Two alternative mechanisms that regulate the presentation of apoptotic cell engulfment signal in Caenorhabditis elegans

Three sorting nexins drive the degradation of apoptotic cells in response to PtdIns(3)P signaling

LST-4/SNX9, SNX-1, and SNX-6 together drive the degradation of apoptotic cells, as PtdIns(3)P effectors, during Caenorhabditis elegans development. By inducing regional membrane curvature and maintaining RAB-7 GTPase on phagosomes, these three sorting nexins stimulate the fusion of endocytic organelles with phagosomes.

Apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Phagosome maturation requires phosphatidylinositol 3-phosphate [PtdIns(3)P], yet how PtdIns(3)P triggers phagosome maturation remains largely unknown. Through a genome-wide PtdIns(3)P effector screen in the nematode Caenorhabditis elegans, we identified LST-4/SNX9, SNX-1, and SNX-6, three BAR domain-containing sorting nexins, that act in two parallel pathways to drive PtdIns(3)P-mediated degradation of apoptotic cells. We found that these proteins were enriched on phagosomal surfaces through association with PtdIns(3)P and through specific protein–protein interaction, and they promoted the fusion of early endosomes and lysosomes to phagosomes, events essential for phagosome maturation. Specifically, LST-4 interacts with DYN-1 (dynamin), an essential phagosome maturation initiator, to strengthen DYN-1’s association to phagosomal surfaces, and facilitates the maintenance of the RAB-7 GTPase on phagosomal surfaces. Furthermore, both LST-4 and SNX-1 promote the extension of phagosomal tubules to facilitate the docking and fusion of intracellular vesicles. Our findings identify the critical and differential functions of two groups of sorting nexins in phagosome maturation and reveal a signaling cascade initiated by phagocytic receptor CED-1, mediated by PtdIns(3)P, and executed through these sorting nexins to degrade apoptotic cells.

A model to die for: signaling to apoptotic cell removal in worm, fly and mouse

Programmed cell death is used during developmental morphogenesis to eliminate superfluous cells or cells with inappropriate developmental potential (e.g., self-reactive immune cells, tumorigenic cells). Recent work in genetic models has led to a number of key observations, revealing signal transduction pathways and identifying new roles for genes previously studied in corpse removal (e.g., removal of broken synapses in the nervous system). Further, studies using mouse models have suggested a role for removal of apoptotic cells in the establishment or maintenance of immune tolerance. In this review, we survey current knowledge of phagocytic pathways derived from studies in the nematode (Caenorhabditis elegans), the fly (Drosophila melanogaster), and mouse (Mus musculus) model systems.

What do we mean when we write "senescence," "apoptosis," "necrosis," or "clearance of dying cells"?

The clearance of dying cells has become an important field of research. Apart from a significant increase in our understanding of the mechanisms for uptake, cell clearance is a basic mechanism in tissue homeostasis, cancer, resolution of inflammation, induction of tolerance, and autoimmunity. Phagocytosis of dying cells is a complex process, involving many interacting molecules on the dying cell and the phagocyte, and in the microenvironment. Although much is known on the subject, there are many questions and unknown variables that remain under investigation. Naturally, different terms were developed, among which some are misused, leading sometimes to pseudoconflicts of understanding. Several receptors were described as "phosphatidylserine receptor: are they all equal?" We will revise terms such as apoptosis, primary and secondary necrosis, lysed cells, senescent cells, clearance of apoptotic cells, efferocytosis, and more. We will try to point out misnomers, misunderstandings, and contradictions, and to define a consensual vocabulary.

Find-me and eat-me signals in apoptotic cell clearance: progress and conundrums

Everyday we turnover billions of cells. The quick, efficient, and immunologically silent disposal of the dying cells requires a coordinated orchestration of multiple steps, through which phagocytes selectively recognize and engulf apoptotic cells. Recent studies have suggested an important role for soluble mediators released by apoptotic cells that attract phagocytes ("find-me" signals). New information has also emerged on multiple receptors that can recognize phosphatidylserine, the key "eat-me" signal exposed on the surface of apoptotic cells. This perspective discusses recent exciting progress, gaps in our understanding, and the conflicting issues that arise from the newly acquired knowledge.

Caenorhabditis elegans transthyretin-like protein TTR-52 mediates recognition of apoptotic cells by the CED-1 phagocyte receptor

During apoptosis, dying cells are swiftly removed by phagocytes. It is not fully understood how apoptotic cells are recognized by phagocytes. Here we report the identification and characterization of the Caenorhabditis elegans ttr-52 gene, which encodes a transthyretin-like protein and is required for efficient cell corpse engulfment. The TTR-52 protein is expressed in, and secreted from, C. elegans endoderm and clusters around apoptotic cells. Genetic analysis indicates that TTR-52 acts in the cell corpse engulfment pathway mediated by CED-1, CED-6 and CED-7 and affects clustering of the phagocyte receptor CED-1 around apoptotic cells. TTR-52 recognizes surface-exposed phosphatidylserine (PtdSer) in vivo and binds to both PtdSer and the extracellular domain of CED-1 in vitro. TTR-52 is therefore the first bridging molecule identified in C. elegans that mediates recognition of apoptotic cells by crosslinking the PtdSer 'eat me' signal with the phagocyte receptor CED-1.

ATP-binding membrane cassette transporter A1 (ABCA1): a possible link between inflammation and reverse cholesterol transport

Atherosclerosis is characterized by a chronic inflammatory condition that involves numerous cellular and molecular inflammatory components. A wide array of inflammatory mediators, such as cytokines and proteins produced by macrophages and other cells, play a critical role in the development and progression of the disease. ATP-binding membrane cassette transporter A1 (ABCA1) is crucial for cellular cholesterol efflux and reverse cholesterol transport (RCT) and is also identified as an important target in antiatherosclerosis treatment. Evidence from several recent studies indicates that inflammation, along with other atherogenic-related mediators, plays distinct regulating roles in ABCA1 expression. Proatherogenic cytokines such as interferon (IFN)-γ and interleukin (IL)-1β have been shown to inhibit the expression of ABCA1, while antiatherogenic cytokines, including IL-10 and transforming growth factor (TGF)-β1, have been shown to promote the expression of ABCA1. Moreover, some cytokines such as tumor necrosis factor (TNF)-α seem to regulate ABCA1 expression in species-specific and dose-dependent manners. Inflammatory proteins such as C-reactive protein (CRP) and cyclooxygenase (COX)-2 are likely to inhibit ABCA1 expression during inflammation, and inflammation induced by lipopolysaccharide (LPS) was also found to block the expression of ABCA1. Interestingly, recent experiments revealed ABCA1 can function as an antiinflammatory receptor to suppress the expression of inflammatory factors, suggesting that ABCA1 may be the molecular basis for the interaction between inflammation and RCT. This review aims to summarize recent findings on the role of inflammatory cytokines, inflammatory proteins, inflammatory lipids, and the endotoxin-mediated inflammatory process in expression of ABCA1. Also covered is the current understanding of the function of ABCA1 in modulating the immune response and inflammation through its direct and indirect antiinflammatory mechanisms including lipid transport, high-density lipoprotein (HDL) formation and apoptosis.

pHMA, a pH-sensitive GFP reporter for cell engulfment, in Drosophila embryos, tissues, and cells

Engulfment of apoptotic cells by phagocytosis ensures the removal of unwanted and defective cells. We developed a genetically encoded marker for cell engulfment, pHMA, which consists of the pH-Sensitive derivative of GFP, pHluorin, fused to the actin-binding domain of Moesin. In healthy cells of Drosophila embryos and cultured cells, pHMA resides at the cell cortex. In dying cells, pHMA loses its cortical localization and reports a modest decrease in pH. In embryos, the dying cells lose their apical contacts, then move basally and are ultimately engulfed by neighboring cells or macrophages. The cell corpse material is strongly acidified soon after engulfment and persists in the phagocytic cell for several hours. Changes in the pHMA signal correlate well with increases or decreases in apoptosis. These data show that pHMA is a useful reporter for cell engulfment and can be used in screening for mutations that affect cell engulfment.

Autoimmunity and the clearance of dead cells

To maintain organismal homeostasis, phagocytes engulf dead cells, which are recognized as dead by virtue of a characteristic "eat me" signal exposed on their surface. The dead cells are then transferred to lysosomes, where their cellular components are degraded for reuse. Inefficient engulfment of dead cells activates the immune system, causing disease such as systemic lupus erythematosus, and if the DNA of the dead cells is not properly degraded, the innate immune response becomes activated, leading to severe anemia and chronic arthritis. Here, we discuss how the endogenous components of dead cells activate the immune system through both extracellular and intracellular pathways.

Engulfment of apoptotic cells in C. elegans is mediated by integrin alpha/SRC signaling

BACKGROUND

Engulfment of apoptotic cells is important for cellular homeostasis and the development of multicellular organisms. Previous studies have shown that more than one engulfment receptors act upstream of the conserved signaling module CED-2/CrkII-CED-5/Dock180-CED-12/ELMO for cell corpse removal in C. elegans, but little is known about their identities, except for PSR-1.

RESULTS

We show that in C. elegans, integrin functions as an engulfment receptor in the recognition and subsequent phagocytosis of apoptotic cells. Mutations in the integrin alpha gene ina-1 result in inefficient engulfment of apoptotic cells. The INA-1 extracellular domain binds to the surface of apoptotic cells in vivo. This binding requires the phospholipid scramblase SCRM-1, which promotes the exposure of phosphatidylserine, a key "eat me" signal in apoptotic cells. Furthermore, we identify an essential role of the nonreceptor tyrosine kinase SRC-1 in INA-1-mediated cell corpse removal. INA-1 and SRC-1 both act in the engulfing cells during the engulfment process and are colocalized in the phagocytic cups extending around apoptotic cells. Finally, our genetic and biochemical data suggest that SRC-1 relays the scrm-1-dependent engulfment signal from INA-1 to the conserved motility-promoting signaling complex CED-2/CrkII-CED-5/Dock180-CED-12/ELMO for CED-10/Rac activation, probably by interactions with CED-2 and the INA-1 cytoplasmic domain, leading to the internalization of apoptotic cells.

CONCLUSIONS

Our findings provide evidence that integrin functions as an engulfment receptor at the whole-organism level and reveal a nonconventional signaling pathway in which SRC provides a FAK-independent linkage between integrin alpha and the common motility-promoting signaling module CED-2/CrkII-CED-5/Dock180-CED-12/ELMO to promote the internalization of apoptotic cells.

The Wnt pathway controls cell death engulfment, spindle orientation, and migration through CED-10/Rac

Specificity in Wnt-mediated developmental processes, such as directional cell cleavage, migration, and engulfment of dead cells in C. elegans, arises from the use of distinct Wnt pathway signalling modules.

Wnt signalling pathways have extremely diverse functions in animals, including induction of cell fates or tumours, guidance of cell movements during gastrulation, and the induction of cell polarity. Wnt can induce polar changes in cellular morphology by a remodelling of the cytoskeleton. However, how activation of the Frizzled receptor induces cytoskeleton rearrangement is not well understood. We show, by an in depth 4-D microscopy analysis, that the Caenorhabditis elegans Wnt pathway signals to CED-10/Rac via two separate branches to regulate modulation of the cytoskeleton in different cellular situations. Apoptotic cell clearance and migration of the distal tip cell require the MOM-5/Fz receptor, GSK-3 kinase, and APC/APR-1, which activate the CED-2/5/12 branch of the engulfment machinery. MOM-5 (Frizzled) thus can function as an engulfment receptor in C. elegans. Our epistatic analyses also suggest that the two partially redundant signalling pathways defined earlier for engulfment may act in a single pathway in early embryos. By contrast, rearrangement of mitotic spindles requires the MOM-5/Fz receptor, GSK-3 kinase, and β-catenins, but not the downstream factors LIT-1/NLK or POP-1/Tcf. Taken together, our results indicate that in multiple developmental processes, CED-10/Rac can link polar signals mediated by the Wnt pathway to rearrangements of the cytoskeleton.

During development, processes such as cell division, fate determination, migration, and removal of dead cells occur in a directional (i.e., polar) manner. For example, cell divisions in the early embryo often occur in a directional manner to maintain specific cell–cell interactions. Later, during organ formation, tissues may be shaped through polar cell migration. During apoptosis, a neighbouring cell engulfing a cell corpse utilizes polar positional information to deform its cytoskeleton and migrate around the corpse. Wnt pathway signalling is a common mechanism by which cells establish polarity during development, but how a single Wnt signal is translated into different outcomes in different cellular or developmental contexts is not clear. Our data on C. elegans suggest that during diverse directional events—cell engulfment, mitotic spindle movement, and cell migration—different Wnt ligands or a specific signal from a cell corpse signals to the Wnt receptor Frizzled (MOM-5 in C. elegans) and on to the downstream factors CED-10/Rac. But specificity of the Wnt signal is mediated by the use of distinct intermediate signal transduction pathways. Our discovery that this pathway is also used for engulfment of cell corpses led us to propose that the two parallel pathways, originally described for engulfment of corpses, are indeed parts of one pathway involving Wnt pathway components.

Pretaporter, a Drosophila protein serving as a ligand for Draper in the phagocytosis of apoptotic cells

Phagocytic removal of cells undergoing apoptosis is necessary for animal development and tissue homeostasis. Draper, a homologue of the Caenorhabditis elegans phagocytosis receptor CED-1, is responsible for the phagocytosis of apoptotic cells in Drosophila, but its ligand presumably present on apoptotic cells remains unknown. An endoplasmic reticulum protein that binds to the extracellular region of Draper was isolated. Loss of this protein, which we name Pretaporter, led to a reduced level of apoptotic cell clearance in embryos, and the overexpression of pretaporter in the mutant flies rescued this defect. Results from genetic analyses suggested that Pretaporter functionally interacts with Draper and the corresponding signal mediators. Pretaporter was exposed at the cell surface after the induction of apoptosis, and cells artificially expressing Pretaporter at their surface became susceptible to Draper-mediated phagocytosis. Finally, the incubation with Pretaporter augmented the tyrosine-phosphorylation of Draper in phagocytic cells. These results collectively suggest that Pretaporter relocates from the endoplasmic reticulum to the cell surface during apoptosis to serve as a ligand for Draper in the phagocytosis of apoptotic cells.

The ins and outs of phospholipid asymmetry in the plasma membrane: roles in health and disease

A common feature of all eukaryotic membranes is the non-random distribution of different lipid species in the lipid bilayer (lipid asymmetry). Lipid asymmetry provides the two sides of the plasma membrane with different biophysical properties and influences numerous cellular functions. Alteration of lipid asymmetry plays a prominent role during cell fusion, activation of the coagulation cascade, and recognition and removal of apoptotic cell corpses by macrophages (programmed cell clearance). Here we discuss the origin and maintenance of phospholipid asymmetry, based on recent studies in mammalian systems as well as in Caenhorhabditis elegans and other model organisms, along with emerging evidence for a conserved role of mitochondria in the loss of lipid asymmetry during apoptosis. The functional significance of lipid asymmetry and its disruption during health and disease is also discussed.

Genetic and biochemical analysis of non-vesicular lipid traffic

Robust lipid traffic within and among membranes is essential for cell growth and membrane biogenesis. Many of these transport reactions occur by nonvesicular pathways, and the genetic and biochemical details of these processes are now beginning to emerge. Intramembrane lipid transport reactions utilize P-type ATPases, ABC transporters, scramblases, and Niemann-Pick type C (NPC) family proteins. The intramembrane processes regulate the establishment and elimination of membrane lipid asymmetry, the cellular influx and efflux of sterols and phospholipids, and the egress of lysosomally deposited lipids. The intermembrane lipid transport processes play important roles in membrane biogenesis, sterol sequestration, and steroid hormone formation. The roles of soluble lipid carriers and membrane-bound lipid-transporting complexes, as well as the mechanisms for regulation of their targeting and assembly, are now becoming apparent. Elucidation of the details of these systems is providing new perspectives on the regulation of lipid traffic within cells.

SED1/MFG-E8: a bi-motif protein that orchestrates diverse cellular interactions

MFG-E8 was initially identified as a principle component of the Milk Fat Globule, a membrane-encased collection of proteins and triglycerides that bud from the apical surface of mammary epithelia during lactation. It has since been independently identified in many species and by many investigators and given a variety of names, including p47, lactadherin, rAGS, PAS6/7, and BA-46. The acronym SED1 was proposed to bring cohesion to this nomenclature based upon it being a Secreted protein that contains two distinct functional domains: an N-terminal domain with two EGF-repeats, the second of which has an integrin-binding RGD motif, and a C-terminal domain with two Discoidin/F5/8C domains that bind to anionic phospholipids and/or extracellular matrices. SED1/MFG-E8 is now known to participate in a wide variety of cellular interactions, including phagocytosis of apoptotic lymphocytes and other apoptotic cells, adhesion between sperm and the egg coat, repair of intestinal mucosa, mammary gland branching morphogenesis, angiogenesis, among others. This article will explore the various roles proposed for SED1/MFG-E8, as well as its provocative therapeutic potential.

Abl kinase inhibits the engulfment of apoptotic [corrected] cells in Caenorhabditis elegans

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 includes the adaptor protein CED-2 CrkII and the small GTPase CED-10 Rac, and acts to rearrange the cytoskeleton of the engulfing cell. The other pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Although many components required for engulfment have been identified, little is known about inhibition of engulfment. The tyrosine kinase Abl regulates the actin cytoskeleton in mammals and Drosophila in multiple ways. For example, Abl inhibits cell migration via phosphorylation of CrkII. We tested whether ABL-1, the C. elegans ortholog of Abl, inhibits the CED-2 CrkII-dependent engulfment of apoptotic cells. Our genetic studies indicate that ABL-1 inhibits apoptotic cell engulfment, but not through CED-2 CrkII, and instead acts in parallel to the two known engulfment pathways. 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 loss of ABL-1 function partially restores normal DTC migration in the CED-10 Rac pathway mutants. We found that ABI-1 the C. elegans homolog of mammalian Abi (Abl interactor) proteins, is required for engulfment of apoptotic cells and proper DTC migration. Like Abl, Abi proteins are cytoskeletal regulators. ABI-1 acts in parallel to the two known engulfment pathways, likely downstream of ABL-1. ABL-1 and ABI-1 interact physically in vitro. We propose that ABL-1 opposes the engulfment of apoptotic cells by inhibiting ABI-1 via a pathway that is distinct from the two known engulfment pathways.

Cell death or apoptosis is a normal part of animal development, as is the engulfment and removal of dead cells by other cells. In the nematode Caenorhabditis elegans, ten highly conserved proteins have been characterized previously for their roles in engulfment and in cell migration, both of which involve the formation of cellular extensions. Little is known, however, about how engulfment is inhibited. In mammals, the tyrosine kinase Abl, which regulates the actin cytoskeleton and which when misexpressed causes two types of leukemia, prevents the CrkII protein from facilitating cell migration. CrkII functions in engulfment in C. elegans and mammals. We tested whether the C. elegans homolog of Abl, ABL-1, could inhibit engulfment. We found that ABL-1 functions as an inhibitor of apoptotic cell engulfment and cell migration. However, our analysis further showed that ABL-1 does not function by inhibiting other known engulfment proteins, including C. elegans CrkII. Our data indicate that ABL-1 blocks ABI-1, the C. elegans homolog of the mammalian and Drosophila Abl-interactor (Abi) cytoskeletal-regulatory proteins. We propose that ABL-1 acts via ABI-1 to inhibit a newly identified pathway during cell corpse engulfment and cell migration.

We show thatC. elegans Abl (ABL-1) inhibits the engulfment of apoptotic cells via a newly defined pathway that includes theC. elegans homolog of the cytoskeletal regulator Abl-interactor.

Detecting apoptotic cells and monitoring their clearance in the nematode Caenorhabditis elegans

Apoptosis is a genetically controlled process of cell suicide that plays an important role in animal development and in maintaining homeostasis. The nematode Caenorhabditis elegans has proven to be an excellent model organism for studying the mechanisms controlling apoptosis and the subsequent clearance of apoptotic cells, aided with cell-biological and genetic tools. In particular, the transparent nature of worm bodies and eggshells makes C. elegans particularly amiable for live cell microscopy. Here we describe a few methods for identifying apoptotic cells in living C. elegans embryos and adults and for monitoring their clearance during embryonic development. These methods are based on Differential Interference Contrast microscopy and on fluorescence microscopy using GFP-based reporters.

Phagosome maturation during the removal of apoptotic cells: receptors lead the way

In metazoan organisms, cells undergoing apoptosis are rapidly engulfed and degraded by phagocytes. Defects in apoptotic-cell clearance result in inflammatory and autoimmune responses. However, little is known about how apoptotic-cell degradation is initiated and regulated and how different phagocytic targets induce different immune responses from their phagocytes. Recent studies in mammalian systems and invertebrate model organisms have led to major progress in identifying new factors involved in the maturation of phagosomes containing apoptotic cells. These studies have delineated signaling pathways that promote the sequential incorporation of intracellular organelles to phagosomes and have also discovered that phagocytic receptors produce the signals that initiate phagosome maturation. Here, we discuss these exciting new findings, focusing on the mechanisms that regulate the interactions between intracellular organelles and phagosomes.

Capitalizing on the immunogenicity of dying tumor cells

Cancer cell death occurs continually during tumor development and progression, whereas the selective killing of surviving cancer cells remains the primary objective of antineoplastic treatments. Recent insights into the immunologic consequences of cancer cell death have begun to elucidate the ways in which host antitumor immunity is shaped during cancer pathogenesis and then modulated by therapeutic intervention. Dying tumor cells evoke a range of host responses, dependent in part upon the mode of cell death, which may either impede or foster additional immune-mediated cancer destruction. Within the tumor microenvironment, the capture of apoptotic tumor cells by macrophages and dendritic cells may trigger tolerance networks that contribute to immune suppression, whereas the uptake of necrotic cancer cells may engender inflammatory pathways that fuel antitumor cytotoxicity. Milk fat globule epidermal growth factor 8, a phosphatidylserine-binding protein, and MHC class I chain-related protein A, an NKG2D ligand, play key roles in these competing outcomes. A deeper understanding of the mechanisms underlying the immunogenicity of dying cells informs the crafting of strategies that exploit endogenous or treatment-induced cancer cell death as the basis for stimulating sustained host antitumor cytotoxic reactions.

Spindle assembly checkpoint gene mdf-1 regulates germ cell proliferation in response to nutrition signals in C. elegans

When newly hatched Caenorhabditis elegans larvae are starved, their primordial germ cells (PGCs) arrest in the post-S phase. This starvation-induced PGC arrest is mediated by the DAF-18/PTEN-AKT-1/PKB nutrient-sensing pathway. Here, we report that the conserved spindle assembly checkpoint (SAC) component MDF-1/MAD1 is required for the PGC arrest. We identified 2 Akt kinase phosphorylation sites on MDF-1. Expression of a non-phosphorylatable mutant MDF-1 partially suppressed the defect in the starvation-induced PGC arrest in L1 larvae lacking DAF-18, suggesting that MDF-1 regulates germ cell proliferation as a downstream target of AKT-1, thereby demonstrating a functional link between cell-cycle regulation by the SAC components and nutrient sensing by DAF-18-AKT-1 during post-embryonic development. The phosphorylation status of MDF-1 affects its binding to another SAC component, MDF-2/MAD2. The loss of MDF-2 or another SAC component also caused inappropriate germ cell proliferation, but the defect was less severe than that caused by mdf-1 hemizygosity, suggesting that MDF-1 causes the PGC arrest by two mechanisms, one involving MDF-2 and another that is independent of other SAC components.

The small GTPase Rab2 functions in the removal of apoptotic cells in Caenorhabditis elegans

We identify here a novel class of loss-of-function alleles of uncoordinated locomotion(unc)-108, which encodes the Caenorhabditis elegans homologue of the mammalian small guanosine triphosphatase Rab2. Like the previously isolated dominant-negative mutants, unc-108 loss-of-function mutant animals are defective in locomotion. In addition, they display unique defects in the removal of apoptotic cells, revealing a previously uncharacterized function for Rab2. unc-108 acts in neurons and engulfing cells to control locomotion and cell corpse removal, respectively, indicating that unc-108 has distinct functions in different cell types. Using time-lapse microscopy, we find that unc-108 promotes the degradation of engulfed cell corpses. It is required for the efficient recruitment and fusion of lysosomes to phagosomes and the acidification of the phagosomal lumen. In engulfing cells, UNC-108 is enriched on the surface of phagosomes. We propose that UNC-108 acts on phagosomal surfaces to promote phagosome maturation and suggest that mammalian Rab2 may have a similar function in the degradation of apoptotic cells.

Engulfment of apoptotic cells: signals for a good meal

The clearance of apoptotic cells by phagocytes is an integral component of normal life, and defects in this process can have significant implications for self tolerance and autoimmunity. Recent studies have provided new insights into the engulfment process, including how phagocytes seek apoptotic cells, how they recognize and ingest these targets and how they maintain cellular homeostasis after the 'meal'. Several new factors that regulate engulfment have been identified, whereas the roles of some of the older players require revision. This Review focuses on these recent developments and attempts to highlight some of the important questions in this field.