Border of Notch activity establishes a boundary between the two dorsal appendage tube cell types

Non-cell autonomous regulation of cell-cell signaling and differentiation by mitochondrial ROS

Mitochondrial reactive oxygen species (ROS) function intrinsically within cells to induce cell damage, regulate transcription, and cause genome instability. However, we know little about how mitochondrial ROS production non-cell autonomously impacts cell-cell signaling. Here, we show that mitochondrial dysfunction inhibits the plasma membrane localization of cell surface receptors that drive cell-cell communication during oogenesis. Within minutes, we found that mitochondrial ROS impairs exocyst membrane binding and leads to defective endosomal recycling. This endosomal defect impairs the trafficking of receptors, such as the Notch ligand Delta, during oogenesis. Remarkably, we found that overexpressing RAB11 restores ligand trafficking and rescues the developmental defects caused by ROS production. ROS production from adjacent cells acutely initiates a transcriptional response associated with growth and migration by suppressing Notch signaling and inducing extra cellualr matrix (ECM) remodeling. Our work reveals a conserved rapid response to ROS production that links mitochondrial dysfunction to the non-cell autonomous regulation of cell-cell signaling.

Maternal Spargel/dPGC-1 is critical for embryonic development and influences chorion gene amplification via Cyclin E activity

The function of spargel/dPGC-1 in Drosophila oogenesis has been unequivocally established. Here, we sought to assess whether Spargel protein or RNA is essential for developmentally competent eggs. The trans-heterozygotic combination of two spargel mutant alleles allowed us to decrease Spargel expression to very low levels. Using this model, we now demonstrated the requirement for Spargel in eggshell patterning and embryonic development, which led us to establish that spargel is a maternal effect gene. Further examination of Spargel's potential mechanism of action in eggshell biogenesis revealed that low levels of Spargel in the adult ovary cause diminished Cyclin E activity, resulting in reduced chorion gene amplification levels, leading to eggshell biogenesis defects. Thus, another novel role for spargel/dPGC-1 is exposed whereby, through Cyclin E activity, this conserved transcriptional coactivator regulates the chorion gene amplification process.

Imaginal disk growth factors are Drosophila chitinase-like proteins with roles in morphogenesis and CO2 response

Chitinase-like proteins (CLPs) are members of the family 18 glycosyl hydrolases, which include chitinases and the enzymatically inactive CLPs. A mutation in the enzyme's catalytic site, conserved in vertebrates and invertebrates, allowed CLPs to evolve independently with functions that do not require chitinase activity. CLPs normally function during inflammatory responses, wound healing, and host defense, but when they persist at excessive levels at sites of chronic inflammation and in tissue-remodeling disorders, they correlate positively with disease progression and poor prognosis. Little is known, however, about their physiological function. Drosophila melanogaster has 6 CLPs, termed Imaginal disk growth factors (Idgfs), encoded by Idgf1, Idgf2, Idgf3, Idgf4, Idgf5, and Idgf6. In this study, we developed tools to facilitate characterization of the physiological roles of the Idgfs by deleting each of the Idgf genes using the CRISPR/Cas9 system and assessing loss-of-function phenotypes. Using null lines, we showed that loss of function for all 6 Idgf proteins significantly lowers viability and fertility. We also showed that Idgfs play roles in epithelial morphogenesis, maintaining proper epithelial architecture and cell shape, regulating E-cadherin and cortical actin, and remarkably, protecting these tissues against CO2 exposure. Defining the normal molecular mechanisms of CLPs is a key to understanding how deviations tip the balance from a physiological to a pathological state.

Lessons from Drosophila: Engineering Genetic Sexing Strains with Temperature-Sensitive Lethality for Sterile Insect Technique Applications

A major obstacle of sterile insect technique (SIT) programs is the availability of robust sex-separation systems for conditional removal of females. Sterilized male-only releases improve SIT efficiency and cost-effectiveness for agricultural pests, whereas it is critical to remove female disease-vector pests prior to release as they maintain the capacity to transmit disease. Some of the most successful Genetic Sexing Strains (GSS) reared and released for SIT control were developed for Mediterranean fruit fly (Medfly), Ceratitis capitata, and carry a temperature sensitive lethal (tsl) mutation that eliminates female but not male embryos when heat treated. The Medfly tsl mutation was generated by random mutagenesis and the genetic mechanism causing this valuable heat sensitive phenotype remains unknown. Conditional temperature sensitive lethal mutations have also been developed using random mutagenesis in the insect model, Drosophila melanogaster, and were used for some of the founding genetic research published in the fields of neuro- and developmental biology. Here we review mutations in select D. melanogaster genes shibire, Notch, RNA polymerase II 215kDa, pale, transformer-2, Dsor1 and CK2α that cause temperature sensitive phenotypes. Precise introduction of orthologous point mutations in pest insect species with CRISPR/Cas9 genome editing technology holds potential to establish GSSs with embryonic lethality to improve and advance SIT pest control.

Broad-complex transcription factor mediates opposing hormonal regulation of two phylogenetically distant arginine kinase genes in Tribolium castaneum

The phosphoarginine-arginine kinase shuttle system plays a critical role in maintaining insect cellular energy homeostasis. Insect molting and metamorphosis are coordinated by fluctuations of the ecdysteroid and juvenile hormone. However, the hormonal regulation of insect arginine kinases remain largely elusive. In this report, we comparatively characterized two arginine kinase genes, TcAK1 and TcAK2, in Tribolium castaneum. Functional analysis using RNAi showed that TcAK1 and TcAK2 play similar roles in adult fertility and stress response. TcAK1 was detected in cytoplasm including mitochondria, whereas TcAK2 was detected in cytoplasm excluding mitochondria. Interestingly, TcAK1 expression was negatively regulated by 20-hydroxyecdysone and positively by juvenile hormone, whereas TcAK2 was regulated by the opposite pattern. RNAi, dual-luciferase reporter assays and electrophoretic mobility shift assay further revealed that the opposite hormonal regulation of TcAK1 and TcAK2 was mediated by transcription factor Broad-Complex. Finally, relatively stable AK activities were observed during larval-pupal metamorphosis, which was generally consistent with the constant ATP levels. These results provide new insights into the mechanisms underlying the ATP homeostasis in insects by revealing opposite hormonal regulation of two phylogenetically distant arginine kinase genes.

Zhang et al. characterize the functions of two distinct arginine kinase genes in flour beetles. Using RNA interference and electophoretic mobility shift assays, they identify Broad-Complex transcription factor as the mediator of opposing hormonal regulation in these genes.

Gene regulation during Drosophila eggshell patterning

A common path to the formation of complex 3D structures starts with an epithelial sheet that is patterned by inductive cues that control the spatiotemporal activities of transcription factors. These activities are then interpreted by the cis-regulatory regions of the genes involved in cell differentiation and tissue morphogenesis. Although this general strategy has been documented in multiple developmental contexts, the range of experimental models in which each of the steps can be examined in detail and evaluated in its effect on the final structure remains very limited. Studies of the Drosophila eggshell patterning provide unique insights into the multiscale mechanisms that connect gene regulation and 3D epithelial morphogenesis. Here we review the current understanding of this system, emphasizing how the recent identification of cis-regulatory regions of genes within the eggshell patterning network enables mechanistic analysis of its spatiotemporal dynamics and evolutionary diversification. It appears that cis-regulatory changes can account for only some aspects of the morphological diversity of Drosophila eggshells, such as the prominent differences in the number of the respiratory dorsal appendages. Other changes, such as the appearance of the respiratory eggshell ridges, are caused by changes in the spatial distribution of inductive signals. Both types of mechanisms are at play in this rapidly evolving system, which provides an excellent model of developmental patterning and morphogenesis.

Epithelial Patterning, Morphogenesis, and Evolution: Drosophila Eggshell as a Model

Understanding the mechanisms driving tissue and organ formation requires knowledge across scales. How do signaling pathways specify distinct tissue types? How does the patterning system control morphogenesis? How do these processes evolve? The Drosophila egg chamber, where EGF and BMP signaling intersect to specify unique cell types that construct epithelial tubes for specialized eggshell structures, has provided a tractable system to ask these questions. Work there has elucidated connections between scales of development, including across evolutionary scales, and fostered the development of quantitative modeling tools. These tools and general principles can be applied to the understanding of other developmental processes across organisms.

Simple Expression Domains Are Regulated by Discrete CRMs During Drosophila Oogenesis

Eggshell patterning has been extensively studied in Drosophila melanogaster. However, the cis-regulatory modules (CRMs), which control spatiotemporal expression of these patterns, are vastly unexplored. The FlyLight collection contains >7000 intergenic and intronic DNA fragments that, if containing CRMs, can drive the transcription factor GAL4. We cross-listed the 84 genes known to be expressed during D. melanogaster oogenesis with the ∼1200 listed genes of the FlyLight collection, and found 22 common genes that are represented by 281 FlyLight fly lines. Of these lines, 54 show expression patterns during oogenesis when crossed to an UAS-GFP reporter. Of the 54 lines, 16 recapitulate the full or partial pattern of the associated gene pattern. Interestingly, while the average DNA fragment size is ∼3 kb in length, the vast majority of fragments show one type of spatiotemporal pattern in oogenesis. Mapping the distribution of all 54 lines, we found a significant enrichment of CRMs in the first intron of the associated genes’ model. In addition, we demonstrate the use of different anteriorly active FlyLight lines as tools to disrupt eggshell patterning in a targeted manner. Our screen provides further evidence that complex gene patterns are assembled combinatorially by different CRMs controlling the expression of genes in simple domains.

The repertoire of epithelial morphogenesis on display: Progressive elaboration of Drosophila egg structure

Epithelial structures are foundational for tissue organization in all metazoans. Sheets of epithelial cells form lateral adhesive junctions and acquire apico-basal polarity perpendicular to the surface of the sheet. Genetic analyses in the insect model, Drosophila melanogaster, have greatly advanced our understanding of how epithelial organization is established, and how it is modulated during tissue morphogenesis. Major insights into collective cell migrations have come from analyses of morphogenetic movements within the adult follicular epithelium that cooperates with female germ cells to build a mature egg. Epithelial follicle cells progress through tightly choreographed phases of proliferation, patterning, reorganization and migrations, before they differentiate to form the elaborate structures of the eggshell. Distinct structural domains are organized by differential adhesion, within which lateral junctions are remodeled to further shape the organized epithelia. During collective cell migrations, adhesive interactions mediate supracellular organization of planar polarized macromolecules, and facilitate crawling over the basement membrane or traction against adjacent cell surfaces. Comparative studies with other insects are revealing the diversification of morphogenetic movements for elaboration of epithelial structures. This review surveys the repertoire of follicle cell morphogenesis, to highlight the coordination of epithelial plasticity with progressive differentiation of a secretory epithelium. Technological advances will keep this tissue at the leading edge for interrogating the precise spatiotemporal regulation of normal epithelial reorganization events, and provide a framework for understanding pathological tissue dysplasia.

Drosophila Cyclin G Is a Regulator of the Notch Signalling Pathway during Wing Development

Notch signalling regulates a multitude of differentiation processes during Drosophila development. For example, Notch activity is required for proper wing vein differentiation which is hampered in mutants of either the receptor Notch, the ligand Delta or the antagonist Hairless. Moreover, the Notch pathway is involved in several aspects of Drosophila oogenesis as well. We have identified Drosophila Cyclin G (CycG) as a molecular interaction partner of Hairless, the major antagonist in the Notch signalling pathway, in vitro and in vivo. Loss of CycG was shown before to cause female sterility and to disturb the architecture of the egg shell. Nevertheless, Notch dependent processes during oogenesis appeared largely unaffected in cycG mutant egg chambers. Loss of CycG modified the dominant wing phenotypes of Notch, Delta and Hairless mutants. Whereas the Notch loss of function phenotype was ameliorated by a loss of CycG, the phenotypes of either Notch gain of function or of Delta or Hairless loss of function were enhanced. In contrast, loss of CycG had only a minor effect on the wing vein phenotype of mutants affecting the EGFR signalling pathway emphasizing the specificity of the interaction of CycG and Notch pathway members.

Regulation of broad by the Notch pathway affects timing of follicle cell development

During Drosophila oogenesis, activation of Notch signaling in the follicular epithelium (FE) around stage 6 of oogenesis is essential for entry into the endocycle and a series of other changes such as cell differentiation and migration of subsets of the follicle cells. Notch induces the expression of zinc finger protein Hindsight and suppresses homeodomain protein Cut to regulate the mitotic/endocycle (ME) switch. Here we report that broad (br), encoding a small group of zinc-finger transcription factors resulting from alternative splicing, is a transcriptional target of Notch nuclear effector Suppressor of Hairless (Su(H)). The early pattern of Br in the FE, uniformly expressed except in the polar cells, is established by Notch signaling around stage 6, through the binding of Su(H) to the br early enhancer (brE) region. Mutation of the Su(H) binding site leads to a significant reduction of brE reporter expression in follicle cells undergoing the endocycle. Chromatin immunoprecipitation results further confirm Su(H) binding to the br early enhancer. Consistent with its expression in follicle cells during midoogenesis, loss of br function results in a delayed entry into the endocycle. Our findings suggest an important role of br in the timing of follicle cell development, and its transcriptional regulation by the Notch pathway.

A discrete model of Drosophila eggshell patterning reveals cell-autonomous and juxtacrine effects

The Drosophila eggshell constitutes a remarkable system for the study of epithelial patterning, both experimentally and through computational modeling. Dorsal eggshell appendages arise from specific regions in the anterior follicular epithelium that covers the oocyte: two groups of cells expressing broad (roof cells) bordered by rhomboid expressing cells (floor cells). Despite the large number of genes known to participate in defining these domains and the important modeling efforts put into this developmental system, key patterning events still lack a proper mechanistic understanding and/or genetic basis, and the literature appears to conflict on some crucial points. We tackle these issues with an original, discrete framework that considers single-cell models that are integrated to construct epithelial models. We first build a phenomenological model that reproduces wild type follicular epithelial patterns, confirming EGF and BMP signaling input as sufficient to establish the major features of this patterning system within the anterior domain. Importantly, this simple model predicts an instructive juxtacrine signal linking the roof and floor domains. To explore this prediction, we define a mechanistic model that integrates the combined effects of cellular genetic networks, cell communication and network adjustment through developmental events. Moreover, we focus on the anterior competence region, and postulate that early BMP signaling participates with early EGF signaling in its specification. This model accurately simulates wild type pattern formation and is able to reproduce, with unprecedented level of precision and completeness, various published gain-of-function and loss-of-function experiments, including perturbations of the BMP pathway previously seen as conflicting results. The result is a coherent model built upon rules that may be generalized to other epithelia and developmental systems.

Dynamic model for the coordination of two enhancers of broad by EGFR signaling

Although it is widely appreciated that a typical developmental control gene is regulated by multiple enhancers, coordination of enhancer activities remains poorly understood. We propose a mechanism for such coordination in Drosophila oogenesis, when the expression of the transcription factor Broad (BR) evolves from a uniform to a two-domain pattern that prefigures the formation of two respiratory eggshell appendages. This change reflects sequential activities of two enhancers of the br gene, early and late, both of which are controlled by the epidermal growth factor receptor (EGFR) pathway. The late enhancer controls br in the appendage-producing cells, but the function of the early enhancer remained unclear. We found that the early enhancer is essential for the activity of the late enhancer and induction of eggshell appendages. This requirement can be explained by a mechanism whereby the BR protein produced by the early enhancer protects the late enhancer from EGFR-dependent repression. We illustrate this complex mechanism using a computational model that correctly predicts the wild-type dynamics of BR expression and its response to genetic perturbations.

Three-dimensional epithelial morphogenesis in the developing Drosophila egg

Morphogenesis of the respiratory appendages on eggshells of Drosophila species provides a powerful experimental system for studying how cell sheets give rise to complex three-dimensional structures. In Drosophila melanogaster, each of the two tubular eggshell appendages is derived from a primordium comprising two distinct cell types. Using live imaging and three-dimensional image reconstruction, we demonstrate that the transformation of this two-dimensional primordium into a tube involves out-of-plane bending followed by a sequence of spatially ordered cell intercalations. These morphological transformations correlate with the appearance of complementary distributions of myosin and Bazooka in the primordium. These distributions suggest that a two-dimensional pattern of line tensions along cell-cell edges on the apical side of the epithelium is sufficient to produce the observed changes in morphology. Computational modeling shows that this mechanism could explain the main features of tissue deformation and cell rearrangements observed during three-dimensional morphogenesis.

The Drosophila BMPRII, wishful thinking, is required for eggshell patterning

The Drosophila eggshell is an elaborate structure that is derived from a monolayer of follicular epithelium surrounding the developing oocyte within the female ovary. The bone morphogenetic protein (BMP) signaling pathway is essential for controlling the patterning and morphogenesis of the eggshell. During oogenesis, the roles of patterning and morphogenesis by the BMP type I receptor thickveins (tkv) have been studied extensively. However, signaling through this pathway requires both type I and II receptors, and the latter has yet to be established in oogenesis. We focus on wishful thinking (wit), the Drosophila homolog to the mammalian BMP type II receptor, BMPRII. We found that wit is expressed dynamically in the FCs of D. melanogaster in an evolutionary conserved pattern. The expression patterns are highly correlated with the dynamics of the BMP signaling, which is consistent with our finding that wit is a target of BMP signaling. Furthermore, we established that WIT is necessary for BMP signaling, and loss of WIT is associated with cell autonomous loss of BMP responses. Of importance, we demonstrated that perturbations in WIT led to changes in eggshell morphologies in domains that are patterned by BMP signaling. Previous studies have shown a role for WIT in BMP signaling during neurogenesis; however, our results reveal a role for WIT in epithelial cells' development.

The lack of autophagy triggers precocious activation of Notch signaling during Drosophila oogenesis

BACKGROUND

The proper balance of autophagy, a lysosome-mediated degradation process, is indispensable for oogenesis in Drosophila. We recently demonstrated that egg development depends on autophagy in the somatic follicle cells (FC), but not in the germline cells (GCs). However, the lack of autophagy only affects oogenesis when FCs are autophagy-deficient but GCs are wild type, indicating that a dysfunctional signaling between soma and germline may be responsible for the oogenesis defects. Thus, autophagy could play an essential role in modulating signal transduction pathways during egg development.

RESULTS

Here, we provide further evidence for the necessity of autophagy during oogenesis and demonstrate that autophagy is especially required in subsets of FCs. Generation of autophagy-deficient FCs leads to a wide range of phenotypes that are similar to mutants with defects in the classical cell-cell signaling pathways in the ovary. Interestingly, we observe that loss of autophagy leads to a precocious activation of the Notch pathway in the FCs as monitored by the expression of Cut and Hindsight, two downstream effectors of Notch signaling.

CONCLUSION

Our findings point to an unexpected function for autophagy in the modulation of the Notch signaling pathway during Drosophila oogenesis and suggest a function for autophagy in proper receptor activation. Egg development is affected by an imbalance of autophagy between signal sending (germline) and signal receiving cell (FC), thus the lack of autophagy in the germline is likely to decrease the amount of active ligand and accordingly compensates for increased signaling in autophagy-defective follicle cells.

EGFR-dependent network interactions that pattern Drosophila eggshell appendages

Similar to other organisms, Drosophila uses its Epidermal Growth Factor Receptor (EGFR) multiple times throughout development. One crucial EGFR-dependent event is patterning of the follicular epithelium during oogenesis. In addition to providing inductive cues necessary for body axes specification, patterning of the follicle cells initiates the formation of two respiratory eggshell appendages. Each appendage is derived from a primordium comprising a patch of cells expressing broad (br) and an adjacent stripe of cells expressing rhomboid (rho). Several mechanisms of eggshell patterning have been proposed in the past, but none of them can explain the highly coordinated expression of br and rho. To address some of the outstanding issues in this system, we synthesized the existing information into a revised mathematical model of follicle cell patterning. Based on the computational model analysis, we propose that dorsal appendage primordia are established by sequential action of feed-forward loops and juxtacrine signals activated by the gradient of EGFR signaling. The model describes pattern formation in a large number of mutants and points to several unanswered questions related to the dynamic interaction of the EGFR and Notch pathways.

Notch Signaling during Oogenesis in Drosophila melanogaster

The Notch signaling pathway is an evolutionarily conserved intercellular signaling mechanism that is required for embryonic development, cell fate specification, and stem cell maintenance. Discovered and studied initially in Drosophila melanogaster, the Notch pathway is conserved and functionally active throughout the animal kingdom. In this paper, we summarize the biochemical mechanisms of Notch signaling and describe its role in regulating one particular developmental pathway, oogenesis in Drosophila.

Transcriptional interpretation of the EGF receptor signaling gradient

Epidermal growth factor receptor (EGFR) controls a wide range of developmental events, from body axes specification in insects to cardiac development in humans. During Drosophila oogenesis, a gradient of EGFR activation patterns the follicular epithelium. Multiple transcriptional targets of EGFR in this tissue have been identified, but their regulatory elements are essentially unknown. We report the regulatory elements of broad (br) and pipe (pip), two important targets of EGFR signaling in Drosophila oogenesis. br is expressed in a complex pattern that prefigures the formation of respiratory eggshell appendages. We found that this pattern is generated by dynamic activities of two regulatory elements, which display different responses to Pointed, Capicua, and Mirror, transcription factors involved in the EGFR-mediated gene expression. One of these elements is active in a pattern similar to pip, a gene repressed by EGFR and essential for establishing the dorsoventral polarity of the embryo. We demonstrate that this similarity of expression depends on a common sequence motif that binds Mirror in vitro and is essential for transcriptional repression in vivo.

Tramtrack is genetically upstream of genes controlling tracheal tube size in Drosophila

The Drosophila transcription factor Tramtrack (Ttk) is involved in a wide range of developmental decisions, ranging from early embryonic patterning to differentiation processes in organogenesis. Given the wide spectrum of functions and pleiotropic effects that hinder a comprehensive characterisation, many of the tissue specific functions of this transcription factor are only poorly understood. We recently discovered multiple roles of Ttk in the development of the tracheal system on the morphogenetic level. Here, we sought to identify some of the underlying genetic components that are responsible for the tracheal phenotypes of Ttk mutants. We therefore profiled gene expression changes after Ttk loss- and gain-of-function in whole embryos and cell populations enriched for tracheal cells. The analysis of the transcriptomes revealed widespread changes in gene expression. Interestingly, one of the most prominent gene classes that showed significant opposing responses to loss- and gain-of-function was annotated with functions in chitin metabolism, along with additional genes that are linked to cellular responses, which are impaired in ttk mutants. The expression changes of these genes were validated by quantitative real-time PCR and further functional analysis of these candidate genes and other genes also expected to control tracheal tube size revealed at least a partial explanation of Ttk's role in tube size regulation. The computational analysis of our tissue-specific gene expression data highlighted the sensitivity of the approach and revealed an interesting set of novel putatively tracheal genes.

Genesis of two-dimensional patterns in cross-gradient fields

Tissue morphogenesis is controlled by the two-dimensional patterning of gene expression in epithelial layers, that determines cell fates. The mechanisms of pattern formation involve intracellular regulatory networks controlled by paracrine and autocrine signaling. We develop a general logical scheme to deduce the morphology of two-dimensional patterns in the field of two crossed morphogen gradients enriched by the action of autocrine signaling that may subdivide expression domains in nontrivial ways. A variety of persistent patterns, either stationary or oscillatory, are generated using the various interaction schemes, some of which have been generated by a special algorithm including random inputs and selected according to suitable criteria. We give the full classification of a variety of stationary and oscillatory expression patterns in the presence of a single autocrine signal based on logical arguments. These results are further confirmed by numerical computations based on reaction-diffusion equations for morphogens and ligands and the discrete (cell-level) description of intracellular dynamics. Model simulations also elucidate transient processes, in particular interaction schemes. Different internal schemes may lead to identical persistent patterns, although relaxation may proceed in distinct ways. As an illustration of the general method, we test a particular scheme suggested by genetic studies of dynamic gene expression patterns in the follicular epithelium of the Drosophila eggshell.

Finding a niche: studies from the Drosophila ovary

Specialized microenvironments called niches help maintain stem cells in an undifferentiated and self-renewing state. The existence of niches has long been predicted from mammalian studies, but identifying stem cells in their native environments in vivo has remained a challenge in most vertebrates. Many of the mechanistic insights into how niches regulate stem cell maintenance have been obtained using invertebrate models such as Drosophila. Here, we focus on the Drosophila ovarian germline stem cell niche and review recent studies that have begun to reveal how intricate crosstalk between various signaling pathways regulates stem cell maintenance, how the extracellular matrix modulates the signaling output of the niche and how epigenetic programming influences cell development and function both inside and outside the niche to ensure proper tissue homeostasis. These insights will probably inform the study of mammalian niches and how their malfunction contributes to human disease.

The putative Notch ligand HyJagged is a transmembrane protein present in all cell types of adult Hydra and upregulated at the boundary between bud and parent

Background

The Notch signalling pathway is conserved in pre-bilaterian animals. In the Cnidarian Hydra it is involved in interstitial stem cell differentiation and in boundary formation during budding. Experimental evidence suggests that in Hydra Notch is activated by presenilin through proteolytic cleavage at the S3 site as in all animals. However, the endogenous ligand for HvNotch has not been described yet.

Results

We have cloned a cDNA from Hydra, which encodes a bona-fide Notch ligand with a conserved domain structure similar to that of Jagged-like Notch ligands from other animals. Hyjagged mRNA is undetectable in adult Hydra by in situ hybridisation but is strongly upregulated and easily visible at the border between bud and parent shortly before bud detachment. In contrast, HyJagged protein is found in all cell types of an adult hydra, where it localises to membranes and endosomes. Co-localisation experiments showed that it is present in the same cells as HvNotch, however not always in the same membrane structures.

Conclusions

The putative Notch ligand HyJagged is conserved in Cnidarians. Together with HvNotch it may be involved in the formation of the parent-bud boundary in Hydra. Moreover, protein distribution of both, HvNotch receptor and HyJagged indicate a more widespread function for these two transmembrane proteins in the adult hydra, which may be regulated by additional factors, possibly involving endocytic pathways.

BMP signaling dynamics in the follicle cells of multiple Drosophila species

The dorsal anterior region of the follicle cells (FCs) in the developing Drosophila egg gives rise to the respiratory eggshell appendages. These tubular structures display a wide range of qualitative and quantitative variations across Drosophila species, providing a remarkable example of a rapidly evolving morphology. In D. melanogaster, the bone morphogenetic protein (BMP) signaling pathway is an important regulator of FCs patterning and dorsal appendages morphology. To explore the mechanisms underlying the diversification of eggshell patterning, we analyzed BMP signaling in the FCs of 16 Drosophila species that span 45 million years of evolution. We found that the spatial patterns of BMP signaling in the FCs are dynamic and exhibit a range of interspecies' variations. In most of the species examined, the dynamics of BMP signaling correlate with the expression of the type I BMP receptor thickveins (tkv). This correlation suggests that interspecies' variations of tkv expression are responsible for the diversification of BMP signaling during oogenesis. This model was supported by genetic manipulations of tkv expression in the FCs of D. melanogaster that successfully recapitulated the signaling diversities found in the other species. Our results suggest that regulation of receptor expression mediates spatial diversification of BMP signaling in Drosophila oogenesis, and they provide insight into a mechanism underlying the evolution of eggshell patterning.

Division of labor: subsets of dorsal-appendage-forming cells control the shape of the entire tube

The function of an organ relies on its form, which in turn depends on the individual shapes of the cells that create it and the interactions between them. Despite remarkable progress in the field of developmental biology, how cells collaborate to make a tissue remains an unsolved mystery. To investigate the mechanisms that determine organ structure, we are studying the cells that form the dorsal appendages (DAs) of the Drosophila melanogaster eggshell. These cells consist of two differentially patterned subtypes: roof cells, which form the outward-facing roof of the lumen, and floor cells, which dive underneath the roof cells to seal off the floor of the tube. In this paper, we present three lines of evidence that reveal a further stratification of the DA-forming epithelium. Laser ablation of only a few cells in the anterior of the region causes a disproportionately severe shortening of the appendage. Genetic alteration through the twin peaks allele of tramtrack69 (ttk(twk)), a female-sterile mutation that leads to severely shortened DAs, causes no such shortening when removed from a majority of the DA-forming cells, but rather, produces short appendages only when removed from cells in the very anterior of the tube-forming tissue. Additionally we show that heterotrimeric G-protein function is required for DA morphogenesis. Like TTK69, Gbeta 13F is not required in all DA-forming follicle cells but only in the floor and leading roof cells. The different phenotypes that result from removal of Gbeta 13F from each region demonstrate a striking division of function between different DA-forming cells. Gbeta mutant floor cells are unable to control the width of the appendage while Gbeta mutant leading roof cells fail to direct the elongation of the appendage and the convergent-extension of the roof-cell population.

Control in time and space: Tramtrack69 cooperates with Notch and Ecdysone to repress ectopic fate and shape changes during Drosophila egg chamber maturation

Organ morphogenesis requires cooperation between cells, which determine their course of action based upon location within a tissue. Just as important, cells must synchronize their activities, which requires awareness of developmental time. To understand how cells coordinate behaviors in time and space, we analyzed Drosophila egg chamber development. We found that the transcription factor Tramtrack69 (TTK69) controls the fates and shapes of all columnar follicle cells by integrating temporal and spatial information, restricting characteristic changes in morphology and expression that occur at stage 10B to appropriate domains. TTK69 is required again later in oogenesis: it controls the volume of the dorsal-appendage (DA) tubes by promoting apical re-expansion and lateral shortening of DA-forming follicle cells. We show that TTK69 and Notch compete to repress each other's expression and that a local Ecdysone signal is required to shift the balance in favor of TTK69. We hypothesize that TTK69 then cooperates with spatially restricted co-factors to define appropriate responses to a globally available (but as yet unidentified) temporal signal that initiates the S10B transformations.

Temporal patterns of broad isoform expression during the development of neuronal lineages in Drosophila

Background

During the development of the central nervous system (CNS) of Drosophila, neuronal stem cells, the neuroblasts (NBs), first generate a set of highly diverse neurons, the primary neurons that mature to control larval behavior, and then more homogeneous sets of neurons that show delayed maturation and are primarily used in the adult. These latter, 'secondary' neurons show a complex pattern of expression of broad, which encodes a transcription factor usually associated with metamorphosis, where it acts as a key regulator in the transitions from larva and pupa.

Results

The Broad-Z3 (Br-Z3) isoform appears transiently in most central neurons during embryogenesis, but persists in a subset of these cells through most of larval growth. Some of the latter are embryonic-born secondary neurons, whose development is arrested until the start of metamorphosis. However, the vast bulk of the secondary neurons are generated during larval growth and bromodeoxyuridine incorporation shows that they begin expressing Br-Z3 about 7 hours after their birth, approximately the time that they have finished outgrowth to their initial targets. By the start of metamorphosis, the oldest secondary neurons have turned off Br-Z3 expression, while the remainder, with the exception of the very youngest, maintain Br-Z3 while they are interacting with potential partners in preparation for neurite elaboration. That Br-Z3 may be involved in early sprouting is suggested by ectopically expressing this isoform in remodeling primary neurons, which do not normally express Br-Z3. These cells now sprout into ectopic locations. The expression of Br-Z3 is transient and seen in all interneurons, but two other isoforms, Br-Z4 and Br-Z1, show a more selective expression. Analysis of MARCM clones shows that the Br-Z4 isoform is expressed by neurons in virtually all lineages, but only in those cells born during a window during the transition from the second to the third larval instar. Br-Z4 expression is then maintained in this temporal cohort of cells into the adult.

Conclusion

These data show the potential for diverse functions of Broad within the developing CNS. The Br-Z3 isoform appears in all interneurons, but not motoneurons, when they first begin to interact with potential targets. Its function during this early sorting phase needs to be defined. Two other Broad isoforms, by contrast, are stably expressed in cohorts of neurons in all lineages and are the first examples of persisting molecular 'time-stamps' for Drosophila postembryonic neurons.

Feedback control of the EGFR signaling gradient: superposition of domain-splitting events in Drosophila oogenesis

The morphogenesis of structures with repeated functional units, such as body segments and appendages, depends on multi-domain patterns of cell signaling and gene expression. We demonstrate that during Drosophila oogenesis, the two-domain expression pattern of Broad, a transcription factor essential for the formation of the two respiratory eggshell appendages, is established by a single gradient of EGFR activation that induces both Broad and Pointed, which mediates repression of Broad. Two negative-feedback loops provided by the intracellular inhibitors of EGFR signaling, Kekkon-1 and Sprouty, control the number and position of Broad-expressing cells and in this way influence eggshell morphology. Later in oogenesis, the gradient of EGFR activation is split into two smaller domains in a process that depends on Argos, a secreted antagonist of EGFR signaling. In contrast to the previously proposed model of eggshell patterning, we show that the two-domain pattern of EGFR signaling is not essential for specifying the number of appendages. Thus, the processes that define the two-domain patterns of Broad and EGFR activation are distinct; their actions are separated in time and have different effects on eggshell morphology.

Tube formation in Drosophila egg chambers

The reorganization of epithelial sheets into tubes is a fundamental process in the formation of many organs, such as the lungs, kidneys, gut, and neural tube. This process involves the patterning of distinct cell types and the coordination of those cells during the shape changes and rearrangements that produce the tube. A better understanding of the cellular and genetic mechanisms that regulate tube formation is necessary for tissue engineers to develop functional organs in vitro. The Drosophila egg chamber has emerged as an outstanding model for studying tubulogenesis. Synthesis of the dorsal respiratory appendages by the follicular epithelium resembles primary neurulation in vertebrates. This review summarizes work on the patterning and morphogenesis of the dorsal-appendage tubes and highlights key areas where mathematical modeling could contribute to our understanding of these processes.

A combinatorial code for pattern formation in Drosophila oogenesis

Two-dimensional patterning of the follicular epithelium in Drosophila oogenesis is required for the formation of three-dimensional eggshell structures. Our analysis of a large number of published gene expression patterns in the follicle cells suggests that they follow a simple combinatorial code based on six spatial building blocks and the operations of union, difference, intersection, and addition. The building blocks are related to the distribution of inductive signals, provided by the highly conserved epidermal growth factor receptor and bone morphogenetic protein signaling pathways. We demonstrate the validity of the code by testing it against a set of patterns obtained in a large-scale transcriptional profiling experiment. Using the proposed code, we distinguish 36 distinct patterns for 81 genes expressed in the follicular epithelium and characterize their joint dynamics over four stages of oogenesis. The proposed combinatorial framework allows systematic analysis of the diversity and dynamics of two-dimensional transcriptional patterns and guides future studies of gene regulation.

Hairless induces cell death by downregulation of EGFR signalling activity

Overexpression of the Notch antagonist Hairless (H) during imaginal development in Drosophila is correlated with tissue loss and cell death. Together with the co-repressors Groucho (Gro) and C-terminal binding protein (CtBP), H assembles a repression complex on Notch target genes, thereby downregulating Notch signalling activity. Here we investigated the mechanisms underlying H-mediated cell death in S2 cell culture and in vivo during imaginal development in Drosophila. First, we mapped the domains within the H protein that are required for apoptosis induction in cell culture. These include the binding sites for the co-repressors, both of which are essential for H-mediated cell death during fly development. Hence, the underlying cause of H-mediated apoptosis seems to be a transcriptional downregulation of Notch target genes involved in cell survival. In a search for potential targets, we observed transcriptional downregulation of rho-lacZ and EGFR signalling output. Moreover, the EGFR antagonists lozenge, klumpfuss and argos were all activated upon H overexpression. This result conforms to the proapoptotic activity of H, as these factors are known to be involved in apoptosis induction. Together, the results indicate that H induces apoptosis by downregulation of EGFR signalling activity. This highlights the importance of a coordinated interplay of Notch and EGFR signalling pathways for cell survival during Drosophila development.

Drosophila phosphopantothenoylcysteine synthetase is required for tissue morphogenesis during oogenesis

Background

Coenzyme A (CoA) is an essential metabolite, synthesized from vitamin B₅ by the subsequent action of five enzymes: PANK, PPCS, PPCDC, PPAT and DPCK. Mutations in Drosophila dPPCS disrupt female fecundity and in this study we analyzed the female sterile phenotype of dPPCS mutants in detail.

Results

We demonstrate that dPPCS is required for various processes that occur during oogenesis including chorion patterning. Our analysis demonstrates that a mutation in dPPCS disrupts the organization of the somatic and germ line cells, affects F-actin organization and results in abnormal PtdIns(4,5)P₂ localization. Improper cell organization coincides with aberrant localization of the membrane molecules Gurken (Grk) and Notch, whose activities are required for specification of the follicle cells that pattern the eggshell. Mutations in dPPCS also induce alterations in scutellar patterning and cause wing vein abnormalities. Interestingly, mutations in dPANK and dPPAT-DPCK result in similar patterning defects.

Conclusion

Together, our results demonstrate that de novo CoA biosynthesis is required for proper tissue morphogenesis.

Distinct activation patterns of EGF receptor signaling in the homoplastic evolution of eggshell morphology in genus Drosophila

Homoplasy is a phenomenon in which organisms in different phylogenetic groups independently acquire similar traits. However, it is largely unknown how developmental mechanisms are altered to give rise to homoplasy. In the genus Drosophila, all species of the subgenus Sophophora, including Drosophila (D.) melanogaster, have eggshells with two dorsal appendages (DAs); most species in the subgenus Drosophila, including D. virilis, and in the subgenus Dorsilopha, have four-DAs. D. melanica belongs to the Drosophila subgenus, but has two-DAs, and phylogenetic analyses suggest that it acquired this characteristic independently. The patterning of the DAs is tightly regulated by epidermal growth factor receptor (EGFR) signaling in D. melanogaster. Previous studies suggested that a change in the EGFR signal activation pattern could have led to the divergence in DA number between D. melanogaster and D. virilis. Here, we compared the patterns of EGFR signal activation across the Drosophila subgenera by immunostaining for anti-activated MAP kinase (MAPK). Our analysis revealed distinct patterns of EGFR signal activation in each subgenus that was consistent with their phylogenetic relationship. In addition, the number of DAs always corresponded to the number of EGFR signaling activation domains in two, three, and four-DA species. Despite their common two-DA characteristic, the EGFR signaling activation pattern in D. melanica diverged significantly from that of species in the subgenus Sophophora. Our results suggest that acquisition of the homoplastic two-DA characteristic could be explained by modifications of the EGFR signaling system in the genus Drosophila that occurred independently and at least twice during evolution.

Cad74A is regulated by BR and is required for robust dorsal appendage formation in Drosophila oogenesis

Drosophila egg development is an established model for studying epithelial patterning and morphogenesis, but the connection between signaling pathways and egg morphology is still incompletely understood. We have identified a non-classical cadherin, Cad74A, as a putative adhesion gene that bridges epithelial patterning and morphogenesis in the follicle cells. Starting in mid-oogenesis, Cad74A is expressed in the follicle cells that contact the oocyte, including the border cells and most of the columnar follicle cells. However, Cad74A is repressed in two dorsolateral patches of follicle cells, which participate in the formation of tubular respiratory appendages. We show genetically that Cad74A is downstream of the EGFR and BMP signaling pathways and is repressed by the Zn-finger transcription factor Broad. The correlation of Cad74A repression in the cells that bend out of the plane of the follicular epithelium is preserved across Drosophila species and mutant backgrounds exhibiting a range of eggshell phenotypes. Complete removal of Cad74A from the follicle cells causes defects in dorsal appendage formation. Ectopic expression of Cad74A in the roof cells results in shortened, flattened appendages due to the hindered migration of the roof cells. Based on these results, we propose that Cad74A is part of the adhesive machinery that enables robust dorsal appendage formation, and as such provides a link between the patterning of the follicle cells and eggshell morphogenesis.

Conditional switches for extracellular matrix patterning in Drosophila melanogaster

An F(1) mutagenesis strategy was developed to identify conditional mutations affecting extracellular matrix (ECM) patterning. Tubulogenesis requires coordinated movement of epithelial cells and deposition of a multilayered ECM. In the Drosophila ovary, an epithelium of follicle cells creates the eggshells, including the paired tubular dorsal appendages (DAs) that act as breathing tubes for the embryo. A P-element mutagenesis strategy allowed for conditional overexpression of hundreds of genes in follicle cells. Conditional phenotypes were scored at the level of individual mutant (F(1)) female flies. ECM pattern regulators were readily identified including MAPK signaling gene ets domain lacking (fused DAs), Wnt pathway genes frizzled 3 and osa (long DAs), Hh pathway gene debra (branched DAs), and transcription factor genes sima/HIF-1alpha, ush, lilli, Tfb1, broad, and foxo. In moving cells the [Ca(2+)]/calcineurin pathway can regulate adhesion to ECM while adherens junctions link cells together. Accordingly, thin eggshell and DA phenotypes were identified for the calcineurin regulator calreticulin and the adherens junction component arc. Finally a tubulogenesis defect phenotype was identified for the gene pterodactyl, homologous to the mammalian serine/threonine receptor-associated protein (STRAP) that integrates the TGF-beta and PI3K/AKT signaling pathways. Because phenotypes can be scored in each mutant fly before and after gene induction, this F(1) conditional mutagenesis strategy should allow for increased scale in screens for mutations affecting repeated (reiterated) events in adult animals, including gametogenesis, movement, behavior, and learning.

Drosophila follicle cells: morphogenesis in an eggshell

Epithelial morphogenesis is important for organogenesis and pivotal for carcinogenesis, but mechanisms that control it are poorly understood. The Drosophila follicular epithelium is a genetically tractable model to understand these mechanisms in vivo. This epithelium of follicle cells encases germline cells to create an egg. In this review, we summarize progress toward understanding mechanisms that maintain the epithelium or permit migrations essential for oogenesis. Cell-cell communication is important, but the same signals are used repeatedly to control distinct events. Understanding intrinsic mechanisms that alter responses to developmental signals will be important to understand regulation of cell shape and organization.

Drosophila eggshell is patterned by sequential action of feedforward and feedback loops

During Drosophila oogenesis, patterning activities of the EGFR and Dpp pathways specify several subpopulations of the follicle cells that give rise to dorsal eggshell structures. The roof of dorsal eggshell appendages is formed by the follicle cells that express Broad (Br), a zinc-finger transcription factor regulated by both pathways. EGFR induces Br in the dorsal follicle cells. This inductive signal is overridden in the dorsal midline cells, which are exposed to high levels of EGFR activation, and in the anterior cells, by Dpp signaling. We show that the resulting changes in the Br pattern affect the expression of Dpp receptor thickveins (tkv), which is essential for Dpp signaling. By controlling tkv, Br controls Dpp signaling in late stages of oogenesis and, as a result, regulates its own repression in a negative-feedback loop. We synthesize these observations into a model, whereby the dynamics of Br expression are driven by the sequential action of feedforward and feedback loops. The feedforward loop controls the spatial pattern of Br expression, while the feedback loop modulates this pattern in time. This mechanism demonstrates how complex patterns of gene expression can emerge from simple inputs, through the interaction of regulatory network motifs.

Krüppel homolog 1 (Kr-h1) mediates juvenile hormone action during metamorphosis of Drosophila melanogaster

Juvenile hormone (JH) given at pupariation inhibits bristle formation and causes pupal cuticle formation in the abdomen of Drosophila melanogaster due to its prolongation of expression of the transcription factor Broad (BR). In a microarray analysis of JH-induced gene expression in abdominal integument, we found that Krüppel homolog 1 (Kr-h1) was up-regulated during most of adult development. Quantitative real-time PCR analyses showed that Kr-h1 up-regulation began at 10h after puparium formation (APF), and Kr-h1 up-regulation occurred in imaginal epidermal cells, persisting larval muscles, and larval oenocytes. Ectopic expression of Kr-h1 in abdominal epidermis using T155-Gal4 to drive UAS-Kr-h1 resulted in missing or short bristles in the dorsal midline. This phenotype was similar to that seen after a low dose of JH or after misexpression of br between 21 and 30 h APF. Ectopic expression of Kr-h1 prolonged the expression of BR protein in the pleura and the dorsal tergite. No Kr-h1 was seen after misexpression of br. Thus, Kr-h1 mediates some of the JH signaling in the adult abdominal epidermis and is upstream of br in this pathway. We also show for the first time that the JH-mediated maintenance of br expression in this epidermis is patterned and that JH delays the fusion of the imaginal cells and the disappearance of Dpp in the dorsal midline.

Notch signaling links interactions between the C/EBP homolog slow border cells and the GILZ homolog bunched during cell migration

In the follicle cell (FC) epithelium that surrounds the Drosophila egg, a complex set of cell signals specifies two cell fates that pattern the eggshell: the anterior centripetal FC that produce the operculum and the posterior columnar FC that produce the main body eggshell structure. We have previously shown that the long-range morphogen DPP represses the expression of the bunched (bun) gene in the anterior-most centripetal FC. bun, which encodes a homolog of vertebrate TSC-22/GILZ, in turn represses anterior gene expression and antagonizes Notch signaling to restrict centripetal FC fates in posterior cells. From a screen for novel targets of bun repression we have identified the C/EBP homolog slow border cells (slbo). At stage 10A, slbo expression overlaps bun in anterior FC; by stage 10B they repress each other's expression to establish a sharp slbo/bun expression boundary. The precise position of the slbo/bun expression boundary is sensitive to Notch signaling, which is required for both slbo activation and bun repression. As centripetal migration proceeds from stages 10B-14, slbo represses its own expression and both slbo loss-of-function mutations and overexpression approaches reveal that slbo is required to coordinate centripetal migration with nurse cell dumping. We propose that in anterior FC exposed to a Dpp morphogen gradient, high and low levels of slbo and bun, respectively, are established by modulation of Notch signaling to direct threshold cell fates. Interactions among Notch, slbo and bun resemble a conserved signaling cassette that regulates mammalian adipocyte differentiation.

Soma-dependent modulations contribute to divergence ofrhomboidexpression during evolution ofDrosophilaeggshell morphology

Patterning of the respiratory dorsal appendages (DAs) on the Drosophila melanogaster eggshell is tightly regulated by epidermal growth factor receptor (EGFR) signaling. Variation in the DA number is observed among Drosophila species; D. melanogaster has two DAs and D. virilis has four. Diversification in the expression pattern of rhomboid (rho), which activates EGFR signaling in somatic follicle cells, could cause the evolutionary divergence of DA numbers. Here we identified a cis-regulatory element of D. virilis rho. A comparison with D. melanogaster rho enhancer and activity studies in homologous and heterologous species suggested that these rho enhancers did not functionally diverge significantly during the evolution of these species. Experiments using chimeric eggs composed of a D. virilis oocyte and D. melanogaster follicle cells showed the evolution of DA number was not attributable to germline Gurken (Grk) signaling, but to divergence in events downstream of Grk signaling affecting the rho enhancer activity in somatic follicle cells. We found that a transcription factor,Mirror, which activates rho, could be one of these downstream factors. Thus, evolution of the trans-regulatory environment that controls rho expression in somatic follicle cells could be a major contributor to the evolutionary changes in DA number.

Spatially localized Kuzbanian required for specific activation of Notch during border cell migration

The transmembrane receptor Notch is used repeatedly during development for a variety of essential functions. During Drosophila oogenesis, Notch activity is required first to specify particular follicle cell fates, then to promote the differentiation of all follicle cell types, to promote border cell migration, and then to form dorsal appendages, raising the question as to how Notch activity is spatially and temporally regulated. Here we show the Notch activity pattern during oogenesis. Notch activation was found in many follicle cells at stage 6 but then at stage 9 was restricted to migrating border cells, despite uniform expression of Delta. Expression of Kuzbanian (KUZ), a metalloproteinase that can activate Notch as well as cleave other substrates, is enriched in border cells at stage 9; and dominant-negative KUZ caused a strong border cell migration defect, without affecting expression of markers of border cell fate or follicle cell differentiation. Constitutively active Notch rescued the migration defect due to dominant-negative KUZ, and conditional alleles of Delta and Notch also exhibited border cell migration defects. Expression of two different reporters of Notch activity was lost upon expression of dominant-negative KUZ. Taken together these results show that Notch activation and KUZ expression are restricted to border cells at stage 9 of oogenesis and are required for migration, but not differentiation, of these cells. This represents a previously unrecognized mechanism for achieving spatial restriction of Notch signaling.

Quantifying the Gurken morphogen gradient in Drosophila oogenesis

Quantitative information about the distribution of morphogens is crucial for understanding their effects on cell-fate determination, yet it is difficult to obtain through direct measurements. We have developed a parameter estimation approach for quantifying the spatial distribution of Gurken, a TGFalpha-like EGFR ligand that acts as a morphogen in Drosophila oogenesis. Modeling of Gurken/EGFR system shows that the shape of the Gurken gradient is controlled by a single dimensionless parameter, the Thiele modulus, which reflects the relative importance of ligand diffusion and degradation. By combining the model with genetic alterations of EGFR levels, we have estimated the value of the Thiele modulus in the wild-type egg chamber. This provides a direct characterization of the shape of the Gurken gradient and demonstrates how parameter estimation techniques can be used to quantify morphogen gradients in development.