C. elegans mitochondrial factor WAH-1 promotes phosphatidylserine externalization in apoptotic cells through phospholipid scramblase SCRM-1

CED-1, CED-7, and TTR-52 regulate surface phosphatidylserine expression on apoptotic and phagocytic cells

BACKGROUND

Phosphatidylserine (PS) normally confined to the cytoplasmic leaflet of plasma membrane (PM) is externalized to the exoplasmic leaflet (exPS) during apoptosis, where it serves as an "eat-me" signal to phagocytes. In addition, some living cells such as macrophages also express exPS.

RESULTS

A secreted Annexin V (sAnxV::GFP) PS sensor reveals that exPS appears early on apoptotic cells in C. elegans embryos and decreases in older or unengulfed apoptotic cells. This decrease in exPS expression is blocked by loss of CED-7, an ATP binding cassette (ABC) transporter, or TTR-52, a secreted PS binding protein. Phagocytic cells also express exPS, which is dependent on the activity of CED-7, TTR-52, and TTR-52-interacting phagocyte receptor CED-1. Interestingly, a secreted lactadherin PS sensor (sGFP::Lact(C1C2)) labels apoptotic cells but not phagocytes, prevents sAnxV::GFP from labeling phagocytes, and compromises phagocytosis. Immuno-electron micrographs of embryos expressing sAnxV::GFP or sGFP::Lact(C1C2) reveal the presence of extracellular PS-containing vesicles between the apoptotic cell and neighboring cells, which are absent or greatly reduced in the ced-7 and ttr-52 mutants, respectively, indicating that CED-7 and TTR-52 promote the generation of extracellular PS vesicles. Loss of the tat-1 gene, which maintains PS asymmetry in the PM, restores phagocyte exPS expression in ced-1, ced-7, and ttr-52 mutants and partially rescues their engulfment defects.

CONCLUSIONS

CED-7 and TTR-52 may promote the efflux of PS from apoptotic cells through the generation of extracellular PS vesicles, which lead to exPS expression on phagocytes via TTR-52 and CED-1 to facilitate cell corpse clearance.

Recently confirmed apoptosis-inducing lead compounds isolated from marine sponge of potential relevance in cancer treatment

Despite intense efforts to develop non-cytotoxic anticancer treatments, effective agents are still not available. Therefore, novel apoptosis-inducing drug leads that may be developed into effective targeted cancer therapies are of interest to the cancer research community. Targeted cancer therapies affect specific aberrant apoptotic pathways that characterize different cancer types and, for this reason, it is a more desirable type of therapy than chemotherapy or radiotherapy, as it is less harmful to normal cells. In this regard, marine sponge derived metabolites that induce apoptosis continue to be a promising source of new drug leads for cancer treatments. A PubMed query from 01/01/2005 to 31/01/2011 combined with hand-curation of the retrieved articles allowed for the identification of 39 recently confirmed apoptosis-inducing anticancer lead compounds isolated from the marine sponge that are selectively discussed in this review.

Phagocytic removal of cells that have become unwanted: implications for animal development and tissue homeostasis

Cells that have become unwanted need to be promptly, selectively, and safely removed. This is made possible by apoptosis-dependent phagocytosis, in which cells unnecessary, obstructive, or dangerous to organisms are induced to undergo apoptosis so that they are earmarked for phagocytosis. The phagocytic elimination occurs so quickly that cells with hallmarks of apoptosis are barely detectable in vivo. The removal of particular types of cells at appropriate stages of development not only contributes to the disposal of spent cells, the creation of space for morphogenesis, and the exclusion of pathogenic or noxious cells, but seems to actively control tissue renewal, tissue remodeling, tissue function, and pathogenic state. This event thus plays an indispensable role in the maintenance of animal development and tissue homeostasis.

Apoptosis-inducing factor: structure, function, and redox regulation

Apoptosis-inducing factor (AIF) is a flavin adenine dinucleotide-containing, NADH-dependent oxidoreductase residing in the mitochondrial intermembrane space whose specific enzymatic activity remains unknown. Upon an apoptotic insult, AIF undergoes proteolysis and translocates to the nucleus, where it triggers chromatin condensation and large-scale DNA degradation in a caspase-independent manner. Besides playing a key role in execution of caspase-independent cell death, AIF has emerged as a protein critical for cell survival. Analysis of in vivo phenotypes associated with AIF deficiency and defects, and identification of its mitochondrial, cytoplasmic, and nuclear partners revealed the complexity and multilevel regulation of AIF-mediated signal transduction and suggested an important role of AIF in the maintenance of mitochondrial morphology and energy metabolism. The redox activity of AIF is essential for optimal oxidative phosphorylation. Additionally, the protein is proposed to regulate the respiratory chain indirectly, through assembly and/or stabilization of complexes I and III. This review discusses accumulated data with respect to the AIF structure and outlines evidence that supports the prevalent mechanistic view on the apoptogenic actions of the flavoprotein, as well as the emerging concept of AIF as a redox sensor capable of linking NAD(H)-dependent metabolic pathways to apoptosis.

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.

Endocytic sorting and recycling require membrane phosphatidylserine asymmetry maintained by TAT-1/CHAT-1

Endocytic sorting is achieved through the formation of morphologically and functionally distinct sub-domains within early endosomes. Cargoes destined for recycling are sorted to and transported through newly-formed tubular membranes, but the processes that regulate membrane tubulation are poorly understood. Here, we identified a novel Caenorhabditis elegans Cdc50 family protein, CHAT-1, which acts as the chaperone of the TAT-1 P4-ATPase to regulate membrane phosphatidylserine (PS) asymmetry and endocytic transport. In chat-1 and tat-1 mutants, the endocytic sorting process is disrupted, leading to defects in both cargo recycling and degradation. TAT-1 and CHAT-1 colocalize to the tubular domain of the early endosome, the tubular endocytic recycling compartment (ERC), and the recycling endosome where PS is enriched on the cytosolic surface. Loss of tat-1 and chat-1 function disrupts membrane PS asymmetry and abrogates the tubular membrane structure. Our data suggest that CHAT-1 and TAT-1 maintain membrane phosphatidylserine asymmetry, thus promoting membrane tubulation and regulating endocytic sorting and recycling.

The process by which cells take up nutrients and other large molecules from the extracellular environment is known as endocytosis. At the cell surface, external molecules become enclosed in membrane spheres called endosomes. Early endosomes serve as a sorting station, directing the contents (cargo molecules) to the correct compartment within the cell. This is thought to be achieved by the formation of membrane structures with distinct shape and function. For example, cargoes destined for recycling and degradation are processed through tubular membrane structures and big vesicular compartments, respectively. However, it is poorly understood how early endosome membranes are shaped into different structures. Here we show that two proteins, CHAT-1 and TAT-1, regulate membrane structure and are important for normal endocytic transport in the nematode worm C. elegans. TAT-1 and CHAT-1 are found in tubular membrane structures along the sorting and recycling pathway, where they enrich the outer membrane layer with a lipid called phosphatidylserine (PS) and probably change the membrane curvature. Loss of tat-1 and chat-1 function disrupts the asymmetric distribution of PS, abolishes tubular membrane structures, and abrogates endocytic sorting/recycling. Our data support a role of TAT-1/CHAT-1–regulated membrane PS asymmetry in promoting membrane tubulation for endocytic cargo sorting and recycling.

Alternative cell death mechanisms in development and beyond

A canonical regulatory pathway involving the members of the Bcl-2 and caspase families has been established to regulate developmental apoptosis in nematodes and flies. However, mutant mice that have major deficiencies in this apoptosis pathway show only relatively minor developmental defects. Recent revelations indicate that multiple mechanisms are involved in regulating cell death during mammalian development, tissue homeostasis, and pathological cell loss. Here, we critically evaluate the evidence demonstrating the existence of alternative cell death mechanisms, including apoptosis of lower organisms in the absence of canonical apoptosis mediators, autophagic cell death, necroptosis, elimination by shedding, keratinocyte death by cornification, and cell-cell cannibalism by entosis. The physiological relevance of alternative cell death mechanisms as primary and backup mechanisms is discussed.

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.

The distribution and function of phosphatidylserine in cellular membranes

Phosphatidylserine (PS) is the most abundant negatively charged phospholipid in eukaryotic membranes. PS directs the binding of proteins that bear C2 or gamma-carboxyglutamic domains and contributes to the electrostatic association of polycationic ligands with cellular membranes. Rather than being evenly distributed, PS is found preferentially in the inner leaflet of the plasma membrane and in endocytic membranes. The loss of PS asymmetry is an early indicator of apoptosis and serves as a signal to initiate blood clotting. This review discusses the determinants and functional implications of the subcellular distribution and membrane topology of PS.

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.

Mechanisms of apoptosis in Crustacea: What conditions induce versus suppress cell death?

Arthropoda is the largest of all animal phyla and includes about 90% of extant species. Our knowledge about regulation of apoptosis in this phylum is largely based on findings for the fruit fly Drosophila melanogaster. Recent work with crustaceans shows that apoptotic proteins, and presumably mechanisms of cell death regulation, are more diverse in arthropods than appreciated based solely on the excellent work with fruit flies. Crustacean homologs exist for many major proteins in the apoptotic networks of mammals and D. melanogaster, but integration of these proteins into the physiology and pathophysiology of crustaceans is far from complete. Whether apoptosis in crustaceans is mainly transcriptionally regulated as in D. melanogaster (e.g., RHG 'killer' proteins), or rather is controlled by pro- and anti-apoptotic Bcl-2 family proteins as in vertebrates needs to be clarified. Some phenomena like the calcium-induced opening of the mitochondrial permeability transition pore (MPTP) are apparently lacking in crustaceans and may represent a vertebrate invention. We speculate that differences in regulation of the intrinsic pathway of crustacean apoptosis might represent a prerequisite for some species to survive harsh environmental insults. Pro-apoptotic stimuli described for crustaceans include UV radiation, environmental toxins, and a diatom-produced chemical that promotes apoptosis in offspring of a copepod. Mechanisms that serve to depress apoptosis include the inhibition of caspase activity by high potassium in energetically healthy cells, alterations in nucleotide abundance during energy-limited states like diapause and anoxia, resistance to opening of the calcium-induced MPTP, and viral accommodation during persistent viral infection. Characterization of the players, pathways, and their significance in the core machinery of crustacean apoptosis is revealing new insights for the field of cell death.

Programmed cell clearance: molecular regulation of the elimination of apoptotic cell corpses and its role in the resolution of inflammation

Programmed cell clearance is a physiological process of elimination of apoptotic cell corpses. Recent studies have disclosed several ligand-receptor interactions that dictate the recognition or non-recognition of cells by macrophages and other phagocytes. The externalization of the anionic phospholipid, phosphatidylserine is effectively recognized by specific receptors on professional phagocytes and facilitates the clearance of apoptotic cells. Macrophage disposal of cells at sites of inflammation is believed to play an important role in the resolution of the inflammatory process, and recent studies have suggested a role for the NADPH oxidase in the process of macrophage elimination of activated neutrophils. The present review will focus on the molecular regulation of programmed cell clearance, and discuss the role of cell elimination in the resolution of inflammation.

AIF promotes chromatinolysis and caspase-independent programmed necrosis by interacting with histone H2AX

Programmed necrosis induced by DNA alkylating agents, such as MNNG, is a caspase-independent mode of cell death mediated by apoptosis-inducing factor (AIF). After poly(ADP-ribose) polymerase 1, calpain, and Bax activation, AIF moves from the mitochondria to the nucleus where it induces chromatinolysis and cell death. The mechanisms underlying the nuclear action of AIF are, however, largely unknown. We show here that, through its C-terminal proline-rich binding domain (PBD, residues 543-559), AIF associates in the nucleus with histone H2AX. This interaction regulates chromatinolysis and programmed necrosis by generating an active DNA-degrading complex with cyclophilin A (CypA). Deletion or directed mutagenesis in the AIF C-terminal PBD abolishes AIF/H2AX interaction and AIF-mediated chromatinolysis. H2AX genetic ablation or CypA downregulation confers resistance to programmed necrosis. AIF fails to induce chromatinolysis in H2AX or CypA-deficient nuclei. We also establish that H2AX is phosphorylated at Ser139 after MNNG treatment and that this phosphorylation is critical for caspase-independent programmed necrosis. Overall, our data shed new light in the mechanisms regulating programmed necrosis, elucidate a key nuclear partner of AIF, and uncover an AIF apoptogenic motif.

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.

The role of mitochondria in apoptosis*

Mitochondria play key roles in activating apoptosis in mammalian cells. Bcl-2 family members regulate the release of proteins from the space between the mitochondrial inner and outer membrane that, once in the cytosol, activate caspase proteases that dismantle cells and signal efficient phagocytosis of cell corpses. Here we review the extensive literature on proteins released from the intermembrane space and consider genetic evidence for and against their roles in apoptosis activation. We also compare and contrast apoptosis pathways in Caenorhabditis elegans, Drosophila melanogaster, and mammals that indicate major mysteries remaining to be solved.

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.

Caenorhabditis elegans myotubularin MTM-1 negatively regulates the engulfment of apoptotic cells

During programmed cell death, apoptotic cells are recognized and rapidly engulfed by phagocytes. Although a number of genes have been identified that promote cell corpse engulfment, it is not well understood how phagocytosis of apoptotic cells is negatively regulated. Here we have identified Caenorhabditis elegans myotubularin MTM-1 as a negative regulator of cell corpse engulfment. Myotubularins (MTMs) constitute a large, highly conserved family of lipid phosphatases. MTM gene mutations are associated with various human diseases, but the cellular functions of MTM proteins are not clearly defined. We found that inactivation of MTM-1 caused significant reduction in cell corpses in strong loss-of-function mutants of ced-1, ced-6, ced-7, and ced-2, but not in animals deficient in the ced-5, ced-12, or ced-10 genes. In contrast, overexpression of MTM-1 resulted in accumulation of cell corpses. This effect is dependent on the lipid phosphatase activity of MTM-1. We show that loss of mtm-1 function accelerates the clearance of cell corpses by promoting their internalization. Importantly, the reduction of cell corpses caused by mtm-1 RNAi not only requires the activities of CED-5, CED-12, and CED-10, but also needs the functions of the phosphatidylinositol 3-kinases (PI3Ks) VPS-34 and PIKI-1. We found that MTM-1 localizes to the plasma membrane in several known engulfing cell types and may modulate the level of phosphatidylinositol 3-phosphate (PtdIns(3)P) in vivo. We propose that MTM-1 negatively regulates cell corpse engulfment through the CED-5/CED-12/CED-10 module by dephosphorylating PtdIns(3)P on the plasma membrane.

Clearance of dead cells is crucial for normal animal development. Cell corpses are recognized, engulfed, and removed by phagocytic cells. However, the mechanisms that regulate phagocytosis are still not well understood, especially the ways in which the process is inhibited (negatively regulated). We screened the nematode worm Caenorhabditis elegans for negative regulators of cell corpse engulfment and identified myotubularin MTM-1. Myotubularins (MTMs) are a family of highly conserved enzymes that remove phosphate groups from membrane lipids. Mutations in human MTM genes are associated with various severe diseases including X-linked myotubular myopathy and Charcot-Marie-Tooth disease, but the normal functions of MTMs are unknown. In this study, we found that MTM-1 inhibits cell corpse engulfment through a series of evolutionarily conserved signaling molecules (the bipartite GEF (CED-5/DOCK180-CED-12/ELMO) and the GTPase CED-10/Rac). The negative regulatory effect of MTM-1 requires both its lipid phosphatase activity and the function of another group of enzymes called PI3-kinases. We propose that MTM-1 acts through Rac GTPase CED-10 by dephosphorylating the lipid PtdIns(3)P on the plasma membrane. We have identified a negative regulatory mechanism of cell corpse engulfment and a previously unknown cellular function of MTM-1, which may provide further insights into the basis of human MTM-related diseases.

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.

Caenorhabditis elegans caspase homolog CSP-2 inhibits CED-3 autoactivation and apoptosis in germ cells

In Caenorhabditis elegans, apoptosis in germ cells is mediated by the same core apoptotic machinery that controls apoptosis in somatic cells. These include the CED-3 caspase, the CED-3 activator CED-4, and the cell death inhibitor CED-9. However, germline apoptosis also differs from somatic apoptosis in its regulation. We found that CSP-3, a caspase homolog that blocks CED-3 autoactivation and apoptosis in somatic cells, does not affect apoptosis in germ cells. Interestingly, the second C. elegans caspase homolog, CSP-2, shares sequence similarity to both catalytic subunits of the CED-3 caspase, and surprisingly, contains a stretch of sequence that is almost identical to that of CSP-3. Unlike CSP-3 that acts specifically in somatic cells, loss of CSP-2 causes increased apoptosis only in germ cells, suggesting that CSP-2 is a germ cell-specific apoptosis inhibitor. Moreover, like CSP-3, CSP-2 associates with the CED-3 zymogen and inhibits its autoactivation, but does not inhibit CED-4-induced CED-3 activation or the activity of the activated CED-3 protease. Thus, two different C. elegans caspase homologs use the same mechanism to prevent caspase autoactivation and apoptosis in different tissues, suggesting that this could be a generally applicable strategy for regulating caspase activation and apoptosis.

Adenine nucleotide translocator cooperates with core cell death machinery to promote apoptosis in Caenorhabditis elegans

In Caenorhabditis elegans, the central cell-killing process is essentially controlled by the interplay of four apoptotic factors: EGL-1/BH3-only protein, CED-9/Bcl2, CED-4/Apaf1, and CED-3/caspase. In cells destined to die, EGL-1 binds to CED-9 and results in the release of CED-4 from the mitochondrion-tethered CED-9-CED-4 complex to the perinucleus, which facilitates processing of the CED-3 caspase to cause apoptosis. However, whether additional factors exist to regulate the cell-killing process remains largely unknown. We have identified here WAN-1, the C. elegans ortholog of mammalian adenine nucleotide translocator, as an important cell death regulator. Genetic inactivation of wan-1 significantly suppressed both somatic and germ line cell deaths in C. elegans. Consistently, chemical inhibition of WAN-1 activity also caused strong reduction of germ line apoptosis. WAN-1 localizes to mitochondria and can form complex with both CED-4 and CED-9. Importantly, the cell death initiator EGL-1 can disrupt the interaction between CED-9 and WAN-1. In addition, overexpression of WAN-1 induced ectopic cell killing dependently on the core cell death pathway. These findings suggest that WAN-1 is involved in the central cell-killing process and cooperates with the core cell death machinery to promote programmed cell death in C. elegans.

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.

Phosphatidylserine externalization in caveolae inhibits Ca2+ efflux through plasma membrane Ca2+-ATPase in ECV304

It has been evidenced that plasma membrane Ca(2+)-ATPase (PMCA) is localized at caveolae. However, the caveolar function of PMCA in living cells has never been demonstrated. In the present study, PMCA is exclusively localized at caveolae from ECV 304 cells demonstrated by sucrose gradient fractionation and the co-localization of PMCA with caveolin-1 was visualized by confocal microscopy. We found that PMCA is the main mechanism involved in Ca(2+) efflux in ECV 304 cells. Treatment of cells with MbetaCD to disrupt caveolae significantly reduced the Ca(2+) efflux, and the rate of decay is 4.45+/-0.14 min(-1) in the absence of MbetaCD and 1.99+/-0.038 min(-1) in the presence of MbetaCD. Moreover, the replenishment of cholesterol restored the reduction of the PMCA-mediated Ca(2+) efflux in the presence of MbetaCD. Consistent with Ca(2+) efflux in living cells, the activity of the reconstituted PMCA in membranes extracted from cells in vitro was decreased in the presence of MbetaCD. It was found that phosphatidylserine, which is normally in the inner leaflet of plasma membranes and is able to stimulate PMCA was relatively enriched in caveolae. Importantly, the treatment of cells with MbetaCD concomitantly increased the phosphatidylserine externalization. Taken together, our results suggest that activation of PMCA in caveolae is modulated by phosphatidylserine, and phosphatidylserine externalization induced by MbetaCD reduced the interaction of phosphatidylserine with PMCA, subsequently PMCA-mediated Ca(2+) efflux in ECV 304 cells.

Caenorhabditis elegans drp-1 and fis-2 regulate distinct cell-death execution pathways downstream of ced-3 and independent of ced-9

The dynamin family of GTPases regulate mitochondrial fission and fusion processes and have been implicated in controlling the release of caspase activators from mitochondria during apoptosis. Here we report that profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans. However, minor proapoptotic roles for drp-1 and fis-2, a homolog of human Fis1, are revealed in sensitized genetic backgrounds. drp-1 and fis-2 function independent of one another and the Bcl-2 homolog CED-9 and downstream of the CED-3 caspase to promote elimination of mitochondria in dying cells, an event that could facilitate cell-death execution. Interestingly, CED-3 can cleave DRP-1, which appears to be important for DRP-1's proapoptotic function, but not its mitochondria fission function. Our findings demonstrate that mitochondria dynamics do not regulate apoptosis activation in C. elegans and reveal distinct roles for drp-1 and fis-2 as mediators of cell-death execution downstream of caspase activation.

Role of C. elegans TAT-1 protein in maintaining plasma membrane phosphatidylserine asymmetry

The asymmetrical distribution of phospholipids on the plasma membrane is critical for maintaining cell integrity and physiology and for regulating intracellular signaling and important cellular events such as clearance of apoptotic cells. How phospholipid asymmetry is established and maintained is not fully understood. We report that the Caenorhabditis elegans P-type adenosine triphosphatase homolog, TAT-1, is critical for maintaining cell surface asymmetry of phosphatidylserine (PS). In animals deficient in tat-1, PS is abnormally exposed on the cell surface, and normally living cells are randomly lost through a mechanism dependent on PSR-1, a PS-recognizing phagocyte receptor, and CED-1, which contributes to recognition and engulfment of apoptotic cells. Thus, tat-1 appears to function in preventing appearance of PS in the outer leaflet of plasma membrane, and ectopic exposure of PS on the cell surface may result in removal of living cells by neighboring phagocytes.

Living with death: the evolution of the mitochondrial pathway of apoptosis in animals

The mitochondrial pathway of cell death, in which apoptosis proceeds following mitochondrial outer membrane permeabilization, release of cytochrome c, and APAF-1 apoptosome-mediated caspase activation, represents the major pathway of physiological apoptosis in vertebrates. However, the well-characterized apoptotic pathways of the invertebrates C. elegans and D. melanogaster indicate that this apoptotic pathway is not universally conserved among animals. This review will compare the role of the mitochondria in the apoptotic programs of mammals, nematodes, and flies, and will survey our knowledge of the apoptotic pathways of other, less familiar model organisms in an effort to explore the evolutionary origins of the mitochondrial pathway of apoptosis.

Membrane lipids: where they are and how they behave

Throughout the biological world, a 30 A hydrophobic film typically delimits the environments that serve as the margin between life and death for individual cells. Biochemical and biophysical findings have provided a detailed model of the composition and structure of membranes, which includes levels of dynamic organization both across the lipid bilayer (lipid asymmetry) and in the lateral dimension (lipid domains) of membranes. How do cells apply anabolic and catabolic enzymes, translocases and transporters, plus the intrinsic physical phase behaviour of lipids and their interactions with membrane proteins, to create the unique compositions and multiple functionalities of their individual membranes?

The molecular archaeology of a mitochondrial death effector: AIF in Drosophila

Apoptosis-inducing factor (AIF) is a phylogenetically conserved redox-active flavoprotein that contributes to cell death and oxidative phosphorylation in Saccharomyces cerevisiae, Caenorhabditis elegans, mouse and humans. AIF has been characterized as a caspase-independent death effector that is activated by its translocation from mitochondria to the cytosol and nucleus. Here, we report the molecular characterization of AIF in Drosophila melanogaster, a species in which most cell deaths occur in a caspase-dependent manner. Interestingly, knockout of zygotic D. melanogaster AIF (DmAIF) expression using gene targeting resulted in decreased embryonic cell death and the persistence of differentiated neuronal cells at late embryonic stages. Although knockout embryos hatch, they undergo growth arrest at early larval stages, accompanied by mitochondrial respiratory dysfunction. Transgenic expression of DmAIF misdirected to the extramitochondrial compartment (DeltaN-DmAIF), but not wild-type DmAIF, triggered ectopic caspase activation and cell death. DeltaN-DmAIF-induced death was not blocked by removal of caspase activator Dark or transgenic expression of baculoviral caspase inhibitor p35, but was partially inhibited by Diap1 overexpression. Knockdown studies revealed that DeltaN-DmAIF interacts genetically with the redox protein thioredoxin-2. In conclusion, we show that Drosophila AIF is a mitochondrial effector of cell death that plays roles in developmentally regulated cell death and normal mitochondrial function.

Therapeutic potential of AIF-mediated caspase-independent programmed cell death

Resistance to anticancer drugs is often related to deficient cell death execution pathways in cancer cells. Apoptosis, which denotes a form of cell death executed by caspases, was traditionally considered as the only physiological and programmed form of cell death. However, recent evidence indicates that programmed cell death (PCD) can occur in complete absence of caspase activation. Indeed, a large number of caspase-independent models are now defined and a key protein implicated in this type of PCD, apoptosis-inducing factor (AIF), has been identified. AIF is a mitochondrial protein with two faces looking in opposite life/death directions. Recently, the identification of five different isoforms allowed a better characterization of AIFs life/mitochondrial versus death/nuclear functions, as well as definition of its pro-apoptotic region and some of its nuclear partners. Importantly, much work on caspase-independent PCD has revealed that AIF participates in more PCD systems than initially thought. A wider molecular knowledge of AIF, and of the caspase-independent PCDs in which it is involved, are key to provide new insights into the role of PCD. There is no doubt that these insights will lead to the development of more selective and efficient drugs against cancer, degenerative diseases, and other pathological disorders implicating AIF.

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.

Fat(al) attraction: oxidized lipids act as "eat-me" signals

Phagocytosis of apoptotic cell corpses is a conserved and well-regulated process and is required to maintain tissue homeostasis within an organism. Evidence suggests that apoptotic cell engulfment by macrophages is dependent upon the externalization of phosphatidylserine (PS) on the plasma membrane of the dying cell. Furthermore, oxidation of PS and other phospholipids may serve to facilitate cell corpse removal. However, our understanding of how these various lipid "eat-me" signals are recognized by macrophages has been limited. Using a combination of cellular and animal models, along with an array of biophysical methods, Hazen and his associates (Greenberg et al., J. Exp. Med., 2006, 203, 2613-2625;Li et al., Biochemistry, 2007, 46, 5009-5017) have now identified the scavenger receptor CD36 as a putative receptor for oxidized PS on apoptotic cells; moreover, they have deduced the conformation of the oxidized lipid ligand that is recognized by this receptor, thus providing structural insight into how phagocytes recognize senescent or apoptotic cells.

Friendly fire against neutrophils: proteolytic enzymes confuse the recognition of apoptotic cells by macrophages

Physiologically the only acceptable fate for almost all damaged or unwanted cells is their apoptotic death, followed by engulfment of the corpses by healthy neighbors or professional phagocytes. Efficient clearance of cells that have succumbed to apoptosis is crucial for normal tissue homeostasis, and for the modulation of immune responses. The disposal of apoptotic cells is finely regulated by a highly redundant system of receptors, bridging molecules and 'eat me' signals. The complexity of the system is reflected by the term: 'engulfment synapse', used to describe the interaction between a phagocytic cell and its target. In healthy humans, dying neutrophils are the most abundant and important targets for such recognition and engulfment. In inflammation the scope and importance of this complicated task is further increased. Paradoxically, despite growing evidence highlighting the priority of neutrophils clearance, the recognition of these cells by phagocytes is not as well understood as the recognition of other apoptotic cell types. New findings indicate that the interaction of phosphatidylserine (PS) on apoptotic neutrophils with its receptor on macrophages is not as critical for the specific clearance of neutrophil corpses it was previously believed. In this review we focus on recent findings regarding alternative, PS-independent "eat me" signals expressed on neutrophils during cell death and activation. Based on our own research, we emphasize the clearance of dying neutrophils, especially at the focus of bacterial infection; and the associated inflammatory reaction, which occurs in a highly proteolytic milieu containing both host and bacteria-derived proteinases. In these environments, eat-me signals expressed by neutrophils are drastically modified; arguing against the phospholipid-based detection of apoptotic cells, but supporting the importance of proteinaceous ligand(s) for the recognition of neutrophils by macrophages. In this context we discuss the effect of the gingipain R (Rgp) proteinases from Porphyromonas gingivalis on neutrophils interactions with macrophages. Since the recognition of apoptotic neutrophils is an important fundamental process, serving multiple functions in the regulation of immunity and homeostasis, we hypothesize that many pathogenic bacteria may have developed similar strategies to confuse macrophage-neutrophil interaction as a common pathogenic strategy.

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

Phosphatidylserine exposed on the surface of apoptotic mammalian cells is considered an "eat-me" signal that attracts phagocytes. The generality of using phosphatidylserine as a clearance signal for apoptotic cells in animals and the regulation of this event remain uncertain. Using ectopically expressed mouse MFG-E8, a secreted phosphatidylserine-binding protein, we detected specific exposure of phosphatidylserine on the surface of apoptotic cells in Caenorhabditis elegans. Masking the surface phosphatidylserine inhibits apoptotic cell engulfment. CED-7, an ATP-binding cassette (ABC) transporter, is necessary for the efficient exposure of phosphatidylserine on apoptotic somatic cells, and for the recognition of these cells by phagocytic receptor CED-1. Alternatively, phosphatidylserine exposure on apoptotic germ cells is not CED-7 dependent, but instead requires phospholipid scramblase PLSC-1, a homologue of mammalian phospholipid scramblases. Moreover, deleting plsc-1 results in the accumulation of apoptotic germ cells but not apoptotic somatic cells. These observations suggest that phosphatidylserine might be recognized by CED-1 and act as a conserved eat-me signal from nematodes to mammals. Furthermore, the two different biochemical activities used in somatic cells (ABC transporter) and germ cells (phospholipid scramblase) suggest an increased complexity in the regulation of phosphatidylserine presentation in response to apoptotic signals in different tissues and during different developmental stages.