Chemical mediation as a structuring element in marine gastropod predator-prey interactions.

Covering: up to 2017Chemical mediation regulates behavioral interactions between species and thus affects population structure, community organization and ecosystem function. Among marine taxa that have developed chemical mediation strategies, gastropods belong to a diverse group of molluscs found worldwide, including species with a coiled, reduced or absent shell. Most gastropods use natural products to mediate a wide range of behaviors such as defense, prey location or interactions with con- and hetero-geners. Their chemically defended diet, such as cyanobacteria, algae, sponges, bryozoans and tunicates, provides them with a considerable opportunity either as shelter from predators, or as a means to enhance their own chemical defense. In addition to improving their defenses, molluscs also use prey secondary metabolites in complex chemical communication including settlement induction, prey detection and feeding preferences. The assimilation of prey secondary metabolites further provides the opportunity for interactions with conspecifics via diet-derived chemical cues or signals. This review intends to provide an overview on the sequestration, detoxification, and biotransformation of diet-derived natural products, as well as the role of these compounds as chemical mediators in gastropod-prey interactions.

Habitat selection by marine larvae in changing chemical environments.

The replenishment and persistence of marine species is contingent on dispersing larvae locating suitable habitat and surviving to a reproductive stage. Pelagic larvae rely on environmental cues to make behavioural decisions with chemical information being important for habitat selection at settlement. We explored the sensory world of crustaceans and fishes focusing on the impact anthropogenic alterations (ocean acidification, red soil, pesticide) have on conspecific chemical signals used by larvae for habitat selection. Crustacean (Stenopus hispidus) and fish (Chromis viridis) larvae recognized their conspecifics via chemical signals under control conditions. In the presence of acidified water, red soil or pesticide, the ability of larvae to chemically recognize conspecific cues was altered. Our study highlights that recruitment potential on coral reefs may decrease due to anthropogenic stressors. If so, populations of fishes and crustaceans will continue their rapid decline; larval recruitment will not replace and sustain the adult populations on degraded reefs.

Secondary Metabolome Variability and Inducible Chemical Defenses in the Mediterranean Sponge Aplysina cavernicola.

Secondary metabolites play a crucial role in marine invertebrate chemical ecology. Thus, it is of great importance to understand factors regulating their production and sources of variability. This work aimed to study the variability of the bromotyrosine derivatives in the Mediterranean sponge Aplysina cavernicola, and also to better understand how biotic (reproductive state) and abiotic factors (seawater temperature) could partly explain this variability. Results showed that the A. cavernicola reproductive cycle has little effect on the variability of the sponges' secondary metabolism, whereas water temperature has a significant influence on the production level of secondary metabolites. Temporal variability analysis of the sponge methanolic extracts showed that bioactivity variability was related to the presence of the minor secondary metabolite dienone, which accounted for 50 % of the bioactivity observed. Further bioassays coupled to HPLC extract fractionation confirmed that dienone was the only compound from Aplysina alkaloids to display a strong bioactivity. Both dienone production and bioactivity showed a notable increase in October 2008, after a late-summer warming episode, indicating that A. cavernicola might be able to induce chemical changes to cope with environmental stressors.

Structures and antimicrobial activities of pyridoacridine alkaloids isolated from different chromotypes of the ascidian Cystodytes dellechiajei.

Three new pentacyclic alkaloids were isolated from different chromotypes of the western Mediterranean ascidian Cystodytes dellechiajei. The purple color morph collected in Catalonia contained the known compounds kuanoniamine D (1), shermilamine B (2), N-deacetylkuanoniamine D (3), and styelsamine C (4) and a new alkaloid named N-deacetylshermilamine B (5). The green color morph collected in the Balearic Islands contained the known compounds 11-hydroxyascididemin (6) and 8,9-dihydro-11-hydroxyascididemin (7) and two new alkaloids named cystodimine A (8) and cystodimine B (9). The blue color morph collected in Catalonia yielded the known compound ascididemin (10). The structures of all compounds were elucidated on the basis of spectroscopic data, mainly 1D and 2D NMR data. The antimicrobial potential of the pyridoacridine alkaloids isolated from each color morph was evaluated and compared.

Temporal variation in the production of four secondary metabolites in a colonial ascidian.

The ecological and applied importance of ascidian secondary metabolites is well reported, but little is known about temporal variation in the production of these compounds. Here, we examined seasonal variation in the production of the four main pyridoacridine alkaloids (shermilamine B, kuanoniamine D, and their deacetylated forms) of the purple morph of the ascidian Cystodytes sp. Five replicate samples were taken monthly from July 2004 to February 2006, and analyzed by high-performance liquid chromatography. Our results showed a high degree of intercolony variability in secondary metabolite concentration and a lack of statistically significant seasonal variation in the production of the four compounds. Minimum values, however, were consistently recorded over the studied years in late summer, after the reproductive period. These minima may be attributable to sexual exhaustion and seasonally varying biotic interactions or abiotic parameters.

Culture of the marine cyanobacterium, Lyngbya majuscula (Oscillatoriaceae), for bioprocess intensified production of cyclic and linear lipopeptides.

Cyanobacteria are an ancient and diverse group of photosynthetic microorganisms, which inhabit many different and extreme environments. This indicates a high degree of biological adaptation, which has enabled these organisms to thrive and compete effectively in nature. The filamentous cyanobacterium, Lyngbya majuscula, produces several promising antifungal and cytotoxic agents, including laxaphycin A and B and curacin A. Samples of L. majuscula collected from Moorea Island, Tahiti (French Polynesia) and from the Culture Collection of Algae and Protozoa (CCAP 1446/4) were studied and adapted to large scale laboratory culture (5 l). This constitutes a 100-fold scale-up for the culture of this particular strain of L. majuscula. The effect of culture vessel configurations, growth conditions and media compositions on growth of L. majuscula was examined. Using optimised culture conditions, two strains of L. majuscula are currently being evaluated for their production of secondary metabolites. Results will be compared with those obtained from four environmental extracts. Comparisons were made by thin layer chromatography (TLC), high performance liquid chromatography (HPLC) and fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). It was shown that varying the culture conditions under which L. majuscula was grown had the greatest effect on secondary metabolite production, thus providing potential for future bioprocess intensified production.

Structure-activity relationship for bromoindole carbaldehydes: effects on the sea urchin embryo cell cycle.

Natural derivatives of indole-3-carbaldehyde were isolated from the tropical marine ascidian Stomoza murravi. A series of 13 derivatives, three natural and 10 synthetic (brominated and N-methylated), were examined for their effects on cell division of sea urchin eggs. These derivatives were shown to inhibit the first mitotic cycle in a concentration-dependent manner. By comparing the IC50 values with the structure of the various molecules, we were able to determine that bromination increased the cytotoxicity of the compound with a maximum occurring when bromine was added to carbon number 2, while addition of N-methylation was shown to markedly reduce the cytotoxicity of these same compounds brominated at carbon 2 only. Biological activity of this family of compounds has been characterized, via detailed study of addition of the most active derivative, 2,5,6-tribromoindole-3-carbaldehyde, on macromolecule synthesis and cytoskeleton reorganization during the first mitotic cycle of fertilized sea urchin eggs. Fluorescence localization of chromatin and microtubules revealed that 2,5,6-tribromoindole-3-carbaldehyde allowed pronuclei migration and fusion but prevented the condensation of chromatin, nuclear envelope breakdown, and bipolar mitotic spindle assembly, inducing an arrest of sea urchin embryogenesis at the beginning of mitosis. It is postulated here that this phenotype is likely to be due to a strong inhibition of DNA replication and protein synthesis.

Matemone, a new bioactive bromine-containing oxindole alkaloid from the indian ocean sponge Iotrochota purpurea.

A new bioactive bromine-containing oxindole alkaloid, matemone (1), was isolated from the Indian Ocean sponge Iotrochota purpurea, together with the known 6-bromoindole-3-carbaldehyde. The structures were elucidated on the basis of spectral and chemical data. Compound 1 shows mild cytotoxicity against three cancer cell lines and marginal antibacterial activity against Staphyloccocus aureus. DNA intercalant screening demonstrated DNA interactions with 1. Both compounds inhibit division of sea-urchin eggs.

Inhibition of topoisomerase II by the marine alkaloid ascididemin and induction of apoptosis in leukemia cells.

Ascididemin (ASC) is a pentacyclic DNA-intercalating agent isolated from the Mediterranean ascidian Cystodytes dellechiajei. This marine alkaloid exhibits marked cytotoxic activities against a range of tumor cells, but its mechanism of action remains poorly understood. We investigated the effects of ASC on DNA cleavage by human topoisomerases I and II. Relaxation assays using supercoiled DNA showed that ASC stimulated double-stranded cleavage of DNA by topoisomerase II, but exerted only a very weak effect on topoisomerase I. ASC is a conventional topoisomerase II poison that significantly promoted DNA cleavage, essentially at sites having a C on the 3' side of the cleaved bond (-1 position), as observed with etoposide. The stimulation of DNA cleavage by topoisomerase I in the presence of ASC was considerably weaker than that observed with camptothecin. Cytotoxicity measurements showed that ASC was even less toxic to P388 leukemia cells than to P388CPT5 cells resistant to camptothecin. In addition, the marine alkaloid was found to be equally toxic to HL-60 leukemia cells sensitive or resistant to mitoxantrone. It is therefore unlikely that topoisomerases are the main cellular targets for ASC. This alkaloid was found to strongly induce apoptosis in HL-60 and P388 leukemia cells. Cell cycle analysis showed that ASC treatment was associated with a loss of cells in the G1 phase accompanied with a large increase in the sub-G1 region. Cleavage experiments with poly(ADP-ribose) polymerase (PARP) revealed that caspase-3 was a mediator of the apoptotic pathway induced by ASC. The DNA of ASC-treated cells was severely fragmented. Collectively, these findings indicate that ASC is a potent inducer of apoptosis in leukemia cells.

Multiwavelength videomicrofluorometric study of cytotoxic properties of a marine peptide, didemnin B, using adriamycin as reference compound.

Didemnin B (DB), a marine natural product, has very encouraging biological activity in vitro (Antineoplastic, immunosuppressive, antiviral). To learn more about its intracellular effects and targets, videomicrofluorometry on single living cells and a protocol of multiple labeling: Hoechst 342 for nuclear DNA, Rhodamine 123 for mitochondria and Nile Red for plasma membrane, have been used. DB behaves differently from Adriamycin, inducing at its IC50 dose of (20 nM) an accumulation of the CEM-WT lymphoblasts in the S phase of the cell cycle while we observed a 50% decrease of the mitochondrial labeling by R123, showing a decrease of the mitochondrial energetic state. Cytostatic dose of DB (250 nM) confirms these observations. However the treatment with a dose reported as apoptotic (1000 nM) induces a much faster effect (corresponding to that of 72 hours at the IC50 dose), 24 hours incubation induced a drastic decrease of nuclear DNA content as well as of the mitochondria energetic state. The evolution of NAD(P)H cellular content exhibited an increase that seems to indicate that the decrease of mitochondrial energetic state was dependent on inhibition of the mitochondrial activity due to an effect of DB at the mitochondrial level, either direct or mediated. Furthermore, the decrease of mitochondrial labeling appears as a very early event in the mechanisms leading to apoptosis.

Binding to DNA and cytotoxic evaluation of ascididemin, the major alkaloid from the Mediterranean ascidian Cystodytes dellechiajei.

The isolation of ascididemin from the Mediterranean ascidian Cystodytes dellechiajei is described. This alkaloid consists of a planar pentacyclic chromophore which was investigated for its DNA-binding and cytotoxic properties. Spectroscopic measurements provided evidence that the drug intercalates into DNA. DNase I footprinting assays indicated that the binding of ascididemin to GC-rich sequences is favoured over binding to AT-rich and mixed sequences. Chemical probes were used to detect ligand-induced structural changes in DNA. The alkaloid induces a hyper-reactivity of the DNA towards potassium permanganate, but not towards diethylpyrocarbonate, just as is the case with ethidium bromide; it has little effect on the catalytic activities of topoisomerases I and II. Ascididemin exhibits marked cytotoxicity towards human leukaemic cells in vitro and appears to be practically equally toxic for drug-sensitive and multidrug-resistant cell lines. The results suggest that DNA, but not topoisomerases, may represent the critical cellular target at which this marine alkaloid exhibits its potent cytotoxic properties in vitro.

(S)-13-hydroxygeranylgeraniol-derived furanoditerpenes from Bifurcaria bifurcata.

Two new diterpenes were isolated from a sample of the brown alga Bifurcaria bifurcata collected from Brittany on the Atlantic coast and their structures established by spectroscopic methods. The new diterpenes are derived from (S)-13-hydroxygeranylgeraniol by terminal cyclization and oxidation leading to a furan-3-yl ring or a beta,gamma-epoxy-gamma-lactone. One of them showed a cytotoxic effect to fertilized sea urchin eggs. The chemical shifts of the methyl groups and quaternary carbons in (S)-13-hydroxygeranylgeraniol have been revised to take account of the results obtained in a 2D NMR long-range C-H correlation experiment and the absolute configuration at C-13 determined for the first time. The geographical variation in the diterpenoid composition of B. bifurcata was also studied.

Eudistomin U and isoeudistomin U, new alkaloids from the Caribbean ascidian Lissoclinum fragile.

Chemical investigations of the Caribbean ascidian Lissoclinum fragile (Didemnidae) have resulted in the isolation of new alkaloids which we have designated eudistomin U [1] and isoeudistomin U [2]. Their structures were determined by spectrometric and chemical means.

Novel meroterpenoids from Cystoseira mediterranea: use of the crown-gall bioassay as a primary screen for lipophilic antineoplastic agents.

Using a slight modification of the crown-gall potato disc bioassay, we were able to apply this test for two previously described antineoplastic lipophilic metabolites, didemnin B and mediterraneol A [1], and to use it as a guide for chromatographic separations of meroterpenoids from Cystoseira mediterranea. An active compound, mediterraneone [3], was isolated, and its structure was found to be a novel norsesquiterpenoid by chemical and spectral methods.