Oyster metallothionein as an oxyradical scavenger: implications for hemocyte defense responses

Hippo dictates signaling for cellular homeostasis and immune defense in Crassostrea hongkongensis hemocytes

Introduction

The Hippo signaling pathway is an evolutionarily conserved signaling cascade that plays a crucial role in regulating cell proliferation, differentiation, and apoptosis. It has been shown to be a key regulator of cell fate and cellular homeostasis in various immune processes. Despite its well-established functions in vertebrate immunity, its roles in marine invertebrate immunity remain poorly understood. Therefore, our present work provides fresh mechanistic insights into how the Hippo pathway orchestrates hemocytic functions in Crassostrea hongkongensis, with implications for studies on its major forms and modifications in animal evolution.

Method

The complete set of Hippo pathway genes, including SAV1, MOB1, LATS, YAP/TAZ, TEAD, and MST, were identified from the C. hongkongensis genome. Quantitative PCR assays were conducted to examine the mRNA expression levels of these genes in different tissues and the levels of these genes in hemocytes before and after bacterial challenges. The study also examined the crosstalk between the Hippo pathway and other immune pathways, such as the AP-1 and p53-dependent p21 signaling cascades. RNA interference was used to knock down MST and TEAD, and MST is a core orchestrator of non-canonical Hippo signaling, to investigate its impact on phagocytosis and bacterial clearance in hemocytes.

Result

The results demonstrated that members of the Hippo pathway were highly expressed in hemocytes, with their expression levels significantly increasing following bacterial challenges. Crosstalk between the Hippo pathway and other immune pathways triggered hemocytic apoptosis, which functioned similarly to the canonical Mst-Lats-Yap signaling pathway in Drosophila and mammals. Knocking down MST resulted in increased phagocytosis and boosted the efficiency of bacterial clearance in hemocytes, presumably due to mobilized antioxidant transcription by Nrf for maintaining immune homeostasis.

Discussion

This study provides novel insights into the regulatory mechanisms underlying the Hippo pathway in immune responses of C. hongkongensis hemocytes. The study highlights the importance of the Hippo pathway in maintaining immune homeostasis and orchestrating hemocytic functions in oysters. Moreover, this study demonstrates the divergence of the Hippo pathway's roles in marine invertebrate immunity from mammalian observations, indicating the need for further comparative studies across species. These findings have significant implications for future research aimed at elucidating the evolutionary trajectory and functional diversity of the Hippo signaling pathway in animal evolution.

Hemocytes of bivalve mollusks as cellular models in toxicological studies of metals and metal-based nanomaterials

Understanding the impacts of environmental pollutants on immune systems is indispensable in ecological and health risk assessments due to the significance of normal immunological functions in all living organisms. Bivalves as sentinel organisms with vital ecological importance are widely distributed in aquatic environments and their innate immune systems are the sensitive targets of environmental pollutants. As the central component of innate immunity, bivalve hemocytes are endowed with specialized endolysosomal systems for particle internalization and metal detoxification. These intrinsic biological features make them a unique cellular model for metal- and nano-immunotoxicology research. In this review, we firstly provided a general overview of bivalve's innate immunity and the classification and immune functions of hemocytes. We then summarized the recent progress on the interactions of metals and nanoparticles with bivalve hemocytes, with emphasis on the involvement of hemocytes in metal regulation and detoxification, the interactions of hemocytes and nanoparticles at eco/bio-nano interface and hemocyte-mediated immune responses to the exposure of metals and nanoparticles. Finally, we proposed the key knowledge gaps and future research priorities in deciphering the fundamental biological processes of the interactions of environmental pollutants with the innate immune system of bivalves as well as in developing bivalve hemocytes into a promising cellular model for nano-immuno-safety assessment.

Low levels of Cd induce persisting epigenetic modifications and acclimation mechanisms in the earthworm Lumbricus terrestris

Toxic effects of cadmium (Cd), a common soil pollutant, are still not very well understood, particularly in regard to its epigenetic impact. Therefore, the aim of this study was to assess DNA methylation changes and their persistence in the earthworm Lumbricus terrestris upon chronic low dose Cd exposure using methylation sensitive amplification polymorphism (MSAP). Moreover, the biomarker response and fitness of the earthworms, as well as the expression of detoxification-related genes (metallothionein (MT) and phytochelatin synthase (PCS)) was evaluated. Low levels of Cd caused an increase in genome-wide DNA methylation, which remained partly modified, even after several months of recovery in unpolluted soil. Increased cellular stress seemed to decrease after two weeks of exposure whereas fitness parameters remained unaffected by Cd, probably as a result from the activation of detoxification mechanisms like the expression of MTs. Interestingly, even though the level of Cd exposure was very low, MT expression levels indicate the development of acclimation mechanisms. Taken together, this study demonstrates that acclimation, as well as epigenetic modifications can occur already in moderately polluted environments. In addition, these effects can have long-lasting impacts on key species of soil invertebrates and might persist long after the actual heavy metal challenge has passed.

Telomere damage and redox status alterations in free-living passerines exposed to metals

Telomere length may reflect the expected life span and possibly individual quality. Environmental stressors are known to increase oxidative stress and accelerate telomere attrition: however the interactions between redox status and telomere dynamics are not fully understood. We investigated whether exposure to heavy metal pollution is associated with oxidative stress and telomere damage in two insectivorous passerines, the Great tit (Parus major) and the Pied flycatcher (Ficedula hypoleuca). We were also interested to know whether within-brood competition could influence the nestling redox status or telomere length. Breeding females and nestlings were sampled near the point pollution source and compared to birds in non-polluted control zone. We measured heavy metal concentrations, calcium, metallothioneins, telomere lengths and redox status (oxidative damage, and enzymatic and non-enzymatic antioxidants) in liver samples. Great tit nestlings in the polluted zone had significantly shorter telomeres compared to those in the unpolluted control zone. In addition, those great tit nestlings that were lighter than their average siblings, had shorter telomeres compared to the heavier ones. In pied flycatchers neither pollution nor growth stress were associated with telomere length, but adult females had significantly shorter telomeres compared to the nestlings. All the results related to redox status varied remarkably among the species and the age groups. In both species antioxidants were related to pollution. There were no significant associations between redox status and telomere length. Our results suggest that wild birds at a young age are vulnerable to pollution and growth stress induced telomere damage. Redox status seems to interact with pollution and growth, but more studies are needed to clarify the underlying physiological mechanisms of telomere attrition. Our study highlights that all the observed associations and differences between the sampling zones varied depending on the species, age, and degree of exposure to pollution.

Oxidation of the N-terminal domain of the wheat metallothionein Ec -1 leads to the formation of three distinct disulfide bridges

Metallothioneins (MTs) are low molecular weight proteins, characterized by a high cysteine content and the ability to coordinate large amounts of d(10) metal ions, for example, Zn(II), Cd(II), and Cu(I), in form of metal-thiolate clusters. Depending on intracellular conditions such as redox potential or metal ion concentrations, MTs can occur in various states ranging from the fully metal-loaded holo- to the metal-free apo-form. The Cys thiolate groups in the apo-form can be either reduced or be involved in disulfide bridges. Although oxidation-mediated Zn(II) release might be a possible mechanism for the regulation of Zn(II) availability by MTs, no concise information regarding the associated pathways and the structure of oxidized apo-MT forms is available. Using the well-studied Zn2 γ-Ec -1 domain of the wheat Zn6 Ec -1 MT we attempt here to answer several question regarding the structure and biophysical properties of oxidized MT forms, such as: (1) does disulfide bond formation increase the stability against proteolysis, (2) is the overall peptide backbone fold similar for the holo- and the oxidized apo-MT form, and (3) are disulfide bridges specifically or randomly formed? Our investigations show that oxidation leads to three distinct disulfide bridges independently of the applied oxidation conditions and of the initial species used for oxidation, that is, the apo- or the holo-form. In addition, the oxidized apo-form is as stable against proteolysis as Zn2 γ-Ec -1, rendering the currently assumed degradation of oxidized MTs unlikely and suggesting a role of the oxidation process for the extension of protein lifetime in absence of sufficient amounts of metal ions. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 295-308, 2016.

Effects of Two Sublethal Concentrations of Mercury Chloride on the Morphology and Metallothionein Activity in the Liver of Zebrafish (Danio rerio)

Mercury (Hg) is a highly hazardous pollutant widely used in industrial, pharmaceutical and agricultural fields. Mercury is found in the environment in several forms, elemental, inorganic (iHg) and organic, all of which are toxic. Considering that the liver is the organ primarily involved in the regulation of metabolic pathways, homeostasis and detoxification we investigated the morphological and ultrastructural effects in Danio rerio liver after 96 h exposure to two low HgCl₂ concentrations (7.7 and 38.5 μg/L). We showed that a short-term exposure to very low concentrations of iHg severely affects liver morphology and ultrastructure. The main effects recorded in this work were: cytoplasm vacuolization, decrease in both lipid droplets and glycogen granules, increase in number of mitochondria, increase of rough endoplasmic reticulum and pyknotic nuclei. Pathological alterations observed were dose dependent. Trough immunohistochemistry, in situ hybridization and real-time PCR analysis, the induction of metallothionein (MT) under stressor conditions was also evaluated. Some of observed alterations could be considered as a general response of tissue to heavy metals, whereas others (such as increased number of mitochondria and increase of RER) may be considered as an adaptive response to mercury.

Localization and Spectroscopic Analysis of the Cu(I) Binding Site in Wheat Metallothionein Ec-1

The early cysteine-labeled metallothionein (MT) from Triticum aestivum (common wheat), denoted Ec-1, features two structurally well-defined domains, γ and βE, coordinating two and four Zn(II) ions, respectively. While the protein is currently assumed to function mainly in zinc homeostasis, a low amount of copper ions was also recently detected in a native Ec-1 sample. To evaluate the observed copper binding in more detail, the recombinant Zn₆Ec-1 form was exposed to different amounts of Cu(I) ions and the resulting species characterized with spectroscopic methods. Data reveal that the first Cu(I) equivalent coordinates exclusively to the N-terminal γ-domain of the protein and replaces one Zn(II) ion. To analyze the ability of the γ-domain for coordination of monovalent metal ions in more detail, the γ-Ec-1 peptide fragment was incubated with increasing amounts of Cu(I) and the process monitored with UV-VIS, circular dichroism, and luminescence spectroscopy. Closely similar spectra are observed regardless if the apo- or the metal ion-loaded and, hence, pre-folded forms, were used for the titration experiments with Cu(I). The results indicate that low amounts of Cu(I) ions displace the two metal ions subsequently and stoichiometrically, despite the different coordination geometry requirements of Cu(I) and Zn(II).

Cyclo(phenylalanine-proline) induces DNA damage in mammalian cells via reactive oxygen species

Cyclo(phenylalanine‐proline) is produced by various organisms such as animals, plants, bacteria and fungi. It has diverse biological functions including anti‐fungal activity, anti‐bacterial activity and molecular signalling. However, a few studies have demonstrated the effect of cyclo(phenylalanine‐proline) on the mammalian cellular processes, such as cell growth and apoptosis. In this study, we investigated whether cyclo(phenylalanine‐proline) affects cellular responses associated with DNA damage in mammalian cells. We found that treatment of 1 mM cyclo(phenylalanine‐proline) induces phosphorylation of H2AX (S139) through ATM‐CHK2 activation as well as DNA double strand breaks. Gene expression analysis revealed that a subset of genes related to regulation of reactive oxygen species (ROS) scavenging and production is suppressed by the cyclo(phenylalanine‐proline) treatment. We also found that cyclo(phenylalanine‐proline) treatment induces perturbation of the mitochondrial membrane, resulting in increased ROS, especially superoxide, production. Collectively, our study suggests that cyclo(phenylalanine‐proline) treatment induces DNA damage via elevation of ROS in mammalian cells. Our findings may help explain the mechanism underlying the bacterial infection‐induced activation of DNA damage response in host mammalian cells.

Effects of subchronic exposure to glyphosate in juvenile oysters (Crassostrea gigas): From molecular to individual levels

Glyphosate-based herbicides are extensively used and can be measured in aquatic ecosystems, including coastal waters. The effect of glyphosate on non-target organisms is an issue of worldwide concern. The aim of this study was to investigate the effects of subchronic exposure to glyphosate in juvenile oysters, Crassostrea gigas. Yearling oysters were exposed to three concentrations of glyphosate (0.1, 1 and 100μgL(-1)) for 56days. Various endpoints were studied, from the individual level (e.g., gametogenesis and tissue alterations) to the molecular level (mRNA quantification), including biochemical endpoints such as glutathione-S-transferase (GST) and catalase activities and malondialdehyde content. No mortality and growth occurred during the experiment, and individual biomarkers revealed only slight effects. The levels of gene expression significantly increased in oysters exposed to the highest glyphosate concentration (GST and metallothioneins) or to all concentrations (multi-xenobiotic resistance). These results suggested an activation of defence mechanisms at the molecular level.

Antioxidant responses of Annelids, Brassicaceae and Fabaceae to pollutants: a review

Pollutants, such as Metal Trace Elements (MTEs) and organic compounds (polycyclic aromatic hydrocarbons, pesticides), can impact DNA structure of living organisms and thus generate damage. For instance, cadmium is a well-known genotoxic and mechanisms explaining its clastogenicity are mainly indirect: inhibition of DNA repair mechanisms and/or induction of Reactive Oxygen Species (ROS). Animal or vegetal cells use antioxidant defense systems to protect themselves against ROS produced during oxidative stress. Because tolerance of organisms depends, at least partially, on their ability to cope with ROS, the mechanisms of production and management of ROS were investigated a lot in Ecotoxicology as markers of biotic and abiotic stress. This was mainly done through the measurement of enzyme activities The present Review focuses on 3 test species living in close contact with soil that are often used in soil ecotoxicology: the worm Eisenia fetida, and two plant species, Trifolium repens (white clover) and Brassica oleracea (cabbage). E. fetida is a soil-dwelling organism commonly used for biomonitoring. T. repens is a symbiotic plant species which forms root nodule with soil bacteria, while B. oleracea is a non-symbiotic plant. In literature, some oxidative stress enzyme activities have already been measured in those species but such analyses do not allow distinction between individual enzyme involvements in oxidative stress. Gene expression studies would allow this distinction at the transcriptomic level. A literature review and a data search in molecular database were carried out on the basis of keywords in Scopus, in PubMed and in Genbank™ for each species. Molecular data regarding E. fetida were already available in databases, but a lack of data regarding oxidative stress related genes was observed for T. repens and B. oleracea. By exploiting the conservation observed between species and using molecular biology techniques, we partially cloned missing candidates involved in oxidative stress and in metal detoxification in E. fetida, T. repens and B. oleracea.

Identification and expression of antioxidant and immune defense genes in the surf clam Mesodesma donacium challenged with Vibrio anguillarum

The immune system in marine invertebrates is mediated through cellular and humoral components, which act together to address the action of potential pathogenic microorganisms. In bivalve mollusks biomolecules implicated in oxidative stress and recognition of pathogens have been involved in the innate immune response. To better understand the molecular basis of the immune response of surf clam Mesodesma donacium, qPCR approaches were used to identify genes related to its immune response against Vibrio anguillarum infection. Genes related to oxidative stress response and recognition of pathogens like superoxide dismutase (MdSOD), catalase (MdCAT), ferritin (MdFER) and filamin (MdFLMN) were identified from 454-pyrosequencing cDNA library of M. donacium and were evaluated in mantle, adductor muscle and gills. The results for transcripts expression indicated that MdSOD, MdFLMN and MdFER were primarily expressed in the muscle, while MdCAT was more expressed in gills. Challenge experiments with the pathogen V. anguillarum had showed that levels of transcript expression for MdSOD, MdCAT, MdFER, and MdFLMN were positively regulated by pathogen, following a time-dependent expression pattern with significant statistical differences between control and challenge group responses (p<0.05). These results suggest that superoxide dismutase, catalase, ferritin and filamin, could be contributing to the innate immune response of M. donacium against the pathogen V. anguillarum.

Heavy metal-induced metallothionein expression is regulated by specific protein phosphatase 2A complexes

Induction of metallothionein (MT) expression is involved in metal homeostasis and detoxification. To identify the key pathways that regulate metal-induced cytotoxicity, we investigate how phosphorylated metal-responsive transcription factor-1 (MTF-1) contributed to induction of MT expression. Immortal human embryonic kidney cells (HEK cells) were treated with seven kinds of metals including cadmium chloride (CdCl2), zinc sulfate (ZnSO4), copper sulfate(CuSO4), lead acetate (PbAc), nickel sulfate (NiSO4), sodium arsenite (NaAsO2), and potassium bichromate (K2Cr2O7). The MT expression was induced in a dose-response and time-dependent manner upon various metal treatments. A cycle of phosphorylation and dephosphorylation was required for translocation of MTF-1 from cytoplasm to nucleus, leading to the up-regulation of MTs expression. Protein phosphatase 2A (PP2A) participated in regulating MT expression through dephosphorylation of MTF-1. A loss-of-function screen revealed that the specific PP2A complexes containing PR110 were involved in metal-induced MT expression. Suppression of PP2A PR110 in HEK cells resulted in the persistent MTF-1 phosphorylation and the disturbance of MTF-1 nuclear translocation, which was concomitant with a significant decrease of MT expression and enhanced cytotoxicity in HEK cells. Notably, MTF-1 was found in complex with specific PP2A complexes containing the PR110 subunit upon metal exposure. Furthermore, we identify that the dephosphorylation of MTF-1 at residue Thr-254 is directly regulated by PP2A PR110 complexes and responsible for MTF-1 activation. Taken together, these findings delineate a novel pathway that determines cytotoxicity in response to metal treatments and provide new insight into the role of PP2A in cellular stress response.

Differential effects of metal contamination on the transcript expression of immune- and stress-response genes in the Sydney Rock oyster, Saccostrea glomerata

Environmental contamination by metals is a serious threat to the biological sustainability of coastal ecosystems. Our current understanding of the potential biological effects of metals in these ecosystems is limited. This study tested the transcriptional expression of immune- and stress-response genes in Sydney Rock oysters (Saccostrea glomerata). Oysters were exposed to four metals (cadmium, copper, lead and zinc) commonly associated with anthropogenic pollution in coastal waterways. Seven target genes (superoxide dismutase, ferritin, ficolin, defensin, HSP70, HSP90 and metallothionein) were selected. Quantitative (real-time) PCR analyses of the transcript expression of these genes showed that each of the different metals elicited unique transcriptional profiles. Significant changes in transcription were found for 18 of the 28 combinations tested (4 metals × 7 genes). Of these, 16 reflected down-regulation of gene transcription. HSP90 was the only gene significantly up-regulated by metal contamination (cadmium and zinc only), while defensin expression was significantly down-regulated by exposure to all four metals. This inhibition could have a significant negative effect on the oyster immune system, promoting susceptibility to opportunistic infections and disease.

Are metallothioneins equally good biomarkers of metal and oxidative stress?

Several researchers investigated the induction of metallothioneins (MTs) in the presence of metals, namely Cadmium (Cd). Fewer studies observed the induction of MTs due to oxidizing agents, and literature comparing the sensitivity of MTs to different stressors is even more scarce or even nonexistent. The role of MTs in metal and oxidative stress and thus their use as a stress biomarker, remains to be clearly elucidated. To better understand the role of MTs as a biomarker in Cerastoderma edule, a bivalve widely used as bioindicator, a laboratory assay was conducted aiming to assess the sensitivity of MTs to metal and oxidative stressors. For this purpose, Cd was used to induce metal stress, whereas hydrogen peroxide (H2O2), being an oxidizing compound, was used to impose oxidative stress. Results showed that induction of MTs occurred at very different levels in metal and oxidative stress. In the presence of the oxidizing agent (H2O2), MTs only increased significantly when the degree of oxidative stress was very high, and mortality rates were higher than 50 percent. On the contrary, C. edule survived to all Cd concentrations used and significant MTs increases, compared to the control, were observed in all Cd exposures. The present work also revealed that the number of ions and the metal bound to MTs varied with the exposure conditions. In the absence of disturbance, MTs bound most (60-70 percent) of the essential metals (Zn and Cu) in solution. In stressful situations, such as the exposure to Cd and H2O2, MTs did not bind to Cu and bound less to Zn. When organisms were exposed to Cd, the total number of ions bound per MT molecule did not change, compared to control. However the sort of ions bound per MT molecule differed; part of the Zn and all Cu ions where displaced by Cd ions. For organisms exposed to H2O2, each MT molecule bound less than half of the ions compared to control and Cd conditions, which indicates a partial oxidation of thiol groups in the cysteine residues through ROS scavenging. The present results suggest that MTs are excellent markers of metal stress, but not of oxidative stress.

Biomarker measurements in Trifolium repens and Eisenia fetida to assess the toxicity of soil contaminated with landfill leachate: a microcosm study

To assess the toxicity of a soil contaminated with landfill leachate, biomarker measurements in two species living in close contact with the soil, i.e. a plant species Trifolium repens and an animal species Eisenia fetida, were conducted. Briefly, both species were studied after simultaneous exposure conducted in microcosms. The organisms were exposed to soil supplemented with pure leachate, leachate diluted to 50%; leachate diluted to 25% and without leachate. After a 10 weeks exposure period, we observed an increase in the Olive Trail Moment in T. repens, compared to the reference, for 50% and pure leachate. The response observed appears to be dose-dependent and linear in our experimental conditions. Addition of the leachate to the reference soil induced an increase in Cd-Metallothionein-coding mRNA quantity in E. fetida. In addition, expression level of another gene implied in detoxification and coding Phytochelatin synthase was significantly induced in worms exposed to the reference soil spiked with the leachate, regardless presence of T. repens. Thus, T. repens and E. fetida can be used in a complementary manner to assess soil quality. Sensitivities of the test species yield sensitive bioassays as both species responded at low doses despite the buffering effect of the soil.

Differential immune response in the hard clam (mercenaria mercenaria) against bacteria and the protistan pathogen QPX (quahog parasite unknown)

The immune response of the hard clam (quahog) Mercenaria mercenaria following challenge with live bacteria (Vibrio alginolyticus) and the protist QPX (Quahog Parasite Unknown) was investigated. The study also compared immune responses following QPX challenge in two different hard clam broodstocks exhibiting different degrees of susceptibility toward this parasite. Different immune and stress-related cellular and humoral factors were assessed including general hemocyte parameters (total and differential hemocyte counts, percentage of dead cells, reactive oxygen production, phagocytosis), parameters geared toward QPX (anti-QPX activity in plasma and hemocyte resistance to the cytotoxicity of QPX extracellular products). Two genes (ferritin and metallothionein) previously shown to be modulated following QPX exposure were molecularly characterized by rapid amplification of cDNA ends (RACE) and their transcription levels were determined in resistant and susceptible clams in response to QPX and bacterial challenge. Results indicated that both V. alginolyticus and QPX challenge triggered significant immune responses in clams with similar trends for most measured parameters. However, specific responses were observed for anti-QPX activity in plasma and hemocyte resistance to QPX products as well as ferritin and metallothionein expression according to each inoculum. Similarly, different response patterns were detected following QPX challenge in susceptible and resistant clam stocks. Resistant clams were able to elicit effective response against the parasite leading to the elimination of QPX and the restoration of constitutive immune status whereas QPX-susceptible clams triggered a strong immune modulation characterized by an acute phase response and associated acute phase protein but appeared to be less active in eliminating the parasite. These results suggest that different signaling pathways are triggered during V. alginolyticus and QPX challenge. Moreover, differences in the immune response toward QPX might be linked to the susceptibility or resistance of different clam stocks to the infection by this parasite.

Effects of vibrio challenge on digestive gland biomarkers and antioxidant gene expression in Mytilus galloprovincialis

In bivalve molluscs, responses to bacterial infection have been largely characterized in terms of both functional responses and gene expression in the immune cells, the hemocytes. The effects of bacterial challenge at the tissue level, where bacterial infection may cause stressful conditions, have not been so far specifically investigated. Biomarkers are widely utilised to evaluate the health status of bivalves, from the molecular to the organism level, in response to both natural and anthropogenic stressors. In this work, the effects of in vivo challenge with heat-killed vibrio species, Vibrio splendidus LGP32 and Vibrio anguillarum (ATCC19264), on different biomarkers in the digestive gland of the marine bivalve Mytilus galloprovincialis were investigated. Mussels were injected with either vibrio and tissues sampled at 3, 6 and 24 h post injection (p.i.). Lysosomal biomarkers, such as lysosomal membrane stability (LMS) and lipofuscin accumulation, as well as specific activities of antioxidant enzymes (catalase and glutathione transferase-GST) were evaluated. Moreover, the expression of antioxidant molecules (catalase, GST-pi and metallothioneins MT10 and MT20) was determined by quantitative RT-PCR. Both V. splendidus and V. anguillarum significantly affected all parameters measured, to a different extent and at different times p.i. Interestingly, whereas both vibrios induced lysosomal membrane destabilisation and increases in the activities of antioxidant enzymes, distinct responses were observed in terms of lysosomal lipofuscin accumulation and expression of antioxidant molecules. In particular, V. splendidus induced a general increase in the transcription of antioxidant genes, indicating that Mytilus digestive gland can mount an efficient antioxidant response towards this vibrio species. On the other hand, a general down-regulation or no effect was observed with V. anguillarum. The lack of this response was reflected in stronger oxidative stress conditions in the digestive gland of mussels challenged with V. anguillarum, as indicated by higher levels of lysosomal lipofuscin observed at longer times p.i. Overall, these data indicate that lysosomal and oxidative stress biomarkers could be usefully applied in order to monitor early changes in the health status of bivalves induced by bacteria. Moreover, the results support the hypothesis that host responses to bacteria may be taken into account when interpreting biomarker data in ecotoxicological studies.

Metal-related oxidative stress in birds

Metals can cause oxidative stress by increasing the formation of reactive oxygen species (ROS), which render antioxidants incapable of defence against growing amounts of free radicals. Metal toxicity is related to their oxidative state and reactivity with other compounds. Our aim is to review the mechanisms on how metals cause oxidative stress and what is known about metal-induced oxidative stress in wildlife. Taking birds as model organisms, we summarize the mechanisms responsible for antioxidant depletion and give a view of how to detect metal-induced oxidative stress in birds by using different biomarkers. The mechanisms producing the harmful effects of oxidative stress are complex with different biomolecular mechanisms associated with ecotoxicological and ecological aspects. The majority of the studies concerning metals and ROS related to oxidative stress have focused on the biomolecular level, but little is known about the effects at the cellular level or at the level of individuals or populations.

Identification and expression of differentially expressed genes in the hard clam, Mercenaria mercenaria, in response to quahog parasite unknown (QPX)

Background

The hard clam, Mercenaria mercenaria, has been affected by severe mortality episodes associated with the protistan parasite QPX (Quahog Parasite Unknown) for several years. Despite the commercial importance of hard clams in the United States, molecular bases of defense mechanisms in M. mercenaria, especially during QPX infection, remain unknown.

Results

Our study used suppression subtractive hybridization (SSH), as well as the construction of cDNA libraries from hemocytes to identify genes related to the defense of the hard clam against its parasite. Hard clams were experimentally infected with QPX and SSH was performed on mRNA samples extracted from mantle and gill tissues at different times post-challenge. A total of 298 clones from SSH libraries and 1352 clones from cDNA libraries were sequenced. Among these sequences, homologies with genes involved in different physiological processes related to signal transduction, stress response, immunity and protein synthesis were identified. Quantitative PCR revealed significant changes in the expression of several of these genes in response to QPX challenge and demonstrated significant correlations in terms of levels of gene expression between intermediates of signalling pathways and humoral defense factors, such as big defensin and lysozyme.

Conclusion

Results of this study allowed the detection of modifications caused by QPX at the transcriptional level providing insight into clam immune response to the infection. These investigations permitted the identification of candidate genes and pathways for further analyses of biological bases of clam resistance to QPX allowing for a better understanding of bivalve immunity in general.

Seasonal changes in mRNA encoding for cell stress markers in the oyster Crassostrea gigas exposed to radioactive discharges in their natural environment

The North Cotentin area (Normandy, France) hosts several nuclear facilities among which the AREVA reprocessing plant of La Hague is responsible for controlled discharges of liquid radioactive wastes into the marine environment. The resulting increase in radioactivity is very small compared to natural radioactivity. However, concerns about environment protection prompted the scientific community to focus on the effects of the chronic exposure to low concentrations of radionuclides in non-human biota. This study contributes to the evaluation of the possible impact of radioactive discharges on the oyster Crassostrea gigas in the field. Real-time polymerase chain reaction was used to quantify the expression levels of genes involved in cell stress in the oyster. They included members of the heat shock protein family (Hsp70, Hsc72, Hsp90), superoxide dismutase (SOD) and metallothionein (MT). Times series measurements were built from periodic samplings in the natural environment in order to characterize the natural variability as well as possible seasonal fluctuations. The genes studied exhibited a general seasonal expression pattern with a peak value in winter. The data inversely correlated with seawater temperature and the nature of the relationship between gene expression and temperature is discussed. In parallel, oysters were collected in four locations on the French shores, exposed or not to radioactive liquid wastes from the nuclear facilities hosted in the North Cotentin. The comparison of data obtained in the reference location on the Atlantic coast (not exposed) and data from oysters of the English Channel (exposed) gave no evidence for any statistical difference. However, because of the complexity of the natural environment, we cannot rule out the possibility that other parameters may have masked the impact of radioactive discharges. This dense set of data is a basis for the use of the expression levels of those genes as biomarkers to address the question of the possible effects of chronic exposure of the oyster to low concentrations of radionuclides in controlled laboratory experimental conditions.

The strong induction of metallothionein gene following cadmium exposure transiently affects the expression of many genes in Eisenia fetida: a trade-off mechanism?

Metal pollution causes disturbances at various levels of biological organization in most species. Important physiological functions could be affected in the exposed individuals and among the main physiological functions, immunity may provide one (or more) effector(s) whose expression can be directly affected by a metal exposure in various macroinvertebrates. Protein expressions were studied in order to test them as molecular biomarkers of metal exposure in Eisenia fetida. Selected effectors were calmodulin, heat shock proteins, superoxide dismutase, catalase, metallothionein, beta-adrenergic receptor kinase, pyruvate carboxylase, transcriptionally controlled tumor protein, protein kinase C, ubiquitin and cyclophilin-A. The level of expression of each gene was analysed in whole organism following exposures to cadmium in soil using real-time PCR. Metallothionein, transcriptionally controlled tumor protein and cyclophilin-A expression were also measured following copper exposures in soil because these genes seemed to be sensitive to copper. This work enabled to distinguish metallothionein and cyclophilin-A among the 15 selected effectors. A strong decrease of the number of transcripts was also detected for most effectors soon after the exposure to cadmium suggesting that a trade-off mechanism occurs.

Structures of PmSOD1 and PmSOD2, two superoxide dismutases from the protozoan parasite Perkinsus marinus

Perkinsus marinus, a facultative intracellular parasite of the eastern oyster Crassostrea virginica, is responsible for mass mortalities of oyster populations. P. marinus trophozoites survive and proliferate within oyster hemocytes, invading most tissues and fluids, thus causing a systemic infection that eventually kills the host. The phagocytosis of P. marinus trophozoites lacks a respiratory burst, suggesting that the parasite has mechanisms that actively abrogate the host's oxidative defense responses. One mechanism and the first line of defense against oxidative damage is the dismutation of superoxide radical to molecular oxygen and hydrogen peroxide by superoxide dismutases (SODs). P. marinus possesses two iron-cofactored SODs, PmSOD1 and PmSOD2. Here, the crystallization and X-ray structures of both PmSOD1 and PmSOD2 are presented.

Temperature-dependent stress response in oysters, Crassostrea virginica: pollution reduces temperature tolerance in oysters

Combined effects of temperature and a toxic metal, cadmium (Cd), on energy metabolism were studied in a model marine bivalve, the eastern oyster Crassostrea virginica, acclimated at 20, 24 and 28 degrees C and exposed to 50microgl(-1) of Cd. Both increasing temperature and Cd exposure led to a rise in standard metabolic rates, and combined stressors appeared to override the capability for aerobic energy production resulting in impaired stress tolerance. Oysters exposed to elevated temperature but not Cd showed no significant change in condition, survival rate and lipid peroxidation, whereas those exposed to both Cd and temperature stress suffered high mortality accompanied by low condition index and elevated lipid peroxidation. Furthermore, RNA/DNA ratios indicative of protein synthesis rate, and levels of glutathione, which is involved in metal detoxification, increased in Cd-exposed oysters at 20 degrees C but not at 28 degrees C. Implications of the synergism between elevated temperatures and cadmium stress on energy metabolism of oysters are discussed in the light of the potential effects of climate change on oyster populations in polluted areas.

Cloning and real-time PCR testing of 14 potential biomarkers in Eisenia fetida following cadmium exposure

Important biological activities could be affected in metal exposed species, and amongthe main physiological functions, immunity may provide one (or more) effector(s) which expression can be directly affected by a metal exposure in various macroinvertebrates. As many proteinic effectors showed a high degree of homology between species, we have developed a PCR approach to characterize partial mRNA sequences of selected effectors in the laboratory model, Eisenia fetida. After cloning, levels of expression of each gene were analyzed following exposures (80 and 800 mg/kg) to cadmium spiked soils using real-time PCR. An implemented approach was allowed to test quickly potential biomarkers in Eisenia fetida. Selected effectors were calmodulin, heat shock proteins, superoxide dismutase, catalase, metallothionein, beta-adrenergic receptor kinase, pyruvate carboxylase, trancriptionally controlled tumor protein, protein kinase C, and ubiquitin. Most of the selected effectors did not show variations of expression level after exposure. Others expressed weak changes of expression as heat shock proteins. At lastfor catalase and metallothionein, early suitable variations of expression were observed.

Involvement of differential metallothionein expression in free radical sensitivity of RTG-2 and CHSE-214 cells

The role of metallothionein (MT) in free radical regulation and scavenging was investigated using two fish cell lines, the rainbow trout gonadal (RTG-2) cell line and the chinook salmon embryonic (CHSE-214) cell line. Exposure of RTG-2 cells to H(2)O(2) resulted in upregulation of both MT mRNA and MT protein and was also demonstrated by immunocytochemistry, confirming that MT was regulated by free radicals. We then compared the H(2)O(2) resistance in RTG-2 and CHSE-214 cells following metal treatment with Zn or Cd to induce MT. Comparison of survival of control cells and metal-exposed cells showed that metal treatment, which induced MT, significantly raised the H(2)O(2) tolerance in a dose-dependent manner in RTG-2 cells, while no increased H(2)O(2) resistance was observed in CHSE-214 cells. Transient over-expression of MT in CHSE-214: 59 cells also resulted in a dose-dependent increase in resistance to H(2)O(2) exposure. The raised resistance against H(2)O(2) in metal treated RTG-2 cells as well as transfected CHSE-214: 59 cells strongly demonstrate that MT is involved in the protection against H(2)O(2) and suggest a physiologically important function for MT when cells or whole organisms are exposed to oxidative stress.