Yong Chun Song

Biology and chemistry of endophytes

This review focuses on new endophyte-related findings in biology and ecology, and also summarises the various metabolites isolated from endophytes.

Unprecedented immunosuppressive polyketides from Daldinia eschscholzii, a mantis-associated fungus.

Immunosuppressants are required for an array of medical purposes, such as organ transplantations and the treatment of autoimmune-associated diseases. However, most of the currently available immunosuppressive drugs have been shown to inevitably possess severe adverse effects, such as hepatotoxicity, nephrotoxicity, and hypertension induction. Therefore, there is an urgent need for new therapeutic agents for modulating the autoimmune response. Some microorganisms are a reliable source of immunocompromising compounds, as exemplified by the discovery of cyclosporin A (CsA), rapamycin, and FK506. Symbionts, a diverse microbial community present in plants, insects, and mammals without the generation of any detectable symptoms, are receiving renewed attention for their production of chemically inspiring and biologically potent metabolites, presumably as a result of their long coevolution with hosts. In particular, some insect-associated fungi might have acquired instinctlike capabilities for synthesizing immunoalleviating metabolites from their initial microbe–host interaction through to the final colonization. In continuation of our characterization of new bioactive metabolites from endophyte cultures, this observation tempted us to explore novel immunosuppressive metabolites that could be produced by fungi inhabiting healthy insect organs, such as the mantis gut, which is clearly an important entrance and shelter for symbionts (including quiescent pathogens) and meal-carried “foreign” microbes. As expected, a preliminary screen recognized the presence of an immunosuppressive substance or immunosuppressive substances in a culture of Daldinia eschscholzii IFB-TL01 residing in the gut of the mantis species Tenodera aridifolia, a common predator of many insects that feed on plants harboring endophyte. Subsequent bioassay-guided fractionation of the extract derived from the scaled-up fermentation of the fungus afforded two polyketides, dalesconols A (1) and B

Cytotoxic chaetoglobosins from the endophyte Chaetomium globosum.

Two new alkaloids chaetoglobosins V (1) and W (2), together with the six known congeners 3-8, were isolated through bioassay-guided fractionations from the EtOAc extract of a solid culture of Chaetomium globosum IFB-E041. The structures were elucidated by spectroscopic methods including mainly 1D and 2D NMR techniques. Chaetoglobosin W (2) was unique in its possession of an oxolane ring formed via an oxygen bridge between C-3 and C-6. The isolated fungal metabolites exhibited moderate cytotoxic activities against four human cancer cell lines (KB, K562, MCF-7, and HepG2) with their IC(50) values in a range of 18-30 µg/mL.

Naphthol radical couplings determine structural features and enantiomeric excess of dalesconols in Daldinia eschscholzii

Understanding how simple molecules are pieced together in organisms may aid biotechnological manipulation and synthetic approaches to complex natural products. The mantis-associated fungus Daldinia eschscholzii IFB-TL01 produces the unusually structured immunosuppressants (±)-dalesconols A and B, along with their congener (±)-dalesconol C, with the (-)-enantiomers in excess. Here we report that these structural and stereochemical peculiarities of dalesconols A-C are a result of promiscuous and atropselective couplings of radicals derived from 1,3,6,8-tetrahydroxynaphthalene, 1,3,8-trihydroxynaphthalene and 1,8-dihydroxynaphthalene. The observed (-)-enantiomeric excess is found to depend on the dominance of particular conformers of naphthol dimer intermediates, which are ligands of laccase.

Bioactive alkaloids from the plant endophytic fungus Aspergillus terreus.

One new alkaloid, named 16 alpha-hydroxy-5 N-acetylardeemin ( 1), along with seven known metabolites ( 2- 8) was isolated from the fermentation broth of an endophytic fungus, ASPERGILLUS TERREUS. The structures of these metabolites were assigned on the basis of detailed spectroscopic analysis and by comparing spectroscopic data with those in the literature. Compound 1 displayed an inhibitory effect against acetylcholinesterase. Compounds 1- 8 also showed moderate or weak cytotoxic activity against KB and HSC-T6 cell lines.

New Phytotoxic Metabolites from Pestalotiopsis sp. HC02, a Fungus Residing in Chondracris rosee Gut

Two new phytotoxic γ‐lactones, pestalotines A and B (1 and 2, resp.), along with 4‐oxo‐4H‐pyran‐3‐acetic acid (3) and 6‐hydroxyramulosin (=3,4,4a,5,6,7‐hexahydro‐6,8‐dihydroxy‐3‐methyl‐1H‐2‐benzopyran‐1‐one; 4), were isolateded from the culture of Pestalotiopsis sp. HC02, a fungus residing in the Chondracris rosee gut. Structures of the new metabolites were elucidated on the basis of their IR, NMR, and MS data. Pestalotines A and B (1 and 2, resp.) significantly inhibited the radical growth of Echinochloa crusgalli with IC50 values of 1.85×10−4 and 2.50×10−4 M, respectively, comparable to that of 2‐(2,4‐dichlorophenoxy)acetic acid (0.94×10−4 M) used as a positive control.

Curvulamine, a New Antibacterial Alkaloid Incorporating Two Undescribed Units from a Curvularia Species

The white croaker (Argyrosomus argentatus) derived Curvularia sp. IFB-Z10 produces curvulamine as a skeletally unprecedented alkaloid incorporating two undescribed extender units. Curvulamine is more selectively antibacterial than tinidazole and biosynthetically unique in the new extenders formed through a decarboxylative condensation between an oligoketide motif and alanine.

Characterization, Synthesis and Self‐Aggregation of (−)‐Alternarlactam: A New Fungal Cytotoxin with Cyclopentenone and Isoquinolinone Scaffolds

(-)-Alternarlactam [(-)-1], a new promising cytotoxin against two human cancer cell lines, was isolated from an endophyte culture and synthesized (along with (+)-1) from readily available starting materials. The absolute configuration, chirality-activity relevance and self-aggregation of (-)-1 were assigned by a combination of synthetic, spectroscopic and computational approaches. The full characterization of the new fungal cytotoxin may provide valuable information in the discovery of new antitumor agents.

Fumigaclavines D-H, new ergot alkaloids from endophytic Aspergillus fumigatus.

Ergot alkaloids are toxins which are produced biotechnologically on an industrial scale. The chemical investigation of endophytic Aspergillus fumigatus resulted in the isolation of five new ergot alkaloids named fumigaclavines D-H (2-6), along with three known analogues, fumigaclavine C (1), festuclavine (7), and fumigaclavine A (8). Their structures were unequivocally elucidated by extensive spectroscopic analyses in association with X-ray single-crystal diffraction. Fumigaclavines D-H are interesting clavine-type ergot alkaloids featuring a reverse prenyl moiety at C-2, with 1-4, 6, and 8 bearing additional substituents, e.g., an OH or OAc group at C-9. Compounds 2, 4, and 6-8 showed a broad spectrum of antimicrobial activity against a panel of anaerobic microorganisms, of which compounds 4 and 6 were the most active against Veillonella parvula with an MIC=16 µg/mL compared to that (0.12 µg/mL) of tinidazole, co-assayed as a positive reference.

Cytotoxic cytochalasin metabolites of endophytic Endothia gyrosa.

In addition to the known metabolites cytochalasin H (1), cytochalasin J (2), and epoxycytochalasin H (3), two new 10-phenyl-(11)-cytochalasans, named cytochalasin Z10 and Z11 (4 and 5, resp.) were isolated from the solid substrate culture of Endothia gyrosa IFB-E023, an endophytic fungus residing inside the healthy leaf of Vatica mangachapo (Dipterocarpaceae). The structure determination of 4 and 5 was accomplished through correlative analyses of their spectral data (UV, ESI-MS, IR, (1)H- and (13)C-NMR, COSY, NOESY, HMQC, and HMBC). Metabolites 1-5 were demonstrated to be substantially cytotoxic to the human leukaemia K562 cell line with the IC(50) values of 10.1, 1.5, 24.5, 28.3, and 24.4 microM, respectively, which are comparable to that of 5-fluorouracil (33.0 microM), co-assayed as the positive reference.