Distribution of BDE-99 and effects on metamorphosis of BDE-99 and -47 after oral exposure in Xenopus tropicalis

Tissue-Specific Distribution and Maternal Transfer of Persistent Organic Halogenated Pollutants in Frogs

Persistent organic pollutants pose a great threat to amphibian populations, but information on the bioaccumulation of contaminants in amphibians remains scarce. To examine the tissue distribution and maternal transfer of organic halogenated pollutants (OHPs) in frogs, seven types of tissues from black-spotted frog (muscle, liver, kidney, stomach, intestine, heart, and egg) were collected from an e-waste-polluted area in South China. Among the seven frog tissues, median total OHP concentrations of 2.3 to 9.7 μg/g lipid weight were found (in 31 polychlorinated biphenyl [PCB] individuals and 15 polybrominated diphenyl ether [PBDE], dechlorane plus [syn-DP and anti-DP], bexabromobenzene [HBB], polybrominated biphenyl] PBB153 and -209], and decabromodiphenyl ethane [DBDPE] individuals). Sex-specific differences in contaminant concentration and compound compositions were observed among the frog tissues, and eggs had a significantly higher contaminant burden on the whole body of female frogs. In addition, a significant sex difference in the concentration ratios of other tissues to the liver was observed in most tissues except for muscle. These results suggest that egg production may involve the mobilization of other maternal tissues besides muscle, which resulted in the sex-specific distribution. Different parental tissues had similar maternal transfer mechanisms; factors other than lipophilicity (e.g., molecular size and proteinophilic characteristics) could influence the maternal transfer of OHPs in frogs. Environ Toxicol Chem 2024;43:1557-1568. © 2024 SETAC.

Biomagnification of persistent organic pollutants (POPs) in detritivorous, phytophagous, and predatory invertebrates: How POPs enter terrestrial food web?

Invertebrates are primary contributors to fluxes of nutrients, energy, and contaminants in terrestrial food webs, but the trophodynamic of contaminants in invertebrate food chains is not fully understood. In this study, occurrence and biomagnification of persistent organic pollutants (POPs) were assessed in detritivorous, phytophagous, and predatory invertebrate food chains. Detritivorous species (earthworm and dung beetle) have higher concentrations of POPs than other species. Different composition patterns and biomagnification factors (BMFs) of POPs were observed for invertebrate species. Negative correlations were found between BMFs and log KOW of POPs for detritivorous and most phytophagous species. In contrast, parabolic relationships between BMFs and log KOW were observed in snails and predatory species, possibly attributed to the efficient digestion and absorption of diet and POPs for them. Bioenergetic characteristics are indicative of the biomagnification potential of POPs in terrestrial wildlife, as suggested by the significant and positive correlation between basal metabolic rates (BMRs) and BMFs of BDE 153 for invertebrates, amphibians, reptiles, birds, and mammals. The estimations of dietary exposure suggest that the terrestrial predators, especially feeding on the underground invertebrates, could be exposed to high level POPs from invertebrates.

Inefficacy of dietary test substance administration in Amphibian Metamorphosis Assay (AMA) studies

Traditionally, the Amphibian Metamorphosis Assay (AMA; OECD TG 231) is performed by exposing Xenopus laevis tadpoles to test substances dissolved in laboratory water. Recently, the use of dietary administration has been proposed to combat poorly soluble test substances in ecotoxicologically-based regulatory endocrine disruption (ED) studies, specifically the AMA warranting an investigation into the efficacy of dietary administration. An efficacy study comprised of two phases: 1) evaluation of the physical influence of the loading process via solvent and 10, 1, and 0.1 mg/l test substance or surrogate (sunflower oil, SFO) on the Sera® Micron Nature (SMN) diet, and 2) performance of a modified AMA in which Nieuwkoop and Faber (NF) stage 51 X. laevis larvae were exposed to dechlorinated tap water using one concentration of the SFO in the diet for 21 days, was performed. In phase 1, the addition of acetone or acetone with bis(2-propylheptyl) phthalate (DPHP) or SFO to SMN with subsequent solvent purge altered the diet reducing the density of the liquified diet and dietary pellet size following centrifugation indicative of alteration of the physical properties of the diet. Treatments used in the modified AMA were acetone alone and 0.1 mg/l SFO dissolved in acetone. These treatments were evaluated against an SMN benchmark using standard AMA endpoints. Both the acetone-treated SMN and 0.1 mg/l SFO-treated diets significantly reduced survival rates, 67 and 70% relative to the SMN benchmark (100%), decreased developmental stage distribution and snout-vent length-normalized hind limb length relative to the SMN benchmark, and slightly increased the prevalence and severity of thyroid follicular cell hypertrophy. Although the acetone-treated diets may have impacted the hypothalamo-pituitary-thyroid axis, clinical signs of gastrointestinal impaction and tail flexure were also observed in the acetone-treated diets, but not the SMN diet alone. Ultimately, test substance exposure via the diet in an AMA study can produce results that may confound data interpretation, which suggests that the traditional aqueous exposure route is generally more appropriate.

Occurrence and Distribution of Persistent Organic Pollutants (POPs) in Amphibian Species: Implications from Biomagnification Factors Based on Quantitative Fatty Acid Signature Analysis

Contaminants pose a great threat to amphibian populations, but the bioaccumulation and distribution of contaminants in amphibians are still unclear. Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) had median concentrations of 468-3560 ng/g lipid weight (lw) and 206-2720 ng/g lw in the muscle of amphibians, respectively. BDE 209 was the predominant PBDE congener, while CBs 118, 138, 153, and 180 were the main PCB congeners. The diet compositions of amphibians were estimated by quantitative fatty acid signature analysis (QFASA). Dragonfly contributed the most to the diet of amphibians. Biomagnification factors (BMFs) based on quantitative amphibian/insect relationships showed more credible results than BMFs based on amphibian/each insect or amphibian/combined prey relationships. BMFs derived from QFASA declined with log K_OW from 5 to 6.5 and then showed a parabolic relationship with log K_OW greater than 6.5. BMFs of PCBs were significantly influenced by the elimination capacity of PCBs in amphibians. Less-hydrophobic PCBs preferentially accumulated in the skin than in muscle, which was probably due to the dermal exposure of less-hydrophobic PCBs for amphibians. The biomagnification and distribution of contaminants may be affected by multiple exposure pathways and the toxicokinetics of contaminants in various life stages of amphibians.

The Effects of Dietary Polybrominated Diphenyl Ether Exposure and Rearing Temperature on Tadpole Growth, Development, and Their Underlying Processes

Depression of growth rate due to polybrominated diphenyl ethers (PBDEs) has been documented in birds, mammals, amphibians, and fish at single temperatures. However, the underlying energetic mechanism for this effect and how it might change in relation to changing environmental temperature remain unstudied. We used a simple energy budget to address hypotheses regarding effects of PBDEs on tadpole (Lithobates pipiens) growth: that reductions in growth are linked to increased respiratory costs, reductions in digestive performance, differences in body composition, reductions in food intake, or a combination of these factors. From 18 days postfertilization (dpf) until 42 dpf, tadpoles were exposed dietarily to a pentabromodiphenyl ether mixture (DE-71^TM ) at a concentration of 100 ng DE-71/g wet mass under a rearing temperature of either 22 or 27 °C. After 20 days of PBDE exposure, total PBDEs in tadpoles averaged 148.4 ng/g wet mass, with no differences by rearing temperature and approximately 50% higher than in their diet; controls not fed PBDE had levels <1 ng/g. Exposure to PBDE resulted in reductions in body length, mass, and development compared to controls, independent of rearing temperature; PBDE had no effect on measures of body composition, dry matter digestibility, or oxygen consumption. A simple energy budget using data from the present study revealed that a 10% decrease in feeding rate could explain the lower mass gain of tadpoles exposed to PBDE. Growth depression by PBDE could be due to (1) direct inhibition of growth processes by PBDE that indirectly decreases total energy demand and food intake, and (2) direct inhibition of food intake. Future studies to disentangle these possible pathways of PBDE effects are warranted. Environ Toxicol Chem 2021;40:3181-3192. © 2021 SETAC.

Effects of Decabrominated Diphenyl Ether Exposure on Growth, Meat Characteristics and Blood Profiles in Broilers

Decabrominated diphenyl ether (BDE-209) is widely used as a flame retardant and is detected at high levels in the environment. Its toxicities have been reported and have attracted attention. In the present study, broilers were used to determine the response in growth performance, carcass traits, meat quality, blood profiles and antioxidant system to BDE-209 exposure at doses of 0, 0.02, 0.4 and 4 mg/kg. The results showed that BDE-209 exposure at levels of 0.02 or 0.4 mg/kg increased feed intake and decreased feed efficiency. BDE-209 altered the blood profiles, such as reducing the numbers of white blood cells, lymphocytes and neutrophilic granulocytes. As compared with the control, BDE-209 exposure significantly increased abdominal fat percentages of broilers at 64.9-159.5% and adversely affected the selected biochemical indicators, including alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine aminotransferase (ALT), creatine (CRE), which indicated its toxicity to liver and kidney functions. Moreover, BDE-209 exposure significantly increased plasma malondialdehyde (MDA) concentrations and decreased the activities of glutathione peroxidase (GSH-px) and superoxide dismutase (SOD), which implied aggravating oxidant stress and decline of antioxidant capacity in broilers. In conclusion, our data demonstrated that the environmental pollutant BDE-209 adversely influenced growth performance, increased the deposition of abdominal fat, impaired antioxidant capacity and the immune system and had potential toxicity to the liver and kidney of broilers.

Amiodarone bioconcentration and suppression of metamorphosis in Xenopus

Trace concentrations of a number of pharmaceutically active compounds have been detected in the aquatic environment in many countries, where they are thought to have the potential to exert adverse effects on non-target organisms. Amiodarone (AMD) is one such high-risk compound commonly used in general hospitals. AMD is known to alter normal thyroid hormone (TH) function, although little information is available regarding the specific mechanism by which this disruption occurs. Anuran tadpole metamorphosis is a TH-controlled developmental process and has proven to be useful as a screening tool for environmental pollutants suspected of disrupting TH functions. In the present study, our objective was to clarify the effects of AMD on Xenopus metamorphosis as well as to assess the bioconcentration of this pharmaceutical in the liver. We found that AMD suppressed spontaneous metamorphosis, including tail regression and hindlimb elongation in pro-metamorphic stage tadpoles, which is controlled by endogenous circulating TH, indicating that AMD is a TH antagonist. In transgenic X. laevis tadpoles carrying plasmid DNA containing TH-responsive element (TRE) and a 5'-upstream promoter region of the TH receptor (TR) βA1 gene linked to a green fluorescent protein (EGFP) gene, triiodothyronine (T₃) exposure induced a strong EGFP expression in the hind limbs, whereas the addition of AMD to T₃ suppressed EGFP expression, suggesting that this drug interferes with the binding of T₃ to TR, leading to the inhibition of TR-mediated gene expression. We also found AMD to be highly bioconcentrated in the liver of pro-metamorphic X. tropicalis tadpoles, and we monitored hepatic accumulation of this drug using mass spectrometry imaging (MSI). Our findings suggest that AMD imposes potential risk to aquatic wildlife by disrupting TH homeostasis, with further possibility of accumulating in organisms higher up in the food chain.

Polybrominated Diphenylether (DE-71) Exposure Skews Phenotypic Sex Ratio, and Alters Steroid Hormone Levels and Steroidogenic Enzyme Activities in Juvenile Silurana tropicalis

The impact of the brominated flame-retardant mixture, DE-71, on gonadal steroidogenesis during sexual differentiation in Silurana tropicalis was examined. A partial lifecycle study exposing S. tropicalis to varying concentrations of DE-71 (0.0, 0.65, 1.3, 2.5, and 5.0 μg/L [nominal]) was conducted from early gastrula-stage embryo to 150 d post-metamorphosis (dpm). Exposure of S. tropicalis to DE-71 induced liver necrosis and induced abnormal ovary development characterized by previtellogenic oocyte necrosis and arrested development of vitellogenic oocytes in females in a concentration-dependent manner. Decreased mean plasma DHT and T, gonad T, and increased mean plasma E2 levels were found in 150 dpm DE-71-treated male S. tropicalis compared to controls. Plasma E2 levels in females were not significantly altered compared to control S. tropicalis, although lower plasma and gonad T were detected. Mean gonadal CYP 19 aromatase activity in both male and female S. tropicalis exposed to DE-71 was not appreciably affected. Decreased mean male 5α-reductase and CYP17 activities in both male and females were observed compared to control frogs. Overall, these studies suggested that PBDE exposure induced liver necrosis and abnormal ovary development; and reduced circulating and gonadal androgens resulting in a phenotypic skew in sex ratio toward the female sex in S. tropicalis.

Effect-based environmental monitoring for thyroid disruption in Swedish amphibian tadpoles

It is well-known that the metamorphosis process in amphibians is dependent on thyroid hormones. Laboratory studies have shown that several environmental contaminants can affect the function of thyroid hormones leading to alterations in the amphibian metamorphosis. The basic idea of the present study was to elucidate if the amphibian metamorphosis might be a useful tool as biomarker for effect-based environmental monitoring, examining wild tadpoles for potential thyroid hormone disruption. A laboratory test was performed to identify the responses from exposure to 6-propylthiouracil (PTU), which has a well-known mechanism on the thyroid system, on Swedish tadpoles from the Rana genus. This was followed by an environmental monitoring study where tadpoles of Rana arvalis, R. temporaria, and Bufo bufo were sampled from various sites in Sweden. Morphological data such as body weight, histopathological measurements of the thyroid glands, and environmental parameters were recorded. The results revealed that Rana tadpoles respond similar as other amphibians to PTU exposure, with interrupted development and increased size relative to the developmental stage. Data on some wild tadpoles showed similar features as the PTU exposed, such as high body weight, thus suggesting potential thyroid disrupting effects. However, histological evaluation of thyroid glands and pesticide analyses of the water revealed no clear evidence of chemical interactions. To a minor degree, the changes in body weight may be explained by natural circumstances such as pH, forest cover, and temperature. The present study cannot fully explain whether the high body weights recorded in some tadpoles have natural or chemical explanations. However, the study reveals that it is clearly achievable to catch tadpoles in suitable stages for the use in this type of biomonitoring and that the use of these biomarkers for assessment of thyroid disruption seems to be highly relevant.

Contaminant and Environmental Influences on Thyroid Hormone Action in Amphibian Metamorphosis

Aquatic and terrestrial environments are increasingly contaminated by anthropogenic sources that include pharmaceuticals, personal care products, and industrial and agricultural chemicals (i. e., pesticides). Many of these substances have the potential to disrupt endocrine function, yet their effect on thyroid hormone (TH) action has garnered relatively little attention. Anuran postembryonic metamorphosis is strictly dependent on TH and perturbation of this process can serve as a sensitive barometer for the detection and mechanistic elucidation of TH disrupting activities of chemical contaminants and their complex mixtures. The ecological threats posed by these contaminants are further exacerbated by changing environmental conditions such as temperature, photoperiod, pond drying, food restriction, and ultraviolet radiation. We review the current knowledge of several chemical and environmental factors that disrupt TH-dependent metamorphosis in amphibian tadpoles as assessed by morphological, thyroid histology, behavioral, and molecular endpoints. Although the molecular mechanisms for TH disruption have yet to be determined for many chemical and environmental factors, several affect TH synthesis, transport or metabolism with subsequent downstream effects. As molecular dysfunction typically precedes phenotypic or histological pathologies, sensitive assays that detect changes in transcript, protein, or metabolite abundance are indispensable for the timely detection of TH disruption. The emergence and application of 'omics techniques-genomics, transcriptomics, proteomics, metabolomics, and epigenomics-on metamorphosing tadpoles are powerful emerging assets for the rapid, proxy assessment of toxicant or environmental damage for all vertebrates including humans. Moreover, these highly informative 'omics techniques will complement morphological, behavioral, and histological assessments, thereby providing a comprehensive understanding of how TH-dependent signal disruption is propagated by environmental contaminants and factors.

Thyroid disruption properties of three indoor dust chemicals tested in Silurana tropicalis tadpoles

Indoor dust contains a multitude of industrial chemicals, and ingestion of dust is considered an important exposure route to organic contaminants. Some of these contaminants have been shown to interfere with the thyroid system, which may result in significant consequences on public health. The amphibian metamorphosis is a thyroid hormone-dependent process, which can be used as an in vivo model for studies on thyroid hormone-disrupting potency. Three contaminants of indoor dust were tested on metamorphosing Silurana (Xenopus) tropicalis tadpoles. The tested chemicals were Tris (1,3-dichloroisopropyl) phosphate (TDCiPP), tetrabromobisphenol-A (TBBPA) and propylparaben (PrP). Measurements reflecting general growth, development progress and thyroid epithelial cell height were performed on the exposed tadpoles as well as chemical analyses of the exposure water. It was shown that TDCiPP acts as a thyroid hormone-disrupting chemical in metamorphosing tadpoles by causing increased epithelial cell height in thyroid glands after exposure to a nominal concentration of 0.010 mg/L and in higher concentrations. TBBPA caused reductions in general growth of tadpoles at the nominal concentration 0.125 mg/L, and PrP caused acute toxicity at the nominal concentration 12.5 mg/L. However, no evident indications of specific thyroid-disrupting effects caused by TBBPA or PrP were observed.

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

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

Warmer temperature modifies effects of polybrominated diphenyl ethers on hormone profiles in leopard frog tadpoles (Lithobates pipiens)

Amphibian populations have been declining, and climate change and exposure to environmental contaminants are thought to be involved. Higher water temperature accelerates larval development; however, its combined effects with contaminants and their influence on hormones during metamorphosis are poorly understood. The authors investigated changes in whole-body triiodothyronine (T3) and corticosterone concentrations in developing leopard frogs reared at 23 °C and 28 °C on diets with 0 ng g^-1 , 6 ng g^-1 , and 37 ng g^-1 of a technical mixture of polybrominated diphenyl ethers (PBDE; DE-71) from 10 d to 44 d (premetamorphosis to late climax; Gosner Stages 28 to 46). Unlike controls, PBDE-exposed tadpoles (6 ng g^-1 ) reared at 23 °C failed to show any increase in T3 concentrations throughout metamorphosis, and exposed tadpoles reared at 28 °C showed a lower peak at climax compared to controls. Corticosterone levels progressively increased throughout metamorphosis, but the levels were higher in PBDE-exposed tadpoles compared to controls at both temperatures. At the warmer temperature, corticosterone increase occurred earlier (at early climax) in controls and exposed tadpoles compared to tadpoles reared at the cooler temperature (late climax), coinciding with the faster development observed at 28 °C. Tadpoles reared at 28 °C were longer and developed faster than tadpoles reared at 23 °C. At both temperatures, PBDE exposure decreased T3 and increased corticosterone concentrations, which can potentially impair developing tadpoles. Environ Toxicol Chem 2017;36:120-127. © 2016 SETAC.

Dietary exposure to polybrominated diphenyl ether 47 (BDE-47) inhibits development and alters thyroid hormone-related gene expression in the brain of Xenopus laevis tadpoles

Few studies have investigated the thyroid-disrupting effects of polybrominated diphenyl ethers (PBDEs) across multiple levels of biological organization in anurans, despite their suitability for the screening of thyroid disruptors. Therefore, the present study evaluated the effects of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) on development, thyroid histology and thyroid hormone-related gene expression in Xenopus laevis exposed to 0 (control), 50 (low), 500 (medium) or 5000μg BDE-47/g food (high) for 21days. Only the high dose of BDE-47 hindered growth and development; however, thyroid hormone-associated gene expression was downregulated in the brains of tadpoles regardless of dose. These results show that BDE-47 disrupts thyroid hormone signaling at the molecular and whole-organism levels and suggest that gene expression in the brain is a more sensitive endpoint than metamorphosis. Furthermore, the altered gene expression patterns among BDE-47-exposed tadpoles provide insight into the mechanisms of PBDE-induced thyroid disruption and highlight the potential for PBDEs to act as neurodevelopmental toxicants.

BDE-47 induces oxidative stress, activates MAPK signaling pathway, and elevates de novo lipogenesis in the copepod Paracyclopina nana

Brominated flame retardant, 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47), has received grave concerns as a persistent organic pollutant, which is toxic to marine organisms, and a suspected link to endocrine abnormalities. Despite the wide distribution in the marine ecosystem, very little is known about the toxic impairments on marine organisms, particularly on invertebrates. Thus, we examined the adverse effects of BDE-47 on life history trait (development), oxidative markers, fatty acid composition, and lipid accumulation in response to BDE-47-induced stress in the marine copepod Paracyclopina nana. Also, activation level of mitogen-activated protein kinase (MAPK) signaling pathways along with the gene expression profile of de novo lipogenesis (DNL) pathways were addressed. As a result, BDE-47 induced oxidative stress (e.g. reactive oxygen species, ROS) mediated activation of extracellular signal-regulated kinase (ERK) and c-Jun-N-terminal kinase (JNK) signaling cascades in MAPK pathways. Activated MAPK pathways, in turn, induced signal molecules that bind to the transcription factors (TFs) responsible for lipogenesis to EcR, SREBP, ChREBP promoters. Also, the stress stimulated the conversion of saturated fatty acids (SFAs) to polyunsaturated fatty acids (PUFAs), a preparedness of the organism to adapt the observed stress, which could be correlated with the elongase and desaturase gene (e.g. ELO3, Δ5-DES, Δ9-DES) expressions, and then extended to the delayed early post-embryonic development and increased accumulation of lipid droplets in P. nana. This study will provide a better understanding of how BDE-47 effects on marine invertebrates particularly on the copepods, an important link in the marine food chain.

Metabolomic approach for identifying and visualizing molecular tissue markers in tadpoles of Xenopus tropicalis by mass spectrometry imaging

In developmental and cell biology it is crucial to evaluate the dynamic profiles of metabolites. An emerging frog model system using Xenopus tropicalis, whose genome sequence and inbred strains are available, is now ready for metabolomics investigation in amphibians. In this study we applied matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry imaging (MSI) analysis to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We detected tissue-specific peaks and visualized their distribution in tissues, and distinguished 19 tissues and their specific peaks. We identified, for the first time, some of their molecular localizations via tandem mass spectrometric analysis: hydrocortisone in artery, L-DOPA in rhombencephalon, taurine in eye, corticosterone in gill, heme in heart, inosine monophosphate and carnosine in muscle, dopamine in nerves, and phosphatidylethanolamine (16:0/20:4) in pharynx. This is the first MALDI-MSI study of X. tropicalis tadpoles, as in small tadpoles it is hard to distinguish and dissect the various organs. Furthermore, until now there has been no data about the metabolomic profile of each organ. Our results suggest that MALDI-MSI is potentially a powerful tool for examining the dynamics of metabolomics in metamorphosis as well as conformational changes due to metabolic changes.

Summary: We applied matrix-assisted laser desorption/ionization−mass spectrometry imaging analyses to identify and visualize metabolomic molecular markers in tadpoles of Xenopus tropicalis. We found new molecular markers in various tissues and cells.

Physiological and biochemical responses and microscopic structure changes of Populus tomentosa Carr seedlings to 4-BDE exposure

Populus species are very effective in remediation of contaminants. Polybrominated diphenyl ethers (PBDEs) are commonly used as flame retardants and are known to be persistent environmental pollutants. Numerous studies have shown that PBDEs are rising in human tissues and biota. 4-Monobrominated diphenyl ether (4-BDE), one of the less brominated PBDEs, was served as a model compound for biodegradation of lower brominated congeners. The present study was designed to clarify the effects of 4-BDE stress on morphological, physiological, and biochemical impacts of Populus tomentosa Carr in a tissue culture condition. Different concentrations of 4-BDE (3 and 30 mg L(-1)) were supplied alone or together with 0.5 mg L(-1) IBA in tissue culture media. With the concentration increased, 4-BDE caused negative effects on the microscopic structure of roots, stem, and leaves. The leaf color became shallow in low concentration of 4-BDE treatments and appeared albinism with 4-BDE concentration increased. The chlorophyll content and the leaf mass per area of albino leaves reduced significantly. 4-BDE also caused positive effects on the adventitious root differentiation and the biomass below 30 mg L(-1). With the 4-BDE treatment time increased (23, 47, and 58 days), the peroxidase (POD) activity displayed the decreasing trend. The proline content decreased first and then increased. Exposure to 4-BDE induced the malondialdehyde (MDA) to increase in leaves. Application of 4-BDE affected the endogenous hormone levels of cuttings in their adventitious roots inducing media. Below 0.3 mg L(-1), 4-BDE caused the faint expression of auxin-sensitive DR5::GUS reporter gene in Arabidopsis thaliana. Additionally, P. tomentosa Carr exhibited the better tolerance against 4-BDE in the range of less than 30 mg L(-1).

In situ effects of pesticides on amphibians in the Sierra Nevada

For more than 20 years, conservationists have agreed that amphibian populations around the world are declining. Results obtained through laboratory or mesocosm studies and measurement of contaminant concentrations in areas experiencing declines have supported a role of contaminants in these declines. The current study examines the effects of contaminant exposure to amphibians in situ in areas actually experiencing declines. Early larval Pseudacris regilla were translocated among Lassen Volcanic, Yosemite and Sequoia National Parks, California, USA and caged in wetlands in 2001 and 2002 until metamorphosis. Twenty contaminants were identified in tadpoles with an average of 1.3-5.9 (maximum = 10) contaminants per animal. Sequoia National Park, which had the greatest variety and concentrations of contaminants in 2001, also had tadpoles that experienced the greatest mortality, slowest developmental rates and lowest cholinesterase activities. Yosemite and Sequoia tadpoles and metamorphs had greater genotoxicity than those in Lassen during 2001, as determined by flow cytometry. In 2001 tadpoles at Yosemite had a significantly higher rate of malformations, characterized as hemimelia (shortened femurs), than those at the other two parks but no significant differences were observed in 2002. Fewer differences in contaminant types and concentrations existed among parks during 2002 compared to 2001. In 2002 Sequoia tadpoles had higher mortality and slower developmental rates but there was no difference among parks in cholinesterase activities. Although concentrations of most contaminants were below known lethal concentrations, simultaneous exposure to multiple chemicals and other stressors may have resulted in lethal and sublethal effects.

Immunomodulation in post-metamorphic northern leopard frogs, Lithobates pipiens, following larval exposure to polybrominated diphenyl ether

Pollutants and disease are factors implicated in amphibian population declines, and it is hypothesized that these factors exert a synergistic adverse effect, which is mediated by pollutant-induced immunosuppression. Polybrominated diphenyl ethers (PBDEs) are ubiquitous pollutants that can exert immunotoxicity, making them of interest to test effects on amphibian immune function. We orally exposed Lithobates (Rana) pipiens tadpoles to environmentally realistic levels (0-634 ng/g wet diet) of a pentabromodiphenyl ether mixture (DE-71) from as soon as they became free-swimming through metamorphic climax. To assess adaptive immune response in juvenile frogs, we used an enzyme-linked immunosorbent assay to measure specific IgY production following immunization with keyhole limpet hemocyanin (KLH). Specific KLH antibody response was significantly decreased in juvenile frogs that had been exposed to PBDEs as tadpoles. When assessing innate immune responses, we found significantly different neutrophil counts among treatments; however, phagocytic activity of neutrophils was not significantly different. Secretion of antimicrobial skin peptides (AMPs) nonsignificantly decreased with increasing PBDE concentrations, and no significant effect of PBDE treatment was observed on efficacy of AMPs to inhibit chytrid fungus (Batrachochytrium dendrobatidis) growth. Our findings demonstrate that environmentally realistic concentrations of PBDEs are able to alter immune function in frogs; however, further research is needed to determine how these alterations impact disease susceptibility in L. pipiens.

Toxicokinetics of polybrominated diphenyl ethers across life stages in the northern leopard frog (Lithobates pipiens)

Polybrominated diphenyl ethers (PBDEs), a class of flame retardants, are bioaccumulative toxins that can biomagnify in food webs. However, little is known about the toxicokinetics of total and congener-specific BDEs in lower vertebrates. The authors exposed northern leopard frog (Lithobates (Rana) pipiens) tadpoles to diets containing DE-71 (a pentabromodiphenyl ether mixture (0 ng/g as control, 71.4 ng/g, and 634 DE-71 ng/g wet mass)) for 50 d, followed by a period of depuration during which they were fed only undosed (control) food. After 28 d, tadpoles eliminated over 94% of the ΣPBDEs from their tissues (t½  = 5.9 ± 1.9 d) with no significant differences in elimination rates for the predominant congeners. Elimination of BDE-99 was independent of dose, indicating first-order kinetics. It did not fit a biexponential model significantly better than a monoexponential model, indicating single-compartment elimination. To compare developmental life-stage kinetics following larval exposure, the authors collected individuals at the beginning and end of metamorphosis and at 70 d postmetamorphosis. During metamorphosis, total-body residues per individual did not significantly change, implying little to no elimination. After 70 d, juvenile frogs eliminated 89.7% of the ΣPBDEs from their tissues, and BDE-47 was eliminated at a faster rate (t½ = 17.3 d) than BDE-99 and BDE-100 (t½  = 63.0 d and 69.3 d, respectively). Because the kinetics of PBDEs in L. pipiens differed among life stages, developmental life stage-especially for species that undergo metamorphosis-should be considered when determining the toxicity of persistent organic pollutants.

Thyroid Hormone-disrupting Effects and the Amphibian Metamorphosis Assay

There are continued concerns about endocrine-disrupting chemical effects, and appropriate vertebrate models for assessment of risk are a high priority. Frog tadpoles are very sensitive to environmental substances because of their habitat and the complex processes of metamorphosis regulated by the endocrine system, mainly thyroid hormones. During metamorphosis, marked alteration in hormonal factors occurs, as well as dramatic structural and functional changes in larval tissues. There are a variety of mechanisms determining thyroid hormone balance or disruption directly or indirectly. Direct-acting agents can cause changes in thyroxine synthesis and/or secretion in thyroid through effects on peroxidases, thyroidal iodide uptake, deiodinase, and proteolysis. At the same time, indirect action may result from biochemical processes such as sulfation, deiodination and glucuronidation. Because their potential to disrupt thyroid hormones has been identified as an important consideration for the regulation of chemicals, the OECD and the EPA have each established guidelines that make use of larval African clawed frogs (Xenopus laevis) and frog metamorphosis for screening and testing of potential endocrine disrupters. The guidelines are based on evaluation of alteration in the hypothalamic-pituitary-thyroid axis. One of the primary endpoints is thyroid gland histopathology. Others are mortality, developmental stage, hind limb length, snout-vent length and wet body weight. Regarding histopathological features, the guidelines include core criteria and additional qualitative parameters along with grading. Taking into account the difficulties in evaluating amphibian thyroid glands, which change continuously throughout metamorphosis, histopathological examination has been shown to be a very sensitive approach.

The hypothalamus-pituitary-thyroid axis in teleosts and amphibians: endocrine disruption and its consequences to natural populations

Teleosts and pond-breeding amphibians may be exposed to a wide variety of anthropogenic, waterborne contaminants that affect the hypothalamus-pituitary-thyroid (HPT) axis. Because thyroid hormone is required for their normal development and reproduction, the potential impact of HPT-disrupting contaminants on natural teleost and amphibian populations raises special concern. There is laboratory evidence indicating that persistent organic pollutants, heavy metals, pharmaceutical and personal care products, agricultural chemicals, and aerospace products may alter HPT activity, development, and reproduction in teleosts and amphibians. However, at present there is no evidence to clearly link contaminant-induced HPT alterations to impairments in teleost or amphibian population health in the field. Also, with the exception of perchlorate for which laboratory studies have shown a direct link between HPT disruption and adverse impacts on development and reproductive physiology, little is known about if or how other HPT-disrupting contaminants affect organismal performance. Future field studies should focus on establishing temporal associations between the presence of HPT-disrupting chemicals, the occurrence of HPT alterations, and adverse effects on development and reproduction in natural populations; as well as determining how complex mixtures of HPT contaminants affect organismal and population health.

Application of mass spectrometry in the analysis of polybrominated diphenyl ethers

This review summarized the applications of mass spectrometric techniques for the analysis of the important flame retardants polybrominated diphenyl ethers (PBDEs) to understand the environmental sources, fate and toxicity of PBDEs that were briefly discussed to give a general idea for the need of analytical methodologies. Specific performance of various mass spectrometers hyphenated with, for example, gas chromatograph, liquid chromatograph, and inductively coupled plasma (GC/MS, LC/MS, and ICP/MS, respectively) for the analysis of PBDEs was compared with an objective to present the information on the evolution of MS techniques for determining PBDEs in environmental and human samples. GC/electron capture negative ionization quadrupole MS (GC/NCI qMS), GC/high resolution MS (GC/HRMS) and GC ion trap MS (GC/ITMS) are most commonly used MS techniques for the determination of PBDEs. New analytical technologies such as fast tandem GC/MS and LC/MS become available to improve analyses of higher PBDEs. The development and application of the tandem MS techniques have helped to understand environmental fate and transformations of PBDEs of which abiotic and biotic degradation of decaBDE is thought to be one major source of Br(1-9)BDEs present in the environment in addition to direct loading from commercial mixtures. MS-based proteomics will offer an insight into the molecular mechanisms of toxicity and potential developmental and neurotoxicity of PBDEs.

Thyroid disruption by technical decabromodiphenyl ether (DE-83R) at low concentrations in Xenopus laevis

Decabromodiphenyl ether (decaBDE), as a flame retardant, is widely produced and used. To study the thyroid disruption by technical decaBDE at low concentrations, Xenopus laevis tadpoles were exposed to technical decaBDE mixture DE-83R (1-1000 ng/L) in water from stage 46/47 (free swimming larvae, system of Nieuwkoop and Faber) to stage 62. DE-83R at concentration of 1000 ng/L significantly delayed the time to metamorphosis (presented by forelimb emergence, FLE). Histological examination showed that DE-83R at all tested concentrations caused histological alterations - multilayer follicular epithelial cell and markedly increased follicle size accompanied by partial colloid depletion and increase in the peripheral colloid vacuolation, in thyroid glands. All tested concentrations of DE-83R also induced a down-regulation of thyroid receptor mRNA expression. These results demonstrated that technical decaBDE disrupted the thyroid system in X. laevis tadpoles. Analysis of polybrominated diphenyl ethers (PBDEs) (sum of 39 congeners) in X. laevis indicated that mean concentrations of total PBDEs in X. laevis exposed to 1, 10, 100, 1000 ng/L were 11.0, 128.1, 412.1, 1400.2 ng/g wet weight, respectively. Considering that PBDEs burden ofX. laevis tadpoles was close to PBDEs levels in amphibians as reported in previous studies, our study has raised new concerns for thyroid disruption in amphibians of technical decaBDE at environmentally relevant concentrations.

Chronic, dietary polybrominated diphenyl ether exposure affects survival, growth, and development of Rana pipiens tadpoles

Levels of polybrominated diphenyl ethers (PBDEs) in the environment have been increasing rapidly over the past two decades; however, the toxicology of these compounds to aquatic organisms is poorly understood. Because amphibians play a role in both aquatic and terrestrial food webs, and are currently undergoing worldwide population declines, it is of interest to determine how PBDEs may affect amphibian health. This is the first study that reports chronic, dietary effects of environmentally relevant levels (7-277 ng/g wet food) of PBDEs in amphibians throughout larval development. Beginning at the free-swimming stage (Gosner Stage [GS] 25), Rana pipiens tadpoles were orally exposed to a technical pentabromodiphenyl ether mixture (DE-71) through metamorphic climax (GS 42). On exposure day 43, a subset of tadpoles was removed for body residue analysis. Sum PBDEs in whole-body tissue correlated linearly to dietary concentrations with BDE-99 represented as the highest contributing congener in both diet and tissue. Survival among all treatments compared to the control was decreased by DE-71 exposure. Further, growth and development were delayed in all but the highest treatment, perhaps indicating greater PBDE tolerance among those individuals that survived the highest treatment. Time to metamorphic climax was delayed, on average, 22 to 36 d in DE-71-treated tadpoles compared to control tadpoles. Additionally, size at metamorphosis was smaller in the highest treatment, suggesting that individuals that survived and metamorphosed similarly to the controls did so with a trade-off in size. At environmentally relevant levels, PBDEs induced mortality as well as sublethal effects on developing tadpoles through dietary exposure.

Residues of polybrominated diphenyl ethers in frogs (Rana limnocharis) from a contaminated site, South China: tissue distribution, biomagnification, and maternal transfer

Environmental pollutants are suspected to be a cause of global declines in amphibian populations, but few data are available on the bioaccumulation of polybrominated diphenyl ethers (PBDEs) in amphibians. To examine the tissue distribution, biomagnification potential, and maternal transfer of PBDEs in frogs, eighteen PBDE congeners were measured in the muscle, liver, and egg tissues of rice frogs (Rana limnocharis) and insects collected from an electronic waste (e-waste) recycling site in South China. PBDE levels in the frogs ranged from 0.63 to 11.6, 4.57 to 56.2, and 10.7 to 125 ng/g wet wt in the muscles, livers, and eggs, respectively. The frogs exhibited a unique congener profile, compared to those in aquatic and terrestrial species, with BDEs 99, 153, 183, 209, and 47 as the dominant congeners, intermediating between aquatic and terrestrial species. Most of the PBDE congeners in general showed higher affinity to liver than to muscle tissue. Except for BDEs 28, 47, 66, 138, and 206, the average biomagnification factors (BMFs) for all PBDE congeners were greater than 1.0, providing clear evidence of their biomagnification from insects to frogs. A parabolic relationship between log BMFs and bromine atom numbers or log Kow of PBDEs was observed, with the maximum BMF values for PBDEs with 6 bromine atoms (or at a log K(ow) of approximately 8.0). Relatively higher levels of 3-MeO-BDE 47 were found in male frogs, suggesting that male frogs in the present study might have higher metabolic capacity for PBDEs compared to female frogs. The ratio of levels in egg/female liver, indicating mother-to-egg transfer capacity, increased with increasing bromine atom numbers up to 7 and then declined as the bromine atom numbers rose. This indicated that the physicochemical properties of the congeners (e.g., K(ow), molecular sizes, and structures), resulting in different affinities to transport proteins, might impact their maternal transfer in frogs.