Melleolides from Honey Mushroom Inhibit 5-Lipoxygenase via Cys159

Nonribosomal peptide synthetase gene clusters and characteristics of predicted NRPS-dependent siderophore synthetases in Armillaria and other species in the Physalacriaceae

Fungal secondary metabolites are often pathogenicity or virulence factors synthesized by genes contained in secondary metabolite gene clusters (SMGCs). Nonribosomal polypeptide synthetase (NRPS) clusters are SMGCs which produce peptides such as siderophores, the high affinity ferric iron chelating compounds required for iron uptake under aerobic conditions. Armillaria spp. are mostly facultative necrotrophs of woody plants. NRPS-dependent siderophore synthetase (NDSS) clusters of Armillaria spp. and selected Physalacriaceae were investigated using a comparative genomics approach. Siderophore biosynthesis by strains of selected Armillaria spp. was evaluated using CAS and split-CAS assays. At least one NRPS cluster and other clusters were detected in the genomes studied. No correlation was observed between the number and types of SMGCs and reported pathogenicity of the species studied. The genomes contained one NDSS cluster each. All NDSSs were multi-modular with the domain architecture (ATC)₃(TC)₂. NDSS clusters of the Armillaria spp. showed a high degree of microsynteny. In the genomes of Desarmillaria spp. and Guyanagaster necrorhizus, NDSS clusters were more syntenic with NDSS clusters of Armillaria spp. than to those of the other Physalacriaceae species studied. Three A-domain orthologous groups were identified in the NDSSs, and atypical Stachelhaus codes were predicted for the A3 orthologous group. In vitro biosynthesis of mainly hydroxamate and some catecholate siderophores was observed. Hence, Armillaria spp. generally contain one highly conserved, NDSS cluster although some interspecific variations in the products of these clusters is expected. Results from this study lays the groundwork for future studies to elucidate the molecular biology of fungal phyto-pathogenicity.

Highly potent and selective 5-lipoxygenase inhibition by new, simple heteroaryl-substituted catechols for treatment of inflammation

5-Lipoxygenase (LO) catalyzes the first steps in the formation of pro-inflammatory leukotrienes (LT) that are pivotal lipid mediators contributing to allergic reactions and inflammatory disorders. Based on its key role in LT biosynthesis, 5-LO is an attractive drug target, demanding for effective and selective inhibitors with efficacy in vivo, which however, are still rare. Encouraged by the recent identification of the catechol 4-(3,4-dihydroxyphenyl)dibenzofuran 1 as 5-LO inhibitor, simple structural modifications were made to yield even more effective and selective catechol derivatives. Within this new series, the two most potent compounds 3,4-dihydroxy-3'-phenoxybiphenyl (6b) and 2-(3,4-dihydroxyphenyl)benzo[b]thiophene (6d) potently inhibited human 5-LO in cell-free (IC506b and 6d = 20 nM) and cell-based assays (IC506b = 70 nM, 6d = 60 nM). Inhibition of 5-LO was reversible, unaffected by exogenously added substrate arachidonic acid, and not primarily mediated via radical scavenging and antioxidant activities. Functional 5-LO mutants expressed in HEK293 cells were still prone to inhibition by 6b and 6d, and docking simulations revealed distinct binding of the catechol moiety to 5-LO at an allosteric site. Analysis of 5-LO nuclear membrane translocation and intracellular Ca2+ mobilization revealed that these 5-LO-activating events are hardly affected by the catechols. Importantly, the high inhibitory potency of 6b and 6d was confirmed in human blood and in a murine zymosan-induced peritonitis model in vivo. Our results enclose these novel catechol derivatives as highly potent, novel type inhibitors of 5-LO with high selectivity and with marked effectiveness under pathophysiological conditions.

A Combination of Genome Mining with an OSMAC Approach Facilitates the Discovery of and Contributions to the Biosynthesis of Melleolides from the Basidiomycete Armillaria tabescens

Genome mining revealed that the genomes of basidiomycetes may include a considerable number of biosynthetic gene clusters (BGCs), yet numerous clusters remain unidentified. Herein, we report a combination of genome mining with an OSMAC (one strain, many compounds) approach to characterize the spectrum of melleolides produced by Armillaria tabescens CPCC 401429. Using F1 fermentation medium, the metabolic pathway of the gene cluster mel was successfully upregulated. From the extracts of the wild-type strain, two new melleolides (1 and 2), along with five new orsellinic acid-derived lactams (10-14), were isolated, and their structures were elucidated by LC-HR-ESIMS/MS and 2D-NMR. Several melleolides exhibited moderate anti-carcinoma (A549, NCI-H520, and H1299) effects with IC₅₀ values of 4.0-48.8 μM. RNA-sequencing based transcriptomic profiling broadened our knowledge of the genetic background, regulation, and mechanisms of melleolide biosynthesis. These results may promote downstream metabolic engineering studies of melleolides. Our study demonstrates the approach is effective for discovering new secondary metabolites from Armillaria sp. and will facilitate the mining of the unexploited biosynthetic potential in other basidiomycetes.

Roles of cysteine in the structure and metabolic function of Mycobacterium tuberculosis CYP142A1

CYP142A1 is a cytochrome P450 (CYP) enzyme expressed in Mycobacterium tuberculosis (Mtb), which supports the growth of Mtb H37Rv relying on cholesterol, in the absence of CYP125A1. Since cysteine residues usually play a fundamental role in maintaining the structure and function of CYP enzymes, in this study, we aimed to determine the potential biochemical functions of six cysteine residues except for the heme-binding cysteine in the amino acid sequence of recombinant Mtb CYP142A1 by replacing each one using site-directed mutagenesis. Recombinant CYP142A1 mutants were heterologously expressed, purified, and analyzed using ESI-MS, far-UV CD spectroscopy, UV-vis spectrophotometric titration, and metabolic function assays. Substitution of the cysteine residues caused various effects on the structure and function of CYP142A1. Separate substitution of the six cysteine residues resulted in numerous changes in the secondary structure, expression level, substrate-binding ability, inhibitor-binding ability, thermal stability and oxidation efficiency of the enzyme. These results contribute to our understanding of the biochemical roles of cysteine residues in the structure and function of Mtb CYP enzymes, especially their effects on the structure and function of CYP142A1.

Substitution of the six cysteine residues resulted in changes in Mtb CYP142A1 structure, binding ability, thermal stability and oxidation efficiency.

The Genomic and Transcriptomic Analyses of Floccularia luteovirens, a Rare Edible Fungus in the Qinghai-Tibet Plateau, Provide Insights into the Taxonomy Placement and Fruiting Body Formation

Floccularia luteovirens is a famous and precious edible mushroom (Huang Mogu) on the Qinghai–Tibet plateau that has a unique flavor and remarkable medical functions. Herein, we report a reference-grade 27 Mb genome of F. luteovirens containing 7068 protein-coding genes. The genome component and gene functions were predicted. Genome ontology enrichment and pathway analyses indicated the potential production capacity for terpenoids, polyketides and polysaccharides. Moreover, 16 putative gene clusters and 145 genes coding for secondary metabolites were obtained, including guadinomine and melleolides. In addition, phylogenetic and comparative genomic analyses shed light on the precise classification of F. luteovirens suggesting that it belongs to the genus Floccularia instead of Armillaria. RNA-sequencing and comparative transcriptomic analysis revealed differentially expressed genes during four developmental stages of F. luteovirens, that of which helps to identify important genes regulating fruiting body formation for strain modification. This study will provide insight into artificial cultivation and increase the production of useful metabolites.

Biochemical Characterization of 13-Lipoxygenases of Arabidopsis thaliana

13-lipoxygenases (13-LOX) catalyze the dioxygenation of various polyunsaturated fatty acids (PUFAs), of which α-linolenic acid (LeA) is converted to 13-S-hydroperoxyoctadeca-9, 11, 15-trienoic acid (13-HPOT), the precursor for the prostaglandin-like plant hormones cis-(+)-12-oxophytodienoic acid (12-OPDA) and methyl jasmonate (MJ). This study aimed for characterizing the four annotated A. thaliana 13-LOX enzymes (LOX2, LOX3, LOX4, and LOX6) focusing on synthesis of 12-OPDA and 4Z,7Z,10Z)-12-[[-(1S,5S)-4-oxo-5-(2Z)-pent-2-en-1yl] cyclopent-2-en-1yl] dodeca-4,7,10-trienoic acid (OCPD). In addition, we performed interaction studies of 13-LOXs with ions and molecules to advance our understanding of 13-LOX. Cell imaging indicated plastid targeting of fluorescent proteins fused to 13-LOXs-N-terminal extensions, supporting the prediction of 13-LOX localization to plastids. The apparent maximal velocity (V_max app) values for LOX-catalyzed LeA oxidation were highest for LOX4 (128 nmol·s⁻¹·mg protein⁻¹), with a K_m value of 5.8 µM. A. thaliana 13-LOXs, in cascade with 12-OPDA pathway enzymes, synthesized 12-OPDA and OCPD from LeA and docosahexaenoic acid, previously shown only for LOX6. The activities of the four isoforms were differently affected by physiologically relevant chemicals, such as Mg²⁺, Ca²⁺, Cu²⁺ and Cd²⁺, and by 12-OPDA and MJ. As demonstrated for LOX4, 12-OPDA inhibited enzymatic LeA hydroperoxidation, with half-maximal enzyme inhibition at 48 µM. Biochemical interactions, such as the sensitivity of LOX toward thiol-reactive agents belonging to cyclopentenone prostaglandins, are suggested to occur in human LOX homologs. Furthermore, we conclude that 13-LOXs are isoforms with rather specific functional and regulatory enzymatic features.

Isolation of a gene cluster from Armillaria gallica for the synthesis of armillyl orsellinate-type sesquiterpenoids

Melleolides and armillyl orsellinates are protoilludene-type aryl esters that are synthesized exclusively by parasitic fungi of the globally distributed genus Armillaria (Agaricomycetes, Physalacriaceae). Several of these compounds show potent antimicrobial and cytotoxic activities, making them promising leads for the development of new antibiotics or drugs for the treatment of cancer. We recently cloned and characterized the Armillaria gallica gene Pro1 encoding protoilludene synthase, a sesquiterpene cyclase catalyzing the pathway-committing step to all protoilludene-type aryl esters. Fungal enzymes representing secondary metabolic pathways are sometimes encoded by gene clusters, so we hypothesized that the missing steps in the pathway to melleolides and armillyl orsellinates might be identified by cloning the genes surrounding Pro1. Here we report the isolation of an A. gallica gene cluster encoding protoilludene synthase and four cytochrome P450 monooxygenases. Heterologous expression and functional analysis resulted in the identification of protoilludene-8α-hydroxylase, which catalyzes the first committed step in the armillyl orsellinate pathway. This confirms that ∆-6-protoilludene is a precursor for the synthesis of both melleolides and armillyl orsellinates, but the two pathways already branch at the level of the first oxygenation step. Our results provide insight into the synthesis of these valuable natural products and pave the way for their production by metabolic engineering. KEY POINTS: • Protoilludene-type aryl esters are bioactive metabolites produced by Armillaria spp. • The pathway-committing step to these compounds is catalyzed by protoilludene synthase. • We characterized CYP-type enzymes in the cluster and identified novel intermediates.

Arachidonic acid promotes the binding of 5-lipoxygenase on nanodiscs containing 5-lipoxygenase activating protein in the absence of calcium-ions

Among the first steps in inflammation is the conversion of arachidonic acid (AA) stored in the cell membranes into leukotrienes. This occurs mainly in leukocytes and depends on the interaction of two proteins: 5-lipoxygenase (5LO), stored away from the nuclear membranes until use and 5-lipoxygenase activating protein (FLAP), a transmembrane, homotrimeric protein, constitutively present in nuclear membrane. We could earlier visualize the binding of 5LO to nanodiscs in the presence of Ca²⁺-ions by the use of transmission electron microscopy (TEM) on samples negatively stained by sodium phosphotungstate. In the absence of Ca²⁺-ions 5LO did not bind to the membrane. In the present communication, FLAP reconstituted in the nanodiscs which could be purified if the His-tag was located on the FLAP C-terminus but not the N-terminus. Our aim was to find out if 1) 5LO would bind in a Ca²⁺-dependent manner also when FLAP is present? 2) Would the substrate (AA) have effects on 5LO binding to FLAP-nanodiscs? TEM was used to assess the complex formation between 5LO and FLAP-nanodiscs along with, sucrose gradient purification, gel-electrophoresis and mass spectrometry. It was found that presence of AA by itself induces complex formation in the absence of added calcium. This finding corroborates that AA is necessary for the complex formation and that a Ca²⁺-flush is mainly needed for the recruitment of 5LO to the membrane. Our results also showed that the addition of Ca²⁺-ions promoted binding of 5LO on the FLAP-nanodiscs as was also the case for nanodiscs without FLAP incorporated. In the absence of added substances no 5LO-FLAP complex was formed. Another finding is that the formation of a 5LO-FLAP complex appears to induce fragmentation of 5LO in vitro.

Rapid Characterization of Chemical Components in Edible Mushroom Sparassis crispa by UPLC-Orbitrap MS Analysis and Potential Inhibitory Effects on Allergic Rhinitis

Sparassis crispa is a kind of edible fungus widely grows in the north temperate zone, which shows various medicinal properties. Due to the complexity of chemical constitutes of this species, few investigations have acquired a comprehensive configuration for the chemical profile of it. In this study, a strategy based on ultra-high performance liquid chromatography (UPLC) combined with Orbitrap mass spectrometer (MS) was established for rapidly characterizing various chemical components in S. crispa. Through the summarized MS/MS fragmentation patterns of reference compounds and systematic identification strategy, a total of 110 components attributed to six categories were identified for the first time. Moreover, allergic rhinitis (AR) is a worldwide inflammatory disease seriously affecting human health, and the development of drugs to treat AR has been a topic of interest. It has been reported that the extracts of S. crispa showed obvious inhibitory effects on degranulation of mast cell- and allergen-induced IgE and proinflammatory mediators, but the active components and specific mechanism were still not clear. Src family kinases (SFKs) participate in the initial stage of allergy occurrence, which are considered the targets of AR treatment. Herein, on the basis of that self-built chemical database, virtual screening was applied to predict the potential SFKs inhibitors in S. crispa, using known crystal structures of Hck, Lyn, Fyn, and Syk as receptors, followed by the anti-inflammatory activity evaluation for screened hits by intracellular calcium mobilization assay. As results, sparoside A was directly confirmed to have strong anti-inflammatory activity with an IC50 value of 5.06 ± 0.60 μM. This study provides a useful elucidation for the chemical composition of S. crispa, and demonstrated its potential inhibitory effects on AR, which could promote the research and development of effective agents from natural resources.

Melleolides impact fungal translation via elongation factor 2

Melleolides from the honey mushroom Armillaria mellea represent a structurally diverse group of polyketide-sesquiterpene hybrids. Among various bioactivites, melleolides show antifungal effects against Aspergillus and other fungi. This bioactivity depends on a Δ2,4-double bond present in dihydroarmillylorsellinate (DAO) or arnamial, for example. Yet, the mode of action of Δ2,4-unsaturated, antifungal melleolides has been unknown. Here, we report on the molecular target of DAO in the fungus Aspergillus nidulans. Using a combination of synthetic chemistry to create a DAO-labelled probe, protein pulldown assays, MALDI-TOF-based peptide analysis and western blotting, we identify the eukaryotic translation elongation factor 2 (eEF2) as a binding partner of DAO. We confirm the inhibition of protein biosynthesis in vivo with an engineered A. nidulans strain producing the red fluorescent protein mCherry. Our work suggests a binding site dissimilar from that of the protein biosynthesis inhibitor sordarin, and highlights translational elongation as a valid antifungal drug target.