Genetic and environmental factors influencing glutathione homeostasis in Saccharomyces cerevisiae

Specific antioxidant enzymes are involved in the freeze-thawing response of industrial baker's yeasts

In this study, the biochemical basis of resistance to slow freezing and thawing (F-T) stress was explored in two baker yeast industrial strains of Saccharomyces cerevisiae that presented differential tolerance to freezing in order to be in the frozen bakery industry. Strain Y8, used commercially in sweet baking doughs, exhibited greater stress tolerance than Y9, a strain employed in regular doughs. Survival of Y8 was higher than that of Y9 (30% vs 12%) after F-T or other reactive oxygen species (ROS) inducing stresses compared to their non-stressed controls. The superior F-T tolerance of Y8 was related to its lower ROS accumulation capacity, determined by fluorometry in cell-free extracts and in vivo, by fluorescence microscopy upon F-T, being Y8 ROS accumulation 2-fold lower than that of Y9. That, in turn, could be positively associated with Y8's higher constitutive activities of cytosolic catalase (CAT) and superoxide dismutase by a significant activation (25%) of Y8 CAT after F-T. That would complement the protective effects of other protectant molecules like trehalose, present at high concentration in this strain.

Accumulation and Enrichment of Trace Elements by Yeast Cells and Their Applications: A Critical Review

Maintaining the homeostasis balance of trace elements is crucial for the health of organisms. Human health is threatened by diseases caused by a lack of trace elements. Saccharomyces cerevisiae has a wide and close relationship with human daily life and industrial applications. It can not only be used as fermentation products and single-cell proteins, but also as a trace elements supplement that is widely used in food, feed, and medicine. Trace-element-enriched yeast, viz., chromium-, iron-, zinc-, and selenium-enriched yeast, as an impactful microelements supplement, is more efficient, more environmentally friendly, and safer than its inorganic and organic counterparts. Over the last few decades, genetic engineering has been developing large-scaled genetic re-design and reconstruction in yeast. It is hoped that engineered yeast will include a higher concentration of trace elements. In this review, we compare the common supplement forms of several key trace elements. The mechanisms of detoxification and transport of trace elements in yeast are also reviewed thoroughly. Moreover, genes involved in the transport and detoxification of trace elements are summarized. A feasible way of metabolic engineering transformation of S. cerevisiae to produce trace-element-enriched yeast is examined. In addition, the economy, safety, and environmental protection of the engineered yeast are explored, and the future research direction of yeast enriched in trace elements is discussed.

Glutathione-Mediated Conjugation of Anticancer Drugs: An Overview of Reaction Mechanisms and Biological Significance for Drug Detoxification and Bioactivation

The effectiveness of many anticancer drugs depends on the creation of specific metabolites that may alter their therapeutic or toxic properties. One significant route of biotransformation is a conjugation of electrophilic compounds with reduced glutathione, which can be non-enzymatic and/or catalyzed by glutathione-dependent enzymes. Glutathione usually combines with anticancer drugs and/or their metabolites to form more polar and water-soluble glutathione S-conjugates, readily excreted outside the body. In this regard, glutathione plays a role in detoxification, decreasing the likelihood that a xenobiotic will react with cellular targets. However, some drugs once transformed into thioethers are more active or toxic than the parent compound. Thus, glutathione conjugation may also lead to pharmacological or toxicological effects through bioactivation reactions. My purpose here is to provide a broad overview of the mechanisms of glutathione-mediated conjugation of anticancer drugs. Additionally, I discuss the biological importance of glutathione conjugation to anticancer drug detoxification and bioactivation pathways. I also consider the potential role of glutathione in the metabolism of unsymmetrical bisacridines, a novel prosperous class of anticancer compounds developed in our laboratory. The knowledge on glutathione-mediated conjugation of anticancer drugs presented in this review may be noteworthy for improving cancer therapy and preventing drug resistance in cancers.

SEA and GATOR 10 Years Later

The SEA complex was described for the first time in yeast Saccharomyces cerevisiae ten years ago, and its human homologue GATOR complex two years later. During the past decade, many advances on the SEA/GATOR biology in different organisms have been made that allowed its role as an essential upstream regulator of the mTORC1 pathway to be defined. In this review, we describe these advances in relation to the identification of multiple functions of the SEA/GATOR complex in nutrient response and beyond and highlight the consequence of GATOR mutations in cancer and neurodegenerative diseases.

An Optimized Competitive-Aging Method Reveals Gene-Drug Interactions Underlying the Chronological Lifespan of Saccharomyces cerevisiae

The chronological lifespan of budding yeast is a model of aging and age-related diseases. This paradigm has recently allowed genome-wide screening of genetic factors underlying post-mitotic viability in a simple unicellular system, which underscores its potential to provide a comprehensive view of the aging process. However, results from different large-scale studies show little overlap and typically lack quantitative resolution to derive interactions among different aging factors. We previously introduced a sensitive, parallelizable approach to measure the chronological-lifespan effects of gene deletions based on the competitive aging of fluorescence-labeled strains. Here, we present a thorough description of the method, including an improved multiple-regression model to estimate the association between death rates and fluorescent signals, which accounts for possible differences in growth rate and experimental batch effects. We illustrate the experimental procedure-from data acquisition to calculation of relative survivorship-for ten deletion strains with known lifespan phenotypes, which is achieved with high technical replicability. We apply our method to screen for gene-drug interactions in an array of yeast deletion strains, which reveals a functional link between protein glycosylation and lifespan extension by metformin. Competitive-aging screening coupled to multiple-regression modeling provides a powerful, straight-forward way to identify aging factors in yeast and their interactions with pharmacological interventions.

The role of low molecular weight thiols in Mycobacterium tuberculosis

Low molecular weight (LMW) thiols are molecules with a functional sulfhydryl group that enable them to detoxify reactive oxygen species, reactive nitrogen species and other free radicals. Their roles range from their ability to modulate the immune system to their ability to prevent damage of biological molecules such as DNA and proteins by protecting against oxidative, nitrosative and acidic stress. LMW thiols are synthesized and found in both eukaryotes and prokaryotes. Due to their beneficial role to both eukaryotes and prokaryotes, their specific functions need to be elucidated, most especially in pathogenic prokaryotes such as Mycobacterium tuberculosis (M.tb), in order to provide a rationale for targeting their biosynthesis for drug development. Ergothioneine (ERG), mycothiol (MSH) and gamma-glutamylcysteine (GGC) are LMW thiols that have been shown to interplay to protect M.tb against cellular stress. Though ERG, MSH and GGC seem to have overlapping functions, studies are gradually revealing their unique physiological roles. Understanding their unique physiological role during the course of tuberculosis (TB) infection, would pave the way for the development of drugs that target their biosynthetic pathway. This review identifies the knowledge gap in the unique physiological roles of LMW thiols and proposes their mechanistic roles based on previous studies. In addition, it gives an update on identified inhibitors of their biosynthetic enzymes.

Identifying novel genetic determinants for oxidative stress tolerance in Candida glabrata via adaptive laboratory evolution

Candida glabrata (C glabrata) is an important yeast of industrial and medical significance. Resistance to oxidative stress is an important trait affecting its robustness as a production host or virulence as a pathogenic agent, but current understanding of resistance mechanisms is still limited in this fungus. In this study, we rapidly evolved C glabrata population to adapt to oxidative challenge (from 80mM to 350mM of H₂ O₂ ) through short-term adaptive laboratory evolution. Adaptive mutants were isolated from evolved populations and subjected to phenotypic and omics analyses to identify potential mechanisms of tolerance to H₂ O₂ . Phenotypic characterizations revealed faster detoxification of H₂ O₂ and ability to initiate growth at a higher concentration of the oxidant in the isolated adaptive mutants compared with the wild type. Genome resequencing and genome-wide transcriptome analysis revealed multiple genetic determinants (eg, CAGL0E01243g, CAGL0F06831g, and CAGL0C00385g) that potentially contribute to enhanced H₂ O₂ resistance. Subsequent experimental verification confirmed that CgCth2 (CAGL0E01243g) and CgMga2 (CAGL0F06831g) are important in C glabrata tolerance to oxidative stress. Transcriptome profiling of adaptive mutants and bioinformatic analysis suggest that NADPH regeneration, modulation of membrane composition, cell wall remodeling, and/or global regulatory changes are involved in C glabrata tolerance to H₂ O₂ .

Glutathione de novo synthesis but not recycling process coordinates with glutamine catabolism to control redox homeostasis and directs murine T cell differentiation

Upon antigen stimulation, T lymphocytes undergo dramatic changes in metabolism to fulfill the bioenergetic, biosynthetic and redox demands of proliferation and differentiation. Glutathione (GSH) plays an essential role in controlling redox balance and cell fate. While GSH can be recycled from Glutathione disulfide (GSSG), the inhibition of this recycling pathway does not impact GSH content and murine T cell fate. By contrast, the inhibition of the de novo synthesis of GSH, by deleting either the catalytic (Gclc) or the modifier (Gclm) subunit of glutamate-cysteine ligase (Gcl), dampens intracellular GSH, increases ROS, and impact T cell differentiation. Moreover, the inhibition of GSH de novo synthesis dampened the pathological progression of experimental autoimmune encephalomyelitis (EAE). We further reveal that glutamine provides essential precursors for GSH biosynthesis. Our findings suggest that glutamine catabolism fuels de novo synthesis of GSH and directs the lineage choice in T cells.

Metabolic Reprogramming in Modulating T Cell Reactive Oxygen Species Generation and Antioxidant Capacity

A robust adaptive immune response requires a phase of proliferative burst which is followed by the polarization of T cells into relevant functional subsets. Both processes are associated with dramatically increased bioenergetics demands, biosynthetic demands, and redox demands. T cells meet these demands by rewiring their central metabolic pathways that generate energy and biosynthetic precursors by catabolizing and oxidizing nutrients into carbon dioxide. Simultaneously, oxidative metabolism also produces reactive oxygen species (ROS), which are tightly controlled by antioxidants and plays important role in regulating T cell functions. In this review, we discuss how metabolic rewiring during T cell activation influence ROS production and antioxidant capacity.

Compounds with Potential Activity against Mycobacterium tuberculosis

The high acquisition rate of drug resistance by Mycobacterium tuberculosis necessitates the ongoing search for new drugs to be incorporated in the tuberculosis (TB) regimen. Compounds used for the treatment of other diseases have the potential to be repurposed for the treatment of TB. In this study, a high-throughput screening of compounds against thiol-deficient Mycobacterium smegmatis strains and subsequent validation with thiol-deficient M. tuberculosis strains revealed that ΔegtA and ΔmshA mutants had increased susceptibility to azaguanine (Aza) and sulfaguanidine (Su); ΔegtB and ΔegtE mutants had increased susceptibility to bacitracin (Ba); and ΔegtA, ΔmshA, and ΔegtB mutants had increased susceptibility to fusaric acid (Fu). Further analyses revealed that some of these compounds were able to modulate the levels of thiols and oxidative stress in M. tuberculosis This study reports the activities of Aza, Su, Fu, and Ba against M. tuberculosis and provides a rationale for further investigations.

Overexpression of rice glutaredoxins (OsGrxs) significantly reduces arsenite accumulation by maintaining glutathione pool and modulating aquaporins in yeast

Arsenic (As) is an acute poison and class I carcinogen, can cause a serious health risk. Staple crops like rice are the primary source of As contamination in human food. Rice grown on As contaminated areas accumulates higher As in their edible parts. Based on our previous transcriptome data, two rice glutaredoxins (OsGrx_C7 and OsGrx_C2.1) were identified that showed up-regulated expression during As stress. Here, we report OsGrx_C7 and OsGrx_C2.1 from rice involved in the regulation of intracellular arsenite (AsIII). To elucidate the mechanism of OsGrx mediated As tolerance, both OsGrxs were cloned and expressed in Escherichia coli (Δars) and Saccharomyces cerevisiae mutant strains (Δycf1, Δacr3). The expression of OsGrxs increased As tolerance in E. coli (Δars) mutant strain (up to 4 mM AsV and up to 0.6 mM AsIII). During AsIII exposure, S. cerevisiae (Δacr3) harboring OsGrx_C7 and OsGrx_C2.1 have lower intracellular AsIII accumulation (up to 30.43% and 24.90%, respectively), compared to vector control. Arsenic accumulation in As-sensitive S. cerevisiae mutant (Δycf1) also reduced significantly on exposure to inorganic As. The expression of OsGrxs in yeast maintained intracellular GSH pool and increased extracellular GSH concentration. Purified OsGrxs displays in vitro GSH-disulfide oxidoreductase, glutathione reductase and arsenate reductase activities. Also, both OsGrxs are involved in AsIII extrusion by altering the Fps1 transcripts in yeast and protect the cell by maintaining cellular GSH pool. Thus, our results strongly suggest that OsGrxs play a crucial role in the maintenance of the intracellular GSH pool and redox status of the cell during both AsV and AsIII stress and might be involved in regulating intracellular AsIII levels by modulation of aquaporin expression and functions.

A transporter for abiotic stress and plant metabolite resistance in the ectomycorrhizal fungus Tricholoma vaccinum

Fungi exposed to toxic substances including heavy metals, xenobiotics, or secondary metabolites formed by co-occurring plants or other microorganisms require a detoxification system provided by exporters of several classes of transmembrane proteins. In case of mycorrhiza, plant metabolites need to be exported at the plant interface, while the extraradical hyphae may prevent heavy metal uptake, thus acting as a biofilter to the host plant at high environmental concentrations. One major family of such transporter proteins is the multidrug and toxic compound extrusion (MATE) class, a member of which, Mte1, was studied in the ectomycorrhizal fungus Tricholoma vaccinum. Phylogenetic analyses placed the protein in a subgroup of basidiomycete MATE sequences. The gene mte1 was found to be induced during symbiotic interaction. It mediated detoxification of xenobiotics and metal ions such as Cu, Li, Al, and Ni, as well as secondary plant metabolites if heterologously expressed in Saccharomyces cerevisiae.

Unravelling glutathione conjugate catabolism in Saccharomyces cerevisiae: the role of glutathione/dipeptide transporters and vacuolar function in the release of volatile sulfur compounds 3-mercaptohexan-1-ol and 4-mercapto-4-methylpentan-2-one

Sulfur-containing aroma compounds are key contributors to the flavour of a diverse range of foods and beverages, such as wine. The tropical fruit characters of Sauvignon Blanc wines are attributed to the presence of the aromatic thiols 3-mercaptohexan-1-ol (3-MH), its acetate ester 3-mercaptohexyl acetate (3-MHA), and 4-mercapto-4-methylpentan-2-one (4-MMP). These aromatic thiols are not detectable in grape juice to any significant extent but are released by yeast during alcoholic fermentation. While the processes involved in the release of 3-MH and 4-MMP from their cysteinylated precursors have been studied extensively, degradation pathways for glutathione S-conjugates (GSH-3-MH and GSH-4-MMP) have not. In this study, a candidate gene approach was taken, focusing on genes known to play a role in glutathione and glutathione-S-conjugate turnover in Saccharomyces cerevisiae. Our results confirm the role of Opt1p as the major transporter responsible for uptake of GSH-3-MH and GSH-4-MMP, and identify vacuolar Ecm38p as a key determinant of 3-MH release from GSH-3-MH. ECM38 was unimportant, on the other hand, for release of 4-MMP, and abolition of vacuolar biogenesis caused an increase in the amount of 4-MMP released. The alternative cytosolic glutathione degradation pathway was not involved in release of either thiol from their glutathionylated precursors. Finally, cycling of GSH-3-MH and/or its breakdown intermediates between the cytosol and the vacuole or extracellular space was implicated in modulation of 3-MH formation. Together, these results provide new targets for development of yeast strains that optimize release of these potent volatile sulfur compounds, and further our understanding of the processes involved in glutathione-S-conjugate turnover.

Newly identified protein Imi1 affects mitochondrial integrity and glutathione homeostasis in Saccharomyces cerevisiae

Glutathione homeostasis is crucial for cell functioning. We describe a novel Imi1 protein of Saccharomyces cerevisiae affecting mitochondrial integrity and involved in controlling glutathione level. Imi1 is cytoplasmic and, except for its N-terminal Flo11 domain, has a distinct solenoid structure. A lack of Imi1 leads to mitochondrial lesions comprising aberrant morphology of cristae and multifarious mtDNA rearrangements and impaired respiration. The mitochondrial malfunctioning is coupled to significantly decrease the level of intracellular reduced glutathione without affecting oxidized glutathione, which decreases the reduced/oxidized glutathione ratio. These defects are accompanied by decreased cadmium sensitivity and increased phytochelatin-2 level.

The ArfGAP2/3 Glo3 and ergosterol collaborate in transport of a subset of cargoes

Proteins reach the plasma membrane through the secretory pathway in which the trans Golgi network (TGN) acts as a sorting station. Transport from the TGN to the plasma membrane is maintained by a number of different pathways that act either directly or via the endosomal system. Here we show that a subset of cargoes depends on the ArfGAP2/3 Glo3 and ergosterol to maintain their proper localization at the plasma membrane. While interfering with neither ArfGAP2/3 activity nor ergosterol biosynthesis individually significantly altered plasma membrane localization of the tryptophan transporter Tat2, the general amino acid permease Gap1 and the v-SNARE Snc1, in a Δglo3 Δerg3 strain those proteins accumulated in internal endosomal structures. Export from the TGN to the plasma membrane and recycling from early endosomes appeared unaffected as the chitin synthase Chs3 that travels along these routes was localized normally. Our data indicate that a subset of proteins can reach the plasma membrane efficiently but after endocytosis becomes trapped in endosomal structures. Our study supports a role for ArfGAP2/3 in recycling from endosomes and in transport to the vacuole/lysosome.

A genomic screen revealing the importance of vesicular trafficking pathways in genome maintenance and protection against genotoxic stress in diploid Saccharomyces cerevisiae cells

The ability to survive stressful conditions is important for every living cell. Certain stresses not only affect the current well-being of cells but may also have far-reaching consequences. Uncurbed oxidative stress can cause DNA damage and decrease cell survival and/or increase mutation rates, and certain substances that generate oxidative damage in the cell mainly act on DNA. Radiomimetic zeocin causes oxidative damage in DNA, predominantly by inducing single- or double-strand breaks. Such lesions can lead to chromosomal rearrangements, especially in diploid cells, in which the two sets of chromosomes facilitate excessive and deleterious recombination. In a global screen for zeocin-oversensitive mutants, we selected 133 genes whose deletion reduces the survival of zeocin-treated diploid Saccharomyces cerevisiae cells. The screen revealed numerous genes associated with stress responses, DNA repair genes, cell cycle progression genes, and chromatin remodeling genes. Notably, the screen also demonstrated the involvement of the vesicular trafficking system in cellular protection against DNA damage. The analyses indicated the importance of vesicular system integrity in various pathways of cellular protection from zeocin-dependent damage, including detoxification and a direct or transitional role in genome maintenance processes that remains unclear. The data showed that deleting genes involved in vesicular trafficking may lead to Rad52 focus accumulation and changes in total DNA content or even cell ploidy alterations, and such deletions may preclude proper DNA repair after zeocin treatment. We postulate that functional vesicular transport is crucial for sustaining an integral genome. We believe that the identification of numerous new genes implicated in genome restoration after genotoxic oxidative stress combined with the detected link between vesicular trafficking and genome integrity will reveal novel molecular processes involved in genome stability in diploid cells.

Formation of hydrogen sulfide from cysteine in Saccharomyces cerevisiae BY4742: genome wide screen reveals a central role of the vacuole

Discoveries on the toxic effects of cysteine accumulation and, particularly, recent findings on the many physiological roles of one of the products of cysteine catabolism, hydrogen sulfide (H₂S), are highlighting the importance of this amino acid and sulfur metabolism in a range of cellular activities. It is also highlighting how little we know about this critical part of cellular metabolism. In the work described here, a genome-wide screen using a deletion collection of Saccharomyces cerevisiae revealed a surprising set of genes associated with this process. In addition, the yeast vacuole, not previously associated with cysteine catabolism, emerged as an important compartment for cysteine degradation. Most prominent among the vacuole-related mutants were those involved in vacuole acidification; we identified each of the eight subunits of a vacuole acidification sub-complex (V₁ of the yeast V-ATPase) as essential for cysteine degradation. Other functions identified included translation, RNA processing, folate-derived one-carbon metabolism, and mitochondrial iron-sulfur homeostasis. This work identified for the first time cellular factors affecting the fundamental process of cysteine catabolism. Results obtained significantly contribute to the understanding of this process and may provide insight into the underlying cause of cysteine accumulation and H₂S generation in eukaryotes.

Complex network-driven view of genomic mechanisms underlying Parkinson's disease: analyses in dorsal motor vagal nucleus, locus coeruleus, and substantia nigra

Parkinson's disease (PD)—classically characterized by severe loss of dopaminergic neurons in the substantia nigra pars compacta—has a caudal-rostral progression, beginning in the dorsal motor vagal nucleus and, in a less extent, in the olfactory system, progressing to the midbrain and eventually to the basal forebrain and the neocortex. About 90% of the cases are idiopathic. To study the molecular mechanisms involved in idiopathic PD we conducted a comparative study of transcriptional interaction networks in the dorsal motor vagal nucleus (VA), locus coeruleus (LC), and substantia nigra (SN) of idiopathic PD in Braak stages 4-5 (PD) and disease-free controls (CT) using postmortem samples. Gene coexpression networks (GCNs) for each brain region (patients and controls) were obtained to identify highly connected relevant genes (hubs) and densely interconnected gene sets (modules). GCN analyses showed differences in topology and module composition between CT and PD networks for each anatomic region. In CT networks, VA, LC, and SN hub modules are predominantly associated with neuroprotection and homeostasis in the ageing brain, whereas in the patient's group, for the three brain regions, hub modules are mostly related to stress response and neuron survival/degeneration mechanisms.

The cellular roles of Ccr4-NOT in model and pathogenic fungi-implications for fungal virulence

The fungal Ccr4-NOT complex has been implicated in orchestrating gene expression networks that impact on pathways key for virulence in pathogenic species. The activity of Ccr4-NOT regulates cell wall integrity, antifungal drug susceptibility, adaptation to host temperature, and the developmental switches that enable the formation of pathogenic structures, such as filamentous hyphae. Moreover, Ccr4-NOT impacts on DNA repair pathways and genome stability, opening the possibility that this gene regulator could control adaptive responses in pathogens that are driven by chromosomal alterations. Here we provide a synthesis of the cellular roles of the fungal Ccr4-NOT, focusing on pathways important for virulence toward animals. Our review is based on studies in models yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, and two species that cause serious human infections, Candida albicans and Cryptococcus neoformans. We hypothesize that the activity of Ccr4-NOT could be targeted for future antifungal drug discovery, a proposition supported by the fact that inactivation of the genes encoding subunits of Ccr4-NOT in C. albicans and C. neoformans reduces virulence in the mouse infection model. We performed bioinformatics analysis to identify similarities and differences between Ccr4-NOT subunits in fungi and animals, and discuss this knowledge in the context of future antifungal strategies.

Identification of gold nanoparticle-resistant mutants of Saccharomyces cerevisiae suggests a role for respiratory metabolism in mediating toxicity

Positively charged gold nanoparticles (0.8-nm core diameter) reduced yeast survival, but not growth, at a concentration of 10 to 100 μg/ml. Among 17 resistant deletion mutants isolated in a genome-wide screen, highly significant enrichment was observed for respiration-deficient mutants lacking genes encoding proteins associated with the mitochondrion.

A genome-wide screen in yeast identifies specific oxidative stress genes required for the maintenance of sub-cellular redox homeostasis

Maintenance of an optimal redox environment is critical for appropriate functioning of cellular processes and cell survival. Despite the importance of maintaining redox homeostasis, it is not clear how the optimal redox potential is sensed and set, and the processes that impact redox on a cellular/organellar level are poorly understood. The genetic bases of cellular redox homeostasis were investigated using a green fluorescent protein (GFP) based redox probe, roGFP2 and a pH sensitive GFP-based probe, pHluorin. The use of roGFP2, in conjunction with pHluorin, enabled determination of pH-adjusted sub-cellular redox potential in a non-invasive and real-time manner. A genome-wide screen using both the non-essential and essential gene collections was carried out in Saccharomyces cerevisiae using cytosolic-roGFP2 to identify factors essential for maintenance of cytosolic redox state under steady-state conditions. 102 genes of diverse function were identified that are required for maintenance of cytosolic redox state. Mutations in these genes led to shifts in the half-cell glutathione redox potential by 75-10 mV. Interestingly, some specific oxidative stress-response processes were identified as over-represented in the data set. Further investigation of the role of oxidative stress-responsive systems in sub-cellular redox homeostasis was conducted using roGFP2 constructs targeted to the mitochondrial matrix and peroxisome and E(GSH) was measured in cells in exponential and stationary phase. Analyses allowed for the identification of key redox systems on a sub-cellular level and the identification of novel genes involved in the regulation of cellular redox homeostasis.

Zap1 regulates zinc homeostasis and modulates virulence in Cryptococcus gattii

Zinc homeostasis is essential for fungal growth, as this metal is a critical structural component of several proteins, including transcription factors. The fungal pathogen Cryptococcus gattii obtains zinc from the stringent zinc-limiting milieu of the host during the infection process. To characterize the zinc metabolism in C. gattii and its relationship to fungal virulence, the zinc finger protein Zap1 was functionally characterized. The C. gattii ZAP1 gene is an ortholog of the master regulatory genes zafA and ZAP1 that are found in Aspergillus fumigatus and Saccharomyces cerevisiae, respectively. There is some evidence to support an association between Zap1 and zinc metabolism in C. gattii: (i) ZAP1 expression is highly induced during zinc deprivation, (ii) ZAP1 knockouts demonstrate impaired growth in zinc-limiting conditions, (iii) Zap1 regulates the expression of ZIP zinc transporters and distinct zinc-binding proteins and (iv) Zap1 regulates the labile pool of intracellular zinc. In addition, the deletion of ZAP1 reduces C. gattii virulence in a murine model of cryptococcosis infection. Based on these observations, we postulate that proper zinc metabolism plays a crucial role in cryptococcal virulence.

Transmembrane glutathione cycling in growing Escherichia coli cells

Glutathione (GSH) plays an important role in bacterial cells, participating in maintenance of redox balance in the cytoplasm and in defense against many toxic compounds and stresses. In this study we demonstrate that in aerobic, exponentially growing Escherichia coli culture endogenous reduced glutathione undergoes continuous transmembrane cycling between the cells and medium. As a result of an establishment of a dynamic balance between GSH efflux and uptake, a constant extracellular concentration of GSH counting per biomass unit is maintained. The magnitude of this concentration strictly depends on external pH. GSH cycling is carried out in respiring cells and disturbed by influences, which change the level of ΔμH(+) and ATP. Export of GSH is modified by phosphate deficiency in the medium.

Sorting nexin Snx41 is essential for conidiation and mediates glutathione-based antioxidant defense during invasive growth in Magnaporthe oryzae

The sorting nexins Atg20/Snx42 and Snx41 regulate membrane traffic and endosomal protein sorting and are essential for Cvt and/or pexophagy in yeast. Previously, we showed that macroautophagy is necessary for conidiation in the rice-blast fungus Magnaporthe oryzae. Here, we analyzed the physiological function(s) of selective autophagy in Magnaporthe through targeted deletion of MGG_12832, an ortholog of yeast SNX41 and ATG20/SNX42. Loss of MGG_12832 (hereafter SNX41) abolished conidia formation and pathogenesis in M. oryzae. Snx41-GFP localized as dynamic puncta or short tubules that are partially associated with autophagosomes and/or autophagic vacuoles. PX domain, but not macroautophagy per se, was required for such localization of Snx41-GFP in Magnaporthe. Although not required for nonselective autophagy, Snx41 was essential for pexophagy in Magnaporthe. We identified Oxp1, an ATP-dependent oxoprolinase in the gamma-glutamyl cycle, as a binding partner and potential retrieval target of Snx41-dependent protein sorting. The substrate of Oxp1, 5-oxoproline, could partially restore conidiation in the snx41Δ. Exogenous glutathione, a product of the gamma-glutamyl cycle, significantly restored pathogenicity in the snx41Δ mutant, likely through counteracting the oxidative stress imposed by the host. We propose that the gamma-glutamyl cycle and glutathione biosynthesis are subject to regulation by Snx41-dependent vesicular trafficking, and mediate antioxidant defense crucial for in planta growth and pathogenic differentiation of Magnaporthe at the onset of blast disease in rice.

Oxidative stresses and ageing

Oxidative damage to cellular constituents has frequently been associated with aging in a wide range of organisms. The power of yeast genetics and biochemistry has provided the opportunity to analyse in some detail how reactive oxygen and nitrogen species arise in cells, how cells respond to the damage that these reactive species cause, and to begin to dissect how these species may be involved in the ageing process. This chapter reviews the major sources of reactive oxygen species that occur in yeast cells, the damage they cause and how cells sense and respond to this damage.

Maximising the yeast chronological lifespan

When investigating aging it is important to focus on the factors that are needed to attain, and which can be manipulated to extend, the longest lifespans. This has long been appreciated by those workers who use Drosophila or Caenorhabditis elegans as model experimental systems to study aging. Often though it seems it is not a consideration in many studies of yeast chronological aging. In this chapter I summarise how recent work has revealed the preconditioning that is needed for yeast to survive for long periods in stationary phase, therefore for it to exhibit a long chronological life span (CLS). Of critical importance in this regard is the nature of the nutrient limitation that, during the earlier growth phase, had forced the cells to undergo growth arrest. I have attempted to highlight those studies that have focussed on the longest CLSs, as this helps to identify investigations that may be addressing - not just factors that can influence chronological longevity - but those factors that are correlated with the authentic processes of chronological aging. Attempting to maximize long-term stationary survival in yeast should also enhance the potential relevance of this organism as an aging model to those who wrestle with the problems of aging in more complex systems. Finally I also give a personal perspective of how studies on the yeast CLS may still yet provide some important new insights into events that are correlated with aging.

Glutathione degradation is a key determinant of glutathione homeostasis

Glutathione (GSH) has several important functions in eukaryotic cells, and its intracellular concentration is tightly controlled. Combining mathematical models and (35)S labeling, we analyzed Saccharomyces cerevisiae sulfur metabolism. This led us to the observation that GSH recycling is markedly faster than previously estimated. We set up additional in vivo assays and concluded that under standard conditions, GSH half-life is around 90 min. Sulfur starvation and growth with GSH as the sole sulfur source strongly increase GSH degradation, whereas cadmium (Cd(2+)) treatment inhibits GSH degradation. Whatever the condition tested, GSH is degraded by the cytosolic Dug complex (composed of the three subunits Dug1, Dug2, and Dug3) but not by the γ-glutamyl-transpeptidase, raising the question of the role of this enzyme. In vivo, both DUG2/3 mRNA levels and Dug activity are quickly induced by sulfur deprivation in a Met4-dependent manner. This suggests that Dug activity is mainly regulated at the transcriptional level. Finally, analysis of dug2Δ and dug3Δ mutant cells shows that GSH degradation activity strongly impacts on GSH intracellular concentration and that GSH intracellular concentration does not affect GSH synthesis rate. Altogether, our data led us to reconsider important aspects of GSH metabolism, challenging notions on GSH synthesis and GSH degradation that were considered as established.

Iron-sulfur clusters: biogenesis, molecular mechanisms, and their functional significance

Iron-sulfur clusters [Fe-S] are small, ubiquitous inorganic cofactors representing one of the earliest catalysts during biomolecule evolution and are involved in fundamental biological reactions, including regulation of enzyme activity, mitochondrial respiration, ribosome biogenesis, cofactor biogenesis, gene expression regulation, and nucleotide metabolism. Although simple in structure, [Fe-S] biogenesis requires complex protein machineries and pathways for assembly. [Fe-S] are assembled from cysteine-derived sulfur and iron onto scaffold proteins followed by transfer to recipient apoproteins. Several predominant iron-sulfur biogenesis systems have been identified, including nitrogen fixation (NIF), sulfur utilization factor (SUF), iron-sulfur cluster (ISC), and cytosolic iron-sulfur protein assembly (CIA), and many protein components have been identified and characterized. In eukaryotes ISC is mainly localized to mitochondria, cytosolic iron-sulfur protein assembly to the cytosol, whereas plant sulfur utilization factor is localized mainly to plastids. Because of this spatial separation, evidence suggests cross-talk mediated by organelle export machineries and dual targeting mechanisms. Although research efforts in understanding iron-sulfur biogenesis has been centered on bacteria, yeast, and plants, recent efforts have implicated inappropriate [Fe-S] biogenesis to underlie many human diseases. In this review we detail our current understanding of [Fe-S] biogenesis across species boundaries highlighting evolutionary conservation and divergence and assembling our knowledge into a cellular context.

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

Optimization of glutathione production in batch and fed-batch cultures by the wild-type and recombinant strains of the methylotrophic yeast Hansenula polymorpha DL-1

Background

Tripeptide glutathione (gamma-glutamyl-L-cysteinyl-glycine) is the most abundant non-protein thiol that protects cells from metabolic and oxidative stresses and is widely used as medicine, food additives and in cosmetic industry. The methylotrophic yeast Hansenula polymorpha is regarded as a rich source of glutathione due to the role of this thiol in detoxifications of key intermediates of methanol metabolism. Cellular and extracellular glutathione production of H. polymorpha DL-1 in the wild type and recombinant strains which overexpress genes of glutathione biosynthesis (GSH2) and its precursor cysteine (MET4) was studied.

Results

Glutathione producing capacity of H. polymorpha DL-1 depending on parameters of cultivation (dissolved oxygen tension, pH, stirrer speed), carbon substrate (glucose, methanol) and type of overexpressed genes of glutathione and its precursor biosynthesis during batch and fed-batch fermentations were studied. Under optimized conditions of glucose fed-batch cultivation, the glutathione productivity of the engineered strains was increased from ~900 up to ~ 2300 mg of Total Intracellular Glutathione (TIG) or GSH+GSSG_in, per liter of culture medium. Meantime, methanol fed-batch cultivation of one of the recombinant strains allowed achieving the extracellular glutathione productivity up to 250 mg of Total Extracellular Glutathione (TEG) or GSH+GSSG_ex, per liter of the culture medium.

Conclusions

H. polymorpha is an competitive glutathione producer as compared to other known yeast and bacteria strains (Saccharomyces cerevisiae, Candida utilis, Escherichia coli, Lactococcus lactis etc.) with good perspectives for further improvement especially for production of extracellular form of glutathione.

The critical role of glutathione in maintenance of the mitochondrial genome

Glutathione (GSH) is a key redox buffer and protectant. Growth (approx. one or two divisions) of cells lacking γ-glutamylcysteine synthetase (gsh1) in the absence of GSH led to irreversible respiratory incompetency in all cells, and after five divisions 75% of cells completely lacked mitochondrial DNA (mtDNA). The level of GSH required to allow continuous growth was distinct from that required to prevent loss of mtDNA. GSH limitation led to a change in the transcript levels of 190 genes, including 30 genes regulated by the Aft1p and/or Aft2p transcription factors, which regulate the cellular response to changes in iron availability. Disruption of AFT1 but not AFT2 in gsh1 cells afforded a protective effect on maintenance of respiratory competency, as did overexpression of GRX3 or GRX4 (encoding monothiol glutaredoxins that act as negative regulators of Aft1p). Importantly, an iron-independent mechanism (~30%) was also observed to mediate GSH-dependent mtDNA loss. Analysis of the redox environment in the cytosol, mitochondrial matrix, and intermembrane space (IMS) found that the cytosol was most severely and rapidly affected by GSH depletion. GSH may also modulate the redox environment of the IMS. The implications of altered GSH homeostasis for maintenance of mtDNA, compartmental redox, and the pathophysiology of certain diseases are discussed.

Non-enzymatic roles for the URE2 glutathione S-transferase in the response of Saccharomyces cerevisiae to arsenic

The response of Saccharomyces cerevisiae to arsenic involves a large ensemble of genes, many of which are associated with glutathione-related metabolism. The role of the glutathione S-transferase (GST) product of the URE2 gene involved in resistance of S. cerevisiae to a broad range of heavy metals was investigated. Glutathione peroxidase activity, previously reported for the Ure2p protein, was unaffected in cell-free extracts of an ure2Δ mutant of S. cerevisiae. Glutathione levels in the ure2Δ mutant were lowered about threefold compared to the isogenic wild-type strain but, as in the wild-type strain, increased 2-2.5-fold upon addition of either arsenate (As(V)) or arsenite (As(III)). However, lack of URE2 specifically caused sensitivity to arsenite but not to arsenate. The protective role of URE2 against arsenite depended solely on the GST-encoding 3'-end portion of the gene. The nitrogen source used for growth was suggested to be an important determinant of arsenite toxicity, in keeping with non-enzymatic roles of the URE2 gene product in GATA-type regulation.

The yeast CLC protein counteracts vesicular acidification during iron starvation

Ion gradients across intracellular membranes contribute to the physicochemical environment inside compartments. CLC anion transport proteins that localise to intracellular organelles are anion-proton exchangers involved in anion sequestration or vesicular acidification. By homology, the only CLC protein of Saccharomyces cerevisiae, Gef1, belongs to this family of intracellular exchangers. Gef1 localises to the late Golgi and prevacuole and is essential in conditions of iron limitation. In the absence of Gef1, a multicopper oxidase involved in iron uptake, Fet3, fails to acquire copper ion cofactors. The precise role of the exchanger in this physiological context is unknown. Here, we show that the Gef1-containing compartment is adjusted to a more alkaline pH under iron limitation. This depends on the antiport function of Gef1, because an uncoupled mutant of Gef1 (E230A) results in the acidification of the lumen and fails to support Fet3 maturation. Furthermore, we found that Gef1 antiport activity correlates with marked effects on cellular glutathione homeostasis, raising the possibility that the effect of Gef1 on Fet3 copper loading is related to the control of compartmental glutathione concentration or redox status. Mutational inactivation of a conserved ATP-binding site in the cytosolic cystathione beta-synthetase domain of Gef1 (D732A) suggests that Gef1 activity is regulated by energy metabolism.

Protein carbonyl and the methionine sulfoxide reductase system

The formation and accumulation of protein-carbonyl by reactive oxygen species may serve as a marker of oxidative stress, aging, and age-related diseases. Enzymatic reversal of the protein-carbonyl modification has not yet been detected. However, an enzymatic reversal of protein-methionine sulfoxide modification exists and is mediated by the methionine sulfoxide reductase (Msr) system. Methionine sulfoxide modifications to proteins may precede the formation of protein-carbonyl adducts because of consequent structural changes that increase the vulnerability of amino acid residues to carbonylation. Supportive evidence for this possibility arises from the elevated protein-carbonyl accumulations observed in organisms, such as yeast and mice, lacking the methionine sulfoxide reductase A (MsrA) enzyme. In addition, advanced age or enhanced oxidative-stress conditions foster the accumulations of protein-carbonyls. This review discusses the possible involvement of methionine sulfoxide formation in the occurrence of protein-carbonyl adducts and their relevance to the aging process and neurodegenerative diseases.

Thioredoxins function as deglutathionylase enzymes in the yeast Saccharomyces cerevisiae

Background

Protein-SH groups are amongst the most easily oxidized residues in proteins, but irreversible oxidation can be prevented by protein glutathionylation, in which protein-SH groups form mixed disulphides with glutathione. Glutaredoxins and thioredoxins are key oxidoreductases which have been implicated in regulating glutathionylation/deglutathionylation in diverse organisms. Glutaredoxins have been proposed to be the predominant deglutathionylase enzymes in many plant and mammalian species, whereas, thioredoxins have generally been thought to be relatively inefficient in deglutathionylation.

Results

We show here that the levels of glutathionylated proteins in yeast are regulated in parallel with the growth cycle, and are maximal during stationary phase growth. This increase in glutathionylation is not a response to increased reactive oxygen species generated from the shift to respiratory metabolism, but appears to be a general response to starvation conditions. Our data indicate that glutathionylation levels are constitutively high in all growth phases in thioredoxin mutants and are unaffected in glutaredoxin mutants. We have confirmed that thioredoxins, but not glutaredoxins, catalyse deglutathionylation of model glutathionylated substrates using purified thioredoxin and glutaredoxin proteins. Furthermore, we show that the deglutathionylase activity of thioredoxins is required to reduce the high levels of glutathionylation in stationary phase cells, which occurs as cells exit stationary phase and resume vegetative growth.

Conclusions

There is increasing evidence that the thioredoxin and glutathione redox systems have overlapping functions and these present data indicate that the thioredoxin system plays a key role in regulating the modification of proteins by the glutathione system.

The thioredoxin-thioredoxin reductase system can function in vivo as an alternative system to reduce oxidized glutathione in Saccharomyces cerevisiae

Cellular mechanisms that maintain redox homeostasis are crucial, providing buffering against oxidative stress. Glutathione, the most abundant low molecular weight thiol, is considered the major cellular redox buffer in most cells. To better understand how cells maintain glutathione redox homeostasis, cells of Saccharomyces cerevisiae were treated with extracellular oxidized glutathione (GSSG), and the effect on intracellular reduced glutathione (GSH) and GSSG were monitored over time. Intriguingly cells lacking GLR1 encoding the GSSG reductase in S. cerevisiae accumulated increased levels of GSH via a mechanism independent of the GSH biosynthetic pathway. Furthermore, residual NADPH-dependent GSSG reductase activity was found in lysate derived from glr1 cell. The cytosolic thioredoxin-thioredoxin reductase system and not the glutaredoxins (Grx1p, Grx2p, Grx6p, and Grx7p) contributes to the reduction of GSSG. Overexpression of the thioredoxins TRX1 or TRX2 in glr1 cells reduced GSSG accumulation, increased GSH levels, and reduced cellular glutathione E(h)'. Conversely, deletion of TRX1 or TRX2 in the glr1 strain led to increased accumulation of GSSG, reduced GSH levels, and increased cellular E(h)'. Furthermore, it was found that purified thioredoxins can reduce GSSG to GSH in the presence of thioredoxin reductase and NADPH in a reconstituted in vitro system. Collectively, these data indicate that the thioredoxin-thioredoxin reductase system can function as an alternative system to reduce GSSG in S. cerevisiae in vivo.

Growth phase-dependent variations in transcript profiles for thioredoxin- and glutathione-dependent redox systems followed by budding and hyphal Candida albicans cultures

We report the absolute transcription profiles of 24 genes coding for putative thioredoxin (Trx)- and glutathione (GSH)-dependent redox system components, accompanying the Candida albicans batch culture growth, under either yeast or hyphal conditions. All mRNAs investigated (plus the housekeeping ACT1) displayed significant alterations in their steady-state copy number. Collectively, these quantifications show that: (1) the most abundant mRNAs during active growth coded for putative thiol peroxidases (TSA1, PRX1, AHP11 and AHP12) and for donor thioredoxin Trx1p, i.e. those five mRNAs represented >74% of all transcript molecules quantified in a late exponential phase; (2) the transcripts under study differed between budding and hyphal cells not only in their abundance but also in their profiles throughout the growth stages; (3) mRNA amounts for four GSH-transferase putative genes (GTT12, GTT13, GTT14 and GST3) increased in the stationary phase in yeast but not under filamentous conditions. Hydrogen peroxide resistance, plus GSH, GSSG and reactive oxygen species contents, throughout yeast and hyphal growth, were also studied, and the differences observed were related to the transcript profiles.

Dissecting the pleiotropic consequences of a quantitative trait nucleotide

The downstream consequences of a single quantitative trait polymorphism can provide important insight into the molecular basis of a trait. However, the molecular consequences of a polymorphism may be complex and only a subset of these may influence the trait of interest. In natural isolates of Saccharomyces cerevisiae, a nonsynonymous polymorphism in cystathione beta-synthase (CYS4) causes a deficiency in both cysteine and glutathione that results in rust-colored colonies and drug-dependent growth defects. Using a single-nucleotide allele replacement, we characterized the effects of this polymorphism on gene expression levels across the genome. To determine whether any of the differentially expressed genes are necessary for the production of rust-colored colonies, we screened the yeast deletion collection for genes that enhance or suppress rust coloration. We found that genes in the sulfur assimilation pathway are required for the production of rust color but not the drug-sensitivity phenotype. Our results show that a single quantitative trait polymorphism can generate a complex set of downstream changes, providing a molecular basis for pleiotropy.

Nonenzymatic, self-elimination degradation mechanism of glutathione

In the present in vitro studies, evidence is provided showing that glutathione (GSH) can undergo spontaneous, nonenzymatic auto-degradation. The initial cleavage of the Glu-Cys bond involves nucleophilic attack of the N-terminal amino group of GSH at the gamma-carbonyl side chain, followed by an unusual cleavage of the Cys-Gly amide bond that is complete within 3-5 weeks in the physiological pH range. These observations may be useful for the development of novel biodegradable polymers and releasable prodrugs for sustained and targeted drug delivery applications.

Cu, Zn superoxide dismutase and NADP(H) homeostasis are required for tolerance of endoplasmic reticulum stress in Saccharomyces cerevisiae

Genome-wide screening for sensitivity to chronic endoplasmic reticulum (ER) stress induced by dithiothreitol and tunicamycin (TM) identified mutants deleted for Cu, Zn superoxide dismutase (SOD) function (SOD1, CCS1) or affected in NADPH generation via the pentose phosphate pathway (TKL1, RPE1). TM-induced ER stress led to an increase in cellular superoxide accumulation and an increase in SOD1 expression and Sod1p activity. Prior adaptation of the hac1 mutant deficient in the unfolded protein response (UPR) to the superoxide-generating agent paraquat reduced cell death under ER stress. Overexpression of the ER oxidoreductase Ero1p known to generate hydrogen peroxide in vitro, did not lead to increased superoxide levels in cells subjected to ER stress. The mutants lacking SOD1, TKL1, or RPE1 exhibited decreased UPR induction under ER stress. Sensitivity of the sod1 mutant to ER stress and decreased UPR induction was partially rescued by overexpression of TKL1 encoding transketolase. These data indicate an important role for SOD and cellular NADP(H) in cell survival during ER stress, and it is proposed that accumulation of superoxide affects NADP(H) homeostasis, leading to reduced UPR induction during ER stress.