Accumulation of Armadillo induced by Wingless, Dishevelled, and dominant-negative Zeste-White 3 leads to elevated DE-cadherin in Drosophila clone 8 wing disc cells

Dishevelled Has Anti-Viral Activity in Rift Valley Fever Virus Infected Aedes aegypti

Mosquitoes in the genera Aedes and Culex are vectors of Rift Valley fever virus (RVFV), which emerges in periodic epidemics in Africa and Saudi Arabia. Factors that influence the transmission dynamics of RVFV are not well characterized. To address this, we interrogated mosquito host-signaling responses through analysis of differentially expressed genes (DEGs) in two mosquito species with marked differences in RVFV vector competence: Aedes aegypti (Aae, low competence) and Culex tarsalis (Cxt, high competence). Mosquito-host transcripts related to three different signaling pathways were investigated. Selected genes from the Wingless (Wg, WNT-beta-catenin) pathway, which is a conserved regulator of cell proliferation and differentiation, were assessed. One of these, dishevelled (DSH), differentially regulates progression/inhibition of the WNT and JNK (c-Jun N-terminal Kinase) pathways. A negative regulator of the JNK-signaling pathway, puckered, was also assessed. Lastly, Janus kinase/signal transducers and activators of transcription (JAK-STAT) are important for innate immunity; in this context, we tested domeless levels. Here, individual Aae and Cxt were exposed to RVFV MP-12 via oral bloodmeals and held for 14 days. Robust decreases in DEGs in both Aae and Cxt were observed. In particular, Aae DSH expression, but not Cxt DSH, was correlated to the presence/absence of viral RNA at 14 days post-challenge (dpc). Moreover, there was an inverse relationship between the viral copy number and aaeDSH expression. DSH silencing resulted in increased viral copy numbers compared to controls at 3 dpc, consistent with a role for aaeDSH in antiviral immunity. Analysis of cis-regulatory regions for the genes of interest revealed clues to upstream regulation of these pathways.

The role of intercellular junction proteins in the penetration resistance of Drosophila larvae to avermectin

Insecticide resistance has become an increasingly serious challenge for agriculture in the world. To reveal the mechanisms of insecticide resistance, majority of studies have been carried out on the insensitivity of insecticide targets and the metabolism of insecticides. However, the mechanism of the insecticide penetration resistance in insects remains unclear. This study aimed to reveal the mechanism underlying the penetration resistance of Drosophila larvae to insecticide avermectin (AVM). Levels of intercellular junction proteins (IJPs) in the larvae were determined by Western blotting analysis and immunofluorescence assay. The result showed that the expression of IJPs septate junction and adherens junction proteins increased in the AVM-resistant insects compared with those in the AVM-susceptible ones, and the upregulation of the IJPs was mediated by the activation of protein kinase C (PKC) pathway. That AVM induced the activation of PKC was found not only in the Drosophila larvae but also in Drosophila S2 cells. These findings revealed that AVM could activate PKC pathway in Drosophila larvae, which mediated the upregulation of the IJPs and then led to the resistance to AVM, suggesting that the chemicals that can disrupt PKC activation may potentially be used to circumvent the resistance to AVM in insects.

Nuclear signaling from cadherin adhesion complexes

The arrival of multicellularity in evolution facilitated cell-cell signaling in conjunction with adhesion. As the ectodomains of cadherins interact with each other directly in trans (as well as in cis), spanning the plasma membrane and associating with multiple other entities, cadherins enable the transduction of "outside-in" or "inside-out" signals. We focus this review on signals that originate from the larger family of cadherins that are inwardly directed to the nucleus, and thus have roles in gene control or nuclear structure-function. The nature of cadherin complexes varies considerably depending on the type of cadherin and its context, and we will address some of these variables for classical cadherins versus other family members. Substantial but still fragmentary progress has been made in understanding the signaling mediators used by varied cadherin complexes to coordinate the state of cell-cell adhesion with gene expression. Evidence that cadherin intracellular binding partners also localize to the nucleus is a major point of interest. In some models, catenins show reduced binding to cadherin cytoplasmic tails favoring their engagement in gene control. When bound, cadherins may serve as stoichiometric competitors of nuclear signals. Cadherins also directly or indirectly affect numerous signaling pathways (e.g., Wnt, receptor tyrosine kinase, Hippo, NFκB, and JAK/STAT), enabling cell-cell contacts to touch upon multiple biological outcomes in embryonic development and tissue homeostasis.

Clinical significance of aberrant Wnt7a promoter methylation in human non-small cell lung cancer in Koreans

The Wnt signaling pathway has regulatory roles in cell proliferation, differentiation, and polarity. Aberrant Wnt pathway regulation can lead to abnormal cell proliferation and cancer, and loss of Wnt7a expression has been demonstrated in lung cancer cell lines. E-cadherin keeps intercellular integrity and prevents metastasis. Therefore, E-cadherin has been known as a prognostic factor in cancer. In the present study, we investigated the E-cadherin expression status by immunohistochemical stain and the Wnt7a promoter methylation status in human non-small cell lung carcinoma (NSCLC) by methylation-specific PCR. We also analyzed their correlations with clinicopathological factors. Methylation of the Wnt7a gene promoter was detected in the lung tissues of 32 of 121 (26.4%) patients with NSCLC. Wnt7a promoter methylation was correlated with advanced tumor stage (P = 0.036) and distant metastasis (P = 0.037). In addition, Wnt7a promoter methylation showed correlation with loss of E-cadherin expression (P < 0.001). However, Wnt7a promoter methylation was not closely related with gender, age, histological type, or smoking habit. Even though Wnt7a methylation could not show significant correlation with the long term survival of the patients with limited follow up data, these findings suggest that loss of the Wnt7a gene induced by promoter methylation might be another prognostic factor for NSCLC and that restoration of Wnt7a may be a promising treatment for NSCLC.

Modulating and measuring Wingless signalling

The main Wnt ligand of Drosophila activates a conserved canonical signalling pathway to regulate a plethora of cellular activities during development, regeneration and nervous system function. Here I first describe experimental means of measuring and modulating Wingless signalling in Drosophila cell culture. Various reporters have been devised by placing TCF-binding sites or DNA fragments from known target genes upstream of luciferase-coding sequences. Signalling can be activated in cells by addition of Wingless conditioned medium, treatment with a chemical inhibitor of Shaggy/GSK3 or transfection with a plasmid encoding activated Armadillo (Drosophila β-catenin). Measuring Wingless signalling in intact tissue is somewhat more challenging than in cell culture. Synthetic transgenic reporters have been devised but further improvements are needed to achieve sensitive responsiveness to Wingless at all times and places. As an alternative, gene traps in frizzled3 and notum/wingful, two context-independent endogenous targets, can be used as reporters. It is hoped that further modification of these loci could lead to more versatile and sensitive means of detecting signalling. Many genetic tools are available to trigger ectopic signalling or prevent endogenous signalling. These mostly rely on RNAi-producing transgenes or the generation of mutant patches by mitotic recombination. New developments in genome engineering are opening further means of manipulating the components of Wingless signalling with exquisite temporal and spatial precision.

Beta-catenin versus the other armadillo catenins: assessing our current view of canonical Wnt signaling

The prevailing view of canonical Wnt signaling emphasizes the role of beta-catenin acting downstream of Wnt activation to regulate transcriptional activity. However, emerging evidence indicates that other armadillo catenins in vertebrates, such as members of the p120 subfamily, convey parallel signals to the nucleus downstream of canonical Wnt pathway activation. Their study is thus needed to appreciate the networked mechanisms of canonical Wnt pathway transduction, especially as they may assist in generating the diversity of Wnt effects observed in development and disease. In this chapter, we outline evidence of direct canonical Wnt effects on p120 subfamily members in vertebrates and speculate upon these catenins' roles in conjunction with or aside from beta-catenin.

Signaling from the adherens junction

The cadherin/catenin complex organizes to form a structural Velcro that joins the cytoskeletal networks of adjacent cells. Functional loss of this complex arrests the development of normal tissue organization, and years of research have gone into teasing out how the physical structure of adhesions conveys information to the cell interior. Evidence that most cadherin-binding partners also localize to the nucleus to regulate transcription supports the view that cadherins serve as simple stoichiometric inhibitors of nuclear signals. However, it is also clear that cadherin-based adhesion initiates a variety of molecular events that can ultimately impact nuclear signaling. This chapter discusses these two modes of cadherin signaling in the context of tissue growth and differentiation.

Drosophila insulin and target of rapamycin (TOR) pathways regulate GSK3 beta activity to control Myc stability and determine Myc expression in vivo

BACKGROUND

Genetic studies in Drosophila melanogaster reveal an important role for Myc in controlling growth. Similar studies have also shown how components of the insulin and target of rapamycin (TOR) pathways are key regulators of growth. Despite a few suggestions that Myc transcriptional activity lies downstream of these pathways, a molecular mechanism linking these signaling pathways to Myc has not been clearly described. Using biochemical and genetic approaches we tried to identify novel mechanisms that control Myc activity upon activation of insulin and TOR signaling pathways.

RESULTS

Our biochemical studies show that insulin induces Myc protein accumulation in Drosophila S2 cells, which correlates with a decrease in the activity of glycogen synthase kinase 3-beta (GSK3β ) a kinase that is responsible for Myc protein degradation. Induction of Myc by insulin is inhibited by the presence of the TOR inhibitor rapamycin, suggesting that insulin-induced Myc protein accumulation depends on the activation of TOR complex 1. Treatment with amino acids that directly activate the TOR pathway results in Myc protein accumulation, which also depends on the ability of S6K kinase to inhibit GSK3β activity. Myc upregulation by insulin and TOR pathways is a mechanism conserved in cells from the wing imaginal disc, where expression of Dp110 and Rheb also induces Myc protein accumulation, while inhibition of insulin and TOR pathways result in the opposite effect. Our functional analysis, aimed at quantifying the relative contribution of Myc to ommatidial growth downstream of insulin and TOR pathways, revealed that Myc activity is necessary to sustain the proliferation of cells from the ommatidia upon Dp110 expression, while its contribution downstream of TOR is significant to control the size of the ommatidia.

CONCLUSIONS

Our study presents novel evidence that Myc activity acts downstream of insulin and TOR pathways to control growth in Drosophila. At the biochemical level we found that both these pathways converge at GSK3β to control Myc protein stability, while our genetic analysis shows that insulin and TOR pathways have different requirements for Myc activity during development of the eye, suggesting that Myc might be differentially induced by these pathways during growth or proliferation of cells that make up the ommatidia.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Direct inhibition of GSK3beta by the phosphorylated cytoplasmic domain of LRP6 in Wnt/beta-catenin signaling

Wnt/β-catenin signaling plays a central role in development and is also involved in a diverse array of diseases. Binding of Wnts to the coreceptors Frizzled and LRP6/5 leads to phosphorylation of PPPSPxS motifs in the LRP6/5 intracellular region and the inhibition of GSK3β bound to the scaffold protein Axin. However, it remains unknown how GSK3β is specifically inhibited upon Wnt stimulation. Here, we show that overexpression of the intracellular region of LRP6 containing a Ser/Thr rich cluster and a PPPSPxS motif impairs the activity of GSK3β in cells. Synthetic peptides containing the PPPSPxS motif strongly inhibit GSK3β in vitro only when they are phosphorylated. Microinjection of these peptides into Xenopus embryos confirms that the phosphorylated PPPSPxS motif potentiates Wnt-induced second body axis formation. In addition, we show that the Ser/Thr rich cluster of LRP6 plays an important role in LRP6 binding to GSK3β. These observations demonstrate that phosphorylated LRP6/5 both recruits and directly inhibits GSK3β using two distinct portions of its cytoplasmic sequence, and suggest a novel mechanism of activation in this signaling pathway.

Applications of high-throughput RNA interference screens to problems in cell and developmental biology

RNA interference (RNAi) in tissue culture cells has emerged as an excellent methodology for identifying gene functions systematically and in an unbiased manner. Here, we describe how RNAi high-throughput screening (HTS) in Drosophila cells are currently being performed and emphasize the strengths and weaknesses of the approach. Further, to demonstrate the versatility of the technology, we provide examples of the various applications of the method to problems in signal transduction and cell and developmental biology. Finally, we discuss emerging technological advances that will extend RNAi-based screening methods.

Wingless/Wnt signal transduction requires distinct initiation and amplification steps that both depend on Arrow/LRP

Members of the Wg/Wnt family provide key intercellular signals during embryonic development and in the maintenance of homeostatic processes, but critical aspects of their signal transduction pathways remain controversial. We have found that canonical Wg signaling in Drosophila involves distinct initiation and amplification steps, both of which require Arrow/LRP. Expressing a chimeric Frizzled2-Arrow protein in flies that lack endogenous Wg or Arrow showed that this construct functions as an activated Wg receptor but is deficient in signal amplification. In contrast, a chimeric Arrow protein containing the dimerization domain of Torso acted as a potent amplifier of Wg signaling but could not initiate Wg signaling on its own. The two chimeric proteins synergized, so that their co-expression largely reconstituted the signaling levels achieved by expressing Wg itself. The amplification function of Arrow/LRP appears to be particularly important for long-range signaling, and may reflect a general mechanism for potentiating signals in the shallow part of a morphogen gradient.

Cooperative activities of drosophila DE-cadherin and DN-cadherin regulate the cell motility process of ommatidial rotation

Ommatidial rotation is a cell motility read-out of planar cell polarity (PCP) signaling in the Drosophila eye. Although the signaling aspects of PCP establishment are beginning to be unraveled, the mechanistic aspects of the associated ommatidial rotation process remain unknown. Here, we demonstrate that the Drosophila DE- and DN-cadherins have opposing effects on rotation. DE-cadherin promotes rotation, as DE-cad mutant ommatidia rotate less than wild type or not at all. By contrast, the two DN-cadherins act to restrict this movement, with ommatidia rotating too fast in the mutants. The opposing effects of DE- and DN-cadherins result in a coordinated cellular movement, enabling ommatidia of the same stage to rotate simultaneously. Genetic interactions, phenotypic analysis and localization studies indicate that EGF-receptor and Frizzled-PCP signaling feed into the regulation of cadherin activity and localization in this context. Thus, DE- and DN-cadherins integrate inputs from at least two signaling pathways, resulting in a coordinated cell movement. A similar input into mammalian E- and N-cadherins might function in the progression of diseases such as metastatic ovarian cancer.

Drosophila IKK-Related Kinase Regulates Nonapoptotic Function of Caspases via Degradation of IAPs

Caspase activation has been extensively studied in the context of apoptosis. However, caspases also control other cellular functions, although the mechanisms regulating caspases in nonapoptotic contexts remain obscure. Drosophila IAP1 (DIAP1) is an endogenous caspase inhibitor that is crucial for regulating cell death during development. Here we describe Drosophila IKK-related kinase (DmIKKvarepsilon) as a regulator of caspase activation in a nonapoptotic context. We show that DmIKKvarepsilon promotes degradation of DIAP1 through direct phosphorylation. Knockdown of DmIKKvarepsilon in the proneural clusters of the wing imaginal disc, in which nonapoptotic caspase activity is required for proper sensory organ precursor (SOP) development, stabilizes endogenous DIAP1 and affects Drosophila SOP development. Our results demonstrate that DmIKKvarepsilon is a determinant of DIAP1 protein levels and that it establishes the threshold of activity required for the execution of nonapoptotic caspase functions.

HES1 cooperates with pRb to activate RUNX2-dependent transcription

The retinoblastoma protein, pRb, can activate the transcription factor RUNX2, an essential regulator of osteogenic differentiation, but the mechanism of this activation is unknown. Here we studied the interaction of pRb and RUNX2 with HES1, previously reported to augment RUNX2 activity. PRb can act to promote RUNX2/HES1 association with concomitant promoter occupancy and transcriptional activation in bone cells.

INTRODUCTION

RUNX2 (also known as OSF2/CBFA1) is a transcription factor required for osteoblast differentiation and bone formation. We have reported that RUNX2 can associate with the retinoblastoma protein pRb, a common tumor suppressor in bone, and the resultant complex can bind and activate transcription from bone-specific promoters. This activity of the pRb/RUNX2 complex may thus link differentiation control with tumor suppressor activity. However, the mechanism through which pRb can activate RUNX2 is unknown. HES1 is a reported co-activator of RUNX2 that shares a binding site on RUNX2 with pRb. Thus, we have tested the cooperativity among these factors in activating transcription from bone specific promoters.

MATERIALS AND METHODS

Coimmunoprecipitation, chromatin immunoprecipitation, and EMSA experiments were used to study the interaction of RUNX2, HES1, and pRb in cell lysates and on DNA. Transcriptional reporter assays were used to analyze the activity of RUNX2 in the presence and absence of HES1 and pRb.

RESULTS

We showed that pRb can associate with HES1, a previously described RUNX2 interactor that can itself augment RUNX2-dependent transcription. The association of HES1 with RUNX2 is augmented by pRb. Furthermore, both pRb and HES1 increase the amount of RUNX2 bound to promoter sites in vivo, pRb and HES1 synergistically activate a RUNX2-dependent reporter gene, and depletion of HES1 reduces RUNX2/pRb activity.

CONCLUSIONS

These data indicate that pRb acts as a RUNX2 co-activator at least in part by recruiting HES1 into the pRb/RUNX2 complex and further elucidate a novel role for pRb as a transcriptional co-activator in osteogenesis.

Wingless signaling modulates cadherin-mediated cell adhesion in Drosophila imaginal disc cells

Armadillo, the Drosophila homolog of beta-catenin, plays a crucial role in both the Wingless signal transduction pathway and cadherin-mediated cell-cell adhesion, raising the possibility that Wg signaling affects cell adhesion. Here, we use a tissue culture system that allows conditional activation of the Wingless signaling pathway and modulation of E-cadherin expression levels. We show that activation of the Wingless signaling pathway leads to the accumulation of hypophosphorylated Armadillo in the cytoplasm and in cellular processes, and to a concomitant reduction of membrane-associated Armadillo. Activation of the Wingless pathway causes a loss of E-cadherin from the cell surface, reduced cell adhesion and increased spreading of the cells on the substratum. After the initial loss of E-cadherin from the cell surface, E-cadherin gene expression is increased by Wingless. We suggest that Wingless signaling causes changes in Armadillo levels and subcellular localization that result in a transient reduction of cadherin-mediated cell adhesion, thus facilitating cell shape changes, division and movement of cells in epithelial tissues.

Fat and Wingless signaling oppositely regulate epithelial cell-cell adhesion and distal wing development inDrosophila

Development of organ-specific size and shape demands tight coordination between tissue growth and cell-cell adhesion. Dynamic regulation of cell adhesion proteins thus plays an important role during organogenesis. In Drosophila, the homophilic cell adhesion protein DE-Cadherin (DE-Cad)regulates epithelial cell-cell adhesion at adherens junctions (AJs). Here, we show that along the proximodistal (PD) axis of the developing wing epithelium,apical cell shapes and expression of DE-Cad are graded in response to Wingless (Wg), a morphogen secreted from the dorsoventral (DV) organizer in distal wing, suggesting a PD gradient of cell-cell adhesion. The Fat (Ft)tumor suppressor, by contrast, represses DE-Cad expression. In genetic tests, ft behaves as a suppressor of Wg signaling. Cytoplasmic pool of β-catenin/Arm, the intracellular transducer of Wg signaling, is negatively correlated with the activity of Ft. Moreover, unlike that of Wg, signaling by Ft negatively regulates the expression of Distalless(Dll) and Vestigial (Vg). Finally, we show that Ft intersects Wnt/Wg signaling, downstream of the Wg ligand. Fat and Wg signaling thus exert opposing regulation to coordinate cell-cell adhesion and patterning along the PD axis of Drosophila wing.

Drosophila caspase transduces Shaggy/GSK-3beta kinase activity in neural precursor development

Caspases are well known for their role in the execution of apoptotic programs, in which they cleave specific target proteins, leading to the elimination of cells, and for their role in cytokine maturation. In this study, we identified a novel substrate, which, through cleavage by caspases, can regulate Drosophila neural precursor development. Shaggy (Sgg)46 protein, an isoform encoded by the sgg gene and essential for the negative regulation of Wingless signaling, is cleaved by the Dark-dependent caspase. This cleavage converts it to an active kinase, which contributes to the formation of neural precursor (sensory organ precursor (SOP)) cells. Our evidence suggests that caspase regulation of the wingless pathway is not associated with apoptotic cell death. These results imply a novel role for caspases in modulating cell signaling pathways through substrate cleavage in neural precursor development.

Axin contains three separable domains that confer intramolecular, homodimeric, and heterodimeric interactions involved in distinct functions

Axin is a major scaffold protein, interacting with diverse molecules involved in a number of signaling pathways. Axin can undergo dimer/oligomerization via its DIX domain. Here we show that whereas deletion of the DIX domain at the C terminus rendered Axin incapable of forming dimer, a larger deletion of the C-terminal region restored the ability of Axin to form dimers. Detailed analyses revealed that Axin actually contains two separate domains (D and I) in addition to the DIX domain for homodimerization. The D, I, and DIX domains alone can form homodimers. Interestingly, D and I domains strongly interact with each other, suggesting that Axin can form an intramolecular structure through D and I interaction in the absence of DIX. We also found that DIX-DIX homodimeric interaction is weak but that point mutations in the DIX domain abolished Axin homodimerization. We propose a model to suggest that Axin forms homodimeric interactions through three domains, D, I, and DIX. More importantly, lack of DIX-DIX interaction caused by point mutations in the DIX domain or deletion causes Axin to form an intramolecular loop through the D and I domains, disallowing homodimer formation. Ccd1 interacts with Axin D domain yet fails to interact with AxinDeltaDIX, confirming that D is masked after D-I looping. The Axin mutants that are defective in homodimer formation fail to activate JNK but have no effect on beta-catenin signaling. Our findings have thus provided a structural basis of conformational changes in Axin, which may underlie the diversity of Axin functions.

Cross-regulation of Wnt signaling and cell adhesion

The complex cross-regulation between Wnt signaling, cell-cell adhesion, and cell-matrix adhesion has revealed a number of regulatory components important in development and cancer progression. In the following, we would like to highlight and summarize some of the steps where pathways converge or diverge in regulating Wnt activity, matrix-induced pathways, and cell adhesion. We would like to focus on the involvement of heparan sulfate proteoglycan-rich proteins (HSPGs), integrin-mediated outside-in signaling, and cadherin-mediated cell-cell adhesion on Wnt pathways and the transcriptional regulation of extracellular matrix components and cell adhesion molecules by Wnt signaling.

WNT7a induces E-cadherin in lung cancer cells

E-cadherin loss in cancer is associated with de-differentiation, invasion, and metastasis. Drosophila DE-cadherin is regulated by Wnt/beta-catenin signaling, although this has not been demonstrated in mammalian cells. We previously reported that expression of WNT7a, encoded on 3p25, was frequently downregulated in lung cancer, and that loss of E-cadherin or beta-catenin was a poor prognostic feature. Here we show that WNT7a both activates E-cadherin expression via a beta-catenin specific mechanism in lung cancer cells and is involved in a positive feedback loop. Li+, a GSK3 beta inhibitor, led to E-cadherin induction in an inositol-independent manner. Similarly, exposure to mWNT7a specifically induced free beta-catenin and E-cadherin. Among known transcriptional suppressors of E-cadherin, ZEB1 was uniquely correlated with E-cadherin loss in lung cancer cell lines, and its inhibition by RNA interference resulted in E-cadherin induction. Pharmacologic reversal of E-cadherin and WNT7a losses was achieved with Li+, histone deacetylase inhibition, or in some cases only with combined inhibitors. Our findings provide support that E-cadherin induction by WNT/beta-catenin signaling is an evolutionarily conserved pathway operative in lung cancer cells, and that loss of WNT7a expression may be important in lung cancer development or progression by its effects on E-cadherin.

Binding site for p120/delta-catenin is not required for Drosophila E-cadherin function in vivo

Homophilic cell adhesion mediated by classical cadherins is important for many developmental processes. Proteins that interact with the cytoplasmic domain of cadherin, in particular the catenins, are thought to regulate the strength and possibly the dynamics of adhesion. beta-catenin links cadherin to the actin cytoskeleton via alpha-catenin. The role of p120/delta-catenin proteins in regulating cadherin function is less clear. Both beta-catenin and p120/delta-catenin are conserved in Drosophila. Here, we address the importance of cadherin-catenin interactions in vivo, using mutant variants of Drosophila epithelial cadherin (DE-cadherin) that are selectively defective in p120ctn (DE-cadherin-AAA) or beta-catenin-armadillo (DE-cadherin-Delta beta) interactions. We have analyzed the ability of these proteins to substitute for endogenous DE-cadherin activity in multiple cadherin-dependent processes during Drosophila development and oogenesis; epithelial integrity, follicle cell sorting, oocyte positioning, as well as the dynamic adhesion required for border cell migration. As expected, DE-cadherin-Delta beta did not substitute for DE-cadherin in these processes, although it retained some residual activity. Surprisingly, DE-cadherin-AAA was able to substitute for the wild-type protein in all contexts with no detectable perturbations. Thus, interaction with p120/delta-catenin does not appear to be required for DE-cadherin function in vivo.

Selection of multipotent stem cells during morphogenesis of small intestinal crypts of Lieberkuhn is perturbed by stimulation of Lef-1/beta-catenin signaling

Studies of chimeric mice have disclosed that the stem cell hierarchy in the small intestinal epithelium is established during formation of its proliferative units (crypts of Lieberkühn). This process involves a selection among several multipotential progenitors so that ultimately only one survives to supply descendants to the fully formed crypt. In this report, we examine the hypothesis that the level of beta-catenin (beta-cat)-mediated signaling is an important factor regulating this stem cell selection. In the canonical Wnt signaling pathway, beta-catenin can partner with Lef-1/Tcf high mobility group (HMG) box transcription factors to control gene expression. Both Lef-1 and Tcf-4 mRNAs are produced in the fetal mouse small intestine. Tcf-4 expression is sustained, whereas Lef-1 levels fall as crypt formation is completed during the first two postnatal weeks. A Tcf-4 gene knockout is known to block intestinal epithelial proliferation in late fetal life. Therefore, to test the hypothesis, we enhanced beta-catenin signaling in a chimeric mouse model in which the stem cell selection could be monitored. A fusion protein containing the HMG box domain of Lef-1 linked to the trans-activation domain of beta-catenin (Lef-1/beta-cat) was constructed to promote direct stimulation of signaling without being retained in the cytoplasm through interactions with E-cadherin and Apc/Axin. Lef-1/beta-cat was expressed in 129/Sv embryonic stem cell-derived small intestinal epithelial progenitors present in developing B6-ROSA26<-->129/Sv chimeras. Lef-1/beta-cat stimulated expression of a known beta-catenin target (E-cadherin), suppressed expression of Apc and Axin, and induced apoptosis in 129/Sv but not in neighboring B6-ROSA26 epithelial cells. This apoptotic response was not associated with any detectable changes in cell division within the Lef-1/beta-cat-expressing epithelium. By the time crypt development was completed, all 129/Sv epithelial cells were lost. These results indicate that developmental changes in beta-catenin-mediated signaling can play an important role in establishing a stem cell hierarchy during crypt morphogenesis.

Casein kinase I phosphorylates the Armadillo protein and induces its degradation in Drosophila

Casein kinase I (CKI) was recently reported as a positive regulator of Wnt signaling in vertebrates and Caenorhabditis elegans. To elucidate the function of Drosophila CKI in the wingless (Wg) pathway, we have disrupted its function by double-stranded RNA-mediated interference (RNAi). While previous findings were mainly based on CKI overexpression, this is the first convincing loss-of-function analysis of CKI. Surprisingly, CKIalpha- or CKIepsilon-RNAi markedly elevated the Armadillo (Arm) protein levels in Drosophila Schneider S2R+ cells, without affecting its mRNA levels. Pulse-chase analysis showed that CKI-RNAi stabilizes Arm protein. Moreover, Drosophila embryos injected with CKIalpha double-stranded RNA showed a naked cuticle phenotype, which is associated with activation of Wg signaling. These results indicate that CKI functions as a negative regulator of Wg/Arm signaling. Overexpression of CKIalpha induced hyper-phosphorylation of both Arm and Dishevelled in S2R+ cells and, conversely, CKIalpha-RNAi reduced the amount of hyper-modified forms. His-tagged Arm was phosphorylated by CKIalpha in vitro on a set of serine and threonine residues that are also phosphorylated by Zeste-white 3. Thus, we propose that CKI phosphorylates Arm and stimulates its degradation.

Intercellular adhesion, signalling and the cytoskeleton

Connections between the cytoskeleton and intercellular junctions profoundly influence cell shape and motility. It is becoming increasingly clear that in addition to structural functions, components of the adhesion apparatus also possess signalling capabilities. Recent studies suggest that their dual function may provide the means to integrate changes in morphology and gene expression during tissue and organ development.

Delta N89 beta-catenin induces precocious development, differentiation, and neoplasia in mammary gland

To investigate the role of beta-catenin in mammary gland development and neoplasia, we expressed a stabilized, transcriptionally active form of beta-catenin lacking the NH(2)-terminal 89 amino acids (Delta N 89 beta-catenin) under the control of the mouse mammary tumor virus long terminal repeat. Our results show that Delta N 89 beta-catenin induces precocious lobuloalveolar development and differentiation in the mammary glands of both male and female mice. Virgin Delta N 89 beta-catenin mammary glands resemble those found in wild-type (wt) pregnant mice and inappropriately express cyclin D1 mRNA. In contrast to wt mammary glands, which resume a virgin appearance after cessation of lactation, transgenic mammary glands involute to a midpregnant status. All transgenic females develop multiple aggressive adenocarcinomas early in life. Surprisingly, the Delta N89 beta-catenin phenotype differs from those elicited by overexpression of Wnt genes in this gland. In particular, Delta N 89 beta-catenin has no effect on ductal side branching. This suggests that Wnt induction of ductal branching involves additional downstream effectors or modulators.

Inhibition of the Wnt signaling pathway by Idax, a novel Dvl-binding protein

In attempting to clarify the roles of Dvl in the Wnt signaling pathway, we identified a novel protein which binds to the PDZ domain of Dvl and named it Idax (for inhibition of the Dvl and Axin complex). Idax and Axin competed with each other for the binding to Dvl. Immunocytochemical analyses showed that Idax was localized to the same place as Dvl in cells and that expression of Axin inhibited the colocalization of Dvl and Idax. Further, Wnt-induced accumulation of beta-catenin and activation of T-cell factor in mammalian cells were suppressed by expression of Idax. Expression of Idax in Xenopus embryos induced ventralization with a reduction in the expression of siamois, a Wnt-inducible gene. Idax inhibited Wnt- and Dvl- but not beta-catenin-induced axis duplication. It is known that Dvl is a positive regulator in the Wnt signaling pathway and that the PDZ domain is important for this activity. Therefore, these results suggest that Idax functions as a negative regulator of the Wnt signaling pathway by directly binding to the PDZ domain of Dvl.

Inhibition of Wnt signaling pathway by a novel axin-binding protein

Axin forms a complex with adenomatous polyposis coli gene product, glycogen synthase kinase-3beta (GSK-3beta), beta-catenin, Dvl, and protein phosphatase 2A and functions as a scaffold protein in the Wnt signaling pathway. In the Axin complex, GSK-3beta efficiently phosphorylates beta-catenin, which is then ubiquitinated and degraded by proteasome. We isolated a novel protein that binds to Axin and named it Axam (for Axin associating molecule). Axam formed a complex with Axin in intact cells and bound directly to Axin. Axam inhibited the complex formation of Dvl with Axin and the activity of Dvl to suppress GSK-3beta-dependent phosphorylation of Axin. Furthermore, Axam induced the degradation of beta-catenin in SW480 cells and inhibited Wnt-dependent axis duplication in Xenopus embryos. These results suggest that Axam regulates the Wnt signaling pathway negatively by inhibiting the binding of Dvl to Axin.

Complex formation of adenomatous polyposis coli gene product and axin facilitates glycogen synthase kinase-3 beta-dependent phosphorylation of beta-catenin and down-regulates beta-catenin

Adenomatous polyposis coli gene product (APC) functions as a tumor suppressor and its mutations in familial adenomatous polyposis and colorectal cancers lead to the accumulation of cytoplasmic beta-catenin. The molecular mechanism by which APC regulates the stability of beta-catenin was investigated. The central region of APC, APC-(1211-2075), has the beta-catenin- and Axin-binding sites and down-regulates beta-catenin. Glycogen synthase kinase-3 beta (GSK-3 beta) phosphorylated beta-catenin slightly in the presence of either APC-(1211-2075) or Axin(delta)(beta)(-catenin), in which the beta-catenin-binding site is deleted, and greatly in the presence of both proteins. The enhancement of the GSK-3 beta-dependent phosphorylation of beta-catenin was eliminated by the APC-binding site of Axin. Axin down-regulated beta-catenin in SW480 cells, but not Axin(delta)(beta)(-catenin). In L cells where APC is intact, Axin(delta)(beta)(-catenin) inhibited Wnt-dependent accumulation of beta-catenin but not Axin-(298-832)(delta)(beta)(-catenin) in which the APC- and beta-catenin-binding sites are deleted. These results indicate that the complex formation of APC and Axin enhances the phosphorylation of beta-catenin by GSK-3 beta, leading to the down-regulation of beta-catenin.

Plakoglobin and beta-catenin: protein interactions, regulation and biological roles

Beta-catenin can play different roles in the cell, including one as a structural protein at cell-cell adherens junctions and another as a transcriptional activator mediating Wnt signal transduction. Plakoglobin (gamma)-catenin), a close homolog of beta-catenin, shares with beta-catenin common protein partners and can fulfill some of the same functions. The complexing of catenins with various protein partners is regulated by phosphorylation and by intramolecular interactions. The competition between different catenin partners for binding to catenins mediates the cross-talk between cadherin-based adhesion, catenin-dependent transcription and Wnt signaling. Although plakoglobin differs from beta-catenin in its functions and is unable to compensate for defects in Wnt signaling resulting from lack of beta-catenin, recent evidence suggests that plakoglobin plays a unique role in Wnt signaling that is different from that of beta-catenin. The functional difference between catenins is reflected in their differential involvement in embryonic development and cancer progression.

Biochemical characterization of the Drosophila axin protein

In the Wnt/Wingless pathway, accumulation of beta-catenin/Armadillo protein is a key regulatory step. Vertebrate Axin is a negative regulator of Wnt signaling, promoting glycogen synthase kinase-3beta-mediated phosphorylation of beta-catenin and thereby destabilizing beta-catenin. Using Drosophila cell culture systems, we demonstrated that a Drosophila homolog of Axin (Daxin) inhibits Wingless-induced Armadillo accumulation and Drosophila T-cell factor-dependent transcription induced by Wingless, Dishevelled, and Armadillo. The carboxy-terminal portion of Daxin is not essential for the inhibitory activity, but a mutant only consisting of this portion behaves as a dominant-negative protein. Moreover, interactions between Daxin and Zeste-white 3, Armadillo, Dishevelled, protein phosphatase 2A and Daxin itself were shown, providing direct evidence that Daxin is a scaffold protein in the Wingless pathway.

Regulation of beta-catenin signaling in the Wnt pathway

beta-Catenin not only regulates cell to cell adhesion as a protein interacting with cadherin, but also functions as a component of the Wnt signaling pathway. The Wnt signaling pathway is conserved in various organisms from worms to mammals, and plays important roles in development, cellular proliferation, and differentiation. Wnt stabilizes cytoplasmic beta-catenin and then beta-catenin is translocated into the nucleus where it stimulates the expression of genes including c-myc, c-jun, fra-1, and cyclin D1. The amounts and functions of beta-catenin are regulated in both the cytoplasm and nucleus. Its molecular mechanisms are becoming increasingly well understood.

Modulation of Wnt signaling by Axin and Axil

The Wnt signaling pathway is conserved in various species from worms to mammals, and plays important roles in development, cellular proliferation, and differentiation. The molecular mechanisms by which the Wnt signal regulates cellular functions are becoming increasingly well understood. Wnt stabilizes cytoplasmic beta-catenin, which stimulates the expression of genes including c-myc, c-jun, fra-1, and cyclin D1. Axin and its homolog Axil, newly recognized as components of the Wnt signaling pathway, negatively regulate this pathway. Other components of the Wnt signaling pathway, including Dvl, glycogen synthase kinase-3beta (GSK-3beta), beta-catenin, and adenomatous polyposis coli (APC), interact with Axin, and the phosphorylation and stability of beta-catenin are regulated in the Axin complex. Axil has similar functions to Axin. Thus, Axin and Axil act as scaffold proteins in the Wnt signaling pathway, thereby modulating the Wnt-dependent cellular functions.

Characterization of mouse dishevelled (Dvl) proteins in Wnt/Wingless signaling pathway

The dishevelled (dsh) gene family encodes cytoplasmic proteins that have been implicated in Wnt/Wingless (Wg) signaling. To demonstrate functional conservation of Dsh family proteins, two mouse homologs of Drosophila Dsh, Dvl-1 and Dvl-2, were biochemically characterized in mouse and Drosophila cell culture systems. We found that treatment with a soluble Wnt-3A leads to hyperphosphorylation of Dvl proteins and a concomitant elevation of the cytoplasmic beta-catenin levels in mouse NIH3T3, L, and C57MG cells. This coincides well with our finding in a Drosophila wing disc cell line, clone-8, that Wg treatment induced hyperphosphorylation of Dsh (Yanagawa, S., van Leeuwen, F., Wodarz, A., Klingensmith, J., and Nusse, R. (1995) Genes Dev. 9, 1087-1097). Furthermore, we showed that mouse Dvl proteins affect downstream components of Drosophila Wg signaling as Dsh does; overexpression of Dvl proteins in clone-8 cells results in elevation of Armadillo (Drosophila homolog of beta-catenin) and Drosophila E-cadherin levels, hyperphosphorylation of Dvl proteins themselves, and inhibition of Zeste-White3 kinase-mediated phosphorylation of a microtubule-binding protein, Tau. In addition, casein kinase II was shown to coimmunoprecipitate with Dvl proteins, and Dvl proteins were phosphorylated in these immune complexes. These results are direct evidence that Dsh family proteins mediate a set of conserved biochemical processes in the Wnt/Wg signaling pathway.

DIX domains of Dvl and axin are necessary for protein interactions and their ability to regulate beta-catenin stability

The N-terminal region of Dvl-1 (a mammalian Dishevelled homolog) shares 37% identity with the C-terminal region of Axin, and this related region is named the DIX domain. The functions of the DIX domains of Dvl-1 and Axin were investigated. By yeast two-hybrid screening, the DIX domain of Dvl-1 was found to interact with Dvl-3, a second mammalian Dishevelled relative. The DIX domains of Dvl-1 and Dvl-3 directly bound one another. Furthermore, Dvl-1 formed a homo-oligomer. Axin also formed a homo-oligomer, and its DIX domain was necessary. The N-terminal region of Dvl-1, including its DIX domain, bound to Axin directly. Dvl-1 inhibited Axin-promoted glycogen synthase kinase 3beta-dependent phosphorylation of beta-catenin, and the DIX domain of Dvl-1 was required for this inhibitory activity. Expression of Dvl-1 in L cells induced the nuclear accumulation of beta-catenin, and deletion of the DIX domain abolished this activity. Although expression of Axin in SW480 cells caused the degradation of beta-catenin and reduced the cell growth rate, expression of an Axin mutant that lacks the DIX domain did not affect the level of beta-catenin or the growth rate. These results indicate that the DIX domains of Dvl-1 and Axin are important for protein-protein interactions and that they are necessary for the ability of Dvl-1 and Axin to regulate the stability of beta-catenin.

Intracisternal type A particle-mediated activation of the Notch4/int3 gene in a mouse mammary tumor: generation of truncated Notch4/int3 mRNAs by retroviral splicing events

The int3 oncogene was discovered as a frequent target in mouse mammary tumor virus-induced mammary tumors and encodes the intracellular domain of a Notch4/int3 protein. In one spontaneous mammary tumor, no. 9, that developed in a BALB/c mouse, we have found an insertion of a 1.2-kb sequence, consisting of a 5' long terminal repeat and gag sequences of an intracisternal type A particle (IAP) as well as an extra copy of the Notch4/int3 genomic sequences containing exons 23 and 24, into the intron between exons 24 and 25 of the Notch4/int3 gene. In this tumor, unique splicing events between the IAP and the Notch4/int3 sequences generated two types of IAP-Notch4/int3 fusion transcripts encoding two different portions of the intracellular domain of Notch4/int3 proteins: one with a RAM domain and the other without. Interestingly, these two proteins showed different subcellular localizations in a mouse mammary epithelial cell line, HC-11.

Dysregulation of beta-catenin is common in canine sporadic colorectal tumors

Human colorectal tumorigenesis is often initiated by APC (adenomatous polyposis coli) or beta-catenin (CTNNB1) mutations, which result in dysregulation of beta-catenin expression, followed by alterations in E-cadherin and/or p53. We examined 32 canine intestinal tumors for expression and intracellular distribution of beta-catenin, E-cadherin, and p53 using immunohistochemistry. beta-Catenin in normal mucosal epithelial cells was restricted to lateral cell membranes, but 13/13 (100%) colorectal adenomas had intense cytoplasmic and/or nuclear reactivity. Three of six (50%) colorectal carcinomas, 2/13 (15%) small intestinal carcinomas, and dysplastic cells in 1/2 focal hyperplastic lesions in the small intestine had a similar pattern of staining; remaining tumors had normal membranous beta-catenin reactivity. There was a correlation (P = 0.007) between abnormal beta-catenin and E-cadherin staining with 11/13 (85%) colorectal adenomas, 3/6 (50%) colorectal carcinomas, and 3/13 (23%) small intestinal carcinomas showing decreased membranous reactivity compared with normal mucosal epithelium. E-cadherin staining was reduced more often in adenomas than in carcinomas (P = 0.04). There were two patterns of nuclear p53 staining: > 60% of nuclei in 2/26 (8%) carcinomas (one colorectal, one small intestinal) were strongly labeled, whereas three colorectal adenomas and one small intestinal carcinoma had fainter staining in 10-20% of cells. Dysregulation of beta-catenin appears to be as important in canine colorectal tumorigenesis as it is in the human disease and could be due to analogous mutations. Malignant progression in canine intestinal tumors does not appear to be dependent on loss of E-cadherin or beta-catenin expression or strongly associated with overexpression of nuclear CMI antibody-reactivity p53.

Coupling assembly of the E-cadherin/beta-catenin complex to efficient endoplasmic reticulum exit and basal-lateral membrane targeting of E-cadherin in polarized MDCK cells

The E-cadherin/catenin complex regulates Ca++-dependent cell-cell adhesion and is localized to the basal-lateral membrane of polarized epithelial cells. Little is known about mechanisms of complex assembly or intracellular trafficking, or how these processes might ultimately regulate adhesion functions of the complex at the cell surface. The cytoplasmic domain of E-cadherin contains two putative basal-lateral sorting motifs, which are homologous to sorting signals in the low density lipoprotein receptor, but an alanine scan across tyrosine residues in these motifs did not affect the fidelity of newly synthesized E-cadherin delivery to the basal-lateral membrane of MDCK cells. Nevertheless, sorting signals are located in the cytoplasmic domain since a chimeric protein (GP2CAD1), comprising the extracellular domain of GP2 (an apical membrane protein) and the transmembrane and cytoplasmic domains of E-cadherin, was efficiently and specifically delivered to the basal-lateral membrane. Systematic deletion and recombination of specific regions of the cytoplasmic domain of GP2CAD1 resulted in delivery of <10% of these newly synthesized proteins to both apical and basal-lateral membrane domains. Significantly, >90% of each mutant protein was retained in the ER. None of these mutants formed a strong interaction with beta-catenin, which normally occurs shortly after E-cadherin synthesis. In addition, a simple deletion mutation of E-cadherin that lacks beta-catenin binding is also localized intracellularly. Thus, beta-catenin binding to the whole cytoplasmic domain of E-cadherin correlates with efficient and targeted delivery of E-cadherin to the lateral plasma membrane. In this capacity, we suggest that beta-catenin acts as a chauffeur, to facilitate transport of E-cadherin out of the ER and the plasma membrane.

Mechanisms of Wnt signaling in development

Wnt genes encode a large family of secreted, cysteine-rich proteins that play key roles as intercellular signaling molecules in development. Genetic studies in Drosophila and Caenorhabditis elegans, ectopic gene expression in Xenopus, and gene knockouts in the mouse have demonstrated the involvement of Wnts in processes as diverse as segmentation, CNS patterning, and control of asymmetric cell divisions. The transduction of Wnt signals between cells proceeds in a complex series of events including post-translational modification and secretion of Wnts, binding to transmembrane receptors, activation of cytoplasmic effectors, and, finally, transcriptional regulation of target genes. Over the past two years our understanding of Wnt signaling has been substantially improved by the identification of Frizzled proteins as cell surface receptors for Wnts and by the finding that beta-catenin, a component downstream of the receptor, can translocate to the nucleus and function as a transcriptional activator. Here we review recent data that have started to unravel the mechanisms of Wnt signaling.

Notes from some crypt watchers: regulation of renewal in the mouse intestinal epithelium

The mouse intestinal epithelium undergoes rapid renewal throughout life, thereby requiring continuous coordination of its cellular proliferation, differentiation, and death programs. Recent advances in our understanding of this process have highlighted some of the molecules that regulate renewal and their potential roles in gut neoplasia.

Identification and characterization of a novel line of Drosophila Schneider S2 cells that respond to wingless signaling

Wingless (Wg) treatment of Drosophila wing disc clone 8 cells leads to Armadillo (Arm) protein elevation, and this effect has been used as the basis of in vitro assays for Wg protein. Previously analyzed stocks of Drosophila Schneider S2 cells could not respond to added Wg, because they lack the Wg receptor, Dfrizzled-2. However, we found that a line of S2 cells obtained from another source express Dfrizzled-2 and Dfrizzled-1. Thus, we designated this cell line as S2R+ (S2 receptor plus). S2R+ cells respond to addition of extracellular Wg by elevating Arm and DE-cadherin protein levels and by hyperphosphorylating Dsh, just as clone 8 cells do. Moreover, overexpression of Wg in S2R+, but not in S2 cells, induced the same changes in Dsh, Arm, and DE-cadherin proteins as induced in clone 8 cells, indicating that these events are common effects of Wg signaling, which occurs in cells expressing functional Wg receptors. In addition, unphosphorylated Dsh protein in S2 cells was phosphorylated as a consequence of expression of Dfrizzled-2 or mouse Frizzled-6, suggesting that basal structures common to various frizzled family proteins trigger this phosphorylation of Dsh. S2R+ cells are as sensitive to Wg as are clone 8 cells but can grow in simpler medium. Therefore, the S2R+ cell line is likely to prove highly useful for in vitro analyses of Wg signaling.

Differential molecular interactions of beta-catenin and plakoglobin in adhesion, signaling and cancer

Plakoglobin and beta-catenin are homologous proteins functioning in cell adhesion and transactivation. Their activities are controlled by three types of interactions: those with cadherins in adherens junctions, linking them to the actin cytoskeleton; interactions in the nucleus, where they bind to transcription factors and stimulate gene expression; interactions of free cytoplasmic beta-catenin with axin and adenomatous polyposis coli (APC) protein which target it for degradation. Studies in the past year have demonstrated the complex interplay between these three types of interactions and the different behavior of beta-catenin and plakoglobin in their involvement in morphogenesis and tumorigenesis strongly suggesting that catenins play key roles in adhesion-mediated signaling.

Effects of forced expression of an NH2-terminal truncated beta-Catenin on mouse intestinal epithelial homeostasis

beta-Catenin functions as a downstream component of the Wnt/Wingless signal transduction pathway and as an effector of cell-cell adhesion through its association with cadherins. To explore the in vivo effects of beta-catenin on proliferation, cell fate specification, adhesion, and migration in a mammalian epithelium, a human NH2-terminal truncation mutant (DeltaN89 beta-catenin) was expressed in the 129/Sv embryonic stem cell-derived component of the small intestine of adult C57Bl/6-ROSA26 left and right arrow 129/Sv chimeric mice. DeltaN89 beta-Catenin was chosen because mutants of this type are more stable than the wild-type protein, and phenocopy activation of the Wnt/Wingless signaling pathway in Xenopus and Drosophila. DeltaN89 beta-Catenin had several effects. Cell division was stimulated fourfold in undifferentiated cells located in the proliferative compartment of the intestine (crypts of Lieberkühn). The proliferative response was not associated with any discernible changes in cell fate specification but was accompanied by a three- to fourfold increase in crypt apoptosis. There was a marked augmentation of E-cadherin at the adherens junctions and basolateral surfaces of 129/Sv (DeltaN89 beta-catenin) intestinal epithelial cells and an accompanying slowing of cellular migration along crypt-villus units. 1-2% of 129/Sv (DeltaN89 beta-catenin) villi exhibited an abnormal branched architecture. Forced expression of DeltaN89 beta-catenin expression did not perturb the level or intracellular distribution of the tumor suppressor adenomatous polyposis coli (APC). The ability of DeltaN89 beta-catenin to interact with normal cellular pools of APC and/or augmented pools of E-cadherin may have helped prevent the 129/Sv gut epithelium from undergoing neoplastic transformation during the 10-mo period that animals were studied. Together, these in vivo studies emphasize the importance of beta-catenin in regulating normal adhesive and signaling functions within this epithelium.

DWnt-2, a Drosophila Wnt gene required for the development of the male reproductive tract, specifies a sexually dimorphic cell fate

The sexually dimorphic characteristics of the reproductive tract in Drosophila require that cells of the gonad and the genital disc be assigned sex-specific fates. We report here that DWnt-2, a secreted glycoprotein related to wingless, is a signal required for cell fate determination and morphogenesis in the developing male reproductive tract. Testes from DWnt-2 null mutant flies lack the male-specific pigment cells of the reproductive tract sheath and the muscle precursors of the sheath fail to migrate normally. However, other cell types of the testis are unaffected. DWnt-2 is expressed in somatic cells of the gonad throughout development, implicating it as a signal that can influence pigment cell fate directly. Indeed, the ectopic expression of DWnt-2 in females results in the appearance of male-specific pigment cells in otherwise morphologically normal ovaries. Thus, the presence of pigment cells is a sexually dimorphic trait that is controlled by DWnt-2 expression. DWnt-2 is also expressed in regions of the male genital disc and gonad, which we have identified as sites of contact with muscle precursor cells, suggesting that secreted DWnt-2 protein is a signal for the migration or attachment of these cells.

Beta-catenin: a key mediator of Wnt signaling

Beta-catenin is a pivotal player in the signaling pathway initiated by Wnt proteins, mediators of several developmental processes. beta-catenin activity is controlled by a large number of binding partners that affect the stability and the localization of beta-catenin and is thereby able to participate in such varying processes as gene expression and cell adhesion. Activating mutations in beta-catenin and in components regulating its stability can contribute to the formation of certain tumors.