wingless expression mediates determination of peripheral nervous system elements in late stages of Drosophila wing disc development

FoxO and rotund form a binding complex governing wing polyphenism in planthoppers

Wing polyphenism is found in a variety of insects and offers an attractive model system for studying the evolutionary significance of dispersal. The Forkhead box O (FoxO) transcription factor (TF) acts as a wing-morph switch that directs wing buds developing into long-winged (LW) or short-winged morphs in wing-dimorphic planthoppers, yet the regulatory mechanism of the FoxO module remains elusive. Here, we identified the zinc finger TF rotund as a potential wing-morph regulator via transcriptomic analysis and phenotypic screening in the brown plathopper, Nilaparvata lugens. RNA interference-mediated knockdown of rotund antagonized the LW development derived from in the context of FoxO depletion or the activation of the insulin/insulin-like growth factor signaling cascade, reversing long wings into intermediate wings. In vitro binding assays indicated that rotund physically binds to FoxO to form the FoxO combinatorial code. These findings broaden our understanding of the complexity of transcriptional regulation governing wing polyphenism in insects.

A single WNT enhancer drives specification and regeneration of the Drosophila wing

Wings have provided an evolutionary advantage to insects and have allowed them to diversify. Here, we have identified in Drosophila a highly robust regulatory mechanism that ensures the specification and growth of the wing not only during normal development but also under stress conditions. We present evidence that a single wing-specific enhancer in the wingless gene is used in two consecutive developmental stages to first drive wing specification and then contribute to mediating the remarkable regenerative capacity of the developing wing upon injury. We identify two evolutionary conserved cis-regulatory modules within this enhancer that are utilized in a redundant manner to mediate these two activities through the use of distinct molecular mechanisms. Whereas Hedgehog and EGFR signalling regulate Wingless expression in early primordia, thus inducing wing specification from body wall precursors, JNK activation in injured tissues induce Wingless expression to promote compensatory proliferation. These results point to evolutionarily linked conservation of wing specification and regeneration to ensure robust development of the wing, perhaps the most relevant evolutionary novelty in insects.

The wing is a remarkable evolutionary novelty in insects. Here the authors demonstrate that the specification and regenerative capacity of the wing relies on a single wing-specific enhancer of the wingless gene in Drosophila.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

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

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

Pegasus, a small extracellular peptide enhancing short-range diffusion of Wingless

Small Open Reading Frames (smORFs) coding for peptides of less than 100 amino-acids are an enigmatic and pervasive gene class, found in the tens of thousands in metazoan genomes. Here we reveal a short 80 amino-acid peptide (Pegasus) which enhances Wingless/Wnt1 protein short-range diffusion and signalling. During Drosophila wing development, Wingless has sequential functions, including late induction of proneural gene expression and wing margin development. Pegasus mutants produce wing margin defects and proneural expression loss similar to those of Wingless. Pegasus is secreted, and co-localizes and co-immunoprecipitates with Wingless, suggesting their physical interaction. Finally, measurements of fixed and in-vivo Wingless gradients support that Pegasus increases Wingless diffusion in order to enhance its signalling. Our results unveil a new element in Wingless signalling and clarify the patterning role of Wingless diffusion, while corroborating the link between small open reading frame peptides, and regulation of known proteins with membrane-related functions.

Wg secreted by conventional Golgi transport diffuses and forms Wg gradient whereas Wg tethered to extracellular vesicles do not diffuse

Wingless (Wg)/Wnt family proteins are essential for animal development and adult homeostasis. Drosophila Wg secreted from the dorsal-ventral (DV) midline in wing discs forms a concentration gradient that is shaped by diffusion rate and stability of Wg. To understand how the gradient of extracellular Wg is generated, we compared the secretion route of NRT-Wg, an artificial membrane-tethered form of Wg that is supposedly not secreted but still supports fly development, to that of wild-type Wg. We found that wild-type Wg is secreted by both conventional Golgi transport and via extracellular vesicles (EVs), and NRT-Wg can be also secreted via EVs. Furthermore, wild-type Wg secreted by Golgi transport diffused and formed Wg gradient but Wg-containing EVs did not diffuse at all. In case of Wg stability, Sol narae (Sona), a metalloprotease that cleaves Wg, contributes to generate a steep Wg gradient. Interestingly, Wg was also produced in the presumptive wing blade region, which indicates that NRT-Wg on EVs expressed in the blade allows the blade cells to proliferate and differentiate without Wg diffused from the DV midline. We propose that EV-associated Wg induces Wg signaling in autocrine and juxtaposed manners whereas Wg secreted by Golgi transport forms gradient and acts in the long-range signaling, and different organs differentially utilize these two types of Wg signaling for their own development.

Genes regulating wing patterning in Drosophila melanogaster show reduced expression under exposure of Daminozide, the fruit ripening retardant

In our previous study we demonstrated that the fruit ripening retardant Daminozide or Alar causes change in life history traits, distortion of adult wing structure, DNA damage in brain cells and mutagenic effects in fruit fly Drosophila melanogaster. As a continuation of the previous study the present work is designed to explore the metabolic modification of Daminozide following ingestion, the effects of Daminozide on the expression of genes which are pivotal for wing development and molecular interactions of Daminozide with those proteins involved in wing patterning. We demonstrated through reporter gene construct assay using X-gal staining method and transgenic Drosophila melanogaster stocks that the vestigial, wingless and decapentaplegic genes in wing imaginal disc from 3^rd instar larvae exhibited reduced expression when exposed to Daminozide in compare to control larvae. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) of those genes confirmed that exposure to Daminozide reduces the transcription level of those genes. In silico approach with molecular docking study revealed Daminozide may bind and interfere with the optimal functioning of expressed wing signaling proteins.

Exocyst-mediated apical Wg secretion activates signaling in the Drosophila wing epithelium

Wnt proteins are secreted signaling factors that regulate cell fate specification and patterning decisions throughout the animal kingdom. In the Drosophila wing epithelium, Wingless (Wg, the homolog of Wnt1) is secreted from a narrow strip of cells at the dorsal-ventral boundary. However, the route of Wg secretion in polarized epithelial cells remains poorly understood and key proteins involved in this process are still unknown. Here, we performed an in vivo RNAi screen and identified members of the exocyst complex to be required for apical but not basolateral Wg secretion. Specifically blocking the apical Wg secretion leads to reduced downstream signaling. Using an in vivo ‘temporal-rescue’ assay, our results further indicate that apically secreted Wg activates target genes that require high signaling activity. In conclusion, our results demonstrate that the exocyst is required for an apical route of Wg secretion from polarized wing epithelial cells.

Regulation of Wnt signaling and the production of Wnt ligands is crucial for proper development and homeostasis, as dysregulation leads to developmental defects and diseases such as cancer. This study addresses the question of how functional Wnt ligands are secreted by epithelial cells. By using the polarized epithelium of the developing Drosophila wing as a model system to study Wnt/Wg secretion, the authors performed a large-scale RNAi screen and identified proteins of the exocyst complex to be required for Wnt signaling. The study shows that exocyst complex preferentially regulates apical secretion of Wg proteins. Taken together, this study identifies routes and regulators for secretion of signaling-active Wnt proteins from polarized epithelial cells.

ADAMTS Sol narae cleaves extracellular Wingless to generate a novel active form that regulates cell proliferation in Drosophila

Wnt/ Wingless (Wg) is essential for embryonic development and adult homeostasis in all metazoans, but the mechanisms by which secreted Wnt/Wg is processed remain largely unknown. A Drosophila Sol narae (Sona) is a member of ADisintegrin And Metalloprotease with ThromboSpondin motif (ADAMTS) family, and positively regulates Wg signaling by promoting Wg secretion. Here we report that Sona and Wg are secreted by both conventional Golgi and exosomal transports, and Sona cleaves extracellular Wg at the two specific sites, leading to the generation of N-terminal domain (NTD) and C-terminal domain (CTD) fragments. The cleaved forms of extracellular Wg were detected in the extracellular region of fly wing discs, and its level was substantially reduced in sona mutants. Transient overexpression of Wg-CTD increased wing size while prolonged overexpression caused lethality and developmental defects. In contrast, Wg-NTD did not induce any phenotype. Moreover, the wing defects and lethality induced by sona RNAi were considerably rescued by Wg-CTD, indicating that a main function of extracellular Sona is the generation of Wg-CTD. Wg-CTD stabilized cytoplasmic Armadillo (Arm) and had genetic interactions with components of canonical Wg signaling. Wg-CTD also induced Wg downstream targets such as Distal-less (Dll) and Vestigial (Vg). Most importantly, Cyclin D (Cyc D) was induced by Wg-CTD but not by full-length Wg. Because Sona also induces Cyc D in a cell non-autonomous manner, Wg-CTD generated by Sona in the extracellular region activates a subset of Wg signaling whose major function is the regulation of cell proliferation.

CCAAT/enhancer-binding protein-β functions as a negative regulator of Wnt/β-catenin signaling through activation of AXIN1 gene expression

Axin1, a concentration-limiting component of the β-catenin destruction complex, negatively regulates the Wnt/β-catenin pathway. Axin1 concentration is reported to be regulated by proteasomal degradation; however, its transcriptional regulation has not yet been reported. Here, we demonstrated that CCAAT/enhancer-binding protein-β (C/EBP-β) activates axis inhibition protein 1 (AXIN1) gene expression, thereby attenuating Wnt/β-catenin signaling. C/EBP-β interacted with cis-regulatory element for C/EBP-β in the 5′-upstream sequences of the AXIN1 gene and increased AXIN1 promoter activity. Functional analysis using Drosophila and zebrafish models established that C/EBP-β negatively regulates the Wnt/β-catenin pathway. Small-molecule-based up-regulation of C/EBP-β induces AXIN1 gene expression and down-regulates the intracellular β-catenin level, thereby inhibiting hepatoma cell growth. Thus, our findings provide a unique mechanistic insight into the regulation of Axin homeostasis and present a novel strategy for the development of anticancer therapeutics targeting Wnt/β-catenin signaling.

Wingless is a positive regulator of eyespot color patterns in Bicyclus anynana butterflies

Eyespot patterns of nymphalid butterflies are an example of a novel trait yet, the developmental origin of eyespots is still not well understood. Several genes have been associated with eyespot development but few have been tested for function. One of these genes is the signaling ligand, wingless, which is expressed in the eyespot centers during early pupation and may function in eyespot signaling and color ring differentiation. Here we tested the function of wingless in wing and eyespot development by down-regulating it in transgenic Bicyclus anynana butterflies via RNAi driven by an inducible heat-shock promoter. Heat-shocks applied during larval and early pupal development led to significant decreases in wingless mRNA levels and to decreases in eyespot size and wing size in adult butterflies. We conclude that wingless is a positive regulator of eyespot and wing development in B. anynana butterflies.

Pentagone internalises glypicans to fine-tune multiple signalling pathways

Tight regulation of signalling activity is crucial for proper tissue patterning and growth. Here we investigate the function of Pentagone (Pent), a secreted protein that acts in a regulatory feedback during establishment and maintenance of BMP/Dpp morphogen signalling during Drosophila wing development. We show that Pent internalises the Dpp co-receptors, the glypicans Dally and Dally-like protein (Dlp), and propose that this internalisation is important in the establishment of a long range Dpp gradient. Pent-induced endocytosis and degradation of glypicans requires dynamin- and Rab5, but not clathrin or active BMP signalling. Thus, Pent modifies the ability of cells to trap and transduce BMP by fine-tuning the levels of the BMP reception system at the plasma membrane. In addition, and in accordance with the role of glypicans in multiple signalling pathways, we establish a requirement of Pent for Wg signalling. Our data propose a novel mechanism by which morphogen signalling is regulated.

dFoxO promotes Wingless signaling in Drosophila

The Wnt/β-catenin signaling is an evolutionarily conserved pathway that regulates a wide range of physiological functions, including embryogenesis, organ maintenance, cell proliferation and cell fate decision. Dysregulation of Wnt/β-catenin signaling has been implicated in various cancers, but its role in cell death has not yet been fully elucidated. Here we show that activation of Wg signaling induces cell death in Drosophila eyes and wings, which depends on dFoxO, a transcription factor known to be involved in cell death. In addition, dFoxO is required for ectopic and endogenous Wg signaling to regulate wing patterning. Moreover, dFoxO is necessary for activated Wg signaling-induced target genes expression. Furthermore, Arm is reciprocally required for dFoxO-induced cell death. Finally, dFoxO physically interacts with Arm both in vitro and in vivo. Thus, we have characterized a previously unknown role of dFoxO in promoting Wg signaling, and that a dFoxO-Arm complex is likely involved in their mutual functions, e.g. cell death.

The Cell Adhesion Molecules Roughest, Hibris, Kin of Irre and Sticks and Stones Are Required for Long Range Spacing of the Drosophila Wing Disc Sensory Sensilla

Most animal tissues and organ systems are comprised of highly ordered arrays of varying cell types. The development of external sensory organs requires complex cell-cell communication in order to give each cell a specific identity and to ensure a regular distributed pattern of the sensory bristles. This involves both long and short range signaling mediated by either diffusible or cell anchored factors. In a variety of processes the heterophilic Irre Cell Recognition Module, consisting of the Neph-like proteins: Roughest, Kin of irre and of the Nephrin-like proteins: Sticks and Stones, Hibris, plays key roles in the recognition events of different cell types throughout development. In the present study these proteins are apically expressed in the adhesive belt of epithelial cells participating in sense organ development in a partially exclusive and asymmetric manner. Using mutant analysis the GAL4/UAS system, RNAi and gain of function we found an involvement of all four Irre Cell Recognition Module-proteins in the development of a highly structured array of sensory organs in the wing disc. The proteins secure the regular spacing of sensory organs showing partial redundancy and may function in early lateral inhibition events as well as in cell sorting processes. Comparisons with other systems suggest that the Irre Cell Recognition module is a key organizer of highly repetitive structures.

Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and Delta-Notch signaling to the air sac primordium

The flight muscles, dorsal air sacs, wing blades, and thoracic cuticle of the Drosophila adult function in concert, and their progenitor cells develop together in the wing imaginal disc. The wing disc orchestrates dorsal air sac development by producing decapentaplegic and fibroblast growth factor that travel via specific cytonemes in order to signal to the air sac primordium (ASP). Here, we report that cytonemes also link flight muscle progenitors (myoblasts) to disc cells and to the ASP, enabling myoblasts to relay signaling between the disc and the ASP. Frizzled (Fz)-containing myoblast cytonemes take up Wingless (Wg) from the disc, and Delta (Dl)-containing myoblast cytonemes contribute to Notch activation in the ASP. Wg signaling negatively regulates Dl expression in the myoblasts. These results reveal an essential role for cytonemes in Wg and Notch signaling and for a signal relay system in the myoblasts.

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.

Wingless gene cloning and its role in manipulating the wing dimorphism in the white-backed planthopper, Sogatella furcifera

BACKGROUND

Wingless gene (Wg) plays a fundamental role in regulating the segment polarity and wing imaginal discs of insects. The rice planthoppers have an obvious wing dimorphism, and the long- and short-winged forms exist normally in natural populations. However, the molecular characteristics and functions of Wg in rice planthoppers are poorly understood, and the relationship between expression level of Wg and wing dimorphism has not been clarified.

RESULTS

In this study, wingless gene (Wg) was cloned from three species of rice planthopper, Sogatella furcifera, Laodelphgax striatellus and Nilaparvata lugens, and its characteristics and role in determining the wing dimorphism of S. furcifera were explored. The results showed that only three different amino acid residuals encoded by Wg were found between S. furcifera and L. striatellus, but more than 10 residuals in N. lugens were different with L. striatellus and S. furcifera. The sequences of amino acids encoded by Wg showed a high degree of identity between these three species of rice planthopper that belong to the same family, Delphacidae. The macropterous and brachypterous lineages of S. furcifera were established by selection experiment. The Wg mRNA expression levels in nymphs were significantly higher in the macropterous lineage than in the brachypterous lineage of S. furcifera. In macropterous adults, the Wg was expressed mainly in wings and legs, and less in body segments. Ingestion of 100 ng/μL double-stranded RNA of Wg from second instar nymphs led to a significant decrease of expression level of Wg during nymphal stage and of body weight of subsequent adults. Moreover, RNAi of Wg resulted in significantly shorter and deformative wings, including shrunken and unfolded wings.

CONCLUSION

Wg has high degree of identity among three species of rice planthopper. Wg is involved in the development and growth of wings in S. furcifera. Expression level of Wg during the nymphal stage manipulates the size and pattern of wings in S. furcifera.

Bipartite recognition of DNA by TCF/Pangolin is remarkably flexible and contributes to transcriptional responsiveness and tissue specificity of wingless signaling

The T-cell factor (TCF) family of transcription factors are major mediators of Wnt/β-catenin signaling in metazoans. All TCFs contain a High Mobility Group (HMG) domain that possesses specific DNA binding activity. In addition, many TCFs contain a second DNA binding domain, the C-clamp, which binds to DNA motifs referred to as Helper sites. While HMG and Helper sites are both important for the activation of several Wnt dependent cis-regulatory modules (W-CRMs), the rules of what constitutes a functional HMG-Helper site pair are unknown. In this report, we employed a combination of in vitro binding, reporter gene analysis and bioinformatics to address this question, using the Drosophila family member TCF/Pangolin (TCF/Pan) as a model. We found that while there were constraints for the orientation and spacing of HMG-Helper pairs, the presence of a Helper site near a HMG site in any orientation increased binding and transcriptional response, with some orientations displaying tissue-specific patterns. We found that altering an HMG-Helper site pair from a sub-optimal to optimal orientation/spacing dramatically increased the responsiveness of a W-CRM in several fly tissues. In addition, we used the knowledge gained to bioinformatically identify two novel W-CRMs, one that was activated by Wnt/β-catenin signaling in the prothoracic gland, a tissue not previously connected to this pathway. In sum, this work extends the importance of Helper sites in fly W-CRMs and suggests that the type of HMG-Helper pair is a major factor in setting the threshold for Wnt activation and tissue-responsiveness.

Regulation of gene expression is controlled in large part by proteins known as transcription factors, which bind to specific DNA sequences in the genome. The DNA binding domains of transcription factors recognize short stretches (5–11 base pairs) of DNA with considerable sequence degeneracy. This means that a single DNA binding domain, on its own, cannot find its targets in the vast excess of genomic sequence. We are studying this question using TCF/Pangolin, a Drosophila transcription factor that mediates Wnt/β-catenin signaling, an important developmental cell-cell communication pathway. TCF/Pangolin contains two DNA binding domains that bind to a pair of DNA motifs known as HMG and Helper sites. We used a combination of biochemistry, genetics and bioinformatics to elucidate the spacing and orientation constraints of HMG-Helper site pairs. We found that HMG-Helper site spacing/orientation influenced the sensitivity of a target to Wnt signaling, as well as its tissue-responsiveness. We used this information to improve our ability to search the Drosophila genome for Wnt targets, one of which was activated by the pathway in the fly ring gland, the major endocrine organ in insects. Our work is relevant to related mammalian TCF family members, which are implicated in development, stem cell biology and the progression of cancer.

Identification of a new stem cell population that generates Drosophila flight muscles

How myoblast populations are regulated for the formation of muscles of different sizes is an essentially unanswered question. The large flight muscles of Drosophila develop from adult muscle progenitor (AMP) cells set-aside embryonically. The thoracic segments are all allotted the same small AMP number, while those associated with the wing-disc proliferate extensively to give rise to over 2500 myoblasts. An initial amplification occurs through symmetric divisions and is followed by a switch to asymmetric divisions in which the AMPs self-renew and generate post-mitotic myoblasts. Notch signaling controls the initial amplification of AMPs, while the switch to asymmetric division additionally requires Wingless, which regulates Numb expression in the AMP lineage. In both cases, the epidermal tissue of the wing imaginal disc acts as a niche expressing the ligands Serrate and Wingless. The disc-associated AMPs are a novel muscle stem cell population that orchestrates the early phases of adult flight muscle development.

Coordination of wing and whole-body development at developmental milestones ensures robustness against environmental and physiological perturbations

Development produces correctly patterned tissues under a wide range of conditions that alter the rate of development in the whole body. We propose two hypotheses through which tissue patterning could be coordinated with whole-body development to generate this robustness. Our first hypothesis states that tissue patterning is tightly coordinated with whole-body development over time. The second hypothesis is that tissue patterning aligns at developmental milestones. To distinguish between our two hypotheses, we developed a staging scheme for the wing imaginal discs of Drosophila larvae using the expression of canonical patterning genes, linking our scheme to three whole-body developmental events: moulting, larval wandering and pupariation. We used our scheme to explore how the progression of pattern changes when developmental time is altered either by changing temperature or by altering the timing of hormone synthesis that drives developmental progression. We found the expression pattern in the wing disc always aligned at moulting and pupariation, indicating that these key developmental events represent milestones. Between these milestones, the progression of pattern showed greater variability in response to changes in temperature and alterations in physiology. Furthermore, our data showed that discs from wandering larvae showed greater variability in patterning stage. Thus for wing disc patterning, wandering does not appear to be a developmental milestone. Our findings reveal that tissue patterning remains robust against environmental and physiological perturbations by aligning at developmental milestones. Furthermore, our work provides an important glimpse into how the development of individual tissues is coordinated with the body as a whole.

The bHLH factors extramacrochaetae and daughterless control cell cycle in Drosophila imaginal discs through the transcriptional regulation of the Cdc25 phosphatase string

One of the major issues in developmental biology is about having a better understanding of the mechanisms that regulate organ growth. Identifying these mechanisms is essential to understand the development processes that occur both in physiological and pathological conditions, such as cancer. The E protein family of basic helix-loop helix (bHLH) transcription factors, and their inhibitors the Id proteins, regulate cell proliferation in metazoans. This notion is further supported because the activity of these factors is frequently deregulated in cancerous cells. The E protein orthologue Daughterless (Da) and the Id orthologue Extramacrochaetae (Emc) are the only members of these classes of bHLH proteins in Drosophila. Although these factors are involved in controlling proliferation, the mechanism underlying this regulatory activity is poorly understood. Through a genetic analysis, we show that during the development of epithelial cells in the imaginal discs, the G2/M transition, and hence cell proliferation, is controlled by Emc via Da. In eukaryotic cells, the main activator of this transition is the Cdc25 phosphatase, string. Our genetic analyses reveal that the ectopic expression of string in cells with reduced levels of Emc or high levels of Da is sufficient to rescue the proliferative defects seen in these mutant cells. Moreover, we present evidence demonstrating a role of Da as a transcriptional repressor of string. Taken together, these findings define a mechanism through which Emc controls cell proliferation by regulating the activity of Da, which transcriptionally represses string.

Precise control of cell proliferation is critical for normal development and tissue homeostasis. Members of the inhibitor of differentiation (Id) family of helix-loop-helix (HLH) proteins are key regulators that coordinate the balance between cell division and differentiation. These proteins exert this function in part by combining with ubiquitously expressed bHLH transcription factors (E proteins), preventing these transcription factors from forming functional hetero- or homodimeric DNA binding complexes. Deregulation of the activity of Id proteins frequently leads to tumour formation. The Daughterless (Da) and Extramacrochaetae (Emc) proteins are the only members of the E and Id families in Drosophila, yet their role in the control of cell proliferation has not been determined. In this study, we show that the elimination of emc or the ectopic expression of da arrests cells in the G2 phase of the cell cycle. Moreover, we demonstrate that emc controls cell proliferation via Da, which acts as a transcriptional repressor of the Cdc25 phosphatase string. These results provide an important insight into the mechanisms through which Id and E protein interactions control cell cycle progression and therefore how the disruption of the function of Id proteins can induce oncogenic transformation.

Distinct DNA binding sites contribute to the TCF transcriptional switch in C. elegans and Drosophila

Regulation of gene expression by signaling pathways often occurs through a transcriptional switch, where the transcription factor responsible for signal-dependent gene activation represses the same targets in the absence of signaling. T-cell factors (TCFs) are transcription factors in the Wnt/ß-catenin pathway, which control numerous cell fate specification events in metazoans. The TCF transcriptional switch is mediated by many co-regulators that contribute to repression or activation of Wnt target genes. It is typically assumed that DNA recognition by TCFs is important for target gene location, but plays no role in the actual switch. TCF/Pangolin (the fly TCF) and some vertebrate TCF isoforms bind DNA through two distinct domains, a High Mobility Group (HMG) domain and a C-clamp, which recognize DNA motifs known as HMG and Helper sites, respectively. Here, we demonstrate that POP-1 (the C. elegans TCF) also activates target genes through HMG and Helper site interactions. Helper sites enhanced the ability of a synthetic enhancer to detect Wnt/ß-catenin signaling in several tissues and revealed an unsuspected role for POP-1 in regulating the C. elegans defecation cycle. Searching for HMG-Helper site clusters allowed the identification of a new POP-1 target gene active in the head muscles and gut. While Helper sites and the C-clamp are essential for activation of worm and fly Wnt targets, they are dispensable for TCF-dependent repression of targets in the absence of Wnt signaling. These data suggest that a fundamental change in TCF-DNA binding contributes to the transcriptional switch that occurs upon Wnt stimulation.

The DNA of cells must be correctly “read” so that the proper genes are expressed. Transcription factors are the primary “DNA readers”, and these proteins bind to specific DNA sequences. Using nematodes as a model system, we investigated the rules of DNA binding for a particular transcription factor, called POP-1, which mediates Wnt signaling, an important cell-cell communication pathway. In addition to its known DNA binding site, we found that POP-1 recognizes additional sequences, termed Helper sites, which are essential for activation of Wnt targets. We used this knowledge to discover that Wnt signaling is active in pacemaker cells in the nematode intestine, which control defecation, a rhythmic behavior with parallels to the vertebrate heartbeat. POP-1 has a dual role in regulating Wnt targets, repressing target genes in the absence of signaling and activating them upon signal stimulation. Surprisingly, we found that Helper sites are only required for activation and not repression, and that this is also the case in the fruit fly Drosophila. This work thus reveals an unexpected complexity in POP-1 DNA binding, which is likely to be relevant for its human counterparts, which play important roles in stem cell biology and cancer.

Bridging Decapentaplegic and Wingless signaling in Drosophila wings through repression of naked cuticle by Brinker

Wnts and bone morphogenetic proteins (BMPs) are signaling elements that are crucial for a variety of events in animal development. In Drosophila, Wingless (Wg, a Wnt ligand) and Decapentaplegic (Dpp, a BMP homolog) are thought to function through distinct signal transduction pathways and independently direct the patterning of the wing. However, recent studies suggest that Mothers against Dpp (Mad), the key transducer of Dpp signaling, might serve as a node for the crosstalk between these two pathways, and both positive and negative roles of Mad in Wg signaling have been suggested. Here, we describe a novel molecular mechanism by which Dpp signaling suppresses Wg outputs. Brinker (Brk), a transcriptional repressor that is downregulated by Dpp, directly represses naked cuticle (nkd), which encodes a feedback inhibitor of Wg signaling, in vitro and in vivo. Through genetic studies, we demonstrate that Brk is required for Wg target gene expression in fly wing imaginal discs and that loss or gain of brk during wing development mimics loss or gain of Wg signaling, respectively. Finally, we show that Dpp positively regulates the expression of nkd and negatively regulates the Wg target gene Distal-less (Dll). These data support a model in which different signaling pathways interact via a negative-feedback mechanism. Such a mechanism might explain how organs coordinate inputs from multiple signaling cues.

The bHLH factors Dpn and members of the E(spl) complex mediate the function of Notch signalling regulating cell proliferation during wing disc development

The Notch signalling pathway plays an essential role in the intricate control of cell proliferation and pattern formation in many organs during animal development. In addition, mutations in most members of this pathway are well characterized and frequently lead to tumour formation. The Drosophila imaginal wing discs have provided a suitable model system for the genetic and molecular analysis of the different pathway functions. During disc development, Notch signalling at the presumptive wing margin is necessary for the restricted activation of genes required for pattern formation control and disc proliferation. Interestingly, in different cellular contexts within the wing disc, Notch can either promote cell proliferation or can block the G1-S transition by negatively regulating the expression of dmyc and bantam micro RNA. The target genes of Notch signalling that are required for these functions have not been identified. Here, we show that the Hes vertebrate homolog, deadpan (dpn), and the Enhancer-of-split complex (E(spl)C) genes act redundantly and cooperatively to mediate the Notch signalling function regulating cell proliferation during wing disc development.

The role of glypicans in Wnt inhibitory factor-1 activity and the structural basis of Wif1's effects on Wnt and Hedgehog signaling

Proper assignment of cellular fates relies on correct interpretation of Wnt and Hedgehog (Hh) signals. Members of the Wnt Inhibitory Factor-1 (WIF1) family are secreted modulators of these extracellular signaling pathways. Vertebrate WIF1 binds Wnts and inhibits their signaling, but its Drosophila melanogaster ortholog Shifted (Shf) binds Hh and extends the range of Hh activity in the developing D. melanogaster wing. Shf activity is thought to depend on reinforcing interactions between Hh and glypican HSPGs. Using zebrafish embryos and the heterologous system provided by D. melanogaster wing, we report on the contribution of glypican HSPGs to the Wnt-inhibiting activity of zebrafish Wif1 and on the protein domains responsible for the differences in Wif1 and Shf specificity. We show that Wif1 strengthens interactions between Wnt and glypicans, modulating the biphasic action of glypicans towards Wnt inhibition; conversely, glypicans and the glypican-binding “EGF-like” domains of Wif1 are required for Wif1's full Wnt-inhibiting activity. Chimeric constructs between Wif1 and Shf were used to investigate their specificities for Wnt and Hh signaling. Full Wnt inhibition required the “WIF” domain of Wif1, and the HSPG-binding EGF-like domains of either Wif1 or Shf. Full promotion of Hh signaling requires both the EGF-like domains of Shf and the WIF domains of either Wif1 or Shf. That the Wif1 WIF domain can increase the Hh promoting activity of Shf's EGF domains suggests it is capable of interacting with Hh. In fact, full-length Wif1 affected distribution and signaling of Hh in D. melanogaster, albeit weakly, suggesting a possible role for Wif1 as a modulator of vertebrate Hh signaling.

In developing organisms, cells choose between alternative fates in order to make appropriately patterned tissues, and misregulation of those choices can underlie both developmental defects and cancers. Cells often make these decisions because of signals received from neighboring cells, such as those mediated by the secreted signaling proteins of the Wnt and Hedgehog (Hh) families. While signaling can be regulated by the levels of signaling or receptor proteins expressed by cells, another level of control is exerted by proteins that bind signaling proteins outside of cells and either inhibit or promote the signaling process. In the fruitfly Drosophilamelanogaster, the secreted Shifted protein has been shown to bind Hh and to increase Hh signaling, likely by reinforcing interactions between Hh and cell surface proteins of the glypican family. We provide evidence that the vertebrate homolog of Shifted, Wnt Inhibitory Factor-1 (Wif1), inhibits Wnt activity by a similar mechanism, reinforcing interactions between Wnts and glypicans in a manner that sequesters Wnts from their receptors. We also examine the structural basis for the specificities of Wif1 and Shifted for Wnt and Hh signaling, respectively, and provide evidence that Wif1, although a potent inhibitor of Wnt activity, influences D. melanogaster Hh signaling.

The structure of Wntch signalling and the resolution of transition states in development

During development, the emergence of different cell fates and their patterning into tissues and organs requires spatio-temporal coordination that controls the relative number of different cell types. Genetic analyses in different systems have revealed that interactions between Wnt and Notch signalling play pervasive roles in these processes. While many of these interactions can be explained in terms of transcriptional cross-talk between the effectors of these pathways, some of them require a different explanation. Experiments in Drosophila, Xenopus and mouse have revealed that Notch plays an important role in the modulation of the transcriptional activity of β-catenin (the main effector of Wnt signalling pathway, independently of its well characterized function as a membrane tethered transcription factor. These studies suggest that rather than two separate pathways, elements of Wnt and Notch signalling configure a single functional module, Wntch, that plays a key role in the resolution of cell fate decisions. Here we review the evidence for Wntch and present a current circuit view of the system, its control and its role in development with a special focus on stem cell populations.

Wnt-Notch signalling: an integrated mechanism regulating transitions between cell states

The activity of Wnt and Notch signalling is central to many cell fate decisions during development and to the maintenance and differentiation of stem cell populations in homeostasis. While classical views refer to these pathways as independent signal transduction devices that co-operate in different systems, recent work has revealed intricate connections between their components. These observations suggest that rather than operating as two separate pathways, elements of Wnt and Notch signalling configure an integrated molecular device whose main function is to regulate transitions between cell states in development and homeostasis. Here, we propose a general framework for the structure and function of the interactions between these signalling systems that is focused on the notion of 'transition states', i.e. intermediates that arise during cell fate decision processes. These intermediates act as checkpoints in cell fate decision processes and are characterised by the mixed molecular identities of the states involved in these processes.

The kinase Sgg modulates temporal development of macrochaetes in Drosophila by phosphorylation of Scute and Pannier

Evolution of novel structures is often made possible by changes in the timing or spatial expression of genes regulating development. Macrochaetes, large sensory bristles arranged into species-specific stereotypical patterns, are an evolutionary novelty of cyclorraphous flies and are associated with changes in both the temporal and spatial expression of the proneural genes achaete (ac) and scute (sc). Changes in spatial expression are associated with the evolution of cis-regulatory sequences, but it is not known how temporal regulation is achieved. One factor required for ac-sc expression, the expression of which coincides temporally with that of ac-sc in the notum, is Wingless (Wg; also known as Wnt). Wingless downregulates the activity of the serine/threonine kinase Shaggy (Sgg; also known as GSK-3). We demonstrate that Scute is phosphorylated by Sgg on a serine residue and that mutation of this residue results in a form of Sc with heightened proneural activity that can rescue the loss of bristles characteristic of wg mutants. We suggest that the phosphorylated form of Sc has reduced transcriptional activity such that sc is unable to autoregulate, an essential function for the segregation of bristle precursors. Sgg also phosphorylates Pannier, a transcriptional activator of ac-sc, the activity of which is similarly dampened when in the phosphorylated state. Furthermore, we show that Wg signalling does not act directly via a cis-regulatory element of the ac-sc genes. We suggest that temporal control of ac-sc activity in cyclorraphous flies is likely to be regulated by permissive factors and might therefore not be encoded at the level of ac-sc gene sequences.

Cell death and selective adhesion reorganize the dorsoventral boundary for zigzag patterning of Drosophila wing margin hairs

Animal tissues and organs are comprised of several types of cells, which are often arranged in a well-ordered pattern. The posterior part of the Drosophila wing margin is covered with a double row of long hairs, which are equally and alternately derived from the dorsal and ventral sides of the wing, exhibiting a zigzag pattern in the lateral view. How this geometrically regular pattern is formed has not been fully understood. In this study, we show that this zigzag pattern is created by rearrangement of wing margin cells along the dorsoventral boundary flanked by the double row of hair cells during metamorphosis. This cell rearrangement is induced by selective apoptosis of wing margin cells that are spatially separated from hair cells. As a result of apoptosis, the remaining wing margin cells are rearranged in a well-ordered manner, which shapes corrugated lateral sides of both dorsal and ventral edges to interlock them for zigzag patterning. We further show that the corrugated topology of the wing edges is achieved by cell-type specific expression and localization of four kinds of NEPH1/nephrin family proteins through heterophilic adhesion between wing margin cells and hair cells. Homophilic E-cadherin adhesion is also required for attachment of the corrugated dorsoventral edges. Taken together, our results demonstrate that sequential coordination of apoptosis and epithelial architecture with selective adhesion creates the zigzag hair alignment. This may be a common mechanism for geometrically ordered repetitive packing of several types of cells in similarly patterned developmental fields such as the mammalian organ of Corti.

The oligomeric state of CtBP determines its role as a transcriptional co-activator and co-repressor of Wingless targets

C-terminal-binding protein (CtBP) is a well-characterized transcriptional co-repressor that requires homo-dimerization for its activity. CtBP can both repress and activate Wingless nuclear targets in Drosophila. Here, we examine the role of CtBP dimerization in these opposing processes. CtBP mutants that cannot dimerize are able to promote Wingless signalling, but are defective in repressing Wingless targets. To further test the role of dimerization in repression, the positions of basic and acidic residues that form inter-molecular salt bridges in the CtBP dimerization interface were swapped. These mutants cannot homo-dimerize and are compromised for repression. However, their co-expression leads to hetero-dimerization and consequent repression of Wingless targets. Our results support a model where CtBP is a gene-specific regulator of Wingless signalling, with some targets requiring CtBP dimers for inhibition while other targets utilize CtBP monomers for activation of their expression. Functional interactions between CtBP and Pygopus, a nuclear protein required for Wingless signalling, support a model where monomeric CtBP acts downstream of Pygopus in activating some Wingless targets.

Sulfated is a negative feedback regulator of wingless in Drosophila

Drosophila Wingless (Wg) acts as a morphogen to control pattern formation in a concentration dependent manner. Previous studies demonstrated important roles of heparan sulfate proteoglycans (HSPGs) in controlling Wg signaling and distribution. Here, we examined the role of Sulfated (Sulf1), a Drosophila homolog of vertebrate heparan sulfate 6-O endosulfatase, in Wg signaling and distribution. We show that sulf1 is specifically up-regulated by Wg signaling in the wing disc. We found that expression of Wg target gene senseless (sens) was elevated in the sulf1 mutant wing discs. Sulf1 also negatively regulate extracellular levels of Wg. Genetic interaction experiments indicate that Wg antagonist Notum may work synergistically with Sulf1 to restrict Wg signaling, and Dally, a member of Drosophila HSPGs, is a potential target of Sulf1. Our results demonstrate that sulf1 is a novel Wg target gene and by a feedback mechanism, it negatively regulated Wg signaling and distribution in vivo.

Integration of RNA processing and expression level control modulates the function of the Drosophila Hox gene Ultrabithorax during adult development

Although most metazoan genes undergo alternative splicing, the functional relevance of the majority of alternative splicing products is still unknown. Here we explore this problem in the Drosophila Hox gene Ultrabithorax (Ubx). Ubx produces a family of six protein isoforms through alternative splicing. To investigate the functional specificity of the Ubx isoforms, we studied their role during the formation of the Drosophila halteres, small dorsal appendages that are essential for normal flight. Our work shows that isoform Ia, which is encoded by all Ubx exons, is more efficient than isoform IVa, which lacks the amino acids coded by two small exons, in controlling haltere development and regulating Ubx downstream targets. However, our experiments also demonstrate that the functional differences among the Ubx isoforms can be compensated for by increasing the expression levels of the less efficient form. The analysis of the DNA-binding profiles of Ubx isoforms to a natural Ubx target, spalt, shows no major differences in isoform DNA-binding activities, suggesting that alternative splicing might primarily affect the regulatory capacity of the isoforms rather than their DNA-binding patterns. Our results suggest that to obtain distinct functional outputs during normal development genes must integrate the generation of qualitative differences by alternative splicing to quantitative processes affecting isoform protein expression levels.

Transcriptional interactions between the pannier isoforms and the cofactor U-shaped during neural development in Drosophila

The pannier (pnr) gene of Drosophila melanogaster encodes two isoforms that belong to the family of GATA transcription factors. The isoforms share an expression domain in the wing discs where they exhibit distinct functions during regulation of the proneural achaete/scute (ac/sc) genes. We previously identified two regions in the pnr locus that drive reporter expression in transgenic lines in patterns that recapitulate the essential features of expression of the two isoforms. Here, we identify promoter regions driving isoform expression, showing that pnr-α regulatory sequences are close to the transcription start site while pnr-β expression requires functional interactions between proximal and distal regulatory elements. We find that the promoter domains necessary for reporter expression also mediate autoregulation of Pnr-β and repression of pnr-α by Pnr-β. The cofactor U-shaped (Ush), which is known to down-regulate the function of Pnr during thorax patterning postranscriptionally, in addition represses pnr-β required for ac/sc activation. Moreover, Ush negatively regulates its own expression, while the pnr isoforms positively regulate ush. Our study uncovers complex transcriptional interactions between the pnr isoforms and the cofactor Ush that may be important for regulation of proneural expression and thorax patterning.

Drosophila heparan sulfate 6-O endosulfatase regulates Wingless morphogen gradient formation

Heparan sulfate proteoglycans (HSPGs) play critical roles in the distribution and signaling of growth factors, but the molecular mechanisms regulating HSPG function are poorly understood. Here, we characterized Sulf1, which is a Drosophila member of the HS 6-O endosulfatase class of HS modifying enzymes. Our genetic and biochemical analyses show that Sulf1 acts as a novel regulator of the Wg morphogen gradient by modulating the sulfation status of HS on the cell surface in the developing wing. Sulf1 affects gradient formation by influencing the stability and distribution of Wg. We also demonstrate that expression of Sulf1 is induced by Wg signaling itself. Thus, Sulf1 participates in a feedback loop, potentially stabilizing the shape of the Wg gradient. Our study shows that the modification of HS fine structure provides a novel mechanism for the regulation of morphogen gradients.

Transcriptional control of stem cell maintenance in the Drosophila intestine

Adult stem cells maintain tissue homeostasis by controlling the proper balance of stem cell self-renewal and differentiation. The adult midgut of Drosophila contains multipotent intestinal stem cells (ISCs) that self-renew and produce differentiated progeny. Control of ISC identity and maintenance is poorly understood. Here we find that transcriptional repression of Notch target genes by a Hairless-Suppressor of Hairless complex is required for ISC maintenance, and identify genes of the Enhancer of split complex [E(spl)-C] as the major targets of this repression. In addition, we find that the bHLH transcription factor Daughterless is essential to maintain ISC identity and that bHLH binding sites promote ISC-specific enhancer activity. We propose that Daughterless-dependent bHLH activity is important for the ISC fate and that E(spl)-C factors inhibit this activity to promote differentiation.

Transcription factor achaete scute-like 2 controls intestinal stem cell fate

The small intestinal epithelium is the most rapidly self-renewing tissue of mammals. Proliferative cells are confined to crypts, while differentiated cell types predominantly occupy the villi. We recently demonstrated the existence of a long-lived pool of cycling stem cells defined by Lgr5 expression and intermingled with post-mitotic Paneth cells at crypt bottoms. We have now determined a gene signature for these Lgr5 stem cells. One of the genes within this stem cell signature is the Wnt target Achaete scute-like 2 (Ascl2). Transgenic expression of the Ascl2 transcription factor throughout the intestinal epithelium induces crypt hyperplasia and ectopic crypts on villi. Induced deletion of the Ascl2 gene in adult small intestine leads to disappearance of the Lgr5 stem cells within days. The combined results from these gain- and loss-of-function experiments imply that Ascl2 controls intestinal stem cell fate.

Osa, a subunit of the BAP chromatin-remodelling complex, participates in the regulation of gene expression in response to EGFR signalling in the Drosophila wing

Gene expression is regulated in part by protein complexes containing ATP-dependent chromatin-remodelling factors of the SWI/SNF family. In Drosophila there is only one SWI/SNF protein, named Brahma, which forms the catalytic subunit of two complexes composed of different proteins. The protein Osa defines the BAP complex, and the proteins Polybromo and Bap170 are only present in the complex named PBAP. In this work we have analysed the functional requirements of Osa during Drosophila wing development, and found that osa is needed for cell growth and survival in the wing imaginal disc, and for the correct patterning of sensory organs, veins and the wing margin. Other members of the BAP complex, such as Snr1, Bap55, Mor and Brm, also share these functions of Osa. We focused on the requirement of Osa during the formation of the wing veins. Genetic interactions between osa alleles and mutations affecting the activity of the EGFR pathway suggest that one aspect of Osa is intimately related to the response to EGFR activity. Thus, loss of osa and EGFR signalling results in similar wing vein phenotypes, and osa alleles enhance the loss of veins caused by reduced EGFR activity. In addition, Osa is required for the expression of several targets of EGFR signalling, such as Delta, rhomboid and argos. We suggest that one role of Osa and Brm in the wing is to establish a chromatin environment in the regulatory regions of EGFR target genes, making them available for both activators and repressors and facilitating transcription in response to EGFR signalling.

Tow (Target of Wingless), a novel repressor of the Hedgehog pathway in Drosophila

Hedgehog (Hh) signalling plays a crucial role in the development and patterning of many tissues in both vertebrates and invertebrates. Aberrations in this pathway lead to severe developmental defects and cancer. Hh signal transduction in receiving cells is a well studied phenomenon; however questions still remain concerning the mechanism of repression of the pathway activator Smoothened (Smo) in the absence of Hh. Here we describe a novel repressor of the Hh pathway, Target of Wingless (Tow). Tow represents the Drosophila homolog of a conserved uncharacterised protein family. We show that Tow acts in Hh receiving cells, where its overexpression represses all levels of Hh signalling, and that this repression occurs upstream or at the level of Smo and downstream of the Hh receptor Patched (Ptc). In addition, we find that like Ptc, overexpression of Tow causes an accumulation of lipophorin in the wing disc. We demonstrate that loss of tow enhances different ptc alleles in a similar manner to another pathway repressor, Suppressor of Fused (SuFu), possibly through mediating Ptc dependant lipophorin internalisation. Combined, these results demonstrate that Tow is an important novel regulator of the Hh pathway in the wing imaginal disc, and may shed light on the mechanism of Ptc repression of Smo.

Interpretation of the wingless gradient requires signaling-induced self-inhibition

In a classical view of development, a cell can acquire positional information by reading the local concentration of a morphogen independently of its neighbors. Accordingly, in Drosophila, the morphogen Wingless produced in the wing's prospective distal region activates target genes in a dose-dependent fashion to organize the proximodistal pattern. Here, we show that, in parallel, Wingless triggers two nonautonomous inhibitory programs that play an important role in the establishment of positional information. Cells flanking the source of Wingless produce a negative signal (encoded by notum) that inhibits Wingless signaling in nearby cells. Additionally, in response to Wingless, all prospective wing cells produce an unidentified signal that dampens target gene expression in surrounding cells. Thus, cells influence each other's response to Wingless through at least two modes of lateral inhibition. Without lateral inhibition, some cells acquire ectopic fates. Lateral inhibition may be a general mechanism behind the interpretation of morphogen gradients.

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.

The chromatin remodelers ISWI and ACF1 directly repress Wingless transcriptional targets

The highly conserved Wingless/Wnt signaling pathway controls many developmental processes by regulating the expression of target genes, most often through members of the TCF family of DNA-binding proteins. In the absence of signaling, many of these targets are silenced, by mechanisms involving TCFs that are not fully understood. Here we report that the chromatin remodeling proteins ISWI and ACF1 are required for basal repression of WG target genes in Drosophila. This regulation is not due to global repression by ISWI and ACF1 and is distinct from their previously reported role in chromatin assembly. While ISWI is localized to the same regions of Wingless target gene chromatin as TCF, we find that ACF1 binds much more broadly to target loci. This broad distribution of ACF1 is dependent on ISWI. ISWI and ACF1 are required for TCF binding to chromatin, while a TCF-independent role of ISWI-ACF1 in repression of Wingless targets is also observed. Finally, we show that Wingless signaling reduces ACF1 binding to WG targets, and ISWI and ACF1 regulate repression by antagonizing histone H4 acetylation. Our results argue that WG signaling activates target gene expression partly by overcoming the chromatin barrier maintained by ISWI and ACF1.

A simulation study of mutations in the genetic regulatory hierarchy for butterfly eyespot focus determination

The color patterns on the wings of butterflies have been an important model system in evolutionary developmental biology. A recent computational model tested genetic regulatory hierarchies hypothesized to underlie the formation of butterfly eyespot foci [Evans, T.M., Marcus, J.M., 2006. A simulation study of the genetic regulatory hierarchy for butterfly eyespot focus determination. Evol. Dev. 8, 273-283]. The computational model demonstrated that one proposed hierarchy was incapable of reproducing the known patterns of gene expression associated with eyespot focus determination in wild-type butterflies, but that two slightly modified alternative hierarchies were capable of reproducing all of the known gene expressions patterns. Here we extend the computational models previously implemented in Delphi 2.0 to two mutants derived from the squinting bush brown butterfly (Bicyclus anynana). These two mutants, comet and Cyclops, have aberrantly shaped eyespot foci that are produced by different mechanisms. The comet mutation appears to produce a modified interaction between the wing margin and the eyespot focus that results in a series of comet-shaped eyespot foci. The Cyclops mutation causes the failure of wing vein formation between two adjacent wing-cells and the fusion of two adjacent eyespot foci to form a single large elongated focus in their place. The computational approach to modeling pattern formation in these mutants allows us to make predictions about patterns of gene expression, which are largely unstudied in butterfly mutants. It also suggests a critical experiment that will allow us to distinguish between two hypothesized genetic regulatory hierarchies that may underlie all butterfly eyespot foci.

A Wingless and Notch double-repression mechanism regulates G1-S transition in the Drosophila wing

The control of tissue growth and patterning is orchestrated in various multicellular tissues by the coordinated activity of the signalling molecules Wnt/Wingless (Wg) and Notch, and mutations in these pathways can cause cancer. The role of these molecules in the control of cell proliferation and the crosstalk between their corresponding pathways remain poorly understood. Crosstalk between Notch and Wg has been proposed to organize pattern and growth in the Drosophila wing primordium. Here we report that Wg and Notch act in a surprisingly linear pathway to control G1-S progression. We present evidence that these molecules exert their function by regulating the expression of the dmyc proto-oncogene and the bantam micro-RNA, which positively modulated the activity of the E2F transcription factor. Our results demonstrate that Notch acts in this cellular context as a repressor of cell-cycle progression and Wg has a permissive role in alleviating Notch-mediated repression of G1-S progression in wing cells.

Prepatterning the Drosophila notum: the three genes of the iroquois complex play intrinsically distinct roles

The Drosophila thorax exhibits 11 pairs of large sensory organs (macrochaetes) identified by their unique position. Remarkably precise, this pattern provides an excellent model system to study the genetic basis of pattern formation. In imaginal wing discs, the achaete-scute proneural genes are expressed in clusters of cells that prefigure the positions of each macrochaete. The activities of prepatterning genes provide positional cues controlling this expression pattern. The three homeobox genes clustered in the iroquois complex (araucan, caupolican and mirror) are such prepattern genes. mirror is generally characterized as performing functions predominantly different from the other iroquois genes. Conversely, araucan and caupolican are described in previous studies as performing redundant functions in most if not all processes in which they are involved. We have addressed the question of the specific role of each iroquois gene in the prepattern of the notum and we clearly demonstrate that they are intrinsically different in their contribution to this process: caupolican and mirror, but not araucan, are required for the neural patterning of the lateral notum. However, when caupolican and/or mirror expression is reduced, araucan loss of function has an effect on thoracic bristles development. Moreover, the overexpression of araucan is able to rescue caupolican loss of function. We conclude that, although retaining some common functionalities, the Drosophila iroquois genes are in the process of diversification. In addition, caupolican and mirror are required for stripe expression and, therefore, to specify the muscular attachment sites prepattern. Thus, caupolican and mirror may act as common prepattern genes for all structures in the lateral notum.

Wing vein patterning in Drosophila and the analysis of intercellular signaling

The positioning and elaboration of ectodermal veins in the wing of Drosophila melanogaster rely on widely utilized developmental signals, including those mediated by EGF, BMP, Hedgehog, Notch, and Wnt. Analysis of vein patterning mutants, using the molecular and genetic mosaic techniques available in Drosophila, has provided important insights into how a combination of short-range and long-range signaling can pattern a simple epidermal tissue. Moreover, venation has become a powerful system for isolating and analyzing novel components in these signaling pathways. I here review the basic events of vein patterning and give examples of how changes in venation have been used to identify important features of cell signaling pathways.

The stars and stripes of animal bodies: evolution of regulatory elements mediating pigment and bristle patterns in Drosophila

Evolution has generated enormous morphological diversity in animals and one of the genetic processes that might have contributed to this is evolution of the cis-regulatory sequences responsible for the temporal and spatial expression of genes regulating embryonic development. This could be particularly relevant to pleiotropic genes with multiple independently acting regulatory modules. Loss or gain of modules enables altered expression without loss of other functions. Here I focus on recent studies correlating differences in morphological traits between related species of Drosophila to changes in cis-regulatory sequences. They show that ancestral regulatory modules have evolved to mediate different transcriptional outputs and suggest that evolution of cis-regulatory sequences might reflect a general mechanism driving evolutionary change.

The balance between the novel protein target of wingless and the Drosophila Rho-associated kinase pathway regulates planar cell polarity in the Drosophila wing

Planar cell polarity (PCP) signaling is mediated by the serpentine receptor Frizzled (Fz) and transduced by Dishevelled (Dsh). Wingless (Wg) signaling utilizes Drosophila Frizzled 2 (DFz2) as a receptor and also requires Dsh for transducing signals to regulate cell proliferation and differentiation in many developmental contexts. Distinct pathways are activated downstream of Dsh in Wg- and Fz-signaling pathways. Recently, a number of genes, which have essential roles as downstream components of PCP signaling, have been identified in Drosophila. They include the small GTPase RhoA/Rho1, its downstream effector Drosophila rho-associated kinase (Drok), and a number of genes such as inturned (in) and fuzzy (fy), whose biochemical functions are unclear. RhoA and Drok provide a link from Fz/Dsh signaling to the modulation of actin cytoskeleton. Here we report the identification of the novel gene target of wingless (tow) by enhancer trap screening. tow expression is negatively regulated by Wg signaling in wing imaginal discs, and the balance between tow and the Drok pathway regulates wing-hair morphogenesis. A loss-of-function mutation in tow does not result in a distinct phenotype. Genetic interaction and gain-of-function studies provide evidence that Tow acts downstream of Fz/Dsh and plays a role in restricting the number of hairs that wing cells form.

The Drosophila tumor suppressors Expanded and Merlin differentially regulate cell cycle exit, apoptosis, and Wingless signaling

Mutations that inactivate either merlin (mer) or expanded (ex) result in increased cell growth and proliferation in Drosophila. Both Mer and Ex are members of the Band 4.1 protein superfamily, and, based on analyses of mer ex double mutants, they are proposed to function together in at least a partially redundant manner upstream of the Hippo (Hpo) and Warts (Wts) proteins to regulate cell growth and division. By individually analyzing ex and mer mutant phenotypes, we have found important qualitative and quantitative differences in the ways Mer and Ex function to regulate cell proliferation and cell survival. Though both mer and ex restrict cell and tissue growth, ex clones exhibit delayed cell cycle exit in the developing eye, while mer clones do not. Conversely, loss of mer substantially compromises normal developmental apoptosis in the pupal retina, while loss of ex has only mild effects. Finally, ex has a role in regulating Wingless protein levels in the eye that is not obviously shared by either mer or hpo. Taken together, our data suggest that Mer and Ex differentially regulate multiple downstream pathways.