A Drosophila Axin homolog, Daxin, inhibits Wnt signaling

Reconstitution of the destruction complex defines roles of AXIN polymers and APC in β-catenin capture, phosphorylation, and ubiquitylation

The Wnt/β-catenin pathway is a highly conserved, frequently mutated developmental and cancer pathway. Its output is defined mainly by β-catenin's phosphorylation- and ubiquitylation-dependent proteasomal degradation, initiated by the multi-protein β-catenin destruction complex. The precise mechanisms underlying destruction complex function have remained unknown, largely because of the lack of suitable in vitro systems. Here we describe the in vitro reconstitution of an active human β-catenin destruction complex from purified components, recapitulating complex assembly, β-catenin modification, and degradation. We reveal that AXIN1 polymerization and APC promote β-catenin capture, phosphorylation, and ubiquitylation. APC facilitates β-catenin's flux through the complex by limiting ubiquitylation processivity and directly interacts with the SCFβ-TrCP E3 ligase complex in a β-TrCP-dependent manner. Oncogenic APC truncation variants, although part of the complex, are functionally impaired. Nonetheless, even the most severely truncated APC variant promotes β-catenin recruitment. These findings exemplify the power of biochemical reconstitution to interrogate the molecular mechanisms of Wnt/β-catenin signaling.

Wnt controls the medial-lateral subdivision of the Drosophila head

In insects, the subdivision of the head into a lateral region, harbouring the compound eyes (CEs), and a dorsal (medial) region, where the ocelli localize, is conserved. This organization might have been already present in the insects' euarthropodan ancestors. In Drosophila, the Wnt-1 homologue wingless (wg) plays a major role in the genetic subdivision of the head. To analyse specifically the role of wg signalling in the development of the dorsal head, we attenuated this pathway specifically in this region by genetic means. We find that loss of wg signalling transforms the dorsal/medial head into lateral head structures, including the development of ectopic CEs. Our genetic analysis further suggests that wg signalling organizes the dorsal head medial–lateral axis by controlling, at least in part, the expression domains of the transcription factors Otd and Ey/Pax6.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Supramolecular assembly of the beta-catenin destruction complex and the effect of Wnt signaling on its localization, molecular size, and activity in vivo

Wnt signaling provides a paradigm for cell-cell signals that regulate embryonic development and stem cell homeostasis and are inappropriately activated in cancers. The tumor suppressors APC and Axin form the core of the multiprotein destruction complex, which targets the Wnt-effector beta-catenin for phosphorylation, ubiquitination and destruction. Based on earlier work, we hypothesize that the destruction complex is a supramolecular entity that self-assembles by Axin and APC polymerization, and that regulating assembly and stability of the destruction complex underlie its function. We tested this hypothesis in Drosophila embryos, a premier model of Wnt signaling. Combining biochemistry, genetic tools to manipulate Axin and APC2 levels, advanced imaging and molecule counting, we defined destruction complex assembly, stoichiometry, and localization in vivo, and its downregulation in response to Wnt signaling. Our findings challenge and revise current models of destruction complex function. Endogenous Axin and APC2 proteins and their antagonist Dishevelled accumulate at roughly similar levels, suggesting competition for binding may be critical. By expressing Axin:GFP at near endogenous levels we found that in the absence of Wnt signals, Axin and APC2 co-assemble into large cytoplasmic complexes containing tens to hundreds of Axin proteins. Wnt signals trigger recruitment of these to the membrane, while cytoplasmic Axin levels increase, suggesting altered assembly/disassembly. Glycogen synthase kinase3 regulates destruction complex recruitment to the membrane and release of Armadillo/beta-catenin from the destruction complex. Manipulating Axin or APC2 levels had no effect on destruction complex activity when Wnt signals were absent, but, surprisingly, had opposite effects on the destruction complex when Wnt signals were present. Elevating Axin made the complex more resistant to inactivation, while elevating APC2 levels enhanced inactivation. Our data suggest both absolute levels and the ratio of these two core components affect destruction complex function, supporting models in which competition among Axin partners determines destruction complex activity.

Complexity of the Wnt/β‑catenin pathway: Searching for an activation model

Wnt signaling refers to a conserved signaling pathway, widely studied due to its roles in cellular communication, cell fate decisions, development and cancer. However, the exact mechanism underlying inhibition of the GSK phosphorylation towards β-catenin and activation of the pathway after biding of Wnt ligand to its cognate receptors at the plasma membrane remains unclear. Wnt target genes are widely spread over several animal phyla. They participate in a plethora of functions during the development of an organism, from axial specification, gastrulation and organogenesis all the way to regeneration and repair in adults. Temporal and spatial oncogenetic re-activation of Wnt signaling almost certainly leads to cancer. Wnt signaling components have been extensively studied as possible targets in anti-cancer therapies. In this review we will discuss one of the most intriguing questions in this field, that is how β-catenin, a major component in this pathway, escapes the destruction complex, gets stabilized in the cytosol and it is translocated to the nucleus where it acts as a co-transcription factor. Four major models have evolved during the past 20years. We dissected each of them along with current views and future perspectives on this pathway. This review will focus on the molecular mechanisms by which Wnt proteins modulate β-catenin cytoplasmic levels and the relevance of this pathway for the development and cancer.

Glycogen synthase kinase-3 (GSK3) regulates TNF production and haemocyte phagocytosis in the immune response of Chinese mitten crab Eriocheir sinensis

Glycogen synthase kinase-3 (GSK3) is a serine/threonine protein kinase firstly identified as a regulator of glycogen synthesis. Recently, it has been proved to be a key regulator of the immune reaction. In the present study, a GSK3 homolog gene (designated as EsGSK3) was cloned from Chinese mitten crab, Eriocheir sinensis. The open reading frame (ORF) was 1824 bp, which encoded a predicted polypeptide of 607 amino acids. There was a conserved Serine/Threonine Kinase domain and a DNA binding domain found in EsGSK3. Phylogenetic analysis showed that EsGSK3 was firstly clustered with GSK3-β from oriental river prawn Macrobrachium nipponense in the invertebrate branch, while GSK3s from vertebrates formed the other distinct branch. EsGSK3 mRNA transcripts could be detected in all tested tissues of the crab including haepatopancreas, eyestalk, muscle, gonad, haemocytes and haematopoietic tissue with the highest expression level in haepatopancreas. And EsGSK3 protein was mostly detected in the cytoplasm of haemocyte by immunofluorescence analysis. The expression levels of EsGSK3 mRNA increased significantly at 6 h after Aeromonas hydrophila challenge (p < 0.05) in comparison with control group, and then gradually decreased to the initial level at 48 h (p > 0.05). The mRNA expression of lipopolysaccharide-induced tumor necrosis factor (TNF)-α factor (EsLITAF) was also induced by A. hydrophila challenge. However, the mRNA expression of EsLITAF and TNF-α production was significantly suppressed after EsGSK3 was blocked in vivo with specific inhibitor lithium, while the phagocytosis of crab haemocytes was significantly promoted. These results collectively demonstrated that EsGSK3 could regulate the innate immune responses of E. sinensis by promoting TNF-α production and inhibiting haemocyte phagocytosis.

Reconstituting regulation of the canonical Wnt pathway by engineering a minimal β-catenin destruction machine

APC and Axin are key negative regulators of Wnt signaling in development and oncogenesis. They form a multiprotein complex targeting the key Wnt effector β-catenin for destruction. Essential components of APC and Axin required for their cooperative function are identified, and the data are used to design a minimal β-catenin–destruction machine.

Negatively regulating key signaling pathways is critical to development and altered in cancer. Wnt signaling is kept off by the destruction complex, which is assembled around the tumor suppressors APC and Axin and targets β-catenin for destruction. Axin and APC are large proteins with many domains and motifs that bind other partners. We hypothesized that if we identified the essential regions required for APC:Axin cooperative function and used these data to design a minimal β-catenin-destruction machine, we would gain new insights into the core mechanisms of destruction complex function. We identified five key domains/motifs in APC or Axin that are essential for their function in reconstituting Wnt regulation. Strikingly, however, certain APC and Axin mutants that are nonfunctional on their own can complement one another in reducing β-catenin, revealing that the APC:Axin complex is a highly robust machine. We used these insights to design a minimal β-catenin-destruction machine, revealing that a minimized chimeric protein covalently linking the five essential regions of APC and Axin reconstitutes destruction complex internal structure, size, and dynamics, restoring efficient β-catenin destruction in colorectal tumor cells. On the basis of our data, we propose a new model of the mechanistic function of the destruction complex as an integrated machine.

Wnt/Wingless Pathway Activation Is Promoted by a Critical Threshold of Axin Maintained by the Tumor Suppressor APC and the ADP-Ribose Polymerase Tankyrase

Wnt/β-catenin signal transduction directs metazoan development and is deregulated in numerous human congenital disorders and cancers. In the absence of Wnt stimulation, a multiprotein "destruction complex," assembled by the scaffold protein Axin, targets the key transcriptional activator β-catenin for proteolysis. Axin is maintained at very low levels that limit destruction complex activity, a property that is currently being exploited in the development of novel therapeutics for Wnt-driven cancers. Here, we use an in vivo approach in Drosophila to determine how tightly basal Axin levels must be controlled for Wnt/Wingless pathway activation, and how Axin stability is regulated. We find that for nearly all Wingless-driven developmental processes, a three- to fourfold increase in Axin is insufficient to inhibit signaling, setting a lower-limit for the threshold level of Axin in the majority of in vivo contexts. Further, we find that both the tumor suppressor adenomatous polyposis coli (APC) and the ADP-ribose polymerase Tankyrase (Tnks) have evolutionarily conserved roles in maintaining basal Axin levels below this in vivo threshold, and we define separable domains in Axin that are important for APC- or Tnks-dependent destabilization. Together, these findings reveal that both APC and Tnks maintain basal Axin levels below a critical in vivo threshold to promote robust pathway activation following Wnt stimulation.

Wnt Signaling in Sexual Dimorphism

The embryonic gonad of Drosophila melanogaster begins to display sexually dimorphic traits soon after its formation. Here we demonstrate the involvement of a wnt family ligand, wnt-2, in the induction of these sex-specific differences. We show that wnt-2 contributes to the survival of a male-specific population of somatic gonadal precursor cells (SGPs), the male-specific SGPs that are located at the posterior of the male gonad. We also show that the Wnt-2 ligand synergizes with the JAK-STAT ligand Upd, which is produced by SGPs at the anterior of the gonad to activate the STAT pathway in male germ cells. We suggest that the use of two spatially separated signaling systems to initiate the JAK-STAT stem cell maintenance pathway in germ cells provides a mechanism for increasing the pool of potential progenitors of the germline stem cells in the adult testes. Finally, we present evidence indicating that, like the JAK-STAT pathway, wnt-2 stimulates germ cells in male embryos to re-enter the cell cycle.

Wnt signaling-mediated redox regulation maintains the germ line stem cell differentiation niche

Adult stem cells continuously undergo self-renewal and generate differentiated cells. In the Drosophila ovary, two separate niches control germ line stem cell (GSC) self-renewal and differentiation processes. Compared to the self-renewing niche, relatively little is known about the maintenance and function of the differentiation niche. In this study, we show that the cellular redox state regulated by Wnt signaling is critical for the maintenance and function of the differentiation niche to promote GSC progeny differentiation. Defective Wnt signaling causes the loss of the differentiation niche and the upregulated BMP signaling in differentiated GSC progeny, thereby disrupting germ cell differentiation. Mechanistically, Wnt signaling controls the expression of multiple glutathione-S-transferase family genes and the cellular redox state. Finally, Wnt2 and Wnt4 function redundantly to maintain active Wnt signaling in the differentiation niche. Therefore, this study has revealed a novel strategy for Wnt signaling in regulating the cellular redox state and maintaining the differentiation niche.

DOI:http://dx.doi.org/10.7554/eLife.08174.001

An animal or plant has many different types of cells that have specific roles in the life of the organism. These cells are organized into tissues. In most tissues in adult animals, small groups of cells called stem cells are responsible for replacing the other cells that have been lost due to disease, injury, or as part of normal body maintenance.

The ‘germ line’ stem cells of female fruit flies—which produce female sex cells (or eggs)—are an effective system for studying how stem cells are regulated. These cells live in an area of the ovary called a stem cell niche. Each time a stem cell divides, it produces one stem cell and one other daughter cell. This daughter cell then moves into another niche called the ‘differentiation’ niche and undergoes a series of divisions that produce the egg cells. The differentiation niche is formed by escort cells and is crucial for producing the egg cells, but it is not clear how the escort cells promote this process, or how the niche is maintained.

Wang et al. have now studied the differentiation niche in more detail. The experiments show that a cell communication system called Wnt signaling maintains the differentiation niche by controlling the ability of the escort cells to grow and divide. If Wnt signaling is defective, the differentiation niche is lost, which disrupts the formation of egg cells.

Further experiments show that two proteins called Wnt2 and Wnt4 in the differentiation niche—which activate Wnt signaling—act as signals to regulate the niche, mainly by controlling the expression of four particular genes. These four genes encode enzymes that remove ‘reactive oxygen species’ from cells. Wang et al.'s findings have revealed an important role for Wnt signaling in maintaining the differentiation niche. The next step is to figure out the details of how this works.

DOI:http://dx.doi.org/10.7554/eLife.08174.002

The intersection of the extrinsic hedgehog and WNT/wingless signals with the intrinsic Hox code underpins branching pattern and tube shape diversity in the drosophila airways

The tubular networks of the Drosophila respiratory system and our vasculature show distinct branching patterns and tube shapes in different body regions. These local variations are crucial for organ function and organismal fitness. Organotypic patterns and tube geometries in branched networks are typically controlled by variations of extrinsic signaling but the impact of intrinsic factors on branch patterns and shapes is not well explored. Here, we show that the intersection of extrinsic hedgehog(hh) and WNT/wingless (wg) signaling with the tube-intrinsic Hox code of distinct segments specifies the tube pattern and shape of the Drosophila airways. In the cephalic part of the airways, hh signaling induces expression of the transcription factor (TF) knirps (kni) in the anterior dorsal trunk (DTa1). kni represses the expression of another TF spalt major (salm), making DTa1 a narrow and long tube. In DTa branches of more posterior metameres, Bithorax Complex (BX-C) Hox genes autonomously divert hh signaling from inducing kni, thereby allowing DTa branches to develop as salm-dependent thick and short tubes. Moreover, the differential expression of BX-C genes is partly responsible for the anterior-to-posterior gradual increase of the DT tube diameter through regulating the expression level of Salm, a transcriptional target of WNT/wg signaling. Thus, our results highlight how tube intrinsic differential competence can diversify tube morphology without changing availabilities of extrinsic factors.

Tubes are common structural elements of many internal organs, facilitating fluid flow and material exchange. To meet the local needs of diverse tissues, the branching patterns and tube shapes vary regionally. Diametric tapering and specialized branch targeting to the brain represent two common examples of variations with organismal benefits in the Drosophila airways and our vascular system. Several extrinsic signals instruct tube diversifications but the impact of intrinsic factors remains underexplored. Here, we show that the local, tube-intrinsic Hox code instructs the pattern and shape of the dorsal trunk (DT), the main Drosophila airway. In the cephalic part (DT1), where Bithorax Complex (BX-C) Hox genes are not expressed, the extrinsic Hedgehog signal is epistatic to WNT/Wingless signals. Hedgehog instructs anterior DT1 cells to take a long and narrow tube fate targeting the brain. In more posterior metameres, BX-C genes make the extrinsic WNT/Wingless signals epistatic over Hedgehog. There, WNT/Wingless instruct all DT cells to take the thick and short tube fate. Moreover, BX-C genes modulate the outputs of WNT/wingless signaling, making the DT tubes thicker in more posterior metameres. We provide a model for how intrinsic factors modify extrinsic signaling to control regional tube morphologies in a network.

Elimination of unfit cells maintains tissue health and prolongs lifespan

Viable yet damaged cells can accumulate during development and aging. Although eliminating those cells may benefit organ function, identification of this less fit cell population remains challenging. Previously, we identified a molecular mechanism, based on “fitness fingerprints” displayed on cell membranes, which allows direct fitness comparison among cells in Drosophila. Here, we study the physiological consequences of efficient cell selection for the whole organism. We find that fitness-based cell culling is naturally used to maintain tissue health, delay aging, and extend lifespan in Drosophila. We identify a gene, azot, which ensures the elimination of less fit cells. Lack of azot increases morphological malformations and susceptibility to random mutations and accelerates tissue degeneration. On the contrary, improving the efficiency of cell selection is beneficial for tissue health and extends lifespan.

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Fitness-based cell culling maintains tissue health

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Azot ensures the elimination of less fit cells

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Lack of azot accelerates tissue degeneration

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Improving the efficiency of cell selection extends lifespan

The Drosophila tankyrase regulates Wg signaling depending on the concentration of Daxin

The canonical Wnt signaling pathway plays critical roles during development and homeostasis. Dysregulation of this pathway can lead to many human diseases, including cancers. A key process in this pathway consists of regulation of β-catenin concentration through an Axin-recruited destruction complex. Previous studies have demonstrated a role for tankyrase (TNKS), a protein with poly(ADP-ribose) polymerase, in the regulation of Axin levels in human cells. However, the role of TNKS in development is still unclear. Here, we have generated a Drosophila tankyrase (DTNKS) mutant and provided compelling evidence that DTNKS is involved in the degradation of Drosophila Axin (Daxin). We show that Daxin physically interacts with DTNKS, and its protein levels are elevated in the absence of DTNKS in the eye discs. In S2 cells, DTNKS suppressed the levels of Daxin. Surprisingly, we found that Daxin in turn down-regulated DTNKS protein level. In vivo study showed that DTNKS regulated Wg signaling and wing patterning at a high Daxin protein level, but not at a normal level. Taken together, our findings identified a conserved role of DTNKS in regulating Daxin levels, and thereby Wg/Wnt signaling during development.

Wnt and lithium: a common destiny in the therapy of nervous system pathologies?

Wnt signaling is required for neurogenesis, the fate of neural progenitors, the formation of neuronal circuits during development, neuron positioning and polarization, axon and dendrite development and finally for synaptogenesis. This signaling pathway is also implicated in the generation and differentiation of glial cells. In this review, we describe the mechanisms of action of Wnt signaling pathways and their implication in the development and correct functioning of the nervous system. We also illustrate how a dysregulated Wnt pathway could lead to psychiatric, neurodegenerative and demyelinating pathologies. Lithium, used for the treatment of bipolar disease, inhibits GSK3β, a central enzyme of the Wnt/β-catenin pathway. Thus, lithium could, to some extent, mimic Wnt pathway. We highlight the possible dialogue between lithium therapy and modulation of Wnt pathway in the treatment of the diseases of the nervous system.

Comparative proteomics and expression analysis of five genes in Epicauta chinensis larvae from the first to fifth instar

Blister beetle is an important insect model for both medicinal and pure research. Previous research has mainly focused on its biology and biochemistry, but very little data is yet available in the molecular biology. This study uses differential proteomics technology to analyze the soluble proteins extracted from each of the 5 instars larvae of Epicauta chinensis. 42 of the differentially-expressed proteins were identified successfully by MALDI-TOF/TOF-MS. Some of these proteins' function and their expression profiles are analyzed. Our analysis revealed dynamics regulation of the following proteins: Axin-like protein pry-1 (APR-1), dihydrolipoyl dehydrogenase (DLD), vitellogenin (Vg) and lysozyme C (Lmz-S). APR-1 negatively regulates the Wnt signaling pathway. Its overexpression could result in embryo, leg, eye and ovary ectopica or malformation. DLD catalyzes the pyruvate into acetyl-CoA, the latter is the starting material of juvenile hormone (JH) and ipsdienol biosynthesis through the MVA pathway in insects. While Vg synthesis can be regulated by JH and stimulated by food factors. So DLD may affect the synthesis of JH, ipsdienol and Vg indirectly. The activity of lysozyme is an indicator of the immunity. Nutrition/food should be taken into account for its potential role during the development of larva in the future. Among the five genes and their corresponding proteins' expression, only hsc70 gene showed a good correspondence with the protein level. This reflects the fluctuating relationship between mRNA and protein levels.

Wnt signal transduction pathways

The Wnt signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning and organogenesis during embryonic development. The Wnts are secreted glycoproteins and comprise a large family of nineteen proteins in humans hinting to a daunting complexity of signaling regulation, function and biological output. To date major signaling branches downstream of the Fz receptor have been identified including a canonical or Wnt/beta-catenin dependent pathway and the non-canonical or beta-catenin-independent pathway which can be further divided into the Planar Cell Polarity and the Wnt/Ca(2+) pathways, and these branches are being actively dissected at the molecular and biochemical levels. In this review, we will summarize the most recent advances in our understanding of these Wnt signaling pathways and the role of these pathways in regulating key events during embryonic patterning and morphogenesis.

Asymmetrically expressed axin required for anterior development in Tribolium

Canonical Wnt signaling has been implicated in an AP axis polarizing mechanism in most animals, despite limited evidence from arthropods. In the long-germ insect, Drosophila, Wnt signaling is not required for global AP patterning, but in short-germ insects including Tribolium castaneum, loss of Wnt signaling affects development of segments in the growth zone but not those defined in the blastoderm. To determine the effects of ectopic Wnt signaling, we analyzed the expression and function of axin, which encodes a highly conserved negative regulator of the pathway. We found Tc-axin transcripts maternally localized to the anterior pole in freshly laid eggs. Expression spread toward the posterior pole during the early cleavage stages, becoming ubiquitous by the time the germ rudiment formed. Tc-axin RNAi produced progeny phenotypes that ranged from mildly affected embryos with cuticles displaying a graded loss of anterior structures, to defective embryos that condensed at the posterior pole in the absence of serosa. Altered expression domains of several blastodermal markers indicated anterior expansion of posterior fates. Analysis of other canonical Wnt pathway components and the expansion of Tc-caudal expression, a Wnt target, suggest that the effects of Tc-axin depletion are mediated through this pathway and that Wnt signaling must be inhibited for proper anterior development in Tribolium. These studies provide unique evidence that canonical Wnt signaling must be carefully regulated along the AP axis in an arthropod, and support an ancestral role for Wnt activity in defining AP polarity and patterning in metazoan development.

Reducing canonical Wingless/Wnt signaling pathway confers protection against mutant Huntingtin toxicity in Drosophila

Huntington's disease (HD) is a genetic neurodegenerative disease characterized by movement disorders, cognitive decline and neuropsychiatric symptoms. HD is caused by expanded CAG tract within the coding region of Huntingtin protein. Despite major insights into the molecular mechanisms leading to HD, no effective cure is yet available. Mutant Huntingtin (mHtt) has been reported to alter the stability and levels of β-Catenin, a key molecule in cell adhesion and signal transduction in Wingless (Wg)/Wnt pathway. However it remains to establish whether manipulation of Wg/Wnt signaling can impact HD pathology. We here investigated the phenotypic interactions between mHtt and Wg/Wnt signaling by using the power of Drosophila genetics. We provide compelling evidence that reducing Armadillo/β-Catenin levels confers protection and that this beneficial effect is correlated with the inactivation of the canonical Wg/Wnt signaling pathway. Knockdowns of Wnt ligands or of the downstream transcription factor Pangolin/TCF both ameliorate the survival of HD flies. Similarly, overexpression of one Armadillo/β-Catenin destruction complex component (Axin, APC2 or Shaggy/GSK-3β) increases the lifespan of HD flies. Loss of functional Armadillo/β-Catenin not only abolishes neuronal intrinsic but also glia-induced alterations in HD flies. Our findings highlight that restoring canonical Wg/Wnt signaling may be of therapeutic value.

Defining components of the ß-catenin destruction complex and exploring its regulation and mechanisms of action during development

Background

A subset of signaling pathways play exceptionally important roles in embryonic and post-embryonic development, and mis-regulation of these pathways occurs in most human cancers. One such pathway is the Wnt pathway. The primary mechanism keeping Wnt signaling off in the absence of ligand is regulated proteasomal destruction of the canonical Wnt effector ßcatenin (or its fly homolog Armadillo). A substantial body of evidence indicates that SCF^βTrCP mediates βcat destruction, however, an essential role for Roc1 has not been demonstrated in this process, as would be predicted. In addition, other E3 ligases have also been proposed to destroy βcat, suggesting that βcat destruction may be regulated differently in different tissues.

Methodology/Principal Findings

Here we used cultured Drosophila cells, human colon cancer cells, and Drosophila embryos and larvae to explore the machinery that targets Armadillo for destruction. Using RNAi in Drosophila S2 cells to examine which SCF components are essential for Armadillo destruction, we find that Roc1/Roc1a is essential for regulating Armadillo stability, and that in these cells the only F-box protein playing a detectable role is Slimb. Second, we find that while embryonic and larval Drosophila tissues use the same destruction complex proteins, the response of these tissues to destruction complex inactivation differs, with Armadillo levels more elevated in embryos. We provide evidence consistent with the possibility that this is due to differences in armadillo mRNA levels. Third, we find that there is no correlation between the ability of different APC2 mutant proteins to negatively regulate Armadillo levels, and their recently described function in positively-regulating Wnt signaling. Finally, we demonstrate that APC proteins lacking the N-terminal Armadillo-repeat domain cannot restore Armadillo destruction but retain residual function in negatively-regulating Wnt signaling.

Conclusions/Significance

We use these data to refine our model for how Wnt signaling is regulated during normal development.

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

Postgenomic technologies targeting the Wnt signaling network

The recent development of high-throughput sequencing technologies and the availability of whole genome sequences of a variety of living organisms, including that of humans, have led to an enormous push in the quest for a comprehensive inquiry for the function of each and every gene discovered in different model organisms. A major conclusion from the sequencing projects was that while forward genetics had been extremely successful in identifying key genes/components of many biological processes, such as signal transduction cascades, the function(s) of the majority of genes in the genome remains a mystery. In this article, we discuss the use of a variety of high-throughput postgenomic tools, including functional genomics, proteomics, and chemical genetics that are being implemented in an exhaustive molecular dissection of a key evolutionarily conserved signal transduction pathway, namely the Wnt/wingless (wg) pathway and its associated signaling network.

Wnt signaling signaling at and above the receptor level

Wnt signaling is one of the most important developmental signaling pathways that controls cell fate decisions and tissue patterning during early embryonic and later development. It is activated by highly conserved Wnt proteins that are secreted as palmitoylated glycoproteins and act as morphogens to form a concentration gradient across a developing tissue. Wnt proteins regulate transcriptional and posttranscriptional processes depending on the distance of their origin and activate distinct intracellular cascades, commonly referred to as canonical (β-catenin-dependent) and noncanonical (β-catenin-independent) pathways. Therefore, the secretion and the diffusion of Wnt proteins needs to be tightly regulated to induce short- and long-range downstream signaling. Even though the Wnt signaling cascade has been studied intensively, key aspects and principle mechanisms, such as transport of Wnt growth factors or regulation of signaling specificity between different Wnt pathways, remain unresolved. Here, we introduce basic principles of Wnt/Wg signal transduction and highlight recent discoveries, such as the involvement of vacuolar ATPases and vesicular acidification in Wnt signaling. We also discuss recent findings regarding posttranslational modifications of Wnts, trafficking through the secretory pathway and developmental consequences of impaired Wnt secretion. Understanding the detailed mechanism and regulation of Wnt protein secretion will provide valuable insights into many human diseases based on overactivated Wnt signaling.

The Sno oncogene antagonizes Wingless signaling during wing development in Drosophila

The Sno oncogene (Snoo or dSno in Drosophila) is a highly conserved protein and a well-established antagonist of Transforming Growth Factor-β signaling in overexpression assays. However, analyses of Sno mutants in flies and mice have proven enigmatic in revealing developmental roles for Sno proteins. Thus, to identify developmental roles for dSno we first reconciled conflicting data on the lethality of dSno mutations. Then we conducted analyses of wing development in dSno loss of function genotypes. These studies revealed ectopic margin bristles and ectopic campaniform sensilla in the anterior compartment of the wing blade suggesting that dSno functions to antagonize Wingless (Wg) signaling. A subsequent series of gain of function analyses yielded the opposite phenotype (loss of bristles and sensilla) and further suggested that dSno antagonizes Wg signal transduction in target cells. To date Sno family proteins have not been reported to influence the Wg pathway during development in any species. Overall our data suggest that dSno functions as a tissue-specific component of the Wg signaling pathway with modest antagonistic activity under normal conditions but capable of blocking significant levels of extraneous Wg, a role that may be conserved in vertebrates.

GSK3beta affects apical-basal polarity and cell-cell adhesion by regulating aPKC levels

The dynamic rearrangement of cell-cell contacts is required for the establishment of functional epithelial cell sheets. However, the signaling pathways and cellular mechanisms that initiate and maintain this polarity are not well understood. We show that loss of the Wnt signaling component GSK3 beta results in increased levels of aPKC and leads to defects in apical-basal polarity. We find that GSK3 beta directly phosphorylates aPKC, which likely promotes its ubiquitin-mediated proteosomal degradation. aPKC increases the levels of Armadillo and stabilizes adherens junctions. These results suggest that the Wnt pathway component GSK3 beta regulates the polarity determinant aPKC, which in turn affects cell-cell contacts during the development of polarized tissues.

The trimeric G protein Go inflicts a double impact on axin in the Wnt/frizzled signaling pathway

The Wnt/Frizzled signaling pathway plays crucial roles in animal development and is deregulated in many cases of carcinogenesis. We and others have previously demonstrated that Frizzled proteins initiating the intracellular signaling are typical G protein-coupled receptors and rely on the trimeric G protein Go for Wnt transduction in Drosophila. However, the mode of action of Go and its interplay with other transducers of the pathway such as Dishevelled and Axin remained unclear. Here we show that the alpha-subunit of Go directly acts on Axin, the multidomain protein playing a negative role in the Wnt signaling. G alpha o physically binds Axin and re-localizes it to the plasma membrane. Furthermore, G alpha o suppresses Axin's inhibitory action on the Wnt pathway in Drosophila wing development. The interaction of G alpha o with Axin critically depends on the RGS domain of the latter. Additionally, we show that the betagamma-component of Go can directly bind and recruit Dishevelled from cytoplasm to the plasma membrane, where activated Dishevelled can act on the DIX domain of Axin. Thus, the two components of the trimeric Go protein mediate a double-direct and indirect-impact on different regions of Axin, which likely serves to ensure a robust inhibition of this protein and transduction of the Wnt signal.

Germ band retraction as a landmark in glucose metabolism during Aedes aegypti embryogenesis

Background

The mosquito A. aegypti is vector of dengue and other viruses. New methods of vector control are needed and can be achieved by a better understanding of the life cycle of this insect. Embryogenesis is a part of A. aegypty life cycle that is poorly understood. In insects in general and in mosquitoes in particular energetic metabolism is well studied during oogenesis, when the oocyte exhibits fast growth, accumulating carbohydrates, lipids and proteins that will meet the regulatory and metabolic needs of the developing embryo. On the other hand, events related with energetic metabolism during A. aegypti embryogenesis are unknown.

Results

Glucose metabolism was investigated throughout Aedes aegypti (Diptera) embryonic development. Both cellular blastoderm formation (CBf, 5 h after egg laying - HAE) and germ band retraction (GBr, 24 HAE) may be considered landmarks regarding glucose 6-phosphate (G6P) destination. We observed high levels of glucose 6-phosphate dehydrogenase (G6PDH) activity at the very beginning of embryogenesis, which nevertheless decreased up to 5 HAE. This activity is correlated with the need for nucleotide precursors generated by the pentose phosphate pathway (PPP), of which G6PDH is the key enzyme. We suggest the synchronism of egg metabolism with carbohydrate distribution based on the decreasing levels of phosphoenolpyruvate carboxykinase (PEPCK) activity and on the elevation observed in protein content up to 24 HAE. Concomitantly, increasing levels of hexokinase (HK) and pyruvate kinase (PK) activity were observed, and PEPCK reached a peak around 48 HAE. Glycogen synthase kinase (GSK3) activity was also monitored and shown to be inversely correlated with glycogen distribution during embryogenesis.

Conclusions

The results herein support the hypothesis that glucose metabolic fate changes according to developmental embryonic stages. Germ band retraction is a moment that was characterized as a landmark in glucose metabolism during Aedes aegypti embryogenesis. Furthermore, the results also suggest a role for GSK3 in glycogen balance/distribution during morphological modifications.

In vivo analysis in Drosophila reveals differential requirements of contact residues in Axin for interactions with GSK3beta or beta-catenin

Proper regulation of the Wingless/Wnt signaling pathway is essential for normal development. The scaffolding protein Axin plays a key role in this process through interactions with Drosophila Shaggy and Armadillo. In the current studies, we used a yeast two-hybrid assay to identify ten amino acids in Axin that are critical for in vitro interaction with Shaggy and two for interaction with Armadillo. We then generated five Axin variants in which individual putative contact amino acids were mutated and compared their activity, as assayed by rescue of axin null mutant flies, to that of Axin lacking the entire Shaggy (AxinDeltaSgg) or Armadillo (AxinDeltaArm) binding domain. Although we expected these mutants to function identically to Axin in which the entire binding domain was deleted, we instead observed a spectrum of phenotypic rescue. Specifically, two point mutants within the Shaggy binding domain showed loss of activity similar to that of AxinDeltaSgg and dominantly interfered with complex function, whereas a third mutant allele, AxinK446E, retained most function. Two Axin point mutants within the Armadillo binding domain were weak alleles and retained most function. These findings demonstrate the importance of in vivo verification of the role of specific amino acids within a protein.

Membrane bound axin is sufficient for Wingless signaling in Drosophila embryos

The Wingless signaling pathway controls various developmental processes in both vertebrates and invertebrates. Here I probe the requirement for nuclear localization of APC2 and Axin in the Wg signal transduction pathway during embryonic development of Drosophila melanogaster. I find that nuclear localization of APC2 appears to be required, but Axin can block signaling when tethered to the membrane. These results support the model where Axin regulates Armadillo localization and activity in the cytoplasm.

Inhibition of Drosophila Wg signaling involves competition between Mad and Armadillo/beta-catenin for dTcf binding

Precisely regulated signal transduction pathways are crucial for the regulation of developmental events and prevention of tumorigenesis. Both the Transforming Growth Factor β (TGFβ)/Bone morphogenetic protein (BMP) and Wnt/Wingless (Wg) pathways play essential roles in organismal patterning and growth, and their deregulation can lead to cancers. We describe a mechanism of interaction between Drosophila Wg and BMP signaling in which Wg target gene expression is antagonized by BMP signaling. In vivo, high levels of both an activated BMP receptor and the BMP effector Mad can inhibit the expression of Wg target genes. Conversely, loss of mad can induce Wg target gene expression. In addition, we find that ectopic expression in vivo of the Wg transcription factor dTcf is able to suppress the inhibitory effect caused by ectopic Mad. In vitro binding studies revealed competition for dTcf binding between Mad and the Wnt effector β-catenin/Armadillo (Arm). Our in vivo genetic analyses and target gene studies support a mechanism consistent with the in vitro binding and competition studies, namely that BMP pathway components can repress Wg target gene expression by influencing the binding of Arm and dTcf.

Unexpectedly robust assembly of the Axin destruction complex regulates Wnt/Wg signaling in Drosophila as revealed by analysis in vivo

Secreted proteins in the Wnt family regulate gene expression in target cells by causing the accumulation of the transcriptional activator beta-catenin. In the absence of Wnt, a protein complex assembled around the scaffold protein Axin targets beta-catenin for destruction, thereby preventing it from transducing inappropriate signals. Loss of Axin or its binding partners APC and GSK3 results in aberrant activation of the Wnt signaling response. We have analyzed the effects of mutant forms of Drosophila Axin with large internal deletions when expressed at physiological levels in vivo, either in the presence or absence of wild type Axin. Surprisingly, even deletions that completely remove the binding sites for fly APC, GSK3 or beta-catenin, though they fail to rescue to viability, these mutant forms of Axin cause only mild developmental defects, indicating largely retained Axin function. Furthermore, two lethal Axin deletion constructs, AxinDeltaRGS and AxinDeltabeta cat(DeltaArm), can complement each other and restore viability. Our findings support a model in which the Axin complex is assembled through cooperative tripartite interactions among the binding partners, making the assembly of functional complexes surprisingly robust.

The RGS gene loco is essential for male reproductive system differentiation in Drosophila melanogaster

BACKGROUND

The loco gene encodes several different isoforms of a regulator of G-protein signalling. These different isoforms of LOCO are part of a pathway enabling cells to respond to external signals. LOCO is known to be required at various developmental stages including neuroblast division, glial cell formation and oogenesis. Less is known about LOCO and its involvement in male development therefore to gain further insight into the role of LOCO in development we carried out a genetic screen and analysed males with reduced fertility.

RESULTS

We identified a number of lethal loco mutants and four semi-lethal lines, which generate males with reduced fertility. We have identified a fifth loco transcript and show that it is differentially expressed in developing pupae. We have characterised the expression pattern of all loco transcripts during pupal development in the adult testes, both in wild type and loco mutant strains. In addition we also show that there are various G-protein alpha subunits expressed in the testis all of which may be potential binding partners of LOCO.

CONCLUSION

We propose that the male sterility in the new loco mutants result from a failure of accurate morphogenesis of the adult reproductive system during metamorphosis, we propose that this is due to a loss of expression of loco c3. Thus, we conclude that specific isoforms of loco are required for the differentiation of the male gonad and genital disc.

Reggie-1/flotillin-2 promotes secretion of the long-range signalling forms of Wingless and Hedgehog in Drosophila

The lipid-modified morphogens Wnt and Hedgehog diffuse poorly in isolation yet can spread over long distances in vivo, predicting existence of two distinct forms of these mophogens. The first is poorly mobile and activates short-range target genes. The second is specifically packed for efficient spreading to induce long-range targets. Subcellular mechanisms involved in the discriminative secretion of these two forms remain elusive. Wnt and Hedgehog can associate with membrane microdomains, but the function of this association was unknown. Here we show that a major protein component of membrane microdomains, reggie-1/flotillin-2, plays important roles in secretion and spreading of Wnt and Hedgehog in Drosophila. Reggie-1 loss-of-function results in reduced spreading of the morphogens, while its overexpression stimulates secretion of Wnt and Hedgehog and expands their diffusion. The resulting changes in the morphogen gradients differently affect the short- and long-range targets. In its action reggie-1 appears specific for Wnt and Hedgehog. These data suggest that reggie-1 is an important component of the Wnt and Hedgehog secretion pathway dedicated to formation of the mobile pool of these morphogens.

Wingless signaling in Drosophila eye development

The secreted morphogen Wingless (Wg) has a variety of functions throughout Drosophila eye development, controlling tissue specification, growth, and patterning. Wg plays a critical role in subdividing the eye imaginal disc into separate primordia that will give rise to the adult retina and the surrounding head capsule. During larval development, wg is expressed in the anterior lateral margins of the eye disc, regions that will give rise to head cuticle; Wg signaling promotes the head fate and prevents these marginal regions from initiating ectopic photoreceptor differentiation. Expression of wg at the dorsal margin is earlier and stronger than at the ventral margin, allowing Wg to contribute to specifying the dorsal domain of the eye disc. Finally, during the pupal stages, wg expression surrounds the entire eye and a concentric gradient of Wg establishes several distinct peripheral retinal cell fates. This chapter reviews these aspects of Wg function and describes how to generate clones of cells mutant for genes encoding components of the Wg signaling pathway in the eye disc and examine their effects on photoreceptor differentiation by immunohistochemistry.

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

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

Expanded and fat regulate growth and differentiation in the Drosophila eye through multiple signaling pathways

Mutations in the expanded gene act as hyperplastic tumor suppressors, interfere with cell competition and elevate Dpp signaling. Unlike Dpp overexpression, ex causes few patterning defects. Our data suggest that patterning effects are partly masked by antagonistic roles of other signaling pathways that are also activated. ex causes proliferation of cells in the posterior eye disc that are normally postmitotic. ex mutations elevate Wg signaling, but Dpp signaling antagonizes patterning effects of Wg. By contrast, if Dpp signaling is blocked in ex mutant cells, the elevated Wg signaling preserves an immature developmental state and prevents retinal differentiation. An effect of ex mutations on vesicle transport is suggested by evidence for altered sterol distribution. Mutations in ft show effects on proliferation, Wg signaling and sterols very similar to those of ex mutations. During disc growth, ex was largely epistatic to ft, and the Warts pathway mutation hippo largely epistatic to ex. Our data suggest that ft and ex act partially through the Warts pathway.

Wnt11/beta-catenin signaling in both oocytes and early embryos acts through LRP6-mediated regulation of axin

Current models of canonical Wnt signaling assume that a pathway is active if beta-catenin becomes nuclearly localized and Wnt target genes are transcribed. We show that, in Xenopus, maternal LRP6 is essential in such a pathway, playing a pivotal role in causing expression of the organizer genes siamois and Xnr3, and in establishing the dorsal axis. We provide evidence that LRP6 acts by degrading axin protein during the early cleavage stage of development. In the full-grown oocyte, before maturation, we find that axin levels are also regulated by Wnt11 and LRP6. In the oocyte, Wnt11 and/or LRP6 regulates axin to maintain beta-catenin at a low level, while in the embryo, asymmetrical Wnt11/LRP6 signaling stabilizes beta-catenin and enriches it on the dorsal side. This suggests that canonical Wnt signaling may not exist in simple off or on states, but may also include a third, steady-state, modality.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Flying at the head of the pack: Wnt biology in Drosophila

The fruitfly, Drosophila melanogaster, has been of central importance in analysing the mechanics of cellular processes. Classic forward genetic screens in the fly have identified many of the genes that define critical cell signaling pathways, for example. Our understanding of the Wnt pathway, in particular, has benefited from the many advantages that the fly offers as a model system. Here, I review the history of these discoveries and highlight the utility of the fly in dissecting the molecular workings of Wnt signal transduction.

The LDL receptor-related protein LRP6 mediates internalization and lethality of anthrax toxin

Toxins produced by Bacillus anthracis and other microbial pathogens require functions of host cell genes to yield toxic effects. Here we show that low density lipoprotein receptor-related protein 6 (LRP6), previously known to be a coreceptor for the Wnt signaling pathway, is required for anthrax toxin lethality in mammalian cells. Downregulation of LRP6 or coexpression of a truncated LRP6 dominant-negative peptide inhibited cellular uptake of complexes containing the protective antigen (PA) carrier of anthrax toxin moieties and protected targeted cells from death, as did antibodies against epitopes in the LRP6 extracellular domain. Fluorescence microscopy and biochemical analyses showed that LRP6 enables toxin internalization by interacting at the cell surface with PA receptors TEM8/ATR and/or CMG2 to form a multicomponent complex that enters cells upon PA binding. Our results, which reveal a previously unsuspected biological role for LRP6, identify LRP6 as a potential target for countermeasures against anthrax toxin lethality.

RanBP3 enhances nuclear export of active (beta)-catenin independently of CRM1

beta-Catenin is the nuclear effector of the Wnt signaling cascade. The mechanism by which nuclear activity of beta-catenin is regulated is not well defined. Therefore, we used the nuclear marker RanGTP to screen for novel nuclear beta-catenin binding proteins. We identified a cofactor of chromosome region maintenance 1 (CRM1)-mediated nuclear export, Ran binding protein 3 (RanBP3), as a novel beta-catenin-interacting protein that binds directly to beta-catenin in a RanGTP-stimulated manner. RanBP3 inhibits beta-catenin-mediated transcriptional activation in both Wnt1- and beta-catenin-stimulated human cells. In Xenopus laevis embryos, RanBP3 interferes with beta-catenin-induced dorsoventral axis formation. Furthermore, RanBP3 depletion stimulates the Wnt pathway in both human cells and Drosophila melanogaster embryos. In human cells, this is accompanied by an increase of dephosphorylated beta-catenin in the nucleus. Conversely, overexpression of RanBP3 leads to a shift of active beta-catenin toward the cytoplasm. Modulation of beta-catenin activity and localization by RanBP3 is independent of adenomatous polyposis coli protein and CRM1. We conclude that RanBP3 is a direct export enhancer for beta-catenin, independent of its role as a CRM1-associated nuclear export cofactor.

Cullin-3 regulates pattern formation, external sensory organ development and cell survival during Drosophila development

Ubiquitin-mediated proteolysis regulates the steady-state abundance of proteins and controls cellular homoeostasis by abrupt elimination of key effector proteins. A multienzyme system targets proteins for destruction through the covalent attachment of a multiubiquitin chain. The specificity and timing of protein ubiquitination is controlled by ubiquitin ligases, such as the Skp1-Cullin-F box protein complex. Cullins are major components of SCF complexes, and have been implicated in degradation of key regulatory molecules including Cyclin E, beta-catenin and Cubitus interruptus. Here, we describe the genetic identification and molecular characterisation of the Drosophila Cullin-3 homologue. Perturbation of Cullin-3 function has pleiotropic effects during development, including defects in external sensory organ development, pattern formation and cell growth and survival. Loss or overexpression of Cullin-3 causes an increase or decrease, respectively, in external sensory organ formation, implicating Cullin-3 function in regulating the commitment of cells to the neural fate. We also find that Cullin-3 function modulates Hedgehog signalling by regulating the stability of full-length Cubitus interruptus (Ci155). Loss of Cullin-3 function in eye discs but not other imaginal discs promotes cell-autonomous accumulation of Ci155. Conversely, overexpression of Cullin-3 results in a cell-autonomous stabilisation of Ci155 in wing, haltere and leg, but not eye, imaginal discs suggesting tissue-specific regulation of Cullin-3 function. The diverse nature of Cullin-3 phenotypes highlights the importance of targeted proteolysis during Drosophila development.

Reorganization of actin cytoskeleton by FRIED, a Frizzled-8 associated protein tyrosine phosphatase

Frizzled receptors transduce signals from the extracellular Wnt ligands through multiple signaling pathways that affect cytoskeletal organization and regulate gene expression. Direct intracellular mediators of Frizzled signaling are largely unknown. We identified FRIED (Frizzled interaction and ectoderm defects) by its association with the C-terminal PDZ-binding motif of Xenopus Frizzled 8. FRIED contains an N-terminal KIND domain, a FERM domain, six PDZ domains, and a tyrosine phosphatase domain, being similar in structure to the protein tyrosine phosphatase PTP-BAS/PTP-BL. We report that FRIED proteins with the FERM domain localize to the apical cortex and can inhibit Wnt8-mediated, but not beta-catenin-mediated, secondary axis induction in Xenopus embryos, suggesting a specific interaction with Wnt signaling. A FRIED construct containing the FERM domain induced reorganization of pigment granules and cortical actin in Xenopus ectoderm. Wnt5a suppressed the depigmentation of ectoderm triggered by FRIED, demonstrating that Wnt5a and FRIED functionally interact to regulate the cytoskeletal organization. Our data are consistent with the possibility that FRIED functions by modulating Rac1 activity. We propose that FRIED is an adaptor protein that serves as a molecular link between Wnt signaling and actin cytoskeleton.

Rethinking WNT signaling

Recent research on the WNT signaling pathway warrants a reassessment of the basic mechanism that transmits signal from the membrane-bound receptor to the nucleus. This article incorporates these findings into a revised model for pathway activation. We propose that the control of Axin stability, rather than the control of ZW3 phosphorylation of the Armadillo protein, is the key step in signaling. Axin degradation is controlled by a stabilizing effect of ZW3-dependent phosphorylation, and a destabilizing effect of active Arrow. Removing Axin enables Armadillo to accumulate and re-localize to the nucleus. We argue that nuclear localization of Armadillo is required for transcriptional pathway activity. Finally, we speculate on the effects this revision will have on the major questions facing the WNT field of research.

The ins and outs of Wingless signaling

Signaling through the highly conserved Wingless/Wnt pathway plays a crucial role in a diverse array of developmental processes, many of which depend upon the precise regulation of Wingless/Wnt signaling levels. Recent evidence has indicated that the intracellular trafficking of Wingless/Wnt signaling components can result in significant changes in the level of signaling. Here, we examine three mechanisms through which intracellular trafficking might regulate Wingless signaling--the degradation of Wingless, its transport and the transduction of its signal. The intracellular trafficking of several Wingless/Wnt signaling components, including LRP5, LRP6, Dishevelled and Axin, as well as the functional implications of protein localization on Wingless/Wnt signaling, will be discussed.

Identification and Functional Analysis of the Drosophila Gene loco

In contrast to vertebrates, the fruit fly Drosophila melanogaster contains only a small number of regulator of G-protein signaling (RGS) domain genes. This article reviews current knowledge on these genes. Although the fruit fly is particularly amenable to genetic analysis and manipulation, not much is known about the functions and mechanisms of action. The best-studied RGS gene in Drosophila is loco, a member of the D/R12 subfamily. The four different protein isoforms all contain RGS, GoLoco, and RBD domains. This article describes the identification and functional analyses of loco in the Drosophila system and discusses some mechanistic models that may underlie loco function.

The roles of APC and Axin derived from experimental and theoretical analysis of the Wnt pathway

Wnt signaling plays an important role in both oncogenesis and development. Activation of the Wnt pathway results in stabilization of the transcriptional coactivator β-catenin. Recent studies have demonstrated that axin, which coordinates β-catenin degradation, is itself degraded. Although the key molecules required for transducing a Wnt signal have been identified, a quantitative understanding of this pathway has been lacking. We have developed a mathematical model for the canonical Wnt pathway that describes the interactions among the core components: Wnt, Frizzled, Dishevelled, GSK3β, APC, axin, β-catenin, and TCF. Using a system of differential equations, the model incorporates the kinetics of protein–protein interactions, protein synthesis/degradation, and phosphorylation/dephosphorylation. We initially defined a reference state of kinetic, thermodynamic, and flux data from experiments using Xenopus extracts. Predictions based on the analysis of the reference state were used iteratively to develop a more refined model from which we analyzed the effects of prolonged and transient Wnt stimulation on β-catenin and axin turnover. We predict several unusual features of the Wnt pathway, some of which we tested experimentally. An insight from our model, which we confirmed experimentally, is that the two scaffold proteins axin and APC promote the formation of degradation complexes in very different ways. We can also explain the importance of axin degradation in amplifying and sharpening the Wnt signal, and we show that the dependence of axin degradation on APC is an essential part of an unappreciated regulatory loop that prevents the accumulation of β-catenin at decreased APC concentrations. By applying control analysis to our mathematical model, we demonstrate the modular design, sensitivity, and robustness of the Wnt pathway and derive an explicit expression for tumor suppression and oncogenicity.

Wnt signaling is important in both oncogenesis and development. Mathematical modeling predicts several unusual features of the Wnt pathway, some of which are tested experimentally

A role of Dishevelled in relocating Axin to the plasma membrane during wingless signaling

Wnt signaling causes changes in gene transcription that are pivotal for normal and malignant development. A key effector of the canonical Wnt pathway is beta-catenin, or Drosophila Armadillo. In the absence of Wnt ligand, beta-catenin is phosphorylated by the Axin complex, which earmarks it for rapid degradation by the ubiquitin system. Axin acts as a scaffold in this complex, to assemble beta-catenin substrate and kinases (casein kinase I [CKI] and glycogen synthase kinase 3 beta [GSK3]). The Adenomatous polyposis coli (APC) tumor suppressor also binds to the Axin complex, thereby promoting the degradation of beta-catenin. In Wnt signaling, this complex is inhibited; as a consequence, beta-catenin accumulates and binds to TCF proteins to stimulate the transcription of Wnt target genes. Wnt-induced inhibition of the Axin complex depends on Dishevelled (Dsh), a cytoplasmic protein that can bind to Axin, but the mechanism of this inhibition is not understood. Here, we show that Wingless signaling causes a striking relocation of Drosophila Axin from the cytoplasm to the plasma membrane. This relocation depends on Dsh. It may permit the subsequent inactivation of the Axin complex by Wingless signaling.