Regulation of Drosophila embryonic tracheogenesis by dVHL and hypoxia

eEF1α2 is required for actin cytoskeleton homeostasis in the aging muscle

The translation elongation factor eEF1α (eukaryotic elongation factor 1α) mediates mRNA translation by delivering aminoacyl-tRNAs to ribosomes. eEF1α also has other reported roles, including the regulation of actin dynamics. However, these distinct roles of eEF1α are often challenging to uncouple and remain poorly understood in aging metazoan tissues. The genomes of mammals and Drosophila encode two eEF1α paralogs, with eEF1α1 expressed ubiquitously and eEF1α2 expression more limited to neurons and muscle cells. Here, we report that eEF1α2 plays a unique role in maintaining myofibril homeostasis during aging in Drosophila. Specifically, we generated an eEF1α2 null allele, which was viable and showed two distinct muscle phenotypes. In young flies, the mutants had thinner myofibrils in indirect flight muscles that could be rescued by expressing eEF1α1. With aging, the muscles of the mutant flies began showing abnormal distribution of actin and myosin in muscles, but without a change in actin and myosin protein levels. This age-related phenotype could not be rescued by eEF1α1 overexpression. These findings support an unconventional role of Drosophila eEF1α2 in age-related homeostasis of muscle myofibers.

Drosophila Accessory Gland: A Complementary In Vivo Model to Bring New Insight to Prostate Cancer

Prostate cancer is the most common cancer in aging men. Despite recent progress, there are still few effective treatments to cure its aggressive and metastatic stages. A better understanding of the molecular mechanisms driving disease initiation and progression appears essential to support the development of more efficient therapies and improve patient care. To do so, multiple research models, such as cell culture and mouse models, have been developed over the years and have improved our comprehension of the biology of the disease. Recently, a new model has been added with the use of the Drosophila accessory gland. With a high level of conservation of major signaling pathways implicated in human disease, this functional equivalent of the prostate represents a powerful, inexpensive, and rapid in vivo model to study epithelial carcinogenesis. The purpose of this review is to quickly overview the existing prostate cancer models, including their strengths and limitations. In particular, we discuss how the Drosophila accessory gland can be integrated as a convenient complementary model by bringing new understanding in the mechanisms driving prostate epithelial tumorigenesis, from initiation to metastatic formation.

The tumor suppressor archipelago E3 ligase is required for spermatid differentiation in Drosophila testis

The human orthologue of the tumor suppressor protein FBW7 is encoded by the Drosophila archipelago (ago) gene. Ago is an F-box protein that gives substrate specificity to its SCF ubiquitin ligase complex. It has a central role in multiple biological processes in a tissue-specific manner such as cell proliferation, cellular differentiation, hypoxia-induced gene expression. Here we present a previously unknown tissue-specific role of Ago in spermatid differentiation. We identified a classical mutant of ago which is semi-lethal and male-sterile. During the characterization of ago function in testis, we found that ago plays role in spermatid development, following meiosis. We confirmed spermatogenesis defects by silencing ago by RNAi in testes. The ago mutants show multiple abnormalities in elongating and elongated spermatids, including aberration of the cyst morphology, malformed mitochondrial structures, and individualization defects. Additionally, we determined the subcellular localization of Ago protein with mCherry-Ago transgene in spermatids. Our findings highlight the potential roles of Ago in different cellular processes of spermatogenesis, like spermatid individualization, and regulation of mitochondrial morphology.

Modeling Neoplastic Growth in Renal Cell Carcinoma and Polycystic Kidney Disease

Renal cell carcinoma (RCC) and autosomal dominant polycystic kidney disease (ADPKD) share several characteristics, including neoplastic cell growth, kidney cysts, and limited therapeutics. As well, both exhibit impaired vasculature and compensatory VEGF activation of angiogenesis. The PI3K/AKT/mTOR and Ras/Raf/ERK pathways play important roles in regulating cystic and tumor cell proliferation and growth. Both RCC and ADPKD result in hypoxia, where HIF-α signaling is activated in response to oxygen deprivation. Primary cilia and altered cell metabolism may play a role in disease progression. Non-coding RNAs may regulate RCC carcinogenesis and ADPKD through their varied effects. Drosophila exhibits remarkable conservation of the pathways involved in RCC and ADPKD. Here, we review the progress towards understanding disease mechanisms, partially overlapping cellular and molecular dysfunctions in RCC and ADPKD and reflect on the potential for the agile Drosophila genetic model to accelerate discovery science, address unresolved mechanistic aspects of these diseases, and perform rapid pharmacological screens.

Single-cell branching morphogenesis in the Drosophila trachea

The terminal cells of the tracheal epithelium in Drosophila melanogaster are one of the few known cell types that undergo subcellular morphogenesis to achieve a stable, branched shape. During the animal's larval stages, the cells repeatedly sprout new cytoplasmic processes. These grow very long, wrapping around target tissues to which the terminal cells adhere, and are hollowed by a gas-filled subcellular tube for oxygen delivery. Our understanding of this ramification process remains rudimentary. This review aims to provide a comprehensive summary of studies on terminal cells to date, and attempts to extrapolate how terminal branches might be formed based on the known genetic and molecular components. Next to this cell-intrinsic branching mechanism, we examine the extrinsic regulation of terminal branching by the target tissue and the animal's environment. Finally, we assess the degree of similarity between the patterns established by the branching programs of terminal cells and other branched cells and tissues from a mathematical and conceptual point of view.

Endocrine regulation of airway clearance in Drosophila

Fluid clearance from the respiratory system during developmental transitions is critically important for achieving optimal gas exchange in animals. During insect development from embryo to adult, airway clearance occurs episodically each time the molt is completed by performance of the ecdysis sequence, coordinated by a peptide-signaling cascade initiated by ecdysis-triggering hormone (ETH). We find that the neuropeptide Kinin (also known as Drosokinin or Leukokinin) is required for normal respiratory fluid clearance or "tracheal air-filling" in Drosophila larvae. Disruption of Kinin signaling leads to defective air-filling during all larval stages. Such defects are observed upon ablation or electrical silencing of Kinin neurons, as well as RNA silencing of the Kinin gene or the ETH receptor in Kinin neurons, indicating that ETH targets Kinin neurons to promote tracheal air-filling. A Kinin receptor mutant fly line (Lkr^f02594 ) also exhibits tracheal air-filling defects in all larval stages. Targeted Kinin receptor silencing in tracheal epithelial cells using breathless or pickpocket (ppk) drivers compromises tracheal air-filling. On the other hand, promotion of Kinin signaling in vivo through peptide injection or Kinin neuron activation through Drosophila TrpA1 (dTrpA1) expression induces premature tracheal collapse and air-filling. Moreover, direct exposure of tracheal epithelial cells in vitro to Kinin leads to calcium mobilization in tracheal epithelial cells. Our findings strongly implicate the neuropeptide Kinin as an important regulator of airway clearance via intracellular calcium mobilization in tracheal epithelial cells of Drosophila.

Building branched tissue structures: from single cell guidance to coordinated construction

Branched networks are ubiquitous throughout nature, particularly found in tissues that require large surface area within a restricted volume. Many tissues with a branched architecture, such as the vasculature, kidney, mammary gland, lung and nervous system, function to exchange fluids, gases and information throughout the body of an organism. The generation of branched tissues requires regulation of branch site specification, initiation and elongation. Branching events often require the coordination of many cells to build a tissue network for material exchange. Recent evidence has emerged suggesting that cell cooperativity scales with the number of cells actively contributing to branching events. Here, we compare mechanisms that regulate branching, focusing on how cell cohorts behave in a coordinated manner to build branched tissues.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

miR-190 Enhances HIF-Dependent Responses to Hypoxia in Drosophila by Inhibiting the Prolyl-4-hydroxylase Fatiga

Cellular and systemic responses to low oxygen levels are principally mediated by Hypoxia Inducible Factors (HIFs), a family of evolutionary conserved heterodimeric transcription factors, whose alpha- and beta-subunits belong to the bHLH-PAS family. In normoxia, HIFα is hydroxylated by specific prolyl-4-hydroxylases, targeting it for proteasomal degradation, while in hypoxia the activity of these hydroxylases decreases due to low oxygen availability, leading to HIFα accumulation and expression of HIF target genes. To identify microRNAs required for maximal HIF activity, we conducted an overexpression screen in Drosophila melanogaster, evaluating the induction of a HIF transcriptional reporter. miR-190 overexpression enhanced HIF-dependent biological responses, including terminal sprouting of the tracheal system, while in miR-190 loss of function embryos the hypoxic response was impaired. In hypoxic conditions, miR-190 expression was upregulated and required for induction of HIF target genes by directly inhibiting the HIF prolyl-4-hydroxylase Fatiga. Thus, miR-190 is a novel regulator of the hypoxia response that represses the oxygen sensor Fatiga, leading to HIFα stabilization and enhancement of hypoxic responses.

Drosophila and the hallmarks of cancer

: Cancer was the disease of the twentieth century. Today it is still a leading cause of death worldwide despite being intensively investigated. Abundant knowledge exists regarding the pathological and molecular mechanisms that drive healthy cells to become malignant and form metastatic tumors. The relation of oncogenes and tumor suppressors to the genetic trigger of carcinogenesis is unquestionable. However, the development of the disease requires many characteristics that due to their proven role in cancer are collectively described as the "hallmarks of cancer." We highlight here the historic discoveries made using the model organism Drosophila melanogaster and its contributions to biomedical and cancer research. Flies are utilized as a model organism for the investigation of each and every aspect of cancer hallmarks. Due to the significant conservation between flies and mammals at the signaling and tissue physiology level it is possible to explore the genes and mechanisms responsible for cancer pathogenesis in flies. Recent Drosophila studies suggest novel aspects of therapeutic intervention and are expected to guide cancer research in the twenty-first century.

The archipelago ubiquitin ligase subunit acts in target tissue to restrict tracheal terminal cell branching and hypoxic-induced gene expression

The Drosophila melanogaster gene archipelago (ago) encodes the F-box/WD-repeat protein substrate specificity factor for an SCF (Skp/Cullin/F-box)-type polyubiquitin ligase that inhibits tumor-like growth by targeting proteins for degradation by the proteasome. The Ago protein is expressed widely in the fly embryo and larva and promotes degradation of pro-proliferative proteins in mitotically active cells. However the requirement for Ago in post-mitotic developmental processes remains largely unexplored. Here we show that Ago is an antagonist of the physiologic response to low oxygen (hypoxia). Reducing Ago activity in larval muscle cells elicits enhanced branching of nearby tracheal terminal cells in normoxia. This tracheogenic phenotype shows a genetic dependence on sima, which encodes the HIF-1α subunit of the hypoxia-inducible transcription factor dHIF and its target the FGF ligand branchless (bnl), and is enhanced by depletion of the Drosophila Von Hippel Lindau (dVHL) factor, which is a subunit of an oxygen-dependent ubiquitin ligase that degrades Sima/HIF-1α protein in metazoan cells. Genetic reduction of ago results in constitutive expression of some hypoxia-inducible genes in normoxia, increases the sensitivity of others to mild hypoxic stimulus, and enhances the ability of adult flies to recover from hypoxic stupor. As a molecular correlate to these genetic data, we find that Ago physically associates with Sima and restricts Sima levels in vivo. Collectively, these findings identify Ago as a required element of a circuit that suppresses the tracheogenic activity of larval muscle cells by antagonizing the Sima-mediated transcriptional response to hypoxia.

HIF- and non-HIF-regulated hypoxic responses require the estrogen-related receptor in Drosophila melanogaster

Low-oxygen tolerance is supported by an adaptive response that includes a coordinate shift in metabolism and the activation of a transcriptional program that is driven by the hypoxia-inducible factor (HIF) pathway. The precise contribution of HIF-1a in the adaptive response, however, has not been determined. Here, we investigate how HIF influences hypoxic adaptation throughout Drosophila melanogaster development. We find that hypoxic-induced transcriptional changes are comprised of HIF-dependent and HIF-independent pathways that are distinct and separable. We show that normoxic set-points of carbohydrate metabolites are significantly altered in sima mutants and that these animals are unable to mobilize glycogen in hypoxia. Furthermore, we find that the estrogen-related receptor (dERR), which is a global regulator of aerobic glycolysis in larvae, is required for a competent hypoxic response. dERR binds to dHIFa and participates in the HIF-dependent transcriptional program in hypoxia. In addition, dERR acts in the absence of dHIFa in hypoxia and a significant portion of HIF-independent transcriptional responses can be attributed to dERR actions, including upregulation of glycolytic transcripts. These results indicate that competent hypoxic responses arise from complex interactions between HIF-dependent and -independent mechanisms, and that dERR plays a central role in both of these programs.

When oxygen levels fall below normal, cells are said to be in a hypoxic state. Once in hypoxia, dramatic changes are induced that allow for adaptation. In particular, energetic metabolism and transcription are highly affected. HIF (hypoxia inducible factor) is a highly conserved factor that is the driving force behind many hypoxia-induced changes—it is inactive in normal conditions and becomes active in hypoxia. Using the fruit fly as a model system, we show that hypoxic responses consist of HIF and non-HIF-dependent pathways. These response programs counteract the impacts of low oxygen by broadly influencing different cellular processes such as the breakdown of sugars, but only at appropriate developmental times. We provide evidence that HIF- and non-HIF-dependent pathways are complemented by the actions of the steroid hormone receptor estrogen-related receptor (ERR), which we show is also essential in hypoxia. Our results place new emphasis on the actions of HIF and suggest that alternative HIF-independent pathways play a more prominent role than previously thought.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Complex cellular functions of the von Hippel-Lindau tumor suppressor gene: insights from model organisms

The von Hippel-Lindau tumor suppressor gene (VHL) has attracted intensive interest not only because its mutations predispose carriers to devastating tumors, but also because it is involved in oxygen sensing under physiological conditions. VHL loss-of-function mutations result in organ-specific tumors, such as hemangioblastoma of the central nervous system and renal cell carcinoma, both untreatable with conventional chemotherapies. The VHL protein is best known as an E3 ubiquitin ligase that targets hypoxia-inducible factor-α (HIF-α), but many diverse, non-canonical cellular functions have also been assigned to VHL, mainly based on studies in cell culture systems. As such, although the HIF-dependent role of VHL is critical, the full spectrum of pathophysiological functions of VHL is still unresolved. Such understanding requires careful cross-referencing with physiologically relevant experimental models. Studies in model systems, such as Caenorhabditis elegans, Drosophila, zebrafish and mouse have provided critical in vivo confirmation of the VHL-HIF pathway, and verification of potentially important cellular functions including microtubule stabilization and epithelial morphogenesis. More recently, animal models have also suggested systemic roles of VHL in hematopoiesis, metabolic homeostasis and inflammation. In this review, the studies performed in model organisms will be summarized and placed in context with existing clinical and in vitro data.

A systematic screen for tube morphogenesis and branching genes in the Drosophila tracheal system

Many signaling proteins and transcription factors that induce and pattern organs have been identified, but relatively few of the downstream effectors that execute morphogenesis programs. Because such morphogenesis genes may function in many organs and developmental processes, mutations in them are expected to be pleiotropic and hence ignored or discarded in most standard genetic screens. Here we describe a systematic screen designed to identify all Drosophila third chromosome genes (∼40% of the genome) that function in development of the tracheal system, a tubular respiratory organ that provides a paradigm for branching morphogenesis. To identify potentially pleiotropic morphogenesis genes, the screen included analysis of marked clones of homozygous mutant tracheal cells in heterozygous animals, plus a secondary screen to exclude mutations in general “house-keeping” genes. From a collection including more than 5,000 lethal mutations, we identified 133 mutations representing ∼70 or more genes that subdivide the tracheal terminal branching program into six genetically separable steps, a previously established cell specification step plus five major morphogenesis and maturation steps: branching, growth, tubulogenesis, gas-filling, and maintenance. Molecular identification of 14 of the 70 genes demonstrates that they include six previously known tracheal genes, each with a novel function revealed by clonal analysis, and two well-known growth suppressors that establish an integral role for cell growth control in branching morphogenesis. The rest are new tracheal genes that function in morphogenesis and maturation, many through cytoskeletal and secretory pathways. The results suggest systematic genetic screens that include clonal analysis can elucidate the full organogenesis program and that over 200 patterning and morphogenesis genes are required to build even a relatively simple organ such as the Drosophila tracheal system.

Elucidating the genetic programs that control formation and maintenance of body organs is a central goal of developmental biology, and understanding how these programs go awry in disease has important implications for medicine. Many such organogenesis genes have been identified, but most are early-acting “patterning genes” encoding signaling proteins and gene regulators that control expression of a poorly characterized set of downstream “morphogenesis genes,” which encode proteins that generate the remarkable organ forms and structures of the constituent cells. We screened ∼40% of the fruit fly Drosophila genome for mutations that affect tracheal (respiratory) system development. We included steps to bypass complexities from mutant effects on other tissues and steps to exclude mutations in general cell “housekeeping genes.” We isolated mutations in ∼70 genes that identify major steps in the organogenesis program including an integral cell growth control step. Many of the new tracheal genes are “morphogenesis genes” that encode proteins involved in cell structure or intracellular transport. The results suggest that genetic screens can elucidate a full organogenesis program and that over 200 patterning and morphogenesis genes are required to build even a relatively simple organ.

Drosophila von Hippel-Lindau tumor suppressor gene function in epithelial tubule morphogenesis

Mutations in the human von Hippel-Lindau (VHL) gene are the cause of VHL disease that displays multiple benign and malignant tumors. The VHL gene has been shown to regulate angiogenic potential and glycolic metabolism via its E3 ubiquitin ligase function against the alpha subunit of hypoxia-inducible factor (HIF-alpha). However, many HIF-independent functions of VHL have been identified. Recent evidence also indicates that the canonical function cannot fully explain the VHL mutant cell phenotypes, although it is still unclear how many of these noncanonical functions relate to the pathophysiological processes because of a lack of tractable genetic systems. Here, we report the first genomic mutant phenotype of Drosophila melanogaster VHL (dVHL) in the epithelial tubule network, the trachea, and show that dVHL regulates branch migration and lumen formation via its endocytic function. The endocytic function regulates the surface level of the chemotactic signaling receptor Breathless and promotes clearing of the lumen matrix during maturation of the tracheal tubes. Importantly, the regulatory function in tubular morphogenesis is conserved in the mammalian system, as conditional knockout of Vhl in mouse kidney also resulted in similar cell motility and lumen phenotypes.

Tracheal remodelling in response to hypoxia

The insect tracheal system is a continuous tubular network that ramifies into progressively thinner branches to provide air directly to every organ and tissue throughout the body. During embryogenesis the basic architecture of the tracheal system develops in a stereotypical and genetically controlled manner. Later, in larval stages, the tracheal system becomes plastic, and adapts to particular oxygen needs of the different tissues of the body. Oxygen sensing is mediated by specific prolyl-4-hydroxylases that regulate protein stability of the alpha subunit of oxygen-responsive transcription factors from the HIF family. Tracheal cells are exquisitely sensitive to oxygen levels, modulating the expression of hypoxia-inducible proteins that mediate sprouting of tracheal branches in direction to oxygen-deprived tissues.

Drosophila VHL tumor-suppressor gene regulates epithelial morphogenesis by promoting microtubule and aPKC stability

Mutations in the human von Hippel-Lindau (VHL) genes are the cause of VHL disease, which displays multiple benign and malignant tumors. The VHL gene has been shown to regulate angiogenic potential and glycolic metabolism via its E3 ubiquitin ligase function against the alpha subunit of hypoxia-inducible factor (HIF). However, many other HIF-independent functions of VHL have been identified and recent evidence indicates that the canonical function cannot fully explain the VHL mutant cell phenotypes. Many of these functions have not been verified in genetically tractable systems. Using an established follicular epithelial model in Drosophila, we show that the Drosophila VHL gene is involved in epithelial morphogenesis via stabilizing microtubule bundles and aPKC. Microtubule defects in VHL mutants lead to mislocalization of aPKC and subsequent loss of epithelial integrity. Destabilizing microtubules in ex vivo culture of wild-type egg chambers can also result in aPKC mislocalization and epithelial defects. Importantly, paclitaxel-induced stabilization of microtubules can rescue the aPKC localization phenotype in Drosophila VHL mutant follicle cells. The results establish a developmental function of the VHL gene that is relevant to its tumor-suppressor activity.

Dose-dependent modulation of HIF-1alpha/sima controls the rate of cell migration and invasion in Drosophila ovary border cells

The role of the hypoxic response during metastasis was analysed in migrating border cells of the Drosophila ovary. Acute exposure to 1% O(2) delayed or blocked border cell migration (BCM), whereas prolonged exposure resulted in the first documented accelerated BCM phenotype. Similarly, manipulating the expression levels of sima, the Drosophila hypoxia-inducible factor (HIF)-1alpha ortholog, revealed that Sima can either block or restore BCM in a dose-dependent manner. In contrast, over-expression of Vhl (Drosophila von Hippel-Lindau) generated a range of phenotypes, including blocked, delayed and accelerated BCM, whereas over-expression of hph (Drosophila HIF prolyl hydroxylase) only accelerated BCM. Mosaic clone analysis of sima or tango (HIF-1beta ortholog) mutants revealed that cells lacking Hif-1 transcriptional activity were preferentially detected in the leading cell position of the cluster, resulting in either a delay or acceleration of BCM. Moreover, in sima mutant cell clones, there was reduced expression of nuclear slow border cells (Slbo) and basolateral DE-cadherin, proteins essential for proper BCM. These results show that Sima levels define the rate of BCM in part through regulation of Slbo and DE-cadherin, and suggest that dynamic regulation of Hif-1 activity is necessary to maintain invasive potential of migrating epithelial cells.

The archipelago tumor suppressor gene limits rb/e2f-regulated apoptosis in developing Drosophila tissues

BACKGROUND

The Drosophila archipelago gene (ago) encodes the specificity component of a ubiquitin ligase that targets the cyclin E and dMyc proteins for degradation. Its human ortholog, Fbw7, is commonly lost in cancers, suggesting that failure to degrade ago/Fbw7 targets drives excess tissue growth.

RESULTS

We find that ago loss induces hyperplasia of some organs but paradoxically reduces the size of the adult eye. This reflects a requirement for ago to restrict apoptotic activity of the rbf1/de2f1 pathway adjacent to the eye-specific morphogenetic furrow (MF): ago mutant cells display elevated de2f1 activity, express the prodeath dE2f1 targets hid and rpr, and undergo high rates of apoptosis. These phenotypes are dependent on rbf1, de2f1, hid, and the rbf1/de2f1 regulators cyclin E and dacapo but are independent of dp53. A transactivation-deficient de2f1 allele blocks MF-associated apoptosis of ago mutant cells but does not retard their clonal overgrowth, indicating that intact de2f1 function is required for the death but not overproliferation of ago cells. Epidermal growth factor receptor (EGFR) and wingless (wg) alleles also modify the ago apoptotic phenotype, indicating that these pathways may modulate the underlying sensitivity of ago mutant cells to apoptotic signals.

CONCLUSIONS

These data show that ago loss requires a collaborating block in cell death to efficiently drive tissue overgrowth and that this conditional phenotype reflects a role for ago in restricting apoptotic output of the rbf1/de2f1 pathway. Moreover, the susceptibility of ago mutant cells to succumb to this apoptotic program appears to depend on local variations in extracellular signaling that could thus determine tissue-specific fates of ago mutant cells.