Gliotoxin effects on fungal growth: Mechanisms and exploitation

The Metabolite Profiling of Aspergillus fumigatus KMM4631 and Its Co-Cultures with Other Marine Fungi

An Aspergillus fumigatus KMM 4631 strain was previously isolated from a Pacific soft coral Sinularia sp. sample and was found to be a source of a number of bioactive secondary metabolites. The aims of this work are the confirmation of this strain' identification based on ITS, BenA, CaM, and RPB2 regions/gene sequences and the investigation of secondary metabolite profiles of Aspergillus fumigatus KMM 4631 culture and its co-cultures with Penicillium hispanicum KMM 4689, Amphichorda sp. KMM 4639, Penicillium sp. KMM 4672, and Asteromyces cruciatus KMM 4696 from the Collection of Marine Microorganisms (PIBOC FEB RAS, Vladivostok, Russia). Moreover, the DPPH-radical scavenging activity, urease inhibition, and cytotoxicity of joint fungal cultures' extracts on HepG2 cells were tested. The detailed UPLC MS qTOF investigation resulted in the identification and annotation of indolediketopiperazine, quinazoline, and tryptoquivaline-related alkaloids as well as a number of polyketides (totally 20 compounds) in the extract of Aspergillus fumigatus KMM 4631. The metabolite profiles of the co-cultures of A. fumigatus with Penicillium hispanicum, Penicillium sp., and Amphichorda sp. were similar to those of Penicillium hispanicum, Penicillium sp., and Amphichorda sp. monocultures. The metabolite profile of the co-culture of A. fumigatus with Asteromyces cruciatus differed from that of each monoculture and may be more promising for the isolation of new compounds.

Exploring the antibacterial action of gliotoxin: Does it induce oxidative stress or protein damage?

The study aimed to investigate the effects of gliotoxin (GTX), a secondary fungal metabolite belonging to the epipolythiodioxopiperazines class, on Gram-positive and Gram-negative bacteria. While the cytotoxic mechanism of GTX on eukaryotes is well understood, its interaction with bacteria is not yet fully comprehended. The study discovered that S. epidermidis displayed a higher uptake rate of GTX than E.coli. However, Gram-negative bacteria required higher doses of GTX than Gram-positive bacteria to experience the bactericidal effect, which occurred within 4 h for both types of bacteria. The treatment of bioluminescent sensor E.coli MG1655 pKatG-lux with GTX resulted in oxidative stress. Pre-incubation with the antioxidant Trolox did not increase the GTX inhibitory dose, however, slightly increased the bacterial growth rate comparing to GTX alone. At the same time, we found that GTX inhibitory dose was significantly increased by the pretreatment of bacteria with 2-mercaptoethanol and reduced glutathione. Using another biosensor, E. coli MG1655 pIpbA-lux, we showed that bacteria treated with GTX exhibited heat shock stress. SDS-page electrophoresis demonstrated protein aggregation under the GTX treatment. In addition, we have found that gliotoxin's action on bacteria was significantly inhibited when zinc salt was added to the growth medium.

Gliotoxin-mediated bacterial growth inhibition is caused by specific metal ion depletion

Overcoming antimicrobial resistance represents a formidable challenge and investigating bacterial growth inhibition by fungal metabolites may yield new strategies. Although the fungal non-ribosomal peptide gliotoxin (GT) is known to exhibit antibacterial activity, the mechanism(s) of action are unknown, although reduced gliotoxin (dithiol gliotoxin; DTG) is a zinc chelator. Furthermore, it has been demonstrated that GT synergises with vancomycin to inhibit growth of Staphylococcus aureus. Here we demonstrate, without precedent, that GT-mediated growth inhibition of both Gram positive and negative bacterial species is reversed by Zn2+ or Cu2+ addition. Both GT, and the known zinc chelator TPEN, mediate growth inhibition of Enterococcus faecalis which is reversed by zinc addition. Moreover, zinc also reverses the synergistic growth inhibition of E. faecalis observed in the presence of both GT and vancomycin (4 µg/ml). As well as zinc chelation, DTG also appears to chelate Cu2+, but not Mn2+ using a 4-(2-pyridylazo)resorcinol assay system and Zn2+ as a positive control. DTG also specifically reacts in Fe3+-containing Siderotec™ assays, most likely by Fe3+ chelation from test reagents. GSH or DTT show no activity in these assays. Confirmatory high resolution mass spectrometry, in negative ion mode, confirmed, for the first time, the presence of both Cu[DTG] and Fe[DTG]2 chelates. Label free quantitative proteomic analysis further revealed major intracellular proteomic remodelling within E. faecalis in response to GT exposure for 30-180 min. Globally, 4.2-7.2% of detectable proteins exhibited evidence of either unique presence/increased abundance or unique absence/decreased abundance (n = 994-1160 total proteins detected), which is the first demonstration that GT affects the bacterial proteome in general, and E. faecalis, specifically. Unique detection of components of the AdcABC and AdcA-II zinc uptake systems was observed, along with apparent ribosomal reprofiling to zinc-free paralogs in the presence of GT. Overall, we hypothesise that GT-mediated bacterial growth inhibition appears to involve intracellular zinc depletion or reduced bioavailability, and based on in vitro chelate formation, may also involve dysregulation of Cu2+ homeostasis.

Gliotoxin and related metabolites as zinc chelators: implications and exploitation to overcome antimicrobial resistance

Antimicrobial resistance (AMR) is a major global problem and threat to humanity. The search for new antibiotics is directed towards targeting of novel microbial systems and enzymes, as well as augmenting the activity of pre-existing antimicrobials. Sulphur-containing metabolites (e.g., auranofin and bacterial dithiolopyrrolones [e.g., holomycin]) and Zn2+-chelating ionophores (PBT2) have emerged as important antimicrobial classes. The sulphur-containing, non-ribosomal peptide gliotoxin, biosynthesised by Aspergillus fumigatus and other fungi exhibits potent antimicrobial activity, especially in the dithiol form (dithiol gliotoxin; DTG). Specifically, it has been revealed that deletion of the enzymes gliotoxin oxidoreductase GliT, bis-thiomethyltransferase GtmA or the transporter GliA dramatically sensitise A. fumigatus to gliotoxin presence. Indeed, the double deletion strain A. fumigatus ΔgliTΔgtmA is especially sensitive to gliotoxin-mediated growth inhibition, which can be reversed by Zn2+ presence. Moreover, DTG is a Zn2+ chelator which can eject zinc from enzymes and inhibit activity. Although multiple studies have demonstrated the potent antibacterial effect of gliotoxin, no mechanistic details are available. Interestingly, reduced holomycin can inhibit metallo-β-lactamases. Since holomycin and gliotoxin can chelate Zn2+, resulting in metalloenzyme inhibition, we propose that this metal-chelating characteristic of these metabolites requires immediate investigation to identify new antibacterial drug targets or to augment the activity of existing antimicrobials. Given that (i) gliotoxin has been shown in vitro to significantly enhance vancomycin activity against Staphylococcus aureus, and (ii) that it has been independently proposed as an ideal probe to dissect the central 'Integrator' role of Zn2+ in bacteria - we contend such studies are immediately undertaken to help address AMR.

Aspergillus fumigatus Supernatants Disrupt Bronchial Epithelial Monolayers: Potential Role for Enhanced Invasion in Cystic Fibrosis

Aspergillus fumigatus is the most commonly isolated fungus in chronic lung diseases, with a prevalence of up to 60% in cystic fibrosis patients. Despite this, the impact of A. fumigatus colonisation on lung epithelia has not been thoroughly explored. We investigated the influence of A. fumigatus supernatants and the secondary metabolite, gliotoxin, on human bronchial epithelial cells (HBE) and CF bronchial epithelial (CFBE) cells. CFBE (F508del CFBE41o^-) and HBE (16HBE14o^-) trans-epithelial electrical resistance (TEER) was measured following exposure to A. fumigatus reference and clinical isolates, a gliotoxin-deficient mutant (ΔgliG) and pure gliotoxin. The impact on tight junction (TJ) proteins, zonula occludens-1 (ZO-1) and junctional adhesion molecule-A (JAM-A) were determined by western blot analysis and confocal microscopy. A. fumigatus conidia and supernatants caused significant disruption to CFBE and HBE TJs within 24 h. Supernatants from later cultures (72 h) caused the greatest disruption while ΔgliG mutant supernatants caused no disruption to TJ integrity. The ZO-1 and JAM-A distribution in epithelial monolayers were altered by A. fumigatus supernatants but not by ΔgliG supernatants, suggesting that gliotoxin is involved in this process. The fact that ΔgliG conidia were still capable of disrupting epithelial monolayers indicates that direct cell-cell contact also plays a role, independently of gliotoxin production. Gliotoxin is capable of disrupting TJ integrity which has the potential to contribute to airway damage, and enhance microbial invasion and sensitisation in CF.

Ten decadal advances in fungal biology leading towards human well-being

Fungi are an understudied resource possessing huge potential for developing products that can greatly improve human well-being. In the current paper, we highlight some important discoveries and developments in applied mycology and interdisciplinary Life Science research. These examples concern recently introduced drugs for the treatment of infections and neurological diseases; application of -OMICS techniques and genetic tools in medical mycology and the regulation of mycotoxin production; as well as some highlights of mushroom cultivaton in Asia. Examples for new diagnostic tools in medical mycology and the exploitation of new candidates for therapeutic drugs, are also given. In addition, two entries illustrating the latest developments in the use of fungi for biodegradation and fungal biomaterial production are provided. Some other areas where there have been and/or will be significant developments are also included. It is our hope that this paper will help realise the importance of fungi as a potential industrial resource and see the next two decades bring forward many new fungal and fungus-derived products.

The Toxic Mechanism of Gliotoxins and Biosynthetic Strategies for Toxicity Prevention

Gliotoxin is a kind of epipolythiodioxopiperazine derived from different fungi that is characterized by a disulfide bridge. Gliotoxins can be biosynthesized by a gli gene cluster and regulated by a positive GliZ regulator. Gliotoxins show cytotoxic effects via the suppression the function of macrophage immune function, inflammation, antiangiogenesis, DNA damage by ROS production, peroxide damage by the inhibition of various enzymes, and apoptosis through different signal pathways. In the other hand, gliotoxins can also be beneficial with different doses. Low doses of gliotoxin can be used as an antioxidant, in the diagnosis and treatment of HIV, and as an anti-tumor agent in the future. Gliotoxins have also been used in the control of plant pathogens, including Pythium ultimum and Sclerotinia sclerotiorum. Thus, it is important to elucidate the toxic mechanism of gliotoxins. The toxic mechanism of gliotoxins and biosynthetic strategies to reduce the toxicity of gliotoxins and their producing strains are summarized in this review.

Aspergillus fumigatus versus Genus Aspergillus: Conservation, Adaptive Evolution and Specific Virulence Genes

Aspergillus is an important fungal genus containing economically important species, as well as pathogenic species of animals and plants. Using eighteen fungal species of the genus Aspergillus, we conducted a comprehensive investigation of conserved genes and their evolution. This also allows us to investigate the selection pressure driving the adaptive evolution in the pathogenic species A. fumigatus. Among single-copy orthologs (SCOs) for A. fumigatus and the closely related species A. fischeri, we identified 122 versus 50 positively selected genes (PSGs), respectively. Moreover, twenty conserved genes of unknown function were established to be positively selected and thus important for adaption. A. fumigatus PSGs interacting with human host proteins show over-representation of adaptive, symbiosis-related, immunomodulatory and virulence-related pathways, such as the TGF-β pathway, insulin receptor signaling, IL1 pathway and interfering with phagosomal GTPase signaling. Additionally, among the virulence factor coding genes, secretory and membrane protein-coding genes in multi-copy gene families, 212 genes underwent positive selection and also suggest increased adaptation, such as fungal immune evasion mechanisms (aspf2), siderophore biosynthesis (sidD), fumarylalanine production (sidE), stress tolerance (atfA) and thermotolerance (sodA). These genes presumably contribute to host adaptation strategies. Genes for the biosynthesis of gliotoxin are shared among all the close relatives of A. fumigatus as an ancient defense mechanism. Positive selection plays a crucial role in the adaptive evolution of A. fumigatus. The genome-wide profile of PSGs provides valuable targets for further research on the mechanisms of immune evasion, antimycotic targeting and understanding fundamental virulence processes.

Characterization of novel gliotoxin biosynthesis-related genes from deep-sea-derived fungus Geosmithia pallida FS140

Gliotoxins are epipolythiodioxopiperazine toxins produced by the filamentous fungi, which show great potential in the treatment of liver and lung cancer because of its cytotoxicity. In this study, three novel genes related to gliotoxin biosynthesis, gliT, gliM and gliK encoding thioredoxin reductase, O-methyltransferase and gamma-glutamyl cyclotransferase, respectively, from the deep-sea-derived fungus Geosmithia pallida were cloned from G. pallida and expressed in Escherichia coli. The recombinant GliT, GliM and GliK proteins were expressed and purified by Ni affinity column, which was demonstrated by SDS-PAGE and Western blot analysis. The inclusion bodies of GliT were renatured and the corresponding enzymatic properties of the two enzymes were further investigated. Using DTNB as a substrate, GliT showed the highest enzymatic activity of 11041 mU/L at pH 7.0, and the optimal reaction temperature was 40 °C. Using EGCG as a substrate, GliM showed the highest enzymatic activity of 239.19 mU/mg at pH 7.0, the optimum temperature was 35 °C. GliK from G. pallida was firstly reported to show bi-function of glutymal cyclotransferase and acetyltransfearse actvity with highest enzymatic activity of 615.5 U/mg in this study. The results suggested the important enzymatic function of GliT, GliM and GliK in the gliotoxin biosynthesis in G. pallida, which would lay a foundation for the mechanism elucidation of the gliotoxin biosynthesis in G. pallida and the exploitation of novel gliotoxin derivaties.

In Vitro and In Vivo Antibacterial Activity of Gliotoxin Alone and in Combination with Antibiotics against Staphylococcus aureus

Multidrug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) is one of the major causes of hospital-acquired and community infections and pose a challenge to the human health care system. Therefore, it is important to find new drugs that show activity against these bacteria, both in monotherapy and in combination with other antimicrobial drugs. Gliotoxin (GT) is a mycotoxin produced by Aspergillus fumigatus and other fungi of the Aspergillus genus. Some evidence suggests that GT shows antimicrobial activity against S. aureus in vitro, albeit its efficacy against multidrug-resistant strains such as MRSA or vancomycin-intermediate S. aureus (VISA) strainsis not known. This work aimed to evaluate the antibiotic efficacy of GT as monotherapy or in combination with other therapeutics against MRSA in vitro and in vivo using a Caenorhabditis elegans infection model.

The functions of glutathione peroxidase in ROS homeostasis and fruiting body development in Hypsizygus marmoreus

Glutathione peroxidase (GPX) is one of the most important antioxidant enzymes for maintaining reactive oxygen species (ROS) homeostasis. Although studies on fungi have suggested many important physiological functions of GPX, few studies have examined the role of this enzyme in Basidiomycetes, particularly its functions in fruiting body developmental processes. In the present study, GPX-silenced (GPxi) strains were obtained by using RNA interference. The GPxi strains of Hypsizygus marmoreus showed defects in mycelial growth and fruiting body development. In addition, the results indicated essential roles of GPX in controlling ROS homeostasis by regulating intracellular H₂O₂ levels, maintaining GSH/GSSG balance, and promoting antioxidant enzyme activity. Furthermore, lignocellulose enzyme activity levels were reduced and the mitochondrial phenotype and mitochondrial complex activity levels were changed in the H. marmoreus GPxi strains, possibly in response to impediments to mycelial growth and fruiting body development. These findings indicate that ROS homeostasis has a complex influence on growth, fruiting body development, GSH/GSSG balance, and carbon metabolism in H. marmoreus.Key points• ROS balance, energy metabolism, fruiting development.

Enlightening Gliotoxin Biological System in Agriculturally Relevant Trichoderma spp

Gliotoxin (GT) is a dual fungal secondary metabolite (SM). It displays pleiotropic activities and possesses medicinal properties and biocontrol abilities but, unfortunately, has toxic properties in humans. Various Trichoderma species are used as fungal biological control agents (BCAs), as a sustainable alternative for crop protection worldwide. Among them is Trichoderma virens, a GT-producing fungus. Since no information was available on the genetically coded prerequisites for the production of GT in other Trichoderma spp., genome analyses were carried out in 10 Trichoderma spp. genomes. Moreover, a real-time PCR assay setup ad hoc and high-performance liquid chromatography (HPLC) analyses were employed to understand the GT-producing biological systems in T. virens GV29-8 (TvGv29-8) and Trichoderma afroharzianum T6776 (TaT6776), two relevant biocontrol fungi. The structure of the GT biosynthesis genes (GT-BG) is polymorphic, with two distinct types associated with the ability to produce GT. GliH, a key protein for GT synthesis, is absent in most of the Trichoderma GT biosynthetic pathways, which may be the reason for their inability to produce GT. The GT-BG are expressed in TvGv29-8 as expected, while they are silent in TaT6776. Interestingly, in the GT-non-producing TaT6776, only gliA (putative GT transporter) and gtmA (putative GT S-methyltransferase) were induced by exogenous GT, underlining the ability of this strain to reduce the deleterious effect of the toxin. This ability is confirmed by growth assays and by the detection of the bis-thiomethylated form of GT catalyzed by GtmA in the culture medium supplemented with GT. To the best of our knowledge, this is the first general description of the GT biological system in different Trichoderma spp. as far as the GT-BG content and organization is concerned and a preliminary insight into their functionality.

The Aspergillus fumigatus Phosphoproteome Reveals Roles of High-Osmolarity Glycerol Mitogen-Activated Protein Kinases in Promoting Cell Wall Damage and Caspofungin Tolerance

Aspergillus fumigatus is an opportunistic human pathogen causing allergic reactions or systemic infections, such as invasive pulmonary aspergillosis in immunocompromised patients. The mitogen-activated protein kinase (MAPK) signaling pathways are essential for fungal adaptation to the human host. Fungal cell survival, fungicide tolerance, and virulence are highly dependent on the organization, composition, and function of the cell wall. Upon cell wall stress, MAPKs phosphorylate multiple target proteins involved in the remodeling of the cell wall. Here, we investigate the global phosphoproteome of the ΔsakA and ΔmpkCA. fumigatus and high-osmolarity glycerol (HOG) pathway MAPK mutants upon cell wall damage. This showed the involvement of the HOG pathway and identified novel protein kinases and transcription factors, which were confirmed by fungal genetics to be involved in promoting tolerance of cell wall damage. Our results provide understanding of how fungal signal transduction networks modulate the cell wall. This may also lead to the discovery of new fungicide drug targets to impact fungal cell wall function, fungicide tolerance, and virulence.

The filamentous fungus Aspergillus fumigatus can cause a distinct set of clinical disorders in humans. Invasive aspergillosis (IA) is the most common life-threatening fungal disease of immunocompromised humans. The mitogen-activated protein kinase (MAPK) signaling pathways are essential to the adaptation to the human host. Fungal cell survival is highly dependent on the organization, composition, and function of the cell wall. Here, an evaluation of the global A. fumigatus phosphoproteome under cell wall stress caused by the cell wall-damaging agent Congo red (CR) revealed 485 proteins potentially involved in the cell wall damage response. Comparative phosphoproteome analyses with the ΔsakA, ΔmpkC, and ΔsakA ΔmpkC mutant strains from the osmotic stress MAPK cascades identify their additional roles during the cell wall stress response. Our phosphoproteomics allowed the identification of novel kinases and transcription factors (TFs) involved in osmotic stress and in the cell wall integrity (CWI) pathway. Our global phosphoproteome network analysis showed an enrichment for protein kinases, RNA recognition motif domains, and the MAPK signaling pathway. In contrast to the wild-type strain, there is an overall decrease of differentially phosphorylated kinases and phosphatases in ΔsakA, ΔmpkC, and ΔsakA ΔmpkC mutants. We constructed phosphomutants for the phosphorylation sites of several proteins differentially phosphorylated in the wild-type and mutant strains. For all the phosphomutants, there is an increase in the sensitivity to cell wall-damaging agents and a reduction in the MpkA phosphorylation upon CR stress, suggesting these phosphosites could be important for the MpkA modulation and CWI pathway regulation.

Involvement of Sulfur in the Biosynthesis of Essential Metabolites in Pathogenic Fungi of Animals, Particularly Aspergillus spp.: Molecular and Therapeutic Implications

Fungal sulfur uptake is required for incorporation into the sidechains of the amino acids cysteine and methionine, and is also essential for the biosynthesis of the antioxidant glutathione (GSH), S-adenosylmethionine (SAM), the key source of methyl groups in cellular transmethylation reactions, and S-adenosylhomocysteine (SAH). Biosynthesis of redox-active gliotoxin in the opportunistic fungal pathogen Aspergillus fumigatus has been elucidated over the past 10 years. Some fungi which produce gliotoxin-like molecular species have undergone unexpected molecular rewiring to accommodate this high-risk biosynthetic process. Specific disruption of gliotoxin biosynthesis, via deletion of gliK, which encodes a γ-glutamyl cyclotransferase, leads to elevated intracellular antioxidant, ergothioneine (EGT), levels, and confirms crosstalk between the biosynthesis of both sulfur-containing moieties. Gliotoxin is ultimately formed by gliotoxin oxidoreductase GliT-mediated oxidation of dithiol gliotoxin (DTG). In fact, DTG is a substrate for both GliT and a bis-thiomethyltransferase, GtmA. GtmA converts DTG to bisdethiobis(methylthio)gliotoxin (BmGT), using 2 mol SAM and resultant SAH must be re-converted to SAM via the action of the Methyl/Met cycle. In the absence of GliT, DTG fluxes via GtmA to BmGT, which results in both SAM depletion and SAH overproduction. Thus, the negative regulation of gliotoxin biosynthesis via GtmA must be counter-balanced by GliT activity to avoid Methyl/Met cycle dysregulation, SAM depletion and trans consequences on global cellular biochemistry in A. fumigatus. DTG also possesses potent Zn²⁺ chelation properties which positions this sulfur-containing metabolite as a putative component of the Zn²⁺ homeostasis system within fungi. EGT plays an essential role in high-level redox homeostasis and its presence requires significant consideration in future oxidative stress studies in pathogenic filamentous fungi. In certain filamentous fungi, sulfur is additionally indirectly required for the formation of EGT and the disulfide-bridge containing non-ribosomal peptide, gliotoxin, and related epipolythiodioxopiperazines. Ultimately, interference with emerging sulfur metabolite functionality may represent a new strategy for antifungal drug development.

Non-lipopeptide fungi-derived peptide antibiotics developed since 2000

The 2,5-diketopiperazines (DKPs) are the smallest cyclopeptides and their basic structure includes a six-membered piperazine nucleus. Typical peptides lack a special functional group in the oligopeptide nucleus. Both are produced by at least 35 representative genera of fungi, and possess huge potential as pharmaceutical drugs and biocontrol agents. To date, only cyclosporin A has been developed into a commercial product. This review summarises 186 fungi-derived compounds reported since 2000. Antibiotic (antibacterial, antifungal, synergistic antifungal, antiviral, antimycobacterial, antimalarial, antileishmanial, insecticidal, antitrypanosomal, nematicidal and antimicroalgal) activities are discussed for 107 of them, including 66 DKPs (14 epipolythiodioxopiperazines, 20 polysulphide bridge-free thiodiketopiperazines, and 32 sulphur-free prenylated indole DKPs), 15 highly N-methylated, and 26 non-highly N-methylated typical peptides. Structure-activity relationships, mechanisms of action, and research methods are covered in detail. Additionally, biosynthases of tardioxopiperazines and neoechinulins are highlighted. These compounds have attracted considerable interest within the pharmaceutical and agrochemical industries.

De Novo Transcriptome Sequencing of the Deep-Sea-Derived Fungus Dichotomomyces cejpii and Analysis of Gliotoxin Biosynthesis Genes

Gliotoxin, produced by fungi, is an epipolythiodioxopiperazine (ETP) toxin with bioactivities such as anti-liver fibrosis, antitumor, antifungus, antivirus, antioxidation, and immunoregulation. Recently, cytotoxic gliotoxins were isolated from a deep-sea-derived fungus, Dichotomomyces cejpii. However, the biosynthetic pathway for gliotoxins in D. cejpii remains unclear. In this study, the transcriptome of D. cejpii was sequenced using an Illumina Hiseq 2000. A total of 19,125 unigenes for D. cejpii were obtained from 9.73 GB of clean reads. Ten genes related to gliotoxin biosynthesis were annotated. The expression levels of gliotoxin-related genes were detected through quantitative real-time polymerase chain reaction (qRT-PCR). The GliG gene, encoding a glutathione S-transferase (DC-GST); GliI, encoding an aminotransferase (DC-AI); and GliO, encoding an aldehyde reductase (DC-AR), were cloned and expressed, purified, and characterized. The results suggested the important roles of DC-GST, DC-AT, and DC-AR in the biosynthesis of gliotoxins. Our study on the genes related to gliotoxin biosynthesis establishes a molecular foundation for the wider application of gliotoxins from D. cejpii in the biomedical industry in the future.

To Construct an Engineered (S)-Equol Resistant E. coli for in Vitro (S)-Equol Production

(S)-equol is one of the major metabolites of daidzein that is produced by human and animal gut bacteria. Most of the physiological functions of soybean isoflavones, such as anti-oxidative activity, anti-cancer activity, and cardiovascular protection have been ascribed to (S)-equol. However, only 30–50% people contain this kind of equol-producing bacteria, and therefore are able to convert daidzein to (S)-equol. Administration of (S)-equol may be more beneficial than soybean isoflavones. The aim of this study was to construct an engineered (S)-equol resistant Escherichia coli to enhance (S)-equol production in vitro. First, transposon mutagenesis libraries were constructed and screened to isolate the (S)-equol resistant mutant E. coli strain BL21 (ydiS) in order to overcome the inhibitory effects of (S)-equol on bacterial growth. Bacterial full genome scan sequencing and in vitro overexpression results revealed that the ydiS gene was responsible for this resistance. Second, the (S)-equol-producing genes L-dznr, L-ddrc, L-dhdr, and L-thdr of Lactococcus strain 20–92 were synthesized and cloned into compatible vectors, pETDuet-1 and pCDFDuet-1. These plasmids were subsequently transformed into BL21 (DE3) and its mutant BL21 (ydiS). Both engineered BL21 (DE3) and BL21 (ydiS) could use daidzein as substrate to produce (S)-equol under both anaerobic and aerobic conditions. As expected, engineered BL21 (ydiS) had faster growth rates than BL21 (DE3) when supplemented with high concentrations of (S)-equol. The yield and the daidzein utilization ratio were higher for engineered BL21 (ydiS). Interestingly, engineered BL21 (ydiS) was able to convert daidzein to (S)-equol efficiently under aerobic conditions, providing a convenient method for (S)-equol production in vitro. In addition, a two-step method was developed to produce (S)-equol using daidzin as substrate.

Mitogen activated protein kinases (MAPK) and protein phosphatases are involved in Aspergillus fumigatus adhesion and biofilm formation

The main characteristic of biofilm formation is extracellular matrix (ECM) production. The cells within the biofilm are surrounded by ECM which provides structural integrity and protection. During an infection, this protection is mainly against cells of the immune system and antifungal drugs. A. fumigatus forms biofilms during static growth on a solid substratum and in chronic aspergillosis infections. It is important to understand how, and which, A. fumigatus signal transduction pathways are important for the adhesion and biofilm formation in a host during infection. Here we investigated the role of MAP kinases and protein phosphatases in biofilm formation. The loss of the MAP kinases MpkA, MpkC and SakA had an impact on the cell surface and the ECM during biofilm formation and reduced the adherence of A. fumigatus to polystyrene and fibronectin-coated plates. The phosphatase null mutants ΔsitA and ΔptcB, involved in regulation of MpkA and SakA phosphorylation, influenced cell wall carbohydrate exposure. Moreover, we characterized the A. fumigatus protein phosphatase PphA. The ΔpphA strain was more sensitive to cell wall-damaging agents, had increased β-(1,3)-glucan and reduced chitin, decreased conidia phagocytosis by Dictyostelium discoideum and reduced adhesion and biofilm formation. Finally, ΔpphA strain was avirulent in a murine model of invasive pulmonary aspergillosis and increased the released of tumor necrosis factor alpha (TNF-α) from bone marrow derived macrophages (BMDMs). These results show that MAP kinases and phosphatases play an important role in signaling pathways that regulate the composition of the cell wall, extracellular matrix production as well as adhesion and biofilm formation in A. fumigatus.

CoIN: co-inducible nitrate expression system for secondary metabolites in Aspergillus nidulans

BACKGROUND

Sequencing of fungal species has demonstrated the existence of thousands of putative secondary metabolite gene clusters, the majority of them harboring a unique set of genes thought to participate in production of distinct small molecules. Despite the ready identification of key enzymes and potential cluster genes by bioinformatics techniques in sequenced genomes, the expression and identification of fungal secondary metabolites in the native host is often hampered as the genes might not be expressed under laboratory conditions and the species might not be amenable to genetic manipulation. To overcome these restrictions, we developed an inducible expression system in the genetic model Aspergillus nidulans.

RESULTS

We genetically engineered a strain of A. nidulans devoid of producing eight of the most abundant endogenous secondary metabolites to express the sterigmatocystin Zn(II)2Cys6 transcription factor-encoding gene aflR and its cofactor aflS under control of the nitrate inducible niiA/niaD promoter. Furthermore, we identified a subset of promoters from the sterigmatocystin gene cluster that are under nitrate-inducible AflR/S control in our production strain in order to yield coordinated expression without the risks from reusing a single inducible promoter. As proof of concept, we used this system to produce β-carotene from the carotenoid gene cluster of Fusarium fujikuroi.

CONCLUSION

Utilizing one-step yeast recombinational cloning, we developed an inducible expression system in the genetic model A. nidulans and show that it can be successfully used to produce commercially valuable metabolites.

Systems impact of zinc chelation by the epipolythiodioxopiperazine dithiol gliotoxin in Aspergillus fumigatus: a new direction in natural product functionality

Dithiol gliotoxin (DTG) is a zinc chelator and an inability to dissipate DTG in Aspergillus fumigatus is associated with multiple impacts which are linked to zinc chelation.

Dysregulated gliotoxin biosynthesis attenuates the production of unrelated biosynthetic gene cluster-encoded metabolites in Aspergillus fumigatus

Gliotoxin is an epipolythiodioxopiperazine (ETP) class toxin, contains a disulfide bridge that mediates its toxic effects via redox cycling and is produced by the opportunistic fungal pathogen Aspergillus fumigatus. The gliotoxin bis-thiomethyltransferase, GtmA, attenuates gliotoxin biosynthesis in A. fumigatus by conversion of dithiol gliotoxin to bis-thiomethylgliotoxin (BmGT). Here we show that disruption of dithiol gliotoxin bis-thiomethylation functionality in A. fumigatus results in significant remodelling of the A. fumigatus secondary metabolome upon extended culture. RP-HPLC and LC-MS/MS analysis revealed the reduced production of a plethora of unrelated biosynthetic gene cluster-encoded metabolites, including pseurotin A, fumagillin, fumitremorgin C and tryprostatin B, occurs in A. fumigatus ΔgtmA upon extended incubation. Parallel quantitative proteomic analysis of A. fumigatus wild-type and ΔgtmA during extended culture revealed cognate abundance alteration of proteins encoded by relevant biosynthetic gene clusters, allied to multiple alterations in hypoxia-related proteins. The data presented herein reveal a previously concealed functionality of GtmA in facilitating the biosynthesis of other BGC-encoded metabolites produced by A. fumigatus.

Structural, mechanistic and functional insight into gliotoxin bis-thiomethylation in Aspergillus fumigatus

Gliotoxin is an epipolythiodioxopiperazine (ETP) class toxin, contains a disulfide bridge that mediates its toxic effects via redox cycling and is produced by the opportunistic fungal pathogen Aspergillus fumigatus Self-resistance against gliotoxin is effected by the gliotoxin oxidase GliT, and attenuation of gliotoxin biosynthesis is catalysed by gliotoxin S-methyltransferase GtmA. Here we describe the X-ray crystal structures of GtmA-apo (1.66 Å), GtmA complexed to S-adenosylhomocysteine (1.33 Å) and GtmA complexed to S-adenosylmethionine (2.28 Å), providing mechanistic insights into this important biotransformation. We further reveal that simultaneous elimination of the ability of A. fumigatus to dissipate highly reactive dithiol gliotoxin, via deletion of GliT and GtmA, results in the most significant hypersensitivity to exogenous gliotoxin observed to date. Indeed, quantitative proteomic analysis of ΔgliT::ΔgtmA reveals an uncontrolled over-activation of the gli-cluster upon gliotoxin exposure. The data presented herein reveal, for the first time, the extreme risk associated with intracellular dithiol gliotoxin biosynthesis-in the absence of an efficient dismutation capacity. Significantly, a previously concealed protective role for GtmA and functionality of ETP bis-thiomethylation as an ancestral protection strategy against dithiol compounds is now evident.

Global gene expression reveals stress-responsive genes in Aspergillus fumigatus mycelia

Background

Aspergillus fumigatus is a human fungal pathogen that causes aspergillosis in immunocompromised hosts. A. fumigatus is believed to be exposed to diverse environmental stresses in the host cells. The adaptation mechanisms are critical for infections in human bodies. Transcriptional networks in response to diverse environmental challenges remain to be elucidated. To gain insights into the adaptation to environmental stresses in A. fumigatus mycelia, we conducted time series transcriptome analyses.

Results

With the aid of RNA-seq, we explored the global gene expression profiles of mycelia in A. fumigatus upon exposure to diverse environmental changes, including heat, superoxide, and osmotic stresses. From the perspective of global transcriptomes, transient responses to superoxide and osmotic stresses were observed while responses to heat stresses were gradual. We identified the stress-responsive genes for particular stresses, and the 266 genes whose expression levels drastically fluctuated upon exposure to all tested stresses. Among these, the 77 environmental stress response genes are conserved in S. cerevisiae, suggesting that these genes might be more general prerequisites for adaptation to environmental stresses. Finally, we revealed the strong correlations among expression profiles of genes related to ‘rRNA processing’.

Conclusions

The time series transcriptome analysis revealed the stress-responsive genes underlying the adaptation mechanisms in A. fumigatus mycelia. These results will shed light on the regulatory networks underpinning the adaptation of the filamentous fungi.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4316-z) contains supplementary material, which is available to authorized users.

Ergothioneine Biosynthesis and Functionality in the Opportunistic Fungal Pathogen, Aspergillus fumigatus

Ergothioneine (EGT; 2-mercaptohistidine trimethylbetaine) is a trimethylated and sulphurised histidine derivative which exhibits antioxidant properties. Here we report that deletion of Aspergillus fumigatus egtA (AFUA_2G15650), which encodes a trimodular enzyme, abrogated EGT biosynthesis in this opportunistic pathogen. EGT biosynthetic deficiency in A. fumigatus significantly reduced resistance to elevated H₂O₂ and menadione, respectively, impaired gliotoxin production and resulted in attenuated conidiation. Quantitative proteomic analysis revealed substantial proteomic remodelling in ΔegtA compared to wild-type under both basal and ROS conditions, whereby the abundance of 290 proteins was altered. Specifically, the reciprocal differential abundance of cystathionine γ-synthase and β-lyase, respectively, influenced cystathionine availability to effect EGT biosynthesis. A combined deficiency in EGT biosynthesis and the oxidative stress response regulator Yap1, which led to extreme oxidative stress susceptibility, decreased resistance to heavy metals and production of the extracellular siderophore triacetylfusarinine C and increased accumulation of the intracellular siderophore ferricrocin. EGT dissipated H₂O₂ in vitro, and elevated intracellular GSH levels accompanied abrogation of EGT biosynthesis. EGT deficiency only decreased resistance to high H₂O₂ levels which suggests functionality as an auxiliary antioxidant, required for growth at elevated oxidative stress conditions. Combined, these data reveal new interactions between cellular redox homeostasis, secondary metabolism and metal ion homeostasis.

Exploration of Sulfur Assimilation of Aspergillus fumigatus Reveals Biosynthesis of Sulfur-Containing Amino Acids as a Virulence Determinant

Fungal infections are of major relevance due to the increased numbers of immunocompromised patients, frequently delayed diagnosis, and limited therapeutics. To date, the growth and nutritional requirements of fungi during infection, which are relevant for invasion of the host, are poorly understood. This is particularly true for invasive pulmonary aspergillosis, as so far, sources of (macro)elements that are exploited during infection have been identified to only a limited extent. Here, we have investigated sulfur (S) utilization by the human-pathogenic mold Aspergillus fumigatus during invasive growth. Our data reveal that inorganic S compounds or taurine is unlikely to serve as an S source during invasive pulmonary aspergillosis since a sulfate transporter mutant strain and a sulfite reductase mutant strain are fully virulent. In contrast, the S-containing amino acid cysteine is limiting for fungal growth, as proven by the reduced virulence of a cysteine auxotroph. Moreover, phenotypic characterization of this strain further revealed the robustness of the subordinate glutathione redox system. Interestingly, we demonstrate that methionine synthase is essential for A. fumigatus virulence, defining the biosynthetic route of this proteinogenic amino acid as a potential antifungal target. In conclusion, we provide novel insights into the nutritional requirements ofA. fumigatus during pathogenesis, a prerequisite to understanding and fighting infection.

Mitogen activated protein kinases SakA(HOG1) and MpkC collaborate for Aspergillus fumigatus virulence

Here, we investigated which stress responses were influenced by the MpkC and SakA mitogen-activated protein kinases of the high-osmolarity glycerol (HOG) pathway in the fungal pathogen Aspergillus fumigatus. The ΔsakA and the double ΔmpkC ΔsakA mutants were more sensitive to osmotic and oxidative stresses, and to cell wall damaging agents. Both MpkC::GFP and SakA::GFP translocated to the nucleus upon osmotic stress and cell wall damage, with SakA::GFP showing a quicker response. The phosphorylation state of MpkA was determined post exposure to high concentrations of congo red and Sorbitol. In the wild-type strain, MpkA phosphorylation levels progressively increased in both treatments. In contrast, the ΔsakA mutant had reduced MpkA phosphorylation, and surprisingly, the double ΔmpkC ΔsakA had no detectable MpkA phosphorylation. A. fumigatus ΔsakA and ΔmpkC were virulent in mouse survival experiments, but they had a 40% reduction in fungal burden. In contrast, the ΔmpkC ΔsakA double mutant showed highly attenuated virulence, with approximately 50% mice surviving and a 75% reduction in fungal burden. We propose that both cell wall integrity (CWI) and HOG pathways collaborate, and that MpkC could act by modulating SakA activity upon exposure to several types of stresses and during CW biosynthesis.

Towards understanding the gliotoxin detoxification mechanism: in vivo thiomethylation protects yeast from gliotoxin cytotoxicity

Gliotoxin (GT) is a mycotoxin produced by some species of ascomycete fungi including the opportunistic human pathogen Aspergillus fumigatus. In order to produce GT the host organism needs to have evolved a self-protection mechanism. GT contains a redox-cycling disulfide bridge that is important in mediating toxicity. Recently is has been demonstrated that A. fumigatus possesses a novel thiomethyltransferase protein called GtmA that has the ability to thiomethylate GT in vivo, which aids the organism in regulating GT biosynthesis. It has been suggested that thiomethylation of GT and similar sulfur-containing toxins may play a role in providing self-protection in host organisms. In this work we have engineered Saccharomyces cerevisiae, a GT-naïve organism, to express A. fumigatus GtmA. We demonstrate that GtmA can readily thiomethylate GT in yeast, which results in protection of the organism from exogenous GT. Our work has implications for understanding the evolution of GT self-protection mechanisms in organisms that are GT producers and non-producers.

Translating biosynthetic gene clusters into fungal armor and weaponry

Filamentous fungi are renowned for the production of a diverse array of secondary metabolites (SMs) where the genetic material required for synthesis of a SM is typically arrayed in a biosynthetic gene cluster (BGC). These natural products are valued for their bioactive properties stemming from their functions in fungal biology, key among those protection from abiotic and biotic stress and establishment of a secure niche. The producing fungus must not only avoid self-harm from endogenous SMs but also deliver specific SMs at the right time to the right tissue requiring biochemical aid. This review highlights functions of BGCs beyond the enzymatic assembly of SMs, considering the timing and location of SM production and other proteins in the clusters that control SM activity. Specifically, self-protection is provided by both BGC-encoded mechanisms and non-BGC subcellular containment of toxic SM precursors; delivery and timing is orchestrated through cellular trafficking patterns and stress- and developmental-responsive transcriptional programs.

Redundant synthesis of a conidial polyketide by two distinct secondary metabolite clusters in Aspergillus fumigatus

Filamentous fungi are renowned for the production of bioactive secondary metabolites. Typically, one distinct metabolite is generated from a specific secondary metabolite cluster. Here, we characterize the newly described trypacidin (tpc) cluster in the opportunistic human pathogen Aspergillus fumigatus. We find that this cluster as well as the previously characterized endocrocin (enc) cluster both contribute to the production of the spore metabolite endocrocin. Whereas trypacidin is eliminated when only tpc cluster genes are deleted, endocrocin production is only eliminated when both the tpc and enc non-reducing polyketide synthase-encoding genes, tpcC and encA, respectively, are deleted. EncC, an anthrone oxidase, converts the product released from EncA to endocrocin as a final product. In contrast, endocrocin synthesis by the tpc cluster likely results from incomplete catalysis by TpcK (a putative decarboxylase), as its deletion results in a nearly 10-fold increase in endocrocin production. We suggest endocrocin is likely a shunt product in all related non-reducing polyketide synthase clusters containing homologues of TpcK and TpcL (a putative anthrone oxidase), e.g. geodin and monodictyphenone. This finding represents an unusual example of two physically discrete secondary metabolite clusters generating the same natural product in one fungal species by distinct routes.

Systematic Global Analysis of Genes Encoding Protein Phosphatases in Aspergillus fumigatus

Aspergillus fumigatus is a fungal pathogen that causes several invasive and noninvasive diseases named aspergillosis. This disease is generally regarded as multifactorial, considering that several pathogenicity determinants are present during the establishment of this illness. It is necessary to obtain an increased knowledge of how, and which, A. fumigatus signal transduction pathways are engaged in the regulation of these processes. Protein phosphatases are essential to several signal transduction pathways. We identified 32 phosphatase catalytic subunit-encoding genes in A. fumigatus, of which we were able to construct 24 viable deletion mutants. The role of nine phosphatase mutants in the HOG (high osmolarity glycerol response) pathway was evaluated by measuring phosphorylation of the p38 MAPK (SakA) and expression of osmo-dependent genes. We were also able to identify 11 phosphatases involved in iron assimilation, six that are related to gliotoxin resistance, and three implicated in gliotoxin production. These results present the creation of a fundamental resource for the study of signaling in A. fumigatus and its implications in the regulation of pathogenicity determinants and virulence in this important pathogen.

Gliotoxin-producing endophytic Acremonium sp. from Zingiber officinale found antagonistic to soft rot pathogen Pythium myriotylum

Soft rot caused by Pythium sp. is a major cause of economic loss in ginger cultivation. Endophytic fungi isolated from Zingiber officinale were screened for its activity against the soft rot pathogen Pythium myriotylum. Among the isolates screened, an endophytic fungus which was identified as Acremonium sp. showed promising activity against the phytopathogen in dual culture. The selected fungus was cultured in large scale on solid rice media and was extracted with ethyl acetate. The crude extract was subjected to column chromatography and preparative HPLC to obtain the fraction with the antifungal activity. LC-QTOF-MS/MS analysis of this fraction done using water-acetonitrile gradient identified a mass of m/z 327 (M + H) corresponding to gliotoxin with specific fragments m/z 263, 245, 227, and 111. The result was reconfirmed in negative mode ionization. Gliotoxin is the major antagonistic peptide produced by the commercially used biocontrol agent, Trichoderma sp., which shows high antagonism against Pythium sp. The gliotoxin production by the isolated endophytic Acremonium sp. of Z. officinale shows the possible natural biocontrol potential of this endophytic fungus.

Resistance is not futile: gliotoxin biosynthesis, functionality and utility

Gliotoxin biosynthesis is encoded by the gli gene cluster in Aspergillus fumigatus. The biosynthesis of gliotoxin is influenced by a suite of transcriptionally-active regulatory proteins and a bis-thiomethyltransferase. A self-protection system against gliotoxin is present in A. fumigatus. Several additional metabolites are also produced via the gliotoxin biosynthetic pathway. Moreover, the biosynthesis of unrelated natural products appears to be influenced either by gliotoxin or by the activity of specific reactions within the biosynthetic pathway. The activity of gliotoxin against animal cells and fungi, often mediated by interference with redox homeostasis or protein modification, is revealing new metabolic interactions within eukaryotic systems. Nature has provided a most useful natural product with which to reveal some of its many molecular secrets.

Redox metabolites signal polymicrobial biofilm development via the NapA oxidative stress cascade in Aspergillus

BACKGROUND

Filamentous fungi and bacteria form mixed-species biofilms in nature and diverse clinical contexts. They secrete a wealth of redox-active small molecule secondary metabolites, which are traditionally viewed as toxins that inhibit growth of competing microbes.

RESULTS

Here, we report that these "toxins" can act as interspecies signals, affecting filamentous fungal development via oxidative stress regulation. Specifically, in coculture biofilms, Pseudomonas aeruginosa phenazine-derived metabolites differentially modulated Aspergillus fumigatus development, shifting from weak vegetative growth to induced asexual sporulation (conidiation) along a decreasing phenazine gradient. The A. fumigatus morphological shift correlated with the production of phenazine radicals and concomitant reactive oxygen species (ROS) production generated by phenazine redox cycling. Phenazine conidiation signaling was conserved in the genetic model A. nidulans and mediated by NapA, a homolog of AP-1-like bZIP transcription factor, which is essential for the response to oxidative stress in humans, yeast, and filamentous fungi. Expression profiling showed phenazine treatment induced a NapA-dependent response of the global oxidative stress metabolome, including the thioredoxin, glutathione, and NADPH-oxidase systems. Conidiation induction in A. nidulans by another microbial redox-active secondary metabolite, gliotoxin, also required NapA.

CONCLUSIONS

This work highlights that microbial redox metabolites are key signals for sporulation in filamentous fungi, which are communicated through an evolutionarily conserved eukaryotic stress response pathway. It provides a foundation for interspecies signaling in environmental and clinical biofilms involving bacteria and filamentous fungi.

Perturbations in small molecule synthesis uncovers an iron-responsive secondary metabolite network in Aspergillus fumigatus

Iron plays a critical role in survival and virulence of the opportunistic pathogen Aspergillus fumigatus. Two transcription factors, the GATA-factor SreA and the bZip-factor HapX oppositely monitor iron homeostasis with HapX activating iron acquisition pathways (e.g., siderophores) and shutting down iron consumptive pathways (and SreA) during iron starvation conditions whereas SreA negatively regulates HapX and corresponding pathways during iron sufficiency. Recently the non-ribosomal peptide, hexadehydroastechrome (HAS; a tryptophan-derived iron (III)-complex), has been found important in A. fumigatus virulence. We found that HAS overproduction caused an iron starvation phenotype, from alteration of siderophore pools to regulation of iron homeostasis gene expression including sreA. Moreover, we uncovered an iron dependent secondary metabolism network where both SreA and HapX oppositely regulate multiple other secondary metabolites including HAS. This circuitry links iron-acquisition and consumption pathways with secondary metabolism-thus placing HAS as part of a metabolic feedback circuitry designed to balance iron pools in the fungus and presenting iron availability as one environmental trigger of secondary metabolism.

RNA-seq reveals the pan-transcriptomic impact of attenuating the gliotoxin self-protection mechanism in Aspergillus fumigatus

Background

Aspergillus fumigatus produces a number of secondary metabolites, one of which, gliotoxin, has been shown to exhibit anti-fungal activity. Thus, A. fumigatus must be able to protect itself against gliotoxin. Indeed one of the genes in the gliotoxin biosynthetic gene cluster in A. fumigatus, gliT, is required for self-protection against the toxin- however the global self-protection mechanism deployed is unclear. RNA-seq was employed to identify genes differentially regulated upon exposure to gliotoxin in A. fumigatus wild-type and A. fumigatus ∆gliT, a strain that is hypersensitive to gliotoxin.

Results

Deletion of A. fumigatus gliT resulted in altered expression of 208 genes (log₂ fold change of 1.5) when compared to A. fumigatus wild-type, of which 175 genes were up-regulated and 33 genes were down-regulated. Expression of 164 genes was differentially regulated (log₂ fold change of 1.5) in A. fumigatus wild-type when exposed to gliotoxin, consisting of 101 genes with up-regulated expression and 63 genes with down-regulated expression. Interestingly, a much larger number of genes, 1700, were found to be differentially regulated (log₂ fold change of 1.5) in A. fumigatus ∆gliT when challenged with gliotoxin. These consisted of 508 genes with up-regulated expression, and 1192 genes with down-regulated expression. Functional Catalogue (FunCat) classification of differentially regulated genes revealed an enrichment of genes involved in both primary metabolic functions and secondary metabolism. Specifically, genes involved in gliotoxin biosynthesis, helvolic acid biosynthesis, siderophore-iron transport genes and also nitrogen metabolism genes and ribosome biogenesis genes underwent altered expression. It was confirmed that gliotoxin biosynthesis is induced upon exposure to exogenous gliotoxin, production of unrelated secondary metabolites is attenuated in A. fumigatus ∆gliT, while quantitative proteomic analysis confirmed disrupted translation in A. fumigatus ∆gliT challenged with exogenous gliotoxin.

Conclusions

This study presents the first global investigation of the transcriptional response to exogenous gliotoxin in A. fumigatus wild-type and the hyper-sensitive strain, ∆gliT. Our data highlight the global and extensive affects of exogenous gliotoxin on a sensitive strain devoid of a self-protection mechanism and infer that GliT functionality is required for the optimal biosynthesis of selected secondary metabolites in A. fumigatus.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-894) contains supplementary material, which is available to authorized users.

A proteomic approach to investigating gene cluster expression and secondary metabolite functionality in Aspergillus fumigatus

A combined proteomics and metabolomics approach was utilised to advance the identification and characterisation of secondary metabolites in Aspergillus fumigatus. Here, implementation of a shotgun proteomic strategy led to the identification of non-redundant mycelial proteins (n = 414) from A. fumigatus including proteins typically under-represented in 2-D proteome maps: proteins with multiple transmembrane regions, hydrophobic proteins and proteins with extremes of molecular mass and pI. Indirect identification of secondary metabolite cluster expression was also achieved, with proteins (n = 18) from LaeA-regulated clusters detected, including GliT encoded within the gliotoxin biosynthetic cluster. Biochemical analysis then revealed that gliotoxin significantly attenuates H2O2-induced oxidative stress in A. fumigatus (p>0.0001), confirming observations from proteomics data. A complementary 2-D/LC-MS/MS approach further elucidated significantly increased abundance (p<0.05) of proliferating cell nuclear antigen (PCNA), NADH-quinone oxidoreductase and the gliotoxin oxidoreductase GliT, along with significantly attenuated abundance (p<0.05) of a heat shock protein, an oxidative stress protein and an autolysis-associated chitinase, when gliotoxin and H2O2 were present, compared to H2O2 alone. Moreover, gliotoxin exposure significantly reduced the abundance of selected proteins (p<0.05) involved in de novo purine biosynthesis. Significantly elevated abundance (p<0.05) of a key enzyme, xanthine-guanine phosphoribosyl transferase Xpt1, utilised in purine salvage, was observed in the presence of H2O2 and gliotoxin. This work provides new insights into the A. fumigatus proteome and experimental strategies, plus mechanistic data pertaining to gliotoxin functionality in the organism.

Extensive proteomic remodeling is induced by eukaryotic translation elongation factor 1Bγ deletion in Aspergillus fumigatus

The opportunistic pathogen Aspergillus fumigatus is ubiquitous in the environment and predominantly infects immunocompromised patients. The functions of many genes remain unknown despite sequencing of the fungal genome. A putative translation elongation factor 1Bγ (eEF1Bγ, termed elfA; 750 bp) is expressed, and exhibits glutathione S-transferase activity, in A. fumigatus. Here, we demonstrate the role of ElfA in the oxidative stress response, as well as a possible involvement in translation and actin cytoskeleton organization, respectively. Comparative proteomics, in addition to phenotypic analysis, under basal and oxidative stress conditions, demonstrated a role for A. fumigatus elfA in the oxidative stress response. An elfA-deficient strain (A. fumigatus ΔelfA) was significantly more sensitive to the oxidants H2O2, diamide, and 4,4'-dipyridyl disulfide (DPS) than the wild-type. This was further supported with the identification of differentially expressed proteins of the oxidative stress response, including; mitochondrial peroxiredoxin Prx1, molecular chaperone Hsp70 and mitochondrial glycerol-3-phosphate dehydrogenase. Phenotypic analysis also revealed that A. fumigatus ΔelfA was significantly more tolerant to voriconazole than the wild-type. The differential expression of two aminoacyl-tRNA synthetases suggests a role for A. fumigatus elfA in translation, while the identification of actin-bundling protein Sac6 and vacuolar dynamin-like GTPase VpsA link A. fumigatus elfA to the actin cytoskeleton. Overall, this work highlights the diverse roles of A. fumigatus elfA, with respect to translation, oxidative stress and actin cytoskeleton organization. In addition to this, the strategy of combining targeted gene deletion with comparative proteomics for elucidating the role of proteins of unknown function is further revealed.

Anticancer and antifungal compounds from Aspergillus, Penicillium and other filamentous fungi

This review covers important anticancer and antifungal compounds reported from filamentous fungi and in particular from Aspergillus, Penicillium and Talaromyces. The taxonomy of these fungi is not trivial, so a focus of this review has been to report the correct identity of the producing organisms based on substantial previous in-house chemotaxonomic studies.

RsmA regulates Aspergillus fumigatus gliotoxin cluster metabolites including cyclo(L-Phe-L-Ser), a potential new diagnostic marker for invasive aspergillosis

Dimeric basic leucine zipper (bZIP) proteins are conserved transcriptional enhancers found in all eukaryotes. A recently reported and novel function for bZIPs is association of these proteins with secondary metabolite production in filamentous fungi. In particular a Yap-like bZIP termed RsmA (restorer of secondary metabolism A) was identified in Aspergillus nidulans that positively regulates the carcinogen sterigmatocystin. To assess for conserved function for RsmA, we examined a role of this protein in secondary metabolism in the pathogen A. fumigatus. RsmA was found to positively regulate gliotoxin where overexpression (OE) of rsmA led to 2–100 fold increases of twelve gli cluster metabolites in culture medium including the newly identified gli metabolite cyclo(L-Phe-L-Ser). Lungs from both wild type and OErsmA infected mice contained gliotoxin (2.3 fold higher in OErsmA treatment) as well as the gliotoxin precursor cyclo(L-Phe-L-Ser) (3.2 fold higher in OErsmA treatment). The data here presents a conserved role for RsmA in secondary metabolite cluster activation and suggests cyclo(L-Phe-L-Ser) may serve as an alternative marker for diagnosis of invasive aspergillosis.

Disparate proteome responses of pathogenic and nonpathogenic aspergilli to human serum measured by activity-based protein profiling (ABPP)

Aspergillus fumigatus is the primary pathogen causing the devastating pulmonary disease Invasive Aspergillosis in immunocompromised individuals. There is high genomic synteny between A. fumigatus and closely related rarely pathogenic Neosartorya fischeri and Aspergillus clavatus genomes. We applied activity-based protein profiling to compare unique or overexpressed activity-based probe-reactive proteins of all three fungi over time in minimal media growth and in response to human serum. We found 360 probe-reactive proteins exclusive to A. fumigatus, including known virulence associated proteins, and 13 proteins associated with stress response exclusive to A. fumigatus culture in serum. Though the fungi are highly orthologous, A. fumigatus has a significantly greater number of ABP-reactive proteins across varied biological process. Only 50% of expected orthologs of measured A. fumigatus reactive proteins were observed in N. fischeri and A. clavatus. Activity-based protein profiling identified a number of processes that were induced by human serum in A. fumigatus relative to N. fischeri and A. clavatus. These included actin organization and assembly, transport, and fatty acid, cell membrane, and cell wall synthesis. Additionally, signaling proteins regulating vegetative growth, conidiation, and cell wall integrity, required for appropriate cellular response to external stimuli, had higher activity-based probe-protein reaction over time in A. fumigatus and N. fisheri, but not in A. clavatus. Together, we show that measured proteins and physiological processes identified solely or significantly over-represented in A. fumigatus reveal a unique adaptive response to human protein not found in closely related, but rarely pathogenic aspergilli. These unique activity-based probe-protein responses to culture condition may reveal how A. fumigatus initiates pulmonary invasion leading to Invasive Aspergillosis.

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.