Amphotericin B induces trehalose synthesis and simultaneously activates an antioxidant enzymatic response in Candida albicans

Insight into Virulence and Mechanisms of Amphotericin B Resistance in the Candida haemulonii Complex

The Candida haemulonii complex includes emerging opportunistic human fungal pathogens with documented multidrug-resistance profiles. It comprises Candida haemulonii sensu stricto, Candida haemulonii var. vulnera, Candida duobushaemulonii, Candida pseudohaemulonii, and Candida vulturna. In recent years, rates of clinical isolation of strains from this complex have increased in multiple countries, including China, Malaysia, and Brazil. Biofilm formation, hydrolytic enzymes, surface interaction properties, phenotype switching and cell aggregation abilities, extracellular vesicles production, stress response, and immune evasion help these fungi to infect the host and exert pathological effects. Multidrug resistance profiles also enhance the threat they pose; they exhibit low susceptibility to echinocandins and azoles and an intrinsic resistance to amphotericin B (AMB), the first fungal-specific antibiotic. AMB is commonly employed in antifungal treatments, and it acts via several known mechanisms. Given the propensity of clinical Candida species to initiate bloodstream infections, clarifying how C. haemulonii resists AMB is of critical clinical importance. This review outlines our present understanding of the C. haemulonii complex's virulence factors, the mechanisms of action of AMB, and the mechanisms underlying AMB resistance.

Polypeptides Targeting Paracoccidioides brasiliensis Drk1

Considering the toxicity of conventional therapeutic approaches and the importance of precise mechanistic targets, it is important to explore signaling pathways implicated in fungal pathobiology. Moreover, treatment of paracoccidioidomycosis, a systemic mycosis caused by a dimorphic fungus, requires prolonged therapeutic regimens. Among the numerous factors underpinning the establishment of Paracoccidioides spp. infection, the capacity to transition from the mycelial to the yeast form is of pivotal importance. The Drk1 protein of Paracoccidioides brasiliensis likely plays a decisive role in this morphological shift and subsequent virulence. We identified peptides with affinity for the PbDrk1 protein using the phage-display method and assessed the effects of these peptides on P. brasiliensis. The peptides were found to inhibit the phase transition of P. brasiliensis. Furthermore, a substantial proportion of these peptides prevented adhesion to pneumocytes. Although these peptides may not possess inherent antifungal properties, they can augment the effects of certain antifungal agents. Notably, the cell wall architecture of P. brasiliensis appears to be modulated by peptide intervention, resulting in a reduced abundance of glycosylated proteins and lipids. These peptides were also evaluated for their efficacy in a Galleria mellonella model and shown to contribute to enhanced larval survival rates. The role of PbDrk1, which is notably absent in mammals, should be further investigated to improve the understanding of its functional role in P. brasiliensis, which may be helpful for designing novel therapeutic modalities.

The antifungal effect induced by itraconazole in Candida parapsilosis largely depends on the oxidative stress generated at the mitochondria

In Candida parapsilosis, homozygous disruption of the two genes encoding trehalase activity increased the susceptibility to Itraconazole compared with the isogenic parental strain. The fungicidal effect of this azole can largely be counteracted by preincubating growing cells with rotenone and the protonophore 2,4-Dinitrophenol. In turn, measurement of endogenous reactive oxygen species formation by flow cytometry confirmed that Itraconazole clearly induced an internal oxidative stress, which can be significantly abolished in rotenone-exposed cells. Analysis of the antioxidant enzymatic activities of catalase and superoxide dismutase pointed to a moderate decrease of catalase in trehalase-deficient mutant cells compared to the wild type, with an additional increase upon addition of rotenone. These enzymatic changes were imperceptible in the case of superoxide dismutase. Alternative assays with Voriconazole led to a similar profile in the results regarding cell growth and antioxidant activities. Collectively, our data suggest that the antifungal action of Itraconazole on C. parapsilosis is dependent on a functional mitochondrial activity. They also suggest that the central metabolic pathways in pathogenic fungi should be considered as preferential antifungal targets in new research.

Functional Characterization of a Bacillus-Derived Novel Broad-Spectrum Antifungal Lipopeptide Variant against Candida tropicalis and Candida auris and Unravelling Its Mode of Action

Limited treatment options, recalcitrance, and resistance to existing therapeutics encourage the discovery of novel antifungal leads for alternative therapeutics. Antifungal lipopeptides have emerged as potential candidates for developing new and alternative antifungal therapies. In our previous studies, we isolated and identified the lipopeptide variant AF4 and purified it to homogeneity via chromatography from the cell-free supernatant of Bacillus subtilis. AF4 was found to have broad-spectrum antifungal activity against more than 110 fungal isolates. In this study, we found that clinical isolates of Candida tropicalis and Candida auris exposed to AF4 exhibited low MICs of 4 to 8 mg/L. Time-kill assays indicated the in vitro pharmacodynamic potential of AF4. Biocompatibility assays demonstrated ~75% cell viability at 8 mg/L of AF4, indicating the lipopeptide's minimally cytotoxic nature. In lipopeptide-treated C. tropicalis and C. auris cells, scanning electron microscopy revealed damage to the cell surface, while confocal microscopy with acridine orange(AO)/propidium iodide (PI) and FUN-1 indicated permeabilization of the cell membrane, and DNA damage upon DAPI (4',6-diamidino-2-phenylindole) staining. These observations were corroborated using flow cytometry (FC) in which propidium iodide, 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA), and rhodamine 123 (Rh123) staining of cells treated with AF4 revealed loss of membrane integrity, increased reactive oxygen species (ROS) production, and mitochondrial membrane dysfunction, respectively. Membrane perturbation was also observed in the 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence study and the interaction with ergosterol was observed by an ergosterol binding assay. Decreased membrane dipole potential also indicated the probable binding of lipopeptide to the cell membrane. Collectively, these findings describe the mode of action of AF4 against fungal isolates by membrane disruption and ROS generation, demonstrating its antifungal potency. IMPORTANCE C. tropicalis is a major concern for candidiasis in India and C. auris has emerged as a resistant yeast causing difficult-to-treat infections. Currently, amphotericin B (AMB) and 5-flucytosine (5-FC) are the main therapeutics for systemic fungal infections; however, the nephrotoxicity of AMB and resistance to 5-FC is a serious concern. Antifungal lead molecules with low adverse effects are the need of the hour. In this study, we briefly describe the antifungal potential of the AF4 lipopeptide and its mode of action using microscopy, flow cytometry, and fluorescence-based assays. Our investigation reveals the basic mode of action of the investigated lipopeptide. This lipopeptide with broad-spectrum antifungal potency is apparently membrane-active, and there is a smaller chance that organisms exposed to such a compound will develop drug resistance. It could potentially act as a lead molecule for the development of an alternative antifungal agent to combat candidiasis.

Reactive oxygen and nitrogen species are crucial for the antifungal activity of amorolfine and ciclopirox olamine against the dermatophyte Trichophyton interdigitale

Onychomycosis is a nail infection caused by Trichophyton interdigitale and other fungi, which can be treated with topical amorolfine (AMR) and ciclopirox olamine (CPX). Although these drugs are widely used, little is known about the role of reactive oxygen (ROS) and nitrogen (RNS) in their mechanism of action. Aiming to better understand the effects of AMR and CPX in dermatophytes, we evaluated whether they act through the production of ROS and peroxynitrite (PRN). We tested a set of strains, all susceptible to AMR and CPX, and these antifungals significantly reduced T. interdigitale viability within 24 hours. This effect occurred concomitantly with reduced ergosterol, increased production of ROS and PRN, and consequently increased lipid peroxidation. Together, these mechanisms lead to cell damage and fungal death. These fungicidal effects were abolished when PRN and superoxide scavengers were used in the assays, demonstrating the role of these species in the mechanism of action. We also studied the antioxidant system when T. interdigitale was exposed to AMR and CPX. Interestingly, superoxide dismutase and catalase inhibition lead to altered ROS and PRN production, lipid peroxidation, and ergosterol levels. In fact, the combination of AMR or CPX with a superoxide dismutase inhibitor was antagonistic. Together, these data demonstrate the importance of ROS and PRN in the antifungal action of AMR and CPX against the evaluated T. interdigitale strains.

Lack of Functional Trehalase Activity in Candida parapsilosis Increases Susceptibility to Itraconazole

Central metabolic pathways may play a major role in the virulence of pathogenic fungi. Here, we have investigated the susceptibility of a Candida parapsilosis mutant deficient in trehalase activity (atc1Δ/ntc1Δ strain) to the azolic compounds fluconazole and itraconazole. A time-course exposure to itraconazole but not fluconazole induced a significant degree of cell killing in mutant cells compared to the parental strain. Flow cytometry determinations indicated that itraconazole was able to induce a marked production of endogenous ROS together with a simultaneous increase in membrane potential, these effects being irrelevant after fluconazole addition. Furthermore, only itraconazole induced a significant synthesis of endogenous trehalose. The recorded impaired capacity of mutant cells to produce structured biofilms was further increased in the presence of both azoles, with itraconazole being more effective than fluconazole. Our results in the opportunistic pathogen yeast C. parapsilosis reinforce the study of trehalose metabolism as an attractive therapeutic target and allow extending the hypothesis that the generation of internal oxidative stress may be a component of the antifungal action exerted by the compounds currently available in medical practice.

The involvement of the Candida glabrata trehalase enzymes in stress resistance and gut colonization

Candida glabrata is an opportunistic human fungal pathogen and is frequently present in the human microbiome. It has a high relative resistance to environmental stresses and several antifungal drugs. An important component involved in microbial stress tolerance is trehalose. In this work, we characterized the three C. glabrata trehalase enzymes Ath1, Nth1 and Nth2. Single, double and triple deletion strains were constructed and characterized both in vitro and in vivo to determine the role of these enzymes in virulence. Ath1 was found to be located in the periplasm and was essential for growth on trehalose as sole carbon source, while Nth1 on the other hand was important for oxidative stress resistance, an observation which was consistent by the lower survival rate of the NTH1 deletion strain in human macrophages. No significant phenotype was observed for Nth2. The triple deletion strain was unable to establish a stable colonization of the gastrointestinal (GI) tract in mice indicating the importance of having trehalase activity for colonization in the gut.

Trehalose as quantitative biomarker for in vivo diagnosis and treatment follow-up in cryptococcomas

Brain lesions caused by Cryptococcus neoformans or C. gattii (cryptococcomas) are typically difficult to diagnose correctly and treat effectively, but rapid differential diagnosis and treatment initiation are crucial for good outcomes. In previous studies, cultured cryptococcal isolates and ex vivo lesion material contained high concentrations of the virulence factor and fungal metabolite trehalose. Here, we studied the in vivo metabolic profile of cryptococcomas in the brain using magnetic resonance spectroscopy (MRS) and assessed the relationship between trehalose concentration, fungal burden, and treatment response in order to validate its suitability as marker for early and noninvasive diagnosis and its potential to monitor treatment in vivo. We investigated the metabolites present in early and late stage cryptococcomas using in vivo ¹H MRS in a murine model and evaluated changes in trehalose concentrations induced by disease progression and antifungal treatment. Animal data were compared to ¹H and ¹³C MR spectra of Cryptococcus cultures and in vivo data from 2 patients with cryptococcomas in the brain. In vivo MRS allowed the noninvasive detection of high concentrations of trehalose in cryptococcomas and showed a comparable metabolic profile of cryptococcomas in the murine model and human cases. Trehalose concentrations correlated strongly with the fungal burden. Treatment studies in cultures and animal models showed that trehalose concentrations decrease following exposure to effective antifungal therapy. Although further cases need to be studied for clinical validation, this translational study indicates that the noninvasive MRS-based detection of trehalose is a promising marker for diagnosis and therapeutic follow-up of cryptococcomas.

Antifungal Drug Resistance: Molecular Mechanisms in Candida albicans and Beyond

Fungal infections are a major contributor to infectious disease-related deaths across the globe. Candida species are among the most common causes of invasive mycotic disease, with Candida albicans reigning as the leading cause of invasive candidiasis. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for antifungal development remains limited. Currently, there are only three major classes of drugs approved for the treatment of invasive mycoses, and the efficacy of these agents is compromised by the development of drug resistance in pathogen populations. Notably, the emergence of additional drug-resistant species, such as Candida auris and Candida glabrata, further threatens the limited armamentarium of antifungals available to treat these serious infections. Here, we describe our current arsenal of antifungals and elaborate on the resistance mechanisms Candida species possess that render them recalcitrant to therapeutic intervention. Finally, we highlight some of the most promising therapeutic strategies that may help combat antifungal resistance, including combination therapy, targeting fungal-virulence traits, and modulating host immunity. Overall, a thorough understanding of the mechanistic principles governing antifungal drug resistance is fundamental for the development of novel therapeutics to combat current and emerging fungal threats.

The Fungicidal Action of Micafungin is Independent on Both Oxidative Stress Generation and HOG Pathway Signaling in Candida albicans

In fungi, the Mitogen-Activated Protein kinase (MAPK) pathways sense a wide variety of environmental stimuli, leading to cell adaptation and survival. The HOG pathway plays an essential role in the pathobiology of Candida albicans, including the colonization of the gastrointestinal tract in a mouse model, virulence, and response to stress. Here, we examined the role of Hog1 in the C. albicans response to the clinically relevant antifungal Micafungin (MF), whose minimum inhibitory concentration (MIC) was identical in the parental strain (RM100) and in the isogenic homozygous mutant hog1 (0.016 mg/L). The cell viability was impaired without significant differences between the parental strain, the isogenic hog1 mutant, and the Hog1⁺ reintegrant. This phenotype was quite similar in a collection of hog1 mutants constructed in a different C. albicans background. MF-treated cells failed to induce a relevant increase of both reactive oxygen species (ROS) formation and activation of the mitochondrial membrane potential in parental and hog1 cells. MF was also unable to trigger any significant activation of the genes coding for the antioxidant activities catalase (CAT1) and superoxide dismutase (SOD2), as well as on the corresponding enzymatic activities, whereas a clear induction was observed in the presence of Amphotericin B (AMB), introduced as a positive control of Hog1 signaling. Furthermore, Hog1 was not phosphorylated by the addition of MF, but, notably, this echinocandin caused Mkc1 phosphorylation. Our results strongly suggest that the toxic effect of MF on C. albicans cells is not mediated by the Hog1 MAPK and is independent of the generation of an internal oxidative stress in C. albicans.

Unmasking the Amphotericin B Resistance Mechanisms in Candida haemulonii Species Complex

The polyene amphotericin B (AMB) exerts a powerful and broad antifungal activity. AMB acts by (i) binding to ergosterol, leading to pore formation at the fungal plasma membrane with subsequent ion leakage, and (ii) inducing the intracellular accumulation of reactive oxygen species (ROS). Herein, we have deciphered the AMB resistance mechanisms in clinical isolates of Candida haemulonii complex (C. haemulonii, C. duobushaemulonii, C. haemulonii var. vulnera) in comparison to other clinically relevant non-albicans Candida species. Membrane gas chromatography-mass spectrometry analysis revealed that the vast majority of sterols were composed of ergosterol pathway intermediates, evidencing the absence of AMB target. Supporting this data, C. haemulonii species complex demonstrated poor membrane permeability after AMB treatment. Regarding the oxidative burst, AMB induced the formation of ROS in all species tested; however, this phenomenon was slightly seen in C. haemulonii complex isolates. Our results indicated that these isolates displayed altered respiratory status, as revealed by their poor growth in nonfermented carbon sources, low consumption of oxygen, and derisive mitochondrial membrane potential. The use of specific inhibitors of mitochondrial respiratory chain (complex I-IV) revealed no effects on the yeast growth, highlighting the metabolic shift to fermentative pathway in C. haemulonii strains. Also, C. haemulonii complex proved to be highly resistant to oxidative burst agents, which can be correlated with a high activity of antioxidant enzymes. Our data demonstrated primary evidence suggesting that ergosterol content, mitochondrial function, and fungal redox homeostasis are involved in AMB fungicidal effects and might explain the resistance presented in this multidrug-resistant, emergent, and opportunistic fungal complex.

The MAPK Hog1 mediates the response to amphotericin B in Candida albicans

The HOG MAP kinase pathway plays a crucial role in the response to different stresses in the opportunistic pathogen Candida albicans. The polyene amphotericin B (AMB) has been reported to trigger oxidative stress in several pathogenic fungi, including C. albicans. In the present work, we have analyzed the role of the MAPK Hog1 in sensing and survival to AMB treatment. Mutants lacking Hog1 are more susceptible to AMB than their parental strains and Hog1 became phosphorylated in the presence of this polyene. A set of mutated versions of Hog1 revealed that both the kinase activity and phosphorylation of Hog1 are required to cope with AMB treatment. Flow cytometry analysis showed that AMB induced intracellular ROS accumulation in both parental and hog1 null mutant strains. In addition, AMB triggered a Hog1-independent synthesis of trehalose. The addition of rotenone to AMB-treated cells improved cell viability, decreased intracellular ROS and prevented intracellular trehalose accumulation, suggesting that AMB-induced ROS is associated to a functional electron transport chain but the presence of rotenone did not impair Hog1 phosphorylation in AMB-treated cells. Our results indicate that Hog1 is necessary during AMB treatment to increase its survival.

Trehalose induced by reactive oxygen species relieved the radial growth defects of Pleurotus ostreatus under heat stress

Trehalose is a nonreducing disaccharide, and it plays an intracellular protective role in organisms under various stress conditions. In this study, the trehalose synthesis and its protective role in Pleurotus ostreatus were investigated. As a signal in metabolic regulation, reactive oxygen species (ROS) accumulated in the mycelia of P. ostreatus under heat stress (HS). Furthermore, mycelial growth was significantly inhibited, and the malondialdehyde (MDA) level significantly increased under HS. First, exogenous addition of H₂O₂ inhibited mycelial growth and elevated the MDA level, while N-acetyl cysteine (NAC) and vitamin C (VC) reduced the MDA level and recovered mycelial growth under HS by scavenging ROS. These results indicated that the mycelial radial growth defect under HS might be partly caused by ROS accumulation. Second, adding NAC and VC to the media resulted in rescued trehalose accumulation, which indicated that ROS has an effect on inducing trehalose synthesis. Third, the mycelial growth was recovered by addition of trehalose to the media after HS, and the MDA level was reduced. This effect was further verified by the overexpression of genes for trehalose-6-phosphate synthase (TPS) and neutral trehalase (NTH), which led to increased and reduced trehalose content, respectively. In addition, adding validamycin A (NTH inhibitor) to the media promoted trehalose accumulation and the recovered mycelial growth after HS. In conclusion, trehalose production was partly induced by ROS accumulation in the mycelia under HS, and the accumulated trehalose could promote the recovery of growth after HS, partly by reducing the MDA level in the mycelia.

Chemogenomic Profiling of the Fungal Pathogen Candida albicans

There is currently a small number of classes of antifungal drugs, and these drugs are known to target a very limited set of cellular functions. We derived a set of approximately 900 nonessential, transactivator-defective disruption strains from the tetracycline-regulated GRACE collection of strains of the fungal pathogen Candida albicans This strain set was screened against classic antifungal drugs to identify gene inactivations that conferred either enhanced sensitivity or increased resistance to the compounds. We examined two azoles, fluconazole and posaconazole; two echinocandins, caspofungin and anidulafungin; and a polyene, amphotericin B. Overall, the chemogenomic profiles within drug classes were highly similar, but there was little overlap between classes, suggesting that the different drug classes interacted with discrete networks of genes in C. albicans We also tested two pyridine amides, designated GPI-LY7 and GPI-C107; these drugs gave very similar profiles that were distinct from those of the echinocandins, azoles, or polyenes, supporting the idea that they target a distinct cellular function. Intriguingly, in cases where these gene sets can be compared to genetic disruptions conferring drug sensitivity in other fungi, we find very little correspondence in genes. Thus, even though the drug targets are the same in the different species, the specific genetic profiles that can lead to drug sensitivity are distinct. This implies that chemogenomic screens of one organism may be poorly predictive of the profiles found in other organisms and that drug sensitivity and resistance profiles can differ significantly among organisms even when the apparent target of the drug is the same.

Targeting Candida spp. to develop antifungal agents

Invasive fungal infections are a complex challenge throughout the world because of their high incidence, mainly in critically ill patients, and high mortality rates. The antifungal agents currently available are limited; thus, there is a need for the rapid development of new drugs. In silico methods are a modern strategy to explore interactions between new compounds and specific fungal targets, but they depend on precise genetic information. Here, we discuss the main Candida spp. target genes, including information about null mutants, virulence, cytolocalization, co-regulatory genes, and compounds that are related to protein expression. These data will provide a basis for the future in silico development of antifungal drugs.

ROS formation is a differential contributory factor to the fungicidal action of Amphotericin B and Micafungin in Candida albicans

The hypothetical role played by the intracellular formation of reactive oxygen species (ROS) in the fungicidal action carried out by Amphotericin B (AmB) and Micafungin (MF) was examined in Candida albicans, which remains the most prevalent fungal pathogen. The clinical MICs for MF and AmB were 0.016 and 0.12μg/ml, respectively. Whereas AmB (0.5-1.0×MIC) induced a marked production of intracellular ROS accompanied by a high degree of cell killing in the C. albicans SC5314 strain, the fungicidal effect of MF was still operative, but ROS generation was slight. Preincubation with thiourea suppressed the formation of ROS and caused a marked increase in cell viability, regardless of the antifungal used. Simultaneous measurement of several well established antioxidant enzymes (catalase, glutathione reductase and superoxide dismutase) revealed strong AmB-induced activation of the three enzymatic activities, whereas MF only had a weak stimulating effect. Likewise, AmB but not MF promoted a conspicuous rise in the mitochondrial membrane potential together with the intracellular synthesis of trehalose, the non-reducing disaccharide which acts as a specific protector against oxidative stress in C. albicans. Optical and electronic microscopy analysis revealed a significant damage to cell integrity and structural alterations caused by both antifungals. Taken together, our results strongly suggest that the induction of an internal oxidative stress in C. albicans through the accumulation of ROS is a preferential contributory factor to the antifungal action of a widely used polyene (AmB) but not of MF (echinocandin).

Role of catalase overproduction in drug resistance and virulence in Candida albicans

Aim: To investigate the role of Cat1 overproduction in Candida albicans. Materials & methods: Strains overproducing the CAT1 gene were constructed. Results: Cells overproducing CAT1 were found to be more resistant to some oxidants and mammalian phagocytic cells. They also showed reduced intracellular reactive oxygen species generated by amphotericin B or ciclopirox olamine. CAT1 overproduction did not change the minimum inhibitory concentration of fungal cells to fungistatic or fungicidal azoles nor to amphotericin B although increased twofold the minimum inhibitory concentration to caspofungin. The role of Cat1 overproduction in virulence and colonization was also analyzed in mouse models. Conclusion: The overproduction of Cat1 protects against oxidants, phagocytes and certain antifungals at subinhibitory concentration but does not increase virulence in a systemic infection mouse model.

Homozygous deletion of ATC1 and NTC1 genes in Candida parapsilosis abolishes trehalase activity and affects cell growth, sugar metabolism, stress resistance, infectivity and biofilm formation

A double homozygous atc1Δ/atc1Δ/ntc1Δ/ntc1Δ mutant (atc1Δ/ntc1Δ KO) was constructed in the pathogen opportunistic yeast Candida parapsilosis by disruption of the two chromosomal alleles coding for NTC1 gene (encoding a neutral trehalase) in a Cpatc1Δ/atc1Δ background (atc1Δ KO strain, deficient in acid trehalase). The Cpatc1Δ/ntc1Δ KO mutant failed to counteract the inability of Cpatc1Δ cells to metabolize exogenous trehalose and showed a similar growth pattern on several monosaccharides and disaccharides. However, upon prolonged incubation in either rich medium (YPD) or nutrient-starved medium the viability of Cpatc1Δ cells exhibited a sensitive phenotype, which was augmented by further CpNTC1/NTC1 disruption. Furthermore, Cpatc1Δ/ntc1Δ KO cells had difficulty in resuming active growth in fresh YPD. This homozygous mutant also lacked any in vitro measurable trehalase activity, whether acid or neutral, suggesting that a single gene codes for each enzyme. By contrast, in Cpatc1Δ/ntc1Δ KO strain the resistance to oxidative and heat stress displayed by atc1Δ mutant was suppressed. Cpatc1Δ/ntc1Δ KO cells showed a significant decrease in virulence as well as in the capacity to form biofilms. These results point to a major role for acid trehalase (Atc1p) in the pathobiology of C. parapsilosis, whereas the activity of neutral trehalase can only partially counteract Atc1p deficiency. They also support the use of ATC1 and NTC1 genes as interesting antifungal targets.

In Candida parapsilosis the ATC1 gene encodes for an acid trehalase involved in trehalose hydrolysis, stress resistance and virulence

An ORF named CPAR2-208980 on contig 005809 was identified by screening a Candida parapsilosis genome data base. Its 67% identity with the acid trehalase sequence from C. albicans (ATC1) led us to designate it CpATC1. Homozygous mutants that lack acid trehalase activity were constructed by gene disruption at the two CpATC1 chromosomal alleles. Phenotypic characterization showed that atc1Δ null cells were unable to grow on exogenous trehalose as carbon source, and also displayed higher resistance to environmental challenges, such as saline exposure (1.2 M NaCl), heat shock (42°C) and both mild and severe oxidative stress (5 and 50 mM H2O2). Significant amounts of intracellular trehalose were specifically stored in response to the thermal upshift in both wild type and mutant strains. Analysis of their antioxidant activities revealed that catalase was only triggered in response to heat shock in atc1Δ cells, whereas glutathione reductase was activated upon mild oxidative stress in wild type and reintegrant strains, and in response to the whole set of stress treatments in the homozygous mutant. Furthermore, yeast cells with double CpATC1 deletion were significantly attenuated in non-mammalian infection models, suggesting that CpATC1 is required for the pathobiology of the fungus. Our results demonstrate the involvement of CpAtc1 protein in the physiological hydrolysis of external trehalose in C. parapsilosis, where it also plays a major role in stress resistance and virulence.

Effect of amphotericin B on the metabolic profiles of Candida albicans

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

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.

Synergism of antifungal activity between mitochondrial respiration inhibitors and kojic acid

Co-application of certain types of compounds to conventional antimicrobial drugs can enhance the efficacy of the drugs through a process termed chemosensitization. We show that kojic acid (KA), a natural pyrone, is a potent chemosensitizing agent of complex III inhibitors disrupting the mitochondrial respiratory chain in fungi. Addition of KA greatly lowered the minimum inhibitory concentrations of complex III inhibitors tested against certain filamentous fungi. Efficacy of KA synergism in decreasing order was pyraclostrobin > kresoxim-methyl > antimycin A. KA was also found to be a chemosensitizer of cells to hydrogen peroxide (H₂O₂), tested as a mimic of reactive oxygen species involved in host defense during infection, against several human fungal pathogens and Penicillium strains infecting crops. In comparison, KA-mediated chemosensitization to complex III inhibitors/H₂O₂ was undetectable in other types of fungi, including Aspergillus flavus, A. parasiticus, and P. griseofulvum, among others. Of note, KA was found to function as an antioxidant, but not as an antifungal chemosensitizer in yeasts. In summary, KA could serve as an antifungal chemosensitizer to complex III inhibitors or H₂O₂ against selected human pathogens or Penicillium species. KA-mediated chemosensitization to H₂O₂ seemed specific for filamentous fungi. Thus, results indicate strain- and/or drug-specificity exist during KA chemosensitization.

Enhancement of commercial antifungal agents by Kojic Acid

Natural compounds that pose no significant medical or environmental side effects are potential sources of antifungal agents, either in their nascent form or as structural backbones for more effective derivatives. Kojic acid (KA) is one such compound. It is a natural by-product of fungal fermentation commonly employed by food and cosmetic industries. We show that KA greatly lowers minimum inhibitory (MIC) or fungicidal (MFC) concentrations of commercial medicinal and agricultural antifungal agents, amphotericin B (AMB) and strobilurin, respectively, against pathogenic yeasts and filamentous fungi. Assays using two mitogen-activated protein kinase (MAPK) mutants, i.e., sakAΔ, mpkCΔ, of Aspergillus fumigatus, an agent for human invasive aspergillosis, with hydrogen peroxide (H₂O₂) or AMB indicate such chemosensitizing activity of KA is most conceivably through disruption of fungal antioxidation systems. KA could be developed as a chemosensitizer to enhance efficacy of certain conventional antifungal drugs or fungicides.

Time course analysis of Candida albicans metabolites during biofilm development

Biofilm-associated infections are difficult to treat because of their decreased susceptibility to antimicrobial therapy. Candida albicans is the most common fungal pathogen associated with colonization and biofilm formation on the surfaces of indwelling medical devices which show intrinsic resistance to many commonly used antifungal agents. In this study, a metabonomic method using gas chromatography-mass spectrometry (GC/MS) was developed to characterize metabolic profiles during the whole biofilm developmental phases compared to the planktonic mode in C. albicans. Thirty-one differentially produced metabolites between the biofilm and planktonic specimens at each time point were identified, and they were mainly involved in the tricarboxylic acid (TCA) cycle, lipid synthesis, amino acid metabolism, glycolysis, and oxidative stress. Further experiments showed that lack of trehalose, one of the metabolites differentially produced between biofilm and planktonic cells, resulted in abnormal biofilm formation and increased sensitivity to amphotericin B and miconazole. This study provides a systemic view of the metabolic pattern during the development of C. albicans biofilms, indicating that multicomponent, phase-specific mechanisms are operative in the process of biofilm formation.

Enhancement of antimycotic activity of amphotericin B by targeting the oxidative stress response of Candida and cryptococcus with natural dihydroxybenzaldehydes

In addition to the fungal cellular membrane, the cellular antioxidant system can also be a viable target in the antifungal action of amphotericin B (AMB). Co-application of certain redox-potent natural compounds with AMB actually increases efficacy of the drug through chemosensitization. Some redox-potent chemosensitizers and AMB perturb common cellular targets, resulting in synergistic inhibition of fungal growth. Chemosensitizing activities of four redox-potent benzaldehydes were tested against clinical and reference strains of Candida albicans, C. krusei, C. tropicalis, and Cryptococcus neoformans in combination with AMB, based on assays outlined by the European Committee on Antimicrobial Susceptibility Testing. Two dihydroxybenzaldehydes (DHBAs), i.e., 2,3-DHBA and 2,5-DHBA, significantly enhanced activity of AMB against most strains, as measured by lower minimum inhibitory concentrations and/or minimum fungicidal concentrations (MFCs). A non-hydroxylated benzaldehyde, trans-cinnamaldehyde, showed chemosensitizing activity through lower MFCs, only. Contrastingly, a methoxylated benzaldehyde (3,5-dimethoxybenzaldehyde) had no chemosensitizing activity, as all strains were hypertolerant to this compound. Bioassays using deletion mutants of the model yeast, Saccharomyces cerevisiae, indicated DHBAs exerted their chemosensitizing activity by targeting mitochondrial superoxide dismutase. This targeting, in turn, disrupted the ability of the yeast strains to respond to AMB-induced oxidative stress. These in vitro results indicate that certain DHBAs are potent chemosensitizing agents to AMB through co-disruption of the oxidative stress response capacity of yeasts. Such redox-potent compounds show promise for enhancing AMB-based antifungal therapy for candidiasis and cryptococcosis.

Resveratrol lacks antifungal activity against Candida albicans

The putative candicidal activity of resveratrol is currently a matter of controversy. Here, the antifungal activity as well as the antioxidant response of resveratrol against Candida albicans, have been tested in a set of strains with a well-established genetic background At the doses usually employed in antifungal tests (10-40 μg/ml), resveratrol has no effect on the exponential growth of the C. albicans CAI.4 strain, a tenfold increase (400 μg/ml) was required in order to record a certain degree of cell killing, which was negligible in comparison with the strong antifungal effect caused by the addition of amphotericin B (5 μg/ml). An identical pattern was recorded in the prototrophic strains of C. albicans SC5314 and RM-100, whereas the oxidative sensitive trehalose-deficient mutant (tps1/tps1 strain) was totally refractory to the presence of resveratrol. In turn, the serum-induced yeast-to-hypha transition remained unaffected upon addition of different concentrations of resveratrol. Determination of endogenous trehalose and catalase activity, two antioxidant markers in C. albicans; revealed no significant changes in their basal contents induced by resveratrol. Collectively, our results seem to dismiss a main antifungal role as well as the therapeutic application of resveratrol against the infections caused by C. albicans.

Targeting the oxidative stress response system of fungi with redox-potent chemosensitizing agents

The cellular antioxidant system is a target in the antifungal action of amphotericin B (AMB) and itraconazole (ITZ), in filamentous fungi. The sakAΔ mutant of Aspergillus fumigatus, a mitogen-activated protein kinase (MAPK) gene deletion mutant in the antioxidant system, was found to be more sensitive to AMB or ITZ than other A. fumigatus strains, a wild type and a mpkCΔ mutant (a MAPK gene deletion mutant in the polyalcohol sugar utilization system). Complete fungal kill (≥99.9%) by ITZ or AMB was also achieved by much lower dosages for the sakAΔ mutant than for the other strains. It appears msnA, an Aspergillus ortholog to Saccharomyces cerevisiaeMSN2 (encoding a stress-responsive C₂H₂-type zinc-finger regulator) and sakA and/or mpkC (upstream MAPKs) are in the same stress response network under tert-butyl hydroperoxide (t-BuOOH)-, hydrogen peroxide (H₂O₂)- or AMB-triggered toxicity. Of note is that ITZ-sensitive yeast pathogens were also sensitive to t-BuOOH, showing a connection between ITZ sensitivity and antioxidant capacity of fungi. Enhanced antifungal activity of AMB or ITZ was achieved when these drugs were co-applied with redox-potent natural compounds, 2,3-dihydroxybenzaldehyde, thymol or salicylaldehyde, as chemosensitizing agents. We concluded that redox-potent compounds, which target the antioxidant system in fungi, possess a chemosensitizing capacity to enhance efficacy of conventional drugs.