Expression of constitutively active Notch arrests follicle cells at a precursor stage during Drosophila oogenesis and disrupts the anterior-posterior axis of the oocyte

Tumor suppressor miR-317 and lncRNA Peony are expressed from a polycistronic non-coding RNA locus that regulates germline differentiation and testis morphology

This research focuses on investigating the impact of non-coding RNAs on stem cell biology and differentiation processes. We found that miR-317 plays a role in germline stem cell progeny differentiation. miR-317 and its neighbor, the lncRNA Peony, originate and are co-expressed from a singular polycistronic non-coding RNA locus. Alternative polyadenylation is implicated in regulation of their differential expression. While the increased expression of the lncRNA Peony results in the disruption of the muscle sheath covering the testis, the absence of miR-317 leads to the emergence of germline tumors in young flies. The deficiency of miR-317 increases Notch signaling activity in the somatic cyst cells, which drives germline tumorigenesis. Germline tumors also arise from upregulation of several predicted targets of miR-317, among which are regulators of the Notch pathway. This implicates miR-317 as a novel tumor suppressor that modulates Notch signaling strength.

Novel Genome-Engineered H Alleles Differentially Affect Lateral Inhibition and Cell Dichotomy Processes during Bristle Organ Development

Hairless (H) encodes the major antagonist in the Notch signaling pathway, which governs cellular differentiation of various tissues in Drosophila. By binding to the Notch signal transducer Suppressor of Hairless (Su(H)), H assembles repressor complexes onto Notch target genes. Using genome engineering, three new H alleles, HFA, HLLAA and HWA were generated and a phenotypic series was established by several parameters, reflecting the residual H-Su(H) binding capacity. Occasionally, homozygous HWA flies develop to adulthood. They were compared with the likewise semi-viable HNN allele affecting H-Su(H) nuclear entry. The H homozygotes were short-lived, sterile and flightless, yet showed largely normal expression of several mitochondrial genes. Typical for H mutants, both HWA and HNN homozygous alleles displayed strong defects in wing venation and mechano-sensory bristle development. Strikingly, however, HWA displayed only a loss of bristles, whereas bristle organs of HNN flies showed a complete shaft-to-socket transformation. Apparently, the impact of HWA is restricted to lateral inhibition, whereas that of HNN also affects the respective cell type specification. Notably, reduction in Su(H) gene dosage only suppressed the HNN bristle phenotype, but amplified that of HWA. We interpret these differences as to the role of H regarding Su(H) stability and availability.

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

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

New mutant alleles for Spargel/dPGC-1 highlights the function of Spargel RRM domain in oogenesis and expands the role of Spargel in embryogenesis and intracellular transport

Energy metabolism in vertebrates is controlled by three members of the PGC-1 (PPAR γ- coactivator 1) family, transcriptional coactivators that shape responses to physiological stimuli by interacting with the nuclear receptors and other transcription factors. Multiple evidence now supports that Spargel protein found in insects and ascidians is the ancestral form of vertebrate PGC-1's. Here, we undertook functional analysis of srl gene in Drosophila, asking about the requirement of Spargel per se during embryogenesis and its RNA binding domains. CRISPR- engineered srl gene deletion turned out to be an amorphic allele that is late embryonic/early larval lethal and Spargel protein missing its RNA binding domain (SrlΔRRM) negatively affects female fertility. Overexpression of wild-type Spargel in transgenic flies expedited the growth of egg chambers. On the other hand, oogenesis is blocked in a dominant-negative fashion in the presence of excess Spargel lacking its RRM domains. Finally, we observed aggregation of Notch proteins in egg chambers of srl mutant flies, suggesting that Spargel is involved in intracellular transport of Notch proteins. Taken together, we claim that these new mutant alleles of spargel are emerging powerful tools for revealing new biological functions for Spargel, an essential transcription coactivator in both Drosophila and mammals.

Tracking Follicle Cell Development

Somatic follicle cells are critical support cells for Drosophila oogenesis, as they provide signals and molecules needed to produce a mature egg. Throughout this process, the follicle cells differentiate into multiple subpopulations and transition between three different cell cycle programs to support nurse cell and oocyte development. The follicle cells are mitotic in early egg chamber development, as they cover the germline cyst. In mid-oogenesis, follicle cells switch from mitosis to endocycling, increasing their ploidy from 2C to 16C. Finally, in late oogenesis, cells transition from endocycling to gene amplification, increasing the copy number of a small subset of genes, including the genes encoding proteins required for egg maturation. In order to explore the genetic regulation of these cell cycle switches and follicle cell development and specification, clonal analysis and the GAL4/UAS system are used frequently to reduce or increase expression of genes of interest. These genetic approaches combined with immunohistochemistry and in situ hybridization are powerful tools for characterizing the mechanisms regulating follicle cell development and the mitosis/endocycle and endocycle/gene amplification transitions. This chapter describes the genetic tools available to manipulate gene expression in follicle cells, as well as the methods and reagents that can be utilized to explore gene expression throughout follicle cell development.

Integrins control epithelial stem cell proliferation in the Drosophila ovary by modulating the Notch pathway

Cell proliferation and differentiation show a remarkable inverse relationship. The temporal coupling between cell cycle withdrawal and differentiation of stem cells (SCs) is crucial for epithelial tissue growth, homeostasis and regeneration. Proliferation vs. differentiation SC decisions are often controlled by the surrounding microenvironment, of which the basement membrane (BM; a specialized form of extracellular matrix surrounding cells and tissues), is one of its main constituents. Years of research have shown that integrin-mediated SC-BM interactions regulate many aspects of SC biology, including the proliferation-to-differentiation switch. However, these studies have also demonstrated that the SC responses to interactions with the BM are extremely diverse and depend on the cell type and state and on the repertoire of BM components and integrins involved. Here, we show that eliminating integrins from the follicle stem cells (FSCs) of the Drosophila ovary and their undifferentiated progeny increases their proliferation capacity. This results in an excess of various differentiated follicle cell types, demonstrating that cell fate determination can occur in the absence of integrins. Because these phenotypes are similar to those found in ovaries with decreased laminin levels, our results point to a role for the integrin-mediated cell-BM interactions in the control of epithelial cell division and subsequent differentiation. Finally, we show that integrins regulate proliferation by restraining the activity of the Notch/Delta pathway during early oogenesis. Our work increases our knowledge of the effects of cell-BM interactions in different SC types and should help improve our understanding of the biology of SCs and exploit their therapeutic potential.

New Mutations in the 5′ Region of the Notch Gene Affect Drosophila melanogaster Oogenesis

The Notch pathway is an important and evolutionarily conserved signaling system involved in the development of multicellular organisms. Notch signaling plays an important role in the regulation of proliferation and differentiation of many cell types. In this study, we report new aspects of Notch gene participation in oogenesis using our previously generated mutations. The mutations consist of an insertion of an auxiliary element of a transgene construct into the first intron of the gene and a series of directed deletions within the 5′ regulatory region of Notch. We showed that some of these mutations affect Drosophila oogenesis. This insertion, either alone or in combination with the deletion of an insulator sequence, led to lower expression of Notch in the ovaries. As a result, the formation of egg chambers was disturbed in middle oogenesis. These abnormalities have not been described previously and imply one more function of Notch in oogenesis. It can be assumed that Notch is associated with not only follicular epithelium morphogenesis but also cellular mechanisms of oocyte growth.

Distant activation of Notch signaling induces stem cell niche assembly

Here we show that multiple modes of Notch signaling activation specify the complexity of spatial cellular interactions necessary for stem cell niche assembly. In particular, we studied the formation of the germline stem cell niche in Drosophila ovaries, which is a two-step process whereby terminal filaments are formed first. Then, terminal filaments signal to the adjacent cap cell precursors, resulting in Notch signaling activation, which is necessary for the lifelong acquisition of stem cell niche cell fate. The genetic data suggest that in order to initiate the process of stem cell niche assembly, Notch signaling is activated among non-equipotent cells via distant induction, where germline Delta is delivered to somatic cells located several diameters away via cellular projections generated by primordial germ cells. At the same time, to ensure the robustness of niche formation, terminal filament cell fate can also be induced by somatic Delta via cis- or trans-inhibition. This exemplifies a double security mechanism that guarantees that the germline stem cell niche is formed, since it is indispensable for the adjacent germline precursor cells to acquire and maintain stemness necessary for successful reproduction. These findings contribute to our understanding of the formation of stem cell niches in their natural environment, which is important for stem cell biology and regenerative medicine.

Adult organs often contain a stem cell niche that maintains stem cells necessary for the replenishment of different types of terminally differentiated cells that are continuously lost. This study reveals that various modes of Notch signaling activation induce the formation of the germline stem cell niche in Drosophila. We show for the first time that even among non-equipotent cells, Notch signaling can be trans-activated via distant induction mode, where the ligand Delta is delivered via cellular protrusions to the somatic stem cell niche precursors located several cell diameters away. Moreover, there is a second security mechanism controlled by the soma that additionally ensures that the stem cell niche is formed. In the stem cell niche precursors, Notch signaling can be locally inhibited by the somatic Delta. While Notch signaling trans-inhibition has been proposed via mathematical modelling, our findings show that a group of cells that have high Delta can be seen in a living organism, confirming that this mode of Notch signaling inhibition by trans-Delta exists in vivo. This work provides significant advances in the understanding of Notch signaling and the stem cell niche formation, which is important for the fields of stem cell biology and regenerative medicine.

Signaling between somatic follicle cells and the germline patterns the egg and embryo of Drosophila

In Drosophila, specification of the embryonic body axes requires signaling between the germline and the somatic follicle cells. These signaling events are necessary to properly localize embryonic patterning determinants in the egg or eggshell during oogenesis. There are three maternal patterning systems that specify the anterior-posterior axis, and one that establishes the dorsal-ventral axis. We will first review oogenesis, focusing on the establishment of the oocyte and nurse cells and patterning of the follicle cells into different subpopulations. We then describe how two coordinated signaling events between the oocyte and follicle cells establish polarity of the oocyte and localize the anterior determinant bicoid, the posterior determinant oskar, and Gurken/epidermal growth factor (EGF), which breaks symmetry to initiate dorsal-ventral axis establishment. Next, we review how dorsal-ventral asymmetry of the follicle cells is transmitted to the embryo. This process also involves Gurken-EGF receptor (EGFR) signaling between the oocyte and follicle cells, leading to ventrally-restricted expression of the sulfotransferase Pipe. These events promote the ventral processing of Spaetzle, a ligand for Toll, which ultimately sets up the embryonic dorsal-ventral axis. We then describe the activation of the terminal patterning system by specialized polar follicle cells. Finally, we present open questions regarding soma-germline signaling during Drosophila oogenesis required for cell identity and embryonic axis formation.

Time-resolved transcriptomics in neural stem cells identifies a v-ATPase/Notch regulatory loop

Drosophila melanogaster neural stem cells (neuroblasts) divide asymmetrically by differentially segregating protein determinants into their daughter cells. Wissel et al. use time-resolved transcriptional profiling to identify a v-ATPase/Notch regulatory loop that acts in multiple stem cell lineages both during nervous system development and in the adult gut.

Drosophila melanogaster neural stem cells (neuroblasts [NBs]) divide asymmetrically by differentially segregating protein determinants into their daughter cells. Although the machinery for asymmetric protein segregation is well understood, the events that reprogram one of the two daughter cells toward terminal differentiation are less clear. In this study, we use time-resolved transcriptional profiling to identify the earliest transcriptional differences between the daughter cells on their way toward distinct fates. By screening for coregulated protein complexes, we identify vacuolar-type H⁺–ATPase (v-ATPase) among the first and most significantly down-regulated complexes in differentiating daughter cells. We show that v-ATPase is essential for NB growth and persistent activity of the Notch signaling pathway. Our data suggest that v-ATPase and Notch form a regulatory loop that acts in multiple stem cell lineages both during nervous system development and in the adult gut. We provide a unique resource for investigating neural stem cell biology and demonstrate that cell fate changes can be induced by transcriptional regulation of basic, cell-essential pathways.

Polar cell fate stimulates Wolbachia intracellular growth

Bacteria are crucial partners in the development and evolution of vertebrates and invertebrates. A large fraction of insects harbor Wolbachia, bacterial endosymbionts that manipulate host reproduction to favor their spreading. Because they are maternally inherited, Wolbachia are under selective pressure to reach the female germline and infect the offspring. However, Wolbachia infection is not limited to the germline. Somatic cell types, including stem cell niches, have higher Wolbachia loads compared with the surrounding tissue. Here, we show a novel Wolbachia tropism to polar cells (PCs), specialized somatic cells in the Drosophila ovary. During oogenesis, all stages of PC development are easily visualized, facilitating the investigation of the kinetics of Wolbachia intracellular growth. Wolbachia accumulation is triggered by particular events of PC morphogenesis, including differentiation from progenitors and between stages 8 and 9 of oogenesis. Moreover, induction of ectopic PC fate is sufficient to promote Wolbachia accumulation. We found that Wolbachia PC tropism is evolutionarily conserved across most Drosophila species, but not in Culex mosquitos. These findings highlight the coordination of endosymbiont tropism with host development and cell differentiation.

Tyrosine phosphorylation and proteolytic cleavage of Notch are required for non-canonical Notch/Abl signaling in Drosophila axon guidance

Notch signaling is required for the development and physiology of nearly every tissue in metazoans. Much of Notch signaling is mediated by transcriptional regulation of downstream target genes, but Notch controls axon patterning in Drosophila by local modulation of Abl tyrosine kinase signaling, via direct interactions with the Abl co-factors Disabled and Trio. Here, we show that Notch-Abl axonal signaling requires both of the proteolytic cleavage events that initiate canonical Notch signaling. We further show that some Notch protein is tyrosine phosphorylated in Drosophila, that this form of the protein is selectively associated with Disabled and Trio, and that relevant tyrosines are essential for Notch-dependent axon patterning but not for canonical Notch-dependent regulation of cell fate. Based on these data, we propose a model for the molecular mechanism by which Notch controls Abl signaling in Drosophila axons.

Quantitative microscopy of the Drosophila ovary shows multiple niche signals specify progenitor cell fate

Adult stem cells commonly give rise to transit-amplifying progenitors, whose progeny differentiate into distinct cell types. It is unclear if stem cell niche signals coordinate fate decisions within the progenitor pool. Here we use quantitative analysis of Wnt, Hh, and Notch signalling reporters and the cell fate markers Eyes Absent (Eya) and Castor (Cas) to study the effects of hyper-activation and loss of niche signals on progenitor development in the Drosophila ovary. Follicle stem cell (FSC) progeny adopt distinct polar, stalk, and main body cell fates. We show that Wnt signalling transiently inhibits expression of the main body cell fate determinant Eya, and Wnt hyperactivity strongly biases cells towards polar and stalk fates. Hh signalling independently controls the proliferation to differentiation transition. Notch is permissive but not instructive for differentiation of multiple cell types. These findings reveal that multiple niche signals coordinate cell fates and differentiation of progenitor cells.

Notch signaling pathway promotes the development of ovine ovarian follicular granulosa cells

The Notch signaling pathway regulates cell proliferation, differentiation and apoptosis involved in development of the organs and tissues such as nervous system, cartilage, lungs, kidneys and prostate as well as the ovarian follicles. This study aimed to investigate the mRNA expression and localization of NOTCH2, as the key factor in Notch signaling pathway. This was determined by PCR, real-time PCR and immunohistochemistry. Additionally, the effects of inhibiting Notch signaling pathway with different concentrations (5μM, 10μM and 20μM) of N-[N-(3, 5-Difuorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT), an inhibitor of Notch signaling pathway, on ovine granulosa cells was determined in vitro by detecting estradiol production using enzyme linked immunosorbent assay and expressions of the genes related to the cell cycle and apoptosis using real-time polymerase chain reaction (PCR). NOTCH2, the key member of Notch signaling pathway, was found in ovine follicles, and the expression of NOTCH2 mRNA was highest in the theca cells of the follicles in medium sizes (3-5mm in diameter) and granulosa cells of the follicles in large sizes (>5mm in diameter). Immunohistochemical results demonstrated that NOTCH2 protein was expressed in granulosa cells of preantral follicles, in both granulosa cells and theca cells of antral follicles. Compared with DAPT-treated groups, the control group had a higher number of granulosa cells (P<0.05) and a higher estradiol production (P<0.05). Compared with the control group, the mRNA abundances of HES1, MYC, BAX, BCL2 and CYP19A1 in DAPT-treated groups was lower (P<0.05), respectively; whereas, the expression of CCND2, CDKN1A and TP53 mRNA showed no remarkable difference compared with control group. Collectively, Notch signaling pathway could be involved in the ovine follicular development by regulating the growth and estradiol production of granulosa cells.

In vivo RNAi in the Drosophila Follicular Epithelium: Analysis of Stem Cell Maintenance, Proliferation, and Differentiation

In vivo RNAi in Drosophila facilitates simple and rapid analysis of gene functions in a cell- or tissue-specific manner. The versatility of the UAS-GAL4 system allows to control exactly where and when during development the function of a gene is depleted. The epithelium of the ovary is a particularly good model to study in a living animal how stem cells are maintained and how their descendants proliferate and differentiate. Here I provide basic information about the publicly available reagents for in vivo RNAi, and I describe how the oogenesis system can be applied to analyze stem cells and epithelial development at a histological level. Moreover, I give helpful hints to optimize the use of the UAS-GAL4 system for RNAi induction in the follicular epithelium. Finally, I provide detailed step-by-step protocols for ovary dissection, antibody stainings, and ovary mounting for microscopic analysis.

Phosphorylated Groucho delays differentiation in the follicle stem cell lineage by providing a molecular memory of EGFR signaling in the niche

In the epithelial follicle stem cells (FSCs) of the Drosophila ovary, Epidermal Growth Factor Receptor (EGFR) signaling promotes self-renewal, whereas Notch signaling promotes differentiation of the prefollicle cell (pFC) daughters. We have identified two proteins, Six4 and Groucho (Gro), that link the activity of these two pathways to regulate the earliest cell fate decision in the FSC lineage. Our data indicate that Six4 and Gro promote differentiation towards the polar cell fate by promoting Notch pathway activity. This activity of Gro is antagonized by EGFR signaling, which inhibits Gro-dependent repression via p-ERK mediated phosphorylation. We have found that the phosphorylated form of Gro persists in newly formed pFCs, which may delay differentiation and provide these cells with a temporary memory of the EGFR signal. Collectively, these findings demonstrate that phosphorylated Gro labels a transition state in the FSC lineage and describe the interplay between Notch and EGFR signaling that governs the differentiation processes during this period.

Increased intracellular pH is necessary for adult epithelial and embryonic stem cell differentiation

Despite extensive knowledge about the transcriptional regulation of stem cell differentiation, less is known about the role of dynamic cytosolic cues. We report that an increase in intracellular pH (pHi) is necessary for the efficient differentiation of Drosophila adult follicle stem cells (FSCs) and mouse embryonic stem cells (mESCs). We show that pHi increases with differentiation from FSCs to prefollicle cells (pFCs) and follicle cells. Loss of the Drosophila Na+-H+ exchanger DNhe2 lowers pHi in differentiating cells, impairs pFC differentiation, disrupts germarium morphology, and decreases fecundity. In contrast, increasing pHi promotes excess pFC cell differentiation toward a polar/stalk cell fate through suppressing Hedgehog pathway activity. Increased pHi also occurs with mESC differentiation and, when prevented, attenuates spontaneous differentiation of naive cells, as determined by expression of microRNA clusters and stage-specific markers. Our findings reveal a previously unrecognized role of pHi dynamics for the differentiation of two distinct types of stem cell lineages, which opens new directions for understanding conserved regulatory mechanisms.

Uncovering Notch pathway in the parasitic flatworm Schistosoma mansoni

Several signaling molecules that govern development in higher animals have been identified in the parasite Schistosoma mansoni, including the transforming growth factor β, protein tyrosine kinases, nuclear hormone receptors, among others. The Notch pathway is a highly conserved signaling mechanism which is involved in a wide variety of developmental processes including embryogenesis and oogenesis in worms and flies. Here we aimed to provide the molecular reconstitution of the Notch pathway in S. mansoni using the available transcriptome and genome databases. Our results also revealed the presence of the transcripts coded for SmNotch, SmSu(H), SmHes, and the gamma-secretase complex (SmNicastrin, SmAph-1, and SmPen-2), throughout all the life stages analyzed. Besides, it was observed that the viability and separation of adult worm pairs were not affected by treatment with N-[N(3,5)-difluorophenacetyl)-L-Alanyl]-S-phenylglycine t-butyl ester (DAPT), a Notch pathway inhibitor. Moreover, DAPT treatment decreased the production of phenotypically normal eggs and arrested their development in culture. Our results also showed a significant decrease in SmHes transcript levels in both adult worms and eggs treated with DAPT. These results provide, for the first time, functional validation of the Notch pathway in S. mansoni and suggest its involvement in parasite oogenesis and embryogenesis. Given the complexity of the Notch pathway, further experiments shall highlight the full repertoire of Notch-mediated cellular processes throughout the S. mansoni life cycle.

Novel tools for genetic manipulation of follicle stem cells in the Drosophila ovary reveal an integrin-dependent transition from quiescence to proliferation

In many tissues, the presence of stem cells is inferred by the capacity of the tissue to maintain homeostasis and undergo repair after injury. Isolation of self-renewing cells with the ability to generate the full array of cells within a given tissue strongly supports this idea, but the identification and genetic manipulation of individual stem cells within their niche remain a challenge. Here we present novel methods for marking and genetically altering epithelial follicle stem cells (FSCs) within the Drosophila ovary. Using these new tools, we define a sequential multistep process that comprises transitioning of FSCs from quiescence to proliferation. We further demonstrate that integrins are cell-autonomously required within FSCs to provide directional signals that are necessary at each step of this process. These methods may be used to define precise roles for specific genes in the sequential events that occur during FSC division after a period of quiescence.

Unlike in Drosophila Meroistic Ovaries, hippo represses notch in Blattella germanica Panoistic ovaries, triggering the mitosis-endocycle switch in the follicular cells

During insect oogenesis, the follicular epithelium undergoes both cell proliferation and apoptosis, thus modulating ovarian follicle growth. The Hippo pathway is key in these processes, and has been thoroughly studied in the meroistic ovaries of Drosophila melanogaster. However, nothing is known about the role of the Hippo pathway in primitive panoistic ovaries. This work examines the mRNA expression levels of the main components of the Hippo pathway in the panoistic ovary of the basal insect species Blattella germanica, and demonstrates the function of Hippo through RNAi. In Hippo-depleted specimens, the follicular cells of the basal ovarian follicles proliferate without arresting cytokinesis; the epithelium therefore becomes bilayered, impairing ovarian follicle growth. This phenotype is accompanied by long stalks between the ovarian follicles. In D. melanogaster loss of function of Notch determines that the stalk is not developed. With this in mind, we tested whether Hippo and Notch pathways are related in B. germanica. In Notch (only)-depleted females, no stalks were formed between the ovarian follicles. Simultaneous depletion of Hippo and Notch rescued partially the stalk to wild-type. Unlike in the meroistic ovaries of D. melanogaster, in panoistic ovaries the Hippo pathway appears to regulate follicular cell proliferation by acting as a repressor of Notch, triggering the switch from mitosis to the endocycle in the follicular cells. The phylogenetically basal position of B. germanica suggests that this might be the ancestral function of Hippo in insect ovaries.

A genome-scale in vivo RNAi analysis of epithelial development in Drosophila identifies new proliferation domains outside of the stem cell niche

The Drosophila oogenesis system provides an excellent model to study the development of epithelial tissues. Here, we report the first genome-scale in vivo RNA interference (RNAi) screen for genes controlling epithelial development. By directly analysing cell and tissue architecture we identified 1125 genes, which we assigned to seven different functions in epithelial formation and homeostasis. We validated the significance of our screen by generating mutants for Vps60, a component of the endosomal sorting complexes required for transport (ESCRT) machinery. This analysis provided new insights into spatiotemporal control of cell proliferation in the follicular epithelium. Previous studies have identified signals controlling divisions in the follicle stem cell niche. However, 99% of cell divisions occur outside of the niche and it is unclear how these divisions are controlled. Our data distinguish two new domains outside of the stem cell niche where there are differing controls on proliferation. One domain abuts the niche and is characterised by ESCRT, Notch and JAK/STAT-mediated control of proliferation. Adjacent to this domain, another domain is defined by loss of the impact of ESCRT on cell division. Thus, during development epithelial cells pass through a variety of microenvironments that exert different modes of proliferation control. The switch between these modes might reflect a decrease in the 'stemness' of epithelial cells over time.

Castor is required for Hedgehog-dependent cell-fate specification and follicle stem cell maintenance in Drosophila oogenesis

Asymmetric division of stem cells results in both self-renewal and differentiation of daughters. Understanding the molecules and mechanisms that govern differentiation of specific cell types from adult tissue stem cells is a major challenge in developmental biology and regenerative medicine. Drosophila follicle stem cells (FSCs) represent an excellent model system to study adult stem cell behavior; however, the earliest stages of follicle cell differentiation remain largely mysterious. Here we identify Castor (Cas) as a nuclear protein that is expressed in FSCs and early follicle cell precursors and then is restricted to differentiated polar and stalk cells once egg chambers form. Cas is required for FSC maintenance and polar and stalk cell fate specification. Eyes absent (Eya) is excluded from polar and stalk cells and represses their fate by inhibiting Cas expression. Hedgehog signaling is essential to repress Eya to allow Cas expression in polar and stalk cells. Finally, we show that the complementary patterns of Cas and Eya reveal the gradual differentiation of polar and stalk precursor cells at the earliest stages of their development. Our studies provide a marker for cell fates in this model and insight into the molecular and cellular mechanisms by which FSC progeny diverge into distinct fates.

Effects of curcumin on the parasite Schistosoma mansoni: a transcriptomic approach

Schistosomiasis remains a severe problem of public health in developing countries. Several reports show that praziquantel, the drug of choice for treating schistosomiasis, can select Schistosoma mansoni strains resistant to the drug. Thus, developing new drugs against this parasitosis is a highly desirable goal. Curcumin, a phenolic compound deriving from the plant Curcuma longa, has been shown to have anticancer, anti-inflammatory and antiparasitic effects. Recently, our group has demonstrated that curcumin causes the separation of S. mansoni adult worm pairs, eggs infertility, decreased oviposition and parasite viability, leading to death. In the present work, we have investigated the effects of curcumin on S. mansoni gene expression in adult worms through microarray analyses. Our results showed 2374 genes that were significantly and differentially expressed, of which 981 were up-regulated and 1393 were down-regulated. Among the differentially expressed genes there were components of important signaling pathways involved in embryogenesis and oogenesis such as Notch and TGF-β. Gene networks most significantly enriched with altered genes were identified, including a network related to Cellular Function and Maintenance, Molecular Transport, Organ Development, which is connected to the TGF-β signaling pathway and might be related to the effect of curcumin on pairing of adult worm pairs, egg production and viability of worms. qPCR validation experiments with 7 genes have confirmed the expression changes detected with arrays. Here we suggest that transcriptional repression observed in Notch and TGF-β pathways could explain the effects on oviposition and egg development described in the literature.

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.

Activation of Notch in lgd mutant cells requires the fusion of late endosomes with the lysosome

The tumorsuppressor Lethal (2) giant discs (Lgd) is a regulator of endosomal trafficking of the Notch signalling receptor as well as other transmembrane proteins in Drosophila. The loss of its function results in an uncontrolled ligand independent activation of of the Notch signalling receptor. Here, we further investigated the consequences of loss of lgd function and the requirements for the activation of Notch. We show that the activation of Notch in lgd cells is independent of Kuz and dependent on γ-secretase. We found that the lgd cells have a defect that delays degradation of transmembrane proteins, which are residents of the plasma membrane. Furthermore, our results show that the activation of Notch in lgd cells occurs in the lysosome. In contrast, the pathway is activated at an earlier phase in mutants of the gene that encodes the ESCRT-III component Shrub, which is an interaction partner of Lgd. We further show that activation of Notch appears to be a general consequence of loss of lgd function. In addition, we describe the EM analysis of lgd cells, which revealed that lgd cells contain enlarged MVBs. The presented results further elucidate the mechanism of uncontrolled Notch activation upon derailed endocytosis.

The Hippo pathway controls polar cell fate through Notch signaling during Drosophila oogenesis

During Drosophila oogenesis, the somatic follicle cells form an epithelial layer surrounding the germline cells to form egg chambers. In this process, follicle cell precursors are specified into polar cells, stalk cells, and main-body follicle cells. Proper specification of these three cell types ensures correct egg chamber formation and polarization of the anterior-posterior axis of the germline cells. Multiple signaling cascades coordinate to control the follicle cell fate determination, including Notch, JAK/STAT, and Hedgehog signaling pathways. Here, we show that the Hippo pathway also participates in polar cell specification. Over-activation of yorkie (yki) leads to egg chamber fusion, possibly through attenuation of polar cell specification. Loss-of-function experiments using RNAi knockdown or generation of mutant clones by mitotic recombination demonstrates that reduction of yki expression promotes polar cell formation in a cell-autonomous manner. Consistently, polar cells mutant for hippo (hpo) or warts (wts) are not properly specified, leading to egg chamber fusion. Furthermore, Notch activity is increased in yki mutant cells and reduction of Notch activity suppresses polar cell formation in yki mutant clones. These results demonstrate that yki represses polar cell fate through Notch signaling. Collectively, our data reveal that the Hippo pathway controls polar cell specification. Through repressing Notch activity, Yki serves as a key repressor in specifying polar cells during Drosophila oogenesis.

Serial specification of diverse neuroblast identities from a neurogenic placode by Notch and Egfr signaling

We used the brain insulin-producing cell (IPC) lineage and its identified neuroblast (IPC NB) as a model to understand a novel example of serial specification of NB identities in the Drosophila dorsomedial protocerebral neuroectoderm. The IPC NB was specified from a small, molecularly identified group of cells comprising an invaginated epithelial placode. By progressive delamination of cells, the placode generated a series of NB identities, including the single IPC NB, a number of other canonical Type I NBs, and a single Type II NB that generates large lineages by transient amplification of neural progenitor cells. Loss of Notch function caused all cells of the placode to form as supernumerary IPC NBs, indicating that the placode is initially a fate equivalence group for the IPC NB fate. Loss of Egfr function caused all placodal cells to apoptose, except for the IPC NB, indicating a requirement of Egfr signaling for specification of alternative NB identities. Indeed, both derepressed Egfr activity in yan mutants and ectopic EGF activity produced supernumerary Type II NBs from the placode. Loss of both Notch and Egfr function caused all placode cells to become IPC NBs and survive, indicating that commitment to NB fate nullified the requirement of Egfr activity for placode cell survival. We discuss the surprising parallels between the serial specification of neural fates from this neurogenic placode and the fly retina.

Rab11 is required for epithelial cell viability, terminal differentiation, and suppression of tumor-like growth in the Drosophila egg chamber

BACKGROUND

The Drosophila egg chamber provides an excellent system in which to study the specification and differentiation of epithelial cell fates because all of the steps, starting with the division of the corresponding stem cells, called follicle stem cells, have been well described and occur many times over in a single ovary.

METHODOLOGY/PRINCIPAL FINDINGS

Here we investigate the role of the small Rab11 GTPase in follicle stem cells (FSCs) and in their differentiating daughters, which include main body epithelial cells, stalk cells and polar cells. We show that rab11-null FSCs maintain their ability to self renew, even though previous studies have shown that FSC self renewal is dependent on maintenance of E-cadherin-based intercellular junctions, which in many cell types, including Drosophila germline stem cells, requires Rab11. We also show that rab11-null FSCs give rise to normal numbers of cells that enter polar, stalk, and epithelial cell differentiation pathways, but that none of the cells complete their differentiation programs and that the epithelial cells undergo premature programmed cell death. Finally we show, through the induction of rab11-null clones at later points in the differentiation program, that Rab11 suppresses tumor-like growth of epithelial cells. Thus, rab11-null epithelial cells arrest differentiation early, assume an aberrant cell morphology, delaminate from the epithelium, and invade the neighboring germline cyst. These phenotypes are associated with defects in E-cadherin localization and a general loss of cell polarity.

CONCLUSIONS/SIGNIFICANCE

While previous studies have revealed tumor suppressor or tumor suppressor-like activity for regulators of endocytosis, our study is the first to identify such activity for regulators of endocytic recycling. Our studies also support the recently emerging view that distinct mechanisms regulate junction stability and plasticity in different tissues.

Notch signaling and developmental cell-cycle arrest in Drosophila polar follicle cells

Temporal and spatial regulation of cell division is critical for proper development of multicellular organisms. An important aspect of this regulation is cell-cycle arrest, which in many cell types is coupled with differentiated status. Here we report that the polar cells--a group of follicle cells differentiated early during Drosophila oogenesis--are arrested at G2 phase and can serve as a model cell type for investigation of developmental regulation of cell-cycle arrest. On examining the effects of String, a mitosis-promoting phosphatase Cdc25 homolog, and Notch signaling in polar cells, we found that misexpression of String can trigger mitosis in existing polar cells to induce extra polar cells. Normally, differentiation of the polar cells requires Notch signaling. We found that the Notch-induced extra polar cells arise through recruitment of the neighboring cells rather than promotion of proliferation, and they are also arrested at G2 phase. Notch signaling is probably involved in down-regulating String in polar cells, thus inducing the G2 cell-cycle arrest.

The cytolinker Pigs is a direct target and a negative regulator of Notch signalling

Gas2-like proteins harbour putative binding sites for both the actin and the microtubule cytoskeleton and could thus mediate crosstalk between these cytoskeletal systems. Family members are highly conserved in all metazoans but their in vivo role is not clear. The sole Drosophila Gas2-like gene, CG3973 (pigs), was recently identified as a transcriptional target of Notch signalling and might therefore link cell fate decisions through Notch activation directly to morphogenetic changes. We have generated a null mutant in CG3973 (pigs): pigs(1) mutants are semi-viable but adult flies are flightless, showing indirect flight muscle degeneration, and females are sterile, showing disrupted oogenesis and severe defects in follicle cell differentiation, similar to phenotypes seen when levels of Notch/Delta signalling are perturbed in these tissues. Loss of Pigs leads to an increase in Notch signalling activity in several tissues. These results indicate that Gas2-like proteins are essential for development and suggest that Pigs acts downstream of Notch as a morphogenetic read-out, and also as part of a regulatory feedback loop to relay back information about the morphogenetic state of cells to restrict Notch activation to appropriate levels in certain target tissues.

Bazooka regulates microtubule organization and spatial restriction of germ plasm assembly in the Drosophila oocyte

Localization of the germ plasm to the posterior of the Drosophila oocyte is required for anteroposterior patterning and germ cell development during embryogenesis. While mechanisms governing the localization of individual germ plasm components have been elucidated, the process by which germ plasm assembly is restricted to the posterior pole is poorly understood. In this study, we identify a novel allele of bazooka (baz), the Drosophila homolog of Par-3, which has allowed the analysis of baz function throughout oogenesis. We demonstrate that baz is required for spatial restriction of the germ plasm and axis patterning, and we uncover multiple requirements for baz in regulating the organization of the oocyte microtubule cytoskeleton. Our results suggest that distinct cortical domains established by Par proteins polarize the oocyte through differential effects on microtubule organization. We further show that microtubule plus-end enrichment is sufficient to drive germ plasm assembly even at a distance from the oocyte cortex, suggesting that control of microtubule organization is critical not only for the localization of germ plasm components to the posterior of the oocyte but also for the restriction of germ plasm assembly to the posterior pole.

Role of Notch signaling in establishing the hemilineages of secondary neurons in Drosophila melanogaster

The secondary neurons generated in the thoracic central nervous system of Drosophila arise from a hemisegmental set of 25 neuronal stem cells, the neuroblasts (NBs). Each NB undergoes repeated asymmetric divisions to produce a series of smaller ganglion mother cells (GMCs), which typically divide once to form two daughter neurons. We find that the two daughters of the GMC consistently have distinct fates. Using both loss-of-function and gain-of-function approaches, we examined the role of Notch signaling in establishing neuronal fates within all of the thoracic secondary lineages. In all cases, the 'A' (Notch(ON)) sibling assumes one fate and the 'B' (Notch(OFF)) sibling assumes another, and this relationship holds throughout the neurogenic period, resulting in two major neuronal classes: the A and B hemilineages. Apparent monotypic lineages typically result from the death of one sibling throughout the lineage, resulting in a single, surviving hemilineage. Projection neurons are predominantly from the B hemilineages, whereas local interneurons are typically from A hemilineages. Although sibling fate is dependent on Notch signaling, it is not necessarily dependent on numb, a gene classically involved in biasing Notch activation. When Numb was removed at the start of larval neurogenesis, both A and B hemilineages were still generated, but by the start of the third larval instar, the removal of Numb resulted in all neurons assuming the A fate. The need for Numb to direct Notch signaling correlated with a decrease in NB cell cycle time and may be a means for coping with multiple sibling pairs simultaneously undergoing fate decisions.

Regulation of epithelial stem cell replacement and follicle formation in the Drosophila ovary

Though much has been learned about the process of ovarian follicle maturation through studies of oogenesis in both vertebrate and invertebrate systems, less is known about how follicles form initially. In Drosophila, two somatic follicle stem cells (FSCs) in each ovariole give rise to all polar cells, stalk cells, and main body cells needed to form each follicle. We show that one daughter from each FSC founds most follicles but that cell type specification is independent of cell lineage, in contrast to previous claims of an early polar/stalk lineage restriction. Instead, key intercellular signals begin early and guide cell behavior. An initial Notch signal from germ cells is required for FSC daughters to migrate across the ovariole and on occasion to replace the opposite stem cell. Both anterior and posterior polar cells arise in region 2b at a time when approximately 16 cells surround the cyst. Later, during budding, stalk cells and additional polar cells are specified in a process that frequently transfers posterior follicle cells onto the anterior surface of the next older follicle. These studies provide new insight into the mechanisms that underlie stem cell replacement and follicle formation during Drosophila oogenesis.

Comparison of queen-specific gene expression in related lower termite species

The molecular mechanisms regulating caste determination and reproductive division of labor, the hallmarks of insect societies, are poorly defined. The identification of key genes involved in these developmentally important processes will be essential to gain a better understanding of the mechanisms controlling one of the most impressive examples of polyphenism, the caste structure of eusocial species. Here, we applied representational difference analysis of cDNAs, to study differential gene expression between queens (female neotenics) and workers in the dry wood termite Cryptotermes cynocephalus and identified 13 genes that were highly expressed in queens. In addition, we partially cloned several homologous genes of the related termite species Cryptotermes secundus and compared the expression profiles of 10 homologous genes. In most cases, the preferential expression in female neotenics was not conserved between species, despite the close phylogenetic relationship of both Cryptotermes species. It is possible that these genes are associated with known species-specific differences in caste development modes. Only three genes (Neofem1, 2, and 3) showed a conserved and highly preferential expression in female neotenics, suggesting that their products may play important roles in female reproductives, in particular in controlling caste determination and reproductive division of labor.

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.

The master switch gene sex-lethal promotes female development by negatively regulating the N-signaling pathway

Notch (N) signaling is used for cell-fate determination in many different developmental contexts. Here, we show that the master control gene for sex determination in Drosophila melanogaster, Sex-lethal (Sxl), negatively regulates the N-signaling pathway in females. In genetic assays, reducing Sxl activity suppresses the phenotypic effects of N mutations, while increasing Sxl activity enhances the effects. Sxl appears to negatively regulate the pathway by reducing N protein accumulation, and higher levels of N are found in Sxl(-) clones than in adjacent wild-type cells. The inhibition of N expression does not depend on the known downstream targets of Sxl; however, we find that Sxl protein can bind to N mRNAs. Finally, our results indicate that downregulation of the N pathway by Sxl contributes to sex-specific differences in morphology and suggest that it may also play an important role in follicle cell specification during oogenesis.

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

Boundaries establish and maintain separate populations of cells critical for organ formation. We show that Notch signaling establishes the boundary between two types of post-mitotic epithelial cells, the Rhomboid- and the Broad-positive cells. These cells will undergo morphogenetic movements to generate the two sides of a simple organ, the dorsal appendage tube of the Drosophila egg chamber. The boundary forms due to a difference in Notch levels in adjacent cells. The Notch expression pattern mimics the boundary; Notch levels are high in Rhomboid cells and low in Broad cells. Notch(-) mutant clones generate an ectopic boundary: ectopic Rhomboid cells arise in Notch(+) cells adjacent to the Notch(-) mutant cells but not further away from the clonal border. Pangolin, a component of the Wingless pathway, is required for Broad expression and for rhomboid repression. We further show that Broad represses rhomboid cell autonomously. Our data provide a foundation for understanding how a single row of Rhomboid cells arises adjacent to the Broad cells in the dorsal appendage primordia. Generating a boundary by the Notch pathway might constitute an evolutionarily conserved first step during organ formation in many tissues.

Bunched sets a boundary for Notch signaling to pattern anterior eggshell structures during Drosophila oogenesis

Organized boundaries between different cell fates are critical in patterning and organogenesis. In some tissues, long-range signals position a boundary, and local Notch signaling maintains it. How Notch activity is restricted to boundary regions is not well understood. During Drosophila oogenesis, the long-range signals EGF and Dpp regulate expression of bunched (bun), which encodes a homolog of mammalian transcription factors TSC-22 and GILZ. Here, we show that bun establishes a boundary for Notch signaling in the follicle cell epithelium. Notch signaling is active in anterior follicle cells and is required for concurrent follicle cell reorganizations including centripetal migration and operculum formation. bun is required in posterior columnar follicle cells to repress the centripetal migration fate, including gene expression, cell shape changes and accumulation of cytoskeletal components. bun mutant clones adjacent to the centripetally migrating follicle cells showed ectopic Notch responses. bun is necessary, but not sufficient, to down-regulate Serrate protein levels throughout the follicular epithelium. These data indicate that Notch signaling is necessary, but not sufficient, for centripetal migration and that bun regulates the level of Notch stimulation to position the boundary between centripetally migrating and stationary columnar follicle cells.

Genome wide analysis of transcript levels after perturbation of the EGFR pathway in the Drosophila ovary

Defects in the epidermal growth factor receptor (EGFR) pathway can lead to aggressive tumor formation. Activation of this pathway during normal development produces multiple outcomes at the cellular level, leading to cellular differentiation and cell cycle activation. To elucidate the downstream events induced by this pathway, we used genome-wide cDNA microarray technology to identify potential EGFR targets in Drosophila oogenesis. We focused on genes for which the transcriptional responses due to EGFR pathway activation and inactivation were in opposite directions, as this is expected for genes that are directly regulated by the pathway in this tissue type. We perturbed the EGFR pathway in epithelial follicle cells using seven different genetic backgrounds. To activate the pathway, we overexpressed an activated form of the EGFR (UAS-caEGFR), and an activated form of the signal transducer Raf (UAS-caRaf); we also over- or ectopically expressed the downstream homeobox transcription factor Mirror (UAS-mirr) and the ligand-activating serine protease Rhomboid (UAS-rho). To reduce pathway activity we used loss-of-function mutations in the ligand (gurken) and receptor (torpedo). From microarrays containing 6,255 genes, we found 454 genes that responded in an opposite manner in gain-of-function and loss-of-function conditions among which are many Wingless signaling pathway components. Further analysis of two such components, sugarless and pangolin, revealed a function for these genes in late follicle cell patterning. Of interest, components of other signaling pathways were also enriched in the EGFR target group, suggesting that one reason for the pleiotropic effects seen with EGFR activity in cancer progression and development may be its ability to regulate many other signaling pathways.

Fringe-dependent notch activation and tramtrack function are required for specification of the polar cells inDrosophilaoogenesis

During Drosophila oogenesis, each egg chamber is encapsulated through the coordinated signaling of multiple pathways, resulting in the formation of polar cells at the termini and a row of stalk cells in between each egg chamber. Notch signaling is required for specification of a precursor group containing both stalk and polar cells. Together, the Notch and JAK/STAT pathways specify the stalk cells as well as a group of prepolar cells, from within that group. The mechanism by which the polar cells differentiate from the prepolar group involves apoptosis, but the pathways which control that process are largely unknown. We now demonstrate that Notch signaling, activated by Delta and transduced by the transcription factor Tramtrack, is involved in the process of refining the prepolar cell group to two polar cells. The glycosyltransferase Fringe is expressed and required cell-autonomously in prepolar cells for this process. However, the transcription factor Mirror, which inhibits fringe expression in other tissues and stages of development, as well as Serrate, one of the two known ligands for Notch, are not required for maturation of prepolar cells. This finding suggests that Fringe is necessary for generating positional information in localizing a high-affinity interaction between Notch and its ligand Delta, even if a second ligand is not essential.

A Notch/Delta-Dependent Relay Mechanism Establishes Anterior-Posterior Polarity in Drosophila

The anterior-posterior axis of Drosophila becomes polarized early in oogenesis, when the oocyte moves to the posterior of the germline cyst because it preferentially adheres to posterior follicle cells. The source of this asymmetry is unclear, however, since anterior and posterior follicle cells are equivalent until midoogenesis, when Gurken signaling from the oocyte induces posterior fate. Here, we show that asymmetry arises because each cyst polarizes the next cyst through a series of posterior to anterior inductions. Delta signaling from the older cyst induces the anterior polar follicle cells, the anterior polar cells signal through the JAK/STAT pathway to induce the formation of the stalk between adjacent cysts, and the stalk polarizes the younger anterior cyst by inducing the shape change and preferential adhesion that position the oocyte at the posterior. The anterior-posterior axis is therefore established by a relay mechanism, which propagates polarity from one cyst to the next.

The polarisation of the anteroposterior axis in Drosophila

The polarisation of the embryonic anteroposterior (AP) axis requires the establishment of positional cues with spatial information, and often involves complex intercellular communications, cell adhesion and cell movement. Recent work on several fronts has begun to shed light on how the initial asymmetries are established and maintained. In this review, I discuss the polarisation of the AP axis during Drosophila oogenesis, focusing on the function of the Notch signalling pathway and its relationship to the activation of the epidermal growth factor receptor. I make special reference to some aspects of Notch activity regulation during oogenesis that appear to depart from the canonical pathway. Finally, I hypothesise on possible similarities between these activities of Notch signalling during Drosophila oogenesis and vertebrate somitogenesis.

General outlines of the molecular genetics of the Notch signalling pathway in Drosophila melanogaster: a review

The Notch signalling pathway appears to be ubiquitous in virtually all cell-cell contacts in all metazoan animals, and is best known and most throughout studied in Drosophila melanogaster. In this species the Notch signalling pathway regulates, with both positive and negative signals, the differentiation of at least central and peripheral nervous system and eye, wing disc, oogenesis, segmental appendages such as antennae and legs, and muscles, through lateral inhibition or induction. In general, the pathway works as follows: Notch is most likely a dimeric transmembrane receptor at the cell surface, where it is activated by its ligands Serrate and or Delta from the neighbouring cell Fringe, discriminating between the two ligands. Then, the receptor is cleaved by a proteolytic mechanism in which Presenilin plays an important role, and the intracellular domain is transferred to the nucleus, where it, together with the Suppressor of Hairless protein, constitutes a transcription factor which activates the Notch target genes, mainly located in the Enhancer of split complex. These target genes then encode repressor proteins.

Translational regulation and RNA localization in Drosophila oocytes and embryos

Translational control is a prevalent means of gene regulation during Drosophila oogenesis and embryogenesis. Multiple maternal mRNAs are localized within the oocyte, and this localization is often coupled to their translational regulation. Subsequently, translational control allows maternally deposited mRNAs to direct the early stages of embryonic development. In this review we outline some general mechanisms of translational regulation and mRNA localization that have been uncovered in various model systems. Then we focus on the posttranscriptional regulation of four maternal transcripts in Drosophila that are localized during oogenesis and are critical for embryonic patterning: bicoid (bcd), nanos (nos), oskar (osk), and gurken (grk). Cis- and trans-acting factors required for the localization and translational control of these mRNAs are discussed along with potential mechanisms for their regulation.

Drosophila oogenesis: coordinating germ line and soma

A new function for Delta-Notch signaling has been discovered in Drosophila oogenesis: Delta expressed in the germ cells activates Notch in the surrounding somatic follicle cells to control their differentiation, proliferation and morphogenesis.

Notch receptors and hematopoiesis

Notch receptors are involved in a variety of cell-fate decisions that affect the development and function of many organs, including hematopoiesis and the immune system. There are four mammalian Notch receptors that have only partially overlapping functions despite sharing similar structures and ligands. The ligands for Notch are transmembrane proteins expressed on adjacent cells, including Jagged and Delta, and it is quite possible that signaling is bidirectional. A large Notch precursor protein is proteolytically cleaved to form the mature cell-surface receptor. Ligand binding induces additional proteolytic events followed by translocation of the intracellular domain to the nucleus. There, Notch interacts with transcription factors such as RBPJ kappa, activating transcription of basic helix-loop-helix genes such as HES1. These in turn regulate expression of tissue-specific transcription factors that influence lineage commitment and other events. In this review, the details of Notch signaling will be discussed, with a focus on what is known about the role of Notch in hematopoiesis.

Delta signaling from the germ line controls the proliferation and differentiation of the somatic follicle cells during Drosophila oogenesis

The body axes of Drosophila are established during oogenesis through reciprocal interactions between the germ line cells and the somatic follicle cells that surround them. The Notch pathway is required at two stages in this process: first, for the migration of the follicle cells around the germ line cyst and, later, for the polarization of the anterior-posterior (A-P) axis of the oocyte. Its function in these events, however, has remained controversial. Using clonal analysis, we show that Notch signaling controls cell proliferation and differentiation in the whole follicular epithelium. Notch mutant follicle cells remain in a precursor state and fail to switch from the mitotic cell cycle to the endocycle. Furthermore, removal of Delta from the germ line produces an identical phenotype, showing that Delta signals from the germ cells to control the timing of follicle cell differentiation. This explains the axis formation defects in Notch mutants, which arise because undifferentiated posterior follicle cells cannot signal to polarize the oocyte. Delta also signals from the germ line to Notch in the soma earlier in oogenesis to control the differentiation of the polar and stalk follicle cells. The germ line therefore regulates the development of the follicle cells through two complementary signaling pathways: Gurken signals twice to control spatial patterning, whereas Delta signals twice to exert temporal control.

Bicaudal-D is essential for egg chamber formation and cytoskeletal organization in drosophila oogenesis

Bicaudal-D (Bic-D) is required for the transport of determinant mRNAs and proteins to the presumptive oocyte, an essential step in the differentiation of the oocyte. Bic-D protein contains four well-defined heptad repeat domains characteristic of intermediate filament proteins. We characterized the ovarian phenotypes of females expressing mutant Bic-D proteins (Bic-D(H)) deleted for each of the heptad repeat domains. The altered migration of follicle cells we observe in mutant ovaries suggests that Bic-D functions in the germline and directs the inward migration of somatic follicle cells. In the germarium Bic-D is required for the organization of the egg chamber and the structural integrity of the oocyte and nurse cells. Examination of the polarized microtubule network in Bic-D(H) ovaries shows that Bic-D function is required for both the establishment of the polarized microtubule network and its maintenance throughout oogenesis. To explain the multiple functions suggested by the pleiotropic Bic-D phenotype, we propose that Bic-D protein could form itself a filamentous structure and represent an integral, essential part of the cytoskeleton.