Identification and characterization of TUP1-regulated genes in Candida albicans

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.

Identification and Functional Analysis of CAP Genes from the Wheat Stripe Rust Fungus Puccinia striiformis f. sp. tritici

Cysteine-rich secretory proteins (C), antigen 5 (A), and pathogenesis-related 1 proteins (P) comprise widespread CAP superfamily proteins, which have been proven to be novel virulence factors of mammalian pathogenic fungi and some plant pathogens. Despite this, the identification and function of CAP proteins in more species of plant pathogens still need to be studied. This work presents the identification and functional analysis of CAP superfamily proteins from Puccinia striiformis f. sp. tritici (Pst), an important fungal pathogen that causes wheat stripe rust on wheat worldwide. A total of six CAP genes were identified in the Pst genome, designated as PsCAP1-PsCAP6. Five PsCAP proteins, including PsCAP1, PsCAP2, PsCAP3, PsCAP4, and PsCAP5, have N-terminal signal peptides secreted with the yeast signal sequence trap assay. Single-nucleotide polymorphism (SNP) analysis indicated that they showed a low level of intraspecies polymorphism. The expression abundance of PsCAP genes at different Pst infection stages was detected by RT-qPCR, and most of them were highly expressed during Pst infection on wheat and also Pst sexual reproduction on barberry (Berberis shensiana). Noticeably, the silencing of these six PsCAP genes by BSMV-mediated HIGS indicated that PsCAP1, PsCAP4, and PsCAP5 contribute significantly to Pst infection in wheat. These results indicate that PsCAP proteins may act as virulence factors during Pst infection, which also provides insights into Pst pathogenicity.

A C. albicans TRAPP Complex-Associated Gene Contributes to Cell Wall Integrity, Hyphal and Biofilm Formation, and Tissue Invasion

While endocytic and secretory pathways are well-studied cellular processes in the model yeast Saccharomyces cerevisiae, they remain understudied in the opportunistic fungal pathogen Candida albicans. We previously found that null mutants of C. albicans homologs of the S. cerevisiae early endocytosis genes ENT2 and END3 not only exhibited delayed endocytosis but also had defects in cell wall integrity, filamentation, biofilm formation, extracellular protease activity, and tissue invasion in an in vitro model. In this study, we focused on a potential C. albicans homolog to S. cerevisiae TCA17, which was discovered in our whole-genome bioinformatics approach aimed at identifying genes involved in endocytosis. In S. cerevisiae, TCA17 encodes a transport protein particle (TRAPP) complex-associated protein. Using a reverse genetics approach with CRISPR-Cas9-mediated gene deletion, we analyzed the function of the TCA17 homolog in C. albicans. Although the C. albicans tca17Δ/Δ null mutant did not have defects in endocytosis, it displayed an enlarged cell and vacuole morphology, impaired filamentation, and reduced biofilm formation. Moreover, the mutant exhibited altered sensitivity to cell wall stressors and antifungal agents. When assayed using an in vitro keratinocyte infection model, virulence properties were also diminished. Our findings indicate that C. albicans TCA17 may be involved in secretion-related vesicle transport and plays a role in cell wall and vacuolar integrity, hyphal and biofilm formation, and virulence. IMPORTANCE The fungal pathogen Candida albicans causes serious opportunistic infections in immunocompromised patients and has become a major cause of hospital-acquired bloodstream infections, catheter-associated infections, and invasive disease. However, due to a limited understanding of Candida molecular pathogenesis, clinical approaches for the prevention, diagnosis, and treatment of invasive candidiasis need significant improvement. In this study, we focus on identifying and characterizing a gene potentially involved in the C. albicans secretory pathway, as intracellular transport is critical for C. albicans virulence. We specifically investigated the role of this gene in filamentation, biofilm formation, and tissue invasion. Ultimately, these findings advance our current understanding of C. albicans biology and may have implications for the diagnosis and treatment of candidiasis.

Production of IgY against iron permease Ftr1 from Candida albicans and evaluation of its antifungal activity using Galleria mellonella as a model of systemic infection

Candida albicans is one of the leading pathological agents of mucosal and deep tissue infections. Considering that the variety of antifungals is restricted and that toxicity limits their use, immunotherapies against pathogenic fungi have been viewed as alternatives with reduced adverse effects. In this context, C. albicans has a protein used to capture iron from the environment and the host, known as the high-affinity iron permease Ftr1. This protein may be a new target of action for novel antifungal therapies, as it influences the virulence of this yeast. Thus, the aim of the present study was to produce and conduct the biological characterization of IgY antibodies against C. albicans Ftr1. Immunization of laying hens with an Ftr1-derived peptide resulted in IgY antibodies extracted from egg yolks capable of binding to the antigen with high affinity (avidity index = 66.6 ± 0.3%). These antibodies reduced the growth and even eliminated C. albicans under iron restriction, a favorable condition for the expression of Ftr1. This also occurred with a mutant strain that does not produce Ftr1 in the presence of iron, a circumstance in which the protein analog of iron permease, Ftr2, is expressed. Furthermore, the survival of G. mellonella larvae infected with C. albicans and treated with the antibodies was 90% higher than the control group, which did not receive treatment (p < 0.0001). Therefore, our data suggest that IgY antibodies against Ftr1 from C. albicans can inhibit yeast propagation by blocking iron uptake.

FonTup1 functions in growth, conidiogenesis and pathogenicity of Fusarium oxysporum f. sp. niveum through modulating the expression of the tricarboxylic acid cycle genes

The Tup1-Cyc8 complex is a highly conserved transcriptional corepressor that regulates intricate genetic network associated with various biological processes in fungi. Here, we report the role and mechanism of FonTup1 in regulating physiological processes and pathogenicity in watermelon Fusarium wilt fungus, Fusarium oxysporum f. sp. niveum (Fon). FonTup1 deletion impairs mycelial growth, asexual reproduction, and macroconidia morphology, but not macroconidial germination in Fon. The ΔFontup1 mutant exhibits altered tolerance to cell wall perturbing agent (congo red) and osmotic stressors (sorbitol or NaCl), but unchanged sensitivity to paraquat. The deletion of FonTup1 significantly decreases the pathogenicity of Fon toward watermelon plants through attenuating the ability to colonize and grow within the host. Transcriptome analysis revealed that FonTup1 regulates primary metabolic pathways, including the tricarboxylic acid (TCA) cycle, via altering the expression of corresponding genes. Downregulation of three malate dehydrogenase genes, FonMDH1-3, occurs in ΔFontup1, and disruption of FonMDH2 causes significant abnormalities in mycelial growth, conidiation, and virulence of Fon. These findings demonstrate that FonTup1, as a global transcriptional corepressor, plays crucial roles in different biological processes and pathogenicity of Fon through regulating various primary metabolic processes, including the TCA cycle. This study highlights the importance and molecular mechanism of the Tup1-Cyc8 complex in multiple basic biological processes and pathogenicity of phytopathogenic fungi.

Morphogenic plasticity: the pathogenic attribute of Candida albicans

Candida albicans is a commensal organism of the human gastrointestinal tract and a prevalent opportunistic pathogen. It exhibits different morphogenic forms to survive in different host niches with distinct environmental conditions (pH, temperature, oxidative stress, nutrients, serum, chemicals, radiation, etc.) and genetic factors (transcription factors and genes). The different morphogenic forms of C. albicans are yeast, hyphal, pseudohyphal, white, opaque, and transient gray cells, planktonic and biofilm forms of cells. These forms differ in the parameters like cellular phenotype, colony morphology, adhesion to solid surfaces, gene expression profile, and the virulent traits. Each form is functionally distinct and responds discretely to the host immune system and antifungal drugs. Hence, morphogenic plasticity is the key to virulence. In this review, we address the characteristics, the pathogenic potential of the different morphogenic forms and the conditions required for morphogenic transitions.

Heme sensing and trafficking in fungi

Fungal pathogens cause life-threatening diseases in humans, and the increasing prevalence of these diseases emphasizes the need for new targets for therapeutic intervention. Nutrient acquisition during infection is a promising target, and recent studies highlight the contributions of endomembrane trafficking, mitochondria, and vacuoles in the sensing and acquisition of heme by fungi. These studies have been facilitated by genetically encoded biosensors and other tools to quantitate heme in subcellular compartments and to investigate the dynamics of trafficking in living cells. In particular, the applications of biosensors in fungi have been extended beyond the detection of metabolites, cofactors, pH, and redox status to include the detection of heme. Here, we focus on studies that make use of biosensors to examine mechanisms of heme uptake and degradation, with guidance from the model fungus Saccharomyces cerevisiae and an emphasis on the pathogenic fungi Candida albicans and Cryptococcus neoformans that threaten human health. These studies emphasize a role for endocytosis in heme uptake, and highlight membrane contact sites involving mitochondria, the endoplasmic reticulum and vacuoles as mediators of intracellular iron and heme trafficking.

Antifungal Properties of Biogenic Selenium Nanoparticles Functionalized with Nystatin for the Inhibition of Candida albicans Biofilm Formation

In the present study, biogenic selenium nanoparticles (SeNPs) have been prepared using Paenibacillus terreus and functionalized with nystatin (SeNP@PVP_Nystatin nanoconjugates) for inhibiting growth, morphogenesis, and a biofilm in Candida albicans. Ultraviolet-visible spectroscopy analysis has shown a characteristic absorption at 289, 303, and 318 nm, and X-ray diffraction analysis has shown characteristic peaks at different 2θ values for SeNPs. Electron microscopy analysis has shown that biogenic SeNPs are spherical in shape with a size in the range of 220-240 nm. Fourier transform infrared spectroscopy has confirmed the functionalization of nystatin on SeNPs (formation of SeNP@PVP_Nystatin nanoconjugates), and the zeta potential has confirmed the negative charge on the nanoconjugates. Biogenic SeNPs are inactive; however, nanoconjugates have shown antifungal activities on C. albicans (inhibited growth, morphogenesis, and a biofilm). The molecular mechanism for the action of nanoconjugates via a real-time polymerase chain reaction has shown that genes involved in the RAS/cAMP/PKA signaling pathway play an important role in antifungal activity. In cytotoxic studies, nanoconjugates have inhibited only 12% growth of the human embryonic kidney cell line 293 cells, indicating that the nanocomposites are not cytotoxic. Thus, the biogenic SeNPs produced by P. terreus can be used as innovative and effective drug carriers to increase the antifungal activity of nystatin.

Metabolomic and Proteomic Changes in Candida albicans Biofilm in Response to Zosteric Acid Treatment

Zosteric acid (ZA) is a secondary metabolite of the seagrass Zostera marina, with antibiofilm activity against fungi. Information concerning its mechanisms of action is lacking and this limits the development of more potent derivatives based on the same target and activity structure. The aim of this work was to investigate the ZA mode of action by analyzing the metabolic status of Candida albicans biofilm and its protein expression profile upon ZA treatment. Fourier-Transform Infrared Spectroscopy confirmed that ZA modified the metabolomic response of treated cells, showing changes in the spectral regions, mainly related to the protein compartment. Nano Liquid Chromatography-High-Resolution Mass Spectrometry highlighted that 10 proteins were differentially expressed in the C. albicans proteome upon ZA treatment. Proteins involved in the biogenesis, structure and integrity of cell walls as well as adhesion and stable attachment of hyphae were found downregulated, whereas some proteins involved in the stress response were found overexpressed. Additionally, ZA was involved in the modulation of non-DNA-based epigenetic regulatory mechanisms triggered by reactive oxygen species. These results partially clarified the ZA mechanism of action against fungi and provided insight into the major C. albicans pathways responsible for biofilm formation.

Transcription Factor VdCf2 Regulates Growth, Pathogenicity, and the Expression of a Putative Secondary Metabolism Gene Cluster in Verticillium dahliae

Verticillium dahliae is an important soilborne phytopathogen which can ruinously attack numerous host plants and cause significant economic losses. Transcription factors (TFs) were reported to be involved in various biological processes, such as hyphal growth and virulence of pathogenic fungi.

Hydroquinones Including Tetrachlorohydroquinone Inhibit Candida albicans Biofilm Formation by Repressing Hyphae-Related Genes

Candida albicans is an opportunistic pathogenic fungus responsible for candidiasis. The pathogen readily forms antifungal agent-resistant biofilms on implanted medical devices or human tissue. Morphologic transition from yeast to filamentous cells and subsequent biofilm formation is a key virulence factor and a prerequisite for biofilm development by C. albicans. We investigated the antibiofilm and antifungal activities of 18 hydroquinones against fluconazole-resistant C. albicans. Tetrachlorohydroquinone (TCHQ) at subinhibitory concentrations (2 to 10 μg/mL) significantly inhibited C. albicans biofilm formation with an MIC of 50 μg/mL, whereas the backbone hydroquinone did not (MIC > 400 μg/mL), and it markedly inhibited cell aggregation and hyphal formation. Transcriptomic analyses showed that TCHQ downregulated the expressions of several hyphae-forming and biofilm-related genes (ALS3, ECE1, HWP1, RBT5, and UME6) but upregulated hyphae- and biofilm-inhibitory genes (IFD6 and YWP1). Furthermore, it prevented C. albicans biofilm development on porcine skin and at concentrations of 20 to 50 μg/mL was nontoxic to the nematode Caenorhabditis elegans and did not adversely affect Brassica rapa seed germination and growth. This study indicates that hydroquinones, particularly TCHQ, diminish the virulence, biofilm formation, and animal tissue adhesion of C. albicans, which suggests hydroquinones should be considered potential candidate antifungal agents against drug-resistant C. albicans strains. IMPORTANCE Persistence in chronic infections by Candida albicans is due to its ability of biofilm formation that endures conventional antifungals and host immune systems. Hence, the inhibition of biofilm formation and virulence characteristics is another mean of addressing infections. This study is a distinctive one since 18 hydroquinone analogues were screened and TCHQ efficiently inhibited the biofilm formation by C. albicans with significantly changed expressional profile of hyphae-forming and biofilm-related genes. The antibiofilm efficacy was confirmed using a porcine skin model and chemical toxicity was investigated using plant seed germination and nematode models. Our findings reveal that TCHQ can efficiently control the C. albicans biofilms and virulence characteristics.

Role of Iron and Iron Overload in the Pathogenesis of Invasive Fungal Infections in Patients with Hematological Malignancies

Iron is an essential trace metal necessary for the reproduction and survival of fungal pathogens. The latter have developed various mechanisms to acquire iron from their mammalian hosts, with whom they participate in a continuous struggle for dominance over iron. Invasive fungal infections are an important problem in the treatment of patients with hematological malignancies, and they are associated with significant morbidity and mortality. The diagnosis of invasive clinical infections in these patients is complex, and the treatment, which must occur as early as possible, is difficult. There are several studies that have shown a possible link between iron overload and an increased susceptibility to infections. This link is also relevant for patients with hematological malignancies and for those treated with allogeneic hematopoietic stem cell transplantation. The role of iron and its metabolism in the virulence and pathogenesis of various invasive fungal infections is intriguing, and so far, there is some evidence linking invasive fungal infections to iron or iron overload. Clarifying the possible association of iron and iron overload with susceptibility to invasive fungal infections could be important for a better prevention and treatment of these infections in patients with hematological malignancies.

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.

Proteomic Analysis Reveals Proteins Involved in the Mode of Action of β-Citronellol Identified From Citrus hystrix DC. Leaf Against Candida albicans

Candida albicans is a fungus that lives primarily on the mucosal surfaces of healthy humans, such as the oral cavity, vagina, and gastrointestinal tract. This commensal organism can be controlled by other microbiota, while certain conditions can increase the risk of C. albicans outgrowth and cause disease. Prevalence of the drug-resistant phenotype, as well as the severity of C. albicans infection in immunocompromised patients, presents a challenge for scientists to develop novel, effective treatment, and prevention strategies. β-Citronellol is an intriguing active compound of several plants that has been linked to antifungal activity, but data on the mechanism of action in terms of proteomic profiling are lacking. Here, β-citronellol identified from Citrus hystrix DC. leaf against C. albicans were evaluated. A proteomic approach was used to identify potential target proteins involved in the mode of action of β-citronellol. This study identified and discussed three protein groups based on the 126 major proteins that were altered in response to β-citronellol treatment, 46 of which were downregulated and 80 of which were upregulated. Significant protein groups include cell wall proteins (e.g., Als2p, Rbt1p, and Pga4p), cellular stress response enzymes (e.g., Sod1p, Gst2p, and Ddr48p), and ATP synthesis-associated proteins (e.g., Atp3p, Atp7p, Cox1p, and Cobp). Results demonstrated the complexities of protein interactions influenced by β-citronellol treatment and highlighted the potential of antifungal activity for future clinical and drug development research.

Phenotypic Switching and Filamentation in Candida haemulonii, an Emerging Opportunistic Pathogen of Humans

Phenotypic plasticity is a common strategy adopted by fungal pathogens to adapt to diverse host environments. Candida haemulonii is an emerging multidrug-resistant human pathogen that is closely related to Candida auris. Until recently, it was assumed that C. haemulonii is incapable of phenotypic switching or filamentous growth. In this study, we report the identification of three distinct phenotypes in C. haemulonii: white, pink, and filament. The white and pink phenotypes differ in cellular size, colony morphology, and coloration on phloxine B- or CuSO₄-containing agar. Switching between the white and pink cell types is heritable and reversible and is referred to as “the primary switching system.” The additional switch phenotype, filament, has been identified and exhibits obviously filamentous morphology when grown on glycerol-containing medium. Several unique characteristics of the filamentous phenotype suggest that switching from or to this phenotype poses as a second yeast-filament switching system. The yeast-filament switch is nonheritable and temperature-dependent. Low temperatures favor the filamentous phenotype, whereas high temperatures promote filament-yeast transition. We further demonstrated that numerous aspects of the distinct cell types differ in numerous biological aspects, including their high temperature response, specific gene expression, CuSO₄ tolerance, secreted aspartyl protease (SAP) activity, and virulence. Therefore, transition among the three phenotypes could enable C. haemulonii to rapidly adapt to, survive, and thrive in certain host niches, thereby contributing to its virulence.

IMPORTANCE The capacity to switch between distinct cell types, known as phenotypic switching, is a common strategy adopted by Candida species to adapt to diverse environments. Despite considerable studies on phenotypic plasticity of various Candida species, Candida haemulonii is considered to be incapable of phenotypic switching or filamentous growth. Here, we report and describe filamentation and three distinct phenotypes (white, pink, and filament) in C. haemulonii. The three cell types differ in cellular and colony appearance, gene expression profiles, CuSO₄ tolerance, and virulence. C. haemulonii cells switch heritably and reversibly between white and pink cell types, which is referred to as the “primary switching system.” Switching between pink and filamentous phenotypes is nonheritable and temperature-dependent, representing a second switching system. As in other Candida species, switching among distinct morphological types may provide C. haemulonii with phenotypic plasticity for rapid responses to the changing host environment, and may contribute to its virulence.

Fungal Cell Wall Proteins and Signaling Pathways Form a Cytoprotective Network to Combat Stresses

Candida species are part of the normal flora of humans, but once the immune system of the host is impaired and they escape from commensal niches, they shift from commensal to pathogen causing candidiasis. Candida albicans remains the primary cause of candidiasis, accounting for about 60% of the global candidiasis burden. The cell wall of C. albicans and related fungal pathogens forms the interface with the host, gives fungal cells their shape, and also provides protection against stresses. The cell wall is a dynamic organelle with great adaptive flexibility that allows remodeling, morphogenesis, and changes in its components in response to the environment. It is mainly composed of the inner polysaccharide rich layer (chitin, and β-glucan) and the outer protein coat (mannoproteins). The highly glycosylated protein coat mediates interactions between C. albicans cells and their environment, including reprograming of wall architecture in response to several conditions, such as carbon source, pH, high temperature, and morphogenesis. The mannoproteins are also associated with C. albicans adherence, drug resistance, and virulence. Vitally, the mannoproteins contribute to cell wall construction and especially cell wall remodeling when cells encounter physical and chemical stresses. This review describes the interconnected cell wall integrity (CWI) and stress-activated pathways (e.g., Hog1, Cek1, and Mkc1 mediated pathways) that regulates cell wall remodeling and the expression of some of the mannoproteins in C. albicans and other species. The mannoproteins of the surface coat is of great importance to pathogen survival, growth, and virulence, thus understanding their structure and function as well as regulatory mechanisms can pave the way for better management of candidiasis.

Rapid Proliferation Compensates for Defective Filamentation in Candida albicans Pathogenesis

A recent study (Dunker et al.) has shown that a Candida albicans mutant, defective for filamentation, is fully virulent due to rapid cellular proliferation in host tissues. These findings challenge the current paradigm in C. albicans pathogenesis and suggest that defects in one virulence property can be compensated for by enhancements in another.

Rapid proliferation due to better metabolic adaptation results in full virulence of a filament-deficient Candida albicans strain

The ability of the fungal pathogen Candida albicans to undergo a yeast-to-hypha transition is believed to be a key virulence factor, as filaments mediate tissue damage. Here, we show that virulence is not necessarily reduced in filament-deficient strains, and the results depend on the infection model used. We generate a filament-deficient strain by deletion or repression of EED1 (known to be required for maintenance of hyphal growth). Consistent with previous studies, the strain is attenuated in damaging epithelial cells and macrophages in vitro and in a mouse model of intraperitoneal infection. However, in a mouse model of systemic infection, the strain is as virulent as the wild type when mice are challenged with intermediate infectious doses, and even more virulent when using low infectious doses. Retained virulence is associated with rapid yeast proliferation, likely the result of metabolic adaptation and improved fitness, leading to high organ fungal loads. Analyses of cytokine responses in vitro and in vivo, as well as systemic infections in immunosuppressed mice, suggest that differences in immunopathology contribute to some extent to retained virulence of the filament-deficient mutant. Our findings challenge the long-standing hypothesis that hyphae are essential for pathogenesis of systemic candidiasis by C. albicans.

The inhibitory activity of 5-aminolevulinic acid photodynamic therapy (ALA-PDT) on Candida albicans biofilms

BACKGROUND

Biofilm-associated Candida albicans (C. albicans) infections are hard to cure due to their high levels of resistance to antifungal agents. Photodynamic therapy (PDT) is a promising approach for controlling infections caused by C. albicans. This study was designed to explore the inhibitory activity of PDT using 5-aminolevulinic acid (ALA) as photosensitizer against C. albicans biofilms.

METHODS

C. albicans cell suspensions were incubated for 48 h to form mature biofilms. ALA solution was diluted to 15 mM and incubated with C. albicans biofilms for 5 h before irradiated by red light semiconductor laser under the light intensity of 300 J/cm² and fluence rate of 100 mW/cm² for 50 min. The inhibitory activity was evaluated from subcellular level, molecular level and transcriptional level using transmission electron microscopy (TEM) observation, flow cytometry analysis and quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) assays, respectively.

RESULTS

From subcellular level, the degraded content of the cytoplasm, nuclear condensation and mitochondrial swelling were observed after ALA-PDT. From molecular level, ALA-PDT resulted in 19.4 % cell apoptosis. From transcriptional level, ALA-PDT significantly reduced the mRNA expressions of hyphae-specific genes (HWP1 and ALS3) and long-term biofilm maintenance genes (UME6 and HGC1), whereas ALA or red light alone had no significant effect.

CONCLUSIONS

The inhibitory activity indicated that ALA-PDT may have the potential to serve as an antifungal strategy in eliminatingC. albicans biofilms.

Adaptive evolution of Moniliophthora PR-1 proteins towards its pathogenic lifestyle

BACKGROUND

Plant pathogenesis related-1 (PR-1) proteins belong to the CAP superfamily and have been characterized as markers of induced defense against pathogens. Moniliophthora perniciosa and Moniliophthora roreri are hemibiotrophic fungi that respectively cause the witches' broom disease and frosty pod rot in Theobroma cacao. Interestingly, a large number of plant PR-1-like genes are present in the genomes of both species and many are up-regulated during the biotrophic interaction. In this study, we investigated the evolution of PR-1 proteins from 22 genomes of Moniliophthora isolates and 16 other Agaricales species, performing genomic investigation, phylogenetic reconstruction, positive selection search and gene expression analysis.

RESULTS

Phylogenetic analysis revealed conserved PR-1 genes (PR-1a, b, d, j), shared by many Agaricales saprotrophic species, that have diversified in new PR-1 genes putatively related to pathogenicity in Moniliophthora (PR-1f, g, h, i), as well as in recent specialization cases within M. perniciosa biotypes (PR-1c, k, l) and M. roreri (PR-1n). PR-1 families in Moniliophthora with higher evolutionary rates exhibit induced expression in the biotrophic interaction and positive selection clues, supporting the hypothesis that these proteins accumulated adaptive changes in response to host-pathogen arms race. Furthermore, although previous work showed that MpPR-1 can detoxify plant antifungal compounds in yeast, we found that in the presence of eugenol M. perniciosa differentially expresses only MpPR-1e, k, d, of which two are not linked to pathogenicity, suggesting that detoxification might not be the main function of most MpPR-1.

CONCLUSIONS

Based on analyses of genomic and expression data, we provided evidence that the evolution of PR-1 in Moniliophthora was adaptive and potentially related to the emergence of the parasitic lifestyle in this genus. Additionally, we also discuss how fungal PR-1 proteins could have adapted from basal conserved functions to possible roles in fungal pathogenesis.

What do we know about the biology of the emerging fungal pathogen of humans Candida auris?

Candida auris is a worrisome fungal pathogen of humans which emerged merely about a decade ago. Ever since then the scientific community worked hard to understand clinically relevant traits, such as virulence factors, antifungal resistance mechanisms, and its ability to adhere to human skin and medical devices. Whole-genome sequencing of clinical isolates and epidemiological studies outlining the path of nosocomial outbreaks have been the focus of research into this pathogenic and multidrug-resistant yeast since its first description in 2009. More recently, work was started by several laboratories to explore the biology of C. auris. Here, we review the insights of studies characterizing the mechanisms underpinning antifungal drug resistance, biofilm formation, morphogenetic switching, cell aggregation, virulence, and pathogenicity of C. auris. We conclude that, although some progress has been made, there is still a long journey ahead of us, before we fully understand this novel pathogen. Critically important is the development of molecular tools for C. auris to make this fungus genetically tractable and traceable. This will allow an in-depth molecular dissection of the life cycle of C. auris, of its characteristics while interacting with the human host, and the mechanisms it employs to avoid being killed by antifungals and the immune system.

Pathways of heme utilization in fungi

Iron acquisition is challenging in most environments. As an alternative to elemental iron, organisms can take up iron-protoporphyrin IX, or heme. Heme can be found in decaying organic matter and is particularly prevalent in animal hosts. Fungi have evolved at least three distinct endocytosis-mediated heme uptake systems, which have been studied in detail in the organisms Candida albicans, Cryptococcus neoformans and Schizosaccharomyces pombe. Here we summarize the known molecular details of these three uptake systems that enable parasitic and saprophytic fungi to take advantage of external heme as either cellular iron or heme sources.

Design and synthesis of new drugs inhibitors of Candida albicans hyphae and biofilm formation by upregulating the expression of TUP1 transcription repressor gene

Candida albicans is a common human fungal pathogen that causes disease ranging from superficial to lethal infections. C. albicans grows as budding yeast which can transform into hyphae in response to various environmental or biological stimuli. Although both forms have been associated with virulence, the hyphae form is responsible for the formation of multi-drug resistance biofilm. Here, new compounds were designed to selectively inhibit C. albicans hyphae formation without affecting human cells to afford sufficient safety. The newly designed 5-[3-substitued-4-(4-substituedbenzyloxy)-benzylidene]-2-thioxo-thiazolidin-4-one derivatives, named SR, showed very specific and effective inhibition activity against C. albicans hyphae formation. SR compounds caused hyphae inhibition activity at concentrations 10-40 fold lower than the concentration required to inhibit Candida yeast and bacterial growths. The anti-hyphae inhibition activities of SR compounds were via activation of the hyphae transcription repressor gene, TUP1. Correlation studies between the expression of TUP1 gene and the activity of SR compounds confirmed that the anti-C. albicans activities of SR compounds were via inhibition of hyphae formation. The newly designed SR compounds showed 10-40% haemolytic activity on human erythrocytes when compared to 100% haemolysis by 0.1% triton employed as positive control. Furthermore, theoretical prediction of absorption, distribution, metabolism, excretion, and toxicity (ADMET) of SR compounds confirmed their safety, efficient metabolism and possible oral bioavailability. With the minimal toxicity and significant activity of the newly-designed SR compounds, a future optimization of pharmaceutical formulation may develop a promising inhibitor of hyphal formation not only for C. albicans but also for other TUP1- dependent dimorphic fungal infections.

Pseudohyphal Growth of the Emerging Pathogen Candida auris Is Triggered by Genotoxic Stress through the S Phase Checkpoint

Candida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

The morphogenetic switching between yeast cells and filaments (true hyphae and pseudohyphae) is a key cellular feature required for full virulence in many polymorphic fungal pathogens, such as Candida albicans. In the recently emerged yeast pathogen Candida auris, occasional elongation of cells has been reported. However, environmental conditions and genetic triggers for filament formation have remained elusive. Here, we report that induction of DNA damage and perturbation of replication forks by treatment with genotoxins, such as hydroxyurea, methyl methanesulfonate, and the clinically relevant fungistatic 5-fluorocytosine, cause filamentation in C. auris. The filaments formed were characteristic of pseudohyphae and not parallel-sided true hyphae. Pseudohyphal growth is apparently signaled through the S phase checkpoint and, interestingly, is Tup1 independent in C. auris. Intriguingly, the morphogenetic switching capability is strain specific in C. auris, highlighting the heterogenous nature of the species as a whole.

IMPORTANCECandida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

Nucleotide Excision Repair Protein Rad23 Regulates Cell Virulence Independent of Rad4 in Candida albicans

Candida albicans remains a significant threat to the lives of immunocompromised people. An understanding of the virulence and infection ability of C. albicans cells in the mammalian host may help with clinical treatment and drug discovery. The DNA damage response pathway is closely related to morphology regulation and virulence, as well as the ability to survive in host cells. In this study, we checked the role of the nucleotide excision repair (NER) pathway, the key repair system that functions to remove a large variety of DNA lesions such as those caused by UV light, but whose function has not been well studied in C. albicans. We found that Rad23, but not Rad4, plays a role in virulence that appears independent of the function of the NER pathway. Our research revealed that the NER pathway represented by Rad4/Rad23 may not play a direct role in virulence but that Rad23 may play a unique role in regulating the transcription of virulence genes that may contribute to the virulence of C. albicans.

In the pathogenic yeast Candida albicans, the DNA damage response contributes to pathogenicity by regulating cell morphology transitions and maintaining survival in response to DNA damage induced by reactive oxygen species (ROS) in host cells. However, the function of nucleotide excision repair (NER) in C. albicans has not been extensively investigated. To better understand the DNA damage response and its role in virulence, we studied the function of the Rad23 nucleotide excision repair protein in detail. The RAD23 deletion strain and overexpression strain both exhibit UV sensitivity, confirming the critical role of RAD23 in the nucleotide excision repair pathway. Genetic interaction assays revealed that the role of RAD23 in the UV response relies on RAD4 but is independent of RAD53, MMS22, and RAD18. RAD4 and RAD23 have similar roles in regulating cell morphogenesis and biofilm formation; however, only RAD23, but not RAD4, plays a negative role in virulence regulation in a mouse model. We found that the RAD23 deletion strain showed decreased survival in a Candida-macrophage interaction assay. Transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) data further revealed that RAD23, but not RAD4, regulates the transcription of a virulence factor, SUN41, suggesting a unique role of RAD23 in virulence regulation. Taking these observations together, our work reveals that the RAD23-related nucleotide excision pathway plays a critical role in the UV response but may not play a direct role in virulence. The virulence-related role of RAD23 may rely on the regulation of several virulence factors, which may give us further understanding about the linkage between DNA damage repair and virulence regulation in C. albicans.

IMPORTANCECandida albicans remains a significant threat to the lives of immunocompromised people. An understanding of the virulence and infection ability of C. albicans cells in the mammalian host may help with clinical treatment and drug discovery. The DNA damage response pathway is closely related to morphology regulation and virulence, as well as the ability to survive in host cells. In this study, we checked the role of the nucleotide excision repair (NER) pathway, the key repair system that functions to remove a large variety of DNA lesions such as those caused by UV light, but whose function has not been well studied in C. albicans. We found that Rad23, but not Rad4, plays a role in virulence that appears independent of the function of the NER pathway. Our research revealed that the NER pathway represented by Rad4/Rad23 may not play a direct role in virulence but that Rad23 may play a unique role in regulating the transcription of virulence genes that may contribute to the virulence of C. albicans.

A Re-Evaluation of the Relationship between Morphology and Pathogenicity in Candida Species

Many pathogenic Candida species possess the ability to undergo a reversible morphological transition from yeast to filamentous cells. In Candida albicans, the most frequently isolated human fungal pathogen, multiple lines of evidence strongly suggest that this transition is associated with virulence and pathogenicity. While it has generally been assumed that non-albicans Candida species (NACS) are less pathogenic than C. albicans, in part, because they do not filament as well, definitive evidence is lacking. Interestingly, however, a recent study suggests that filamentation of NACS is associated with reduced, rather than increased, pathogenicity. These findings, in turn, challenge conventional views and suggest that there are fundamental evolutionary differences in the morphology–pathogenicity relationship in C. albicans vs. NACS. The findings also raise many new and intriguing questions and open new avenues for future research, which are discussed.

Filamentation Is Associated with Reduced Pathogenicity of Multiple Non-albicans Candida Species

Many immunocompromised individuals, including HIV/AIDS and cancer patients, are susceptible to candidiasis. About half of all cases are caused by the major fungal pathogen Candida albicans, whereas the remainder are due to less pathogenic non-albicans Candida species (NACS). Generation of filamentous cells represents a major virulence property of C. albicans, and the NACS are believed to be less pathogenic, in part, because they do not filament as well as C. albicans does. To address this question, we determined the pathogenicity of two NACS strains that have been genetically engineered to promote filamentation during infection. Surprisingly, these strains showed a dramatic reduction in pathogenicity. The host immune response did not appear to be affected. However, unlike C. albicans, filamentation of the NACS was associated with downregulation of several genes important for pathogenicity processes. Our results suggest that there are fundamental evolutionary differences in the relationship between filamentation and pathogenesis in NACS compared to C. albicans.

Candidiasis affects a wide variety of immunocompromised and medically compromised patients. Candida albicans, a major human fungal pathogen, accounts for about 50% of all cases, while the remainder are caused by the less pathogenic non-albicans Candida species (NACS). These species are believed to be less pathogenic, in part, because they do not filament as readily or robustly as C. albicans, although definitive evidence is lacking. To address this question, we used strains for two NACS, Candida tropicalis and Candida parapsilosis, which were genetically engineered to constitutively express the key transcriptional regulator UME6 and drive strong filamentation both in vitro and during infection in vivo. Unexpectedly, both strains showed a dramatic reduction in organ fungal burden in response to UME6 expression. Consistent with these findings, we observed that a C. tropicalis hyperfilamentous mutant was significantly reduced and a filamentation-defective mutant was slightly increased for organ fungal burden. Comprehensive immune profiling generally did not reveal any significant changes in the host response to UME6 expression in the NACS that could explain the increased clearance of infection. Interestingly, whole-genome transcriptional profiling indicated that while genes important for filamentation were induced by UME6 expression in C. tropicalis and C. parapsilosis, other genes involved in a variety of processes important for pathogenesis were strongly downregulated. These findings suggest that there are fundamental evolutionary differences in the relationship between morphology and pathogenicity among Candida species and that NACS do not necessarily possess the same virulence properties as C. albicans.

IMPORTANCE Many immunocompromised individuals, including HIV/AIDS and cancer patients, are susceptible to candidiasis. About half of all cases are caused by the major fungal pathogen Candida albicans, whereas the remainder are due to less pathogenic non-albicans Candida species (NACS). Generation of filamentous cells represents a major virulence property of C. albicans, and the NACS are believed to be less pathogenic, in part, because they do not filament as well as C. albicans does. To address this question, we determined the pathogenicity of two NACS strains that have been genetically engineered to promote filamentation during infection. Surprisingly, these strains showed a dramatic reduction in pathogenicity. The host immune response did not appear to be affected. However, unlike C. albicans, filamentation of the NACS was associated with downregulation of several genes important for pathogenicity processes. Our results suggest that there are fundamental evolutionary differences in the relationship between filamentation and pathogenesis in NACS compared to C. albicans.

Regulatory mechanisms controlling morphology and pathogenesis in Candida albicans

Candida albicans, a major human fungal pathogen, can cause a wide variety of both mucosal and systemic infections, particularly in immunocompromised individuals. Multiple lines of evidence suggest a strong association between virulence and the ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous cells in response to host environmental cues. Most previous studies on mechanisms important for controlling the C. albicans morphological transition have focused on signaling pathways and sequence-specific transcription factors. However, in recent years a variety of novel mechanisms have been reported, including those involving global transcriptional regulation and translational control. A large-scale functional genomics screen has also revealed new roles in filamentation for certain key biosynthesis pathways. This review article will highlight several of these exciting recent discoveries and discuss how they are relevant to the development of novel antifungal strategies. Ultimately, components of mechanisms that control C. albicans morphogenesis and pathogenicity could potentially serve as viable antifungal targets.

Regulation of Candida albicans Hyphal Morphogenesis by Endogenous Signals

Candida albicans is a human commensal fungus that is able to assume several morphologies, including yeast, hyphal, and pseudohyphal. Under a range of conditions, C. albicans performs a regulated switch to the filamentous morphology, characterized by the emergence of a germ tube from the yeast cell, followed by a mold-like growth of branching hyphae. This transition from yeast to hyphal growth has attracted particular attention, as it has been linked to the virulence of C. albicans as an opportunistic human pathogen. Signal transduction pathways that mediate the induction of the hyphal transcription program upon the imposition of external stimuli have been extensively investigated. However, the hyphal morphogenesis transcription program can also be induced by internal cellular signals, such as inhibition of cell cycle progression, and conversely, the inhibition of hyphal extension can repress hyphal-specific gene expression, suggesting that endogenous cellular signals are able to modulate hyphal gene expression as well. Here we review recent developments in the regulation of the hyphal morphogenesis of C. albicans, with emphasis on endogenous morphogenetic signals.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

Genetic Analysis of Candida auris Implicates Hsp90 in Morphogenesis and Azole Tolerance and Cdr1 in Azole Resistance

Candida auris is an emerging fungal pathogen and a serious global health threat as the majority of clinical isolates display elevated resistance to currently available antifungal drugs. Despite the increased prevalence of C. auris infections, the mechanisms governing drug resistance remain largely elusive. In diverse fungi, the evolution of drug resistance is enabled by the essential molecular chaperone Hsp90, which stabilizes key regulators of cellular responses to drug-induced stress. Hsp90 also orchestrates temperature-dependent morphogenesis in Candida albicans, a key virulence trait. However, the role of Hsp90 in the pathobiology of C. auris remains unknown. In order to study regulatory functions of Hsp90 in C. auris, we placed HSP90 under the control of a doxycycline-repressible promoter to enable transcriptional repression. We found that Hsp90 is essential for growth in C. auris and that it enables tolerance of clinical isolates with respect to the azoles, which inhibit biosynthesis of the membrane sterol ergosterol. High-level azole resistance was independent of Hsp90 but dependent on the ABC transporter CDR1, deletion of which resulted in abrogated resistance. Strikingly, we discovered that C. auris undergoes a morphogenetic transition from yeast to filamentous growth in response to HSP90 depletion or cell cycle arrest but not in response to other cues that induce C. albicans filamentation. Finally, we observed that this developmental transition is associated with global transcriptional changes, including the induction of cell wall-related genes. Overall, this report provides a novel insight into mechanisms of drug tolerance and resistance in C. auris and describes a developmental transition in response to perturbation of a core regulator of protein homeostasis.IMPORTANCE Fungal pathogens pose a serious threat to public health. Candida auris is an emerging fungal pathogen that is often resistant to commonly used antifungal drugs. However, the mechanisms governing drug resistance and virulence in this organism remain largely unexplored. In this study, we adapted a conditional expression system to modulate the transcription of an essential gene, HSP90, which regulates antifungal resistance and virulence in diverse fungal pathogens. We showed that Hsp90 is essential for growth in C. auris and is important for tolerance of the clinically important azole antifungals, which block ergosterol biosynthesis. Further, we established that the Cdr1 efflux transporter regulates azole resistance. Finally, we discovered that C. auris transitions from yeast to filamentous growth in response to Hsp90 inhibition, accompanied by global transcriptional remodeling. Overall, this work provides a novel insight into mechanisms regulating azole resistance in C. auris and uncovers a distinct developmental program regulated by Hsp90.

Proteomic Analysis of the Responses of Candida albicans during Infection of Galleria mellonella Larvae

This study assessed the development of disseminated candidiasis within Galleria mellonella larvae and characterized the proteomic responses of Candida albicans to larval hemolymph. Infection of larvae with an inoculum of 1 × 10⁶ yeast cells reduced larval viability 24 (53.33 ± 3.33%), 48 (33.33 ± 3.33%) and 72 (6.66 ± 3.33%) h post infection. C. albicans infection quickly disseminated from the site of inoculation and the presence of yeast and hyphal forms were found in nodules extracted from infected larvae at 6 and 24 h. A range of proteins secreted during infection of G. mellonella by C. albicans were detected in larval hemolymph and these were enriched for biological processes such as interaction with host and pathogenesis. The candicidal activity of hemolymph after immediate incubation of yeast cells resulted in a decrease in yeast cell viability (0.23 ± 0.03 × 10⁶ yeast cells/mL), p < 0.05) as compared to control (0.99 ± 0.01 × 10⁶ yeast cells/mL). C. albicans responded to incubation in hemolymph ex vivo by the induction of an oxidative stress response, a decrease in proteins associated with protein synthesis and an increase in glycolytic proteins. The results presented here indicate that C. albicans can overcome the fungicidal activity of hemolymph by altering protein synthesis and cellular respiration, and commence invasion and dissemination throughout the host.

(1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives suppress Candida albicans virulence by interfering with morphological transition

Clinical treatment of Candida albicans infections has become more difficult due to the limited development of antifungal agents and the rapid emergence of drug resistance. In this study, we demonstrate the synthesis of a series of piperazine derivatives and the evaluation of their inhibitory activity against C. albicans virulence. Thirty-four (1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives, including 25 new compounds, were synthesized and assessed for their efficacy against the physiology and pathogenesis of C. albicans. Several compounds strongly inhibited the morphological transition and virulence of C. albicans cells, although they did not influence the growth rate of the fungal pathogen. A leading novel compound, (1-(4-ethoxyphenyl)-4-(1-biphenylol-2-hydroxypropyl)-piperazine), significantly attenuated C. albicans virulence by interfering with the process of hyphal development, but it showed no cytotoxicity against human cells at a micromolar level. These findings suggest that (1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives could potentially be developed as novel therapeutic agents for the clinical treatment of C. albicans infections by interfering with morphological transition and virulence.

Localization and functional characterization of the pathogenesis-related proteins Rbe1p and Rbt4p in Candida albicans

Members of the Cysteine-rich secretory protein, Antigen 5 and Pathogenesis-related 1 (CAP) protein superfamily are important virulence factors in fungi but remain poorly characterized on molecular level. Here, we investigate the cellular localization and molecular function of Rbe1p and Rbt4p, two CAP family members from the human pathogen Candida albicans. We unexpectedly found that Rbe1p localizes to budding sites of yeast cells in a disulfide bond-dependent manner. Furthermore, we show that Rbe1p and Rbt4p bind free cholesterol in vitro and export cholesteryl acetate in vivo. These findings suggest a previously undescribed role for Rbe1p in cell wall-associated processes and a possible connection between the virulence attributes of fungal CAP proteins and sterol binding.

Release of transcriptional repression through the HCR promoter region confers uniform expression of HWP1 on surfaces of Candida albicans germ tubes

The mechanisms that fungi use to co-regulate subsets of genes specifically associated with morphogenic states represent a basic unsolved problem in fungal biology. Candida albicans is an important model of fungal differentiation both for rapid interconversion between yeast and hyphal growth forms and for white/opaque switching mechanisms. The Sundstrom lab is interested in mechanisms regulating hypha-specific expression of adhesin genes that are critical for C. albicans hyphal growth phenotypes and pathogenicity. Early studies on hypha-specific genes such as HWP1 and ALS3 reported 5’ intergenic regions that are larger than those typically found in an average promoter and are associated with hypha-specific expression. In the case of HWP1, activation and repression involves a 368 bp region, denoted the HWP1 control region (HCR), located 1410 bp upstream of its transcription start site. In previous work we showed that HCR confers developmental regulation to a heterologous ENO1 promoter, indicating that HCR by itself contains sufficient information to couple gene expression to morphology. Here we show that the activation and repression mediated by HCR are localized to distinct HCR regions that are targeted by the transcription factors Nrg1p and Efg1p. The finding that Efg1p mediates both repression via HCR under yeast morphological conditions and activation conditions positions Efg1p as playing a central role in coupling HWP1 expression to morphogenesis through the HCR region. These localization studies revealed that the 120 terminal base pairs of HCR confer Efg1p-dependent repressive activity in addition to the Nrg1p repressive activity mediated by DNA upstream of this subregion. The 120 terminal base pair subregion of HCR also contained an initiation site for an HWP1 transcript that is specific to yeast growth conditions (HCR-Y) and may function in the repression of downstream DNA. The detection of an HWP1 mRNA isoform specific to hyphal growth conditions (HWP1-H) showed that morphology-specific mRNA isoforms occur under both yeast and hyphal growth conditions. Similar results were found at the ALS3 locus. Taken together, these results, suggest that the long 5’ intergenic regions upstream of hypha-specific genes function in generating mRNA isoforms that are important for morphology-specific gene expression. Additional complexity in the HWP1 promoter involving HCR-independent activation was discovered by creating a strain lacking HCR that exhibited variable HWP1 expression during hyphal growth conditions. These results show that while HCR is important for ensuring uniform HWP1 expression in cell populations, HCR independent expression also exists. Overall, these results elucidate HCR-dependent mechanisms for coupling HWP1-dependent gene expression to morphology uniformly in cell populations and prompt the hypothesis that mRNA isoforms may play a role in coupling gene expression to morphology in C. albicans.

Analysis of gene expression in filamentous cells of Candida albicans grown on agar plates

Candida albicans (C. albicans) is a commensal organism of the human gastrointestinal and genitourinary tracts. C. albicans is also a major human pathogen, causing disease ranging from cutaneous infections to lethal systemic disease. The ability of this fungus to switch between yeast and filamentous forms of growth has long been linked to its pathogenesis. Filamentation can be induced by a variety of distinct environmental cues and can occur in either liquid or solid media. While some evidence suggests that there are differences between filamentation in solid and liquid media, gene expression analysis of filamentation in C. albicans has focused strictly on cells grown in liquid media. We have developed a method for analyzing gene expression of filamentous cells grown on solid induction media at early stages of filamentation, establishing cell plating densities, ideal collection times, and collection techniques. We have also demonstrated the utility of the approach not only in qRT-PCR assays, but high-throughput RNAseq assays as well. These assays will allow for comparison studies of C. albicans filamentation initiation in solid and liquid media.

Investigation of Candida parapsilosis virulence regulatory factors during host-pathogen interaction

Invasive candidiasis is among the most life-threatening infections in patients in intensive care units. Although Candida albicans is the leading cause of candidaemia, the incidence of Candida parapsilosis infections is also rising, particularly among the neonates. Due to differences in their biology, these species employ different antifungal resistance and virulence mechanisms and also induce dissimilar immune responses. Previously, it has been suggested that core virulence effecting transcription regulators could be attractive ligands for future antifungal drugs. Although the virulence regulatory mechanisms of C. albicans are well studied, less is known about similar mechanisms in C. parapsilosis. In order to search for potential targets for future antifungal drugs against this species, we analyzed the fungal transcriptome during host-pathogen interaction using an in vitro infection model. Selected genes with high expression levels were further examined through their respective null mutant strains, under conditions that mimic the host environment or influence pathogenicity. As a result, we identified several mutants with relevant pathogenicity affecting phenotypes. During the study we highlight three potentially tractable signaling regulators that influence C. parapsilosis pathogenicity in distinct mechanisms. During infection, CPAR2_100540 is responsible for nutrient acquisition, CPAR2_200390 for cell wall assembly and morphology switching and CPAR2_303700 for fungal viability.

Wor1 establishes opaque cell fate through inhibition of the general co-repressor Tup1 in Candida albicans

The pathogenic fungus Candida albicans can undergo phenotypic switching between two heritable states: white and opaque. This phenotypic plasticity facilitates its colonization in distinct host niches. The master regulator WOR1 is exclusively expressed in opaque phase cells. Positive feedback regulation by Wor1 on the WOR1 promoter is essential for opaque formation, however the underlying mechanism of how Wor1 functions is not clear. Here, we use tandem affinity purification coupled with mass spectrometry to identify Wor1-interacting proteins. Tup1 and its associated complex proteins are found as the major factors associated with Wor1. Tup1 occupies the same regions of the WOR1 promoter as Wor1 preferentially in opaque cells. Loss of Tup1 is sufficient to induce the opaque phase, even in the absence of Wor1. This is the first such report of a bypass of Wor1 in opaque formation. These genetic analyses suggest that Tup1 is a key repressor of the opaque state, and Wor1 functions via alleviating Tup1 repression at the WOR1 promoter. Opaque cells convert to white en masse at 37°C. We show that this conversion occurs only in the presence of glycolytic carbon sources. The opaque state is stabilized when cells are cultured on non-glycolytic carbon sources, even in a MTLa/α background. We further show that temperature and carbon source affect opaque stability by altering the levels of Wor1 and Tup1 at the WOR1 promoter. We propose that Wor1 and Tup1 form the core regulatory circuit controlling the opaque transcriptional program. This model provides molecular insights on how C. albicans adapts to different host signals to undergo phenotypic switching for colonization in distinct host niches.

Ras signalling in pathogenic yeasts

The small GTPase Ras acts as a master regulator of growth, stress response and cell death in eukaryotic cells. The control of Ras activity is fundamental, as highlighted by the oncogenic properties of constitutive forms of Ras proteins. Ras also plays a crucial role in the pathogenicity of fungal pathogens where it has been found to regulate a number of adaptions required for virulence. The importance of Ras in fungal disease raises the possibility that it may provide a useful target for the development of new treatments at a time when resistance to available antifungals is increasing. New findings suggest that important regulatory sequences found within fungal Ras proteins that are not conserved may prove useful in the development of new antifungals. Here we review the roles of Ras protein function and signalling in the major human yeast pathogens Candida albicans and Cryptococcus neoformans and discuss the potential for targeting Ras as a novel approach to anti-fungal therapy.

Iron acquisition in fungal pathogens of humans

The devastating infections that fungal pathogens cause in humans are underappreciated relative to viral, bacterial and parasitic diseases. In recent years, the contributions to virulence of reductive iron uptake, siderophore-mediated uptake and heme acquisition have been identified in the best studied and most life-threatening fungal pathogens: Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. In particular, exciting new work illustrates the importance of iron acquisition from heme and hemoglobin in the virulence of pathogenic yeasts. However, the challenge of establishing how these fungi gain access to hemoglobin in blood and to other sources of heme remains to be fully addressed. Recent studies are also expanding our knowledge of iron uptake in less-well studied fungal pathogens, including dimorphic fungi where new information reveals an integration of iron acquisition with morphogenesis and cell-surface properties for adhesion to host cells. Overall, the accumulating information provides opportunities to exploit iron acquisition for antifungal therapy, and new work highlights the development of specific inhibitors of siderophore biosynthesis and metal chelators for therapeutic use alone or in conjunction with existing antifungal drugs. It is clear that iron-related therapies will need to be customized for specific diseases because the emerging view is that fungal pathogens use different combinations of strategies for iron acquisition in the varied niches of vertebrate hosts.

Proteus vulgaris and Proteus mirabilis Decrease Candida albicans Biofilm Formation by Suppressing Morphological Transition to Its Hyphal Form

PURPOSE

Candida albicans (C. albicans) and Proteus species are causative agents in a variety of opportunistic nosocomial infections, and their ability to form biofilms is known to be a virulence factor. In this study, the influence of co-cultivation with Proteus vulgaris (P. vulgaris) and Proteus mirabilis (P. mirabilis) on C. albicans biofilm formation and its underlying mechanisms were examined.

MATERIALS AND METHODS

XTT reduction assays were adopted to measure biofilm formation, and viable colony counts were performed to quantify yeast growth. Real-time reverse transcriptase polymerase chain reaction was used to evaluate the expression of yeast-specific genes (rhd1 and rbe1), filament formation inhibiting genes (tup1 and nrg1), and hyphae-related genes (als3, ece1, hwp1, and sap5).

RESULTS

Candida biofilm formation was markedly inhibited by treatment with either living or heat-killed P. vulgaris and P. mirabilis. Proteus-cultured supernatant also inhibited Candida biofilm formation. Likewise, treatment with live P. vulgaris or P. mirabilis or with Proteus-cultured supernatant decreased expression of hyphae-related C. albicans genes, while the expression of yeast-specific genes and the filament formation inhibiting genes of C. albicans were increased. Heat-killed P. vulgaris and P. mirabilis treatment, however, did not affect the expression of C. albicans morphology-related genes.

CONCLUSION

These results suggest that secretory products from P. vulgaris and P. mirabilis regulate the expression of genes related to morphologic changes in C. albicans such that transition from the yeast form to the hyphal form can be inhibited.

Alizarin and Chrysazin Inhibit Biofilm and Hyphal Formation by Candida albicans

Candida albicans is one of the most common pathogen causes fungal infections. This opportunistic pathogen can form biofilms comprised of yeast, hyphae and pseudo hyphal elements, and the hyphal form C. albicans considered as probable virulence factor. We investigated the antibiofilm activities of 13 quinones and anthraquinones related compounds against C. albicans biofilms by using crystal violet and 2,3-bis (2-Methoxy-4-Nitro-5-Sulfo-phenyl)-2H-Tetrazolium-5-Carboxanilide (XTT) reduction assays to assess inhibitions of biofilm growth. Morphological changes in biofilms and biofilm thicknesses were determined by scanning electron microscopy and confocal laser scanning microscopy, respectively. It was found alizarin (1,2-dihydroxyanthraquinone) and chrysazin (1,8-dihydroxyanthraquinone) suppressed C. albicans biofilm formation. Interestingly, alizarin and chrysazin at only 2 μg/ml effectively inhibited hyphal formation and prolonged the survival of C. albicans infected Caenorhabditis elegans, thus showing a distinct antivirulent potential. A structural activity relationship study of alizarin and 6 other anthraquinones showed the presence of a hydroxyl group at C-1 position which is important for antibiofilm and antifilamentation activities. Transcriptomic analyses revealed that alizarin downregulated the expression of several hypha-specific and biofilm related genes (ALS3, ECE1, ECE2, and RBT1). Furthermore, unlike the commercial antifungal drug fluconazole, no acute toxic effect was observed when uninfected nematodes were exposed to alizarin at concentrations up to 1 mg/ml. The results of this study indicate alizarin suppresses the virulence of C. albicans in vivo which suggests alizarin may be considered as a potential candidate for further investigations to develop antifungal agent against fungal pathogen in vivo.

Antibiofilm and Antihyphal Activities of Cedar Leaf Essential Oil, Camphor, and Fenchone Derivatives against Candida albicans

Candida albicans can form biofilms composed of yeast, hyphal, and pseudohyphal elements, and C. albicans cells in the hyphal stage could be a virulence factor. The present study describes the chemical composition, antibiofilm, and antihyphal activities of cedar leaf essential oil (CLEO), which was found to possess remarkable antibiofilm activity against C. albicans but not to affect its planktonic cell growth. Nineteen components were identified in CLEO by gas chromatography/mass spectrometry, and phenolics were the main constituents. Of these, camphor, fenchone, fenchyl alcohol, α-thujone, and borneol significantly reduced C. albicans biofilm formation. Notably, treatments with CLEO, camphor, or fenchyl alcohol at 0.01% clearly inhibited hyphal formation, and this inhibition appeared to be largely responsible for their antibiofilm effects. Transcriptomic analyses indicated that camphor and fenchyl alcohol downregulated some hypha-specific and biofilm related genes (ECE1, ECE2, RBT1, and EED1). Furthermore, camphor and fenchyl alcohol reduced C. albicans virulence in a Caenorhabditis elegans nematode model. These results demonstrate CLEO, camphor, and fenchyl alcohol might be useful for controlling C. albicans infections.

Grf10 and Bas1 Regulate Transcription of Adenylate and One-Carbon Biosynthesis Genes and Affect Virulence in the Human Fungal Pathogen Candida albicans

Candida albicans is an opportunistic human fungal pathogen that causes superficial fungal infections and lethal systemic infections. To colonize and establish infections, C. albicans coordinates the expression of virulence and metabolic genes. Previous work showed that the homeodomain transcription factor Grf10 is required for formation of hyphae, a virulence factor. Here we report global gene expression analysis of a grf10Δ strain using a DNA microarray and identify genes for de novo adenylate biosynthesis (ADE genes), one-carbon metabolism, and a nucleoside permease (NUP). Upregulation of these genes in response to adenine limitation required both Grf10 and the myb protein Bas1, as shown by quantitative real-time PCR (qRT-PCR). Phenotypic analysis showed that both mutants exhibited growth defects when grown in the absence of adenine, and the doubling time was slower for the bas1Δ mutant. Bas1 is required for basal expression of these genes, whereas NUP expression is more dependent upon Grf10. Disruption of BAS1 led to only modest defects in hypha formation and weak attenuation of virulence in a systemic mouse model of infection, as opposed to the previously reported strong effects found in the grf10Δ mutant. Our data are consistent with a model in which Grf10 coordinates metabolic effects on nucleotide metabolism by interaction with Bas1 and indicate that AMP biosynthesis and its regulation are important for C. albicans growth and virulence. IMPORTANCECandida albicans is a commensal and a common constituent of the human microbiota; however, it can become pathogenic and cause infections in both immunocompetent and immunocompromised people. C. albicans exhibits remarkable metabolic versatility as it can colonize multiple body sites as a commensal or pathogen. Understanding how C. albicans adapts metabolically to each ecological niche is essential for developing novel therapeutic approaches. Purine metabolism has been targeted pharmaceutically in several diseases; however, the regulation of this pathway has not been fully elucidated in C. albicans. Here, we report how C. albicans controls the AMP de novo biosynthesis pathway in response to purine availability. We show that the lack of the transcription factors Grf10 and Bas1 leads to purine metabolic dysfunction, and this dysfunction affects the ability of C. albicans to establish infections.

Staurosporine Induces Filamentation in the Human Fungal Pathogen Candida albicans via Signaling through Cyr1 and Protein Kinase A

The impact of fungal pathogens on human health is devastating. One of the most pervasive fungal pathogens is Candida albicans, which kills ~40% of people suffering from bloodstream infections. Treatment of these infections is extremely difficult, as fungi are closely related to humans, and there are limited drugs that kill the fungus without host toxicity. The capacity of C. albicans to transition between yeast and filamentous forms is a key virulence trait. Thus, understanding the genetic pathways that regulate morphogenesis could provide novel therapeutic targets to treat C. albicans infections. Here, we establish the small molecule staurosporine as an inducer of filamentous growth. We unveil distinct regulatory circuitry required for staurosporine-induced filamentation that appears to be unique to this filament-inducing cue. Thus, this work highlights the fact that small molecules, such as staurosporine, can improve our understanding of the pathways required for key virulence programs, which may lead to the development of novel therapeutics.

Protein kinases are key regulators of signal transduction pathways that participate in diverse cellular processes. In fungal pathogens, kinases regulate signaling pathways that govern drug resistance, stress adaptation, and pathogenesis. The impact of kinases on the fungal regulatory circuitry has recently garnered considerable attention in the opportunistic fungal pathogen Candida albicans, which is a leading cause of human morbidity and mortality. Complex regulatory circuitry governs the C. albicans morphogenetic transition between yeast and filamentous growth, which is a key virulence trait. Here, we report that staurosporine, a promiscuous kinase inhibitor that abrogates fungal drug resistance, also influences C. albicans morphogenesis by inducing filamentation in the absence of any other inducing cue. We further establish that staurosporine exerts its effect via the adenylyl cyclase Cyr1 and the cyclic AMP (cAMP)-dependent protein kinase A (PKA). Strikingly, filamentation induced by staurosporine does not require the known upstream regulators of Cyr1, Ras1 or Pkc1, or effectors downstream of PKA, including Efg1. We further demonstrate that Cyr1 is capable of activating PKA to enable filamentation in response to staurosporine through a mechanism that does not require degradation of the transcriptional repressor Nrg1. We establish that staurosporine-induced filamentation is accompanied by a defect in septin ring formation, implicating cell cycle kinases as potential staurosporine targets underpinning this cellular response. Thus, we establish staurosporine as a chemical probe to elucidate the architecture of cellular signaling governing fungal morphogenesis and highlight the existence of novel circuitry through which the Cyr1 and PKA govern a key virulence trait.

IMPORTANCE The impact of fungal pathogens on human health is devastating. One of the most pervasive fungal pathogens is Candida albicans, which kills ~40% of people suffering from bloodstream infections. Treatment of these infections is extremely difficult, as fungi are closely related to humans, and there are limited drugs that kill the fungus without host toxicity. The capacity of C. albicans to transition between yeast and filamentous forms is a key virulence trait. Thus, understanding the genetic pathways that regulate morphogenesis could provide novel therapeutic targets to treat C. albicans infections. Here, we establish the small molecule staurosporine as an inducer of filamentous growth. We unveil distinct regulatory circuitry required for staurosporine-induced filamentation that appears to be unique to this filament-inducing cue. Thus, this work highlights the fact that small molecules, such as staurosporine, can improve our understanding of the pathways required for key virulence programs, which may lead to the development of novel therapeutics.

Nutritional Immunity and Fungal Pathogenesis: The Struggle for Micronutrients at the Host-Pathogen Interface

All living organisms require certain micronutrients such as iron, zinc, manganese and copper for cellular function and growth. For human pathogens however, the maintenance of metal ion homeostasis is particularly challenging. This is because the mammalian host actively enforces extremes of micronutrient availability on potential microbial invaders-processes collectively termed nutritional immunity. The role of iron sequestration in controlling microbial infections is well established and, more recently, the importance of other metals including zinc, manganese and copper has been recognised. In this chapter, we explore the nutritional immune mechanisms that defend the human body against fungal infections and the strategies that these important pathogens exploit to counteract nutritional immunity and thrive in the infected host.