Caspases function in autophagic programmed cell death in Drosophila

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.

Autophagy, not apoptosis, is essential for midgut cell death in Drosophila

Most developmentally programmed cell death in metazoans is mediated by caspases. During Drosophila metamorphosis, obsolete tissues, including the midgut and salivary glands, are removed by programmed cell death [1]. The initiator caspase Dronc and its activator Ark are required for the death of salivary glands, but not for midgut removal [2, 3]. In addition to caspases, complete removal of salivary glands requires autophagy [4]. However, the contribution of autophagy to midgut cell death has not been explored. Examination of combined mutants of the main initiator and effector caspases revealed that the canonical apoptotic pathway is not required for midgut cell death. Further analyses revealed that the caspase Decay is responsible for most of the caspase activity in dying midguts, yet inhibition of this activity has no effect on midgut removal. By contrast, midgut degradation was severely delayed by inhibition of autophagy, and this occurred without a decrease in caspase activity. Surprisingly, the combined inhibition of caspases and autophagy did not result in an additional delay in midgut removal. Together, our results indicate that autophagy, not caspases, is essential for midgut programmed cell death, providing the first in vivo evidence of caspase-independent programmed cell death that requires autophagy despite the presence of high caspase activity.

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.

Autophagy and plant innate immunity: Defense through degradation

Autophagy is a process of bulk degradation and nutrient sequestration that occurs in all eukaryotes. In plants, autophagy is activated during development, environmental stress, starvation, and senescence. Recent evidence suggests that autophagy is also necessary for the proper regulation of hypersensitive response programmed cell death (HR-PCD) during the plant innate immune response. We review autophagy in plants with emphasis on the role of autophagy during innate immunity. We hypothesize a role for autophagy in the degradation of pro-death signals during HR-PCD, with specific focus on reactive oxygen species and their sources. We propose that the plant chloroplasts are an important source of pro-death signals during HR-PCD, and that the chloroplast itself may be targeted for autophagosomal degradation by a process called chlorophagy.

Endocycling cells do not apoptose in response to DNA rereplication genotoxic stress

Initiation of DNA replication at origins more than once per cell cycle results in rereplication and has been implicated in cancer. Here we use Drosophila to examine the checkpoint responses to rereplication in a developmental context. We find that increased Double-parked (Dup), the Drosophila ortholog of Cdt1, results in rereplication and DNA damage. In most cells, this rereplication triggers caspase activation and apoptotic cell death mediated by both p53-dependent and -independent pathways. Elevated Dup also caused DNA damage in endocycling cells, which switch to a G/S cycle during normal development, indicating that rereplication and the endocycling DNA reduplication program are distinct processes. Unexpectedly, however, endocycling cells do not apoptose regardless of tissue type. Our combined evidence suggests that endocycling apoptosis is repressed in part because proapoptotic gene promoters are silenced. Normal endocycling cells had DNA lesions near heterochromatin, which increased after rereplication, explaining why endocycling cells must constantly repress the genotoxic apoptotic response. Our results reveal a novel regulation of apoptosis in development and new insights into the little-understood endocycle. Similar mechanisms may operate during vertebrate development, with implications for cancer predisposition in certain tissues.

Autophagy in Drosophila melanogaster

Macroautophagy (autophagy) is a bulk cytoplasmic degradation process that is conserved from yeast to mammals. Autophagy is an important cellular response to starvation and stress, and plays critical roles in development, cell death, aging, immunity, and cancer. The fruit fly Drosophila melanogaster provides an excellent model system to study autophagy in vivo, in the context of a developing organism. Autophagy (atg) genes and their regulators are conserved in Drosophila, and autophagy is induced in response to nutrient starvation and hormones during development. In this review we provide an overview of how Drosophila research has contributed to our understanding of the role and regulation of autophagy in cell survival, growth, nutrient utilization, and cell death. Recent Drosophila research has also provided important mechanistic information about the role of autophagy in protein aggregation disorders, neurodegeneration, aging, and innate immunity. Differences in the role of autophagy in specific contexts and/or cell types suggest that there may be cell-context-specific regulators of autophagy, and studies in Drosophila are well-suited to yield discoveries about this specificity.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

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.

Morphological changes and patterns of ecdysone receptor B1 immunolocalization in the anterior silk gland undergoing programmed cell death in the silkworm, Bombyx mori

The silk gland is a specific larval tissue of Lepidopteran insects and begins to degenerate shortly before pupation. The steroid hormone ecdysone triggers the stage specific programmed cell death of the anterior silk glands during metamorphosis in the silkworm, Bombyx mori. The anterior silk gland expresses ecdysone receptors, which are involved in regulation processes in response to ecdysone. In this study, the morphological changes, immunohistochemical localization and protein levels of ecdysone receptor B1 (EcR-B1) in the anterior silk gland of B. mori were investigated during programmed cell death. Morphological changes observed during the degeneration process involve the appearance of large vacuoles, probably autophagic vacuoles, which increase in number in pupal anterior silk glands. No macrophages were found in the silk gland during the prepupal and pupal stage unlike in apoptosis, which strongly suggests that programmed cell death of the anterior silk gland is carried out by autophagy. Morphological changes of the silk glands were accompanied by changes in the immunolocalization and protein levels of EcR-B1. The differences in tissue distribution and protein levels of EcR-B1 during the programmed cell death indicate that the receptor plays a major role in the modulation and function of ecdysone activity in Bombyx anterior silk glands. Our results indicate that EcR-B1 expression may be important for the process of programmed cell death in the anterior silk glands.

Warts is required for PI3K-regulated growth arrest, autophagy, and autophagic cell death in Drosophila

BACKGROUND

Cell growth arrest and autophagy are required for autophagic cell death in Drosophila. Maintenance of growth by expression of either activated Ras, Dp110, or Akt is sufficient to inhibit autophagy and cell death in Drosophila salivary glands, but the mechanism that controls growth arrest is unknown. Although the Warts (Wts) tumor suppressor is a critical regulator of tissue growth in animals, it is not clear how this signaling pathway controls cell growth.

RESULTS

Here, we show that genes in the Wts pathway are required for salivary gland degradation and that wts mutants have defects in cell growth arrest, caspase activity, and autophagy. Expression of Atg1, a regulator of autophagy, in salivary glands is sufficient to rescue wts mutant salivary gland destruction. Surprisingly, expression of Yorkie (Yki) and Scalloped (Sd) in salivary glands fails to phenocopy wts mutants. By contrast, misexpression of the Yki target bantam was able to inhibit salivary gland cell death, even though mutations in bantam fail to suppress the wts mutant salivary gland-persistence phenotype. Significantly, wts mutant salivary glands possess altered phosphoinositide signaling, and decreased function of the class I PI3K-pathway genes chico and TOR suppressed wts defects in cell death.

CONCLUSIONS

Although we have previously shown that salivary gland degradation requires genes in the Wts pathway, this study provides the first evidence that Wts influences autophagy. Our data indicate that the Wts-pathway components Yki, Sd, and bantam fail to function in salivary glands and that Wts regulates salivary gland cell death in a PI3K-dependent manner.

Classification of cell death: recommendations of the Nomenclature Committee on Cell Death 2009

Different types of cell death are often defined by morphological criteria, without a clear reference to precise biochemical mechanisms. The Nomenclature Committee on Cell Death (NCCD) proposes unified criteria for the definition of cell death and of its different morphologies, while formulating several caveats against the misuse of words and concepts that slow down progress in the area of cell death research. Authors, reviewers and editors of scientific periodicals are invited to abandon expressions like 'percentage apoptosis' and to replace them with more accurate descriptions of the biochemical and cellular parameters that are actually measured. Moreover, at the present stage, it should be accepted that caspase-independent mechanisms can cooperate with (or substitute for) caspases in the execution of lethal signaling pathways and that 'autophagic cell death' is a type of cell death occurring together with (but not necessarily by) autophagic vacuolization. This study details the 2009 recommendations of the NCCD on the use of cell death-related terminology including 'entosis', 'mitotic catastrophe', 'necrosis', 'necroptosis' and 'pyroptosis'.

The cell biology of autophagy in metazoans: a developing story

The cell biological phenomenon of autophagy (or ;self-eating') has attracted increasing attention in recent years. In this review, we first address the cell biological functions of autophagy, and then discuss recent insights into the role of autophagy in animal development, particularly in C. elegans, Drosophila and mouse. Work in these and other model systems has also provided evidence for the involvement of autophagy in disease processes, such as neurodegeneration, tumorigenesis, pathogenic infection and aging. Insights gained from investigating the functions of autophagy in normal development should increase our understanding of its roles in human disease and its potential as a target for therapeutic intervention.

Combined apoptosis and autophagy, the process that eliminates the oocytes of atretic follicles in immature rats

We studied the alterations of dying oocytes in 1-28 days old rats using TUNEL method, immunolocalizations of active caspase 3, lamp1, localization of acid phosphatase, and DAPI staining. All procedures were performed in adjacent sections of each oocyte. In most dying oocytes exist simultaneously features of apoptosis as active caspase 3 and DNA breaks, and a large increase of lamp1 and acid phosphatase characteristic of autophagy. Large clumps of compact chromatin and membrane blebbing were absent. Electron microscope observations demonstrated the presence of small clear vesicles and autophagolysosomes. All these features indicate that a large number of oocytes are eliminated by a process sharing features of apoptosis and autophagy. In dying oocytes of new born rats the markers of apoptosis predominate over those of autophagy. However, fragmentation and apoptotic bodies were not found. These features suggest that in different cytophysiological conditions the processes of cell death may be differently modulated.

The role of autophagy in mammalian development: cell makeover rather than cell death

Autophagy is important for the degradation of bulk cytoplasm, long-lived proteins, and entire organelles. In lower eukaryotes, autophagy functions as a cell death mechanism or as a stress response during development. However, autophagy's significance in vertebrate development, and the role (if any) of vertebrate-specific factors in its regulation, remains unexplained. Through careful analysis of the current autophagy gene mutant mouse models, we propose that in mammals, autophagy may be involved in specific cytosolic rearrangements needed for proliferation, death, and differentiation during embryogenesis and postnatal development. Thus, autophagy is a process of cytosolic "renovation," crucial in cell fate decisions.

Organ renewal and cell divisions by differentiated cells in Drosophila

For many organs, the processes of renewal and regeneration recruit stem cells to replace differentiated, postmitotic cells, but the capacity of an organ's differentiated cells to divide and contribute is uncertain. Most cells of the Drosophila adult are the descendants of dedicated precursors that divide and replace larval cells that are histolyzed during metamorphosis. We investigated the provenance of cells that reconstitute the second thoracic metamere of the tracheal system (Tr2). These cells contribute the precursors for Branchless(FGF)-dependent growth of the dorsal air sacs, the major tracheal organs of the adult fly. We found that, in contrast to the cells in other tracheal metameres that proceed through many cycles of endoreplication, the cells that constitute the Tr2 branches in young larvae do not. Like the cells in other tracheal metameres, these cells arrest mitotic cycling in the embryo and form differentiated, air-filled tracheal branches of the larva. We report here that they reinitiate cell divisions during the third instar (L3) to increase the Tr2 population by approximately 10-fold with multipotent cells.

Experimental control and characterization of autophagy in Drosophila

Insects such as the fruit fly Drosophila melanogaster, which fundamentally reorganize their body plan during metamorphosis, make extensive use of autophagy for their normal development and physiology. In the fruit fly, the hepatic/adipose organ known as the fat body accumulates nutrient stores during the larval feeding stage. Upon entering metamorphosis, as well as in response to starvation, these nutrients are mobilized through a massive induction of autophagy, providing support to other tissues and organs during periods of nutrient deprivation. High levels of autophagy are also observed in larval tissues destined for elimination, such as the salivary glands and larval gut. Drosophila is emerging as an important system for studying the functions and regulation of autophagy in an in vivo setting. In this chapter we describe reagents and methods for monitoring autophagy in Drosophila, focusing on the larval fat body. We also describe methods for experimentally activating and inhibiting autophagy in this system and discuss the potential for genetic analysis in Drosophila to identify novel genes involved in autophagy.

Apoptosis and autophagy: regulatory connections between two supposedly different processes

Apoptosis and autophagy are genetically-regulated, evolutionarily-conserved processes that regulate cell fate. Both apoptosis and autophagy are important in development and normal physiology and in a wide range of diseases. Recent studies show that despite the marked differences between these two processes, their regulation is intimately connected and the same regulators can sometimes control both apoptosis and autophagy. In this review, I discuss some of these findings, which provide possible molecular mechanisms for crosstalk between apoptosis and autophagy and suggest that it may be useful to think of these processes as different facets of the same cell death continuum rather than completely separate processes.

Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Mutations in ATM (Ataxia telangiectasia mutated) result in Ataxia telangiectasia (A-T), a disorder characterized by progressive neurodegeneration. Despite advances in understanding how ATM signals cell cycle arrest, DNA repair, and apoptosis in response to DNA damage, it remains unclear why loss of ATM causes degeneration of post-mitotic neurons and why the neurological phenotype of ATM-null individuals varies in severity. To address these issues, we generated a Drosophila model of A-T. RNAi knockdown of ATM in the eye caused progressive degeneration of adult neurons in the absence of exogenously induced DNA damage. Heterozygous mutations in select genes modified the neurodegeneration phenotype, suggesting that genetic background underlies variable neurodegeneration in A-T. The neuroprotective activity of ATM may be negatively regulated by deacetylation since mutations in a protein deacetylase gene, RPD3, suppressed neurodegeneration, and a human homolog of RPD3, histone deacetylase 2, bound ATM and abrogated ATM activation in cell culture. Moreover, knockdown of ATM in post-mitotic neurons caused cell cycle re-entry, and heterozygous mutations in the cell cycle activator gene String/CDC25 inhibited cell cycle re-entry and neurodegeneration. Thus, we hypothesize that ATM performs a cell cycle checkpoint function to protect post-mitotic neurons from degeneration and that cell cycle re-entry causes neurodegeneration in A-T.

Different sets of genes are activated by p53 upon UV or ionizing radiation in Drosophila melanogaster

The p53 tumour suppressor plays central role in the maintenance of genome integrity. P53 deficient fruit flies are highly sensitive to ionizing radiation (IR) and show genome instability suggesting that the Drosophila melanogaster p53 (Dmp53) is necessary for the proper damage response upon IR. We found that Dmp53 null fruit flies are highly sensitive to ultraviolet radiation (UV) as well. We analyzed the expression levels of apoptotic genes in wild type and Dmp53 null mutant animals after UV or IR using quantitative real-time RT-PCR. Ark (Apaf-1 related killer) was induced in a Dmp53-dependent way upon UV treatment but not by IR, hid (head involution defective/wrinkled) was induced upon both types of DNA damage, while reaper was induced only upon IR but not UV treatment. Using microarray analysis we identified several further genes that are activated upon UV irradiation in the presence of wild type Dmp53 only. Some but not all of these genes show Dmp53-dependent activation upon IR treatment as well. These results suggest that Dmp53 activates distinct cellular pathways through regulation of different target genes after different types of DNA damage.

Alternative pathways of programmed cell death are activated in cells with defective caspase-dependent apoptosis

Loss of programmed cell death pathways is one of the features of malignancy that complicate the response of cancer cells to a therapy. Activation of alternative cell death pathways offers a promising approach to enhance efficiency of cancer chemotherapy. We analysed programmed cell death pathways of v-myb-transformed BM2 monoblasts induced by arsenic trioxide, cycloheximide and camptothecin with U937 promonocytes as a reference cell line. We show that induced death of BM2 cells is not executed by caspases but rather by alternative cell death pathways. Camptothecin induces the lysosome-dependent cell death, arsenic trioxide induces autophagy, and most of cycloheximide-treated BM2 cells die by necrosis. The fact that alternative cell death pathways can be switched in cells with defects in activation and/or function of caspases suggests that understanding and targeting of these pathways could improve therapy of cancer cells suffering from defective apoptosis.

Induction of autophagy during extracellular matrix detachment promotes cell survival

Autophagy has been proposed to promote cell death during lumen formation in three-dimensional mammary epithelial acini because numerous autophagic vacuoles are observed in the dying central cells during morphogenesis. Because these central cells die due to extracellular matrix (ECM) deprivation (anoikis), we have directly interrogated how matrix detachment regulates autophagy. Detachment induces autophagy in both nontumorigenic epithelial lines and in primary epithelial cells. RNA interference-mediated depletion of autophagy regulators (ATGs) inhibits detachment-induced autophagy, enhances apoptosis, and reduces clonogenic recovery after anoikis. Remarkably, matrix-detached cells still exhibit autophagy when apoptosis is blocked by Bcl-2 overexpression, and ATG depletion reduces the clonogenic survival of Bcl-2-expressing cells after detachment. Finally, stable reduction of ATG5 or ATG7 in MCF-10A acini enhances luminal apoptosis during morphogenesis and fails to elicit long-term luminal filling, even when combined with apoptotic inhibition mediated by Bcl-2 overexpression. Thus, autophagy promotes epithelial cell survival during anoikis, including detached cells harboring antiapoptotic lesions.

Programmed cell death in fission yeast Schizosaccharomyces pombe

Yeasts have proven to be invaluable, genetically tractable systems to study various fundamental biological processes including programmed cell death. Recent advances in the elucidation of the molecular pathways underlying apoptotic cell death in yeasts have revealed remarkable similarities to mammalian apoptosis at cellular, organelle and macromolecular levels, thus making a strong case for the relevance of yeast models of regulated cell death. Programmed cell death has been reported in fission yeast Schizosaccharomyces pombe, primarily in the contexts of perturbed intracellular lipid metabolism, defective DNA replication, improper mitotic entry, chronological and replicative aging. Here we review the current understanding of the programmed cell death in fission yeast, paying particular attention to lipid-induced cell death. We discuss our recent findings that fission yeast exhibits plasticity of apoptotic and non-apoptotic modes of cell death in response to different lipid stimuli and growth conditions, and that mitochondria, reactive oxygen species and novel cell death mediators including metacaspase Pca1, SpRad9 and Pck1 are involved in the lipotoxic cell death. We also present perspectives on how various aspects of the cell and molecular biology of this organism can be explored to shed light on the governing principles underlying lipid-mediated signaling and cell demise.

Growth arrest and autophagy are required for salivary gland cell degradation in Drosophila

Autophagy is a catabolic process that is negatively regulated by growth and has been implicated in cell death. We find that autophagy is induced following growth arrest and precedes developmental autophagic cell death of Drosophila salivary glands. Maintaining growth by expression of either activated Ras or positive regulators of the class I phosphoinositide 3-kinase (PI3K) pathway inhibits autophagy and blocks salivary gland cell degradation. Developmental degradation of salivary glands is also inhibited in autophagy gene (atg) mutants. Caspases are active in PI3K-expressing and atg mutant salivary glands, and combined inhibition of both autophagy and caspases increases suppression of gland degradation. Further, induction of autophagy is sufficient to induce premature cell death in a caspase-independent manner. Our results provide in vivo evidence that growth arrest, autophagy, and atg genes are required for physiological autophagic cell death and that multiple degradation pathways cooperate in the efficient clearance of cells during development.

Inhibition of ERK attenuates autophagy and potentiates tumour necrosis factor-alpha-induced cell death in MCF-7 cells

The role of autophagy in cell death is under considerable debate. The process of autophagy has been shown to lead to either cell survival or cell death depending on cell type and stimulus. In the present study, we determined the contribution of ERK1/2 signalling to autophagy and cell death induced by tumour necrosis factor-α (TNF) in MCF-7 breast cancer cells. Treatment of MCF-7 cells with TNF caused a time-dependent increase in ERK1/2 activity. There was an induction of autophagy and cleavage of caspase-7, -8, -9 and PARP. Pharmacological inhibition of ERK1/2 phosphorylation with U0126 or PD98059 resulted in a decrease in TNF-induced autophagy that was accompanied by an increase in cleavage of caspase-7, -8, -9 and PARP Furthermore, inhibition of ERK1/2 signalling resulted in decreased clonogenic capacity of MCF-7 cells. These data suggest that TNF-induces autophagy through ERK1/2 and that inhibition of autophagy increases cellular sensitivity to TNF.

Autophagy functions in programmed cell death

Autophagic cell death is a prominent morphological form of cell death that occurs in diverse animals. Autophagosomes are abundant during autophagic cell death, yet the functional role of autophagy in cell death has been enigmatic. We find that autophagy and the Atg genes are required for autophagic cell death of Drosophila salivary glands. Although caspases are present in dying salivary glands, autophagy is required for complete cell degradation. Further, induction of high levels of autophagy results in caspase-independent autophagic cell death. Our results provide the first in vivo evidence that autophagy and the Atg genes are required for autophagic cell death and confirm that autophagic cell death is a physiological death program that occurs during development.

In vitro methods to monitor autophagy in Lepidoptera

Autophagy is attracting growing interest, especially in relation to increasing evidence of the importance of autophagic processes in animal development, as well as in human cancer progression. In holometabolous insects (i.e., that undergo four distinct life cycle stages, including embryo, larva, pupa and imago), such as flies, butterflies, bees and beetles, autophagy has been found to play a fundamental role in metamorphosis, and given the high degree of conservation of the genes and the basic mechanisms of autophagy, attention to these relatively simple models has increased significantly. Together with Drosophila, Lepidoptera larvae are among the most common invertebrate models in studies concerning the protective action of starvation-induced autophagy or the possible role of autophagy as a programmed cell death process. In this chapter, we provide experimental methods developed for, or applicable to, the study of the autophagic process in the IPLB-LdFB cell line derived from the fat body of the caterpillar of the gypsy moth, Lymantria dispar.

Autophagy in cardiovascular disease

Autophagy is a major cytoprotective pathway that eukaryotic cells use to degrade and recycle cytoplasmic contents. Recent evidence indicates that autophagy under baseline conditions represents an important homeostatic mechanism for the maintenance of normal cardiovascular function and morphology. By contrast, excessive induction of the autophagic process by environmental or intracellular stress has an important role in several types of cardiomyopathy by functioning as a death pathway. As a consequence, enhanced autophagy represents one of the mechanisms underlying the cardiomyocyte dropout responsible for the worsening of heart failure. Successful therapeutic approaches that regulate autophagy have been reported recently, suggesting that the autophagic machinery can be manipulated to treat heart failure or to prevent rupture of atherosclerotic plaques and sudden death.

Self-eating and self-killing: crosstalk between autophagy and apoptosis

The functional relationship between apoptosis ('self-killing') and autophagy ('self-eating') is complex in the sense that, under certain circumstances, autophagy constitutes a stress adaptation that avoids cell death (and suppresses apoptosis), whereas in other cellular settings, it constitutes an alternative cell-death pathway. Autophagy and apoptosis may be triggered by common upstream signals, and sometimes this results in combined autophagy and apoptosis; in other instances, the cell switches between the two responses in a mutually exclusive manner. On a molecular level, this means that the apoptotic and autophagic response machineries share common pathways that either link or polarize the cellular responses.

Pupal remodeling and the evolution and development of alternative male morphologies in horned beetles

BACKGROUND

How novel morphological traits originate and diversify represents a major frontier in evolutionary biology. Horned beetles are emerging as an increasingly popular model system to explore the genetic, developmental, and ecological mechanisms, as well as the interplay between them, in the genesis of novelty and diversity. The horns of beetles originate during a rapid growth phase during the prepupal stage of larval development. Differential growth during this period is either implicitly or explicitly assumed to be the sole mechanism underlying differences in horn expression within and between species. Here I focus on male horn dimorphisms, a phenomenon at the center of many studies in behavioral ecology and evolutionary development, and quantify the relative contributions of a previously ignored developmental process, pupal remodeling, to the expression of male dimorphism in three horned beetle species.

RESULTS

Prepupal growth is not the only determinant of differences in male horn expression. Instead, following their initial prepupal growth phase, beetles may be extensively remodeled during the subsequent pupal stage in a sex and size-dependent manner. Specifically, male dimorphism in the three Onthophagus species studied here was shaped not at all, partly or entirely by such pupal remodeling rather than differential growth, suggesting that pupal remodeling is phylogenetically widespread, evolutionarily labile, and developmentally flexible.

CONCLUSION

This study is the first to document that male dimorphism in horned beetles is the product of two developmentaly dissociated processes: prepupal growth and pupal remodeling. More generally, adult morphology alone appears to provide few clues, if any, as to the relative contributions of both processes to the expression of alternative male morphs, underscoring the importance of developmental studies in efforts aimed at understanding the evolution of adult diversity patterns.

Local initiation of caspase activation in Drosophila salivary gland programmed cell death in vivo

Programmed cell death, or apoptosis, is an essential event in animal development. Spatiotemporal analysis of caspase activation in vivo could provide new insights into programmed cell death occurring during development. Here, using the FRET-based caspase-3 indicator, SCAT3, we report the results of live-imaging analysis of caspase activation in developing Drosophila in vivo. In Drosophila, the salivary gland is sculpted by caspase-mediated programmed cell death initiated by the steroid hormone 20-hydroxyecdysone (ecdysone). Using a SCAT3 probe, we observed that caspase activation in the salivary glands begins in the anterior cells and is then propagated to the posterior cells in vivo. In vitro salivary gland culture experiments indicated that local exposure of ecdysone to the anterior salivary gland reproduces the caspase activation gradient as observed in vivo. In betaFTZ-F1 mutants, caspase activation was delayed and occurred in a random pattern in vivo. In contrast to the in vivo response, the salivary glands from betaFTZ-F1 mutants showed a normal in vitro response to ecdysone, suggesting that betaFTZ-F1 may be involved in ecdysteroid biosynthesis and secretion of ecdysone from the ring gland for local initiation of programmed cell death. These results imply a role of betaFTZ-F1 in coordinating the initiation of salivary gland apoptosis in development.

Proteomic Analysis of Silk Gland Programmed Cell Death during Metamorphosis of the Silkworm Bombyx mori

The silk gland of the silkworm Bombyx mori undergoes programmed cell death (PCD) during pupal metamorphosis. On the basis of their morphological changes and the occurrence of a DNA ladder, the tissue cells were categorized into three groups: intact, committed, and dying. To identify the proteins involved in this process, we conducted a comparative proteomic analysis. Protein expression changes among the three different cell types were examined by two-dimensional gel electrophoresis. Among approximately 1000 reproducibly detected protein spots on each gel, 43 were down-regulated and 34 were up-regulated in PCD process. Mass spectrometry identified 17 differentially expressed proteins, including some well-studied proteins as well as some novel PCD related proteins, such as caspases, proteasome subunit, elongation factor, heat shock protein, and hypothetical proteins. Our results suggest that these proteins may participate in the silk gland PCD process of B. mori and, thus, provide new insights for this mechanism.

Influence of glucose starvation on the pathway of death in insect cell line Sl: apoptosis follows autophagy

The relation between autophagy and apoptosis has not been clearly elucidated. Here, we reported that apoptosis followed autophagy in insect Spodoptera litura cells (Sl) undergoing glucose starvation. Sl cells have been adapted to Leibovitz-15 medium supplemented with glucose (1.0 g/l) and 5% fetal bovine serum (FBS), used for mammalian cell cultures. If glucose (1 g/l) or glutamine (1.6 g/l) had not been supplemented in L-15 medium with 5% FBS, Sl cells began to form many vacuoles and these vacuoles gradually enlarged in the cytoplasm, which were autophagic vacuoles. However, these large vacuoles began to disappear gradually after 48 h of glucose starvation, accompanied with remarkable apoptosis without apoptotic bodies, which was demonstrated by DNA fragmentation and activation of caspase-3-like. During glucose starvation, Sl cell ATP concentrations gradually decreased. Interestingly, if the conditioned L-15 medium without glucose was replaced with fresh L-15 medium supplemented with glucose or glutamine after the cultures had been starved seriously for 48 h or longer, the formation of apoptotic bodies was initiated. These data suggested that the partial depletion of cell ATP triggered apoptosis following autophagy in glucose-starved Sl cells and the formation of apoptotic bodies required higher level of ATP than DNA fragmentation and activation of caspase-3-like activity. Additionally, the disappearance of autophagic vacuoles, negative staining of neutral red, green staining of acridine orange and diffusion of acid phosphatase activity in Sl cells at the late stage of starvation (over 48 h) suggested that the dysfunction of lysosome was more likely to involve in apoptosis. The facts that Actinomycin D-induced apoptosis was partially inhibited and cyclosporin A, blocking the opening of mitochondrial permeability transition (MPT) pores, inhibited partially apoptosis in glucose-starved Sl cells, suggested the pathway of glucose starvation-induced apoptosis seemed to be different from that induced by actinomycin D and the opening of MPT pores on mitochondria probably involved in apoptosis triggered by glucose starvation, respectively.

EAST and Chromator control the destruction and remodeling of muscles during Drosophila metamorphosis

Metamorphosis involves the destruction of larval, the formation of adult and the transformation of larval into adult tissues. In this study, we demonstrate the role of the Drosophila nuclear proteins EAST and Chromator in tissue destruction and remodeling. To better understand the function of east, we performed a yeast two-hybrid screen and identified the euchromatin associated protein Chromator as a candidate interactor. To analyze the functional significance of our two-hybrid data, we generated a set of novel pupal lethal Chro alleles by P-element excision. The pupal lethal Chro mutants resemble lethal east alleles as homozygous mutants develop into pharates with normal looking body parts, but fail to eclose. The eclosion defect of the Chro alleles is rescued in an east heterozygous background, indicating antagonistic genetic interactions between the two genes. Live cell imaging was applied to study muscle development during metamorphosis. Consistent with the eclosion defects, mutant pharates of both genes show loss and abnormal differentiation of adult eclosion muscles. The two genes have opposite effects on the destruction of larval muscles in metamorphosis. While Chro mutants show incomplete histolysis, muscles degenerate prematurely in east mutants. Moreover east mutants affect the remodeling of abdominal larval muscles into adult eclosion muscles. During this process, loss of east interferes with the spatial coordination of thinning of the larval muscles. Overexpression of EAST-GFP can prevent the disintegration of polytene chromosomes during programmed cell death. We propose that Chro activates and east inhibits processes and genes involved in tissue destruction and remodeling.

The extrachromosomal East protein of Drosophila can associate with polytene chromosomes and regulate gene expression

The EAST protein of Drosophila is a component of an expandable extrachromosomal domain of the nucleus. To better understand its function, we studied the dynamics and localization of GFP-tagged EAST. In live larval salivary glands, EAST-GFP is highly mobile and localizes to the extrachromosomal nucleoplasm. When these cells are permeabilized, EAST-GFP rapidly associated with polytene chromosomes. The affinity to chromatin increases and mobility decreases with decreasing salt concentration. Deleting the C-terminal residues 1535 to 2301 of EAST strongly reduces the affinity to polytene chromosomes. The bulk of EAST-GFP co-localizes with heterochromatin and is absent from transcriptionally active chromosomal regions. The predominantly chromosomal localization of EAST-GFP can be detected in non-detergent treated salivary glands of pupae as they undergo apoptosis, however not in earlier stages of development. Consistent with this chromosomal pattern of localization, genetic evidence indicates a role for EAST in the repression of gene expression, since a lethal east mutation is allelic to the viable mutation suppressor of white-spotted. We propose that EAST acts as an ion sensor that modulates gene expression in response to changing intracellular ion concentrations.

Direct induction of autophagy by Atg1 inhibits cell growth and induces apoptotic cell death

BACKGROUND

To survive starvation and other forms of stress, eukaryotic cells undergo a lysosomal process of cytoplasmic degradation known as autophagy. Autophagy has been implicated in a number of cellular and developmental processes, including cell-growth control and programmed cell death. However, direct evidence of a causal role for autophagy in these processes is lacking, resulting in part from the pleiotropic effects of signaling molecules such as TOR that regulate autophagy. Here, we circumvent this difficulty by directly manipulating autophagy rates in Drosophila through the autophagy-specific protein kinase Atg1.

RESULTS

We find that overexpression of Atg1 is sufficient to induce high levels of autophagy, the first such demonstration among wild-type Atg proteins. In contrast to findings in yeast, induction of autophagy by Atg1 is dependent on its kinase activity. We find that cells with high levels of Atg1-induced autophagy are rapidly eliminated, demonstrating that autophagy is capable of inducing cell death. However, this cell death is caspase dependent and displays DNA fragmentation, suggesting that autophagy represents an alternative induction of apoptosis, rather than a distinct form of cell death. In addition, we demonstrate that Atg1-induced autophagy strongly inhibits cell growth and that Atg1 mutant cells have a relative growth advantage under conditions of reduced TOR signaling. Finally, we show that Atg1 expression results in negative feedback on the activity of TOR itself.

CONCLUSIONS

Our results reveal a central role for Atg1 in mounting a coordinated autophagic response and demonstrate that autophagy has the capacity to induce cell death. Furthermore, this work identifies autophagy as a critical mechanism by which inhibition of TOR signaling leads to reduced cell growth.

Proteomic analysis of steroid-triggered autophagic programmed cell death during Drosophila development

Two morphological forms of programmed cell death, apoptosis and autophagic cell death, remove unneeded or damaged cells during animal development. Although the mechanisms that regulate apoptosis are well studied, little is known about autophagic cell death. A shotgun proteome analysis of purified dying larval salivary glands in Drosophila was used to identify proteins that are expressed during autophagic programmed cell death. A total of 5661 proteins were identified from stages before and after the onset of cell death. Analyses of these data enabled us to identify proteins from a number of interesting categories including regulators of transcription, the apoptosis, autophagy, lysosomal, and ubiquitin proteasome degradation pathways, and proteins involved in growth control. Several of the identified proteins, including the serine/threonine kinase warts (Wts), were not detected using whole-genome DNA microarrays, providing support for the importance of such high-throughput proteomic technology. Wts regulates cell-cycle arrest and apoptosis, and significantly, mutations in wts prevent destruction of salivary glands.

NF-kappaB inhibition sensitizes to starvation-induced cell death in high-risk myelodysplastic syndrome and acute myeloid leukemia

CD34(+) bone marrow blasts from high-risk myelodysplastic syndrome (MDS) patients as well as MDS patient-derived cell lines (P39 and MOLM13) constitutively activate the nuclear factor-kappaB (NF-kappaB) pathway and undergo apoptosis when NF-kappaB is inhibited. Here, we show that the combination of conventional chemotherapeutic agents (daunorubicin, mitoxantrone, 5-azacytidine or camptothecin) with the NF-kappaB inhibitor BAY11-7082 did not yield a synergistic cytotoxicity. In contrast, BAY11-7082 (which targets the NF-kappaB-activating I-kappaB kinase (IKK) complex) or knockdown of essential components of the NF-kappaB system (such as the IKK1 and IKK2 subunits of the IKK complex and the p65 subunit of NF-kappaB), by small interfering RNAs sensitized MDS cell lines to starvation-induced apoptosis. The combination of BAY11-7082 and nutrient depletion synergistically killed the acute myeloid leukemia (AML) cell line U937 as well as primary CD34(+) bone marrow blasts from AML and high-risk MDS patients. The synergistic killing by BAY11-7082, combined with nutrient depletion, led to cell death accompanied by all hallmarks of apoptosis, including an early loss of the mitochondrial transmembrane potential, the release of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria, activation of caspase-3, phosphatidylserine exposure on the plasma membrane surface and nuclear chromatin condensation. Transmission electron microscopy revealed the presence of numerous autophagic vacuoles in the cytoplasm before cells underwent nuclear apoptosis. Nonetheless, cell death was neither inhibited by the pan-caspase inhibitor z-VAD-fmk nor by knockdown of AIF or of essential components of the autophagy pathway (ATG5, ATG6/Beclin-1, ATG10, ATG12). In contrast, external supply of glucose, insulin or insulin-like growth factor-I could retard the cell death induced by BAY11-7082 combined with starvation. These results suggest that in MDS cells, NF-kappaB inhibition can precipitate a bioenergetic crisis that leads to an autophagic stress response followed by apoptotic cell death.

Autophagy and cell death

Autophagy is a physiological and evolutionarily conserved phenomenon maintaining homeostatic functions like protein degradation and organelle turnover. It is rapidly upregulated under conditions leading to cellular stress, such as nutrient or growth factor deprivation, providing an alternative source of intracellular building blocks and substrates for energy generation to enable continuous cell survival. Yet accumulating data provide evidence that the autophagic machinery can be also recruited to kill cells under certain conditions generating a caspase-independent form of programed cell death (PCD), named autophagic cell death. Due to increasing interest in nonapoptotic PCD forms and the development of mammalian genetic tools to study autophagy, autophagic cell death has achieved major prominence, and is recognized now as a legitimate alternative death pathway to apoptosis. This chapter aims at summarizing the recent data in the field of autophagy signaling and autophagic cell death.

Caspase-dependent cell death in Drosophila

Cell death plays many roles during development, in the adult, and in the genesis of many pathological states. Much of this death is apoptotic in nature and requires the activity of members of the caspase family of proteases. It is now possible uniquely in Drosophila to carry out genetic screens for genes that determine the fate-life or death-of any population of cells during development and adulthood. This, in conjunction with the ability to obtain biochemical quantities of material, has made Drosophila a useful organism for exploring the mechanisms by which apoptosis is carried out and regulated. This review summarizes our knowledge of caspase-dependent cell death in Drosophila and compares that knowledge with what is known in worms and mammals. We also discuss the significance of recent work showing that a number of key cell death activators also play nonapoptotic roles. We highlight opportunities and outstanding questions along the way.

Inducible cell death in plant immunity

Programmed cell death (PCD) occurs during vegetative and reproductive plant growth, as typified by autumnal leaf senescence and the terminal differentiation of the endosperm of cereals which provide our major source of food. PCD also occurs in response to environmental stress and pathogen attack, and these inducible PCD forms are intensively studied due their experimental tractability. In general, evidence exists for plant cell death pathways which have similarities to the apoptotic, autophagic and necrotic forms described in yeast and metazoans. Recent research aiming to understand these pathways and their molecular components in plants are reviewed here.

Pupal Remodeling and the Development and Evolution of Sexual Dimorphism in Horned Beetles

Horns or hornlike structures in beetles have become an increasingly popular study system for exploring the evolution and development of secondary sexual trait diversity and sexual dimorphisms. The horns of adult beetles originate during a rapid growth phase during the prepupal stage of larval development, and differential activation of growth during this time is either implicitly or explicitly assumed to be the sole mechanism underlying intra- and interspecific differences in adult horn expression. Here I show that this assumption is not based on developmental reality. Instead, after their initial prepupal growth phase, beetle horns are extensively remodeled during the subsequent pupal stage via sex- and size-dependent resorption of horn tissue. I show that adult sexual dimorphism in four Onthophagus species is shaped partly or entirely by such pupal remodeling rather than by differential growth. Specifically, I show that after a sexually monomorphic growth phase, differential pupal horn resorption can generate both regular and reversed sexual dimorphism. Furthermore, I show that in cases in which initial growth is already dimorphic, pupal horn resorption can both magnify and reverse initial dimorphism resulting from differential growth. Finally, I show that complete resorption of pupal horns in both sexes can remove any trace of horn expression from all resulting adults. In such species, examination of adults only would result in the false conclusion that this species lacks the ability to develop a horn. Instead, such species appear to differ from those with sexually dimorphic adults merely in that they activate pupal horn resorption in both sexes rather than in just one. Combined, these results suggest that pupal remodeling of secondary trait expression is taxonomically widespread, at least among Onthophagus species, and is developmentally extensive and remarkably evolutionarily labile. These results have immediate implications for reconstructing the evolutionary history of horned beetles and the role of developmental processes in guiding evolutionary trajectories. I use these results to revise current understanding of the evolutionary developmental biology of secondary sexual traits in horned beetles in particular and holometabolous insects in general. The results presented here seriously call into question whether descriptions of adult diversity patterns alone suffice for meaningful inferences toward understanding the developmental and evolutionary origin of these patterns. These results illustrate that a lasting integration of development into an evolutionary framework must integrate development as a process rather than define it solely by some of its products.

Expression of Two Ecdysteroid-Regulated Genes,Broad-ComplexandE75, in the Brain and Ovary of the Honeybee (Apis mellifera L.)

We previously demonstrated that two ecdysteroid-regulated genes, Mblk-1/E93 and E74, are expressed selectively in Kenyon cell subtypes in the mushroom bodies of the honeybee (Apis mellifera L.) brain. To further examine the possible involvement of ecdysteroid-regulated genes in brain function as well as in oogenesis in the honeybee, we isolated cDNAs for two other ecdysteroid-regulated genes, Broad-Complex (BR-C) and E75, and analyzed their expression in the worker brain as well as in the queen abdomen. In situ hybridization revealed that BR-C, like Mblk-1/ E93, is expressed selectively in the large-type Kenyon cells of the mushroom bodies in the worker brain, whereas E75 is expressed in all mushroom body neuron subtypes, suggesting a difference in the mode of response to ecdysteroid among Kenyon cell subtypes. In the queen ovary, both BR-C and E75 are expressed preferentially in the follicle cells that surround egg cells at the late stage, suggesting their role in oogenesis. These results suggest that BR-C and E75 are involved in the regulation of brain function as well as in reproductive physiology in the adult honeybee.

Calpain is required for macroautophagy in mammalian cells

Ubiquitously expressed micro- and millicalpain, which both require the calpain small 1 (CAPNS1) regulatory subunit for function, play important roles in numerous biological and pathological phenomena. We have previously shown that the product of GAS2, a gene specifically induced at growth arrest, is an inhibitor of millicalpain and that its overexpression sensitizes cells to apoptosis in a p53-dependent manner (Benetti, R., G. Del Sal, M. Monte, G. Paroni, C. Brancolini, and C. Schneider. 2001. EMBO J. 20:2702–2714). More recently, we have shown that calpain is also involved in nuclear factor κB activation and its relative prosurvival function in response to ceramide, in which calpain deficiency strengthens the proapoptotic effect of ceramide (Demarchi, F., C. Bertoli, P.A. Greer, and C. Schneider. 2005. Cell Death Differ. 12:512–522). Here, we further explore the involvement of calpain in the apoptotic switch and find that in calpain-deficient cells, autophagy is impaired with a resulting dramatic increase in apoptotic cell death. Immunostaining of the endogenous autophagosome marker LC3 and electron microscopy experiments demonstrate that autophagy is impaired in CAPNS1-deficient cells. Accordingly, the enhancement of lysosomal activity and long-lived protein degradation, which normally occur upon starvation, is also reduced. In CAPNS1-depleted cells, ectopic LC3 accumulates in early endosome-like vesicles that may represent a salvage pathway for protein degradation when autophagy is defective.

Activation of Apoptotic Caspase Cascade During the Transition to Pressure Overload-Induced Heart Failure

OBJECTIVES

A pressure overload model was developed to simulate aortic stenosis and assess caspase activity during the transition to heart failure.

BACKGROUND

Cardiomyocyte apoptosis is implicated in the pathogenesis of heart failure, and caspase activation is central to this pathophysiological process.

METHODS

A total of 10 sheep were banded with variable aortic constriction devices, progressively inflated to increase left ventricular (LV) afterload. Serial LV endomyocardial biopsy samples were obtained to measure caspase activity and presence of apoptosis.

RESULTS

Over the first 3 to 4 weeks, hypertrophy developed in the sheep (LV mass index 90.8 +/- 4.9 g/m2 vs. 44.0 +/- 3.0 g/m2, p < 0.01), followed by gradual dilatation of the left ventricle (diastolic LV internal diameter 4.23 +/- 0.08 cm vs. 3.39 +/- 0.07 cm, p < 0.01). Ventricular function remained stable until 7 to 8 weeks after banding, when there was significant deterioration (fractional shortening 18.3 +/- 2.4% vs. 46.9 +/- 2.6%, p < 0.01), associated with clinical heart failure. Serial LV endomyocardial biopsy samples were obtained at each echocardiographically defined stage (LV hypertrophy, LV dilation, and LV failure). Activity of caspases-3, -8, and -9 (measured by specific fluorogenic peptide substrates and immunohistochemistry) increased progressively, particularly with the onset of myocardial dysfunction (caspase-3 7.92 +/- 1.19 vs. 1.00 +/- 0.15, caspase-8 1.94 +/- 0.21 vs. 1.00 +/- 0.04, caspase-9 5.87 +/- 0.97 vs. 1.00 +/- 0.18 relative fluorescent units, p < 0.05). No evidence of deoxyribonucleic acid (DNA) fragmentation, however, was identified by immunohistochemical assays.

CONCLUSIONS

Activation of cardiomyocyte caspase enzymes occurs during the transition to heart failure, without completion of apoptotic DNA fragmentation. Increased activity of caspase-8 and -9 suggests both mitochondrial and death-receptor mediated pathways are involved in this pathological process. Further knowledge of these pathways may stimulate development of apoptosis-based strategies for slowing progression of heart failure in aortic stenosis patients.

Mechanisms of programmed cell death during oogenesis in Drosophila virilis

We describe the features of programmed cell death occurring in the egg chambers of Drosophila virilis during mid-oogenesis and late oogenesis. During mid-oogenesis, the spontaneously degenerating egg chambers exhibit typical characteristics of apoptotic cell death. As revealed by propidium iodide, rhodamine-conjugated phalloidin staining, and the TUNEL assay, respectively, the nurse cells contain condensed chromatin, altered actin cytoskeleton, and fragmented DNA. In vitro caspase activity assays and immunostaining procedures demonstrate that the atretic egg chambers possess high levels of caspase activity. Features of autophagic cell death are also observed during D. virilis mid-oogenesis, as shown by monodansylcadaverine staining, together with an ultrastructural examination by transmission electron microscopy. During the late stages of oogenesis in D. virilis, once again, the two mechanisms, viz., nurse cell cluster apoptosis and autophagy, operate together, manifesting features of cell death similar to those detailed above. Moreover, an altered form of cytochrome c seems to be released from the mitochondria in the nurse cells proximal to the oocyte. We propose that apoptosis and autophagy function synergistically during oogenesis in D. virilis in order to achieve a more efficient elimination of the degenerated nurse cells and abnormal egg chambers.

Programmed cell death of the ovarian nurse cells during oogenesis of the silkmoth Bombyx mori

In the present study, we describe the features of programmed cell death of the ovarian nurse cells occurring during vitellogenesis of the silkmoth Bombyx mori. At developmental stage 5, the nurse cells occupy one-half of the follicular volume and obtain a rather spherical shape, while the nurse cell nuclei appear large and elongated, forming impressive projections. At the following stage, stage 6, the nurse cells decrease in size and their shape becomes elliptic. The nuclei remain elongated, being also characterized by large lobes. The lobes of the ramified nurse cell nuclei seem to retain the nucleus in the center of the cell during the dumping of the nurse cell cytoplasm into the growing oocyte. At stage 7, membrane enclosed vacuoles can be easily detected into the nurse cells cytoplasm. Ultrastructural analysis and fluorescent microscopy using mono-dansyl-cadaverine staining of these vacuoles also reveal that they represent autolysosomes. Caspase activity is detected during stage 7, as it is demonstrated by using the Red-VAD-FMK staining reagent. At developmental stages 8 and 9, the nurse cells exhibit chromatin condensation, DNA fragmentation and caspase activity. Finally, during the following stage 10, the nuclear remnants are assembled into apoptotic vesicles, which, after being phagocytosed, are observed in the cytoplasm of adjacent follicle cells. We propose that apoptosis and autophagy operate synergistically during vitellogenesis of B. mori, in order to achieve an efficient and rapid clearance of the degenerated nurse cell cluster.