Natural products from Antarctic colonial ascidians of the genera Aplidium and Synoicum: variability and defensive role

Tunicates as Sources of High-Quality Nutrients and Bioactive Compounds for Food/Feed and Pharmaceutical Applications: A Review

Tunicates are widely distributed worldwide and are recognized as abundant marine bioresources with many potential applications. In this review, state-of-the-art studies on chemical composition analyses of various tunicate species were summarized; these studies confirmed that tunicates contain nutrients similar to fish (such as abundant cellulose, protein, and ω-3 fatty acid (FA)-rich lipids), indicating their practical and feasible uses for food or animal feed exploration. However, the presence of certain toxic elements should be evaluated in terms of safety. Moreover, recent studies on bioactive substances extracted from tunicates (such as toxins, sphingomyelins, and tunichromes) were analyzed, and their biological properties were comprehensively reviewed, including antimicrobial, anticancer, antioxidant, antidiabetic, and anti-inflammatory activities. In addition, some insights and prospects for the future exploration of tunicates are provided which are expected to guide their further application in the food, animal feed, and pharmaceutical industries. This review is critical to providing a new pathway for converting the common pollution issues of hydroponic nutrients into valuable marine bioresources.

Natural Alkaloids as Multi-Target Compounds towards Factors Implicated in Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia in elderly people; currently, there is no efficient treatment. Considering the increase in life expectancy worldwide AD rates are predicted to increase enormously, and thus the search for new AD drugs is urgently needed. A great amount of experimental and clinical evidence indicated that AD is a complex disorder characterized by widespread neurodegeneration of the CNS, with major involvement of the cholinergic system, causing progressive cognitive decline and dementia. The current treatment, based on the cholinergic hypothesis, is only symptomatic and mainly involves the restoration of acetylcholine (ACh) levels through the inhibition of acetylcholinesterase (AChE). Since the introduction of the Amaryllidaceae alkaloid galanthamine as an antidementia drug in 2001, alkaloids have been one of the most attractive groups for searching for new AD drugs. The present review aims to comprehensively summarize alkaloids of various origins as multi-target compounds for AD. From this point of view, the most promising compounds seem to be the β-carboline alkaloid harmine and several isoquinoline alkaloids since they can simultaneously inhibit several key enzymes of AD's pathophysiology. However, this topic remains open for further research on detailed mechanisms of action and the synthesis of potentially better semi-synthetic analogues.

A Review: Meridianins and Meridianins Derivatives

Meridianins are a family of indole alkaloids derived from Antarctic tunicates with extensive pharmacological activities. A series of meridianin derivatives had been synthesized by drug researchers. This article reviews the extraction and purification methods, biological activities and pharmacological applications, pharmacokinetic characters and chemical synthesis of meridianins and their derivatives. And prospects on discovering new bioactivities of meridianins and optimizing their structure for the improvement of the ADMET properties are provided.

Would Antarctic Marine Benthos Survive Alien Species Invasions? What Chemical Ecology May Tell Us

Many Antarctic marine benthic macroinvertebrates are chemically protected against predation by marine natural products of different types. Antarctic potential predators mostly include sea stars (macropredators) and amphipod crustaceans (micropredators) living in the same areas (sympatric). Recently, alien species (allopatric) have been reported to reach the Antarctic coasts, while deep-water crabs are suggested to be more often present in shallower waters. We decided to investigate the effect of the chemical defenses of 29 representative Antarctic marine benthic macroinvertebrates from seven different phyla against predation by using non-native allopatric generalist predators as a proxy for potential alien species. The Antarctic species tested included 14 Porifera, two Cnidaria, two Annelida, one Nemertea, two Bryozooa, three Echinodermata, and five Chordata (Tunicata). Most of these Antarctic marine benthic macroinvertebrates were chemically protected against an allopatric generalist amphipod but not against an allopatric generalist crab from temperate waters. Therefore, both a possible recolonization of large crabs from deep waters or an invasion of non-native generalist crab species could potentially alter the fundamental nature of these communities forever since chemical defenses would not be effective against them. This, together with the increasing temperatures that elevate the probability of alien species surviving, is a huge threat to Antarctic marine benthos.

Meridianins Rescue Cognitive Deficits, Spine Density and Neuroinflammation in the 5xFAD Model of Alzheimer's Disease

Glycogen synthase kinase 3β (GSK3β) is a core protein, with a relevant role in many neurodegenerative disorders including Alzheimer’s disease. The enzyme has been largely studied as a potential therapeutic target for several neurological diseases. Unfortunately, preclinical and clinical studies with several GSK3β inhibitors have failed due to many reasons such as excessive toxicity or lack of effects in human subjects. We previously reported that meridianins are potent GSK3β inhibitors without altering neuronal viability. In the present work, we examine whether meridianins are capable to inhibit neural GSK3β in vivo and if such inhibition induces improvements in the 5xFAD mouse model of Alzheimer’s Disease. Direct administration of meridianins in the third ventricle of 5xFAD mice induced robust improvements of recognition memory and cognitive flexibility as well as a rescue of the synaptic loss and an amelioration of neuroinflammatory processes. In summary, our study points out meridianins as a potential compound to treat neurodegenerative disorders associated with an hyperactivation of GSK3β such as Alzheimer’s disease.

Australindolones, New Aminopyrimidine Substituted Indolone Alkaloids from an Antarctic Tunicate Synoicum sp

Five new alkaloids have been isolated from the lipophilic extract of the Antarctic tunicate Synoicum sp. Deep-sea specimens of Synoicum sp. were collected during a 2011 cruise of the R/V Nathanial B. Palmer to the southern Scotia Arc, Antarctica. Crude extracts from the invertebrates obtained during the cruise were screened in a zebrafish-based phenotypic assay. The Synoicum sp. extract induced embryonic dysmorphology characterized by axis truncation, leading to the isolation of aminopyrimidine substituted indolone (1–4) and indole (5–12) alkaloids. While the primary bioactivity tracked with previously reported meridianins A–G (5–11), further investigation resulted in the isolation and characterization of australindolones A–D (1–4) and the previously unreported meridianin H (12).

Exploitation of Marine Molecules to Manage Alzheimer's Disease

Neurodegenerative diseases are sociosanitary challenges of today, as a result of increased average life expectancy, with Alzheimer’s disease being one of the most prevalent. This pathology is characterized by brain impairment linked to a neurodegenerative process culminating in cognitive decline and behavioral disorders. Though the etiology of this pathology is still unknown, it is usually associated with the appearance of senile plaques and neurofibrillary tangles. The most used prophylaxis relies on anticholinesterase drugs and NMDA receptor antagonists, whose main action is to relieve symptoms and not to treat or prevent the disease. Currently, the scientific community is gathering efforts to disclose new natural compounds effective against Alzheimer’s disease and other neurodegenerative pathologies. Marine natural products have been shown to be promising candidates, and some have been proven to exert a high neuroprotection effect, constituting a large reservoir of potential drugs and nutraceutical agents. The present article attempts to describe the processes of extraction and isolation of bioactive compounds derived from sponges, algae, marine bacteria, invertebrates, crustaceans, and tunicates as drug candidates against AD, with a focus on the success of pharmacological activity in the process of finding new and effective drug compounds.

Marine Natural Products from Tunicates and Their Associated Microbes

Marine tunicates are identified as a potential source of marine natural products (MNPs), demonstrating a wide range of biological properties, like antimicrobial and anticancer activities. The symbiotic relationship between tunicates and specific microbial groups has revealed the acquisition of microbial compounds by tunicates for defensive purpose. For instance, yellow pigmented compounds, "tambjamines", produced by the tunicate, Sigillina signifera (Sluiter, 1909), primarily originated from their bacterial symbionts, which are involved in their chemical defense function, indicating the ecological role of symbiotic microbial association with tunicates. This review has garnered comprehensive literature on MNPs produced by tunicates and their symbiotic microbionts. Various sections covered in this review include tunicates' ecological functions, biological activities, such as antimicrobial, antitumor, and anticancer activities, metabolic origins, utilization of invasive tunicates, and research gaps. Apart from the literature content, 20 different chemical databases were explored to identify tunicates-derived MNPs. In addition, the management and exploitation of tunicate resources in the global oceans are detailed for their ecological and biotechnological implications.

Evaluation of the Physicochemical, Antioxidant, and Antibacterial Properties of Tunichrome Released from Phallusia nigra Persian Gulf Marine Tunicate

The aim of this study was to evaluate the physicochemical, nutraceutical, antioxidant, and antibacterial properties of tunichrome released from Persian Gulf tunicate (Phallusia nigra). For this purpose, molecular weight (SDS-PAGE), amino acid profile, chemical composition (GC-MS), mineral composition, functional groups (FTIR), total phenol content (TPC), total flavonoid content (TFC), antioxidant activity, and antimicrobial properties were investigated. The results showed that tunichrome contained a high amount of essential amino acids (i.e., Lys = 32.24 mg/100 g) and essential minerals. According to GC-MS results, tunichrome had different antioxidant and antimicrobial components. The TPC and TFC of tunichrome were 0.55 mg GA/g and 0.21 mg quercetin/100 g, respectively. Tunichrome showed higher antioxidant activity than ascorbic acid, and its radical scavenging activity values were increased from 30.28 to 82.08% by increasing concentration from 50 to 200 ppm. Inhibition zones of Staphylococcus aureus, Bacillus cereus, Salmonella enterica, and Escherichia coli O157:H7 were 14, 18, 17, and 15 mm, respectively. Moreover, the minimum inhibitory concentration values of tunichrome for S. aureus, Bacillus cereus, S. enterica, and E. coli O157:H7 were 1.17, 0.59, 0.59, and 1.17 mg/ml, respectively. The minimum bacterial concentrations were 2.34, 1.17, 1.17, and 2.34 mg/ml for S. aureus, Bacillus cereus, S. enterica, and E. coli O157:H7, respectively. These results showed that tunichrome of Phallusia nigra has excellent biological effects as a bioactive compound for food fortification.

Antifouling Activity of Meroterpenes Isolated from the Ascidian Aplidium aff. densum

The settlement and growth of fouling organisms on man-made surfaces can be prevented by the application of antifouling paints containing active compounds (biocides, heavy metals), most of which are toxic to non-target organisms. As part of our research program in chemical ecology and blue biotechnology, we are conducting studies to investigate the natural defence mechanisms of marine organisms that are free from epibionts, with the aim of isolating molecules involved in surface defence that could be good candidates as antifouling agents. Ascidians were selected for our investigation because previous studies have shown that they contain abundant and diverse secondary metabolites, which play a defensive role and have been applied to drug discovery. It is therefore relevant to study the role of such secondary metabolites in surface protection. In this study, 5 meroterpenoids (cordiachromene A, didehydroconicol, epiconicol, methoxyconidiol, conidione) from Aplidium aff. densum (ascidian) were investigated as potential antifoulants towards the inhibition of bacterial growth and settlement inhibition of barnacles. Cardiochromene A (IC₅₀ barnacle settlement = 6.04 μg/mL; MIC Gram positive = 125 μg/mL; MIC Gram negative = 32 μg/mL) and epiconicol (IC₅₀ barnacle settlement = 8.05 μg/mL; MIC Bacillus = 63 μg/mL; MIC other strains = 32 μg/mL) were the most promising compounds among those tested in this study.

The Ascidian-Derived Metabolites with Antimicrobial Properties

Among the sub-phylum of Tunicate, ascidians represent the most abundant class of marine invertebrates, with 3000 species by heterogeneous habitat, that is, from shallow water to deep sea, already reported. The chemistry of these sessile filter-feeding organisms is an attractive reservoir of varied and peculiar bioactive compounds. Most secondary metabolites isolated from ascidians stand out for their potential as putative therapeutic agents in the treatment of several illnesses like microbial infections. In this review, we present and discuss the antibacterial activity shown by the main groups of ascidian-derived products, such as sulfur-containing compounds, meroterpenes, alkaloids, peptides, furanones, and their derivatives. Moreover, the direct evidence of a symbiotic association between marine ascidians and microorganisms shed light on the real producers of many extremely potent marine natural compounds. Hence, we also report the antibacterial potential, joined to antifungal and antiviral activity, of metabolites isolated from ascidian-associate microorganisms by culture-dependent methods.

Natural indole-containing alkaloids and their antibacterial activities

As the growth in resistance to bacterial infection treatments poses a grave threat to global health in the 21st century, there is a constant need to explore novel antibacterial agents that have the ability to overcome drug resistance. Indole-containing alkaloids are widely distributed in nature, and a variety of indole-containing alkaloids have already been applied in clinical practice, proving that indole-containing alkaloids are fascinating and privileged scaffolds for the development of novel drugs. Moreover, indole-containing alkaloids could exert their antibacterial activity through the inhibition of efflux pumps, the biofilm, filamentous temperature-sensitive protein Z, and methicillin-resistant Staphylococcus aureus pyruvate kinase; so, indole-containing alkaloids constitute an important source of novel antibacterial agents. This review is an endeavor to highlight the advances in the development of indole-containing alkaloids with antibacterial potential, covering articles published in the recent 10 years.

Meridianins and Lignarenone B as Potential GSK3β Inhibitors and Inductors of Structural Neuronal Plasticity

Glycogen Synthase Kinase 3 (GSK3) is an essential protein, with a relevant role in many diseases such as diabetes, cancer and neurodegenerative disorders. Particularly, the isoform GSK3β is related to pathologies such as Alzheimer's disease (AD). This enzyme constitutes a very interesting target for the discovery and/or design of new therapeutic agents against AD due to its relation to the hyperphosphorylation of the microtubule-associated protein tau (MAPT), and therefore, its contribution to neurofibrillary tangles (NFT) formation. An in silico target profiling study identified two marine molecular families, the indole alkaloids meridianins from the tunicate genus Aplidium, and lignarenones, the secondary metabolites of the shelled cephalaspidean mollusc Scaphander lignarius, as possible GSK3β inhibitors. The analysis of the surface of GSK3β, aimed to find possible binding regions, and the subsequent in silico binding studies revealed that both marine molecular families can act over the ATP and/or substrate binding regions. The predicted inhibitory potential of the molecules from these two chemical families was experimentally validated in vitro by showing a ~50% of increased Ser9 phosphorylation levels of the GSK3β protein. Furthermore, we determined that molecules from both molecular families potentiate structural neuronal plasticity in vitro. These results allow us to suggest that meridianins and lignarenone B could be used as possible therapeutic candidates for the treatment of GSK3β involved pathologies, such as AD.

Bioactive Prenyl- and Terpenyl-Quinones/Hydroquinones of Marine Origin †

The sea is a rich source of biological active compounds, among which terpenyl-quinones/hydroquinones constitute a family of secondary metabolites with diverse pharmacological properties. The chemical diversity and bioactivity of those isolated from marine organisms in the last 10 years are summarized in this review. Aspects related to synthetic approaches towards the preparation of improved bioactive analogues from inactive terpenoids are also outlined.

Meridianin D Analogues Display Antibiofilm Activity against MRSA and Increase Colistin Efficacy in Gram-Negative Bacteria

In the last 30 years, development of new classes of antibiotics has slowed, increasing the necessity for new options to treat multidrug resistant bacterial infections. Development of antibiotic adjuvants that increase the effectiveness of currently available antibiotics is a promising alternative approach to classical antibiotic development. Reports of the ability of the natural product meridianin D to modulate bacterial behavior have been rare. Herein, we describe the ability of meridianin D to inhibit biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) and to increase the potency of colistin against colistin-resistant and sensitive Gram-negative bacteria. Analogues were identified that are capable of inhibiting and dispersing MRSA biofilms and lowering the colistin MIC to below the CLSI breakpoint against Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli.

Chemical Ecology of Chemosensation in Asteroidea: Insights Towards Management Strategies of Pest Species

Within the Phylum Echinodermata, the class Asteroidea, commonly known as starfish and sea stars, encompasses a large number of benthos inhabiting genera and species with various feeding modalities including herbivores, carnivores, omnivores and detritivores. The Asteroidea rely on chemosensation throughout their life histories including hunting prey, avoiding or deterring predators, in the formation of spawning aggregations, synchronizing gamete release and targeting appropriate locations for larval settlement. The identities of many of the chemical stimuli that mediate these physiological and behavioural processes remain unresolved even though evidence indicates they play pivotal roles in the functionality of benthic communities. Aspects of chemosensation, as well as putative chemically-mediated behaviours and the molecular mechanisms of chemoreception, within the Asteroidea are reviewed here, with particular reference to the coral reef pest the Crown-of-Thorns starfish Acanthaster planci species complex, in the context of mitigation of population outbreaks.

Computer-Aided Drug Design Applied to Marine Drug Discovery: Meridianins as Alzheimer's Disease Therapeutic Agents

Computer-aided drug discovery/design (CADD) techniques allow the identification of natural products that are capable of modulating protein functions in pathogenesis-related pathways, constituting one of the most promising lines followed in drug discovery. In this paper, we computationally evaluated and reported the inhibitory activity found in meridianins A-G, a group of marine indole alkaloids isolated from the marine tunicate Aplidium, against various protein kinases involved in Alzheimer's disease (AD), a neurodegenerative pathology characterized by the presence of neurofibrillary tangles (NFT). Balance splitting between tau kinase and phosphate activities caused tau hyperphosphorylation and, thereby, its aggregation and NTF formation. Inhibition of specific kinases involved in its phosphorylation pathway could be one of the key strategies to reverse tau hyperphosphorylation and would represent an approach to develop drugs to palliate AD symptoms. Meridianins bind to the adenosine triphosphate (ATP) binding site of certain protein kinases, acting as ATP competitive inhibitors. These compounds show very promising scaffolds to design new drugs against AD, which could act over tau protein kinases Glycogen synthetase kinase-3 Beta (GSK3β) and Casein kinase 1 delta (CK1δ, CK1D or KC1D), and dual specificity kinases as dual specificity tyrosine phosphorylation regulated kinase 1 (DYRK1A) and cdc2-like kinases (CLK1). This work is aimed to highlight the role of CADD techniques in marine drug discovery and to provide precise information regarding the binding mode and strength of meridianins against several protein kinases that could help in the future development of anti-AD drugs.

Structural analysis of a sulfated galactan from the tunic of the ascidian Microcosmus exasperatus and its inhibitory effect of the intrinsic coagulation pathway

Several bioactive sulfated galactans have been isolated from the tunic of different species of ascidians. The biological activity of this kind of polysaccharides has been related with the presence and position of sulfate groups, and by the chemical composition of this kind of polysaccharides. A sulfated galactan (1000RS) was isolated from the tunic of the Brazilian ascidia Microcosmus exasperatus through proteolytic digestion, ethanol precipitation, dialysis and freeze-thaw cycles. Homogeneity and molecular weight were estimated by using size exclusion chromatography. Monosaccharide composition and type of linkage were assessed by Gas chromatography coupled to mass spectrometry and the sulfate content was quantified through gelatin/BaCl₂ method. These experiments along with NMR and FTIR analysis allowed to claim that the galactan backbone is mainly composed of 4-linked α-l-Galp units. In addition, they permitted to establish that some of the galactose residues are sulfated at the 3-position. This sulfated polysaccharide, which has an average molecular mass of 439.5kDa, presents anticoagulant effect in a dose-dependent manner through the inhibition of the intrinsic coagulation pathway.

Experimental evidence of chemical defence mechanisms in Antarctic bryozoans

Bryozoans are among the most abundant and diverse members of the Antarctic benthos, however the role of bioactive metabolites in ecological interactions has been scarcely studied. To extend our knowledge about the chemical ecology of Antarctic bryozoans, crude ether extracts (EE) and butanol extracts (BE) obtained from two Antarctic common species (Cornucopina pectogemma and Nematoflustra flagellata), were tested for antibacterial and repellent activities. The extracts were screened for quorum quenching and antibacterial activities against four Antarctic bacterial strains (Bacillus aquimaris, Micrococcus sp., Oceanobacillus sp. and Paracoccus sp.). The Antarctic amphipod Cheirimedon femoratus and the sea star Odontaster validus were selected as sympatric predators to perform anti-predatory and substrate preference assays. No quorum quenching activity was detected in any of the extracts, while all EE exhibited growth inhibition towards at least one bacterium strain. Although the species were not repellent against the sea star, they caused repellence to the amphipods in both extracts, suggesting that defence activities against predation derive from both lipophilic and hydrophilic metabolites. In the substrate preference assays, one EE and one BE deriving from different specimens of the species C. pectogemma were active. This study reveals intraspecific variability of chemical defences and supports the fact that chemically mediated interactions are common in Antarctic bryozoans as means of protection against fouling and predation.

Secondary Metabolites from Polar Organisms

Polar organisms have been found to develop unique defences against the extreme environment environment, leading to the biosynthesis of novel molecules with diverse bioactivities. This review covers the 219 novel natural products described since 2001, from the Arctic and the Antarctic microoganisms, lichen, moss and marine faunas. The structures of the new compounds and details of the source organism, along with any relevant biological activities are presented. Where reported, synthetic and biosynthetic studies on the polar metabolites have also been included.

Natural Products Diversity of Marine Ascidians (Tunicates; Ascidiacea) and Successful Drugs in Clinical Development

This present study reviewed the chemical diversity of marine ascidians and their pharmacological applications, challenges and recent developments in marine drug discovery reported during 1994-2014, highlighting the structural activity of compounds produced by these specimens. Till date only 5% of living ascidian species were studied from <3000 species, this study represented from family didemnidae (32%), polyclinidae (22%), styelidae and polycitoridae (11-12%) exhibiting the highest number of promising MNPs. Close to 580 compound structures are here discussed in terms of their occurrence, structural type and reported biological activity. Anti-cancer drugs are the main area of interest in the screening of MNPs from ascidians (64%), followed by anti-malarial (6%) and remaining others. FDA approved ascidian compounds mechanism of action along with other compounds status of clinical trials (phase 1 to phase 3) are discussed here in. This review highlights recent developments in the area of natural products chemistry and biotechnological approaches are emphasized.

Natural products mediating ecological interactions in Antarctic benthic communities: a mini-review of the known molecules

Out of the many bioactive compounds described from the oceans, only a small fraction have been studied for their ecological significance. Similarly, most chemically mediated interactions are not well understood, because the molecules involved remain unrevealed. In Antarctica, this gap in knowledge is even more acute in comparison to tropical or temperate regions, even though polar organisms are also prolific producers of chemical defenses, and pharmacologically relevant products are being reported from the Southern Ocean. The extreme and unique marine environments surrounding Antarctica along with the numerous unusual interactions taking place in benthic communities are expected to select for novel functional secondary metabolites. There is an urgent need to comprehend the evolutionary role of marine derived substances in general, and particularly at the Poles, since molecules of keystone significance are vital in species survival, and therefore, in structuring the communities. Here we provide a mini-review on the identified marine natural products proven to have an ecological function in Antarctic ecosystems. This report recapitulates some of the bibliography from original Antarctic reviews, and updates the new literature in the field from 2009 to the present.

Deep sea as a source of novel-anticancer drugs: update on discovery and preclinical/clinical evaluation in a systems medicine perspective

The deep-sea habitat is a source of very potent marine-derived agents that may inhibit the growth of human cancer cells "in vitro" and "in vivo". Salinosporamide-A, Marizomib, by Salinispora species is a proteasome inhibitor with promising anticancer activity (Phase I/II trials). Different deep-sea-derived drugs are under preclinical evaluation. Cancer is a complex disease that may be represented by network medicine. A simple consequence is the change of the concept of target entity from a single protein to a whole molecular pathway and or cellular network. Deep-sea-derived drugs fit well to this new concept.

Deep sea as a source of novel-anticancer drugs: update on discovery and preclinical/clinical evaluation in a systems medicine perspective

The deep-sea habitat is a source of very potent marine-derived agents that may inhibit the growth of human cancer cells "in vitro" and "in vivo". Salinosporamide-A, Marizomib, by Salinispora species is a proteasome inhibitor with promising anticancer activity (Phase I/II trials). Different deep-sea-derived drugs are under preclinical evaluation. Cancer is a complex disease that may be represented by network medicine. A simple consequence is the change of the concept of target entity from a single protein to a whole molecular pathway and or cellular network. Deep-sea-derived drugs fit well to this new concept.

Mass spectrometry detection of minor new meridianins from the Antarctic colonial ascidians Aplidium falklandicum and Aplidium meridianum

Taking into account the broad biological activities found in the meridianin indole alkaloids isolated to date, we have re-examined the organic extracts of an Antarctic collection of the tunicates Aplidium meridianum and A. falklandicum (Chordata: Ascidiacea) by HPLC in conjunction with a high-resolution mass spectrometer (HPLC-MS). A new set of analogs of meridianins A-G has been detected, and their structures are proposed on the basis of the molecular formulae identified by LC-HRMS analysis using a C18 column with a gradient of water/acetonitrile and an LTQ-FT-MS Orbitrap detector. Remarkably, dimers derived from meridianin A and from meridianin B or E were also detected. Our findings provide further evidence of the broad variability within the meridianin-like derivatives of this highly bioactive alkaloid family.

Antifouling compounds from the sub-arctic ascidian Synoicum pulmonaria: synoxazolidinones A and C, pulmonarins A and B, and synthetic analogues

The current study describes the antifouling properties of four members belonging to the recently discovered synoxazolidinone and pulmonarin families, isolated from the sub-Arctic sessile ascidian Synoicum pulmonaria collected off the Norwegian coast. Four simplified synthetic analogues were also prepared and included in the study. Several of the studied compounds displayed MIC values in the micro-nanomolar range against 16 relevant marine species involved in both the micro- and macrofouling process. Settlement studies on Balanus improvisus cyprids indicated a deterrent effect and a low toxicity for selected compounds. The two synoxazolidinones displayed broad activity and are shown to be among the most active natural antifouling bromotyrosine derivatives described. Synoxazolidinone C displayed selected antifouling properties comparable to the commercial antifouling product Sea-Nine-211. The pulmonarins prevented the growth of several bacterial strains at nanomolar concentrations but displayed a lower activity toward microalgae and no effect on barnacles. The linear and cyclic synthetic peptidic mimics also displayed potent antifouling activities mainly directed against bacterial adhesion and growth.

Recent advances in deep-sea natural products

Covering: 2009 to 2013. This review covers the 188 novel marine natural products described since 2008, from deep-water (50->5000 m) marine fauna including bryozoa, chordata, cnidaria, echinodermata, microorganisms, mollusca and porifera. The structures of the new compounds and details of the source organism, depth of collection and country of origin are presented, along with any relevant biological activities of the metabolites. Where reported, synthetic studies on the deep-sea natural products have also been included. Most strikingly, 75% of the compounds were reported to possess bioactivity, with almost half exhibiting low micromolar cytotoxicity towards a range of human cancer cell lines, along with a significant increase in the number of microbial deep-sea natural products reported.

Defensive metabolites from Antarctic invertebrates: does energetic content interfere with feeding repellence?

Many bioactive products from benthic invertebrates mediating ecological interactions have proved to reduce predation, but their mechanisms of action, and their molecular identities, are usually unknown. It was suggested, yet scarcely investigated, that nutritional quality interferes with defensive metabolites. This means that antifeedants would be less effective when combined with energetically rich prey, and that higher amounts of defensive compounds would be needed for predator avoidance. We evaluated the effects of five types of repellents obtained from Antarctic invertebrates, in combination with diets of different energetic values. The compounds came from soft corals, ascidians and hexactinellid sponges; they included wax esters, alkaloids, a meroterpenoid, a steroid, and the recently described organic acid, glassponsine. Feeding repellency was tested through preference assays by preparing diets (alginate pearls) combining different energetic content and inorganic material. Experimental diets contained various concentrations of each repellent product, and were offered along with control compound-free pearls, to the Antarctic omnivore amphipod Cheirimedon femoratus. Meridianin alkaloids were the most active repellents, and wax esters were the least active when combined with foods of distinct energetic content. Our data show that levels of repellency vary for each compound, and that they perform differently when mixed with distinct assay foods. The natural products that interacted the most with energetic content were those occurring in nature at higher concentrations. The bioactivity of the remaining metabolites tested was found to depend on a threshold concentration, enough to elicit feeding repellence, independently from nutritional quality.

Quinone and hydroquinone metabolites from the ascidians of the genus Aplidium

Ascidians of the genus Aplidium are recognized as an important source of chemical diversity and bioactive natural products. Among the compounds produced by this genus are non-nitrogenous metabolites, mainly prenylated quinones and hydroquinones. This review discusses the isolation, structural elucidation, and biological activities of quinones, hydroquinones, rossinones, longithorones, longithorols, floresolides, scabellones, conicaquinones, aplidinones, thiaplidiaquinones, and conithiaquinones. A compilation of the ¹³C-NMR spectral data of these compounds is also presented.

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.

XAS spectroscopy, sulfur, and the brew within blood cells from Ascidia ceratodes

We report the first use of K-edge X-ray absorption spectroscopy (XAS) as a direct spectroscopic probe of pH and cytosolic emf within living cells. A new accuracy metric of model-based fits to K-edge spectra is further developed. Sulfur functional groups in three collections of living blood cells and one sample of cleared blood plasma from the tunicate Ascidia ceratodes were speciated using K-edge XAS. Cysteine and cystine, the preferred thiol-disulfide model, averaged about 12% of total sulfur. Sulfate monoesters and cyclic diesters unexpectedly constituted 36% of blood cell sulfur. Soluble sulfate averaged about 25% across the three blood cell samples, while the ratio of SO4(2-) to HSO4(-) implied average signet ring vacuolar pH values of 0.85, 1.4, or 3.1. Intracellular (VSO4)(+) was unobserved, while [V(RSO3)n]((3-n)+) was detected in the two lowest pH blood cell samples. About 5% of sulfur was distributed as mono- or dibenzothiophene or ethylene-epi-sulfide, or as a thiadiazole reminiscent of the polycarpathiamines. Blood plasma was dominated by sulfate (83%), but with 15% of an alkylsulfate ester and about 2% of low-valent sulfur. Gravimetric analysis of soluble sulfate yielded average concentrations of blood cell sulfur. Average [cysteine] and [cystine] (ranging ~10-30 mM and ~20-90 mM, respectively) implied blood-cell cytosolic emf values of approximately -0.20 V. High cellular [cysteine] is consistent with the proposed model for enzymatic reduction of vanadate by endogenous thiol, wherein the trajectory of metal site-symmetry is controlled and directed through to a thermodynamically favored 7-coordinate V(III) product.

Recent advances in deep-sea natural products

Review of deep-sea natural products covering the five-year period 2009–2013.