Nine new cytochalasan alkaloids from Chaetomium globosum TW1-1 (Ascomycota, Sordariales)

Naming of new natural products: Standard, pitfalls and tips-and-tricks

Naming a newly discovered natural product (NP) is a pleasant but difficult exercise. In most cases, the NP name will be given with reference to the species of origin, be it a plant, a marine organism, a mammalian or microbial species. For a long time, the use of biologically-based trivial names has been recommended to identify the parental linkage between the product and the originating genus or species. But the recommendation is not always followed and a multiplicity of trivial names have been attributed to NP, based on locations (country, region, city), foods, music, animals, forenames, etc. Tips-and-tricks associated with the naming of NP are underlined here. Usually, NP are differentiated across a homogeneous chemical series with a letter (from the Latin or Greek alphabet), followed or not with a number. In other cases, the change of a single letter distinguishes a series of NP. Common pitfalls associated with the naming of NP are enumerated, including the complexity of names, use of synonyms, duplicated names, confusing names and inappropriate terminology. The difficulties regularly encountered with the naming of NP are discussed. Four essential recommendations are recalled: (i) a thorough analysis of the existing products to avoid duplicated names and confusion, (ii) the use of a biologically-based trivial name to retrace the origin of the product, (iii) the strict adherence to the codes of chemical nomenclature, and (iv) the preference for simple names to facilitate transmission. Naming a new NP is a rewarding task, which shall be performed with all due skill, care and diligence.

Rubichaetoglobin A, a new cytochalasan alkaloid isolated from the plant endophytic fungus Chaetomium tectifimeti S104

Rubichaetoglobin A (1), a new cytochalasan alkaloid, together with nine closely related known ones (2-10), were isolated from the ethyl acetate extracts of the endophytic fungus Chaetomium tectifimeti S104 harbored in the root of Rubia podantha Diels. Their structures were elucidated based on comprehensive spectroscopic analysis. All isolated compounds were tested for cytotoxic, antibacterial, and nitric oxide inhibitory activities. The results showed that 2, 4, and 5 possessed moderate cytotoxicity against MDA-MB-231 cells with the IC₅₀ values of 19.14, 11.43, and 10.27 μM, respectively.

Diversified Chaetoglobosins from the Marine-Derived Fungus Emericellopsis sp. SCSIO41202

Two undescribed cytochalasins, emeriglobosins A (1) and B (2), together with nine previously reported analogues (3–11) and two known tetramic acid derivatives (12, 13) were isolated from the solid culture of Emericellopsis sp. SCSIO41202. Their structures, including the absolute configurations of their stereogenic carbons, were fully elucidated based on spectroscopic analysis and the calculated ECD. Some of the isolated compounds were evaluated for their cytotoxicity and enzyme inhibitory activity against acetylcholinesterase (AChE) in vitro. Among them, 8 showed potent AChE inhibitory activity, with an IC₅₀ value of 1.31 μM, and 5 showed significant cytotoxicity against PC-3 cells, with an IC₅₀ value of 2.32 μM.

Endophytic Fungi: An Effective Alternative Source of Plant-Derived Bioactive Compounds for Pharmacological Studies

Plant-associated fungi (endophytic fungi) are a biodiversity-rich group of microorganisms that are normally found asymptomatically within plant tissues or in the intercellular spaces. Endophytic fungi promote the growth of host plants by directly producing secondary metabolites, which enhances the plant's resistance to biotic and abiotic stresses. Additionally, they are capable of biosynthesizing medically important "phytochemicals" that were initially thought to be produced only by the host plant. In this review, we summarized some compounds from endophyte fungi with novel structures and diverse biological activities published between 2011 and 2021, with a focus on the origin of endophytic fungi, the structural and biological activity of the compounds they produce, and special attention paid to the exploration of pharmacological activities and mechanisms of action of certain compounds. This review revealed that endophytic fungi had high potential to be harnessed as an alternative source of secondary metabolites for pharmacological studies.

Cytochalasans and azaphilones: suitable chemotaxonomic markers for the Chaetomium species

The accurate taxonomic concept of the fungal Chaetomium species has been a hard work due to morphological similarity. Chemotaxonomy based on secondary metabolites is a powerful tool for taxonomical purposes, which could be used as an auxiliary reference to solve the problems encountered in the classification of Chaetomium. Among secondary metabolites produced by Chaetomium, cytochalasans and azaphilones exhibited a pattern of distribution and frequency of occurrence that establish them as chemotaxonomic markers for the Chaetomium species. This review attempted to elucidate the composition of the Chaetomium species and its relationship with classical taxonomy by summarizing the pattern of cytochalasans and azaphilones distribution and biosynthesis in the Chaetomium species. KEY POINTS: • Secondary metabolites from the genus Chaetomium are summarized. • Cytochalasans and azaphilones could be characteristic metabolites of the Chaetomium species. • Cytochalasans and azaphilones could be used to analyze for taxonomical purposes.

Chaetomium and Chaetomium-like Species from European Indoor Environments Include Dichotomopilus finlandicus sp. nov

The genus Chaetomium is a frequently occurring fungal taxon world-wide. Chaetomium and Chaetomium-like species occur in indoor environments, where they can degrade cellulose-based building materials, thereby causing structural damage. Furthermore, several species of this genus may also cause adverse effects on human health. The aims of this research were to identify Chaetomium and Chaetomium-like strains isolated from indoor environments in Hungary and Finland, two geographically distant regions of Europe with drier and wetter continental climates, respectively, and to study their morphological and physiological properties, as well as their extracellular enzyme activities, thereby comparing the Chaetomium and Chaetomium-like species isolated from these two different regions of Europe and their properties. Chaetomium and Chaetomium-like strains were isolated from flats and offices in Hungary, as well as from schools, flats, and offices in Finland. Fragments of the translation elongation factor 1α (tef1α), the second largest subunit of RNA polymerase II (rpb2) and β-tubulin (tub2) genes, as well as the internal transcribed spacer (ITS) region of the ribosomal RNA gene cluster were sequenced, and phylogenetic analysis of the sequences performed. Morphological examinations were performed by stereomicroscopy and scanning electron microscopy. Thirty-one Chaetomium sp. strains (15 from Hungary and 16 from Finland) were examined during the study. The most abundant species was Ch. globosum in both countries. In Hungary, 13 strains were identified as Ch. globosum, 1 as Ch. cochliodes, and 1 as Ch. interruptum. In Finland, 10 strains were Ch. globosum, 2 strains were Ch. cochliodes, 2 were Ch. rectangulare, and 2 isolates (SZMC 26527, SZMC 26529) proved to be representatives of a yet undescribed phylogenetic species from the closely related genus Dichotomopilus, which we formally describe here as the new species Dichotomopilus finlandicus. Growth of the isolates was examined at different temperatures (4, 15, 20, 25, 30, 37, 35, 40, and 45 °C), while their extracellular enzyme production was determined spectrophotometrically.

Progress in the Chemistry of Cytochalasans

Cytochalasans are a group of fungal-derived natural products characterized by a perhydro-isoindolone core fused with a macrocyclic ring, and they exhibit a high structural diversity and a broad spectrum of bioactivities. Cytochalasans have attracted significant attention from the chemical and pharmacological communities and have been reviewed previously from various perspectives in recent years. However, continued interest in the cytochalasans and the number of laboratory investigations on these compounds are both growing rapidly. This contribution provides a general overview of the isolation, structural determination, biological activities, biosynthesis, and total synthesis of cytochalasans. In total, 477 cytochalasans are covered, including "merocytochalasans" that arise by the dimerization or polymerization of one or more cytochalasan molecules with one or more other natural product units. This contribution provides a comprehensive treatment of the cytochalasans, and it is hoped that it may stimulate further work on these interesting natural products.

Armochaetoglasins J and K: new cytochalasans from Chaetomium globosum

Two novel cytochalasans, armochaetoglasin J (1) and armochaetoglasin K (2), along with 14 known analogues (3-16) were isolated from Chaetomium globosum. Their structures were elucidated by HRESIMS, NMR spectroscopy, single-crystal X-ray crystallography, and ECD spectra. Armochaetoglasins J and K were found to be inactive against the HepG2, HT-29, K562, HL-60, and A549 cancer cell lines.

Identification of the Antifungal Metabolite Chaetoglobosin P From Discosia rubi Using a Cryptococcus neoformans Inhibition Assay: Insights Into Mode of Action and Biosynthesis

Cryptococcus neoformans is an important human pathogen with limited options for treatments. We have interrogated extracts from fungal fermentations to find Cryptococcus-inhibiting natural products using assays for growth inhibition, differential thermosensitivity, and synergy with existing antifungal drugs. Extracts from fermentations of strains of Discosia rubi from eastern Texas showed anticryptococcal bioactivity with preferential activity in agar zone of inhibition assays against C. neoformans at 37°C versus 25°C. Assay-guided fractionation led to the purification and identification of chaetoglobosin P as the active component of these extracts. Genome sequencing of these strains revealed a biosynthetic gene cluster consistent with chaetoglobosin biosynthesis and β-methylation of the tryptophan residue. Proximity of genes of the actin-binding protein twinfilin-1 to the chaetoglobosin P and K gene clusters suggested a possible self-resistance mechanism involving twinfilin-1 which is consistent with the predicted mechanism of action involving interference with the polymerization of the capping process of filamentous actin. A C. neoformans mutant lacking twinfilin-1 was hypersensitive to chaetoglobosin P. Chaetoglobosins also potentiated the effects of amphotericin B and caspofungin on C. neoformans.

Bioactivities and Future Perspectives of Chaetoglobosins

Chaetoglobosins belonging to cytochalasan alkaloids represent a large class of fungal secondary metabolites. To date, around 100 chaetoglobosins and their analogues have been isolated and identified over the years from a variety of fungi, mainly from the fungus Chaetomium globosum. Studies have found that chaetoglobosins possess a broad range of biological activities, including antitumor, antifungal, phytotoxic, fibrinolytic, antibacterial, nematicidal, anti-inflammatory, and anti-HIV activities. This review will comprehensively summarize the biological activities and mechanisms of action of nature-derived chaetoglobosins.

HPLC-DAD-Guided Isolation of Diversified Chaetoglobosins from the Coral-Associated Fungus Chaetomium globosum C2F17

Cytochalasans have continuously aroused considerable attention among the chemistry and pharmacology communities due to their structural complexities and pharmacological significances. Sixteen structurally diverse chaetoglobosins, 10-(indol-3-yl)-[13]cytochalasans, including a new one, 6-O-methyl-chaetoglobosin Q (1), were isolated from the coral-associated fungus Chaetomium globosum C2F17. Their structures were accomplished by extensive spectroscopic analysis combined with single-crystal X-ray crystallography and ECD calculations. Meanwhile, the structures and absolute configurations of the previously reported compounds 6, 12, and 13 were confirmed by single-crystal X-ray analysis for the first time. Chaetoglobosins E (6) and Fex (11) showed significant cytotoxicity against a panel of cancer cell lines, K562, A549, Huh7, H1975, MCF-7, U937, BGC823, HL60, Hela, and MOLT-4, with the IC₅₀ values ranging from 1.4 μM to 9.2 μM.

Diagnostically analyzing 1H NMR spectra of sub-types in chaetoglobosins for dereplication

¹H-NMR spectra provide abundant diagnostic information including chemical shift values, splitting patterns, coupling constants, and integrals. Thus some key functional groups, and even planar structures could be elucidated on the basis of carefully analyzing the corresponding ¹H NMR spectrum. In this paper, the different sub-types of chaetoglobosins are classified according to the structural features, of which the ¹H NMR spectra are systematically summed up. Thus diagnostically analyzing the ¹H-NMR spectra could identify possible sub-types of chaetoglobosins, which could be used for dereplication. According to the analysis of this report, it implies that different new sub-types or new sub-type combinations in the key skeleton of chaetoglobosins might exist in nature. More importantly, dereplication based on ¹H NMR spectral analysis will not only provide a useful approach to determine the chaetoglobosins structures quickly, but also could set a good example for structural dereplication of other NPs.

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.

Revision of the structure of isochaetoglobosin Db based on NMR analysis and biosynthetic consideration

Isochaetoglobosin D_b is a new chaetoglobosin possessing a unique 3,4-substituted pyrrole ring isolated and named by Qiu et al., and it is different from any one of the 14 sub-types in the macrocyclic ring of chaetoglobosins classified in our previous work. Its chemical shift values, coupling constants and biosynthetic consideration implied that the proposed structure of isochaetoglobosin D_b was incorrect. In this report, based on detailed NMR data analysis together with biosynthetic consideration, the structure of isochaetoglobosin D_b is suggested to be revised to that of penochalasin C. The NMR spectra of penochalasin C measured in the same solvent (DMSO-d₆) as that of isochaetoglobosin D_b supported the above conclusion. The results imply that reasonable biosynthetic consideration could complement spectroscopic structural determination, and also support that the ¹H-NMR rule of chaetoglobosin summarized in our previous work can provide help for dereplication and rectification.

Isochaetoglobosin D_b is a new chaetoglobosin possessing a unique 3,4-substituted pyrrole ring isolated and named by Qiu et al., and it is different from any one of the 14 sub-types in the macrocyclic ring of chaetoglobosins classified in our previous work.

Chaetomadrasins A and B, Two New Cytotoxic Cytochalasans from Desert Soil-Derived Fungus Chaetomium madrasense 375

Two new cytochalasans, Chaetomadrasins A (1) and B (2), along with six known analogues (3-8), were isolated from the solid-state fermented culture of desert soil-derived Chaetomium madrasense 375. Their structures were clarified by comprehensive spectroscopic analyses, and the absolute configurations of Compounds 1 and 2 were confirmed by electronic circular dichroism (ECD) and calculated ECD. For the first time, Chaetomadrasins A (1), which belongs to the chaetoglobosin family, is characterized by the presence of all oxygen atoms in the form of Carbonyl. Chaetomadrasin B (2) represents the first example of chaetoglobosin type cytochalasan characterized by a hydroxy unit and carbonyl group fused to the indole ring. Compounds 1 and 2 displayed moderate cytotoxicity against HepG2 human hepatocellular carcinoma cells.

Mollicellins O⁻R, Four New Depsidones Isolated from the Endophytic Fungus Chaetomium sp. Eef-10

Four new depsidones, mollicellins O⁻R (compounds 1⁻4), along with three known compounds 5⁻7, were isolated from cultures of the fungus Chaetomium sp. Eef-10, an endophyte isolated from Eucalyptus exserta. The structures of the new compounds were elucidated by analysis of the 1D and 2D NMR and HR-ESI-MS spectra. The known compounds were identified by comparison of their spectral data with published values. Compounds 1⁻7 were evaluated for antibacterial activities against Staphylococcus aureus (sensitive and resistant strains), Escherichia coli, Agrobacterium tumefaciens, Salmonella typhimurium, Pseudomonas lachrymans, Ralstonia solanacearum, Xanthomonas vesicatoria and cytotoxic activities against two human cancer cell lines (HepG2 and Hela). Mollicellin H (6) displayed best antibacterial activity, with IC₅₀ values of 5.14 µg/mL against S. aureus ATCC29213 and 6.21 µg/mL against S. aureus N50, MRSA, respectively. Mollicellin O (1) and mollicellin I (7) also exhibited antibacterial activities against S. aureus ATCC29213 and S. aureus N50. Mollicellin G (5) was active against both two human cancer cell lines, with IC₅₀ values of 19.64 and 13.97 µg/mL while compounds 6 and 7 only showed cytotoxic activity against one cell line. In addition, mollicellin O (1) showed antioxidant activity based on DPPH radical scavenging, with an IC₅₀ value of 71.92 µg/mL.

Armochaetoglasins A-I: Cytochalasan alkaloids from fermentation broth of Chaetomium globosum TW1-1 by feeding L-tyrosine

By feeding L-tyrosine into the culture medium, nine undescribed compounds, termed as armochaetoglasins A-I, together with three known analogues, namely armochaetoglobin E, chaetoglobosin V, and chaetoglobosin J, were isolated and identified from the medicinal terrestrial arthropod-derived fungus Chaetomium globosum TW1-1. Their structures were elucidated by means of NMR spectroscopy, single-crystal X-ray crystallography, and comparison of their electronic circular dichroism (ECD) spectra. Structurally, armochaetoglasin A represented the first tyrosine-derived cytochalasan alkaloid characterized by a 13-membered carbocyclic ring system; armochaetoglasins B and C possessed a rare 19,20-seco-chaetoglobosin skeleton. Armochaetoglasin B, chaetoglobosin V, and chaetoglobosin J showed weak cytotoxic activity with IC₅₀ values ranging from 19.5 to 34.72 μM.

New Metabolites from Endophytic Fungus Chaetomium globosum CDW7

Five metabolites including two new ones, prochaetoviridin A (1) and chaetoindolin A (2), were isolated from the endophytic fungus Chaetomium globosum CDW7. Compounds 1 and 2 were characterized as an isocoumarin and an indole alkaloid derivative, respectively, with their structures elucidated by comprehensive spectroscopic analyses including high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), NMR, and circular dichroism (CD) comparison. Compounds 3–5 were identified as chaetoviridin A, chaetoglobosin R, and chaetoglobosin T, respectively. Chaetoviridin A (3) exhibited antifungal activity against Sclerotinia sclerotiorum with an EC₅₀ value of 1.97 μg/mL. In vivo test showed that 3 displayed a protective efficacy of 64.3% against rape Sclerotinia rot at the dosage of 200 μg/mL, comparable to that of carbendazim (69.2%).

Two Novel Aspochalasins from the Gut Fungus Aspergillus sp. Z4

Two novel aspochalasins, tricochalasin A (1) and aspochalasin A2 (2), along with three known compounds (3–5) have been isolated from the different culture broth of Aspergillus sp., which was found in the gut of a marine isopod Ligia oceanica. Compound 1 contains a rare 5/6/6 tricyclic ring fused with the aspochalasin skeleton. The structures were determined on the basis of electrospray ionisation mass spectroscopy (ESIMS), nuclear magnetic resonance (NMR) spectral data, and the absolute configurations were further confirmed by modified Mosher’s method. Cytotoxicity against the prostate cancer PC3 cell line were assayed by the MTT method. Compound 3 showed strong activity while the remaining compounds showed weak activity.

Two Novel Aspochalasins from the Gut Fungus Aspergillus sp. Z4

Two novel aspochalasins, tricochalasin A (1) and aspochalasin A2 (2), along with three known compounds (3–5) have been isolated from the different culture broth of Aspergillus sp., which was found in the gut of a marine isopod Ligia oceanica. Compound 1 contains a rare 5/6/6 tricyclic ring fused with the aspochalasin skeleton. The structures were determined on the basis of electrospray ionisation mass spectroscopy (ESIMS), nuclear magnetic resonance (NMR) spectral data, and the absolute configurations were further confirmed by modified Mosher’s method. Cytotoxicity against the prostate cancer PC3 cell line were assayed by the MTT method. Compound 3 showed strong activity while the remaining compounds showed weak activity.

Cytochalasans Produced by the Coculture of Aspergillus flavipes and Chaetomium globosum

The cocultivation of Aspergillus flavipes and Chaetomium globosum, rich sources of cytochalasans, on solid rice medium, resulted in the production of 13 new, highly oxygenated cytochalasans, aspochalasinols A-D (1-4) and oxichaetoglobosins A-I (5-13), as well as seven known compounds (14-20). Of these compounds, 13 is a novel cytochalasan with an unexpected 2-norindole group. The isolated compounds were characterized by NMR spectroscopy, single-crystal X-ray crystallography, and ECD experiments. Compounds 1-4 represent the first examples of Asp-type cytochalasans with C-12 hydroxy groups, which may be a result of the coculture, as hydroxylated Me-12 groups are frequently found in Chae-type cytochalasans from C. globosum. In addition, 5-10 are unusual cytochalasans with an oxygenated C-10. Interestingly, 13 is the first example of a naturally occurring cytochalasan possessing a uniquely degraded indole ring that is derived from chaetoglobosin W, with 11 and 12 both serving as its biosynthetic intermediates. In the coculture of A. flavipes and C. globosum, most of these cytochalasans are more functionalized than normal cytochalasans, and the underlying causes may attract substantial attention from synthetic biologists. The cytotoxicities against five human cancer cell lines (SW480, HL-60, A549, MCF-7, and SMMC-7721) and the immunomodulatory activities of these new compounds were evaluated in vitro.

The biosynthesis of cytochalasans

This highlight summarises the recent advances in elucidating and engineering the biosynthesis of cytochalasan natural products.

Novel small molecule 11β-HSD1 inhibitor from the endophytic fungus Penicillium commune

Two new phenone derivatives penicophenones A (1) and B (2), a new cyclic tetrapeptide penicopeptide A (3), and five known compounds were isolated from the culture broth of Penicillium commune, an endophytic fungus derived from Vitis vinifera. Compounds 1–3 were elucidated by extensive spectroscopic analyses including 1D and 2D NMR and HRESIMS. The absolute configurations of 1 and 3 were determined by comparing its ECD with related molecules and modified Marfey’s analysis, respectively. Penicophenone A (1) possesses a rare benzannulated 6,6-spiroketal moiety, which is a new member of the unusual structural class with peniphenone A as the representative. Compound 3 exhibited significant inhibition activities against 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) in vitro and showed strong binding affinity to 11β-HSD1. Moreover, compound 3 treatments decreased the lipid droplet accumulation associate with the inhibition of 11β-HSD1 expression in differentiate-induced 3T3-L1 preadipocytes. Furthermore, the molecular docking demonstrated that compound 3 coordinated in the active site of 11β-HSD1 is essential for the ability of diminishing the enzyme activity.