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

The many roles of sulfur in the fungal-host interaction

Sulfur is an essential macronutrient for life, and consequently, all living organisms must acquire it from external sources to thrive and grow. Sulfur is a constituent of a multitude of crucial molecules, such as the S-containing proteinogenic amino acids cysteine and methionine; cofactors and prosthetic groups, such as coenzyme-A and iron-sulfur (Fe-S) clusters; and other essential organic molecules, such as glutathione or S-adenosylmethionine. Additionally, sulfur in cysteine thiols is an active redox group that plays paramount roles in protein stability, enzyme catalysis, and redox homeostasis. Furthermore, H2S is gaining more attention as a crucial signaling molecule that influences metabolism and physiological functions. Given its importance, it is not surprising that sulfur plays key roles in the host-pathogen interaction. However, in contrast to its well-recognized involvement in the plant-pathogen interaction, the specific contributions of sulfur to the human-fungal interaction are much less understood. In this short review, I highlight some of the most important known mechanisms and propose directions for further research.

Characterization of Aspergillus fumigatus secretome during sublethal infection of Galleria mellonella larvae

Introduction. The fungal pathogen Aspergillus fumigatus can induce prolonged colonization of the lungs of susceptible patients, resulting in conditions such as allergic bronchopulmonary aspergillosis and chronic pulmonary aspergillosis.Hypothesis. Analysis of the A. fumigatus secretome released during sub-lethal infection of G. mellonella larvae may give an insight into products released during prolonged human colonisation.Methodology. Galleria mellonella larvae were infected with A. fumigatus, and the metabolism of host carbohydrate and proteins and production of fungal virulence factors were analysed. Label-free qualitative proteomic analysis was performed to identify fungal proteins in larvae at 96 hours post-infection and also to identify changes in the Galleria proteome as a result of infection.Results. Infected larvae demonstrated increasing concentrations of gliotoxin and siderophore and displayed reduced amounts of haemolymph carbohydrate and protein. Fungal proteins (399) were detected by qualitative proteomic analysis in cell-free haemolymph at 96 hours and could be categorized into seven groups, including virulence (n = 25), stress response (n = 34), DNA repair and replication (n = 39), translation (n = 22), metabolism (n = 42), released intracellular (n = 28) and cellular development and cell cycle (n = 53). Analysis of the Gallerial proteome at 96 hours post-infection revealed changes in the abundance of proteins associated with immune function, metabolism, cellular structure, insect development, transcription/translation and detoxification.Conclusion. Characterizing the impact of the fungal secretome on the host may provide an insight into how A. fumigatus damages tissue and suppresses the immune response during long-term pulmonary colonization.

Chemical Genetics in C. elegans Identifies Anticancer Mycotoxins Chaetocin and Chetomin as Potent Inducers of a Nuclear Metal Homeostasis Response

C. elegans numr-1/2 (nuclear-localized metal-responsive) is an identical gene pair encoding a nuclear protein previously shown to be activated by cadmium and disruption of the integrator RNA metabolism complex. We took a chemical genetic approach to further characterize regulation of this novel metal response by screening 41,716 compounds and extracts for numr-1p::GFP activation. The most potent activator was chaetocin, a fungal 3,6-epidithiodiketopiperazine (ETP) with promising anticancer activity. Chaetocin activates numr-1/2 strongly in the alimentary canal but is distinct from metal exposure, because it represses canonical cadmium-responsive metallothionine genes. Chaetocin has diverse targets in cancer cells including thioredoxin reductase, histone lysine methyltransferase, and acetyltransferase p300/CBP; further work is needed to identify the mechanism in C. elegans as genetic disruption and RNAi screening of homologues did not induce numr-1/2 in the alimentary canal and chaetocin did not affect markers of integrator dysfunction. We demonstrate that disulfides in chaetocin and chetomin, a dimeric ETP analog, are required to induce numr-1/2. ETP monomer gliotoxin, despite possessing a disulfide linkage, had almost no effect on numr-1/2, suggesting a dimer requirement. Chetomin inhibits C. elegans growth at low micromolar levels, and loss of numr-1/2 increases sensitivity; C. elegans and Chaetomiaceae fungi inhabit similar environments raising the possibility that numr-1/2 functions as a defense mechanism. There is no direct orthologue of numr-1/2 in humans, but RNaseq suggests that chaetocin affects expression of cellular processes linked to stress response and metal homeostasis in colorectal cancer cells. Our results reveal interactions between metal response gene regulation and ETPs and identify a potential mechanism of resistance to this versatile class of preclinical compounds.

Aspergillus fumigatus ZrfC Zn(II) transporter scavengers zincophore-bound Zn(II)

Aspergillus fumigatus is an opportunistic pathogen that is able to invade and grow in the lungs of immunosuppressed patients and cause invasive pulmonary aspergillosis. The concentration of free Zn(II) in living tissues is much lower than that required for optimal fungal growth; thus, to obtain Zn(II) from the host, Aspergillus fumigatus uses highly specified Zn(II) transporters: ZrfA, ZrfB and ZrfC. The ZrfC transporter plays the main role in Zn(II) acquisition from the host in neutral and mildly alkaline environment via interacting with the secreted Aspf2 zincophore. Understanding the Aspf2-ZrfC interactions is therefore necessary for explaining the process of Zn(II) acquisition by Aspergillus fumigatus, and identifying Zn(II) binding sites in its transporter and describing the thermodynamics of such binding are the fundamental steps to achieve this goal. We focus on two probable ZrfC Zn(II) binding sites and show that the Ac-MNCHFHAGVEHCIGAGESESGSSQ-NH2 region binds Zn(II) with higher affinity than the Ac-TGCHSHGS-NH2 one and that this binding is much stronger than the binding of Zn(II) to the Aspf2 zincophore, allowing efficient Zn(II) transport from the Aspf2 zincophore to the ZrfC transporter. The same ZrfC fragments also able to bind Ni(II), another metal ion essential for fungi that could also compete with Zn(II) binding, with comparable affinity.

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.

Proteomic dissection of the role of GliZ in gliotoxin biosynthesis in Aspergillus fumigatus

Gliotoxin (GT) biosynthesis in fungi is encoded by the gli biosynthetic gene cluster. While GT addition autoinduces biosynthesis, Zn²⁺ has been shown to attenuate cluster activity, and it was speculated that identification of Zn2Cys6 binuclear transcription factor GliZ binding partners might provide insight into this observation. Using the Tet-ON induction system, doxycycline (DOX) presence induced GliZ fusion protein expression in, and recovery of GT biosynthesis by, A. fumigatus ΔgliZ::HA-gliZ and ΔgliZ::TAP-gliZ strains, respectively. Quantitative RT-PCR confirmed that DOX induces gli cluster gene expression (n = 5) in both A. fumigatus HA-GliZ and TAP-GliZ strains. GT biosynthesis was evident in Czapek-Dox and in Sabouraud media, however tagged GliZ protein expression was more readily detected in Sabouraud media. Unexpectedly, Zn²⁺ was essential for GliZ fusion protein expression in vivo, following 3 h DOX induction. Moreover, HA-GliZ abundance was significantly higher in either DOX/GT or DOX/Zn²⁺, compared to DOX-only. This suggests that while GT induction is still intact, Zn²⁺ inhibition of HA-GliZ production in vivo is lost. Co-immunoprecipitation revealed that GT oxidoreductase GliT associates with GliZ in the presence of GT, suggesting a potential protective role. Additional putative HA-GliZ interacting proteins included cystathionine gamma lyase, ribosomal protein L15 and serine hydroxymethyltransferase (SHMT). Total mycelial quantitative proteomic data revealed that GliT and GtmA, as well as several other gli cluster proteins, are increased in abundance or uniquely expressed with GT addition. Proteins involved in sulphur metabolism are also differentially expressed with GT or Zn²⁺ presence. Overall, we disclose that under DOX induction GliZ functionality is unexpectedly evident in zinc-replete media, subject to GT induction and that GliT appears to associate with GliZ, potentially to prevent DTG-mediated GliZ inactivation by zinc ejection.

Regulation of pseurotin A biosynthesis by GliZ and zinc in Aspergillus fumigatus

Recently, we reported that zinc regulates gliotoxin biosynthesis via ZafA, which is a zinc-responsive transcriptional activator. From an HPLC analysis of culture media of Aspergillus fumigatus, we found a trend of decreasing gliotoxin production but increasing pseurotin A and fumagillin production in proportion to the zinc concentration. The expression of the genes involved in pseurotin A biosynthesis was upregulated under high zinc concentrations. Furthermore, upregulated expression of pseurotin A biosynthetic genes and higher production of pseurotin A were observed in the zafA deletion strain. Interestingly, the deletion of gliZ, a transcriptional activator of gliotoxin biosynthesis genes, resulted in upregulated expression of pseurotin A biosynthetic genes and increased production of pseurotin A. We detected upregulation of fumR expression in the gliZ and zafA deletion mutants. The overexpression of gliZ observed in the zafA deletion mutant resulted in the failure of the mutant to increase pseurotin A production, which is a phenotype of the zafA deletion mutant. These results suggest that ZafA sequentially regulates pseurotin A biosynthesis through GliZ. Finally, we found through a murine virulence test that the gliZ and fumR double-deletion mutants showed a delayed death rate compared with the single-deletion mutants of either gliZ or fumR. Taken together, these results suggested that the biosynthesis of gliotoxin and pseurotin A are regulated in opposite ways by zinc utilization and that each secondary metabolite is synthesized when the synthesis of another secondary metabolite fails to protect it against the defense system of the host.

Exposure of Aspergillus fumigatus to Klebsiella pneumoniae Culture Filtrate Inhibits Growth and Stimulates Gliotoxin Production

Aspergillus fumigatus is an opportunistic fungal pathogen capable of inducing chronic and acute infection in susceptible patients. A. fumigatus interacts with numerous bacteria that compose the microbiota of the lung, including Pseudomonas aeruginosa and Klebsiella pneumoniae, both of which are common isolates from cystic fibrosis sputum. Exposure of A. fumigatus to K. pneumoniae culture filtrate reduced fungal growth and increased gliotoxin production. Qualitative proteomic analysis of the K. pneumoniae culture filtrate identified proteins associated with metal sequestering, enzymatic degradation and redox activity, which may impact fungal growth and development. Quantitative proteomic analysis of A. fumigatus following exposure to K. pneumoniae culture filtrate (25% v/v) for 24 h revealed a reduced abundance of 1,3-beta-glucanosyltransferase (-3.97 fold), methyl sterol monooxygenase erg25B (-2.9 fold) and calcium/calmodulin-dependent protein kinase (-4.2 fold) involved in fungal development, and increased abundance of glutathione S-transferase GliG (+6.17 fold), non-ribosomal peptide synthase GliP (+3.67 fold), O-methyltransferase GliM (+3.5 fold), gamma-glutamyl acyltransferase GliK (+2.89 fold) and thioredoxin reductase GliT (+2.33 fold) involved in gliotoxin production. These results reveal that exposure of A. fumigatus to K. pneumoniae in vivo could exacerbate infection and negatively impact patient prognosis.

Fungal Zinc Homeostasis and Its Potential as an Antifungal Target: A Focus on the Human Pathogen Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic airborne fungus that causes severe invasive aspergillosis in immunocompromised patients. Zinc is an essential micronutrient for the growth of A. fumigatus and even for all microorganisms. An increasing number of studies have reported that fungal zinc acquisition ability plays a key role in fungal survival in hosts with an extremely zinc-limited microenvironment. The ability to fight scarcity and excess of zinc are tightly related to fungal virulence and may be used as new potential targets. Because the regulation of zinc homeostasis is important, a thorough understanding of the functional genes involved in the regulatory network for zinc homeostasis is required for fungal pathogens. The current mini-review summarized potential zinc homeostasis regulators in A. fumigatus and classified these regulators according to localization and function, which were identified or predicted based on A. fumigatus or deduced from homologs in model yeasts. Future perspectives for zinc homeostasis regulators as potential antifungal targets to treat invasive aspergillosis are also discussed.

Quantitative Proteomic Analysis Reveals Yeast Cell Wall Products Influence the Serum Proteome Composition of Broiler Chickens

With an ever-growing market and continual financial pressures associated with the prohibition of antibiotic growth promoters, the poultry industry has had to rapidly develop non-antibiotic alternatives to increase production yields. A possible alternative is yeast and its derivatives, such as the yeast cell wall (YCW), which have been proposed to confer selected beneficial effects on the host animal. Here, the effect of YCW supplementation on the broiler chicken was investigated using a quantitative proteomic strategy, whereby serum was obtained from three groups of broilers fed with distinct YCW-based Gut Health Products (GHP) or a control basal diet. Development of a novel reagent enabled application of ProteoMiner™ technology for sample preparation and subsequent comparative quantitative proteomic analysis revealed proteins which showed a significant change in abundance (n = 167 individual proteins; p < 0.05); as well as proteins which were uniquely identified (n = 52) in, or absent (n = 37) from, GHP-fed treatment groups versus controls. An average of 7.1% of proteins showed changes in abundance with GHP supplementation. Several effects of these GHPs including immunostimulation (via elevated complement protein detection), potential alterations in the oxidative status of the animal (e.g., glutathione peroxidase and catalase), stimulation of metabolic processes (e.g., differential abundance of glyceraldehyde-3-phosphate dehydrogenase), as well as evidence of a possible hepatoprotective effect (attenuated levels of serum α-glutathione s-transferase) by one GHP feed supplement, were observed. It is proposed that specific protein detection may be indicative of GHP efficacy to stimulate broiler immune status, i.e., may be biomarkers of GHP efficacy. In summary, this work has developed a novel technology for the preparation of high dynamic range proteomic samples for LC-MS/MS analysis, is part of the growing area of livestock proteomics and, importantly, provides evidential support for beneficial effects that GHP supplementation has on the broiler chicken.

A Single Aspergillus fumigatus Gene Enables Ergothioneine Biosynthesis and Secretion by Saccharomyces cerevisiae

The naturally occurring sulphur-containing histidine derivative, ergothioneine (EGT), exhibits potent antioxidant properties and has been proposed to confer human health benefits. Although it is only produced by select fungi and prokaryotes, likely to protect against environmental stress, the GRAS organism Saccharomyces cerevisiae does not produce EGT naturally. Herein, it is demonstrated that the recombinant expression of a single gene, Aspergillus fumigatus egtA, in S. cerevisiae results in EgtA protein presence which unexpectedly confers complete EGT biosynthetic capacity. Both High Performance Liquid Chromatography (HPLC) and LC−mass spectrometry (MS) analysis were deployed to detect and confirm EGT production in S. cerevisiae. The localisation and quantification of the resultant EGT revealed a significantly (p < 0.0001) larger quantity of EGT was extracellularly present in culture supernatants than intracellularly accumulated in 96 h yeast cultures. Methionine addition to cultures improved EGT production. The additional expression of two candidate cysteine desulfurases from A. fumigatus was thought to be required to complete EGT biosynthesis, namely AFUA_2G13295 and AFUA_3G14240, termed egt2a and egt2b in this study. However, the co-expression of egtA and egt2a in S. cerevisiae resulted in a significant decrease in the observed EGT levels (p < 0.05). The AlphaFold prediction of A. fumigatus EgtA 3-Dimensional structure illuminates the bidomain structure of the enzyme and the opposing locations of both active sites. Overall, we clearly show that recombinant S. cerevisiae can biosynthesise and secrete EGT in an EgtA-dependent manner which presents a facile means of producing EGT for biotechnological and biomedical use.

Regulation of gliotoxin biosynthesis and protection in Aspergillus species

Aspergillus fumigatus causes a range of human and animal diseases collectively known as aspergillosis. A. fumigatus possesses and expresses a range of genetic determinants of virulence, which facilitate colonisation and disease progression, including the secretion of mycotoxins. Gliotoxin (GT) is the best studied A. fumigatus mycotoxin with a wide range of known toxic effects that impair human immune cell function. GT is also highly toxic to A. fumigatus and this fungus has evolved self-protection mechanisms that include (i) the GT efflux pump GliA, (ii) the GT neutralising enzyme GliT, and (iii) the negative regulation of GT biosynthesis by the bis-thiomethyltransferase GtmA. The transcription factor (TF) RglT is the main regulator of GliT and this GT protection mechanism also occurs in the non-GT producing fungus A. nidulans. However, the A. nidulans genome does not encode GtmA and GliA. This work aimed at analysing the transcriptional response to exogenous GT in A. fumigatus and A. nidulans, two distantly related Aspergillus species, and to identify additional components required for GT protection. RNA-sequencing shows a highly different transcriptional response to exogenous GT with the RglT-dependent regulon also significantly differing between A. fumigatus and A. nidulans. However, we were able to observe homologs whose expression pattern was similar in both species (43 RglT-independent and 11 RglT-dependent). Based on this approach, we identified a novel RglT-dependent methyltranferase, MtrA, involved in GT protection. Taking into consideration the occurrence of RglT-independent modulated genes, we screened an A. fumigatus deletion library of 484 transcription factors (TFs) for sensitivity to GT and identified 15 TFs important for GT self-protection. Of these, the TF KojR, which is essential for kojic acid biosynthesis in Aspergillus oryzae, was also essential for virulence and GT biosynthesis in A. fumigatus, and for GT protection in A. fumigatus, A. nidulans, and A. oryzae. KojR regulates rglT, gliT, gliJ expression and sulfur metabolism in Aspergillus species. Together, this study identified conserved components required for GT protection in Aspergillus species.

A. fumigatus secretes mycotoxins that are essential for its virulence and pathogenicity. Gliotoxin (GT) is a sulfur-containing mycotoxin, which is known to impair several aspects of the human immune response. GT is also toxic to different fungal species, which have evolved several GT protection strategies. To further decipher these responses, we used transcriptional profiling aiming to compare the response to GT in the GT producer A. fumigatus and the GT non-producer A. nidulans. This analysis allowed us to identify additional genes with a potential role in GT protection. We also identified 15 transcription factors (TFs) encoded in the A. fumigatus genome that are important for conferring resistance to exogenous gliotoxin. One of these TFs, KojR, which is essential for A. oryzae kojic acid production, is also important for virulence in A. fumigatus and GT protection in A. fumigatus, A. nidulans and A. oryzae. KojR regulates the expression of genes important for gliotoxin biosynthesis and protection, and sulfur metabolism. Together, this work identified conserved components required for gliotoxin protection in Aspergillus species.

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.