Dose-responsive gene expression changes in juvenile and adult mummichogs (Fundulus heteroclitus) after arsenic exposure

Long-term exposure of the binary mixture of cadmium and mercury damages the developed ovary of adult zebrafish

The toxicity of the binary mixture of cadmium (Cd) and mercury (Hg) on the ovary of adult zebrafish was evaluated in the present study. Adult female zebrafish were exposed to cadmium chloride (1 mg/L), mercury chloride (30 µg/L), and a binary mixture of both metals for 21 days. The toxic effects of both metals on the ovary were investigated by evaluating the oxidative stress markers and related gene expression in ovarian tissue along with the histopathological examination. The significantly decreased level of GSH and increased level of MDA in ovarian tissue of adult female zebrafish exposed to Cd + Hg indicated that the exposure of binary mixture of Cd and Hg caused more lipid peroxidation in the ovary. The significant changes in expression of mRNA of catalase (CAT) and nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2) were not observed in the ovary of zebrafish exposed to the binary mixture. Upon histological evaluation, a decreased number of full-growth (mature) oocytes along with degenerative changes due to Cd exposure were noticed, while ovary of zebrafish of the Hg-exposed group had shown a decreased number of pre-and early vitellogenic oocytes along with atretic previtellogenic oocytes compared to the control group. The ovary of zebrafish of the Cd + Hg-exposed group had shown a decreased number of previtellogenic oocytes with marked pathological changes in mature oocytes. Present findings elucidate that simultaneous long-term exposure of Cd and Hg compared to individual exposure significantly damaged the various stages of oocytes of an ovary of adult zebrafish.

Synonymous single nucleotide polymorphism in arsenic (+3) methyltransferase of the Western mosquitofish (Gambusia affinis) and its gene expression among field populations

Naturally occurring arsenic is toxic at extremely low concentrations, yet some species persist even in high arsenic environments. We wanted to test if these species show evidence of evolution associated with arsenic exposure. To do this, we compared allelic variation across 872 coding nucleotides of arsenic (+3) methyltransferase (as3mt) and whole fish as3mt gene expression from three field populations of Gambusia affinis, from water sources containing low (1.9 ppb), medium-low (3.3 ppb), and high (15.7 ppb) levels of arsenic. The high arsenic site exceeds the US EPA's Maximum Contamination Level for drinking water. Medium-low and high populations exhibited homozygosity, and no sequence variation across all animals sampled. Eleven of 24 fish examined (45.8%) in the low arsenic population harbored synonymous single nucleotide polymorphisms (SNPs) in exons 4 and/or 10. SNP presence in the low arsenic population was not associated with differences in as3mt transcript levels compared to fish from the medium-low site, where SNPs were noted; however, as3mt expression in fish from the high arsenic concentration site was significantly lower than the other two sites. Low sequence variation in fish populations from sites with medium-low and high arsenic concentrations suggests greater selective pressure on this allele, while higher variation in the low population suggests a relaxed selection. Our results suggest gene regulation associated with arsenic detoxification may play a more crucial role in influencing responses to arsenic than polymorphic gene sequence. Understanding microevolutionary processes to various contaminants require the evaluation of multiple populations across a wide range of pollution exposures.

Arsenic alters transcriptional responses to Pseudomonas aeruginosa infection and decreases antimicrobial defense of human airway epithelial cells

Arsenic contamination of drinking water and food threatens the health of hundreds of millions of people worldwide by increasing the risk of numerous diseases. Arsenic exposure has been associated with infectious lung disease in epidemiological studies, but it is not yet understood how ingestion of low levels of arsenic increases susceptibility to bacterial infection. Accordingly, the goal of this study was to examine the effect of arsenic on gene expression in primary human bronchial epithelial (HBE) cells and to determine if arsenic altered epithelial cell responses to Pseudomonas aeruginosa, an opportunistic pathogen. Bronchial epithelial cells line the airway surface, providing a physical barrier and serving critical roles in antimicrobial defense and signaling to professional immune cells. We used RNA-seq to define the transcriptional response of HBE cells to Pseudomonas aeruginosa, and investigated how arsenic affected HBE gene networks in the presence and absence of the bacterial challenge. Environmentally relevant levels of arsenic significantly changed the expression of genes involved in cellular redox homeostasis and host defense to bacterial infection, and decreased genes that code for secreted antimicrobial factors such as lysozyme. Using pathway analysis, we identified Sox4 and Nrf2-regulated gene networks that are predicted to mediate the arsenic-induced decrease in lysozyme secretion. In addition, we demonstrated that arsenic decreased lysozyme in the airway surface liquid, resulting in reduced lysis of Microccocus luteus. Thus, arsenic alters the expression of genes and proteins in innate host defense pathways, thereby decreasing the ability of the lung epithelium to fight bacterial infection.

The immune responses and expression of metallothionein (MT) gene and heat shock protein 70 (HSP 70) in juvenile rockfish, Sebastes schlegelii, exposed to waterborne arsenic (As3+)

Juvenile rockfish, Sebastes schlegelii (mean length 16.4±1.9cm, and mean weight 71.6±6.4g) were exposed for 20days with the different levels of waterborne arsenic concentration (0, 50, 100, 200 and 400μg/L). The plasma cortisol of S. schlegelii was significantly increased by the waterborne arsenit exposure. In the immune responses, the immunoglobulin M (Ig M) and lysozyme activity of S. schlegelii were significantly increased by the waterborne arsenic exposure. The acetylcholinesterase (AChE) activity of S. schlegelii was inhibited by the waterborne arsenic exposure. The substantial increases in the gene expression such as metallothionein (MT) and heat shock protein 70 (HSP 70) were observed by the waterborne arsenic exposure. The results demonstrated that waterborne arsenic exposure can induce the significant alterations in the immune responses and specific gene expression of S. schlegelii.

Effects of low arsenic concentration exposure on freshwater fish in the presence of fluvial biofilms

Arsenic (As) is a highly toxic element and its carcinogenic effect on living organisms is well known. However, predicting real effects in the environment requires an ecological approach since toxicity is influenced by many environmental and biological factors. The purpose of this paper was to evaluate if environmentally-realistic arsenic exposure causes toxicity to fish. An experiment with four different treatments (control (C), biofilm (B), arsenic (+As) and biofilm with arsenic (B+As)) was conducted and each one included sediment to enhance environmental realism, allowing the testing of the interactive effects of biofilm and arsenic on the toxicity to fish. Average arsenic exposure to Eastern mosquitofish (Gambusia holbrooki) was 40.5 ± 7.5 μg/L for +As treatment and 34.4 ± 1.4 μg/L for B+As treatment for 56 days. Fish were affected directly and indirectly by this low arsenic concentration since exposure did not only affect fish but also the function of periphytic biofilms. Arsenic effects on the superoxide dismutase (SOD) and glutathione reductase (GR) activities in the liver of mosquitofish were ameliorated in the presence of biofilms at the beginning of exposure (day 9). Moreover, fish weight gaining was only affected in the treatment without biofilm. After longer exposure (56 days), effects of exposure were clearly seen. Fish showed a marked increase in the catalase (CAT) activity in the liver but the interactive influence of biofilms was not further observed since the arsenic-affected biofilm had lost its role in water purification. Our results highlight the interest and application of incorporating some of the complexity of natural systems in ecotoxicology and support the use of criterion continuous concentration (CCC) for arsenic lower than 150 μg/L and closer to the water quality criteria to protect aquatic life recommended by the Canadian government which is 5 μg As/L.

Functional genomics to assess biological responses to marine pollution at physiological and evolutionary timescales: toward a vision of predictive ecotoxicology

Marine pollution is ubiquitous, and is one of the key factors influencing contemporary marine biodiversity worldwide. To protect marine biodiversity, how do we surveil, document and predict the short- and long-term impacts of pollutants on at-risk species? Modern genomics tools offer high-throughput, information-rich and increasingly cost-effective approaches for characterizing biological responses to environmental stress, and are important tools within an increasing sophisticated kit for surveiling and assessing impacts of pollutants on marine species. Through the lens of recent research in marine killifish, we illustrate how genomics tools may be useful for screening chemicals and pollutants for biological activity and to reveal specific mechanisms of action. The high dimensionality of transcriptomic responses enables their usage as highly specific fingerprints of exposure, and these fingerprints can be used to diagnose environmental problems. We also emphasize that molecular pathways recruited to respond at physiological timescales are the same pathways that may be targets for natural selection during chronic exposure to pollutants. Gene complement and sequence variation in those pathways can be related to variation in sensitivity to environmental pollutants within and among species. Furthermore, allelic variation associated with evolved tolerance in those pathways could be tracked to estimate the pace of environmental health decline and recovery. We finish by integrating these paradigms into a vision of how genomics approaches could anchor a modernized framework for advancing the predictive capacity of environmental and ecotoxicological science.

Modulatory role of dietary Chlorella vulgaris powder against arsenic-induced immunotoxicity and oxidative stress in Nile tilapia (Oreochromis niloticus)

Arsenic intoxicant have long been regarded as an impending carcinogenic, genotoxic, and immunotoxic heavy metal to human and animals as well. In this respect, we evaluated biomarkers of the innate immune response and oxidative stress metabolism in gills and liver of Nile tilapia (Oreochromis niloticus) after arsenic exposure, and the protective role of Chlorella vulgaris (Ch) dietary supplementation were elucidated. Protective role of C. vulgaris (Ch), as supplementary feeds (5% and 10% of the diet) was studied in Nile tilapia (O. niloticus) against arsenic induced toxicity (NaAsO2 at 7 ppm) for 21 days exposure period. A significant down-regulation in innate immune response; including, respiratory burst, lysozyme, and bactericidal activity followed due to deliberately As(+3) exposure. Similarly, oxidative stress response; like nitric oxide (NO), catalase (CAT), glutathione (GSH), glutathione peroxidase (GPx), malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels were significantly decreased. Combined treatment of Ch and As(+3) significantly enhanced the innate immune response and antioxidant activity. Strikingly, Ch supplementation at 10% has been considered the optimum for Nile tilapia since it exhibited enhancement of innate immune response and antioxidant activity over the level 5%, and even better than that of control level. Thus, our results concluded that dietary Ch supplementation could protect Nile tilapia against arsenic induced immunosuppression and oxidative stresses.

Low dose of arsenic trioxide triggers oxidative stress in zebrafish brain: expression of antioxidant genes

Occurrence of arsenic in the aquatic environment of West Bengal (India), Bangladesh and other countries are of immediate environmental concern. In the present study, zebrafish (Danio rerio) was used as a model to investigate oxidative stress related enzyme activities and expression of antioxidant genes in the brain to 50µg/L arsenic trioxide for 90 days. In treated fish, generation of reactive oxygen species (ROS), malondialdehyde (MDA) and conjugated diene (CD) showed a triphasic response attaining a peak at the end of the exposure. In addition, a gradual increase in GSH level was noted until 60 days and at 90 days, a sudden fall was recorded which heightened arsenic toxicity. However, GSH level does not correlate well with the glutathione reductase (GR) activity. Generation of ROS in zebrafish brain due to As2O3 exposure was further evidenced by significant alteration of glutathione peroxidase (GPx) and catalase (CAT) activity, which converts H2O2 to water and helps in detoxication. Moreover, enhanced mRNA level of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in As2O3 exposed zebrafish indicates a protective role of Nrf2. kelch-like ECH-associated protein 1 (Keap1), a negative regulator of Nrf2, inversely correlates with the mRNA expression of Nrf2. As2O3 induced toxicity was also validated by the alteration in NRF2 and NRF2 dependent expression of proteins such as heme oxygenase1 (HO1) and NAD(P)H dehydrogenase quinone1 (NQO1). The mRNA expression of glutathione peroxidase (Gpx1), catalase (Cat), manganese superoxide dismutase (Mn-Sod), copper/zinc superoxide dismutase (Cu/Zn Sod) and cytochrome c oxidase1 (Cox1) were also up regulated. The expression of uncoupling protein 2 (Ucp2), an important mitochondrial enzyme was also subdued in arsenic exposed zebrafish. The oxidative stress induced by arsenic also cause reduced mRNA expression of B-cell lymphoma 2 (Bcl2) present in the inner mitochondrial membrane and thereby indicating onset of apoptosis in treated fish. It is concluded that even a low dose of arsenic trioxide is toxic enough to induce significant oxidative stress in zebrafish brain.

Arsenic exposure alters expression of cell cycle and lipid metabolism genes in the liver of adult zebrafish (Danio rerio)

Adult zebrafish (Danio rerio) were used to investigate mRNA expression in the liver following 7-day and 21-day exposures to 0, 10, 50, or 500 ppb sodium arsenite. Arsenic exposure has been linked to several human disorders including cancers and cardiovascular and metabolic diseases. Quantitative PCR was employed to determine the mRNA expression of genes involved in cell cycle regulation [cyclin E1 (ccne1), WEE1 A kinase (wee1)], DNA damage repair [breast cancer 2 (brca2)] and lipid transport and metabolism [carnitine O-octanoyltransferase (crot), fatty acid binding protein-3 (fabp3) and 3-hydroxy-3-methylglutaryl-CoA synthase 1 (hmgcs1)]. Results from the 7-day exposure showed sex- and dose-specific changes in expression of wee1, brca2, crot and hmgcs1. No significant differences from controls were observed in fish exposed for 21 days. Expression of all genes, except ccne1, was significantly different between the 7- and 21-day exposures. The results presented here correlate with prior findings from our lab and others, and offer further insight into potential mechanisms of low-dose arsenic exposure.

A novel electrochemical method to evaluate the cytotoxicity of heavy metals

There is an ongoing search to develop techniques for detection of heavy metals which are highly toxic and can cause damaging effects even at very low concentrations. In this present study, we report a label-free electrochemical method based on the direct voltammetric response of human cervical carcinoma (HeLa) cells on a highly sensitive graphene modified electrode. Five heavy metals were tested with the method and the results were validated by the traditional methyl tetrazolium (MTT) assay. The results revealed that the most toxic metal was Cr, followed by Cd, Cu, Pb and Zn. A good correlation between the two methods was observed. This work will be beneficial in providing a novel monitoring method to detect hazardous pollutants in the field of environmental toxicology.

Embryonic arsenic exposure reduces the number of muscle fibers in killifish (Fundulus heteroclitus)

Arsenic is a contaminant of drinking water and has been correlated with adverse developmental outcomes such as low birth weight, reduced weight gain, and altered locomotor activity. Previous research has shown that killifish (Fundulus heteroclitus) exposed to high arsenic levels during embryogenesis had smaller muscle fiber diameters. The current study was designed to determine whether changes in muscle fibers persisted, were exacerbated, or resolved over time. Killifish embryos were exposed to 0-5 ppm arsenite and, upon hatching, were transferred into either clean water or continued receiving the same exposure to arsenic for up to 16 weeks. Arsenic significantly decreased the weight of both embryonic-only exposed juveniles and continuously exposed juveniles between 4 and 16 weeks of development at concentrations as low as 0.8 ppm. Although arsenite exposure increased the percentage of small diameter fibers during the early weeks, fiber diameters returned to control levels in the embryonic-only exposed fish. However, muscle fiber density was still reduced to 85.7%, 80.3%, and 73.8% of control for the 0.8, 2, and 5 ppm embryonic-only exposure groups, respectively, even after 16 weeks of development. These results indicate that while continuous exposure to arsenic may alter the size of muscle fibers, embryonic-only exposure to arsenic has lasting effects on the number of muscle fibers formed.

Trophic status and metal bioaccumulation differences in multiple fish species exposed to coal ash-associated metals

On December 22, 2008 a dike containing coal fly ash from the Tennessee Valley Authority Kingston Fossil Plant near Kingston Tennessee USA failed and resulted in the largest coal ash spill in U.S. history. Coal ash, a by-product of coal combustion, is known to contain multiple contaminants of concern, including arsenic and selenium. The purpose of this study was to investigate species differences in the bioaccumulation of arsenic and selenium and potential factors contributing to these differences (i.e., trophic dynamics and gut pH) in the vicinity of the Kingston coal ash spill. Elevated levels of arsenic and selenium were observed in various tissues of largemouth bass, white crappie, bluegill and redear sunfish from sites associated with the Kingston coal ash spill. Highest concentrations of selenium were found in redear sunfish with liver concentrations as high as 24.83mg/kg dry weight and ovary concentrations up to 10.40mg/kg dry weight at coal ash-associated sites. Investigations into the gut pH and trophic dynamics of redear sunfish and bluegill demonstrated a large difference in the gut physiology between these two species. Redear sunfish stomach and intestinal pH was found to be 1.1 and 0.16 pH units higher than in bluegill, respectively. In addition, fish from coal ash-associated sites showed enrichment differences ((15)N and (13)C) compared to no ash sites, indicating differences in food web dynamics between sites. These results imply the incorporation of coal ash-associated compounds into local food webs and/or a shift in diet at ash sites compared to the no ash reference sites. Based on these results, further investigation into a broader food web at ash-associated sites is warranted.

Arsenic exposure to killifish during embryogenesis alters muscle development

Epidemiological studies have correlated arsenic exposure in drinking water with adverse developmental outcomes such as stillbirths, spontaneous abortions, neonatal mortality, low birth weight, delays in the use of musculature, and altered locomotor activity. Killifish (Fundulus heteroclitus) were used as a model to help to determine the mechanisms by which arsenic could impact development. Killifish embryos were exposed to three different sodium arsenite concentrations and were collected at 32 h post-fertilization (hpf), 42 hpf, 168 hpf, or < 24 h post-hatch. A killifish oligo microarray was developed and used to examine gene expression changes between control and 25-ppm arsenic-exposed hatchlings. With artificial neural network analysis of the transcriptomic data, accurate prediction of each group (control vs. arsenic-exposed embryos) was obtained using a small subset of only 332 genes. The genes differentially expressed include those involved in cell cycle, development, ubiquitination, and the musculature. Several of the genes involved in cell cycle regulation and muscle formation, such as fetuin B, cyclin D-binding protein 1, and CapZ, were differentially expressed in the embryos in a time- and dose-dependent manner. Examining muscle structure in the hatchlings showed that arsenic exposure during embryogenesis significantly reduces the average muscle fiber size, which is coupled with a significant 2.1- and 1.6-fold upregulation of skeletal myosin light and heavy chains, respectively. These findings collectively indicate that arsenic exposure during embryogenesis can initiate molecular changes that appear to lead to aberrant muscle formation.

MRP proteins as potential mediators of heavy metal resistance in zebrafish cells

Acquired resistance of mammalian cells to heavy metals is closely relevant to enhanced expression of several multidrug resistance-associated proteins (MRP), but it remains unclear whether MRP proteins confer resistance to heavy metals in zebrafish. In this study, we obtained zebrafish (Danio rerio) fibroblast-like ZF4 cells with resistance to toxic heavy metals after chronic cadmium exposure and selection for 6months. These cadmium-resistant cells (ZF4-Cd) were maintained in 5μM cadmium and displayed cross-resistance to cadmium, mercury, arsenite and arsenate. ZF4-Cd cells remained the resistance to heavy metals after protracted culture in cadmium-free medium. In comparison with ZF4-WT cells, ZF4-Cd cells exhibited accelerated rate of cadmium excretion, enhanced activity of MRP-like transport, elevated expression of abcc2, abcc4 and mt2 genes, and increased content of cellular GSH. Inhibition of MRP-like transport activity, GSH biosynthesis and GST activity significantly attenuated the resistance of ZF4-Cd cells to heavy metals. The results indicate that some of MRP transporters are involved in the efflux of heavy metals conjugated with cellular GSH and thus play crucial roles in heavy metal detoxification of zebrafish cells.

Hepatic gene expression profile in brown trout (Salmo trutta) exposed to traffic related contaminants

In recent decades there has been growing concern about highway runoff as a potential threat and a significant source of diffuse pollution to the aquatic environment. However, identifying ecotoxicological effects might be challenging, especially at sites where the traffic density is modest to low. Hence, there is a need for alternatives e.g. small-scale toxicity tests using conventional endpoints such as mortality and growth. The present paper presents result from a transcriptional (microarray) screening performed on liver from brown trout (Salmo trutta) acutely exposed (4h) to traffic-related contaminants during washing of a highway tunnel outside the city of Oslo, Norway. The results demonstrated that traffic-related contaminants caused a plethora of molecular changes that persisted several hours after the exposure (i.e. during recovery). Beside an evident transcriptional up-regulation of e.g. cytochrome P450 1A1 (CYP1A1), cytochrome P450 1B1 (CYP1B1), and cytosolic sulfotransferase (SULT) involved in xenobiotic biotransformation, the observed responses were predominantly associated with immunosuppression, oxidative damage, and endocrine modulation. The observed responses were likely caused by an interaction of several contaminants including trace metals and organic micro-pollutants such as PAHs.