Gene regulation and host adaptation mechanisms in Candida albicans

Mms21: A Putative SUMO E3 Ligase in Candida albicans That Negatively Regulates Invasiveness and Filamentation, and Is Required for the Genotoxic and Cellular Stress Response

In the life cycle of the fungal pathogen Candida albicans, the formation of filamentous cells is a differentiation process that is critically involved in host tissue invasion, and in adaptation to host cell and environmental stresses. Here, we have used the Gene Replacement And Conditional Expression library to identify genes controlling invasiveness and filamentation; conditional repression of the library revealed 69 mutants that triggered these processes. Intriguingly, the genes encoding the small ubiquitin-like modifier (SUMO) E3 ligase Mms21, and all other tested members of the sumoylation pathway, were both nonessential and capable of triggering filamentation upon repression, suggesting an important role for sumoylation in controlling filamentation in C. albicans We have investigated Mms21 in detail. Both Mms21 nulls (mms21Δ/Δ) and SP [Siz/Pias (protein inhibitor of activated signal transducer and activator of transcription)] domain (SUMO E3 ligase domain)-deleted mutants displayed invasiveness, filamentation, and abnormal nuclear segregation; filament formation occurred even in the absence of the hyphal transcription factor Efg1. Transcriptional analysis of mms21Δ/Δ showed an increase in expression from two- to eightfold above that of the wild-type for hyphal-specific genes, including ECE1, PGA13, PGA26, HWP1, ALS1, ALS3, SOD4, SOD5, UME6, and HGC1 The Mms21-deleted mutants were unable to recover from DNA-damaging agents like methyl methane sulfonate, hydroxyurea, hydrogen peroxide, and UV radiation, suggesting that the protein is important for genotoxic stress responses. In addition, the mms21Δ/Δ mutant displayed sensitivity to cell wall and thermal stresses, and to different antifungal drugs. All these findings suggest that Mms21 plays important roles in cellular differentiation, DNA damage and cellular stress responses, and in response to antifungal drugs.

Rapid Phenotypic and Genotypic Diversification After Exposure to the Oral Host Niche in Candida albicans

In vitro studies suggest that stress may generate random standing variation and that different cellular and ploidy states may evolve more rapidly under stress. Yet this idea has not been tested with pathogenic fungi growing within their host niche in vivo Here, we analyzed the generation of both genotypic and phenotypic diversity during exposure of Candida albicans to the mouse oral cavity. Ploidy, aneuploidy, loss of heterozygosity (LOH), and recombination were determined using flow cytometry and double digest restriction site-associated DNA sequencing. Colony phenotypic changes in size and filamentous growth were evident without selection and were enriched among colonies selected for LOH of the GAL1 marker. Aneuploidy and LOH occurred on all chromosomes (Chrs), with aneuploidy more frequent for smaller Chrs and whole Chr LOH more frequent for larger Chrs. Large genome shifts in ploidy to haploidy often maintained one or more heterozygous disomic Chrs, consistent with random Chr missegregation events. Most isolates displayed several different types of genomic changes, suggesting that the oral environment rapidly generates diversity de novo In sharp contrast, following in vitro propagation, isolates were not enriched for multiple LOH events, except in those that underwent haploidization and/or had high levels of Chr loss. The frequency of events was overall 100 times higher for C. albicans populations following in vivo passage compared with in vitro These hyper-diverse in vivo isolates likely provide C. albicans with the ability to adapt rapidly to the diversity of stress environments it encounters inside the host.

Organic modification and subsequent biofunctionalization of porous anodic alumina using terminal alkynes

Porous anodic alumina (PAA) is a well-defined material that has found many applications. The range of applications toward sensing and recognition can be greatly expanded if the alumina surface is covalently modified with an organic monolayer. Here, we present a new method for the organic modification of PAA based on the reaction of terminal alkynes with the alumina surface. The reaction results in the the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. Characterization with X-ray photoelectron spectroscopy and infrared spectroscopy indicates formation of a well-defined monolayer in which the adsorbed species is an oxidation product of the 1-alkyne, namely, its α-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures, as was shown by monitoring the water contact angle. Modification with 1,15-hexadecadiyne resulted in a surface that has alkyne end groups available for further reaction, as was demonstrated by the subsequent reaction of N-(11-azido-3,6,9-trioxaundecyl)trifluoroacetamide with the modified surface. Biofunctionalization was explored by coupling 11-azidoundecyl lactoside to the surface and studying the subsequent adsorption of the lectin peanut agglutinin (PNA) and the yeast Candida albicans, respectively. Selective and reversible binding of PNA to the lactosylated surfaces was demonstrated. Moreover, PNA adsorption was higher on surfaces that exposed the β-lactoside than on those that displayed the α anomer, which was attributed to surface-associated steric hindrance. Likewise, the lactosylated surfaces showed increased colonization of C. albicans compared to unmodified surfaces, presumably due to interactions involving the cell wall β-glucan. Thus, this study provides a new modification method for PAA surfaces and shows that it can be used to induce selective adsorption of proteins and microorganisms.

New cis-configured aziridine-2-carboxylates as aspartic acid protease inhibitors

A series of 52 cis-configured 1-alkyl-3-phenylaziridine-2-carboxylates were synthesized as new pseudo-irreversible inhibitors of Candida albicans secreted aspartic acid protease 1 (SAP1), SAP2, SAP3, and SAP8. Some of the compounds, which were obtained as diastereomers with S,S- and R,R-configured aziridine rings by Cromwell synthesis of racemic (2R,3S+2S,3R)-dibromophenylpropionic acid ester with amines, followed by ester hydrolysis and coupling to hydrophobic amino acid esters, were separated by preparative HPLC. The absolute configuration of the aziridine ring was assigned by a combination of experimental circular dichroism (CD) investigations and quantum chemical CD calculations. In agreement with previous docking studies, the diastereomers all exhibit similar activity. The compounds were found to be more active against the related mammalian enzyme cathepsin D, presumably due to productive interactions of the N-alkyl substituent with the highly lipophilic S2 pocket. The most active inhibitors (5, 9, 10, 21, and 28), characterized by benzyl, cyclohexylmethyl, tert-butyl, or 1,4-dimethylpentyl moieties at the aziridine nitrogen atom, exhibit k(2nd) values between 500 and 900×10³ M⁻¹ min⁻¹ and K(i) values near or below 1 μM for cathepsin D.

Candida tropicalis in human disease

Candida tropicalis is one of the more common Candida causing human disease in tropical countries; the frequency of invasive disease varies by geography causing 3--66% of candidaemia. C. tropicalis is taxonomically close to C. albicans and shares many pathogenic traits. C. tropicalis is particularly virulent in neutropenic hosts commonly with hematogenous seeding to peripheral organs. For candidaemia and invasive candidiasis amphotericin B or an echinocandin are recommended as first-line treatment, with extended-spectrum triazoles acceptable alternatives. Primary fluconazole resistance is uncommon but may be induced on exposure. Physicians in regions where C. tropicalis is common need to be mindful of this lesser-described pathogen.

Candida albicans RFX2 encodes a DNA binding protein involved in DNA damage responses, morphogenesis, and virulence

We previously showed that Candida albicans orf19.4590, which we have renamed RFX2, expresses a protein that is reactive with antibodies in persons with candidiasis. In this study, we demonstrate that C. albicans RFX2 shares some functional redundancy with Saccharomyces cerevisiae RFX1. Complementation of an S. cerevisiae rfx1 mutant with C. albicans RFX2 partially restored UV susceptibility and the repression of DNA damage response genes. DNA damage- and UV-induced genes RAD6 and DDR48 were derepressed in a C. albicans rfx2 null mutant strain under basal conditions, and the mutant was significantly more resistant to UV irradiation, heat shock, and ethanol than wild-type strain SC5314. The rfx2 mutant was hyperfilamentous on solid media and constitutively expressed hypha-specific genes HWP1, ALS3, HYR1, ECE1, and CEK1. The mutant also demonstrated increased invasion of solid agar and significantly increased adherence to human buccal epithelial cells. During hematogenously disseminated candidiasis, mice infected with the mutant had a significantly delayed time to death compared to the wild type. During oropharyngeal candidiasis, mice infected with the mutant had significantly lower tissue burdens in the oral cavity and esophagus at 7 days and they were less likely to develop disseminated infections because of mucosal translocation. The data demonstrate that C. albicans Rfx2p regulates DNA damage responses, morphogenesis, and virulence.

Physiological and molecular characterization of atypical isolates of Malassezia furfur

The species constituting the genus Malassezia are considered to be emergent opportunistic yeasts of great importance. Characterized as lipophilic yeasts, they are found in normal human skin flora and sometimes are associated with different dermatological pathologies. We have isolated seven Malassezia species strains that have a different Tween assimilation pattern from the one typically used to differentiate M. furfur, M. sympodialis, and M. slooffiae from other Malassezia species. In order to characterize these isolates of Malassezia spp., we studied their physiological features and conducted morphological and molecular characterization by PCR-restriction fragment length polymorphism and sequencing of the 26S and 5.8S ribosomal DNA-internal transcribed spacer 2 regions in three strains from healthy individuals, four clinical strains, and eight reference strains. The sequence analysis of the ribosomal region was based on the Blastn algorithm and revealed that the sequences of our isolates were homologous to M. furfur sequences. To support these findings, we carried out phylogenetic analyses to establish the relationship of the isolates to M. furfur and other reported species. All of our results confirm that all seven strains are M. furfur; the atypical assimilation of Tween 80 was found to be a new physiological pattern characteristic of some strains isolated in Colombia.

Phenotypic switching in Candida lusitaniae on copper sulfate indicator agar: association with amphotericin B resistance and filamentation

Candida lusitaniae is an opportunistic yeast pathogen that has the ability to develop resistance to amphotericin B (AmB). The mechanism(s) for this resistance is not well understood, although there are data supporting mutations in sterol pathways and other data supporting phenotypic switching (PS). The goal of this study was to determine whether C. lusitaniae has a PS system and to characterize any phenotypes, including any changes in AmB MICs. When 10(4) CFU of an AmB-resistant (MIC of 16 to 32 microg/ml) clinical strain was plated on yeast-peptone-dextrose (YPD) agar with 1 mM CuSO(4), three colony colors were observed: light brown (LB) >> dark brown (DB) > white (W), similar to the result for Candida glabrata. Switching did occur with high AmB resistance (MIC of 256 microg/ml) being associated with W, whereas LB and DB colonies had MICs of 2 to 8 microg/ml and 2 to 16 microg/ml, respectively. Filamentation (pseudohyphae) was associated with DB colonies. All phenotypes occurred spontaneously with greater frequency ( approximately 10(-2) to 10(-4)) than spontaneous mutations, and all phenotypes were reversible, fulfilling the two PS criteria. High AmB MICs were always associated with W colonies but not with all W colonies. Detection of PS on YPD-CuSO(4) is also similar to that in Candida glabrata, and we hypothesize that this is due to similarities in metallothionein gene expression. Phenotypic switching represents a key strategy in C. lusitaniae that confers a selective advantage during environmental challenges, including the ability to switch to AmB resistance.

Biosensor-based screening and characterization of HIV-1 inhibitor interactions with Sap 1, Sap 2, and Sap 3 from Candida albicans

A surface plasmon resonance (SPR) biosensor-based strategy for identification and characterization of compounds has been devised as a tool for the discovery of specific drugs for treatment of Candida albicans infections. Three secreted aspartic proteases (Saps 1-3) from C. albicans were used as parallel targets. The stepwise procedure involved screening of 104 HIV-1 pro-tease inhibitors at a single concentration for binding to the targets. Twenty-four compounds that appeared to interact with the targets were identified in the screen. False positives and compounds with low affinities or very fast dissociation rates could be removed after a series of additional measurements of these compounds at 3 different concentrations. Kinetic characterization was performed with 13 compounds, giving information about the interaction mechanism and interaction kinetic parameters (k(on), k(off), and K(D)). The pH dependence of the interaction and the inhibitory effect of a final small set of compounds were also evaluated. The strategy resulted in the identification of ritonavir as the compound generally exhibiting the highest affinity for the Candida enzymes. It had similar interaction kinetic characteristics for Sap 1 and Sap 2 but a lower affinity for Sap 3 due to a slower association rate. Several additional compounds with high affinity and/or slow dissociation rates for the targets were identified, revealing 2 other structural scaffolds for Sap inhibitors. In addition, important differences in the specificity for these types of compounds by the Saps were identified. The stepwise biosensor-based strategy was consequently efficient for identification and characterization of new lead compounds for 3 important drug targets.

Genetic relatedness of oral yeasts within and between patients with marginal periodontitis and subjects with oral health

BACKGROUND

Yeasts are found in periodontal pockets at a frequency of 15-21%. However, the genetic relatedness of oral yeasts within and between patients with marginal periodontitis is not clear.

OBJECTIVES

Assay genetic relatedness of oral yeasts from marginal periodontitis patients and oral health subjects, as well as genetic relatedness of yeasts from different oral sites in these two groups of participants.

MATERIAL AND METHODS

Yeast isolates were collected from 23 marginal periodontitis patients and 19 oral health subjects. Random amplified polymorphic DNA (RAPD) fingerprinting and the Dendron computer-assisted program for gel analyses were applied for estimation of genetic relatedness of yeasts.

RESULTS

The similarity coefficient (S(AB)) of the marginal periodontitis group ranged from 0.49 to 1.00 with an average of 0.64 +/- 0.11, whereas the S(AB) of the oral health group ranged from 0.62 to 1.00 with an average of 0.72 +/- 0.07. Three genetic clusters and 73 genotypes were obtained from the marginal periodontitis group, whereas three genetic clusters and 55 genotypes were found in the oral health group. In the pooled dendrogram, 57% of the yeast isolates and the type strain of Candida albicans fell in a major cluster V. There were no significant differences between the frequencies of clusters from the different oral sites within the two participant groups.

CONCLUSIONS

Genetically heterogeneous yeasts were found in the oral cavities of marginal periodontitis patients and oral health subjects. Similar genetic clustering patterns were obtained from the yeasts of the two groups, with cluster V being most predominant. Yeasts of the marginal periodontitis group were more genetically diverse than yeasts of the oral health group, and some yeasts of the marginal periodontitis group exhibited unique genetic patterns. There was no clear association between yeast genetic clusters and oral sites in the two participant groups.

Sensing the host environment: recognition of hemoglobin by the pathogenic yeast Candida albicans

Adhesion to host cells and tissues is important for several steps in the pathogenesis of disseminated Candida albicans infections. Although such adhesion is evident in vivo and for C. albicans grown in vitro in complex medium, some adhesive activities are absent when cultures are grown in defined media. However, addition of hemoglobin to defined media restores binding and adhesion to several host proteins. This activity of hemoglobin is independent of iron acquisition and is mediated by a cell surface hemoglobin receptor. In addition to regulating expression of adhesion receptors, hemoglobin rapidly induces expression of several genes. One of these, a heme oxygenase, allows the pathogen to utilize exogenous heme or hemoglobin to acquire iron and to produce the cytoprotective molecules alpha-biliverdin and carbon monoxide. The specific recognition of and responses to hemoglobin demonstrate a unique adaptation of C. albicans to be both a commensal and an opportunistic pathogen in humans.

Ammonia signaling in yeast colony formation

Multicellular structures formed by microorganisms possess various properties, which make them interesting in terms of processes that occur in tissues of higher eukaryotes. These include processes important for morphogenesis and development of multicellular structures as well as those evoked by stress, starvation, and aging. Investigation of colonies created by simple nonmotile yeast cells revealed the existence of various regulators involved in their development. One of the identified signaling compounds, unprotonated volatile ammonia, is produced by colonies in pulses and seems to represent a long-distance signal notifying the colony population of incoming nutrient starvation. This alarm evokes changes in colonies that are important for their long-term survival. Models of the action of ammonia on yeast cells as well as the routes of its production are proposed. Interestingly, ammonia/ammonium also act as a signaling molecule in other organisms. Ammonia regulates several steps of the multicellular development of Dictyostelium discoideum and evidence indicates that ammonia/ammonium plays a role in neural tissues of higher eukaryotes.

Domestication of wild Saccharomyces cerevisiae is accompanied by changes in gene expression and colony morphology

Although colonies from Saccharomyces cerevisiae laboratory strains are smooth, those isolated from nature exhibit a structured fluffy pattern. Environmental scanning electron microscopy shows that the cells within wild fluffy colonies are connected by extracellular matrix (ECM) material. This material contains a protein of about 200 kDa unrelated to the flocculins, proteins involved in cell-cell adhesion in liquid media. The matrix material binds to concanavalin A. Within a few passages on rich agar medium, the wild strains switch from the fluffy to the smooth colony morphology. This domestication is accompanied by loss of the ECM and by extensive changes in gene expression as detected by DNA microarrays. The expression of about 320 genes was changed in smooth colonies. The major changes comprise carbohydrate metabolism, cell wall, water channels, Ty-transposons and subtelomeric genes, iron homeostasis, vitamin metabolism and cell cycle and polarity. The growth in fluffy colonies may represent a metabolic strategy for survival of yeast under unfavourable conditions that is switched off under felicitous laboratory conditions.

Carvone and perillaldehyde interfere with the serum-induced formation of filamentous structures in Candida albicans at substantially lower concentrations than those causing significant inhibition of growth

Carvone and perillaldehyde were shown to inhibit the transformation of Candida albicans to a filamentous form at concentrations far lower and more biologically relevant than the concentrations necessary to inhibit growth. This morphological transformation is associated with C. albicans pathogenicity; hence these naturally occurring monoterpenes are potential lead compounds in the development of therapeutic agents against C. albicans infection.

Secreted aspartic proteases as virulence factors of Candida species

Candida infections have emerged as a significant medical problem during the last few decades. Among the different virulence traits of C. albicans, secreted proteolytic activity has been intensively investigated. Pathogenesis of the various forms of candidiasis was shown to be associated with the differential and temporal regulation of the expression of genes coding for secreted aspartic proteases (Sap). These enzymes act as cytolysins in macrophages after phagocytosis of Candida, are present in tissue penetration and are also involved in adherence to epithelial cells. Since the introduction of new antiretroviral therapeutics such as HIV protease inhibitors, oropharyngeal candidiasis is less often observed in AIDS patients. Different HIV aspartic protease inhibitors were able to inhibit the C. albicans Saps involved in adherence. The lower rates of oropharyngeal candidiasis observed in individuals receiving antiretroviral combination therapy could reflect not only an improvement in the immune system but also direct inhibition of Candida Saps by HIV protease inhibitors. Therefore, the development of specific aspartic protease inhibitors might be of interest for the inhibition of candidiasis.