Sequence and promoter regulation of the PCK1 gene encoding phosphoenolpyruvate carboxykinase of the fungal pathogen Candida albicans

Comparative dynamics of gene expression during in vitro and in vivo Candida albicans filamentation

Candida albicans is one of them most common causes of fungal disease in humans and is a commensal member of the human microbiome. The ability of C. albicans to cause disease is tightly correlated with its ability to undergo a morphological transition from budding yeast to a filamentous form (hyphae and pseudohyphae). This morphological transition is accompanied by the induction of a set of well characterized hyphae-associated genes and transcriptional regulators. To date, the vast majority of data regarding this process has been based on in vitro studies of filamentation using a range of inducing conditions. Recently, we developed an in vivo imaging approach that allows the direct characterization of morphological transition during mammalian infection. Here, we couple this imaging assay with in vivo expression profiling to characterize the time course of in vivo filamentation and the accompanying changes in gene expression. We also compare in vivo observations to in vitro filamentation using a medium (RPMI 1640 tissue culture medium with 10% bovine calf serum) widely used to mimic host conditions. From these data, we make the following conclusions regarding in vivo and in vitro filamentation. First, the transcriptional programs regulating filamentation are rapidly induced in vitro and in vivo. Second, the tempo of filamentation in vivo is prolonged relative to in vitro filamentation and the period of high expression of genes associated with that process is also prolonged. Third, hyphae are adapting to changing infection environments after filamentation has reached steady-state.

Candida albicans Oropharyngeal Infection Is an Exception to Iron-Based Nutritional Immunity

Candida albicans is a commensal of the human gastrointestinal tract and a common cause of human fungal disease, including mucosal infections, such as oropharyngeal candidiasis and disseminated infections of the bloodstream and deep organs. We directly compared the in vivo transcriptional profile of C. albicans during oral infection and disseminated infection of the kidney to identify niche specific features. Overall, 97 genes were differentially expressed between the 2 infection sites. Virulence-associated genes, such as hyphae-specific transcripts, were expressed similarly in the 2 sites. Genes expressed during growth in a poor carbon source (ACS1 and PCK1) were upregulated in oral tissue relative to kidney. Most strikingly, C. albicans in oral tissue shows the transcriptional hallmarks of an iron replete state while in the kidney it is in the expected iron starved state. Interestingly, C. albicans expresses genes associated with a low zinc environment in both niches. Consistent with these expression data, strains lacking transcription factors that regulate iron responsive genes (SEF1, HAP5) have no effect on virulence in a mouse model of oral candidiasis. During microbial infection, the host sequesters iron, zinc, and other metal nutrients to suppress growth of the pathogen in a process called nutritional immunity. Our results indicate that C. albicans is subject to iron and zinc nutritional immunity during disseminated infection but not to iron nutritional immunity during oral infection. IMPORTANCE Nutritional immunity is a response by which infected host tissue sequesters nutrients, such as iron, to prevent the microbe from efficiently replicating. Microbial pathogens subjected to iron nutritional immunity express specific genes to compensate for low iron availability. By comparing the gene expression profiles of the common human fungal pathogen Candida albicans in 2 infection sites, we found that C. albicans infecting the kidney has the transcriptional profile of iron starvation. By contrast, the C. albicans expression profile during oropharyngeal infection indicates the fungus is not iron starved. Two transcription factors that activate the transcriptional response to iron starvation are not required for C. albicans virulence during oral infection but are required for disseminated infection of the kidney. Thus, our results indicate that C. albicans is subject to nutritional iron immunity during disseminated infection but not during oropharyngeal infection, and highlight niche specific differences in the host-Candida albicans interaction.

Candida albicans oropharyngeal infection is an exception to iron-based nutritional immunity

Candida albicans is a commensal of the human gastrointestinal tract and one of the most causes of human fungal disease, including mucosal infections such as oropharyngeal candidiasis and disseminated infections of the bloodstream and deep organs. We directly compared the in vivo transcriptional profile of C. albicans during oral infection and disseminated infection of the kidney to identify niche specific features. Although the expression of a set of environmentally responsive genes were correlated in the two infection sites (Pearson R ² , 0.6), XXX genes were differentially expressed. Virulence associated genes such as hyphae-specific transcripts were expressed similarly in the two sites. Genes expressed during growth in a poor carbon source ( ACS1 and PCK1 ) were upregulated in oral tissue relative to kidney. Most strikingly, C. albicans in oral tissue shows the transcriptional hallmarks of an iron-replete state while in the kidney it is in the expected iron starved state. Interestingly, C. albicans expresses genes associated with a low zinc environment in both niches. Consistent with these expression data, deletion of two transcription factors that activate iron uptake genes ( SEF1 , HAP5 ) have no effect on virulence in a mouse model of oral candidiasis. During microbial infection, the host sequesters iron and other metal nutrients to suppress growth of the pathogen in a process called nutritional immunity. Our results indicate that C. albicans is subject to iron and zinc nutritional immunity during disseminated infection but is exempted from iron nutritional immunity during oral infection.

Use of the Iron-Responsive RBT5 Promoter for Regulated Expression in Candida albicans

Engineered conditional gene expression is used in appraisal of gene function and pathway relationships. For pathogens like the fungus Candida albicans, conditional expression systems are most useful if they are active in the infection environment and if they can be utilized in multiple clinical isolates. Here, we describe such a system. It employs the RBT5 promoter and can be implemented with a few PCRs. We validated the system with RBT5 promoter fusions to two genes that promote filamentation and polarized growth, UME6 and HGC1, and with efg1Δ/Δ mutants, which are defective in an activator of filamentous growth. An RBT5 promoter fusion to either gene enabled filamentous growth of an efg1Δ/Δ mutant of strain SC5314 in iron-limited media, including RPMI with serum and yeast extract-peptone-dextrose with bathophenanthrolinedisulfonic acid. The RBT5-UME6 fusion promoted filamentation of efg1Δ/Δ mutants in RPMI with serum of four other clinical C. albicans isolates as well. In a mouse model of disseminated candidiasis, the RBT5-UME6 fusion promoted filamentation of the SC5314 efg1Δ/Δ mutant in kidney tissue, an indication that the RBT5 promoter is active in the iron-limited host environment. The RBT5 promoter expands the conditional expression toolkit for C. albicans genetics. IMPORTANCE Genetic strategies have been vital for mechanistic analysis of biological processes. Here, we describe a genetic tool for the fungal pathogen Candida 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.

The requirement of phosphoenolpyruvate carboxykinase 1 for angiogenesis in vitro and in vivo

We here examined the potential biological function of phosphoenolpyruvate carboxykinase 1 (PCK1) in angiogenesis. shRNA- or CRISPR-Cas9-induced PCK1 depletion potently inhibited endothelial cell proliferation, migration, sprouting, and tube formation, whereas ectopic PCK1 overexpression exerted opposite activity. In HUVECs, Gα_i3 expression and Akt activation were decreased following PCK1 depletion, but were augmented by ectopic PCK1 overexpression. In vivo, retinal expression of PCK1 gradually increased from postnatal day 1 (P1) to P5. The intravitreous injection of endothelial-specific PCK1 shRNA adenovirus at P1 potently inhibited the radial extension of vascular plexus at P5. Conditional endothelial knockdown of PCK1 in adult mouse retina increased vascular leakage and the number of acellular capillaries while decreasing the number of RGCs in murine retinas. In diabetic retinopathy patients, PCK1 mRNA and protein levels were up-regulated in retinal tissues. Together, PCK1 is essential for angiogenesis possibly by mediating Gα_i3 expression and Akt activation.

Overexpression approaches to advance understanding of Candida albicans

Candida albicans is an opportunistic fungal pathogen that is responsible for infections linked to high mortality. Loss‐of‐function approaches, taking advantage of gene knockouts or inducible down‐regulation, have been successfully used in this species in order to understand gene function. However, overexpression of a gene provides an alternative, powerful tool to elucidate gene function and identify novel phenotypes. Notably, overexpression can identify pathway components that might remain undetected using loss‐of‐function approaches. Several repressible or inducible promoters have been developed which allow to shut off or turn on the expression of a gene in C. albicans upon growth in the presence of a repressor or inducer. In this review, we summarize recent overexpression approaches used to study different aspects of C. albicans biology, including morphogenesis, biofilm formation, drug tolerance, and commensalism.

Metabolic, structural, and proteomic changes in Candida albicans cells induced by the protein-carbohydrate fraction of Dendrobaena veneta coelomic fluid

The isolated protein-polysaccharide fraction (AAF) from the coelomic fluid of Dendrobaena veneta earthworm shows effective activity against Candida albicans yeast. Fungal cells of the clinical strain after incubation with the active fraction were characterized by disturbed cell division and different morphological forms due to the inability to separate the cells from each other. Staining of the cells with acridine orange revealed a change in the pH of the AAF-treated cells. It was observed that, after the AAF treatment, the mitochondrial DNA migrated towards the nuclear DNA, whereupon both merged into a single nuclear structure, which preceded the apoptotic process. Cells with a large nucleus were imaged with the scanning electron cryomicroscopy (Cryo-SEM) technique, while enlarged mitochondria and the degeneration of cell structures were shown by transmission electron microscopy (TEM). The loss of the correct cell shape and cell wall integrity was visualized by both the TEM and SEM techniques. Mass spectrometry and relative quantitative SWATH MS analysis were used to determine the reaction of the C. albicans proteome to the components of the AAF fraction. AAF was observed to influence the expression of mitochondrial and oxidative stress proteins. The oxidative stress in C. albicans cells caused by the action of AAF was demonstrated by fluorescence microscopy, proteomic methods, and XPS spectroscopy. The secondary structure of AAF proteins was characterized by Raman spectroscopy. Analysis of the elemental composition of AAF confirmed the homogeneity of the preparation. The observed action of AAF, which targets not only the cell wall but also the mitochondria, makes the preparation a potential antifungal drug killing the cells of the C. albicans pathogen through apoptosis.

Potential Antifungal Targets Based on Glucose Metabolism Pathways of Candida albicans

In recent years, fungal infections have become a serious health problem. Candida albicans are considered as the fourth most common isolates associated with approximately 40% mortality in bloodstream infections among hospitalized patients. Due to various limitations of classical antifungals used currently, such as limited kinds of drugs, inevitable toxicities, and high price, there is an urgent need to explore new antifungal agents based on novel targets. Generally, nutrient metabolism is involved with fungal virulence, and glucose is one of the important nutrients in C. albicans. C. albicans can obtain and metabolize glucose through a variety of pathways; in theory, many enzymes in these pathways can be potential targets for developing new antifungal agents, and several studies have confirmed that compounds which interfere with alpha-glucosidase, acid trehalase, trehalose-6-phosphate synthase, class II fructose bisphosphate aldolases, and glucosamine-6-phosphate synthase in these pathways do have antifungal activities. In this review, the glucose metabolism pathways in C. albicans, the potential antifungal targets based on these pathways, and some compounds which have antifungal activities by inhibiting several enzymes in these pathways are summarized. We believe that our review will be helpful to the exploration of new antifungal drugs with novel antifungal targets.

Aurora kinase Ipl1 facilitates bilobed distribution of clustered kinetochores to ensure error-free chromosome segregation in Candida albicans

Candida albicans, an ascomycete, has an ability to switch to diverse morphological forms. While C. albicans is predominatly diploid, it can tolerate aneuploidy as a survival strategy under stress. Aurora kinase B homolog Ipl1 is a critical ploidy regulator that controls microtubule dynamics and chromosome segregation in Saccharomyces cerevisiae. In this study, we show that Ipl1 in C. albicans has a longer activation loop than that of the well-studied ascomycete S. cerevisiae. Ipl1 localizes to the kinetochores during the G1/S phase and associates with the spindle during mitosis. Ipl1 regulates cell morphogenesis and is required for cell viability. Ipl1 monitors microtubule dynamics which is mediated by separation of spindle pole bodies. While Ipl1 is dispensable for maintaining structural integrity and clustering of kinetochores in C. albicans, it is required for the maintenance of bilobed distribution of clustered kinetochores along the mitotic spindle. Depletion of Ipl1 results in erroneous kinetochore-microtubule attachments leading to aneuploidy due to which the organism can survive better in the presence of fluconazole. Taking together, we suggest that Ipl1 spatiotemporally ensures bilobed kinetochore distribution to facilitate bipolar spindle assembly crucial for ploidy maintenance in C. albicans.

Serum responsive proteome reveals correlation between oxidative phosphorylation and morphogenesis in Candida albicans ATCC10231

To understand the impact of fetal bovine serum (FBS) on metabolism and cellular architecture in addition to morphogenesis, we have identified FBS responsive proteome of Candida albicans. FBS induced 34% hyphae and 60% pseudohyphae in C. albicans at 30 °C while 98% hyphae at 37 °C. LC-MS/MS analysis revealed that 285 proteins modulated significantly in response to FBS at 30 °C and 37 °C. Out of which 152 were upregulated and 62 were downregulated at 30 °C while 18 were up and 53 were downregulated at 37 °C. Functional annotation suggests that FBS may inhibit glycolysis and fermentative pathway and enhance oxidative phosphorylation (OxPhos), TCA cycle, amino acid and fatty acid metabolism indicating a use of alternative energy source by C. albicans. OxPhos inhibition assay using sodium azide corroborated the correlation between inhibition of glycolysis and enhanced OxPhos with pseudohyphae formation. C. albicans induced hyphae in response to FBS irrespective of down regulation of Ras1,Asr1/Asr2, indicates the possible involvement of MAPK and cAMP-PKA independent pathway. The Cell wall of cells grown in presence of FBS at 30 °C was rich in mannan, Beta 1,3-glucan and chitin while membranes were rich in ergosterol compared to those grown at 37 °C.

SIGNIFICANCE OF THE STUDY

This is the first study suggesting a correlation between OxPhos and morphogenesis especially pseudohyphae formation in C. albicans. Our data also indicate that fetal bovine serum (FBS) induced morphogenesis is multifactorial and may involve MAPK and cAMP-PKA independent pathway. In addition to morphogenesis, our study provides an insight in to the modulation of metabolism and cellular architecture of C. albicans in response to FBS.

Identification of Differentially Expressed Micrornas Associate with Glucose Metabolism in Different Organs of Blunt Snout Bream (Megalobrama amblycephala)

Blunt snout bream (Megalobrama amblycephala) is a widely favored herbivorous fish species and is a frequentlyused fish model for studying the metabolism physiology. This study aimed to provide a comprehensive illustration of the mechanisms of a high-starch diet (HSD) induced lipid metabolic disorder by identifying microRNAs (miRNAs) controlled pathways in glucose and lipid metabolism in fish using high-throughput sequencing technologies. Small RNA libraries derived from intestines, livers, and brains of HSD and normal-starch diet (NSD) treated M. amblycephala were sequenced and 79, 124 and 77 differentially expressed miRNAs (DEMs) in intestines, livers, and brains of HSD treated fish were identified, respectively. Bioinformatics analyses showed that these DEMs targeted hundreds of predicted genes were enriched into metabolic pathways and biosynthetic processes, including peroxisome proliferator-activated receptor (PPAR), glycolysis/gluconeogenesis, and insulin signaling pathway. These analyses confirmed that miRNAs play crucial roles in glucose and lipid metabolism related to high wheat starch treatment. These results provide information on further investigation of a DEM-related mechanism dysregulated by a high carbohydrate diet.

Bypass of Candida albicans Filamentation/Biofilm Regulators through Diminished Expression of Protein Kinase Cak1

Biofilm formation on implanted medical devices is a major source of lethal invasive infection by Candida albicans. Filamentous growth of this fungus is tied to biofilm formation because many filamentation-associated genes are required for surface adherence. Cell cycle or cell growth defects can induce filamentation, but we have limited information about the coupling between filamentation and filamentation-associated gene expression after cell cycle/cell growth inhibition. Here we identified the CDK activating protein kinase Cak1 as a determinant of filamentation and filamentation-associated gene expression through a screen of mutations that diminish expression of protein kinase-related genes implicated in cell cycle/cell growth control. A cak1diminished expression (DX) strain displays filamentous growth and expresses filamentation-associated genes in the absence of typical inducing signals. In a wild-type background, expression of filamentation-associated genes depends upon the transcription factors Bcr1, Brg1, Efg1, Tec1, and Ume6. In the cak1 DX background, the dependence of filamentation-associated gene expression on each transcription factor is substantially relieved. The unexpected bypass of filamentation-associated gene expression activators has the functional consequence of enabling biofilm formation in the absence of Bcr1, Brg1, Tec1, Ume6, or in the absence of both Brg1 and Ume6. It also enables filamentous cell morphogenesis, though not biofilm formation, in the absence of Efg1. Because these transcription factors are known to have shared target genes, we suggest that cell cycle/cell growth limitation leads to activation of several transcription factors, thus relieving dependence on any one.

The ability of the pathogen Candida albicans to grow on surfaces as biofilms is a determinant of infection ability, because biofilms on implanted medical devices seed infections. Biofilm formation by this organism requires growth in the form of filamentous cells and the expression of filamentation-associated genes. Inhibition of cell proliferation can induce filamentous cell formation, as we find here for strains that express greatly reduced levels of the cell cycle regulator Cak1. Surprisingly, biofilm formation occurs independently of many central biofilm regulatory genes when Cak1 levels are reduced. This response to proliferation inhibition may reflect the activation of numerous biofilm regulators, thus relieving the dependence on any one regulator. The stimulation of biofilm formation by proliferation inhibition, a property of many bacterial pathogens as well, may contribute to the limited effectiveness of antimicrobials against biofilms.

Chromatin Immunoprecipitation (ChIP) Assay in Candida albicans

Chromatin immunoprecipitation (ChIP) is a widely used technique which can determine the in vivo association of a specific protein on a particular DNA locus in the genome. In this method cross-linked chromatin is sheared and immunoprecipitated with antibodies raised against a target protein of interest. The end result of this process is the enrichment of DNA fragments associated with the desired protein. Thus, interactions between proteins and genomic loci in cellular context can be determined by this technique. Here, we are describing a ChIP protocol that is optimized for Candida albicans. The protocol requires 4-5 days for completion of the assay and has been used to produce robust ChIP results for diverse proteins in this organism and its related species including Candida dubliniensis and Candida tropicalis.

Budding off: bringing functional genomics to Candida albicans

Candida species are the most prevalent human fungal pathogens, with Candida albicans being the most clinically relevant species. Candida albicans resides as a commensal of the human gastrointestinal tract but is a frequent cause of opportunistic mucosal and systemic infections. Investigation of C. albicans virulence has traditionally relied on candidate gene approaches, but recent advances in functional genomics have now facilitated global, unbiased studies of gene function. Such studies include comparative genomics (both between and within Candida species), analysis of total RNA expression, and regulation and delineation of protein-DNA interactions. Additionally, large collections of mutant strains have begun to aid systematic screening of clinically relevant phenotypes. Here, we will highlight the development of functional genomics in C. albicans and discuss the use of these approaches to addressing both commensalism and pathogenesis in this species.

Carbon source dependent promoters in yeasts

Budding yeasts are important expression hosts for the production of recombinant proteins.

The choice of the right promoter is a crucial point for efficient gene expression, as most regulations take place at the transcriptional level. A wide and constantly increasing range of inducible, derepressed and constitutive promoters have been applied for gene expression in yeasts in the past; their different behaviours were a reflection of the different needs of individual processes.

Within this review we summarize the majority of the large available set of carbon source dependent promoters for protein expression in yeasts, either induced or derepressed by the particular carbon source provided. We examined the most common derepressed promoters for Saccharomyces cerevisiae and other yeasts, and described carbon source inducible promoters and promoters induced by non-sugar carbon sources. A special focus is given to promoters that are activated as soon as glucose is depleted, since such promoters can be very effective and offer an uncomplicated and scalable cultivation procedure.

Normal adaptation of Candida albicans to the murine gastrointestinal tract requires Efg1p-dependent regulation of metabolic and host defense genes

Although gastrointestinal colonization by the opportunistic fungal pathogen Candida albicans is generally benign, severe systemic infections are thought to arise due to escape of commensal C. albicans from the gastrointestinal (GI) tract. The C. albicans transcription factor Efg1p is a major regulator of GI colonization, hyphal morphogenesis, and virulence. The goals of this study were to identify the Efg1p regulon during GI tract colonization and to compare C. albicans gene expression during colonization of different organs of the GI tract. Our results identified significant differences in gene expression between cells colonizing the cecum and ileum. During colonization, efg1(-) null mutant cells expressed higher levels of genes involved in lipid catabolism, carnitine biosynthesis, and carnitine utilization than did colonizing wild-type (WT) cells. In addition, during laboratory growth, efg1(-) null mutant cells grew to a higher density than WT cells. The efg1(-) null mutant grew in depleted medium, while WT cells could grow only if the depleted medium was supplemented with carnitine, a compound that promotes the metabolism of fatty acids. Altered gene expression and altered growth capability support the ability of efg1(-) cells to hypercolonize naïve mice. Also, Efg1p was shown to be important for transcriptional responses to the stresses present in the cecum environment. For example, during colonization, SOD5, encoding a superoxide dismutase, was highly upregulated in an Efg1p-dependent manner. Ectopic expression of SOD5 in an efg1(-) null mutant increased the fitness of the efg1(-) null mutant cells during colonization. These data show that EFG1 is an important regulator of GI colonization.

A versatile overexpression strategy in the pathogenic yeast Candida albicans: identification of regulators of morphogenesis and fitness

Candida albicans is the most frequently encountered human fungal pathogen, causing both superficial infections and life-threatening systemic diseases. Functional genomic studies performed in this organism have mainly used knock-out mutants and extensive collections of overexpression mutants are still lacking. Here, we report the development of a first generation C. albicans ORFeome, the improvement of overexpression systems and the construction of two new libraries of C. albicans strains overexpressing genes for components of signaling networks, in particular protein kinases, protein phosphatases and transcription factors. As a proof of concept, we screened these collections for genes whose overexpression impacts morphogenesis or growth rates in C. albicans. Our screens identified genes previously described for their role in these biological processes, demonstrating the functionality of our strategy, as well as genes that have not been previously associated to these processes. This article emphasizes the potential of systematic overexpression strategies to improve our knowledge of regulatory networks in C. albicans. The C. albicans plasmid and strain collections described here are available at the Fungal Genetics Stock Center. Their extension to a genome-wide scale will represent important resources for the C. albicans community.

Potent synergistic effect of doxycycline with fluconazole against Candida albicans is mediated by interference with iron homeostasis

Doxycycline was found to act synergistically with the antifungal fluconazole against Candida albicans. Combination with doxycycline converts fluconazole from fungistatic to fungicidal, prevents the onset of drug resistance, and is also effective against a clinical isolate characterized by elevated resistance to fluconazole. Investigation of the interactions between the two drugs by way of checkerboard assays indicated that doxycycline had an influence on the MIC for fluconazole, as defined by CLSI standards, only at high concentrations (200 μg/ml). However, lower concentrations were effective at eliminating residual cell growth at supra-MICs of fluconazole. Using MIC-0, defined as a drug combination resulting in optically clear wells, as an endpoint, doxycycline was found to be synergistic with fluconazole at a concentration as low as 25 μg/ml, with a fractional inhibitory concentration index of <0.5. Doxycycline-mediated growth inhibition can be reversed by externally added iron, indicating that iron depletion may account for the synergism. Consistently, we confirmed old literature data about iron-chelating activity of doxycycline. Synergism of fluconazole with doxycycline does not appear to be mediated by calcineurin, since doxycycline further aggravates the susceptibility to fluconazole of mutants lacking the catalytic or the regulatory subunits of calcineurin. Growth in the presence of fluconazole and doxycycline is restored by an elevated dosage of ERG11 in Saccharomyces cerevisiae but not in C. albicans, despite the full competence of the pathogen's protein to act as a suppressor in baker's yeast.

A coordinated interdependent protein circuitry stabilizes the kinetochore ensemble to protect CENP-A in the human pathogenic yeast Candida albicans

Unlike most eukaryotes, a kinetochore is fully assembled early in the cell cycle in budding yeasts Saccharomyces cerevisiae and Candida albicans. These kinetochores are clustered together throughout the cell cycle. Kinetochore assembly on point centromeres of S. cerevisiae is considered to be a step-wise process that initiates with binding of inner kinetochore proteins on specific centromere DNA sequence motifs. In contrast, kinetochore formation in C. albicans, that carries regional centromeres of 3-5 kb long, has been shown to be a sequence independent but an epigenetically regulated event. In this study, we investigated the process of kinetochore assembly/disassembly in C. albicans. Localization dependence of various kinetochore proteins studied by confocal microscopy and chromatin immunoprecipitation (ChIP) assays revealed that assembly of a kinetochore is a highly coordinated and interdependent event. Partial depletion of an essential kinetochore protein affects integrity of the kinetochore cluster. Further protein depletion results in complete collapse of the kinetochore architecture. In addition, GFP-tagged kinetochore proteins confirmed similar time-dependent disintegration upon gradual depletion of an outer kinetochore protein (Dam1). The loss of integrity of a kinetochore formed on centromeric chromatin was demonstrated by reduced binding of CENP-A and CENP-C at the centromeres. Most strikingly, Western blot analysis revealed that gradual depletion of any of these essential kinetochore proteins results in concomitant reduction in cellular protein levels of CENP-A. We further demonstrated that centromere bound CENP-A is protected from the proteosomal mediated degradation. Based on these results, we propose that a coordinated interdependent circuitry of several evolutionarily conserved essential kinetochore proteins ensures integrity of a kinetochore formed on the foundation of CENP-A containing centromeric chromatin.

Tetracycline-inducible gene expression in Candida albicans

In addition to gene inactivation, the manipulation of gene expression is another highly useful tool for the analysis of gene function. Several regulatable promoters are available that enable researchers to shut off or turn on the expression of a target gene in Candida albicans, usually by growing the cells in inducing or repressing media. In this chapter, we describe a tetracycline-inducible gene expression system (Tet-On) that allows forced expression of endogenous or heterologous genes in C. albicans by the addition of the small-molecule inducer doxycycline in a growth medium-independent manner. The system is based on a cassette in which a gene of interest can be placed under the control of a Tet-inducible promoter in a single cloning step and integrated into the C. albicans genome with the help of a dominant selection marker. As the cassette contains all necessary components for Tet-inducible gene expression, it can be used to study the effect of forced gene expression on the phenotype of C. albicans cells in any strain without a requirement of additional genetic manipulations.

Modular gene over-expression strategies for Candida albicans

Over-expression is a valid functional genomics approach to characterise genes of unknown function on a genome-wide scale. Strains are engineered to over-express a specific gene and the resulting gain-of-function phenotype assessed. Here, we describe the strategy we are adopting to synthesise a Candida albicans ORFeome collection and the options available to create over-expressing strains from this collection.

Endocytosis-mediated vacuolar accumulation of the human ApoE apolipoprotein-derived ApoEdpL-W antimicrobial peptide contributes to its antifungal activity in Candida albicans

The 18-amino-acid cationic, tryptophan-rich ApoEdpL-W peptide derived from human ApoE apolipoprotein was shown to have antifungal activity against pathogenic yeasts of the Candida genus (except C. glabrata). ApoEdpL-W was active against planktonic cells and early-stage biofilms but less active against mature biofilms, possibly because of its affinity for extracellular matrix beta-glucans. Moreover, ApoEdpL-W absorbed to medically relevant materials partially prevented the formation of biofilms on these materials. The exposure of C. albicans cells to sublethal doses of ApoEdpL-W triggered a transcriptional response reminiscent of that associated with the inactivation of the MYO5 gene required for endocytosis as well as the upregulation of amino acid transporter genes. A fluorescent derivative of ApoEdpL-W accumulated at the cytoplasmic membrane and subsequently was translocated to the vacuole. Strikingly, the inactivation of MYO5 or addition of latrunculin, an inhibitor of endocytosis, prevented the vacuolar accumulation of fluorescein-labeled ApoEdpL-W and reduced the antifungal activity of ApoEdpL-W. This, together with the insensitivity of ApoEdpL-W to alterations in membrane fluidity and high salt, suggested that the ApoEdpL-W mode of action was dependent upon vacuolar targeting and differed significantly from that of other antifungal peptides, such as Histatin-5 and Magainin 2.

The essentiality of the fungus-specific Dam1 complex is correlated with a one-kinetochore-one-microtubule interaction present throughout the cell cycle, independent of the nature of a centromere

A fungus-specific outer kinetochore complex, the Dam1 complex, is essential in Saccharomyces cerevisiae, nonessential in fission yeast, and absent from metazoans. The reason for the reductive evolution of the functionality of this complex remains unknown. Both Candida albicans and Schizosaccharomyces pombe have regional centromeres as opposed to the short-point centromeres of S. cerevisiae. The interaction of one microtubule per kinetochore is established both in S. cerevisiae and C. albicans early during the cell cycle, which is in contrast to the multiple microtubules that bind to a kinetochore only during mitosis in S. pombe. Moreover, the Dam1 complex is associated with the kinetochore throughout the cell cycle in S. cerevisiae and C. albicans but only during mitosis in S. pombe. Here, we show that the Dam1 complex is essential for viability and indispensable for proper mitotic chromosome segregation in C. albicans. The kinetochore localization of the Dam1 complex is independent of the kinetochore-microtubule interaction, but the function of this complex is monitored by a spindle assembly checkpoint. Strikingly, the Dam1 complex is required to prevent precocious spindle elongation in premitotic phases. Thus, constitutive kinetochore localization associated with a one-microtubule-one kinetochore type of interaction, but not the length of a centromere, is correlated with the essentiality of the Dam1 complex.

The requirement for the Dam1 complex is dependent upon the number of kinetochore proteins and microtubules

The Dam1 complex attaches the kinetochore to spindle microtubules and is a processivity factor in vitro. In Saccharomyces cerevisiae, which has point centromeres that attach to a single microtubule, deletion of any Dam1 complex member results in chromosome segregation failures and cell death. In Schizosaccharomyces pombe, which has epigenetically defined regional centromeres that each attach to 3-5 kinetochore microtubules, Dam1 complex homologs are not essential. To determine why the complex is essential in some organisms and not in others, we used Candida albicans, a multimorphic yeast with regional centromeres that attach to a single microtubule. Interestingly, the Dam1 complex was essential in C. albicans, suggesting that the number of microtubules per centromere is critical for its requirement. Importantly, by increasing CENP-A expression levels, more kinetochore proteins and microtubules were recruited to the centromeres, which remained fully functional. Furthermore, Dam1 complex members became less crucial for growth in cells with extra kinetochore proteins and microtubules. Thus, the requirement for the Dam1 complex is not due to the DNA-specific nature of point centromeres. Rather, the Dam1 complex is less critical when chromosomes have multiple kinetochore complexes and microtubules per centromere, implying that it functions as a processivity factor in vivo as well as in vitro.

Damage to the glycoshield activates PMT-directed O-mannosylation via the Msb2-Cek1 pathway in Candida albicans

Protein-O-mannosyltransferases (Pmt) transfer mannosyl residues to secretory proteins. Five isoforms of Pmt proteins in the human fungal pathogen Candida albicans have distinct functions in growth, morphogenesis and antifungal resistance. We found that PMT genes encoding the major isoforms Pmt1, Pmt2, Pmt4 are regulated differently in response to impaired glycostructures. While the PMT1 transcript level increased in cell wall mutants and under inhibition of N-glycosylation by tunicamycin, PMT2 and PMT4 transcripts were upregulated only by inhibition of Pmt1 activity. Reporter fusions revealed specific promoter sequences to be required for PMT1 repression in undamaged cells, which was de-repressed by tunicamycin. Constitutive PMT1 de-repression was observed in mutants lacking the Cek1 MAP kinase and its upstream sensor Msb2. In contrast, in msb2 and cek1 mutants, upregulation of PMT2/PMT4 by Pmt1 inhibition did not occur and basal expression of both transcripts were decreased. We identified Ace2 as a novel transcription factor, which upregulates PMT basal expression and induction in response to glycostructure damage. Mutants lacking Msb2, Cek1 and Ace2 were supersensitive to glycosylation and cell wall inhibitors. We propose that a Msb2, Cek1 and Ace2 signalling pathway addresses PMT genes as downstream targets and that different modes of regulation have evolved for PMT1 and PMT2/PMT4 genes.

CaMtw1, a member of the evolutionarily conserved Mis12 kinetochore protein family, is required for efficient inner kinetochore assembly in the pathogenic yeast Candida albicans

Proper assembly of the kinetochore, a multi-protein complex that mediates attachment of centromere DNA to spindle microtubules on each chromosome, is required for faithful chromosome segregation. Each previously characterized member of the Mis12/Mtw1 protein family is part of an essential subcomplex in the kinetochore. In this work, we identify and characterize CaMTW1, which encodes the homologue of the human Mis12 protein in the pathogenic budding yeast Candida albicans. Subcellular localization and chromatin immunoprecipitation assays confirmed CaMtw1 is a kinetochore protein. CaMtw1 is essential for viability. CaMtw1-depleted cells and cells in which CaMtw1 was inactivated with a temperature-sensitive mutation had reduced viability, accumulated at the G2/M stage of the cell cycle, and exhibited increased chromosome missegregation. CaMtw1 depletion also affected spindle length and alignment. Interestingly, in C. albicans, CaMtw1 and the centromeric histone, CaCse4, influence each other for kinetochore localization. In addition, CaMtw1 is required for efficient kinetochore recruitment of another inner kinetochore protein, the CENP-C homologue, CaMif2. Mis12/Mtw1 proteins have well-established roles in the recruitment and maintenance of outer kinetochore proteins. We propose that Mis12/Mtw1 proteins also have important co-dependent interactions with inner kinetochore proteins and that these interactions may increase the fidelity of kinetochore formation.

Glucose promotes stress resistance in the fungal pathogen Candida albicans

Metabolic adaptation, and in particular the modulation of carbon assimilatory pathways during disease progression, is thought to contribute to the pathogenicity of Candida albicans. Therefore, we have examined the global impact of glucose upon the C. albicans transcriptome, testing the sensitivity of this pathogen to wide-ranging glucose levels (0.01, 0.1, and 1.0%). We show that, like Saccharomyces cerevisiae, C. albicans is exquisitely sensitive to glucose, regulating central metabolic genes even in response to 0.01% glucose. This indicates that glucose concentrations in the bloodstream (approximate range 0.05-0.1%) have a significant impact upon C. albicans gene regulation. However, in contrast to S. cerevisiae where glucose down-regulates stress responses, some stress genes were induced by glucose in C. albicans. This was reflected in elevated resistance to oxidative and cationic stresses and resistance to an azole antifungal agent. Cap1 and Hog1 probably mediate glucose-enhanced resistance to oxidative stress, but neither is essential for this effect. However, Hog1 is phosphorylated in response to glucose and is essential for glucose-enhanced resistance to cationic stress. The data suggest that, upon entering the bloodstream, C. albicans cells respond to glucose increasing their resistance to the oxidative and cationic stresses central to the armory of immunoprotective phagocytic cells.

A proteomic view of Candida albicans yeast cell metabolism in exponential and stationary growth phases

The facultative pathogenic fungus Candida albicans has to come up with dynamic metabolic adaptation programs in order to be able to survive within a variety of niches in the human host, each of which has its different nutrient availability. Using a large-scale two-dimensional (2-D) protein gel electrophoresis approach, we analyzed the adaptation mechanisms to nutrient limitation in a batch culture in complex medium with glucose as carbon source. To this end, we constructed a 2-D reference map of cytoplasmic proteins and quantitatively compared protein accumulation of growing yeast cells with those from the stationary phase. This yielded characteristic proteome signatures for each physiological state. During exponential growth, proteins required for the synthesis of RNA, DNA, and proteins, including components of purine and pyrimidine synthesis pathways and ribosomal proteins, were over-represented. The stationary-phase signature revealed a complex reprogramming of metabolic networks: Up-regulation of glyoxylate cycle, gluconeogenesis, and glutamate degradation signaled a switch to the utilization of alternative carbon sources instead of the exhausted glucose. Induction of proteins involved in defense against oxidative and heat stress indicates a change in redox balance and reactive oxygen species concentrations.

Functional mapping of the Candida albicans Efg1 regulator

Efg1p is a key transcriptional regulator in Candida albicans which controls various aspects of morphogenesis and metabolism in this organism. Efg1p contains a central basic helix-loop-helix (bHLH) domain, flanked by sequences highly conserved in fungal APSES proteins, as well as polyglutamine stretches at the N- and C-terminal ends. A systematic deletion approach to specify functional domains of Efg1p revealed that the APSES domain is essential for morphogenesis of the normal yeast and true hyphal cell forms and that bHLH flanking sequences are needed for Efg1p stability. Additional C-terminal sequences were required for hyphal formation on some inducing media, and most Efg1p sequences were needed for chlamydospore morphogenesis. Overexpression of EFG1 led to pseudohypha formation only if a functional APSES domain was present, while a switch from the opaque to the white cell type in addition depended on the presence of certain N- and C-terminal segments. Yeast two-hybrid experiments revealed that binding of Efg1p to its antagonist Czf1p required two regions outside of the APSES domain, which did not coincide with Efg1p sequences needed for its transcriptional repressor activity. Binding of the Flo8 transcription factor to Efg1p did not require the APSES domain but appeared to occur at two or more redundant domains. In contrast, DNA binding of Efg1p to an MluI cell cycle box (MCB) element solely required the APSES domain. Overall, these results suggest that functional domains of Efg1p are spread throughout most of its sequences, including the central APSES domain involved in DNA binding, as well as flanking regions required for various protein interactions and regulatory activities.

Multiple functions of DOA1 in Candida albicans

While searching for regulators of virulence attributes of the human-pathogenic fungus Candida albicans, a gene was identified similar to the genes encoding the mammalian phospholipase A2-activating protein (PLAP) and the Saccharomyces cerevisiae protein Doa1, which is known to play a key role during ubiquitin (Ub)-dependent protein degradation. All three proteins contain WD-repeats. Both PLAP and CaDoa1 contain a mellitin-like sequence with a central 'KVL'. This mellitin-like sequence was shown to be necessary for full function of CaDoa1. CaDOA1 was expressed under all conditions investigated. Gene disruption of CaDOA1 caused phenotypes including modified colony morphologies, temperature sensitivity, reduced secretion of hydrolytic enzymes and hypersensitivity to various compounds such as propranolol, butanol, caffeine, chelators, azoles, nocodazole and cadmium. Strikingly, mutants lacking DOA1 were filamentous and grew as pseudohyphae and true hyphae under conditions that normally support yeast growth. Transcriptional profiling of Deltadoa1 indicated that several genes associated with Ub-mediated proteolysis, including CDC48 and UBI4, are upregulated. These data suggest that DOA1 of C. albicans, like its orthologue in S. cerevisiae, is associated with Ub-mediated proteolysis and has multiple functions. However, some functions of CaDoa1 seem to be unique for C. albicans. These results support the hypothesis that Ub-mediated proteolysis plays an important role in the regulation of morphology in C. albicans.

Niche-specific regulation of central metabolic pathways in a fungal pathogen

To establish an infection, the pathogen Candida albicans must assimilate carbon and grow in its mammalian host. This fungus assimilates six-carbon compounds via the glycolytic pathway, and two-carbon compounds via the glyoxylate cycle and gluconeogenesis. We address a paradox regarding the roles of these central metabolic pathways in C. albicans pathogenesis: the glyoxylate cycle is apparently required for virulence although glyoxylate cycle genes are repressed by glucose at concentrations present in the bloodstream. Using GFP fusions, we confirm that glyoxylate cycle and gluconeogenic genes in C. albicans are repressed by physiologically relevant concentrations of glucose, and show that these genes are inactive in the majority of fungal cells infecting the mouse kidney. However, these pathways are induced following phagocytosis by macrophages or neutrophils. In contrast, glycolytic genes are not induced following phagocytosis and are expressed in infected kidney. Mutations in all three pathways attenuate the virulence of this fungus, highlighting the importance of central carbon metabolism for the establishment of C. albicans infections. We conclude that C. albicans displays a metabolic program whereby the glyoxylate cycle and gluconeogenesis are activated early, when the pathogen is phagocytosed by host cells, while the subsequent progression of systemic disease is dependent upon glycolysis.

Conserved fungal genes as potential targets for broad-spectrum antifungal drug discovery

The discovery of novel classes of antifungal drugs depends to a certain extent on the identification of new, unexplored targets that are essential for growth of fungal pathogens. Likewise, the broad-spectrum capacity of future antifungals requires the target gene(s) to be conserved among key fungal pathogens. Using a genome comparison (or concordance) tool, we identified 240 conserved genes as candidates for potential antifungal targets in 10 fungal genomes. To facilitate the identification of essential genes in Candida albicans, we developed a repressible C. albicans MET3 (CaMET3) promoter system capable of evaluating gene essentiality on a genome-wide scale. The CaMET3 promoter was found to be highly amenable to controlled gene expression, a prerequisite for use in target-based whole-cell screening. When the expression of the known antifungal target C. albicans ERG1 was reduced via down-regulation of the CaMET3 promoter, the CaERG1 conditional mutant strain became hypersensitive, specifically to its inhibitor, terbinafine. Furthermore, parallel screening against a small compound library using the CaERG1 conditional mutant under normal and repressed conditions uncovered several hypersensitive compound hits. This work therefore demonstrates a streamlined process for proceeding from selection and validation of candidate antifungal targets to screening for specific inhibitors.

Tetracycline-inducible gene expression and gene deletion in Candida albicans

The genetic analysis of Candida albicans, the major fungal pathogen of humans, is hampered by its diploid genome, the absence of a normal sexual cycle, and a nonstandard codon usage. Although effective methods to study gene function have been developed in the past years, systems to control gene expression in C. albicans are limited. We have established a system that allows induction of gene expression in C. albicans by the addition of tetracycline (Tet). By fusing genetically modified versions of the reverse Tet repressor from Escherichia coli and the transcription activation domain of the Gal4 protein from Saccharomyces cerevisiae, a C. albicans-adapted reverse Tet-dependent transactivator (rtTA) was created that was expressed from the constitutive ADH1 or the opaque-specific OP4 promoter. To monitor Tet-inducible gene expression, the caGFP reporter gene was placed under the control of a Tet-dependent promoter, obtained by fusing a minimal promoter from C. albicans to seven copies of the Tet operator sequence. Fluorescence of the cells demonstrated that gene expression could be efficiently induced by the addition of doxycycline in yeast, hyphal, and opaque cells of C. albicans. The Tet-inducible gene expression system was then used to manipulate the behavior of the various growth forms of C. albicans. Tet-induced expression of a dominant-negative CDC42 allele resulted in growth arrest as large, multinucleate cells. Filamentous growth was efficiently inhibited under all tested hyphal-growth-promoting conditions by Tet-inducible expression of the NRG1 repressor. Tet-induced expression of the MTLa1 gene in opaque cells of an MTLalpha strain forced the cells to switch to the white phase, whereas Tet-induced expression of the MTLa2 transcription factor induced shmooing. When the ecaFLP gene, encoding the site-specific recombinase FLP, was placed under the control of the Tet-dependent promoter, Tet-inducible deletion of genes which were flanked by the FLP target sequences was achieved with high efficiency to generate conditional null mutants. In combination with the dominant selection marker caSAT1, the Tet-inducible gene expression system was also applied in C. albicans wild-type strains, including drug-resistant clinical isolates that overexpressed the MDR1, CDR1, and CDR2 multidrug efflux pumps. This system, therefore, allows a growth medium-independent, Tet-inducible expression and deletion of genes in C. albicans and provides a convenient, versatile new tool to study gene function and manipulate cellular behavior in this model pathogenic fungus.

Functional analysis of the phospholipase C gene CaPLC1 and two unusual phospholipase C genes, CaPLC2 and CaPLC3, of Candida albicans

Phospholipases C are known to be important regulators of cellular processes but may also act as virulence factors of pathogenic microbes. At least three genes in the genome of the human-pathogenic fungus Candida albicans encode phospholipases with conserved phospholipase C (Plc) motifs. None of the deduced protein sequences contain N-terminal signal peptides, suggesting that these phospholipases are not secreted. In contrast to its orthologue in Sacharomyces cerevisiae, CaPLC1 seems to be an essential gene. However, a conditional mutant with reduced transcript levels of CaPLC1 had phenotypes similar to Plc1p-deficient mutants in S. cerevisiae, including reduced growth on media causing increased osmotic stress, on media with a non-glucose carbon source, or at elevated or lower temperatures, suggesting that CaPlc1p, like the Plc1p counterpart in S. cerevisiae, may be involved in multiple cellular processes. Furthermore, phenotypic screening of the heterozygous DeltaCaplc1/CaPLC1 mutant showed additional defects in hyphal formation. The loss of CaPLC1 cannot be compensated by two additional PLC genes of C. albicans (CaPLC2 and CaPLC3) encoding two almost identical phospholipases C with no counterpart in S. cerevisiae but containing structural elements found in bacterial phospholipases C. Although the promoter sequences of CaPLC2 and CaPLC3 differed dramatically, the transcriptional pattern of both genes was similar. In contrast to CaPLC1, CaPLC2 and CaPLC3 are not essential. Although Caplc2/3 mutants had reduced abilities to produce hyphae on solid media, these mutants were as virulent as the wild-type in a model of systemic infection. These data suggest that C. albicans contains two different classes of phospholipases C which are involved in cellular processes but which have no specific functions in pathogenicity.

An update on antifungal targets and mechanisms of resistance in Candida albicans

Much progress has been made in the last decade in identifying genes responsible for antifungal resistance in Candida albicans. Attention has focused on five major C. albicans genes: ABC transporter genes CDR1 and CDR2, major facilitator efflux gene MDR1, and ergosterol biosynthesis genes ERG11 and ERG3. Resistance involves mutations in 14C-lanosterol demethylase, targeted by fluconazole (FLZ) and encoded by ERG11, and mutations that up-regulate efflux genes that probably efflux the antifungals. Mutations that affect ERG3 mutations have been understudied as mechanism resistance among clinical isolates. In vitro resistance in clinical isolates typically involves step-wise mutations affecting more than one of these genes, and often unidentified genes. Different approaches are needed to identify these other genes. Very little is understood about reversible adaptive resistance of C. albicans despite its potential clinical significance; most clinical failures to control infections other than oropharyngeal candidiasis (OPC) occur with in vitro susceptible strains. Tolerance of C. albicans to azoles has been attributed to the calcineurin stress-response pathway, offering new potential targets for next generation antifungals. Recent studies have identified genes that regulate CDR1 or ERG genes. The focus of this review is C. albicans, although information on Saccharomyces cerevisiae or Candida glabrata is provided in areas in where Candida research is underdeveloped. With the completion of the C. albicans genomic sequence, and new methods for high throughput gene overexpression and disruption, rapid progress towards understanding the regulation of resistance, novel resistance mechanisms, and adaptive resistance is expected in the near future.

Cell cycle arrest during S or M phase generates polarized growth via distinct signals in Candida albicans

Treatments that perturb DNA synthesis or mitosis will activate checkpoints that prevent cell cycle progression and cell proliferation. In yeast-form cells of the fungal pathogen Candida albicans, exposure to hydroxyurea (HU) or shutting off expression of the polo-like kinase CaCDC5 blocked nuclear division and spindle elongation, but activated a highly polarized growth mode. We have used transcription profiling both to characterize the initiation and progression of this polar growth pattern and to determine how cell elongation may be linked to the cell cycle in C. albicans. Different gene expression patterns during early stages of cell elongation support the concept that CaCdc5p-depleted and HU-exposed cells were blocked at different stages of the cell cycle, and suggest that different signals may generate the common polarized growth phenotype. Consistent with this, BUB2 expression was modulated in CaCdc5p-depleted cells, and absence of BUB2 prevented the maintenance of cell polarization, resulting in multibudded, pseudohyphal cells with constrictions. In contrast, HU-induced filaments did not modulate or require BUB2, but were dependent on the GTPase Ras1p. However, at later stages of cell elongation, transcription profiles were more similar, and comparisons with serum-induced hyphae revealed that the cell cycle-arrested filaments expressed several targets of the hyphal signalling pathways. Thus, arresting the yeast cell cycle in S or M phase generates a polarized growth pattern through different mechanisms in C. albicans, and maintenance of the polar growth mode can ultimately lead to the expression of hyphal-associated cell wall and virulence-related factors, in the absence of any external stimuli.

Pmt-mediated O mannosylation stabilizes an essential component of the secretory apparatus, Sec20p, in Candida albicans

Sec20p is an essential endoplasmic reticulum (ER) membrane protein in yeasts, functioning as a tSNARE component in retrograde vesicle traffic. We show that Sec20p in the human fungal pathogen Candida albicans is extensively O mannosylated by protein mannosyltransferases (Pmt proteins). Surprisingly, Sec20p occurs at wild-type levels in a pmt6 mutant but at very low levels in pmt1 and pmt4 mutants and also after replacement of specific Ser/Thr residues in the lumenal domain of Sec20p. Pulse-chase experiments revealed rapid degradation of unmodified Sec20p (38.6 kDa) following its biosynthesis, while the stable O-glycosylated form (50 kDa) was not formed in a pmt1 mutant. These results suggest a novel function of O mannosylation in eukaryotes, in that modification by specific Pmt proteins will prevent degradation of ER-resident membrane proteins via ER-associated degradation or a proteasome-independent pathway.

The G protein-coupled receptor Gpr1 and the Galpha protein Gpa2 act through the cAMP-protein kinase A pathway to induce morphogenesis in Candida albicans

We investigated the role in cell morphogenesis and pathogenicity of the Candida albicans GPR1 gene, encoding the G protein-coupled receptor Gpr1. Deletion of C. albicans GPR1 has only minor effects in liquid hypha-inducing media but results in strong defects in the yeast-to-hypha transition on solid hypha-inducing media. Addition of cAMP, expression of a constitutively active allele of the Galpha protein Gpa2 or of the catalytic protein kinase A subunit TPK1 restores the wild-type phenotype of the CaGPR1-deleted strain. Overexpression of HST7, encoding a component of the mitogen-activated protein kinase pathway, does not suppress the defect in filamentation. These results indicate that CaGpr1 functions upstream in the cAMP-protein kinase A (PKA) pathway. We also show that, in the presence of glucose, CaGpr1 is important for amino acid-induced transition from yeast to hyphal cells. Finally, as opposed to previous reports, we show that CaGpa2 acts downstream of CaGpr1 as activator of the cAMP-PKA pathway but that deletion of neither CaGpr1 nor CaGpa2 affects glucose-induced cAMP signaling. In contrast, the latter is abolished in strains lacking CaCdc25 or CaRas1, suggesting that the CaCdc25-CaRas1 rather than the CaGpr1-CaGpa2 module mediates glucose-induced cAMP signaling in C. albicans.

Centromeric DNA sequences in the pathogenic yeast Candida albicans are all different and unique

In an approach to clone and characterize centromeric DNA sequences of Candida albicans by chromatin immunoprecipitation, we have used antibodies directed against an evolutionarily conserved histone H3-like protein, CaCse4p (CENP-A homolog). Sequence analysis of clones obtained by this procedure reveals that only eight relatively small regions (approximately 3 kb each) of the Can. albicans genome are selectively enriched. These CaCse4-bound sequences are located within 4- to 18-kb regions lacking ORFs and occur once in each of the eight chromosomes of Can. albicans. Binding of another evolutionarily conserved kinetochore protein, CaMif2p (CENP-C homolog), colocalizes with CaCse4p. Deletion of the CaCse4p-binding region of chromosome 7 results in a high rate of loss of the altered chromosome, confirming that CaCse4p, a centromeric histone in the CENP-A family, indeed identifies the functional centromeric DNA of Can. albicans. The CaCse4p-rich regions not only lack conserved DNA motifs of point (<400 bp) centromeres and repeated elements of regional (>40 kb) centromeres, but also each chromosome of Can. albicans contains a different and unique CaCse4p-rich centromeric DNA sequence, a centromeric property previously unobserved in other organisms.

Dosage-dependent functions of fatty acid desaturase Ole1p in growth and morphogenesis of Candida albicans

Conditions in the infected human host trigger virulence attributes of the fungal pathogenCandida albicans. Specific inducers and elevated temperatures lead to hyphal development or regulate chlamydospore development. To explore if these processes are affected by membrane lipids, an investigation of the functions of the Ole1 fatty acid desaturase (stearoyl-CoA desaturase) inC. albicans, which synthesizes oleic acid, was undertaken. A conditional strain expressingOLE1from the regulatableMET3promoter was unable to grow in repressing conditions, indicating thatOLE1is an essential gene. In contrast, a mutant lacking both alleles ofOLE2, encoding a Ole1p homologue, was viable and had no apparent phenotypes. Partial repression ofMET3p–OLE1slightly lowered oleic acid levels and decreased membrane fluidity; these conditions permitted growth in the yeast form, but prevented hyphal development in aerobic conditions and blocked the formation of chlamydospores. In contrast, in hypoxic conditions, which trigger an alternative morphogenetic pathway, hyphal morphogenesis was unaffected. Because aerobic morphogenetic signalling and oleic acid biosynthesis require oxygen, it is proposed that oleic acid may function as a sensor activating specific morphogenetic pathways in normoxic conditions.

Ciclopirox olamine treatment affects the expression pattern of Candida albicans genes encoding virulence factors, iron metabolism proteins, and drug resistance factors

The hydroxypyridone ciclopirox olamine belongs to the antimycotic drugs used for the treatment of superficial mycoses. In contrast to the azoles and other antimycotic drugs, its specific mode of action is only poorly understood. To investigate the mode of action of ciclopirox olamine on fungal viability, pathogenicity, and drug resistance, we examined the expression patterns of 47 Candida albicans genes in cells grown in the presence of a subinhibitory concentration (0.6 micro g/ml) of ciclopirox olamine by reverse transcription-PCR. In addition, we used suppression-subtractive hybridization to further identify genes that are up-regulated in the presence of ciclopirox olamine. The expression of essential genes such as ACT1 was not significantly modified in cells exposed to ciclopirox olamine. Most putative and known virulence genes such as genes encoding secreted proteinases or lipases had no or only moderately reduced expression levels. In contrast, exposure of cells to ciclopirox olamine led to a distinct up- or down-regulation of genes encoding iron permeases or transporters (FTR1, FTR2, FTH1), a copper permease (CCC2), an iron reductase (CFL1), and a siderophore transporter (SIT1); these effects resembled those found under iron-limited conditions. Addition of FeCl(3) to ciclopirox olamine-treated cells reversed the effect of the drug. Addition of the iron chelator bipyridine to the growth medium induced similar patterns of expression of distinct iron-regulated genes (FTR1, FTR2). While serum-induced yeast-to-hyphal phase transition of C. albicans was not affected in ciclopirox olamine-treated cells in the presence of subinhibitory conditions, a dramatic increase in sensitivity to oxidative stress was noted, which may indicate the reduced activities of iron-containing gene products responsible for detoxification. Although the Candida drug resistance genes CDR1 and CDR2 were up-regulated, no change in resistance or increased tolerance could be observed even after an incubation period of 6 months. This was in contrast to control experiments with fluconazole, in which the MICs for cells incubated with this drug had noticeably increased after 2 months. These data support the view that the antifungal activity of ciclopirox olamine may at least be partially caused by iron limitation. Furthermore, neither the expression of certain multiple-drug resistance genes nor other resistance mechanisms caused C. albicans resistance to this drug even after long-term exposure.

Depletion of a polo-like kinase in Candida albicans activates cyclase-dependent hyphal-like growth

Morphogenesis in the fungal pathogen Candida albicans is an important virulence-determining factor, as a dimorphic switch between yeast and hyphal growth forms can increase pathogenesis. We identified CaCDC5, a cell cycle regulatory polo-like kinase (PLK) in C. albicans and demonstrate that shutting off its expression induced cell cycle defects and dramatic changes in morphology. Cells lacking CaCdc5p were blocked early in nuclear division with very short spindles and unseparated chromatin. GFP-tagged CaCdc5p localized to unseparated spindle pole bodies, the spindle, and chromatin, consistent with a role in spindle elongation at an earlier point in the cell cycle than that described for the homologue Cdc5p in yeast. Strikingly, the cell cycle defects were accompanied by the formation of hyphal-like filaments under yeast growth conditions. Filament growth was determinate, as the filaments started to die after 24 h. The filaments resembled serum-induced hyphae with respect to morphology, organization of cytoplasmic microtubules, localization of nuclei, and expression of hyphal-specific components. Filament formation required CaCDC35, but not EFG1 or CPH1. Similar defects in spindle elongation and a corresponding induction of filaments occurred when yeast cells were exposed to hydroxyurea. Because CaCdc5p does not appear to act as a direct repressor of hyphal growth, the data suggest that a target of CaCdc5p function is associated with hyphal-like development. Thus, an internal, cell cycle-related cue can activate hyphal regulatory networks in Candida.

Strategies for the identification of virulence determinants in human pathogenic fungi

The incidence of fungal infections is increasing in different countries. The current available therapy of these infections does not satisfy all requirements in terms of specificity and therapeutic index, a fact that has stimulated the scientific community to identify fungal virulence determinants. Several pathogenic fungi are opportunistic and, therefore, identification of virulence genes is difficult, given their close relationship with host cells. In recent years, the development of genetic tools in several pathogenic fungi has enabled the development of genetic strategies for their identification. These include several strategies based on the phenotypic analysis of strains or environmental conditions in which the expression of the putative gene(s) is either altered or deleted; and this is accomplished through the development of in vitro or in vivo systems. In the near future, this research will produce a better picture of fungal pathogenesis and therefore define novel promising targets in antifungal therapy.

Ssn6, an important factor of morphological conversion and virulence in Candida albicans

Candida albicans, the major fungal pathogen in humans, undergoes morphological conversion from yeasts to filamentous growth forms depending upon various environmental conditions. Here, we have identified a C. albicans gene, namely SSN6, encoding a putative global transcriptional co-repressor that is highly homologous to the Saccharomyces cerevisiae Ssn6. The isolated C. albicans SSN6 complemented the pleiotropic phenotypes of S. cerevisiae ssn6 mutation, and its expression levels declined significantly in response to a strong true hyphal inducer, serum. The mutant lacking C. albicans Ssn6 displayed a stubby pseudohyphal growth pattern, derepressed filament-specific genes in response to elevated temperature 37 degrees C and failed to develop true hyphae, extensive filamentation and virulence. Such morphological defects of ssn6/ssn6 mutant were not rescued by overexpression of Tup1, Cph1 or Efg1. Moreover, epistatic analysis showed that, as far as cell morphology was concerned, Ssn6 was epistatic to Tup1 at the higher temperature but that, at the lower temperature, the ssn6/ssn6 tup1/tup1 double mutant grew in a stubby form of pseudohyphae distinct from the phenotypes of either single mutant. Furthermore, overexpression of SSN6 in C. albicans led to enhanced filamentous growth and attenuated virulence. These findings suggest that Ssn6 may function as an activator as well as a repressor of filamentous growth and be a target for candidacidal drugs, as its excess or deficiency resulted in impaired virulence.