Biosynthesis and mass spectral fragmentation pathways of (13)C and (15)N labeled cytochalasin D produced by Xylaria arbuscula

Highly diverse endophytic fungi from Serra do Amolar-Pantanal (Brazil) producing bioactive secondary metabolites against phytopathogens

Introduction

The exploration of new bioactive compounds for agricultural applications is critical for sustainable development. Endophytic fungi, particularly those from underexplored biomes in Brazil, represent a promising source of natural compounds. This study focused on isolation and bioprospecting endophytic fungi from the medicinal plant Vochysia divergens (Pohl), grown in Serra do Amolar (Brazilian Pantanal Biome), with an additional emphasis on conserving microbial biodiversity.

Methods and results

Leaves and petioles were collected from 18 V. divergens specimens, from which 293 endophytes were isolated and grouped by morphological characteristics into 91 phenotypes. One representative of each phenotype was selected for secondary metabolite extraction and taxonomic identification. Fungi belonging to 27 families and 32 different genera were identified, with Diaporthe, Phyllosticta, and Pseudofusicoccum as the most predominant. We also introduce and describe a new endophytic species, Diaporthe amolarensis. Multiple extracts inhibited mycelial growth of the phytopathogenic fungus Colletotrichum abscissum, with a superior effect compared to the fungicide control. These extracts were produced by Diaporthe amolarensis, Xylaria arbuscula, and Nemania primolutea. Additionally, the extract from one X. arbuscula isolate displayed moderate activity against the phytopathogen Phyllosticta citricarpa. HPLC-UV and HPLC-MS analyses of these most inhibitory extracts revealed natural products with beneficial potential that need characterization and to have their modes of action elucidated.

Discussion

Finally, a very important contribution of this study was the ex situ conservation of the biodiversity of the Serra do Amolar, allowing future studies and biotechnological applications involving endophytes from this region.

Dereplication of Cytochalasans and Octaketides in Cytotoxic Extracts of Endophytic Fungi from Casearia arborea (Salicaceae)

Endophytes have been shown to be a source of novel drug prototypes. The Casearia genus is known for presenting cytotoxic clerodane diterpenes; however, there are few reports on secondary metabolites produced by its fungal microbiota. Thus, in the present study endophytic fungi obtained from the fresh leaves of C. arborea were grown in potato dextrose broth and rice to perform a secondary metabolite prospection study. The cytotoxic profile of the crude extracts at 10 µg/mL was determined by a colorimetric assay on tumor cell lines. The endophytes producing cytotoxic extracts were identified through phylogenetic analysis and belong to Diaporthe and Colletotrichum species. Metabolites present in these extracts were organized in molecular networking format based on HRMS-MS, and a dereplication process was performed to target compounds for chromatographic purification. Metabolic classes, such as lipids, peptides, alkaloids, and polyketides were annotated, and octaketide and cytochalasin derivatives were investigated. Cytochalasin H was purified from the cytotoxic Diaporthe sp. CarGL8 extract and its cytotoxic activity was determined on human cancer cell lines A549, MCF-7, and HepG2. The data collected in the present study showed that molecular networking is useful to understand the chemical profile of complex matrices to target compounds, minimizing the cost and time spent in purification processes.

LC-PDA-MS/MS-Based Dereplication Guided Isolation of a New Optical Isomer of 19,20-Epoxycytochalasin-N and Its Cytotoxic Activity

The Rosellinia sanctae-cruciana extract was subjected to detailed liquid chromatography tandem mass spectrometry studies. A total of 38 peaks were annotated to m/z 508.26, m/z 510.28, m/z 524.26, m/z 526.28, m/z 540.26, m/z 542.27, and m/z 584.28 [M + H]+. The accurate mass, mutually supported UV/vis spectra, and database search identified these compounds as cytochalasins. Systematic dereplication helped identify a peak at m/z 540.26 [M + H]+ as the new compound. Further, the identified compound was purified by high-performance liquid chromatography and characterized by 2D NMR to be 19,20-epoxycytochalasin N1, a new optical isomer of 19,20-epoxycytochalasin-N. It exhibited substantial cytotoxicity with IC50 values ranging from 1.34 to 19.02 μM. This study shows a fast approach for dereplicating and identifying novel cytochalasin metabolites in crude extracts.

Target specific tight junction modulators

Intercellular tight junctions represent a formidable barrier against paracellular drug absorption at epithelia (e.g., nasal, intestinal) and the endothelium (e.g., blood-brain barrier). In order to enhance paracellular transport of drugs and increase their bioavailability and organ deposition, active excipients modulating tight junctions have been applied. First-generation of permeation enhancers (PEs) acted by unspecific interactions, while recently developed PEs address specific physiological mechanisms. Such target specific tight junction modulators (TJMs) have the advantage of a defined specific mechanism of action. To date, merely a few of these novel active excipients has entered into clinical trials, as their lack in safety and efficiency in vivo often impedes their commercialisation. A stronger focus on the development of such active excipients would result in an economic and therapeutic improvement of current and future drugs.

Tandem MS-Based Metabolite Profiling of 19,20-Epoxycytochalasin C Reveals the Importance of a Hydroxy Group at the C7 Position for Biological Activity

Seven cytochalasins, 19,20-epoxycytochalasin N, cytochalasin P1, deacetyl 19,20-epoxycytochalasin C, 19,20-epoxycytochalasin D, 19,20-epoxycytochalasin C, cytochalasin D, and cytochalasin C, were isolated from a fungal (Rosellinia sanctae-cruciana) crude extract. A cytotoxicity assay (sulforhodamine B) was performed on a series of cancer cell lines: HT-29, A-549, PC-3, HCT-116, SW-620, and MCF-7. Simultaneously, the liquid chromatography-mass spectrometry (LC-MS)/MS profile of 19,20-epoxycytochalasin C-treated cell lines revealed that 19,20-epoxycytochalasin C (m/z 524.25) oxidized to a metabolite of m/z 522.25 Da (-2 Da (-2H) from 19,20-epoxycytochalasin C). Further chemical oxidation of 19,20-epoxycytochalasin C using the Dess-Martin reagent produced an identical metabolite. It has been noticed that the parent molecule (19,20-epoxycytochalasin C) showed an IC₅₀ of 650 nM (on HT-29), whereas for the oxidized metabolite (m/z 522.24) of 19,20-epoxycytochalasin C, the IC₅₀ was >10 μM. It is clear that the parent molecule had 16 times higher cytotoxic potential as compared to the oxidized metabolite. The spectroscopic investigation indicated that the oxidation of the hydroxyl (-OH) group occurred at the C7 position in 19,20-epoxycyctochalsin C and led to the inactivation of 19,20-epoxycytochalasin C. Further, cell cycle analysis and histopathological evidence support the findings, and CDK2 could be a possible target of 19,20-epoxycyctochalasin C.

Diagnostic fragmentation filtering for the discovery of new chaetoglobosins and cytochalasins

RATIONALE

Microbial natural products are often biosynthesized as classes of structurally related compounds that have similar tandem mass spectrometry (MS/MS) fragmentation patterns. Mining MS/MS datasets for precursor ions that share diagnostic or common features enables entire chemical classes to be identified, including novel derivatives that have previously been unreported. Analytical data analysis tools that can facilitate a class-targeted approach to rapidly dereplicate known compounds and identify structural variants within complex matrices would be useful for the discovery of new natural products.

METHODS

A diagnostic fragmentation filtering (DFF) module was developed for MZmine to enable the efficient screening of MS/MS datasets for class-specific product ions(s) and/or neutral loss(es). This approach was applied to series of the structurally related chaetoglobosin and cytochalasin classes of compounds. These were identified from the culture filtrates of three fungal genera: Chaetomium globosum, a putative new species of Penicillium (called here P. cf. discolor: closely related to P. discolor), and Xylaria sp. Extracts were subjected to LC/MS/MS analysis under positive electrospray ionization and operating in a data-dependent acquisition mode, performed using a Thermo Q-Exactive mass spectrometer. All MS/MS datasets were processed using the DFF module and screened for diagnostic product ions at m/z 130.0648 and 185.0704 for chaetoglobosins, and m/z 120.0808 and 146.0598 for cytochalasins.

RESULTS

Extracts of C. globosum and P. cf. discolor strains revealed different mixtures of chaetoglobosins, whereas the Xylaria sp. produced only cytochalasins; none of the strains studied produced both classes of compounds. The dominant chaetoglobosins produced by both C. globosum and P. cf. discolor were chaetoglobosins A, C, and F. Tetrahydrochaetoglobosin A was identified from P. cf. discolor extracts and is reported here for the first time as a natural product. The major cytochalasins produced by the Xylaria sp. were cytochalasin D and epoxy cytochalasin D. A larger unknown "cytochalasin-like" molecule with the molecular formula C₃₈ H₄₇ NO₁₀ was detected from Xylaria sp. culture filtrate extracts and is a current target for isolation and structural characterization.

CONCLUSIONS

DFF is an effective LC/MS data analysis approach for rapidly identifying entire classes of compounds from complex mixtures. DFF has proved useful in the identification of new natural products and allowing for their partial characterization without the need for isolation.

Optimization of sample preparation for intact cell mass spectrometry (matrix-assisted laser desorption/ionization linear time-of-flight mass spectrometry) of endophytic Xylaria

RATIONALE

Although the fruiting-body of the fungi of the genus Xylaria shows a great variety of morphological characteristics, their mycelial forms are always very similar, imposing difficulties for their identification. Intact cell mass spectrometry (ICMS) using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) can be a fast and reliable strategy to support the differentiation/identification of Xylaria species in those cases where fruit-bodies are not available.

METHODS

Many experimental parameters such as sample preparation and culture media are crucial for filamentous fungi analysis by MALDI-TOFMS. For the purposes of this study, we used four matrices (CHCA, DHB, FA and SA) with five different concentrations (0.1, 0.3, 0.5, 1.0 and 2.5%) of TFA in the matrix, the influence of six different culture media (solid and liquid), and three mycelium peptide/protein extraction protocols (acid, basic and thymol-supported solution) to optimize the sample preparation of the endophytic fungus X. arbuscula.

RESULTS

It was observed that sinapinic acid (30 mg/mL) dissolved in acetonitrile/0.1% TFA and PDA were the best matrix solution and culture medium, respectively, for the ICMS of X. arbuscula. The formic acid and ammonium bicarbonate (AB) protocols provided similar mass spectra; however, a higher number of peaks were observed using AB extraction. Mass spectra obtained from different thymol-containing solutions (EtOH/aqueous 0.1% TFA and ACN/aqueous 0.1% TFA) show increasing peak abundances at m/z 3000-6500.

CONCLUSIONS

X. arbuscula could be analyzed by ICMS. However, an extraction step was required to provide suitable MALDI mass spectra. Formic acid-, AB- and thymol-containing solutions were demonstrated to be good cocktails for the extraction of peptide/protein biomarkers from these fungi.