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

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.

Enhancing gliotoxins production in deep-sea derived fungus Dichotomocyes cejpii by engineering the biosynthetic pathway

Gliotoxin can be developed as potent biopesticide. In this study, the positive transcriptional factor gliZ, glutathione-S transferase encoding gene gliG and gliN were firstly deleted by CRISPR/Cas9 system, which abolished the production of gliotoxin-like compounds in Dichotomomyces cejpii. CRISPR/dCas9 system targeting promoter of gliG was used to activate the biosynthetic genes in gli cluster. The overexpression of gliZ, gliN and gliG can significantly improve the yield of gliotoxin-like compunds. The gliotoxin yields was improved by 16.38 ± 1.36 fold, 18.98 ± 1.28 fold through gliZ overexpression and gliM deletion in D. cejpii FS110. In addtion, gliN was heterologously expressed in E. coli, the purified GliN can catalyze gliotoxin into methyl-gliotoxin. Furthermore, the binding sequences of GliZ in the promoters of gliG was determined by Dnase footprinting. This study firstly illustrated the transcriptional regulatory mechanism of DcGliZ for the gliotoxin biosynthesis in D. cejpii, and improved the yields of gliotoxins significantly in D. cejpii via biosynthetic approaches.

Exposure to the Pseudomonas aeruginosa secretome alters the proteome and secondary metabolite production of Aspergillus fumigatus

The fungal pathogen Aspergillus fumigatus is frequently cultured from the sputum of cystic fibrosis (CF) patients along with the bacterium Pseudomonas aeruginosa. A. fumigatus secretes a range of secondary metabolites, and one of these, gliotoxin, has inhibitory effects on the host immune response. The effect of P. aeruginosa culture filtrate (CuF) on fungal growth and gliotoxin production was investigated. Exposure of A. fumigatus hyphae to P. aeruginosa cells induced increased production of gliotoxin and a decrease in fungal growth. In contrast, exposure of A. fumigatus hyphae to P. aeruginosa CuF led to increased growth and decreased gliotoxin production. Quantitative proteomic analysis was used to characterize the proteomic response of A. fumigatus upon exposure to P. aeruginosa CuF. Changes in the profile of proteins involved in secondary metabolite biosynthesis (e.g. gliotoxin, fumagillin, pseurotin A), and changes to the abundance of proteins involved in oxidative stress (e.g. formate dehydrogenase) and detoxification (e.g. thioredoxin reductase) were observed, indicating that the bacterial secretome had a profound effect on the fungal proteome. Alterations in the abundance of proteins involved in detoxification and oxidative stress highlight the ability of A. fumigatus to differentially regulate protein synthesis in response to environmental stresses imposed by competitors such as P. aeruginosa. Such responses may ultimately have serious detrimental effects on the host.

Chromatin profiling reveals heterogeneity in clinical isolates of the human pathogen Aspergillus fumigatus

Invasive Pulmonary Aspergillosis, which is caused by the filamentous fungus Aspergillus fumigatus, is a life-threatening infection for immunosuppressed patients. Chromatin structure regulation is important for genome stability maintenance and has the potential to drive genome rearrangements and affect virulence and pathogenesis of pathogens. Here, we performed the first A. fumigatus global chromatin profiling of two histone modifications, H3K4me3 and H3K9me3, focusing on the two most investigated A. fumigatus clinical isolates, Af293 and CEA17. In eukaryotes, H3K4me3 is associated with active transcription, while H3K9me3 often marks silent genes, DNA repeats, and transposons. We found that H3K4me3 deposition is similar between the two isolates, while H3K9me3 is more variable and does not always represent transcriptional silencing. Our work uncovered striking differences in the number, locations, and expression of transposable elements between Af293 and CEA17, and the differences are correlated with H3K9me3 modifications and higher genomic variations among strains of Af293 background. Moreover, we further showed that the Af293 strains from different laboratories actually differ in their genome contents and found a frequently lost region in chromosome VIII. For one such Af293 variant, we identified the chromosomal changes and demonstrated their impacts on its secondary metabolites production, growth and virulence. Overall, our findings not only emphasize the influence of genome heterogeneity on A. fumigatus fitness, but also caution about unnoticed chromosomal variations among common laboratory strains.

At the metal-metabolite interface in Aspergillus fumigatus: towards untangling the intersecting roles of zinc and gliotoxin

Cryptic links between apparently unrelated metabolic systems represent potential new drug targets in fungi. Evidence of such a link between zinc and gliotoxin (GT) biosynthesis in Aspergillus fumigatus is emerging. Expression of some genes of the GT biosynthetic gene cluster gli is influenced by the zinc-dependent transcription activator ZafA, zinc may relieve GT-mediated fungal growth inhibition and, surprisingly, GT biosynthesis is influenced by zinc availability. In A. fumigatus, dithiol gliotoxin (DTG), which has zinc-chelating properties, is converted to either GT or bis-dethiobis(methylthio)gliotoxin (BmGT) by oxidoreductase GliT and methyltransferase GtmA, respectively. A double deletion mutant lacking both GliT and GtmA was previously observed to be hypersensitive to exogenous GT exposure. Here we show that compared to wild-type exposure, exogenous GT and the zinc chelator N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN) inhibit A. fumigatus ΔgliTΔgtmA growth, specifically under zinc-limiting conditions, which can be reversed by zinc addition. While GT biosynthesis is evident in zinc-depleted medium, addition of zinc (1 µM) suppressed GT and activated BmGT production. In addition, secretion of the unferrated siderophore, triacetylfusarinine C (TAFC), was evident by A. fumigatus wild-type (at >5 µM zinc) and ΔgtmA (at >1 µM zinc) in a low-iron medium. TAFC secretion suggests that differential zinc-sensing between both strains may influence fungal Fe3+ requirement. Label-free quantitative proteomic analysis of both strains under equivalent differential zinc conditions revealed protein abundance alterations in accordance with altered metabolomic observations, in addition to increased GliT abundance in ΔgtmA at 5 µM zinc, compared to wild-type, supporting a zinc-sensing deficiency in the mutant strain. The relative abundance of a range of oxidoreductase- and secondary metabolism-related enzymes was also evident in a zinc- and strain-dependent manner. Overall, we elaborate new linkages between zinc availability, natural product biosynthesis and oxidative stress homeostasis in A. fumigatus.

Production, stability and degradation of Trichoderma gliotoxin in growth medium, irrigation water and agricultural soil

Gliotoxin produced by Trichoderma virens is inhibitory against various phytopathogenic fungi and bacteria. However, its stability in soil-ecosystem has not yet been well-defined. This study aimed to decipher its persistence and behaviour in growth media, irrigation water and soil ecosystems. Gliotoxin production was noticed at logarithmic growth phase and converted into bis-thiomethyl gliotoxin at late stationary growth phase of T. virens in acidic growth medium. But, no gliotoxin production was observed in neutral and alkaline growth medium. Gliotoxin was stable for several days in acidic water but degraded in alkaline water. Degradation of gliotoxin was more in unsterile soil than sterile soil and also that was higher under wet soil than dry soil. Degradation of gliotoxin was hastened by alkaline pH in wet soil but not in dry soil. Under unsterile soil conditions, high soil moisture increased the degradation of gliotoxin and the degradation of gliotoxin occurred quickly in alkaline soil (in 5 days) compared to acidic soil (in 10 days). Under sterile soil conditions, high soil moisture also enhanced the degradation of gliotoxin but level of degradation was less compared to unsterile conditions. Thus, gliotoxin stability is influenced mainly by the soil wetness, soil microbial community and pH conditions.

Telomeric and Sub-Telomeric Structure and Implications in Fungal Opportunistic Pathogens

Telomeres are long non-coding regions found at the ends of eukaryotic linear chromosomes. Although they have traditionally been associated with the protection of linear DNA ends to avoid gene losses during each round of DNA replication, recent studies have demonstrated that the role of these sequences and their adjacent regions go beyond just protecting chromosomal ends. Regions nearby to telomeric sequences have now been identified as having increased variability in the form of duplications and rearrangements that result in new functional abilities and biodiversity. Furthermore, unique fungal telomeric and chromatin structures have now extended clinical capabilities and understanding of pathogenicity levels. In this review, telomere structure, as well as functional implications, will be examined in opportunistic fungal pathogens, including Aspergillus fumigatus, Candida albicans, Candida glabrata, and Pneumocystis jirovecii.

The Aspergillus fumigatus transcription factor RglT is important for gliotoxin biosynthesis and self-protection, and virulence

Aspergillus fumigatus is an opportunistic fungal pathogen that secretes an array of immune-modulatory molecules, including secondary metabolites (SMs), which contribute to enhancing fungal fitness and growth within the mammalian host. Gliotoxin (GT) is a SM that interferes with the function and recruitment of innate immune cells, which are essential for eliminating A. fumigatus during invasive infections. We identified a C6 Zn cluster-type transcription factor (TF), subsequently named RglT, important for A. fumigatus oxidative stress resistance, GT biosynthesis and self-protection. RglT regulates the expression of several gli genes of the GT biosynthetic gene cluster, including the oxidoreductase-encoding gene gliT, by directly binding to their respective promoter regions. Subsequently, RglT was shown to be important for virulence in a chemotherapeutic murine model of invasive pulmonary aspergillosis (IPA). Homologues of RglT and GliT are present in eurotiomycete and sordariomycete fungi, including the non-GT-producing fungus A. nidulans, where a conservation of function was described. Phylogenetically informed model testing led to an evolutionary scenario in which the GliT-based resistance mechanism is ancestral and RglT-mediated regulation of GliT occurred subsequently. In conclusion, this work describes the function of a previously uncharacterised TF in oxidative stress resistance, GT biosynthesis and self-protection in both GT-producing and non-producing Aspergillus species.

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.

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.