Wnt-5a inhibits the canonical Wnt pathway by promoting GSK-3-independent beta-catenin degradation

Gαq negatively regulates the Wnt-β-catenin pathway and dorsal embryonicXenopus laevis development

The non-canonical Wnt/Ca2+ signaling pathway has been implicated in the regulation of axis formation and gastrulation movements during early Xenopus laevis embryo development, by antagonizing the canonical Wnt/beta-catenin dorsalizing pathway and specifying ventral cell fate. However, the molecular mechanisms involved in this antagonist crosstalk are not known. Since Galphaq is the main regulator of Ca2+ signaling in vertebrates and from this perspective probably involved in the events elicited by the non-canonical Wnt/Ca2+ pathway, we decided to study the effect of wild-type Xenopus Gq (xGalphaq) in dorso-ventral axis embryo patterning. Overexpression of xGalphaq or its endogenous activation at the dorsal animal region of Xenopus embryo both induced a strong ventralized phenotype and inhibited the expression of dorsal-specific mesoderm markers goosecoid and chordin. Dorsal expression of an xGalphaq dominant-negative mutant reverted the xGalphaq-induced ventralized phenotype. Finally, we observed that the Wnt8-induced secondary axis formation is reverted by endogenous xGalphaq activation, indicating that it is negatively regulating the Wnt/beta-catenin pathway.

Wnt4 action in gonadal development and sex determination

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

Distinct Wnt signaling pathways have opposing roles in appendage regeneration

In contrast to mammals, lower vertebrates have a remarkable capacity to regenerate complex structures damaged by injury or disease. This process, termed epimorphic regeneration, involves progenitor cells created through the reprogramming of differentiated cells or through the activation of resident stem cells. Wnt/beta-catenin signaling regulates progenitor cell fate and proliferation during embryonic development and stem cell function in adults, but its functional involvement in epimorphic regeneration has not been addressed. Using transgenic fish lines, we show that Wnt/beta-catenin signaling is activated in the regenerating zebrafish tail fin and is required for formation and subsequent proliferation of the progenitor cells of the blastema. Wnt/beta-catenin signaling appears to act upstream of FGF signaling, which has recently been found to be essential for fin regeneration. Intriguingly, increased Wnt/beta-catenin signaling is sufficient to augment regeneration, as tail fins regenerate faster in fish heterozygous for a loss-of-function mutation in axin1, a negative regulator of the pathway. Likewise, activation of Wnt/beta-catenin signaling by overexpression of wnt8 increases proliferation of progenitor cells in the regenerating fin. By contrast, overexpression of wnt5b (pipetail) reduces expression of Wnt/beta-catenin target genes, impairs proliferation of progenitors and inhibits fin regeneration. Importantly, fin regeneration is accelerated in wnt5b mutant fish. These data suggest that Wnt/beta-catenin signaling promotes regeneration, whereas a distinct pathway activated by wnt5b acts in a negative-feedback loop to limit regeneration.

Gene profiling on mixed embryonic stem cell populations reveals a biphasic role for beta-catenin in osteogenic differentiation

The differentiation of embryonic stem cells (ESCs) into osteoblasts is enhanced to 60% when exposed to vitamin D3 (VD3) but leaves a remainder of one half of the cell population unidentified. To increase differentiation outcome, the known osteoinducers retinoic acid (RA) and bone morphogenetic protein-2 (BMP-2) were evaluated. Initial studies using RA and BMP-2 during early osteogenesis in addition to VD3 increased osteogenic yield in the case of RA, but surprisingly decreased osteogenesis when BMP-2 was administered together with VD3 or RA. This paper describes a comprehensive microarray study examining the gene expression profile of differentiating osteoblasts in these mixed ESC populations. In addition to five other families of signaling molecules (insulin growth factors, prostaglandin, follistatin, TGFbeta2, and Wnt molecules), we identified an endogenous expression pattern for BMPs and RA that differed from our previous exogenous administration of these molecules. By mimicking the change in expression of the RA and BMP-2 families with exogenous supplementation at the correct time, it was then possible to increase the number of ESC-derived osteoblasts to 90%. This effect was mediated through alteration in beta-catenin (CatnB) expression levels and nuclear CatnB activity, both of which are modulated by VD3, RA, and BMP-2. Our results suggest that blockage of CatnB activity by VD3 and RA is opposed by induction of CatnB activity through BMP-2 when administered together. Hence, osteoinduction, in vitro, is an intricate process involving both temporal and quantitative changes in gene expression and CatnB activity.

The effects of dickkopf 1 on gene expression and Wnt signaling by melanocytes: mechanisms underlying its suppression of melanocyte function and proliferation

Dickkopf 1 (DKK1), which is expressed at high mRNA levels by fibroblasts in the dermis of human skin on the palms and soles, inhibits the function and proliferation of melanocytes in the epidermis of those areas via the suppression of beta-catenin and microphthalmia-associated transcription factor (MITF). In this study, we investigated the protein expression levels of DKK1 between palmoplantar and non-palmoplantar areas and the effects of DKK1 on melanocyte gene expression profiles and on Wnt signaling pathways using DNA microarray technology, reverse transcriptase-PCR, Western blot, 3-dimensional reconstructed skin, immunocytochemistry, and immunohistochemistry. DKK1-responsive genes included those encoding proteins involved in the regulation of melanocyte development, growth, differentiation, and apoptosis (including Kremen 1, G-coupled receptor 51, lipoprotein receptor-related protein 6, low-density lipoprotein receptor, tumor necrosis factor receptor super-family 10, growth arrest and DNA-damage-inducible gene 45beta, and MITF). Of special interest was the rapid decrease in expression of MITF in melanocytes treated with DKK1, which is concurrent with the decreased activities of beta-catenin and of glucose-synthase kinase 3beta via phosphorylation at Ser9 and with the upregulated expression of protein kinase C alpha. These results further clarify the mechanism by which DKK1 suppresses melanocyte density and differentiation, and help explain why DKK1-rich palmoplantar epidermis is paler than non-palmoplantar epidermis via mesenchymal-epithelial interactions.

Expression of Wnt-5a is correlated with aggressiveness of gastric cancer by stimulating cell migration and invasion

Wnt-5a is a representative ligand that activates a beta-catenin-independent pathway in the Wnt signaling. Although abnormal activation of beta-catenin-dependent pathway is often observed in human cancer, the relationship between beta-catenin-independent pathway and tumorigenesis is not clear. We sought to clarify how Wnt-5a is involved in aggressiveness of gastric cancer. Abnormal expression of Wnt-5a was observed in 71 of 237 gastric cancer cases by means of immunohistochemistry. The positivity of Wnt-5a expression was correlated with advanced stages and poor prognosis of gastric cancer. Wnt-5a had the abilities to stimulate cell migration and invasion in gastric cancer cells. Wnt-5a activated focal adhesion kinase and small GTP-binding protein Rac, both of which are known to play a role in cell migration. Cell migration, membrane ruffling, and turnover of paxillin were suppressed in Wnt-5a knockdown cells. Furthermore, anti-Wnt-5a antibody suppressed gastric cancer cell migration. These results suggest that Wnt-5a stimulates cell migration by regulating focal adhesion complexes and that Wnt-5a is not only a prognostic factor but also a good therapeutic target for gastric cancer.

Protein-kinase-C-mediated β-catenin phosphorylation negatively regulates the Wnt/β-catenin pathway

Normally, the Wnt/β-catenin pathway controls developmental processes and homeostasis, but abnormal activation of this pathway is a frequent event during the development of cancer. The key mechanism in regulation of the Wnt/β-catenin pathway is the amino-terminal phosphorylation of β-catenin, marking it for proteasomal degradation. Here we present small-molecule-based identification of protein kinase C (PKC)-mediated β-catenin phosphorylation as a novel mechanism regulating the Wnt/β-catenin pathway. We used a cell-based chemical screen to identify A23187, which inhibits the Wnt/β-catenin pathway. PKC was activated by A23187 treatment and subsequently phosphorylated N-terminal serine (Ser) residues of β-catenin, which promoted β-catenin degradation. Moreover, the depletion of PKCα inhibited the phosphorylation and degradation of β-catenin. Therefore, our findings suggest that the PKC pathway negatively regulates the β-catenin level outside of the Wnt/β-catenin pathway.

Canonical Wnt signaling transiently stimulates proliferation and enhances neurogenesis in neonatal neural progenitor cultures

Canonical Wnt signaling triggers the formation of heterodimeric transcription factor complexes consisting of beta-catenin and T cell factors, and thereby controls the execution of specific genetic programs. During the expansion and neurogenic phases of embryonic neural development canonical Wnt signaling initially controls proliferation of neural progenitor cells, and later neuronal differentiation. Whether Wnt growth factors affect neural progenitor cells postnatally is not known. Therefore, we have analyzed the impact of Wnt signaling on neural progenitors isolated from cerebral cortices of newborn mice. Expression profiling of pathway components revealed that these cells are fully equipped to respond to Wnt signals. However, Wnt pathway activation affected only a subset of neonatal progenitors and elicited a limited increase in proliferation and neuronal differentiation in distinct subsets of cells. Moreover, Wnt pathway activation only transiently stimulated S-phase entry but did not support long-term proliferation of progenitor cultures. The dampened nature of the Wnt response correlates with the predominant expression of inhibitory pathway components and the rapid actuation of negative feedback mechanisms. Interestingly, in differentiating cell cultures activation of canonical Wnt signaling reduced Hes1 and Hes5 expression suggesting that during postnatal neural development, Wnt/beta-catenin signaling enhances neurogenesis from progenitor cells by interfering with Notch pathway activity.

Jun NH2-terminal kinase (JNK) prevents nuclear beta-catenin accumulation and regulates axis formation in Xenopus embryos

Jun NH(2)-terminal kinases (JNKs) regulate convergent extension movements in Xenopus embryos through the noncanonical Wnt/planar cell polarity pathway. In addition, there is a high level of maternal JNK activity spanning from oocyte maturation until the onset of gastrulation that has no defined functions. Here, we show that maternal JNK activation requires Dishevelled and JNK is enriched in the nucleus of Xenopus embryos. Although JNK activity is not required for the glycogen synthase kinase-3-mediated degradation of beta-catenin, inhibition of the maternal JNK signaling by morpholino-antisense oligos causes hyperdorsalization of Xenopus embryos and ectopic expression of the Wnt/beta-catenin target genes. These effects are associated with an increased level of nuclear and nonmembrane-bound beta-catenin. Moreover, ventral injection of the constitutive-active Jnk mRNA blocks beta-catenin-induced axis duplication, and dorsal injection of active Jnk mRNA into Xenopus embryos decreases the dorsal marker gene expression. In mammalian cells, activation of JNK signaling reduces Wnt3A-induced and beta-catenin-mediated gene expression. Furthermore, activation of JNK signaling rapidly induces the nuclear export of beta-catenin. Taken together, these results suggest that JNK antagonizes the canonical Wnt pathway by regulating the nucleocytoplasmic transport of beta-catenin rather than its cytoplasmic stability. Thus, the high level of sustained maternal JNK activity in early Xenopus embryos may provide a timing mechanism for controlling the dorsal axis formation.

Wnt5a signaling induces proliferation and survival of endothelial cells in vitro and expression of MMP-1 and Tie-2

Wnts are lipid-modified secreted glycoproteins that regulate diverse biological processes. We report that Wnt5a, which functions in noncanonical Wnt signaling, has activity on endothelial cells. Wnt5a is endogenously expressed in human primary endothelial cells and is expressed in murine vasculature at several sites in mouse embryos and tissues. Expression of exogenous Wnt5a in human endothelial cells promoted angiogenesis. Wnt5a induced noncanonical Wnt signaling in endothelial cells, as measured by Dishevelled and ERK1/2 phosphorylation, and inhibition of canonical Wnt signaling, a known property of Wnt5a. Wnt5a induced endothelial cell proliferation and enhanced cell survival under serum-deprived conditions. The Wnt5a-mediated proliferation was blocked by Frizzled-4 extracellular domain. Wnt5a expression enhanced capillary-like network formation, whereas reduction of Wnt5a expression decreased network formation. Reduced Wnt5a expression inhibited endothelial cell migration. Screening for Wnt5a-regulated genes in cultured endothelial cells identified several encoding angiogenic regulators, including matrix metalloproteinase-1, an interstitial collagenase, and Tie-2, a receptor for angiopoietins. Thus, Wnt5a acts through noncanonical Wnt signaling to promote angiogenesis.

Wnt signal pathways and neural stem cell differentiation

Self-renewal, migration and differentiation of neural progenitor cells are controlled by a variety of pleiotropic signal molecules. Members of the morphogen family of Wnt molecules play a crucial role for developmental and repair mechanisms in the embryonic and adult nervous system. A strategy of disclosure of the role of different canonical (glycogen synthase kinase-3beta/beta-catenin-dependent) and noncanonical (Ca2+- and JNK-dependent) signal pathways for progenitor cell expansion and differentiations is illustrated at the example of the rat striatal progenitor cell line ST14A that is immortalized by stable retroviral transfection with a temperature-sensitive mutant of the SV40 large T antigen. A shift from permissive 33 degrees C to nonpermissive 39 degrees C leads to proliferation stop and start of differentiation into glial and neuronal cells. Investigation of expression of Wnts, Wnt receptors and Wnt-dependent signal pathway assay point to a stage-dependent involvement of canonical and noncanonical signaling in proliferation and differentiation of ST14A cells, whereby a mutual suppression of pathway activities is likely. Canonical Wnt molecules are not detected in proliferating and differentiating ST14A cells except Wnt2. The noncanonical Wnt molecules Wnt4, Wnt5a and Wnt11 are expressed in proliferating cells and increase during differentiation, whereas cellular beta-catenin decreases in the early phase and is restored in the late phase of differentiation. Accumulation of beta-catenin at the membrane in undifferentiated proliferating cells and its nuclear localization in nondividing undifferentiated cells under differentiation conditions argues for a distinct spatially regulated role of the molecule in the proliferation and early differentiation phase. Ca2+-dependent and JNK-dependent noncanonical Wnt signaling is not detected during differentiation of ST14A cells. Complete exploration of the role of Wnt pathways, for differentiation of the neural progenitor cells ST14A will require Wnt overexpression and exposure of ST14A cells to exogenous Wnts either with purified Wnts or by co-cultures with Wnt producers.

Hexachlorophene inhibits Wnt/beta-catenin pathway by promoting Siah-mediated beta-catenin degradation

Aberrant activation of Wnt/beta-catenin signaling and subsequent up-regulation of beta-catenin response transcription (CRT) is a critical event in the development of human colon cancer. Thus, Wnt/beta-catenin signaling is an attractive target for the development of anticancer therapeutics. In this study, we identified hexachlorophene as an inhibitor of Wnt/beta-catenin signaling from cell-based small-molecule screening. Hexachlorophene antagonized CRT that was stimulated by Wnt3a-conditioned medium by promoting the degradation of beta-catenin. This degradation pathway is Siah-1 and adenomatous polyposis colidependent, but glycogen synthase kinase-3beta and F-box beta-transducin repeat-containing protein-independent. In addition, hexachlorophene represses the expression of cyclin D1, which is a known beta-catenin target gene, and inhibits the growth of colon cancer cells. Our findings suggest that hexachlorophene attenuates Wnt/beta-catenin signaling through the Siah-1-mediated beta-catenin degradation.

Constitutively Active β-Catenin Promotes Expansion of Multipotent Hematopoietic Progenitors in Culture

This study was designed to investigate one component of the Wnt/beta-catenin signaling pathway that has been implicated in stem cell self-renewal. Retroviral-mediated introduction of stable beta-catenin to primitive murine bone marrow cells allowed the expansion of multipotential c-Kit(low)Sca-1(low/-)CD19(-) CD11b/Mac-1(-)Flk-2(-)CD43(+)AA4.1(+)NK1.1(-)CD3(-)CD11c(-)Gr-1(-)CD45R/B220(+) cells in the presence of stromal cells and cytokines. They generated myeloid, T, and B lineage lymphoid cells in culture, but had no T lymphopoietic potential when transplanted. Stem cell factor and IL-6 were found to be minimal requirements for long-term, stromal-free propagation, and a beta-catenin-transduced cell line was maintained for 5 mo with these defined conditions. Although multipotential and responsive to many normal stimuli in culture, it was unable to engraft several types of irradiated recipients. These findings support previous studies that have implicated the canonical Wnt pathway signaling in regulation of multipotent progenitors. In addition, we demonstrate how it may be experimentally manipulated to generate valuable cell lines.

Wnt-5a/Ca2+-induced NFAT activity is counteracted by Wnt-5a/Yes-Cdc42-casein kinase 1alpha signaling in human mammary epithelial cells

Wnt-5a has been shown to influence the metastatic behavior of human breast cancer cells, and the loss of Wnt-5a expression is associated with metastatic disease. We show here that NFAT1, a transcription factor connected with breast cancer metastasis, is activated by Wnt-5a through a Ca2+ signaling pathway in human breast epithelial cells. This activation was simultaneously counteracted by a Wnt-5a-induced Yes/Cdc42 signaling pathway. The observation that inhibition of the Wnt-5a/Yes/Cdc42 signal prolonged the duration of ionomycin-induced NFAT1 activation revealed the general importance of this pathway. The Wnt-5a-induced inhibition of NFAT1 did not require glycogen synthase kinase 3beta, JNK, or Pak1 activity or modulation of the cytoskeleton. Instead, we observed that Wnt-5a induced a complex formation of NFAT1/casein kinase 1alpha, even upon treatment with ionomycin, which was blocked upon inhibition of the Wnt-5a/Yes/Cdc42 signaling pathway. Our results explain why Wnt-5a/Ca2+-induced NFAT activity is hard to detect and suggest a novel mechanism by which Wnt-5a can suppress tumor-specific, agonist-induced NFAT activity and thus the metastatic behavior of breast cancer cells.

Increased expression of Wnt5a in psoriatic plaques

Psoriasis vulgaris is characterized by hyperproliferation and incomplete terminal differentiation of epidermal keratinocytes. Despite the established role of Wnt pathways in the regulation of stem cell proliferation and differentiation, they have not yet been associated with the pathophysiology of psoriasis. Here, we took biopsies from uninvolved and from lesional skin of 20 patients with plaque-type psoriasis. The biopsies were used for microarray RNA expression profiling. Based on paired samples from 13 patients, we defined 179 genes that were more than 2-fold differentially expressed in lesional skin. This list included 16 genes with known or possible association to the canonical Wnt/beta-catenin or the non-canonical Wnt/Ca2+ pathway. The expression of Wnt5a was 4-fold higher in lesional skin. Other Wnt molecules were largely unchanged (Wnt4 and Wnt16), or tended to be expressed at lower levels (Wnt7b). The mRNA expression levels of two inhibitory factors related to Wnt signaling, frizzled-related protein, and dickkopf homolog 2, were reduced in lesional skin, as was mRNA expression of cyclin D1. These findings were confirmed by quantitative reverse transcription-PCR experiments. We conclude that Wnt5a and other Wnt pathway genes are differentially expressed in psoriatic plaques. Their functional contribution to the pathophysiology of psoriasis needs to be elaborated.

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

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

Differential inhibition of Wnt-3a by Sfrp-1, Sfrp-2, and Sfrp-3

Secreted frizzled related proteins (Sfrps) are extracellular attenuators of Wnt signaling that play important roles in both embryogenesis and oncogenesis. Although Sfrps are generally thought to bind and sequester Wnts away from active receptor complexes, very little is known about the specificity of Sfrp family members for various Wnts. In the developing chick neural tube, sfrp-1, 2, and 3 transcripts are expressed in and adjacent to the dorsal neural tube, where Wnt-1 and Wnt-3a are expressed. To better define the possible roles of Sfrp-1, 2, and 3 in the neural tube, we first tested the ability of purified Sfrps to inhibit Wnt-3a-induced accumulation of beta-catenin in L cells. We find that both Sfrp-1 and Sfrp-2 can inhibit Wnt-3a activity while Sfrp-3 cannot. To determine where Sfrp-1 and Sfrp-2 impinge on the Wnt signaling pathway, we tested the ability of these Sfrps to inhibit Wnt signaling induced by the addition of LiCl, an inhibitor of GSK-3. Sfrp-1 and Sfrp-2 are unable to inhibit the accumulation of beta-catenin in LiCl-treated cells, suggesting that the ability of Sfrps to inhibit the accumulation of beta-catenin is GSK-3 dependent. We have further shown that Sfrp-2 inhibits the ability of ectopic Wnt-3a to stimulate proliferation in the developing chick neural tube. These results provide the framework for understanding how Sfrps function to regulate Wnt-3a activity in developing embryos and in cancer.

Regulation of bone mass by Wnt signaling

Wnt proteins are a family of secreted proteins that regulate many aspects of cell growth, differentiation, function, and death. Considerable progress has been made in our understanding of the molecular links between Wnt signaling and bone development and remodeling since initial reports that mutations in the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) are causally linked to alterations in human bone mass. Of the pathways activated by Wnts, it is signaling through the canonical (i.e., Wnt/beta-catenin) pathway that increases bone mass through a number of mechanisms including renewal of stem cells, stimulation of preosteoblast replication, induction of osteoblastogenesis, and inhibition of osteoblast and osteocyte apoptosis. This pathway is an enticing target for developing drugs to battle skeletal diseases as Wnt/beta-catenin signaling is composed of a series of molecular interactions that offer potential places for pharmacological intervention. In considering opportunities for anabolic drug discovery in this area, one must consider multiple factors, including (a) the roles of Wnt signaling for development, remodeling, and pathology of bone; (b) how pharmacological interventions that target this pathway may specifically treat osteoporosis and other aspects of skeletal health; and (c) whether the targets within this pathway are amenable to drug intervention. In this Review we discuss the current understanding of this pathway in terms of bone biology and assess whether targeting this pathway might yield novel therapeutics to treat typical bone disorders.

Expression and function of Wnt5a in the development of the glandular stomach in the chicken embryo

The epithelium of the chicken embryonic glandular stomach (proventriculus) differentiates into both a glandular and a luminal epithelium, the cells of which express specific marker genes. The subsequent formation and differentiation of the glands then proceed under the influence of the mesenchyme. To search for possible candidates for the mesenchymal factors involved, we have now investigated the expression and function of Wnt5a in this process. Our current results show that Wnt5a is expressed in the mesenchyme during active gland formation and that overexpression of this gene in ovo results in the increased and ectopic expression of some of the marker genes of the luminal and glandular epithelia. In particular, the overexpression of Wnt5a markedly enhances the expression of the embryonic chicken pepsinogen gene, a marker of the glandular epithelium, indicating its role as a mesenchymal factor that regulates the differentiation of the proventricular epithelium.

The C. elegans Frizzled CFZ-2 is required for cell migration and interacts with multiple Wnt signaling pathways

Members of the Frizzled family of integral membrane proteins are implicated in many developmental events, including specifying cell fate, orienting cell and planar polarity, and directing cell migration. Frizzleds function as cell surface receptors for secreted Wnt proteins. We report here the isolation of a mutation in cfz-2, a Caenorhabditis elegans Frizzled gene. Mutation of cfz-2 causes defective cell migration, disorganization of head neurons, and can cause ectopic axon outgrowth. Analysis of mosaic animals shows that CFZ-2 functions cell nonautonomously, but does not rule out an autonomous role. CFZ-2 is expressed primarily in the anterior of embryos and in several cells in the head of adults. Our analysis of interactions between CFZ-2 and other Wnt pathways reveals that three Wnts, CWN-1, CWN-2 and EGL-20, and a Frizzled, MOM-5, function redundantly with one another and with CFZ-2 for specific cell migrations. In contrast, CWN-1, CWN-2, EGL-20, CFZ-2, and MOM-5 antagonize one another for other migrations. Therefore, CFZ-2 functions by collaborating with and/or antagonizing other Wnt signaling pathways to regulate specific cell migrations.

Purified Wnt5a protein activates or inhibits beta-catenin-TCF signaling depending on receptor context

The Wnts comprise a large class of secreted proteins that control essential developmental processes such as embryonic patterning, cell growth, migration, and differentiation. In the most well-understood “canonical” Wnt signaling pathway, Wnt binding to Frizzled receptors induces β-catenin protein stabilization and entry into the nucleus, where it complexes with T-cell factor/lymphoid enhancer factor transcription factors to affect the transcription of target genes. In addition to the canonical pathway, evidence for several other Wnt signaling pathways has accumulated, in particular for Wnt5a, which has therefore been classified as a noncanonical Wnt family member. To study the alternative mechanisms by which Wnt proteins signal, we purified the Wnt5a protein to homogeneity. We find that purified Wnt5a inhibits Wnt3a protein–induced canonical Wnt signaling in a dose-dependent manner, not by influencing β-catenin levels but by downregulating β-catenin–induced reporter gene expression. The Wnt5a signal is mediated by the orphan tyrosine kinase Ror2, is pertussis toxin insensitive, and does not influence cellular calcium levels. We show that in addition to its inhibitory function, Wnt5a can also activate β-catenin signaling in the presence of the appropriate Frizzled receptor, Frizzled 4. Thus, this study shows for the first time that a single Wnt ligand can initiate discrete signaling pathways through the activation of two distinct receptors. Based on these and additional observations, we propose a model wherein receptor context dictates Wnt signaling output. In this model, signaling by different Wnt family members is not intrinsically regulated by the Wnt proteins themselves but by receptor availability.

Understanding signaling by Wnt proteins has been hampered by a history of conflicting data. The authors reconcile previous findings concerning Wnt signaling by using purified Wnt5a to probe the signaling pathways it activates.

Developmental pathways and specification of intrapulmonary stem cells

Tissues have the capacity to maintain a homeostatic balance between wear-and-tear and regeneration. Repair of non-lethal injury also activates cell proliferation to repopulate the injured sites with appropriate cell types and to restore function. Although controversial, the source of the material appears to be at least partly from pools of unique, multipotent stem cells that reside in specialized locations referred to as "niches." Molecular interactions between the niche and the intracellular factors within stem cells are crucial in maintaining stem cell functions, particularly the balance between self-renewal and differentiation. Many of the mediators of the stem cell-niche interactions are similar or identical to those that control developmental pathways during organogenesis. In this review, we present a systematic discussion and evaluation of the relevant literature with a focused emphasis on three primary signaling pathways, WNT, SHH and BMP with potentially overlapping roles during both development and stem cell maintenance.

Engrailed-1 negatively regulates beta-catenin transcriptional activity by destabilizing beta-catenin via a glycogen synthase kinase-3beta-independent pathway

The Wnt signaling pathway plays a major role in development, and upon deregulation it is implicated in neoplasia. The hallmark of the canonical Wnt signal is the protection of beta-catenin from ubiquitination and proteasomal degradation induced by glycogen synthase kinase (GSK)-3beta inhibition. The stabilized beta-catenin translocates to the nucleus where it binds to T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors, activating the expression of Wnt target genes. In the absence of Wnt signal, TCF/LEF bind to Groucho (Gro)/TLE corepressors and repress Wnt target genes. Gro/TLE bind also to Engrailed (En) transcription factors mediating En-repressive activity on En target genes. Here, we present data suggesting that En-1 serves also as a negative regulator of beta-catenin transcriptional activity; however, its repressive effect is independent of Gro/TLE. Our data suggest that En-1 acts by destabilizing beta-catenin via a proteasomal degradation pathway that is GSK-3beta-independent. Moreover, because En-1-mediated beta-catenin degradation is also Siah independent, our data imply that En-1 exerts its repressive effect by a novel mechanism negatively controlling the level of beta-catenin.

Wnt 5a signaling is critical for macrophage-induced invasion of breast cancer cell lines

Interactions between neoplastic and stromal cells contribute to tumor progression. Wnt genes, involved in cell migration and often deregulated in cancers, are attractive candidates to regulate these effects. We have recently shown that coculture of breast cancer cells with macrophages enhances invasiveness via matrix metalloproteases and TNF-alpha. Here we demonstrate that coculture of MCF-7 cells and macrophages leads to up-regulation of Wnt 5a in the latter. This was accompanied by activation of AP-1/c-Jun in MCF-7. Recombinant Wnt 5a mimicked the coculture effect. Wnt 5a was also detectable in tumor-associated macrophages in primary breast cancers. Experiments with agonists and antagonists of Wnt signaling revealed that a functional canonical pathway in the tumor cells was a necessary prerequisite; however, noncanonical signaling via Wnt 5a and the Jun-N-terminal kinase pathway was critical for invasiveness. It was also responsible for induction of matrix metalloprotease-7, known to release TNF-alpha. All these effects could be antagonized by dickkopf-1. Our results indicate that Wnt 5a is essential for macrophage-induced invasiveness, because it regulates tumor cell migration as well as proteolytic activity of the macrophages. The function of Wnt 5a as either a suppressor or promoter of malignant progression seems to be modulated by intercellular interactions. Wnt 5a detection in tumor-associated macrophages in breast cancer biopsies supports the assumption that similar events play a role in vivo.

Wnt signal transduction and the formation of the myocardium

Soon after fertilization, vertebrate embryos grow very rapidly. Thus, early in gestation, a sizeable yet underdeveloped organism requires circulating blood. This need dictates the early appearance of a contractile heart, which is the first functional organ in both the avian and mammalian embryo. The heart arises from paired mesodermal regions within the anterior half of the embryo. As development proceeds, these bilateral precardiac fields merge at the midline to give rise to the primary heart tube. How specific areas of nondifferentiated mesoderm organize into myocardial tissue has been a question that has long intrigued developmental biologists. In recent years, the regulation of Wnt signal transduction has been implicated as an important event that initiates cardiac development. While initial reports in Drosophila and the bird had implicated Wnt proteins as promoters of cardiac tissue formation, subsequent findings that the WNT inhibitors Dkk1 and crescent possess cardiac-inducing activities led to the contrary hypothesis that WNTs actively inhibit cardiogenesis. This seeming contradiction has been resolved, in part, by more recent information indicating that Wnts stimulate multiple signal transduction pathways. In this review, we will examine what is presently known about the importance of regulated Wnt activity for the formation of the heart and the development of the myocardium and discuss this information in context of the emerging complexity of Wnt signal transduction.

Wnt signalling in osteoblasts regulates expression of the receptor activator of NFkappaB ligand and inhibits osteoclastogenesis in vitro

Reports implicating Wnt signalling in the regulation of bone mass have prompted widespread interest in the use of Wnt mimetics for the treatment of skeletal disorders. To date much of this work has focused on their anabolic effects acting on cells of the osteoblast lineage. In this study we provide evidence that Wnts also regulate osteoclast formation and bone resorption, through a mechanism involving transcriptional repression of the gene encoding the osteoclastogenic cytokine receptor activator of NFkappaB ligand (RANKL or TNFSF11) expressed by osteoblasts. In co-cultures of mouse mononuclear spleen cells and osteoblasts, inhibition of GSK3beta with LiCl or exposure to Wnt3a inhibited the formation of tartrate-resistant acid phosphatase-positive multinucleated cells compared with controls. However, these treatments had no consistent effect on the differentiation, survival or activity of osteoclasts generated in the absence of supporting stromal cells. Activation of Wnt signalling downregulated RANKL mRNA and protein expression, and overexpression of fulllength beta-catenin, but not transcriptionally inactive beta-catenin DeltaC(695-781), inhibited RANKL promoter activity. Since previous studies have demonstrated an absence of resorptive phenotype in mice lacking LRP5, we determined expression of a second Wnt co-receptor LRP6 in human osteoblasts, CD14(+) osteoclast progenitors and mature osteoclasts. LRP5 expression was undetectable in CD14-enriched cells and mature human osteoclasts, although LRP6 was expressed at high levels by these cells. Our evidence of Wnt-dependent regulation of osteoclastogenesis adds to the growing complexity of Wnt signalling mechanisms that are now known to influence skeletal function and highlights the requirement to develop novel therapeutics that differentially target anabolic and catabolic Wnt effects in bone.

Foxf1andFoxf2control murine gut development by limiting mesenchymal Wnt signaling and promoting extracellular matrix production

Development of the vertebrate gut is controlled by paracrine crosstalk between the endodermal epithelium and the associated splanchnic mesoderm. In the adult, the same types of signals control epithelial proliferation and survival, which account for the importance of the stroma in colon carcinoma progression. Here, we show that targeting murine Foxf1 and Foxf2, encoding forkhead transcription factors, has pleiotropic effects on intestinal paracrine signaling. Inactivation of both Foxf2alleles, or one allele each of Foxf1 and Foxf2, cause a range of defects, including megacolon, colorectal muscle hypoplasia and agangliosis. Foxf expression in the splanchnic mesoderm is activated by Indian and sonic hedgehog secreted by the epithelium. In Foxf mutants, mesenchymal expression of Bmp4 is reduced, whereas Wnt5a expression is increased. Activation of the canonical Wnt pathway – with nuclear localization of β-catenin in epithelial cells – is associated with over-proliferation and resistance to apoptosis. Extracellular matrix,particularly collagens, is severely reduced in Foxf mutant intestine, which causes epithelial depolarization and tissue disintegration. Thus, Foxf proteins are mesenchymal factors that control epithelial proliferation and survival, and link hedgehog to Bmp and Wnt signaling.

Multiple roles of mesenchymal beta-catenin during murine limb patterning

Recently canonical Wnt signaling in the ectoderm has been shown to be required for maintenance of the apical ectodermal ridge (AER) and for dorsoventral signaling. Using conditional gain- and loss-of-function beta-catenin alleles, we have studied the role of mesenchymal beta-catenin activity during limb development. Here, we show that loss of beta-catenin results in limb truncations due to a defect in AER maintenance. Stabilization of beta-catenin also results in truncated limbs, caused by a premature regression of the AER. Concomitantly, in these limbs, the expression of Bmp2, Bmp4 and Bmp7, and of the Bmp target genes Msx1, Msx2 and gremlin, is expanded in the mesenchyme. Furthermore, we found that the expression of Lmx1b, a gene exclusively expressed in the dorsal limb mesenchyme and involved in dorsoventral patterning, is reduced upon loss of beta-catenin activity and is expanded ventrally in gain-of-function limbs. However, the known ectodermal regulators Wnt7a and engrailed 1 are expressed normally. This suggests that Lmx1b is also regulated, in part, by a beta-catenin-mediated Wnt signal, independent of the non-canoncial Wnt7a signaling pathway. In addition, loss of beta-catenin results in a severe agenesis of the scapula. Concurrently, the expression of two genes, Pax1 and Emx2, which have been implicated in scapula development, is lost in beta-catenin loss-of-function limbs; however, only Emx2 is upregulated in gain-of-function limbs. Mesenchymal beta-catenin activity is therefore required for AER maintenance, and for normal expression of Lmx1b and Emx2.

Sfrp1 and Sfrp2 regulate anteroposterior axis elongation and somite segmentation during mouse embryogenesis

Regulation of Wnt signaling is essential for embryonic patterning. Sfrps are secreted Wnt antagonists that directly interact with the Wnt ligand to inhibit signaling. Here, we show that Sfrp1 and Sfrp2 are required for anteroposterior (AP) axis elongation and somitogenesis in the thoracic region during mouse embryogenesis. Double homozygous mutations in Sfrp1 and Sfrp2 lead to severe shortening of the thoracic region. By contrast, a homozygous mutation in one or the other exerts no effect on embryogenesis, indicating that Sfrp1 and Sfrp2 are functionally redundant. The defect of a shortened thoracic region appears to be the consequence of AP axis reduction and incomplete somite segmentation. The reduction in the AP axis is partially due to abnormalities in cell migration of pre-somitic mesoderm from the end of gastrulation. Aberrant somite segmentation is associated with altered oscillations of Notch signaling, as evidenced by abnormal Lfng and Hes7 expression during somitogenesis in the thoracic region. This study suggests that Wnt regulation by Sfrp1 and Sfrp2 is required for embryonic patterning.

Importance of P-cadherin, beta-catenin, and Wnt5a/frizzled for progression of melanocytic tumors and prognosis in cutaneous melanoma

PURPOSE

It has been proposed that melanoma cells shift from E-cadherin to N-cadherin expression during tumor development, and recent gene profiling has shown increased expression of Wnt5a/Frizzled in aggressive melanomas possibly by interactions with beta-catenin. We therefore wanted to investigate the role of cadherin subtypes, beta-catenin, and Wnt5a/Frizzled in melanocytic tumors, with focus on prognosis in nodular melanomas.

EXPERIMENTAL DESIGN

The immunohistochemical expression of E-cadherin, N-cadherin, P-cadherin, beta-catenin, and Wnt5a/Frizzled was examined using tissue microarrays of 312 melanocytic tumors.

RESULTS

Cytoplasmic expression of P-cadherin was associated with increasing tumor thickness (P=0.005) and level of invasion (P=0.019), whereas membranous staining was associated with thinner (P=0.012) and more superficial (P=0.018) tumors. Increased cytoplasmic P-cadherin was associated with reduced survival (P=0.047). Lack of nuclear beta-catenin expression was related to increased tumor thickness (P=0.002) and poor patient survival in univariate (P=0.0072) and multivariate (P=0.004) analyses. Membranous expression of N-cadherin was significantly increased from primary tumors to metastatic lesions, whereas E-cadherin staining tended to be decreased. Wnt5a and its receptor Frizzled were highly coexpressed, and nuclear expression of both markers was significantly reduced from benign nevi to melanomas, with a shift from nuclear to cytoplasmic expression in malignant tumors. In addition, Wnt5a expression was significantly associated with nuclear beta-catenin expression.

CONCLUSIONS

Alterations in the expression and subcellular localization of cell adhesion markers are important in the development and progression of melanocytic tumors, and strong cytoplasmic P-cadherin expression and loss of nuclear beta-catenin staining were associated with aggressive melanoma behavior and reduced patient survival.

WNT pathways in the neonatal ovine uterus: potential specification of endometrial gland morphogenesis by SFRP2

Endometrial glands are critical for uterine function and develop between birth (Postnatal Day [P] 0) and P56 in the neonatal ewe. Endometrial gland morphogenesis or adenogenesis involves the site-specific budding differentiation of the glandular epithelium from the luminal epithelium followed by their coiling/branching development within the stroma of the intercaruncular areas of the endometrium. To determine whether WNT signaling regulates endometrial adenogenesis, the WNT signaling system was studied in the neonatal ovine uterus. WNT5A, WNT7A, and WNT11 were expressed in the uterine epithelia, whereas WNT2B was in the stroma. The WNT receptors FZD2 and FZD6 and coreceptor LRP6 were detected in all uterine cells, and FZD6 was particularly abundant in the endometrial epithelia. Secreted FZD-related protein-2 (SFRP2), a WNT antagonist, was not detected in the P0 uterus, but was abundant in the aglandular caruncular areas of the endometrium between P7 and P56. Exposure of ewes to estrogens during critical developmental periods inhibits or retards endometrial adenogenesis. Estrogen-induced disruption of endometrial adenogenesis was associated with reduction or ablation of WNT2B, WNT7A, and WNT11, and with an increase in WNT2 and SFRP2 mRNA, depending on exposure period. Collectively, results implicate the canonical and noncanonical WNT pathways in regulation of postnatal ovine uterine development and endometrial adenogenesis. Expression of SFRP2 in aglandular caruncular areas may inhibit the WNT signaling pathway, thereby concentrating WNT signaling and restricting endometrial adenogenesis in the intercaruncular areas of the uterus. Further, estrogen-induced inhibition of adenogenesis may be mediated by a reduction in WNT signaling caused by aberrant induction of SFRP2 and loss of several critical WNTs.

Comparative ontogeny and phylogeny of the upper jaw skeleton in amniotes

The morphology, position, and presence of the upper jaw bones vary greatly across amniote taxa. In this review, we compare the development and anatomy of upper jaw bones from the three living amniote groups: reptiles, birds, and mammals. The study of reptiles is particularly important as comparatively little is known about the embryogenesis of the jaw in this group. Our review covers the ontogeny and phylogeny of membranous bones in the face. The aim is to identify conserved embryonic processes that may exist among the three major amniote groups. Finally, we discuss how temporal and spatial regulation of preosseous condensations and ossification centers can lead to variation in the morphology of amniote upper jaw bones.

Wnt-5a protein expression in primary dukes B colon cancers identifies a subgroup of patients with good prognosis

Oncogenic Wnt/beta-catenin signaling occurs in a majority of colorectal cancers. In contrast, very little is known about the role of the nontransforming Wnt protein family member Wnt-5a in those tumors. In the most common of the three colon cancer stages, Dukes B or lymph node-negative, the outcome is the hardest to predict. We searched for a predictive marker in this group and observed loss of or reduced Wnt-5a expression in 50% of Dukes B tumors. Such Wnt-5a negativity was a strong predictor of adverse outcome, with a relative risk of death of 3.007 (95% confidence interval, 1.336-6.769; P = 0.008) after 5 years in Wnt-5a-negative patients. Furthermore, the median survival time after diagnosis was 109.1 months for patients with Wnt-5a-positive primary tumors but only 58 months for those with Wnt-5a-negative primary tumors. To find a possible biological explanation for these results, we studied the invasive and poorly differentiated human colon cancer cell line, SW480, which does not express Wnt-5a protein and the Wnt-5a-expressing and moderately differentiated Caco2 colon cancer cell line. We found that the addition of recombinant/purified Wnt-5a significantly reduced the migratory capacity of SW480 cells. By comparison, equivalent treatment did not significantly alter migration in the Wnt-5a-expressing Caco2 colon cancer cell line. These findings indicate that the expression of Wnt-5a in primary Dukes B colon cancer tissue constitutes a good prognostic marker for longer survival, which can be explained by the ability of Wnt-5a to impair tumor cell migration and thus reduce invasiveness and metastasis.

Up-regulation of beta-catenin by a viral oncogene correlates with inhibition of the seven in absentia homolog 1 in B lymphoma cells

The protein levels of beta-catenin are tightly regulated by the ubiquitin/proteasome system. We provide evidence that two distinct ubiquitin-dependent degradation pathways for beta-catenin are active in the same Burkitt's lymphoma cells: Along with the classical glycogen-synthase kinase 3beta-dependent destruction machinery, degradation of beta-catenin through seven in absentia homolog 1 (Siah-1) ubiquitin ligase is functional in these cells. We show that inhibition of endogenous Siah-1 stabilizes and activates beta-catenin in B cells. The principal Epstein-Barr virus oncoprotein, latent membrane protein 1, is involved in beta-catenin up-regulation, and expression of latent membrane protein 1 in B lymphoma cells is associated with decreased Siah-1 RNA and protein levels. Thus, we demonstrate the significance of the endogenous Siah-1-dependent ubiquitin/proteasome pathway for beta-catenin degradation in malignant human cells and its regulation by a viral oncogene.

Glycogen synthase kinase 3beta functions to specify gene-specific, NF-kappaB-dependent transcription

Loss of glycogen synthase kinase 3beta (GSK-3beta) in mice results in embryonic lethality via hepatocyte apoptosis. Consistent with this result, cells from these mice have diminished nuclear factor kappaB (NF-kappaB) activity, implying a functional role for GSK-3beta in regulating NF-kappaB. Here, we have explored mechanisms by which GSK-3beta may control NF-kappaB function. We show that cytokine-induced IkappaB kinase activity and subsequent phosphorylation of IkappaBalpha, p105, and p65 are not affected by the absence of GSK-3beta activity. Furthermore, nuclear accumulation of p65 following tumor necrosis factor treatment is unaffected by the loss of GSK-3beta. However, NF-kappaB DNA binding activity is reduced in GSK-3beta null cells and in cells treated with a pharmacological inhibitor of GSK-3. Expression of certain NF-kappaB-regulated genes, such as IkappaBalpha and macrophage inflammatory protein 2, is minimally affected by the absence of GSK-3beta. Conversely, we have identified a subset of NF-kappaB-regulated genes, including those for interleukin-6 and monocyte chemoattractant protein 1, that require GSK-3beta for efficient expression. We show that efficient localization of p65 to the promoter regions of the interleukin-6 and monocyte chemoattractant protein 1 genes following tumor necrosis factor alpha treatment requires GSK-3beta. Therefore, GSK-3beta has profound effects on transcription in a gene-specific manner through a mechanism involving control of promoter-specific recruitment of NF-kappaB.

The Wnt-inducible transcription factor Twist1 inhibits chondrogenesis

Wnt signaling is essential for many developmental processes, including skeletogenesis. To investigate the effects of Wnt signaling during skeletogenesis we studied the effects of Wnt on cultured chondrocytic cells and differentiating limb-bud mesenchyme. We showed that Wnt3a strongly repressed chondrogenesis and chondrocyte gene expression. Canonical Wnt signaling was responsible for the repression of differentiation, as evidenced by results showing that inhibition of glycogen synthase kinase 3 or expression of beta-catenin caused similar repression of differentiation. Significantly, we showed that the transcription repressor Twist1 is induced by canonical Wnt signaling. Expression of Twist1 strongly inhibited chondrocyte gene expression and short hairpin RNA knockdown of Twist1 transcript levels caused increased expression of the chondrocyte-specific genes aggrecan and type II collagen. Interestingly, Twist1 interfered with BMP2-induced expression of aggrecan and type II collagen expression and knockdown of Twist1 augmented BMP2-induced aggrecan and type II collagen expression. These data support the conclusions that Twist1 contributes to the repression of chondrogenesis and chondrocyte gene expression resulting from canonical Wnt signaling and that Twist1 interferes with BMP-dependent signaling.

Wnt proteins prevent apoptosis of both uncommitted osteoblast progenitors and differentiated osteoblasts by beta-catenin-dependent and -independent signaling cascades involving Src/ERK and phosphatidylinositol 3-kinase/AKT

Genetic studies in humans and mice have revealed an important role of the Wnt signaling pathway in the regulation of bone mass, resulting from potent effects on the control of osteoblast progenitor proliferation, commitment, differentiation, and perhaps osteoblast apoptosis. To establish the linkage between Wnts and osteoblast survival and to elucidate the molecular pathways that link the two, we have utilized three cell models: the uncommitted bipotential C2C12 cells, the pre-osteoblastic cell line MC3T3-E1, and bone marrow-derived OB-6 osteoblasts. Serum withdrawal-induced apoptosis was prevented by the canonical Wnts (Wnt3a and Wnt1) and the noncanonical Wnt5a in all cell types. Wnt3a induced LRP5-independent transient phosphorylation and nuclear accumulation of ERKs and phosphorylation of Src and Akt. The anti-apoptotic effect of Wnt3a was abrogated by inhibitors of canonical Wnt signaling, as well as by inhibitors of MEK, Src, phosphatidylinositol 3-kinase (PI3K), or Akt kinases, or by the addition of cycloheximide to the culture medium. Wnt3a-induced phosphorylation of GSK-3beta and downstream activation of beta-catenin-mediated transcription required ERK, PI3K, and Akt signaling. Wnt3a increased the expression of the anti-apoptotic protein Bcl-2 in an ERK-dependent manner. Beta-catenin-mediated transcription was permissive for the anti-apoptotic actions of Wnt1 and Wnt3a but was dispensable for the anti-apoptotic action of Wnt5a. However, Src, ERKs, PI3K, and Akt kinases were required for the anti-apoptotic effects of Wnt5a. These results demonstrate for the first time that Wnt proteins, irrespective of their ability to stimulate canonical Wnt signaling, prolong the survival of osteoblasts and uncommitted osteoblast progenitors via activation of the Src/ERK and PI3K/Akt signaling cascades.

Wnt5 signaling in vertebrate pancreas development

Background

Signaling by the Wnt family of secreted glycoproteins through their receptors, the frizzled (Fz) family of seven-pass transmembrane proteins, is critical for numerous cell fate and tissue polarity decisions during development.

Results

We report a novel role of Wnt signaling in organogenesis using the formation of the islet during pancreatic development as a model tissue. We used the advantages of the zebrafish to visualize and document this process in living embryos and demonstrated that insulin-positive cells actively migrate to form an islet. We used morpholinos (MOs), sequence-specific translational inhibitors, and time-lapse imaging analysis to show that the Wnt-5 ligand and the Fz-2 receptor are required for proper insulin-cell migration in zebrafish. Histological analyses of islets in Wnt5a^-/- mouse embryos showed that Wnt5a signaling is also critical for murine pancreatic insulin-cell migration.

Conclusion

Our results implicate a conserved role of a Wnt5/Fz2 signaling pathway in islet formation during pancreatic development. This study opens the door for further investigation into a role of Wnt signaling in vertebrate organ development and disease.

Morphing into cancer: the role of developmental signaling pathways in brain tumor formation

Morphogens play a critical role in most aspects of development, including expansion and patterning of the central nervous system. Activating germline mutations in components of the Hedgehog and Wnt pathways have provided evidence for the important roles morphogens play in the genesis of brain tumors such as cerebellar medulloblastoma. In addition, aberrant expression of transforming growth factor-beta (TGF-beta) superfamily members has been demonstrated to contribute to progression of malignant gliomas. This review summarizes our current knowledge about the roles of morphogens in central nervous system tumorigenesis.

Wnt-mediated regulation of chondrocyte maturation: modulation by TGF-beta

Wnt proteins are expressed during limb morphogenesis, yet their role and mechanism of action remains unclear during long bone growth. Wnt expression, effects and modulation of signaling events by BMP and transforming growth factor-beta (TGF-beta) were evaluated in chick embryonic chondrocytes. Chondrocyte cell cultures underwent spontaneous maturation with increased expression of colX and this was associated with an increase in the expression of multiple Wnts, including Wnt 4, 5a, 8c, and 9a. Both parathyroid hormone related peptide (PTHrP) and TGF-beta inhibited colX, but had disparate effects on Wnt expression. While TGF-beta strongly inhibited all Wnts, PTHrP did not inhibit either Wnt8c or Wnt9a and had lesser effects on the expression of the other Wnts. BMP-2 induced colX expression, and also markedly increased Wnt8c expression. Overexpression of beta-catenin and/or T cell factor (TCF)-4 also induced the type X collagen promoter. Overexpression of Wnt8c induced maturation, as did overexpression of beta-catenin. The Wnt8c/beta-catenin maturational effects were enhanced by BMP-2 and inhibited by TGF-beta. TGF-beta also inhibited activation of the Topflash reporter by beta-catenin, suggesting a direct inhibitory effect since the Topflash reporter contains only beta-catenin binding sequences. In turn beta-catenin inhibited activation of the p3TP-Luc reporter by TGF-beta, although the effect was partial. Thus, Wnt/beta-catenin signaling is a critical regulator of the rate of chondrocyte differentiation. Moreover, this pathway is modulated by members of the TGF-beta family and demonstrates the highly integrated nature of signals controlling endochondral ossification.

Global detection of molecular changes reveals concurrent alteration of several biological pathways in nonsmall cell lung cancer cells

To identify the molecular changes that occur in non-small cell lung carcinoma (NSCLC), we compared the gene expression profile of the NCI-H292 (H292) NSCLC cell line with that of normal human tracheobronchial epithelial (NHTBE) cells. The NHTBE cells were grown in a three-dimensional organotypic culture system that permits maintenance of the normal pseudostratified mucociliary phenotype characteristic of bronchial epithelium in vivo. Microarray analysis using the Affymetrix oligonucleotide chip U95Av2 revealed that 1,683 genes showed a >1.5-fold change in expression in the H292 cell line relative to the NHTBE cells. Specifically, 418 genes were downregulated and 1,265 were upregulated in the H292 cells. The expression data for selected genes were validated in several different NSCLC cell lines using quantitative real-time PCR and Western analysis. Further analysis of the differentially expressed genes indicated that WNT responses, apoptosis, cell cycle regulation and cell proliferation were significantly altered in the H292 cells. Functional analysis using fluorescence-activated cell sorting confirmed concurrent changes in the activity of these pathways in the H292 line. These findings show that (1) NSCLC cells display deregulation of the WNT, apoptosis, proliferation and cell cycle pathways, as has been found in many other types of cancer cells, and (2) that organotypically cultured NHTBE cells can be used as a reference to identify genes and pathways that are differentially expressed in tumor cells derived from bronchogenic epithelium.

hecate, a zebrafish maternal effect gene, affects dorsal organizer induction and intracellular calcium transient frequency

A zebrafish maternal effect mutation, in the gene hecate, results in embryos that have defects in the formation of dorsoanterior structures and altered calcium release. hecate mutant embryos lack nuclear accumulation of beta-catenin and have reduced expression of genes specific to the dorsal organizer. We found that hecate mutant embryos exhibit a nearly 10-fold increase in the frequency of intracellular Ca2+ transients normally present in the enveloping layer during the blastula stages. Inhibition of Ca2+ release leads to ectopic expression of dorsal genes in mutant embryos suggesting that Ca2+ transients are important in mediating dorsal gene expression. Inhibition of Ca2+ release also results in the expression of dorsal-specific genes in the enveloping layer in a beta-catenin-independent manner, which suggests an additional function for the Ca2+ transients in this cellular layer. The mutant phenotype can be reversed by the expression of factors that activate Wnt/beta-catenin signaling, suggesting that the Wnt/beta-catenin pathway, at least as activated by an exogenous Wnt ligand, is intact in hec mutant embryos. Our results are consistent with a role for the hecate gene in the regulation of Ca2+ release during the cleavage stages, which in turn influences dorsal gene expression in both marginal cells along the dorsoventral axis and in the enveloping layer.

Wnt signalling pathway: a new target for the treatment of osteoporosis

The prevention and treatment of osteoporosis traditionally involves the use of antiresorptive agents that target osteoclast function. Antiresorptive therapy is not associated with a significant increase in bone mass and, thus, only partially reduces the risk of fractures. For that reason, the search for anabolic agents, which target osteoblast function, represents an urgent medical need. The first approved bone anabolic drug for the treatment of osteoporosis was teriparatide (human parathyroid hormone 1-34). Recently, both human genetics and animal studies have pointed out the role of the Wnt/LRP5 pathway as a major regulator of bone mass accrual. Wnts are secreted glycoproteins that bind to receptor complexes including low-density lipoprotein receptor-related protein (LRP)-5/6 and Frizzled proteins. A subsequent intracellular cascade of events stabilises beta-catenin, leading to its translocation into the nucleus where, associated with Tcf/Lef transcription factors, it triggers gene expression. The existence of many potential pharmacological targets in this pathway makes it attractive for bone anabolic drug discovery.

SFRP1 regulates the growth of retinal ganglion cell axons through the Fz2 receptor

Axon growth is governed by the ability of growth cones to interpret attractive and repulsive guidance cues. Recent studies have shown that secreted signaling molecules known as morphogens can also act as axon guidance cues. Of the large family of Wnt signaling components, only Wnt4 and Wnt5 seem to participate directly in axon guidance. Here we show that secreted Frizzled-related protein 1 (SFRP1), a proposed Wnt signaling inhibitor, can directly modify and reorient the growth of chick and Xenopus laevis retinal ganglion cell axons. This activity does not require Wnt inhibition and is modulated by extracellular matrix molecules. Intracellularly, SFRP1 function requires G(alpha) protein activation, protein synthesis and degradation, and it is modulated by cyclic nucleotide levels. Because SFRP1 interacts with Frizzled-2 (Fz2) and interference with Fz2 expression abolishes growth cone responses to SFRP1, we propose a previously unknown function for this molecule: the ability to guide growth cone movement via the Fz2 receptor.

xBtg-x regulates Wnt/beta-Catenin signaling during early Xenopus development

In Xenopus, two signaling systems, maternal beta-Catenin and Nodal-related, are required for induction of the Spemann organizer and establishment of the body plan. By screening cDNA macroarrays for genes activated by these two signaling pathways, we identified Xenopus xBtg-x, a novel member of the Btg/Tob gene family of antiproliferative proteins. We show that xBtg-x is expressed in the dorsal mesendoderm (Spemann organizer tissue) of gastrula stage embryos and that its expression is regulated by both beta-Catenin and Nodal-related signals. Microinjection of synthetic xBtg-x mRNA into Xenopus embryos induced axis duplication and completely rescued the ventralizing effects of UV irradiation through the activation of the canonical Wnt/beta-Catenin signaling pathway. Interestingly, xBtg-x stimulated beta-Catenin-dependent transcription without affecting the stability of beta-Catenin protein. These data suggest that xBtg-x is a novel component of the Wnt/beta-Catenin signaling pathway regulating early embryonic patterning.

Estrogen down-regulation of the corepressor N-CoR: mechanism and implications for estrogen derepression of N-CoR-regulated genes

The nuclear receptor corepressor N-CoR plays a crucial role in the repressive activity of diverse transcription factors, yet little is known about what regulates its cellular level. We have found that estrogen markedly down-regulates N-CoR protein levels in estrogen receptor (ER)-positive breast cancer cells without affecting N-CoR mRNA levels, whereas levels of the related corepressor SMRT are unaffected. This effect is attributable to estrogen up-regulation of the ubiquitin ligase Siah2, which is a rapid and primary transcriptional response mediated by the ER, and precedes the loss of N-CoR. Treatment with proteasomal inhibitor or with small interfering RNA against Siah2 prevented the down-regulation of N-CoR by estrogen. Furthermore, the expression of 24-hydroxylase, a gene repressed by unliganded vitamin D receptor through its interaction with N-CoR, was up-regulated by estrogen and required Siah2. Our results illustrate a mechanism by which the estrogen-ER complex markedly reduces the level of N-CoR through a process involving the up-regulation of Siah2 and the subsequent targeting of N-CoR for proteasomal degradation. These findings reveal that, although estrogen directly regulates the transcription of many genes, by regulating a gene such as Siah2 it can exert profound "secondary" effects on cellular activity through mechanisms such as targeting regulatory proteins for degradation. This estrogen-evoked down-regulation of N-CoR could have a global derepressive effect on genes whose repression depends on N-CoR and thereby have broad impact on the activity of transcription factors and nuclear receptors whose actions involve N-CoR.

Canonical Wnt signaling: high-throughput RNAi widens the path

The canonical Wnt signaling pathway is highly conserved in evolution, widely used throughout animal development, and frequently hyperactive in cancer. Although Wnt signaling has been the subject of extensive genetic analysis in the past, some 200 genes have now been identified as candidate modulators of this pathway by a recent study using high-throughput RNAi screening.

Signalling pathways regulating the invasive differentiation of human trophoblasts: a review

The invasive differentiation pathway of trophoblasts is an indispensable physiological process of early human placental development. Formation of anchoring villi, proliferation of cell columns and invasion of extravillous cytotrophoblasts into maternal decidual stroma and vessels induce vascular changes ensuring an adequate blood supply to the growing fetus. Extravillous trophoblast differentiation is regulated by numerous growth factors as well as by extracellular matrix proteins and adhesion molecules expressed at the fetal-maternal interface. These regulatory molecules control cell invasion by modulating activities of matrix-degrading protease systems and ECM adhesion. The differentiation process involves numerous signalling cascades/proteins such as the GTPases RhoA, the protein kinases ROCK, ERK1, ERK2, FAK, PI3K, Akt/protein kinase B and mTOR as well as TGF-beta-dependent SMAD factors. While an increasing number of signalling pathways regulating trophoblast differentiation are being unravelled, downstream effectors such as executing transcription factors remain largely elusive. Here, we summarise our current knowledge on signal transduction cascades regulating invasive trophoblast differentiation. We will focus on cell model systems which are used to study the particular differentiation process and discuss signalling pathways which regulate trophoblast proliferation and motility.