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Byline: Shazmin. Mohamed, Muhammad. Asif, Mansoureh. Nazari, Hussein. Baharetha, Syed. Mahmood, Abdul. M. Yatim, Aman. Abdul Majid, Amin. Abdul Majid OBJECTIVES: Sophorolipids (SLs) are a group of surface-active glycolipids produced [...]

ABSTRACT Candida albicans is an opportunistic human fungal pathogen which causes disease mainly in immuno-compromised patients. Activity of hydrolytic enzymes is essential for virulence of C. albicans and so is [...]

The pathogenic fungus Candida albicans is able to cover its most potent proinflammatory cell wall molecules, the [beta]-glucans, underneath a dense mannan layer, so that the pathogen becomes partly invisible for immune cells such as phagocytes. As the C. albicans histidine kinases Chk1 p, Cos1 p and CaSIn1p had been reported to be involved in virulence and cell wall biosynthesis, we investigated whether deletion of the respective genes influences the activity of phagocytes against C. albicans. We found that among all histidine kinase genes, CHK1 plays a prominent role in phagocyte activation. Uptake of the deletion mutant [DELTA]chk1 as well as the acidification of [DELTA]chk1-carrying phagosomes was significantly increased compared with the parental strain. These improved activities could be correlated with an enhanced accessibility of the mutant [beta]-1,3-glucans for immunolabelling. In addition, any inhibition of [beta]-1,3-glucan-mediated phagocytosis resulted in a reduced uptake of [DELTA]chk1, while ingestion of the parental strain was hardly affected. Moreover, deletion of CHK1 caused an enhanced release of interleukins 6 and 10, indicating a stronger activation of the [beta]-1,3-glucan receptor dectin-1. In conclusion, the Chk1 p protein is likely to be involved in masking [beta]-1,3-glucans from immune recognition. As there are no homologues of fungal histidine kinases in mammals, Chk1p has to be considered as a promising target for new antifungal agents. DOI 10.1099/mic.0.040006-0

Among the most important classes of regulatory proteins are the sequence-specific DNA-binding proteins that control transcription through the occupancy of discrete DNA sequences within genomes. Currently, this class of proteins encompasses at least 37 distinct structural superfamilies and more than 100 distinct structural motifs. In this paper, we examine the transcriptional regulator Wor1, a master regulator of white-opaque switching in the human fungal pathogen Candida albicans. As assessed by a variety of algorithms, this protein has no sequence or structural similarity to any known DNA-binding protein. It is, however, conserved across the vast fungal lineage, with a 300aa region of sequence conservation. Here, we show that this 300aa region of Wor1 exhibits sequence-specific DNA binding and therefore represents a new superfamily of DNA-binding proteins. We identify the 14-nucleotide-pair DNA sequence recognized by Wor1, characterize the site through mutational analysis, and demonstrate that this sequence is sufficient for the Wor1-dependent activation of transcription in vivo. Within the 300aa DNA-binding conserved region, which we have termed the WOPR box, are two domains (WOPRa and WOPRb), dissimilar to each other but especially well-conserved across the fungal lineage. We show that the WOPR box binds DNA as a monomer and that neither domain, when expressed and purified separately, exhibits sequence-specific binding. DNA binding is restored, however, when the two isolated domains are added together. These results indicate that the WOPR family of DNA-binding proteins involves an unusual coupling between two dissimilar, covalently linked domains. DNA-protein interaction | transcription factor | gene expression | DNA motif | heritable states of transcription doi/ 10.1073/pnas.1005911107

The fungal cell wall plays a crucial role in host-pathogen interactions. Its formation is the result of the coordinated activity of several extracellutar enzymes, which assemble the constituents, and remodel and hydrolyse them in the extracellular space. Canclida albicans Phrl and Phr2 proteins belong to family GH72 of the [beta]-(1,)-glucanosyltransferases and play a crucial role in cell wall assembly. PHR1 and PHR2, homologues of Saccharomyces cerevisiae GAS1, are differently regulated by extracellular pH. PHR1 is expressed when ambient pH is 5.5 or higher, whereas PHR2 has the reverse expression pattern. Their deletion causes a pH-conditional defect in morphogenesis and virulence. In this work we explored whether PHR1 deletion affects the ability of C. albicans to adhere to and invade human epithelia. PHR1 null mutants exhibited a marked reduction in adhesion to both abiotic surfaces and epithelial cell monolayers. In addition, the mutant was unable to penetrate and invade reconstituted human epithelia. Transcription profiling of selected hyphal-specific and adhesin-encoding genes indicated that in the PHR1 null mutant, HWP1 and ECE1 transcript levels were similarly reduced in both adhesion and suspension conditions. These results, combined with microscopy analysis of the septum position, suggest that PHR1 is not required for the induction of hyphal development but plays a key role in the maintenance of hyphal growth. Thus, the [beta]-(1,)-glucan processing catalysed by Phrl p is of fundamental importance in the maintenance of the morphological state on which the adhesive and invasive properties of C. albicans greatly depend. DOI 10.1099/mic.0.038000-0

Signal-mediated interactions between the human opportunistic pathogens Pseudomonas aeruginosa and Candida albicans affect virulence traits in both organisms. Phenotypic studies revealed that bacterial supernatant from four P. aeruginosa strains strongly reduced the ability of C. albicans to form biofilms on silicone. This was largely a consequence of inhibition of biofilm maturation, a phenomenon also observed with supernatant prepared from non-clinical bacterial species. The effects of supernatant on biofilm formation were not mediated via interference with the yeast--hyphal morphological switch and occurred regardless of the level of homoserine lactone (HSL) produced, indicating that the effect is HSL-independent. A transcriptome analysis to dissect the effects of the P. aeruginosa supernatants on gene expression in the early stages of C. albicans biofilm formation identified 238 genes that exhibited reproducible changes in expression in response to all four supernatants. In particular, there was a strong increase in the expression of genes related to drug or toxin efflux and a decrease in expression of genes associated with adhesion and biofilm formation. Furthermore, expression of YWP1, which encodes a protein known to inhibit biofilm formation, was significantly increased. Biofilm formation is a key aspect of C. albicans infections, therefore the capacity of P. aeruginosa to antagonize this has clear biomedical implications. DOI 10.1099/mic.0.037549-0

C9-methylated glucosylceramide is a fungus-specific sphingolipid. This lipid is a major membrane component in the cell and is thought to play important roles in the growth and virulence of several fungal species. To investigate the importance of the methyl branch of the long-chain base in glucosylceramides in pathogenic fungi, we identified and characterized a sphingolipid C9-methyltransferase gene (MTS1, C9-MethylTransferase for Sphingolipid 1) in the pathogenic yeast Candida albicans. The mts1 disruptant lacked (E,E)-9-methylsphinga-4,8-dienine in its glucosylceramides and contained (E)-sphing-4-enine and (E,E)-sphinga-4,8-dienine. Reintroducing the MTS1 gene into the mts1 disruptant restored the synthesis of (E,E)-9-methylsphinga-4,8-dienine in the glucosylceramides. We also created a disruptant of the HSX11 gene, encoding glucosylceramide synthase, which catalyses the final step of glucosylceramide synthesis, in C. albicans and compared this mutant with the mts1 disruptant. The C. albicans mts1 and hsx11 disruptants both had a decreased hyphal growth rate compared to the wild-type strain. The hsx11 disruptant showed increased susceptibility to SDS and fluconazole, similar to a previously reported sld1 disruptant that contained only (E)-sphing-4-enine in its glucosylceramides, suggesting that these strains have defects in their cell membrane structures. In contrast, the mts1 disruptant grew similarly to wild-type in medium containing SDS or fluconazole. These results suggest that the C9-methyl group of a long-chain base in glucosylceramides plays an important role in the hyphal elongation of C. albicans independent of lipid membrane disruption. DOI 10.1099/mic.0.033985-0

The isoleucine and valine biosynthetic enzyme acetolactate synthase (Ilv2p) is an attractive antifungal drug target, since the isoleucine and valine biosynthetic pathway is not present in mammals, Saccharomyces cerevisiae ilv2[DELTA] mutants do not survive in vivo, Cryptococcus neoformans ilv2 mutants are avirulent, and both S. cerevisiae and Cr. neoformans ilv2 mutants die upon isoleucine and valine starvation. To further explore the potential of Ilv2p as an antifungal drug target, we disrupted Candida albicans ILV2, and demonstrated that Ca. albicans ilv2[DELTA] mutants were significantly attenuated in virulence, and were also profoundly starvation-cidal, with a greater than 100-fold reduction in viability after only 4 h of isoleucine and valine starvation. As fungicidal starvation would be advantageous for drug design, we explored the basis of the starvation-cidal phenotype in both S. cerevisiae and Ca. albicans ilv2[DELTA] mutants. Since the mutation of ILV1, required for the first step of isoleucine biosynthesis, did not suppress the ilv2[DELTA] starvation-cidal defects in either species, the cidal phenotype was not due to [alpha]-ketobutyrate accumulation. We found that starvation for isoleucine alone was more deleterious in Ca. albicans than in S. cerevisiae, and starvation for valine was more deleterious than for isoleucine in both species. Interestingly, while the target of rapamycin (TOR) pathway inhibitor rapamycin further reduced S. cerevisiae ilv2[DELTA] starvation viability, it increased Ca. albicans ilv1[DELTA] and ilv2[DELTA] viability. Furthermore, the recovery from starvation was dependent on the carbon source present during recovery for S. cerevisiae ilv2[DELTA] mutants, reminiscent of isoleucine and valine starvation inducing a viable but non-culturable-Like state in this species, while Ca. albicans ilv1[DELTA] and ilv2[DELTA] viability was influenced by the carbon source present during starvation, supporting a role for glucose wasting in the Ca. albicans cidal phenotype. DOI 10.1099/mic.0.034348-0

Device-associated microbial growth, including Candida biofilms, represents more than half of all human microbial infections and, despite a relatively small risk of implant-associated diseases, this type of infection usually leads to high morbidity, increased health-care costs and prolonged antimicrobial therapy. Animal models are needed to elucidate the complex host--pathogen interactions that occur during the development of attached and structured biofilm populations. We describe here a new in vivo model to study Candida biofilm, based on the avascular implantation of small catheters in rats. Polyurethane biomaterials challenged with Candida cells were placed underneath the skin of immunosuppressed animals following only minor surgery. The model allowed the study of up to ten biofilms at once, and the recovery of mature biofilms from 2 days after implantation. The adhering inoculum was adjusted to the standard threshold of positive diagnosis of fungal infection in materials recovered from patients. Wild-type biofilms were mainly formed of hyphal cells, and they were unevenly distributed across the catheter length as observed in infected materials in clinical cases. The hyphal multilayered structure of the biofilms of wild-type strains was observed by confocal microscopy and compared to the monolayer of yeast or hyphal cells of two well-known biofilm-deficient strains, efg1[DELTA]/efg1[DELTA] cph1[DELTA]/cph1[DELTA] and bcr1[DELTA]/ bcr1[DELTA], respectively. The subcutaneous Candida biofilm model relies on the use of implanted catheters with accessible, fast and minor surgery to the animals. This model can be used to characterize the ability of antimicrobial agents to eliminate biofilms, and to evaluate the prophylactic effect of antifungal drugs and biomaterial coatings. DOI 10.1099/mic.0.033530-0

Using genetically matched azole-susceptible (AS) and azole-resistant (AR) clinical isolates of Candida albicans, we recently demonstrated that CDR1 overexpression in AR isolates is due to its enhanced transcriptional activation and mRNA stability. This study examines the molecular mechanisms underlying enhanced CDR1 mRNA stability in AR isolates. Mapping of the 3' untranslated region (3' UTR) of CDR1 revealed that it was rich in adenylate/uridylate (AU) elements, possessed heterogeneous polyadenylation sites, and had putative consensus sequences for RNA-binding proteins. Swapping of heterologous and chimeric lacZ-CDR1 3' UTR transcriptional reporter fusion constructs did not alter the reporter activity in AS and AR isolates, indicating that cis-acting sequences within the CDR1 3' UTR itself are not sufficient to confer the observed differential mRNA decay. Interestingly, the poly(A) tail of the CDR1 mRNA of AR isolates was ~35-50 % hyperadenylated as compared with AS isolates. C. albicans poly(A) polymerase (PAP1), responsible for mRNA adenylation, resides on chromosome 5 in close proximity to the mating type-like (MTL) locus. Two different PAP1 alleles, PAP1-a/PAP1-[alpha], were recovered from AS (MTL-a/MTL-[alpha]), while a single type of PAP1 allele (PAP1-[alpha]) was recovered from AR isolates (MTL-[alpha]/MTL-[alpha]). Among the heterozygous deletions of PAP1-a ([DELTA]pap1-a/PAP1-[alpha]) and PAP1-[alpha] (PAP1-a/[DELTA]pap1-[alpha]), only the former led to relatively enhanced drug resistance, to polyadenylation and to transcript stability of CDR1 in the AS isolate. This suggests a dominant negative role of PAP1-a in CDR1 transcript polyadenylation and stability. Taken together, our study provides the first evidence, to our knowledge, that loss of heterozygosity at the PAP1 locus is linked to hyperadenylation and subsequent increased stability of CDR1 transcripts, thus contributing to enhanced drug resistance. DOI 10.1099/mic.0.035154-0

Steam distillation of Lavandula angustifolia cv.Okamurasaki cultivated in Akita, Japan gave two kinds of hydrosols from the floret and stalk origins. The floret hydrosol gave linalool as a main constituent with little content of coumarin, while the stalk hydrosol gave coumarin as a main constituent with linalool as a minor constituent. The result revealed that coumarin often detected in lavender oil and hydrosol as a constituent in various amount is originated from the stalks but not from the florets. Furthermore, the composition and content of constituents in the floret and stalk hydrosols were considerably different from that of commercially available lavender hydrosol, though the composition of essential oil was similar. Both hydrosols showed potent and comparable inhibitory activity against filament formation of Candida albicans, but the stalk hydrosol gave higher scavenging activity than the floret hydrosol against 1,1 -diphenyl-2- picrylhydrazyl radical. Key words: Lavender, floret hydrosol, stalk hydrosol, essential oil, anti-Candida activity, radical scavenging activity, coumarin

The early endocytic patch protein Sla2 is important for morphogenesis and growth rates in Saccharomyces cerevisiae and Candida albicans, but the mechanism that connects these processes is not clear. Here we report that growth defects in cells lacking CaSLA2 or ScSLA2 are associated with a cell cycle delay that is influenced by Swe1, a morphogenesis checkpoint kinase. To establish how Swe1 monitors Sla2 function, we compared actin organization and cell cycle dynamics in strains lacking other components of early endocytic patches (Sla1 and Abp1) with those in strains lacking Sla2. Only sla2 strains had defects in actin cables, a known trigger of the morphogenesis checkpoint, yet all three strains exhibited Swe1-dependent phenotypes. Thus, Swe1 appears to monitor actin patch in addition to actin cable function. Furthermore, Swe1 contributed to virulence in a mouse model of disseminated candidiasis, implying a role for the morphogenesis checkpoint during the pathogenesis of C. albicans infections. DOI 10.1099/mic.0.033233-0

The fungal cell wall is essential in maintaining cellular integrity and plays key roles in the interplay between fungal pathogens and their hosts. The PGA59 and PGA62 genes encode two short and related glycosylphosphatidylinositol-anchored cell wall proteins and their expression has been previously shown to be strongly upregulated when the human pathogen Candida albicans grows as biofilms. Using GFP fusion proteins, we have shown that Pga59 and Pga62 are cell-wall-located, N- and O-glycosylated proteins. The characterization of C. albicans pga59[DELTA]/pga59[DELTA], pga62[DELTA]/pga62[DELTA] and pga59[DELTA]/pga59[DELTA] pga62A/pga62[DELTA] mutants suggested a minor role of these two proteins in hyphal morphogenesis and that they are not critical to biofilm formation. Importantly, the sensitivity to different cell-wall-perturbing agents was altered in these mutants. In particular, simultaneous inactivation of PGA59 and PGA62 resulted in high sensitivity to Calcofluor white, Congo red and nikkomicin Z and in resistance to caspofungin. Furthermore, cell wall composition and observation by transmission electron microscopy indicated an altered cell wall structure in the mutant strains. Collectively, these data suggest that the cell wall proteins Pga59 and Pga62 contribute to cell wall stability and structure.

Candida albicans is the most frequently isolated human fungal pathogen among species causing biofilm-related clinical infections. Mechanical properties of Candida biofilms have hitherto been given no attention, despite the fact that mechanical properties are important for selection of treatment or dispersal of biofilm organisms due to a bodily fluid flow. The aim of this study was to identify the factors that determine the compression strength of Candida biofilms. Biofilms of C. albicans wild-type parental strain Caf2-1, mutant strain Chk24 lacking Chklp [known to be involved in regulation of morphogenesis (yeast-to-hyphae transition)] and gene-reconstructed strain Chk23 were evaluated for their resistance to compression, along with biofilms of Candida tropicalis GB 9/9 and Candida parapsilosis GB 2/8, derived from used voice prosthetic biofilms. Additionally, cell morphologies within the biofilm, cell-surface hydrophobicities and extracellular polymeric substance composition were determined. Our results suggest that the hyphae-to-yeast ratio influences the compression strength of C. albicans biofilms. Biofilms with a hyphal content >50 % possessed significantly higher compressive strength and were more difficult to destroy by vortexing and sonication than biofilms with a lower hyphal content. However, when the amount of extracellular DNA (eDNA) in biofilms of C. albicans Caf2-1 and Chk24 increased, biofilm strength declined, suggesting that eDNA may influence biofilm integrity adversely.

The detection of protein-protein interactions through two-hybrid assays has revolutionized our understanding of biology. The remarkable impact of two-hybrid assay platforms derives from their speed, simplicity, and broad applicability. Yet for many organisms, the need to express test proteins in Saccharomyces cerevisiae or Escherichia coli presents a substantial barrier because variations in codon specificity or bias may result in aberrant protein expression. In particular, nonstandard genetic codes are characteristic of several eukaryotic pathogens, for which there are currently no genetically based systems for detection of protein-protein interactions. We have developed a protein-protein interaction assay that is carried out in native host cells by using GFP as the only foreign protein moiety, thus circumventing these problems. We show that interaction can be detected between two protein pairs in both the model yeast S. cerevisiae and the fungal pathogen Candida albicans. We use computational analysis of microscopic images to provide a quantitative and automated assessment of confidence. DOI 10.1534/genetics.109.101162

Candida albicans accumulates large amounts of the polyols glycerol and D-arabitol when the cells are exposed to physiological conditions relevant to stress and virulence in animals. Intracellular concentrations of glycerol are determined by rates of glycerol production and catabolism and of glycerol uptake and efflux through the plasma membrane. We and others have studied glycerol production in C. albicans, but glycerol uptake by C. albicans has not been studied. In the present study, we found that [[sup.14]C]glycerol uptake by C. albicans SC5314 was (i) accumulative; (ii) dependent on proton-motive force; (iii) unaffected by carbon source; and (iv) unaffected by large molar excesses of D-arabitol or other polyols. The respective [K.sub.m] and [V.sub.max] values were 2.1 mM and 460 [micro]mol [h.sup.-1] [(g dry wt).sup.-1] in glucose medium and 2.6 mM and 268 [micro]mol [h.sup.-1] [(g dry wt).sup.-1] in glycerol medium. To identify the C. albicans glycerol uptake protein(s), we cloned the C. albicans homologues of the Saccharomyces cerevisiae genes GUP1 and STL1, both of which are known to be involved in glycerol transport. When multicopy plasmids encoding C. albicans STL1, C. albicans STL2 and C. albicans GUP1 were introduced into the corresponding S. cerevisiae null mutants, the transformants all acquired the ability to grow on minimal glycerol medium; however, only S. cerevisiae stl1 null mutants transformed with C. albicans STL1 actively took up extracellular [[sup.14]C]glycerol. When both chromosomal alleles of C. albicans STL1 were deleted from C. albicans BWP17, the resulting stl1 null mutants grew poorly on minimal glycerol medium, and their ability to transport [[sup.14]C]glycerol into the cell was markedly reduced. In contrast, deletion of both chromosomal alleles of C. albicans STL2 or of C. albicans GUP1 had no significant effects on [[sup.14]C]glycerol uptake or the ability to grow on minimal glycerol medium. Northern blot analysis indicated that C. albicans STL1 was expressed in both glucose and glycerol media, conditions under which we detected wild-type active glycerol uptake. Furthermore, STL1 was highly expressed in salt-stressed cells; however, the stl1 null mutant was no more sensitive to salt stress than wild-type controls. We also detected high levels of STL2 expression in glycerol-grown cells, even though deletion of this gene did not influence glycerol uptake activity in glycerol-grown cells. We conclude from the results above that a plasma-membrane [H.sup.+] symporter encoded by C. albicans STL1 actively transports glycerol into C. albicans cells.

The ability of the pathogenic fungus Candida albicans to cause disease requires rapid adaptation to changes in the host environment and to an evolving host immune response. The identification of 'virulence factors' using in vitro characterization of mutant strains has traditionally relied on a common set of phenotypic and biochemical assays (most often performed at 30[degrees]C) and the subsequent correlation with their corresponding virulence in mouse models of disease. Utilizing a panel of isogenic mutants for the multifunctional signal-modulating 14-3-3 protein (Bmh1p), we have found that specific mutations affect a variety of different pathways currently associated with virulence, including those involved with the formation of filaments, as well as interaction with host immune cells. Surprisingly, our studies revealed that deficiencies in many of these pathways do not always correlate with virulence in a mouse model of disseminated infection. Mutations within the binding pocket of Bmh1p that affect the ability of the protein to efficiently bind ligand had varying effects on the results of a number of in vitro and in vivo assays. The capability, in vitro, to filament in embedment conditions, and to filament and form chlamydospores under microaerophilic conditions on cornmeal agar, does not correlate with virulence. It is likely that only a subset of hyphal signalling pathways is actually required for the establishment of infection in the disseminated mouse model. Most importantly, our results suggest that the delayed onset of lag-phase growth in vitro at 37[degrees]C, and not at 30[degrees]C, results in an inability of these mutants to rapidly adjust to environmental changes in vivo and may be responsible for their increased clearance and reduced virulence. It is critical, therefore, that future in vitro studies of putative virulence factors in C. albicans include careful characterization at physiological temperatures.

Mitochondria of eukaryotic organisms contain populations of DNA molecules that are packed into higher-order structures called mitochondrial nucleoids (mt-nucleoids). In Saccharomyces cerevisiae, the compaction of mitochondrial DNA (mtDNA) into mt-nucleoids is mediated primarily by the high-mobility group (HMG) box-containing protein Abf2, which is an important player in stabilization and metabolism of mtDNA. Although it is evident that analogous proteins must exist in other yeast species, an apparently fast divergence rate has precluded their identification, characterization and comparative analysis. Using in silico analysis of the complete genome sequence of the pathogenic yeast Candida albicans we predicted that the ORF 19.400/19.8030 assigned as GCF1 encodes a putative mitochondrial HMG box-containing protein. In contrast to Abf2p, which contains two HMG boxes, Gcfl p contains only one C-terminal HMG box. In addition, it contains one putative coiled-coil domain with a potential role in protein dimerization. Fluorescence microscopy analysis of a C-terminally tagged Gcf1p with green fluorescent protein (GFP) revealed its mitochondrial localization in both heterologous (S. cerevisiae) and native (C. albicans) hosts. Biochemical analyses of DNA-binding properties indicate that Gcf1p is, similarly to Abf2p, a non-specific DNA-binding protein. To analyse the role of Gcf1p in mtDNA metabolism, we constructed strains lacking one functional allele of the GCF1 gene and carrying one GCF1 allele under the control of the MET3 promoter. Under repressible conditions this strain exhibited a more than 3000-fold decrease in levels of GCF1 mRNA, which was correlated with a substantial decrease in the number of mtDNA copies as well as recombination intermediates. The dramatic effect of reduced levels of Gcf1p on mtDNA metabolism indicates that the protein is involved in essential molecular transactions that relate to the mitochondrial genome.

The outermost layer of the Candida albicans cell wall is enriched with mannosylated glycoproteins. We have used a range of isogenic glycosylation mutants of C. albicans, which are defective to varying degrees in cell wall protein mannosylation, to investigate the role of the outermost layer of the yeast cell wail in mediating the fungicidal action of the cationic, [alpha]-helical antimicrobial peptide dermaseptin S3(1-16) [DsS3(1-16)]. The degree of phosphomannan loss, and concomitant reduction in surface negative charge, from the series of glycosylation mutants correlated with reduced levels of peptide binding to the cells. In turn, the reduced peptide binding correlated with enhanced resistance to DsS3(1-16). To ascertain whether DsS3(1-16) binds to negatively charged phosphate, we studied the effect of exogenous glucosamine 6-phosphate, and glucosamine hydrochloride as a negative control, on the antifungal efficacy of DsS3(1-16). Glucosamine 6-phosphate retarded the efficacy of DsS3(1-16), and this was attributed to the presence of phosphate, because addition of identical concentrations of glucosamine hydrochloride had little detrimental effect on peptide efficacy. Fluorescence microscopy with DsS3(1-16) tagged with fluorescein revealed that the peptide binds to the outer surface of the yeast cell, supporting our previous conclusion that the presence of exterior phosphomannan is a major determinant of the antifungal potency of DsS3(1-16). The binding of the peptide to the cell surface was a transient event that was followed by apparent localization of DsS3(1-16) in the vacuole or dissemination throughout the entire cytosol. The presence of glucosamine 6-phosphate clearly reduced the proportion of cells in the population that showed complete cytosolic staining, implying that the binding and entry of the peptide into the cytosol is significantly reduced due to the exogenous phosphate sequestering the peptide and reducing the amount of peptide able to bind to the surface phosphomannan. In conclusion, we present evidence that an antimicrobial peptide, similar to those employed by cells of the human immune system, has evolved to recognize molecular patterns on the surface of pathogens in order to maximize efficacy.

Signal transduction pathways mediated by mitogen-activated protein kinases (MAPKs) play crucial roles in eukaryotic cells. In the pathogenic fungus Candida albicans the HOG MAPK pathway regulates the response to external stresses (osmotic and oxidative among others) and is involved in morphogenesis and virulence. We show here that the lack of the Hog1 MAPK increases the sensitivity of this fungus to inhibitors of the respiratory chain, hog1 mutants also show an enhanced basal respiratory rate compared to parental strains, and higher levels of intracellular reactive oxygen species despite an increased expression of detoxifying enzymes. We also demonstrate that although oxidative phosphorylation is essentially unaffected, hog1 mutants have an altered mitochondrial membrane potential. Data indicate that hog1-defective mutants are more dependent on mitochondrial ATP synthesis, probably due to an increased cellular ATP demand. Our results therefore link a MAPK pathway with respiratory metabolism in pathogenic fungi.

A well-known virulence attribute of the human-pathogenic yeast Candida albicans is the secretion of aspartic proteases (Saps), which may contribute to colonization and infection of different host niches by degrading tissue barriers, destroying host defence molecules, or digesting proteins for nutrient supply. The role of individual Sap isoenzymes, which are encoded by a large gene family, for the pathogenicity of C. albicans has been investigated by assessing the virulence of mutants lacking specific SAP genes and by studying the expression pattern of the SAP genes in various models of superficial and systemic infections. We used a recombination-based genetic reporter system to detect the induction of the SAP1-SAP6 genes during infection of reconstituted human vaginal epithelium. Only SAP5, but none of the other tested SAP genes, was detectably activated in this in vitro infection model. To directly address the importance of the SAP1-SAP6 genes for invasion of reconstituted human epithelia (RHE), we constructed a set of mutants of the wild-type C. albicans model strain SC5314 in which either single or multiple SAP genes were specifically deleted. Even mutants lacking all of the SAP1-SAP3 or the SAP4-SAP6 genes displayed the same capacity to invade and damage both oral and vaginal RHE as their wild-type parental strain, in contrast to a nonfilamentous efg1[DELTA] mutant that was avirulent under these conditions. We therefore conclude from these results that the secreted aspartic proteases Sap1p-Sap6p are not required for invasion of RHE by C. albicans.

A quantitative real-time RT-PCR system was established to identify which secreted aspartyl proteinase (SAP) genes are most highly expressed and potentially contribute to Candida albicans infection of human epithelium in vitro and in vivo. C. albicans SC5314 SAP1-10 gene expression was monitored in organotypic reconstituted human epithelium (RHE) models, monolayers of oral epithelial cells, and patients with oral (n = 17) or vaginal (n= 17) candidiasis. SAP gene expression was also analysed in [DELTA]sap1-3, [DELTA]sap4-6, [DELTA]efg1 and [DELTA]efg1/cph1 mutants to determine whether compensatory SAP gene regulation occurs in the absence of distinct proteinase gene subfamilies. In monolayers, RHE models and patient samples SAP9 was consistently the most highly expressed gene in wild-type cells. SAP5 was the only gene significantly upregulated as infection progressed in both RHE models and was also highly expressed in patient samples. Interestingly, the SAP4-6 subfamily was generally more highly expressed in oral monolayers than in RHE models. SAP1 and SAP2 expression was largely unchanged in all model systems, and SAP3, SAP7 and SAP8 were expressed at low levels throughout. In [DELTA]sap1-3, expression was compensated for by increased expression of SAP5, and in [DELTA]sap4-6, expression was compensated for by SAP2: both were observed only in the oral RHE. Both [DELTA]sap1-3 and [DELTA]sap4-6 mutants caused RHE tissue damage comparable to the wild-type. However, addition of pepstatin A reduced tissue damage, indicating a role for the Sap family as a whole in inducing epithelial damage. With the hypha-deficient mutants, RHE tissue damage was significantly reduced in both [DELTA]efg1/cph1 and [DELTA]efg1, but SAP5 expression was only dramatically reduced in [DELTA]efg1/cph1 despite the absence of hyphal growth in both mutants. This indicates that hypha formation is the predominant cause of tissue damage, and that SAP5 expression can be hypha-independent and is not solely controlled by the Efg1 pathway but also by the Cph1 pathway. This is believed to be the first study to fully quantify SAP gene expression levels during human mucosal infections; the results suggest that SAP5 and SAP9 are the most highly expressed proteinase genes in vivo. However, the overall contribution of the Sap1-3 and Sap4-6 subfamilies individually in inducing epithelial damage in the RHE models appears to be low.

We previously identified Candida albicans Irs4p as an epidermal growth factor substrate 15 homology (EH) domain-containing protein that is reactive with antibodies in the sera of patients with candidiasis and contributes to cell wall integrity, hyphal formation and virulence. In this study, we use a yeast two-hybrid method and co-immunoprecipitation to show that Irs4p physically interacts with the phosphatase Inp51p. Disruption of the Inp51p Asn-Pro-Phe (NPF) motif eliminates the interaction, suggesting that this motif is targeted by Irs4p. Both inp51 and irs4 null mutants exhibit significantly increased levels of phosphatidylinositol-4,5-bisphosphate [PI(4,5)[P.sub.2]] without changes in levels of other phosphoinositides. Like the irs4 mutant, the inp51 mutant demonstrates increased susceptibility to cell wall-active agents, impaired hyphal formation and abnormal chitin distribution along hyphal walls during growth within solid agar. Moreover, the inp51 and irs4 mutants overactivate the cell wall integrity pathway as measured by Mkc1p phosphorylation. As anticipated, mortality due to disseminated candidiasis is significantly attenuated among mice infected with the inp51 mutant, and tissue burdens and inflammation within the kidneys are reduced. Hyphal formation and chitin distribution in vivo are also impaired, consistent with observations of embedded growth in vitro. All phenotypes exhibited by the inp51 and irs4 mutants are rescued by complementation with the respective genes. In conclusion, our findings suggest that Irs4p binds and activates Inp51p to negatively regulate PI(4,5)[P.sub.2] levels and the cell integrity pathway, and that PI(4,5)[P.sub.2] homeostasis is important for coordinating cell wall integrity, hyphal growth and virulence under conditions of cell wall stress.

The glyoxylate cycle, a metabolic pathway required for generating [C.sub.4] units from [C.sub.2] compounds, is an important factor in virulence, in both animal and plant pathogens. Here, we report the localization of the key enzymes of this cycle, isocitrate lyase (Icl1; EC 4.1.3.1) and malate synthase (Mls1; EC 2.3.3.9), in the human fungal pathogen Candida albicans. Immunocytochemistry in combination with subcetlular fractionation showed that both Icl1 and Mls1 are localized to peroxisomes, independent of the carbon source used. Although Icl1 and Mls1 lack a consensus type I peroxisomal targeting signal (PTS1), their import into peroxisomes was dependent on the PTS1 receptor Pex5p, suggesting the presence of non-canonical targeting signals in both proteins. Peroxisomal compartmentalization of the glyoxylate cycle is not essential for proper functioning of this metabolic pathway because a pex5[DELTA]/[DELTA] strain, in which Icl1 and Mls1 were localized to the cytosol, grew equally as well as the wild-type strain on acetate and ethanol. Previously, we reported that a fox2[DELTA]/[DELTA] strain that is completely deficient in fatty acid [beta]- oxidation, but has no peroxisomal protein import defect, displayed strongly reduced growth on non-fermentable carbon sources such as acetate and ethanol. Here, we show that growth of the fox2[DELTA]/[DELTA] strain on these carbon compounds can be restored when Icl1 and Mls1 are relocated to the cytosol by deleting the PEX5 gene. We hypothesize that the fox2[DELTA]/[DELTA] strain is disturbed in the transport of glyoxylate cycle products and/or acetyI-CoA across the peroxisomal membrane and discuss the possible relationship between such a transport defect and the presence of giant peroxisomes in the fox2[DELTA]/[DELTA] mutant.

Candida albicans, clinically the most important human fungal pathogen, rapidly develops resistance to antifungal drugs. The acquisition of resistance has been linked to various types of genome changes. As part of an ongoing study of this problem, we investigated mutation, genome stability and drug resistance acquisition in C. albicans strains with deletions in the base excision repair (BER) genes NTG1, APN1 and QGG1, and in the nucleotide excision repair (NER) genes RAD2 and RADIO. The BER mutants did not exhibit any change in their susceptibility to DNA-damaging agents, but the NER mutants were extremely sensitive to UV-induced DNA damage. We did not observe any significant change in mutation, genome stability and antifungal drug sensitivity in the mutant strains we tested. However, we detected a number of intriguing phenotypic differences between strains bearing deletions in equivalent C. albicans and Saccharomyces cerevisiae BER and NER genes, which may be related to differences in the life cycles of these two fungi.

The ability to visualize cellular events by linking them to color or fluorescence changes has been an invaluable tool for biology. We describe a novel plasmid-borne color marker whose expression in yeast leads to purple-colored cells that are also brightly fluorescent. This dominant marker provides a useful tool for rapidly screening plasmid maintenance using a visual or fluorescence assay in both Saccharomyces cerevisiae and Candida albicans.

The physiological role of Candida albicans Cnh1, a member of the [Na.sup.+]/[H.sup.+] antiporter family, was characterized. Though CaCnh1p had broad substrate specificity and mediated efflux of at least four alkali metal cations upon heterologous expression in Saccharomyces cerevisiae, its presence in C. albicans cells was important especially for potassium homeostasis. In C. albicans, CaCnh1p tagged with GFP was localized in the plasma membrane of cells growing as both yeasts and hyphae. Deletion of CNH1 alleles did not affect tolerance to NaCl, LiCl or CsCl, but resulted in increased sensitivity to high external concentrations of KCl and RbCl. The potassium and rubidium tolerance of a cnh1 homozygous mutant was fully restored by reintegration of CNH1 into the genome. The higher sensitivity of the cnh1/cnh1 mutant to external KCl was caused by a lower [K.sup.+] efflux from these cells. Together, the functional characterization of the CaCnh1 antiporter in C. albicans revealed that this antiporter plays a significant role in C. albicans physiology. It ensures potassium and rubidium tolerance and participates in the regulation of intracellular potassium content of C. albicans cells.

Microarrays were used to identify changes in gene expression associated with Candida albicans biofilm development. Two biofilm substrates (denture and catheter), and two C. albicans strains for each substrate, were tested to remove model- and strain-dependent variability from the overall dataset. Three biofilm developmental phases were examined: early (6 h), intermediate (12 h), and mature (48 h). Planktonic specimens were collected at the same time points. Data analysis focused primarily on gene expression changes over the time-course of biofilm development. Glycolytic and non-glycolytic carbohydrate assimilation, amino acid metabolism, and intracellular transport mechanisms were important during the early phase of biofilm formation. These early events increase intracellular pools of pyruvate, pentoses and amino acids, which prepare the biofilm for the large biomass increase that begins around 12 h of development. This developmental stage demands energy and utilizes specific transporters for amino acids, sugars, ions, oligopeptides and lactate/pyruvate. At mature phase (48 h), few genes were differentially expressed compared with the 12 h time point, suggesting a relative lack of initiation of new metabolic activity. Data analysis to assess biofilm model-specific gene expression showed more dynamic changes in the denture model than in the catheter model. Data analysis to identify gene expression changes that are associated with each strain/substrate combination identified the same types of genes that were identified in the analysis of the entire dataset. Collectively, these data suggest that genes belonging to different, but interconnected, functional categories regulate the morphology and phenotype of C. albicans biofilm.

The Candida albicans ALS (agglutinin-like sequence) family includes eight genes (ALS1 to ALS7, and ALS9) that share a common general organization, consisting of a relatively conserved 5' domain, a central domain of tandemly repeated sequence units, and a 3' domain of relatively variable length and sequence. To test the hypothesis that the cell-surface glycoproteins encoded by the ALS genes mediate contact between the fungal cell and host surfaces, a set of C. albicans mutant strains was systematically constructed, each lacking one of the ALS sequences. Phenotypes of the mutant strains were evaluated, primarily using adhesion assays. ALS9 is unique within the ALS family due to extensive allelic sequence variation within the 5' domain that may result in functional differences between proteins encoded by ALS9-1 and ALS9-2. Deletion of ALS9 significantly reduces C. albicans adhesion to human vascular endothelial cell monolayers. The mutation was complemented by reintegration of a wild-type copy of ALS9-2, but not ALS9-1, suggesting allelic functional differences. Complementation of the mutation with a gene fusion between the 5' domain of ALS9-2 and the tandem repeats and 3' domain of ALS9-1 also restored wild-type adhesion levels. Analysis of the als9[DELTA]/als9[DELTA] mutant phenotype in other assays demonstrated no significant difference from a control strain for adhesion to buccal epithelial cells or laminin-coated plastic plates. The als9[DELTA]/als9[DELTA] mutant did not show significant differences from the control for adhesion to or destruction of cells in the reconstituted human epithelium (RHE) disease model, or for cell-wall defects, germ-tube formation or biofilm formation in a catheter model. Analysis of ALS9 allelic frequency in a collection of geographically diverse clinical isolates showed a distinct preference for ALS9-2 allelic sequences, within both the 5' and the 3' domain of the ALS9 coding region. These data suggest greater selective pressure to maintain the ALS9-2 allele in C. albicans isolates and imply its greater relative importance in host-pathogen interactions.

The phosphatidylinositol (Ptdlns) 3-kinase Vps34p of the human pathogenic yeast Candida albicans participates in virulence and in protein transport. In order to dissect these two functions, a search for proteins interacting with C. albicans Vps34p was performed using a yeast two-hybrid system. This study demonstrates the physical interaction between Vps34p and Ade5,7p, which is the bifunctional enzyme of the de novo purine nucleotide biosynthetic pathway. The interaction initially observed in a yeast two-hybrid system was confirmed in vitro with recombinant proteins. Given the complex formation between Ade5,7p and the virulence-regulating Vps34p, it was of interest to characterize the function of Ade5,7p in C. albicans. To this end, ade5,7 null mutants were generated. The resulting mutants were adenine deficient, and sensitive to the presence of metal ions. In addition, the ade5,7 null mutants were avirulent in a mouse model of systemic candidiasis, and showed reduced hyphal growth in an agar matrix under embedded conditions. In summary, Ade5,7p interacts with the multifunctional virulence regulator Ptdlns 3-kinase Vps34p, and ade5,7 and vps34 null mutant strains show similar phenotypes regarding sensitivity to metal ions, hyphal growth and virulence.

In Candida albicans, the ATC1 gene, encoding a cell wall-associated acid trehalase, has been considered as a potentially interesting target in the search for new antifungal compounds. A phenotypic characterization of the double disruptant atc1[DELTA]/atc1[delta] mutant showed that it was unable to grow on exogenous trehalose as sole carbon source. Unlike actively growing cells from the parental strain (CAI4), the atc1[DELTA] null mutant displayed higher resistance to environmental insults, such as heat shock (42[degrees]C) or saline exposure (0.5 M NaCl), and to both mild and severe oxidative stress (5 and 50 mM [H.sub.2][O.sub.2]), which are relevant during in vivo infections. Parallel measurements of intracellular trehalose and trehalose-metabolizing enzymes revealed that significant amounts of the disaccharide were stored in response to thermal and oxidative challenge in the two cell types. The antioxidant activities of catalase and glutathione reductase were triggered by moderate oxidative exposure (5 mM [H.sub.2][O.sub.2]), whereas superoxide dismutase was inhibited dramatically by [H.sub.2][O.sub.2], where a more marked decrease was observed in atc1[DELTA] cells. In turn, the atc1[DELTA] mutant exhibited a decreased capacity of hypha and pseudohypha formation tested in different media. Finally, the homozygous null mutant in a mouse model of systemic candidiasis displayed strongly reduced pathogenicity compared with parental or heterozygous strains. These results suggest not only a novel role for the ATC1 gene in dimorphism and infectivity, but also that an interconnection between stress resistance, dimorphic conversion and virulence in C. albicans may be reconsidered. They also support the hypothesis that Atc1 p is not involved in the physiological hydrolysis of endogenous trehalose. DOI 10.1099/mic.02006/003921-0

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 Doal, 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 [DELTA]Adoa1 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.

The major repeat sequence (MRS) is known to play a role in karyotypic variation in Candida albicans. The MRS affects karyotypic variation by expanding and contracting internal repeats, by altering the frequency of chromosome loss, and by serving as a hotspot for chromosome translocation. We proposed that the effects of the MRS on translocation could be better understood by examination of the effect of the MRS on a similar event, mitotic recombination between two chromosome homologs. We examined the frequency of mitotic recombination across an MRS of average size (~50 kb) as well as the rate of recombination in a 325-kb stretch of DNA adjacent to the MRS. Our results indicate that mitotic recombination frequencies across the MRS were not enhanced compared to the frequencies measured across the 325-kb region adjacent to the MRS. Mitotic recombination events were found to occur throughout the 325-kb region analyzed as well as within the MRS itself. This analysis of mitotic recombination frequencies across a large portion of chromosome 5 is the first large-scale analysis of mitotic recombination done in C. albicans and indicates that mitotic recombination frequencies are similar to the rates found in Saccharomyces cerevisiae.

Candida albicans, a commensal organism and a pathogen of humans, can switch stochastically between a white phase and an opaque phase without an intermediate phase. The white and opaque phases have distinct cell shapes and gene expression programs. Once switched, each phase is stable for many cell divisions. White--opaque switching is under a1-[alpha]2 repression and therefore only happens in a or [alpha] cells. Mechanisms that control the switching are unknown. Here, we identify Wor1 (white--opaque regulator 1) as a master regulator of white--opaque switching. The deletion of WOR1 blocks opaque cell formation. The ectopic expression of WOR1 converts all cells to stable opaque cells in a or cells. In addition, the ectopic expression of WOR1 in a/[alpha] cells is sufficient to induce opaque cell formation. Importantly, WOR1 expression displays an all-or-none pattern. It is undetectable in white cells, and it is highly expressed in opaque cells. The ectopic expression of Wor1 induces the transcription of WOR1 from the WOR1 locus, which correlates with the switch to opaque phase. We present genetic evidence for feedback regulation of WOR1 transcription. The feedback regulation explains the bistable and stochastic nature of white--opaque switching. feedback regulation | a1-[alpha]2 repression

White--opaque switching in the human fungal pathogen Candida albicans is an alternation between two distinct types of cells, white and opaque. White and opaque cells differ in their appearance under the microscope, the genes they express, their mating behaviors, and the host tissues for which they are best suited. Each state is heritable for many generations, and switching between states occurs stochastically, at low frequency. In this article, we identify a master regulator of white--opaque switching (Wor1), and we show that this protein is a transcriptional regulator that is needed to both establish and maintain the opaque state. We show that in opaque cells, Wor1 forms a positive feedback loop: It binds its own DNA regulatory region and activates its own transcription leading to the accumulation of high levels of Wor1. We further show that this feedback loop is self-sustaining: Once activated, it persists for many generations. We propose that this Wor1 feedback loop accounts, at least in part, for the heritability of the opaque state. In contrast, white cells (and their descendents) lack appreciable levels of Wor1, and the feedback loop remains inactive. Thus, this simple model can account for both the heritability of the white and opaque states and the stochastic nature of the switching between them. phenotypic switching | transcriptional regulation | WOR1

Abstract: A method for determining mRNA half-lives in the polymorphic fungus Candida albicans is described. It employs growth in a defined medium, the inhibition of transcription with thiolutin (10-20 [micro]g/mL), [...]