Ras signaling is required for serum-induced hyphal differentiation in Candida albicans

2-Chloromethyl anthraquinone inhibits Candida albicans biofilm formation by inhibiting the Ras1-cAMP-Efg1 pathway

Candida albicans is an opportunistic pathogen, and the formation of its biofilm makes it resistant to traditional antifungal therapy. Anthraquinones have universal antibacterial activity. We evaluated the inhibitory effects of 2-chloromethyl anthraquinone on C. albicans adhesion, mycelial morphology transformation, and biofilm formation. The results showed that 2-chloromethyl anthraquinone could inhibit C. albicans adhesion, mycelium formation, and biofilm formation in a dose-dependent manner at 2 μg/mL. In addition, 2-chloromethyl anthraquinone significantly inhibited the expression of biofilm formation-related genes in C. albicans, including ALS1, CPH1, ECE1, HWP1, TEC1, BCR1, and UME6. In addition, Ras1-cAMP-Efg1 pathway-related genes (RAC1, CYR1, and TPK2) were also significantly down-regulated, indicating that the inhibitory effect of 2-chloromethyl anthraquinone on C. albicans biofilms may be related to the Ras1-cAMP-Efg1 signaling pathway. In summary, the results of this study confirmed the inhibitory mechanism of 2-chloromethyl anthraquinone on the virulence factors of C. albicans, which laid a theoretical foundation for its use as an anti-biofilm agent against C. albicans.

The role of serum albumin in Candida albicans filamentation, germ tube formation, and farnesol sequestration

Candida albicans is an opportunistic pathogen and colonizer of the human gut and mucosal membranes. C. albicans exhibits morphological plasticity, which is crucial for its fitness within the host and virulence. Morphogenesis in C. albicans is regulated, in part, by its production of farnesol, an autoregulatory molecule that inhibits filamentation. Morphogenesis is also regulated in response to external cues, such as serum, which stimulates hyphal formation by C. albicans. The precise mechanism by which serum stimulates hyphal formation is unknown. The most abundant serum protein is albumin. The binding affinity of albumin for nonpolar, fatty-acid-like molecules suggests that it may interact directly with exogenous farnesol and influence morphogenesis through sequestration of free farnesol. To test this hypothesis, we assessed whether albumin and albumin devoid of fatty acids (i) stimulated farnesol secretion and (ii) influenced the farnesol threshold required to inhibit filamentation. We found that albumin promoted farnesol secretion and filamentation, and the extent of its ability to do so was based on the presence or absence of bound fatty acids. We hypothesize that albumin not bound to fatty acids has the capacity to bind to farnesol and sequester it from C. albicans, encouraging filamentation.IMPORTANCEFor at least 50 years, researchers have wondered about the mechanisms by which serum stimulates germ tube formation (GTF) and hyphal growth in C. albicans. Here, we tested a model (Nickerson et al., Microbiol Mol Biol Rev 88:e00081-22, 2024, https://doi.org/10.1128/mmbr.00081-22) that serum promotes GTF and farnesol synthesis in part by extracting internal farnesol (Fi) from the cells toward the excess binding capacity of the albumins. The data presented here suggests that albumin not bound by fatty acids sequesters free farnesol thereby modulating filamentation and farnesol secretion by altering the equilibrium of internal vs external farnesol. We expect that the influence of secreted farnesol on cell morphology will differ during pathogenesis depending on location within the body, but sequestration of farnesol in the blood could mediate immune cell recruitment and promote hyphal formation.

Pullulan nanoparticles inhibit the pathogenicity of Candida albicans by regulating hypha-related gene expression

Candida albicans is a prevalent opportunistic pathogenic fungus that resides in the skin and gastrointestinal (GI) tract of humans. Under specific conditions, C. albicans cells transition from a commensal to a pathogenic state, leading to both superficial and invasive infections. Although systemic candidiasis poses a life-threatening risk, a limited number of antifungal drugs are employed for its treatment. Moreover, the emergence of resistant strains to antifungal agents underscores the pressing need for new treatment options. In this study, we propose the use of polysaccharide nanoparticles as a strategy for treating candidiasis. We synthesized phthalic pullulan nanoparticles (PPNPs) and examined their ability to inhibit the pathogenicity of C. albicans. We observed that PPNPs inhibit hyphal growth, adhesion to abiotic surfaces, and biofilm formation of C. albicans in a dose-dependent manner. This inhibitory effect is mediated by transcriptional modulation, particularly the downregulation of hypha-related genes and the upregulation of stress-responsive genes, involving the Ras/cAMP/PKA signaling pathway. Furthermore, we observed that PPNPs inhibit the adhesion of C. albicans to human epithelial cells without inducing toxicity in human cells. In addition, PPNPs inhibited the in vivo pathogenicity of C. albicans in Caenorhabditis elegans, suggesting an antagonistic effect on candidiasis. Our findings suggest that PPNPs exhibit inhibitory effects on C. albicans biofilm formation and in vivo pathogenicity, indicating their potential as a novel therapeutic agent for candidiasis.

IMPORTANCE

The pathogenic process of Candida albicans, the primary causative species of candidiasis, involves hyphal growth, biofilm formation, and secretion of virulence factors. Of these factors, the biofilm, created by the secretion of extracellular matrix from adherent cells, shields cells from external threats, enabling them to withstand high concentrations of antifungal agents. Therefore, suppressing biofilm formation is a crucial aspect of combating candidiasis. This study developed phthalic pullulan nanoparticles (PPNPs) as a novel material for inhibiting C. albicans' pathogenicity. PPNPs were internalized within Candida cells and reduced pathogenicity at the gene expression level, resulting in reduced in vitro biofilm formation, adhesion to human cells, and mortality of infected Caenorhabditis elegans. Moreover, PPNPs exhibited these effects without toxicity to human cells and host animals. These findings not only indicate that PPNPs can be employed to hinder in vitro biofilm formation but also suggest their potential as a novel treatment for candidiasis.

Gain- and loss-of-function alleles within signaling pathways lead to phenotypic diversity among individuals

Understanding how phenotypic diversity is generated is an important question in biology. We explored phenotypic diversity among wild yeast isolates (Saccharomyces cerevisiae) and found variation in the activity of MAPK signaling pathways as a contributing mechanism. To uncover the genetic basis of this mechanism, we identified 1957 SNPs in 62 candidate genes encoding signaling proteins from a MAPK signaling module within a large collection of yeast (>1500 individuals). Follow-up testing identified functionally relevant variants in key signaling proteins. Loss-of-function (LOF) alleles in a PAK kinase impacted protein stability and pathway specificity decreasing filamentous growth and mating phenotypes. In contrast, gain-of-function (GOF) alleles in G-proteins that were hyperactivating induced filamentous growth. Similar amino acid substitutions in G-proteins were identified in metazoans that in some cases were fixed in multicellular lineages including humans, suggesting hyperactivating GOF alleles may play roles in generating phenotypic diversity across eukaryotes. A mucin signaler that regulates MAPK activity was also found to contain a prevalance of presumed GOF alleles amoung individuals based on changes in mucin repeat numbers. Thus, genetic variation in signaling pathways may act as a reservoir for generating phenotypic diversity across eukaryotes.

A new model of endotracheal tube biofilm identifies combinations of matrix-degrading enzymes and antimicrobials able to eradicate biofilms of pathogens that cause ventilator-associated pneumonia

Ventilator-associated pneumonia is defined as pneumonia that develops in a patient who has been on mechanical ventilation for more than 48 hours through an endotracheal tube. It is caused by biofilm formation on the indwelling tube, which introduces pathogenic microbes such as Pseudomonas aeruginosa, Klebsiella pneumoniae and Candida albicans into the patient's lower airways. Currently, there is a lack of accurate in vitro models of ventilator-associated pneumonia development. This greatly limits our understanding of how the in-host environment alters pathogen physiology and the efficacy of ventilator-associated pneumonia prevention or treatment strategies. Here, we showcase a reproducible model that simulates the biofilm formation of these pathogens in a host-mimicking environment and demonstrate that the biofilm matrix produced differs from that observed in standard laboratory growth medium. In our model, pathogens are grown on endotracheal tube segments in the presence of a novel synthetic ventilated airway mucus medium that simulates the in-host environment. Matrix-degrading enzymes and cryo-scanning electron microscopy were employed to characterize the system in terms of biofilm matrix composition and structure, as compared to standard laboratory growth medium. As seen in patients, the biofilms of ventilator-associated pneumonia pathogens in our model either required very high concentrations of antimicrobials for eradication or could not be eradicated. However, combining matrix-degrading enzymes with antimicrobials greatly improved the biofilm eradication of all pathogens. Our in vitro endotracheal tube model informs on fundamental microbiology in the ventilator-associated pneumonia context and has broad applicability as a screening platform for antibiofilm measures including the use of matrix-degrading enzymes as antimicrobial adjuvants.

The Vps21 signalling pathway regulates white-opaque switching and mating in Candida albicans

Candida albicans is able to switch between two epigenetic cell types, namely white and opaque. Multiple conserved signalling pathways control the switch between white and opaque cell types in response to environmental changes. Here, we report the regulatory roles of the endosomal Rab family GTPase Vps21 and associated key components of the Vps21 signalling pathway in white-opaque switching and mating in C. albicans. Deletion of VPS21 promoted a switch from the white to the opaque phenotype in the presence of N-acetyl-glucosamine (GlcNAc). Consistently, inactivation of the guanine nucleotide exchange factor of Vps21 (Vps9) and downstream components in the Vps21 pathway (Vps3, Vac1, and Pep12) had similar promoting effects on phenotypic switching. The mating efficiency of opaque cells is much higher than that of white cells under standard laboratory culture conditions. However, compared to the wildtype strain, the vps21/vps21, vps9/vps9, vps3/vps3, vac1/vac1, and pep12/pep12 mutant strains exhibited dramatically reduced mating efficiencies. Quantitative RT-PCR assays demonstrated that inactivation of the Vps21 signalling pathway led to downregulation of pheromone expression and mating response pathway associated genes. Taken together, our findings indicate that the conserved Vps21 signalling pathway plays critical roles in the regulation of cell-type switching and mating in C. albicans.

A highly conserved tRNA modification contributes to C. albicans filamentation and virulence

tRNA modifications play important roles in maintaining translation accuracy in all domains of life. Disruptions in the tRNA modification machinery, especially of the anticodon stem loop, can be lethal for many bacteria and lead to a broad range of phenotypes in baker's yeast. Very little is known about the function of tRNA modifications in host-pathogen interactions, where rapidly changing environments and stresses require fast adaptations. We found that two closely related fungal pathogens of humans, the highly pathogenic Candida albicans and its much less pathogenic sister species, Candida dubliniensis, differ in the function of a tRNA-modifying enzyme. This enzyme, Hma1, exhibits species-specific effects on the ability of the two fungi to grow in the hypha morphology, which is central to their virulence potential. We show that Hma1 has tRNA-threonylcarbamoyladenosine dehydratase activity, and its deletion alters ribosome occupancy, especially at 37°C-the body temperature of the human host. A C. albicans HMA1 deletion mutant also shows defects in adhesion to and invasion into human epithelial cells and shows reduced virulence in a fungal infection model. This links tRNA modifications to host-induced filamentation and virulence of one of the most important fungal pathogens of humans.IMPORTANCEFungal infections are on the rise worldwide, and their global burden on human life and health is frequently underestimated. Among them, the human commensal and opportunistic pathogen, Candida albicans, is one of the major causative agents of severe infections. Its virulence is closely linked to its ability to change morphologies from yeasts to hyphae. Here, this ability is linked-to our knowledge for the first time-to modifications of tRNA and translational efficiency. One tRNA-modifying enzyme, Hma1, plays a specific role in C. albicans and its ability to invade the host. This adds a so-far unknown layer of regulation to the fungal virulence program and offers new potential therapeutic targets to fight fungal infections.

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Macrophage pyroptosis induced by Candida albicans

Candida albicans (C. albicans) is a prevalent opportunistic pathogen that causes mucocutaneous and systemic infections, particularly in immunocompromised individuals. Macrophages play a crucial role in eliminating C. albicans in local and bloodstream contexts, while also regulating antifungal immune responses. However, C. albicans can induce macrophage lysis through pyroptosis, a type of regulated cell death. This process can enable C. albicans to escape from immune cells and trigger the release of IL-1β and IL-18, which can impact both the host and the pathogen. Nevertheless, the mechanisms by which C. albicans triggers pyroptosis in macrophages and the key factors involved in this process remain unclear. In this review, we will explore various factors that may influence or trigger pyroptosis in macrophages induced by C. albicans, such as hypha, ergosterol, cell wall remodeling, and other virulence factors. We will also examine the possible immune response following macrophage pyroptosis.

Multi-Omics Profiling of Candida albicans Grown on Solid Versus Liquid Media

Candida albicans is a common pathogenic fungus that presents a challenge to healthcare facilities. It can switch between a yeast cell form that diffuses through the bloodstream to colonize internal organs and a filamentous form that penetrates host mucosa. Understanding the pathogen's strategies for environmental adaptation and, ultimately, survival, is crucial. As a complementary study, herein, a multi-omics analysis was performed using high-resolution timsTOF MS to compare the proteomes and metabolomes of Wild Type (WT) Candida albicans (strain DK318) grown on agar plates versus liquid media. Proteomic analysis revealed a total of 1793 proteins and 15,013 peptides. Out of the 1403 identified proteins, 313 proteins were significantly differentially abundant with a p-value < 0.05. Of these, 156 and 157 proteins were significantly increased in liquid and solid media, respectively. Metabolomics analysis identified 192 metabolites in total. The majority (42/48) of the significantly altered metabolites (p-value 0.05 FDR, FC 1.5), mainly amino acids, were significantly higher in solid media, while only 2 metabolites were significantly higher in liquid media. The combined multi-omics analysis provides insight into adaptative morphological changes supporting Candida albicans' life cycle and identifies crucial virulence factors during biofilm formation and bloodstream infection.

Artemisinins inhibit oral candidiasis caused by Candida albicans through the repression on its hyphal development

Candida albicans is the most abundant fungal species in oral cavity. As a smart opportunistic pathogen, it increases the virulence by switching its forms from yeasts to hyphae and becomes the major pathogenic agent for oral candidiasis. However, the overuse of current clinical antifungals and lack of new types of drugs highlight the challenges in the antifungal treatments because of the drug resistance and side effects. Anti-virulence strategy is proved as a practical way to develop new types of anti-infective drugs. Here, seven artemisinins, including artemisinin, dihydroartemisinin, artemisinic acid, dihydroartemisinic acid, artesunate, artemether and arteether, were employed to target at the hyphal development, the most important virulence factor of C. albicans. Artemisinins failed to affect the growth, but significantly inhibited the hyphal development of C. albicans, including the clinical azole resistant isolates, and reduced their damage to oral epithelial cells, while arteether showed the strongest activities. The transcriptome suggested that arteether could affect the energy metabolism of C. albicans. Seven artemisinins were then proved to significantly inhibit the productions of ATP and cAMP, while reduced the hyphal inhibition on RAS1 overexpression strain indicating that artemisinins regulated the Ras1-cAMP-Efg1 pathway to inhibit the hyphal development. Importantly, arteether significantly inhibited the fungal burden and infections with no systemic toxicity in the murine oropharyngeal candidiasis models in vivo caused by both fluconazole sensitive and resistant strains. Our results for the first time indicated that artemisinins can be potential antifungal compounds against C. albicans infections by targeting at its hyphal development.

Ras2 Is Responsible for the Environmental Responses, Melanin Metabolism, and Virulence of Botrytis cinerea

Ras proteins are monomeric G proteins that are ubiquitous in fungal cells and play important roles in fungal growth, virulence, and environmental responses. Botrytis cinerea is a phytopathogenic fungus that infects various crops. However, under specific environmental conditions, the overripe grapes infected by B. cinerea can be used to brew valuable noble rot wine. As a Ras protein, the role of Bcras2 in the environmental responses of B. cinerea is poorly understood. In this study, we deleted the Bcras2 gene using homologous recombination and examined its functions. Downstream genes regulated by Bcras2 were explored using RNA sequencing transcriptomics. It was found that ΔBcras2 deletion mutants showed significantly reduced growth rate, increased sclerotia production, decreased resistance to oxidative stress, and enhanced resistance to cell wall stress. Additionally, Bcras2 deletion promoted the expression of melanin-related genes in sclerotia and decreased the expression of melanin-related genes in conidia. The above results indicate that Bcras2 positively regulates growth, oxidative stress resistance, and conidial melanin-related genes expression, and negatively regulates sclerotia production, cell wall stress resistance and sclerotial melanin-related genes expression. These results revealed previously unknown functions of Bcras2 in environmental responses and melanin metabolism in B. cinerea.

An integrated transcriptomic and metabolomic approach to investigate the heterogeneous Candida albicans biofilm phenotype

Candida albicans is the most prevalent and notorious of the Candida species involved in bloodstream infections, which is characterised by its capacity to form robust biofilms. Biofilm formation is an important clinical entity shown to be highly variable among clinical isolates. There are various environmental and physiological factors, including nutrient availability which influence the phenotype of Candida species. However, mechanisms underpinning adaptive biofilm heterogeneity have not yet been fully explored. Within this study we have profiled previously characterised and phenotypically distinct C. albicans bloodstream isolates. We assessed the dynamic susceptibility of these differing populations to antifungal treatments using population analysis profiling in addition to assessing biofilm formation and morphological changes. High throughput methodologies of RNA-Seq and LC-MS were employed to map and integrate the transcriptional and metabolic reprogramming undertaken by heterogenous C. albicans isolates in response to biofilm and hyphal inducing serum. We found a significant relationship between biofilm heterogeneity and azole resistance (P < 0.05). In addition, we observed that in response to serum our low biofilm forming (LBF) C. albicans exhibited a significant increase in biofilm formation and hyphal elongation. The transcriptional reprogramming of LBF strains compared to high biofilm forming (HBF) was distinct, indicating a high level of plasticity and variation in stress responses by heterogenous strains. The metabolic responses, although variable between LBF and HBF, shared many of the same responses to serum. Notably, a high upregulation of the arachidonic acid cascade, part of the COX pathway, was observed and this pathway was found to induce biofilm formation in LBF 3-fold. C. albicans is a highly heterogenous bloodstream pathogen with clinical isolates varying in antifungal tolerance and biofilm formation. In addition to this, C. albicans is capable of highly complex and variable regulation of transcription and metabolic pathways and heterogeneity across isolates further increases the complexity of these pathways. Here we have shown with a dual and integrated approach, the importance of studying a diverse panel of C. albicans isolates, which has the potential to reveal distinct pathways that can harnessed for drug discovery.

The Cdc25 and Ras1 Proteins of Candida albicans Influence Epithelial Toxicity in a Niche-Specific Way

The PKA pathway is a signaling pathway involved in virulence in Candida albicans. This mechanism can be activated via addition of glucose and activation involves at least two proteins, namely Cdc25 and Ras1. Both proteins are involved in specific virulence traits. However, it is not clear if Cdc25 and Ras1 also affect virulence independently of PKA. C. albicans holds a second, atypical, Ras protein, Ras2, but its function in PKA activation is still unclear. We investigated the role of Cdc25, Ras1, and Ras2 for different in vitro and ex vivo virulence characteristics. We show that deletion of CDC25 and RAS1 result in less toxicity towards oral epithelial cells, while deletion of RAS2 has no effect. However, toxicity towards cervical cells increases in both the ras2 and the cdc25 mutants while it decreases in a ras1 mutant compared to the WT. Toxicity assays using mutants of the transcription factors downstream of the PKA pathway (Efg1) or the MAPK pathway (Cph1) show that the ras1 mutant shows similar phenotypes as the efg1 mutant, whereas the ras2 mutant shows similar phenotypes as the cph1 mutant. These data show niche-specific roles for different upstream components in regulating virulence through both signal transduction pathways.

In Silico and In Vitro Analysis of Sulforaphane Anti-Candida Activity

Oropharyngeal candidiasis/candidosis is a common and recurrent opportunistic fungal infection. Fluconazole (FLZ), one of the most used and effective antifungal agents, has been associated with a rise of resistant Candida species in immunocompromised patients undergoing prophylactic therapy. Sulforaphane (SFN), a compound from cruciferous vegetables, is an antimicrobial with yet controversial activities and mechanisms on fungi. Herein, the in silico and antifungal activities of SFN against C. albicans were investigated. In silico analyzes for the prediction of the biological activities and oral bioavailability of SFN, its possible toxicity and pharmacokinetic parameters, as well as the estimates of its gastrointestinal absorption, permeability to the blood-brain barrier and skin, and similarities to drugs, were performed by using different software. SFN in vitro anti-Candida activities alone and in combination with fluconazole (FLZ) were determined by the broth microdilution method and the checkerboard, biofilm and hyphae formation tests. Amongst the identified probable biological activities of SFN, nine indicated an antimicrobial potential. SFN was predicted to be highly absorbable by the gastrointestinal tract, to present good oral availability, and not to be irritant and/or hepatotoxic. SFN presented antifungal activity against C. albicans and prevented both biofilm and hyphae formation by this microorganism. SFN was additive/synergistic to FLZ. Overall, the data highlights the anti-Candida activity of SFN and its potential to be used as an adjuvant therapy to FLZ in clinical settings.

Targeting Virulence Factors of Candida albicans with Natural Products

Natural products derived from natural resources, including nutritional functional food, play an important role in human health. In recent years, the study of anti-fungal and other properties of agri-foods and derived functional compounds has been a hot research topic. Candida albicans is a parasitic fungus that thrives on human mucosal surfaces, which are colonized through opportunistic infection. It is the most prevalent cause of invasive fungal infection in immunocompromised individuals, resulting in a wide variety of clinical symptoms. Moreover, the efficacy of classical therapeutic medications such as fluconazole is often limited by the development of resistance. There is an ongoing need for the development of novel and effective antifungal therapy and medications. Infection of C. albicans is influenced by a great quantity of virulence factors, like adhesion, invasion-promoting enzymes, mycelial growth, and phenotypic change, and among others. Furthermore, various natural products especially from food sources that target C. albicans virulence factors have been researched, providing promising prospects for C. albicans prevention and treatment. In this review, we discuss the virulence factors of C. albicans and how functional foods and derived functional compounds affect them. Our hope is that this review will stimulate additional thoughts and suggestions regarding nutritional functional food and therapeutic development for patients afflicted with C. albicans.

Regulation of heme utilization and homeostasis in Candida albicans

Heme (iron-protoporphyrin IX) is an essential but potentially toxic cellular cofactor. While most organisms are heme prototrophs, many microorganisms can utilize environmental heme as iron source. The pathogenic yeast Candida albicans can utilize host heme in the iron-poor host environment, using an extracellular cascade of soluble and anchored hemophores, and plasma membrane ferric reductase-like proteins. To gain additional insight into the C. albicans heme uptake pathway, we performed an unbiased genetic selection for mutants resistant to the toxic heme analog Ga3+-protoporphyrin IX at neutral pH, and a secondary screen for inability to utilize heme as iron source. Among the mutants isolated were the genes of the pH-responsive RIM pathway, and a zinc finger transcription factor related to S. cerevisiae HAP1. In the presence of hemin in the medium, C. albicans HAP1 is induced, the Hap1 protein is stabilized and Hap1-GFP localizes to the nucleus. In the hap1 mutant, cytoplasmic heme levels are elevated, while influx of extracellular heme is lower. Gene expression analysis indicated that in the presence of extracellular hemin, Hap1 activates the heme oxygenase HMX1, which breaks down excess cytoplasmic heme, while at the same time it also activates all the known heme uptake genes. These results indicate that Hap1 is a heme-responsive transcription factor that plays a role both in cytoplasmic heme homeostasis and in utilization of extracellular heme. The induction of heme uptake genes by C. albicans Hap1 under iron satiety indicates that preferential utilization of host heme can be a dietary strategy in a heme prototroph.

A mathematical model captures the role of adenyl cyclase Cyr1 and guanidine exchange factor Ira2 in creating a growth-to-hyphal bistable switch in Candida albicans

Recent biochemical experiments have indicated that in Candida albicans, a commensal fungal pathogen, the Ras signaling pathway plays a significant role in the yeast-to-hyphal transition; specifically, two enzymes in this pathway, Adenyl Cyclase Cyr1 and GTPase activating protein Ira2, facilitate this transition, in the presence of energy sensor ATP. However, the precise mechanism by which protein interactions between Ira2 and Cyr1 and the energy sensor ATP result in the yeast-to-hyphal transition and create a switch-like process are unknown. We propose a new set of biochemical reaction steps that captures all the essential interactions between Ira2, Cyr1, and ATP in the Ras pathway. With the help of chemical reaction network theory, we demonstrate that this set of biochemical reaction steps results in bistability. Further, bifurcation analysis of the differential equations based on this set of reaction steps supports the existence of a bistable switch, and this switch may act as a checkpoint mechanism for the promotion of growth-to-hyphal transition in C. albicans.

Mucin O-glycans are natural inhibitors of Candida albicans pathogenicity

Mucins are large gel-forming polymers inside the mucus barrier that inhibit the yeast-to-hyphal transition of Candida albicans, a key virulence trait of this important human fungal pathogen. However, the molecular motifs in mucins that inhibit filamentation remain unclear despite their potential for therapeutic interventions. Here, we determined that mucins display an abundance of virulence-attenuating molecules in the form of mucin O-glycans. We isolated and cataloged >100 mucin O-glycans from three major mucosal surfaces and established that they suppress filamentation and related phenotypes relevant to infection, including surface adhesion, biofilm formation and cross-kingdom competition between C. albicans and the bacterium Pseudomonas aeruginosa. Using synthetic O-glycans, we identified three structures (core 1, core 1 + fucose and core 2 + galactose) that are sufficient to inhibit filamentation with potency comparable to the complex O-glycan pool. Overall, this work identifies mucin O-glycans as host molecules with untapped therapeutic potential to manage fungal pathogens.

Biofilm and hyphal inhibitory synergistic effects of phytoactives piperine and cinnamaldehyde against Candida albicans

Oral candidiasis, the most common mycotic infection of the human oral cavity is non-life-threatening yet if untreated may advance as systemic infections. Ability of Candida albicans to adapt sessile lifestyle imparts resistance to drugs and host immunity. Consequently, due to limited effectiveness of conventional antifungal treatment, novel therapeutic strategies are required. In the present study, synergistic interaction of phytochemicals, piperine and cinnamaldehyde against the biofilm and hyphal of C. albicans was evaluated. Minimum inhibitory concentration (MIC) and biofilm inhibitory concentration (BIC) of piperine and cinnamaldehyde against C. albicans were analysed through microbroth dilution assay and crystal violet staining method, respectively. Combinatorial biofilm and hyphal inhibitory effect were investigated through checkerboard assay. In vitro results were validated through gene expression analysis. BIC of piperine and cinnamaldehyde was determined to be 32 µg/mL and 64 µg/mL, respectively. Interaction between these two phytocomponents was found to be synergistic and six different synergistic antibiofilm combinations were identified. Microscopic analysis of biofilm architecture also evidenced the biofilm and surface adherence inhibitory potential of piperine and cinnamaldehyde combinations. Phenotypic switching between yeast and hyphal morphological forms was influenced by synergistic combinations. qPCR analysis corroborated the results of in vitro activities. nrg1 and trp1, the negative transcriptional regulators of filamentous growth were upregulated whereas other genes that are involved in biofilm formation, filamentous growth, adhesion etc were found to be downregulated. These proficient phytochemical combinations provide a new therapeutic avenue for the treatment of biofilm associated oral candidiasis and to combat the recurrent infections due to antibiotic resistance.

Antifungal and Antibiofilm Efficacy of Paeonol Treatment Against Biofilms Comprising Candida albicans and/or Cryptococcus neoformans

Fungal populations are commonly found in natural environments and present enormous health care challenges, due to increased resistance to antifungal agents. Paeonol exhibits antifungal activities; nevertheless, the antifungal and antibiofilm activities of paeonol against Candida albicans and Cryptococcus neoformans remain largely unexplored. Here, we aimed to evaluate the antifungal and antibiofilm activities of paeonol against C. albicans and/or C. neoformans (i.e., against mono- or dual-species). The minimum inhibitory concentrations (MICs) of paeonol for mono-species comprising C. albicans or C. neoformans were 250 μg ml⁻¹, whereas the MIC values of paeonol for dual-species were 500 μg ml⁻¹. Paeonol disrupted cell membrane integrity and increased the influx of gatifloxacin into cells of mono- and dual-species cells, indicating an antifungal mode of action. Moreover, paeonol at 8 times the MIC damaged mono- and dual-species cells within C. albicans and C. neoformans biofilms, as it did planktonic cells. In particular, at 4 and 8 mg ml⁻¹, paeonol efficiently dispersed preformed 48-h biofilms formed by mono- and dual-species cells, respectively. Paeonol inhibited effectively the yeast-to-hyphal-form transition of C. albicans and impaired capsule and melanin production of C. neoformans. The addition of 10 MIC paeonol to the medium did not shorten the lifespan of C. elegans, and 2 MIC paeonol could effectively protect the growth of C. albicans and C. neoformans-infected C. elegans. Furthermore, RNA sequencing was employed to examine the transcript profiling of C. albicans and C. neoformans biofilm cells in response to 1/2 MIC paeonol. RNA sequencing data revealed that paeonol treatment impaired biofilm formation of C. albicans by presumably downregulating the expression level of initial filamentation, adhesion, and growth-related genes, as well as biofilm biosynthesis genes, whereas paeonol inhibited biofilm formation of C. neoformans by presumably upregulating the expression level of ergosterol biosynthesis-related genes. Together, the findings of this study indicate that paeonol can be explored as a candidate antifungal agent for combating serious single and mixed infections caused by C. albicans and C. neoformans.

Roles of the pro-apoptotic factors CaNma111 and CaYbh3 in apoptosis and virulence of Candida albicans

Candida albicans, a commensal and opportunistic pathogen, undergoes apoptosis in response to various stimuli, including hydrogen peroxide, acetic acid, and antifungal agents. Apoptotic processes are highly conserved among mammals, plants, and fungi, but little is known about the apoptosis-regulating factors in C. albicans. In this study, C. albicans homologs of the putative apoptosis factors were identified by database screening followed by overexpression analysis. CaNma111, a homolog of the pro-apoptotic mammalian HtrA2/Omi, and CaYbh3, a homolog of BH3-only protein, yielded increased apoptotic phenotypes upon overexpression. We showed that CaNma111 and CaYbh3 functions as pro-apoptotic regulators by examining intracellular ROS accumulation, DNA end breaks (TUNEL assay), and cell survival in Canma111/Canma111 and Caybh3/Caybh3 deletion strains. We found that the protein level of CaBir1, an inhibitor-of-apoptosis (IAP) protein, was down-regulated by CaNma111. Interestingly, the Canma111/Canma111 and Caybh3/Caybh3 deletion strains showed hyperfilamentation phenotypes and increased virulence in a mouse infection model. Together, our results suggest that CaNma111 and CaYbh3 play key regulatory roles in the apoptosis and virulence of C. albicans.

Characterization of the Growth and Morphology of a BSL-2 Coccidioides posadasii Strain That Persists in the Parasitic Life Cycle at Ambient CO2

Coccidioides is a dimorphic fungus responsible for Valley Fever and is the cause of severe morbidity and mortality in the infected population. Although there is some insight into the genes, pathways, and growth media involved in the parasitic to saprophytic growth transition, the exact determinants that govern this switch are largely unknown. In this work, we examined the growth and morphology of a Coccidioides posadasii strain (C. posadasii S/E) that efficiently produces spherules and endospores and persists in the parasitic life cycle at ambient CO2. We demonstrated that C. posadasii S/E remains virulent in an insect infection model. Surprisingly, under spherule-inducing conditions, the C. posadasii S/E culture was found to be completely hyphal. Differential interference contrast (DIC) and transmission electron microscopy (TEM) revealed unexpected cellular changes in this strain including cell wall remodeling and formation of septal pores with Woronin bodies. Our study suggests that the C. posadasii S/E strain is a useful BSL-2 model for studying mechanisms underlying the parasitic to saprophytic growth transition—a morphological switch that can impact the pathogenicity of the organism in the host.

Control of β-glucan exposure by the endo-1,3-glucanase Eng1 in Candida albicans modulates virulence

Candida albicans is a major opportunistic pathogen of humans. It can grow as morphologically distinct yeast, pseudohyphae and hyphae, and the ability to switch reversibly among different forms is critical for its virulence. The relationship between morphogenesis and innate immune recognition is not quite clear. Dectin-1 is a major C-type lectin receptor that recognizes β-glucan in the fungal cell wall. C. albicans β-glucan is usually masked by the outer mannan layer of the cell wall. Whether and how β-glucan masking is differentially regulated during hyphal morphogenesis is not fully understood. Here we show that the endo-1,3-glucanase Eng1 is differentially expressed in yeast, and together with Yeast Wall Protein 1 (Ywp1), regulates β-glucan exposure and Dectin-1-dependent immune activation of macrophage by yeast cells. ENG1 deletion results in enhanced Dectin-1 binding at the septa of yeast cells; while eng1 ywp1 yeast cells show strong overall Dectin-1 binding similar to hyphae of wild-type and eng1 mutants. Correlatively, hyphae of wild-type and eng1 induced similar levels of cytokines in macrophage. ENG1 expression and Eng1-mediated β-glucan trimming are also regulated by antifungal drugs, lactate and N-acetylglucosamine. Deletion of ENG1 modulates virulence in the mouse model of hematogenously disseminated candidiasis in a Dectin-1-dependent manner. The eng1 mutant exhibited attenuated lethality in male mice, but enhanced lethality in female mice, which was associated with a stronger renal immune response and lower fungal burden. Thus, Eng1-regulated β-glucan exposure in yeast cells modulates the balance between immune protection and immunopathogenesis during disseminated candidiasis.

Phytolectin nanoconjugates in combination with standard antifungals curb multi-species biofilms and virulence of Vulvovaginal Candidiasis (VVC) causing Candida albicans and Non albicans Candida

Vulvovaginal Candidiasis (VVC) is commonly occurring yeast infection caused by Candida species in women. Among Candida species, C. albicans is the predominant member that causes vaginal candidiasis followed by Candida glabrata. Biofilm formation by Candida albicans on the vaginal mucosal tissue leads to VVC infection and is one of the factors for a commensal organism to get into virulent form leading to disease. In addition to that, morphological switching from yeast to hyphal form increases the risk of pathogenesis as it aids in tissue invasion. In this study, jacalin, a phyto-lectin complexed Copper sulfide nanoparticles (NPs) have been explored to eradicate the mono and mixed species biofilms formed by fluconazole resistant C. albicans and C. glabrata isolated from VVC patients. NPs along with standard antifungals like micafungin and amphotericin B have been evaluated to explore interaction behaviour and we observed synergistic interactions between them. Microscopic techniques like light microscopy, phase contrast microscopy, scanning electron microscopy, confocal laser scanning microscopy were used to visualize the inhibition of biofilm by NPs and in synergistic combinations with standard antifungals. Real time PCR analysis was carried out to study the expression pattern of the highly virulent genes which are responsible for yeast to hyphal switch, drug resistance and biofilm formation upon treatment with NPs in combination with standard antifungals. The current study shows that lectin conjugated NPs with standard antifungals might be a different means to disrupt the mixed species population of Candida spp. that causes VVC.

Inhibition of morphological transition and hyphae extension in Candida spp. by occidiofungin

AIMS

To assess the efficacy of the antifungal, occidiofungin, against Candida albicans and Candida tropicalis morphological transformation.

METHODS AND RESULTS

Susceptibility assays and morphological data were used to demonstrate that occidiofungin effectively targets C. albicans and C. tropicalis undergoing morphological transformation. Susceptibility assays found that cell sensitivity to occidiofungin varied with the media conditions used for morphological switching. Microscopy data showed that occidiofungin inhibited hyphae formation when added at the time of morphological induction and hyphal extension when added within the first hour following hyphae induction. Immunoblot analysis demonstrated that occidiofungin addition prevented activation of Cek1p MAPK signalling.

CONCLUSIONS

The data indicated that the antimicrobial compound, occidiofungin, effectively targets hyphae elongation in Candida spp. and suggests the biological target of occidiofungin is necessary for the morphological changes associated with yeast-to-hyphae switching.

SIGNIFICANCE AND IMPACT OF THE STUDY

Findings from this study demonstrated that occidiofungin effectively targets the invasive growth of dimorphic Candida which suggests this compound may also inhibit the heterogenous population of cells present in a clinical setting. This presents occidiofungin as a promising candidate for the treatment of Candida associated infections.

N-acetylglucosamine transporter, Ngt1, undergoes sugar-responsive endosomal trafficking in Candida albicans

N-acetylglucosamine (GlcNAc), an important amino sugar at the infection sites of the fungal pathogen Candida albicans, triggers multiple cellular processes. GlcNAc import at the cell surface is mediated by GlcNAc transporter, Ngt1 which seems to play a critical role during GlcNAc signaling. We have investigated the Ngt1 dynamics that provide a platform for further studies aimed at understanding the mechanistic insights of regulating process(es) in C. albicans. The expression of this transporter is prolific and highly sensitive to even very low levels (˂2 µM) of GlcNAc. Under these conditions, Ngt1 undergoes phosphorylation-associated ubiquitylation as a code for internalization. This ubiquitylation process involves the triggering proteins like protein kinase Snf1, arrestin-related trafficking adaptors (ART) protein Rod1, and yeast ubiquitin ligase Rsp5. Interestingly, analysis of ∆snf1 and ∆rsp5 mutants revealed that while Rsp5 is promoting the endosomal trafficking of Ngt1-GFPɤ, Snf1 hinders the process. Furthermore, colocalization experiments of Ngt1 with Vps17 (an endosomal marker), Sec7 (a trans-Golgi marker), and a vacuolar marker revealed the fate of Ngt1 during sugar-responsive endosomal trafficking. ∆ras1 and ∆ubi4 mutants showed decreased ubiquitylation and delayed endocytosis of Ngt1. According to our knowledge, this is the first report which illustrates the mechanistic insights that are responsible for endosomal trafficking of a GlcNAc transporter in an eukaryotic organism.

Development of Carbazole Derivatives Compounds against Candida albicans: Candidates to Prevent Hyphal Formation via the Ras1-MAPK Pathway

Morphogenesis contributes to the virulence of the opportunistic human fungal pathogen Candida albicans. Ras1-MAPK pathways play a critical role in the virulence of C. albicans by regulating cell growth, morphogenesis, and biofilm formation. Ume6 acts as a transcription factor, and Nrg1 is a transcriptional repressor for the expression of hyphal-specific genes in morphogenesis. Azoles or echinocandin drugs have been extensively prescribed for C. albicans infections, which has led to the development of drug-resistant strains. Therefore, it is necessary to develop new molecules to effectively treat fungal infections. Here, we showed that Molecule B and Molecule C, which contained a carbazole structure, attenuated the pathogenicity of C. albicans through inhibition of the Ras1/MAPK pathway. We found that Molecule B and Molecule C inhibit morphogenesis through repressing protein and RNA levels of Ras/MAPK-related genes, including UME6 and NRG1. Furthermore, we determined the antifungal effects of Molecule B and Molecule C in vivo using a candidiasis murine model. We anticipate our findings are that Molecule B and Molecule C, which inhibits the Ras1/MAPK pathway, are promising compounds for the development of new antifungal agents for the treatment of systemic candidiasis and possibly for other fungal diseases.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Suppression of hyphal formation and virulence of Candida albicans by natural and synthetic compounds

Candida albicans undergoes a morphological yeast-to-hyphal transition during infection, which plays a significant role in its pathogenesis. The filamentous morphology of the hyphal form has been identified as a virulence factor as it facilitates surface adherence, intertwining with biofilm, invasion, and damage to host tissues and organs. Hence, inhibition of filamentation in addition to biofilm formation is considered a viable strategy against C. albicans infections. Furthermore, a good understanding of the signaling pathways involved in response to environmental cues driving hyphal growth is also critical to an understanding of C. albicans pathogenicity and to develop novel therapies. In this review, first the clinical significance and transcriptional control of C. albicans hyphal morphogenesis are addressed. Then, various strategies employed to suppress filamentation, prevent biofilm formation, and reduce virulence are discussed. These strategies include the inhibition of C. albicans filament formation using natural or synthetic compounds, and their combination with other agents or nanoformulations.

CaBir1 functions as an inhibitor-of-apoptosis and affects caspase-like activitiy in Candida albicans

CaMca1 is the only metacaspase in Candida albicans, which shows structural homology to the mammalian caspases. CaMca1 consists of the caspase domain, the P20 and P10 regions, and the conserved catalytic histidine-cysteine dyad that is required for executing apoptosis in C. albicans. However, little is known about the proteolytic processing of CaMca1 or its activation under apoptosis-inducing conditions. To understand the regulation of this process, we characterized CaBir1 which is the single IAP (inhibitor-of-apoptosis protein) in C. albicans. IAPs are a family of proteins whose members all harbor a BIR (baculovirus IAP repeat) domain and negatively regulate apoptosis by inhibiting caspases. We found that the Cabir1/Cabir1 deletion mutant exhibited increased apoptotic phenotypes, such as ROS accumulation, nuclear segmentation, and cell survival, under apoptosis-inducing conditions. Examination of CaMca1 cleavage patterns in response to various apoptotic stresses revealed that these cleavages were stress-specific and dependent on the catalytic histidine-cysteine residues of CaMca1. The Cabir1/Cabir1 mutation was not associated with altered CaMca1 processing with or without apoptotic stimuli, but the Cabir1/Cabir1 mutant exhibited significantly increased caspase-like activities. These results suggest that CaBir1 acts as an apoptosis inhibitor by regulating caspase-like activity, but not CaMca1 processing.

Albumin Neutralizes Hydrophobic Toxins and Modulates Candida albicans Pathogenicity

Albumin is abundant in serum but is also excreted at mucosal surfaces and enters tissues when inflammation increases vascular permeability. Host-associated opportunistic pathogens encounter albumin during commensalism and when causing infections. Considering the ubiquitous presence of albumin, we investigated its role in the pathogenesis of infections with the model human fungal pathogen, Candida albicans. Albumin was introduced in various in vitro models that mimic different stages of systemic or mucosal candidiasis, where it reduced the ability of C. albicans to damage host cells. The amphipathic toxin candidalysin mediates necrotic host cell damage induced by C. albicans. Using cellular and biophysical assays, we determined that albumin functions by neutralizing candidalysin through hydrophobic interactions. We discovered that albumin, similarly, can neutralize a variety of fungal (α-amanitin), bacterial (streptolysin O and staurosporin), and insect (melittin) hydrophobic toxins. These data suggest albumin as a defense mechanism against toxins, which can play a role in the pathogenesis of microbial infections. IMPORTANCE Albumin is the most abundant serum protein in humans. During inflammation, serum albumin levels decrease drastically, and low albumin levels are associated with poor patient outcome. Thus, albumin may have specific functions during infection. Here, we describe the ability of albumin to neutralize hydrophobic microbial toxins. We show that albumin can protect against damage induced by the pathogenic yeast C. albicans by neutralizing its cytolytic toxin candidalysin. These findings suggest that albumin is a toxin-neutralizing protein that may play a role during infections with toxin-producing microorganisms.

Transcriptional control of hyphal morphogenesis in Candida albicans

Candida albicans is a multimorphic commensal organism and opportunistic fungal pathogen in humans. A morphological switch between unicellular budding yeast and multicellular filamentous hyphal growth forms plays a vital role in the virulence of C. albicans, and this transition is regulated in response to a range of environmental cues that are encountered in distinct host niches. Many unique transcription factors contribute to the transcriptional regulatory network that integrates these distinct environmental cues and determines which phenotypic state will be expressed. These hyphal morphogenesis regulators have been extensively investigated, and represent an increasingly important focus of study, due to their central role in controlling a key C. albicans virulence attribute. This review provides a succinct summary of the transcriptional regulatory factors and environmental signals that control hyphal morphogenesis in C. albicans.

Underlying mechanisms of the effect of minocycline against Candida albicans biofilms

Minocycline (MH) is a broad-spectrum antimicrobial agent and semisynthetic tetracycline derivative, which has been widely used in the clinic due to its efficacy. Having the strongest anti-microbial effect, MH exceeded the traditional scope of antibiotics and its previously unknown antifungal activity is also gradually being discovered. To preliminarily investigate the inhibitory effect of MH on Candida albicans (C. albicans), changes of cell growth, hyphal formation and transition, biofilm production and signaling pathway gene expression of C. albicans in the presence of MH were assessed in the present study. An XTT reduction assay was performed to quantitatively detect the metabolic activity of biofilms and evaluate the inhibition of MH on this. The results suggested that biofilm formation was clearly inhibited by 67% (P<0.0001) in the presence of 250 µg/ml MH, while mature biofilms were not significantly affected. In addition, MH inhibited the transition from yeast to hypha in a dose-dependent manner. Furthermore, reverse transcription-quantitative PCR revealed that several hyphae- and adhesion-specific genes associated with the Ras/cyclic (c)AMP/protein kinase A (PKA) pathway were differentially expressed following MH treatment, including downregulation of ras family GTPase (RAS1), adenylyl cyclase-associated protein 1 (CAP1), thiamin pyrophosphokinase 1 (TPK1), adenylate cyclase (CDC35), transcription factor (TEC1), agglutinin-like protein 3 (ALS3) and hyphal wall protein 1 (HWP1) and upregulation of EFG1 (enhanced filamentous growth protein 1 gene) and PDE2 (high-affinity phosphodiesterase gene). The most obviously changed genes were TPK1, HWP1 and RAS1, downregulated by 0.33-, 0.48- and 0.55-fold, respectively. It was suggested that MH is associated with alterations in the morphology of C. albicans, such as the repression of hypha and biofilm formation of cells, and MH affected the Ras/cAMP pathway to regulate the expression of cAMP-associated genes.

Wor1-regulated ferroxidases contribute to pigment formation in opaque cells of Candida albicans

Candida albicans is a harmless commensal resident in the human gut and a prevalent opportunistic pathogen. A key part of its commensalism and pathogenesis is its ability to switch between different morphological forms, including white‐to‐opaque switching. The Wor1 protein was previously identified as a master regulator of white‐to‐opaque switching in mating type locus (MTL) homozygous cells. The mechanisms by which the dark color of the opaque colonies is controlled and the pimpled surface of opaque cells is formed remain unknown. Candida albicans produces melanin pigment in vitro and during infection. However, the molecular mechanism underlying the regulation of melanin production is unclear. In this study, we demonstrated that ferroxidases (Fets) function as pigment multicopper oxidases and regulate the production of dark‐pigmented melanin in opaque cells. The FET genes presented distinct regulation patterns in response to different extracellular stimuli. In YPD (1% yeast extract, 2% peptone and 2% dextrose)‐rich medium, four of the five FET genes were up‐regulated by Wor1, especially at the human body temperature of 37 °C. In minimal medium with low ammonium concentrations, all five FET genes were up‐regulated by Wor1. However, at high ammonium concentrations, some FET genes were down‐regulated by Wor1. Wor1‐up‐regulated Fets contributed to dark pigment formation in opaque colonies, but not to the elongated shape of these opaque cells. Increased melanin externalization was associated with the pimpled surface of the opaque cells. Melanized C. albicans cells were more resistant to fungal clearance. Deletion of the five FET genes completely blocked melanin production in opaque cells and resulted in the generation of white elongated ‘opaque’ cells. In addition, the up‐regulated Fets are important for defense against oxidant attacks. The functional diversity of Fets may reflect the multiple strategies of C. albicans to rapidly adapt to diverse host niches.

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

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

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

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

Inhibition of Distinct Proline- or N-Acetylglucosamine-Induced Hyphal Formation Pathways by Proline Analogs in Candida albicans

Candida albicans undergoes a yeast-to-hyphal transition that has been recognized as a virulence property as well as a turning point leading to biofilm formation associated with candidiasis. It is known that yeast-to-hyphal transition is induced under complex environmental conditions including temperature (above 35°C), pH (greater than 6.5), CO₂, N-acetylglucosamine (GlcNAc), amino acids, RPMI-1640 synthetic culture medium, and blood serum. To identify the hyphal induction factor in the RPMI-1640 medium, we examined each component of RPMI-1640 and established a simple hyphal induction condition, that is, incubation in L-proline solution at 37°C. Incubation in GlcNAc solution alone, which is not contained in RPMI-1640, without any other materials was also identified as another simple hyphal induction condition. To inhibit hyphal formation, proline and GlcNAc analogs were examined. Among the proline analogs used, L-azetidine-2-carboxylic acid (AZC) inhibited hyphal induction under both induction conditions, but L-4-thiazolidinecarboxylic acid (T4C) specifically inhibited proline-induced hyphal formation only, while α-N-methyl-L-proline (mPro) selectively inhibited GlcNAc-induced hyphal formation. Hyphal formation in fetal bovine serum was also inhibited by AZC or T4C together with mPro without affecting the proliferation of yeast form. These results indicate that these proline analogs are ideal inhibitors of yeast-to-hyphal transition in C. albicans.

Using genetically encoded heme sensors to probe the mechanisms of heme uptake and homeostasis in Candida albicans

Candida albicans is a major fungal pathogen that can utilise hemin and haemoglobin as iron sources in the iron-scarce host environment. While C. albicans is a heme prototroph, we show here that it can also efficiently utilise external heme as a cellular heme source. Using genetically encoded ratiometric fluorescent heme sensors, we show that heme extracted from haemoglobin and free hemin enter the cells with different kinetics. Heme supplied as haemoglobin is taken up via the Common in Fungal Extracellular Membrane (CFEM) hemophore cascade, and reaches the cytoplasm over several hours, whereas entry of free hemin via CFEM-dependent and independent pathways is much faster, less than an hour. To prevent an influx of extracellular heme from reaching toxic levels in the cytoplasm, the cells deploy Hmx1, a heme oxygenase. Hmx1 was previously suggested to be involved in utilisation of haemoglobin and hemin as iron sources, but we find that it is primarily required to prevent heme toxicity. Taken together, the combination of novel heme sensors with genetic analysis revealed new details of the fungal mechanisms of heme import and homeostasis, necessary to balance the uses of heme as essential cofactor and potential iron source against its toxicity.

Interactions of microorganisms with host mucins: a focus on Candida albicans

Mucus is an important host innate defense factor that lines most epithelial cell layers of the body and provides crucial physical and biological protection against pathogenic microorganisms. Mucins are the main glycoproteins of mucus that are responsible for interacting with microorganisms and are critical for the antimicrobial properties of mucus. The mechanisms by which microorganisms interact with mucins are poorly understood, especially in terms of fungi, and these interactions are continually evolving. Work in bacterial pathogens has shown that mucins inhibit bacterial virulence traits, including quorum sensing, toxin secretion and biofilm formation. Among the fungal clade, the common opportunistic human fungal pathogen and commensal Candida albicans engages in constant battle with the host innate immune system. This battle creates strong selective pressures for C. albicans to evolve in response to the host. Recent work in C. albicans found that mucins inhibit specific virulence traits, such as surface adherence, filamentation, biofilm formation and the production of secreted proteases. Here we review the current knowledge of microbial interactions with mucins, with a special emphasis on the interactions between C. albicans and mucins.

Anandamide prevents the adhesion of filamentous Candida albicans to cervical epithelial cells

Candidiasis is a fungal infection caused by Candida species that have formed a biofilm on epithelial linings of the body. The most frequently affected areas include the vagina, oral cavity and the intestine. In severe cases, the fungi penetrate the epithelium and cause systemic infections. One approach to combat candidiasis is to prevent the adhesion of the fungal hyphae to the epithelium. Here we demonstrate that the endocannabinoid anandamide (AEA) and the endocannabinoid-like N-arachidonoyl serine (AraS) strongly prevent the adherence of C. albicans hyphae to cervical epithelial cells, while the endocannabinoid 2-arachidonoylglycerol (2-AG) has only a minor inhibitory effect. In addition, we observed that both AEA and AraS prevent the yeast-hypha transition and perturb hyphal growth. Real-time PCR analysis showed that AEA represses the expression of the HWP1 and ALS3 adhesins involved in Candida adhesion to epithelial cells and the HGC1, RAS1, EFG1 and ZAP1 regulators of hyphal morphogenesis and cell adherence. On the other hand, AEA increased the expression of NRG1, a transcriptional repressor of filamentous growth. Altogether, our data show that AEA and AraS have potential anti-fungal activities by inhibiting hyphal growth and preventing hyphal adherence to epithelial cells.

Proteomic profiling of the monothiol glutaredoxin Grx3 reveals its global role in the regulation of iron dependent processes

Iron is an essential nutrient required as a cofactor for many biological processes. As a fungal commensal-pathogen of humans, Candida albicans encounters a range of bioavailable iron levels in the human host and maintains homeostasis with a conserved regulatory circuit. How C. albicans senses and responds to iron availability is unknown. In model yeasts, regulation of the iron homeostasis circuit requires monothiol glutaredoxins (Grxs), but their functions beyond the regulatory circuit are unclear. Here, we show Grx3 is required for virulence and growth on low iron for C. albicans. To explore the global roles of Grx3, we applied a proteomic approach and performed in vivo cross-linked tandem affinity purification coupled with mass spectrometry. We identified a large number of Grx3 interacting proteins that function in diverse biological processes. This included Fra1 and Bol2/Fra2, which function with Grxs in intracellular iron trafficking in other organisms. Grx3 interacts with and regulates the activity of Sfu1 and Hap43, components of the C. albicans iron regulatory circuit. Unlike the regulatory circuit, which determines expression or repression of target genes in response to iron availability, Grx3 amplifies levels of gene expression or repression. Consistent with the proteomic data, the grx3 mutant is sensitive to heat shock, oxidative, nitrosative, and genotoxic stresses, and shows growth dependence on histidine, leucine, and tryptophan. We suggest Grx3 is a conserved global regulator of iron-dependent processes occurring within the cell.

Mammalian pathogens occupy a diverse set of niches within the host organism. These niches vary in iron and oxygen availability. As a commensal and pathogen of humans, its ability to regulate iron uptake and utilization in response to bioavailable iron level is critical for its survival in different host environments encompassing a broad range of iron levels. This study aims to understand how C. albicans senses and responds to iron level to regulate multiple aspects of its biology. The cytosolic monothiol glutaredoxin Grx3 is a critical regulator of C. albicans iron homeostasis and virulence. Taking a proteomic approach, we identified a large list of Grx3 associated proteins of diverse functions, including iron-sulfur trafficking, iron homeostasis, metabolism redox homeostasis, protein translation, DNA maintenance and repair. In support of these protein associations, Grx3 is important for all these processes. Thus, Grx3 is a global regulator of iron homeostasis and other iron dependent cellular processes.

Antifungal activity of hypocrellin compounds and their synergistic effects with antimicrobial agents against Candida albicans

Candida albicans is a common human fungal pathogen. The previous study revealed that quinone compounds showed antimicrobial activity against C. albicans by inhibiting cell growth. However, it was unclear whether quinones have other antifungal effects against C. albicans in addition to fungicidal effects. In this study, we assessed the inhibitory activity of a total of 25 quinone compounds against C. albicans morphological transition, which is essential for the pathogenicity of C. albicans. Several quinones exhibited strong inhibition of mycelium formation by C. albicans SC5314. Three leading compounds, namely hypocrellins A, B and C, also exhibited marked attenuation of C. albicans SC5314 virulence in both human cell lines and mouse infection models. These three compounds significantly suppressed the proliferation of C. albicans SC5314 cells in a mouse mucosal infection model. Intriguingly, hypocrellins not only attenuated the cytotoxicity of a nystatin-resistant C. albicans strain but also showed excellent synergistic effects with antifungal agents against both wild-type C. albicans SC5314 and the drug-resistant mutant strains. In addition, hypocrellins A, B and C interfered with the biological functions and virulence of various clinical Candida species, suggesting the promising potential of these compounds for development as new therapeutic agents against infections caused by Candida pathogens.

Dimorphic Mechanism on cAMP Mediated Signal Pathway in Mucor circinelloides

Mucor circinelloides is a dimorphic fungus that is a non-pathogen strain belonging to zygomycetes. In this research, a part of hypothetical mechanism on yeast-like cell induction of M. circinelloides in CO₂ atmosphere was reported from the viewpoint of gene expression. To explain the relation between the change and the expressions of some genes involved in morphological changes of the strain, these were analyzed on the filamentous and yeast cell by real-time qPCR. The compared genes were Nce103, Ras3, Cyr1, Pde, and Efg1 encoding carbonic anhydrase, GTPase, adenylate cyclase, phosphodiesterase, and elongation factor G1, respectively. In anaerobic grown yeast cell with 70%N₂ + 30%CO₂, the Nce103 and Ras3 gene expressions decreased to 24 h whereas that of the filamentous cell increased. However, a downstream gene of Cyr1 expression level in the yeast cell was higher than that of filamentous cell. A lower level of Pde in the yeast cell than that of the filamentous cell indicated intracellular cAMP accumulation. The actual cAMP in the yeast cell remained whereas that of the filamentous cell decreased with cultivation. The Efg1 expression level controlling hyphal elongation was suppressed in the yeast cell. The intracellular cAMP accumulation and Efg1 expression regulate hyphal elongation or yeast forming.

Genetic regulation of the development of mating projections in Candida albicans

Candida albicans is a major human fungal pathogen, capable of switching among a range of morphological types, such as the yeast form, including white and opaque cell types and the GUT (gastrointestinally induced transition) cell type, the filamentous form, including hyphal and pseudohyphal cell types, and chlamydospores. This ability is associated with its commensal and pathogenic life styles. In response to pheromone, C. albicans cells are able to form long mating projections resembling filaments. This filamentous morphology is required for efficient sexual mating. In the current study, we report the genetic regulatory mechanisms controlling the development of mating projections in C. albicans. Ectopic expression of MTLα1 in “a” cells induces the secretion of α-pheromone and promotes the development of mating projections. Using this inducible system, we reveal that members of the pheromone-sensing pathway (including the pheromone receptor), the Ste11-Hst7-Cek1/2 mediated MAPK signalling cascade, and the RAM pathway are essential for the development of mating projections. However, the cAMP/PKA signalling pathway and a number of key regulators of filamentous growth such as Hgc1, Efg1, Flo8, Tec1, Ume6, and Rfg1 are not required for mating projection formation. Therefore, despite the phenotypic similarities between filaments and mating projections in C. albicans, distinct mechanisms are involved in the regulation of these two morphologies.

Inhibition of Candida albicans and Staphylococcus aureus biofilms by centipede oil and linoleic acid

Microbial biofilms are associated with persistent infections because of their high tolerance to antimicrobial agents and host defenses. The effects of centipede oil from Scolopendra subspinipes mutilans and its main components were investigated to identify non-toxic biofilm inhibitors. Centipede oil and linoleic acid at 20 µg ml^-1 markedly inhibited biofilm formation by two fluconazole-resistant Candida albicans strains and three Staphylococcus aureus strains without affecting their planktonic cell growth. Also, both centipede oil and linoleic acid inhibited hyphal growth and cell aggregation by C. albicans. In addition, centipede oil and linoleic acid showed anti-biofilm activities against mixed C. albicans and S. aureus biofilms. Transcriptomic analysis showed that centipede oil and linoleic acid downregulated the expressions of several hypha/biofilm-related genes in C. albicans and α-hemolysin in S. aureus. Furthermore, both compounds effectively reduced C. albicans virulence in a nematode infection model with minimal toxicity.

Genome-wide screening and in silico gene knockout to predict potential candidates for drug designing against Candida albicans

C. albicans infections are increasingly becoming a threat to public health with emergence of drug resistant strains. It emphasizes the need to look for alternate drug targets through genome-wide screening. In the present study, whole proteome of C. albicans SC5314 was analyzed in single click target mining workflow of TiDv2. A protein-protein interaction network (PPI) for the resulting putative targets was generated based on String database. Ninety four proteins with higher connectivity (degree ≥ 10) in the network are noted as hub genes. Among them, 24 are observed to be known targets while 70 are novel ones. Further, chokepoint analysis revealed FAS2, FOL1 and ERG5 as chokepoint enzymes in their respective pathways. Subsequently, the chokepoints were selected as prior interest for in silico gene knockout via MATLAB and COBRA Toolbox. In silico gene knockout pointed that FAS2 inhibition reduced the growth rate of pathogen from 0.2879 mmol.gDW^-1.h^-1 to zero. Furthermore, enzyme inhibition assay of FAS2 with cerulenin strengthen the computational outcome with MIC 1.25 μg/mL. Hence, the study establishes FAS2 as a promising target to design therapeutics against C. albicans.

Lrg1 Regulates β (1,3)-Glucan Masking in Candida albicans through the Cek1 MAP Kinase Pathway

Candida albicans is among the most prevalent opportunistic human fungal pathogens. The ability to mask the immunogenic polysaccharide β (1,3)-glucan from immune detection via a layer of mannosylated proteins is a key virulence factor of C. albicans We previously reported that hyperactivation of the Cek1 mitogen-activated protein (MAP) kinase pathway promotes β (1,3)-glucan exposure. In this communication, we report a novel upstream regulator of Cek1 activation and characterize the impact of Cek1 activity on fungal virulence. Lrg1 encodes a GTPase-activating protein (GAP) that has been suggested to inhibit the GTPase Rho1. We found that disruption of LRG1 causes Cek1 hyperactivation and β (1,3)-glucan unmasking. However, when GTPase activation was measured for a panel of GTPases, the lrg1ΔΔ mutant exhibited increased activation of Cdc42 and Ras1 but not Rho1 or Rac1. Unmasking and Cek1 activation in the lrg1ΔΔ mutant can be blocked by inhibition of the Ste11 MAP kinase kinase kinase (MAPKKK), indicating that the lrg1ΔΔ mutant acts through the canonical Cek1 MAP kinase cascade. In order to determine how Cek1 hyperactivation specifically impacts virulence, a doxycycline-repressible hyperactive STE11ΔN467 allele was expressed in C. albicans In the absence of doxycycline, this allele overexpressed STE11ΔN467 , which induced production of proinflammatory tumor necrosis factor alpha (TNF-α) from murine macrophages. This in vitro phenotype correlates with decreased colonization and virulence in a mouse model of systemic infection. The mechanism by which Ste11ΔN467 causes unmasking was explored with RNA sequencing (RNA-Seq) analysis. Overexpression of Ste11ΔN467 caused upregulation of the Cph1 transcription factor and of a group of cell wall-modifying proteins which are predicted to impact cell wall architecture.IMPORTANCECandida albicans is an important source of systemic infections in humans. The ability to mask the immunogenic cell wall polymer β (1,3)-glucan from host immune surveillance contributes to fungal virulence. We previously reported that the hyperactivation of the Cek1 MAP kinase cascade promotes cell wall unmasking, thus increasing strain immunogenicity. In this study, we identified a novel regulator of the Cek1 pathway called Lrg1. Lrg1 is a predicted GTPase-activating protein (GAP) that represses Cek1 activity by downregulating the GTPase Cdc42 and its downstream MAPKKK, Ste11. Upregulation of Cek1 activity diminished fungal virulence in the mouse model of infection, and this correlates with increased cytokine responses from macrophages. We also analyzed the transcriptional profile determined during β (1,3)-glucan exposure driven by Cek1 hyperactivation. Our report provides a model where Cek1 hyperactivation causes β (1,3)-glucan exposure by upregulation of cell wall proteins and leads to more robust immune detection in vivo, promoting more effective clearance.