Notch signalling coordinates tissue growth and wing fate specification in Drosophila

The embryonic role of juvenile hormone in the firebrat, Thermobia domestica, reveals its function before its involvement in metamorphosis

To gain insights into how juvenile hormone (JH) came to regulate insect metamorphosis, we studied its function in the ametabolous firebrat, Thermobia domestica. Highest levels of JH occur during late embryogenesis, with only low levels thereafter. Loss-of-function and gain-of-function experiments show that JH acts on embryonic tissues to suppress morphogenesis and cell determination and to promote their terminal differentiation. Similar embryonic actions of JH on hemimetabolous insects with short germ band embryos indicate that JH's embryonic role preceded its derived function as the postembryonic regulator of metamorphosis. The postembryonic expansion of JH function likely followed the evolution of flight. Archaic flying insects were considered to lack metamorphosis because tiny, movable wings were evident on the thoraces of young juveniles and their positive allometric growth eventually allowed them to support flight in late juveniles. Like in Thermobia, we assume that these juveniles lacked JH. However, a postembryonic reappearance of JH during wing morphogenesis in the young juvenile likely redirected wing development to make a wing pad rather than a wing. Maintenance of JH then allowed wing pad growth and its disappearance in the mature juvenile then allowed wing differentiation. Subsequent modification of JH action for hemi- and holometabolous lifestyles are discussed.

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.

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.

Use of FLP/FRT System to Screen for Notch Signaling Regulators in the Drosophila Wing

Mutations of genes encoding key components of the Notch signaling pathways often result in lethality at early developmental stages, making it difficult to decipher how they regulate the formation of specific cell types or organs. Mosaic analysis using the FLP/FRT system allows investigating the roles of essential genes during wing development in Drosophila melanogaster. This chapter describes the practical methods to isolate Notch signaling regulators by somatic mosaic screen. The fly stocks, cross schemes, and screen parameters are summarized. We also explain how to validate the roles of potential Notch signaling regulators.

Cell proliferation control by Notch signalling during imaginal discs development in Drosophila

The Notch signalling pathway is evolutionary conserved and participates in numerous developmental processes, including the control of cell proliferation. However, Notch signalling can promote or restrain cell division depending on the developmental context, as has been observed in human cancer where Notch can function as a tumor suppressor or an oncogene. Thus, the outcome of Notch signalling can be influenced by the cross-talk between Notch and other signalling pathways. The use of model organisms such as Drosophila has been proven to be very valuable to understand the developmental role of the Notch pathway in different tissues and its relationship with other signalling pathways during cell proliferation control. Here we review recent studies in Drosophila that shed light in the developmental control of cell proliferation by the Notch pathway in different contexts such as the eye, wing and leg imaginal discs. We also discuss the autonomous and non-autonomous effects of the Notch pathway on cell proliferation and its interactions with different signalling pathways.

Interplay between sex determination cascade and major signaling pathways during Drosophila eye development: Perspectives for future research

Understanding molecular mechanisms of sexually dimorphic organ growth is a fundamental problem of developmental biology. Recent quantitative studies showed that the Drosophila compound eye is a convenient model to study the determination of the final organ size. In Drosophila, females have larger eyes than males and this is evident even after correction for the larger body size. Moreover, female eyes include more ommatidia (photosensitive units) than male eyes and this difference is specified at the third larval instar in the eye primordia called eye imaginal discs. This may result in different visual capabilities between the two sexes and have behavioral consequences. Despite growing evidence on the genetic bases of eye size variation between different Drosophila species and strains, mechanisms responsible for within-species sexual dimorphism still remain elusive. Here, we discuss a presumptive crosstalk between the sex determination cascade and major signaling pathways during dimorphic eye development. Male- and female-specific isoforms of Doublesex (Dsx) protein are known to control sex-specific differentiation in the somatic tissues. However, no data on Dsx function during eye disc growth and patterning are currently available. Remarkably, Sex lethal (Sxl), the sex determination switch protein, was shown to directly affect Hedgehog (Hh) and Notch (N) signaling in the Drosophila wing disc. The similarity of signaling pathways involved in the wing and eye disc growth suggests that Sxl might be integrated into regulation of eye development. Dsx role in the eye disc requires further investigation. We discuss currently available data on sex-biased gene expression in the Drosophila eye and highlight perspectives for future studies.

Specification and Patterning of Drosophila Appendages

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophilamelanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.

JAK/STAT controls organ size and fate specification by regulating morphogen production and signalling

A stable pool of morphogen-producing cells is critical for the development of any organ or tissue. Here we present evidence that JAK/STAT signalling in the Drosophila wing promotes the cycling and survival of Hedgehog-producing cells, thereby allowing the stable localization of the nearby BMP/Dpp-organizing centre in the developing wing appendage. We identify the inhibitor of apoptosis dIAP1 and Cyclin A as two critical genes regulated by JAK/STAT and contributing to the growth of the Hedgehog-expressing cell population. We also unravel an early role of JAK/STAT in guaranteeing Wingless-mediated appendage specification, and a later one in restricting the Dpp-organizing activity to the appendage itself. These results unveil a fundamental role of the conserved JAK/STAT pathway in limb specification and growth by regulating morphogen production and signalling, and a function of pro-survival cues and mitogenic signals in the regulation of the pool of morphogen-producing cells in a developing organ.

The Homeodomain Iroquois Proteins Control Cell Cycle Progression and Regulate the Size of Developmental Fields

During development, proper differentiation and final organ size rely on the control of territorial specification and cell proliferation. Although many regulators of these processes have been identified, how both are coordinated remains largely unknown. The homeodomain Iroquois/Irx proteins play a key, evolutionarily conserved, role in territorial specification. Here we show that in the imaginal discs, reduced function of Iroquois genes promotes cell proliferation by accelerating the G1 to S transition. Conversely, their increased expression causes cell-cycle arrest, down-regulating the activity of the Cyclin E/Cdk2 complex. We demonstrate that physical interaction of the Iroquois protein Caupolican with Cyclin E-containing protein complexes, through its IRO box and Cyclin-binding domains, underlies its activity in cell-cycle control. Thus, Drosophila Iroquois proteins are able to regulate cell-autonomously the growth of the territories they specify. Moreover, our results provide a molecular mechanism for a role of Iroquois/Irx genes as tumour suppressors.

The correct development of body organs, with their characteristic size and shape, requires the coordination of cell division and cell differentiation. Here we show that the Iroquois proteins (Irx in vertebrates) slow down cell division in the Drosophila imaginal discs, in addition to their well-known role in cell fate and territorial specification. In humans, inactivating mutations at the Irx genes are associated to several types of cancer, thus allowing their classification as tumour suppressor genes. We have observed that Drosophila Iroquois genes similarly behave as tumour suppressor genes. Iroquois proteins belong to a family of homeodomain-containing transcriptional regulators. However, our results indicate that they control cell division by a transcription independent mechanism based on their physical interaction with Cyclin E containing complexes, a key player in cell-cycle progression. We have identified two evolutionary conserved domains of Iroquois proteins, different from the homeodomain, involved in that interaction. This new function of Iroquois proteins places them in a key position to coordinate growth and differentiation during normal development. Our results further suggest a molecular mechanism for their role in tumour suppression. Future studies of Irx genes should help to determine if a similar mechanism could operate to help cancer progression when Irx activity is compromised.

Drosophila p53 controls Notch expression and balances apoptosis and proliferation

A balance between cell proliferation and apoptosis is important for normal development and tissue homeostasis. Under stress conditions, the conserved tumor suppressor and transcription factor Dp53 induces apoptosis to contribute to the maintenance of homeostasis. However, in some cases Dp53-induced apoptosis results in the proliferation of surrounding non-apoptotic cells. To gain insight into the Dp53 function in the control of apoptosis and proliferation, we studied the interaction between the Drosophila Dp53 and Notch genes. We present evidence that simultaneous reduction of Dp53 and Notch function synergistically increases the wing phenotype of Notch heterozygous mutant flies. Further, we found that a Notch cis-regulatory element is responsive to loss and gain of Dp53 function and that over-expression of Dp53 up-regulates Notch mRNA and protein expression. These findings suggest not only that Dp53 and Notch act together to control wing development but also indicate that Dp53 transcriptionally regulates Notch expression. Moreover, using Notch  gain and loss of function mutations we examined the relevance of Dp53 and Notch interactions in the process of Dp53-apoptosis induced proliferation. Results show that proliferation induced by Dp53 over-expression is dependent on Notch, thus identifying Notch as a new player in Dp53-induced proliferation. Interestingly, we found that Dp53-induced Notch activation and proliferation occurs even under conditions where apoptosis was inhibited. Our findings highlight the conservation between flies and vertebrates of the Dp53 and Notch cross-talk and suggest that Dp53 has a dual role regulating cell death and proliferation gene networks to control the homeostatic balance between apoptosis and proliferation.

De novo transcriptome analysis of wing development-related signaling pathways in Locusta migratoria manilensis and Ostrinia furnacalis (Guenée)

Background

Orthopteran migratory locust, Locusta migratoria, and lepidopteran Asian corn borer, Ostrinia furnacalis, are two types of insects undergoing incomplete and complete metamorphosis, respectively. Identification of candidate genes regulating wing development in these two insects would provide insights into the further study about the molecular mechanisms controlling metamorphosis development. We have sequenced the transcriptome of O. furnacalis larvae previously. Here we sequenced and characterized the transcriptome of L. migratoria wing discs with special emphasis on wing development-related signaling pathways.

Methodology/Principal Findings

Illumina Hiseq2000 was used to sequence 8.38 Gb of the transcriptome from dissected nymphal wing discs. De novo assembly generated 91,907 unigenes with mean length of 610 nt. All unigenes were searched against five databases including Nt, Nr, Swiss-Prot, COG, and KEGG for annotations using blastn or blastx algorithm with an cut-off E-value of 10⁻⁵. A total of 23,359 (25.4%) unigenes have homologs within at least one database. Based on sequence similarity to homologs known to regulate Drosophila melanogaster wing development, we identified 50 and 46 potential wing development-related unigenes from L. migratoria and O. furnacalis transcriptome, respectively. The identified unigenes encode putative orthologs for nearly all components of the Hedgehog (Hh), Decapentaplegic (Dpp), Notch (N), and Wingless (Wg) signaling pathways, which are essential for growth and pattern formation during wing development. We investigated the expression profiles of the component genes involved in these signaling pathways in forewings and hind wings of L. migratoria and O. furnacalis. The results revealed the tested genes had different expression patterns in two insects.

Conclusions/Significance

This study provides the comprehensive sequence resource of the wing development-related signaling pathways of L. migratoria. The obtained data gives an insight into better understanding the molecular mechanisms involved in the wing development in L. migratoria and O. furnacalis, two insect species with different metamorphosis types.

Notch inhibits Yorkie activity in Drosophila wing discs

During development, tissues and organs must coordinate growth and patterning so they reach the right size and shape. During larval stages, a dramatic increase in size and cell number of Drosophila wing imaginal discs is controlled by the action of several signaling pathways. Complex cross-talk between these pathways also pattern these discs to specify different regions with different fates and growth potentials. We show that the Notch signaling pathway is both required and sufficient to inhibit the activity of Yorkie (Yki), the Salvador/Warts/Hippo (SWH) pathway terminal transcription activator, but only in the central regions of the wing disc, where the TEAD factor and Yki partner Scalloped (Sd) is expressed. We show that this cross-talk between the Notch and SWH pathways is mediated, at least in part, by the Notch target and Sd partner Vestigial (Vg). We propose that, by altering the ratios between Yki, Sd and Vg, Notch pathway activation restricts the effects of Yki mediated transcription, therefore contributing to define a zone of low proliferation in the central wing discs.

Dpp-induced Egfr signaling triggers postembryonic wing development in Drosophila

The acquisition of flight contributed to the success of insects and winged forms are present in most orders. Key to understanding the origin of wings will be knowledge of the earliest postembryonic events promoting wing outgrowth. The Drosophila melanogaster wing is intensely studied as a model appendage, and yet little is known about the beginning of wing outgrowth. Vein (Vn) is a neuregulin-like ligand for the EGF receptor (Egfr), which is necessary for global development of the early Drosophila wing disc. vn is not expressed in the embryonic wing primordium and thus has to be induced de novo in the nascent larval wing disc. We find that Decapentaplegic (Dpp), a Bone Morphogenetic Protein (BMP) family member, provides the instructive signal for initiating vn expression. The signaling involves paracrine communication between two epithelia in the early disc. Once initiated, vn expression is amplified and maintained by autocrine signaling mediated by the E-twenty six (ETS)-factor PointedP2 (PntP2). This interplay of paracrine and autocrine signaling underlies the spatial and temporal pattern of induction of Vn/Egfr target genes and explains both body wall development and wing outgrowth. It is possible this gene regulatory network governing expression of an EGF ligand is conserved and reflects a common origin of insect wings.

New perspectives on pharyngeal dorsoventral patterning in development and evolution of the vertebrate jaw

Patterning of the vertebrate facial skeleton involves the progressive partitioning of neural-crest-derived skeletal precursors into distinct subpopulations along the anteroposterior (AP) and dorsoventral (DV) axes. Recent evidence suggests that complex interactions between multiple signaling pathways, in particular Endothelin-1 (Edn1), Bone Morphogenetic Protein (BMP), and Jagged-Notch, are needed to pattern skeletal precursors along the DV axis. Rather than directly determining the morphology of individual skeletal elements, these signals appear to act through several families of transcription factors, including Dlx, Msx, and Hand, to establish dynamic zones of skeletal differentiation. Provocatively, this patterning mechanism is largely conserved from mouse and zebrafish to the jawless vertebrate, lamprey. This implies that the diversification of the vertebrate facial skeleton, including the evolution of the jaw, was driven largely by modifications downstream of a conversed pharyngeal DV patterning program.

The roles of cell size and cell number in determining ovariole number in Drosophila

All insect ovaries are composed of functional units called ovarioles, which contain sequentially developing egg chambers. The number of ovarioles varies between and within species. Ovariole number is an important determinant of fecundity and thus affects individual fitness. Although Drosophila oogenesis has been intensively studied, the genetic and cellular basis for determination of ovariole number remains unknown. Ovariole formation begins during larval development with the morphogenesis of terminal filament cells (TFCs) into stacks called terminal filaments (TFs). We induced changes in ovariole number in Drosophila melanogaster by genetically altering cell size and cell number in the TFC population, and analyzed TF morphogenesis in these ovaries to understand the cellular basis for the changes in ovariole number. Increasing TFC size contributed to higher ovariole number by increasing TF number. Similarly, increasing total TFC number led to higher ovariole number via an increase in TF number. By analyzing ovarian morphogenesis in another Drosophila species we showed that TFC number regulation is a target of evolutionary change that affects ovariole number. In contrast, temperature-dependent plasticity in ovariole number was due to changes in cell-cell sorting during TF morphogenesis, rather than changes in cell size or cell number. We have thus identified two distinct developmental processes that regulate ovariole number: establishment of total TFC number, and TFC sorting during TF morphogenesis. Our data suggest that the genetic changes underlying species-specific ovariole number may alter the total number of TFCs available to contribute to TF formation. This work provides for the first time specific and quantitative developmental tools to investigate the evolution of a highly conserved reproductive structure.

The interplay between morphogens and tissue growth

Morphogens are conserved, secreted signalling molecules that regulate the size, shape and patterning of animal tissues and organs. Recent experimental evidence has emphasized the fundamental role of tissue growth in expanding the expression domains of morphogens and their target genes, in generating morphogen gradients and in modulating the response of cells to morphogens. Moreover, the classic view of how morphogens, particularly through their concentration gradient, regulate tissue size during development has been revisited recently. In this review, we discuss how morphogens and tissue growth affect each other, and we attempt to integrate genetic and molecular evidence from vertebrate and invertebrate model systems to put forward the idea that the interaction between growth and morphogens is a general feature of highly proliferative tissues.

The miRNA machinery targets Mei-P26 and regulates Myc protein levels in the Drosophila wing

MicroRNAs (miRNAs) have been implicated in cell-cycle regulation and in some cases shown to have a role in tissue growth control. Depletion of miRNAs was found to have an effect on tissue growth rates in the wing primordium of Drosophila, a highly proliferative epithelium. Dicer-1 (Dcr-1) is a double-stranded RNAseIII essential for miRNA biogenesis. Adult cells lacking dcr-1, or with reduced dcr-1 activity, were smaller than normal cells and gave rise to smaller wings. dcr-1 mutant cells showed evidence of being susceptible to competition by faster growing cells in vivo and the miRNA machinery was shown to promote G(1)-S transition. We present evidence that Dcr-1 acts by regulating the TRIM-NHL protein Mei-P26, which in turn regulates dMyc protein levels. Mei-P26 is a direct target of miRNAs, including the growth-promoting bantam miRNA. Thus, regulation of tissue growth by the miRNA pathway involves a double repression mechanism to control dMyc protein levels in a highly proliferative and growing epithelium.

Hrp48 attenuates Sxl expression to allow for proper notch expression and signaling in wing development

Different signaling pathways are deployed in specific developmental contexts to generate sexually dimorphic traits. Recently, Sex-lethal (Sxl), the female determinant in Drosophila melanogaster, was shown to down-regulate Notch (N) signaling to accomplish sex-specific patterning. Paradoxically, however, both Sxl and N are ubiquitously expressed in all of the female cells. This raises a key question as to how, during monomorphic female development, N signaling escapes the negative impact of Sxl. Here, we uncover a regulatory loop involving Hrp48, an abundant Drosophila hnRNP, Sxl and N. Phenotypic consequences of the partial loss of hrp48 resemble that of N but are more pronounced in females than in males. Likewise, N levels are drastically diminished only in females. Interestingly, monomorphic female tissues including wing, eye and antennal discs display considerable increase in Sxl amounts. Finally, female-specific attenuation of N signaling is rescued upon simultaneous removal of Sxl. Thus, our data demonstrate that in monomorphic contexts, Hrp48 functions as a moderator of Sxl expression to achieve adequate levels of N receptor production and signaling. We propose that it is critical to modulate the activities of the master determinant underling sexual dimorphism, to ensure that it does not function inappropriately in monomorphic tissues and disrupt their development.

Expression profiling of prospero in the Drosophila larval chemosensory organ: Between growth and outgrowth

Background

The antenno-maxilary complex (AMC) forms the chemosensory system of the Drosophila larva and is involved in gustatory and olfactory perception. We have previously shown that a mutant allele of the homeodomain transcription factor Prospero (prosVoila1, V1), presents several developmental defects including abnormal growth and altered taste responses. In addition, many neural tracts connecting the AMC to the central nervous system (CNS) were affected. Our earlier reports on larval AMC did not argue in favour of a role of pros in cell fate decision, but strongly suggested that pros could be involved in the control of other aspect of neuronal development. In order to identify these functions, we used microarray analysis of larval AMC and CNS tissue isolated from the wild type, and three other previously characterised prospero alleles, including the V1 mutant, considered as a null allele for the AMC.

Results

A total of 17 samples were first analysed with hierarchical clustering. To determine those genes affected by loss of pros function, we calculated a discriminating score reflecting the differential expression between V1 mutant and other pros alleles. We identified a total of 64 genes in the AMC. Additional manual annotation using all the computed information on the attributed role of these genes in the Drosophila larvae nervous system, enabled us to identify one functional category of potential Prospero target genes known to be involved in neurite outgrowth, synaptic transmission and more specifically in neuronal connectivity remodelling. The second category of genes found to be differentially expressed between the null mutant AMC and the other alleles concerned the development of the sensory organs and more particularly the larval olfactory system. Surprisingly, a third category emerged from our analyses and suggests an association of pros with the genes that regulate autophagy, growth and insulin pathways. Interestingly, EGFR and Notch pathways were represented in all of these three functional categories. We now propose that Pros could perform all of these different functions through the modulation of these two antagonistic and synergic pathways.

Conclusions

The current data contribute to the clarification of the prospero function in the larval AMC and show that pros regulates different function in larvae as compared to those controlled by this gene in embryos. In the future, the possible mechanism by which Pros could achieve its function in the AMC will be explored in detail.