The Drosophila female sterile mutation twin peaks is a novel allele of tramtrack and reveals a requirement for Ttk69 in epithelial morphogenesis

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Noncanonical function of Capicua as a growth termination signal in Drosophila oogenesis

Capicua (Cic) proteins are conserved HMG-box transcriptional repressors that control receptor tyrosine kinase (RTK) signaling responses and are implicated in human neurological syndromes and cancer. While Cic is known to exist as short (Cic-S) and long (Cic-L) isoforms with identical HMG-box and associated core regions but distinct N termini, most previous studies have focused on Cic-S, leaving the function of Cic-L unexplored. Here we show that Cic-L acts in two capacities during Drosophila oogenesis: 1) as a canonical sensor of RTK signaling in somatic follicle cells, and 2) as a regulator of postmitotic growth in germline nurse cells. In these latter cells, Cic-L behaves as a temporal signal that terminates endoreplicative growth before they dump their contents into the oocyte. We show that Cic-L is necessary and sufficient for nurse cell endoreplication arrest and induces both stabilization of CycE and down-regulation of Myc. Surprisingly, this function depends mainly on the Cic-L-specific N-terminal module, which is capable of acting independently of the Cic HMG-box-containing core. Mirroring these observations, basal metazoans possess truncated Cic-like proteins composed only of Cic-L N-terminal sequences, suggesting that this module plays unique, ancient roles unrelated to the canonical function of Cic.

Developmental regulation of oocyte lipid intake through 'patent' follicular epithelium in Drosophila melanogaster

Epithelia form protective permeability barriers that selectively allow the exchange of material while maintaining tissue integrity under extreme mechanical, chemical, and bacterial loads. Here, we report in the Drosophila follicular epithelium a developmentally regulated and evolutionarily conserved process "patency", wherein a breach is created in the epithelium at tricellular contacts during mid-vitellogenesis. In Drosophila, patency exhibits a strict temporal range potentially delimited by the transcription factor Tramtrack69 and a spatial pattern influenced by the dorsal-anterior signals of the follicular epithelium. Crucial for growth and lipid uptake by the oocyte, patency is also exploited by endosymbionts such as Spiroplasma pulsonii. Our findings reveal an evolutionarily conserved and developmentally regulated non-typical epithelial function in a classic model system.

Detecting New Allies: Modifier Screen Identifies a Genetic Interaction Between Imaginal disc growth factor 3 and combover, a Rho-kinase Substrate, During Dorsal Appendage Tube Formation in Drosophila

Biological tube formation underlies organ development and, when disrupted, can cause severe birth defects. To investigate the genetic basis of tubulogenesis, we study the formation of Drosophila melanogaster eggshell structures, called dorsal appendages, which are produced by epithelial tubes. Previously we found that precise levels of Drosophila Chitinase-Like Proteins (CLPs), encoded by the Imaginal disc growth factor (Idgf) gene family, are needed to regulate dorsal-appendage tube closure and tube migration. To identify factors that act in the Idgf pathway, we developed a genetic modifier screen based on the finding that overexpressing Idgf3 causes dorsal appendage defects with ∼50% frequency. Using a library of partially overlapping heterozygous deficiencies, we scanned chromosome 3L and found regions that enhanced or suppressed the Idgf3-overexpression phenotype. Using smaller deletions, RNAi, and mutant alleles, we further mapped five regions and refined the interactions to 58 candidate genes. Importantly, mutant alleles identified combover (cmb), a substrate of Rho-kinase (Rok) and a component of the Planar Cell Polarity (PCP) pathway, as an Idgf3-interacting gene: loss of function enhanced while gain of function suppressed the dorsal appendage defects. Since PCP drives cell intercalation in other systems, we asked if cmb/+ affected cell intercalation in our model, but we found no evidence of its involvement in this step. Instead, we found that loss of cmb dominantly enhanced tube defects associated with Idgf3 overexpression by expanding the apical area of dorsal appendage cells. Apical surface area determines tube volume and shape; in this way, Idgf3 and cmb regulate tube morphology.

Differentiating Drosophila female germ cells initiate Polycomb silencing by regulating PRC2-interacting proteins

Polycomb silencing represses gene expression and provides a molecular memory of chromatin state that is essential for animal development. We show that Drosophila female germline stem cells (GSCs) provide a powerful system for studying Polycomb silencing. GSCs have a non-canonical distribution of PRC2 activity and lack silenced chromatin like embryonic progenitors. As GSC daughters differentiate into nurse cells and oocytes, nurse cells, like embryonic somatic cells, silence genes in traditional Polycomb domains and in generally inactive chromatin. Developmentally controlled expression of two Polycomb repressive complex 2 (PRC2)-interacting proteins, Pcl and Scm, initiate silencing during differentiation. In GSCs, abundant Pcl inhibits PRC2-dependent silencing globally, while in nurse cells Pcl declines and newly induced Scm concentrates PRC2 activity on traditional Polycomb domains. Our results suggest that PRC2-dependent silencing is developmentally regulated by accessory proteins that either increase the concentration of PRC2 at target sites or inhibit the rate that PRC2 samples chromatin.

Downregulation of homeodomain protein Cut is essential for Drosophila follicle maturation and ovulation

Proper development and maturation of a follicle is essential for successful ovulation and reproduction; however, the molecular mechanisms for follicle maturation, particularly for somatic follicle cell differentiation, are poorly understood. During Drosophila oogenesis, the somatic follicle cells encasing oocytes undergo two distinct well-established transitions: the mitotic to endocycle switch at stage 6/7 and the endocycle to gene amplification switch at stage10A/10B. Here, we identify a novel third follicle cell transition that occurs in the final stages of oogenesis (stage 13/14). This late follicle cell transition is characterized by upregulation of the transcription factor Hindsight (Hnt), and downregulation of the homeodomain transcription factor Cut and the zinc-finger transcription factor Tramtrack-69 (Ttk69). We demonstrate that inducing expression of Cut in stage 14 follicle cells is sufficient to inhibit follicle rupture and ovulation through its negative regulation of Hnt and promotion of Ttk69 expression. Our work illustrates the importance of the stage13/14 transition for follicle maturation and demonstrates the complex regulation required for somatic follicle cells to differentiate into a state primed for follicle rupture and ovulation.

The indispensable contribution of s38 protein to ovarian-eggshell morphogenesis in Drosophila melanogaster

Drosophila chorion represents a remarkable model system for the in vivo study of complex extracellular-matrix architectures. For its organization and structure, s38 protein is considered as a component of major importance, since it is synthesized and secreted during early choriogenesis. However, there is no evidence that proves its essential, or redundant, role in chorion biogenesis. Hence, we show that targeted downregulation of s38 protein, specifically in the ovarian follicle-cell compartment, via employment of an RNAi-mediated strategy, causes generation of diverse dysmorphic phenotypes, regarding eggshell’s regionally and radially specialized structures. Downregulation of s38 protein severely impairs fly’s fertility and is unable to be compensated by the s36 homologous family member, thus unveiling s38 protein’s essential contribution to chorion’s assembly and function. Altogether, s38 acts as a key skeletal protein being critically implicated in the patterning establishment of a highly structured tripartite endochorion. Furthermore, it seems that s38 loss may sensitize choriogenesis to stochastic variation in its coordination and timing.

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.

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.

Proteomics Analysis Identifies Orthologs of Human Chitinase-Like Proteins as Inducers of Tube Morphogenesis Defects in Drosophila melanogaster

Elevated levels of human chitinase-like proteins (CLPs) are associated with numerous chronic inflammatory diseases and several cancers, often correlating with poor prognosis. Nevertheless, there is scant knowledge of their function. The CLPs normally mediate immune responses and wound healing and, when upregulated, they can promote disease progression by remodeling tissue, activating signaling cascades, stimulating proliferation and migration, and by regulating adhesion. We identified Imaginal disc growth factors (Idgfs), orthologs of human CLPs CHI3L1, CHI3L2, and OVGP1, in a proteomics analysis designed to discover factors that regulate tube morphogenesis in a Drosophila melanogaster model of tube formation. We implemented a novel approach that uses magnetic beads to isolate a small population of specialized ovarian cells, cells that nonautonomously regulate morphogenesis of epithelial tubes that form and secrete eggshell structures called dorsal appendages (DAs). Differential mass spectrometry analysis of these cells detected elevated levels of four of the six Idgf family members (Idgf1, Idgf2, Idgf4, and Idgf6) in flies mutant for bullwinkle (bwk), which encodes a transcription factor and is a known regulator of DA-tube morphogenesis. We show that, during oogenesis, dysregulation of Idgfs (either gain or loss of function) disrupts the formation of the DA tubes. Previous studies demonstrate roles for Drosophila Idgfs in innate immunity, wound healing, and cell proliferation and motility in cell culture. Here, we identify a novel role for Idgfs in both normal and aberrant tubulogenesis processes.

Targeted Downregulation of s36 Protein Unearths its Cardinal Role in Chorion Biogenesis and Architecture during Drosophila melanogaster Oogenesis

Drosophila chorion represents a model biological system for the in vivo study of gene activity, epithelial development, extracellular-matrix assembly and morphogenetic-patterning control. It is produced during the late stages of oogenesis by epithelial follicle cells and develops into a highly organized multi-layered structure that exhibits regional specialization and radial complexity. Among the six major proteins involved in chorion’s formation, the s36 and s38 ones are synthesized first and regulated in a cell type-specific and developmental stage-dependent manner. In our study, an RNAi-mediated silencing of s36 chorionic-gene expression specifically in the follicle-cell compartment of Drosophila ovary unearths the essential, and far from redundant, role of s36 protein in patterning establishment of chorion’s regional specialization and radial complexity. Without perturbing the developmental courses of follicle- and nurse-cell clusters, the absence of s36 not only promotes chorion’s fragility but also induces severe structural irregularities on chorion’s surface and entirely impairs fly’s fertility. Moreover, we herein unveil a novel function of s36 chorionic protein in the regulation of number and morphogenetic integrity of dorsal appendages in follicles sporadically undergoing aged fly-dependent stress.

The ecdysone receptor signalling regulates microvilli formation in follicular epithelial cells

Epithelial morphogenesis contributes greatly to the development and homeostasis of the organs and body parts. Here, we analysed the consequences of impaired ecdysone receptor (EcR) signalling in the Drosophila follicular epithelium. Besides governing cell growth, the three EcR isoforms act redundantly in controlling follicle cell positioning. Flattening of the microvilli and an aberrant actin cytoskeleton arise from defective EcR signalling in follicle cells, and these defects impact on the organisation of the oocyte membrane. We found that this signalling governs a complex molecular network since its impairment affects key molecules as atypical protein kinase C and activated Moesin. Interestingly, the activity of the transcription factor Tramtrack69 isoform is required for microvilli and their actin core morphogenesis as well as for follicle cell positioning. In conclusion, our findings provide evidence of novel roles for EcR signalling and Tramtrack69 transcription factor in controlling stage-specific differentiation events that take place in the follicular epithelium.

Diversity of epithelial morphogenesis during eggshell formation in drosophilids

The eggshells of drosophilid species provide a powerful model for studying the origins of morphological diversity. The dorsal appendages, or respiratory filaments, of these eggshells display a remarkable interspecies variation in number and shape, and the epithelial patterning underlying the formation of these structures is an area of active research. To extend the analysis of dorsal appendage formation to include morphogenesis, we developed an improved 3D image reconstruction approach. This approach revealed considerable interspecies variation in the cell shape changes and neighbor exchanges underlying appendage formation. Specifically, although the appendage floor in Drosophila melanogaster is formed through spatially ordered neighbor exchanges, the same structure in Scaptodrosophila pattersoni is formed through extreme changes in cell shape, whereas Drosophila funebris appears to display a combination of both cellular mechanisms. Furthermore, localization patterns of Par3/Bazooka suggest a self-organized, cell polarity-based origin for the variability of appendage number in S. pattersoni. Our results suggest that species deploy different combinations of apically and basally driven mechanisms to convert a two-dimensional primordium into a three-dimensional structure, and provide new directions for exploring the molecular origins of interspecies morphological variation.

Regulation of pattern formation and gene amplification during Drosophila oogenesis by the miR-318 microRNA

Pattern formation during epithelial development requires the coordination of multiple signaling pathways. Here, we investigate the functions of an ovary-enriched miRNA, miR-318, in epithelial development during Drosophila oogenesis. mir-318 maternal loss-of-function mutants were female-sterile and laid eggs with abnormal morphology. Removal of mir-318 disrupted the dorsal-anterior follicle cell patterning, resulting in abnormal dorsal appendages. mir-318 mutant females also produced thin and fragile eggshells due to impaired chorion gene amplification. We provide evidence that the ecdysone signaling pathway activates expression of miR-318 and that miR-318 cooperates with Tramtrack69 to control the switch from endocycling to chorion gene amplification during differentiation of the follicular epithelium. The multiple functions of miR-318 in oogenesis illustrate the importance of miRNAs in maintaining cell fate and in promoting the developmental transition in the female follicular epithelium.

The Drosophila BTB domain protein Jim Lovell has roles in multiple larval and adult behaviors

Innate behaviors have their origins in the specification of neural fates during development. Within Drosophila, BTB (Bric-a-brac,Tramtrack, Broad) domain proteins such as Fruitless are known to play key roles in the neural differentiation underlying such responses. We previously identified a gene, which we have termed jim lovell (lov), encoding a BTB protein with a role in gravity responses. To understand more fully the behavioral roles of this gene we have investigated its function through several approaches. Transcript and protein expression patterns have been examined and behavioral phenotypes of new lov mutations have been characterized. Lov is a nuclear protein, suggesting a role as a transcriptional regulator, as for other BTB proteins. In late embryogenesis, Lov is expressed in many CNS and PNS neurons. An examination of the PNS expression indicates that lov functions in the late specification of several classes of sensory neurons. In particular, only two of the five abdominal lateral chordotonal neurons express Lov, predicting functional variation within this highly similar group. Surprisingly, Lov is also expressed very early in embryogenesis in ways that suggests roles in morphogenetic movements, amnioserosa function and head neurogenesis. The phenotypes of two new lov mutations that delete adjacent non-coding DNA regions are strikingly different suggesting removal of different regulatory elements. In lov⁴⁷, Lov expression is lost in many embryonic neurons including the two lateral chordotonal neurons. lov⁴⁷ mutant larvae show feeding and locomotor defects including spontaneous backward movement. Adult lov⁴⁷ males perform aberrant courtship behavior distinguished by courtship displays that are not directed at the female. lov⁴⁷ adults also show more defective negative gravitaxis than the previously isolated lov^91Y mutant. In contrast, lov⁶⁶ produces largely normal behavior but severe female sterility associated with ectopic lov expression in the ovary. We propose a negative regulatory role for the DNA deleted in lov⁶⁶.

Following the 'tracks': Tramtrack69 regulates epithelial tube expansion in the Drosophila ovary through Paxillin, Dynamin, and the homeobox protein Mirror

Epithelial tubes are the infrastructure for organs and tissues, and tube morphogenesis requires precise orchestration of cell signaling, shape, migration, and adhesion. Follicle cells in the Drosophila ovary form a pair of epithelial tubes whose lumens act as molds for the eggshell respiratory filaments, or dorsal appendages (DAs). DA formation is a robust and accessible model for studying the patterning, formation, and expansion of epithelial tubes. Tramtrack69 (TTK69), a transcription factor that exhibits a variable embryonic DNA-binding preference, controls DA lumen volume and shape by promoting tube expansion; the tramtrack mutation twin peaks (ttk(twk)) reduces TTK69 levels late in oogenesis, inhibiting this expansion. Microarray analysis of wild-type and ttk(twk) ovaries, followed by in situ hybridization and RNAi of candidate genes, identified the Phospholipase B-like protein Lamina ancestor (LAMA), the scaffold protein Paxillin, the endocytotic regulator Shibire (Dynamin), and the homeodomain transcription factor Mirror, as TTK69 effectors of DA-tube expansion. These genes displayed enriched expression in DA-tube cells, except lama, which was expressed in all follicle cells. All four genes showed reduced expression in ttk(twk) mutants and exhibited RNAi phenotypes that were enhanced in a ttk(twk)/+ background, indicating ttk(twk) genetic interactions. Although previous studies show that Mirror patterns the follicular epithelium prior to DA tubulogenesis, we show that Mirror has an independent, novel role in tube expansion, involving positive regulation of Paxillin. Thus, characterization of ttk(twk)-differentially expressed genes expands the network of TTK69 effectors, identifies novel epithelial tube-expansion regulators, and significantly advances our understanding of this vital developmental process.

R7 photoreceptor axon growth is temporally controlled by the transcription factor Ttk69, which inhibits growth in part by promoting transforming growth factor-β/activin signaling

Work on axon growth has classically focused on understanding how extrinsic cues control growth cone dynamics independent of the cell body. However, more recently, neuron-intrinsic transcription factors have been shown to influence both normal and regenerative axon growth, suggesting that understanding their mechanism of action is of clinical importance. We are studying axon targeting in the Drosophila visual system and here show that the BTB/POZ zinc-finger transcription factor Tramtrack69 (Ttk69) plays an instructive role in inhibiting the growth of R7 photoreceptor axon terminals. Although ttk69 mutant R7 axons project to the correct medullar target layer, M6, their terminals fail to remain retinotopically restricted and instead grow laterally within M6. This overgrowth is not caused by an inability to be repelled by neighboring R7 axons or by an inability to recognize and initiate synapse formation with postsynaptic targets. The overgrowth is progressive and occurs even if contact between ttk69 mutant R7 axons and their normal target layer is disrupted. Ttk69 is first expressed in wild-type R7s after their axons have reached the medulla; ttk69 mutant R7 axon terminal overgrowth begins shortly after this time point. We find that expressing Ttk69 prematurely in R7s collapses their growth cones and disrupts axon extension, indicating that Ttk69 plays an instructive role in this process. A TGF-β/Activin pathway was shown previously to inhibit R7 axon terminal growth. We find that Ttk69 is required for normal activation of this pathway but that Ttk69 likely also inhibits R7 axon growth by a TGF-β/Activin-independent mechanism.

The microRNA miR-7 regulates Tramtrack69 in a developmental switch in Drosophila follicle cells

Development in multicellular organisms includes both small incremental changes and major switches of cell differentiation and proliferation status. During Drosophila oogenesis, the follicular epithelial cells undergo two major developmental switches that cause global changes in the cell-cycle program. One, the switch from the endoreplication cycle to a gene-amplification phase, during which special genomic regions undergo repeated site-specific replication, is attributed to Notch downregulation, ecdysone signaling activation and upregulation of the zinc-finger protein Tramtrack69 (Ttk69). Here, we report that the microRNA miR-7 exerts an additional layer of regulation in this developmental switch by regulating Ttk69 transcripts. miR-7 recognizes the 3' UTR of ttk69 transcripts and regulates Ttk69 expression in a dose-dependent manner. Overexpression of miR-7 effectively blocks the switch from the endocycle to gene amplification through its regulation of ttk69. miR-7 and Ttk69 also coordinate other cell differentiation events, such as vitelline membrane protein expression, that lead to the formation of the mature egg. Our studies reveal the important role miR-7 plays in developmental decision-making in association with signal-transduction pathways.

CoREST acts as a positive regulator of Notch signaling in the follicle cells of Drosophila melanogaster

The Notch signaling pathway plays important roles in a variety of developmental events. The context-dependent activities of positive and negative modulators dramatically increase the diversity of cellular responses to Notch signaling. In a screen for mutations affecting the Drosophila melanogaster follicular epithelium, we isolated a mutation in CoREST that disrupts the Notch-dependent mitotic-to-endocycle switch of follicle cells at stage 6 of oogenesis. We show that Drosophila CoREST positively regulates Notch signaling, acting downstream of the proteolytic cleavage of Notch but upstream of Hindsight activity; the Hindsight gene is a Notch target that coordinates responses in the follicle cells. We show that CoREST genetically interacts with components of the Notch repressor complex, Hairless, C-terminal Binding Protein and Groucho. In addition, we demonstrate that levels of H3K27me3 and H4K16 acetylation are dramatically increased in CoREST mutant follicle cells. Our data indicate that CoREST acts as a positive modulator of the Notch pathway in the follicular epithelium as well as in wing tissue, and suggests a previously unidentified role for CoREST in the regulation of Notch signaling. Given its high degree of conservation among species, CoREST probably also functions as a regulator of Notch-dependent cellular events in other organisms.

The transcriptional corepressor SMRTER influences both Notch and ecdysone signaling during Drosophila development

SMRTER (SMRT-related and ecdysone receptor interacting factor) is the Drosophila homologue of the vertebrate proteins SMRT and N-CoR, and forms with them a well-conserved family of transcriptional corepressors. Molecular characterization of SMRT-family proteins in cultured cells has implicated them in a wide range of transcriptional regulatory pathways. However, little is currently known about how this conserved class of transcriptional corepressors regulates the development of particular tissues via specific pathways. In this study, through our characterization of multiple Smrter (Smr) mutant lines, mosaic analysis of a loss-of-function Smr allele, and studies of two independent Smr RNAi fly lines, we report that SMRTER is required for the development of both ovarian follicle cells and the wing. In these two tissues, SMRTER inhibits not only the ecdysone pathway, but also the Notch pathway. We differentiate SMRTER's influence on these two signaling pathways by showing that SMRTER inhibits the Notch pathway, but not the ecdysone pathway, in a spatiotemporally restricted manner. We further confirm the likely involvement of SMRTER in the Notch pathway by demonstrating a direct interaction between SMRTER and Suppressor of Hairless [Su(H)], a DNA-binding transcription factor pivotal in the Notch pathway, and the colocalization of both proteins at many chromosomal regions in salivary glands. Based on our results, we propose that SMRTER regulates the Notch pathway through its association with Su(H), and that overcoming a SMRTER-mediated transcriptional repression barrier may represent a key mechanism used by the Notch pathway to control the precise timing of events and the formation of sharp boundaries between cells in multiple tissues during development.

Molecular mechanisms of EGF signaling-dependent regulation of pipe, a gene crucial for dorsoventral axis formation in Drosophila

During Drosophila oogenesis the expression of the sulfotransferase Pipe in ventral follicle cells is crucial for dorsoventral axis formation. Pipe modifies proteins that are incorporated in the ventral eggshell and activate Toll signaling which in turn initiates embryonic dorsoventral patterning. Ventral pipe expression is the result of an oocyte-derived EGF signal which down-regulates pipe in dorsal follicle cells. The analysis of mutant follicle cell clones reveals that none of the transcription factors known to act downstream of EGF signaling in Drosophila is required or sufficient for pipe regulation. However, the pipe cis-regulatory region harbors a 31-bp element which is essential for pipe repression, and ovarian extracts contain a protein that binds this element. Thus, EGF signaling does not act by down-regulating an activator of pipe as previously suggested but rather by activating a repressor. Surprisingly, this repressor acts independent of the common co-repressors Groucho or CtBP.

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.

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.

Ttk69-dependent repression of lozenge prevents the ectopic development of R7 cells in the Drosophila larval eye disc

Background

During the development of the Drosophila eye, specific cell types differentiate from an initially equipotent group of uncommitted precursor cells. The lozenge (lz) gene, which is a member of the Runt family of transcriptional regulators, plays a pivotal role in mediating this process through regulating the expression of several fate-specifying transcription factors. However, the regulation of lz, and the control of lz expression levels in different cell types is not fully understood.

Results

Here, we show a genetic interaction between Tramtrack69 (Ttk69) a key transcriptional repressor and an inhibitor of neuronal fate specification, and lz, the master patterning gene of cells posterior to the morphogenetic furrow in the Drosophila eye disc. Loss of Ttk69 expression causes the development of ectopic R7 cells in the third instar eye disc, with these cells being dependent upon Lz for their development. Using the binary UAS Gal4 system, we show that overexpression of Ttk69 causes the loss of lz-dependent differentiating cells, and a down-regulation of Lz expression in the developing eye. The loss of lz-dependent cells can be rescued by overexpressing lz via a GMR-lz transgene. We provide additional data showing that factors functioning upstream of Ttk69 in eye development regulate lz in a Ttk69-dependent manner.

Conclusions

Our results lead us to conclude that Ttk69 can either directly or indirectly repress lz gene expression to prevent the premature development of R7 precursor cells in the developing eye of Drosophila. We therefore define a mechanism for the tight regulatory control of the master pre-patterning gene, lz, in early Drosophila eye development and provide insight into how differential levels of lz expression can be achieved to effect specific cell fate outcomes.

Regulation of the endocycle/gene amplification switch by Notch and ecdysone signaling

The developmental signals that regulate the switch from genome-wide DNA replication to site-specific amplification remain largely unknown. Drosophila melanogaster epithelial follicle cells, which begin synchronized chorion gene amplification after three rounds of endocycle, provide an excellent model for study of the endocycle/gene amplification (E/A) switch. Here, we report that down-regulation of Notch signaling and activation of ecdysone receptor (EcR) are required for the E/A switch in these cells. Extended Notch activity suppresses EcR activation and prevents exit from the endocycle. Tramtrack (Ttk), a zinc-finger protein essential for the switch, is regulated negatively by Notch and positively by EcR. Ttk overexpression stops endoreplication prematurely and alleviates the endocycle exit defect caused by extended Notch activity or removal of EcR function. Our results reveal a developmental pathway that includes down-regulation of Notch, activation of the EcR, up-regulation of Ttk to execute the E/A switch, and, for the first time, the genetic interaction between Notch and ecdysone signaling in regulation of cell cycle programs and differentiation.

Tramtrack regulates different morphogenetic events duringDrosophilatracheal development

Tramtrack (Ttk) is a widely expressed transcription factor, the function of which has been analysed in different adult and embryonic tissues in Drosophila. So far, the described roles of Ttk have been mainly related to cell fate specification, cell proliferation and cell cycle regulation. Using the tracheal system of Drosophila as a morphogenetic model, we have undertaken a detailed analysis of Ttk function. Ttk is autonomously and non-autonomously required during embryonic tracheal formation. Remarkably, besides a role in the specification of different tracheal cell identities, we have found that Ttk is directly involved and required for different cellular responses and morphogenetic events. In particular, Ttk appears to be a new positive regulator of tracheal cell intercalation. Analysis of this process in ttk mutants has unveiled cell shape changes as a key requirement for intercalation and has identified Ttk as a novel regulator of its progression. Moreover, we define Ttk as the first identified regulator of intracellular lumen formation and show that it is autonomously involved in the control of tracheal tube size by regulating septate junction activity and cuticle formation. In summary, the involvement of Ttk in different steps of tube morphogenesis identifies it as a key player in tracheal development.

Analysis of Cell Migration Using Whole-Genome Expression Profiling of Migratory Cells in the Drosophila Ovary

Cell migration contributes to normal development and homeostasis as well as to pathological processes such as inflammation and tumor metastasis. Previous genetic screens have revealed signaling pathways that govern follicle cell migrations in the Drosophila ovary, but few downstream targets of the critical transcriptional regulators have been identified. To characterize the gene expression profile of two migratory cell populations and identify Slbo targets, we purified border cells and centripetal cells expressing the mouse CD8 antigen and carried out whole-genome microarray analysis. Genes predicted to control actin dynamics and the endocytic and secretory pathways were overrepresented in the migratory cell transcriptome. Mutations in five genes, including ttk, failed to complement previously isolated mutations that cause cell migration defects in mosaic clones. Functional analysis revealed a role for the Notch-activating protease Kuzbanian in border cell migration and identified Tie as a guidance receptor for the border cells.

Recurrent early pregnancy loss and endothelial nitric oxide synthase gene polymorphisms

OBJECTIVE(S)

Studies on the relation between endothelial nitric oxide synthase (eNOS) activity in implantation and maintenance of pregnancy highlights the importance of eNOS gene polymorphisms in recurrent early pregnancy loss (REPL). We investigated the relationship between idiopathic REPL and polymorphisms in eNOS among South Indian women.

METHODS

A case-control study comprising 145 females with REPL and 99 control females. The polymorphisms studied include a 27 bp intron 4 repeat, Glu298Asp variation of exon 7 and a novel 140 A --> G polymorphism in intron 6. A polymerase chain reaction-based di-deoxy dye terminator sequencing method was used for genotyping.

RESULTS

A novel A --> G polymorphism was identified in intron 6. The more frequent b allele of intron 4 repeat was present at a frequency of 0.84 in cases as compared to 0.86 in controls (O.R 1.17); the G allele of exon 7 coding for the wild-type glutamate containing isoform was present at a frequency of 0.79 in cases and 0.83 in controls (O.R 1.30, CI 0.6-2.8). The intron 6 variant A allele was present at a frequency of 0.58 in cases and 0.45 in controls (O.R 0.59, CI 0.33-1.08). Overall, the polymorphism in intron 6, in homozygous condition, exhibited a significant association to the risk of REPL (O.R 0.43, CI 0.21-0.89), P: 0.021).

CONCLUSIONS

The present study identifies and validates a novel polymorphism in the eNOS gene which was found associated with the risk of REPL.

Juxtaposition between two cell types is necessary for dorsal appendage tube formation

The Drosophila egg chamber provides an excellent model for studying the link between patterning and morphogenesis. Late in oogenesis, a portion of the flat follicular epithelium remodels to form two tubes; secretion of eggshell proteins into the tube lumens creates the dorsal appendages. Two distinct cell types contribute to dorsal appendage formation: cells expressing the rhomboid-lacZ (rho-lacZ) marker form the ventral floor of the tube and cells expressing high levels of the transcription factor Broad form a roof over the rho-lacZ cells. In mutants that produce defective dorsal appendages (K10, Ras and ectopic decapentaplegic) both cell types are specified and reorganize to occupy their stereotypical locations within the otherwise defective tubes. Although the rho-lacZ and Broad cells rearrange to form a tube in wild type and mutant egg chambers, they never intermingle, suggesting that a boundary exists that prevents mixing between these two cell types. Consistent with this hypothesis, the Broad and rho-lacZ cells express different levels of the homophilic adhesion molecule Fasciclin 3. Furthermore, in the anterior of the egg, ectopic rhomboid is sufficient to induce both cell types, which reorganize appropriately to form an ectopic tube. We propose that signaling across a boundary separating the rho-lacZ and Broad cells choreographs the cell shape-changes and rearrangements necessary to transform an initially flat epithelium into a tube.

Mass transit: Epithelial morphogenesis in theDrosophila egg chamber

Epithelial cells use a striking array of morphogenetic behaviors to sculpt organs and body plans during development. Although it is clear that epithelial morphogenesis is largely driven by cytoskeletal rearrangements and changes in cell adhesion, little is known about how these processes are coordinated to construct complex biological structures from simple sheets of cells. The follicle cell epithelium of the Drosophila egg chamber exhibits a diverse range of epithelial movements in a genetically accessible tissue, making it an outstanding system for the study of epithelial morphogenesis. In this review, we move chronologically through the process of oogenesis, highlighting the dynamic movements of the follicle cells. We discuss the cellular architecture and patterning events that set the stage for morphogenesis, detail individual cellular movements, and focus on current knowledge of the cellular processes that drive follicle cell behavior.

bullwinkle is required for epithelial morphogenesis during Drosophila oogenesis

Many organs, such as the liver, neural tube, and lung, form by the precise remodeling of flat epithelial sheets into tubes. Here we investigate epithelial tubulogenesis in Drosophila melanogaster by examining the development of the dorsal respiratory appendages of the eggshell. We employ a culture system that permits confocal analysis of stage 10-14 egg chambers. Time-lapse imaging of GFP-Moesin-expressing egg chambers reveals three phases of morphogenesis: tube formation, anterior extension, and paddle maturation. The dorsal-appendage-forming cells, previously thought to represent a single cell fate, consist of two subpopulations, those forming the tube roof and those forming the tube floor. These two cell types exhibit distinct morphological and molecular features. Roof-forming cells constrict apically and express high levels of Broad protein. Floor cells lack Broad, express the rhomboid-lacZ marker, and form the floor by directed cell elongation. We examine the morphogenetic phenotype of the bullwinkle (bwk) mutant and identify defects in both roof and floor formation. Dorsal appendage formation is an excellent system in which cell biological, molecular, and genetic tools facilitate the study of epithelial morphogenesis.

bullwinkle and shark regulate dorsal-appendage morphogenesis in Drosophila oogenesis

bullwinkle (bwk) regulates embryonic anteroposterior patterning and, through a novel germline-to-soma signal, morphogenesis of the eggshell dorsal appendages. We screened for dominant modifiers of the bullwinkle mooseantler eggshell phenotype and identified shark, which encodes an SH2-domain, ankyrin-repeat tyrosine kinase. At the onset of dorsal-appendage formation, shark is expressed in a punctate pattern in the squamous stretch cells overlying the nurse cells. Confocal microscopy with cell-type-specific markers demonstrates that the stretch cells act as a substrate for the migrating dorsal-appendage-forming cells and extend cellular projections towards them. Mosaic analyses reveal that shark is required in follicle cells for cell migration and chorion deposition. Proper shark RNA expression in the stretch cells requires bwk activity, while restoration of shark expression in the stretch cells suppresses the bwk dorsal-appendage phenotype. These results suggest that shark plays an important downstream role in the bwk-signaling pathway. Candidate testing implicates Src42A in a similar role, suggesting conservation with a vertebrate signaling pathway involving non-receptor tyrosine kinases.