Structures and biomimetic synthesis of novel α-pyrone polyketides of an endophytic Penicillium sp. in Catharanthus roseus

Metabolomic Strategy to Characterize the Profile of Secondary Metabolites in Aspergillus aculeatus DL1011 Regulated by Chemical Epigenetic Agents

Chemical epigenetic regulation (CER) is an effective method to activate the silent pathway of fungal secondary metabolite synthesis. However, conventional methods for CER study are laborious and time-consuming. In the meantime, the overall profile of the secondary metabolites in the fungi treated by the CER reagent is not well characterized. In this study, suberohydroxamic acid (SBHA), a histone deacetylase inhibitor, was added to a culture of Aspergillus aculeatus DL1011 and a new strategy based on LC-MS/MS analysis integrated with various metabolomic tools (MetaboAnalyst, MS-DIAL, SIRIUS and GNPS) was developed to characterize the profile of induced metabolites. As a result, 13.6%, 29.5% and 27.2% of metabolites were identified as newly biosynthesized, increasing and decreasing in abundance by CER, respectively. The structures of the 18 newly induced secondary metabolites were further identified by the new strategy to demonstrate that 72.2% of them (1 novel compound and 12 known compounds) were first discovered in A. aculeatus upon SBHA treatment. The accuracy of the new approach was confirmed by purification and NMR data analysis of major newly biosynthesized secondary metabolites. The bioassay showed that the newly biosynthesized compounds, roseopurpurin analogues, showed selective activities against DPPH scavenging, cytotoxicity and SHP1 inhibition. Our research demonstrated that CER was beneficial for changing the secondary metabolic profile of fungi and was an effective means of increasing the diversity of active metabolites. Our work also supplied a metabolomic strategy to characterize the profile changes and determine the newly induced compounds in the secondary metabolites of fungi treated with the chemical epigenetic regulator.

Exploring Verrucosidin Derivatives with Glucose-Uptake-Stimulatory Activity from Penicillium cellarum Using MS/MS-Based Molecular Networking

Under the guidance of LC-MS/MS-based molecular networking, seven new verrucosidin derivatives, penicicellarusins A-G (3–9), were isolated together with three known analogues from the fungus Penicillium cellarum. The structures of the new compounds were determined by a combination of NMR, mass and electronic circular dichroism spectral data analysis. The absolute configuration of penicyrone A (10) was corrected based on X-ray diffraction analyses. Bioactivity screening indicated that compounds 1, 2, and 4 showed much stronger promising hypoglycemic activity than the positive drug (rosiglitazone) in the range of 25–100 μM, which represents a potential new class of hypoglycemic agents. Preliminary structure-activity relationship analysis indicates that the formation of epoxy ring on C₆-C₇ in the structures is important for the glucose uptake-stimulating activity. The gene cluster for the biosynthesis of 1–12 is identified by sequencing the genome of P. cellarum and similarity analysis with the gene cluster of verrucosidins in P. polonicum.

New dimeric chromanone derivatives from the mutant strains of Penicillium oxalicum and their bioactivities

Three new chromanone dimer derivatives, paecilins F–H (1–3) and ten known compounds (4–13), were obtained from the mutant strains of Penicillium oxalicum 114-2. Their structures were elucidated by extensive analysis of spectroscopic data and comparison with reported data, and the configurations of 1–3 were resolved by quantum chemical calculations of NMR shifts and ECD spectra. Compounds 5 and 11 showed significant anti-influenza A virus activities with IC₅₀ values of 5.6 and 6.9 μM, respectively. Compounds 8 and 9 displayed cytotoxic activities against the MIA-PaCa-2 cell line with IC₅₀ values of 2.6 and 2.1 μM, respectively. Compound 10 exhibited antibacterial activities against Bacillus cereus with a MIC value of 4 μg mL⁻¹.

Three new chromanone dimers, paecilins F–H (1–3) and ten known compounds (4–13), were obtained from the mutant strains of Penicillium oxalicum 114-2, and some of them showed significant antiviral activities.

Regulation of Secondary Metabolism in the Penicillium Genus

Penicillium, one of the most common fungi occurring in a diverse range of habitats, has a worldwide distribution and a large economic impact on human health. Hundreds of the species belonging to this genus cause disastrous decay in food crops and are able to produce a varied range of secondary metabolites, from which we can distinguish harmful mycotoxins. Some Penicillium species are considered to be important producers of patulin and ochratoxin A, two well-known mycotoxins. The production of these mycotoxins and other secondary metabolites is controlled and regulated by different mechanisms. The aim of this review is to highlight the different levels of regulation of secondary metabolites in the Penicillium genus.

Natural Polypropionates in 1999-2020: An Overview of Chemical and Biological Diversity

Natural polypropionates (PPs) are a large subgroup of polyketides with diverse structural features and bioactivities. Most of the PPs are discovered from marine organisms including mollusks, fungi and actinomycetes, while some of them are also isolated from terrestrial resources. An increasing number of studies about PPs have been carried out in the past two decades and an updated review is needed. In this current review, we summarize the chemical structures and biological activities of 164 natural PPs reported in 67 research papers from 1999 to 2020. The isolation, structural features and bioactivities of these PPs are discussed in detail. The chemical diversity, bioactive diversity, biodiversity and the relationship between chemical classes and the bioactivities are also concluded.

Shimalactone Biosynthesis Involves Spontaneous Double Bicyclo-Ring Formation with 8π-6π Electrocyclization

Shimalactones A and B are neuritogenic polyketides possessing characteristic oxabicyclo[2.2.1]heptane and bicyclo[4.2.0]octadiene ring systems that are produced by the marine fungus Emericella variecolor GF10. We identified a candidate biosynthetic gene cluster and conducted heterologous expression analysis. Expression of ShmA polyketide synthase in Aspergillus oryzae resulted in the production of preshimalactone. Aspergillus oryzae and Saccharomyces cerevisiae transformants expressing ShmA and ShmB produced shimalactones A and B, thus suggesting that the double bicyclo-ring formation reactions proceed non-enzymatically from preshimalactone epoxide. DFT calculations strongly support the idea that oxabicyclo-ring formation and 8π-6π electrocyclization proceed spontaneously after opening of the preshimalactone epoxide ring through protonation. We confirmed the formation of preshimalactone epoxide in vitro, followed by its non-enzymatic conversion to shimalactones in the dark.

Fungi: outstanding source of novel chemical scaffolds

A large number of remarkable studies on the secondary metabolites of fungi have been conducted in recent years. This review gives an overview of one hundred and sixty-seven molecules with novel skeletons and their bioactivities that have been reported in seventy-nine articles published from 2013 to 2017. Our statistical data showed that endophytic fungi and marine-derived fungi are the major sources of novel bioactive secondary metabolites.

Functional expression of a highly-reducing polyketide synthase of Emericella variecolor IFM42010, an asteltoxin-producing strain, resulted in production of two polyenoic β-ketolactones with opposite stereochemistry

The asteltoxin-producing fungus Emericella variecolor IFM42010 possesses 22 highly-reducing polyketide synthase (HR-PKS) genes. Of these, an HR-PKS with a methyltransferase domain but lacking an enoylreductase domain could be involved in the biosynthesis of asteltoxin and related compounds. From six such candidate HR-PKS genes, Ev460pks was analyzed by gene disruption in E. variecolor and heterologous expression in Aspergillus oryzae. The Ev460pks-disrupted strain retained asteltoxin production ability, indicating that Ev460pks is not involved in asteltoxin biosynthesis. The A. oryzae transformant harboring the Ev460pks gene produced compounds 1 and 2, along with several unidentified products possibly decomposed from 2. Spectroscopic analyses revealed that 1 was a 4-methyl-β-ketolactone with a methylheptatriene side-chain at the C-5 position, and 2 was also a 4-methyl-β-ketolactone, bearing a dimethyltetradecahexaene side-chain at the same position. The relative configuration at C-4 in compounds 1 and 2 was opposite.

Broad-Spectrum Antiviral Natural Products from the Marine-Derived Penicillium sp. IMB17-046

A new pyrazine derivative, trypilepyrazinol (1), a new α-pyrone polyketide, (+)-neocitreoviridin (2), and a new ergostane analogue, 3β-hydroxyergosta-8,14,24(28)-trien-7-one (3), were isolated and characterized along with five known compounds from the marine-derived fungus Penicillium sp. IMB17-046. The structures of these new compounds were determined using spectroscopic data analyses (HRESIMS, 1D- and 2D-NMR), X-ray crystallography analysis, and TDDFT ECD calculation. Compounds 1 and 3 exhibited broad-spectrum antiviral activities against different types of viruses, including human immunodeficiency virus (HIV), hepatitis C virus (HCV), and influenza A virus (IAV), with IC₅₀ values ranging from 0.5 to 7.7 μM. Compounds 1 and 2 showed antibacterial activities against Helicobacter pylori, a causative pathogen of various gastric diseases, with minimum inhibitory concentration (MIC) values of 1–16 μg/mL.

Diversity and biological activities of endophytic fungi associated with Catharanthus roseus

BACKGROUND

The present study involves diversity and bioactivity of the endophytic fungal community from Catharanthus roseus inhabiting the coastal region. This study has been conducted hypothesizing that the microbial communities in the coastal regions would tolerate a range of abiotic stress such as salinity, humidity, temperature and soil composition, and it may produce new metabolites, which may possess bioactive property. Therefore in the current study, the cytotoxicity and free radical scavenging potential of the fungal organic extracts have been investigated. Moreover, the apoptotic and the antioxidant potential of the fungus that exhibited the best activity in preliminary screening has also been demonstrated.

RESULTS

Twenty endophytic fungal isolates were obtained from different parts of the plant, and identified using internal transcribed spacer region analysis. Based on the colonization frequency, the dominant genera were found to be Colletotrichum, Alternaria and Chaetomium with colonization frequency % of 8.66, 7.00 and 6.33, respectively. It was observed that the species diversity and richness was the highest in bark followed by leaf and stem regions of the plant. On screening the fungal ethyl acetate extracts for cytotoxicity against the HeLa cells, the Chaetomium nigricolor extract exhibited potent cytotoxic activity of 92.20% at 100 μg mL^- 1 concentration. Comparison between the different organic extracts (ethyl acetate, chloroform, dichloromethane and hexane) of Chaetomium nigricolor mycelial and culture filtrate, it was observed that the mycelial as well the culture filtrate ethyl acetate extracts and the culture filtrate hexane extract showed significant cytotoxic potential against the HeLa and MCF-7 cells, respectively. The apoptotic- and mitochondrial membrane depolarisation-induction potential of the Chaetomium nigricolor ethyl acetate extract has also been demonstrated in this study. Further the screening of antioxidant potential of the ethyl acetate fungal extracts using DPPH scavenging assay showed that Chaetomium nigricolor extract exhibited potential activity with a significant EC₅₀ value of 22 μg mL^- 1. The ethyl acetate extract of Chaetomium nigricolor also exhibited superoxide radical scavenging potential.

CONCLUSION

These results indicated that diverse endophytic fungal population inhabits Catharanthus roseus. One of the fungal isolate Chaetomium nigricolor exhibited significant cytotoxic, apoptotic and antioxidant potential.

Structural Diversity and Biological Activities of Novel Secondary Metabolites from Endophytes

Exploration of structurally novel natural products greatly facilitates the discovery of biologically active pharmacophores that are biologically validated starting points for the development of new drugs. Endophytes that colonize the internal tissues of plant species, have been proven to produce a large number of structurally diverse secondary metabolites. These molecules exhibit remarkable biological activities, including antimicrobial, anticancer, anti-inflammatory and antiviral properties, to name but a few. This review surveys the structurally diverse natural products with new carbon skeletons, unusual ring systems, or rare structural moieties that have been isolated from endophytes between 1996 and 2016. It covers their structures and bioactivities. Biosynthesis and/or total syntheses of some important compounds are also highlighted. Some novel secondary metabolites with marked biological activities might deserve more attention from chemists and biologists in further studies.

Epigenetic Modulation of Endophytic Eupenicillium sp. LG41 by a Histone Deacetylase Inhibitor for Production of Decalin-Containing Compounds

An endophytic fungus, Eupenicillium sp. LG41, isolated from the Chinese medicinal plant Xanthium sibiricum, was subjected to epigenetic modulation using an NAD⁺-dependent histone deacetylase (HDAC) inhibitor, nicotinamide. Epigenetic stimulation of the endophyte led to enhanced production of two new decalin-containing compounds, eupenicinicols C and D (3 and 4), along with two biosynthetically related known compounds, eujavanicol A (1) and eupenicinicol A (2). The structures and stereochemistry of the new compounds were elucidated by extensive spectroscopic analysis using LC-HRMS, NMR, optical rotation, and ECD calculations, as well as single-crystal X-ray diffraction. Compounds 3 and 4 exist in chemical equilibrium with two and three cis/trans isomers, respectively, as revealed by LC-MS analysis. Compound 4 was active against Staphylococcus aureus with an MIC of 0.1 μg/mL and demonstrated marked cytotoxicity against the human acute monocytic leukemia cell line (THP-1). We have shown that the HDAC inhibitor, nicotinamide, enhanced the production of compounds 3 and 4 by endophytic Eupenicillium sp. LG41, facilitating their isolation, structure elucidation, and evaluation of their biological activities.

Epigenetic stimulation of polyketide production in Chaetomium cancroideum by an NAD(+)-dependent HDAC inhibitor

Exposure of the fungus Chaetomium cancroideum to an NAD(+)-dependent HDAC inhibitor, nicotinamide, enhanced the production of aromatic and branched aliphatic polyketides, which allowed us to isolate new secondary metabolites, chaetophenol G and cancrolides A and B. Their structures were determined using spectroscopic analyses, and their absolute configuration was elucidated by electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and chemical transformations. Biosynthesis of the branched aliphatic polyketide skeletons in cancrolides A and B was evidenced by conducting a feeding experiment using compounds labeled with a (13)C stable isotope.

Expanding the species and chemical diversity of Penicillium section Cinnamopurpurea

A set of isolates very similar to or potentially conspecific with an unidentified Penicillium isolate NRRL 735, was assembled using a BLAST search of ITS similarity among described (GenBank) and undescribed Penicillium isolates in our laboratories. DNA was amplified from six loci of the assembled isolates and sequenced. Two species in section Cinnamopurpurea are self-compatible sexual species, but the asexual species had polymorphic loci suggestive of sexual reproduction and variation in conidium size suggestive of ploidy level differences typical of heterothallism. Accordingly we use genealogical concordance analysis, a technique valid only in heterothallic organisms, for putatively asexual species. Seven new species were revealed in the analysis and are described here. Extrolite analysis showed that two of the new species, P. colei and P. monsserratidens produce the mycotoxin citreoviridin that has demonstrated pharmacological activity against human lung tumors. These isolates could provide leads in pharmaceutical research.

A simple base-mediated synthesis of diverse functionalized ring-fluorinated 4H-pyrans via double direct C–F substitutions

A simple base-mediated synthesis of diverse substituted ring-fluorinated 4H-pyrans (monofluorinated 4H-pyrans) from trifluoromethylated alkenes and 1,3-dicarbonyl compounds was developed.

Benzophenones from an endophytic fungus, Graphiopsis chlorocephala, from Paeonia lactiflora cultivated in the presence of an NAD+-dependent HDAC inhibitor

Graphiopsis chlorocephala was separated from the surface-sterilized healthy leaves of Paeonia lactiflora (Paeoniaceae) and cultivated with nicotinamide (an NAD(+)-dependent HDAC inhibitor). The culture conditions significantly enhanced secondary metabolite production in the fungus and led to the isolation of a structurally diverse set of new benzophenones, cephalanones A-F (1-6), and a known 2-(2,6-dihydroxy-4-methylbenzoyl)-6-hydroxybenzoic acid (7). The structures of 1-6 were determined from NMR data, single crystal X-ray diffraction, and chemical transformations.