Apoptosis in yeast--a monocellular organism exhibits altruistic behaviour

The meaning of death: evolution and ecology of apoptosis in protozoan parasites

The discovery that an apoptosis-like, programmed cell death (PCD) occurs in a broad range of protozoan parasites offers novel therapeutic tools to treat some of the most serious infectious diseases of humans, companion animals, wildlife, and livestock. Whilst apoptosis is an essential part of normal development, maintenance, and defence in multicellular organisms, its occurrence in unicellular parasites appears counter-intuitive and has proved highly controversial: according to the Darwinian notion of “survival of the fittest”, parasites are expected to evolve strategies to maximise their proliferation, not death. The prevailing, and untested, opinion in the literature is that parasites employ apoptosis to “altruistically” self-regulate the intensity of infection in the host/vector. However, evolutionary theory tells us that at most, this can only be part of the explanation, and other non-mutually exclusive hypotheses must also be tested. Here, we explain the evolutionary concepts that can explain apoptosis in unicellular parasites, highlight the key questions, and outline the approaches required to resolve the controversy over whether parasites “commit suicide”. We highlight the need for integration of proximate and functional approaches into an evolutionary framework to understand apoptosis in unicellular parasites. Understanding how, when, and why parasites employ apoptosis is central to targeting this process with interventions that are sustainable in the face of parasite evolution.

The small molecular mass antifungal protein of Penicillium chrysogenum--a mechanism of action oriented review

The β-lactam producing filamentous fungus Penicillium chrysogenum secretes a 6.25 kDa small molecular mass antifungal protein, PAF, which has a highly stable, compact 3D structure and is effective against a wide spectrum of plant and zoo pathogenic fungi. Its precise physiological functions and mode of action need to be elucidated before considering possible biomedical, agricultural or food technological applications. According to some more recent experimental data, PAF plays an important role in the fine-tuning of conidiogenesis in Penicillium chrysogenum. PAF triggers apoptotic cell death in sensitive fungi, and cell death signaling may be transmitted through two-component systems, heterotrimeric G protein coupled signal transduction and regulatory networks as well as via alteration of the Ca(2+) -homeostasis of the cells. Possible biotechnological applications of PAF are also outlined in the review.

Allicin disrupts the cell's electrochemical potential and induces apoptosis in yeast

The volatile substance allicin gives crushed garlic (Allium sativum) its characteristic odor and is a pro-oxidant that undergoes thiol-disulfide exchange reactions with -SH groups in proteins and glutathione. The antimicrobial activity of allicin is suspected to be due to the oxidative inactivation of essential thiol-containing enzymes. We investigated the hypothesis that at threshold inhibitory levels allicin can shunt yeast cells into apoptosis by altering their overall redox status. Yeast cells were treated either with chemically synthesized, pure allicin or with allicin in garlic juice. Allicin-dependent cell oxidation was demonstrated with a redox-sensitive GFP construct and the shift in cellular electrochemical potential (E(hc)) from less than -215 to -181mV was calculated using the Nernst equation after the glutathione/glutathione disulfide couple (2GSH/GSSG) in the cell was quantified. Caspase activation occurred after allicin treatment, and yeast expressing a human antiapoptotic Bcl-XL construct was rendered more resistant to allicin. Also, a yeast apoptosis-inducing factor deletion mutant was more resistant to allicin than wild-type cells. We conclude that allicin in garlic juice can activate apoptosis in yeast cells through its oxidizing properties and that this presents an alternative cell-killing mechanism to the previously proposed specific oxidative inactivation of essential enzymes.

Investigating the evolution of apoptosis in malaria parasites: the importance of ecology

Apoptosis is a precisely regulated process of cell death which occurs widely in multicellular organisms and is essential for normal development and immune defences. In recent years, interest has grown in the occurrence of apoptosis in unicellular organisms. In particular, as apoptosis has been reported in a wide range of species, including protozoan malaria parasites and trypanosomes, it may provide a novel target for intervention. However, it is important to understand when and why parasites employ an apoptosis strategy before the likely long- and short-term success of such an intervention can be evaluated. The occurrence of apoptosis in unicellular parasites provides a challenge for evolutionary theory to explain as organisms are expected to have evolved to maximise their own proliferation, not death. One possible explanation is that protozoan parasites undergo apoptosis in order to gain a group benefit from controlling their density as this prevents premature vector mortality. However, experimental manipulations to examine the ultimate causes behind apoptosis in parasites are lacking. In this review, we focus on malaria parasites to outline how an evolutionary framework can help make predictions about the ecological circumstances under which apoptosis could evolve. We then highlight the ecological considerations that should be taken into account when designing evolutionary experiments involving markers of cell death, and we call for collaboration between researchers in different fields to identify and develop appropriate markers in reference to parasite ecology and to resolve debates on terminology.

On the paradigm of altruistic suicide in the unicellular world

Altruistic suicide is best known in the context of programmed cell death (PCD) in multicellular individuals, which is understood as an adaptive process that contributes to the development and functionality of the organism. After the realization that PCD-like processes can also be induced in single-celled lineages, the paradigm of altruistic cell death has been extended to include these active cell death processes in unicellular organisms. Here, we critically evaluate the current conceptual framework and the experimental data used to support the notion of altruistic suicide in unicellular lineages, and propose new perspectives. We argue that importing the paradigm of altruistic cell death from multicellular organisms to explain active death in unicellular lineages has the potential to limit the types of questions we ask, thus biasing our understanding of the nature, origin, and maintenance of this trait. We also emphasize the need to distinguish between the benefits and the adaptive role of a trait. Lastly, we provide an alternative framework that allows for the possibility that active death in single-celled organisms is a maladaptive trait maintained as a byproduct of selection on pro-survival functions, but that could-under conditions in which kin/group selection can act-be co-opted into an altruistic trait.

The redox-sensing quinone reductase Lot6p acts as an inducer of yeast apoptosis

Mammalian NAD(P)H:quinone oxidoreductases such as human NQO1 act as inducers of apoptosis. Quinone reductases generated interest over the last decade due to their proposed function in the oxidative stress response. Furthermore, human NQO1 was reported to regulate p53 stability and p53-dependent apoptosis through regulation of cellular oxidation-reduction events. In this study, we have used low concentrations of hydrogen peroxide (0.4 and 0.6 mM) to induce apoptosis-like cell death in wild type, an LOT6 overexpressing and a Deltalot6 yeast strain to monitor cell survival. Using this approach, we demonstrate that yeast quinone reductase Lot6p, an orthologue of mammalian quinone reductases, plays a pivotal role in apoptosis-like cell death in Saccharomyces cerevisiae. Overexpression of LOT6 results in enhanced cell death, as shown by an investigation of the morphological hallmarks of apoptosis-like fragmentation of DNA and externalization of phosphatidylserine, whereas the deletion strain displays a deficiency in apoptosis-like cell death as compared with the wild type. Thus, we propose that Lot6p is directly involved in the control of the apoptosis-like cell death in yeast.

Involvement of mitochondria and metacaspase elevation in harpin Pss-induced cell death of Saccharomyces cerevisiae

Expression of a proteinaceous elicitor harpin(Pss,) encoded by hrpZ of Pseudomonas syringae pv. syringae 61, under GAL1 promoter in Saccharomyces cerevisiae Y187 resulted in galactose-inducible yeast cell death (YCD). Extracellular treatment of harpin did not affect the growth of yeast. The observed YCD was independent of the stage of cell cycle. "Petite" mutant of S. cerevisiae Y187 pYEUT-hrpZ was insensitive to cell death indicating the involvement of mitochondria in this YCD. Loss in mitochondrial potential, but no leakage of Cytochrome c from mitochondria into the cytosol, were notable features in harpin(Pss)-induced YCD. Cyclosporin A had no effect on hrpZ expressing yeast cells, further confirmed that there was no release of Cytochrome c. Elevation of caspase activity has been reported for the first time in this form of cell death induced by harpin expression. Release of reactive oxygen species and clear loss of membrane integrity were evident with the absence of nuclear fragmentation and chromosomal condensation, while annexin V and propidium iodide staining showed features typical of necrosis.

INHIBITION OF CASPASE-LIKE ACTIVITIES PREVENTS THE APPEARANCE OF REACTIVE OXYGEN SPECIES AND DARK-INDUCED APOPTOSIS IN THE UNICELLULAR CHLOROPHYTE DUNALIELLA TERTIOLECTA(1)

When the chlorophyte alga Dunaliella tertiolecta Butcher is placed in darkness, a form of programmed cell death with many similarities to apoptosis is induced, including the induction of caspase-like proteases. Many uncertainties about the regulation and mediators that participate in the process remain. To examine the relationship between caspase-like activities and different apoptotic events (i.e., phosphatidylserine [PS] translocation), increases in membrane permeability and numbers of dead cells revealed by SYTOX-green staining, and the generation of reactive oxygen species (ROS), we used the broad-range caspase inhibitor Boc-D-FMK to block the activity of the whole class of caspase-like proteins simultaneously. In the presence of the inhibitor, ROS were not produced, and cells did not die. Loss of membrane asymmetry, indicated by external labeling of PS by annexin V, was apparent at midstages of light deprivation, although it did not conform to the typical pattern for PS exposure observed in metazoans or vascular plants, which occurs at early stages of the apoptotic event. Thus, we have evidence for a link between ROS and cell death involving caspase-like enzymes in an alga. The fact that caspase-like inhibitors prevent not only cell death, but also ROS and loss of cell membrane integrity and asymmetry, suggests that caspase-like proteases might have regulatory roles early in cell death, in addition to dismantling functions.

Fungal apoptosis: function, genes and gene function

Cells of all living organisms are programmed to self-destruct under certain conditions. The most well known form of programmed cell death is apoptosis, which is essential for proper development in higher eukaryotes. In fungi, apoptotic-like cell death occurs naturally during aging and reproduction, and can be induced by environmental stresses and exposure to toxic metabolites. The core apoptotic machinery in fungi is similar to that in mammals, but the apoptotic network is less complex and of more ancient origin. Only some of the mammalian apoptosis-regulating proteins have fungal homologs, and the number of protein families is drastically reduced. Expression in fungi of animal proteins that do not have fungal homologs often affects apoptosis, suggesting functional conservation of these components despite the absence of protein-sequence similarity. Functional analysis of Saccharomyces cerevisiae apoptotic genes, and more recently of those in some filamentous species, has revealed partial conservation, along with substantial differences in function and mode of action between fungal and human proteins. It has been suggested that apoptotic proteins might be suitable targets for novel antifungal treatments. However, implementation of this approach requires a better understanding of fungal apoptotic networks and identification of the key proteins regulating apoptotic-like cell death in fungi.

Different ways to die: cell death modes of the unicellular chlorophyte Dunaliella viridis exposed to various environmental stresses are mediated by the caspase-like activity DEVDase

Programmed cell death is necessary for homeostasis in multicellular organisms and it is also widely recognized to occur in unicellular organisms. However, the mechanisms through which it occurs in unicells, and the enzymes involved within the final response is still the subject of heated debate. It is shown here that exposure of the unicellular microalga Dunaliella viridis to several environmental stresses, induced different cell death morphotypes, depending on the stimulus received. Senescent cells demonstrated classical and unambiguous apoptotic-like characteristics such as chromatin condensation, DNA fragmentation, intact organelles, and blebbing of the cell membrane. Acute heat shock caused general swelling and altered plasma membrane, but the presence of chromatin clusters and DNA strand breaks suggested a necrotic-like event. UV irradiated cells presented changes typical for necrosis, together with apoptotic characteristics resembling an intermediate cell-death phenotype termed aponecrosis-like. Cells subjected to hyperosmotic shock revealed chromatin spotting without DNA fragmentation, and extensive cytoplasmic swelling and vacuolization, comparable to a paraptotic-like cell death phenotype. Nitrogen-starved cells showed pyknosis, blebbing, and cytoplasmic consumption, indicating a similarity to autophagic/vacuolar-like cell death. The caspase-like activity DEVDase was measured by using the fluorescent substrate Ac-DEVD-AMC and antibodies against the human caspase-3 active enzyme cross-reacted with bands, the intensity of which paralleled the activity. All the environmental stresses tested produced a substantial increase in both DEVDase activity and protein levels. The irreversible caspase-3 inhibitor Z-DEVD-FMK completely inhibited the enzymatic activity whereas serine and aspartyl proteases inhibitors did not. These results show that cell death in D. viridis does not conform to a single pattern and that environmental stimuli may produce different types of cell death depending on the type and intensity of the stimulus, all of which help to understand the cell death-dependent and cell death-independent functions of caspase-like proteins. Hence, these data support the theory that alternative, non-apoptotic programmed cell death (PCDs), exist either in parallel or in an independent manner with apoptosis and were already present in single-celled organisms that evolved some 1.2-1.6 billion years ago.

A non-death role of the yeast metacaspase: Yca1p alters cell cycle dynamics

Caspase proteases are a conserved protein family predominantly known for engaging and executing apoptotic cell death. Nevertheless, in higher eukaryotes, caspases also influence a variety of cell behaviors including differentiation, proliferation and growth control. S. cerevisiae expresses a primordial caspase, yca1, and exhibits apoptosis-like death under certain stresses; however, the benefit of a dedicated death program to single cell organisms is controversial. In the absence of a clear rationale to justify the evolutionary retention of a death only pathway, we hypothesize that yca1 also influences non-apoptotic events. We report that genetic ablation and/or catalytic inactivation of Yca1p leads to a longer G1/S transition accompanied by slower growth in fermentation conditions. Downregulation of Yca1p proteolytic activity also results in failure to arrest during nocodazole treatment, indicating that Yca1p participates in the G2/M mitotic checkpoint. 20s proteasome activity and ROS staining of the Δyca1 strain is indistinguishable from its isogenic control suggesting that putative regulation of the oxidative stress response by Yca1p does not instigate the cell cycle phenotype. Our results demonstrate multiple non-death roles for yca1 in the cell cycle.

Diallyl disulphide depletes glutathione in Candida albicans: oxidative stress-mediated cell death studied by two-photon microscopy

Using two-photon scanning laser microscopy, we investigated the effect of an Allium sativum (garlic) constituent, diallyl disulphide (DADS), on key physiological functions of the opportunistic pathogen Candida albicans. A short 30 min exposure to 0.5 mM DADS followed by removal induced 70% cell death (50% necrotic, 20% apoptotic) within 2 h, increasing to 75% after 4 h. The early intracellular events associated with DADS-induced cell death were monitored with two-photon fluorescence microscopy to track mitochondrial membrane potential (Deltapsi(m)), reactive oxygen species (ROS) and NADH or reduced glutathione (GSH) under aerobic conditions. DADS treatment decreased intracellular GSH and elevated intracellular ROS levels. Additionally, DADS induced a marked decrease of Deltapsi(m) and lowered respiration in cell suspensions and isolated mitochondria. In vitro kinetic experiments in cell-free extracts suggest that glutathione-S-transferase (GST) is one of the intracellular targets of DADS. Additional targets were also identified, including inhibition of a site or sites between complexes II-IV in the electron transport chain, as well as the mitochondrial ATP-synthase. The results indicate that DADS is an effective antifungal agent able to trigger cell death in Candida, most probably by eliciting oxidative stress as a consequence of thiol depletion and impaired mitochondrial function.

Mitochondria-independent morphological and biochemical apoptotic alterations promoted by the anti-tumor agent bleomycin in Saccharomyces cerevisiae

Bleomycin is a highly potent cytotoxic and genotoxic agent used in the chemotherapy of various types of tumors. It is a radiomimetic anticancer drug that produces single- and double-stranded DNA breaks in a catalytic way. Using Saccharomyces cerevisiae as a model system, we show that when a high amount of bleomycin molecules is internalized (100 micromol/L), morphological changes identical to those usually associated with apoptosis, i.e., a sub-G1 region peak, chromatin condensation, and very rapid DNA fragmentation into oligonucleosomal-sized fragments, are observed. The known bleomycin inhibitors cobalt and EDTA were able to prevent bleomycin nucleasic activity and thus apoptotic cell death. However, both oligomycin, a potent inhibitor of the mitochondrial F0F1-ATPase, and antimycin, a drug affecting mitochondria respiration, were unable to prevent the bleomycin-induced apoptotic-like cell death. These results suggest that high bleomycin concentrations induce an apoptosis-like mitochondria-independent cell death in yeast.

Yeast cells lacking the CIT1-encoded mitochondrial citrate synthase are hypersusceptible to heat- or aging-induced apoptosis

In Saccharomyces cerevisiae, the initial reaction of the tricarboxylic acid cycle is catalyzed by the mitochondrial citrate synthase Cit1. The function of Cit1 has previously been studied mainly in terms of acetate utilization and metabolon construction. Here, we report the relationship between the function of Cit1 and apoptosis. Yeast cells with cit1 deletion showed a temperature-sensitive growth phenotype, and they displayed a rapid loss in viability associated with typical apoptotic hallmarks, i.e., reactive oxygen species (ROS) accumulation and nuclear fragmentation, DNA breakage, and phosphatidylserine translocation, when exposed to heat stress. On long-term cultivation, cit1 null strains showed increased potentials for both aging-induced apoptosis and adaptive regrowth. Activation of the metacaspase Yca1 was detected during heat- or aging-induced apoptosis in cit1 null strains, and accordingly, deletion of YCA1 suppressed the apoptotic phenotype caused by cit1 null mutation. Cells with cit1 deletion showed higher tendency toward glutathione (GSH) depletion and subsequent ROS accumulation than the wild type, which was rescued by exogenous GSH, glutamate, or glutathione disulfide (GSSG). These results led us to conclude that GSH deficiency in cit1 null cells is caused by an insufficient supply of glutamate necessary for biosynthesis of GSH rather than the depletion of reducing power required for reduction of GSSG to GSH.

BimS-induced apoptosis requires mitochondrial localization but not interaction with anti-apoptotic Bcl-2 proteins

Release of apoptogenic proteins such as cytochrome c from mitochondria is regulated by pro- and anti-apoptotic Bcl-2 family proteins, with pro-apoptotic BH3-only proteins activating Bax and Bak. Current models assume that apoptosis induction occurs via the binding and inactivation of anti-apoptotic Bcl-2 proteins by BH3-only proteins or by direct binding to Bax. Here, we analyze apoptosis induction by the BH3-only protein Bim(S). Regulated expression of Bim(S) in epithelial cells was followed by its rapid mitochondrial translocation and mitochondrial membrane insertion in the absence of detectable binding to anti-apoptotic Bcl-2 proteins. This caused mitochondrial recruitment and activation of Bax and apoptosis. Mutational analysis of Bim(S) showed that mitochondrial targeting, but not binding to Bcl-2 or Mcl-1, was required for apoptosis induction. In yeast, Bim(S) enhanced the killing activity of Bax in the absence of anti-apoptotic Bcl-2 proteins. Thus, cell death induction by a BH3-only protein can occur through a process that is independent of anti-apoptotic Bcl-2 proteins but requires mitochondrial targeting.

alpha-Tomatine, the major saponin in tomato, induces programmed cell death mediated by reactive oxygen species in the fungal pathogen Fusarium oxysporum

The tomato saponin alpha-tomatine has been proposed to kill sensitive cells by binding to cell membranes followed by leakage of cell components. However, details of the modes of action of the compound on fungal cells are poorly understood. In the present study, mechanisms involved in alpha-tomatine-induced cell death of fungi were examined using a filamentous pathogenic fungus Fusarium oxysporum. alpha-Tomatine-induced cell death of F. oxysporum (TICDF) occurred only under aerobic conditions and was blocked by the mitochondrial F(0)F(1)-ATPase inhibitor oligomycin, the caspase inhibitor D-VAD-fmk, and protein synthesis inhibitor cycloheximide. Fungal cells exposed to alpha-tomatine showed TUNEL-positive nuclei, depolarization of transmembrane potential of mitochondria, and reactive oxygen species (ROS) accumulation. These results suggest that TICDF occurs through a programmed cell death process in which mitochondria play a pivotal role. Pharmacological studies using inhibitors suggest that alpha-tomatine activates phosphotyrosine kinase and monomeric G-protein signaling pathways leading to Ca(2+) elevation and ROS burst in F. oxysporum cells.

Synchronization of cell death in a dinoflagellate population is mediated by an excreted thiol protease

Regulated programmed cell death (PCD) processes have been documented in several phytoplankton species and are hypothesized to play a role in population dynamics. However, the mechanisms leading to the coordinated collapse of phytoplankton blooms are poorly understood. We showed that the collapse of the annual bloom of Peridinium gatunense, an abundant dinoflagellate in Lake Kinneret, Israel, is initiated by CO2 limitation followed by oxidative stress that triggers a PCD-like cascade. We provide evidences that a protease excreted by senescing P. gatunense cells sensitizes younger cells to oxidative stress and may consequently trigger synchronized cell death of the population. Ageing of the P. gatunense cultures was characterized by a remarkable rise in DNA fragmentation and enhanced sensitivity to H2O2. Exposure of logarithmic phase (young) cultures to conditioning media from stationary phase (old) cells sensitized them to H2O2 and led to premature massive cell death. We detected the induction of specific extracellular protease activity, leupeptin-sensitive, in ageing cultures and in lake waters during the succession of the P. gatunense bloom. Partial purification of the conditioned media revealed that this protease activity is responsible for the higher susceptibility of young cells to oxidative stress. Inhibition of the protease activity lowered the sensitivity to oxidative stress, whereas application of papain to logarithmic phase P. gatunense cultures mimicked the effect of the spent media and enhanced cell death. We propose a novel mechanistic framework by which a population of unicellular phytoplankton orchestrates a coordinated response to stress, thereby determine the fate of its individuals.

Caspases in yeast apoptosis-like death: facts and artefacts

Various findings suggest that programmed cell death (PCD) is induced in yeast as a response to the impact of a deleterious environment and/or an intracellular defect. Moreover, the specifically localized PCD within multicellular colonies seems to be important for the safe degradation of cell subpopulations to simple compounds that can be used as nutrients by healthy survivors occurring in propitious colony areas, being thus important for proper development and survival of the yeast population. In spite of this, the question remains whether yeast dies by real apoptosis, i.e. death involving caspases, or by other kinds of PCD. A large group of mammalian caspases includes those that are responsible for monitoring of the stimulus and initiating the dying process, as well as those involved in the execution of death. Additionally, paracaspases and metacaspases, that share some homology with real caspases, but possibly differ in substrate specificity, have been identified in plants, fungi, Dictyostelium and metazoa. In yeast, one homologue of caspases, metacaspase Mca1p/Yca1p, has been identified so far, although there are several indications of the presence of other caspase-like activities in yeast. In this minireview, we summarize various data on the possible involvement of Mca1p and other caspase-like activities in yeast PCD.

Bcl-2 proteins link programmed cell death with growth and morphogenetic adaptations in the fungal plant pathogen Colletotrichum gloeosporioides

Proteins belonging to the Bcl-2 family regulate apoptosis in mammals by controlling mitochondria efflux of cytochrome c and other apoptosis-related proteins. Homologues of human Bcl-2 proteins are found in different metazoan organisms where they play a similar role, while they seem to be absent in plants and fungi. Nonetheless, Bcl-2 protein members can induce or prevent yeast cell death, suggesting that enough functional conservation exists between apoptotic machineries of mammals and fungi. Here we show that induction or prevention of apoptosis by Bcl-2 proteins in the fungal plant pathogen Colletotrichum gloeosporioides is tightly linked with growth and morphogenetic adaptation that occur throughout the entire fungal life cycle. Isolates expressing the pro-apoptotic Bax protein underwent cell death with apoptotic characteristics, and showed alterations in conidial germination that are associated with pathogenic and non-pathogenic life styles. Isolates expressing the anti-apoptotic Bcl-2 protein had prolonged longevity, were protected from Bax-induced cell death, and exhibited enhanced stress resistance. These isolates also had enhanced mycelium and conidia production, and were hyper virulent to host plants. Our findings show that apoptotic-associated machinery regulates morphogenetic switches, which are critical for proper responses and adaptation fungi to different environments.

Leishmania major metacaspase can replace yeast metacaspase in programmed cell death and has arginine-specific cysteine peptidase activity

The human protozoan parasite Leishmania major has been shown to exhibit several morphological and biochemical features characteristic of a cell death program when differentiating into infectious stages and under a variety of stress conditions. Although some caspase-like peptidase activity has been reported in dying parasites, no caspase gene is present in the genome. However, a single metacaspase gene is present in L. major whose encoded protein harbors the predicted secondary structure and the catalytic dyad histidine/cysteine described for caspases and other metacaspases identified in plants and yeast. The Saccharomyces cerevisiae metacaspase YCA1 has been implicated in the death of aging cells, cells defective in some biological functions, and cells exposed to different environmental stresses. In this study, we describe the functional heterologous complementation of a S. cerevisiae yca1 null mutant with the L. major metacaspase (LmjMCA) in cell death induced by oxidative stress. We show that LmjMCA is involved in yeast cell death, similar to YCA1, and that this function depends on its catalytic activity. LmjMCA was found to be auto-processed as occurs for caspases, however LmjMCA did not exhibit any activity with caspase substrates. In contrast and similarly to Arabidopsis thaliana metacaspases, LmjMCA was active towards substrates with arginine in the P1 position, with the activity being abolished following H147A and C202A catalytic site mutations. These results suggest that metacaspases are members of a family of peptidases with a role in cell death conserved in evolution notwithstanding possible differences in their catalytic activity.

Programmed cell death in African trypanosomes

Until recently it had generally been assumed that apoptosis and other forms of programmed cell death evolved during evolution of the metazoans to regulate growth and development in these multicellular organisms. However, recent research is adding strength to the original phenotypic observations described almost a decade ago which indicated that some parasitic protozoa may have evolved a cell death pathway analogous to the process described as apoptosis in metazoa. Here we explore the implications of a programmed cell death pathway in the African tsetse-transmitted trypanosomes.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Chronic heat-shock treatment driven differentiation induces apoptosis in Leishmania donovani

The present study investigates the role of apoptosis in the regulation of cell numbers of Leishmania donovani during the in vitro differentiation of promastigote stage to amastigote stage in axenic conditions. We report that apoptosis is induced in Leishmania donovani due to chronic heat-shock treatment of 37 ( degrees )C that also mediates the differentiation of promastigotes to amastigotes. This is characterized by the fragmentation of DNA, blebbing in the parasite cell membrane, nuclear condensation, formation of preapoptotic bodies and involvement of Ca(++) in the apoptotic process. The flowcytometric analysis shows an early and steep rise in percentage apoptotic nuclei till 48-hour stage of differentiation and then a gradual decline, suggesting synergistic action of Ca(++) ATPase and probably Hsp70. Hsp70 might be rescuing cells from apoptosis in the death signaling pathway. Incubation of the culture with Ca(++) chelator EGTA (1 mM) brings down the percentage of apoptotic nuclei considerably showing thereby that calcium is needed for the process of cell death here that occurs by apoptosis. The survival of the infective individuals appears to be decided by the parasite in the early stages of its differentiation. Our studies show the potential of the physiological temperature of 37 ( degrees )C in inducing apoptosis in Leishmania donovani and the therapeutic use it can be put to.

Programmed and altruistic ageing

Ageing is widely believed to be a non-adaptive process that results from a decline in the force of natural selection. However, recent studies in Saccharomyces cerevisiae are consistent with the existence of a programme of altruistic ageing and death. We suggest that the similarities between the molecular pathways that regulate ageing in yeast, worms, flies and mice, together with evidence that is consistent with programmed death in salmon and other organisms, raise the possibility that programmed ageing or death can also occur in higher eukaryotes.

Expression of alternative oxidase inhibits programmed cell death-like phenomenon in bloodstream form of Trypanosoma brucei rhodesiense

Trypanosoma brucei rhodesiense is one of the causative agents of African Trypanosomiasis. Programmed cell death (PCD) is fundamental in the development, homeostasis and immune mechanisms of multicellular organisms. It has been shown that, other than occurring in multicellular organisms, the PCD phenomenon also takes place in unicellular organisms. In the present study, we have found that under high-density axenic culture conditions, bloodstream form of T. b. rhodesiense depicts a PCD-like phenomenon. We investigated the association of the PCD-like phenomenon with expression of trypanosome alternative oxidase (TAO) under low-temperature stress conditions. We observed that bloodstream form of T. b. rhodesiense did not show any PCD but had up-regulated expression of TAO. Inhibition of TAO by the addition of ascofranone caused the development of PCD in bloodstream T. b. rhodesiense under low-temperature stress, implying that expression of TAO may contribute to the inhibition of PCD.

Prostaglandin D2 induces programmed cell death in Trypanosoma brucei bloodstream form

African trypanosomes produce some prostanoids, especially PGD2, PGE2 and PGF2alpha (Kubata et al. 2000, J. Exp. Med. 192: 1327-1338), probably to interfere with the host's physiological response. However, addition of prostaglandin D2 (but not PGE2 or PGF2alpha) to cultured bloodstream form trypanosomes led also to a significant inhibition of cell growth. Based on morphological alterations and specific staining methods using vital dyes, necrosis and autophagy were excluded. Here, we report that in bloodstream form trypanosomes PGD2 induces an apoptosis-like programmed cell death, which includes maintenance of plasma membrane integrity, phosphatidylserine exposure, loss of mitochondrial membrane potential, nuclear chromatin condensation and DNA degradation. The use of caspase inhibitors cannot prevent the cell death, indicating that the process is caspase-independent. Based on these results, we suggest that PGD2-induced programmed cell death is part of the population density regulation as observed in infected animals.

Note on the origin and history of the term "apoptosis"

This brief essay offers a perspective concerning the etymon of the term "apoptosis," a term that is currently and widely recognized as a synonym for programmed cell death. The origin of the term from the Greek and a historical perspective of how the concept of cell death was viewed in the 1950s to the 1970s are discussed. Studies in such diverse systems as cork oak bark, embryonic neuronal development, hepatology, and insect metamorphosis ultimately described processes similar to what we now call apoptosis.

Galectin-1 induces nuclear translocation of endonuclease G in caspase- and cytochrome c-independent T cell death

Galectin-1, a mammalian lectin expressed in many tissues, induces death of diverse cell types, including lymphocytes and tumor cells. The galectin-1 T cell death pathway is novel and distinct from other death pathways, including those initiated by Fas and corticosteroids. We have found that galectin-1 binding to human T cell lines triggered rapid translocation of endonuclease G from mitochondria to nuclei. However, endonuclease G nuclear translocation occurred without cytochrome c release from mitochondria, without nuclear translocation of apoptosis-inducing factor, and prior to loss of mitochondrial membrane potential. Galectin-1 treatment did not result in caspase activation, nor was death blocked by caspase inhibitors. However, galectin-1 cell death was inhibited by intracellular expression of galectin-3, and galectin-3 expression inhibited the eventual loss of mitochondrial membrane potential. Galectin-1-induced cell death proceeds via a caspase-independent pathway that involves a unique pattern of mitochondrial events, and different galectin family members can coordinately regulate susceptibility to cell death.

Metabolic aspects of aspirin-induced apoptosis in yeast

We have previously shown that aspirin induces apoptosis in manganese superoxide dismutase (MnSOD)-deficient Saccharomyces cerevisiae cells cultivated in ethanol medium, and that it exhibits a significant antioxidant effect until the onset of overt apoptosis. We here report that glucose-6-phosphate dehydrogenase activity in these cells is not inhibited by aspirin. However, the reducing power, as measured by the NADPH/NADP(+) concentration ratio, is significantly lower than in wild-type cells. With aspirin, the levels of NADPH, NADP(+) and catalase in MnSOD-deficient cells decrease significantly after 72 h of cultivation, without significant decrease of the NADPH/NADP(+) ratio. This ratio is higher when the cells are grown in glycerol or acetate medium. This seems to prevent loss in viability and induction of apoptosis on treatment with aspirin. Additionally, the glutathione (GSH) level is maintained, but the level of oxidized glutathione (GSSG) increases, leading to a significant decrease in the GSH/GSSG ratio in aspirin-treated cells. This decrease in the GSH/GSSG ratio is much less in cells grown in glycerol medium, while there is an increase in the GSH/GSSG ratio of cells grown in acetate medium. Consequently, the decreased reducing power may be linked to apoptotic induction by aspirin. This occurs independently of the level of reactive oxygen species which, as shown in our previous studies, do not play a primary role in the apoptosis of cells exposed to aspirin. The protective effect of MnSOD appears to be related to the cellular reducing power.

Identification of an upstream regulatory pathway controlling actin-mediated apoptosis in yeast

The build up of reactive oxygen species (ROS) is known to contribute to a reduction in the lifespan of a cell and to their degeneration in diseases such as Alzheimer's and tissue ischaemia. It is therefore important to elucidate pathways that regulate cellular oxidative stress. We have previously shown that actin dynamics can affect the oxidative-stress burden on a yeast cell and thereby its potential lifespan. To elucidate further the connection between actin dynamics and oxidative stress, we sought to identify regulators of this process. The actin regulatory proteins Sla1p and End3p are important in maintaining a rapid turnover of F-actin in cortical patches. We show that cells expressing a mutated form of Sla1p or lacking End3p display markers of apoptosis such as depolarized mitochondrial membranes and elevated levels of reactive oxygen species. Overexpression of the ubiquitin ligase RSP5 can alleviate the oxidative-stress phenotype observed in cells lacking End3p by targeting Sla1p to the cortex and restoring actin remodelling capability. We also demonstrate that overexpression of PDE2, a negative regulator of the Ras/cAMP pathway rescues actin dynamics, reduces oxidative stress sensitivity and restores viability in deltaend3 cells. Our data suggest, for the first time, that a physiological link exists between actin regulation and cAMP signalling that regulates apoptosis in yeast.

Programmed cell death in Trypanosoma cruzi induced by Bothrops jararaca venom

Cells die through a programmed process or accidental death, know as apoptosis or necrosis, respectively. Bothrops jararaca is a snake whose venom inhibits the growth of Trypanosoma cruzi epimastigote forms causing mitochondrion swelling and cell death. The aim of the present work was to determine the type of death induced in epimastigotes of T. cruzi by this venom. Parasite growth was inhibited after venom treatment, and 50% growth inhibition was obtained with 10 microg/ml. Ultrastructural observations confirmed mitochondrion swelling and kinetoplast disorganization. Furthermore, cytoplasmic condensation, loss of mitochondrion membrane potential, time-dependent increase in phosphatidylserine exposure at the outer leaflet plasma membrane followed by permeabilization, activation of caspase like protein and DNA fragmentation were observed in epimastigotes throughout a 24 h period of venom treatment. Taken together, these results indicate that the stress induced in epimastigote by this venom, triggers a programmed cell death process, similar to metazoan apoptosis, which leads to parasite death.

Induction of DNA damage and apoptosis in Saccharomyces cerevisiae by a yeast killer toxin

The cellular response of Saccharomyces cerevisiae to a linear plasmid encoded killer toxin from Pichia acaciae was analysed. As for the Kluyveromyces lactis zymocin, such toxin was recently shown to bind to the target cell's chitin and probably acts by facilitating the import of a toxin subunit. However, as distinct from zymocin, which arrests cells in G1, it provokes S-phase arrest and concomitant DNA damage checkpoint activation. Here, we report that such novel toxin type causes cell death in a two-step process. Within 4 h in toxin, viability of cells is immediately reduced to approximately 30%. Elevated mutation rates at the CAN1 locus prove DNA damaging mediated by the toxin. Cells arrested artificially in G1 or G2/M are very rapidly affected, while cells arrested in S loose their viability at a slower rate. S-phase arrest is, thus, a response of target cells to cope with DNA damage induced by the toxin. A second decline in viability requiring metabolically active target cells emerges upon toxin exposure over 10 h. During this phase, toxin treated cells develop abnormal nuclear morphology and react positive to terminal deoxynucleotidyl transferase-mediated nick end-labelling (TUNEL), indicative of DNA fragmentation. Furthermore, as judged from staining with fluorescein conjugated annexinV, cells expose phosphatidylserine at the outer membrane face and the formation of reactive oxygen species (ROS) is increased. ROS formation and concomitant cell death was heavily suppressed in a rho- derivative of the tester strain, while immediate reduction of viability was indistinguishable from the wild type. As a strain lacking the cellular target because of defects in the major chitinsynthase (Chs3) did not display such characteristic changes, the chitin binding and DNA-damaging P. acaciae toxin constitutes an apoptosis inducing protein. Both, DNA-damaging and apoptosis induction are unique features of this novel toxin type.

Involvement of the yeast metacaspase Yca1 in ubp10Delta-programmed cell death

UBP10 encodes a deubiquitinating enzyme of Saccharomyces cerevisiae. Its inactivation results in a complex phenotype characterized by a subpopulation of cells that exhibits the typical cellular markers of apoptosis. Here, we show that additional deletion of YCA1, coding for the yeast metacaspase, suppressed the ubp10 disruptant phenotype. Moreover, YCA1 overexpression, without any external stimulus, had a detrimental effect on growth and viability of ubp10 cells accompanied by an increase of apoptotic cells. This response was completely abrogated by ascorbic acid addition. We also observed that cells lacking UBP10 had an endogenous caspase activity, revealed by incubation in vivo with FITC-labeled VAD-fmk. All these results argue in favour of an involvement of the yeast metacaspase in the active cell death triggered by loss of UBP10 function.

The role of programmed cell death in Plasmodium-mosquito interactions

Many host-parasite interactions are regulated in part by the programmed cell death of host cells or the parasite. Here we review evidence suggesting that programmed cell death occurs during the early stages of the development of the malaria parasite in its vector. Zygotes and ookinetes of Plasmodium berghei have been shown to die by programmed cell death (apoptosis) in the midgut lumen of the vector Anopheles stephensi, or whilst developing in vitro. Several morphological markers, indicative of apoptosis, are described and evidence for the involvement of a biochemical pathway involving cysteine proteases discussed in relationship to other protozoan parasites. Malaria infection induces apoptosis in the cells of two mosquito tissues, the midgut and the follicular epithelium. Observations on cell death in both these tissues are reviewed including the role of caspases as effector molecules and the rescue of resorbing follicles resulting from inhibition of caspases. Putative signal molecules that might induce parasite and vector apoptosis are suggested including nitric oxide, reactive nitrogen intermediates, oxygen radicals and endocrine balances. Finally, we suggest that programmed cell death may play a critical role in regulation of infection by the parasite and the host, and contribute to the success or not of parasite establishment and host survival.

Dissection of the mitochondrial import and assembly pathway for human Tom40

Tom40 is the channel-forming subunit of the translocase of the mitochondrial outer membrane (TOM complex), essential for protein import into mitochondria. Tom40 is synthesized in the cytosol and contains information for its mitochondrial targeting and assembly. A number of stable import intermediates have been identified for Tom40 precursors in fungi, the first being an association with the sorting and assembly machinery (SAM) of the outer membrane. By examining the import pathway of human Tom40, we have been able to elucidate additional features in its import. We identify that Hsp90 is involved in delivery of the Tom40 precursor to mitochondria in an ATP-dependent manner. The precursor then forms its first stable intermediate with the outer face of the TOM complex before its membrane integration and assembly. Deletion of an evolutionary conserved region within Tom40 disrupts the TOM complex intermediate and causes it to stall at a new complex in the intermembrane space that we identify to be the mammalian SAM. Unlike its fungal counterparts, the human Tom40 precursor is not found stably arrested at a SAM intermediate. Nevertheless, we show that Tom40 assembly is reduced in mitochondria depleted of human Sam50. These findings are discussed in context with current models from fungal studies.

Interactions between malaria and mosquitoes: the role of apoptosis in parasite establishment and vector response to infection

Malaria parasites of the genus Plasmodium are transmitted from host to host by mosquitoes. Sexual reproduction occurs in the blood meal and the resultant motile zygote, the ookinete, migrates through the midgut epithelium and transforms to an oocyst under the basal lamina. After sporogony, sporozoites are released into the mosquito haemocoel and invade the salivary gland before injection when next the mosquito feeds on a host. Interactions between parasite and vector occur at all stages of the establishment and development of the parasite and some of these result in the death of parasite and host cells by apoptosis. Infection-induced programmed cell death occurs in patches of follicular epithelial cells in the ovary, resulting in follicle resorption and thus a reduction in egg production. We argue that fecundity reduction will result in a change in resource partitioning that may benefit the parasite. Apoptosis also occurs in cells of the midgut epithelium that have been invaded by the parasite and are subsequently expelled into the midgut. In addition, the parasite itself dies by a process of programmed cell death (PCD) in the lumen of the midgut before invasion has occurred. Caspase-like activity has been detected in the cytoplasm of the ookinetes, despite the absence of genes homologous to caspases in the genome of this, or any, unicellular eukaryote. The putative involvement of other cysteine proteases in ancient apoptotic pathways is discussed. Potential signal pathways for induction of apoptosis in the host and parasite are reviewed and we consider the evidence that nitric oxide may play a role in this induction. Finally, we consider the hypothesis that death of some parasites in the midgut will limit infection and thus prevent vector death before the parasites have developed into mature sporozoites.

Expression of anti-apoptotic factors in cells parasitized by second-generation schizonts of Eimeria tenella and Eimeria necatrix

Intracellular infections by parasites require a functional anti-apoptotic mechanism for parasite survival within the host cell. The intracellular cycle of Eimeria tenella and Eimeria necatrix in chicken intestinal cells involves the maturation of schizonts within the epithelial cells lining the crypt lumen of the ceca (E. tenella) and jejunum (E. necatrix). After invasion, these cells detach from the epithelial layer and migrate into the underlying connective tissue, where maturation of second-generation schizonts takes place. However, the detached epithelial cells that harbour the parasite and localize in the lamina propia do not undergo apoptosis despite the fact that they are parasitized cells and are located in an inappropriate microenvironment. In this study we consider the hypothesis that E. tenella and E. necatrix may inhibit the host cell apoptosis that accompanies parasite-mediated transformation during late schizogony. To that end, the expression of both NF-kappaB, a transcriptional factor that blocks parasite-induced apoptosis, and bcl-xL, an anti-apoptotic protein induced by NF-kappaB, were studied in the host cell during the maturation of second-generation schizonts. In addition, the expression of the phosphorylated inhibitor of NF-kappaB, p-IkBalpha, was also studied to further confirm NF-kappaB activation. Immunocytochemical techniques, flow cytometric and blott analysis were applied by using polyclonal antibodies that specifically react with bcl-xL, p-IkBalpha, and NF-kappaB to detect these anti-apoptotic proteins in the parasitized cell. Our results offer evidence that both these coccidial species first induce NF-kappaB activation to protect the transformed parasitized cells from apoptosis, allowing the second-generation schizonts to mature, and later, after complete schizonts maturation, cause NF-kappaB inhibition to trigger host cell apoptosis in order to facilitate the escape of merozoites. To determine whether inhibition of the NF-kappaB pathway would induce apoptosis of the host cell, a protease inhibitor (TPCK), which induces apoptosis by mediating inhibition of IkB phosphorylation, was administered to parasitized chickens.