Characterisation of the laccase-encoding gene abr2 of the dihydroxynaphthalene-like melanin gene cluster of Aspergillus fumigatus

Fungal melanin suppresses airway epithelial chemokine secretion through blockade of calcium fluxing

Respiratory infections caused by the human fungal pathogen Aspergillus fumigatus are a major cause of mortality for immunocompromised patients. Exposure to these pathogens occurs through inhalation, although the role of the respiratory epithelium in disease pathogenesis has not been fully defined. Employing a primary human airway epithelial model, we demonstrate that fungal melanins potently block the post-translational secretion of the chemokines CXCL1 and CXCL8 independent of transcription or the requirement of melanin to be phagocytosed, leading to a significant reduction in neutrophil recruitment to the apical airway both in vitro and in vivo . Aspergillus -derived melanin, a major constituent of the fungal cell wall, dampened airway epithelial chemokine secretion in response to fungi, bacteria, and exogenous cytokines. Furthermore, melanin muted pathogen-mediated calcium fluxing and hindered actin filamentation. Taken together, our results reveal a critical role for melanin interaction with airway epithelium in shaping the host response to fungal and bacterial pathogens.

Targeting of Specialized Metabolites Biosynthetic Enzymes to Membranes and Vesicles by Posttranslational Palmitoylation: A Mechanism of Non-Conventional Traffic and Secretion of Fungal Metabolites

In nature, the formation of specialized (secondary) metabolites is associated with the late stages of fungal development. Enzymes involved in the biosynthesis of secondary metabolites in fungi are located in distinct subcellular compartments including the cytosol, peroxisomes, endosomes, endoplasmic reticulum, different types of vesicles, the plasma membrane and the cell wall space. The enzymes traffic between these subcellular compartments and the secretion through the plasma membrane are still unclear in the biosynthetic processes of most of these metabolites. Recent reports indicate that some of these enzymes initially located in the cytosol are later modified by posttranslational acylation and these modifications may target them to membrane vesicle systems. Many posttranslational modifications play key roles in the enzymatic function of different proteins in the cell. These modifications are very important in the modulation of regulatory proteins, in targeting of proteins, intracellular traffic and metabolites secretion. Particularly interesting are the protein modifications by palmitoylation, prenylation and miristoylation. Palmitoylation is a thiol group-acylation (S-acylation) of proteins by palmitic acid (C16) that is attached to the SH group of a conserved cysteine in proteins. Palmitoylation serves to target acylated proteins to the cytosolic surface of cell membranes, e.g., to the smooth endoplasmic reticulum, whereas the so-called toxisomes are formed in trichothecene biosynthesis. Palmitoylation of the initial enzymes involved in the biosynthesis of melanin serves to target them to endosomes and later to the conidia, whereas other non-palmitoylated laccases are secreted directly by the conventional secretory pathway to the cell wall space where they perform the last step(s) of melanin biosynthesis. Six other enzymes involved in the biosynthesis of endocrosin, gliotoxin and fumitremorgin believed to be cytosolic are also targeted to vesicles, although it is unclear if they are palmitoylated. Bioinformatic analysis suggests that palmitoylation may be frequent in the modification and targeting of polyketide synthetases and non-ribosomal peptide synthetases. The endosomes may integrate other small vesicles with different cargo proteins, forming multivesicular bodies that finally fuse with the plasma membrane during secretion. Another important effect of palmitoylation is that it regulates calcium metabolism by posttranslational modification of the phosphatase calcineurin. Mutants defective in the Akr1 palmitoyl transferase in several fungi are affected in calcium transport and homeostasis, thus impacting on the biosynthesis of calcium-regulated specialized metabolites. The palmitoylation of secondary metabolites biosynthetic enzymes and their temporal distribution respond to the conidiation signaling mechanism. In summary, this posttranslational modification drives the spatial traffic of the biosynthetic enzymes between the subcellular organelles and the plasma membrane. This article reviews the molecular mechanism of palmitoylation and the known fungal palmitoyl transferases. This novel information opens new ways to improve the biosynthesis of the bioactive metabolites and to increase its secretion in fungi.

Synergistic degradation of Azure B and sulfanilamide antibiotics by the white-rot fungus Trametes versicolor with an activated ligninolytic enzyme system

The treatment of complex polluted wastewater has become an increasingly critical concern for the various types of hazardous organic compounds, including synthetic dyes and pharmaceuticals. Due to their efficient and eco-friendly advantages, the white-rot fungi (WRF) have been applied to degrade environmental pollutants. This study aimed to investigate the removal ability of WRF (i.e., Trametes versicolor WH21) in the co-contamination system composed of Azure B dye and sulfacetamide (SCT). Our study discovered that the decolorization of Azure B (300 mg/L) by strain WH21 was significantly improved (from 30.5% to 86.5%) by the addition of SCT (30 mg/L), while the degradation of SCT was also increased from 76.4% to 96.2% in the co-contamination system. Transcriptomic and biochemical analyses indicated that the ligninolytic enzyme system was activated by the enhanced enzymatic activities of MnPs and laccases, generating higher concentration of extracellular H₂O₂ and organic acids in strain WH21 in response to SCT stress. Purified MnP and laccase of strain WH21 were revealed with remarkable degradation effect on both Azure B and SCT. These findings significantly expanded the existing knowledge on the biological treatment of organic pollutants, indicating the strong promise of WRF in the treatment of complex polluted wastewater.

Biocatalytic characterization of Hericium erinaceus laccase isoenzymes for the oxidation of lignin derivative substrates

Mushroom laccases are biocatalysts that oxidize various substrates. To identify a novel enzyme involved in lignin valorization, we isolated and characterized laccase isoenzymes from the mushroom Hericium erinaceus. The laccase cDNAs (Lac1a and Lac1b) cloned from the mushroom mycelia consisted of 1536 bp and each encoded a protein with 511 amino acids, containing a 21-amino-acid signal peptide. Comparative phylogenetic analysis revealed high homology between the deduced amino acid sequences of Lac1a and Lac1b and those from basidiomycetous fungi. In the Pichia pastoris expression system, high extracellular production of Lac1a, a glycoprotein, was achieved, whereas Lac1b was not expressed as a secreted protein because of hyper-glycosylation. Biochemical characterization of the purified recombinant Lac1a (rLac1a) protein revealed its oxidizing efficacy toward 14 aromatic substrates. The highly substrate-specific rLac1a showed catalytic efficiencies of 877 s^-1 mM^-1, 829 s^-1 mM^-1, 520 s^-1 mM^-1, and 467 s^-1 mM^-1 toward 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid), hydroquinone, guaiacol, and 2,6-dimethylphenol, respectively. Moreover, rLac1a showed approximately 10 % higher activity in non-ionic detergents and >50 % higher residual activity in various organic solvents. These results indicate that rLac1a is a novel oxidase biocatalyst for the bioconversion of lignin into value-added products.

Modulation of Virulence-Associated Traits in Aspergillus fumigatus by BET Inhibitor JQ1

Aspergillus fumigatus is a disease-causing, opportunistic fungus that can establish infection due to its capacity to respond to a wide range of environmental conditions. Secreted proteins and metabolites, which play a critical role in fungal–host interactions and pathogenesis, are modulated by epigenetic players, such as bromodomain and extraterminal domain (BET) proteins. In this study, we evaluated the in vitro and in vivo capability of the BET inhibitor JQ1 to modulate the extracellular proteins and virulence of A. fumigatus. The abundance of 25 of the 76 extracellular proteins identified through LC-MS/MS proteomic analysis changed following JQ1 treatment. Among them, a ribonuclease, a chitinase, and a superoxide dismutase were dramatically downregulated. Moreover, the proteomic analysis of A. fumigatus intracellular proteins indicated that Abr2, an intracellular laccase involved in the last step of melanin synthesis, was absent in the JQ1-treated group. To investigate at which level this downregulation occurred and considering the ability of JQ1 to modulate gene expression we checked the level of ABR2, Chitinase, and Superoxide dismutase mRNA expression by qRT-PCR. Finally, the capacity of JQ1 to reduce the virulence of A. fumigatus has been proved using Galleria mellonella larvae, which are an in vivo model to evaluate fungal virulence. Overall, the promising activity exhibited by JQ1 suggests that A. fumigatus is sensitive to BET inhibition and BET proteins may be a viable target for developing antifungal agents.

Manipulation of the Global Regulator mcrA Upregulates Secondary Metabolite Production in Aspergillus wentii Using CRISPR-Cas9 with In Vitro Assembled Ribonucleoproteins

Genome sequencing of filamentous fungi has demonstrated that most secondary metabolite biosynthetic gene clusters (BGCs) are silent under standard laboratory conditions. In this work, we have established an in vitro CRISPR-Cas9 system in Aspergillus wentii. To activate otherwise silent BGCs, we deleted the negative transcriptional regulator mcrA. Deletion of mcrA (mcrAΔ) resulted in differential production of 17 SMs in total when the strain was cultivated on potato dextrose media (PDA). Nine out of fifteen of these SMs were fully characterized, including emodin (1), physcion (2), sulochrin (3), physcion bianthrone (4), 14-O-demethylsulochrin (5), (trans/cis)-emodin bianthrone (6 and 7), and (trans/cis)-emodin physcion bianthrone (8 and 9). These compounds were all found to be produced by the same polyketide synthase (PKS) BGC. We then performed a secondary knockout targeting this PKS cluster in the mcrAΔ background. The metabolite profile of the dual-knockout strain revealed new metabolites that were not previously detected in the mcrAΔ parent strain. Two additional SMs were purified from the dual-knockout strain and were characterized as aspergillus acid B (16) and a structurally related but previously unidentified compound (17). For the first time, this work presents a facile genetic system capable of targeted gene editing in A. wentii. This work also illustrates the utility of performing a dual knockout to eliminate major metabolic products, enabling additional SM discovery.

Design and synthesis of eugenol/isoeugenol glycoconjugates and other analogues as antifungal agents against Aspergillus fumigatus

Glycoconjugates are biologically significant molecules as they tend to serve a wide range of intra- and extra-cellular processes depending on their size and complexity. The secondary metabolites of the plant Myristica fragrans, eugenol and isoeugenol, have shown antifungal activities (IC50 1900 μM). Therefore, we envisioned that glycoconjugates based on these two scaffolds could prove to be potent antifungal agents. Triazole-containing compounds have shown prominent activities as antifungal agents. Based on this, we opined that a Cu(i) catalyzed click reaction could serve as the bridging tool between a eugenol/isoeugenol moiety and sugars to synthesize eugenol/isoeugenol based glycoconjugates. In our present work, we have coupled propargylated eugenol/isoeugenol and azido sugar to furnish eugenol/isoeugenol based glycoconjugates. In another approach, we have carried out hydroxylation of the double bond of eugenol and subsequent azidation of a primary alcohol followed by intramolecular coupling reactions leading to various other analogues. All the synthesized compounds were assayed against an opportunistic pathogenic fungus, Aspergillus fumigatus. Among the synthesized compounds, two analogues have exhibited significant antifungal activities with IC50 values of 5.42 and 9.39 μM, respectively. The study suggested that these two analogues inhibit cell wall-associated melanin hydrophobicity along with the number of conidia. The synthesized compounds were found to be non-cytotoxic to an untransformed cell line.

Comparative Genomics Reveals a Single Nucleotide Deletion in pksP That Results in White-Spore Phenotype in Natural Variants of Aspergillus fumigatus

Aspergillus fumigatus is a potentially deadly opportunistic human pathogen. A. fumigatus has evolved a variety of mechanisms to evade detection by the immune system. For example, the conidium surface is covered in a layer of 1,8-dihydroxynaphthalene (DHN) melanin which masks the antigen macrophages use for recognition. DHN melanin also protects conidia from ultraviolet radiation and gives A. fumigatus conidia their characteristic green-grayish color. Here, we conducted genomic analysis of two closely related white-spore natural variants of A. fumigatus in comparison to two closely related green-spore isolates to identify a genetic basis of the white-spore phenotype. Illumina whole-genome resequencing data of the four isolates was used to identify variants that were shared in the white-spore isolates and different from both the green-spore isolates and the Af293 reference genome (which is also a green-spore isolate). We identified 4,279 single nucleotide variants and 1,785 insertion/deletions fitting this pattern. Among these, we identified 64 variants predicted to be high impact, loss-of-function mutations. One of these variants is a single nucleotide deletion that results in a frameshift in pksP (Afu2g17600), the core biosynthetic gene in the DHN melanin encoding gene cluster. The frameshift mutation in the white-spore isolates leads to a truncated protein in which a phosphopantetheine attachment site (PP-binding domain) is interrupted and an additional PP-binding domain and a thioesterase domain are omitted. Growth rate analysis of white-spore and green-spore isolates at 37°C and 48°C revealed that white-spore isolates are thermosensitive. Growth rate of A. fumigatus Af293 and a pksP null mutant in the Af293 background suggests pksP is not directly involved in the thermosensitivity phenotype. Further, our study identified a mutation in a gene (Afu4g04740) associated with thermal sensitivity in yeasts which could also be responsible for the thermosensitivity of the white-spore mutants. Overall, we used comparative genomics to identify the mutation and protein alterations responsible for the white-spore phenotype of environmental isolates of A. fumigatus.

Subcellular localization of fungal specialized metabolites

Fungal specialized metabolites play an important role in the environment and have impacted human health and survival significantly. These specialized metabolites are often the end product of a series of sequential and collaborating biosynthetic enzymes that reside within different subcellular compartments. A wide variety of methods have been developed to understand fungal specialized metabolite biosynthesis in terms of the chemical conversions and the biosynthetic enzymes required, however there are far fewer studies elucidating the compartmentalization of the same enzymes. This review illustrates the biosynthesis of specialized metabolites where the localization of all, or some, of the biosynthetic enzymes have been determined and describes the methods used to identify the sub-cellular localization.

Fungal Melanin Biosynthesis Pathway as Source for Fungal Toxins

Contamination of food and feed with toxin-producing fungi is a major threat in agriculture and for human health. The filamentous fungus Alternaria alternata is one of the most widespread postharvest contaminants and a weak plant pathogen. It produces a large variety of secondary metabolites with alternariol and its derivatives as characteristic mycotoxin. Other important phyto- and mycotoxins are perylene quinones (PQs), some of which have anticancer properties. Here, we discovered that the PQ altertoxin (ATX) biosynthesis shares most enzymes with the 1,8-dihydroxynaphthalene (1,8-DHN) melanin pathway. However, melanin was formed in aerial hyphae and spores, and ATXs were synthesized in substrate hyphae. This spatial separation is achieved through the promiscuity of a polyketide synthase, presumably producing a pentaketide (T4HN), a hexaketide (AT4HN), and a heptaketide (YWA1) as products. T4HN directly enters the altertoxin and DHN melanin pathway, whereas AT4HN and YWA1 can be converted only in aerial hyphae, which probably leads to a higher T4HN concentration, favoring 1,8-DHN melanin formation. Whereas the production of ATXs was strictly dependent on the CmrA transcription factor, melanin could still be produced in the absence of CmrA to some extent. This suggests that different cues regulate melanin and toxin formation. Since DHN melanin is produced by many fungi, PQs or related compounds may be produced in many more fungi than so far assumed.

Transcriptome Profiling Reveals Differential Gene Expression of Laccase Genes in Aspergillus terreus KC462061 during Biodegradation of Crude Oil

Fungal laccases have high catalytic efficiency and are utilized for the removal of crude oil because they oxidize various aliphatic and aromatic hydrocarbons and convert them into harmless compounds or less toxic compounds, thus accelerating the biodegradation potential of crude oil. Laccases are important gene families and the function of laccases genes varied widely based on transcription and function. Biodegradation of crude oil using Aspergillus terreus KC462061 was studied in the current study beside the transcription level of eight laccase (Lcc) genes have participated in biodegradation in the presence of aromatic compounds, and metal ions. Time-course profiles of laccase activity in the presence of crude oil indicated that the five inducers individual or combined have a very positive on laccase activity. In the status of the existence of crude oil, the synergistic effect of Cu-ABTS compound caused an increase in laccase yields up to 22-fold after 10 days than control. The biodegradation efficiencies of A. terreus KC462061 for aliphatic and aromatic hydrocarbons of crude oil were 82.1 ± 0.2% and 77.4 ± 0.6%, respectively. The crude oil biodegradation efficiency was improved by the supplemented Cu-ABTS compound in A. terreus KC462061. Gas chromatography-mass spectrometry was a very accurate tool to demonstrate the biodegradation efficiencies of A. terreus KC462061 for crude oil. Significant differences were observed in the SDS-PAGE of A. terreus KC462061 band intensities of laccase proteins after the addition of five inducers, but the Cu-ABTS compound highly affects very particular laccase electrophoresis. Quantitative real-time polymerase chain reaction (qPCR) was used for the analysis of transcription profile of eight laccase genes in A. terreus KC462061 with a verified reference gene. Cu²⁺ ions and Cu-ABTS were highly effective for efficient laccase expression profiling, mainly via Lcc11 and 12 transcription induction. The current study will explain the theoretical foundation for laccase transcription in A. terreus KC462061, paving the road for commercialization and usage.

Fungal Pathogenesis-Related Cell Wall Biogenesis, with Emphasis on the Maize Anthracnose Fungus Colletotrichum graminicola

The genus Colletotrichum harbors many plant pathogenic species, several of which cause significant yield losses in the field and post harvest. Typically, in order to infect their host plants, spores germinate, differentiate a pressurized infection cell, and display a hemibiotrophic lifestyle after plant invasion. Several factors required for virulence or pathogenicity have been identified in different Colletotrichum species, and adaptation of cell wall biogenesis to distinct stages of pathogenesis has been identified as a major pre-requisite for the establishment of a compatible parasitic fungus-plant interaction. Here, we highlight aspects of fungal cell wall biogenesis during plant infection, with emphasis on the maize leaf anthracnose and stalk rot fungus, Colletotrichum graminicola.

The Protective Role of 1,8-Dihydroxynaphthalene-Melanin on Conidia of the Opportunistic Human Pathogen Aspergillus fumigatus Revisited: No Role in Protection against Hydrogen Peroxide and Superoxides

Previously, 1,8-dihydroxynaphthalene (DHN)-melanin was described to protect Aspergillus fumigatus against hydrogen peroxide (H₂O₂), thereby protecting this opportunistic human pathogen from reactive oxygen species generated by the immune system. This was based on the finding that the ATCC 46645 mutant with mutations in the pksP gene of the DHN-melanin synthesis pathway showed increased sensitivity to reactive oxygen species compared to the wild type. Here, it is shown that deletion of the pksP gene in A. fumigatus strain CEA10 did not affect sensitivity for H₂O₂ and superoxide in a plate stress assay. In addition, direct exposure of the dormant white conidia of the pksP deletion strains to H₂O₂ did not result in increased sensitivity. Moreover, complementation of the ATCC 46645 pksP mutant strain with the wild-type pksP gene did result in pigmented conidia but did not rescue the H₂O₂-sensitive phenotype observed in the plate stress assay. Genome sequencing of the ATCC 46645 pksP mutant strain and its complemented strain revealed a mutation in the cat1 gene, likely due to the UV mutagenesis procedure used previously, which could explain the increased sensitivity toward H₂O₂. In summary, DHN-melanin is not involved in protection against H₂O₂ or superoxide and, thus, has no role in survival of conidia when attacked by these reactive oxygen species. IMPORTANCE Opportunistic pathogens like Aspergillus fumigatus have strategies to protect themselves against reactive oxygen species like hydrogen peroxides and superoxides that are produced by immune cells. DHN-melanin is the green pigment on conidia of Aspergillus fumigatus and more than 2 decades ago was reported to protect conidia against hydrogen peroxide. Here, we correct this misinterpretation by showing that DHN-melanin actually is not involved in protection of conidia against hydrogen peroxide. We show that UV mutagenesis that was previously used to select a pksP mutant generated many more genome-wide mutations. We discovered that a mutation in the mycelial catalase gene cat1 could explain the observed phenotype of increased hydrogen peroxide sensitivity. Our work shows that UV mutagenesis is not the preferred methodology to be used for generating mutants. It requires genome sequencing with single-nucleotide polymorphism analysis as well as additional validations to discard unwanted and confirm correct phenotypes.

Coprinopsis cinerea uses laccase Lcc9 as a defense strategy to eliminate oxidative stress during fungal-fungal interactions

Frequently, laccases are triggered during fungal cocultivation for overexpression. The function of these activated laccases during coculture has not been clarified. Previously, we reported that Gongronella sp. w5 (w5) (Mucoromycota, Mucoromycetes) specifically triggered the laccase Lcc9 overexpression in Coprinopsis cinerea (Basidiomycota, Agaricomycetes). To systematically analyze the function of the overexpressed laccase during fungal interaction, C. cinerea mycelia before and after the initial Lcc9 overexpression were chosen for transcriptome analysis. Results showed that accompanied by specific utilization of fructose as carbohydrate substrate, oxidative stress derived from antagonistic compounds secreted by w5 appears to be a signal critical for laccase production in C. cinerea. Reactive oxygen species (ROS) decrease in the C. cinerea wild-type strain followed the increase in laccase production and then, lcc9 transcription and laccase activity stopped. By comparison, increased H₂O₂ content and mycelial ROS levels were observed during the entire cocultivation in lcc9 silenced C. cinerea strains. Moreover, lcc9 silencing slowed down the C. cinerea mycelial growth, affected hyphal morphology, and decreased the asexual sporulation in coculture. Our results showed that intracellular ROS acted as signal molecules to stimulate defense responses by C. cinerea with the expression of oxidative stress response regulator Skn7 and various detoxification proteins. Lcc9 takes part as a defense strategy to eliminate oxidative stress during the interspecific interaction with w5. Importance: The overproduction of laccase during interspecific fungal interactions is notoriously known. However, the exact role of the up-regulated laccases remains underexplored. Based on comparative transcriptomic analysis of C. cinerea and gene silencing of laccase Lcc9, here we show that oxidative stress derived from antagonistic compounds secreted by Gongronella sp. w5 was a signal critical for laccase Lcc9 production in Coprinopsis cinerea. Intracellular ROS acted as signal molecules to stimulate defense responses by C. cinerea with the expression of oxidative stress response regulator Skn7 and various detoxification proteins. Ultimately, Lcc9 takes part as a defense strategy to eliminate oxidative stress and help cell growth and development during the interspecific interaction with Gongronella sp. w5. These findings deepened our understanding of fungal interactions in their natural population and communities.

Natural products in the predatory defence of the filamentous fungal pathogen Aspergillus fumigatus

The kingdom of fungi comprises a large and highly diverse group of organisms that thrive in diverse natural environments. One factor to successfully confront challenges in their natural habitats is the capability to synthesize defensive secondary metabolites. The genetic potential for the production of secondary metabolites in fungi is high and numerous potential secondary metabolite gene clusters have been identified in sequenced fungal genomes. Their production may well be regulated by specific ecological conditions, such as the presence of microbial competitors, symbionts or predators. Here we exemplarily summarize our current knowledge on identified secondary metabolites of the pathogenic fungus Aspergillus fumigatus and their defensive function against (microbial) predators.

Variation of virulence of five Aspergillus fumigatus isolates in four different infection models

Conidia of Aspergillus fumigatus are inhaled by humans on daily basis. As a consequence, these conidia can cause infections that differ in severity ranging from allergic bronchopulmonary aspergillosis to invasive aspergillosis. In this study we compared virulence of five A. fumigatus isolates in four different infection models to address the predictive value of different model systems. Two of the A. fumigatus strains were isolated from dogs with a non-invasive sino-nasal aspergillosis (DTO271-B5 and DTO303-F3), while three strains were isolated from human patients with invasive aspergillosis (Af293, ATCC46645 and CEA10). Infection models used encompassed cultured type II A549 lung epithelial cells, Protostelium aurantium amoeba, Galleria melonella larvae and zebrafish embryos. No major differences in virulence between these five strains were observed in the lung epithelial cell model. In contrast, strain ATCC46645 was most virulent in the amoeba and zebrafish model, whereas it was much less virulent in the Galleria infection model. DTO303-F3 was most virulent in the latter model. In general, reference strain Af293 was less virulent as compared to the other strains. Genome sequence analysis showed that this latter strain differed from the other four strains in 136 SNPs in virulence-related genes. Together, our results show that virulence of individual A. fumigatus strains show significant differences between infection models. We conclude that the predictive value of different model systems varies since the relative virulence across fungal strains does not hold up across different infection model systems.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.

Sensing the threat posed by Aspergillus infection

The mammalian immune system can tune its inflammatory response to the threat level posed by an invading pathogen. It is well established that the host utilizes numerous 'patterns of pathogenicity', such as microbial growth, invasion, and viability, to achieve this tuning during bacterial infections. This review discusses how this notion fits during fungal infection, particularly regarding Aspergillus fumigatus infection. Moreover, how the environmental niches filled by A. fumigatus may drive the evolution of the fungal traits responsible for inducing the strain-specific inflammatory responses that have been experimentally observed will be discussed. Moving forward understanding the mechanisms of the fungal strain-specific inflammatory response due to the initial interactions with the host innate immune system will be essential for enhancing our therapeutic options for the treatment of invasive fungal infections.

A Fungal Ascorbate Oxidase with Unexpected Laccase Activity

Ascorbate oxidases are an enzyme group that has not been explored to a large extent. So far, mainly ascorbate oxidases from plants and only a few from fungi have been described. Although ascorbate oxidases belong to the well-studied enzyme family of multi-copper oxidases, their function is still unclear. In this study, Af_AO1, an enzyme from the fungus Aspergillus flavus, was characterized. Sequence analyses and copper content determination demonstrated Af_AO1 to belong to the multi-copper oxidase family. Biochemical characterization and 3D-modeling revealed a similarity to ascorbate oxidases, but also to laccases. Af_AO1 had a 10-fold higher affinity to ascorbic acid (K_M = 0.16 ± 0.03 mM) than to ABTS (K_M = 1.89 ± 0.12 mM). Furthermore, the best fitting 3D-model was based on the ascorbate oxidase from Cucurbita pepo var. melopepo. The laccase-like activity of Af_AO1 on ABTS (V_max = 11.56 ± 0.15 µM/min/mg) was, however, not negligible. On the other hand, other typical laccase substrates, such as syringaldezine and guaiacol, were not oxidized by Af_AO1. According to the biochemical and structural characterization, Af_AO1 was classified as ascorbate oxidase with unusual, laccase-like activity.

Basidiospores from Wood-Decay Fungi Transform Laccase Substrates in the Absence of Glucose and Nitrogen Supplements

Preparations of bacterial endospores and fungal conidia are applied in biocontrols, biocatalyses, and lignocellulose fermentations. The biocatalytic abilities of basidiospores from mushrooms of the order Agaricales are unknown. To assess their potential in colonizing recalcitrant substrates solely with their inherent resources, spores of the white-rot fungi Stropharia rugoso-annulata (Stru) and Kuehneromyces mutabilis (Kmt, Strophariaceae) were analyzed for surface-bound and internal total carbohydrates, phenols, proteins, minerals, and oxidoreductases to estimate their chemistry and the preconditions to transform the laccase substrates guaiacol and 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) independent of external glucose and nitrogen. Surfaces of Stru/Kmt spores released (mg kg-1) hexoses, 7300/9700; phenols, >62/220; proteins, 21/168; and laccases, 42/0-0.15 µmol ABTS•+ kg-1 min-1 that mimicked oxidative activities of the resting spores. Milled-spore extracts contained pentoses, 96,600/6750; hexoses, 160,000/15,130; phenols, 452/767; protein, 12,600/924; true laccase, 688/0.30; and enzyme-protein-activating transition metals such as Cu in concentrations typical of wheat grains. Independent of external N and C supply, spores (<1‱) germinated in bideionized water, supported by their surface resources. Kmt spores germinated, too, at comparable rates in N-free solutions of glucose and the not immediately metabolizable ABTS and guaiacol. The release of proteins and oxidoreductase(s) by Kmt spores starting upon germination was higher in guaiacol-incubated idiophase- than in glucose-incubated trophophase-spores and led to the 3-4-fold formation of guaiacol polymerizates and ABTS•+. Constitutive aromatic ring-cleaving dioxygenases in the dormant spore that could be involved in the intrinsic metabolization of guaiacol were not detected. It is concluded that intrinsic resources enable (germinating) spores to release the highly efficient laccases of basidiomycetes and to transform aromatic compounds in the absence of sugar amendments. Spores show therefore plant seed-like autonomy in nutrient modification and acquisition during the early stages of the colonization of inert substrates.

Multicopper Oxidases in Saccharomyces cerevisiae and Human Pathogenic Fungi

Multicopper oxidases (MCOs) are produced by microscopic and macroscopic fungal species and are involved in various physiological processes such as morphogenesis, lignin degradation, and defense mechanisms to stress inducing environmental conditions as well as fungal virulence. This review will summarize our current understanding regarding the functions of MCOs present in Saccharomyces cerevisiae and in different human fungal pathogens. Of the two main MCO groups, the first group of MCOs is involved in iron homoeostasis and the second includes laccases. This review will also discuss their role in the pathogenesis of human fungal pathogens.

Aspergillus fumigatus Cell Wall Promotes Apical Airway Epithelial Recruitment of Human Neutrophils

Aspergillus fumigatus is a ubiquitous fungal pathogen capable of causing multiple pulmonary diseases, including invasive aspergillosis, chronic necrotizing aspergillosis, fungal colonization, and allergic bronchopulmonary aspergillosis. Intact mucociliary barrier function and early airway neutrophil responses are critical for clearing fungal conidia from the host airways prior to establishing disease. Following inhalation, Aspergillus conidia deposit in the small airways, where they are likely to make their initial host encounter with epithelial cells. Challenges in airway infection models have limited the ability to explore early steps in the interactions between A. fumigatus and the human airway epithelium. Here, we use inverted air-liquid interface cultures to demonstrate that the human airway epithelium responds to apical stimulation by A. fumigatus to promote the transepithelial migration of neutrophils from the basolateral membrane surface to the apical airway surface. Promoting epithelial transmigration with Aspergillus required prolonged exposure with live resting conidia. Swollen conidia did not expedite epithelial transmigration. Using A. fumigatus strains containing deletions of genes for cell wall components, we identified that deletion of the hydrophobic rodlet layer or dihydroxynaphthalene-melanin in the conidial cell wall amplified the epithelial transmigration of neutrophils, using primary human airway epithelium. Ultimately, we show that an as-yet-unidentified nonsecreted cell wall protein is required to promote the early epithelial transmigration of human neutrophils into the airspace in response to A. fumigatus Together, these data provide critical insight into the initial epithelial host response to Aspergillus.

The Genetics and Biochemistry of Cell Wall Structure and Synthesis in Neurospora crassa, a Model Filamentous Fungus

This review discusses the wealth of information available for the N. crassa cell wall. The basic organization and structure of the cell wall is presented and how the wall changes during the N. crassa life cycle is discussed. Over forty cell wall glycoproteins have been identified by proteomic analyses. Genetic and biochemical studies have identified many of the key enzymes needed for cell wall biogenesis, and the roles these enzymes play in cell wall biogenesis are discussed. The review includes a discussion of how the major cell wall components (chitin, β-1,3-glucan, mixed β-1,3-/ β-1,4- glucans, glycoproteins, and melanin) are synthesized and incorporated into the cell wall. We present a four-step model for how cell wall glycoproteins are covalently incorporated into the cell wall. In N. crassa, the covalent incorporation of cell wall glycoproteins into the wall occurs through a glycosidic linkage between lichenin (a mixed β-1,3-/β-1,4- glucan) and a "processed" galactomannan that has been attached to the glycoprotein N-linked oligosaccharides. The first step is the addition of the galactomannan to the N-linked oligosaccharide. Mutants affected in galactomannan formation are unable to incorporate glycoproteins into their cell walls. The second step is carried out by the enzymes from the GH76 family of α-1,6-mannanases, which cleave the galactomannan to generate a processed galactomannan. The model suggests that the third and fourth steps are carried out by members of the GH72 family of glucanosyltransferases. In the third step the glucanosyltransferases cleave lichenin and generate enzyme/substrate intermediates in which the lichenin is covalently attached to the active site of the glucanosyltransferases. In the final step, the glucanosyltransferases attach the lichenin onto the processed galactomannans, which creates new glycosidic bonds and effectively incorporates the glycoproteins into the cross-linked cell wall glucan/chitin matrix.

The Role of Melanin in Fungal Pathogenesis for Animal Hosts

Melanins are a class of pigments that are ubiquitous throughout biology. They play incredibly diverse and important roles ranging from radiation protection to immune defense, camouflage, and virulence. Fungi have evolved to use melanin to be able to persist in the environment and within organisms. Fungal melanins are often located within the cell well and are able to neutralize reactive oxygen species and other radicals, defend against UV radiation, bind and sequester non-specific peptides and compounds, and produce a physical barrier that defends the cell. For this reason, melanized fungi are often well-suited to be human pathogens-melanin allows fungi to neutralize the microbicidal oxidative bursts of our innate immune system, bind and inactivate to antimicrobial peptides and enzymes, sequester antifungal pharmaceuticals, and create a shield to block immune recognition of the fungus. Due to the importance and pervasiveness of melanin in fungal virulence, mammalian immune systems have evolved antifungal strategies that involve directly detecting and binding to fungal melanins. Such strategies include the use of melanin-specific antibody responses and C-type lectins like the newly discovered melanin-specific MelLec receptor.

Two transcription factors cooperatively regulate DHN melanin biosynthesis and development in Pestalotiopsis fici

Fungal 1,8-dihydroxynaphthalene (DHN) melanin plays important roles in UV protection, oxidative stress and pathogenesis. However, knowledge of the regulatory mechanisms of its biosynthesis is limited. Previous studies showed two transcription factors, PfmaF and PfmaH, located in the DHN melanin biosynthetic gene cluster (Pfma) in Pestalotiopsis fici. In this study, deletion of PfmaH resulted in loss of melanin and affected conidia cell wall integrity. Specifically, PfmaH directly regulates the expression of scytalone dehydratase, which catalyzes the transition of scytalone to T₃ HN. However, PfmaF disruption using CRISPR/Cas9 system affected neither DHN melanin distribution nor conidia cell wall integrity in P. fici. Unexpectedly, overexpression of PfmaF leads to heavy pigment accumulation in P. fici hyphae. Transcriptome and qRT-PCR analyses provide insight into the roles of PfmaF and PfmaH in DHN melanin regulation. PfmaH, as a pathway specific regulator, mainly regulates melanin biosynthesis that contributes to cell wall development. Furthermore, PfmaF functions as a broad regulator to stimulate PfmaH expression in melanin production, secondary metabolism as well as fungal development.

SH3-class Ras guanine nucleotide exchange factors are essential for Aspergillus fumigatus invasive growth

Proper hyphal morphogenesis is essential for the establishment and progression of invasive disease caused by filamentous fungi. In the human pathogen Aspergillus fumigatus, signalling cascades driven by Ras and Ras‐like proteins orchestrate a wide variety of cellular processes required for hyphal growth. For activation, these proteins require interactions with Ras‐subfamily‐specific guanine nucleotide exchange factors (RasGEFs). Although Ras‐protein networks are essential for virulence in all pathogenic fungi, the importance of RasGEF proteins is largely unexplored. A. fumigatus encodes four putative RasGEFs that represent three separate classes of RasGEF proteins (SH3‐, Ras guanyl nucleotide‐releasing protein [RasGRP]–, and LTE‐class), each with fungus‐specific attributes. Here, we show that the SH3‐class and RasGRP‐class RasGEFs are required for properly timed polarity establishment during early growth and branch emergence as well as for cell wall stability. Further, we show that SH3‐class RasGEF activity is essential for polarity establishment and maintenance, a phenotype that is, at least, partially independent of the major A. fumigatus Ras proteins, RasA and RasB. Finally, loss of both SH3‐class RasGEFs resulted in avirulence in multiple models of invasive aspergillosis. Together, our findings suggest that RasGEF activity is essential for the integration of multiple signalling networks to drive invasive growth in A. fumigatus.

RgsD negatively controls development, toxigenesis, stress response, and virulence in Aspergillus fumigatus

The regulator of G protein signaling (RGS) domain proteins generally attenuate heterotrimeric G protein signaling, thereby fine-tune the duration and strength of signal transduction. In this study, we characterize the functions of RgsD, one of the six RGS domain proteins present in the human pathogenic fungus Aspergillus fumigatus. The deletion (Δ) of rgsD results in enhanced asexual sporulation coupled with increased mRNA levels of key developmental activators. Moreover, ΔrgsD leads to increased spore tolerance to UV and oxidative stress, which might be associated with the enhanced expression of melanin biosynthetic genes and increased amount of melanin. Yeast two-hybrid assays reveal that RgsD can interact with the three Gα proteins GpaB, GanA, and GpaA, showing the highest interaction potential with GpaB. Importantly, the ΔrgsD mutant shows elevated expression of genes in the cAMP-dependent protein kinase A (PKA) pathway and PKA catalytic activity. The ΔrgsD mutant also display increased gliotoxin production and elevated virulence toward Galleria mellonella wax moth larvae. Transcriptomic analyses using RNA-seq reveal the expression changes associated with the diverse phenotypic outcomes caused by ΔrgsD. Collectively, we conclude that RgsD attenuates cAMP-PKA signaling pathway and negatively regulates asexual development, toxigenesis, melanin production, and virulence in A. fumigatus.

Assignment of a dubious gene cluster to melanin biosynthesis in the tomato fungal pathogen Cladosporium fulvum

Pigments and phytotoxins are crucial for the survival and spread of plant pathogenic fungi. The genome of the tomato biotrophic fungal pathogen Cladosporium fulvum contains a predicted gene cluster (CfPKS1, CfPRF1, CfRDT1 and CfTSF1) that is syntenic with the characterized elsinochrome toxin gene cluster in the citrus pathogen Elsinoë fawcettii. However, a previous phylogenetic analysis suggested that CfPks1 might instead be involved in pigment production. Here, we report the characterization of the CfPKS1 gene cluster to resolve this ambiguity. Activation of the regulator CfTSF1 specifically induced the expression of CfPKS1 and CfRDT1, but not of CfPRF1. These co-regulated genes that define the CfPKS1 gene cluster are orthologous to genes involved in 1,3-dihydroxynaphthalene (DHN) melanin biosynthesis in other fungi. Heterologous expression of CfPKS1 in Aspergillus oryzae yielded 1,3,6,8-tetrahydroxynaphthalene, a typical precursor of DHN melanin. Δcfpks1 deletion mutants showed similar altered pigmentation to wild type treated with DHN melanin inhibitors. These mutants remained virulent on tomato, showing this gene cluster is not involved in pathogenicity. Altogether, our results showed that the CfPKS1 gene cluster is involved in the production of DHN melanin and suggests that elsinochrome production in E. fawcettii likely involves another gene cluster.

Characterization of the Neurospora crassa DHN melanin biosynthetic pathway in developing ascospores and peridium cells

Neurospora crassa contains all four enzymes for the synthesis of DHN (dihydroxynaphthalene), the substrate for melanin formation. We show that the DHN melanin pathway functions during N. crassa female development to generate melanized peridium and ascospore cell walls. N. crassa contains one polyketide synthase (PER-1), two polyketide hydrolases (PKH-1 and PKH-2), two THN (tetrahydroxynaphthalene) reductases (PKR-1 and PKR-2), and one scytalone dehydratase (SCY-1). We show that the PER-1, PKH-1, PKR-1 and SCY-1 are required for ascospoer melanization. We also identified the laccase that functions in the conversion of DHN into melanin via a free radical oxidative polymerization reaction, and have named the gene lacm-1 (laccase for melanin formation-1). In maturing perithecia, we show that LACM-1 is localized to the peridium cell wall space while the DHN pathway enzymes are localized to intracellular vesicles. We present a model for melanin formation in which melanin is formed within the cell wall space and the cell wall structure is similar to "reinforced concrete" with the cell wall glucan, chitin, and glycoproteins encased within the melanin polymer. This arrangement provides for a very strong and resilient cell wall and protects the glucan/chitin/glycoprotein matrix from digestion from enzymes and damage from free radicals.

Comparative proteomic analysis reveals the regulatory network of the veA gene during asexual and sexual spore development of Aspergillus cristatus

Aspergillus cristatus is the predominant fungal population during fermentation of Chinese Fuzhuan brick tea, and belongs to the homothallic fungal group that undergoes a sexual stage without asexual conidiation under hypotonic conditions, while hypertonic medium induces initiation of the asexual stage and completely blocks sexual development. However, the veA deletion mutant only produces conidia in hypotonic medium after a 24-h culture, but both asexual and sexual spores are observed after 72 h. The veA gene is one of the key genes that positively regulates sexual and negatively regulates asexual development in A. cristatus. To elucidate the molecular mechanism of how VeA regulates asexual and sexual spore development in A. cristatus, 2D electrophoresis (2-DE) combined with MALDI-tandem ToF MS analysis were applied to identify 173 differentially expressed proteins (DEPs) by comparing the agamotype (24 h) and teleomorph (72 h) with wild-type (WT) A. cristatus strains. Further analysis revealed that the changed expression pattern of Pmk1-MAPK and Ser/Thr phosphatase signaling, heat shock protein (Hsp) 90 (HSP90), protein degradation associated, sulphur-containing amino acid biosynthesis associated, valine, leucine, isoleucine, and arginine biosynthesis involved, CYP450 and cytoskeletal formation associated proteins were involved in the production of conidia in agamotype of A. cristatus. Furthermore, the deletion of veA in A. cristatus resulted in disturbed process of transcription, translation, protein folding, amino acid metabolism, and secondary metabolism. The carbohydrate and energy metabolism were also greatly changed, which lied in the suppression of anabolism through pentose phosphate pathway (PPP) but promotion of catabolism through glycolysis and tricarboxylic acid (TCA) cycle. The energy compounds produced in the agamotype were mainly ATP and NADH, whereas they were NADPH and FAD in the teleomorph. These results will contribute to the existing knowledge on the complex role of VeA in the regulation of spore development in Aspergillus and provide a framework for functional investigations on the identified proteins.

Characterization of three multicopper oxidases in the filamentous fungus Podospora anserina: A new role of an ABR1-like protein in fungal development?

The Podospora anserina genome contains a large family of 15 multicopper oxidases (MCOs), including three genes encoding a FET3-like protein, an ABR1-like protein and an ascorbate oxidase (AO)-like protein. FET3, ABR1 and AO1 are involved in global laccase-like activity since deletion of the relevant genes led to a decrease of activity when laccase substrate (ABTS) was used as substrate. However, contrary to the P. anserina MCO proteins previously characterized, none of these three MCOs seemed to be involved in lignocellulose degradation and in resistance to phenolic compounds and oxidative stress. We showed that the bulk of ferroxidase activity was clearly due to ABR1, and only in minor part to FET3, although ABR1 does not contain all the residues typical of FET3 proteins. Moreover, we showed that ABR1, related to the Aspergillus fumigatus ABR1 protein, was clearly and specifically involved in pigmentation of ascospores. Surprisingly, phenotypes were more severe in mutants lacking both abr1 and ao1. Deletion of the ao1 gene led to an almost total loss of AO activity. No direct involvement of AO1 in fungal developmental process in P. anserina was evidenced, except in a abr1^Δ background. Overall, unlike other previously characterized MCOs, we thus evidence a clear involvement of ABR1 protein in fungal development.

Functional Characterization of the alb1 Orthologue Gene in the Ochratoxigenic Fungus Aspergillus carbonarius (AC49 strain)

Aspergillus carbonarius, belonging to the group Nigri, is the main species responsible for contamination by ochratoxin A (OTA) in grapes and derivative products. OTA can accumulate in the mycelium and in black conidia of the fungus and released into the matrix. Here, we have deleted in A. carbonarius the alb1 orthologue gene of A. fumigatus, involved in melanin biosynthesis. Three A. carbonarius Δalb1 mutants were characterized for morphologic traits and OTA production on different media and temperatures. Δalb1 mutants showed a fawn color of conidia associated with a significant reduction of the conidiogenesis and a statistically significant increase (p ≤ 0.01) of total OTA production as compared to the wild type (WT) strain. The alb1 gene somehow affected OTA partitioning since in Δalb1 mutants OTA amount was lower in conidia and was more abundantly secreted into the medium as compared to the WT. On grape berries the Δalb1 mutants and the WT caused lesions with similar sizes but OTA amount in berry tissues was higher for the mutants. These results demonstrate that A. carbonarius conidia pigmentation is largely dependent on polyketide biosynthesis. The gene is not directly involved in virulence and its deletion affects morphological features and OTA production in the fungus.

The StLAC2 gene is required for cell wall integrity, DHN-melanin synthesis and the pathogenicity of Setosphaeria turcica

Laccases are blue multicopper oxidases, play important roles in various biological processes. These processes include fungal dihydroxynaphthalene (DHN)-melanin biosynthesis and pathogenicity, cellular growth, morphogenesis, and differentiation. This study investigated functions of the laccase gene StLAC2 in Setosphaeria turcica. The Δlac2 mutant colony color was distinct from that of the S. turcica wild-type (WT) isolate, and the mutants exhibited defective conidial formation. In contrast to the WT, the mutants exhibited a lighter color on the 2, 2-azino-di-[3-ethylbenzo-thia-zolin-sulphonate] (ABTS) plates, and the intracellular laccase activity was lower. Notably, StLAC2 gene loss correlated with decreased DHN-melanin biosynthesis and affected the integrity of the cell wall, where the StLAC2 gene mutants showed thinner, more transparent walls with a higher number of mitochondria than the WT. The Δlac2 mutants also lost their pathogenicity in maize. The results indicated that the StLAC2 gene involved in cell wall integrity, melanin biosynthesis and appressorial and conidial formation.

Dirhamnolipids secreted from Pseudomonas aeruginosa modify anjpegungal susceptibility of Aspergillus fumigatus by inhibiting β1,3 glucan synthase activity

Pseudomonas aeruginosa and Aspergillus fumigatus are the two microorganisms responsible for most of the chronic infections in cystic fibrosis patients. P. aeruginosa is known to produce quorum-sensing controlled rhamnolipids during chronic infections. Here we show that the dirhamnolipids secreted from P. aeruginosa (i) induce A. fumigatus to produce an extracellular matrix, rich in galactosaminogalactan, 1,8-dihydroxynaphthalene (DHN)- and pyo-melanin, surrounding their hyphae, which facilitates P. aeruginosa binding and (ii) inhibit A. fumigatus growth by blocking β1,3 glucan synthase (GS) activity, thus altering the cell wall architecture. A. fumigatus in the presence of diRhls resulted in a growth phenotype similar to that upon its treatment with anjpegungal echinocandins, showing multibranched hyphae and thicker cell wall rich in chitin. The diRhl structure containing two rhamnose moieties attached to fatty acyl chain is essential for the interaction with β1,3 GS; however, the site of action of diRhls on GS is different from that of echinocandins, and showed synergistic anjpegungal effect with azoles.

Molecular Insights into Pathogenesis and Infection with Aspergillus Fumigatus

The virulence of fungi is dependent on multiple factors, including the immune status of patients and biological features of fungi. In particular, the virulence of Aspergillus fumigatus is due to the complex interaction among various molecules involved in thermotolerance (such as ribosomal biogenesis proteins, α-mannosyltransferase and heat shock proteins), pigment production (DHN-melanin), immune evasion (like melanin and hydrophobin) and nutrient uptake (such as siderophores and zinc transporters). Other molecules also play important roles in the virulence of A. fumigatus, including cell wall components and those which maintain its integrity (for instance β-1-3 glucan, α-1-3 glucan, chitin, galactomannan and mannoproteins) and adhesion (such as hydrophobins), as well as various hydrolytic enzymes (such as serine and aspartic protease, phospholipases, metalloproteinase and dipeptidyl peptidases). Signalling molecules (including G-protein, cAMP, Ras protein and calcineurin) also increase the virulence through altering the metabolic response to stress conditions and toxins (such as gliotoxin, fumitremorgins, fumagatin and helvolic acid).

Transcription factor Afmac1 controls copper import machinery in Aspergillus fumigatus

Copper (Cu) is an essential metal for all living organisms, although it is toxic in excess. Filamentous fungus must acquire copper from its environment for growth. Despite its essentiality for growth, the mechanisms that maintain copper homeostasis are not fully understood in filamentous fungus. To gain insights into copper homeostasis, we investigated the roles of a copper transcription factor Afmac1 in the life-threatening fungus Aspergillus fumigatus, a homolog of the yeast MAC1. We observed that the Afmac1 deletion mutant exhibited not only significantly slower growth, but also incomplete conidiation including a short chain of conidia and defective melanin. Moreover, the expressions of the copper transporters, ctrA1, ctrA2, and ctrC, and metalloreductases, Afu8g01310 and fre7, were repressed in ∆Afmac1 cells, while those expressions were induced under copper depletion conditions in wild-type. The expressions of pksP and wetA, which are, respectively, involved in biosynthesis of conidia-specific melanin and the late stage of conidiogenesis, were decreased in the ∆Afmac1 strain under minimal media condition. Taken together, these results indicate that copper acquisition through AfMac1 functions in growth as well as conidiation.

Talaromyces marneffei laccase modifies THP-1 macrophage responses

Talaromyces (Penicillium) marneffei is an emerging opportunistic pathogen associated with HIV infection, particularly in Southeast Asia and southern China. The rapid uptake and killing of T. marneffei conidia by phagocytic cells along with the effective induction of an inflammatory response by the host is essential for disease control. T. marneffei produces a number of different laccases linked to fungal virulence. To understand the role of the various laccases in T. marneffei, laccase-encoding genes were investigated. Targeted single, double and triple gene deletions of laccases encoding lacA, lacB, and lacC showed no significant phenotypic effects suggesting redundancy of function. When a fourth laccase-encoding gene, pbrB, was deleted in the ΔlacA ΔlacB ΔlacC background, the quadruple mutant displayed delayed conidiation and the conidia were more sensitive to H2O2, sodium dodecyl sulfate (SDS), and antifungal agents than wild-type and other transformants. Conidia of the quadruple mutant showed marked differences in their interaction with the human monocyte cell line, THP-1 such that phagocytosis was significantly higher when compared with the wild-type at one and 2 hours of incubation while the phagocytic index was significantly different from 15 to 120 minutes. In addition, killing of the quadruple mutant by THP-1 cells was more efficient at 2 and 4 hours of incubation. The levels of the proinflammatory cytokines TNF-α, IL-1β and IL-6 from THP-1 cells infected with the quadruple mutant were also significantly increased in comparison with wild-type. The results demonstrate that production of laccases by T. marneffei actually promotes the pathogen's resistance to innate host defenses.

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.

Subcellular Compartmentalization and Trafficking of the Biosynthetic Machinery for Fungal Melanin

Protection by melanin depends on its subcellular location. Although most filamentous fungi synthesize melanin via a polyketide synthase pathway, where and how melanin biosynthesis occurs and how it is deposited as extracellular granules remain elusive. Using a forward genetic screen in the pathogen Aspergillus fumigatus, we find that mutations in an endosomal sorting nexin abolish melanin cell-wall deposition. We find that all enzymes involved in the early steps of melanin biosynthesis are recruited to endosomes through a non-conventional secretory pathway. In contrast, late melanin enzymes accumulate in the cell wall. Such subcellular compartmentalization of the melanin biosynthetic machinery occurs in both A. fumigatus and A. nidulans. Thus, fungal melanin biosynthesis appears to be initiated in endosomes with exocytosis leading to melanin extracellular deposition, much like the synthesis and trafficking of mammalian melanin in endosomally derived melanosomes.

Cloning and functional analysis of a laccase gene during fruiting body formation in Hypsizygus marmoreus

The Hypsizygus marmoreus laccase gene (lcc1) sequence was cloned and analyzed. The genomic DNA of lcc1 is 2336 bp, comprising 13 introns and 14 exons. The 1626-bp full-length cDNA encodes a mature laccase protein containing 542 amino acids, with a 21-amino acid signal peptide. Phylogenetic analysis showed that the lcc1 amino acid sequence is homologous to basidiomycete laccases and shares the highest similarity with Flammulina velutipes laccase. A 2021-bp promoter sequence containing a TATA box, CAAT box, and several putative cis-acting elements was also identified. To study the function of lcc1, we first overexpressed lcc1 in H. marmoreus and found that the transgenic fungus producing recombinant laccase displayed faster mycelial growth than the wild-type (wt) strain. Additionally, primordium initiation was induced 3-5 days earlier in the transgenic fungus, and fruiting body maturation was also promoted approximately five days earlier than in the wt strain. Furthermore, we detected that lcc1 was sustainably overexpressed and that laccase activity was also higher in the transgenic strains compared with the wt strain during development in H. marmoreus. These results indicate that the H. marmoreus lcc1 gene is involved in mycelial growth and fruiting body initiation by increasing laccase activity.

The pbrB gene encodes a laccase required for DHN-melanin synthesis in conidia of Talaromyces (Penicillium) marneffei

Talaromyces marneffei (Basionym: Penicillium marneffei) is a significant opportunistic fungal pathogen in patients infected with human immunodeficiency virus in Southeast Asia. T. marneffei cells have been shown to become melanized in vivo. Melanins are pigment biopolymers which act as a non-specific protectant against various stressors and which play an important role during virulence in fungi. The synthesis of the two most commonly found melanins in fungi, the eumelanin DOPA-melanin and the allomelanin DHN-melanin, requires the action of laccase enzymes. The T. marneffei genome encodes a number of laccases and this study describes the characterization of one of these, pbrB, during growth and development. A strain carrying a PbrB-GFP fusion shows that pbrB is expressed at high levels during asexual development (conidiation) but not in cells growing vegetatively. The pbrB gene is required for the synthesis of DHN-melanin in conidia and when deleted results in brown pigmented conidia, in contrast to the green conidia of the wild type.

Exposure of Aspergillus fumigatus to T-2 toxin results in a stress response associated with exacerbation of aspergillosis in poultry

Aspergillus fumigatus is a ubiquitous airborne pathogen. Saprophytic growth in the presence of environmental mycotoxins might affect its fitness and virulence. T-2 toxin (T-2) is a trichothecene mycotoxin produced by Fusarium spp. in various substrates. This study aimed to evaluate the effects of T-2 on the fitness of A. fumigatus in vitro and its virulence in experimentally inoculated chickens. We cultured A. fumigatus on agar media containing T-2, and examined the changes in viability, morphology, growth rate, proteome expression, and susceptibility to antimycotics and oxidative stress of this fungus. Results showed that exposure to 1000 ng/ml T-2 in the substrate did not reduce the viability of A. fumigatus, but its growth was inhibited, with wrinkling and depigmentation of the colonies. Proteomic analysis revealed 21 upregulated proteins and 33 downregulated proteins, including those involved in stress response, pathogenesis, metabolism, transcription. The proteome seems to have shifted to enhance the glycolysis, catabolism of lipids, and amino acid conversion. Assays on fungal susceptibility to antimycotics and oxidative stress showed that T-2 exposure did not affect the minimal inhibitory concentrations of amphotericin B, itraconazole, voriconazole and terbinafine against A. fumigatus, but increased the susceptibility of A. fumigatus to H2O2 and menadione. Experimental inoculation of chickens with A. fumigatus showed that exposure of A. fumigatus to T-2 significantly exacerbated aspergillosis in chickens exposed to dietary T-2. In conclusion, A. fumigatus is capable of surviving and growing on substrates containing levels of T-2 up to 1000 ng/ml. Growth in presence of T-2 induces a stress response in A. fumigatus, which is associated with exacerbation of aspergillosis in vivo.

Transcriptome Analysis of a Ustilago maydis ust1 Deletion Mutant Uncovers Involvement of Laccase and Polyketide Synthase Genes in Spore Development

Ustilago maydis, causal agent of corn smut disease, is a dimorphic fungus alternating between a saprobic budding haploid and an obligate pathogenic filamentous dikaryon. Maize responds to U. maydis colonization by producing tumorous structures, and only within these does the fungus sporulate, producing melanized sexual teliospores. Previously we identified Ust1, an APSES (Asm1p, Phd1p, Sok2p, Efg1p, and StuAp) transcription factor, whose deletion led to filamentous haploid growth and the production of highly pigmented teliospore-like structures in culture. In this study, we analyzed the transcriptome of a ust1 deletion mutant and functionally characterized two highly upregulated genes with potential roles in melanin biosynthesis: um05361, encoding a putative laccase (lac1), and um06414, encoding a polyketide synthase (pks1). The Δlac1 mutant strains showed dramatically reduced virulence on maize seedlings and fewer, less-pigmented teliospores in adult plants. The Δpks1 mutant was unaffected in seedling virulence but adult plant tumors generated hyaline, nonmelanized teliospores. Thus, whereas pks1 appeared to be restricted to the synthesis of melanin, lac1 showed a broader role in virulence. In conclusion, the ust1 deletion mutant provided an in vitro model for sporulation in U. maydis, and functional analysis supports the efficacy of this in vitro mutant analysis for identification of genes involved in in planta teliosporogenesis.

Bilirubin oxidase-like proteins from Podospora anserina: promising thermostable enzymes for application in transformation of plant biomass

Plant biomass degradation by fungi is a critical step for production of biofuels, and laccases are common ligninolytic enzymes envisioned for ligninolysis. Bilirubin oxidases (BODs)-like are related to laccases, but their roles during lignocellulose degradation have not yet been fully investigated. The two BODs of the ascomycete fungus Podospora anserina were characterized by targeted gene deletions. Enzymatic assay revealed that the bod1(Δ) and bod2(Δ) mutants lost partly a thermostable laccase activity. A triple mutant inactivated for bod1, bod2 and mco, a previously investigated multicopper oxidase gene distantly related to laccases, had no thermostable laccase activity. The pattern of fruiting body production in the bod1(Δ) bod2(Δ) double mutant was changed. The bod1(Δ) and bod2(Δ) mutants were reduced in their ability to grow on ligneous and cellulosic materials. Furthermore, bod1(Δ) and bod2(Δ) mutants were defective towards resistance to phenolic substrates and H2 O2 , which may also impact lignocellulose breakdown. Double and triple mutants were more affected than single mutants, evidencing redundancy of function among BODs and mco. Overall, the data show that bod1, bod2 and mco code for non-canonical thermostable laccases that participate in the degradation of lignocellulose. Thanks to their thermal stability, these enzymes may be more promising candidate for biotechnological application than canonical laccases.

Surface structure characterization of Aspergillus fumigatus conidia mutated in the melanin synthesis pathway and their human cellular immune response

In Aspergillus fumigatus, the conidial surface contains dihydroxynaphthalene (DHN)-melanin. Six-clustered gene products have been identified that mediate sequential catalysis of DHN-melanin biosynthesis. Melanin thus produced is known to be a virulence factor, protecting the fungus from the host defense mechanisms. In the present study, individual deletion of the genes involved in the initial three steps of melanin biosynthesis resulted in an altered conidial surface with masked surface rodlet layer, leaky cell wall allowing the deposition of proteins on the cell surface and exposing the otherwise-masked cell wall polysaccharides at the surface. Melanin as such was immunologically inert; however, deletion mutant conidia with modified surfaces could activate human dendritic cells and the subsequent cytokine production in contrast to the wild-type conidia. Cell surface defects were rectified in the conidia mutated in downstream melanin biosynthetic pathway, and maximum immune inertness was observed upon synthesis of vermelone onward. These observations suggest that although melanin as such is an immunologically inert material, it confers virulence by facilitating proper formation of the A. fumigatus conidial surface.

Cloning, characterization and expression of a novel laccase gene Pclac2 from Phytophthora capsici

Laccases are blue copper oxidases (E.C. 1.10.3.2) that catalyze the one-electron oxidation of phenolics, aromatic amines, and other electron-rich substrates with the concomitant reduction of O2 to H2O. A novel laccase gene pclac2 and its corresponding full-length cDNA were cloned and characterized from Phytophthora capsici for the first time. The 1683 bp full-length cDNA of pclac2 encoded a mature laccase protein containing 560 amino acids preceded by a signal peptide of 23 amino acids. The deduced protein sequence of PCLAC2 showed high similarity with other known fungal laccases and contained four copper-binding conserved domains of typical laccase protein. In order to achieve a high level secretion and full activity expression of PCLAC2, expression vector pPIC9K with the Pichia pastoris expression system was used. The recombinant PCLAC2 protein was purified and showed on SDS-PAGE as a single band with an apparent molecular weight ca. 68 kDa. The high activity of purified PCLAC2, 84 U/mL, at the seventh day induced with methanol, was observed with 2,2'-azino-di-(3-ethylbenzothialozin-6-sulfonic acid) (ABTS) as substrate. The optimum pH and temperature for ABTS were 4.0 and 30 °C, respectively. The reported data add a new piece to the knowledge about P. Capsici laccase multigene family and shed light on potential function about biotechnological and industrial applications of the individual laccase isoforms in oomycetes.