates regulation of Ihh expression in prehypertrophic chondrocytes by Wnt9a, because embryos double-heterozygous for Wnt9a and beta-catenin show reduced Ihh expression, and in vivo chromatin immunoprecipitation demonstrates a direct interaction between the beta-catenin/Lef1 complex and the Ihh promoter.

Wnt signaling activation during bone regeneration and the role of Dishevelled in chondrocyte proliferation and differentiation

Wnt signaling is intrinsically involved in diverse cellular activities during cell differentiation, early embryonic development and organogenesis. Although much is known regarding the effects of Wnt signaling in the developing skeletal system, its role during regeneration remains unclear. Herein, we show transcriptional activation of specific members and target genes of the Wnt signaling pathway. Specifically, all of the Wnt signaling members and target genes analyzed were found to be upregulated during the early stages of fracture repair, with the exception of LEF1 whose expression was downregulated. In addition, spatial expression analysis of Dishevelled (Dvl) and beta-catenin in the fracture callus revealed an identical pattern of expression with both proteins localizing in osteoprogenitor cells of the periosteum, osteoblasts and proliferating/pre-hypertrophic chondrocytes. Further, in vitro knockdown of all three Dvl isoforms in chondrocytes using small interfering RNAs (siRNA) leads to partial inhibition of cell proliferation and differentiation, decreased expression of chondrogenic markers (ColII, ColX, Sox9) and suppressed nuclear accumulation of unphosphorylated beta-catenin. Taken together, these data verify our previous finding that the Wnt signaling pathway is activated during bone regeneration, by characterizing the temporal and spatial expression of a broad spectrum of Wnt-signaling molecules. Our data also suggest that all three Dvl isoforms, acting through the Wnt canonical pathway, are critical regulatory molecules for chondrocyte proliferation and differentiation.

Wdr5, a WD-40 protein, regulates osteoblast differentiation during embryonic bone development

Wdr5 accelerates osteoblast and chondrocyte differentiation in vitro, and is developmentally expressed in osteoblasts as well as in proliferating and hypertrophic chondrocytes. To investigate the role of Wdr5 during endochondral bone development, transgenic mice overexpressing Wdr5 under the control of the 2.3-kb fragment of the mouse alpha(1) I collagen promoter were generated. The transgene was specifically expressed in the osteoblasts of transgene positive mice and was absent in the growth plate. Histological analyses at embryonic day 14.5 demonstrated that the humeri of transgene positive embryos were longer than those isolated from wild-type littermates largely due to an expansion of the hypertrophic chondrocyte layer. Acceleration of osteoblast differentiation was observed with greater and more extensive expression of type I collagen and more extensive mineral deposition in the bone collar of transgene positive embryos. Acceleration of vascular invasion was also observed in transgene positive mice. Postnatal analyses of transgenic mice confirmed persistent acceleration of osteoblast differentiation. Targeted expression of Wdr5 to osteoblasts resulted in earlier activation of the canonical Wnt signaling pathway in the bone collar as well as in primary calvarial osteoblast cultures. In addition, overexpression of Wdr5 increased the expression of OPG, a target of the canonical Wnt signaling pathway. Overall, our findings suggest that Wdr5 accelerates osteoblast differentiation in association with activation of the canonical Wnt pathway.