Exploration of Indole Alkaloids from Marine Fungus Pseudallescheria boydii F44-1 Using an Amino Acid-Directed Strategy

OSMAC Strategy: A promising way to explore microbial cyclic peptides

Microbial secondary metabolites are pivotal for the development of novel drugs. However, conventional culture techniques, have left a vast array of unexpressed biosynthetic gene clusters (BGCs) in microorganisms, hindering the discovery of metabolites with distinct structural features and diverse biological functions. To address this limitation, several innovative strategies have been emerged. The "One Strain Many Compounds" (OSMAC) strategy, which involves altering microbial culture conditions, has proven to be particularly effective in mining numerous novel secondary metabolites for the past few years. Among these, microbial cyclic peptides stand out. These peptides often comprise rare amino acids, unique chemical structures, and remarkable biological function. With the advancement of the OSMAC strategy, a plethora of new cyclic peptides have been identified from diverse microbial genera. This work reviews the progress in mining novel compounds using the OSMAC strategy and the applications of this strategy in discovering 284 microbial cyclic peptides from 63 endophytic strains, aiming to offer insights for the further explorations into novel active cyclic peptides.

The current scenario of naturally occurring indole alkaloids with anticancer potential

Naturally occurring indole alkaloids are ubiquitously present in nature and possess extensive biological properties and structural diversity. Mechanistically, naturally occurring indole alkaloids have the potential to inhibit cancer cell proliferation, arrest cell cycle and induce apoptosis. Accordingly, naturally occurring indole alkaloids exhibit promising activity against both drug-sensitive and drug-resistant cancers including multidrug-resistant forms. Therefore, naturally occurring indole alkaloids constitute an important source of anticancer drug leads and candidates. The goal of this review is to highlight the current scenario of naturally occurring indole alkaloids with anticancer potential, covering articles published from 2018 to present. The names, sources, and antiproliferative activity are discussed to continuously open up a map for the remarkable exploration of more effective candidates.

Indole alkaloids fusarindoles A-E from marine-derived fungus Fusarium equiseti LJ-1

Five undescribed indole alkaloids, fusarindoles A-E, together with seven known compounds were obtained from the marine-derived fungus Fusarium equiseti LJ-1. Their chemical structures and absolute configurations were determined by comprehensive analysis of the NMR, HRMS, UV, IR, ECD calculation and single-crystal X-ray diffraction data. The possible biosynthetic pathways of fusarindoles C-E were proposed. The cytotoxicities of eleven compounds, including fusarindoles A-E and six known compounds, against five human cancer cell lines A549, CNE2, SUNE1, HepG2 and QGY7701 were evaluated.

Extracellularly Released Molecules by the Multidrug-Resistant Fungal Pathogens Belonging to the Scedosporium Genus: An Overview Focused on Their Ecological Significance and Pathogenic Relevance

The multidrug-resistant species belonging to the Scedosporium genus are well recognized as saprophytic filamentous fungi found mainly in human impacted areas and that emerged as human pathogens in both immunocompetent and immunocompromised individuals. It is well recognized that some fungi are ubiquitous organisms that produce an enormous amount of extracellular molecules, including enzymes and secondary metabolites, as part of their basic physiology in order to satisfy their several biological processes. In this context, the molecules secreted by Scedosporium species are key weapons for successful colonization, nutrition and maintenance in both host and environmental sites. These biologically active released molecules have central relevance on fungal survival when colonizing ecological places contaminated with hydrocarbons, as well as during human infection, particularly contributing to the invasion/evasion of host cells and tissues, besides escaping from the cellular and humoral host immune responses. Based on these relevant premises, the present review compiled the published data reporting the main secreted molecules by Scedosporium species, which operate important physiopathological events associated with pathogenesis, diagnosis, antimicrobial activity and bioremediation of polluted environments.

Bisindole natural products: A vital source for the development of new anticancer drugs

Currently, the number of new cancer cases and deaths worldwide is increasing year on year. In addition to the requirement for cancer prevention, the top priority is still to seek the effective cure of cancer. In over a half century of constant exploration, increasing attention has been paid to the excellent anticancer activity of natural products, with more and more natural products isolated, identified and detected. For this study, the focus lies the natural products of bisindole, where two indole molecules are indirectly linked or directly polymerized, developing the diversity of structure and mechanism, accompanied with the better anticancer activity than monomers. There has been a long history of applying indirubin and vincristine in cancer treatment, verifying the anticancer effect of bisindoles. Vincribine, midostaurin and other anticancer drugs have also been developed and commercialized. In this paper, a review regarding the potential therapeutic effect of bisindole alkaloids extracted from various natural products was carried out, in which the progress made in research of 242 bisindole alkaloids for cancer treatment was introduced. These compounds may be applicable as medicinal products for clinical research in the future.

Cryptic Metabolites from Marine-Derived Microorganisms Using OSMAC and Epigenetic Approaches

Marine microorganisms have proven to be a source of new natural products with a wide spectrum of biological activities relevant in different industrial sectors. The ever-increasing number of sequenced microbial genomes has highlighted a discrepancy between the number of gene clusters potentially encoding the production of natural products and the actual number of chemically characterized metabolites for a given microorganism. Homologous and heterologous expression of these biosynthetic genes, which are often silent under experimental laboratory culture conditions, may lead to the discovery of new cryptic natural products of medical and biotechnological interest. Several new genetic and cultivation-based strategies have been developed to meet this challenge. The OSMAC approach (one strain-many compounds), based on modification of growth conditions, has proven to be a powerful strategy for the discovery of new cryptic natural products. As a direct extension of this approach, the addition of chemical elicitors or epigenetic modifiers have also been used to activate silent genes. This review looks at the structures and biological activities of new cryptic metabolites from marine-derived microorganisms obtained using the OSMAC approach, the addition of chemical elicitors, and enzymatic inhibitors and epigenetic modifiers. It covers works published up to June 2021.

Natural Indole Alkaloids from Marine Fungi: Chemical Diversity and Biological Activities

The indole scaffold is one of the most important heterocyclic ring systems for pharmaceutical development, and serves as an active moiety in several clinical drugs. Fungi derived from marine origin are more liable to produce novel indole-containing natural products due to their extreme living environments. The indole alkaloids from marine fungi have drawn considerable attention for their unique chemical structures and significant biological activities. This review attempts to provide a summary of the structural diversity of marine fungal indole alkaloids including prenylated indoles, diketopiperazine indoles, bisindoles or trisindoles, quinazoline-containing indoles, indole-diterpenoids, and other indoles, as well as their known biological activities, mainly focusing on cytotoxic, kinase inhibitory, antiinflammatory, antimicrobial, anti-insecticidal, and brine shrimp lethal effects. A total of 306 indole alkaloids from marine fungi have been summarized, covering the references published from 1995 to early 2021, expecting to be beneficial for drug discovery in the future.

Marine-Derived Indole Alkaloids and Their Biological and Pharmacological Activities

Novel secondary metabolites from marine macroorganisms and marine-derived microorganisms have been intensively investigated in the last few decades. Several classes of compounds, especially indole alkaloids, have been a target for evaluating biological and pharmacological activities. As one of the most promising classes of compounds, indole alkaloids possess not only intriguing structural features but also a wide range of biological/pharmacological activities including antimicrobial, anti-inflammatory, anticancer, antidiabetic, and antiparasitic activities. This review reports the indole alkaloids isolated during the period of 2016–2021 and their relevant biological/pharmacological activities. The marine-derived indole alkaloids reported from 2016 to 2021 were collected from various scientific databases. A total of 186 indole alkaloids from various marine organisms including fungi, bacteria, sponges, bryozoans, mangroves, and algae, are described. Despite the described bioactivities, further evaluation including their mechanisms of action and biological targets is needed to determine which of these indole alkaloids are worth studying to obtain lead compounds for the development of new drugs.

Marine Indole Alkaloids-Isolation, Structure and Bioactivities

Indole alkaloids are heterocyclic natural products with extensive pharmacological activities. As an important source of lead compounds, many clinical drugs have been derived from natural indole compounds. Marine indole alkaloids, from unique marine environments with high pressure, high salt and low temperature, exhibit structural diversity with various bioactivities, which attracts the attention of drug researchers. This article is a continuation of the previous two comprehensive reviews and covers the literature on marine indole alkaloids published from 2015 to 2021, with 472 new or structure-revised compounds categorized by sources into marine microorganisms, invertebrates, and plant-derived. The structures and bioactivities demonstrated in this article will benefit the synthesis and pharmacological activity study for marine indole alkaloids on their way to clinical drugs.

Alkaloids From the Marine Fungus Lecanicillium fusisporum Using an Amino Acid-Directed Strategy

An amino acid-directed strategy has been developed to explore the potential of marine fungi to produce alkaloids. The marine fungus Lecanicillium fusisporum was cultured in glucose-peptone-yeast (GPY) medium to which were added L-tryptophan, L-phenylalanine, L-threonine, D, L-methionine, L-serine, L-lysine and L-valine. A new indole alkaloid, lecasporinoid (1), along with five known alkaloids (2−6) were discovered from the culture broth. The planar structure of lecasporinoid (1) was determined by HR-ESIMS, and 1D and 2D NMR spectroscopic data. The absolute configuration was established by optical rotation and 13C NMR calculations combining with a chemical synthetic approach.

Species Diversity and Secondary Metabolites of Sarcophyton-Associated Marine Fungi

Soft corals are widely distributed across the globe, especially in the Indo-Pacific region, with Sarcophyton being one of the most abundant genera. To date, there have been 50 species of identified Sarcophyton. These soft corals host a diverse range of marine fungi, which produce chemically diverse, bioactive secondary metabolites as part of their symbiotic nature with the soft coral hosts. The most prolific groups of compounds are terpenoids and indole alkaloids. Annually, there are more bio-active compounds being isolated and characterised. Thus, the importance of the metabolite compilation is very much important for future reference. This paper compiles the diversity of Sarcophyton species and metabolites produced by their associated marine fungi, as well as the bioactivity of these identified compounds. A total of 88 metabolites of structural diversity are highlighted, indicating the huge potential these symbiotic relationships hold for future research.

Monascuslactams A-D, cytotoxic γ-lactams from marine fungus Monascus albidus BB3

Amino acid-directed strategy becomes an efficient way to explore the alkaloids' biosynthetic potential of marine fungi. The metabolites of marine fungus Monascus albidus BB3 were regulated obviously when cultured in GPY medium supplemented with L-tryptophan, L-phenylalanine, D,L-methionine, L-threonine, L-lysine, L-serine and L-valine. Four new γ-lactams, monascuslactams A-D (1-4), together with two known compounds pulchellalactam (5) and O-acetylperlolyrine (6) were obtained. Their structures were determined by comprehensive analysis on the 1 D and 2 D NMR, HRESIMS, UV and IR data, and their absolute configurations were assigned by the experimental and calculated ECD data analysis. Compounds 3, 4 and 6 showed moderate cytotoxicity against human cancer cell lines SUNE1, HepG2, QGY7701, GLC82, HCT116 and MDA-MB-231.

Marine-derived fungi as a source of bioactive indole alkaloids with diversified structures

Marine-derived fungi are well known as rich sources of bioactive natural products. Growing evidences indicated that indole alkaloids, isolated from a variety of marine-derived fungi, have attracted considerable attention for their diverse, challenging structural complexity and promising bioactivities, and therefore, indole alkaloids have potential to be pharmaceutical lead compounds. Systemic compilation of the relevant literature. In this review, we demonstrated a comprehensive overview of 431 new indole alkaloids from 21 genera of marine-derived fungi with an emphasis on their structures and bioactivities, covering literatures published during 1982-2019.

Marine natural products

This review covers the literature published in 2019 for marine natural products (MNPs), with 719 citations (701 for the period January to December 2019) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 440 papers for 2019), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Methods used to study marine fungi and their chemical diversity have also been discussed.

Antioxidant Molecules from Marine Fungi: Methodologies and Perspectives

The marine environment represents a prosperous existing resource for bioprospecting, covering 70% of the planet earth, and hosting a huge biodiversity. Advances in the research are progressively uncovering the presence of unknown microorganisms, which have evolved unique metabolic and genetic pathways for the production of uncommon secondary metabolites. Fungi have a leading role in marine bioprospecting since they represent a prolific source of structurally diverse bioactive metabolites. Several bioactive compounds from marine fungi have already been characterized including antibiotics, anticancer, antioxidants and antivirals. Nowadays, the search for natural antioxidant molecules capable of replacing those synthetic currently used, is an aspect that is receiving significant attention. Antioxidants can inactivate reactive oxygen and nitrogen species, preventing the insurgence of several degenerative diseases including cancer, autoimmune disorders, cardiovascular and neurodegenerative diseases. Moreover, they also find applications in different fields, including food preservation, healthcare and cosmetics. This review focuses on the production of antioxidants from marine fungi. We begin by proposing a survey of the available tools suitable for the evaluation of antioxidants, followed by the description of various classes of marine fungi antioxidants together with their extraction strategies. In addition, a view of the future perspectives and trends of these natural products within the "blue economy" is also presented.

l-Tryptophan Induces a Marine-Derived Fusarium sp. to Produce Indole Alkaloids with Activity against the Zika Virus

The effects of l-tryptophan supplementation on secondary metabolite production in the marine-derived fungus Fusarium sp. L1 were investigated by culturing the fungus in GPY medium with and without the amino acid. HPLC analysis of the products showed distinct metabolite profiles between the two cultures. The ¹H NMR spectrum of the EtOAc extract of the culture supplemented with l-tryptophan displayed a series of characteristic aromatic proton signals (δ_H 6.50-8.50) and NH signals (δ_H 10.50-11.50) that were not observed in those from cultures not supplemented with l-tryptophan. Subsequently, 23 distinct indole alkaloids, including six new compounds, fusaindoterpenes A and B (1 and 2), fusariumindoles A-C (3-5), and (±)-isoalternatine A (6), together with 17 known compounds, were obtained from this culture. Fusaindoterpene A (1) contains a 6/9/6/6/5 heterocyclic system. Their chemical structures were determined by analysis of HRMS, NMR spectroscopy, optical rotation calculation, ECD calculation, and single-crystal X-ray diffraction data. Compounds 2, 9, and 15 displayed inhibitory activity against the Zika virus (ZIKV) in a standard plaque assay with EC₅₀ values of 7.5, 4.2, and 5.0 μM, respectively, while not showing significant cell cytotoxicity against the A549 adenocarcinomic human alveolar basal epithelial cell line.

Anticancer activity of bisindole alkaloids derived from natural sources and synthetic bisindole hybrids

The bisindole moiety, as a versatile pharmacophore, is one of the widespread heterocycles in naturally occurring and synthetic bioactive compounds. The bisindole alkaloids derived from natural sources possess structural and mechanistic diversity, and they were found to be generally more active than monoindole alkaloids against various cancer cell lines. Moreover, some bisindole alkaloids such as the tubulin inhibitors, vinorelbine and vinblastine, have already been approved for cancer therapy, suggesting that bisindole alkaloids are a significant source of anticancer agents and lead hits. Bisindole hybrids have the potential to overcome drug resistance, enhance efficiency, and reduce severe side effects. The bisindole-lactam hybrid midostaurin has already been approved for the treatment of adult patients with newly diagnosed acute myeloid leukemia who are FLT3 mutation-positive, highlighting the importance of bisindole hybrids in the development of novel anticancer agents. In this review, we present a brief account of the bisindole alkaloids derived from nature and of synthetic hybrids with potential anticancer activity developed in the recent 10 years.

Monarubins A-C from the Marine Shellfish-Associated Fungus Monascus ruber BB5

Three new compounds, monarubins A-C (1, 6 and 13), together with ten known compounds, including four alkaloids (2-5), two isocoumarins (7 and 8) and four polyketides (9-12), were isolated from marine shellfish-associated fungus Monascus ruber BB5. The structures were determined on the basis of the 1D and 2D NMR, MS, UV and IR data. The absolute configurations of compounds 3, 6 and 13 were determined by ECD calculations. The NMR data of compounds deoxyhydroxyaspergillic acid (3) and 2-hydroxy-6-(1-hydroxy-1-methylpropyl)-3-sec-buthylpyrazine (4) were first reported. All of the isolated compounds were evaluated for their cytotoxic activities against human nasopharyngeal carcinoma cell lines CNE1, CNE2, SUNE1 and HONE1 and hepatocellular carcinoma cell lines QGY7701 and HepG2. Monarubin B (6) displayed potent cytotoxicities against the cancer cell lines HepG2 and QGY7701 with IC₅₀ values of 1.72 and 0.71 μΜ, respectively; lunatinin (7) showed moderate cytotoxic activities against the cancer cell lines HepG2, QGY7701 and SUNE1 with the IC₅₀ values of 9.60, 7.12 and 28.12 μΜ, respectively.

Cytotoxic alkaloids from the marine shellfish-associated fungus Aspergillus sp. XBB-4 induced by an amino acid-directed strategy

Eight different culture media were used to culture shellfish Panopea abbreviate associated fungus Aspergillus sp. XBB-4. In a glucose-peptone-yeast (GPY) culture medium supplied with amino acids, this fungus can produce chemodiversity metabolites. Four new alkaloids including three β-carboline alkaloids, aspercarbolines A–C (1–3) and one piperazinedione, asperdione A (13) along with nine known compounds were isolated. The structures were elucidated mainly based on the NMR, MS, ECD and X-ray single-crystal diffraction data. The possible biosynthetic pathways of aspercarbolines A–C (1–3) were proposed. All compounds (1–13) were evaluated for their cytotoxicity against six cancer cell lines, including human nasopharyngeal carcinoma cell lines CNE1, CNE2, HONE1 and SUNE1, and human hepatocellular carcinoma cell lines hepG2 and QGY7701.

Cytotoxic alkaloids from marine fungus Aspergillus sp. XBB-4 induced by an amino acid-directed strategy.

A pair of novel bisindole alkaloid enantiomers from marine fungus Fusarium sp. XBB-9

A preliminary research on the marine fungus Fusarium sp. XBB-9 resulted in a pair of novel bisindole alkaloid enantiomers, (+)- and (-)-fusaspoid A (1a/1b) and 12 diverse compounds. One strain many compound (OSMAC) method was used to enhance as many biosynthetic pathways as possible. The structures of the compounds were elucidated by spectroscopic analysis, and the absolute configuration of 1a was determined by X-ray single-crystal diffraction analysis. Compounds 1a and 1b were tested for cytotoxic activity against HCT-15, RKO cell lines, but were inactive. Compounds 1a and 1b were the first example of bisindole alkaloids isolated from fungus Fusarium sp. XBB-9.