The role of segment polarity genes during early oogenesis in Drosophila

Transcriptome profiling of gonad-stimulating factors in thoracic ganglia and a potential role of Indian hedgehog gene in vitellogenesis of banana shrimp Fenneropenaeus merguiensis

Shrimp reproduction is controlled by several factors. Central nervous tissues, especially thoracic ganglia and brain, are known sources of gonad stimulating factors (GSFs) in crustaceans, but the GSFs in shrimp have not yet been clarified. Hence, we aimed to characterize and study putative GSFs from thoracic ganglia of adult female Fenneropenaeus merguiensis. An analysis of thoracic ganglia transcriptome revealed 3224 putative GSFs of a total 77,681 unigenes. Only 376 putative GSFs were differentially expressed during ovarian developmental stages. Eight candidate GSFs were validated for their expression patterns in thoracic ganglia, including the Indian hedgehog gene. F. merguiensis Indian hedgehog (FmIHH) was then investigated for its role in vitellogenesis. The obtained full-length cDNA of FmIHH was similar to other crustacean IHHs rather than Sonic and Desert HHs. The FmIHH was dominantly expressed in thoracic ganglia, and its expression was significantly increased in the vitellogenic stages before being downregulated at the mature stage of ovarian development. Injection of the recombinant FmIHH (His-TF-IHH) protein stimulated vitellogenin expression in ovaries on day 3 and 7, and also increased the gonadosomatic index. In addition, crustacean hyperglycemic hormone expression and total sugar were significantly decreased in eyestalks and hemolymph, respectively, after injection of His-TF-IHH, while lactic acid was increased. Both total sugar and lactic acid were unchanged in ovaries of His-TF-IHH injected shrimp. These results suggested that FmIHH plays a crucial role in vitellogenesis and regulate sugar uptake during ovarian development.

Single-cell expression profile of Drosophila ovarian follicle stem cells illuminates spatial differentiation in the germarium

BACKGROUND

How stem cell populations are organized and regulated within adult tissues is important for understanding cancer origins and for developing cell replacement strategies. Paradigms such as mammalian gut stem cells and Drosophila ovarian follicle stem cells (FSC) are characterized by population asymmetry, in which stem cell division and differentiation are separately regulated processes. These stem cells behave stochastically regarding their contributions to derivative cells and also exhibit dynamic spatial heterogeneity. Drosophila FSCs provide an excellent model for understanding how a community of active stem cells maintained by population asymmetry is regulated. Here, we use single-cell RNA sequencing to profile the gene expression patterns of FSCs and their immediate derivatives to investigate heterogeneity within the stem cell population and changes associated with differentiation.

RESULTS

We describe single-cell RNA sequencing studies of a pre-sorted population of cells that include FSCs and the neighboring cell types, escort cells (ECs) and follicle cells (FCs), which they support. Cell-type assignment relies on anterior-posterior (AP) location within the germarium. We clarify the previously determined location of FSCs and use spatially targeted lineage studies as further confirmation. The scRNA profiles among four clusters are consistent with an AP progression from anterior ECs through posterior ECs and then FSCs, to early FCs. The relative proportion of EC and FSC clusters are in good agreement with the prevalence of those cell types in a germarium. Several genes with graded profiles from ECs to FCs are highlighted as candidate effectors of the inverse gradients of the two principal signaling pathways, Wnt and JAK-STAT, that guide FSC differentiation and division.

CONCLUSIONS

Our data establishes an important resource of scRNA-seq profiles for FSCs and their immediate derivatives that is based on precise spatial location and functionally established stem cell identity, and facilitates future genetic investigation of regulatory interactions guiding FSC behavior.

Analysis of a rare progeria variant of Barrier-to-autointegration factor in Drosophila connects centromere function to tissue homeostasis

Barrier-to-autointegration factor (BAF/BANF) is a nuclear lamina protein essential for nuclear integrity, chromatin structure, and genome stability. Whereas complete loss of BAF causes lethality in multiple organisms, the A12T missense mutation of the BANF1 gene in humans causes a premature aging syndrome, called Néstor-Guillermo Progeria Syndrome (NGPS). Here, we report the first in vivo animal investigation of progeroid BAF, using CRISPR editing to introduce the NGPS mutation into the endogenous Drosophila baf gene. Progeroid BAF adults are born at expected frequencies, demonstrating that this BAF variant retains some function. However, tissue homeostasis is affected, supported by studies of the ovary, a tissue that depends upon BAF for stem cell survival and continuous oocyte production. We find that progeroid BAF causes defects in germline stem cell mitosis that delay anaphase progression and compromise chromosome segregation. We link these defects to decreased recruitment of centromeric proteins of the kinetochore, indicating dysfunction of cenBAF, a localized pool of dephosphorylated BAF produced by Protein Phosphatase PP4. We show that DNA damage increases in progenitor germ cells, which causes germ cell death due to activation of the DNA damage transducer kinase Chk2. Mitotic defects appear widespread, as aberrant chromosome segregation and increased apoptosis occur in another tissue. Together, these data highlight the importance of BAF in establishing centromeric structures critical for mitosis. Further, these studies link defects in cenBAF function to activation of a checkpoint that depletes progenitor reserves critical for tissue homeostasis, aligning with phenotypes of NGPS patients.

Canonical Wnt Signaling Promotes Formation of Somatic Permeability Barrier for Proper Germ Cell Differentiation

Morphogen-mediated signaling is critical for proper organ development and stem cell function, and well-characterized mechanisms spatiotemporally limit the expression of ligands, receptors, and ligand-binding cell-surface glypicans. Here, we show that in the developing Drosophila ovary, canonical Wnt signaling promotes the formation of somatic escort cells (ECs) and their protrusions, which establish a physical permeability barrier to define morphogen territories for proper germ cell differentiation. The protrusions shield germ cells from Dpp and Wingless morphogens produced by the germline stem cell (GSC) niche and normally only received by GSCs. Genetic disruption of EC protrusions allows GSC progeny to also receive Dpp and Wingless, which subsequently disrupt germ cell differentiation. Our results reveal a role for canonical Wnt signaling in specifying the ovarian somatic cells necessary for germ cell differentiation. Additionally, we demonstrate the morphogen-limiting function of this physical permeability barrier, which may be a common mechanism in other organs across species.

A single-cell atlas reveals unanticipated cell type complexity in Drosophila ovaries

Organ function relies on the spatial organization and functional coordination of numerous cell types. The Drosophila ovary is a widely used model system to study the cellular activities underlying organ function, including stem cell regulation, cell signaling and epithelial morphogenesis. However, the relative paucity of cell type-specific reagents hinders investigation of molecular functions at the appropriate cellular resolution. Here, we used single-cell RNA sequencing to characterize all cell types of the stem cell compartment and early follicles of the Drosophila ovary. We computed transcriptional signatures and identified specific markers for nine states of germ cell differentiation, and 23 somatic cell types and subtypes. We uncovered an unanticipated diversity of escort cells, the somatic cells that directly interact with differentiating germline cysts. Three escort cell subtypes reside in discrete anatomical positions, and express distinct sets of secreted and transmembrane proteins, suggesting that diverse micro-environments support the progressive differentiation of germ cells. Finally, we identified 17 follicle cell subtypes, and characterized their transcriptional profiles. Altogether, we provide a comprehensive resource of gene expression, cell type-specific markers, spatial coordinates and functional predictions for 34 ovarian cell types and subtypes.

Extracellular spreading of Wingless is required for Drosophila oogenesis

Recent studies have investigated whether the Wnt family of extracellular ligands can signal at long range, spreading from their source and acting as morphogens, or whether they signal only in a juxtacrine manner to neighboring cells. The original evidence for long-range Wnt signaling arose from studies of Wg, a Drosophila Wnt protein, which patterns the wing disc over several cell diameters from a central source of Wg ligand. However, the requirement of long-range Wg for patterning was called into question when it was reported that replacing the secreted protein Wg with a membrane-tethered version, NRT-Wg, results in flies with normally patterned wings. We and others previously reported that Wg spreads in the ovary about 50 μm or 5 cell diameters, from the cap cells to the follicle stem cells (FSCs) and that Wg stimulates FSC proliferation. We used the NRT-wg flies to analyze the consequence of tethering Wg to the cap cells. NRT-wg homozygous flies are sickly, but we found that hemizygous NRT-wg/null flies, carrying only one copy of tethered Wingless, were significantly healthier. Despite their overall improved health, these hemizygous flies displayed dramatic reductions in fertility and in FSC proliferation. Further, FSC proliferation was nearly undetectable when the wg locus was converted to NRT-wg only in adults, and the resulting germarium phenotype was consistent with a previously reported wg loss-of-function phenotype. We conclude that Wg protein spreads from its source cells in the germarium to promote FSC proliferation.

A single-cell atlas and lineage analysis of the adult Drosophila ovary

The Drosophila ovary is a widely used model for germ cell and somatic tissue biology. Here we use single-cell RNA-sequencing (scRNA-seq) to build a comprehensive cell atlas of the adult Drosophila ovary that contains transcriptional profiles for every major cell type in the ovary, including the germline stem cells and their niche cells, follicle stem cells, and previously undescribed subpopulations of escort cells. In addition, we identify Gal4 lines with specific expression patterns and perform lineage tracing of subpopulations of escort cells and follicle cells. We discover that a distinct subpopulation of escort cells is able to convert to follicle stem cells in response to starvation or upon genetic manipulation, including knockdown of escargot, or overactivation of mTor or Toll signalling.

Analysis of Gal4 Expression Patterns in Adult Drosophila Females

Precise genetic manipulation of specific cell types or tissues to pinpoint gene function requirement is a critical step in studies aimed at unraveling the intricacies of organismal physiology. Drosophila researchers heavily rely on the UAS/Gal4/Gal80 system for tissue-specific manipulations; however, it is often unclear whether the reported Gal4 expression patterns are indeed specific to the tissue of interest such that experimental results are not confounded by secondary sites of Gal4 expression. Here, we surveyed the expression patterns of commonly used Gal4 drivers in adult Drosophila female tissues under optimal conditions and found that multiple drivers have unreported secondary sites of expression beyond their published cell type/tissue expression pattern. These results underscore the importance of thoroughly characterizing Gal4 tools as part of a rigorous experimental design that avoids potential misinterpretation of results as we strive for understanding how the function of a specific gene/pathway in one tissue contributes to whole-body physiology.

A single-cell atlas of the developing Drosophila ovary identifies follicle stem cell progenitors

Addressing the complexity of organogenesis at a system-wide level requires a complete understanding of adult cell types, their origin, and precursor relationships. The Drosophila ovary has been a model to study how coordinated stem cell units, germline, and somatic follicle stem cells maintain and renew an organ. However, lack of cell type-specific tools have limited our ability to study the origin of individual cell types and stem cell units. Here, we used a single-cell RNA sequencing approach to uncover all known cell types of the developing ovary, reveal transcriptional signatures, and identify cell type-specific markers for lineage tracing. Our study identifies a novel cell type corresponding to the elusive follicle stem cell precursors and predicts subtypes of known cell types. Altogether, we reveal a previously unanticipated complexity of the developing ovary and provide a comprehensive resource for the systematic analysis of ovary morphogenesis.

Signal transduction in the early Drosophila follicle stem cell lineage

The follicle stem cell (FSC) lineage in the Drosophila ovary is a highly informative model of in vivo epithelial stem cell biology. Studies over the past 30 years have identified roles for every major signaling pathway in the early FSC lineage. These pathways regulate a wide variety of cell behaviors, including self-renewal, proliferation, survival and differentiation. Studies of cell signaling in the follicle epithelium have provided new insights into how these cell behaviors are coordinated within an epithelial stem cell lineage and how signaling pathways interact with each other in the native, in vivo context of a living tissue. Here, we review these studies, with a particular focus on how these pathways specify differences between the FSCs and their daughter cells. We also describe common themes that have emerged from these studies, and highlight new research directions that have been made possible by the detailed understanding of the follicle epithelium.

The regulation of Drosophila ovarian stem cell niches by signaling crosstalk

The Drosophila female ovary is an excellent model for investigating how multiple stem cell types are coordinately regulated in vivo. The ovary contains at least two stem cell types, germline stem cells (GSCs) and somatic follicular stem cells (FSCs). Although GSCs and FSCs are maintained within a distinct extra-cellular microenvironment, known as a niche, they share some common signaling molecules to generate their own niche. To properly maintain these stem cell types, understanding how signaling molecules are regulated is essential. In this review, we summarize the recent understanding of the mechanisms maintaining GSCs and FSCs from the perspective of growth factor regulation and discuss how these regulatory mechanisms contribute to stem cell maintenance, competition, and survival.

The Plk1 kinase negatively regulates the Hedgehog signaling pathway by phosphorylating Gli1

Hedgehog (Hh) signaling is a highly conserved cell signaling pathway important for cell life, development and tumorigenesis. Increasing evidence suggests that the Hh signaling pathway functions in certain phases of the cell cycle. However, the coordination between Hh signaling and cell cycle control remains poorly understood. Here, we show that polo-like kinase-1 (Plk1), a critical protein kinase regulating many processes during the cell cycle, also regulates Hh signaling by phosphorylating and inhibiting Gli1, a downstream transcription factor of the Hh signaling pathway. Gli1 expression increases along with Hh signaling activation, leading to upregulation of Hh target genes, including cyclin E, during the G1 and S phases. Gli1 is phosphorylated at S481 by Plk1, and this phosphorylation facilitates the nuclear export and binding of Gli1 with its negative regulator Sufu, leading to a reduction in Hh signaling activity. Inhibition of Plk1 kinase activity led to Gli1 maintaining is role in promoting downstream gene expression. Collectively, our data reveal a novel mechanism regarding the crosstalk between Hh signaling and cell cycle control.

Wnt Signaling in Stem Cell Maintenance and Differentiation in the Drosophila Germarium

Wnt signaling is a conserved regulator of stem cell behaviors, and the Drosophila germarium has been an important model tissue for the study of stem cell maintenance, differentiation, and proliferation. Here we review Wnt signaling in the germarium, which houses two distinct types of ovarian stem cells: the anteriorly located germline stem cells (GSCs), which give rise to oocytes; and the mid-posteriorly located follicle stem cells (FSCs), which give rise to the somatic follicle cells that cover a developing oocyte. The maintenance and proliferation of GSCs and FSCs is regulated by the stem cell niches, whereas differentiation of the germline is regulated by the differentiation niche. Four distinct Wnt ligands are localized in the germarium, and we focus review on how these Wnt ligands and Wnt signaling affects maintenance and differentiation of both germline and follicle stem cells in their respective niches.

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.

Engrailed acts with Nejire to control decapentaplegic expression in the Drosophila ovarian stem cell niche

Homeostasis of adult tissues is maintained by a small number of stem cells, which are sustained by their niches. In the Drosophila female germline stem cell (GSC) niche, Decapentaplegic (Dpp) is the primary factor that promotes GSC self-renewal. However, the mechanism regulating dpp expression in the niche is largely unknown. Here, we identify a 2.0 kb fragment located in a 5' cis-regulatory region of the dpp locus containing enhancer activity that drives its expression in the niche. This region is distinct from a previously characterized 3' cis-regulatory enhancer responsible for dpp expression in imaginal discs. Our data demonstrate that Engrailed, a homeodomain-containing transcription factor that serves as a cap cell marker, binds to this region and regulates dpp expression in cap cells. Further data suggest that En forms a complex with Nejire (Nej), the Drosophila ortholog of histone acetyltransferase CBP/p300, and directs Nej to this cis-regulatory region where Nej functions as the co-activator for dpp expression. Therefore, our study defines the molecular pathway controlling dpp expression in the Drosophila ovarian stem cell niche.

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.

Alternative direct stem cell derivatives defined by stem cell location and graded Wnt signalling

Adult stem cells provide a renewable source of differentiated cells for a wide variety of tissues and generally give rise to multiple cell types. Basic principles of stem cell organization and regulation underlying this behavior are emerging. Local niche signals maintain stem cells, while different sets of signals act outside the niche to diversify initially equivalent stem cell progeny. Here we show that Drosophila ovarian Follicle Stem Cells (FSCs) produced two distinct cell types directly. This cell fate choice was determined by the A/P position of an FSC and by the magnitude of spatially graded Wnt pathway activity. These findings reveal a paradigm of immediate diversification of stem cell derivatives according to stem cell position within a larger population, guided by a graded niche signal. We also found that FSCs strongly resemble mammalian intestinal stem cells in many aspects of their organization, including population asymmetry and dynamic heterogeneity.

Wnt5a Signaling in Normal and Cancer Stem Cells

Wnt5a is involved in activating several noncanonical Wnt signaling pathways, which can inhibit or activate canonical Wnt/β-catenin signaling pathway in a receptor context-dependent manner. Wnt5a signaling is critical for regulating normal developmental processes, including stem cell self-renewal, proliferation, differentiation, migration, adhesion, and polarity. Moreover, the aberrant activation or inhibition of Wnt5a signaling is emerging as an important event in cancer progression, exerting both oncogenic and tumor suppressive effects. Recent studies show the involvement of Wnt5a signaling in regulating normal and cancer stem cell self-renewal, cancer cell proliferation, migration, and invasion. In this article, we review recent findings regarding the molecular mechanisms and roles of Wnt5a signaling in stem cells in embryogenesis and in the normal or neoplastic breast or ovary, highlighting that Wnt5a may have different effects on target cells depending on the surface receptors expressed by the target cell.

Combgap Promotes Ovarian Niche Development and Chromatin Association of EcR-Binding Regions in BR-C

The development of niches for tissue-specific stem cells is an important aspect of stem cell biology. Determination of niche size and niche numbers during organogenesis involves precise control of gene expression. How this is achieved in the context of a complex chromatin landscape is largely unknown. Here we show that the nuclear protein Combgap (Cg) supports correct ovarian niche formation in Drosophila by controlling ecdysone-Receptor (EcR)- mediated transcription and long-range chromatin contacts in the broad locus (BR-C). Both cg and BR-C promote ovarian growth and the development of niches for germ line stem cells. BR-C levels were lower when Combgap was either reduced or over-expressed, indicating an intricate regulation of the BR-C locus by Combgap. Polytene chromosome stains showed that Cg co-localizes with EcR, the major regulator of BR-C, at the BR-C locus and that EcR binding to chromatin was sensitive to changes in Cg levels. Proximity ligation assay indicated that the two proteins could reside in the same complex. Finally, chromatin conformation analysis revealed that EcR-bound regions within BR-C, which span ~30 KBs, contacted each other. Significantly, these contacts were stabilized in an ecdysone- and Combgap-dependent manner. Together, these results highlight Combgap as a novel regulator of chromatin structure that promotes transcription of ecdysone target genes and ovarian niche formation.

Germ line stem cells (GSCs) supply either eggs or sperm throughout the life- time of many organisms, including mammals. For their function, GSCs require input from somatic niche cells. Understanding how niches form during development is an important initial step in understanding how stem cell units form, and by extension, how they may regenerate. In this work we describe a new function for the chromatin binding protein Combgap in ovarian niche formation of the model organism Drosophila melanogaster. Combgap is required for the correct expression of another factor, BR-C, in somatic ovarian cells. BR-C is one of the central target genes of the steroid hormone ecdysone, and its expression is controlled by the ecdysone receptor (EcR). Interestingly, EcR-enriched regions within the BR-C locus are engaged in long-range contacts that are stabilized by ecdysone in a Combgap-depended manner. We also found that EcR binding to chromatin depends on WT levels of Combgap. BR-C regulates GSC unit establishment, intestinal stem cells, immune responses, and many other processes. Understanding Combgaps’ function in shaping the BR-C chromatin landscape is a first step towards better appreciation of how this important locus is controlled, and the general machinery coupling gene expression to 3D chromatin structure.

The Wnt pathway limits BMP signaling outside of the germline stem cell niche in Drosophila ovaries

The mechanisms that modulate and limit the signaling output of adult stem cell niches remain poorly understood. To gain further insights into how these microenvironments are regulated in vivo, we performed a candidate gene screen designed to identify factors that restrict BMP signal production to the cap cells that comprise the germline stem cell (GSC) niche of Drosophila ovaries. Through these efforts, we found that disruption of Wnt4 and components of the canonical Wnt pathway results in a complex germ cell phenotype marked by an expansion of GSC-like cells, pre-cystoblasts and cystoblasts in young females. This phenotype correlates with an increase of decapentaplegic (dpp) mRNA levels within escort cells and varying levels of BMP responsiveness in the germline. Further genetic experiments show that Wnt4, which exhibits graded expression in somatic cells of germaria, activates the Wnt pathway in posteriorly positioned escort cells. The activation of the Wnt pathway appears to be limited by the BMP pathway itself, as loss of Mad in escort cells results in the expansion of Wnt pathway activation. Wnt pathway activity changes within germaria during the course of aging, coincident with changes in dpp production. These data suggest that mutual antagonism between the BMP and Wnt pathways in somatic cells helps to regulate germ cell differentiation.

Hedgehog Signaling Strength Is Orchestrated by the mir-310 Cluster of MicroRNAs in Response to Diet

Since the discovery of microRNAs (miRNAs) only two decades ago, they have emerged as an essential component of the gene regulatory machinery. miRNAs have seemingly paradoxical features: a single miRNA is able to simultaneously target hundreds of genes, while its presence is mostly dispensable for animal viability under normal conditions. It is known that miRNAs act as stress response factors; however, it remains challenging to determine their relevant targets and the conditions under which they function. To address this challenge, we propose a new workflow for miRNA function analysis, by which we found that the evolutionarily young miRNA family, the mir-310s (mir-310/mir-311/mir-312/mir-313), are important regulators of Drosophila metabolic status. mir-310s-deficient animals have an abnormal diet-dependent expression profile for numerous diet-sensitive components, accumulate fats, and show various physiological defects. We found that the mir-310s simultaneously repress the production of several regulatory factors (Rab23, DHR96, and Ttk) of the evolutionarily conserved Hedgehog (Hh) pathway to sharpen dietary response. As the mir-310s expression is highly dynamic and nutrition sensitive, this signal relay model helps to explain the molecular mechanism governing quick and robust Hh signaling responses to nutritional changes. Additionally, we discovered a new component of the Hh signaling pathway in Drosophila, Rab23, which cell autonomously regulates Hh ligand trafficking in the germline stem cell niche. How organisms adjust to dietary fluctuations to sustain healthy homeostasis is an intriguing research topic. These data are the first to report that miRNAs can act as executives that transduce nutritional signals to an essential signaling pathway. This suggests miRNAs as plausible therapeutic agents that can be used in combination with low calorie and cholesterol diets to manage quick and precise tissue-specific responses to nutritional changes.

Antibody Staining in Drosophila Germaria

Drosophila oogenesis is a powerful model for studying a wide spectrum of cellular and developmental processes in vivo. Oogenesis starts in a specialized structure called the germarium, which harbors the stem cells for both germ and somatic cells. The germarium produces egg chambers, each of which will develop into an egg. Active areas of research in Drosophila germaria include stem cell self-renewal, division, and maintenance, cell cycle control and differentiation, oocyte specification, intercellular communication, and signaling, among others. The solid knowledge base, the genetic tractability of the Drosophila model, as well as the availability and fast development of tools and imaging techniques for oogenesis research ensure that studies in this model will keep being instrumental for novel discoveries within cell and developmental biology also in the future. This chapter focuses on antibody staining in Drosophila germaria and provides a protocol for immunostaining as well as an overview of commonly used antibodies for visualization of different cell types and cellular structures. The protocol is well-suited for subsequent confocal microscopy analyses, and in addition we present key adaptations of the protocol that are useful when performing structured illumination microscopy (SIM) super-resolution imaging.

Hedgehog signaling and steroidogenesis

Since its discovery nearly 30 years ago, the Hedgehog (Hh) signaling pathway has been shown to be pivotal in many developmental and pathophysiological processes in several steroidogenic tissues, including the testis, ovary, adrenal cortex, and placenta. New evidence links the evolutionarily conserved Hh pathway to the steroidogenic organs, demonstrating how Hh signaling can influence their development and homeostasis and can act in concert with steroids to mediate physiological functions. In this review, we highlight the role of the components of the Hh signaling pathway in steroidogenesis of endocrine tissues.

Wnt ligands regulate Tkv expression to constrain Dpp activity in the Drosophila ovarian stem cell niche

Multiple Wnt ligands produced by cap cells regulate the expression of Tkv, which acts as a receptor sink to remove excess cap cell–expressed Dpp and to restrict niche-associated Dpp activity, in escort cells.

Stem cell self-renewal versus differentiation is regulated by the niche, which provides localized molecules that favor self-renewal. In the Drosophila melanogaster female germline stem cell (GSC) niche, Decapentaplegic (Dpp), a fly transforming growth factor β molecule and well-established long-range morphogen, acts over one cell diameter to maintain the GSCs. Here, we show that Thickveins (Tkv; a type I receptor of Dpp) is highly expressed in stromal cells next to Dpp-producing cells and functions to remove excess Dpp outside the niche, thereby spatially restricting its activity. Interestingly, Tkv expression in these stromal cells is regulated by multiple Wnt ligands that are produced by the niche. Our data demonstrate a self-restraining mechanism by which the Drosophila ovarian GSC niche acts to define its own boundary.

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.

A Pak-regulated cell intercalation event leading to a novel radial cell polarity is involved in positioning of the follicle stem cell niche in the Drosophila ovary

In the germarium of the Drosophila ovary, germline cysts are encapsulated one at a time by a follicular epithelium derived from two follicle stem cells (FSCs). Ovaries in flies mutant for the serine/threonine kinase Pak exhibit a novel phenotype, in which two side-by-side cysts are encapsulated at a time, generating paired egg chambers. This striking phenotype originates in the pupal ovary, where the developing germarium is shaped by the basal stalk, a stack of cells formed by cell intercalation. The process of basal stalk formation is not well understood, and we provide evidence that the cell intercalation is driven by actomyosin contractility of DE-Cadherin-adhered cells, leading to a column of disk-shaped cells exhibiting a novel radial cell polarity. Cell intercalation fails in Pak mutant ovaries, leading to abnormally wide basal stalks and consequently wide germaria with side-by-side cysts. We present evidence that Pak mutant germaria have extra FSCs, and we propose that contact of a germline cyst with the basal stalk in the pupal ovary contributes to FSC niche formation. The wide basal stalk in Pak mutants enables the formation of extra FSC niches which are mispositioned and yet functional, indicating that the FSC niche can be established in diverse locations.

Soma influences GSC progeny differentiation via the cell adhesion-mediated steroid-let-7-Wingless signaling cascade that regulates chromatin dynamics

It is known that signaling from the germline stem cell niche is required to maintain germline stem cell identity in Drosophila. However, it is not clear whether the germline stem-cell daughters differentiate by default (because they are physically distant from the niche) or whether additional signaling is necessary to initiate the differentiation program. Previously, we showed that ecdysteroid signaling cell non-autonomously regulates early germline differentiation via its soma-specific co-activator and co-repressor, Taiman and Abrupt. Now, we demonstrate that this regulation is modulated by the miRNA let-7, which acts in a positive feedback loop to confer ecdysone signaling robustness via targeting its repressor, the transcription factor Abrupt. This feedback loop adjusts ecdysteroid signaling in response to some stressful alterations in the external and internal conditions, which include temperature stress and aging, but not nutritional deprivation. Upon let-7 deficit, escort cells fail to properly differentiate: their shape, division, and cell adhesive characteristics are perturbed. These cells have confused cellular identity and form columnar-like rather than squamous epithelium and fail to send protrusions in between differentiating germline cysts, affecting soma-germline communication. Particularly, levels of the homophilic cell adhesion protein Cadherin, which recruits Wg signaling transducer β-catenin, are increased in mutant escort cells and, correspondingly, in the adjacent germline cells. Readjustment of heterotypic (soma-germline) cell adhesion modulates Wg signaling intensity in the germline, which in turn regulates histone modifications that promote expression of the genes necessary to trigger early germline differentiation. Thus, our data first show the intrinsic role for Wg signaling in the germline and support a model where the soma influences the tempo of germline differentiation in response to external conditions.

A matrix metalloproteinase mediates long-distance attenuation of stem cell proliferation

Long-range signaling by Wingless in the Drosophila ovary requires the glypican Dally-like and is antagonized by Dally-like cleavage by the extracellular metalloproteinase Mmp2.

Ligand-based signaling can potentiate communication between neighboring cells and between cells separated by large distances. In the Drosophila melanogaster ovary, Wingless (Wg) promotes proliferation of follicle stem cells located ∼50 µm or five cell diameters away from the Wg source. How Wg traverses this distance is unclear. We find that this long-range signaling requires Division abnormally delayed (Dally)-like (Dlp), a glypican known to extend the range of Wg ligand in the wing disc by binding Wg. Dlp-mediated spreading of Wg to follicle stem cells is opposed by the extracellular protease Mmp2, which cleaved Dlp in cell culture, triggering its relocalization such that Dlp no longer contacted Wg protein. Mmp2-deficient ovaries displayed increased Wg distribution, activity, and stem cell proliferation. Mmp2 protein is expressed in the same cells that produce Wg; thus, niche cells produce both a long-range stem cell proliferation factor and a negative regulator of its spreading. This system could allow for spatial control of Wg signaling to targets at different distances from the source.

Transcriptional properties and splicing of the flamenco piRNA cluster

In Drosophila, the piRNA cluster, flamenco, produces most of the piRNAs (PIWI-interacting RNAs) that silence transposable elements in the somatic follicle cells during oogenesis. These piRNAs are thought to be processed from a long single-stranded precursor transcript. Here, we demonstrate that flamenco transcription is initiated from an RNA polymerase II promoter containing an initiator motif (Inr) and downstream promoter element (DPE) and requires the transcription factor, Cubitus interruptus. We show that the flamenco precursor transcript undergoes differential alternative splicing to generate diverse RNA precursors that are processed to piRNAs. Our data reveal dynamic processing steps giving rise to piRNA cluster precursors.

Debra-mediated Ci degradation controls tissue homeostasis in Drosophila adult midgut

Adult tissue homeostasis is maintained by resident stem cells and their progeny. However, the underlying mechanisms that control tissue homeostasis are not fully understood. Here, we demonstrate that Debra-mediated Ci degradation is important for intestinal stem cell (ISC) proliferation in Drosophila adult midgut. Debra inhibition leads to increased ISC activity and tissue homeostasis loss, phenocopying defects observed in aging flies. These defects can be suppressed by depleting Ci, suggesting that increased Hedgehog (Hh) signaling contributes to ISC proliferation and tissue homeostasis loss. Consistently, Hh signaling activation causes the same defects, whereas depletion of Hh signaling suppresses these defects. Furthermore, the Hh ligand from multiple sources is involved in ISC proliferation and tissue homeostasis. Finally, we show that the JNK pathway acts downstream of Hh signaling to regulate ISC proliferation. Together, our results provide insights into the mechanisms of stem cell proliferation and tissue homeostasis control.

A dynamic population of stromal cells contributes to the follicle stem cell niche in the Drosophila ovary

Epithelial stem cells are maintained within niches that promote self-renewal by providing signals that specify the stem cell fate. In the Drosophila ovary, epithelial follicle stem cells (FSCs) reside in niches at the anterior tip of the tissue and support continuous growth of the ovarian follicle epithelium. Here, we demonstrate that a neighboring dynamic population of stromal cells, called escort cells, are FSC niche cells. We show that escort cells produce both Wingless and Hedgehog ligands for the FSC lineage, and that Wingless signaling is specific for the FSC niche whereas Hedgehog signaling is active in both FSCs and daughter cells. In addition, we show that multiple escort cells simultaneously encapsulate germ cell cysts and contact FSCs. Thus, FSCs are maintained in a dynamic niche by a non-dedicated population of niche cells.

The Drosophila nuclear lamina protein otefin is required for germline stem cell survival

LEM domain (LEM-D) proteins are components of an extensive protein network that assembles beneath the inner nuclear envelope. Defects in LEM-D proteins cause tissue-restricted human diseases associated with altered stem cell homeostasis. Otefin (Ote) is a Drosophila LEM-D protein that is intrinsically required for female germline stem cell (GSC) maintenance. Previous studies linked Ote loss with transcriptional activation of the key differentiation gene bag-of-marbles (bam), leading to the model in which Ote tethers the bam gene to the nuclear periphery for gene silencing. Using genetic and phenotypic analyses of multiple ote(-/-) backgrounds, we obtained evidence that is inconsistent with this model. We show that bam repression is maintained in ote(-/-) GSCs and that germ cell loss persists in ote(-/-), bam(-/-) mutants, together demonstrating that GSC loss is independent of bam transcription. We show that the primary defect in ote(-/-) GSCs is a block of differentiation, which ultimately leads to germ cell death.

Hedgehog in the Drosophila testis niche: what does it do there?

Stem cell niche is a specialized microenvironment crucial to self-renewal. The testis in Drosophila contains two different types of stem cells, the germline stem cells and the somatic cyst stem cells that are sustained by their respective niche signals, thus is a good system for studying the interaction between the stem cells and their hosting niche. The JAK-STAT and BMP pathways are known to play critical roles in the self-renewal of different kinds of stem cells, but the roles of several other pathways have emerged recently in a complex signaling network in the testis niche. Reports of independent observations from three research groups have uncovered an important role of Hedgehog (Hh) in the Drosophila testis niche. In this review, we summarize these recent findings and discuss the interplay between the Hh signaling mechanisms and those of the JAK-STAT and BMP pathways. We also discuss directions for further investigation.

Diet controls Drosophila follicle stem cell proliferation via Hedgehog sequestration and release

Dietary cholesterol levels control follicle stem cell proliferation in the Drosophila ovary via regulation of Hedgehog protein localization.

A healthy diet improves adult stem cell function and delays diseases such as cancer, heart disease, and neurodegeneration. Defining molecular mechanisms by which nutrients dictate stem cell behavior is a key step toward understanding the role of diet in tissue homeostasis. In this paper, we elucidate the mechanism by which dietary cholesterol controls epithelial follicle stem cell (FSC) proliferation in the fly ovary. In nutrient-restricted flies, the transmembrane protein Boi sequesters Hedgehog (Hh) ligand at the surface of Hh-producing cells within the ovary, limiting FSC proliferation. Upon feeding, dietary cholesterol stimulates S6 kinase–mediated phosphorylation of the Boi cytoplasmic domain, triggering Hh release and FSC proliferation. This mechanism enables a rapid, tissue-specific response to nutritional changes, tailoring stem cell divisions and egg production to environmental conditions sufficient for progeny survival. If conserved in other systems, this mechanism will likely have important implications for studies on molecular control of stem cell function, in which the benefits of low calorie and low cholesterol diets are beginning to emerge.

Unscrambling butterfly oogenesis

Background

Butterflies are popular model organisms to study physiological mechanisms underlying variability in oogenesis and egg provisioning in response to environmental conditions. Nothing is known, however, about; the developmental mechanisms governing butterfly oogenesis, how polarity in the oocyte is established, or which particular maternal effect genes regulate early embryogenesis. To gain insights into these developmental mechanisms and to identify the conserved and divergent aspects of butterfly oogenesis, we analysed a de novo ovarian transcriptome of the Speckled Wood butterfly Pararge aegeria (L.), and compared the results with known model organisms such as Drosophila melanogaster and Bombyx mori.

Results

A total of 17306 contigs were annotated, with 30% possibly novel or highly divergent sequences observed. Pararge aegeria females expressed 74.5% of the genes that are known to be essential for D. melanogaster oogenesis. We discuss the genes involved in all aspects of oogenesis, including vitellogenesis and choriogenesis, plus those implicated in hormonal control of oogenesis and transgenerational hormonal effects in great detail. Compared to other insects, a number of significant differences were observed in; the genes involved in stem cell maintenance and differentiation in the germarium, establishment of oocyte polarity, and in several aspects of maternal regulation of zygotic development.

Conclusions

This study provides valuable resources to investigate a number of divergent aspects of butterfly oogenesis requiring further research. In order to fully unscramble butterfly oogenesis, we also now also have the resources to investigate expression patterns of oogenesis genes under a range of environmental conditions, and to establish their function.

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.

Genetic interaction screens identify a role for hedgehog signaling in Drosophila border cell migration

BACKGROUND

Cell motility is essential for embryonic development and physiological processes such as the immune response, but also contributes to pathological conditions such as tumor progression and inflammation. However, our understanding of the mechanisms underlying migratory processes is incomplete. Drosophila border cells provide a powerful genetic model to identify the roles of genes that contribute to cell migration.

RESULTS

Members of the Hedgehog signaling pathway were uncovered in two independent screens for interactions with the small GTPase Rac and the polarity protein Par-1 in border cell migration. Consistent with a role in migration, multiple Hh signaling components were enriched in the migratory border cells. Interference with Hh signaling by several different methods resulted in incomplete cell migration. Moreover, the polarized distribution of E-Cadherin and a marker of tyrosine kinase activity were altered when Hh signaling was disrupted. Conservation of Hh-Rac and Hh-Par-1 signaling was illustrated in the wing, in which Hh-dependent phenotypes were enhanced by loss of Rac or par-1.

CONCLUSIONS

We identified a pathway by which Hh signaling connects to Rac and Par-1 in cell migration. These results further highlight the importance of modifier screens in the identification of new genes that function in developmental pathways.

Regulation of stem cells by intersecting gradients of long-range niche signals

We have used Drosophila ovarian follicle stem cells (FSCs) to study how stem cells are regulated by external signals and draw three main conclusions. First, the spatial definition of supportive niche positions for FSCs depends on gradients of Hh and JAK-STAT pathway ligands, which emanate from opposite, distant sites. FSC position may be further refined by a preference for low-level Wnt signaling. Second, hyperactivity of supportive signaling pathways can compensate for the absence of the otherwise essential adhesion molecule, DE-cadherin, suggesting a close regulatory connection between niche adhesion and niche signals. Third, FSC behavior is determined largely by summing the inputs of multiple signaling pathways of unequal potencies. Altogether, our findings indicate that a stem cell niche need not be defined by short-range signals and invariant cell contacts; rather, for FSCs, the intersection of gradients of long-range niche signals regulates the longevity, position, number, and competitive behavior of stem cells.

RNA localization in the honeybee (Apis mellifera) oocyte reveals insights about the evolution of RNA localization mechanisms

Subcellular localization of RNAs is a critical biological process for generation of cellular asymmetries for many cell types and a critical step in axis determination during the early development of animals. We have identified transcripts localized to the anterior and posterior of honeybee oocyte using laser capture microscopy and microarray analysis. Analysis of orthologous transcripts in Drosophila indicates that many do not show a conserved pattern of localization. By microinjecting fluorescently labeled honeybee transcripts into Drosophila egg chambers we show that these RNAs become localized in a similar manner to their localization in honeybee oocytes, indicating conservation of the localization machinery. Thus while the mechanisms for localizing RNA are conserved, the complement of localized RNAs are not. We propose that this complement of localized RNAs may change relatively rapidly through the loss or evolution of signal sequences detected by the conserved localization machinery, and show this has occurred in one transcript that is localized in a novel way in the honeybee. Our proposal, that the acquisition of novel RNA localization is relatively easy to evolve, has implications for the evolution of symmetry breaking mechanisms that trigger axis formation and development in animal embryos.

Control of germline stem cell self-renewal and differentiation in the Drosophila ovary: concerted actions of niche signals and intrinsic factors

In the Drosophila ovary, germline stem cells (GSCs) physically interact with their niche composed of terminal filament cells, cap cells, and possibly GSC-contacting escort cells (ECs). A GSC divides to generate a self-renewing stem cell that remains in the niche and a differentiating daughter that moves away from the niche. The GSC niche provides a bone morphogenetic protein (BMP) signal that maintains GSC self-renewal by preventing stem cell differentiation via repression of the differentiation-promoting gene bag of marbles (bam). In addition, it expresses E-cadherin, which mediates cell adhesion for anchoring GSCs in the niche, enabling continuous self-renewal. GSCs themselves also express different classes of intrinsic factors, including signal transducers, transcription factors, chromatin remodeling factors, translation regulators, and miRNAs, which control self-renewal by strengthening interactions with the niche and repressing various differentiation pathways. Differentiated GSC daughters, known as cystoblasts (CBs), also express distinct classes of intrinsic factors to inhibit self-renewal and promote germ cell differentiation. Surprisingly, GSC progeny are also dependent on their surrounding ECs for proper differentiation at least partly by preventing BMP from diffusing to the differentiated germ cell zone and by repressing ectopic BMP expression. Therefore, both GSC self-renewal and CB differentiation are controlled by collaborative actions of extrinsic signals and intrinsic factors.

Caprin controls follicle stem cell fate in the Drosophila ovary

Adult stem cells must balance self-renewal and differentiation for tissue homeostasis. The Drosophila ovary has provided a wealth of information about the extrinsic niche signals and intrinsic molecular processes required to ensure appropriate germline stem cell renewal and differentiation. The factors controlling behavior of the more recently identified follicle stem cells of the ovary are less well-understood but equally important for fertility. Here we report that translational regulators play a critical role in controlling these cells. Specifically, the translational regulator Caprin (Capr) is required in the follicle stem cell lineage to ensure maintenance of this stem cell population and proper encapsulation of developing germ cells by follicle stem cell progeny. In addition, reduction of one copy of the gene fmr1, encoding the translational regulator Fragile X Mental Retardation Protein, exacerbates the Capr encapsulation phenotype, suggesting Capr and fmr1 are regulating a common process. Caprin was previously characterized in vertebrates as Cytoplasmic Activation/Proliferation-Associated Protein. Significantly, we find that loss of Caprin alters the dynamics of the cell cycle, and we present evidence that misregulation of CycB contributes to the disruption in behavior of follicle stem cell progeny. Our findings support the idea that translational regulators may provide a conserved mechanism for oversight of developmentally critical cell cycles such as those in stem cell populations.

Somatic gonadal cells: the supporting cast for the germline

Cell-cell signaling and adhesion are critical for establishing tissue architecture during development and for maintaining tissue architecture and function in the adult. Defects in adhesion and signaling can result in mislocalization of cells, uncontrolled proliferation and improper differentiation, leading to tissue overgrowth, tumor formation, and cancer metastasis. An important example is found in the germline. Germ cells that are not incorporated into the gonad exhibit a greater propensity for forming germ cell tumors, and defects in germline development can reduce fertility. While much attention is given to germ cells, their development into functional gametes depends upon somatic gonadal cells. The study of model organisms has provided great insights into how somatic gonadal cells are specified, the molecular mechanisms that regulate gonad morphogenesis, and the role of germline-soma communication in the establishment and maintenance of the germline stem cell niche. This work will be discussed in the context of Drosophila melanogaster.

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

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

Finding a niche: studies from the Drosophila ovary

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

BMP and Hh signaling affects primordial germ cell division in Drosophila

The germline is segregated from the remainder of the soma during early embryonic development in metazoan species. In Drosophila, female primordial germ cells (PGCs) continue to proliferate during larval development, and become germline stem cells at the early pupal stage. To elucidate the roles of growth factors in larval PGC division, we examined expression patterns of a bone morphogenetic protein (BMP) growth factor, Decapentaplegic (Dpp), and Hedgehog (Hh), along with factors downstream of each, in the ovary during larval development. Dpp signaling appeared in the ovarian soma from early larval development, and was prominent in the terminal filament cells at late larval stage, whereas Hh appeared in the ovarian soma and PGCs from the third instar larval stage. The number of PGCs decreased when components of these signal transduction pathways were abrogated by RNAi in the PGCs, indicating that both Dpp and Hh signals directly regulate PGC proliferation. Experiments on the up- and down-regulation of Dpp and Hh with a tissue-specific Gal4 driver indicated that Dpp and Hh act as extrinsic and autocrine growth factors. Furthermore, heat-pulse experiments with hs-Gal4 showed that Dpp is active in PGC proliferation throughout larval development, whereas Hh has effects only during late larval development. In addition to Dpp, the reduction of Glass bottom boat (Gbb), another BMP molecule, caused a decrease in the number of PGCs and initiation of larval PGCs differentiation into cystocytes, indicating that Gbb functions to promote PGC division and repress differentiation.

Drosophila Boi limits Hedgehog levels to suppress follicle stem cell proliferation

The Boi receptor regulates stem cell function by sequestering the diffusible hedgehog ligand.

Stem cells depend on signals from cells within their microenvironment, or niche, as well as factors secreted by distant cells to regulate their maintenance and function. Here we show that Boi, a Hedgehog (Hh)-binding protein, is a novel suppressor of proliferation of follicle stem cells (FSCs) in the Drosophila ovary. Hh is expressed in apical cells, distant from the FSC niche, and diffuses to reach FSCs, where it promotes FSC proliferation. We show that Boi is expressed in apical cells and exerts its suppressive effect on FSC proliferation by binding to and sequestering Hh on the apical cell surface, thereby inhibiting Hh diffusion. Our studies demonstrate that cells distant from the local niche can regulate stem cell function through ligand sequestration, a mechanism that likely is conserved in other epithelial tissues.

Mutations in the polycomb group gene polyhomeotic lead to epithelial instability in both the ovary and wing imaginal disc in Drosophila

BACKGROUND

Most human cancers originate from epithelial tissues and cell polarity and adhesion defects can lead to metastasis. The Polycomb-Group of chromatin factors were first characterized in Drosophila as repressors of homeotic genes during development, while studies in mammals indicate a conserved role in body plan organization, as well as an implication in other processes such as stem cell maintenance, cell proliferation, and tumorigenesis. We have analyzed the function of the Drosophila Polycomb-Group gene polyhomeotic in epithelial cells of two different organs, the ovary and the wing imaginal disc.

RESULTS

Clonal analysis of loss and gain of function of polyhomeotic resulted in segregation between mutant and wild-type cells in both the follicular and wing imaginal disc epithelia, without excessive cell proliferation. Both basal and apical expulsion of mutant cells was observed, the former characterized by specific reorganization of cell adhesion and polarity proteins, the latter by complete cytoplasmic diffusion of these proteins. Among several candidate target genes tested, only the homeotic gene Abdominal-B was a target of PH in both ovarian and wing disc cells. Although overexpression of Abdominal-B was sufficient to cause cell segregation in the wing disc, epistatic analysis indicated that the presence of Abdominal-B is not necessary for expulsion of polyhomeotic mutant epithelial cells suggesting that additional polyhomeotic targets are implicated in this phenomenon.

CONCLUSION

Our results indicate that polyhomeotic mutations have a direct effect on epithelial integrity that can be uncoupled from overproliferation. We show that cells in an epithelium expressing different levels of polyhomeotic sort out indicating differential adhesive properties between the cell populations. Interestingly, we found distinct modalities between apical and basal expulsion of ph mutant cells and further studies of this phenomenon should allow parallels to be made with the modified adhesive and polarity properties of different types of epithelial tumors.