Pseudoxylallemycins A-F, Cyclic Tetrapeptides with Rare Allenyl Modifications Isolated from Pseudoxylaria sp. X802: A Competitor of Fungus-Growing Termite Cultivars

Investigation into how Odontotermes obesus maintains a predominantly Termitomyces monoculture in their fungus combs suggests a potential partnership with both fungi and bacteria

Fungus-growing termites, like Odontotermes obesus, cultivate Termitomyces as their sole food source on fungus combs which are continuously maintained with foraged plant materials. This necessary augmentation also increases the threat of introducing non-specific fungi capable of displacing Termitomyces. The magnitude of this threat and how termites prevent the invasion of such fungi remain largely unknown. This study identifies these non-specific fungi by establishing the pan-mycobiota of O. obesus from the fungus comb and termite castes. Furthermore, to maximize the identification of such fungi, the mycobiota of the decaying stages of the unattended fungus comb were also assessed. The simultaneous assessment of the microbiota and the mycobiota of these stages identified possible interactions between the fungal and bacterial members of this community. Based on these findings, we propose possible interactions among the crop fungus Termitomyces, the weedy fungus Pseudoxylaria and some bacterial symbiotes. These possibilities were then tested with in vitro interaction assays which suggest that Termitomyces, Pseudoxylaria and certain potential bacterial symbiotes possess anti-fungal capabilities. We propose a multifactorial interaction model of these microbes, under the care of the termites, to explain how their interactions can maintain a predominantly Termitomyces monoculture.

The insect microbiome is a vast source of bioactive small molecules

Covering: September 1964 to June 2023Bacteria and fungi living in symbiosis with insects have been studied over the last sixty years and found to be important sources of bioactive natural products. Not only classic producers of secondary metabolites such as Streptomyces and other members of the phylum Actinobacteria but also numerous bacteria from the phyla Proteobacteria and Firmicutes and an impressive array of fungi (usually pathogenic) serve as the source of a structurally diverse number of small molecules with important biological activities including antimicrobial, cytotoxic, antiparasitic and specific enzyme inhibitors. The insect niche is often the exclusive provider of microbes producing unique types of biologically active compounds such as gerumycins, pederin, dinactin, and formicamycins. However, numerous insects still have not been described taxonomically, and in most cases, the study of their microbiota is completely unexplored. In this review, we present a comprehensive survey of 553 natural products produced by microorganisms isolated from insects by collating and classifying all the data according to the type of compound (rather than the insect or microbial source). The analysis of the correlations among the metadata related to insects, microbial partners, and their produced compounds provides valuable insights into the intricate dynamics between insects and their symbionts as well as the impact of their metabolites on these relationships. Herein, we focus on the chemical structure, biosynthesis, and biological activities of the most relevant compounds.

Molecular networking and computational NMR analyses uncover six polyketide-terpene hybrids from termite-associated Xylaria isolates

Fungi constitute the Earth's second most diverse kingdom, however only a small percentage of these have been thoroughly examined and categorized for their secondary metabolites, which still limits our understanding of the ecological chemical and pharmacological potential of fungi. In this study, we explored members of the co-evolved termite-associated fungal genus Xylaria and identified a family of highly oxygenated polyketide-terpene hybrid natural products using an MS/MS molecular networking-based dereplication approach. Overall, we isolated six no yet reported xylasporin derivatives, of which xylasporin A (1) features a rare cyclic-carbonate moiety. Extensive comparative spectrometric (HRMS2) and spectroscopic (1D and 2D NMR) studies allowed to determine the relative configuration across the xylasporin family, which was supported by chemical shift calculations of more than 50 stereoisomers and DP4+ probability analyses. The absolute configuration of xylasporin A (1) was also proposed based on TDDFT-ECD calculations. Additionally, we were able to revise the relative and absolute configurations of co-secreted xylacremolide B produced by single x-ray crystallography. Comparative genomic and transcriptomic analysis allowed us to deduce the putative biosynthetic assembly line of xylasporins in the producer strain X802, and could guide future engineering efforts of the biosynthetic pathway.

Genome mining for macrolactam-encoding gene clusters allowed for the network-guided isolation of β-amino acid-containing cyclic derivatives and heterologous production of ciromicin A

β-Amino acid-containing macrolactams represent a structurally diverse group of bioactive natural products derived from polyketides; however we are currently lacking a comprehensive overview about their abundance across bacterial families and the underlying biosynthetic diversity. In this study, we employed a targeted β-amino acid-specific homology-based multi-query search to identify potential bacterial macrolactam producers. Here we demonstrate that approximately 10% of each of the identified actinobacterial genera harbor a biosynthetic gene cluster (BGC) encoding macrolactam production. Based on our comparative study, we propose that mutations occurring in specific regions of polyketide synthases (PKS) are the primary drivers behind the variation in macrolactam ring sizes. We successfully validated two producers of ciromicin A from the genus Amycolatopsis, revised the composition of the biosynthetic gene cluster region mte of macrotermycins, and confirmed the ciromicin biosynthetic pathway through heterologous expression. Additionally, network-based metabolomic analysis uncovered three previously unreported macrotermycin congeners from Amycolatopsis sp. M39. The combination of targeted mining and network-based analysis serves as a powerful tool for identifying macrolactam producers and our studies will catalyze the future discovery of yet unreported macrolactams.

Adaptations of Pseudoxylaria towards a comb-associated lifestyle in fungus-farming termite colonies

Characterizing ancient clades of fungal symbionts is necessary for understanding the evolutionary process underlying symbiosis development. In this study, we investigated a distinct subgeneric taxon of Xylaria (Xylariaceae), named Pseudoxylaria, whose members have solely been isolated from the fungus garden of farming termites. Pseudoxylaria are inconspicuously present in active fungus gardens of termite colonies and only emerge in the form of vegetative stromata, when the fungus comb is no longer attended ("sit and wait" strategy). Insights into the genomic and metabolic consequences of their association, however, have remained sparse. Capitalizing on viable Pseudoxylaria cultures from different termite colonies, we obtained genomes of seven and transcriptomes of two Pseudoxylaria isolates. Using a whole-genome-based comparison with free-living members of the genus Xylaria, we document that the association has been accompanied by significant reductions in genome size, protein-coding gene content, and reduced functional capacities related to oxidative lignin degradation, oxidative stress responses and secondary metabolite production. Functional studies based on growth assays and fungus-fungus co-cultivations, coupled with isotope fractionation analysis, showed that Pseudoxylaria only moderately antagonizes growth of the termite food fungus Termitomyces, and instead extracts nutrients from the food fungus biomass for its own growth. We also uncovered that Pseudoxylaria is still capable of producing structurally unique metabolites, which was exemplified by the isolation of two novel metabolites, and that the natural product repertoire correlated with antimicrobial and insect antifeedant activity.

Diastereoselective synthesis of cyclic tetrapeptide pseudoxylallemycin A illuminates the impact of base during macrolactamization

Therapeutic agents with unique molecular structures and new mechanisms of action are needed to confront the phenomenon of multidrug resistance among bacteria. Pseudoxylallemycins, cyclic tetrapeptide (CTP) natural products, have exhibited modest antibiotic activity, but their synthesis has proven challenging. Inherent ring strain in CTPs decreases the rate of cyclization in lieu of polymerization and racemization pathways, which has resulted in previous syntheses describing mixtures of diastereomers containing predominantly an undesired epimer. We have optimized the cyclization step of pseudoxylallemycin A to favor production of the natural diastereomer; notably, variation of the base, temperature, and solvent with peptide coupling reagent propylphosphonic anhydride (T3P) afforded exquisite selectivity for the natural product in as high as 97 : 3 DR, and our conditions can provide the natural product in up to 32% overall yield through 8 steps. Employing weaker bases than those typically used in peptide coupling reactions led to the greatest improvement in diastereoselectivity, and these studies demonstrated that the identity of the amine base has enormous impact on the rate of C-terminal epimerization when T3P is used, a variable usually considered of lesser consequence when combined with typical amide coupling reagents. Toward fully characterizing pseudoxylallemycin stereoisomers, variable temperature NMR was described as a tool to more clearly analyze CTPs that exhibit multiple conformational states. These synthetic and spectroscopic insights were applied toward synthesizing several natural product analogues, and their antibacterial activity was examined using microdilution assays.

Insights into the Metabolomic Capacity of Podaxis and Isolation of Podaxisterols A-D, Ergosterol Derivatives Carrying Nitrosyl Cyanide-Derived Modifications

Cultures of a termite-associated and a free-living member of the fungal genus Podaxis, revived from spores maintained in century-old herbarium collections, were analyzed for their insecticidal and antimicrobial effects. Their secondary metabolomes were explored to uncover possible adaptive mechanisms of termite association, and dereplication of LC-HRMS/MS data sets led to the isolation of podaxisterols A-D (1-4), modified ergosterol derivatives that result from a Diels-Alder reaction with endogenous nitrosyl cyanide. Chemical structures were determined based on HRMS/MS and NMR analyses as well as X-ray crystallography. The putative origin of the endogenous fungal nitrosyl cyanide and ergosterol derivatives is discussed based on results obtained from stable isotope experiments and in silico analysis. Our "omics"-driven analysis of this underexplored yet worldwide distributed fungal genus builds a foundation for studies on a potential metabolic adaptations to diverse lifestyles.

Fungal-fungal co-culture: a primer for generating chemical diversity

Covering: 2002 to 2020In their natural environment, fungi must compete for resources. It has been hypothesized that this competition likely induces the biosynthesis of secondary metabolites for defence. In a quest to discover new chemical diversity from fungal cultures, a growing trend has been to recapitulate this competitive environment in the laboratory, essentially growing fungi in co-culture. This review covers fungal-fungal co-culture studies beginning with the first literature report in 2002. Since then, there has been a growing number of new secondary metabolites reported as a result of fungal co-culture studies. Specifically, this review discusses and provides insights into (1) rationale for pairing fungal strains, (2) ways to grow fungi for co-culture, (3) different approaches to screening fungal co-cultures for chemical diversity, (4) determining the secondary metabolite-producing strain, and (5) final thoughts regarding the fungal-fungal co-culture approach. Our goal is to provide a set of practical strategies for fungal co-culture studies to generate unique chemical diversity that the natural products research community can utilize.

The chemical ecology of the fungus-farming termite symbiosis

Covering: September 1972 to December 2020Explorations of complex symbioses have often elucidated a plethora of previously undescribed chemical compounds that may serve ecological functions in signalling, communication or defence. A case in point is the subfamily of termites that cultivate a fungus as their primary food source and maintain complex bacterial communities, from which a series of novel compound discoveries have been made. Here, we summarise the origins and types of 375 compounds that have been discovered from the symbiosis over the past four decades and discuss the potential for synergistic actions between compounds within the complex chemical mixtures in which they exist. We go on to highlight how vastly underexplored the diversity and geographic distribution of the symbiosis is, which leaves ample potential for natural product discovery of compounds of both ecological and medical importance.

Cu(i)- and Pd(ii)-catalyzed decarboxylative cross-couplings of alkynyl carboxylic acids with N-tosylhydrazones: access to trisubstituted allenes and conjugated enynes

This paper reports the copper- and palladium-catalyzed decarboxylative cross-coupling reactions of alkynyl carboxylic acids and N-tosylhydrazones, affording trisubstituted allenes and conjugated enynes, respectively.

Antioxidant Activity and Cytotoxicity against Cancer Cell Lines of the Extracts from Novel Xylaria Species Associated with Termite Nests and LC-MS Analysis

Xylaria species associated with termite nests or soil have been considered rare species in nature and the few which have been reported upon have been found to act as a rich source of bioactive metabolites. This study evaluated 10 ethyl acetate extracts of five new Xylaria species associated with termite nests or soil for their antioxidant activity, and cytotoxicity against different cancer and normal cell lines. DPPH and ABTS radical scavenging activities of the extracts demonstrated strong capacity with low IC₅₀ values. The highest observed activities belonged to X. vinacea SWUF18-2.3 having IC₅₀ values of 0.194 ± 0.031 mg/mL for DPPH assay and 0.020 ± 0.004 mg/mL for ABTS assay. Total phenolic content ranged from 0.826 ± 0.123 to 3.629 ± 0.381 g GAE/g crude extract which correlated with antioxidant activities. The high total phenolic content could contribute to the high antioxidant activities. Cytotoxicity was recorded against A549, HepG2, HeLa and PNT2 and resulted in broad spectrum to specific activity depending on the cell lines. The highest activities were observed with X. subintraflava SWUF16-11.1 which resulted in 11.15 ± 0.32 to 13.17 ± 2.37% cell viability at a concentration of 100 µg/mL. Moreover, LC-MS fingerprints indicated over 61 peaks from all isolates. There were 18 identified and 43 unidentified compounds compared to mass databases. The identified compounds were from various groups of diterpenoids, diterpenes, cytochalasin, flavones, flavonoids, polyphenols, steroids and derivatives, triterpenoids and tropones. These results indicate that Xylaria spp. has abundant secondary metabolites that could be further explored for their therapeutic properties.

Fungal Guttation, a Source of Bioactive Compounds, and Its Ecological Role-A Review

Guttation is a common phenomenon in the fungal kingdom. Its occurrence and intensity depend largely on culture conditions, such as growth medium composition or incubation temperature. As filamentous fungi are a rich source of compounds, possessing various biological activities, guttation exudates could also contain bioactive substances. Among such molecules, researchers have already found numerous mycotoxins, antimicrobials, insecticides, bioherbicides, antiviral, and anticancer agents in exudate droplets. They belong to either secondary metabolites (SMs) or proteins and are secreted with different intensities. The background of guttation, in terms of its biological role, in vivo, and promoting factors, has been explored only partially. In this review, we describe the metabolites present in fungal exudates, their diversity, and bioactivities. Pointing to the significance of fungal ecology and natural products discovery, selected aspects of guttation in the fungi are discussed.

A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has been demonstrated through a series of transformations of the allene moiety. By applying this reaction as the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.

GNPS-guided discovery of xylacremolide C and D, evaluation of their putative biosynthetic origin and bioactivity studies of xylacremolide A and B

Targeted HRMS2-GNPS-based metabolomic analysis of Pseudoxylaria sp. X187, a fungal antagonist of the fungus-growing termite symbiosis, resulted in the identification of two lipopeptidic congeners of xylacremolides, named xylacremolide C and D, which are built from d-phenylalanine, l-proline and an acetyl-CoA starter unit elongated by four malonyl-CoA derived ketide units. The putative xya gene cluster was identified from a draft genome generated by Illumina and PacBio sequencing and RNAseq studies. Biological activities of xylacremolide A and B were evaluated and revealed weak histone deacetylase inhibitory (HDACi) and antifungal activities, as well as moderate protease inhibition activity across a panel of nine human, viral and bacterial proteases.

An Overview of Antimicrobial Compounds from African Edible Insects and Their Associated Microbiota

The need for easily biodegradable and less toxic chemicals in drug development and pest control continues to fuel the exploration and discovery of new natural molecules. Like certain plants, some insects can also respond rapidly to microbial infections by producing a plethora of immune-induced molecules that include antibacterial and antifungal peptides/polypeptides (AMPs), among other structurally diverse small molecules. The recent recognition that new natural product-derived scaffolds are urgently needed to tackle life-threatening pathogenic infections has been prompted by the health threats posed by multidrug resistance. Although many researchers have concentrated on the discovery of AMPs, surprisingly, edible insect-produced AMPs/small molecules have received little attention. This review will discuss the recent advances in the identification and bioactivity analysis of insect AMPs, with a focus on small molecules associated with the microbiota of selected African edible insects. These molecules could be used as templates for developing next-generation drugs to combat multidrug-resistant pathogens.

Targeted Isolation of Saalfelduracin B-D from Amycolatopsis saalfeldensis Using LC-MS/MS-Based Molecular Networking

High-resolution tandem mass spectrometry (HR-MS²)-based metabolomic studies of Amycolatopsis saalfeldensis, isolated from the "Saalfelder Feengrotten" caves in Germany, led to the isolation of three ribosomally synthesized and post-translationally modified type II thiopeptides, saalfelduracin B-D (1-3) and the known saalfelduracin A (4). The structures of all four compounds were determined by comparative two-dimensional NMR analysis and high-resolution tandem mass spectrometry.

Polyene-Producing Streptomyces spp. From the Fungus-Growing Termite Macrotermes barneyi Exhibit High Inhibitory Activity Against the Antagonistic Fungus Xylaria

Fungus-growing termites are engaged in a tripartite mutualism with intestinal microbes and a monocultivar (Termitomyces sp.) in the fungus garden. The termites are often plagued by entomopathogen (Metarhizium anisopliae) and fungus garden is always threatened by competitors (Xylaria spp.). Here, we aim to understand the defensive role of intestinal microbes, the actinomycetes which were isolated from the gut of Macrotermes barneyi. We obtained 44 antifungal isolates, which showed moderate to strong inhibition to Xylaria sp. HPLC analysis indicated that different types of polyenes (tetraene, pentene, and heptaene) existed in the metabolites of 10 strong antifungal Streptomyces strains. Two pentene macrolides (pentamycin and 1′14-dihydroxyisochainin) were firstly purified from Streptomyces strain HF10, both exhibiting higher activity against Xylaria sp. and M. anisopliae than cultivar Termitomyces. Subsequently, tetraene and heptaene related gene disruption assay showed that the mutant strains lost the ability to produce corresponding polyenes, and they also had significantly decreased activities against Xylaria sp. and M. anisopliae compared to that of wild type strains. These results indicate that polyene-producing Streptomyces from the guts of M. barneyi have strong inhibition to competitor fungus and polyenes contribute to inhibitory effects on Xylaria sp.

Natural Cyclopeptides as Anticancer Agents in the Last 20 Years

Cyclopeptides or cyclic peptides are polypeptides formed by ring closing of terminal amino acids. A large number of natural cyclopeptides have been reported to be highly effective against different cancer cells, some of which are renowned for their clinical uses. Compared to linear peptides, cyclopeptides have absolute advantages of structural rigidity, biochemical stability, binding affinity as well as membrane permeability, which contribute greatly to their anticancer potency. Therefore, the discovery and development of natural cyclopeptides as anticancer agents remains attractive to academic researchers and pharmaceutical companies. Herein, we provide an overview of anticancer cyclopeptides that were discovered in the past 20 years. The present review mainly focuses on the anticancer efficacies, mechanisms of action and chemical structures of cyclopeptides with natural origins. Additionally, studies of the structure-activity relationship, total synthetic strategies as well as bioactivities of natural cyclopeptides are also included in this article. In conclusion, due to their characteristic structural features, natural cyclopeptides have great potential to be developed as anticancer agents. Indeed, they can also serve as excellent scaffolds for the synthesis of novel derivatives for combating cancerous pathologies.

Combining NMR-Based Metabolic Profiling and Genome Mining for the Accelerated Discovery of Archangiumide, an Allenic Macrolide from the Myxobacterium Archangium violaceum SDU8

An unprecedented 19-membered allenic macrolide archangiumide (1) was discovered from the myxobacterium Archangium violaceum SDU8 by integrating NMR-based metabolic profiling and genome mining. Its biosynthesis pathway was proposed based on the architectural analysis of the encoding trans-AT PKS genes and validated by isotope labeling. The methodology of combing 2D NMR-based metabolic profiling and bioinformatics-aided structure prediction, as exemplified by this study, is anticipated to improve discovery efficiency of a broader range of microbial "dark matter".

Total Synthesis of Allene-Containing Cyclic Tetrapeptide Pseudoxylallemycin C

The first total synthesis of the allene-containing cyclic tetrapeptide pseudoxylallemycin C is reported. The Tyr(t-Bu)-protected linear peptide was prepared on-resin and cyclized in solution phase to yield the protected cyclic precursor. Upon deprotection of Tyr(t-Bu), the desired phenolic cyclic tetrapeptide was separated by RP-HPLC from its epimer that also formed during the macrocyclisation step. Subsequent alkylation with 4-bromobuta-1,2-diene yielded pseudoxylallemycin C.

Targeted Discovery of Tetrapeptides and Cyclic Polyketide-Peptide Hybrids from a Fungal Antagonist of Farming Termites

Herein, we report the targeted isolation and characterization of four linear nonribosomally synthesized tetrapeptides (pseudoxylaramide A-D) and two cyclic nonribosomal peptide synthetase-polyketide synthase-derived natural products (xylacremolide A and B) from the termite-associated stowaway fungus Pseudoxylaria sp. X187. The fungal strain was prioritized for further metabolic analysis based on its taxonomical position and morphological and bioassay data. Metabolic data were dereplicated based on high-resolution tandem mass spectrometry data and global molecular networking analysis. The structure of all six new natural products was elucidated based on a combination of 1D and 2D NMR analysis, Marfey's analysis and X-ray crystallography.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

Entomogenous fungi isolated from Cryptotympana atrata with antibacterial and antifungal activity

Although many entomogenous fungi have been discovered over the years, few studies on the crude extracts of fungi isolated from Cryptotympana atrata with antibacterial and antifungal activity were reported. In this study, total twenty entomogenous fungi were isolated for the first time. And among of them, two pure cultures were identified as Purpureocillium lilacinum and Aspergillus fumigatus with apparent morphology, microscopic identification and 18S rRNA gene sequence. The active strains were fermented to optimize in six different culture media at three different pH values. The antibacterial and antifungal activities of the metabolites were more potent and efficient in Fungal medium 3^# at a pH of 6.2 than in the other tested media or at the other tested pH values. Total seven human pathogens and one insect pathogen were used to evaluate the antibacterial and antifungal activity of crude extracts, among which 25% of the extracts exhibited antifungal activity against Verticillium lecanii, while 33.3% and 47.2% of the extracts exhibited antibacterial activity against the important human pathogens Staphylococcus aureus and Bacillus cereus, respectively. The range of the MICs was from 15.6 to 250 μg mL^-1, and 35% of the fungal metabolites exhibited antibacterial activity against Pseudomonas aeruginosa, Bacillus thuringiensis and Enterobacter aerogenes at 1000 μg mL^-1 except the previously described antibacterial activities. Furthermore, the phylogenetic relationships of the two identified fungi were also constructed. In brief, it is the first reporting about enthompathogenic fungi from Cryptotympana atrata and provides candidate strains with potential use as biological agents and against multidrug-resistant organisms.

Xyloneside A: A New Glycosylated Incisterol Derivative from Xylaria sp. FB

Xylaria species are prolific natural product producers. Here, we report the characterization of a new glycosylated incisterol derivative, called xyloneside A (1) and two known lignans (2 and 3) from the ascomycetous Xylaria sp. FB. The structure of xyloneside A (1) was determined by 1D and 2D NMR spectroscopy, high-resolution electrospray ionization mass spectrometry and electronic circular dichroism measurements. Xyloneside A is composed of a 1,2,3,4,5,10,19-heptanorergosterane skeleton and a β-D-mannopyranose moiety. This is the first report of an incisterol derivative from an Ascomycete. The biological effects of the isolated metabolites on cytotoxicity, autophagy, cell-migration, and angiogenesis were evaluated.

Identification and bioactivity evaluation of secondary metabolites from Antarctic-derived Penicillium chrysogenum CCTCC M 2020019

Extracts from Antarctic-derived Penicillium chrysogenum CCTCC M 2020019 showed potent antibacterial bioactivities. We report herein the isolation of chrysonin (1), a new compound containing a pair of enantiomers 6S- and 6R-chrysonin (1a and 1b) featuring an unprecedented eight-membered heterocycle fused with a benzene ring. Compound 2, a mixture consisting of a new zwitterionic compound chrysomamide (2a) and N-[2-trans-(4-hydroxyphenyl) ethenyl] formamide (2b) in a ratio around 1 : 2.8, was isolated together with seven known compounds 3-9. Chemical structures of all compounds were determined by comprehensive spectroscopic analyses. The isolated compounds were evaluated for antimicrobial, cytotoxic and alpha-glucosidase inhibition activities. Chrysonin (1) showed moderate alpha-glucosidase inhibitory activity. The dominant product xanthocillin X (4) displayed potent inhibition activities against Gram-negative pathogens Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa with MIC values at 0.125 μg mL-1. Xanthocillins X (4) and Y1 (5) also showed significant cytotoxicities against four cancer cell lines with IC50 values ranging from 0.26 to 5.04 μM. This study highlights that microorganisms from polar regions are emerging as a new resource for the discovery of natural products combating human pathogens.

Research Tales from the Clardy Laboratory: Function-Driven Natural Product Discovery

Over the past 70 years, the search for small molecules from nature has transformed biomedical research: natural products are the basis for half of all pharmaceuticals; the quest for total synthesis of natural products fueled development of methodologies for organic synthesis; and their biosynthesis presented unprecedented biochemical transformations, expanding our chemo-enzymatic toolkit. Initially, the discovery of small molecules was driven by bioactivity-guided fractionation. However, this approach yielded the frequent rediscovery of already known metabolites. As a result, focus shifted to identifying novel scaffolds through either structure-first methods or genome mining, relegating function as a secondary concern. Over the past two decades, the laboratory of Jon Clardy has taken an alternative route and focused on an ecology-driven, function-first approach in pursuit of uncovering bacterial small molecules with biological activity. In this review, we highlight several examples that showcase this ecology-first approach. Though the highlighted systems are diverse, unifying themes are (1) to understand how microbes interact with their host or environment, (2) to gain insights into the environmental roles of microbial metabolites, and (3) to explore pharmaceutical potential from these ecologically relevant metabolites.

Fungi: outstanding source of novel chemical scaffolds

A large number of remarkable studies on the secondary metabolites of fungi have been conducted in recent years. This review gives an overview of one hundred and sixty-seven molecules with novel skeletons and their bioactivities that have been reported in seventy-nine articles published from 2013 to 2017. Our statistical data showed that endophytic fungi and marine-derived fungi are the major sources of novel bioactive secondary metabolites.

Investigations of the key macrolactamisation step in the synthesis of cyclic tetrapeptide pseudoxylallemycin A

The total synthesis and structural confirmation of naturally occurring all l-cyclic tetrapeptide pseudoxylallemycin A is reported. X-ray crystallography revealed that the linear precursor adopted an all-trans (ttt) extended linear conformation, while its cyclic derivative adopts a trans,cis,trans,cis (tctc) conformation. Two kinetically favoured cyclic conformers prone to hydrolysis initially formed rapidly during cyclisation, with subsequent conversion to the thermodynamically stable tctc macrocycle taking place slowly. We postulate the initial unstable cyclic product undergoes an unprecedented nucleophilic ring opening with either the T3P or PyAOP by-products to give the linear ttt structure as a reactivated species and through a series of equilibria is slowly consumed by cyclisation to the thermodynamic product pseudoxylallemycin A. Consumption of the reactivated species by formation of pseudoxylallemycin A requires a trans-cis isomerism to occur and necessitates moderately increased reaction temperatures. Cyclisation with T3P was found to provide the greatest stereoretention. Synthesis and X-ray crystallography of the C-terminal epimer demonstrated its cyclisation to be kinetically favoured and to proceed without epimerisation despite also bearing an all-trans backbone.

Expanding the chemical diversity through microorganisms co-culture: Current status and outlook

Natural products (NPs) are considered as a cornerstone for the generation of bioactive leads in drug discovery programs. However, one of the major limitations of NP drug discovery program is "rediscovery" of known compounds, thereby hindering the rate of drug discovery efficiency. Therefore, in recent years, to overcome these limitations, a great deal of attention has been drawn towards understanding the role of microorganisms' co-culture in inducing novel chemical entities. Such induction could be related to activation of genes which might be silent or expressed at very low levels (below detection limit) in pure-strain cultures under normal laboratory conditions. In this review, chemical diversity of compounds isolated from microbial co-cultures, is discussed. For this purpose, chemodiversity has been represented as a chemical-structure network based on the "Tanimoto Structural Similarity Index". This highlights the huge structural diversity induced by microbial co-culture. In addition, the current trends in microbial co-culture research are highlighted. Finally, the current challenges (1 - induction monitoring, 2 - reproducibility, 3 - growth time effect and 4 - up-scaling for isolation purposes) are discussed. The information in this review will support researchers to design microbial co-culture strategies for future research efforts. In addition, guidelines for co-culture induction reporting are also provided to strengthen future reporting in this NP field.

Biosynthetic Pathways to Nonproteinogenic α-Amino Acids

Natural nonproteinogenic amino acids vastly outnumber the well-known 22 proteinogenic amino acids. Such amino acids are generated in specialized metabolic pathways. In these pathways, diverse biosynthetic transformations, ranging from isomerizations to the stereospecific functionalization of C-H bonds, are employed to generate structural diversity. The resulting nonproteinogenic amino acids can be integrated into more complex natural products. Here we review recently discovered biosynthetic routes to freestanding nonproteinogenic α-amino acids, with an emphasis on work reported between 2013 and mid-2019.

Total Synthesis of Endolides A and B

The total synthesis of endolides A and B has been achieved in a concise, highly stereoselective fashion (12 steps, 16.2% and 16.0% overall yields, respectively). Key features of the route include a modified Negishi coupling between 3-bromofuran and an organozinc reagent derived from an iodoalanine derivative for the synthesis of 3-(3-furyl)-alanine derivative, and a judicious choice of reaction conditions to surmount the conformational constraints placed by converting a linear peptide into the corresponding macrocycle.

Cyclic Tetrapeptides from Nature and Design: A Review of Synthetic Methodologies, Structure, and Function

Small cyclic peptides possess a wide range of biological properties and unique structures that make them attractive to scientists working in a range of areas from medicinal to materials chemistry. However, cyclic tetrapeptides (CTPs), which are important members of this family, are notoriously difficult to synthesize. Various synthetic methodologies have been developed that enable access to natural product CTPs and their rationally designed synthetic analogues having novel molecular structures. These methodologies include the use of reversible protecting groups such as pseudoprolines that restrict conformational freedom, ring contraction strategies, on-resin cyclization approaches, and optimization of coupling reagents and reaction conditions such as temperature and dilution factors. Several fundamental studies have documented the impacts of amino acid configurations, N-alkylation, and steric bulk on both synthetic success and ensuing conformations. Carefully executed retrosynthetic ring dissection and the unique structural features of the linear precursor sequences that result from the ring dissection are crucial for the success of the cyclization step. Other factors that influence the outcome of the cyclization step include reaction temperature, solvent, reagents used as well as dilution levels. The purpose of this review is to highlight the current state of affairs on naturally occurring and rationally designed cyclic tetrapeptides, including strategies investigated for their syntheses in the literature, the conformations adopted by these molecules, and specific examples of their function. Using selected examples from the literature, an in-depth discussion of the synthetic techniques and reaction parameters applied for the successful syntheses of 12-, 13-, and 14-membered natural product CTPs and their novel analogues are presented, with particular focus on the cyclization step. Selected examples of the three-dimensional structures of cyclic tetrapeptides studied by NMR, and X-ray crystallography are also included.

Diversity of biologically active secondary metabolites from endophytic and saprotrophic fungi of the ascomycete order Xylariales

Covering: up to December 2017 The diversity of secondary metabolites in the fungal order Xylariales is reviewed with special emphasis on correlations between chemical diversity and biodiversity as inferred from recent taxonomic and phylogenetic studies. The Xylariales are arguably among the predominant fungal endophytes, which are the producer organisms of pharmaceutical lead compounds including the antimycotic sordarins and the antiparasitic nodulisporic acids, as well as the marketed drug, emodepside. Many Xylariales are "macromycetes", which form conspicuous fruiting bodies (stromata), and the metabolite profiles that are predominant in the stromata are often complementary to those encountered in corresponding mycelial cultures of a given species. Secondary metabolite profiles have recently been proven highly informative as additional parameters to support classical morphology and molecular phylogenetic approaches in order to reconstruct evolutionary relationships among these fungi. Even the recent taxonomic rearrangement of the Xylariales has been relying on such approaches, since certain groups of metabolites seem to have significance at the species, genus or family level, respectively, while others are only produced in certain taxa and their production is highly dependent on the culture conditions. The vast metabolic diversity that may be encountered in a single species or strain is illustrated based on examples like Daldinia eschscholtzii, Hypoxylon rickii, and Pestalotiopsis fici. In the future, it appears feasible to increase our knowledge of secondary metabolite diversity by embarking on certain genera that have so far been neglected, as well as by studying the volatile secondary metabolites more intensively. Methods of bioinformatics, phylogenomics and transcriptomics, which have been developed to study other fungi, are readily available for use in such scenarios.

Non-lipopeptide fungi-derived peptide antibiotics developed since 2000

The 2,5-diketopiperazines (DKPs) are the smallest cyclopeptides and their basic structure includes a six-membered piperazine nucleus. Typical peptides lack a special functional group in the oligopeptide nucleus. Both are produced by at least 35 representative genera of fungi, and possess huge potential as pharmaceutical drugs and biocontrol agents. To date, only cyclosporin A has been developed into a commercial product. This review summarises 186 fungi-derived compounds reported since 2000. Antibiotic (antibacterial, antifungal, synergistic antifungal, antiviral, antimycobacterial, antimalarial, antileishmanial, insecticidal, antitrypanosomal, nematicidal and antimicroalgal) activities are discussed for 107 of them, including 66 DKPs (14 epipolythiodioxopiperazines, 20 polysulphide bridge-free thiodiketopiperazines, and 32 sulphur-free prenylated indole DKPs), 15 highly N-methylated, and 26 non-highly N-methylated typical peptides. Structure-activity relationships, mechanisms of action, and research methods are covered in detail. Additionally, biosynthases of tardioxopiperazines and neoechinulins are highlighted. These compounds have attracted considerable interest within the pharmaceutical and agrochemical industries.

Genomics-Driven Discovery of a Symbiont-Specific Cyclopeptide from Bacteria Residing in the Rice Seedling Blight Fungus

The rice seedling blight fungus Rhizopus microsporus harbors endosymbiotic bacteria (Burkholderia rhizoxinica) that produce the virulence factor rhizoxin and control host development. Genome mining indicated a massive inventory of cryptic nonribosomal peptide synthetase (NRPS) genes, which have not yet been linked to any natural products. The discovery and full characterization of a novel cyclopeptide from endofungal bacteria is reported. In silico analysis of an orphan, symbiont-specific NRPS predicted the structure of a nonribosomal peptide, which was targeted by LC-MS/MS profiling of wild-type and engineered null mutants. NMR spectroscopy and chemical derivatization elucidated the structure of the bacterial cyclopeptide. Phylogenetic analyses revealed the relationship of starter C domains for rare N-acetyl-capped peptides. Heptarhizin is produced under symbiotic conditions in geographically constrained strains from the Pacific clade; this indicates a potential ecological role of the peptide.

Natural Cyclic Peptides as an Attractive Modality for Therapeutics: A Mini Review

Peptides are important biomolecules which facilitate the understanding of complex biological processes, which in turn could be serendipitous biological targets for future drugs. They are classified as a unique therapeutic niche and will play an important role as fascinating agents in the pharmaceutical landscape. Until now, more than 40 cyclic peptide drugs are currently in the market, and approximately one new cyclopeptide drug enters the market annually on average. Interestingly, the majority of clinically approved cyclic peptides are derived from natural sources, such as peptide antibiotics and human peptide hormones. In this report, the importance of cyclic peptides is discussed, and their role in drug discovery as interesting therapeutic biomolecules will be highlighted. Recently isolated naturally occurring cyclic peptides from microorganisms, sponges, and other sources with a wide range of pharmacological properties are reviewed herein.

Defense contracts: molecular protection in insect-microbe symbioses

Insects frequently host microbes that produce defensive molecules: a successful protective strategy and also an opportunity for antibiotic discovery

Structural Diversity and Biological Activities of Fungal Cyclic Peptides, Excluding Cyclodipeptides

Cyclic peptides are cyclic compounds formed mainly by the amide bonds between either proteinogenic or non-proteinogenic amino acids. This review highlights the occurrence, structures and biological activities of fungal cyclic peptides (excluding cyclodipeptides, and peptides containing ester bonds in the core ring) reported until August 2017. About 293 cyclic peptides belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, undeca-, dodeca-, tetradeca-, and octadecapeptides as well as cyclic peptides containing ether bonds in the core ring have been isolated from fungi. They were mainly isolated from the genera Aspergillus, Penicillium, Fusarium, Acremonium and Amanita. Some of them were screened to have antimicrobial, antiviral, cytotoxic, phytotoxic, insecticidal, nematicidal, immunosuppressive and enzyme-inhibitory activities to show their potential applications. Some fungal cyclic peptides such as the echinocandins, pneumocandins and cyclosporin A have been developed as pharmaceuticals.

Gunnilactams A-C, Macrocyclic Tetralactams from the Mycelial Culture of the Entomogenous Fungus Paecilomyces gunnii

Three novel macrocyclic tetralactams, gunnilactam A (1), gunnilactam B (2), and gunnilactam C (3), were isolated from the submerged fermentation broth of Paecilomyces gunnii, an entomogenous fungus identified as the anamorph of Cordyceps gunnii. Their structures were determined using NMR data, HREIMS, and single-crystal X-ray crystallography. Gunnilactam A exhibited selective cytotoxic activity against human prostate cancer C42B cells with an IC₅₀ value of 5.4 μM.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as chrysamide A from a deep-sea fungus Penicillium chrysogenum.