Bioactive sulfur-containing sulochrin dimers and other metabolites from an Alternaria sp. isolate from a Hawaiian soil sample

Naturally Occurring Asterric Acid Analogs: Chemistry and Biology

Asterric acid and its analogs belong to diphenyl ethers (DPEs) with multiple substitutions on A/B aromatic rings. This member of DPEs originates from the polyketide pathway and displays a wide range of biological effects. Though the structures of asterric acid analogs are not complex, there were only more than 50 asterric acid analogs found in nature from 1960 to 2023. In this review, the structures, bioactivities, and biosynthesis of asterric acid analogs are summarized. More importantly, the empirical rule about the shielding effect of B-ring on H-6 is suggested, and this provides a convenient and useful way to analyze the NMR spectral data of asterric acid analogs, based on which the chemical shift values of the A-ring in some asterric acid analogs are revised.

Antibacterial and Antibiofilm Activity of Endophytic Alternaria sp. Isolated from Eremophila longifolia

The threat to public health resulting from the emergence of antimicrobial resistance (AMR) is ever rising. One of the major bacterial pathogens at the forefront of this problem is methicillin-resistant Staphylococcus aureus, or MRSA, for which there is a great need to find alternative treatments. One of the most promising alternatives is endophytic fungi, which were shown to produce a vast array of bioactive compounds, including many novel antibacterial compounds. In this study, two endophytic Alternaria sp., EL 24 and EL 35, were identified from the leaves of Eremophila longifolia. Ethyl acetate (EtOAc) extracts of their culture filtrates were found to inhibit both methicillin-sensitive S. aureus ATCC 25923 and MRSA strains M173525 and M180920. The activity of each extract was shown to be greatly affected by the growth medium, with considerable reductions in minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) observed when tested in tryptic soy broth with glucose (TSBG) compared with Mueller-Hinton broth (MHB). Both extracts displayed significant (p ≤ 0.05) antibiofilm activity against all three S. aureus strains, the greatest of which was that of EL 35, which reduced biofilm formation by M180920 by 72%, while that of EL 24 resulted in a 57% reduction against ATCC 25923. Both extracts also disrupted established biofilms, of which the most effective was EL 35, which reduced the M180920 biofilm by 64%, while EL 24 also performed best against M180920, reducing biofilm by 54%. Gas chromatography-mass spectrometry (GC-MS) analysis of the EL 24 EtOAc extract revealed five known compounds. This study highlights the promise of endophytic fungi from Australian plants as a potential source of substances effective against important bacterial pathogens. Further understanding of the responsible compounds and their mechanisms could lead to the development of treatments effective against MRSA, as well as novel biofilm-resistant biomedical materials, contributing towards reducing the burden of AMR.

Anti-Aβ42 Aggregative Polyketides from the Antarctic Psychrophilic Fungus Pseudogymnoascus sp. OUCMDZ-3578

Seven new polyketides, diphenyl ketone (1), diphenyl ketone glycosides (2-4), diphenyl ketone-diphenyl ether dimer (6), and anthraquinone-diphenyl ketone dimers (7 and 8), together with compound 5, were isolated from the psychrophilic fungus Pseudogymnoascus sp. OUCMDZ-3578 fermented at 16 °C and identified by spectroscopic analysis. The absolute configurations of 2-4 were determined by acid hydrolysis and 1-phenyl-3-methyl-5-pyrazolone precolumn derivatization. The configuration of 5 was first determined by X-ray diffraction analysis. Compounds 6 and 8 showed the highest activity against amyloid beta (Aβ₄₂) aggregation with half-maximal inhibitory concentrations (IC₅₀) of 0.10 and 0.18 μM, respectively. They also showed strong abilities to chelate with metal ions, especially iron, were sensitive to Aβ₄₂ aggregation induced by metal ions, and displayed depolymerizing activity. Compounds 6 and 8 show potential as leads for the treatment of Alzheimer's disease to prevent Aβ₄₂ aggregation.

Eumitrins I-K: three new xanthone dimers from the lichen Usnea baileyi

In the continuing discovery and structure elucidation of natural xanthone dimers, which are still rarely reported in absolute configuration, three new xanthone dimers, eumitrins I-K (1-3) were isolated from the lichen Usnea baileyi, a rich source of natural xanthone dimers. Their structures were elucidated unambiguously by spectroscopic analyses, including high-resolution electrospray ionization mass spectrometry (HRESIMS), 1D and 2D nuclear magnetic resonance spectroscopy (1D and 2D NMR). The absolute configuration of all three compounds was established through DP4 probability and ECD calculation. All compounds revealed weak activity for their enzymatic inhibition against α-glucosidase and tyrosinase, as well as antibacterial activity.

Aculeaxanthones A-E, new xanthones from the marine-derived fungus Aspergillus aculeatinus WHUF0198

Introduction

Dimeric natural products are widespread in plants and microorganisms, which usually have complex structures and exhibit greater bioactivities than their corresponding monomers. In this study, we report five new dimeric tetrahydroxanthones, aculeaxanthones A-E (4-8), along with the homodimeric tetrahydroxanthone secalonic acid D (1), chrysoxanthones B and C (2 and 3), and 4-4'-secalonic acid D (9), from different fermentation batches of the title fungus.

Methods

A part of the culture was added to a total of 60 flasks containing 300 ml each of number II fungus liquid medium and culture 4 weeks in a static state at 28˚C. The liquid phase (18 L) and mycelia was separated from the fungal culture by filtering. A crude extract was obtained from the mycelia by ultrasound using acetone. To obtain a dry extract (18 g), the liquid phase combined with the crude extract were further extracted by EtOAc and concentrated in vacuo. The MIC of anaerobic bacteria was examined by a broth microdilution assay. To obtain MICs for aerobic bacteria, the agar dilution streak method recommended in Clinical and Laboratory Standards Institute document (CLSI) M07-A10 was used. Compounds 1-9 was tested against the Bel-7402, A-549 and HCT-116 cell lines according to MTT assay.

Results and Discussion

The structures of these compounds were elucidated on the base of 1D and 2D NMR and HR-ESIMS data, and the absolute configurations of the new xanthones 4-8 were determined by conformational analysis and time-dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculations. Compounds 1-9 were tested for cytotoxicity against the Bel-7402, A549, and HCT-116 cancer cell lines. Of the dimeric tetrahydroxanthone derivatives, only compound 6 provided cytotoxicity effect against Bel-7402 cell line (IC50, 1.96 µM). Additionally, antimicrobial activity was evaluated for all dimeric tetrahydroxanthones, including four Gram-positive bacteria including Enterococcus faecium ATCC 19434, Bacillus subtilis 168, Staphylococcus aureus ATCC 25923 and MRSA USA300; four Gram-negative bacteria, including Helicobacter pylori 129, G27, as well as 26,695, and multi drug-resistant strain H. pylori 159, and one Mycobacterium M. smegmatis ATCC 607. However, only compound 1 performed activities against H. pylori G27, H. pylori 26695, H. pylori 129, H. pylori 159, S. aureus USA300, and B. subtilis 168 with MIC values of 4.0, 4.0, 2.0, 2.0, 2.0 and 1.0 μg/mL, respectively.

Secondary metabolites of Alternaria: A comprehensive review of chemical diversity and pharmacological properties

Fungi are considered to be one of the wealthiest sources of bio-metabolites that can be employed for yielding novel biomedical agents. Alternaria, including parasitic, saprophytic, and endophytic species, is a kind of dark fungi that can produce a broad array of secondary metabolites (SMs) widely distributed in many ecosystems. These are categorized into polyketides, nitrogen-containing compounds, quinones, terpenes, and others based on the unique structural features of the metabolites. New natural products derived from Alternaria exhibit excellent bioactivities characterized by antibacterial, antitumor, antioxidative, phytotoxic, and enzyme inhibitory properties. Thus, the bio-metabolites of Alternaria species are significantly meaningful for pharmaceutical, industrial, biotechnological, and medicinal applications. To update the catalog of secondary metabolites synthesized by Alternaria fungi, 216 newly described metabolites isolated from Alternaria fungi were summarized with their diverse chemical structures, pharmacological activity, and possible biosynthetic pathway. In addition, possible insights, avenues, and challenges for future research and development of Alternaria are discussed.

Sulfur-Containing Metabolites from Marine and Terrestrial Fungal Sources: Origin, Structures, and Bioactivities

Organosulfur natural products (NPs) refer to the different kinds of small molecular-containing sulfur (S) elements. Sulfur-containing NPs tightly link to the biochemical processes and play an important role in the pharmaceutical industry. The majority of S-containing NPs are generally isolated from Alliaceae plants or bacteria, and those from fungi are still relatively rare. In recent years, an increasing number of S-containing metabolites have been discovered in marine and terrestrial fungi, but there is no comprehensive and targeted review to summarize the studies. In order to make it more straightforward to better grasp the fungal-derived S-containing NPs and understand the particularity of marine S-containing NPs compared to those from terrestrial fungi, we summarized the chemical structures and biological activities of 89 new fungal-derived S-containing metabolites from 1929 when the penicillin was discovered to the present in this current review. The structural and bioactive diversity of these S-containing metabolites were concluded in detail, and the preliminary mechanism for C-S bond formation in fungi was also discussed briefly.

The Novel Compounds with Biological Activity Derived from Soil Fungi in the Past Decade

The secondary metabolites isolated from soil fungi have received more and more attention, especially new compounds that exhibited good biological activities. In this review, a total of 546 new compounds are included in the relevant literature since 2011. The new compounds are isolated from soil fungi, We divided these compounds into seven categories, including alkaloids, terpenoids, steroids, ketones, phenylpropanoids, quinones, esters, lactones, etc. In addition, the biological activities and structure-activity relationships of these compounds have also been fully discussed. The activities of these compounds are roughly divided into eight categories, including anticancer activity, antimicrobial activity, anti-inflammatory activity, antioxidant activity, antiviral activity, antimalarial activity, immunosuppressive activity and other activities. Since natural products are an important source of new drugs, this review may have a positive guiding effect on drug screening.

Identification of Antibacterial Metabolites from Endophytic Fungus Aspergillus fumigatus, Isolated from Albizia lucidior Leaves (Fabaceae), Utilizing Metabolomic and Molecular Docking Techniques

The rapid spread of bacterial infection caused by Staphylococcus aureus has become a problem to public health despite the presence of past trials devoted to controlling the infection. Thus, the current study aimed to explore the chemical composition of the extract of endophytic fungus Aspergillus fumigatus, isolated from Albizia lucidior leaves, and investigate the antimicrobial activity of isolated metabolites and their probable mode of actions. The chemical investigation of the fungal extract via UPLC/MS/MS led to the identification of at least forty-two metabolites, as well as the isolation and complete characterization of eight reported metabolites. The antibacterial activities of isolated metabolites were assessed against S. aureus using agar disc diffusion and microplate dilution methods. Compounds ergosterol, helvolic acid and monomethyl sulochrin-4-sulphate showed minimal inhibitory concentration (MIC) values of 15.63, 1.95 and 3.90 µg/mL, respectively, compared to ciprofloxacin. We also report the inhibitory activity of the fungal extract on DNA gyrase and topoisomerase IV, which led us to perform molecular docking using the three most active compounds isolated from the extract against both enzymes. These active compounds had the required structural features for S. aureus DNA gyrase and topoisomerase IV inhibition, evidenced via molecular docking.

Building Natural Product Libraries Using Quantitative Clade-Based and Chemical Clustering Strategies

The success of natural product-based drug discovery is predicated on having chemical collections that offer broad coverage of metabolite diversity. We propose a simple set of tools combining genetic barcoding and metabolomics to help investigators build natural product libraries aimed at achieving predetermined levels of chemical coverage. It was found that such tools aided in identifying overlooked pockets of chemical diversity within taxa, which could be useful for refocusing collection strategies. We have used fungal isolates identified as Alternaria from a citizen-science-based soil collection to demonstrate the application of these tools for assessing and carrying out predictive measurements of chemical diversity in a natural product collection. Within Alternaria, different subclades were found to contain nonequivalent levels of chemical diversity. It was also determined that a surprisingly modest number of isolates (195 isolates) was sufficient to afford nearly 99% of Alternaria chemical features in the data set. However, this result must be considered in the context that 17.9% of chemical features appeared in single isolates, suggesting that fungi like Alternaria might be engaged in an ongoing process of actively exploring nature's metabolic landscape. Our results demonstrate that combining modest investments in securing internal transcribed spacer (ITS)-based sequence information (i.e., establishing gene-based clades) with data from liquid chromatography-mass spectrometry (i.e., generating feature accumulation curves) offers a useful route to obtaining actionable insights into chemical diversity coverage trends in a natural product library. It is anticipated that these outcomes could be used to improve opportunities for accessing bioactive molecules that serve as the cornerstone of natural product-based drug discovery. IMPORTANCE Natural product drug discovery efforts rely on libraries of organisms to provide access to diverse pools of compounds. Actionable strategies to rationally maximize chemical diversity, rather than relying on serendipity, can add value to such efforts. Readily implementable biological (i.e., ITS sequence analysis) and chemical (i.e., mass spectrometry-based feature and scaffold measurements) diversity assessment tools can be employed to monitor and adjust library development tactics in real time. In summary, metabolomics-driven technologies and simple gene-based specimen barcoding approaches have broad applicability to building chemically diverse natural product libraries.

Tetrahydroxanthone-chromanone heterodimers from lichen Usnea aciculifera and their cytotoxic activity against human cancer cell lines

Four new tetrahydroxanthone-chromanone heterodimers, usneaxanthones E-H (1-4) together with eleven known compounds (5-15) were isolated from lichen Usnea aciculifera Vain (Parmeliaceae). Their structures and absolute configurations, particularly the central and axial chiralities, were unambiguously demonstrated by a combination of spectroscopic data (1D, 2D NMR, HRESIMS), electronic circular dichroism (ECD) experiments, and single-crystal X-ray crystallographic analyses. The cytotoxicity of new compounds was evaluated on four human cancer cell lines including HCT116 colorectal cancer, MCF-7 breast cancer, A549 lung cancer, and OVCAR-3 ovarian cancer. Compounds 1-4 exhibited good cytotoxicity against all tested cancer cell lines, except ovarian cancer, with the best IC₅₀ value of 3.37 μM. All compounds showed potent cytotoxicity against HCT116 colon cancer with IC₅₀ value from 3.37 to 4.53 μM.

Determination of Alternaria toxins in drinking water by ultra-performance liquid chromatography tandem mass spectrometry

A sensitive and reliable analytical method has been developed and validated for the determination of five Alternaria toxins, including tenuazonic acid (TeA), alternariol (AOH), alternariol monomethyl ether (AME), altenuene (ALT), and tentoxin (TEN), in drinking water using a one-step enrichment and clean-up strategy followed by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Drinking water samples were preprocessed using excess sodium sulfite to remove residual chlorine, and the pH was adjusted by formic acid. Analytes were concentrated and purified from the water samples using hydrophilic-lipophilic balanced (HLB) solid-phase extraction (SPE) cartridges. Chromatographic separation was performed on an Acquity HSS C₁₈ column using 0.1 mM ammonium carbonate and methanol as the mobile phase. The average recoveries at three spiked levels (0.1, 0.5, and 1 ng/L for TeA, AOH, and ALT; 0.01, 0.05, and 0.1 ng/L for TEN and AME) were 76.1-106.5%, with an intra-day precision less than 15.5% and inter-day precision of 11.8-16.5%. The limits of detection (LODs) were 0.05 ng/L for TeA, AOH, and ALT and 0.005 ng/L for TEN and AME. The limits of quantification (LOQs) were 0.1 ng/L for TeA, AOH, and ALT and 0.01 ng/L for TEN and AME. The developed method was applied to monitor 289 drinking water samples collected in Beijing from 2015 to 2017, and TeA and TEN were found in 28 samples, with concentrations ranging from 0.16 to 2.7 ng/L and 0.21 to 2.2 ng/L, respectively.

Cytotoxic and neuroprotective activities of constituents from Alternaria alternate, a fungal endophyte of Psidium littorale

Chemical investigation of the EtOAc extract of the plant-associated fungus Alternaria alternate in rice culture led to the isolation of a novel liphatic polyketone, alternin A (1), a new indole alkaloid (8), and a new sesquiterpene (11), together with 12 known compounds. Their structures were elucidated by the interpretation of extensive spectroscopic data, and the absolute configurations of 1-3 were established using calculations of ECD spectra, NMR data, and optical rotation values. Compound 1 possesses an unprecedented C25 liphatic polyketone skeleton. Compounds 5 and 10 exhibited potential cytotoxic activities against MCF-7 and HepG cells, and compounds 2, 7, and 9 exhibited potential neuroprotective activities in glutamate induced-PC12 injured cells.

Bacillus amyloliquefaciens induces production of a novel blennolide K in coculture of Setophoma terrestris

AIMS

The discovery of known bioactive chemical leads from microbial monocultures hinders the efficiency of drug discovery programmes. Therefore, in recent years, the use of fungal-bacterial coculture experiments has gained considerable attention due to their ability to generate new bioactive leads. In this work, fungal strain Setophoma terrestris was cocultured with Bacillus amyloliquifaciens to discover novel bioactive compounds.

MATERIALS AND METHODS

The bioactive methanolic coculture extract was chosen for the isolation of compounds by chromatographic methods. The isolated compounds were characterized by NMR and mass spectrometric techniques.

CONCLUSION

Coculture extract has resulted in the production of five blennolides. The novel compound, blennolide K was found active against PC-3 (prostate) and MCF-7 (breast) cell lines with an IC₅₀ value of 3·7 ± 0·6 and 4·8 ± 0·4 μmol l^-1 respectively. Furthermore, the nuclear morphology study in PC-3 cells after treatment with blennolide K, demonstrated chromatin condensation, formation of apoptotic bodies and shrinkage of cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our knowledge, only few studies have reported the induction of bioactive compounds by coculture having long-distance inhibition morphology. This is principally due to the low occurrences of such morphology. Our study demonstrates the impact of coculture on production of new chemical leads in drug discovery programmes.

Enantioselective Total Synthesis of Chromanone Lactone Homo- and Heterodimers

A one pot borylation/Suzuki-Miyaura reaction of the 4-bromochromanone lactones 21 and 23, respectively, followed by cleavage of the methyl ether moieties gave the homodimeric chromanone lactones 10 and 11. Reaction of a 1:1 mixture of 21 and 23 under otherwise identical conditions gave a 1:1:2-mixture of the two homodimers 10 and 11 and the heterodimer 12. This is the first example of the preparation of a heterodimeric chromanone lactone. For the enantioselective synthesis of the starting material, phenol 17 was transformed into the chromane 18 using a Wacker-type cyclisation with 99 % ee and 80 % yield.

Enantioselective Total Synthesis of Blennolide H and Phomopsis-H76 A and Determination of Their Structure

This work reports on the enantioselective total synthesis of the two dimeric natural chromanone lactones phomopsis-H76 A (5) and blennolide H (6). Both syntheses could be achieved from chromane 11, which was obtained by an enantioselective Wacker-type cyclization with >99 % ee. The dimerization of the corresponding monomers was performed using a palladium-catalyzed Suzuki reaction. Moreover, within this work it was possible to revise the absolute configuration of phomopsis-H76 A and determine the relative as well as absolute configuration of blennolide H.

Versixanthones A-F, Cytotoxic Xanthone-Chromanone Dimers from the Marine-Derived Fungus Aspergillus versicolor HDN1009

Six unusual xanthone-chromanone dimers, versixanthones A-F (1-6), featuring different formal linkages of tetrahydroxanthone and 2,2-disubstituted chroman-4-one monomers, were isolated from a culture of the mangrove-derived fungus Aspergillus versicolor HDN1009. The absolute configurations of 1-6, representing the central and axial chirality elements or preferred helicities, were established by a combination of X-ray diffraction analysis, chemical conversions, and TDDFT-ECD calculations. The interconversion of different biaryl linkages between 1 and 4 and between 2 and 3 in DMSO by a retro-oxa-Michael mechanism provided insight into the formation of the xanthone-chromanone dimers and supported the assignments of their absolute configurations. Compounds 1-6 exhibited cytotoxicities against the seven tested cancer cell lines, with the best IC50 value of 0.7 μM. Compound 5 showed further inhibitory activity against topoisomerase I.

Targeting mosquito FREP1 with a fungal metabolite blocks malaria transmission

Inhibiting Plasmodium development in mosquitoes will block malaria transmission. Fibrinogen-related protein 1 (FREP1) is critical for parasite infection in Anopheles gambiae and facilitates Plasmodium invasion in mosquitoes through interacting with gametocytes and ookinetes. To test the hypothesis that small molecules that disrupt this interaction will prevent parasites from infecting mosquitoes, we developed an ELISA-based method to screen a fungal extract library. We obtained a candidate fungal extract of Aspergillus niger that inhibited the interaction between FREP1 and P. falciparum infected cells by about 92%. The inhibition specificity was confirmed by immunofluorescence assays. Notably, feeding mosquitoes with the candidate fungal extract significantly inhibited P. falciparum infection in the midgut without cytotoxicity or inhibition of the development of P. falciparum gametocytes or ookinetes. A bioactive natural product that prevents FREP1 from binding to gametocytes or ookinetes was isolated and identified as P-orlandin. Importantly, the nontoxic orlandin significantly reduced P. falciparum infection intensity in mosquitoes. Therefore, disruption of the interaction between FREP1 and parasites effectively reduces Plasmodium infection in mosquitoes. Targeting FREP1 with small molecules is thus an effective novel approach to block malaria transmission.

Transferring Fungi to a Deuterium-Enriched Medium Results in Assorted, Conditional Changes in Secondary Metabolite Production

Deuterium is one of the few stable isotopes that have the capacity to significantly alter a compound's chemical and biological properties. The addition of a single neutron to a protium atom results in the near doubling of its mass, which gives rise to deuterium's characteristic isotope effects. Since the incorporation of deuterium into organic substrates is known to alter enzyme/protein-substrate interactions, we tested the extent to which deuterium enrichment would modify fungal secondary metabolite production. Several fungal cultures were tested, and in all cases their secondary metabolomes were marked by changes in natural product production. Workup of one Aspergillus sp. grown under deuterium-enrichment conditions resulted in the production of several secondary metabolites not previously detected from the fungus. Bioassay testing revealed that in comparison to the inactive crude fungal extract derived from growing the fungus under non-deuterium-enriched conditions, an extract derived from the same isolate cultured in a deuterium-enriched medium inhibited methicillin-resistant Staphylococcus aureus. Using an assortment of NMR and mass spectrometry experiments, we were able to identify the bacterial inhibitor as an isotope-labeled version of pigmentosin A (6). Five additional isotopically labeled metabolites were also obtained from the fungus including brevianamide F (1), stephacidin A (2), notoamide D (3), notoamide L (4), and notoamide C (5). Given the assorted changes observed in the secondary metabolite profiles of this and other fungi grown in deuterium-enriched environments, as well as the fact that 1 and 3-6 had not been previously observed from the Aspergillus sp. isolate used in this study, we propose that deuterium enrichment might offer an effective method for further expanding a fungus's chemical diversity potential.