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

Wg/Wnt1 and Erasp link ER stress to proapoptotic signaling in an autosomal dominant retinitis pigmentosa model

The endoplasmic reticulum (ER) is a subcellular organelle essential for cellular homeostasis. Perturbation of ER functions due to various conditions can induce apoptosis. Chronic ER stress has been implicated in a wide range of diseases, including autosomal dominant retinitis pigmentosa (ADRP), which is characterized by age-dependent retinal degeneration caused by mutant rhodopsin alleles. However, the signaling pathways that mediate apoptosis in response to ER stress remain poorly understood. In this study, we performed an unbiased in vivo RNAi screen with a Drosophila ADRP model and found that Wg/Wnt1 mediated apoptosis. Subsequent transcriptome analysis revealed that ER stress-associated serine protease (Erasp), which has been predicted to show serine-type endopeptidase activity, was a downstream target of Wg/Wnt1 during ER stress. Furthermore, knocking down Erasp via RNAi suppressed apoptosis induced by mutant rhodopsin-1 (Rh-1^P37H) toxicity, alleviating retinal degeneration in the Drosophila ADRP model. In contrast, overexpression of Erasp resulted in enhanced caspase activity in Drosophila S2 cells treated with apoptotic inducers and the stabilization of the initiator caspase Dronc (Death regulator Nedd2-like caspase) by stimulating DIAP1 (Drosophila inhibitor of apoptosis protein 1) degradation. These findings helped identify a novel cell death signaling pathway involved in retinal degeneration in an autosomal dominant retinitis pigmentosa model.

Long-term sevoflurane exposure resulted in temporary rather than lasting cognitive impairment in Drosophila

Sevoflurane is the primary inhaled anesthetic used in pediatric surgery. It has been the focus of research since animal models studies found that it was neurotoxic to the developing brain two decades ago. However, whether pediatric general anesthesia can lead to permanent cognitive deficits remained a subject of heated debate. Therefore, our study aims to determine the lifetime neurotoxicity of early long-time sevoflurane exposure using a short-life-cycle animal model, Drosophila melanogaster. To investigate this question, we measured the lifetime changes of two-day-old flies' learning and memory abilities after anesthesia with 3 % sevoflurane for 6 h by the T-maze memory assay. We evaluated the apoptosis, levels of ATP and ROS, and related genes in the fly head. Our results suggest that 6 h 3 % sevoflurane exposure at a young age can only induce transient neuroapoptosis and cognitive deficits around the first week after anesthesia. But this brain damage recedes with time and vanishes in late life. We also found that the mRNA level of caspases and Bcl-2, ROS level, and ATP level increased during this temporary neuroapoptosis process. And mRNA levels of antioxidants, such as SOD2 and CAT, increased and decreased simultaneously with the rise and fall of the ROS level, indicating a possible contribution to the recovery from the sevoflurane impairment. In conclusion, our results suggest that one early prolonged sevoflurane-based general anesthesia can induce neuroapoptosis and learning and memory deficit transiently but not permanently in Drosophila.

Caspase Inhibition Modulates Monocyte-Derived Macrophage Polarization in Damaged Tissues

Circulating monocytes are recruited in damaged tissues to generate macrophages that modulate disease progression. Colony-stimulating factor-1 (CSF-1) promotes the generation of monocyte-derived macrophages, which involves caspase activation. Here, we demonstrate that activated caspase-3 and caspase-7 are located to the vicinity of the mitochondria in CSF1-treated human monocytes. Active caspase-7 cleaves p47^PHOX at aspartate 34, which promotes the formation of the NADPH (nicotinamide adenine dinucleotide phosphate) oxidase complex NOX2 and the production of cytosolic superoxide anions. Monocyte response to CSF-1 is altered in patients with a chronic granulomatous disease, which are constitutively defective in NOX2. Both caspase-7 down-regulation and radical oxygen species scavenging decrease the migration of CSF-1-induced macrophages. Inhibition or deletion of caspases prevents the development of lung fibrosis in mice exposed to bleomycin. Altogether, a non-conventional pathway that involves caspases and activates NOX2 is involved in CSF1-driven monocyte differentiation and could be therapeutically targeted to modulate macrophage polarization in damaged tissues.

Actin remodeling mediates ROS production and JNK activation to drive apoptosis-induced proliferation

Stress-induced cell death, mainly apoptosis, and its subsequent tissue repair is interlinked although our knowledge of this connection is still very limited. An intriguing finding is apoptosis-induced proliferation (AiP), an evolutionary conserved mechanism employed by apoptotic cells to trigger compensatory proliferation of their neighboring cells. Studies using Drosophila as a model organism have revealed that apoptotic caspases and c-Jun N-terminal kinase (JNK) signaling play critical roles to activate AiP. For example, the initiator caspase Dronc, the caspase-9 ortholog in Drosophila, promotes activation of JNK leading to release of mitogenic signals and AiP. Recent studies further revealed that Dronc relocates to the cell cortex via Myo1D, an unconventional myosin, and stimulates production of reactive oxygen species (ROS) to trigger AiP. During this process, ROS can attract hemocytes, the Drosophila macrophages, which further amplify JNK signaling cell non-autonomously. However, the intrinsic components connecting Dronc, ROS and JNK within the stressed signal-producing cells remain elusive. Here, we identified LIM domain kinase 1 (LIMK1), a kinase promoting cellular F-actin polymerization, as a novel regulator of AiP. F-actin accumulates in a Dronc-dependent manner in response to apoptotic stress. Suppression of F-actin polymerization in stressed cells by knocking down LIMK1 or expressing Cofilin, an inhibitor of F-actin elongation, blocks ROS production and JNK activation, hence AiP. Furthermore, Dronc and LIMK1 genetically interact. Co-expression of Dronc and LIMK1 drives F-actin accumulation, ROS production and JNK activation. Interestingly, these synergistic effects between Dronc and LIMK1 depend on Myo1D. Therefore, F-actin remodeling plays an important role mediating caspase-driven ROS production and JNK activation in the process of AiP.

In Vitro Cleavage Assays using Purified Recombinant Drosophila Caspases for Substrate Screening

Caspases are very specific cell death proteases that are involved in apoptotic and non-apoptotic processes. While the role of caspases during apoptosis has been very well defined and many apoptotic proteolytic substrates of caspases have been identified and characterized, the role of caspases for non-apoptotic processes is not well understood. In particular, few non-apoptotic substrates of caspases have been identified thus far. Here, in order to facilitate the identification and characterization of potential caspase substrates, a protocol that allows the testing of candidate substrates in caspase cleavage assays in vitro is described. This protocol includes the production and purification of recombinant caspase proteins, the production of the candidate substrates either recombinantly or in a cell-free expression system, and the actual in vitro cleavage reaction followed by SDS-PAGE and immunoblotting. This protocol is tailored for the Drosophila caspases Dronc and Drice but can easily be adapted for caspases from other organisms, including mammals.

A Mini Review on Molecules Inducing Caspase-Independent Cell Death: A New Route to Cancer Therapy

Most anticancer treatments trigger tumor cell death through apoptosis, where initiation of proteolytic action of caspase protein is a basic need. But under certain circumstances, apoptosis is prevented by the apoptosis inhibitor proteins, survivin and Hsp70. Several drugs focusing on classical programmed death of the cell have been reported to have low anti-tumorogenic potency due to mutations in proteins involved in the caspase-dependent programmed cell death with intrinsic and extrinsic pathways. This review concentrates on the role of anti-cancer drug molecules targeting alternative pathways of cancer cell death for treatment, by providing a molecular basis for the new strategies of novel anti-cancer treatment. Under these conditions, active agents targeting alternative cell death pathways can be considered as potent chemotherapeutic drugs. Many natural compounds and other small molecules, such as inorganic and synthetic compounds, including several repurposing drugs, are reported to cause caspase-independent cell death in the system. However, few molecules indicated both caspase-dependent as well caspase-free cell death in specific cancer lines. Cancer cells have alternative methods of caspase-independent programmed cell death which are equally promising for being targeted by small molecules. These small molecules may be useful leads for rational therapeutic drug design, and can be of potential interest for future cancer-preventive strategies.

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-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.

Infection of Mammals and Mosquitoes by Alphaviruses: Involvement of Cell Death

Alphaviruses, such as the chikungunya virus, are emerging and re-emerging viruses that pose a global public health threat. They are transmitted by blood-feeding arthropods, mainly mosquitoes, to humans and animals. Although alphaviruses cause debilitating diseases in mammalian hosts, it appears that they have no pathological effect on the mosquito vector. Alphavirus/host interactions are increasingly studied at cellular and molecular levels. While it seems clear that apoptosis plays a key role in some human pathologies, the role of cell death in determining the outcome of infections in mosquitoes remains to be fully understood. Here, we review the current knowledge on alphavirus-induced regulated cell death in hosts and vectors and the possible role they play in determining tolerance or resistance of mosquitoes.

Transiently "Undead" Enterocytes Mediate Homeostatic Tissue Turnover in the Adult Drosophila Midgut

We reveal surprising similarities between homeostatic cell turnover in adult Drosophila midguts and "undead" apoptosis-induced compensatory proliferation (AiP) in imaginal discs. During undead AiP, immortalized cells signal for AiP, allowing its analysis. Critical for undead AiP is the Myo1D-dependent localization of the initiator caspase Dronc to the plasma membrane. Here, we show that Myo1D functions in mature enterocytes (ECs) to control mitotic activity of intestinal stem cells (ISCs). In Myo1D mutant midguts, many signaling events involved in AiP (ROS generation, hemocyte recruitment, and JNK signaling) are affected. Importantly, similar to AiP, Myo1D is required for membrane localization of Dronc in ECs. We propose that ECs destined to die transiently enter an undead-like state through Myo1D-dependent membrane localization of Dronc, which enables them to generate signals for ISC activity and their replacement. The concept of transiently "undead" cells may be relevant for other stem cell models in flies and mammals.

Intersections between Regulated Cell Death and Autophagy

In multicellular organisms, cell death is an essential aspect of life. Over the past decade, the spectrum of different forms of regulated cell death (RCD) has expanded dramatically with relevance in several pathologies such as inflammatory and neurodegenerative diseases. This has been paralleled by the growing awareness of the central importance of autophagy as a stress response that influences decisions of cell life and cell death. Here, we first introduce criteria and methodologies for correct identification of the different RCD forms. We then discuss how the autophagy machinery is directly associated with specific cell death forms and dissect the complex interactions between autophagy and apoptotic and necrotic cell death. This highlights how the balance of the relationship between other cell death pathways and autophagy presides over life and death in specific cellular contexts.

Two Sides of the Same Coin - Compensatory Proliferation in Regeneration and Cancer

Apoptosis has long been regarded as a tumor suppressor mechanism and evasion from apoptosis is considered to be one hallmark of cancer. However, this principle is not always consistent with clinical data which often illustrate a correlation between apoptosis and poor prognosis. Work in the last 15 years has provided an explanation for this apparent paradox. Apoptotic cells communicate with their environment and can produce signals which promote compensatory proliferation of surviving cells. This behavior of apoptotic cells is important for tissue regeneration in several model organisms, ranging from hydra to mammals. However, it may also play an important feature for tumorigenesis and tumor relapse. Several distinct forms of apoptosis-induced compensatory proliferation (AiP) have been identified, many of which involve reactive oxygen species (ROS) and immune cells. One type of AiP, "undead" AiP, in which apoptotic cells are kept in an immortalized state and continuously divide, may have particular relevance for tumorigenesis. Furthermore, given that chemo- and radiotherapy often aim to kill tumor cells, an improved understanding of the effects of apoptotic cells on the tumor and the tumor environment is of critical importance for the well-being of the patient. In this review, we summarize the current knowledge of AiP and focus our attention on recent findings obtained in Drosophila and other model organisms, and relate them to tumorigenesis.

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.

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.

Caspases maintain tissue integrity by an apoptosis-independent inhibition of cell migration and invasion

Maintenance of tissue integrity during development and homeostasis requires the precise coordination of several cell-based processes, including cell death. In animals, the majority of such cell death occurs by apoptosis, a process mediated by caspase proteases. To elucidate the role of caspases in tissue integrity, we investigated the behavior of Drosophila epithelial cells that are severely compromised for caspase activity. We show that these cells acquire migratory and invasive capacities, either within 1–2 days following irradiation or spontaneously during development. Importantly, low levels of effector caspase activity, which are far below the threshold required to induce apoptosis, can potently inhibit this process, as well as a distinct, developmental paradigm of primordial germ cell migration. These findings may have implications for radiation therapy in cancer treatment. Furthermore, given the presence of caspases throughout metazoa, our results could imply that preventing unwanted cell migration constitutes an ancient non-apoptotic function of these proteases.

In addition to regulating programmed cell death, caspases also have non-apoptotic roles. Here, the authors show that low level caspase activity prevents cell migration to maintain tissue integrity.

Ubiquitin-dependent proteolysis of CXCL7 leads to posterior longitudinal ligament ossification

Ossification of the posterior longitudinal ligament (OPLL), a spinal ligament, reduces the range of motion in limbs. No treatment is currently available for OPLL, which is why therapies are urgently needed. OPLL occurs in obesity, is more common in men, and has an onset after 40 years of age. The mechanisms underlying OPLL remain unclear. In this study, we performed a serum proteomic analysis in both OPLL patients and healthy subjects to identify factors potentially involved in the development of OPLL, and found reduced levels of a protein that might underlie the pathology of OPLL. We isolated the protein, determined its amino acid sequence, and identified it as chemokine (C-X-C motif) ligand 7 (CXCL7). Based on these proteomics findings, we generated a CXCL7 knockout mouse model to study the molecular mechanisms underlying OPLL. CXCL7-null mice presented with a phenotype of OPLL, showing motor impairment, heterotopic ossification in the posterior ligament tissue, and osteoporosis in vertebrate tissue. To identify the mechanisms of CXCL7 deficiency in OPLL, we searched for single nucleotide polymorphisms and altered DNA exons, but no abnormalities were found. Although miR-340 levels were found to be high in an miRNA array, they were insufficient to reduce CXCL7 levels. Ubiquitin C-terminal hydrolase1 (UCHL1) was found to be overexpressed in CXCL7-null mice and in the sera of patients with OPLL, and was correlated with OPLL severity. Post-translational modifications of proteins with ubiquitin and ubiquitin-like modifiers, orchestrated by a cascade of specialized ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and ubiquitin ligase (E3) enzymes, are thought to control a wide range of cellular processes, and alterations in the ubiquitin–proteasome system have been associated with several degenerative disorders. In addition, the OPLL tissue of CXCL7-null mouse and its primary cells expressed the antibody to ubiquitin (linkage-specific K48). Our data clearly show decreased CXCL7 levels in patients with OPLL, and that OPLL developed in mice lacking CXCL7. Tumor necrosis factor receptor-associated factor (TRAF)6 expression was decreased in CXCL7-null mouse primary cells. Furthermore, K48 polyubiquitination was found in posterior longitudinal ligament ossified tissue and primary cells from CXCL7-null mice. We performed a phosphoproteomics analysis in CXCL7-deficient mice and identified increased phosphorylation of mitogen-activated protein kinase kinase (ME3K)15, ubiquitin protein ligase E3C (UBE3C) and protein kinase C (PKC) alpha, suggesting that ubiquitin-dependent degradation is involved in CXCL7 deficiency. Future studies in the CXCL7-null mouse model are, therefore, warranted to investigate the role of ubiquitination in the onset of OPLL. In conclusion, CXCL7 levels may be useful as a serum marker for the progression of OPLL. This study also suggests that increasing CXCL7 levels in patients can serve as an effective therapeutic strategy for the treatment of OPLL.

Plasma Membrane Localization of Apoptotic Caspases for Non-apoptotic Functions

Caspases are best characterized for their function in apoptosis. However, they also have non-apoptotic functions such as apoptosis-induced proliferation (AiP), where caspases release mitogens for compensatory proliferation independently of their apoptotic role. Here, we report that the unconventional myosin, Myo1D, which is known for its involvement in left/right development, is an important mediator of AiP in Drosophila. Mechanistically, Myo1D translocates the initiator caspase Dronc to the basal side of the plasma membrane of epithelial cells where Dronc promotes the activation of the NADPH-oxidase Duox for reactive oxygen species generation and AiP in a non-apoptotic manner. We propose that the basal side of the plasma membrane constitutes a non-apoptotic compartment for caspases. Finally, Myo1D promotes tumor growth and invasiveness of the neoplastic scrib RasV12 model. Together, we identified a new function of Myo1D for AiP and tumorigenesis, and reveal a mechanism by which cells sequester apoptotic caspases in a non-apoptotic compartment at the plasma membrane.

Learning on the Fly: The Interplay between Caspases and Cancer

The ease of genetic manipulation, as well as the evolutionary conservation of gene function, has placed Drosophila melanogaster as one of the leading model organisms used to understand the implication of many proteins with disease development, including caspases and their relation to cancer. The family of proteases referred to as caspases have been studied over the years as the major regulators of apoptosis: the most common cellular mechanism involved in eliminating unwanted or defective cells, such as cancerous cells. Indeed, the evasion of the apoptotic programme resulting from caspase downregulation is considered one of the hallmarks of cancer. Recent investigations have also shown an instrumental role for caspases in non-lethal biological processes, such as cell proliferation, cell differentiation, intercellular communication, and cell migration. Importantly, malfunction of these essential biological tasks can deeply impact the initiation and progression of cancer. Here, we provide an extensive review of the literature surrounding caspase biology and its interplay with many aspects of cancer, emphasising some of the key findings obtained from Drosophila studies. We also briefly describe the therapeutic potential of caspase modulation in relation to cancer, highlighting shortcomings and hopeful promises.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

p53-dependent programmed necrosis controls germ cell homeostasis during spermatogenesis

The importance of regulated necrosis in pathologies such as cerebral stroke and myocardial infarction is now fully recognized. However, the physiological relevance of regulated necrosis remains unclear. Here, we report a conserved role for p53 in regulating necrosis in Drosophila and mammalian spermatogenesis. We found that Drosophila p53 is required for the programmed necrosis that occurs spontaneously in mitotic germ cells during spermatogenesis. This form of necrosis involved an atypical function of the initiator caspase Dronc/Caspase 9, independent of its catalytic activity. Prevention of p53-dependent necrosis resulted in testicular hyperplasia, which was reversed by restoring necrosis in spermatogonia. In mouse testes, p53 was required for heat-induced germ cell necrosis, indicating that regulation of necrosis is a primordial function of p53 conserved from invertebrates to vertebrates. Drosophila and mouse spermatogenesis will thus be useful models to identify inducers of necrosis to treat cancers that are refractory to apoptosis.

Tango7 regulates cortical activity of caspases during reaper-triggered changes in tissue elasticity

Caspases perform critical functions in both living and dying cells; however, how caspases perform physiological functions without killing the cell remains unclear. Here we identify a novel physiological function of caspases at the cortex of Drosophila salivary glands. In living glands, activation of the initiator caspase dronc triggers cortical F-actin dismantling, enabling the glands to stretch as they accumulate secreted products in the lumen. We demonstrate that tango7, not the canonical Apaf-1-adaptor dark, regulates dronc activity at the cortex; in contrast, dark is required for cytoplasmic activity of dronc during salivary gland death. Therefore, tango7 and dark define distinct subcellular domains of caspase activity. Furthermore, tango7-dependent cortical dronc activity is initiated by a sublethal pulse of the inhibitor of apoptosis protein (IAP) antagonist reaper. Our results support a model in which biological outcomes of caspase activation are regulated by differential amplification of IAP antagonists, unique caspase adaptor proteins, and mutually exclusive subcellular domains of caspase activity.

Caspases are known for their role in cell death, but they can also participate in other physiological functions without killing the cells. Here the authors show that unique caspase adaptor proteins can regulate caspase activity within mutually-exclusive and independently regulated subcellular domains.

Proapoptotic function of deubiquitinase DUSP31 in Drosophila

Drosophila have been used to identify new components in apoptosis regulation. The Drosophila protein Dark forms an octameric apoptosome complex that induces the initiator caspase Dronc to trigger the caspase cell death pathway and, therefore, plays an important role in controlling apoptosis. Caspases and Dark are constantly expressed in cells, but their activity is blocked by DIAP1 E3 ligase-mediated ubiquitination and subsequent inactivation or proteasomal degradation. One of the regulatory mechanisms that stabilize proapoptotic factors is the removal of ubiquitin chains by deubiquitinases. In this study performed a modified genetic screen for deubiquitinases (dsRNA lines) to identify those involved in stabilizing proapoptotic components. Loss-of-function alleles of deubiquitinase DUSP31 were identified as suppressors of the Dronc overexpression phenotype. DUSP31 deficiency also suppresses apoptosis induced by the RHG protein, Grim. Genetic analysis revealed for the first time that DUSP31 deficiency sufficiently suppresses the Dark phenotype, indicating its involvement in the control of Dark/Dronc apoptosome function in invertebrate apoptosis.

Killers creating new life: caspases drive apoptosis-induced proliferation in tissue repair and disease

Apoptosis is a carefully orchestrated and tightly controlled form of cell death, conserved across metazoans. As the executioners of apoptotic cell death, cysteine-dependent aspartate-directed proteases (caspases) are critical drivers of this cellular disassembly. Early studies of genetically programmed cell death demonstrated that the selective activation of caspases induces apoptosis and the precise elimination of excess cells, thereby sculpting structures and refining tissues. However, over the past decade there has been a fundamental shift in our understanding of the roles of caspases during cell death-a shift precipitated by the revelation that apoptotic cells actively engage with their surrounding environment throughout the death process, and caspases can trigger a myriad of signals, some of which drive concurrent cell proliferation regenerating damaged structures and building up lost tissues. This caspase-driven compensatory proliferation is referred to as apoptosis-induced proliferation (AiP). Diverse mechanisms of AiP have been found across species, ranging from planaria to mammals. In this review, we summarize the current knowledge of AiP and we highlight recent advances in the field including the involvement of reactive oxygen species and macrophage-like immune cells in one form of AiP, novel regulatory mechanisms affecting caspases during AiP, and emerging clinical data demonstrating the critical importance of AiP in cancer.