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

Glutathione is required for growth and cadmium tolerance in the amphibian chytrid fungus, Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a lethal amphibian pathogen, partly due to its ability to evade the immune system of susceptible frog species. In many pathogenic fungi, the antioxidant glutathione is a virulence factor that helps neutralise oxidative stressors generated from host immune cells, as well as other environmental stressors such as heavy metals. The role of glutathione in stress tolerance in Bd has not been investigated. Here, we examine the changes in the glutathione pool after stress exposure and quantify the effect of glutathione depletion on cell growth and stress tolerance. Depletion of glutathione repressed growth and release of zoospores, suggesting that glutathione is essential for life cycle completion in Bd. Supplementation with <2 mM exogenous glutathione accelerated zoospore development, but concentrations >2 mM were strongly inhibitory to Bd cells. While hydrogen peroxide exposure lowered the total cellular glutathione levels by 42 %, glutathione depletion did not increase the sensitivity to hydrogen peroxide. Exposure to cadmium increased total cellular glutathione levels by 93 %. Glutathione-depleted cells were more sensitive to cadmium, and this effect was attenuated by glutathione supplementation, suggesting that glutathione plays an important role in cadmium tolerance. The effects of heat and salt were exacerbated by the addition of exogenous glutathione. The impact of glutathione levels on Bd stress sensitivity may help explain differences in host susceptibility to chytridiomycosis and may provide opportunities for synergistic therapeutics.

The Role of the Glutathione System in Stress Adaptation, Morphogenesis and Virulence of Pathogenic Fungi

Morphogenesis and stress adaptation are key attributes that allow fungal pathogens to thrive and infect human hosts. During infection, many fungal pathogens undergo morphological changes, and this ability is highly linked to virulence. Furthermore, pathogenic fungi have developed multiple antioxidant defenses to cope with the host-derived oxidative stress produced by phagocytes. Glutathione is a major antioxidant that can prevent cellular damage caused by various oxidative stressors. While the role of glutathione in stress detoxification is known, studies of the glutathione system in fungal morphological switching and virulence are lacking. This review explores the role of glutathione metabolism in fungal adaptation to stress, morphogenesis, and virulence. Our comprehensive analysis of the fungal glutathione metabolism reveals that the role of glutathione extends beyond stressful conditions. Collectively, glutathione and glutathione-related proteins are necessary for vitality, cellular development and pathogenesis.

Methylglyoxal-Scavenging Enzyme Activities Trigger Erythroascorbate Peroxidase and Cytochrome c Peroxidase in Glutathione-Depleted Candida albicans

γ-Glutamylcysteine synthetase (Gcs1) and glutathione reductase (Glr1) activity maintains minimal levels of cellular methylglyoxal in Candida albicans. In glutathione-depleted Δgcs1, we previously saw that NAD(H)-linked methylglyoxal oxidoreductase (Mgd1) and alcohol dehydrogenase (Adh1) are the most active methylglyoxal scavengers. With methylglyoxal accumulation, disruptants lacking MGD1 or ADH1 exhibit a poor redox state. However, there is little convincing evidence for a reciprocal relationship between methylglyoxal scavenger genes-disrupted mutants and changes in glutathione-(in)dependent redox regulation. Herein, we attempt to demonstrate a functional role for methylglyoxal scavengers, modeled on a triple disruptant (Δmgd1/Δadh1/Δgcs1), to link between antioxidative enzyme activities and their metabolites in glutathione-depleted conditions. Despite seeing elevated methylglyoxal in all of the disruptants, the result saw a decrease in pyruvate content in Δmgd1/Δadh1/Δgcs1 which was not observed in double gene-disrupted strains such as Δmgd1/Δgcs1 and Δadh1/Δgcs1. Interestingly, Δmgd1/Δadh1/Δgcs1 exhibited a significantly decrease in H₂O₂ and superoxide which was also unobserved in Δmgd1/Δgcs1 and Δadh1/Δgcs1. The activities of the antioxidative enzymes erythroascorbate peroxidase and cytochrome c peroxidase were noticeably higher in Δmgd1/Δadh1/Δgcs1 than in the other disruptants. Meanwhile, Glr1 activity severely diminished in Δmgd1/Δadh1/Δgcs1. Monitoring complementary gene transcripts between double gene-disrupted Δmgd1/Δgcs1 and Δadh1/Δgcs1 supported the concept of an unbalanced redox state independent of the Glr1 activity for Δmgd1/Δadh1/Δgcs1. Our data demonstrate the reciprocal use of Eapx1 and Ccp1 in the absence of both methylglyoxal scavengers; that being pivotal for viability in non-filamentous budding yeast.

Alcohol dehydrogenase 1 and NAD(H)-linked methylglyoxal oxidoreductase reciprocally regulate glutathione-dependent enzyme activities in Candida albicans

Glutathione reductase (Glr1) activity controls cellular glutathione and reactive oxygen species (ROS). We previously demonstrated two predominant methylglyoxal scavengers-NAD(H)-linked methylglyoxal oxidoreductase (Mgd1) and alcohol dehydrogenase 1 (Adh1)-in glutathione-depleted γ-glutamyl cysteinyl synthetase-disrupted Candida albicans. However, experimental evidence for Candida pathophysiology lacking the enzyme activities of Mgd1 and Adh1 on glutathione-dependent redox regulation remains unclear. Herein, we have aimed to demonstrate that glutathione-dependent enzyme activities coupled with cellular ROS changes is regulated by methylglyoxal accumulation in Δmgd1/Δadh1 double disruptants. Δmgd1/Δadh1 showed severe growth defects and G1-phase cell cycle arrest. The observed complementary and reciprocal methylglyoxal-oxidizing and methylglyoxalreducing activities between Δmgd1 and Δadh1 were not always exhibited in Δmgd1/Δadh1. Although intracellular accumulation of methylglyoxal and pyruvate was shown in all disruptants, to a greater or lesser degree, methylglyoxal was particularly accumulated in the Δmgd1/Δadh1 double disruptant. While cellular ROS significantly increased in Δmgd1 and Δadh1 as compared to the wild-type, Δmgd1/Δadh1 underwent a decrease in ROS in contrast to Δadh1. Despite the experimental findings underlining the importance of the undergoing unbalanced redox state of Δmgd1/Δadh1, glutathione-independent antioxidative enzyme activities did not change during proliferation and filamentation. Contrary to the significantly lowered glutathione content and Glr1 enzyme activity, the activity staining-based glutathione peroxidase activities concomitantly increased in this mutant. Additionally, the enhanced GLR1 transcript supported our results in Δmgd1/Δadh1, indicating that deficiencies of both Adh1 and Mgd1 activities stimulate specific glutathione-dependent enzyme activities. This suggests that glutathione-dependent redox regulation is evidently linked to C. albicans pathogenicity under the control of methylglyoxal-scavenging activities.

Fungistatic Action of N-Acetylcysteine on Candida albicans Biofilms and Its Interaction with Antifungal Agents

Therapies targeted to fungal biofilms, mainly against the matrix, and therapies that do not induce microbial resistance are relevant. N-acetylcysteine (NAC), a mucolytic agent, has shown antimicrobial action. This study evaluated the effect of NAC against fluconazole-susceptible (CaS) and -resistant (CaR) Candida albicans. The susceptibility of planktonic cultures to NAC, the effect of NAC on biofilms and their matrix, the interaction of NAC with antifungal agents, and confocal microscopy were evaluated. Data were analyzed descriptively and by the ANOVA/Welch and Tukey/Gomes-Howell tests. The minimum inhibitory concentration (MIC) of NAC was 25 mg/mL for both strains. NAC significantly reduced the viability of both fungal strains. Concentrations higher than the MIC (100 and 50 mg/mL) reduced the viability and the biomass. NAC at 12.5 mg/mL increased the fungal viability. NAC also reduced the soluble components of the biofilm matrix, and showed synergism with caspofungin against planktonic cultures of CaS, but not against biofilms. Confocal images demonstrated that NAC reduced the biofilm thickness and the fluorescence intensity of most fluorochromes used. High concentrations of NAC had similar fungistatic effects against both strains, while a low concentration showed the opposite result. The antibiofilm action of NAC was due to its fungistatic action.

Metabolism-induced oxidative stress and DNA damage selectively trigger genome instability in polyploid fungal cells

Understanding how cellular activities impact genome stability is critical to multiple biological processes including tumorigenesis and reproductive biology. The fungal pathogen Candida albicans displays striking genome dynamics during its parasexual cycle as tetraploid cells, but not diploid cells, exhibit genome instability and reduce their ploidy when grown on a glucose-rich "pre-sporulation" medium. Here, we reveal that C. albicans tetraploid cells are metabolically hyperactive on this medium with higher rates of fermentation and oxidative respiration relative to diploid cells. This heightened metabolism results in elevated levels of reactive oxygen species (ROS), activation of the ROS-responsive transcription factor Cap1, and the formation of DNA double-strand breaks. Genetic or chemical suppression of ROS levels suppresses each of these phenotypes and also protects against genome instability. These studies reveal how endogenous metabolic processes can generate sufficient ROS to trigger genome instability in polyploid C. albicans cells. We also discuss potential parallels with metabolism-induced instability in cancer cells and speculate that ROS-induced DNA damage could have facilitated ploidy cycling prior to a conventional meiosis in eukaryotes.

Regulated Cell Death as a Therapeutic Target for Novel Antifungal Peptides and Biologics

The rise of microbial pathogens refractory to conventional antibiotics represents one of the most urgent and global public health concerns for the 21st century. Emergence of Candida auris isolates and the persistence of invasive mold infections that resist existing treatment and cause severe illness has underscored the threat of drug-resistant fungal infections. To meet these growing challenges, mechanistically novel agents and strategies are needed that surpass the conventional fungistatic or fungicidal drug actions. Host defense peptides have long been misunderstood as indiscriminant membrane detergents. However, evidence gathered over the past decade clearly points to their sophisticated and selective mechanisms of action, including exploiting regulated cell death pathways of their target pathogens. Such peptides perturb transmembrane potential and mitochondrial energetics, inducing phosphatidylserine accessibility and metacaspase activation in fungi. These mechanisms are often multimodal, affording target pathogens fewer resistance options as compared to traditional small molecule drugs. Here, recent advances in the field are examined regarding regulated cell death subroutines as potential therapeutic targets for innovative anti-infective peptides against pathogenic fungi. Furthering knowledge of protective host defense peptide interactions with target pathogens is key to advancing and applying novel prophylactic and therapeutic countermeasures to fungal resistance and pathogenesis.

Schiff bases of putrescine with methylglyoxal protect from cellular damage caused by accumulation of methylglyoxal and reactive oxygen species in Dictyostelium discoideum

Polyamines protect protein glycation in cells against the advanced glycation end product precursor methylglyoxal, which is inevitably produced during glycolysis, and the enzymes that detoxify this α-ketoaldehyde have been widely studied. Nonetheless, nonenzymatic methylglyoxal-scavenging molecules have not been sufficiently studied either in vitro or in vivo. Here, we hypothesized reciprocal regulation between polyamines and methylglyoxal modeled in Dictyostelium grown in a high-glucose medium. We based our hypothesis on the reaction between putrescine and methylglyoxal in putrescine-deficient (odc^-) or putrescine-overexpressing (odc^oe) cells. In these strains, growth and cell cycle were found to be dependent on cellular methylglyoxal and putrescine contents. The odc^- cells showed growth defects and underwent G1 phase cell cycle arrest, which was efficiently reversed by exogenous putrescine. Cellular methylglyoxal, reactive oxygen species (ROS), and glutathione levels were remarkably changed in odc^oe cells and odc̄ cells. These results revealed that putrescine may act as an intracellular scavenger of methylglyoxal and ROS. Herein, we observed interactions of putrescine and methylglyoxal via formation of a Schiff base complex, by UV-vis spectroscopy, and confirmed this adduct by liquid chromatography with mass spectrometry via electrospray ionization. Schiff bases were isolated, analyzed, and predicted to have molecular masses ranging from 124 to 130. We showed that cellular putrescine-methylglyoxal Schiff bases were downregulated in proportion to the levels of endogenous or exogenous putrescine and glutathione in the odc mutants. The putrescine-methylglyoxal Schiff base affected endogenous metabolite levels. This is the first report showing that cellular methylglyoxal functions as a signaling molecule through reciprocal interactions with polyamines by forming Schiff bases.

Candida albicans glutathione reductase downregulates Efg1-mediated cyclic AMP/protein kinase A pathway and leads to defective hyphal growth and virulence upon decreased cellular methylglyoxal content accompanied by activating alcohol dehydrogenase and glycolytic enzymes

BACKGROUND

Glutathione reductase maintains the glutathione level in a reduced state. As previously demonstrated, glutathione is required for cell growth/division and its biosynthesizing-enzyme deficiency causes methylglyoxal accumulation. However, experimental evidences for reciprocal relationships between Cph1-/Efg1-mediated signaling pathway regulation and methylglyoxal production exerted by glutathione reductase on yeast morphology remain unclear.

METHODS

Glutathione reductase (GLR1) disruption/overexpression were performed to investigate aspects of pathological/morphological alterations in Candida albicans. These assumptions were proved by observations of cellular susceptibility to oxidants and thiols, and measurements of methylglyoxal and glutathione content in hyphal-inducing conditions mainly through the activity of GLR1-overexpressing cells. Additionally, the transcriptional/translational levels of bioenergetic enzymes and dimorphism-regulating protein kinases were examined in the strain.

RESULTS

The GLR1-deficient strain was non-viable when GLR1 expression under the control of a CaMAL2 promoter was conditionally repressed, despite partial rescue of growth by exogenous thiols. During filamentation, non-growing hyphal GLR1-overexpressing cells exhibited resistance against oxidants and cellular methylglyoxal was significantly decreased, which concomitantly increased expressions of genes encoding energy-generating enzymes, including fructose-1,6-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase (ADH1), with remarkable repression of Efg1-signaling cascades.

CONCLUSIONS

This is the first report that GLR1-triggered Efg1-mediated signal transduction repression strictly reduces dimorphic switching and virulence by maintaining the basal level of methylglyoxal following the enhanced gene expressions of glycolytic enzymes and ADH1.

GENERAL SIGNIFICANCE

The Efg1 downregulatory mechanism by GLR1 expression has possibilities to involve in other complex network of signal pathways. Understanding how GLR1 overexpression affects multiple signaling pathways can help identify attractive targets for antifungal drugs.

Biochemical and biophysical characterization of Leishmania donovani gamma-glutamylcysteine synthetase

γ-glutamylcysteine synthetase (Gcs) is a vital enzyme catalyzing the first and rate limiting step in the trypanothione biosynthesis pathway, the ATP-dependent ligation of L-Glutamate and L-Cysteine to form gamma-glutamylcysteine. The Trypanothione biosynthesis pathway is unique metabolic pathway essential for trypanosomatid survival rendering Gcs as a potential drug target. Here we report the cloning, expression, purification and characterization of L. donovani Gcs. Three other constructs of Gcs (GcsN, GcsC and GcsT) were designed on the basis of S. cerevisiae and E. coli Gcs crystal structures. The study shows Gcs possesses ATPase activity even in the absence of substrates L-glutamate and L-Cysteine. Divalent ions however plays an indispensable role in LdGcs ATPase activity. Isothermal titration calorimetry and fluorescence studies illustrates that L. donovani Gcs binds substrate in order ATP >L-glutamate>L-cysteine with Glu 92 and Arg 498 involved in ATP hydrolysis and Glu 92, Glu 55 and Arg 498 involved in glutamate binding. Homology modeling and molecular dynamic simulation studies provided the structural rationale of LdGcs catalytic activity and emphasized on the possibility of involvement of three Mg²⁺ ions along with Glutamates 52, 55, 92, 99, Met 322, Gln 328, Tyr 397, Lys 483, Arg 494 and Arg 498 in the catalytic function of L. donovani Gcs.

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L. donovani Gamma glutamylcysteine synthetase is a divalent dependent ATPase.

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Substrate binds in order ATP>> L-Glutamate> L-cysteine.

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Glu 92 and Arg 498 involved in ATP hydrolysis.

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Glu 92, Glu 55 and Arg 498 involved in glutamate binding.

Polyamines regulate cell growth and cellular methylglyoxal in high-glucose medium independently of intracellular glutathione

Polyamines can presumably inhibit protein glycation, when associated with the methylglyoxal inevitably produced during glycolysis. Herein, we hypothesized a nonenzymatic interaction between putrescine and methylglyoxal in putrescine-deficient or -overexpressing Dictyostelium cells in high-glucose medium, which can control methylglyoxal production. Putrescine was essentially required for growth rescue accompanying methylglyoxal detoxification when cells underwent growth defect and cell cycle G1-arrest when supplemented with high glucose. Furthermore, methylglyoxal regulation by putrescine seemed to be a parallel pathway independent of the changes in cellular glutathione content in high-glucose medium. Consequently, we suggest that Dictyostelium cells need polyamines for normal growth and cellular methylglyoxal regulation.

Effects of fluconazole on the metabolomic profile of Candida albicans

BACKGROUND

Little is known about the effects of fluconazole on the metabolism of Candida albicans. We performed LC/MS-based metabolomic profiling of the response of C. albicans cells to increasing doses of fluconazole.

METHODS

C. albicans cells were cultured to mid-logarithmic growth phase in liquid medium and then inoculated in replicate on to nitrocellulose filters under vacuum filtration. Organisms were cultured to mid-logarithmic growth phase and treated with 0-4 mg/L fluconazole. Following metabolic quenching at mid-logarithmic growth phase, intracellular metabolites were extracted and analysed by LC/MS. Changes in pool sizes of individual metabolites were verified by Student's t-test, adjusted for multiple hypothesis testing by Benjamini-Hochberg correction. Distribution of metabolites was analysed by the Kyoto Encyclopedia of Genes and Genomes metabolic pathways database.

RESULTS

We reproducibly detected 64 metabolites whose identities were confirmed by comparison against a pure standard and a library of accurate mass-retention time pairs. These 64 metabolites were broadly representative of eukaryotic central metabolic pathways. Among them 12 had their mean abundance significantly altered in response to increasing fluconazole concentrations. Pool sizes of four intermediates of central carbon metabolism (α-ketoglutarate, glucose-6-phosphate, phenylpyruvate and ribose-5-phosphate) and mevalonate were increased by 0.5-1.5-fold (P ≤ 0.05). Five amino acids (glycine, proline, tryptophan, aminoisobutanoate and asparagine) and guanine were decreased by 0.5-0.75-fold (P ≤ 0.05).

CONCLUSIONS

Fluconazole treatment of C. albicans resulted in increased central carbon and decreased amino acid synthesis intermediates, suggesting a rerouting of metabolic pathways. The function of these metabolomic changes remains to be elucidated; however, they may represent previously unrecognized mechanisms of metabolic injury induced by fluconazole against C. albicans.

The Candida albicans Histone Acetyltransferase Hat1 Regulates Stress Resistance and Virulence via Distinct Chromatin Assembly Pathways

Human fungal pathogens like Candida albicans respond to host immune surveillance by rapidly adapting their transcriptional programs. Chromatin assembly factors are involved in the regulation of stress genes by modulating the histone density at these loci. Here, we report a novel role for the chromatin assembly-associated histone acetyltransferase complex NuB4 in regulating oxidative stress resistance, antifungal drug tolerance and virulence in C. albicans. Strikingly, depletion of the NuB4 catalytic subunit, the histone acetyltransferase Hat1, markedly increases resistance to oxidative stress and tolerance to azole antifungals. Hydrogen peroxide resistance in cells lacking Hat1 results from higher induction rates of oxidative stress gene expression, accompanied by reduced histone density as well as subsequent increased RNA polymerase recruitment. Furthermore, hat1Δ/Δ cells, despite showing growth defects in vitro, display reduced susceptibility to reactive oxygen-mediated killing by innate immune cells. Thus, clearance from infected mice is delayed although cells lacking Hat1 are severely compromised in killing the host. Interestingly, increased oxidative stress resistance and azole tolerance are phenocopied by the loss of histone chaperone complexes CAF-1 and HIR, respectively, suggesting a central role for NuB4 in the delivery of histones destined for chromatin assembly via distinct pathways. Remarkably, the oxidative stress phenotype of hat1Δ/Δ cells is a species-specific trait only found in C. albicans and members of the CTG clade. The reduced azole susceptibility appears to be conserved in a wider range of fungi. Thus, our work demonstrates how highly conserved chromatin assembly pathways can acquire new functions in pathogenic fungi during coevolution with the host.

Candida albicans is the most prevalent fungal pathogen infecting humans, causing life-threatening infections in immunocompromised individuals. Host immune surveillance imposes stress conditions upon C. albicans, to which it has to adapt quickly to escape host killing. This can involve regulation of specific genes requiring disassembly and reassembly of histone proteins, around which DNA is wrapped to form the basic repeat unit of eukaryotic chromatin—the nucleosome. Here, we discover a novel function for the chromatin assembly-associated histone acetyltransferase complex NuB4 in oxidative stress response, antifungal drug tolerance as well as in fungal virulence. The NuB4 complex modulates the induction kinetics of hydrogen peroxide-induced genes. Furthermore, NuB4 negatively regulates susceptibility to killing by immune cells and thereby slowing the clearing from infected mice in vivo. Remarkably, the oxidative stress resistance seems restricted to C. albicans and closely related species, which might have acquired this function during coevolution with the host.

PsANT, the adenine nucleotide translocase of Puccinia striiformis, promotes cell death and fungal growth

Adenine nucleotide translocase (ANT) is a constitutive mitochondrial component that is involved in ADP/ATP exchange and mitochondrion-mediated apoptosis in yeast and mammals. However, little is known about the function of ANT in pathogenic fungi. In this study, we identified an ANT gene of Puccinia striiformis f. sp. tritici (Pst), designated PsANT. The PsANT protein contains three typical conserved mitochondrion-carrier-protein (mito-carr) domains and shares more than 70% identity with its orthologs from other fungi, suggesting that ANT is conserved in fungi. Immuno-cytochemical localization confirmed the mitochondrial localization of PsANT in normal Pst hyphal cells or collapsed cells. Over-expression of PsANT indicated that PsANT promotes cell death in tobacco, wheat and fission yeast cells. Further study showed that the three mito-carr domains are all needed to induce cell death. qRT-PCR analyses revealed an in-planta induced expression of PsANT during infection. Knockdown of PsANT using a host-induced gene silencing system (HIGS) attenuated the growth and development of virulent Pst at the early infection stage but not enough to alter its pathogenicity. These results provide new insight into the function of PsANT in fungal cell death and growth and might be useful in the search for and design of novel disease control strategies.

Role and Regulation of Glutathione Metabolism in Plasmodium falciparum

Malaria in humans is caused by one of five species of obligate intracellular protozoan parasites of the genus Plasmodium. P. falciparum causes the most severe disease and is responsible for 600,000 deaths annually, primarily in Sub-Saharan Africa. It has long been suggested that during their development, malaria parasites are exposed to environmental and metabolic stresses. One strategy to drug discovery was to increase these stresses by interfering with the parasites’ antioxidant and redox systems, which may be a valuable approach to disease intervention. Plasmodium possesses two redox systems—the thioredoxin and the glutathione system—with overlapping but also distinct functions. Glutathione is the most abundant low molecular weight redox active thiol in the parasites existing primarily in its reduced form representing an excellent thiol redox buffer. This allows for an efficient maintenance of the intracellular reducing environment of the parasite cytoplasm and its organelles. This review will highlight the mechanisms that are responsible for sustaining an adequate concentration of glutathione and maintaining its redox state in Plasmodium. It will provide a summary of the functions of the tripeptide and will discuss the potential of glutathione metabolism for drug discovery against human malaria parasites.

Contribution of Fdh3 and Glr1 to Glutathione Redox State, Stress Adaptation and Virulence in Candida albicans

The major fungal pathogen of humans, Candida albicans, is exposed to reactive nitrogen and oxygen species following phagocytosis by host immune cells. In response to these toxins, this fungus activates potent anti-stress responses that include scavenging of reactive nitrosative and oxidative species via the glutathione system. Here we examine the differential roles of two glutathione recycling enzymes in redox homeostasis, stress adaptation and virulence in C. albicans: glutathione reductase (Glr1) and the S-nitrosoglutathione reductase (GSNOR), Fdh3. We show that the NADPH-dependent Glr1 recycles GSSG to GSH, is induced in response to oxidative stress and is required for resistance to macrophage killing. GLR1 deletion increases the sensitivity of C. albicans cells to H2O2, but not to formaldehyde or NO. In contrast, Fdh3 detoxifies GSNO to GSSG and NH3, and FDH3 inactivation delays NO adaptation and increases NO sensitivity. C. albicans fdh3⎔ cells are also sensitive to formaldehyde, suggesting that Fdh3 also contributes to formaldehyde detoxification. FDH3 is induced in response to nitrosative, oxidative and formaldehyde stress, and fdh3Δ cells are more sensitive to killing by macrophages. Both Glr1 and Fdh3 contribute to virulence in the Galleria mellonella and mouse models of systemic infection. We conclude that Glr1 and Fdh3 play differential roles during the adaptation of C. albicans cells to oxidative, nitrosative and formaldehyde stress, and hence during the colonisation of the host. Our findings emphasise the importance of the glutathione system and the maintenance of intracellular redox homeostasis in this major pathogen.

Oxidative stress protection and glutathione metabolism in response to hydrogen peroxide and menadione in riboflavinogenic fungus Ashbya gossypii

Ashbya gossypii is a plant pathogen and a natural overproducer of riboflavin and is used for industrial riboflavin production. A few literature reports depict a link between riboflavin overproduction and stress in this fungus. However, the stress protection mechanisms and glutathione metabolism are not much explored in A. gossypii. In the present study, an increase in the activity of catalase and superoxide dismutase was observed in response to hydrogen peroxide and menadione. The lipid peroxide and membrane lipid peroxide levels were increased by H2O2 and menadione, indicating oxidative damage. The glutathione metabolism was altered with a significant increase in oxidized glutathione (GSSG), glutathione peroxidase (GPX), glutathione S transferase (GST), and glutathione reductase (GR) and a decrease in reduced glutathione (GSH) levels in the presence of H2O2 and menadione. Expression of the genes involved in stress mechanism was analyzed in response to the stressors by semiquantitative RT-PCR. The messenger RNA (mRNA) levels of CTT1, SOD1, GSH1, YAP1, and RIB3 were increased by H2O2 and menadione, indicating the effect of stress at the transcriptional level. A preliminary bioinformatics study for the presence of stress response elements (STRE)/Yap response elements (YRE) depicted that the glutathione metabolic genes, stress genes, and the RIB genes hosted either STRE/YRE, which may enable induction of these genes during stress.

Mechanisms underlying the exquisite sensitivity of Candida albicans to combinatorial cationic and oxidative stress that enhances the potent fungicidal activity of phagocytes

Immune cells exploit reactive oxygen species (ROS) and cationic fluxes to kill microbial pathogens, such as the fungus Candida albicans. Yet, C. albicans is resistant to these stresses in vitro. Therefore, what accounts for the potent antifungal activity of neutrophils? We show that simultaneous exposure to oxidative and cationic stresses is much more potent than the individual stresses themselves and that this combinatorial stress kills C. albicans synergistically in vitro. We also show that the high fungicidal activity of human neutrophils is dependent on the combinatorial effects of the oxidative burst and cationic fluxes, as their pharmacological attenuation with apocynin or glibenclamide reduced phagocytic potency to a similar extent. The mechanistic basis for the extreme potency of combinatorial cationic plus oxidative stress—a phenomenon we term stress pathway interference—lies with the inhibition of hydrogen peroxide detoxification by the cations. In C. albicans this causes the intracellular accumulation of ROS, the inhibition of Cap1 (a transcriptional activator that normally drives the transcriptional response to oxidative stress), and altered readouts of the stress-activated protein kinase Hog1. This leads to a loss of oxidative and cationic stress transcriptional outputs, a precipitous collapse in stress adaptation, and cell death. This stress pathway interference can be suppressed by ectopic catalase (Cat1) expression, which inhibits the intracellular accumulation of ROS and the synergistic killing of C. albicans cells by combinatorial cationic plus oxidative stress. Stress pathway interference represents a powerful fungicidal mechanism employed by the host that suggests novel approaches to potentiate antifungal therapy.

The immune system combats infection via phagocytic cells that recognize and kill pathogenic microbes. Human neutrophils combat Candida infections by killing this fungus with a potent mix of chemicals that includes reactive oxygen species (ROS) and cations. Yet, Candida albicans is relatively resistant to these stresses in vitro. We show that it is the combination of oxidative plus cationic stresses that kills yeasts so effectively, and we define the molecular mechanisms that underlie this potency. Cations inhibit catalase. This leads to the accumulation of intracellular ROS and inhibits the transcription factor Cap1, which is critical for the oxidative stress response in C. albicans. This triggers a dramatic collapse in fungal stress adaptation and cell death. Blocking either the oxidative burst or cationic fluxes in human neutrophils significantly reduces their ability to kill this fungal pathogen, indicating that combinatorial stress is pivotal to immune surveillance.

Changes in glutathione-dependent redox status and mitochondrial energetic strategies are part of the adaptive response during the filamentation process in Candida albicans

Candida albicans is an opportunist pathogen responsible for a large spectrum of infections, from superficial mycosis to systemic diseases called candidiasis. Its ability to grow in various morphological forms, such as unicellular budding yeast, filamentous pseudohyphae and hyphae, contributes to its survival in the diverse microenvironments it encounters in the host. During infection in vivo, C. albicans is faced with high levels of reactive oxygen species (ROS) generated by phagocytes, and the thiol-dependent redox status of the cells reflects their levels of oxidative stress. We investigated the role of glutathione during the transition between the yeast and hyphal forms of the pathogen, in relation to possible changes in mitochondrial bioenergetic pathways. Using various growth media and selective mutations affecting the filamentation process, we showed that C. albicans filamentation was always associated with a depletion of intracellular glutathione levels. Moreover, the induction of hypha formation resulted in general changes in thiol metabolism, including the oxidation of cell surface -SH groups and glutathione excretion. Metabolic adaptation involved tricarboxylic acid (TCA) cycle activation, acceleration of mitochondrial respiration and a redistribution of electron transfer pathways, with an increase in the contribution of the alternative oxidase and rotenone-insensitive dehydrogenase. Changes in redox status and apparent oxidative stress may be necessary to the shift to adaptive metabolic pathways, ensuring normal mitochondrial function and adenosine triphosphate (ATP) levels. The consumption of intracellular glutathione levels during the filamentation process may thus be the price paid by C. albicans for survival in the conditions encountered in the host.

Glutathione initiates the development of Dictyostelium discoideum through the regulation of YakA

Reduced glutathione (GSH) is an essential metabolite that performs multiple indispensable roles during the development of Dictyostelium. We show here that disruption of the gene (gcsA-) encoding y-glutamylcysteine synthetase, an essential enzyme in GSH biosynthesis, inhibited aggregation, and that this developmental defect was rescued by exogenous GSH, but not by other thiols or antioxidants. In GSH-depleted gcsA- cells, the expression ofa growth-stage-specific gene (cprD) was not inhibited, and we did not detect the expression of genes that encode proteins required for early development (cAMP receptor, carA/cAR1; adenylyl cyclase, acaA/ACA; and the catalytic subunit of protein kinase A, pkaC/PKA-C). The defects in gcsA cells were not restored by cAMP stimulation or by cAR1 expression. Further, the expression of yakA, which initiates development and induces the expression of PKA-C, ACA, and cAR1, was regulated by the intracellular concentration of GSH. Constitutive expression of YakA in gcsA- cells (YakA(OE)/gcsA-) rescued the defects in developmental initiation and the expression of early developmental genes in the absence of GSH. Taken together, these findings suggest that GSH plays an essential role in the transition from growth to development by modulating the expression of the genes encoding YakA as well as components thatact downstream in the YakA signaling pathway.

NAD(+)-linked alcohol dehydrogenase 1 regulates methylglyoxal concentration in Candida albicans

We purified a fraction that showed NAD(+)-linked methylglyoxal dehydrogenase activity, directly catalyzing methylglyoxal oxidation to pyruvate, which was significantly increased in glutathione-depleted Candida albicans. It also showed NADH-linked methylglyoxal-reducing activity. The fraction was identified as a NAD(+)-linked alcohol dehydrogenase (ADH1) through mass spectrometric analyses. In ADH1-disruptants of both the wild type and glutathione-depleted cells, the intracellular methylglyoxal concentration increased significantly; defects in growth, differentiation, and virulence were observed; and G2-phase arrest was induced.

Effect of amphotericin B on the metabolic profiles of Candida albicans

Amphotericin B (AmB) is a polyene antifungal drug widely used for systemic fungal infections. In this study, a metabonomic method using gas chromatography-mass spectrometry (GC/MS) was developed to characterize the metabolic profiles of Candida albicans cells exposed to AmB. Thirty-one differentially produced metabolites between AmB-treated and the control groups were identified, among which 10 metabolites were upregulated and 21 metabolites were downregulated. These differentially produced metabolites were mainly involved in polyamines synthesis, tricarboxylic acid (TCA) cycle, oxidative stress, glutathione metabolism, lipid synthesis and glycolysis. Further experiments showed that the polyamines including putrescine, spermidine, and spermine played an important role in the sensitivity of C. albicans cells upon AmB treatment, and combined use of AmB and inhibitors of polyamine biosynthesis pathway might be a potential antifungal strategy. This study provided a systemic view of the metabolic pattern in C. albicans upon exposure to AmB, which shed new light on the mechanisms of action of antifungal agents.

Cap1p attenuates the apoptosis of Candida albicans

Candida albicans is the most common opportunistic fungal pathogen and its apoptosis is inducible by environmental stress. Based on our previous finding that transcription factor Cap1p was involved in baicalein-induced apoptosis, the present study aimed to further clarify the role of Cap1p in apoptosis by observing the impact of CAP1 deletion on cell fate. It was found that apoptotic stimulation with amphotericin B, acetic acid and hydrogen peroxide increased the number of apoptotic and necrotic cells, caspase activity and the accumulation of reactive oxygen species, whereas it decreased the mitochondrial membrane potential and intracellular ATP level in the cap1Δ/Δ mutant. The cell fate was, at least partly, caused by glutathione depletion and attenuation of the expression of the glutathione reductase gene in the cap1Δ/Δ mutant. Collectively, our data suggest that Cap1p participated in the apoptosis of C. albicans by regulating the expression of the glutathione reductase gene and glutathione content.

Hsp90 is involved in apoptosis of Candida albicans by regulating the calcineurin-caspase apoptotic pathway

Candida albicans is the most common human fungal pathogen. Recent evidence has revealed the occurrence of apoptosis in C. albicans that is inducible by environmental stresses such as hydrogen peroxide, acetic acid, and amphotericin B. Apoptosis is regulated by the calcineurin-caspase pathway in C. albicans, and calcineurin is under the control of Hsp90 in echinocandin resistance. However, the role of Hsp90 in apoptosis of C. albicans remains unclear. In this study, we investigated the role of Hsp90 in apoptosis of C. albicans by using an Hsp90-compromised strain tetO-HSP90/hsp90 and found that upon apoptotic stimuli, including hydrogen peroxide, acetic acid or amphotericin B treatment, less apoptosis occurred, less ROS was produced, and more cells survived in the Hsp90-compromised strain compared with the Hsp90/Hsp90 wild-type strain. In addition, Hsp90-compromised cells were defective in up-regulating caspase-encoding gene CaMCA1 expression and activating caspase activity upon the apoptotic stimuli. Investigations on the relationship between Hsp90 and calcineurin revealed that activation of calcineurin could up-regulate apoptosis but could not further down-regulate apoptosis in Hsp90-compromised cells, indicating that calcineurin was downstream of Hsp90. Hsp90 inhibitor geldanamycin (GdA) could further decrease the apoptosis in calcineurin-pathway-defect strains, indicating that compromising Hsp90 function had a stronger effect than compromising calcineurin function on apoptosis. Collectively, this study demonstrated that compromised Hsp90 reduced apoptosis in C. albicans, partially through downregulating the calcineurin-caspase pathway.

Farnesol-induced apoptosis in Candida albicans is mediated by Cdr1-p extrusion and depletion of intracellular glutathione

Farnesol is a key derivative in the sterol biosynthesis pathway in eukaryotic cells previously identified as a quorum sensing molecule in the human fungal pathogen Candida albicans. Recently, we demonstrated that above threshold concentrations, farnesol is capable of triggering apoptosis in C. albicans. However, the exact mechanism of farnesol cytotoxicity is not fully elucidated. Lipophilic compounds such as farnesol are known to conjugate with glutathione, an antioxidant crucial for cellular detoxification against damaging compounds. Glutathione conjugates act as substrates for ATP-dependent ABC transporters and are extruded from the cell. To that end, this current study was undertaken to validate the hypothesis that farnesol conjugation with intracellular glutathione coupled with Cdr1p-mediated extrusion of glutathione conjugates, results in total glutathione depletion, oxidative stress and ultimately fungal cell death. The combined findings demonstrated a significant decrease in intracellular glutathione levels concomitant with up-regulation of CDR1 and decreased cell viability. However, addition of exogenous reduced glutathione maintained intracellular glutathione levels and enhanced viability. In contrast, farnesol toxicity was decreased in a mutant lacking CDR1, whereas it was increased in a CDR1-overexpressing strain. Further, gene expression studies demonstrated significant up-regulation of the SOD genes, primary enzymes responsible for defense against oxidative stress, with no changes in expression in CDR1. This is the first study describing the involvement of Cdr1p-mediated glutathione efflux as a mechanism preceding the farnesol-induced apoptotic process in C. albicans. Understanding of the mechanisms underlying farnesol-cytotoxicity in C. albicans may lead to the development of this redox-cycling agent as an alternative antifungal agent.

Dissecting the role of glutathione biosynthesis in Plasmodium falciparum

Glutathione (γ-glutamylcysteinyl-glycine, GSH) has vital functions as thiol redox buffer and cofactor of antioxidant and detoxification enzymes. Plasmodium falciparum possesses a functional GSH biosynthesis pathway and contains mM concentrations of the tripeptide. It was impossible to delete in P. falciparum the genes encoding γ-glutamylcysteine synthetase (γGCS) or glutathione synthetase (GS), the two enzymes synthesizing GSH, although both gene loci were not refractory to recombination. Our data show that the parasites cannot compensate for the loss of GSH biosynthesis via GSH uptake. This suggests an important if not essential function of GSH biosynthesis pathway for the parasites. Treatment with the irreversible inhibitor of γGCS L-buthionine sulfoximine (BSO) reduced intracellular GSH levels in P. falciparum and was lethal for their intra-erythrocytic development, corroborating the suggestion that GSH biosynthesis is important for parasite survival. Episomal expression of γgcs in P. falciparum increased tolerance to BSO attributable to increased levels of γGCS. Concomitantly expression of glutathione reductase was reduced leading to an increased GSH efflux. Together these data indicate that GSH levels are tightly regulated by a functional GSH biosynthesis and the reduction of GSSG.

Glutathione biosynthesis in the yeast pathogens Candida glabrata and Candida albicans: essential in C. glabrata, and essential for virulence in C. albicans

Redox pathways play a key role in pathogenesis. Glutathione, a central molecule in redox homeostasis in yeasts, is an essential metabolite, but its requirements can be met either from endogenous biosynthesis or from the extracellular milieu. In this report we have examined the importance of glutathione biosynthesis in two major human opportunistic fungal pathogens, Candida albicans and Candida glabrata. As the genome sequence of C. glabrata had suggested the absence of glutathione transporters, we initially investigated exogenous glutathione utilization in C. glabrata by disruption of the MET15 gene, involved in methionine biosynthesis. We observed an organic sulphur auxotrophy in a C. glabrata met15Δ strain; however, unlike its Saccharomyces cerevisiae counterpart, the C. glabrata met15Δ strain was unable to grow on exogenous glutathione. This inability to grow on exogenous glutathione was demonstrated to be due to the lack of a functional glutathione transporter, despite the presence of a functional glutathione degradation machinery (the Dug pathway). In the absence of the ability to obtain glutathione from the extracellular medium, we examined and could demonstrate that γ-glutamyl cysteine synthase, the first enzyme of glutathione biosynthesis, was essential in C. glabrata. Further, although γ-glutamyl cysteine synthase has been reported to be non-essential in C. albicans, we report here for what is believed to be the first time that the enzyme is required for survival in human macrophages in vitro, as well as for virulence in a murine model of disseminated candidiasis. The essentiality of γ-glutamyl cysteine synthase in C. glabrata, and its essentiality for virulence in C. albicans, make the enzyme a strong candidate for antifungal development.

Candida albicans AGE3, the ortholog of the S. cerevisiae ARF-GAP-encoding gene GCS1, is required for hyphal growth and drug resistance

BACKGROUND

Hyphal growth and multidrug resistance of C. albicans are important features for virulence and antifungal therapy of this pathogenic fungus.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show by phenotypic complementation analysis that the C. albicans gene AGE3 is the functional ortholog of the yeast ARF-GAP-encoding gene GCS1. The finding that the gene is required for efficient endocytosis points to an important functional role of Age3p in endosomal compartments. Most C. albicans age3Delta mutant cells which grew as cell clusters under yeast growth conditions showed defects in filamentation under different hyphal growth conditions and were almost completely disabled for invasive filamentous growth. Under hyphal growth conditions only a fraction of age3Delta cells shows a wild-type-like polarization pattern of the actin cytoskeleton and lipid rafts. Moreover, age3Delta cells were highly susceptible to several unrelated toxic compounds including antifungal azole drugs. Irrespective of the AGE3 genotype, C-terminal fusions of GFP to the drug efflux pumps Cdr1p and Mdr1p were predominantly localized in the plasma membrane. Moreover, the plasma membranes of wild-type and age3Delta mutant cells contained similar amounts of Cdr1p, Cdr2p and Mdr1p.

CONCLUSIONS/SIGNIFICANCE

The results indicate that the defect in sustaining filament elongation is probably caused by the failure of age3Delta cells to polarize the actin cytoskeleton and possibly of inefficient endocytosis. The high susceptibility of age3Delta cells to azoles is not caused by inefficient transport of efflux pumps to the cell membrane. A possible role of a vacuolar defect of age3Delta cells in drug susceptibility is proposed and discussed. In conclusion, our study shows that the ARF-GAP Age3p is required for hyphal growth which is an important virulence factor of C. albicans and essential for detoxification of azole drugs which are routinely used for antifungal therapy. Thus, it represents a promising antifungal drug target.

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.

The antifungal plant defensin RsAFP2 from radish induces apoptosis in a metacaspase independent way in Candida albicans

We show that the antifungal plant defensin Raphanus sativus antifungal protein 2 (RsAFP2) from radish induces apoptosis and concomitantly triggers activation of caspases or caspase-like proteases in the human pathogen Candida albicans. Furthermore, we demonstrate that deletion of C. albicans metacaspase 1, encoding the only reported (putative) caspase in C. albicans, significantly affects caspase activation by the apoptotic stimulus acetic acid, but not by RsAFP2. To our knowledge, this is the first report on the induction of apoptosis with concomitant caspase activation by a defensin in this pathogen. Moreover, our data point to the existence of at least two different types of caspases or caspase-like proteases in C. albicans.

Fungal apoptosis: function, genes and gene function

Cells of all living organisms are programmed to self-destruct under certain conditions. The most well known form of programmed cell death is apoptosis, which is essential for proper development in higher eukaryotes. In fungi, apoptotic-like cell death occurs naturally during aging and reproduction, and can be induced by environmental stresses and exposure to toxic metabolites. The core apoptotic machinery in fungi is similar to that in mammals, but the apoptotic network is less complex and of more ancient origin. Only some of the mammalian apoptosis-regulating proteins have fungal homologs, and the number of protein families is drastically reduced. Expression in fungi of animal proteins that do not have fungal homologs often affects apoptosis, suggesting functional conservation of these components despite the absence of protein-sequence similarity. Functional analysis of Saccharomyces cerevisiae apoptotic genes, and more recently of those in some filamentous species, has revealed partial conservation, along with substantial differences in function and mode of action between fungal and human proteins. It has been suggested that apoptotic proteins might be suitable targets for novel antifungal treatments. However, implementation of this approach requires a better understanding of fungal apoptotic networks and identification of the key proteins regulating apoptotic-like cell death in fungi.

Negative regulation of apoptosis in yeast

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

Key function for the CCAAT-binding factor Php4 to regulate gene expression in response to iron deficiency in fission yeast

The fission yeast Schizosaccharomyces pombe responds to the deprivation of iron by inducing the expression of the php4+ gene, which encodes a negative regulatory subunit of the heteromeric CCAAT-binding factor. Once formed, the Php2/3/4/5 transcription complex is required to inactivate a subset of genes encoding iron-using proteins. Here, we used a pan-S. pombe microarray to study the transcriptional response to iron starvation and identified 86 genes that exhibit php4+-dependent changes on a genome-wide scale. One of these genes encodes the iron-responsive transcriptional repressor Fep1, whose mRNA levels were decreased after treatment with the permeant iron chelator 2,2'-dipyridyl. In addition, several genes encoding the components of iron-dependent biochemical pathways, including the tricarboxylic acid cycle, mitochondrial respiration, amino acid biosynthesis, and oxidative stress defense, were downregulated in response to iron deficiency. Furthermore, Php4 repressed transcription when brought to a promoter using a yeast DNA-binding domain, and iron deprivation was required for this repression. On the other hand, Php4 was constitutively active when glutathione levels were depleted within the cell. Based on these and previous results, we propose that iron-dependent inactivation of Php4 is regulated at two distinct levels: first, at the transcriptional level by the iron-responsive GATA factor Fep1 and second, at the posttranscriptional level by a mechanism yet to be identified, which inhibits Php4-mediated repressive function when iron is abundant.

Diallyl disulphide depletes glutathione in Candida albicans: oxidative stress-mediated cell death studied by two-photon microscopy

Using two-photon scanning laser microscopy, we investigated the effect of an Allium sativum (garlic) constituent, diallyl disulphide (DADS), on key physiological functions of the opportunistic pathogen Candida albicans. A short 30 min exposure to 0.5 mM DADS followed by removal induced 70% cell death (50% necrotic, 20% apoptotic) within 2 h, increasing to 75% after 4 h. The early intracellular events associated with DADS-induced cell death were monitored with two-photon fluorescence microscopy to track mitochondrial membrane potential (Deltapsi(m)), reactive oxygen species (ROS) and NADH or reduced glutathione (GSH) under aerobic conditions. DADS treatment decreased intracellular GSH and elevated intracellular ROS levels. Additionally, DADS induced a marked decrease of Deltapsi(m) and lowered respiration in cell suspensions and isolated mitochondria. In vitro kinetic experiments in cell-free extracts suggest that glutathione-S-transferase (GST) is one of the intracellular targets of DADS. Additional targets were also identified, including inhibition of a site or sites between complexes II-IV in the electron transport chain, as well as the mitochondrial ATP-synthase. The results indicate that DADS is an effective antifungal agent able to trigger cell death in Candida, most probably by eliciting oxidative stress as a consequence of thiol depletion and impaired mitochondrial function.

Genome-wide transcription profiling of the early phase of biofilm formation by Candida albicans

The ability to adhere to surfaces and develop as a multicellular community is an adaptation used by most microorganisms to survive in changing environments. Biofilm formation proceeds through distinct developmental phases and impacts not only medicine but also industry and evolution. In organisms such as the opportunistic pathogen Candida albicans, the ability to grow as biofilms is also an important mechanism for persistence, facilitating its growth on different tissues and a broad range of abiotic surfaces used in medical devices. The early stage of C. albicans biofilm is characterized by the adhesion of single cells to the substratum, followed by the formation of an intricate network of hyphae and the beginning of a dense structure. Changes in the transcriptome begin within 30 min of contact with the substrate and include expression of genes related to sulfur metabolism, in particular MET3, and the equivalent gene homologues of the Ribi regulon in Saccharomyces cerevisiae. Some of these changes are initiated early and maintained throughout the process; others are restricted to the earliest stages of biofilm formation. We identify here a potential alternative pathway for cysteine metabolism and the biofilm-associated expression of genes involved in glutathione production in C. albicans.

Glutathione is required for growth and prespore cell differentiation in Dictyostelium

Glutathione (GSH) is the most abundant non-protein thiol in eukaryotic cells and acts as reducing equivalent in many cellular processes. We investigated the role of glutathione in Dictyostelium development by disruption of gamma-glutamylcysteine synthetase (GCS), an essential enzyme in glutathione biosynthesis. GCS-null strain showed glutathione auxotrophy and could not grow in medium containing other thiol compounds. The developmental progress of GCS-null strain was determined by GSH concentration contained in preincubated media before development. GCS-null strain preincubated with 0.2 mM GSH was arrested at mound stage or formed bent stalk-like structure during development. GCS-null strain preincubated with more than 0.5 mM GSH formed fruiting body with spores, but spore viability was significantly reduced. In GCS-null strain precultured with 0.2 mM GSH, prestalk-specific gene expression was delayed, while prespore-specific gene and spore-specific gene expressions were not detected. In addition, GCS-null strain precultured with 0.2 mM GSH showed prestalk tendency and extended G1 phase of cell cycle. Since G1 phase cells at starvation differentiate into prestalk cells, developmental defect of GCS-null strain precultured with 0.2 mM GSH may result from altered cell cycle. These results suggest that glutathione itself is essential for growth and differentiation to prespore in Dictyostelium.

Correlation between the intracellular content of glutathione and the formation of germ-tubes induced by human serum in Candida albicans

The physiological role of the tripeptide glutathione (GSH) and its oxidized form (GSSG) was investigated during the initial steps of dimorphism (formation of germ-tubes), which is induced by human serum in exponential yeast-like cells (blastoconidia) of the Candida albicans strain CAI-4 (wild type) and its congenic tps1/tps1 mutant, deficient in trehalose synthesis. The content of glutathione, measured both as GSH and the ratio GSH/GSSG, underwent a moderate drop in parallel with the induction of a significant degree of germ-tube emergence. Whereas the supply of exogenous glutathione did not affect the degree of dimorphic transition, depletion of intracellular glutathione by addition of 1-chloro-2,4 dinitrobenzene (CDNB) caused a clear reduction in the percentage of hyphae formation; although this effect must be due to the severe cell mortality produced by CDNB. Simultaneous measurements of GSH-metabolizing activities revealed a moderate decrease of glutathione reductase concomitant with the activation of glutathione peroxidase. In turn, catalase activity did not show noticeable changes. The putative correlation between the redox status of glutathione and the dimorphic conversion in C. albicans is discussed.