Gender-specific modulation of immune system complement gene expression in marine medaka Oryzias melastigma following dietary exposure of BDE-47

Weathered polyethylene microplastics induced immunomodulation in zebrafish

Microplastics are pollutants of emerging concern and the aquatic biota consumes microplastics (MPs), which has a range of toxicological and environmental effects on aquatic organisms that are not the intended targets. The current study looked into how weathered polyethylene (wPE) MPs affected Danio albolineatus immunological and haematological markers. In this experiment, fish of both sexes were placed in control and exposure groups, and they were exposed for 40 d at the sublethal level (1μgL-1) of fragmented wPE, which contained 1074 ± 52 MPs per litre. Similarly, fish exposed to wPE MPs showed significant modifications in lysozyme, antimicrobial, and antiprotease activity, as well as differential counts. Results of the present study show that the male fish were more susceptible than female fish after 40 d of chronic exposure. Further studies are needed to ascertain how the innate and humoral immune systems of the fish respond to MPs exposure.

Transcriptomic response patterns of hornyhead turbot (Pleuronichthys verticalis) dosed with polychlorinated biphenyls and polybrominated diphenyl ethers

To evaluate the impact of environmental contaminants on aquatic health, extensive surveys of fish populations have been conducted using bioaccumulation as an indicator of impairment. While these studies have reported mixtures of chemicals in fish tissues, the relationship between specific contaminants and observed adverse impacts remains poorly understood. The present study aimed to characterize the toxicological responses induced by persistent organic pollutants in wild-caught hornyhead turbot (P. verticalis). To do so, hornyhead turbot were interperitoneally injected with a single dose of PCB or PBDE congeners prepared using environmentally realistic mixture proportions. After 96-hour exposure, the livers were excised and analyzed using transcriptomic approaches and analytical chemistry. Concentrations of PCBs and PBDEs measured in the livers indicated clear differences across treatments, and congener profiles closely mirrored our expectations. Distinct gene profiles were characterized for PCB and PBDE exposed fish, with significant differences observed in the expression of genes associated with immune responses, endocrine-related functions, and lipid metabolism. Our findings highlight the key role that transcriptomics can play in monitoring programs to assess chemical-induced toxicity in heterogeneous group of fish (mixed gender and life stage) as is typically found during field surveys. Altogether, the present study provides further evidence of the potential of transcriptomic tools to improve aquatic health assessment and identify causative agents.

Advancing the fathead minnow (Pimephales promelas) as a model for immunotoxicity testing: Characterization of the renal transcriptome following Yersinia ruckeri infection

Recent studies have utilized the fathead minnow (Pimephales promelas) to explore the immunotoxic effects associated with a variety of environmental contaminants in the absence of immunological stimuli. Though this approach allows for alterations in the resting immune system to be detected, previous evidence suggests that many immunotoxic effects may only manifest in the activated immune system. However, basic immune responses to pathogens have not been well described in this species. To expand the utility of the fathead minnow as a model for immunotoxicity testing, a more comprehensive understanding of the activated immune system is required. As such, the main goal of this study was to characterize the transcriptomic response to pathogen infection in the fathead minnow using RNA sequencing. To achieve this goal, female fathead minnows were intraperitoneally injected with either Hank's Balanced Salt Solution (sham-injected) or Yersinia ruckeri (pathogen-injected). Eight hours following injection, fish were sacrificed for the assessment of general morphological (i.e., mass, length, condition factor, hepatic index) and immunological (i.e., leukocyte counts, spleen index) endpoints. To assess the molecular immune response to Y. ruckeri, kidney tissue was collected for transcriptomic analysis. A comparison of sham- and pathogen-injected fish revealed that >1800 genes and >500 gene networks were differentially expressed.Gene networks associated with inflammation, innate immunity, complement, hemorrhaging and iron absorption are highlighted and their utility within the context of immunotoxicity is discussed. These data reveal pathogen-related molecular endpoints to improve data interpretation of future studies utilizing the fathead minnow as a model for immunotoxicity.

Effects of dietary exposure to ciguatoxin P-CTX-1 on the reproductive performance in marine medaka (Oryzias melastigma)

Ciguatoxins are natural compounds produced by benthic dinoflagellates Gambierdiscus and Fukuyoa spp., which cause fish intoxication by ciguatera fish poisoning. This study aimed to assess the dietary exposure effects of ciguatoxin P-CTX-1 on the reproductive performance in marine medaka (Oryzias melastigma). Fish which ingested >1.16 pg·day^-1 for 21 days exhibited abnormal behaviors including diarrhea, abnormal swimming, loss of appetite and decreased egg production. After 7-day exposure to P-CTX-1 at a dose of 1.93 pg·day^-1, significant gender-specific differences in reproductive performance and decreased hatching rate of the offspring were observed. Chemical analysis of P-CTX-1 showed that the P-CTX-1 accumulation rates were 24.1 ± 1.4% in female fish and 9.9 ± 0.4% in male fish, and 0.05 pg·egg^-1 was detected. The results illustrate that dietary exposure to P-CTX-1 affected the reproductive performance and survival of offspring, and caused bioaccumulation and maternal transfer of P-CTX-1 in marine medaka.

Transcriptomic analysis revealing hepcidin expression in Oryzias melastigma regulated through the JAK-STAT signaling pathway upon exposure to BaP

Our previous study revealed that an antimicrobial peptide hepcidin, can be significantly up-regulated either with LPS challenge or upon exposure to Benzo[a]pyrene (BaP) in red sea bream, but the molecular mechanism involved in whether the transcriptional expression of hepcidin induced by LPS or BaP is regulated through a similar signaling pathway is not yet known. To elucidate the underlying molecular mechanism, the marine model fish Oryzias melastigma was exposed to 1 μg/L BaP as well as challenged with 5 μg of LPS per fish. Samples at 3 h post-LPS challenge, and 2 d and 3 d post-BaP exposure were separately collected for transcriptome analysis. General analysis of the predicted immune-associated unigenes based on the transcriptomic data showed that the percentages of modulated immune-associated genes were 7% with LPS challenge, and 3% and 7% with BaP exposure at 2 and 3 days, respectively. Genes involved in functions like antimicrobial activity, neutrophil activation, and leukocyte chemotaxis were up-regulated with LPS challenge, whereas more than half of the immune associated genes including the KLF family were down-regulated upon BaP exposure, indicating a difference in the modulated immune genes between LPS challenge and BaP exposure. Specific comparative analyses of the immune-associated signal pathways NOD, TOLL, NF-κB and JAK-STAT with LPS challenge or upon exposure to BaP, indicated that most of the modulated genes in association with the NOD, TOLL and NF-κB pathways were induced with LPS challenge but only a few after exposure to BaP, suggesting that BaP exposure was generally not associated with any of the three signal pathways. Interestingly, further transcriptomic analysis revealed that 5 of the 8 modulated genes associated with the JAK-STAT pathway were down-regulated, while 2 inhibiting genes were up-regulated after BaP exposure for 2 days whereas LPS challenge resulted in only less than half modulated, suggesting the possibility of down-regulation caused by BaP exposure through JAK-STAT pathway. Further testing using an EPC cell culture demonstrated that expression of the hepcidin1 gene was less involved in the known signal pathways, such as c/EBP, BMP, and NF-κB, but instead mostly in association with the JAK-STAT pathway upon BaP exposure.

Sex-specific immunomodulatory action of the environmentalestrogen 17α-ethynylestradiol alongside with reproductive impairment in fish

Estrogenic endocrine disrupting chemicals (EEDCs) are present ubiquitously in sediments and aquatic ecosystems worldwide. The detrimental impact of EEDCs on the reproduction of wildlife is widely recognized. Increasing evidence shows the immunosuppressive effects of EEDCs in vertebrates. Yet, no studies have considered concomitantly EEDC-induced impacts on reproductive impairment and immune suppression in vivo, which are deemed essential for risk assessment and environmental monitoring. In this study, EE2 was used as a representative EEDC, for parallel evaluation of EEDC-induced immune suppression (immune marker gene expression, leukocyte numbers, host resistance assay, and immune competence index) and reproductive impairment (estrogen responsive gene expression, fecundity, fertilization success, hatching success, and reproductive competence index) in an established fish model (marine medaka Oryzias melastigma), considering sex-specific induction and adaptation and recovery responses under different EE2 exposure scenarios. The findings in marine medaka reveal distinct sex differences in the EE2-mediated biological responses. For female fish, low concentration of exogenous EE2 (33 ng/L) could induce hormesis (immune enhancement), enable adaptation (restored reproduction) and even boost fish resistance to bacterial challenge after abatement of EE2. However, a prolonged exposure to high levels of EE2 (113 ng/L) not only impaired F0 immune function, but also perturbed females recovering from reproductive impairment, resulting in a persistent impact on the F1 generation output. Thus, for female fish, the exposure concentration of EE2 is more critical than the dose of EE2 in determining the impacts of EE2 on immune function and reproduction. Conversely, male fish are far more sensitive than females to the presence of low levels of exogenous EE2 in water and the EE2-mediated biological impacts are clearly dose-dependent. It is also evident in male fish that direct contact of EE2 is essential to sustain impairments of immune competence and reproductive output as well as deregulation of immune function genes in vivo. The immunomodulatory pathways altered by EE2 were deciphered for male and female fish, separately. Downregulation of hepatic tlr3 and c3 (in female) and tlr3, tlr5 and c3 (in male) may be indicative of impaired fish immune competence. Taken together, impaired immune competence in the EE2-exposed fish poses an immediate thread on the survival of F0 population. Impaired reproduction in the EE2-exposed fish can directly affect F1 output. Parallel evaluation of immune competence and reproduction are important considerations when assessing the risk of sublethal levels of EE2/EEDCs in aquatic environments.

Embryo-larval BDE-47 exposure causes decreased pathogen resistance in adult male fathead minnows (Pimephales promelas)

Exposures to polybrominated diphenyl ethers (PBDEs) have been shown to alter immune function in adult organisms across a variety of taxa. However, few if any studies have investigated the long-term consequences of early life stage PBDE exposures on immune function in fish. This study sought to determine the effects of early life stage BDE-47 exposure on pathogen resistance in the fathead minnow (Pimephales promelas) following an extended depuration period (≥180 d). Minnows were exposed to BDE-47 via a combination of maternal transfer and diet through 34 days post fertilization (dpf), raised to adulthood (>215 dpf) on a clean diet, then subjected to pathogen resistance trials. Early life stage exposures to BDE-47 did not affect the ability of females to survive from Yersinia ruckeri infection. However, the survival of BDE-47 exposed males was significantly reduced relative to controls, indicating that developmental exposures to BDE-47 altered male immunity. Because BDE-47 is a known thyroid hormone disruptor and thyroid hormone disruptors have the potential to adversely impact immune development and function, metrics indicative of thyroid disruption were evaluated, as were immune parameters known to be altered in response to thyroid disruption. BDE-47 exposed minnows exhibited signs of thyroid disruption (i.e., reduced growth); however, no alterations were observed in immune parameters known to be influenced by thyroid hormones (i.e., thymus size, expression of genes associated with lymphoid development) suggesting that the observed alterations in immunocompetence may occur through alternative mechanisms. Regardless of the mechanisms responsible, the results of this study demonstrate the potential for early life stage PBDE exposures to adversely impact immunity and illustrate that the immunological consequences of PBDE exposures are sex dependent.

The genome of the marine medaka Oryzias melastigma

Marine medaka (Oryzias melastigma) is considered to be a useful fish model for marine and estuarine ecotoxicology studies and has good potential for field-based population genomics because of its geographical distribution in Asian estuarine and coastal areas. In this study, we present the first whole-genome draft of O. melastigma. The genome assembly consists of 8,602 scaffolds (N50 = 23.737 Mb) and a total genome length of 779.4 Mb. A total of 23,528 genes were predicted, and 12,670 gene families shared with three teleost species (Japanese medaka, mangrove killifish and zebrafish) were identified. Genome analyses revealed that the O. melastigma genome is highly heterozygous and contains a large number of repeat sequences. This assembly represents a useful genomic resource for fish scientists.

Immune competence assessment in marine medaka (Orzyias melastigma)-a holistic approach for immunotoxicology

Many anthropogenic pollutants in coastal marine environments can induce immune impairments in wild fish and reduce their survival fitness. There is a pressing need to establish sensitive and high throughput in vivo tools to systematically evaluate the immunosuppressive effects of contaminants in marine teleosts. This study reviewed a battery of in vivo immune function detection technologies established for different biological hierarchies at molecular (immune function pathways and genes by next generation sequencing (NGS)), cellular (leukocytes profiles by flow cytometry), tissues/organ system (whole adult histo-array), and organism (host resistance assays (HRAs)) levels, to assess the immune competence of marine medaka Oryzias melastigma. This approach enables a holistic assessment of fish immune competence under different chemical exposure or environmental scenarios. The data obtained will also be useful to unravel the underlying immunotoxic mechanisms. Intriguingly, NGS analysis of hepatic immune gene expression profiles (male > female) are in support of the bacterial HRA findings, in which infection-induced mortality was consistently higher in females than in males. As such, reproductive stages and gender-specific responses must be taken into consideration when assessing the risk of immunotoxicants in the aquatic environment. The distinct phenotypic sexual dimorphism and short generation time (3 months) of marine medaka offer additional advantages for sex-related immunotoxicological investigation.

Sex-dependent telomere shortening, telomerase activity and oxidative damage in marine medaka Oryzias melastigma during aging

Marine medaka Oryzias melastigma at 4months (young), 8months (middle-aged) and 12months old (senior) were employed to determine age-associated change of sex ratios, sex hormones, telomere length (TL), telomerase activity (TA), telomerase transcription (omTERT) and oxidative damage in the liver. Overall, O. melastigma exhibited gradual senescence, sex differences in longevity (F>M), TL (F>M) and oxidative damage (F<M) during aging. In females, the plasma E2 level was positively correlated with TL (TRF>5kb), TA and omTERT expression (p≤0.01), and negatively correlated with liver DNA oxidation (p≤0.05). The results suggest high levels of E2 in female O. melastigma may retard TL shortening by enhancing TA via TERT transcription and/or reducing oxidative DNA damage. The findings support TL shortening as a biomarker of aging and further development of accelerated TL shortening, abnormal suppression of TA and excessive oxidative DNA damage as early molecular endpoints, indicative of advanced/premature aging in marine medaka/fish.

Modification of the plasma complement protein profile by exogenous estrogens is indicative of a compromised immune competence in marine medaka (Oryzias melastigma)

Growing evidence suggests that the immune system of teleost is vulnerable to xenoestrogens, which are ubiquitous in the marine environment. This study detected and identified the major circulatory immune proteins deregulated by 17α-ethinylestradiol (EE2), which may be linked to fish susceptibility to pathogens in the marine medaka, Oryzias melastigma. Fish immune competence was determined using a host resistance assay to pathogenic bacteria Edwardsiella tarda. Females were consistently more susceptible to infection-induced mortality than males. Exposure to EE2 could narrow the sex gap of mortality by increasing infection-induced death in male fish. Proteomic analysis revealed that the major plasma immune proteins of adult fish were highly sexually dimorphic. EE2 induced pronounced sex-specific changes in the plasma proteome, with the male plasma composition clearly becoming "feminised". Male plasma was found to contain a higher level of fibrinogens, WAP63 and ependymin-2-like protein, which are involved in coagulation, inflammation and regeneration. For the first time, we demonstrated that expression of C1q subunit B (C1Q), an initiating factor of the classical complement pathway, was higher in males and was suppressed in both sexes in response to EE2 and bacterial challenge. Moreover, cleavage and post-translational modification of C3, the central component of the complement system, could be altered by EE2 treatment in males (C3dg down; C3g up). Multiple regression analysis indicated that C1Q is possibly an indicator of fish survival, which warrants further confirmation. The findings support the potential application of plasma immune proteins for prognosis/diagnosis of fish immune competence. Moreover, this study provides the first biochemical basis of the sex-differences in fish immunity and how these differences might be modified by xenoestrogens.

20 Years of fish immunotoxicology - what we know and where we are

Despite frequent field observations of impaired immune response and increased disease incidence in contaminant-exposed wildlife populations, immunotoxic effects are rarely considered in ecotoxicological risk assessment. The aim of this study was to review the literature on immunotoxic effects of chemicals in fish to quantitatively evaluate (i) which experimental approaches were used to assess immunotoxic effects, (ii) whether immune markers exist to screen for potential immunotoxic activities of chemicals, and (iii) how predictive those parameters are for adverse alterations of fish immunocompetence and disease resistance. A total of 241 publications on fish immunotoxicity were quantitatively analyzed. The main conclusions included: (i) To date, fish immunotoxicology focused mainly on innate immune responses and immunosuppressive effects. (ii) In numerous studies, the experimental conditions are poorly documented, as for instance age or sex of the fish or the rationale for the selected exposure conditions is often missing. (iii) Although a broad variety of parameters were used to assess immunotoxicity, the rationale for the choice of measured parameters was often not given, remaining unclear how they link to the suspected immunotoxic mode of action of the chemicals. (iv) At the current state of knowledge, it is impossible to identify a set of immune parameters that could reliably screen for immunotoxic potentials of chemicals. (v) Similarly, in fish immunotoxicology there is insufficient understanding of how and when chemical-induced modulations of molecular/cellular immune changes relate to adverse alterations of fish immunocompetence, although this would be crucial to include immunotoxicity in ecotoxicological risk assessment.

Characterization of basic immune function parameters in the fathead minnow (Pimephales promelas), a common model in environmental toxicity testing

The fathead minnow (Pimephales promelas) is an environmental sentinel species, commonly used in toxicity testing. However, there is a lack of data regarding basic immune function in this species. To improve the usefulness of the fathead minnow as a model for basic immune function and immunotoxicity, this study sought to 1) compare the differential expression of immune function genes in naïve fathead minnows and 2) determine the effects of pathogen exposure on immune gene expression and spleen index. To accomplish this, kidney, spleen and liver tissue were collected three days post injection (dpi) from adult male fathead minnows from each of the following groups: 1) uninjected control 2) sham-injected (Hank's balanced salt solution) and 3) pathogen-injected (Yersinia ruckeri). Spleen tissue was also collected at seven and 14 dpi. Differential tissue expression of immune function genes was evaluated in naïve minnows and expression patterns were similar to those found in other fish species, with liver tissue generally having the highest amount of expression. Following pathogen injection, the expression of complement component 3 (c3) (4.4-fold, kidney; 2.5-fold, liver), interleukin 11 (il11) (4.8-fold, kidney; 15.2-fold, liver) and interleukin 1β (il1β) (8.2-fold, kidney; 17.2-fold, spleen; 2.6-fold, liver) were significantly upregulated. Elastase 2 (elas2) was significantly downregulated (5.8-fold) in liver tissue. A significant increase in spleen index at seven dpi was also observed in pathogen-injected minnows. This study has identified endpoints that are part of the normal response to pathogen in fathead minnows, an essential step toward the development of the fathead minnow as a model for immunotoxicity evaluations.

Profiling of Selected Functional Metabolites in the Central Nervous System of Marine Medaka (Oryzias melastigma) for Environmental Neurotoxicological Assessments

The simultaneous profiling of 43 functional metabolites in the brain of the small model vertebrate organism, marine medaka (Oryzais melastigma), has been accomplished via dansyl chloride derivatization and LC-MS/MS quantification. This technique was applied to examine effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), one of the most abundant polybrominated diphenyl ether flame retardants in the natural environment, on the central nervous system (CNS) of vertebrates. The model teleosts were fed with bioencapsulated Artemia nauplii for up to 21 days. Multivariate statistical analysis has demonstrated that levels of numerous classical neurotransmitters and their metabolites in the CNS of the fish were perturbed even at the early phase of dietary exposure. Subsequent metabolic pathway analysis further implied potential impairment of the arginine and proline metabolism; glycine, serine and threonine metabolism; D-glutamine and D-glutamate metabolism; alanine, aspartate, and glutamate metabolism; valine, leucine, and isoleucine biosynthesis, and the cysteine and methionine metabolism in the brain of the test organism. Our results demonstrate that targeted profiling of functional metabolites in the CNS may shed light on how the various neurological pathways of vertebrates, including humans, are affected by toxicant/stress exposure.

Bioconcentration, metabolism, and biomarker responses in marine medaka (Oryzias melastigma) exposed to sulfamethazine

The antibiotic sulfamethazine (SM₂) is commonly used in agriculture and livestock for its broad-spectrum antibacterial properties. Due to its widespread application, SM₂ is frequently detected in surface water and sediments. The objective of this study was to investigate the bioconcentration, distribution and biomarker responses of SM₂ and its main metabolite, acetylated sulfamethazine (N-SM₂) in medaka (Oryzias melastigma). Two treated groups of medaka were exposed to concentrations of 40μg/L and 200μg/L of SM₂ for 24h to simulate the habitual use of those antibiotics in aquiculture activities. SM₂ and its main metabolite, N-SM₂, were measured in several tissues during the 24h uptake period by UPLC/MS/MS. The bile exhibited the highest SM₂ concentration followed by the liver, gonad, gills, and muscle and the bioconcenration factor (BCF) was 10.69-42.95 in female fish and 2.78-145.36 in male fish. N-SM₂ showed a different distribution pattern from the parent compound, accumulating mainly in the gonad, and its BCF was much higher in the male group. Gender-related differences were also observed in the bioconcentration, transform rate and biomarkers of SM₂. Biomarkers (SOD, CAT) in the liver changed significantly after 2, 12, and 24h of exposure (P<0.05), and presented a double-peak phenomenon. These results indicated that SM₂ can be absorbed and metabolized through multiple routes by fish in a short time. Interactions between biological systems and SM₂ or its metabolites may induce biochemical disturbances in fish.

Effects of dietary 2,2', 4,4'-tetrabromodiphenyl ether (BDE-47) exposure in growing medaka fish (Oryzias latipes)

In this research work, we addressed the effects of a diet fortified with BDE-47 (0, 10, 100, 1000ng/g) dosed to 4-7 day-old post-hatch medaka fish for 40 days, followed by an 80-day depuration period. BDE-47 accumulation and overall growth were evaluated throughout the dosing period, and its elimination was quantified over the following 60 days. The histological condition of the thyroid gland, liver and gonads from the 1000ng BDE-47-treated fish were assessed 5 and 70days after exposures finished. The phenotypic males to females ratio was also quantified 70days after treatments finished. Sixty days after the BDE-47 exposures, reproductive capacity (i.e. fecundity, fertility and hatchability) was evaluated in mating groups for a 20-day period. BDE-47 exposure via food from larval through juvenile life stages of medaka fish resulted in steady accumulation with time dose-dependently. This accumulation tendency rapidly decreased after dosing ended. The growth rates showed a significant increase only at the highest concentration 70days after exposures finished. The histological survey did not reveal BDE-47-related alterations in the condition of the potential target organs. However, a morphometrical approach suggested BDE-47-related differences in the thickness of the epithelium that lines thyroid follicles. The reproduction studies showed comparable values for the fecundity, fertility and hatching rates. Dietary BDE-47 dosed for 40days to growing medaka fish did not alter the phenotypic sex ratios at maturity. The dietary approach used herein could not provide conclusive evidence of effects on medaka development and thriving despite the fact that BDE-47 underwent rapid accumulation in whole fish during the 40-day treatment.

Omics of the marine medaka (Oryzias melastigma) and its relevance to marine environmental research

In recent years, the marine medaka (Oryzias melastigma), also known as the Indian medaka or brackish medaka, has been recognized as a model fish species for ecotoxicology and environmental research in the Asian region. O. melastigma has several promising features for research, which include a short generation period (3-4 months), daily spawning, small size (3-4 cm), transparent embryos, sexual dimorphism, and ease of mass culture in the laboratory. There have been extensive transcriptome and genome studies on the marine medaka in the past decade. Such omics data can be useful in understanding the signal transduction pathways of small teleosts in response to environmental stressors. An omics-integrated approach in the study of the marine medaka is important for strengthening its role as a small fish model for marine environmental studies. In this review, we present current omics information about the marine medaka and discuss its potential applications in the study of various molecular pathways that can be targets of marine environmental stressors, such as chemical pollutants. We believe that this review will encourage the use of this small fish as a model species in marine environmental research.

Assessment of the European flounder responses to chemical stress in the English Channel, considering biomarkers and life history traits

A multi-biomarker approach was developed to evaluate responses of European flounder (Platichthys flesus) in three contrasted estuaries over the English Channel: the Canche (pristine site), Tamar (heavy metals and PAHs contamination) and Seine (heavily pollution with a complex cocktail of contaminants). The condition factor and several biomarkers of the immune system, antioxidant enzymes, energetic metabolism and detoxification processes were investigated in young-of-the-year (0+) and one-year-old (1+) flounder. Results underlined the difference between the pristine site and the Seine estuary which showed a lower condition factor, a modulation of the immune system, a higher Cytochrome C oxidase activity, and an up-regulation of BHMT expression. The moderate biomarker responses in the Tamar fish could be linked to the specific contamination context of this estuary. Flounder life history traits were analyzed by otolith microchemistry, in order to depict how the fish use their habitat and thus respond to chemical stress in estuaries.

Dietary Exposure to Individual Polybrominated Diphenyl Ether Congeners BDE-47 and BDE-99 Alters Innate Immunity and Disease Susceptibility in Juvenile Chinook Salmon

Polybrominated diphenyl ethers (PBDEs), used as commercial flame-retardants, are bioaccumulating in threatened Pacific salmon. However, little is known of PBDE effects on critical physiological functions required for optimal health and survival. BDE-47 and BDE-99 are the predominant PBDE congeners found in Chinook salmon collected from the Pacific Northwest. In the present study, both innate immunity (phagocytosis and production of superoxide anion) and pathogen challenge were used to evaluate health and survival in groups of juvenile Chinook salmon exposed orally to either BDE-47 or BDE-99 at environmentally relevant concentrations. Head kidney macrophages from Chinook salmon exposed to BDE-99, but not those exposed to BDE-47, were found to have a reduced ability in vitro to engulf foreign particles. However, both congeners increased the in vitro production of superoxide anion in head kidney macrophages. Salmon exposed to either congener had reduced survival during challenge with the pathogenic marine bacteria Listonella anguillarum. The concentration response curves generated for these end points were nonmonotonic and demonstrated a requirement for using multiple environmentally relevant PBDE concentrations for effect studies. Consequently, predicting risk from toxicity reference values traditionally generated with monotonic concentration responses may underestimate PBDE effect on critical physiological functions required for optimal health and survival in salmon.

Tissue-specific transcriptome assemblies of the marine medaka Oryzias melastigma and comparative analysis with the freshwater medaka Oryzias latipes

Background

The marine medaka Oryzias melastigma has been demonstrated as a novel model for marine ecotoxicological studies. However, the lack of genome and transcriptome reference has largely restricted the use of O. melastigma in the assessment of in vivo molecular responses to environmental stresses and the analysis of biological toxicity in the marine environment. Although O. melastigma is believed to be phylogenetically closely related to Oryzias latipes, the divergence between these two species is still largely unknown. Using Illumina high-throughput RNA sequencing followed by de novo assembly and comprehensive gene annotation, we provided transcriptomic resources for the brain, liver, ovary and testis of O. melastigma. We also investigated the possible extent of divergence between O. melastigma and O. latipes at the transcriptome level.

Results

More than 14,000 transcripts across brain, liver, ovary and testis in marine medaka were annotated, of which 5880 transcripts were orthologous between O. melastigma and O. latipes. Tissue-enriched genes were identified in O. melastigma, and Gene Ontology analysis demonstrated the functional specificity of the annotated genes in respective tissue. Lastly, the identification of marine medaka-enriched transcripts suggested the necessity of generating transcriptome dataset of O. melastigma.

Conclusions

Orthologous transcripts between O. melastigma and O. latipes, tissue-enriched genes and O. melastigma-enriched transcripts were identified. Genome-wide expression studies of marine medaka require an assembled transcriptome, and this sequencing effort has generated a valuable resource of coding DNA for a non-model species. This transcriptome resource will aid future studies assessing in vivo molecular responses to environmental stresses and those analyzing biological toxicity in the marine environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1325-7) contains supplementary material, which is available to authorized users.

In vitro immune toxicity of polybrominated diphenyl ethers on murine peritoneal macrophages: apoptosis and immune cell dysfunction

Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants and are often detected in the environment, wildlife, and humans, presenting potential threats to ecosystem and human health. PBDEs can cause neurotoxicity, hepatotoxicity, and endocrine disruption. However, data on PBDE immunotoxicity are limited, and the toxicity mechanisms remain largely unknown. Both immune cell death and dysfunction can modulate the responses of the immune system. This study examined the toxic effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and decabromodiphenyl ether (BDE-209) on the immune system by using peritoneal macrophages as the model. The macrophages were exposed to PBDEs, and cell death was determined through flow cytometry and immunochemical blot. The results showed that after 24h of exposure, BDE-47 (>5 μM) and BDE-209 (>20 μM) induced cell apoptosis, increased intracellular reactive oxygen species (ROS) formation and depleted glutathione. BDE-47 was more potent than BDE-209; the cytotoxic concentrations for BDE-47 and BDE-209 were determined to be 5 μM and 20 μM, respectively, during 24h of exposure. However, pretreatment with n-acetyl-l-cysteine (ROS scavenger) partially reversed the cytotoxic effects. Further gene expression analyses on Caspase-3,-8,-9, TNFR1, and Bax revealed that both intrinsic and extrinsic apoptotic pathways were activated. More importantly, non-cytotoxic concentrations BDE-47 (<2 μM) and BDE-209 (<10 μM) could impair macrophage accessory cell function in a concentration-dependent manner, but no effects were observed on phagocytic responses. These revealed effects of PBDEs on macrophages may shed light on the toxicity mechanisms of PBDEs and suggest the necessity of evaluating cellular functionality during the risk assessment of PBDE immunotoxicity.

Proteomic changes in brain tissues of marine medaka (Oryzias melastigma) after chronic exposure to two antifouling compounds: butenolide and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT)

SeaNine 211 with active ingredient of 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) has been used as a "green" antifouling agent worldwide but has raised serious biosafety concerns in coastal environments. DCOIT has the potential to disrupt the neurotransmission in nervous system, but the underlying mechanism has not been clarified. In the present study, we used TMT six-plex labeling coupled with two-dimensional LC-MS/MS analysis to investigate the protein expression profiles in brain tissues of the marine medaka (Oryzias melastigma) after a 28-day exposure to environmentally-realistic concentration of DCOIT at 2.55 μg/L (0.009 μM) or butenolide, one promising antifouling compound, at 2.31 μg/L (0.012 μM). DCOIT and butenolide induced differential expression of 26 and 18 proteins in male brains and of 27 and 23 proteins in female brains, respectively. Distinct mechanisms of toxicity were initiated by DCOIT and butenolide in males, whereas the protein expression profiles were largely similar in females treated by these two compounds. In males, DCOIT exposure mainly led to disruption of mitogen-activated protein kinase (MAPK) signaling pathway, while butenolide affected proteins related to the cytoskeletal disorganization that is considered as a general response to toxicant stress. Furthermore, a sex-dependent protein expression profile was also noted between male and female fish, as evident by the inverse changes in the expressions of common proteins (5 proteins for butenolide- and 2 proteins for DCOIT-exposed fish). Overall, this study provided insight into the molecular mechanisms underlying the toxicity of DCOIT and butenolide. The extremely low concentrations used in this study highlighted the ecological relevance, arguing for thorough assessments of their ecological risks before the commercialization of any new antifouling compound.

Responses of the European flounder (Platichthys flesus) to a mixture of PAHs and PCBs in experimental conditions

A multibiomarker approach was developed to evaluate the juvenile European flounder responses to a complex mixture of 9 polycyclic aromatic hydrocarbons (PAHs) and 12 polychlorinated biphenyls (PCBs). Exposure was performed through contaminated food pellets displaying: (1) PAH and PCB levels similar to those detected in the heavily polluted Seine estuary, respectively in sediments and in flatfish and (2) ten times these concentrations. Several biomarkers of the immune system (e.g., lysozyme concentration and gene expression of complement component C3 and TNF-receptor), DNA damage (e.g., Comet assay), energetic metabolism (e.g., activity of cytochrome C oxidase), detoxification process (e.g., cytochrome P450 1A1 expression level: CYP1A1; betaine homocysteine methyl transferase expression level: BHMT) were investigated after 14 and 29 days of contamination, followed by a 14-days recovery period. After 29 days of contamination, the detoxification activity (CYP1A1 expression level) was positively correlated with DNA damages; the increase of the BHMT expression level could also be related to the detoxification process. Furthermore, after the recovery period, some biomarkers were still upregulated (i.e., CYP1A1 and BHMT expression levels). The immune system was significantly modulated by the chemical stress at the two concentration levels, and the lysozyme appeared to be the most sensitive marker of the mixture impact.

Influence of persistent organic pollutants on the complement system in a population-based human sample

BACKGROUND

Persistent organic pollutants (POPs) are toxic compounds generated through various industrial activities and have adverse effects on human health. Studies performed in cell cultures and animals have revealed that POPs can alter immune-system functioning. The complement system is part of innate immune system that helps to clear pathogens from the body. We performed a large-scale population-based study to find out associations between summary measures of different POPs and different complement system markers.

METHODS

In this cross-sectional study, 16 polychlorinated biphenyls (PCBs), 3 organochlorine (OC) pesticides, octachloro-p-dibenzodioxin, and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) were analyzed for their association with levels of protein complement 3 (C3), 3a (C3a), 4 (C4) and C3a/C3 ratio. A total of 992 individuals (all aged 70 years, 50% females) were recruited from the Prospective Investigation of the Vasculature in Uppsala Seniors cohort. Regression analysis adjusting for a variety of confounders was performed to study the associations of different POP exposures (total toxic equivalency value or TEQ and sum of 16 PCBs) with protein complements.

RESULTS

The TEQ values were found to be positively associated with C3a (β=0.07, 95% CI=0.017-0.131, p=0.01) and C3a/C3 ratio (β=0.07, 95% CI=0.015-0.126, p=0.01) taking possible confounders into account. The association observed was mainly driven by PCB-126.

CONCLUSION

In this study involving 992 elderly individuals from the general population, we showed that POPs, mainly PCB-126, were associated with levels of complement system markers indicating that the association of these toxic compounds with downstream disease could be mediated by activation of immune system.

iTRAQ-based proteomic profiling of the marine medaka (Oryzias melastigma) gonad exposed to BDE-47

A recent study demonstrated that 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) may have an adverse effect on the reproduction in marine medaka (Oryzias melastigma), but the molecular mechanisms remain largely unknown. In this study, we investigated the protein expression profiles of male and female gonads of O. melastigma exposed to dietary BDE-47 at two dosages (0.65 and 1.30 μg/g/day, respectively) for 21 days. Extracted proteins were labeled with iTRAQ and analyzed on a MALDI TOF/TOF analyzer, as results, 133 and 144 unique proteins were identified in testis and ovary, respective, and they exerted dose- and sex-dependent expression patterns. In testis, among the 42 differentially expressed proteins; down-regulation of histone variants and parvalbumins implicated BDE-47 may disrupt the spermatogenesis and induce sterility in fishes. In ovary, 38 proteins were differentially expressed; the elevation of vitellogenins and apolipoprotein A-I expression indicated BDE-47 acts as an estrogen-mimicking compound and led to reproductive impairment in O. melastigma.

Comparative safety of the antifouling compound butenolide and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) to the marine medaka (Oryzias melastigma)

This study evaluated the potential adverse effects of butenolide, a promising antifouling compound, using the marine medaka (Oryzias melastigma), a model fish for marine ecotoxicology. The active ingredient used in the commercial antifoulant SeaNine 211, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) was employed as the positive control. Adult marine medaka (4-month-old) were exposed to various concentrations of butenolide or DCOIT for 28 days and then depurated in clean seawater for 14 days (recovery). A suite of sensitive biomarkers, including hepatic oxidative stress, neuronal signal transmission, endocrine disruption, and reproductive function, was used to measure significant biological effects induced by the chemicals. Compared to DCOIT, chronic exposure to butenolide induced a lower extent of oxidative stress in the liver of male and female medaka. Furthermore, butenolide-exposed fish could recover faster from oxidative stress than fish exposed to DCOIT. Regarding neurotransmission, DCOIT significantly inhibited acetylcholinesterase (AChE) activity in the brain of both male and female medaka, whereas this was not significant for butenolide. In addition, plasma estradiol (E2) level was elevated and testosterone (T) level was decreased in male medaka exposed to DCOIT. This greatly imbalanced sex hormones ratio (E2/T) in exposed males, indicating that DCOIT is a potent endocrine disruptive chemical. In contrast, butenolide induced only moderate effects on sex hormone levels in exposed males, which could be gradually recovered during depuration. Moreover, the endocrine disruptive effect induced by butenolide did not affect normal development of offspring. In contrast, DCOIT-exposed fish exhibited a decrease of egg production and impaired reproductive success. Overall, the above findings demonstrated that chronic exposure to butenolide induced transient, reversible biological effect on marine medaka, while DCOIT could impair reproductive success of fish, as evident by clear alterations of the E2/T ratio. The relatively low toxicity of butenolide on marine biota highlights its promising application in the antifouling industry. The present findings also emphasize gender difference in fish susceptibility to chemical treatment (male>female), which is an important consideration for ecological risk assessment.

Development of a promising fish model (Oryzias melastigma) for assessing multiple responses to stresses in the marine environment

With the increasing number of contaminants in the marine environment, various experimental organisms have been “taken into labs” by investigators to find the most suitable environmentally relevant models for toxicity testing. The marine medaka, Oryzias melastigma, has a number of advantages that make it a prime candidate for these tests. Recently, many studies have been conducted on marine medaka, especially in terms of their physiological, biochemical, and molecular responses after exposure to contaminants and other environmental stressors. This review provides a literature survey highlighting the steady increase of ecotoxicological research on marine medaka, summarizes the advantages of using O. melastigma as a tool for toxicological research, and promotes the utilization of this organism in future studies.

A characterization of the ZFL cell line and primary hepatocytes as in vitro liver cell models for the zebrafish (Danio rerio)

The zebrafish (Danio rerio) is a widely used model species in biomedical research. The ZFL cell line, established from zebrafish liver, and freshly isolated primary hepatocytes from zebrafish have been used in several toxicological studies. However, no previous report has compared and characterized these two systems at the level of gene expression. The aim of this study was to evaluate the ZFL cell line in comparison to primary hepatocytes as in vitro models for studying effects of environmental contaminants in zebrafish liver. Using quantitative real-time PCR, the basal level and transcriptional induction potential of key genes involved in toxic responses in the ZFL cell line, primary hepatocytes and whole liver from zebrafish were compared. The study showed that the ZFL cells have lower levels of mRNA of most selected genes compared to zebrafish liver. The induced gene transcription following exposure to ligand was much lower in ZFL cells compared to zebrafish primary hepatocytes at the doses tested. Importantly, oestrogen receptor and vitellogenin genes showed low basal transcription and no induction response in the ZFL cell line. In conclusion, it appears that primary hepatocytes are well suited for studying environmental contaminants including xenoestrogens, but may show large sex-dependent differences in gene transcription. The ZFL cell line shows potential in toxicological studies involving the aryl hydrocarbon receptor pathway. However, low potential for transcriptional induction of genes in general should be expected, especially notable when studying estrogenic responses.

PBDE-47 exposure causes gender specific effects on apoptosis and heat shock protein expression in marine medaka, Oryzias melastigma

Marine medaka (Oryzias melastigma) was fed with a low and high dose of dietary 2,2',4,4'-tetra-bromodiphenyl ether (PBDE-47), over 21 days. Gender specific changes in caspases 3 and 8 in medaka were found as activities in male medaka were significantly increased in both liver and muscle at both low and high exposure levels whereas caspase activity in female medaka tissue remained unchanged. Results of HSP90 and HSP70 immunoassays also showed gender specific related changes as both HSP families were unchanged in liver and muscle of male medaka but significantly increased in liver and muscle of female medaka, following PBDE-47 exposure. The gender specific effects of PBDE-47 on HSP expression profiles could not be explained by inherent differences in the heat shock response of male and female marine medaka, as the HSP profiles in liver and muscle, induced by acute heat shock, were similar in both sexes. The findings from this study provide evidence that PBDE-47 can cause gender specific modulatory effects on mechanisms critical to the apoptotic cascade as well as HSP regulation and expression.

Involvement of reactive oxygen species in brominated diphenyl ether-47-induced inflammatory cytokine release from human extravillous trophoblasts in vitro

Polybrominated diphenyl ethers (PBDEs) are widely used flame retardant compounds. Brominated diphenyl ether (BDE)-47 is one of the most prevalent PBDE congeners found in human breast milk, serum and placenta. Despite the presence of PBDEs in human placenta, effects of PBDEs on placental cell function are poorly understood. The present study investigated BDE-47-induced reactive oxygen species (ROS) formation and its role in BDE-47-stimulated proinflammatory cytokine release in a first trimester human extravillous trophoblast cell line, HTR-8/SVneo. Exposure of HTR-8/SVneo cells for 4h to 20μM BDE-47 increased ROS generation 1.7 fold as measured by the dichlorofluorescein (DCF) assay. Likewise, superoxide anion production increased approximately 5 fold at 10 and 15μM and 9 fold at 20μM BDE-47 with a 1-h exposure, as measured by cytochrome c reduction. BDE-47 (10, 15 and 20μM) decreased the mitochondrial membrane potential by 47-64.5% at 4, 8 and 24h as assessed with the fluorescent probe Rh123. Treatment with 15 and 20μM BDE-47 stimulated cellular release and mRNA expression of IL-6 and IL-8 after 12 and 24-h exposures: the greatest increases were a 35-fold increased mRNA expression at 12h and a 12-fold increased protein concentration at 24h for IL-6. Antioxidant treatments (deferoxamine mesylate, (±)α-tocopherol, or tempol) suppressed BDE-47-stimulated IL-6 release by 54.1%, 56.3% and 37.7%, respectively, implicating a role for ROS in the regulation of inflammatory pathways in HTR-8/SVneo cells. Solvent (DMSO) controls exhibited statistically significantly decreased responses compared with non-treated controls for IL-6 release and IL-8 mRNA expression, but these responses were not consistent across experiments and times. Nonetheless, it is possible that DMSO (used to dissolve BDE-47) may have attenuated the stimulatory actions of BDE-47 on cytokine responses. Because abnormal activation of proinflammatory responses can disrupt trophoblast functions necessary for placental development and successful pregnancy, further investigation is warranted of the impact of ROS and BDE-47 on trophoblast cytokine responses.

Modulation of benzo[a]pyrene-induced toxic effects in Japanese medaka (Oryzias latipes) by 2,2',4,4'-tetrabromodiphenyl ether

Because polycyclic aromatic hydrocarbons (PAHs) and polybrominated diphenylethers (PBDEs) are ubiquitous and coexist in the environment and in wildlife, there are potential interactions between them that could cause toxic effects. In this study, the modulating effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE47) and benzo[a]pyrene (BaP)-induced reproductive and developmental toxic effects in Japanese medaka (Oryzias latipes) were investigated by exposing adult Japanese medaka to BaP alone, BDE47 alone, and coexposing them with both BaP and BDE47 at different concentrations, respectively. Exposure to BaP alone significantly suppressed fecundity and egg protein content and markedly induced skeletal deformation in F1 generation eleutheroembryos. BDE47 significantly recovered reproductive functions, fecundity, and egg protein content, suppressed by BaP when the concentration of BDE47 increased to 0.44 μg/L. Such effects can be at least partly explained by the decreased BaP levels in the coexposure groups and the accompanying increase in the circulating level of 17β-estradiol in female medaka fish. The prevalence of skeletal deformations markedly increased to 19.3 ± 2.4% and 16.0 ± 1.6% in fish coexposed to BaP and BDE47 at 0.44 and 2.58 μg/L compared with that of fish exposed to 1.21 μg/L BaP alone (9.7 ± 1.7%), and the impacts on male medaka fish in the coexposure groups would be the crucial reason leading to these effects. Considering that the measured water concentrations of BaP and BDE47 in the present study were comparable with those reported in rivers and harbors, BaP and BDE47 contamination in the real world would have a significant level of interactive effects on wild fish.

The Toxic Effects of Polybrominated Diphenyl Ethers on Rotifer Branchionus plicatilis

In the current study, two of the most environmentally relevant Polybrominated diphenyl ethers (PBDEs) were individually evaluated for toxic effects on rotifer Brachionus plicatilis. The results showed that the life span declined significantly in all treatment groups (0.05mg/L, 0.1mg/L, 0.2mg/L BDE-47 and BDE-209) compared to the control, and BDE-47 caused significant decrease compared to BDE-209 at 0.2mg/L treatment group. Significant increases of intracellular levels of reactive oxygen species (ROS) occurred in all treatment groups except for 0.05mg/L BDE-209 treatment group compared to the control, and significant increases were observed in all BDE-47 treatment groups compared to the equivalent treatment groups of BDE-209. Meanwhile significant increases of intracellular calcium levels ([Ca2+]in) occurred in 0.1mg/L, 0.2mg/L BDE-47 and 0.2mg/L BDE-209 treatment groups compared to the control, and BDE-47 cause significant increase compared to BDE-209 at 0.2mg/L treatment group. The present study demonstrated that life span, ROS and Ca2+ were involved in PBDEs toxic effects, and toxicities of BDE-47 were higher than BDE-209. Otherwise, the toxic effects in both BDE-47 and BDE-209 were similar, which suggest that the toxic effects of two PBDEs congeners may be caused by the same toxic mechanism of action.

Gender-specific transcriptional profiling of marine medaka (Oryzias melastigma) liver upon BDE-47 exposure

Marine medaka (Oryzias melastigma) were exposed to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) to investigate the gender-specific transcriptional profiles of liver tissue in response to this flame retardant. A cDNA library of O. melastigma was constructed, and 2304 clones were amplified from the library to fabricate a cDNA microarray. Sequences of these genes were assembled into 1800 sequences using Geneious, a bioinformatics software. Corresponding expressed sequence tags were blasted against the National Centre for Biotechnology Information non-redundant database and further classified into various biological categories according to the Gene Ontology project. Male and female three-month-old were fed a diet of BDE-47 contaminated Artemia at low dosage (290.3±172.3ng BDE-47/day) and high dosage (580.5±344.6ng BDE-47/day) for 5 and 21 days, respectively. The transcriptional profiles of O. melastigma liver were then generated by the species-specific cDNA microrarray. The results from microarray analysis suggested very different gene expression patterns between males and females for both BDE-47 exposure-dose and exposure-time, with male livers having stronger gene regulatory responses than female livers. Importantly, our results revealed that in male O. melastigma only, BDE-47 exposure may activate phosphoinositide-3-kinase and mitogen-activated protein kinase, proteins that play importance roles in cell growth, proliferation and survival.