Control of the cell death pathway by Dapaf-1, a Drosophila Apaf-1/CED-4-related caspase activator

Dark and Dronc activation in Drosophila melanogaster

The onset of apoptosis is characterized by a cascade of caspase activation, where initiator caspases are activated by a multimeric adaptor complex known as the apoptosome. In Drosophila melanogaster, the initiator caspase Dronc undergoes autocatalytic activation in the presence of the Dark apoptosome. Despite rigorous investigations, the activation mechanism for Dronc remains elusive. Here, we report the cryo-EM structures of an auto-inhibited Dark monomer and a single-layered, multimeric Dark/Dronc complex. Our biochemical analysis suggests that the auto-inhibited Dark oligomerizes upon binding to Dronc, which is sufficient for the activation of both Dark and Dronc. In contrast, the previously observed double-ring Dark apoptosome may represent a non-functional or "off-pathway" conformation. These findings expand our understanding on the molecular mechanism of apoptosis in Drosophila.

The appearance of cytoplasmic cytochrome C precedes apoptosis during Drosophila salivary gland degradation

Apoptosis is an important process for organism development that functions to eliminate cell damage, maintain homeostasis, and remove obsolete tissues during morphogenesis. In mammals, apoptosis is accompanied by the release of cytochrome C (Cyt-c) from mitochondria to the cytoplasm. However, whether this process is conserved in the fruit fly, Drosophila melanogaster, remains controversial. In this study, we discovered that during the degradation of Drosophila salivary gland, the transcription of mitochondria apoptosis factors (MAPFs), Cyt-c, and death-associated APAF1-related killer (Dark) encoding genes are all upregulated antecedent to initiator and effector caspases encoding genes. The proteins Cyt-c and the active caspase 3 appear gradually in the cytoplasm during salivary gland degradation. Meanwhile, the Cyt-c protein colocates with mito-GFP, the marker indicating cytoplasmic mitochondria, and the change in mitochondrial membrane potential coincides with the appearance of Cyt-c in the cytoplasm. Moreover, impeding or promoting 20E-induced transcription factor E93 suppresses or enhances the staining of Cyt-c and the active caspase 3 in the cytoplasm of salivary gland, and accordingly decreases or increases the mitochondrial membrane potential, respectively. Our research provides evidence that cytoplasmic Cyt-c appears before apoptosis during Drosophila salivary gland degradation, shedding light on partial conserved mechanism in apoptosis between insects and mammals.

Drosophila as a model to explore secondary injury cascades after traumatic brain injury

Drosophilae are emerging as a valuable model to study traumatic brain injury (TBI)-induced secondary injury cascades that drive persisting neuroinflammation and neurodegenerative pathology that imposes significant risk for long-term neurological deficits. As in mammals, TBI in Drosophila triggers axonal injury, metabolic crisis, oxidative stress, and a robust innate immune response. Subsequent neurodegeneration stresses quality control systems and perpetuates an environment for neuroprotection, regeneration, and delayed cell death via highly conserved cell signaling pathways. Fly injury models continue to be developed and validated for both whole-body and head-specific injury to isolate, evaluate, and modulate these parallel pathways. In conjunction with powerful genetic tools, the ability for longitudinal evaluation, and associated neurological deficits that can be tested with established behavioral tasks, Drosophilae are an attractive model to explore secondary injury cascades and therapeutic intervention after TBI. Here, we review similarities and differences between mammalian and fly pathophysiology and highlight strategies for their use in translational neurotrauma research.

Non-apoptotic enteroblast-specific role of the initiator caspase Dronc for development and homeostasis of the Drosophila intestine

The initiator caspase Dronc is the only CARD-domain containing caspase in Drosophila and is essential for apoptosis. Here, we report that homozygous dronc mutant adult animals are short-lived due to the presence of a poorly developed, defective and leaky intestine. Interestingly, this mutant phenotype can be significantly rescued by enteroblast-specific expression of dronc⁺ in dronc mutant animals, suggesting that proper Dronc function specifically in enteroblasts, one of four cell types in the intestine, is critical for normal development of the intestine. Furthermore, enteroblast-specific knockdown of dronc in adult intestines triggers hyperplasia and differentiation defects. These enteroblast-specific functions of Dronc do not require the apoptotic pathway and thus occur in a non-apoptotic manner. In summary, we demonstrate that an apoptotic initiator caspase has a very critical non-apoptotic function for normal development and for the control of the cell lineage in the adult midgut and therefore for proper physiology and homeostasis.

Salvador-Warts-Hippo pathway regulates sensory organ development via caspase-dependent nonapoptotic signaling

The fundamental roles for the Salvador-Warts-Hippo (SWH) pathway are widely characterized in growth regulation and organ size control. However, the function of SWH pathway is less known in cell fate determination. Here we uncover a novel role of the SWH signaling pathway in determination of cell fate during neural precursor (sensory organ precursor, SOP) development. Inactivation of the SWH pathway in SOP of the wing imaginal discs affects caspase-dependent bristle patterning in an apoptosis-independent process. Such nonapoptotic functions of caspases have been implicated in inflammation, proliferation, cellular remodeling, and cell fate determination. Our data indicate an effect on the Wingless (Wg)/Wnt pathway. Previously, caspases were proposed to cleave and activate a negative regulator of Wg/Wnt signaling, Shaggy (Sgg)/GSK3β. Surprisingly, we found that a noncleavable form of Sgg encoded from the endogenous locus after CRISPR-Cas9 modification supported almost normal bristle patterning, indicating that Sgg might not be the main target of the caspase-dependent nonapoptotic process. Collectively, our results outline a new function of SWH signaling that crosstalks to caspase-dependent nonapoptotic signaling and Wg/Wnt signaling in neural precursor development, which might be implicated in neuronal pathogenesis.

Cullin-4B E3 ubiquitin ligase mediates Apaf-1 ubiquitination to regulate caspase-9 activity

Apoptotic protease-activating factor 1 (Apaf-1) is a component of apoptosome, which regulates caspase-9 activity. In addition to apoptosis, Apaf-1 plays critical roles in the intra-S-phase checkpoint; therefore, impaired expression of Apaf-1 has been demonstrated in chemotherapy-resistant malignant melanoma and nuclear translocation of Apaf-1 has represented a favorable prognosis of patients with non-small cell lung cancer. In contrast, increased levels of Apaf-1 protein are observed in the brain in Huntington’s disease. The regulation of Apaf-1 protein is not yet fully understood. In this study, we show that etoposide triggers the interaction of Apaf-1 with Cullin-4B, resulting in enhanced Apaf-1 ubiquitination. Ubiquitinated Apaf-1, which was degraded in healthy cells, binds p62 and forms aggregates in the cytosol. This complex of ubiquitinated Apaf-1 and p62 induces caspase-9 activation following MG132 treatment of HEK293T cells that stably express bcl-xl. These results show that ubiquitinated Apaf-1 may activate caspase-9 under conditions of proteasome impairment.

New insights into apoptosome structure and function

The apoptosome is a platform that activates apical procaspases in response to intrinsic cell death signals. Biochemical and structural studies in the past two decades have extended our understanding of apoptosome composition and structure, while illuminating the requirements for initiator procaspase activation. A number of studies have now provided high-resolution structures for apoptosomes from C. elegans (CED-4), D. melanogaster (Dark), and H. sapiens (Apaf-1), which define critical protein interfaces, including intra and interdomain interactions. This work also reveals interactions of apoptosomes with their respective initiator caspases, CED-3, Dronc and procaspase-9. Structures of the human apoptosome have defined the requirements for cytochrome c binding, which triggers the conversion of inactive Apaf-1 molecules to an extended, assembly competent state. While recent data have provided a detailed understanding of apoptosome formation and procaspase activation, they also highlight important evolutionary differences with functional implications for caspase activation. Comparison of the CARD/CARD disks and apoptosomes formed by CED-4, Dark and Apaf-1. Cartoons of the active states of the CARD-CARD disks, illustrating the two CED-4 CARD tetrameric ring layers (CED4a and CED4b; top row) and the binding of 8 Dronc CARDs and between 3-4 pc-9 CARDs, to the Dark and Apaf-1 CARD disk respectively (middle and lower rows). Ribbon diagrams of the active CED-4, Dark and Apaf-1 apoptosomes are shown (right column).

Starfish Apaf-1 activates effector caspase-3/9 upon apoptosis of aged eggs

Caspase-3-related DEVDase activity is initiated upon apoptosis in unfertilized starfish eggs. In this study, we cloned a starfish procaspase-3 corresponding to mammalian effector caspase containing a CARD that is similar to the amino terminal CARD of mammalian capsase-9, and we named it procaspase-3/9. Recombinant procaspase-3/9 expressed at 15 °C was cleaved to form active caspase-3/9 which has DEVDase activity. Microinjection of the active caspase-3/9 into starfish oocytes/eggs induced apoptosis. An antibody against the recombinant protein recognized endogenous procaspase-3/9 in starfish oocytes, which was cleaved upon apoptosis in aged unfertilized eggs. These results indicate that caspase-3/9 is an effector caspase in starfish. To verify the mechanism of caspase-3/9 activation, we cloned starfish Apaf-1 containing a CARD, a NOD, and 11 WD40 repeat regions, and we named it sfApaf-1. Recombinant sfApaf-1 CARD interacts with recombinant caspase-3/9 CARD and with endogenous procaspase-3/9 in cell-free preparations made from starfish oocytes, causing the formation of active caspase-3/9. When the cell-free preparation without mitochondria was incubated with inactive recombinant procaspase-3/9 expressed at 37 °C, DEVDase activity increased and apoptosome-like complexes were formed in the high molecular weight fractions containing both sfApaf-1 and cleaved caspase-3/9. These results suggest that sfApaf-1 activation is not dependent on cytochrome c.

Caspase-dependent non-apoptotic processes in development

Caspases are at the core of executing apoptosis by orchestrating cellular destruction with proteolytic cascades. Caspase-mediated proteolysis also controls diverse nonlethal cellular activities such as proliferation, differentiation, cell fate decision, and cytoskeletal reorganization. During the last decade or so, genetic studies of Drosophila have contributed to our understanding of the in vivo mechanism of the non-apoptotic cellular responses in developmental contexts. Furthermore, recent studies using C. elegans suggest that apoptotic signaling may play unexpected roles, which influence ageing and normal development at the organism level. In this review, we describe how the caspase activity is elaborately controlled during vital cellular processes at the level of subcellular localization, the duration and timing to avoid full apoptotic consequences, and also discuss the novel roles of non-apoptotic caspase signaling in adult homeostasis and physiology.

An inhibitory mono-ubiquitylation of the Drosophila initiator caspase Dronc functions in both apoptotic and non-apoptotic pathways

Apoptosis is an evolutionary conserved cell death mechanism, which requires activation of initiator and effector caspases. The Drosophila initiator caspase Dronc, the ortholog of mammalian Caspase-2 and Caspase-9, has an N-terminal CARD domain that recruits Dronc into the apoptosome for activation. In addition to its role in apoptosis, Dronc also has non-apoptotic functions such as compensatory proliferation. One mechanism to control the activation of Dronc is ubiquitylation. However, the mechanistic details of ubiquitylation of Dronc are less clear. For example, monomeric inactive Dronc is subject to non-degradative ubiquitylation in living cells, while ubiquitylation of active apoptosome-bound Dronc triggers its proteolytic degradation in apoptotic cells. Here, we examined the role of non-degradative ubiquitylation of Dronc in living cells in vivo, i.e. in the context of a multi-cellular organism. Our in vivo data suggest that in living cells Dronc is mono-ubiquitylated on Lys78 (K78) in its CARD domain. This ubiquitylation prevents activation of Dronc in the apoptosome and protects cells from apoptosis. Furthermore, K78 ubiquitylation plays an inhibitory role for non-apoptotic functions of Dronc. We provide evidence that not all of the non-apoptotic functions of Dronc require its catalytic activity. In conclusion, we demonstrate a mechanism whereby Dronc’s apoptotic and non-apoptotic activities can be kept silenced in a non-degradative manner through a single ubiquitylation event in living cells.

Apoptosis is a programmed cell death mechanism which is conserved from flies to humans. Apoptosis is mediated by proteases, termed caspases that cleave cellular proteins and trigger the death of the cell. Activation of caspases is regulated at various levels such as protein-protein interaction for initiator caspases and ubiquitylation. Caspase 9 in mammals and its Drosophila ortholog Dronc carry a protein-protein interaction domain (CARD) in their prodomain which interacts with scaffolding proteins to form the apoptosome, a cell-death platform. Here, we show that Dronc is mono-ubiquitylated at Lysine 78 in its CARD domain. This ubiquitylation interferes with the formation of the apoptosome, causing inhibition of apoptosis. In addition to its apoptotic function, Dronc also participates in events where caspase activity is not required for cell killing, but for regulating other functions, so-called non-apoptotic functions of caspases such as apoptosis-induced proliferation. We found that mono-ubiquitylation of Lysine 78 plays an inhibitory role for these non-apoptotic functions of Dronc. Interestingly, we demonstrate that the catalytic activity of Dronc is not strictly required in these processes. Our in vivo study sheds light on how a single mono-ubiquitylation event could inhibit both apoptotic and non-apoptotic functions of a caspase.

A Near-Atomic Structure of the Dark Apoptosome Provides Insight into Assembly and Activation

In Drosophila, the Apaf-1-related killer (Dark) forms an apoptosome that activates procaspases. To investigate function, we have determined a near-atomic structure of Dark double rings using cryo-electron microscopy. We then built a nearly complete model of the apoptosome that includes 7- and 8-blade β-propellers. We find that the preference for dATP during Dark assembly may be governed by Ser325, which is in close proximity to the 2' carbon of the deoxyribose ring. Interestingly, β-propellers in V-shaped domains of the Dark apoptosome are more widely separated, relative to these features in the Apaf-1 apoptosome. This wider spacing may be responsible for the lack of cytochrome c binding to β-propellers in the Dark apoptosome. Our structure also highlights the roles of two loss-of-function mutations that may block Dark assembly. Finally, the improved model provides a framework to understand apical procaspase activation in the intrinsic cell death pathway.

Nerve-racking - apoptotic and non-apoptotic roles of caspases in the nervous system of Drosophila

Studies using Drosophila as a model system have contributed enormously to our knowledge of caspase function and regulation. Caspases are best known as central executioners of apoptosis but also control essential physiological processes in a non-apoptotic manner. The Drosophila genome codes for seven caspases and in this review we provide an overview of current knowledge about caspase function in the nervous system. Caspases regulate neuronal death at all developmental stages and in various neuronal populations. In contrast, non-apoptotic roles are less well understood. The development of new genetically encoded sensors for caspase activity provides unprecedented opportunities to study caspase function in the nervous system in more detail. In light of these new tools we discuss the potential of Drosophila as a model to discover new apoptotic and non-apoptotic neuronal roles of caspases.

Cell Death Signaling

In multicellular organisms, cell death is a critical and active process that maintains tissue homeostasis and eliminates potentially harmful cells. There are three major types of morphologically distinct cell death: apoptosis (type I cell death), autophagic cell death (type II), and necrosis (type III). All three can be executed through distinct, and sometimes overlapping, signaling pathways that are engaged in response to specific stimuli. Apoptosis is triggered when cell-surface death receptors such as Fas are bound by their ligands (the extrinsic pathway) or when Bcl2-family proapoptotic proteins cause the permeabilization of the mitochondrial outer membrane (the intrinsic pathway). Both pathways converge on the activation of the caspase protease family, which is ultimately responsible for the dismantling of the cell. Autophagy defines a catabolic process in which parts of the cytosol and specific organelles are engulfed by a double-membrane structure, known as the autophagosome, and eventually degraded. Autophagy is mostly a survival mechanism; nevertheless, there are a few examples of autophagic cell death in which components of the autophagic signaling pathway actively promote cell death. Necrotic cell death is characterized by the rapid loss of plasma membrane integrity. This form of cell death can result from active signaling pathways, the best characterized of which is dependent on the activity of the protein kinase RIP3.

The multifaceted activity of insect caspases

Caspases are frequently considered synonymous with apoptotic cell death. Increasing evidence demonstrates that these proteases may exert their activities in non-apoptotic functions. The non-apoptotic roles of caspases may include developmentally regulated autophagy during insect metamorphosis, as well as neuroblast self-renewal and the immune response. Here, we summarize the established knowledge and the recent advances in the multiple roles of insect caspases to highlight their relevance for physiological processes and survival.

Structure of the apoptosome: mechanistic insights into activation of an initiator caspase from Drosophila

The autocatalytic activation of an initiator caspase, exemplified by caspase-9 in mammals or its ortholog, Dronc, in fruit flies, is facilitated by a multimeric adaptor complex known as the apoptosome. Pang et al. report two cryo-EM structures: the complete Dark apoptosome at an overall resolution of 4.0 Å and a complex between the Dark apoptosome and the CARD of Dronc at 4.1 Å resolution. The structural findings, together with structure-guided biochemical analyses, allow delineation of the molecular mechanisms for Dronc activation.

Apoptosis is executed by a cascade of caspase activation. The autocatalytic activation of an initiator caspase, exemplified by caspase-9 in mammals or its ortholog, Dronc, in fruit flies, is facilitated by a multimeric adaptor complex known as the apoptosome. The underlying mechanism by which caspase-9 or Dronc is activated by the apoptosome remains unknown. Here we report the electron cryomicroscopic (cryo-EM) structure of the intact apoptosome from Drosophila melanogaster at 4.0 Å resolution. Analysis of the Drosophila apoptosome, which comprises 16 molecules of the Dark protein (Apaf-1 ortholog), reveals molecular determinants that support the assembly of the 2.5-MDa complex. In the absence of dATP or ATP, Dronc zymogen potently induces formation of the Dark apoptosome, within which Dronc is efficiently activated. At 4.1 Å resolution, the cryo-EM structure of the Dark apoptosome bound to the caspase recruitment domain (CARD) of Dronc (Dronc-CARD) reveals two stacked rings of Dronc-CARD that are sandwiched between two octameric rings of the Dark protein. The specific interactions between Dronc-CARD and both the CARD and the WD40 repeats of a nearby Dark protomer are indispensable for Dronc activation. These findings reveal important mechanistic insights into the activation of initiator caspase by the apoptosome.

Fipronil induces apoptosis through caspase-dependent mitochondrial pathways in Drosophila S2 cells

Fipronil is the first phenylpyrazole insecticide widely used in controlling pests, including pyrethroid, organophosphate and carbamate insecticides. It is generally accepted that fipronil elicits neurotoxicity via interactions with GABA and glutamate receptors, although alternative mechanisms have recently been proposed. This study evaluates the genotoxicity of fipronil and its likely mode of action in Drosophila S2 cells, as an in vitro model. Fipronil administrated the concentration- and time-dependent S2 cell proliferation. Intracellular biochemical assays showed that fipronil-induced S2 cell apoptosis coincided with a decrease in the mitochondrial membrane potential and an increase reactive oxygen species generation, a significant decrease of Bcl-2 and DIAP1, and a marked augmentation of Cyt c and caspase-3. Because caspase-3 is the major executioner caspase downstream of caspase-9 in Drosophila, enzyme activity assays were used to determine the activities of caspase-3 and caspase-9. Our results indicated that fipronil effectively induced apoptosis in Drosophila S2 cells through caspase-dependent mitochondrial pathways.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Essential versus accessory aspects of cell death: recommendations of the NCCD 2015

Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death' (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death' (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.

Apoptosome and inflammasome: conserved machineries for caspase activation

Apoptosome and inflammasome are multimeric protein complexes that mediate the activation of specific caspases at the onset of apoptosis and inflammation. The central component of apoptosome or inflammasome is a tripartite scaffold protein, exemplified by Apaf-1 and NLRC4, which contains an amino-terminal homotypic interaction motif, a central nucleotide-binding oligomerization domain and a carboxyl-terminal ligand-sensing domain. In the absence of death cue or an inflammatory signal, Apaf-1 or NLRC4 exists in an auto-inhibited, monomeric state, which is stabilized by adenosine diphosphate (ADP). Binding to an apoptosis- or inflammation-inducing ligand, together with replacement of ADP by adenosine triphosphate (ATP), results in the formation of a multimeric apoptosome or inflammasome. The assembled apoptosome and inflammasome serve as dedicated machineries to facilitate the activation of specific caspases. In this review, we describe the structure and functional mechanisms of mammalian inflammasome and apoptosomes from three representative organisms. Emphasis is placed on the molecular mechanism of caspase activation and the shared features of apoptosomes and inflammasomes.

Gene expression and reproductive abilities of male Drosophila melanogaster subjected to ELF-EMF exposure

Extremely low frequency electromagnetic field (ELF-EMF) exposure is attracting increased attention as a possible disease-inducing factor. The in vivo effects of short-term and long-term ELF-EMF exposure on male Drosophila melanogaster were studied using transcriptomic analysis for preliminary screening and QRT-PCR for further verification. Transcriptomic analysis indicated that 439 genes were up-regulated and 874 genes were down-regulated following short-term exposures and that 514 genes were up-regulated and 1206 genes were down-regulated following long-term exposures (expression >2- or <0.5-fold, respectively). In addition, there are 238 up-regulated genes and 598 down-regulated genes in the intersection of short-term and long-term exposure (expression >2- or <0.5-fold). The DEGs (differentially expressed genes) in D. melanogaster following short-term exposures were involved in metabolic processes, cytoskeletal organization, mitotic spindle organization, cell death, protein modification and proteolysis. Long-term exposure let to changes in expression of genes involved in metabolic processes, response to stress, mitotic spindle organization, aging, cell death and cellular respiration. In the intersection of short-term and long-term exposure, a series of DEGs were related to apoptosis, aging, immunological stress and reproduction. To check the ELF-EMF effects on reproduction, some experiments on male reproduction ability were performed. Their results indicated that short-term ELF-EMF exposure may decrease the reproductive ability of males, but long-term exposures had no effect on reproductive ability. Down-regulation of ark gene in the exposed males suggests that the decrease in reproductive capacity may be induced by the effects of ELF-EMF exposure on spermatogenesis through the caspase pathway. QRT-PCR analysis confirmed that jra, ark and decay genes were down regulated in males exposed for 1 Generation (1G) and 72 h, which suggests that apoptosis may be inhibited in vivo. ELF-EMF exposure may have accelerated cell senescence, as suggested by the down-regulation of both cat and jra genes and the up-regulation of hsp22 gene. Up-regulation of totA and hsp22 genes during exposure suggests that exposed flies might induce an in vivo immune response to counter the adverse effects encountered during ELF-EMF exposure. Down-regulation of cat genes suggests that the partial oxidative protection system might be restrained, especially during short-term exposures. This study demonstrates the bioeffects of ELF-EMF exposure and provides evidence for understanding the in vivo mechanisms of ELF-EMF exposure on male D. melanogaster.

Radioresistant Sf9 insect cells undergo an atypical form of Bax-dependent apoptosis at very high doses of γ-radiation

PURPOSE

To investigate the underlying mechanisms of cell-death at extremely high doses of radiation in radioresistant Spodoptera frugiperda-9 (Sf9) insect cells.

MATERIALS AND METHODS

Morphology, cell proliferation and DNA-fragmentation analysis was performed at 500-2000 Gy. Changes in intracellular reactive oxygen species (ROS), mitochondrial membrane potential (MMP), cardiolipin oxidation and Annexin-V externalization were studied using flow-cytometry. Cytochrome-c release was measured using immunofluorescence microscopy. Inhibitors of apoptosis, i.e., Bongkrekic acid (BKA), Caspase-9 inhibitor (C9i), 5-(4-fluorosulfonylbenzoyl) adenosine hydrochloride (FSBA) and Cyclosporin-A (CsA) were used to dissect apoptotic mechanism at many classical steps. Caspase-3 activity was measured using a caspase-activity assay kit.

RESULTS

A dose-dependent induction of typical apoptosis was observed at extremely high doses, marked by extensive apoptotic body formation. However, certain atypical responses such as cellular hypertrophy and the lack of phosphatidylserine-externalization were observed during the initial hours after radiation. Loss of mitochondrial membrane potential observed at 48 h following a 2000 Gy dose was accompanied by an increase in ROS that caused significant cardiolipin oxidation leading to cytochrome-c release, caspase activation and internucleosomal DNA fragmentation. Inhibitors of B-cell lymphoma-2 (Bcl-2)-associated X protein (Bax)-mediated cytochrome-c release, apoptosome formation and caspase-9 effectively prevented radiation-induced apoptosis, strongly suggesting the role of Bax-dependent cell death mechanism.

CONCLUSIONS

Our study demonstrates that the Sf9 insect cells display good homology with human cells in the mitochondria-dependent events during radiation-induced apoptosis, although doses eliciting similar responses were 50-200 times higher than human cells. Factors upstream to mitochondrial damage remain pertinent for a thorough understanding of this extreme radioresistance displayed by lepidopteran cells.

SfDronc, an initiator caspase involved in apoptosis in the fall armyworm Spodoptera frugiperda

Initiator caspases are the first caspases that are activated following an apoptotic stimulus, and are responsible for cleaving and activating downstream effector caspases, which directly cause apoptosis. We have cloned a cDNA encoding an ortholog of the initiator caspase Dronc in the lepidopteran insect Spodoptera frugiperda. The SfDronc cDNA encodes a predicted protein of 447 amino acids with a molecular weight of 51 kDa. Overexpression of SfDronc induced apoptosis in Sf9 cells, while partial silencing of SfDronc expression in Sf9 cells reduced apoptosis induced by baculovirus infection or by treatment with UV or actinomycin D. Recombinant SfDronc exhibited several expected biochemical characteristics of an apoptotic initiator caspase: 1) SfDronc efficiently cleaved synthetic initiator caspase substrates, but had very little activity against effector caspase substrates; 2) mutation of a predicted cleavage site at position D340 blocked autoprocessing of recombinant SfDronc and reduced enzyme activity by approximately 10-fold; 3) SfDronc cleaved the effector caspase Sf-caspase-1 at the expected cleavage site, resulting in Sf-caspase-1 activation; and 4) SfDronc was strongly inhibited by the baculovirus caspase inhibitor SpliP49, but not by the related protein AcP35. These results indicate that SfDronc is an initiator caspase involved in caspase-dependent apoptosis in S. frugiperda, and as such is likely to be responsible for the initiator caspase activity in S. frugiperda cells known as Sf-caspase-X.

Apoptosome structure, assembly, and procaspase activation

Apaf-1-like molecules assemble into a ring-like platform known as the apoptosome. This cell death platform then activates procaspases in the intrinsic cell death pathway. In this review, crystal structures of Apaf-1 monomers and CED-4 dimers have been combined with apoptosome structures to provide insights into the assembly of cell death platforms in humans, nematodes, and flies. In humans, the caspase recognition domains (CARDs) of procaspase-9 and Apaf-1 interact with each other to form a CARD-CARD disk, which interacts with the platform to create an asymmetric proteolysis machine. The disk tethers multiple pc-9 catalytic domains to the platform to raise their local concentration, and this leads to zymogen activation. These findings have now set the stage for further studies of this critical activation process on the apoptosome.

Homeostatic epithelial renewal in the gut is required for dampening a fatal systemic wound response in Drosophila

Effective defense responses involve the entire organism. To maintain body homeostasis after tissue damage, a systemic wound response is induced in which the response of each tissue is tightly orchestrated to avoid incomplete recovery or an excessive, damaging response. Here, we provide evidence that in the systemic response to wounding, an apoptotic caspase pathway is activated downstream of reactive oxygen species in the midgut enterocytes (ECs), cells distant from the wound site, in Drosophila. We show that a caspase-pathway mutant has defects in homeostatic gut cell renewal and that inhibiting caspase activity in fly ECs results in the production of systemic lethal factors after wounding. Our results indicate that wounding remotely controls caspase activity in ECs, which activates the tissue stem cell regeneration pathway in the gut to dampen the dangerous systemic wound reaction.

De-regulation of JNK and JAK/STAT signaling in ESCRT-II mutant tissues cooperatively contributes to neoplastic tumorigenesis

Multiple genes involved in endocytosis and endosomal protein trafficking in Drosophila have been shown to function as neoplastic tumor suppressor genes (nTSGs), including Endosomal Sorting Complex Required for Transport-II (ESCRT-II) components vacuolar protein sorting 22 (vps22), vps25, and vps36. However, most studies of endocytic nTSGs have been done in mosaic tissues containing both mutant and non-mutant populations of cells, and interactions among mutant and non-mutant cells greatly influence the final phenotype. Thus, the true autonomous phenotype of tissues mutant for endocytic nTSGs remains unclear. Here, we show that tissues predominantly mutant for ESCRT-II components display characteristics of neoplastic transformation and then undergo apoptosis. These neoplastic tissues show upregulation of c-Jun N-terminal Kinase (JNK), Notch, and Janus Kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) signaling. Significantly, while inhibition of JNK signaling in mutant tissues partially inhibits proliferation, inhibition of JAK/STAT signaling rescues other aspects of the neoplastic phenotype. This is the first rigorous study of tissues predominantly mutant for endocytic nTSGs and provides clear evidence for cooperation among de-regulated signaling pathways leading to tumorigenesis.

Beyond apoptosis: caspase regulatory mechanisms and functions in vivo

The caspases, a family of cysteine proteases, function as central regulators of cell death. Recently, caspase activity and caspase substrates identified in the absence of cell death have sparked strong interest in caspase functions in nonapoptotic cellular responses; these functions suggest that caspases may be activated without inducing or before apoptosis, thus leading to the cleavage of a specific subset of substrates. This review focuses primarily on the caspase enzymatic activity. Detailed genetic analyses of caspase-deficient Caenorhabditis elegans, Drosophila, and mice have shown that caspases are essential, not only for controlling the number of cells involved in sculpting or deleting structures in developing animals, but also for dynamic, nonapoptotic cell processes, such as innate immune response, tissue regeneration, cell-fate determination, stem-cell differentiation and neural activation. Our understanding of the spatio-temporal caspase activation mechanisms has advanced, primarily through the study of Drosophila developmental processes. This review will discuss current findings regarding caspase functions in cytoskeletal modification, morphogenetic regulation of cell shape, cell migration and the production of mechanical force during embryogenesis.

Caspase-3 in the central nervous system: beyond apoptosis

Caspase-3 has been identified as a key mediator of neuronal programmed cell death. This protease plays a central role in the developing nervous system and its activation is observed early in neural tube formation and persists during postnatal differentiation of the neural network. Caspase-3 activation, a crucial event of neuronal cell death program, is also a feature of many chronic neurodegenerative diseases. This traditional apoptotic function of caspase-3 is challenged by recent studies that reveal new cell death-independent roles for mitochondrial-activated caspase-3 in neurite pruning and synaptic plasticity. These findings underscore the need for further research into the mechanism of action and functions of caspase-3 that may prove useful in the development of novel pharmacological treatments for a diverse range of neurological disorders.

The role of cytochrome c on apoptosis induced by Anagrapha falcifera multiple nuclear polyhedrosis virus in insect Spodoptera litura cells

There are conflicting reports on the role of cytochrome c during insect apoptosis. Our previous studies have showed that cytochrome c released from the mitochondria was an early event by western blot analysis and caspase-3 activation was closely related to cytochrome c release during apoptosis induced by baculovirus in Spodoptera litura cells (Sl-1 cell line). In the present study, alteration in mitochondrial morphology was observed by transmission electron microscopy, and cytochrome c release from mitochondria in apoptotic Sl-1 cells induced with Anagrapha falcifera multiple nuclear polyhedrosis virus (AfMNPV) has further been confirmed by immunofluoresence staining protocol, suggesting that structural disruption of mitochondria and the release of cytochrome c are important events during Lepidoptera insect cell apoptosis. We also used Sl-1 cell-free extract system and the technique of RNA interference to further investigate the role of cytochrome c in apoptotic Sl-1 cells induced by AfMNPV. Caspase-3 activity in cell- free extracts supplemented with exogenous cytochrome c was determined and showed an increase with the extension of incubation time. DsRNA-mediated silencing of cytochrome c resulted in the inhibition of apoptosis and protected the cells from AfMNPV-induced cell death. Silencing of expression of cytochrome c had a remarkable effect on pro-caspase-3 and pro-caspase-9 activation and resulted in the reduction of caspase-3 and caspase-9 activity in Sl-1 cells undergoing apoptosis. Caspase-9 inhibitor could inhibit activation of pro-caspase-3, and the inhibition of the function of Apaf-1 with FSBA blocked apoptosis, hinting that Apaf-1 could be involved in Sl-1 cell apoptosis induced by AfMNPV. Taken together, these results strongly demonstrate that cytochrome c plays an important role in apoptotic signaling pathways in Lepidopteran insect cells.

Regulation of Yorkie activity in Drosophila imaginal discs by the Hedgehog receptor gene patched

The Hedgehog (Hh) pathway was first defined by its role in segment polarity in the Drosophila melanogaster embryonic epidermis and has since been linked to many aspects of vertebrate development and disease. In humans, mutation of the Patched1 (PTCH1) gene, which encodes an inhibitor of Hh signaling, leads to tumors of the skin and pediatric brain. Despite the high level of conservation between the vertebrate and invertebrate Hh pathways, studies in Drosophila have yet to find direct evidence that ptc limits organ size. Here we report identification of Drosophila ptc in a screen for mutations that require a synergistic apoptotic block in order to drive overgrowth. Developing imaginal discs containing clones of ptc mutant cells immortalized by the concurrent loss of the Apaf-1-related killer (Ark) gene are overgrown due, in large part, to the overgrowth of wild type portions of these discs. This phenotype correlates with overexpression of the morphogen Dpp in ptc,Ark double-mutant cells, leading to elevated phosphorylation of the Dpp pathway effector Mad (p-Mad) in cells surrounding ptc,Ark mutant clones. p-Mad functions with the Hippo pathway oncoprotein Yorkie (Yki) to induce expression of the pro-growth/anti-apoptotic microRNA bantam. Accordingly, Yki activity is elevated among wild type cells surrounding ptc,Ark clones and alleles of bantam and yki dominantly suppress the enlarged-disc phenotype produced by loss of ptc. These data suggest that ptc can regulate Yki in a non-cell autonomous manner and reveal an intercellular link between the Hh and Hippo pathways that may contribute to growth-regulatory properties of the Hh pathway in development and disease.

A molecular view on signal transduction by the apoptosome

Apoptosomes are signaling platforms that initiate the dismantling of a cell during apoptosis. In mammals, assembly of the apoptosome is the pivotal point in the mitochondrial pathway of apoptosis, and is prompted by binding of cytochrome c to the apoptotic protease-activating factor 1 (Apaf-1) in the presence of ATP. The resulting wheel-like heptamer of seven molecules Apaf-1 and seven molecules cytochrome c binds and activates the initiator caspase-9, which in turn ignites the downstream caspase cascade. In this review we discuss the molecular determinants for the formation of the mammalian apoptosome and caspase activation and describe the related signaling platforms in flies and nematodes.

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

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

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

Caspase signaling in animal development

The caspases are a family of cysteine proteases that function as central regulators of cell death. Recent investigations in Caenorhabditis elegans, Drosophila, and mice indicate that caspases are essential not only in controlling the number of cells involved in sculpting or deleting structures in developing animals, but also in dynamic cell processes such as cell-fate determination, compensatory proliferation of neighboring cells, and actin cytoskeleton reorganization, in a non-apoptotic context during development. This review focuses primarily on caspase functions involving their enzymatic activity.

Who lives and who dies: Role of apoptosis in quashing developmental errors

Apoptosis is essential for normal development. Large numbers of cells are eliminated by apoptosis in early neural development and during the formation of neural connections. However, our understanding of this life-or-death decision is incomplete, because it is difficult to identify dying cells by conventional strategies. Live imaging is powerful for studying apoptosis, because it can trace a death-fated cell throughout its lifetime. The Drosophila sensory organ development is a convenient system for studying neural-cell selection via lateral inhibition. We recently showed that about 20% of the differentiating neuronal cells die during sensory organ development, which results in the characteristic spatial patterning of the sensory organs. The eliminated differentiating neurons expressed neurogenic genes and high levels of activated Notch. Thus, live imaging allowed us to document the role of apoptosis in neural progenitor selection, and revealed that Notch activation is the mechanism determining which cells die during sensory organ development.

Structure of the Drosophila apoptosome at 6.9 å resolution

The Drosophila Apaf-1 related killer forms an apoptosome in the intrinsic cell death pathway. In this study we show that Dark forms a single ring when initiator procaspases are bound. This Dark-Dronc complex cleaves DrICE efficiently; hence, a single ring represents the Drosophila apoptosome. We then determined the 3D structure of a double ring at ∼6.9 Å resolution and created a model of the apoptosome. Subunit interactions in the Dark complex are similar to those in Apaf-1 and CED-4 apoptosomes, but there are significant differences. In particular, Dark has "lost" a loop in the nucleotide-binding pocket, which opens a path for possible dATP exchange in the apoptosome. In addition, caspase recruitment domains (CARDs) form a crown on the central hub of the Dark apoptosome. This CARD geometry suggests that conformational changes will be required to form active Dark-Dronc complexes. When taken together, these data provide insights into apoptosome structure, function, and evolution.

Identification of dAven, a Drosophila melanogaster ortholog of the cell cycle regulator Aven

Aven is a regulator of the DNA-damage response and G2/M cell cycle progression. Overexpression of Aven is associated with poor prognosis in patients with childhood acute lymphoblastic leukemia and acute myeloid leukemia, and altered intracellular Aven distribution is associated with infiltrating ductal carcinoma and papillary carcinoma breast cancer subtypes. Although Aven orthologs have been identified in most vertebrate species, no Aven gene has been reported in invertebrates. Here, we describe a Drosophila melanogaster open reading frame (ORF) that shares sequence and functional similarities with vertebrate Aven genes. The protein encoded by this ORF, which we named dAven, contains several domains that are highly conserved among Aven proteins of fish, amphibian, bird and mammalian origins. In flies, knockdown of dAven by RNA interference (RNAi) resulted in lethality when its expression was reduced either ubiquitously or in fat cells using Gal4 drivers. Animals undergoing moderate dAven knockdown in the fat body had smaller fat cells displaying condensed chromosomes and increased levels of the mitotic marker phosphorylated histone H3 (PHH3), suggesting that dAven was required for normal cell cycle progression in this tissue. Remarkably, expression of dAven in Xenopus egg extracts resulted in G2/M arrest that was comparable to that caused by human Aven. Taken together, these results suggest that, like its vertebrate counterparts, dAven plays a role in cell cycle regulation. Drosophila could be an excellent model for studying the function of Aven and identifying cellular factors that influence its activity, revealing information that may be relevant to human disease.

Mitochondrial involvement in cell death of non-mammalian eukaryotes

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

A Drosophila model for primary coenzyme Q deficiency and dietary rescue in the developing nervous system

Coenzyme Q (CoQ) or ubiquinone is a lipid component of the electron transport chain required for ATP generation in mitochondria. Mutations in CoQ biosynthetic genes are associated with rare but severe infantile multisystemic diseases. CoQ itself is a popular over-the-counter dietary supplement that some clinical and rodent studies suggest might be beneficial for neurodegenerative diseases. Here, we identify mutations in the Drosophila qless gene, which encodes an orthologue of the human PDSS1 prenyl transferase that synthesizes the isoprenoid side chain of CoQ. We show that neurons lacking qless activity upregulate markers of mitochondrial stress and undergo caspase-dependent apoptosis. Surprisingly, even though experimental inhibition of caspase activity did not prevent mitochondrial disruption, it was sufficient to rescue the size of neural progenitor clones. This demonstrates that, within the developing larval CNS, qless activity is required primarily for cell survival rather than for cell growth and proliferation. Full rescue of the qless neural phenotype was achieved by dietary supplementation with CoQ4, CoQ9 or CoQ10, indicating that a side chain as short as four isoprenoid units can provide in vivo activity. Together, these findings show that Drosophila qless provides a useful model for studying the neural effects of CoQ deficiency and dietary supplementation.

Apoptogenic factors released from mitochondria

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

Metabolic regulation of Drosophila apoptosis through inhibitory phosphorylation of Dronc

Apoptosis ensures tissue homeostasis in response to developmental cues or cellular damage. Recently reported genome-wide RNAi screens have suggested that several metabolic regulators can modulate caspase activation in Drosophila. Here, we establish a previously unrecognized link between metabolism and Drosophila apoptosis by showing that cellular NADPH levels modulate the initiator caspase Dronc through its phosphorylation at S130. Depletion of NADPH removed this inhibitory phosphorylation, resulting in the activation of Dronc and subsequent cell death. Conversely, upregulation of NADPH prevented Dronc-mediated apoptosis upon DIAP1 RNAi or cycloheximide treatment. Furthermore, this CaMKII-mediated phosphorylation of Dronc hindered Dronc activation, but not its catalytic activity. Blockade of NADPH production aggravated the death-inducing activity of Dronc in specific neurons, but not in the photoreceptor cells of the eyes of transgenic flies; similarly, non-phosphorylatable Dronc was more potent than wild type in triggering specific neuronal apoptosis. Our observations reveal a novel regulatory circuitry in Drosophila apoptosis, and, as NADPH levels are elevated in cancer cells, also provide a genetic model to understand aberrations in cancer cell apoptosis resulting from metabolic alterations.

Crystal structure of the Caenorhabditis elegans apoptosome reveals an octameric assembly of CED-4

The CED-4 homo-oligomer or apoptosome is required for initiation of programmed cell death in Caenorhabditis elegans by facilitating autocatalytic activation of the CED-3 caspase zymogen. How the CED-4 apoptosome assembles and activates CED-3 remains enigmatic. Here we report the crystal structure of the complete CED-4 apoptosome and show that it consists of eight CED-4 molecules, organized as a tetramer of an asymmetric dimer via a previously unreported interface among AAA(+) ATPases. These eight CED-4 molecules form a funnel-shaped structure. The mature CED-3 protease is monomeric in solution and forms an active holoenzyme with the CED-4 apoptosome, within which the protease activity of CED-3 is markedly stimulated. Unexpectedly, the octameric CED-4 apoptosome appears to bind only two, not eight, molecules of mature CED-3. The structure of the CED-4 apoptosome reveals shared principles for the NB-ARC family of AAA(+) ATPases and suggests a mechanism for the activation of CED-3.

The role of ubiquitylation for the control of cell death in Drosophila

Ubiquitylation describes a process in which ubiquitin, a 76-amino-acid polypeptide, is covalently attached to target proteins. Traditionally, ubiquitin-conjugated proteins are targeted for degradation by the 26S proteasome. However, non-proteolytic roles in histone regulation, DNA repair and signal transduction have been reported. Here, the role of ubiquitylation in the cell death pathway in Drosophila is reviewed. Interestingly, ubiquitylation serves both pro- and anti-apoptotic functions. Although pro-apoptotic ubiquitylation leads to proteolytic degradation, recent evidence suggests that anti-apoptotic ubiquitylation may involve, at least in part, non-proteolytic functions.

The cleaved-Caspase-3 antibody is a marker of Caspase-9-like DRONC activity in Drosophila

The cleaved-Caspase-3 antibody is a popular tool in apoptosis research in Drosophila. As the antibody was raised against cleaved human Caspase-3, it was assumed that it detects cleaved DRICE and DCP-1, Caspase-3-like effector caspases in Drosophila. However, as shown here, strong immunoreactivity persists in apoptotic models doubly mutant for drICE and dcp-1. In contrast, mutants of the apoptosome components DRONC (Caspase-9-like) and ARK (Apaf-1 related) do not label with the cleaved-Caspase-3 antibody. By peptide blocking experiments and further genetic studies, we provide evidence that the cleaved-Caspase-3 antibody recognizes multiple proteins including DCP-1 and likely DRICE, but also at least one additional unknown protein, all of which require DRONC for epitope exposure. The unknown substrate may be involved in non-apoptotic functions of DRONC. Because the cleaved-Caspase-3 antibody not only detects cleaved Caspase-3-like proteins in Drosophila, but also other proteins in a DRONC-dependent manner, it is more accurate to consider the cleaved-Caspase-3 antibody as a marker for DRONC activity, rather than effector caspase activity.

Dual roles of Drosophila p53 in cell death and cell differentiation

The mammalian p53-family consists of p53, p63 and p73. While p53 accounts for tumor suppression through cell cycle arrest and apoptosis, the functions of p63 and p73 are more diverse and also include control of cell differentiation. The Drosophila genome contains only one p53 homolog, Dp53. Previous work has established that Dp53 induces apoptosis, but not cell cycle arrest. Here, by using the developing eye as a model, we show that Dp53-induced apoptosis is primarily dependent on the pro-apoptotic gene hid, but not reaper, and occurs through the canonical apoptosis pathway. Importantly, similar to p63 and p73, expression of Dp53 also inhibits cellular differentiation of photoreceptor neurons and cone cells in the eye independently of its apoptotic function. Intriguingly, expression of the human cell cycle inhibitor p21 or its Drosophila homolog dacapo can suppress both Dp53-induced cell death and differentiation defects in Drosophila eyes. These findings provide new insights into the pathways activated by Dp53 and reveal that Dp53 incorporates functions of multiple p53-family members.