Involvement of extracellular calcium in phosphatidylserine exposure during apoptosis

How do necrotic cells expose phosphatidylserine to attract their predators—What’s unique and what’s in common with apoptotic cells

Phosphatidylserine (PS) is a lipid component of the plasma membrane. It is asymmetrically distributed to the inner leaflet in live cells. In cells undergoing apoptosis, phosphatidylserine is exposed to the outer surfaces. The exposed phosphatidylserine acts as an evolutionarily conserved “eat-me” signal that attracts neighboring engulfing cells in metazoan organisms, including the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and mammals. During apoptosis, the exposure of phosphatidylserine to the outer surface of a cell is driven by the membrane scramblases and flippases, the activities of which are regulated by caspases. Cells undergoing necrosis, a kind of cell death frequently associated with cellular injuries and morphologically distinct from apoptosis, were initially believed to allow passive exposure of phosphatidylserine through membrane rupture. Later studies revealed that necrotic cells actively expose phosphatidylserine before any rupture occurs. A recent study in C. elegans further reported that the calcium ion (Ca2+) plays an essential role in promoting the exposure of phosphatidylserine on the surfaces of necrotic cells. These findings indicate that necrotic and apoptotic cells, which die through different molecular mechanisms, use common and unique mechanisms for promoting the exposure of the same “eat me” signal. This article will review the mechanisms regulating the exposure of phosphatidylserine on the surfaces of necrotic and apoptotic cells and highlight their similarities and differences.

Design, Synthesis, and Anticancer Activity of Triptycene-Peptide Hybrids that Induce Paraptotic Cell Death in Cancer Cells

We report on the design and synthesis of triptycene-peptide hybrids (TPHs), 5, syn-6, and anti-6, which are conjugates of a triptycene core unit with two or three cationic KKKGG peptides (K: lysine and G: glycine) through a C₈ alkyl chain. It was discovered that syn-6 and anti-6 induce paraptosis, a type of programmed cell death (PCD), in Jurkat cells (leukemia T-lymphocytes). Mechanistic studies indicate that these TPHs induce the transfer of Ca²⁺ from the endoplasmic reticulum (ER) to mitochondria, a loss of mitochondrial membrane potential (ΔΨ_m), tethering of the ER and mitochondria, and cytoplasmic vacuolization in the paraptosis processes.

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

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

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

ABT-737 Triggers Caspase-Dependent Inhibition of Platelet Procoagulant Extracellular Vesicle Release during Apoptosis and Secondary Necrosis In Vitro

Platelet lifespan is limited by activation of intrinsic apoptosis. Apoptotic platelets are rapidly cleared from the circulation in vivo. ABT-737 triggers platelet apoptosis and is a useful tool for studying this process. However, in vitro experiments lack clearance mechanisms for apoptotic platelets. To determine whether apoptotic platelets progress to secondary necrosis, apoptosis was triggered in human platelets with ABT-737, a BH3 mimetic. Platelet annexin V (AnV) binding, release of AnV⁺ extracellular vesicles (EVs), and loss of plasma membrane integrity were monitored by flow cytometry. ABT-737 triggered AnV binding, indicating phosphatidylserine exposure, release of AnV⁺ EVs, and a slow loss of plasma membrane integrity. The latter suggests that apoptotic platelets progress to secondary necrosis in vitro. These responses were dependent on caspase activation and Ca²⁺ entry. Surprisingly, although intracellular Ca²⁺ concentration increased, AnV⁺ EV release was not dependent on the Ca²⁺-dependent protease, calpain. On the contrary, ABT-737 downregulated the ability of the Ca²⁺ ionophore, A23187, to trigger calpain-dependent release of AnV⁺ EVs. This was dependent on caspase activity as, when caspases were inhibited, ABT-737 increased the ability of A23187 to trigger AnV⁺ EV release. These data suggest that apoptotic platelets progress to secondary necrosis unless they are cleared. This may affect the interpretation of ABT-737-triggered signaling in platelets in vitro. Ca²⁺-dependent AnV⁺ EV release is downregulated during apoptosis in a caspase-dependent manner, which may limit the potential consequences of secondary necrotic platelets.

Membrane Remodeling of Giant Vesicles in Response to Localized Calcium Ion Gradients

In a wide variety of fundamental cell processes, such as membrane trafficking and apoptosis, cell membrane shape transitions occur concurrently with local variations in calcium ion concentration. The main molecular components involved in these processes have been identified; however, the specific interplay between calcium ion gradients and the lipids within the cell membrane is far less known, mainly due to the complex nature of biological cells and the difficultly of observation schemes. To bridge this gap, a synthetic approach is successfully implemented to reveal the localized effect of calcium ions on cell membrane mimics. Establishing a mimic to resemble the conditions within a cell is a severalfold problem. First, an adequate biomimetic model with appropriate dimensions and membrane composition is required to capture the physical properties of cells. Second, a micromanipulation setup is needed to deliver a small amount of calcium ions to a particular membrane location. Finally, an observation scheme is required to detect and record the response of the lipid membrane to the external stimulation. This article offers a detailed biomimetic approach for studying the calcium ion-membrane interaction, where a lipid vesicle system, consisting of a giant unilamellar vesicle (GUV) connected to a multilamellar vesicle (MLV), is exposed to a localized calcium gradient formed using a microinjection system. The dynamics of the ionic influence on the membrane were observed using fluorescence microscopy and recorded at video frame rates. As a result of the membrane stimulation, highly curved membrane tubular protrusions (MTPs) formed inside the GUV, oriented away from the membrane. The described approach induces the remodeling of the lipid membrane and MTP production in an entirely contactless and controlled manner. This approach introduces a means to address the details of calcium ion-membrane interactions, providing new avenues to study the mechanisms of cell membrane reshaping.

Sensitivity of peripheral membrane proteins to the membrane context: A case study of phosphatidylserine and the TIM proteins

There is a diverse class of peripheral membrane-binding proteins that specifically bind phosphatidylserine (PS), a lipid that signals apoptosis or cell fusion depending on the membrane context of its presentation. PS-receptors are specialized for particular PS-presenting pathways, indicating that they might be sensitive to the membrane context. In this review, we describe a combination of thermodynamic, structural, and computational techniques that can be used to investigate the mechanisms underlying this sensitivity. As an example, we focus on three PS-receptors of the T-cell Immunoglobulin and Mucin containing (TIM) protein family, which we have previously shown to differ in their sensitivity to PS surface density.

Unconventional apoptosis of polymorphonuclear neutrophils (PMN): staurosporine delays exposure of phosphatidylserine and prevents phagocytosis by MΦ-2 macrophages of PMN

Apoptosis of polymorphonuclear neutrophils (PMN) and subsequent 'silent' removal represents an important check-point for the resolution of inflammation. Failure in PMN clearance resulting in secondary necrosis-driven tissue damage has been implicated in conditions of chronic inflammation and autoimmunity. Apoptotic PMN undergo profound biophysical changes that warrant their efficient recognition and uptake by phagocytes before fading to secondary necrosis. In this study, we demonstrate that staurosporine (STS), a non-selective but potent inhibitor of cyclin-dependent kinase and protein kinase C, exerts a drastic impact on PMN apoptosis. PMN treated with STS underwent an unconventional form of cell death characterized by a delayed exposure of aminophospholipids, including phosphatidylserine (PS) and phosphatidylethanolamine and an increased exposure of neo-glycans. STS caused an impaired cellular fragmentation and accelerated DNA fragmentation. Phagocytosis of STS-treated PMN lacking PS on their surfaces was decreased significantly, which highlights the importance of PS for the clearance of apoptotic PMN. Specific opsonization with immune complexes completely restored phagocytosis of STS-treated PMN, demonstrating the efficiency of back-up clearance pathways in the absence of PS exposure.

Pioglitazone restores phagocyte mitochondrial oxidants and bactericidal capacity in chronic granulomatous disease

BACKGROUND

Deficient production of reactive oxygen species (ROS) by the phagocyte nicotinamide adenine dinucleotide (NADPH) oxidase in patients with chronic granulomatous disease (CGD) results in susceptibility to certain pathogens secondary to impaired oxidative killing and mobilization of other phagocyte defenses. Peroxisome proliferator-activated receptor (PPAR) γ agonists, including pioglitazone, approved for type 2 diabetes therapy alter cellular metabolism and can heighten ROS production. It was hypothesized that pioglitazone treatment of gp91(phox-/-) mice, a murine model of human CGD, would enhance phagocyte oxidant production and killing of Staphylococcus aureus, a significant pathogen in patients with this disorder.

OBJECTIVES

We sought to determine whether pioglitazone treatment of gp91(phox-/-) mice enhanced phagocyte oxidant production and host defense.

METHODS

Wild-type and gp91(phox-/-) mice were treated with the PPARγ agonist pioglitazone, and phagocyte ROS and killing of S aureus were investigated.

RESULTS

As demonstrated by 3 different ROS-sensing probes, short-term treatment of gp91(phox-/-) mice with pioglitazone enhanced stimulated ROS production in neutrophils and monocytes from blood and neutrophils and inflammatory macrophages recruited to tissues. Mitochondria were identified as the source of ROS. Findings were replicated in human monocytes from patients with CGD after ex vivo pioglitazone treatment. Importantly, although mitochondrial (mt)ROS were deficient in gp91(phox-/-) phagocytes, their restoration with treatment significantly enabled killing of S aureus both ex vivo and in vivo.

CONCLUSIONS

Together, the data support the hypothesis that signaling from the NADPH oxidase under normal circumstances governs phagocyte mtROS production and that such signaling is lacking in the absence of a functioning phagocyte oxidase. PPARγ agonism appears to bypass the need for the NADPH oxidase for enhanced mtROS production and partially restores host defense in CGD.

Phosphatidylserine-mediated cellular signaling

Phosphatidylserine (PS), a phospholipid with a negatively charged head group, is an important constituent of eukaryotic membranes. Rather than being a passive component of cellular membranes, PS plays an important role in a number of signaling pathways. Signaling is mediated by proteins that are recruited and/or activated by PS in one of two ways: via domains that stereospecifically recognize the head group, or by electrostatic interactions with membranes that are rich in PS and therefore display negative surface charge. Such interactions are key to both intracellular and extracellular signaling cascades. PS, exposed extracellularly, is instrumental in triggering blood clotting and also serves as an "eat me" signal for the clearance of apoptotic cells. Inside the cell, a number of pathways depend of PS; these include kinases, small GTPases and fusogenic proteins. This review will discuss the generation and distribution of PS, current methods of phospholipid visualization within live cells, as well as the current understanding of the role of PS in both extracellular and intracellular signaling events.

Calcium-dependent phospholipid scramblase activity of TMEM16 protein family members

BACKGROUND

TMEM16A and 16B work as Cl(-) channel, whereas 16F works as phospholipid scramblase. The function of other TMEM16 members is unknown.

RESULTS

Using TMEM16F(-/-) cells, TMEM16C, 16D, 16F, 16G, and 16J were shown to be lipid scramblases.

CONCLUSION

Some TMEM16 members are divided into two Cl(-) channels and five lipid scramblases.

SIGNIFICANCE

Learning the biochemical function ofTMEM16family members is essential to understand their physiological role. Asymmetrical distribution of phospholipids between the inner and outer plasma membrane leaflets is disrupted in various biological processes. We recently identified TMEM16F, an eight-transmembrane protein, as a Ca(2+)-dependent phospholipid scramblase that exposes phosphatidylserine (PS) to the cell surface. In this study, we established a mouse lymphocyte cell line with a floxed allele in the TMEM16F gene. When TMEM16F was deleted, these cells failed to expose PS in response to Ca(2+) ionophore, but PS exposure was elicited by Fas ligand treatment. We expressed other TMEM16 proteins in the TMEM16F(-/-) cells and found that not only TMEM16F, but also 16C, 16D, 16G, and 16J work as lipid scramblases with different preference to lipid substrates. On the other hand, a patch clamp analysis in 293T cells indicated that TMEM16A and 16B, but not other family members, acted as Ca(2+)-dependent Cl(-) channels. These results indicated that among 10 TMEM16 family members, 7 members could be divided into two subfamilies, Ca(2+)-dependent Cl(-) channels (16A and 16B) and Ca(2+)-dependent lipid scramblases (16C, 16D, 16F, 16G, and 16J).

Calcium signalling and the regulation of apoptosis

Apoptotic cell death is characterized by cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies and phagocytosis (Kerr et al., 1972). At the molecular level, activation of a family of cysteine proteases, caspases, related to interleukin-1beta-converting enzyme is believed to be a crucial event in apoptosis. This is associated with the proteolysis of nuclear and cytoskeletal proteins, cell shrinkage, glutathione efflux, exposure of phosphatidylserine on the cell surface, membrane blebbing, etc. In CD95- or TNF-mediated apoptosis, the proteolytic cascade is believed to be triggered directly by caspase binding to the activated plasma membrane receptor complex. In other forms of apoptosis, the mechanisms of activation of the proteolytic cascade are less well established but may involve imported proteases, such as granzyme B, or factors released from the mitochondria and, possibly, other organelles. Recently, the possibility that cytochrome c released from the mitochondria may serve to activate dormant caspases in the cytosol, and thereby to propagate the apoptotic process, has attracted considerable attention. A perturbation of intracellular Ca(2+) homeostasis has been found to trigger apoptosis in many experimental systems, and the apoptotic process has been related to either a sustained increase in cytosolic free Ca(2+) level or a depletion of intracellular Ca(2+) stores. Although many of the biochemical events involved in the apoptotic process are Ca(2+) dependent, the exact mechanism by which Ca(2+) triggers apoptosis remains unknown. The bcl-2 gene family, which includes both inhibitors and inducers of apoptosis, appears to regulate intracellular Ca(2+) compartmentalization. The induction of apoptosis by Ca(2+)-mobilizing agents results in caspase activation, which is similar to what is seen with other inducers of apoptosis. In addition, Ca(2+)-dependent proteases, such as calpain and a Ca(2+)-dependent nuclear scaffold-associated serine protease, are also activated by Ca(2+) signalling in some cell types where they appear to be involved in alpha-fodrin and lamin beta cleavage, respectively. Thus, a spectrum of proteases are activated during apoptosis depending on both cell type and inducer. This proteolytic cascade can involve both caspases and Ca(2+)-dependent proteases, which seem to interact during the apoptotic process.

Alterations in Lipid Levels of Mitochondrial Membranes Induced by Amyloid-β: A Protective Role of Melatonin

Alzheimer pathogenesis involves mitochondrial dysfunction, which is closely related to amyloid-β (Aβ) generation, abnormal tau phosphorylation, oxidative stress, and apoptosis. Alterations in membranal components, including cholesterol and fatty acids, their characteristics, disposition, and distribution along the membranes, have been studied as evidence of cell membrane alterations in AD brain. The majority of these studies have been focused on the cytoplasmic membrane; meanwhile the mitochondrial membranes have been less explored. In this work, we studied lipids and mitochondrial membranes in vivo, following intracerebral injection of fibrillar amyloid-β (Aβ). The purpose was to determine how Aβ may be responsible for beginning of a vicious cycle where oxidative stress and alterations in cholesterol, lipids and fatty acids, feed back on each other to cause mitochondrial dysfunction. We observed changes in mitochondrial membrane lipids, and fatty acids, following intracerebral injection of fibrillar Aβ in aged Wistar rats. Melatonin, a well-known antioxidant and neuroimmunomodulator indoleamine, reversed some of these alterations and protected mitochondrial membranes from obvious damage. Additionally, melatonin increased the levels of linolenic and n-3 eicosapentaenoic acid, in the same site where amyloid β was injected, favoring an endogenous anti-inflammatory pathway.

CD95 triggers Orai1-mediated localized Ca2+ entry, regulates recruitment of protein kinase C (PKC) β2, and prevents death-inducing signaling complex formation

The death receptor CD95 plays a pivotal role in immune surveillance and immune tolerance. Binding of CD95L to CD95 leads to recruitment of the adaptor protein Fas-associated death domain protein (FADD), which in turn aggregates caspase-8 and caspase-10. Efficient formation of the CD95/FADD/caspase complex, known as the death-inducing signaling complex (DISC), culminates in the induction of apoptosis. We show that cells exposed to CD95L undergo a reorganization of the plasma membrane in which the Ca(2+) release-activated Ca(2+) channel Orai1 and the endoplasmic reticulum-resident activator stromal interaction molecule 1 colocalize with CD95 into a micrometer-sized cluster in which the channel elicits a polarized entry of calcium. Orai1 knockdown and expression of a dominant negative construct (Orai1E106A) reveal that on CD95 engagement, the Orai1-driven localized Ca(2+) influx is fundamental to recruiting the Ca(2+)-dependent protein kinase C (PKC) β2 to the DISC. PKCβ2 in turn transiently holds the complex in an inactive status, preventing caspase activation and transmission of the apoptotic signal. This study identifies a biological role of Ca(2+) and the Orai1 channel that drives a transient negative feedback loop, introducing a lag phase in the early steps of the CD95 signal. We suggest that these localized events provide a time of decision to prevent accidental cell death.

Phosphatidylserine as an anchor for plasminogen and its plasminogen receptor, histone H2B, to the macrophage surface

BACKGROUND

Plasminogen (Plg) binding to cell surface Plg receptors (Plg-Rs) on the surface of macrophages facilitates Plg activation and migration of these cells. Histone H2B (H2B) acts as a Plg-R and its cell surface expression is up-regulated when monocytes are differentiated to macrophages via a pathway dependent on L-type Ca(2+) channels and intracellular Ca(2+).

OBJECTIVES

We sought to investigate the mechanism by which H2B, a protein without a transmembrane domain, is retained on the macrophage surface.

METHODS

THP-1 monocytoid cells were induced to differentiate with interferon gamma + Vitamin D3 or to undergo apoptosis by treatment with camptothecin. Flow cytometry and cell surface biotinylation followed by Western blotting were used to measure the interrelationship between Plg binding, cell surface expression of H2B and outer membrane exposure of phosphatidylserine (PS).

RESULTS

H2B interacted directly with PS via an electrostatic interaction. Anti-PS or PS binding proteins, annexin V and protein S, diminished H2B interaction with PS on the surface of differentiated or apoptotic cells and these same reagents inhibited Plg binding to these cells. L-type Ca(2+) channels played a significant role in PS exposure, H2B surface expression and Plg binding induced either by differentiation or apoptosis.

CONCLUSIONS

These data suggest that H2B tethers to the surface of cells by interacting with PS on differentiated or apoptotic monocytoid cells. L-type Ca(2+) channels regulate PS exposure on the surface of these cells. The exposed PS interacts directly with H2B and hence provides sites for Plg to bind to.

Dose-dependent thresholds of 10-ns electric pulse induced plasma membrane disruption and cytotoxicity in multiple cell lines

In this study, we determined the LD₅₀ (50% lethal dose) for cell death, and the ED₅₀ (50% of cell population staining positive) for propidium (Pr) iodide uptake, and phosphatidylserine (PS) externalization for several commonly studied cell lines (HeLa, Jurkat, U937, CHO-K1, and GH3) exposed to 10-ns electric pulses (EP). We found that the LD₅₀ varied substantially across the cell lines studied, increasing from 51 J/g for Jurkat to 1861 J/g for HeLa. PS externalized at doses equal or lower than that required for death in all cell lines ranging from 51 J/g in Jurkat, to 199 J/g in CHO-K1. Pr uptake occurred at doses lower than required for death in three of the cell lines: 656 J/g for CHO-K1, 634 J/g for HeLa, and 142 J/g for GH3. Both Jurkat and U937 had a LD₅₀ lower than the ED₅₀ for Pr uptake at 780 J/g and 1274 J/g, respectively. The mechanism responsible for these differences was explored by evaluating cell size, calcium concentration in the exposure medium, and effect of trypsin treatment prior to exposure. None of the studied parameters correlated with the observed results suggesting that cellular susceptibility to injury and death by 10-ns EP was largely determined by cell physiology. In contrast to previous studies, our findings suggest that permeabilization of internal membranes may not necessarily be responsible for cell death by 10-ns EP. Additionally, a mixture of Jurkat and HeLa cells was exposed to 10-ns EP at a dose of 280 J/g. Death was observed only in Jurkat cells suggesting that 10-ns EP may selectively kill cells within a heterogeneous tissue.

Site-specific 68Ga-labeled Annexin A5 as a PET imaging agent for apoptosis

PURPOSE

Two variants of Annexin A5 (Cys2-AnxA5 and Cys165-AnxA5) were labelled with Gallium-68 in order to evaluate their biological properties.

PROCEDURES

Biodistribution and pharmacokinetics of the radiotracers were studied with μPET in healthy mice and in a mouse model of hepatic apoptosis. μPET imaging after IV injection of the tracers in combination with μMRI was performed in Daudi tumor bearing mice before and after treatment with a combination of chemotherapy and radiotherapy.

RESULTS

The biodistribution data indicated a fast urinary clearance with only minor hepatobilliary clearance, although a high retention in the kidneys was observed. Animals treated with anti-Fas showed a 3 to 8 times higher liver uptake as compared to healthy animals. Tumor uptake of (68)Ga-Cys2-AnxA5 and (68)Ga-Cys165-AnxA5 was low but significantly increased after therapy.

CONCLUSION

Both (68)Ga-Cys2-AnxA5 and (68)Ga-Cys165-AnxA5 show a clear binding to apoptotic cells and are promising tracers for rapid evaluation of cancer therapy.

Apoptosis and autoimmune diseases

Every day billions of cells die in our bodies to eliminate those that are harmful, useless, or senescent. The process can be divided into two steps: cell dying and cell clearance. In the first step, death machinery is activated in the cells and quickly kills them. During the second step, dead cells are engulfed by phagocytes, and their components are degraded in the lysosomes of the phagocytes. The death mechanism and the clearance of dead cells have been extensively studied. Mouse lines that are deficient in the death or clearance process have been established, and human patients carrying a mutation in the death machinery have been identified. Data from these mutant mice and human patients indicate that defects in cell death or dead-cell clearance leads to autoimmunity. This review examines the cell death and clearance processes and briefly discusses the diseases they cause.

Cleavage of sphingosine kinase 2 by caspase-1 provokes its release from apoptotic cells

Execution of physiologic cell death known as apoptosis is tightly regulated and transfers immunologically relevant information. This ensures efficient clearance of dying cells and shapes the phenotype of their "captors" toward anti-inflammatory. Here, we identify a mechanism of sphingosine-1-phosphate production by apoptotic cells. During cell death, sphingosine kinase 2 (SphK2) is cleaved at its N-terminus in a caspase-1-dependent manner. Thereupon, a truncated but enzymatically active fragment of SphK2 is released from cells. This step is coupled to phosphatidylserine exposure, which is a hallmark of apoptosis and a crucial signal for phagocyte/apoptotic cell interaction. Our data link signaling events during apoptosis to the extracellular production of a lipid mediator that affects immune cell attraction and activation.

Site-specific labeling of 'second generation' annexin V with 99mTc(CO)3 for improved imaging of apoptosis in vivo

In this study 'second generation' AnxV was specifically labeled with (99m)Tc in three different ways outside the binding region of the protein to obtain an improved target-to-background activity ratio. The compounds were tested in vitro and in vivo in normal mice and in a model of hepatic apoptosis (anti-Fas mAb). The apoptosis binding was most prominent for the HIS-tagged 'second generation' AnxV labeled with (99m)Tc(CO)(3) in comparison to (99m)Tc-HYNIC-cys-AnxV and (99m)Tc(CO)(3)-DTPA-cys-AnxV.

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.

Mobilization of lysosomal calcium regulates the externalization of phosphatidylserine during apoptosis

A hallmark of apoptotic cells is the Ca2+-dependent appearance of phosphatidylserine (PS) at the cell surface as a result of its redistribution from the inner-to-outer plasma membrane leaflet. Although endoplasmic reticulum and mitochondrial Ca2+ are known to participate in apoptosis, their role in PS externalization has not been established. In this study, several organelle-specific fluorescent markers and Ca2+-sensitive probes were used to identify the source of Ca2+ critical to PS externalization. By employing Rhod-2AM, fluorescein-labeled high molecular weight dextran, and Calcium Green 1, we provide evidence that lysosomes respond to apoptotic stimuli by releasing their luminal Ca2+ to the cytosol. Cells treated with the cytosolic phospholipase A2 inhibitor, cPLA2alpha, had no effect on caspase activation but exhibited a significant decrease in lysosomal Ca2+ release and externalization of PS in response to apoptotic stimuli. Similarly, cells depleted of lysosomal Ca2+ underwent programmed cell death yet failed to externalize PS. These data indicate that although Ca2+ release from other intracellular organelles to the cytosol is adequate for apoptosis, the release of Ca2+ from lysosomes is critical for PS externalization.

Long-chain ceramide is a potent inhibitor of the mitochondrial permeability transition pore

The sphingolipid ceramide has been implicated in mediating cell death that is accompanied by mitochondrial functional alterations. Moreover, ceramide has been shown to accumulate in mitochondria upon induction of apoptotic processes. In this study, we sought to evaluate the effects of natural, highly hydrophobic long-chain ceramides on mitochondrial function in vitro. Ceramide in a dodecane/ethanol delivery system inhibited the opening of the mitochondrial permeability transition pore (PTP) induced by either oxidative stress, SH group cross-linking, or high Ca2+ load, suggesting that the inhibitory point is at a level at which major PTP regulatory pathways converge. Moreover, ceramide had no effect on well known mitochondrial components that modulate PTP activity, such as cyclophilin D, voltage-dependent anion channel, adenine nucleotide transporter, and ATP synthase. The inhibitory effect of ceramide on PTP was not stereospecific, nor was there a preference for ceramide over dihydroceramide. However, the effect of ceramide on PTP was significantly influenced by the fatty acid moiety chain length. These studies are the first to show that long-chain ceramide can influence PTP at physiologically relevant concentrations, suggesting that it is the only known potent natural inhibitor of PTP. These results suggest a novel mechanism of ceramide regulation of mitochondrial function.

Cholesterol and anionic phospholipids increase the binding of amyloidogenic transthyretin to lipid membranes

Deposition of transthyretin (TTR) amyloid is a pathological hallmark of familial amyloidotic polyneuropathy (FAP). Recently we showed that TTR binds to membrane lipids via electrostatic interactions and that membrane binding is correlated with the cytotoxicity induced by amyloidogenic TTR. In the present study, we examined the role of lipid composition in membrane binding of TTR by a surface plasmon resonance (SPR) approach. TTR bound to lipid bilayers through both high- and low-affinity interactions. Increasing the mole fraction of cholesterol in the bilayer led to an increase in the amount of high-affinity binding of an amyloidogenic mutant (L55P) TTR. In addition, a greater amount of L55P TTR bound with high affinity to membranes made from anionic phospholipids, phosphatidylglycerol (PG) and phosphatidylserine (PS), than to membranes made from zwitterionic phospholipid phosphatidylcholine (PC). The anionic phospholipids (PS and PG) promoted the aggregation of L55P TTR by accelerating the nucleation phase of aggregation, whereas the zwitterionic phospholipid PC had little effect. These results suggest that cholesterol and anionic phospholipids may be important for TTR aggregation and TTR-induced cytotoxicity.

Apical phosphatidylserine externalization in auditory hair cells

In hair cells of the inner ear, phosphatidylserine (PS), detected with fluorescent annexin V labeling, was rapidly exposed on the external leaflet of apical plasma membranes upon dissection of the organ of Corti. PS externalization was unchanged by caspase inhibition, suggesting that externalization did not portend apoptosis or necrosis. Consistent with that conclusion, mitochondrial membrane potential and hair-cell nuclear structure remained normal during externalization. PS externalization was triggered by forskolin, which raises cAMP, and blocked by inhibitors of adenylyl cyclase. Blocking Na(+) influx by inhibiting the mechanoelectrical transduction channels and P2X ATP channels also inhibited external PS externalization. Diminished PS externalization was also seen in cells exposed to LY 294002, which blocks membrane recycling in hair cells by inhibiting phosphatidylinositol 3-kinase. These results indicate that PS exposure on the external leaflet, presumably requiring vesicular transport, results from elevation of intracellular cAMP, which can be triggered by Na(+) entry into hair cells.

Insect cytokine, growth-blocking peptide, is a primary regulator of melanin-synthesis enzymes in armyworm larval cuticle

The cuticles of most insect larvae have a variety of melanin patterns that function in the insects' interactions with their biotic and abiotic environments. Larvae of the armyworm Pseudaletia separata have black and white stripes running longitudinally along the body axis. This pattern is emphasized after the last larval molt by an increase in the contrast between the lines. We have previously shown that 3,4-dihydroxy-L-phenylalanine (Dopa) decarboxylase (DDC) is activated during the molt period by preferential enhancement of its transcription in the epidermal cells beneath the black stripes. This study demonstrated that tyrosine hydroxylase (TH) expression is activated synchronously with DDC. Furthermore, enhancement of DDC and TH transcription is due to an increase in cyotoplasmic Ca(2+), which is induced by the insect cytokine, growth-blocking peptide (GBP). Enhanced gene expression for both enzymes was induced by substitution of the calcium ionophore A23187, and completely blocked by EGTA. A GBP-induced increase in cytoplasmic Ca(2+) was seen in epidermal cells under the black stripes but not those beneath the white stripes, suggesting that a difference in Ca(2+) concentration in stripe cells leads to the specific expression of DDC and TH genes. Based on the fact that epidermal cells beneath the white stripes contain abundant granules composed mainly of uric acid, which can form a complex with Ca(2+) and hence decrease its free concentration, we discuss the possibility that uric acid, as well as GBP, contributes to the difference in cytoplasmic Ca(2+) within the epidermal cells.

The apoptotic microtubule network preserves plasma membrane integrity during the execution phase of apoptosis

It has recently been shown that the microtubule cytoskeleton is reformed during the execution phase of apoptosis. We demonstrate that this microtubule reformation occurs in many cell types and under different apoptotic stimuli. We confirm that the apoptotic microtubule network possesses a novel organization, whose nucleation appears independent of conventional gamma-tubulin ring complex containing structures. Our analysis suggests that microtubules are closely associated with the plasma membrane, forming a cortical ring or cellular "cocoon". Concomitantly other components of the cytoskeleton, such as actin and cytokeratins disassemble. We found that colchicine-mediated disruption of apoptotic microtubule network results in enhanced plasma membrane permeability and secondary necrosis, suggesting that the reformation of a microtubule cytoskeleton plays an important role in preserving plasma membrane integrity during apoptosis. Significantly, cells induced to enter apoptosis in the presence of the pan-caspase inhibitor z-VAD, nevertheless form microtubule-like structures suggesting that microtubule formation is not dependent on caspase activation. In contrast we found that treatment with EGTA-AM, an intracellular calcium chelator, prevents apoptotic microtubule network formation, suggesting that intracellular calcium may play an essential role in the microtubule reformation. We propose that apoptotic microtubule network is required to maintain plasma membrane integrity during the execution phase of apoptosis.

Involvement of the Na+/H+ exchanger in membrane phosphatidylserine exposure during human platelet activation

Platelet membrane phosphatidylserine (PS) exposure that regulates the production of thrombin represents an important link between platelet activation and the coagulation cascade. Here, we have evaluated the involvement of the Na+/H+ exchanger (NHE) in this process in human platelets. PS exposure induced in human platelets by thrombin, TRAP, collagen or TRAP+ collagen was abolished in a Na+ -free medium. Inhibition of the Na+/H+ exchanger (NHE) by 5-(N-Ethyl-N-Isopropyl) Amiloride (EIPA) reduced significantly PS exposure, whereas monensin or nigericin, which mimic or cause activation of NHE, respectively, reproduced the agonist effect. These data suggest a role for Na+ influx through NHE activation in the mechanism of PS exposure. This newly identified pathway does not discount a role for Ca2+, whose cytosolic concentration varies together with that of Na+ after agonist stimulation. Ca2+ deprivation from the incubation medium only attenuated PS exposure induced by thrombin, measured from the uptake of FM1-43 (a marker of phospholipid scrambling independent of external Ca2+). Surprisingly, removal of external Ca2+ partially reduced FM1-43 uptake induced by A23187, known as a Ca2+ ionophore. The residual effect can be attributed to an increase in [Na+]i mediated by the ionophore due to a lack of its specificity. Finally, phosphatidylinositol 4,5-bisphosphate (PIP2), previously reported as a target for Ca2+ in the induction of phospholipid scrambling, was involved in PS exposure through a regulation of NHE activity. All these results would indicate that the mechanism that results in PS exposure uses redundant pathways inextricably linked to the physio-pathological requirements of this process.

Contributions of Ca2+ to galectin-1-induced exposure of phosphatidylserine on activated neutrophils

Apoptotic cells redistribute phosphatidylserine (PS) to the cell surface by both Ca(2+)-dependent and -independent mechanisms. Binding of dimeric galectin-1 (dGal-1) to glycoconjugates on N-formyl-Met-Leu-Phe (fMLP)-activated neutrophils exposes PS and facilitates neutrophil phagocytosis by macrophages, yet it does not initiate apoptosis. We asked whether dGal-1 initiated Ca(2+) fluxes that are required to redistribute PS to the surface of activated neutrophils. Prolonged occupancy by dGal-1 was required to maximally mobilize PS to the surfaces of fMLP-activated neutrophils. Like fMLP, dGal-1 rapidly elevated cytosolic Ca(2+) levels in Fluo-4-loaded neutrophils. An initial Ca(2+) mobilization from intracellular stores was followed by movement of extracellular Ca(2+) to the cytosolic compartment, with return to basal Ca(2+) levels within 10 min. Chelation of extracellular Ca(2+) did not prevent PS mobilization. Chelation of intracellular Ca(2+) revealed that fMLP and dGal-1 independently release Ca(2+) from intracellular stores that cooperate to induce optimal redistribution of PS. Ca(2+) mobilization by ionomycin did not permit dGal-1 to mobilize PS, indicating that fMLP initiated both Ca(2+)-dependent and -independent signals that facilitated dGal-1-induced exposure of PS. dGal-1 elevated cytosolic Ca(2+) and mobilized PS through a pathway that required action of Src kinases and phospholipase Cgamma. These results demonstrate that transient Ca(2+) fluxes contribute to a sustained redistribution of PS on neutrophils activated with fMLP and dGal-1.

Proteome Analysis Associated with Cadmium Adaptation in U937 Cells

Cadmium is a well known environmental toxicant and carcinogen. To identify proteins involved in cellular adaptive responses to cadmium, we established cadmium-adapted U937 cells that exhibit resistance to cadmium-induced apoptosis, and we performed comparative proteome analysis of these cells with parental cells that were either untreated or treated with cadmium. Newly identified proteins that were changed in expression level in both adapted cells and cadmium-treated parental cells included proteins implicated in cell proliferation and malignant transformation. Most interesting, a calcium-binding protein calbindin-D(28k) was increased only in the adapted cells but not in cadmium-exposed parental cells. The level of calbindin-D(28k) increased by the degree of cadmium adaptation and was stably maintained without selective pressure of cadmium. Cadmium-adapted U937 cells were resistant to the toxic effects of cytosolic calcium rise by cadmium treatment and by depletion of intracellular calcium stores, suggesting that enhanced calcium buffering by up-regulated calbindin-D(28k) may be responsible for acquiring resistance to cadmium-induced apoptosis. We demonstrated that overexpression of calbindin-D(28k) in MN9D neuronal cells resulted in reduced cadmium-induced apoptosis. Our study documents for the first time that cells respond to long term cadmium exposure by increasing calbindin-D(28k) expression, thereby attenuating cadmium-induced apoptosis.

Rapid, noninflammatory and PS-dependent phagocytic clearance of necrotic cells

In pathological situations, different modes of cell death are observed, and information on the role and uptake of nonapoptotic corpses is scarce. Here, we modeled two distinct forms of death in human Jurkat T cells treated with staurosporine: classical apoptosis under normal culture conditions and programmed death with necrotic morphology under ATP-depleting conditions (necPCD). When offered to phagocytes, both types of cell corpses (but not heat-killed unscheduled necrotic cells) reduced the release of the proinflammatory cytokine TNF from the macrophages. The necPCD cells were efficiently engulfed by macrophages and microglia, and from mixtures of necPCD and apoptotic cells macrophages preferentially engulfed the necrotic cells. Using a newly developed assay, we demonstrated that phosphatidylserine is translocated to the surface of such necrotic cells. We demonstrate that this can occur independently of calcium signals, and that surface phosphatidylserine is essential for the uptake of necrotic cells by both human macrophages and murine microglia.

Mitochondrial dysfunction as an early event in the process of apoptosis induced by woodfordin I in human leukemia K562 cells

Tannins are a group of widely distributed plant polyphenols, some of which are beneficial to health because of their chemopreventive activities. In the present study, we investigated the effects and action mechanisms of woodfordin I, a macrocyclic ellagitannin dimer, on human chronic myelogenous leukemia (CML) K562 cells. The results showed that woodfordin I was able to suppress the proliferation and induce apoptosis in K562 cells. Apoptosis was evaluated by cytomorphology, internucleosomal DNA fragmentation, and externalization of phosphatidylserine. Woodfordin I treatment caused a rapid and sustained loss of mitochondrial transmembrane potential (MMP), transient generation of reactive oxygen species (ROS), transient elevation of intracellular Ca2+ concentration, and cytosolic accumulation of cytochrome c. The activation of caspase-9 and 3, but not caspase-8, was also demonstrated, indicating that the apoptotic signaling triggered by woodfordin I was mediated through the intrinsic mitochondria-dependent pathway. Western blot and immunofluorescence analysis revealed that the anti-apoptotic Bcl-2 and Bcl-xL levels were downregulated, together with the pro-apoptotic Bax protein. Significantly, woodfordin I-induced apoptosis was associated with a decline in the levels of c-Abl, Bcr-Abl, and cellular protein tyrosine phosphorylation. Considering the consequence of all the events in the process of woodfordin I-induced apoptosis, the mitochondrial dysfunction is directly responsible for the pro-apoptotic effects on K562 cells. Furthermore, because CML is a malignancy of pleuripotent hematopoietic cells caused by the dysregulated tyrosine kinase activity of Bcr-Abl, these findings suggest that woodfordin I may be a potential lead compound against CML.

Involvement of sodium in early phosphatidylserine exposure and phospholipid scrambling induced by P2X7 purinoceptor activation in thymocytes

Extracellular ATP (ATP(ec)), a possible effector in thymocyte selection, induces thymocyte death via purinoceptor activation. We show that ATP(ec) induced cell death by apoptosis, rather than lysis, and early phosphatidylserine (PS) exposure and phospholipid scrambling in a limited thymocyte population (35-40%). PS externalization resulted from the activation of the cationic channel P2X7 (formerly P2Z) receptor and was triggered in all thymocyte subsets although to different proportions in each one. Phospholipid movement was dependent on ATP(ec)-induced Ca(2+) and/or Na(+) influx. At physiological external Na(+) concentration, without external Ca(2+), PS was exposed in all ATP(ec)-responsive cells. In contrast, without external Na(+), physiological external Ca(2+) concentration promoted a submaximal response. Altogether these data show that Na(+) influx plays a major role in the rapid PS exposure induced by P2X7 receptor activation in thymocytes.

Scott syndrome, a bleeding disorder caused by defective scrambling of membrane phospholipids

Normal quescent cells maintain membrane lipid asymmetry by ATP-dependent membrane lipid transporters, which shuttle different phospholipids from one leaflet to the other against their respective concentration gradients. When cells are challenged, membrane lipid asymmetry can be perturbed resulting in exposure of phosphatidylserine [PS] at the outer cell surface. Translocation of PS from the inner to outer membrane leaflet of activated blood platelets and platelet-derived microvesicles provides a catalytic surface for interacting coagulation factors. This process is dramatically impaired in Scott syndrome, a rare congenital bleeding disorder, underscoring the indispensible role of PS in hemostasis. This also testifies to a defect of a protein-catalyzed scrambling of membrane phospholipids. The Scott phenotype is not restricted to platelets, but can be demonstrated in other blood cells as well. The functional aberrations observed in Scott syndrome have increased our understanding of transmembrane lipid movements, and may help to identify the molecular elements that promote the collapse of phospholipid asymmetry during cell activation and apoptosis.

Potentiation of trichothecene-induced leukocyte cytotoxicity and apoptosis by TNF-alpha and Fas activation

Trichothecene mycotoxins cause immunosuppression by inducing apoptosis in lymphoid tissue. Trichothecene-induced leukocyte apoptosis can be augmented by bacterial lipopolysaccharide (LPS) but the mechanisms involved in this potentiating effect are not completely understood. The objective of this study was to test the hypothesis that the trichothecene deoxynivalenol (DON, vomitoxin) can interact with LPS directly and other mediators or agonists associated with immune/inflammatory responses to induce apoptosis in primary murine leukocyte cultures. Primary leukocyte suspensions were prepared from murine thymus (TH), spleen (SP), bone marrow (BM) and Peyer's patches (PP) and then cultured with DON in the absence or presence of LPS, prostaglandin E2 (PGE2), anti-immunoglobulin (as antigen mimic), dexamethasone, Fas ligand, or TNF-alpha. Cytotoxicity and apoptosis were evaluated by MTT assay and morphologic assays, respectively. DON was found to inhibit LPS-induced proliferation and dexamethasone-induced apoptosis in SP cultures. In contrast, potentiation of DON-induced apoptosis and cytotoxicity was observed in BM cultures treated with anti-Fas and in TH cultures treated with TNF-alpha. When potentiation of DON-induced apoptosis by TNF-alpha was assessed using pharmacological inhibitors, generation of ROS, intracellular Ca2+, p38/SAPK, and caspase-3 activation were found to play roles. Taken together, these data demonstrate that LPS and its downstream mediators can interact with trichothecenes to modulate proliferative, cytotoxic and apoptotic outcomes in leukocytes in a tissue-specific manner.

CD69 expression induced by thapsigargin, phorbol ester and ouabain on thymocytes is dependent on external Ca2+ entry

In the present work murine thymocytes exposed to Thapsigargin (TG 10, 20 and 50 nM), Phorbol-12,13,20-triacetate (TPA16 nM) and Ouabain (OUA100 nM) exhibited an increased expression of CD69, a molecule related to cellular activation and associated to Ca(++) influx in other systems. The kinetics of CD69 appearance depended on the stimuli and dose used. TG 50 nM induced an increased expression by 6 h whereas with lower doses (10 and 20 nM) an increase was detected at 18 h. TPA maximal increase was evident at 6 h. OUA lead to an observable increase at 18 h. However, in the case of TPA or TG the presence of the stimuli was only necessary for the first 2 h of culture, whereas OUA needed to be present during the whole assay. It was also demonstrated that Ca(++) influx was an essential feature, as EGTA diminished or abolished CD69 increased expression. Nevertheless, EGTA was only capable of this effect when present at the time of the stimuli. No correlation of CD69 expression with thymocyte death was observed. Similarly, the agents under study did not promote the maturation from double-positive into single-positive thymocytes. TPA and Thapsigargin were capable of decreasing the level of CD4 molecules on the cell surface, probably due to the loss of these molecules. OUA, on the other hand, did not modify CD4/CD8 expression on these cells.

Regulation of cell death: the calcium-apoptosis link

To live or to die? This crucial question eloquently reflects the dual role of Ca2+ in living organisms--survival factor or ruthless killer. It has long been known that Ca2+ signals govern a host of vital cell functions and so are necessary for cell survival. However, more recently it has become clear that cellular Ca2+ overload, or perturbation of intracellular Ca2+ compartmentalization, can cause cytotoxicity and trigger either apoptotic or necrotic cell death.

Appetizing rancidity of apoptotic cells for macrophages: oxidation, externalization, and recognition of phosphatidylserine

Programmed cell death (apoptosis) functions as a mechanism to eliminate unwanted or irreparably damaged cells ultimately leading to their orderly phagocytosis in the absence of calamitous inflammatory responses. Recent studies have demonstrated that the generation of free radical intermediates and subsequent oxidative stress are implicated as part of the apoptotic execution process. Oxidative stress may simply be an unavoidable yet trivial byproduct of the apoptotic machinery; alternatively, intermediates or products of oxidative stress may act as essential signals for the execution of the apoptotic program. This review is focused on the specific role of oxidative stress in apoptotic signaling, which is realized via phosphatidylserine-dependent pathways leading to recognition of apoptotic cells and their effective clearance. In particular, the mechanisms involved in selective phosphatidylserine oxidation in the plasma membrane during apoptosis and its association with disturbances of phospholipid asymmetry leading to phosphatidylserine externalization and recognition by macrophage receptors are at the center of our discussion. The putative importance of this oxidative phosphatidylserine signaling in lung physiology and disease are also discussed.

Phosphatidylserine exposure in Fas type I cells is mitochondria-dependent

Previous studies have demonstrated that Fas-triggered activation of effector caspases and subsequent nuclear apoptosis either is mitochondria-independent (type I cells) or relies on mitochondrial amplification of the initial stimulus (type II cells). We show herein that Bcl-2 overexpression in a prototypic type I cell line (SKW6.4) promotes mitochondrial generation of ATP and blocks Fas-triggered plasma membrane externalization of phosphatidylserine (PS). Moreover, overexpression of Bcl-2 attenuates macrophage engulfment of Fas-triggered cells. Fas-mediated DNA fragmentation, on the other hand, remains unaffected in SKW6.4-bcl-2 cells. These studies thus demonstrate that PS externalization and clearance of cell corpses are mitochondria-dependent events, and show that these events can be dissociated from other features of the apoptotic program, in Fas type I cells.

Transbilayer phospholipid movement and the clearance of apoptotic cells

When lymphocytes (and other cells) die by apoptosis, they orchestrate their own orderly removal by macrophages, and thereby prevent the inflammation that would otherwise attend cell lysis. As part of their demise, apoptotic cells disrupt the normal asymmetric distribution of phospholipids across their plasma membranes, an asymmetry normally maintained by an aminophospholipid translocase. This disruption of asymmetry, mediated by an activity known as the scramblase, generates ligands on the cell surface that trigger phagocytosis of the dying cell before lysis can occur. This crucial alteration of the plasma membrane is not dependent on caspase-mediated proteolysis, but quite unexpectedly, it is required both on the apoptotic target cell and on the phagocyte that engulfs it. At least in the phagocyte, this rearrangement may depend on the activity of an ABC ATPase, termed ABC1 in mammals and ced-7 in C. elegans.

Fas-triggered phosphatidylserine exposure is modulated by intracellular ATP

Recognition signals are displayed on the cell surface during apoptosis that enable macrophages to engulf and dispose of the dying cell. A common signal is the externalization of phosphatidylserine (PS). Studies in erythrocytes and platelets have suggested that PS exposure requires the concomitant activation of a phospholipid scramblase (PLS) and inhibition of an adenosine triphosphate (ATP)-dependent aminophospholipid translocase. However, the molecular mechanism underlying PS exposure during apoptosis remains poorly understood. In this study, we provide evidence that expression of PLS is neither necessary nor sufficient for PS exposure during Fas-triggered apoptosis. On the other hand, egress of PS is shown to correlate with a decline in intracellular ATP and inhibition of aminophospholipid translocase activity upon Fas stimulation. Moreover, suppression of intracellular ATP levels by the glucose anti-metabolite, 2-deoxyglucose, alone or in combination with glucose-free medium, potentiates Fas-induced PS exposure in the PLS-expressing Jurkat cell line and enables PLS-defective Raji cells to externalize PS in response to Fas ligation. These studies suggest that intracellular ATP levels can modulate the externalization of PS during apoptosis, and implicate the ATP-dependent aminophospholipid translocase in this process.

Evidence that cytosolic calcium increases are not sufficient to stimulate phospholipid scrambling in human T-lymphocytes

Phospholipid scrambling, the disruption of normal plasma-membrane asymmetry, occurs during apoptotic and necrotic cell death and during the activation of platelets and neutrophils. It is currently believed that phospholipid scrambling is triggered simply by increases in bulk cytosolic [Ca(2+)]. We have presented evidence previously that the styryl dye FM1-43 is sensitive to phospholipid scrambling in Jurkat human leukaemic T-lymphocytes. Here we have used FM1-43, in combination with fura 2 and the Ca(2+)-elevating agents ionomycin and thapsigargin, in imaging experiments to test the idea that increases in bulk cytosolic [Ca(2+)] stimulate scrambling. Intracellular Ca(2+) increases of approximately 2 microM accompanied ionomycin-stimulated scrambling in approximately 50% of cells, and scrambling occurred in >99% of cells in which intracellular Ca(2+) rose to 4 microM. Chelating intracellular Ca(2+) with bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid or EGTA suppressed both ionomycin-stimulated intra cellular Ca(2+) increases and scrambling, demonstrating that intracellular Ca(2+) increases are necessary for ionomycin-stimulated scrambling. However, elevating intracellular Ca(2+) to 2-4 microM with thapsigargin, a drug that depletes intracellular Ca(2+) stores and triggers Ca(2+) entry via Ca(2+)-release-activated Ca(2+) channels, did not trigger scrambling, as assessed with either FM1-43 or FITC-labelled annexin V. These results suggest that increases in intracellular [Ca(2+)] are necessary but not sufficient to stimulate scrambling in lymphoyctes, and indicate that ionomycin has an additional effect that is required to stimulate scrambling.

Adenovirus E3-6.7K maintains calcium homeostasis and prevents apoptosis and arachidonic acid release

E3-6.7K is a small and hydrophobic membrane glycoprotein encoded by the E3 region of subgroup C adenovirus. Recently, E3-6.7K has been shown to be required for the downregulation of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptors by the adenovirus E3/10.4K and E3/14.5K complex of proteins. We demonstrate here that E3-6.7K has additional protective roles, independent of other virus proteins. In transfected Jurkat T-cell lymphoma cells, E3-6.7K was found to maintain endoplasmic reticulum-Ca(2+) homeostasis and inhibit the induction of apoptosis by thapsigargin. The presence of E3-6.7K also lead to a reduction in the TNF-induced release of arachidonic acid from transfected U937 human histiocytic lymphoma cells. In addition, E3-6.7K protected cells against apoptosis induced through Fas, TNF receptor, and TRAIL receptors. Therefore, E3-6.7K confers a wide range of protective effects against both Ca(2+) flux-induced and death receptor-mediated apoptosis.