Changes in cyclic nucleotide levels and dimorphic transition in Candida albicans

Molecular Docking Reveals Critical Residues in Candida albicans Cyr1 for Peptidoglycan Recognition and Hyphal Growth

The key virulent characteristic of Candida albicans, the major fungal pathogen in humans, lies in its ability to switch between the benign yeast state and the invasive hyphal form upon exposure to specific stimuli. Among the numerous hyphal-inducing signals, bacterial peptidoglycan fragments (PGNs) represent the most potent inducers of C. albicans hyphal growth. The sole adenylyl cyclase Cyr1 in C. albicans is a known sensor for PGNs and activates downstream signaling of hyphal growth, yet the molecular details of PGN-Cyr1 interactions have remained unclear. In this study, we performed in silico docking of a PGN motif to the modeled structure of the Cyr1 leucine-rich repeat (LRR) domain and uncovered four putative PGN-interacting residues in Cyr1_LRR. The critical roles of these residues in PGN binding and supporting C. albicans hyphal growth were demonstrated by in-gel fluorescence binding assay and hyphal induction assay, respectively. Remarkably, the C. albicans mutant harboring the cyr1 variant allele that is defective for PGN recognition exhibits significantly reduced cytotoxicity in macrophage infection assay. Overall, our work offered important insights into the molecular recognition of PGNs by C. albicans Cyr1 sensor protein, establishing that disruption of PGN recognition by Cyr1 results in defective hyphal growth and reduced virulence of C. albicans. Our findings provide an exciting starting point for the future development of Cyr1 antagonists as novel anti-virulence therapeutics to combat C. albicans invasive growth and infection.

Expression pattern and phenotypic characterization of the mutant strain reveals target genes and processes regulated by pka1 in the dimorphic fission yeast Schizosaccharomyces japonicus

The cAMP cascade plays an important role in several biological processes. Thus, study of its molecular details can contribute to a better understanding of these processes, treatment of diseases, or even finding antifungal drug targets. To gain further information about the PKA pathway, and its evolutionarily conserved and species-specific features, the central regulator pka1 gene, which encodes the cAMP-dependent protein kinase catalytic subunit, was studied in the less known haplontic, dimorphic fission yeast Schizosaccharomyces japonicus. Namely, this species belongs to a highly divergent phylogenetic branch of fungi. Furthermore, S. japonicus had only a single copy pka1 gene in contrast to the budding yeasts. Therefore, the pka1 deleted mutant was created, whose RNA sequencing and phenotypic studies revealed that the Pka1 regulated at least 373 genes, among them further kinases, phosphatases and transcriptional regulators. It regulated elongation of hyphae, cell size, aging and stress response. Furthermore, half of the pka1 target genes seemed to be conserved in Schizosaccharomyces pombe and S. japonicus. However, there were oppositely regulated genes in the two closely related species. The target genes suggest that this single gene must be able to fulfill all the functions of TPK1-3 of Saccharomyces cerevisiae. Thus, our results shed light on certain similarities and differences of the PKA pathway of S. japonicus compared to the budding yeasts and confirmed the multifunctionality of the pka1 gene, but further experiments are needed to prove its involvement in the metabolic processes and transport.

Candida albicans HSP12 is co-regulated by physiological CO2 and pH

Global transcriptional analysis of Candida albicans exposed to elevated ambient CO(2) revealed a statistically significant differential regulation of 14 genes. Subsequent RNA hybridisation analysis of one gene, HSP12, confirmed CO(2)-regulation via a cAMP-dependent mechanism. Additionally, Northern analyses and gel mobility shift assays demonstrate the co-regulation of HSP12 by environmental pH via a Rim101-dependent mechanism.

Histone deacetylase inhibitors may reduce pathogenicity and virulence inCandida albicans

Candida albicans is able to establish mucosal and invasive diseases by means of different virulence factors that are frequently regulated by epigenetic mechanisms, including the acetylation-deacetylation of histones and of other regulatory proteins. The focus of our work was on understanding the possible effects of several histone deacetylase inhibitors (HDACi) on the expression of phenotypes that are associated with virulence and pathogenicity in C. albicans, such as adhesion to epithelial cells and the yeast to hypha transition. Some of the HDACi used for experiments caused a 90% reduction in the adherence of C. albicans to human cultured pneumocytes and significantly inhibited serum-induced germination. Inhibition of germination was correlated with a significant reduction in transcription of EFG1. Inhibition appeared less evident when an HDA1-deficient strain was tested. These results suggest that selective and specific HDACi could prove to be a valid approach for selected at-risk patients in the combined treatment of infections caused by C. albicans.

Integrative, multifunctional plasmids for hypha-specific or constitutive expression of green fluorescent protein in Candida albicans

The authors have engineered plasmid constructs for developmental and constitutive expression of yeast-enhanced green fluorescent protein (yEGFP3) in Candida albicans. The promoter for the hyphae-specific gene Hyphal Wall Protein 1 (HWP1) conferred developmental expression of yEGFP3 in germ tubes and hyphae but not in yeasts or pseudohyphae when targeted to the ENO1 (enolase) locus in single copy. The pHWP1GFP3 construct allows for the easy visualization of HWP1 promoter activity in individual cells expressing true hyphae without having to prepare RNA for analysis. Constitutive expression of yEGFP was seen in all cell morphologies when the HWP1 promoter was replaced with the ENO1 promoter region. The use of the plasmids for expression of genes other than yEGFP3 was examined by substituting the putative C. albicans BCY1 (SRA1) gene, a component of the cAMP signalling pathway involved in yeast to hyphae transitions, for yEGFP3. Strains overexpressing BCY1 from the ENO1 promoter were inhibited in germ tube formation and filamentation in both liquid and solid media, a phenotype consistent with keeping protein kinase A in its inactive form by association with Bcy1p. The plasmids are suitable for studies of germ tube induction or assessing germ tube formation by measuring yEGFP3 expression, for inducible expression of genes concomitant with germ tube formation by the HWP1 promoter, for constitutive expression of genes by the ENO1 promoter, and for expressing yEGFP3 using a promoter of choice.

The cAMP phosphodiesterase encoded by CaPDE2 is required for hyphal development in Candida albicans

The cAMP-dependent pathway, which regulates yeast-to-hypha morphogenesis in Candida albicans, is controlled by changes in cAMP levels determined by the processes of synthesis and hydrolysis. Both low- and high-affinity cAMP phosphodiesterases are encoded in the C. albicans genome. CaPDE2, encoding the high-affinity cAMP phosphodiesterase, has been cloned and shown to be toxic in Saccharomyces cerevisiae upon overexpression under pGAL1, but functional under the moderate pMET3. Deletion of CaPDE2 causes elevated cAMP levels and responsiveness to exogenous cAMP, higher sensitivity to heat shock, severe growth defects at 42 degrees C and highly reduced levels of EFG1 transcription. In vitro in hypha-inducing liquid medium CaPDE2, deletion prohibits normal hyphal, but not pseudohyphal growth. On solid medium capde2 mutants form aberrant hyphae, with fewer branches and almost no lateral buds, which are deficient in hypha-to-yeast reversion. The phenotypic defects of capde2 mutants show that the cAMP-dependent pathway plays specific roles in hyphal and pseudohyphal development, its regulatory role however, being greater in liquid than on solid medium in vitro. The increased expression of CaPDE2 after serum addition correlates well with a drop in cAMP levels following the initial rise in response to the hyphal inducer. These results suggest that Capde2p mediates a desensitization mechanism by lowering basal cAMP levels in response to environmental stimuli in C. albicans.

Gpa2, a G-protein alpha subunit required for hyphal development in Candida albicans

Candida albicans is able to respond to environmental changes by inducing a distinct morphological program, which is related to the ability to infect mammalian hosts. Although some of the signal transduction pathways involved in this response are known, it is not clear how the environmental signals are sensed and transmitted to these transduction cascades. In this work, we have studied the function of GPA2, a new gene from C. albicans, which encodes a G-protein alpha-subunit homologue. We demonstrate that Gpa2 plays an important role in the yeast-hypha dimorphic transition in the response of C. albicans to some environmental inducers. Deletion of both alleles of the GPA2 gene causes in vitro defects in morphological transitions in Spider medium and SLAD medium and in embedded conditions but not in medium containing serum. These defects cannot be reversed by exogenous addition of cyclic AMP. However, overexpression of HST7, which encodes a component of the filament-inducing mitogen-activated protein kinase (MAPK) cascade, bypasses the Gpa2 requirement. We have obtained different gain-of-function and loss-of-function mutant alleles of the GPA2 gene, which we have introduced in several C. albicans genetic backgrounds. Our results indicate that, in response to environmental cues, Gpa2 is required for the regulation of a MAPK signaling pathway.

Signaling through adenylyl cyclase is essential for hyphal growth and virulence in the pathogenic fungus Candida albicans

The human fungal pathogen Candida albicans switches from a budding yeast form to a polarized hyphal form in response to various external signals. This morphogenetic switching has been implicated in the development of pathogenicity. We have cloned the CaCDC35 gene encoding C. albicans adenylyl cyclase by functional complementation of the conditional growth defect of Saccharomyces cerevisiae cells with mutations in Ras1p and Ras2p. It has previously been shown that these Ras homologues regulate adenylyl cyclase in yeast. The C. albicans adenylyl cyclase is highly homologous to other fungal adenylyl cyclases but has less sequence similarity with the mammalian enzymes. C. albicans cells deleted for both alleles of CaCDC35 had no detectable cAMP levels, suggesting that this gene encodes the only adenylyl cyclase in C. albicans. The homozygous mutant cells were viable but grew more slowly than wild-type cells and were unable to switch from the yeast to the hyphal form under all environmental conditions that we analyzed in vitro. Moreover, this morphogenetic switch was completely blocked in mutant cells undergoing phagocytosis by macrophages. However, morphogenetic switching was restored by exogenous cAMP. On the basis of epistasis experiments, we propose that CaCdc35p acts downstream of the Ras homologue CaRas1p. These epistasis experiments also suggest that the putative transcription factor Efg1p and components of the hyphal-inducing MAP kinase pathway depend on the function of CaCdc35p in their ability to induce morphogenetic switching. Homozygous cacdc35 Delta cells were unable to establish vaginal infection in a mucosal membrane mouse model and were avirulent in a mouse model for systemic infections. These findings suggest that fungal adenylyl cyclases and other regulators of the cAMP signaling pathway may be useful targets for antifungal drugs.

CAP1, an adenylate cyclase-associated protein gene, regulates bud-hypha transitions, filamentous growth, and cyclic AMP levels and is required for virulence of Candida albicans

In response to a wide variety of environmental stimuli, the opportunistic fungal pathogen Candida albicans exits the budding cycle, producing germ tubes and hyphae concomitant with expression of virulence genes, such as that encoding hyphal wall protein 1 (HWP1). Biochemical studies implicate cyclic AMP (cAMP) increases in promoting bud-hypha transitions, but genetic evidence relating genes that control cAMP levels to bud-hypha transitions has not been reported. Adenylate cyclase-associated proteins (CAPs) of nonpathogenic fungi interact with Ras and adenylate cyclase to increase cAMP levels under specific environmental conditions. To initiate studies on the relationship between cAMP signaling and bud-hypha transitions in C. albicans, we identified, cloned, characterized, and disrupted the C. albicans CAP1 gene. C. albicans strains with inactivated CAP1 budded in conditions that led to germ tube formation in isogenic strains with CAP1. The addition of 10 mM cAMP and dibutyryl cAMP promoted bud-hypha transitions and filamentous growth in the cap1/cap1 mutant in liquid and solid media, respectively, showing clearly that cAMP promotes hypha formation in C. albicans. Increases in cytoplasmic cAMP preceding germ tube emergence in strains having CAP1 were markedly diminished in the budding cap1/cap1 mutant. C. albicans strains with deletions of both alleles of CAP1 were avirulent in a mouse model of systemic candidiasis. The avirulence of a germ tube-deficient cap1/cap1 mutant coupled with the role of Cap1 in regulating cAMP levels shows that the Cap1-mediated cAMP signaling pathway is required for bud-hypha transitions, filamentous growth, and the pathogenesis of candidiasis.

Control of filament formation in Candida albicans by polyamine levels

Candida albicans, the most common fungal pathogen, regulates its cellular morphology in response to environmental conditions. The ODC gene, which encodes ornithine decarboxylase, a key enzyme in polyamine biosynthesis, was isolated and disrupted. Homozygous null Candida mutants behaved as polyamine auxotrophs and grew exclusively in the yeast form at low polyamine levels (0.01 mM putrescine) under all conditions tested. An increase in the polyamine concentration (10 mM putrescine) restored the capacity to switch from the yeast to the filamentous form. The strain with a deletion mutation also showed increased sensitivity to salts and calcofluor white. This Candida odc/odc mutant was virulent in a mouse model. The results suggest a model in which polyamine levels exert a pleiotrophic effect on transcriptional activity.

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.

N-acetyl-D-glucosamine induces germination in Candida albicans through a mechanism sensitive to inhibitors of cAMP-dependent protein kinase

The present study examines the involvement of cAMP-dependent protein kinase (PKA) in the dimorphic transition of Candida albicans by assessing the in vivo effect of two permeable PKA inhibitors on N-acetyl-D-glucosamine (GlcNAc)- and serum-induced differentiation. The permeable myristoylated derivative of the heat-stable PKA inhibitor (MyrPKI), which inhibited C. albicans PKA in vitro, caused a concentration-dependent inhibition of germ-tube formation in cultures induced to germinate by GlcNAc; germination halted irrespective of the time of addition of the inhibitor. MyrPKI also blocked dibutyryl-cAMP (dbcAMP)- and glucagon-stimulated germination but did not affect serum-induced germination. H-89, another highly specific PKA inhibitor, displayed the same effect on germination. Neither MyrPKI nor H-89 had any effect on budding of yeast cells. In conclusion, our results indicate that cAMP-mediated activation of PKA plays a pivotal role in the biochemical mechanism underlying morphogenesis.

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.

Characterization and molecular genetic complementation of mutants affecting dimorphism in the fungus ustilago maydis

Ustilago maydis, the causal agent of corn smut disease, displays dimorphic growth in which it alternates between a unicellular, nonpathogenic yeast-like form and a dikaryotic, pathogenic filamentous form. Previously, a constitutively filamentous haploid mutant was obtained. Complementation of this mutant led to the isolation of the gene encoding adenylate cyclase, uac1. Secondary mutagenesis of a uac1 disruption strain allowed the isolation of a large number of suppressor mutants, termed ubc, for Ustilago bypass of cyclase, lacking the filamentous phenotype. Analysis of one of these suppressor mutants previously led to the identification of the ubc1 gene, encoding the regulatory subunit of cAMP-dependent protein kinase. In this report we describe the isolation of cosmids containing three new ubc genes, termed ubc2, ubc3, and ubc4. We also describe the morphology of the ubc2, ubc3, and ubc4 mutants in a uac1- background as well as in a background with a functional uac1 gene. In addition, we describe several mutant strains not complemented with any of the genes currently in hand and that are thus presumed to possess mutations in additional ubc genes. Copyright 1998 Academic Press.

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.

Yeast pseudohyphal growth is regulated by GPA2, a G protein alpha homolog

Pseudohyphal differentiation, a filamentous growth form of the budding yeast Saccharomyces cerevisiae, is induced by nitrogen starvation. The mechanisms by which nitrogen limitation regulates this process are currently unknown. We have found that GPA2, one of the two heterotrimeric G protein alpha subunit homologs in yeast, regulates pseudohyphal differentiation. Deltagpa2/Deltagpa2 mutant strains have a defect in pseudohyphal growth. In contrast, a constitutively active allele of GPA2 stimulates filamentation, even on nitrogen-rich media. Moreover, a dominant negative GPA2 allele inhibits filamentation of wild-type strains. Several findings, including epistasis analysis and reporter gene studies, indicate that GPA2 does not regulate the MAP kinase cascade known to regulate filamentous growth. Previous studies have implicated GPA2 in the control of intracellular cAMP levels; we find that expression of the dominant RAS2(Gly19Val) mutant or exogenous cAMP suppresses the Deltagpa2 pseudohyphal defect. cAMP also stimulates filamentation in strains lacking the cAMP phosphodiesterase PDE2, even in the absence of nitrogen starvation. Our findings suggest that GPA2 is an element of the nitrogen sensing machinery that regulates pseudohyphal differentiation by modulating cAMP levels.

Control of filamentous growth by mating and cyclic-AMP inUstilago

Mating between compatible haploid cells of the corn smut fungus, Ustilago maydis, results in a switch from budding to filamentous growth. We are analyzing the multiallelic b incompatibility locus that governs maintenance of the infectious, filamentous dikaryon. Specificity regions have been identified in the N-terminal portions of the bE and bW genes at the b locus. In addition, we have found that heterozygosity at the b locus attenuates fusion. This result suggests that b gene products may exert a negative influence on some processes, in addition to their generally recognized role in maintaining filamentous growth. The b genes have also been characterized in Ustilago hordei. This species has a bipolar mating system in which the b genes are linked to genes required for pheromone production and response to form one large mating-type region (MAT locus) with two allelic specificities. In a separate study, we have discovered that defects in adenylate cyclase result in constitutive filamentous growth and greatly reduce the virulence of U. maydis on corn seedlings. Mutations have been identified that suppress the adenylate cyclase defect and restore budding growth. Sequence analysis revealed that one of the suppressor mutations is in the gene encoding the regulatory subunit of cAMP-dependent protein kinase (PKA). Mutants altered in PKA activity have the interesting phenotype of multiple budding and frequent mislocalization of the bud site. Key words: smut, dimorphism, b locus, sex.

cAMP regulates morphogenesis in the fungal pathogen Ustilago maydis

The fungal pathogen Ustilago maydis exhibits a dimorphic switch from budding to filamentous growth in response to mating interactions and environmental conditions. We have found that disruption of the uac1 gene, encoding adenylate cyclase, results in a constitutively filamentous phenotype. Budding is restored to the uac1 mutant upon growth in the presence of cAMP or by extragenic suppression because of a mutation in the ubc1 gene. The ubc1 gene encodes a type II regulatory subunit of cAMP-dependent protein kinase (PKA); defects in this gene attenuate the filamentous growth that normally occurs in response to mating and exposure to air. Growth of wild-type cells in cAMP and mutation of the ubc1 gene also cause defects in the separation of mother and daughter cells (cytokinesis) and alter bud site selection. These results indicate a key role for cAMP and PKA in morphogenesis in U. maydis; this role may be common among dimorphic fungal pathogens.

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.

Induction and substrate specificity of the lanosterol 14 alpha-demethylase from Saccharomyces cerevisiae Y222

The potential inducibility of the lanosterol 14 alpha-demethylase (P-45014DM) from Saccharomyces cerevisiae Y222 by xenobiotics was investigated. This enzyme and NADPH-cytochrome P-450 reductase were unaffected by a number of compounds known to induce mammalian and some yeast cytochrome P-450 monooxygenases. Furthermore, dibutyryl cyclic AMP did not affect P-45014DM or P-450 reductase levels, while growth at 37 degrees C resulted in a slight decrease. P-45014DM was found to be specific for lanosterol and did not metabolize a number of P-450 substrates including benzo[a]pyrene.

Prostaglandin E2 enhances and gamma interferon inhibits germ tube formation in Candida albicans

Prostaglandin E2 (PGE2), an immunosuppressive monokine that increases intracellular cyclic AMP (cAMP) levels, stimulated Candida albicans germ tube formation. Dibutyryl cAMP (dB-cAMP) and isoproterenol, other compounds that increase cAMP levels, also stimulated germination. Gamma interferon (IFN-gamma), a product of cellular immune system activation, inhibited Candida germ tube formation, even in the presence of PGE2, dB-cAMP, and isoproterenol. Thus, PGE2 and IFN-gamma as well as having opposing roles in the suppression or activation of cell-mediated immunity, are also antagonists for the yeast-to-hyphal transition of C. albicans.

Current trends in Candida albicans research

Candida albicans is an opportunistic pathogen of human beings and other mammals. Two other features, besides its pathogenicity, have made it a popular organism of study. It exists in different cellular forms and can change from one form to another, depending on growth conditions. Thus, it is being used as a model system to study cellular differentiation. It can also heritably and reversibly switch its cellular and colony morphologies. The yeast is diploid and lacks a sexual cycle. Thus, it has not been possible to apply the powerful methods of genetic analysis to understand morphogenesis or pathogenesis. Few clinical isolates are haploid, but they do not form hyphae and are not yet well characterized. Recombinant DNA techniques are increasingly being applied to C. albicans to solve many of the unanswered questions of morphogenesis and pathogenesis. Genetic transformation and gene-disruption techniques were recently developed for the yeast. Thus it is possible to study the role of any cloned gene through directed mutagenesis. However, the difficulty is to clone the putative genes involved in morphogenesis or pathogenesis. Candida albicans exists in four different cellular forms, namely blastospores, pseudohyphae, hyphae and chlamydospores. Blastospore-to-hypha conversion is well studied. A variety of conditions can induce this transition. It is not clear how cells sense such varied conditions and respond appropriately. In other systems where differentiation is well understood, regulatory genes which control differentiation have been uncovered. These genes cause differential expression of other genes, and ultimately differentiated phenotypes. Thus, it is likely that differential gene expression is involved in the bud-to-hypha transition in C. albicans. Certain proteins are expressed exclusively on the cell surface of hyphae. It should be possible to clone genes coding for these proteins. A study of the expression of these genes might allow us to identify the regulatory gene which determines differentiation. Another approach to understanding morphogenesis is to study how the difference in the shape of buds and hyphae is generated. This difference appears to be due to the differential activity of apical and general growth zones, which determine growth of the cell wall. Activity of these growth zones is apparently determined by actin localization. It remains a possibility that conditions which induce hyphae formation may directly affect actin localization or cell-wall growth zones and cause differences in cell shape. Candida albicans can also heritably switch its cellular phenotype. This has come to light from a study of colony-morphology switching. Some strains can switch their colony morphology, both heritably and reversibly.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Importance of some factors on the dimorphism of Candida albicans

The passage between the yeast and mycelial forms of Candida albicans B 311-10 was studied by using the minimal synthetic medium of Shepherd et al. modified without biotin and with low glucose concentrations. It was observed that biotin, aminoacids and particularly pH are not important factors in the dimorphism of C. albicans. The only factor of notable importance in the passage of yeast form to mycelial form in C. albicans was glucose concentration.

Characterization of phosphomannan-protein complexes isolated from viable cells of yeast and mycelial forms of Candida albicans NIH B-792 strain by the action of Zymolyase-100T

The isolation of phosphomannan-protein complexes from the viable cells of yeast (Y) and mycelial (M) forms of Candida albicans NIH B-792 strain was conducted by treatment with Zymolyase-100T followed by fractional precipitation with cetyltrimethylammonium bromide. The M-form complex was found to contain smaller amount of phosphate (1.3%) than that of the Y-form complex (1.6%). Proton magnetic resonance (PMR) spectra of these complexes indicated that the content of beta-1,2-linked oligomannosyl and nonreducing terminal alpha-1,3-linked mannopyranosyl residues in the M-form complex was lower than that of the Y-form complex. With hot 10 mM HCl, the Y-form complex released a mixture of oligosaccharides ranging from mannose to mannoheptaose, while the M-form complex produced lower oligosaccharides, from mannose to mannotetraose. Upon acetolysis, the acid-modified complex of the M form gave mainly mannotetraose, while that of the Y form produced mainly mannopentaose and mannohexaose in addition to mannotetraose. The average length of branching moieties of the mannan of Y-form cells was therefore longer than that of M-form cells. These results indicate that the Y to M transformation of this C. albicans strain accompanies the suppression of enzyme activity concerning the biosynthesis of mannan such as beta-1,2- and alpha-1,3-mannosyltransferases to synthesize the phosphomannan-protein complex containing mannan moiety with incomplete structure.

Regulation of mannitol catabolism in Candida albicans: evidence for cyclic AMP-independent glucose effect

Candida albicans was examined for a glucose effect and showed typical diauxic growth on a mixture of glucose and mannitol, in which mannitol utilization occurred only after exhaustion of glucose. The activity of NAD-linked mannitol dehydrogenase was very low while glucose was present in the medium, but started to increase after consumption of glucose. This increase in activity was fully prevented by trichodermin, an inhibitor of protein synthesis. The uptake of mannitol was detected in the cells grown on mannitol, but not in those grown on glucose with or without mannitol. Mannitol uptake by mannitol-grown cells was not affected by the presence of glucose (0.2 g l-1). These findings indicate that in C. albicans glucose represses the inducible syntheses of mannitol dehydrogenase and a mannitol transport system, and that the involvement of inducer exclusion in this effect is unlikely. Fructose, and to lesser extents galactose, mannose and sucrose, also exhibited similar effects on mannitol metabolism. No correlation was found between the intracellular cyclic AMP levels and the glucose effect.

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.

The role of zinc in Candida dimorphism

By analyzing the effects of zinc on growth and dimorphism, it has become clear that there exists at least two modes, or "pathways," of mycelium formation in C. albicans (7). Paradoxically, even though the characteristics for the two modes appear to be opposite in nature, the mycelium that form appear to be superficially similar. Unfortunately, it may be difficult to compare the two modes unambiguously at the molecular level for two reasons. First, the physiology of cells resuming growth after release from stationary phase will undoubtedly differ drastically from the physiology of cells exiting from the growth cycle, regardless of phenotype. Therefore, most molecular or physiologic differences probably will represent differences in growth rate or position in the cell cycle, rather than alternate molecular mechanisms that are basic to the alternate modes of mycelium formation. Second, it has been observed that during release from stationary phase, a prescribed program of gene expression accompanies commitment to the mycelial and budding forms (11). This program was demonstrable because of the excellent synchrony and homogeneity of released cultures (60), which is a characteristic lacking in cultures entering stationary phase in the M10 mode. Even so, a comparison at the molecular level between the two modes of mycelium formation should be undertaken with the above reservations in mind. Perhaps the most attractive aspect of alternate modes of mycelium formation in Candida is at the genetic level of analysis. The hypothesis of homozygosis in the expression of the M10 phenotype is testable, as is the possible role of the M10 phenotype in tissue penetration. If the hypothesis is true and if the M10 phenotype predominates in infected tissue, it would represent a new mechanism of opportunism in infectious fungi that may be used by other systems as well as Candida. If it is not true, a detailed analysis of the differences between the two modes of mycelium formation will still be valuable in our understanding of both the mechanisms regulating phenotypic transitions in Candida and the more general question of cell divergence in developing systems.

Morphogenesis in Candida albicans

This review will survey environmental controls on the morphology of Candida albicans, describe the cellular and ultrastructural events associated with morphological transitions in this fungus, and attempt to relate biochemical phenomena that have been reported to be associated with dimorphic change to C. albicans cell biology. The synthesis of the cell wall of C. albicans and its control remain largely undiscovered, but it is clear that the cell wall is the principal component involved in shape determination. Possible models for C. albicans dimorphism will be critically reviewed.

Trehalase activity and cyclic AMP content during early development of Mucor rouxii spores

Incubation of Mucor rouxii sporangiospores in complex medium under aerobic conditions resulted in a transient 20-fold increase in trehalase activity. Maximum activity was reached after 15 min. Simultaneously, the cyclic AMP (cAMP) content increased approximately eightfold, reaching a maximum within 10 min. Increases in trehalase activity and cAMP content were also observed under anaerobic conditions (CO2). The extent of trehalase activation and the changes in cAMP content, during both aerobic and anaerobic incubation, varied with the medium used. Trehalase was activated in vitro by a cAMP- and ATP-dependent process. An even faster activation was obtained when cAMP was replaced by the catalytic subunit of beef heart protein kinase. The coincidence of, and the correlation between, increased cAMP contents and trehalase activities support the involvement of a cAMP-dependent phosphorylation in the in vivo regulation of trehalase activity.

Effects of caffeine, cyclic 3', 5' adenosine monophosphate and cyclic 3', 5' guanosine monophosphate in the development of the mycelial form of Sporothrix schenckii

The kinetics of the development of the mycelial form of Sporothrix schenckii from yeast cells and conidia in a minimal basal medium with glucose at pH 4.0 and 25 degrees C were established. Germ tube formation was used as the index of germination for both yeast cells and conidia. Yeast cells were first observed to develop germ tubes after 3 h of incubation, reaching 92 +/- 5%, after 12 h of incubation. Germ tubes were first detected in conidia after 9 h of incubation, and 12 h after inoculation 92 +/- 6% of the conidia had germ tubes. After 24 h of incubation, fully developed, sporulating mycelia were observed from both yeast cells and conidia. A delay in germ tube formation from yeast cells was observed when But2cAMP (10 mM) and But2cGMP (10 mM) were added to the medium. Also the addition of caffeine, a cyclic nucleotide phosphodiesterase inhibitor, inhibited the yeast to mycelial transition. Conidial germination into the mycelial form was also inhibited when cAMP, But2cAMP and caffeine were added to the medium. These results suggest the possible involvement of cyclic nucleotides in the control of dimorphism in S. schenckii.

The control of morphogenesis in Candida albicans

Morphogenesis (germ-tube formation) in Candida albicans was induced gratuitously by N-acetylhexosamine derivatives (N-acetyl-glucosamine covalently linked to agarose, N-acetylmannosamine, hyaluronic acid, colloidal chitin, and mucin). These compounds were not taken up by the yeast cells and did not support growth. 2-Deoxyglucose was a potent inhibitor of germ-tube formation (50 microM), but did not affect the yeast growth yield at a concentration of 2.5 mM. 2-Deoxyglucose covalently linked to agarose did not affect germ-tube formation, and the inhibition by free 2-deoxyglucose was overcome by the addition of glucose to the germ-tube-forming cells. Glucose competitively inhibited 2-deoxyglucose uptake (Ki = 0.14 mM), and these data indicate that 2-deoxyglucose acted intracellularly.

Molecular aspects of fungal dimorphism

The approach to this article will be the review of experimental developments achieved in recent years in the fields of biochemistry, molecular biology, ultrastructure, etc. and that have suggested models and hypothesis in order to understand the regulation of the molecular mechanisms involved in fungal differentiation. Since fungal dimorphism has been seen by many investigators as a useful model of differentiation in eukaryotic systems, and also because of the prevalence of dimorphism among human pathogenic fungi, we have considered relevant to review the work done in Mucor, but also in dimorphic pathogenic fungi such as Paracoccidioides brasiliensis, Histoplasma capsulatum, Candida albicans, Blastomyces dermatitidis, and Sporothrix Schenckii. From this point of view, the involvements of cyclic nucleotides, nucleic acid polymerases, synthesis of macromolecules, synthesis of cell wall and less studied factors will be reviewed.

[Microbial utilization of mixed substrates]

Decomposition of substrates by heterotrophic microorganisms is accomplished in natural biotopes such as in soil and in waters, on or in macroorganisms but also in laboratory and industrial biotopes. The interest of man in these processes is manifold. Starting with the division of substrates into three groups of simple substrates, complex and mixed substrates with or without solid particles their qualitative and quantitative occurrence in nature and their significance in biotechnology will be discussed. In the decomposition of these substrates their utilization by pure cultures or mixed populations is to be exactly distinguished. Simple growth curves, di- or polyauxy, sequences of decomposition of simple substrates of a mixed substrate, population changes and successions are only some of the phenomena occurring in this process. The pathways of catabolism are subjected to manifold regulations on the three levels of stoichiometric regulation, the regulation of enzyme activity and the regulation of enzyme synthesis. In natural biotopes there is hardly a constant substrate supply over a longer period. That's why certain mechanisms of regulation are permanently acting. Thus the "normal" physiological state for microorganisms is characterized by permanent transition situations--called "transients". These reactions are also applied to many biotechnological processes.

Induction of germ tube formation by N-acetyl-D-glucosamine in Candida albicans: uptake of inducer and germinative response

A number of strains of Candida albicans were tested for germ tube formation after induction by N-acetyl-D-glucosamine (GlcNAc) and other simple (proline, glucose plus glutamine) or complex (serum) compounds. A proportion of strains (high responders) were induced to form germ tubes evolving to true hyphae by GlcNAc alone or by proline or glucose plus glutamine mixture. The majority of strains were low responders because they could be induced only by serum or GlcNAc-serum medium. Two strains were found to be nonresponders: they grew as pseudohyphae in serum. Despite minor quantitative differences, all strains efficiently utilized GlcNAc for growth under the yeast form at 28 degrees C. They also had comparable active, inducible, and constitutive uptake systems for GlcNAc. During germ tube formation in GlcNAc, the inducible uptake system was modulated, as expected from induction and decay of GlcNAc kinase. Uranyl acetate, at a concentration of 0.01 mM, inhibited both GlcNAc uptake and germ tube formation and was reversed by phosphates. Germinating and nongerminating cells differed in the rapidity and extent of GlcNAc incorporation into acid-insoluble and alkali-acid-insoluble cell fractions. During germ tube formation induced by proline, GlcNAc was almost totally incorporated into the acid-insoluble fraction after 60 min. Moreover, hyphal development on induction by either GlcNAc or proline was characterized by an apparent "uncoupling" between protein and polysaccharide metabolism, the ratio between the two main cellular constituents falling from more than 1 to less than 0.5 after 270 min of development. The data suggest that utilization of the inducer for wall synthesis is a determinant of germ tube formation C. albicans but that the nature and extent of inducer uptake is not a key event for this phenomenon to occur.

Temperature- and cyclic nucleotide-induced phase transitions of Histoplasma capsulatum

The transition from yeast to mycelia of Histoplasma capsulatum could be accomplished by shifting the temperature of incubation from 37 to 25 degrees C. It was accompanied by many changes in cellular metabolism, including changes in respiration, intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels, and activities of two enzymes specific for the yeast phase, cystine reductase (EC 1.6.4.1) and cysteine oxidase (EC 1.13.11.20). Even at 37 degrees C, the yeast to mycelial transition could be induced by cAMP and agents which raise the intracellular levels of cAMP (theophylline, acetylsalicylic acid, prostaglandin E1, and nerve growth factor). During this morphogenesis the same pattern of changes occurred as in the temperature-induced transition. Therefore, these changes were not simply dependent on a shift in temperature, but rather were part of the process of the phase transition.

Regulation of dimorphism in Histoplasma capsulatum by cyclic adenosine 3',5'-monophosphate

During temperature-induced transition of the dimorphic pathogenic fungus Histoplasma capsulatum from the single yeast cell form to the multicellular mycelial form, there was an increase in intracellular cyclic adenosine 3',5'-monophosphate (cAMP) levels as well as a striking accumulation of cAMP in the medium. cAMP levels also changed during the reverse mycelium-to-yeast transition.