The origin of programmed cell death

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.

Regulation of caspase activation in apoptosis: implications for transformation and drug resistance

Recent developments in the apoptosis field have uncovered a family of cysteine proteases, the Caspases, that act as signalling components as well as effectors of the cell death machinery. Caspases are constitutively present as inactive precursors within most cells and undergo proteolytic processing in response to diverse death-inducing stimuli to initiate the death programme. Active caspases can process other caspases of the same type as well as process caspases further downstream in the pathway that ultimately leads to collapse of the cell. This cellular collapse is thought to occur as a consequence of caspase-mediated cleavage of a diverse array of cellular substrates. Regulation of entry into the death programme is controlled at a number of levels by members of the Bcl-2 family, as well as by other cell death regulatory proteins. Recent data has shed light upon the mechanism of action of these regulatory molecules and suggests that the point of caspase activation is a major checkpoint in the cell death programme. Because many transformed cell populations possess derangements in cell death-regulatory genes, such as bcl-2, such cells frequently exhibit elevated resistance to cytotoxic chemotherapy. Thus, a deeper understanding of how apoptosis is normally regulated has therapeutic implications for disease states where the normal controls on the cell death machinery have been subverted.

Cell death upon H(2)O(2) induction in the unicellular green alga Micrasterias

In the present study, we investigated whether the unicellular green alga Micrasterias denticulata is capable of executing programmed cell death (PCD) upon experimental induction, and which morphological, molecular and physiological hallmarks characterise this. This is particularly interesting as unicellular freshwater green algae growing in shallow bog ponds are exposed to extreme environmental conditions, and the capacity to perform PCD may be an important strategy to guarantee survival of the population. The theoretically 'immortal' alga Micrasterias is an ideal object for such investigations as it has served as a cell biological model system for many years and details on its growth properties, physiology and ultrastructure throughout the cell cycle are well known. Treatments with low concentrations of H(2)O(2) are known to induce PCD in other organisms, resulting in severe ultrastructural changes to organelles, as observed in TEM. These include deformation and part disintegration of mitochondria, abnormal dilatation of cisternal rims of dictyosomes, occurrence of multivesicular bodies, an increase in the number of ER compartments, and slight condensation of chromatin. Additionally, a statistically significant increase in caspase-3-like activity was detected, which was abrogated by a caspase-3 inhibitor. Photosynthetic activity measured by fast chlorophyll fluorescence decreased as a consequence of H(2)O(2) exposure, whereas pigment composition, except for a reduction in carotenoids, was the same as in untreated controls. TUNEL positive staining and ladder-like degradation of DNA, both frequently regarded as a hallmark of PCD in higher plants, could only be detected in dead Micrasterias cells.

p,p'-DDE induces mitochondria-mediated apoptosis of cultured rat Sertoli cells

p,p'-Dichlorodiphenoxydichloroethylene (p,p'-DDE), the major metabolite of dichlorodiphenoxytrichloroethane (DDT), is a known persistent organic pollutant and male reproductive toxicant. However, the mechanism underlying male reproductive toxicity of p,p'-DDE remains limited. In the present study, Sertoli cells were used to investigate the molecular mechanism involved in p,p'-DDE's male reproductive toxicity. Results showed that p,p'-DDE exposure at over 30 microM showed induction of apoptotic cell death. p,p'-DDE could induce mitochondria-mediated apoptotic changes including elevation in reactive oxygen species (ROS) generation, decrease in mitochondrial membrane potential (DeltaPsi(m)), and release of cytochrome c into the cytosol, which could be blocked by antioxidant agent N-acetyl-l-cysteine (NAC). In addition, elevated ratios of Bax/Bcl-w and Bak/Bcl-w and cleavages of procaspase-3 and -9 were induced by p,p'-DDE treatment. All of the results suggested that ROS generation may play a critical role in the initiation of p,p'-DDE-induced apoptosis by mediation of the disruption of DeltaPsi(m), the release of cytochrome c into the cytosol and further the activation of caspase cascade.

Apoptotic-like changes in equine spermatozoa separated by density-gradient centrifugation or after cryopreservation

The objective was to evaluate apoptotic markers in ejaculated equine spermatozoa after separation by density-gradient centrifugation and after cryopreservation. Subpopulations of percoll-separated equine spermatozoa differed (P<0.05) in the percentage of live, caspase-activated spermatozoa (2.9+/-0.7% vs 14.2+/-6.4%; mean+/-S.E.M.), low mitochondrial membrane potential (MMP; 6.8+/-1.1 vs 23.8+/-3.7), altered plasma membrane permeability (1.3+/-0.2 vs 3.0+/-0.5), DNA fragmentation (2.0+/-1.3 vs 14.3+/-3.6), total motility (81.8+/-3.3 vs 35.1+/-5.4), and progressive motility (66.3+/-4.3 vs 24.1+/-4.5) for high-density versus low-density subpopulations, respectively. Phosphatidylserine externalization did not differ (P=0.67) between the high- and low-density subpopulations (2.6+/-0.7 vs 3.1+/-0.9). After cryopreservation, equine spermatozoa differed (P<0.01) in the percentage of active caspases (19.1+/-1.6 vs 52.1+/-2.8), low MMP (18.2+/-2.5 vs 48.7+/-2.6), altered plasma membrane permeability (6.8+/-1.7 vs 17.6+/-2.0), total motility (75.5+/-2.4 vs 45.2+/-5.6), and progressive motility (53.9+/-3.1 vs 28.3+/-4.5) for pre-freeze versus cryopreserved spermatozoa. There was no difference (P=0.21) in percentage of DNA fragmented cells before (5.5+/-1.2) versus after cryopreservation (6.6+/-1.1). We concluded that apoptotic-like changes were detectable in ejaculated equine spermatozoa and were more prevalent after cryopreservation.

Removal of dying cells and systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a very heterogeneous systemic autoimmune disease, in which autoantibody synthesis against nuclear constituents is the main immunological characteristic. These autoantibodies underwent affinity maturation and isotype switching. Additionally, T-cell tolerance against nuclear autoantigens should be affected in these autoimmune patients. Nuclear material derived from apoptotic and/or necrotic cells may serve as an important source of autoantigens. However, dead and dying cells as well as cellular debris are rapidly removed from tissues by phagocytes without eliciting inflammation or immune responses under healthy conditions. During apoptosis nuclear components are strongly modified through enzymatic reactions. If these cells are not timely cleared, those autoantigens may be released, taken up, and presented by dendritic cells in tissues or presented by follicular dendritic cells in lymph nodes to T and B cells, respectively. This could be a mechanism for breaking the peripheral self-tolerance. In this article we focus on the deficient clearance of apoptotic cells in SLE patients and its importance in development of this autoimmune disease.

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.

Formation of archegonium chamber is associated with nucellar-cell programmed cell death in Ginkgo biloba

The archegonium chamber in Ginkgo biloba L. is a pathway for spermatozoids swimming towards the archegonium for fertilization. The objective of this study was to elucidate the mechanism of archegonium chamber formation. The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay and DNA ladder demonstrated that the nucellar cell death, coordinated with the archegonium chamber formation, was a process of programmed cell death. Cytochemical localization of Ca(2+) in these nucellar cells was determined by means of in situ precipitation with potassium pyroantimonate and electron microscopic visualization, in order to study the relation between Ca(2+) and programmed cell death. The results showed an early uptake of the mitochondrial calcium particles in the nucellar cells undergoing programmed cell death. Together with other dynamic changes in Ca(2+) subcellular distribution, this indicates that Ca(2+) may play a role in the regulation of mitochondria-mediated programmed events in the nucellar cells.

Mitochondrial biology and disease in Dictyostelium

The cellular slime mold Dictyostelium discoideum has become an increasingly useful model for the study of mitochondrial biology and disease. Dictyostelium is an amoebazoan, a sister clade to the animal and fungal lineages. The mitochondrial biology of Dictyostelium exhibits some features which are unique, others which are common to all eukaryotes, and still others that are otherwise found only in the plant or the animal lineages. The AT-rich mitochondrial genome of Dictyostelium is larger than its mammalian counterpart and contains 56kb (compared to 17kb in mammals) encoding tRNAs, rRNAs, and 33 polypeptides (compared to 13 in mammals). It produces a single primary transcript that is cotranscriptionally processed into multiple monocistronic, dicistronic, and tricistronic mRNAs, tRNAs, and rRNAs. The mitochondrial fission mechanism employed by Dictyostelium involves both the extramitochondrial dynamin-based system used by plant, animal, and fungal mitochondria and the ancient FtsZ-based intramitochondrial fission process inherited from the bacterial ancestor. The mitochondrial protein-import apparatus is homologous to that of other eukaryote, and mitochondria in Dictyostelium play an important role in the programmed cell death pathways. Mitochondrial disease in Dictyostelium has been created both by targeted gene disruptions and by antisense RNA and RNAi inhibition of expression of essential nucleus-encoded mitochondrial proteins. This has revealed a regular pattern of aberrant mitochondrial disease phenotypes caused not by ATP insufficiency per se, but by chronic activation of the universal eukaryotic energy-sensing protein kinase AMPK. This novel insight into the cytopathological mechanisms of mitochondrial dysfunction suggests new possibilities for therapeutic intervention in mitochondrial and neurodegenerative diseases.

Bacterial programmed cell death and multicellular behavior in bacteria

Traditionally, programmed cell death (PCD) is associated with eukaryotic multicellular organisms. However, recently, PCD systems have also been observed in bacteria. Here we review recent research on two kinds of genetic programs that promote bacterial cell death. The first is mediated by mazEF, a toxin–antitoxin module found in the chromosomes of many kinds of bacteria, and mainly studied in Escherichia coli. The second program is found in Bacillus subtilis, in which the skf and sdp operons mediate the death of a subpopulation of sporulating bacterial cells. We relate these two bacterial PCD systems to the ways in which bacterial populations resemble multicellular organisms.

Is caspase-dependent apoptosis only cell differentiation taken to the extreme?

The benefits of apoptosis for a multicellular organism are obvious and fit the current dogma that the maintenance and viability of such organisms are dependent on the selective elimination of unneeded or deleterious cell types. However, self destruction at the level of the individual cell defies the most basic precepts of biology (sustaining life). If apoptosis is viewed through this construct then one question becomes paramount, i.e., why would an individual cell and its progeny develop, retain, or evolve a mechanism the sole purpose of which is to eliminate itself? In consideration of such a paradox, it is reasonable to postulate that prospective apoptotic pathways coevolved with and or were co-opted from another basic cell function(s) that did not involve the death of the cell per se. In the following article, we present the hypothesis that the conserved biochemical pathways of apoptosis are integral components of terminal cell differentiation and it is the time of engagement and activity level of these pathways that ultimately determines the choice between cell death or cell maturation.

Pro- and anti-apoptotic activities of protozoan parasites

During infection, programmed cell death, i.e. apoptosis, is an important effector mechanism of innate and adaptive host responses to parasites. In addition, it fulfils essential functions in regulating host immunity and tissue homeostasis. Not surprisingly, however, adaptation of parasitic protozoa to their hosts also involves modulation or even exploitation of cell death in order to facilitate parasite survival in a hostile environment. During recent years, considerable progress has been made in our understanding of apoptosis during parasitic infections and there is now convincing evidence that apoptosis and its modulation by protozoan parasites has a major impact on the parasite-host interaction and on the pathogenesis of disease. This review updates our current knowledge on the diverse functions apoptosis may fulfil during infections with diverse protozoan parasites including apicomplexans, kinetoplastids and amoebae. Furthermore, we also summarize common mechanistic themes of the pro- and anti-apoptotic activities of protozoan parasites. The diverse and complex effects which parasitic protozoa exert on apoptotic cell death within the host highlight fascinating interactions of parasites and their hosts. Importantly, they also stress the importance of further investigations before the modulation of host cell apoptosis can be exploited to combat parasitic infections.

Mechanistically identified suitable biomarkers of exposure, effect, and susceptibility for silicosis and coal-worker's pneumoconiosis: a comprehensive review

Clinical detection of silicosis is currently dependent on radiological and lung function abnormalities, both late manifestations of disease. Markers of prediction and early detection of pneumoconiosis are imperative for the implementation of timely intervention strategies. Understanding the underlying mechanisms of the etiology of coal workers pneumoconiosis (CWP) and silicosis was essential in proposing numerous biomarkers that have been evaluated to assess effects following exposure to crystalline silica and/or coal mine dust. Human validation studies have substantiated some of these proposed biomarkers and argued in favor of their use as biomarkers for crystalline silica- and CWP-induced pneumoconiosis. A number of "ideal" biological markers of effect were identified, namely, Clara cell protein-16 (CC16) (serum), tumor necrosis factor-alpha (TNF-alpha) (monocyte release), interleukin-8 (IL-8) (monocyte release), reactive oxygen species (ROS) measurement by chemiluminescence (neutrophil release), 8-isoprostanes (serum), total antioxidant levels measured by total equivalent antioxidant capacity (TEAC), glutathione, glutathione peroxidase activity, glutathione S-transferase activity, and platelet-derived growth factor (PDGF) (serum). TNF-alpha polymorphism (blood cellular DNA) was identified as a biomarker of susceptibility. Further studies are planned to test the validity and feasibility of these biomarkers to detect either high exposure to crystalline silica and early silicosis or susceptibility to silicosis in gold miners in South Africa.

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.

Octylphenol induces apoptosis in cultured rat Sertoli cells

In this study, the effects of 4-tert-octylphenol (OP) were examined on the viability of rat cultured Sertoli cells using the MTT assay and OP-induced apoptosis was detected by transmission electron microscope (TEM), flow cytometric analysis and Hoechst staining. In addition, RT-PCR was used to analyze the levels of Bcl-2 and Bax mRNA. Bcl-2, Bax and caspase-3 protein expressions were determined by Western blot analysis. Sertoli cells were treated with OP from 30 to 60microM for 6-24h. Decreased viability of Sertoli cells and increased apoptosis occurred in a concentration- and a time-dependent manner. The expression of Bcl-2 was down-regulated, while the expression of Bax up-regulated. OP also down-regulated the expression of 32kDa procaspase-3, which was cleaved to generate active subunit (17kDa). These results suggest that OP may induce Sertoli cell apoptosis by regulation of Bcl-2/Bax and caspase-3 activation.

Expression of a mitochondrial peroxiredoxin prevents programmed cell death in Leishmania donovani

Leishmania promastigote cells transmitted by the insect vector get phagocytosed by macrophages and convert into the amastigote form. During development and transformation, the parasites are exposed to various concentrations of reactive oxygen species, which can induce programmed cell death (PCD). We show that a mitochondrial peroxiredoxin (LdmPrx) protects Leishmania donovani from PCD. Whereas this peroxiredoxin is restricted to the kinetoplast area in promastigotes, it covers the entire mitochondrion in amastigotes, accompanied by dramatically increased expression. A similar change in the expression pattern was observed during the growth of Leishmania from the early to the late logarithmic phase. Recombinant LdmPrx shows typical peroxiredoxin-like enzyme activity. It is able to detoxify organic and inorganic peroxides and prevents DNA from hydroxyl radical-induced damage. Most notably, Leishmania parasites overexpressing this peroxiredoxin are protected from hydrogen peroxide-induced PCD. This protection is also seen in promastigotes grown to the late logarithmic phase, also characterized by high expression of this peroxiredoxin. Apparently, the physiological role of this peroxiredoxin is stabilization of the mitochondrial membrane potential and, as a consequence, inhibition of PCD through removal of peroxides.

Bloodstream form Trypanosoma brucei depend upon multiple metacaspases associated with RAB11-positive endosomes

Trypanosoma brucei possesses five metacaspase genes. Of these, MCA2 and MCA3 are expressed only in the mammalian bloodstream form of the parasite, whereas MCA5 is expressed also in the insect procyclic form. Triple RNAi analysis showed MCA2, MCA3 and MCA5 to be essential in the bloodstream form, with parasites accumulating pre-cytokinesis. Nevertheless, triple null mutants (deltamca2/3deltamca5) could be isolated after sequential gene deletion. Thereafter, deltamca2/3deltamca5 mutants were found to grow well both in vitro in culture and in vivo in mice. We hypothesise that metacaspases are essential for bloodstream form parasites, but they have overlapping functions and their progressive loss can be compensated for by activation of alternative biochemical pathways. Analysis of deltamca2/3deltamca5 revealed no greater or lesser susceptibility to stresses reported to initiate programmed cell death, such as treatment with prostaglandin D2. The metacaspases were found to colocalise with RAB11, a marker for recycling endosomes. However, variant surface glycoprotein (VSG) recycling processes and the degradation of internalised anti-VSG antibody were found to occur similarly in wild type, deltamca2/3deltamca5 and triple RNAi induced parasites. Thus, the data provide no support for the direct involvement of T. brucei metacaspases in programmed cell death and suggest that the proteins have a function associated with RAB11 vesicles that is independent of known recycling processes of RAB11-positive endosomes.

Yeast AMID homologue Ndi1p displays respiration-restricted apoptotic activity and is involved in chronological aging

Apoptosis-inducing factor (AIF) and AIF-homologous mitochondrion-associated inducer of death (AMID) are both mitochondrial flavoproteins that trigger caspase-independent apoptosis. Phylogenetic analysis suggests that these two proteins evolutionarily diverge back from their common prokaryote ancestor. Compared with AIF, the proapoptotic nature of AMID and its mode of action are much less clarified. Here, we show that overexpression of yeast AMID homologue internal NADH dehydrogenase (NDI1), but not external NADH dehydrogenase (NDE1), can cause apoptosis-like cell death, and this effect can be repressed by increased respiration on glucose-limited media. This result indicates that the regulatory network of energy metabolism, in particular the cross-talk between mitochondria and the rest of the cell, is involved in Ndi1p-induced yeast cell apoptosis. The apoptotic effect of NDI1 overexpression is associated with increased production of reactive oxygen species (ROS) in mitochondria. In addition, NDI1 overexpression in sod2 background causes cell lethality in both fermentable and semifermentable media. Interruption of certain components in the electron transport chain can suppress the growth inhibition from Ndi1p overexpression. We finally show that disruption of NDI1 or NDE1 decreases ROS production and elongates the chronological life span of yeast, accompanied by the loss of survival fitness. Implication of these findings for Ndi1p-induced apoptosis is discussed.

Acid stress adaptation protects Saccharomyces cerevisiae from acetic acid-induced programmed cell death

In this work evidence is presented that acid stress adaptation protects Saccharomyces cerevisiae from acetic acid-mediated programmed cell death. Exponential-phase yeast cells, non-adapted or adapted to acid stress by 30 min incubation in rich medium set at pH 3.0 with HCl, have been exposed to increasing concentrations of acetic acid and time course changes of cell viability have been assessed. Adapted cells, in contrast to non-adapted cells, when exposed to 80 mM acetic acid for 200 min did not display loss of cell viability associated to morphological alterations typical of apoptosis. Thus, 80 mM acetic acid death-inducing conditions were selected to further characterize the early molecular events leading to such active cell death process. Catalase was specifically activated during acid stress adaptation and protection against acetic acid-induced death was associated with maintenance of its activity during treatment with 80 mM acetic acid. On the other hand, intracellular superoxide dismutase activity was found present at comparable levels both in adapted and in dying yeast cells, excepting in non-adapted cells which displayed a maximum activity value after 15 min acetic acid exposure, corresponding to more than 80% cell viability. This study gives first experimental evidence that H2O2, rather than superoxide, detoxification may have a major role in preventing yeast cell death in response to acetic acid. The results, as a whole, suggest that commitment of S. cerevisiae to a programmed cell death process in response to acetic acid is mediated through a ROS-dependent apoptotic pathway.

Apoptosis and lipoapoptosis in the fission yeast Schizosaccharomyces pombe

Yeasts being simple eukaryotes are established genetic systems that are often employed to solve important biological questions. Recently, it has become evident that certain cell death programs exist in these unicellular organisms. For example, it has been shown recently that strains of the fission yeast Schizosaccharomyces pombe deficient in triacylglycerol synthesis undergo cell death with prominent apoptotic markers. This minireview is intended to discuss key developments that have rendered fission yeast useful both as a tool and as a model for apoptosis and lipoapoptosis research. It is attempted to delineate a putative signaling pathway leading to the execution of lipoapoptosis in the fission yeast. Although in its infancy, apoptosis research in the fission yeast promises exciting breakthroughs in the near future.

Role of mitochondria in the pheromone- and amiodarone-induced programmed death of yeast

Although programmed cell death (PCD) is extensively studied in multicellular organisms, in recent years it has been shown that a unicellular organism, yeast Saccharomyces cerevisiae, also possesses death program(s). In particular, we have found that a high doses of yeast pheromone is a natural stimulus inducing PCD. Here, we show that the death cascades triggered by pheromone and by a drug amiodarone are very similar. We focused on the role of mitochondria during the pheromone/amiodarone-induced PCD. For the first time, a functional chain of the mitochondria-related events required for a particular case of yeast PCD has been revealed: an enhancement of mitochondrial respiration and of its energy coupling, a strong increase of mitochondrial membrane potential, both events triggered by the rise of cytoplasmic [Ca²⁺], a burst in generation of reactive oxygen species in center o of the respiratory chain complex III, mitochondrial thread-grain transition, and cytochrome c release from mitochondria. A novel mitochondrial protein required for thread-grain transition is identified.

Novobiocin induces apoptosis-like cell death in topoisomerase II over-expressing arsenite resistant Leishmania donovani

Leishmaniasis affects millions of people worldwide every year. Lack of effective vaccination, co-infection with other dreaded diseases like AIDS and generation of drug resistant strains demand immediate attention into this neglected area of research. The sodium m-arsenite (NaAsO2) resistant Leishmania donovani used in this study is resistant to 20 microM NaAsO2, which shows a 13-fold increase in resistance compared with wild type. Here we report that the arsenite resistant strain of L. donovani promastigotes shows cross-resistance to novobiocin, a catalytic inhibitor of topoisomerase II, with IC50 value of 320 microg ml-1 as compared with 242 microg ml-1 for wild type L. donovani. Leishmanicidal action of novobiocin induces dose- and time-dependent increase in cell death. Treatment with IC50 of novobiocin caused morphological and biochemical changes which lead to induction of cell death exhibiting characteristic features of metazoan apoptosis. Phosphatidylserine externalization, cytochrome C release to cytoplasm, activation of caspases, oligonucleosomal DNA fragmentation and in situ labelling of condensed and fragmented nuclei in both wild type and arsenite resistant L. donovani promastigotes strongly suggest the apoptosis-like mode of cell death. Cross-resistance to novobiocin in arsenite resistant strain has been correlated to over-expression of topoisomerase II and substantiated by differential inhibition of enzyme activity in wild type and arsenite resistant L. donovani.

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.

Mammalian apoptosis-inducing protein, HAP, induces bacterial cell death

In attempting to produce the HAP, endoplasmic reticulum (ER) targeted apoptosis-inducing protein, as a GST-fusion protein we found that the expression of HAP, but not GST alone, induced bacterial cell death. The HAP protein inhibited the bacterial growth within 30 min after inducting HAP expression. The transmission electron microscopic examination revealed that the morphology of the bacterial cells expressing hap was changed dramatically: unusually elongated phenotype compared with those of controls and finally leading to cell death. The lethality of HAP was relieved by the addition of vitamin E as a reducing agent and under anaerobic growth conditions. These results suggest that a trace amount of HAP induces bacterial cell death and the death is related with reactive oxygen species (ROS).

Programmed cell death in trypanosomatids: a way to maximize their biological fitness?

Programmed cell death (PCD) is a biochemical process that plays an essential role in the development of multicellular organisms. However, accumulating evidence indicates that PCD is also present in single-celled eukaryotes. Thus, trypanosomatids might be endowed with a PCD mechanism that is derived from ancestral death machinery. PCD in trypanosomatids could be a process without a defined function, inherited through eukaryotic cell evolution, which might be triggered in response to diverse stimuli and stress conditions. However, recent observations suggest that PCD might be used by trypanosomatids to maximize their biological fitness. Therefore, PCD could represent a potential pharmacological target for protozoan control.

Starvation for an essential amino acid induces apoptosis and oxidative stress in yeast

Protracted starvation of auxotrophic Saccharomyces cerevisiae strains for an essential amino acid is commonly used to allow investigation of adaptive mutation mechanisms during starvation-induced cell cycle arrest. Under these conditions, the majority of cells dies during the first 6 days. We investigated starving cells for markers of programmed cell death and for the production of reactive oxygen species (ROS). We observed that protracted starvation for lysine or histidine resulted in an increasing number of cells exhibiting DNA fragmentation and chromatin condensation, thus an apoptotic phenotype. Not only respiration-competent cells but also respiratory deficient rho0 cells were able to undergo programmed cell death. In addition the starving cells rapidly exhibited indicators of oxidative stress, independently of their respiratory competence. These results indicate that starvation for an essential amino acid results in severe cell stress, which may finally be the trigger of programmed cell death.

Possible mechanism of miltefosine-mediated death of Leishmania donovani

Miltefosine causes leishmanial death, but the possible mechanism(s) of action is not known. The mode of action of miltefosine was investigated in vitro in Leishmania donovani promastigotes as well as in extra- and intracellular amastigotes. Here, we demonstrate that miltefosine induces apoptosis-like death in L. donovani based on observed phenomena such as nuclear DNA condensation, DNA fragmentation with accompanying ladder formation, and in situ labeling of DNA fragments by the terminal deoxyribonucleotidyltransferase-mediated dUTP-biotin nick end labeling method. Understanding of miltefosine-mediated death will facilitate the design of new therapeutic strategies against Leishmania parasites.

Death's toolbox: examining the molecular components of bacterial programmed cell death

Programmed cell death (PCD) is a genetically determined process of cellular suicide that is activated in response to cellular stress or damage, as well as in response to the developmental signals in multicellular organisms. Although historically studied in eukaryotes, it has been proposed that PCD also functions in prokaryotes, either during the developmental life cycle of certain bacteria or to remove damaged cells from a population in response to a wide variety of stresses. This review will examine several putative examples of bacterial PCD and summarize what is known about the molecular components of these systems.

Apoptosis induced by environmental stresses and amphotericin B in Candida albicans

New antifungal agents are urgently required to combat life-threatening infections caused by opportunistic fungal pathogens like Candida albicans. The manipulation of endogenous fungal programmed cell death responses could provide a basis for future therapies. Here we assess the physiology of death in C. albicans in response to environmental stresses (acetic acid and hydrogen peroxide) and an antifungal agent (amphotericin B). Exposure of C. albicans to 40-60 mM acetic acid, 5-10 mM hydrogen peroxide, or 4-8 microg.ml-1 amphotericin B produced cellular changes reminiscent of mammalian apoptosis. Nonviable cells that excluded propidium iodide displayed the apoptotic marker phosphatidylserine (as shown by annexin-V-FITC labeling), were terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL)-positive (indicating nuclease-mediated double-strand DNA breakage), and produced reactive oxygen species. Ultrastructural changes in apoptotic cells included chromatin condensation and margination, separation of the nuclear envelope, and nuclear fragmentation. C. albicans cells treated at higher doses of these compounds showed cellular changes characteristic of necrosis. Necrotic cells displayed reduced TUNEL staining, a lack of surface phosphatidylserine, limited reactive oxygen species production, and an inability to exclude propidium iodide. Necrotic cells lacked defined nuclei and showed extensive intracellular vacuolization. Apoptosis in C. albicans was associated with an accumulation of cells in the G2/M phase of the cell cycle, and under some apoptosis-inducing conditions, significant proportions of yeast cells switched to hyphal growth before dying. This is a demonstration of apoptosis in a medically important fungal pathogen.

Ischemia and stroke

Cell death following cerebral ischemia is mediated by a complex pathophysiologic interaction of different mechanisms. In this Chapter we will outline the basic principles as well as introduce in vitro and in vivo models of cerebral ischemia. Mechanistically, excitotoxicity, peri-infarct depolarization, inflammation and apoptosis seem to be the most relevant mediators of damage and are promising targets for neuroprotective strategies.

Programmed cell death in trypanosomatids and other unicellular organisms

In multicellular organisms, cellular growth and development can be controlled by programmed cell death (PCD), which is defined by a sequence of regulated events. However, PCD is thought to have evolved not only to regulate growth and development in multicellular organisms but also to have a functional role in the biology of unicellular organisms. In protozoan parasites and in other unicellular organisms, features of PCD similar to those in multicellular organisms have been reported, suggesting some commonality in the PCD pathway between unicellular and multicellular organisms. However, more extensive studies are needed to fully characterise the PCD pathway and to define the factors that control PCD in the unicellular organisms. The understanding of the PCD pathway in unicellular organisms could delineate the evolutionary origin of this pathway. Further characterisation of the PCD pathway in the unicellular parasites could provide information regarding their pathogenesis, which could be exploited to target new drugs to limit their growth and treat the disease they cause.

Single-strand-specific nucleases

Single-strand-specific nucleases are multifunctional enzymes and widespread in distribution. Their ability to act selectively on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids has led to their extensive application as probes for the structural determination of nucleic acids. Intracellularly, they have been implicated in recombination, repair and replication, whereas extracellular enzymes have a role in nutrition. Although more than 30 single-strand-specific nucleases from various sources have been isolated till now, only a few enzymes (S1 nuclease from Aspergillus oryzae, P1 nuclease from Penicillium citrinum and nucleases from Alteromonas espejiana, Neurospora crassa, Ustilago maydis and mung bean) have been characterized to a significant extent. Recently, some of these enzymes have been cloned, their crystal structures solved and their interactions with different substrates have been established. The detection, purification, characteristics, structure-function correlations, biological role and applications of single-strand-specific nucleases are reviewed.

Programmed death in yeast as adaptation?

During recent years, several pieces of indirect evidence of a programmed death in yeast have been published. Among them there are observations that some mammalian pro- or anti-apoptotic proteins induce or prevent the death of yeast; some toxic compounds kill yeast at lower concentrations if protein synthesis is operative; this death, as well as the death due to certain mutations, shows some apoptotic markers. In April 2002, the yeast programmed death concept received direct support. Madeo et al. [Madeo et al., Mol. Cell 9 (2002) 911-917] disclosed a caspase which is activated by H(2)O(2) or aging and is required for the protein-synthesis-dependent death of yeast. Thus, a specific apoptosis-mediating protein was identified for the first time in Saccharomyces cerevisiae. Independently, Severin and Hyman [Severin, F.F., Hyman, A.A., Curr. Biol. 12 (2002) R233-R235] discovered that death of yeast, induced by a high level of a pheromone, is programmed. In particular, the death was found to be prevented by cycloheximide and cyclosporin A. It required mitochondrial DNA, cytochrome c and the pheromone-initiated protein kinase cascade. When haploids of opposite mating types were mixed, some cells died, the inhibitory pattern being the same as in the case of the killing by pheromone. Inhibition of mating proved to be favorable for death. Thus, pheromone not only activates mating but also eliminates yeast cells failing to mate. Such an effect should (i) stimulate switch of the yeast population from vegetative to sexual reproduction, and (ii) shorten the life span and, hence, accelerate changing of generations. As a result, the probability of appearance of new traits could be enhanced when ambient conditions turned for the worse.

Cell death in Leishmania induced by stress and differentiation: programmed cell death or necrosis?

Unicellular organisms, such as the protozoan parasite Leishmania, can be stimulated to show some morphological and biochemical features characteristic of mammalian apoptosis. This study demonstrates that under a variety of stress conditions such as serum deprivation, heat shock and nitric oxide, cell death can be induced leading to genomic DNA fragmentation into oligonucleosomes. DNA fragmentation was observed, without induction, in the infectious stages of the parasite, and correlated with the presence of internucleosomal nuclease activity, visualisation of 45 to 59 kDa nucleases and detection of TUNEL-positive nuclei. DNA fragmentation was not dependent on active effector downstream caspases nor on the lysosomal cathepsin L-like enzymes CPA and CPB. These data are consistent with the presence of a caspase-independent cell death mechanism in Leishmania, induced by stress and differentiation that differs significantly from metazoa.

Salt stress induces programmed cell death in prokaryotic organism Anabaena

AIMS

Our main interest is to check if programmed cell death (PCD) can occur in prokaryotic algae and if the morphological and biochemical features of PCD are conserved.

METHODS AND RESULTS

Using TUNEL labelling, fluorescence and light microscopy and DNA gel electrophoresis, we found that cell death with features similar to those in metazoan PCD could be induced in different Anabaena strains after exposure to univalent-cation salts at moderate concentration. These features included specific DNA fragmentation, cytoplasmic vacuolation, and the progressive disorganization, fragmentation and subsequent autolysis of the cell corpse. Further analyses of cell viability and proteinase activity revealed that increased protease activities, decreased DNA content, and loss of plasmalemma integrity were related to the PCD process.

CONCLUSIONS

The results showed that like PCD in eukaryotes, PCD in Anabaena is an active process, and is an adaptation to adverse environments. The features of PCD shared between eukaryotes and Anabaena suggest that PCD mechanisms are conserved during evolution.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results will contribute greatly to our understanding of PCD origin and evolution, and are potentially useful in controlling the deluge of algae in some lakes.

Correlates of developmental cell death in Dictyostelium discoideum

We have studied the correlates of cell death during stalk cell differentiation in Dictyostelium discoideum. Our main findings are four. (i) There is a gradual increase in the number of cells with exposed phosphatidyl serine residues, an indicator of membrane asymmetry loss and increased permeability. Only presumptive stalk cells show this change in membrane asymmetry. Cells also show an increase in cell membrane permeability under conditions of calcium-induced stalk cell differentiation in cell monolayers. (ii) There is a gradual fall in mitochondrial membrane potential during development, again restricted to the presumptive stalk cells. (iii) The fraction of cells showing caspase-3 activity increases as development proceeds and then declines in the terminally differentiated fruiting body. (iv) There is no internucleosomal cleavage of DNA, or DNA fragmentation, in D. discoideum nor is there any calcium- and magnesium-dependent endonucleolytic activity in nuclear extracts from various developmental stages. However, nuclear condensation and peripheralization does occur in stalk cells. Thus, cell death in D. discoideum shows some, but not all, features of apoptotic cell death as recognized in other multicellular systems. These findings argue against the emergence of a single mechanism of 'programmed cell death (PCD)' before multicellularity arose during evolution.

Nitric oxide-mediated proteasome-dependent oligonucleosomal DNA fragmentation in Leishmania amazonensis amastigotes

Resistance to leishmanial infections depends on intracellular parasite killing by activated host macrophages through the L-arginine-nitric oxide (NO) metabolic pathway. Here we investigate the cell death process induced by NO for the intracellular protozoan Leishmania amazonensis. Exposure of amastigotes to moderate concentrations of NO-donating compounds (acidified sodium nitrite NaNO(2) or nitrosylated albumin) or to endogenous NO produced by lipopolysaccharide or gamma interferon treatment of infected macrophages resulted in a dramatic time-dependent cell death. The combined use of several standard DNA status analysis techniques (including electrophoresis ladder banding patterns, YOPRO-1 staining in flow cytofluorometry, and in situ recognition of DNA strand breaks by TUNEL [terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling] assay) revealed a rapid and extensive fragmentation of nuclear DNA in both axenic and intracellular NO-treated amastigotes of L. amazonensis. Despite some similarities to apoptosis, the nuclease activation responsible for characteristic DNA degradation was not under the control of caspase activity as indicated by the lack of involvement of cell-permeable inhibitors of caspases and cysteine proteases. In contrast, exposure of NO-treated amastigotes with specific proteasome inhibitors, such as lactacystin or calpain inhibitor I, markedly reduced the induction of the NO-mediated apoptosis-like process. These data strongly suggest that NO-induced oligonucleosomal DNA fragmentation in Leishmania amastigotes is, at least in part, regulated by noncaspase proteases of the proteasome. The determination of biochemical pathways leading up to cell death might ultimately allow the identification of new therapeutic targets.

A form of cell death with some features resembling apoptosis in the amitochondrial unicellular organism Trichomonas vaginalis

One of hallmarks of apoptosis is the degradation and concomitant compaction of chromatin. It is assumed that caspases and caspase-independent pathways are rate limiting for the development of nuclear apoptosis. The caspase-independent pathway involves apoptosis-inducing factor (AIF) and leads to DNA fragmentation and peripheral chromatin condensation. Both pathways are the result of activation of death signals that the mitochondrion receives, integrates, and responds to with the release of various molecules (e.g., cytochrome c and AIF). In fact, both pathways have in common the final point of the DNA fragmentation and the mitochondrial origin of molecules that initiate the apoptotic events. Here, we examine the question of whether apoptosis or apoptotic-like processes exist in a unicellular organism that lacks mitochondria. We herein show that a form of cell death with some features resembling apoptosis is indeed present in Trichomonas vaginalis. Characterization of morphological aspects implicated in this event together with the preliminary biochemical data provided may lead to new insight about the evolutionary relationships between the different forms of programmed cell death identified so far.

A metacaspase of Trypanosoma brucei causes loss of respiration competence and clonal death in the yeast Saccharomyces cerevisiae

Metacaspases constitute a new group of cysteine proteases homologous to caspases. Heterologous expression of Trypanosoma brucei metacaspase TbMCA4 in the budding yeast Saccharomyces cerevisiae resulted in growth inhibition, mitochondrial dysfunction and clonal death. The metacaspase orthologue of yeast, ScMCA1 (YOR197w), exhibited genetic interaction with WWM1 (YFL010c), which encodes a small WW domain protein. WWM1 overexpression resulted in growth arrest and clonal death, which was suppressed by concomitant overexpression of ScMCA1. GFP-fusion reporters of WWM1, ScMCA1 and TbMCA4 localized to the nucleus. Taken together, we suggest that metacaspases may play a role in nuclear function controlling cellular proliferation coupled to mitochondrial biogenesis.