Isolation of a Cyclic Depsipetide, Aspergillicin F, and Synthesis of Aspergillicins with Innate Immune-Modulating Activity

A Molecular Networking Based Discovery of Diketopiperazine Heterodimers and Aspergillicins from Aspergillus caelatus

The number of species in Aspergillus section Flavi has recently increased to 36 and includes some of the most important and well-known species in the genus Aspergillus. Numerous secondary metabolites, especially mycotoxins, have been reported from species such as A. flavus; however many of the more recently described species are less studied from a chemical point of view. This paper describes the use of MS/MS-based molecular networking to investigate the metabolome of A. caelatus leading to the discovery of several new diketopiperazine dimers and aspergillicins. An MS-guided isolation procedure yielded six new compounds, including asperazines D-H (1-5) and aspergillicin H (6). Asperazines G and H are artifacts derived from asperazines E and F formed during the separation process by formic acid. Two known compounds, aspergillicins A and C (7 and 8), were isolated from the same strain. Structures were elucidated by analyzing their HR-MS/MS and NMR spectroscopic data. The absolute configuration of asperazines D-F and aspergillicin H were deduced from the combination of NMR, Marfey's method, and ECD analyses.

Apoptotic Activity of New Oxisterigmatocystin Derivatives from the Marine-Derived Fungus Aspergillus nomius NC06

Sponge-derived fungi have recently attracted attention as an important source of interesting bioactive compounds. Aspergillus nomius NC06 was isolated from the marine sponge Neopetrosia chaliniformis. This fungus was cultured on rice medium and yielded four compounds including three new oxisterigmatocystins, namely, J, K, and L (1, 2, and 3), and one known compound, aspergillicin A (4). Structures of the compounds were elucidated by 1D and 2D NMR spectroscopy and by high-resolution mass spectrometry. The isolated compounds were tested for cytotoxic activity against HT 29 colon cancer cells, where compounds 1, 2, and 4 exhibited IC50 values of 6.28, 15.14, and 1.63 µM, respectively. Under the fluorescence microscope by using a double staining method, HT 29 cells were observed to be viable, apoptotic, and necrotic after treatment with the cytotoxic compounds 1, 2, and 4. The result shows that compounds 1 and 2 were able to induce apoptosis and cell death in HT 29 cells.

New chloptosins B and C from an Embleya strain exhibit synergistic activity against methicillin-resistant Staphylococcus aureus when combined with co-producing compound L-156,602

Two new dimeric cyclohexapeptides, chloptosins B and C, were discovered from the culture broth of Embleya sp. MM621-AF10 together with the known compounds chloptosin and L-156,602. The structures of the new chloptosins were determined by spectroscopic studies and advanced Marfey's methods. The stereo structure of the constituent isoleucine was determined by C3 Marfey's analysis. Chloptosins demonstrated potent antimicrobial activity against Gram-positive bacteria including drug-resistant strains of methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci with MICs of 0.5-2 µg ml-1. The antimicrobial activities of chloptosins were enhanced by addition of co-producing compound L-156,602, as shown by checkerboard analysis.

Chemical repertoire and biosynthetic machinery of the Aspergillus flavus secondary metabolome: A review

Filamentous fungi represent a rich source of extrolites, including secondary metabolites (SMs) comprising a great variety of astonishing structures and interesting bioactivities. State-of-the-art techniques in genome mining, genetic manipulation, and secondary metabolomics have enabled the scientific community to better elucidate and more deeply appreciate the genetic and biosynthetic chemical arsenal of these microorganisms. Aspergillus flavus is best known as a contaminant of food and feed commodities and a producer of the carcinogenic family of SMs, aflatoxins. This fungus produces many SMs including polyketides, ribosomal and nonribosomal peptides, terpenoids, and other hybrid molecules. This review will discuss the chemical diversity, biosynthetic pathways, and biological/ecological role of A. flavus SMs, as well as their significance concerning food safety and security.

The tetrameric pheromone module SteC-MkkB-MpkB-SteD regulates asexual sporulation, sclerotia formation and aflatoxin production in Aspergillus flavus

For eukaryotes like fungi to regulate biological responses to environmental stimuli, various signalling cascades are utilized, like the highly conserved mitogen‐activated protein kinase (MAPK) pathways. In the model fungus Aspergillus nidulans, a MAPK pathway known as the pheromone module regulates development and the production of secondary metabolites (SMs). This pathway consists five proteins, the three kinases SteC, MkkB and MpkB, the adaptor SteD and the scaffold HamE. In this study, homologs of these five pheromone module proteins have been identified in the plant and human pathogenic fungus Aspergillus flavus. We have shown that a tetrameric complex consisting of the three kinases and the SteD adaptor is assembled in this species. It was observed that this complex assembles in the cytoplasm and that MpkB translocates into the nucleus. Deletion of steC, mkkB, mpkB or steD results in abolishment of both asexual sporulation and sclerotia production. This complex is required for the positive regulation of aflatoxin production and negative regulation of various SMs, including leporin B and cyclopiazonic acid (CPA), likely via MpkB interactions in the nucleus. These data highlight the conservation of the pheromone module in Aspergillus species, signifying the importance of this pathway in regulating fungal development and secondary metabolism.

Depsipeptide Aspergillicins Revealed by Chromatin Reader Protein Deletion

Expression of biosynthetic gene clusters (BGCs) in filamentous fungi is highly regulated by epigenetic remodeling of chromatin structure. Two classes of histone modifying proteins, writers (which place modifications on histone tails) and erasers (which remove the modifications), have been used extensively to activate cryptic BGCs in fungi. Here, for the first time, we present activation of a cryptic BGC by a third category of histone modifying proteins, reader proteins that recognize histone tail modifications and commonly mediate writer and eraser activity. Loss of the reader SntB (Δ sntB) resulted in the synthesis of two cryptic cyclic hexa-depsipeptides, aspergillicin A and aspergillicin F, in the fungus Aspergillus flavus. Liquid chromatography, high resolution mass spectrometry, and NMR analysis coupled with bioinformatic analysis and gene deletion experiments revealed that a six adenylation (A) domain nonribosomal peptide synthetase (NRPS, called AgiA) and O-methyltransferase (AgiB) were required for metabolite formation. A proposed biosynthetic scheme illustrates the requirement for unusual NRPS domains, such as a starting condensation domain and a thiolesterase domain proposed to cyclize the depsipeptides. This latter activity has only been found in bacterial but not fungal NRPS. The agi BGC-unique to A. flavus and some closely related species (e.g., A. oryzae, A. arachidicola)-is located next to a conserved Aspergillus siderophore BGC syntenic to other fungi.

Structural Diversity and Biological Activities of Cyclic Depsipeptides from Fungi

Cyclic depsipeptides (CDPs) are cyclopeptides in which amide groups are replaced by corresponding lactone bonds due to the presence of a hydroxylated carboxylic acid in the peptide structure. These peptides sometimes display additional chemical modifications, including unusual amino acid residues in their structures. This review highlights the occurrence, structures and biological activities of the fungal CDPs reported until October 2017. About 352 fungal CDPs belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, and tridecadepsipeptides have been isolated from fungi. These metabolites are mainly reported from the genera Acremonium, Alternaria, Aspergillus, Beauveria, Fusarium, Isaria, Metarhizium, Penicillium, and Rosellina. They are known to exhibit various biological activities such as cytotoxic, phytotoxic, antimicrobial, antiviral, anthelmintic, insecticidal, antimalarial, antitumoral and enzyme-inhibitory activities. Some CDPs (i.e., PF1022A, enniatins and destruxins) have been applied as pharmaceuticals and agrochemicals.

C3 and 2D C3 Marfey's Methods for Amino Acid Analysis in Natural Products

We validate the improved resolution and sensitivity of the C3 Marfey's method, including an ability to resolve all Ile isomers, against an array of amino acids commonly encountered in natural products and by comparison to an existing Marfey's method. We also describe an innovative 2D C3 Marfey's method as an analytical approach for determining the regiochemistry of enantiomeric amino acid residues in natural products. The C3 and 2D C3 Marfey's methods represent valuable tools for probing and defining the stereocomplexity of hydrolytically accessible amino acid residues in natural products.