Role of mitochondria and C-terminal membrane anchor of Bcl-2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae

Apoptotic Factors Are Evolutionarily Conserved Since Mitochondrial Domestication

The mechanisms initiating apoptotic programmed cell death in diverse eukaryotes are very similar. Basically, the mitochondrial permeability transition activates apoptotic proteases, DNases, and flavoproteins such as apoptosis-inducing factors (AIFs). According to the hypothesis of the endosymbiotic origin of apoptosis, these mechanisms evolved during mitochondrial domestication. Various phylogenetic analyses, including ours, have suggested that apoptotic factors were eubacterial protomitochondrial toxins used for killing protoeukaryotic hosts. Here, we tested whether the function of yeast Saccharomyces cerevisiae apoptotic proteases (metacaspases Mca1 and Nma111), DNase Nuc1, and flavoprotein Ndi1 can be substituted with orthologs from remotely related eukaryotes such as plants, protists, and eubacteria. We found that orthologs of remotely related eukaryotic and even eubacterial proteins can initiate apoptosis in yeast when triggered by chemical stresses. This observation suggests that apoptotic mechanisms have been maintained since mitochondrial domestication, which occurred approximately 1,800 Mya. Additionally, it supports the hypothesis that some of these apoptotic factors could be modified eubacterial toxins.

Protective Effects of Cirsium japonicum var. maackii Flower on Amyloid Beta25-35-Treated C6 Glial Cells

Amyloid beta (Aβ) is a neurotoxic peptide and a key factor causing Alzheimer's disease. Cirsium japonicum var. maackii (CJM) has neuroprotective effects, but the protective effects of the flower from CJM (FCJM) on the neural system remain unclear. This study aimed to identify the fraction of FCJM with the highest neuroprotective potential and investigate its protective mechanisms against Aβ_25-35-induced inflammation in C6 glial cells. The cell viability and generation of reactive oxygen species (ROS) were measured to investigate the positive effect of FCJM on oxidative stress. Treatment with the FCJM extract or fractions increased the cell viability to 60-70% compared with 52% in the Aβ_25-35-treated control group and decreased ROS production to 84% compared with 100% in the control group. The ethyl acetate fraction of FCJM (EFCJM) was the most effective among all the extracts and fractions. We analyzed the protective mechanisms of EFCJM on Aβ_25-35-induced inflammation in C6 glial cells using Western blot. EFCJM downregulated amyloidogenic pathway-related proteins, such as Aβ precursor protein, β-secretase, presenilin 1, and presenilin 2. Moreover, EFCJM attenuated the Bax/Bcl-2 ratio, an index of apoptosis, and upregulated the oxidative stress-related protein, heme oxygenase-1. Therefore, this study demonstrated that FCJM improves cell viability and inhibits ROS in Aβ_25-35-treated C6 glial cells. Furthermore, EFCJM exhibits neuroprotective effects in Aβ_25-35-induced inflammation in C6 glial cells by modulating oxidative stress and amyloidogenic and apoptosis signaling pathways. FCJM, especially EFCJM, can be a promising agent for neurodegenerative disease prevention.

Acetic acid triggers cytochrome c release in yeast heterologously expressing human Bax

Proteins of the Bcl-2 protein family, including pro-apoptotic Bax and anti-apoptotic Bcl-xL, are critical for mitochondrial-mediated apoptosis regulation. Since yeast lacks obvious orthologs of Bcl-2 family members, heterologous expression of these proteins has been used to investigate their molecular and functional aspects. Active Bax is involved in the formation of mitochondrial outer membrane pores, through which cytochrome c (cyt c) is released, triggering a cascade of downstream apoptotic events. However, when in its inactive form, Bax is largely cytosolic or weakly bound to mitochondria. Given the central role of Bax in apoptosis, studies aiming to understand its regulation are of paramount importance towards its exploitation as a therapeutic target. So far, studies taking advantage of heterologous expression of human Bax in yeast to unveil regulation of Bax activation have relied on the use of artificial mutated or mitochondrial tagged Bax for its activation, rather than the wild type Bax (Bax α). Here, we found that cell death could be triggered in yeast cells heterologoulsy expressing Bax α with concentrations of acetic acid that are not lethal to wild type cells. This was associated with Bax mitochondrial translocation and cyt c release, closely resembling the natural Bax function in the cellular context. This regulated cell death process was reverted by co-expression with Bcl-xL, but not with Bcl-xLΔC, and in the absence of Rim11p, the yeast ortholog of mammalian GSK3β. This novel system mimics human Bax α regulation by GSK3β and can therefore be used as a platform to uncover novel Bax regulators and explore its therapeutic modulation.

Contribution of Yeast Studies to the Understanding of BCL-2 Family Intracellular Trafficking

BCL-2 family members are major regulators of apoptotic cell death in mammals. They form an intricate regulatory network that ultimately regulates the release of apoptogenic factors from mitochondria to the cytosol. The ectopic expression of mammalian BCL-2 family members in the yeast Saccharomyces cerevisiae, which lacks BCL-2 homologs, has been long established as a useful addition to the available models to study their function and regulation. In yeast, individual proteins can be studied independently from the whole interaction network, thus providing insight into the molecular mechanisms underlying their function in a living context. Furthermore, one can take advantage of the powerful tools available in yeast to probe intracellular trafficking processes such as mitochondrial sorting and interactions/exchanges between mitochondria and other compartments, such as the endoplasmic reticulum that are largely conserved between yeast and mammals. Yeast molecular genetics thus allows the investigation of the role of these processes on the dynamic equilibrium of BCL-2 family members between mitochondria and extramitochondrial compartments. Here we propose a model of dynamic regulation of BCL-2 family member localization, based on available evidence from ectopic expression in yeast.

EDS1-Dependent Cell Death and the Antioxidant System in Arabidopsis Leaves is Deregulated by the Mammalian Bax

Cell death is the ultimate end of a cell cycle that occurs in all living organisms during development or responses to biotic and abiotic stresses. In the course of evolution, plants and animals evolve various molecular mechanisms to regulate cell death; however, some of them are conserved among both these kingdoms. It was found that mammalian proapoptotic BCL-2 associated X (Bax) protein, when expressed in plants, induces cell death, similar to hypersensitive response (HR). It was also shown that changes in the expression level of genes encoding proteins involved in stress response or oxidative status regulation mitigate Bax-induced plant cell death. In our study, we focused on the evolutional compatibility of animal and plant cell death molecular mechanisms. Therefore, we studied the deregulation of reactive oxygen species burst and HR-like propagation in Arabidopsis thaliana expressing mammalian Bax. We were able to diminish Bax-induced oxidative stress and HR progression through the genetic cross with plants mutated in ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), which is a plant-positive HR regulator. Plants expressing the mouse Bax gene in eds1-1 null mutant background demonstrated less pronounced cell death and exhibited higher antioxidant system efficiency compared to Bax-expressing plants. Moreover, eds1/Bax plants did not show HR marker genes induction, as in the case of the Bax-expressing line. The present study indicates some common molecular features between animal and plant cell death regulation and can be useful to better understand the evolution of cell death mechanisms in plants and animals.

Apoptosis, Induced by Human α-Synuclein in Yeast, Can Occur Independent of Functional Mitochondria

Human α-synuclein expression in baker’s yeast reportedly induces mitochondria-dependent apoptosis. Surprisingly, we find that, under de-repressing conditions of the inducible MET25/GAL1 promoters, yeast cells expressing chromosomally-integrated copies of the human α-synuclein gene are not killed, but spontaneously form respiration-deficient rho-minus (ρ⁻) petites. Although yeast cells can undergo cell death (apoptosis) from loss of mitochondrial function, they can also survive without functional mitochondria. Such cells are referred to as ρ⁰ or ρ⁻ petites. This study reports that minimal expression of human α-synuclein in yeast, from MET25/GAL1 promoter, gives rise to ρ⁻ petites. Interestingly, the full expression of α-synuclein, from the same promoters, in α-synuclein-triggered ρ⁻ petites and also in ρ⁰ petites (produced by treating ρ⁺ cells with the mutagen ethidium bromide) initiates apoptosis. The percentages of petites increase with increasing α-synuclein gene copy-number. ρ⁻ petites expressing α-synuclein from fully-induced MET25/GAL1 promoters exhibit increased ROS levels, loss of mitochondrial membrane potential, and nuclear DNA fragmentation, with increasing copies of α-synuclein. Our results indicate that, for the first time in yeast, α-synuclein-triggered apoptosis can occur independently of functional mitochondria. The observation that α-synuclein naturally forms petites and that they can undergo apoptosis may have important implications in understanding the pathogenesis of Parkinson’s disease.

Sensing the Generation of Intracellular Free Electrons Using the Inactive Catalytic Subunit of Cytochrome P450s as a Sink

Cytochrome P450 reductase (CPR) abstracts electrons from Nicotinamide adenine dinucleotide phosphate H (NADPH), transferring them to an active Cytochrome P450 (CYP) site to provide a functional CYP. In the present study, a yeast strain was genetically engineered to delete the endogenous CPR gene. A human CYP expressed in a CPR-null (yRD⁻) strain was inactive. It was queried if Bax—which induces apoptosis in yeast and human cells by generating reactive oxygen species (ROS)—substituted for the absence of CPR. Since Bax-generated ROS stems from an initial release of electrons, is it possible for these released electrons to be captured by an inactive CYP to make it active once again? In this study, yeast cells that did not contain any CPR activity (i.e., because the yeasts’ CPR gene was completely deleted) were used to show that (a) human CYPs produced within CPR-null (yRD-) yeast cells were inactive and (b) low levels of the pro-apoptotic human Bax protein could activate inactive human CYPs within this yeast cells. Surprisingly, Bax activated three inactive CYP proteins, confirming that it could compensate for CPR’s absence within yeast cells. These findings could be useful in research, development of bioassays, bioreactors, biosensors, and disease diagnosis, among others.

Classical swine fever virus Npro antagonises IRF3 to prevent IFN-independent TLR3 and RIG-I-mediated apoptosis

Classical swine fever virus (CSFV) is the causative agent of classical swine fever, a notifiable disease of economic importance that causes severe leukopenia, fever and haemorrhagic disease in domesticated pigs and wild boar across the globe. CSFV has been shown to antagonise the induction of type I IFN, partly through a function of its N-terminal protease (N^pro) which binds IRF3 and targets it for proteasomal degradation. Additionally, N^pro has been shown to antagonise apoptosis triggered by the dsRNA-homolog poly(I:C), however the exact mechanism by which this is achieved has not been fully elucidated. In this study we confirm the ability of N^pro to inhibit dsRNA-mediated apoptosis and show that N^pro is also able to antagonise Sendai virus-mediated apoptosis in PK-15 cells. Gene edited PK-15 cell lines were used to show the dsRNA-sensing pathogen recognition receptors (PRRs) TLR3 and RIG-I specifically respond to poly(I:C) and SeV respectively, subsequently triggering apoptosis through pathways that converge on IRF3 and culminate in the cleavage of caspase-3. Importantly, this IRF3-mediated apoptosis was found to be dependent on transcription-independent functions of IRF3 and also on Bax, a pro-apoptotic Bcl-2 family protein, through a direct interaction between the two proteins. Deletion of IRF3, stable expression of N^pro and infection with wild-type CSFV were found to antagonise the mitochondrial localisation of Bax, a key hallmark of the intrinsic, mitochondrial pathway of apoptosis. Together, these findings show that N^pro's putative interaction with IRF3 is involved not only in its antagonism of type I IFN, but also dsRNA-mediated mitochondrial apoptosis.Importance Responsible for severe haemorrhagic disease in domestic pigs and wild boar, classical swine fever is recognised by the World Organisation for Animal Health (OIE) and European Union as a notifiable disease of economic importance. Persistent infection, immunotolerance and early dissemination of the virus at local sites of infection have been linked to the antagonism of type I IFN induction by N^pro This protein may further contribute to these phenomena by antagonising the induction of dsRNA-mediated apoptosis. Ultimately, apoptosis is an important innate mechanism by which cells counter viruses at local sites of infection, thus preventing wider spread and dissemination within the host, potentially also contributing to the onset of persistence. Elucidation of the mechanism by which Npro antagonises the apoptotic response will help inform the development of rationally-designed live-attenuated vaccines and antivirals for control of outbreaks in typically CSFV-free countries.

Reconstituting the Mammalian Apoptotic Switch in Yeast

Proteins of the Bcl-2 family regulate the permeabilization of the mitochondrial outer membrane that represents a crucial irreversible step in the process of induction of apoptosis in mammalian cells. The family consists of both proapoptotic proteins that facilitate the membrane permeabilization and antiapoptotic proteins that prevent it in the absence of an apoptotic signal. The molecular mechanisms, by which these proteins interact with each other and with the mitochondrial membranes, however, remain under dispute. Although yeast do not have apparent homologues of these apoptotic regulators, yeast cells expressing mammalian members of the Bcl-2 family have proved to be a valuable model system, in which action of these proteins can be effectively studied. This review focuses on modeling the activity of proapoptotic as well as antiapoptotic proteins of the Bcl-2 family in yeast.

Overexpression of the transcription factor HAC1 improves nerolidol production in engineered yeast

Increasing the metabolic flux of the mevalonate pathway, reducing the metabolic flux of competing pathway and utilizing the diauxie-inducible system constructed by GAL promoters are strategies commonly used in yeast metabolic engineering for the production of terpenoids. Using these strategies, we constructed a series of yeast strains with a strengthened mevalonate pathway and finally produced 336.5 mg/L nerolidol in a shake flask. The spliced HAC1 mRNA assay indicated that the unfolded protein response (UPR) occurred in the strains that we constructed. UPR strains exhibited the low transcriptional activities of GAL1 promoter. HAC1-overexpressing strain exhibited dramatically enhanced transcriptional activity of GAL1 promoter at 72 h of fermentation in flasks. HAC1 overexpression also increased the nerolidol titer by 47.7 %, reaching 497.0 mg/L and increased cell vitality. RNA-seq showed that the genes whose transcription responded to HAC1-overexpression were involved in the regulation of monocarboxylic acid metabolic processes and cellular amino acid biosynthetic process, indicating that the metabolic regulation may be part of the reason of the improved nerolidol synthesis. Our findings enrich the knowledge of the relationship between the construction of sesquiterpene-producing cell factories and UPR regulation. This study provides an effective strategy for sesquiterpene production in yeast.

Biotechnological Potential of LSD1, EDS1, and PAD4 in the Improvement of Crops and Industrial Plants

Lesion Simulating Disease 1 (LSD1), Enhanced Disease Susceptibility (EDS1) and Phytoalexin Deficient 4 (PAD4) were discovered a quarter century ago as regulators of programmed cell death and biotic stress responses in Arabidopsis thaliana. Recent studies have demonstrated that these proteins are also required for acclimation responses to various abiotic stresses, such as high light, UV radiation, drought and cold, and that their function is mediated through secondary messengers, such as salicylic acid (SA), reactive oxygen species (ROS), ethylene (ET) and other signaling molecules. Furthermore, LSD1, EDS1 and PAD4 were recently shown to be involved in the modification of cell walls, and the regulation of seed yield, biomass production and water use efficiency. The function of these proteins was not only demonstrated in model plants, such as Arabidopsis thaliana or Nicotiana benthamiana, but also in the woody plant Populus tremula x tremuloides. In addition, orthologs of LSD1, EDS1, and PAD4 were found in other plant species, including different crop species. In this review, we focus on specific LSD1, EDS1 and PAD4 features that make them potentially important for agricultural and industrial use.

Radicicol rescues yeast cell death triggered by expression of human α-synuclein and its A53T mutant, but not by human βA4 peptide and proapoptotic protein bax

Aggregation/misfolding of α-synuclein and βA4 proteins cause neuronal cell death (NCD) associated with Parkinson's and Alzheimer's disease. It has been suggested that a heat shock protein-90 (Hsp90) inhibitor can prevent NCD by activating the heat shock transcription factor-1 which, in turn, upregulates molecular chaperones such as Hsp70 that targets aggregated/misfolded proteins for refolding/degradation. We have isolated radicicol, an Hsp90 inhibitor, from a fungus occurring in the crevices of marble rocks of Central India. Radicicol, which was found to be a strong antioxidant, was tested for its ability to rescue yeast cells from death induced by expression of wild-type α-synuclein, its more toxic A53T mutant, and βA4. It effectively overcomes wild-type/mutant α-synuclein mediated yeast cell death, concomitantly diminishes ROS levels, reverses mitochondrial dysfunction and prevents nuclear DNA-fragmentation, a hallmark of apoptosis. Surprisingly however, radicicol is unable to rescue yeast cells from death triggered by expression of secreted βA4. Moreover, although radicicol acts as an antioxidant it fails to prevent yeast cell death inflicted by the proapoptotic protein, Bax. Our results indicate that radicicol specifically targets aggregated/misfolded α-synuclein's toxicity and opens up the possibility of using multiple yeast assays to screen natural product libraries for compounds that would unambiguously target α-synuclein aggregation/misfolding.

Nonantioxidant Tetramethoxystilbene Abrogates α-Synuclein-Induced Yeast Cell Death but Not That Triggered by the Bax or βA4 Peptide

The overexpression of α-synuclein (α-syn) and its aggregation is the hallmark of Parkinson's disease. The α-syn aggregation results in the formation of Lewy bodies that causes neuronal cell death. Therefore, the small molecules that can protect neuronal cells from α-syn toxicity or inhibit the aggregation of α-syn could emerge as anti-Parkinson agents. Herein, a library of methoxy-stilbenes was screened for their ability to restore the cell growth from α-syn toxicity, using a yeast strain that stably expresses two copies of a chromosomally integrated human α-syn gene. Tetramethoxy-stilbene 4s, a nonantioxidant, was the most capable of restoring cell growth. It also rescues the more toxic cells that bear three copies of wild-type or A53T-mutant α-syn, from cell growth block. Its EC₅₀ values for growth restoration of the 2-copy wild-type and the 3-copy mutant α-syn strains are 0.95 and 0.35 μM, respectively. Stilbene 4s mitigates mitochondrial membrane potential loss, negates ROS production, and prevents nuclear DNA-fragmentation, all hallmarks of apoptosis. However, 4s does not rescue cells from the death-inducing effects of Bax and βA4, which suggest that 4s specifically inhibits α-syn-mediated toxicity in the yeast. Our results signify that simultaneous use of multiple yeast-cell-based screens can facilitate revelation of compounds that may have the potential for further investigation as anti-Parkinson's agents.

Biotransformation, Using Recombinant CYP450-Expressing Baker's Yeast Cells, Identifies a Novel CYP2D6.10A122V Variant Which Is a Superior Metabolizer of Codeine to Morphine Than the Wild-Type Enzyme

CYP2D6, a cytochrome P450 (CYP) enzyme, metabolizes codeine to morphine. Within the human body, 0-15% of codeine undergoes O-demethylation by CYP2D6 to form morphine, a far stronger analgesic than codeine. Genetic polymorphisms in wild-type CYP2D6 (CYP2D6-wt) are known to cause poor-to-extensive metabolism of codeine and other CYP2D6 substrates. We have established a platform technology that allows stable expression of human CYP genes from chromosomal loci of baker's yeast cells. Four CYP2D6 alleles, (i) chemically synthesized CYP2D6.1, (ii) chemically synthesized CYP2D6-wt, (iii) chemically synthesized CYP2D6.10, and (iv) a novel CYP2D6.10 variant CYP2D6-C (i.e., CYP2D6.10^A122V) isolated from a liver cDNA library, were cloned for chromosomal integration in yeast cells. When expressed in yeast, CYP2D6.10 enzyme shows weak activity compared with CYP2D6-wt and CYP2D6.1 which have moderate activity, as reported earlier. Surprisingly, however, the CYP2D6-C enzyme is far more active than CYP2D6.10. More surprisingly, although CYP2D6.10 is a known low metabolizer of codeine, yeast cells expressing CYP2D6-C transform >70% of codeine to morphine, which is more than twice that of cells expressing the extensive metabolizers, CYP2D6.1, and CYP2D6-wt. The latter two enzymes predominantly catalyze formation of codeine's N-demethylation product, norcodeine, with >55% yield. Molecular modeling studies explain the specificity of CYP2D6-C for O-demethylation, validating observed experimental results. The yeast-based CYP2D6 expression systems, described here, could find generic use in CYP2D6-mediated drug metabolism and also in high-yield chemical reactions that allow the formation of regio-specific dealkylation products.

'Traceless' tracing of proteins - high-affinity trans-splicing directed by a minimal interaction pair

Using a minimal lock-and-key element the affinity between the intein fragments for N-terminal protein trans-splicing was significantly increased, allowing for site-specific, ‘traceless’ covalent protein labeling in living mammalian cells at nanomolar probe concentrations.

Protein trans-splicing mediated by split inteins is a powerful technique for site-specific protein modification. Despite recent developments there is still an urgent need for ultra-small high-affinity intein tags for in vitro and in vivo approaches. To date, only very few in-cell applications of protein trans-splicing have been reported, all limited to C-terminal protein modifications. Here, we developed a strategy for covalent N-terminal intein-mediated protein labeling at (sub) nanomolar probe concentrations. Combined with a minimal synthetic lock-and-key element, the affinity between the intein fragments was increased more than 50-fold to 10 nM. Site-specific and efficient ‘traceless’ protein modification by high-affinity trans-splicing is demonstrated at nanomolar concentrations in living mammalian cells.

Yeast as a tool for studying proteins of the Bcl-2 family

Permeabilization of the outer mitochondrial membrane that leads to the release of cytochrome c and several other apoptogenic proteins from mitochondria into cytosol represents a commitment point of apoptotic pathway in mammalian cells. This crucial event is governed by proteins of the Bcl-2 family. Molecular mechanisms, by which Bcl-2 family proteins permeabilize mitochondrial membrane, remain under dispute. Although yeast does not have apparent homologues of these proteins, when mammalian members of Bcl-2 family are expressed in yeast, they retain their activity, making yeast an attractive model system, in which to study their action. This review focuses on using yeast expressing mammalian proteins of the Bcl-2 family as a tool to investigate mechanisms, by which these proteins permeabilize mitochondrial membranes, mechanisms, by which pro- and antiapoptotic members of this family interact, and involvement of other cellular components in the regulation of programmed cell death by Bcl-2 family proteins.

Phenoptosis in yeasts

The current view on phenoptosis and apoptosis as genetic programs aimed at eliminating potentially dangerous organisms and cells, respectively, is given. Special emphasis is placed on apoptosis (phenoptosis) in yeasts: intracellular defects and a plethora of external stimuli inducing apoptosis in yeasts; distinctive morphological and biochemical hallmarks accompanying apoptosis in yeasts; pro- and antiapoptotic factors involved in yeast apoptosis signaling; consecutive stages of apoptosis from external stimulus to the cell death; a prominent role of mitochondria and other organelles in yeast apoptosis; possible pathways for release of apoptotic factors from the intermembrane mitochondrial space into the cytosol are described. Using some concrete examples, the obvious physiological importance and expediency of altruistic death of yeast cells is shown. Poorly known aspects of yeast apoptosis and prospects for yeast apoptosis study are defined.

Site-specific N-terminal labeling of proteins using sortase-mediated reactions

This protocol describes the use of sortase-mediated reactions to label the N terminus of any given protein of interest. The sortase recognition sequence, LPXTG (for Streptococcus aureus sortase A) or LPXTA (for Staphylococcus pyogenes sortase A), can be appended to a variety of probes such as fluorophores, biotin or even to other proteins. The protein to be labeled acts as a nucleophile by attacking the intermediate formed between the probe containing the LPXTG/A motif and the sortase enzyme. If sortase, the protein of interest and a suitably functionalized label are available, the reactions usually require less than 3 h.

Boolean model of yeast apoptosis as a tool to study yeast and human apoptotic regulations

Programmed cell death (PCD) is an essential cellular mechanism that is evolutionary conserved, mediated through various pathways and acts by integrating different stimuli. Many diseases such as neurodegenerative diseases and cancers are found to be caused by, or associated with, regulations in the cell death pathways. Yeast Saccharomyces cerevisiae, is a unicellular eukaryotic organism that shares with human cells components and pathways of the PCD and is therefore used as a model organism. Boolean modeling is becoming promising approach to capture qualitative behavior and describe essential properties of such complex networks. Here we present large literature-based and to our knowledge first Boolean model that combines pathways leading to apoptosis (a type of PCD) in yeast. Analysis of the yeast model confirmed experimental findings of anti-apoptotic role of Bir1p and pro-apoptotic role of Stm1p and revealed activation of the stress protein kinase Hog proposing the maximal level of activation upon heat stress. In addition we extended the yeast model and created an in silico humanized yeast in which human pro- and anti-apoptotic regulators Bcl-2 family and Valosin-contain protein (VCP) are included in the model. We showed that accumulation of Bax in silico humanized yeast shows apoptotic markers and that VCP is essential target of Akt Signaling. The presented Boolean model provides comprehensive description of yeast apoptosis network behavior. Extended model of humanized yeast gives new insights of how complex human disease like neurodegeneration can initially be tested.

Untangling the Roles of Anti-Apoptosis in Regulating Programmed Cell Death using Humanized Yeast Cells

Genetically programmed cell death (PCD) mechanisms, including apoptosis, are important for the survival of metazoans since it allows, among things, the removal of damaged cells that interfere with normal function. Cell death due to PCD is observed in normal processes such as aging and in a number of pathophysiologies including hypoxia (common causes of heart attacks and strokes) and subsequent tissue reperfusion. Conversely, the loss of normal apoptotic responses is associated with the development of tumors. So far, limited success in preventing unwanted PCD has been reported with current therapeutic approaches despite the fact that inhibitors of key apoptotic inducers such as caspases have been developed. Alternative approaches have focused on mimicking anti-apoptotic processes observed in cells displaying increased resistance to apoptotic stimuli. Hormesis and pre-conditioning are commonly observed cellular strategies where sub-lethal levels of pro-apoptotic stimuli lead to increased resistance to higher or lethal levels of stress. Increased expression of anti-apoptotic sequences is a common mechanism mediating these protective effects. The relevance of the latter observation is exemplified by the observation that transgenic mice overexpressing anti-apoptotic genes show significant reductions in tissue damage following ischemia. Thus strategies aimed at increasing the levels of anti-apoptotic proteins, using gene therapy or cell penetrating recombinant proteins are being evaluated as novel therapeutics to decrease cell death following acute periods of cell death inducing stress. In spite of its functional and therapeutic importance, more is known regarding the processes involved in apoptosis than anti-apoptosis. The genetically tractable yeast Saccharomyces cerevisiae has emerged as an exceptional model to study multiple aspects of PCD including the mitochondrial mediated apoptosis observed in metazoans. To increase our knowledge of the process of anti-apoptosis, we screened a human heart cDNA expression library in yeast cells undergoing PCD due to the conditional expression of a mammalian pro-apoptotic Bax cDNA. Analysis of the multiple Bax suppressors identified revealed several previously known as well as a large number of clones representing potential novel anti-apoptotic sequences. The focus of this review is to report on recent achievements in the use of humanized yeast in genetic screens to identify novel stress-induced PCD suppressors, supporting the use of yeast as a unicellular model organism to elucidate anti-apoptotic and cell survival mechanisms.

Reconstitution of interactions of Murine gammaherpesvirus 68 M11 with Bcl-2 family proteins in yeast

One of the mechanisms of defense against viral infection is induction of apoptosis in infected cells. To escape this line of protection, genomes of many viruses encode for proteins that inhibit apoptosis. Murid herpesvirus 4 gene M11 encodes for homologue of cellular Bcl-2 proteins that inhibits apoptosis and autophagy in infected cell. To study a role of M11 in regulation of apoptosis we have established a yeast model system in which the action of M11 together with proapoptotic proteins Bax, Bak and Bid can be studied. When expressed in yeast, M11 did not inhibit autophagic pathway, so only effects of expression of M11 on activity of coexpressed proapoptotic proteins could be observed. In this experimental setting M11 potently inhibited both proapoptotic multidomain proteins Bax and Bak. The antiapoptotic activity of M11 was suppressed by coexpression of proapoptotic BH3-only protein tBid, indicating that M11 inhibits apoptosis likely by the same mechanism as cellular antiapoptotic proteins Bcl-2 or Bcl-XL.

Deerpox virus encodes an inhibitor of apoptosis that regulates Bak and Bax

Poxviruses encode numerous proteins that inhibit apoptosis, a form of cell death critical to the elimination of virally infected cells. Sequencing of the deerpox virus genome revealed DPV022, a protein that lacks obvious homology to cellular members of the Bcl-2 family but shares limited regions of amino acid identity with two unique poxviral inhibitors of apoptosis, M11L and F1L. Given the limited homology, we sought to determine whether DPV022 could inhibit apoptosis. Here we show that DPV022 localized to the mitochondria, where it inhibited apoptosis. We used a Saccharomyces cerevisiae model system to demonstrate that in the absence of all other Bcl-2 family proteins, DPV022 interacted directly with Bak and Bax. We confirmed the ability of DPV022 to interact with Bak and Bax by immunoprecipitation and showed that DPV022 prevented apoptosis induced by Bak and Bax overexpression. Moreover, we showed that DPV022 blocked apoptosis even when all the endogenous mammalian antiapoptotic proteins were neutralized by a combination of selective BH3 ligands. During virus infection, DPV022 interacted with endogenous Bak and Bax and prevented the conformational activation of both of them. Thus, we have characterized a novel poxviral inhibitor of apoptosis with intriguing amino acid differences from the well-studied proteins M11L and F1L.

Anti-apoptosis and cell survival: a review

Type I programmed cell death (PCD) or apoptosis is critical for cellular self-destruction for a variety of processes such as development or the prevention of oncogenic transformation. Alternative forms, including type II (autophagy) and type III (necrotic) represent the other major types of PCD that also serve to trigger cell death. PCD must be tightly controlled since disregulated cell death is involved in the development of a large number of different pathologies. To counter the multitude of processes that are capable of triggering death, cells have devised a large number of cellular processes that serve to prevent inappropriate or premature PCD. These cell survival strategies involve a myriad of coordinated and systematic physiological and genetic changes that serve to ward off death. Here we will discuss the different strategies that are used to prevent cell death and focus on illustrating that although anti-apoptosis and cellular survival serve to counteract PCD, they are nevertheless mechanistically distinct from the processes that regulate cell death.

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

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

Combining chemical genomics screens in yeast to reveal spectrum of effects of chemical inhibition of sphingolipid biosynthesis

BACKGROUND

Single genome-wide screens for the effect of altered gene dosage on drug sensitivity in the model organism Saccharomyces cerevisiae provide only a partial picture of the mechanism of action of a drug.

RESULTS

Using the example of the tumor cell invasion inhibitor dihydromotuporamine C, we show that a more complete picture of drug action can be obtained by combining different chemical genomics approaches--analysis of the sensitivity of rho0 cells lacking mitochondrial DNA, drug-induced haploinsufficiency, suppression of drug sensitivity by gene overexpression and chemical-genetic synthetic lethality screening using strains deleted of nonessential genes. Killing of yeast by this chemical requires a functional mitochondrial electron-transport chain and cytochrome c heme lyase function. However, we find that it does not require genes associated with programmed cell death in yeast. The chemical also inhibits endocytosis and intracellular vesicle trafficking and interferes with vacuolar acidification in yeast and in human cancer cells. These effects can all be ascribed to inhibition of sphingolipid biosynthesis by dihydromotuporamine C.

CONCLUSION

Despite their similar conceptual basis, namely altering drug sensitivity by modifying gene dosage, each of the screening approaches provided a distinct set of information that, when integrated, revealed a more complete picture of the mechanism of action of a drug on cells.

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.

Drug-induced apoptosis in yeast

In order to alter the impact of diseases on human society, drug development has been one of the most invested research fields. Nowadays, cancer and infectious diseases are leading targets for the design of effective drugs, in which the primary mechanism of action relies on the modulation of programmed cell death (PCD). Due to the high degree of conservation of basic cellular processes between yeast and higher eukaryotes, and to the existence of an ancestral PCD machinery in yeast, yeasts are an attractive tool for the study of affected pathways that give insights into the mode of action of both antitumour and antifungal drugs. Therefore, we covered some of the leading reports on drug-induced apoptosis in yeast, revealing that in common with mammalian cells, antitumour drugs induce apoptosis through reactive oxygen species (ROS) generation and altered mitochondrial functions. The evidence presented suggests that yeasts may be a powerful model for the screening/development of PCD-directed drugs, overcoming the problem of cellular specificity in the design of antitumour drugs, but also enabling the design of efficient antifungal drugs, targeted to fungal-specific apoptotic regulators that do not have major consequences for human cells.

Suppression of reactive oxygen species by glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a classical glycolytic enzyme, is involved in cellular energy production and has important housekeeping functions. In this report, we show that a GAPDH from Arabidopsis, GAPDHa, has a novel function involved in H(2)O(2)-mediated cell death in yeast and Arabidopsis protoplasts. GAPDHa was cloned along with other plant genes that suppress Bax-induced cell death in yeast. Flow cytometry analyses with dihydrorhodamine 123 indicated that H(2)O(2) production mediated by Bax expression in yeast cells was greatly reduced when Bax was coexpressed with GAPDHa. In plants, GAPDHa transcript levels were greatly increased by H(2)O(2) treatment. Furthermore, transformation of GAPDHa into Arabidopsis protoplasts strongly suppressed heat shock-induced H(2)O(2) production and cell death. Together, our results indicate that GAPDH controls generation of H(2)O(2) by Bax and heat shock, which in turn suppresses cell death in yeast and plant cells.

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.

Preprotein Transport Machineries of Yeast Mitochondrial Outer Membrane Are not Required for Bax-induced Release of Intermembrane Space Proteins

The mitochondrial outer membrane contains protein import machineries, the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been speculated that TOM or SAM are required for Bax-induced release of intermembrane space (IMS) proteins; however, experimental evidence has been scarce. We used isolated yeast mitochondria as a model system and report that Bax promoted an efficient release of soluble IMS proteins while preproteins were still imported, excluding an unspecific damage of mitochondria. Removal of import receptors by protease treatment did not inhibit the release of IMS proteins by Bax. Yeast mutants of each Tom receptor and the Tom40 channel were not impaired in Bax-induced protein release. We analyzed a large collection of mutants of mitochondrial outer membrane proteins, including SAM, fusion and fission components, but none of these components was required for Bax-induced protein release. The released proteins included complexes up to a size of 230 kDa. We conclude that Bax promotes efficient release of IMS proteins through the outer membrane of yeast mitochondria while the inner membrane remains intact. Inactivation of the known protein import and sorting machineries of the outer membrane does not impair the function of Bax at the mitochondria.

The mitochondrial pathway in yeast apoptosis

Mitochondria are not only important for the energetic status of the cell, but are also the fatal organelles deciding about cellular life and death. Complex mitochondrial features decisive for cell death execution in mammals are present and functional in yeast: AIF and cytochrome c release to the cytosol, mitochondrial fragmentation as well as mitochondrial hyperpolarisation followed by an oxidative burst, and breakdown of mitochondrial membrane potential. The easy accessibility of mitochondrial manipulations such as repression of respiration by growing yeast on glucose or deletion of mitochondrial DNA (rho(0)) on the one hand and the unique ability of yeast cells to grow on non-fermentable carbon sources by switching on mitochondrial respiration on the other hand have made yeast an excellent tool to delineate the necessity for mitochondria in cell death execution. Yeast research indicates that the connection between mitochondria and apoptosis is intricate, as abrogation of mitochondrial function can be either deleterious or beneficial for the cell depending on the specific context of the death scenario. Surprisingly, mitochondrion dependent yeast apoptosis currently helps to understand the aetiology (or the complex biology) of lethal cytoskeletal alterations, ageing and neurodegeneration. For example, mutation of mitochondrial superoxide dismutase or CDC48/VCP mutations, both implicated in several neurodegenerative disorders, are associated with mitochondrial impairment and apoptosis in yeast.

Prion protein prevents Bax-mediated cell death in the absence of other Bcl-2 family members inSaccharomyces cerevisiae

Although there is no consensus regarding the normal function of the prion protein, increasing evidence points towards a role in cellular protection against cell death. We have previously shown that prion protein is a potent inhibitor of Bax-induced apoptosis in human primary neurons and in the breast carcinoma MCF-7 cells. Here, we used the yeast Saccharomyces cerevisiae to investigate if the neuroprotective function of prion protein requires other members of the Bcl-2 family given that S. cerevisiae lacks Bcl-2 genes but undergoes a mitochondrial-dependent apoptotic cell death upon exogenous expression of Bax protein. We show that Bax induces cell death and growth inhibition in S. cerevisiae. Prion protein prevents Bax-mediated cell death. Prion protein overcomes Bax-mediated growth arrest in S phase but cannot overcome population growth inhibition because the cells then accumulate in G(2)/M phase. We conclude that prion protein does not require other Bcl-2 family proteins to protect against Bax-mediated cell death.

Tomato QM-like protein protects Saccharomyces cerevisiae cells against oxidative stress by regulating intracellular proline levels

Exogenous proline can protect cells of Saccharomyces cerevisiae from oxidative stress. We altered intracellular proline levels by overexpressing the proline dehydrogenase gene (PUT1) of S. cerevisiae. Put1p performs the first enzymatic step of proline degradation in S. cerevisiae. Overexpression of Put1p results in low proline levels and hypersensitivity to oxidants, such as hydrogen peroxide and paraquat. A put1-disrupted yeast mutant deficient in Put1p activity has increased protection from oxidative stress and increased proline levels. Following a conditional life/death screen in yeast, we identified a tomato (Lycopersicon esculentum) gene encoding a QM-like protein (tQM) and found that stable expression of tQM in the Put1p-overexpressing strain conferred protection against oxidative damage from H2O2, paraquat, and heat. This protection was correlated with reactive oxygen species (ROS) reduction and increased proline accumulation. A yeast two-hybrid system assay was used to show that tQM physically interacts with Put1p in yeast, suggesting that tQM is directly involved in modulating proline levels. tQM also can rescue yeast from the lethality mediated by the mammalian proapoptotic protein Bax, through the inhibition of ROS generation. Our results suggest that tQM is a component of various stress response pathways and may function in proline-mediated stress tolerance in plants.

Defects in N-glycosylation induce apoptosis in yeast

N-glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to man. Defects of N-glycosylation in humans lead to congenital disorders. The pivotal step of this pathway is the transfer of the evolutionarily conserved lipid-linked core-oligosaccharide to the nascent polypeptide chain, catalysed by the oligosaccharyltransferase. One of its nine subunits, Ost2, has homology to DAD1, originally characterized in hamster cells as a defender against apoptotic death. Here we show that ost mutants, such as ost2 and wbp1-1, display morphological and biochemical features of apoptosis upon induction of the glycosylation defect. We observe nuclear condensation, DNA fragmentation as well as externalization of phosphatidylserine. We also demonstrate induction of caspase-like activity, both determined by flow cytometric analysis and in cell-free extracts. Similarly, the N-glycosylation inhibitor tunicamycin in combination with elevated temperature is able to challenge the apoptotic cascade. Heterologous expression of anti-apoptotic human Bcl-2 diminishes caspase activation, improves survival of cells and suppresses the temperature-sensitive growth defect of wbp1-1. Furthermore, accumulation of reactive oxygen species occurs in response to defective glycosylation. As deletion of the metacaspase YCA1 does not seem to abrogate glycosylation-induced apoptosis, we postulate a different proteolytic process to be involved in this death pathway.

Old yellow enzyme interferes with Bax-induced NADPH loss and lipid peroxidation in yeast

The yeast transcriptional response to murine Bax expression was compared with the changes induced by H(2)O(2) treatment via microarray technology. Although most of the Bax-responsive genes were also triggered by H(2)O(2) treatment, OYE3, ICY2, MLS1 and BTN2 were validated to have a Bax-specific transcriptional response not shared with the oxidative stress trigger. In knockout experiments, only deletion of OYE3, coding for yeast Old yellow enzyme, attenuated the rate of Bax-induced growth arrest, cell death and NADPH decrease. Lipid peroxidation was completely absent in DeltaOYE3 expressing Bax. However, the absence of OYE3 sensitized yeast cells to H(2)O(2)-induced cell death, and increased the rate of NADPH decrease and lipid peroxidation. Our results clearly indicate that OYE3 interferes with Bax- and H(2)O(2)-induced lipid peroxidation and cell death in Saccharomyces cerevisiae.

Are yeasts free-living unicellular eukaryotes?

Yeasts are defined as unicellular fungi, yet many recent observations suggest their whole lifestyle is anything but unicellular. This review surveys the evidence that yeasts are really social organisms with cell-to-cell communication.

Depletion of glutathione as a cause of the promotive effects of polygodial, a sesquiterpene on the production of reactive oxygen species in Saccharomyces cerevisiae

The pungent sesquiterpenoid unsaturated dialdehyde, polygodial, exhibited a strong yeastcidal activity against the cells of Saccharomyces cerevisiae, in which production of reactive oxygen species (ROS) at a significant level could be detected with a fluorescent probe. The production of ROS in polygodial-treated cells was further confirmed by its elimination and the accompanying protection against yeastcidal effects in the presence of antioxidants such as L-ascorbate and alpha-tocopherol (alpha-TOH). Polygodial could accelerate ROS production only in cells of the wild-type grande strain but not in those of the respiratory-deficient petite mutant (rho0), indicating the role of the mitochondrial electron transport chain in the production of ROS. Unlike the case with antimycin A which accelerates ROS production by directly targeting the mitochondrial electron flow, polygodial caused depletion of cytoplasmic and mitochondrial glutathione which functions in estiminating ROS inevitably generated during aerobic growth. Polygodial-mediated depletion of intracellular glutathione was possibly dependent on a direct interaction between its enal moiety and the sulfhydryl group of the cysteine in glutathione by a Michael-type reaction.

Lactate utilization in mitochondria prevents Bax cytotoxicity in yeast Kluyveromyces lactis

In a search for the physiological conditions able to suppress the disruption of electron transport through the inner mitochondrial membrane induced by Bax, we found that respiratory substrate - lactate completely abolished Bax toxicity in yeast Kluyveromyces lactis. The effect of lactate was dependent on the presence of cytochrome c, as no effect was observed in the cytochrome c null strain. The investigation of lactate effect on markers of Bax toxicity showed that: (i) oxidation of lactate did not affect the decrease in oxygen consumption, but (ii) lactate was able to diminish the generation of reactive oxygen species and simultaneously to suppress Bax-induced cell death. We show that suppression of Bax lethality in K. lactis can be, in addition to anti-apoptotic proteins, achieved also by the utilization of lactate in the mitochondria.

Functional analysis of Arabidopsis ethylene-responsive element binding protein conferring resistance to Bax and abiotic stress-induced plant cell death

Arabidopsis (Arabidopsis thaliana) ethylene-responsive element binding protein (AtEBP) gene was isolated as a suppressor of Bax-induced cell death by functional screening in yeast (Saccharomyces cerevisiae). To further examine the cell death suppressive action of AtEBP in plant cells, we established transgenic tobacco (Nicotiana tabacum) plants overexpressing AtEBP as well as transgenic tobacco plants ectopically expressing mouse Bax protein under a dexamethasone-inducible promoter. We prepared the crosses of the selective lines of each transgenic plant, which were evaluated in terms of cell death suppression activity. Results indicate that AtEBP suppressed Bax-induced cell death in tobacco plants, an action also associated with a lowered level of ion leakage. Furthermore, tobacco Bright Yellow-2 cells overexpressing AtEBP conferred resistance to hydrogen peroxide (H(2)O(2)) and heat treatments. AtEBP protein localized in the nucleus and functioned as an in vivo transcription activator as confirmed in transient assays and experiments using stable transgenic system. Up-regulation of defense genes was observed in transgenic Arabidopsis plants overexpressing AtEBP. Based on the analysis of mRNA accumulation in ethylene-related mutants, the position of AtEBP in signaling pathway is presented.

Apoptosis in yeast

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. However, dysfunction of apoptosis also leads to several diseases. Yeast undergoes apoptosis after application of acetic acid, sugar- or salt-stress, plant antifungal peptides, or hydrogen peroxide. Oxygen radicals seem to be key elements of apoptotic execution, conserved during evolution. Furthermore, several yeast orthologues of central metazoan apoptotic regulators have been identified, such as a caspase and a caspase-regulating serine protease. In addition, physiological occurrence of cell death has been detected during aging and mating in yeast. The finding of apoptosis in yeast, other fungi and parasites is not only of great medical relevance but will also help to understand some of the still unknown molecular mechanisms at the core of apoptotic execution.

Studies of the Interaction of Substituted Mutants of BAX with Yeast Mitochondria Reveal That the C-terminal Hydrophobic α-Helix Is a Second ART Sequence and Plays a Role in the Interaction with Anti-apoptotic BCL-xL

The role of the two ends of the pro-apoptotic protein BAX in its interaction with mitochondria was challenged by assaying substituted mutants in yeast cells for the ability to bind and insert into the mitochondrial membrane and to promote the release of cytochrome c. Mutations at the N-terminal end confirmed the inhibitory function of this zone, known as apoptotic regulation of targeting (ART). On the other hand, mutations at the C-terminal end of the protein support the hypothesis that the hydrophobic helix alpha9 is not required for the insertion of BAX. In addition, three mutations (a T174D single substitution in the helix alpha9, a K189E/K190E double substitution at the end of the protein, and a P168A mutation in the loop before alpha9) exhibited a strong binding capacity, a strong insertion, as well as high ability to induce cytochrome c release. Considering the positions of these mutations and their potential effect on the movement of helix alpha9, we propose that the C-terminal end of the protein behaves like a second ART. Also, opposite to a mutation that changes the conformation of the N-terminal ART, the mutations in the C-terminal part of the protein impaired the inhibitory effect of anti-apoptotic BCL-xL over BAX insertion, suggesting that the conformation of the alpha9-helix plays a significant role in BAX/BCL-xL interaction.

Bax-induced cell death of Arabidopsis is meditated through reactive oxygen-dependent and -independent processes

An Arabidopsis protoplast system was developed for dissecting plant cell death in individual cells. Bax, a mammalian pro-apoptotic member of the Bcl-2 family, induces apoptotic-like cell death in Arabidopsis. Bax accumulation in Arabidopsis mesophyll protoplasts expressing murine Bax cDNA from a glucocorticoid-inducible promoter results in cytological characteristics of apoptosis, namely DNA fragmentation, increased vacuolation, and loss of plasma membrane integrity. In vivo targeting analysis monitored using jellyfish green fluorescent protein (GFP) reporter indicated full-length Bax was localized to the mitochondria, as it does in animal cells. Deletion of the carboxyl-terminal transmembrane domain of Bax completely abolished targeting to mitochondria. Bax expression was followed by reactive oxygen species (ROS) accumulation. Treatment of protoplasts with the antioxidant N -acetyl- -cysteine (NAC) during induction of Bax expression strongly suppressed Bax-mediated ROS production and the cell death phenotype. However, some population of the ROS depleted cells still induced cell death, indicating that there is a process that Bax-mediated plant cell death is independent of ROS accumulation. Accordingly, suppression of Bax-mediated plant cell death also takes place in two different processes. Over-expression of a key redox-regulator, Arabidopsis nucleoside diphosphate kinase 2 (AtNDPK2) down-regulated ROS accumulation and suppressed Bax-mediated cell death and transient expression of Arabidopsis Bax inhibitor-1 (AtBI-1) substantially suppressed Bax-induced cell death without altering cellular ROS level. Taken together, our results collectively suggest that the Bax-mediated cell death and its suppression in plants is mediated by ROS-dependent and -independent processes.

Utilization of yeast to investigate the role of lipid oxidation in cell death

The yeast Saccharomyces cerevisiae is a powerful tool to investigate several aspects of the molecular mechanisms underlying programmed cell death, namely, the events involving mitochondria. Yeast has allowed new information to be gained about crucial aspects of the interaction between proapoptotic proteins Bax and Bid and mitochondria, namely, their addressing and insertion into the mitochondrial outer membrane and their ability to induce the relocalization of cytochrome c. Going one step further, the prooxidant effect of Bax can also be studied in yeast. Bax expression induces both the accumulation of reactive oxygen species and lipid oxidation. Lipid oxidation is involved in Bax-induced cell death and may be required for optimal insertion of Bax in mitochondria. The easy modulation of lipid composition in yeast is a powerful tool to investigate this process further, and studies can be extended to other regulators of apoptosis, such as proapoptotic Bid or antiapoptotic Bcl-x(L). Also, yeast is a model for the study of other types of cell death, such as autophagy-related forms of death, for which a role of lipid oxidation has also been evidenced.

Disruption of gamma-glutamylcysteine synthetase results in absolute glutathione auxotrophy and apoptosis in Candida albicans

Glutathione is the most abundant non-protein thiol and a major source of reducing equivalents in eukaryotes. We examined the role of glutathione in Candida albicans by the disruption of gamma-glutamylcysteine synthetase (GCS1), an essential enzyme in glutathione biosynthesis. The gcs1/gcs1 null mutants exhibited glutathione auxotrophy, which could be rescued by supplementing with reduced and oxidized glutathione and gamma-glutamylcysteine. When the mutants were depleted of glutathione, they showed typical markers of apoptosis. These results suggest that glutathione itself is an essential metabolite and C. albicans lacking GCS1 undergoes apoptosis.