C. elegans Dynamin mediates the signaling of phagocytic receptor CED-1 for the engulfment and degradation of apoptotic cells

CED-6/GULP and components of the clathrin-mediated endocytosis machinery act redundantly to correctly display CED-1 on the cell membrane in Caenorhabditis elegans

CED-1 (cell death abnormal) is a transmembrane receptor involved in the recognition of "eat-me" signals displayed on the surface of apoptotic cells and thus central for the subsequent engulfment of the cell corpse in Caenorhabditis elegans. The roles of CED-1 in engulfment are well established, as are its downstream effectors. The latter include the adapter protein CED-6/GULP and the ATP-binding cassette family homolog CED-7. However, how CED-1 is maintained on the plasma membrane in the absence of engulfment is currently unknown. Here, we show that CED-6 and CED-7 have a novel role in maintaining CED-1 correctly on the plasma membrane. We propose that the underlying mechanism is via endocytosis as CED-6 and CED-7 act redundantly with clathrin and its adaptor, the Adaptor protein 2 complex, in ensuring correct CED-1 localization. In conclusion, CED-6 and CED-7 impact other cellular processes than engulfment of apoptotic cells.

Tracking Chlamydia - Host interactions and antichlamydial activity in Caenorhabditis elegans

The fading efficacy of antibiotics is a growing global health concern due to its life-threatening consequences and increased healthcare costs. Non-genetic mechanisms of antimicrobial resistance, such as those employed by Chlamydia pneumoniae and Chlamydia trachomatis, complicate treatment as these bacteria can enter a non-replicative, persistent state under stress, evading antibiotics and linking to inflammatory conditions. Understanding chlamydial persistence at the molecular level is challenging, and new models for studying Chlamydia-host interactions in vivo are urgently needed. Caenorhabditis elegans offers an alternative given its immune system and numerous orthologues of human genes. This study established C. elegans as an in vivo model for chlamydial infection. Both Chlamydia species reduced the worm's lifespan, their DNA being detectable at three- and six-days post-infection. Azithromycin at its MIC (25 nM) failed to prevent the infection-induced lifespan reduction, indicating a persister phenotype. In contrast, the methanolic extract of Schisandra chinensis berries showed anti-chlamydial activity both in vitro (in THP-1 macrophages) and in vivo, significantly extending the lifespan of infected C. elegans and reducing the bacterial load. Moreover, S. chinensis increased the transcriptional activity of SKN-1 in the worms, but was unable to impact the bacterial load or lifespan in a sek-1 defective C. elegans strain. In summary, this study validated C. elegans as a chlamydial infection model and showcased S. chinensis berries' in vivo anti-chlamydial potential, possibly through SEK/SKN-1 signaling modulation.

Apoptosis and beyond: A new era for programmed cell death in Caenorhabditis elegans

Programmed cell death (PCD) is crucial for normal development and homeostasis. Our first insights into the genetic regulation of apoptotic cell death came from in vivo studies in the powerful genetic model system of C. elegans. More recently, novel developmental cell death programs occurring both embryonically and post-embryonically, and sex-specifically, have been elucidated. Recent studies in the apoptotic setting have also shed new light on the intricacies of phagocytosis in particular. This review provides a brief historical perspective of the origins of PCD studies in C. elegans, followed by a more detailed description of non-canonical apoptotic and non-apoptotic death programs. We conclude by posing open questions and commenting on our outlook on the future of PCD studies in C. elegans, highlighting the importance of advanced imaging tools and the continued leveraging of C. elegans genetics both with classical and modern cutting-edge approaches.

Regulation and Functions of Autophagy During Animal Development

Autophagy is used to degrade cytoplasmic materials, and is critical to maintain cell and organismal health in diverse animals. Here we discuss the regulation, utilization and impact of autophagy on development, including roles in oogenesis, spermatogenesis and embryogenesis in animals. We also describe how autophagy influences postembryonic development in the context of neuronal and cardiac development, wound healing, and tissue regeneration. We describe recent studies of selective autophagy during development, including mitochondria-selective autophagy and endoplasmic reticulum (ER)-selective autophagy. Studies of developing model systems have also been used to discover novel regulators of autophagy, and we explain how studies of autophagy in these physiologically relevant systems are advancing our understanding of this important catabolic process.

The dynamin GTPase mediates regenerative axonal fusion in Caenorhabditis elegans by regulating fusogen levels

Axonal fusion is a neuronal repair mechanism that results in the reconnection of severed axon fragments, leading to the restoration of cytoplasmic continuity and neuronal function. While synaptic vesicle recycling has been linked to axonal regeneration, its role in axonal fusion remains unknown. Dynamin proteins are large GTPases that hydrolyze lipid-binding membranes to carry out clathrin-mediated synaptic vesicle recycling. Here, we show that the Caenorhabditis elegans dynamin protein DYN-1 is a key component of the axonal fusion machinery. Animals carrying a temperature-sensitive allele of dyn-1(ky51) displayed wild-type levels of axonal fusion at the permissive temperature (15°C) but presented strongly reduced levels at the restrictive temperature (25°C). Furthermore, the average length of regrowth was significantly diminished in dyn-1(ky51) animals at the restrictive temperature. The expression of wild-type DYN-1 cell-autonomously into dyn-1(ky51) mutant animals rescued both the axonal fusion and regrowth defects. Furthermore, DYN-1 was not required prior to axonal injury, suggesting that it functions specifically after injury to control axonal fusion. Finally, using epistatic analyses and superresolution imaging, we demonstrate that DYN-1 regulates the levels of the fusogen protein EFF-1 post-injury to mediate axonal fusion. Together, these results establish DYN-1 as a novel regulator of axonal fusion.

Effect of gamma-ray irradiated reduced graphene oxide (rGO) on environmental health: An in-vitro and in-vivo studies

The unique properties of reduced graphene oxide (rGO) have drawn the attention of scientists worldwide since the last decade and it is explored for a wide range of applications. However, the rapid expansion of rGO use in various products will eventually lead to environenal exposure and rises a safety concern on the environment and humal health risk. Moreover, the utilization of toxic chemicals for the reduction of graphene oxide (GO) into rGO is not environmentally friendly, warranting the exploration of non-toxic approaches. In the present work, rGO was synthesized using a different dose of gamma-ray irradiation and characterized. The in-vitro and in-vivo analysis indicated that the gamma-irradiated rGO induced toxicity depending on its degree of reduction and dosage. In the L929 cells, rGO-30 KGy significantly induced cytotoxicity even at low concentration (1 mg L^-1) by inducing reactive oxygen species (ROS), lactate dehydrogenase (LDH) enzyme production, nuclear fragmentation and apoptosis. The change in morphology of the cells like membrane blebbing and cell rounding was also observed via FESEM. In the in-vivo model Caenorhabditis elegans, rGO-30 KGy significantly affected the functioning of primary and secondary targeted organs and also negatively influenced the nuclear accumulation of transcription factors (DAF-16/FOXO and SKN-1/Nrf2), neuronal health, and antioxidant defense mechanism of the nematodes. The real-time PCR analysis showed significant up-regulation (ced-3, ced-4, cep-1, egl-1, and hus-1) and down-regulation (ced-9) of the gene involved in germ-line and DNA damage-induced apoptosis. The detailed toxicity mechanism of gamma irradiated rGO has been elucidated. This work highlights the toxicity of rGO prepared by gamma-ray radiation and paves way for understating the toxicity mechanism.

Microglial efferocytosis: Diving into the Alzheimer's disease gene pool

Genome-wide association studies and functional genomics studies have linked specific cell types, genes, and pathways to Alzheimer's disease (AD) risk. In particular, AD risk alleles primarily affect the abundance or structure, and thus the activity, of genes expressed in macrophages, strongly implicating microglia (the brain-resident macrophages) in the etiology of AD. These genes converge on pathways (endocytosis/phagocytosis, cholesterol metabolism, and immune response) with critical roles in core macrophage functions such as efferocytosis. Here, we review these pathways, highlighting relevant genes identified in the latest AD genetics and genomics studies, and describe how they may contribute to AD pathogenesis. Investigating the functional impact of AD-associated variants and genes in microglia is essential for elucidating disease risk mechanisms and developing effective therapeutic approaches.

trim-21 promotes proteasomal degradation of CED-1 for apoptotic cell clearance in C. elegans

The phagocytic receptor CED-1 mediates apoptotic cell recognition by phagocytic cells, enabling cell corpse clearance in Caenorhabditis elegans. Whether appropriate levels of CED-1 are maintained for executing the engulfment function remains unknown. Here, we identified the C. elegans E3 ubiquitin ligase tripartite motif containing-21 (TRIM-21) as a component of the CED-1 pathway for apoptotic cell clearance. When the NPXY motif of CED-1 was bound to the adaptor protein CED-6 or the YXXL motif of CED-1 was phosphorylated by tyrosine kinase SRC-1 and subsequently bound to the adaptor protein NCK-1 containing the SH2 domain, TRIM-21 functioned in conjunction with UBC-21 to catalyze K48-linked poly-ubiquitination on CED-1, targeting it for proteasomal degradation. In the absence of TRIM-21, CED-1 accumulated post-translationally and drove cell corpse degradation defects, as evidenced by direct binding to VHA-10. These findings reveal a unique mechanism for the maintenance of appropriate levels of CED-1 to regulate apoptotic cell clearance.

Involvement of phosphatidylserine receptors in the skeletal muscle regeneration: therapeutic implications

Sarcopenia is a progressive loss of muscle mass and strength with a risk of adverse outcomes such as disability, poor quality of life, and death. Increasing evidence indicates that diminished ability of the muscle to activate satellite cell-dependent regeneration is one of the factors that might contribute to its development. Skeletal muscle regeneration following myogenic cell death results from the proliferation and differentiation of myogenic stem cells, called satellite cells, located beneath the basal lamina of the muscle fibres. Satellite cell differentiation is not a satellite cell-autonomous process but depends on signals provided by the surrounding cells. Infiltrating macrophages play a key role in the process partly by clearing the necrotic cell debris, partly by producing cytokines and growth factors that guide myogenesis. At the beginning of the muscle regeneration process, macrophages are pro-inflammatory, and the cytokines produced by them trigger the proliferation and differentiation of satellite cells. Following the uptake of dead cells, however, a transcriptionally regulated phenotypic change (macrophage polarization) is induced in them resulting in their transformation into healing macrophages that guide resolution of inflammation, completion of myoblast differentiation, myoblast fusion and growth, and return to homeostasis. Impaired efferocytosis results in delayed cell death clearance, delayed macrophage polarization, prolonged inflammation, and impaired muscle regeneration. Thus, proper efferocytosis by macrophages is a determining factor during muscle repair. Here we review that both efferocytosis and myogenesis are dependent on the cell surface phosphatidylserine (PS), and surprisingly, these two processes share a number of common PS receptors and signalling pathways. Based on these findings, we propose that stimulating the function of PS receptors for facilitating muscle repair following injury could be a successful approach, as it would enhance efferocytosis and myogenesis simultaneously. Because increasing evidence indicates a pathophysiological role of impaired efferocytosis in the development of chronic inflammatory conditions, as well as in impaired muscle regeneration both contributing to the development of sarcopenia, improving efferocytosis should be considered also in its management. Again applying or combining those treatments that target PS receptors would be expected to be the most effective, because they would also promote myogenesis. A potential PS receptor-triggering candidate molecule is milk fat globule-EGF-factor 8 (MFG-E8), which not only stimulates PS-dependent efferocytosis and myoblast fusion but also promotes extracellular signal-regulated kinase (ERK) and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

Autophagosomes fuse to phagosomes and facilitate the degradation of apoptotic cells in Caenorhabditis elegans

Autophagosomes are double-membrane intracellular vesicles that degrade protein aggregates, intracellular organelles, and other cellular components. During the development of the nematode Caenorhabditis elegans, many somatic and germ cells undergo apoptosis. These cells are engulfed and degraded by their neighboring cells. We discovered a novel role of autophagosomes in facilitating the degradation of apoptotic cells using a real-time imaging technique. Specifically, the double-membrane autophagosomes in engulfing cells are recruited to the surfaces of phagosomes containing apoptotic cells and subsequently fuse to phagosomes, allowing the inner vesicle to enter the phagosomal lumen. Mutants defective in the production of autophagosomes display significant defects in the degradation of apoptotic cells, demonstrating the importance of autophagosomes to this process. The signaling pathway led by the phagocytic receptor CED-1, the adaptor protein CED-6, and the large GTPase dynamin (DYN-1) promotes the recruitment of autophagosomes to phagosomes. Moreover, the subsequent fusion of autophagosomes with phagosomes requires the functions of the small GTPase RAB-7 and the HOPS complex. Further observations suggest that autophagosomes provide apoptotic cell-degradation activities in addition to and in parallel of lysosomes. Our findings reveal that, unlike the single-membrane, LC3-associated phagocytosis (LAP) vesicles reported to facilitate phagocytosis in mammals, it is the canonical double-membrane autophagosomes that facilitate the clearance of C. elegans apoptotic cells. These findings add autophagosomes to the collection of intracellular organelles that contribute to phagosome maturation, identify novel crosstalk between the autophagy and phagosome maturation pathways, and discover the upstream signaling molecules that initiate this crosstalk.

Conserved and Distinct Elements of Phagocytosis in Human and C. elegans

Endocytosis provides the cellular nutrition and homeostasis of organisms, but pathogens often take advantage of this entry point to infect host cells. This is counteracted by phagocytosis that plays a key role in the protection against invading microbes both during the initial engulfment of pathogens and in the clearance of infected cells. Phagocytic cells balance two vital functions: preventing the accumulation of cell corpses to avoid pathological inflammation and autoimmunity, whilst maintaining host defence. In this review, we compare elements of phagocytosis in mammals and the nematode Caenorhabditis elegans. Initial recognition of infection requires different mechanisms. In mammals, pattern recognition receptors bind pathogens directly, whereas activation of the innate immune response in the nematode rather relies on the detection of cellular damage. In contrast, molecules involved in efferocytosis—the engulfment and elimination of dying cells and cell debris—are highly conserved between the two species. Therefore, C. elegans is a powerful model to research mechanisms of the phagocytic machinery. Finally, we show that both mammalian and worm studies help to understand how the two phagocytic functions are interconnected: emerging data suggest the activation of innate immunity as a consequence of defective apoptotic cell clearance.

Response of G protein-coupled receptor CED-1 in germline to polystyrene nanoparticles in Caenorhabditis elegans

The deposition of a certain amount of nanopolystyrene (NPS) can be observed in the gonad of Caenorhabditis elegans. However, we still know little about the response of germline towards NPS exposure. In the germline of C. elegans, NPS (1-1000 μg L^-1) increased the expression levels of two G protein-coupled receptors (GPCRs), namely PAQR-2 and CED-1. Moreover, susceptibility to NPS toxicity was observed in ced-1(RNAi) worms, which suggested that the protective response of germline was mediated by GPCR CED-1. In the germline, five proteins (CED-10, VPS-34, SNX-1, RAB-7, and RAB-14) functioned as downstream targets of GPCR CED-1 in controlling NPS toxicity. Furthermore, these five targets in the germline regulated NPS toxicity by affecting the activities of p38 MAPK and insulin signaling pathways in intestinal cells. Therefore, we raised a GPCR CED-1-mediated signaling cascade in the germline in response to NPS exposure, which is helpful for understanding the molecular basis of the germline in response to NPS exposure.

Glia actively sculpt sensory neurons by controlled phagocytosis to tune animal behavior

Glia in the central nervous system engulf neuron fragments to remodel synapses and recycle photoreceptor outer segments. Whether glia passively clear shed neuronal debris or actively prune neuron fragments is unknown. How pruning of single-neuron endings impacts animal behavior is also unclear. Here, we report our discovery of glia-directed neuron pruning in Caenorhabditis elegans. Adult C. elegans AMsh glia engulf sensory endings of the AFD thermosensory neuron by repurposing components of the conserved apoptotic corpse phagocytosis machinery. The phosphatidylserine (PS) flippase TAT-1/ATP8A functions with glial PS-receptor PSR-1/PSR and PAT-2/α-integrin to initiate engulfment. This activates glial CED-10/Rac1 GTPase through the ternary GEF complex of CED-2/CrkII, CED-5/DOCK180, CED-12/ELMO. Execution of phagocytosis uses the actin-remodeler WSP-1/nWASp. This process dynamically tracks AFD activity and is regulated by temperature, the AFD sensory input. Importantly, glial CED-10 levels regulate engulfment rates downstream of neuron activity, and engulfment-defective mutants exhibit altered AFD-ending shape and thermosensory behavior. Our findings reveal a molecular pathway underlying glia-dependent engulfment in a peripheral sense-organ and demonstrate that glia actively engulf neuron fragments, with profound consequences on neuron shape and animal sensory behavior.

Neurons are tree-shaped cells that receive information through endings connected to neighbouring cells or the environment. Controlling the size, number and location of these endings is necessary to ensure that circuits of neurons get precisely the right amount of input from their surroundings.

Glial cells form a large portion of the nervous system, and they are tasked with supporting, cleaning and protecting neurons. In humans, part of their duties is to ‘eat’ (or prune) unnecessary neuron endings. In fact, this role is so important that defects in glial pruning are associated with conditions such as Alzheimer’s disease. Yet it is still unknown how pruning takes place, and in particular whether it is the neuron or the glial cell that initiates the process.

To investigate this question, Raiders et al. enlisted the common laboratory animal Caenorhabditis elegans, a tiny worm with a simple nervous system where each neuron has been meticulously mapped out. First, the experiments showed that glial cells in C. elegans actually prune the endings of sensory neurons. Focusing on a single glia-neuron pair then revealed that the glial cell could trim the endings of a living neuron by redeploying the same molecular machinery it uses to clear dead cell debris. Compared to this debris-clearing activity, however, the glial cell takes a more nuanced approach to pruning: specifically, it can adjust the amount of trimming based on the activity load of the neuron.

When Raiders et al. disrupted the glial pruning for a single temperature-sensing neuron, the worm lost its normal temperature preferences; this demonstrated how the pruning activity of a single glial cell can be linked to behavior.

Taken together the experiments showcase how C. elegans can be used to study glial pruning. Further work using this model could help to understand how disease emerges when glial cells cannot perform their role, and to spot the genetic factors that put certain individuals at increased risk for neurological and sensory disorders.

Worms, Fat, and Death: Caenorhabditis elegans Lipid Metabolites Regulate Cell Death

Caenorhabditis elegans is well-known as the model organism used to elucidate the genetic pathways underlying the first described form of regulated cell death, apoptosis. Since then, C. elegans investigations have contributed to the further understanding of lipids in apoptosis, especially the roles of phosphatidylserines and phosphatidylinositols. More recently, studies in C. elegans have shown that dietary polyunsaturated fatty acids can induce the non-apoptotic, iron-dependent form of cell death, ferroptosis. In this review, we examine the roles of various lipids in specific aspects of regulated cell death, emphasizing recent work in C. elegans.

Calcium ions trigger the exposure of phosphatidylserine on the surface of necrotic cells

Intracellular Ca²⁺ level is under strict regulation through calcium channels and storage pools including the endoplasmic reticulum (ER). Mutations in certain ion channel subunits, which cause mis-regulated Ca²⁺ influx, induce the excitotoxic necrosis of neurons. In the nematode Caenorhabditis elegans, dominant mutations in the DEG/ENaC sodium channel subunit MEC-4 induce six mechanosensory (touch) neurons to undergo excitotoxic necrosis. These necrotic neurons are subsequently engulfed and digested by neighboring hypodermal cells. We previously reported that necrotic touch neurons actively expose phosphatidylserine (PS), an “eat-me” signal, to attract engulfing cells. However, the upstream signal that triggers PS externalization remained elusive. Here we report that a robust and transient increase of cytoplasmic Ca²⁺ level occurs prior to the exposure of PS on necrotic touch neurons. Inhibiting the release of Ca²⁺ from the ER, either pharmacologically or genetically, specifically impairs PS exposure on necrotic but not apoptotic cells. On the contrary, inhibiting the reuptake of cytoplasmic Ca²⁺ into the ER induces ectopic necrosis and PS exposure. Remarkably, PS exposure occurs independently of other necrosis events. Furthermore, unlike in mutants of DEG/ENaC channels, in dominant mutants of deg-3 and trp-4, which encode Ca²⁺ channels, PS exposure on necrotic neurons does not rely on the ER Ca²⁺ pool. Our findings indicate that high levels of cytoplasmic Ca²⁺ are necessary and sufficient for PS exposure. They further reveal two Ca²⁺-dependent, necrosis-specific pathways that promote PS exposure, a “two-step” pathway initiated by a modest influx of Ca²⁺ and further boosted by the release of Ca²⁺ from the ER, and another, ER-independent, pathway. Moreover, we found that ANOH-1, the worm homolog of mammalian phospholipid scramblase TMEM16F, is necessary for efficient PS exposure in thapsgargin-treated worms and trp-4 mutants, like in mec-4 mutants. We propose that both the ER-mediated and ER-independent Ca²⁺ pathways promote PS externalization through activating ANOH-1.

Necrosis is a type of cell death that exhibits distinct morphological features such as cell and organelle swelling. Necrotic cells expose phosphatidylserine (PS)–a type of phospholipid—on their outer surfaces. Receptor molecules on phagocytes detect PS on necrotic cells and subsequently initiate the engulfment process. As necrosis is associated with stroke, cancer, neurodegenerative diseases, and heart diseases, studying necrotic cell clearance has important medical relevance. In the model organism the nematode C. elegans, we previously identified membrane proteins that promote the exposure of PS on necrotic cell surfaces by studying neurons that are induced to undergo necrosis by dominant mutations in ion channels. Here, in C. elegans, we have discovered that the necrotic insults trigger an increase of the cytoplasmic calcium ion (Ca²⁺), which in turn promotes PS externalization on necrotic cell surfaces. Furthermore, we have identified two different mechanisms that increase cytoplasmic Ca²⁺ levels, one dependent on the Ca²⁺ contribution from the endoplasmic reticulum (ER), the other independent of the ER. The Ca²⁺ signal targets ANOH-1, a worm homolog of mammalian proteins capable of externalizing PS, for promoting PS exposure on necrotic cells. Our findings reveal novel upstream regulatory mechanisms that promote necrotic cell clearance in animals.

SUMO modification in apoptosis

Apoptosis and clearance of dead cells is highly evolutionarily conserved from nematode to humans, which is crucial to the growth and development of multicellular organism. Fail to remove apoptotic cells often lead to homeostasis imbalance, fatal autoimmune diseases, and neurodegenerative diseases. Small ubiquitin-related modifiers (SUMOs) modification is a post-translational modification of ubiquitin proteins mediated by the sentrin-specific proteases (SENPs) family. SUMO modification is widely involved in many cellular biological process, and abnormal SUMO modification is also closely related to many major human diseases. Recent researches have revealed that SUMO modification event occurs during apoptosis and clearance of apoptotic cells, and plays an important role in the regulation of apoptotic signaling pathways. This review summarizes some recent progress in the revelation of regulatory mechanisms of these pathways and provides some potential researching hotpots of the SUMO modification regulation to apoptosis.

Involvement of the synaptotagmin/stonin2 system in vesicular transport regulated by semaphorins in Caenorhabditis elegans epidermal cells

Vesicular transport serves as an important mechanism for cell shape regulation during development. Although the semaphorin signaling molecule, a well-known regulator of axon guidance, induces endocytosis in the growth cone and the axonal transport of vertebrate neurons, the underlying molecular mechanisms remain largely unclear. Here, we show that the Caenorhabditis elegans SNT-1/synaptotagmin-UNC-41/stonin2 system, whose role in synaptic vesicle recycling in neurons has been studied extensively, is involved in semaphorin-regulated vesicular transport in larval epidermal cells. Mutations in the snt-1/unc-41 genes strongly suppressed the cell shape defects of semaphorin mutants. The null mutation in the semaphorin receptor gene, plx-1, altered the expression and localization pattern of endocytic and exocytic markers in the epidermal cells while repressing the transport of SNT-1-containing vesicles toward late endosome/lysosome pathways. Our findings suggest that the nematode semaphorins regulate the vesicular transport in epidermal cells in a manner distinct from that of vertebrate semaphorins in neurons.

The small GTPase RAB-35 facilitates the initiation of phagosome maturation and acts as a robustness factor for apoptotic cell clearance

We recently identified the novel function of the small GTPase RAB-35 in apoptotic cell clearance in Caenorhabditis elegans, a process in which dying cells are engulfed and degraded inside phagosomes. We have found that RAB-35 functions in two separate steps of cell corpse clearance, cell corpse recognition and the initiation of phagosome maturation. During the latter process, RAB-35 facilitates the removal of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) from the membranes of nascent phagosomes and the simultaneous production of phosphatidylinositol-3-P (PI(3)P) on these same membranes, a process that we have coined the PI(4,5)P₂ to PI(3)P shift. RAB-35 also promotes the recruitment of the small GTPase RAB-5 to the phagosomal surface. During these processes, the activity of RAB-35 is controlled by the candidate GTPase-activating protein (GAP) TBC-10 and the candidate guanine nucleotide exchange factor (GEF) FLCN-1. Overall, RAB-35 leads a third pathway during cell corpse clearance that functions in parallel to the two known pathways, one led by the phagocytic receptor CED-1 and the other led by the CED-10/Rac1 GTPase. Here, we further report that RAB-35 acts as a robustness factor that maintains the clearance activity and embryonic viability under conditions of heat stress. Moreover, we obtained additional evidence suggesting that RAB-35 acts upstream of RAB-5 and RAB-7. To establish a precise temporal pattern for its own dissociation from phagosomal surfaces, RAB-35 controls the removal of its own GAP. We propose that RAB-35 defines a largely unexplored initial phase of phagosome maturation.

Gulp1 controls Eph/ephrin trogocytosis and is important for cell rearrangements during development

Trogocytosis, intercellular cannibalism distinct from phagocytosis, occurs when cells rearrange during development. Here, Gong et al. reveal that trogocytosis induced by ephrins and Eph receptors involves phagocytic adaptor protein Gulp1, Rac-specific guanine nucleotide exchange factor Tiam2, and endocytic GTPase dynamin. These results suggest that ephrin/Eph-induced trogocytosis uses phagocytosis-like mechanisms.

Trogocytosis, in which cells nibble away parts of neighboring cells, is an intercellular cannibalism process conserved from protozoa to mammals. Its underlying molecular mechanisms are not well understood and are likely distinct from phagocytosis, a process that clears entire cells. Bi-directional contact repulsion induced by Eph/ephrin signaling involves transfer of membrane patches and full-length Eph/ephrin protein complexes between opposing cells, resembling trogocytosis. Here, we show that the phagocytic adaptor protein Gulp1 regulates EphB/ephrinB trogocytosis to achieve efficient cell rearrangements of cultured cells and during embryonic development. Gulp1 mediates trogocytosis bi-directionally by dynamic engagement with EphB/ephrinB protein clusters in cooperation with the Rac-specific guanine nucleotide exchange factor Tiam2. Ultimately, Gulp1’s presence at the Eph/ephrin cluster is a prerequisite for recruiting the endocytic GTPase dynamin. These results suggest that EphB/ephrinB trogocytosis, unlike other trogocytosis events, uses a phagocytosis-like mechanism to achieve efficient membrane scission and engulfment.

The nucleoside diphosphate kinase NDK-1/NME1 promotes phagocytosis in concert with DYN-1/Dynamin

Phagocytosis of various targets, such as apoptotic cells or opsonized pathogens, by macrophages is coordinated by a complex signaling network initiated by distinct phagocytic receptors. Despite the different initial signaling pathways, each pathway ends up regulating the actin cytoskeletal network, phagosome formation and closure, and phagosome maturation leading to degradation of the engulfed particle. Herein, we describe a new phagocytic function for the nucleoside diphosphate kinase 1 (NDK-1), the nematode counterpart of the first identified metastasis inhibitor NM23-H1 (nonmetastatic clone number 23) nonmetastatic clone number 23 or nonmetastatic isoform 1 (NME1). We reveal by coimmunoprecipitation, Duolink proximity ligation assay, and mass spectrometry that NDK-1/NME1 works in a complex with DYN-1/Dynamin (Caenorhabditis elegans/human homolog proteins), which is essential for engulfment and phagosome maturation. Time-lapse microscopy shows that NDK-1 is expressed on phagosomal surfaces during cell corpse clearance in the same time window as DYN-1. Silencing of NM23-M1 in mouse bone marrow–derived macrophages resulted in decreased phagocytosis of apoptotic thymocytes. In human macrophages, NM23-H1 and Dynamin are corecruited at sites of phagosome formation in F-actin–rich cups. In addition, NM23-H1 was required for efficient phagocytosis. Together, our data demonstrate that NDK-1/NME1 is an evolutionarily conserved element of successful phagocytosis.—Farkas, Z., Petric, M., Liu, X., Herit, F., Rajnavölgyi, É., Szondy, Z., Budai, Z., Orbán, T. I., Sándor, S., Mehta, A., Bajtay, Z., Kovács, T., Jung, S. Y., Afaq Shakir, M., Qin, J., Zhou, Z., Niedergang, F., Boissan, M., Takács-Vellai, K. The nucleoside diphosphate kinase NDK-1/NME1 promotes phagocytosis in concert with DYN-1/dynamin.

Proteomic Analysis of Novel Components of Nemopilema nomurai Jellyfish Venom: Deciphering the Mode of Action

Nowadays, proliferation of jellyfish has become a severe matter in many coastal areas around the world. Jellyfish Nemopilema nomurai is one of the most perilous organisms and leads to significant deleterious outcomes such as harm to the fishery, damage the coastal equipment, and moreover, its envenomation can be hazardous to the victims. Till now, the components of Nemopilema nomurai venom (NnV) are unknown owing to scant transcriptomics and genomic data. In the current research, we have explored a proteomic approach to identify NnV components and their interrelation with pathological effects caused by the jellyfish sting. Altogether, 150 proteins were identified, comprising toxins and other distinct proteins that are substantial in nematocyst genesis and nematocyte growth by employing two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI/TOF/MS). The identified toxins are phospholipase A2, phospholipase D Li Sic Tox beta IDI, a serine protease, putative Kunitz-type serine protease inhibitor, disintegrin and metalloproteinase, hemolysin, leukotoxin, three finger toxin MALT0044C, allergens, venom prothrombin activator trocarin D, tripeptide Gsp 9.1, and along with other toxin proteins. These toxins are relatively well characterized in the venoms of other poisonous species to induce pathogenesis, hemolysis, inflammation, proteolysis, blood coagulation, cytolysis, hemorrhagic activity, and type 1 hypersensitivity, suggesting that these toxins in NnV can also cause similar deleterious consequences. Our proteomic works indicate that NnV protein profile represents valuable source which leads to better understanding the clinical features of the jellyfish stings. As one of the largest jellyfish in the world, Nemopilema nomurai sting is considered to be harmful to humans due to its potent toxicity. The identification and functional characterization of its venom components have been poorly described and are beyond our knowledge. Here is the first report demonstrating the methodical overview of NnV proteomics research, providing significant information to understand the mechanism of NnV envenomation. Our proteomics findings can provide a platform for novel protein discovery and development of practical ways to deal with jellyfish stings on human beings.

Axonal fusion: An alternative and efficient mechanism of nerve repair

Injuries to the nervous system can cause lifelong morbidity due to the disconnect that occurs between nerve cells and their cellular targets. Re-establishing these lost connections is the ultimate goal of endogenous regenerative mechanisms, as well as those induced by exogenous manipulations in a laboratory or clinical setting. Reconnection between severed neuronal fibers occurs spontaneously in some invertebrate species and can be induced in mammalian systems. This process, known as axonal fusion, represents a highly efficient means of repair after injury. Recent progress has greatly enhanced our understanding of the molecular control of axonal fusion, demonstrating that the machinery required for the engulfment of apoptotic cells is repurposed to mediate the reconnection between severed axon fragments, which are subsequently merged by fusogen proteins. Here, we review our current understanding of naturally occurring axonal fusion events, as well as those being ectopically produced with the aim of achieving better clinical outcomes.

Engulfing cells promote neuronal regeneration and remove neuronal debris through distinct biochemical functions of CED-1

Two important biological events happen coincidently soon after nerve injury in the peripheral nervous system in C. elegans: removal of axon debris and initiation of axon regeneration. But, it is not known how these two events are co-regulated. Mutants of ced-1, a homolog of Draper and MEGF10, display defects in both events. One model is that those events could be related. But our data suggest that they are actually separable. CED-1 functions in the muscle-type engulfing cells in both events and is enriched in muscle protrusions in close contact with axon debris and regenerating axons. Its two functions occur through distinct biochemical mechanisms; extracellular domain-mediated adhesion for regeneration and extracellular domain binding-induced intracellular domain signaling for debris removal. These studies identify CED-1 in engulfing cells as a receptor in debris removal but as an adhesion molecule in neuronal regeneration, and have important implications for understanding neural circuit repair after injury.

RAB-35 and ARF-6 GTPases Mediate Engulfment and Clearance Following Linker Cell-Type Death

Clearance of dying cells is essential for development and homeostasis. Conserved genes mediate apoptotic cell removal, but whether these genes control non-apoptotic cell removal is a major open question. Linker cell-type death (LCD) is a prevalent non-apoptotic developmental cell death process with features conserved from C. elegans to vertebrates. Using microfluidics-based long-term in vivo imaging, we show that unlike apoptotic cells, the C. elegans linker cell, which dies by LCD, is competitively phagocytosed by two neighboring cells, resulting in cell splitting. Subsequent cell elimination does not require apoptotic engulfment genes. Rather, we find that RAB-35 GTPase is a key coordinator of competitive phagocytosis onset and cell degradation. RAB-35 binds CNT-1, an ARF-6 GTPase activating protein, and removes ARF-6, a degradation inhibitor, from phagosome membranes. This facilitates phosphatidylinositol-4,5-bisphosphate removal from phagosome membranes, promoting phagolysosome maturation. Our studies suggest that RAB-35 and ARF-6 drive a conserved program eliminating cells dying by LCD.

Autophagy mediates phosphatidylserine exposure and phagosome degradation during apoptosis through specific functions of GABARAP/LGG-1 and LC3/LGG-2

Phagocytosis and macroautophagy/autophagy are 2 processes involved in lysosome-mediated clearance of extracellular and intracellular components, respectively. Recent studies have identified the recruitment of the autophagic protein LC3 during phagocytosis of apoptotic corpses in what is now called LC3-associated phagocytosis (LAP). LAP is a distinct process from autophagy but it relies on some members of the autophagy pathway to allow efficient degradation of the phagocytosed cargo. We investigated whether both LC3/LGG-2 and GABARAP/LGG-1 are involved in phagocytosis of apoptotic corpses during embryonic development of Caenorhabditis elegans. We discovered that both LGG-1 and LGG-2 are involved in the correct elimination of apoptotic corpses, but that they have different functions. lgg-1 and lgg-2 mutants present a delay in phagocytosis of apoptotic cells but genetic analyses indicate that LGG-1 and LGG-2 act upstream and downstream of the engulfment pathways, respectively. Moreover, LGG-1 and LGG-2 display different cellular localizations with enrichment in apoptotic corpses and phagocytic cells, respectively. For both LGG-1 and LGG-2, subcellular localization is vesicular and dependent on UNC-51/ULK1, BEC-1/BECN1 and the lipidation machinery, indicating that their functions during phagocytosis of apoptotic corpses mainly rely on autophagy. Finally, we show that LGG-1 is involved in the exposure of the 'eat-me signal' phosphatidylserine at the surface of the apoptotic cell to allow its recognition by the phagocytic cell, whereas LGG-2 is involved in later steps of phagocytosis to allow efficient cell corpse clearance by mediating the maturation/degradation of the phagosome.

The small GTPase RAB-35 defines a third pathway that is required for the recognition and degradation of apoptotic cells

In metazoans, apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Multiple small GTPases in the Rab family are known to function in phagosome maturation by regulating vesicle trafficking. We discovered rab-35 as a new gene important for apoptotic cell clearance from a genetic screen targeting putative Rab GTPases in Caenorhabditis elegans. We further identified TBC-10 as a putative GTPase-activating protein (GAP), and FLCN-1 and RME-4 as two putative Guanine Nucleotide Exchange Factors (GEFs), for RAB-35. We found that RAB-35 was required for the efficient incorporation of early endosomes to phagosomes and for the timely degradation of apoptotic cell corpses. More specifically, RAB-35 promotes two essential events that initiate phagosome maturation: the switch of phagosomal membrane phosphatidylinositol species from PtdIns(4,5)P2 to PtdIns(3)P, and the recruitment of the small GTPase RAB-5 to phagosomal surfaces. These functions of RAB-35 were previously unknown. Remarkably, although the phagocytic receptor CED-1 regulates these same events, RAB-35 and CED-1 appear to function independently. Upstream of degradation, RAB-35 also facilitates the recognition of apoptotic cells independently of the known CED-1 and CED-5 pathways. RAB-35 localizes to extending pseudopods and is further enriched on nascent phagosomes, consistent with its dual roles in regulating apoptotic cell-recognition and phagosome maturation. Epistasis analyses indicate that rab-35 acts in parallel to both of the canonical ced-1/6/7 and ced-2/5/10/12 clearance pathways. We propose that RAB-35 acts as a robustness factor, defining a novel pathway that aids these canonical pathways in both the recognition and degradation of apoptotic cells.

The ABC Transporter Eato Promotes Cell Clearance in the Drosophila melanogaster Ovary

The clearance of dead cells is a fundamental process in the maintenance of tissue homeostasis. Genetic studies in Drosophila melanogaster, Caenorhabditis elegans, and mammals have identified two evolutionarily conserved signaling pathways that act redundantly to regulate this engulfment process: the ced-1/-6/-7 and ced-2/-5/-12 pathways. Of these engulfment genes, only the ced-7/ABCA1 ortholog remains to be identified in D. melanogaster Homology searches have revealed a family of putative ced-7/ABCA1 homologs encoding ATP-binding cassette (ABC) transporters in D. melanogaster To determine which of these genes functions similarly to ced-7/ABCA1, we analyzed mutants for engulfment phenotypes in oogenesis, during which nurse cells (NCs) in each egg chamber undergo programmed cell death (PCD) and are removed by neighboring phagocytic follicle cells (FCs). Our genetic analyses indicate that one of the ABC transporter genes, which we have named Eato (Engulfment ABC Transporter in the ovary), is required for NC clearance in the ovary and acts in the same pathways as drpr, the ced-1 ortholog, and in parallel to Ced-12 in the FCs. Additionally, we show that Eato acts in the FCs to promote accumulation of the transmembrane receptor Drpr, and promote membrane extensions around the NCs for their clearance. Since ABCA class transporters, such as CED-7 and ABCA1, are known to be involved in lipid trafficking, we propose that Eato acts to transport membrane material to the growing phagocytic cup for cell corpse clearance. Our work presented here identifies Eato as the ced-7/ABCA1 ortholog in D. melanogaster, and demonstrates a role for Eato in Drpr accumulation and phagocytic membrane extensions during NC clearance in the ovary.

Scrambled Eggs: Apoptotic Cell Clearance by Non-Professional Phagocytes in the Drosophila Ovary

For half of a century, it has been known that non-professional phagocytes, such as fibroblasts, endothelial, and epithelial cells, are capable of efferocytosis (engulfment of apoptotic cells). Non-professional phagocytes differ from professional phagocytes in the range and efficiency of engulfment. Much of the recognition and underlying signaling machinery between non-professional and professional phagocytes is the same, but it is not known how the engulfment capacity of non-professional phagocytes is controlled. Moreover, the signaling networks involved in cell corpse recognition, engulfment, and phagosome maturation are only partially understood. The Drosophila ovary provides an excellent system to investigate the regulation of phagocytic activity by epithelial cells, a major class of non-professional phagocytes. During Drosophila oogenesis, mid-stage egg chambers undergo apoptosis of the germline in response to nutrient deprivation. Epithelial follicle cells then undergo major cell shape changes and concomitantly engulf the germline material. Our previous work has established that Draper and the integrin α-PS3/β-PS heterodimer are required in follicle cells for germline cell clearance. In addition, we have characterized phagosome maturation pathways, and found that the JNK pathway amplifies the engulfment response. In this review, we discuss recent advances on the interplay between engulfment pathways in the follicular epithelium for cell clearance in the Drosophila ovary. We also provide a comparison to apoptotic cell clearance mechanisms in C. elegans and mammals, illustrating strong conservation of efferocytosis mechanisms by non-professional phagocytes.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

Control of developmental networks by Rac/Rho small GTPases: How cytoskeletal changes during embryogenesis are orchestrated

Small GTPases in the Rho family act as major nodes with functions beyond cytoskeletal rearrangements shaping the Caenorhabditis elegans embryo during development. These small GTPases are key signal transducers that integrate diverse developmental signals to produce a coordinated response in the cell. In C. elegans, the best studied members of these highly conserved Rho family small GTPases, RHO-1/RhoA, CED-10/Rac, and CDC-42, are crucial in several cellular processes dealing with cytoskeletal reorganization. In this review, we update the functions described for the Rho family small GTPases in spindle orientation and cell division, engulfment, and cellular movements during C. elegans embryogenesis, focusing on the Rho subfamily Rac. Please also see the video abstract here.

Developmentally programmed germ cell remodelling by endodermal cell cannibalism

Primordial germ cells (PGCs) in many species associate intimately with endodermal cells, but the significance of such interactions is largely unexplored. Here, we show that Caenorhabditis elegans PGCs form lobes that are removed and digested by endodermal cells, dramatically altering PGC size and mitochondrial content. We demonstrate that endodermal cells do not scavenge lobes PGCs shed, but rather, actively remove lobes from the cell body. CED-10 (Rac)-induced actin, DYN-1 (dynamin) and LST-4 (SNX9) transiently surround lobe necks and are required within endodermal cells for lobe scission, suggesting that scission occurs through a mechanism resembling vesicle endocytosis. These findings reveal an unexpected role for endoderm in altering the contents of embryonic PGCs, and define a form of developmentally programmed cell remodelling involving intercellular cannibalism. Active roles for engulfing cells have been proposed in several neuronal remodelling events, suggesting that intercellular cannibalism may be a more widespread method used to shape cells than previously thought.

Extracellular RNA is transported from one generation to the next in Caenorhabditis elegans

Experiences during the lifetime of an animal have been proposed to have consequences for subsequent generations. Although it is unclear how such intergenerational transfer of information occurs, RNAs found extracellularly in animals are candidate molecules that can transfer gene-specific regulatory information from one generation to the next because they can enter cells and regulate gene expression. In support of this idea, when double-stranded RNA (dsRNA) is introduced into some animals, the dsRNA can silence genes of matching sequence and the silencing can persist in progeny. Such persistent gene silencing is thought to result from sequence-specific interaction of the RNA within parents to generate chromatin modifications, DNA methylation, and/or secondary RNAs, which are then inherited by progeny. Here, we show that dsRNA can be directly transferred between generations in the worm Caenorhabditis elegans Intergenerational transfer of dsRNA occurs even in animals that lack any DNA of matching sequence, and dsRNA that reaches progeny can spread between cells to cause gene silencing. Surprisingly, extracellular dsRNA can also reach progeny without entry into the cytosol, presumably within intracellular vesicles. Fluorescently labeled dsRNA is imported from extracellular space into oocytes along with yolk and accumulates in punctate structures within embryos. Subsequent entry into the cytosol of early embryos causes gene silencing in progeny. These results demonstrate the transport of extracellular RNA from one generation to the next to regulate gene expression in an animal and thus suggest a mechanism for the transmission of experience-dependent effects between generations.

Components of the Engulfment Machinery Have Distinct Roles in Corpse Processing

Billions of cells die in our bodies on a daily basis and are engulfed by phagocytes. Engulfment, or phagocytosis, can be broken down into five basic steps: attraction of the phagocyte, recognition of the dying cell, internalization, phagosome maturation, and acidification. In this study, we focus on the last two steps, which can collectively be considered corpse processing, in which the engulfed material is degraded. We use the Drosophila ovarian follicle cells as a model for engulfment of apoptotic cells by epithelial cells. We show that engulfed material is processed using the canonical corpse processing pathway involving the small GTPases Rab5 and Rab7. The phagocytic receptor Draper is present on the phagocytic cup and on nascent, phosphatidylinositol 3-phosphate (PI(3)P)- and Rab7-positive phagosomes, whereas integrins are maintained on the cell surface during engulfment. Due to the difference in subcellular localization, we investigated the role of Draper, integrins, and downstream signaling components in corpse processing. We found that some proteins were required for internalization only, while others had defects in corpse processing as well. This suggests that several of the core engulfment proteins are required for distinct steps of engulfment. We also performed double mutant analysis and found that combined loss of draper and αPS3 still resulted in a small number of engulfed vesicles. Therefore, we investigated another known engulfment receptor, Crq. We found that loss of all three receptors did not inhibit engulfment any further, suggesting that Crq does not play a role in engulfment by the follicle cells. A more complete understanding of how the engulfment and corpse processing machinery interact may enable better understanding and treatment of diseases associated with defects in engulfment by epithelial cells.

Programmed cell death and clearance of cell corpses in Caenorhabditis elegans

Programmed cell death is critical to the development of diverse animal species from C. elegans to humans. In C. elegans, the cell death program has three genetically distinguishable phases. During the cell suicide phase, the core cell death machinery is activated through a protein interaction cascade. This activates the caspase CED-3, which promotes numerous pro-apoptotic activities including DNA degradation and exposure of the phosphatidylserine "eat me" signal on the cell corpse surface. Specification of the cell death fate involves transcriptional activation of the cell death initiator EGL-1 or the caspase CED-3 by coordinated actions of specific transcription factors in distinct cell types. In the cell corpse clearance stage, recognition of cell corpses by phagocytes triggers several signaling pathways to induce phagocytosis of apoptotic cell corpses. Cell corpse-enclosing phagosomes ultimately fuse with lysosomes for digestion of phagosomal contents. This article summarizes our current knowledge about programmed cell death and clearance of cell corpses in C. elegans.

PtdIns(4,5)P₂ and PtdIns3P coordinate to regulate phagosomal sealing for apoptotic cell clearance

A coincidence detection mechanism regulates phagosomal sealing and couples it with phosphoinositide conversion from PtdIns(4,5)P₂ enrichment on unsealed phagosomes to PtdIns3P enrichment on fully sealed phagosomes.

Phagocytosis requires phosphoinositides (PIs) as both signaling molecules and localization cues. How PIs coordinate to control phagosomal sealing and the accompanying switch of organelle identity is unclear. In this study, we followed dynamic changes in PIs during apoptotic cell clearance in Caenorhabditis elegans. We found that phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) and phosphatidylinositol-3-phosphate (PtdIns3P), which accumulate transiently on unsealed and fully sealed phagosomes, respectively, are both involved in phagosome closure. We identified PtdIns3P phosphatase MTM-1 as an effector of PtdIns(4,5)P₂ to promote phagosomal sealing. MTM-1 coordinates with the class II PI3 kinase PIKI-1 to control PtdIns3P levels on unsealed phagosomes. The SNX9 family protein LST-4 is required for sealing, and its association with unsealed phagosomes is regulated by PtdIns(4,5)P₂, PIKI-1, and MTM-1. Loss of LST-4 or its retention on phagosomes disrupts sealing and suppresses PtdIns3P accumulation, indicating close coupling of the two events. Our findings support a coincidence detection mechanism by which phagosomal sealing is regulated and coupled with conversion from PtdIns(4,5)P₂ enrichment on unsealed phagosomes to PtdIns3P enrichment on fully sealed phagosomes.

Loss of Acetylcholine Signaling Reduces Cell Clearance Deficiencies in Caenorhabditis elegans

The ability to eliminate undesired cells by apoptosis is a key mechanism to maintain organismal health and homeostasis. Failure to clear apoptotic cells efficiently can cause autoimmune diseases in mammals. Genetic studies in Caenorhabditis elegans have greatly helped to decipher the regulation of apoptotic cell clearance. In this study, we show that the loss of levamisole-sensitive acetylcholine receptor, but not of a typical neuronal acetylcholine receptor causes a reduction in the number of persistent cell corpses in worms suffering from an engulfment deficiency. This reduction is not caused by impaired or delayed cell death but rather by a partial restoration of the cell clearance capacity. Mutants in acetylcholine turn-over elicit a similar phenotype, implying that acetylcholine signaling is the process responsible for these observations. Surprisingly, tissue specific RNAi suggests that UNC-38, a major component of the levamisole-sensitive receptor, functions in the dying germ cell to influence engulfment efficiency. Animals with loss of acetylcholine receptor exhibit a higher fraction of cell corpses positive for the “eat-me” signal phosphatidylserine. Our results suggest that modulation by ion channels of ion flow across plasma membrane in dying cells can influence the dynamics of phosphatidylserine exposure and thus clearance efficiency.

Engulfment pathways promote programmed cell death by enhancing the unequal segregation of apoptotic potential

Components of the conserved engulfment pathways promote programmed cell death in Caenorhabditis elegans (C. elegans) through an unknown mechanism. Here we report that the phagocytic receptor CED-1 mEGF10 is required for the formation of a dorsal–ventral gradient of CED-3 caspase activity within the mother of a cell programmed to die and an increase in the level of CED-3 protein within its dying daughter. Furthermore, CED-1 becomes enriched on plasma membrane regions of neighbouring cells that appose the dorsal side of the mother, which later forms the dying daughter. Therefore, we propose that components of the engulfment pathways promote programmed cell death by enhancing the polar localization of apoptotic factors in mothers of cells programmed to die and the unequal segregation of apoptotic potential into dying and surviving daughters. Our findings reveal a novel function of the engulfment pathways and provide a better understanding of how apoptosis is initiated during C. elegans development.

Programed cell death occurs in a stereotypic fashion during C. elegans development, and it is thought that engulfment promotes programmed cell death. Here the authors present evidence that a signaling function of the conserved engulfment pathways, not the process of engulfment itself, promotes apoptotic cell death.

Changes in apoptotic microRNA and mRNA expression profiling in Caenorhabditis elegans during the Shenzhou-8 mission

Radiation and microgravity exposure have been proven to induce abnormal apoptosis in microRNA (miRNA) and mRNA expression, but whether space conditions, including radiation and microgravity, activate miRNAs to regulate the apoptosis is undetermined. For that purpose, we investigated miRNome and mRNA expression in the ced-1 Caenorhabditis elegans mutant vs the wild-type, both of which underwent spaceflight, spaceflight 1g-centrifuge control and ground control conditions during the Shenzhou-8 mission. Results showed that no morphological changes in the worms were detected, but differential miRNA expression increased from 43 (ground control condition) to 57 and 91 in spaceflight and spaceflight control conditions, respectively. Microgravity altered miRNA expression profiling by decreasing the number and significance of differentially expressed miRNA compared with 1 g incubation during spaceflight. Alterations in the miRNAs were involved in alterations in apoptosis, neurogenesis larval development, ATP metabolism and GTPase-mediated signal transduction. Among these, 17 altered miRNAs potentially involved in apoptosis were screened and showed obviously different expression signatures between space conditions. By integrated analysis of miRNA and mRNA, miR-797 and miR-81 may be involved in apoptosis by targeting the genes ced-10 and both drp-1 and hsp-1, respectively. Compared with ground condition, space conditions regulated apoptosis though a different manner on transcription, by altering expression of seven core apoptotic genes in spaceflight condition, and eight in spaceflight control condition. Results indicate that, miRNA of Caenorhabditis elegans probably regulates apoptotic gene expression in response to space environmental stress, and shows different behavior under microgravity condition compared with 1 g condition in the presence of space radiation.

Cell Death in C. elegans Development

Cell death is a common and important feature of animal development, and cell death defects underlie many human disease states. The nematode Caenorhabditis elegans has proven fertile ground for uncovering molecular and cellular processes controlling programmed cell death. A core pathway consisting of the conserved proteins EGL-1/BH3-only, CED-9/BCL2, CED-4/APAF1, and CED-3/caspase promotes most cell death in the nematode, and a conserved set of proteins ensures the engulfment and degradation of dying cells. Multiple regulatory pathways control cell death onset in C. elegans, and many reveal similarities with tumor formation pathways in mammals, supporting the idea that cell death plays key roles in malignant progression. Nonetheless, a number of observations suggest that our understanding of developmental cell death in C. elegans is incomplete. The interaction between dying and engulfing cells seems to be more complex than originally appreciated, and it appears that key aspects of cell death initiation are not fully understood. It has also become apparent that the conserved apoptotic pathway is dispensable for the demise of the C. elegans linker cell, leading to the discovery of a previously unexplored gene program promoting cell death. Here, we review studies that formed the foundation of cell death research in C. elegans and describe new observations that expand, and in some cases remodel, this edifice. We raise the possibility that, in some cells, more than one death program may be needed to ensure cell death fidelity.

A systems toxicology approach on the mechanism of uptake and toxicity of MWCNT in Caenorhabditis elegans

The increased volumes of carbon nanotubes (CNTs) being utilized in industrial and biomedical processes carries with it an increased risk of unintentional release into the environment, requiring a thorough hazard and risk assessment. In this study, the toxicity of pristine and hydroxylated (OH-) multiwall CNTs (MWCNTs) was investigated in the nematode Caenorhabditis elegans using an integrated systems toxicology approach. To gain an insight into the toxic mechanism of MWCNTs, microarray and proteomics were conducted for C. elegans followed by pathway analyses. The results of pathway analyses suggested endocytosis, phagocytosis, oxidative stress and endoplasmic reticulum (ER) stress, as potential mechanisms of uptake and toxicity, which were subsequently investigated using loss-of-function mutants of genes of those pathways. The expression of phagocytosis related genes (i.e. ced-10 and rab-7) were significantly increased upon exposure to OH-MWCNT, concomitantly with the rescued toxicity by loss-of-function mutants of those genes, such as ced-10(n3246) and rab-7(ok511). An increased sensitivity of the hsp-4(gk514) mutant by OH-MWCNT, along with a decreased expression of hsp-4 at both gene and protein level suggests that MWCNTs may affect ER stress response in C. elegans. Collectively, the results implied phagocytosis to be a potential mechanism of uptake of MWCNTs, and ER and oxidative stress as potential mechanisms of toxicity. The integrated systems toxicology approach applied in this study provided a comprehensive insight into the toxic mechanism of MWCNTs in C. elegans, which may eventually be used to develop an "Adverse Outcome Pathway (AOP)", a recently introduced concept as a conceptual framework to link molecular level responses to higher level effects.

Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca(2+)-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common "eat me" signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Nucleoside diphosphate kinases (NDPKs) in animal development

In textbooks of biochemistry, nucleoside diphosphate conversion to a triphosphate by nucleoside diphosphate 'kinases' (NDPKs, also named NME or NM23 proteins) merits a few lines of text. Yet this essential metabolic function, mediated by a multimeric phosphotransferase protein, has effects that lie beyond a simple housekeeping role. NDPKs attracted more attention when NM23-H1 was identified as the first metastasis suppressor gene. In this review, we examine these NDPK enzymes from a developmental perspective because of the tractable phenotypes found in simple animal models that point to common themes. The data suggest that NDPK enzymes control the availability of surface receptors to regulate cell-sensing cues during cell migration. NDPKs regulate different forms of membrane enclosure that engulf dying cells during development. We suggest that NDPK enzymes have been essential for the regulated uptake of objects such as bacteria or micronutrients, and this evolutionarily conserved endocytic function contributes to their activity towards the regulation of metastasis.

JNK pathway activation is able to synchronize neuronal death and glial phagocytosis in Drosophila

Glial phagocytosis of superfluous neurons and damaged or aberrant neuronal material is crucial for normal development and maintenance of the CNS. However, the molecular mechanisms underlying the relationship between neuronal death and glial phagocytosis are poorly understood. We describe a novel mechanism that is able to synchronize neuronal cell death and glial phagocytosis of dying neurons in the Drosophila embryonic CNS. This mechanism involves c-Jun N-terminal kinase (JNK) signaling, which is required for developmental apoptosis of specific neurons during embryogenesis. We demonstrate that the dJNK pathway gain-of-function in neurons leads to dJNK signaling in glia, which results in upregulation of glial phagocytosis. Importantly, this promotion of phagocytosis is not mediated by upregulation of the glial phagocytic receptors SIMU and DRPR, but by increasing glial capacity to degrade apoptotic particles inside phagosomes. The proposed mechanism may be important for removal of damaged neurons in the developing and mature CNS.

Signaling by the engulfment receptor draper: a screen in Drosophila melanogaster implicates cytoskeletal regulators, Jun N-terminal Kinase, and Yorkie

Draper, the Drosophila melanogaster homolog of the Ced-1 protein of Caenorhabditis elegans, is a cell-surface receptor required for the recognition and engulfment of apoptotic cells, glial clearance of axon fragments and dendritic pruning, and salivary gland autophagy. To further elucidate mechanisms of Draper signaling, we screened chromosomal deficiencies to identify loci that dominantly modify the phenotype of overexpression of Draper isoform II (suppressed differentiation of the posterior crossvein in the wing). We found evidence for 43 genetic modifiers of Draper II. Twenty-four of the 37 suppressor loci and 3 of the 6 enhancer loci were identified. An additional 5 suppressors and 2 enhancers were identified among mutations in functionally related genes. These studies reveal positive contributions to Drpr signaling for the Jun N-terminal Kinase pathway, supported by genetic interactions with hemipterous, basket, jun, and puckered, and for cytoskeleton regulation as indicated by genetic interactions with rac1, rac2, RhoA, myoblast city, Wiskcott-Aldrich syndrome protein, and the formin CG32138, and for yorkie and expanded. These findings indicate that Jun N-terminal Kinase activation and cytoskeletal remodeling collaborate in Draper signaling. Relationships between Draper signaling and Decapentaplegic signaling, insulin signaling, Salvador/Warts/Hippo signaling, apical-basal cell polarity, and cellular responses to mechanical forces are also discussed.

NDK-1, the homolog of NM23-H1/H2 regulates cell migration and apoptotic engulfment in C. elegans

Abnormal regulation of cell migration and altered rearrangement of cytoskeleton are characteristic of metastatic cells. The first described suppressor of metastatic processes is NM23-H1, which displays NDPK (nucleoside-diphosphate kinase) activity. To better understand the role of nm23 genes in cell migration, we investigated the function of NDK-1, the sole Caenorhabditis elegans homolog of group I NDPKs in distal tip cell (DTC) migration. Dorsal phase of DTC migration is regulated by integrin mediated signaling. We find that ndk-1 loss of function mutants show defects in this phase. Epistasis analysis using mutants of the α-integrin ina-1 and the downstream functioning motility-promoting signaling module (referred to as CED-10 pathway) placed NDK-1 downstream of CED-10/Rac. As DTC migration and engulfment of apoptotic corpses are analogous processes, both partially regulated by the CED-10 pathway, we investigated defects of apoptosis in ndk-1 mutants. Embryos and germ cells defective for NDK-1 showed an accumulation of apoptotic cell corpses. Furthermore, NDK-1::GFP is expressed in gonadal sheath cells, specialized cells for engulfment and clearence of apoptotic corpses in germ line, which indicates a role for NDK-1 in apoptotic corpse removal. In addition to the CED-10 pathway, engulfment in the worm is also mediated by the CED-1 pathway. abl-1/Abl and abi-1/Abi, which function in parallel to both CED-10/CED-1 pathways, also regulate engulfment and DTC migration. ndk-1(-);abi-1(-) double mutant embryos display an additive phenotype (e. g. enhanced number of apoptotic corpses) which suggests that ndk-1 acts in parallel to abi-1. Corpse number in ndk-1(-);ced-10(-) double mutants, however, is similar to ced-10(-) single mutants, suggesting that ndk-1 acts downstream of ced-10 during engulfment. In addition, NDK-1 shows a genetic interaction with DYN-1/dynamin, a downstream component of the CED-1 pathway. In summary, we propose that NDK-1/NDPK might represent a converging point of CED-10 and CED-1 pathways in the process of cytoskeleton rearrangement.