Apoptosis induced by environmental stresses and amphotericin B in Candida albicans

Amphiphilic Tobramycin Analogues as Antibacterial and Antifungal Agents

In this study, we investigated the in vitro antifungal activities, cytotoxicities, and membrane-disruptive actions of amphiphilic tobramycin (TOB) analogues. The antifungal activities were established by determination of MIC values and in time-kill studies. Cytotoxicity was evaluated in mammalian cell lines. The fungal membrane-disruptive action of these analogues was studied by using the membrane-impermeable dye propidium iodide. TOB analogues bearing a linear alkyl chain at their 6″-position in a thioether linkage exhibited chain length-dependent antifungal activities. Analogues with C12 and C14 chains showed promising antifungal activities against tested fungal strains, with MIC values ranging from 1.95 to 62.5 mg/liter and 1.95 to 7.8 mg/liter, respectively. However, C4, C6, and C8 TOB analogues and TOB itself exhibited little to no antifungal activity. Fifty percent inhibitory concentrations (IC50s) for the most potent TOB analogues (C12 and C14) against A549 and Beas 2B cells were 4- to 64-fold and 32- to 64-fold higher, respectively, than their antifungal MIC values against various fungi. Unlike conventional aminoglycoside antibiotics, TOB analogues with alkyl chain lengths of C12 and C14 appear to inhibit fungi by inducing apoptosis and disrupting the fungal membrane as a novel mechanism of action. Amphiphilic TOB analogues showed broad-spectrum antifungal activities with minimal mammalian cell cytotoxicity. This study provides novel lead compounds for the development of antifungal drugs.

Digital microfluidics for time-resolved cytotoxicity studies on single non-adherent yeast cells

Single cell analysis (SCA) has gained increased popularity for elucidating cellular heterogeneity at genomic, proteomic and cellular levels. Flow cytometry is considered as one of the most widely used techniques to characterize single cell responses; however, its inability to analyse cells with spatio-temporal resolution poses a major drawback. Here, we introduce a digital microfluidic (DMF) platform as a useful tool for conducting studies on isolated yeast cells in a high-throughput fashion. The reported system exhibits (i) a microwell array for trapping single non-adherent cells by shuttling a cell-containing droplet over the array, and allows (ii) implementation of high-throughput cytotoxicity assays with enhanced spatio-temporal resolution. The system was tested for five different concentrations of the antifungal drug Amphotericin B, and the cell responses were monitored over time by time lapse fluorescence microscopy. The DMF platform was validated by bulk experiments, which mimicked the DMF experimental design. A correlation analysis revealed that the results obtained on the DMF platform are not significantly different from those obtained in bulk; hence, the DMF platform can be used as a tool to perform SCA on non-adherent cells, with spatio-temporal resolution. In addition, no external forces, other than the physical forces generated by moving the droplet, were used to capture single cells, thereby avoiding cell damage. As such, the information on cellular behaviour during treatment could be obtained for every single cell over time making this platform noteworthy in the field of SCA.

Enhancement of the antibiofilm activity of amphotericin B by polyamine biosynthesis inhibitors

The aim of this study was to investigate the effect of polyamine biosynthesis inhibitors on the activity of amphotericin B (AmB) against Candida albicans biofilms and to clarify the underlying mechanisms. The antibiofilm activity of AmB was significantly enhanced when used in combination with the polyamine biosynthesis inhibitors 1,4-diamino-2-butanone (DAB) and α-difluoromethylornithine (DFMO). Further study showed that DAB and DFMO also enhanced the antibiofilm activity of several other antifungal agents. Moreover, the combination of AmB and polyamine biosynthesis inhibitors resulted in an increase in intracellular levels of reactive oxygen species. In addition, caspase activity and transcription of the caspase-encoding gene CaMCA1 were greatly increased upon combined treatment with polyamine biosynthesis inhibitors and AmB. Consistently, the biofilm formed by a Δcamca1 mutant exhibited greater viability and lower caspase activity than that of the wild-type strain upon combined treatment. These data provide useful information for the development of new strategies to enhance the antibiofilm activities of antifungal agents.

Oxidative stress responses in the human fungal pathogen, Candida albicans

Candida albicans is a major fungal pathogen of humans, causing approximately 400,000 life-threatening systemic infections world-wide each year in severely immunocompromised patients. An important fungicidal mechanism employed by innate immune cells involves the generation of toxic reactive oxygen species (ROS), such as superoxide and hydrogen peroxide. Consequently, there is much interest in the strategies employed by C. albicans to evade the oxidative killing by macrophages and neutrophils. Our understanding of how C. albicans senses and responds to ROS has significantly increased in recent years. Key findings include the observations that hydrogen peroxide triggers the filamentation of this polymorphic fungus and that a superoxide dismutase enzyme with a novel mode of action is expressed at the cell surface of C. albicans. Furthermore, recent studies have indicated that combinations of the chemical stresses generated by phagocytes can actively prevent C. albicans oxidative stress responses through a mechanism termed the stress pathway interference. In this review, we present an up-date of our current understanding of the role and regulation of oxidative stress responses in this important human fungal pathogen.

Deletion analysis of LSm, FDF, and YjeF domains of Candida albicans Edc3 in hyphal growth and oxidative-stress response

Candida albicans is an opportunistic fungal pathogen whose responses to environmental changes are associated with the virulence attributes. Edc3 is known to be an enhancer of the mRNA decapping reactions and a scaffold protein of cytoplasmic processing bodies (P-bodies). Recent studies of C. albicans Edc3 suggested its critical roles in filamentous growth and stress-induced apoptotic cell death. The edc3/edc3 deletion mutant strain showed increased cell survival and less ROS accumulation upon treatment with hydrogen peroxide. To investigate the diverse involvement of Edc3 in the cellular processes, deletion mutations of LSm, FDF, or YjeF domain of Edc3 were constructed. The edc3-LSmΔ or edc3-YjeFΔ mutation showed the filamentation defect, resistance to oxidative stress, and decreased ROS accumulation. In contrast, the edc3-FDFΔ mutation exhibited a wild-type level of filamentous growth and a mild defect in ROS accumulation. These results suggest that Lsm and YjeF domains of Edc3 are critical in hyphal growth and oxidative stress response.

A new look at the antibiotic amphotericin B effect on Candida albicans plasma membrane permeability and cell viability functions

Amphotericin B (AmB) is an antifungal polyene for which the most accepted mode of action is formation of protein-like ion channels in the cell membrane. Patch-clamp research on Candida albicans protoplasts carried out in the outside-out configuration showed that application of 0.05 and 0.1 μM AmB caused a decrease in seal resistance. Such a phenomenon can be correlated with a decrease in membrane tightness. AmB applied at a 0.05 μM concentration also caused a decrease in the number of active TOK1 (two-pore outward rectifiers) potassium channels, but did not significantly change their open probability. The results indicate that in C. albicans protoplast AmB causes a decrease in cell membrane integrity by interaction with its lipid phase but not with ion channels. Fluorescence microscopy techniques showed that AmB treatment, in clinical concentrations, had no effect on the percentage of PI-positive protoplasts. AmB treatment in the concentrations tested did not cause a rapid reduction of the number of C. albicans protoplasts. However, there was a significant loss of replication competency and numerous morphological and physiological disorders, including cytoplasm shrinking, abnormal morphology of the nucleus and mitochondria, a sudden decrease in the MTT reduction level and oxidative stress. Our results show that the induction of yeast cell death by AmB, at therapeutic doses, is a multistage and long-term process involving multiple intracellular pathways.

Roles of Edc3 in the oxidative stress response and CaMCA1-encoded metacaspase expression in Candida albicans

The Edc3 protein is an enhancer of mRNA decapping, and acts as a scaffold protein for the mRNA granules that are known as processing bodies in yeast. In the pathogenic yeast Candida albicans, various stresses, such as glucose depletion, oxidative stress, and filamentation defects, induce the accumulation of processing bodies. Here, we report that the edc3/edc3 deletion strain showed increased resistance to various stresses, including hydrogen peroxide, acetic acid, and high temperature. Oxidative stress is known to induce the intracellular accumulation of reactive oxygen species (ROS) and apoptotic cell death in C. albicans. We found that the ROS level was lower in edc3/edc3 cells than in wild-type cells following oxidative stress. We also observed that expression of the metacaspase gene CaMCA1 was decreased in edc3/edc3 cells. Overexpression of CaMCA1 suppressed the decreased accumulation of ROS and the increased resistance to hydrogen peroxide in edc3/edc3 cells. The catalase Cat1 and the superoxide dismutase Sod1 were upregulated in edc3/edc3 cells as compared with wild-type cells. On the basis of these findings, we suggest that EDC3 plays a critical role in the expression of CaMCA1 and the oxidative stress response in C. albicans.

Stress adaptation in a pathogenic fungus

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

The production of reactive oxygen species is a universal action mechanism of Amphotericin B against pathogenic yeasts and contributes to the fungicidal effect of this drug

Amphotericin B (AMB) is an antifungal drug that binds to ergosterol and forms pores at the cell membrane, causing the loss of ions. In addition, AMB induces the accumulation of reactive oxygen species (ROS), and although these molecules have multiple deleterious effects on fungal cells, their specific role in the action mechanism of AMB remains unknown. In this work, we studied the role of ROS in the action mechanism of AMB. We determined the intracellular induction of ROS in 44 isolates of different pathogenic yeast species (Candida albicans, Candida parapsilosis, Candida glabrata, Candida tropicalis, Candida krusei, Cryptococcus neoformans, and Cryptococcus gattii). We also characterized the production of ROS in AMB-resistant isolates. We found that AMB induces the formation of ROS in all the species tested. The inhibition of the mitochondrial respiratory chain by rotenone blocked the induction of ROS by AMB and provided protection from the killing action of the antifungal. Moreover, this phenomenon was absent in strains that displayed resistance to AMB. These strains showed an alteration in the respiration rate and mitochondrial membrane potential and also had higher catalase activity than that of the AMB-susceptible strains. Consistently, AMB failed to induce protein carbonylation in the resistant strains. Our data demonstrate that the production of ROS by AMB is a universal and important action mechanism that is correlated with the fungicidal effect and might explain the low rate of resistance to the molecule. Finally, these data provide an opportunity to design new strategies to improve the efficacy of this antifungal.

Ascorbate peroxidase, a key molecule regulating amphotericin B resistance in clinical isolates of Leishmania donovani

Amphotericin B (AmB), a polyene macrolide, is now a first-line treatment of visceral leishmaniasis cases refractory to antimonials in India. AmB relapse cases and the emergence of secondary resistance have now been reported. To understand the mechanism of AmB, differentially expressed genes in AmB resistance strains were identified by a DNA microarray and real-time reverse transcriptase PCR (RT-PCR) approach. Of the many genes functionally overexpressed in the presence of AmB, the ascorbate peroxidase gene from a resistant Leishmania donovani strain (LdAPx gene) was selected because the gene is present only in Leishmania, not in humans. Apoptosis-like cell death after exposure to AmB was investigated in a wild-type (WT) strain in which the LdAPx gene was overexpressed and in AmB-sensitive and -resistant strains. A higher percentage of apoptosis-like cell death after AmB treatment was noticed in the sensitive strain than in both the resistant isolate and the strain sensitive to LdAPx overexpression. This event is preceded by AmB-induced formation of reactive oxygen species and elevation of the cytosolic calcium level. Enhanced cytosolic calcium was found to be responsible for depolarization of the mitochondrial membrane potential and the release of cytochrome c (Cyt c) into the cytosol. The redox behavior of Cyt c showed that it has a role in the regulation of apoptosis-like cell death by activating metacaspase- and caspase-like proteins and causing concomitant nuclear alterations, as determined by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and DNA fragmentation in the resistant strain. The present study suggests that constitutive overexpression of LdAPx in the L. donovani AmB-resistant strain prevents cells from the deleterious effect of oxidative stress, i.e., mitochondrial dysfunction and cellular death induced by AmB.

A novel mechanism for the antibacterial effect of silver nanoparticles on Escherichia coli

Silver nanoparticles are known to have antimicrobial properties and have been used extensively in medicine, although the mechanism(s) of action have not yet been clearly established. In the present study, the findings suggest a novel mechanism for the antibacterial effect of silver nanoparticles on Escherichia coli, namely, the induction of a bacterial apoptosis-like response. We propose a possible mechanism for the bacterial apoptosis-like response that includes the following: accumulation of reactive oxygen species (ROS) (detected with H2DCFDA staining), increased intracellular calcium levels (detected with Fura-2 AM), phosphatidylserine exposure in the outer membrane (detected with Annexin V) which is the hallmarks of early apoptosis, disruption of the membrane potential [detected with DiBAC4(3)], activation of a bacterial caspase-like protein (detected by FITC-VAD-FMK staining) and DNA degradation (detected with TUNEL assay) which is the hallmarks of late apoptosis in bacterial cells treated with silver nanoparticles. We also performed RecA expression assay with western blotting and observed activation of SOS response to repair the damaged DNA. To summarize, silver nanoparticles are involved in the apoptosis-like response in E. coli and the novel mechanisms which were identified in this study, suggest that silver nanoparticles may be an effective antimicrobial agent with far lower propensity for inducing microbial resistance than antibiotics.

Ca(2+)-CaM regulating viability of Candida guilliermondii under oxidative stress by acting on detergent resistant membrane proteins

Reactive oxygen species (ROS) play a vital role in reducing viability of yeast cells. The Ca(2+)-CaM signaling pathways are involved in regulating the intracellular ROS level in yeast cells under stress. Detergent resistant membranes (DRMs), the sterol-rich microdomains, participate in a wide range of cellular processes including growth, trafficking and death in yeast cells. In the present study, we found that Trifluoperazine (TFP), an antagonist of CaM, could increase the viability of Candida guilliermondii cells under H2O2 stress. Based on comparative analysis of DRM sub proteomics, a total number of 29 differentially expressed protein spots were identified, among which 8 protein spots belong to the electron transport chain and 7 protein spots belong to transporters. It is suggested that TFP treatment could modulate the intracellular ROS generation in yeast cells. We additionally ascertained that TFP treatment could effectively alleviate the ROS accumulation and protein damage in C. guilliermondii cells under H2O2 stress, via investigating the intracellular ROS levels and protein oxidative damage in yeast cells. These findings firstly revealed that the Ca(2+)-CaM signaling pathway is related to the viability of yeast cells under H2O2 stress, and provide novel evidences for exploring Ca(2+)-CaM's role in regulating this viability via acting on DRM proteins.

BIOLOGICAL SIGNIFICANCE

Detergent-resistant membranes (DRMs), which are more resistant to extraction with cold non-ionic detergents, have been considered the functional microdomains in the plasma membrane. In yeast, DRMs are involved in a wide range of additional cellular processes including cell growth and death. The Ca(2+)-CaM signaling pathways could regulate the stress tolerance of yeast cells by modulating the intracellular ROS generation. In this study, we found that trifluoperazine (TFP), a calmodulin antagonist, could increase the viability of C. guilliermondii under H2O2 stress. Based on comparative analysis of DRM sub proteomics, electron transport chain proteins and transporters were identified to be associated with the Ca(2+)-CaM transduction. We proved that TFP treatment decreases the intracellular ROS accumulation and alleviates oxidative damage to cellular proteins. These results ascertain that Ca(2+)-CaM is involved in regulating the viability of C. guilliermondii under oxidative stress via acting on the DRM proteins.

Synergistic effect of fluconazole and doxycycline against Candida albicans biofilms resulting from calcium fluctuation and downregulation of fluconazole-inducible efflux pump gene overexpression

Candida albicans biofilms are intrinsically resistant to antimicrobial agents. Previous work demonstrated that the antifungal activity of fluconazole against C. albicans biofilms is notably enhanced by doxycycline. In order to explore the synergistic mechanism of fluconazole and doxycycline, we investigated the changes of efflux pump gene expression, intracellular calcium concentration and cell cycle distribution after drug intervention in this study. The expression levels of CDR1, CDR2 and MDR1 were determined by real-time PCR, and the results showed that fluconazole alone could stimulate the high expression of CDR1, CDR2 and MDR1, and the combination of doxycycline and fluconazole downregulated the gene overexpression induced by fluconazole. Intracellular calcium concentration was determined using Fluo-3/AM by observing the fluorescence with flow cytometry. A calcium fluctuation, which started 4 h and peaked 8 h after the treatment with fluconazole, was observed. The combined drugs also initiated a calcium fluctuation after 4 h treatment and showed a peak at 16 h, and the peak was higher than that stimulated by fluconazole alone. The cell cycle was measured using flow cytometry. Fluconazole alone and the combined drugs both induced a reduction in the percentages of S-phase cells and an elevation in the percentages of cells in the G2/M phase. The results of this research showed that the synergism of fluconazole and doxycycline against C. albicans biofilms is associated with blockade of the efflux pump genes CDR1, CDR2 and MDR1, and stimulation of high intracellular calcium concentration. The findings of this study are of great significance in the search for new antifungal mechanisms.

Heat stress induces apoptotic-like cell death in two Pleurotus species

High temperature is an important environmental factor that affects the growth and development of most edible fungi, however, the mechanism(s) for resistance to high temperature remains elusive. Nitric oxide is known to be able to effectively alleviate oxidative damage and plays an important role in the regulation of trehalose accumulation during heat stress in mycelia of Pleurotus eryngii var. tuoliensis. In this paper, we investigated whether heat stress can activate apoptosis-like cell death in mycelia of Pleurotus. Two Pleurotus species were used to detect morphological features characteristic of apoptosis including nuclear condensation, reactive oxygen species accumulation, and DNA fragmentation when exposed to heat stress (42 °C). The results showed that these classical apoptosis markers were apparent in Pleurotus strains after heat treatment. The heat-induced apoptosis-like cell death in Pleurotus was further probed using oligomycin and N-acetylcysteine, both of which were shown to block processes leading to apoptosis. This is the first report that apoptosis-like cell death occurs in Pleurotus species as a result of abiotic stress, and that this process can be inhibited with chemicals that block mitochondrial-induced apoptotic pathways and/or with ROS-scavenging compounds.

The lack of upstream elements of the Cek1 and Hog1 mediated pathways leads to a synthetic lethal phenotype upon osmotic stress in Candida albicans

Different signal transduction pathways mediated by MAP kinases have been described in Candida albicans. These pathways sense different stimuli and, therefore, elaborate specific responses. Hog1 was identified as the MAPK that is primarily involved in stress response and virulence, while Cek1 was more specific to cell wall biogenesis, mating and biofilm formation. In the present work, mutants defective in both pathways have been characterized under osmotic stress. Both routes are required for a full response against high osmotic challenge, since mutants defective in both pathways displayed aberrant morphology, cell polarity defects and abnormal chitin deposition, which correlate with loss of viability and appearance of apoptotic markers. These alterations occurred in spite of proper Hog1 and Cek1 phosphorylation and increased intra-cellular glycerol accumulation. The relevance of both routes in virulence is shown as ssk1 msb2 sho1 opy2 mutants are avirulent in a mouse systemic model of infection and display reduced virulence in the Galleria mellonella model.

Ocimum sanctum (L.) essential oil and its lead molecules induce apoptosis in Candida albicans

Manipulation of endogenous responses during programmed cell death (PCD) in fungi can lead to development of effective therapeutic strategies. In the present study, we evaluated the physiology of cell death in Candida albicans in response to Ocimum sanctum essential oil (OSEO) and its two major constituents - methyl chavicol (MET CHAV) and linalool (LIN) at varying inhibitory concentrations. Apoptotic cell death was studied on the basis of externalization of membrane phosphatidylserine (PS) revealed by annexin-V-FITC labeling, morphological alterations revealed by transmission electron microscopy and DNA fragmentation by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay. Exposure of fungal cells to MIC/4 of OSEO, MET CHAV and LIN resulted in morphological features characteristic of apoptosis, while necrosis was observed at higher concentrations. Necrotic cells displayed reduced TUNEL staining and an inability to exclude propidium iodide. In addition, they lacked a defined nucleus and an intact external morphology. Exposed cells were TUNEL-positive, showed chromatin condensation and margination, nuclear envelope separation, nuclear fragmentation, cytoplasmic shrinkage and plasma membrane blebbing. A dose-dependent decrease in cytochrome c oxidase activity was observed with each compound, but the decrease was not comparable to that elicited by H2O2, eliminating the primary involvement of cytochrome c release in apoptosis thus induced. Previously reported data revealed induction of apoptosis at low concentrations as a result of oxidative insult. Studies aimed at identifying other mitochondrial factors activated during this course to mediate apoptosis will further elucidate the mechanism of antifungal action of these natural products.

The polyene antifungals, amphotericin B and nystatin, cause cell death in Saccharomyces cerevisiae by a distinct mechanism to amphibian-derived antimicrobial peptides

Background

There is a pressing need to identify novel antifungal drug targets to aid in the therapy of life-threatening mycoses and overcome increasing drug resistance. Identifying specific mechanisms of action of membrane-interacting antimicrobial drugs on the model fungus Saccharomyces cerevisiae is one avenue towards addressing this issue. The S. cerevisiae deletion mutants Δizh2, Δizh3, Δaif1 and Δstm1 were demonstrated to be resistant to amphibian-derived antimicrobial peptides (AMPs). The purpose of this study was to examine whether AMPs and polyene antifungals have a similar mode of action; this was done by comparing the relative tolerance of the mutants listed above to both classes of antifungal.

Findings

In support of previous findings on solid media it was shown that Δizh2 and Δizh3 mutants had increased resistance to both amphotericin B (1–2 μg ml⁻¹) and nystatin (2.5 – 5 μg ml⁻¹) in liquid culture, after acute exposure. However, Δaif1 and Δstm1 had wild-type levels of susceptibility to these polyenes. The generation of reactive oxygen species (ROS) after exposure to amphotericin B was also reduced in Δizh2 and Δizh3. These data indicated that polyene antifungal and AMPs may act via distinct mechanisms of inducing cell death in S. cerevisiae.

Conclusions

Further understanding of the mechanism(s) involved in causing cell death and the roles of IZH2 and IZH3 in drug susceptibility may help to inform improved drug design and treatment of fungal pathogens.

The type II Ca2+/calmodulin-dependent protein kinases are involved in the regulation of cell wall integrity and oxidative stress response in Candida albicans

The type II Ca(2+)/calmodulin-dependent protein kinases (CaMKs) are thought to play a vital role in cellular regulation in mammalian cells. Two genes CMK1 and CMK2 in the Candida albicans genome encode homologues of mammalian CaMKs. In this work, we constructed the cmk1Δ/Δ, the cmk2Δ/Δ and the cmk1Δ/Δcmk2Δ/Δ mutants and found that CaMKs function in cell wall integrity (CWI) and cellular redox regulation. Loss of either CMK1 or CMK2, or both resulted in increased expression of CWI-related genes under Calcofluor white (CFW) treatment. Besides, CaMKs are essential for the maintenance of cellular redox balance. Disruption of either CMK1 or CMK2, or both not only led to a significant increase of intracellular ROS levels, but also led to a decrease of the mitochondrial membrane potential (MMP), suggesting the important roles that CaMKs play in the maintenance of the mitochondrial function.

Increased oxidative stress tolerance results in general stress tolerance in Candida albicans independently of stress-elicited morphological transitions

A selection of tert-butylhydroperoxide (tBOOH)-tolerant Candida albicans mutants showed increased tolerances to 19 different stress conditions. These mutants are characterized by a constitutively upregulated antioxidative defense system and, therefore, adaptation to oxidative stress may play an important role in gaining general stress tolerance in C. albicans. Although C. albicans cells may undergo morphological transitions under various stress treatments, this ability shows considerable stress-specific and strain-specific variability and, hence, it is independent of mounting stress cross protections.

Inhibition of nucleotide biosynthesis potentiates the antifungal activity of amphotericin B

The polyene antifungal agent Amphotericin B exhibits potent and broad spectrum fungicidal activity. However, high nephrotoxicity can hinder its administration in resource poor settings. Quantification of early fungicidal activity in studies of HIV patients with cryptococcosis demonstrate that 5-Fluorocytosine therapy in combination with Amphotericin B results in faster clearance than with Amphotericin B alone. In vitro synergy between the two drugs has also been reported but the mechanism by which 5-Fluorocytosine synergizes with Amphotericin B has not been delineated. In this study we set out to investigate the effect of genetic mutation or pharmacologic repression of de novo pyrimidine and purine biosynthesis pathways on the Amphotericin B susceptibility of Cryptococcus neoformans. We demonstrate that a ura- derivative of wild type Cryptococcus neoformans strain H99 is hypersensitive to Amphotericin B. This sensitivity is remediated by re-introduction of a wild type URA5 gene, but not by addition of exogenous uracil to supplement the auxotrophy. Repression of guanine biosynthesis by treatment with the inosine monophosphate dehydrogenase inhibitor, mycophenolic acid, was synergistic with Amphotericin B as determined by checkerboard analysis. As in Cryptococcus neoformans, a ura⁻ derivative of Candida albicans was also hypersensitive to Amphotericin B, and treatment of Candida albicans with mycophenolic acid was likewise synergistic with Amphotericin B. In contrast, neither mycophenolic acid nor 5-FC had an effect on the Amphotericin B susceptibility of Aspergillus fumigatus. These studies suggest that pharmacological targeting of nucleotide biosynthesis pathways has potential to lower the effective dose of Amphotericin B for both C. neoformans and C. albicans. Given the requirement of nucleotide and nucleotide sugars for growth and pathogenesis of Cryptococcus neoformans, disrupting nucleotide metabolic pathways might thus be an effective mechanism for the development of novel antifungal drugs.

Reactive oxygen species-inducing antifungal agents and their activity against fungal biofilms

Invasive fungal infections are associated with very high mortality rates ranging from 20–90% for opportunistic fungal pathogens such as Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. Fungal resistance to antimycotic treatment can be genotypic (due to resistant strains) as well as phenotypic (due to more resistant fungal lifestyles, such as biofilms). With regard to the latter, biofilms are considered to be critical in the development of invasive fungal infections. However, there are only very few antimycotics, such as miconazole (azoles), echinocandins and liposomal formulations of amphotericin B (polyenes), which are also effective against fungal biofilms. Interestingly, these antimycotics all induce reactive oxygen species (ROS) in fungal (biofilm) cells. This review provides an overview of the different classes of antimycotics and novel antifungal compounds that induce ROS in fungal planktonic and biofilm cells. Moreover, different strategies to further enhance the antibiofilm activity of such ROS-inducing antimycotics will be discussed.

Purpurin triggers caspase-independent apoptosis in Candida dubliniensis biofilms

Candida dubliniensis is an important human fungal pathogen that causes oral infections in patients with AIDS and diabetes mellitus. However, C. Dubliniensis has been frequently reported in bloodstream infections in clinical settings. Like its phylogenetically related virulent species C. albicans, C. Dubliniensis is able to grow and switch between yeast form and filamentous form (hyphae) and develops biofilms on both abiotic and biotic surfaces. Biofilms are recalcitrant to antifungal therapies and C. Dubliniensis readily turns drug resistant upon repeated exposure. More than 80% of infections are associated with biofilms. Suppression of fungal biofilms may therefore represent a viable antifungal strategy with clinical relevance. Here, we report that C. dubliniensis biofilms were inhibited by purpurin, a natural anthraquinone pigment isolated from madder root. Purpurin inhibited C. dubliniensis biofilm formation in a concentration-dependent manner; while mature biofilms were less susceptible to purpurin. Scanning electron microscopy (SEM) analysis revealed scanty structure consisting of yeast cells in purpurin-treated C. dubliniensis biofilms. We sought to delineate the mechanisms of the anti-biofilm activity of purpurin on C. Dubliniensis. Intracellular ROS levels were significantly elevated in fungal biofilms and depolarization of MMP was evident upon purpurin treatment in a concentration-dependent manner. DNA degradation was evident. However, no activated metacaspase could be detected. Together, purpurin triggered metacaspase-independent apoptosis in C. dubliniensis biofilms.

Synergistic effect of the flavonoid catechin, quercetin, or epigallocatechin gallate with fluconazole induces apoptosis in Candida tropicalis resistant to fluconazole

Flavonoids are a class of phenolic compounds commonly found in fruits, vegetables, grains, flowers, tea, and wine. They differ in their chemical structures and characteristics. Such compounds show various biological functions and have antioxidant, antimicrobial, anti-inflammatory, and antiapoptotic properties. The aim of this study was to evaluate the in vitro interactions of flavonoids with fluconazole against Candida tropicalis strains resistant to fluconazole, investigating the mechanism of synergism. Three combinations formed by the flavonoids (+)-catechin hydrated, hydrated quercetin, and (-)-epigallocatechin gallate at a fixed concentration with fluconazole were tested. Flavonoids alone had no antifungal activity within the concentration range tested, but when they were used as a cotreatment with fluconazole, there was significant synergistic activity. From this result, we set out to evaluate the possible mechanisms of cell death involved in this synergism. Isolated flavonoids did not induce morphological changes or changes in membrane integrity in the strains tested, but when they were used as a cotreatment with fluconazole, these changes were quite significant. When evaluating mitochondrial damage and the production of reactive oxygen species (ROS) only in the cotreatment, changes were observed. Flavonoids combined with fluconazole were shown to cause a significant increase in the rate of damage and the frequency of DNA damage in the tested strains. The cotreatment also induced an increase in the externalization of phosphatidylserine, an important marker of early apoptosis. It is concluded that flavonoids, when combined with fluconazole, show activity against strains of C. tropicalis resistant to fluconazole, promoting apoptosis by exposure of phosphatidylserine in the plasma membrane and morphological changes, mitochondrial depolarization, intracellular accumulation of ROS, condensation, and DNA fragmentation.

Fungicidal photodynamic effect of a twofold positively charged porphyrin against Candida albicans planktonic cells and biofilms

BACKGROUND

Antimicrobial photodynamic therapy is an interesting alternative for the treatment of superficial mucocutaneous mycoses. In immunodeficient patients, these infections are frequently recurrent and resistant to the most commonly used antifungal medications. Candida albicans biofilms frequently cause such infections that can even evolve to deep-seated mycoses.

MATERIALS & METHODS

The efficiency of a photodynamic therapy was investigated against C. albicans using a twofold positively charged porphyrin (XF-73) in comparison with the well-known fourfold positively charged porphyrin (5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine, tetra-p-tosylate salt).

RESULTS

After incubation with 0.5 µM of XF-73 for 15 min and irradiation with blue light (12.1 J/cm(2)), the viability of C. albicans planktonic cells decreased by over 6 log10. For biofilm cells, a longer incubation time (4 h) with 1 µM of XF-73 and a light dose of 48.2 J/cm(2) was necessary to achieve over 5 log10 cell killing. Cell killing was mediated by singlet oxygen that was directly detected via its luminescence at 1270 nm in XF-73-incubated C. albicans biofilms for the first time. Antimicrobial photodynamic therapy yielded better results for XF-73 compared with 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine, tetra-p-tosylate salt when using the same conditions.

CONCLUSION

This study provides evidence that XF-73 is a highly efficient photosensitizer to kill C. albicans and it would be worthwhile to test this photosensitizer in clinical studies for both prophylaxis and treatment of infections caused by this microorganism, preventing the spread of C. albicans throughout the bloodstream.

Functional linkage between genes that regulate osmotic stress responses and multidrug resistance transporters: challenges and opportunities for antibiotic discovery

All cells need to protect themselves against the osmotic challenges of their environment by maintaining low permeability to ions across their cell membranes. This is a basic principle of cellular function, which is reflected in the interactions among ion transport and drug efflux genes that have arisen during cellular evolution. Thus, upon exposure to pore-forming antibiotics such as amphotericin B (AmB) or daptomycin (Dap), sensitive cells overexpress common resistance genes to protect themselves from added osmotic challenges. These genes share pathway interactions with the various types of multidrug resistance (MDR) transporter genes, which both preserve the native lipid membrane composition and at the same time eliminate disruptive hydrophobic molecules that partition excessively within the lipid bilayer. An increased understanding of the relationships between the genes (and their products) that regulate osmotic stress responses and MDR transporters will help to identify novel strategies and targets to overcome the current stalemate in drug discovery.

Genome-wide identification of genes involved in the positive and negative regulation of acetic acid-induced programmed cell death in Saccharomyces cerevisiae

BACKGROUND

Acetic acid is mostly known as a toxic by-product of alcoholic fermentation carried out by Saccharomyces cerevisiae, which it frequently impairs. The more recent finding that acetic acid triggers apoptotic programmed cell death (PCD) in yeast sparked an interest to develop strategies to modulate this process, to improve several biotechnological applications, but also for biomedical research. Indeed, acetate can trigger apoptosis in cancer cells, suggesting its exploitation as an anticancer compound. Therefore, we aimed to identify genes involved in the positive and negative regulation of acetic acid-induced PCD by optimizing a functional analysis of a yeast Euroscarf knock-out mutant collection.

RESULTS

The screen consisted of exposing the mutant strains to acetic acid in YPD medium, pH 3.0, in 96-well plates, and subsequently evaluating the presence of culturable cells at different time points. Several functional categories emerged as greatly relevant for modulation of acetic acid-induced PCD (e.g.: mitochondrial function, transcription of glucose-repressed genes, protein synthesis and modifications, and vesicular traffic for protection, or amino acid transport and biosynthesis, oxidative stress response, cell growth and differentiation, protein phosphorylation and histone deacetylation for its execution). Known pro-apoptotic and anti-apoptotic genes were found, validating the approach developed. Metabolism stood out as a main regulator of this process, since impairment of major carbohydrate metabolic pathways conferred resistance to acetic acid-induced PCD. Among these, lipid catabolism arose as one of the most significant new functions identified. The results also showed that many of the cellular and metabolic features that constitute hallmarks of tumour cells (such as higher glycolytic energetic dependence, lower mitochondrial functionality, increased cell division and metabolite synthesis) confer sensitivity to acetic acid-induced PCD, potentially explaining why tumour cells are more susceptible to acetate than untransformed cells and reinforcing the interest in exploiting this acid in cancer therapy. Furthermore, our results clearly establish a connection between cell proliferation and cell death regulation, evidencing a conserved developmental role of programmed cell death in unicellular eukaryotes.

CONCLUSIONS

This work advanced the characterization of acetic acid-induced PCD, providing a wealth of new information on putative molecular targets for its control with impact both in biotechnology and biomedicine.

Aging and cell death in the other yeasts, Schizosaccharomyces pombe and Candida albicans

How do cells age and die? For the past 20 years, the budding yeast, Saccharomyces cerevisiae, has been used as a model organism to uncover the genes that regulate lifespan and cell death. More recently, investigators have begun to interrogate the other yeasts, the fission yeast, Schizosaccharomyces pombe, and the human fungal pathogen, Candida albicans, to determine if similar longevity and cell death pathways exist in these organisms. After summarizing the longevity and cell death phenotypes in S. cerevisiae, this mini-review surveys the progress made in the study of both aging and programed cell death (PCD) in the yeast models, with a focus on the biology of S. pombe and C. albicans. Particular emphasis is placed on the similarities and differences between the two types of aging, replicative aging, and chronological aging, and between the three types of cell death, intrinsic apoptosis, autophagic cell death, and regulated necrosis, found in these yeasts. The development of the additional microbial models for aging and PCD in the other yeasts may help further elucidate the mechanisms of longevity and cell death regulation in eukaryotes.

An antimicrobial peptidomimetic induces Mucorales cell death through mitochondria-mediated apoptosis

The incidence of mucormycosis has dramatically increased in immunocompromised patients. Moreover, the array of cellular targets whose inhibition results in fungal cell death is rather limited. Mitochondria have been mechanistically identified as central regulators of detoxification and virulence in fungi. Our group has previously designed and developed a proteolytically-resistant peptidomimetic motif D(KLAKLAK)2 with pleiotropic action ranging from targeted (i.e., ligand-directed) activity against cancer and obesity to non-targeted activity against antibiotic resistant gram-negative rods. Here we evaluated whether this non-targeted peptidomimetic motif is active against Mucorales. We show that D(KLAKLAK)2 has marked fungicidal action, inhibits germination, and reduces hyphal viability. We have also observed cellular changes characteristic of apoptosis in D(KLAKLAK)2-treated Mucorales cells. Moreover, the fungicidal activity was directly correlated with vacuolar injury, mitochondrial swelling and mitochondrial membrane depolarization, intracellular reactive oxygen species accumulation (ROS), and increased caspase-like enzymatic activity. Finally, these apoptotic features were prevented by the addition of the ROS scavenger N-acetyl-cysteine indicating mechanistic pathway specificity. Together, these findings indicate that D(KLAKLAK)2 makes Mucorales exquisitely susceptible via mitochondrial injury-induced apoptosis. This prototype may serve as a candidate drug for the development of translational applications against mucormycosis and perhaps other fungal infections.

Potentiation of antibiofilm activity of amphotericin B by superoxide dismutase inhibition

This study demonstrates a role for superoxide dismutases (Sods) in governing tolerance of Candida albicans biofilms to amphotericin B (AmB). Coincubation of C. albicans biofilms with AmB and the Sod inhibitors N,N'-diethyldithiocarbamate (DDC) or ammonium tetrathiomolybdate (ATM) resulted in reduced viable biofilm cells and increased intracellular reactive oxygen species levels as compared to incubation of biofilm cells with AmB, DDC, or ATM alone. Hence, Sod inhibitors can be used to potentiate the activity of AmB against C. albicans biofilms.

Mechanism of action of novel synthetic dodecapeptides against Candida albicans

BACKGROUND

Three de novo designed low molecular weight cationic peptides (IJ2, IJ3 and IJ4) containing an unnatural amino acid α,β-didehydrophenylalanine (∆Phe) exhibited potent antifungal activity against fluconazole (FLC) sensitive and resistant clinical isolates of Candida albicans as well as non-albicans and other yeast and filamentous pathogenic fungi. In the present study, their synthesis, susceptibility of different fungi and the mechanism of anti-candidal action have been elucidated.

METHODS

The antimicrobial peptides (AMPs) were synthesized by solid-phase method and checked for antifungal activity against different yeasts and fungi by broth microdilution method. Anti-candidal mode of action of the peptides was investigated through detecting membrane permeabilization by confocal microscopy, Reactive Oxygen Species (ROS) generation by fluorometry, apoptosis and necrosis by flow cytometry and cell wall damage using Scanning and Transmission Electron Microscopy.

RESULTS AND CONCLUSIONS

The MIC of the peptides against C. albicans and other yeast and filamentous fungal pathogens ranged between 3.91 and 250μM. All three peptides exhibited effect on multiple targets in C. albicans including disruption of cell wall structures, compromised cell membrane permeability leading to their enhanced entry into the cells, accumulation of ROS and induction of apoptosis. The peptides also showed synergistic effect when used in combination with fluconazole (FLC) and caspofungin (CAS) against C. albicans.

GENERAL SIGNIFICANCE

The study suggests that the AMPs alone or in combination with conventional antifungals hold promise for the control of fungal pathogens, and need to be further explored for treatment of fungal infections.

Identification of the cell targets important for propolis-induced cell death in Candida albicans

Candida albicans is the most common fungal pathogen of humans, forming both commensal and opportunistic pathogenic interactions, causing a variety of skin and soft tissue infections in healthy people. In immunocompromised patients C. albicans can result in invasive, systemic infections that are associated with a high incidence of mortality. Propolis is a complex mixture of several resinous substances which are collected from plants by bees. Here, we demonstrated the fungicidal activity of propolis against all three morphogenetic types of C. albicans and that propolis-induced cell death was mediated via metacaspase and Ras signaling. To identify genes that were involved in propolis tolerance, we screened ~800 C. albicans homozygous deletion mutants for decreased tolerance to propolis. Fifty-one mutant strains were identified as being hypersensitive to propolis including seventeen genes involved in cell adhesion, biofilm formation, filamentous growth, phenotypic switching and pathogenesis (HST7, GIN4, VPS34, HOG1, ISW2, SUV3, MDS3, HDA2, KAR3, YHB1, NUP85, CDC10, MNN9, ACE2, FKH2, and SNF5). We validated these results by showing that propolis inhibited the transition from yeast-like to hyphal growth. Propolis was shown to contain compounds that conferred fluorescent properties to C. albicans cells. Moreover, we have shown that a topical pharmaceutical preparation, based upon propolis, was able to control C. albicans infections in a mouse model for vulvovaginal candidiasis. Our results strongly indicate that propolis could be used as a strategy for controlling candidiasis.

The calcineurin pathway inhibitor tacrolimus enhances the in vitro activity of azoles against Mucorales via apoptosis

The calcineurin pathway regulates antifungal drug resistance and the virulence of several major human-pathogenic fungi, including the recalcitrant Mucorales. We hypothesized that the fungistatic triazoles posaconazole (PCZ) and itraconazole (ICZ) become fungicidal in the setting of the calcineurin inhibitor tacrolimus (TCR) and that such an effect is mediated through apoptosis. Fungicidal activity and apoptosis were studied using standard microbiological techniques and hyphal metabolic and vital dye reduction assays at 37°C in RPMI 1640. Apoptosis was characterized by detecting intracellular Ca(2+), phosphatidylserine (PS) externalization, DNA fragmentation, plasma membrane integrity, chromatin condensation, reactive oxygen species (ROS) generation, caspase-like activity, ATP, and cytochrome c release. MICs for PCZ and ICZ alone were significantly higher (8 to 128 μg/ml) than those of PCZ or ICZ plus TCR (0.25 to 4 μg/ml) for Rhizopus oryzae, Cunninghamella bertholletiae, and Mucor circinelloides. Both PCZ and ICZ in combination with TCR became fungicidal, and their activity was mediated through increased apoptotic cell death of R. oryzae (10 to 50%), C. bertholletiae (5 to 50%), and M. circinelloides (5 to 55%) germlings, with morphological apoptotic changes characterized by externalization of PS, nuclear condensation, and DNA fragmentation. Moreover, activation of the caspase-like activity was correlated with cell death induced by TCR plus PCZ or ICZ. These changes correlated with elevated intracellular Ca(2+) and ROS levels and disturbance of mitochondrial potential. We found that PCZ or ICZ in combination with TCR renders Mucorales sensitive to triazoles via apoptotic death. These observations could serve as a new paradigm for the development of new therapeutic strategies.

A component of gamma-radiation-induced cell death in E. coli is programmed and interlinked with activation of caspase-3 and SOS response

The current study deals with the molecular mechanism of radiation-induced cell death (RICD) in Escherichia coli. Irradiated E. coli cells displayed markers similar to those found in eukaryotic programmed cell death (PCD) such as caspase-3 activation and phosphatidylserine externalization. RICD was found to be suppressed upon pretreatment with sublethal concentrations of rifampicin or chloramphenicol, indicating the requirement of de novo gene expression. RICD was also found to be inhibited by cell permeable inhibitors of caspase-3 or poly (ADP-ribose) polymerase, indicating the involvement of PCD during RICD in E. coli. Radiation-induced SOS response was alleviated as observed with decrease in LexA level and also reduced cell filamentation frequency in the presence of caspase inhibitor. Further, the inhibitor-mediated rescue was not observed in single-gene knockouts of umuC, umuD, recB and ruvA, the genes which are associated with SOS response. This implies a linkage between SOS response and PCD in radiation-exposed E. coli cells.

The antifungal occidiofungin triggers an apoptotic mechanism of cell death in yeast

Occidiofungin is a nonribosomally synthesized cyclic peptide having a base mass of 1200 Da. It is naturally produced by the soil bacterium Burkholderia contaminans MS14 and possesses potent broad-spectrum antifungal properties. The mechanism of action of occidiofungin is unknown. Viability, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), reactive oxygen species (ROS) detection, membrane and cell wall stability, and membrane mimetic assays were used to characterize the effect of occidiofungin on yeast cells. Confocal and electron microscopy experiments were used to visualize morphological changes within treated cells. TUNEL and ROS detection assays revealed an increase in fluorescence with increasing concentrations of the antifungal. Yeast cells appeared to shrink in size and showed the presence of 'dancing bodies' at low drug concentrations (1 μg/mL). A screen carried out on Saccharomyces cerevisiae gene deletion mutants in the apoptotic and autophagy pathways identified the apoptotic gene for YCA1, as having an important role in occidiofungin response as cells deleted for this gene exhibit a 2-fold increase in resistance. Results from our experiments demonstrate that the mechanism of action for occidiofungin in yeast is different from that of the common classes of antifungals used in the clinic, such as azoles, polyenes, and echinocandins. Our study also indicates that occidiofungin causes cell death in yeast through an apoptotic mechanism of action.

Mitochondrial respiratory pathways inhibition in Rhizopus oryzae potentiates activity of posaconazole and itraconazole via apoptosis

The incidence of mucormycosis has increased drastically in immunocompromised patients. Also the array of targets whose inhibition results in Mucorales death is limited. Recently, researchers identified mitochondria as important regulators of detoxification and virulence mechanisms in fungi. In this context, targeting the mitochondrial respiratory chain may provide a new platform for antifungal development. We hypothesized that targeting respiratory pathways potentiates triazoles activity via apoptosis. We found that simultaneous administration of antimycin A (AA) and benzohydroxamate (BHAM), inhibitors of classical and alternative mitochondrial pathways respectively, resulted in potent activity of posaconazole (PCZ) and itraconazole (ICZ) against Rhizopus oryzae. We observed cellular changes characteristic of apoptosis in R. oryzae cells treated with PCZ or ICZ in combination with AA and BHAM. The fungicidal activity of this combination against R. oryzae was correlated with intracellular reactive oxygen species accumulation (ROS), phosphatidylserine externalization, mitochondrial membrane depolarization, and increased caspase like activity. DNA fragmentation and condensation assays also revealed apoptosis of R. oryzae cells. These apoptotic features were prevented by the addition of the ROS scavenger N-acetyl-cysteine. Taken together, these findings suggest that the use of PCZ or ICZ in combination with AA and BHAM makes R. oryzae exquisitely sensitive to treatment with triazoles via apoptosis. This strategy may serve as a new model for the development of improved or novel antifungal agents.

CaIPF19998 reduces drug susceptibility by enhancing the ability of biofilm formation and regulating redox homeostasis in Candida albicans

CaIPF19998, a functionally unknown gene in Candida albicans, was identified by its homology to Saccharomyces cerevisiae AIF1 gene, which is involved in cell apoptosis. In this study, ipf19998 null mutant was generated with the URA-blaster method and the construction of overexpression of CaIPF19998 was measured by quantitative RT-PCR. Minimal inhibitory concentrations determination showed that the ipf19998 overexpressed strains was more resistant to the antifungals tested than the wildtype (strain CAI4). The 2,3-bis (2-methoxy-4-nitro-5-sulfo-phenyl)-2Htetrazolium-5-carboxanilide reduction assay showed that CaIPF19998 could enhance the capacity of C. albicans biofilms formation. On Candida biofilms mode, intracellular levels of reactive oxygen species were significantly decreased and real-time RT-PCR showed that some important redox-related genes, including ALD5, CIT1, PIL1, AHP1, TRX1 and TSA1, were up-regulated in the CaIPF19998 overexpressed strains. These results demonstrate that CaIPF19998 played an important role in C. albicans biofilms formation and intracellular redox homeostasis, therefore led to a close relationship between CaIPF19998 and drug susceptibility in C. albicans.

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.

Reactive oxygen species homeostasis and virulence of the fungal pathogen Cryptococcus neoformans requires an intact proline catabolism pathway

Degradation of the multifunctional amino acid proline is associated with mitochondrial oxidative respiration. The two-step oxidation of proline is catalyzed by proline oxidase and Δ(1)-pyrroline-5-carboxylate (P5C) dehydrogenase, which produce P5C and glutamate, respectively. In animal and plant cells, impairment of P5C dehydrogenase activity results in P5C-proline cycling when exogenous proline is supplied via the actions of proline oxidase and P5C reductase (the enzyme that converts P5C to proline). This proline is oxidized by the proline oxidase-FAD complex that delivers electrons to the electron transport chain and to O2, leading to mitochondrial reactive oxygen species (ROS) overproduction. Coupled activity of proline oxidase and P5C dehydrogenase is therefore important for maintaining ROS homeostasis. In the genome of the fungal pathogen Cryptococcus neoformans, there are two paralogs (PUT1 and PUT5) that encode proline oxidases and a single ortholog (PUT2) that encodes P5C dehydrogenase. Transcription of all three catabolic genes is inducible by the presence of proline. However, through the creation of deletion mutants, only Put5 and Put2 were found to be required for proline utilization. The put2Δ mutant also generates excessive mitochondrial superoxide when exposed to proline. Intracellular accumulation of ROS is a critical feature of cell death; consistent with this fact, the put2Δ mutant exhibits a slight, general growth defect. Furthermore, Put2 is required for optimal production of the major cryptococcal virulence factors. During murine infection, the put2Δ mutant was discovered to be avirulent; this is the first report highlighting the importance of P5C dehydrogenase in enabling pathogenesis of a microorganism.

Fungicidal drugs induce a common oxidative-damage cellular death pathway

Amphotericin, miconazole, and ciclopirox are antifungal agents from three different drug classes that can effectively kill planktonic yeast, yet their complete fungicidal mechanisms are not fully understood. Here, we employ a systems biology approach to identify a common oxidative-damage cellular death pathway triggered by these representative fungicides in Candida albicans and Saccharomyces cerevisiae. This mechanism utilizes a signaling cascade involving the GTPases Ras1 and Ras2 and protein kinase A, and it culminates in death through the production of toxic reactive oxygen species in a tricarboxylic-acid-cycle- and respiratory-chain-dependent manner. We also show that the metabolome of C. albicans is altered by antifungal drug treatment, exhibiting a shift from fermentation to respiration, a jump in the AMP/ATP ratio, and elevated production of sugars; this coincides with elevated mitochondrial activity. Lastly, we demonstrate that DNA damage plays a critical role in antifungal-induced cellular death and that blocking DNA-repair mechanisms potentiates fungicidal activity.

S-Adenosyl-L-methionine protects the probiotic yeast, Saccharomyces boulardii, from acid-induced cell death

Background

Saccharomyces boulardii is a probiotic yeast routinely used to prevent and to treat gastrointestinal disorders, including the antibiotic-associated diarrhea caused by Clostridium difficile infections. However, only 1-3% of the yeast administered orally is recovered alive in the feces suggesting that this yeast is unable to survive the acidic environment of the gastrointestinal tract.

Results

We provide evidence that suggests that S. boulardii undergoes programmed cell death (PCD) in acidic environments, which is accompanied by the generation of reactive oxygen species and the appearance of caspase-like activity. To better understand the mechanism of cell death at the molecular level, we generated microarray gene expression profiles of S. boulardii cells cultured in an acidic environment. Significantly, functional annotation revealed that the up-regulated genes were significantly over-represented in cell death pathways Finally, we show that S-adenosyl-L-methionine (AdoMet), a commercially available, FDA-approved dietary supplement, enhances the viability of S. boulardii in acidic environments, most likely by preventing programmed cell death.

Conclusions

In toto, given the observation that many of the proven health benefits of S. boulardii are dependent on cell viability, our data suggests that taking S. boulardii and AdoMet together may be a more effective treatment for gastrointestinal disorders than taking the probiotic yeast alone.

Variation in adhesion and germ tube formation of oral Candida using Egyptian propolis

Adhesion of Candida cells to surfaces is considered the first step in colonization. Some natural products, such as propolis, could be used to block cell adhesion and therefore preventing colonization. In this study, Egyptian propolis ethanol extract concentrations in the range of 25 to 125 ng/μL were used to inhibit the adhesion of oral Candida. The exopolysaccharides showed a 2.5-fold decrease, while the surface-bound exopolysaccharides showed only about 1.15-fold decrease. On the other hand, surface-bound proteins decreased by 7.5-fold upon the addition of 75 ng/μL propolis. The inhibition of adhesion was detected by scanning electron microscopy. The non-slip incubation assay showed a significant decrease in germ tube formation (GTF) compared with an increase upon the addition of fetal bovine serum and corn meal, both of which had a positive effect on GTF compared with a negative GTF effect when using propolis, suggesting that propolis could be considered a quorum-sensing molecule. The use of propolis would help in maintaining the cleanliness of dental fixtures and (or) treating recurrent candidiasis as a complementary and alternative treatment, especially in elders and immunocompromised patients.