Phagocytosis promotes programmed cell death in C. elegans

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Mechanisms and consequences of entosis

Multiple mechanisms have emerged where the engulfment of whole live cells, leading to the formation of what are called 'cell-in-cell' structures, induces cell death. Entosis is one such mechanism that drives cell-in-cell formation during carcinogenesis and development. Curiously, entotic cells participate actively in their own engulfment, by invading into their hosts, and are then killed non-cell-autonomously. Here we review the mechanisms of entosis and entotic cell death and the consequences of entosis on cell populations.

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.

Both the apoptotic suicide pathway and phagocytosis are required for a programmed cell death in Caenorhabditis elegans

Background

Programmed cell deaths in the nematode Caenorhabditis elegans are generally considered suicides. Dying cells are engulfed by neighboring cells in a process of phagocytosis. To better understand the interaction between the engulfment and death processes, we analyzed B.al/rapaav cell death, which has been previously described as engulfment-dependent and hence as a possible murder.

Results

We found that B.al/rapaav is resistant to caspase-pathway activation: the caspase-mediated suicide pathway initiates the cell-death process but is insufficient to cause B.al/rapaav death without the subsequent assistance of engulfment. When the engulfing cell P12.pa is absent, other typically non-phagocytic cells can display cryptic engulfment potential and facilitate this death.

Conclusions

We term this death an “assisted suicide” and propose that assisted suicides likely occur in other organisms. The study of assisted suicides might provide insight into non-cell autonomous influences on cell death. Understanding the mechanism that causes B.al/rapaav to be resistant to activation of the caspase pathway might reveal the basis of differences in the sensitivity to apoptotic stimuli of tumor and normal cells, a key issue in the field of cancer therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0262-5) contains supplementary material, which is available to authorized users.

Your neighbours matter - non-autonomous control of apoptosis in development and disease

Traditionally, the regulation of apoptosis has been thought of as an autonomous process in which the dying cell dictates its own demise. However, emerging studies in genetically tractable multicellular organisms, such as Caenorhabditis elegans and Drosophila, have revealed that death is often a communal event. Here, we review the current literature on non-autonomous mechanisms governing apoptosis in multiple cellular contexts. The importance of the cellular community in dictating the funeral arrangements of apoptotic cells has profound implications in development and disease.

CasExpress reveals widespread and diverse patterns of cell survival of caspase-3 activation during development in vivo

Caspase-3 carries out the executioner phase of apoptosis, however under special circumstances, cells can survive its activity. To document systematically where and when cells survive caspase-3 activation in vivo, we designed a system, CasExpress, which drives fluorescent protein expression, transiently or permanently, in cells that survive caspase-3 activation in Drosophila. We discovered widespread survival of caspase-3 activity. Distinct spatial and temporal patterns emerged in different tissues. Some cells activated caspase-3 during their normal development in every cell and in every animal without evidence of apoptosis. In other tissues, such as the brain, expression was sporadic both temporally and spatially and overlapped with periods of apoptosis. In adults, reporter expression was evident in a large fraction of cells in most tissues of every animal; however the precise patterns varied. Inhibition of caspase activity in wing discs reduced wing size demonstrating functional significance. The implications of these patterns are discussed.

DOI:http://dx.doi.org/10.7554/eLife.10936.001

Every day, individual cells in our body actively decide whether to live or die. There are enzymes called executioner caspases that help cells to die in a carefully controlled process called apoptosis. Although the activation of executioner caspases generally leads to apoptosis, there are some circumstances in which cells are able to survive.

Fruit flies are often used in research as models of how animals grow and develop. Ding, Sun et al. set out to find out more about the circumstances in which cells manage to survive caspase activation in fruit flies. The experiments used a new method that results in cells that survive caspase activity producing a fluorescent marker protein. This allowed Ding, Sun et al. to track when and where these events occurred in the flies.

Few cells in fruit fly embryos survive the activation of executioner caspase. However, in later stages of development, more and more cells survive this process. Cells in different parts of the body responded differently. For some types of cells, every cell seemed to survive caspase activity with no evidence of apoptosis. In other tissues like the central brain, in which a few cells normally choose to die, some cells occasionally managed to survive the activation of caspases. This rescue from the brink of death was more common than Ding, Sun et al. had anticipated.

The next step will be to uncover the molecular mechanisms that enable the cells to survive caspase activity. This knowledge may help us to develop treatments that can promote the survival of useful cells like heart muscle cells and brain cells, or trigger the death of cancer cells.

DOI:http://dx.doi.org/10.7554/eLife.10936.002

Phagocytosis genes nonautonomously promote developmental cell death in the Drosophila ovary

Programmed cell death (PCD) is usually considered a cell-autonomous suicide program, synonymous with apoptosis. Recent research has revealed that PCD is complex, with at least a dozen cell death modalities. Here, we demonstrate that the large-scale nonapoptotic developmental PCD in the Drosophila ovary occurs by an alternative cell death program where the surrounding follicle cells nonautonomously promote death of the germ line. The phagocytic machinery of the follicle cells, including Draper, cell death abnormality (Ced)-12, and c-Jun N-terminal kinase (JNK), is essential for the death and removal of germ-line-derived nurse cells during late oogenesis. Cell death events including acidification, nuclear envelope permeabilization, and DNA fragmentation of the nurse cells are impaired when phagocytosis is inhibited. Moreover, elimination of a small subset of follicle cells prevents nurse cell death and cytoplasmic dumping. Developmental PCD in the Drosophila ovary is an intriguing example of nonapoptotic, nonautonomous PCD, providing insight on the diversity of cell death mechanisms.

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.

Activated microglia cause reversible apoptosis of pheochromocytoma cells, inducing their cell death by phagocytosis

Some apoptotic processes, such as phosphatidylserine exposure, are potentially reversible and do not necessarily lead to cell death. However, phosphatidylserine exposure can induce phagocytosis of a cell, resulting in cell death by phagocytosis: phagoptosis. Phagoptosis of neurons by microglia might contribute to neuropathology, whereas phagoptosis of tumour cells by macrophages might limit cancer. Here, we examined the mechanisms by which BV-2 microglia killed co-cultured pheochromocytoma (PC12) cells that were either undifferentiated or differentiated into neuronal cells. We found that microglia activated by lipopolysaccharide rapidly phagocytosed PC12 cells. Activated microglia caused reversible phosphatidylserine exposure on and reversible caspase activation in PC12 cells, and caspase inhibition prevented phosphatidylserine exposur and decreased subsequent phagocytosis. Nitric oxide was necessary and sufficient to induce the reversible phosphatidylserine exposure and phagocytosis. The PC12 cells were not dead at the time they were phagocytised, and inhibition of their phagocytosis left viable cells. Cell loss was inhibited by blocking phagocytosis mediated by phosphatidylserine, MFG-E8, vitronectin receptors or P2Y6 receptors. Thus, activated microglia can induce reversible apoptosis of target cells, which is insufficient to cause apoptotic cell death, but sufficient to induce their phagocytosis and therefore cell death by phagoptosis.

The Effect of Hypoxic Preconditioning on Induced Schwann Cells under Hypoxic Conditions

OBJECT

Our objective was to explore the protective effects of hypoxic preconditioning on induced Schwann cells exposed to an environment with low concentrations of oxygen. It has been observed that hypoxic preconditioning of induced Schwann cells can promote axonal regeneration under low oxygen conditions.

METHOD

Rat bone marrow mesenchymal stem cells (MSCs) were differentiated into Schwann cells and divided into a normal oxygen control group, a hypoxia-preconditioning group and a hypoxia group. The ultrastructure of each of these groups of cells was observed by electron microscopy. In addition, flow cytometry was used to measure changes in mitochondrial membrane potential. Annexin V-FITC/PI staining was used to detect apoptosis, and Western blots were used to detect the expression of Bcl-2/Bax. Fluorescence microscopic observations of axonal growth in NG-108 cells under hypoxic conditions were also performed.

RESULTS

The hypoxia-preconditioning group maintained mitochondrial cell membrane and crista integrity, and these cells exhibited less edema than the hypoxia group. In addition, the cells in the hypoxia-preconditioning group were found to be in early stages of apoptosis, whereas cells from the hypoxia group were in the later stages of apoptosis. The hypoxia-preconditioning group also had higher levels of Bcl-2/Bax expression and longer NG-108 cell axons than were observed in the hypoxia group.

CONCLUSION

Hypoxic preconditioning can improve the physiological state of Schwann cells in a severe hypoxia environment and improve the ability to promote neurite outgrowth.

Apaf1 inhibition promotes cell recovery from apoptosis

The protein apoptotic protease activating factor 1 (Apaf1) is the central component of the apoptosome, a multiprotein complex that activates procaspase-9 after cytochrome c release from the mitochondria in the intrinsic pathway of apoptosis. We have developed a vital method that allows fluorescence-activated cell sorting of cells at different stages of the apoptotic pathway and demonstrated that upon pharmacological inhibition of Apaf1, cells recover from doxorubicin- or hypoxia-induced early apoptosis to normal healthy cell. Inhibiting Apaf1 not only prevents procaspase-9 activation but delays massive mitochondrial damage allowing cell recovery.

The End of the Beginning: Cell Death in the Germline

Programmed cell death occurs in the germline of many organisms, both as an essential part of development and throughout adult life. Germline cell death can be apoptotic or nonapoptotic, depending on the stimulus or stage of development. Here, we focus on the Drosophila ovary, which is a powerful model for studying diverse types of cell death. In Drosophila, the death of primordial germ cells occurs normally during embryonic development, and germline nurse cells are programmed to die during oocyte development in adult flies. Cell death of previtellogenic egg chambers in adults can also be induced by starvation or other environmental cues. Mid-oogenesis seems to be particularly sensitive to such cues and has been proposed to serve as a checkpoint to avoid the energetically expensive cost of egg production. After the germline dies in mid-oogenesis, the remnants are engulfed by an epithelial layer of follicle cells; thus, the fly ovary also serves as a highly tractable model for engulfment by epithelial cells. These examples of cell death in the fly ovary share many similarities to the types of cell death seen in the mammalian germline. Recent progress in elucidating the molecular mechanisms of cell death in the germline is discussed.

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.

Caspase-3 promotes genetic instability and carcinogenesis

Apoptosis is typically considered an anti-oncogenic process since caspase activation can promote the elimination of genetically unstable or damaged cells. We report that a central effector of apoptosis, caspase-3, facilitates rather than suppresses chemical- and radiation-induced genetic instability and carcinogenesis. We found that a significant fraction of mammalian cells treated with ionizing radiation can survive despite caspase-3 activation. Moreover, this sublethal activation of caspase-3 promoted persistent DNA damage and oncogenic transformation. In addition, chemically induced skin carcinogenesis was significantly reduced in mice genetically deficient in caspase-3. Furthermore, attenuation of EndoG activity significantly reduced radiation-induced DNA damage and oncogenic transformation, identifying EndoG as a downstream effector of caspase-3 in this pathway. Our findings suggest that rather than acting as a broad inhibitor of carcinogenesis, caspase-3 activation may contribute to genome instability and play a pivotal role in tumor formation following damage.

The role of Drosophila TNF Eiger in developmental and damage-induced neuronal apoptosis

Eiger, the sole Drosophila TNF-alpha homolog, causes ectopic apoptosis through JNK pathway activation. Yet, its role in developmental apoptosis remains unclear. eiger mutant flies are viable and fertile but display compromised elimination of oncogenic cells and extracellular bacteria. Here we show that Eiger, specifically expressed in embryonic neurons and glia, is not involved in developmental neuronal apoptosis or in apoptotic cell clearance. Instead, we provide evidence that Eiger is required for damage-induced apoptosis in the embryonic CNS through regulation of the pro-apoptotic gene hid independently of the JNK pathway. Our study thus reveals a new requirement for Eiger in eliminating damaged cells during development.

Strategies for tracking anastasis, a cell survival phenomenon that reverses apoptosis

Anastasis (Greek for "rising to life") refers to the recovery of dying cells. Before these cells recover, they have passed through important checkpoints of apoptosis, including mitochondrial fragmentation, release of mitochondrial cytochrome c into the cytosol, activation of caspases, chromatin condensation, DNA damage, nuclear fragmentation, plasma membrane blebbing, cell shrinkage, cell surface exposure of phosphatidylserine, and formation of apoptotic bodies. Anastasis can occur when apoptotic stimuli are removed prior to death, thereby allowing dying cells to reverse apoptosis and potentially other death mechanisms. Therefore, anastasis appears to involve physiological healing processes that could also sustain damaged cells inappropriately. The functions and mechanisms of anastasis are still unclear, hampered in part by the limited tools for detecting past events after the recovery of apparently healthy cells. Strategies to detect anastasis will enable studies of the physiological mechanisms, the hazards of undead cells in disease pathology, and potential therapeutics to modulate anastasis. Here, we describe effective strategies using live cell microscopy and a mammalian caspase biosensor for identifying and tracking anastasis in mammalian cells.

SRGP-1 regulation, targets, and contribution to cell killing in C. elegans

The engulfment of apoptotic cell corpses is an evolutionary conserved process used by multicellular systems to remove cells with inappropriate potential (e.g., self-reactive T-cells, potentially cancerous cells). Neighboring or specialized phagocytic cells remove cell corpses through distinct steps: they first recognize the cell on the verge of death, then reorchestrate their cellular architecture toward it, actively contribute to cell killing, and eventually engulf the corpse. Thus engulfment signaling must be tightly controlled to maintain tissue homeostasis. Signaling cascades mediating cell corpse clearance likely converge at the level of the small GTPase CED-10 (Rac1); given this key position, CED-10 must be subject to a tight regulatory mechanism to prevent inappropriate phagocytic events. Here, we discuss recent work characterizing srgp-1 (nematode ortholog of mammalian srGAP), a candidate GTPase activating protein (GAP) for CED-10 involved in cell corpse clearance and "sick" cell killing in C. elegans. We additionally discuss several possible determinants of SRGP-1 function, contributing to either SRGP-1 localization and/or activation. We also survey other potential candidate GTPases that might contribute to cell corpse clearance in C. elegans, and eventually recapitulate the role of engulfment during cell killing.

Induction of entosis by epithelial cadherin expression

Cell engulfment typically targets dead or dying cells for clearance from metazoan tissues. However, recent evidence demonstrates that live cells can also be targeted and that engulfment can cause cell death. Entosis is one mechanism proposed to mediate the engulfment and killing of live tumor cells by their neighbors, an activity often referred to as cell cannibalism. Here we report that the expression of exogenous epithelial cadherin proteins (E- or P-cadherin) in human breast tumor cells lacking endogenous expression of epithelial cadherins induces entosis and inhibits transformed growth. Entosis induced by cadherin expression is associated with the polarized distribution of Rho and Rho-kinase (ROCK) activity within entotic cells, which is dependent on p190A RhoGAP activity. ROCK inhibition or downregulation of p190A RhoGAP expression reduces entosis and increases the transformed growth of epithelial cadherin-expressing tumor cells. These data define new cell systems for the study of entosis, and identify entosis as a mechanism of cell cannibalism that is induced by the establishment of epithelial adhesion and inhibits transformed growth.

The interaction between microglia and neural stem/precursor cells

In the adult mammalian brain, neurogenesis from neural stem/precursor cell occurs within two regions, the subgranular zone (SGZ) in the dentate gyrus (DG) and the subventricular zone (SVZ) lining the lateral ventricles. The function of neural stem cell is enhanced by external stimuli, such as injury and inflammation. Microglia, as the main immune modulating cells, play important roles in the central nervous system (CNS). Recently, select discoveries reported that microglia might influence the proliferation, differentiation and survival of neural precursor cells (NPCs). Other studies revealed that NPCs might reversibly regulate the function of microglia. Accordingly, in this review we focus on the interaction between microglia and NPCs.

Impaired cell death and mammary gland involution in the absence of Dock1 and Rac1 signaling

Throughout life, the tight equilibrium between cell death and the prompt clearance of dead corpses is required to maintain a proper tissue homeostasis and prevent inflammation. Following lactation, mammary gland involution is triggered and results in the death of excessive epithelial cells that are rapidly cleared by phagocytes to ensure that the gland returns to its prepregnant state. Orthologs of Dock1 (dedicator of cytokinesis 1), Elmo and Rac1 (ras-related C3 botulinum toxin substrate 1) in Caenorhabditis elegans are part of a signaling module in phagocytes that is linking apoptotic cell recognition to cytoskeletal reorganization required for engulfment. In mammals, Elmo1 was shown to interact with the phosphatidylserine receptor Bai1 and relay signals to promote phagocytosis of apoptotic cells. Still, the role of the RacGEF Dock1 in the clearance of dying cells in mammals was never directly addressed. We generated two mouse models with conditional inactivation of Dock1 and Rac1 and revealed that the expression of these genes is not essential in the mammary gland during puberty, pregnancy and lactation. We induced mammary gland involution in these mice to investigate the role of Dock1/Rac1 signaling in the engulfment of cell corpses. Unpredictably, activation of Stat3 (signal transducer and activator of transcription 3), a key regulator of mammary gland involution, was impaired in the absence of Rac1 and Dock1 expression. Likewise, failure to activate properly Stat3 was coinciding with a significant delay in the initiation and progression of mammary gland involution in mutant animals. By using an in vitro phagocytosis assay, we observed that Dock1 and Rac1 are essential to mediate engulfment in epithelial phagocytes. In vivo, cell corpses accumulated at late time points of involution in Dock1 and Rac1 mutant mammary glands. Overall, our study demonstrated an unsuspected role for Dock1/Rac1 signaling in the initiation of mammary gland involution, and also suggested a role for this pathway in the clearance of dead cells by epithelial phagocytes.

The translational regulators GCN-1 and ABCF-3 act together to promote apoptosis in C. elegans

The proper regulation of apoptosis requires precise spatial and temporal control of gene expression. While the transcriptional and translational activation of pro-apoptotic genes is known to be crucial to triggering apoptosis, how different mechanisms cooperate to drive apoptosis is largely unexplored. Here we report that pro-apoptotic transcriptional and translational regulators act in distinct pathways to promote programmed cell death. We show that the evolutionarily conserved C. elegans translational regulators GCN-1 and ABCF-3 contribute to promoting the deaths of most somatic cells during development. GCN-1 and ABCF-3 are not obviously involved in the physiological germ-cell deaths that occur during oocyte maturation. By striking contrast, these proteins play an essential role in the deaths of germ cells in response to ionizing irradiation. GCN-1 and ABCF-3 are similarly co-expressed in many somatic and germ cells and physically interact in vivo, suggesting that GCN-1 and ABCF-3 function as members of a protein complex. GCN-1 and ABCF-3 are required for the basal level of phosphorylation of eukaryotic initiation factor 2α (eIF2α), an evolutionarily conserved regulator of mRNA translation. The S. cerevisiae homologs of GCN-1 and ABCF-3, which are known to control eIF2α phosphorylation, can substitute for the worm proteins in promoting somatic cell deaths in C. elegans. We conclude that GCN-1 and ABCF-3 likely control translational initiation in C. elegans. GCN-1 and ABCF-3 act independently of the anti-apoptotic BCL-2 homolog CED-9 and of transcriptional regulators that upregulate the pro-apoptotic BH3-only gene egl-1. Our results suggest that GCN-1 and ABCF-3 function in a pathway distinct from the canonical CED-9-regulated cell-death execution pathway. We propose that the translational regulators GCN-1 and ABCF-3 maternally contribute to general apoptosis in C. elegans via a novel pathway and that the function of GCN-1 and ABCF-3 in apoptosis might be evolutionarily conserved.

Apoptosis, also referred to as programmed cell death, is a crucial cellular process that eliminates unwanted cells during animal development and tissue homeostasis. Abnormal regulation of apoptosis can cause developmental defects and a variety of other human disorders, including cancer, neurodegenerative diseases and autoimmune diseases. Therefore, it is important to identify regulatory mechanisms that control apoptosis. Previous studies have demonstrated that the transcriptional induction of apoptotic genes can be crucial to initiating an apoptotic program. Less is known about translational controls of apoptosis. Here we report that the evolutionarily conserved C. elegans translational regulators GCN-1 and ABCF-3 promote apoptosis generally and act independently of the anti-apoptotic BCL-2 homolog CED-9. GCN-1 and ABCF-3 physically interact and maintain the phosphorylation level of eukaryotic initiation factor 2α, suggesting that GCN-1 and ABCF-3 act together to regulate the initiation of translation. We propose that the translational regulators GCN-1 and ABCF-3 maternally contribute to the proper execution of the apoptotic program.

Microglial phagocytosis of live neurons

Microglia, the brain's professional phagocytes, can remove dead and dying neurons as well as synapses and the processes of live neurons. However, we and others have recently shown that microglia can also execute neuronal death by phagocytosing stressed-but-viable neurons - a process that we have termed phagoptosis. In this Progress article, we discuss evidence suggesting that phagoptosis may contribute to neuronal loss during brain development, inflammation, ischaemia and neurodegeneration.

Small GTPase CDC-42 promotes apoptotic cell corpse clearance in response to PAT-2 and CED-1 in C. elegans

The rapid clearance of dying cells is important for the well-being of multicellular organisms. In C. elegans, cell corpse removal is mainly mediated by three parallel engulfment signaling cascades. These pathways include two small GTPases, MIG-2/RhoG and CED-10/Rac1. Here we present the identification and characterization of CDC-42 as a third GTPase involved in the regulation of cell corpse clearance. Genetic analyses performed by both loss of cdc-42 function and cdc-42 overexpression place cdc-42 in parallel to the ced-2/5/12 signaling module, in parallel to or upstream of the ced-10 module, and downstream of the ced-1/6/7 module. CDC-42 accumulates in engulfing cells at membranes surrounding apoptotic corpses. The formation of such halos depends on the integrins PAT-2/PAT-3, UNC-112 and the GEF protein UIG-1, but not on the canonical ced-1/6/7 or ced-2/5/12 signaling modules. Together, our results suggest that the small GTPase CDC-42 regulates apoptotic cell engulfment possibly upstream of the canonical Rac GTPase CED-10, by polarizing the engulfing cell toward the apoptotic corpse in response to integrin signaling and ced-1/6/7 signaling in C. elegans.

PDR-1/hParkin negatively regulates the phagocytosis of apoptotic cell corpses in Caenorhabditis elegans

Apoptotic cell death is an integral part of cell turnover in many tissues, and proper corpse clearance is vital to maintaining tissue homeostasis in all multicellular organisms. Even in tissues with high cellular turnover, apoptotic cells are rarely seen because of efficient clearance mechanisms in healthy individuals. In Caenorhabditis elegans, two parallel and partly redundant conserved pathways act in cell corpse engulfment. The pathway for cytoskeletal rearrangement requires the small GTPase CED-10 Rac1 acting for an efficient surround of the dead cell. The CED-10 Rac pathway is also required for the proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. Parkin, the mammalian homolog of the C. elegans PDR-1, interacts with Rac1 in aged human brain and it is also implicated with actin dynamics and cytoskeletal rearrangements in Parkinsons's disease, suggesting that it might act on engulfment. Our genetic and biochemical studies indicate that PDR-1 inhibits apoptotic cell engulfment and DTC migration by ubiquitylating CED-10 for degradation.

Cell biology in neuroscience: Architects in neural circuit design: glia control neuron numbers and connectivity

Glia serve many important functions in the mature nervous system. In addition, these diverse cells have emerged as essential participants in nearly all aspects of neural development. Improved techniques to study neurons in the absence of glia, and to visualize and manipulate glia in vivo, have greatly expanded our knowledge of glial biology and neuron-glia interactions during development. Exciting studies in the last decade have begun to identify the cellular and molecular mechanisms by which glia exert control over neuronal circuit formation. Recent findings illustrate the importance of glial cells in shaping the nervous system by controlling the number and connectivity of neurons.

Noncanonical cell death in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has served as a fruitful setting for cell death research for over three decades. A conserved pathway of four genes, egl-1/BH3-only, ced-9/Bcl-2, ced-4/Apaf-1, and ced-3/caspase, coordinates most developmental cell deaths in C. elegans. However, other cell death forms, programmed and pathological, have also been described in this animal. Some of these share morphological and/or molecular similarities with the canonical apoptotic pathway, while others do not. Indeed, recent studies suggest the existence of an entirely novel mode of programmed developmental cell destruction that may also be conserved beyond nematodes. Here, we review evidence for these noncanonical pathways. We propose that different cell death modalities can function as backup mechanisms for apoptosis, or as tailor-made programs that allow specific dying cells to be efficiently cleared from the animal.

You are what you eat: multifaceted functions of autophagy during C. elegans development

Autophagy involves the sequestration of a portion of the cytosolic contents in an enclosed double-membrane autophagosomal structure and its subsequent delivery to lysosomes for degradation. Autophagy activity functions in multiple biological processes during Caenorhabditis elegans development. The basal level of autophagy in embryos removes aggregate-prone proteins, paternal mitochondria and spermatid-specific membranous organelles (MOs). Autophagy also contributes to the efficient removal of embryonic apoptotic cell corpses by promoting phagosome maturation. During larval development, autophagy modulates miRNA-mediated gene silencing by selectively degrading AIN-1, a component of miRNA-induced silencing complex, and thus participates in the specification of multiple cell fates controlled by miRNAs. During development of the hermaphrodite germline, autophagy acts coordinately with the core apoptotic machinery to execute genotoxic stress-induced germline cell death and also cell death when caspase activity is partially compromised. Autophagy is also involved in the utilization of lipid droplets in the aging process in adult animals. Studies in C. elegans provide valuable insights into the physiological functions of autophagy in the development of multicellular organisms.

Astrocytes engage unique molecular programs to engulf pruned neuronal debris from distinct subsets of neurons

Neural circuit assembly involves an initial overproduction of neurons and synaptic connections, followed by their selective elimination. Tasdemir-Yilmaz and Freeman show that Drosophila larval astrocytes are the primary phagocytic cell type in the pupal CNS neuropil. MB γ neuron axons are engulfed by astrocytes using the Draper and Crk/Mbc/dCed-12 signaling pathways, while Crk/Mbc/dCed-12, but not Draper, mediates the elimination of vCrz⁺ neurites. Eliminating Draper signaling delays early vCrz⁺ neurite degeneration, suggesting that glial cells promote neurite destruction through engulfment signaling.

Precise neural circuit assembly is achieved by initial overproduction of neurons and synapses, followed by refinement through elimination of exuberant neurons and synapses. Glial cells are the primary cells responsible for clearing neuronal debris, but the cellular and molecular basis of glial pruning is poorly defined. Here we show that Drosophila larval astrocytes transform into phagocytes through activation of a cell-autonomous, steroid-dependent program at the initiation of metamorphosis and are the primary phagocytic cell type in the pupal neuropil. We examined the developmental elimination of two neuron populations—mushroom body (MB) γ neurons and vCrz⁺ neurons (expressing Corazonin [Crz] neuropeptide in the ventral nerve cord [VNC])—where only neurites are pruned or entire cells are eliminated, respectively. We found that MB γ axons are engulfed by astrocytes using the Draper and Crk/Mbc/dCed-12 signaling pathways in a partially redundant manner. In contrast, while elimination of vCrz⁺ cell bodies requires Draper, elimination of vCrz⁺ neurites is mediated by Crk/Mbc/dCed-12 but not Draper. Intriguingly, we also found that elimination of Draper delayed vCrz⁺ neurite degeneration, suggesting that glia promote neurite destruction through engulfment signaling. This study identifies a novel role for astrocytes in the clearance of synaptic and neuronal debris and for Crk/Mbc/dCed-12 as a new glial pathway mediating pruning and reveals, unexpectedly, that the engulfment signaling pathways engaged by glia depend on whether neuronal debris was generated through cell death or local pruning.

Phagocytosis executes delayed neuronal death after focal brain ischemia

Delayed neuronal loss and brain atrophy after cerebral ischemia contribute to stroke and dementia pathology, but the mechanisms are poorly understood. Phagocytic removal of neurons is generally assumed to be beneficial and to occur only after neuronal death. However, we report herein that inhibition of phagocytosis can prevent delayed loss and death of functional neurons after transient brain ischemia. Two phagocytic proteins, Mer receptor tyrosine kinase (MerTK) and Milk fat globule EGF-like factor 8 (MFG-E8), were transiently up-regulated by macrophages/microglia after focal brain ischemia in vivo. Strikingly, deficiency in either protein completely prevented long-term functional motor deficits after cerebral ischemia and strongly reduced brain atrophy as a result of inhibiting phagocytosis of neurons. Correspondingly, in vitro glutamate-stressed neurons reversibly exposed the "eat-me" signal phosphatidylserine, leading to their phagocytosis by microglia; this neuronal loss was prevented in the absence of microglia and reduced if microglia were genetically deficient in MerTK or MFG-E8, both of which mediate phosphatidylserine-recognition. Thus, phagocytosis of viable neurons contributes to brain pathology and, surprisingly, blocking this process is strongly beneficial. Therefore, inhibition of specific phagocytic pathways may present therapeutic targets for preventing delayed neuronal loss after transient cerebral ischemia.

Combined inhibition of autophagy and caspases fails to prevent developmental nurse cell death in the Drosophila melanogaster ovary

During the final stages of Drosophila melanogaster oogenesis fifteen nurse cells, sister cells to the oocyte, degenerate as part of normal development. This process involves at least two cell death mechanisms, caspase-dependent cell death and autophagy, as indicated by apoptosis and autophagy markers. In addition, mutations affecting either caspases or autophagy partially reduce nurse cell removal, leaving behind end-stage egg chambers with persisting nurse cell nuclei. To determine whether apoptosis and autophagy work in parallel to degrade and remove these cells as is the case with salivary glands during pupariation, we generated mutants doubly affecting caspases and autophagy. We found no significant increase in either the number of late stage egg chambers containing persisting nuclei or in the number of persisting nuclei per egg chamber in the double mutants compared to single mutants. These findings suggest that there is another cell death mechanism functioning in the ovary to remove all nurse cell remnants from late stage egg chambers.

A shift to organismal stress resistance in programmed cell death mutants

Animals have many ways of protecting themselves against stress; for example, they can induce animal-wide, stress-protective pathways and they can kill damaged cells via apoptosis. We have discovered an unexpected regulatory relationship between these two types of stress responses. We find that C. elegans mutations blocking the normal course of programmed cell death and clearance confer animal-wide resistance to a specific set of environmental stressors; namely, ER, heat and osmotic stress. Remarkably, this pattern of stress resistance is induced by mutations that affect cell death in different ways, including ced-3 (cell death defective) mutations, which block programmed cell death, ced-1 and ced-2 mutations, which prevent the engulfment of dying cells, and progranulin (pgrn-1) mutations, which accelerate the clearance of apoptotic cells. Stress resistance conferred by ced and pgrn-1 mutations is not additive and these mutants share altered patterns of gene expression, suggesting that they may act within the same pathway to achieve stress resistance. Together, our findings demonstrate that programmed cell death effectors influence the degree to which C. elegans tolerates environmental stress. While the mechanism is not entirely clear, it is intriguing that animals lacking the ability to efficiently and correctly remove dying cells should switch to a more global animal-wide system of stress resistance.

As an animal interacts with its environment, it invariably encounters stressful conditions such as extreme temperatures, drought, UV exposure and harmful xenobiotics. Since the ability to respond appropriately to stressful stimuli is paramount to survival, organisms have developed sophisticated stress response programs. Some stressful conditions cause damaged cells to commit suicide (undergo apoptosis), whereas others cause the entire organism to develop mechanisms to resist environmental stress. Studying the small roundworm C. elegans, we find that these two responses are somehow linked: perturbing the mechanisms that allow cells to undergo apoptosis changes the whole animal's response to environmental stress. In fact, perturbing the apoptosis machinery in any way—through mutations that prevent apoptosis altogether, or through mutations that either slow or accelerate the clearance of dying cells—causes the animal to become more stress resistant. Together our findings raise the possibility that the animal may have a way of detecting defects in the normal programmed cell death pathway, and that in response it induces a new program that protects itself from a harsh environment.

The scavenger receptor SCARF1 mediates the clearance of apoptotic cells and prevents autoimmunity

Clearance of apoptotic cells is critical for control of tissue homeostasis however the full range of receptor(s) on phagocytes responsible for recognition of apoptotic cells remains to be identified. Here we show that dendritic cells (DCs), macrophages and endothelial cells use scavenger receptor type F family member 1 (SCARF1) to recognize and engulf apoptotic cells via C1q. Loss of SCARF1 impairs uptake of apoptotic cells. Consequently, in SCARF1-deficient mice, dying cells accumulate in tissues leading to a lupus-like disease with the spontaneous generation of autoantibodies to DNA-containing antigens, immune cell activation, dermatitis and nephritis. The discovery of SCARF1 interactions with C1q and apoptotic cells provides insights into molecular mechanisms involved in maintenance of tolerance and prevention of autoimmune disease.

Methods for detection and analysis of apoptosis signaling in the C. elegans germline

This review assesses current and emerging methods for the detection, and analysis of apoptosis in the Caenorhabditis elegans germline. The nematode worm C. elegans is highly tractable to genetic manipulation, making it an excellent model for elucidating mechanisms of apoptosis signaling in a multicellular setting. Here we profile the most efficacious fluorescent tools to visualize and quantify germline apoptosis. We focus specifically on the application of fluorescent markers to screen by RNAi for genes and pathways that regulate germline apoptosis under normal conditions or in response to genotoxic stress. We also present the limitations of these methods, and suggest complimentary techniques in order that researchers new to the field can comprehensively assess apoptosis phenotypes in the C. elegans germline.

Both the caspase CSP-1 and a caspase-independent pathway promote programmed cell death in parallel to the canonical pathway for apoptosis in Caenorhabditis elegans

Caspases are cysteine proteases that can drive apoptosis in metazoans and have critical functions in the elimination of cells during development, the maintenance of tissue homeostasis, and responses to cellular damage. Although a growing body of research suggests that programmed cell death can occur in the absence of caspases, mammalian studies of caspase-independent apoptosis are confounded by the existence of at least seven caspase homologs that can function redundantly to promote cell death. Caspase-independent programmed cell death is also thought to occur in the invertebrate nematode Caenorhabditis elegans. The C. elegans genome contains four caspase genes (ced-3, csp-1, csp-2, and csp-3), of which only ced-3 has been demonstrated to promote apoptosis. Here, we show that CSP-1 is a pro-apoptotic caspase that promotes programmed cell death in a subset of cells fated to die during C. elegans embryogenesis. csp-1 is expressed robustly in late pachytene nuclei of the germline and is required maternally for its role in embryonic programmed cell deaths. Unlike CED-3, CSP-1 is not regulated by the APAF-1 homolog CED-4 or the BCL-2 homolog CED-9, revealing that csp-1 functions independently of the canonical genetic pathway for apoptosis. Previously we demonstrated that embryos lacking all four caspases can eliminate cells through an extrusion mechanism and that these cells are apoptotic. Extruded cells differ from cells that normally undergo programmed cell death not only by being extruded but also by not being engulfed by neighboring cells. In this study, we identify in csp-3; csp-1; csp-2 ced-3 quadruple mutants apoptotic cell corpses that fully resemble wild-type cell corpses: these caspase-deficient cell corpses are morphologically apoptotic, are not extruded, and are internalized by engulfing cells. We conclude that both caspase-dependent and caspase-independent pathways promote apoptotic programmed cell death and the phagocytosis of cell corpses in parallel to the canonical apoptosis pathway involving CED-3 activation.

Caspases are cysteine proteases that in many cases drive apoptosis, an evolutionarily conserved and highly stereotyped form of cellular suicide with functions in animal development and tissue maintenance. The dysregulation of apoptosis can contribute to diseases as diverse as cancer, autoimmunity, and neurodegeneration. Caspases are often thought to be required for, or even to define, apoptosis. Although there is evidence that apoptosis can occur in the absence of caspase activity, caspase-independence can be difficult to prove, as most animals have multiple caspases. The nematode Caenorhabditis elegans has four caspases, CED-3, CSP-1, CSP-2, and CSP-3. CED-3 has a well-established role in apoptosis, but less is known about the functions of the CSP caspases. In this study, we show that CSP-1 promotes apoptosis in the developing C. elegans embryo and that CSP-1 is regulated differently than its homolog CED-3. Furthermore, we show that apoptosis and the engulfment of dying cells can occur in mutants lacking all four caspases, proving that neither apoptosis nor cell-corpse engulfment require caspase function and that caspase-independent activities can contribute to apoptosis of some cells during animal development.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Clearing the dead: apoptotic cell sensing, recognition, engulfment, and digestion

Clearance of apoptotic cells is the final stage of programmed cell death. Uncleared corpses can become secondarily necrotic, promoting inflammation and autoimmunity. Remarkably, even in tissues with high cellular turnover, apoptotic cells are rarely seen because of efficient clearance mechanisms in healthy individuals. Recently, significant progress has been made in understanding the steps involved in prompt cell clearance in vivo. These include the sensing of corpses via "find me" signals, the recognition of corpses via "eat me" signals and their cognate receptors, the signaling pathways that regulate cytoskeletal rearrangement necessary for engulfment, and the responses of the phagocyte that keep cell clearance events "immunologically silent." This study focuses on our understanding of these steps.

SLI-1 Cbl inhibits the engulfment of apoptotic cells in C. elegans through a ligase-independent function

The engulfment of apoptotic cells is required for normal metazoan development and tissue remodeling. In Caenorhabditis elegans, two parallel and partially redundant conserved pathways act in cell-corpse engulfment. One pathway, which includes the small GTPase CED-10 Rac and the cytoskeletal regulator ABI-1, acts to rearrange the cytoskeleton of the engulfing cell. The CED-10 Rac pathway is also required for proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. The second pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Cbl, the mammalian homolog of the C. elegans E3 ubiquitin ligase and adaptor protein SLI-1, interacts with Rac and Abi2 and modulates the actin cytoskeleton, suggesting it might act in engulfment. Our genetic studies indicate that SLI-1 inhibits apoptotic cell engulfment and DTC migration independently of the CED-10 Rac and CED-1 pathways. We found that the RING finger domain of SLI-1 is not essential to rescue the effects of SLI-1 deletion on cell migration, suggesting that its role in this process is ubiquitin ligase-independent. We propose that SLI-1 opposes the engulfment of apoptotic cells via a previously unidentified pathway.

Different patterns of regional Purkinje cell loss in the cerebellar vermis as a function of the timing of prenatal ethanol exposure in an ovine model

Studies in rat models of fetal alcohol spectrum disorders have indicated that the cerebellum is particularly vulnerable to ethanol-induced Purkinje cell loss during the third trimester-equivalent, with striking regional differences in vulnerability in which early-maturing regions in the vermis show significantly more loss than the late-maturing regions. The current study tested the hypothesis that the sheep model will show similar regional differences in fetal cerebellar Purkinje cell loss when prenatal binge ethanol exposure is restricted to the prenatal period of brain development equivalent to the third trimester and also compared the pattern of loss to that produced by exposure during the first trimester-equivalent. Pregnant Suffolk sheep were assigned to four groups: first trimester-equivalent saline control group, first trimester-equivalent ethanol group (1.75 g/kg/day), third trimester-equivalent saline control group, and third trimester-equivalent ethanol group (1.75 g/kg/day). Ethanol was administered as an intravenous infusion on 3 consecutive days followed by a 4-day ethanol-free interval, to mimic a weekend binge drinking pattern. Animals from all four groups were sacrificed and fetal brains were harvested on gestation day 133. Fetal cerebellar Purkinje cell counts were performed in an early-maturing region (lobules I-X) and a late-maturing region (lobules VIc-VII) from mid-sagittal sections of the cerebellar vermis. As predicted, the third trimester-equivalent ethanol exposure caused a significant reduction in the fetal cerebellar Purkinje cell volume density and Purkinje cell number in the early-maturing region, but not in the late-maturing region. In contrast, the first trimester-equivalent ethanol exposure resulted in significant reductions in both the early and late-maturing regions. These data confirmed that the previous findings in rat models that third trimester-equivalent prenatal ethanol exposure resulted in regionally-specific Purkinje cell loss in the early-maturing region of the vermis, and further demonstrated that first trimester ethanol exposure caused more generalized fetal cerebellar Purkinje cell loss, independent of the cerebellar vermal region. These findings support the idea that prenatal ethanol exposure in the first trimester interferes with the genesis of Purkinje cells in an unselective manner, whereas exposure during the third trimester selectively kills post-mitotic Purkinje cells in specific vermal regions during a vulnerable period of differentiation and synaptogenesis.

Draper acts through the JNK pathway to control synchronous engulfment of dying germline cells by follicular epithelial cells

The efficient removal of dead cells is an important process in animal development and homeostasis. Cell corpses are often engulfed by professional phagocytes such as macrophages. However, in some tissues with limited accessibility to circulating cells, engulfment is carried out by neighboring non-professional phagocytes such as epithelial cells. Here, we investigate the mechanism of corpse clearance in the Drosophila melanogaster ovary, a tissue that is closed to circulating cells. In degenerating egg chambers, dying germline cells are engulfed by the surrounding somatic follicular epithelium by unknown mechanisms. We show that the JNK pathway is activated and required in engulfing follicle cells. We find that the receptor Draper is also required in engulfing follicle cells, and activates the JNK pathway. Overexpression of Draper or the JNK pathway in follicle cells is sufficient to induce death of the underlying germline, suggesting that there is coordination between the germline and follicular epithelium to promote germline cell death. Furthermore, activation of JNK bypasses the need for Draper in engulfment. The induction of JNK and Draper in follicle cells occurs independently of caspase activity in the germline, indicating that at least two pathways are necessary to coordinate germline cell death with engulfment by the somatic epithelium.

Primary phagocytosis of viable neurons by microglia activated with LPS or Aβ is dependent on calreticulin/LRP phagocytic signalling

Background

Microglia are resident brain macrophages that can phagocytose dead, dying or viable neurons, which may be beneficial or detrimental in inflammatory, ischaemic and neurodegenerative brain pathologies. Cell death caused by phagocytosis of an otherwise viable cell is called ‘primary phagocytosis’ or ‘phagoptosis’. Calreticulin (CRT) exposure on the surface of cancer cells can promote their phagocytosis via LRP (low-density lipoprotein receptor-related protein) on macrophages, but it is not known whether this occurs with neurons and microglia.

Methods

We used primary cultures of cerebellar neurons, astrocytes and microglia to investigate the potential role of CRT/LRP phagocytic signalling in the phagocytosis of viable neurons by microglia stimulated with lipopolysaccharide (LPS) or nanomolar concentrations of amyloid-β peptide_1-42 (Aβ). Exposure of CRT on the neuronal surface was investigated using surface biotinylation and western blotting. A phagocytosis assay was also developed using BV2 and PC12 cell lines to investigate CRT/LRP signalling in microglial phagocytosis of apoptotic cells.

Results

We found that BV2 microglia readily phagocytosed apoptotic PC12 cells, but this was inhibited by a CRT-blocking antibody or LRP-blocking protein (receptor-associated protein: RAP). Activation of primary rat microglia with LPS or Aβ resulted in loss of co-cultured cerebellar granule neurons, and this was blocked by RAP or antibodies against CRT or against LRP, preventing all neuronal loss and death. CRT was present on the surface of viable neurons, and this exposure did not change in inflammatory conditions. CRT antibodies prevented microglia-induced neuronal loss when added to neurons, while LRP antibodies prevented neuronal loss when added to the microglia. Pre-binding of CRT to neurons promoted neuronal loss if activated microglia were added, but pre-binding of CRT to microglia or both cell types prevented microglia-induced neuronal loss.

Conclusions

CRT exposure on the surface of viable or apoptotic neurons appears to be required for their phagocytosis via LRP receptors on activated microglia, but free CRT can block microglial phagocytosis of neurons by acting on microglia. Phagocytosis of CRT-exposing neurons by microglia can be a direct cause of neuronal death during inflammation, and might therefore contribute to neurodegeneration and be prevented by blocking the CRT/LRP pathway.

Eaten alive! Cell death by primary phagocytosis: 'phagoptosis'

Phagoptosis, also called primary phagocytosis, is a recently recognised form of cell death caused by phagocytosis of viable cells, resulting in their destruction. It is provoked by exposure of 'eat-me' signals and/or loss of 'don't-eat-me' signals by viable cells, causing their phagocytosis by phagocytes. Phagoptosis mediates turnover of erythrocytes, neutrophils and other cells, and thus is quantitatively one of the main forms of cell death in the body. It defends against pathogens and regulates inflammation and immunity. However, recent results indicate that inflamed microglia eat viable brain neurons in models of neurodegeneration, and cancer cells can evade phagocytosis by expressing a 'don't-eat-me' signal, suggesting that too much or too little phagoptosis can contribute to pathology. This review provides an overview of the molecular signals that regulate phagoptosis and the physiological and pathological circumstances in which it has been observed.

Somatic gonad sheath cells and Eph receptor signaling promote germ-cell death in C. elegans

Programmed cell death eliminates unwanted cells during normal development and physiological homeostasis. While cell interactions can influence apoptosis as they do other types of cell fate, outside of the adaptive immune system little is known about the intercellular cues that actively promote cell death in healthy cells. We used the Caenorhabditis elegans germline as a model to investigate the extrinsic regulators of physiological apoptosis. Using genetic and cell biological methods, we show that somatic gonad sheath cells, which also act as phagocytes of dying germ cells, promote death in the C. elegans germline through VAB-1/Eph receptor signaling. We report that the germline apoptosis function of VAB-1 impacts specific cell death pathways, and may act in parallel to extracellular signal-regulated kinase MAPK signaling. This work defines a non-autonomous, pro-apoptotic signaling for efficient physiological cell death, and highlights the dynamic nature of intercellular communication between dying cells and the phagocytes that remove them.