Apoptosis induced by environmental stresses and amphotericin B in Candida albicans

Activity of tannins from Stryphnodendron adstringens on Cryptococcus neoformans: effects on growth, capsule size and pigmentation

BACKGROUND

Stryphnodendron adstringens (Mart.) Coville, Leguminosae, also known in Brazil as barbatimão, is rich in tannins and many flavan-3-ols and proanthocyanidins such as prodelphinidins and prorobinetinidins. Previous studies have demonstrated several pharmacological properties of tannins from barbatimão, including anti-candidal activity.

METHODS

The antifungal activity of proanthocyanidin polymeric tannins from Stryphnodendron adstringens (subfraction F2.4) was evaluated against three strains of Cryptococcus neoformans with different capsule expressions, using the broth microdilution technique, light microscopy and transmission electron microscopy. The effect of subfraction F2.4 on C. neoformans and melanoma mammalian cells pigmentation was also evaluated.

RESULTS

Although susceptibility assays revealed MIC values quite similar (between 2.5 and 5.0 microg/ml), analyses of MFC values revealing that the acapsular mutant Cap 67 was more susceptible to be killed by the subfraction F2.4 (MFC = 20 microg/ml) than the two tested capsular strains (T1-444 and ATCC 28957) (MFC > 160 microg/ml). Optical and electron microscopy experiments revealed relevant alterations in cell shape and size in all strains treated with 1 and 2.5 microg/ml of subfraction F2.4. Capsule size of the capsular strains decreased drastically after subfraction F2.4 treatment. In addition, ultrastructural alterations such as cell wall disruption, cytoplasm extraction, mitochondria swelling, increase in the number of cytoplasmic vacuoles and formation of membranous structures in the cytoplasm were also observed in treated yeasts. Incubation with subfraction F2.4 also decreased C. neoformans pigmentation, however, did not interfere in melanization of B16F10 mammalian cells.

CONCLUSION

Our data indicate that tannins extracted from S. adstringens interfered with growth, capsule size and pigmentation, all important virulence factors of C. neoformans, and may be considered as a putative candidate for the development of new antifungal agents.

Apoptosis in Candida biofilms exposed to amphotericin B

Candida biofilms are resistant to a range of antifungal agents in current clinical use. The basis of this drug resistance is not clear, but in some cases it could be due to the presence of a small number of drug-tolerant or persister cells. In this study, specific staining methods were used to investigate the existence of persisters and apoptosis in Candida biofilms subjected to different concentrations of amphotericin B. Fluorescein diacetate staining revealed the presence of persisters in biofilms of one of two strains of Candida albicans tested, and in biofilms of Candida krusei and Candida parapsilosis. Caspase activity, indicative of apoptosis, was detected with SR-FLICA and (aspartyl)(2)-rhodamine 110 fluorochrome-based staining reagents in all of these biofilms. The general inhibitor of mammalian caspases, Z-VAD-FMK, when used at a low concentration (2.5 microM), increased the viability of drug-treated biofilms up to 11.5-fold (P <0.001 %). Seven specific caspase inhibitors had different effects on C. albicans biofilm viability, but inhibitors of caspases-1, -9, -5, -3 and -2 all significantly increased cell survival (40-fold, 8-fold, 3.5-fold, 1.9-fold and 1.7-fold, respectively). However, histone deacetylase (HDA) inhibitors enhanced the activity of amphotericin B for biofilms of all three Candida species. Sodium butyrate and sodium valproate, for example, when added concurrently with amphotericin B, completely eliminated biofilm populations of C. albicans. Overall, our results demonstrate an apoptotic process in amphotericin-treated biofilms of three Candida species. They also indicate that HDA inhibitors can enhance the action of the drug and in some cases even eradicate persister subpopulations, suggesting that histone acetylation might activate apoptosis in these cells.

Ras1 and Ras2 play antagonistic roles in regulating cellular cAMP level, stationary-phase entry and stress response in Candida albicans

The GTPase Ras1 activates the yeast-to-hypha transition in Candida albicans by activating cAMP synthesis. Here, we have characterized Ras2. Ras2 belongs to a group of atypical Ras proteins in some fungal species that share poor identity with other Ras GTPases with many variations in conserved motifs thought to be crucial for Ras-associated activities. We find that recombinant Ras2 is enzymatically as active as Ras1. However, only RAS1 can rescue the lethality of the Saccharomyces cerevisiae ras1 ras2 mutant, suggesting functional divergence of the two genes. ras2Delta is normal in hyphal growth, but deleting RAS2 in the ras1Delta background greatly aggravates the hyphal defect, indicating that Ras2 also has a role in hyphal development. Strikingly, while RAS1 deletion causes a approximately 20-fold decrease in cellular cAMP, further deletion of RAS2 restores it to approximately 30% of the wild-type level. Consistently, while the ras1Delta mutant enters the stationary phase prematurely, the double mutant does so normally. Moreover, ras1Delta cells exhibit increased resistance to H(2)O(2) and higher sensitivity to the heavy metal Co(2+), whereas ras2Delta cells show the opposite phenotypes. Together, our data reveal a novel regulatory mechanism by which two antagonizing Ras GTPases balance each other in regulating multiple cellular processes in C. albicans.

Transcriptional profiling and functional analysis of heterokaryon incompatibility in Neurospora crassa reveals that reactive oxygen species, but not metacaspases, are associated with programmed cell death

Heterokaryon incompatibility (HI) is a nonself recognition phenomenon occurring in filamentous fungi that is important for limiting resource plundering and restricting viral transfer between strains. Nonself recognition and HI occurs during hyphal fusion between strains that differ at het loci. If two strains undergo hyphal fusion, but differ in allelic specificity at a het locus, the fusion cell is compartmentalized and undergoes a rapid programmed cell death (PCD). Incompatible heterokaryons show a macroscopic phenotype of slow growth and diminished conidiation, and a microscopic phenotype of hyphal compartmentation and cell death. To understand processes associated with HI and PCD, we used whole-genome microarrays for Neurospora crassa to assess transcriptional differences associated with induction of HI mediated by differences in het-c pin-c haplotype. Our data show that HI is a dynamic and transcriptionally active process. The production of reactive oxygen species is implicated in the execution of HI and PCD in N. crassa, as are several genes involved in phosphatidylinositol and calcium signalling pathways. However, genes encoding mammalian homologues of caspases or apoptosis-inducing factor (AIF) are not required for HI or programmed cell death. These data indicate that PCD during HI occurs via a novel and possibly fungal-specific mechanism, making this pathway an attractive drug target for control of fungal infections.

Antifungal drug susceptibility of oral Candida albicans isolates may be associated with apoptotic responses to Amphotericin B

BACKGROUND

Candida albicans is the important opportunistic fungal pathogens which can cause oral Candidiasis and even more seriously systemic infection. Apoptosis of C. albicans induced by environmental factor such as weak acid and antifungal drugs were studied recently. Illustrating the phenomenon of apoptosis in C. albicans may help us to discover new antifungal therapy by activating the fungal cells to suicide.

METHODS

Two oral C. albians clinical isolates which isolated respectively from healthy host [Strain 23C: minimal inhibition concentration (MIC) is 0.125 microg/ml for Amphotericin B (AmB)] and advanced cancer patient (Strain 28A: MIC is 2 microg/ml for AmB), were induced by 1 microg/ml AmB in vitro for 200 min, and then studied the apoptosis markers using terminal deoxynucletidyltransferase-mediated dUTP nick end labeling (TUNEL) (shown by diaminobenzidine and fluorescent isothiocyanate), and the ultrastructure of cell nuclear using transmission electron microscope (TEM), quantitative analysis using flow cytometry for the rapid exposure of phosphatidylserine at the outer membrane and propodium iodide (PI) double staining. C. albicans conference strain YEM30 was used as the control strain.

RESULTS

With TUNEL assay and TEM, we detected the typical characteristics of apoptosis. Strain 23C (with low MIC) showed significantly higher percentage of apoptosis (19.92%) compared with Strain 28A (with high MIC) which was isolated from the cancer patient (7.29%) (P < 0.01). In addition, 7.3% of early apoptosis cells of Strain 23C can form colonies on the plates, while 15% for Strain 28A. None of the PI+ cells can form colony.

CONCLUSIONS

Apoptosis of oral C. albicans isolates can be induced by AmB. The feature of antifungal drug susceptibility of the oral C. albicans clinical isolates may associate with the response of apoptosis inducing.

Nitrosative and oxidative stress responses in fungal pathogenicity

Fungal pathogenicity has arisen in polyphyletic manner during evolution, yielding fungal pathogens with diverse infection strategies and with differing degrees of evolutionary adaptation to their human host. Not surprisingly, these fungal pathogens display differing degrees of resistance to the reactive oxygen and nitrogen species used by human cells to counteract infection. Furthermore, whilst evolutionarily conserved regulators, such as Hog1, are central to such stress responses in many fungal pathogens, species-specific differences in their roles and regulation abound. In contrast, there is a high degree of commonality in the cellular responses to reactive oxygen and nitrogen species evoked in evolutionarily divergent fungal pathogens.

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.

The Acinetobacter baumannii 19606 OmpA protein plays a role in biofilm formation on abiotic surfaces and in the interaction of this pathogen with eukaryotic cells

The ability of Acinetobacter baumannii to adhere to and persist on surfaces as biofilms could be central to its pathogenicity. The production of pili and a biofilm-associated protein and the expression of antibiotic resistance are needed for robust biofilm formation on abiotic and biotic surfaces. This multistep process also depends on the expression of transcriptional regulatory functions, some of which could sense nutrients available to cells. This report extends previous observations by showing that although outer membrane protein A (OmpA) of A. baumannii 19606 plays a partial role in the development of robust biofilms on plastic, it is essential for bacterial attachment to Candida albicans filaments and A549 human alveolar epithelial cells. In contrast to abiotic surfaces, the interaction with biotic surfaces is independent of the CsuA/BABCDE-mediated pili. The interaction of A. baumannii 19606 with fungal and epithelial cells also results in their apoptotic death, a response that depends on the direct contact of bacteria with these two types of eukaryotic cells. Furthermore, the bacterial adhesion phenotype correlates with the ability of bacteria to invade A549 epithelial cells. Interestingly, the killing activity of cell-free culture supernatants proved to be protease and temperature sensitive, suggesting that its cytotoxic activity is due to secreted proteins, some of which are different from OmpA.

Growth inhibition and ultrastructural alterations induced by Delta24(25)-sterol methyltransferase inhibitors in Candida spp. isolates, including non-albicans organisms

Background

Although Candida species are commensal microorganisms, they can cause many invasive fungal infections. In addition, antifungal resistance can contribute to failure of treatment.

The purpose of this study was to evaluate the antifungal activity of inhibitors of Δ²⁴⁽²⁵⁾-sterol methyltransferase (24-SMTI), 20-piperidin-2-yl-5α-pregnan-3β-20(R)-diol (AZA), and 24(R,S),25-epiminolanosterol (EIL), against clinical isolates of Candida spp., analysing the ultrastructural changes.

Results

AZA and EIL were found to be potent growth inhibitors of Candida spp. isolates. The median MIC₅₀ was 0.5 μg.ml^-1 for AZA and 2 μg.ml^-1 for EIL, and the MIC₉₀ was 2 μg.ml^-1 for both compounds. All strains used in this study were susceptible to amphotericin B; however, some isolates were fluconazole- and itraconazole-resistant. Most of the azole-resistant isolates were Candida non-albicans (CNA) species, but several of them, such as C. guilliermondii, C. zeylanoides, and C. lipolytica, were susceptible to 24-SMTI, indicating a lack of cross-resistance. Reference strain C. krusei (ATCC 6258, FLC-resistant) was consistently susceptible to AZA, although not to EIL. The fungicidal activity of 24-SMTI was particularly high against CNA isolates. Treatment with sub-inhibitory concentrations of AZA and EIL induced several ultrastructural alterations, including changes in the cell-wall shape and thickness, a pronounced disconnection between the cell wall and cytoplasm with an electron-lucent zone between them, mitochondrial swelling, and the presence of electron-dense vacuoles. Fluorescence microscopy analyses indicated an accumulation of lipid bodies and alterations in the cell cycle of the yeasts. The selectivity of 24-SMTI for fungal cells versus mammalian cells was assessed by the sulforhodamine B viability assay.

Conclusion

Taken together, these results suggest that inhibition of 24-SMT may be a novel approach to control Candida spp. infections, including those caused by azole-resistant strains.

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.

Farnesol-induced apoptosis in Candida albicans

Farnesol, a precursor in the isoprenoid/sterol pathway, was recently identified as a quorum-sensing molecule produced by the fungal pathogen Candida albicans. Farnesol is involved in the inhibition of germination and biofilm formation by C. albicans and can be cytotoxic at certain concentrations. In addition, we have shown that farnesol can trigger apoptosis in mammalian cells via the classical apoptotic pathways. In order to elucidate the mechanism behind farnesol cytotoxicity in C. albicans, the response to farnesol was investigated, using proteomic analysis. Global protein expression profiles demonstrated significant changes in protein expression resulting from farnesol exposure. Among the downregulated proteins were those involved in metabolism, glycolysis, protein synthesis, and mitochondrial electron transport and the respiratory chain, whereas proteins involved in folding, protection against environmental and oxidative stress, actin cytoskeleton reorganization, and apoptosis were upregulated. Cellular changes that accompany apoptosis (regulated cell death) were further analyzed using fluorescent microscopy and gene expression analysis. The results indicated reactive oxygen species accumulation, mitochondrial degradation, and positive terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) in the farnesol-exposed cells concurrent with increased expression of antioxidant-encoding and drug response genes. More importantly, the results demonstrated farnesol-induced upregulation of the caspase gene MCA1 and the intracellular presence of activated caspases. In conclusion, this study demonstrated that farnesol promotes apoptosis in C. albicans through caspase activation, implying an important physiological role for farnesol in the fungal cell life cycle with important implications for adaptation and survival.

Comparative neuroprotective effects of rasagiline and aminoindan with selegiline on dexamethasone-induced brain cell apoptosis

Stress can affect the brain and lead to depression; however, the molecular pathogenesis is unclear. An association between stress and stress-induced hypersecretion of glucocorticoids occurs during stress. Dexamethasone (a synthetic glucocorticoid steroid) has been reported to induce apoptosis and increase the activity of monoamine oxidase (MAO) (Youdim et al. 1989). MAO is an enzyme for the degradation of aminergic neurotransmitters; dopamine, noradrenaline and serotonin and dietary amines and MAO inhibitors are classical antidepressant drugs. In this study, we have compared the ability of rasagiline (Azilect) and its main metabolite, R-aminoindan with selegiline (Deprenyl) in prevention of dexamethasone-induced brain cell death employing human neuroblastoma SH-SY5Y cells and glioblastoma 1242-MG cells. Dexamethasone reduced cell viability as measured by MTT test, but rasagiline, selegiline, and 1-R-aminoindan could significantly prevent dexamethasone-induced brain cell death. Among three drugs, rasagiline had the highest neuroprotective effect. Furthermore, the inhibitory effects of these drugs on MAO B catalytic activity and on apoptotic DNA damage (TUNEL staining) were examined. Rasagiline exhibited highest inhibition on MAO B enzymatic activity and prevention on DNA damage as compared to selegiline and 1-R-aminoindan. In summary, the greater neuroprotective effect of rasagiline may be associated with the combination of the parent drug and its metabolite 1-R-aminoindan.

Whi2p links nutritional sensing to actin-dependent Ras-cAMP-PKA regulation and apoptosis in yeast

Elucidating the mechanisms by which eukaryotic cells coordinate environmental signals with intracellular ;fate' decisions, such as apoptosis, remains one of the important challenges facing cell biologists. It has recently emerged that the dynamic nature of the actin cytoskeleton is an important factor in the linkage of sensation of extracellular stimuli to signalling mechanisms that regulate programmed cell death. In yeast, actin has been shown to play a role in the regulation of apoptosis as cells prepare themselves for quiescence in the face of nutritional exhaustion, by facilitating the shutdown of Ras-cAMP-PKA pathway activity. Here, we demonstrate that the loss of Whi2p function, a protein known to influence cell cycle exit under conditions of nutritional stress, leads to cell death in yeast that displays the hallmarks of actin-mediated apoptosis. We show that actin-mediated apoptosis occurs as a result of inappropriate Ras-cAMP-PKA activity in Deltawhi2 cells. Cells lacking Whi2p function exhibit an aberrant accumulation of activated Ras2 at the mitochondria in response to nutritional depletion. This study provides evidence that the shutdown of cAMP-PKA signalling activity in wild-type cells involves Whi2p-dependent targeting of Ras2p to the vacuole for proteolysis. We also demonstrate for the first time that Whi2p-dependent regulation of cAMP-PKA signalling plays a physiological role in the differentiation of yeast colonies by facilitating elaboration of distinct zones of cell death.

Pharmacotherapy of yeast infections

The rise of immunocompromised individuals in our society has provoked a significant emergence in the number of patients affected by opportunistic pathogenic yeast. The microorganisms with a major clinical incidence are species from the genera Candida (especially Candida albicans) and Cryptococcus (particularly Cryptococcus neoformans), although there has been a significant increase in other pathogenic yeasts, such as Trichosporon spp. and Rhodotorula spp. In addition, there are an increasing number of patients infected by yeasts that were not previously considered as pathogenic, such as Saccharomyces cerevisiae. The management of these infections is complicated and is highly dependent on the susceptibility profile not only of the species but also of the strain. The available antifungal compounds belong mainly to the polyene, azole and candin families, which show a distinct spectrum of activity. This review summarizes the current knowledge about the use of the main antifungals for treating infections caused by the yeast species with the most significant clinical relevance, including the susceptibility profiles exhibited by these species in vitro.

Candida albicans–macrophage interactions: genomic and proteomic insights

Candida albicans infection is a significant cause of morbidity and mortality in immunocompromised patients. In vivo and in vitro models have been developed to study both the fungal and the mammalian immune responses. Phagocytic cells (i.e., macrophages) play a key role in innate immunity against C. albicans by capturing, killing and processing the pathogen for presentation to T cells. The use of microarray technology to study global fungal transcriptional changes after interaction with different host cells has revealed how C. albicans adapts to its environment. Proteomic tools complement molecular approaches and computational methods enable the formulation of relevant biological hypotheses. Therefore, the combination of genomics, proteomics and bioinformatics tools (i.e., network analyses) is a powerful strategy to better understand the biological situation of the fungus inside macrophages; part of the fungal population is killed while a significantly high percentage survives.

Candida albicans cells lacking CaMCA1-encoded metacaspase show resistance to oxidative stress-induced death and change in energy metabolism

Candida albicans, an opportunistic pathogen, can undergo programmed cell death upon various stimuli, including oxidative stress. In this study, we showed that deletion of CaMCA1, a homologue of Saccharomyces cerevisiae metacaspase YCA1, could both attenuated oxidative stress-induced cell death and caspase activation. Compared to wild-type strain, Camca1Delta mutant showed higher accumulation of trehalose and transcription of the genes related to trehalose biosynthesis (TPS2 and TPS3) under the condition of oxidative stress. Furthermore, lower intracellular ATP concentration and mitochondrial membrane potential, less endogenous reactive oxygen species (ROS) generation were observed in Camca1Delta mutant. Our results suggest that CaMCA1 might mediate the sensitiveness to oxidative stress by affecting energy metabolism in C. albicans.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Human lactoferrin induces apoptosis-like cell death in Candida albicans: critical role of K+-channel-mediated K+ efflux

Human lactoferrin (hLf) induced an apoptosis-like phenotype in Candida albicans cells, which includes phosphatidylserine externalization, nuclear chromatin condensation, DNA degradation, and increased reactive oxygen species (ROS) production. Intracellular ROS accumulation was seen to correlate with candidacidal activity in hLf-treated cells. Mitochondrial activity was involved as indicated by mitochondrial depolarization and increased hLf resistance of cells preincubated with sordarin or erythromycin, the latter of which inhibits protein synthesis in mitoribosomes. Interestingly, Cl(-)- and K(+)-channel blockers prevented the hLf antimicrobial activity, but only when cells were pretreated with the blocking agent (tetraethylammonium) prior to the hLf-induced K(+)-release period. These results indicate for the first time that K(+)-channel-mediated K(+) efflux is required for the progression of apoptosis-like process in yeast, suggesting that this essential apoptotic event of higher eukaryotes has been evolutionary conserved among species ranging from yeasts to humans.

DNA fragmentation in microorganisms assessed in situ

Chromosomal DNA fragmentation may be a direct or indirect outcome of cell death. Unlike DNA fragmentation in higher eukaryotic cells, DNA fragmentation in microorganisms is rarely studied. We report an adaptation of a diffusion-based assay, developed as a kit, which allows for simple and rapid discrimination of bacteria with fragmented DNA. Intact cells were embedded in an agarose microgel on a slide, incubated in a lysis buffer to partially remove the cell walls, membranes, and proteins, and then stained with a DNA fluorochrome, SYBR Gold. Identifying cells with fragmented DNA uses peripheral diffusion of DNA fragments. Cells without DNA fragmentation show only limited spreading of DNA fiber loops. These results have been seen in several gram-negative and gram-positive bacteria, as well as in yeasts. Detection of DNA fragmentation was confirmed by fluoroquinolone treatment and by DNA breakage detection-fluorescence in situ hybridization. Proteus mirabilis with spontaneously fragmented DNA during exponential and stationary growth or Escherichia coli with DNA damaged after exposure to hydrogen peroxide or antibiotics, such as ciprofloxacin or ampicillin, was clearly detected. Similarly, fragmented DNA was detected in Saccharomyces cerevisiae after amphotericin B treatment. Our assay may be useful for the simple and rapid evaluation of DNA damage and repair as well as cell death, either spontaneous or induced by exogenous stimuli, including antimicrobial agents or environmental conditions.

MNL1 regulates weak acid-induced stress responses of the fungal pathogen Candida albicans

MNL1, the Candida albicans homologue of an orphan Msn2-like gene (YER130c in Saccharomyces cerevisiae) has no known function. Here we report that MNL1 regulates weak acid stress responses. Deletion of MNL1 prevents the long-term adaptation of C. albicans cells to weak acid stresses and compromises their global transcriptional response under these conditions. The promoters of Mnl1-dependent genes contain a novel STRE-like element (SLE) that imposes Mnl1-dependent, weak acid stress-induced transcription upon a lacZ reporter in C. albicans. The SLE (HHYYCCCCTTYTY) is related to the Nrg1 response element (NRE) element recognized by the transcriptional repressor Nrg1. Deletion of NRG1 partially restores the ability of C. albicans mnl1 cells to adapt to weak acid stress, indicating that Mnl1 and Nrg1 act antagonistically to regulate this response. Molecular, microarray, and proteomic analyses revealed that Mnl1-dependent adaptation does not occur in cells exposed to proapoptotic or pronecrotic doses of weak acid, suggesting that Ras-pathway activation might suppress the Mnl1-dependent weak acid response in dying cells. Our work defines a role for this YER130c orthologue in stress adaptation and cell death.

Increased resistance of complex I mutants to phytosphingosine-induced programmed cell death

We have studied the effects of phytosphingosine (PHS) on cells of the filamentous fungus Neurospora crassa. Highly reduced viability, impairment of asexual spore germination, DNA condensation and fragmentation, and production of reactive oxygen species were observed in conidia treated with the drug, suggesting that PHS induces an apoptosis-like death in this fungus. Interestingly, we found that complex I mutants are more resistant to PHS treatment than the wild type strain. This effect appears to be specific because it was not observed in mutants defective in other components of the mitochondrial respiratory chain, pointing to a particular involvement of complex I in cell death. The response of the mutant strains to PHS correlated with their response to hydrogen peroxide. The fact that complex I mutants generate fewer reactive oxygen species than the wild type strain when exposed to PHS likely explains the PHS-resistant phenotype. As compared with the wild type strain, we also found that a strain containing a deletion in the gene encoding an AIF (apoptosis-inducing factor)-like protein is more resistant to PHS and H2O2. In contrast, a strain containing a deletion in a gene encoding an AMID (AIF-homologous mitochondrion-associated inducer of death)-like polypeptide is more sensitive to both drugs. These results indicate that N. crassa has the potential to be a model organism to investigate the molecular basis of programmed cell death in eukaryotic species.

Apoptosis pathways in fungal growth, development and ageing

Apoptosis is one type of programmed cell death with great importance for development and homeostasis of multicellular organisms. Unexpectedly, during the past decade, evidence has been obtained for the existence of a basal apoptosis machinery in yeast, as unicellular fungus, and in some filamentous fungi, a group of microorganisms that are neither true unicellular nor true multicellular biological systems but something in between. Here, we review evidence for a role of apoptotic processes in fungal pathogenicity, competitiveness, propagation, ageing and lifespan control.

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.

Programmed cell death in fission yeast Schizosaccharomyces pombe

Yeasts have proven to be invaluable, genetically tractable systems to study various fundamental biological processes including programmed cell death. Recent advances in the elucidation of the molecular pathways underlying apoptotic cell death in yeasts have revealed remarkable similarities to mammalian apoptosis at cellular, organelle and macromolecular levels, thus making a strong case for the relevance of yeast models of regulated cell death. Programmed cell death has been reported in fission yeast Schizosaccharomyces pombe, primarily in the contexts of perturbed intracellular lipid metabolism, defective DNA replication, improper mitotic entry, chronological and replicative aging. Here we review the current understanding of the programmed cell death in fission yeast, paying particular attention to lipid-induced cell death. We discuss our recent findings that fission yeast exhibits plasticity of apoptotic and non-apoptotic modes of cell death in response to different lipid stimuli and growth conditions, and that mitochondria, reactive oxygen species and novel cell death mediators including metacaspase Pca1, SpRad9 and Pck1 are involved in the lipotoxic cell death. We also present perspectives on how various aspects of the cell and molecular biology of this organism can be explored to shed light on the governing principles underlying lipid-mediated signaling and cell demise.

Reactive oxygen species and yeast apoptosis

Apoptosis is associated in many cases with the generation of reactive oxygen species (ROS) in cells across a wide range of organisms including lower eukaryotes such as the yeast Saccharomyces cerevisiae. Currently there are many unresolved questions concerning the relationship between apoptosis and the generation of ROS. These include which ROS are involved in apoptosis, what mechanisms and targets are important and whether apoptosis is triggered by ROS damage or ROS are generated as a consequence or part of the cellular disruption that occurs during cell death. Here we review the nature of the ROS involved, the damage they cause to cells, summarise the responses of S. cerevisiae to ROS and discuss those aspects in which ROS affect cell integrity that may be relevant to the apoptotic process.

Drug-induced apoptosis in yeast

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

Autophagy in Candida albicans

Candida albicans is considered a commensal organism of humans, colonizing the oral cavity, gastrointestinal, and reproductive tracts. However, when host defenses are compromised C. albicans can transform into a tissue invasive pathogen. Infections fall into two broad categories, those of mucosal tissue and serious disseminated disease, which involves transport through the bloodstream and invasion of the deeper organs. The ability of C. albicans to colonize diverse host microenvironments and switch from benign commensal to invasive pathogen suggests that C. albicans is able to undergo rapid and highly specialized adaptive responses in vivo. To date the role played by autophagy in facilitating asymptomatic host colonization, persistence, and transition of C. albicans into its pathogenic form have not been fully explored. Also the therapeutic potential of manipulating autophagic degradation within an invading fungus is yet to be established. In this chapter we describe several methods that have been adapted to detect autophagy within the opportunistic pathogen C. albicans.

The antifungal activity of RsAFP2, a plant defensin from raphanus sativus, involves the induction of reactive oxygen species in Candida albicans

RsAFP2 (Raphanus sativus antifungal peptide 2), an antifungal plant defensin isolated from seed of R. sativus, interacts with glucosylceramides (GlcCer) in membranes of susceptible yeast and fungi and induces membrane permeabilization and fungal cell death. However, using carboxyfluorescein-containing small unilamellar vesicles containing purified GlcCer, we could not observe permeabilization as a consequence of insertion of RsAFP2 in such vesicles. Therefore, we focused on a putative RsAFP2-induced signaling cascade downstream of RsAFP2-binding to GlcCer in fungal membranes. We show that RsAFP2 induces reactive oxygen species (ROS) in Candida albicans wild type in a dose-dependent manner, but not at all in an RsAFP2-resistant DeltagcsC. albicans mutant that lacks the RsAFP2-binding site in its membranes. These findings indicate that upstream binding of RsAFP2 to GlcCer is needed for ROS production leading to yeast cell death. Moreover, the antioxidant ascorbic acid blocks RsAFP2-induced ROS generation, as well as RsAFP2 antifungal activity. These data point to the presence of an intracellular plant defensin-induced signaling cascade, which involves ROS generation and leads to fungal cell growth arrest.

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.

Role of actin cytoskeletal dynamics in activation of the cyclic AMP pathway and HWP1 gene expression in Candida albicans

Changes in gene expression during reversible bud-hypha transitions of the opportunistic fungal pathogen Candida albicans permit adaptation to environmental conditions that are critical for proliferation in host tissues. Our previous work has shown that the hypha-specific adhesin gene HWP1 is up-regulated by the cyclic AMP (cAMP) signaling pathway. However, little is known about the potential influences of determinants of cell morphology on HWP1 gene expression. We found that blocking hypha formation with cytochalasin A, which destabilizes actin filaments, and with latrunculin A, which sequesters actin monomers, led to a loss of HWP1 gene expression. In contrast, high levels of HWP1 gene expression were observed when the F-actin stabilizer jasplakinolide was used to block hypha formation, suggesting that HWP1 expression could be regulated by actin structures. Mutants defective in formin-mediated nucleation of F-actin were reduced in HWP1 gene expression, providing genetic support for the importance of actin structures. Kinetic experiments with wild-type and actin-deficient cells revealed two distinct phases of HWP1 gene expression, with a slow, actin-independent phase preceding a fast, actin-dependent phase. Low levels of HWP1 gene expression that appeared to be independent of stabilized actin and cAMP signaling were detected using indirect immunofluorescence. A connection between actin structures and the cAMP signaling pathway was shown using hyper- and hypomorphic cAMP mutants, providing a possible mechanism for up-regulation of HWP1 gene expression by stabilized actin. The results reveal a new role for F-actin as a regulatory agent of hypha-specific gene expression at the bud-hypha transition.

Histatin-induced alterations in Candida albicans: a microscopic and submicroscopic comparison

Despite the numerous studies performed in an attempt to clarify the issue, the mechanism of action of salivary histatins remains unclear. The aim of the present study was to correlate histatin-induced morphological changes in Candida albicans by fluorescence microscopy (FM), transmission electron microscopy (TEM), and high resolution scanning electron microscopy (HRSEM). Each of the fluorescent dyes used by FM (i.e., tetramethylrhodamine methyl ester perchlorate for mitochondrial potential, Lysotracker for lysosome acidic compartment, and 4',6-diamino-2-phenylindole dihydrochloride for DNA) exhibited a specific staining in control cells. Following histatin treatment, we observed a recurring staining pattern, corresponding to fluorescence concentration along the cell periphery, suggesting a loss of dye specificity. To assess histatin-induced cytoplasmic modifications, ultrastructural analysis was then carried out. After treatments with histatins, TEM revealed characteristic intracellular modifications including: vacuole overgrowth, nuclear disappearance, loss of organelle identity, as well as the appearance of electron-dense membranes, likely of mitochondrial origin. Additionally, structures resembling autophagosomes were occasionally observed. By HRSEM, mitochondrial swelling was invariably the first sign of a histatin-induced effect. Other modifications included intracellular membrane disarrangement, organelles in disarray, and a large central cavity with deformed bodies displaced to the cell periphery, similar to what was detected by TEM. In summary, our study illustrates the occurrence of ultrastructural modifications following administration of histatins. Observations made with FM, TEM, and HRSEM provided different views of the same signs, demonstrating a definite action of histatins on C. albicans morphology. The possible functional meanings of these morphological results is discussed in light of the most recent biochemical data on histatin fungicidal activity.

SpxB is a suicide gene of Streptococcus pneumoniae and confers a selective advantage in an in vivo competitive colonization model

The human bacterial pathogen Streptococcus pneumoniae dies spontaneously upon reaching stationary phase. The extent of S. pneumoniae death at stationary phase is unusual in bacteria and has been conventionally attributed to autolysis by the LytA amidase. In this study, we show that spontaneous pneumococcal death is due to hydrogen peroxide (H(2)O(2)), not LytA, and that the gene responsible for H(2)O(2) production (spxB) also confers a survival advantage in colonization. Survival of S. pneumoniae in stationary phase was significantly prolonged by eliminating H(2)O(2) in any of three ways: chemically by supplementing the media with catalase, metabolically by growing the bacteria under anaerobic conditions, or genetically by constructing DeltaspxB mutants that do not produce H(2)O(2). Likewise, addition of H(2)O(2) to exponentially growing S. pneumoniae resulted in a death rate similar to that of cells in stationary phase. While DeltalytA mutants did not lyse at stationary phase, they died at a rate similar to that of the wild-type strain. Furthermore, we show that the death process induced by H(2)O(2) has features of apoptosis, as evidenced by increased annexin V staining, decreased DNA content, and appearance as assessed by transmission electron microscopy. Finally, in an in vivo rat model of competitive colonization, the presence of spxB conferred a selective advantage over the DeltaspxB mutant, suggesting an explanation for the persistence of this gene. We conclude that a suicide gene of pneumococcus is spxB, which induces an apoptosis-like death in pneumococci and confers a selective advantage in nasopharyngeal cocolonization.

Heat stress: an inducer of programmed cell death in Chlorella saccharophila

Programmed cell death (PCD) has been recognized as a fundamental cellular process conserved in metazoans, plants and yeast. However, the cellular mechanisms leading to PCD have not been fully elucidated in unicellular organisms. Evidence is presented that heat stress induces PCD in Chlorella saccharophila cells. Our results demonstrate that heat shock triggers a PCD pathway occurring with characteristics features such as chromatin condensation, DNA fragmentation, cell shrinkage and detachment of the plasma membrane from the cell wall, and suggest the presence of caspase 3-like activity. The caspase 3 inhibitor Ac-DEVD-CHO gave significant protection against heat shock-induced cell death. Moreover, a reduction in photosynthetic pigment contents associated with alteration of chloroplast morphology and a fairly rapid disappearance of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit and the light-harvesting complex of PSII have been observed. The timing of events in the signaling cascade associated with the C. saccharophila heat shock PCD response is discussed. Insights into this field may have general implications for understanding the pathway of cell death in unicellular green algae.

alpha-Tomatine, the major saponin in tomato, induces programmed cell death mediated by reactive oxygen species in the fungal pathogen Fusarium oxysporum

The tomato saponin alpha-tomatine has been proposed to kill sensitive cells by binding to cell membranes followed by leakage of cell components. However, details of the modes of action of the compound on fungal cells are poorly understood. In the present study, mechanisms involved in alpha-tomatine-induced cell death of fungi were examined using a filamentous pathogenic fungus Fusarium oxysporum. alpha-Tomatine-induced cell death of F. oxysporum (TICDF) occurred only under aerobic conditions and was blocked by the mitochondrial F(0)F(1)-ATPase inhibitor oligomycin, the caspase inhibitor D-VAD-fmk, and protein synthesis inhibitor cycloheximide. Fungal cells exposed to alpha-tomatine showed TUNEL-positive nuclei, depolarization of transmembrane potential of mitochondria, and reactive oxygen species (ROS) accumulation. These results suggest that TICDF occurs through a programmed cell death process in which mitochondria play a pivotal role. Pharmacological studies using inhibitors suggest that alpha-tomatine activates phosphotyrosine kinase and monomeric G-protein signaling pathways leading to Ca(2+) elevation and ROS burst in F. oxysporum cells.

Mitochondrial-derived ROS in edelfosine-induced apoptosis in yeasts and tumor cells

AIM

To investigate whether a similar process mediates cytotoxicity of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3, edelfosine) in both yeasts and human tumor cells.

METHODS

A modified version of a previously described assay for the intracellular conversion of nitro blue tetrazolium to formazan by superoxide anion was used to measure the generation of reactive oxygen species (ROS). Apoptotic yeast cells were detected using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay. DNA fragmentation and the generation of ROS were measured by cytofluorimetric analysis in Jurkat cells.

RESULTS

Edelfosine induced apoptosis in Saccharomyces cerevisiae, as assessed by TUNEL assay. Meanwhile, edelfosine induced a time- and concentration-dependent generation of ROS in yeasts. Rotenone, an inhibitor of the mitochondrial electron transport chain, prevented ROS generation and apoptosis in response to edelfosine in S cerevisiae. alpha-Tocopherol abrogated the edelfosine-induced generation of intracellular ROS and apoptosis. Edelfosine also induced an increase of ROS in human leukemic cells that preceded apoptosis. The overexpression of Bcl-2 by gene transfer abrogated both ROS generation and apoptosis induced by edelfosine in leukemic cells. Changes in the relative mitochondrial membrane potential were detected in both yeasts and Jurkat cells.

CONCLUSION

These results indicate that edelfosine induces apoptosis in yeasts in addition to human tumor cells, and this apoptotic process involves mitochondria, likely through mitochondrial-derived ROS. These data also suggest that yeasts can be used as a suitable cell model in elucidating the antitumor mechanism of action of edelfosine.

CaIPF7817 is involved in the regulation of redox homeostasis in Candida albicans

CaIPF7817, a functionally unknown gene in Candida albicans, was suggested to be involved in the redox system previously, but its exact role is unknown. In this study, ipf7817 null mutant was generated with the URA-blaster method. After the deletion of CaIPF7817, intracellular levels of reactive oxygen species were significantly increased; mitochondrial membrane potential, a direct indicator of mitochondrial function, was elevated; some important redox-related genes, including GLR1, SOD2, and TRR1, were up-regulated; and the GSH/GSSG ratio was raised. These changes indicated that CaIPF7817 played important roles in the regulation of redox homeostasis in C. albicans.

Contribution of the mannan backbone of cryptococcal glucuronoxylomannan and a glycolytic enzyme of Staphylococcus aureus to contact-mediated killing of Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans is killed by the bacterium Staphylococcus aureus, and the killing is inhibited by soluble capsular polysaccharides. To investigate the mechanism of killing, cells in coculture were examined by scanning and transmission electron microscopy. S. aureus attached to the capsule of C. neoformans, and the ultrastructure of the attached C. neoformans cells was characteristic of dead cells. To identify the molecules that contributed to the fungal-bacterial interaction, we treated each with NaIO(4) or protease. Treatment of C. neoformans with NaIO(4) promoted adherence. It was inferred that cleavage of xylose and glucuronic acid side chains of glucuronoxylomannan (GXM) allowed S. aureus to recognize mannose residues in the backbone, which resisted periodate oxidation. On the other hand, treatment of S. aureus with protease decreased adherence, suggesting that protein contributed to attachment in S. aureus. In confirmation, side chain-cleaved polysaccharide or defined alpha-(1-->3)-mannan inhibited the killing at lower concentrations than native GXM did. Also, these polysaccharides reduced the adherence of the two species and induced clumping of pure S. aureus cells. alpha-(1-->3)-Mannooligosaccharides with a degree of polymerization (DP) of >/=3 induced cluster formation of S. aureus in a dose-dependent manner. Surface plasmon resonance analyses showed interaction of GXM and surface protein from S. aureus; the interaction was inhibited by oligosaccharides with a DP of > or =3. Conformations of alpha-(1-->3) oligosaccharides were predicted. The three-dimensional structures of mannooligosaccharides larger than triose appeared curved and could be imagined to be recognized by a hypothetical staphylococcal lectin. Native polyacrylamide gel electrophoresis of staphylococcal protein followed by electroblotting, enzyme-linked immunolectin assay, protein staining, and N-terminal amino acid sequencing suggested that the candidate protein was triosephosphate isomerase (TPI). The enzymatic activities were confirmed by using whole cells of S. aureus. TPI point mutants of S. aureus decreased the ability to interact with C. neoformans. Thus, TPI on S. aureus adheres to the capsule of C. neoformans by recognizing the structure of mannotriose units in the backbone of GXM; we suggest that this contact is required for killing of C. neoformans.

Lovastatin possesses a fungistatic effect against Candida albicans, but does not trigger apoptosis in this opportunistic human pathogen

Lovastatin inhibited the growth of Candida albicans in a fungistatic way. Although it triggers apoptosis in a great variety of eukaryotic cells, including many tumour cell lines, lovastatin failed to provoke apoptotic events in this human pathogen. The fungistatic behaviour of this statin might arise from its negative influence on membrane fluidity. Because yeast-->pseudomycelium and hyphae morphogenetic transitions took place under exposure to lovastatin morphogenetic switch and apoptotic cell death must be regulated independently in C. albicans.

Yeast apoptosis—From genes to pathways

Yeast are eukaryotic unicellular organisms that are easy to cultivate and offer a wide spectrum of genetic and cytological tools for research. Yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have successfully been used as models for human cell division cycle. Stress conditions, cellular ageing, failed mating, certain mutations or heterologous expression of proapoptotic genes induce yeast cell death with the characteristic markers of apoptosis. Several crucial regulators of apoptosis are conserved between metazoans and yeast. This simple model organism offers the possibility to identify conserved and new components of the apoptotic machinery and to elucidate the regulatory pathways beyond.

An atypical active cell death process underlies the fungicidal activity of ciclopirox olamine against the yeast Saccharomyces cerevisiae

Ciclopirox olamine (CPO), a fungicidal agent widely used in clinical practice, induced in Saccharomyces cerevisiae an active cell death (ACD) process characterized by changes in nuclear morphology and chromatin condensation associated with the appearance of a population in the sub-G(0)/G(1) cell cycle phase and an arrest delay in the G(2)/M phases. This ACD was associated neither with intracellular reactive oxygen species (ROS) signaling, as revealed by the use of different classes of ROS scavengers, nor with a terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive phenotype. Furthermore, CPO-induced cell death seems to be dependent on unknown protease activity but independent of the apoptotic regulators Aif1p and Yca1p and of autophagic pathways involving Apg5p, Apg8p and Uth1p. Our results show that CPO triggers in S. cerevisiae an atypical nonapoptotic, nonautophagic ACD with as yet unknown regulators.

Extraction of cell surface-associated proteins from living yeast cells

To extract cell surface-associated proteins from living fungal cells, reducing agents such as beta-mercaptoethanol and dithiothreitol are often used. We show here that both compounds are moderately lipophilic and may perturb the plasma membrane, thus causing the release of cytosolic proteins, especially at high extraction temperatures. To avoid artifacts, we recommend using (a) a low concentration of the reducing agent for only a short period of time, and (b) an extraction temperature of 4 degrees C to protect the integrity of the plasma membrane. Similarly, biotinylation of cell surface proteins should be carried out at low temperatures in the absence of dimethylsulphoxide.

TheAspergillus fumigatusmetacaspases CasA and CasB facilitate growth under conditions of endoplasmic reticulum stress

We have examined the contribution of metacaspases to the growth and stress response of the opportunistic human mould pathogen, Aspergillus fumigatus, based on increasing evidence implicating the yeast metacaspase Yca1p in apoptotic-like programmed cell death. Single metacaspase-deficient mutants were constructed by targeted disruption of each of the two metacaspase genes in A. fumigatus, casA and casB, and a metacaspase-deficient mutant, DeltacasA/DeltacasB, was constructed by disrupting both genes. Stationary phase cultures of wild-type A. fumigatus were associated with the appearance of typical markers of apoptosis, including elevated proteolytic activity against caspase substrates, phosphatidylserine exposure on the outer leaflet of the membrane, and loss of viability. By contrast, phosphatidylserine exposure was not observed in stationary phase cultures of the DeltacasA/DeltacasB mutant, although caspase activity and viability was indistinguishable from wild type. The mutant retained wild-type virulence and showed no difference in sensitivity to a range of pro-apoptotic stimuli that have been reported to initiate yeast apoptosis. However, the DeltacasA/DeltacasB mutant showed a growth detriment in the presence of agents that disrupt endoplasmic reticulum homeostasis. These findings demonstrate that metacaspase activity in A. fumigatus contributes to the apoptotic-like loss of membrane phospholipid asymmetry at stationary phase, and suggest that CasA and CasB have functions that support growth under conditions of endoplasmic reticulum stress.