Salinipyrones and pacificanones, mixed-precursor polyketides from the marine actinomycete Salinispora pacifica

Biologically Active Antimicrobial Compounds from Marine Microorganisms (2005-2019)

INTRODUCTION

The increase in contagious diseases like nosocomial infections, urinary tract infections, and meningitis has led to the emergence of antimicrobial resistance urgently needs new antimicrobial medication with new modes of action. Some of the antibiotics present in the market have been obtained from terrestrial plants, or extracted semisynthetically from materials which can be fermented.

METHODS

Marine microorganisms account for approximately 80% of sea biomass. They are essential for the survival and well-being of aquatic habitats due to their indispensable contribution to biogeochemical cycles and biological processes. In marine ecosystems, microorganisms live as microbial communities in seawater, where symbiotic relationships are formed, and their ecological functions are fulfilled.

RESULTS

Marine microorganisms remain the largest, most diverse and most exciting source of structurally and functionally complex antimicrobial agents. They are extremely involved in their structure and functions. Enormous biological wealth lies in marine habitats. These microorganisms are potential sources of novel antimicrobial compounds to combat the most infectious diseases like nosocomial infections, and urinary tract infections.

CONCLUSION

This study deals with biologically active antimicrobial compounds taken from marine microorganism source, which was reported between the years 2005 and 2019. This review highlights their chemical groups, their bioactivities and sources. Marine microorganism exploitation techniques have also been reported by the authors.

Piperazine-2,5-dione derivatives and an α-pyrone polyketide from Penicillium griseofulvum and their immunosuppression activity

Four undescribed piperazine-2,5-dione derivatives designated janthinolides C-F, and an α-pyrone-containing polyketide namely trichopyrone C, were isolated from the extract of the fungus Penicillium griseofulvum along with four known products. Among them, janthinolide C represents the first naturally occured piperazine-2,5-dione analogue featuring a cleavaged piperazinedione ring with an oxime group, while the structure of janthinolide D possesses a rare N-methoxy group in natural products. Their structures and absolute stereochemistry were elucidated based on spectroscopic data, theoretical NMR and ECD calculations, Snatzke's method, and modified Mosher's method. All compounds were evaluated for in vitro immunosuppression activity in murine splenocytes stimulated by anti-CD3/anti-CD28 mAbs, of which janthinolides B and C showed potential inhibitory activity with IC₅₀ values at 9.3 and 1.3 μM, respectively.

Coupling Mass Spectral and Genomic Information to Improve Bacterial Natural Product Discovery Workflows

Bacterial natural products possess potent bioactivities and high structural diversity and are typically encoded in biosynthetic gene clusters. Traditional natural product discovery approaches rely on UV- and bioassay-guided fractionation and are limited in terms of dereplication. Recent advances in mass spectrometry, sequencing and bioinformatics have led to large-scale accumulation of genomic and mass spectral data that is increasingly used for signature-based or correlation-based mass spectrometry genome mining approaches that enable rapid linking of metabolomic and genomic information to accelerate and rationalize natural product discovery. In this mini-review, these approaches are presented, and discovery examples provided. Finally, future opportunities and challenges for paired omics-based natural products discovery workflows are discussed.

Antiangiogenic molecules from marine actinomycetes and the importance of using zebrafish model in cancer research

Blood vessel sprouting from pre-existing vessels or angiogenesis plays a significant role in tumour progression. Development of novel biomolecules from marine natural sources has a promising role in drug discovery specifically in the area of antiangiogenic chemotherapeutics. Symbiotic actinomycetes from marine origin proved to be potent and valuable sources of antiangiogenic compounds. Zebrafish represent a well-established model for small molecular screening and employed to study tumour angiogenesis over the last decade. Use of zebrafish has increased in the laboratory due to its various advantages like rapid embryo development, optically transparent embryos, large clutch size of embryos and most importantly high genetic conservation comparable to humans. Zebrafish also shares similar physiopathology of tumour angiogenesis with humans and with these advantages, zebrafish has become a popular model in the past decade to study on angiogenesis related disorders like diabetic retinopathy and cancer. This review focuses on the importance of antiangiogenic compounds from marine actinomycetes and utility of zebrafish in cancer angiogenesis research.

Bioactive natural products from the genus Salinospora: a review

Actinomycetes are an important source for bioactive secondary metabolites. Among them, the genus Salinispora is one of the first salt obligatory marine species worldwide and is typically found in various types of substrates in tropical and subtropical marine environments including sediments and marine organisms. This genus produces a wide range of chemical scaffolds and bioactive compounds such as lomaiviticins, cyclomarins, rifamycins, salinaphthoquinones, and salinosporamides. This review arranged Salinispora derived secondary metabolites according to the three species that comprise the genus. Moreover, muta- and semi-synthesis analogs derived from salinosporamide were also described in this review.

Natural Polypropionates in 1999-2020: An Overview of Chemical and Biological Diversity

Natural polypropionates (PPs) are a large subgroup of polyketides with diverse structural features and bioactivities. Most of the PPs are discovered from marine organisms including mollusks, fungi and actinomycetes, while some of them are also isolated from terrestrial resources. An increasing number of studies about PPs have been carried out in the past two decades and an updated review is needed. In this current review, we summarize the chemical structures and biological activities of 164 natural PPs reported in 67 research papers from 1999 to 2020. The isolation, structural features and bioactivities of these PPs are discussed in detail. The chemical diversity, bioactive diversity, biodiversity and the relationship between chemical classes and the bioactivities are also concluded.

Novel Bioactive Polyketides Isolated from Marine Actinomycetes: An Update Review from 2013 to 2019

Marine organism-associated actinobacteria represent a valuable resource for marine drugs due to their abundant secondary metabolites. The special environments in the ocean, for instance, high salt, high pressure, low temperature and oligotrophy, not only adapt to survival of actinomycetes but also enhance molecular diversity of actinomycete secondary metabolites production, thus making marine actinomycetes important sources of marine-based bioactive compounds, especially polyketides. Herein, we summarized the structures and pharmacological activities of polyketides from actinobacteria associated with marine organisms from 2013 to 2019; moreover, the main source species of actinomycetes were discussed as well. We expected that this review would be helpful for future in-depth research and development of marine-based bioactive polyketides.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Bacteria and bacterial anticancer agents as a promising alternative for cancer therapeutics

Cancer is the leading cause of deaths worldwide, though significant advances have occurred in its diagnosis and treatment. The development of resistance against chemotherapeutic agents, their side effects, and non-specific toxicity urge to screen for the novel anticancer agent. Hence, the development of novel anticancer agents with a new mechanism of action has become a major scientific challenge. Bacteria and bacterially produced bioactive compounds have recently emerged as a promising alternative for cancer therapeutics. Bacterial anticancer agents such as antibiotics, bacteriocins, non-ribosomal peptides, polyketides, toxins, etc. These are adopted different mechanisms of actions such as apoptosis, necrosis, reduced angiogenesis, inhibition of translation and splicing, and obstructing essential signaling pathways to kill cancer cells. Also, live tumor-targeting bacteria provided a unique therapeutic alternative for cancer treatment. This review summarizes the anticancer properties and mechanism of actions of the anticancer agents of bacterial origin and antitumor bacteria along with their possible future applications in cancer therapeutics.

Microbial Natural Products in Drug Discovery

Over a long period of time, humans have explored many natural resources looking for remedies of various ailments. Traditional medicines have played an intrinsic role in human life for thousands of years, with people depending on medicinal plants and their products as dietary supplements as well as using them therapeutically for treatment of chronic disorders, such as cancer, malaria, diabetes, arthritis, inflammation, and liver and cardiac disorders. However, plant resources are not sufficient for treatment of recently emerging diseases. In addition, the seasonal availability and other political factors put constrains on some rare plant species. The actual breakthrough in drug discovery came concurrently with the discovery of penicillin from Penicillium notatum in 1929. This discovery dramatically changed the research of natural products and positioned microbial natural products as one of the most important clues in drug discovery due to availability, variability, great biodiversity, unique structures, and the bioactivities produced. The number of commercially available therapeutically active compounds from microbial sources to date exceeds those discovered from other sources. In this review, we introduce a short history of microbial drug discovery as well as certain features and recent research approaches, specifying the microbial origin, their featured molecules, and the diversity of the producing species. Moreover, we discuss some bioactivities as well as new approaches and trends in research in this field.

Identification of Novel α-Pyrones from Conexibacter woesei Serving as Sulfate Shuttles

Pyrones comprise a structurally diverse class of compounds. Although they are widespread in nature, their specific physiological functions remain unknown in most cases. We recently described that triketide pyrones mediate the sulfotransfer in caprazamycin biosynthesis. Herein, we report the identification of conexipyrones A-C, three previously unrecognized tetra-substituted α-pyrones, from the soil actinobacterium Conexibacter woesei. Insights into their biosynthesis via a type III polyketide synthase were obtained by feeding studies using isotope-enriched precursors. In vitro assays employing the genetically associated 3'-phosphoadenosine-5'-phosphosulfate (PAPS)-dependent sulfotransferase CwoeST revealed conexipyrones as the enzymes' genuine sulfate acceptor substrates. Furthermore, conexipyrones were determined to function as sulfate shuttles in a two-enzyme assay, because their sulfated derivatives were accepted as donor molecules by the PAPS-independent arylsulfate sulfotransferase (ASST) Cpz4 to yield sulfated caprazamycin intermediates.

Discovery of Stealthin Derivatives and Implication of the Amidotransferase FlsN3 in the Biosynthesis of Nitrogen-Containing Fluostatins

Diazobenzofluorene-containing atypical angucyclines exhibit promising biological activities. Here we report the inactivation of an amidotransferase-encoding gene flsN3 in Micromonospora rosaria SCSIO N160, a producer of fluostatins. Bioinformatics analysis indicated that FlsN3 was involved in the diazo formation. Chemical investigation of the flsN3-inactivation mutant resulted in the isolation of a variety of angucycline aromatic polyketides, including four racemic aminobenzo[b]fluorenes stealthins D–G (9–12) harboring a stealthin C-like core skeleton with an acetone or butanone-like side chain. Their structures were elucidated on the basis of nuclear magnetic resonance (NMR) spectroscopic data and X-ray diffraction analysis. A plausible mechanism for the formation of stealthins D–G (9–12) was proposed. These results suggested a functional role of FlsN3 in the formation/modification of N–N bond-containing fluostatins.

Bioactivity assessment of the Saudi Arabian Marine Streptomyces sp. Al-Dhabi-90, metabolic profiling and its in vitro inhibitory property against multidrug resistant and extended-spectrum beta-lactamase clinical bacterial pathogens

BACKGROUND

Metabolites obtained from the marine microorganisms were known for their important role in microbial inhibition. Interestingly, bioprospecting of secondary metabolites from marine derived actinomycetes has huge demand especially in the treatment of multi drug resistant clinical pathogens. The present study subjected towards the identification of promising antimicrobial actinomycetes from the Arabian Gulf regions and metabolic profiling of the crude organic solvent extract by chromatographic techniques.

METHODS

The strains were characterized by 16S rRNA sequencing. Extracellular metabolites were profiled by performing GC-MS analysis. MIC values of the compounds were detected using broth dilution technique.

RESULTS

A Gram positive, spore forming filamentous Streptomyces sp. Al-Dhabi-90 possessed good antibacterial activities against the drug resistant pathogens were confirmed by 16S rRNA gene sequencing. Further, the gas chromatography coupled with mass spectrum analysis data revealed that the organic solvent extract of the fermented Streptomyces sp. Al-Dhabi-90 contained major components such as 3-methylpyridazine, n-hexadecanoic acid, indazol-4-one, octadecanoic acid and 3a-methyl-6-((4-methylphenyl) sul respectively. The Minimum Inhibitory Concentration (MIC) of the extract against Staphylococcus aureus and Klebsiella pneumoniae were 12.5 and 50μg/ml respectively. Against drug resistant ESBL pathogens such as Escherichia coli, Pseudomonsa aeroginosa and Proteus mirabilis were 12.5, and 25μg/ml respectively. Interestingly, the extract showed promising activity against the vancomycin resistant Enterococcus faecium at 50μg/ml. The increased level of cellular constituents after the extract treatment evidenced that the metabolites altered the membrane integrity of the pathogens.

CONCLUSION

Conclusively, the marine Streptomyces sp. Al-Dhabi-90 is an ideal source for the treatment of multi drug resistant clinical pathogens.

Bioprospecting from marine coastal sediments of Colombian Caribbean: screening and study of antimicrobial activity

AIMS

To isolate micro-organisms associated with marine coastal sediments of Colombian Caribbean Sea and for evaluating its antimicrobial activity in order to identify the most active strains.

METHODS AND RESULTS

One hundred and four strains were isolated from sediment samples of the Colombian Caribbean Sea. First at all, an antimicrobial activity screening was made using agar well diffusion method against the pathogens: Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Candida albicans, Candida tropicalis and Pseudomonas aeruginosa. Seventeen strains showed strong antimicrobial activity and were identified as members of the Streptomyces, Micrococcus and Bacillus genera. Organic extracts were produced by liquid-liquid extraction and HPLC profiles of the most active extracts were obtained. Then, the antimicrobial activity of the extracts was evaluated with the broth microdilution test, finding antimicrobial activities superior to 90% against S. aureus MRSA and C. albicans. HPLC profiles indicated the presence of different antimicrobial compounds.

CONCLUSION

This study demonstrates that the microorganisms isolated from the Colombian Caribbean Sea are possible sources of antimicrobial compounds against pathogenic strains.

SIGNIFICANCE AND IMPACT OF STUDY

These results contribute to the knowledge of the biotechnological potential of the Colombian biodiversity for the development of pharmaceutical products that can counteract the increasing problem of pathogen resistance.

Identification of a Thioesterase Bottleneck in the Pikromycin Pathway through Full-Module Processing of Unnatural Pentaketides

Polyketide biosynthetic pathways have been engineered to generate natural product analogs for over two decades. However, manipulation of modular type I polyketide synthases (PKSs) to make unnatural metabolites commonly results in attenuated yields or entirely inactive pathways, and the mechanistic basis for compromised production is rarely elucidated since rate-limiting or inactive domain(s) remain unidentified. Accordingly, we synthesized and assayed a series of modified pikromycin (Pik) pentaketides that mimic early pathway engineering to probe the substrate tolerance of the PikAIII-TE module in vitro. Truncated pentaketides were processed with varying efficiencies to corresponding macrolactones, while pentaketides with epimerized chiral centers were poorly processed by PikAIII-TE and failed to generate 12-membered ring products. Isolation and identification of extended but prematurely offloaded shunt products suggested that the Pik thioesterase (TE) domain has limited substrate flexibility and functions as a gatekeeper in the processing of unnatural substrates. Synthesis of an analogous hexaketide with an epimerized nucleophilic hydroxyl group allowed for direct evaluation of the substrate stereoselectivity of the excised TE domain. The epimerized hexaketide failed to undergo cyclization and was exclusively hydrolyzed, confirming the TE domain as a key catalytic bottleneck. In an accompanying paper , we engineer the standalone Pik thioesterase to yield a thioesterase (TE_S148C) and module (PikAIII-TE_S148C) that display gain-of-function processing of substrates with inverted hydroxyl groups.

Marine natural products

Covering: 2015. Previous review: Nat. Prod. Rep., 2016, 33, 382-431This review covers the literature published in 2015 for marine natural products (MNPs), with 1220 citations (792 for the period January to December 2015) 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 (1340 in 429 papers for 2015), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Emerging biopharmaceuticals from marine actinobacteria

Actinobacteria are quotidian microorganisms in the marine world, playing a crucial ecological role in the recycling of refractory biomaterials and producing novel secondary metabolites with pharmaceutical applications. Actinobacteria have been isolated from the huge area of marine organisms including sponges, tunicates, corals, mollusks, crabs, mangroves and seaweeds. Natural products investigation of the marine actinobacteria revealed that they can synthesize numerous natural products including alkaloids, polyketides, peptides, isoprenoids, phenazines, sterols, and others. These natural products have a potential to provide future drugs against crucial diseases like cancer, HIV, microbial and protozoal infections and severe inflammations. Therefore, marine actinobacteria portray as a pivotal resource for marine drugs. It is an upcoming field of research to probe a novel and pharmaceutically important secondary metabolites from marine actinobacteria. In this review, we attempt to summarize the present knowledge on the diversity, chemistry and mechanism of action of marine actinobacteria-derived secondary metabolites from 2007 to 2016.

Salinipyrone and Pacificanone Are Biosynthetic By-products of the Rosamicin Polyketide Synthase

Salinipyrones and pacificanones are structurally related polyketides from Salinispora pacifica CNS-237 that are proposed to arise from the same modular polyketide synthase (PKS) assembly line. Genome sequencing revealed a large macrolide PKS gene cluster that codes for the biosynthesis of rosamicin A and a series of new macrolide antibiotics. Mutagenesis experiments unexpectedly correlated salinipyrone and pacificanone biosynthesis to the rosamicin octamodule Spr PKS. Remarkably, this bifurcated polyketide pathway illuminates a series of enzymatic domain- and module-skipping reactions that give rise to natural polyketide product diversity. Our findings enlarge the growing knowledge of polyketide biochemistry and illuminate potential challenges in PKS bioengineering.

The marine actinomycete genus Salinispora: a model organism for secondary metabolite discovery

This review covers the initial discovery of the marine actinomycete genus Salinispora through its development as a model for natural product research. A focus is placed on the novel chemical structures reported with reference to their biological activities and the synthetic and biosynthetic studies they have inspired. The time line of discoveries progresses from more traditional bioassay-guided approaches through the application of genome mining and genetic engineering techniques that target the products of specific biosynthetic gene clusters. This overview exemplifies the extraordinary biosynthetic diversity that can emanate from a narrowly defined genus and supports future efforts to explore marine taxa in the search for novel natural products.

Molecular networking and pattern-based genome mining improves discovery of biosynthetic gene clusters and their products from Salinispora species

Genome sequencing has revealed that bacteria contain many more biosynthetic gene clusters than predicted based on the number of secondary metabolites discovered to date. While this biosynthetic reservoir has fostered interest in new tools for natural product discovery, there remains a gap between gene cluster detection and compound discovery. Here we apply molecular networking and the new concept of pattern-based genome mining to 35 Salinispora strains, including 30 for which draft genome sequences were either available or obtained for this study. The results provide a method to simultaneously compare large numbers of complex microbial extracts, which facilitated the identification of media components, known compounds and their derivatives, and new compounds that could be prioritized for structure elucidation. These efforts revealed considerable metabolite diversity and led to several molecular family-gene cluster pairings, of which the quinomycin-type depsipeptide retimycin A was characterized and linked to gene cluster NRPS40 using pattern-based bioinformatic approaches.

Indimicins A-E, Bisindole Alkaloids from the Deep-Sea-Derived Streptomyces sp. SCSIO 03032

Five new bisindole alkaloids, indimicins A-E (1-5), bearing a unique 1',3'-dimethyl-2'-hydroindole moiety, were isolated from the marine-derived Streptomyces sp. SCSIO 03032, along with two new compounds, lynamicins F and G (6 and 7). Their planar structures were elucidated by detailed interpretation of their MS and NMR spectroscopic data, and the absolute configurations were determined by X-ray crystallographic analysis (for 1), comparison of CD spectra (for 2-4), and quantum chemical calculations (for 5). Indimicin B (2) exhibited moderate cytotoxic activity toward the MCF-7 cell line.

Marine actinobacteria: an important source of bioactive natural products

Marine environment is largely an untapped source for deriving actinobacteria, having potential to produce novel, bioactive natural products. Actinobacteria are the prolific producers of pharmaceutically active secondary metabolites, accounting for about 70% of the naturally derived compounds that are currently in clinical use. Among the various actinobacterial genera, Actinomadura, Actinoplanes, Amycolatopsis, Marinispora, Micromonospora, Nocardiopsis, Saccharopolyspora, Salinispora, Streptomyces and Verrucosispora are the major potential producers of commercially important bioactive natural products. In this respect, Streptomyces ranks first with a large number of bioactive natural products. Marine actinobacteria are unique enhancing quite different biological properties including antimicrobial, anticancer, antiviral, insecticidal and enzyme inhibitory activities. They have attracted global in the last ten years for their ability to produce pharmaceutically active compounds. In this review, we have focused attention on the bioactive natural products isolated from marine actinobacteria, possessing unique chemical structures that may form the basis for synthesis of novel drugs that could be used to combat resistant pathogenic microorganisms.

Actinomycetes from Red Sea sponges: sources for chemical and phylogenetic diversity

The diversity of actinomycetes associated with marine sponges collected off Fsar Reef (Saudi Arabia) was investigated in the present study. Forty-seven actinomycetes were cultivated and phylogenetically identified based on 16S rRNA gene sequencing and were assigned to 10 different actinomycete genera. Eight putatively novel species belonging to genera Kocuria, Mycobacterium, Nocardia, and Rhodococcus were identified based on sequence similarity values below 98.2% to other 16S rRNA gene sequences available in the NCBI database. PCR-based screening for biosynthetic genes including type I and type II polyketide synthases (PKS-I, PKS-II) as well as nonribosomal peptide synthetases (NRPS) showed that 20 actinomycete isolates encoded each at least one type of biosynthetic gene. The organic extracts of nine isolates displayed bioactivity against at least one of the test pathogens, which were Gram-positive and Gram-negative bacteria, fungi, human parasites, as well as in a West Nile Virus protease enzymatic assay. These results emphasize that marine sponges are a prolific resource for novel bioactive actinomycetes with potential for drug discovery.

Marine natural products

This review covers the literature published in 2012 for marine natural products, with 1035 citations (673 for the period January to December 2012) 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 (1241 for 2012), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Pharmaceutically active secondary metabolites of marine actinobacteria

Marine actinobacteria are one of the most efficient groups of secondary metabolite producers and are very important from an industrial point of view. Many representatives of the order Actinomycetales are prolific producers of thousands of biologically active secondary metabolites. Actinobacteria from terrestrial sources have been studied and screened since the 1950s, for many important antibiotics, anticancer, antitumor and immunosuppressive agents. However, frequent rediscovery of the same compounds from the terrestrial actinobacteria has made them less attractive for screening programs in the recent years. At the same time, actinobacteria isolated from the marine environment have currently received considerable attention due to the structural diversity and unique biological activities of their secondary metabolites. They are efficient producers of new secondary metabolites that show a range of biological activities including antibacterial, antifungal, anticancer, antitumor, cytotoxic, cytostatic, anti-inflammatory, anti-parasitic, anti-malaria, antiviral, antioxidant, anti-angiogenesis, etc. In this review, an evaluation is made on the current status of research on marine actinobacteria yielding pharmaceutically active secondary metabolites. Bioactive compounds from marine actinobacteria possess distinct chemical structures that may form the basis for synthesis of new drugs that could be used to combat resistant pathogens. With the increasing advancement in science and technology, there would be a greater demand for new bioactive compounds synthesized by actinobacteria from various marine sources in future.

A novel natural phenyl alkene with cytotoxic activity

A novel phenyl alkene (1) was isolated from a mixture of three Florida sponges, Smenospongia aurea, Smenospongia cerebriformis, and Verongula rigida. Unlike terpenoids or amino acid derivatives, which are commonly known classes of secondary metabolites from these genera, the chemical structure of 1 showed an unprecedented linear phenyl alkene skeleton. Through comprehensive analyses of NMR and MS data, the gross structure of 1 was determined to be (E)-10-benzyl-5,7-dimethylundeca-1,5,10-trien-4-ol. The absolute configuration at C-4 was established as R by a modified Mosher's method. Based on the relative configuration between C-4 and C-7, the absolute configuration at C-7 was assigned as S. Compound 1 showed in vitro cytotoxic activity against HL-60 human leukemia cancer cells with an IC₅₀ value of 8.1 µM. Molecular docking study suggests that the structure of compound 1 matches the pharmacophore of eribulin required to display cytotoxic activity through the inhibition of microtubule activity.

Computational Methods for Identification of Novel Secondary Metabolite Biosynthetic Pathways by Genome Analysis

Secondary metabolites belonging to polyketide and nonribosomal peptide families constitute a major class of natural products with diverse biological functions and a variety of pharmaceutically important properties. Experimental studies have shown that the biosynthetic machinery for polyketide and nonribosomal peptides involves multi-functional megasynthases like Polyketide Synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) which utilize a thiotemplate mechanism similar to that for fatty acid biosynthesis. Availability of complete genome sequences for an increasing number of microbial organisms has provided opportunities for using in silico genome mining to decipher the secondary metabolite natural product repertoire encoded by these organisms. Therefore, in recent years there have been major advances in development of computational methods which can analyze genome sequences to identify genes involved in secondary metabolite biosynthesis and help in deciphering the putative chemical structures of their biosynthetic products based on analysis of the sequence and structural features of the proteins encoded by these genes. These computational methods for deciphering the secondary metabolite biosynthetic code essentially involve identification of various catalytic domains present in this PKS/NRPS family of enzymes; a prediction of various reactions in these enzymatic domains and their substrate specificities and also precise identification of the order in which these domains would catalyze various biosynthetic steps. Structural bioinformatics analysis of known secondary metabolite biosynthetic clusters has helped in formulation of predictive rules for deciphering domain organization, substrate specificity, and order of substrate channeling. In this chapter, the progress in development of various computational methods is discussed by different research groups, and specifically, the utility in identification of novel metabolites by genome mining and rational design of natural product analogs by biosynthetic engineering studies.

RETRACTED: Marine actinobacterial metabolites: current status and future perspectives

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor. Authors and Editor agreed to retract this article because substantial parts of the text were copied from the following sources without proper attribution: Lam, K.S. (2006), Discovery of novel metabolites from marine actinomycetes. Current Opinion in Microbiology 9(3), pp. 245–251; Subramani, R., Aalbersberg, W. (2012), Marine actinomycetes: An ongoing source of novel bioactive metabolites. Microbiological Research 167(10), pp. 571–580; Dharmaraj, S. (2010), Marine Streptomyces as a novel source of bioactive substances. World Journal of Microbiology and Biotechnology 26(12), pp. 2123–2139. The authors apologize for this oversight and any inconvenience caused.

Marine actinomycetes: an ongoing source of novel bioactive metabolites

Actinomycetes are virtually unlimited sources of novel compounds with many therapeutic applications and hold a prominent position due to their diversity and proven ability to produce novel bioactive compounds. There are more than 22,000 known microbial secondary metabolites, 70% of which are produced by actinomycetes, 20% from fungi, 7% from Bacillus spp. and 1-2% by other bacteria. Among the actinomycetes, streptomycetes group are considered economically important because out of the approximately more than 10,000 known antibiotics, 50-55% are produced by this genus. The ecological role of actinomycetes in the marine ecosystem is largely neglected and various assumptions meant there was little incentive to isolate marine strains for search and discovery of new drugs. The search for and discovery of rare and new actinomycetes is of significant interest to drug discovery due to a growing need for the development of new and potent therapeutic agents. Modern molecular technologies are adding strength to the target-directed search for detection and isolation of bioactive actinomycetes, and continued development of improved cultivation methods and molecular technologies for accessing the marine environment promises to provide access to this significant new source of chemical diversity with novel/rare actinomycetes including new species of previously reported actinomycetes.

Efficacy of albendazole combined with a marine fungal extract (m2-9) against Angiostrongylus cantonensis-induced meningitis in mice

The pathogenesis of angiostrongyliasis, resulting from Angiostrongylus cantonensis invasion of the human central nervous system, remains elusive. Anthelmintics are usually used to kill worms, although dead worms in the brain may cause severe inflammation which will lead to central nervous system damage. Therefore, combination therapy with anthelmintics and anti-inflammatory drugs in the treatment of human angiostrongyliasis needs further study. To evaluate the efficacy of albendazole combined with a marine fungal extract (m2-9) in A. cantonensis infection, BALB/c mice infected by the third-stage larvae of A. cantonensis were divided into three groups: mice treated with albendazole or m2-9 alone or in combination from day 5 post-inoculation (PI). Several efficacy parameters were recorded, including weight change, worm recovery, neurological function, behavioural analysis, eosinophil and leucocyte counts. The results showed that combination therapy increased body weight, reduced worm burden, improved learning ability, memory and action, decreased neurological dysfunction and leucocyte response in these mice. The combination of albendazole and m2-9 treatment significantly decreased leucocyte response and increased the frequency of rearing, compared to infected mice treated with either drug alone. Therefore, m2-9 is a natural product with potentially significant therapeutic value for angiostrongyliasis and is worthy of further study.

Structures and biosynthesis of the pyridinopyrones, polyenepyrones from a marine-derived Streptomyces species

Three polyenylpyrone metabolites, pyridinopyrones A to C (1-3), have been isolated from the culture broth of a marine-derived Streptomyces sp., strain CNQ-301. The structures of the pyridinopyrones were assigned on the basis of chemical modification and combined spectroscopic methods, focusing on interpretation of 1D and 2D NMR data. Pyridinopyrones B and C (2, 3), examined as an inseparable mixture of methyl positional isomers, were ultimately defined by hydrogenation and NMR analysis of a saturated derivative. The biosynthesis of these metabolites was defined by the incorporation of stable isotope-labeled precursors, revealing that the biosynthetic starter unit is nicotinic acid, while the polyene chain and pendant methyl groups are acetate- and methionine-derived, respectively.

Cyclopenicillone, a unique cyclopentenone from the cultures of Penicillium decumbens

Cyclopenicillone (1), possessing a unique 2,5-dimethylcyclopent-2-enone carbon skeleton, has been isolated from the cultures broth of the fungus Penicillium decumbens. The structure and absolute configuration of 1 were elucidated using a combination of NMR spectroscopy, CD data and computational approaches. Cyclopenicillone (1) demonstrated a dose-dependent (10-100 μM) inhibition against LPS-induced NO production in RAW264.7 macrophages.

Gombapyrones E and F, New α-Pyrone Polyenes Produced by Streptomyces sp. KMC-002

Microorganism-derived polyene polyketides have been shown to display a variety of biological activities and have attracted great interest due to their structurally intriguing chemical diversity. Two new polyenes were isolated from a culture broth of Streptomyces sp. KMC-002 obtained from a soil sample in an abandoned mine. The structures of these compounds were determined to be α-pyrone-containing polyene analogues through analyses of HRFABMS, UV and NMR data, and were named Gombapyrones E (1) and F (2). Gombapyrone E (1) showed antibacterial activity against Micrococcus luteus, Enterococcus hirae, Staphylococcus aureus and MRSA.

Marine pharmacology in 2007-8: Marine compounds with antibacterial, anticoagulant, antifungal, anti-inflammatory, antimalarial, antiprotozoal, antituberculosis, and antiviral activities; affecting the immune and nervous system, and other miscellaneous mechanisms of action

The peer-reviewed marine pharmacology literature in 2007-8 is covered in this review, which follows a similar format to the previous 1998-2006 reviews of this series. The preclinical pharmacology of structurally characterized marine compounds isolated from marine animals, algae, fungi and bacteria is discussed in a comprehensive manner. Antibacterial, anticoagulant, antifungal, antimalarial, antiprotozoal, antituberculosis and antiviral activities were reported for 74 marine natural products. Additionally, 59 marine compounds were reported to affect the cardiovascular, immune and nervous systems as well as to possess anti-inflammatory effects. Finally, 65 marine metabolites were shown to bind to a variety of receptors and miscellaneous molecular targets, and thus upon further completion of mechanism of action studies, will contribute to several pharmacological classes. Marine pharmacology research during 2007-8 remained a global enterprise, with researchers from 26 countries, and the United States, contributing to the preclinical pharmacology of 197 marine compounds which are part of the preclinical marine pharmaceuticals pipeline. Sustained preclinical research with marine natural products demonstrating novel pharmacological activities, will probably result in the expansion of the current marine pharmaceutical clinical pipeline, which currently consists of 13 marine natural products, analogs or derivatives targeting a limited number of disease categories.

Secondary metabolites from a marine-derived endophytic fungus Penicillium chrysogenum QEN-24S

Penicillium chrysogenum QEN-24S, an endophytic fungus isolated from an unidentified marine red algal species of the genus Laurencia, displayed inhibitory activity against the growth of pathogen Alternaria brassicae in dual culture test. Chemical investigation of this fungal strain resulted in the isolation of four new (1–3 and 5) and one known (4) secondary metabolites. Their structures were identified as two polyketide derivatives penicitides A and B (1 and 2), two glycerol derivatives 2-(2,4-dihydroxy-6-methylbenzoyl)-glycerol (3) and 1-(2,4-dihydroxy-6-methylbenzoyl)- glycerol (4), and one monoterpene derivative penicimonoterpene (5). Penicitides A and B (1 and 2) feature a unique 10-hydroxy- or 7,10-dihydroxy-5,7-dimethylundecyl moiety substituting at C-5 of the α-tetrahydropyrone ring, which is not reported previously among natural products. Compound 5 displayed potent activity against the pathogen A. brassicae, while compound 1 exhibited moderate cytotoxic activity against the human hepatocellular liver carcinoma cell line.