Expression of a constitutively active Cdc42 homologue promotes development of sclerotic bodies but represses hyphal growth in the zoopathogenic fungus Wangiella (Exophiala) dermatitidis

The Transcriptome of Exophiala dermatitidis during Ex-vivo Skin Model Infection

The black yeast Exophiala dermatitidis is a widespread polyextremophile and human pathogen, that is found in extreme natural habitats and man-made environments such as dishwashers. It can cause various diseases ranging from phaeohyphomycosis and systemic infections, with fatality rates reaching 40%. While the number of cases in immunocompromised patients are increasing, knowledge of the infections, virulence factors and host response is still scarce. In this study, for the first time, an artificial infection of an ex-vivo skin model with Exophiala dermatitidis was monitored microscopically and transcriptomically. Results show that Exophiala dermatitidis is able to actively grow and penetrate the skin. The analysis of the genomic and RNA-sequencing data delivers a rich and complex transcriptome where circular RNAs, fusion transcripts, long non-coding RNAs and antisense transcripts are found. Changes in transcription strongly affect pathways related to nutrients acquisition, energy metabolism, cell wall, morphological switch, and known virulence factors. The L-Tyrosine melanin pathway is specifically upregulated during infection. Moreover the production of secondary metabolites, especially alkaloids, is increased. Our study is the first that gives an insight into the complexity of the transcriptome of Exophiala dermatitidis during artificial skin infections and reveals new virulence factors.

Black yeasts and their filamentous relatives: principles of pathogenesis and host defense

Among the melanized fungi, the so-called "black yeasts" and their filamentous relatives are particularly significant as agents of severe phaeohyphomycosis, chromoblastomycosis, and mycetoma in humans and animals. The pathogenicity and virulence of these fungi may differ significantly between closely related species. The factors which probably are of significance for pathogenicity include the presence of melanin and carotene, formation of thick cell walls and meristematic growth, presence of yeast-like phases, thermo- and perhaps also osmotolerance, adhesion, hydrophobicity, assimilation of aromatic hydrocarbons, and production of siderophores. Host defense has been shown to rely mainly on the ingestion and elimination of fungal cells by cells of the innate immune system, especially neutrophils and macrophages. However, there is increasing evidence supporting a role of T-cell-mediated immune responses, with increased interleukin-10 (IL-10) and low levels of gamma interferon (IFN-γ) being deleterious during the infection. There are no standardized therapies for treatment. It is therefore important to obtain in vitro susceptibilities of individual patients' fungal isolates in order to provide useful information for selection of appropriate treatment protocols. This article discusses the pathogenesis and host defense factors for these fungi and their severity, chronicity, and subsequent impact on treatment and prevention of diseases in human or animal hosts.

Comparative genomic and transcriptomic analysis of wangiella dermatitidis, a major cause of phaeohyphomycosis and a model black yeast human pathogen

Black or dark brown (phaeoid) fungi cause cutaneous, subcutaneous, and systemic infections in humans. Black fungi thrive in stressful conditions such as intense light, high radiation, and very low pH. Wangiella (Exophiala) dermatitidis is arguably the most studied phaeoid fungal pathogen of humans. Here, we report our comparative analysis of the genome of W. dermatitidis and the transcriptional response to low pH stress. This revealed that W. dermatitidis has lost the ability to synthesize alpha-glucan, a cell wall compound many pathogenic fungi use to evade the host immune system. In contrast, W. dermatitidis contains a similar profile of chitin synthase genes as related fungi and strongly induces genes involved in cell wall synthesis in response to pH stress. The large portfolio of transporters may provide W. dermatitidis with an enhanced ability to remove harmful products as well as to survive on diverse nutrient sources. The genome encodes three independent pathways for producing melanin, an ability linked to pathogenesis; these are active during pH stress, potentially to produce a barrier to accumulated oxidative damage that might occur under stress conditions. In addition, a full set of fungal light-sensing genes is present, including as part of a carotenoid biosynthesis gene cluster. Finally, we identify a two-gene cluster involved in nucleotide sugar metabolism conserved with a subset of fungi and characterize a horizontal transfer event of this cluster between fungi and algal viruses. This work reveals how W. dermatitidis has adapted to stress and survives in diverse environments, including during human infections.

Molecular characterization of pathogenic members of the genus Fonsecaea using multilocus analysis

Members of the fungal genus Fonsecaea causing human chromoblastomycosis show substantial geographic structuring. Genetic identity of clinical and environmental strains suggests transmission from plant debris, while the evolutionary processes that have led to spatially separated populations have remained unexplained. Sequences of ITS, BT2, ACT1, Cdc42, Lac and HmgA were analyzed, either by direct sequencing or by cloning. Thirty-seven clinical and environmental Fonsecaea strains from Central and South America, Asia, Africa and Europe were sequenced and possible recombination events were calculated. Phylogenetic trees of Cdc42, Lac and HmgA were statistically supported, but ITS, BT2 and ACT1 trees were not. The Standardized Index of Association (I_A^S) did not detect recombination (I_A^S = 0.4778), neither did the Phi-test for separate genes. In Fonsecaea nubica non-synonymous mutations causing functional changes were observed in Lac gene, even though no selection pressures were detected with the neutrality test (Tajima D test, p>0.05). Genetic differentiation of populations for each gene showed separation of American, African and Asian populations. Strains of clinical vs. environmental origin showed genetic distances that were comparable or lower than found in geographic differentiation. In conclusion, here we demonstrated clonality of sibling species using multilocus data, geographic structuring of populations, and a low functional and structural selective constraint during evolution of the genus Fonsecaea.

Evolution of CDC42, a putative virulence factor triggering meristematic growth in black yeasts

The cell division cycle gene (CDC42) controlling cellular polarization was studied in members of Chaetothyriales. Based on ribosomal genes, ancestral members of the order exhibit meristematic growth in view of their colonization of inert surfaces such as rock, whereas in derived members of the order the gene is a putative virulence factor involved in expression of the muriform cell, the invasive phase in human chromoblastomycosis. Specific primers were developed to amplify a portion of the gene of 32 members of the order with known position according to ribosomal phylogeny. Phylogeny of CDC42 proved to be very different. In all members of Chaetohyriales the protein sequence is highly conserved. In most species, distributed all over the phylogenetic tree, introns and 3^rd codon positions are also invariant. However, a number of species had paralogues with considerable deviation in non-coding exon positions, and synchronous variation in introns, although non-synonomous variation had remained very limited. In some strains both orthologues and paralogues were present. It is concluded that CDC42 does not show any orthologous evolution, and that its paralogues haves the same function but are structurally relaxed. The variation or absence thereof could not be linked to ecological changes, from rock-inhabiting to pathogenic life style. It is concluded that eventual pathogenicity in Chaetothyriales is not expressed at the DNA level in CDC42 evolution.

Molecular cloning, characterization and differential expression of Cdc42 in Fonsecaea monophora

The cell divisions cycle 42 (Cdc42) gene has been characterized in the fungi, such as Candida albicans, Penicillium marneffei, and Wangiella (Exophiala) dermatitidis, which plays important roles during growth and development. The partial cDNA sequence of Cdc42 of Fonsecaea monophora (F. monophora), designated FmCdc42, was obtained using degenerate primers based on the conserved domain of the other fungi Cdc42. Then the complete cDNA sequence of FmCdc42 was obtained by 5' and 3' RACE. The full-length cDNA is 1,510 bp in size which had an open reading frame (ORF) of 582 bp, encoding 193 amino acid residues. The predicted molecular mass of FmCdc42 is 21.5 kDa with an estimated theoretical isoelectric point of 5.67. The deduced amino acid sequence of FmCdc42 shows 99% identity to that of Wangiella (Exophiala) dermatitidis. 5 exons and 4 introns are identified within the 1,617 bp FmCdc42 genomic DNA sequence of F. monophora. The ORF could be subcloned into the pCDNA6/myc-His B expression vector. The recombinant protein about 27.5 kD infusion protein had high expression level in Vero cells with SDS-PAGE and Western blot analysis. Quantitative real time RT-PCR revealed that FmCdc42 was the highest expression in the sclerotic bodies' stage compared with that in the mycelia and conidia stages, which indicated that the FmCdc42 may be involved in formation of F. monophora sclerotic bodies.

Roles of the pH signaling transcription factor PacC in Wangiella (Exophiala) dermatitidis

To study the function of the PacC transcription factor in Wangiella dermatitidis, a black, polymorphic fungal pathogen of humans with yeast-phase predominance, the PACC gene was cloned, sequenced, disrupted and expressed. Three zinc finger DNA-binding motifs were found at the N-terminus, and a signaling protease cleavage site at the C-terminus. PACC was more expressed at neutral-alkaline pH than at acidic pH. Truncation at about 40 residues of the coding sequence upstream of the conserved protease processing cleavage site of PacC affected growth on a nutrient-rich medium, increased sensitivity to Na(+) stress, decreased yeast growth at neutral-alkaline pH, and repressed hyphal growth on a nutrient-poor medium at 25 degrees C. Truncation at the coding sequence for the conserved signaling protease box of PacC impaired growth and reduced RNA expression of the class II chitin synthase gene at acidic pH. The results suggested that PacC is important not only for the adaptation of W. dermatitidis to different ambient pH conditions and Na(+) stress conditions, but also for influencing yeast-hyphal transitions in this agent of phaeohyphomycosis.

A Cdc42 ortholog is required for penetration and virulence of Magnaporthe grisea

Cdc42, a member of the Rho-family small GTP-binding proteins, is a pivotal signaling switch that cycles between active GTP-bound and inactive GDP-bound forms, controlling actin cytoskeleton organization and cell polarity. In this report, we show that MgCdc42, a Cdc42 ortholog in Magnaporthe grisea, is required for its plant penetration. Consequently, the deletion mutants show dramatically decreased virulence to rice due to the arrest of penetration and infectious growth, which may be attributed to the defect of turgor and superoxide generation during the appressorial development in Mgcdc42 deletion mutants. In addition, the mutants also exhibit pleotropic defects including gherkin-shaped conidia, delayed germination as well as decreased sporulation. Furthermore, dominant negative mutation leads to a similar phenotype to that of the deletion mutants, lending further support to the conclusion that MgCdc42 is required for the penetration and virulence of M. grisea.

Cytolocalization of the class V chitin synthase in the yeast, hyphal and sclerotic morphotypes of Wangiella (Exophiala) dermatitidis

Wangiella (Exophiala) dermatitidis is a polymorphic fungus that produces polarized yeast and hyphae, as well as a number of non-polarized sclerotic morphotypes. The phenotypic malleability of this agent of human phaeohyphomycosis allows detailed study of its biology, virulence and the regulatory mechanisms responsible for the transitions among the morphotypes. Our prior studies have demonstrated the existence of seven chitin synthase structural genes in W. dermatitidis, each of which encodes an isoenzyme of a different class. Among them, the class V chitin synthase (WdChs5p) is most unique in terms of protein structure, because it has an N-terminal myosin motor-like domain with a P-loop (MMD) fused to its C-terminal chitin synthase catalytic domain (CSCD). However, the exact role played by WdChs5p in the different morphotypes remains undefined beyond the knowledge that it is the only single chitin synthase required for sustained cell growth at 37 degrees C and consequently virulence. This report describes the expression in Escherichia coli of a 12kDa polypeptide (WdMyo12p) of WdChs5p, which was used to raise in rabbits a polyclonal antibody that recognized exclusively its MMD region. Results from the use of the antibody in immunocytolocalization studies supported our previous findings that WdChs5p is critically important at infection temperatures for maintaining the cell wall integrity of developing yeast buds, elongating tips of hyphae, and random sites of expansion in sclerotic forms. The results also suggested that WdChs5p localizes to the regions of cell wall growth in an actin-dependent fashion.

The small GTPase Rac and the p21-activated kinase Cla4 in Claviceps purpurea: interaction and impact on polarity, development and pathogenicity

Claviceps purpurea, the ergot fungus, is a highly specialized pathogen of grasses; its colonization of host ovarian tissue requires an extended period of strictly polarized, oriented growth towards the vascular tissue. To understand this process, we study the role of signalling factors affecting polarity and differentiation. We showed that the small GTPase Cdc42 is involved in polarity, sporulation and in planta growth in C. purpurea. Here we present evidence that the GTPase Rac has an even stronger and, in some aspects, inverse impact on growth and development: Deltarac mutants form coralline-like colonies, show hyper-branching, loss of polarity, sporulation and ability to penetrate. Functional analyses and yeast two-hybrid studies prove that the p21-activated kinase Cla4 is a major downstream partner of Rac. Phosphorylation assays of MAP kinases and expression studies of genes encoding reactive oxygen species (ROS)-scavenging and -generating enzymes indicate a function of Rac and Cla4 in fungal ROS homoeostasis which could contribute to their drastic impact on differentiation.

Molecular cloning and characterization of WdTUP1, a gene that encodes a potential transcriptional repressor important for yeast-hyphal transitions in Wangiella (Exophiala) dermatitidis

The general transcriptional repressor Tup1p is known to influence cell development in many fungi. To determine whether the Tup1p ortholog (WdTup1p) of Wangiella dermatitidis also influences cellular development in this melanized, polymorphic human pathogen, the gene (WdTUP1) that encodes this transcription factor was isolated, sequenced and disrupted. Phylogenetic analysis showed that the WdTup1p sequence was closely related to homologues in other polymorphic, conidiogenous fungi. Disruption of WdTUP1 produced mutants (wdtup1Delta) with pronounced growth and cellular abnormalities, including slow growth on various agar media and exclusively as a filamentous morphotype in liquid media. We concluded that WdTup1p represents an important switch regulator that controls the yeast-to-filamentous growth transition. However, detailed observations of the filamentous growth of the disruption mutant showed that the hyphae produced by the wdtup1Delta mutants, unlike those of the wild-type, were arrested at a stage prior to the formation of true hyphae and subsequent conidia production.

A Rho3 homolog is essential for appressorium development and pathogenicity of Magnaporthe grisea

The small GTPase Rho3 is conserved in fungi and plays a key role in the control of cell polarity and exocytosis in yeast. In this report, we show that a Rho3 homolog, MgRho3, is dispensable for polarized hyphal growth in the rice blast fungus Magnaporthe grisea. However, MgRho3 is required for plant infection. Appressoria formed by the Mgrho3 deletion mutants are morphologically abnormal and defective in plant penetration. Conidia of the Mgrho3 deletion mutants are narrower than those of the wild-type strain and delayed in germination. Transformants expressing a dominant negative Mgrho3 allele exhibit similar phenotypes as the Mgrho3 deletion mutant, while transformants expressing a constitutively active allele of MgRho3 can produce normal conidia but remain defective in appressorium formation and plant infection. In contrast, overexpression of wild-type MgRho3 increases the infectivity of M. grisea. Our results reveal a new role for the conserved Rho3 as a critical regulator of developmental processes and pathogenicity of M. grisea.

WdStuAp, an APSES transcription factor, is a regulator of yeast-hyphal transitions in Wangiella (Exophiala) dermatitidis

APSES transcription factors are well-known regulators of fungal cellular development and differentiation. To study the function of an APSES protein in the fungus Wangiella dermatitidis, a conidiogenous and polymorphic agent of human phaeohyphomycosis with yeast predominance, the APSES transcription factor gene WdSTUA was cloned, sequenced, disrupted, and overexpressed. Analysis showed that its derived protein was most similar to the APSES proteins of other conidiogenous molds and had its APSES DNA-binding domain located in the amino-terminal half. Deletion of WdSTUA in W. dermatitidis induced convoluted instead of normal smooth colony surface growth on the rich yeast maintenance agar medium yeast extract-peptone-dextrose agar (YPDA) at 37 degrees C. Additionally, deletion of WdSTUA repressed aerial hyphal growth, conidiation, and invasive hyphal growth on the nitrogen-poor, hypha-inducing agar medium potato dextrose agar (PDA) at 25 degrees C. Ectopic overexpression of WdSTUA repressed the convoluted colony surface growth on YPDA at 37 degrees C, and also strongly repressed hyphal growth on PDA at 25 degrees C and 37 degrees C. These new results provide additional insights into the diverse roles played by APSES factors in fungi. They also suggest that the transcription factor encoded by WdSTUA is both a positive and negative morphotype regulator in W. dermatitidis and possibly other of the numerous human pathogenic, conidiogenous fungi capable of yeast growth.

A Ras1-Cdc24 signal transduction pathway mediates thermotolerance in the fungal pathogen Cryptococcus neoformans

Pathogenic microorganisms must precisely regulate morphogenesis to survive and proliferate within an infected host. This regulation is often controlled by conserved signal transduction pathways that direct morphological changes in varied species. One such pathway, whose components include Ras proteins and the PAK kinase Ste20, allows the human fungal pathogen Cryptococcus neoformans to grow at high temperature. Previously, we found that Ras1 signalling is required for differentiation, thermotolerance and pathogenesis in C. neoformans. We show here that the guanine nucleotide exchange factor Cdc24 is a Ras1 effector in C. neoformans to mediate the ability of this fungus to grow at high temperature and to cause disease. In addition, we provide evidence that the Ras1-Cdc24 signalling cascade functions specifically through one of the three Cdc42/Rac1 homologues in C. neoformans. In conclusion, our studies illustrate how components of conserved signalling cascades can be specialized for different downstream functions, such as pathogenesis.

Rac1 and Cdc42 regulate hyphal growth and cytokinesis in the dimorphic fungus Ustilago maydis

Small GTP-binding proteins of the highly conserved Rho family act as molecular switches regulating cell signalling, cytoskeletal organization and vesicle trafficking in eukaryotic cells. Here we show that in the dimorphic plant pathogenic fungus Ustilago maydis deletion of either cdc42 or rac1 results in loss of virulence but does not interfere with viability. Cells deleted for cdc42 display a cell separation defect during budding. We have previously shown that the Rho-specific guanine nucleotide exchange factor (GEF) Don1 is required for cell separation in U. maydis. Expression of constitutive active Cdc42 rescues the phenotype of don1 mutant cells indicating that Don1 triggers cell separation by activating Cdc42. Deletion of rac1 affects cellular morphology and interferes with hyphal growth, whereas overexpression of wild-type Rac1 induces filament formation in haploid cells. This indicates that Rac1 is both necessary and sufficient for the dimorphic switch from budding to hyphal growth. Cdc42 and Rac1 share at least one common essential function because depletion of both Rac1 and Cdc42 is lethal. Expression of constitutively active Rac1(Q61L) is lethal and results in swollen cells with a large vacuole. The morphological phenotype, but not lethality is suppressed in cla4 mutant cells suggesting that the PAK family kinase Cla4 acts as a downstream effector of Rac1.

WdChs1p, a class II chitin synthase, is more responsible than WdChs2p (Class I) for normal yeast reproductive growth in the polymorphic, pathogenic fungus Wangiella (Exophiala) dermatitidis

The chitin synthase gene WdCHS1 was isolated from a partial genomic DNA library of the pathogenic polymorphic fungus Wangiella dermatitidis. Sequencing showed that WdCHS1 encoded a class II chitin synthase composed of 988 amino acids. Disruption of WdCHS1 produced strains that were hyperpigmented in rich media, grew as yeast at wild-type rates at both 25 and 37 degrees C and were as virulent as the wild type in a mouse model. However, detailed morphological and cytological studies of the wdchs1Delta mutants showed that yeast cells often failed to separate, tended to be enriched with chitin in septal regions and, sometimes, were enlarged with multiple nuclei, had broader mother cell-daughter bud regions and had other cell wall defects seen considerably less often than in the wild type or wdchs2 Delta strains. Disruption of WdCHS1 and WdCHS2 in the same background revealed that WdChs1p had functions synergistic to those of WdChs2p, because mutants devoid of both isozymes produced growth that was very abnormal at 25 degrees C and was not viable at 37 degrees C unless osmotically stabilized. These results suggested that WdChs1p was more responsible than WdChs2p for normal yeast cell reproductive growth because strains with defects in the latter exhibited no morphological abnormalities, whereas those with defects in WdChs1p were frequently impaired in one or more yeast developmental processes.

Cdc42 is required for proper growth and development in the fungal pathogen Colletotrichum trifolii

Cdc42 is a highly conserved small GTP-binding protein that is involved in regulating morphogenesis in eukaryotes. In this study, we isolated and characterized a highly conserved Cdc42 gene from Colletotrichum trifolii (CtCdc42), a fungal pathogen of alfalfa. CtCdc42 is, at least in part, functionally equivalent to Saccharomyces cerevisiae Cdc42p, since it restores the temperature-sensitive phenotype of a yeast Cdc42p mutant. Inhibition of CtCdc42 by expression of an antisense CtCdc42 or a dominant negative form of CtCdc42 (DN Cdc42) resulted in appressorium differentiation under noninductive conditions, suggesting that CtCdc42 negatively regulates pathogenic development in this fungus. We also examined the possible linkage between CtCdc42 and Ras signaling. Expression of a dominant active Cdc42 (DA Cdc42) in C. trifolii leads to aberrant hyphal growth under nutrient-limiting conditions. This phenotype was similar to that of our previously reported dominant active Ras (DA Ras) mutant. Also consistent with our observations of the DA Ras mutant, high levels of reactive oxygen species (ROS) were observed in the DA Cdc42 mutant, and proline restored the wild-type phenotype. Moreover, overexpression of DN Cdc42 resulted in a significant decrease in spore germination, virtually no hyphal branching, and earlier sporulation, again similar to what we observed in a dominant negative Ras (DN Ras) mutant strain. Interestingly, coexpression of DA Cdc42 with DN Ras resulted in germination rates close to wild-type levels, while coexpression of DN Cdc42 with the DA Ras mutant restored the wild-type phenotype. These data suggest that CtCdc42 is positioned as a downstream effector of CtRas to regulate spore germination and pathogenic development.

A CDC42 homologue in Claviceps purpurea is involved in vegetative differentiation and is essential for pathogenicity

Claviceps purpurea, a biotrophic pathogen of cereals, has developed a unique pathogenic strategy including an extended period of unbranched directed growth in the host's style and ovarian tissue to tap the vascular system. Since the small GTPase Cdc42 has been shown to be involved in cytoskeleton organization and polarity in other fungi, we investigated the role of Cdc42 in the development and pathogenicity of C. purpurea. Expression of heterologous dominant-active (DA) and dominant-negative (DN) alleles of Colletotrichum trifolii in a wild strain of C. purpurea had significant impact on vegetative differentiation: whereas DA Ctcdc42 resulted in loss of conidiation and in aberrant cell shape, expression of DN Ctcdc42 stimulated branching and conidiation. Deletion of the endogenous Cpcdc42 gene was not lethal but led to a phenotype comparable to that of DN Ctcdc42 transformants. DeltaCpcdc42 mutants were nonpathogenic; i.e., they induced no disease symptoms. Cytological analysis (light microscopy and electron microscopy) revealed that the mutants can penetrate and invade the stylar tissue. However, invasive growth was arrested in an early stage, presumably induced by plant defense reactions (necrosis or increased production of reactive oxygen species), which were never observed in wild-type infection. The data show a significant impact of Cpcdc42 on vegetative differentiation and pathogenicity in C. purpurea.

Ectopic expression of a constitutively active Cdc42 small GTPase alters the morphology of haploid and dikaryotic hyphae in the filamentous homobasidiomycete Schizophyllum commune

Cloning of the Cdc42 gene from Schizophyllum commune enabled investigation of the role of ScCdc42 in the regulation of vegetative growth and sexual reproduction in this fungus, which has a well-characterized hyphal cell structure, cytoskeleton, and mating system. Ectopic expression of the constitutively active Sccdc42(G12V) or Sccdc42(Q61L) alleles from native or inducible ScCel1 promoters in haploid hyphae had dramatic effects on hyphal morphology, cytoskeletal structure, and Cdc42 localization. For transformants with constitutively active Sccdc42, polar tip growth of apical cells in the leading hyphae was normal but polar tip growth in side branches was altered, implying different regulation of polarity establishment in the two groups of apical cells. Branch emergence at exceptional sites and isotropic growth of cells near the septum indicated that ScCdc42 regulates branch site selection and subsequent hyphal development. Poor dikaryotization along with irregular clamp connections in mates expressing Sccdc42(G12V) or Sccdc42(Q61L) suggested that Cdc42 also contributes to efficient mating in S. commune.

Wangiella (Exophiala) dermatitidis WdChs5p, a class V chitin synthase, is essential for sustained cell growth at temperature of infection

The chitin synthase structural gene WdCHS5 was isolated from the black fungal pathogen of humans Wangiella dermatitidis. Sequence analysis revealed that the gene has a myosin motor-like-encoding region at its 5' end and a chitin synthase (class V)-encoding region at its 3' end. Northern blotting showed that WdCHS5 is expressed at high levels under conditions of stress. Analysis of the 5' upstream region of WdCHS5 fused to a reporter gene indicated that one or more of the potential regulatory elements present may have contributed to the high expression levels. Disruption of WdCHS5 produced mutants that grow normally at 25 degrees C but have severe growth and cellular abnormalities at 37 degrees C. Osmotic stabilizers, such as sorbitol and sucrose, rescued the wild-type phenotype, which indicated that the loss of WdChs5p causes cell wall integrity defects. Animal survival tests with a mouse model of acute infection showed that all wdchs5Delta mutants are less virulent than the parental strain. Reintroduction of the WdCHS5 gene into the wdchs5Delta mutants abolished the temperature-sensitive phenotype and reestablished their virulence. We conclude that the product of WdCHS5 is required for the sustained growth of W. dermatitidis at 37 degrees C and is of critical importance to its virulence.

Characterization of two polyketide synthase genes in Exophiala lecanii-corni, a melanized fungus with bioremediation potential

Exophiala lecanii-corni has significant bioremediation potential because it can degrade a wide range of volatile organic compounds. In order to identify sites for the insertion of genes that might enhance this potential, a genetic analysis of E. lecanii-corni was undertaken. Two polyketide synthase genes, ElPKS1 and ElPKS2, have now been discovered by a PCR-based strategy. ElPKS1 was isolated by a marker rescue technique. The nucleotide sequence of ElPKS1 consists of a 6576-bp open reading frame encoding a protein with 2192 amino acids, which was interrupted by a 60-bp intron near the 5' end and a 54-bp intron near the 3' end. Sequence analysis, results from disruption experiments, and physiological tests showed that ElPKS1 encoded a polyketide synthase required for melanin biosynthesis. Since ElPKS1 is non-essential, it is a desirable bioengineering target site for the insertion of native and foreign genes. The successful expression of these genes could enhance the bioremediation capability of the organism. ElPKS2 was cloned by colony hybridization screening of a partial genomic library with an ElPKS2 PCR product. ElPKS2 had a 6465-bp open reading frame that encoded 2155 amino acids and had introns of 56, 67, 54, and 71 bp. Although sequence analysis of the derived protein of ElPKS2 confirmed the polyketide synthase nature of its protein product, the function of that product remains unclear.

Polarity in filamentous fungi: moving beyond the yeast paradigm

Filamentous fungi grow by the polar extension of hyphae. This polar growth requires the specification of sites of germ tube or branch emergence, followed by the recruitment of the morphogenetic machinery to those sites for localized cell wall deposition. Researchers attempting to understand hyphal morphogenesis have relied upon the powerful paradigm of bud emergence in the yeast Saccharomyces cerevisiae. The yeast paradigm has provided a useful framework, however several features of hyphal morphogenesis, such as the ability to maintain multiple axes of polarity and an extremely rapid extension rate, cannot be explained by simple extrapolation from yeast models. We discuss recent polarity research from filamentous fungi focusing on the position of germ tube emergence, the relaying of positional information via RhoGTPase modules, and the recruitment of morphogenetic machinery components including cytoskeleton, polarisome and ARP2/3 complexes, and the vesicle trafficking system.

Cdc24, the GDP-GTP exchange factor for Cdc42, is required for invasive hyphal growth of Candida albicans

Candida albicans, the most common human fungal pathogen, is particularly problematic for immunocompromised individuals. The reversible transition of this fungal pathogen to a filamentous form that invades host tissue is important for its virulence. Although different signaling pathways such as a mitogen-activated protein kinase and a protein kinase A cascade are critical for this morphological transition, the function of polarity establishment proteins in this process has not been determined. We examined the role of four different polarity establishment proteins in C. albicans invasive growth and virulence by using strains in which one copy of each gene was deleted and the other copy expressed behind the regulatable promoter MET3. Strikingly, mutants with ectopic expression of either the Rho G-protein Cdc42 or its exchange factor Cdc24 are unable to form invasive hyphal filaments and germ tubes in response to serum or elevated temperature and yet grow normally as a budding yeast. Furthermore, these mutants are avirulent in a mouse model for systemic infection. This function of the Cdc42 GTPase module is not simply a general feature of polarity establishment proteins. Mutants with ectopic expression of the SH3 domain containing protein Bem1 or the Ras-like G-protein Bud1 can grow in an invasive fashion and are virulent in mice, albeit with reduced efficiency. These results indicate that a specific regulation of Cdc24/Cdc42 activity is required for invasive hyphal growth and suggest that these proteins are required for pathogenicity of C. albicans.

Molecular genetic studies of the model dematiaceous pathogen Wangiella dermatitidis

The rapidly improving molecular genetic tractability of Wangiella (Exophiala) dermatitidis significantly enhances its usefulness as a model for the more than 100 other dematiaceous (melanized) agents of human disease. Previously this model was based almost exclusively on its vegetative polymorphism, which at the simplest level is expressed as three well-characterized modes of growth (e.g., blastic, apical and isotropic) that produce myriad yeast, hyphal and sclerotic phenotypes. This cellular plasticity is important for a dematiaceous model pathogen because some are hyphal in nature but exist almost exclusively as sclerotic bodies in infected tissue, whereas others are hyphal both in nature and in tissue, and still others exist in nature predominantly as yeast, but as mixtures of yeast, hyphae and sclerotic bodies in tissue. By exploiting the polymorphism of W. dermatitidis, any phenotype of another dematiaceous pathogen can be produced for study of the regulation of its development and its contribution to pathogenicity and virulence. The coupling of this asset with the recent finding that its haploid, uninucleate yeast cell is easily transformed molecularly, and the even more recent development of systems for both random and targeted gene disruptions and for site-specific, integrative gene overexpression studies suggest that it will continue as the preferred model for the dematiaceous fungi and irrespective of the mycosis involved. The results reviewed here aim to confirm this contention, stimulate others to study this fungus, and demonstrate that W. dermatitidis is exceptionally useful for discovering by molecular genetic techniques cell wall-associated virulence factors in fungi, and in particular in the dematiaceous fungi.

Control of morphogenesis in the human fungal pathogen Penicillium marneffei

Fungal pathogens are an increasing threat to human health due to the increasing population of immunocompromised individuals and the increased incidence of treatment-derived infections. Penicillium marneffei is an emerging fungal pathogen endemic to South-east Asia, where it is AIDS defining. Like many other fungal pathogens, P. marneffei is capable of alternating between a filamentous and a yeast growth form, known as dimorphic switching, in response to environmental stimuli. P. marneffei grows in the filamentous form at 25 degrees C and in the yeast form at 37 degrees C. During filamentous growth and in response to environmental cues, P. marneffei undergoes asexual development to form complex multicellular structures from which the infectious agents, the conidia, are produced. At 37 degrees C, P. marneffei undergoes the dimorphic switching program to produce the pathogenic yeast cells. These yeast cells are found intracellularly in the mononuclear phagocyte system of the host and divide by fission, in contrast to the budding mode of division exhibited by most other fungal pathogens. In addition, P. marneffei is evolutionarily distinct from most other dimorphic fungal pathogens and is the only known Penicillium species which exhibits dimorphic growth. The unique evolutionary history of P. marneffei and the rapidly increasing incidence of infection, coupled with the presence of both complex asexual development and dimorphic switching programs in one organism, makes this system a valuable one for the study of morphogenesis and pathogenicity. Recent development of molecular genetic techniques for P. marneffei, including DNA-mediated transformation, have greatly facilitated the study of these two important morphogenetic programs, asexual development and dimorphic switching, and we are beginning to uncover important determinants which control these events. Understand these programs is providing insights into the biology of P. marneffei and its pathogenic capacity.

Compensatory expression of five chitin synthase genes, a response to stress stimuli, in Wangiella (Exophiala) dermatitidis, a melanized fungal pathogen of humans

Numerous chitin synthase structural (CHS) genes have been identified in fungi, and usually there are several CHS genes per species. Compensatory expression of one CHS gene in response to defects in other CHS genes has not been reported. Five chitin synthase structural (WdCHS) genes have been identified in the melanized human pathogen Wangiella dermatitidis: WdCHS1, WdCHS2, WdCHS3, WdCHS4 and WdCHS5. This study showed that increased WdCHS expression existed as a compensatory mechanism in response to stress induced by chitin synthase gene disruptions, or by exposure of the wild-type or two temperature-sensitive morphological mutants, for short or long periods, to 37 degrees C. In general, the compensatory responses varied with each WdCHS gene, and in accordance with the hypothesized functions of the chitin synthase (WdChsp) encoded. It is suggested that these compensatory responses indicate that WdCHS gene transcription in W. dermatitidis functions as part of a cell-wall integrity pathway in a manner similar to that recently described for Saccharomyces cerevisiae.

CDC42 is required for polarized growth in human pathogen Candida albicans

Cdc42p is a member of the RAS superfamily of GTPases and plays an essential role in polarized growth in many eukaryotic cells. We cloned the Candida albicans CaCDC42 by functional complementation in Saccharomyces cerevisiae and analyzed its function in C. albicans. A double deletion of CaCDC42 was made in a C. albicans strain containing CaCDC42 under the control of the PCK1 promoter. When expression of the heterologous copy of CaCDC42 was repressed in this strain, the cells ceased proliferation. These arrested cells were large, round, and unbudded and contained predominantly two nuclei. The PCK1-mediated overexpression of wild-type CaCdc42p had no effect on cells. However, in cells overexpressing CaCdc42p containing the dominant-negative D118A substitution, proliferation was blocked and the arrested cells were large, round, unbudded, and multinucleated, similar to the phenotype of the cdc42 double-deletion strain. Cells overexpressing CaCdc42p containing the hyperactive G12V substitution also ceased proliferation in yeast growth medium; in this case the arrested cells were multinucleated and multibudded. An intact CAAX box is essential for the phenotypes associated with either CaCdc42p(G12V) or CaCdc42p(D118A) ectopic expression, suggesting that membrane attachment is involved in CaCdc42p function. In addition, the lethality caused by ectopic expression of CaCdc42p(G12V) was suppressed by deletion of CST20 but not by deletion of CaCLA4. CaCdc42p function was also examined under hypha-inducing conditions. Cdc42p depletion prior to hyphal induction trapped cells in a round, unbudded state, while depletion triggered at the same time as hyphal induction permitted the initiation of germ tubes that failed to be extended. Ectopic expression of either the G12V or D118A substitution protein modified hyphal formation in a CAAX box-dependent manner. Thus, CaCdc42p function appears important for polarized growth of both the yeast and hyphal forms of C. albicans.

WdChs2p, a class I chitin synthase, together with WdChs3p (class III) contributes to virulence in Wangiella (Exophiala) dermatitidis

The chitin synthase structural gene WdCHS2 was isolated by screening a subgenomic DNA library of Wangiella dermatitidis by using a 0.6-kb PCR product of the gene as a probe. The nucleotide sequence revealed a 2,784-bp open reading frame, which encoded 928 amino acids, with a 59-bp intron near its 5' end. Derived protein sequences showed highest amino acid identities with those derived from the CiCHS1 gene of Coccidioides immitis and the AnCHSC gene of Aspergillus nidulans. The derived sequence also indicated that WdChs2p is an orthologous enzyme of Chs1p of Saccharomyces cerevisiae, which defines the class I chitin synthases. Disruptions of WdCHS2 produced strains that showed no obvious morphological defects in yeast vegetative growth or in ability to carry out polymorphic transitions from yeast cells to hyphae or to isotropic forms. However, assays showed that membranes of wdchs2Delta mutants were drastically reduced in chitin synthase activity. Other assays of membranes from a wdchs1Deltawdchs3Deltawdchs4Delta triple mutant showed that their residual chitin synthase activity was extremely sensitive to trypsin activation and was responsible for the majority of zymogenic activity. Although no loss of virulence was detected when wdchs2Delta strains were tested in a mouse model of acute infection, wdchs2Deltawdchs3Delta disruptants were considerably less virulent in the same model, even though wdchs3Delta strains also had previously shown no loss of virulence. This virulence attenuation in the wdchs2Deltawdchs3Delta mutants was similarly documented in a limited fashion in more-sensitive cyclophosphamide-induced immunocompromised mice. The importance of WdChs2p and WdChs3p to the virulence of W. dermatitidis was then confirmed by reconstituting virulence in the double mutant by the reintroduction of either WdCHS2 or WdCHS3 into the wdchs2Deltawdchs3Delta mutant background.

The CDC42 homolog of the dimorphic fungus Penicillium marneffei is required for correct cell polarization during growth but not development

The opportunistic human pathogenic fungus Penicillium marneffei is dimorphic and is thereby capable of growth either as filamentous multinucleate hyphae or as uninucleate yeast cells which divide by fission. The dimorphic switch is temperature dependent and requires regulated changes in morphology and cell shape. Cdc42p is a Rho family GTPase which in Saccharomyces cerevisiae is required for changes in polarized growth during mating and pseudohyphal development. Cdc42p homologs in higher organisms are also associated with changes in cell shape and polarity. We have cloned a highly conserved CDC42 homolog from P. marneffei named cflA. By the generation of dominant-negative and dominant-activated cflA transformants, we have shown that CflA initiates polarized growth and extension of the germ tube and subsequently maintains polarized growth in the vegetative mycelium. CflA is also required for polarization and determination of correct cell shape during yeast-like growth, and active CflA is required for the separation of yeast cells. However, correct cflA function is not required for dimorphic switching and does not appear to play a role during the generation of specialized structures during asexual development. In contrast, heterologous expression of cflA alleles in Aspergillus nidulans prevented conidiation.