Chemical diversity and biological activities of specialized metabolites from the genus Chaetomium: 2013-2022

Chaetomium (Chaetomiaceae), a large fungal genus consisting of at least 400 species, has been acknowledged as a promising resource for the exploration of novel compounds with potential bioactivities. Over the past decades, emerging chemical and biological investigations have suggested the structural diversity and extensive potent bioactivity of the specialized metabolites in the Chaetomium species. To date, over 500 compounds with diverse chemical types have been isolated and identified from this genus, including azaphilones, cytochalasans, pyrones, alkaloids, diketopiperazines, anthraquinones, polyketides, and steroids. Biological research has indicated that these compounds possess a broad range of bioactivities, including antitumor, anti-inflammatory, antimicrobial, antioxidant, enzyme inhibitory, phytotoxic, and plant growth inhibitory activities. This paper summarizes current knowledge referring to the chemical structure, biological activity, and pharmacologic potency of the specialized metabolites in the Chaetomium species from 2013 to 2022, which might provide insights for the exploration and utilization of bioactive compounds in this genus both in the scientific field and pharmaceutical industry.

Chemical characterization and microencapsulation of extracellular fungal pigments

In this work, extracellular colored metabolites obtained from the filamentous fungi Talaromyces australis and Penicillium murcianum, isolated in the Andean-Patagonian native forests of Chile, were studied as prospect compounds to increase the sustainability of cosmetic products. The chemical and antioxidant properties of these natural pigments were characterized and strategies for their microencapsulation were also studied. UHPLC/MS-MS analyses indicated that the predominant metabolites detected in the cultures of P. murcianum were monascin (m/z = 411.15) and monashexenone (m/z = 319.10), while athrorosin H (m/z = 458.20) and damnacanthal (m/z = 281.05) were detected in cultures of T. australis. ORAC tests revealed that P. murcianum's metabolites had the greatest antioxidant properties with values higher than 2000 μmol of trolox equivalents/g. The fungal metabolites were successfully microencapsulated by ionic gelation into structures made of 1.3% sodium alginate, 0.2% chitosan, and 0.07% hyaluronic acid. The microencapsulation process generated structures of 543.57 ± 0.13 µm of mean diameter (d₅₀) with an efficiency of 30% for P. murcianum, and 329.59 ± 0.15 µm of mean diameter (d₅₀) and 40% efficiency, for T. australis. The chemical and biological characterization show the biotechnological potential of these fungal species to obtain pigments with antioxidant activity that could be useful in the cosmetic industry. The encapsulation process enables the production of easy-to-handle dry powder from the fungal metabolites, which could be potentially marketed as a functional cosmetic ingredient. KEY POINTS: • The predominant fungal pigments were of azaphilone and anthraquinoid classes. • The fungal pigments showed high antioxidant activity by ORAC assay. • Fungal pigment microcapsules obtained by ionic gelation were characterized.

Azaphilone derivatives with anti-inflammatory activity from the mangrove endophytic fungus Penicillium sclerotiorum ZJHJJ-18

Nine undescribed azaphilone derivatives, sclerazaphilones A-H (1-9), and three known analogues (10-12), were obtained and identified from the fermented rice cultures of a mangrove endophytic fungus Penicillium sclerotiorum ZJHJJ-18. 1D and 2D NMR, HRESIMS and spectral data indicated the chemical structures of 1-9, and their absolute configurations were assigned by experimental and computational analyses of electronic circular dichroism (ECD) spectra, and application of the chemical transformations. Compounds 1-4 were the first reported N-containing azaphilone derivatives with 5/6 dicyclic core. The bioassay results showed that compounds 3-5 exhibited effective inhibitory effects on the nitric oxide (NO) production in lipopolysaccharide (LPS)-induced RAW264.7 macrophage cells with IC₅₀ values in the range of 6.30-9.45 μM. Moreover, a molecular docking study was conducted to investigate the probable binding interaction of 3-5 with inducible nitric oxide synthase (iNOS).

Isolation, absolute configurations and bioactivities of pestaphilones A-I: Undescribed methylated side chain containing-azaphilones from Pestalotiopsis oxyanthi

Nine undescribed side chain containing azaphilones, pestaphilones A-I, were isolated from the Anoectochilus roxburghii endophytic fungus Pestalotiopsis oxyanthi. The structures of these isolates were identified by spectroscopic data, electronic circular dichroism (ECD) calculations and comparisons, quantum-chemical ¹³C NMR calculations with DP4+ probability analysis, Rh₂(OCOCF₃)₄-induced ECD, acetonide formation, selective oxidation reaction and X-ray crystallographic data. Structurally, pestaphilones A-I were the first azaphilones characteristically formed via a methyl group at C-9 in the C₇ side chain. More importantly, a selective oxidation reaction was firstly set up to resolve the absolute configuration of flexible side chain containing azaphilones, and an acetonide formation based Rh₂(OCOCF₃)₄-induced ECD experiment was performed to identify the configurations of the oxygenated pyranoquinone core in the azaphilones. In bioassay, pestaphilones A-F displayed potential immunosuppressive activity in concanavalin A (Con A)-induced T lymphocyte proliferation, with IC₅₀ values ranging from (9.36 ± 1.14) μM to (35.21 ± 3.25) μM.

The grass root endophytic fungus Flavomyces fulophazii: An abundant source of tetramic acid and chlorinated azaphilone derivatives

Fungal endophytes are remarkable sources of biologically active metabolites of ecological and pharmacological significance. In this study, fungal isolates producing yellow pigments and originating from grass roots, were identified as the recently described grass root colonizing dark septate endophyte (DSE), Flavomyces fulophazii (Periconiaceae, Pleosporales). While analyzing the metabolite composition of 17 isolates of this fungus, 11 previously undescribed compounds, including four tetramic acids (dihydroxyvermelhotin, hydroxyvermelhotin, methoxyvermelhotin, oxovermelhotin), and seven chlorinated azaphilones (flavochlorines A-G), together with the known tetramic acid vermelhotin, were tentatively identified by high performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS). Among them, flavochlorine A, flavochlorine G, hydroxyvermelhotin and vermelhotin could be isolated by preparative HPLC, thus their structures were also confirmed by nuclear magnetic resonance (NMR) spectroscopy. Vermelhotin was found to be the main compound, reaching its maximum level of 5.5 mg/g in the in vitro cultures of a selected F. fulophazii isolate. A significant amount of vermelhotin was isolated by preparative HPLC from these cultures (4.8 mg from 1.0 g lyophilized culture), confirming the practical utility of F. fulophazii in high-yield vermelhotin production. The main compounds of this endophyte expressed no activity in standardized plant bioassays (i.e., in the Lactuca sativa seed germination and Lemna minor growth tests). An antiproliferative study of the isolated compounds confirmed moderate activity of vermelhotin against a panel of twelve cancer cell lines, with IC₅₀ ranges of 10.1-37.0 μM, without inhibiting the non-cancer Vero cells, suggesting its selectivity towards cancers.

Secondary metabolite biosynthetic diversity in the fungal family Hypoxylaceae and Xylaria hypoxylon

To date little is known about the genetic background that drives the production and diversification of secondary metabolites in the Hypoxylaceae. With the recent availability of high-quality genome sequences for 13 representative species and one relative (Xylaria hypoxylon) we attempted to survey the diversity of biosynthetic pathways in these organisms to investigate their true potential as secondary metabolite producers. Manual search strategies based on the accumulated knowledge on biosynthesis in fungi enabled us to identify 783 biosynthetic pathways across 14 studied species, the majority of which were arranged in biosynthetic gene clusters (BGC). The similarity of BGCs was analysed with the BiG-SCAPE engine which organised the BGCs into 375 gene cluster families (GCF). Only ten GCFs were conserved across all of these fungi indicating that speciation is accompanied by changes in secondary metabolism. From the known compounds produced by the family members some can be directly correlated with identified BGCs which is highlighted herein by the azaphilone, dihydroxynaphthalene, tropolone, cytochalasan, terrequinone, terphenyl and brasilane pathways giving insights into the evolution and diversification of those compound classes. Vice versa, products of various BGCs can be predicted through homology analysis with known pathways from other fungi as shown for the identified ergot alkaloid, trigazaphilone, curvupallide, viridicatumtoxin and swainsonine BGCs. However, the majority of BGCs had no obvious links to known products from the Hypoxylaceae or other well-studied biosynthetic pathways from fungi. These findings highlight that the number of known compounds strongly underrepresents the biosynthetic potential in these fungi and that a tremendous number of unidentified secondary metabolites is still hidden. Moreover, with increasing numbers of genomes for further Hypoxylaceae species becoming available, the likelihood of revealing new biosynthetic pathways that encode new, potentially useful compounds will significantly improve. Reaching a better understanding of the biology of these producers, and further development of genetic methods for their manipulation, will be crucial to access their treasures.

Genome mining combined metabolic shunting and OSMAC strategy of an endophytic fungus leads to the production of diverse natural products

Endophytic fungi are promising producers of bioactive small molecules. Bioinformatic analysis of the genome of an endophytic fungus Penicillium dangeardii revealed 43 biosynthetic gene clusters, exhibited its strong ability to produce numbers of secondary metabolites. However, this strain mainly produce rubratoxins alone with high yield in varied culture conditions, suggested most gene clusters are silent. Efforts for mining the cryptic gene clusters in P. dangeardii, including epigenetic regulation and one-strain-many-compounds (OSMAC) approach were failed probably due to the high yield of rubratoxins. A metabolic shunting strategy by deleting the key gene for rubratoxins biosynthesis combining with optimization of culture condition successfully activated multiple silent genes encoding for other polyketide synthases (PKSs), and led to the trace compounds detectable. As a result, a total of 23 new compounds including azaphilone monomers, dimers, trimers with unprecedented polycyclic bridged heterocycle and spiral structures, as well as siderophores were identified. Some compounds showed significant cytotoxicities, anti-inflammatory or antioxidant activities. The attractive dual PKSs gene clusters for azaphilones biosynthesis were mined by bioinformatic analysis and overexpression of a pathway specific transcription factor. Our work therefor provides an efficient approach to mine the chemical diversity of endophytic fungi.

New azaphilones from Aspergillus neoglaber

Three new azaphilones, sassafrin E (1), sassafrin F (2), and sassafrinamine A (3), were isolated from the filamentous fungus Aspergillus neoglaber. The structures of the compounds were determined by nuclear magnetic resonance spectroscopy, and were found to be novel analogues of two already known compound classes; sassafrins and berkchaetoazaphilones. Sassafrin E and F were both oxygen containing, while sassafrinamine A additionally contained a nitrogen atom, originating from an aminoethanol moiety, as well as extensive conjugation resulting in an intense purple colour of the pure compound. The structure of sassafrin E was further confirmed using deuterium exchange experiments coupled with high-resolution tandem mass spectrometry.

Azaphilones with protein tyrosine phosphatase inhibitory activity isolated from the fungus Aspergillus deflectus

Six undescribed azaphilones, deflectins C1-C3, deflectins D1-D2, and deflectin E, along with five known azaphilones were obtained from a solid culture of the wild fungus Aspergillus deflectus NCC0415. Their structures were determined by HRESIMS, NMR and ECD analyses, together with the GIAO ¹³C NMR calculation method. All compounds displayed strong or moderate inhibitory activity against protein tyrosine phosphatases SHP2 and PTP1B. Structure-activity relationship analysis of these azaphilones suggested that the length of the ketone aliphatic side chain would affect their SHP2 and PTP1B inhibitory activity. In addition, the presence of a Δ⁸⁽¹²⁾ double bond on γ-lactone ring and the presence of CH₃-2' in fatty chains may increase their inhibitory activity.

Chaetoglobosins and azaphilones from Chaetomium globosum associated with Apostichopus japonicus

Increasing attention has recently been focused on complex symbiotic associations, for instance coral and its symbionts. Sea cucumber, harboring diverse fungi, has also attracted more and more attention for their functional diversity. Here, secondary metabolites produced by Chaetomium globosum associated with sea cucumber, Apostichopus japonicus, were investigated using gene mining with third-generation sequencing technology (PacBio SMRT). Nine compounds, including one new compound cytoglobosin X (1), were isolated from cultures of Chaetomium globosum. Compound 1 was identified based on NMR data, HRESIMS, and ECD, and the absolute configurations were identified as 3S, 4R, 7S, 8R, 9R, 16S, 19S, 20S, and 23S. In an antimicrobial assay, compound 4 showed moderate activity against Staphylococcus aureus and methicillin-resistant Staphylococcus aureus with MICs of 47.3 and 94.6 μM, respectively. Our results suggest that the microbiomes associated with sea cucumber could be an important resource for biodiversity and structural novelty, and the bioactive compounds may protect the host from pathogen microbial.

Sequential fungal fermentation-biotransformation process to produce a red pigment from sclerotiorin

The fungus Penicillium sclerotiorum produces sclerotiorin, an orange compound closely related to the useful food coloring pigments produced by Monascus species. The high productivity, together with several biological activities reported for sclerotiorin highlights its potential application in food industry. In this work, sclerotiorin was obtained as the major metabolite produced in liquid fermentation by P. sclerotiorum standing for 30% of the fungal dry extract. Modulation of sclerotiorin color was accomplished by biotransformation using Beauveria bassiana generating a red derivative with 13.8% yield. Color modification was caused by fungal-mediated substitution of oxygen by nitrogen in the pyrone ring changing the molecule's chromophore. A derivative, 1-methyl sclerotiorin was synthesized from sclerotiorin using diazomethane and fed to B. bassiana. In this case, substituent at C-1 avoided heteroatom substitution. Sclerotiorin derivatives obtained in the present show the great potential of sclerotiorin derivatives as food colorants.