Apoptosis in yeast: a new model for aging research

Oxidative stress related effect of xenobiotics on eukaryotic model organism, Saccharomyces cerevisiae

Ecotoxicological assays have traditionally focused on the effects of chemicals at the individual level by exploiting mortality and reproduction as endpoints. Although these two parameters are ecologically relevant, they rarely provide information regarding the elemental toxic mechanisms. Obviously, the number of xenobiotics used has been rapidly increased. Thus, any established measurement that shortens the actual outcome and, simultaneously provides information about toxic mechanisms is desirable. This research focused on the study of oxidative stress response as a biomarker in the eukaryotic model organism, Saccharomyces cerevisiae. For this, yeast cells were exposed to a set of selected environmentally relevant chemicals via different approaches, including cellular diagnostics, gene expression analysis and chemo-genetic screening. The results demonstrated that at the cellular level, model organisms reacted to different chemicals in distinct manner. For each xenobiotic, the correlation between toxic response of molecular and cellular levels are presented. Namely, the expression of target genes after chemical exposure affected the cellular alteration as evidenced by an elevated level of superoxide dismutase and a reduced amount of glutathione. Furthermore, the results derived from chemo-genetic screening, in which mutants lacking of gene of interest were employed, exhibited more susceptibility to test chemicals in comparison to the wildtype. The response of oxidative stress upon chemical exposure in budding yeast from this study is potentially useful for an establishment of a proper bio-test system which can eventually be linked to adverse effects at an individual level in higher eukaryotes.

Cell death of Acanthamoeba castellanii following exposure to antimicrobial agents commonly included in contact lens disinfecting solutions

Several antimicrobial agents are commonly included in contact lens disinfectant solutions including chlorhexidine diacetate (CHX), polyhexamethylene biguanide (PHMB) or myristamidopropyl dimethylamine (MAPD); however, their mode of action, i.e. necrosis versus apoptosis is incompletely understood. Here, we determined whether a mechanism of cell death resembling that of apoptosis was present in Acanthamoeba castellanii of the T4 genotype (NEFF) following exposure to the aforementioned antimicrobials using the anticoagulant annexin V that undergoes rapid high affinity binding to phosphatidylserine in the presence of calcium, making it a sensitive probe for phosphatidylserine exposure. The results revealed that under the conditions employed in this study, an apoptotic pathway of cell death in this organism at the tested conditions does not occur. Our findings suggest that necrosis is the likely mode of action; however, future mechanistic studies should be accomplished in additional experimental conditions to further comprehend the molecular mechanisms of cell death in Acanthamoeba.

Studying Huntington's Disease in Yeast: From Mechanisms to Pharmacological Approaches

Huntington's disease (HD) is a neurodegenerative disorder that leads to progressive neuronal loss, provoking impaired motor control, cognitive decline, and dementia. So far, HD remains incurable, and available drugs are effective only for symptomatic management. HD is caused by a mutant form of the huntingtin protein, which harbors an elongated polyglutamine domain and is highly prone to aggregation. However, many aspects underlying the cytotoxicity of mutant huntingtin (mHTT) remain elusive, hindering the efficient development of applicable interventions to counteract HD. An important strategy to obtain molecular insights into human disorders in general is the use of eukaryotic model organisms, which are easy to genetically manipulate and display a high degree of conservation regarding disease-relevant cellular processes. The budding yeast Saccharomyces cerevisiae has a long-standing and successful history in modeling a plethora of human maladies and has recently emerged as an effective tool to study neurodegenerative disorders, including HD. Here, we summarize some of the most important contributions of yeast to HD research, specifically concerning the elucidation of mechanistic features of mHTT cytotoxicity and the potential of yeast as a platform to screen for pharmacological agents against HD.

Yeast polyubiquitin gene UBI4 deficiency leads to early induction of apoptosis and shortened replicative lifespan

Ubiquitin is a 76-amino acid protein that is highly conserved among higher and lower eukaryotes. The polyubiquitin gene UBI4 encodes a unique precursor protein that contains five ubiquitin repeats organized in a head-to-tail arrangement. Although the involvement of the yeast polyubiquitin gene UBI4 in the stress response was reported long ago, there are no reports regarding the underlying mechanism of this involvement. In this study, we used UBI4-deletion and UBI4-overexpressing yeast strains as models to explore the potential mechanism by which UBI4 protects yeast cells against paraquat-induced oxidative stress. Here, we show that ubi4Δ cells exhibit oxidative stress, an apoptotic phenotype, and a decreased replicative lifespan. Additionally, the reduced resistance of ubi4Δ cells to paraquat that was observed in this study was rescued by overexpression of either the catalase or the mitochondrial superoxide dismutase SOD2. We also demonstrated that only SOD2 overexpression restored the replicative lifespan of ubi4Δ cells. In contrast to the case of ubi4Δ cells, UBI4 overexpression in wild-type yeast increases the yeast's resistance to paraquat, and this overexpression is associated with large pools of expressed ubiquitin and increased levels of ubiquitinated proteins. Collectively, these findings highlight the role of the polyubiquitin gene UBI4 in apoptosis and implicate UBI4 as a modulator of the replicative lifespan.

Toxicological effects of graphene oxide on Saccharomyces cerevisiae

Using Saccharomyces cerevisiae as an experimental model, the potential toxicity of graphene oxide (GO) was evaluated following exposure to 0-600 mg L-1 for 24 h. The results showed that cell proliferation was observably inhibited and the IC50 value was 352.704 mg L-1. Mortality showed a concentration-dependent increase, and was 19.3% at 600 mg L-1. A small number of cells were deformed and shrunken after exposure. The percentage of late apoptosis/necrosis showed a significant increase (p < 0.01) at 600 mg L-1 (19.16%) compared with the control (1.14%). The mitochondrial transmembrane potential was significantly decreased (p < 0.01) at 50-600 mg L-1, indicating that the apoptosis was related to mitochondrial impairment. Moreover, ROS was observably increased (p < 0.01) at 200, 400 and 600 mg L-1. The expressions of apoptosis-related genes (SOD, Yca1, Nma111 and Nuc1) were significantly changed. The results presented so far indicate that GO has the potential to cause adverse effects on organisms when released into the environment.

Chronological aging in conidia of pathogenic Aspergillus: Comparison between species

Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus and Aspergillus niger are common airborne fungi, and the most frequent causative agents of human fungal infections. However, the resistance and lifetime persistence of these fungi in the atmosphere, and the mechanism of aging of Aspergillus conidia are unknown.With this work, we intended to study the processes underlying conidial aging of these four relevant and pathogenic Aspergillus species. Chronological aging was therefore evaluated in A. fumigatus, A. flavus, A. terreus and A. niger conidia exposed to environmental and human body temperatures. The results showed that the aging process in Aspergillus conidia involves apoptosis,with metacaspase activation, DNA fragmentation, and reactive oxygen species production, associated with secondary necrosis. Distinct results were observed for the selected pathogenic species. At environmental conditions, A. niger was the species with the highest resistance to aging, indicating a higher adaption to environmental conditions, whereas A. flavus followed by A. terreus were the most sensitive species. At higher temperatures (37 °C), A. fumigatus presented the longest lifespan, in accordance with its good adaptation to the human body temperature. Altogether,with this work new insights regarding conidia aging are provided, which may be useful when designing treatments for aspergillosis.

Calorie restriction-mediated replicative lifespan extension in yeast is non-cell autonomous

Calorie-restriction extends lifespan in many multicellular organisms; here substances secreted by calorie-restricted yeast are found to induce longer life in other yeast cells, suggesting that cellular communication is a component of this phenomenon even in a single-celled organism.

In laboratory yeast strains with Sir2 and Fob1 function, wild-type NAD⁺ salvage is required for calorie restriction (CR) to extend replicative lifespan. CR does not significantly alter steady state levels of intracellular NAD⁺ metabolites. However, levels of Sir2 and Pnc1, two enzymes that sequentially convert NAD⁺ to nicotinic acid (NA), are up-regulated during CR. To test whether factors such as NA might be exported by glucose-restricted mother cells to survive later generations, we developed a replicative longevity paradigm in which mother cells are moved after 15 generations on defined media. The experiment reveals that CR mother cells lose the longevity benefit of CR when evacuated from their local environment to fresh CR media. Addition of NA or nicotinamide riboside (NR) allows a moved mother to maintain replicative longevity despite the move. Moreover, conditioned medium from CR-treated cells transmits the longevity benefit of CR to moved mother cells. Evidence suggests the existence of a longevity factor that is dialyzable but is neither NA nor NR, and indicates that Sir2 is not required for the longevity factor to be produced or to act. Data indicate that the benefit of glucose-restriction is transmitted from cell to cell in budding yeast, suggesting that glucose restriction may benefit neighboring cells and not only an individual cell.

Though calorie restriction extends lifespan and healthspan in multiple model organisms, the intrinsic mechanisms remain unclear. In budding yeast Saccharomyces cerevisiae, manipulation of nicotinamide adenine dinucleotide (NAD⁺)—a central metabolic cofactor—can restrict or extend replicative lifespan, suggesting that NAD⁺-dependent targets might be mediators of extended longevity. However, although treating cells with the NAD⁺ precursor nicotinamide riboside extends lifespan, intracellular NAD⁺ metabolites levels are not altered by glucose restriction. This suggests the potential involvement of extracellular factors in replicative lifespan extension. Here we show that though yeast cells display a longevity benefit upon glucose restriction, these cells surprisingly lose the longevity benefit if moved from their local environment to fresh glucose-restricted media. They are, however, able to regain the longevity benefit, despite the change in environment, if the new environment is supplemented with conditioned medium from glucose restricted cells. Our results suggest that calorie restriction-induced longevity is not cell autonomous and, instead, appears to be transmitted from cell to cell in S. cerevisiae via a dialyzable extracellular factor.

Evolution of aging and death: what insights bacteria can provide

Several unresolved issues, paradoxes, and information voids characterize the field of evolution of aging. The recent discovery of aging-like phenomenon in Escherichia coli, marked by asymmetric segregation of damaged components, particularly protein aggregates, has created a number of new possibilities that remain underexplored. Bacterial systems can potentially throw light on issues such as: whether evolution of aging and evolution of death are different phenomena; whether aging is inevitable for life or is an evolved strategy; whether there could be selection for aging or aging is a pleiotropic effect of some other selection; what are the possible mechanisms of antagonistic pleiotropy, if any; and whether there are mechanisms of aging that are conserved throughout the hierarchy of life. Bacterial aging itself is underexplored and least understood as of now, but even scratching the surface appears to reveal things that may compel us to revise some of the classical concepts about evolution of aging. This warrants more focused and innovative inquiry into aging-like processes in bacteria.

Salt stress causes cell wall damage in yeast cells lacking mitochondrial DNA

The yeast cell wall plays an important role in maintaining cell morphology, cell integrity and response to environmental stresses. Here, we report that salt stress causes cell wall damage in yeast cells lacking mitochondrial DNA (ρ⁰). Upon salt treatment, the cell wall is thickened, broken and becomes more sensitive to the cell wall-perturbing agent sodium dodecyl sulfate (SDS). Also, SCW11 mRNA levels are elevated in ρ⁰ cells. Deletion of SCW11 significantly decreases the sensitivity of ρ⁰ cells to SDS after salt treatment, while overexpression of SCW11 results in higher sensitivity. In addition, salt stress in ρ⁰ cells induces high levels of reactive oxygen species (ROS), which further damages the cell wall, causing cells to become more sensitive towards the cell wall-perturbing agent.

Old Cryptococcus neoformans cells contribute to virulence in chronic cryptococcosis

Does cell age matter in virulence? The emergence of persister cells during chronic infections is critical for persistence of infection, but little is known how this occurs. Here, we demonstrate for the first time that the replicative age of the fungal pathogen Cryptococcus neoformans contributes to persistence during chronic meningoencephalitis. Generationally older C. neoformans cells are more resistant to hydrogen peroxide stress, macrophage intracellular killing, and antifungal agents. Older cells accumulate in both experimental rat infection and in human cryptococcosis. Mathematical modeling supports the concept that the presence of older C. neoformans cells emerges from in vivo selection pressures. We propose that advanced replicative aging is a new unanticipated virulence trait that emerges during chronic fungal infection and facilitates persistence. Therapeutic interventions that target old cells could help in the clearance of chronic infections.

Our findings that the generational age of Cryptococcus neoformans cells matters in pathogenesis introduces a novel concept to eukaryotic pathogenesis research. We propose that emerging properties of aging C. neoformans cells and possibly also other fungal pathogens contribute to persistence and virulence. Whereas the replicative life span of strains may not matter for virulence per se, age-related resilience and thus the generational age of individual C. neoformans cells within a pathogen population could greatly affect persistence of the pathogen population and therefore impact outcome.

Oxidative stress and programmed cell death in yeast

Yeasts, such as Saccharomyces cerevisiae, have long served as useful models for the study of oxidative stress, an event associated with cell death and severe human pathologies. This review will discuss oxidative stress in yeast, in terms of sources of reactive oxygen species (ROS), their molecular targets, and the metabolic responses elicited by cellular ROS accumulation. Responses of yeast to accumulated ROS include upregulation of antioxidants mediated by complex transcriptional changes, activation of pro-survival pathways such as mitophagy, and programmed cell death (PCD) which, apart from apoptosis, includes pathways such as autophagy and necrosis, a form of cell death long considered accidental and uncoordinated. The role of ROS in yeast aging will also be discussed.

Stm1p alters the ribosome association of eukaryotic elongation factor 3 and affects translation elongation

Stm1p is a Saccharomyces cerevisiae protein that is primarily associated with cytosolic 80S ribosomes and polysomes. Several lines of evidence suggest that Stm1p plays a role in translation under nutrient stress conditions, although its mechanism of action is not yet known. In this study, we show that yeast lacking Stm1p (stm1Delta) are hypersensitive to the translation inhibitor anisomycin, which affects the peptidyl transferase reaction in translation elongation, but show little hypersensitivity to other translation inhibitors such as paromomycin and hygromycin B, which affect translation fidelity. Ribosomes isolated from stm1Delta yeast have intrinsically elevated levels of eukaryotic elongation factor 3 (eEF3) associated with them. Overexpression of eEF3 in cells lacking Stm1p results in a growth defect phenotype and increased anisomycin sensitivity. In addition, ribosomes with increased levels of Stm1p exhibit decreased association with eEF3. Taken together, our data indicate that Stm1p plays a complementary role to eEF3 in translation.

Multiple signaling pathways regulate yeast cell death during the response to mating pheromones

Mating pheromones promote cellular differentiation and fusion of yeast cells with those of the opposite mating type. In the absence of a suitable partner, high concentrations of mating pheromones induced rapid cell death in approximately 25% of the population of clonal cultures independent of cell age. Rapid cell death required Fig1, a transmembrane protein homologous to PMP-22/EMP/MP20/Claudin proteins, but did not require its Ca2+ influx activity. Rapid cell death also required cell wall degradation, which was inhibited in some surviving cells by the activation of a negative feedback loop involving the MAP kinase Slt2/Mpk1. Mutants lacking Slt2/Mpk1 or its upstream regulators also underwent a second slower wave of cell death that was independent of Fig1 and dependent on much lower concentrations of pheromones. A third wave of cell death that was independent of Fig1 and Slt2/Mpk1 was observed in mutants and conditions that eliminate calcineurin signaling. All three waves of cell death appeared independent of the caspase-like protein Mca1 and lacked certain "hallmarks" of apoptosis. Though all three waves of cell death were preceded by accumulation of reactive oxygen species, mitochondrial respiration was only required for the slowest wave in calcineurin-deficient cells. These findings suggest that yeast cells can die by necrosis-like mechanisms during the response to mating pheromones if essential response pathways are lacking or if mating is attempted in the absence of a partner.

Stm1p, a ribosome-associated protein, is important for protein synthesis in Saccharomyces cerevisiae under nutritional stress conditions

Stm1p is a Saccharomyces cerevisiae protein that has been implicated in several biological processes, ranging from apoptosis to telomere biosynthesis. Likewise, Stm1p has been identified as a protein associated with supramolecular structures, including ribosomes and nuclear telomere cap complexes. Using a variety of biochemical methods, we found that the vast majority of cellular Stm1p is associated with free cytosolic 80S ribosomes and polysomes. In its association with ribosomes, Stm1p interacts in an equimolar complex with both ribosomal subunits and is not associated with mRNA. Functionally, targeted disruption of the STM1 gene results in rapamycin hypersensitivity and a defect in recovery following nitrogen starvation and replenishment. These effects coincide with severe polysome depletion and reduced total protein synthesis. Taken together, our data indicate that Stm1p plays a critical role in facilitating translation under nutrient stress conditions and suggest that Stm1p acts in concert with the target of rapamycin (TOR) signaling pathway.

Bud selection and apoptosis-like degradation of nuclei in yeast heterokaryons: a KAR1 effect

It has been shown that defects in cell fusion during mating can trigger programmed cell death in the yeast Saccharomyces cerevisiae. We wished to test whether defects in nuclear migration during cell fusion have the same effect. A partial pedigree analysis of nine kar1 x KAR1 crosses of two different types (four alpha KAR1 x a kar1 and five alpha kar1 x a KAR1 crosses) was carried out, and quantitative estimates of the frequencies of different mother/daughter (m/d) classes were obtained. The kar1 mutation affects nuclear congression and delays nuclear fusion. In each cross tested, the nucleus that entered the first bud tended to be the one contributed by the cell that carried the wild-type allele of KAR1. If budding was delayed by nutrient limitation, the kar1 nucleus could be rescued, indicating that the primary effect of the kar1 mutation is that it slows spindle action. Many m/d classes appear as a result of the degradation of one of the nuclei in the heterokaryon. Loss of nuclei in heterokaryons was accompanied by an accumulation of reactive oxygen species (ROS), and by abnormalities in nuclear structure revealed by TUNEL (terminal transferase-mediated dUTP nick end-labeling) analysis, DAPI staining and by histone-GFP fluorescence patterns which suggested an apoptosis-like process. Often only one nucleus was degraded, and ROS accumulation was restricted to one half of the zygote. We therefore suggest that the data obtained can be explained by apoptosis-like death of a half-cell (cell body).

Involvement of oxidative stress response genes in redox homeostasis, the level of reactive oxygen species, and ageing in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants lacking oxidative stress response genes were used to investigate which genes are required under normal aerobic conditions to maintain cellular redox homeostasis, using intracellular glutathione redox potential (glutathione E(h)) to indicate the redox environment of the cells. Levels of reactive oxygen species (ROS) and mitochondrial membrane potentials (MMP) were also assessed by FACS using dihydroethidium and rhodamine 123 as fluorescent probes. Cells became more oxidised as strains shifted from exponential growth to stationary phase. During both phases the presence of reduced thioredoxin and the activity of glutathione reductase were important for redox homeostasis. Thioredoxin reductase contributed less during exponential phase when there was a strong requirement for active Yap1p transcription factor, but was critical during stationary phase. The absence of ROS detoxification systems, such as catalases or superoxide dismutases, had a lesser effect on glutathione E(h), but a more pronounced effect on ROS levels and MMP. These results reflect the major shift in ROS generation as cells switch from fermentative to respiratory metabolism and also showed that there was not a strong correlation between ROS production, MMP and cellular redox environment. Heterogeneity was detected in populations of strains with compromised anti-oxidant defences, and as cells aged they shifted from one cell type with low ROS content to another with much higher intracellular ROS.

The actin cytoskeleton: a key regulator of apoptosis and ageing?

Evidence from many organisms has shown that the accumulation of reactive oxygen species (ROS) has a detrimental effect on cell well-being. High levels of ROS have been linked to programmed cell death pathways and to ageing. Recent reports have implicated changes to the dynamics of the actin cytoskeleton in the release of ROS from mitochondria and subsequent cell death.

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

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

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

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

Search for apoptotic nucleases in yeast: role of Tat-D nuclease in apoptotic DNA degradation

DNA fragmentation/degradation is an important step for apoptosis. However, in unicellular organisms such as yeast, this process has rarely been investigated. In the current study, we revealed eight apoptotic nuclease candidates in Saccharyomyces cerevisiae, analogous to the Caenorhabditis elegans apoptotic nucleases. One of them is Tat-D. Sequence comparison indicates that Tat-D is conserved across kingdoms, implicating that it is evolutionarily and functionally indispensable. In order to better understand the biochemical and biological functions of Tat-D, we have overexpressed, purified, and characterized the S. cerevisiae Tat-D (scTat-D). Our biochemical assays revealed that scTat-D is an endo-/exonuclease. It incises the double-stranded DNA without obvious specificity via its endonuclease activity and excises the DNA from the 3'- to 5'-end by its exonuclease activity. The enzyme activities are metal-dependent with Mg(2+) as an optimal metal ion and an optimal pH around 5. We have also identified three amino acid residues, His(185), Asp(325), and Glu(327), important for its catalysis. In addition, our study demonstrated that knock-out of TAT-D in S. cerevisiae increases the TUNEL-positive cells and cell survival in response to hydrogen hyperoxide treatment, whereas overexpression of Tat-D facilitates cell death. These results suggest a role of Tat-D in yeast apoptosis.

Advances in measuring lifespan in the yeast Saccharomyces cerevisiae

Much research aimed at discovering the genetic bases of longevity focuses on the budding yeast Saccharomyces cerevisiae. Unfortunately, yeast researchers use a definition of longevity not applied to other species. We propose here a method that makes it possible to estimate for yeast the same measures of longevity calculated for other species. We also show that the conventional method (equating longevity with the number of offspring) is only an approximate measure of true chronological lifespan. Our method will allow results for yeast to be compared more correctly with those for other species.

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

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

Chromatin and cell death

HMGB1, a very mobile chromatin protein, leaks out from necrotic cells and signals to neighbouring cells that tissue damage has occurred. At least one receptor for extracellular HMGB1 exists, and signals to different cells to divide, migrate, activate inflammation or start an immune response. Remarkably, apoptotic chromatin binds HMGB1 irreversibly, thereby ensuring that it will not diffuse away to activate responses from neighbouring cells. Thus, dying cells use their own chromatin to signal how they have died. We argue that the nuclear events in apoptosis serve to control the molecular signals that dying cells send out.

The impact of manipulations with cytoplasmically inherited factors on nuclear transmission and degradation in yeast heterokaryons

Heterokaryotic zygotes in yeast provide a unique possibility to study the survival and transmission of two genetically diverse nuclei in one cell. Using partial pedigree analysis, we show that various treatments used to change cytoplasmic hereditary determinants can essentially affect nuclear transmission in yeast heterokaryons. This includes choice of nucleus to enter the first bud and incidence of various classes of mother/daughter pairs demonstrating nuclear degradation patterns in heterokaryotic zygotes. These treatments include guanidine hydrochloride, a prion-curing agent, ethidium bromide, an agent causing elimination of mitochondrial DNA, and cytoplasm replacement by cytoduction, which leads to mtDNA replacement and transfer of some other cytoplasmically inherited determinants. The genetic and cytological evidence obtained favors prion involvement in nuclear transmission and suggests apoptotic features in nuclear degradation in yeast heterokaryotic zygotes.

A role for the actin cytoskeleton in cell death and aging in yeast

Several determinants of aging, including metabolic capacity and genetic stability, are recognized in both yeast and humans. However, many aspects of the pathways leading to cell death remain to be elucidated. Here we report a role for the actin cytoskeleton both in cell death and in promoting longevity. We have analyzed yeast strains expressing mutants with either increased or decreased actin dynamics. We show that decreased actin dynamics causes depolarization of the mitochondrial membrane and an increase in reactive oxygen species (ROS) production, resulting in cell death. Important, however, is the demonstration that increasing actin dynamics, either by a specific actin allele or by deletion of a gene encoding the actin-bundling protein Scp1p, can increase lifespan by over 65%. Increased longevity appears to be due to these cells producing lower than wild-type levels of ROS. Homology between Scp1p and mammalian SM22/transgelin, which itself has been isolated in senescence screens, suggests a conserved mechanism linking aging to actin stability.

Yeast lacking the SRO7/SOP1-encoded tumor suppressor homologue show increased susceptibility to apoptosis-like cell death on exposure to NaCl stress

Yeast cells deleted for the SRO7/SOP1 encoded tumor suppressor homologue show increased sensitivity to NaCl stress. On exposure to growth-inhibiting NaCl concentrations, sro7Delta mutants display a rapid loss in viability that is associated with markers of apoptosis: accumulation of reactive oxygen species, DNA breakage, and nuclear fragmentation. Additional deletion of the yeast metacaspase gene YCA1 prevents the primary fast drop in viability and diminishes nuclear fragmentation and DNA breakage. We also observed that NaCl induced loss in viability of wild-type cells is Yca1p dependent. However, a yeast strain deleted for both SRO7 and its homologue SRO77 exhibits NaCl-induced cell death that is independent on YCA1. Likewise, sro77Delta single mutants do not survive better after additional deletion of the YCA1 gene, and both sro77Delta and sro77Deltayca1Delta mutants display apoptotic characteristics when exposed to growth-inhibiting salinity, suggesting that yeast possesses Yca1p-independent pathway(s) for apoptosis-like cell death. The activity of Yca1p increases with increasing NaCl stress and sro7Delta mutants achieve levels that are higher than in wild-type cells. However, mutants lacking SRO77 do not enhance caspase activity when subject to NaCl stress, suggesting that Sro7p and Sro77p exert opposing effects on the cellular activity of Yca1p.

Nuclear DNA degradation during heterokaryon incompatibility in Neurospora crassa

Many filamentous fungi are capable of undergoing conspecific hyphal fusion with a genetically different individual to form a heterokaryon. However, the viability of such heterokaryons is dependent upon vegetative (heterokaryon) incompatibility (het) loci. If two individuals undergo hyphal anastomosis, but differ in allelic specificity at one or more het loci, the fusion cell is usually compartmentalized and self-destructs. Many of the microscopic features associated with vegetative incompatibility resemble apoptosis in metazoans and plants. To test the hypothesis whether vegetative incompatibility results in nuclear degradation, a characteristic of apoptosis, the cytology of hyphal fusions between incompatible Neurospora crassa strains that differed at three het loci, mat, het-c and het-6, and the cytology of transformants containing incompatible het-c alleles were examined using fluorescent DNA stains and terminal deoxynucleotidyl transferase-mediated dUTP-X nick end labeling (TUNEL). Hyphal fusion cells between het incompatible strains and hyphal segments in het-c incompatible transformants were compartmentalized by septal plugging and contained heavily degraded nuclear DNA. Hyphal fusion cells in compatible self-pairings and hyphal cells in het-c compatible transformants were not compartmentalized and rarely showed TUNEL-positive nuclei. Cell death events also were observed in senescent, older hyphae. Morphological features of hyphal compartmentation and death during vegetative incompatibility and the extent to which it is genetically controlled can best be described as a form of programmed cell death.

Understanding fragile X syndrome: insights from animal models

The fragile X mental retardation syndrome is caused by large methylated expansions of a CGG repeat in the FMR1 gene leading to the loss of expression of FMRP, an RNA-binding protein. FMRP is proposed to act as a regulator of mRNA transport or translation that plays a role in synaptic maturation and function. To study the physiological function of the FMR1 protein, mouse and Drosophila models have been developed. The loss-of-function mouse model shows slightly enlarged testes, a subtle behavioral phenotype, and discrete anomalies of dendrite spines similar to those observed in brains of patients. Studies in Drosophila indicate that FXMR plays an important role in synaptogenesis and axonal arborization, which may underlie the observed deficits in flight ability and circadian behavior of FXR mutant flies. The relevance of these studies to our understanding of fragile X syndrome is discussed.

Longevity regulation in Saccharomyces cerevisiae: linking metabolism, genome stability, and heterochromatin

When it was first proposed that the budding yeast Saccharomyces cerevisiae might serve as a model for human aging in 1959, the suggestion was met with considerable skepticism. Although yeast had proved a valuable model for understanding basic cellular processes in humans, it was difficult to accept that such a simple unicellular organism could provide information about human aging, one of the most complex of biological phenomena. While it is true that causes of aging are likely to be multifarious, there is a growing realization that all eukaryotes possess surprisingly conserved longevity pathways that govern the pace of aging. This realization has come, in part, from studies of S. cerevisiae, which has emerged as a highly informative and respected model for the study of life span regulation. Genomic instability has been identified as a major cause of aging, and over a dozen longevity genes have now been identified that suppress it. Here we present the key discoveries in the yeast-aging field, regarding both the replicative and chronological measures of life span in this organism. We discuss the implications of these findings not only for mammalian longevity but also for other key aspects of cell biology, including cell survival, the relationship between chromatin structure and genome stability, and the effect of internal and external environments on cellular defense pathways. We focus on the regulation of replicative life span, since recent findings have shed considerable light on the mechanisms controlling this process. We also present the specific methods used to study aging and longevity regulation in S. cerevisiae.

The evolution of cell death programs as prerequisites of multicellularity

One of the hallmarks of multicellularity is that the individual cellular fate is sacrificed for the benefit of a higher order of life-the organism. The accidental death of cells in a multicellular organism results in swelling and membrane-rupture and inevitably spills cell contents into the surrounding tissue with deleterious effects for the organism. To avoid this form of necrotic death the cells of metazoans have developed complex self-destruction mechanisms, collectively called programmed cell death, which see to an orderly removal of superfluous cells. Since evolution never invents new genes but plays variations on old themes by DNA mutations, it is not surprising, that some of the genes involved in metazoan death pathways apparently have evolved from homologues in unicellular organisms, where they originally had different functions. Interestingly some unicellular protozoans have developed a primitive form of non-necrotic cell death themselves, which could mean that the idea of an altruistic death for the benefit of genetically identical cells predated the invention of multicellularity. The cell death pathways of protozoans, however, show no homology to those in metazoans, where several death pathways seem to have evolved in parallel. Mitochondria stands at the beginning of several death pathways and also determines, whether a cell has sufficient energy to complete a death program. However, the endosymbiotic bacterial ancestors of mitochondria are unlikely to have contributed to the recent mitochondrial death machinery and therefore, these components may derive from mutated eukaryotic precursors and might have invaded the respective mitochondrial compartments. Although there is no direct evidence, it seems that the prokaryotic-eukaryotic symbiosis created the space necessary for sophisticated death mechanisms on command, which in their distinct forms are major factors for the evolution of multicellular organisms.

Programmed cell death in trypanosomatids and other unicellular organisms

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

Yeast and apoptosis

Even though yeast lack much of the molecular machinery that is responsible for apoptosis in metazoans, they can be a powerful tool in apoptosis research. The ectopic expression of several animal apoptosis proteins in yeast can help us to discover new genes -- and chemical compounds -- that modulate the cell-death pathways of higher eukaryotes.