Drosophila homologs of baculovirus inhibitor of apoptosis proteins function to block cell death

X-linked inhibitor of apoptosis protein and active caspase-3 expression patterns in antral follicles in the sheep ovary

X-linked inhibitor of apoptosis protein (XIAP) interacts with caspases to inhibit their activity, thereby providing a potential mechanism for regulation of granulosa cell apoptosis occurring during follicular atresia. The aim of this study was to determine the presence and localization of XIAP mRNA and protein content in the sheep ovary and compare these expression patterns with active caspase-3 protein in the same antral follicles. Romney ewe estrous cycles (n=25) were synchronized with 2–3 Estrumate injections and ovarian tissue collected during the luteal and follicular phases of the cycle. The presence ofXIAPmRNA was confirmed by RT-PCR using laser capture microdissected ovarian cell samples.XIAPmRNA was subsequently localized byin situhybridization histochemistry and XIAP and active caspase-3 protein visualized by immunohistochemistry. In antral follicles extensive XIAP localization was evident in both granulosa and thecal cells. In contrast, mRNA expression was widespread in granulosa cells and only detected in thecal tissue from a small proportion of antral follicles. Active caspase-3 and XIAP comparative expression analysis showed positiveXIAPmRNA expression in all late luteal phase (day 14) follicles, despite varying levels of active caspase-3 protein. A proportion of follicular phase (days 15 and 16) follicles, however, showed an inverse expression relationship at the protein and mRNA levels in both granulosa and thecal tissue, as did XIAP protein in day 14 follicles. These results suggest high XIAP may prevent activation of caspase-3, thereby regulating follicular atresia in antral follicles and could potentially be utilized as a marker of follicular health.

Maintenance of imaginal disc plasticity and regenerative potential in Drosophila by p53

Following irradiation (IR), the DNA damage response (DDR) activates p53, which triggers death of cells in which repair cannot be completed. Lost tissue is then replaced and re-patterned through regeneration. We have examined the role of p53 in co-regulation of the DDR and tissue regeneration following IR damage in Drosophila. We find that after IR, p53 is required for imaginal disc cells to repair DNA, and in its absence the damage marker, γ-H2AX is persistently expressed. p53 is also required for the compensatory proliferation and re-patterning of the damaged discs, and our results indicate that cell death is not required to trigger these processes. We identify an IR-induced delay in developmental patterning in wing discs that accompanies an animal-wide delay of the juvenile-adult transition, and demonstrate that both of these delays require p53. In p53 mutants, the lack of developmental delays and of damage resolution leads to anueploidy and tissue defects, and ultimately to morphological abnormalities and adult inviability. We propose that p53 maintains plasticity of imaginal discs by co-regulating the maintenance of genome integrity and disc regeneration, and coordinating these processes with the physiology of the animal. These findings place p53 in a role as master coordinator of DNA and tissue repair following IR.

Inhibitor of apoptosis (IAP)-like protein lacks a baculovirus IAP repeat (BIR) domain and attenuates cell death in plant and animal systems

A novel Arabidopsis thaliana inhibitor of apoptosis was identified by sequence homology to other known inhibitor of apoptosis (IAP) proteins. Arabidopsis IAP-like protein (AtILP) contained a C-terminal RING finger domain but lacked a baculovirus IAP repeat (BIR) domain, which is essential for anti-apoptotic activity in other IAP family members. The expression of AtILP in HeLa cells conferred resistance against tumor necrosis factor (TNF)-α/ActD-induced apoptosis through the inactivation of caspase activity. In contrast to the C-terminal RING domain of AtILP, which did not inhibit the activity of caspase-3, the N-terminal region, despite displaying no homology to known BIR domains, potently inhibited the activity of caspase-3 in vitro and blocked TNF-α/ActD-induced apoptosis. The anti-apoptotic activity of the AtILP N-terminal domain observed in plants was reproduced in an animal system. Transgenic Arabidopsis lines overexpressing AtILP exhibited anti-apoptotic activity when challenged with the fungal toxin fumonisin B1, an agent that induces apoptosis-like cell death in plants. In AtIPL transgenic plants, suppression of cell death was accompanied by inhibition of caspase activation and DNA fragmentation. Overexpression of AtILP also attenuated effector protein-induced cell death and increased the growth of an avirulent bacterial pathogen. The current results demonstrated the existence of a novel plant IAP-like protein that prevents caspase activation in Arabidopsis and showed that a plant anti-apoptosis gene functions similarly in plant and animal systems.

Drosophila IAP1-mediated ubiquitylation controls activation of the initiator caspase DRONC independent of protein degradation

Ubiquitylation targets proteins for proteasome-mediated degradation and plays important roles in many biological processes including apoptosis. However, non-proteolytic functions of ubiquitylation are also known. In Drosophila, the inhibitor of apoptosis protein 1 (DIAP1) is known to ubiquitylate the initiator caspase DRONC in vitro. Because DRONC protein accumulates in diap1 mutant cells that are kept alive by caspase inhibition (“undead” cells), it is thought that DIAP1-mediated ubiquitylation causes proteasomal degradation of DRONC, protecting cells from apoptosis. However, contrary to this model, we show here that DIAP1-mediated ubiquitylation does not trigger proteasomal degradation of full-length DRONC, but serves a non-proteolytic function. Our data suggest that DIAP1-mediated ubiquitylation blocks processing and activation of DRONC. Interestingly, while full-length DRONC is not subject to DIAP1-induced degradation, once it is processed and activated it has reduced protein stability. Finally, we show that DRONC protein accumulates in “undead” cells due to increased transcription of dronc in these cells. These data refine current models of caspase regulation by IAPs.

The Drosophila inhibitor of apoptosis 1 (DIAP1) readily promotes ubiquitylation of the CASPASE-9–like initiator caspase DRONC in vitro and in vivo. Because DRONC protein accumulates in diap1 mutant cells that are kept alive by effector caspase inhibition—producing so-called “undead” cells—it has been proposed that DIAP1-mediated ubiquitylation would target full-length DRONC for proteasomal degradation, ensuring survival of normal cells. However, this has never been tested rigorously in vivo. By examining loss and gain of diap1 function, we show that DIAP1-mediated ubiquitylation does not trigger degradation of full-length DRONC. Our analysis demonstrates that DIAP1-mediated ubiquitylation controls DRONC processing and activation in a non-proteolytic manner. Interestingly, once DRONC is processed and activated, it has reduced protein stability. We also demonstrate that “undead” cells induce transcription of dronc, explaining increased protein levels of DRONC in these cells. This study re-defines the mechanism by which IAP-mediated ubiquitylation regulates caspase activity.

Molecular genetic analyses of polytene chromosome region 72A-D in Drosophila melanogaster reveal a gene desert in 72D

We have investigated a region of ∼310 kb of genomic DNA within polytene chromosome subdivisions 72A to 72D of Drosophila melanogaster. This region includes 57 predicted protein-coding genes. Seventeen of these genes are in six clusters that appear to have arisen by tandem duplication. Within this region we found 23 complementation groups that are essential for zygotic viability, and we have identified the transcription units for 18 of the 23. We also found a 55 kb region in 72D that is nonessential. Flies deficient for this region are viable and fertile. Within this nonessential region are 48 DNA sequences of 12 to 33 base pairs that are completely conserved among 12 distantly related Drosophila species. These sequences do not have the evolutionary signature of conserved protein-coding DNA sequences, nor do they appear to encode microRNAs, however, the strong selection suggests functions in wild populations that are not apparent in laboratory cultures. This region resembles dispensable gene deserts previously characterized in the mouse genome.

A screen for conditional growth suppressor genes identifies the Drosophila homolog of HD-PTP as a regulator of the oncoprotein Yorkie

Mammalian cancers depend on "multiple hits," some of which promote growth and some of which block apoptosis. We screened for mutations that require a synergistic block in apoptosis to promote tissue overgrowth and identified myopic (mop), the Drosophila homolog of the candidate tumor-suppressor and endosomal regulator His-domain protein tyrosine phosphatase (HD-PTP). We find that Myopic regulates the Salvador/Warts/Hippo (SWH) tumor suppressor pathway: Myopic PPxY motifs bind conserved residues in the WW domains of the transcriptional coactivator Yorkie, and Myopic colocalizes with Yorkie at endosomes. Myopic controls Yorkie endosomal association and protein levels, ultimately influencing expression of some Yorkie target genes. However, the antiapoptotic gene diap1 is not affected, which may explain the conditional nature of the myopic growth phenotype. These data establish Myopic as a Yorkie regulator and implicate Myopic-dependent association of Yorkie with endosomal compartments as a regulatory step in nuclear outputs of the SWH pathway.

Neuronal remodeling and apoptosis require VCP-dependent degradation of the apoptosis inhibitor DIAP1

The regulated degeneration of axons or dendrites (pruning) and neuronal apoptosis are widely used during development to determine the specificity of neuronal connections. Pruning and apoptosis often share similar mechanisms; for example, developmental dendrite pruning of Drosophila class IV dendritic arborization (da) neurons is induced by local caspase activation triggered by ubiquitin-mediated degradation of the caspase inhibitor DIAP1. Here, we examined the function of Valosin-containing protein (VCP), a ubiquitin-selective AAA chaperone involved in endoplasmic reticulum-associated degradation, autophagy and neurodegenerative disease, in Drosophila da neurons. Strong VCP inhibition is cell lethal, but milder inhibition interferes with dendrite pruning and developmental apoptosis. These defects are associated with impaired caspase activation and high DIAP1 levels. In cultured cells, VCP binds to DIAP1 in a ubiquitin- and BIR domain-dependent manner and facilitates its degradation. Our results establish a new link between ubiquitin, dendrite pruning and the apoptosis machinery.

Steroid-triggered, cell-autonomous death of a Drosophila motoneuron during metamorphosis

BACKGROUND

The metamorphosis of Drosophila melanogaster is accompanied by elimination of obsolete neurons via programmed cell death (PCD). Metamorphosis is regulated by ecdysteroids, including 20-hydroxyecdysone (20E), but the roles and modes of action of hormones in regulating neuronal PCD are incompletely understood.

RESULTS

We used targeted expression of GFP to track the fate of a larval motoneuron, RP2, in ventral ganglia. RP2s in abdominal neuromeres two through seven (A2 to A7) exhibited fragmented DNA by 15 hours after puparium formation (h-APF) and were missing by 20 h-APF. RP2 death began shortly after the 'prepupal pulse' of ecdysteroids, during which time RP2s expressed ecdysteroid receptors (EcRs). Genetic manipulations showed that RP2 death required the function of EcR-B isoforms, the death-activating gene, reaper (but not hid), and the apoptosome component, Dark. PCD was blocked by expression of the caspase inhibitor p35 but unaffected by manipulating Diap1. In contrast, aCC motoneurons in neuromeres A2 to A7, and RP2s in neuromere A1, expressed EcRs during the prepupal pulse but survived into the pupal stage under all conditions tested. To test the hypothesis that ecdysteroids trigger RP2's death directly, we placed abdominal GFP-expressing neurons in cell culture immediately prior to the prepupal pulse, with or without 20E. 20E induced significant PCD in putative RP2s, but not in control neurons, as assessed by morphological criteria and propidium iodide staining.

CONCLUSIONS

These findings suggest that the rise of ecdysteroids during the prepupal pulse acts directly, via EcR-B isoforms, to activate PCD in RP2 motoneurons in abdominal neuromeres A2 to A7, while sparing RP2s in A1. Genetic manipulations suggest that RP2's death requires Reaper function, apoptosome assembly and Diap1-independent caspase activation. RP2s offer a valuable 'single cell' approach to the molecular understanding of neuronal death during insect metamorphosis and, potentially, of neurodegeneration in other contexts.

Actin-Capping Protein and the Hippo pathway regulate F-actin and tissue growth in Drosophila

The conserved Hippo tumor suppressor pathway is a key kinase cascade that controls tissue growth by regulating the nuclear import and activity of the transcription co-activator Yorkie. Here, we report that the actin-Capping Protein αβ heterodimer, which regulates actin polymerization, also functions to suppress inappropriate tissue growth by inhibiting Yorkie activity. Loss of Capping Protein activity results in abnormal accumulation of apical F-actin, reduced Hippo pathway activity and the ectopic expression of several Yorkie target genes that promote cell survival and proliferation. Reduction of two other actin-regulatory proteins, Cofilin and the cyclase-associated protein Capulet, cause abnormal F-actin accumulation, but only the loss of Capulet, like that of Capping Protein, induces ectopic Yorkie activity. Interestingly, F-actin also accumulates abnormally when Hippo pathway activity is reduced or abolished, independently of Yorkie activity, whereas overexpression of the Hippo pathway component expanded can partially reverse the abnormal accumulation of F-actin in cells depleted for Capping Protein. Taken together, these findings indicate a novel interplay between Hippo pathway activity and actin filament dynamics that is essential for normal growth control.

Drosophila PI4KIIIalpha is required in follicle cells for oocyte polarization and Hippo signaling

In a genetic screen we isolated mutations in CG10260, which encodes a phosphatidylinositol 4-kinase (PI4KIIIalpha), and found that PI4KIIIalpha is required for Hippo signaling in Drosophila ovarian follicle cells. PI4KIIIalpha mutations in the posterior follicle cells lead to oocyte polarization defects similar to those caused by mutations in the Hippo signaling pathway. PI4KIIIalpha mutations also cause misexpression of well-established Hippo signaling targets. The Merlin-Expanded-Kibra complex is required at the apical membrane for Hippo activity. In PI4KIIIalpha mutant follicle cells, Merlin fails to localize to the apical domain. Our analysis of PI4KIIIalpha mutants provides a new link in Hippo signal transduction from the cell membrane to its core kinase cascade.

Modeling cancers in Drosophila

The basic cellular processes deregulated during carcinogenesis and the vast majority of the genes implicated in cancer appear conserved from humans to flies. This conservation, together with an ever-expanding fly genetic toolbox, has made of Drosophila melanogaster a remarkably profitable model to study many fundamental aspects of carcinogenesis. In particular, Drosophila has played a major role in the identification of genes and pathways implicated in cancer and in disclosing novel functional relationships between cancer genes. It has also proved to be a genetically tractable system where to mimic cancer-like situations and characterize the mode of action of human oncogenes. Here, we outline some advances in the study of cancer, both at the basic and more translational levels, which have benefited from research carried out in flies.

The Salvador/Warts/Hippo pathway controls regenerative tissue growth in Drosophila melanogaster

During tissue regeneration, cell proliferation replaces missing structures to restore organ function. Regenerative potential differs greatly between organs and organisms; for example some amphibians can regrow entire limbs whereas mammals cannot. The process of regeneration relies on several signaling pathways that control developmental tissue growth, and implies the existence of organ size-control checkpoints that regulate both developmental, and regenerative, growth. Here we explore the role of one such checkpoint, the Salvador-Warts-Hippo pathway, in tissue regeneration. The Salvador-Warts-Hippo pathway limits tissue growth by repressing the Yorkie transcriptional co-activator. Several proteins serve as upstream modulators of this pathway including the atypical cadherins, Dachsous and Fat, whilst the atypical myosin, Dachs, functions downstream of Fat to activate Yorkie. Using Drosophila melanogaster imaginal discs we show that Salvador-Warts-Hippo pathway activity is repressed in regenerating tissue and that Yorkie is rate-limiting for regeneration of the developing wing. We show that regeneration is compromised in dachs mutant wing discs, but that proteins in addition to Fat and Dachs are likely to modulate Yorkie activity in regenerating cells. In conclusion our data reveal the importance of Yorkie hyperactivation for tissue regeneration and suggest that multiple upstream inputs, including Fat-Dachsous signaling, sense tissue damage and regulate Yorkie activity during regeneration of epithelial tissues.

Mitochondrial involvement in cell death of non-mammalian eukaryotes

Although mitochondria are essential organelles for long-term survival of eukaryotic cells, recent discoveries in biochemistry and genetics have advanced our understanding of the requirements for mitochondria in cell death. Much of what we understand about cell death is based on the identification of conserved cell death genes in Drosophila melanogaster and Caenorhabditis elegans. However, the role of mitochondria in cell death in these models has been much less clear. Considering the active role that mitochondria play in apoptosis in mammalian cells, the mitochondrial contribution to cell death in non-mammalian systems has been an area of active investigation. In this article, we review the current research on this topic in three non-mammalian models, C. elegans, Drosophila, and Saccharomyces cerevisiae. In addition, we discuss how non-mammalian models have provided important insight into the mechanisms of human disease as they relate to the mitochondrial pathway of cell death. The unique perspective derived from each of these model systems provides a more complete understanding of mitochondria in programmed cell death. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Flower forms an extracellular code that reveals the fitness of a cell to its neighbors in Drosophila

Cell competition promotes the elimination of weaker cells from a growing population. Here we investigate how cells of Drosophila wing imaginal discs distinguish "winners" from "losers" during cell competition. Using genomic and functional assays, we have identified several factors implicated in the process, including Flower (Fwe), a cell membrane protein conserved in multicellular animals. Our results suggest that Fwe is a component of the cell competition response that is required and sufficient to label cells as "winners" or "losers." In Drosophila, the fwe locus produces three isoforms, fwe(ubi), fwe(Lose-A), and fwe(Lose-B). Basal levels of fwe(ubi) are constantly produced. During competition, the fwe(Lose) isoforms are upregulated in prospective loser cells. Cell-cell comparison of relative fwe(Lose) and fwe(ubi) levels ultimately determines which cell undergoes apoptosis. This "extracellular code" may constitute an ancient mechanism to terminate competitive conflicts among cells.

Apoptogenic factors released from mitochondria

When cells kill themselves, they usually do so by activating mechanisms that have evolved specifically for that purpose. These mechanisms, which are broadly conserved throughout the metazoa, involve two processes: activation in the cytosol of latent cysteine proteases (termed caspases), and disruption of mitochondrial functions. These processes are linked in a number of different ways. While active caspases can cleave proteins in the mitochondrial outer membrane, and cleave and thereby activate certain pro-apoptotic members of the Bcl-2 family, proteins released from the mitochondria can trigger caspase activation and antagonise IAP family proteins. This review will focus on the pro-apoptotic molecules that are released from the mitochondria of cells endeavouring to kill themselves. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Analysis of expressed sequence tags for Frankliniella occidentalis, the western flower thrips

Thrips are members of the insect order Thysanoptera and Frankliniella occidentalis (the western flower thrips) is the most economically important pest within this order. F. occidentalis is both a direct pest of crops and an efficient vector of plant viruses, including Tomato spotted wilt virus (TSWV). Despite the world-wide importance of thrips in agriculture, there is little knowledge of the F. occidentalis genome or gene functions at this time. A normalized cDNA library was constructed from first instar thrips and 13 839 expressed sequence tags (ESTs) were obtained. Our EST data assembled into 894 contigs and 11 806 singletons (12 700 nonredundant sequences). We found that 31% of these sequences had significant similarity (E< or = 10(-10)) to protein sequences in the National Center for Biotechnology Information nonredundant (nr) protein database, and 25% were functionally annotated using Blast 2GO. We identified 74 sequences with putative homology to proteins associated with insect innate immunity. Sixteen sequences had significant similarity to proteins associated with small RNA-mediated gene silencing pathways (RNA interference; RNAi), including the antiviral pathway (short interfering RNA-mediated pathway). Our EST collection provides new sequence resources for characterizing gene functions in F. occidentalis and other thrips species with regards to vital biological processes, studying the mechanism of interactions with the viruses harboured and transmitted by the vector, and identifying new insect gene-centred targets for plant disease and insect control.

Pharmacokinetics and derivation of an anticancer dosing regimen for PAC-1, a preferential small molecule activator of procaspase-3, in healthy dogs

PAC-1 is a preferential small molecule activator of procaspase-3 and has potential to become a novel and effective anticancer agent. The rational development of PAC-1 for translational oncologic applications would be advanced by coupling relevant in vitro cytotoxicity studies with pharmacokinetic investigations conducted in large mammalian models possessing similar metabolism and physiology as people. In the present study, we investigated whether concentrations and exposure durations of PAC-1 that induce cytotoxicity in lymphoma cell lines in vitro can be achievable in healthy dogs through a constant rate infusion (CRI) intravenous delivery strategy. Time- and dose-dependent procaspase-3 activation by PAC-1 with subsequent cytotoxicity was determined in a panel of B-cell lymphoma cells in vitro. The pharmacokinetics of PAC-1 administered orally or intravenously was studied in 6 healthy dogs using a crossover design. The feasibility of maintaining steady state plasma concentration of PAC-1 for 24 or 48 h that paralleled in vitro cytotoxic concentrations was investigated in 4 healthy dogs. In vitro, PAC-1 induced apoptosis in lymphoma cell lines in a time- and dose-dependent manner. The oral bioavailability of PAC-1 was relatively low and highly variable (17.8 ± 9.5%). The achievement and maintenance of predicted PAC-1 cytotoxic concentrations in normal dogs was safely attained via intravenous CRI lasting for 24 or 48 h in duration. Using the dog as a large mammalian model, PAC-1 can be safely administered as an intravenous CRI while achieving predicted in vitro cytotoxic concentrations.

A novel F-box protein is required for caspase activation during cellular remodeling in Drosophila

Terminal differentiation of male germ cells in Drosophila and mammals requires extensive cytoarchitectural remodeling, the elimination of many organelles, and a large reduction in cell volume. The associated process, termed spermatid individualization, is facilitated by the apoptotic machinery, including caspases, but does not result in cell death. From a screen for genes defective in caspase activation in this system, we isolated a novel F-box protein, which we termed Nutcracker, that is strictly required for caspase activation and sperm differentiation. Nutcracker interacts through its F-box domain with members of a Cullin-1-based ubiquitin ligase complex (SCF): Cullin-1 and SkpA. This ubiquitin ligase does not regulate the stability of the caspase inhibitors DIAP1 and DIAP2, but physically binds Bruce, a BIR-containing giant protein involved in apoptosis regulation. Furthermore, nutcracker mutants disrupt proteasome activity without affecting their distribution. These findings define a new SCF complex required for caspase activation during sperm differentiation and highlight the role of regulated proteolysis during this process.

Phenotypic interactions of spinster with the genes encoding proteins for cell death control in Drosophila melanogaster

The spin gene was first identified by its mutant phenotype, which is characterized by extremely strong mate refusal by females in response to male courtship in Drosophila. Spin mutants are also known to be accompanied by a remarkable reduction in programmed cell death in the reproductive and nervous systems. To better understand the molecular functions of spin, we searched for its genetic modifiers. Forced expression of spin(+) in somatic cells as driven by ptc-Gal4 in the testis resulted in the invasion of mature sperm into the anterior testes tip, which is otherwise occupied only by immature germ cells. To obtain genes that modulate spin's effect, the gain-of-function spin phenotype was observed in the presence of a chromosome harboring an EP or GS P-element insertion, which initiates transcription of the genomic sequence neighboring the insertion site. We isolated th and emc as suppressors of spin and atg8a as a gene that reproduces the spin phenotype on its own. th encodes Inhibitor of apoptosis-1, and mammalian Id genes homologous to emc are known to inhibit apoptosis. atg8a encodes a protein essential for autophagy. These results suggest that spin promotes cell death mechanisms that are regulated negatively by th and emc and positively by atg8a.

Spatial discontinuity of optomotor-blind expression in the Drosophila wing imaginal disc disrupts epithelial architecture and promotes cell sorting

Background

Decapentaplegic (Dpp) is one of the best characterized morphogens, required for dorso-ventral patterning of the Drosophila embryo and for anterior-posterior (A/P) patterning of the wing imaginal disc. In the larval wing pouch, the Dpp target gene optomotor-blind (omb) is generally assumed to be expressed in a step function above a certain threshold of Dpp signaling activity.

Results

We show that the transcription factor Omb forms, in fact, a symmetrical gradient on both sides of the A/P compartment boundary. Disruptions of the Omb gradient lead to a re-organization of the epithelial cytoskeleton and to a retraction of cells toward the basal membrane suggesting that the Omb gradient is required for correct epithelial morphology. Moreover, by analysing the shape of omb gain- and loss-of-function clones, we find that Omb promotes cell sorting along the A/P axis in a concentration-dependent manner.

Conclusions

Our findings show that Omb distribution in the wing imaginal disc is described by a gradient rather than a step function. Graded Omb expression is necessary for normal cell morphogenesis and cell affinity and sharp spatial discontinuities must be avoided to allow normal wing development.

Differential requirement of Salvador-Warts-Hippo pathway members for organ size control in Drosophila melanogaster

The Salvador-Warts-Hippo (SWH) pathway contains multiple growth-inhibitory proteins that control organ size during development by limiting activity of the Yorkie oncoprotein. Increasing evidence indicates that these growth inhibitors act in a complex network upstream of Yorkie. This complexity is emphasised by the distinct phenotypes of tissue lacking different SWH pathway genes. For example, eye tissue lacking the core SWH pathway components salvador, warts or hippo is highly overgrown and resistant to developmental apoptosis, whereas tissue lacking fat or expanded is not. Here we explore the relative contribution of SWH pathway proteins to organ size control by determining their temporal activity profile throughout Drosophila melanogaster eye development. We show that eye tissue lacking fat, expanded or discs overgrown displays elevated Yorkie activity during the larval growth phase of development, but not in the pupal eye when apoptosis ensues. Fat and Expanded do possess Yorkie-repressive activity in the pupal eye, but loss of fat or expanded at this stage of development can be compensated for by Merlin. Fat appears to repress Yorkie independently of Dachs in the pupal eye, which would contrast with the mode of action of Fat during larval development. Fat is more likely to restrict Yorkie activity in the pupal eye together with Expanded, given that pupal eye tissue lacking both these genes resembles that of tissue lacking either gene. This study highlights the complexity employed by different SWH pathway proteins to control organ size at different stages of development.

The chromatin-remodeling protein Osa interacts with CyclinE in Drosophila eye imaginal discs

Coordinating cell proliferation and differentiation is essential during organogenesis. In Drosophila, the photoreceptor, pigment, and support cells of the eye are specified in an orchestrated wave as the morphogenetic furrow passes across the eye imaginal disc. Cells anterior of the furrow are not yet differentiated and remain mitotically active, while most cells in the furrow arrest at G(1) and adopt specific ommatidial fates. We used microarray expression analysis to monitor changes in transcription at the furrow and identified genes whose expression correlates with either proliferation or fate specification. Some of these are members of the Polycomb and Trithorax families that encode epigenetic regulators. Osa is one; it associates with components of the Drosophila SWI/SNF chromatin-remodeling complex. Our studies of this Trithorax factor in eye development implicate Osa as a regulator of the cell cycle: Osa overexpression caused a small-eye phenotype, a reduced number of M- and S-phase cells in eye imaginal discs, and a delay in morphogenetic furrow progression. In addition, we present evidence that Osa interacts genetically and biochemically with CyclinE. Our results suggest a dual mechanism of Osa function in transcriptional regulation and cell cycle control.

Precise temporal regulation of roughest is required for correct salivary gland autophagic cell death in Drosophila

The Drosophila roughest (rst) locus encodes an immunoglobulin superfamily transmembrane glycoprotein implicated in a variety of embryonic and postembryonic developmental processes. Here we demonstrate a previously unnoticed role for this gene in the autophagic elimination of larval salivary glands during early pupal stages by showing that overexpression of the Rst protein ectodomain in early pupa leads to persistence of salivary glands up to at least 12 hours after head eversion, although with variable penetrance. The same phenotype is observed in individuals carrying the dominant regulatory allele rst(D), but not in loss of function alleles. Analysis of persistent glands at the ultrastructural level showed that programmed cell death starts at the right time but is arrested at an early stage of the process. Finally we describe the expression pattern and intracellular distribution of Rst in wild type and rst(D) mutants, showing that its downregulation in salivary glands at the beginning of pupal stage is an important factor in the correct implementation of the autophagic program of this tissue in space and time.

Cross-species RNAi rescue platform in Drosophila melanogaster

RNAi-mediated gene knockdown in Drosophila melanogaster is a powerful method to analyze loss-of-function phenotypes both in cell culture and in vivo. However, it has also become clear that false positives caused by off-target effects are prevalent, requiring careful validation of RNAi-induced phenotypes. The most rigorous proof that an RNAi-induced phenotype is due to loss of its intended target is to rescue the phenotype by a transgene impervious to RNAi. For large-scale validations in the mouse and Caenorhabditis elegans, this has been accomplished by using bacterial artificial chromosomes (BACs) of related species. However, in Drosophila, this approach is not feasible because transformation of large BACs is inefficient. We have therefore developed a general RNAi rescue approach for Drosophila that employs Cre/loxP-mediated recombination to rapidly retrofit existing fosmid clones into rescue constructs. Retrofitted fosmid clones carry a selection marker and a phiC31 attB site, which facilitates the production of transgenic animals. Here, we describe our approach and demonstrate proof-of-principle experiments showing that D. pseudoobscura fosmids can successfully rescue RNAi-induced phenotypes in D. melanogaster, both in cell culture and in vivo. Altogether, the tools and method that we have developed provide a gold standard for validation of Drosophila RNAi experiments.

Inhibitor of Apoptosis (IAP) proteins as drug targets for the treatment of cancer

Three companies, Genentech, Aegera Therapeutics/Human Genome Sciences, and Novartis, have commenced phase 1 clinical trials of inhibitor of apoptosis (IAP) antagonist ‘Smac mimetic’ compounds for the treatment of cancer. These trials represent the culmination of a line of research that commenced with analysis of how insect viruses stop host cells from killing themselves and led to the discovery of a family of proteins that regulate development in insects and signalling by tumour necrosis factor superfamily members in mammals, which prompted development of drugs that mimic natural IAP-binding proteins to promote cell death.

Syntrophin-2 is required for eye development in Drosophila

Syntrophins are components of the dystrophin glycoprotein complex (DGC), which is encoded by causative genes of muscular dystrophies. The DGC is thought to play roles not only in linking the actin cytoskeleton to the extracellular matrix, providing stability to the cell membrane, but also in signal transduction. Because of their binding to a variety of different molecules, it has been suggested that syntrophins are adaptor proteins recruiting signaling proteins to membranes and the DGC. However, critical roles in vivo remain elusive. Drosophila Syntrophin-2 (Syn2) is an orthologue of human gamma 1/gamma 2-syntrophins. Western immunoblot analysis here showed Syn2 to be expressed throughout development, with especially high levels in the adult head. Morphological aberrations were observed in Syn2 knockdown adult flies, with lack of retinal elongation and malformation of rhabdomeres. Furthermore, Syn2 knockdown flies exhibited excessive apoptosis in third instar larvae and alterations in the actin localization in the pupal retinae. Genetic crosses with a collection of Drosophila deficiency stocks allowed us to identify seven genomic regions, deletions of which caused enhancement of the rough eye phenotype induced by Syn2 knockdown. This information should facilitate identification of Syn2 regulators in Drosophila and clarification of roles of Syn2 in eye development.

Temporal regulation of Drosophila IAP1 determines caspase functions in sensory organ development

Caspase activation is regulated by the turnover of E3 ubiquitin ligase, DIAP1, and depends on cell type and maturity.

The caspases comprise a family of cysteine proteases that function in various cellular processes, including apoptosis. However, how the balance is struck between the caspases’ role in cell death and their nonapoptotic functions is unclear. To address this issue, we monitored the protein turnover of an endogenous caspase inhibitor, Drosophila IAP1 (DIAP1). DIAP1 is an E3 ubiquitin ligase that promotes the ubiquitination of caspases and thereby prevents caspase activation. For this study, we developed a fluorescent probe to monitor DIAP1 turnover in the external sensory organ precursor (SOP) lineage of living Drosophila. The SOP divides asymmetrically to make the shaft, socket, and sheath cells, and the neuron that comprise each sensory organ. We found that the quantity of DIAP1 changed dramatically depending on the cell type and maturity, and that the temporal regulation of DIAP1 turnover determines whether caspases function nonapoptotically in cellular morphogenesis or cause cell death.

Zfh1 promotes survival of a peripheral glia subtype by antagonizing a Jun N-terminal kinase-dependent apoptotic pathway

In Drosophila subperineurial glia (SPG) ensheath and insulate the nerve. SPG is under strict cell cycle and survival control because cell division or death of such a cell type would compromise the integrity of the blood-nerve barrier. The mechanisms underlying the survival of SPG remain unknown. Here, we show that the embryonic peripheral glia expresses the Zfh1 transcription factor, and in zfh1 mutants a particular SPG subtype, ePG10, undergoes apoptosis. Our findings show that in ePG10, Zfh1 represses the pro-apoptotic RHG-motif gene reaper in a cell-autonomous manner. Zfh1 also blocks the activation of the Jun N-terminal kinase (JNK) pathway, and reducing or enhancing JNK signalling in zfh1 mutants prevents or promotes ePG10 apoptosis. Our study shows a novel function for Zfh1 as an anti-apoptotic molecule and uncovers a cryptic JNK-dependent apoptotic programme in ePG10, which is normally blocked by Zfh1. We propose that, in cells such as SPG that do not undergo self-renewal and survive long periods, transcriptional control of RHG-motif gene expression together with fine tuning of JNK signalling is crucial for cell survival.

The developmentally active and stress-inducible noncoding hsromega gene is a novel regulator of apoptosis in Drosophila

The large nucleus limited noncoding hsromega-n RNA of Drosophila melanogaster is known to associate with a variety of heterogeneous nuclear RNA-binding proteins (hnRNPs) and certain other RNA-binding proteins to assemble the nucleoplasmic omega speckles. In this article, we show that RNAi-mediated depletion of this noncoding RNA dominantly suppresses apoptosis, in eye and other imaginal discs, triggered by induced expression of Rpr, Grim, or caspases (initiator as well as effector), all of which are key regulators/effectors of the canonical caspase-mediated cell death pathway. We also show, for the first time, a genetic interaction between the noncoding hsromega transcripts and the c-Jun N-terminal kinase (JNK) signaling pathway since downregulation of hsromega transcripts suppressed JNK activation. In addition, hsromega-RNAi also augmented the levels of Drosophila Inhibitor of Apoptosis Protein 1 (DIAP1) when apoptosis was activated. Suppression of induced cell death following depletion of hsromega transcripts was abrogated when the DIAP1-RNAi transgene was coexpressed. Our results suggest that the hsromega transcripts regulate cellular levels of DIAP1 via the hnRNP Hrb57A, which physically interacts with DIAP1, and any alteration in levels of the hsromega transcripts in eye disc cells enhances association between these two proteins. Our studies thus reveal a novel regulatory role of the hsromega noncoding RNA on the apoptotic cell death cascade through multiple paths. These observations add to the diversity of regulatory functions that the large noncoding RNAs carry out in the cells' life.

Mutation in Rab11 results in abnormal organization of ommatidial cells and activation of JNK signaling in the Drosophila eye

Rab11(mo), a P insertion line of Rab11 showed degenerated ommatidia and excess cell death in larval/pupal eyes. Here, we demonstrate that Rab11 is essential for normal organization of ommatidial cells and their survival in Drosophila, and a mutation in this gene results in cytoskeleton disruption and activation of JNK signaling in the eye. The spatial organization of various cell types in compound eye, viz., cone, photoreceptor, pigment and bristle cells, were disrupted in Rab11 mutants as revealed by immunostaining of F-actin and adherens and septate junction proteins. Genetic interaction studies indicated that mutation in Rab11 upregulates Drosophila apoptotic genes, rpr, hid and grim. In order to study the pathway that causes excessive cell death in Rab11 mutants, the JNK pathway was chosen and genetic interaction analyses were carried out between Rab11 and candidates of the JNK signaling pathway. A downregulation of JNK signaling rescued the phenotype in Rab11 mutant eyes significantly while overexpression of JNK in the eyes using UAS-eiger, UAS-dtak1 or EP(2)0578, resulted in enhancement of the eye phenotype indicating a link between Rab11 and the JNK signaling pathway.

High levels of dRYBP induce apoptosis in Drosophila imaginal cells through the activation of reaper and the requirement of trithorax, dredd and dFADD

Drosophila RYBP (dRYBP; Ring1 and YY1 Binding Protein) is a Polycomb and trithorax interacting protein, whose homologous RYBP/DEDAF mammalian counterparts exhibit tumor cell-specific killing activity. Here we show that although endogenous dRYBP is not involved in developmental apoptosis, high levels of exogenous dRYBP induce apoptosis in all the imaginal discs of the fly, indicating that dRYBP apoptotic activity is not specific to tumor cells. We also show that dRYBP-induced apoptosis is inhibited by high levels of either p35 or DIAP1 (Drosophila Inhibitor of Apoptosis Protein 1), and requires the function of the pro-apoptotic REAPER, HID and GRIM proteins, the apical caspase DREDD, the adaptor dFADD protein as well as TRITHORAX (TRX), an epigenetic transcriptional regulator. Furthermore, we demonstrate that overexpression of TRX also induces apoptosis in the imaginal discs. Finally, we show that the expression of reaper-lacZ is upregulated both upon dRYBP-induced apoptosis and upon TRX-induced apoptosis in imaginal discs and that the reaper gene is a direct target of dRYBP in Drosophila embryos. Our results indicate that dRYBP triggers in a receptor-mediated apoptotic pathway that also includes TRX-dependent epigenetic regulation of gene expression.

Inhibitor of apoptosis proteins in Drosophila: gatekeepers of death

Regulation of apoptosis is crucial to ensure cellular viability, and failure to do so is linked to several human pathologies. The apoptotic cell death programme culminates in the activation of caspases, a family of highly specific cysteine proteases essential for the destruction of the cell. Although best known for their role in executing apoptosis, caspases also play important signalling roles in non-apoptotic processes, such as regulation of actin dynamics, innate immunity, cell proliferation, differentiation and survival. Under such conditions, caspases are activated without killing the cell. Caspase activation and activity is subject to complex regulation, and various cellular and viral inhibitors have been identified that control the activity of caspases in their apoptotic and non-apoptotic roles. Members of the Inhibitor of APoptosis (IAP) protein family ensure cell viability in Drosophila by directly binding to caspases and regulating their activities in a ubiquitin-dependent manner. The observation that IAPs are essential for cell survival in Drosophila, and are frequently deregulated in human cancer, contributing to tumourigenesis, chemoresistance, disease progression and poor patient survival, highlights the importance of this family of caspase regulators in health and disease. Here we summarise recent advances from Drosophila that start to elucidate how the cellular response to caspase activation is modulated by IAPs and their regulators.

A lepidopteran orthologue of reaper reveals functional conservation and evolution of IAP antagonists

Genetic studies in Drosophila melanogaster have revealed that inhibitor of apoptosis (IAP) proteins and IAP antagonists such as reaper play a pivotal role in controlling cell death in insects. Interestingly, although the sequences and structures of IAPs are highly conserved, the sequence of IAP antagonists diverged very rapidly during evolution, making their identification difficult. Using a customized bioinformatics approach, we identified an IAP antagonist, IAP-binding motif 1 (Ibm1), from the genome of the silkworm Bombyx mori. This is the first reaper/grim orthologue identified in a nondipteran insect. Previous analysis indicated that both Reaper and Grim induce cell death through their N-terminal IBM as well as the Grim_helix3 (GH3) domain. Functional studies indicated that Ibm1 binds to an IAP protein from B. mori, BmIAP1, and induces apoptosis in insect cells via the IAP-binding motif, a seven amino acid sequence that is highly conserved in all IAP antagonists. Interestingly, Ibm1 also contains a region that is a statistically significant match to the GH3 domain. Mutational analysis indicated that the GH3-like motif in Ibm1 has an important supportive role in IAP-antagonist function and can trigger cell death under certain conditions.

The ion channel activity of the SARS-coronavirus 3a protein is linked to its pro-apoptotic function

The severe acute respiratory syndrome-coronavirus (SARS-CoV) caused an outbreak of atypical pneumonia in 2003. The SARS-CoV viral genome encodes several proteins which have no homology to proteins in any other coronaviruses, and a number of these proteins have been implicated in viral cytopathies. One such protein is 3a, which is also known as X1, ORF3 and U274. 3a expression is detected in both SARS-CoV infected cultured cells and patients. Among the different functions identified, 3a is a capable of inducing apoptosis. We previously showed that caspase pathways are involved in 3a-induced apoptosis. In this study, we attempted to find out protein domains on 3a that are essential for its pro-apoptotic function. Protein sequence analysis reveals that 3a possesses three major protein signatures, the cysteine-rich, Yxx phi and diacidic domains. We showed that 3a proteins carrying respective mutations in these protein domains exhibit reduced pro-apoptotic activities, indicating the importance of these domains on 3a's pro-apoptotic function. It was previously reported that 3a possesses potassium ion channel activity. We further demonstrated that the blockade of 3a's potassium channel activity abolished caspase-dependent apoptosis. This report provides the first evidence that ion channel activity of 3a is required for its pro-apoptotic function. As ion channel activity has been reported to regulate apoptosis in different pathologic conditions, finding ways to modulate the ion channel activity may offer a new direction toward the inhibition of apoptosis triggered by SARS-CoV.

ER stress protects from retinal degeneration

The unfolded protein response (UPR) is a specific cellular process that allows the cell to cope with the overload of unfolded/misfolded proteins in the endoplasmic reticulum (ER). ER stress is commonly associated with degenerative pathologies, but its role in disease progression is still a matter for debate. Here, we found that mutations in the ER-resident chaperone, neither inactivation nor afterpotential A (NinaA), lead to mild ER stress, protecting photoreceptor neurons from various death stimuli in adult Drosophila. In addition, Drosophila S2 cultured cells, when pre-exposed to mild ER stress, are protected from H(2)O(2), cycloheximide- or ultraviolet-induced cell death. We show that a specific ER-mediated signal promotes antioxidant defences and inhibits caspase-dependent cell death. We propose that an immediate consequence of the UPR not only limits the accumulation of misfolded proteins but also protects tissues from harmful exogenous stresses.

Bicaudal is a conserved substrate for Drosophila and mammalian caspases and is essential for cell survival

Members of the caspase family of cysteine proteases coordinate cell death through restricted proteolysis of diverse protein substrates and play a conserved role in apoptosis from nematodes to man. However, while numerous substrates for the mammalian cell death-associated caspases have now been described, few caspase substrates have been identified in other organisms. Here, we have utilized a proteomics-based approach to identify proteins that are cleaved by caspases during apoptosis in Drosophila D-Mel2 cells, a subline of the Schneider S2 cell line. This approach identified multiple novel substrates for the fly caspases and revealed that bicaudal/βNAC is a conserved substrate for Drosophila and mammalian caspases. RNAi-mediated silencing of bicaudal expression in Drosophila D-Mel2 cells resulted in a block to proliferation, followed by spontaneous apoptosis. Similarly, silencing of expression of the mammalian bicaudal homologue, βNAC, in HeLa, HEK293T, MCF-7 and MRC5 cells also resulted in spontaneous apoptosis. These data suggest that bicaudal/βNAC is essential for cell survival and is a conserved target of caspases from flies to man.

Patterns of variation in the inhibitor of apoptosis 1 gene of Aedes triseriatus, a transovarial vector of La Crosse virus

Aedes triseriatus mosquitoes transovarially transmit (TOT) La Crosse virus (LACV) to their offspring with minimal damage to infected ovaries. Ae. triseriatus inhibitor of apoptosis 1 (AtIAP1) is a candidate gene conditioning the ability to vertically transmit LACV. AtIAP1 was amplified and sequenced in adult mosquitoes reared from field-collected eggs. Sequence analysis showed that AtIAP1 has much higher levels of genetic diversity than genes found in other mosquitoes. Despite this large amount of diversity, strong purifying selection of polymorphisms located in the Baculovirus inhibitor of apoptosis repeat (BIR) domains and, to a lesser extent, in the 5' untranslated region seems to indicate that these portions of AtIAP1 are the most important. These results indicate that the 5'UTR plays an important role in transcription and translation and that the BIR domains are important functional domains in the protein. Single nucleotide polymorphisms (SNPs) were compared between LACV-positive and -negative mosquitoes to test for associations between segregating sites and the ability to be transovarially infected with LACV. Initial results indicated that five SNPs were associated with TOT of LACV; however, these results were not replicable with larger sample sizes.

RNAi-mediated knockdown showing impaired cell survival in Drosophila wing imaginal disc

The genetically amenable organism Drosophila melanogaster has been estimated to have 14,076 protein coding genes in the genome, according to the flybase release note R5.13 (http://flybase.bio.indiana.edu/static_pages/docs/release_notes.html). Recent application of RNA interference (RNAi) to the study of developmental biology in Drosophila has enabled us to carry out a systematic investigation of genes affecting various specific phenotypes. In order to search for genes supporting cell survival, we conducted an immunohistochemical examination in which the RNAi of 2,497 genes was independently induced within the dorsal compartment of the wing imaginal disc. Under these conditions, the activities of a stress-activated protein kinase JNK (c-Jun N-terminal kinase) and apoptosis-executing factor Caspase-3 were monitored. Approximately half of the genes displayed a strong JNK or Caspase-3 activation when their RNAi was induced. Most of the JNK activation accompanied Caspase-3 activation, while the opposite did not hold true. Interestingly, the area activating Caspase-3 was more broadly seen than that activating JNK, suggesting that JNK is crucial for induction of non-autonomous apoptosis in many cases. Furthermore, the RNAi of essential factors commonly regulating transcription and translation showed a severe and cell-autonomous apoptosis but also elicited another apoptosis at an adjacent area in a non-autonomous way. We also found that the frequency of apoptosis varies depending on the tissues.

Persistent competition among stem cells and their daughters in the Drosophila ovary germline niche

Cell competition is a short-range cell-cell interaction leading to the proliferation of winner cells at the expense of losers, although either cell type shows normal growth in homotypic environments. Drosophila Myc (dMyc; Dm-FlyBase) is a potent inducer of cell competition in wing epithelia, but its role in the ovary germline stem cell niche is unknown. Here, we show that germline stem cells (GSCs) with relative lower levels of dMyc are replaced by GSCs with higher levels of dMyc. By contrast, dMyc-overexpressing GSCs outcompete wild-type stem cells without affecting total stem cell numbers. We also provide evidence for a naturally occurring cell competition border formed by high dMyc-expressing stem cells and low dMyc-expressing progeny, which may facilitate the concentration of the niche-provided self-renewal factor BMP/Dpp in metabolically active high dMyc stem cells. Genetic manipulations that impose uniform dMyc levels across the germline produce an extended Dpp signaling domain and cause uncoordinated differentiation events. We propose that dMyc-induced competition plays a dual role in regulating optimal stem cell pools and sharp differentiation boundaries, but is potentially harmful in the case of emerging dmyc duplications that facilitate niche occupancy by pre-cancerous stem cells. Moreover, competitive interactions among stem cells may be relevant for the successful application of stem cell therapies in humans.

Genetic control of programmed cell death (apoptosis) in Drosophila

Programmed cell death, or apoptosis, is a highly conserved cellular process that has been intensively investigated in nematodes, flies and mammals. The genetic conservation, the low redundancy, the feasibility for high-throughput genetic screens and the identification of temporally and spatially regulated apoptotic responses make Drosophila melanogaster a great model for the study of apoptosis. Here, we review the key players of the cell death pathway in Drosophila and discuss their roles in apoptotic and non-apoptotic processes.

Detection of cell death in Drosophila

Drosophila is a powerful model system for the identification of cell death genes and understanding the role of cell death in development. In this chapter, we describe three methods typically used for the detection of cell death in Drosophila. The TUNEL and acridine orange methods are used to detect dead or dying cells in a variety of tissues. We focus on methods for the embryo and the ovary, but these techniques can be used on other tissues as well. The third method is the detection of genetic interactions by expressing cell death genes in the Drosophila eye.

Hsp60D is essential for caspase-mediated induced apoptosis in Drosophila melanogaster

Apart from their roles as chaperones, heat shock proteins are involved in other vital activities including apoptosis with mammalian Hsp60 being ascribed proapoptotic as well as antiapoptotic roles. Using conditional RNAi or overexpression of Hsp60D, a member of the Hsp60 family in Drosophila melanogaster, we show that the downregulation of this protein blocks caspase-dependent induced apoptosis. GMR-Gal4-driven RNAi for Hsp60D in developing eyes dominantly suppressed cell death caused by expression of Reaper, Hid, or Grim (RHG), the key activators of canonical cell death pathway. Likewise, Hsp60D-RNAi rescued cell death induced by GMR-Gal4-directed expression of full-length and activated DRONC. Overexpression of Hsp60D enhanced cell death induced either by directed expression of RHG or DRONC. However, the downregulation of Hsp60D failed to suppress apoptosis caused by unguarded caspases in DIAP1-RNAi flies. Furthermore, in DIAP1-RNAi background, Hsp60D-RNAi also failed to inhibit apoptosis induced by RHG expression. The Hsp60 and DIAP1 show diffuse and distinct granular overlapping distributions in the photoreceptor cells with the bulk of both proteins being outside the mitochondria. Depletion of either of these proteins disrupts the granular distribution of the other. We suggest that in the absence of Hsp60D, DIAP1 is unable to dissociate from effecter and executioner caspases, which thus remain inactive.

Inhibitor of apoptosis proteins in eukaryotic evolution and development: a model of thematic conservation

The past decade and a half has witnessed the discovery of a large, evolutionarily conserved family of cellular genes bearing homology to the prototype baculovirus Inhibitor of Apoptosis (IAP). The logical decision in the field to also refer to these cellular proteins as IAPs fails to do justice to this versatile group of factors that play a wide range of roles in eukaryotic development and homeostasis which include, but are not limited to, the regulation of programmed cell death. Here we describe the shared functional characteristics of several well-characterized IAPs whose defining motifs place them more in the category of multifunctional modular protein interaction domains.

The role of the RING-finger protein Elfless in Drosophila spermatogenesis and apoptosis

elfless (CG15150, FBgn0032660) maps to polytene region 36DE 5' (left) of reduced ocelli/Pray for Elves (PFE) on chromosome 2L and is predicted to encode a 187 amino acid RING finger E3 ubiquitin ligase that is putatively involved in programmed cell death (PCD, e.g., apoptosis). Several experimental approaches were used to characterize CG15150/elfless and test whether defects in this gene underlie the male sterile phenotype associated with overlapping chromosomal deficiencies of region 36DE. elfless expression is greatly enhanced in the testes and the expression pattern of UAS-elfless-EGFP driven by elfless-Gal4 is restricted to the tail cyst cell nuclei of the testes. Despite this, elfless transgenes failed to rescue the male sterile phenotype in Df/Df flies. Furthermore, null alleles of elfless, generated either by imprecise excision of an upstream P-element or by FLP-FRT deletion between two flanking piggyBac elements, are fertile. In a gain-of-function setting in the eye, we found that elfless genetically interacts with key members of the apoptotic pathway including the initiator caspase Dronc and the ubiquitin conjugating enzyme UbcD1. DIAP1, but not UbcD1, protein levels are increased in heads of flies expressing Elfless-EGFP in the eye, and in testes of flies expressing elfless-Gal4 driven Elfless-EGFP. Based on these findings, we speculate that Elfless may regulate tail cyst cell degradation to provide an advantageous, though not essential, function in the testis.