Nonfilamentous C. albicans mutants are avirulent

Invasive lesions containing filamentous forms produced by a Candida albicans mutant that is defective in filamentous growth in culture

A Candida albicans efg1 cph1 double mutant is nonfilamentous under standard laboratory conditions and avirulent in mice. However, this mutant produced filaments in the tongues of immunosuppressed gnotobiotic piglets and when embedded in agar, demonstrating that an Efg1p- and Cph1p-independent pathway for promotion of filamentous growth exists.

Cyclic AMP-dependent protein kinase regulates pseudohyphal differentiation in Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the yeast Saccharomyces cerevisiae differentiate to a filamentous growth form known as pseudohyphal differentiation. Filamentous growth is regulated by elements of the pheromone mitogen-activated protein (MAP) kinase cascade and a second signaling cascade involving the receptor Gpr1, the Galpha protein Gpa2, Ras2, and cyclic AMP (cAMP). We show here that the Gpr1-Gpa2-cAMP pathway signals via the cAMP-dependent protein kinase, protein kinase A (PKA), to regulate pseudohyphal differentiation. Activation of PKA by mutation of the regulatory subunit Bcy1 enhances filamentous growth. Mutation and overexpression of the PKA catalytic subunits reveal that the Tpk2 catalytic subunit activates filamentous growth, whereas the Tpk1 and Tpk3 catalytic subunits inhibit filamentous growth. The PKA pathway regulates unipolar budding and agar invasion, whereas the MAP kinase cascade regulates cell elongation and invasion. Epistasis analysis supports a model in which PKA functions downstream of the Gpr1 receptor and the Gpa2 and Ras2 G proteins. Activation of filamentous growth by PKA does not require the transcription factors Ste12 and Tec1 of the MAP kinase cascade, Phd1, or the PKA targets Msn2 and Msn4. PKA signals pseudohyphal growth, in part, by regulating Flo8-dependent expression of the cell surface flocculin Flo11. In summary, the cAMP-dependent protein kinase plays an intimate positive and negative role in regulating filamentous growth, and these findings may provide insight into the roles of PKA in mating, morphogenesis, and virulence in other yeasts and pathogenic fungi.

Unanticipated heterogeneity in growth rate and virulence among Candida albicans AAF1 null mutants

The disruption of a specific gene in Candida albicans is commonly used to determine the function of the gene product. We disrupted AAF1, a gene of C. albicans that causes Saccharomyces cerevisiae to flocculate and adhere to endothelial cells. We then characterized multiple heterozygous and homozygous mutants. These null mutants adhered to endothelial cells to the same extent as did the parent organism. However, mutants with presumably the same genotype revealed significant heterogeneity in their growth rates in vitro. This heterogeneity was not the result of the transformation procedure per se, nor was it caused by differences in the expression or function of URA3, a marker used in the process of gene disruption. The growth rate among the different heterozygous and homozygous null mutants was positively correlated with in vivo virulence in mice. It is possible that the variable phenotypes of C. albicans were due to mutations outside of the AAF1 coding region that were introduced during the gene disruption process. These results indicate that careful phenotypic characterization of mutants of C. albicans generated through targeted gene disruption should be performed to exclude the introduction of unexpected mutations that may influence pathogenicity in mice.

A G1 cyclin is necessary for maintenance of filamentous growth in Candida albicans

Candida albicans undergoes a dramatic morphological transition in response to various growth conditions. This ability to switch from a yeast form to a hyphal form is required for its pathogenicity. The intractability of Candida to traditional genetic approaches has hampered the study of the molecular mechanism governing this developmental switch. Our approach is to use the more genetically tractable yeast Saccharomyces cerevisiae to yield clues about the molecular control of filamentation for further studies in Candida. G1 cyclins Cln1 and Cln2 have been implicated in the control of morphogenesis in S. cerevisiae. We show that C. albicans CLN1 (CaCLN1) has the same cell cycle-specific expression pattern as CLN1 and CLN2 of S. cerevisiae. To investigate whether G1 cyclins are similarly involved in the regulation of cell morphogenesis during the yeast-to-hypha transition of C. albicans, we mutated CaCLN1. Cacln1/Cacln1 cells were found to be slower than wild-type cells in cell cycle progression. The Cacln1/Cacln1 mutants were also defective in hyphal colony formation on several solid media. Furthermore, while mutant strains developed germ tubes under several hypha-inducing conditions, they were unable to maintain the hyphal growth mode in a synthetic hypha-inducing liquid medium and were deficient in the expression of hypha-specific genes in this medium. Our results suggest that CaCln1 may coordinately regulate hyphal development with signal transduction pathways in response to various environmental cues.

Phenotypic switching in Candida albicans is controlled by a SIR2 gene

We report the cloning of a gene from the human fungal pathogen Candida albicans with sequence and functional similarity to the Saccharomyces cerevisiae SIR2 gene. Deletion of the gene in C. albicans produces a dramatic phenotype: variant colony morphologies arise at frequencies as high as 1 in 10. The morphologies resemble those described previously as part of a phenotypic switching system proposed to contribute to pathogenesis. Deletion of SIR2 also produces a high frequency of karyotypic changes. These and other results are consistent with a model whereby Sir2 controls phenotypic switching and chromosome stability in C.albicans by organizing chromatin structure.

Host-induced, stage-specific virulence gene activation in Candida albicans during infection

An understanding of the complex interactions between pathogenic microbes and their host must include the identification of gene expression patterns during infection. To detect the activation of virulence genes in the opportunistic fungal pathogen Candida albicans in vivo by host signals, we devised a reporter system that is based on FLP-mediated genetic recombination. The FLP gene, encoding the site-specific recombinase FLP, was genetically modified for expression in C. albicans and fused to the promoter of the SAP2 gene that codes for one of the secreted aspartic proteinases, which are putative virulence factors of C. albicans. The SAP2P-FLP fusion was integrated into one of the SAP2 alleles in a strain that contained a deletable marker that conferred resistance to mycophenolic acid and was flanked by direct repeats of the FLP recognition target (FRT). Using this reporter system, a transient gene induction could be monitored at the level of single cells by the mycophenolic acid-sensitive phenotype of the colonies generated from such cells after FLP-mediated marker excision. In two mouse models of disseminated candidiasis, SAP2 expression was not observed in the initial phase of infection, but the SAP2 gene was strongly induced after dissemination into deep organs. In contrast, in a mouse model of oesophageal candidiasis in which dissemination into internal organs did not occur, no SAP2 expression was detected at any time. Our results support a role of the SAP2 gene in the late stages of an infection, after fungal spread into deep tissue. This new in vivo expression technology (IVET) for a human fungal pathogen allows the detection of virulence gene induction at different stages of an infection, and therefore provides clues about the role of these genes in the disease process.

Role of the mitogen-activated protein kinase Hog1p in morphogenesis and virulence of Candida albicans

The relevance of the mitogen-activated protein (MAP) kinase Hog1p in Candida albicans was addressed through the characterization of C. albicans strains without a functional HOG1 gene. Analysis of the phenotype of hog1 mutants under osmostressing conditions revealed that this mutant displays a set of morphological alterations as the result of a failure to complete the final stages of cytokinesis, with parallel defects in the budding pattern. Even under permissive conditions, hog1 mutants displayed a different susceptibility to some compounds such as nikkomycin Z or Congo red, which interfere with cell wall functionality. In addition, the hog1 mutant displayed a colony morphology different from that of the wild-type strain on some media which promote morphological transitions in C. albicans. We show that C. albicans hog1 mutants are derepressed in the serum-induced hyphal formation and, consistently with this behavior, that HOG1 overexpression in Saccharomyces cerevisiae represses the pseudodimorphic transition. Most interestingly, deletion of HOG1 resulted in a drastic increase in the mean survival time of systemically infected mice, supporting a role for this MAP kinase pathway in virulence of pathogenic fungi. This finding has potential implications in antifungal therapy.

Molecular and functional study of AQY1 from Saccharomyces cerevisiae: role of the C-terminal domain

The yeast YPR192w gene, which encodes a protein (Aqy1p) with strong homology to aquaporins (AQPs), was cloned from nine S. cerevisiae strains. The osmotic water permeability coefficient (Pf) of X. laevis oocytes expressing the gene cloned from the Sigma1278b strain (AQY1-1) was 5.7 times higher than the Pf of oocytes expressing the gene cloned from other strains (AQY1-2). Aqy1-1p, initially cloned without its C-terminus (Aqy1-1DeltaCp), mediated an approximately 3 times higher water permeability than the full-length protein. This corresponds to a 3-fold higher protein density in the oocyte plasma membrane, as shown by freeze-fracture electron microscopy. Pf measurements in yeast spheroplasts confirmed the presence of functional water channels in Sigma1278b and a pharmacological study indicated that this strain contains at least a second functional aquaporin.

MAP kinase and cAMP filamentation signaling pathways converge on the unusually large promoter of the yeast FLO11 gene

In Saccharomyces cerevisiae, two major signal transduction pathways, the Kss1 MAPK pathway and the cAMP-regulated pathway, are critical for the differentiation of round yeast form cells to multicellular, invasive pseudohyphae. Here we report that these parallel pathways converge on the promoter of a gene, FLO11, which encodes a cell surface protein required for pseudohyphal formation. The FLO11 promoter is unusually large, containing at least four upstream activation sequences (UASs) and nine repression elements which together span at least 2.8 kb. Several lines of evidence indicate that the MAPK and cAMP signals are received by distinct transcription factors and promoter elements. First, regulation via the MAPK pathway requires the transcription factors Ste12p/Tec1p, whereas cAMP-mediated activation requires a distinct factor, Flo8p. Secondly, mutations in either pathway block FLO11 transcription. Overexpression of STE12 can suppress the loss of FLO8, and overexpression of FLO8 can suppress the loss of STE12. Finally, multiple distinct promoter regions of the FLO11 promoter are required for its activation by either Flo8p or Ste12p/ Tec1p. Thus, like the promoters of the key developmental genes, HO and IME1, the FLO11 promoter is large and complex, endowing it with the ability to integrate multiple inputs.

Oligomeric structure and regulation of Candida albicans glucosamine-6-phosphate synthase

Candida albicans glucosamine-6-phosphate (GlcN-6-P) synthase was purified to apparent homogeneity with 52% yield from recombinant yeast YRSC-65 cells efficiently overexpressing the GFA1 gene. The pure enzyme exhibited Km(Gln) = 1.56 mM and Km(Fru-6-P) = 1.41 mM and catalyzed GlcN-6-P formation with kcat = 1150 min-1. The isoelectric point of 4.6 +/- 0.05 was estimated from isoelectric chromatofocusing. Gel filtration, native polyacrylamide gel electrophoresis, subunit cross-linking, and SDS-polyacrylamide gel electrophoresis showed that the native enzyme was a homotetramer of 79.5-kDa subunits, with an apparent molecular mass of 330-340 kDa. Results of chemical modification of the enzyme by group-specific reagents established an essential role of a cysteinyl residue at the glutamine-binding site and histidyl, lysyl, arginyl, and tyrosyl moieties at the Fru-6-P-binding site. GlcN-6-P synthase in crude extract was effectively inhibited by UDP-GlcNAc (IC50 = 0.67 mM). Purification of the enzyme markedly decreased the sensitivity to the inhibitor, but this could be restored by addition of another effector, glucose 6-phosphate. Binding of UDP-GlcNAc to the pure enzyme in the presence of Glc-6-P showed strong negative cooperativity, with nH = 0.54, whereas in the absence of this sugar phosphate no cooperative effect was observed. Pure enzyme was a substrate for cAMP-dependent protein kinase, the action of which led to the substantial increase of GlcN-6-P synthase activity, correlated with an extent of protein phosphorylation. The maximal level of activity was observed for the enzyme molecules containing 1. 21 +/- 0.08 mol of phosphate/mol of GlcN-6-P synthase. Monitoring of GlcN-6-P synthase activity and its sensitivity to UDP-GlcNAc during yeast --> mycelia transformation of C. albicans cells, under in situ conditions, revealed a marked increase of the former and a substantial fall of the latter.

Genetic and biochemical characterization of phosphofructokinase from the opportunistic pathogenic yeast Candida albicans

We have used the two PFK genes of Saccharomyces cerevisiae encoding the alpha and beta-subunit of the enzyme phosphofructokinase (Pfk) as heterologous probes to isolate fragments of the respective genes from the dimorphic pathogenic fungus Candida albicans. The complete coding sequences were obtained by combining sequences of chromosomal fragments and fragments obtained by inverse polymerase chain reaction (PCR). The CaPFK1 and CaPFK2 comprise open reading frames of 2961 bp and 2838 bp, respectively, encoding Pfk subunits with deduced molecular masses of 109 kDa and 104 kDa. The genes presumably evolved by a duplication event from a prokaryotic type ancestor, followed by another duplication. Heterologous expression in S. cerevisiae revealed that each gene alone was able to complement the glucose-negative phenotype of a pfk1 pfk2 double mutant. In vitro Pfk activity in S. cerevisiae was not only obtained after coexpression of both genes, but also in conjunction with the respective complementary subunits from S. cerevisiae. This indicates the formation of functional hetero-oligomers consisting of C. albicans and S. cerevisiae Pfk subunits. In C. albicans, specific Pfk activity was shown to decrease twofold upon induction of hyphal growth. CaPfk cross-reacts with a polyclonal antiserum raised against ScPfk and displays similar allosteric properties, i.e. inhibition by ATP and activation by AMP and fructose 2,6-bisphosphate.

The URA5 gene is necessary for histoplasma capsulatum growth during infection of mouse and human cells

The Histoplasma capsulatum URA5 gene, which has recently been cloned and disrupted by allelic replacement, encodes orotidine-5'-monophosphate pyrophosphorylase. Inactivation of URA5 by either targeted or UV mutagenesis results in disruption of the pyrimidine biosynthetic pathway and uracil auxotrophy. We examined the effect of uracil auxotrophy due to a ura5 mutation on H. capsulatum virulence in both cell culture and whole-animal models. Uracil auxotrophs of two H. capsulatum restriction fragment length polymorphism classes were found to be avirulent in cultured murine and human cells, as well as in mice. Moreover, virulence could be restored either by supplying a functional URA5 gene in trans or by supplying exogenous uracil during infection in vitro. These experiments demonstrate that the pyrimidine biosynthetic pathway is essential for H. capsulatum growth and virulence.

Over-expression of Candida albicans mitochondrial ribosomal protein S9 (MrpS9p) disturbs mitochondrial function in Saccharomyces cerevisiae

A Candida albicans mitochondrial ribosomal protein S9 (MRPS9) cDNA was identified in a screen for sequences whose expression induce galactose lethality in Saccharomyces cerevisiae. MRPS9 appears to encode a protein of 346 amino acids with an N-terminal mitochondrial targeting sequence and an internal S9 signature that is conserved amongst eukaryotic mitochondrial and prokaryotic ribosomal protein S9 sequences. Expression of a GAL1-CaMRPS9 fusion in S. cerevisiae caused the slow development of a galactose-negative phenotype upon repeated subculturing, and this correlated with an increased frequency of petite mutant formation. Therefore, over-expression of CaMRPS9 interferes with S. cerevisiae mitochondrial function, which accounts for the inhibition of growth on galactose.

Differential regulation of transcription: repression by unactivated mitogen-activated protein kinase Kss1 requires the Dig1 and Dig2 proteins

Kss1, a yeast mitogen-activated protein kinase (MAPK), in its unphosphorylated (unactivated) state binds directly to and represses Ste12, a transcription factor necessary for expression of genes whose promoters contain filamentous response elements (FREs) and genes whose promoters contain pheromone response elements (PREs). Herein we show that two nuclear proteins, Dig1 and Dig2, are required cofactors in Kss1-imposed repression. Dig1 and Dig2 cooperate with Kss1 to repress Ste12 action at FREs and regulate invasive growth in a naturally invasive strain. Kss1-imposed Dig-dependent repression of Ste12 also occurs at PREs. However, maintenance of repression at PREs is more dependent on Dig1 and/or Dig2 and less dependent on Kss1 than repression at FREs. In addition, derepression at PREs is more dependent on MAPK-mediated phosphorylation than is derepression at FREs. Differential utilization of two types of MAPK-mediated regulation (binding-imposed repression and phosphorylation-dependent activation), in combination with distinct Ste12-containing complexes, contributes to the mechanisms by which separate extracellular stimuli that use the same MAPK cascade can elicit two different transcriptional responses.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Dimorphism and virulence in Candida albicans

Two regulatory pathways govern filamentation in the pathogenic fungus Candida albicans. Recent virulence studies of filamentation regulatory mutants argue that both yeast and filamentous forms have roles in infection. Filamentation control pathways seem closely related in C. albicans and in Saccharomyces cerevisiae, thus permitting speculation about C. albicans filamentation genes not yet discovered.

Regulators of pseudohyphal differentiation in Saccharomyces cerevisiae identified through multicopy suppressor analysis in ammonium permease mutant strains

Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Galpha subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Deltamep2/Deltamep2 and Deltagpa2/Deltagpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltamep1 Deltamep2/Deltamep2 and Deltamep1/Deltamep1 Deltagpa2/Deltagpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Deltamep1/Deltamep1 Deltamep2/Deltamep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth.

A Candida albicans chaperonin subunit (CaCct8p) as a suppressor of morphogenesis and Ras phenotypes in C. albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and the pathogen Candida albicans can be induced to undergo morphogenesis from a yeast to a filamentous form. A C. albicans gene (CaCCT8) was identified encoding a subunit of the Cct chaperonin complex, whose expression prevents filament formation in both fungi without interfering with growth of the yeast form. In S. cerevisiae, pseudohyphal growth induced by Ras2Val19, by overproduction of Phd1p or by expression of the C. albicans EFG1 gene, was blocked by CaCct8p and its N-terminally deleted derivative CaCct8-delta1p; in contrast, pseudohyphal induction by other components (Cph1p, Cdc42p) could not be suppressed, indicating that morphogenesis per se is not inhibited. CaCCT8 expression also interfered with other Ras2pVal19 phenotypes, including heat sensitivity, lack of glycogen accumulation and lack of sporulation. In C. albicans, overproduction of CaCct8p effectively blocked hyphal morphogenesis induced by starvation conditions and by serum. The results suggest that the activity of a component in the Ras2p signal transduction pathway is suppressed by excess chaperonin subunits. This component may be a novel folding target for the Cct complex. In agreement with this hypothesis, disruption of one of the two CaCCT8 alleles in C. albicans led to defective hyphal morphogenesis.

Reduced pathogenicity of a Candida albicans MAP kinase phosphatase (CPP1) mutant in the murine mastitis model

Candida albicans strains with a deletion of the mitogen-activated protein kinase tyrosine phosphatase gene (CPP1) are derepressed in the yeast-to-hyphal transition on solid surfaces in vitro at ambient temperatures and this gene is therefore required for repression of the yeast-to-hyphal switch. The pathology caused by a CPP1 null mutant strain was compared with that of the null mutant into which the wild-type CPP1 gene was introduced by homologous recombination and with the wild-type parent strain in a murine mycotic mastitis model. The mammary glands of lactating mice (at day 5 postpartum) were infected for 2, 4 and 6 days with 1 x 10(5), 1 x 10(6) and 1 x 10(7) cell-forming units before euthanasia. Infected and non-infected control glands were evaluated histopathologically. The null mutant strains showed less severe pathology than the two control strains. The Cpplp tyrosine phosphatase may thus be considered a virulence determinant during localized infection in C. albicans.

Altered expression of selectable marker URA3 in gene-disrupted Candida albicans strains complicates interpretation of virulence studies

The ura-blaster technique for the disruption of Candida albicans genes has been employed in a number of studies to identify possible genes encoding virulence factors of this fungal pathogen. In this study, the URA3-encoded orotidine 5'-monophosphate (OMP) decarboxylase enzyme activities of C. albicans strains with ura-blaster-mediated genetic disruptions were measured. All strains harboring genetic lesions via the ura-blaster construct showed reduced OMP decarboxylase activities compared to that of the wild type when assayed. The activity levels in different gene disruptions varied, suggesting a positional effect on the level of gene expression. Because the URA3 gene of C. albicans has previously been identified as a virulence factor for this microorganism, our results suggest that decreased virulence observed in strains constructed with the ura-blaster cassette cannot accurately be attributed, in all cases, to the targeted genetic disruption. Although revised methods for validating a URA3-disrupted gene as a target for antifungal drug development could be devised, it is clearly desirable to replace URA3 with a different selectable marker that does not influence virulence.

Signal transduction pathways regulating differentiation and pathogenicity of Cryptococcus neoformans

The basidiomycetous yeast Cryptococcus neoformans is a human pathogen. Several phenotypes of this organism are defined as virulence traits including the polysaccharide capsule, melanin, and the ability to grow at 37 degreesC. The signaling pathways regulating the expression of these phenotypes and other important cellular processes are being defined on a molecular level. For example, the highly conserved signaling molecule calcineurin regulates high temperature growth in C. neoformans. A cryptococcal homolog of Saccharomyces cerevisiae STE12, the gene for a transcriptional regulator activated by the MAP kinase cascade, has also been identified. Additionally, the C. neoformans Galpha protein GPA1 and cAMP regulate mating, melanin production, encapsulation, and pathogenicity. This fungus is an excellent model to further dissect virulence-associated signaling pathways. The conserved role of Galpha proteins and cAMP-associated signaling pathways in fungal differentiation and pathogenicity is also reviewed.

Molecular cloning and characterization of a Candida albicans gene coding for cytochrome c haem lyase and a cell wall-related protein

Immunoscreening of a Candida albicans cDNA library with a monoclonal antibody (mAb 4C12) recognizing an epitope present in high-molecular-weight mannoprotein (HMWM) components specific for the mycelial cell walls (a 180 kDa component and a poly-dispersed 260 kDa species) resulted in the isolation of the gene CaCYC3 encoding for cytochrome c haem lyase (CCHL). The CaCYC3 gene was transcribed preferentially in mycelial cells in which two mRNA transcripts of 0.8 and 1 kb were found. The nucleotide and the deduced amino acid sequences of this gene displayed 45% homology and 46% identity, respectively, to the Saccharomyces cerevisiae CYC3 gene and shared common features with other reported genes encoding for CCHL. The CaCYC3 gene restored the respiratory activity when transformed in a S. cerevisiae cyc3- mutant strain. A C. albicans CYC3 null mutant was constructed after sequential disruption using the hisG::URA3::hisG ('ura-blaster') cassette. Null mutant cells were unable to use lactate as a sole carbon source and had a reduced ability to form germ tubes. Western immunoblotting analysis of subcellular fractions from wild-type and null mutant strains demonstrated the presence of two gene products, a 33kDa mitochondrial protein and a 40 kDa cell wall-associated moiety reacting with antibodies against CCHL, in both yeast cells and germ tubes. mAb 4C12 still reacted with the CaCYC3 null mutant (by immunofluorescence and immunoblotting) but showed an altered pattern of immunoreactivity against cell wall HMWM species, indicating a relationship between these moieties and the CaCYC3 gene products. The results suggest that the CaCYC3 gene encodes two proteins, one targeted to the mitochondria and the other to the cell wall.

Repression of yeast Ste12 transcription factor by direct binding of unphosphorylated Kss1 MAPK and its regulation by the Ste7 MEK

The mitogen-activated protein kinase (MAPK) Kss1 has a dual role in regulating filamentous (invasive) growth of the yeast Saccharomyces cerevisiae. The stimulatory function of Kss1 requires both its catalytic activity and its activation by the MAPK/ERK kinase (MEK) Ste7; in contrast, the inhibitory function of Kss1 requires neither. This study examines the mechanism by which Kss1 inhibits invasive growth, and how Ste7 action overcomes this inhibition. We found that unphosphorylated Kss1 binds directly to the transcription factor Ste12, that this binding is necessary for Kss1-mediated repression of Ste12, and that Ste7-mediated phosphorylation of Kss1 weakens Kss1-Ste12 interaction and relieves Kss1-mediated repression. Relative to Kss1, the MAPK Fus3 binds less strongly to Ste12 and is correspondingly a weaker inhibitor of invasive growth. Analysis of Kss1 mutants indicated that the activation loop of Kss1 controls binding to Ste12. Potent repression of a transcription factor by its physical interaction with the unactivated isoform of a protein kinase, and relief of this repression by activation of the kinase, is a novel mechanism for signal-dependent regulation of gene expression.

The control of filamentous differentiation and virulence in fungi

Many members of the fungal kingdom have a distinguishing feature, dimorphism, which is the ability to switch between two morphological forms: a cellular yeast form and a multicellular invasive filamentous form. At least three pathways are involved in regulating the transition between these two forms in the budding yeast Saccharomyces cerevisiae, and evidence is now emerging that homologous signalling modules are involved in regulating filament formation and virulence in a range of human and plant fungal pathogens. Strikingly, components used to signal sexual differentiation in the response to mating pheromones are often reutilized to regulate dimorphic development, suggesting an ancient link between these processes.

Reporters for the analysis of gene regulation in fungi pathogenic to man

In the past few years, highly sensitive gene reporters have been developed for the infectious fungi including gene reporters with altered codon usage. The tools are, therefore, now at hand for functionally characterizing the promoters of genes regulated by the bud-hypha transition, high frequency switching and cues from the cellular environment.

Insertional mutagenesis of pathogenic fungi

Screening insertional mutants for loss of virulence is an effective method for investigating the molecular genetic basis of bacterial pathogenesis, but has only recently been applied to fungal pathogens. For many pathogenic fungi transformation with heterologous plasmid DNA results in complex integration events. This problem can now be circumvented for some species using restriction enzyme mediated integration. Insertional mutagenesis of Fusarium oxysporum using the naturally occurring fungal transposon impala has been described, but transposon tagging for other fungi has yet to be developed. Although insertional mutagenesis has recently identified important virulence determinants of fungal phytopathogens, the lack of suitable screening strategies has so far limited its applicability for fungal pathogens of humans.

Gene disruption to evaluate the role of fungal candidate virulence genes

Gene disruption is a powerful genetic tool that can define pathogenic or virulence factors. In the past two years gene disruption approaches have been used to identify fungal virulence genes. The capsule genes, an alpha subunit of G protein and certain kinases of Cryptococcus neoformans have clearly been demonstrated to be associated with pathogenicity. In Candida albicans at least four genes involved in hyphal formation have been disrupted and tested for virulence. In other fungi, such as Histoplasma capsulatum, however, more efficient gene disruption methods need to be developed before such approaches can be regularly used for identifying virulence genes.

Lack of consistent short sequence repeat polymorphisms in genetically homologous colonizing and invasive Candida albicans strains

Short sequence repeats (SSRs), potentially representing variable numbers of tandem repeat (VNTR) loci, were identified for the human-pathogenic yeast species Candida albicans by computerized DNA sequence scanning. The individual SSR regions were investigated in different clinical isolates of C. albicans. Most of the C. albicans SSRs were identified as genuine VNTRs. They appeared to be present in multiple allelic variants and were demonstrated to be diverse in length among nonrelated strains. As such, these loci provide adequate targets for the molecular typing of C. albicans strains. VNTRs encountered in other microbial species sometimes participate in regulation of gene expression and function as molecular switches at the transcriptional or translational level. Interestingly, the VNTRs identified here often encode polyglutamine stretches and are frequently located within genes potentially involved in the regulation of transcription. DNA sequencing of these VNTRs demonstrated that the length variability was restricted to the CAA/CAG repeats encoding the polyglutamine stretches. For these reasons, paired C. albicans isolates of similar genotype, either found as noninvasive colonizers or encountered in an invasive state in the same individual, were studied with respect to potentially invasion-related alterations in the VNTR profiles. However, none of the VNTRs analyzed thus far varied systematically with the transition from colonization to invasion. In contrast to the situation described for some prokaryotic species, this finding suggests that VNTRs of C. albicans may not simply function as contingency loci related to straightforward on/off regulation of invasion-related gene expression.

A yeast glutamine tRNA signals nitrogen status for regulation of dimorphic growth and sporulation

Dimorphic growth of the budding yeast Saccharomyces cerevisiae is regulated by the quality of the nitrogen supply. On a preferred nitrogen source diploid cells grow as ellipsoidal cells by using a bipolar pattern of budding, whereas on a poor nitrogen source a unipolar pattern of budding is adopted, resulting in extended pseudohyphal chains of filamentous cells. Here we report that the quality of the nitrogen source is signaled by the glutamine tRNA isoform with a 5'-CUG anticodon (tRNACUG). Mutations that alter this tRNA impair assessment of the nitrogen supply without measurably affecting protein synthesis, so that mutant cells display pseudohyphal growth even on a preferred nitrogen source. The nitrogen status for other nitrogen-responsive processes such as catabolic gene expression and sporulation also is signaled by this tRNA: mutant cells inappropriately induce the nitrogen-repressed gene CAR1 and undergo precocious sporulation in nitrogen-rich media. Therefore, in addition to its role in mRNA translation, this tRNA also transduces nitrogen signals that regulate development.

The pH of the host niche controls gene expression in and virulence of Candida albicans

Little is known of the biological attributes conferring pathogenicity on the opportunistic fungal pathogen Candida albicans. Infection by this pathogen, as for bacterial pathogens, may rely upon environmental signals within the host niche to regulate the expression of virulence determinants. To determine if C. albicans responds to the pH of the host niche, we tested the virulence of strains with mutations in either of two pH-regulated genes, PHR1 and PHR2. In vitro, PHR1 is expressed when the ambient pH is at 5.5 or higher and deletion of the gene results in growth and morphological defects at neutral to alkaline pHs. Conversely, PHR2 is expressed at an ambient pH below 5.5, and the growth and morphology of the null mutant is compromised below this pH. A PHR1 null mutant was avirulent in a mouse model of systemic infection but uncompromised in its ability to cause vaginal infection in rats. Since systemic pH is near neutrality and vaginal pH is around 4.5, the virulence phenotype paralleled the pH dependence of the in vitro phenotypes. The virulence phenotype of a PHR2 null mutant was the inverse. The mutant was virulent in a systemic-infection model but avirulent in a vaginal-infection model. Heterozygous mutants exhibited partial reductions in their pathogenic potential, suggesting a gene dosage effect. Unexpectedly, deletion of PHR2 did not prevent hyphal development in vaginal tissue, suggesting that it is not essential for hyphal development in this host niche. The results suggest that the pH of the infection site regulates the expression of genes essential to survival within that niche. This implies that the study of environmentally regulated genes may provide a rationale for understanding the pathobiology of C. albicans.

COS1, a two-component histidine kinase that is involved in hyphal development in the opportunistic pathogen Candida albicans

Two-component histidine kinases recently have been found in eukaryotic organisms including fungi, slime molds, and plants. We describe the identification of a gene, COS1, from the opportunistic pathogen Candida albicans by using a PCR-based screening strategy. The sequence of COS1 indicates that it encodes a homolog of the histidine kinase Nik-1 from the filamentous fungus Neurospora crassa. COS1 is also identical to a gene called CaNIK1 identified in C. albicans by low stringency hybridization using CaSLN1 as a probe [Nagahashi, S., Mio, T., Yamada-Okabe, T., Arisawa, M., Bussey, H. & Yamada-Okabe, H. (1998) Microbiol. 44, 425-432]. We assess the function of COS1/CaNIK1 by constructing a diploid deletion mutant. Mutants lacking both copies of COS1 appear normal when grown as yeast cells; however, they exhibit defective hyphal formation when placed on solid agar media, either in response to nutrient deprivation or serum. In constrast to the Deltanik-1 mutant, the Deltacos1/Deltacos1 mutant does not demonstrate deleterious effects when grown in media of high osmolarity; however both Deltanik-1 and Deltacos1/Deltacos1 mutants show defective hyphal formation. Thus, as predicted for Nik-1, Cos1p may be involved in some aspect of hyphal morphogenesis and may play a role in virulence properties of the organism.

Imaging techniques in microbiology

Recent advances in optical imaging have dramatically expanded the capabilities of the light microscope and its usefulness in microbiology research. Some of these advances include improved fluorescent probes, better cameras, new techniques such as confocal and deconvolution microscopy, and the use of computers in imaging and image analysis. These new technologies have now been applied to microbiological problems with resounding success.

Roles of the Candida albicans mitogen-activated protein kinase homolog, Cek1p, in hyphal development and systemic candidiasis

Extracellular signal-regulated protein kinase (ERK, or mitogen-activated protein kinase [MAPK]) regulatory cascades in fungi turn on transcription factors that control developmental processes, stress responses, and cell wall integrity. CEK1 encodes a Candida albicans MAPK homolog (Cek1p), isolated by its ability to interfere with the Saccharomyces cerevisiae MAPK mating pathway. C. albicans cells with a deletion of the CEK1 gene are defective in shifting from a unicellular budding colonial growth mode to an agar-invasive hyphal growth mode when nutrients become limiting on solid medium with mannitol as a carbon source or on glucose when nitrogen is severely limited. The same phenotype is seen in C. albicans mutants in which the homologs (CST20, HST7, and CPH1) of the S. cerevisiae STE20, STE7, and STE12 genes are disrupted. In S. cerevisiae, the products of these genes function as part of a MAPK cascade required for mating and invasiveness of haploid cells and for pseudohyphal development of diploid cells. Epistasis studies revealed that the C. albicans CST20, HST7, CEK1, and CPH1 gene products lie in an equivalent, canonical, MAPK cascade. While Cek1p acts as part of the MAPK cascade involved in starvation-specific hyphal development, it may also play independent roles in C. albicans. In contrast to disruptions of the HST7 and CPH1 genes, disruption of the CEK1 gene adversely affects the growth of serum-induced mycelial colonies and attenuates virulence in a mouse model for systemic candidiasis.

Ash1, a daughter cell-specific protein, is required for pseudohyphal growth of Saccharomyces cerevisiae

Ash1 (for asymmetric synthesis of HO) was first uncovered in genetic screens that revealed its role in mating-type switching. Ash1 prevents HO expression in daughter cells. Because Ash1 has a zinc finger-like domain related to that of the GATA family of transcription factors, it presumably acts by repressing HO transcription. Nonswitching diploid cells also express Ash1, suggesting it could have functions in addition to regulation of HO expression. We show here that Ash1 has an essential function for pseudohyphal growth. Our epistasis analyses are consistent with the deduction that Ash1 acts separately from the mitogen-activated protein kinase cascade and Ste12. Similarly to the case in yeast form cells, Ash1 is asymmetrically localized to the nuclei of daughter cells during pseudohyphal growth. This asymmetric localization reveals that there is a previously unsuspected daughter cell-specific function necessary for pseudohyphal growth.

The expression of the secreted aspartyl proteinases Sap4 to Sap6 from Candida albicans in murine macrophages

Medically important yeasts of the genus Candida secrete aspartyl proteinases (Sap), which are of particular interest as virulence factors. Six closely related gene sequences, SAP1 to SAP6, for secreted proteinases are present in Candida albicans. The methylotrophic yeast Pichia pastoris was chosen as an expression system for preparing substantial amounts of each Sap isoenzyme. Interestingly, Sap4, Sap5 and Sap6, which have not yet been detected in C. albicans cultures in vitro, were produced as active recombinant enzymes. Different Sap polyclonal antibodies were raised in rabbits and tested before further application by enzyme-linked immunosorbent assay (ELISA) against each recombinant Sap. Two antisera recognized only Sap4 to Sap6. Using these antisera, together with sap null mutants obtained by targeted mutagenesis, we could demonstrate a high production of Sap4, Sap5 and Sap6 by C. albicans cells after phagocytosis by murine peritoneal macrophages. Furthermore, a delta sap4,5,6 null mutant was killed 53% more effectively after contact with macrophages than the wild-type strain. These results support a role for Sap4 to Sap6 in pathogenicity.

Signal transduction by MAP kinase cascades in budding yeast

Budding yeast contain at least four distinct MAPK (mitogen activated protein kinase) cascades that transduce a variety of intracellular signals: mating-pheromone response, pseudohyphal/invasive growth, cell wall integrity, and high osmolarity adaptation. Although each MAPK cascade contains a conserved set of three protein kinases, the upstream activation mechanisms for these cascades are diverse, including a trimeric G protein, monomeric small G proteins, and a prokaryotic-like two-component system. Recently, it became apparent that there is extensive sharing of signaling elements among the MAPK pathways; however, little undesirable cross-talk occurs between various cascades. The formation of multi-protein signaling complexes is probably centrally important for this insulation of individual MAPK cascades.

The MEP2 ammonium permease regulates pseudohyphal differentiation in Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the budding yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. This dimorphic transition is regulated by the Galpha protein GPA2, by RAS2, and by elements of the pheromone-responsive MAP kinase cascade, yet the mechanisms by which nitrogen starvation is sensed remain unclear. We have found that MEP2, a high affinity ammonium permease, is required for pseudohyphal differentiation in response to ammonium limitation. In contrast, MEP1 and MEP3, which are lower affinity ammonium permeases, are not required for filamentous growth. Deltamep2 mutant strains had no defects in growth rates or ammonium uptake, even at limiting ammonium concentrations. The pseudohyphal defect of Deltamep2/Deltamep2 strains was suppressed by dominant active GPA2 or RAS2 mutations and by addition of exogenous cAMP, but was not suppressed by activated alleles of the MAP kinase pathway. Analysis of MEP1/MEP2 hybrid proteins identified a small intracellular loop of MEP2 involved in the pseudohyphal regulatory function. In addition, mutations in GLN3, URE2 and NPR1, which abrogate MEP2 expression or stability, also conferred pseudohyphal growth defects. We propose that MEP2 is an ammonium sensor, generating a signal to regulate filamentous growth in response to ammonium starvation.

Linkage of adhesion, filamentous growth, and virulence in Candida albicans to a single gene, INT1

Adhesion and the ability to form filaments are thought to contribute to the pathogenicity of Candida albicans, the leading cause of fungal disease in immunocompromised patients. Int1p is a C. albicans surface protein with limited similarity to vertebrate integrins. INT1 expression in Saccharomyces cerevisiae was sufficient to direct the adhesion of this normally nonadherent yeast to human epithelial cells. Furthermore, disruption of INT1 in C. albicans suppressed hyphal growth, adhesion to epithelial cells, and virulence in mice. Thus, INT1 links adhesion, filamentous growth, and pathogenicity in C. albicans and Int1p may be an attractive target for the development of antifungal therapies.

Altered adherence in strains of Candida albicans harbouring null mutations in secreted aspartic proteinase genes

The aspartate proteinase inhibitor pepstatin A has been shown previously to reduce the adherence of Candida albicans yeast cells to human surfaces. This suggests that in addition to their presumed function facilitating tissue penetration, the secreted aspartate proteinases (Saps) of this fungal pathogen may have auxiliary roles as cellular adhesins. We therefore examined the relative adherence of yeast cells of a parental wild-type strain of C. albicans in relation to yeast cells of strains harbouring specific disruptions in various members of the SAP gene family in an otherwise isogenic background. The adhesiveness of delta sap1, delta sap2, delta sap3 null mutants and a triple delta sap 4-6 disruptant was examined on three surfaces--glass coated with poly-L-lysine or a commercial cell-free basement membrane preparation (Matrigel) and on human buccal epithelial cells. Pepstatin A reduced adherence to all surfaces. Adherence of the each of the single SAP null mutants to these three substrates was either reduced or not affected significantly compared to that of the parental strain. The adherence of the delta sap4-6 mutant was reduced on poly-L-lysine and Matrigel, but increased on buccal cells. The results suggest that in addition to a primary enzymatic role, various SAPs may also act singly or synergistically to enhance the adhesiveness to C. albicans cells to certain human tissues.

17-beta-estradiol upregulates the stress response in Candida albicans: implications for microbial virulence

OBJECTIVE

The influence of 17-beta-estradiol on the stress response of Candida albicans was studied.

METHODS

The survival of clinical isolates of C. albicans treated with 17-beta-estradiol after heat and oxidative stress was measured by viable plate counts. Cellular proteins were analyzed via SDS-PAGE.

RESULTS

The heat stress response induced by 17-beta-estradiol in C. albicans grown at 25 degrees C protected the organisms against the lethal temperature of 48.5 degrees C, as shown by viable plate counts. 17-beta-estradiol also enhanced protection of C. albicans against oxidative stress (menadione exposure). SDS-PAGE analysis of cytoplasmic extracts revealed proteins induced by 17-beta-estradiol were similar to those induced by heat.

CONCLUSION

17-beta-estradiol enhances survival of C. albicans under heat and oxidative stresses. The proteins induced by 17-beta-estradiol are probably heat shock proteins. Because heat shock proteins are considered to be virulence factors, 17-beta-estradiol may function to promote in vivo survival.

Candida pathogenesis: unravelling the threads of infection

Recent studies are beginning to delineate those pathways by which the important pathogen Candida albicans switches from one growth form to another; at the same time, insights are being gained into the importance of growth form in pathogenesis.