Effects of okadaic acid on haemocytes from Mytilus galloprovincialis: a comparison between field and laboratory studies

Responses of ABCB and ABCC transporters to the toxic dinoflagellate Prorocentrum lima in the mussel Perna viridis

Bivalve mollusks can accumulate diarrheic shellfish poisoning (DSP) toxins through filter-feeding, but they exhibit some resistance to the toxins. Previous studies have suggested that the ABC transporters may have an important role in the resistance to DSP toxins, but comprehensive studies are lacking. In this study, we comprehensively analyzed the distribution of ABC transporters in the mussel Perna viridis, and observed responses of ABCB and ABCC transporters to the DSP toxins-producing dinoflagellate Prorocentrum lima. Total 39 members of ABC transporters were identified in P. viridis, including 3 full PvABCBs, 3 half PvABCBs, and 7 PvABCCs transporters. We found that PvABCBs and PvABCCs subfamilies were expressed in hemocytes, gills and digestive gland with some difference, especially in hemocytes. After exposure to P. lima, PvABCBs and PvABCCs displayed different expression changes in different tissues. The short-term (3 h) exposure to P. lima induced the transcription of PvABCB1_like1, PvABCB6, PvABCC1, PvABCC1_like and PvABCC1/3, and the longer-term (96 h) exposure increased the transcription of PvABCB1, PvABCB1_like, PvABCB10, PvABCC1 and PvABCC1_like1 in gills and PvABCC10 in digestive gland. These results suggest that different types of PvABCBs and PvABCCs in P. viridis may contribute to the detoxification of DSP toxins in different tissues at different time after exposure to DSP toxins. Our finding provides new evidence for further understanding the role of ABC transporters in the tolerance of mussel to DSP toxins.

Responses of JNK signaling pathway to the toxic dinoflagellate Prorocentrum lima in the mussel Perna viridis

Diarrheic shellfish poisoning (DSP) toxins are widely distributed over the world, causing diarrhea, vomiting, and even tumor in human. However, bivalves, the main carrier of the DSP toxins, have some tolerant mechanisms to DSP toxins, though it remains unclear. In this study, we scrutinized the role of Jun N-terminal kinases (JNK) in tolerance of DSP toxins and the relationship between JNK, apoptosis and nuclear factor E2-related factor/antioxidant response element (Nrf2/ARE) pathways. We found that the phosphorylated level of JNK protein was significantly increased both in hemocytes (6 h) and gills (3 h) of the mussel Perna viridis after short-term exposure to DSP toxins-producing dinoflagellate Prorocentrum lima. Exposure of P. lima induced oxidative stress in mussels. Hemocytes and gills displayed different sensitivities to the cytotoxicity of DSP toxins. Exposure of P. lima activated caspase-3 and induced apoptosis in gills but did not induce caspase-3 and apoptosis in hemocytes. The short-term exposure of P. lima could activate Nrf2/ARE signaling pathway in hemocytes (6 h), while longer-term exposure could induce glutathione reductase (GR) expression in hemocytes (96 h) and glutathione-S-transferases (GST) in gills (96 h). Based on the phylogenetic tree of Nrf2, Nrf2 in P. viridis was closely related to that in other mussels, especially Mytilus coruscus, but far from that in Mus musculus. The most likely phosphorylated site of Nrf2 in the mussels P. viridis is threonine 504 for JNK, which is different from that in M. musculus. Taken all together, the tolerant mechanism of P. viridis to DSP toxins might be involved in JNK and Nrf2/ARE signaling pathways, and JNK play a key role in the mechanism. Our findings provide a new clue to further understand tolerant mechanisms of bivalves to DSP toxins.

In vitro Evaluation of Programmed Cell Death in the Immune System of Pacific Oyster Crassostrea gigas by the Effect of Marine Toxins

Programmed cell death (PCD) is an essential process for the immune system's development and homeostasis, enabling the remotion of infected or unnecessary cells. There are several PCD's types, depending on the molecular mechanisms, such as non-inflammatory or pro-inflammatory. Hemocytes are the main component of cellular immunity in bivalve mollusks. Numerous infectious microorganisms produce toxins that impair hemocytes functions, but there is little knowledge on the role of PCD in these cells. This study aims to evaluate in vitro whether marine toxins induce a particular type of PCD in hemocytes of the bivalve mollusk Crassostrea gigas during 4 h at 25°C. Hemocytes were incubated with two types of marine toxins: non-proteinaceous toxins from microalgae (saxitoxin, STX; gonyautoxins 2 and 3, GTX2/3; okadaic acid/dynophysistoxin-1, OA/DTX-1; brevetoxins 2 and 3, PbTx-2,-3; brevetoxin 2, PbTx-2), and proteinaceous extracts from bacteria (Vibrio parahaemolyticus, Vp; V. campbellii, Vc). Also, we used the apoptosis inducers, staurosporine (STP), and camptothecin (CPT). STP, CPT, STX, and GTX 2/3, provoked high hemocyte mortality characterized by apoptosis hallmarks such as phosphatidylserine translocation into the outer leaflet of the cell membrane, exacerbated chromatin condensation, DNA oligonucleosomal fragments, and variation in gene expression levels of apoptotic caspases 2, 3, 7, and 8. The mixture of PbTx-2,-3 also showed many apoptosis features; however, they did not show apoptotic DNA oligonucleosomal fragments. Likewise, PbTx-2, OA/DTX-1, and proteinaceous extracts from bacteria Vp, and Vc, induced a minor degree of cell death with high gene expression of the pro-inflammatory initiator caspase-1, which could indicate a process of pyroptosis-like PCD. Hemocytes could carry out both PCD types simultaneously. Therefore, marine toxins trigger PCD's signaling pathways in C. gigas hemocytes, depending on the toxin's nature, which appears to be highly conserved both structurally and functionally.

Cultures of Dinophysis sacculus, D. acuminata and pectenotoxin 2 affect gametes and fertilization success of the Pacific oyster, Crassostrea gigas

Harmful algal blooms (HABs) of toxic species of the dinoflagellate genus Dinophysis are a threat to human health as they are mainly responsible for diarrheic shellfish poisoning (DSP) in the consumers of contaminated shellfish. Such contamination leads to shellfish farm closures causing major economic and social issues. The direct effects of numerous HAB species have been demonstrated on adult bivalves, whereas the effects on critical early life stages remain relatively unexplored. The present study aimed to determine the in vitro effects of either cultivated strains of D. sacculus and D. acuminata isolated from France or their associated toxins (i.e. okadaic acid (OA) and pectenotoxin 2 (PTX2)) on the quality of the gametes of the Pacific oyster Crassostrea gigas. This was performed by assessing the ROS production and viability of the gametes using flow cytometry, and fertilization success using microscopic counts. Oocytes were more affected than spermatozoa and their mortality and ROS production increased in the presence of D. sacculus and PTX2, respectively. A decrease in fertilization success was observed at concentrations as low as 0.5 cell mL^-1 of Dinophysis spp. and 5 nM of PTX2, whereas no effect of OA could be observed. The effect on fertilization success was higher when both gamete types were concomitantly exposed compared to separate exposures, suggesting a synergistic effect. Our results also suggest that the effects could be due to cell-to-cell contact. These results highlight a potential effect of Dinophysis spp. and PTX2 on reproduction and recruitment of the Pacific oyster.

Effects of marine harmful algal blooms on bivalve cellular immunity and infectious diseases: A review

Bivalves were long thought to be "symptomless carriers" of marine microalgal toxins to human seafood consumers. In the past three decades, science has come to recognize that harmful algae and their toxins can be harmful to grazers, including bivalves. Indeed, studies have shown conclusively that some microalgal toxins function as active grazing deterrents. When responding to marine Harmful Algal Bloom (HAB) events, bivalves can reject toxic cells to minimize toxin and bioactive extracellular compound (BEC) exposure, or ingest and digest cells, incorporating nutritional components and toxins. Several studies have reported modulation of bivalve hemocyte variables in response to HAB exposure. Hemocytes are specialized cells involved in many functions in bivalves, particularly in immunological defense mechanisms. Hemocytes protect tissues by engulfing or encapsulating living pathogens and repair tissue damage caused by injury, poisoning, and infections through inflammatory processes. The effects of HAB exposure observed on bivalve cellular immune variables have raised the question of possible effects on susceptibility to infectious disease. As science has described a previously unrecognized diversity in microalgal bioactive substances, and also found a growing list of infectious diseases in bivalves, episodic reports of interactions between harmful algae and disease in bivalves have been published. Only recently, studies directed to understand the physiological and metabolic bases of these interactions have been undertaken. This review compiles evidence from studies of harmful algal effects upon bivalve shellfish that establishes a framework for recent efforts to understand how harmful algae can alter infectious disease, and particularly the fundamental role of cellular immunity, in modulating these interactions. Experimental studies reviewed here indicate that HABs can modulate bivalve-pathogen interactions in various ways, either by increasing bivalve susceptibility to disease or conversely by lessening infection proliferation or transmission. Alteration of immune defense and global physiological distress caused by HAB exposure have been the most frequent reasons identified for these effects on disease. Only few studies, however, have addressed these effects so far and a general pattern cannot be established. Other mechanisms are likely involved but are under-studied thus far and will need more attention in the future. In particular, the inhibition of bivalve filtration by HABs and direct interaction between HABs and infectious agents in the seawater likely interfere with pathogen transmission. The study of these interactions in the field and at the population level also are needed to establish the ecological and economical significance of the effects of HABs upon bivalve diseases. A more thorough understanding of these interactions will assist in development of more effective management of bivalve shellfisheries and aquaculture in oceans subjected to increasing HAB and disease pressures.

De novo transcriptome analysis of the mussel Perna viridis after exposure to the toxic dinoflagellate Prorocentrum lima

Diarrheic shellfish poisoning (DSP) toxins are produced by harmful microalgae and accumulate in bivalve mollusks, causing various toxicity. These toxic effects appear to abate with increasing DSP concentration and longer exposure time, however, the underlying mechanisms remain unclear. To explore the underlying molecular mechanisms, de novo transcriptome analysis of the digestive gland of Perna viridis was performed after Prorocentrum lima exposure. RNA-seq analysis showed that 1886 and 237 genes were up- and down-regulated, respectively after 6 h exposure to P. lima, while 265 genes were up-regulated and 217 genes were down-regulated after 96 h compared to the control. These differentially expressed genes mainly involved in Nrf2 signing pathways, immune stress, apoptosis and cytoskeleton, etc. Combined with qPCR results, we speculated that the mussel P. viridis might mainly rely on glutathione S-transferase (GST) and ABC transporters to counteract DSP toxins during short-term exposure. However, longer exposure of P. lima could activate the Nrf2 signaling pathway and inhibitors of apoptosis protein (IAP), which in turn reduced the damage of DSP toxins to the mussel. DSP toxins could induce cytoskeleton destabilization and had some negative impact on the immune system of bivalves. Collectively, our findings uncovered the crucial molecular mechanisms and the regulatory metabolic nodes that underpin the defense mechanism of bivalves against DSP toxins and also advanced our current understanding of bivalve defense mechanisms.

Up-regulation of Nrf2-dependent antioxidant defenses in Perna viridis after exposed to Prorocentrum lima

It is well documented that diarrhetic shellfish poisoning (DSP) toxins have strong genetic toxicity, cytotoxicity and oxidative damage to bivalve species. However, these toxic effects seem to decrease with the extension of exposure time and the increment of the toxin concentration, the mechanism involved remained unclear, though. In this paper, we found that expression of the genes related to cytoskeleton and Nrf2 signaling pathway displayed different changes over time in the gill of Perna viridis after exposure to DSP toxins-producing microalga Prorocentrum lima. During the short-term exposure (3 h and 6 h), KEAP1 gene expression was significantly up-regulated, coupled with up-regulation of MRP, ABCB1 and CAT transcriptions and down-regulation of GPx1 and NQO1 mRNA. After longer exposure to high density of P. lima, Nrf2 was significantly up-regulated, accompanied with up-regulation of Nrf2 pathway related genes such as NQO1, SOD, GST-ω and ABCB1, whereas KEAP1 was down-regulated. TUBA1C and TUBB1 transcripts were significantly down-regulated after short-term exposure of P. lima, but both of them were up-regulated at 96 h after exposure to high density of P. lima. Paraffin section demonstrated that P. lima had a strong damage on the gill of mussels during the short-term exposure. However, the negative effect to the gill decreased, and the gill restored after longer exposure (96 h). Taking together, we proposed that P. lima had a negative impact on cytoskeleton of mussel gill tissue, could cause oxidative damage to the gills. However, longer exposure of P. lima in high density could activate Nrf2 signaling pathway, thereby reducing the influence of toxin on mussel. Our study might provide a novel clue for the resistance mechanism of shellfish to DSP toxins.

RNA-Seq Analysis for Assessing the Early Response to DSP Toxins in Mytilus galloprovincialis Digestive Gland and Gill

The harmful effects of diarrhetic shellfish poisoning (DSP) toxins on mammalian cell lines have been widely assessed. Studies in bivalves suggest that mussels display a resistance to the cytogenotoxic effects of DSP toxins. Further, it seems that the bigger the exposure, the more resistant mussels become. To elucidate the early genetic response of mussels against these toxins, the digestive gland and the gill transcriptomes of Mytilus galloprovincialis after Prorocentrum lima exposure (100,000 cells/L, 48 h) were de novo assembled based on the sequencing of 8 cDNA libraries obtained using an Illumina HiSeq 2000 platform. The assembly provided 95,702 contigs. A total of 2286 and 4523 differentially expressed transcripts were obtained in the digestive gland and the gill, respectively, indicating tissue-specific transcriptome responses. These transcripts were annotated and functionally enriched, showing 44 and 60 significant Pfam families in the digestive gland and the gill, respectively. Quantitative PCR (qPCR) was performed to validate the differential expression patterns of several genes related to lipid and carbohydrate metabolism, energy production, genome integrity and defense, suggesting their participation in the protective mechanism. This work provides knowledge of the early response against DSP toxins in the mussel M. galloprovincialis and useful information for further research on the molecular mechanisms of the bivalve resistance to these toxins.

Transcriptional and biochemical analysis of antioxidant enzymes in the mussel Mytilus galloprovincialis during experimental exposures to the toxic dinoflagellate Prorocentrum lima

The genotoxic and cytotoxic effects of Diarrhetic Shellfish Poisoning (DSP) toxins have been widely investigated in bivalve molluscs, representing the main vectors of these compounds in the Atlantic coast of Europe. DSP toxins are produced by Harmful Algal Blooms (HABs) of Dinophysis and Prorocentrum dinoflagellates, being subsequently accumulated by marine organisms and biomagnified throughout trophic webs. Yet, bivalves display increased resistance to the harmful effects of these toxins during HAB episodes. While previous reports have suggested that such resilience might be the result of an increased activity in the bivalve antioxidant system, very little is still known about the specific mechanism underlying the protective effect observed in these organisms. The present work aims to fill this gap by studying transcriptional expression levels and biochemical activities of antioxidant enzymes in different tissues the mussel Mytilus galloprovincialis during experimental exposures to DSP toxins produced by the dinoflagellate Prorocentrum lima. Results are consistent with the presence of a compensatory mechanism involving a down-regulation in the expression of specific genes encoding antioxidant enzymes [i.e., SuperOxide Dismutase (SOD) and CATalase (CAT)] which is counterbalanced by the up-regulation of other antioxidant genes such as Glutathione S-Transferase pi-1 (GST-pi) and Selenium-dependent Glutathione PeroXidase (Se-GPx), respectively. Enzymatic activity analyses mirror gene expression results, revealing high antioxidant activity levels (consistent with a protective role for the antioxidant system) along with reduced lipid peroxidation (increasing the defense against oxidative stress).

Effects of algal toxin okadaic acid on the non-specific immune and antioxidant response of bay scallop (Argopecten irradians)

Okadaic acid (OA) is produced by dinoflagellates during harmful algal blooms and is a diarrhetic shellfish-poisoning (DSP) toxin. This toxin is particularly problematic for bivalves that are cultured for human consumption. This study aimed to reveal the effects of exposure to OA on the non-specific immune responses of bay scallop, Argopecten irradians. Various immunological parameters (superoxide dismutase (SOD), acid phosphatase (ACP), alkaline phosphatase (ALP), lysozyme activities, and total protein level) were assessed in the hemolymph of bay scallops at 3, 6, 12, 24, and 48 h post-exposure (hpe) to different concentrations (50, 100, and 500 nM) of OA. Moreover, the expression of immune system-related genes (MnSOD, PrxV, PGRP, and BD) was also measured. Results showed that SOD and ACP activities were decreased between 12 and 48 hpe. The ALP, lysozyme activities, and total protein levels were also modulated after exposure to different concentrations of OA. The expression of immune-system-related genes was also assessed at different time points during the exposure period. Overall, our results suggest that the exposure to OA had negative effects on the antioxidant and non-specific immune responses, and even disrupted the metabolism of bay scallops, making them more vulnerable to environmental stress-inducing agents; they provide a better understanding of the response status of bivalves against DSP toxins.

Exposure of okadaic acid alters the angiogenesis in developing chick embryos

Okadaic acid (OA) is a common phycotoxin, which concerns diarrheic shellfish poisoning (DSP) in human being. It has been known that OA can induce disorganization in cytoskeletal architecture and cell-cell contact, cause chromosome loss, apoptosis, DNA damage and inhibit phosphatases, suggesting its potential embryotoxicity. In this paper, we found that low concentration of OA (50 nM, 100 nM and 200 nM) significantly reduced the density of vascular plexus in yolk-sac membrane (YSM) of chick embryo, while high concentration of OA (500 nM) distinctly depressed the blood vessel density in chorioallantoic membrane (CAM). After exposed to OA, MDA level and SOD activity increased significantly in CAM tissues. However, addition of vitamin C could rescue OA-suppressed angiogenesis in CAM of chick embryo. After exposure of OA, Ang-2 expression was down-regulated in CAM tissues. Taking together, we proposed that OA interfered with angiogenesis in developing chick embryo, through, at least partly, the induction of excessive ROS generation.

Crassostrea gigas exposure to the dinoflagellate Prorocentrum lima: Histological and gene expression effects on the digestive gland

Bivalve mollusks bioaccumulate toxins via ingestion of toxic dinoflagellates. In this study, Crassostrea gigas was used to investigate the effects related to Prorocentrum lima exposure. Oysters were fed with three diets Isochrysis galbana (2 × 10(6) cell mL(-1)) control treatment; algal mix of I. galbana (2 × 10(6)) and P. lima (3 × 10(3) cell mL(-1)); and P. lima alone (3 × 10(3) cell mL(-1)). Feeding behavior changes, histopathological alterations, and expression patterns changes of genes involved in cell cycle (p21, cafp55, p53), cytoskeleton (tub, act), and inflammatory process (casp1) were evaluated. Results indicated that the presence of diarrheic shellfish poisoning by P. lima cells decreased the clearance rate (p < 0.05), induced structural loss, significantly decreased tubule area of the digestive gland (p < 0.05), and up-regulated in expression all gene (p < 0.05), suggesting that toxic cells might trigger inflammatory tissue process, disturb cell cycle and cytoskeleton representing a risk to oysters integrity.

Early Genotoxic and Cytotoxic Effects of the Toxic Dinoflagellate Prorocentrum lima in the Mussel Mytilus galloprovincialis

Okadaic acid (OA) and dinophysistoxins (DTXs) are the main toxins responsible for diarrhetic shellfish poisoning (DSP) intoxications during harmful algal blooms (HABs). Although the genotoxic and cytotoxic responses to OA have been evaluated in vitro, the in vivo effects of these toxins have not yet been fully explored. The present work fills this gap by evaluating the in vivo effects of the exposure to the DSP-toxin-producing dinoflagellate Prorocentrum lima during the simulation of an early HAB episode in the mussel Mytilus galloprovincialis. The obtained results revealed that in vivo exposure to this toxic microalgae induced early genotoxicity in hemocytes, as a consequence of oxidative DNA damage. In addition, the DNA damage observed in gill cells seems to be mainly influenced by exposure time and P. lima concentration, similarly to the case of the oxidative damage found in hemocytes exposed in vitro to OA. In both cell types, the absence of DNA damage at low toxin concentrations is consistent with the notion suggesting that this level of toxicity does not disturb the antioxidant balance. Lastly, in vivo exposure to growing P. lima cell densities increased apoptosis but not necrosis, probably due to the presence of a high number of protein apoptosis inhibitors in molluscs. Overall, this work sheds light into the in vivo genotoxic and cytotoxic effects of P. lima. In doing so, it also demonstrates for the first time the potential of the modified (OGG1) comet assay for assessing oxidative DNA damage caused by marine toxins in marine invertebrates.

Unbiased high-throughput characterization of mussel transcriptomic responses to sublethal concentrations of the biotoxin okadaic acid

Background. Harmful Algal Blooms (HABs) responsible for Diarrhetic Shellfish Poisoning (DSP) represent a major threat for human consumers of shellfish. The biotoxin Okadaic Acid (OA), a well-known phosphatase inhibitor and tumor promoter, is the primary cause of acute DSP intoxications. Although several studies have described the molecular effects of high OA concentrations on sentinel organisms (e.g., bivalve molluscs), the effect of prolonged exposures to low (sublethal) OA concentrations is still unknown. In order to fill this gap, this work combines Next-Generation sequencing and custom-made microarray technologies to develop an unbiased characterization of the transcriptomic response of mussels during early stages of a DSP bloom.

Methods. Mussel specimens were exposed to a HAB episode simulating an early stage DSP bloom (200 cells/L of the dinoflagellate Prorocentrum lima for 24 h). The unbiased characterization of the transcriptomic responses triggered by OA was carried out using two complementary methods of cDNA library preparation: normalized and Suppression Subtractive Hybridization (SSH). Libraries were sequenced and read datasets were mapped to Gene Ontology and KEGG databases. A custom-made oligonucleotide microarray was developed based on these data, completing the expression analysis of digestive gland and gill tissues.

Results. Our findings show that exposure to sublethal concentrations of OA is enough to induce gene expression modifications in the mussel Mytilus. Transcriptomic analyses revealed an increase in proteasomal activity, molecular transport, cell cycle regulation, energy production and immune activity in mussels. Oppositely, a number of transcripts hypothesized to be responsive to OA (notably the Serine/Threonine phosphatases PP1 and PP2A) failed to show substantial modifications. Both digestive gland and gill tissues responded similarly to OA, although expression modifications were more dramatic in the former, supporting the choice of this tissue for future biomonitoring studies.

Discussion. Exposure to OA concentrations within legal limits for safe consumption of shellfish is enough to disrupt important cellular processes in mussels, eliciting sharp transcriptional changes as a result. By combining the study of cDNA libraries and a custom-made OA-specific microarray, our work provides a comprehensive characterization of the OA-specific transcriptome, improving the accuracy of the analysis of expresion profiles compared to single-replicated RNA-seq methods. The combination of our data with related studies helps understanding the molecular mechanisms underlying molecular responses to DSP episodes in marine organisms, providing useful information to develop a new generation of tools for the monitoring of OA pollution.

In Vitro Analysis of Early Genotoxic and Cytotoxic Effects of Okadaic Acid in Different Cell Types of the Mussel Mytilus galloprovincialis

Okadaic acid (OA) is the predominant biotoxin responsible for diarrhetic shellfish poisoning (DSP) syndrome in humans. While its harmful effects have been extensively studied in mammalian cell lines, the impact on marine organisms routinely exposed to OA is still not fully known. Few investigations available on bivalve molluscs suggest less genotoxic and cytotoxic effects of OA at high concentrations during long exposure times. In contrast, no apparent information is available on how sublethal concentrations of OA affect these organisms over short exposure times. In order to fill this gap, this study addressed for the first time in vitro analysis of early genotoxic and cytotoxic effects attributed to OA in two cell types of the mussel Mytilus galloprovincialis. Accordingly, hemocytes and gill cells were exposed to low OA concentrations (10, 50, 100, 200, or 500 nM) for short periods of time (1 or 2 h). The resulting DNA damage, as apoptosis and necrosis, was subsequently quantified using the comet assay and flow cytometry, respectively. Data demonstrated that (1) mussel hemocytes seem to display a resistance mechanism against early genotoxic and cytotoxic OA-induced effects, (2) mussel gill cells display higher sensitivity to early OA-mediated genotoxicity than hemocytes, and (3) mussel gill cells constitute more suitable systems to evaluate the genotoxic effect of low OA concentrations in short exposure studies. Taken together, this investigation provides evidence supporting the more reliable suitability of mussel gill cells compared to hemocytes to evaluate the genotoxic effect of low short-duration exposure to OA.

Proteomic profile in Perna viridis after exposed to Prorocentrum lima, a dinoflagellate producing DSP toxins

In the current study, we compared protein profiles in gills of Perna viridis after exposure to Prorocentrumlima, a dinoflagellate producing DSP toxins, and identified the differential abundances of protein spots using 2D-electrophoresis. After exposure to P. lima, the level of okadaic acid (a main component of DSP toxins) in gills of P. viridis significantly increased at 6 h, but mussels were all apparently healthy without death. Among the 28 identified protein spots by MALDI TOF/TOF-MS, 12 proteins were up-regulated and 16 were down-regulated in the P. lima-exposed mussels. These identified proteins were involved in various biological activities, such as metabolism, cytoskeleton, signal transduction, response to oxidative stress and detoxification. Taken together, our results indicated that the presence of P. lima caused DSP toxins accumulation in mussel gill, and might consequently induce cytoskeletonal disorganization,oxidative stress, a dysfunction in metabolism and ubiquitination/proteasome activity.

Does the marine biotoxin okadaic acid cause DNA fragmentation in the blue mussel and the pacific oyster?

Two bivalve species of global economic importance: the blue mussel, Mytilus edulis and the pacific oyster, Crassostrea gigas were exposed in vivo, to the diarrhoetic shellfish toxin okadaic acid (OA), and impacts on DNA fragmentation were measured. Shellfish were exposed using two different regimes, the first was a single (24 h) exposure of 2.5 nM OA (∼0.1 μg/shellfish) and algal feed at the beginning of the trial (T0), after which shellfish were only fed algae. The second was daily exposure of shellfish to two different concentrations of OA mixed with the algal feed over 7 days; 1.2 nM OA (∼0.05 μg OA/shellfish/day) and 50 nM OA (∼2 μg OA/shellfish/day). Haemolymph and hepatopancreas cells were extracted following 1, 3 and 7 days exposure. Cell viability was measured using the trypan blue exclusion assay and remained above 85% for both cell types. DNA fragmentation was examined using the single-cell gel electrophoresis (comet) assay. A significant increase in DNA fragmentation was observed in the two cell types from both species relative to the controls. This increase was greater in the pacific oyster at the higher toxin concentration. However, there was no difference in the proportion of damage measured between the two cell types, and a classic dose response was not observed, increasing toxin concentration did not correspond to increased DNA fragmentation.

P-glycoprotein expression in Perna viridis after exposure to Prorocentrum lima, a dinoflagellate producing DSP toxins

Bivalves naturally exposed to toxic algae have mechanisms to prevent from harmful effects of diarrhetic shellfish poisoning (DSP) toxins. However, quite few studies have examined the mechanisms associated, and the information currently available is still insufficient. Multixenobiotic resistance (MXR) is ubiquitous in aquatic invertebrates and plays an important role in defense against xenobiotics. Here, to explore the roles of P-glycoprotein (P-gp) in the DSP toxins resistance in shellfish, complete cDNA of P-gp gene in the mussel Perna viridis was cloned and analyzed. The accumulation of okadaic acid (OA), a main component of DSP toxins, MXR activity and expression of P-gp in gills of P. viridis were detected after exposure to Prorocentrum lima, a dinoflagellate producing DSP toxins in the presence or absence of P-gp inhibitors PGP-4008, verapamil (VER) and cyclosporin A (CsA). The mussel P. viridis P-gp closely matches MDR/P-gp/ABCB protein from various organisms, having a typical sequence organization as full transporters from the ABCB family. After exposure to P. lima, OA accumulation, MXR activity and P-gp expression significantly increased in gills of P. viridis. The addition of P-gp-specific inhibitors PGP-4008 and VER decreased MXR activity induced by P. lima, but had no effect on the OA accumulation in gills of P. viridis. However, CsA, a broad-spectrum inhibitor of ABC transporter not only decreased MXR activity, but also increased OA accumulation in gills of P. viridis. Together with the ubiquitous presence of other ABC transporters such as MRP/ABCC in bivalves and potential compensatory mechanism in P-gp and MRP-mediated resistance, we speculated that besides P-gp, other ABC transporters, especially MRP might be involved in the resistance mechanisms to DSP toxins.

Apoptosis of hemocytes from lions-paw scallop Nodipecten subnodosus induced with paralyzing shellfish poison from Gymnodinium catenatum

The toxic dinoflagellate Gymnodinium catenatum produces paralyzing shellfish poisons (PSPs) that are consumed and accumulated by bivalves. Previously, we recorded a decrease in hemocytes 24h after injection of PSPs (gonyautoxin 2/3 epimers, GTX2/3) in the adductor muscle in the lions-paw scallop Nodipecten subnodosus. In this work, qualitative and quantitative analyses, in in vivo and in vitro experiments, revealed that the lower count of hemocytes results from cells undergoing typical apoptosis when exposed to GTX 2/3 epimers. This includes visible morphological alterations of the cytoplasmic membrane, damage to the nuclear membrane, condensation of chromatin, DNA fragmentation, and release of DNA fragments into the cytoplasm. Induction of apoptosis was accompanied by phosphatidylserine exposure to the outer cell membrane and activation of cysteine-aspartic proteases, caspase 3 and caspase 8. Addition of an inhibitor of caspase to the medium suppressed activation in hemocytes exposed to the toxins, suggesting that cell death was induced by a caspase-dependent apoptotic pathway. The results are important for future investigation of the scallop's immune system and should provide new insights into apoptotic processes in immune cells of scallops exposed to PSPs.

Okadaic acid meet and greet: an insight into detection methods, response strategies and genotoxic effects in marine invertebrates

Harmful Algal Blooms (HABs) constitute one of the most important sources of contamination in the oceans, producing high concentrations of potentially harmful biotoxins that are accumulated across the food chains. One such biotoxin, Okadaic Acid (OA), is produced by marine dinoflagellates and subsequently accumulated within the tissues of filtering marine organisms feeding on HABs, rapidly spreading to their predators in the food chain and eventually reaching human consumers causing Diarrhetic Shellfish Poisoning (DSP) syndrome. While numerous studies have thoroughly evaluated the effects of OA in mammals, the attention drawn to marine organisms in this regard has been scarce, even though they constitute primary targets for this biotoxin. With this in mind, the present work aimed to provide a timely and comprehensive insight into the current literature on the effect of OA in marine invertebrates, along with the strategies developed by these organisms to respond to its toxic effect together with the most important methods and techniques used for OA detection and evaluation.