Bioaccumulation and transformation of methylmercury and selenite using zebrafish (Danio Rerio) larvae as a model

Toxicological Mechanisms of Emerging Per-/poly-fluoroalkyl Substances: Focusing on Transcriptional Activity and Gene Expression Disruption

Environmental and human monitoring studies have witnessed increasing occurrence of emerging per-/poly-fluoroalkyl substances (ePFASs) worldwide. Three classes of ePFASs, namely chlorinated polyfluoroalkylether sulfonic acids, hexafluoropropylene oxide homologues and short-chain perfluoroalkyl acids attracted the most attention. It is, therefore, the goal of this review to systematically and critically analyse the toxicity and toxicological mechanisms of these ePFASs based on the papers published between 2017 and 2022. The review summarized the main findings from both in vivo and in vitro studies, covering the hepatotoxicity of ePFASs and their interference with the endocrine system, including reproductive, developmental and thyroid toxicity. It also summarized the changes in gene expression in the hypothalamic-pituitary-thyroid axis and hypothalamic-pituitary-gonad axis of the model organisms after ePFASs exposure. The changes in gene expression in vitro and in vivo provide a clearer understanding of the toxicological mechanisms of ePFASs interference on hormonal levels (i.e., estradiol, testosterone, and thyroid hormones), developmental disturbance (e.g., swim bladder dysfunction) and lipid metabolism disruption (e.g., lipid droplet accumulation and hepatomegaly). In the end, future research directions on the toxicological mechanisms of ePFASs are suggested.

Uptake, elimination, and toxicokinetics of selected pharmaceuticals in multiple tissues of Nile tilapia (Oreochromis niloticus) exposed to environmentally relevant concentrations

Pharmaceuticals in aquatic environment displayed adverse effects to fish. The effects are usually related to the internal levels of pharmaceuticals accumulated in specific fish tissues. In this study, we investigated the uptake, elimination, and toxicokinetics of six pharmaceuticals, e.g. naproxen (NAX), diclofenac (DCF), ibuprofen (IBU), carbamazepine (CBZ), fluoxetine (FLX), and sertraline (SER), in 11 fish tissues of Nile tilapia. The experiments were conducted in a flow-through system with an 8-day uptake/8-day elimination periods. The fish exposure groups involved the control, single FLX, and mixture of six pharmaceuticals at environmentally relevant concentration of 4 μg/L. FLX and SER showed the maximum concentrations of 145 and 201 ng/g wet weight, respectively, in fish spleen tissue, while NAX and IBU were not detected in any tissue. The mean concentrations for the pharmaceuticals in Nile tilapia tissues generally followed the order: bile> kidney, gut, stomach, liver> brain, gill, spleen> plasma, skin, muscle. The steady-state bioconcentration factors in various tissues generally range at 0.74-437.58 L/kg. The uptake and elimination toxicokinetics illustrated the rapid accumulation and depuration of pharmaceuticals in fish tissues. The results help to understand the internal bioconcentration, tissue distribution, and toxicokinetics of pharmaceuticals in multiple fish biological compartments.

Alleviation of copper toxicity in Daphnia magna by hydrogen nanobubble water

As a novel antioxidant, hydrogen water has been widely used to alleviate oxidative stress in plants as well as in the medical field. However, the function of hydrogen water in environmental toxicology remains unknown. In this study, combining nanobubbles (NBs) and hydrogen water, we investigate the effect and mechanism of hydrogen NB water on copper induced acute toxicity to water fleas (Daphnia magna). The 24-h lethal Cu concentrations at which 50 % of the population die were 84 μg/L in hydrogen NB water and 45 μg/L in control water, confirming that hydrogen NB water effectively alleviated acute Cu toxicity in D. magna. The results were consistent with a significant reduction of Cu uptake and decrease of Cu accumulation in D. magna. As confirmed in fluorescence spectrophotometry and high-content screening system analysis, the hydrogen NB water also significantly reduced the oxidative damage and improved Cu tolerance in D. magna. From the results, it can be inferred that hydrogen NB water alleviates Cu stress in D. magna by depressing Cu bioaccumulation and reducing oxidative stress. The results provide basic data of hydrogen NB water for environmental toxicologists, and also a reference for the application of hydrogen NB water in the environment.

Impact of selenium co-administration on methylmercury exposed eleutheroembryos and adult zebrafish (Danio rerio): Changes in bioaccumulation and gene expression

Mercury still represents one of the most hazardous threats for the aquatic ecosystem due to its high toxicity, and the fact that it can be easily incorporated into the food chain by accumulation in fish as MeHg. On the other hand, selenium is a micronutrient that is part of different antioxidant enzymes that regulate the cellular redox state, and whose complex interaction with Hg has been extensively studied from a toxicological point of view. In order to evaluate the protective effect of Se(IV) co-administration against MeHg accumulation and toxicity, we have selected an in-vivo model at two developmental stages: zebrafish eleutheroembryos and adult fish. Embryos were exposed during 48 h to MeHg (5 or 25 μg/l) and a concentration of Se (IV) representing a molar ratio close to one (2.5 or 12.5 μg/l), while adult zebrafish were exposed during 72 h to either 25 μg/l of MeHg alone or co-exposed with 12.5 μg/l of Se (IV). A significant decrease in MeHg bioaccumulation factor was observed in eleutheroembryos co-exposed to Se(IV). A time-dependent accumulation of MeHg was observed in all the analyzed organs and tissues of adult fish, which was significantly reduced in the muscular tissue and the intestine by Se(IV) co-administration. However, such protection against MeHg bioaccumulation was not maintained in the brain and liver. The data derived from the gene expression analysis also demonstrated the protective effect of Se(IV) against MeHg-induced oxidative stress and the activation of different defense mechanisms by Se(IV) co-administration.

Toxicokinetics and toxic effects of a Chinese PFOS alternative F-53B in adult zebrafish

6:2 chlorinated polyfluorinated ether sulfonate (F-53B), a Chinese PFOS alternative, has recently been identified in river water, sewage sludge, wildlife and humans, causing great concerns about its potential toxic effects. Here, we report the first investigation of the toxicokinetics and oxidative stress of F-53B in adult zebrafish. Adult male and female zebrafish were exposed to 10 and 100 μg/L of F-53B for 7 days followed by a 5-d depuration period to examine bioaccumulation, distribution, and depuration of F-53B in fish. The results showed that F-53B was readily accumulated in fish tissues with log BCF values of 2.36-3.65, but was eliminated slowly (t_1/2 = 152.4-358.5 h). F-53B accumulation was greater in males than in females and the concentration in tissues decreased in the following order: gonad ≈ liver ≫ gill ≫ brain in females and liver ≈ gill ≫ gonad ≫ brain in males, showing sex- and tissue- specific accumulation of F-53B in fish. After chronic exposure to F-53B for 28 days, a significant dose-dependent increase in histopathological changes in the liver were mainly manifested by vacuolation. Furthermore, F-53B also significantly reduced the enzyme activity (or content) of most of the measured oxidative stress-related markers (e.g., SOD, CAT and MDA) except for an increase in GSH-Px activity, indicating that oxidative stress was induced in zebrafish after treatment with F-53B. The results of this study provide important information on the toxicokinetics and toxic effects of F-53B, which will contribute to the ecological risk assessments of F-53B released into surface waters.

In vivo bioconcentration of a metal mixture by Danio rerio eleutheroembryos

Exposure to heavy metals has represented one of the most serious health risks of environmental pollution over the last 50 years. Most of the bioconcentration studies that have been carried out to date explored only individual contaminants, unlike the real situations that occur in the environment. In this work, zebrafish eleutheroembryos were exposed to a mixture of CH₃Hg(II), iAs(III), Ag(I) and Cd(II), and new BCFs were calculated and compared with those calculated from single metal exposures. In both cases, experimental conditions meet the OECD Test 305 conditions established for aquatic systems. In addition, spatial imaging obtained by laser ablation coupled to inductively plasma mass spectrometry (LA-ICP/MS), has been directly performed in these samples providing complementary information. The new BCF's have revealed some differences compared to single metal exposures when eleutheroembryos were exposed to the metal mixture, especially for iAs(III) and Cd(II). LA-ICP/MS images are in good agreement with the BFC's found, representing an interesting approach to get spatial distribution of metals that reinforces the toxicokinetic information.

High bioconcentration of titanium dioxide nanoparticles in Daphnia magna determined by kinetic approach

The environmental risk assessments of titanium dioxide nanoparticles (TiO2 NPs) have drawn wide attention and one of the required critical information is the bioconcentration potentials of these nanoparticles in aquatic organisms. In the present study, the bioconcentration of six commercially available TiO2 NPs with different sizes and surface properties were quantified in a freshwater cladoceran Daphnia magna using kinetic modeling approach. We first calculated the uptake rate constant (ku) and depuration rate constant (ke) of TiO2 NPs and then employed a first-order kinetic model to predict the bioconcentration factors (BCF) at different TiO2 NPs concentrations. Both the ku and ke of TiO2 NPs were significantly affected by the exposure concentration and the nanoparticle property. The predicted BCF values in D. magna of six TiO2 NPs ranged from 2.40×10(5)L/kg to 1.52×10(6)L/kg, and had no clear correlation with the exposure concentration. Large nominal size resulted in a lower BCF of TiO2 NPs at lower exposure concentration. Higher hydrophobicity and Al(OH)3 coating also resulted in a higher BCF. All the six TiO2 NPs in this study were therefore considered very bioaccumulative. More attention should be paid to bioconcentration in the environmental risk assessments of TiO2 NPs, and the physicochemical properties of TiO2 NPs should be taken into account.

Bioconcentration of ionic cadmium and cadmium selenide quantum dots in zebrafish larvae

The concern related to the use of nanomaterials is growing nowadays, especially the risk associated with their emission or exposure. One type of nanomaterials that has attracted much attention is quantum dots (QDs). QDs incorporation in consumer goods increases the probability of their entering in the environment and then into living organisms and human. In order to evaluate their potential to be bioconcentrated, zebrafish larvae have been exposed to SeCd/ZnS QDs, after performing an exhaustive characterization of these nanoparticles under the assay conditions. These data were compared with those obtained when zebrafish larvae were exposed to ionic cadmium. Finally, distribution of ionic Cd and QDs in exposed zebrafish larvae have been evaluated by Laser Ablation ICP-MS.

Zebrafish (Danio rerio) eleutheroembryo-based procedure for assessing bioaccumulation

This paper reports on the development and preliminary evaluation of a new bioaccumulation test based on the use of zebrafish (Danio renio) eleutheroembryos (72 h after hatching, corresponding to 144 h post fertilization, hpf) as an alternative to adult fish-based procedures for regulatory purposes regarding REACH application. The proposed test accomplished the OECD 305 guideline and consists of a 48 h uptake period followed by a 24 h depuration step. Bioaccumulation experiments were performed for a selected of hyper hydrophobic chemicals (log Kow> 7.6), that is, PCB 136 and PBDE 154 at two concentration levels corresponding roughly to 1% and 0.1% the chemicaĺs LC50(nominal concentrations of 4.0 and 12.0 μg/L for PCB 136, and 1.0 and 5.0 μg/L PBDE 154, respectively). Toxicokinetic models were used to calculate the bioconcentration factors (BCFs) based on of the chemical concentrations found in the contaminated eleutheroembryos and their surrounding media. The experimentally determined accumulation profiles show bioaccumulation by zebrafish eleutheroembryos of both chemicals, and that the process is more complex than simple water-lipid partition. Calculated log BCFs using a first-order accumulation model(3.97 and 3.73 for PCB 136, and 3.95 and 4.29 for PBDE 154) were in the range of those previously reported in the literature. The suitability of this new nonprotected life stage bioaccumulation protocol for BCF estimation was evaluated by application to widely divergent micropollutants with different accumulation mechanisms. The results were compared with those in the MITE-NITE database for adult rice fish (Oryzias latipes).

Dynamics of uptake and elimination of pyrethroid insecticides in zebrafish (Danio rerio) eleutheroembryos

Synthetic pyrethroids (SPs) are among the most heavily used insecticides for residential and agricultural applications. Their residues have frequently been detected in aquatic ecosystems. Despite their high aquatic toxicity, their toxicokinetics are still unclear. In this study, the kinetics of uptake and depuration of three SPs, permethrin (PM), bifenthrin (BF) and λ-cyhalothrin (λ-CH), were determined for the first time using zebrafish eleutheroembryo assays. The diastereoisomer selectivity of PM in eleutheroembryos was further examined. The results indicated that three SPs were quickly taken up by eleutheroembryos. The bioaccumulation factors of the SPs ranged from 125.4 to 708.4. The depuration of SPs in zebrafish eleutheroembryos followed the first-order process. The elimination rate constants (k2) of SPs in eleutheroembryos ranged from 0.018 h(-1) to 0.0533 h(-1). The half-lives (t1/2) were in the range 13.0-38.5h. The diastereoisomer fraction (DF) values for PM in the eleutheroembryos estimated at different uptake and depuration times were all significantly greater than the original value (DF=0.43), indicating selective enrichment and elimination of cis-PM relative to trans-PM. These results reveal a high capacity for SP bioconcentration by zebrafish eleutheroembryos, suggesting that SPs possess a highly cumulative risk to fish.