Recent progress in neuroactive marine natural products

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.

Chemistry and bioactivities of alkaloids isolated from marine fungi (covering 2016-2022)

The ocean is a treasure house with rich resources of new chemical and biological molecules. A growing body of evidence suggests that marine fungi represent a huge and largely untapped resource of natural products that have been optimized by evolution for biological and ecological relevance. Alkaloids, the important components of natural products, have attracted much attention from medicinal and natural product chemists due to their unique structures and biological potential. The number and variety of alkaloids from marine fungi discovered in recent years maintain an upward trend. This review would give a systematic overview of the structures and bioactivities of marine fungal alkaloids obtained over the past six years and inspire the development of novel pharmaceutical agents.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

Benefits under the Sea: The Role of Marine Compounds in Neurodegenerative Disorders

Marine habitats offer a rich reservoir of new bioactive compounds with great pharmaceutical potential; the variety of these molecules is unique, and its production is favored by the chemical and physical conditions of the sea. It is known that marine organisms can synthesize bioactive molecules to survive from atypical environmental conditions, such as oxidative stress, photodynamic damage, and extreme temperature. Recent evidence proposed a beneficial role of these compounds for human health. In particular, xanthines, bryostatin, and 11-dehydrosinulariolide displayed encouraging neuroprotective effects in neurodegenerative disorders. This review will focus on the most promising marine drugs' neuroprotective potential for neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases. We will describe these marine compounds' potential as adjuvant therapies for neurodegenerative diseases, based on their antioxidant, anti-inflammatory, and anti-apoptotic properties.

Biologically active compounds from marine organisms in the strategies for combating coronaviruses

Despite the progress made in immunization and drug development, so far there are no prophylactic vaccines and effective therapies for many viral infections, including infections caused by coronaviruses. In this regard, the search for new antiviral substances continues to be relevant, and the enormous potential of marine resources are a stimulus for the study of marine compounds with antiviral activity in experiments and clinical trials. The highly pathogenic human coronaviruses-severe acute respiratory syndrome-related coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) remain a serious threat to human health. In this review, the authors hope to bring the attention of researchers to the use of biologically active substances of marine origin as potential broad-spectrum antiviral agents targeting common cellular pathways and various stages of the life cycle of different viruses, including coronaviruses. The review has been compiled using references from major databases such as Web of Science, PubMed, Scopus, Elsevier, Springer and Google Scholar (up to June 2020) and keywords such as 'coronaviruses', 'marine organisms', 'biologically active substances', 'antiviral drugs', 'SARS-CoV', 'MERS-CoV', 'SARS-CoV-2', '3CLpro', 'TMPRSS2', 'ACE2'. After obtaining all reports from the databases, the papers were carefully analysed in order to find data related to the topic of this review (98 references). Biologically active substances of marine origin, such as flavonoids, phlorotannins, alkaloids, terpenoids, peptides, lectins, polysaccharides, lipids and others substances, can affect coronaviruses at the stages of penetration and entry of the viral particle into the cell, replication of the viral nucleic acid and release of the virion from the cell; they also can act on the host's cellular targets. These natural compounds could be a vital resource in the fight against coronaviruses.

Diverse Aromatic Metabolites in the Solitary Tunicate Cnemidocarpa irene

Fourteen aromatic metabolites (6-19) were isolated from an aqueous extract of the solitary tunicate Cnemidocarpa irene collected in Hokkaido, Japan. The structures of the metabolites were determined based on the spectroscopic interpretations, including one- and two-dimensional NMR, mass spectra, UV, and circular dichroism data. The biopterin analogue 10 modulated the behavior of mice after intracerebroventricular injection and showed a weak affinity to ionotropic glutamate receptor subtypes. Analyses of fluorescent coelomic fluid of the tunicate revealed that pterin 12 was responsible for the fluorescence of the blood cells, while β-carbolines 1 and 3 were fluorescent compounds in the serum. The metabolic profiles in adults, juveniles, larvae, and eggs of the animal differed substantially, suggesting that the metabolism of the animal, especially biosynthesis of aromatic secondary metabolites, changes over different life stages.

Bioactive molecules from protists: Perspectives in biotechnology

For hundreds of years, mankind has benefited from the natural metabolic processes of microorganisms to obtain basic products such as fermented foods and alcoholic beverages. More recently, microorganisms have been exploited for the production of antibiotics, vitamins and enzymes to be used in medicine and chemical industries. Additionally, several modern drugs, including those for cancer therapy, are natural products or their derivatives. Protists are a still underexplored source of natural products potentially of interest for biotechnological and biomedical applications. This paper focuses on some examples of bioactive molecules from protists and associated bacteria and their possible use in biotechnology.

Palladium-catalysed dearomative aryl/cycloimidoylation of indoles

The first example of a dearomative palladium-catalysed isocyanide insertion reaction has been developed using functionalized isocyanides as the reaction partner of N-(2-bromobenzoyl)indoles. The imidoyl-palladium intermediate generated by tandem indole double bond/isocyanide insertion reactions could be trapped by intramolecular functional groups such as the C(sp2)-H bond and alkenes, affording diversified indoline derivatives bearing C3 imine-containing heterocycles. The dearomative aryl/cycloimidoylation of indoles proceeded smoothly in good to excellent yields with a wide functional group tolerance.

Electrochemically Enabled C3-Formylation and -Acylation of Indoles with Aldehydes

Reported herein is an effective strategy for oxidative cross-coupling of indoles with various aldehydes. The strategy is based on a two-step transformation via a well-known Mannich-type reaction and a C-N bond cleavage for carbonyl introduction. The key step-the C-N bond cleavage of the Mannich product-was enabled by electrochemistry. This strategy (with over 40 examples) ensures excellent functional-group tolerance as well as late-stage functionalization of pharmaceutical molecules.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Susceptibility of erythrocytes from different sources to xenobiotics-induced lysis

As erythrocytes are continuously exposed to plenty of metabolites and toxicants, the aim of the present work is to show whether erythrocytes from different sources may exhibit different susceptibility to hemolysis induced by two classes of xenobiotics, Cnidaria venoms, acting via pore forming mechanism on cell membrane, and Cd²⁺, inducing cell damage mainly via lipid peroxidation. To this end, the hemolytic power of crude venom from stinging cells of the anthozoan Aiptasia mutabilis and from the scyphozoan Pelagia noctiluca has been tested, along with Cd²⁺ (5 to 20 mM), on erythrocytes (0.05% v/v) obtained from either rabbit, or dog, or chicken, or human source. Our results show a more significant susceptibility of rabbit erythrocytes to hemolysis induced by both crude venom and Cd²⁺ than erythrocytes from other sources. This difference seems to rely neither on the different specimens used to extract crude venom, nor on the different mechanism of toxicants. In this light, the present study may contribute: i) to confirm hemolytic test as a suitable biological assay to verify erythrocytes resistance to toxicants; ii) to show variability in hemolytic response to xenobiotics; iii) to propose rabbit erythrocytes as more sensitive to the lytic action of xenobiotics, adding more knowledge to the field of toxicology.

Soritesidine, a Novel Proteinous Toxin from the Okinawan Marine Sponge Spongosorites sp

A novel protein, soritesidine (SOR) with potent toxicity was isolated from the marine sponge Spongosorites sp. SOR exhibited wide range of toxicities over various organisms and cells including brine shrimp (Artemia salina) larvae, sea hare (Aplysia kurodai) eggs, mice, and cultured mammalian cells. Toxicities of SOR were extraordinary potent. It killed mice at 5 ng/mouse after intracerebroventricular (i.c.v.) injection, and brine shrimp and at 0.34 µg/mL. Cytotoxicity for cultured mammalian cancer cell lines against HeLa and L1210 cells were determined to be 0.062 and 12.11 ng/mL, respectively. The SOR-containing fraction cleaved plasmid DNA in a metal ion dependent manner showing genotoxicity of SOR. Purified SOR exhibited molecular weight of 108.7 kDa in MALDI-TOF MS data and isoelectric point of approximately 4.5. N-terminal amino acid sequence up to the 25th residue was determined by Edman degradation. Internal amino acid sequences for fifteen peptides isolated from the enzyme digest of SOR were also determined. None of those amino acid sequences showed similarity to existing proteins, suggesting that SOR is a new proteinous toxin.

Drug Discovery on Natural Products: From Ion Channels to nAChRs, from Nature to Libraries, from Analytics to Assays

Natural extracts are complex mixtures that may be rich in useful bioactive compounds and therefore are attractive sources for new leads in drug discovery. This review describes drug discovery from natural products and in explaining this process puts the focus on ion-channel drug discovery. In particular, the identification of bioactives from natural products targeting nicotinic acetylcholine receptors (nAChRs) and serotonin type 3 receptors (5-HT3Rs) is discussed. The review is divided into three parts: "Targets," "Sources," and "Approaches." The "Targets" part will discuss the importance of ion-channel drug targets in general, and the α7-nAChR and 5-HT3Rs in particular. The "Sources" part will discuss the relevance for drug discovery of finding bioactive compounds from various natural sources such as venoms and plant extracts. The "Approaches" part will give an overview of classical and new analytical approaches that are used for the identification of new bioactive compounds with the focus on targeting ion channels. In addition, a selected overview is given of traditional venom-based drug discovery approaches and of diverse hyphenated analytical systems used for screening complex bioactive mixtures including venoms.

Recent Status and Advancements in the Development of Antifungal Agents: Highlights on Plant and Marine Based Antifungals

The developing resistance in fungi has become a key challenge, which is being faced nowadays with the available antifungal agents in the market. Further search for novel compounds from different sources has been explored to meet this problem. The current review describes and highlights recent advancement in the antifungal drug aspects from plant and marine based sources. The current available antifungal agents act on specific targets on the fungal cell wall, like ergosterol synthesis, chitin biosynthesis, sphingolipid synthesis, glucan synthesis etc. We discuss some of the important anti-fungal agents like azole, polyene and allylamine classes that inhibit the ergosterol biosynthesis. Echinocandins inhibit β-1, 3 glucan synthesis in the fungal cell wall. The antifungals poloxins and nikkomycins inhibit fungal cell wall component chitin. Apart from these classes of drugs, several combinatorial therapies have been carried out to treat diseases due to fungal resistance. Recently, many antifungal agents derived from plant and marine sources showed potent activity. The renewed interest in plant and marine derived compounds for the fungal diseases created a new way to treat these resistant strains which are evident from the numerous literature publications in the recent years. Moreover, the compounds derived from both plant and marine sources showed promising results against fungal diseases. Altogether, this review article discusses the current antifungal agents and highlights the plant and marine based compounds as a potential promising antifungal agents.

Total Synthesis of Pulmonarin B and Design of Brominated Phenylacetic Acid/Tacrine Hybrids: Marine Pharmacophore Inspired Discovery of New ChE and Aβ Aggregation Inhibitors

A marine natural product, pulmonarin B (1), and a series of related tacrine hybrid analogues were synthesized and evaluated as cholinesterase (ChE) inhibitors. The in vitro ChE assay results revealed that 1 showed moderate dual acetylcholinesterase (AChE)/ butyrylcholinesterase (BChE) inhibitory activity, while the hybrid 12j proved to be the most potent dual inhibitor among the designed derivatives, being almost as active as tacrine. Molecular modeling studies together with kinetic analysis suggested that 12j interacted with both the catalytic active site and peripheral anionic site of AChE. Compounds 1 and 12j could also inhibit self-induced and AChE-induced Aβ aggregation. In addition, the cell-based assay against the human hepatoma cell line (HepG2) revealed that 1 and 12j did not show significant hepatotoxicity compared with tacrine and donepezil. Taken together, the present study confirmed that compound 1 was a potential anti-Alzheimer's disease (AD) hit, and 12j could be highlighted as a multifunctional lead compound for anti-AD drug development.

Zebrafish-Based Discovery of Antiseizure Compounds from the Red Sea: Pseurotin A2 and Azaspirofuran A

In search for novel antiseizure drugs (ASDs), the European FP7-funded PharmaSea project used zebrafish embryos and larvae as a drug discovery platform to screen marine natural products to identify promising antiseizure hits in vivo for further development. Within the framework of this project, seven known heterospirocyclic γ-lactams, namely, pseurotin A, pseurotin A₂, pseurotin F1, 11- O-methylpseurotin A, pseurotin D, azaspirofuran A, and azaspirofuran B, were isolated from the bioactive marine fungus Aspergillus fumigatus, and their antiseizure activity was evaluated in the larval zebrafish pentylenetetrazole (PTZ) seizure model. Pseurotin A₂ and azaspirofuran A were identified as antiseizure hits, while their close chemical analogues were inactive. Besides, electrophysiological analysis from the zebrafish midbrain demonstrated that pseurotin A₂ and azaspirofuran A also ameliorate PTZ-induced epileptiform discharges. Next, to determine whether these findings translate to mammalians, both compounds were analyzed in the mouse 6 Hz (44 mA) psychomotor seizure model. They lowered the seizure duration dose-dependently, thereby confirming their antiseizure properties and suggesting activity against drug-resistant seizures. Finally, in a thorough ADMET assessment, pseurotin A₂ and azaspirofuran A were found to be drug-like. Based on the prominent antiseizure activity in both species and the drug-likeness, we propose pseurotin A₂ and azaspirofuran A as lead compounds that are worth further investigation for the treatment of epileptic seizures. This study not only provides the first evidence of antiseizure activity of pseurotins and azaspirofurans, but also demonstrates the value of the zebrafish model in (marine) natural product drug discovery in general, and for ASD discovery in particular.

Neuronal Nicotinic Acetylcholine Receptor Modulators from Cone Snails

Marine cone snails are a large family of gastropods that have evolved highly potent venoms for predation and defense. The cone snail venom has exceptional molecular diversity in neuropharmacologically active compounds, targeting a range of receptors, ion channels, and transporters. These conotoxins have helped to dissect the structure and function of many of these therapeutically significant targets in the central and peripheral nervous systems, as well as unravelling the complex cellular mechanisms modulated by these receptors and ion channels. This review provides an overview of α-conotoxins targeting neuronal nicotinic acetylcholine receptors. The structure and activity of both classical and non-classical α-conotoxins are discussed, along with their contributions towards understanding nicotinic acetylcholine receptor (nAChR) structure and function.

Sterols from the Green Alga Ulva australis

Three new sterols, (24R)-5,28-stigmastadiene-3β,24-diol-7-one (1), (24S)-5,28-stigmastadiene-3β,24-diol-7-one (2), and 24R and 24S-vinylcholesta-3β,5α,6β,24-tetraol (3), together with three known sterols (4–6) were isolated from the green alga Ulva australis. The structures of the new compounds (1–3) were elucidated through 1D and 2D nuclear magnetic resonance spectroscopy as well as mass spectrometry. Compounds 4–6 were identified as isofucoterol (4), 24R,28S and 24S,28R-epoxy-24-ethylcholesterol (5), and (24S)-stigmastadiene-3β,24-diol (6) on the basis of spectroscopic data analyses and comparison with those reported in the literature. Compounds 4–6 were isolated from U. australis for the first time. These compounds, together with the previously isolated secondary metabolites of this alga, were investigated for their inhibitory effects on human recombinant aldose reductase in vitro. Of the compounds, 24R,28S and 24S,28R-epoxy-24-ethylcholesterol (5), 1-O-palmitoyl-3-O-(6′-sulfo-α-d-quinovopyranosyl) glycerol, (2S)-1-O-palmitoyl-3-O-[α-d-galactopyranosyl(1→2)β-d-galactopyranosyl] glycerol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, and 8-hydroxy-(6E)-octenoic acid weakly inhibited the enzyme, while the three new sterols, 1–3, were almost inactive.

Immuno-Modulatory and Anti-Inflammatory Effects of Dihydrogracilin A, a Terpene Derived from the Marine Sponge Dendrilla membranosa

We assessed the immunomodulatory and anti-inflammatory effects of 9,11-dihydrogracilin A (DHG), a molecule derived from the Antarctic marine sponge Dendrilla membranosa. We used in vitro and in vivo approaches to establish DHG properties. Human peripheral blood mononuclear cells (PBMC) and human keratinocytes cell line (HaCaT cells) were used as in vitro system, whereas a model of murine cutaneous irritation was adopted for in vivo studies. We observed that DHG reduces dose dependently the proliferative response and viability of mitogen stimulated PBMC. In addition, DHG induces apoptosis as revealed by AnnexinV staining and downregulates the phosphorylation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), signal transducer and activator of transcription (STAT) and extracellular signal–regulated kinase (ERK) at late time points. These effects were accompanied by down-regulation of interleukin 6 (IL-6) production, slight decrease of IL-10 and no inhibition of tumor necrosis factor-alpha (TNF-α) secretion. To assess potential properties of DHG in epidermal inflammation we used HaCaT cells; this compound reduces cell growth, viability and migration. Finally, we adopted for the in vivo study the croton oil-induced ear dermatitis murine model of inflammation. Of note, topical use of DHG significantly decreased mouse ear edema. These results suggest that DHG exerts anti-inflammatory effects and its anti-edema activity in vivo strongly supports its potential therapeutic application in inflammatory cutaneous diseases.

The Marine Guanidine Alkaloid Crambescidin 816 Induces Calcium Influx and Cytotoxicity in Primary Cultures of Cortical Neurons through Glutamate Receptors

Crambescidin 816 is a guanidine alkaloid produced by the sponge Crambe crambe with known antitumoral activity. While the information describing the effects of this alkaloid in central neurons is scarce, Cramb816 is known to block voltage dependent calcium channels being selective for L-type channels. Moreover, Cramb816 reduced neuronal viability through an unknown mechanism. Here, we aimed to describe the toxic activity of Cramb816 in cortical neurons. Since calcium influx is considered the main mechanism responsible for neuronal cell death, the effects of Cramb816 in the cytosolic calcium concentration of cortical neurons were studied. The alkaloid decreased neuronal viability and induced a dose-dependent increase in cytosolic calcium that was also related to the presence of calcium in the extracellular media. The increase in calcium influx was age dependent, being higher in younger neurons. Moreover, this effect was prevented by glutamate receptor antagonists, which did not fully block the cytotoxic effect of Cramb816 after 24 h of treatment but completely prevented Cramb816 cytotoxicity after 10 min exposure. Therefore, the findings presented herein provide new insights into the cytotoxic effect of Cramb816 in cortical neurons.

N-Methyl-β-carbolinium Salts and an N-Methylated 8-Oxoisoguanine as Acetylcholinesterase Inhibitors from a Solitary Ascidian, Cnemidocarpa irene

New brominated β-carbolines irenecarbolines A (1) and B (4) along with known β-carbolines 2 and 3 and a new 8-oxoisoguanine derivative, 5, were isolated from a solitary ascidian, Cnemidocarpa irene. The structures of these compounds were determined on the basis of their spectral data. All, except for 3, inhibited the action of acetylcholinesterase (AchE). The activities of 1 and 5 were comparable to those of galantamine, a clinically used AchE inhibitor. Compounds 1 and 2 were found to be present in high concentrations in blood, and fluorescence was observed in certain types of cells found in the blood of the tunicate.

Identification of the First Marine-Derived Opioid Receptor "Balanced" Agonist with a Signaling Profile That Resembles the Endorphins

Opioid therapeutics are excellent analgesics, whose utility is compromised by dependence. Morphine (1) and its clinically relevant derivatives such as OxyContin (2), Vicodin (3), and Dilaudid (4) are "biased" agonists at the μ opioid receptor (OR), wherein they engage G protein signaling but poorly engage β-arrestin and the endocytic machinery. In contrast, endorphins, the endogenous peptide agonists for ORs, are potent analgesics, show reduced liability for tolerance and dependence, and engage both G protein and β-arrestin pathways as "balanced" agonists. We set out to determine if marine-derived alkaloids could serve as novel OR agonist chemotypes with a signaling profile distinct from morphine and more similar to the endorphins. Screening of 96 sponge-derived extracts followed by LC-MS-based purification to pinpoint the active compounds and subsequent evaluation of a mini library of related alkaloids identified two structural classes that modulate the ORs. These included the following: aaptamine (10), 9-demethyl aaptamine (11), demethyl (oxy)-aaptamine (12) with activity at the δ-OR (EC₅₀: 5.1, 4.1, 2.3 μM, respectively) and fascaplysin (17), and 10-bromo fascaplysin (18) with activity at the μ-OR (EC₅₀: 6.3, 4.2 μM respectively). An in vivo evaluation of 10 using δ-KO mice indicated its previously reported antidepressant-like effects are dependent on the δ-OR. Importantly, 17 functioned as a balanced agonist promoting both G protein signaling and β-arrestin recruitment along with receptor endocytosis similar to the endorphins. Collectively these results demonstrate the burgeoning potential for marine natural products to serve as novel lead compounds for therapeutic targets in neuroscience research.

Antifungal potential of marine natural products

Fungal diseases represent an increasing threat to human health worldwide which in some cases might be associated with substantial morbidity and mortality. However, only few antifungal drugs are currently available for the treatment of life-threatening fungal infections. Furthermore, plant diseases caused by fungal pathogens represent a worldwide economic problem for the agriculture industry. The marine environment continues to provide structurally diverse and biologically active secondary metabolites, several of which have inspired the development of new classes of therapeutic agents. Among these secondary metabolites, several compounds with noteworthy antifungal activities have been isolated from marine microorganisms, invertebrates, and algae. During the last fifteen years, around 65% of marine natural products possessing antifungal activities have been isolated from sponges and bacteria. This review gives an overview of natural products from diverse marine organisms that have shown in vitro and/or in vivo potential as antifungal agents, with their mechanism of action whenever applicable. The natural products literature is covered from January 2000 until June 2015, and we are reporting the chemical structures together with their biological activities, as well as the isolation source.

Marine AChE inhibitors isolated from Geodia barretti: natural compounds and their synthetic analogs

Barettin, 8,9-dihydrobarettin, bromoconicamin and a novel brominated marine indole were isolated from the boreal sponge Geodia barretti collected off the Norwegian coast. The compounds were evaluated as inhibitors of electric eel acetylcholinesterase. Barettin and 8,9-dihydrobarettin displayed significant inhibition of the enzyme, with inhibition constants (Ki) of 29 and 19 μM respectively towards acetylcholinesterase via a reversible noncompetitive mechanism. These activities are comparable to those of several other natural acetylcholinesterase inhibitors of marine origin. Bromoconicamin was less potent against acetylcholinesterase, and the novel compound was inactive. Based on the inhibitory activity, a library of 22 simplified synthetic analogs was designed and prepared to probe the role of the brominated indole, common to all the isolated compounds. From the structure-activity investigation it was shown that the brominated indole motif is not sufficient to generate a high acetylcholinesterase inhibitory activity, even when combined with natural cationic ligands for the acetylcholinesterase active site. The four natural compounds were also analysed for their butyrylcholinesterase inhibitory activity in addition and shown to display comparable activities. The study illustrates how both barettin and 8,9-dihydrobarettin display additional bioactivities which may help to explain their biological role in the producing organism. The findings also provide new insights into the structure-activity relationship of both natural and synthetic acetylcholinesterase inhibitors.

Dimethylsulfoniopropionate Promotes Process Outgrowth in Neural Cells and Exerts Protective Effects against Tropodithietic Acid

The marine environment harbors a plethora of bioactive substances, including drug candidates of potential value in the field of neuroscience. The present study was undertaken to investigate the effects of dimethylsulfoniopropionate (DMSP), produced by several algae, corals and higher plants, on cells of the mammalian nervous system, i.e., neuronal N2a and OLN-93 cells as model system for nerve cells and glia, respectively. Additionally, the protective capabilities of DMSP were assessed in cells treated with tropodithietic acid (TDA), a marine metabolite produced by several Roseobacter clade bacteria. Both cell lines, N2a and OLN-93, have previously been shown to be a sensitive target for the action of TDA, and cytotoxic effects of TDA have been connected to the induction of oxidative stress. Our data shows that DMSP promotes process outgrowth and microtubule reorganization and bundling, accompanied by an increase in alpha-tubulin acetylation. Furthermore, DMSP was able to prevent the cytotoxic effects exerted by TDA, including the breakdown of the mitochondrial membrane potential, upregulation of heat shock protein Hsp32 and activation of the extracellular signal-regulated kinases 1/2 (ERK1/2). Our study points to the conclusion that DMSP provides an antioxidant defense, not only in algae but also in mammalian neural cells.

Contemporary Strategies for the Synthesis of Tetrahydropyran Derivatives: Application to Total Synthesis of Neopeltolide, a Marine Macrolide Natural Product

Tetrahydropyrans are structural motifs that are abundantly present in a range of biologically important marine natural products. As such, significant efforts have been paid to the development of efficient and versatile methods for the synthesis of tetrahydropyran derivatives. Neopeltolide, a potent antiproliferative marine natural product, has been an attractive target compound for synthetic chemists because of its complex structure comprised of a 14-membered macrolactone embedded with a tetrahydropyran ring, and twenty total and formal syntheses of this natural product have been reported so far. This review summarizes the total and formal syntheses of neopeltolide and its analogues, highlighting the synthetic strategies exploited for constructing the tetrahydropyran ring.

Synthetic analogs of stryphnusin isolated from the marine sponge Stryphnus fortis inhibit acetylcholinesterase with no effect on muscle function or neuromuscular transmission

The current study describes the AChE inhibitory effect of natural marine neuroactive compounds.

The chemistry and biology of guanidine natural products

The present review discusses the isolation, structure determination, synthesis, biosynthesis and biological activities of secondary metabolites bearing a guanidine group.

Cytotoxic Effects of Tropodithietic Acid on Mammalian Clonal Cell Lines of Neuronal and Glial Origin

The marine metabolite tropodithietic acid (TDA), produced by several Roseobacter clade bacteria, is known for its broad antimicrobial activity. TDA is of interest not only as a probiotic in aquaculture, but also because it might be of use as an antibacterial agent in non-marine or non-aquatic environments, and thus the potentially cytotoxic influences on eukaryotic cells need to be evaluated. The present study was undertaken to investigate its effects on cells of the mammalian nervous system, i.e., neuronal N2a cells and OLN-93 cells as model systems for nerve cells and glia. The data show that in both cell lines TDA exerted morphological changes and cytotoxic effects at a concentration of 0.3–0.5 µg/mL (1.4–2.4 µM). Furthermore, TDA caused a breakdown of the mitochondrial membrane potential, the activation of extracellular signal-regulated kinases ERK1/2, and the induction of the small heat shock protein HSP32/HO-1, which is considered as a sensor of oxidative stress. The cytotoxic effects were accompanied by an increase in intracellular Ca²⁺-levels, the disturbance of the microtubule network, and the reorganization of the microfilament system. Hence, mammalian cells are a sensitive target for the action of TDA and react by the activation of a stress response resulting in cell death.

Tetrodotoxin, an Extremely Potent Marine Neurotoxin: Distribution, Toxicity, Origin and Therapeutical Uses

Tetrodotoxin (TTX) is a potent neurotoxin responsible for many human intoxications and fatalities each year. The origin of TTX is unknown, but in the pufferfish, it seems to be produced by endosymbiotic bacteria that often seem to be passed down the food chain. The ingestion of contaminated pufferfish, considered the most delicious fish in Japan, is the usual route of toxicity. This neurotoxin, reported as a threat to human health in Asian countries, has spread to the Pacific and Mediterranean, due to the increase of temperature waters worldwide. TTX, for which there is no known antidote, inhibits sodium channel producing heart failure in many cases and consequently death. In Japan, a regulatory limit of 2 mg eq TTX/kg was established, although the restaurant preparation of “fugu” is strictly controlled by law and only chefs qualified are allowed to prepare the fish. Due to its paralysis effect, this neurotoxin could be used in the medical field as an analgesic to treat some cancer pains.

Synthesis and biology of cyclic imine toxins, an emerging class of potent, globally distributed marine toxins

From a small group of exotic compounds isolated only two decades ago, Cyclic Imine (CI) toxins have become a major class of marine toxins with global distribution. Their distinct chemical structure, biological mechanism of action, and intricate chemistry ensures that CI toxins will continue to be the subject of fascinating fundamental studies in the broad fields of chemistry, chemical biology, and toxicology. The worldwide occurrence of potent CI toxins in marine environments, their accumulation in shellfish, and chemical stability are important considerations in assessing risk factors for human health. This review article aims to provide an account of chemistry, biology, and toxicology of CI toxins from their discovery to the present day.

Bioactive marine drugs and marine biomaterials for brain diseases

Marine invertebrates produce a plethora of bioactive compounds, which serve as inspiration for marine biotechnology, particularly in drug discovery programs and biomaterials development. This review aims to summarize the potential of drugs derived from marine invertebrates in the field of neuroscience. Therefore, some examples of neuroprotective drugs and neurotoxins will be discussed. Their role in neuroscience research and development of new therapies targeting the central nervous system will be addressed, with particular focus on neuroinflammation and neurodegeneration. In addition, the neuronal growth promoted by marine drugs, as well as the recent advances in neural tissue engineering, will be highlighted.