wingless signaling in the Drosophila eye and embryonic epidermis

Notch Signaling in Insect Development: A Simple Pathway with Diverse Functions

Notch signaling is an evolutionarily conserved pathway which functions between adjacent cells to establish their distinct identities. Despite operating in a simple mechanism, Notch signaling plays remarkably diverse roles in development to regulate cell fate determination, organ growth and tissue patterning. While initially discovered and characterized in the model insect Drosophila melanogaster, recent studies across various insect species have revealed the broad involvement of Notch signaling in shaping insect tissues. This review focuses on providing a comprehensive picture regarding the roles of the Notch pathway in insect development. The roles of Notch in the formation and patterning of the insect embryo, wing, leg, ovary and several specific structures, as well as in physiological responses, are summarized. These results are discussed within the developmental context, aiming to deepen our understanding of the diversified functions of the Notch signaling pathway in different insect species.

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.

Hippo pathway and Bonus control developmental cell fate decisions in the Drosophila eye

The canonical function of the Hippo signaling pathway is the regulation of organ growth. How this pathway controls cell-fate determination is less well understood. Here, we identify a function of the Hippo pathway in cell-fate decisions in the developing Drosophila eye, exerted through the interaction of Yorkie (Yki) with the transcriptional regulator Bonus (Bon), an ortholog of mammalian transcriptional intermediary factor 1/tripartite motif (TIF1/TRIM) family proteins. Instead of controlling tissue growth, Yki and Bon promote epidermal and antennal fates at the expense of the eye fate. Proteomic, transcriptomic, and genetic analyses reveal that Yki and Bon control these cell-fate decisions by recruiting transcriptional and post-transcriptional co-regulators and by repressing Notch target genes and activating epidermal differentiation genes. Our work expands the range of functions and regulatory mechanisms under Hippo pathway control.

Ultrastructure and morphology of the compound eyes of the predatory bug Montandoniola moraguesi (Insecta: Hemiptera: Anthocoridae)

The morphology and ultrastructure of the compound eye of the predatory bug, Montandoniola moraguesi (Puton, 1986) was investigated using scanning and transmission electron microscopy. Its compound eyes, which contain ∼195 ommatidia per eye, have the following characteristics: each ommatidium possesses a laminated corneal lens measuring ∼9 μm in diameter and ∼7 μm in thickness, a tetrapartite eucone crystalline cone, which is approximately 5.5 μm long, like a dumbbell with the distal end larger than the proximal end, eight clustered retinula cells ∼25.6 μm in length, two primary pigment cells and eight secondary primary pigment cells. The rhabdomeres of the eight retinula cells form a circular, tiered rhabdom of two elongated and six peripheral retinula cells. The rhabdomeres of cells R7 and R8 are distributed along the basolateral surface of the cone and form a centrally-fused rhabdom that spans nearly the full length of the ommatidium. The microvilli of the peripheral rhabdom (R1-R6) are radially arranged and form a bilobed, V-like shape in the central rhabdom. Based on the similarity of the compound eye of M. moraguesi to the eyes of other predatory insect species, the evolution and function of eyes in predators are briefly discussed.

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.

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.

Control of apoptosis by Drosophila DCAF12

Regulated Apoptosis (Programmed Cell Death, PCD) maintains tissue homeostasis in adults, and ensures proper growth and morphogenesis of tissues during development of metazoans. Accordingly, defects in cellular processes triggering or executing apoptotic programs have been implicated in a variety of degenerative and neoplastic diseases. Here, we report the identification of DCAF12, an evolutionary conserved member of the WD40-motif repeat family of proteins, as a new regulator of apoptosis in Drosophila. We find that DCAF12 is required for Diap1 cleavage in response to pro-apoptotic signals, and is thus necessary and sufficient for RHG (Reaper, Hid, and Grim)-mediated apoptosis. Loss of DCAF12 perturbs the elimination of supernumerary or proliferation-impaired cells during development, and enhances tumor growth induced by loss of neoplastic tumor suppressors, highlighting the wide requirement for DCAF12 in PCD.

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.

The Drosophila T-box transcription factor Midline functions within the Notch-Delta signaling pathway to specify sensory organ precursor cell fates and regulates cell survival within the eye imaginal disc

We report that the T-box transcription factor Midline (Mid), an evolutionary conserved homolog of the vertebrate Tbx20 protein, functions within the Notch-Delta signaling pathway essential for specifying the fates of sensory organ precursor (SOP) cells. These findings complement an established history of research showing that Mid regulates the cell-fate specification of diverse cell types within the developing heart, epidermis and central nervous system. Tbx20 has been detected in unique neuronal and epithelial cells of embryonic eye tissues in both mice and humans. However, the mechanisms by which either Mid or Tbx20 function to regulate cell-fate specification or other critical aspects of eye development including cell survival have not yet been elucidated. We have also gathered preliminary evidence suggesting that Mid may play an indirect, but vital role in selecting SOP cells within the third-instar larval eye disc by regulating the expression of the proneural gene atonal. During subsequent pupal stages, Mid specifies SOP cell fates as a member of the Notch-Delta signaling hierarchy and is essential for maintaining cell viability by inhibiting apoptotic pathways. We present several new hypotheses that seek to understand the role of Mid in regulating developmental processes downstream of the Notch receptor that are critical for specifying unique cell fates, patterning the adult eye and maintaining cellular homeostasis during eye disc morphogenesis.

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.

Computer simulation of cellular patterning within the Drosophila pupal eye

We present a computer simulation and associated experimental validation of assembly of glial-like support cells into the interweaving hexagonal lattice that spans the Drosophila pupal eye. This process of cell movements organizes the ommatidial array into a functional pattern. Unlike earlier simulations that focused on the arrangements of cells within individual ommatidia, here we examine the local movements that lead to large-scale organization of the emerging eye field. Simulations based on our experimental observations of cell adhesion, cell death, and cell movement successfully patterned a tracing of an emerging wild-type pupal eye. Surprisingly, altering cell adhesion had only a mild effect on patterning, contradicting our previous hypothesis that the patterning was primarily the result of preferential adhesion between IRM-class surface proteins. Instead, our simulations highlighted the importance of programmed cell death (PCD) as well as a previously unappreciated variable: the expansion of cells' apical surface areas, which promoted rearrangement of neighboring cells. We tested this prediction experimentally by preventing expansion in the apical area of individual cells: patterning was disrupted in a manner predicted by our simulations. Our work demonstrates the value of combining computer simulation with in vivo experiments to uncover novel mechanisms that are perpetuated throughout the eye field. It also demonstrates the utility of the Glazier-Graner-Hogeweg model (GGH) for modeling the links between local cellular interactions and emergent properties of developing epithelia as well as predicting unanticipated results in vivo.

The microRNA miR-8 is a conserved negative regulator of Wnt signaling

Wnt signaling plays many important roles in animal development. This evolutionarily conserved signaling pathway is highly regulated at all levels. To identify regulators of the Wnt/Wingless (Wg) pathway, we performed a genetic screen in Drosophila. We identified the microRNA miR-8 as an inhibitor of Wg signaling. Expression of miR-8 potently antagonizes Wg signaling in vivo, in part by directly targeting wntless, a gene required for Wg secretion. In addition, miR-8 inhibits the pathway downstream of the Wg signal by repressing TCF protein levels. Another positive regulator of the pathway, CG32767, is also targeted by miR-8. Our data suggest that miR-8 potently antagonizes the Wg pathway at multiple levels, from secretion of the ligand to transcription of target genes. In addition, mammalian homologues of miR-8 promote adipogenesis of marrow stromal cells by inhibiting Wnt signaling. These findings indicate that miR-8 family members play an evolutionarily conserved role in regulating the Wnt signaling pathway.

Wingless signaling in Drosophila eye development

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

Regulation of wingless signaling by the CKI family in Drosophila limb development

The Wingless (Wg)/Wnt signaling pathway regulates a myriad of developmental processes and its malfunction leads to human disorders including cancer. Recent studies suggest that casein kinase I (CKI) family members play pivotal roles in the Wg/Wnt pathway. However, genetic evidence for the involvement of CKI family members in physiological Wg/Wnt signaling events is lacking. In addition, there are conflicting reports regarding whether a given CKI family member functions as a positive or negative regulator of the pathway. Here we examine the roles of seven CKI family members in Wg signaling during Drosophila limb development. We find that increased CKIepsilon stimulates whereas dominant-negative or a null CKIepsilon mutation inhibits Wg signaling. In contrast, inactivation of CKIalpha by RNA interference (RNAi) leads to ectopic Wg signaling. Interestingly, hypomorphic CKIepsilon mutations synergize with CKIalpha RNAi to induce ectopic Wg signaling, revealing a negative role for CKIepsilon. Conversely, CKIalpha RNAi enhances the loss-of-Wg phenotypes caused by CKIepsilon null mutation, suggesting a positive role for CKIalpha. While none of the other five CKI isoforms can substitute for CKIalpha in its inhibitory role in the Wg pathway, several CKI isoforms including CG12147 exhibit a positive role based on overexpression. Moreover, loss of Gilgamesh (Gish)/CKIgamma attenuates Wg signaling activity. Finally, we provide evidence that several CKI isoforms including CKIalpha and Gish/CKIgamma can phosphorylate the Wg coreceptor Arrow (Arr), which may account, at least in part, for their positive roles in the Wg pathway.

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.

Drosophila CK2 regulates lateral-inhibition during eye and bristle development

Lateral inhibition is critical for cell fate determination and involves the functions of Notch (N) and its effectors, the Enhancer of Split Complex, E(spl)C repressors. Although E(spl) proteins mediate the repressive effects of N in diverse contexts, the role of phosphorylation was unclear. The studies we describe implicate a common role for the highly conserved Ser/Thr protein kinase CK2 during eye and bristle development. Compromising the functions of the catalytic (alpha) subunit of CK2 elicits a rough eye and defects in the interommatidial bristles (IOBs). These phenotypes are exacerbated by mutations in CK2 and suppressed by an increase in the dosage of this protein kinase. The appearance of the rough eye correlates, in time and space, to the specification and refinement of the 'founding' R8 photoreceptor. Consistent with this observation, compromising CK2 elicits supernumerary R8's at the posterior margin of the morphogenetic furrow (MF), a phenotype characteristic of loss of E(spl)C and impaired lateral inhibition. We also show that compromising CK2 elicits ectopic and split bristles. The former reflects the specification of excess bristle SOPs, while the latter suggests roles during asymmetric divisions that drive morphogenesis of this sensory organ. In addition, these phenotypes are exacerbated by mutations in CK2 or E(spl), indicating genetic interactions between these two loci. Given the centrality of E(spl) to the repressive effects of N, our studies suggest conserved roles for this protein kinase during lateral inhibition. Candidates for this regulation are the E(spl) repressors, the terminal effectors of this pathway.

Identification of the death zone: a spatially restricted region for programmed cell death that sculpts the fly eye

Programmed cell death (PCD) sculpts many developing tissues. The final patterning step of the Drosophila retina is the elimination, through PCD, of a subset of interommatidial lattice cells during pupation. It is not understood how this process is spatially regulated to ensure that cells die in the proper positions. To address this, we observed PCD of lattice cells in the pupal retina in real time. This live-visualization method demonstrates that lattice cell apoptosis is a highly specific process. In all, 85% of lattice cells die in exclusive 'death zone' positions between adjacent ommatidia. In contrast, cells that make specific contacts with primary pigment cells are protected from death. Two signaling pathways, Drosophila epidermal growth factor receptor (dEgfr) and Notch, that are thought to be central to the regulation of lattice cell survival and death, are not sufficient to establish the death zone. Thus, application of live visualization to the fly eye gives new insight into a dynamic developmental process.

Notch signaling through tramtrack bypasses the mitosis promoting activity of the JNK pathway in the mitotic-to-endocycle transition of Drosophila follicle cells

Background

The follicle cells of the Drosophila egg chamber provide an excellent model in which to study modulation of the cell cycle. During mid-oogenesis, the follicle cells undergo a variation of the cell cycle, endocycle, in which the cells replicate their DNA, but do not go through mitosis. Previously, we showed that Notch signaling is required for the mitotic-to-endocycle transition, through downregulating String/Cdc25, and Dacapo/p21 and upregulating Fizzy-related/Cdh1.

Results

In this paper, we show that Notch signaling is modulated by Shaggy and temporally induced by the ligand Delta, at the mitotic-to-endocycle transition. In addition, a downstream target of Notch, tramtrack, acts at the mitotic-to-endocycle transition. We also demonstrate that the JNK pathway is required to promote mitosis prior to the transition, independent of the cell cycle components acted on by the Notch pathway.

Conclusion

This work reveals new insights into the regulation of Notch-dependent mitotic-to-endocycle switch.

Notch signaling controls proliferation through cell-autonomous and non-autonomous mechanisms in the Drosophila eye

During Drosophila eye development, localized Notch signaling at the dorsal ventral (DV)-midline promotes growth of the entire eye field. This long-range action of Notch signaling may be mediated through the diffusible ligand of the Jak/STAT pathway, Unpaired (Upd), which was recently identified as a downstream target of Notch. However, Notch activity has not been shown to be cell-autonomously required for Upd expression and therefore yet another diffusible signal may be required for Notch activation of Upd. Our results clarify the Notch requirement, demonstrating that Notch activity at the DV-midline leads to cell-autonomous expression of Upd as monitored in loss and gain-of-function Notch clones. In addition, mutations in the Jak/STAT pathway interact genetically with the Notch pathway by suppressing Notch mediated overgrowth. N(act) clones show non-autonomous effects on the cell cycle anterior to the furrow, indicating function of the Jak/STAT pathway. However, cell-autonomous effects of Notch within and posterior to the furrow are independent of Upd. Here, Notch autonomously maintains cells in a proliferative state and blocks photoreceptor differentiation.

Organ specification-growth control connection: new in-sights from the Drosophila eye-antennal disc

The eye-antennal disc of Drosophila is serving a guiding role in the studies of how eye identity is specified, as well as how the retina is patterned. However, this system also holds a great potential for studying the coordination between organ growth and specification when various distinct organs form from a common primordium. The eye-antennal disc gives origin not only to the compound eye but also to the head capsule, ocelli, maxillary palp, and antenna, and these organs develop bearing constant size proportions with each other. Here, we review recent results that have shed light on the mechanisms that control the specification and growth of organs of the eye-antennal disc and discuss how these controls are intertwined during the development of neighboring organs to ensure their constant shape and relative sizes.

A genetic screen identifies putative targets and binding partners of CREB-binding protein in the developing Drosophila eye

Drosophila CREB-binding protein (dCBP) is a very large multidomain protein, which belongs to the CBP/p300 family of proteins that were first identified by their ability to bind the CREB transcription factor and the adenoviral protein E1. Since then CBP has been shown to bind to >100 additional proteins and functions in a multitude of different developmental contexts. Among other activities, CBP is known to influence development by remodeling chromatin, by serving as a transcriptional coactivator, and by interacting with terminal members of several signaling transduction cascades. Reductions in CBP activity are the underlying cause of Rubinstein-Taybi syndrome, which is, in part, characterized by several eye defects, including strabismus, cataracts, juvenile glaucoma, and coloboma of the eyelid, iris, and lens. Development of the Drosophila melanogaster compound eye is also inhibited in flies that are mutant for CBP. However, the vast array of putative protein interactions and the wide-ranging roles played by CBP within a single tissue such as the retina can often complicate the analysis of CBP loss-of-function mutants. Through a series of genetic screens we have identified several genes that could either serve as downstream transcriptional targets or encode for potential CBP-binding partners and whose association with eye development has hitherto been unknown. The identification of these new components may provide new insight into the roles that CBP plays in retinal development. Of particular interest is the identification that the CREB transcription factor appears to function with CBP at multiple stages of retinal development.

Comparative analysis of Wingless patterning in the embryonic grasshopper eye

The signaling factor Wingless regulates multiple steps during the postembryonic development of the Drosophila eye. To obtain insight into the molecular regulation of embryonic eye development in primitive insects, we studied the expression of wg and genes projected to interact with wg in the grasshopper Schistocerca americana. We find that the dynamic and complex expression of wg in the early grasshopper procephalon results in three paired expression domains with relevance to eye primordium development. By comparison with Drosophila, these domains are compatible with a conserved function of wg during anteroposterior and dorsoventral axis formation by repression of retinal differentiation and stimulation of tissue proliferation. This is further supported by the expression of grasshopper orthologs of the retina determination genes sine oculis and eyes absent, and by inhibition of retina differentiation in grasshopper eye primordia cultured with LiCl. Surprisingly, the expression of wg and the grasshopper orthologs of pannier, fringe, Delta, and Iroquois complex is inconsistent with induction of midline centered Notch signaling activity, which is essential for Drosophila retina development. Similarly substantial evolutionary divergence is found concerning the control of retina versus head epidermis specification. The transcription factor Extradenticle (Exd), which cooperates with wg in specifying the Drosophila head epidermis, is not detected outside the labral and antennal primordia in the embryonic grasshopper head. Our results, which provide the first insight into the molecular control of eye primordium formation in primitive insects, suggest substantial modification of this process during the evolution of the Drosophila mode of postembryonic eye development.

Developmental roles of the Mi-2/NURD-associated protein p66 in Drosophila

The NURD and Sin3 histone deacetylase complexes are involved in transcriptional repression through global deacetylation of chromatin. Both complexes contain many different components that may control how histone deacetylase complexes are regulated and interact with other transcription factors. In a genetic screen for modifiers of wingless signaling in the Drosophila eye, we isolated mutations in the Drosophila homolog of p66, a protein previously purified as part of the Xenopus NURD/Mi-2 complex. p66 encodes a highly conserved nuclear zinc-finger protein that is required for development and we propose that the p66 protein acts as a regulatory component of the NURD complex. Animals homozygous mutant for p66 display defects during metamorphosis possibly caused by misregulation of ecdysone-regulated expression. Although heterozygosity for p66 enhances a wingless phenotype in the eye, loss-of-function clones in the wing and the eye discs do not have any detectable phenotype, possibly due to redundancy with the Sin3 complex. Overexpression of p66, on the other hand, can repress wingless-dependent phenotypes. Furthermore, p66 expression can repress multiple reporters in a cell culture assay, including a Wnt-responsive TCF reporter construct, implicating the NURD complex in repression of Wnt target genes. By co-immunoprecipitation, p66 associates with dMi-2, a known NURD complex member.

The relative role of the T-domain and flanking sequences for developmental control and transcriptional regulation in protein chimeras of Drosophila OMB and ORG-1

optomotor-blind (omb) and optomotor-blind related-1 (org-1) encode T-domain DNA binding proteins in Drosophila. Members of this family of transcription factors play widely varying roles during early development and organogenesis in both vertebrates and invertebrates. Functional specificity differs in spite of similar DNA binding preferences of all family members. Using a series of domain swap chimeras, in which different parts of OMB and ORG-1 were mutually exchanged, we investigated the relevance of individual domains in vitro and in vivo. In cell culture transfection assays, ORG-1 was a strong transcriptional activator, whereas OMB appeared neutral. The main transcriptional activation function was identified in the C-terminal part of ORG-1. Also in vivo, OMB and ORG-1 showed qualitative differences when the proteins were ectopically expressed during development. Gain-of-function expression of OMB is known to counteract eye formation and resulted in the loss of the arista, whereas ORG-1 had little effect on eye development but caused antenna-to-leg transformations and shortened legs in the corresponding gain-of-function situations. The functional properties of OMB/ORG-1 chimeras in several developmental contexts was dominated by the origin of the C-terminal region, suggesting that the transcriptional activation potential can be one major determinant of developmental specificity. In late eye development, we observed, however, a strong influence of the T-domain on ommatidial differentiation. The specificity of chimeric omb/org-1transgenes, thus, depended on the cellular context in which they were expressed. This suggests that both transcriptional activation/repression properties as well as intrinsic DNA binding specificity can contribute to the functional characteristics of T-domain factors.

Senseless is required for pupal retinal development inDrosophila

Drosophila sensory organs are specified by a family of proneural genes which induce the expression of several common targets. One such target is senseless, which encodes a zinc finger transcription factor. We analyzed the function of senseless during pupal retinal development and found that senseless is required for recruitment of both cone and pigment cells, the pupal-derived ommatidial support cells. We also found that Senseless is expressed in neural precursors shortly after the larval-pupal transition and is both necessary and sufficient for interommatidial bristle development. Furthermore, senseless is the primary target of achaete and scute during interommatidial bristle development. We also identified several differences between the development of interommatidial bristles and other macrochaete. In particular, EGFR signaling is not required for interommatidial bristle development, nor is positive feedback regulation of proneural genes by senseless. A model for interommatidial bristle specification is presented.

Patterning the peripheral retina of the fly: decoding a gradient

The peripheral regions of the fly eye show a number of specializations. First, immediately interior to the circumscribing head capsule and completely encircling the rest of the eye lies a thick band of pigment cells (pigment rim; PR). Second, in the dorsal periphery of the eye directly interior to the PR lie the dorsal rim (DR) ommatidia that are specialized polarized light detectors. The equivalent position in the ventral eye is occupied by standard ommatidia. Third, ommatidia characteristically project mechanosensory hairs above their lenses, but in the most peripheral rows (including the DR) the ommatidia are bald. Wingless secreted from the head capsule appears to organize all these peripheral specializations. Higher Wg levels induce PR, intermediate levels induce DR, and lower levels induce baldness. The predisposition of dorsal cells to generate DR ommatidia appears to be endowed by the exclusive dorsal expression of Iroquois genes.

Immunohistochemical study of Ca2+/calmodulin-dependent protein kinase II in the Drosophila brain using a specific monoclonal antibody

To analyze the distribution of Drosophila calcium/calmodulin-dependent protein kinase II (dCaMKII) in the adult brain, we generated monoclonal antibodies against the bacterially expressed 490-amino acid (a.a.) form of dCaMKII. One of those, named #18 antibody, was used for this study. Western blot analysis of the adult head extracts showed that the antibody specifically detects multiple bands between 55 and 60 kDa corresponding to the molecular weights of the splicing isoforms of dCaMKII. Epitope mapping revealed that it was in the region between 199 and 283 a.a. of dCaMKII. Preferential dCaMKII immunoreactivity in the embryonic nervous system, adult thoracic ganglion and gut, and larval neuro-muscular junction (NMJ) was consistent with previous observations by in situ hybridization and immunostaining with a polyclonal antibody at the NMJ, indicating that the antibody is applicable to immunohistochemistry. Although dCaMKII immunoreactive signal was low in the retina, it was found at regular intervals in the outer margin of the compound eye. These signals were most likely to be interommatidial bristle mechanosensory neurons. dCaMKII immunoreactivity in the brain was observed in almost all regions and relatively higher staining was found in the neuropilar region than in the cortex. Higher dCaMKII immunoreactivity in the mushroom body (MB) was found in the entire gamma lobe including the heel, and dorsal tips of the alpha and alpha' lobes, while cores of alpha and beta lobes were stained light. Finding abundant dCaMKII accumulation in the gamma lobe suggested that this lobe might especially require high levels of dCaMKII expression to function properly among MB lobes.

Patterning the fly eye: the role of apoptosis

Early development in many tissues is characterized by a rapid expansion in cell number. Excess cells are removed through activation of their intrinsic apoptotic machinery. This over-expansion followed by selective removal is important for the sculpting of these tissues, and how specific cells are selected to die is one of the central questions in development. The Drosophila eye is a unique example of such patterning through cell death. Because of its remarkable reiterative design, the fly eye lends itself to studies of mutants with increased or decreased apoptosis. We know that the process of elimination of lattice cells is highly regulated. And we have learned that each ommatidial unit is involved in the life-death decision of lattice cells through cell-cell signaling. But, we have yet to understand how this signaling is regulated spatially to result in such precision. In this article, we describe and speculate on the role of selective cell death during maturation of the fly eye.

Runnin' with the Dvl: proteins that associate with Dsh/Dvl and their significance to Wnt signal transduction

Wnt proteins transmit myriad intercellular signals crucial for the development and homeostasis of metazoan animals from Hydra to human. Abnormal Wnt signaling causes a growing number of diseases, including cancer and osteoporosis. Depending on the context, a given Wnt signal may denote: cell proliferation or apoptosis; cell fate determination, differentiation, or stem cell maintenance; a variety of changes in cell behavior; and/or coordinated interactions with its neighbors. Which event(s) occur in Wnt-responsive cells depends critically on the ability of Dishevelled (Dsh)/Dvl proteins to interpret distinct types of intracellular, receptor-generated stimuli and transmit them to at least two distinct sets of effector molecules, all while apparently ignoring a third type of Wnt-generated Ca(2+) signal. The three conserved domains present in Dsh/Dvl proteins uniquely function in each Wnt pathway, in part by association with 18 (and counting) Dsh/Dvl-associated proteins. The latest data suggest that Dsh/Dvl proteins organize dynamic, pathway-specific subcellular signaling complexes that ensure correct information routing, signal amplification, and dynamic control through feedback regulation. The biochemical and cell biological mechanisms by which Dsh/Dvl proteins accomplish these remarkable tasks remain obscure.

Modulation of the Ras/MAPK signalling pathway by the redox function of selenoproteins in Drosophila melanogaster

Modulation of reactive oxygen species (ROS) plays a key role in signal transduction pathways. Selenoproteins act controlling the redox balance of the cell. We have studied how the alteration of the redox balance caused by patufet (selD(ptuf)), a null mutation in the Drosophila melanogaster selenophosphate synthetase 1 (sps1) gene, which codes for the SelD enzyme of the selenoprotein biosynthesis, affects the Ras/MAPK signalling pathway. The selD(ptuf) mutation dominantly suppresses the phenotypes in the eye and the wing caused by hyperactivation of the Ras/MAPK cassette and the activated forms of the Drosophila EGF receptor (DER) and Sevenless (Sev) receptor tyrosine kinases (RTKs), which signal in the eye and wing, respectively. No dominant interaction is observed with sensitized conditions in the Wnt, Notch, Insulin-Pi3K, and DPP signalling pathways. Our current hypothesis is that selenoproteins selectively modulate the Ras/MAPK signalling pathway through their antioxidant function. This is further supported by the fact that a selenoprotein-independent increase in ROS caused by the catalase amorphic Cat(n1) allele also reduces Ras/MAPK signalling. Here, we present the first evidence for the role of intracellular redox environment in signalling pathways in Drosophila as a whole organism.

DWnt4 and wingless elicit similar cellular responses during imaginal development

Wnt genes encode evolutionarily conserved secreted proteins that provide critical functions during development. Although Wnt proteins share highly conserved features, they also show sequence divergence, which almost certainly contributes to the variety of their signaling activities. We previously reported that DWnt4 and wingless (wg), two divergent clustered Wnt genes, can have either antagonist or distinct functions during Drosophila embryogenesis. Here we provide evidence that both genes can elicit similar cellular responses during imaginal development. Ectopic expression of DWnt4 along the anterior/posterior (A/P) boundary of imaginal discs alters morphogenesis of adult appendages. In the wing disc, DWnt4 phenocopies ectopic Wg activity by inducing notum to wing transformation, suggesting similar signaling capabilities of both molecules. In support of this, we demonstrate that DWnt4 can rescue wg loss-of-function phenotypes in the antenna and haltere and is able to substitute for Wg in wing field specification. We also show that both genes are transcribed in overlapping domains in imaginal discs, suggesting that DWnt4 may cooperate with wg during limb patterning.

Naked cuticle targets dishevelled to antagonize Wnt signal transduction

In Drosophila embryos the protein Naked cuticle (Nkd) limits the effects of the Wnt signal Wingless (Wg) during early segmentation. nkd loss of function results in segment polarity defects and embryonic death, but how nkd affects Wnt signaling is unknown. Using ectopic expression, we find that Nkd affects, in a cell-autonomous manner, a transduction step between the Wnt signaling components Dishevelled (Dsh) and Zeste-white 3 kinase (Zw3). Zw3 is essential for repressing Wg target-gene transcription in the absence of a Wg signal, and the role of Wg is to relieve this inhibition. Our double-mutant analysis shows that, in contrast to Zw3, Nkd acts when the Wg pathway is active to restrain signal transduction. Yeast two hybrid and in vitro experiments indicate that Nkd directly binds to the basic-PDZ region of Dsh. Specially timed Nkd overexpression is capable of abolishing Dsh function in a distinct signaling pathway that controls planar-cell polarity. Our results suggest that Nkd acts directly through Dsh to limit Wg activity and thus determines how efficiently Wnt signals stabilize Armadillo (Arm)/beta-catenin and activate downstream genes.

Heparan sulfate proteoglycans are critical for the organization of the extracellular distribution of Wingless

Recent studies in Drosophila have shown that heparan sulfate proteoglycans (HSPGs) are required for Wingless (Wg/Wnt) signaling. In addition, genetic and phenotypic analyses have implicated the glypican gene dally in this process. Here, we report the identification of another Drosophila glypican gene, dally-like (dly) and show that it is also involved in Wg signaling. Inhibition of dly gene activity implicates a function for DLY in Wg reception and we show that overexpression of DLY leads to an accumulation of extracellular Wg. We propose that DLY plays a role in the extracellular distribution of Wg. Consistent with this model, a dramatic decrease of extracellular Wg was detected in clones of cells that are deficient in proper glycosaminoglycan biosynthesis. We conclude that HSPGs play an important role in organizing the extracellular distribution of Wg.

A screen for identifying genes interacting with armadillo, the Drosophila homolog of beta-catenin

Drosophila Armadillo is a multifunctional protein implicated in both cell adhesion, as a catenin, and cell signaling, as part of the Wingless signal transduction pathway. We have generated viable fly stocks with alterations in the level of Armadillo available for signaling. Flies from one stock overexpress Armadillo and, as a result, have increased vein material and bristles in the wings. Flies from the other stock have reduced cytoplasmic Armadillo following overexpression of the intracellular domain of DE-cadherin. These flies display a wing-notching phenotype typical of wingless mutations. Both misexpression phenotypes can be dominantly modified by removing one copy of genes known to encode members of the wingless pathway. Here we describe the identification of further mutations that dominantly modify the Armadillo misexpression phenotypes. These mutations are in genes encoding three different functions: establishment and maintenance of adherens junctions, cell cycle control, and Egfr signaling.

Notch and wingless regulate expression of cuticle patterning genes

The cell surface receptor Notch is required during Drosophila embryogenesis for production of epidermal precursor cells. The secreted factor Wingless is required for specifying different types of cells during differentiation of tissues from these epidermal precursor cells. The results reported here show that the full-length Notch and a form of Notch truncated in the amino terminus associate with Wingless in S2 cells and in embryos. In S2 cells, Wingless and the two different forms of Notch regulate expression of Dfrizzled 2, a receptor of Wg; hairy, a negative regulator of achaete expression; shaggy, a negative regulator of engrailed expression; and patched, a negative regulator of wingless expression. Analyses of expression of the same genes in mutant N embryos indicate that the pattern of gene regulations observed in vitro reflects regulations in vivo. These results suggest that the strong genetic interactions observed between Notch and wingless genes during development of Drosophila is at least partly due to regulation of expression of cuticle patterning genes by Wingless and the two forms of Notch.

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

Dominant mutations have served as invaluable tools for Drosophila geneticists. Here we analyze the dominant eye mutation Glazed (Gla) that was described by T. H. Morgan more than 50 years ago. We show that Gla causes the loss of photoreceptor cells during pupal stages, in a process reminiscent of apoptosis, with a concomitant overproduction of eye pigment. This phenotype is very similar to that caused by the loss of D-APC, a negative regulator of Wingless (Wg) signal transduction. Genetic analyses reveal however that the Gla gain-of-function phenotype can be reverted to wild-type. By generating a P-element-induced revertant of Gla we demonstrate that Gla is allelic to wg. The molecular lesion in Gla indicates that the insertion of a roo retrotransposon leads to ectopic expression of wg during pupal stages. We show that the Gla phenotype is similar to that caused by ectopic expression of Wg driven by the sevenless (sev) enhancer. In both cases Wg exerts its effect, at least in part, by negatively regulating the expression of the Pax2 homolog sparkling (spa). Gla represents not only the first dominant allele of wg, but it may also be the first allele ever described for wg.

Mechanisms of Wnt signaling in development

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

Interactions between Wingless and DFz2 during Drosophila wing development

Drosophila Wingless (Wg) is a secreted signaling protein of the Wnt family. Mutations in the wg gene disrupt the patterning of embryonic segments and their adult derivatives. Wg protein has been shown in cell culture to functionally interact with DFz2, a receptor that is structurally related to the tissue polarity protein Frizzled (Fz). However, it has not been determined if DFz2 functions in the Wg signaling pathway during fly development. Here we demonstrate that overexpression of DFz2 increases Wg-dependent signaling to induce ectopic margin bristle formation in developing Drosophila wings. Overexpression of a truncated form of DFz2 acts in a dominant-negative manner to block Wg signaling at the wing margin, and this block is rescued by co-expression of full-length DFz2 but not full-length Fz. Our results suggest that DFz2 and not Fz acts in the Wg signaling pathway for wing margin development. However, a truncated form of Fz also blocks Wg signaling in embryo and wing margin development, and the truncated form of DFz2 affects ommatidial polarity during eye development. These observations suggest that a single dominant-negative form of Fz or DFz2 can block more than one type of Wnt signaling pathway and imply that truncated proteins of the Fz family lose some aspect of signaling specificity.

Local induction of patterning and programmed cell death in the developing Drosophila retina

Local cell signaling can pattern the nervous system by directing cell fates, including programmed cell death. In the developing Drosophila retina, programmed cell death is used to remove excess cells between ommatidia. Cell ablation revealed the source and position of signals required for regulating the pattern of programmed cell death among these interommatidial cells. Two types of signals regulate this patterning event. Notch-mediated signals between interommatidial precursors result in removal of unneeded cells. Cone cells and primary pigment cells oppose this signal by supplying a ‘life’-promoting activity; evidence is provided that this signal occurs through localized activation of the EGF Receptor/Ras pathway. Together, these signals refine the highly regular pattern observed in the adult retina.

Wingless repression of Drosophila frizzled 2 expression shapes the Wingless morphogen gradient in the wing

In Drosophila wing imaginal discs, the Wingless (Wg) protein acts as a morphogen, emanating from the dorsal/ventral (D/V) boundary of the disc to directly define cell identities along the D/V axis at short and long range. Here, we show that high levels of a Wg receptor, Drosophila frizzled 2 (Dfz2), stabilize Wg, allowing it to reach cells far from its site of synthesis. Wg signaling represses Dfz2 expression, creating a gradient of decreasing Wg stability moving toward the D/V boundary. This repression of Dfz2 is crucial for the normal shape of Wg morphogen gradient as well as the response of cells to the Wg signal. In contrast to other ligand-receptor relationships where the receptor limits diffusion of the ligand, Dfz2 broadens the range of Wg action by protecting it from degradation.

Secreted forms of DELTA and SERRATE define antagonists of Notch signaling in Drosophila

We examined the function of secreted forms of the two known Drosophila Notch ligands, DELTA and SERRATE, by expressing them under various promoters in the Drosophila developing eye and wing. The phenotypes associated with the expression of secreted Delta (DlS) or secreted Serrate (SerS) forms mimic loss-of-function mutations in the Notch pathway. Both genetic interactions between DlS or SerS transgenics and duplications or loss-of-function mutations of Delta or Serrate indicate that DlS and SerS behave as dominant negative mutations. These observations were extended to the molecular level by demonstrating that the expression of Enhancer of split mdelta, a target of Notch signaling, is down-regulated by SERS. The antagonistic nature of the two mutant secreted ligand forms in the eye is consistent with their behavior in the wing, where they are capable of down-regulating wing margin specific genes opposite to the effects of the endogenous ligands. This analysis uncovers secreted molecular antagonists of Notch signaling and provides evidence of qualitative differences in the actions of the two ligands DLS and SERS.

pangolin encodes a Lef-1 homologue that acts downstream of Armadillo to transduce the Wingless signal in Drosophila

Members of the Wnt/Wingless (Wg) family of signalling proteins organize many aspects of animal development by regulating the expression of particular target genes in responding cells. Recent biochemical studies indicate that the vertebrate HMG-domain proteins Lef-1 and XTcf-3 can physically interact with beta-catenin, a homologue of Drosophila Armadillo (Arm), the most downstream component known in the Wnt signal transduction pathway. However, these studies do not address whether the endogenous Lef/Tcf family members are required in vivo to transduce Wnt signals. Using genetic methods in Drosophila, we define a new segment polarity gene, pangolin (pan), and show that its product is required in vivo for Wg signal transduction in embryos and in developing adult tissues. In addition, we show that pan encodes a Lef/Tcf homologue and provide evidence that its protein product binds to the beta-catenin homologue Armadillo in vivo. Finally, we demonstrate that Pan functions downstream of Arm to transduce the Wg signal. Thus, our results indicate that Pan is an essential component of the Wg transduction pathway and suggest that it acts directly to regulate gene transcription in response to Wg signalling.