Serial analysis of gene expression in the skeletal muscles of zebrafish fed with a methylmercury-contaminated diet

Resveratrol alleviates perinatal methylmercury-induced neurobehavioral impairments by modulating the gut microbiota composition and neurotransmitter disturbances

Methylmercury (MeHg), a potent neurotoxic substance, causes adverse health outcomes by modulating metabolites through altered gut microbiota patterns. Among the many metabolites, neurotransmitters play a particularly important role in the nervous system and behavior. Resveratrol (RSV) has been investigated as an antiaging, antioxidant, anti-inflammatory, and neuroprotective agent. The current study evaluated that RSV is protective of neurodevelopmental toxicity induced by MeHg and further explored the underlying mechanisms. Sprague-Dawley rats were treated with 1.2 mg/kg/d of MeHg, and the effects were evaluated after supplementation with RSV (20 mg/kg/d). The results indicated that MeHg had adverse effects on early neurodevelopmental indicators in the experimental group offspring as compared to control pups. Interestingly, RSV significantly improved the MeHg-induced delays in the neurobehavioral reflexes and reduced the total mercury (THg) concentration in the colons of the offspring rats. In agreement, RSV administration improved the gut microbiota diversity and structure by increasing the abundance of probiotics and upregulating the expression of tight junction proteins. It also ameliorated the MeHg-induced abnormalities in the expression profiles of neurotransmitters. Furthermore, eight key bacteria that were strongly linked with the neurotransmitters and neuroreflex parameters were identified. Taken together, these results demonstrate that RSV treatment effectively reduces the occurrence of neurodevelopmental toxicity caused by perinatal MeHg exposure by modulating the intestinal flora and neurotransmitter metabolism. These findings provide a new therapeutic approach for treating MeHg-induced neurotoxicity. The cover image is based on the Research Article Resveratrol alleviates perinatal methylmercury-induced neurobehavioral impairments by modulating the gut microbiota composition and neurotransmitter disturbances by Fang Chen et al., https://doi.org/10.1002/tox.23973.

Bioaccumulation of mercury and transcriptional responses in tusk (Brosme brosme), a deep-water fish from a Norwegian fjord

High concentrations of mercury (Hg) have been documented in deep-water fish species from some Norwegian fjords. In this study, tusk (Brosme brosme) was sampled from four locations in the innermost parts of Sognefjorden in Western Norway. Total Hg and methylmercury (MeHg) levels were measured in liver tissue. To search for potential sublethal effects of Hg, we characterized the hepatic transcriptome in tusk with high and low levels of Hg bioaccumulation using global transcriptomics analysis (RNA-seq). The results showed that there was a significant correlation between fish weight and accumulated concentrations of MeHg but not total Hg. MeHg accounted for 30-40% of total Hg in liver of most of the fish, although at concentrations above 2-3 mg Hg/kg wet weight the percentage of MeHg dropped considerably. Transcriptome analysis resulted in hundreds of differentially expressed genes in the liver of tusk with high Hg levels. Functional enrichment analysis suggested that the top affected pathways are associated with protein folding, adipogenesis, notch signaling, and lipid metabolism (beta-oxidation and phospholipids). Based on transcriptional responses pointing to well-known effects of Hg compounds in fish, the study suggests that tusk in Sognefjorden could be negatively impacted by Hg bioaccumulation.

RNA sequencing and proteomic profiling reveal different alterations by dietary methylmercury in the hippocampal transcriptome and proteome in BALB/c mice

Methylmercury (MeHg) is a highly neurotoxic form of mercury (Hg) present in seafood. Here, we recorded and compared proteomic and transcriptomic changes in hippocampus of male BALB/c mice exposed to two doses of MeHg. Mice were fed diets spiked with 0.28 mg MeHg kg-1, 5 mg MeHg kg-1, or an unspiked control diet for 77 days. Total mercury content was significantly (P < 0.05) increased in brain tissue of both MeHg-exposed groups (18 ± 2 mg Hg kg-1 and 0.56 ± 0.06 mg Hg kg-1). Hippocampal protein and ribonucleic acid (RNA) expression levels were significantly altered both in tissues from mice receiving a low dose MeHg (20 proteins/294 RNA transcripts) and a high dose MeHg (61 proteins/876 RNA transcripts). The majority but not all the differentially expressed features in hippocampus were dose dependent. The combined use of transcriptomic and proteomic profiling data provided insight on the influence of MeHg on neurotoxicity, energy metabolism, and oxidative stress through several regulated features and pathways, including RXR function and superoxide radical degradation.

Methylmercury displays pro-adipogenic properties in rainbow trout preadipocytes

Methylmercury (MeHg) is a ubiquitous contaminant largely found in aquatic environments, especially in species at high trophic level such as salmonids. The aim of this study was to evaluate the effects of MeHg on adipocyte differentiation and lipid metabolism in rainbow trout. Primary cultured preadipocytes were exposed to increasing concentrations of MeHg during six days with or without a hormonal cocktail. Main results showed a dose-dependent intracellular accumulation of neutral lipids with a preferential uptake of n-3 polyunsaturated fatty acids. Interestingly, this accumulation occurred after a fairly low uptake of MeHg by preadipocytes and was maintained after the cellular exposure to MeHg. In membrane phospholipids, arachidonic acid (20:4 n-6) was released in a dose-dependent manner. At the transcriptional level, the expression of several adipocyte-specific genes (perilipin 2 and apolipoprotein Eb) as well as lipid-related genes (fatty acid synthase and fatty acid binding protein 11a) was up-regulated in preadipocytes exposed to MeHg. These results highlight for the first time the disrupting effect of MeHg in trout adipocyte metabolism, providing new insights regarding the role of environmental pollutants in adipose tissue dysfunction and related pathologies.

New insights into methylmercury induced behavioral and energy-related gene transcriptional responses in European glass eel (Anguilla anguilla)

The effect of methylmercury (MeHg) was investigated in glass eel migration behavior and metabolism. To migrate up estuary, glass eels synchronize their swimming activity to the flood tide and remain on or in the substratum during ebb tide. Following seven days of exposure to MeHg (100 ng L^-1), glass eels migration behavior was expressed by their swimming synchronization to the water current reversal every 6.2 h (mimicking the alternation of flood and ebb tides) and their swimming activity level. In relation to their behavior, we then analyzed the energy-related gene expression levels in individual head, viscera and muscle. Results showed that MeHg decreased the number of glass eels synchronized to the change in water current direction and their swimming activity level. This last effect was more pronounced in non-synchronized fish than in synchronized ones, supporting the idea that non-synchronized glass eels could be more vulnerable to stress. As regard the expression of energy-related genes, no significant difference was observed between control and MeHg-exposed fish. In contrast, when the swimming activity levels were plotted against transcriptional responses, positive correlations were evidenced in viscera and especially in the head of exposed glass eels but not in control. Finally, it is noteworthy that non-synchronized glass eels displayed lower expression level of metabolism genes than their synchronized counterpart, but only in the head. Altogether, these results support the interest of focusing on the head to investigate the facultative migration behavior in glass eels and the effect of environmental stressors on this rhythmic behavior.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Current basis and future directions of zebrafish nutrigenomics

This review investigates the current state of nutrigenomics in the zebrafish animal models. The zebrafish animal model has been used extensively in the study of disease onset and progression and associated molecular changes. In this review, we provide a synopsis of nutrigenomics using the zebrafish animal model. Obesity and dyslipidemia studies describe the genomics of dietary-induced obesity in relation to high-fat/high-calorie diets. Inflammation and cardiovascular studies describe dietary effects on the expression of acute inflammatory markers and resulting chronic inflammatory issues including atherosclerosis. We also evaluated the genomic response to bioactive dietary compounds associated with metabolic disorders. Carbohydrate metabolism and β-cell function studies describe the impacts of high-carbohydrate dietary challenges on nutritional programming. We also report tumorigenesis in relation to dietary carcinogen exposure studies that can result in permanent genomic changes. Vitamin and mineral deficiency studies demonstrate transgenerational genomic impacts of micronutrients in the diet and temporal expression changes. Circadian rhythm studies describe the relation between metabolism and natural temporal cycles of gene expression that impacts health. Bone formation studies describe the role of dietary composition that influences bone reabsorption regulation. Finally, this review provides future directions in the use of the zebrafish model for nutrigenomic and nutrigenetic research.

Methylmercury modifies temporally expressed myogenic regulatory factors to inhibit myoblast differentiation

Methylmercury (MeHg) is a pervasive environmental toxicant, with known detrimental effects on neurodevelopment. Despite a longstanding paradigm of neurotoxicity, where motor deficits are prevalent among those developmentally exposed, consideration of muscle as a MeHg target has received minimal investigation. Recent evidence has identified muscle-specific gene networks that modulate developmental sensitivity to MeHg toxicity. One such network is muscle cell differentiation. Muscle cell differentiation is a coordinated process regulated by the myogenic regulatory factors (MRFs): Myf5, MyoD, MyoG, and MRF4. A previous study demonstrated that MeHg inhibits muscle cell differentiation in vitro, concurrent with reduced MyoG expression. The potential for MeHg to modify the temporal expression of the MRFs to alter differentiation, however, has yet to be fully explored. Using the C2C12 mouse myoblast model, we examined MRF expression profiles at various stages subsequent to MeHg exposure to proliferating myoblasts. MeHg was seen to persistently alter myoblast differentiation capacity, as myod, myog, and mrf4 gene expression were all affected. Myog exhibited the most robust changes in expression across the various culture conditions, while myf5 was unaffected. Following MeHg exposure to myoblasts, where elevated p21 expression indicated departure from proliferation, cells failed to subsequently differentiate, even in the absence of MeHg, as reflected by a concurrent reduction in MRF4 and myosin heavy chain (MHC), markers of terminal differentiation. Our results indicate that within a brief window of exposure MeHg can disrupt the intrinsic myogenic differentiation program of proliferative myoblasts.

Functional roles of eriocalyxin B in zebrafish revealed by transcriptome analysis

Eriocalyxin B (EriB) is a naturalent-kaurane diterpenoid obtained fromIsodon eriocalyxvar.laxiflora(family Lamiaceae), which exerted multiple biological activities (e.g.anti-tumor and anti-inflammatory)viathe alteration of gene expression and signaling transduction.

Mercury accumulation and its effects on molecular, physiological, and histopathological responses in the peacock blenny Salaria pavo

For humans, fish consumption is the major source of mercury (Hg) exposure. The aim of this study was to assess the effect of Hg in the peacock blenny Salaria pavo, a species of the family of blennies that was used as indicator of water pollution. We performed a sublethal contamination of fish to 66 μg HgCl₂ L^-1 during 1, 4, 10 and 15 days but Hg concentration measured in the experimental water was much lower than the nominal concentration. Hg was also measured in both gill and liver tissues and displays a significant increase of its concentration in gills after 1 day of exposure followed by a decrease throughout the experiment. In the liver, Hg burden reaches its maximum at day 4 followed also by a decrease. Partial-length cDNA of mt1, mt2, gpx, cat, mnsod and cuznsod was characterized. Results from mRNA expression levels displayed an up-regulation of mt1, gpx and mnsod while a downregulation of cat was observed. Several biomarker activities were determined in gills and liver and exposure to Hg affected all antioxidant enzymes in gills. EROD, GST and GPx significantly decreased, while CAT levels increased from 4 days of Hg exposure. No lipid peroxidation (LPO) induction was observed in gills of exposed fish. Regarding the liver, the activity of all enzymes increased significantly from the beginning of the experiment. LPO induction was, however, induced after 4 days only. The histological analysis also performed indicated that fish exhibited several damages in gills and liver, mainly in relation to circulatory disturbances in the gills and regressive changes in the liver. All biomarkers assessed showed that peacock blennies are able to detoxify Hg from gill and liver tissues by developing various defense mechanisms.

In vitro Assessment of Hg Toxicity in Hepatocytes from Heat-Stressed Atlantic Salmon

Global warming may alter the bioavailability of contaminants in aquatic environments. In this work, mercury (Hg²⁺) toxicity was studied in cells obtained from Atlantic salmon smolt kept at 15 °C (optimal growth temperature) for 3 months or at a stepwise increase to 20 °C (temperature-stress) during 3 months prior to cell harvest to evaluate whether acclimation temperature affects Hg toxicity. To examine possible altered dietary requirements in warmer seas, one group of fish following the stepwise temperature regimes was fed a diet spiked with antioxidants. Atlantic salmon hepatocytes were exposed in vitro to 0, 1.0, or 100 μM Hg²⁺ for 48 h. Cytotoxicity, determined as electrical impedance changes with the xCELLigence system, and transcriptional responses, determined with RT-qPCR, were assessed as measures of toxicity. The results showed that inorganic Hg at a concentration up to 100 μM is not cytotoxic to Atlantic salmon hepatocytes. Significance and directional responses of the 18 evaluated target genes suggest that both Hg and temperature stress affected the transcription of genes encoding proteins involved in the protection against ROS-generated oxidative stress. Both stressors also affected the transcription of genes linked to lipid metabolism. Spiking the diet with antioxidants resulted in higher concentrations of Se and vitamin C and reduced concentration of Hg in the liver in vivo, but no interactions were seen between the dietary supplementation of antioxidants and Hg toxicity in vitro. In conclusion, no evidence was found suggesting that inorganic Hg is more toxic in cells harvested from temperature-stressed fish.

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.

Transcriptomic, Biochemical, and Histopathological Responses of the Clam Ruditapes decussatus from a Metal-Contaminated Tunis Lagoon

This study was designed to investigate the molecular (transcriptional expression), biochemical (oxidative stress and neurotoxicity), and histopathological effects of metal contamination in the gill of clams (Ruditapes decussatus) sampled from the Tunis lagoon. The concentrations of five heavy metals (Cd, Pb, Hg, Cu, and Zn) in surface sediments and their accumulation in soft tissues of R. decussatus were evaluated in three sites (Z1, Z2, and Z3). A metal contamination state of Tunis lagoon sediments was noted with spatial variations with relatively high levels at Z2. Biomarker analyses showed an increase in glutathione S-transferase and catalase activities and lipid peroxidation levels and a decrease in acetylcholinesterase activity in the studied sites. Molecular investigation showed a significant overexpression of: cytochrome c oxidase subunit I, ribosomal RNA 16S, Cu/Zn superoxide dismutase, heat shock protein 70, and metallothioneins in the three sampling sites. Moreover, our data were correlated to severe and diverse histopathological alterations in the clam gills. The principal component analysis showed that the Z2 region is more affected by metal contamination than Z1 and Z3 regions. Current field results suggest the use of several combined biomarkers at different cell levels instead of individual ones in monitoring programs.

Specific Effects of Dietary Methylmercury and Inorganic Mercury in Zebrafish (Danio rerio) Determined by Genetic, Histological, and Metallothionein Responses

A multidisciplinary approach is proposed here to compare toxicity mechanisms of methylmercury (MeHg) and inorganic mercury (iHg) in muscle, liver, and brain from zebrafish (Danio rerio). Animals were dietary exposed to (1) 50 ng Hg g(-1), 80% as MeHg; (2) diet enriched in MeHg 10000 ng Hg g(-1), 95% as MeHg; (3) diet enriched in iHg 10000 ng Hg g(-1), 99% as iHg, for two months. Hg species specific bioaccumulation pathways were highlighted, with a preferential bioaccumulation of MeHg in brain and iHg in liver. In the same way, differences in genetic pattern were observed for both Hg species, (an early genetic response (7 days) for both species in the three organs and a late genetic response (62 days) for iHg) and revealed a dissimilar metabolization of both Hg species. Among the 18 studied genes involved in key metabolic pathways of the cell, major genetic responses were observed in muscle. Electron microscopy revealed damage mainly because of MeHg in muscle and also in liver tissue. In brain, high MeHg and iHg concentrations induced metallothionein production. Finally, the importance of the fish origin in ecotoxicological studies, here the seventh descent of a zebrafish line, is discussed.

Recovery capabilities of Xenopus laevis after exposure to Cadmium and Zinc

The present investigation evaluates the recovery capabilities of Xenopus laevis following 12days of exposure to 30μg CdL(-1) and 1000μg ZnL(-1) alone or mixed, followed by a depuration phase in laboratory conditions. Focused endpoints, which were investigated at different times of depuration, are bioaccumulation of Cd and Zn, micronucleus induction, quantification of metallothioneins (MTs), and expression of genes involved in metal toxicity mechanisms. The results show that at the end of the contamination phase, there was higher metal bioaccumulation capability and MT synthesis in remaining tissues than in the liver. An increased expression of genes involved in detoxification and oxidative stress mechanisms was observed, suggesting an additive effect of both metals and a higher Zn regulation in the liver. During the depuration phase, the results show the recovery capability of Xenopus from 7days of depuration related to metamorphosis processes, which were observed at the end of the experiment. The results confirm the relevance of the amphibian model and the complementarities between a marker of genotoxicity, MT production, bioaccumulation and transcriptional analysis in the evaluation of the ecotoxicological impact. The results also highlight the reversible effects of Cd and Zn toxicity.

Isolation of Novel CreERT2-Driver Lines in Zebrafish Using an Unbiased Gene Trap Approach

Gene manipulation using the Cre/loxP-recombinase system has been successfully employed in zebrafish to study gene functions and lineage relationships. Recently, gene trapping approaches have been applied to produce large collections of transgenic fish expressing conditional alleles in various tissues. However, the limited number of available cell- and tissue-specific Cre/CreERT2-driver lines still constrains widespread application in this model organism. To enlarge the pool of existing CreERT2-driver lines, we performed a genome-wide gene trap screen using a Tol2-based mCherry-T2a-CreERT2 (mCT2aC) gene trap vector. This cassette consists of a splice acceptor and a mCherry-tagged variant of CreERT2 which enables simultaneous labeling of the trapping event, as well as CreERT2 expression from the endogenous promoter. Using this strategy, we generated 27 novel functional CreERT2-driver lines expressing in a cell- and tissue-specific manner during development and adulthood. This study summarizes the analysis of the generated CreERT2-driver lines with respect to functionality, expression, integration, as well as associated phenotypes. Our results significantly enlarge the existing pool of CreERT2-driver lines in zebrafish and combined with Cre-dependent effector lines, the new CreERT2-driver lines will be important tools to manipulate the zebrafish genome.

Methylmercury effects on migratory behaviour in glass eels (Anguilla anguilla): an experimental study using isotopic tracers

The effect of methylmercury (MeHg) on glass eels' propensity to migrate, mitochondrial activity and antioxidative defence systems was investigated. Marine glass eels were first sorted in an experimental flume according to their response to dusk. Fish responding to the decrease in light intensity by ascending in the water column and moving with or against the flow were considered as having a high propensity to migrate (migrant). Glass eels still sheltering at the end of the 24 h catching period were considered as having a low propensity to migrate and were called non-migrant. Migrant and non-migrant glass eels were then individually tagged and exposed to isotopically enriched (201)MeHg (50 ng L(-1)) for 11 days. The effect of contamination was studied on muscle fibre structure, and the expression level of genes involved in mitochondrial activity and antioxidative defence systems. To investigate the effect of MeHg on glass eel behaviour, migrant and non-migrant glass eels were sorted again and the bioaccumulation of (201)MeHg and its demethylation product ((201)Hg(II)) were determined for each individual. MeHg exposure increased activity in non-migrant glass eels but not migratory behaviour. Contamination affected mitochondrial structure and metabolism and suggests a higher oxidative stress and activation of antioxidative defence systems in non-migrant glass eels. Overall, our results suggest that exposure to MeHg might induce an increase in energy expenditure and a higher vulnerability to predation in non-migrant glass eels in the wild.

Impact of dietary selenium on methylmercury toxicity in juvenile Atlantic cod: a transcriptional survey

Selenium (Se) and its derivatives are known to have protective effects against mercury (Hg) toxicity in mammals. In this study we wanted to evaluate whether Se co-exposure affect the transcription of methylmercury (MeHg) toxicity-relevant genes in early life stages of fish. Juvenile Atlantic cod were exposed to regular feed (control), Se-spiked feed (3mg Se kg(-1)), MeHg-spiked feed (10mg Hg kg(-1)) or to Se- and MeHg-spiked feed (3mg Se kg(-1) and 10mg Hg kg(-1), respectively) for ten weeks. Liver tissue was harvested for transcriptional analysis when the fish were weighing 11.4 ± 3.2g. Accumulated levels of Hg in liver of the two groups of fish exposed to MeHg were 1.5mg Hg kg(-1) wet weight, or 44-fold higher than in the control group, while the Se concentrations differed with less than 2-fold between the fish groups. Selenium co-exposure had no effect on the accumulated levels of Hg in liver tissue; however, MeHg co-exposure reduced the accumulated level of Se. Dietary exposure to MeHg had no effect on fish growth. Interaction effects between Se and MeHg exposure were observed for the transcriptional levels of CAT, GPX1, GPX3, NFE2L2, UBA52, SEPP1 and DNMT1. Significant effects of MeHg exposure were seen for DNMT1 and PPARG, while effects of Se exposure were seen for GPX4B and SEPP1A, as well as for DNA methyltransferase activity. The transcriptional results suggest, by considering up-regulation as a proxy for negative impact and at the tested concentrations, a pro-oxidative effect of Se co-exposure with MeHg, rather than an antioxidative effect.

Impacts of TCDD and MeHg on DNA methylation in zebrafish (Danio rerio) across two generations

This study aimed to investigate whether dioxin (TCDD) and methylmercury (MeHg) pose a threat to offspring of fish exposed to elevated concentrations of these chemicals via epigenetic-based mechanisms. Adult female zebrafish were fed diets added either 20 μg/kg 2,3,7,8 TCDD or 10 mg/kg MeHg for 47 days, or 10 mg/kg 5-aza-2'-deoxycytidine (5-AZA), a hypomethylating agent, for 32 days, and bred with unexposed males in clean water to produce F1 and F2 offspring. Global DNA methylation, promoter CpG island methylation and target gene transcription in liver of adult females and in 3 days post fertilization (dpf) F1 and F2 embryos were determined with HPLC, a novel CpG island tiling array containing 54,933 different probes and RT-qPCR, respectively. The results showed that chemical treatment had no significant effect on global DNA methylation levels in F1 (MeHg and TCDD) and F2 (MeHg) embryos and only a limited number of genes were identified with altered methylation levels at their promoter regions. CYP1A1 transcription, an established marker of TCDD exposure, was elevated 27-fold in F1 embryos compared to the controls, matching the high levels of CYP1A1 expression observed in F0 TCDD-treated females. This suggests that maternal transfer of TCDD is a significant route of exposure for the F1 offspring. In conclusion, the selected doses of TCDD and MeHg, two chemicals often found in high concentrations in fish, appear to have only modest effects on DNA methylation in F1 (MeHg and TCDD) and F2 (MeHg) embryos of treated F0 females.

RNA-Seq technology and its application in fish transcriptomics

High-throughput sequencing technologies, also known as next-generation sequencing (NGS) technologies, have revolutionized the way that genomic research is advancing. In addition to the static genome, these state-of-art technologies have been recently exploited to analyze the dynamic transcriptome, and the resulting technology is termed RNA sequencing (RNA-seq). RNA-seq is free from many limitations of other transcriptomic approaches, such as microarray and tag-based sequencing method. Although RNA-seq has only been available for a short time, studies using this method have completely changed our perspective of the breadth and depth of eukaryotic transcriptomes. In terms of the transcriptomics of teleost fishes, both model and non-model species have benefited from the RNA-seq approach and have undergone tremendous advances in the past several years. RNA-seq has helped not only in mapping and annotating fish transcriptome but also in our understanding of many biological processes in fish, such as development, adaptive evolution, host immune response, and stress response. In this review, we first provide an overview of each step of RNA-seq from library construction to the bioinformatic analysis of the data. We then summarize and discuss the recent biological insights obtained from the RNA-seq studies in a variety of fish species.

Mercury contamination in deep-water fish: transcriptional responses in tusk (Brosme brosme) from a fjord gradient

Recent findings have shown that deep-water fish from coastal areas may contain elevated levels of mercury (Hg). Tusk (Brosme brosme) was collected from six locations in Hardangerfjord, a fjord system where the inner parts are contaminated by metals due to historic industrial activity. ICPMS was used to determine the accumulated levels of metals (Hg, MeHg, Cd, Pb, As, and Se) in the fish, whereas oxidative status of the liver was assessed by measuring TBARS, vitamin C, vitamin E and catalase activity. To find out whether accumulated Hg triggers toxicologically relevant transcriptional responses and in order to gain genomic knowledge from a non-model species, the liver transcriptome of the gadoid fish was sequenced and assembled, and RNA-seq and RT-qPCR were used to screen for effects of Hg. The results showed high levels of accumulated Hg in tusk liver, probably reflecting an adaptation to deep-water life history, and only a weak declining outward fjord gradient of Hg concentration in tusk liver. MeHg only accounted for about 17% of total Hg in liver, suggesting hepatotoxicity of both inorganic and organic Hg. Pathway analysis suggested an effect of Hg exposure on lipid metabolism and beta-oxidation in liver. Oxidative stress markers glutathione peroxidase 1 and ferritin mRNA, as well as vitamin C and vitamin E (alpha and gamma tocopherol) showed a significant correlation with accumulated levels of Hg. Many transcripts of genes encoding established markers for Hg exposure were co-regulated in the fish. In conclusion, tusk from Hardangerfjord contains high levels of Hg, with possible hepatic effects on lipid metabolism and oxidative stress.

Differential gene expression associated with dietary methylmercury (MeHg) exposure in rainbow trout (Oncorhynchus mykiss) and zebrafish (Danio rerio)

The objective of this study was to identify and evaluate conserved biomarkers that could be used in most species of teleost fish at most life-stages. We investigated the effects of sublethal methylmercury (MeHg) exposure on developing rainbow trout and zebrafish. Juvenile rainbow trout and young adult zebrafish were fed food with MeHg added at 0, 0.5, 5, and 50 ppm. Atomic absorption spectrometry was applied to measure whole body total Hg levels, and pathologic analysis was performed to identify MeHg-induced toxicity. Fish at 6 weeks were sampled from each group for microarray analysis using RNA from whole fish. MeHg-exposed trout and zebrafish did not show overt signs of toxicity or pathology, nor were significant differences seen in mortality, length, mass, or condition factor. The accumulation of MeHg in trout and zebrafish exhibited dose- and time-dependent patterns during 6 weeks, and zebrafish exhibited greater assimilation of total Hg than rainbow trout. The dysregulated genes in MeHg-treated fish have multiple functional annotations, such as iron ion homeostasis, glutathione transferase activity, regulation of muscle contraction, troponin I binding and calcium-dependent protein binding. Genes were selected as biomarker candidates based on their microarray data and their expression was evaluated by QPCR. Unfortunately, these genes are not good consistent biomarkers for both rainbow trout and zebrafish from QPCR evaluation using individual fish. Our conclusion is that biomarker analysis for aquatic toxicant assessment using fish needs to be based on tissue-, sex- and species-specific consideration.

Gene responses in the central nervous system of zebrafish embryos exposed to the neurotoxicant methyl mercury

Methyl mercury (MeHg) is a neurotoxicant with adverse effects on the development of the nervous system from fish to man. Despite a detailed understanding of the molecular mechanisms by which MeHg affects cellular homeostasis, it is still not clear how MeHg causes developmental neurotoxicity. We performed here a genome-wide transcriptional analysis of MeHg-exposed zebrafish embryos and combined this with a whole-mount in situ expression analysis of 88 MeHg-affected genes. The majority of the analyzed genes showed tissue- and region-restricted responses in various organs and tissues. The genes were linked to gene ontology terms like oxidative stress, transport and cell protection. Areas even within the central nervous system (CNS) are affected differently resulting in distinct cellular stress responses. Our study revealed an unexpected heterogeneity in gene responses to MeHg exposure in different tissues and neuronal subregions, even though the known molecular action of MeHg would predict a similar burden of exposed cells. The overall structure of the developing brain of MeHg-exposed embryos appeared normal, suggesting that the mechanism leading to differentiation of the CNS is not overtly affected by exposure to MeHg. We propose that MeHg disturbs the function of the CNS by disturbing the cellular homeostasis. As these cellular stress responses comprise genes that are also involved in normal neuronal activity and learning, MeHg may affect the developing CNS in a subtle manner that manifests itself in behavioral deficits.

Comparison of neurobehavioral effects of methylmercury exposure in older and younger adult zebrafish (Danio rerio)

It is widely recognized that the nature and severity of responses to toxic exposure are age-dependent. Using active avoidance conditioning as the behavioral paradigm, the present study examined the effect of short-term methylmercury (MeHg) exposure on two adult age classes, 1- and 2-year-olds to coincide with zebrafish in relatively peak vs. declining health conditions. In Experiment 1, 2-year-old zebrafish were randomly divided into groups and were exposed to no MeHg, 0.15% ethanol (EtOH), 0.01, 0.03, 0.1, or 0.3 μM of MeHg (in 0.15% ethanol) for 2 weeks. The groups were then trained and tested for avoidance responses. The results showed that older zebrafish exposed to no MeHg or EtOH learned and retained avoidance responses. However, 0.01 μM or higher concentrations of MeHg exposure impaired avoidance learning in a dose-dependent manner with 0.3 μM of MeHg exposure producing death during the exposure period or shortly after the exposure but before the avoidance training. In Experiment 2, 1-year-old zebrafish were randomly divided into groups and were exposed to the same concentrations of MeHg used in Experiment 1 for 2 weeks. The groups were then trained and tested for avoidance responses. The results showed that younger zebrafish exposed to no MeHg, EtOH, or 0.01 μM of MeHg learned and retained avoidance responses, while 0.1 or 0.3 μM of MeHg exposure impaired avoidance learning in a dose-dependent manner. The study suggested that MeHg exposure produced learning impairments at a much lower concentration of MeHg exposure and more severely in older adult compared against younger adult zebrafish even after short exposure times.

Hormetic effects of acute methylmercury exposure on grp78 expression in rat brain cortex

This study aims to explore the expression of GRP78, a marker of endoplasmic reticulum (ER) stress, in the cortex of rat brains acutely exposed to methylmercury (MeHg). Thirty Sprague-Dawley (SD) rats were randomly divided into six groups, and decapitated 6 hours (h) after intraperitoneal (i.p.) injection of MeHg (2, 4, 6, 8 or 10 mg/kg body weight) or normal saline. Protein and mRNA expression of Grp78 were detected by western blotting and real-time PCR, respectively. The results showed that a gradual increase in GRP78 protein expression was observed in the cortex of rats acutely exposed to MeHg (2, 4 or 6 mg/kg). Protein levels peaked in the 6 mg/kg group (p < 0.05 vs. controls), decreased in the 8 mg/kg group, and bottomed below the control level in the 10 mg/kg group. Parallel changes were noted for Grp78 mRNA expression. It may be implied that acute exposure to MeHg induced hormetic dose-dependent changes in Grp78 mRNA and protein expression, suggesting that activation of ER stress is involved in MeHg-induced neurotoxicity. Low level MeHg exposure may induce GRP78 protein expression to stimulate endogenous cytoprotective mechanisms.

Progress and promises in toxicogenomics in aquatic toxicology: is technical innovation driving scientific innovation?

In the last decade, new technologies have been invented to analyze large amounts of information such as gene transcripts (transcriptomics), proteins (proteomics) and small cellular molecules (metabolomics). Many studies have been performed in the last few years applying these technologies to aquatic toxicology, mainly in fish. In this article, we summarize the current state of knowledge and question whether the application of modern technology for descriptive purposes truly represents scientific advancement in aquatic toxicology. We critically discuss the advantages and disadvantages of these technologies and emphasize the importance of these critical aspects. To date, these techniques have been used mainly as a proof of principle, demonstrating effects of model compounds. The potential to use these techniques to better analyze the mode-of-action of a toxicant or the effects of a compound within organisms has rarely been met. This is partly due to a lack of baseline data and the fact that the expression of mRNA and protein profiles is rarely linked to physiology or toxicologically meaningful outcomes. It seems premature to analyze mixtures or environmental samples until more is known about the expression profiles of individual toxicants. Gene transcription, protein, or metabolic data give only a partial view of these effects. Thus, we emphasize that data obtained by these technologies must be linked to physiological changes to fully understand their significance. The use of these techniques in aquatic toxicology is still in its infancy, data cannot yet be applied to environmental risk assessment or regulation until more emphasis is placed on interpreting the data within their physiological and toxicological contexts.

Effects of dietary methylmercury on the zebrafish brain: histological, mitochondrial, and gene transcription analyses

The neurotoxic compound methylmercury (MeHg) is a commonly encountered pollutant in the environment, and constitutes a hazard for wildlife and human health through fish consumption. To study the neurotoxic impact of MeHg on piscivorous fish, we contaminated the model fish species Danio rerio for 25 and 50 days with food containing 13.5 μg/g dry weight (dw) of MeHg (0.6 μg MeHg/fish/day), an environmentally relevant dose leading to brain mercury concentrations of 30 ± 4 μg of Hg g(-1) (dw) after 25 days of exposure and 46 ± 7 μg of Hg g(-1) (dw) after 50 days. Brain mitochondrial respiration was not modified by exposure to MeHg, contrary to what happens in skeletal muscles. A 6-fold increase in the expression of the sdh gene encoding the succinate dehydrogenase Fe/S protein subunit was detected in the contaminated brain after 50 days of exposure. An up regulation of 3 genes, atp2b3a, atp2b3b, and slc8a2b, encoding for calcium transporters was noticed after 25 days of exposure but the atp2b3a and atp2b3b were repressed and the slc8a2b gene expression returned to its basal level after 50 days, suggesting a perturbation of calcium homeostasis. After 50 days, we detected the up regulation of glial fibrillary acidic protein and glutathione S-transferase genes (gfap and gst), along with a repression of the glutathione peroxidase gene gpx1. These results match well with a MeHg-induced onset of oxidative stress and inflammation. A transmission electron microscopic observation confirmed an impairment of the optical tectum integrity, with a decrease of the nucleal area in contaminated granular cells compared to control cells, and a lower density of cells in the contaminated tissue. A potential functional significance of such changes observed in optical tectum when considering wild fish contaminated in their natural habitat might be an impaired vision and therefore a lowered adaptability to their environment.

Gene expression changes in female zebrafish (Danio rerio) brain in response to acute exposure to methylmercury

Methylmercury (MeHg) is a potent neurotoxicant and endocrine disruptor that accumulates in aquatic systems. Previous studies have shown suppression of hormone levels in both male and female fish, suggesting effects on gonadotropin regulation in the brain. The gene expression profile in adult female zebrafish whole brain induced by acute (96 h) MeHg exposure was investigated. Fish were exposed by injection to 0 or 0.5 µg MeHg/g. Gene expression changes in the brain were examined using a 22,000-feature zebrafish microarray. At a significance level of p < 0.01, 79 genes were up-regulated and 76 genes were down-regulated in response to MeHg exposure. Individual genes exhibiting altered expression in response to MeHg exposure implicate effects on glutathione metabolism in the mechanism of MeHg neurotoxicity. Gene ontology (GO) terms significantly enriched among altered genes included protein folding, cell redox homeostasis, and steroid biosynthetic process. The most affected biological functions were related to nervous system development and function, as well as lipid metabolism and molecular transport. These results support the involvement of oxidative stress and effects on protein structure in the mechanism of action of MeHg in the female brain. Future studies will compare the gene expression profile induced in response to MeHg with that induced by other toxicants and will investigate responsive genes as potential biomarkers of MeHg exposure.

Gene set enrichment analysis of microarray data from Pimephales promelas (Rafinesque), a non-mammalian model organism

Background

Methods for gene-class testing, such as Gene Set Enrichment Analysis (GSEA), incorporate biological knowledge into the analysis and interpretation of microarray data by comparing gene expression patterns to pathways, systems and emergent phenotypes. However, to use GSEA to its full capability with non-mammalian model organisms, a microarray platform must be annotated with human gene symbols. Doing so enables the ability to relate a model organism's gene expression, in response to a given treatment, to potential human health consequences of that treatment. We enhanced the annotation of a microarray platform from a non-mammalian model organism, and then used the GSEA approach in a reanalysis of a study examining the biological significance of acute and chronic methylmercury exposure on liver tissue of fathead minnow (Pimephales promelas). Using GSEA, we tested the hypothesis that fathead livers, in response to methylmercury exposure, would exhibit gene expression patterns similar to diseased human livers.

Results

We describe an enhanced annotation of the fathead minnow microarray platform with human gene symbols. This resource is now compatible with the GSEA approach for gene-class testing. We confirmed that GSEA, using this enhanced microarray platform, is able to recover results consistent with a previous analysis of fathead minnow exposure to methylmercury using standard analytical approaches. Using GSEA to compare fathead gene expression profiles to human phenotypes, we also found that fathead methylmercury-treated livers exhibited expression profiles that are homologous to human systems & pathways and results in damage that is similar to those of human liver damage associated with hepatocellular carcinoma and hepatitis B.

Conclusions

This study describes a powerful resource for enabling the use of non-mammalian model organisms in the study of human health significance. Results of microarray gene expression studies involving fathead minnow, typically used for aquatic ecological toxicology studies, can now be used to generate hypotheses regarding consequences of contaminants and other stressors on humans. The same approach can be used with other model organisms with microarray platforms annotated in a similar manner.