The dominant mutation Glazed is a gain-of-function allele of wingless that, similar to loss of APC, interferes with normal eye development

Mutant APC reshapes Wnt signaling plasma membrane nanodomains by altering cholesterol levels via oncogenic β-catenin

Although the role of the Wnt pathway in colon carcinogenesis has been described previously, it has been recently demonstrated that Wnt signaling originates from highly dynamic nano-assemblies at the plasma membrane. However, little is known regarding the role of oncogenic APC in reshaping Wnt nanodomains. This is noteworthy, because oncogenic APC does not act autonomously and requires activation of Wnt effectors upstream of APC to drive aberrant Wnt signaling. Here, we demonstrate the role of oncogenic APC in increasing plasma membrane free cholesterol and rigidity, thereby modulating Wnt signaling hubs. This results in an overactivation of Wnt signaling in the colon. Finally, using the Drosophila sterol auxotroph model, we demonstrate the unique ability of exogenous free cholesterol to disrupt plasma membrane homeostasis and drive Wnt signaling in a wildtype APC background. Collectively, these findings provide a link between oncogenic APC, loss of plasma membrane homeostasis and CRC development.

The Drosophila functional Smad suppressing element fuss, a homologue of the human Skor genes, retains pro-oncogenic properties of the Ski/Sno family

Over the years Ski and Sno have been found to be involved in cancer progression e.g. in oesophageal squamous cell carcinoma, melanoma, oestrogen receptor-positive breast carcinoma, colorectal carcinoma, and leukaemia. Often, their prooncogenic features have been linked to their ability of inhibiting the anti-proliferative action of TGF-ß signalling. Recently, not only pro-oncogenic but also anti-oncogenic functions of Ski/Sno proteins have been revealed. Besides Ski and Sno, which are ubiquitously expressed other members of Ski/Sno proteins exist which show highly specific neuronal expression, the SKI Family Transcriptional Corepressors (Skor). Among others Skor1 and Skor2 are involved in the development of Purkinje neurons and a mutation of Skor1 has been found to be associated with restless legs syndrome. But neither Skor1 nor Skor2 have been reported to be involved in cancer progression. Using overexpression studies in the Drosophila eye imaginal disc, we analysed if the Drosophila Skor homologue Fuss has retained the potential to inhibit differentiation and induce increased proliferation. Fuss expressed in cells posterior to the morphogenetic furrow, impairs photoreceptor axon pathfinding and inhibits differentiation of accessory cells. However, if its expression is induced prior to eye differentiation, Fuss might inhibit the differentiating function of Dpp signalling and might maintain proliferative action of Wg signalling, which is reminiscent of the Ski/Sno protein function in cancer.

The Emerging Mechanisms of Wnt Secretion and Signaling in Development

Wnts are highly-conserved lipid-modified secreted proteins that activate multiple signaling pathways. These pathways regulate crucial processes during various stages of development and maintain tissue homeostasis in adults. One of the most fascinating aspects of Wnt protein is that despite being hydrophobic, they are known to travel several cell distances in the extracellular space. Research on Wnts in the past four decades has identified several factors and uncovered mechanisms regulating their expression, secretion, and mode of extracellular travel. More recently, analyses on the importance of Wnt protein gradients in the growth and patterning of developing tissues have recognized the complex interplay of signaling mechanisms that help in maintaining tissue homeostasis. This review aims to present an overview of the evidence for the various modes of Wnt protein secretion and signaling and discuss mechanisms providing precision and robustness to the developing tissues.

HumanaFly: high-throughput transgenesis and expression of breast cancer transcripts in Drosophila eye discovers the RPS12-Wingless signaling axis

Drosophila melanogaster has been a model for multiple human disease conditions, including cancer. Among Drosophila tissues, the eye development is particularly sensitive to perturbations of the embryonic signaling pathways, whose improper activation in humans underlies various forms of cancer. We have launched the HumanaFly project, whereas human genes expressed in breast cancer patients are screened for their ability to aberrate development of the Drosophila eye, hoping to thus identify novel oncogenes. Here we report identification of a breast cancer transgene, which upon expression in Drosophila produces eye malformation similar to the famous Glazed phenotype discovered by Thomas Morgan and decades later dissected to originate from mis-expression of Wingless (Wg). Wg is the ortholog of human Wnt proteins serving as ligands to initiate the developmental/oncogenic Wnt signaling pathway. Through genetic experiments we identified that this transgene interacted with the Wg production machinery, rather than with Wg signal transduction. In Drosophila imaginal discs, we directly show that the transgene promoted long-range diffusion of Wg, affecting expression of the Wg target genes. The transgene emerged to encode RPS12—a protein of the small ribosomal subunit overexpressed in several cancer types and known to also possess extra-ribosomal functions. Our work identifies RPS12 as an unexpected regulator of secretion and activity of Wnts. As Wnt signaling is particularly important in the context of breast cancer initiation and progression, RPS12 might be implicated in tumorigenesis in this and other Wnt-dependent cancers. Continuation of our HumanaFly project may bring further discoveries on oncogenic mechanisms.

The Classic Lobe Eye Phenotype of Drosophila Is Caused by Transposon Insertion-Induced Misexpression of a Zinc-Finger Transcription Factor

Drosophila Lobe (L) alleles were first discovered ∼100 years ago as spontaneous dominant mutants with characteristic developmental eye defects. However, the molecular basis for L dominant eye phenotypes has not been clearly understood. A previous work reported identification of CG10109/PRAS40 as the L gene, but subsequent analyses suggested that PRAS40 may not be related to L Here, we revisited the L gene to clarify this discrepancy and understand the basis for the dominance of L mutations. Genetic analysis localized the L gene to Oaz, which encodes a homolog of the vertebrate zinc finger protein 423 (Zfp423) family transcriptional regulators. We demonstrate that RNAi knockdown of Oaz almost completely restores all L dominant alleles tested. Lrev6-3 , a revertant allele of the L2 dominant eye phenotype, has an inframe deletion in the Oaz coding sequence. Molecular analysis of L dominant mutants identified allele-specific insertions of natural transposons (roo[ ]L1 , hopper[ ]L5 , and roo[ ]Lr ) or alterations of a preexisting transposon (L2 -specific mutations in roo[ ]Mohr) in the Oaz region. In addition, we generated additional L2 -reversion alleles by CRISPR targeting at Oaz These new loss-of-function Oaz mutations suppress the dominant L eye phenotype. Oaz protein is not expressed in wild-type eye disc but is expressed ectopically in L2/+ mutant eye disc. We induced male recombination between Oaz-GAL4 insertions and the L2 mutation through homologous recombination. By using the L2 -recombined GAL4 reporters, we show that Oaz-GAL4 is expressed ectopically in L2 eye imaginal disc. Taken together, our data suggest that neomorphic L eye phenotypes are likely due to misregulation of Oaz by spontaneous transposon insertions.

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.

Wnt signalling: conquering complexity

The history of the Wnt pathway is an adventure that takes us from mice and flies to frogs, zebrafish and beyond, sketching the outlines of a molecular signalling cascade along the way. Here, we specifically highlight the instrumental role that developmental biology has played throughout. We take the reader on a journey, starting with developmental genetics studies that identified some of the main molecular players, through developmental model organisms that helped unravel their biochemical function and cell biological activities. Culminating in complex analyses of stem cell fate and dynamic tissue growth, these efforts beautifully illustrate how different disciplines provided missing pieces of a puzzle. Together, they have shaped our mechanistic understanding of the Wnt pathway as a conserved signalling process in development and disease. Today, researchers are still uncovering additional roles for Wnts and other members of this multifaceted signal transduction pathway, opening up promising new avenues for clinical applications.

Drosophila DDX3/Belle Exerts Its Function Outside of the Wnt/Wingless Signaling Pathway

The helicases human DDX3 and Drosophila Belle (Bel) are part of a well-defined subfamily of the DEAD-box helicases. Individual subfamily-members perform a myriad of functions in nuclear and cytosolic RNA metabolism. It has also been reported that DDX3X is involved in cell signaling, including IFN-α and IFN-β inducing pathways upon viral infection as well as in Wnt signaling. Here we used a collection of EMS-induced bel alleles recovered from a Wingless (Wg) suppressor screen to analyze the role of the Drosophila homolog of DDX3 in Wg/Wnt signaling. These EMS alleles, as well as a P-element induced null allele and RNAi-mediated knock down of bel, all suppressed the phenotype of ectopic Wg signaling in the eye. However, they did not affect the expression of known Wg target genes like senseless, Distalless or wingful/Notum. Ectopic Wg signaling in eye imaginal discs induces apoptosis by increasing grim expression. Mutations in bel revert grim expression to wild-type levels. Together, these results indicate that Bel does not function as a core component in the Drosophila Wg pathway, and that mutations affecting its helicase function suppress the effects of ectopic Wg signaling downstream of the canonical pathway.

The pugilistDominant Mutation of Drosophila melanogaster: A Simple-Sequence Repeat Disorder Reveals Localized Transport in the Eye

The pugilist-Dominant mutation results from fusion of a portion of the gene encoding the tri-functional Methylene Tetrahydrofolate Dehydrogenase (E.C.1.5.1.5, E.C.3.5.4.9, E.C.6.3.4.3) to approximately one kb of a heterochromatic satellite repeat. Expression of this fusion gene results in an unusual ring pattern of pigmentation around the eye. We carried out experiments to determine the mechanism for this pattern. By using FLP-mediated DNA mobilization to place different pug^D transgenes at pre-selected sites we found that variation in repeat length makes a strong contribution to variability of the pug phenotype. This variation is manifest primarily as differences in the thickness of the pigmented ring. We show that similar phenotypic variation can also be achieved by changing gene copy number. We found that the pug^D pattern is not controlled by wingless, which is normally expressed in a similar ring pattern. Finally, we found that physical injury to a pug^D eye can lead to pigment deposition in parts of the eye that would not have been pigmented in the absence of injury. Our results are consistent with a model in which a metabolite vital for pigment formation is imported from the periphery of the eye, and pug^D limits the extent of its transport towards the center of the eye, thus revealing the existence of a hitherto unknown mechanism of localized transport in the eye.

Powerful Drosophila screens that paved the wingless pathway

The Wnt/Wingless (Wg) signaling cascade controls a number of biological processes in animal development and adult life; aberrant Wnt/Wg signaling can cause diseases. In the 1980s genes were discovered that encode core Wnt/Wg pathway components: their mutant phenotypes were similar and an outline of a signaling cascade emerged. Over the years our knowledge of this important signaling system increased and more components were uncovered that are instrumental for Wnt/Wg secretion and transduction. Here we provide an overview of these discoveries, the technologies involved, with a particular focus on the important role Drosophila screens played in this process.

Compartmentalized zinc deficiency and toxicities caused by ZnT and Zip gene over expression result in specific phenotypes in Drosophila

Movement of zinc ions across cellular membranes is achieved mainly by two families of zinc transport genes encoding multi-transmembrane domain proteins. Members of the Zip family generally transport zinc into the cytosol, either from outside the cell or from the lumen of subcellular organelles such as the endoplasmic reticulum, Golgi, endosomes or storage vacuoles. ZnT proteins move zinc in the opposite direction, resulting in efflux from the cell or uptake into organelles. Zinc homeostasis at both the cellular and systemic level is achieved by the coordinated action of numerous Zip and ZnT proteins, twenty-four in mammals and seventeen in the vinegar fly Drosophila melanogaster. Previously, we have identified a zinc toxicity phenotype in the Drosophila eye, caused by targeted over expression of dZip42C.1 (dZip1) combined with knockdown of dZnT63C (dZnT1). In general, this phenotype was rescued by increased zinc efflux or decreased uptake and was exacerbated by decreased efflux or increased uptake. Now we have identified three additional zinc dyshomeostasis phenotypes caused by over expression of dZnT86D, dZnT86D(eGFP) and dZip71B(FLAG). Genetic and dietary manipulation experiments showed that these three phenotypes all differ both from each other and from our original zinc toxicity phenotype. Based on these data and the approximate subcellular localization of each zinc transport protein, we propose that each phenotype represents a different redistribution of zinc within these cells, resulting in a Golgi zinc toxicity, a Golgi zinc deficiency and a combined Golgi/other organelle zinc toxicity respectively. We are able to group the remaining Drosophila Zip and ZnT genes into several functional categories based on their interaction with the three novel zinc dyshomeostasis phenotypes, allowing the role of each zinc transport protein to be defined in greater detail. This research highlights the differential effects that redistribution of zinc can have within a particular tissue and identifies the Golgi as being particularly sensitive to both excess and insufficient zinc.

Protection of armadillo/β-Catenin by armless, a novel positive regulator of wingless signaling

This study uses an RNAi screen in Drosophila to identify a UBX protein, Armless, as a novel positive regulator of the important Wingless/Wnt signaling pathway, acting to stabilize Armadillo/?-Catenin by antagonizing its turnover.

The Wingless (Wg/Wnt) signaling pathway is essential for metazoan development, where it is central to tissue growth and cellular differentiation. Deregulated Wg pathway activation underlies severe developmental abnormalities, as well as carcinogenesis. Armadillo/β-Catenin plays a key role in the Wg transduction cascade; its cytoplasmic and nuclear levels directly determine the output activity of Wg signaling and are thus tightly controlled. In all current models, once Arm is targeted for degradation by the Arm/β-Catenin destruction complex, its fate is viewed as set. We identified a novel Wg/Wnt pathway component, Armless (Als), which is required for Wg target gene expression in a cell-autonomous manner. We found by genetic and biochemical analyses that Als functions downstream of the destruction complex, at the level of the SCF/Slimb/βTRCP E3 Ub ligase. In the absence of Als, Arm levels are severely reduced. We show by biochemical and in vivo studies that Als interacts directly with Ter94, an AAA ATPase known to associate with E3 ligases and to drive protein turnover. We suggest that Als antagonizes Ter94's positive effect on E3 ligase function and propose that Als promotes Wg signaling by rescuing Arm from proteolytic degradation, spotlighting an unexpected step where the Wg pathway signal is modulated.

The Wg/Wnt signaling pathway, found in most animals, is essential for regulating tissue growth and the formation of different cell types during development. Defects in the Wg/Wnt signaling relay can have serious consequences, ranging from aberrant organ patterning to malignant tumor formation. A pivotal step in the transmission of the Wg/Wnt signal is the stabilization of the protein Armadillo/β-Catenin, a key component of the pathway. However, the means by which the levels of this protein are regulated remain unclear. Here, we describe a novel control point of Armadillo/β-Catenin levels. Using RNA interference, we performed a screen in the fruit fly Drosophila melanogaster and identified Armless, a protein whose biological function was previously unknown, as a novel regulator of Wg/Wnt signaling, essential for the Wg/Wnt-dependent expression of downstream target genes. Our experiments suggest that Armless interferes with the tagging of Armadillo/β-Catenin with ubiquitin, thereby sparing it from proteasomal degradation. We also show that Armless directly interacts in vivo with Ter94, a ubiquitous ATPase involved in protein turnover. Our results suggest that Armless antagonizes Ter94's function in protein turnover, thereby acting as a positive regulator of Wg/Wnt signaling by promoting the stabilization of Armadillo/β-Catenin.

Extramacrochaetae imposes order on the Drosophila eye by refining the activity of the Hedgehog signaling gradient

The compound eye of Drosophila melanogaster is configured by a differentiating wave, the morphogenetic furrow, that sweeps across the eye imaginal disc and transforms thousands of undifferentiated cells into a precisely ordered repetitive array of 800 ommatidia. The initiation of the furrow at the posterior margin of the epithelium and its subsequent movement across the eye field is controlled by the activity of the Hedgehog (Hh) signaling pathway. Differentiating photoreceptors that lie behind the furrow produce and secrete the Hh morphogen, which is captured by cells within the furrow itself. This leads to the stabilization of the full-length form of the zinc-finger transcription factor Cubitus interruptus (Ci(155)), the main effector of Hh signaling. Ci(155) functions as a transcriptional activator of a number of downstream targets, including decapentaplegic (dpp), a TGFβ homolog. In this report, we describe a mechanism that is in place within the fly retina to limit Hh pathway activity within and ahead of the furrow. We demonstrate that the helix-loop-helix (HLH) protein Extramacrochaetae (Emc) regulates Ci(155) levels. Loss of emc leads to an increase in Ci(155) levels, nuclear migration, apical cell constriction and an acceleration of the furrow. We find that these roles are distinct from the bHLH protein Hairy (H), which we show restricts atonal (ato) expression ahead of the furrow. Secondary furrow initiation along the dorsal and ventral margins is blocked by the activity of the Wingless (Wg) pathway. We also show that Emc regulates and cooperates with Wg signaling to inhibit lateral furrow initiation.

Analysis of micro- and nano-structures of the corneal surface of Drosophila and its mutants by atomic force microscopy and optical diffraction

Drosophila melanogaster is a model organism instrumental for numerous biological studies. The compound eye of this insect consists of some eight hundred individual ommatidia or facets, ca. 15 µm in cross-section. Each ommatidium contains eighteen cells including four cone cells secreting the lens material (cornea). High-resolution imaging of the cornea of different insects has demonstrated that each lens is covered by the nipple arrays - small outgrowths of ca. 200 nm in diameter. Here we for the first time utilize atomic force microscopy (AFM) to investigate nipple arrays of the Drosophila lens, achieving an unprecedented visualization of the architecture of these nanostructures. We find by Fourier analysis that the nipple arrays of Drosophila are disordered, and that the seemingly ordered appearance is a consequence of dense packing of the nipples. In contrast, Fourier analysis confirms the visibly ordered nature of the eye microstructures - the individual lenses. This is different in the frizzled mutants of Drosophila, where both Fourier analysis and optical imaging detect disorder in lens packing. AFM reveals intercalations of the lens material between individual lenses in frizzled mutants, providing explanation for this disorder. In contrast, nanostructures of the mutant lens show the same organization as in wild-type flies. Thus, frizzled mutants display abnormal organization of the corneal micro-, but not nano-structures. At the same time, nipples of the mutant flies are shorter than those of the wild-type. We also analyze corneal surface of glossy-appearing eyes overexpressing Wingless - the lipoprotein ligand of Frizzled receptors, and find the catastrophic aberration in nipple arrays, providing experimental evidence in favor of the major anti-reflective function of these insect eye nanostructures. The combination of the easily tractable genetic model organism and robust AFM analysis represents a novel methodology to analyze development and architecture of these surface formations.

Canonical wingless signaling regulates cone cell specification in the Drosophila retina

Correct tissue patterning during development involves multiple morphogenetic events that include specification of different cell fates, cell proliferation, cell death, and coordinated changes in cell shape, position, and adhesion. Here, we use the Drosophila retina to explore the molecular mechanisms that regulate and integrate these various events. In a previous report, we found that wingless (wg) was required to induce a previously unknown surge of cell death ("early death") in the pupal retina. Here, we show that wg is also required to induce the more widely studied mid-pupal cell death ("late death") in a process that involves regulation of DIAP1. Furthermore, our data suggest that wg has a previously unreported role in specifying the glial-like cone cells. This activity requires canonical Wg signaling and is linked with Notch pathway activity. Our work broadens the role of canonical Wg signaling to encompass multiple patterning steps in the emerging Drosophila retina.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Null mutations in Drosophila Optomotor-blind affect T-domain residues conserved in all Tbx proteins

The T-box transcription factors TBX2 and TBX3 are overexpressed in many human cancers raising the need for a thorough understanding of the cellular function of these proteins. In Drosophila, there is one corresponding ortholog, Optomotor-blind (Omb). Currently, only two missense mutations are known for the two human proteins. Making use of the developmental defects caused by inactivation of omb, we have isolated and molecularly characterized four new omb mutations, three of them are missense mutations of amino acids fully conserved in all Tbx proteins. We interpret the functional defects in the framework of the known structure of the human TBX3 protein and provide evidence for loss of Omb DNA-binding activity in all three newly identified missense mutations.

Evolutionary dynamics of the LTR retrotransposons roo and rooA inferred from twelve complete Drosophila genomes

Background

Roo is the most abundant retrotransposon in the fruit fly Drosophila melanogaster. Its evolutionary origins and dynamics are thus of special interest for understanding the evolutionary history of Drosophila genome organization. We here study the phylogenetic distribution and evolution of roo, and its highly diverged relative rooA in 12 completely sequenced genomes of the genus Drosophila.

Results

We identify a total of 164 roo copies, 57 of which were previously unidentified copies that occur in 9 of the 12 genomes. Additionally we find 66 rooA copies in four genomes and remnants of this element in two additional genomes. We further increased the number of elements by searching for individual roo/rooA sequence domains. Most of our roo and rooA elements have been recently inserted. Most elements within a genome are highly similar. A comparison of the phylogenetic tree of our roo and rooA elements shows that the split between roo and rooA took place early in Drosophila evolution. Furthermore there is one incongruency between the species tree and the phylogenetic tree of the roo element. This incongruency regards the placement of elements from D. mojavensis, which are more closely related to D. melanogaster than elements from D. willistoni.

Conclusion

Within genomes, the evolutionary dynamics of roo and rooA range from recent transpositional activity to slow decay and extinction. Among genomes, the balance of phylogenetic evidence, sequence divergence distribution, and the occurrence of solo-LTR elements suggests an origin of roo/rooA within the Drosophila clade. We discuss the possibility of a horizontal gene transfer of roo within this clade.

The adenomatous polyposis coli tumor suppressor and Wnt signaling in the regulation of apoptosis

The adenomatous polyposis coli (APC) tumor suppressor is an essential negative regulator in the evolutionarily conserved Wnt/Wingless (Wg) signal transduction pathway. During normal development, Wnt signaling is required not only to induce cell proliferation and cell fate specification, but also to induce apoptotic cell death. However in some malignant states triggered by APC loss, inappropriate activation of Wnt signaling promotes cell survival and inhibits cell death, indicating that the cellular response to APC loss and Wnt signaling is highly dependent on cell context. This chapter summarizes our current understanding of the role of APC and Wnt signaling in the regulation of apoptosis, based upon studies from fly and mouse in vivo models, as well as cultured carcinoma cells.

Drosophila Myosin II, Zipper, is essential for ommatidial rotation

The adult Drosophila retina is a highly polarized epithelium derived from a precursor tissue that is initially symmetric across its dorsoventral axis. Specialized 90 degrees rotational movements of subsets of cells, the ommatidial precursors, establish mirror symmetry in the retinal epithelium. Myosin II, or Zipper (Zip), a motor protein, regulates the rate at which ommatidia rotate: in zip mutants, the rate of rotation is significantly slowed. Zip is concentrated in the cells that we show to be at the likely interface between rotating and non-rotating cells: the boundary between differentiated and undifferentiated cells. Zip is also robust in newly added ommatidial cells, consistent with our model that the machinery that drives rotation should shift to newly recruited cells as they are added to the growing ommatidium. Finally, cell death genes and canonical Wnt signaling pathway members genetically modify the zip phenotype.

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.

An unconventional nuclear localization motif is crucial for function of the Drosophila Wnt/wingless antagonist Naked cuticle

Wnt/beta-catenin signals orchestrate cell fate and behavior throughout the animal kingdom. Aberrant Wnt signaling impacts nearly the entire spectrum of human disease, including birth defects, cancer, and osteoporosis. If Wnt signaling is to be effectively manipulated for therapeutic advantage, we first must understand how Wnt signals are normally controlled. Naked cuticle (Nkd) is a novel and evolutionarily conserved inducible antagonist of Wnt/beta-catenin signaling that is crucial for segmentation in the model genetic organism, the fruit fly Drosophila melanogaster. Nkd can bind and inhibit the Wnt signal transducer Dishevelled (Dsh), but the mechanism by which Nkd limits Wnt signaling in the fly embryo is not understood. Here we show that nkd mutants exhibit elevated levels of the beta-catenin homolog Armadillo but no alteration in Dsh abundance or distribution. In the fly embryo, Nkd and Dsh are predominantly cytoplasmic, although a recent report suggests that vertebrate Dsh requires nuclear localization for activity in gain-of-function assays. While Dsh-binding regions of Nkd contribute to its activity, we identify a conserved 30-amino-acid motif, separable from Dsh-binding regions, that is essential for Nkd function and nuclear localization. Replacement of the 30-aa motif with a conventional nuclear localization sequence rescued a small fraction of nkd mutant animals to adulthood. Our studies suggest that Nkd targets Dsh-dependent signal transduction steps in both cytoplasmic and nuclear compartments of cells receiving the Wnt signal.

The genetics of Drosophila transgenics

In Drosophila, the genetic approach is still the method of choice for answering fundamental questions on cell biology, signal transduction, development, physiology and behavior. In this approach, a gene's function is ascertained by altering either the amount or quality of the gene product, and then observing the consequences. The genetic approach is itself polymorphous, encompassing new and more complex techniques that typically employ the growing collections of transgenes. The keystone of these modern Drosophila transgenic techniques has been the Gal4 binary system. Recently, several new techniques have modified this binary system to offer greater control over the timing, tissue specificity and magnitude of gene expression. Additionally, the advances in post-transcriptional gene silencing, or RNAi, have greatly expanded the ability to knockdown almost any gene's function. Regardless of the growing experimental intricacy, the application of these advances to modify gene activity still obeys the fundamental principles of genetic analysis. Several of these transgenic techniques, which offer more precise control over a gene's activity, will be reviewed here with a discussion on how they may be used for determining a gene's function.

Drosophila tufted is a gain-of-function allele of the proneural gene amos

Tufted is a classical Drosophila mutant characterized by a large number of ectopic mechanosensory bristles on the dorsal mesothorax. Unlike other ectopic bristle mutants, Tufted is epistatic to achaete and scute, the proneural genes that normally control the development of these sensory organs. In this report, I present genetic and molecular evidence that Tufted is a gain-of-function allele of the proneural gene amos that ectopically activates mechanosensory neurogenesis. I also systematically examine the ability of the various proneural bHLH proteins to cross-activate each other and find that their ability to do so is in general relatively limited, despite their common ability to induce the formation of mechanosensory bristles. This phenomenon seems instead to be related to their shared ability to activate Asense and Senseless.

Expression of the PAX2 oncogene in human breast cancer and its role in progesterone-dependent mammary growth

In this study, we first describe expression of the paired domain transcription factor PAX2 in the normal and cancerous human breast, then demonstrate in a murine model a novel function for PAX2 in the regulation of progesterone stimulation of secondary ductal growth. In human mammary tissue, PAX2 expression was coincident with sub-populations of mammary ductal cells, some of which possessed an undifferentiated histiotype, and was also found in >50% of the human breast tumors surveyed (n=38). In the mouse, mammary parenchyma with a targeted deletion of PAX2 developed normal ductal systems when grafted into wild-type host mammary fat pads, but failed to undergo higher order side-branching and lobular development in response to progesterone. A previously unsuspected PAX2/WT1 (Wilms' tumor suppressor gene) regulatory axis in the mammary gland was also indicated. Using RT-PCR, a significant reduction in WT1 mRNA expression was detected in the PAX2 mutant glands compared to wild-type counterparts and double-antibody immunohistochemistry detected the co-localization of PAX2 and WT1 in the nuclei of normal and cancerous breast cells. These data indicate a role for PAX2 (and possibly WT1) in the regulation of the progesterone response of the mature mammary gland. The potential contribution of PAX2 to breast tumor pathogenesis is discussed.

Dominant Drop mutants are gain-of-function alleles of the muscle segment homeobox gene (msh) whose overexpression leads to the arrest of eye development

Dominant Drop (Dr) mutations are nearly eyeless and have additional recessive phenotypes including lethality and patterning defects in eye and sensory bristles due to cis-regulatory lesions in the cell cycle regulator string (stg). Genetic analysis demonstrates that the dominant small eye phenotype is the result of separate gain-of-function mutations in the closely linked muscle segment homeobox (msh) gene, encoding a homeodomain transcription factor required for patterning of muscle and nervous system. Reversion of the Dr(Mio) allele was coincident with the generation of lethal loss-of-function mutations in msh in cis, suggesting that the dominant eye phenotype is the result of ectopic expression. Molecular genetic analysis revealed that two dominant Dr alleles contain lesions upstream of the msh transcription start site. In the Dr(Mio) mutant, a 3S18 retrotransposon insertion is the target of second-site mutations (P-element insertions or deletions) which suppress the dominant eye phenotype following reversion. The pattern of 3S18 expression and the absence of msh in eye imaginal discs suggest that transcriptional activation of the msh promoter accounts for ectopic expression. Dr dominant mutations arrest eye development by blocking the progression of the morphogenetic furrow leading to photoreceptor cell loss via apoptosis. Gal4-mediated ubiquitous expression of msh in third-instar larvae was sufficient to arrest the morphogenetic furrow in the eye imaginal disc and resulted in lethality prior to eclosion. Dominant mutations in the human msx2 gene, one of the vertebrate homologs of msh, are associated with craniosynostosis, a disease affecting cranial development. The Dr mutations are the first example of gain-of-function mutations in the msh/msx gene family identified in a genetically tractible model organism and may serve as a useful tool to identify additional genes that regulate this class of homeodomain proteins.

Activation tagging in Arabidopsis

Activation tagging using T-DNA vectors that contain multimerized transcriptional enhancers from the cauliflower mosaic virus (CaMV) 35S gene has been applied to Arabidopsis plants. New activation-tagging vectors that confer resistance to the antibiotic kanamycin or the herbicide glufosinate have been used to generate several tens of thousands of transformed plants. From these, over 30 dominant mutants with various phenotypes have been isolated. Analysis of a subset of mutants has shown that overexpressed genes are almost always found immediately adjacent to the inserted CaMV 35S enhancers, at distances ranging from 380 bp to 3.6 kb. In at least one case, the CaMV 35S enhancers led primarily to an enhancement of the endogenous expression pattern rather than to constitutive ectopic expression, suggesting that the CaMV 35S enhancers used here act differently than the complete CaMV 35S promoter. This has important implications for the spectrum of genes that will be discovered by this method.