A complex of Armadillo, Legless, and Pygopus coactivates dTCF to activate wingless target genes

Structural basis of the interaction between BCL9-Pygo and LDB-SSBP complexes in assembling the Wnt enhanceosome

The Wnt enhanceosome is responsible for transactivation of Wnt-responsive genes and a promising therapeutic target for treatment of numerous cancers with Adenomatous Polyposis Coli (APC) or β-catenin mutations. How the Wnt enhanceosome is assembled remains poorly understood. Here we show that B-cell lymphoma 9 protein (BCL9), Pygopus (Pygo), LIM domain-binding protein 1 (LDB1) and single-stranded DNA-binding protein (SSBP) form a stable core complex within the Wnt enhanceosome. Their mutual interactions rely on a highly conserved N-terminal asparagine proline phenylalanine (NPF) motif of Pygo, through which the BCL9-Pygo complex binds to the LDB-SSBP core complex. Our crystal structure of a ternary complex comprising the N-terminus of human Pygo2, LDB1 and SSBP2 reveals a single LDB1-SSBP2 complex binding simultaneously to two Pygo2 molecules via their NPF motifs. These interactions critically depend on the NPF motifs which bind to a deep groove formed between LDB1 and SSBP2, potentially constituting a binding site for drugs blocking Wnt/β-catenin signaling. Analysis of human cell lines lacking LDB or Pygo supports the functional relevance of the Pygo-LDB1-SSBP2 interaction for Wnt/β-catenin-dependent transcription.

Arf6 is necessary for senseless expression in response to wingless signalling during Drosophila wing development

Wnt signalling is a core pathway involved in a wide range of developmental processes throughout the metazoa. In vitro studies have suggested that the small GTP binding protein Arf6 regulates upstream steps of Wnt transduction, by promoting the phosphorylation of the Wnt co-receptor, LRP6, and the release of β-catenin from the adherens junctions. To assess the relevance of these previous findings in vivo, we analysed the consequence of the absence of Arf6 activity on Drosophila wing patterning, a developmental model of Wnt/Wingless signalling. We observed a dominant loss of wing margin bristles and Senseless expression in Arf6 mutant flies, phenotypes characteristic of a defect in high level Wingless signalling. In contrast to previous findings, we show that Arf6 is required downstream of Armadillo/β-catenin stabilisation in Wingless signal transduction. Our data suggest that Arf6 modulates the activity of a downstream nuclear regulator of Pangolin activity in order to control the induction of high level Wingless signalling. Our findings represent a novel regulatory role for Arf6 in Wingless signalling.

USP9X-mediated deubiquitination of B-cell CLL/lymphoma 9 potentiates Wnt signaling and promotes breast carcinogenesis

Hyperactivation of the canonical Wnt-signaling pathway is a prominent feature of a number of human malignancies. Transcriptional activation of this signaling cascade depends on the formation of the β-catenin-B-cell CLL/lymphoma 9 (BCL9)-pygopus (PYGO) family plant homeodomain finger 1 complex, yet how the assembly of this complex is regulated remains to be investigated. Here, using MCF-7, HeLa, HEK293T, MDA-MB-231, and Sf9 cells, along with immunoblotting and immunofluorescence, nano-HPLC-MS/MS, deubiquitination, immunoprecipitation, and chromatin immunoprecipitation (ChIP) assays, we report that BCL9 physically associates with a protein deubiquitinase, ubiquitin-specific peptidase 9, X-linked (USP9X), and that USP9X removes Lys-63-linked polyubiquitin on Lys-212 of BCL9. Importantly, the USP9X-mediated BCL9 deubiquitination facilitated the formation of the β-catenin-BCL9-PYGO complex, thereby potentiating the transcriptional activation of Wnt/β-catenin target genes. We also show that USP9X-mediated BCL9 deubiquitination promotes the proliferation and invasion of breast cancer cells. Together, these results uncover USP9X as a deubiquitinase of BCL9, implicating USP9X in Wnt/β-catenin signaling and breast carcinogenesis.

Structure-Based Optimization of Small-Molecule Inhibitors for the β-Catenin/B-Cell Lymphoma 9 Protein-Protein Interaction

Structure-based optimization was conducted to improve the potency, selectivity, and cell-based activities of β-catenin/B-cell lymphoma 9 (BCL9) inhibitors based on the 4'-fluoro- N-phenyl-[1,1'-biphenyl]-3-carboxamide scaffold, which was designed to mimic the side chains of the hydrophobic α-helical hot spots at positions i, i + 3, and i + 7. Compound 29 was found to disrupt the β-catenin/BCL9 protein-protein interaction (PPI) with a K_i of 0.47 μM and >1900-fold selectivity for β-catenin/BCL9 over β-catenin/E-cadherin PPIs. The proposed binding mode of new inhibitors was consistent with the results of site-directed mutagenesis and structure-activity relationship studies. Cell-based studies indicated that 29 disrupted the β-catenin/BCL9 interaction without affecting the β-catenin/E-cadherin interaction, selectively suppressed transactivation of Wnt/β-catenin signaling, downregulated expression of Wnt target genes, and inhibited viability of Wnt/β-catenin-dependent cancer cells in dose-dependent manners. A comparison of the biochemical and cell-based assay results offered the directions for future inhibitor optimization.

Role of BCL9L in transforming growth factor-β (TGF-β)-induced epithelial-to-mesenchymal-transition (EMT) and metastasis of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) has a low overall survival rate, which is approximately 20% during the first year and decreases to less than 6% within five years of the disease. This is due to premature dissemination accompanied by a lack of disease-specific symptoms during the initial stages. Additionally, to date there are no biomarkers for an early prognosis available.A growing number of studies indicate that epithelial to mesenchymal transition (EMT), triggered by WNT-, TGF-β- and other signaling pathways is crucial for the initiation of the metastatic process in PDAC. Here we show, that BCL9L is up-regulated in PDAC cell lines and patient tissue compared to non-cancer controls. RNAi-induced BCL9L knockdown negatively affected proliferation, migration and invasion of pancreatic cancer cells. On a molecular basis, BCL9L depletion provoked an increment of E-cadherin protein levels, with concomitant increase of β-catenin retention at the plasma membrane. This is linked to the induction of a strong epithelial phenotype in pancreatic cancer cells upon BCL9L knockdown even in the presence of the EMT-inducer TGF-β. Finally, xenograft mouse models of pancreatic cancer revealed a highly significant reduction in the number of liver metastases upon BCL9L knockdown. Taken together, our findings underline the key importance of BCL9L for EMT and thus progression and metastasis of pancreatic cancer cells. Direct targeting of this protein might be a valuable approach to effectively antagonize invasion and metastasis of PDAC.

Of Wnts and Ribosomes

Wnt proteins are secreted glycoproteins that activate different intracellular signal transduction pathways. They regulate cell proliferation and are required for proper embryonic development. Misregulation of Wnt signaling can result in various diseases including cancer. In most circumstances, cell growth is essential for cell division and thus cell proliferation. Therefore, several reports have highlighted the key role of Wnt proteins for cell growth. Ribosomes represent the cellular protein synthesis machinery and cells need to be equipped with an appropriate number of ribosomes to allow cell growth. Recent findings suggest a role for Wnt proteins in regulating ribosome biogenesis and we here summarize these findings representing a previously unknown function of Wnt proteins. Understanding this role of Wnt signaling might open new avenues to slow down proliferation by drugs for instance in cancer therapy.

KDM3 epigenetically controls tumorigenic potentials of human colorectal cancer stem cells through Wnt/β-catenin signalling

Human colorectal cancer stem cells (CSCs) are tumour initiating cells that can self-renew and are highly tumorigenic and chemoresistant. While genetic mutations associated with human colorectal cancer development are well-known, little is known about how and whether epigenetic factors specifically contribute to the functional properties of human colorectal CSCs. Here we report that the KDM3 family of histone demethylases plays an important role in tumorigenic potential and survival of human colorectal CSCs by epigenetically activating Wnt target gene transcription. The depletion of KDM3 inhibits tumorigenic growth and chemoresistance of human colorectal CSCs. Mechanistically, KDM3 not only directly erases repressive H3K9me2 marks, but also helps to recruit histone methyltransferase MLL1 to promote H3K4 methylation, thereby promoting Wnt target gene transcription. Our results suggest that KDM3 is a critical epigenetic factor in Wnt signalling that orchestrates chromatin changes and transcription in human colorectal CSCs, identifying potential therapeutic targets for effective elimination of CSCs.

Role of Pygo2 in tumors

Pygo2, a vital component of the Wnt signaling pathway that was recently discovered, has been reported to be closely related to the tumorigenesis of several types of malignant tumors. There are two highly conservative domains named NHD in the N terminus and PHD in the C terminus of Pygo2. Previous studies have shown that the Pygo2 PHD domain can act as a protein code reader to link the chromatin remodeling complex to specific changes in gene transcription, as demonstrated for the Wnt target genes. Furthermore, the activity of the chromatin remodeling is further facilitated by the recruiting of histone methyltransferase and acetyltransferase through the interaction with the Pygo2 NHD domain. However, the molecular mechanism of Pygo2 in the tumor development is still poorly understood. In the present study, we intend to review the structure and role of Pygo2 in tumor progression.

A co-dependent requirement of xBcl9 and Pygopus for embryonic body axis development in Xenopus

The Wnt/beta-catenin transcriptional activation complex requires the adapter protein Pygopus (Pygo), which links the basal transcription machinery to beta-catenin, by its association with legless (Lgs)/ B-cell lymphoma-9 (Bcl9). Pygo was shown to be required for development in vertebrates, but the role of Lgs/Bcl9 is unknown. We identified an amphibian orthologue of Lgs/Bcl9, XBcl9, which interacted biochemically with Xbeta-catenin and XPygo2. The body axis promoting ability of Xbeta-catenin was diminished when residues required for its interaction with XBcl9 were mutated. In blastula embryos, XBcl9 was transiently preferentially expressed in nuclei of dorsoanterior cells and ectopically expressed XBcl9 required XPygo2 to localize to nuclei. Furthermore, while neither XBcl9 nor XPygo2 alone affected development when ectopically expressed, both were required to induce supernumerary axis and dorsal gene activation. Like XPygo2, depletion of maternal XBcl9 alone caused dorsal defects. These results indicated an essential role of the Pygo-Bcl9 duet in vertebrate body axis formation.

Decreased pygopus 2 expression suppresses glioblastoma U251 cell growth

Gliomas are common malignant tumors of the human neural system, and Wnt signaling activation is closely related to glioma malignancy. Human Pygopus 2 (Pygo2) was recently discovered to be a component of the Wnt signaling pathway, which is required for β-catenin/Tcf-dependent transcription. However, the role of Pygo2 in glioblastoma cell growth and survival remains uncertain. In the present study, Pygo2 expression was evaluated in 80 glioma tissue samples. Results demonstrated that tumor grade exhibited a positive correlation with overexpression of Pygo2. In addition, small hairpin RNA (shRNA) was used to specifically knockdown Pygo2 expression in human glioblastoma U251 cell lines. Results showed that inhibition of Pygo2 expression resulted in inhibited cell proliferation and invasiveness, as well as increased cell cycle arrest at the G(1) stage and decreased expression of the Wnt target gene cyclin D1. These results demonstrated that Pygo2 was highly expressed in glioma tissue and required for growth of glioblastoma cells.

The ISWI-containing NURF complex regulates the output of the canonical Wingless pathway

Wingless (Wg) signalling regulates the expression of its target genes through Pangolin, Armadillo and their interacting co-factors. In a genetic screen for Wg signalling components, we found that imitation switch (ISWI), a chromatin-remodelling ATPase, had a positive role in transducing the canonical Wg signal, promoting the expression of the Wg target senseless. ISWI is found in several chromatin-remodelling complexes, including nucleosome remodelling factor (NURF). The effect of interfering with the function of other components of the NURF complex in vivo mimics that of ISWI. The NURF complex is also required for the efficient expression of other Wg target genes. Armadillo interacts directly with the NURF complex in vitro and recruits it to Wg targets in cultured cells. Together, our results suggest that the ISWI-containing NURF complex functions as a co-activator of Armadillo to promote Wg-mediated transcription.

The transcriptional activity of Pygopus is enhanced by its interaction with cAMP-response-element-binding protein (CREB)-binding protein

Pygopus is a core component of the beta-catenin/TCF (T-cell factor) transcriptional activation complex required for the expression of canonical Wnt target genes. Recent evidence suggests that Pygopus could interpret histone methylation associated with target genes and it was shown to be required for histone acetylation. The involvement of a specific acetyltransferase, however, was not determined. In this report, we demonstrate that Pygopus can interact with the HAT (histone acetyltransferase) CBP [CREB (cAMP-responsive-element-binding protein)-binding protein]. The interaction is via the NHD (N-terminal homology domain) of Pygopus, which binds to two regions in the vicinity of the HAT domain of CBP. Transfected and endogenous hPygo2 (human Pygopus2) and CBP proteins co-immunoprecipitate in HEK-293 (human embryonic kidney 293) cells and both proteins co-localize in SW480 colorectal cancer cells. The interaction with CBP also enhances both DNA-tethered and TCF/LEF1 (lymphoid enhancing factor 1)-dependent transcriptional activity of Pygopus. Furthermore, immunoprecipitated Pygopus protein complexes displayed CBP-dependent histone acetyltransferase activity. Our data support a model in which the NHD region of Pygopus is required to augment TCF/beta-catenin-mediated transcriptional activation by a mechanism that includes both transcriptional activation and histone acetylation resulting from the recruitment of the CBP histone acetyltransferase.

Epigenetics, Wnt signaling, and stem cells: the Pygo2 connection

Stem cells use both transcriptional and epigenetic mechanisms to control gene expression and regulate tissue development and homeostasis. In this issue, Gu et al. (Gu, B., P. Sun, Y. Yuan, R.C. Moraes, A. Li, A. Teng, A. Agrawal, C. Rhéaume, V. Bilanchone, J.M. Veltmaat, et al. 2009. J. Cell Biol. 185:811–826) reveal an important link between these two mechanisms in mammary epithelial stem cells by showing that transcriptional activation of β-catenin downstream of Wnt signaling can be regulated epigenetically through a chromatin remodeling factor, Pygo2.

The PHD domain is required to link Drosophila Pygopus to Legless/beta-catenin and not to histone H3

In Drosophila Pygopus (Pygo) and Legless (Lgs)/BCL9 are integral components of the nuclear Wnt/Wg signaling machine. Despite intense research, ideas that account for their mode of action remain speculative. One proposition, based on a recently discovered function of PHD fingers, is that Pygo, through its PHD, may decipher the histone code. We found that human, but not Drosophila, Pygo robustly interacts with a histone-H3 peptide methylated at lysine-4. The different binding behavior is due to a single amino acid change that appears unique to Drosophilidae Pygo proteins. Rescue experiments with predicted histone binding mutants showed that in Drosophila the ability to bind histones is not essential. Further experiments with Pygo-Lgs fusions instead demonstrated that the crucial role of the PHD is to provide an interaction motif to bind Lgs. Our results reveal an interesting evolutionary dichotomy in Pygo structure-function, as well as evidence underpinning the chain of adaptors model.

A role of Pygopus as an anti-repressor in facilitating Wnt-dependent transcription

Wnt/beta-catenin signaling controls animal development and tissue homeostasis, and is also an important cancer pathway. Pygopus (Pygo) is a conserved nuclear Wnt signaling component that is essential for Wingless-induced transcription throughout Drosophila development. It associates with Armadillo/beta-catenin and T cell factor (TCF) through the Legless/BCL9 adaptor, but its molecular function in TCF-mediated transcription is unknown. Here, we use a groucho-null allele to show that Groucho represses Wingless target genes during Drosophila development. Interestingly, groucho pygo double-mutants revealed that Pygo is not obligatory for transcriptional and phenotypic Wingless signaling outputs if the interaction between Groucho and Drosophila TCF is compromised genetically. Pygo function is also non-essential for Wingless outputs in the absence of other transcriptional antagonists of Wingless signaling. This indicates an anti-repressor function of Pygo: we propose that Pygo predisposes Drosophila TCF target genes for rapid Wingless-induced transcription, or that it protects them against premature shut-down.

Low concentrations of resveratrol inhibit Wnt signal throughput in colon-derived cells: implications for colon cancer prevention

Resveratrol is a bioflavonoid which is known to inhibit cell proliferation and induce apoptosis in cancer cell lines at concentrations above 50 muM. It also has colon cancer prevention activity in mouse models and possibly in humans. We have examined the effects of low concentrations of resveratrol on a specific signaling pathway, the Wnt pathway, which is activated in over 85% of sporadic colon cancers. Two colon cancer (HT29 and RKO) and one normal mucosa-derived (NCM460) cell lines were utilized. Cell proliferation was not affected by resveratrol at < or =40 microM for HT29 and NCM460 and <20 microM for RKO though Wnt signal throughput, as measured by a reporter construct, was reduced in RKO and NCM460 at concentrations as low as 10 microM (p < 0.001). This effect was most easily appreciated following Wnt pathway stimulation with Wnt3a conditioned medium and LEF1 or LEF1/beta-catenin transfection. Resveratrol did not inhibit Wnt throughput in mutationally activated HT29. Low concentrations of resveratrol significantly decreased the amount and proportion of beta-catenin in the nucleus in RKO (p = 0.002) and reduced the expression of lgs and pygoI, regulators of beta-catenin localization, in all cells lines. Thus, at low concentrations, in the absence of effects on cell proliferation, resveratrol significantly inhibits Wnt signaling in colon-derived cells which do not have a basally activated Wnt pathway. This inhibitory effect may be due in part to regulation of intracellular beta-catenin localization.

The function of BCL9 in Wnt/beta-catenin signaling and colorectal cancer cells

BACKGROUND

Most cases of colorectal cancer are initiated by hyperactivation of the Wnt/beta-catenin pathway due to mutations in the APC tumour suppressor, or in beta-catenin itself. A recently discovered component of this pathway is Legless, which is essential for Wnt-induced transcription during Drosophila development. Limited functional information is available for its two mammalian relatives, BCL9 and B9L/BCL9-2: like Legless, these proteins bind to beta-catenin, and RNAi-mediated depletion of B9L/BCL9-2 has revealed that this protein is required for efficient beta-catenin-mediated transcription in mammalian cell lines. No loss-of-function data are available for BCL9.

METHODS

We have used overexpression of dominant-negative forms of BCL9, and RNAi-mediated depletion, to study its function in human cell lines with elevated Wnt pathway activity, including colorectal cancer cells.

RESULTS

We found that BCL9 is required for efficient beta-catenin-mediated transcription in Wnt-stimulated HEK 293 cells, and in the SW480 colorectal cancer cell line whose Wnt pathway is active due to APC mutation. Dominant-negative mutants of BCL9 indicated that its function depends not only on its beta-catenin ligand, but also on an unknown ligand of its C-terminus. Finally, we show that BCL9 and B9L are both Wnt-inducible genes, hyperexpressed in colorectal cancer cell lines, indicating that they are part of a positive feedback loop.

CONCLUSION

BCL9 is required for efficient beta-catenin-mediated transcription in human cell lines whose Wnt pathway is active, including colorectal cancer cells, indicating its potential as a drug target in colorectal cancer.

Decoding of methylated histone H3 tail by the Pygo-BCL9 Wnt signaling complex

Pygo and BCL9/Legless transduce the Wnt signal by promoting the transcriptional activity of β-catenin/Armadillo in normal and malignant cells. We show that human and Drosophila Pygo PHD fingers associate with their cognate HD1 domains from BCL9/Legless to bind specifically to the histone H3 tail methylated at lysine 4 (H3K4me). The crystal structures of ternary complexes between PHD, HD1, and two different H3K4me peptides reveal a unique mode of histone tail recognition: efficient histone binding requires HD1 association, and the PHD-HD1 complex binds preferentially to H3K4me2 while displaying insensitivity to methylation of H3R2. Therefore, this is a prime example of histone tail binding by a PHD finger (of Pygo) being modulated by a cofactor (BCL9/Legless). Rescue experiments in Drosophila indicate that Wnt signaling outputs depend on histone decoding. The specificity of this process provided by the Pygo-BCL9/Legless complex suggests that this complex facilitates an early step in the transition from gene silence to Wnt-induced transcription.

Nuclear regulator Pygo2 controls spermiogenesis and histone H3 acetylation

Mammalian spermiogenesis, a process where haploid male germ cells differentiate to become mature spermatozoa, entails dramatic morphological and biochemical changes including remodeling of the germ cell chromatin. Proteins that contain one or more plant homeodomain (PHD) fingers have been implicated in the regulation of chromatin structure and function. Pygopus 2 (Pygo2) belongs to a family of evolutionarily conserved PHD finger proteins thought to act as co-activators of Wnt signaling effector complexes composed of beta-catenin and LEF/TCF transcription factor. Here we analyze mice containing hypomorphic alleles of pygopus 2 (Pygo2 or mpygo2) and uncover a beta-catenin-independent involvement of the Pygo2 protein in spermiogenesis. Pygo2 is expressed in elongating spermatids at stages when chromatin remodeling occurs, and block of Pygo2 function leads to spermiogenesis arrest and consequent infertility. Analysis of spermiogenesis in Pygo2 mutants reveals reduced expression of select post-meiotic genes including protamines, transition protein 2, and H1fnt, all of which are required for germ cell chromatin condensation, and drastically altered pattern of histone H3 hyperacetylation. These findings suggest that Pygo2 is involved in the chromatin remodeling events that lead to nuclear compaction of male germ cells.

Pygopus and the Wnt signaling pathway: a diverse set of connections

Identification of Pygopus in Drosophila as a dedicated component of the Wg (fly homolog of mammalian Wnt) signaling cascade initiated many inquiries into the mechanism of its function. Surprisingly, the nearly exclusive role for Pygopus in Wg signal transduction in flies is not seen in mice, where Pygopus appears to have both Wnt-related and Wnt-independent functions. This review addresses the initial findings of Pygopus as a Wg/Wnt co-activator in light of recent data from both fly and mammalian studies. We compare and contrast the developmental phenotypes of pygopus mutants to those characterized for known Wg/Wnt transducers and explore the data regarding a role for mammalian Pygopus 2 in tumorigenesis. We further analyze the roles of the two conserved domains of Pygopus proteins in transcription, and propose a model for the molecular mechanism of Pygopus function in both Wg/Wnt signaling and Wnt-independent transcriptional regulation.

Pygopus activates Wingless target gene transcription through the mediator complex subunits Med12 and Med13

Wnt target gene transcription is mediated by nuclear translocation of stabilized beta-catenin, which binds to TCF and recruits Pygopus, a cofactor with an unknown mechanism of action. The mediator complex is essential for the transcription of RNA polymerase II-dependent genes; it associates with an accessory subcomplex consisting of the Med12, Med13, Cdk8, and Cyclin C subunits. We show here that the Med12 and Med13 subunits of the Drosophila mediator complex, encoded by kohtalo and skuld, are essential for the transcription of Wingless target genes. kohtalo and skuld act downstream of beta-catenin stabilization both in vivo and in cell culture. They are required for transcriptional activation by the N-terminal domain of Pygopus, and their physical interaction with Pygopus depends on this domain. We propose that Pygopus promotes Wnt target gene transcription by recruiting the mediator complex through interactions with Med12 and Med13.

Analysis of mPygo2 mutant mice suggests a requirement for mesenchymal Wnt signaling in pancreatic growth and differentiation

Pygopus has recently been identified in Drosophila as an essential component of the nuclear complex required for canonical Wnt signaling. Here, we have investigated the role of the mammalian pygopus ortholog, mPygo2, in pancreas development. We show that a null mutation of mPygo2 in mice causes pancreas hypoplasia due to decreased progenitor cell proliferation after embryonic day (e) 12.5. During the same time window, mPygo2-deficient embryos begin to display a reduction in endocrine progenitors and consequently a decrease in islet endocrine cell mass. Consistent with its function after e12.5, late-developing endocrine cell types, such as beta, delta and PP cells, are specifically reduced, while the earlier-forming alpha cells develop normally. We find canonical Wnt signaling to be predominantly active in the mesenchyme at the time when mPygo2 is required and demonstrate the dependence of Wnt signal transduction on mPygo2. Furthermore, conditional deletion of mPygo2(flox) allele in the pancreatic epithelium does not phenocopy the defects in mPygo2-null mutants. Since mPygo2 is expressed in the pancreatic mesenchyme and the role of the mesenchyme in epithelial progenitor cell expansion is well documented, our findings suggest an indirect role for mPygo2 in epithelial growth and differentiation through regulation of mesenchymal signals. Together, our data suggest a previously unappreciated role for mesenchymal Wnt signaling in regulating pancreatic organ growth and cell differentiation.

Cell-type-specific function of BCL9 involves a transcriptional activation domain that synergizes with beta-catenin

Transcriptional regulation by the canonical Wnt pathway involves the stabilization and nuclear accumulation of beta-catenin, which assembles with LEF1/TCF transcription factors and cofactors to activate Wnt target genes. Recently, the nuclear beta-catenin complex has been shown to contain BCL9, which interacts with beta-catenin and recruits Pygopus as a transcriptional coactivator. However, the presumed general functions of Pygopus and BCL9, which has been proposed to act as a scaffolding protein for Pygopus, have been challenged by the rather specific and modest developmental defects of targeted inactivations of both the Pygo1 and the Pygo2 genes. Here, we analyze the function of BCL9 in transcriptional activation by beta-catenin. We find that BCL9 acts in a cell-type-specific manner and, in part, independent of Pygopus. We show that BCL9 itself contains a transcriptional activation domain in the C terminus, which functionally synergizes in lymphoid cells with the C-terminal transactivation domain of beta-catenin. Finally, we identify amino acids in the transactivation domain of beta-catenin that are important for its function and association with the histone acetyltransferases CBP/p300 and TRRAP/GCN5. Thus, BCL9 may serve to modulate and diversify the transcriptional responses to Wnt signaling in a cell-type-specific manner.

Mastermind-like 1 Is a specific coactivator of beta-catenin transcription activation and is essential for colon carcinoma cell survival

Misregulation of the Wnt signaling pathway has been linked to many human cancers including colon carcinoma and melanoma. The primary mediator of the oncogenic effects of the Wnt signaling pathway is beta-catenin. Accumulation of nuclear beta-catenin and transcription activation of lymphoid enhancer factor 1 (LEF1)/T-cell factor (TCF) target genes underlie the oncogenic activity. However, the mechanism of beta-catenin-mediated transcriptional activation remains poorly understood. In this study, we identified Mastermind-like 1 (Maml1), which is thought to be a specific coactivator for the Notch pathway, as a coactivator for beta-catenin. We found that Maml1 participates in the Wnt signaling by modulating the beta-catenin/TCF activity. We show in vivo that Maml1 is recruited by beta-catenin on the cyclin D1 and c-Myc promoters. Importantly, we show that Maml1 functions in the Wnt/beta-catenin pathway independently of Notch signaling. Finally, we show that the knockdown of Mastermind-like family proteins in colonic carcinoma cells results in cell death by affecting beta-catenin-induced expression of cyclin D1 and c-Myc. This is the first demonstration of a role for the Mastermind-like family in another signaling pathway and that the knockdown of Mastermind-like family function leads to tumor cell death.

Defining the roles of beta-catenin and plakoglobin in LEF/T-cell factor-dependent transcription using beta-catenin/plakoglobin-null F9 cells

beta-Catenin functions as a transcriptional regulator in Wnt signaling. Its function is regulated by a specific destruction system. Plakoglobin is a close homologue of beta-catenin in mammalian cells and is regulated in a similar fashion. When beta-catenin or plakoglobin is exogenously expressed in cells, endogenous beta-catenin is stabilized, which complicates estimation of the transcriptional activities of exogenously expressed proteins. To facilitate the design of experiments aimed at investigating the transcriptional activities of beta-catenin and plakoglobin, we utilized F9 cells in which we knocked out endogenous beta-catenin and/or plakoglobin by gene deletion and exogenously expressed wild-type and mutant beta-catenin and/or plakoglobin. We show that C-terminally deleted beta-catenin, but not plakoglobin, has a strong dominant-negative effect on transcription without altering the nuclear accumulation of beta-catenin. Moreover, we show that Wnt-3a activation of LEF/T-cell factor (TCF)-dependent transcription depends on beta-catenin but not on plakoglobin. Using chimeras of beta-catenin and plakoglobin, we demonstrate that plakoglobin has the potential to function in transcriptional regulation but is not responsible for Wnt-3a signaling in F9 cells. Our data show that preferential nuclear accumulation of beta-catenin is not necessarily linked to its transcriptional activity. We also clearly demonstrate that plakoglobin is insufficient for LEF/TCF-dependent transcriptional activation by Wnt-3a in F9 cells.

Developmental phenotypes and reduced Wnt signaling in mice deficient for pygopus 2

Canonical Wnt signaling involves complex intracellular events culminating in the stabilization of beta-catenin, which enters the nucleus and binds to LEF/TCF transcription factors to stimulate gene expression. Pygopus was identified as a genetic modifier of Wg (Wnt homolog) signaling in Drosophila, and encodes a PHD domain protein that associates with the beta-catenin/LEF/TCF complex. Two murine pygopus paralogs, mpygo1 and mpygo2, have been identified, but their roles in development and Wnt signaling remain elusive. In this study, we report that ablation of mpygo2 expression in mice causes defects in morphogenesis of both ectodermally and endodermally derived tissues, including brain, eyes, hair follicles, and lung. However, no gross abnormality was observed in embryonic intestine. Using a BAT-gal reporter, we found Wnt signaling at most body sites to be reduced in the absence of mpygo2. Taken together, our studies show for the first time that mpygo2 deletion affects embryonic development of some but not all Wnt-requiring tissues.

The emerging role of Wnt signaling in the pathogenesis of acute myeloid leukemia

Wnt signaling plays an important role in stem cell self-renewal and proliferation. Aberrant activation of Wnt signaling and its downstream targets are intimately linked with several types of cancer with colon cancer being the best-studied example. However, recent results also suggest an important role of Wnt signaling in normal as well as leukemic hematopoietic stem cells. Aberrant activation of Wnt signaling and downstream effectors has been demonstrated in acute myeloid leukemia. Here, mutant receptor tyrosine kinases, such as Flt3 and chimeric transcription factors such as promyelocytic leukemia-retinoic acid receptor-alpha and acute myeloid leukemia1-ETO, induce downstream Wnt signaling events. These findings suggest that the Wnt signaling pathway is an important target in several leukemogenic pathways and may provide a novel opportunity for targeting leukemic stem cells.

Crystal Structure Analysis of the PHD Domain of the Transcription Co-activator Pygopus

The Wnt/beta-catenin signaling pathway plays important roles in animal development and cancer. Pygopus (Pygo) and Legless (Lgs) are recently discovered core components of the Wnt/beta-catenin transcription machinery complex, and are crucially involved in the regulation of the transcription of the Arm/beta-catenin and T cell factors (TCF). Lgs/Bcl9 functions as an adaptor between Pygo and Arm/beta-catenin. Here, we report the first crystal structure of the plant homeodomain (PHD) finger of Pygopus (Pygo1 PHD), a Pygo family member, which is essential for the association with Lgs/Bcl9. The Pygo1 PHD structure forms a canonical PHD finger motif, stabilized by two Zn ions coordinated in a cross-brace scheme. Surprisingly, the Pygo1 PHD domain forms a dimer in both the crystals and solution. This is the first structural evidence for dimerization among the known PHD domain structures. The dimer formation occurs by the interactions of antiparallel beta-sheets between the symmetry-related beta3 strands of the monomers. The Pygo1 PHD dimer interface mainly comprises hydrophobic residues. Interestingly, some of the interface residues, such as Met372, Thr373, Ala376 and Leu380, are reportedly important for the association with Lgs/Bcl9 and are also critical for transcriptional activation. The M372A and L380D mutants, and several surrounding mutants such as S385A and A386D, showed decreased ability to form dimers and to interact with the homology domain 1 (HD1) of Lgs/Bcl9. These results suggest that the Pygo1 PHD dimerization is functionally important for Lgs/Bcl9 recognition as well as for the regulation of the Wnt/beta-catenin signaling pathway.

pygopus 2 has a crucial, Wnt pathway-independent function in lens induction

Drosophila Pygopus was originally identified as a core component of the canonical Wnt signaling pathway and a transcriptional coactivator. Here we have investigated the microophthalmia that arises in mice with a germline null mutation of pygopus 2. We show that this phenotype is a consequence of defective lens development at inductive stages. Using a series of regionally limited Cre recombinase transgenes for conditional deletion of Pygo2flox, we show that Pygo2 activity in pre-placodal presumptive lens ectoderm, placodal ectoderm and ocular mesenchyme all contribute to lens development. In each case, Pygo2 is required for normal expression levels of the crucial transcription factor Pax6. Finally, we provide multiple lines of evidence that although Pygo2 can function in the Wnt pathway, its activity in lens development is Wnt pathway-independent.

CBP/p300 are bimodal regulators of Wnt signaling

Many Wnts influence cell behavior by a conserved signaling cascade that promotes the stabilization and nuclear accumulation of beta-catenin (beta-cat), which then associates with TCF family members to activate target genes. The histone acetyltransferase CREB binding protein (CBP) can bind to TCF and inhibit Wnt signaling in Drosophila. In contrast, studies in vertebrates indicate a positive role for CBP and the closely related protein p300 as beta-cat binding transcriptional co-activators. We address this discrepancy by demonstrating that in addition to its negative role, CBP has an essential positive role in Wnt signaling in flies. CBP binds directly to the C-terminus of Armadillo (Arm, the fly beta-cat) and is recruited to a Wnt-regulated enhancer (WRE) in a Wnt- and Arm-dependent manner. In a human colorectal cancer cell line, we show that CBP and p300 can inhibit Wnt signaling and demonstrate that human p300 can bind directly to TCF4 in vitro. Our results argue that CBP/p300 has an evolutionarily conserved role as a buffer regulating TCF-beta-cat/Arm binding. Subsequent to this interaction, it also has an essential role in mediating the transactivation activity of beta-cat/Arm.

Wingless-independent association of Pygopus with dTCF target genes

The Wnt signaling pathway controls numerous cell fates during animal development. Its inappropriate activity can lead to cancer in many human tissues. A key effector of the canonical Wnt pathway is beta-catenin (or Drosophila Armadillo), a highly unstable phosphorylated protein that shuttles rapidly between nucleus and cytoplasm. Wnt signaling inhibits its phosphorylation and degradation; this allows it to associate with TCF/LEF factors bound to Wnt target genes and to stimulate their transcription by recruiting chromatin modifying and remodeling factors. The transcriptional activity of Armadillo/beta-catenin also depends on Pygopus (Pygo), a nuclear protein with which it associates through the Legless/BCL9 adaptor. It has been proposed that Pygo associates with TCF target genes during Wnt signaling through Armadillo and Legless to recruit a transcriptional coactivator through its Nbox motif. Here, we report that Pygo is associated constitutively with dTCF target genes in Drosophila salivary glands and tissue-culture cells. Our evidence indicates that this association depends on dTCF and on the Nbox motif of Pygo, but not on Legless. We thus propose an alternative model according to which Pygo functions at the onset of Wnt signaling, or at low signaling levels, to capture Armadillo at dTCF target genes, thus enabling the interaction between Armadillo and dTCF and, consequently, the Armadillo-mediated recruitment of transcriptional coactivators.

BCL9-2 binds Arm/beta-catenin in a Tyr142-independent manner and requires Pygopus for its function in Wg/Wnt signaling

The Wingless (Wg)/Wnt signal transduction pathway controls fundamental processes during animal development. Deregulation of the Wg/Wnt pathway has been causally linked to several forms of cancer, most notably to colorectal cancer. In response to Wg/Wnt signaling, Armadillo/beta-catenin associates in the nucleus with DNA bound TCF and several co-factors, among them Legless/BCL9, which provides a link to Pygopus. Recently, the second vertebrate homologue of Legless, BCL9-2 (or B9L), was characterized and proposed to mediate Wnt signaling in a Pygopus-independent manner, by binding to a Tyrosine-142-phosphorylated form of beta-catenin. Here we examine the role of Tyrosine-142 phosphorylation in several assays and find that it is neither important for the recruitment of BCL9-2, nor for the transcriptional activity of beta-catenin in cultured mammalian cells, nor in Drosophila for Wg signaling activity in vivo. Furthermore, we demonstrate that BCL9-2 can functionally replace Lgs both in cultured cells as well as in vivo and that this rescue activity depends on the ability of BCL9-2 to bind Pygo. Our results do not show a significant functional difference between BCL9-2 and BCL9 but rather suggest that the two proteins represent evolutionary duplicates of Legless, which have acquired distinct expression patterns while acting in a largely redundant manner.

Transcription under the control of nuclear Arm/beta-catenin

The Wingless/Wnt pathway controls cell fates during animal development and regulates tissue homeostasis as well as stem cell number and differentiation in epithelia. Deregulation of Wnt signaling has been associated with cancer in humans. In the nucleus, the Wingless/Wnt signal is transmitted via the key effector protein Armadillo/beta-catenin. The recent identification and functional analysis of novel Armadillo/beta-catenin interaction partners provide new and exciting insights into the highly complex mechanism of Wingless/Wnt target gene activation.

The specification of a highly derived arthropod appendage, the Drosophila labial palps, requires the joint action of selectors and signaling pathways

The remarkable diversity of form in arthropods reflects flexible genetic programs deploying many conserved genes. In the insect model Drosophila melanogaster, diversity of form can be observed between serially homologous appendages or when a single appendage is transformed by homeotic mutations, such as the adult labial mouthparts that can present alternative antennal, prothoracic, or maxillary identities. We have examined the roles of the Hox selector genes proboscipedia (pb) and Sex combs reduced (Scr), and the antennal selectors homothorax (hth) and spineless (ss) in labial specification, by tissue-directed mitotic recombination. Whereas loss of pb function transforms labium to prothoracic leg, loss of Scr, hth, or ss functions results in little or no change in labial specification. Results of analysis of single and multiple mutant combinations support a genetic hierarchy in which the homeotic pb gene possesses a primary role. It is surprising to note that while loss of ss activity alone had no detected effect, all mutant combinations lacking both pb and ss yielded the most severe phenotype observed: stunted, apparently tripartite legs that may correspond to a default state. The roles of the four selector genes are functionally linked to a cell nonautonomous mechanism involving the coupled activities of the decapentaplegic (dpp)/TGF-beta and wingless (wg)/Wnt signaling pathways. Accordingly, several mutant combinations impaired in dpp signaling were seen to reorient labial-to-leg transformations toward antennal aristae. A crucial aspect of selector function in development and evolution may be in regulating diffusible signals, including those emitted by dpp and wg.

Wnt signaling: is the party in the nucleus?

The Wnt signaling pathway controls cell proliferation and body patterning throughout development. A surprising number of cytoplasmic Wnt regulators (e.g., beta-catenin, Bcl-9/Lgs, APC, Axin) also appear, often transiently, in the nucleus. beta-Catenin is an integral component of E-cadherin complexes at intercellular adherens junctions, but also recruits chromatin remodeling complexes to activate transcription in the nucleus. The APC tumor suppressor is a part of the cytoplasmic beta-catenin destruction complex, yet also counteracts beta-catenin transactivation and histone H3K4 methylation at Wnt target genes. Furthermore, APC coordinates the cyclic exchange of Wnt coregulator complexes at the DNA. These opposing roles of APC and beta-catenin enable a rapid coordination of gene expression and cytoskeletal organization throughout the cell in response to signaling.

C-terminal-binding protein directly activates and represses Wnt transcriptional targets in Drosophila

Regulation of Wnt transcriptional targets is thought to occur by a transcriptional switch. In the absence of Wnt signaling, sequence-specific DNA-binding proteins of the TCF family repress Wnt target genes. Upon Wnt stimulation, stabilized beta-catenin binds to TCFs, converting them into transcriptional activators. C-terminal-binding protein (CtBP) is a transcriptional corepressor that has been reported to inhibit Wnt signaling by binding to TCFs or by preventing beta-catenin from binding to TCF. Here, we show that CtBP is also required for the activation of some Wnt targets in Drosophila. CtBP is recruited to Wnt-regulated enhancers in a Wnt-dependent manner, where it augments Armadillo (the fly beta-catenin) transcriptional activation. We also found that CtBP is required for repression of a subset of Wnt targets in the absence of Wnt stimulation, but in a manner distinct from previously reported mechanisms. CtBP binds to Wnt-regulated enhancers in a TCF-independent manner and represses target genes in parallel with TCF. Our data indicate dual roles for CtBP as a gene-specific activator and repressor of Wnt target gene transcription.

Balancing cell adhesion and Wnt signaling, the key role of beta-catenin

Controlled regulation of cell proliferation and differentiation is essential for embryonic development and requires the coordinated regulation of cell-cell adhesion and gene transcription. The armadillo repeat protein beta-catenin is an important integrator of both processes. Beta-catenin acts in the Wnt signaling pathway, activating the transcription of crucial target genes responsible for cellular proliferation and differentiation. Beta-catenin also controls E-cadherin-mediated cell adhesion at the plasma membrane and mediates the interplay of adherens junction molecules with the actin cytoskeleton. Both functions of beta-catenin are de-regulated in human malignancies, thereby leading both to the loss of cell-cell adhesion and to the increased transcription of Wnt target genes.

Two Sides of the Same Coin: Wnt Signaling in Neurodegeneration and Neuro-Oncology

Wnts function through the activation of at least three intracellular signal transduction pathways, of which the canonical β-catenin mediated pathway is the best understood. Aberrant canonical Wnt signaling has been involved in both neurodegeneration and cancer. An impairment of Wnt signals appears to be associated with aspects of neurodegenerative pathologies while overactivation of Wnt signaling is a common theme in several types of human tumors. Therefore, although therapeutic approaches aimed at modulating Wnt signaling in neurodegenerative and hyperproliferative diseases might impinge on the same molecular mechanisms, different pharmacological outcomes are required. Here we review recent developments on the understanding of the role of Wnt signaling in Alzheimer's disease and CNS tumors, and identify possible avenues for therapeutic intervention within a complex and multi-faceted signaling pathway.

Dissecting nuclear Wingless signalling: recruitment of the transcriptional co-activator Pygopus by a chain of adaptor proteins

Members of the Wingless (Wg)/Wnt family of secreted glycoproteins control cell fate during embryonic development and adult homeostasis. Wnt signals regulate the expression of target genes by activating a conserved signal transduction pathway. Upon receptor activation, the signal is transmitted intracellularly by stabilization of Armadillo (Arm)/beta-catenin. Arm/beta-catenin translocates to the nucleus, interacts with DNA-binding factors of the Pangolin (Pan)/TCF/LEF class and activates transcription of target genes in cooperation with the recently identified proteins Legless/BCL9 (Lgs) and Pygopus (Pygo). Here, we analyse the mode of action of Pan, Arm, Lgs, and Pygo in Drosophila cultured cells. We provide evidence that together these four proteins form a 'chain of adaptors' linking the NH2-terminal homology domain (NHD) of Pygo to the DNA-binding domain of Pan. We show that the NHD has potent transcriptional activation capacity, which differs from that of acidic activator domains and depends on a conserved NPF tripeptide. A single point mutation within this NPF motif abolishes the transcriptional activity of the Pygo NHD in vitro and strongly reduces Wg signalling in vivo. Together, our results suggest that the transcriptional output of Wg pathway activity largely relies on a 'chain of adaptors' design to direct the Pygo NHD to Wg target promoters in an Arm-dependent manner.

Pygopus and Legless Provide Essential Transcriptional Coactivator Functions to Armadillo/β-Catenin

Wnt signaling controls important aspects of animal development, and its deregulation has been causally linked to cancer. Transduction of Wnt signals entails the association of beta-catenin with nuclear TCF DNA binding proteins and the subsequent activation of target genes. The transcriptional activity of Armadillo (Arm, the Drosophila beta-catenin homolog) largely depends on two recently discovered components, Legless (Lgs) and Pygopus (Pygo). Lgs functions as an adaptor between Arm/beta-catenin and Pygo, but different mechanisms have been proposed as to how Arm/beta-catenin is controlled by Lgs and Pygo. Although Lgs and Pygo were originally thought to serve as nuclear cofactors for Arm/beta-catenin to enhance its transactivation capacity, a recent analysis argued that they function instead to target Arm/beta-catenin to the nucleus. Here, we used genetic assays in cultured cells and in vivo to discriminate between the two paradigms. Regardless of the measures taken to maintain the nuclear presence of Arm/beta-catenin, a transcriptional-activation function of Pygo could not be bypassed. Our findings therefore indicate that Arm/beta-catenin depends on Lgs and Pygo primarily for its transcriptional output rather than for its nuclear import.

WNT signalling and haematopoiesis: a WNT-WNT situation

The evolutionarily conserved WNT-signalling pathway has pivotal roles during the development of many organ systems, and dysregulated WNT signalling is a key factor in the initiation of various tumours. Recent studies have implicated a role for WNT signal transduction at several stages of lymphocyte development and in the self-renewal of haematopoietic stem cells. Here, we outline new insights into the WNT-signalling pathway, review its role in the self-renewal of haematopoietic stem cells and in the development of T and B cells, and discuss controversies and future developments with regard to WNT signalling in the thymus.

The canonical Wnt pathway in early mammalian embryogenesis and stem cell maintenance/differentiation

The Wnt signaling pathway plays essential roles in embryogenesis of higher eukaryotes from diploblastic, radially symmetrical cnidarians to mice and humans. Whereas studies in model organisms such as Drosophila and Xenopus continue to elucidate how the Wnt pathway is regulated, studies of mouse mutants and cultured mammalian cells start to reveal how the Wnt pathway controls development and differentiation in mammals. Here we review some of the recent progresses in our understanding of the Wnt pathway. We focus on how this pathway, through regulating transcription of its downstream target genes, specifies pattern formation during mammalian embryogenesis and functions in the differentiation and maintenance of stem cells both in vivo and in vitro.

Pygopus and Legless target Armadillo/beta-catenin to the nucleus to enable its transcriptional co-activator function

Wnt signalling controls the transcription of genes that function during normal and malignant development. Stimulation by canonical Wnt ligands activates beta-catenin (or Drosophila melanogaster Armadillo) by blocking its phosphorylation, resulting in its stabilization and translocation to the nucleus. Here, Armadillo/beta-catenin binds to TCF/LEF transcription factors and recruits chromatin-modifying and -remodelling complexes to transcribe Wnt target genes. The transcriptional activity of Armadillo/beta-catenin depends on two conserved nuclear proteins recently discovered in Drosophila, Pygopus (Pygo) and Legless/BCL-9 (Lgs). Lgs functions as an adaptor between Pygo and Armadillo/beta-catenin, but how Armadillo/beta-catenin is controlled by Pygo and Lgs is not known. Here, we show that the nuclear localization of Lgs entirely depends on Pygo, which itself is constitutively localized to the nucleus; thus, Pygo functions as a nuclear anchor. Pygo is also required for high nuclear Armadillo levels during Wingless signalling, and together with Lgs increases the transcriptional activity of beta-catenin in APC mutant cancer cells. Notably, linking Armadillo to a nuclear localization sequence rescues pygo and lgs mutant fly embryos. This indicates that Pygo and Lgs function in targeting Armadillo/beta-catenin to the nucleus, thus ensuring its availability to TCF during Wnt signalling.