The role of Axin2 in calvarial morphogenesis and craniosynostosis

Two functionally distinct Axin-like proteins regulate canonical Wnt signaling in C. elegans

Axin is a central component of the canonical Wnt signaling pathway that interacts with the adenomatous polyposis coli protein APC and the kinase GSK3beta to downregulate the effector beta-catenin. In the nematode Caenorhabditis elegans, canonical Wnt signaling is negatively regulated by the highly divergent Axin ortholog PRY-1. Mutation of pry-1 leads to constitutive activation of BAR-1/beta-catenin-dependent Wnt signaling and results in a range of developmental defects. The pry-1 null phenotype is however not fully penetrant, indicating that additional factors may partially compensate for PRY-1 function. Here, we report the cloning and functional analysis of a second Axin-like protein, which we named AXL-1. We show that despite considerable sequence divergence with PRY-1 and other Axin family members, AXL-1 is a functional Axin ortholog. AXL-1 functions redundantly with PRY-1 in negatively regulating BAR-1/beta-catenin signaling in the developing vulva and the Q neuroblast lineage. In addition, AXL-1 functions independently of PRY-1 in negatively regulating canonical Wnt signaling during excretory cell development. In contrast to vertebrate Axin and the related protein Conductin, AXL-1 and PRY-1 are not functionally equivalent. We conclude that Axin function in C. elegans is divided over two different Axin orthologs that have specific functions in negatively regulating canonical Wnt signaling.

In vivo analysis of Wnt signaling in bone

Bone remodeling requires osteoblasts and osteoclasts working in concert to maintain a constant bone mass. The dysregulation of signaling pathways that affect osteoblast or osteoclast differentiation or function leads to either osteopenia or high bone mass. The discovery that activating and inactivating mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, led to high bone mass and low bone mass in human beings, respectively, generated a tremendous amount of interest in the possible role of the Wnt signaling pathway in the regulation of bone remodeling. A number of mouse models have been generated to study a collection of Wnt signaling molecules that have been identified as regulators of bone mass. These mouse models help establish the canonical Wnt signaling pathway as a major regulator of chondrogenesis, osteoblastogenesis, and osteoclastogenesis. This review will summarize these advances.

And the segmentation clock keeps ticking

The vertebrate body is organized in segments, easily visible in the consecutive vertebrae of the skeleton. These are first defined in the embryo by the formation of somites. Somites are generated at regular intervals from the presomitic mesoderm by a combination of oscillating signals, known as the segmentation clock, which establish the pace at which new somites are formed, and signaling gradients that set the location of new intersomitic borders. Using a microarray approach, Dequéant et al. have now shown that the segmentation clock is more complex than previously thought and includes oscillating expression of genes from at least three signaling pathways organized in coordinated networks.

Viable mice with compound mutations in the Wnt/Dvl pathway antagonists nkd1 and nkd2

Gradients of Wnt/beta-catenin signaling coordinate development and physiological homeostasis in metazoan animals. Proper embryonic development of the fruit fly Drosophila melanogaster requires the Naked cuticle (Nkd) protein to attenuate a gradient of Wnt/beta-catenin signaling across each segmental anlage. Nkd inhibits Wnt signaling by binding the intracellular protein Dishevelled (Dsh). Mice and humans have two nkd homologs, nkd1 and nkd2, whose encoded proteins can bind Dsh homologs (the Dvl proteins) and inhibit Wnt signaling. To determine whether nkd genes are necessary for murine development, we replaced nkd exons that encode Dvl-binding sequences with IRES-lacZ/neomycin cassettes. Mutants homozygous for each nkd(lacZ) allele are viable with slightly reduced mean litter sizes. Surprisingly, double-knockout mice are viable, with subtle alterations in cranial bone morphology that are reminiscent of mutation in another Wnt/beta-catenin antagonist, axin2. Our data show that nkd function in the mouse is dispensable for embryonic development.

Bmp and Wnt/β-catenin signals control expression of the transcription factor Olig3 and the specification of spinal cord neurons

In the developing spinal cord, signals of the roof plate pattern the dorsal progenitor domain and control the specification of three neuron types, dorsal interneurons dI1, dI2, and dI3. Bmp and Wnt/beta-catenin signals as well as transcription factors like Olig3 or Ngn1/2 are essential in this process. We have studied the epistatic relationship between Bmp and Wnt/beta-catenin signals and the transcription factor Olig3 in dorsal spinal cord patterning. Using beta-catenin gain-of-function and compound beta-catenin gain-of-function/Olig3 loss-of-function mutations in mice, we could show that Wnt/beta-catenin signals act upstream of Olig3 in the specification of dI2 and dI3 neurons. The analysis of such compound mutant mice allowed us to distinguish between the two functions of Wnt/beta-catenin signaling in proliferation and patterning of dorsal progenitors. Using electroporation of chick spinal cords, we further demonstrate that Bmp signals act upstream of Wnt/beta-catenin in the regulation of Olig3 and that Wnt/beta-catenin signals play an instructive role in controlling Olig3 expression. We conclude that Wnt/beta-catenin and BMP signals coordinately control the specification of dorsal neurons in the spinal cord.

Wnt signaling and human diseases: what are the therapeutic implications?

Wnt signaling plays an important role in regulating cell proliferation and differentiation. De-regulation of these signaling pathways has been implicated in many human diseases, ranging from cancers to skeletal disorders. Wnt proteins are a large family of secreted factors that bind to the Frizzled receptors and LRP5/6 co-receptors and initiate complex signaling cascades. Over the past two decades, our understanding of Wnt signaling has been significantly improved due to the identification of many key regulators and mediators of these pathways. Given that Wnt signaling is tightly regulated at multiple cellular levels, these pathways themselves offer ample nodal points for targeted therapeutics. Here, we focus on our current understanding of these pathways, the associations of Wnt signaling with human disorders, and the opportunities to target key components of Wnt signaling for rational drug discovery.

Wnt/beta-catenin signaling in development and disease

A remarkable interdisciplinary effort has unraveled the WNT (Wingless and INT-1) signal transduction cascade over the last two decades. Wnt genes encode small secreted proteins that are found in all animal genomes. Wnt signaling is involved in virtually every aspect of embryonic development and also controls homeostatic self-renewal in a number of adult tissues. Germline mutations in the Wnt pathway cause several hereditary diseases, and somatic mutations are associated with cancer of the intestine and a variety of other tissues.

Dact1presomitic mesoderm expression oscillates in phase withAxin2in the somitogenesis clock of mice

During segmentation (somitogenesis) in vertebrate embryos, somites form in a rostral-to-caudal sequence according to a species-specific rhythm called the somitogenesis clock. The expression of genes participating in somitogenesis oscillates in the presomitic mesoderm (PSM) in time with this clock. We previously reported that the Dact1 gene (aka Dpr1/Frd1/ThyEx3), which encodes a Dishevelled-binding intracellular regulator of Wnt signaling, is prominently expressed in the PSM as well as in a caudal-rostral gradient across the somites of mouse embryos. This observation led us to examine whether Dact1 expression oscillates in the PSM. We have found that Dact1 PSM expression does indeed oscillate in time with the somitogenesis clock. Consistent with its known signaling functions and with the "clock and wavefront" model of signal regulation during somitogenesis, the oscillation of Dact1 occurs in phase with the Wnt signaling component Axin2, and out of phase with the Notch signaling component Lfng.

A Wnt-Axin2-GSK3beta cascade regulates Snail1 activity in breast cancer cells

Accumulating evidence indicates that hyperactive Wnt signalling occurs in association with the development and progression of human breast cancer. As a consequence of engaging the canonical Wnt pathway, a beta-catenin-T-cell factor (TCF) transcriptional complex is generated, which has been postulated to trigger the epithelial-mesenchymal transition (EMT) that characterizes the tissue-invasive phenotype. However, the molecular mechanisms by which the beta-catenin-TCF complex induces EMT-like programmes remain undefined. Here, we demonstrate that canonical Wnt signalling engages tumour cell dedifferentiation and tissue-invasive activity through an Axin2-dependent pathway that stabilizes the Snail1 zinc-transcription factor, a key regulator of normal and neoplastic EMT programmes. Axin2 regulates EMT by acting as a nucleocytoplasmic chaperone for GSK3beta, the dominant kinase responsible for controlling Snail1 protein turnover and activity. As dysregulated Wnt signalling marks a diverse array of cancerous tissue types, the identification of a beta-catenin-TCF-regulated Axin2-GSK3beta-Snail1 axis provides new mechanistic insights into cancer-associated EMT programmes.

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.

Impaired neural development caused by inducible expression of Axin in transgenic mice

Ablations of the Axin family genes demonstrated that they modulate Wnt signaling in key processes of mammalian development. The ubiquitously expressed Axin1 plays an important role in formation of the embryonic neural axis, while Axin2 is essential for craniofacial skeletogenesis. Although Axin2 is also highly expressed during early neural development, including the neural tube and neural crest, it is not essential for these processes, apparently due to functional redundancy with Axin1. To further investigate the role of Wnt signaling during early neural development, and its potential regulation by Axins, we developed a mouse model for conditional gene activation in the Axin2-expressing domains. We show that gene expression can be successfully targeted to the Axin2-expressing cells in a spatially and temporally specific fashion. High levels of Axin in this domain induce a region-specific effect on the patterning of neural tube. In the mutant embryos, only the development of midbrain is severely impaired even though the transgene is expressed throughout the neural tube. Axin apparently regulates beta-catenin in coordinating cell cycle progression, cell adhesion and survival of neuroepithelial precursors during development of ventricles. Our data support the conclusion that the development of embryonic neural axis is highly sensitive to the level of Wnt signaling.

On periodicity and directionality of somitogenesis

It is currently thought that the mechanism underlying somitogenesis is linked to a molecular oscillator, the segmentation clock, and to gradients of signaling molecules within the paraxial mesoderm. Here, we review the current picture of this segmentation clock and gradients, and use this knowledge to critically ask: What is the basis for periodicity and directionality of somitogenesis?

The regulator of G protein signaling domain of axin selectively interacts with Galpha12 but not Galpha13

Axin, a negative regulator of the Wnt signaling pathway, contains a canonical regulator of G protein signaling (RGS) core domain. Herein, we demonstrate both in vitro and in cells that this domain interacts with the alpha subunit of the heterotrimeric G protein G12 but not with the closely related Galpha13 or with several other heterotrimeric G proteins. Axin preferentially binds the activated form of Galpha12, a behavior consistent with other RGS proteins. However, unlike other RGS proteins, that of axin (axinRGS) does not affect intrinsic GTP hydrolysis by Galpha12. Despite its inability to act as a GTPase-activating protein, we demonstrate that in cells, axinRGS can compete for Galpha12 binding with the RGS domain of p115RhoGEF, a known G12-interacting protein that links G12 signaling to activation of the small G protein Rho. Moreover, ectopic expression of axinRGS specifically inhibits Galpha12-directed activation of the Rho pathway in MDA-MB 231 breast cancer cells. These findings establish that the RGS domain of axin is able to directly interact with the alpha subunit of heterotrimeric G protein G12 and provide a unique tool to interdict Galpha12-mediated signaling processes.

A novel c.581C>T transition localized in a highly conserved homeobox sequence of MSX1: is it responsible for oligodontia?

Even though selective tooth agenesis is the most common developmental anomaly of human dentition, its genetic background still remains poorly understood. To date, familial as well as sporadic forms of both hypodontia and oligodontia have been associated with mutations or polymorphisms of MSX1, PAX9, AXIN2 and TGFa, whose protein products play a crucial role in odontogenesis. In the present report we described a novel mutation of MSX1, which might be responsible for the lack of 14 permanent teeth in our proband. However, this c.581C>T transition, localized in a highly conserved homeobox sequence of MSX1, was identified also in 2 healthy individuals from the proband's family. Our finding suggests that this transition might be the first described mutation of MSX1 that might be responsible for oligodontia and showing incomplete penetrance. It may also support the view that this common anomaly of human dentition might be an oligogenic trait caused by simultaneous mutations of different genes.

Wnt signaling and osteoblastogenesis

Wnts are a large family of growth factors that mediate fundamental biological processes like embryogenesis, organogenesis and tumorigenesis. These proteins bind to a membrane receptor complex comprised of a frizzled (FZD) G-protein-coupled receptor (GPCRs) and a low-density lipoprotein (LDL) receptor-related protein (LRP). The formation of this ligand-receptor complex initiates a number of intracellular signaling cascades that includes the canonical/beta-catenin pathway, as well as several GPCR-mediated noncanonical pathways. In recent years, canonical Wnt signaling has been shown to play a substantial role in the control of bone formation. Clinical investigations have found that mutations in LRP-5 are associated with bone mineral density and fractures. For example, loss-of-function mutations in LRP-5 cause osteoporosis pseudoglioma syndrome, while gain-of-function mutations lead to high bone mass phenotypes. Studies of knockout and transgenic mouse models for Wnt pathway components like Wnt-10b, LRP-5/6, secreted frizzled-related protein-1, dickkopf-2, Axin-2 and beta-catenin have demonstrated that canonical signaling modulates most aspects of osteoblast physiology including proliferation, differentiation, bone matrix formation/mineralization and apoptosis as well as coupling to osteoclastogenesis and bone resorption. Future studies in this rapidly growing area of research should focus on elucidating Wnt/FZD specificity in the control of bone cell function, the role of noncanonical pathways in skeletal remodeling, and direct effects of Wnts on cells of the osteoclast lineage.

Networks and hubs for the transcriptional control of osteoblastogenesis

We present an overview of the concepts of tissue-specific transcriptional control mechanisms essential for development of the bone cell phenotype. BMP2 induced transcription factors constitute a network of activities and molecular switches for bone development and osteoblast differentiation. Among these regulators are Runx2 (Cbfa1/AML3), the principal osteogenic master gene for bone formation, as well as homeodomain proteins and osterix. Runx2 has multiple regulatory activities, including activation or repression of gene expression, and integration of biological signals from developmental cues, such as BMP/TGFbeta, Wnt and Src signaling pathways. Runx2 provides a new paradigm for transcriptional control by functioning as a principal scaffolding protein in nuclear microenvironments to control gene expression in response to physiologic signals (growth factors, cytokines and hormones). The protein serves as a hub for the coordination of activities essential for the expansion and differentiation of osteogenic lineage cells through the formation of co-regulatory protein complexes organized in subnuclear domains. Mechanisms by which Runx2 supports commitment to osteogenesis and determines cell fate involve its retention on mitotic chromosomes. Disruption of a unique protein module, the subnuclear targeting signal of Runx2, has profound effects on osteoblast differentiation and metastasis of cancer cells in the bone microenvironment. Runx2 target genes include regulators of cell growth control, components of the bone extracellular matrix, angiogenesis, and signaling proteins for development of the osteoblast phenotype and bone turnover. The specificity of Runx2 regulatory activities provides a basis for novel therapeutic strategies to correct bone disorders.

A Wnt canon orchestrating osteoblastogenesis

Several transcription factors have been identified that control the differentiation of osteoblasts; however, relatively little is known about the signaling pathways involved in regulating the differentiation process. Recently, the canonical Wnt-beta-catenin pathway has been implicated in osteoblastogenesis. This review focuses on the role of the canonical Wnt-beta-catenin pathway during embryonic development, where it is required for the differentiation of osteoblasts from a precursor that is shared with the chondrocyte lineage and the requirement of this pathway during postnatal life in bone homeostasis. The recent findings covered in this review are major advances in our understanding of skeletal development and promise new therapeutic avenues for tissue engineering and treatment of osteoporosis.

Molecular bases of the regulation of bone remodeling by the canonical Wnt signaling pathway

Osteoporosis is a common, prevalent, and debilitating condition, particularly in postmenopausal women. Genetics play a major role in determining peak bone mass and fracture risk, but few genes have been demonstrated conclusively to be involved, much less the signaling pathways with which they are affiliated. The identification of mutations in the gene Lrp5, a Wnt coreceptor, as the cause for both osteoporotic and high-bone mass disorders implicated the canonical Wnt signaling pathway in bone mass regulation. Since Lrp5, other Wnt components have been identified as being regulators of bone mass, and Wnt target genes affecting bone homeostasis have begun to be elucidated. This chapter looks at the various components of the canonical Wnt signaling pathway and the data indicating that this pathway plays a major role in the control of both bone formation and bone resorption, the two key aspects of bone remodeling.

Canonical WNT signaling promotes osteogenesis by directly stimulating Runx2 gene expression

Both activating and null mutations of proteins required for canonical WNT signaling have revealed the importance of this pathway for normal skeletal development. However, tissue-specific transcriptional mechanisms through which WNT signaling promotes the differentiation of bone-forming cells have yet to be identified. Here, we address the hypothesis that canonical WNT signaling and the bone-related transcription factor RUNX2/CBFA1/AML3 are functionally linked components of a pathway required for the onset of osteoblast differentiation. Our findings show that, in bone of the SFRP1 (secreted frizzled-related protein-1)-null mouse, which exhibits activated WNT signaling and a high bone mass phenotype, there is a significant increase in expression of T-cell factor (TCF)-1, Runx2, and the RUNX2 target gene osteocalcin. We demonstrate by mutational analysis that a functional TCF regulatory element responsive to canonical WNT signaling resides in the promoter of the Runx2 gene (-97 to -93). By chromatin immunoprecipitation, recruitment of beta-catenin and TCF1 to the endogenous Runx2 gene is shown. Coexpression of TCF1 with canonical WNT proteins resulted in a 2-5-fold activation of Runx2 promoter activity and a 7-8-fold induction of endogenous mRNA in mouse pluripotent mesenchymal and osteoprogenitor cells. This enhancement was abrogated by SFRP1. Taken together, our results provide evidence for direct regulation of Runx2 by canonical WNT signaling and suggest that Runx2 is a target of beta-catenin/TCF1 for the stimulation of bone formation. We propose that WNT/TCF1 signaling, like bone morphogenetic protein/transforming growth factor-beta signaling, activates Runx2 gene expression in mesenchymal cells for the control of osteoblast differentiation and skeletal development.

Mouse axin and axin2/conductin proteins are functionally equivalent in vivo

Axin is a central component of the canonical Wnt signal transduction machinery, serving as a scaffold for the beta-catenin destruction complex. The related protein Axin2/Conductin, although less extensively studied, is thought to perform similar functions. Loss of Axin causes early embryonic lethality, while Axin2-null mice are viable but have craniofacial defects. Mutations in either gene contribute to cancer in humans. The lack of redundancy between Axin and Axin2 could be due to their different modes of expression: while Axin is expressed ubiquitously, Axin2 is expressed in tissue- and developmental-stage-specific patterns, and its transcription is induced by canonical Wnt signaling. Alternatively, the two proteins might have partially different functions, a hypothesis supported by the observation that they differ in their subcellular localizations in colon epithelial cells. To test the functional equivalence of Axin and Axin2 in vivo, we generated knockin mice in which the Axin gene was replaced with Myc-tagged Axin or Axin2 cDNA. Mice homozygous for the resulting alleles, Axin(Ax) or Axin(Ax2), express no endogenous Axin but express either Myc-Axin or Myc-Axin2 under the control of the Axin locus. Both Axin(Ax/Ax) and Axin(Ax2/Ax2) homozygotes are apparently normal and fertile, demonstrating that the Axin and Axin2 proteins are functionally equivalent.

Development of a unique system for spatiotemporal and lineage-specific gene expression in mice

We have developed an advanced method for conditional gene expression in mice that integrates the Cre-mediated and tetracycline-dependent expression systems. An rtTA gene, preceded by a loxP-flanked STOP sequence, was inserted into the ROSA26 locus to create a R26STOPrtTA mouse strain. When the STOP sequence is excised by Cre-mediated recombination, the rtTA is expressed in the Cre-expressing cells and all of their derivatives. Therefore, cell type-, tissue-, or lineage-specific expression of rtTA is achieved by the use of an appropriate Cre transgenic strain. In mice also carrying a target gene under the control of the tetracycline response element, inducible expression of the target gene is temporally regulated by administration of doxycycline. Our results demonstrate that this universal system is uniquely suited for spatiotemporal and lineage-specific gene expression in an inducible fashion. Gene expression can be manipulated in specific cell types and lineages with a flexibility that is difficult to achieve with conventional methods.