Exposure to a PBDE/OH-BDE mixture alters juvenile zebrafish (Danio rerio) development

Association between brominated flame retardants (PBDEs and PBB153) exposure and hypertension in U.S. adults: results from NHANES 2005-2016

BACKGROUND

Brominated Flame Retardants (BFRs) have attracted widespread concern due to their environmental persistence and potential toxicity. This study aims to examine the association between BFRs exposure and hypertension.

METHODS

We used data from the National Health and Nutrition Examination Survey (NHANES) spanning 2005 to 2016 for the cross-sectional analysis. To evaluate the individual and combined impacts of BFRs exposure on hypertension, we utilized multivariate models, including generalized additive models, weighted quantile sum (WQS) regression, and Bayesian kernel machine regression (BKMR) models.

RESULTS

9882 individuals (48% male) aged ≥ 20 were included in the final analysis, of whom 4114 had hypertension. After controlling for potential covariates, higher serum concentrations of PBDE100 (OR: 1.26; 95% CI: 1.01, 1.57) and PBDE153 (OR: 1.50; 95% CI: 1.18, 1.88) were significantly associated with hypertension. A nonlinear relationship between PBDE28 and hypertension was observed (P = 0.03). Moreover, BFRs mixture were positively associated with the prevalence of hypertension in both the WQS (β:1.09; 95% CI: 1.02, 1.17; P = 0.02) and BKMR models.

CONCLUSION

Our study suggested that BFRs exposure is positively associated with hypertension in the general population. To confirm this association and elucidate the mechanisms, further research is required.

Reviewing the use of zebrafish for the detection of neurotoxicity induced by chemical mixtures through the analysis of behaviour

The knowledge and assessment of mixtures of chemical pollutants in the aquatic environment is a complex issue that is often challenging to address. In this review, we focused on the use of zebrafish (Danio rerio), a vertebrate widely used in biomedical research, as a model for detecting the effects of chemical mixtures with a focus on behaviour. Our aim was to summarize the current status of the ecotoxicological research in this sector. Specifically, we limited our research to the period between January 2012 and September 2023, including only those works aimed at detecting neurotoxicity through behavioural endpoints, utilizing zebrafish at one or more developmental stages, from egg to adult. Additionally, we gathered the findings for every group of chemicals involved and summarised data from all the works we included. At the end of the screening process 101 papers were considered eligible for inclusion. Results show a growing interest in zebrafish at all life stages for this kind of research in the last decade. Also, a wide variety of different assays, involving different senses, was used in the works we surveyed, with exposures ranging from acute to chronic. In conclusion, the results of this study show the versatility of zebrafish as a model for the detection of mixture toxicity although, for what concerns behavioural analysis, the lack of standardisation of methods and endpoints might still be limiting.

Ex vivo exposure to polybrominated diphenyl ether (PBDE) selectively affects the immune response in autistic children

Children on the autism spectrum have been shown to have immune dysregulation that often correlates with behavioral deficits. The role of the post-natal environment in this dysregulation is an area of active investigation. We examined the association between plasma levels of polybrominated diphenyl ether (PBDE) and immune cell function in age-matched autistic children and non-autistic controls. Plasma from children on the autism spectrum (n = 38) and typically developing controls (TD; n = 60) were analyzed for 14 major PBDE congeners. Cytokine/chemokine production was measured in peripheral blood mononuclear cell (PBMC) supernatants with and without ex vivo BDE-49 exposure. Total plasma concentration (∑PBDE14) and individual congener levels were also correlated with T cell function. ∑PBDE14 did not differ between diagnostic groups but correlated with reduced immune function in children on the autism spectrum. In autistic children, IL-2 and IFN-γ production was reduced in association with several individual BDE congeners, especially BDE-49 (p = 0.001). Furthermore, when PBMCs were exposed ex vivo to BDE-49, cells from autistic children produced elevated levels of IL-6, TNF-α, IL-1β, MIP-1α and MCP-1 (p < 0.05). Therefore, despite similar plasma levels of PBDE, these data suggest that PBMC function was differentially impacted in the context of several PBDE congeners in autistic children relative to TD children where increased body burden of PBDE significantly correlated with a suppressed immune response in autistic children but not TD controls. Further, acute ex vivo exposure of PBMCs to BDE-49 stimulates an elevated cytokine response in AU cases versus a depressed response in TD controls. These data suggest that exposure to the toxicant BDE-49 differentially impacts immune cell function in autistic children relative to TD children providing evidence for an underlying association between susceptibility to PBDE exposure and immune anomalies in children on the autism spectrum.

Skeletal effects following developmental flame-retardant exposure are specific to sex and chemical class in the adult Wistar rat

Introduction: Accumulating evidence reveals that endocrine disrupting chemicals (EDCs) can disrupt aspects of metabolic programming, suggesting that skeletal development may be at risk, a possibility that is rarely examined. The commercial flame retardant (FR) mixture, Firemaster 550 (FM 550), has repeatedly been shown to negatively influence metabolic programming, raising concerns that skeletal integrity may consequently be impaired. We have previously shown that gestational and lactational exposure to 1,000 µg FM 550 negatively affected sex-specific skeletal traits in male, but not female, rats assessed at 6 months of age. Whether this outcome is primarily driven by the brominated (BFR) or organophosphate ester (OPFR) portions of the mixture or the effects persist to older ages is unknown. Materials and methods: To address this, in the present study, dams were orally exposed throughout gestation and lactation to either 1,000 μg BFR, 1,000 µg OPFR, or 2,000 µg FM 550. Offspring (n = 8/sex/exposure) were weaned at PND 21 and assessed for femoral cortical and trabecular bone parameters at 8 months of age by high-resolution X-ray micro-computed tomography (micro-CT). Serum levels of serotonin, osteocalcin, alkaline phosphatase, and calcium were quantified. Results: FM 550 affected both sexes, but the females were more appreciably impacted by the OPFRs, while the males were more vulnerable to the BFRs. Conclusion: Although sex specificity was expected due to the sexual dimorphic nature of skeletal physiology, the mechanisms accounting for the male- and female-specific phenotypes remain to be determined. Future work aims to clarify these unresolved issues.

Dinitroaniline herbicide pendimethalin affects development and induces biochemical and histological alterations in zebrafish early-life stages

Pendimethalin (PND) is a dinitroaniline preemergent herbicide widely used to control grasses and weeds. The present study aimed to evaluate the PND potential effects on the development of zebrafish early-life stages. The research focuses first on acute toxicity, followed by the integration of toxicity results through histopathology, oxidative status, and neurotoxicity evaluation of sublethal and environmentally relevant concentrations. Zebrafish larvae exposed to PND showed mortality and developed sublethal alterations including impaired fin development, lordosis, scoliosis, blood congestion, impaired blood flow, and reduced heartbeat. PND exposure (0.5 mg/L) affects musculoskeletal development leading to delayed and reduced ossification of the vertebral centra in the developing vertebral column and disruption of muscle morphology. Herbicide exposure (0.5 mg/L and 0.05 mg/L) led also to biochemical changes of antioxidant enzymes, increasing the activity of CAT, GR, and GPx, while no effects were observed on the activity of SOD and GST in zebrafish larvae. Lastly, AChE activity, a biochemical marker of neurotoxicity, was also increased in zebrafish larvae exposed to 0.5 mg/L of PND. These results confirm the developmental toxicity of PND in zebrafish early-life stages, pointing out the potential role of oxidative stress in the onset of sublethal alterations.

Understanding exposures and latent disease risk within the National Institute of Environmental Health Sciences Superfund Research Program

Understanding the health effects of exposures when there is a lag between exposure and the onset of disease is an important and challenging topic in environmental health research. The National Institute of Environmental Health Sciences (NIEHS) Superfund Basic Research and Training Program (SRP) is a National Institutes of Health (NIH) grant program that uses a multidisciplinary approach to support biomedical and environmental science and engineering research. Because of the multidisciplinary nature of the program, SRP grantees are well-positioned to study exposure and latent disease risk across humans, animal models, and various life stages. SRP-funded scientists are working to address the challenge of connecting exposures that occur early in life and prior to conception with diseases that manifest much later, including developing new tools and approaches to predict how chemicals may affect long-term health. Here, we highlight research from the SRP focused on understanding the health effects of exposures with a lag between exposure and the onset of the disease as well as provide future directions for addressing knowledge gaps for this highly complex and challenging topic. Advancing the knowledge of latency to disease will require a multidisciplinary approach to research, the need for data sharing and integration, and new tools and computation approaches to make better predications about the timing of disease onset. A better understanding of exposures that may contribute to later-life diseases is essential to supporting the implementation of prevention and intervention strategies to reduce or modulate exposures to reduce disease burden.

Emerging environmental pollutants hydroxylated polybrominated diphenyl ethers: From analytical methods to toxicology research

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are of particular concern due to their ubiquitous distribution and adverse health effects. Significant progress has been made in the characterization of OH-PBDEs by using mass spectrometry (MS). In this review, we summarize applications of MS-based techniques in detection, environmental and biota distribution, and potential health risk effects, hoping to unfold an overall picture on account of current knowledge of OH-PBDEs. The analytical methodologies are discussed from sample pretreatment to MS analysis. The methods including gas chromatography-MS (GC-MS), liquid chromatography-MS (LC-MS), and ion mobility spectrometry-MS (IMS-MS) are discussed. GC-MS is the most frequently adopted method in the analysis of OH-PBDEs due to its excellent chromatographic resolution, high sensitivity, and strong ability for unknown identification. LC-MS has been widely used for its high sensitivity and capability of direct analysis. As a newly developed technique, IMS-MS provides high specificity, which greatly facilitates the identification of isomers. OH-PBDEs pervasively existed in both abiotic and biotic samples, including humans, animals, and environmental matrices. Multiple adverse health effects have been reported, such as thyroid hormone disruption, estrogen effects, and neurotoxicity. The reported potential pathological mechanisms are also reviewed. Additionally, MS-based metabolomics, lipidomics, and proteomics have been shown as promising tools to unveil the molecular mechanisms of the toxicity of OH-PBDEs. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Behavioral change and transcriptomics reveal the effects of 2, 2', 4, 4'-tetrabromodiphenyl ether exposure on neurodevelopmental toxicity to zebrafish (Danio rerio) in early life stage

Polybrominated diphenyl ethers (PBDEs) are a class of widely used flame retardants, and their residue in the environment may threaten the ecosystem and human health. The neurodevelopmental toxic effects of PBDEs have been verified in previous studies, but the mechanisms are still unclear. Behavioral analysis and transcriptomics were performed in this study to assess the neurodevelopmental toxic effects of PBDEs on zebrafish embryos and larvae, and the potential mechanisms. The embryos were collected after fertilization and exposed to control (0.05% DMSO), 10, 50, 100 (ug/L) 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) for 7 days. The locomotion parameters of larvae were recorded and analyzed by a behavioral analysis system (EthoVision XT, Noldus). Enrichment of functions and signaling pathways of differentially expressed genes (DEGs) were analyzed by GO and DAVID database. The comparison with the control group showed adverse developments such as low hatching rate, high mortality rate, alterative heart rate, and abnormal spontaneous tail coiling frequency of embryos (24hpf). For the zebrafish larvae, behavioral analyses results suggested decreased activities and movements of the treatment in the light-dark period at 120, 144 and 168hpf, especially the 50 and 100μg/L groups. The affected functions included steroid hormone regulation, neuro regulation, circadian regulation, cardioblast differentiation, immune-related regulation. The enrichment of KEGG pathways were Hedgehog signaling (Shh), Toll-like receptor signaling, FoxO signaling, and Steroid biosynthesis pathway. Hedgehog signaling pathway was further verified via RT-qPCR for its major role in the development of neurogenesis. The mRNA levels of Shh pathway indicated the inhibition of Shh signal in our study since shha, patched1, gli1 and gli2 genes were significantly down-regulated. In summary, PBDEs might influence the neurodevelopment of zebrafish in the early life stage by multiple toxic signaling pathways alteration.

A comparison of developmental toxicity of brominated and halogen-free flame retardant on zebrafish

Brominated diphenyl ethers (BDEs) are halogenated flame retardants. Several concerns related to persistence and toxicity of BDEs have been resulted in a growing need of BDEs replacement. The use of halogen-free flame retardants (HFFR) has increased as a safer alternative, but little information is available on their toxic potential for environmental health and for developing organisms. Therefore, the aim of this study was to evaluate and compare the toxicity of three congeners of BDEs (BDE-47, BDE-99 and BDE-154) with an HFFR (aluminum diethylphosphinate, ALPI) on zebrafish (Danio rerio) by assessing endpoints of lethality, sub-lethality and teratogenicity at the earlier stages of development. The highest tested concentration of BDE-47 (12.1 mg/L) induced pericardium and yolk sac edemas that first appeared at 48 h post-fertilization (hpf) and then were mostly reabsorbed until 144 hpf. BDE-47 also showed a slight but non-significant tendency to affect swim bladder inflation. The rate of edemas increased in a concentration-dependent manner after exposure to BDE-99, but there were no significant differences. In addition, the congener BDE-99 also presented a slight and non-significant effect on swim bladder inflation, but only at the highest concentration tested. Regarding BDE-154 exposure, the rate of edemas and swim bladder inflation were not affected. Finally, in all ALPI exposure concentrations (0.003 up to 30 mg/L), no sub-lethal or teratogenic effects were observed on developing organisms until 96 hpf. Although further studies are needed, our results demonstrate that when comparing the developmental toxicity induced by flame retardants in zebrafish, the HFFR ALPI may be considered a more suitable alternative to BDE-47.

Lactational exposure of polybrominated diphenyl ethers and its association with infant developmental measurements

This study was designed to assess the effect of lactational polybrominated diphenyl ether (PBDE) exposure on early physical development of both healthy and fetal growth restriction (FGR) infants. Z scores of head circumference-for-age (ZHC), length-for-age (ZLEN), weight-for-age (ZWEI) and weight-for-length (ZWFL) were calculated according to the WHO Child Growth Standards. FGR infants had a higher PBDE exposure level and faster growth speed from 42 days to 6 months compared with healthy infants. Exposure of lower brominated BDEs (BDE28∼154) and the sum of 18 BDE congeners (BDE28∼209) negatively associated with ZHC [mean difference estimate (95 % CI): -0.71 (-1.22, -0.22) and -0.81 (-1.31, -0.33)] in FGR boys. Both BDE153 and BDE196 exposure had a significant correlation with ZLEN of boys in FGR group [mean difference estimate (95 % CI): -0.28 (-0.48, -0.07) and -0.52 (-0.91, -0.14)]. Each 10 ng/g lipid increase in the concentrations of BDE154 were respectively associated with 0.16 increase in ZWEI among boys in healthy group. No significant association was found in girls. Lactational PBDE exposure had effects on the early growth of both FGR and healthy infants. The encouragement of breastfeeding should therefore be considered in conjunction with PBDE contamination levels.

Selenomethionine exposure affects chondrogenic differentiation and bone formation in Japanese medaka (Oryzias latipes)

Excess selenium entering the aquatic environment from anthropogenic activities has been associated with developmental abnormalities in fish including skeletal deformities of the head and spine. However, mechanisms of this developmental toxicity have not been well-characterized. In this study, Japanese medaka (Oryzias latipes) embryos were exposed to seleno-l-methionine (Se-Met) in a range of concentrations. Gene expression was evaluated for sex-determining region Y (SRY)-related box (Sox9a and Sox9b), runt-related transcription factor 2 (Runx2), and melatonin receptor (Mtr). Alterations in the length of Meckel's cartilage, tail curvature, and decreased calcification were observed in skeletal stains at 10- and 22-days post-fertilization (dpf). Embryonic exposure of Osterix-mCherry transgenic medaka resulted in fewer teeth. Sox9a and Sox9b were up-regulated, while Runx2 and Mtr were down-regulated by Se-Met prior to hatch. Whole mount in situ hybridization (WISH) localized gene expression to areas observed to be affected in vivo. In addition, Se-Met exposures of a Mtr morpholino (Mtr-MO) as well as Luzindole exposed embryos developed similar skeletal malformations, supporting involvement of Mtr. These findings demonstrate that Se-Met modulates expression of key genes involved in chondrogenic differentiation and bone formation during development.

Effects of Organophosphate Ester Flame Retardants on Endochondral Ossification in Ex Vivo Murine Limb Bud Cultures

Phasing out the usage of polybrominated diphenyl ether (PBDE) flame retardants (FRs) in consumer products led to their widespread replacement with organophosphate ester (OPE) FRs, despite scarce safety data. PBDE exposures were associated with the suppression of endochondral ossification but little is known about the effects of OPEs on bones. Here, we used a novel ex vivo murine limb bud culture system to compare the effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) with those of several OPEs. Gestation day 13 embryos were collected from transgenic CD1 mice expressing fluorescent markers for the major stages of endochondral ossification: COL2A1-ECFP (chondrogenesis), COL10A1-mCherry (early osteogenesis), and COL1A1-YFP (late osteogenesis). Limbs were excised and cultured for 6 days in the presence of vehicle, BDE-47, or an OPE FR: triphenyl phosphate (TPHP), tert-butylphenyl diphenyl phosphate (BPDP), tris(methylphenyl) phosphate (TMPP), or isopropylated triphenyl phosphate (IPPP). BDE-47 (50 μM) decreased the extent of chondrogenesis in the digits and COL1A1-YFP expression in the radius and ulna relative to control. In comparison, concentrations of ≥1 μM of all 4 OPEs limited chondrogenesis; osteogenesis (both COL10A1-mCherry and COL1A1-YFP fluorescence) was markedly inhibited at concentrations ≥3 μM. The expression of Sox9, the master regulator of chondrogenesis, was altered by BDE-47, TPHP, and BPDP. BDE-47 exposure had minimal impact on the expression of Runx2 and Sp7, which drive osteogenesis, whereas TPHP and BPDP both suppressed the expression of these transcription factors. These data suggest that OPE FRs may be more detrimental to bone formation than their brominated predecessors.

Toxicological, gene expression and histopathological evaluations of environmentally realistic concentrations of polybrominated diphenyl ethers PBDE- 47, PBDE-99 and PBDE-209 on zebrafish embryos

Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants. Biomonitoring studies have shown widespread presence of PBDEs in humans and their accumulation in food chain cause concern to human health, especially for foetus and infant development. The early-life stages are generally considered more sensitive to exposure to toxic compounds than juvenile or adults. For this reason the aim of this study was to evaluate the effects of the three most environmentally relevant BDE (BDE- 47, 99 and 209) on zebrafish embryos. The fish embryo toxicity (FET) OECD tests on zebrafish were performed followed by histopathogical examination to assess morphological changes. The gene expression of the thyroid stimulating hormone β (Tshβ), the transport proteins transthyretin (Ttr) and thyroxine-binding globulin (Tbg) as well as the enzyme iodothyronine deiodinase 1 (Dio1) was also assessed by Real-time PCR. BDE-47 and BDE-99 showed an increase of the severity of the effects at the lower concentrations while for the BDE-209 the effects were higher to the high concentrations. Although all compounds did not show any acute toxicity for none of the concentrations tested, they reported interesting sub-acute lesions, including yolk and pericardial edema, tail and head malformation, reduced and extremely reduced heart beat rate, blood stasis and spinal curvature, with the highest percentage recorded for BDE-209. Cardiac edema, damage of eye structure and hydrocephaly were confirmed also by histophatological examination. Furthermore, a toxic and dose-dependent liver vacuolization in BDE-209 was observed in all experimental groups. Although no statistically significant difference in gene expression was observed, BDE-209 up-regulated only Dio1 while the other congeners induced Tshβ, Ttr, Tbg and Dio1. Overall, this research highlighted that exposure to BDE-47, BDE-99 and BDE-209 at realistic concentrations caused lethal and sub-lethal alterations and impaired genes involved in thyroid hormones homeostasis leading to abnormal development of zebrafish embryos.

Complex Interplay Among Nuclear Receptor Ligands, Cytosine Methylation, and the Metabolome in Driving Tris(1,3-dichloro-2-propyl)phosphate-Induced Epiboly Defects in Zebrafish

Tris(1,3-dichloro-2-propyl)phosphate (TDCIPP) is a high-production-volume organophosphate flame retardant (OPFR) that induces epiboly defects during zebrafish embryogenesis, leading to the disruption of dorsoventral patterning. Therefore, the objectives of this study were to (1) identify the potential mechanisms involved in TDCIPP-induced epiboly defects and (2) determine whether coexposure to triphenyl phosphate (TPHP)-an OPFR commonly detected with TDCIPP-enhances or mitigates epiboly defects. Although TDCIPP-induced epiboly defects were not associated with adverse impacts on cytoskeletal protein abundance in situ, the coexposure of embryos to TPHP partially blocked TDCIPP-induced epiboly defects. As nuclear receptors are targets for both TPHP and TDCIPP, we exposed the embryos to TDCIPP in the presence or absence of 69 nuclear receptor ligands and, similar to TPHP, found that ciglitazone (a peroxisome proliferator-activated receptor γ agonist) and 17β-estradiol (E2; an estrogen receptor α agonist) nearly abolished TDCIPP-induced epiboly defects. Moreover, E2 and ciglitazone mitigated TDCIPP-induced effects on CpG hypomethylation within the target loci prior to epiboly, and ciglitazone altered TDCIPP-induced effects on the abundance of two polar metabolites (acetylcarnitine and cytidine-5-diphosphocholine) during epiboly. Overall, our results point to a complex interplay among nuclear receptor ligands, cytosine methylation, and the metabolome in both the induction and mitigation of epiboly defects induced by TDCIPP.

Developmental exposure to BDE-99 hinders cerebrovascular growth and disturbs vascular barrier formation in zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) are distributed throughout the environment. Despite a moratorium on their use, concentrations of PBDEs in the atmosphere and in residential environments remain high due to their persistence. The environmental health risks remain concerning and one of the major adverse effects is neurodevelopmental toxicity. However, the early response and effects of PBDEs exposure on the developing brain remain unknown. In the present study, we investigated the impacts of 2,2',4,4',5-pentabrominated diphenyl ether (BDE-99) on vascular growth and vascular barrier function with an emphasis on cerebral blood vessels, in the early life stages, using a zebrafish model. No general toxicity was observed in exposing zebrafish larvae to 0-0.5 μM BDE-99 at 72 hpf. BDE-99 exposure resulted in neither general toxicity nor pronounced developmental impairment in somatic blood vessels, including intersegmental vessels (ISV) and common cardinal veins (CCV). Meanwhile, both 0.05 μM and 0.5 μM of BDE-99 reduced cerebrovascular density as well as down-regulation of VEGFA and VEGFR2 in the head. In addition, BDE-99 exposure increased vascular leakage, both in cerebral and truncal vasculature at 72 hpf. The accentuated vascular permeability was observed in the head. The mRNA levels of genes encoding tight junction molecules decreased in the BDE-99-exposed larvae, and more robust reductions in Cldn5, Zo1 and Jam were detected in the head than in the trunk. Moreover, proinflammatory factors including TNF-α, IL-1β and ICAM-1 were induced, and the expression of neurodevelopment-related genes was suppressed in the head following BDE-99 exposure. Taken together, these results reveal that developmental exposure to BDE-99 impedes cerebrovascular growth and disturbs vascular barrier formation. The cerebral vasculature in developing zebrafish, a more sensitive target for BDE-99, may be a promising tool for the assessment of the early neurodevelopmental effects due to PBDEs exposure.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

Effect of polybrominated diphenyl ethers on sand-bentonite liner material

Polybrominated diphenyl ethers (PBDEs) are present in biosolids and other solid wastes, as well as being detected in landfill leachates. As sand-bentonite mixtures are extensively used as basal liner materials for landfills, a sand-bentonite mixture was investigated by swelling tests and leaching column tests to determine whether it can effectively contain and/or immobilize PBDEs in landfill leachate. Leaching column tests were conducted with permeants consisting of biosolids' leachates diluted to 50% by volume and spiked with 50 μg/mL of a pentaBDE mixture solution. The results showed that the sand-bentonite retained up to 45-66% of the total PBDEs in the permeant; however, the concentration of PBDEs in the effluent increased continuously and reached a significant level during a 3-week period. PBDEs probably sorbed onto both fine and ultra-fine organic particles. During leaching, a compacted sand-bentonite admix could stop fine particles from passing, but at the same time, ultra-fine organic particles carried PBDEs through the barrier materials. The hydraulic conductivity, k, of the sand-bentonite was negatively affected by shrinkage of the clay interlayer caused by the permeant hydrophobicity. However, the hydraulic conductivity changed only to a limited extent, remaining at a magnitude of 10^-9 cm/s, probably because the PBDE concentrations were low. Therefore, caution is needed when sand-bentonite is applied to landfill liners as a barrier for PBDEs.

Dietary exposure to a binary mixture of polybrominated diphenyl ethers alters innate immunity and disease susceptibility in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Polybrominated diphenyl ethers (PBDEs) have been used as flame retardants in consumer products and are now found in the aquatic environment. The presence of PBDEs puts the health and survival of aquatic species at risk due to the various toxic effects associated with exposure to these compounds. The effects of a binary dietary mixture of PBDEs on innate immunity and disease susceptibility of juvenile Chinook salmon (Oncorhynchus tshawytscha) were examined in the present study. Salmon were fed roughly 1:1 mixtures of two environmentally predominant PBDE congeners, BDE-47 and BDE-99. The six resulting whole body total PBDE concentrations ranged from less than the limit of quantification to 184 ng/g, wet weight (ww). The innate immune system was assessed by using two in vitro macrophage function assays. Specifically, assays that examined the ability of head kidney macrophages to: (1) engulf sheep red blood cells (SRBCs); and (2) produce a respiratory burst, as determined by the production of a reactive oxygen species, superoxide anion. Macrophages from salmon fed the BDE-47/99 mixture diets engulfed more SRBCs and produced greater superoxide anion than salmon fed the control diet. An increase in macrophage function was observed in fish with whole body total PBDE concentrations ranging from 2.81 ng/g, ww to 184 ng/g, ww. The mechanism for this increase in macrophage function due to PBDE exposure is currently unknown, but may be due to the ability of PBDEs to act as an endocrine receptor agonist and/or antagonist. Salmon exposed to the BDE-47/99 mixture diets were also challenged with the pathogenic bacteria, Vibrio (Listonella) anguillarum to determine disease susceptibility. Kaplan-Meier survival curves of fish exposed to the BDE-47/99 mixture and control diets were significantly different. The Cox proportional hazard risk ratios of disease-induced mortality in juvenile Chinook salmon with whole body concentrations of total PBDEs of 10.9, 36.8, and 184 ng/g, ww were significantly greater than the fish fed the control diet by 1.56, 1.83 and 1.50 times, respectively. Not all concentrations of the binary mixture diets had significant hazard ratios relative to the control diet, due to a non-monotonic concentration response curve. The mixture of PBDE congeners resulted in interactive effects that were generally non-additive and dependent upon the congener concentrations and metric examined. Consequently, predicting the interactive effects in juvenile Chinook salmon exposed to mixtures of PBDE congeners on innate immunity and disease susceptibility cannot be readily determined from the adverse effects of individual PBDE congeners.

Bioconcentration and effects of hexabromocyclododecane exposure in crucian carp (Carassius auratus)

As a cycloaliphatic brominated flame retardant, hexabromocyclododecane (HBCD) has been widely used in building thermal insulation and fireproof materials. However, there is little information on the bioconcentration as well as effects with respect to HBCD exposure in the aquatic environment. To investigate the bioconcentration of HBCD in tissues (muscle and liver) and its biochemical and behavioural effects, juvenile crucian carp (Carassius auratus) were exposed to different concentrations of technical HBCD (nominal concentrations, 2, 20, 200 μg/L) for 7 days, using a flow-through exposure system. HBCD was found to concentrate in the liver and muscle with a terminal concentration of 0.60 ± 0.22 μg/g lw (lipid weight) and 0.18 ± 0.02 μg/g lw, respectively, at an environmentally-relevant concentration (2 μg/L). The total thyroxine and total triiodothyronine in the fish plasma were lowered as a result of exposure to the HBCD. Acetylcholinesterase activity in the brain was increased, while swimming activity was inhibited and shoaling inclination was enhanced after exposure to 200 μg/L HBCD. Feeding rate was suppressed in the 20 and 200 μg/L treatment groups. In summary, HBCD concentrations 10-100× higher than the current environmentally-relevant exposures induced adverse effects in the fish species tested in this study. These results suggest that increasing environmental concentrations and/or species with higher sensitivity than carp might be adversely affected by HBCD.

Assessing the ecotoxicity of potentially neurotoxic substances - Evaluation of a behavioural parameter in the embryogenesis of Danio rerio

The pollution of the aquatic environment is currently characterised by a large number of contaminants, especially by mixtures of micro-pollutants including neurotoxins. The ecotoxicological consequences of this burden are not yet assessable. Within the present study, a new test method was applied which evaluates behavioural changes in zebrafish (Danio rerio) embryos to quantify the neurotoxic effect of selected chemicals. Changes in the frequency of spontaneous tail movements - a parameter of locomotion - occurring during embryogenesis was assessed as parameter for neurotoxic effects. Embryos were exposed in 24-microwell plates to neurotoxic compounds. Behaviour was examined after 24 h of exposure by videotaping and quantifying spontaneous locomotion. Additionally, acute toxicity was determined after 48 h of exposure by utilising the fish embryo toxicity test. Abamectin, emamectin benzoate, chlorpyrifos-oxon and carbamazepine were analysed using both: the new and classic acute test methods. The results showed the neurotoxic effect of the substances. Furthermore, an increased sensitivity compared to acute toxicity data was shown. The aim of the present study, to illustrate the sensitivity and specificity of the established endpoint to reliably assess adverse, neurotoxic effects of compounds on the model organism Danio rerio and to apply a new test method was achieved. Therefore, the present study is a substantial contribution to an effect-based risk assessment of contaminants in aquatic ecosystems.

Molecular Mechanisms of Developmental Toxicity Induced by Graphene Oxide at Predicted Environmental Concentrations

Developmental toxicity is a critical issue in nanotoxicity. However, very little is known about the effects of graphene oxide (GO, a widely used carbon material) at predicted environmental concentrations on biological development or the specific molecular mechanisms. The present study established that the development of zebrafish embryos exposed to trace concentrations (1-100 μg/L) of GO was impaired because of DNA modification, protein carbonylation and excessive generation of reactive oxygen species (ROS), especially the superoxide radical. Noticeably, there was a nonmonotonic response of zebrafish developmental toxicity to GO at μg/L to mg/L levels. Transcriptomics analysis revealed that disturbing collagen- and matrix metalloproteinase (MMP)-related genes affected the skeletal and cardiac development of zebrafish. Moreover, metabolomics analysis showed that the inhibition of amino acid metabolism and the ratios of unsaturated fatty acids (UFAs) to saturated fatty acids (SFAs) contributed to the above developmental toxicity. The present work verifies the developmental toxicity of GO at trace concentrations and illustrates for the first time the specific molecular mechanisms thereof. Because of the potential developmental toxicity of GO at trace concentrations, government administrators and nanomaterial producers should consider its potential risks prior to the widespread environmental exposure to GO.