Mitochondrial Cochaperone Mge1 Is Involved in Regulating Susceptibility to Fluconazole in Saccharomyces cerevisiae and Candida Species

Sterol Biosynthesis Contributes to Brefeldin-A-Induced Endoplasmic Reticulum Stress Resistance in Chlamydomonas reinhardtii

The endoplasmic reticulum (ER) stress response is an evolutionarily conserved mechanism in most eukaryotes. In this response, sterols in the phospholipid bilayer play a crucial role in controlling membrane fluidity and homeostasis. Despite the significance of both the ER stress response and sterols in maintaining ER homeostasis, their relationship remains poorly explored. Our investigation focused on Chlamydomonas strain CC-4533 and revealed that free sterol biosynthesis increased in response to ER stress, except in mutants of the ER stress sensor Inositol-requiring enzyme 1 (IRE1). Transcript analysis of Chlamydomonas experiencing ER stress unveiled the regulatory role of the IRE1/basic leucine zipper 1 pathway in inducing the expression of ERG5, which encodes C-22 sterol desaturase. Through the isolation of three erg5 mutant alleles, we observed a defect in the synthesis of Chlamydomonas' sterol end products, ergosterol and 7-dehydroporiferasterol. Furthermore, these erg5 mutants also exhibited increased sensitivity to ER stress induced by brefeldin A (BFA, an inhibitor of ER-Golgi trafficking), whereas tunicamycin (an inhibitor of N-glycosylation) and dithiothreitol (an inhibitor of disulfide-bond formation) had no such effect. Intriguingly, the sterol biosynthesis inhibitors fenpropimorph and fenhexamid, which impede steps upstream of the ERG5 enzyme in sterol biosynthesis, rescued BFA hypersensitivity in CC-4533 cells. Collectively, our findings support the conclusion that the accumulation of intermediates in the sterol biosynthetic pathway influences ER stress in a complex manner. This study highlights the significance and complexity of regulating sterol biosynthesis during the ER stress response in microalgae.

The intricate link between iron, mitochondria and azoles in Candida species

Invasive fungal infections are rapidly increasing, and the opportunistic pathogenic Candida species are the fourth most common cause of nosocomial systemic infections. The current antifungal classes, of which azoles are the most widely used, all have shortcomings. Azoles are generally considered fungistatic rather than fungicidal, they do not actively kill fungal cells and therefore resistance against azoles can be rapidly acquired. Combination therapies with azoles provide an interesting therapeutic outlook and agents limiting iron are excellent candidates. We summarize how iron is acquired by the host and transported towards both storage and iron-utilizing organelles. We indicate whether these pathways alter azole susceptibility and/or tolerance, to finally link these transport mechanisms to mitochondrial iron availability. In this review, we highlight putative novel intracellular iron shuffling mechanisms and indicate that mitochondrial iron dynamics in relation to azole treatment and iron limitation is a significant knowledge gap.

Microdomain Protein Nce102 Is a Local Sensor of Plasma Membrane Sphingolipid Balance

Sphingolipids are essential building blocks of eukaryotic membranes and important signaling molecules that are regulated tightly in response to environmental and physiological inputs. While their biosynthetic pathway has been well-described, the mechanisms that facilitate the perception of sphingolipid levels at the plasma membrane remain to be uncovered. In Saccharomyces cerevisiae, the Nce102 protein has been proposed to function as a sphingolipid sensor as it changes its plasma membrane distribution in response to sphingolipid biosynthesis inhibition. We show that Nce102 redistributes specifically in regions of increased sphingolipid demand, e.g., membranes of nascent buds. Furthermore, we report that the production of Nce102 increases following sphingolipid biosynthesis inhibition and that Nce102 is internalized when excess sphingolipid precursors are supplied. This finding suggests that the total amount of Nce102 in the plasma membrane is a measure of the current need for sphingolipids, whereas its local distribution marks sites of high sphingolipid demand. The physiological role of Nce102 in the regulation of sphingolipid synthesis is demonstrated by mass spectrometry analysis showing reduced levels of hydroxylated complex sphingolipids in response to heat stress in the nce102Δ deletion mutant. We also demonstrate that Nce102 behaves analogously in the widespread human fungal pathogen Candida albicans, suggesting a conserved principle of local sphingolipid control across species. IMPORTANCE Microorganisms are challenged constantly by their rapidly changing environment. To survive, they have developed diverse mechanisms to quickly perceive stressful situations and adapt to them appropriately. The primary site of both stress sensing and adaptation is the plasma membrane. We identified the yeast protein Nce102 as a marker of local sphingolipid levels and fluidity in the plasma membrane. Nce102 is an important structural and functional component of the membrane compartment Can1 (MCC), a plasma membrane microdomain stabilized by a large cytosolic hemitubular protein scaffold, the eisosome. The MCC/eisosomes are widely conserved among fungi and unicellular algae. To determine if Nce102 carries out similar functions in other organisms, we analyzed the human fungal pathogen Candida albicans and found that Nce102 responds to sphingolipid levels also in this organism, which has potential applications for the development of novel therapeutic approaches. The presented study represents a valuable model for how organisms regulate plasma membrane sphingolipids.

Copper Acts Synergistically With Fluconazole in Candida glabrata by Compromising Drug Efflux, Sterol Metabolism, and Zinc Homeostasis

The synergistic combinations of drugs are promising strategies to boost the effectiveness of current antifungals and thus prevent the emergence of resistance. In this work, we show that copper and the antifungal fluconazole act synergistically against Candida glabrata, an opportunistic pathogenic yeast intrinsically tolerant to fluconazole. Analyses of the transcriptomic profile of C. glabrata after the combination of copper and fluconazole showed that the expression of the multidrug transporter gene CDR1 was decreased, suggesting that fluconazole efflux could be affected. In agreement, we observed that copper inhibits the transactivation of Pdr1, the transcription regulator of multidrug transporters and leads to the intracellular accumulation of fluconazole. Copper also decreases the transcriptional induction of ergosterol biosynthesis (ERG) genes by fluconazole, which culminates in the accumulation of toxic sterols. Co-treatment of cells with copper and fluconazole should affect the function of proteins located in the plasma membrane, as several ultrastructural alterations, including irregular cell wall and plasma membrane and loss of cell wall integrity, were observed. Finally, we show that the combination of copper and fluconazole downregulates the expression of the gene encoding the zinc-responsive transcription regulator Zap1, which possibly, together with the membrane transporters malfunction, generates zinc depletion. Supplementation with zinc reverts the toxic effect of combining copper with fluconazole, underscoring the importance of this metal in the observed synergistic effect. Overall, this work, while unveiling the molecular basis that supports the use of copper to enhance the effectiveness of fluconazole, paves the way for the development of new metal-based antifungal strategies.

Pseudomonas Synergizes with Fluconazole against Candida during Treatment of Polymicrobial Infection

Polymicrobial infections are challenging to treat because we don’t fully understand how pathogens interact during infection and how these interactions affect drug efficacy. Candida albicans and Pseudomonas aeruginosa are opportunistic pathogens that can be found in similar sites of infection such as in burn wounds and most importantly in the lungs of CF and mechanically ventilated patients. C. albicans is particularly difficult to treat because of the paucity of antifungal agents, some of which lack fungicidal activity. In this study, we investigated the efficacy of anti-fungal treatment during C. albicans-P. aeruginosa coculture in vitro and co-infection in the mucosal zebrafish infection model analogous to the lung. We find that P. aeruginosa enhances the activity of fluconazole (FLC), an anti-fungal drug that is fungistatic in vitro, to promote both clearance of C. albicans during co-infection in vivo and fungal killing in vitro. This synergy between FLC treatment and bacterial antagonism is partly due to iron piracy, as it is reduced upon iron supplementation and knockout of bacterial siderophores. Our work demonstrates that FLC has enhanced activity in clinically relevant contexts and highlights the need to understand antimicrobial effectiveness in the complex environment of the host with its associated microbial communities.

What 'Omics Can Tell Us About Antifungal Adaptation

Invasive candidiasis, the most frequent healthcare-associated invasive fungal infection, is commonly caused by Candida albicans. However, in recent years other antifungal-resistant Candida species—namely Candida glabrata and Candidaauris—have emerged as a serious matter of concern. Much of our understanding of the mechanisms regulating antifungal resistance and tolerance relies on studies utilizing C. albicans, C. glabrataand the model yeast Saccharomyces cerevisiae. ‘Omics studies have been used to describe alterations in metabolic, genomic and transcriptomic expression profiles upon antifungal treatment of fungal cells. The physiological changes identified by these approaches could significantly affect fungal fitness in the host and survival during antifungal challenge, as well as provide further understanding of clinical resistance. Thus, this review aims to comparatively address ‘omics data for C. albicans, C. glabrata andS. cerevisiae published from 2000 to 2021 to identify what these technologies can tell us regarding cellular responses to antifungal therapy. We will also highlight possible effects on pathogen survival and identify future avenues for antifungal research.

Mitochondrial perturbation reduces susceptibility to xenobiotics through altered efflux in Candida albicans

Candida albicans is a leading human fungal pathogen, which can cause superficial infections or life-threatening systemic disease in immunocompromised individuals. The ability to transition between yeast and filamentous forms is a major virulence trait of C. albicans, and a key regulator of this morphogenetic transition is the molecular chaperone Hsp90. To explore the mechanisms governing C. albicans morphogenesis in response to Hsp90 inhibition, we performed a functional genomic screen using the gene replacement and conditional expression collection to identify mutants that are defective in filamentation in response to the Hsp90 inhibitor, geldanamycin. We found that transcriptional repression of genes involved in mitochondrial function blocked filamentous growth in response to the concentration of the Hsp90 inhibitor used in the screen, and this was attributable to increased resistance to the compound. Further exploration revealed that perturbation of mitochondrial function reduced susceptibility to two structurally distinct Hsp90 inhibitors, geldanamycin and radicicol, such that filamentous growth was restored in the mitochondrial mutants by increasing the compound concentration. Deletion of two representative mitochondrial genes, MSU1 and SHY1, enhanced cellular efflux and reduced susceptibility to diverse intracellularly acting compounds. Additionally, screening a C. albicans efflux pump gene deletion library implicated Yor1 in the efflux of geldanamycin and Cdr1, in the efflux of radicicol. Deletion of these transporter genes restored sensitivity to Hsp90 inhibitors in MSU1 and SHY1 homozygous deletion mutants, thereby enabling filamentation. Taken together, our findings suggest that mitochondrial dysregulation elevates cellular efflux and consequently reduces susceptibility to xenobiotics in C. albicans.

Fluorescent toys 'n' tools lighting the way in fungal research

Although largely overlooked compared to bacterial infections, fungal infections pose a significant threat to the health of humans and other organisms. Many pathogenic fungi, especially Candida species, are extremely versatile and flexible in adapting to various host niches and stressful situations. This leads to high pathogenicity and increasing resistance to existing drugs. Due to the high level of conservation between fungi and mammalian cells, it is hard to find fungus-specific drug targets for novel therapy development. In this respect, it is vital to understand how these fungi function on a molecular, cellular as well as organismal level. Fluorescence imaging allows for detailed analysis of molecular mechanisms, cellular structures and interactions on different levels. In this manuscript, we provide researchers with an elaborate and contemporary overview of fluorescence techniques that can be used to study fungal pathogens. We focus on the available fluorescent labelling techniques and guide our readers through the different relevant applications of fluorescent imaging, from subcellular events to multispecies interactions and diagnostics. As well as cautioning researchers for potential challenges and obstacles, we offer hands-on tips and tricks for efficient experimentation and share our expert-view on future developments and possible improvements.

Photochromic Fluorophores Enable Imaging of Lowly Expressed Proteins in the Autofluorescent Fungus Candida albicans

Fluorescence microscopy is a standard research tool in many fields, although collecting reliable images can be difficult in systems characterized by low expression levels and/or high background fluorescence. We present the combination of a photochromic fluorescent protein and stochastic optical fluctuation imaging (SOFI) to deliver suppression of the background fluorescence. This strategy makes it possible to resolve lowly or endogenously expressed proteins, as we demonstrate for Gcn5, a histone acetyltransferase required for complete virulence, and Erg11, the target of the azole antifungal agents in the fungal pathogen Candida albicans We expect that our method can be readily used for sensitive fluorescence measurements in systems characterized by high background fluorescence.IMPORTANCE Understanding the spatial and temporal organization of proteins of interest is key to unraveling cellular processes and identifying novel possible antifungal targets. Only a few therapeutic targets have been discovered in Candida albicans, and resistance mechanisms against these therapeutic agents are rapidly acquired. Fluorescence microscopy is a valuable tool to investigate molecular processes and assess the localization of possible antifungal targets. Unfortunately, fluorescence microscopy of C. albicans suffers from extensive autofluorescence. In this work, we present the use of a photochromic fluorescent protein and stochastic optical fluctuation imaging to enable the imaging of lowly expressed proteins in C. albicans through the suppression of autofluorescence. This method can be applied in C. albicans research or adapted for other fungal systems, allowing the visualization of intricate processes.

Mitochondria-Mediated Azole Drug Resistance and Fungal Pathogenicity: Opportunities for Therapeutic Development

In recent years, the role of mitochondria in pathogenic fungi in terms of azole resistance and fungal pathogenicity has been a rapidly developing field. In this review, we describe the molecular mechanisms by which mitochondria are involved in regulating azole resistance and fungal pathogenicity. Mitochondrial function is involved in the regulation of drug efflux pumps at the transcriptional and posttranslational levels. On the one hand, defects in mitochondrial function can serve as the signal leading to activation of calcium signaling and the pleiotropic drug resistance pathway and, therefore, can globally upregulate the expression of drug efflux pump genes, leading to azole drug resistance. On the other hand, mitochondria also contribute to azole resistance through modulation of drug efflux pump localization and activity. Mitochondria further contribute to azole resistance through participating in iron homeostasis and lipid biosynthesis. Additionally, mitochondrial dynamics play an important role in azole resistance. Meanwhile, mitochondrial morphology is important for fungal virulence, playing roles in growth in stressful conditions in a host. Furthermore, there is a close link between mitochondrial respiration and fungal virulence, and mitochondrial respiration plays an important role in morphogenetic transition, hypoxia adaptation, and cell wall biosynthesis. Finally, we discuss the possibility for targeting mitochondrial factors for the development of antifungal therapies.

Molecular Elucidation of Riboflavin Production and Regulation in Candida albicans, toward a Novel Antifungal Drug Target

Candida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

Candida albicans is a major cause of fungal infections, both superficial and invasive. The economic costs as well as consequences for patient welfare are substantial. Only a few treatment options are available due to the high resemblance between fungal targets and host molecules, as both are eukaryotes. Riboflavin is a yellow pigment, also termed vitamin B₂. Unlike animals, fungi can synthesize this essential component themselves, thereby leading us to appreciate that targeting riboflavin production is a promising novel strategy against fungal infections. Here, we report that the GTP cyclohydrolase encoded by C. albicansRIB1 (CaRIB1) is essential and rate-limiting for production of riboflavin in the fungal pathogen. We confirm the high potential of CaRib1 as an antifungal drug target, as its deletion completely impairs in vivo infectibility by C. albicans in model systems. Furthermore, the stimulating effect of iron deprivation and PKA activation on riboflavin production seems to involve CaRib1 and the upstream transcription factor CaSef1. Gathering insights in the synthesis mechanism of riboflavin in pathogenic fungi, like C. albicans, will allow us to design a novel strategy and specifically target this process to combat fungal infections.

IMPORTANCECandida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

Regulation of Ergosterol Biosynthesis in Saccharomyces cerevisiae

Ergosterol is an essential component of fungal cell membranes that determines the fluidity, permeability and activity of membrane-associated proteins. Ergosterol biosynthesis is a complex and highly energy-consuming pathway that involves the participation of many enzymes. Deficiencies in sterol biosynthesis cause pleiotropic defects that limit cellular proliferation and adaptation to stress. Thereby, fungal ergosterol levels are tightly controlled by the bioavailability of particular metabolites (e.g., sterols, oxygen and iron) and environmental conditions. The regulation of ergosterol synthesis is achieved by overlapping mechanisms that include transcriptional expression, feedback inhibition of enzymes and changes in their subcellular localization. In the budding yeast Saccharomyces cerevisiae, the sterol regulatory element (SRE)-binding proteins Upc2 and Ecm22, the heme-binding protein Hap1 and the repressor factors Rox1 and Mot3 coordinate ergosterol biosynthesis (ERG) gene expression. Here, we summarize the sterol biosynthesis, transport and detoxification systems of S. cerevisiae, as well as its adaptive response to sterol depletion, low oxygen, hyperosmotic stress and iron deficiency. Because of the large number of ERG genes and the crosstalk between different environmental signals and pathways, many aspects of ergosterol regulation are still unknown. The study of sterol metabolism and its regulation is highly relevant due to its wide applications in antifungal treatments, as well as in food and pharmaceutical industries.

Candida albicans reprioritizes metal handling during fluconazole stress

Maintenance of metal homeostasis is critical to cell survival due to the multitude of cellular processes that depend on one or more metal cofactors. Here, we show that the opportunistic fungal pathogen Candida albicans extensively remodels its metal homeostasis networks to respond to treatment with the antifungal drug fluconazole. Disruption of the ergosterol biosynthetic pathway by fluconazole requires C. albicans adaptation, including increased Cu import and storage, increased retention of Fe, Mn, and Zn, altered utilization of Cu- and Mn-dependent enzymes, mobilization of Fe stores, and increased production of the heme prosthetic group utilized by the enzyme target of fluconazole. The findings offer a new perspective for thinking about fungal response to drug stress that pushes cells out of their metal homeostatic zones, leading them to enact metal-associated adaptation mechanisms to restore homeostasis to survive.

Sugar Phosphorylation Controls Carbon Source Utilization and Virulence of Candida albicans

Candida albicans is an opportunistic human fungal pathogen that relies upon different virulence traits, including morphogenesis, invasion, biofilm formation, and nutrient acquisition from host sources as well as metabolic adaptations during host invasion. In this study, we show how sugar kinases at the start of glycolysis modulate virulence of C. albicans. Sequence comparison with Saccharomyces cerevisiae identified four enzymes (Hxk1, Hxk2, Glk1, and Glk4) in C. albicans with putative roles in sugar phosphorylation. Hxk2, Glk1, and Glk4 demonstrate a critical role in glucose metabolism, while Hxk2 is the only kinase important for fructose metabolism. Additionally, we show that Hxk1 controls HXK2, GLK1, and GLK4 expression in the presence of fermentable as well as non-fermentable carbon sources, thereby indirectly controlling glycolysis. Moreover, these sugar kinases are important during virulence. Disabling the glycolytic pathway reduces adhesion capacity, while deletion of HXK1 decreases biofilm formation. Finally, we demonstrate that hxk2Δ/Δ glk1Δ/Δ glk4Δ/Δ and hxk1Δ/Δ hxk2Δ/Δ glk1Δ/Δ glk4Δ/Δ have attenuated virulence upon systemic infections in mice. These results indicate a regulatory role for Hxk1 during sugar phosphorylation. Furthermore, these kinases are essential during growth on glucose or fructose, and C. albicans relies on a functional glycolytic pathway for maximal virulence.

Three-Dimensional Visualization of APEX2-Tagged Erg11 in Saccharomyces cerevisiae Using Focused Ion Beam Scanning Electron Microscopy

The determination of the exact location of a protein in the cell is essential to the understanding of biological processes. Here, we report for the first time the visualization of a protein of interest in Saccharomyces cerevisiae using focused ion beam scanning electron microscopy (FIB-SEM). As a proof of concept, the integral endoplasmic reticulum (ER) membrane protein Erg11 has been C-terminally tagged with APEX2, which is an engineered peroxidase that catalyzes an electron-dense deposition of 3,3'-diaminobenzidine (DAB), as such marking the location of the fused protein of interest in electron microscopic images. As DAB is unable to cross the yeast cell wall to react with APEX2, cell walls have been partly removed by the formation of spheroplasts. This has resulted in a clear electron-dense ER signal for the Erg11 protein using FIB-SEM. With this study, we have validated the use of the APEX2 tag for visualization of yeast proteins in electron microscopy. Furthermore, we have introduced a methodology that enables precise and three-dimensional (3D) localization studies in yeast, with nanometer resolution and without the need for antibody staining. Because of these properties, the described technique can offer valuable information on the molecular functions of studied proteins.IMPORTANCE With this study, we have validated the use of the APEX2 tag to define the localization of proteins in the model yeast S. cerevisiae As such, FIB-SEM can identify the exact 3D location of a protein of interest in the cell with nanometer-scale resolution. Such detailed imaging could provide essential information on the elucidation of various biological processes. APEX2, which adds electron density to a fused protein of interest upon addition of the substrate DAB, originally was used in mammalian studies. With this study, we expand its use to protein localization studies in one of the most important models in molecular biology.

Transcriptional responses of Candida glabrata biofilm cells to fluconazole are modulated by the carbon source

Candida glabrata is an important human fungal pathogen known to trigger serious infections in immune-compromised individuals. Its ability to form biofilms, which exhibit high tolerance to antifungal treatments, has been considered as an important virulence factor. However, the mechanisms involving antifungal resistance in biofilms and the impact of host niche environments on these processes are still poorly defined. In this study, we performed a whole-transcriptome analysis of C. glabrata biofilm cells exposed to different environmental conditions and constraints in order to identify the molecular pathways involved in fluconazole resistance and understand how acidic pH niches, associated with the presence of acetic acid, are able to modulate these responses. We show that fluconazole treatment induces gene expression reprogramming in a carbon source and pH-dependent manner. This is particularly relevant for a set of genes involved in DNA replication, ergosterol, and ubiquinone biosynthesis. We also provide additional evidence that the loss of mitochondrial function is associated with fluconazole resistance, independently of the growth condition. Lastly, we propose that C. glabrata Mge1, a cochaperone involved in iron metabolism and protein import into the mitochondria, is a key regulator of fluconazole susceptibility during carbon and pH adaptation by reducing the metabolic flux towards toxic sterol formation. These new findings suggest that different host microenvironments influence directly the physiology of C. glabrata, with implications on how this pathogen responds to antifungal treatment. Our analyses identify several pathways that can be targeted and will potentially prove to be useful for developing new antifungals to treat biofilm-based infections.

Sef1-Regulated Iron Regulon Responds to Mitochondria-Dependent Iron-Sulfur Cluster Biosynthesis in Candida albicans

Iron homeostasis mechanisms allow the prime commensal-pathogen Candida albicans to cope with the profound shift in iron levels in the mammalian host. The regulators, Sef1 and Sfu1 influence activation and repression of genes required for iron uptake and acquisition by inducing the expression of iron regulon genes in iron-deplete conditions and inactivating them in iron-replete condition. Our study for the first time shows that C. albicans coordinates the activation of the iron regulon with the mitochondrial use of iron for Fe–S cluster biosynthesis, a cellular process that is connected to cellular iron metabolism. We took advantage of a mutant defective in mitochondrial biogenesis (fzo1Δ/Δ) to assess the aforesaid link as this mutant exhibited sustained expression of the Sef1 iron regulon, signifying an iron-starved state in the mutant. Our analysis demonstrates that mitochondrion is pivotal for regulation of Fe–S cluster synthesis such that the disruption of this cellular process in fzo1Δ/Δ cells lead to excessive mitochondrial iron accumulation and reduced activity of the Fe–S cluster-containing enzyme aconitase. Sef1 responds to defective Fe–S cluster synthesis by regulated changes in its subcellular localization; it was retained in the nucleus resulting in the induced expression of the iron regulon. We predict that the mitochondrial Fe–S assembly generates a molecule that is critical for ensuring iron-responsive transcriptional activation of the Sef1 regulon. All told, our data marks Fe–S biogenesis as a mechanism that meshes cellular iron procurement with mitochondrial iron metabolism resulting in regulating the Sef1 regulon in C. albicans.

Inhibition of Vesicular Transport Influences Fungal Susceptibility to Fluconazole

Fungal infections pose a substantial threat to the human population. They can cause either mild and relatively harmless infections or invasive and often lethal diseases in patients with a weakened immune system. The majority of these human fungal infections are caused by Candida species. The limited amount of available therapies, together with the development of resistance against these drugs, strongly emphasizes the need for novel therapeutic strategies. As it is quite time-consuming to introduce completely new drugs to the market, potentiating the efficacy of existing drugs would be a better strategy. Therefore, it is important to identify cellular pathways involved in the development of drug resistance. We found that vesicular transport is involved in fungal susceptibility to the most widely used antifungal drug, fluconazole. We identified specific complexes in the vesicular transport pathway which contribute to fluconazole resistance or tolerance in the model organism Saccharomyces cerevisiae Furthermore, we confirmed our findings in the clinically relevant fungi Candida albicans and Candida glabrata Finally, we show that the combination of fluconazole with a specific inhibitor of the vesicular transport pathway increases the susceptibility of Candida species, indicating the potential of using vesicular transport as a target in combination therapy.

Can Saccharomyces cerevisiae keep up as a model system in fungal azole susceptibility research?

The difficulty of manipulation and limited availability of genetic tools for use in many pathogenic fungi hamper fast and adequate investigation of cellular metabolism and consequent possibilities for antifungal therapies. S. cerevisiae is a model organism that is used to study many eukaryotic systems. In this review, we analyse the potency and relevance of this model system in investigating fungal susceptibility to azole drugs. Although many of the concepts apply to multiple pathogenic fungi, for the sake of simplicity, we will focus on the validity of using S. cerevisiae as a model organism for two Candida species, C. albicans and C. glabrata. Apart from the general benefits, we explore how S. cerevisiae can specifically be used to improve our knowledge on azole drug resistance and enables fast and efficient screening for novel drug targets in combinatorial therapy. We consider the shortcomings of the model system, yet conclude that it is still opportune to use S. cerevisiae as a model system for pathogenic fungi in this era.

Dysfunction of Prohibitin 2 Results in Reduced Susceptibility to Multiple Antifungal Drugs via Activation of the Oxidative Stress-Responsive Transcription Factor Pap1 in Fission Yeast

The fight against resistance to antifungal drugs requires a better understanding of the underlying cellular mechanisms. In order to gain insight into the mechanisms leading to antifungal drug resistance, we performed a genetic screen on a model organism, Schizosaccharomyces pombe, to identify genes whose overexpression caused resistance to antifungal drugs, including clotrimazole and terbinafine. We identified the phb2 ⁺ gene, encoding a highly conserved mitochondrial protein, prohibitin (Phb2), as a novel determinant of reduced susceptibility to multiple antifungal drugs. Unexpectedly, deletion of the phb2 ⁺ gene also exhibited antifungal drug resistance. Overexpression of the phb2 ⁺ gene failed to cause drug resistance when the pap1 ⁺ gene, encoding an oxidative stress-responsive transcription factor, was deleted. Furthermore, pap1⁺ mRNA expression was significantly increased when the phb2 ⁺ gene was overexpressed or deleted. Importantly, either overexpression or deletion of the phb2 ⁺ gene stimulated the synthesis of NO and reactive oxygen species (ROS), as measured by the cell-permeant fluorescent NO probe DAF-FM DA (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate) and the ROS probe DCFH-DA (2',7'-dichlorodihydrofluorescein diacetate), respectively. Taken together, these results suggest that Phb2 dysfunction results in reduced susceptibility to multiple antifungal drugs by increasing NO and ROS synthesis due to dysfunctional mitochondria, thereby activating the transcription factor Pap1 in fission yeast.

A phenotypic small-molecule screen identifies halogenated salicylanilides as inhibitors of fungal morphogenesis, biofilm formation and host cell invasion

A poorly exploited paradigm in the antimicrobial therapy field is to target virulence traits for drug development. In contrast to target-focused approaches, antivirulence phenotypic screens enable identification of bioactive molecules that induce a desirable biological readout without making a priori assumption about the cellular target. Here, we screened a chemical library of 678 small molecules against the invasive hyphal growth of the human opportunistic yeast Candida albicans. We found that a halogenated salicylanilide (N1-(3,5-dichlorophenyl)-5-chloro-2-hydroxybenzamide) and one of its analogs, Niclosamide, an FDA-approved anthelmintic in humans, exhibited both antifilamentation and antibiofilm activities against C. albicans and the multi-resistant yeast C. auris. The antivirulence activity of halogenated salicylanilides were also expanded to C. albicans resistant strains with different resistance mechanisms. We also found that Niclosamide protected the intestinal epithelial cells against invasion by C. albicans. Transcriptional profiling of C. albicans challenged with Niclosamide exhibited a signature that is characteristic of the mitochondria-to-nucleus retrograde response. Our chemogenomic analysis showed that halogenated salicylanilides compromise the potential-dependant mitochondrial protein translocon machinery. Given the fact that the safety of Niclosamide is well established in humans, this molecule could represent the first clinically approved antivirulence agent against a pathogenic fungus.

Identification of genome-wide binding sites of heat shock factor 1, Hsf1, under basal conditions in the human pathogenic yeast, Candida albicans

The master regulator of thermal stress response, Hsf1, is also an essential determinant for viability and virulence in Candida albicans. Our recent studies highlighted that apart from ubiquitous roles of Hsf1 at higher temperatures, it also has myriad non-heat shock responsive roles essential under iron deprivation and drug defense. Here, we further explored its implications in the normal cellular functioning, by profiling its genome-wide occupancy using chromatin immuno-precipitation coupled to high-density tiling arrays under basal and iron deprived conditions. Hsf1 recruitment profiles revealed that it binds to promoters of 660 genes of varied functions, under both the conditions, however, elicited variability in intensity of binding. For instance, Hsf1 binding was observed on several genes of oxidative and osmotic stress response, cell wall integrity, iron homeostasis, mitochondrial, hyphal and multidrug transporters. Additionally, the present study divulged a novel motif under basal conditions comprising, -GTGn₃GTGn₃GTG- where, Hsf1 displays strong occupancy at significant number of sites on several promoters distinct from the heat induced motif. Hence, by binding to and regulating major chaperones, stress responsive genes and drug resistance regulators, Hsf1 is imperative in regulating various cellular machineries. The current study provides a framework for understanding novel aspects of how Hsf1 coordinates diverse cellular functions.

Altered sterol metabolism in budding yeast affects mitochondrial iron-sulfur (Fe-S) cluster synthesis

Ergosterol synthesis is essential for cellular growth and viability of the budding yeast Saccharomyces cerevisiae, and intracellular sterol distribution and homeostasis are therefore highly regulated in this species. Erg25 is an iron-containing C4-methyl sterol oxidase that contributes to the conversion of 4,4-dimethylzymosterol to zymosterol, a precursor of ergosterol. The ERG29 gene encodes an endoplasmic reticulum (ER)-associated protein, and here we identified a role for Erg29 in the methyl sterol oxidase step of ergosterol synthesis. ERG29 deletion resulted in lethality in respiring cells, but respiration-incompetent (Rho^- or Rho⁰) cells survived, suggesting that Erg29 loss leads to accumulation of oxidized sterol metabolites that affect cell viability. Down-regulation of ERG29 expression in Δerg29 cells indeed led to accumulation of methyl sterol metabolites, resulting in increased mitochondrial oxidants and a decreased ability of mitochondria to synthesize iron-sulfur (Fe-S) clusters due to reduced levels of Yfh1, the mammalian frataxin homolog, which is involved in mitochondrial iron metabolism. Using a high-copy genomic library, we identified suppressor genes that permitted growth of Δerg29 cells on respiratory substrates, and these included genes encoding the mitochondrial proteins Yfh1, Mmt1, Mmt2, and Pet20, which reversed all phenotypes associated with loss of ERG29 Of note, loss of Erg25 also resulted in accumulation of methyl sterol metabolites and also increased mitochondrial oxidants and degradation of Yfh1. We propose that accumulation of toxic intermediates of the methyl sterol oxidase reaction increases mitochondrial oxidants, which affect Yfh1 protein stability. These results indicate an interaction between sterols generated by ER proteins and mitochondrial iron metabolism.