Plasma Membrane Localization of Apoptotic Caspases for Non-apoptotic Functions

Regeneration following tissue necrosis is mediated by non-apoptotic caspase activity

Tissue necrosis is a devastating complication for many human diseases and injuries. Unfortunately, our understanding of necrosis and how it impacts surrounding healthy tissue - an essential consideration when developing methods to treat such injuries - has been limited by a lack of robust genetically tractable models. Our lab previously established a method to study necrosis-induced regeneration in the Drosophila wing imaginal disc, which revealed a unique phenomenon whereby cells at a distance from the injury upregulate caspase activity in a process called Necrosis-induced Apoptosis (NiA) that is vital for regeneration. Here we have further investigated this phenomenon, showing that NiA is predominantly associated with the highly regenerative pouch region of the disc, shaped by genetic factors present in the presumptive hinge. Furthermore, we find that a proportion of NiA fail to undergo apoptosis, instead surviving effector caspase activation to persist within the tissue and stimulate reparative proliferation late in regeneration. This proliferation relies on the initiator caspase Dronc, and occurs independent of JNK, ROS or mitogens associated with the previously characterized Apoptosis-induced Proliferation (AiP) mechanism. These data reveal a new means by which non-apoptotic Dronc signaling promotes regenerative proliferation in response to necrotic damage.

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.

A Non-Apoptotic Pattern of Caspase-9/Caspase-3 Activation During Differentiation of Human Embryonic Stem Cells into Cardiomyocytes

In vitro studies have demonstrated that the differentiation of embryonic stem cells (ESCs) into cardiomyocytes requires activation of caspases through the mitochondrial pathway. These studies have relied on synthetic substrates for activity measurements, which can be misleading due to potential none-specific hydrolysis of these substrates by proteases other than caspases. Hence, caspase-9 and caspase-3 activation are investigated during the differentiation of human ESCs (hESCs) by directly assessing caspase-9 and -3 cleavage. Western blot reveals the presence of the cleaved caspase-9 prior to and during the differentiation of human ESCs (hESCs) into cardiomyocytes at early stages, which diminishes as the differentiation progresses, without cleavage and activation of endogenous procaspase-3. Activation of exogenous procaspase-3 by endogenous caspase-9 and subsequent cleavage of chromogenic caspase-3 substrate i.e. DEVD-pNA during the course of differentiation confirmes that endogenous caspase-9 has the potency to recognize and activate procaspase-3, but for reasons that are unknown to us fails to do so. These observations suggest the existence of distinct mechanisms of caspase regulation in differentiation as compared to apoptosis. Bioinformatics analysis suggests the presence of caspase-9 regulators, which may influence proteolytic function under specific conditions.

Proteomic analysis of holocarboxylase synthetase deficient-MDA-MB-231 breast cancer cells revealed the biochemical changes associated with cell death, impaired growth signaling, and metabolism

We have previously shown that the holocarboxylase synthetase (HLCS) is overexpressed in breast cancer tissue of patients, and silencing of its expression in triple-negative cancer cell line inhibits growth and migration. Here we investigated the global biochemical changes associated with HLCS knockdown in MDA-MB-231 cells to discern the pathways that involve HLCS. Proteomic analysis of two independent HLCS knockdown cell lines identified 347 differentially expressed proteins (DEPs) whose expression change > 2-fold (p < 0.05) relative to the control cell line. GO enrichment analysis showed that these DEPs were mainly associated with the cellular process such as cellular metabolic process, cellular response to stimulus, and cellular component organization or biogenesis, metabolic process, biological regulation, response to stimuli, localization, and signaling. Among the 347 identified DEPs, 64 proteins were commonly found in both HLCS knockdown clones, confirming their authenticity. Validation of some of these DEPs by Western blot analysis showed that plasminogen activator inhibitor type 2 (SerpinB2) and interstitial collagenase (MMP1) were approximately 90% decreased in HLCS knockdown cells, consistent with a 50%-60% decrease in invasion ability of knockdown cells. Notably, argininosuccinate synthase 1 (ASS1), one of the enzymes in the urea cycle, showed approximately a 10-fold increase in the knockdown cells, suggesting the crucial role of HLCS in supporting the urea cycle in the triple-negative cancer cell line. Collectively, our proteomic data provide biochemical insights into how suppression of HLCS expression perturbs global changes in cellular processes and metabolic pathways, impairing cell growth and invasion.

Chromosomal instability-induced cell invasion through caspase-driven DNA damage

Chromosomal instability (CIN), an increased rate of changes in chromosome structure and number, is observed in most sporadic human carcinomas with high metastatic activity. Here, we use a Drosophila epithelial model to show that DNA damage, as a result of the production of lagging chromosomes during mitosis and aneuploidy-induced replicative stress, contributes to CIN-induced invasiveness. We unravel a sub-lethal role of effector caspases in invasiveness by enhancing CIN-induced DNA damage and identify the JAK/STAT signaling pathway as an activator of apoptotic caspases through transcriptional induction of pro-apoptotic genes. We provide evidence that an autocrine feedforward amplification loop mediated by Upd3-a cytokine with homology to interleukin-6 and a ligand of the JAK/STAT signaling pathway-contributes to amplifying the activation levels of the apoptotic pathway in migrating cells, thus promoting CIN-induced invasiveness. This work sheds new light on the chromosome-signature-independent effects of CIN in metastasis.

Hijacking homeostasis: Regulation of the tumor microenvironment by apoptosis

Cancers are genetically driven, rogue tissues which generate dysfunctional, obdurate organs by hijacking normal, homeostatic programs. Apoptosis is an evolutionarily conserved regulated cell death program and a profoundly important homeostatic mechanism that is common (alongside tumor cell proliferation) in actively growing cancers, as well as in tumors responding to cytotoxic anti-cancer therapies. Although well known for its cell-autonomous tumor-suppressive qualities, apoptosis harbors pro-oncogenic properties which are deployed through non-cell-autonomous mechanisms and which generally remain poorly defined. Here, the roles of apoptosis in tumor biology are reviewed, with particular focus on the secreted and fragmentation products of apoptotic tumor cells and their effects on tumor-associated macrophages, key supportive cells in the aberrant homeostasis of the tumor microenvironment. Historical aspects of cell loss in tumor growth kinetics are considered and the impact (and potential impact) on tumor growth of apoptotic-cell clearance (efferocytosis) as well as released soluble and extracellular vesicle-associated factors are discussed from the perspectives of inflammation, tissue repair, and regeneration programs. An "apoptosis-centric" view is proposed in which dying tumor cells provide an important platform for intricate intercellular communication networks in growing cancers. The perspective has implications for future research and for improving cancer diagnosis and therapy.

Caspases help to spread the message via extracellular vesicles

Cell-cell communication is an essential aspect of multicellular life, key for coordinating cell proliferation, growth, and death in response to environmental changes. Whilst caspases are well-known for facilitating apoptotic and pyroptotic cell death, several recent investigations are uncovering new roles for these enzymes in biological scenarios requiring long-range intercellular signalling mediated by extracellular vesicles (EVs). EVs are small membrane-bound nanoparticles released from cells that may carry and deliver cargo between distant cells, thus helping to coordinate their behaviour. Intriguingly, there is emerging evidence indicating a key contribution of caspases in the biogenesis of EVs, the selection of their cargo content, and EV uptake/function in recipient cells. Here, we discuss the latest findings supporting the interplay between caspases and EVs, and the biological relevance of this molecular convergence for cellular signalling, principally in non-apoptotic scenarios.

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.

Ghost messages: cell death signals spread

Cell death is a mystery in various forms. Whichever type of cell death, this is always accompanied by active or passive molecules release. The recent years marked the renaissance of the study of these molecules showing they can signal to and communicate with recipient cells and regulate physio- or pathological events. This review summarizes the defined forms of messages cells could spread while dying, the effects of these signals on the target tissue/cells, and how these types of communications regulate physio- or pathological processes. By doing so, this review hopes to identify major unresolved questions in the field, formulate new hypothesis worthy of further investigation, and when possible, provide references for the search of novel diagnostic/therapeutics agents. Video abstract.

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.

Toll-9 interacts with Toll-1 to mediate a feedback loop during apoptosis-induced proliferation in Drosophila

Drosophila Toll-1 and all mammalian Toll-like receptors regulate innate immunity. However, the functions of the remaining eight Toll-related proteins in Drosophila are not fully understood. Here, we show that Drosophila Toll-9 is necessary and sufficient for a special form of compensatory proliferation after apoptotic cell loss (undead apoptosis-induced proliferation [AiP]). Mechanistically, for AiP, Toll-9 interacts with Toll-1 to activate the intracellular Toll-1 pathway for nuclear translocation of the NF-κB-like transcription factor Dorsal, which induces expression of the pro-apoptotic genes reaper and hid. This activity contributes to the feedback amplification loop that operates in undead cells. Given that Toll-9 also functions in loser cells during cell competition, we define a general role of Toll-9 in cellular stress situations leading to the expression of pro-apoptotic genes that trigger apoptosis and apoptosis-induced processes such as AiP. This work identifies conceptual similarities between cell competition and AiP.

Evolutionary Diversity and Function of Metacaspases in Plants: Similar to but Not Caspases

Caspase is a well-studied metazoan protease involved in programmed cell death and immunity in animals. Obviously, homologues of caspases with evolutionarily similar sequences and functions should exist in plants, and yet, they do not exist in plants. Plants contain structural homologues of caspases called metacaspases, which differ from animal caspases in a rather distinct way. Metacaspases, a family of cysteine proteases, play critical roles in programmed cell death during plant development and defense responses. Plant metacaspases are further subdivided into types I, II, and III. In the type I Arabidopsis MCs, AtMC1 and AtMC2 have similar structures, but antagonistically regulate hypersensitive response cell death upon immune receptor activation. This regulatory action is similar to caspase-1 inhibition by caspase-12 in animals. However, so far very little is known about the biological function of the other plant metacaspases. From the increased availability of genomic data, the number of metacaspases in the genomes of various plant species varies from 1 in green algae to 15 in Glycine max. It is implied that the functions of plant metacaspases will vary due to these diverse evolutions. This review is presented to comparatively analyze the evolution and function of plant metacaspases compared to caspases.

Non-apoptotic activation of Drosophila caspase-2/9 modulates JNK signaling, the tumor microenvironment, and growth of wound-like tumors

Resistance to apoptosis due to caspase deregulation is considered one of the main hallmarks of cancer. However, the discovery of novel non-apoptotic caspase functions has revealed unknown intricacies about the interplay between these enzymes and tumor progression. To investigate this biological problem, we capitalized on a Drosophila tumor model with human relevance based on the simultaneous overactivation of the EGFR and the JAK/STAT signaling pathways. Our data indicate that widespread non-apoptotic activation of initiator caspases limits JNK signaling and facilitates cell fate commitment in these tumors, thus preventing the overgrowth and exacerbation of malignant features of transformed cells. Intriguingly, caspase activity also reduces the presence of macrophage-like cells with tumor-promoting properties in the tumor microenvironment. These findings assign tumor-suppressing activities to caspases independent of apoptosis, while providing molecular details to better understand the contribution of these enzymes to tumor progression.

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.

Necrosis-induced apoptosis promotes regeneration in Drosophila wing imaginal discs

Regeneration is a complex process that requires a coordinated genetic response to tissue loss. Signals from dying cells are crucial to this process and are best understood in the context of regeneration following programmed cell death, like apoptosis. Conversely, regeneration following unregulated forms of death, such as necrosis, have yet to be fully explored. Here, we have developed a method to investigate regeneration following necrosis using the Drosophila wing imaginal disc. We show that necrosis stimulates regeneration at an equivalent level to that of apoptosis-mediated cell death and activates a similar response at the wound edge involving localized JNK signaling. Unexpectedly, however, necrosis also results in significant apoptosis far from the site of ablation, which we have termed necrosis-induced apoptosis (NiA). This apoptosis occurs independent of changes at the wound edge and importantly does not rely on JNK signaling. Furthermore, we find that blocking NiA limits proliferation and subsequently inhibits regeneration, suggesting that tissues damaged by necrosis can activate programmed cell death at a distance from the injury to promote regeneration.

The Duality of Caspases in Cancer, as Told through the Fly

Caspases, a family of cysteine-aspartic proteases, have an established role as critical components in the activation and initiation of apoptosis. Alongside this a variety of non-apoptotic caspase functions in proliferation, differentiation, cellular plasticity and cell migration have been reported. The activity level and context are important factors in determining caspase function. As a consequence of their critical role in apoptosis and beyond, caspases are uniquely situated to have pathological roles, including in cancer. Altered caspase function is a common trait in a variety of cancers, with apoptotic evasion defined as a "hallmark of cancer". However, the role that caspases play in cancer is much more complex, acting both to prevent and to promote tumourigenesis. This review focuses on the major findings in Drosophila on the dual role of caspases in tumourigenesis. This has major implications for cancer treatments, including chemotherapy and radiotherapy, with the activation of apoptosis being the end goal. However, such treatments may inadvertently have adverse effects on promoting tumour progression and acerbating the cancer. A comprehensive understanding of the dual role of caspases will aid in the development of successful cancer therapeutic approaches.

Apoptosis - Fueling the oncogenic fire

Apoptosis, the most extensively studied form of programmed cell death, is essential for organismal homeostasis. Apoptotic cell death has widely been reported as a tumor suppressor mechanism. However, recent studies have shown that apoptosis exerts noncanonical functions and may paradoxically promote tumor growth and metastasis. The hijacking of apoptosis by cancer cells may arise at different levels, either via the interaction of apoptotic cells with their local or distant microenvironment, or through the abnormal pro-oncogenic roles of the main apoptosis effectors, namely caspases and mitochondria, particularly upon failed apoptosis. In this review, we highlight some of the recently described mechanisms by which apoptosis and these effectors may promote cancer aggressiveness. We believe that a better understanding of the noncanonical roles of apoptosis may be crucial for developing more efficient cancer therapies.

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.

Interplay between caspase, Yes-associated protein, and mechanics: A possible switch between life and death?

Organism development requires fine-tuning of the cell number by apoptosis and cell division, as well as proper cell fate specification. These processes are achieved through the integration of intracellular signals and intercellular interactions with neighboring cells as well as the extracellular environment. Apoptosis, a form of cell death typically associated with development and homeostasis, is mainly regulated by the caspase family of proteases. Although caspases are known to initiate and execute apoptosis, it is also known that low caspase levels have a broad spectrum of nonapoptotic functions, including differentiation and organ growth. These different roles of caspases raise intriguing questions: how are caspase levels regulated and what defines the balance between life and death? In this review, we focus on some recent findings that highlight how nonlethal levels of caspase activity, transcriptional coregulator Yes-associated protein (YAP), and mechanical factors influence each other in determining cell fate. We further discuss a possibility that the mechanical signals encountered by cells could regulate the level of caspase activity by mechanics through YAP and, in turn, how this determines whether a cell is susceptible or resistant to undergoing apoptosis in response to cell death stimuli.

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.

Sometimes even apoptosis fails: implications for cancer

Apoptosis is vital for the correct morphogenesis of multi-cellular organisms. However, like most physiological programs, the cell's ability to commit suicide is hijacked by cancer in its own proliferative and invasive interest. We recently showed that inefficient execution of apoptosis (or failed apoptosis) is used by cancer to boost invasiveness.

Basement membrane damage by ROS- and JNK-mediated Mmp2 activation drives macrophage recruitment to overgrown tissue

Macrophages are a major immune cell type infiltrating tumors and promoting tumor growth and metastasis. To elucidate the mechanism of macrophage recruitment, we utilize an overgrowth tumor model ("undead" model) in larval Drosophila imaginal discs that are attached by numerous macrophages. Here we report that changes to the microenvironment of the overgrown tissue are important for recruiting macrophages. First, we describe a correlation between generation of reactive oxygen species (ROS) and damage of the basement membrane (BM) in all neoplastic, but not hyperplastic, models examined. ROS and the stress kinase JNK mediate the accumulation of matrix metalloproteinase 2 (Mmp2), damaging the BM, which recruits macrophages to the tissue. We propose a model where macrophage recruitment to and activation at overgrowing tissue is a multi-step process requiring ROS- and JNK-mediated Mmp2 upregulation and BM damage. These findings have implications for understanding the role of the tumor microenvironment for macrophage activation.

Reduction of endocytic activity accelerates cell elimination during tissue remodeling of the Drosophila epidermal epithelium

Epithelial tissues undergo cell turnover both during development and for homeostatic maintenance. Cells that are no longer needed are quickly removed without compromising the barrier function of the tissue. During metamorphosis, insects undergo developmentally programmed tissue remodeling. However, the mechanisms that regulate this rapid tissue remodeling are not precisely understood. Here, we show that the temporal dynamics of endocytosis modulate physiological cell properties to prime larval epidermal cells for cell elimination. Endocytic activity gradually reduces as tissue remodeling progresses. This reduced endocytic activity accelerates cell elimination through the regulation of Myosin II subcellular reorganization, junctional E-cadherin levels, and caspase activation. Whereas the increased Myosin II dynamics accelerates cell elimination, E-cadherin plays a protective role against cell elimination. Reduced E-cadherin is involved in the amplification of caspase activation by forming a positive-feedback loop with caspase. These findings reveal the role of endocytosis in preventing cell elimination and in the cell-property switching initiated by the temporal dynamics of endocytic activity to achieve rapid cell elimination during tissue remodeling.

Brainiac Caspases: Beyond the Wall of Apoptosis

For the last two decades, caspases, a family of cysteine-aspartic proteases, have evolved from being considered solely as regulators of apoptosis or inflammation to having a wider range of functions. In this mini review, we focus on the most recent “non-apoptotic” roles of caspases in the CNS, particularly in neurons, astrocytes and oligodendrocytes. Non-apoptotic caspase functions in microglia have already been reviewed extensively elsewhere. Here we discuss the involvement of caspases in the activation of the inflammasome, autophagy, and non-apoptotic forms of cell death such as necroptosis and pyroptosis. Also, we review the involvement of caspases in synapses and the processing of aggregates key to neurodegenerative diseases such as Parkinson’s, Alzheimer’s and Huntington’s diseases. Likewise, we mention the recently described involvement of caspases in mitochondrial biogenesis, which is a function independent of the enzymatic activity. We conclude discussing the relevance that “new” functions of caspases have in the CNS and the future of this field of research.

Dronc-independent basal executioner caspase activity sustains Drosophila imaginal tissue growth

Caspase is the enzyme involved in cell death, and its activation via the apoptosome is thought to represent irreversible cellular destruction. Furthermore, accumulating evidence suggests increasingly diverse functions of caspase beyond apoptosis. Here, using Drosophila wing as a model, we reveal that the specific executioner caspases, Dcp-1 and Decay, promote, rather than suppress by inducing apoptosis, tissue growth. These executioner caspases act independently of initiator caspase Dronc and apoptosis. We further show that the caspase-mediated cleavage of Acinus is important for sustaining tissue growth. Our research highlights the importance of executioner caspase-mediated basal proteolytic cleavage of substrates during tissue growth, and the findings hint at the original function of caspase—not apoptosis, but basal proteolytic cleavages for cell vigor.

Caspase is best known as an enzyme involved in programmed cell death, which is conserved among multicellular organisms. In addition to its role in cell death, caspase is emerging as an indispensable enzyme in a wide range of cellular functions, which have recently been termed caspase-dependent nonlethal cellular processes (CDPs). In this study, we examined the involvement of cell death signaling in tissue-size determination using Drosophila wing as a model. We found that the Drosophila executioner caspases Dcp-1 and Decay, but not Drice, promoted wing growth independently of apoptosis. Most of the reports on CDPs argue the importance of the spatiotemporal regulation of the initiator caspase, Dronc; however, this sublethal caspase function was independent of Dronc, suggesting a more diverse array of CDP regulatory mechanisms. Tagging of TurboID, an improved promiscuous biotin ligase that biotinylates neighboring proteins, to the C terminus of caspases revealed the differences among the neighbors of executioner caspases. Furthermore, we found that the cleavage of Acinus, a substrate of the executioner caspase, was important in promoting wing growth. These results demonstrate the importance of executioner caspase-mediated basal proteolytic cleavage of substrates in sustaining tissue growth. Given the existence of caspase-like DEVDase activity in a unicellular alga, our results likely highlight the original function of caspase—not cell death, but basal proteolytic cleavages for cell vigor.

Characterization of TNF-induced cell death in Drosophila reveals caspase- and JNK-dependent necrosis and its role in tumor suppression

Tumor-necrosis factor (TNF) and its superfamily members are pleiotropic cytokines. Activation of TNF can lead to distinct cellular outcomes including inflammation, cell survival, and different forms of cell death, such as apoptosis and necrosis in a context-dependent manner. However, our understanding of what determines the versatile functions of TNF is far from complete. Here, we examined the molecular mechanisms that distinguish the forms of cell death induced by Eiger (Egr), the sole homolog of TNF in Drosophila. We show that expression of Egr in the developing Drosophila eye simultaneously induces apoptosis and apoptosis-independent developmental defects indicated by cellular disorganization, both of which rely on the c-Jun N-terminal kinase (JNK) signaling activity. Intriguingly, when effector caspases DrICE and Dcp-1 are defective or inhibited, expression of Egr triggers necrosis which is characterized by loss of cell membrane integrity, translucent cytoplasm, and aggregation of cellular organelles. Moreover, such Egr-induced necrosis depends on the catalytic activity of the initiator caspase Dronc and the input from JNK signaling but is independent of their roles in apoptosis. Further mosaic analysis with mutants of scribble (scrib), an evolutionarily conserved tumor suppressor gene regulating cell polarity, suggests that Egr/JNK-mediated apoptosis and necrosis establish a two-layered defense system to inhibit the oncogenic growth of scrib mutant cells. Together, we have identified caspase- and JNK-dependent mechanisms underlying Egr-induced apoptosis versus necrosis and their fail-safe roles in tumor suppression in an intact organism in vivo.

The Pancreatic Cancer-Initiating Cell Marker CD44v6 Affects Transcription, Translation, and Signaling: Consequences for Exosome Composition and Delivery

Pancreatic cancer-initiating cells (PaCIC) express CD44v6 and Tspan8. A knockdown (kd) of these markers hinders the metastatic capacity, which can be rescued, if the cells are exposed to CIC-exosomes (TEX). Additional evidence that CD44v6 regulates Tspan8 expression prompted us to explore the impact of these PaCIC markers on nonmetastatic PaCa and PaCIC-TEX. We performed proteome, miRNA, and mRNA deep sequencing analyses on wild-type, CD44v6kd, and Tspan8kd human PaCIC and TEX. Database comparative analyses were controlled by qRT-PCR, Western blot, flow cytometry, and confocal microscopy. Transcriptome analysis of CD44 versus CD44v6 coimmunoprecipitating proteins in cells and TEX revealed that Tspan8, several signal-transducing molecules including RTK, EMT-related transcription factors, and proteins engaged in mRNA processing selectively associate with CD44v6 and that the membrane-attached CD44 intracytoplasmic tail supports Tspan8 and NOTCH transcription. Deep sequencing uncovered a CD44v6 contribution to miRNA processing. Due to the association of CD44v6 with Tspan8 in internalization prone tetraspanin-enriched membrane domains (TEM) and the engagement of Tspan8 in exosome biogenesis, most CD44v6-dependent changes were transferred into TEX such that the input of CD44v6 to TEX activities becomes largely waved in both a CD44v6kd and a Tspan8kd. Few differences between CD44v6kd- and Tspan8kd-TEX rely on CD44v6 being also recovered in non-TEM derived TEX, highlighting distinct TEX delivery from individual cells that jointly account for TEX-promoted target modulation. This leads us to propose a model in which CD44v6 strongly supports tumor progression by cooperating with signaling molecules, altering transcription of key molecules, and through its association with the mRNA processing machinery. The association of CD44v6 with Tspan8, which plays a crucial role in vesicle biogenesis, promotes metastases by transferring CD44v6 activities into TEM and TEM-independently derived TEX. Further investigations of the lead position of CD44v6 in shifting metastasis-promoting activities into CIC-TEX may offer a means of targeting TEX-CD44v6 in therapeutic applications.

Cells with Broken Left–Right Symmetry: Roles of Intrinsic Cell Chirality in Left–Right Asymmetric Epithelial Morphogenesis

Chirality is a fundamental feature in biology, from the molecular to the organismal level. An animal has chirality in the left–right asymmetric structure and function of its body. In general, chirality occurring at the molecular and organ/organism scales has been studied separately. However, recently, chirality was found at the cellular level in various species. This “cell chirality” can serve as a link between molecular chirality and that of an organ or animal. Cell chirality is observed in the structure, motility, and cytoplasmic dynamics of cells and the mechanisms of cell chirality formation are beginning to be understood. In all cases studied so far, proteins that interact chirally with F-actin, such as formin and myosin I, play essential roles in cell chirality formation or the switching of a cell’s enantiomorphic state. Thus, the chirality of F-actin may represent the ultimate origin of cell chirality. Links between cell chirality and left–right body asymmetry are also starting to be revealed in various animal species. In this review, the mechanisms of cell chirality formation and its roles in left–right asymmetric development are discussed, with a focus on the fruit fly Drosophila, in which many of the pioneering studies were conducted.

The Caspase-3 homolog DrICE regulates endocytic trafficking during Drosophila tracheal morphogenesis

Although well known for its role in apoptosis, the executioner caspase DrICE has a non-apoptotic function that is required for elongation of the epithelial tubes of the Drosophila tracheal system. Here, we show that DrICE acts downstream of the Hippo Network to regulate endocytic trafficking of at least four cell polarity, cell junction and apical extracellular matrix proteins involved in tracheal tube size control: Crumbs, Uninflatable, Kune-Kune and Serpentine. We further show that tracheal cells are competent to undergo apoptosis, even though developmentally-regulated DrICE function rarely kills tracheal cells. Our results reveal a developmental role for caspases, a pool of DrICE that co-localizes with Clathrin, and a mechanism by which the Hippo Network controls endocytic trafficking. Given reports of in vitro regulation of endocytosis by mammalian caspases during apoptosis, we propose that caspase-mediated regulation of endocytic trafficking is an evolutionarily conserved function of caspases that can be deployed during morphogenesis.

Caspases are well-known drivers of apoptosis, although recent studies suggest potential non-apoptotic functions. Here, McSharry and Beitel show that the Drosophila executioner caspase DrICE regulates endocytic trafficking of key proteins downstream of Hippo during tracheal morphogenesis.

Non-apoptotic function of Drosophila caspase activation in epithelial thorax closure and wound healing

Non-apoptotic caspase activation involves multiple cellular events. However, the link between visible non-apoptotic caspase activation and its function in living organisms has not yet been revealed. Here, we visualized sub-lethal activation of apoptotic signaling with the combination of a sensitive indicator for caspase 3 activation and in vivo live-imaging analysis of Drosophila. During thorax closure in pupal development, caspase 3 activation was specifically observed at the leading edge cells, with no signs of apoptosis. Inhibition of caspase activation led to an increase in thorax closing speed, which suggests a role of non-apoptotic caspase activity in cell motility. Importantly, sub-lethal activation of caspase 3 was also observed during wound closure at the fusion sites at which thorax closure had previously taken place. Further genetic analysis revealed that the activation of the initiator caspase Dronc is coupled with the generation of reactive oxygen species. The activation of Dronc also regulates myosin levels and delays wound healing. Our findings suggest a possible function for non-apoptotic caspase activation in the fine-tuning of cell migratory behavior during epithelial closure.

Using Nanobodies to Study Protein Function in Developing Organisms

Polyclonal and monoclonal antibodies have been invaluable tools to study proteins over the past decades. While indispensable for most biological studies including developmental biology, antibodies have been used mostly in fixed tissues or as binding reagents in the extracellular milieu. For functional studies and for clinical applications, antibodies have been functionalized by covalently fusing them to heterologous partners (i.e., chemicals, proteins or other moieties). Such functionalized antibodies have been less widely used in developmental biology studies. In the past few years, the discovery and application of small functional binding fragments derived from single-chain antibodies, so-called nanobodies, has resulted in novel approaches to study proteins during the development of multicellular animals in vivo. Expression of functionalized nanobody fusions from integrated transgenes allows manipulating proteins of interest in the extracellular and the intracellular milieu in a tissue- and time-dependent manner in an unprecedented manner. Here, we describe how nanobodies have been used in the field of developmental biology and look into the future to imagine how else nanobody-based reagents could be further developed to study the proteome in living organisms.

Proper direction of male genitalia is prerequisite for copulation in Drosophila, implying cooperative evolution between genitalia rotation and mating behavior

Animal morphology and behavior often appear to evolve cooperatively. However, it is difficult to assess how strictly these two traits depend on each other. The genitalia morphologies and courtship behaviors in insects, which vary widely, may be a good model for addressing this issue. In Diptera, phylogenetic analyses of mating positions suggested that the male-above position evolved from an end-to-end one. However, with this change in mating position, the dorsoventral direction of the male genitalia became upside down with respect to that of the female genitalia. It was proposed that to compensate for this incompatibility, the male genitalia rotated an additional 180° during evolution, implying evolutionary cooperativity between the mating position and genitalia direction. According to this scenario, the proper direction of male genitalia is critical for successful mating. Here, we tested this hypothesis using a Drosophila Myosin31DF (Myo31DF) mutant, in which the rotation of the male genitalia terminates prematurely, resulting in various deviations in genitalia direction. We found that the proper dorsoventral direction of the male genitalia was a prerequisite for successful copulation, but it did not affect the other courtship behaviors. Therefore, our results suggested that the male genitalia rotation and mating position evolved cooperatively in Drosophila.

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.

To the edge of cell death and back

Programmed cell death plays a central role in maintaining homeostasis. Various studies have demonstrated that programmed cell death is not a one-way street; cells can survive even when the core cell death processes are underway. Cell death initiation, prevention, and recovery function in a coordinated fashion to establish and maintain a homeostatic environment. In this review, we discuss how dying cells can be rescued from death's grip and the subsequent physiological consequences. We suggest a fundamental question to be answered-at least at the single cell level is, can we predict if a certain cell is more or less likely to survive or die? And importantly, what physiological and pathological consequences, as well as therapeutic approaches can we delineate from this ability to predict cell death versus survival.

Novel initiator caspase reporters uncover previously unknown features of caspase-activating cells

The caspase-mediated regulation of many cellular processes, including apoptosis, justifies the substantial interest in understanding all of the biological features of these enzymes. To complement functional assays, it is crucial to identify caspase-activating cells in live tissues. Our work describes novel initiator caspase reporters that, for the first time, provide direct information concerning the initial steps of the caspase activation cascade in Drosophila tissues. One of our caspase sensors capitalises on the rapid subcellular localisation change of a fluorescent marker to uncover novel cellular apoptotic events relating to the actin-mediated positioning of the nucleus before cell delamination. The other construct benefits from caspase-induced nuclear translocation of a QF transcription factor. This feature enables the genetic manipulation of caspase-activating cells and reveals the spatiotemporal patterns of initiator caspase activity. Collectively, our sensors offer experimental opportunities not available by using previous reporters and have proven useful to illuminate previously unknown aspects of caspase-dependent processes in apoptotic and non-apoptotic cellular scenarios.

Cellular plasticity, caspases and autophagy; that which does not kill us, well, makes us different

The ability to regenerate is a fundamental requirement for tissue homeostasis. Regeneration draws on three sources of cells. First and best-studied are dedicated stem/progenitor cells. Second, existing cells may proliferate to compensate for the lost cells of the same type. Third, a different cell type may change fate to compensate for the lost cells. This review focuses on regeneration of the third type and will discuss the contributions by post-transcriptional mechanisms including the emerging evidence for cell-autonomous and non-lethal roles of cell death pathways.

Hydrogen Peroxide and Redox Regulation of Developments

Reactive oxygen species (ROS), which were originally classified as exclusively deleterious compounds, have gained increasing interest in the recent years given their action as bona fide signalling molecules. The main target of ROS action is the reversible oxidation of cysteines, leading to the formation of disulfide bonds, which modulate protein conformation and activity. ROS, endowed with signalling properties, are mainly produced by NADPH oxidases (NOXs) at the plasma membrane, but their action also involves a complex machinery of multiple redox-sensitive protein families that differ in their subcellular localization and their activity. Given that the levels and distribution of ROS are highly dynamic, in part due to their limited stability, the development of various fluorescent ROS sensors, some of which are quantitative (ratiometric), represents a clear breakthrough in the field and have been adapted to both ex vivo and in vivo applications. The physiological implication of ROS signalling will be presented mainly in the frame of morphogenetic processes, embryogenesis, regeneration, and stem cell differentiation. Gain and loss of function, as well as pharmacological strategies, have demonstrated the wide but specific requirement of ROS signalling at multiple stages of these processes and its intricate relationship with other well-known signalling pathways.