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

Vacuole-mitochondrial cross-talk during apoptosis in yeast: a model for understanding lysosome-mitochondria-mediated apoptosis in mammals

The yeast apoptosis field emerged with the finding that key components of the apoptotic machinery are conserved in these simple eukaryotes. Thus it became possible to exploit these genetically tractable organisms to improve our understanding of the intricate mechanisms of cell death in higher eukaryotes and of severe human diseases associated with apoptosis dysfunctions. Early on, it was recognized that a mitochondria-mediated apoptotic pathway showing similarities to the mammalian intrinsic pathway was conserved in yeast. Recently, lysosomes have also emerged as central players in mammalian apoptosis. Following LMP (lysosomal membrane permeabilization), lysosomal proteases such as cathepsins B, D and L are released into the cytosol and can trigger a mitochondrial apoptotic cascade. CatD (cathepsin D) can also have anti-apoptotic effects in some cellular types and specific contexts. Nonetheless, the mechanisms underlying LMP and the specific role of cathepsins after their release into the cytosol remain poorly understood. We have recently shown that yeast vacuoles, membrane-bound acidic organelles, which share many similarities to plant vacuoles and mammalian lysosomes, are also involved in the regulation of apoptosis and that the vacuolar protease Pep4p, orthologue of the human CatD, is released from the vacuole into the cytosol in response to acetic acid. Here, we discuss how the conservation of cell-death regulation mechanisms in yeast by the lysosome-like organelle and mitochondria may provide new insights into the understanding of the complex interplay between the mitochondria and lysosome-mediated signalling routes during mammalian apoptosis.

mRNA stability and control of cell proliferation

Most of the studies on cell proliferation examine the control of gene expression by specific transcription factors that act on transcriptional initiation. In the last few years, it became evident that mRNA stability/turnover provides an important mechanism for post-transcriptional control of gene expression. In eukaryotes, mRNAs are mainly degraded after deadenylation by decapping and exosome pathways. Mechanisms of mRNA surveillance comprise deadenylation-independent pathways such as NMD (nonsense-mediated decay), when mRNAs harbour a PTC (premature termination codon), NSD (non-stop decay, when mRNAs lack a termination codon, and NGD (no-go decay), when mRNA translation elongation stalls. Many proteins involved in these processes are conserved from bacteria to yeast and humans. Recent papers showed the involvement of proteins deputed to decapping in controlling cell proliferation, virus replication and cell death. In this paper, we will review the newest findings in this field.

Experimental evolution of multicellularity

Multicellularity was one of the most significant innovations in the history of life, but its initial evolution remains poorly understood. Using experimental evolution, we show that key steps in this transition could have occurred quickly. We subjected the unicellular yeast Saccharomyces cerevisiae to an environment in which we expected multicellularity to be adaptive. We observed the rapid evolution of clustering genotypes that display a novel multicellular life history characterized by reproduction via multicellular propagules, a juvenile phase, and determinate growth. The multicellular clusters are uniclonal, minimizing within-cluster genetic conflicts of interest. Simple among-cell division of labor rapidly evolved. Early multicellular strains were composed of physiologically similar cells, but these subsequently evolved higher rates of programmed cell death (apoptosis), an adaptation that increases propagule production. These results show that key aspects of multicellular complexity, a subject of central importance to biology, can readily evolve from unicellular eukaryotes.

Encapsulation of yeast cells in colloidosomes

Polymeric colloidosomes encapsulating viable Baker's yeast cells were prepared. To make the capsules, an aqueous suspension of 153 nm poly(methyl methacrylate-co-butyl acrylate) latex particles plus yeast cells is emulsified in a continuous phase of sunflower oil. By adding a small amount of ethanol to the oil phase, the latex particles at the surface of the emulsion droplets aggregate, forming the colloidosome shells. The microcapsules have been examined using optical, confocal, and scanning electron microscopies. The viability of the yeast cells was tested using fluorescent molecular probes. The encapsulated Baker's yeast cells were able to metabolize glucose from solution, although at a slower rate compared to nonencapsulated yeast. This demonstrates diffusion limitation through the colloidosome shell. The diffusive resistance could be increased by manufacturing colloidosomes with a double latex shell.

Yeast aging and apoptosis

A concerted balance between proliferation and apoptosis is essential to the survival of multicellular organisms. Thus, apoptosis per se, although it is a destructive process leading to the death of single cells, also serves as a pro-survival mechanism pro-survival mechanism that ensures healthy organismal development and acts as a life-prolonging or anti-aging anti-aging program. The discovery that yeast also possess a functional and, in many cases, highly conserved apoptotic machinery has made it possible to study the relationships between aging and apoptosis in depth using a well-established genetic system and the powerful tools available to yeast researchers for investigating complex physiological and cytological interactions. The aging process of yeast, be it replicative replicative or chronological chronological aging, is closely related to apoptosis, although it remains unclear whether apoptosis is a causal feature of the aging process or vice versa. Nevertheless, experimental results obtained during the past several years clearly demonstrate that yeast serve as a powerful and versatile experimental system for understanding the interconnections between these two fundamentally important cellular and physiological pathways.

A novel role for protein kinase Gcn2 in yeast tolerance to intracellular acid stress

Intracellular pH conditions many cellular systems, but its mechanisms of regulation and perception are mostly unknown. We have identified two yeast genes important for tolerance to intracellular acidification caused by weak permeable acids. One corresponded to LEU2 and functions by removing the dependency of the leu2 mutant host strain on uptake of extracellular leucine. Leucine transport is inhibited by intracellular acidification, and either leucine oversupplementation or overexpression of the transporter gene BAP2 improved acid growth. Another acid-tolerance gene is GCN2, encoding a protein kinase activated by uncharged tRNAs during amino acid starvation. Gcn2 phosphorylates eIF2α (eukaryotic initiation factor 2α) (Sui2) at Ser51 and this inhibits general translation, but activates that of Gcn4, a transcription factor for amino acid biosynthetic genes. Intracellular acidification activates Gcn2 probably by inhibition of aminoacyl-tRNA synthetases because we observed accumulation of uncharged tRNAleu without leucine depletion. Gcn2 is required for leucine transport and a gcn2-null mutant is sensitive to acid stress if auxotrophic for leucine. Gcn4 is required for neither leucine transport nor acid tolerance, but a S51A sui2 mutant is acid-sensitive. This suggests that Gcn2, by phosphorylating eIF2α, may activate translation of an unknown regulator of amino acid transporters different from Gcn4.

Trichothecene mycotoxins inhibit mitochondrial translation--implication for the mechanism of toxicity

Fusarium head blight (FHB) reduces crop yield and results in contamination of grains with trichothecene mycotoxins. We previously showed that mitochondria play a critical role in the toxicity of a type B trichothecene. Here, we investigated the direct effects of type A and type B trichothecenes on mitochondrial translation and membrane integrity in Saccharomyces cerevisiae. Sensitivity to trichothecenes increased when functional mitochondria were required for growth, and trichothecenes inhibited mitochondrial translation at concentrations, which did not inhibit total translation. In organello translation in isolated mitochondria was inhibited by type A and B trichothecenes, demonstrating that these toxins have a direct effect on mitochondrial translation. In intact yeast cells trichothecenes showed dose-dependent inhibition of mitochondrial membrane potential and reactive oxygen species, but only at doses higher than those affecting mitochondrial translation. These results demonstrate that inhibition of mitochondrial translation is a primary target of trichothecenes and is not secondary to the disruption of mitochondrial membranes.

Achievements and perspectives in yeast acetic acid-induced programmed cell death pathways

The use of non-mammalian model organisms, including yeast Saccharomyces cerevisiae, can provide new insights into eukaryotic PCD (programmed cell death) pathways. In the present paper, we report recent achievements in the elucidation of the events leading to PCD that occur as a response to yeast treatment with AA (acetic acid). In particular, ROS (reactive oxygen species) generation, cyt c (cytochrome c) release and mitochondrial function and proteolytic activity will be dealt with as they vary along the AA-PCD time course by using both wild-type and mutant yeast cells. Two AA-PCD pathways are described sharing common features, but distinct from one another with respect to the role of ROS and mitochondria, the former in which YCA1 acts upstream of cyt c release and caspase-like activation in a ROS-dependent manner and the latter in which cyt c release does not occur, but caspase-like activity increases, in a ROS-independent manner.

Acetate but not propionate induces oxidative stress in bakers' yeast Saccharomyces cerevisiae

The influence of acetic and propionic acids on baker's yeast was investigated in order to expand our understanding of the effect of weak organic acid food preservatives on eukaryotic cells. Both acids decreased yeast survival in a concentration-dependent manner, but with different efficiencies. The acids inhibited the fluorescein efflux from yeast cells. The inhibition constant of fluorescein extrusion from cells treated with acetate was significantly lower in parental strain than in either PDR12 (ABC-transporter Pdr12p) or WAR1 (transcriptional factor of Pdr12p) defective mutants. The constants of inhibition by propionate were virtually the same in all strains used. Yeast exposure to acetate increased the level of oxidized proteins and the activity of antioxidant enzymes, while propionate did not change these parameters. This suggests that various mechanisms underlie the yeast toxicity by acetic and propionic acids. Our studies with mutant cells clearly indicated the involvement of Yap1p transcriptional regulator and de novo protein synthesis in superoxide dismutase up-regulation by acetate. The up-regulation of catalase was Yap1p independent. Yeast pre-incubation with low concentrations of H₂O₂ caused cellular cross-protection against high concentrations of acetate. The results are discussed from the point of view that acetate induces a prooxidant effect in vivo, whereas propionate does not.

ROS production and apoptosis induction by formation of Gts1p-mediated protein aggregates

GTS1 of Saccharomyces cerevisiae is a pleiotropic gene. Its induction leads to a variety of biological phenomena represented by cell aggregation. The C-terminal polyglutamine sequence in Gts1p is indispensable for its pleiotropy and nuclear localization. This sequence is often observed in polyglutamine diseases, such as Huntington disease, and is believed to induce protein aggregation, leading to cell death. In this study, protein aggregates were formed in a polyglutamine-dependent manner in cells inducing GTS1, and heat-shock protein family, translation elongation factor, and mitochondrial proteins were trapped in Gts1p-mediated protein aggregates. Moreover, the polyglutamine sequence of Gts1p was indispensable to the induction of reactive oxygen species (ROS) production and apoptosis. Deletion of the genes encoding Por1p and Yhb1p altered the profiles of ROS production and apoptosis caused by GTS1 induction, suggesting that the trapping of these proteins in Gts1p-mediated protein aggregates inhibits the intrinsic functions of these proteins.

A genomic analysis of chronological longevity factors in budding yeast

Chronological life span (CLS) has been studied as an aging paradigm in yeast. A few conserved aging genes have been identified that modulate both chronological and replicative longevity in yeast as well as longevity in the nematode Caenorhabditis elegans; however, a comprehensive analysis of the relationship between genetic control of chronological longevity and aging in other model systems has yet to be reported. To address this question, we performed a functional genomic analysis of chronological longevity for 550 single-gene deletion strains, which accounts for approximately 12% of the viable homozygous diploid deletion strains in the yeast ORF deletion collection. This study identified 33 previously unknown determinants of CLS. We found no significant enrichment for enhanced CLS among deletions corresponding to yeast orthologs of worm aging genes or among replicatively long-lived deletion strains, although a trend toward overlap was noted. In contrast, a subset of gene deletions identified from a screen for reduced acidification of culture media during growth to stationary phase was enriched for increased CLS. These results suggest that genetic control of CLS under the most commonly utilized assay conditions does not strongly overlap with longevity determinants in C. elegans, with the existing confined to a small number of genetic pathways. These data also further support the model that acidification of the culture medium plays an important role in survival during chronological aging in synthetic medium, and suggest that chronological aging studies using alternate medium conditions may be more informative with regard to aging of multicellular eukaryotes.

Identification of autophagy genes participating in zinc-induced necrotic cell death in Saccharomyces cerevisiae

Eukaryotes use a common set of genes to perform two mechanistically similar autophagic processes. Bulk autophagy harvests proteins nonselectively and reuses their constitutents when nutrients are scarce. In contrast, different forms of selective autophagy target protein aggregates or damaged organelles that threaten to interfere with growth. Yeast uses one form of selective autophagy, called cytoplasm-to-vacuole targeting (Cvt), to engulf two vacuolar enzymes in Cvt vesicles ("CVT-somes") within which they are transported to vacuoles for maturation. While both are dispensable normally, bulk and selective autophagy help sustain life under stressful conditions. Consistent with this view, knocking out several genes participating in Cvt and specialized autophagic pathways heightened the sensitivity of Saccharomyces cerevisiae to inhibitory levels of Zn(2+). The loss of other autophagic genes, and genes responsible for apoptotic cell death, had no such effect. Unexpectedly, the loss of members of a third set of autophagy genes heightened cellular resistance to zinc as if they encoded proteins that actively contributed to zinc-induced cell death. Further studies showed that both sensitive and resistant strains accumulated similar amounts of H2O2 during zinc treatments, but that more sensitive strains showed signs of necrosis sooner. Although zinc lethality depended on autophagic proteins, studies with several reporter genes failed to reveal increased autophagic activity. In fact, microscopy analysis indicated that Zn(2+) partially inhibited fusion of Cvt vesicles with vacuoles. Further studies into how the loss of autophagic processes suppressed necrosis in yeast might reveal whether a similar process could occur in plants and animals.

Impacts of variations in elemental nutrient concentration of Chardonnay musts on Saccharomyces cerevisiae fermentation kinetics and wine composition

Chardonnay, being the predominant white wine-grape cultivar in the Australian wine sector, is subject to widely varying winemaking processes with the aim of producing a variety of wine styles. Therefore, juice composition might not always be ideal for optimal fermentation outcomes. Our aim was to better understand the composition of Chardonnay juice and how compositional parameters impact on fermentation outcomes. This was achieved through a survey of 96 commercially prepared Chardonnay juices during the 2009 vintage. Common juice variables were estimated using near infrared spectroscopy, and elemental composition was determined using radial view inductively coupled plasma optical emission spectrometry. The influence of elemental composition on fermentation outcomes was assessed by fermentation of a defined medium formulated to reflect the composition and range of concentrations as determined by the juice survey. Yeast (Saccharomyces cerevisiae) strain effects were also assessed. Key parameters influencing fermentation outcomes were verified by laboratory scale fermentation of Chardonnay juice. This exploration of Chardonnay juice identified interactions between juice pH and potassium concentration as key factors impacting on fermentation performance and wine quality. Outcomes differed depending on yeast strain.

Molecular characterization of propolis-induced cell death in Saccharomyces cerevisiae

Propolis, a natural product of plant resins, is used by the bees to seal holes in their honeycombs and protect the hive entrance. However, propolis has also been used in folk medicine for centuries. Here, we apply the power of Saccharomyces cerevisiae as a model organism for studies of genetics, cell biology, and genomics to determine how propolis affects fungi at the cellular level. Propolis is able to induce an apoptosis cell death response. However, increased exposure to propolis provides a corresponding increase in the necrosis response. We showed that cytochrome c but not endonuclease G (Nuc1p) is involved in propolis-mediated cell death in S. cerevisiae. We also observed that the metacaspase YCA1 gene is important for propolis-mediated cell death. To elucidate the gene functions that may be required for propolis sensitivity in eukaryotes, the full collection of about 4,800 haploid S. cerevisiae deletion strains was screened for propolis sensitivity. We were able to identify 138 deletion strains that have different degrees of propolis sensitivity compared to the corresponding wild-type strains. Systems biology revealed enrichment for genes involved in the mitochondrial electron transport chain, vacuolar acidification, negative regulation of transcription from RNA polymerase II promoter, regulation of macroautophagy associated with protein targeting to vacuoles, and cellular response to starvation. Validation studies indicated that propolis sensitivity is dependent on the mitochondrial function and that vacuolar acidification and autophagy are important for yeast cell death caused by propolis.

Over-Expression of YLR162W in Saccharomyces cerevisiae Inhibits Cell Proliferation and Renders Cells Susceptible to the Hypoxic Conditions Induced by Cobalt Chloride

YLR162W is an uncharacterized Saccharomyces cerevisiae ORF whose transcript level is elevated in cells under environmental stress, during α-factor response and in stationary phase. We obtained a partial cDNA clone of YLR162W by subtractive hybridization cloning of genes that were not expressed in a CoCl(2) resistant DNA synthesis mutant but expressed in its wild type counterpart. Our studies demonstrated that YLR162W transcript level was reduced in BY4741 cells upon exposure to the hypoxia mimetic agent CoCl(2), and continuous expression of full length YLR162W from a plasmid borne copy of the gene rendered BY4741 cells extremely susceptible to the hypoxic conditions induced by CoCl(2). At initial time points following the induction of YLR162W expression, cell cycle progression was inhibited with the emergence of a distinct sub-G1 peak indicative of apoptotic cells, mitochondrial membrane potential was also decreased along with an increase in the fraction of cells permeable to propidium iodide; none of the above was further affected by CoCl(2). The up-regulation of Ylr162wp in cells exposed to environmental stress and in non-replicating cells appears to be related to its growth inhibitory properties presented in this report.

Resistance of yeasts to weak organic acid food preservatives

Carboxylate weak acids are invaluable for large-scale food and beverage preservation. However, in response to safety concerns, there is now desire to reduce the use of these additives. The resistance to these compounds displayed by spoilage yeasts and fungi is a major reason why these preservatives often have to be used in millimolar levels. This chapter summarizes the mechanisms whereby yeasts are rendered resistant to acetate, propionate, sorbate, and benzoate. In baker's yeast (Saccharomyces cerevisiae), resistance to high acetic acid is acquired partly by loss of the plasma membrane aquaglyceroporin that facilitates the passive diffusional entry of undissociated acid into cells (Fps1), and partly through a transcriptional response mediated by the transcription factor Haa1. Other carboxylate preservatives are too large to enter cells through the Fps1 channel but instead penetrate at appreciable rates by passive diffusion across the plasma membrane. In Saccharomyces and Candida albicans though not, it seems, in the Zygosaccharomyces, resistance to the latter acids involves activation of the War1 transcription factor, which in turn generates strong induction of a specific plasma membrane ATP-binding cassette transporter (Pdr12). The latter actively pumps the preservative anion from the cell. Other contributors to weak acid resistance include enzymes that allow preservative degradation, members of the Tpo family of major facilitator superfamily transporters, and changes to the cell envelope that minimize the diffusional entry of the preservative into the cell.

Genome-wide analysis of yeast aging

In the past several decades the budding yeast Saccharomyces cerevisiae has emerged as a prominent model for aging research. The creation of a single-gene deletion collection covering the majority of open reading frames in the yeast genome and advances in genomic technologies have opened yeast research to genome-scale screens for a variety of phenotypes. A number of screens have been performed looking for genes that modify secondary age-associated phenotypes such as stress resistance or growth rate. More recently, moderate-throughput methods for measuring replicative life span and high-throughput methods for measuring chronological life span have allowed for the first unbiased screens aimed at directly identifying genes involved in determining yeast longevity. In this chapter we discuss large-scale life span studies performed in yeast and their implications for research related to the basic biology of aging.

Chronological aging in Saccharomyces cerevisiae

The two paradigms to study aging in Saccharomyces cerevisiae are the chronological life span (CLS) and the replicative life span (RLS). The chronological life span is a measure of the mean and maximum survival time of non-dividing yeast populations while the replicative life span is based on the mean and maximum number of daughter cells generated by an individual mother cell before cell division stops irreversibly. Here we review the principal discoveries associated with yeast chronological aging and how they are contributing to the understanding of the aging process and of the molecular mechanisms that may lead to healthy aging in mammals. We will focus on the mechanisms of life span regulation by the Tor/Sch9 and the Ras/adenylate Ras/adenylate cyclase/PKA pathways with particular emphasis on those implicating age-dependent oxidative oxidative stress stress and DNA damage/repair.

Lead induces oxidative stress and phenotypic markers of apoptosis in Saccharomyces cerevisiae

In the present work, the mode of cell death induced by Pb in Saccharomyces cerevisiae was studied. Yeast cells Pb-exposed, up to 6 h, loss progressively the capacity to proliferate and maintained the membrane integrity evaluated by the fluorescent probes bis(1,3-dibutylbarbituric acid trimethine oxonol) and propidium iodide. Pb-induced death is an active process, requiring the participation of cellular metabolism, since the simultaneous addition of cycloheximide attenuated the loss of cell proliferation capacity. Cells exposed to Pb accumulated intracellularly reactive oxygen species (ROS), evaluated by 2',7'-dichlorodihydrofluorescein diacetate. The addition of ascorbic acid (a ROS scavenger) strongly reduced the oxidative stress and impaired the loss of proliferation capacity in Pb-treated cells. Pb-exposed cells displayed nuclear morphological alterations, like chromatin fragmentation, as revealed by diaminophenylindole staining. Together, the data obtained indicate that yeast cells exposition to 1 mmol/l Pb results in severe oxidative stress which can be the trigger of programmed cell death by apoptosis.

Secondary necrosis: the natural outcome of the complete apoptotic program

The predominant definition of apoptosis considers that the elimination of the apoptosing cell is by heterolytic degradation following phagocytosis by an assisting scavenger (efferocytosis). However, an alternative and largely underestimated outcome of apoptosis is secondary necrosis, an autolytic process of cell disintegration with release of cell components that occurs when there is no intervention of scavengers and the full apoptotic program is completed. Secondary necrosis is the typical outcome of apoptosis in unicellular eukaryotes but, importantly, it may also occur in multicellular animals and has been implicated in the genesis of important human pathologies. Secondary necrosis is a mode of cell elimination with specific molecular and morphological features and should be considered the natural outcome of the complete apoptotic program.

Drug resistance marker-aided genome shuffling to improve acetic acid tolerance in Saccharomyces cerevisiae

Acetic acid existing in a culture medium is one of the most limiting constraints in yeast growth and viability during ethanol fermentation. To improve acetic acid tolerance in Saccharomyces cerevisiae strains, a drug resistance marker-aided genome shuffling approach with higher screen efficiency of shuffled mutants was developed in this work. Through two rounds of genome shuffling of ultraviolet mutants derived from the original strain 308, we obtained a shuffled strain YZ2, which shows significantly faster growth and higher cell viability under acetic acid stress. Ethanol production of YZ2 (within 60 h) was 21.6% higher than that of 308 when 0.5% (v/v) acetic acid was added to fermentation medium. Membrane integrity, higher in vivo activity of the H+-ATPase, and lower oxidative damage after acetic acid treatment are the possible reasons for the acetic acid-tolerance phenotype of YZ2. These results indicated that this novel genome shuffling approach is powerful to rapidly improve the complex traits of industrial yeast strains.

Global transcriptional analysis of yeast cell death induced by mutation of sister chromatid cohesin

Cohesin is a protein complex that regulates sister chromatid cohesin during cell division. Malfunction in chromatid cohesin results in chromosome missegregation and aneuploidy. Here, we report that mutations of MCD1 and PDS5, two major components of cohesin in budding yeast, cause apoptotic cell death, which is characterized by externalization of phosphatidylserine at cytoplasmic membrane, chromatin condensation and fragmentation, and ROS production. Microarray analysis suggests that the cell death caused by mutation of MCD1 or PDS5 is due to the internal stress response, contrasting to the environmental or external stress response induced by external stimuli, such as hydrogen peroxide. A common feature shared by the internal stress response and external stress response is the response to stimulus, including response to DNA damage, mitochondria functions, and oxidative stress, which play an important role in yeast apoptotic cell death.

Bre1p-mediated histone H2B ubiquitylation regulates apoptosis in Saccharomyces cerevisiae

BRE1 encodes an E3 ubiquitin protein ligase that is required for the ubiquitylation of histone H2B at lysine 123 (K123). Ubiquitylation of this histone residue is involved in a variety of cellular processes including gene activation and gene silencing. Abolishing histone H2B ubiquitylation also confers X-ray sensitivity and abrogates checkpoint activation after DNA damage. Here we show that Saccharomyces cerevisiae Bre1p exhibits anti-apoptotic activity in yeast and that this is linked to histone H2B ubiquitylation. We found that enhanced levels of Bre1p protect from hydrogen-peroxide-induced cell death, whereas deletion of BRE1 enhances cell death. Moreover, cells lacking Bre1p show reduced lifespan during chronological ageing, a physiological apoptotic condition in yeast. Importantly, the resistance against apoptosis is conferred by histone H2B ubiquitylation mediated by the E3 ligase activity of Bre1p. Furthermore, we found that the death of Δbre1 cells depends on the yeast caspase Yca1p, because Δbre1 cells exhibit increased caspase activity when compared with wild-type cells, and deletion of YCA1 leads to reduced apoptosis sensitivity of cells lacking Bre1p.

Exit from the Golgi is required for the expansion of the autophagosomal phagophore in yeast Saccharomyces cerevisiae

The delivery of proteins and organelles to the vacuole by autophagy involves membrane rearrangements that result in the formation of autophagosomes. We have investigated the role of the Golgi in autophagy and found that, in yeast, this organelle plays a crucial role in supplying lipid bilayers necessary for autophagosome biogenesis.

The delivery of proteins and organelles to the vacuole by autophagy involves membrane rearrangements that result in the formation of large vesicles called autophagosomes. The mechanism underlying autophagosome biogenesis and the origin of the membranes composing these vesicles remains largely unclear. We have investigated the role of the Golgi complex in autophagy and have determined that in yeast, activation of ADP-ribosylation factor (Arf)1 and Arf2 GTPases by Sec7, Gea1, and Gea2 is essential for this catabolic process. The two main events catalyzed by these components, the biogenesis of COPI- and clathrin-coated vesicles, do not play a critical role in autophagy. Analysis of the sec7 strain under starvation conditions revealed that the autophagy machinery is correctly assembled and the precursor membrane cisterna of autophagosomes, the phagophore, is normally formed. However, the expansion of the phagophore into an autophagosome is severely impaired. Our data show that the Golgi complex plays a crucial role in supplying the lipid bilayers necessary for the biogenesis of double-membrane vesicles possibly through a new class of transport carriers or a new mechanism.

Cr-(III)-organic compounds treatment causes genotoxicity and changes in DNA and protein level in Saccharomyces cerevisiae

Natural Cr-(III)-organic species are being known as the part of natural biogeochemical cycle of chromium, but unfortunately, their mechanism of toxicity as well as genotoxic potentiality is still unknown. To evaluate the characteristic toxic effect exerted by natural Cr-(III)-organic species on the cellular macromolecules, changes in DNA and protein level was observed. Besides, Comet assay was applied to measure genotoxic potentiality of Cr-(III)-organic species in the target organism Saccharomyces cerevisiae exposed to Cr-(III)-citrate and Cr-(III)-histidine. It has been observed that both of the Cr-(III)-organic compounds are responsible for diminution in macromolecules concentration. Cr-(III)-citrate showed ladder pattern of DNA fragmentation in support of apoptosis. Two new protein bands appeared in protein profile of the Saccharomyces cerevisiae treated with Cr-(III)-organic compounds. Thus it supports the possibility of the synthesis of stress proteins. Comet assay proved positive correlation between Cr-(III)-organic compounds' concentration and DNA damage. The Cr-(III)-citrate causes DNA damage at the concentrations ranging from 50 to 150 mg L(-1), whereas the DNA damaging capacity of Cr-(III)-histidine was found insignificant, except at highest concentration (150 mg L(-1)). These results can throw light on the mechanism of the toxic effect as well as genotoxicity exerted by natural Cr-(III)-organic species.

The anthracenedione compound bostrycin induces mitochondria-mediated apoptosis in the yeast Saccharomyces cerevisiae

Bostrycin is an anthracenedione with phytotoxic and antibacterial activity that belongs to the large family of quinones. We have isolated bostrycin from the secondary metabolites of a mangrove endophytic fungus, no. 1403, collected from the South China Sea. Using the yeast Saccharomyces cerevisiae as a model, we show that bostrycin inhibits cell proliferation by blocking the cell cycle at G1 phase and ultimately leads to cell death in a time- and dose-dependent manner. Bostrycin-induced lethal cytotoxicity is accompanied with increased levels of intracellular reactive oxygen species and hallmarks of apoptosis such as chromatin condensation, DNA fragmentation and externalization of phosphatidylserine. We further show that bostrycin decreases mitochondrial membrane electric potential and causes mitochondrial destruction during the progression of cell death. Bostrycin-induced cell death was promoted in YCA1 null yeast strain but was partially rescued in AIF1 null mutant both in fermentative and respiratory media, strongly indicating that bostrycin induces apoptosis in yeast cells through a mitochondria-mediated but caspase-independent pathway.

Characterization of human VDAC isoforms: a peculiar function for VDAC3?

VDACs are a family of pore-forming proteins mainly located in the mitochondrial outer membrane. In mammals three isoforms exist. In this work we review the information available about them with the addition of new results. We have compared the human VDACs transformed in a yeast strain lacking the endogenous porin. VDAC1 and 2 are able to complement the lack of porin in mitochondrial respiration and modulation of ROS. VDAC3 has a limited ability to support the mitochondrial respiration and has no influence in the control of ROS production. The over-expression of VDAC isoforms in wild type yeast strain led to a dramatic sensitivity to oxidative stress, especially for VDAC3, and a shorter lifespan in respiratory conditions. Real-time PCR comparison of the isoforms indicated that in HeLa cells VDAC1 is 10 times more abundant than VDAC2 and 100 times than VDAC3. The over-expression of any single isoform caused a 10 times increase of the transcripts of VDAC2 and VDAC3, while VDAC1 is not changed by the over-expression of the other isoforms. Models of VDAC2 and VDAC3 isoform structure showed that they could be made of a 19-strand beta-barrel and an N-terminal sequence with variable features. In this work we show for the first time a functional characterization of VDAC3 in a cellular context.

Nuclear localisation is crucial for the proapoptotic activity of the HtrA-like serine protease Nma111p

Programmed cell death is induced by the activation of a subset of intracellular proteins in response to specific extra- and intracellular signals. In the yeast Saccharomyces cerevisiae, Nma111p functions as a nuclear serine protease that is necessary for apoptosis under cellular stress conditions, such as elevated temperature or treatment of cells with hydrogen peroxide to induce cell death. We have examined the role of nuclear protein import in the function of Nma111p in apoptosis. Nma111p contains two small clusters of basic residues towards its N-terminus, both of which are necessary for efficient translocation into the nucleus. Nma111p does not shuttle between the nucleus and cytoplasm during either normal growth conditions or under environmental stresses that induce apoptosis. The N-terminal half of Nma111p is sufficient to provide the apoptosis-inducing activity of the protein, and the nuclear-localisation signal (NLS) sequences and catalytic serine 235 are both necessary for this function. We provide compelling evidence that intranuclear Nma111p activity is necessary for apoptosis in yeast.

Controlled mixed culture fermentation: a new perspective on the use of non-Saccharomyces yeasts in winemaking

Mixed fermentations using controlled inoculation of Saccharomyces cerevisiae starter cultures and non-Saccharomyces yeasts represent a feasible way towards improving the complexity and enhancing the particular and specific characteristics of wines. The profusion of selected starter cultures has allowed the more widespread use of inoculated fermentations, with consequent improvements to the control of the fermentation process, and the use of new biotechnological processes in winemaking. Over the last few years, as a consequence of the re-evaluation of the role of non-Saccharomyces yeasts in winemaking, there have been several studies that have evaluated the use of controlled mixed fermentations using Saccharomyces and different non-Saccharomyces yeast species from the wine environment. The combined use of different species often results in unpredictable compounds and/or different levels of fermentation products being produced, which can affect both the chemical and the aromatic composition of wines. Moreover, possible synergistic interactions between different yeasts might provide a tool for the implementation of new fermentation technologies. Thus, knowledge of the Saccharomyces and non-Saccharomyces wine yeast interactions during wine fermentation needs to be improved. To reach this goal, further investigations into the genetic and physiological background of such non-Saccharomyces wine yeasts are needed, so as to apply '-omics' approaches to mixed culture fermentations.

Calnexin regulates apoptosis induced by inositol starvation in fission yeast

Inositol is a precursor of numerous phospholipids and signalling molecules essential for the cell. Schizosaccharomyces pombe is naturally auxotroph for inositol as its genome does not have a homologue of the INO1 gene encoding inositol-1-phosphate synthase, the enzyme responsible for inositol biosynthesis. In this work, we demonstrate that inositol starvation in S. pombe causes cell death with apoptotic features. This apoptotic death is dependent on the metacaspase Pca1p and is affected by the UPR transducer Ire1p. Previously, we demonstrated that calnexin is involved in apoptosis induced by ER stress. Here, we show that cells expressing a lumenal version of calnexin exhibit a 2-fold increase in the levels of apoptosis provoked by inositol starvation. This increase is reversed by co-expression of a calnexin mutant spanning the transmembrane domain and C-terminal cytosolic tail. Coherently, calnexin is physiologically cleaved at the end of its lumenal domain, under normal growth conditions when cells approach stationary phase. This cleavage suggests that the two naturally produced calnexin fragments are needed to continue growth into stationary phase and to prevent cell death. Collectively, our observations indicate that calnexin takes part in at least two apoptotic pathways in S. pombe, and suggest that the cleavage of calnexin has regulatory roles in apoptotic processes involving calnexin.

Presence of the Fps1p aquaglyceroporin channel is essential for Hog1p activation, but suppresses Slt2(Mpk1)p activation, with acetic acid stress of yeast

When grown at pH 4.5, Saccharomyces cerevisiae acquires a resistance to inhibitory acetic acid levels ( approximately 0.1 M) by destabilizing Fps1p, the plasma membrane aquaglyceroporin that provides the main route for passive diffusional entry of this acid into the cell. Acetic acid stress transiently activates Hog1p mitogen-activated protein (MAP) kinase, which, in turn, phosphorylates Fps1p in order to target this channel for endocytosis and degradation in the vacuole. This activation of Hog1p is abolished with the loss of Fps1p, but is more sustained when cells express an open Fps1p channel refractory to destabilization. At neutral pH, much higher levels of acetate ( approximately 0.5 M) are needed to inhibit growth. Under such conditions, the loss of Fps1p does not abolish, but merely slows, the activation of Hog1p. Acetate stress also activates the Slt2(Mpk1)p cell integrity MAP kinase, possibly by causing inhibition of glucan synthase activity. In pH 4.5 cultures, this acetate activation of Slt2p is strongly enhanced by the loss of Fps1p and is dependent upon the cell surface sensor Wsc1p. Lack of Fps1p therefore exerts opposing effects on the activation of Hog1p and Slt2p in yeast exposed to acetic acid stress.

Bax Inhibitor-1, a conserved cell death suppressor, is a key molecular switch downstream from a variety of biotic and abiotic stress signals in plants

In Nature plants are constantly challenged by a variety of environmental stresses that could lead to disruptions in cellular homeostasis. Programmed cell death (PCD) is a fundamental cellular process that is often associated with defense responses to pathogens, during development and in response to abiotic stresses in fungi, animals and plants. Although there are many characteristics shared between different types of PCD events, it remains unknown whether a common mechanism drives various types of PCD in eukaryotes. One candidate regulator for such a mechanism is Bax Inhibitor-1 (BI-1), an evolutionary conserved, endoplasmic reticulum (ER)-resident protein that represents an ancient cell death regulator that potentially regulates PCD in all eukaryotes. Recent findings strongly suggested that BI-1 plays an important role in the conserved ER stress response pathway to modulate cell death induction in response to multiple types of cell death signals. As ER stress signaling pathways has been suggested to play important roles not only in the control of ER homeostasis but also in other biological processes such as the response to pathogens and abiotic stress in plants, BI-1 might function to control the convergence point that modulates the level of the “pro-survival and pro-death” signals under multiple stress conditions.

A molecular mechanism of chronological aging in yeast

The molecular mechanisms that cause organismal aging are a topic of intense scrutiny and debate. Dietary restriction extends the life span of many organisms, including yeast, and efforts are underway to understand the biochemical and genetic pathways that regulate this life span extension in model organisms. Here we describe the mechanism by which dietary restriction extends yeast chronological life span, defined as the length of time stationary yeast cells remain viable in a quiescent state. We find that aging under standard culture conditions is the result of a cell-extrinsic component that is linked to the pH of the culture medium. We identify acetic acid as a cell-extrinsic mediator of cell death during chronological aging, and demonstrate that dietary restriction, growth in a non-fermentable carbon source, or transferring cells to water increases chronological life span by reducing or eliminating extracellular acetic acid. Other life span extending environmental and genetic interventions, such as growth in high osmolarity media, deletion of SCH9 or RAS2, increase cellular resistance to acetic acid. We conclude that acetic acid induced mortality is the primary mechanism of chronological aging in yeast under standard conditions.

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.

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.