Saccharomyces cerevisiae commits to a programmed cell death process in response to acetic acid

PGK1, the gene encoding the glycolitic enzyme phosphoglycerate kinase, acts as a multicopy suppressor of apoptotic phenotypes in S. cerevisiae

In a previous paper we reported the construction of a S. cerevisiae strain lacking the essential gene LSM4, which could survive by the introduction of a truncated form of the orthologous gene from Kluyveromyces lactis. This strain showed apoptotic hallmarks and other phenotypes, including an increased sensitivity to caffeine and acetic acid. The suppression of the latter phenotype by overexpressing yeast genes allowed the isolation of PGK1, the gene encoding the glycolytic enzyme phosphoglycerate kinase. This gene restored normal ageing, oxygen peroxide resistance and nuclear integrity in the mutant. Other phenotypes, such as caffeine sensitivity and glycerol utilization, were also suppressed.

A newly identified essential complex, Dre2-Tah18, controls mitochondria integrity and cell death after oxidative stress in yeast

A mutated allele of the essential gene TAH18 was previously identified in our laboratory in a genetic screen for new proteins interacting with the DNA polymerase delta in yeast [1]. The present work shows that Tah18 plays a role in response to oxidative stress. After exposure to lethal doses of H₂O₂, GFP-Tah18 relocalizes to the mitochondria and controls mitochondria integrity and cell death. Dre2, an essential Fe/S cluster protein and homologue of human anti-apoptotic Ciapin1, was identified as a molecular partner of Tah18 in the absence of stress. Moreover, Ciapin1 is able to replace yeast Dre2 in vivo and physically interacts with Tah18. Our results are in favour of an oxidative stress-induced cell death in yeast that involves mitochondria and is controlled by the newly identified Dre2-Tah18 complex.

Human p53 induces cell death and downregulates thioredoxin expression in Saccharomyces cerevisiae

The p53 tumour suppressor protein has a crucial role in controlling cell cycle and apoptosis in human cells and its inactivation by selective point mutations is associated with human cancers. Here we show that overexpression of the human wild-type (wt) p53 in Saccharomyces cerevisiae completely inhibits yeast growth under minimal media conditions. In contrast, the R248W 'hot spot' p53 mutant (one of the most frequent p53 mutations encountered in human cancers) does not impair yeast growth. Moreover, we report, for the first time, that the human wt p53 induces yeast cell death with characteristic markers of apoptosis: exposure of phosphatidylserine and DNA strand cleavage as shown by Annexin V staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay, respectively. In addition, p53 also has an impact on the expression of yeast genes. Using differential display and Northern blot analysis, we demonstrated that human wt p53 expression in yeast leads to gene repression of thioredoxin (TRX1/2), a highly conserved multifunctional antioxidative and antiapoptotic protein family. Accordingly, we demonstrated that reactive oxygen species (ROS) are highly produced in p53 yeast induced cell death as shown by dihydrorhodamine 123 staining. These results suggest that the generation of ROS is a key event in p53 yeast induced cell death.

Reliable method for detection of programmed cell death in yeast

Accumulating evidence suggests that yeasts are capable of undergoing programmed cell death (PCD) to benefit long-term survival of the species, and that yeast and mammals may share at least partially conserved PCD pathways. In our experience, mammalian apoptosis assays have not been readily applicable to yeast. Therefore, to take advantage of yeast as a genetic tool to study PCD, we developed a yeast cell death assay that can reliably reveal viability differences between wild-type strains and strains lacking the mitochondrial fission genes DNM1/Drp1 and FIS1, orthologs of mammalian cell death regulators. Cell viability following treatment with acetic acid is quantified by colony formation and vital dye (FUN1) staining to reproducibly detect dose-dependent, genetically programmed yeast cell death.

Vacuolar functions determine the mode of cell death

The yeast vacuole plays a crucial role in cell homeostasis including pH regulation and degradation of proteins and organelles. Class C VPS genes code for proteins essential for vacuolar and endosomal vesicle fusion, their deletion results in the absence of a detectable vacuole. We found that single gene deletions of class C VPS genes result in a drastically enhanced sensitivity to treatment with acetic acid whereas sensitivity towards H2O2 remains largely unaffected. Interestingly acetic acid treatment known as an established inducer of yeast apoptosis leads to necrosis in class C VPS deletion strains. Their intracellular pH drops from 6.7 to 5.5 after acetic acid treatment, while in wild type the pH drops to just 6.3. When the intracellular pH in wild type is lowered below pH 5.5 using a higher concentration of acetic acid, the survival rate is similarly low as in the class C VPS mutants, however, the death phenotype is predominantly apoptotic. Hence, the vacuole not only prevents acetic acid induced cell death by buffering the cytosolic pH, but it also has a proapoptotic function.

Oxidative stress and chronological aging in glycogen-phosphorylase-deleted yeast

Chronological aging in yeast resembles aging in mammalian, postmitotic tissues. Such chronological aging begins with entrance into the stationary phase after the nutrients are exhausted. Many changes in metabolism take place at this moment, and survival in this phase strongly depends on oxidative-stress resistance. In this study, hypo- and hyperglycogenic phenotypes of Saccharomyces cerevisiae strains with deletions of carbohydrate-metabolism enzymes were selected, and a comparison of their chronological longevities was made. Stress sensitivity, ROS, and apoptosis markers during aging were analyzed in the emerged candidates. Among the strains that accumulated greater amounts of glycogen, the deletion of glycogen phosphorylase, gph1delta, was unique in showing a shortened life span, stress intolerance, and higher levels of ROS during its survival. The transcription of superoxide dismutase genes during survival was three- to fourfold lower in gph1delta. Extra copies of SOD1/2 counteracted the stress sensitivity and the accelerated aging of gph1delta. In conclusion, the lack of gph1 produced a rapidly aging strain, which could be attributed, at least in part, to the weakened stress resistance associated with the decreased expression of both SODs. Gph1p seems to be a candidate in a scenario that could link early metabolic changes with other targets of the stress response during stationary-phase survival.

Influence of pH conditions on the viability of Saccharomyces boulardii yeast

Saccharomyces boulardii is a probiotic with proven health benefits. However its survival is challenged by gastrointestinal transit, and a ratio between 1 and 3% of living yeast is recovered in the feces after oral administration. The aim of the study was to determine to what extent the yeast was sensitive to gastrointestinal pH conditions. Therefore we explored the survival of different concentrations of S. boulardii in conditions mimicking the stomach pH (pH 1.1 0.1 N HCl) and the intestinal pH (pH 6.8 phosphate buffer) in vitro. The probiotic being commercialized as a freeze-dried powder obtained from an aqueous suspension, both forms were evaluated. In phosphate buffer pH 6.8, the viability remained stable for both forms of S. boulardii for 6 h. In HCl pH 1.1, viability of both forms (200 mg L(-1)) significantly decreased from 5 min. Observation under scanning/transmission electron microscopy showed morphological damages and rupture of the yeast wall. Threshold value from which S. boulardii viability was unaltered was pH 4. At the highest concentration of 200 g L(-1), the initial pH value of 1.1 rose to 3.2, exerting a protective effect. In conclusion, although the yeast in aqueous suspension was less sensitive than the freeze-dried yeast to acidic conditions, a gastric protection for improvement of oral bioavailability of viable S. boulardii appears necessary.

Apoptotic death of ageing yeast

Yeast has been a valuable model to study replicative and chronological ageing processes. Replicative ageing is defined by the number of daughter cells a mother can give birth to and hence reflects the ageing situation in proliferating cells, whereas chronological ageing is widely accepted as a model for postmitotic tissue ageing. Since both ageing forms end in yeast programmed death (necrotic and apoptotic), and abrogation of cell death by deletion of the apoptotic machinery or diminishment of oxidative radicals leads to longevity, apoptosis and ageing seem closely connected. This review focuses on ageing as a physiological way to induce yeast apoptosis, which unexpectedly defines apoptosis as a pro- and not an anti-ageing mechanism.

Human lactoferrin induces apoptosis-like cell death in Candida albicans: critical role of K+-channel-mediated K+ efflux

Human lactoferrin (hLf) induced an apoptosis-like phenotype in Candida albicans cells, which includes phosphatidylserine externalization, nuclear chromatin condensation, DNA degradation, and increased reactive oxygen species (ROS) production. Intracellular ROS accumulation was seen to correlate with candidacidal activity in hLf-treated cells. Mitochondrial activity was involved as indicated by mitochondrial depolarization and increased hLf resistance of cells preincubated with sordarin or erythromycin, the latter of which inhibits protein synthesis in mitoribosomes. Interestingly, Cl(-)- and K(+)-channel blockers prevented the hLf antimicrobial activity, but only when cells were pretreated with the blocking agent (tetraethylammonium) prior to the hLf-induced K(+)-release period. These results indicate for the first time that K(+)-channel-mediated K(+) efflux is required for the progression of apoptosis-like process in yeast, suggesting that this essential apoptotic event of higher eukaryotes has been evolutionary conserved among species ranging from yeasts to humans.

Loss of peroxisome function triggers necrosis

Disturbance of peroxisome function can lead to various degenerative diseases during ageing. Here, we show that in yeast deletion of PEX6, encoding a protein involved in a key step of peroxisomal protein import, results in an increased accumulation of reactive oxygen species and an enhanced loss of viability upon acetic acid treatment and during early stationary phase. Cell death of ageing-like yeast cells lacking PEX6 does not depend on the apoptotic key players Yca1p and Aif1p, but instead shows markers of necrosis. Thus, we conclude that loss of peroxisomal function leads to a form of necrotic cell death.

Kex1 protease is involved in yeast cell death induced by defective N-glycosylation, acetic acid, and chronological aging

N-glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to humans. The key step of this pathway is the transfer of the lipid-linked core oligosaccharide to the nascent polypeptide chain, catalyzed by the oligosaccharyltransferase complex. Temperature-sensitive oligosaccharyltransferase mutants of Saccharomyces cerevisiae at the restrictive temperature, such as wbp1-1, as well as wild-type cells in the presence of the N-glycosylation inhibitor tunicamycin display typical apoptotic phenotypes like nuclear condensation, DNA fragmentation, phosphatidylserine translocation, caspase-like activity, and reactive oxygen species accumulation. Since deletion of the yeast metacaspase YCA1 did not abrogate this death pathway, we postulated a different proteolytic process to be responsible. Here, we show that Kex1 protease is involved in the programmed cell death caused by defective N-glycosylation. Its disruption decreases caspase-like activity, production of reactive oxygen species, and fragmentation of mitochondria and, conversely, improves growth and survival of cells. Moreover, we demonstrate that Kex1 contributes also to the active cell death program induced by acetic acid stress or during chronological aging, suggesting that Kex1 plays a more general role in cellular suicide of yeast.

Novel stress responses facilitate Saccharomyces cerevisiae growth in the presence of the monocarboxylate preservatives

Certain yeasts are relatively resistant to the small number of monocarboxylic acids allowed in food preservation, with the result that these preservatives often have to be used in high concentrations in order to prevent spoilage. When grown at slightly acid pH, Saccharomyces cerevisiae acquires elevated resistance to these acids by means of discrete stress responses. Acquisition of resistance to acetic acid involves loss of Fps1p, the aquaglyceroporin of the plasma membrane that facilitates the passive diffusional entry of this acid into cells. Acetic acid stress transiently activates Hog1p mitogen-activated protein kinase, which then directly phosphorylates Fps1p in order to target this channel for endocytosis and degradation in the vacuole. Other carboxylate preservatives (propionate, sorbate or benzoate) are too large to traverse the Fps1p pore. Instead, being more lipophilic than acetic acid, they enter cells mainly by a process of non-facilitated diffusion across the plasma membrane. Once inside the cell, these acids activate War1p, a transcription factor that induces the gene for the Pdr12p plasma membrane ATP-binding cassette transporter. Pdr12p lowers the intracellular levels of propionate, sorbate or benzoate by catalysing the active efflux of the preservative anion from the cell. Still other mechanisms of weak acid resistance are found in Zygosaccharomyces, including a capacity for the oxidative degradation of sorbic and benzoic acids conferred by a mitochondrial monooxygenase, a system absent in S. cerevisiae.

Mitochondrial death pathways in yeast and mammalian cells

In mammals, mitochondria are important mediators of programmed cell death, and this process is often regulated by Bcl-2 family proteins. However, a role for mitochondria-mediated cell death in non-mammalian species is more controversial. New evidence from a variety of sources suggests that mammalian mitochondrial fission/division proteins also have the capacity to promote programmed cell death, which may involve interactions with Bcl-2 family proteins. Homologues of these fission factors and several additional mammalian cell death regulators are conserved in flies, worms and yeast, and have been suggested to regulate programmed cell death in these species as well. However, the molecular mechanisms by which these phylogenetically conserved proteins contribute to cell death are not known for any species. Some have taken the conserved pro-death activity of mitochondrial fission factors to mean that mitochondrial fission per se, or failed attempts to undergo fission, are directly involved in cell death. Other evidence suggests that the fission function and the cell death function of these factors are separable. Here we consider the evidence for these arguments and their implications regarding the origins of programmed cell death.

Apoptotic signals induce specific degradation of ribosomal RNA in yeast

Organisms exposed to reactive oxygen species, generated endogenously during respiration or by environmental conditions, undergo oxidative stress. Stress response can either repair the damage or activate one of the programmed cell death (PCD) mechanisms, for example apoptosis, and finally end in cell death. One striking characteristic, which accompanies apoptosis in both vertebrates and yeast, is a fragmentation of cellular DNA and mammalian apoptosis is often associated with degradation of different RNAs. We show that in yeast exposed to stimuli known to induce apoptosis, such as hydrogen peroxide, acetic acid, hyperosmotic stress and ageing, two large subunit ribosomal RNAs, 25S and 5.8S, became extensively degraded with accumulation of specific intermediates that differ slightly depending on cell death conditions. This process is most likely endonucleolytic, is correlated with stress response, and depends on the mitochondrial respiratory status: rRNA is less susceptible to degradation in respiring cells with functional defence against oxidative stress. In addition, RNA fragmentation is independent of two yeast apoptotic factors, metacaspase Yca1 and apoptosis-inducing factor Aif1, but it relies on the apoptotic chromatin condensation induced by histone H2B modifications. These data describe a novel phenotype for certain stress- and ageing-related PCD pathways in yeast.

Formic acid induces Yca1p-independent apoptosis-like cell death in the yeast Saccharomyces cerevisiae

Formic acid disrupts mitochondrial electron transport and sequentially causes cell death in mammalian ocular cells by an unidentified molecular mechanism. Here, we show that a low concentration of formic acid induces apoptosis-like cell death in the budding yeast Saccharomyces cerevisiae, with several morphological and biochemical changes that are typical of apoptosis, including chromatin condensation, DNA fragmentation, externalization of phosphatidylserine, reactive oxygen species (ROS) production, loss of mitochondrial membrane potential and mitochondrion destruction. This process may not be dependent on the activation of Yca1p, the yeast caspase counterpart. In addition, the cell death induced by formic acid is associated with ROS burst,while intracellular ROS accumulate more rapidly and to a higher level in the YCA1 disruptant than in the wild-type strain during the progression of cell death. Our data indicate that formic acid induces yeast apoptosis via an Yca1p-independent pathway and it could be used as an extrinsic inducer for identifying the regulators downstream of ROS production in yeast.

Apoptosis pathways in fungal growth, development and ageing

Apoptosis is one type of programmed cell death with great importance for development and homeostasis of multicellular organisms. Unexpectedly, during the past decade, evidence has been obtained for the existence of a basal apoptosis machinery in yeast, as unicellular fungus, and in some filamentous fungi, a group of microorganisms that are neither true unicellular nor true multicellular biological systems but something in between. Here, we review evidence for a role of apoptotic processes in fungal pathogenicity, competitiveness, propagation, ageing and lifespan control.

Chronological aging-induced apoptosis in yeast

Saccharomyces cerevisiae is the simplest among the major eukaryotic model organisms for aging and diseases. Longevity in the chronological life span paradigm is measured as the mean and maximum survival period of populations of non-dividing yeast. This paradigm has been used successfully to identify several life-regulatory genes and three evolutionary conserved pro-aging pathways. More recently, Schizosaccharomyces pombe has been shown to age chronologically in a manner that resembles that of S. cerevisiae and that depends on the activity of the homologues of two pro-aging proteins previously identified in the budding yeast. Both yeast show features of apoptotic death during chronological aging. Here, we review some fundamental aspects of the genetics of chronological aging and the overlap between yeast aging and apoptotic processes with particular emphasis on the identification of an aging/death program that favors the dedifferentiation and regrowth of a few better adapted mutants generated within populations of aging S. cerevisiae. We also describe the use of a genome-wide screening technique to gain further insights into the mechanisms of programmed death in populations of chronologically aging S. cerevisiae.

Negative regulation of apoptosis in yeast

In recent years, yeast has been proven to be a useful model organism for studying programmed cell death. It not only exhibits characteristic markers of apoptotic cell death when heterologous inducers of apoptosis are expressed or when treated with apoptosis inducing drugs such as hydrogen peroxide (H(2)O(2)) or acetic acid, but contains homologues of several components of the apoptotic machinery identified in mammals, flies and nematodes, such as caspases, apoptosis inducing factor (AIF), Omi/HtrA2 and inhibitor-of-apoptosis proteins (IAPs). In this review, we focus on the role of negative regulators of apoptosis in yeasts. Bir1p is the only IAP protein in Saccharomyces cerevisiae and has long been known to play a role in cell cycle progression by acting as kinetochore and chromosomal passenger protein. Recent data established Bir1p's protective function against programmed cell death induced by H(2)O(2) treatment and in chronological ageing. Other factors that have a direct or indirect influence on intracellular levels of reactive oxygen species (ROS) and thus lead to apoptosis if they are misregulated or non-functional will be discussed.

Formulations for protecting the probiotic Saccharomyces boulardii from degradation in acidic condition

Saccharomyces boulardii is a nonpathogenic yeast with proven health benefits, some of them depending on its viability. However, the living yeast is sensitive to environmental conditions and its viability is less than 1% in the faeces after oral administration. Therefore, we assessed the survival conditions of S. boulardii in aqueous suspension and in its freeze-dried form and we formulated microspheres with the former and tablets with the latter in order to preserve the viability of the probiotic. While the viability of the yeast in aqueous suspension could be maintained for one year at -20 degrees C and +5 degrees C, increasing the temperature led to almost total mortality within 14 d at +40 degrees C and 4 d at +60 degrees C. The viability of the freeze-dried yeast was preserved for one year at +25 degrees C without moisture. With 75% relative humidity, the mortality was significant at 28 d at +25 degrees C and almost total within 1 d at +60 degrees C. In vitro, whereas less than 1% of non-encapsulated or non-tabletted S. boulardii survived after 120 min at pH 1.1, both formulations in microspheres and direct compression enabled to protect the yeast from degradation in HCl and to release it viable at pH 6.8. However, despite a similar release profile from both dosage forms, the compression led to a significant decrease in the viability of the freeze-dried yeast. In conclusion, although both formulations are efficient in protecting S. boulardii in acidic condition, microspheres provide a higher entrapment efficiency and a faster release of the viable probiotic in intestinal condition than matrix tablets.

Caspase-independent apoptosis in yeast

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. Yeast counterparts of central metazoan apoptotic regulators, such as metacaspase Yca1p, have been identified. In spite of the importance of Yca1p in yeast apoptotic process, many other factors such as Aif1p, orthologs of EndoG, AMID and cyclophilin D play important roles in caspase-independent apoptotic pathways. This review summarized recent progress about studies of various intrinsic and extrinsic apoptotic stimuli that may induce yeast cell death via caspase-independent apoptosis.

Caspase-dependent apoptosis in yeast

Damaging environment, certain intracellular defects or heterologous expression of pro-apoptotic genes induce death in yeast cells exhibiting typical markers of apoptosis. In mammals, apoptosis can be directed by the activation of groups of proteases, called caspases, that cleave specific substrates and trigger cell death. In addition, in plants, fungi, Dictyostelium and metazoa, paracaspases and metacaspases have been identified that share some homologies with caspases but showing different substrate specificity. In the yeast Saccharomyces cerevisiae, a gene (MCA1/YCA1) has been identified coding for a metacaspase involved in the induction of cell death. Metacaspases are not biochemical, but sequence and functional homologes of caspases, as deletion of them rescues entirely different death scenarios. In this review we will summarize the current knowledge in S. cerevisiae on apoptotic processes, induced by internal and external triggers, which are dependent on the metacaspase gene YCA1.

Reactive oxygen species and yeast apoptosis

Apoptosis is associated in many cases with the generation of reactive oxygen species (ROS) in cells across a wide range of organisms including lower eukaryotes such as the yeast Saccharomyces cerevisiae. Currently there are many unresolved questions concerning the relationship between apoptosis and the generation of ROS. These include which ROS are involved in apoptosis, what mechanisms and targets are important and whether apoptosis is triggered by ROS damage or ROS are generated as a consequence or part of the cellular disruption that occurs during cell death. Here we review the nature of the ROS involved, the damage they cause to cells, summarise the responses of S. cerevisiae to ROS and discuss those aspects in which ROS affect cell integrity that may be relevant to the apoptotic process.

Alterations in mitochondrial morphology of Schizosaccharomyces pombe induced by cell-death promoting agents

The effect of the yeast cell-death inducing agents, Bax and acetic acid, on mitochondrial structure of Schizosaccharomyces pombe was studied. Comparison of mitochondrial structures in cells grown on different substrates and visualized with different probes revealed variations in their morphology. Cells grown on respiratory C sources as well as in the presence of antimycin A exhibited punctuated mitochondria when visualized with mitochondrially targeted green fluorescent protein, while they still appeared as tubular structures when stained with DiOC6(3). Both expression of Bax and acetic acid treatment induced fragmentation and aggregation of mitochondrial network, which could be prevented by coexpression of Bcl-XL. Aberrant mitochondrial morphology generated by either Bax or acetic acid was not accompanied with the loss of mitochondrial genome (mtDNA), indicating that alterations of mitochondrial morphology following death stimuli follow different mechanisms than those involved in mitochondrial inheritance mutants.

A transient proteasome activation is needed for acetic acid-induced programmed cell death to occur in Saccharomyces cerevisiae

To gain further insight into the mechanism by which yeast programmed cell death (PCD) occurs, we investigated whether and how proteasome activity changes in Saccharomyces cerevisiae cells undergoing PCD as a result of treatment with acetic acid (AA-PCD). We show that proteasome activation starts 60 min after AA-PCD induction, with a maximum at 90 min, and decreases at 150 min. Moreover, cell survival measurements carried out in the absence or presence of MG132, which inhibits proteasome function, show that the inhibition of proteasome activity partially prevents AA-PCD, thus indicating that a transient proteasome activation is needed for AA-PCD to occur.

Isc1p plays a key role in hydrogen peroxide resistance and chronological lifespan through modulation of iron levels and apoptosis

The inositolphosphosphingolipid phospholipase C (Isc1p) of Saccharomyces cerevisiae belongs to the family of neutral sphingomyelinases that generates the bioactive sphingolipid ceramide. In this work the role of Isc1p in oxidative stress resistance and chronological lifespan was investigated. Loss of Isc1p resulted in a higher sensitivity to hydrogen peroxide that was associated with an increase in oxidative stress markers, namely intracellular oxidation, protein carbonylation, and lipid peroxidation. Microarray analysis showed that Isc1p deficiency up-regulated the iron regulon leading to increased levels of iron, which is known to catalyze the production of the highly reactive hydroxyl radicals via the Fenton reaction. In agreement, iron chelation suppressed hydrogen peroxide sensitivity of isc1Delta mutants. Cells lacking Isc1p also displayed a shortened chronological lifespan associated with oxidative stress markers and aging of parental cells was correlated with a decrease in Isc1p activity. The analysis of DNA fragmentation and caspase-like activity showed that Isc1p deficiency increased apoptotic cell death associated with oxidative stress and aging. Furthermore, deletion of Yca1p metacaspase suppressed the oxidative stress sensitivity and premature aging phenotypes of isc1Delta mutants. These results indicate that Isc1p plays an important role in the regulation of cellular redox homeostasis, through modulation of iron levels, and of apoptosis.

Catalase T and Cu,Zn-superoxide dismutase in the acetic acid-induced programmed cell death in Saccharomyces cerevisiae

To investigate the role of catalase and superoxide dismutase (SOD) in the acetic acid (AA) induced yeast programmed cell death (AA-PCD), we compared Saccharomyces cerevisiae cells (C-Y) and cells individually over-expressing catalase T (CTT1-Y) and Cu,Zn-SOD (SOD1-Y) with respect to cell survival, hydrogen peroxide (H2O2) levels and enzyme activity as measured up to 200 min after AA treatment. AA-PCD does not occur in CTT1-Y, where H2O2 levels were lower than in C-Y and the over-expressed catalase activity decreased with time. In SOD1-Y, AA-PCD was exacerbated; high H2O2 levels were found, SOD activity increased early, remaining constant en route to AA-PCD, but catalase activity was strongly reduced.

Old yellow enzymes, highly homologous FMN oxidoreductases with modulating roles in oxidative stress and programmed cell death in yeast

In a genetic screen to identify modifiers of Bax-dependent lethality in yeast, the C terminus of OYE2 was isolated based on its capacity to restore sensitivity to a Bax-resistant yeast mutant strain. Overexpression of full-length OYE2 suppresses Bax lethality in yeast, lowers endogenous reactive oxygen species (ROS), increases resistance to H(2)O(2)-induced programmed cell death (PCD), and significantly lowers ROS levels generated by organic prooxidants. Reciprocally, Delta oye2 yeast strains are sensitive to prooxidant-induced PCD. Overexpression and knock-out analysis indicate these OYE2 antioxidant activities are opposed by OYE3, a highly homologous heterodimerizing protein, which functions as a prooxidant promoting H(2)O(2)-induced PCD in wild type yeast. To exert its effect OYE3 requires the presence of OYE2. Deletion of the 12 C-terminal amino acids and catalytic inactivation of OYE2 by a Y197F mutation enhance significantly survival upon H(2)O(2)-induced PCD in wild type cells, but accelerate PCD in Delta oye3 cells, implicating the oye2p-oye3p heterodimer for promoting cell death upon oxidative stress. Unexpectedly, a strain with a double knock-out of these genes (Delta oye2 oye3) is highly resistant to H(2)O(2)-induced PCD, exhibits increased respiratory capacity, and undergoes less cell death during the adaptive response in chronological aging. Simultaneous deletion of OYE2 and other antioxidant genes hyperinduces endogenous levels of ROS, promoting H(2)O(2)-induced cell death: in Delta oye2 glr1 yeast high levels of oxidized glutathione elicited gross morphological aberrations involving the actin cytoskeleton and defects in organelle partitioning. Altering the ratio of reduced to oxidized glutathione by exogenous addition of GSH fully reversed these alterations. Based on this work, OYE proteins are firmly placed in the signaling network connecting ROS generation, PCD modulation, and cytoskeletal dynamics in yeast.

Hydrogen peroxide and superoxide anion production during acetic acid-induced yeast programmed cell death

Hydrogen peroxide production in yeast cells undergoing programmed cell death in response to acetic acid occurred in the majority of live cells 15 min after death induction and was no longer detectable after 60 min. Superoxide anion production was found later, 60 and 90 min after death induction when cells viability was 60 and 30%, respectively. In cells protected from death due to acid stress adaptation neither hydrogen peroxide nor superoxide anion could be observed after acetic acid treatment. The early production of hydrogen peroxide in cells in which survival was 100% could play a major role in acetic acid-induced programmed cell death signaling. Superoxide anion is assumed to be generated in cells already en route to acetic acid-induced programmed cell death.

Copper and manganese induce yeast apoptosis via different pathways

Metal ions are essential as well as toxic to the cell. The mechanism of metal-induced toxicity is not well established. Here, for the first time we studied two essential nutritional elements, copper and manganese, for their apoptotic effects in yeast Saccharomyces cerevisiae. Although beneficial at subtoxic levels, we demonstrated that at moderately toxic levels, both metals induce extensive apoptosis in yeast cells. At even higher concentrations, necrosis takes over. Furthermore, we investigated the molecular pathways mediating Cu- and Mn-mediated apoptotic action. Mitochondria-defective yeast exhibit a much reduced apoptotic marker expression and better survival under Cu and Mn stress, indicating mitochondria are involved in both Cu- and Mn-induced apoptosis. Reactive oxygen species (ROS) are generated in high amounts in Cu- but not in Mn-induced cell death, and Cu toxicity can be alleviated by overexpression of superoxide dismutase 2, suggesting ROS mediate Cu but not Mn toxicity. Yeast metacaspase Yca1p is not involved in Cu-induced apoptosis, although it plays an important role in the Mn-induced process. A genetic screen identified Cpr3p, a yeast cyclophilin D homologue, as mediating the Cu-induced apoptotic program. Cpr3p mutant seems to eliminate Cu-induced apoptosis without affecting ROS production, while leaving necrosis intact. These results may provide important insight into a detailed understanding at the molecular and cellular level of metal toxicity and metal accumulation diseases.

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.

ADP/ATP carrier is required for mitochondrial outer membrane permeabilization and cytochrome c release in yeast apoptosis

Adenine nucleotide translocator (ANT) is a mitochondrial inner membrane protein involved in the ADP/ATP exchange and is a component of the mitochondrial permeability transition pore (PTP). In mammalian apoptosis, the PTP can mediate mitochondrial outer membrane permeabilization (MOMP), which is suspected to be responsible for the release of apoptogenic factors, including cytochrome c. Although release of cytochrome c in yeast apoptosis has previously been reported, it is not known how it occurs. Herein we used yeast genetics to investigate whether depletion of proteins putatively involved in MOMP and cytochrome c release affects these processes in yeast. While deletion of POR1 (yeast voltage-dependent anion channel) enhances apoptosis triggered by acetic acid, H(2)O(2) and diamide, CPR3 (mitochondrial cyclophilin) deletion had no effect. Absence of ADP/ATP carrier (AAC) proteins, yeast orthologues of ANT, protects cells exposed to acetic acid and diamide but not to H(2)O(2). Expression of a mutated form of Aac2p (op1) exhibiting very low ADP/ATP translocase activity indicates that AAC's pro-death role does not require translocase activity. Absence of AAC proteins impairs MOMP and release of cytochrome c, which, together with other mitochondrial inner membrane proteins, is degraded. Our findings point to a crucial role of AAC in yeast apoptosis.

Generic and specific transcriptional responses to different weak organic acids in anaerobic chemostat cultures ofSaccharomyces cerevisiae

Transcriptional responses to four weak organic acids (benzoate, sorbate, acetate and propionate) were investigated in anaerobic, glucose-limited chemostat cultures of Saccharomyces cerevisiae. To enable quantitative comparison of the responses to the acids, their concentrations were chosen such that they caused a 50% decrease of the biomass yield on glucose. The concentration of each acid required to achieve this yield was negatively correlated with membrane affinity. Microarray analysis revealed that each acid caused hundreds of transcripts to change by over twofold relative to reference cultures without added organic acids. However, only 14 genes were consistently upregulated in response to all acids. The moderately lipophilic compounds benzoate and sorbate and, to a lesser extent, the less lipophilic acids acetate and propionate showed overlapping transcriptional responses. Statistical analysis for overrepresented functional categories and upstream regulatory elements indicated that responses to the strongly lipophilic acids were focused on genes related to the cell wall, while acetate and propionate had a stronger impact on membrane-associated transport processes. The fact that S. cerevisiae exhibits a minimal generic transcriptional response to weak organic acids along with extensive specific responses is relevant for interpreting and controlling weak acid toxicity in food products and in industrial fermentation processes.

Protein expression profiles in Saccharomyces cerevisiae during apoptosis induced by H2O2

We identified the proteins involved during apoptosis induced by H2O2 in Saccharomyces cerevisiae, and analyzed the global protein pattern by 2-DE. We analyzed classical parameters of apoptosis such as chromatin condensation, DNA fragmentation, and morphology changes of cells. Exposure of yeast cells to nonphysiological doses of peroxides decreases the expression (or increases degradation) of enzymes involved in protection against oxidative stress. This leads the yeast cells to a reduction of their antioxidant defense and makes the cells more prone to apoptosis. In our data the down expression of peroxiredoxin II and GST I, could induce a perturbation of mitochondrial function with an alteration of permeability of the membrane leading to the mitochondria-mediated apoptosis. Moreover, we identified a new spot of a classical glycolytic enzyme: the glyceraldehyde 3-phosphate dehydrogenase during apoptosis. It is known that GAPDH is an extremely abundant glycolytic enzyme with multiple functions and that its overexpression is evident during apoptosis induced by a variety of stimuli. Our results confirm that it is a major intracellular messenger mediating apoptotic death and that this new spot of GAPDH could be an intracellular sensor of oxidative stress during apoptosis induced by H2O2 in S. cerevisiae.

Induction of apoptosis in Saccharomyces cerevisiae results in the spontaneous maturation of tetravirus procapsids in vivo

The Tetraviridae are a family of small, non-enveloped, insect RNA viruses consisting of one or two single-stranded, positive-sense genomic RNAs encapsidated in an icosahedral capsid with T=4 symmetry. Tetravirus procapsids undergo maturation when exposed to a low pH environment in vitro. While the structural biology of the conformational changes that mediate acid-dependent maturation is well understood, little is known about the significance of acid-dependent maturation in vivo. To address this question, the capsid-coding sequence of the tetravirus Helicoverpa armigera stunt virus was expressed in Saccharomyces cerevisiae cells. Virus-like particles were shown to assemble as procapsids that matured spontaneously in vivo as the cells began to age. Growth in the presence of hydrogen peroxide or acetic acid, which induced apoptosis or programmed cell death in the yeast cells, resulted in virus-like particle maturation. The results demonstrate that assembly-dependent maturation of tetravirus procapsids in vivo is linked to the onset of apoptosis in yeast cells. We propose that the reduction in pH required for tetraviral maturation may be the result of cytosolic acidification, which is associated with the early onset of programmed cell death in infected cells.

Arsenic induces caspase- and mitochondria-mediated apoptosis in Saccharomyces cerevisiae

In recent years, it has been shown that yeast, a unicellular organism, undergoes apoptosis in response to various factors. Here we demonstrate that the highly effective anticancer agent arsenic induces apoptotic process in yeast cells. Reactive oxygen species (ROS) production was observed in the process. Moreover, mitochondrial membrane potential decreased after arsenic treatment. Resistance of the rho(0) mutant strain (lacking mtDNA) to arsenic provides further evidence that this death process involves mitochondria. In addition, hypersensitivity of Deltasod1 to arsenic suggests the critical role of ROS. Cell death and DNA fragmentation decreased in a Deltayca1 deletion mutant, indicating the participation of yeast caspase-1 protein in apoptosis. The implications of these findings for arsenic-induced apoptosis are discussed.

Kinetics and metabolic behavior of a composite culture of Kloeckera apiculata and Saccharomyces cerevisiae wine related strains

The kinetics and metabolic behavior of Kloeckera apiculata mc1 and Saccharomyces cerevisiae mc2 in composite culture was investigated. K. apiculata showed a higher viability through the fermentation; however the maximum cell density of both yeasts decreased. This behavior was not due to ethanol concentration, killer toxins production or competition for assimilable nitrogenous compounds between both yeasts. Despite the consistent production of secondary products by single culture of K. apiculata, an increase of these compounds was not observed in mixed culture. These results contribute to a better understanding of the behavior of non-Saccharomyces yeasts and their potential application in the wine industry.

Hog1p mitogen-activated protein kinase determines acetic acid resistance in Saccharomyces cerevisiae

When glucose-repressed, Saccharomyces cerevisiae cannot use acetic acid as a carbon source and is inhibited in growth by high levels of this compound, especially at low pH. Cultures exposed to a 100 mM acetate stress activate both the Hog1p and Slt2p stress-activated MAP kinases. Nevertheless, only active Hog1p, not Slt2p, is needed for the acquisition of acetate resistance. Hog1p undergoes more rapid activation by acetate in pH 4.5, than in pH 6.8 cultures, an indication that the acid may have to enter the cells in order to generate the Hog1p activatory signal. Acetate activation of Hog1p is absent in the ssk1Delta and pbs2Delta mutants, but is present in sho1Delta and ste11Delta, showing that it involves the Sln1p branch of the high-osmolarity glycerol (HOG) pathway signaling to Pbs2p. In low-pH (pH 4.5) cultures, the acetate-activated Hog1p, although conferring acetate resistance, does not generate the GPD1 gene or intracellular glycerol inductions that are hallmarks of activation of the HOG pathway by hyperosmotic stress.

Yeast apoptosis—From genes to pathways

Yeast are eukaryotic unicellular organisms that are easy to cultivate and offer a wide spectrum of genetic and cytological tools for research. Yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have successfully been used as models for human cell division cycle. Stress conditions, cellular ageing, failed mating, certain mutations or heterologous expression of proapoptotic genes induce yeast cell death with the characteristic markers of apoptosis. Several crucial regulators of apoptosis are conserved between metazoans and yeast. This simple model organism offers the possibility to identify conserved and new components of the apoptotic machinery and to elucidate the regulatory pathways beyond.

Endonuclease G regulates budding yeast life and death

Endonuclease G (EndoG) is located in mitochondria yet translocates into the nucleus of apoptotic cells during human degenerative diseases. Nonetheless, a direct involvement of EndoG in cell-death execution remains equivocal, and the mechanism for mitochondrio-nuclear translocation is not known. Here, we show that the yeast homolog of EndoG (Nuc1p) can efficiently trigger apoptotic cell death when excluded from mitochondria. Nuc1p induces apoptosis in yeast independently of metacaspase or of apoptosis inducing factor. Instead, the permeability transition pore, karyopherin Kap123p, and histone H2B interact with Nuc1p and are required for cell death upon Nuc1p overexpression, suggesting a pathway in which mitochondrial pore opening, nuclear import, and chromatin association are successively involved in EndoG-mediated death. Deletion of NUC1 diminishes apoptotic death when mitochondrial respiration is increased but enhances necrotic death when oxidative phosphorylation is repressed, pointing to dual--lethal and vital--roles for EndoG.