The Eng1 β-Glucanase Enhances Histoplasma Virulence by Reducing β-Glucan Exposure

The fungal pathogen Histoplasma capsulatum parasitizes host phagocytes. To avoid antimicrobial immune responses, Histoplasma yeasts must minimize their detection by host receptors while simultaneously interacting with the phagocyte. Pathogenic Histoplasma yeast cells, but not avirulent mycelial cells, secrete the Eng1 protein, which is a member of the glycosylhydrolase 81 (GH81) family. We show that Histoplasma Eng1 is a glucanase that hydrolyzes β-(1,3)-glycosyl linkages but is not required for Histoplasma growth in vitro or for cell separation. However, Histoplasma yeasts lacking Eng1 function have attenuated virulence in vivo, particularly during the cell-mediated immunity stage. Histoplasma yeasts deficient for Eng1 show increased exposure of cell wall β-glucans, which results in enhanced binding to the Dectin-1 β-glucan receptor. Consistent with this, Eng1-deficient yeasts trigger increased tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) cytokine production from macrophages and dendritic cells. While not responsible for large-scale cell wall structure and function, the secreted Eng1 reduces levels of exposed β-glucans at the yeast cell wall, thereby diminishing potential recognition by Dectin-1 and proinflammatory cytokine production by phagocytes. In α-glucan-producing Histoplasma strains, Eng1 acts in concert with α-glucan to minimize β-glucan exposure: α-glucan provides a masking function by covering the β-glucan-rich cell wall, while Eng1 removes any remaining exposed β-glucans. Thus, Histoplasma Eng1 has evolved a specialized pathogenesis function to remove exposed β-glucans, thereby enhancing the ability of yeasts to escape detection by host phagocytes.

IMPORTANCE

The success of Histoplasma capsulatum as an intracellular pathogen results, in part, from an ability to minimize its detection by receptors on phagocytic cells of the immune system. In this study, we showed that Histoplasma pathogenic yeast cells, but not avirulent mycelia, secrete a β-glucanase, Eng1, which reduces recognition of fungal cell wall β-glucans. We demonstrated that the Eng1 β-glucanase promotes Histoplasma virulence by reducing levels of surface-exposed β-glucans on yeast cells, thereby enabling Histoplasma yeasts to escape detection by the host β-glucan receptor, Dectin-1. As a consequence, phagocyte recognition of Histoplasma yeasts is reduced, leading to less proinflammatory cytokine production by phagocytes and less control of Histoplasma infection in vivo Thus, Histoplasma yeasts express two mechanisms to avoid phagocyte detection: masking of cell wall β-glucans by α-glucan and enzymatic removal of exposed β-glucans by the Eng1 β-glucanase.

Isotopic fractionation by a fungal P450 nitric oxide reductase during the production of N2O

Nitrous oxide (N2O) is a potent greenhouse gas with a 100-year global warming potential approximately 300 times that of CO2. Because microbes account for over 75% of the N2O released in the U.S., understanding the biochemical processes by which N2O is produced is critical to our efforts to mitigate climate change. In the current study, we used gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to measure the δ(15)N, δ(18)O, δ(15)N(α), and δ(15)N(β) of N2O generated by purified fungal nitric oxide reductase (P450nor) from Histoplasma capsulatum. The isotope values were used to calculate site preference (SP) values (difference in δ(15)N between the central (α) and terminal (β) N atoms in N2O), enrichment factors (ε), and kinetic isotope effects (KIEs). Both oxygen and N(α) displayed normal isotope effects during enzymatic NO reduction with ε values of -25.7‰ (KIE = 1.0264) and -12.6‰ (KIE = 1.0127), respectively. However, bulk nitrogen (average δ(15)N of N(α) and N(β)) and N(β) exhibited inverse isotope effects with ε values of 14.0‰ (KIE = 0.9862) and 36.1‰ (KIE = 0.9651), respectively. The observed inverse isotope effect in δ(15)N(β) is consistent with reversible binding of the first NO in the P450nor reaction mechanism. In contrast to the constant SP observed during NO reduction in microbial cultures, the site preference measured for purified H. capsulatum P450nor was not constant, increasing from ∼ 15‰ to ∼ 29‰ during the course of the reaction. This indicates that SP for microbial cultures can vary depending on the growth conditions, which may complicate source tracing during microbial denitrification.

Expression of Paracoccidioides brasiliensis AMY1 in a Histoplasma capsulatum amy1 mutant, relates an α-(1,4)-amylase to cell wall α-(1,3)-glucan synthesis

In the cell walls of the pathogenic yeast phases of Paracoccidioides brasiliensis, Blastomyces dermatitidis and Histoplasma capsulatum, the outer α-(1,3)-glucan layer behaves as a virulence factor. In H. capsulatum, an α-(1,4)-amylase gene (AMY1) is essential for the synthesis of this polysaccharide, hence related to virulence. An orthologous gene to H. capsulatum AMY1 was identified in P. brasiliensis and also labeled AMY1. P. brasiliensis AMY1 transcriptional levels were increased during the yeast phase, which correlates with the presence of α-(1,3)-glucan as the major yeast cell wall polysaccharide. Complementation of a H. capsulatum amy1 mutant strain with P. brasiliensis AMY1, suggests that P. brasiliensis Amy1p may play a role in the synthesis of cell wall α-(1,3)-glucan. To study some biochemical properties of P. brasiliensis Amy1p, the enzyme was overexpressed, purified and studied its activity profile with starch and amylopeptin. It showed a relatively higher hydrolyzing activity on amylopeptin than starch, producing oligosaccharides from 4 to 5 glucose residues. Our findings show that P. brasiliensis Amy1p produces maltooligosaccharides which may act as a primer molecule for the fungal cell wall α-(1,3)-glucan biosynthesis by Ags1p.

[Significantly increase of glycolytic flux and pyruvate productivity in Torulopsis glabrata by heterologous expression of NADH alternative oxidase]

OBJECTIVE

This study aimed at increasing the glycolytic flux and pyruvate productivity in Torulopsis glabrata with glucose as carbon source.

METHODS

For this target, we introduced a mitochondrial located alternative oxidase encoded by Histoplasma capsulatum AOX1 gene into T. glabrata, and a mutant strain named as AOX with total NADH oxidase activity was 1.8-fold higher than that of the parent stain, was achieved.

RESULTS

The heterologous expression of NADH alternative oxidase resulted in decrease of the dry cell weight and fermentation time by 20.3% and 10.7%, but the specific rate of glucose consumption and pyruvate production increased 34.7% and 54.1% higher, respectively. The reasons for high glycolytic flux were the intracellular NADH/NAD+ ratio and ATP concentration decreased 74.7% and 52.9% respectively, the specific activity of phosphofructokinase, pyruvate kinase increased 185.0% and 28.1%.

CONCLUSION

Introduction of a novel NADH oxidation pathway by alternative oxidase can efficiently increase the rate of glucose consumption and the target metabolite productivity.

Histoplasma capsulatum encodes a dipeptidyl peptidase active against the mammalian immunoregulatory peptide, substance P

The pathogenic fungus Histoplasma capsulatum secretes dipeptidyl peptidase (Dpp) IV enzyme activity and has two putative DPPIV homologs (HcDPPIVA and HcDPPIVB). We previously showed that HcDPPIVB is the gene responsible for the majority of secreted DppIV activity in H. capsulatum culture supernatant, while we could not detect any functional contribution from HcDPPIVA. In order to determine whether HcDPPIVA encodes a functional DppIV enzyme, we expressed HcDPPIVA in Pichia pastoris and purified the recombinant protein. The recombinant enzyme cleaved synthetic DppIV substrates and had similar biochemical properties to other described DppIV enzymes, with temperature and pH optima of 42°C and 8, respectively. Recombinant HcDppIVA cleaved the host immunoregulatory peptide substance P, indicating the enzyme has the potential to affect the immune response during infection. Expression of HcDPPIVA under heterologous regulatory sequences in H. capsulatum resulted in increased secreted DppIV activity, indicating that the encoded protein can be expressed and secreted by its native organism. However, HcDPPIVA was not required for virulence in a murine model of histoplasmosis. This work reports a fungal enzyme that can function to cleave the immunomodulatory host peptide substance P.

Histoplasma requires SID1, a member of an iron-regulated siderophore gene cluster, for host colonization

The macrophage is the primary host cell for the fungal pathogen Histoplasma capsulatum during mammalian infections, yet little is known about fungal genes required for intracellular replication in the host. Since the ability to scavenge iron from the host is important for the virulence of most pathogens, we investigated the role of iron acquisition in H. capsulatum pathogenesis. H. capsulatum acquires iron through the action of ferric reductases and the production of siderophores, but the genes responsible for these activities and their role in virulence have not been determined. We identified a discrete set of co-regulated genes whose transcription is induced under low iron conditions. These genes all appeared to be involved in the synthesis, secretion, and utilization of siderophores. Surprisingly, the majority of these transcriptionally co-regulated genes were found clustered adjacent to each other in the genome of the three sequenced strains of H. capsulatum, suggesting that their proximity might foster coordinate gene regulation. Additionally, we identified a consensus sequence in the promoters of all of these genes that may contribute to iron-regulated gene expression. The gene set included L-ornithine monooxygenase (SID1), the enzyme that catalyzes the first committed step in siderophore production in other fungi. Disruption of SID1 by allelic replacement resulted in poor growth under low iron conditions, as well as a loss of siderophore production. Strains deficient in SID1 showed a significant growth defect in murine bone-marrow-derived macrophages and attenuation in the mouse model of infection. These data indicated that H. capsulatum utilizes siderophores in addition to other iron acquisition mechanisms for optimal growth during infection.

Fungal infections are a growing public health threat, particularly for immunocompromised individuals such as people with AIDS, organ transplant recipients, and cancer patients. Present antifungal therapies are often highly toxic and resistance to these therapies continues to rise. Histoplasma capsulatum is a pathogenic fungus that infects humans, causing pulmonary and systemic disease. It is the most common cause of fungal respiratory infection in the world, and is endemic to the Mississippi and Ohio River valleys of the United States. H. capsulatum produces small molecules, called siderophores, to acquire iron, an essential nutrient. We have identified genes that are involved in the synthesis of siderophores in this fungus and have found that siderophore production in H. capsulatum is important for its virulence. Since siderophore production is confined to microbes and plays no role in human biology, it is an excellent target for rational drug design.

Expression of hygromycin phosphotransferase alters virulence of Histoplasma capsulatum

The Escherichia coli hygromycin phosphotransferase (hph) gene, which confers hygromycin resistance, is commonly used as a dominant selectable marker in genetically modified bacteria, fungi, plants, insects, and mammalian cells. Expression of the hph gene has rarely been reported to induce effects other than those expected. Hygromycin B is the most common dominant selectable marker used in the molecular manipulation of Histoplasma capsulatum in the generation of knockout strains of H. capsulatum or as a marker in mutant strains. hph-expressing organisms appear to have no defect in long-term in vitro growth and survival and have been successfully used to exploit host-parasite interaction in short-term cell culture systems and animal experiments. We introduced the hph gene as a selectable marker together with the gene encoding green fluorescent protein into wild-type strains of H. capsulatum. Infection of mice with hph-expressing H. capsulatum yeast cells at sublethal doses resulted in lethality. The lethality was not attributable to the site of integration of the hph construct into the genomes or to the method of integration and was not H. capsulatum strain related. Death of mice was not caused by altered cytokine profiles or an overwhelming fungal burden. The lethality was dependent on the kinase activity of hygromycin phosphotransferase. These results should raise awareness of the potential detrimental effects of the hph gene.

An alpha-(1,4)-amylase is essential for alpha-(1,3)-glucan production and virulence in Histoplasma capsulatum

Histoplasma capsulatum is a dimorphic fungus that causes respiratory and systemic disease and is capable of surviving and replicating within macrophages. The virulence of Histoplasma has been linked to cell wall alpha-(1,3)-glucan; however, the role of this polysaccharide during infection, its organization within the cell wall, and its synthesis and regulation remain poorly understood. To identify genes involved in the biosynthesis of alpha-(1,3)-glucan, we employed a forward genetics strategy to isolate physically marked mutants with reduced alpha-(1,3)-glucan. Insertional mutants were generated in a virulent strain of H. capsulatum by optimization of Agrobacterium tumefaciens-mediated transformation. Approximately 90% of these mutants possessed single insertions with no chromosomal rearrangements or deletions in the host genome. To confirm the role and specificity of identified candidate genes, we phenocopied the disrupted locus by either RNA interference or targeted gene deletion. Our findings indicate alpha-(1,3)-glucan production requires the function of the AMY1 gene product, a novel protein with homology to the alpha-amylase family of glycosyl hydrolases, and UGP1, a UTP-glucose-1-phosphate uridylyltransferase which synthesizes UDP-glucose monomers. Loss of AMY1 function attenuated the ability of Histoplasma to kill macrophages and to colonize murine lungs.

Global control of dimorphism and virulence in fungi

Microbial pathogens that normally inhabit our environment can adapt to thrive inside mammalian hosts. There are six dimorphic fungi that cause disease worldwide, which switch from nonpathogenic molds in soil to pathogenic yeast after spores are inhaled and exposed to elevated temperature. Mechanisms that regulate this switch remain obscure. We show that a hybrid histidine kinase senses host signals and triggers the transition from mold to yeast. The kinase also regulates cell-wall integrity, sporulation, and expression of virulence genes in vivo. This global regulator shapes how dimorphic fungal pathogens adapt to the mammalian host, which has broad implications for treating and preventing systemic fungal disease.

Characterization of an alternative oxidase activity of Histoplasma capsulatum

Histoplasma capsulatum possesses a branched mitochondrial electron transport chain, with both cyanide-sensitive and -insensitive oxygen-consuming activities. The latter, carried out by a single subunit enzyme termed 'alternative oxidase', is the focus of this report. AOX1 cDNA clones were isolated and direct evidence that the cDNA ORF encodes functional alternative oxidase is reported. Also reported are the generation of an antiserum to the AOX1 protein product, and specific detection in vivo of the mRNA and protein products of the AOX1 gene. Finally, initial studies of regulation of H. capsulatum AOX1 gene expression demonstrated that RNA abundance was increased after hydrogen peroxide-mediated oxidative stress and after inhibition of mitochondrial electron transport enzymes with antimycin A or sodium cyanide. This pattern of regulation is consistent with the hypothesis that alternative oxidase contributes to survival of H. capsulatum after oxidative or metabolic stress and may be important for virulence of this pathogenic organism. The GenBank Accession Nos for the cDNA sequences reported in this paper are AF133236, AF133237 (AOX1).

Redundancy, phylogeny and differential expression of Histoplasma capsulatum catalases

Histoplasma capsulatum produces an extracellular catalase termed M antigen, which is similar to catalase B of Aspergillus and Emericella species. Evidence is presented here for two additional catalase isozymes in H. capsulatum. Catalase A is highly similar to a large-subunit catalase in Aspergillus and Emericella species, while catalase P is a small-subunit catalase protein with greatest similarity to known peroxisomal catalases of animals and Saccharomycotina yeasts. Complete cDNAs for the CATA and CATP genes (encoding catalases A and P, respectively) were isolated. The transcriptional expression of the H. capsulatum CATA, CATB (M antigen) and CATP genes was assessed by Northern blot hybridizations on total RNA. Results at the transcript levels for these genes are shown for three conditions: cell morphology (mycelial versus yeast phase cells), oxidative stress (in response to a challenge with H(2)O(2)) and carbon source (glucose vs glycerol). Collectively, these results demonstrated regulation of CATA by both cell morphology and oxidative stress, but not by carbon source, and regulation of CATB and CATP by carbon source but not cell morphology or oxidative stress. A phylogenetic analysis of presently available catalase sequences and intron residences was done. The results support a model for evolution of eukaryotic monofunctional catalase genes from prokaryotic genes.

Determination of beta-glucosidase enzymatic function of the histoplasma capsulatum H antigen using a native expression system

The Histoplasma capsulatum H antigen is a major secreted glycoprotein of this pathogenic fungus that is a target of humoral and cell-mediated host responses. Its predicted protein sequence displays homology to beta-glucosidases of other organisms, but a recombinant antigen expressed in a prokaryotic system showed no enzymatic activity. We expressed a recombinant form of the protein carrying a carboxyl-terminus oligohistidine tag in the native fungal background to facilitate proper glycosylation and folding of a product that could then be purified from culture supernatants using nickel affinity chromatography. The recombinant protein was expressed and secreted by a transformant carrying the modified gene under the control of its native promoter. The purified protein from the native expression system showed beta-glucosidase enzymatic activity in substrate gels and quantitative microplate assays. This activity was blocked by glucosidase-specific inhibitors. These results are the first direct demonstration of the function of this protein, and show the utility of expression in a native system to achieve post-translational modification necessary for structural and functional integrity.

Applying in vivo expression technology (IVET) to the fungal pathogen Histoplasma capsulatum

Understanding how pathogens survive within the host cell is of paramount importance in the development of vaccines and therapeutic agents. This task has been particularly daunting in the study of fungal pathogens due to the lack of easily manipulated genetic systems. In recent years several molecular genetic reporter systems have been developed to identify genes expressed during the infection process and potential virulence determinants. The development of one method in particular, in vivo expression technology (IVET), has led to the discovery of several genes from various bacterial pathogens necessary for survival during infection. The recent development of molecular genetic tools for Histoplasma capsulatum has enabled us to adapt the IVET technology for this pathogenic fungus utilizing the URA5 gene, which is essential for H. capsulatum survival in mice and in cultured macrophages, as a reporter of in vivo gene expression. We report the first successful application of IVET screening of a fungal pathogen for genes expressed exclusively during infection.

Isolation of a Histoplasma capsulatum cDNA that complements a mitochondrial NAD(+)-isocitrate dehydrogenase subunit I-deficient mutant of Saccharomyces cerevisiae

A cDNA library was prepared from Histoplasma capsulatum strain G-217B yeast cells and an apparently full-length cDNA for a subunit of the citric acid cycle enzyme NAD(+)-isocitrate dehydrogenase was identified by sequence analysis. Its predicted amino acid sequence is more similar to the IDH1 regulatory subunit of S. cerevisiae NAD(+)-isocitrate dehydrogenase than to the IDH2 catalytic subunit. After expression in S. cerevisiae from an S. cerevisiae promoter, it was shown to functionally complement an S. cerevisiae idh1 mutant, but not an idh2 mutant, for growth on acetate as a carbon source and for production of NAD(+)-isocitrate dehydrogenase enzyme activity. These results confirm that the H. capsulatum cDNA encodes a homologue of subunit I of the S. cerevisiae mitochondrial isocitrate dehydrogenase isozyme that functions in the citric acid cycle.

Cloning and sequence analysis of Histoplasma capsulatum glucosamine-6-phosphate synthase gene fragment

The 3' part of the glucosamine-6-phosphate synthase gene from Histoplasma capsulatum was PCR amplified using degenerate primers designed from the known glucosamine-6-phosphate synthase gene sequences, cloned and sequenced. The computer analysis of the 676 bp sequence revealed the presence of two introns. The identities of the deduced amino acid sequence to the corresponding Saccharomyces cerevisiae and Candida albicans fragment are 65 and 63.8%, respectively.

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

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

Rare homologous gene targeting in Histoplasma capsulatum: disruption of the URA5Hc gene by allelic replacement

URA5 genes encode orotidine-5'-monophosphate pyrophosphorylase (OMPpase), an enzyme involved in pyrimidine biosynthesis. We cloned the Histoplasma capsulatum URA5 gene (URA5Hc) by using a probe generated by PCR with inosine-rich primers based on relatively conserved sequences in OMPpases from other organisms. Transformation with this gene restored uracil prototrophy and OMPpase activity to UV-mutagenized ura5 strains of H. capsulatum. We attempted to target the genomic URA5 locus in this haploid organism to demonstrate homologous allelic replacement with transforming DNA, which has not been previously done in H. capsulatum and has been challenging in some other pathogenic fungi. Several strategies commonly used in Saccharomyces cerevisiae and other eukaryotes were unsuccessful, due to the frequent occurrence of ectopic integration, linear plasmid formation, and spontaneous resistance to 5-fluoroorotic acid, which is a selective agent for URA5 gene inactivation. Recent development of an efficient electrotransformation system and of a second selectable marker (hph, conferring hygromycin B resistance) for this fungus enabled us to achieve allelic replacement by using transformation with an insertionally inactivated Deltaura5Hc::hph plasmid, followed by dual selection with hygromycin B and 5-fluoroorotic acid, or by screening hygromycin B-resistant transformants for uracil auxotrophy. The relative frequency of homologous gene targeting was approximately one allelic replacement event per thousand transformants. This work demonstrates the feasibility but also the potential challenge of gene disruption in this organism. To our knowledge, it represents the first example of experimentally directed allelic replacement in H. capsulatum, or in any dimorphic systemic fungal pathogen of humans.

An AP1 element is involved in transcriptional regulation of delta9-desaturase gene of Histoplasma capsulatum

We have characterized a region of the promoter of a cloned delta9-desaturase gene (Ole1) of Histoplasma capsulatum, a dimorphic pathogenic fungus of humans. The product of the delta9-desaturase gene is involved in regulating membrane fluid state in animal cells and microorganisms. To identify sequences critical for Ole1 expression in both the saprobic mycelial and parasitic yeast phases of this organism, we performed a deletion analysis. Evidence is presented that a 240 nt region of the proximal promoter is involved in a phase-specific binding in vitro. By sequence analysis we have identified one likely regulatory element that coincides with an AP1 binding site (TGACTAA) that is located at -740 nt of 5'-upstream from the ATG. Using gel mobility shift assays, we show that this cis-acting element binds nuclear proteins extracted from the yeast and mycelial phases of H. capsulatum that may participate in control of expression of the delta9-desaturase gene.

Extracellular proteolytic enzyme activity of Histoplasma capsulatum var. duboisii

Histoplasma capsulatum var. duboisii is the etiological agent of African histoplasmosis, an important deep mycosis in West Africa. Not much is known about the physiological properties of this fungus. This communication reports on the extracellular proteolytic enzyme activity of this fungus. Five isolates of this fungus tested hydrolyzed azocasein and bovine serum albumin at pH 6.8 and 8.0. Assay of the crude enzyme showed that proteolytic activity increased with age and peaked on the 10th day and then again on the 13th day for the yeast form, and on the 11th day of growth for the mycelial form. The optimum temperature and pH for maximum enzyme activity were 35 degrees C and 6.8 respectively. The proteinase activity was more pronounced with the yeast form than with the mycelial form. The action of enzyme inhibitors suggested the presence of an aspartyl proteinase.

A temperature-sensitive strain of Histoplasma capsulatum has an altered delta 9-fatty acid desaturase gene

We have isolated and characterized the delta 9-desaturase gene (Ole1), which codes for a key enzyme involved in regulating membrane fluidity in animal cells and microorganisms, from two strains of Histoplasma capsulatum, one that is temperature-tolerant (G217B) and the other temperature-susceptible (Downs). These pathogenic fungi are dimorphic in that they undergo a morphologic transition from the mycelial to yeast-like form when the temperature of incubation is switched from 25 to 37 degrees C or when they infect a susceptible host. The coding sequences of the two genes, both containing an intron of 93 nucleotides, are virtually identical and analogous to the delta 9-desaturase gene of Saccharomyces cerevisiae and those of the rat, mouse and human. Ole1 transcription of the thermotolerant G217B and thermosensitive Downs strains is similar in yeast phase cells and during the temperature shift down from 34, 37, or 40 to 25 degrees C (yeast-to-mycelia transition). Nevertheless, the delta 9-desaturase gene is transcriptionally inactive in mycelia of G217B at 25 degrees C while it is actively transcribed in the Downs strain at the same temperature. These results are in agreement with the finding that membranes of the Downs strain have a higher level of oleic acid. The differential expression of delta 9-desaturase genes is discussed in relationship to differences in thermosensitivity in the fungal isolates and in regulating the level of expression of heat shock genes.

Effects of histoplasmin M antigen chemical and enzymatic deglycosylation on cross-reactivity in the enzyme-linked immunoelectrotransfer blot method

The enzyme-linked immunoelectrotransfer blot (EITB) method was evaluated as a suitable method for detecting antibodies against M antigen of Histoplasma capsulatum by use of both glycosylated and deglycosylated M protein of histoplasmin (HMIN). Sera from patients with histoplasmosis, paracoccidioidomycosis, blastomycosis, coccidioidomycosis, and aspergillosis were tested by the EITB with glycosylated M protein of HMIN. This assay demonstrated 100% sensitivity with histoplasmosis serum samples, all of which reacted with the 94-kDa glycoprotein (M antigen). Although the EITB is highly sensitive, it is not specific for histoplasmosis when glycosylated M protein is used as an antigen. A total of 81% of paracoccidioidomycosis, 25% of blastomycosis, 33% of coccidioidomycosis, 73% of aspergillosis, and 16% of tuberculosis serum samples cross-reacted with M protein of HMIN and yielded patterns indistinguishable from those obtained with histoplasmosis serum samples. The EITB reactions with both untreated M antigen and M antigen altered by periodate oxidation or by deglycosylation with endoglycosidases were compared. Cross-reactions with heterologous sera in the EITB could be attributed to periodate-sensitive carbohydrate epitopes, as reflected by the increase in the test specificity from 46.1 to 91.2% after periodate treatment of M protein. The EITB for the detection of antibodies to M antigen is a potential diagnostic test for histoplasmosis, provided that periodate-treated M protein is used as an antigen.

The Histoplasma capsulatum cdc2 gene is transcriptionally regulated during the morphologic transition

To understand the molecular mechanisms that control the reversible morphologic transition from mycelia to yeast in dimorphic fungi, we have isolated and characterized a cdc2 gene from Histoplasma capsulatum. This organism is a dimorphic pathogenic fungus that grows as a filamentous saprobic mold in soil and as a unicellular pathogenic yeast in human tissue. The cloned gene, whose protein product has a high degree of homology with other members of the cdc2 family, is split into four exons and three introns of 95, 52 and 85 nucleotides. Analyses of cDNA clones confirm the presence of the eukaryotic splice donor (GT) and acceptor (AG) sites. The spliced gene codes for a protein of 324 amino acids (aa) with a predicted molecular mass of 36.9 kDa. The H. capsulatum cdc2 product has 71% aa identity with Saccharomyces cerevisiae and 70% with Schizosaccharomyces pombe. The deduced protein contains the sequence, PSTAIRE, that is normally found in most p34cdc2 proteins. H. capsulatum cdc2 is transcriptionally regulated during the morphologic mycelium<==>yeast transitions and is more actively transcribed in the yeast than in the mycelial phase.

Comparison of myristoyl-CoA:protein N-myristoyltransferases from three pathogenic fungi: Cryptococcus neoformans, Histoplasma capsulatum, and Candida albicans

Myristoyl-CoA:protein N-myristoyltransferase (Nmt) transfers myristate from CoA to the N-terminal Gly residue of cellular proteins in an ordered reaction mechanism that first involves binding of myristoyl-CoA to the apoenzyme. The gene encoding Saccharomyces cerevisiae Nmt1p (NMT1) is essential for vegetative growth. Candida albicans, Cryptococcus neoformans var. neoformans, and Histoplasma capsulatum are the principal causes of systemic fungal infections in immunocompromised humans. Metabolic labeling studies indicate that they synthesize a small set of cellular N-myristoylproteins during exponential growth on rich media, the most prominent of which co-migrate with two essential functionally interchangeable S. cerevisiae N-myristoylproteins, ADP ribosylation factor-1 (Arf1p) and Arf2p. NMT and ARF genes have been recovered from C. neoformans and H. capsulatum using the polymerase chain reaction. They are single copy genes, interrupted by multiple introns. C. neoformans and H. capsulatum Nmts have approximately 50% amino acid sequence identity with the orthologous S. cerevisiae, C. albicans, and Homo sapiens N-myristoyltransferases, whereas C. neoformans and H. capsulatum Arfs are approximately 80% identical with C. albicans Arf and S. cerevisiae Arf1p and Arf2p. Functional studies of C. neoformans and C. albicans Nmts conducted in Escherichia coli reveal that (i) both efficiently acylate S. cerevisiae Arf2p; (ii) C. neoformans Arf is a substrate for C. neoformans Nmt; and (iii) substitution of an Asp for a Gly located 5 residues from the C terminus of these two enzymes causes marked temperature-dependent reductions in their catalytic efficiency, just as it does with S. cerevisiae and H. sapiens Nmts. Wild type C. neoformans, C. albicans, and H. sapiens NMTs can fully complement the lethal phenotype of a S. cerevisiae nmt1 null allele at 24 and 37 degrees C when the GAL1-10 promoter controlling their expression is induced by galactose. Only the C. albicans enzyme is able to do so when the promoter is repressed with glucose. This complementation profile likely arises, at least in part, from differences in the protein substrate specificities of the orthologous Nmts. A Gly-->Asp mutation in S. cerevisiae, C. neoformans, C. albicans, and H. sapiens Nmts produces temperature-sensitive growth arrest in isogenic S. cerevisiae strains with a nmt1 null allele. Growth of strains producing the mutant C. albicans or H. sapiens, but not the C. neoformans, enzyme can be rescued by myristate at the non-permissive temperature (37 degrees C) even in the presence of cerulenin, an inhibitor of fatty acid synthetase.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro production of extracellular elastolytic proteinase by Histoplasma capsulatum var. duboisii and Histoplasma capsulatum var. capsulatum in the yeast phase

Extracellular elastolytic proteinase was produced by yeast cultures of Histoplasma capsulatum var. duboisii and H. capsulatum var. capsulatum in three different induction media, assayed using elastin-orcein as the substrate. Medium-dependent variations in the time-course for enzyme production were observed and no peak was recorded. The proteinases hydrolysed both casein and bovine serum albumin indicating a broad substrate specificity. Both elastolytic proteinases had similar optimum pH (pH 8) and temperature (35 degrees C); over 90% residual elastolytic activity was measured in the crude enzymes of v. duboisii and v. capsulatum after incubation in the pH ranges 4.5-8.0 and 4.0-7.0 respectively, at 4 degrees C for 24 h. The proteinases were not significantly inhibited by any of the tested proteinase inhibitors. The molecular weights, estimated by column chromatography, were 23 kD. A probable role in the pathogenesis of histoplasmosis is discussed.

Production of extracellular collagenolytic proteinases by Histoplasma capsulatum var. duboisii and Histoplasma capsulatum var. capsulatum in the yeast phase

Yeast cultures of Histoplasma capsulatum var. duboisii and H. capsulatum var. capsulatum in collagen containing defined, semi-defined and complex media produced extracellular collagenolytic proteinases, assayed using 4-phenylazo-benzyloxycarbonyl-L-propyl-L-leucyl- glycyl-L-propyl-D-arginine, a specific collagenase substrate. Significant levels of hydroxyproline were measured in the cultures and clear zones of hydrolysis were produced in collagen buffer agar by the crude enzyme preparations. Hydrolysis of casein and bovine serum albumin at pH 8 suggests the presence, in the crude enzymes, of multiple proteinases rather than a collagenase with broad substrate specificity since collagenolytic activity was not detected at pH 5 and above. Collagenolytic activities in the crude enzymes of both fungi were optimal at pH 4, 40 degrees C and were inhibited by EDTA, phosphoramidion and aprotinin indicating a metallo-serine nature. The molecular weights, estimated by column chromatography, were both 17 kD. The enzymes probably constitute a shared antigen. A probable role in the pathogenesis of histoplasmosis is discussed.

Development of a genetic transformation system for Histoplasma capsulatum: complementation of uracil auxotrophy

We have developed a simple and efficient transformation system for the dimorphic fungus Histoplasma capsulatum. Mutants of H. capsulatum defective in orotidine-5'-monophosphate pyrophosphorylase were transformed to prototrophy by the cloned URA5 gene of the filamentous fungus Podospora anserina. Abortive and mitotically stable transformants were obtained. The stable transformants had integrated copies of the plasmid, some in tandem head-to-tail orientation. Free plasmid identical to the transforming plasmid was present in some of the transformants. We obtained a transformation efficiency of up to 30 transformants/micrograms DNA for plasmid pPAura5-1 (9.2 kb). pPW2001, a smaller plasmid (4.7 kb) derived from pPAura5-1, transformed H. capsulatum more efficiently (up to 155 transformants/micrograms DNA).

Studies on the catalase of Histoplasma capsulatum

Factors which control the levels of catalase within yeast cells of Histoplasma capsulatum were studied. Only a small fraction of the total catalase activity could be detected in whole cells. The bulk of the activity was revealed in cell-free extracts or in cells permeabilized with acetone. The formation of the enzyme was regulated by glucose and by oxygen. There were large, consistent differences in the levels of catalase among strains of H. capsulatum. The sensitivity of the strains to H2O2 toxicity also varied remarkably. Peroxidase activity could not be detected in cell-free extracts of the strains. Resistance to H2O2 did not correspond to levels of catalase. There was no obvious correlation of H2O2 sensitivity and virulence among the strains.

Purification and characterization of a cysteine dioxygenase from the yeast phase of Histoplasma capsulatum

A cysteine dioxygenase, cysteine oxidase (EC 1.13.11.20), has been purified from the cytosolic fraction of yeast phase cells of the dimorphic fungus Histoplasma capsulatum. The cysteine oxidase is an iron-containing dioxygenase with a molecular weight of 10500 (+/- 1500) and is present only in the yeast phase of the fungus. The enzyme is highly specific for L-cysteine, with a Km of 2 X 10(-5) M in vitro. The product of cysteine oxidation is cysteinesulfinic acid, as analyzed by thin-layer chromatography and mass spectroscopy. To our knowledge, this is the first cysteine oxidase isolated from a fungus, and it probably plays an important role in the mycelial to yeast phase transition of H. capsulatum during which redox potential and cysteine levels are crucial factors.

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.

Electron cytochemical evidence for lysosomal-like equivalents in Histoplasma capsulatum

Electron cytochemical localizations of acid phosphatase, aryl sulfatase, deoxyribonuclease, adenylate cyclase, and c-AMP phosphodiesterase activity sites in thin sections of cells of the two growth phases of the zoopathogenic Histoplasma capsulatum are described and illustrated by transmission electron micrographs. Various activity sites of these enzymes included the cytomembranes of the nucleus, mitochondria, and endoplasmic reticulum. At the same time, electron opaque reaction products were sequestered within membrane-bound, vacuolar regions of the cytosol. These vacuoles may be ontogenically related to membranous or vesicular inclusions commonly seen in thin sections of glutaraldehyde osmium tetroxide-fixed cells. These enzymatically-active vacuoles are believed consistent with previous descriptions of fungal lysosomal-like structures found in certain other fungi. Lysosomal-like vacuoles of H. capsulatum may provide a means of compartmentalization of various hydrolytic enzymes involved in catabolism and mobilization of storage reserves, and perhaps to function as well in other aspects of the life cycle of this important pathogenic dimorphic fungus.

Comparison of the ribonucleic acid polymerases from both phases of Histoplasma capsulatum

The three ribonucleic acid (RNA) polymerases (ribonucleoside triphosphate RNA nucleotidyltransferases, EC 2.7.7.6) of the two phases (yeast and mycelial) of the dimorphic fungus Histoplasma capsulatum have been purified and characterized. The corresponding enzymes from the two phases differ in sensitivity to alpha-amanitin, ion and salt requirements, temperature sensitivity, and subunit structure. This is the first case in which such qualitative differences in RNA polymerases have been demonstrated in two growth states of the same organism.

Cystine reductase in the dimorphic fungus Histoplasma capsulatum

Organo-sulfur compounds favor the transition of mycelia of Histoplasma capsulatum to the yeast form (6, 8). Investigation of the role of cystine in the transition revealed that the two phases concentrated this amino acid at comparable rates and that mutants defective in the uptake of cystine were still able to undergo the transition normally. Uptake of cystine is therefore probably not a requirement for transition to or maintenance of the yeast phase. Both phases contained a reduced nicotinamide adenine dinucleotide phosphate-dependent glutathione reductase; but a reduced nicotinamide adenine dinucleotide-dependent cystine reductase was detectable only in the yeast phase. The cystine reductase appeared early in the transition of mycelium to yeast. Treatment of mycelia with p-chloromercuriphenylsulfonic acid, which prevented the transition to yeast, had no effect on cystine uptake but strongly inhibited the cystine reductase. These results suggest that cystine reductase may provide reduced sulfhydryl groups involved in the transition of mycelium to yeast.

Isolation of aminopeptidases from Histoplasma capsulatum

Two aminopeptidases (arylamidases) were isolated and partially purified from Histoplasma capsulatum. The larger molecular weight enzyme was a proline iminopeptidase and hydrolyzed primarily a synthetic substrate, L-prolyl-beta-napthylamide. The other aminopeptidase was less substrate specific and hydrolyzed rapidly the following amino acid beta-napthylamides (beta NA): L-arginyl-beta NA greater than L-lysyl-beta NA greater than -L-4-methoxy-leucyl-beta NA greater than L-leucyl-beta NA greater than L-phenylalanyl-beta NA greater than L-alanyl-beta NA. The proline iminopeptidase was purified 1420 fold while the leucine aminopeptidase was purified 650 fold with good recovery.

Inhibition of isolated yeast and mycelial phase RNA polymerases of Histoplasma capsulatum by rifamycin derivatives

Various derivatives of rifamycin were shown to inhibit the RNA polymerases of the yeast and mycelial phases of Histoplasma capsulatum. The relative potency of each of the derivatives against the isolated polymerases was the same as the potency of each against the viable organism. RNA polymerase PC III from the yeast phase was more susceptible to the rifamycin derivatives than yeast phase polymerases PC I and PC II and the biggest differences in susceptibility were seen with the derivative AF/ABDP (2,6-dimethyl-4-benzyl-4-demethyl-rifamycin). The susceptibility pattern of the mycelial polymerase activity was identical to the yeast polymerase PC III.

Mechanism of the inhibition by RNA of the RNA polymerases of Histoplasma capsulatum

4 S RNA isolated from the dimorphic fungus Histoplasma capsulatum inhibited the DNA-dependent RNA polymerase activity of the yeast phase of this fungus. Inhibition was specific for initiation, and resulted from binding of the RNA to the enzyme. Among a variety of synthetic polynucleotides tested, only poly(G) and oligo(dG) were effective inhibitors, suggesting a role for guanines or guanine-rich sequences of RNA in the inhibition reaction.

Inhibition of Histoplasma capsulatum ribonucleic acid polymerases by homologous and heterologous ribonucleic acid

The ribonucleic acid (RNA) polymerases from the yeast phase of Histoplasma capsulatum are differentially sensitive to RNA isolated from the yeast and mycelial phases of this fungus and from Escherichia coli. Low-molecular-weight RNA from H. capsulatum was the most effective inhibitor.

Cell wall studies of Histoplasma capsulatum and Blastomyces dermatitidis using autologous and heterologous enzymes

Enzymes capable of hydrolyzing cell walls of Blastomyces dermatitidis and chemotypes I and II of Histoplasma capsulatum were prepared in the laboratory or obtained from commercial sources. They included chitinases, beta-1,3-glucanases, beta-1,6-glucanase, and Pronase. Monosaccharides and disaccharides of glucose released from the cell walls by the enzymes were determined qualitatively by paper and gas-liquid chromatography, and monosaccharides were quantitated by the latter technique as well. An enzyme system isolated from Streptomyces sp. containing both chitinase and glucanase released maximum amounts of glucose and N-acetylglucosamine from the cell walls of H. capsulatum chemotype I. A chitinase preparation, free of glucanase, from Serratia marcescens released only chitobiose and N-acetylglucosamine from chemotype I cell walls, but the total quantity of N-acetylglucosamine released was about 60% less than that released by the Streptomyces system. A beta-1,3-glucanase from Bacillus circulans hydrolyzed the cell walls of H. capsulatum chemotype I, but a beta-1,6-glucanase failed to release glucose from the same walls. Autolytic enzymes, viz., beta-1,3-glucanases and several glycosidases were detected as constitutive enzymes in both yeast and mycelial phases of B. dermatitidis and H. capsulatum chemotypes I and II. No difference in the amount of activity was found between cell sap and culture filtrate preparations. The beta-glucanases prepared from the Histoplasma and Blastomyces strains were active on the cell walls of the yeast phases of H. capsulatum chemotypes I and II, releasing laminaribiose and glucose, but were essentially inactive on the cell walls of B. dermatitidis. Chitinase, beta-1,6-glucanase, alpha-glucanase, and alpha-glucosidase activities were absent from these fungal enzyme preparations.

Identification of yeast phase of pathogenic fungi by the specificity of their aminopeptidase(s)

Specificity of aminopeptidase(s) was fluorimetrically determined in the yeast phase of Histoplasma capsulatum, H. duboisii, H. farciminosum, Blastomyces dermatitidis, Paracoccidioides brasiliensis, Candida albicans and Crytococcus neoformans. After individually incubating each of 26 amino acid-beta-naphthylamides with each yeast, the amount of each amino acid-beta-naphthylamide hydrolyzed was determined by measuring the free naphythylamine. This resulted in a reproducible profile of the aminopeptidase(s) for each fungus when medium, growth time, size of inoculum and incubation period were standardized. This technique provided a rapid and specific means of identification and differentiation among these yeasts. Specific amino acids were identified from the profile of each yeast as those rapidly liberated by the aminopeptidase(s) of that yeast. These amino acids delineated the amino acid requirement for the normal growth of each yeast.

Amino acid synthetic media for fungal pathogens based on aminopeptidase specificities: Histoplasma capsulatum, Blastomyces dermititidis, Paracoccidioides brasiliensis and Cryptococcus neoformans

The development of simple and chemically defined liquid media for Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides brasiliensis and Crypto-occus neoformans according to their aminopeptidases profiles as amino acid requirement was described. When 1.5% purified agar was added, these media also supported excellent mycelial growth and sporulation of the deep mycoses. H. capsulatum was converted to and maintained in yeast phase when 0.1% L-cystine was added to the solid medium incubated at 37 degrees C.

Characterization of an inhibitor of ribonucleic acid polymerase from the mycelial phase of Histoplasma capsulatum

An inhibitor of ribonucleic acid polymerases has been obtained from the mycelial phase of Histoplasma capsulatum and partially characterized. The inhibitor, called histin, was purified 200-fold by heat treatment at 100 C and electrophoresis on polyacrylamide gels. Histin moved in electrophoresis as if negatively charged; it was insensitive to treatment with ribonuclease of deoxyribonuclease but was completely digested by Pronase. Sucrose gradient centrifugation suggests a molecular weight of 24,000. The possibility of a regulatory role for histin in the life cycle of H. capsulatum is discussed.

Ribonucleic acid polymerases of the yeast phase of Histoplasma capsulatum

Ribonucleic acid (RNA) polymerases of Histoplasma capsulatum (yeast phase) were fractionated by phosphocellulose chromatography and partially characterized. Three distinct, active fractions were seen. The major RNA polymerase species was inhibited strongly by alpha-amanitin, whereas the other two were resistant. When either slightly purified (HSE) extract or the major active component was assayed at 37 C, the incorporation of tritiated uridine monophosphate into RNA stopped after 10 to 15 min. In contrast, the synthesis continued for at least 1 h at 23 C. The other two RNA polymerase species exhibited higher rates of incorporation when tested at 37 C, and continued to synthesize RNA even after 60 min. However, by that time the levels of incorporation at 23 C were higher than at 37 C for all three enzymes. The temperature sensitivity was not affected by changing substrate concentration or employing either native or denatured calf thymus deoxyribonucleic acid as a template. These results are compared with the data obtained with RNA polymerases from different fungi and other organisms. A possible involvement of RNA polymerase(s) in morphological differentiation of H. capsulatum is discussed.

Comparative physiologial studies of the yeast and mycelial forms of Histoplasma capsulaum: uptake and incorporation of L-leucine

l-Leucine entered the cells of both morphological forms of Histoplasma capsulatum by a permease-like system at low external concentrations of substrate. However, at levels greater than 5 x 10(-5)m l-leucine, the amino acid entered the cells both through a simple diffusion-like process and the permease-like system. The rate of the amino acid diffusion into yeast and mycelial forms appeared to be the same, whereas the initial rate of accumulation through the permease-like system was 5 to 10 times faster in the mycelial phase than it was in the yeast phase. The Michaelis constants were 2.2 x 10(-5)m in yeast phase and 2 x 10(-5)m in mycelial phase cells. Transport of l-leucine at an external concentration of 10(-5)m showed all of the characteristics of a system of active transport, which was dependent on temperature and pH. Displacement or removal of the alpha-amino group, or modification of the alpha-carboxyl group abolished amino acid uptake. The process was competitively inhibited by neutral aliphatic side-chain amino acids (inhibition constants ranged from 1.5 x 10(-5) to 6.2 x 10(-5)m). Neutral aromatic side-chain amino acids and the d-isomers of leucine and valine did not inhibit l-leucine uptake. These data were interpreted to mean that the l-leucine transport system is stereospecific and is highly specific for neutral aliphatic side-chain amino acids. Incorporation of l-leucine into macromolecules occurred at almost the same rate in both morphological forms of the fungus. The mycelial phase but not the yeast phase showed a slight initial lag in incorporation. In both morphological forms the intracellular pool of l-leucine was of limited capacity, and the total uptake of the amino acid was a function of intracellular pool size. The initial rate of l-leucine uptake was independent of the level of intracellular pool. Both morphological forms deaminated and degraded only a minor fraction of the accumulated leucine.