Molecular aspects of fungal dimorphism

Paracoccidioidomycosis: Detection of Paracoccidioides brasiliensis´ genome in biological samples by quantitative chain reaction polymerase (qPCR)

Paracoccidioidomycosis (PCM) is a chronic mycosis caused by the saprobic and dimorphic species Paracoccidioides brasiliensis and P. lutzii. This disease is prevalent in Latin American countries. PCM appears as a relevant concern and challenge for the mycologists, since until now there is no a methodology suitable for an efficient and safe diagnosis and species identification. Thus, the present study aimed to validate a methodology for PCM´s diagnosis, using quantitative Polymerase Chain Reaction (qPCR) through target amplification of the gene encoding the recombinant protein Pb27, a common protein to the both species Paracoccidioides brasiliensis and P. lutzii. The experiments were performed in vitro to determine the specificity, efficiency and detection limit of qPCR assay, using specific primers and probe, which sequences were subject to a patent deposited in Brazilian CTIT, under the registration number: BR1020160078830. According to the results the technique showed sensitivity of 94% and specificity of 100%, demonstrating that it will be possible to develop a new fast and safe diagnostic PCM and can be standardized in order to present a low cost, accessible to the patient served by the public health system in Brazil and Latin America.

Paracoccidioidomycosis: Current Perspectives from Brazil

BACKGROUND

This review article summarizes and updates the knowledge on paracoccidioidomycosis. P lutzii and the cryptic species of P. brasiliensis and their geographical distribution in Latin America, explaining the difficulties observed in the serological diagnosis.

OBJECTIVES

Emphasis has been placed on some genetic factors as predisposing condition for paracoccidioidomycosis. Veterinary aspects were focused, showing the wide distribution of infection among animals. The cell-mediated immunity was better characterized, incorporating the recent findings.

METHODS

Serological methods for diagnosis were also compared for their parameters of accuracy, including the analysis of relapse.

RESULTS

Clinical forms have been better classified in order to include the pictures less frequently observesiod.

CONCLUSION

Itraconazole and the trimethoprim-sulfamethoxazole combination was compared regarding efficacy, effectiveness and safety, demonstrating that azole should be the first choice in the treatment of paracoccidioidomycosis.

Comparing the sugar profiles and primary structures of alkali-extracted water-soluble polysaccharides in cell wall between the yeast and mycelial phases from Tremella fuciformis

To gain insights into dimorphism, cell wall polysaccharides from Tremella fuciformis strains were obtained from alkali-extracted water-soluble fractions PTF-M38 (from the mycelial form), PTF-Y3 and PTF-Y8 (from the yeast form) of T. fuciformis strains were used to gain some insights into dimorphism study. Their chemical properties and structural features were investigated using gel permeation chromatography, gas chromatography, UV and IR spectrophotometry and Congo red binding reactions. The results indicated that the backbones of PTF-M38, PTF-Y3 and PTF-Y8 were configured with α-linkages with average molecular weights of 1.24, 1.08, and 1.19 kDa, respectively. PTF-M38 was mainly composed of xylose, mannose, glucose, and galactose in a ratio of 1:1.47:0.48:0.34, while PTF-Y3 and PTF-Y8 were mainly composed of xylose, mannose and glucose in a ratio of 1:1.65:4.06 and 1:1.21:0.44, respectively. The sugar profiles of PTF-M38, PTF-Y3 and PTF-Y8 were also established for further comparison. These profiles showed that all three polysaccharides contained the same sugars but in different ratios, and the carbon sources (xylose, mannose, glucose, and galactose) affected the sugar ratios within the polysaccharides.

Characterization of Lipids and Proteins Associated to the Cell Wall of the Acapsular Mutant Cryptococcus neoformans Cap 67

Cryptococcus neoformans is an opportunistic human pathogen that causes life-threatening meningitis. In this fungus, the cell wall is exceptionally not the outermost structure due to the presence of a surrounding polysaccharide capsule, which has been highly studied. Considering that there is little information about C. neoformans cell wall composition, we aimed at describing proteins and lipids extractable from this organelle, using as model the acapsular mutant C. neoformans cap 67. Purified cell wall preparations were extracted with either chloroform/methanol or hot sodium dodecyl sulfate. Total lipids fractionated in silica gel 60 were analyzed by electrospray ionization tandem mass spectrometry (ESI-MS/MS), while trypsin digested proteins were analyzed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). We detected 25 phospholipid species among phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and phosphatidic acid. Two glycolipid species were identified as monohexosyl ceramides. We identified 192 noncovalently linked proteins belonging to different metabolic processes. Most proteins were classified as secretory, mainly via nonclassical mechanisms, suggesting a role for extracellular vesicles (EV) in transwall transportation. In concert with that, orthologs from 86% of these proteins have previously been reported both in fungal cell wall and/or in EV. The possible role of the presently described structures in fungal-host relationship is discussed.

Description of Taphrina antarctica f.a. sp. nov., a new anamorphic ascomycetous yeast species associated with Antarctic endolithic microbial communities and transfer of four Lalaria species in the genus Taphrina

In the framework of a large-scale rock sampling in Continental Antarctica, a number of yeasts have been isolated. Two strains that are unable to grow above 20 °C and that have low ITS sequence similarities with available data in the public domain were found. The D1/D2 LSU molecular phylogeny placed them in an isolated position in the genus Taphrina, supporting their affiliation to a not yet described species. Because the new species is able to grow in its anamorphic state only, the species Taphrina antarctica f.a. (forma asexualis) sp. nov. has been proposed to accommodate both strains (type strain DBVPG 5268(T), DSM 27485(T) and CBS 13532(T)). Lalaria and Taphrina species are dimorphic ascomycetes, where the anamorphic yeast represents the saprotrophic state and the teleomorph is the parasitic counterpart on plants. This is the first record for this genus in Antarctica; since plants are absent on the continent, we hypothesize that the fungus may have focused on the saprotrophic part of its life cycle to overcome the absence of its natural host and adapt environmental constrains. Following the new International Code of Nomenclature for Algae, Fungi and Plants (Melbourne Code 2011) the reorganization of Taphrina-Lalaria species in the teleomorphic genus Taphrina is proposed. We emend the diagnosis of the genus Taphrina to accommodate asexual saprobic states of these fungi. Taphrina antarctica was registered in MycoBank under MB 808028.

Recombinant paracoccin reproduces the biological properties of the native protein and induces protective Th1 immunity against Paracoccidioides brasiliensis infection

BACKGROUND

Paracoccin is a dual-function protein of the yeast Paracoccidioides brasiliensis that has lectin properties and N-acetylglucosaminidase activities. Proteomic analysis of a paracoccin preparation from P. brasiliensis revealed that the sequence matched that of the hypothetical protein encoded by PADG-3347 of isolate Pb-18, with a polypeptide sequence similar to the family 18 endochitinases. These endochitinases are multi-functional proteins, with distinct lectin and enzymatic domains.

METHODOLOGY/PRINCIPAL FINDINGS

The multi-exon assembly and the largest exon of the predicted ORF (PADG-3347), was cloned and expressed in Escherichia coli cells, and the features of the recombinant proteins were compared to those of the native paracoccin. The multi-exon protein was also used for protection assays in a mouse model of paracoccidioidomycosis.

CONCLUSIONS/SIGNIFICANCE

Our results showed that the recombinant protein reproduced the biological properties described for the native protein-including binding to laminin in a manner that is dependent on carbohydrate recognition-showed N-acetylglucosaminidase activity, and stimulated murine peritoneal macrophages to produce high levels of TNF-α and nitric oxide. Considering the immunomodulatory potential of glycan-binding proteins, we also investigated whether prophylactic administration of recombinant paracoccin affected the course of experimental paracoccidioidomycosis in mice. In comparison to animals injected with vehicle (controls), mice treated with recombinant paracoccin displayed lower pulmonary fungal burdens and reduced pulmonary granulomas. These protective effects were associated with augmented pulmonary levels of IL-12 and IFN-γ. We also observed that injection of paracoccin three days before challenge was the most efficient administration protocol, as the induced Th1 immunity was balanced by high levels of pulmonary IL-10, which may prevent the tissue damage caused by exacerbated inflammation. The results indicated that paracoccin is the protein encoded by PADG-3347, and we propose that this gene and homologous proteins in other P. brasiliensis strains be called paracoccin. We also concluded that recombinant paracoccin confers resistance to murine P. brasiliensis infection by exerting immunomodulatory effects.

Morphological and metabolic shifts of Yarrowia lipolytica induced by alteration of the dissolved oxygen concentration in the growth environment

Yarrowia lipolytica, an ascomycete with biotechnological potential, is able to form either yeast cells or hyphae and pseudohyphae in response to environmental conditions. This study shows that the morphology of Y. lipolytica, cultivated in batch cultures on hydrophilic (glucose and glycerol) and hydrophobic (olive oil) media, was not affected by the nature of the carbon source, nor by the nature or the concentration of the nitrogen source. By contrast, dissolved oxygen concentration (DOC) should be considered as the major factor affecting yeast morphology. Specifically, when growth occurred at low or zero DOC the mycelial and/or pseudomycelial forms predominated over the yeast form independently of the carbon and nitrogen sources used. Experimental data obtained from a continuous culture of Y. lipolytica on glycerol, being used as carbon and energy source, demonstrated that the mycelium-to-yeast form transition occurs when DOC increases from 0.1 to 1.5 mg l(-1). DOC also affected the yeast physiology, as the activity of enzymes implicated in lipid biosynthesis (i.e. ATP-citrate lyase, malic enzyme) was upregulated at high DOC whereas the activity of enzymes implicated in glycerol assimilation (such as glycerol dehydrogenase and kinase) remained fundamentally unaffected in the cell-free extract.

Effect of Extracellular Factors on Growth and Dimorphism of Rhizopus oryzae with Multiple Enzyme Synthesizing Ability

Rhizopus oryzae PR7 MTCC 9642 was a dimorphic fungus that showed a regular 90 days cycle of filament (mycelium) to pellet (yeast) transformation through a distinct bottom dwelling intermediate state and the pellets never revert back to filamentous form. Apart from the normal cycle, high temperature (37°C and above) and extreme pH also induced the yeast formation. Among the ions tested, calcium and chloride ions were found to restore the filamentous morphology, even in extreme pH and temperature. Cysteine HCl also played noteworthy role in maintaining mycelial growth even at adverse condition. Immobilized spores showed the appearance of intermediate form instead of typical yeast form even at high temperature. The strain could produce a number of extracellular hydrolytic enzymes like cellulolytic, xylanolytic, pectinolytic and amylolytic enzymes. The pellet and mycelial forms were found to be a better producer of cellulase-lignocellulase enzymes and amylolytic enzymes respectively, which might be correlated with their infectivity. Increase in inoculum size, agitation during cultivation, change in carbon and nitrogen source failed to induce mycelial growth in extreme conditions, which might be explained as irreversible change of configuration of protein responsible for mycelial development.

Characterization of cell wall lipids from the pathogenic phase of Paracoccidioides brasiliensis cultivated in the presence or absence of human plasma

BACKGROUND

The fungal cell wall is a complex and dynamic outer structure. In pathogenic fungi its components interact with the host, determining the infection fate. The present work aimed to characterize cell wall lipids from P. brasiliensis grown in the presence and absence of human plasma. We compared the results from isolates Pb3 and Pb18, which represent different phylogenetic species that evoke distinct patterns of experimental paracoccidioidomycosis.

METHODOLOGY/PRINCIPAL FINDINGS

We comparatively characterized cell wall phospholipids, fatty acids, sterols, and neutral glycolipids by using both electrospray ionization- and gas chromatography-mass spectrometry analyses of lipids extracted with organic solvents followed by fractionation in silica-gel-60. We detected 49 phospholipid species in Pb3 and 38 in Pb18, including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol, and phosphatidic acid. In both Pb3 and Pb18, PC and PE had the most numerous species. Among the fatty acids, C18:1 and C18:2 were the most abundant species in both isolates, although C18:2 was more abundant in Pb18. There was a different effect of plasma supplementation on fatty acids depending on the fungal isolate. The prevalent glycolipid species was Hex-C18:0-OH/d19:2-Cer, although other four minor species were also detected. The most abundant sterol in all samples was brassicasterol. Distinct profiles of cell wall and total yeast sterols suggested that the preparations were enriched for cell wall components. The presence of plasma in the culture medium specially increased cell wall brassicasterol abundance and also other lipids.

CONCLUSIONS/SIGNIFICANCE

We here report an original comparative lipidomic analysis of P. brasiliensis cell wall. Our results open doors to understanding the role of cell wall lipids in fungal biology, and interaction with anti-fungal drugs and the host.

Regulation of genes involved in cell wall synthesis and structure during Ustilago maydis dimorphism

The cell wall is the structure that provides the shape to fungal cells and protects them from the difference in osmotic pressure existing between the cytosol and the external medium. Accordingly, changes in structure and composition of the fungal wall must occur during cell differentiation, including the dimorphic transition of fungi. We analyzed, by use of microarrays, the transcriptional regulation of the 639 genes identified to be involved in cell wall synthesis and structure plus the secretome of the Basidiomycota species Ustilago maydis during its dimorphic transition induced by a change in pH. Of these, 189 were differentially expressed during the process, and using as control two monomorphic mutants, one yeast like and the other mycelium constitutive, 66 genes specific of dimorphism were identified. Most of these genes were up-regulated in the mycelial phase. These included CHS genes, genes involved in β-1,6-glucan synthesis, N-glycosylation, and proteins containing a residue of glycosylphosphatidylinositol, and a number of genes from the secretome. The possible significance of these data on cell wall plasticity is discussed.

The paracoccidioides cell wall: past and present layers toward understanding interaction with the host

The cell wall of pathogenic fungi plays import roles in the interaction with the host, so that its composition and structure may determine the course of infection. Here we present an overview of the current and past knowledge on the cell wall constituents of Paracoccidioides brasiliensis and P. lutzii. These are temperature-dependent dimorphic fungi that cause paracoccidioidomycosis, a systemic granulomatous, and debilitating disease. Focus is given on cell wall carbohydrate and protein contents, their immune-stimulatory features, adhesion properties, drug target characteristics, and morphological phase specificity. We offer a journey toward the future understanding of the dynamic nature of the cell wall and of the changes that may occur when the fungus infects the human host.

Discordant influence of Blastomyces dermatitidis yeast-phase-specific gene BYS1 on morphogenesis and virulence

Blastomyces dermatitidis is a thermally induced dimorphic fungus capable of causing lung and systemic infections in immunocompetent animal hosts. With the publication of genomic sequences from three different strains of B. dermatitidis and the development of RNA interference as a gene-silencing tool, it has become possible to easily ascertain the virulence and morphological effects of knocking down the expression of candidate genes of interest. BYS1 (Blastomyces yeast-phase-specific 1), first identified by Burg and Smith, is expressed at high levels in yeast cells and is undetectable in mold. The deduced protein sequence of BYS1 has a putative signal sequence at its N terminus, opening the possibility that the BYS1-encoded protein is associated with the yeast cell wall. Herein, strains of B. dermatitidis with silenced expression of BYS1 were engineered and tested for morphology and virulence. The silenced strains produced rough-surfaced cultures on agar medium and demonstrated a propensity to form pseudohyphal cells on prolonged culture in vitro and in vivo, as measured in the mouse lung. Tests using a mouse model of blastomycosis with either yeast or spore inocula showed that the bys1-silenced strains were as virulent as control strains. Thus, although silencing of BYS1 alters morphology at 37 degrees C, it does not appear to impair the pathogenicity of B. dermatitidis.

The effect of temperature on Natural Antisense Transcript (NAT) expression in Aspergillus flavus

Naturally occurring Antisense Transcripts (NATs) compose an emerging group of regulatory RNAs. These regulatory elements appear in all organisms examined, but little is known about global expression of NATs in fungi. Analysis of currently available EST sequences suggests that 352 cis NATs are present in Aspergillus flavus. An Affymetrix GeneChip microarray containing probes for these cis NATs, as well as all predicted genes in A. flavus, allowed a whole genome expression analysis of these elements in response to two ecologically important temperatures for the fungus. RNA expression analysis showed that 32 NATs and 2,709 genes were differentially expressed between 37 degrees C, the optimum temperature for growth, and 28 degrees C, the conducive temperature for the biosynthesis of aflatoxin (AF) and many other secondary metabolites. These NATs correspond to sense genes with diverse functions including transcription initiation, carbohydrate processing and binding, temperature sensitive morphogenesis, and secondary metabolism. This is the first report of a whole genome transcriptional analysis of NAT expression in a fungus.

Detection and measurement of two-component systems that control dimorphism and virulence in fungi

Systemic dimorphic fungi include six phylogenetically related ascomycetes. These organisms grow in a mold form in the soil on most continents around the world. After the mold spores, which are the infectious particles, are inhaled into the lung of a susceptible mammalian host, they undergo a morphological change into a pathogenic yeast form. The ability to convert to the yeast form is essential for this class of fungal agents to be pathogenic and produce disease. Temperature change is one key stimulus that triggers the phase transition from mold (25 degrees ) to yeast (37 degrees ). Genes that are expressed only in the pathogenic yeast form of these fungi have been identified to help explain how and why this phase transition is required for virulence. However, the regulators of yeast-phase specific genes, especially of phase transition from mold to yeast, have remained poorly understood. We used Agrobacterium-mediated gene transfer for insertional mutagenesis to create mutants that are defective in the phase transition and to identify genes that regulate this critical event. We discovered that a hybrid histidine kinase senses environmental signals such as temperature and regulates phase transition, dimorphism, and virulence in members of this fungal family. This chapter describes our approach to the identification and analysis of this global regulator.

Yeast-like cell formation and glutathione metabolism in autolysing cultures of Penicillium chrysogenum

The bulk formation of yeast-like (arthrospore-like) cells were typical in carbon-depleted submerged cultures of the high beta-lactam producer Penicillium chrysogenum NCAIM 00237 strain independently of the nitrogen-content of the culture medium. This morphogenetic switch was still quite common in carbon-starving cultures of the low-penicillin-producer strain P. chrysogenum ATCC 28089 (Wis 54-1255) when the nitrogen-content of the medium was low but was a very rare event in wild-type P. chrysogenum cultures. The mycelium-->yeast-like cell transition correlated well with a relatively high glutathione concentration and a reductive glutathione/glutathione disulfite (GSH/GSSG) redox balance in autolysing cultures, which was a consequence of industrial strain development. Paradoxically, the development of high beta-lactam productivity resulted in a high intracellular GSH level and, concomitantly, in an increased y-glutamyltranspeptidase (i.e. GSH-decomposing) activity in the autolytic phase of growth of P. chrysogenum NCAIM 00237. The hypothesized causal connection between GSH metabolism and cell morphology, if verified, may help us in future metabolic engineering of industrially important filamentous fungi.

Dimorphism and virulence in fungi

The signature feature of systemic dimorphic fungi - a family of six primary fungal pathogens of humans - is a temperature-induced phase transition. These fungi grow as a mold in soil at ambient temperature and convert to yeast after infectious spores are inhaled into the lungs of a mammalian host. Seminal work 20 years ago established that a temperature-induced phase transition from mold to yeast is required for virulence. Several yeast-phase specific genes, identified one-by-one and studied by reverse genetics, have revealed mechanisms by which the phase transition promotes disease pathogenesis. Transcriptional profiling of microarrays built with genomic elements of Histoplasma capsulatum and ESTs of Paracoccidioides brasiliensis that represent partial genomes has identified 500 genes and 328 genes, respectively, that are differentially expressed upon the phase transition. The genomes of most of the dimorphic fungi are now in varying stages of being sequenced. The creation of additional microarrays and the application of new reverse genetic tools promise fresh insight into genes and mechanisms that regulate pathogenesis and morphogenesis. The use of insertional mutagenesis by Agrobacterium has uncovered a hybrid histidine kinase that regulates dimorphism and pathogenicity in Blastomyces dermatitidis and H. capsulatum. Two-component signaling appears to be a common strategy for model and pathogenic fungi to sense and respond to environmental stresses.

Development and application of a green fluorescent protein sentinel system for identification of RNA interference in Blastomyces dermatitidis illuminates the role of septin in morphogenesis and sporulation

A high-throughput strategy for testing gene function would accelerate progress in our understanding of disease pathogenesis for the dimorphic fungus Blastomyces dermatitidis, whose genome is being completed. We developed a green fluorescent protein (GFP) sentinel system of gene silencing to rapidly study genes of unknown function. Using Gateway technology to efficiently generate RNA interference plasmids, we cloned a target gene, "X," next to GFP to create one hairpin to knock down the expression of both genes so that diminished GFP reports target gene expression. To test this approach in B. dermatitidis, we first used LACZ and the virulence gene BAD1 as targets. The level of GFP reliably reported interference of their expression, leading to rapid detection of gene-silenced transformants. We next investigated a previously unstudied gene encoding septin and explored its possible role in morphogenesis and sporulation. A CDC11 septin homolog in B. dermatitidis localized to the neck of budding yeast cells. CDC11-silenced transformants identified with the sentinel system grew slowly as flat or rough colonies on agar. Microscopically, they formed ballooned, distorted yeast cells that failed to bud, and they sporulated poorly as mold. Hence, this GFP sentinel system enables rapid detection of gene silencing and has revealed a pronounced role for septin in morphogenesis, budding, and sporulation of B. dermatitidis.

Functional genome of the human pathogenic fungus Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is a dimorphic and thermo-regulated fungus which is the causative agent of paracoccidioidomycosis, an endemic disease widespread in Latin America. Pathogenicity is assumed to be a consequence of the cellular differentiation process that this fungus undergoes from mycelium to yeast cells during human infection. In an effort to elucidate the molecular mechanisms involved in this process a network of Brazilian laboratories carried out a transcriptome project for both cell types. This review focuses on the data analysis yielding a comprehensive view of the fungal metabolism and the molecular adaptations during dimorphism in P. brasiliensis from analysis of 6022 groups, related to expressed genes, which were generated from both mycelium and yeast phases.

Interaction of Blastomyces dermatitidis, Sporothrix schenckii, and Histoplasma capsulatum with Acanthamoeba castellanii

Several dimorphic fungi are important human pathogens, but the origin and maintenance of virulence in these organisms is enigmatic, since an interaction with a mammalian host is not a requisite for fungal survival. Recently, Cryptococcus neoformans was shown to interact with macrophages, slime molds, and amoebae in a similar manner, suggesting that fungal pathogenic strategies may arise from environmental interactions with phagocytic microorganisms. In this study, we examined the interactions of three dimorphic fungi with the soil amoeba Acanthameobae castellanii. Yeast forms of Blastomyces dermatitidis, Sporothrix schenckii, and Histoplasma capsulatum were each ingested by amoebae and macrophages, and phagocytosis of yeast cells resulted in amoeba death and fungal growth. H. capsulatum conidia were also cytotoxic to amoebae. For each fungal species, exposure of yeast cells to amoebae resulted in an increase in hyphal cells. Exposure of an avirulent laboratory strain of H. capsulatum to A. castellanii selected for, or induced, a phenotype of H. capsulatum that caused a persistent murine lung infection. These results are consistent with the view that soil amoebae may contribute to the selection and maintenance of certain traits in pathogenic dimorphic fungi that confer on these microbes the capacity for virulence in mammals.

Isolation and characterization of YlBEM1, a gene required for cell polarization and differentiation in the dimorphic yeast Yarrowia lipolytica

The ability to switch between a unicellular yeast form and different filamentous forms (fungal dimorphism) is an important attribute of most pathogenic fungi. Dimorphism involves a series of events that ultimately result in dramatic changes in the polarity of cell growth in response to environmental factors. We have isolated and characterized YlBEM1, a gene encoding a protein of 639 amino acids that is essential for the yeast-to-hypha transition in the yeast Yarrowia lipolytica and whose transcription is significantly increased during this event. Cells with deletions of YlBEM1 are viable but show substantial alterations in morphology, disorganization of the actin cytoskeleton, delocalization of cortical actin and chitin deposition, multinucleation, and loss of mating ability, thus pointing to a major role for YlBEM1 in the regulation of cell polarity and morphogenesis in this fungus. This role is further supported by the localization of YlBemlp, which, like cortical actin, appears to be particularly abundant at sites of growth of yeast, hyphal, and pseudohyphal cells. In addition, the potential involvement of YlBem1p in septum formation and/or cytokinesis is suggested by the concentration of a green fluorescent protein-tagged version of this protein at the mother-bud neck during the last stages of cell division. Interestingly, overexpression of MHY1, YlRAC1, or YlSEC31, three genes involved in filamentous growth of Y. lipolytica, induced hyphal growth of bem1 null mutant cells.

Selective expression of the virulence factor BAD1 upon morphogenesis to the pathogenic yeast form of Blastomyces dermatitidis: evidence for transcriptional regulation by a conserved mechanism

Most dimorphic fungal pathogens grow as non-pathogenic moulds in soil and convert to pathogenic yeast in the host, suggesting that virulence factors are upregulated during phase transition. Such factors have been difficult to identify. We analysed BAD1 (formerly WI-1), a virulence factor in the dimorphic fungus Blastomyces dermatitidis, for expression in yeast and mycelial morphotypes. BAD1 was expressed in yeast but not in mycelia of North American strains of B. dermatitidis, and this expression pattern was confirmed for BAD1 transcript. BAD1 under the control of its promoter was transferred into African B. dermatitidis lacking a native BAD1 locus, and phase-specific expression was conserved. Sequence similarity was identified between the BAD1 promoter and the promoters of two yeast phase-specific genes in Histoplasma capsulatum. In H. capsulatum BAD1 transformants, yeast phase-specific expression of BAD1 was conserved, and no transcript was detected in mycelia. BAD1 beta-galactosidase reporter fusions analysed in B. dermatitidis and H. capsulatum confirmed that BAD1 is transcriptionally regulated in both fungi. BAD1 promoter activity and surface BAD1 expression were detected 6 h after shifting mycelia to 37 degrees C. Thus, BAD1 is expressed after transition to the pathogenic yeast morphotype and is regulated by a mechanism for phase-specific gene expression that appears to be conserved.

Characterization of the ABC transporter genes MgAtr1 and MgAtr2 from the wheat pathogen Mycosphaerella graminicola

ATP-binding cassette (ABC) transporters are membrane-bound transporters involved in various physiological processes. In this paper we describe the cloning of the ABC transporter encoding genes MgAtr1 and MgAtr2 from the wheat pathogen Mycosphaerella graminicola (anamorph Septoria tritici). Both deduced proteins MgAtr1 and MgAtr2 are highly homologous to other fungal ABC transporters. RT-PCR revealed that the MgAtr2 mRNA population consists of partially and fully spliced transcripts. Putative substrates of ABC transporters, modulators of ABC transporter activity, and inducers of ABC transporter gene transcription were analyzed for their potential to induce expression of MgAtr1 and MgAtr2 in m. graminicola. The genes are differently upregulated by compounds such as the plant secondary metabolites eugenol and reserpine. Similar results are obtained for several antibiotics and the azole fungicides cyproconazole and imazalil. Moreover, a different expression pattern between yeast-like cells and mycelium of this dimorphic fungus was observed. These results indicate that MgAtr1 and MgAtr2 play a role in protection of m. graminicola against natural toxic compounds and xenobiotics. A putative role in protection against plant defense compounds during pathogenesis is suggested.

MHY1 encodes a C2H2-type zinc finger protein that promotes dimorphic transition in the yeast Yarrowia lipolytica

The yeast-to-hypha morphological transition (dimorphism) is typical of many pathogenic fungi. Dimorphism has been attributed to changes in temperature and nutritional status and is believed to constitute a mechanism of response to adverse conditions. We have isolated and characterized a gene, MHY1, whose transcription is dramatically increased during the yeast-to-hypha transition in Yarrowia lipolytica. Deletion of MHY1 is viable and has no effect on mating, but it does result in a complete inability of cells to undergo mycelial growth. MHY1 encodes a C2H2-type zinc finger protein, Mhy1p, which can bind putative cis-acting DNA stress response elements, suggesting that Mhy1p may act as a transcription factor. Interestingly, Mhy1p tagged with a hemagglutinin epitope was concentrated in the nuclei of actively growing cells found at the hyphal tip.

HOY1, a homeo gene required for hyphal formation in Yarrowia lipolytica

The dimorphic fungus Yarrowia lipolytica grows to form hyphae either in rich media or in media with GlcNAc as a carbon source. A visual screening, called FIL (filamentation minus), for Y. lipolytica yeast growth mutants has been developed. The FIL screen was used to identify three Y. lipolytica genes that abolish hypha formation in all media assayed. Y. lipolytica HOY1, a gene whose deletion prevents the yeast-hypha transition both in liquid and solid media, was characterized. HOY1 is predicted to encode a 509-amino-acid protein with a homeodomain homologous to that found in the chicken Hox4.8 gene. Analysis of the protein predicts a nuclear location. These observations suggest that Hoy1p may function as a transcriptional regulatory protein. In disrupted strains, reintroduction of HOY1 restored the capacity for hypha formation. Northern blot hybridization revealed the HOY1 transcript to be approximately 1.6 kb. Expression of this gene was detected when Y. lipolytica grew as a budding yeast, but an increase in its expression was observed by 1 h after cells had been induced to form hyphae. The possible functions of HOY1 in hyphal growth and the uses of the FIL screen to identify morphogenetic regulatory genes from heterologous organisms are discussed.

Dimorphism and haploid fruiting in Cryptococcus neoformans: association with the alpha-mating type

Cryptococcus neoformans is a major opportunistic fungal pathogen in AIDS and other immunosuppressed patients. We have shown that wild-type haploid C. neoformans can develop an extensive hyphal phase under appropriate conditions. Hyphae produced under these conditions are monokaryotic, possess unfused clamp connections, and develop basidia with viable basidiospores. The ability to undergo this transition is determined by the presence of the alpha-mating type locus and is independent of serotype. The association of the hyphal phase with the alpha-mating type may explain the preponderance of this mating type in the environment and the nature of the infectious propagule of C. neoformans.

Tumour necrosis factor production in vivo and in vitro in response to Paracoccidioides brasiliensis and the cell wall fractions thereof

Tumour necrosis factor (TNF) was detected in serum from mice challenged with Paracoccidioides brasiliensis. The serum TNF levels of mice challenged with an avirulent strain were significantly higher than those of mice challenged with a virulent strain, and the same was observed for the TNF levels of mice challenged with a cell wall fraction (F1) from the two fungal strains. Fraction F1 consisted of chitin and beta-glucan; but although the chitin contents were similar for the two strains, the avirulent strain allowed a greater content of beta-glucan. The beta-glucan, purified from both strains, increased serum TNF levels in an identical dose-dependent manner, whereas purified chitin did not induce serum TNF levels. P. brasiliensis, the F1 fractions and beta-glucan induced macrophages to secrete TNF in vitro. The differences in TNF levels, induced by the different fungal strains, were correlated with the beta-glucan concentrations in the cell walls of both the avirulent and virulent strains of P. brasiliensis. These findings support a role for TNF in the pathogenicity of P. brasiliensis.

Paracoccidioidomycosis: an update

This review summarizes knowledge on various aspects of paracoccidioidomycosis. Mycelial propagules, chlamydospores, and arthroconidia exhibit thermal dimorphism; arthroconidia are infectious in animals and, by electron microscopy, appear well provided for survival. The mycelial-to-yeast-phase transformation requires a strict control of glucan synthesis probably mediated by membrane enzymes. Hormonal influences on the transformation of the fungus (mycelium or conidium to yeast phase) have been demonstrated. Estrogen-binding proteins have been detected in the fungal cytosol, and during the transformation novel proteins are produced as a result of estradiol incorporation. Clinical forms have been better defined on the basis of better experimental models. Emphasis has been placed on the lungs as the portal of entry and on the existence of silent pulmonary infections. A specific Paracoccidioides brasiliensis antigen, the 43-kDa glycoprotein (Gp43), has been identified, characterized, and cloned. This has led to improved reproducibility and specificity of serologic tests. The depression of cell-mediated immune responses has been associated with severe disease in humans and in the experimental host. T-cell subsets in patients' tissues were characterized by means of monoclonal antibodies, and a reduced CD4/CD8 ratio was demonstrated. This has been related to alterations in lymphokine and tumor necrosis factor production, production of antigen-antibody complexes, etc. Amphotericin B has provided effective therapy. Azole derivatives have also improved prognosis and facilitated therapy. Itraconazole is presently the drug of choice, yet incapacitating sequelae (mainly pulmonary fibrosis) still constitute major problems.

Effect of nucleosides and nucleotides and the relationship between cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) and germ tube formation in Candida albicans

A yeast-mycelium (Y-M) transition in Candida albicans was induced by exogenous yeast extract, adenosine, adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), adenosine 3':5' cyclic monophosphate (cAMP) and its analogue N6, O2'-dibutyryl adenosine 3':5'-cyclic monophosphate (dbcAMP) in defined liquid medium at 25 degrees C. Adenosine 5'-triphosphate (ATP) was found to delay germ tube formation in yeast cells, whereas the cAMP phosphodiesterase inhibitors, theophylline and caffeine, induced a Y-M transition. Intracellular and extracellular cyclic AMP levels increased during the yeast-mycelium transition and maximum levels of intracellular cyclic AMP coincided with maximum germ tube formation. Of the many inducers and inhibitors of germ tube and mycelium formation in C. albicans tested, including incubation at 37 degrees C or in the presence of 1.5 mM CaCl2, the calmodulin inhibitor calmidazolium (R24571) added together with CaCl2 induced the highest intra- and extracellular cyclic AMP levels. These results confirm the involvement of cyclic AMP in the yeast-mycelium transition of C. albicans.

Utilização de aminoácidos no estudo do crescimento do Paracoccidioides brasiliensis: Influência sobre o dimorfismo

Utilizamos 15 amostras de Paracoccidioides brasiliensis nas formas miceliana (M) e leveduriforme (L), cultivadas em meio mínimo (MM) e adaptadas ao mesmo meio suplementado com a solução de aminoácidos (MMS). Para a realização do estudo auxológico das amostras, foram preparadas soluções complementares das quais foram retirados um aminoácido de cada vez. Nove amostras foram prototróficas nas formas M e/ou L e as demais auxotróficas para os diferentes aminoácidos e bases nitrogenadas. A heterogeneidade dos resultados apresentados não permitiu a caracterização auxológica das 15 amostras de P. brasiliensis estudadas. Nenhum dos compostos nitrogenados demonstrou ser essencial para o crescimento ou para a manutenção da morfogênese do fungo. Alterações morfológicas (macro e microscópicas) também foram observadas, mas somente entre as amostras prototróficas, sugerindo a ativação de um mecanismo de adaptação desenvolvido pelo fungo mediante a ausência de substratos nitrogenados no meio de cultura (MM).

Dimorphism of Benjaminiella poitrasii: isolation and biochemical studies of morphological mutants

The yeast-mycelium dimorphism of the genus Benjaminiella poitrasii has been investigated. To understand the mechanism of dimorphism two stable yeast-phase mutants (Y-1 & Y-2) and one slow growing mycelial mutant (M-1) of B. poitrasii were isolated after NTG treatment of parent strain spores and studied for their biochemical characteristics. Effects of (i) kind and concentration of carbon source, (ii) presence of complex organic nitrogen and (iii) C:N ratio in the growth medium on the morphology of parent and mutant strains were carried out at 28 degrees C under shaking conditions. Ethanol induced morphological change and its reversal were studied in all the strains in order to elucidate the possible mechanism of morphogenesis.

Dimorphism in Histoplasma capsulatum: a model for the study of cell differentiation in pathogenic fungi

Several fungi can assume either a filamentous or a unicellular morphology in response to changes in environmental conditions. This process, known as dimorphism, is a characteristic of several pathogenic fungi, e.g., Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis, and appears to be directly related to adaptation from a saprobic to a parasitic existence. H. capsulatum is the most extensively studied of the dimorphic fungi, with a parasitic phase consisting of yeast cells and a saprobic mycelial phase. In culture, the transition of H. capsulatum from one phase to the other can be triggered reversibly by shifting the temperature of incubation between 25 degrees C (mycelia) and 37 degrees C (yeast phase). Mycelia are found in soil and never in infected tissue, in contrast to the yeast phase, which is the only form present in patients. The temperature-induced phase transition and the events in establishment of the disease state are very likely to be intimately related. Furthermore, the temperature-induced phase transition implies that each growth phase is an adaptation to two critically different environments. A fundamental question concerning dimorphism is the nature of the signal(s) that responds to temperature shifts. So far, both the responding cell component(s) and the mechanism(s) remain unclear. This review describes the work done in the last several years at the biochemical and molecular levels on the mechanisms involved in the mycelium to yeast phase transition and speculates on possible models of regulation of morphogenesis in dimorphic pathogenic fungi.

Host-parasite relationships in paracoccidioidomycosis

A viewpoint of host-parasite relationships in paracoccidioidomycosis is presented. The characteristics of the fungus which are important to the host-parasite interaction are discussed. Aspects of inhibition of mycelium-to-yeast transformation by estrogens acting at receptors on the fungal wall and in the cytoplasm, and the role of polysaccharide components of the cell wall in virulence are reviewed. The natural mechanisms of host defense are also examined, including phagocytosis, complement system, natural-killer cells and genetic control of resistance and susceptibility. Finally, a discussion of granuloma morphogenesis and its relationship to the humoral and cellular anti-P. brasiliensis immune response is presented.

Cyclic adenosine 3',5' monophosphate (cAMP) and dimorphism in the pathogenic fungus Paracoccidioides brasiliensis

Exogenous cAMP or its analogs inhibit the mycelium transformation of yeast and induce bulging of the apex of mycelia. But intracellular cAMP levels of yeast and mycelial cells are not significantly different.