Expression of P-glycoprotein in southeastern oysters, Crassostrea virginica

Toxic Responses of Different Shellfish Species after Exposure to Prorocentrum lima, a DSP Toxins Producing Dinoflagellate

Prorocentrum lima is a global benthic dinoflagellate that produces diarrhetic shellfish poisoning (DSP) toxins, which can be ingested by filter-feeding bivalves, and eventually pose a great threat to human health through food chain. After being exposed to P. lima, different bivalves may accumulate various levels of DSP toxins and display different toxic responses. However, the underlying mechanism remains unclear. Here, we found that the content of okadaic acid-equivalents (OA-eq) varied in the digestive glands of the three bivalves including Crassostrea gigas, Mytilus coruscus and Tegillarca granosa after P. lima exposure. The degree of esterification of OA-eq in the three bivalves were opposite to the accumulation of OA-eq. The digestive gland tissues of the three bivalve species were damaged to different degrees. The transcriptional induction of Nrf2 targeted genes such as ABCB1 and GPx indicates the functionality of Nrf2 pathway against DSP toxins in bivalves. The oyster could protect against DSP toxins mainly through ABC transporters and esterification, while the mussel and clam reduce the damage induced by DSP toxins mainly by regulating the expression of antioxidant genes. Our findings may provide some explanations for the difference in toxic response to DSP toxins in different shellfish.

Effect of temperature on the toxicokinetics and gene expression of the pacific cupped oyster Crassostrea gigas exposed to cadmium

In this study, we investigated the influence of temperature on the bioaccumulation and depuration of Crassostrea gigas exposed to Cd associated with its molecular responses. Oysters were acclimatized to different temperatures (10 °C, 15 °C, 20 °C, 25 °C, and 30 °C) for 14 d and then exposed to 10 μg/L Cd for 28 d, followed by a depuration period of 35 d. Oysters were sampled for chemical analysis by inductively coupled plasma mass spectrometry (ICP-MS) and for mRNA quantification by qPCR. In the digestive gland, gill, and mantle, the cadmium concentration at 10 °C was significantly lower than that at 25 °C and 30 °C in both the whole experiments. The use of a two-compartment model showed that the uptake rate k₁ in the above three tissues increased with increasing temperatures ranging from 15 to 25 °C. The fastest elimination rates and shortest half-lives were observed at 15-25 °C. The induction of metallothionein (MT) only occurred in the digestive gland at 15 °C and 20 °C at the end of the accumulation phase. In the mantle and gills, the expression of P-glycoprotein (P-gp) was significantly induced at the end of the accumulation phase and significantly inhibited at the end of the depuration phase. In the digestive gland, the expression of P-gp was induced at the end of both the accumulation and depuration phases. Heat shock protein (hsp70) expression exhibited an overall increasing trend throughout the experiment.

Effects of the toxic dinoflagellates Prorocentrum lima and Ostreopsis cf. ovata on immune responses of cultured oysters Crassostrea gasar

Oyster production in Brazil has been highlighted as an important economic activity and is directly impacted by the quality of the environment, which is largely the result of human interference and climate change. Harmful algal blooms occur in aquatic ecosystems worldwide, including coastal marine environments which have been increasing over the last decades as a result of global change and anthropogenic activities. In this study, the native oysters Crassostrea gasar from Northeast of Brazil were exposed to two toxic benthic dinoflagellate species, Prorocentrum lima and Ostreopsis cf. ovata. Their respective effects on C. gasar physiology and defense mechanisms were investigated. Oyster hemocytes were first exposed in vitro to different concentrations of both dinoflagellate species to assess their effects on hemocyte functions, such as phagocytosis, production of reactive oxygen species, as well as mortality. Results highlighted an alteration of hemocyte phagocytosis and viability in presence of O. cf. ovata, whereas P. lima did not affect the measured hemocyte functions. In a second experiment, oysters were exposed for 4 days in vivo to toxic culture of O. cf. ovata to assess its effects on hemocyte parameters, tissues damages and pathogenic Perkinsus spp. infection. An increase in hemocyte mortality was also observed in vivo, associated with a decrease of ROS production. Histopathological analyses demonstrated a thinning of the epithelium of the digestive tubules of the digestive gland, inflammatory reaction and a significant increase in the level of infection by Perkinsus spp. in oysters exposed to O. cf. ovata. These results indicate that oysters C. gasar seem to be pretty resilient to an exposure to P. lima and may be more susceptible to O. cf. ovata. Furthermore, the latter clearly impaired oyster physiology and defense mechanisms, thus highlighting that harmful algal blooms of O. cf. ovata could potentially lead to increased susceptibility of C. gasar oysters to parasite infections.

MXR response in sea anemones: Effect of temperature, salinity and copper

Multixenobiotic resistance (MXR) phenotype is a cellular defense which can eliminate toxic substances from cells. Several studies describe the MXR activity after pollutant exposure, but little is known about the interference of abiotic factors in this mechanism. The present study aimed to evaluate MXR activity in sea anemones Bunodosoma cangicum after in vivo and in vitro exposures to different temperatures (15, 20 and 25C) and salinities (15, 30 and 45‰) associated or not with copper (0, 7.8 and 15.6 μg/L). Results showed that low temperature inhibited the MXR activity in vivo and in vitro, while salinity did not alter this activity. Copper could change the response, mainly at different temperatures (15 and 25 °C) - 7.8 μg/L Cu activated in vivo and in vitro and 15.6 μg/L Cu in vitro inhibited MXR activity in relation to same copper concentrations at 20 °C. Results for MXR activity found between in vivo and in vitro exposures were similar among temperature treatments and salinities; however, under hyperosmotic shock, in vivo exposure showed that animals has different response than isolated cells. The animals exposed to salinity 45‰ produced a mucus layer as a defense mechanism, because of this protection the response was different between in vivo and in vitro exposures. Concluding, temperature affects MXR activity independently of the presence of copper and each model of exposure contributes with different type of knowledge (cellular mechanism/systemic response).

Effect of salinity on the bioaccumulation and depuration of cadmium in the pacific cupped oyster, Crassostrea gigas

As a euryhaline species, the oyster Crassostrea gigas can adapt rapid and dramatic salinity fluctuations, and show physiological mechanisms of adaption to tolerant salinity changes. They are continuously exposed to Cd because they are filter feeders and their mobility is poor. In order to understand the influence of salinity on the molecular responses of C. gigas exposed to Cd, oysters were acclimatized to different salinities (13, 20, 27, and 34) for 14 days and then exposed to 10 μg/L Cd for 28 d, followed by a depuration period of 35 d. Control groups were kept at the same salinities without Cd. Oysters were sampled for chemical analysis by inductively coupled plasma mass spectrometry (ICP-MS) and for mRNA quantification by qPCR. The rank order of the mean concentrations of Cd in oyster tissues was digestive gland > mantle > adductor muscle. Use of a two-compartment model showed that in the three tissues, Cd uptake rates (k₁) in digestive gland (13.525-35.430 d^-1) also increased as salinity decreased. However, no difference was observed in Cd uptake rates of C.gigas in the three higher salinities in mantle (11.703-17.250 d^-1). Cd depuration rates (k₂) (0.0139 - 0.0127 d^-1 in mantle and 0.0111-0.0134 d^-1 in digestive gland) followed a reverse trend. There was not a relationship between k₂ and salinity in adductor muscle. In response to Cd contamination, MT was significantly up-regulated by Cd at all salinities, and P-gp was significantly up-regulated in mantle, while down-regulated in digestive gland, which means a disruption of the protein synthesis at high concentration. At depuration phase, MT level was higher in digestive gland and mantle, and its expression was higher at S₁₃ than that at S₃₄. No relationship was found between the P-gp gene expression level and concentrations of cadmium in tissues in either accumulation phase or depuration phase. In accumulation phase, the MT gene expression level was positively correlated with the concentration of cadmium in both the digestive gland and the mantle, while the relationship was weakened in depuration phase, suggesting an effort to create a detoxification mechanism.

Transcriptomic evaluation of the American oyster, Crassostrea virginica, deployed during the Deepwater Horizon oil spill: Evidence of an active hydrocarbon response pathway

Estuarine organisms were impacted by the Deepwater Horizon oil spill which released ∼5 million barrels of crude oil into the Gulf of Mexico in the spring and summer of 2010. Crassostrea virginica, the American oyster, is a keystone species in these coastal estuaries and is routinely used for environmental monitoring purposes. However, very little is known about their cellular and molecular responses to hydrocarbon exposure. In response to the spill, a monitoring program was initiated by deploying hatchery-reared oysters at three sites along the Alabama and Mississippi coast (Grand Bay, MS, Fort Morgan, AL, and Orange Beach, AL). Oysters were deployed for 2-month periods at five different time points from May 2010 to May 2011. Gill and digestive gland tissues were harvested for gene expression analysis and determination of aliphatic and polycyclic aromatic hydrocarbon (PAH) concentrations. To facilitate identification of stress response genes that may be involved in the hydrocarbon response, a nearly complete transcriptome was assembled using Roche 454 and Illumina high-throughput sequencing from RNA samples obtained from the gill and digestive gland tissues of deployed oysters. This effort resulted in the assembly and annotation of 27,227 transcripts comprised of a large assortment of stress response genes, including members of the aryl hydrocarbon receptor (AHR) pathway, Phase I and II biotransformation enzymes, antioxidant enzymes and xenobiotic transporters. From this assembly several potential biomarkers of hydrocarbon exposure were chosen for expression profiling, including the AHR, two cytochrome P450 1A genes (CYP1A-like 1 and CYP1A-like 2), Cu/Zn superoxide dismutase (CuZnSOD), glutathione S-transferase theta (GST theta) and multidrug resistance protein 3 (MRP3). Higher expression levels of GST theta and MRP3 were observed in gill tissues from all three sites during the summer to early fall 2010 deployments. Linear regression analysis indicated a statistically significant relationship between total PAH levels in digestive gland tissue samples with CYP1A-like 2, CuZnSOD, GST theta and MRP3 induction. These observations provide evidence of a potentially conserved AHR pathway in invertebrates and yield new insight into the development of novel biomarkers for use in environmental monitoring activities.

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.

Separating natural from anthropogenic causes of impairment in Zebra mussel (Dreissena polymorpha) populations living across a pollution gradient

The relationship between the reproductive stage, the total lipid content and eight broadly used biochemical stress responses were used to assess seasonal and pollutant effects across eleven different zebra mussel (Dreissena polymorpha) populations from the Ebro and Mijares river basin, Spain. Biochemical markers included superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), glutathione S transferase (GST), multixenobiotic transporter activity (MXR), lactate dehydrogenase (LDH), lipid peroxidation (LPO) and single strand DNA breaks. Principal component analyses of zebra mussel responses across an annual cycle, showed a marked gonad stage component in total lipid content and biochemical responses. The same response pattern was observed across the populations sampled along a broad geographical and pollution gradient. Population differences on the gonad developmental stage were highly correlated with most of the measured responses and unrelated with the pollution gradient. Conversely, bioaccumulation of organic and inorganic contaminant residues was more related to pollution sources than with the reproductive cycle. These results indicate that the reproductive cycle is the major factor affecting the temporal and spatial variation of the studied markers in D. polymorpha.

P-glycoprotein and CYP1A protein expression patterns in Nile tilapia (Oreochromis niloticus) tissues after waterborne exposure to benzo(a)pyrene (BaP)

The protein levels and tissue distribution patterns of P-glycoprotein (Pgp) and cytochrome P450 (CYP1A) were investigated in Nile tilapia (Oreochromis niloticus) after waterborne exposure to different benzo(a)pyrene (BaP) concentrations, using immunochemical approaches. The Pgp mammalian monoclonal antibody (mAb) C219 cross reacted with a ∼170kDa protein, almost exclusively localized to the bile canaliculi, while probing with the Pgp mammalian mAb C494, resulted in a positive reaction in liver, gills and intestine of Nile tilapia and covered a wider set of cell types. Levels of Pgp expression were not altered after in vivo exposure to BaP. CYP1A, detected with the mAb C10-7, reacted positively in liver, gills and intestine and followed a BaP dose-dependent fold induction. Taken together, these results indicate that CYP1A is involved in BaP metabolism in liver, gills and intestine, however, further studies are needed to elucidate the possible interaction of the efflux protein Pgp with BaP and/or its metabolites.

Effects of cadmium exposure on expression and activity of P-glycoprotein in eastern oysters, Crassostrea virginica Gmelin

Heavy metal pollution is a worldwide problem, and cadmium (Cd) is one of the most noxious pollutants in aquatic environments. We studied P-glycoprotein (P-gp) expression and function in control and Cd exposed (50microgL(-1) Cd, 30-40 days) oysters Crassostrea virginica as a possible mechanism of cell protection against Cd. Our data show that P-gp is expressed on cell membrane and in mitochondria of oyster gills and hepatopancreas. Inhibitor studies with verapamil, cyclosporine A and JS-2190 suggest that in the gills, mitochondrial P-gp pumps substrates from cytosol into the mitochondria, while cell membrane P-gp pumps substrates from cytosol out of the cell. Cd exposure resulted in a 2-2.5-fold increase in P-gp protein expression in cell membranes and a 3.5-7-fold increase in transport activity measured as the inhibitor-sensitive rhodamine B extrusion rate. In contrast, p-gp mRNA levels were similar in control and Cd-exposed oysters. No difference in P-gp protein expression was observed between mitochondria of control and Cd-exposed oysters but the apparent transport activity was higher in mitochondria from Cd-exposed oysters. Overall, a stronger increase in substrate transport activity in Cd-exposed oysters compared to a relatively weaker change in P-gp protein levels suggests that P-gp activity is post-translationally regulated. Our data show that direct determination of P-gp transport activity may be the best measure of the xenobiotic-resistant phenotype, whereas p-gp mRNA levels are not a good marker due to the likely involvement of multiple post-transcriptional regulatory steps. Cd exposure resulted in a significantly elevated rate of oxygen consumption of isolated oyster gills by 46%. Specific inhibitors of ATPase function of P-gp (cyclosporine A and JS-2190) had no significant effect on tissue oxygen consumption indicating that P-gp contribution to energy budget is negligible and supporting indirect estimates based on the ATP stoichiometry of substrate transport that also suggest low energy demand for P-gp function.

ATP-binding cassette multidrug transporters in Indian-rock oyster Saccostrea forskali and their role in the export of an environmental organic pollutant tributyltin

ATP-binding cassette (ABC) multidrug transporters confer resistance in human cancer cells as well as in pathogenic microorganisms by mediating the extrusion of various chemotherapeutic drugs out of the cell. In aquatic invertebrates, the presence of ABC transporters which are involved in the multi-xenobiotic resistance has been demonstrated. However, most studies have been confined to the MDR1 subfamily. In the present study, we characterized the expression and localization of the ABC multidrug transporters including MDR1, MRP1 and BCRP subfamily in the Indian-rock oyster Saccostrea forskali. To our knowledge, these data represent one of the first reports on the orthologues of MRP1 and BCRP in marine invertebrates. Furthermore, the observations of (i) the expression of the ABC multidrug proteins in detoxifying tissues; (ii) the induction of these transporters upon exposure to an environmental organic pollutant tributyltin (TBT); and (iii) the concentration-dependent inhibition of rhodamine efflux by TBT imply a possible role of these proteins in the export of TBT. Our findings along with previous studies suggest that the ABC multidrug transporters act as a detoxifying mechanism of various toxic agents including TBT in aquatic organisms.

Biomarker versus environmental factors: seasonal variations and modelling multixenobiotic defence (MXD) transport activity in transplanted zebra mussels

The occurrence of biomarker temporal variations linked to environmental factors makes it difficult to distinguish the specific effect of pollution. The present work aims to investigate the seasonal variations of the transport activity of the multixenobiotic defence (MXD), which is used as a biological tool for the monitoring of pollution in aquatic ecosystems. The MXD transport activity was monitored monthly from August 2001 to October 2002 in zebra mussels (Dreissena polymorpha) transplanted to three sites in the Moselle River. The 'efflux method' was used to evaluate functional activity of MXD by assessing rhodamine B efflux with or without an inhibitor (verapamil). Environmental parameters were provided by a French regulatory agency (Water Agency) that monitors river water quality. The results of a principal components analysis describe the seasonal cycle of water characteristics and demonstrate that MXD activity is subjected to significant temporal variations. These data were described with a generalised linear model that enables it to link MXD variability to the seasonal variations of environmental parameters such as temperature or levels of organic contamination. This work proposes a modelling approach and highlights that the occurrence of seasonal variations in MXD response has to be taken into account in the interpretation of in situ monitoring studies.

Application of Pharmacological Approaches to Plant–Mammal Interactions

The dominant theory in the field of mammalian herbivore-plant interactions is that intake, and therefore tolerance, of plant secondary metabolites (PSMs) is regulated by mechanisms that reduce absorption and increase detoxification of PSMs. Methods designed by pharmacologists to measure detoxification enzyme activity, metabolite excretion, and most recently, drug absorption, have been successfully applied by ecologists to study PSM intake in a variety of mammalian study systems. Here, we describe several pharmacological and molecular techniques used to investigate the fate of drugs in human that have potential to further advance knowledge of mammalian herbivore-plant interactions.

Accumulation of ivermectin in the brain of sea bream, Sparus aurata after intraperitoneal administration

Ivermectin, which is widely used in veterinary and human, has been considered safe due to its inability to penetrate into the central nervous system of higher vertebrates. This paper presents data on the ability of the drug to cross the blood-brain barrier of the marine teleost sea bream, Sparus aurata and accumulate in the brain. The concentration of the drug in the brain and the serum of the fish was assessed by the use of a direct competitive ELISA commercial kit. Our results showed a rapid uptake of the substance by the brain of the fish reaching a maximum concentration of 98.9ngg(-1) 8h post treatment. The trend of the absorption of the drug in brain followed that of the blood. Concentration of the drug in the brain remained high at each sampling point over the 24h duration of the experiment. In view of these findings, the need of study of the role of the blood-brain barrier and particularly the multidrug resistance mechanism in sea bream is outlined.

Response of the Pacific oyster Crassostrea gigas to hydrocarbon contamination under experimental conditions

Hydrocarbon contamination perturbs the metabolism of the marine bivalve Crassostrea gigas. To understand the response of this organism to hydrocarbon exposure, a suppression subtractive hybridisation method was employed to characterise up- and down-regulated genes during hydrocarbon exposure. The number of differentially expressed gene sequences obtained via this method was 258. The expression of genes involved in hydrocarbon detoxification (cytochrome p4501A1-like protein, cytochrome b(5), flavin-containing monooxygenase 2 and glutathione S-transferase omega class), protection against oxidative stress (copper/zinc superoxide dismutase) and cell protection (heat shock protein 70 family) was analysed by RT-PCR. An increase in the mRNA level of all genes studied was observed. A quantification of HSP70 by Enzyme Linked Immunosorbent Assay (ELISA) showed a significant increase of this protein during exposure. This study provides a basis for studying hydrocarbon detoxification processes in marine bivalves, especially C. gigas.

Le mécanisme de défense multixénobiotique (MDMX) chez les bivalves

Multixenobiotic defence mechanism (MXDM) consists in a cellular system that functions as membrane extrusion pumps effluxing organic compounds out of the cells. In bivalves, it represents a primordial protection against toxic effects of organic xenobiotics in preventing their cellular accumulation. It has raised attention during the last decade for its potential to be used as a biomarker of pollution. This article reviews the fundamental knowledge on the MXDM system in bivalves and the methods proposed to assess its activity. Finally, it reviews the major results of laboratory and field studies that enabled to hypothesise that MXDM could be used as a biomarker of environmental stress rather than of pollutant exposure.

Expression and activity of a multixenobiotic resistance system in the Pacific oyster Crassostrea gigas

The multidrug resistance (MDR) mechanism corresponds to a defence system relying on the expression of high molecular membrane proteins that can actively lower the intracellular concentration of a wide variety of toxins, thus maintaining them below their toxic level. Using RT-PCR, expression levels of a gene belonging to the class I of mammalian mdr genes, has been assessed in different developmental stages of the oyster Crassostrea gigas. While no expression was found in the oocyte or the trocophore stage, a rise of mRNA content was observed from the veliger stage to the juvenile stage, thus indicating the induction of the system as the animal is developing in the environment. The incubation of gill fragments in the dye rhodamine B and subsequent measurements of intracellular fluorescence using a microplate reader indicates that the system can effectively decrease the accumulation of the test compound in a competitive manner with known inhibitors or environmental contaminants as observed in vertebrate cells. The oyster MXR system is thus becoming active in adult oyster and could be of importance in environmentally contaminated areas.

Genetic and immunological characterisation of a multixenobiotic resistance system in the turbot (Scophthalmus maximus)

Pleiotropic resistance driven by transport proteins constitutes a very ubiquitous protection mechanism against natural or synthetic toxic compounds. The multidrug (MDR) or multixenobiotic (MXR) system has been identified in many different species, and may be used as a biomarker for pollution assessment. Here we report the existence of a gene encoding a MXR-related protein in a benthic fish species, the turbot Scophthalmus maximus, and its constitutive expression in several tissues. A 433bp cDNA fragment has been cloned by RT-PCR. The deduced amino-acid sequence shares close to 80% homology with class I or class II mammalian MDR proteins. This cDNA corresponds to a major mRNA of 5.6 kb and encodes a protein having an apparent molecular weight of 83 kDa. Constitutive expression levels assessed by semi-quantitative RT-PCR and Western blot, revealed that the kidney and the brain, and to a lesser extent, the heart, gills and intestine, are the organs which contain the highest amount of both MXR mRNAs or proteins. This tissue specific expression suggests a role for the identified mechanism in protection against endogenous or exogenous toxic compounds.

Expression of P-glycoprotein in the gills of oysters, Crassostrea virginica: seasonal and pollutant related effects

The expression of p-glycoprotein (p-gp) in aquatic organisms has been proposed as a biomarker of pollution exposure. Previous research has provided evidence that p-gp is inducible by organic xenobiotics and that p-gp is overexpressed in mussels from degraded areas. However, seasonal changes in expression at polluted sites has not been described previously. The purposes of these studies were to evaluate the expression of p-gp in polluted and unpolluted sites in oysters (Crassostrea virginica) and to determine if there were seasonal differences. P-gp expression and total protein concentrations were measured seasonally in the gills of southeastern oysters, Crassostrea virginica, at degraded and undegraded sites in Charleston Harbor, South Carolina, USA. At all sites, p-gp expression was generally higher during the warmer months and lower in the colder months. Polluted sites tended to show a decrease in p-gp expression in June and September, suggesting that p-gp inhibition may have occurred. P-gp expression was not significantly related to specific classes of sediment contaminants or to overall sediment contaminant loading. Total gill protein concentrations at all sites were lower during the warmer months and higher during the colder months. In general, all sites tended to show decreased total gill protein concentrations when compared to the control site. Total protein concentrations were significantly related to water temperature, specific classes of sediment contaminants and overall sediment contaminant loads, but there was no relationship with salinity.