Apoptosis in late stage Drosophila nurse cells does not require genes within the H99 deficiency

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

A model of replicating coupled oscillators generates naturally occurring cell networks

When a founder cell and its progeny divide with incomplete cytokinesis, a network forms in which each intercellular bridge corresponds to a past mitotic event. Such networks are required for gamete production in many animals, and different species have evolved diverse final network topologies. Although mechanisms regulating network assembly have been identified in particular organisms, we lack a quantitative framework to understand network assembly and inter-species variability. Motivated by cell networks responsible for oocyte production in invertebrates, where the final topology is typically invariant within each species, we devised a mathematical model for generating cell networks, in which each node is an oscillator and, after a full cycle, the node produces a daughter to which it remains connected. These cell cycle oscillations are transient and coupled via diffusion over the edges of the network. By variation of three biologically motivated parameters, our model generates nearly all such networks currently reported across invertebrates. Furthermore, small parameter variations can rationalize cases of intra-species variation. Because cell networks outside of the ovary often form less deterministically, we propose model generalizations to account for sources of stochasticity.

Changes in life history parameters and transcriptome profile of Serangium japonicum associated with feeding on natural prey (Bemisia tabaci) and alternate host (Corcyra cephalonica eggs)

BACKGROUND

The mass production of natural predators with prolonged shelf life is a prerequisite for their field application as pest control agents. The traditional methods used for the mass production of Serangium japonicum rely heavily on the consistent supply of natural prey. This study explains the effects of B. tabaci (natural prey) and C. cephalonica eggs (alternative food) on life history and transcriptome profile of S. japanicum.

METHODS

This study compares the effects of B. tabaci (natural prey) and C. cephalonica eggs (alternative food) on biology, reproduction, and predatory efficacy, and transcriptome profile of S. japanicum.

RESULTS

This study revealed that S. japonicum was able to successfully complete its life cycle while feeding on B. tabaci (natural prey) and C. cephalonica eggs (alternative food). The C. cephalonica eggs fed S. japonicum individuals had longer developmental period and lower fecundity as compared to those feeding on whitefly but the survival rates (3rd instar nymphs, 4th instar nymphs and pupae) and predatory efficacy of C. cephalonica eggs fed S. japonicum individuals were significantly similar to to those feeding on whitefly.Transcriptome analysis showed that when faced with dietary changes, S. japanicum could successfully feed on C. cephalonica eggs by regulating genes related to nutrient transport, metabolism, and detoxification. Moreover, S. japanicum degraded excess cellular components through ribosomal autophagy and apoptosis, which provided sufficient materials and energy for survival and basic metabolism.

CONCLUSION

Corcyra cephalonica eggs can be used as an alternate host for the predator, Serangium japonicum, as the survival rates and predatory efficacy of the predator are similar to those feeding on the natural host (B.tabaci). When faced with dietary changes, S. japanicum could successfully feed on C. cephalonica eggs as revealed by upregulation of genes related to nutrient transport, metabolism, and detoxification. These findings are of great significance for studying the functional evolution of S. japonicum in response to dietary changes.

Tracking Follicle Cell Development

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

Frazzled/Dcc acts independently of Netrin to promote germline survival during Drosophila oogenesis

The Netrin receptor Frazzled/Dcc (Fra in Drosophila) functions in diverse tissue contexts to regulate cell migration, axon guidance and cell survival. Fra signals in response to Netrin to regulate the cytoskeleton and also acts independently of Netrin to directly regulate transcription during axon guidance in Drosophila. In other contexts, Dcc acts as a tumor suppressor by directly promoting apoptosis. In this study, we report that Fra is required in the Drosophila female germline for the progression of egg chambers through mid-oogenesis. Loss of Fra in the germline, but not the somatic cells of the ovary, results in the degeneration of egg chambers. Although a failure in nutrient sensing and disruptions in egg chamber polarity can result in degeneration at mid-oogenesis, these factors do not appear to be affected in fra germline mutants. However, similar to the degeneration that occurs in those contexts, the cell death effector Dcp-1 is activated in fra germline mutants. The function of Fra in the female germline is independent of Netrin and requires the transcriptional activation domain of Fra. In contrast to the role of Dcc in promoting cell death, our observations reveal a role for Fra in regulating germline survival by inhibiting apoptosis.

High-resolution in situ analysis of Cas9 germline transcript distributions in gene-drive Anopheles mosquitoes

Gene drives are programmable genetic elements that can spread beneficial traits into wild populations to aid in vector-borne pathogen control. Two different drives have been developed for population modification of mosquito vectors. The Reckh drive (vasa-Cas9) in Anopheles stephensi displays efficient allelic conversion through males but generates frequent drive-resistant mutant alleles when passed through females. In contrast, the AgNos-Cd1 drive (nos-Cas9) in An. gambiae achieves almost complete allelic conversion through both genders. Here, we examined the subcellular localization of RNA transcripts in the mosquito germline. In both transgenic lines, Cas9 is strictly co-expressed with endogenous genes in stem and pre-meiotic cells of the testes, where both drives display highly efficient conversion. However, we observed distinct co-localization patterns for the two drives in female reproductive tissues. These studies suggest potential determinants underlying efficient drive through the female germline. We also evaluated expression patterns of alternative germline genes for future gene-drive designs.

Elimination of nurse cell nuclei that shuttle into oocytes during oogenesis

Drosophila oocytes develop together with 15 sister germline nurse cells (NCs), which pass products to the oocyte through intercellular bridges. The NCs are completely eliminated during stages 12-14, but we discovered that at stage 10B, two specific NCs fuse with the oocyte and extrude their nuclei through a channel that opens in the anterior face of the oocyte. These nuclei extinguish in the ooplasm, leaving 2 enucleated and 13 nucleated NCs. At stage 11, the cell boundaries of the oocyte are mostly restored. Oocytes in egg chambers that fail to eliminate NC nuclei at stage 10B develop with abnormal morphology. These findings show that stage 10B NCs are distinguished by position and identity, and that NC elimination proceeds in two stages: first at stage 10B and later at stages 12-14.

A region of SLBP outside the mRNA-processing domain is essential for deposition of histone mRNA into the Drosophila egg

Replication-dependent histone mRNAs are the only cellular mRNAs that are not polyadenylated, ending in a stemloop instead of a polyA tail, and are normally regulated coordinately with DNA replication. Stemloop-binding protein (SLBP) binds the 3' end of histone mRNA, and is required for processing and translation. During Drosophila oogenesis, large amounts of histone mRNAs and proteins are deposited in the developing oocyte. The maternally deposited histone mRNA is synthesized in stage 10B oocytes after the nurse cells complete endoreduplication. We report that in wild-type stage 10B oocytes, the histone locus bodies (HLBs), formed on the histone genes, produce histone mRNAs in the absence of phosphorylation of Mxc, which is normally required for histone gene expression in S-phase cells. Two mutants of SLBP, one with reduced expression and another with a 10-amino-acid deletion, fail to deposit sufficient histone mRNA in the oocyte, and do not transcribe the histone genes in stage 10B. Mutations in a putative SLBP nuclear localization sequence overlapping the deletion phenocopy the deletion. We conclude that a high concentration of SLBP in the nucleus of stage 10B oocytes is essential for histone gene transcription.This article has an associated First Person interview with the first author of the paper.

Communal living: the role of polyploidy and syncytia in tissue biology

Multicellular organisms are composed of tissues with diverse cell sizes. Whether a tissue primarily consists of numerous, small cells as opposed to fewer, large cells can impact tissue development and function. The addition of nuclear genome copies within a common cytoplasm is a recurring strategy to manipulate cellular size within a tissue. Cells with more than two genomes can exist transiently, such as in developing germlines or embryos, or can be part of mature somatic tissues. Such nuclear collectives span multiple levels of organization, from mononuclear or binuclear polyploid cells to highly multinucleate structures known as syncytia. Here, we review the diversity of polyploid and syncytial tissues found throughout nature. We summarize current literature concerning tissue construction through syncytia and/or polyploidy and speculate why one or both strategies are advantageous.

Apoptotic Bodies: Mechanism of Formation, Isolation and Functional Relevance

In the final stages of apoptosis, apoptotic cells can generate a variety of membrane-bound vesicles known as apoptotic extracellular vesicles (ApoEVs). Apoptotic bodies (ApoBDs), a major subset of ApoEVs, are formed through a process termed apoptotic cell disassembly characterised by a series of tightly regulated morphological steps including plasma membrane blebbing, apoptotic membrane protrusion formation and fragmentation into ApoBDs. To better characterise the properties of ApoBDs and elucidate their function, a number of methods including differential centrifugation, filtration and fluorescence-activated cell sorting were developed to isolate ApoBDs. Furthermore, it has become increasingly clear that ApoBD formation can contribute to various biological processes such as apoptotic cell clearance and intercellular communication. Together, recent literature demonstrates that apoptotic cell disassembly and thus, ApoBD formation, is an important process downstream of apoptotic cell death. In this chapter, we discuss the current understandings of the molecular mechanisms involved in regulating apoptotic cell disassembly, techniques for ApoBD isolation, and the functional roles of ApoBDs in physiological and pathological settings.

Vps13 is required for timely removal of nurse cell corpses

Programmed cell death and consecutive removal of cellular remnants is essential for development. During late stages of Drosophila melanogaster oogenesis, the small somatic follicle cells that surround the large nurse cells promote non-apoptotic nurse cell death, subsequently engulf them, and contribute to the timely removal of nurse cell corpses. Here, we identify a role for Vps13 in the timely removal of nurse cell corpses downstream of developmental programmed cell death. Vps13 is an evolutionarily conserved peripheral membrane protein associated with membrane contact sites and lipid transfer. It is expressed in late nurse cells, and persistent nurse cell remnants are observed when Vps13 is depleted from nurse cells but not from follicle cells. Microscopic analysis revealed enrichment of Vps13 in close proximity to the plasma membrane and the endoplasmic reticulum in nurse cells undergoing degradation. Ultrastructural analysis uncovered the presence of an underlying Vps13-dependent membranous structure in close association with the plasma membrane. The newly identified structure and function suggests the presence of a Vps13-dependent process required for complete degradation of bulky remnants of dying cells.

Lysosomal Machinery Drives Extracellular Acidification to Direct Non-apoptotic Cell Death

Cell death is a fundamental aspect of development, homeostasis, and disease; yet, our understanding of non-apoptotic forms of cell death is limited. One such form is phagoptosis, in which one cell utilizes phagocytosis machinery to kill another cell that would otherwise continue living. We have previously identified a non-autonomous requirement of phagocytosis machinery for the developmental programmed cell death of germline nurse cells in the Drosophila ovary; however, the precise mechanism of death remained elusive. Here, we show that lysosomal machinery acting in epithelial follicle cells is used to non-autonomously induce the death of nearby germline cells. Stretch follicle cells recruit V-ATPases and chloride channels to their plasma membrane to extracellularly acidify the germline and release cathepsins that destroy the nurse cells. Our results reveal a role for lysosomal machinery acting at the plasma membrane to cause the death of neighboring cells, providing insight into mechanisms driving non-autonomous cell death.

Mondragon et al. show that V-ATPase proton pumps localize to the plasma membrane of follicle cells and promote extracellular acidification to eliminate adjacent nurse cells in the Drosophila ovary. The follicle cells subsequently release cathepsins by exocytosis into the nurse cells to promote their final degradation.

Quantitative methods for assessing local and bodywide contributions to Wolbachia titer in maternal germline cells of Drosophila

Background

Little is known about how bacterial endosymbionts colonize host tissues. Because many insect endosymbionts are maternally transmitted, egg colonization is critical for endosymbiont success. Wolbachia bacteria, carried by approximately half of all insect species, provide an excellent model for characterizing endosymbiont infection dynamics. To date, technical limitations have precluded stepwise analysis of germline colonization by Wolbachia. It is not clear to what extent titer-altering effects are primarily mediated by growth rates of Wolbachia within cell lineages or migration of Wolbachia between cells.

Results

The objective of this work is to inform mechanisms of germline colonization through use of optimized methodology. The approaches are framed in terms of nutritional impacts on Wolbachia. Yeast-rich diets in particular have been shown to suppress Wolbachia titer in the Drosophila melanogaster germline. To determine the extent of Wolbachia sensitivity to diet, we optimized 3-dimensional, multi-stage quantification of Wolbachia titer in maternal germline cells. Technical and statistical validation confirmed the identity of Wolbachia in vivo, the reproducibility of Wolbachia quantification and the statistical power to detect these effects. The data from adult feeding experiments demonstrated that germline Wolbachia titer is distinctly sensitive to yeast-rich host diets in late oogenesis. To investigate the physiological basis for these nutritional impacts, we optimized methodology for absolute Wolbachia quantification by real-time qPCR. We found that yeast-rich diets exerted no significant effect on bodywide Wolbachia titer, although ovarian titers were significantly reduced. This suggests that host diets affects Wolbachia distribution between the soma and late stage germline cells. Notably, relative qPCR methods distorted apparent wsp abundance, due to altered host DNA copy number in yeast-rich conditions. This highlights the importance of absolute quantification data for testing mechanistic hypotheses.

Conclusions

We demonstrate that absolute quantification of Wolbachia, using well-controlled cytological and qPCR-based methods, creates new opportunities to determine how bacterial abundance within the germline relates to bacterial distribution within the body. This methodology can be applied to further test germline infection dynamics in response to chemical treatments, genetic conditions, new host/endosymbiont combinations, or potentially adapted to analyze other cell and tissue types.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1579-3) contains supplementary material, which is available to authorized users.

Disassembly of dying cells in diverse organisms

Programmed cell death (PCD) is a conserved phenomenon in multicellular organisms required to maintain homeostasis. Among the regulated cell death pathways, apoptosis is a well-described form of PCD in mammalian cells. One of the characteristic features of apoptosis is the change in cellular morphology, often leading to the fragmentation of the cell into smaller membrane-bound vesicles through a process called apoptotic cell disassembly. Interestingly, some of these morphological changes and cell disassembly are also noted in cells of other organisms including plants, fungi and protists while undergoing 'apoptosis-like PCD'. This review will describe morphologic features leading to apoptotic cell disassembly, as well as its regulation and function in mammalian cells. The occurrence of cell disassembly during cell death in other organisms namely zebrafish, fly and worm, as well as in other eukaryotic cells will also be discussed.

I Spy in the Developing Fly a Multitude of Ways to Die

Cell proliferation and cell death are two opposing, yet complementary fundamental processes in development. Cell proliferation provides new cells, while developmental programmed cell death adjusts cell numbers and refines structures as an organism grows. Apoptosis is the best-characterized form of programmed cell death; however, there are many other non-apoptotic forms of cell death that occur throughout development. Drosophila is an excellent model for studying these varied forms of cell death given the array of cellular, molecular, and genetic techniques available. In this review, we discuss select examples of apoptotic and non-apoptotic cell death that occur in different tissues and at different stages of Drosophila development. For example, apoptosis occurs throughout the nervous system to achieve an appropriate number of neurons. Elsewhere in the fly, non-apoptotic modes of developmental cell death are employed, such as in the elimination of larval salivary glands and midgut during metamorphosis. These and other examples discussed here demonstrate the versatility of Drosophila as a model organism for elucidating the diverse modes of programmed cell death.

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.

Specialization of CDK1 and cyclin B paralog functions in a coenocystic mode of oogenic meiosis

Oogenesis in the urochordate, Oikopleura dioica, occurs in a large coenocyst in which vitellogenesis precedes oocyte selection in order to adapt oocyte production to nutrient conditions. The animal has expanded Cyclin-Dependant Kinase 1 (CDK1) and Cyclin B paralog complements, with several expressed during oogenesis. Here, we addressed functional redundancy and specialization of CDK1 and cyclin B paralogs during oogenesis and early embryogenesis through spatiotemporal analyses and knockdown assays. CDK1a translocated from organizing centres (OCs) to selected meiotic nuclei at the beginning of the P4 phase of oogenesis, and its knockdown impaired vitellogenesis, nurse nuclear dumping, and entry of nurse nuclei into apoptosis. CDK1d-Cyclin Ba translocated from OCs to selected meiotic nuclei in P4, drove meiosis resumption and promoted nuclear envelope breakdown (NEBD). CDK1d-Cyclin Ba was also involved in histone H3S28 phosphorylation on centromeres and meiotic spindle assembly through regulating Aurora B localization to centromeres during prometaphase I. In other studied species, Cyclin B3 commonly promotes anaphase entry, but we found O. dioica Cyclin B3a to be non-essential for anaphase entry during oogenic meiosis. Instead, Cyclin B3a contributed to meiotic spindle assembly though its loss could be compensated by Cyclin Ba.

Non-apoptotic cell death in animal development

Programmed cell death (PCD) is an important process in the development of multicellular organisms. Apoptosis, a form of PCD characterized morphologically by chromatin condensation, membrane blebbing, and cytoplasm compaction, and molecularly by the activation of caspase proteases, has been extensively investigated. Studies in Caenorhabditis elegans, Drosophila, mice, and the developing chick have revealed, however, that developmental PCD also occurs through other mechanisms, morphologically and molecularly distinct from apoptosis. Some non-apoptotic PCD pathways, including those regulating germ cell death in Drosophila, still appear to employ caspases. However, another prominent cell death program, linker cell-type death (LCD), is morphologically conserved, and independent of the key genes that drive apoptosis, functioning, at least in part, through the ubiquitin proteasome system. These non-apoptotic processes may serve as backup programs when caspases are inactivated or unavailable, or, more likely, as freestanding cell culling programs. Non-apoptotic PCD has been documented extensively in the developing nervous system, and during the formation of germline and somatic gonadal structures, suggesting that preservation of these mechanisms is likely under strong selective pressure. Here, we discuss our current understanding of non-apoptotic PCD in animal development, and explore possible roles for LCD and other non-apoptotic developmental pathways in vertebrates. We raise the possibility that during vertebrate development, apoptosis may not be the major PCD mechanism.

Control of non-apoptotic nurse cell death by engulfment genes in Drosophila

Programmed cell death occurs as a normal part of oocyte development in Drosophila. For each egg that is formed, 15 germline-derived nurse cells transfer their cytoplasmic contents into the oocyte and die. Disruption of apoptosis or autophagy only partially inhibits the death of the nurse cells, indicating that other mechanisms significantly contribute to nurse cell death. Recently, we demonstrated that the surrounding stretch follicle cells non-autonomously promote nurse cell death during late oogenesis and that phagocytosis genes including draper, ced-12, and the JNK pathway are crucial for this process. When phagocytosis genes are inhibited in the follicle cells, events specifically associated with death of the nurse cells are impaired. Death of the nurse cells is not completely blocked in draper mutants, suggesting that other engulfment receptors are involved. Indeed, we found that the integrin subunit, αPS3, is enriched on stretch follicle cells during late oogenesis and is required for elimination of the nurse cells. Moreover, double mutant analysis revealed that integrins act in parallel to draper. Death of nurse cells in the Drosophila ovary is a unique example of programmed cell death that is both non-apoptotic and non-cell autonomously controlled.

Polo Kinase Phosphorylates Miro to Control ER-Mitochondria Contact Sites and Mitochondrial Ca(2+) Homeostasis in Neural Stem Cell Development

Mitochondria play central roles in buffering intracellular Ca²⁺ transients. While basal mitochondrial Ca²⁺ (Ca²⁺ mito) is needed to maintain organellar physiology, Ca²⁺ mito overload can lead to cell death. How Ca²⁺ mito homeostasis is regulated is not well understood. Here we show that Miro, a known component of the mitochondrial transport machinery, regulates Drosophila neural stem cell (NSC) development through Ca²⁺ mito homeostasis control, independent of its role in mitochondrial transport. Miro interacts with Ca²⁺ transporters at the ER-mitochondria contact site (ERMCS). Its inactivation causes Ca²⁺ mito depletion and metabolic impairment, whereas its overexpression results in Ca²⁺ mito overload, mitochondrial morphology change, and apoptotic response. Both conditions impaired NSC lineage progression. Ca²⁺ mito homeostasis is influenced by Polo-mediated phosphorylation of a conserved residue in Miro, which positively regulates Miro localization to, and the integrity of, ERMCS. Our results elucidate a regulatory mechanism underlying Ca²⁺ mito homeostasis and how its dysregulation may affect NSC metabolism/development and contribute to disease.

Phagocytosis genes nonautonomously promote developmental cell death in the Drosophila ovary

Programmed cell death (PCD) is usually considered a cell-autonomous suicide program, synonymous with apoptosis. Recent research has revealed that PCD is complex, with at least a dozen cell death modalities. Here, we demonstrate that the large-scale nonapoptotic developmental PCD in the Drosophila ovary occurs by an alternative cell death program where the surrounding follicle cells nonautonomously promote death of the germ line. The phagocytic machinery of the follicle cells, including Draper, cell death abnormality (Ced)-12, and c-Jun N-terminal kinase (JNK), is essential for the death and removal of germ-line-derived nurse cells during late oogenesis. Cell death events including acidification, nuclear envelope permeabilization, and DNA fragmentation of the nurse cells are impaired when phagocytosis is inhibited. Moreover, elimination of a small subset of follicle cells prevents nurse cell death and cytoplasmic dumping. Developmental PCD in the Drosophila ovary is an intriguing example of nonapoptotic, nonautonomous PCD, providing insight on the diversity of cell death mechanisms.

The End of the Beginning: Cell Death in the Germline

Programmed cell death occurs in the germline of many organisms, both as an essential part of development and throughout adult life. Germline cell death can be apoptotic or nonapoptotic, depending on the stimulus or stage of development. Here, we focus on the Drosophila ovary, which is a powerful model for studying diverse types of cell death. In Drosophila, the death of primordial germ cells occurs normally during embryonic development, and germline nurse cells are programmed to die during oocyte development in adult flies. Cell death of previtellogenic egg chambers in adults can also be induced by starvation or other environmental cues. Mid-oogenesis seems to be particularly sensitive to such cues and has been proposed to serve as a checkpoint to avoid the energetically expensive cost of egg production. After the germline dies in mid-oogenesis, the remnants are engulfed by an epithelial layer of follicle cells; thus, the fly ovary also serves as a highly tractable model for engulfment by epithelial cells. These examples of cell death in the fly ovary share many similarities to the types of cell death seen in the mammalian germline. Recent progress in elucidating the molecular mechanisms of cell death in the germline is discussed.

Genetics of gonadal stem cell renewal

Stem cells are necessary for the maintenance of many adult tissues. Signals within the stem cell microenvironment, or niche, regulate the self-renewal and differentiation capability of these cells. Misregulation of these signals through mutation or damage can lead to overgrowth or depletion of different stem cell pools. In this review, we focus on the Drosophila testis and ovary, both of which contain well-defined niches, as well as the mouse testis, which has become a more approachable stem cell system with recent technical advances. We discuss the signals that regulate gonadal stem cells in their niches, how these signals mediate self-renewal and differentiation under homeostatic conditions, and how stress, whether from mutations or damage, can cause changes in cell fate and drive stem cell competition.

Lin-28 regulates oogenesis and muscle formation in Drosophila melanogaster

Understanding the control of stem cell (SC) differentiation is important to comprehend developmental processes as well as to develop clinical applications. Lin28 is a conserved molecule that is involved in SC maintenance and differentiation by regulating let-7 miRNA maturation. However, little is known about the in vivo function of Lin28. Here, we report critical roles for lin-28 during oogenesis. We found that let-7 maturation was increased in lin-28 null mutant fly ovaries. We showed that lin-28 null mutant female flies displayed reduced fecundity, due to defects in egg chamber formation. More specifically, we demonstrated that in mutant ovaries, the egg chambers fuse during early oogenesis resulting in abnormal late egg chambers. We also showed that this phenotype is the combined result of impaired germline SC differentiation and follicle SC differentiation. We suggest a model in which these multiple oogenesis defects result from a misregulation of the ecdysone signaling network, through the fine-tuning of Abrupt and Fasciclin2 expression. Our results give a better understanding of the evolutionarily conserved role of lin-28 on GSC maintenance and differentiation.

Nuclear positioning by actin cables and perinuclear actin: Special and general?

Nuclear positioning is an important process during development and homeostasis. Depending on the affected tissue, mislocalized nuclei can alter cellular processes such as polarization, differentiation, or migration and lead ultimately to diseases. Many cells actively control the position of their nucleus using their cytoskeleton and motor proteins. We have recently shown that during Drosophila oogenesis, nurse cells employ cytoplasmic actin cables in association with perinuclear actin to position their nucleus. Here, we briefly summarize our work and discuss why nuclear positioning in nurse cells is specialized but the molecular mechanisms are likely to be more generally used.

Detecting apoptosis in Drosophila tissues and cells

In this chapter we discuss methods that can be used to study apoptotic cell death in the Drosophila embryo, ovary, as well as in cultured cell lines. These methods assay various aspects of the cell death process, from mitochondrial changes to caspase activation and DNA cleavage. The assays are useful for examining apoptosis in normal development and in response to developmental perturbations and external stresses. These techniques include Acridine Orange staining, TUNEL, cleaved caspase staining, caspase activity assays and assays for mitochondrial fission and permeabilization.

Epigenetic regulation of oogenesis and germ stem cell maintenance by the Drosophila histone methyltransferase Eggless/dSetDB1

The Drosophila melanogaster histone lysine methyltransferase (HKMT) Eggless (Egg/dSETDB1) catalyzes methylation of Histone H3 lysine 9 (H3K9), a signature of repressive heterochromatin. Our previous studies showed that H3K9 methylation by Egg is required for oogenesis. Here we analyze a set of EMS-induced mutations in the egg gene, identify the molecular lesions of these mutations, and compare the effects on oogenesis of both strong loss-of-function and weak hypomorphic alleles. These studies show that H3K9 methylation by Egg is required for multiple stages of oogenesis. Mosaic expression experiments show that the egg gene is not required intrinsically in the germ cells for their early differentiation, but is required in the germ cells for their survival past stage 5 of oogenesis. egg is also required in germ stem cells for their maintenance, since egg- germ stem cells initially survive but are not maintained as females age. Mosaic analysis also reveals that the early egg chamber budding defects in egg- ovaries are due to an intrinsic requirement for egg in follicle stem cells and their descendents, and that egg plays a non-autonomous role in somatic cells in the germarium to influence the differentiation of early germ cells.

Methods for studying oogenesis

Drosophila oogenesis is an excellent system for the study of developmental cell biology. Active areas of research include stem cell maintenance, gamete development, pattern formation, cytoskeletal regulation, intercellular communication, intercellular transport, cell polarity, cell migration, cell death, morphogenesis, cell cycle control, and many more. The large size and relatively simple organization of egg chambers make them ideally suited for microscopy of both living and fixed whole mount tissue. A wide range of tools is available for oogenesis research. Newly available shRNA transgenic lines provide an alternative to classic loss-of-function F2 screens and clonal screens. Gene expression can be specifically controlled in either germline or somatic cells using the Gal4/UAS system. Protein trap lines provide fluorescent tags of proteins expressed at endogenous levels for live imaging and screening backgrounds. This review provides information on many available reagents and key methods for research in oogenesis.

Induction of endocycles represses apoptosis independently of differentiation and predisposes cells to genome instability

The endocycle is a common developmental cell cycle variation wherein cells become polyploid through repeated genome duplication without mitosis. We previously showed that Drosophila endocycling cells repress the apoptotic cell death response to genotoxic stress. Here, we investigate whether it is differentiation or endocycle remodeling that promotes apoptotic repression. We find that when nurse and follicle cells switch into endocycles during oogenesis they repress the apoptotic response to DNA damage caused by ionizing radiation, and that this repression has been conserved in the genus Drosophila over 40 million years of evolution. Follicle cells defective for Notch signaling failed to switch into endocycles or differentiate and remained apoptotic competent. However, genetic ablation of mitosis by knockdown of Cyclin A or overexpression of fzr/Cdh1 induced follicle cell endocycles and repressed apoptosis independently of Notch signaling and differentiation. Cells recovering from these induced endocycles regained apoptotic competence, showing that repression is reversible. Recovery from fzr/Cdh1 overexpression also resulted in an error-prone mitosis with amplified centrosomes and high levels of chromosome loss and fragmentation. Our results reveal an unanticipated link between endocycles and the repression of apoptosis, with broader implications for how endocycles may contribute to genome instability and oncogenesis.

Diversity of cell death pathways: insight from the fly ovary

Multiple types of cell death exist including necrosis, apoptosis, and autophagic cell death. The Drosophila ovary provides a valuable model to study the diversity of cell death modalities, and we review recent progress to elucidate these pathways. At least five distinct types of cell death occur in the ovary, and we focus on two that have been studied extensively. Cell death of mid-stage egg chambers occurs through a novel caspase-dependent pathway that involves autophagy and triggers phagocytosis by surrounding somatic epithelial cells. For every egg, 15 germline nurse cells undergo developmental programmed cell death, which occurs independently of most known cell death genes. These forms of cell death are strikingly similar to cell death observed in the germlines of other organisms.

Combined inhibition of autophagy and caspases fails to prevent developmental nurse cell death in the Drosophila melanogaster ovary

During the final stages of Drosophila melanogaster oogenesis fifteen nurse cells, sister cells to the oocyte, degenerate as part of normal development. This process involves at least two cell death mechanisms, caspase-dependent cell death and autophagy, as indicated by apoptosis and autophagy markers. In addition, mutations affecting either caspases or autophagy partially reduce nurse cell removal, leaving behind end-stage egg chambers with persisting nurse cell nuclei. To determine whether apoptosis and autophagy work in parallel to degrade and remove these cells as is the case with salivary glands during pupariation, we generated mutants doubly affecting caspases and autophagy. We found no significant increase in either the number of late stage egg chambers containing persisting nuclei or in the number of persisting nuclei per egg chamber in the double mutants compared to single mutants. These findings suggest that there is another cell death mechanism functioning in the ovary to remove all nurse cell remnants from late stage egg chambers.

Detrimental effects of proteasome inhibition activity in Drosophila melanogaster: implication of ER stress, autophagy, and apoptosis

In eukaryotes, the ubiquitin-proteasome machinery regulates a number of fundamental cellular processes through accurate and tightly controlled protein degradation pathways. We have, herein, examined the effects of proteasome functional disruption in Dmp53 (+/+) (wild-type) and Dmp53 (-/-) Drosophila melanogaster fly strains through utilization of Bortezomib, a proteasome-specific inhibitor. We report that proteasome inhibition drastically shortens fly life-span and impairs climbing performance, while it also causes larval lethality and activates developmentally irregular cell death programs during oogenesis. Interestingly, Dmp53 gene seems to play a role in fly longevity and climbing ability. Moreover, Bortezomib proved to induce endoplasmic reticulum (ER) stress that was able to result in the engagement of unfolded protein response (UPR) signaling pathway, as respectively indicated by fly Xbp1 activation and Ref(2)P-containing protein aggregate formation. Larva salivary gland and adult brain both underwent strong ER stress in response to Bortezomib, thus underscoring the detrimental role of proteasome inhibition in larval development and brain function. We also propose that the observed upregulation of autophagy operates as a protective mechanism to "counterbalance" Bortezomib-induced systemic toxicity, which is tightly associated, besides ER stress, with activation of apoptosis, mainly mediated by functional Drice caspase and deregulated dAkt kinase. The reduced life-span of exposed to Bortezomib flies overexpressing Atg1_RNAi or Atg18_RNAi supports the protective nature of autophagy against proteasome inhibition-induced stress. Our data reveal the in vivo significance of proteasome functional integrity as a major defensive system against cellular toxicity likely occurring during critical biological processes and morphogenetic courses.

Oocyte destruction is activated during viral infection

Viral infection has been associated with a starvation-like state in Drosophila melanogaster. Because starvation and inhibiting TOR kinase activity in vivo result in blocked oocyte production, we hypothesized that viral infection would also result in compromised oogenesis. Wild-type flies were injected with flock house virus (FHV) and survival and embryo production were monitored. Infected flies had a dose-responsive loss of fecundity that corresponded to a global reduction in Akt/TOR signaling. Highly penetrant egg chamber destruction mid-way through oogenesis was noted and FHV coat protein was detected within developing egg chambers. As seen with in vivo TOR inhibition, oogenesis was partially rescued in loss of function discs large and merlin mutants. As expected, mutants in genes known to be involved in virus internalization and trafficking [Clathrin heavy chain (chc) and synaptotagmin] survive longer during infection. However, oogenesis was rescued only in chc mutants. This suggests that viral response mechanisms that control fly survival and egg chamber survival are separable. The genetic and signaling requirements for oocyte destruction delineated here represent a novel host-virus interaction with implications for the control of both fly and virus populations.

Differences in the relative timing of developmental events during oogenesis in lower dipterans (Nematocera) reveal the autonomy of follicular cells' differentiation program

Although the ovaries of Nematocera are of the same meroistic-polytrophic type, they show significant differences in the activity of germ cells (oocytes, nurse cells) and their relative contribution to ribosome synthesis and storage during oogenesis. These different activities result in the different growth rate of the germ cells and may determine the life span of the nurse cells. Comparative analysis revealed that with reference to germ cell activity, two basic types of oogenesis in Nematocera can be distinguished. In the Tinearia type, the nurse cells grow considerably and are active until advanced stages of oogenesis, whereas the oocyte is transcriptionally inert. Conversely, in the Tipula type of oogenesis, the oocyte nucleus contains transcriptionally active multiple nucleoli, while nurse cells probably do not contribute to ribosome synthesis, remain relatively small and degenerate early in oogenesis. We studied and compared the process of somatic follicular cell differentiation in nematoceran species representing both types of oogenesis. Our observations indicate that morphogenesis of the follicular cells is at least partly independent of the nurse cell activity, while the execution of their differentiation does not require direct contacts between the follicular cells and the oocyte.

Proteasome inhibition induces developmentally deregulated programs of apoptotic and autophagic cell death during Drosophila melanogaster oogenesis

Ubiquitin/proteasome-mediated degradation of eukaryotic proteins is critically implicated in a number of signalling pathways and cellular processes. To specifically impair proteasome activities, in vitro developing Drosophila melanogaster egg chambers were exposed to the MG132 or epoxomicin proteasome inhibitors, while a GAL4/UAS binary genetic system was employed to generate double transgenic flies overexpressing β2 and β6 conditional mutant proteasome subunits in a cell type-specific manner. MG132 and epoxomicin administration resulted in severe deregulation of in vitro developing egg chambers, which was tightly associated with precocious induction of nurse cell-specific apoptotic and autophagic death programmes, featured by actin cytoskeleton disorganization, nuclear chromatin condensation, DRICE caspase activation and autophagosome accumulation. In vivo targeted overexpression of β2 and β6 conditional mutants, specifically in the nurse cell compartment, led to a notable up-regulation of sporadic apoptosis potency during early and mid-oogenesis 'checkpoints', thus reasonably justifying the observed reduction in eclosion efficiency. Furthermore, in response to the intracellular abundance of β2 and β6 conditional mutant forms, specifically in numerous tissues of third instar larval stage, the developmental course was arrested, and lethal phenotypes were obtained at this particular embryonic period, with the double transgenic heterozygote embryos being unable to further proceed to complete maturation to adult flies. Our data demonstrate that physiological proteasome function is required to ensure normal oogenesis and embryogenesis in D. melanogaster, since targeted and cell type-dependent proteasome inactivation initiates developmentally deregulated apoptotic and autophagic mechanisms.

Genetic control of necrosis - another type of programmed cell death

Necrosis has been thought to be an accidental or uncontrolled type of cell death rather than programmed. Recent studies from diverse organisms show that necrosis follows a stereotypical series of cellular and molecular events: swelling of organelles, increases in reactive oxygen species and cytoplasmic calcium, a decrease in ATP, activation of calpain and cathepsin proteases, and finally rupture of organelles and plasma membrane. Genetic and chemical manipulations demonstrate that necrosis can be inhibited, indicating that necrosis can indeed be controlled and follows a specific 'program.' This review highlights recent findings from C. elegans, yeast, Dictyostelium, Drosophila, and mammals that collectively provide evidence for conserved mechanisms of necrosis.

Bcl-2 proteins and autophagy regulate mitochondrial dynamics during programmed cell death in the Drosophila ovary

The Bcl-2 family has been shown to regulate mitochondrial dynamics during cell death in mammals and C. elegans, but evidence for this in Drosophila has been elusive. Here, we investigate the regulation of mitochondrial dynamics during germline cell death in the Drosophila melanogaster ovary. We find that mitochondria undergo a series of events during the progression of cell death, with remodeling, cluster formation and uptake of clusters by somatic follicle cells. These mitochondrial dynamics are dependent on caspases, the Bcl-2 family, the mitochondrial fission and fusion machinery, and the autophagy machinery. Furthermore, Bcl-2 family mutants show a striking defect in cell death in the ovary. These data indicate that a mitochondrial pathway is a major mechanism for activation of cell death in Drosophila oogenesis.

Physiological apoptosis of polar cells during Drosophila oogenesis is mediated by Hid-dependent regulation of Diap1

Although much has been learned in recent years about the apoptotic machinery, the mechanisms underlying survival and death choices during development of metazoans remain less clearly understood. During early oogenesis in Drosophila, a small excess in the number of specialized somatic cells, called polar cells (PCs), produced at follicle extremities is reduced to exactly two cells through apoptosis by mid-oogenesis. We have found that PCs destined to die first lose their apical contacts and then round up and shrink progressively until they disappear. Caspases are activated only once the cells have begun to shrink, suggesting that they are implicated in this part of the process, but not in the initial loss of cell polarity. Loss-of-function analyses based on mutant, clonal and RNAi approaches show that among the RHG family of pro-apoptotic factors, Hid is specifically necessary for PC apoptosis, as well as the initiator caspase Dronc and its adaptor Dark/Apaf-1, and likely several effector caspases, in particular Drice. In addition, we show that Hid protein and transcripts accumulate specifically in PCs destined to die, while the anti-apoptotic factor Diap1 is downregulated in these cells in a hid-dependent manner. Therefore, our results implicate the Hid-Diap1 module as an important regulatory point in a developmental case of apoptosis.

Identification and expression profiling of Ceratitis capitata genes coding for β-hexosaminidases

The goal of this study was to identify the genes coding for β-N-acetylhexosaminidases in the Mediterranean fruit fly (medfly) Ceratitis capitata, one of the most destructive agricultural pests, belonging to the Tephritidae family, order Diptera. Two dimeric β-N-acetylhexosaminidases, HEXA and HEXB, have been recently identified on Drosophila sperm. These enzymes are involved in egg binding through interactions with complementary carbohydrates on the surface of the egg shell. Three genes, Hexosaminidase 1 (Hexo1), Hexosaminidase 2 (Hexo2) and fused lobes (fdl), encode for HEXA and HEXB subunits. The availability of C. capitata EST libraries derived from embryos and adult heads allowed us to identify three sequences homologous to the D. melanogaster Hexo1, Hexo2 and fdl genes. Here, we report the expression profile analysis of CcHexo1, CcHexo2 and Ccfdld in several tissues, organs and stages. Ccfdl expression was highest in heads of both sexes and in whole adult females. In the testis and ovary the three genes showed distinct spatial and temporal expression patterns. All the mRNAs were detectable in early stages of spermatogenesis; CcHexo2 and Ccfdl were also expressed in early elongating spermatid cysts. All three genes are expressed in the ovarian nurse cells. CcHexo1 and Ccfdl are stage specific, since they have been observed in stages 12 and 13 during oocyte growth, when programmed cell death occurs in nurse cells. The expression pattern of the three genes in medfly gonads suggests that, as their Drosophila counterparts, they may encode for proteins involved in gametogenesis and fertilization.

The proteasome-ubiquitin pathway in the Schistosoma mansoni egg has development- and morphology-specific characteristics

Schistosoma mansoni eggs, consisting of an ovum surrounded by nutritive vitelline cells packaged in a tanned protein shell, are produced by paired worms residing in the mesenteric veins of the human host. The vitelline cells are degraded as the larval miracidium matures, the fully developed egg either crossing the gut wall to escape the host or becoming lodged in the host's tissues where it dies and disintegrates, inducing a potentially pathological immune response. Thus, the egg is central to both the transmission of the parasite and the aetiology of the disease. Here we present the first study investigating protein turnover in the egg. We establish that the ubiquitin-proteasome pathway (UPP) changes with egg development and furthermore, that the morphological components of the fully developed egg (the miracidium and the subshell envelope) also exhibit different proteasome subunit expression profiles. We conclude that the UPP is responsible not only for degrading the vitelline cells but is also more highly developed in the envelope than in the miracidium. The envelope is involved in the defence of the miracidium and produces the proteins that the egg secretes, presumably to facilitate its escape from the host, so the UPP probably has a multi-faceted role in the egg's biology.

Autophagic degradation of dBruce controls DNA fragmentation in nurse cells during late Drosophila melanogaster oogenesis

Blocking autophagy protects the apoptosis inhibitor dBruce from destruction and promotes nurse cell survival in developing egg chambers.

Autophagy is an evolutionarily conserved pathway responsible for degradation of cytoplasmic material via the lysosome. Although autophagy has been reported to contribute to cell death, the underlying mechanisms remain largely unknown. In this study, we show that autophagy controls DNA fragmentation during late oogenesis in Drosophila melanogaster. Inhibition of autophagy by genetically removing the function of the autophagy genes atg1, atg13, and vps34 resulted in late stage egg chambers that contained persisting nurse cell nuclei without fragmented DNA and attenuation of caspase-3 cleavage. The Drosophila inhibitor of apoptosis (IAP) dBruce was found to colocalize with the autophagic marker GFP-Atg8a and accumulated in autophagy mutants. Nurse cells lacking Atg1 or Vps34 in addition to dBruce contained persisting nurse cell nuclei with fragmented DNA. This indicates that autophagic degradation of dBruce controls DNA fragmentation in nurse cells. Our results reveal autophagic degradation of an IAP as a novel mechanism of triggering cell death and thereby provide a mechanistic link between autophagy and cell death.

Cracking open cell death in the Drosophila ovary

The Drosophila melanogaster ovary is a powerful yet simple system with only a few cell types. Cell death in the ovary can be induced in response to multiple developmental and environmental signals. These cell deaths occur at distinct stages of oogenesis and involve unique mechanisms utilizing apoptotic, autophagic and perhaps necrotic processes. In this review, we summarize recent progress characterizing cell death mechanisms in the fly ovary.

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.