Structure and Biosynthesis of Mutolide, a Novel Macrolide from a UV Mutant of the Fungus F-24′707

Endophytic fungi as a potential source of anti-cancer drug

Endophytes are considered one of the major sources of bioactive compounds used in different aspects of health care including cancer treatment. When colonized, they either synthesize these bioactive compounds as a part of their secondary metabolite production or augment the host plant machinery in synthesising such bioactive compounds. Hence, the study of endophytes has drawn the attention of the scientific community in the last few decades. Among the endophytes, endophytic fungi constitute a major portion of endophytic microbiota. This review deals with a plethora of anti-cancer compounds derived from endophytic fungi, highlighting alkaloids, lignans, terpenes, polyketides, polyphenols, quinones, xanthenes, tetralones, peptides, and spirobisnaphthalenes. Further, this review emphasizes modern methodologies, particularly omics-based techniques, asymmetric dihydroxylation, and biotic elicitors, showcasing the dynamic and evolving landscape of research in this field and describing the potential of endophytic fungi as a source of anticancer drugs in the future.

Total Synthesis and Biological Evaluation of Mutolide and Analogues

The convergent total syntheses of three 14-membered macrolide natural products, mutolide, nigrosporolide and (4S,7S,13S)-4,7-dihydroxy-13-tetradeca-2,5,8-trienolide have been achieved. The key synthetic features include Shiina macrolactonization to assemble the 14-membered macrocyclic core, Wittig or Still-Gennari olefination and selective reduction of propargylic alcohol to construct the E- or Z-olefins. Cross metathesis was also highlighted as an efficient tool to forge the formation of E-olefin. The three synthetic macrolides were evaluated for their cytotoxic activity against three human cancer cell lines as well as for inhibitory effect on CFTR-mediated chloride secretion in human intestinal epithelial (T84) cells. Mutolide displayed significant cytotoxic activity against HCT116 colon cancer cells with an IC₅₀ of ∼12 μM as well as a potent CTFR inhibitory effect with an IC₅₀ value of ∼1 μM.

Total synthesis and cytotoxic activity of 7-O-methylnigrosporolide and pestalotioprolide D

A convergent total synthesis of 7-O-methylnigrosporolide and pestalotioprolide D has been accomplished in 17 longest linear steps starting with 1.7% and 2.6% overall yields, respectively, starting from (S)-propylene oxide and known (S)-benzyl glycidyl ether. Our synthesis exploited acetylide addition and Shiina macrolactonization to assemble the macrocycle, Lindlar reduction, Wittig and Still–Gennari olefination to construct the three olefins as well as a Jacobsen hydrolytic kinetic resolution to install the stereogenic center. The selection of silyl protecting group of C-4 alcohol was crucial for final deprotection step. Our synthesis also led to a hypothesis that pestalotioprolide D could be an artifact of 7-O-methylnigrosporolide. Cytotoxic activity of both synthetic compounds against six human cancer cell lines was evaluated. Synthetic pestalotioprolide D showed more potent cytotoxic activity against all cancer cell lines tested than 7-O-methylnigrosporolide, and the SiHa cervical cancer cell line was the most sensitive cell to both synthetic compounds.

Investigating the biosynthesis of Sch-642305 in the fungus Phomopsis sp. CMU-LMA

Sch-642305 is an unusual bicyclic 10-membered macrolide produced by the filamentous fungus Phomopsis sp. CMU-LMA for which no biosynthetic evidence exists. Here, we generate a draft genome sequence of the producing organism and discover the biosynthetic gene cluster responsible for formation of Sch-642305. Targeted gene disruptions together with reconstitution of the pathway in the heterologous host Aspergillus oryzae dissect key chemical steps and shed light on a series of oxidoreductions occuring in the pathway.

Sesterterpenes and macrolide derivatives from the endophytic fungus Aplosporella javeedii

Five sesterterpenes (1-5) including two new compounds (1 and 2), as well as a new (6) and a known macrolide (7) were isolated from the endophytic fungus Aplosporella javeedii. The structures of the new compounds were elucidated by analysis of their 1D and 2D NMR and HRMS data as well as by comparison with the literature. Compound 4 and its acetyl derivatives 4a, 4b, 4c which were prepared by acetylation of 4 exhibited moderate cytotoxicity against the mouse lymphoma cell line L5178Y with IC₅₀ values ranging from 6.2 to 12.8 μM, respectively. Moreover, 4a and 4c exhibited also cytotoxicity against human leukemia (Jurkat J16) and lymphoma (Ramos) cell lines. Compound 7 showed strong cytotoxicity against the L5178Y cell line, as well as against human Jurkat J16 and Ramos cells with IC₅₀ values of 0.4, 5.8, and 4.4 μM, respectively. Mechanistic studies indicated that 7 induces apoptotic cell death. In addition, compounds 3, 4 and 7 showed low antibacterial activities against Mycobacterium tuberculosis H37Rv and compound 6 against Staphylococcus aureus, respectively, with MICs of 100 μM. Preliminary structure-activity relationships are discussed.

Cladocladosin A, an unusual macrolide with bicyclo 5/9 ring system, and two thiomacrolides from the marine mangrove-derived endophytic fungus, Cladosporium cladosporioides MA-299

A simple but previously undescribed macrolide with unprecedented bicyclo 5/9 ring system, namely, cladocladosin A (1), along with two new sulfur-containing macrolides, namely, thiocladospolides F and G (2 and 3), were characterized from the mangrove-derived endophytic fungus Cladosporium cladosporioides MA-299. The structures of these compounds were established on the basis of spectroscopic interpretation, and the absolute configurations of compounds 1-3 were determined by X-ray crystallographic analysis, Mosher's method, and by a biogenetic point of view. The possible biogenetic pathway for compounds 1-3 as well as their congeners thiocladospolides A-D and pandangolide 3 was proposed, providing a role in distinguishing the position of sulfur substitution in thiomacrolides. Compounds 1-3 were evaluated for antimicrobial activities against human-, aquatic-, and plant-pathogenic microbes.

From genomics to metabolomics, moving toward an integrated strategy for the discovery of fungal secondary metabolites

Fungal secondary metabolites are defined by bioactive properties that ensure adaptation of the fungus to its environment. Although some of these natural products are promising sources of new lead compounds especially for the pharmaceutical industry, others pose risks to human and animal health. The identification of secondary metabolites is critical to assessing both the utility and risks of these compounds. Since fungi present biological specificities different from other microorganisms, this review covers the different strategies specifically used in fungal studies to perform this critical identification. Strategies focused on the direct detection of the secondary metabolites are firstly reported. Particularly, advances in high-throughput untargeted metabolomics have led to the generation of large datasets whose exploitation and interpretation generally require bioinformatics tools. Then, the genome-based methods used to study the entire fungal metabolic potential are reported. Transcriptomic and proteomic tools used in the discovery of fungal secondary metabolites are presented as links between genomic methods and metabolomic experiments. Finally, the influence of the culture environment on the synthesis of secondary metabolites by fungi is highlighted as a major factor to consider in research on fungal secondary metabolites. Through this review, we seek to emphasize that the discovery of natural products should integrate all of these valuable tools. Attention is also drawn to emerging technologies that will certainly revolutionize fungal research and to the use of computational tools that are necessary but whose results should be interpreted carefully.

A Novel Steroid Derivative and a New Steroidal Saponin from Endophytic Fungus Xylaria sp

Two new steroids, (24 R)-22, 23-dihydroxy-ergosta-4,6,8(14)-trien-3-one 23-β-D-glucopyranoside (1), and xylarester (2), together with 16 known compounds were isolated from the extract of endophytic Xylaria sp. solid culture. Their structures were elucidated by spectroscopic methods. Compound 2 has an unprecedent ergosta skeleton with a six-membered lactonic group in A ring. The X-ray single crystal diffraction data of 4,7-dihydroxy-13-tetradeca-2,5,8-trienolide (4) were also reported for the first time. The antibacterial, anti-acetylcholinesterase, nitric oxide inhibition and cytotoxic activities of 1–3 were investigated. Compound 1 showed cytotoxicity to MCF-7 with the ratio of inhibition at 72 % for a concentration at 40 μM.

Chemically Induced Cryptic Sesquiterpenoids and Expression of Sesquiterpene Cyclases in Botrytis cinerea Revealed New Sporogenic (+)-4-Epieremophil-9-en-11-ols

The sequencing of the genomes of the B05.10 and T4 strains of the fungus Botrytis cinerea revealed an abundance of novel biosynthetic gene clusters, the majority of which were unexpected on the basis of the previous analyses of the fermentation of these and closely related species. By systematic alteration of easy accessible cultivation parameters, using chemical induction with copper sulfate, we have found a cryptic sesquiterpenoid family with new structures related to eremophil-9-ene, which had the basic structure of the sesquiterpene (+)-5-epiaristolochene ((+)-4-epieremophil-9-ene). An expression study of the sesquiterpene cyclase genes present in the Botrytis cinerea genome, under culture conditions, is reported. In general, a 3 day delay and a higher BcSTC genes expression were observed when copper (5 ppm) was fed to the fermentation broth. In addition, to the observed effect on the BcBOT2 (BcSTC1) gene, involved in the biosynthesis of the botrydial toxin, a higher expression level for BcSTC3 and BcSTC4 was observed with respect to the control in the strain B05.10. Interestingly, under copper conditions, the BcSTC4 gene was the most expressed gene in the Botrytis cinerea UCA992 strain. In vitro evaluation of the biological role of these metabolites indicates that they contributed to the conidial development in B. cinerea and appear to be involved in self-regulation of the production of asexual spores. Furthermore, they promoted the formation of complex appressoria or infection cushions.

Anti-inflammatory properties of mutolide isolated from the fungus Lepidosphaeria species (PM0651419)

Mutolide an anti-inflammatory compound was isolated from the coprophilous fungus Lepidosphaeria sp. (PM0651419). The compound mitigated LPS-induced secretion of pro-inflammatory cytokines TNF-α and IL-6 from THP-1 cells as well as human peripheral blood mononuclear cells (hPBMCs). Mutolide also inhibited secretion of another pro-inflammatory cytokine IL-17 from anti-hCD3/anti-hCD28 stimulated hPBMCs. NF-κB is the major transcription factor involved in the secretion of pro-inflammatory cytokines including IL-17. Mechanistic evaluations revealed that mutolide inhibited induced NF-κB activation and translocation from cytoplasm into the nucleus. However, mutolide did not significantly affect activity of p38 MAPK enzyme, a serine/threonine kinase involved in cell cycle proliferation and cytokine secretion. These results indicate that mutolide may exert its anti-inflammatory effect via NF-κB inhibition. Oral administration of mutolide at 100 mg/kg showed significant inhibition of LPS-induced release of TNF-α from Balb/c mice in an acute model of inflammation. Our results highlight the anti-inflammatory properties of mutolide and suggest that further evaluation in a chronic model of inflammation is required to confirm the potential of mutolide as a druggable candidate for the treatment of inflammatory diseases.

Effect of Culture Conditions on Metabolite Production of Xylaria sp

Seeking a strategy for triggering the cryptic natural product biosynthesis to yield novel compounds in the plant-associated fungus Xylaria sp., the effect of culture conditions on metabolite production was investigated. A shift in the production of five known cytochalasin-type analogues 1-5 to six new α-pyrone derivatives, xylapyrones A-F (compounds 6-11), from a solid to a liquid medium was observed. These compounds were identified by analysis of 1D and 2D NMR and HRMS data. Compounds 1-3 showed moderate cytotoxicity against HepG2 and Caski cancer cell lines with IC50 values ranging from 25 to 63 μM and compounds 4-11 were found to be inactive, with IC50 values>100 μM.

Phylum-specific regulation of resistomycin production in a Streptomyces sp. via microbial coculture

Actinomycete genomes are encoded with immense potential to produce secondary metabolites, however standard laboratory culture experiments rarely provide the conditions under which associated biosynthetic pathways are expressed. Despite years of research attempting to access these pathways and aside from a few well-studied bacterial quorum sensing systems, little is known about the specificity of secondary metabolite regulation in bacteria, such as the conditions under which a bacterium produces an antibiotic and the extent to which it does so in recognition of a particular species in the immediate environment. In the current study, we observed that the cocultivation of a Streptomyces sp. (strain B033) with four pathogenic strains of the phylum Proteobacteria resulted in the production of the antibiotic resistomycin. After further coculture experiments, we determined that Proteobacteria induced the production of resistomycin in B033 at significantly higher rates (65%) than strains from the phyla Firmicutes (5.9%) and Actinobacteria (9.1%), supporting that the regulation of secondary metabolism in bacteria can be dependent on the species present in the immediate environment. These results suggest a lack of promiscuity of antibiotic biosynthetic pathway regulation and indicate that it is feasible to mine existing microbial strain libraries for antibiotics in a phylum-specific manner.

Mining the metabiome: identifying novel natural products from microbial communities

Microbial-derived natural products provide the foundation for most of the chemotherapeutic arsenal available to contemporary medicine. In the face of a dwindling pipeline of new lead structures identified by traditional culturing techniques and an increasing need for new therapeutics, surveys of microbial biosynthetic diversity across environmental metabiomes have revealed enormous reservoirs of as yet untapped natural products chemistry. In this review, we touch on the historical context of microbial natural product discovery and discuss innovations and technological advances that are facilitating culture-dependent and culture-independent access to new chemistry from environmental microbiomes with the goal of reinvigorating the small molecule therapeutics discovery pipeline. We highlight the successful strategies that have emerged and some of the challenges that must be overcome to enable the development of high-throughput methods for natural product discovery from complex microbial communities.

Caryophyllene sesquiterpenes from the marine-derived fungus Ascotricha sp. ZJ-M-5 by the one strain-many compounds strategy

The marine-derived fungus Ascotricha sp. ZJ-M-5, which can produce cyclonerodiol analogues, a 3,4-seco lanostane triterpenoid, and diketopiperazines in an eutrophic medium, was subjected to a one strain-many compounds (OSMAC) analysis. It was found to produce three new caryophyllene derivatives (1-3) and the known 1,3,6-trihydroxy-8-methylxanthone (4) in an oligotrophic medium, Czapek Dox broth with or without Mg(2+). (+)-6-O-Demethylpestalotiopsin A (1) and (+)-6-O-demethylpestalotiopsin C (2), which have a five-membered hemiacetal structural moiety, showed growth inhibitory effects against HL-60 and K562 leukemia cell lines with the lowest GI50 value of 6.9 ± 0.4 μM. It can be concluded that modification of the culture media is still effective in the discovery of novel bioactive fungal secondary metabolites.

Metabolite induction via microorganism co-culture: a potential way to enhance chemical diversity for drug discovery

Microorganisms have a long track record as important sources of novel bioactive natural products, particularly in the field of drug discovery. While microbes have been shown to biosynthesize a wide array of molecules, recent advances in genome sequencing have revealed that such organisms have the potential to yield even more structurally diverse secondary metabolites. Thus, many microbial gene clusters may be silent under standard laboratory growth conditions. In the last ten years, several methods have been developed to aid in the activation of these cryptic biosynthetic pathways. In addition to the techniques that demand prior knowledge of the genome sequences of the studied microorganisms, several genome sequence-independent tools have been developed. One of these approaches is microorganism co-culture, involving the cultivation of two or more microorganisms in the same confined environment. Microorganism co-culture is inspired by the natural microbe communities that are omnipresent in nature. Within these communities, microbes interact through signaling or defense molecules. Such compounds, produced dynamically, are of potential interest as new leads for drug discovery. Microorganism co-culture can be achieved in either solid or liquid media and has recently been used increasingly extensively to study natural interactions and discover new bioactive metabolites. Because of the complexity of microbial extracts, advanced analytical methods (e.g., mass spectrometry methods and metabolomics) are key for the successful detection and identification of co-culture-induced metabolites. This review focuses on co-culture studies that aim to increase the diversity of metabolites obtained from microbes. The various strategies are summarized with a special emphasis on the multiple methods of performing co-culture experiments. The analytical approaches for studying these interaction phenomena are discussed, and the chemical diversity and biological activity observed among the induced metabolites are described.

Isolation, structure elucidation and biological activity of metabolites from Sch-642305-producing endophytic fungus Phomopsis sp. CMU-LMA

Eight polyketide compounds were isolated from the cultivation broth of Phomopsis sp. CMU-LMA. We have recently described LMA-P1, a bicyclic 10-membered macrolide, obtained as a bioconversion derivative of Sch-642305, the major compound isolated in this study. Benquinol is the ethyl ester derivative of the 13-dihydroxytetradeca-2,4,8-trienoic acid produced by Valsa ambiens. This compound is concomitantly produced with the 6,13-dihydroxytetradeca-2,4,8-trienoic acid (DHTTA) previously isolated from Mycosphaerellarubella. The absolute configuration of the new compound, (2R,3R,4S,5R)-3-hydroxy-2,4-dimethyl-5-[(S,Z)-3-methylpentenyl]-tetrahydro-pyranone LMA-P2 was confirmed by X-ray crystallography. The δ-lactone 2,3-dihydroxytetradecan-5-olide (DHTO) was previously isolated from Seiridium unicorne. This compound may form through the cyclization of the methyl-2,3,5-trihydroxytridecanoate LMA-P3, a new linear polyketide isolated in this study. Benquoine, a new 14-membered lactone generated from the cyclization of benquinol, is proposed as the key precursor for the biosynthesis of Sch-642305. Antimicrobial activity and cancer cell viability inhibition by the new compounds were investigated. Benquoine exhibits antimicrobial activity against Gram positive bacteria, and cytotoxicity against HCT-116 cancer cell line.

Small-molecule elicitation of microbial secondary metabolites

Microbial natural products continue to be an unparalleled resource for pharmaceutical lead discovery, but the rediscovery rate is high. Bacterial and fungal sequencing studies indicate that the biosynthetic potential of many strains is much greater than that observed by fermentation. Prodding the expression of such silent (cryptic) pathways will allow us to maximize the chemical diversity available from microorganisms. Cryptic metabolic pathways can be accessed in the laboratory using molecular or cultivation‐based approaches. A targeted approach related to cultivation‐based methods is the application of small‐molecule elicitors to specifically affect transcription of secondary metabolite gene clusters. With the isolation of the novel secondary metabolites lunalides A and B, oxylipins, cladochromes F and G, nygerone A, chaetoglobosin‐542, ‐540 and ‐510, sphaerolone, dihydrosphaerolone, mutolide and pestalone, and the enhanced production of known secondary metabolites like penicillin and bacitracin, chemical elicitation is proving to be an effective way to augment natural product libraries.

Triggering cryptic natural product biosynthesis in microorganisms

Natural products from microorganisms are a crucial source for novel therapeutics. Even so, it seems that many valuable compounds are overlooked when culturing microbes under standardized laboratory conditions. Many biosynthesis genes remain silent and such "cryptic" or "orphan" pathways are only activated under specific conditions. This report gives an overview on the strategies to trigger biosynthetic pathways to yield "cryptic natural products" through external cues, co-cultivation and genomic approaches such as genome-mining, epigenetic remodeling, and engineered pathway activation.

Novel Macrolide from Wild Strains of the Phytopathogen Fungus Colletotrichum Acutatum

Three wild strains of C. acutatum have been investigated for their phytotoxic secondary metabolites involved in anthracnose disease. In addition to known compounds, a new macrolide 6 has been isolated and characterized by spectroscopic analysis.

Synthesis of (+)-Sch 642305 by a Biomimetic Transannular Michael Reaction

[reaction: see text] The synthesis of (+)-Sch 642305 (1) has been completed in 17 steps in 1.6% overall yield. Transannular Michael reaction of 2b with NaH in THF provided cyclohexenone 23 stereospecifically. Heating 23 in TFA/CDCl(3) provided a 3:1 equilibrium mixture of 23 and 25, which was hydrolyzed to give (+)-6-epi-Sch 642305 (24) and (+)-Sch 642305 (1), respectively.

UV mutagenesis and enzyme inhibitors as tools to elucidate the late biosynthesis of the spirobisnaphthalenes

The metabolite pattern of UV mutants of the spirobisnaphthalene producing fungus F-24'707 by TLC and HPLC analysis has been investigated. Mutants with differences in colony morphology or colour compared to the parent strain were isolated. Cultivation in shaking flasks and P flasks showed differences in the metabolite pattern of some of the strains. Furthermore, enzyme inhibitors were used to block the spirobisnaphthalene biosynthesis of the parent strain at different steps. Feeding of precursors and intermediates of cladospirone bisepoxide (15) led to a two-fold increase of the production of 15. From these data and preceding biosynthetic studies we deduced a general pathway for the biosynthesis of all spirobisnaphthalenes of the fungus F-24'707. This enables us to present the hypothesis that all bisnaphthalenes described so far are produced using a common pathway with only a few intermediates as central branching points.

Sphaerolone and dihydrosphaerolone, two bisnaphthyl-pigments from the fungus Sphaeropsidales sp. F-24'707

Two new bisnaphthalene compounds, sphaerolone (1) and dihydrosphaerolone (2), together with 2-hydroxyjuglone (9), were isolated from the culture broth of a Sphaeropsidales sp. (strain F-24'707) after inhibition of the regular proceeding 1,8-dihydroxynaphthalene (DHN) biosynthesis with tricyclazole. The structures of 1 and 2 were established by detailed spectroscopic analysis and present novel bisnaphthalenes. The biosynthetic origin of 1 and 2 as dimerization products of 1,3,8-trihydroxynaphthalene, an intermediate of the DHN biosynthesis, is discussed.