The role of Axin2 in calvarial morphogenesis and craniosynostosis

Origin matters: differences in embryonic tissue origin and Wnt signaling determine the osteogenic potential and healing capacity of frontal and parietal calvarial bones

Calvarial bones arise from two embryonic tissues, namely, the neural crest and the mesoderm. In this study we have addressed the important question of whether disparate embryonic tissue origins impart variable osteogenic potential and regenerative capacity to calvarial bones, as well as what the underlying molecular mechanism(s). Thus, by performing in vitro and in vivo studies, we have investigated whether differences exist between neural crest-derived frontal and paraxial mesodermal-derived parietal bone. Of interest, our data indicate that calvarial bone osteoblasts of neural crest origin have superior potential for osteogenic differentiation. Furthermore, neural crest-derived frontal bone displays a superior capacity to undergo osseous healing compared with calvarial bone of paraxial mesoderm origin. Our study identified both in vitro and in vivo enhanced endogenous canonical Wnt signaling in frontal bone compared with parietal bone. In addition, we demonstrate that constitutive activation of canonical Wnt signaling in paraxial mesodermal-derived parietal osteoblasts mimics the osteogenic potential of frontal osteoblasts, whereas knockdown of canonical Wnt signaling dramatically impairs the greater osteogenic potential of neural crest-derived frontal osteoblasts. Moreover, fibroblast growth factor 2 (FGF-2) treatment induces phosphorylation of GSK-3beta and increases the nuclear levels of beta-catenin in osteoblasts, suggesting that enhanced activation of Wnt signaling might be mediated by FGF. Taken together, our data provide compelling evidence that indeed embryonic tissue origin makes a difference and that active canonical Wnt signaling plays a major role in contributing to the superior intrinsic osteogenic potential and tissue regeneration observed in neural crest-derived frontal bone.

Differential activation of canonical Wnt signaling determines cranial sutures fate: a novel mechanism for sagittal suture craniosynostosis

Premature closure of cranial sutures, which serve as growth centers for the skull vault, result in craniosynostosis. In the mouse posterior frontal (PF) suture closes by endochondral ossification, whereas sagittal (SAG) remain patent life time, although both are neural crest tissue derived. We therefore, investigated why cranial sutures of same tissue origin adopt a different fate. We demonstrated that closure of the PF suture is tightly regulated by canonical Wnt signaling, whereas patency of the SAG suture is achieved by constantly activated canonical Wnt signaling. Importantly, the fate of PF and SAG sutures can be reversed by manipulating Wnt signaling. Continuous activation of canonical Wnt signaling in the PF suture inhibits endochondral ossification and therefore, suture closure, In contrast, inhibition of canonical Wnt signaling in the SAG suture, upon treatment with Wnt antagonists results in endochondral ossification and suture closure. Thus, inhibition of canonical Wnt signaling in the SAG suture phenocopies craniosynostosis. Moreover, mice haploinsufficient for Twist1, a target gene of canonical Wnt signaling which inhibits chondrogenesis, have sagittal craniosynostosis. We propose that regulation of canonical Wnt signaling is of crucial importance during the physiological patterning of PF and SAG sutures. Importantly, dysregulation of this pathway may lead to craniosynostosis.

beta-Catenin initiates tooth neogenesis in adult rodent incisors

beta-Catenin signaling is required for embryonic tooth morphogenesis and promotes continuous tooth development when activated in embryos. To determine whether activation of this pathway in the adult oral cavity could promote tooth development, we induced mutation of epithelial beta-catenin to a stabilized form in adult mice. This caused increased proliferation of the incisor tooth cervical loop, outpouching of incisor epithelium, abnormal morphology of the epithelial-mesenchymal junction, and enhanced expression of genes associated with embryonic tooth development. Ectopic dental-like structures were formed from the incisor region following implantation into immunodeficient mice. Thus, forced activation of beta-catenin signaling can initiate an embryonic-like program of tooth development in adult rodent incisor teeth.

Redox regulation of Wnt signalling via nucleoredoxin

Numerous studies indicate that reactive oxygen species (ROS) are not merely cellular by-products of respiration, but are able to modulate various signalling pathways and play certain physiological roles. Recent studies have revealed the importance of translating ROS-generation to activation/suppression of specific signalling pathways. The Wnt signalling pathway, which is essential for early development and stem cell maintenance, is also regulated by ROS. A thioredoxin-related protein, nucleoredoxin (NRX), governs ROS-stimulated Wnt signalling in a temporal manner. NRX usually interacts with Dishevelled (Dvl), an essential adaptor protein for Wnt signalling, and blocks the activation of the Wnt pathway. Oxidative stress causes dissociation of NRX from Dvl, which enables Dvl to activate the downstream Wnt signalling pathway. This study also presents the latest research findings on NRX and its related molecules.

rBMP represses Wnt signaling and influences skeletal progenitor cell fate specification during bone repair

Bone morphogenetic proteins (BMPs) participate in multiple stages of the fetal skeletogenic program from promoting cell condensation to regulating chondrogenesis and bone formation through endochondral ossification. Here, we show that these pleiotropic functions are recapitulated when recombinant BMPs are used to augment skeletal tissue repair. In addition to their well-documented ability to stimulate chondrogenesis in a skeletal injury, we show that recombinant BMPs (rBMPs) simultaneously suppress the differentiation of skeletal progenitor cells in the endosteum and bone marrow cavity to an osteoblast lineage. Both the prochondrogenic and antiosteogenic effects are achieved because rBMP inhibits endogenous beta-catenin-dependent Wnt signaling. In the injured periosteum, this repression of Wnt activity results in sox9 upregulation; consequently, cells in the injured periosteum adopt a chondrogenic fate. In the injured endosteum, rBMP also inhibits Wnt signaling, which results in the runx2 and collagen type I downregulation; consequently, cells in this region fail to differentiate into osteoblasts. In muscle surrounding the skeletal injury site, rBMP treatment induces Smad phosphorylation followed by exuberant cell proliferation, an increase in alkaline phosphatase activity, and chondrogenic differentiation. Thus different populations of adult skeletal progenitor cells interpret the same rBMP stimulus in unique ways, and these responses mirror the pleiotropic effects of BMPs during fetal skeletogenesis. These mechanistic insights may be particularly useful for optimizing the reparative potential of rBMPs while simultaneously minimizing their adverse outcomes.

The balance of WNT and FGF signaling influences mesenchymal stem cell fate during skeletal development

Craniosynostosis, a developmental disorder resulting from premature closure of the gaps (sutures) between skull bones, can be caused by excessive intramembranous ossification, a type of bone formation that does not involve formation of a cartilage template (chondrogenesis). Here, we show that endochondral ossification, a type of bone formation that proceeds through a cartilage intermediate, caused by switching the fate of mesenchymal stem cells to chondrocytes, can also result in craniosynostosis. Simultaneous knockout of Axin2, a negative regulator of the WNT-beta-catenin pathway, and decreased activity of fibroblast growth factor (FGF) receptor 1 (FGFR1) in mice induced ectopic chondrogenesis, leading to abnormal suture morphogenesis and fusion. Genetic analyses revealed that activation of beta-catenin cooperated with FGFR1 to alter the lineage commitment of mesenchymal stem cells to differentiate into chondrocytes, from which cartilage is formed. We showed that the WNT-beta-catenin pathway directly controlled the stem cell population by regulating its renewal and proliferation, and indirectly modulated lineage specification by setting the balance of the FGF and bone morphogenetic protein pathways. This study identifies endochondral ossification as a mechanism of suture closure during development and implicates this process in craniosynostosis.

Crosstalk between Wnt and bone morphogenic protein signaling: a turbulent relationship

The Wnt and the bone morphogenic protein (BMP) pathways are evolutionarily conserved and essentially independent signaling mechanisms, which, however, often regulate similar biological processes. Wnt and BMP signaling are functionally integrated in many biological processes, such as embryonic patterning in Drosophila and vertebrates, formation of kidney, limb, teeth and bones, maintenance of stem cells, and cancer progression. Detailed inspection of regulation in these and other tissues reveals that Wnt and BMP signaling are functionally integrated in four fundamentally different ways. The molecular mechanism evolved to mediate this integration can also be summarized in four different ways. However, a fundamental aspect of functional and mechanistic interaction between these pathways relies on tissue-specific mechanisms, which are often not conserved and cannot be extrapolated to other tissues. Integration of the two pathways contributes toward the sophisticated means necessary for creating the complexity of our bodies and the reliable and healthy function of its tissues and organs.

Manipulating gene activity in Wnt1-expressing precursors of neural epithelial and neural crest cells

Targeted gene disruption or expression often encounters lethality. Conditional approaches, permitting manipulation at desired stages, are required to overcome this problem in order to analyze gene function in later developmental processes. Wnt1 has been shown to be expressed in neural crest precursors at the dorsal midline region. However, its expression was not detected in emigrated neural crest cells, the descendants of Wnt1-expressing precursors. We have developed mouse transgenic systems to manipulate gene activity in the Wnt1-expressing precursors and their derivatives by integrating the tetracycline-dependent activation and Cre-mediated recombination methods. A new Wnt1-rtTA strain, carrying rtTA under control of Wnt1 regulatory elements, has been created for gene manipulation in a spatiotemporal-specific fashion. Together with our previously developed Wnt1-Cre;R26STOPrtTA model, these systems permit conditional gene expression and ablation in pre-migratory and/or post-migratory neural crest cells. This study demonstrated the versatility of our mouse models to achieve gene manipulation in early neural development.

Patterns of Wnt pathway activity in the mouse incisor indicate absence of Wnt/beta-catenin signaling in the epithelial stem cells

The Wnt pathway is crucial for tooth development as shown by dental defects caused by impaired Wnt signaling in mouse and human. We investigated Wnt signaling in continuously growing mouse incisors focusing on epithelial stem cells. Ten Wnt ligands were expressed both in the dental epithelium and mesenchyme, and were associated mainly with odontoblast and ameloblast differentiation. Wnt/beta-catenin activity was detected in mesenchyme in BATgal and TOPgal reporter mice while Axin2, also a reporter of Wnt/beta-catenin signaling, was expressed additionally in the epithelium. Axin2 was, however, excluded from the epithelial stem cells in the cervical loop. Interestingly, these cells expressed specifically Lgr5, a Wnt target gene and stem cell marker in the intestine, suggesting that Lgr5 is a marker of incisor stem cells but is not regulated by Wnt signaling in the incisor. We conclude that epithelial stem cells in the mouse incisors are not regulated directly by Wnt/beta-catenin signaling.

Role of Wnt signaling in the biology of the periodontium

Continuously erupting teeth have associated with them a continuously regenerating periodontal ligament, but the factors that control this amazing regenerative potential are unknown. We used genetic strategies to show that the periodontal ligament arises from the cranial neural crest. Despite their histological similarity, the periodontal ligament of continuously erupting incisor teeth differs dramatically from the periodontal ligament of molar teeth. The most notable difference was in the distribution of Wnt responsive cells in the incisor periodontal ligament, which coincided with regions of periodontal ligament cell proliferation. We discuss these findings in the context of dental tissue regeneration.

A Wnt oscillator model for somitogenesis

We propose a model for the segmentation clock in vertebrate somitogenesis, based on the Wnt signaling pathway. The core of the model is a negative feedback loop centered around the Axin2 protein. Axin2 is activated by beta-catenin, which in turn is degraded by a complex of GSK3beta and Axin2. The model produces oscillatory states of the involved constituents with typical time periods of a few hours (ultradian oscillations). The oscillations are robust to changes in parameter values and are often spiky, where low concentration values of beta-catenin are interrupted by sharp peaks. Necessary for the oscillations is the saturated degradation of Axin2. Somite formation in chick and mouse embryos is controlled by a spatial Wnt gradient which we introduce in the model through a time-dependent decrease in Wnt3a ligand level. We find that the oscillations disappear as the ligand concentration decreases, in agreement with observations on embryos.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Activation of p38 MAPK pathway in the skull abnormalities of Apert syndrome Fgfr2(+P253R) mice

Background

Apert syndrome is characterized by craniosynostosis and limb abnormalities and is primarily caused by FGFR2 +/P253R and +/S252W mutations. The former mutation is present in approximately one third whereas the latter mutation is present in two-thirds of the patients with this condition. We previously reported an inbred transgenic mouse model with the Fgfr2 +/S252W mutation on the C57BL/6J background for Apert syndrome. Here we present a mouse model for the Fgfr2+/P253R mutation.

Results

We generated inbred Fgfr2^+/P253R mice on the same C56BL/6J genetic background and analyzed their skeletal abnormalities. 3D micro-CT scans of the skulls of the Fgfr2^+/P253R mice revealed that the skull length was shortened with the length of the anterior cranial base significantly shorter than that of the Fgfr2^+/S252W mice at P0. The Fgfr2^+/P253R mice presented with synostosis of the coronal suture and proximate fronts with disorganized cellularity in sagittal and lambdoid sutures. Abnormal osteogenesis and proliferation were observed at the developing coronal suture and long bones of the Fgfr2^+/P253R mice as in the Fgfr2^+/S252W mice. Activation of mitogen-activated protein kinases (MAPK) was observed in the Fgfr2^+/P253R neurocranium with an increase in phosphorylated p38 as well as ERK1/2, whereas phosphorylated AKT and PKCα were not obviously changed as compared to those of wild-type controls. There were localized phenotypic and molecular variations among individual embryos with different mutations and among those with the same mutation.

Conclusions

Our in vivo studies demonstrated that the Fgfr2 +/P253R mutation resulted in mice with cranial features that resemble those of the Fgfr2^+/S252W mice and human Apert syndrome. Activated p38 in addition to the ERK1/2 signaling pathways may mediate the mutant neurocranial phenotype. Though Apert syndrome is traditionally thought to be a consistent phenotype, our results suggest localized and regional variations in the phenotypes that characterize Apert syndrome.

Macrophage Wnt7b is critical for kidney repair and regeneration

Macrophages are required for tissue homeostasis through their role in regulation of the immune response and the resolution of injury. Here we show, using the kidney as a model, that the Wnt pathway ligand Wnt7b is produced by macrophages to stimulate repair and regeneration. When macrophages are inducibly ablated from the injured kidney, the canonical Wnt pathway response in kidney epithelial cells is reduced. Furthermore, when Wnt7b is somatically deleted in macrophages, repair of injury is greatly diminished. Finally, injection of the Wnt pathway regulator Dkk2 enhances the repair process and suggests a therapeutic option. Because Wnt7b is known to stimulate epithelial responses during kidney development, these findings suggest that macrophages are able to rapidly invade an injured tissue and reestablish a developmental program that is beneficial for repair and regeneration.

Axin2 regulates chondrocyte maturation and axial skeletal development

Axis inhibition proteins 1 and 2 (Axin1 and Axin2) are scaffolding proteins that modulate at least two signaling pathways that are crucial in skeletogenesis: the Wnt/beta-catenin and TGF-beta signaling pathways. To determine whether Axin2 is important in skeletogenesis, we examined the skeletal phenotype of Axin2-null mice in a wild-type or Axin1(+/-) background. Animals with disrupted Axin2 expression displayed a runt phenotype when compared to heterozygous littermates. Whole-mount and tissue beta-galactosidase staining of Axin2(LacZ/LacZ) mice revealed that Axin2 is expressed in cartilage tissue, and histological sections from knockout animals showed shorter hypertrophic zones in the growth plate. Primary chondrocytes were isolated from Axin2-null and wild-type mice, cultured, and assayed for type X collagen gene expression. While type II collagen levels were depressed in cells from Axin2-deficient animals, type X collagen gene expression was enhanced. There was no difference in BrdU incorporation between null and heterozygous mice, suggesting that loss of Axin2 does not alter chondrocyte proliferation. Taken together, these findings reveal that disruption of Axin2 expression results in accelerated chondrocyte maturation. In the presence of a heterozygous deficiency of Axin1, Axin2 was also shown to play a critical role in craniofacial and axial skeleton development.

Beta-catenin/T-cell factor signaling is activated during lung injury and promotes the survival and migration of alveolar epithelial cells

The Wnt/beta-catenin signaling cascade activates genes that allow cells to adopt particular identities throughout development. In adult self-renewing tissues like intestine and blood, activation of the Wnt pathway maintains a progenitor phenotype, whereas forced inhibition of this pathway promotes differentiation. In the lung alveolus, type 2 epithelial cells (AT2) have been described as progenitors for the type 1 cell (AT1), but whether AT2 progenitors use the same signaling mechanisms to control differentiation as rapidly renewing tissues is not known. We show that adult AT2 cells do not exhibit constitutive beta-catenin signaling in vivo, using the AXIN2(+/LacZ) reporter mouse, or after fresh isolation of an enriched population of AT2 cells. Rather, this pathway is activated in lungs subjected to bleomycin-induced injury, as well as upon placement of AT2 cells in culture. Forced inhibition of beta-catenin/T-cell factor signaling in AT2 cultures leads to increased cell death. Cells that survive show reduced migration after wounding and reduced expression of AT1 cell markers (T1alpha and RAGE). These results suggest that AT2 cells may function as facultative progenitors, where activation of Wnt/beta-catenin signaling during lung injury promotes alveolar epithelial survival, migration, and differentiation toward an AT1-like phenotype.

Reciprocal regulation of Wnt and Gpr177/mouse Wntless is required for embryonic axis formation

Members of the Wnt family are secreted glycoproteins that trigger cellular signals essential for proper development of organisms. Cellular signaling induced by Wnt proteins is involved in diverse developmental processes and human diseases. Previous studies have generated an enormous wealth of knowledge on the events in signal-receiving cells. However, relatively little is known about the making of Wnt in signal-producing cells. Here, we describe that Gpr177, the mouse orthologue of Drosophila Wls, is expressed during formation of embryonic axes. Embryos with deficient Gpr177 exhibit defects in establishment of the body axis, a phenotype highly reminiscent to the loss of Wnt3. Although many different mammalian Wnt proteins are required for a wide range of developmental processes, the Wnt3 ablation exhibits the earliest developmental abnormality. This suggests that the Gpr177-mediated Wnt production cannot be substituted. As a direct target of Wnt, Gpr177 is activated by beta-catenin and LEF/TCF-dependent transcription. This activation alters the cellular distributions of Gpr177 which binds to Wnt proteins and assists their sorting and secretion in a feedback regulatory mechanism. Our findings demonstrate that the loss of Gpr177 affects Wnt production in the signal-producing cells, leading to alterations of Wnt signaling in the signal-receiving cells. A reciprocal regulation of Wnt and Gpr177 is essential for the patterning of the anterior-posterior axis during mammalian development.

Co-activator activator (CoAA) prevents the transcriptional activity of Runt domain transcription factors

Runx proteins are essential for a number of developmental processes and are aberrantly expressed in many human cancers. Runx factors bind DNA and co-factors to activate or repress genes crucial for bone formation, hematopoiesis, and neuronal development. Co-activator activator (CoAA) is a nuclear protein that regulates gene expression, RNA splicing and is overexpressed in many human tumors. In this study, we identified CoAA as a Runx2 binding protein. CoAA repressed Runx factor-dependent activation of reporter genes in a histone deacetylase-independent manner. CoAA also blocked Runx2-mediated repression of the Axin2 promoter, a novel Runx target gene. The carboxy-terminus of CoAA is essential for binding the Runt domains of Runx1 and Runx2. In electophoretic mobility shift assays, CoAA inhibited Runx2 interactions with DNA. These data indicate that CoAA is an inhibitor of Runx factors and can negate Runx factor regulation of gene expression. CoAA is expressed at high levels in human fetal osteoblasts and osteosarcoma cell lines. Suppression of CoAA expression by RNA interference reduced osteosarcoma cell viability in vitro, suggesting that it contributes to the proliferation and/or survival of osteoblast lineage cells.

Axin2 controls bone remodeling through the beta-catenin-BMP signaling pathway in adult mice

To investigate the role of Wnt-beta-catenin signaling in bone remodeling, we analyzed the bone phenotype of female Axin2-lacZ knockout (KO) mice. We found that trabecular bone mass was significantly increased in 6- and 12-month-old Axin2 KO mice and that bone formation rates were also significantly increased in 6-month-old Axin2 KO mice compared with wild-type (WT) littermates. In vitro studies were performed using bone marrow stromal (BMS) cells isolated from 6-month-old WT and Axin2 KO mice. Osteoblast proliferation and differentiation were significantly increased and osteoclast formation was significantly reduced in Axin2 KO mice. Nuclear beta-catenin protein levels were significantly increased in BMS cells derived from Axin2 KO mice. In vitro deletion of the beta-catenin gene under Axin2 KO background significantly reversed the increased alkaline phosphatase activity and the expression of osteoblast marker genes observed in Axin2 KO BMS cells. We also found that mRNA expression of Bmp2 and Bmp4 and phosphorylated Smad1/5 protein levels were significantly increased in BMS cells derived from Axin2 KO mice. The chemical compound BIO, an inhibitor of glycogen synthase kinase 3beta, was utilized for in vitro signaling studies in which upregulated Bmp2 and Bmp4 expression was measured in primary calvarial osteoblasts. Primary calvarial osteoblasts were isolated from Bmp2(fx/fx);Bmp4(fx/fx) mice and infected with adenovirus-expressing Cre recombinase. BIO induced Osx, Col1, Alp and Oc mRNA expression in WT cells and these effects were significantly inhibited in Bmp2/4-deleted osteoblasts, suggesting that BIO-induced Osx and marker gene expression were Bmp2/4-dependent. We further demonstrated that BIO-induced osteoblast marker gene expression was significantly inhibited by Osx siRNA. Taken together, our findings demonstrate that Axin2 is a key negative regulator in bone remodeling in adult mice and regulates osteoblast differentiation through the beta-catenin-BMP2/4-Osx signaling pathway in osteoblasts.

Reciprocal requirements for EDA/EDAR/NF-kappaB and Wnt/beta-catenin signaling pathways in hair follicle induction

Wnt/beta-catenin and NF-kappaB signaling mechanisms provide central controls in development and disease, but how these pathways intersect is unclear. Using hair follicle induction as a model system, we show that patterning of dermal Wnt/beta-catenin signaling requires epithelial beta-catenin activity. We find that Wnt/beta-catenin signaling is absolutely required for NF-kappaB activation, and that Edar is a direct Wnt target gene. Wnt/beta-catenin signaling is initially activated independently of EDA/EDAR/NF-kappaB activity in primary hair follicle primordia. However, Eda/Edar/NF-kappaB signaling is required to refine the pattern of Wnt/beta-catenin activity, and to maintain this activity at later stages of placode development. We show that maintenance of localized expression of Wnt10b and Wnt10a requires NF-kappaB signaling, providing a molecular explanation for the latter observation, and identify Wnt10b as a direct NF-kappaB target. These data reveal a complex interplay and interdependence of Wnt/beta-catenin and EDA/EDAR/NF-kappaB signaling pathways in initiation and maintenance of primary hair follicle placodes.

Genetic basis of tooth agenesis

Tooth agenesis or hypodontia, failure to develop all normally developing teeth, is one of the most common developmental anomalies in man. Common forms, including third molar agenesis and hypodontia of one or more of the incisors and premolars, constitute the great majority of cases. They typically affect those teeth that develop latest in each tooth class and these teeth are also most commonly affected in more severe and rare types of tooth agenesis. Specific vulnerability of the last developing teeth suggests that agenesis reflects quantitative defects during dental development. So far molecular genetics has revealed the genetic background of only rare forms of tooth agenesis. Mutations in MSX1, PAX9, AXIN2 and EDA have been identified in familial severe agenesis (oligodontia) and mutations in many other genes have been identified in syndromes in which tooth agenesis is a regular feature. Heterozygous loss of function mutations in many genes reduce the gene dose, whereas e.g. in hypohidrotic ectodermal dysplasia (EDA) the complete inactivation of the partially redundant signaling pathway reduces the signaling centers. Although these mechanisms involve quantitative disturbances, the phenotypes associated with mutations in different genes indicate that in addition to an overall reduction of odontogenic potential, tooth class-specific and more complex mechanisms are also involved. Although several of the genes so far identified in rare forms of tooth agenesis are being studied as candidate genes of common third molar agenesis and incisor and premolar hypodontia, it is plausible that novel genes that contribute to these phenotypes will also become identified.

Ror2 receptor requires tyrosine kinase activity to mediate Wnt5A signaling

The Wnts include a large family of secreted proteins that serve as important signals during embryonic development and adult homeostasis. In the most well understood Wnt signaling pathway, Wnt binding to Frizzled and low density lipoprotein receptor-related protein induces β-catenin protein stabilization and entry into the nucleus, resulting in changes in target gene transcription. Emerging evidence suggests that Wnt5a can inhibit Wnt/β-catenin signaling through interaction with the receptor Ror2. The Ror2 protein belongs to the receptor tyrosine kinase superfamily and contains several recognizable structural motifs. However, limited information is available regarding which specific domains are required for the inhibitory signaling activity of Wnt5a. Through mutation and deletion analysis, we have analyzed which specific domains and residues, including those necessary for tyrosine kinase activity, mediate the Wnt5a signal. To determine whether Ror2 can inhibit canonical Wnt signaling in vivo, we examined the effect of Ror2 loss on the expression of the Wnt reporter Axin2^LacZ, finding increased reporter activity in Ror2 null mice, demonstrating that Ror2 can also inhibit Wnt/β-catenin signaling in the context of intact tissues.

Wnt signaling as a therapeutic target for bone diseases

BACKGROUND

There is a need to develop new bone anabolic agents because current bone regeneration regimens have limitations. The Wingless-type MMTV integration site (Wnt) pathway has emerged as a regulator of bone formation and regeneration.

OBJECTIVE

To review the molecular basis for Wnt pathway modulation and discuss strategies that target it and improve bone mass.

METHODS

Data in peer-reviewed reports and meeting abstracts are discussed.

RESULTS/CONCLUSIONS

Neutralizing inhibitors of Wnt signaling have emerged as promising strategies. Small-molecule inhibitors of glycogen synthase kinase 3beta increase bone mass, lower adiposity and reduce fracture risk. Neutralizing antibodies to Dickkopf 1, secreted Frizzled-related protein 1 and sclerostin produce similar outcomes in animal models. These drugs are exciting breakthroughs but are not without risks. The challenges include tissue-specific targeting and consequently, long-term safety.

Glucocorticoid-dependent Wnt signaling by mature osteoblasts is a key regulator of cranial skeletal development in mice

Glucocorticoids are important regulators of bone cell differentiation and mesenchymal lineage commitment. Using a cell-specific approach of osteoblast-targeted transgenic disruption of intracellular glucocorticoid signaling, we discovered a novel molecular pathway by which glucocorticoids, mainly through the mature osteoblast, regulate the cellular mechanisms that govern cranial skeleton development. Embryonic and neonatal transgenic mice revealed a distinct phenotype characterized by hypoplasia and osteopenia of the cranial skeleton; disorganized frontal, parietal and interparietal bones; increased suture patency; ectopic differentiation of cartilage in the sagittal suture; and disturbed postnatal removal of parietal cartilage. Concurrently, expression of Mmp14, an enzyme essential for calvarial cartilage removal, was markedly reduced in parietal bone and cartilage of transgenic animals. Expression of Wnt9a and Wnt10b was significantly reduced in osteoblasts with disrupted glucocorticoid signaling, and accumulation of beta-catenin, the upstream regulator of Mmp14 expression, was decreased in osteoblasts, chondrocytes and mesenchymal progenitors of transgenic mice. Supracalvarial injection of Wnt3a protein rescued the transgenic cranial phenotype. These results define novel roles for glucocorticoids in skeletal development and delineate how osteoblasts--under steroid hormone control--orchestrate the intricate process of intramembranous bone formation by directing mesenchymal cell commitment towards osteoblastic differentiation while simultaneously initiating and controlling cartilage dissolution in the postnatal mouse.

EphA4 as an effector of Twist1 in the guidance of osteogenic precursor cells during calvarial bone growth and in craniosynostosis

Heterozygous loss of Twist1 function causes coronal synostosis in both mice and humans. We showed previously that in mice this phenotype is associated with a defect in the neural crest-mesoderm boundary within the coronal suture, as well as with a reduction in the expression of ephrin A2 (Efna2), ephrin A4 (Efna4) and EphA4 in the coronal suture. We also demonstrated that mutations in human EFNA4 are a cause of non-syndromic coronal synostosis. Here we investigate the cellular mechanisms by which Twist1, acting through Eph-ephrin signaling, regulates coronal suture development. We show that EphA4 mutant mice exhibit defects in the coronal suture and neural crest-mesoderm boundary that phenocopy those of Twist1(+/-) mice. Further, we demonstrate that Twist1 and EphA4 interact genetically: EphA4 expression in the coronal suture is reduced in Twist1 mutants, and compound Twist1-EphA4 heterozygotes have suture defects of greater severity than those of individual heterozygotes. Thus, EphA4 is a Twist1 effector in coronal suture development. Finally, by DiI labeling of migratory osteogenic precursor cells that contribute to the frontal and parietal bones, we show that Twist1 and EphA4 are required for the exclusion of such cells from the coronal suture. We suggest that the failure of this process in Twist1 and EphA4 mutants is the cause of craniosynostosis.

Both the RGS domain and the six C-terminal amino acids of mouse Axin are required for normal embryogenesis

Axin is a negative regulator of canonical Wnt signaling, which promotes the degradation of beta-catenin, the major effector in this signaling cascade. While many protein-binding domains of Axin have been identified, their significance has not been evaluated in vivo. Here, we report the generation and analysis of mice carrying modified Axin alleles in which either the RGS domain or the six C-terminal amino acids (C6 motif) were deleted. The RGS domain is required for APC-binding, while the C6 motif has been implicated in the activation of c-Jun N-terminal kinase, but is not required for the effects of Axin on the Wnt/beta-catenin pathway, in vitro. Both mutant Axin alleles caused recessive embryonic lethality at E9.5-E10.5, with defects indistinguishable from those caused by a null allele. As Axin-DeltaRGS protein was produced at normal levels, its inability to support embryogenesis confirms the importance of interactions between Axin and APC. In contrast, Axin-DeltaC6 protein was expressed at only 25-30% of the normal level, which may account for the recessive lethality of this allele. Furthermore, many Axin(DeltaC6/DeltaC6) embryos that were heterozygous for a beta-catenin null mutation survived to term, demonstrating that early lethality was due to failure to negatively regulate beta-catenin.

Issues associated with assessing nuclear localization of N-terminally unphosphorylated beta-catenin with monoclonal antibody 8E7

BACKGROUND

Beta-catenin is a dual function adhesion/transcriptional co-activator protein, and both functions are critical for normal tissue homeostasis. Since the transcriptional functions of beta-catenin are more often implicated in various disease processes, there is much interest in the development and use of reagents to interrogate spatial and temporal evidence of beta-catenin nuclear signaling in cells and tissues. An important study demonstrated that the signaling form of beta-catenin is specifically unphosphorylated at residues S37 and T41, and suggested that this form exhibits a propensity for cytosolic/nuclear accumulation relative to the total pool of beta-catenin.

RESULTS

We show that monoclonal antibody, 8E7, which recognizes the signaling form of beta-catenin specifically unphosphorylated at S37 and T41 (Active B-Catenin, ABC), also cross-reacts with a widely expressed, variably accessible nuclear antigen that is not beta-catenin. In cell types commonly used to study Wnt activation, this non-specific nuclear staining can be robust, obscuring the ABC signal. Definitive detection of nuclear localized ABC can be confirmed through an ability of classical cadherins to sequester ABC to cell junctions. In tissues, milder antigen retrieval methods can reduce the accessibility of mAb 8E7 to this cross-reacting nuclear antigen.

CONCLUSION

These findings reveal that interpretation of nuclear, signaling active beta-catenin using monoclonal antibody 8E7 should be considered judiciously, and in conjunction with independent methods.

SUMO-specific protease 2 is essential for modulating p53-Mdm2 in development of trophoblast stem cell niches and lineages

SUMO-specific protease 2 (SENP2) modifies proteins by removing SUMO from its substrates. Although SUMO-specific proteases are known to reverse sumoylation in many defined systems, their importance in mammalian development and pathogenesis remains largely elusive. Here we report that SENP2 is highly expressed in trophoblast cells that are required for placentation. Targeted disruption of SENP2 in mice reveals its essential role in development of all three trophoblast layers. The mutation causes a deficiency in cell cycle progression. SENP2 has a specific role in the G-S transition, which is required for mitotic and endoreduplication cell cycles in trophoblast proliferation and differentiation, respectively. SENP2 ablation disturbs the p53-Mdm2 pathway, affecting the expansion of trophoblast progenitors and their maturation. Reintroducing SENP2 into the mutants can reduce the sumoylation of Mdm2, diminish the p53 level and promote trophoblast development. Furthermore, downregulation of p53 alleviates the SENP2-null phenotypes and stimulation of p53 causes abnormalities in trophoblast proliferation and differentiation, resembling those of the SENP2 mutants. Our data reveal a key genetic pathway, SENP2-Mdm2-p53, underlying trophoblast lineage development, suggesting its pivotal role in cell cycle progression of mitosis and endoreduplication.

Co-opted JNK/SAPK signaling in Wnt/beta-catenin-induced tumorigenesis

Aberrant stimulation of the canonical Wnt pathway induces mammary tumorigenesis in mice. It has been well documented that two types of tumors, adenocarcinoma and adenocarcinoma with squamous metaplasia, develop in these mutants. However, the molecular mechanism underlying the induction of squamous transdifferentiation remains largely unknown. Here, we show that JNK/SAPK signaling plays an important role in Wnt-dependent mammary development and malignant transformation. The JNK/SAPK pathway is stimulated in pregnancy-mediated lobulo-alveolar morphogenesis, a process highly dependent on Wnt/beta-catenin signaling. Strong elevations of JNK/SAPK signaling are associated with squamous metaplasia of the Wnt-induced adenocarcinoma. Reconstitution of beta-catenin and JNK/SAPK signaling activities also promotes expression of the squamous cell marker in cultured epithelial cells. Furthermore, a synergistic activation of these two pathways can be identified in the malignant squamous cells of human endometrial and lung cancers. This is potentially a significant discovery in modern cancer therapy because of the effectiveness of an angiogenesis inhibitor, Avastin, for the treatment of adenocarcinoma, but not squamous cell carcinoma, in human lung cancers. Our finding may improve the usage of biomarkers to distinguish these two poorly differentiated tumor types, sharing similar histologic features.

BMP/Wnt antagonists are upregulated by dexamethasone in osteoblasts and reversed by alendronate and PTH: potential therapeutic targets for glucocorticoid-induced osteoporosis

We used osteoblastic MC3T3-E1 cells to clarify the mechanisms by which dexamethasone (Dex) suppresses osteoblast function, or alendronate or parathyroid hormone (PTH) alleviate it. Dex (10(-7)M) increased mRNA expression of bone morphogenetic protein (BMP) antagonists, follistatin and Dan, and of a Wnt antagonist, secreted frizzled-related protein-1 (sFRP-1) and a Wnt signal inhibitor, axin-2, while concomitantly decreased the expression of downstream molecules, Runx2 mRNA and beta-catenin protein. Pretreatments with alendronate (10(-8)M) or human PTH-(1-34) (10(-8)M) totally or partially antagonized not only the Dex-induced enhancement in mRNA expression of follistatin/Dan and sFRP-1/axin-2 but also the Dex-induced reduction in Runx2 mRNA expression and mineralization. These findings suggest that Dex suppresses the Wnt and BMP pathways as well as osteoblast function by enhancing the expression of BMP and Wnt antagonists, and bisphosphonate and PTH exert pharmacologic effects by canceling these processes.

Ovarian development in mice requires the GATA4-FOG2 transcription complex

We have demonstrated previously that mammalian sexual differentiation requires both the GATA4 and FOG2 transcriptional regulators to assemble the functioning testis. Here we have determined that the sexual development of female mice is profoundly affected by the loss of GATA4-FOG2 interaction. We have also identified the Dkk1 gene, which encodes a secreted inhibitor of canonical β-catenin signaling, as a target of GATA4-FOG2 repression in the developing ovary. The tissue-specific ablation of theβ-catenin gene in the gonads disrupts female development. In Gata4ki/ki; Dkk1-/- or Fog2-/-;Dkk1-/- embryos, the normal ovarian gene expression pattern is partially restored. Control of ovarian development by the GATA4-FOG2 complex presents a novel insight into the cross-talk between transcriptional regulation and extracellular signaling that occurs in ovarian development.

Translating insights from development into regenerative medicine: the function of Wnts in bone biology

The Wnt pathway constitutes one of the most attractive candidates for modulating skeletal tissue regeneration based on its functions during skeletal development and homeostasis. Wnts participate in every stage of skeletogenesis, from the self-renewal and proliferation of skeletal stem cells to the specification of osteochondroprogenitor cells and the maturation of chondrocytes and osteoblasts. We propose that the function of Wnts depend upon a skeletogenic cell's state of differentiation. In this review we summarize recent data with a focus on the roles of Wnt signaling in mesenchymal stem cell fate, osteoprogenitor cell differentiation, chondrocyte maturation, bone remodeling, and bone regeneration.

Osteogenic tumours in Lkb1-deficient mice

Germline mutation in LKB1 is the cause of Peutz-Jeghers Syndrome in humans, a rare disorder predisposing to cancer and multiple gastrointestinal hamartomous polyps. Mice harboring a germline inactivating Lkb1 mutation develop similar gastrointestinal polyps and liver neoplasia. We observed paralysis in approximately 2% of Lkb1(+/-) mice on two genetic backgrounds, C57BL/6J and 129/sv, at around 300 days of age. Stepped serial sectioning of the whole spinal column found multiple osteogenic tumours that were lobulated, showed osteoid formation and had an infiltrative growth pattern, which extended into the surrounding muscle. Osteogenic tumours were also present in asymptomatic Lkb1(+/-) mice (n=12) in the lateral spinous processes, spinous vertebral bodies and the bodies of sacral tail vertebrae. Although asymptomatic, the proliferation in several mice caused a narrowing and compression of the spinal canal. The long bones of Lkb1(+/-) mice had osteoblastosis within the femur and tibia indicating that the process is multi-focal; bone remodelling was accompanied by angiogenesis. No wild type Lkb1(+/+) siblings (n=12) showed aberrant osteoblastosis or bone remodelling. This is the first report of multifocal osteoblastic tumours in Lkb1(+/-) mice and our observations indicate that Lkb1, like Pten, may have a distinct role in controlling osteoblast proliferation in the mouse.

Mechanical influences on suture development and patency

In addition to their role in skull growth, sutures are sites of flexibility between the more rigid bones. Depending on the suture, predominant loading during life may be either tensile or compressive. Loads are transmitted across sutures via collagenous fibers and a fluid-rich extracellular matrix and can be quasi-static (growth of neighboring tissues) or intermittent (mastication). The mechanical properties of sutures, while always viscoelastic, are therefore quite different for tensile versus compressive loading. The morphology of individual sutures reflects the nature of local loading, evidently by a process of developmental adaptation. In vivo or ex vivo, sutural cells respond to tensile or cyclic loading by expressing markers of proliferation and differentiation, whereas compressive loading appears to favor osteogenesis. Braincase and facial sutures exhibit similar mechanical behavior and reactions despite their different natural environments.

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.

Segmental patterning of the vertebrate embryonic axis

The body axis of vertebrates is composed of a serial repetition of similar anatomical modules that are called segments or metameres. This particular mode of organization is especially conspicuous at the level of the periodic arrangement of vertebrae in the spine. The segmental pattern is established during embryogenesis when the somites--the embryonic segments of vertebrates--are rhythmically produced from the paraxial mesoderm. This process involves the segmentation clock, which is a travelling oscillator that interacts with a maturation wave called the wavefront to produce the periodic series of somites. Here, we review our current understanding of the segmentation process in vertebrates.

Cranial sutures: a brief review

Craniosynostosis, or the premature fusion of one or more cranial sutures, is a relatively common congenital defect that causes a number of morphologic and functional abnormalities. With advances in genetics and molecular biology, research of craniosynostosis has progressed from describing gross abnormalities to understanding the molecular interactions that underlie these cranial deformities. Animal models have been extremely valuable in improving our comprehension of human craniofacial morphogenesis, primarily by human genetic linkage analysis and the development of knock-out animals. This article provides a brief review of perisutural tissue interactions, embryonic origins, signaling molecules and their receptors, and transcription factors in maintaining the delicate balance between proliferation and differentiation of cells within the suture complex that determines suture fate. Finally, this article discusses the potential implications for developing novel therapies for craniosynostosis.

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

Chondrocyte maturation during endochondral bone formation is regulated by a number of signals that either promote or inhibit maturation. Among these, two well-studied signaling pathways play crucial roles in modulating chondrocyte maturation: transforming growth factor-beta (TGF-beta)/Smad3 signaling slows the rate of chondrocyte maturation, while Wingless/INT-1-related (Wnt)/beta-catenin signaling enhances the rate of chondrocyte maturation. Axin1 and Axin2 are functionally equivalent and have been shown to inhibit Wnt/beta-catenin signaling and stimulate TGF-beta signaling. Here we show that while Wnt3a stimulates Axin2 in a negative feedback loop, TGF-beta suppresses the expression of both Axin1 and Axin2 and stimulates beta-catenin signaling. In Axin2 -/- chondrocytes, TGF-beta treatment results in a sustained increase in beta-catenin levels compared to wild-type chondrocytes. In contrast, overexpression of Axin enhanced TGF-beta signaling while overexpression of beta-catenin inhibited the ability of TGF-beta to induce Smad3-sensitive reporters. Finally, the suppression of the Axins is Smad3-dependent since the effect is absent in Smad3 -/- chondrocytes. Altogether these findings show that the Axins act to integrate signals between the Wnt/beta-catenin and TGF-beta/Smad pathways. Since the suppression Axin1 and Axin2 expression by TGF-beta reduces TGF-beta signaling and enhances Wnt/beta-catenin signaling, the overall effect is a shift from TGF-beta toward Wnt/beta-catenin signaling and an acceleration of chondrocyte maturation.

Cranial suture biology and dental development: genetic and clinical perspectives

Premature fusion of the calvarial bones at the sutures, or craniosynostosis (CS), is a relatively common birth defect (1:2000-3000) frequently associated with limb deformity. Patients with CS may present oral defects, such as cleft soft palate, hypodontia, hyperdontia, and delayed tooth eruption, but also unusual associations of major dental anomalies such as taurodontism, microdontia, multiple dens invaginatus, and dentin dysplasia. The list of genes that are involved in CS includes those coding for the different fibroblast growth factor receptors and a ligand of ephrin receptors, but also genes encoding transcription factors, such as MSX2 and TWIST. Most of these genes are equally involved in odontogenesis, providing a pausible explanation for clinical associations of CS with dental agenesis or tooth malformations. On the basis of the present knowledge on genes and transcription factors that are involved in craniofacial morphogenesis, and from dental clinics of CS syndromes, the molecular mechanisms that control suture formation and suture closure are expected to play key roles in patterning events and development of teeth. The purpose of this article is to review and merge the recent advances in the field of suture research at the genetic and cellular levels with those of tooth development, and to apply them to the dental clinics of CS syndromes. These new perspectives and future challenges in the field of both dental clinics and molecular genetics, more in particular the identification of possible candidate genes involved in both CS and dental defects, are discussed.

Wnt3a/beta-catenin signaling controls posterior body development by coordinating mesoderm formation and segmentation

Somitogenesis is thought to be controlled by a segmentation clock, which consists of molecular oscillators in the Wnt3a, Fgf8 and Notch pathways. Using conditional alleles of Ctnnb1 (beta-catenin), we show that the canonical Wnt3a/beta-catenin pathway is necessary for molecular oscillations in all three signaling pathways but does not function as an integral component of the oscillator. Small, irregular somites persist in abnormally posterior locations in the absence of beta-catenin and cycling clock gene expression. Conversely, Notch pathway genes continue to oscillate in the presence of stabilized beta-catenin but boundary formation is delayed and anteriorized. Together, these results suggest that the Wnt3a/beta-catenin pathway is permissive but not instructive for oscillating clock genes and that it controls the anterior-posterior positioning of boundary formation in the presomitic mesoderm (PSM). The Wnt3a/beta-catenin pathway does so by regulating the activation of the segment boundary determination genes Mesp2 and Ripply2 in the PSM through the activation of the Notch ligand Dll1 and the mesodermal transcription factors T and Tbx6. Spatial restriction of Ripply2 to the anterior PSM is ensured by the Wnt3a/beta-catenin-mediated repression of Ripply2 in posterior PSM. Thus, Wnt3a regulates somitogenesis by activating a network of interacting target genes that promote mesodermal fates, activate the segmentation clock, and position boundary determination genes in the anterior PSM.

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.

The pattern of beta-catenin responsiveness within the mammary gland is regulated by progesterone receptor

Experiments involving beta-catenin loss- and gain-of-function in the mammary gland have decisively demonstrated the role of this protein in normal alveologenesis. However, the relationship between hormonal and beta-catenin signaling has not been investigated. In this study, we demonstrate that activated beta-catenin rescues alveologenesis in progesterone receptor (PR; Pgr)-null mice during pregnancy. Two distinct subsets of mammary cells respond to expression of DeltaN89beta-catenin. Cells at ductal tips are inherently beta-catenin-responsive and form alveoli in the absence of PR. However, PR activity confers beta-catenin responsiveness to progenitor cells along the lateral ductal borders in the virgin gland. Once activated by beta-catenin, responding cells switch on an alveolar differentiation program that is indistinguishable from that observed in pregnancy and is curtailed by PR signaling.

Wnt signaling mediates regional specification in the vertebrate face

At early stages of development, the faces of vertebrate embryos look remarkably similar, yet within a very short timeframe they adopt species-specific facial characteristics. What are the mechanisms underlying this regional specification of the vertebrate face? Using transgenic Wnt reporter embryos we found a highly conserved pattern of Wnt responsiveness in the developing mouse face that later corresponded to derivatives of the frontonasal and maxillary prominences. We explored the consequences of disrupting Wnt signaling, first using a genetic approach. Mice carrying compound null mutations in the nuclear mediators Lef1 and Tcf4 exhibited radically altered facial features that culminated in a hyperteloric appearance and a foreshortened midface. We also used a biochemical approach to perturb Wnt signaling and found that in utero delivery of a Wnt antagonist, Dkk1,produced similar midfacial malformations. We tested the hypothesis that Wnt signaling is an evolutionarily conserved mechanism controlling facial morphogenesis by determining the pattern of Wnt responsiveness in avian faces,and then by evaluating the consequences of Wnt inhibition in the chick face. Collectively, these data elucidate a new role for Wnt signaling in regional specification of the vertebrate face, and suggest possible mechanisms whereby species-specific facial features are generated.

Independent functions and mechanisms for homeobox gene Barx1 in patterning mouse stomach and spleen

Homeobox genes convey positional information in embryos and their role in patterning the mammalian gut is a topic of considerable interest. Barx1 is expressed selectively in fetal stomach mesenchyme and directs differentiation of overlying endoderm. Recombinant tissue cultures and study of young mouse embryos previously suggested that Barx1 controls expression of secreted Wnt antagonists, which suppress endodermal Wnt signaling, to enable stomach epithelial differentiation. We overcame mid-gestational lethality of Barx1(-/-) mouse embryos and report here the spectrum of anomalies in a distinctive and unprecedented model of gastrointestinal homeotic transformation. Using various mouse models, we confirm the importance of attenuated Wnt signaling in stomach development and the role of Barx1 in suppressing endodermal Wnt activity. Absence of Barx1 also results in fully penetrant defects in positioning and expansion of the spleen, an organ that originates within the mesothelial lining of the stomach. Barx1 is absent from the spleen primordium but highly expressed in the mesogastrium, indicating an indirect effect on spleen development. However, our results argue against a role for Wnt antagonism in genesis of the spleen. Mouse spleen development relies on several homeodomain transcriptional regulators that are expressed in the spleen primordium. Loss of Barx1 does not affect expression of any of these genes but notably reduces expression of Wt1, a transcription factor implicated in spleen morphogenesis and expressed in the mesothelium. These observations place Barx1 proximally within a Wt1 pathway of spleen development and reveal how a homeotic regulator employs different molecular mechanisms to mold neighboring organs.

Molecular clocks underlying vertebrate embryo segmentation: A 10-year-old hairy-go-round

Segmentation of the vertebrate embryo body is a fundamental developmental process that occurs with strict temporal precision. Temporal control of this process is achieved through molecular segmentation clocks, evidenced by oscillations of gene expression in the unsegmented presomitic mesoderm (PSM, precursor tissue of the axial skeleton) and in the distal limb mesenchyme (limb chondrogenic precursor cells). The first segmentation clock gene, hairy1, was identified in the chick embryo PSM in 1997. Ten years later, chick hairy2 expression unveils a molecular clock operating during limb development. This review revisits vertebrate embryo segmentation with special emphasis on the current knowledge on somitogenesis and limb molecular clocks. A compilation of human congenital disorders that may arise from deregulated embryo clock mechanisms is presented here, in an attempt to reconcile different sources of information regarding vertebrate embryo development. Challenging open questions concerning the somitogenesis clock are presented and discussed, such as When?, Where?, How?, and What for? Hopefully the next decade will be equally rich in answers.

Phases of canonical Wnt signaling during the development of mouse intestinal epithelium

BACKGROUND AND AIMS

Intestinal crypts constitute a niche in which epithelial progenitors respond to Wnt signals, replicate, and prepare to differentiate. Because mutations in Wnt pathway genes lead to intestinal cancer, the role of Wnt signaling in gut epithelial homeostasis is a subject of intense investigation. We studied how Wnt signaling is established during intestine development.

METHODS

We studied spatiotemporal features of Wnt signaling at formative stages in mouse embryos, when villous projections appear and crypt precursors occupy intervillus regions. We used TOP-GAL transgenic and Axin2(LacZ) mice, which report faithfully on canonical Wnt activity, relevant molecular markers, and embryos with aberrant beta-catenin activation.

RESULTS

Developing intestines first display evidence for Wnt signaling after appearance of villi. During villus morphogenesis, intervillus cells proliferate actively but lack signs of canonical Wnt signaling. Surprisingly, in late gestation and briefly thereafter, conspicuous Wnt activity is evident in differentiated, postmitotic villus epithelium. Neither Tcf4, a principal transcriptional effector of intestinal Wnt signals, nor candidate Wnt targets CD44 and cyclinD1 are expressed in late fetal villus cells that show high Wnt activity. Instead, those cells express the related factor Tcf3 and a different Wnt target, c-Myc. Premature and deregulated beta-catenin activation causes severe villus dysmorphogenesis in transgenic mice.

CONCLUSIONS

Relationships among Wnt signaling, epithelial proliferation, and tissue differentiation are reversed in the developing and adult gut. The canonical Wnt pathway has independent, albeit possibly overlapping, functions in early intestinal villi and adult crypts. These observations advance understanding of Wnt functions in intestinal development and disease.

Understanding the somitogenesis clock: what's missing?

The segmentation of vertebrate embryos depends on a complex genetic network that generates highly dynamic gene expression. Many of the elements of the network have been identified, but their interaction and their influence on segmentation remain poorly understood. A few mathematical models have been proposed to explain the dynamics of subsets of the network, but the mechanistic bases remain controversial. This review focuses on outstanding problems with the generation of somitogenesis clock oscillations, and the ways they could regulate segmentation. Proposals that oscillations are generated by a negative feedback loop formed by Lunatic fringe and Notch signaling are weighed against a model based on positive feedback, and the experimental basis for models of simple negative feedback involving Her/Hes genes or Wnt targets is evaluated. Differences are then made explicit between the many 'clock and wavefront' model variants that have been proposed to explain how the clock regulates segmentation. An understanding of the somitogenesis clock will require addressing experimentally the many questions that arise from the study of simple models.