A proliferative role for Wnt-3a in chick somites

The proper patterning of somites to give rise to sclerotome, dermomyotome, and myotome involves the coordination of many different cellular processes, including lineage specification, cell proliferation, cell death, and differentiation, by intercellular signals. One such family of secreted signaling proteins known to influence somite patterning is the Wnt family. Although the participation of Wnt-3a in the patterning of dorsal structures in the somite is well established, no clear consensus has emerged about the cellular processes that are governed by Wnt-3a in the somite. The recent demonstration that Wnt-3a has a proliferative role in the neural tube [Development 129 (2002) 2087] suggested that Wnt-3a might also act to regulate proliferation in somites. To test this hypothesis, we first analyzed the effects of Wnt-3a on segmental plate and somite explants (from Hamburger and Hamilton stage 10 chick embryos) grown in culture. These studies indicate that Wnt-3a is capable of maintaining and/or inducing expression of both Pax-3 and Pax-7, transcription factors that have been implicated in proliferation. To directly test for a role in proliferation, explants were immunostained with antibodies against phospho-histone H3. Explants treated with Wnt-3a show an increase in the percentage of cells expressing phospho-histone H3 as compared to controls. To test the proliferative effect of Wnt-3a in vivo, we ectopically expressed Wnt-3a in chick neural tubes via electroporation. Consistent with previous studies, ectopic expression of Wnt-3a in vivo results in a mediolateral expansion of the dermomyotome and myotome. We now show that proliferation of dorsal/dermomyotomal cells is significantly enhanced by ectopic Wnt-3a. Collectively, our explant and in vivo studies indicate that an increase in proliferation plays an important role in the expansion of the dermomyotome and myotome in Wnt-3a-treated embryos. Furthermore, our results demonstrate that small changes in proliferation can dramatically influence patterning and morphogenesis.

Differential regulation of osteogenic marker gene expression by Wnt-3a in embryonic mesenchymal multipotential progenitor cells

The Wnt family of secreted glycoproteins plays an integral role in embryonic development and differentiation. To explore the role of Wnt's in one aspect of differentiation, namely osteogenesis, we employed a retroviral gene transfer approach to express Wnt-3a in the multipotent murine embryonic mesenchymal cell line C3H10T1/2. We found that expression of Wnt-3a in these cells had a significant, positive effect on cell growth in serum-containing medium, in that the cells grew to very high densities compared to the control cells. Additionally, apoptosis was markedly inhibited by Wnt-3a. However, when the cells were grown in serum-deficient medium, the Wnt-3a-expressing cells arrested efficiently in G1 phase, indicating that serum growth factors were needed in addition to Wnt-3a for enhanced proliferation. Wnt-3a-expressing cells exhibited high levels of alkaline phosphatase gene expression and enzymatic activity, but did not show any matrix mineralization. Unexpectedly, basal expression of bone sialoprotein, osteocalcin, and osteopontin were markedly inhibited by Wnt-3a, as were other known target genes of Wnt-3a, such as Brachyury, FGF-10, and Cdx1. When Wnt-3a-expressing cells were treated with osteogenic supplements in the presence of BMP-2, alkaline phosphatase gene expression and activity were further elevated. Additionally, BMP-2 was able to reverse the inhibitory effect of Wnt-3a on osteocalcin and osteopontin gene expression. These results indicate that while Wnt-3a represses basal expression of some osteogenic genes, this repression can be partially reversed by BMP-2. Finally, the enhanced gene expression of alkaline phosphatase induced by Wnt-3a could be effectively suppressed by the combined action of dexamethasone and 1,25-dihydroxyvitamin D(3). These data show for the first time that Wnt-3a has an unusual effect on multipotential embryonic cells, in that it enhances cellular proliferation and expression of alkaline phosphatase, while it represses most other marker genes of osteogenic differentiation.

Combinatorial Wnt control of zebrafish midbrain–hindbrain boundary formation

Wnt signaling is known to be required for the normal development of the vertebrate midbrain and hindbrain, but genetic loss of function analyses in the mouse and zebrafish yield differing results regarding the relative importance of specific Wnt loci. In the zebrafish, Wnt1 and Wnt10b functionally overlap in their control of gene expression in the ventral midbrain-hindbrain boundary (MHB), but they are not required for the formation of the MHB constriction. Whether other wnt loci are involved in zebrafish MHB development is unclear, although the expression of at least two wnts, wnt3a and wnt8b, is maintained in wnt1/wnt10b mutants. In order to address the role of wnt3a in zebrafish, we have isolated a full length cDNA and examined its expression and function via knockdown by morpholino antisense oligonucleotide (MO)-mediated knockdown. The expression pattern of wnt3a appears to be evolutionarily conserved between zebrafish and mouse, and MO knockdown shows that Wnt3a, while not uniquely required for MHB development, is required in the absence of Wnt1 and Wnt10b for the formation of the MHB constriction. In zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b, the expression of engrailed orthologs, pax2a and fgf8 is not maintained after mid-somitogenesis. In contrast to acerebellar and no isthmus mutants, in which midbrain and hindbrain cells acquire new fates but cell number is not significantly affected until late in embryogenesis, zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b undergo extensive apoptosis in the midbrain and cerebellum anlagen beginning in mid-somitogenesis, which results in the absence of a significant portion of the midbrain and cerebellum. Thus, the requirement for Wnt signaling in forming the MHB constriction is evolutionarily conserved in vertebrates and it is possible in zebrafish to dissect the relative impact of multiple Wnt loci in midbrain and hindbrain development.

Wnt signaling through the beta-catenin pathway is sufficient to maintain, but not restore, anagen-phase characteristics of dermal papilla cells

Dermal papilla cells of the hair follicle can be maintained in an active, hair-inducing state in vitro when cocultured with cells secreting Wnt3a. By inducing cultured dermal papilla cells to secrete Wnt themselves, we demonstrate that this activity is a direct effect of Wnt signaling to dermal papilla cells. We further demonstrate that the effects of Wnt3a are exerted through activation of the beta-catenin signal transduction pathway and do not require alternative Wnt transduction cascades. Once dermal papilla cells have lost hair-inducing properties in vitro, neither treatment with Wnt nor expression of a truncated and activating form of beta-catenin is sufficient to restore these properties to the cultured cells.

Biochemical characterization of the Drosophila wingless signaling pathway based on RNA interference

Regulation of Armadillo (Arm) protein levels through ubiquitin-mediated degradation plays a central role in the Wingless (Wg) signaling. Although zeste-white3 (Zw3)-mediated Arm phosphorylation has been implicated in its degradation, we have recently shown that casein kinase Ialpha (CKIalpha) also phosphorylates Arm and induces its degradation. However, it remains unclear how CKIalpha and Zw3, as well as other components of the Arm degradation complex, regulate Arm phosphorylation in response to Wg. In particular, whether Wg signaling suppresses CKIalpha- or Zw3-mediated Arm phosphorylation in vivo is unknown. To clarify these issues, we performed a series of RNA interference (RNAi)-based analyses in Drosophila S2R+ cells by using antibodies that specifically recognize Arm phosphorylated at different serine residues. These analyses revealed that Arm phosphorylation at serine-56 and at threonine-52, serine-48, and serine-44, is mediated by CKIalpha and Zw3, respectively, and that Zw3-directed Arm phosphorylation requires CKIalpha-mediated priming phosphorylation. Daxin stimulates Zw3- but not CKIalpha-mediated Arm phosphorylation. Wg suppresses Zw3- but not CKIalpha-mediated Arm phosphorylation, indicating that a vital regulatory step in Wg signaling is Zw3-mediated Arm phosphorylation. In addition, further RNAi-based analyses of the other aspects of the Wg pathway clarified that Wg-induced Dishevelled phosphorylation is due to CKIalpha and that presenilin and protein kinase A play little part in the regulation of Arm protein levels in Drosophila tissue culture cells.

Newly expressed proteins of mouse embryonic fibroblasts irradiated to be inactive

It has been found that post-radiation mouse embryonic fibroblasts can well maintain the pluripotency in human embryonic stem cells. However, the molecular mechanism remains unclear. In the present study, the new protein expression profile of post-radiation mouse embryonic fibroblasts was analyzed by immobilized pH gradient 2-dimensional polyacrylamide gel electrophoresis. Image analysis following silver staining revealed (969+/-57) vs. (1085+/-107) spots from post-radiation mouse embryonic fibroblasts and pre-radiation ones, respectively. Some newly expressed proteins, which were only abundantly present after irradiation, were subjected to peptide mass fingerprint analysis and identified using MALDI-TOF-MS, SWISS-PROT database, and RT-PCR. Several of those proteins were preliminarily identified to participate in cytokine secretion, cell signal transduction, transcriptional regulation, and apoptosis, etc., which suggested that inactive post-radiation mouse embryonic fibroblasts expressed some new proteins that may underlie the molecular mechanisms to maintain the pluripotency in human embryonic stem cells.

Wnt proteins promote neuronal differentiation in neural stem cell culture

Wnt signaling is implicated in the control of cell growth and differentiation during CNS development from studies of mouse and chick models, but its action at the cellular level has been poorly understand. In this study, we examine the in vitro function of Wnt signaling in embryonic neural stem cells, dissociated from neurospheres derived from E11.5 mouse telencephalon. Conditioned media containing active Wnt-3a proteins are added to the neural stem cells and its effect on regeneration of neurospheres and differentiation into neuronal and glial cells was examined. Wnt-3a proteins inhibit regeneration of neurospheres, but promote differentiation into MAP2-positive neuronal cells. Wnt-3a proteins also increase the number of GFAP-positive astrocytes but suppress the number of oligodendroglial lineage cells expressing PDGFR or O4. These results indicate that Wnt-3a signaling can inhibit the maintenance of neural stem cells, but rather promote the differentiation of neural stem cells into several cell lineages.

Homozygous WNT3 mutation causes tetra-amelia in a large consanguineous family

Tetra-amelia is a rare human genetic disorder characterized by complete absence of all four limbs and other anomalies. We studied a consanguineous family with four affected fetuses displaying autosomal recessive tetra-amelia and craniofacial and urogenital defects. By homozygosity mapping, the disease locus was assigned to chromosome 17q21, with a maximum multipoint LOD score of 2.9 at markers D17S931, D17S1785, D17SS1827, and D17S1868. Further fine mapping defined a critical interval of approximately 8.9 Mb between D17S1299 and D17S797. We identified a homozygous nonsense mutation (Q83X) in the WNT3 gene in affected fetuses of the family. WNT3, a human homologue of the Drosophila wingless gene, encodes a member of the WNT family known to play key roles in embryonic development. The Q83X mutation truncates WNT3 at its amino terminus, suggesting that loss of function is the most likely cause of the disorder. Our findings contrast with the observation of early lethality in mice homozygous for null alleles of Wnt3. To our knowledge, this is the first report of a mutation in a WNT gene associated with a Mendelian disorder. The identification of a WNT3 mutation in tetra-amelia indicates that WNT3 is required at the earliest stages of human limb formation and for craniofacial and urogenital development.

Two new mouse chromosome 11 balancers

Segmental inversions causing recombination suppression are an essential feature of balancer chromosomes. Meiotic crossing over between homologous chromosomes within an inversion interval will lead to nonviable gametes, while gametes generated from recombination events elsewhere on the chromosome will be unaffected. This apparent recombination suppression has been widely exploited in genetic studies in Drosophila to maintain and analyze stocks carrying recessive lethal mutations. Balancers are particularly useful in mutagenesis screens since they help to establish the approximate genomic location of alleles of genes causing phenotypes. Using the Cre-loxP recombination system, we have constructed two mouse balancer chromosomes carrying 8- and 30-cM inversions between Wnt3 and D11Mit69 and between Trp53 and EgfR loci, respectively. The Wnt3-D11Mit69 inversion mutates the Wnt3 locus and is therefore homozygous lethal. The Trp53-EgfR inversion is homozygous viable, since the EgfR locus is intact and mutations in p53 are homozygous viable. A dominantly acting K14-agouti minigene tags both rearrangements, which enables these balancer chromosomes to be visibly tracked in mouse stocks. With the addition of these balancers to the previously reported Trp53-Wnt3 balancer, most of mouse chromosome 11 is now available in balancer stocks.

Beta-catenin regulates Cripto- and Wnt3-dependent gene expression programs in mouse axis and mesoderm formation

Gene expression profiling of beta-catenin, Cripto and Wnt3 mutant mouse embryos has been used to characterise the genetic networks that regulate early embryonic development. We have defined genes whose expression is regulated by beta-catenin during formation of the anteroposterior axis and the mesoderm, and have identified Cripto, which encodes a Nodal co-receptor, as a primary target of beta-catenin signals both in embryogenesis as well as in colon carcinoma cell lines and tissues. We have also defined groups of genes regulated by Wnt3/beta-catenin signalling during primitive streak and mesoderm formation. Our data assign a key role to beta-catenin upstream of two distinct gene expression programs during anteroposterior axis and mesoderm formation.

The human Frizzled 6 (HFz6) acts as a negative regulator of the canonical Wnt. beta-catenin signaling cascade

Previously we have cloned the human Frizzled 1 (HFz1) and shown that it transmits the Wnt-3a-induced canonical pathway. We also cloned the human Frizzled 6 (HFz6) and show in the present study that, as opposed to HFz1, HFz6 did not activate the canonical Wnt pathway following exposure to various Wnts, whether belonging to the Wnt-1 or to the Wnt-5a group. Moreover we show that HFz6 repressed Wnt-3a-induced canonical signaling when co-expressed with HFz1. HFz6 repressed the canonical Wnt cascade activated also by various Wnt signaling intracellular mediators such as Dishevelled-1, a stabilized beta-catenin(S33Y) mutant, and LiCl-mediated repression of glycogen synthase kinase-3beta activity. Removal of HFz6 N'- or C'-terminal sequences abolished HFz6 repressive activity. As the HFz6 repressive effect was not associated with a decrease in the level of beta-catenin, it is suggested that HFz6 does not affect beta-catenin stabilization, implying that HFz6 transmits a repressive signaling that cross-talks with and inhibits the canonical Wnt pathway downstream of beta-catenin destruction complex. HFz6 did not affect the level of nuclear T-cell factor 4 (TCF4) nor did it affect beta-catenin.TCF4 complex formation. However, electrophoretic mobility shift assays indicated that HFz6 repressed the binding of TCF/lymphoid enhancer factor transcription factors to target DNA. Moreover we present data suggesting that HFz6 activates the transforming growth factor-beta-activated kinase-NEMO-like kinase pathway that blocks TCF/lymphoid enhancer factor binding to target promoters, thereby inhibiting the ability of beta-catenin to activate transcription of Wnt target genes.

The low density lipoprotein receptor-1, LRP1, interacts with the human frizzled-1 (HFz1) and down-regulates the canonical Wnt signaling pathway

Members of the low density lipoprotein receptor family (LDLR), LRP5/6, were shown to interact with the Frizzled (Fz) receptors and to function as Wnt coreceptors. Here we show that mLRP4T100, a minireceptor of LRP1, another member of the LDLR family, interacts with the human Fz-1 (HFz1), previously shown to serve as a receptor transmitting the canonical Wnt-3a-induced signaling cascade. However, in contrast to LRP5/6, mLRP4T100, as well as the full-length LRP1, did not cooperate with HFz1 in transmitting the Wnt-3a signaling but rather repressed it. mLRP4T100 inhibitory effect was displayed also by endocytosis-defective mLRP4T100 mutants, suggesting that LRP1 repressive effect is not attributable to LRP1-mediated enhanced HFz1 internalization and subsequent degradation. Enforced expression of mLRP4T100 decreased the capacity of HFz1 cysteine-rich domain (CRD) to interact with LRP6, in contrast to HFz1-CRD/Wnt-3a interaction that was not disrupted by overexpressing mLRP4T100. These data suggest that LRP1, by sequestering HFz1, disrupts the receptor/coreceptor complex formation, leading to the repression of the canonical Wnt signaling. Thus, this study implies that the ability to interact with Fz receptors is shared by several members of the LDLR family. However, whereas some members of the LDLR family, such as LRP5/6, interact with Fz and serve as Wnt coreceptors, others negatively regulate Wnt signaling, presumably by sequestering Fz.

Wnt Target Genes Identified by DNA Microarrays in Immature CD34+Thymocytes Regulate Proliferation and Cell Adhesion

The thymus is seeded by very small numbers of progenitor cells that undergo massive proliferation before differentiation and rearrangement of TCR genes occurs. Various signals mediate proliferation and differentiation of these cells, including Wnt signals. Wnt signals induce the interaction of the cytoplasmic cofactor beta-catenin with nuclear T cell factor (TCF) transcription factors. We identified target genes of the Wnt/beta-catenin/TCF pathway in the most immature (CD4-CD8-CD34+) thymocytes using Affymetrix DNA microarrays in combination with three different functional assays for in vitro induction of Wnt signaling. A relatively small number (approximately 30) of genes changed expression, including several proliferation-inducing transcription factors such as c-fos and c-jun, protein phosphatases, and adhesion molecules, but no genes involved in differentiation to mature T cell stages. The adhesion molecules likely confine the proliferating immature thymocytes to the appropriate anatomical sites in the thymus. For several of these target genes, we validated that they are true Wnt/beta-catenin/TCF target genes using real-time quantitative PCR and reporter gene assays. The same core set of genes was repressed in Tcf-1-null mice, explaining the block in early thymocyte development in these mice. In conclusion, Wnt signals mediate proliferation and cell adhesion, but not differentiation of the immature thymic progenitor pool.

Sonic hedgehog patterning in chick neural plate is antagonized by a Wnt3-like signal

Sonic hedgehog (Shh) patterns the dorsal-ventral axis of the neural tube by promoting the differentiation of ventral neural cell types while suppressing dorsal neural fates. Other signals impinge upon the Shh response, biasing the differentiation of a cell. Three dorsally expressed transforming Wnts, of which the most broadly expressed is Wnt3, may be among the signals that influence the Shh response. We demonstrate that activation of Wnt signaling results in an inhibition of the Shh response in neural tissue. Additionally, we show that the expression pattern of chick Wnt3 is consistent with a role in neural patterning. These results indicate that differentiating neural tube cells, besides integrating signals from Hedgehogs and BMPs, may also incorporate a Wnt response to make cell fate decisions.

Identification and characterization of a novel Dvl-binding protein that suppresses Wnt signalling pathway

BACKGROUND

Dvl is a cytoplasmic protein to regulate the stability of beta-catenin in the Wnt signalling pathway. However, the molecular mechanism by which Dvl regulates the Wnt signalling pathway is not fully understood.

RESULTS

We identified a novel protein that binds to Dvl and named it Daple. Daple consisted of 2009 amino acids with a high frequency of leucine residues and formed a homo-oligomer. The C-terminal three amino acids of Daple were necessary for binding to the region containing the PDZ domain of Dvl. Expression of Daple in mouse fibroblast L cells inhibited Wnt-3a-induced accumulation of beta-catenin. Furthermore, Daple inhibited Wnt-3a-dependent activation of T-cell factor (Tcf) transcriptional activity. Expression of Daple in the dorsal region of Xenopus embryos inhibited axis formation, which is known to be regulated by the Wnt signalling pathway. Daple also inhibited Dvl-induced secondary axis formation in Xenopus embryos.

CONCLUSIONS

Daple binds to Dvl and functions as a negative regulator of the Wnt signalling pathway.

Murine Frizzled-1 behaves as an antagonist of the canonical Wnt/beta-catenin signaling

Activation of the Wnt signaling cascade provides key signals during development and in disease. Wnt signals are transduced by seven-transmembrane Frizzleds (Fzs) and the single transmembrane low density lipoprotein receptor-related proteins 5 or 6. In the course of the analysis of genes regulated by bone morphogenetic protein 2 in mesenchymal cells we found a significant induction of murine Frizzled-1 (mFz1) gene expression. Unexpectedly overexpression of mFz1 dramatically repressed the induction of alkaline phosphatase mediated by either bone morphogenetic protein 2 or Wnt3a in these cells. Moreover mFz1 overexpression significantly repressed both beta-catenin translocation into the nucleus and T cell factor signaling mediated by Wnt3a. Importantly microinjection of mFz1 transcript in Xenopus embryo inhibited the ability of Wnt1 to induce the expression of the Wnt/beta-catenin target gene Siamois in animal cap assay and secondary axis formation in whole embryo. By using chimeric constructs in which N- and C-terminal segments of mFz1 were replaced by the corresponding parts of Xfz3 we demonstrated that the antagonistic activity resides in the cysteine-rich domain of the N-terminal part. The antagonist activity of mFz1 could be prevented by overexpression of Galphaq-(305-359), which specifically uncouples Gq-coupled receptors, suggesting that Galphaq signaling contributes to the inhibition of Wnt/beta-catenin pathway by mFz1. This is the first time that a Frizzled receptor has been reported to antagonize Wnt/beta-catenin.

Essential role of glycosaminoglycans in Fgf signaling during mouse gastrulation

In vitro studies have suggested that proteoglycans facilitate signaling by mammalian growth factors, but genetic evidence supporting this role has been lacking. Here, we characterize the ENU-induced mutation lazy mesoderm (lzme), which disrupts the single mouse gene encoding UDP-glucose dehydrogenase (Ugdh), an enzyme required for the synthesis of the glycosaminoglycan (GAG) side chains of proteoglycans. lzme mutants arrest during gastrulation with defects in migration of mesoderm and endoderm, a phenotype similar to that of mutants in the fibroblast growth factor (Fgf) pathway. Analysis of the expression of molecular markers indicates that Fgf signaling is blocked in lzme mutant embryos. In contrast, signaling by the growth factors Nodal and Wnt3, which are also essential during mouse gastrulation, appears to be normal in lzme embryos. The results demonstrate that proteoglycans are required during mouse gastrulation specifically to promote Fgf signaling.

A restrictive role for Hedgehog signalling during otic specification in Xenopus

Vertebrate inner ear development is initiated by the specification of the otic placode, an ectodermal structure induced by signals from neighboring tissue. Although several signaling molecules have been identified as candidate otic inducers, many details of the process of inner ear induction remain elusive. Here, we report that otic induction is responsive to the level of Hedgehog (Hh) signaling activity in Xenopus, making use of both gain- and loss-of-function approaches. Ectopic activation of Hedgehog signaling resulted in the development of ectopic vesicular structures expressing the otic marker genes XPax-2, Xdll-3, and Xwnt-3A, thus revealing otic identity. Induction of ectopic otic vesicles was also achieved by misexpression of two different inhibitors of Hh signaling: the putative Hh antagonist mHIP and XPtc1deltaLoop2, a dominant-negative form of the Hh receptor Patched. In addition, misexpression of XPtc1deltaLoop2 as well as treatment of Xenopus embryos with the specific Hh signaling antagonist cyclopamine resulted in the formation of enlarged otic vesicles. In summary, our observations suggest that a defined level of Hh signaling provides a restrictive environment for otic fate in Xenopus embryos.

A disrupted balance between Bmp/Wnt and Fgf signaling underlies the ventralization of the Gli3 mutant telencephalon

Regionalization of the neural plate and the early neural tube is controlled by several signaling centers that direct the generation of molecularly distinct domains. In the developing telencephalon, the anterior neural ridge (ANR) and the roof and floor plate act as such organizing centers via the production of Fgfs, Bmps/Wnts, and Shh, respectively. It remains largely unknown, however, how the combination of these different signals is used to coordinate the generation of different telencephalic territories. In the present study, we report on telencephalic development in Pdn mutant mice, which carry an integration of a retrotransposon in the Gli3 locus. Homozygous mutant animals are characterized by a partial dorsal-to-ventral transformation of the telencephalon and by an increased size of the septum. On a molecular level, these alterations correlate with a reduction and/or loss of Bmp/Wnt expression and a concomitant expansion of Fgf8 transcription. Finally, we provide evidence that the ectopic activation of Fgf signaling in the dorsal telencephalon provides an explanation for the ventralization of the Gli3 mutant telencephalon as application of Fgf8-soaked beads to dorsal telencephalic explants led to the specific induction and repression of ventral marker and dorsal marker genes, respectively.

Characterization of Wnt signaling components and activation of the Wnt canonical pathway in the murine retina

The neuroepithelial layer of the developing eyecup contains multipotential precursor cells that give rise to all of the neurons and the one glial cell type present in the adult retina. Patterning within the retinal neuroepithelium is regulated by cell intrinsic as well as cell extrinsic mechanisms. Although the identity of some of the signaling molecules that regulate retinal development is known, the function of many others, especially members of the Wnt family, has yet to be characterized in the context of retinal development. We undertook a comprehensive in situ hybridization analysis to examine the expression of Wnt pathway components in the developing and adult mouse neural retina. Our findings confirm and extend previous expression studies in mice and other vertebrates, as we show that Wnt-3, -5a, -5b, and -7b are expressed in the neural retina and that there is a dynamic pattern of Wnt receptor (Mouse frizzled [Mfz]) and Wnt antagonist (Secreted-frizzled-related protein [Sfrp]) gene expression in the embryonic and perinatal neural retina. Moreover, we show that Wnt-13 is expressed in the pigment epithelium overlying the distal part of the eyecup and the ciliary margin and that Mfz-4, -6, and -7 are expressed in different regions within the ciliary margin. To determine where activation of canonical Wnt signaling is occurring in the retina, we examined reporter gene expression in TCF/Lef-LacZ mice and we demonstrate that the highest levels of beta-gal activity are found in the ciliary margin, adjacent to and within the Wnt-13 expression domain, implicating Wnt-13 signaling in the development of the ciliary margin and its derivatives.

Regulation of the Wnt signaling pathway by disabled-2 (Dab2)

The adaptor molecule Disabled-2 (Dab2) has been shown to link cell surface receptors to downstream signaling pathways. Using a small-pool cDNA screening strategy, we identify that the N-terminal domain of Dab2 interacts with Dishevelled-3 (Dvl-3), a signaling mediator of the Wnt pathway. Ectopic expression of Dab2 in NIH-3T3 mouse fibroblasts attenuates canonical Wnt/beta-catenin-mediated signaling, including accumulation of beta-catenin, activation of beta-catenin/T-cell-specific factor/lymphoid enhancer-binding factor 1-dependent reporter constructs, and endogenous cyclin D1 induction. Wnt stimulation leads to a time-dependent dissociation of endogenous Dab2-Dvl-3 and Dvl-3-axin interactions in NIH-3T3 cells, while Dab2 overexpression leads to maintenance of Dab2-Dvl-3 association and subsequent loss of Dvl-3-axin interactions. In addition, we find that Dab2 can associate with axin in vitro and stabilize axin expression in vivo. Mouse embryo fibroblasts which lack Dab2 exhibit constitutive Wnt signaling as evidenced by increased levels of nuclear beta-catenin and cyclin D1 protein levels. Based on these results, we propose that Dab2 functions as a negative regulator of canonical Wnt signaling by stabilizing the beta-catenin degradation complex, which may contribute to its proposed role as a tumor suppressor.

[Regulation of development of oligodendrocyte]

Oligodendrocyte (OL) progenitors in the mouse spinal cord are generated from a strictly restricted region in the ventricular zone of the ventral cord as early as on embryonic day 11 (E11). We previously reported that one of the factors that restrict this ventral specific appearance of OLs is an inhibitory factor secreted from the dorsal spinal cord, in addition to well-known stimulatory ventral factors such as sonic hedgehog. We characterized the developmental change of the inhibitory activities. They were very strong at E11, gradually reduced, and disappeared by E14. This pattern seemed to be well correlated with the developmental profile of Wnt3a expression at/near the roof plate. A conditioned medium of L cells that stably express Wnt3a showed significant reduction of O4 positive OLs in the ventral spinal cord explants, indicating that Wnt3a is one of the dorsal factors that inhibit OL development. Addition of Wnt3a supernatant to CG4 cells, an OL progenitor strain, and to the dissociated primary cultured cells suggested that Wnt3a directly acts on OL lineage cells and inhibits a differentiation step from OL progenitor to O4-positive stage. Thus, Wnt3a may directly control the timing of OL differentiation and the motility of OL lineage cells. A population of myelinating OLS in the dorsal area of telencephalon was further demonstrated to be ventral origin by the newly established cell marking system using in utero DNA electroporation.

Wnt proteins are lipid-modified and can act as stem cell growth factors

Wnt signalling is involved in numerous events in animal development, including the proliferation of stem cells and the specification of the neural crest. Wnt proteins are potentially important reagents in expanding specific cell types, but in contrast to other developmental signalling molecules such as hedgehog proteins and the bone morphogenetic proteins, Wnt proteins have never been isolated in an active form. Although Wnt proteins are secreted from cells, secretion is usually inefficient and previous attempts to characterize Wnt proteins have been hampered by their high degree of insolubility. Here we have isolated active Wnt molecules, including the product of the mouse Wnt3a gene. By mass spectrometry, we found the proteins to be palmitoylated on a conserved cysteine. Enzymatic removal of the palmitate or site-directed and natural mutations of the modified cysteine result in loss of activity, and indicate that the lipid is important for signalling. The purified Wnt3a protein induces self-renewal of haematopoietic stem cells, signifying its potential use in tissue engineering.

A Wnt- and beta -catenin-dependent pathway for mammalian cardiac myogenesis

Acquisition of a cardiac fate by embryonic mesodermal cells is a fundamental step in heart formation. Heart development in frogs and avians requires positive signals from adjacent endoderm, including bone morphogenic proteins, and is antagonized by a second secreted signal, Wnt proteins, from neural tube. By contrast, mechanisms of mesodermal commitment to create heart muscle in mammals are largely unknown. In addition, Wnt-dependent signals can involve either a canonical beta-catenin pathway or other, alternative mediators. Here, we tested the involvement of Wnts and beta-catenin in mammalian cardiac myogenesis by using a pluripotent mouse cell line (P19CL6) that recapitulates early steps for cardiac specification. In this system, early and late cardiac genes are up-regulated by 1% DMSO, and spontaneous beating occurs. Notably, Wnt3A and Wnt8A were induced days before even the earliest cardiogenic transcription factors. DMSO induced biochemical mediators of Wnt signaling (decreased phosphorylation and increased levels of beta-catenin), which were suppressed by Frizzled-8Fc, a soluble Wnt antagonist. DMSO provoked T cell factor-dependent transcriptional activity; thus, induction of Wnt proteins by DMSO was functionally coupled. Frizzled-8Fc inhibited the induction of cardiogenic transcription factors, cardiogenic growth factors, and sarcomeric myosin heavy chains. Likewise, differentiation was blocked by constitutively active glycogen synthase kinase 3beta, an intracellular inhibitor of the Wntbeta-catenin pathway. Conversely, lithium chloride, which inhibits glycogen synthase kinase 3beta, and Wnt3A-conditioned medium up-regulated early cardiac markers and the proportion of differentiated cells. Thus, Wntbeta-catenin signaling is activated at the inception of mammalian cardiac myogenesis and is indispensable for cardiac differentiation, at least in this pluripotent model system.

Casein kinase I epsilon enhances the binding of Dvl-1 to Frat-1 and is essential for Wnt-3a-induced accumulation of beta-catenin

We demonstrate that Dvl-1, casein kinase I epsilon (CKI epsilon), and Frat-1 activate the Wnt signaling pathway cooperatively. The amino acid region 228-250 of Dvl-1 was necessary for its binding to Frat-1, and the interaction of Dvl-1 with Frat-1 was enhanced by CKI epsilon. Coexpression of Dvl-1 and Frat-1 caused accumulation of beta-catenin synergistically in L cells. Both proteins also activated the transcriptional activity of T-cell factor-4 (Tcf-4) synergistically in human embryonic kidney 293 cells, but coexpression of Dvl-1-(Delta 228-250), which lacks the amino acid region 228-250 from Dvl-1, and Frat-1 did not. Dvl-1, but not Dvl-1-(Delta 228-250), acted synergistically with CKI epsilon to activate Tcf-4. Depletion of CKI epsilon by double-stranded RNA interference in HeLa S3 cells led to the inhibition of Wnt-3a-induced phosphorylation of Dvl and the binding of Dvl-1 to Frat-1. Furthermore, depletion of CKI epsilon reduced the Wnt-3a-induced accumulation of beta-catenin, although it did not affect the basal level of beta-catenin. These results indicate that CKI epsilon-dependent phosphorylation of Dvl enhances the formation of a complex of Dvl-1 with Frat-1 and that this complex leads to the activation of the Wnt signaling pathway.

Wnt signaling in B-cell neoplasia

Wnts comprise a family of secreted proteins that interact with receptors consisting of a Frizzled (Fz) family member alone or complexed with LDL receptor-related proteins (LRP5/6). Wnt signaling plays a crucial role in both development and differentiation, and activation of a 'canonical' Wnt pathway resulting in beta-catenin stabilization is associated with several types of human cancers. To date, little is known about potential Wnt signaling in mature lymphocytes or lymphoid neoplasia. Herein, we have analysed Wnt signaling in mature B cells (lymphomas) and plasma cells (multiple myeloma). Both Fz and LRP5/6 mRNAs were expressed in myeloma lines, but LRP5/6 were not observed in lymphomas. In myelomas, a canonical Wnt signaling pathway was activated following treatment with Wnt-3a as assessed by accumulation of beta-catenin, but beta-catenin levels actually decreased in lymphoma cells. Wnt-3a treatment further led to striking morphological changes in myeloma cells accompanied by rearrangement of the actin cytoskeleton. Morphological changes were associated with a second Wnt pathway dependent on Rho activation. These results suggest that Wnt responsiveness is a stage-specific phenomenon in B-cell development and that the morphological changes associated with Wnt signaling may play a role in the motility and metastatic potential of myeloma cells.

Wnt3a plays a major role in the segmentation clock controlling somitogenesis

The vertebral column derives from somites generated by segmentation of presomitic mesoderm (PSM). Somitogenesis involves a molecular oscillator, the segmentation clock, controlling periodic Notch signaling in the PSM. Here, we establish a novel link between Wnt/beta-catenin signaling and the segmentation clock. Axin2, a negative regulator of the Wnt pathway, is directly controlled by Wnt/beta-catenin and shows oscillating expression in the PSM, even when Notch signaling is impaired, alternating with Lfng expression. Moreover, Wnt3a is required for oscillating Notch signaling activity in the PSM. We propose that the segmentation clock is established by Wnt/beta-catenin signaling via a negative-feedback mechanism and that Wnt3a controls the segmentation process in vertebrates.

Mesd encodes an LRP5/6 chaperone essential for specification of mouse embryonic polarity

Specification of embryonic polarity and pattern formation in multicellular organisms requires inductive signals from neighboring cells. One approach toward understanding these interactions is to study mutations that disrupt development. Here, we demonstrate that mesd, a gene identified in the mesoderm development (mesd) deletion interval on mouse chromosome 7, is essential for specification of embryonic polarity and mesoderm induction. MESD functions in the endoplasmic reticulum as a specific chaperone for LRP5 and LRP6, which in conjunction with Frizzled, are coreceptors for canonical WNT signal transduction. Disruption of embryonic polarity and mesoderm differentiation in mesd-deficient embryos likely results from a primary defect in WNT signaling. However, phenotypic differences between mesd-deficient and wnt3(-)(/)(-) embryos suggest that MESD may function on related members of the low-density lipoprotein receptor (LDLR) family, whose members mediate diverse cellular processes ranging from cargo transport to signaling.

Ectodermal Wnt3/beta-catenin signaling is required for the establishment and maintenance of the apical ectodermal ridge

The formation of the apical ectodermal ridge (AER) is critical for the distal outgrowth and patterning of the vertebrate limb. Recent work in the chick has demonstrated that interplay between the Wnt and Fgf signaling pathways is essential in the limb mesenchyme and ectoderm in the establishment and perhaps the maintenance of the AER. In the mouse, whereas a role for Fgfs for AER establishment and function has been clearly demonstrated, the role of Wnt/beta-catenin signaling, although known to be important, is obscure. In this study, we demonstrate that Wnt3, which is expressed ubiquitously throughout the limb ectoderm, is essential for normal limb development and plays a critical role in the establishment of the AER. We also show that the conditional removal of beta-catenin in the ventral ectodermal cells is sufficient to elicit the mutant limb phenotype. In addition, removing beta-catenin after the induction of the ridge results in the disappearance of the AER, demonstrating the requirement for continued beta-catenin signaling for the maintenance of this structure. Finally, we demonstrate that Wnt/beta-catenin signaling lies upstream of the Bmp signaling pathway in establishment of the AER and regulation of the dorsoventral polarity of the limb.

Activation of Wnt signaling rescues neurodegeneration and behavioral impairments induced by beta-amyloid fibrils

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, which is probably caused by the cytotoxic effect of the amyloid beta-peptide (Abeta). We report here molecular changes induced by Abeta, both in neuronal cells in culture and in rats injected in the dorsal hippocampus with preformed Abeta fibrils, as an in vivo model of the disease. Results indicate that in both systems, Abeta neurotoxicity resulted in the destabilization of endogenous levels of beta-catenin, a key transducer of the Wnt signaling pathway. Lithium chloride, which mimics Wnt signaling by inhibiting glycogen synthase kinase-3beta promoted the survival of post-mitotic neurons against Abeta neurotoxicity and recovered cytosolic beta-catenin to control levels. Moreover, the neurotoxic effect of Abeta fibrils was also modulated with protein kinase C agonists/inhibitors and reversed with conditioned medium containing the Wnt-3a ligand. We also examined the spatial memory performance of rats injected with preformed Abeta fibrils in the Morris water maze paradigm, and found that chronic lithium treatment protected neurodegeneration by rescuing beta-catenin levels and improved the deficit in spatial learning induced by Abeta. Our results are consistent with the idea that Abeta-dependent neurotoxicity induces a loss of function of Wnt signaling components and indicate that lithium or compounds that mimic this signaling cascade may be putative candidates for therapeutic intervention in Alzheimer's patients.

Wnt glycoproteins regulate the expression of FoxN1, the gene defective in nude mice

T cell development and selection require the fully mature and diverse epithelial microenvironment of the thymus. Acquisition of these characteristics is dependent on expression of the forkhead (also known as winged-helix) transcription factor FoxN1, as a lack of functional FoxN1 results in aberrant epithelial morphogenesis and an inability to attract lymphoid precursors to the thymus primordium. However, the transcriptional control of Foxn1 expression has not been elucidated. Here we report that secreted Wnt glycoproteins, expressed by thymic epithelial cells and thymocytes, regulate epithelial Foxn1 expression in both autocrine and paracrine fashions. Wnt molecules therefore provide regulatory signals critical for thymic function.

A sonic hedgehog-dependent signaling relay regulates growth of diencephalic and mesencephalic primordia in the early mouse embryo

Sonic hedgehog (Shh) is a key signal in the specification of ventral cell identities along the length of the developing vertebrate neural tube. In the presumptive hindbrain and spinal cord, dorsal development is largely Shh independent. By contrast, we show that Shh is required for cyclin D1 expression and the subsequent growth of both ventral and dorsal regions of the diencephalon and midbrain in early somite-stage mouse embryos. We propose that a Shh-dependent signaling relay regulates proliferation and survival of dorsal cell populations in the diencephalon and midbrain. We present evidence that Fgf15 shows Shh-dependent expression in the diencephalon and may participate in this interaction, at least in part, by regulating the ability of dorsal neural precursors to respond to dorsally secreted Wnt mitogens.

Inhibition by Wnt-1 or Wnt-3a of nerve growth factor-induced differentiation of PC12 cells is reversed by bisindolylmaleimide-I but not by several other PKC inhibitors

Wnt-1 and Wnt-3a exhibit redundancy in neural crest development. We have found that they do not produce the same effects on PC12 cells, which were obtained from the adrenal medulla, a neural crest derivative. However, both Wnt-1 or Wnt-3a inhibit nerve growth factor (NGF)-induced neurite outgrowth. The inhibition is reversed by the protein kinase C (PKC) inhibitor, bisindolylmaleimide-I, but it did not reverse Wnt-1-induced activation of the canonical Wnt pathway. The Wnt-1 inhibitory effect was not reversed by several other PKC inhibitors, by phorbol ester-induced down-regulation of PKC, or by pertussis toxin, which is known to inhibit another Wnt signaling pathway, the Wnt/Ca(2+) pathway. We suggest that bisindolylmaleimide-I acts by affecting either a pathway downstream from Lef-1/Tcf in the canonical pathway or a Wnt signaling pathway other than the canonical pathway. In either case, the bisindolylmaleimide-I sensitivity of this pathway should aid in its identification.

WNT-3, expressed by motoneurons, regulates terminal arborization of neurotrophin-3-responsive spinal sensory neurons

Sensory axons from dorsal root ganglia neurons are guided to spinal targets by molecules differentially expressed along the dorso-ventral axis of the neural tube. NT-3-responsive muscle afferents project ventrally, cease extending, and branch upon contact with motoneurons (MNs), their synaptic partners. We have identified WNT-3 as a candidate molecule that regulates this process. Wnt-3 is expressed by MNs of the lateral motor column at the time when MNs form synapses with sensory neurons. WNT-3 increases branching and growth cone size while inhibiting axonal extension in NT-3- but not NGF-responsive axons. Ventral spinal cord secretes factors with axonal remodeling activity for NT-3-responsive neurons. This activity is present at limb levels and is blocked by a WNT antagonist. We propose that WNT-3, expressed by MNs, acts as a retrograde signal that controls terminal arborization of muscle afferents.

Wnt-3A/beta-catenin signaling induces transcription from the LEF-1 promoter

Members of the Wnt family of secreted molecules have been established as key factors in determining cell fate and morphogenic signaling. It has long been recognized that Wnt induces morphogenic signaling through the Tcf/LEF-1 cascade by regulating free intracellular levels of beta-catenin, a co-factor for Tcf/LEF-1 transcription factors. In the present study, we have demonstrated that Wnt-3A can also directly induce transcription from the LEF-1 promoter. This induction was dependent on glycogen synthase kinase 3beta inactivation, a rise in free intracellular beta-catenin, and a short 110-bp Wnt-responsive element (WRE) in the LEF-1 promoter. Linear and internal deletion of this WRE led to a dramatic increase in constitutive LEF-1 promoter activity and loss of Wnt-3A responsiveness. In isolation, the 110-bp WRE conferred context-independent Wnt-3A or beta-catenin(S37A) responsiveness to a heterologous SV40 promoter. Studies expressing dominant active and negative forms of LEF-1, beta-catenin, GSK-3beta, and beta-catenin/LEF-1 fusions suggest that Wnt-3A activates the LEF-1 promoter through a beta-catenin-dependent and LEF-1-independent process. Wnt-3A expression also induced multiple changes in the binding of factors to the WRE and suggests that regulatory mechanisms may involve modulation of a multiprotein complex. In summary, these results provide evidence for transcriptional regulation of the LEF-1 promoter by Wnt and enhance the mechanistic understanding of Wnt/beta-catenin signaling in the regulation of LEF-1-dependent developmental processes.

Maternal diabetes increases the risk of caudal regression caused by retinoic acid

Maternal diabetes increases the risk of congenital malformations in the offspring of affected pregnancies. This increase arises from the teratogenic effect of the maternal diabetic milieu on the developing embryo, although the mechanism of this action is poorly understood. In the present study, we examined whether the vitamin A metabolite retinoic acid (RA), a common drug with well-known teratogenic properties, may interact with maternal diabetes to alter the incidence of congenital malformations in mice. Our results show that when treated with RA, embryos of diabetic mice are significantly more prone than embryos of nondiabetic mice to develop caudal regression, a defect that is highly associated with diabetic pregnancy in humans. By studying the vestigial tail (Wnt-3a(vt)) mutant, we provide evidence that Wnt-3a, a gene that controls the development of the caudal region, is directly involved in the pathogenic pathway of RA-induced caudal regression. We further show that the molecular basis of the increased susceptibility of embryos of diabetic mice to RA involves enhanced downregulation of Wnt-3a expression. This positive interaction between RA and maternal diabetes may have implications for humans in suggesting increased susceptibility to environmental teratogens during diabetic pregnancy.

Wnt-3A enhances bone morphogenetic protein-2-mediated chondrogenesis of murine C3H10T1/2 mesenchymal cells

We have recently reported the chondrogenic effect of bone morphogenetic protein-2 (BMP-2) in high density cultures of the mouse multipotent mesenchymal C3H10T1/2 cell line and have shown the functional requirement of the cell-cell adhesion molecule N-cadherin in BMP-2-induced chondrogenesis in vitro (Denker, A. E., Nicoll, S. B., and Tuan, R. S. (1995) Differentiation 59, 25-34; Haas, A. R., and Tuan, R. S. (1999) Differentiation 64, 77-89). Furthermore, BMP-2 treatment also results in an increased protein level of beta-catenin, a known N-cadherin-associated Wnt signal transducer (Fischer, L., Haas, A., and Tuan, R. S. (2001) Signal Transduction 2, 66-78), suggesting functional cross-talk between the BMP-2 and Wnt signaling pathways. We have observed previously that BMP-2 treatment up-regulates expression of Wnt-3A in high density cultures of C3H10T1/2 cells. To assess the contribution of Wnt-3A to BMP-2-mediated chondrogenesis, we have generated C3H10T1/2 cell lines overexpressing Wnt-3A and various forms of glycogen synthase kinase-3beta (GSK-3beta), an immediate cytosolic component of the Wnt signaling pathway, and examined their response to BMP-2. We show that overexpression of either Wnt-3A or kinase-dead GSK-3beta enhances BMP-2-mediated chondrogenesis. Furthermore, Wnt-3A overexpression results in decreases in both N-cadherin and GSK-3beta protein levels, whereas Wnt-3A as well as kinase-dead GSK-3beta overexpression increase total and nuclear levels of both beta-catenin and LEF-1. Direct cross-talk between Wnts and BMP-2 was also indicated by the up-regulated interaction between beta-catenin and SMAD-4 in response to BMP-2. These results suggest that Wnt-3A acts in a manner opposite to that of other Wnts, such as Wnt-7A, which were previously identified as inhibitory to chondrogenesis, and is the first BMP-2-regulated, chondrogenesis-enhancing member of the Wnt family.

Beta-catenin-dependent Wnt signaling in apical ectodermal ridge induction and FGF8 expression in normal and limbless mutant chick limbs

The fibroblast growth factor (FGF) and beta-catenin-dependent Wnt signaling pathways are key regulators of vertebrate limb development. FGF10 induces expression of Wnt3a, which regulates the formation and FGF8 expression of the apical ectodermal ridge (AER). In amelic limbless limbs, an AER fails to form and FGF8 is not expressed, despite expression of FGF10. It has been found that Wnt3a is initially expressed in limbless ectoderm, although subsequently is drastically reduced. In addition, changes in the expression pattern or level of several Frizzled receptors, Axin, Lef1/Tcf1 and beta-catenin have been found in limbless limbs. Notably, while normal wing buds respond to LiCl-stimulated activation of beta-catenin-dependent signaling by forming ectopic, FGF8-expressing AER, LiCl was unable to induce an AER in limbless wing buds. The results of this study suggest that the limbless gene is required for beta-catenin-dependent Wnt signaling in limb ectoderm leading to FGF8 expression and AER formation.

Wnt/beta-catenin signaling suppresses apoptosis in low serum medium and induces morphologic change in rodent fibroblasts

Wnt/beta-catenin signaling plays important roles in tumorigenesis in certain tumors as well as during development. However, the mechanism of tumorigenesis mediated by this signaling remains to be elucidated. We investigated the response of rodent fibroblasts to activation of Wnt/beta-catenin signaling by treatment with conditioned medium containing soluble Wnt-3a protein (W3a-CM) and by expression of a constitutive active beta-catenin gene harbored by an adenovirus vector. W3a-CM induced transcriptional activation of a beta-catenin/T-cell factor (Tcf)-responsive promoter in rodent fibroblasts such as NIH3T3, Rat-1, Swiss3T3 and Balb3T3 cells. In these cells, an increase in saturation density and an inhibition of apoptosis and/or promotion of growth in low-serum medium were induced by treatment with W3a-CM. In Rat-1 cells, morphologic changes were also induced. All these alterations were reversible. Moreover, the inhibition of apoptosis of NIH3T3 cells in low-serum medium and the morphologic changes in Rat-1 cells, but not the increase in saturation density, were also induced by ectopic expression of a constitutive active beta-catenin gene. These results suggested that activation of Wnt/beta-catenin signaling induces inhibition of apoptosis and morphologic changes in these cells.

WNT3-WNT14B and WNT3A-WNT14 gene clusters (Review)

WNT signals are transduced to beta-catenin - TCF pathway, JNK pathway, or Ca2+-releasing pathway through WNT receptors. FRAT1, FRAT2, and PAR-1 are positive regulators of WNT - beta-catenin pathway. APC, AXIN, NKD1, NKD2, and Strabismus (STB1, STB2) are negative regulators of WNT - beta-catenin pathway. Here, biological significance of WNT3-WNT14B/WNT15 gene cluster (human chromosome 17q21) and WNT3A-WNT14 gene cluster (human chromosome 1q42) will be reviewed. Total-amino-acid identity between WNT3 and WNT3A is 84.2%, and that between WNT14 and WNT14B is 61.4%. WNT3A and WNT14B show reciprocal regulation by all-trans retinoic acid in NT2 cells and by beta-estradiol in MCF-7 cells. Exon-intron structures are well conserved between WNT3-WNT14B gene cluster and WNT3A-WNT14 gene cluster, except for the existence of an additional intron in 3'-UTR of WNT3. Capicua pseudogene and AK024248-related sequence are located within intergenic region of human WNT3A-WNT14 gene cluster, but not within intergenic regions of human WNT3-WNT14B gene cluster and mouse Wnt3a-Wnt14 gene cluster. Integration of mouse mammary tumor virus (MMTV) into mouse Wnt3-Wnt14b gene cluster leads to carcinogenesis. Because these WNT gene clusters might be fragile sites in the human genome, implication of WNT3 or WNT3A in cancer as well as implication of WNT14 or WNT14B in connective tissue disease and congenital joint malformation should be elucidated in the future. WNT3, WNT3A, WNT14, and WNT14B might be applicable to tissue engineering of neuron and joint in the field of regenerative medicine, and as an early diagnostic marker in the field of clinical oncology.

Wnt signaling during BMP-2 stimulation of mesenchymal chondrogenesis

Members of both the Wnt and bone morphogenetic protein (BMP) families of signaling molecules have been implicated in the regulation of cartilage development. A key component of the Wnt signaling pathway is the cytosolic protein, beta-catenin. We have recently shown that the chondrogenic activity of BMP-2 in vitro involves the action of the cell-cell adhesion protein, N-cadherin, which functionally complexes with beta-catenin. The aim of this study is to test the hypothesis that Wnts may be involved in BMP-2 induced chondrogenesis, using an in vitro model of high-density micromass cultures of the murine multipotent mesenchymal cell line, C3H10T1/2. Expression of a number of Wnt members was detected in these cultures, including Wnt-3A and Wnt-7A, whose levels were up- and downregulated, respectively, by BMP-2. To assess the functional involvement of Wnt signaling in BMP-2 induced chondrogenesis, cultures were treated with lithium chloride, a Wnt-7A mimetic that acts by inhibiting the serine/threonine phosphorylation activity of glycogen synthase kinase-3beta (GSK-3beta). Lithium treatment significantly inhibited BMP-2 stimulation of chondrogenesis as well as GSK-3beta enzymatic activity, and decreased the levels of N-cadherin protein and mRNA. Furthermore, lithium decreased BMP-2 upregulation of total and nuclear levels of LEF-1 and beta-catenin as well as their interaction during later chondrogenesis; similarly, the interaction of beta-catenin with N-cadherin was also decreased. Interestingly, lithium treatment did not affect the ability of BMP-2 to decrease ubiquitination of beta-catenin, although it did reduce the interaction of beta-catenin with GSK-3beta during late chondrogenesis (days 9-13). We suggest that the chondro-inhibitory effect of lithium on BMP-2 induced chondrogenesis indicates antagonism between lithium-like Wnts and BMP-2 during mesenchymal condensation.

Beta-catenin is essential and sufficient for skeletal myogenesis in P19 cells

Wnt1 and Wnt3a are signaling factors known to play a role in the induction of myogenesis in the myotome of the differentiating somite. Both factors may transduce their signal by a conserved pathway that leads to transcriptional regulation by beta-catenin/Lef1. beta-Catenin and Lef1 are found in the myotome prior to MyoD expression. We have utilized the P19 cell system to study the mechanisms by which Wnt3a may activate MyoD expression and subsequent skeletal muscle development. We have isolated P19 cell lines that stably express either Wnt3a or activated beta-catenin and found that aggregation of these cells results in the induction of myogenesis compared with control cells. Pax3, Gli2, Mox1, and Six1 were expressed during Wnt3a and beta-catenin-induced differentiation prior to MyoD expression. Furthermore, we have shown that the nuclear function of beta-catenin was essential for skeletal myogenesis in P19 cells by overexpression of a dominant negative beta-catenin/engrailed chimera. Primitive streak factors were present, but expression of Pax3, Mox1, Gli2, and Six1 was lost in these cells, indicating that nuclear beta-catenin is essential for specification of mesodermal precursors to the myogenic lineage. Therefore, Wnt signaling, acting via beta-catenin, is necessary and sufficient for skeletal myogenesis in P19 cells.

A mitogen gradient of dorsal midline Wnts organizes growth in the CNS

Cell cycle progression and exit must be precisely patterned during development to generate tissues of the correct size, shape and symmetry. Here we present evidence that dorsal-ventral growth of the developing spinal cord is regulated by a Wnt mitogen gradient. Wnt signaling through the β-catenin/TCF pathway positively regulates cell cycle progression and negatively regulates cell cycle exit of spinal neural precursors in part through transcriptional regulation of cyclin D1 and cyclin D2. Wnts expressed at the dorsal midline of the spinal cord, Wnt1 and Wnt3a, have mitogenic activity while more broadly expressed Wnts do not. We present several lines of evidence suggesting that dorsal midline Wnts form a dorsal to ventral concentration gradient. A growth gradient that correlates with the predicted gradient of mitogenic Wnts emerges as the neural tube grows with the proliferation rate highest dorsally and the differentiation rate highest ventrally. These data are rationalized in a ‘mitogen gradient model’ that explains how proliferation and differentiation can be patterned across a growing field of cells. Computer modeling demonstrates this model is a robust and self-regulating mechanism for patterning cell cycle regulation in a growing tissue.

Supplemental data available on-line

Axin-mediated CKI phosphorylation of beta-catenin at Ser 45: a molecular switch for the Wnt pathway

The Wnt pathway controls numerous developmental processes via the beta-catenin-TCF/LEF transcription complex. Deregulation of the pathway results in the aberrant accumulation of beta-catenin in the nucleus, often leading to cancer. Normally, cytoplasmic beta-catenin associates with APC and axin and is continuously phosphorylated by GSK-3beta, marking it for proteasomal degradation. Wnt signaling is considered to prevent GSK-3beta from phosphorylating beta-catenin, thus causing its stabilization. However, the Wnt mechanism of action has not been resolved. Here we study the regulation of beta-catenin phosphorylation and degradation by the Wnt pathway. Using mass spectrometry and phosphopeptide-specific antibodies, we show that a complex of axin and casein kinase I (CKI) induces beta-catenin phosphorylation at a single site: serine 45 (S45). Immunopurified axin and recombinant CKI phosphorylate beta-catenin in vitro at S45; CKI inhibition suppresses this phosphorylation in vivo. CKI phosphorylation creates a priming site for GSK-3beta and is both necessary and sufficient to initiate the beta-catenin phosphorylation-degradation cascade. Wnt3A signaling and Dvl overexpression suppress S45 phosphorylation, thereby precluding the initiation of the cascade. Thus, a single, CKI-dependent phosphorylation event serves as a molecular switch for the Wnt pathway.

Wnt-1 and Wnt-4 regulate thymic cellularity

Thymic primordium, formed by cells derived from the endoderm, the ectoderm and the neural crest-derived mesenchyme, receive fetal liver derived lymphoid precursors. Reciprocal cell-cell interactions between thymic stromal cells and lymphoid precursors are critical in the expansion and maturation of thymocytes. Transcription factor TCF-1 is critical for the expansion of thymocytes because deletion of TCF-1 results in a significant decrease in the number of thymocytes without affecting the developmental pattern. In this report we show that Wnt-1 and Wnt-4 are expressed in the thymus and the deletion of Wnt-1 or Wnt-4 result in a substantial decrease in the number of thymocytes without affecting the pattern of maturation. Wnt-1 and Wnt-4 both regulate developing thymocytes because a double deficiency results in a significantly greater decrease of immature and mature thymocytes compared to deficiency in either Wnt-1 or Wnt-4.

Molecular cloning and expression of mouse Wnt14, and structural comparison between mouse Wnt14-Wnt3a gene cluster and human WNT14-WNT3A gene cluster

Glycoprotein WNTs play key roles in carcinogenesis and embryogenesis. Human WNT14 and WNT3A genes are clustered in human chromosome 1q42 region with an interval of about 58 kb. Here, mouse Wnt14 was isolated to compare the structure of human WNT14-WNT3A gene cluster with that of mouse Wnt14-Wnt3a gene cluster. Mouse Wnt14 showed 98.1% total-amino-acid identity with human WNT14, and 61.9% total-amino-acid identity with human WNT14B/WNT15. Mouse Wnt14 mRNA was expressed in adult brain, lung, skeletal muscle, heart, and 17-day embryo. Mouse Wnt14 and Wnt3a genes were clustered in head-to-head manner with an interval of about 16 kb. Exon-intron structures were well conserved between human WNT14-WNT3A gene cluster and mouse Wnt14-Wnt3a gene cluster. Capicua-related sequence and AK024248-related sequence were identified in the intergenic region of human Wnt14-Wnt3a gene cluster as well as in other human chromosomal loci, but not in that of mouse Wnt14-Wnt3a gene cluster. Capicua-related sequences were pseudogenes derived from Capicua gene on human chromosome 19q13. Capicua pseudogene and AK024248-related sequence were clustered in tail-to-tail manner with interval ranging from 2.2 to 11.0 kb. AK024248-related sequences in several human genome draft sequences were truncated in the 3'-portion compared with that in the intergenic region of human WNT14-WNT3A gene cluster. This is the first report on structural comparison of WNT gene clusters in human genome and in mouse genome.

Wnt signaling plays an essential role in neuronal specification of the dorsal spinal cord

In the developing spinal cord, signals from the roof plate are required for the development of three classes of dorsal interneuron: D1, D2, and D3, listed from dorsal to ventral. Here, we demonstrate that absence of Wnt1 and Wnt3a, normally expressed in the roof plate, leads to diminished development of D1 and D2 neurons and a compensatory increase in D3 neuron populations. This occurs without significantly altered expression of BMP and related genes in the roof plate. Moreover, Wnt3a protein induces expression of D1 and D2 markers in the isolated medial region of the chick neural plate, and Noggin does not interfere with this induction. Thus, Wnt signaling plays a critical role in the specification of cell types for dorsal interneurons.

Regulation of WNT3 and WNT3A mRNAs in human cancer cell lines NT2, MCF-7, and MKN45

We have previously cloned and characterized human WNT2B/WNT13, WNT3, WNT3A, WNT5B, WNT6, WNT7B, WNT8A, WNT10A, WNT10B, WNT11, WNT14, and WNT14B/WNT15 by using bioinformatics, cDNA-library screening, cDNA-PCR, and RACE. WNT3 and WNT3A genes are two human paralogues of mouse proto-oncogene Wnt3, which induces carcinogenesis through activation of the beta-catenin - TCF signaling pathway. Here, regulation of WNT3 and WNT3A mRNAs in human cancer cell lines was investigated. WNT3 and WNT3A mRNAs were co-expressed in an embryonal carcinoma cell line NT2, which is reported to differentiate into postmitotic CNS neurons by treatment with retinoic acid for two weeks. Expression level of WNT3 mRNA in NT2 cells was not changed during 72 h after retinoic acid treatment, while expression of WNT3A mRNA was down-regulated in NT2 cells by retinoic acid. WNT3 and WNT3A mRNAs were also co-expressed in a breast cancer cell line MCF-7, and were down-regulated together by beta-estradiol in MCF-7 cells. Expression of WNT3 mRNA in a gastric cancer cell line MKN45 was not changed after treatment with tumor necrosis factor alpha (TNFalpha) or interferon gamma (IFNgamma), and that of WNT3A mRNA was undetectable before and after treatment with TNFalpha or IFNgamma. WNT3A, down-regulated by retinoic acid in NT2 cells, might play key roles in the maintenance of NT2 cells in the undifferentiated proliferation stage through activation of the beta-catenin - TCF signaling pathway.

Molecular cloning and characterization of mouse Wnt14b, clustered with mouse Wnt3 in mouse chromosome 11

WNTs are a family of secreted-type glycoproteins implicated in embryogenesis and carcinogenesis. We have previously cloned and characterized human WNT2B/WNT13, WNT3, WNT3A, WNT5B, WNT6, WNT7B, WNT8A, WNT8B, WNT10A, WNT10B, WNT11, WNT14, and WNT14B/WNT15. WNT14B gene is clustered with WNT3 gene in human chromosome 17q21, and mRNA expression of WNT14B is significantly up-regulated by retinoic acid during the early phase of neuronal differentiation in human NT2 cells. Here, we identified mouse Wnt14b gene fragments in mouse genome draft sequence AL596108.5 by using bioinformatics, and isolated mouse Wnt14b cDNAs by using cDNA-PCR. Mouse Wnt14b was found to encode a 359-amino-acid WNT family protein with the N-terminal signal peptide, an N-linked glycosylation site, and 24 conserved cysteine residues. Mouse Wnt14b showed 92.5% total-amino-acid identity with human WNT14B, and 64.2% total-amino-acid identity with human WNT14. Mouse Wnt14b gene, consisting of 4 exons, was clustered with mouse Wnt3 gene in mouse chromosome 11. Mouse Wnt14b mRNA was relatively highly expressed in 17-day embryo, and also expressed in adult brain, kidney, liver, 7-day embryo, and 11-day embryo. This is the first report on molecular cloning and characterization of mouse Wnt14b.