Drosophila IKK-Related Kinase Regulates Nonapoptotic Function of Caspases via Degradation of IAPs

How does caspases regulation play role in cell decisions? apoptosis and beyond

Caspases are a family of cysteine proteases, and the key factors behind the cellular events which occur during apoptosis and inflammation. However, increasing evidence shows the non-conventional pro-survival action of apoptotic caspases in crucial processes. These cellular events include cell proliferation, differentiation, and migration, which may appear in the form of metastasis, and chemotherapy resistance in cancerous situations. Therefore, there should be a precise and strict control of caspases activity, perhaps through maintaining the threshold below the required levels for apoptosis. Thus, understanding the regulators of caspase activities that render apoptotic caspases as non-apoptotic is of paramount importance both mechanistically and clinically. Furthermore, the functions of apoptotic caspases are affected by numerous post-translational modifications. In the present mini-review, we highlight the various mechanisms that directly impact caspases with respect to their anti- or non-apoptotic functions. In this regard, post-translational modifications (PTMs), isoforms, subcellular localization, transient activity, substrate availability, substrate selection, and interaction-mediated regulations are discussed.

Perish in the Attempt: Regulated Cell Death in Regenerative and Nonregenerative Tissue

Significance: A cell plays its roles throughout its life span, even during its demise. Regulated cell death (RCD) is one of the key topics in modern biomedical studies. It is considered the main approach for removing stressed and/or damaged cells. Research during the past two decades revealed more roles of RCD, such as coordinating tissue development and driving compensatory proliferation during tissue repair. Recent Advances: Compensatory proliferation, initially identified in primitive organisms during the regeneration of lost tissue, is an evolutionarily conserved process that also functions in mammals. Among various types of RCD, apoptosis is considered the top candidate to induce compensatory proliferation in damaged tissue. Critical Issues: The roles of apoptosis in the recovery of nonregenerative tissue are still vague. The roles of other types of RCD, such as necroptosis and ferroptosis, have not been well characterized in the context of tissue regeneration. Future Directions: In this review article, we attempt to summarize the recent insights on the role of RCD in tissue repair. We focus on apoptosis, with expansion to ferroptosis and necroptosis, in primitive organisms with significant regenerative capacity as well as common mammalian research models. After gathering hints from regenerative tissue, in the second half of the review, we take a notoriously nonregenerative tissue, the myocardium, as an example to discuss the role of RCD in terminally differentiated quiescent cells. Antioxid. Redox Signal. 39, 1053-1069.

IKKβ regulates antimicrobial innate immune responses in the yellow mealworm, Tenebrio molitor

Toll and IMD pathways regulate antimicrobial innate immune responses in insect model systems. The transcriptional activation of antimicrobial peptides (AMPs) confers humoral immunity in the host against invaded pathogens. The IKK kinase complex (IKKα, IKKβ, and the regulatory subunit IKKγ/NEMO) centrally regulates the NF-κB response to various stimuli. It triggers an appropriate antimicrobial immune response in the host. In this study, a TmIKKβ (or TmIrd5) homolog was screened from the RNA-seq database of the coleopteran beetle, Tenebrio molitor. A single exon characterizes the TmIKKβ gene, and the open reading frame (ORF) comprises of 2112 bp that putatively encodes a polypeptide of 703 amino acid residues. TmIKKβ contains a serine/threonine kinase domain and is phylogenetically close to Tribolium castaneum IKKβ homolog (TcIKKβ). TmIKKβ transcripts were highly expressed in the early pupal (P1) and adult (A5) stages. Among the tissues, TmIKKβ showed higher expression in the integument of the last instar larvae and the fat body and hemocytes of 5-day-old adults. TmIKKβ mRNA was upregulated post-E. coli challenge to the host. Moreover, RNAi-based TmIKKβ mRNA silencing increased host larvae' susceptibility against E. coli, S. aureus and C. albicans. TmIKKβ RNAi in the fat body led to a downregulation in mRNA expression of ten out of fourteen AMP genes, including TmTenecin1, -2, and -4; TmDefensin, and -like; TmColeoptericinA, and -B; and TmAttacin1a, -1b, and -2, suggesting the requirement of the gene in antimicrobial innate immune responses. Further, a decrease in the mRNA expression of NF-κB factors such as TmRelish, TmDorsal1, and TmDorsal2 in the fat body of T. molitor larvae was observed post-microorganisms challenge. Thus, TmIKKβ regulates antimicrobial innate immune responses in T. molitor.

A caspase-RhoGEF axis contributes to the cell size threshold for apoptotic death in developing Caenorhabditis elegans

A cell's size affects the likelihood that it will die. But how is cell size controlled in this context and how does cell size impact commitment to the cell death fate? We present evidence that the caspase CED-3 interacts with the RhoGEF ECT-2 in Caenorhabditis elegans neuroblasts that generate "unwanted" cells. We propose that this interaction promotes polar actomyosin contractility, which leads to unequal neuroblast division and the generation of a daughter cell that is below the critical "lethal" size threshold. Furthermore, we find that hyperactivation of ECT-2 RhoGEF reduces the sizes of unwanted cells. Importantly, this suppresses the "cell death abnormal" phenotype caused by the partial loss of ced-3 caspase and therefore increases the likelihood that unwanted cells die. A putative null mutation of ced-3 caspase, however, is not suppressed, which indicates that cell size affects CED-3 caspase activation and/or activity. Therefore, we have uncovered novel sequential and reciprocal interactions between the apoptosis pathway and cell size that impact a cell's commitment to the cell death fate.

Non-Apoptotic Role of Apoptotic Caspases in the Drosophila Nervous System

An increasing number of studies demonstrate that cells can activate apoptotic caspases but not die and, instead, display profound changes in cellular structure and function. In this minireview, we will discuss observations in the nervous system of Drosophila melanogaster that illustrate non-apoptotic roles of apoptotic caspases. We will preface these examples with similar observations in other experimental systems and end with a discussion of how apoptotic caspase activity might be constrained to provide non-lethal functions without killing the cell.

Non-Canonical Roles of Apoptotic Caspases in the Nervous System

Caspases are a family of cysteine proteases that predominantly cleave their substrates after aspartic acid residues. Much of what we know of caspases emerged from investigation a highly conserved form of programmed cell death called apoptosis. This form of cell death is regulated by several caspases, including caspase-2, caspase-3, caspase-7, caspase-8 and caspase-9. However, these “killer” apoptotic caspases have emerged as versatile enzymes that play key roles in a wide range of non-apoptotic processes. Much of what we understand about these non-apoptotic roles is built on work investigating how “killer” caspases control a range of neuronal cell behaviors. This review will attempt to provide an up to date synopsis of these roles.

TmIKKε Is Required to Confer Protection Against Gram-Negative Bacteria, E. coli by the Regulation of Antimicrobial Peptide Production in the Tenebrio molitor Fat Body

The inhibitor of nuclear factor-kappa B (NF-κB) kinase (IKK) is the core regulator of the NF-κB pathway against pathogenic invasion in vertebrates or invertebrates. IKKβ, -ε and -γ have pivotal roles in the Toll and immune deficiency (IMD) pathways. In this study, a homolog of IKKε (TmIKKε) was identified from Tenebrio molitor RNA sequence database and functionally characterized for its role in regulating immune signaling pathways in insects. The TmIKKε gene is characterized by two exons and one intron comprising an open reading frame (ORF) of 2,196 bp that putatively encodes a polypeptide of 731 amino acid residues. TmIKKε contains a serine/threonine protein kinases catalytic domain. Phylogenetic analysis established the close homology of TmIKKε to Tribolium castaneum IKKε (TcIKKε) and its proximity with other IKK-related kinases. The expression of TmIKKε mRNA was elevated in the gut, integument, and hemocytes of the last-instar larva and the fat body, Malpighian tubules, and testis of 5-day-old adults. TmIKKε expression was significantly induced by Escherichia coli, Staphylococcus aureus, and Candida albicans challenge in whole larvae and tissues, such as hemocytes, gut, and fat body. The knockdown of the TmIKKε messenger RNA (mRNA) expression significantly reduced the survival of the larvae against microbial challenges. Further, we investigated the induction patterns of 14 T. molitor antimicrobial peptides (AMPs) genes in TmIKKε gene-silencing model after microbial challenges. While in hemocytes, the transcriptional regulation of most AMPs was negatively regulated in the gut and fat body tissue of T. molitor, AMPs, such as TmTenecin 1, TmTenecin 4, TmDefensin, TmColeoptericin A, TmColeoptericin B, TmAttacin 1a, and TmAttacin 2, were positively regulated in TmIKKε-silenced individuals after microbial challenge. Collectively, the results implicate TmIKKε as an important factor in antimicrobial innate immune responses in T. molitor.

Orally acquired cyclic dinucleotides drive dSTING-dependent antiviral immunity in enterocytes

Enteric pathogens overcome barrier immunity within the intestinal environment that includes the endogenous flora. The microbiota produces diverse ligands, and the full spectrum of microbial products that are sensed by the epithelium and prime protective immunity is unknown. Using Drosophila, we find that the gut presents a high barrier to infection, which is partially due to signals from the microbiota, as loss of the microbiota enhances oral viral infection. We report cyclic dinucleotide (CDN) feeding is sufficient to protect microbiota-deficient flies from enhanced oral infection, suggesting that bacterial-derived CDNs induce immunity. Mechanistically, we find CDN protection is dSTING- and dTBK1-dependent, leading to NF-kB-dependent gene expression. Furthermore, we identify the apical nucleoside transporter, CNT2, as required for oral CDN protection. Altogether, our studies define a role for bacterial products in priming immune defenses in the gut.

Non-lethal message from the Holy Land: The first international conference on nonapoptotic roles of apoptotic proteins

Apoptosis is a major form of programmed cell death (PCD) that eliminates unnecessary and potentially dangerous cells in all metazoan organisms, thus ensuring tissue homeostasis and many developmental processes. Accordingly, defects in the activation of the apoptotic pathway often pave the way to disease. After several decades of intensive research, the molecular details controlling the apoptosis program have largely been unraveled, as well as the regulatory mechanisms of caspase activation during apoptosis. Nevertheless, an ever-growing list of studies is suggesting the essential role of caspases and other apoptotic proteins in ensuring nonlethal cellular functions during normal development, tissue repair, and regeneration. Moreover, if deregulated, these novel nonapoptotic functions can also instigate diseases. The difficulty of identifying and manipulating the caspase-dependent nonlethal cellular processes (CDPs), as well as the nonlethal functions of other cell death proteins (NLF-CDPs), meant that CDPs and NLF-CDPs have been only curiosities within the apoptotic field; however, the recent technical advancements and the latest biological findings are assigning an unanticipated biological significance to these nonapoptotic functions. Here, we summarize the various talks presented in the first international conference fully dedicated to discuss CDPs and NFL-CDPs and named 'The Batsheva de Rothschild Seminar on Non-Apoptotic Roles of Apoptotic Proteins'. The conference was organized between September 22, 2019, and 25, 2019, by Eli Arama (Weizmann Institute of Science), Luis Alberto Baena-Lopez (University of Oxford), and Howard O. Fearnhead (NUI Galway) at the Weizmann Institute of Science in Israel, and hosted a large international group of researchers.

Non-apoptotic roles of apoptotic proteases: new tricks for an old dog

This Open Question article highlights current advances in the study of non-apoptotic roles of apoptotic proteins. Apoptosis is a highly regulated and energy-requiring process in which cells actively kill themselves. Apoptosis helps remove extra cells to sculpt organs during embryo development and culls damaged cells throughout the body. Apoptosis relies on evolutionarily conserved proteins that include a family of proteases called caspases. Caspases activity has long been considered a hallmark of apoptosis. Yet an emerging body of literature indicates that caspase activity is required for a number of non-lethal processes that range from sculpting cells, removing protein aggregates, changing cell identity during differentiation or de-differentiation, and rebuilding tissues. Failure in each of these processes is associated with human disease. This article is not meant to be an exhaustive review but an introduction to the subject for an educated public, with caspases as a gateway example. I propose that it is time to explore non-apoptotic roles of caspases and other apoptotic proteins, in order to better understand their non-apoptosis function and to leverage new knowledge into new therapies.

Rab11 activation by Ik2 kinase is required for dendrite pruning in Drosophila sensory neurons

Neuronal pruning is a commonly observed phenomenon for the developing nervous systems to ensure precise wiring of neural circuits. The function of Ik2 kinase and its downstream mediator, Spindle-F (Spn-F), are essential for dendrite pruning of Drosophila sensory neurons during development. However, little is known about how Ik2/Spn-F signaling is transduced in neurons and ultimately results in dendrite pruning. Our genetic analyses and rescue experiments demonstrated that the small GTPase Rab11, especially the active GTP-bound form, is required for dendrite pruning. We also found that Rab11 shows genetic interactions with spn-F and ik2 on pruning. Live imaging of single neurons and antibody staining reveal normal Ik2 kinase activation in Rab11 mutant neurons, suggesting that Rab11 could have a functional connection downstream of and/or parallel to the Ik2 kinase signaling. Moreover, we provide biochemical evidence that both the Ik2 kinase activity and the formation of Ik2/Spn-F/Rab11 complexes are central to promote Rab11 activation in cells. Together, our studies reveal that a critical role of Ik2/Spn-F signaling in neuronal pruning is to promote Rab11 activation, which is crucial for dendrite pruning in neurons.

During metamorphosis in Drosophila, both the central and peripheral nervous systems undergo substantial neuronal remodeling, such as the cell death of most larval neurons and regeneration of adult neurons, while few larval neurons remain alive and prune their branches. Pruning is a self-destruction program, and thus requires to be tightly controlled within single neurons spatially and temporally during development. Recent studies have shown a strong correlation between pruning and human psychiatric disorders, such as schizophrenia and autism. Drosophila sensory neurons that undergo dendrite pruning provide us an opportunity to study the regulatory mechanism of neuronal pruning. Previously, we identified an IKK-related kinase Ik2 that is essential and sufficient for dendrite pruning, and a coiled-coil protein Spindle-F that mediates Ik2-dependent pruning activity in neurons. However, what are the downstream targets of Ik2/Spindle-F signaling in dendrite pruning remains unclear. In this study, we found that the small GTPase Rab11, especially the active GTP-bound form, is required for dendrite pruning in neurons. We further demonstrated that both the Ik2 kinase activity and Ik2/Spindle-F complexes are essential to enhance Rab11 activation in neurons during dendrite pruning.

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.

Oyster Versatile IKKα/βs Are Involved in Toll-Like Receptor and RIG-I-Like Receptor Signaling for Innate Immune Response

IκB kinases (IKKs) play critical roles in innate immunity through signal-induced activation of the key transcription factors nuclear factor-κB (NF-κB) and interferon regulatory factors (IRFs). However, studies of invertebrate IKK functions remain scarce. In this study, we performed phylogenetic analysis of IKKs and IKK-related kinases encoded in the Pacific oyster genome. We then cloned and characterized the oyster IKKα/β-2 gene. We found that oyster IKKα/β-2, a homolog of human IKKα/IKKβ, responded to challenge with lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid [poly(I:C)]. As a versatile immune molecule, IKKα/β-2 activated the promoters of NF-κB, TNFα, and IFNβ, as well as IFN-stimulated response element (ISRE)-containing promoters, initiating an antibacterial or antiviral immune state in mammalian cells. Importantly, together with the cloned oyster IKKα/β-1, we investigated the signal transduction pathways mediated by these two IKKα/β proteins. Our results showed that IKKα/β-1 and IKKα/β-2 could interact with the oyster TNF receptor-associated factor 6 (TRAF6) and that IKKα/β-2 could also bind to the oyster myeloid differentiation factor 88 (MyD88) protein directly, suggesting that oyster IKKα/βs participate in both RIG-I-like receptor (RLR) and Toll-like receptor (TLR) signaling for the reception of upstream immune signals. The fact that IKKα/β-1 and IKKα/β-2 formed homodimers by interacting with themselves and heterodimers by interacting with each other, along with the fact that both oyster IKKα/β proteins interacted with NEMO protein, indicates that oyster IKKα/βs and the scaffold protein NEMO form an IKK complex, which may be a key step in phosphorylating IκB proteins and activating NF-κB. Moreover, we found that oyster IKKα/βs could interact with IRF8, and this may be related to the IKK-mediated activation of ISRE promotors and their involvement in the oyster "interferon (IFN)-like" antiviral pathway. Moreover, the expression of oyster IKKα/β-1 and IKKα/β-2 may induce the phosphorylation of IκB proteins to activate NF-κB. These results reveal the immune function of oyster IKKα/β-2 and establish the existence of mollusk TLR and RLR signaling mediated by IKKα/β proteins for the first time. Our findings should be helpful in deciphering the immune mechanisms of invertebrates and understanding the development of the vertebrate innate immunity network.

Penaeus monodon IKKs Participate in Regulation of Cytokine-Like System and Antiviral Responses of Innate Immune System

The IKK-NF-κB signaling cascade is one of the crucial responsive mechanisms in inflammatory and immune responses. The key kinase proteins called inhibitor of kappa B kinases (IKKs) serve as the core elements involved in cascade activation. Here, the complete ORFs of IKK homologs, PmIKKβ, PmIKKε1, and PmIKKε2, from the black tiger shrimp Penaeus monodon were identified and characterized for their functions in shrimp antiviral responses. The PmIKK transcripts were widely expressed in various examined tissues and the PmIKKε protein was detected in all three types of shrimp hemocytes. Only the PmIKKε1 and PmIKKε2 were responsive to white spot syndrome virus (WSSV), yellow head virus (YHV) and a bacterium Vibrio harveyi infection, while the PmIKKβ exhibited no significant response to pathogen infection. On the contrary, suppression of PmIKKβ and PmIKKε by dsRNA-mediated RNA interference (RNAi) resulted in a rapid death of WSSV-infected shrimp and the significant reduction of an IFN-like PmVago4 transcript. Whereas, the mRNA levels of the antimicrobial peptides, ALFPm3 and CrustinPm5, and a transcription factor, PmDorsal were significantly increased, those of ALFPm6, CrustinPm1, CrustinPm7, PmVago1, PmRelish, and PmCactus were unaffected. Overexpression of PmIKKβ and PmIKKε in HEK293T cells differentially activated the NF-κB and IFNβ promoter activities, respectively. These results suggest that the PmIKKβ and PmIKKε may act as common factors regulating the expression of immune-related genes from various signaling pathways. Interestingly, the PmIKKs may also contribute a possible role in shrimp cytokine-like system and cross-talking between signaling transductions in innate immune responses.

Ionizing radiation induces stem cell-like properties in a caspase-dependent manner in Drosophila

Cancer treatments including ionizing radiation (IR) can induce cancer stem cell-like properties in non-stem cancer cells, an outcome that can interfere with therapeutic success. Yet, we understand little about what consequences of IR induces stem cell like properties and why some cancer cells show this response but not others. In previous studies, we identified a pool of epithelial cells in Drosophila larval wing discs that display IR-induced stem cell-like properties. These cells are resistant to killing by IR and, after radiation damage, change fate and translocate to regenerate parts of the disc that suffered more cell death. Here, we report the identification of two new pools of cells with IR-induced regenerative capability. We addressed how IR exposure results in the induction of stem cell-like behavior, and found a requirement for IR-induced caspase activity and for Zfh2, a transcription factor and an effector in the JAK/STAT pathway. Unexpectedly, the requirement for caspase activity was cell-autonomous within cell populations that display regenerative behavior. We propose a model in which the requirement for caspase activity and Zfh2 can be explained by apoptotic and non-apoptotic functions of caspases in the induction of stem cell-like behavior.

Ionizing Radiation (IR), alone or in combination with other therapies, is used to treat an estimated half of all cancer patients. Yet, we understand little about why some tumors cells respond to treatment while others grow back (regenerate). We identified specific pools of cells within a Drosophila organ that are capable of regeneration after damage by IR. We also identified what it is about IR damage that allows these cells to regenerate. These results help us understand how tissues regenerate after IR damage and will aid in designing better therapies that involve radiation.

The Role of Apoptotic Signaling in Axon Guidance

Navigating growth cones are exposed to multiple signals simultaneously and have to integrate competing cues into a coherent navigational response. Integration of guidance cues is traditionally thought to occur at the level of cytoskeletal dynamics. Drosophila studies indicate that cells exhibit a low level of continuous caspase protease activation, and that axon guidance cues can activate or suppress caspase activity. We base a model for axon guidance on these observations. By analogy with other systems in which caspase signaling has non-apoptotic functions, we propose that caspase signaling can either reinforce repulsion or negate attraction in response to external guidance cues by cleaving cytoskeletal proteins. Over the course of an entire trajectory, incorrectly navigating axons may pass the threshold for apoptosis and be eliminated, whereas axons making correct decisions will survive. These observations would also explain why neurotrophic factors can act as axon guidance cues and why axon guidance systems such as Slit/Robo signaling may act as tumor suppressors in cancer.

Non-apoptotic Caspase regulation of stem cell properties

The evolutionarily conserved family of proteins called caspases are the main factors mediating the orchestrated programme of cell suicide known as apoptosis. Since this protein family was associated with this essential biological function, the majority of scientific efforts were focused towards understanding their molecular activation and function during cell death. However, an emerging body of evidence has highlighted a repertoire of non-lethal roles within a large variety of cell types, including stem cells. Here we intend to provide a comprehensive overview of the key role of caspases as regulators of stem cell properties. Finally, we briefly discuss the possible pathological consequences of caspase malfunction in stem cells, and the therapeutic potential of caspase regulation applied to this context.

Validating upstream regulators of Yorkie activity in Hippo signaling through scalloped-based genetic epistasis

Genetic studies in Drosophila have been instrumental in characterizing the Hippo pathway, which converges on the co-activator Yorkie to regulate target gene transcription. A routinely used strategy to interrogate upstream regulators of Yorkie involves the examination of selected Hippo target genes upon loss or gain of function of a suspected pathway regulator. A caveat with this strategy is that aberrant expression of a given Hippo target per se does not distinguish whether it is caused by changes in Yorkie or Yorkie-independent inputs converging on the same target gene. Building on previous findings that the DNA-binding transcription factor Scalloped mediates both Yorkie overexpression and loss-of-function phenotypes yet is itself dispensable for normal eye development, we describe a simple strategy to distinguish these possibilities by analyzing double-mutant clones of scalloped and a suspected Yorkie regulator. We provide proof of principle that this strategy can be used effectively to validate canonical Yorkie regulators and to exclude proteins that impact target expression independent of Yorkie. The described methodology and reagents should facilitate efforts to assess the expanding repertoire of proteins implicated in regulation of Yorkie activity.

When dying is not the end: Apoptotic caspases as drivers of proliferation

Caspases are well known for their role as executioners of apoptosis. However, recent studies have revealed that these lethal enzymes also have important mitogenic functions. Caspases can promote proliferation through autonomous regulation of the cell cycle, as well as by induction of secreted signals, which have a profound impact in neighboring tissues. Here, I review the proliferative role of caspases during development and homeostasis, in addition to their key regenerative function during tissue repair upon injury. Furthermore, the emerging properties of apoptotic caspases as drivers of carcinogenesis are discussed, as well as their involvement in other diseases. Finally, I examine further effects of caspases regulating death and survival in a non-autonomous manner.

Physiological functions of non-apoptotic caspase activity in the nervous system

Caspases are cysteine proteases that play important and well-defined roles in apoptosis and inflammation. Increasing evidence point to alternative functions of caspases where restricted and localized caspase activation within neurons allows for a variety of non-apoptotic and non-inflammatory processes required for brain development and function. In this review, we highlight sublethal caspase functions in axon and dendrite pruning, neurite outgrowth and dendrite branches formation, as well as in long-term depression and synaptic plasticity. Importantly, as non-apoptotic activity of caspases is often confined in space and time in neurons, we also discuss the mechanisms that restrict caspase activity in order to maintain the neuronal networks in a healthy and functional state.

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.

Non-apoptotic functions of caspases in myeloid cell differentiation

Subtle caspase activation is associated with the differentiation of several myeloid lineages. A tightly orchestrated dance between caspase-3 activation and the chaperone HSP70 that migrates to the nucleus to protect the master regulator GATA-1 from cleavage transiently occurs in basophilic erythroblasts and may prepare nucleus and organelle expel that occurs at the terminal phase of erythroid differentiation. A spatially restricted activation of caspase-3 occurs in maturing megakaryocytes to promote proplatelet maturation and platelet shedding in the bloodstream. In a situation of acute platelet need, caspase-3 could be activated in response to IL-1α and promote megakaryocyte rupture. In peripheral blood monocytes, colony-stimulating factor-1 provokes the formation of a molecular platform in which caspase-8 is activated, which downregulates nuclear factor-kappa B (NF-κB) activity and activates downstream caspases whose target fragments such as those generated by nucleophosmin (NPM1) cleavage contribute to the generation of resting macrophages. Human monocytes secrete mature IL-1β in response to lipopolysaccharide through an alternative inflammasome activation that involves caspase-8, a pathway that does not lead to cell death. Finally, active caspase-3 is part of the proteases contained in secretory granules of mast cells. Many questions remain on how these proteases are activated in myeloid cell lineages, which target proteins are cleaved, whereas other are protected from proteolysis, the precise role of cleaved proteins in cell differentiation and functions, and the link between these non-apoptotic functions of caspases and the death of these diverse cell types. Better understanding of these functions may generate therapeutic strategies to control cytopenias or modulate myeloid cell functions in various pathological situations.

The de-ubiquitylating enzyme DUBA is essential for spermatogenesis in Drosophila

De-ubiquitylating enzymes (DUBs) reverse protein ubiquitylation and thereby control essential cellular functions. Screening for a DUB that counteracts caspase ubiquitylation to regulate cell survival, we identified the Drosophila ovarian tumour-type DUB DUBA (CG6091). DUBA physically interacts with the initiator caspase death regulator Nedd2-like caspase (Dronc) and de-ubiquitylates it, thereby contributing to efficient inhibitor of apoptosis-antagonist-induced apoptosis in the fly eye. Searching also for non-apoptotic functions of DUBA, we found that Duba-null mutants are male sterile and display defects in spermatid individualisation, a process that depends on non-apoptotic caspase activity. Spermatids of DUBA-deficient flies showed reduced caspase activity and lack critical structures of the individualisation process. Biochemical characterisation revealed an obligate activation step of DUBA by phosphorylation. With genetic rescue experiments we demonstrate that DUBA phosphorylation and catalytic activity are crucial in vivo for DUBA function in spermatogenesis. Our results demonstrate for the first time the importance of de-ubiquitylation for fly spermatogenesis.

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.

The unconventional myosin CRINKLED and its mammalian orthologue MYO7A regulate caspases in their signalling roles

Caspases provide vital links in non-apoptotic regulatory networks controlling inflammation, compensatory proliferation, morphology and cell migration. How caspases are activated under non-apoptotic conditions and process a selective set of substrates without killing the cell remain enigmatic. Here we find that the Drosophila unconventional myosin CRINKLED (CK) selectively interacts with the initiator caspase DRONC and regulates some of its non-apoptotic functions. Loss of CK in the arista, border cells or proneural clusters of the wing imaginal discs affects DRONC-dependent patterning. Our data indicate that CK acts as substrate adaptor, recruiting SHAGGY46/GSK3-β to DRONC, thereby facilitating caspase-mediated cleavage and localized modulation of kinase activity. Similarly, the mammalian CK counterpart, MYO7A, binds to and impinges on CASPASE-8, revealing a new regulatory axis affecting receptor interacting protein kinase-1 (RIPK1)>CASPASE-8 signalling. Together, our results expose a conserved role for unconventional myosins in transducing caspase-dependent regulation of kinases, allowing them to take part in specific signalling events.

Drosophila SETDB1 and caspase cooperatively fine-tune cell fate determination of sensory organ precursor

Drosophila produce a constant number of mechanosensory bristles called macrochaetae (MC), which develop from sensory organ precursor (SOP) cells within a proneural cluster (PNC). However, what ensures the precise determination of SOP cells remains to be elucidated. In this study, we conducted RNAi screening in PNC for genes involved in epigenetic regulation. We identified a H3K9 histone methyltransferase, SETDB1/eggless, as a regulator of SOP development. Knockdown of SETDB1 in PNC led to additional SOPs. We further tested the relationship between SETDB1 and non-apoptotic function of caspase on SOP development. Reinforcing caspase activation by heterozygous Drosophila inhibitor of apoptosis protein 1 (DIAP1) mutation rescued ectopic SOP development caused by SETDB1 knockdown. Knockdown of SETDB1, however, had little effect on caspase activity. Simultaneous loss of SETDB1 and caspase activity resulted in further increase in MC, indicating that the two components work cooperatively. Our study suggests the fine-tuning mechanisms for SOP development by epigenetic methyltransferase and non-apoptotic caspase function.

New Views on the Misconstrued: Executioner Caspases and Their Diverse Non-apoptotic Roles

Initially characterized for their roles in apoptosis, executioner caspases have emerged as important regulators of an array of cellular activities. This is especially true in the nervous system, where sublethal caspase activity has been implicated in axonal pathfinding and branching, axonal degeneration, dendrite pruning, regeneration, long-term depression, and metaplasticity. Here we examine the roles of sublethal executioner caspase activity in nervous system development and maintenance, consider the mechanisms that locally activate and restrain these potential killers, and discuss how their activity be subverted in neurodegenerative disease.

Protection of Insects against Viral Infection by Apoptosis-Dependent Phagocytosis

We investigated whether phagocytosis participates in the protection of insects from viral infection using the natural host-virus interaction between Drosophila melanogaster and Drosophila C virus (DCV). Drosophila S2 cells were induced to undergo apoptotic cell death upon DCV infection. However, UV-inactivated virus was unable to cause apoptosis, indicating the need for productive infection for apoptosis induction. S2 cells became susceptible to phagocytosis by hemocyte-derived l(2)mbn cells after viral infection, and the presence of phagocytes in S2 cell cultures reduced viral proliferation. Phagocytosis depended, in part, on caspase activity in S2 cells, as well as the engulfment receptors Draper and integrin βν in phagocytes. To validate the in vivo situation, adult flies were abdominally infected with DCV, followed by the analysis of fly death and viral growth. DCV infection killed flies in a dose-responding manner, and the activation of effector caspases was evident, as revealed by the cleavage of a target protein ectopically expressed in flies. Furthermore, hemocytes isolated from infected flies contained DCV-infected cells, and preinjection of latex beads to inhibit the phagocytic activity of hemocytes accelerated fly death after viral infection. Likewise, viral virulence was exaggerated in flies lacking the engulfment receptors, and was accompanied by the augmented proliferation of virus. Finally, phagocytosis of DCV-infected cells in vitro was inhibited by phosphatidylserine-containing liposome, and virus-infected flies died early when a phosphatidylserine-binding protein was ectopically expressed. Collectively, our study demonstrates that the apoptosis-dependent, phosphatidylserine-mediated phagocytosis of virus-infected cells plays an important role in innate immune responses against viral infection in Drosophila.

Spindle-F Is the Central Mediator of Ik2 Kinase-Dependent Dendrite Pruning in Drosophila Sensory Neurons

During development, certain Drosophila sensory neurons undergo dendrite pruning that selectively eliminates their dendrites but leaves the axons intact. How these neurons regulate pruning activity in the dendrites remains unknown. Here, we identify a coiled-coil protein Spindle-F (Spn-F) that is required for dendrite pruning in Drosophila sensory neurons. Spn-F acts downstream of IKK-related kinase Ik2 in the same pathway for dendrite pruning. Spn-F exhibits a punctate pattern in larval neurons, whereas these Spn-F puncta become redistributed in pupal neurons, a step that is essential for dendrite pruning. The redistribution of Spn-F from puncta in pupal neurons requires the phosphorylation of Spn-F by Ik2 kinase to decrease Spn-F self-association, and depends on the function of microtubule motor dynein complex. Spn-F is a key component to link Ik2 kinase to dynein motor complex, and the formation of Ik2/Spn-F/dynein complex is critical for Spn-F redistribution and for dendrite pruning. Our findings reveal a novel regulatory mechanism for dendrite pruning achieved by temporal activation of Ik2 kinase and dynein-mediated redistribution of Ik2/Spn-F complex in neurons.

Baculovirus Inhibitor-of-Apoptosis Op-IAP3 Blocks Apoptosis by Interaction with and Stabilization of a Host Insect Cellular IAP

Baculovirus-encoded inhibitor of apoptosis (IAP) proteins likely evolved from their host cell IAP homologs, which function as critical regulators of cell death. Despite their striking relatedness to cellular IAPs, including the conservation of two baculovirus IAP repeat (BIR) domains and a C-terminal RING, viral IAPs use an unresolved mechanism to suppress apoptosis in insects. To define this mechanism, we investigated Op-IAP3, the prototypical IAP from baculovirus OpMNPV. We found that Op-IAP3 forms a stable complex with SfIAP, the native, short-lived IAP of host insect Spodoptera frugiperda. Long-lived Op-IAP3 prevented virus-induced SfIAP degradation, which normally causes caspase activation and apoptosis. In uninfected cells, Op-IAP3 also increased SfIAP steady-state levels and extended SfIAP's half-life. Conversely, SfIAP stabilization was lost or reversed in the presence of mutated Op-IAP3 that was engineered for reduced stability. Thus, Op-IAP3 stabilizes SfIAP and preserves its antiapoptotic function. In contrast to SfIAP, Op-IAP3 failed to bind or inhibit native Spodoptera caspases. Furthermore, BIR mutations that abrogate binding of well-conserved IAP antagonists did not affect Op-IAP3's capacity to prevent virus-induced apoptosis. Remarkably, Op-IAP3 also failed to prevent apoptosis when endogenous SfIAP was ablated by RNA silencing. Thus, Op-IAP3 requires SfIAP as a cofactor. Our findings suggest a new model wherein Op-IAP3 interacts directly with SfIAP to maintain its intracellular level, thereby suppressing virus-induced apoptosis indirectly. Consistent with this model, Op-IAP3 has evolved an intrinsic stability that may serve to repress signal-induced turnover and autoubiquitination when bound to its targeted cellular IAP.

IMPORTANCE

The IAPs were first discovered in baculoviruses because of their potency for preventing apoptosis. However, the antiapoptotic mechanism of viral IAPs in host insects has been elusive. We show here that the prototypical viral IAP, Op-IAP3, blocks apoptosis indirectly by associating with unstable, autoubiquitinating host IAP in such a way that cellular IAP levels and antiapoptotic activities are maintained. This mechanism explains Op-IAP3's requirement for native cellular IAP as a cofactor and the dispensability of caspase inhibition. Viral IAP-mediated preservation of the host IAP homolog capitalizes on normal IAP-IAP interactions and is likely the result of viral IAP evolution in which degron-mediated destabilization and ubiquitination potential have been reduced. This mechanism illustrates another novel means by which DNA viruses incorporate host death regulators that are modified for resistance to host regulatory controls for the purpose of suppressing host cell apoptosis and acquiring replication advantages.

Regulation of Cell Death by IAPs and Their Antagonists

Inhibitors of apoptosis (IAPs) family of genes encode baculovirus IAP-repeat domain-containing proteins with antiapoptotic function. These proteins also contain RING or UBC domains and act by binding to major proapoptotic factors and ubiquitylating them. High levels of IAPs inhibit caspase-mediated apoptosis. For these cells to undergo apoptosis, IAP function must be neutralized by IAP-antagonists. Mammalian IAP knockouts do not exhibit obvious developmental phenotypes, but the cells are more sensitized to apoptosis in response to injury. Loss of the mammalian IAP-antagonist ARTS results in reduced stem cell apoptosis. In addition to the antiapoptotic properties, IAPs regulate the innate immune response, and the loss of IAP function in humans is associated with immunodeficiency. The roles of IAPs in Drosophila apoptosis regulation are more apparent, where the loss of IAP1, or the expression of IAP-antagonists in Drosophila cells, is sufficient to trigger apoptosis. In this organism, apoptosis as a fate is conferred by the transcriptional induction of the IAP-antagonists. Many signaling pathways often converge on shared enhancer regions of IAP-antagonists. Cell death sensitivity is further regulated by posttranscriptional mechanisms, including those regulated by kinases, miRs, and ubiquitin ligases. These mechanisms are employed to eliminate damaged or virus-infected cells, limit neuroblast (neural stem cell) numbers, generate neuronal diversity, and sculpt tissue morphogenesis.

Surviving apoptosis: life-death signaling in single cells

Tissue development and homeostasis are regulated by opposing pro-survival and pro-death signals. An interesting feature of the Tumor Necrosis Factor (TNF) family of ligands is that they simultaneously activate opposing signals within a single cell via the same ligand-receptor complex. The magnitude of pro-death events such as caspase activation and pro-survival events such as Nuclear Factor (NF)-κB activation vary not only from one cell type to the next but also among individual cells of the same type due to intrinsic and extrinsic noise. The molecules involved in these pro-survival and/or pro-death pathways, and the different phenotypes that result from their activities, have been recently reviewed. Here we focus on the impact of cell-to-cell variability in the strength of these opposing signals on shaping cell fate decisions.

A transport and retention mechanism for the sustained distal localization of Spn-F-IKKε during Drosophila bristle elongation

Stable localization of the signaling complex is essential for the robust morphogenesis of polarized cells. Cell elongation involves molecular signaling centers that coordinately regulate intracellular transport and cytoskeletal structures. In Drosophila bristle elongation, the protein kinase IKKε is activated at the distal tip of the growing bristle and regulates the shuttling movement of recycling endosomes and cytoskeletal organization. However, how the distal tip localization of IKKε is established and maintained during bristle elongation is unknown. Here, we demonstrate that IKKε distal tip localization is regulated by Spindle-F (Spn-F), which is stably retained at the distal tip and functions as an adaptor linking IKKε to cytoplasmic dynein. We found that Javelin-like (Jvl) is a key regulator of Spn-F retention. In jvl mutant bristles, IKKε and Spn-F initially localize to the distal tip but fail to be retained there. In S2 cells, particles that stain positively for Jvl or Spn-F move in a microtubule-dependent manner, whereas Jvl and Spn-F double-positive particles are immobile, indicating that Jvl and Spn-F are transported separately and, upon forming a complex, immobilize each other. These results suggest that polarized transport and selective retention regulate the distal tip localization of the Spn-F–IKKε complex during bristle cell elongation.

Summary: In the Drosophila bristle, the microtubule binding protein Jvl, the adaptor Spn-F and cytoplasmic dynein are required for localised transport and retention of polarised signalling factors.

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.

Insect inhibitor-of-apoptosis (IAP) proteins are negatively regulated by signal-induced N-terminal degrons absent within viral IAP proteins

Inhibitor-of-apoptosis (IAP) proteins are key regulators of the innate antiviral response by virtue of their capacity to respond to signals affecting cell survival. In insects, wherein the host IAP provides a primary restriction to apoptosis, diverse viruses trigger rapid IAP depletion that initiates caspase-mediated apoptosis, thereby limiting virus multiplication. We report here that the N-terminal leader of two insect IAPs, Spodoptera frugiperda SfIAP and Drosophila melanogaster DIAP1, contain distinct instability motifs that regulate IAP turnover and apoptotic consequences. Functioning as a protein degron, the cellular IAP leader dramatically shortened the life span of a long-lived viral IAP (Op-IAP3) when fused to its N terminus. The SfIAP degron contains mitogen-activated kinase (MAPK)-like regulatory sites, responsible for MAPK inhibitor-sensitive phosphorylation of SfIAP. Hyperphosphorylation correlated with increased SfIAP turnover independent of the E3 ubiquitin-ligase activity of the SfIAP RING, which also regulated IAP stability. Together, our findings suggest that the SfIAP phospho-degron responds rapidly to a signal-activated kinase cascade, which regulates SfIAP levels and thus apoptosis. The N-terminal leader of dipteran DIAP1 also conferred virus-induced IAP depletion by a caspase-independent mechanism. DIAP1 instability mapped to previously unrecognized motifs that are not found in lepidopteran IAPs. Thus, the leaders of cellular IAPs from diverse insects carry unique signal-responsive degrons that control IAP turnover. Rapid response pathways that trigger IAP degradation and initiate apoptosis independent of canonical prodeath gene (Reaper-Grim-Hid) expression may provide important innate immune advantages. Furthermore, the elimination of these response motifs within viral IAPs, including those of baculoviruses, explains their unusual stability and their potent antiapoptotic activity.

IMPORTANCE

Apoptosis is an effective means by which a host controls virus infection. In insects, inhibitor-of-apoptosis (IAP) proteins act as regulatory sentinels by responding to cellular signals that determine the fate of infected cells. We discovered that lepidopteran (moth and butterfly) IAPs, which are degraded upon baculovirus infection, are controlled by a conserved phosphorylation-sensitive degron within the IAP N-terminal leader. The degron likely responds to virus-induced kinase-specific signals for degradation through SKP1/Cullin/F-box complex-mediated ubiquitination. Such signal-induced destruction of cellular IAPs is distinct from degradation caused by well-known IAP antagonists, which act to expel IAP-bound caspases. The major implication of this study is that insects have multiple signal-responsive mechanisms by which the sentinel IAPs are actively degraded to initiate host apoptosis. Such diversity of pathways likely provides insects with rapid and efficient strategies for pathogen control. Furthermore, the absence of analogous degrons in virus-encoded IAPs explains their relative stability and antiapoptotic potency.

Early development of the neural plate: new roles for apoptosis and for one of its main effectors caspase-3

Despite its tremendous complexity, the vertebrate nervous system emerges from a homogenous layer of neuroepithelial cells, the neural plate. Its formation relies on the time- and space-controlled progression of developmental programs. Apoptosis is a biological process that removes superfluous and potentially dangerous cells and is implemented through the activation of a molecular pathway conserved during evolution. Apoptosis and an unconventional function of one of its main effectors, caspase-3, contribute to the patterning and growth of the neuroepithelium. Little is known about the intrinsic and extrinsic cues controlling activities of the apoptotic machinery during development. The BarH-like (Barhl) proteins are homeodomain-containing transcription factors. The observations in Caenorhabditis elegans, Xenopus, and mice document that Barhl proteins act in cell survival and as cell type-specific regulators of a caspase-3 function that limits neural progenitor proliferation. In this review, we discuss the roles and regulatory modes of the apoptotic machinery in the development of the neural plate. We focus on the Barhl2, the Sonic Hedgehog, and the Wnt pathways and their activities in neural progenitor survival and proliferation.

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.

Multiple mechanisms modulate distinct cellular susceptibilities toward apoptosis in the developing Drosophila eye

Although apoptosis is mechanistically well understood, a comprehensive understanding of how cells modulate their susceptibility toward apoptosis in a developing tissue is lacking. Here, we reveal striking dynamics in the apoptotic susceptibilities of different cell types in the Drosophila retina over a period of only 24 hr. Mitotic cells are extremely susceptible to apoptotic signals, while postmitotic cells have developed several strategies to promote survival. For example, photoreceptor neurons accumulate the inhibitor of apoptosis, Diap1. In unspecified cells, Cullin-3-mediated degradation keeps Diap1 levels low. These cells depend on EGFR signaling for survival. As development proceeds, developmentally older photoreceptors degrade Diap1, resulting in increased apoptosis susceptibility. Finally, R8 photoreceptors have very efficient survival mechanisms independent of EGFR or Diap1. These examples illustrate how complex cellular susceptibility toward apoptosis is regulated in a developing organ. Similar complexities may regulate apoptosis susceptibilities in mammalian development, and tumor cells may take advantage of it.

Local caspase activation interacts with Slit-Robo signaling to restrict axonal arborization

In addition to being critical for apoptosis, components of the apoptotic pathway, such as caspases, are involved in other physiological processes in many types of cells, including neurons. However, very little is known about their role in dynamic, nonphysically destructive processes, such as axonal arborization and synaptogenesis. We show that caspases were locally active in vivo at the branch points of young, dynamic retinal ganglion cell axonal arbors but not in the cell body or in stable mature arbors. Caspase activation, dependent on Caspase-3, Caspase-9, and p38 mitogen-activated protein kinase (MAPK), rapidly increased at branch points corresponding with branch tip addition. Time-lapse imaging revealed that knockdown of Caspase-3 and Caspase-9 led to more stable arbors and presynaptic sites. Genetic analysis showed that Caspase-3, Caspase-9, and p38 MAPK interacted with Slit1a-Robo2 signaling, suggesting that localized activation of caspases lie downstream of a ligand receptor system, acting as key promoters of axonal branch tip and synaptic dynamics to restrict arbor growth in vivo in the central nervous system.

A steroid-controlled global switch in sensitivity to apoptosis during Drosophila development

Precise control over activation of the apoptotic machinery is critical for development, tissue homeostasis and disease. In Drosophila, the decision to trigger apoptosis--whether in response to developmental cues or to DNA damage--converges on transcription of inhibitor of apoptosis protein (IAP) antagonists reaper, hid and grim. Here we describe a parallel process that regulates the sensitivity to, rather than the execution of, apoptosis. This process establishes developmental windows that are permissive or restrictive for triggering apoptosis, where the status of cells determines their capacity to die. We characterize one switch in the sensitivity to apoptotic triggers, from restrictive to permissive, that occurs during third-instar larval (L3) development. Early L3 animals are highly resistant to induction of apoptosis by expression of IAP-antagonists, DNA-damaging agents and even knockdown of the IAP diap1. This resistance to apoptosis, however, is lost in wandering L3 animals after acquiring a heightened sensitivity to apoptotic triggers. This switch in sensitivity to death activators is mediated by a change in mechanisms available for activating endogenous caspases, from an apoptosome-independent to an apoptosome-dependent pathway. This switch in apoptotic pathways is regulated in a cell-autonomous manner by the steroid hormone ecdysone, through changes in expression of critical pro-, but not anti-, apoptotic genes. This steroid-controlled switch defines a novel, physiologically-regulated, mechanism for controlling sensitivity to apoptosis and provides new insights into the control of apoptosis during development.

Drosophila oocyte polarity and cytoskeleton organization require regulation of Ik2 activity by Spn-F and Javelin-like

The Drosophila melanogaster Spn-F, Ik2, and Javelin-like (Jvl) proteins interact to regulate oocyte mRNA localization and cytoskeleton organization. However, the mechanism by which these proteins interact remains unclear. Using antibodies to activated Ik2, we showed that this protein is found at the region of oocyte and follicle cell where microtubule minus ends are enriched. We demonstrate that germ line Ik2 activation is diminished both in jvl and in spn-F mutant ovaries. Structure-function analysis of Spn-F revealed that the C-terminal end is critical for protein function, since it alone was able to rescue spn-F sterility. On the other hand, germ line expression of Spn-F lacking its conserved C-terminal region (Spn-FΔC) phenocopied ik2, leading to production of ventralized eggshell and bicaudal embryos. In Spn-FΔC-expressing oocytes, Gurken protein is mislocalized and oskar mRNA and protein localization is disrupted. Expression of Ik2 rescued Spn-FΔC ovarian phenotypes. We found that whereas Spn-F physically interacts with Ik2 and Jvl, Spn-FΔC physically interacts with Ik2 but not with Jvl. Thus, expression of Spn-FΔC, which lacks the Jvl-interacting domain, probably interferes with interaction of Ik2 and Jvl. In summary, our results demonstrate that Spn-F mediates the interaction between Ik2 and Jvl to control Ik2 activity.

The shrimp IKK-NF-κB signaling pathway regulates antimicrobial peptide expression and may be subverted by white spot syndrome virus to facilitate viral gene expression

The IκB kinases IKKα and IKKβ and the IKK-related kinases TANK-binding kinase 1 (TBK1) and IKKε are the master regulators of the NF-κB signaling pathway. Although this pathway has been extensively studied in mammals, less attention has been paid in crustaceans, which have significant economic value. Here, we report the cloning and functional studies of two IKK homologs, LvIKKβ and LvIKKε, from Pacific white shrimp, Litopenaeus vannamei. LvIKKβ and LvIKKε mRNAs are widely expressed in different tissues and are responsive to white spot syndrome virus (WSSV) infection. When overexpressed in Drosophila S2 cells, LvIKKβ but not LvIKKε activates the promoters of NF-κB pathway-controlled antimicrobial peptide genes (AMPs), such as the Penaeidins (PENs). In HEK 293T cells, both LvIKKβ and LvIKKε activate an NF-κB reporter. The silencing of LvIKKβ or LvIKKε using double-stranded RNA (dsRNA)-mediated RNA interference (RNAi) decreases the expression of L. vannamei AMPs, including PENs, lysozyme and crustins. Intriguingly, LvIKKβ- or LvIKKε-silenced L. vannamei are resistant to WSSV infection. We hypothesized that successful infection with WSSV requires the activation of the IKK–NF-κB signaling pathway to modulate viral gene expression. We constructed luciferase reporters for 147 WSSV genes. By screening, we found that the WSV051, WSV059, WSV069, WSV083, WSV090, WSV107, WSV244, WSV303, WSV371 and WSV445 promoters can be activated by LvIKKβ or LvIKKε in Drosophila S2 cells. Taken together, our results reveal that LvIKKβ and LvIKKε may participate in the regulation of shrimp AMPs and that WSSV may subvert the L. vannamei IKK–NF-κB signaling pathway to facilitate viral gene expression.

Caspase-3 is involved in the signalling in erythroid differentiation by targeting late progenitors

A role for caspase activation in erythroid differentiation has been established, yet its precise mode of action remains elusive. A drawback of all previous investigations on caspase activation in ex vivo erythroid differentiation is the lack of an in vitro model producing full enucleation of erythroid cells. Using a culture system which renders nearly 100% enucleated red cells from human CD34(+) cells, we investigated the role of active caspase-3 in erythropoiesis. Profound effects of caspase-3 inhibition were found on erythroid cell growth and differentiation when inhibitors were added to CD34(+) cells at the start of the culture and showed dose-response to the concentration of inhibitor employed. Enucleation was only reduced as a function of the reduced maturity of the culture and the increased cell death of mature cells while the majority of cells retained their ability to extrude their nuclei. Cell cycle analysis after caspase-3 inhibition showed caspase-3 to play a critical role in cell proliferation and highlighted a novel function of this protease in erythroid differentiation, i.e. its contribution to cell cycle regulation at the mitotic phase. While the effect of caspase-3 inhibitor treatment on CD34(+) derived cells was not specific to the erythroid lineage, showing a similar reduction of cell expansion in myeloid cultures, the mechanism of action in both lineages appeared to be distinct with a strong induction of apoptosis causing the decreased yield of myeloid cells. Using a series of colony-forming assays we were able to pinpoint the stage at which cells were most sensitive to caspase-3 inhibition and found activated caspase-3 to play a signalling role in erythroid differentiation by targeting mature BFU-E and CFU-E but not early BFU-E.

Blocking apoptotic signaling rescues axon guidance in Netrin mutants

Netrins are guidance cues that form gradients to guide growing axons. We uncover a mechanism for axon guidance by demonstrating that axons can accurately navigate in the absence of a Netrin gradient if apoptotic signaling is blocked. Deletion of the two Drosophila NetA and NetB genes leads to guidance defects and increased apoptosis, and expression of either gene at the midline is sufficient to rescue the connectivity defects and cell death. Surprisingly, pan-neuronal expression of NetB rescues equally well, even though no Netrin gradient has been established. Furthermore, NetB expression blocks apoptosis, suggesting that NetB acts as a neurotrophic factor. In contrast, neuronal expression of NetA increases axon defects. Simply blocking apoptosis in NetAB mutants is sufficient to rescue connectivity, and inhibition of caspase activity in subsets of neurons rescues guidance independently of survival. In contrast to the traditional role of Netrin as simply a guidance cue, our results demonstrate that guidance and survival activities may be functionally related.

Polypeptide chain release factor eRF3 is a novel molecular partner of survivin

The eukaryotic class II polypeptide chain release factor (eRF3) is an eRF1- and ribosome-dependent GTPase involved in translation termination of protein biosynthesis. eRF3 is a multifunctional protein that is also involved in chromosomal segregation and cytokinesis during mitosis. Survivin is a member of the inhibitor of apoptosis protein (IAP) family that is involved in the organisation of spindle and cell apoptosis. Interaction between survivin and eRF3a-F3 or eRF3b, encoded by the GSPT1 and GSPT2 genes, respectively, was confirmed using yeast two-hybrid (Y2H) and pull-down assays in vitro, and co-immunoprecipitation in vivo. The domains involved in the formation of the survivin-eRF3s complex have been identified. The sites on survivin that interact with eRF3 are located in the baculovirus IAP repeat domain (residues 65-76), which forms a beta-strand structure with an overall negative charge. The sites on eRF3 that interact with survivin were localised to the N-terminal domain(NTD; residues 131-200). Cell localisation experiments indicate that both factors are in the nucleus, suggesting that they cooperatively function in nuclear processes.