Polycomb group genes Psc and Su(z)2 restrict follicle stem cell self-renewal and extrusion by controlling canonical and noncanonical Wnt signaling

Chromatin state transitions in the Drosophila intestinal lineage identify principles of cell-type specification

Tissue homeostasis relies on rewiring of stem cell transcriptional programs into those of differentiated cells. Here, we investigate changes in chromatin occurring in a bipotent adult stem cells. Combining mapping of chromatin-associated factors with statistical modeling, we identify genome-wide transitions during differentiation in the adult Drosophila intestinal stem cell (ISC) lineage. Active, stem-cell-enriched genes transition to a repressive heterochromatin protein-1-enriched state more prominently in enteroendocrine cells (EEs) than in enterocytes (ECs), in which the histone H1-enriched Black state is preeminent. In contrast, terminal differentiation genes associated with metabolic functions follow a common path from a repressive, primed, histone H1-enriched Black state in ISCs to active chromatin states in EE and EC cells. Furthermore, we find that lineage priming has an important function in adult ISCs, and we identify histone H1 as a mediator of this process. These data define underlying principles of chromatin changes during adult multipotent stem cell differentiation.

Polycomb group genes are required for neuronal pruning in Drosophila

BACKGROUND

Pruning that selectively eliminates unnecessary or incorrect neurites is required for proper wiring of the mature nervous system. During Drosophila metamorphosis, dendritic arbourization sensory neurons (ddaCs) and mushroom body (MB) γ neurons can selectively prune their larval dendrites and/or axons in response to the steroid hormone ecdysone. An ecdysone-induced transcriptional cascade plays a key role in initiating neuronal pruning. However, how downstream components of ecdysone signalling are induced remains not entirely understood.

RESULTS

Here, we identify that Scm, a component of Polycomb group (PcG) complexes, is required for dendrite pruning of ddaC neurons. We show that two PcG complexes, PRC1 and PRC2, are important for dendrite pruning. Interestingly, depletion of PRC1 strongly enhances ectopic expression of Abdominal B (Abd-B) and Sex combs reduced, whereas loss of PRC2 causes mild upregulation of Ultrabithorax and Abdominal A in ddaC neurons. Among these Hox genes, overexpression of Abd-B causes the most severe pruning defects, suggesting its dominant effect. Knockdown of the core PRC1 component Polyhomeotic (Ph) or Abd-B overexpression selectively downregulates Mical expression, thereby inhibiting ecdysone signalling. Finally, Ph is also required for axon pruning and Abd-B silencing in MB γ neurons, indicating a conserved function of PRC1 in two types of pruning.

CONCLUSIONS

This study demonstrates important roles of PcG and Hox genes in regulating ecdysone signalling and neuronal pruning in Drosophila. Moreover, our findings suggest a non-canonical and PRC2-independent role of PRC1 in Hox gene silencing during neuronal pruning.

Drosophila SERTAD domain protein Taranis is required in somatic cells for maintenance of male germline stem cells

BACKGROUND

Polycomb proteins are essential for maintaining stem cell identity across different stem cell niches. However, how they function to maintain stem cell niches is not fully understood.

RESULTS

Here we show that the SERTAD protein Taranis (Tara), which is a Polycomb-trithorax group protein, is expressed in the adult testis niche and plays a role in its maintenance in Drosophila. We found that tara is expressed in early cyst cells, likely including somatic cyst stem cells (CySCs) of Drosophila male testis tip region, which houses both germline and somatic cyst stem cells along with the hub cells, forming the stem cell niche. Consistent with its expression, we found that, while loss of tara in germline cells only had minimal effects, tara knockdown in all cells or only in somatic cells of the niche reduced the number of not only somatic cells, but also germline stem cells (GSCs). We further found that Tara might antagonize Notch signaling in CySCs to maintain the stem cell niche.

CONCLUSIONS

Our studies suggest that Tara might function in somatic CySCs for GSC maintenance in the Drosophila testis.

Characteristics of in Vivo Model Systems for Ovarian Cancer Studies

An understanding of the molecular pathogenesis and heterogeneity of ovarian cancer holds promise for the development of early detection strategies and novel, efficient therapies. In this review, we discuss the advantages and limitations of animal models available for basic and preclinical studies. The fruit fly model is suitable mainly for basic research on cellular migration, invasiveness, adhesion, and the epithelial-to-mesenchymal transition. Higher-animal models allow to recapitulate the architecture and microenvironment of the tumor. We discuss a syngeneic mice model and the patient derived xenograft model (PDX), both useful for preclinical studies. Conditional knock-in and knock-out methodology allows to manipulate selected genes at a given time and in a certain tissue. Such models have built our knowledge about tumor-initiating genetic events and cell-of-origin of ovarian cancers; it has been shown that high-grade serous ovarian cancer may be initiated in both the ovarian surface and tubal epithelium. It is postulated that clawed frog models could be developed, enabling studies on tumor immunity and anticancer immune response. In laying hen, ovarian cancer develops spontaneously, which provides the opportunity to study the genetic, biochemical, and environmental risk factors, as well as tumor initiation, progression, and histological origin; this model can also be used for drug testing. The chick embryo chorioallantoic membrane is another attractive model and allows the study of drug response.

Wingless promotes EGFR signaling in follicle stem cells to maintain self-renewal

Adult stem cell niche boundaries must be precisely maintained to facilitate the segregation of stem cell and daughter cell fates. However, the mechanisms that govern this process in epithelial tissues are not fully understood. In this study, we investigated the relationship between two signals, Wnt and EGFR, that are necessary for self-renewal of the epithelial follicle stem cells (FSCs) in the Drosophila ovary, but must be downregulated in cells that have exited the niche to allow for differentiation. We found that Wingless produced by inner germarial sheath (IGS) cells acts over a short distance to activate Wnt signaling in FSCs, and that movement across the FSC niche boundary is limited. In addition, we show that Wnt signaling functions genetically upstream of EGFR signaling by activating the expression of the EGFR ligand, Spitz, and that constitutive activation of EGFR partially rescues the self-renewal defect caused by loss of Wnt signaling. Collectively, our findings support a model in which the Wnt and EGFR pathways operate in a signaling hierarchy to promote FSC self-renewal.

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

Fsh-Pc-Sce complex mediates active transcription of Cubitus interruptus (Ci)

The Hedgehog (Hh) signaling pathway plays important roles in both embryonic development and adult tissue homeostasis. Such biological functions are mediated by the transcription factor Cubitus interruptus (Ci). Yet the transcriptional regulation of the effector Ci itself is poorly investigated. Through an RNAi-based genetic screen, we identified that female sterile (1) homeotic (Fsh), a transcription co-activator, directly activates Ci transcription. Biochemistry assays demonstrated physical interactions among Fsh, Sex combs extra (Sce), and Polycomb (Pc). Functional assays further showed that both Pc and Sce are required for Ci expression, which is not likely mediated by the derepression of Engrailed (En), a repressor of Ci, in Pc or Sce mutant cells. Finally, we provide evidence showing that Pc/Sce facilitates the binding of Fsh at Ci locus and that the physical interaction between Fsh and Pc is essential for Fsh-mediated Ci transcription. Taken together, we not only uncover that Ci is transcriptionally regulated by Fsh-Pc-Sce complex but also provide evidence for the coordination between Fsh and PcG proteins in transcriptional regulation.

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.

Repression of Abd-B by Polycomb is critical for cell identity maintenance in adult Drosophila testis

Hox genes play a fundamental role in regulating animal development. However, less is known about their functions on homeostasis maintenance in adult stem cells. Here, we report that the repression of an important axial Hox gene, Abdominal-B (Abd-B), in cyst stem cells (CySCs) is essential for the homeostasis and cell identity maintenance in the adult Drosophila testis. Derepression of Abd-B in CySCs disrupts the proper self-renewal of both germline stem cells (GSCs) and CySCs, and leads to an excessive expansion of early stage somatic cells, which originate from both lineages. We further demonstrate that canonical Polycomb (Pc) and functional pathway of Polycomb group (PcG) proteins are responsible for maintaining the germline cell identity non-autonomously via repressing Abd-B in CySCs in the adult Drosophila testis.

Role of Chromatin Modifications in Drosophila Germline Stem Cell Differentiation

During Drosophila oogenesis, germline stem cells (GSCs) self-renew and differentiate to give rise to a mature egg. Self-renewal and differentiation of GSCs are regulated by both intrinsic mechanisms such as regulation of gene expression in the germ line and extrinsic signaling pathways from the surrounding somatic niche. Epigenetic mechanisms, including histone-modifying proteins, nucleosome remodeling complexes, and histone variants, play a critical role in regulating intrinsic gene expression and extrinsic signaling cues from the somatic niche. In the GSCs, intrinsic epigenetic modifiers are required to maintain a stem cell fate by promoting expression of self-renewal factors and repressing the differentiation program. Subsequently, in the GSC daughters, epigenetic regulators activate the differentiation program to promote GSC differentiation. During differentiation, the GSC daughter undergoes meiosis to give rise to the developing egg, containing a compacted chromatin architecture called the karyosome. Epigenetic modifiers control the attachment of chromosomes to the nuclear lamina to aid in meiotic recombination and the release from the lamina for karyosome formation. The germ line is in close contact with the soma for the entirety of this developmental process. This proximity facilitates signaling from the somatic niche to the developing germ line. Epigenetic modifiers play a critical role in the somatic niche, modulating signaling pathways in order to coordinate the transition of GSC to an egg. Together, intrinsic and extrinsic epigenetic mechanisms modulate this exquisitely balanced program.

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.

Control of germline stem cell differentiation by Polycomb and Trithorax group genes in the niche microenvironment

Polycomb and Trithorax group (PcG and TrxG) genes function to regulate gene transcription by maintaining a repressive or active chromatin state, respectively. This antagonistic activity is important for body patterning during embryonic development, but whether this function module has a role in adult tissues is unclear. Here, we report that in the Drosophila ovary, disruption of the Polycomb repressive complex 1 (PRC1), specifically in the supporting escort cells, causes blockage of cystoblast differentiation and germline stem cell-like tumor formation. Tumors are caused by derepression of decapentaplegic (dpp), which prevents cystoblast differentiation. Interestingly, activation of dpp in escort cells requires the function of the TrxG gene brahma (brm), suggesting that loss of PRC1 in escort cells causes Brm-dependent dpp expression. Our study suggests a requirement for balanced activity between PcG and TrxG in an adult stem cell niche, and disruption of this balance could lead to the loss of tissue homeostasis and tumorigenesis.

A positive role for polycomb in transcriptional regulation via H4K20me1

The highly conserved polycomb group (PcG) proteins maintain heritable transcription repression of the genes essential for development from fly to mammals. However, sporadic reports imply a potential role of PcGs in positive regulation of gene transcription, although systematic investigation of such function and the underlying mechanism has rarely been reported. Here, we report a Pc-mediated, H3K27me3-dependent positive transcriptional regulation of Senseless (Sens), a key transcription factor required for development. Mechanistic studies show that Pc regulates Sens expression by promoting H4K20me1 at the Sens locus. Further bioinformatic analysis at genome-wide level indicates that the existence of H4K20me1 acts as a selective mark for positive transcriptional regulation by Pc/H3K27me3. Both the intensities and specific patterns of Pc and H3K27me3 are important for the fates of target gene transcription. Moreover, binding of transcription factor Broad (Br), which physically interacts with Pc and positively regulates the transcription of Sens, is observed in Pc⁺H3K27me3⁺H4K20me1⁺ genes, but not in Pc⁺H3K27me3⁺H4K20me1⁻ genes. Taken together, our study reveals that, coupling with the transcription factor Br, Pc positively regulates transcription of Pc⁺H3K27me3⁺H4K20me1⁺ genes in developing Drosophila wing disc.

The RNA surveillance complex Pelo-Hbs1 is required for transposon silencing in the Drosophila germline

Silencing of transposable elements (TEs) in the metazoan germline is critical for genome integrity and is primarily dependent on Piwi proteins and associated RNAs, which exert their function through both transcriptional and posttranscriptional mechanisms. Here, we report that the evolutionarily conserved Pelo (Dom34)-Hbs1 mRNA surveillance complex is required for transposon silencing in the Drosophila germline. In pelo mutant gonads, mRNAs and proteins of some selective TEs are up-regulated. Pelo is not required for piRNA biogenesis, and our studies suggest that Pelo may function at the translational level to silence TEs: This function requires interaction with Hbs1, and overexpression of RpS30a partially reverts TE-silencing defects in pelo mutants. Interestingly, TE silencing and spermatogenesis defects in pelo mutants can also effectively be rescued by expressing the mammalian ortholog of Pelo. We propose that the Pelo-Hbs1 surveillance complex provides another level of defense against the expression of TEs in the germline of Drosophila and possibly all metazoa.

The transcriptional response to tumorigenic polarity loss in Drosophila

Loss of polarity correlates with progression of epithelial cancers, but how plasma membrane misorganization drives oncogenic transcriptional events remains unclear. The polarity regulators of the Drosophila Scribble (Scrib) module are potent tumor suppressors and provide a model for mechanistic investigation. RNA profiling of Scrib mutant tumors reveals multiple signatures of neoplasia, including altered metabolism and dedifferentiation. Prominent among these is upregulation of cytokine-like Unpaired (Upd) ligands, which drive tumor overgrowth. We identified a polarity-responsive enhancer in upd3, which is activated in a coincident manner by both JNK-dependent Fos and aPKC-mediated Yki transcription. This enhancer, and Scrib mutant overgrowth in general, are also sensitive to activity of the Polycomb Group (PcG), suggesting that PcG attenuation upon polarity loss potentiates select targets for activation by JNK and Yki. Our results link epithelial organization to signaling and epigenetic regulators that control tissue repair programs, and provide insight into why epithelial polarity is tumor-suppressive.

DOI:http://dx.doi.org/10.7554/eLife.03189.001

The cavities and organs within our body are lined with epithelial cells, which connect to each other to form continuous barriers. These cells have a highly polarized structure in which different components are found at the top and bottom of cells. In the fruit fly and most other animals, three genes known as the Scribble module control the polarity of epithelial cells. If these genes are faulty, the cells lose their polarity, break the epithelial barrier, and grow rapidly to form a tumor. Most malignant tumors that form from epithelial cells have lost normal cell polarity, so understanding how the organization and growth of epithelial cells are linked is a critical question.

It is not clear how the loss of cell polarity can drive tumor formation. Here, Bunker et al. used a technique called RNA sequencing to study the expression of genes in tumor cells that have mutations in the Scribble module. Hundreds of genes in the tumor cells had different levels of expression from the levels seen in normal fly cells. One of these is a gene called upd3, which was expressed much more highly in tumor cells than in normal cells. This gene activates a signaling pathway—called the JAK/STAT pathway—that promotes cell growth and division in many animals. Bunker et al. found that experimentally lowering the activity of the JAK/STAT pathway reduced the growth of the tumor cells that had lost normal polarity.

Further experiments show that disrupting the layer of epithelial cells activates two other signaling pathways that work together to switch on the upd3 gene when cell polarity is lost. Proteins belonging to the Polycomb Group also control the expression of upd3 and other genes involved in cell growth by altering how genetic material is packaged in cells.

The similarities between this response and the response to tissue damage suggest that the loss of polarity drives tumor formation through an unstoppable wound-healing reaction. Therefore, Bunker et al.'s findings link the formation of epithelial tumors to the signaling pathways that control the repair of damaged tissues.

DOI:http://dx.doi.org/10.7554/eLife.03189.002

Genetics and mechanisms of ovarian cancer: parallels between Drosophila and humans

Considering the degree of detail available at the genetic and cellular levels, the Drosophila ovary stands out as a powerful system to identify new players in the regulation of key aspects of cancer progression. In this review, we will comment on how the use of the Drosophila ovary has helped to elucidate some of the molecular bases of ovarian malignancies and to identify and characterize critical tumour suppressor genes and oncogenes with an impact in human pathologies.

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.

PRC2 controls Drosophila oocyte cell fate by repressing cell cycle genes

The oocyte is a unique cell type that undergoes extensive chromosome changes on its way to fertilization, but the chromatin determinants of its fate are unknown. Here, we show that Polycomb group (PcG) proteins of the Polycomb repressive complex 2 (PRC2) determine the fate of the oocyte in Drosophila. Mutation of the enzymatic PRC2 subunit Enhancer of zeste (E(z)) in the germline abolishes spatial and temporal control of the cell cycle and induces sterility via transdetermination of the oocyte into a nurse-like cell. This fate switch depends on loss of silencing of two PRC2 target genes, Cyclin E and the cyclin-dependent kinase inhibitor dacapo. By contrast, the PRC1 component Polycomb (Pc) plays no role in this process. Our results demonstrate that PRC2 plays an exquisite role in the determination of the oocyte fate by preventing its switching into an endoreplicative program.

From neural development to cognition: unexpected roles for chromatin

Recent genome-sequencing studies in human neurodevelopmental and psychiatric disorders have uncovered mutations in many chromatin regulators. These human genetic studies, along with studies in model organisms, are providing insight into chromatin regulatory mechanisms in neural development and how alterations to these mechanisms can cause cognitive deficits, such as intellectual disability. We discuss several implicated chromatin regulators, including BAF (also known as SWI/SNF) and CHD8 chromatin remodellers, HDAC4 and the Polycomb component EZH2. Interestingly, mutations in EZH2 and certain BAF complex components have roles in both neurodevelopmental disorders and cancer, and overlapping point mutations are suggesting functionally important residues and domains. We speculate on the contribution of these similar mutations to disparate disorders.

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.

APC loss-induced intestinal tumorigenesis in Drosophila: Roles of Ras in Wnt signaling activation and tumor progression

Adenomatous polyposis coli (APC) and K-ras are the two most frequently mutated genes found in human colorectal cancers. In human colorectal cancers, Wnt signaling activation after the loss of APC is hypothesized to be the key event for adenoma initiation, whereas additional mutations such as Ras activation are required for the progression from adenoma to carcinoma. However, accumulating data have led to conflicting views regarding the precise role of Ras in APC loss-induced tumorigenesis. Here, using Drosophila midgut as a model system, we show that in the absence of Ras, APC mutant epithelial cells cannot initiate hyperplasia, suggesting that Ras plays an essential role in tumor initiation. Conversely, activating Ras by expressing oncogenic Ras or Raf in APC-deficient cells led to a blockage of cell differentiation and to preinvasive tumor outgrowth, characteristics that are shared by advanced colorectal carcinoma in humans. Mechanistically, we find that Ras is not required for Wnt signaling activation after APC loss, although Ras hyperactivation is able to potentiate Wnt signaling by increasing the cytoplasmic and nuclear accumulation of Armadillo/β-catenin via mechanisms independent of JNK/Rac1 or PI3K-Akt signaling, partly owing to the downregulation of DE-cadherin. Together with the data from gene expression analyses, our results indicate that both parallel and cooperative mechanisms of Wnt and Ras signaling are responsible for the initiation and progression of intestinal tumorigenesis after APC loss.

Wnt signaling in stem and cancer stem cells

The functional versatility of Wnt/β-catenin signaling can be seen by its ability to act in stem cells of the embryo and of the adult as well as in cancer stem cells. During embryogenesis, stem cells demonstrate a requirement for β-catenin in mediating the response to Wnt signaling for their maintenance and transition from a pluripotent state. In adult stem cells, Wnt signaling functions at various hierarchical levels to contribute to specification of different tissues. This has raised the possibility that the tightly regulated self-renewal mediated by Wnt signaling in stem and progenitor cells is subverted in cancer cells to allow malignant progression. Intensive work is currently being performed to resolve how intrinsic and extrinsic factors that regulate Wnt/β-catenin signaling coordinate the stem and cancer stem cell states.

Polycomb group genes Psc and Su(z)2 maintain somatic stem cell identity and activity in Drosophila

Adult stem cells are essential for the proper function of many tissues, yet the mechanisms that maintain the proper identity and regulate proliferative capacity in stem cell lineages are not well understood. Polycomb group (PcG) proteins are transcriptional repressors that have recently emerged as important regulators of stem cell maintenance and differentiation. Here we describe the role of Polycomb Repressive Complex 1 (PRC1) genes Posterior sex combs (Psc) and Suppressor of zeste two (Su(z)2) in restricting the proliferation and maintaining the identity of the Cyst Stem Cell (CySC) lineage in the Drosophila testis. In contrast, Psc and Su(z)2 seem to be dispensable for both germline stem cell (GSC) maintenance and germ cell development. We show that loss of Psc and Su(z)2 function in the CySC lineage results in the formation of aggregates of mutant cells that proliferate abnormally, and display abnormal somatic identity correlated with derepression of the Hox gene Abdominal-B. Furthermore, we show that tumorigenesis in the CySC lineage interferes non-cell autonomously with maintenance of GSCs most likely by displacing them from their niche.

Chromatin modification by PSC occurs at one PSC per nucleosome and does not require the acidic patch of histone H2A

Chromatin architecture is regulated through both enzymatic and non-enzymatic activities. For example, the Polycomb Group (PcG) proteins maintain developmental gene silencing using an array of chromatin-based mechanisms. The essential Drosophila PcG protein, Posterior Sex Combs (PSC), compacts chromatin and inhibits chromatin remodeling and transcription through a non-enzymatic mechanism involving nucleosome bridging. Nucleosome bridging is achieved through a combination of nucleosome binding and self-interaction. Precisely how PSC interacts with chromatin to bridge nucleosomes is not known and is the subject of this work. We determine the stoichiometry of PSC-chromatin interactions in compact chromatin (in which nucleosomes are bridged) using Scanning Transmission Electron Microscopy (STEM). We find that full compaction occurs with one PSC per nucleosome. In addition to compacting chromatin, we show that PSC oligomerizes nucleosome arrays. PSC-mediated oligomerization of chromatin occurs at similar stoichiometry as compaction suggesting it may also involve nucleosome bridging. Interactions between the tail of histone H4 and the acidic patch of histone H2A are important for chromatin folding and oligomerization, and several chromatin proteins bind the histone H2A acidic patch. However, mutation of the acidic patch of histone H2A does not affect PSC’s ability to inhibit chromatin remodeling or bridge nucleosomes. In fact, PSC does not require nucleosomes for bridging activity but can bridge naked DNA segments. PSC clusters nucleosomes on sparsely assembled templates, suggesting it interacts preferentially with nucleosomes over bare DNA. This may be due to the ability of PSC to bind free histones. Our data are consistent with a model in which each PSC binds a nucleosome and at least one other PSC to directly bridge nucleosomes and compact chromatin, but also suggest that naked DNA can be included in compacted structures. We discuss how our data highlight the diversity of mechanisms used to modify chromatin architecture.

Stem cells: a view from the roots

In both plants and animals, regeneration requires the activation of stem cells. This is possibly related to the origin and requirements of multicellularity. Although long diverged from a common ancestry, plant and animal models such as Arabidopsis, Drosophila and mouse share considerable similarities in stem cell regulation. This includes stem cell niche organisation, epigenetic modification of DNA and histones, and the role of small RNA machinery in differentiation and pluripotency states. Dysregulation of any of these can lead to premature ageing, patterning and specification defects, as well as cancers. Moreover, emerging basal animal and plant systems are beginning to provide important clues concerning the diversity and evolutionary history of stem cell regulatory mechanisms in eukaryotes. This review provides a comparative framework, highlighting both the commonalities and differences among groups, which should promote the intelligent design of artificial stem cell systems, and thereby fuel the field of biomaterials science.

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.

Drosophila models of epithelial stem cells and their niches

Epithelial stem cells are regulated through a complex interplay of signals from diffusible ligands, cellular interactions, and attachment to the extracellular matrix. The development of Drosophila models of epithelial stem cells and their associated niche has made it possible to dissect the contribution of each of these factors in vivo, during both basal homeostasis and in response to acute damage such as infection. Studies of Drosophila epithelial stem cells have also provided insight into the mechanisms by which a healthy population of stem cells are maintained throughout adulthood by demonstrating, for example, that stem cells have a finite lifespan and may be displaced by replacement cells competing for niche occupancy. Here, we summarize the literature on each of the known Drosophila epithelial stem cells, with a focus on the two most well-characterized types, the follicle stem cells (FSCs) in the ovary and the intestinal stem cells (ISCs) in the posterior midgut. Several themes have emerged from these studies, which suggest that there may be a common set of features among niches in a variety of epithelia. For example, unlike the simpler Drosophila germline stem cell niches, both the FSC and ISC niches produce multiple, partially redundant, niche signals, some of which activate pathways such as Wnt/Wingless, Hedgehog, and epidermal growth factor (EGF) that also regulate mammalian epithelial tissue renewal. Further study into these relatively new stem cell models will be of use in understanding both the specifics of epithelial regeneration and the diversity of mechanisms that regulate adult stem cells in general.

Outgrowth of single oncogene-expressing cells from suppressive epithelial environments

Tumorigenesis is a clonal evolution process initiated from single cells within otherwise histologically normal tissue¹. How single, sporadic mutant cells that have sustained oncogenic alterations evolve within tightly regulated tissue environment remains elusive. Here, we investigated the effects of inducing oncogene expression in single cells within organotypic mammary acini as a model to elucidate the processes by which oncogenic alterations initiate clonal progression from organized epithelial environments. We found that sporadic cells induced to overexpress oncogenes that specifically perturb cell-cycle checkpoints (HPV16-E7 and cyclinD1), deregulate c-Myc transcription, or activate AKT signaling, remain quiescent within growth-arrested acini. In contrast, single ErbB2-overexpressing cells initiate a cellular cascade involving cell translocation from the epithelial layer and luminal outgrowth characteristic of neoplastic progression in early-stage epithelial tumors. ErbB2-mediated luminal cell translocation is dependent on ERK and matrix metalloproteinase (MMP) activities, and genetic alterations that perturb local cell-matrix adhesion can drive cell translocation. We further provide evidence that luminal cell translocation may drive clonal selection by promoting either death or expansion of quiescent oncogene-expressing cells depending on whether preexisting alterations allow anchorage-independent survival and growth. Our data show that the initial outgrowth of single oncogene-expressing cells from organized epithelial structures is a highly regulated process, and we propose that a cell translocation mechanism allows sporadic mutant cells to evade suppressive microenvironments and provokes clonal selection for survival and proliferative expansion outside their native niches.

The biological function of the WD40 repeat-containing protein p55/Caf1 in Drosophila

BACKGROUND

The p55 family WD40 repeat-containing histone chaperone proteins are components of several chromatin regulatory complexes (such as PRC2, NURF and CAF-1) and interact with histone H4, yet their functional relevance in vivo is unclear.

RESULTS

Here we use Drosophila as a genetic model to dissect the function of p55/Caf1 during development. In agree with a recent report, we find that p55 is essential for Drosophila development and is required for cell proliferation and viability. However, our data further demonstrate that histone H3K27 di-/tri-methylation and PRC2-mediated gene silencing still occur normally when p55 is missing. p55 is also implicated in bridging chromatin regulatory complexes to the chromatin by binding to histone H4, but we find that a transgene of p55 whose binding pocket is disrupted could still functionally substitute the wild-type p55 for the survival.

CONCLUSIONS

Our studies suggest that p55 is not crucial for PRC2-mediated gene silencing in vivo, and the vital function of p55 is probably not dependent on its interaction with histone H4.

The kidney and planar cell polarity

Planar cell polarity (PCP) or tissue polarity describes a coordinated polarity at the plane of the tissue where most or all cells within a tissue are polarized in one direction. It is perpendicular to the apical-basal polarity of the cell. PCP is manifested readily in the Drosophila wing and cuticle bristles, Drosophila eye ommatidia, and mammalian hair and inner ear hair bundles, and less evidently, in cellular processes such as in the coordinated, directional cell movements, and oriented cell divisions that are important for tissue morphogenesis. Several distinct molecular and cellular processes have been implicated in the regulation of PCP. Here, we review potential roles for PCP during mouse kidney development and maintenance, including ureteric bud branching morphogenesis, renal medulla elongation, tubule diameter establishment/maintenance, glomerulogenesis, and response to injury. The potential mechanisms underlying these processes, including oriented cell division and coordinated cell migration/cell intercalation, are discussed. In addition, we discuss some unaddressed research topics related to PCP in the kidney that we hope will spur further discussion and investigation.

Identification and characterization of Polycomb group genes in the silkworm, Bombyx mori

Polycomb group (PcG) proteins are involved in chromatin modifications for maintaining gene repression that play important roles in the regulation of gene expression, tumorigenesis, chromosome X-inactivation, and genomic imprinting in Drosophila melanogaster, mammals, and even plants. To characterize the orthologs of PcG genes in the silkworm, Bombyx mori, 13 candidates were identified from the updated silkworm genome sequence by using the fruit fly PcG genes as queries. Comparison of the silkworm PcG proteins with those from other insect species revealed that the insect PcG proteins shared high sequence similarity. High-level expressions of all the silkworm PcG genes were maintained through day 2 to day 7 of embryogenesis, and tissue microarray data on day 3 of the fifth instar larvae showed that their expression levels were relatively low in somatic tissues, except for Enhancer of zeste (E(Z)). In addition, knockdown of each PRC2 component, such as E(Z), Extra sex combs (ESC), and Suppressor of zeste 12 (SU(Z)12), considerably decreased the global levels of H3K27me3 but not of H3K27me2. Taken together, these results suggest that insect PcG proteins are highly conserved during evolution and might play similar roles in embryogenesis.

Drosophila stem cell niches: a decade of discovery suggests a unified view of stem cell regulation

The past decade of research on Drosophila stem cells and niches has provided key insights. Fly stem cells do not occupy a special "state" based on novel "stem cell genes" but resemble transiently arrested tissue progenitors. Moreover, individual stem cells and downstream progenitors are highly dynamic and dispensable, not tissue bulwarks. Niches, rather than fixed cell lineages, ensure tissue health by holding stem cells and repressing cell differentiation inside, but not outside. We review the five best-understood adult Drosophila stem cells and argue that the fundamental biology of stem cells and niches is conserved between Drosophila and mice.

Extra sex combs, chromatin, and cancer: exploring epigenetic regulation and tumorigenesis in Drosophila

Developmental genetic studies in Drosophila unraveled the importance of Polycomb group (PcG) and Trithorax group (TrxG) genes in controlling cellular identity. PcG and TrxG proteins form histone modifying complexes that catalyze repressive or activating histone modifications, respectively, and thus maintaining the expression status of homeotic genes. Human orthologs of PcG and TrxG genes are implicated in tumorigenesis as well as in determining the prognosis of individual cancers. Recent whole genome analyses of cancers also highlighted the importance of histone modifying proteins in controlling tumorigenesis. Comprehensive understanding of the mechanistic relationship between histone regulation and tumorigenesis holds the promise of significantly advancing our understanding and management of cancer. It is anticipated that Drosophila melanogaster, the model organism that contributed significantly to our understanding of the functional role of histone regulation in development, could also provide unique insight for our understanding of how histone dysregulation can lead to cancer. In this review, we will discuss several recent advances in this regard.

Planar cell polarity in kidney development and disease

Planar cell polarity (PCP) describes the coordinated polarization of tissue cells in a direction that is orthogonal to their apical/basal axis. In the last several years, studies in flies and vertebrates have defined evolutionarily conserved pathways that establish and maintain PCP in various cellular contexts. Defective responses to the polarizing signal(s) have deleterious effects on the development and repair of a wide variety of organs/tissues. In this review, we cover the known and hypothesized roles for PCP in the metanephric kidney. We highlight the similarities and differences in PCP establishment in this organ compared with flies, especially the role of Wnt signaling in this process. Finally, we present a model whereby the signal(s) that organizes PCP in the kidney epithelium, at least in part, comes from the adjacent stromal fibroblasts.

At the crossroads of differentiation and proliferation: precise control of cell-cycle changes by multiple signaling pathways in Drosophila follicle cells

Here, we discuss the findings to date about genes and pathways required for regulation of somatic follicle-cell proliferation and differentiation during Drosophila oogenesis and demonstrate how loss of these genes contributes to the tumorigenic potential of mutant cells. Follicle cells undergo cell-fate determination through stepwise activation of multiple signaling pathways, including the Notch, Hedgehog, Wingless, janus kinase/STAT, and JNK pathways. In addition, changes in DNA replication and cellular growth depend on the spatial and temporal activation of the mitotic cycle-endocycle and endocycle-gene amplification cell-cycle switches and insulin-dependent monitoring of cellular health; systemic loss of these pathways contributes to loss of controlled cellular proliferation, loss of differentiation/growth, and aberrant cell polarity in follicle cells. We also highlight the effects of the neoplastic and Hippo pathways on the cell cycle and cellular proliferation in promoting normal development and conclude that lack of coordination of multiple signaling pathways promotes conditions favorable for tumorigenesis.