Disruption of a nonribosomal peptide synthetase in Aspergillus fumigatus eliminates gliotoxin production

The Aspergillus fumigatus protein GliK protects against oxidative stress and is essential for gliotoxin biosynthesis

The function of a number of genes in the gliotoxin biosynthetic cluster (gli) in Aspergillus fumigatus remains unknown. Here, we demonstrate that gliK deletion from two strains of A. fumigatus completely abolished gliotoxin biosynthesis. Furthermore, exogenous H(2)O(2) (1 mM), but not gliotoxin, significantly induced A. fumigatus gliK expression (P = 0.0101). While both mutants exhibited significant sensitivity to both exogenous gliotoxin (P < 0.001) and H(2)O(2) (P < 0.01), unexpectedly, exogenous gliotoxin relieved H(2)O(2)-induced growth inhibition in a dose-dependent manner (0 to 10 μg/ml). Gliotoxin-containing organic extracts derived from A. fumigatus ATCC 26933 significantly inhibited (P < 0.05) the growth of the ΔgliK(26933) deletion mutant. The A. fumigatus ΔgliK(26933) mutant secreted metabolites, devoid of disulfide linkages or free thiols, that were detectable by reverse-phase high-performance liquid chromatography and liquid chromatography-mass spectrometry with m/z 394 to 396. These metabolites (m/z 394 to 396) were present at significantly higher levels in the culture supernatants of the A. fumigatus ΔgliK(26933) mutant than in those of the wild type (P = 0.0024 [fold difference, 24] and P = 0.0003 [fold difference, 9.6], respectively) and were absent from A. fumigatus ΔgliG. Significantly elevated levels of ergothioneine were present in aqueous mycelial extracts of the A. fumigatus ΔgliK(26933) mutant compared to the wild type (P < 0.001). Determination of the gliotoxin uptake rate revealed a significant difference (P = 0.0045) between that of A. fumigatus ATCC 46645 (9.3 pg/mg mycelium/min) and the ΔgliK(46645) mutant (31.4 pg/mg mycelium/min), strongly suggesting that gliK absence and the presence of elevated ergothioneine levels impede exogenously added gliotoxin efflux. Our results confirm a role for gliK in gliotoxin biosynthesis and reveal new insights into gliotoxin functionality in A. fumigatus.

Genome-based cluster deletion reveals an endocrocin biosynthetic pathway in Aspergillus fumigatus

Endocrocin is a simple anthraquinone frequently identified in extracts of numerous fungi. Several biosynthetic schemes for endocrocin synthesis have been hypothesized, but to date, no dedicated secondary metabolite gene cluster that produces this polyketide as its major metabolite has been identified. Here we describe our biosynthetic and regulatory characterization of the endocrocin gene cluster in Aspergillus fumigatus. This is the first report of this anthraquinone in this species. The biosynthetic genes required for endocrocin production are regulated by the global regulator of secondary metabolism, LaeA, and encode an iterative nonreducing polyketide synthase (encA), a physically discrete metallo-β-lactamase type thioesterase (encB), and a monooxygenase (encC). Interestingly, the deletion of a gene immediately adjacent to encC, termed encD and encoding a putative 2-oxoglutarate-Fe(II) type oxidoreductase, resulted in higher levels of endocrocin production than in the wild-type strain, whereas overexpression of encD eliminated endocrocin accumulation. We found that overexpression of the encA transcript resulted in higher transcript levels of encA-D and higher production of endocrocin. We discuss a model of the enc cluster as one evolutionary origin of fungal anthraquinones derived from a nonreducing polyketide synthase and a discrete metallo-β-lactamase-type thioesterase.

Nonribosomal peptide synthetase genes pesL and pes1 are essential for Fumigaclavine C production in Aspergillus fumigatus

The identity of metabolites encoded by the majority of nonribosomal peptide synthetases in the opportunistic pathogen, Aspergillus fumigatus, remains outstanding. We found that the nonribosomal peptide (NRP) synthetases PesL and Pes1 were essential for fumigaclavine C biosynthesis, the end product of the complex ergot alkaloid (EA) pathway in A. fumigatus. Deletion of either pesL (ΔpesL) or pes1 (Δpes1) resulted in complete loss of fumigaclavine C biosynthesis, relatively increased production of fumitremorgins such as TR-2, fumitremorgin C and verruculogen, increased sensitivity to H(2)O(2), and increased sensitivity to the antifungals, voriconazole, and amphotericin B. Deletion of pesL resulted in severely reduced virulence in an invertebrate infection model (P < 0.001). These findings indicate that NRP synthesis plays an essential role in mediating the final prenylation step of the EA pathway, despite the apparent absence of NRP synthetases in the proposed EA biosynthetic cluster for A. fumigatus. Liquid chromatography/diode array detection/mass spectrometry analysis also revealed the presence of fumiquinazolines A to F in both A. fumigatus wild-type and ΔpesL strains. This observation suggests that alternative NRP synthetases can also function in fumiquinazoline biosynthesis, since PesL has been shown to mediate fumiquinazoline biosynthesis in vitro. Furthermore, we provide here the first direct link between EA biosynthesis and virulence, in agreement with the observed toxicity associated with EA exposure. Finally, we demonstrate a possible cluster cross-talk phenomenon, a theme which is beginning to emerge in the literature.

Advances in Aspergillus secondary metabolite research in the post-genomic era

This review studies the impact of whole genome sequencing on Aspergillus secondary metabolite research. There has been a proliferation of many new, intriguing discoveries since sequencing data became widely available. What is more, the genomes disclosed the surprising finding that there are many more secondary metabolite biosynthetic pathways than laboratory research had suggested. Activating these pathways has been met with some success, but many more dormant genes remain to be awakened.

Toward awakening cryptic secondary metabolite gene clusters in filamentous fungi

Mining for novel natural compounds is of eminent importance owing to the continuous need for new pharmaceuticals. Filamentous fungi are historically known to harbor the genetic capacity for an arsenal of natural compounds, both beneficial and detrimental to humans. The majority of these metabolites are still cryptic or silent under standard laboratory culture conditions. Mining for these cryptic natural products can be an excellent source for identifying new compound classes. Capitalizing on the current knowledge on how secondary metabolite gene clusters are regulated has allowed the research community to unlock many hidden fungal treasures, as described in this chapter.

Biosynthesis and function of gliotoxin in Aspergillus fumigatus

Gliotoxin (GT) is the prototype of the epidithiodioxopiperazine (ETP)-type fungal toxins. GT plays a critical role in the pathobiology of Aspergillus fumigatus. It modulates the immune response and induces apoptosis in different cell types. The toxicity has been attributed to the unusual intramolecular disulfide bridge, which is the functional motif of all ETPs. Because of the extraordinary structure and activity of GT, this fungal metabolite has been the subject of many investigations. The biosynthesis of GT involves unprecedented reactions catalysed by recently discovered enzymes. Here, we summarize the recent progress in elucidating the GT biosynthetic pathway and its role in virulence.

Immune responses against Aspergillus fumigatus: what have we learned?

PURPOSE OF REVIEW

Aspergillus fumigatus causes invasive and allergenic disease. Host defense relies on the ability of the respiratory immune system to restrict spore germination into invasive hyphae and to limit fungus-induced or inflammation-induced damage in infected tissues. This review covers the molecular and cellular events that mediate innate and CD4 T-cell responses to A. fumigatus and fungal attributes that counter hostile microenvironments and, in turn, affect host responses.

RECENT FINDINGS

Host recognition of fungal cell wall components is critical for fungal uptake, killing, and the formation of protective innate and CD4 T-cell effector populations. Beyond the known role of neutrophils and macrophages, circulating monocytes, dendritic cells, and natural killer cells contribute to optimal defense against A. fumigatus. Genetic and pharmacologic manipulation of A. fumigatus reveals that hypoxia adaptation, cell wall assembly, and secondary metabolite production in mammalian tissues contribute to fungal pathogenesis and the outcome of infection.

SUMMARY

Greater understanding of the immune mechanisms that underlie protective responses and fungal pathways that promote microbial adaptation and growth in mammalian tissue provide a conceptual framework for improving current antifungal therapies.

Targeted disruption of nonribosomal peptide synthetase pes3 augments the virulence of Aspergillus fumigatus

Nonribosomal peptide synthesis (NRPS) is a documented virulence factor for the opportunistic pathogen Aspergillus fumigatus and other fungi. Secreted or intracellularly located NRP products include the toxic molecule gliotoxin and the iron-chelating siderophores triacetylfusarinine C and ferricrocin. No structural or immunologically relevant NRP products have been identified in the organism. We investigated the function of the largest gene in A. fumigatus, which encodes the NRP synthetase Pes3 (AFUA_5G12730), by targeted gene deletion and extensive phenotypic analysis. It was observed that in contrast to other NRP synthetases, deletion of pes3 significantly increases the virulence of A. fumigatus, whereby the pes3 deletion strain (A. fumigatus Δpes3) exhibited heightened virulence (increased killing) in invertebrate (P < 0.001) and increased fungal burden (P = 0.008) in a corticosteroid model of murine pulmonary aspergillosis. Complementation restored the wild-type phenotype in the invertebrate model. Deletion of pes3 also resulted in increased susceptibility to the antifungal, voriconazole (P < 0.01), shorter germlings, and significantly reduced surface β-glucan (P = 0.0325). Extensive metabolite profiling revealed that Pes3 does not produce a secreted or intracellularly stored NRP in A. fumigatus. Macrophage infections and histological analysis of infected murine tissue indicate that Δpes3 heightened virulence appears to be mediated by aberrant innate immune recognition of the fungus. Proteome alterations in A. fumigatus Δpes3 strongly suggest impaired germination capacity. Uniquely, our data strongly indicate a structural role for the Pes3-encoded NRP, a finding that appears to be novel for an NRP synthetase.

The role of glutathione S-transferase GliG in gliotoxin biosynthesis in Aspergillus fumigatus

Gliotoxin, a redox-active metabolite, is produced by the opportunistic fungal pathogen Aspergillus fumigatus, and its biosynthesis is directed by the gli gene cluster. Knowledge of the biosynthetic pathway to gliotoxin, which contains a disulfide bridge of unknown origin, is limited, although L-Phe and L-Ser are known biosynthetic precursors. Deletion of gliG from the gli cluster, herein functionally confirmed as a glutathione S-transferase, results in abrogation of gliotoxin biosynthesis and accumulation of 6-benzyl-6-hydroxy-1-methoxy-3-methylenepiperazine-2,5-dione. This putative shunt metabolite from the gliotoxin biosynthetic pathway contains an intriguing hydroxyl group at C-6, consistent with a gliotoxin biosynthetic pathway involving thiolation via addition of the glutathione thiol group to a reactive acyl imine intermediate. Complementation of gliG restored gliotoxin production and, unlike gliT, gliG was found not to be involved in fungal self-protection against gliotoxin.

The cross-pathway control system regulates production of the secondary metabolite toxin, sirodesmin PL, in the ascomycete, Leptosphaeria maculans

BACKGROUND

Sirodesmin PL is a secondary metabolite toxin made by the ascomycetous plant pathogen, Leptosphaeria maculans. The sirodesmin biosynthetic genes are clustered in the genome. The key genes are a non-ribosomal peptide synthetase, sirP, and a pathway-specific transcription factor, sirZ. Little is known about regulation of sirodesmin production.

RESULTS

Genes involved in regulation of sirodesmin PL in L. maculans have been identified. Two hundred random insertional T-DNA mutants were screened with an antibacterial assay for ones producing low levels of sirodesmin PL. Three such mutants were isolated and each transcribed sirZ at very low levels. One of the affected genes had high sequence similarity to Aspergillus fumigatus cpcA, which regulates the cross-pathway control system in response to amino acid availability. This gene was silenced in L. maculans and the resultant mutant characterised. When amino acid starvation was artificially-induced by addition of 3-aminotriazole for 5 h, transcript levels of sirP and sirZ did not change in the wild type. In contrast, levels of sirP and sirZ transcripts increased in the silenced cpcA mutant. After prolonged amino acid starvation the silenced cpcA mutant produced much higher amounts of sirodesmin PL than the wild type.

CONCLUSIONS

Production of sirodesmin PL in L. maculans is regulated by the cross pathway control gene, cpcA, either directly or indirectly via the pathway-specific transcription factor, sirZ.

A dedicated glutathione S-transferase mediates carbon-sulfur bond formation in gliotoxin biosynthesis

Gliotoxin is a virulence factor of the human pathogen Aspergillus fumigatus , the leading cause of invasive aspergillosis. Its toxicity is mediated by the unusual transannular disulfide bridge of the epidithiodiketopiperazine (ETP) scaffold. Here we disclose the critical role of a specialized glutathione S-transferase (GST), GliG, in enzymatic sulfurization. Furthermore, we show that bishydroxylation of the diketopiperazine by the oxygenase GliC is a prerequisite for glutathione adduct formation. This is the first report of the involvement of a GST in enzymatic C-S bond formation in microbial secondary metabolism.

Identification of cryptic products of the gliotoxin gene cluster using NMR-based comparative metabolomics and a model for gliotoxin biosynthesis

Gliotoxin, a major product of the gli non-ribosomal peptide synthetase gene cluster, is strongly associated with virulence of the opportunistic human pathogen Aspergillus fumigatus. Despite identification of the gli cluster, the pathway of gliotoxin biosynthesis has remained elusive, in part because few potential intermediates have been identified. In addition, previous studies suggest that knowledge of gli-dependent metabolites is incomplete. Here we use differential analysis by 2D NMR spectroscopy (DANS) of metabolite extracts derived from gli knock-out and wild-type (WT) strains to obtain a detailed inventory of gli-dependent metabolites. DANS-based comparison of the WT metabolome with that of ΔgliZ, a knock-out strain devoid of the gene encoding the transcriptional regulator of the gli cluster, revealed nine novel gliZ-dependent metabolites including unexpected structural motifs. Their identification provides insight into gliotoxin biosynthesis and may benefit studies of the role of the gli cluster in A. fumigatus virulence. Our study demonstrates the utility of DANS for correlating gene expression and metabolite biosynthesis in microorganisms.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Streptomyces clavuligerus HlmI is an intramolecular disulfide-forming dithiol oxidase in holomycin biosynthesis

Holomycin and related dithiolopyrrolone antibiotics display broad-spectrum antimicrobial activities and contain a unique 5,5-bicyclic ring structure with an N-acylated aminopyrrolone fused to a cyclic ene-disulfide. Here we show that the intramolecular disulfide bridge is constructed from the acyclic ene-dithiol at a late stage in the pathway by a thioredoxin oxidoreductase-like enzyme HlmI from the holomycin producer Streptomyces clavuligerus. Recombinant HlmI was purified from E. coli with bound flavin adenine dinucleotide (FAD) and converts reduced holomycin to holomycin utilizing O(2) as cosubstrate. As a dithiol oxidase, HlmI is functionally homologous to GliT and DepH, which perform a similar dithiol to disulfide oxidation in the biosynthesis of fungal natural product gliotoxin and epigenetic regulator compound FK228, respectively. Deletion of the hlmI gene in the wild type S. clavuligerus and in a holomycin-overproducing mutant resulted in decreased level of holomycin production and increased sensitivity toward holomycin, suggesting a self-protection role of HlmI in the holomycin biosynthetic pathway. HlmI belongs to a new clade of uncharacterized thioredoxin oxidoreductase-like enzymes, distinctive from the GliT-like enzymes and the DepH-like enzymes, and represents a third example of oxidoreductases that catalyzes disulfide formation in the biosynthesis of small molecules.

Clinical utility and development of biomarkers in invasive aspergillosis

The diagnosis of invasive aspergillosis remains very difficult, and there are limited treatment options for the disease. Pre-clinical models have been used to evaluate the diagnosis and treatment of Aspergillus infection and to assess the pathogenicity and virulence of the organism. Extensive efforts in Aspergillus research have significantly expanded the genomic information about this microorganism. The standardization of animal models of invasive aspergillosis can be used to enhance the evaluation of genomic information about the organism to improve the diagnosis and treatment of invasive aspergillosis. One approach to this process has been the award of a contract by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health to establish and standardize animal models of invasive aspergillosis for the development of new diagnostic technologies for both pulmonary and disseminated Aspergillus infection. This work utilizes molecular approaches for the genetic manipulation of Aspergillus strains that can be tested in animal-model systems to establish new diagnostic targets and tools. Studies have evaluated the performance characteristics of assays for cell-wall antigens of Aspergillus including galactomannan and beta-D-glucan, as well as for DNA targets in the organism, through PCR. New targets, such as proteomic and genomic approaches, and novel detection methods, such as point-of-care lateral-flow devices, have also been evaluated. The goal of this paper is to provide a framework for evaluating genomic targets in animal models to improve the diagnosis and treatment of invasive aspergillosis toward ultimately improving the outcomes for patients with this frequently fatal infection.

The small GTPase RacA mediates intracellular reactive oxygen species production, polarized growth, and virulence in the human fungal pathogen Aspergillus fumigatus

Aspergillus fumigatus is the predominant mold pathogen in immunocompromised patients. In this study, we present the first characterization of the small GTPase RacA in A. fumigatus. To gain insight into the function of racA in the growth and pathogenesis of A. fumigatus, we constructed a strain that lacks a functional racA gene. The ΔracA strain showed significant morphological defects, including a reduced growth rate and abnormal conidiogenesis on glucose minimal medium. In the ΔracA strain, apical dominance in the leading hyphae is lost and, instead, multiple axes of polarity emerge. Intriguingly, superoxide production at the hyphal tips was reduced by 25% in the ΔracA strain. Treatment of wild-type hyphae with diphenylene iodonium, an inhibitor of NADPH oxidase, resulted in phenotypes similar to that of the ΔracA strain. These data suggest that ΔracA strain phenotypes may be due to a reduction or alteration in the production of reactive oxygen species. Most surprisingly, despite these developmental and growth abnormalities, the ΔracA strain retained at least wild-type virulence in both an insect model and two immunologically distinct murine models of invasive pulmonary aspergillosis. These results demonstrate that in vitro growth phenotypes do not always correlate with in vivo virulence and raise intriguing questions about the role of RacA in Aspergillus virulence.

Transannular disulfide formation in gliotoxin biosynthesis and its role in self-resistance of the human pathogen Aspergillus fumigatus

Gliotoxin (1), the infamous representative of the group of epipolythiodioxopiperazines (ETPs), is a virulence factor of the human pathogenic fungus Aspergillus fumigatus. The unique redox-sensitive transannular disulfide bridge is critical for deleterious effects caused by redox cycling and protein conjugation in the host. Through a combination of genetic, biochemical, and chemical analyses, we found that 1 results from GliT-mediated oxidation of the corresponding dithiol. In vitro studies using purified GliT demonstrate that the FAD-dependent, homodimeric enzyme utilizes molecular oxygen as terminal electron acceptor with concomitant formation of H(2)O(2). In analogy to the thiol-disulfide oxidoreductase superfamily, a model for dithiol-disulfide exchange involving the conserved CxxC motif is proposed. Notably, while all studied disulfide oxidases invariably form intra- or interchenar disulfide bonds in peptides, GliT is the first studied enzyme producing an epidithio bond. Furthermore, through sensitivity assays using wild type, Delta gliT mutant, and complemented strain, we found that GliT confers resistance to the producing organism. A phylogenetic study revealed that GliT falls into a clade of yet fully uncharacterized fungal gene products deduced from putative ETP biosynthesis gene loci. GliT thus not only represents the prototype of ETP-forming enzymes in eukaryotes but also delineates a novel mechanism for self-resistance.

What makes Aspergillus fumigatus a successful pathogen? Genes and molecules involved in invasive aspergillosis

Aspergillus fumigatus is an opportunistic pathogen that causes 90% of invasive aspergillosis (IA) due to Aspergillus genus, with a 50-95% mortality rate. It has been postulated that certain virulence factors are characteristic of A. fumigatus, but the "non-classical" virulence factors seem to be highly variable. Overall, published studies have demonstrated that the virulence of this fungus is multifactorial, associated with its structure, its capacity for growth and adaptation to stress conditions, its mechanisms for evading the immune system and its ability to cause damage to the host. In this review we intend to give a general overview of the genes and molecules involved in the development of IA. The thermotolerance section focuses on five genes related with the capacity of the fungus to grow at temperatures above 30°C (thtA, cgrA, afpmt1, kre2/afmnt1, and hsp1/asp f 12). The following sections discuss molecules and genes related to interaction with the host and with the immune responses. These sections include β-glucan, α-glucan, chitin, galactomannan, galactomannoproteins (afmp1/asp f 17 and afmp2), hydrophobins (rodA/hyp1 and rodB), DHN-melanin, their respective synthases (fks1, rho1-4, ags1-3, chsA-G, och1-4, mnn9, van1, anp1, glfA, pksP/alb1, arp1, arp2, abr1, abr2, and ayg1), and modifying enzymes (gel1-7, bgt1, eng1, ecm33, afpigA, afpmt1-2, afpmt4, kre2/afmnt1, afmnt2-3, afcwh41 and pmi); several enzymes related to oxidative stress protection such as catalases (catA, cat1/catB, cat2/katG, catC, and catE), superoxide dismutases (sod1, sod2, sod3/asp f 6, and sod4), fatty acid oxygenases (ppoA-C), glutathione tranferases (gstA-E), and others (afyap1, skn7, and pes1); and efflux transporters (mdr1-4, atrF, abcA-E, and msfA-E). In addition, this review considers toxins and related genes, such as a diffusible toxic substance from conidia, gliotoxin (gliP and gliZ), mitogillin (res/mitF/asp f 1), hemolysin (aspHS), festuclavine and fumigaclavine A-C, fumitremorgin A-C, verruculogen, fumagillin, helvolic acid, aflatoxin B1 and G1, and laeA. Two sections cover genes and molecules related with nutrient uptake, signaling and metabolic regulations involved in virulence, including enzymes, such as serine proteases (alp/asp f 13, alp2, and asp f 18), metalloproteases (mep/asp f 5, mepB, and mep20), aspartic proteases (pep/asp f 10, pep2, and ctsD), dipeptidylpeptidases (dppIV and dppV), and phospholipases (plb1-3 and phospholipase C); siderophores and iron acquisition (sidA-G, sreA, ftrA, fetC, mirB-C, and amcA); zinc acquisition (zrfA-H, zafA, and pacC); amino acid biosynthesis, nitrogen uptake, and cross-pathways control (areA, rhbA, mcsA, lysF, cpcA/gcn4p, and cpcC/gcn2p); general biosynthetic pathway (pyrG, hcsA, and pabaA), trehalose biosynthesis (tpsA and tpsB), and other regulation pathways such as those of the MAP kinases (sakA/hogA, mpkA-C, ste7, pbs2, mkk2, steC/ste11, bck1, ssk2, and sho1), G-proteins (gpaA, sfaD, and cpgA), cAMP-PKA signaling (acyA, gpaB, pkaC1, and pkaR), His kinases (fos1 and tcsB), Ca(2+) signaling (calA/cnaA, crzA, gprC and gprD), and Ras family (rasA, rasB, and rhbA), and others (ace2, medA, and srbA). Finally, we also comment on the effect of A. fumigatus allergens (Asp f 1-Asp f 34) on IA. The data gathered generate a complex puzzle, the pieces representing virulence factors or the different activities of the fungus, and these need to be arranged to obtain a comprehensive vision of the virulence of A. fumigatus. The most recent gene expression studies using DNA-microarrays may be help us to understand this complex virulence, and to detect targets to develop rapid diagnostic methods and new antifungal agents.

Self-protection against gliotoxin--a component of the gliotoxin biosynthetic cluster, GliT, completely protects Aspergillus fumigatus against exogenous gliotoxin

Gliotoxin, and other related molecules, are encoded by multi-gene clusters and biosynthesized by fungi using non-ribosomal biosynthetic mechanisms. Almost universally described in terms of its toxicity towards mammalian cells, gliotoxin has come to be considered as a component of the virulence arsenal of Aspergillus fumigatus. Here we show that deletion of a single gene, gliT, in the gliotoxin biosynthetic cluster of two A. fumigatus strains, rendered the organism highly sensitive to exogenous gliotoxin and completely disrupted gliotoxin secretion. Addition of glutathione to both A. fumigatus ΔgliT strains relieved gliotoxin inhibition. Moreover, expression of gliT appears to be independently regulated compared to all other cluster components and is up-regulated by exogenous gliotoxin presence, at both the transcript and protein level. Upon gliotoxin exposure, gliT is also expressed in A. fumigatus ΔgliZ, which cannot express any other genes in the gliotoxin biosynthetic cluster, indicating that gliT is primarily responsible for protecting this strain against exogenous gliotoxin. GliT exhibits a gliotoxin reductase activity up to 9 µM gliotoxin and appears to prevent irreversible depletion of intracellular glutathione stores by reduction of the oxidized form of gliotoxin. Cross-species resistance to exogenous gliotoxin is acquired by A. nidulans and Saccharomyces cerevisiae, respectively, when transformed with gliT. We hypothesise that the primary role of gliotoxin may be as an antioxidant and that in addition to GliT functionality, gliotoxin secretion may be a component of an auto-protective mechanism, deployed by A. fumigatus to protect itself against this potent biomolecule.

The pathogenic fungus Aspergillus fumigatus causes disease in immunocompromised individuals such as cancer patients. The fungus makes a small molecule called gliotoxin which helps A. fumigatus bypass the immune system in ill people, and cause disease. Although a small molecule, gliotoxin biosynthesis is enabled by a complex series of enzymes, one of which is called GliT, in A. fumigatus. Amazingly, nobody has really considered that gliotoxin might be toxic to A. fumigatus itself. Here we show that absence of GliT makes A. fumigatus highly sensitive to added gliotoxin and inhibits fungal growth, both of which can be reversed by restoring GliT. Neither can the fungus make or release its own gliotoxin when GliT is missing. We also show that gliotoxin sensitivity can be totally overcome by adding glutathione, which is an important anti-oxidant within cells. We demonstrate that gliotoxin addition increases the production of GliT, and that GliT breaks the disulphide bond in gliotoxin which may be a step in the pathway for gliotoxin protection or release from A. fumigatus. We conclude that gliotoxin may mainly be involved in protecting A. fumigatus against oxidative stress and that it is an accidental toxin.

Trehalose 6-phosphate phosphatase is required for cell wall integrity and fungal virulence but not trehalose biosynthesis in the human fungal pathogen Aspergillus fumigatus

The trehalose biosynthesis pathway is critical for virulence in human and plant fungal pathogens. In this study, we tested the hypothesis that trehalose 6-phosphate phosphatase (T6PP) is required for Aspergillus fumigatus virulence. A mutant of the A. fumigatus T6PP, OrlA, displayed severe morphological defects related to asexual reproduction when grown on glucose (1%) minimal media. These defects could be rescued by addition of osmotic stabilizers, reduction in incubation temperature or increase in glucose levels (> 4%). Subsequent examination of the mutant with cell wall perturbing agents revealed a link between cell wall biosynthesis and trehalose 6-phosphate (T6P) levels. As expected, high levels of T6P accumulated in the absence of OrlA resulting in depletion of free inorganic phosphate and inhibition of hexokinase activity. Surprisingly, trehalose production persisted in the absence of OrlA. Further analyses revealed that A. fumigatus contains two trehalose phosphorylases that may be responsible for trehalose production in the absence of OrlA. Despite a normal growth rate under in vitro growth conditions, the orlA mutant was virtually avirulent in two distinct murine models of invasive pulmonary aspergillosis. Our results suggest that further study of this pathway will lead to new insights into regulation of fungal cell wall biosynthesis and virulence.

Spotlights on advances in mycotoxin research

A remarkable feature of filamentous fungi is their ability to produce small yet structurally complex and often bioactive natural products. In this mini-review, we cover advances in the research on fungal secondary metabolites, particularly mycotoxins, and focus on biosynthetic aspects as well as on the complex regulatory mechanisms which control the expression of biosynthetic genes. We also highlight the increasing impact of genomics and transcriptomics, which help explore the realm of secondary metabolism of fungi.

Aspergillus fumigatus MedA governs adherence, host cell interactions and virulence

In medically important fungi, regulatory elements that control development and asexual reproduction often govern the expression of virulence traits. We therefore cloned the Aspergillus fumigatus developmental modifier MedA and characterized its role in conidiation, host cell interactions and virulence. As in the model organism Aspergillus nidulans, disruption of medA in A. fumigatus dramatically reduced conidiation. However, the conidiophore morphology was markedly different between the two species. Further, gene expression analysis suggested that MedA governs conidiation through different pathways in A. fumigatus compared with A. nidulans. The A. fumigatusDeltamedA strain was impaired in biofilm production and adherence to plastic, as well as adherence to pulmonary epithelial cells, endothelial cells and fibronectin in vitro. The DeltamedA strain also had reduced capacity to damage pulmonary epithelial cells, and stimulate pro-inflammatory cytokine mRNA and protein expression. Consistent with these results, the A. fumigatusDeltamedA strain also exhibited reduced virulence in both an invertebrate and a mammalian model of invasive aspergillosis. Collectively, these results suggest that the downstream targets of A. fumigatus MedA mediate virulence, and may provide novel therapeutic targets for invasive aspergillosis.

Evolution of aromatic prenyltransferases in the biosynthesis of indole derivatives

A series of putative indole prenyltransferase genes could be identified in the genome sequences of different fungal strains including Aspergillus fumigatus and Neosartorya fischeri. The gene products show significant sequence similarities to dimethylallyltryptophan synthases from different fungi. We have cloned and overexpressed seven of these genes, fgaPT1, fgaPT2, ftmPT1, ftmPT2, 7-dmats, cdpNPT and anaPT in Escherichia coli and Saccharomyces cerevisiae. The overproduced enzymes were characterised biochemically. Three additional indole prenyltransferases, DmaW-Cs, TdiB and MaPT were also identified and characterised in the last years. Sequence analysis and comparison with known aromatic prenyltransferases as well as biochemical investigation revealed that these enzymes belong to a group of aromatic prenyltransferases. The characterised prenyltransferases are soluble proteins, catalyse different prenyl transfer reactions on indole moieties of various substrates and do not require divalent metal ions for their prenyl transfer reactions. In addition, indole prenyltransferases carry tryptophan aminopeptidase activity, which strengths their relationship in the evolution. These properties differ clearly from membrane-bound aromatic prenyltransferases from different sources and soluble prenyltransferases from bacteria. All of the indole prenyltransferases accepted only dimethylallyl diphosphate as prenyl donor. On the other hand, they showed broad substrate specificity towards their aromatic substrates. Diverse simple tryptophan derivatives and tryptophan-containing cyclic dipeptides were accepted by these enzymes, providing a strategy for convenient production of biologically active substances, e.g. by chemoenzymatic synthesis.

Pathogenesis of Aspergillus fumigatus in Invasive Aspergillosis

Aspergillus species are globally ubiquitous saprophytes found in a variety of ecological niches. Almost 200 species of aspergilli have been identified, less than 20 of which are known to cause human disease. Among them, Aspergillus fumigatus is the most prevalent and is largely responsible for the increased incidence of invasive aspergillosis (IA) in the immunocompromised patient population. IA is a devastating illness, with mortality rates in some patient groups reaching as high as 90%. Studies identifying and assessing the roles of specific factors of A. fumigatus that contribute to the pathogenesis of IA have traditionally focused on single-gene deletion and mutant characterization. In combination with recent large-scale approaches analyzing global fungal responses to distinct environmental or host conditions, these studies have identified many factors that contribute to the overall pathogenic potential of A. fumigatus. Here, we provide an overview of the significant findings regarding A. fumigatus pathogenesis as it pertains to invasive disease.

Aspergillus fumigatus calcipressin CbpA is involved in hyphal growth and calcium homeostasis

Calcineurin is a conserved protein phosphatase that plays a critical role in Ca(2+) signaling and stress responses. Previously, a new class of conserved calcineurin-binding proteins, the calcipressins, was identified. However, the role of these proteins remains controversial, and both inhibitory and stimulatory effects on calcineurin were observed. In this study, we investigate the role of CbpA, the Aspergillus fumigatus member of the calcipressin family, and report that deletion of the cbpA gene resulted in reduced hyphal growth and limited attenuated virulence. Interestingly, under high-calcium-level conditions, the DeltacbpA strain displayed improved Ca(2+) tolerance compared to the wild-type strain and revealed increased expression of vcxA, chsA, and cnaA, which encode the vacuolar Ca(2+)/H(+) exchanger VcxA, chitin synthase A, and the calcineurin catalytic subunit CnaA, respectively. The increased transcript levels of these three genes were reversed in the presence of the calcineurin inhibitor FK506, indicating a calcineurin-dependent mechanism. Overexpression of cbpA resulted in decreased transcription of vcxA, chsA, and cnaA, associated with wild-type sensitivity to Ca(2+). Taken together, our study highlights the importance of CbpA in the regulation of hyphal growth and calcium adaptation of A. fumigatus and provides evidence that CbpA may serve as a feedback inhibitor in some aspects of calcineurin functions.

Aspergillus genomes and the Aspergillus cloud

Aspergillus Genomes is a public resource for viewing annotated genes predicted by various Aspergillus sequencing projects. It has arisen from the union of two significant resources: the Aspergillus/Aspergillosis website and the Central Aspergillus Data REpository (CADRE). The former has primarily served the medical community, providing information about Aspergillus and associated diseases to medics, patients and scientists; the latter has focused on the fungal genomic community, providing a central repository for sequences and annotation extracted from Aspergillus Genomes. By merging these databases, genomes benefit from extensive cross-linking with medical information to create a unique resource, spanning genomics and clinical aspects of the genus. Aspergillus Genomes is accessible from http://www.aspergillus-genomes.org.uk.

A sterol-regulatory element binding protein is required for cell polarity, hypoxia adaptation, azole drug resistance, and virulence in Aspergillus fumigatus

At the site of microbial infections, the significant influx of immune effector cells and the necrosis of tissue by the invading pathogen generate hypoxic microenvironments in which both the pathogen and host cells must survive. Currently, whether hypoxia adaptation is an important virulence attribute of opportunistic pathogenic molds is unknown. Here we report the characterization of a sterol-regulatory element binding protein, SrbA, in the opportunistic pathogenic mold, Aspergillus fumigatus. Loss of SrbA results in a mutant strain of the fungus that is incapable of growth in a hypoxic environment and consequently incapable of causing disease in two distinct murine models of invasive pulmonary aspergillosis (IPA). Transcriptional profiling revealed 87 genes that are affected by loss of SrbA function. Annotation of these genes implicated SrbA in maintaining sterol biosynthesis and hyphal morphology. Further examination of the SrbA null mutant consequently revealed that SrbA plays a critical role in ergosterol biosynthesis, resistance to the azole class of antifungal drugs, and in maintenance of cell polarity in A. fumigatus. Significantly, the SrbA null mutant was highly susceptible to fluconazole and voriconazole. Thus, these findings present a new function of SREBP proteins in filamentous fungi, and demonstrate for the first time that hypoxia adaptation is likely an important virulence attribute of pathogenic molds.

The incidence of potentially lethal infections caused by normally benign molds has increased tremendously over the last two decades. One disease in particular, invasive pulmonary aspergillosis (IPA), caused by the common mold Aspergillus fumigatus, has become the leading cause of death due to invasive mycoses. Currently, we have a limited understanding of how this opportunistic pathogen causes disease in immunocompromised patients. In this study, we discover a previously unexplored mechanism required by this mold to cause disease, hypoxia (low oxygen) adaptation. We report that hypoxia adaptation in A. fumigatus is mediated in part by a highly conserved transcription factor, SrbA, a protein in the sterol regulatory element binding protein family. A null mutant of SrbA was unable to grow in hypoxia, displayed increased susceptibility to the azole class of antifungal drugs, and was avirulent in two distinct murine models of IPA. Importantly, we report the discovery of a novel function of SrbA in molds related to maintenance of cell polarity. The finding that SrbA regulates resistance to the azole class of antifungal drugs presents an opportunity to uncover new mechanisms of antifungal drug resistance in A. fumigatus.

Sub-telomere directed gene expression during initiation of invasive aspergillosis

Aspergillus fumigatus is a common mould whose spores are a component of the normal airborne flora. Immune dysfunction permits developmental growth of inhaled spores in the human lung causing aspergillosis, a significant threat to human health in the form of allergic, and life-threatening invasive infections. The success of A. fumigatus as a pathogen is unique among close phylogenetic relatives and is poorly characterised at the molecular level. Recent genome sequencing of several Aspergillus species provides an exceptional opportunity to analyse fungal virulence attributes within a genomic and evolutionary context. To identify genes preferentially expressed during adaptation to the mammalian host niche, we generated multiple gene expression profiles from minute samplings of A. fumigatus germlings during initiation of murine infection. They reveal a highly co-ordinated A. fumigatus gene expression programme, governing metabolic and physiological adaptation, which allows the organism to prosper within the mammalian niche. As functions of phylogenetic conservation and genetic locus, 28% and 30%, respectively, of the A. fumigatus subtelomeric and lineage-specific gene repertoires are induced relative to laboratory culture, and physically clustered genes including loci directing pseurotin, gliotoxin and siderophore biosyntheses are a prominent feature. Locationally biased A. fumigatus gene expression is not prompted by in vitro iron limitation, acid, alkaline, anaerobic or oxidative stress. However, subtelomeric gene expression is favoured following ex vivo neutrophil exposure and in comparative analyses of richly and poorly nourished laboratory cultured germlings. We found remarkable concordance between the A. fumigatus host-adaptation transcriptome and those resulting from in vitro iron depletion, alkaline shift, nitrogen starvation and loss of the methyltransferase LaeA. This first transcriptional snapshot of a fungal genome during initiation of mammalian infection provides the global perspective required to direct much-needed diagnostic and therapeutic strategies and reveals genome organisation and subtelomeric diversity as potential driving forces in the evolution of pathogenicity in the genus Aspergillus.

Airborne spores of the fungus Aspergillus fumigatus are present in significant quantities worldwide and are responsible for a range of illnesses from allergy to deadly invasive lung infection. A number of fungal properties are likely required for germination and growth of the fungus in the host, and now that the genome sequence of A. fumigatus is available it is possible to address which genes become important during initiation of infection. Understanding this might lead to new therapeutics and diagnostic tools. We have compared A. fumigatus gene activation during infection in a murine model to that in a laboratory culture to identify fungal attributes preferentially employed during disease. Our analysis entailed measurement of activity from most of the >9000 A. fumigatus genes, identifying iron limitation, alkaline stress, and nitrogen starvation as prominent stresses imposed by the host environment. We also found that genes preferentially employed for infection occur in clusters and are more likely to reside near the end of chromosomes, otherwise known as telomeres.

Metabolomics of Aspergillus fumigatus

Aspergillus fumigatus is the most important species in Aspergillus causing infective lung diseases. This species has been reported to produce a large number of extrolites, including secondary metabolites, acids, and proteins such as hydrophobins and extracellular enzymes. At least 226 potentially bioactive secondary metabolites have been reported from A. fumigatus that can be ordered into 24 biosynthetic families. Of these families we have detected representatives from the following families of secondary metabolites: fumigatins, fumigaclavines, fumiquinazolines, trypacidin and monomethylsulochrin, fumagillins, gliotoxins, pseurotins, chloroanthraquinones, fumitremorgins, verruculogen, helvolic acids, and pyripyropenes by HPLC with diode array detection and mass spectrometric detection. There is still doubt whether A. fumigatus can produce tryptoquivalins, but all isolates produce the related fumiquinazolines. We also tentatively detected sphingofungins in A. fumigatus Af293 and in an isolate of A. lentulus. The sphingofungins may have a similar role as the toxic fumonisins, found in A. niger. A further number of mycotoxins, including ochratoxin A, and other secondary metabolites have been reported from A. fumigatus, but in those cases either the fungus or its metabolite appear to be misidentified.

Calcineurin target CrzA regulates conidial germination, hyphal growth, and pathogenesis of Aspergillus fumigatus

The calcineurin pathway is a critical signal transduction pathway in fungi that mediates growth, morphology, stress responses, and pathogenicity. The importance of the calcineurin pathway in fungal physiology creates an opportunity for the development of new antifungal therapies that target this critical signaling pathway. In this study, we examined the role of the zinc finger transcription factor Crz1 homolog (CrzA) in the physiology and pathogenicity of the opportunistic human fungal pathogen Aspergillus fumigatus. Genetic replacement of the crzA locus in A. fumigatus resulted in a strain with significant defects in conidial germination, polarized hyphal growth, cell wall structure, and asexual development that are similar to but with differences from defects seen in the A. fumigatus DeltacnaA (calcineurin A) strain. Like the DeltacnaA strain, the DeltacrzA strain was incapable of causing disease in an experimental persistently neutropenic inhalational murine model of invasive pulmonary aspergillosis. Our results suggest that CrzA is an important downstream effector of calcineurin that controls morphology in A. fumigatus, but additional downstream effectors that mediate calcineurin signal transduction are likely present in this opportunistic fungal pathogen. In addition, the importance of CrzA to the production of disease is critical, and thus CrzA is an attractive fungus-specific antifungal target for the treatment of invasive aspergillosis.

Aspergillus fumigatus: virulence genes in a street-smart mold

Infections with the filamentous fungus Aspergillus fumigatus are among the most devastating of the systemic mycoses. Unlike most primary pathogens, which possess virulence traits that developed in association with a host organism, evidence suggests that the virulence of A. fumigatus entails a collection of 'street-smart' attributes that have evolved to resist the adverse selection pressures encountered in decaying vegetation. These features enhance the overall competitiveness of the organism in its environmental niche but are also thought to promote growth and survival in a human host. Although many of the genes that are responsible for these characteristics do not fit into the classical definition of a virulence factor, they are nonetheless important to the pathogenesis of aspergillosis and may therefore provide novel opportunities for antifungal development.

Identification of the nonribosomal peptide synthetase gene responsible for bassianolide synthesis in wood-decaying fungus Xylaria sp. BCC1067

Intensive study of gene diversity of bioactive compounds in a wood-rot fungus, Xylaria sp. BCC1067, has made it possible to identify polyketides and nonribosomal peptides (NRPs) unaccounted for by conventional chemical screening methods. Here we report the complete nonribosomal peptide synthetase (NRPS) gene responsible for the biosynthesis of an NRP, bassianolide, using a genetic approach. Isolation of the bassianolide biosynthetic gene, nrpsxy, was achieved using degenerate primers specific to the adenylation domain of NRPS. The complete ORF of nrpsxy is 10.6 kb in length. Based on comparisons with other known NRPSs, the domain arrangement of NRPSXY is most likely to be C-A-T-C-A-M-T-T-C-R. The other ORF found upstream of nrpsxy, designated efxy, is 1.8 kb in length and shows high similarity to members of the major facilitator superfamily of transporters. Functional analysis of the nrpsxy gene was conducted by gene disruption, and the missing metabolite in the mutant was identified. Chemical analysis revealed the structure of the metabolite to be a cyclooctadepsipeptide, bassianolide, which has been found in other fungi. A bioassay of bassianolide revealed a wide range of biological activities other than insecticidal uses, which have been previously reported, thus making bassianolide an interesting candidate for future structural modification. This study is the first evidence for a gene involved in the biosynthesis of bassianolide.

In vivo analysis of Aspergillus fumigatus developmental gene expression determined by real-time reverse transcription-PCR

Very little is known about the developmental stages of Aspergillus fumigatus during invasive aspergillosis. We performed real-time reverse transcription-PCR analysis on lung samples from mice with invasive pulmonary aspergillosis to determine the expression of A. fumigatus genes that are expressed at specific stages of development. In established infection, A. fumigatus exhibited mRNA expression of genes specific to developmentally competent hyphae, such as stuA. In contrast, mRNA of genes expressed by conidia and precompetent hyphae was not detected. Many genes required for mycotoxin synthesis, including aspHS, gliP, mitF, and metAP, are known to be expressed by developmentally competent hyphae in vitro. Interestingly, each of these genes was expressed at significantly higher levels during invasive infection than in vitro. The expression of gliP mRNA in vitro was found to be highly dependent on culture conditions. Furthermore, gliP expression was found to be dependent on the transcription factor StuA both in vitro and in vivo. Therefore, developmentally competent hyphae predominate during established invasive infection, and many mycotoxin genes are expressed at high levels in vivo. These results highlight the importance of the evaluation of putative virulence factors expressed by competent hyphae and analysis of gene expression levels during invasive infection rather than in vitro alone.

What do we know about the role of gliotoxin in the pathobiology of Aspergillus fumigatus?

Gliotoxin is a member of the epipolythiodioxopiperazine class of toxins and is both the major and the most potent toxin produced by Aspergillus fumigatus. Since the discovery of the putative gliotoxin biosynthetic 12-gene cluster in the genome of A. fumigatus, five different laboratories have attempted to determine the role of this toxin in the virulence of A. fumigatus. The genes in the cluster that have been disrupted to study the pathobiological importance of gliotoxin include gliZ that encodes a transcription factor and gliP that encodes a nonribosomal peptide synthase. Two of the five laboratories have reported gliotoxin to be an important virulence determinant of A. fumigatus, while the other three laboratories have shown it to be unimportant. Comparisons of the data generated among the five laboratories revealed that the immunosuppressive regimen used for mice was the key factor that contributed to the observed disparity. Regardless of either the mouse strains used or the route of infection, immunosuppression with a combination of cyclophosphamide and corticosteroids (neutropenic mice) showed gliotoxin to be unimportant. The mice immunosuppressed with corticosteroids alone, however, revealed that gliotoxin is an important virulence determinant of A. fumigatus. These studies indicate that the neutropenic mice model is inadequate to reveal the pathobiological importance of fungal secondary metabolites in invasive pulmonary aspergillosis.

A Zn(II)2Cys6 DNA binding protein regulates the sirodesmin PL biosynthetic gene cluster in Leptosphaeria maculans

A gene, sirZ, encoding a Zn(II)(2)Cys(6) DNA binding protein is present in a cluster of genes responsible for the biosynthesis of the epipolythiodioxopiperazine (ETP) toxin, sirodesmin PL in the ascomycete plant pathogen, Leptosphaeria maculans. RNA-mediated silencing of sirZ gives rise to transformants that produce only residual amounts of sirodesmin PL and display a decrease in the transcription of several sirodesmin PL biosynthetic genes. This indicates that SirZ is a major regulator of this gene cluster. Proteins similar to SirZ are encoded in the gliotoxin biosynthetic gene cluster of Aspergillus fumigatus (gliZ) and in an ETP-like cluster in Penicillium lilacinoechinulatum (PlgliZ). Despite its high level of sequence similarity to gliZ, PlgliZ is unable to complement the gliotoxin-deficiency of a mutant of gliZ in A. fumigatus. Putative binding sites for these regulatory proteins in the promoters of genes in these clusters were predicted using bioinformatic analysis. These sites are similar to those commonly bound by other proteins with Zn(II)(2)Cys(6) DNA binding domains.

Gliotoxin production in Aspergillus fumigatus contributes to host-specific differences in virulence

BACKGROUND

Gliotoxin is a epipolythiodioxopiperazine toxin that is made by the filamentous fungus Aspergillus fumigatus. Gliotoxin has a wide range of effects on metazoan cells in culture, including induction of apoptosis through inhibition of Nf-kappaB, and inhibition of superoxide production by phagocytes. These activities have led to the proposal that gliotoxin contributes to pathogenesis during invasive aspergillosis. We tested this hypothesis by creating isogenic strains of gliotoxin-producing and nonproducing strains.

METHODS

We deleted gliP, the gene that encodes the nonribosomal peptide synthetase GliP. GliP catalyzes the first biosynthetic step in the synthesis of gliotoxin. We then tested for gliotoxin production and virulence in different animal models.

RESULTS

Deletion of gliP resulted in strains that were wild type for growth, but they did not synthesize gliotoxin. Transformation of gliP deletion mutants with a full copy of gliP restored gliotoxin production. The gliP deletion strain had attenuated virulence in nonneutropenic mice immunosuppressed with corticosteroids, but had normal virulence in neutropenic mice. It also had reduced virulence in a Drosophila melanogaster model.

CONCLUSIONS

Gliotoxin only contributes to the virulence of A. fumigatus in nonneutropenic mice and in fruit flies with functional phagocytes. These results suggest that the principal targets of gliotoxin are neutrophils or other phagocytes.

Nonribosomal peptide synthetases involved in the production of medically relevant natural products

Natural products biosynthesized wholly or in part by nonribosomal peptide synthetases (NRPSs) are some of the most important drugs currently used clinically for the treatment of a variety of diseases. Since the initial research into NRPSs in the early 1960s, we have gained considerable insights into the mechanism by which these enzymes assemble these natural products. This review will present a brief history of how the basic mechanistic steps of NRPSs were initially deciphered and how this information has led us to understand how nature modified these systems to generate the enormous structural diversity seen in nonribosomal peptides. This review will also briefly discuss how drug development and discovery are being influenced by what we have learned from nature about nonribosomal peptide biosynthesis.

Genomewide screening for genes associated with gliotoxin resistance and sensitivity in Saccharomyces cerevisiae

Gliotoxin (GT) is a secondary fungal metabolite with pleiotropic immunosuppressive properties that have been implicated in Aspergillus virulence. However, the mechanisms of GT cytotoxicity and its molecular targets in eukaryotic cells have not been fully characterized. We screened a haploid library of Saccharomyces cerevisiae single-gene deletion mutants (4,787 strains in EUROSCARF) to identify nonessential genes associated with GT increased resistance (GT-IR) and increased sensitivity (GT-IS). The susceptibility of the wild-type parental strain BY4741 to GT was initially assessed by broth microdilution methods using different media. GT-IR and GT-IS were defined as a fourfold increase and decrease, respectively, in MIC, and this was additionally confirmed by susceptibility testing on agar yeast extract-peptone-glucose plates. The specificity of GT-IR and GT-IS mutants exhibiting normal growth compared with the wild-type strain was further tested in studies of their susceptibility to conventional antifungal agents, cycloheximide, and H2O2. GT-IR was associated with the disruption of genes acting in general metabolism (OPI1, SNF1, IFA38), mitochondrial function (RTG2), DNA damage repair (RAD18), and vesicular transport (APL2) and genes of unknown function (YGL235W, YOR345C, YLR456W, YGL072C). The disruption of three genes encoding transsulfuration (CYS3), mitochondrial function (MEF2), and an unknown function (YKL037W) led to GT-IS. Specificity for GT-IR and GT-IS was observed in all mutants. Importantly, the majority (69%) of genes implicated in GT-IR (6/10) and GT-IS (2/3) have human homologs. We identified novel Saccharomyces genes specifically implicated in GT-IR or GT-IS. Because most of these genes are evolutionarily conserved, further characterization of their function could improve our understanding of GT cytotoxicity mechanisms in humans.

Identification of a hybrid PKS/NRPS required for pseurotin A biosynthesis in the human pathogen Aspergillus fumigatus

The genome sequence of Aspergillus fumigatus revealed the presence of a single hybrid polyketide synthase-non-ribosomal peptide synthetase (PKS/NRPS) gene that is present within a cluster of five genes suggestive of its involvement in secondary metabolism. Here, we present evidence that it is required for the biosynthesis of pseurotin A, a compound with an unusual heterospirocyclic gamma-lactam structure. We have confirmed that the genome reference strain A. fumigatus Af293 produces pseurotin A, a compound previously reported to be a competitive inhibitor of chitin synthase and an inducer of nerve-cell proliferation. Deletion or overexpression of the PKS/NRPS gene psoA in A. fumigatus leads to the absence or accumulation of pseurotin A, respectively; this indicates that this gene is essential for the biosynthesis of pseurotin A. It is likely that the first product of psoA is converted to pseurotin A by the products of other genes in this cluster.

Iron and siderophores in fungal-host interactions

Most fungi and bacteria express specific mechanisms for the acquisition of iron from the hosts they infect for their own survival. This is primarily because iron plays a key catalytic role in various vital cellular reactions in conjunction with the fact that iron is not freely available in these environments due to host sequestration. High-affinity iron uptake systems, such as siderophore-mediated iron uptake and reductive iron assimilation, enable fungi to acquire limited iron from animal or plant hosts. Regulating iron uptake is crucial to maintain iron homeostasis, a state necessary to avoid iron-induced toxicity from iron abundance, while simultaneously supplying iron required for biochemical demand. Siderophores play diverse roles in fungal-host interactions, many of which have been principally delineated from gene deletions in non-ribosomal peptide synthetases, enzymes required for siderophore biosynthesis. These analyses have demonstrated that siderophores are required for virulence, resistance to oxidative stress, asexual/sexual development, iron storage, and protection against iron-induced toxicity in some fungal organisms. In this review, the strategies fungi employ to obtain iron, siderophore biosynthesis, and the regulatory mechanisms governing iron homeostasis will be discussed with an emphasis on siderophore function and relevance for fungal organisms in their interactions with their hosts.