A rapid, physiologic protocol for testing transcriptional effects of thyroid-disrupting agents in premetamorphic Xenopus tadpoles

Pollution characteristics, ecological and health risks of herbicides in a drinking water source and its inflowing rivers in North China

This study measured the pollution characteristics and ecological and health risks of 19 herbicides found in drinking water sources and their inflowing rivers. The targeted herbicides were prevalent in the study area, but most concentrations were well below 10 ng L-1. Acetochlor and atrazine were the dominant herbicides, although their levels were much lower than previously reported. Total herbicide residual levels were greater in April than in December and increased from upstream to downstream, resulting in the highest pollution levels found in the reservoirs, likely due to herbicides delivered from upstream and dense agricultural planting in the surrounding areas. Only atrazine and ametryn presented moderate ecological risks, while the summed risk quotients (ΣRQs) of each sample were >0.1, indicated that the total herbicide levels represented a moderate risk in all samples. For the human health risks, the risk quotients (RQ) of all target herbicides, the total RQs of each sample, and estimated life-stage RQs were far smaller than the 0.2 threshold, indicating the absence of human health risks when the water was consumed at any stage of life. However, early life stages exhibited 3-6 times higher RQ values than adulthood and should not be overlooked. And crucially, the synergistic or antagonistic effects of mixed herbicides are not well understood, and further research is needed to understand the impact of these herbicides on the ecosystem and human health, particularly possible affects in early life stages, such as infants and children.

Short- and Long-Term Effects of Chlorpyrifos on Thyroid Hormone Axis and Brain Development in Xenopus laevis

INTRODUCTION

The extensive use of the insecticide chlorpyrifos (CPF) throughout the world has brought increased scrutiny on its environmental and health impact. CPF is a cholinergic neurotoxicant; however, exposure to low noncholinergic doses is associated with numerous neurodevelopmental effects in animal models. In this study, we aimed to assess CPF for its potential to disrupt thyroid hormone signalling and investigate the short- and long-term effects on neurodevelopment by using Xenopus laevis.

METHODS

The thyroid hormone (TH) disrupting potential of CPF was assessed using TH-sensitive transgenic Tg(thibz:eGFP) tadpoles. The consequences of early embryonic exposure were examined by exposing fertilized eggs for 72 h to environmentally relevant CPF concentrations (10-10 M and 10-8 M). Three endpoints were evaluated: (1) gene expression in whole embryonic brains immediately after exposure, (2) mobility and brain morphology 1 week after exposure, and (3) brain morphology and axon diameters at the end of metamorphosis (2 months after the exposure).

RESULTS

CPF disrupted TH signalling in Tg(thibz:eGFP) tadpoles. The expression of genes klf9, cntn4, oatp1c1, and tubb2b was downregulated in response to CPF. Tadpoles exposed to CPF exhibited increased mobility and altered brain morphology compared to control tadpoles. Early embryonic exposure of CPF affected myelinated axon diameter, with exposed animals exhibiting shifted frequency distributions of myelinated axons diameters towards smaller diameters in the hindbrain of froglets.

DISCUSSION/CONCLUSION

This study provides more evidence of the endocrine and neurodevelopment disrupting activity of CPF. Further experimental and epidemiological studies are warranted to determine the long-term consequences of early CPF exposure on brain development.

Towards regulation of Endocrine Disrupting chemicals (EDCs) in water resources using bioassays - A guide to developing a testing strategy

Endocrine disrupting chemicals (EDCs) are found in every environmental medium and are chemically diverse. Their presence in water resources can negatively impact the health of both human and wildlife. Currently, there are no mandatory screening mandates or regulations for EDC levels in complex water samples globally. Bioassays, which allow quantifying in vivo or in vitro biological effects of chemicals are used commonly to assess acute toxicity in water. The existing OECD framework to identify single-compound EDCs offers a set of bioassays that are validated for the Estrogen-, Androgen-, and Thyroid hormones, and for Steroidogenesis pathways (EATS). In this review, we discussed bioassays that could be potentially used to screen EDCs in water resources, including in vivo and in vitro bioassays using invertebrates, fish, amphibians, and/or mammalians species. Strengths and weaknesses of samples preparation for complex water samples are discussed. We also review how to calculate the Effect-Based Trigger values, which could serve as thresholds to determine if a given water sample poses a risk based on existing quality standards. This work aims to assist governments and regulatory agencies in developing a testing strategy towards regulation of EDCs in water resources worldwide. The main recommendations include 1) opting for internationally validated cell reporter in vitro bioassays to reduce animal use & cost; 2) testing for cell viability (a critical parameter) when using in vitro bioassays; and 3) evaluating the recovery of the water sample preparation method selected. This review also highlights future research avenues for the EDC screening revolution (e.g., 3D tissue culture, transgenic animals, OMICs, and Adverse Outcome Pathways (AOPs)).

Agrochemicals disrupt multiple endocrine axes in amphibians

Concern over global amphibian declines and possible links to agrochemical use has led to research on the endocrine disrupting actions of agrochemicals, such as fertilizers, fungicides, insecticides, acaricides, herbicides, metals, and mixtures. Amphibians, like other species, have to partition resources for body maintenance, growth, and reproduction. Recent studies suggest that metabolic impairments induced by endocrine disrupting chemicals, and more particularly agrichemicals, may disrupt physiological constraints associated with these limited resources and could cause deleterious effects on growth and reproduction. Metabolic disruption has hardly been considered for amphibian species following agrichemical exposure. As for metamorphosis, the key thyroid hormone-dependent developmental phase for amphibians, it can either be advanced or delayed by agrichemicals with consequences for juvenile and adult health and survival. While numerous agrichemicals affect anuran sexual development, including sex reversal and intersex in several species, little is known about the mechanisms involved in dysregulation of the sex differentiation processes. Adult anurans display stereotypical male mating calls and female phonotaxis responses leading to successful amplexus and spawning. These are hormone-dependent behaviours at the foundation of reproductive success. Therefore, male vocalizations are highly ecologically-relevant and may be a non-invasive low-cost method for the assessment of endocrine disruption at the population level. While it is clear that agrochemicals disrupt multiple endocrine systems in frogs, very little has been uncovered regarding the molecular and cellular mechanisms at the basis of these actions. This is surprising, given the importance of the frog models to our deep understanding of developmental biology and thyroid hormone action to understand human health. Several agrochemicals were found to have multiple endocrine effects at once (e.g., targeting both the thyroid and gonadal axes); therefore, the assessment of agrochemicals that alter cross-talk between hormonal systems must be further addressed. Given the diversity of life-history traits in Anura, Caudata, and the Gymnophiona, it is essential that studies on endocrine disruption expand to include the lesser known taxa. Research under ecologically-relevant conditions will also be paramount. Closer collaboration between molecular and cellular endocrinologists and ecotoxicologists and ecologists is thus recommended.

Testing for thyroid hormone disruptors, a review of non-mammalian in vivo models

Thyroid hormones (THs) play critical roles in profound changes in many vertebrates, notably in mammalian neurodevelopment, although the precise molecular mechanisms of these fundamental biological processes are still being unravelled. Environmental and health concerns prompted the development of chemical safety testing and, in the context of endocrine disruption, identification of thyroid hormone axis disrupting chemicals (THADCs) remains particularly challenging. As various molecules are known to interfere with different levels of TH signalling, screening tests for THADCs may not rely solely on in vitro ligand/receptor binding to TH receptors. Therefore, alternatives to mammalian in vivo assays featuring TH-related endpoints that are more sensitive than circulatory THs and more rapid than thyroid histopathology are needed to fulfil the ambition of higher throughput screening of the myriad of environmental chemicals. After a detailed introduction of the context, we have listed current assays and parameters to assess thyroid disruption following a literature search of recent publications referring to non-mammalian models. Potential THADCs were mostly investigated in zebrafish and the frog Xenopus laevis, an amphibian model extensively used to study TH signalling.

Endocrine disrupting chemicals: exposure, effects on human health, mechanism of action, models for testing and strategies for prevention

Endocrine Disrupting Chemicals (EDCs) are a global problem for environmental and human health. They are defined as "an exogenous chemical, or mixture of chemicals, that can interfere with any aspect of hormone action". It is estimated that there are about 1000 chemicals with endocrine-acting properties. EDCs comprise pesticides, fungicides, industrial chemicals, plasticizers, nonylphenols, metals, pharmaceutical agents and phytoestrogens. Human exposure to EDCs mainly occurs by ingestion and to some extent by inhalation and dermal uptake. Most EDCs are lipophilic and bioaccumulate in the adipose tissue, thus they have a very long half-life in the body. It is difficult to assess the full impact of human exposure to EDCs because adverse effects develop latently and manifest at later ages, and in some people do not present. Timing of exposure is of importance. Developing fetus and neonates are the most vulnerable to endocrine disruption. EDCs may interfere with synthesis, action and metabolism of sex steroid hormones that in turn cause developmental and fertility problems, infertility and hormone-sensitive cancers in women and men. Some EDCs exert obesogenic effects that result in disturbance in energy homeostasis. Interference with hypothalamo-pituitary-thyroid and adrenal axes has also been reported. In this review, potential EDCs, their effects and mechanisms of action, epidemiological studies to analyze their effects on human health, bio-detection and chemical identification methods, difficulties in extrapolating experimental findings and studying endocrine disruptors in humans and recommendations for endocrinologists, individuals and policy makers will be discussed in view of the relevant literature.

Following Endocrine-Disrupting Effects on Gene Expression in Xenopus laevis

Endocrine-disrupting chemicals (EDCs), found in all categories of chemicals, are suspected to be a cause of declining well-being and human health, both as single molecules and as mixtures. It is therefore necessary to develop high throughput methods to assess the endocrine-disrupting potential of multiple chemicals currently on the market that are as yet untested. An advantage of in vivo chemical screening is that it provides a full spectrum of physiological impacts exerted by a given chemical. Xenopus laevis is an ideal model organism to test thyroid axis disruption in vivo as thyroid hormones (THs) are highly conserved across vertebrates and orchestrate tadpole metamorphosis. In particular, NF stage 45 Xenopus laevis are most apt for in vivo screening as at this stage the tadpoles possess all the main elements of thyroid hormone signaling (thyroid receptors, deiodinases transporters) and are metabolically competent, while fitting into multiple well plates, allowing the use of small amounts of test chemicals. One way to assess the endocrine-disrupting potential of chemicals or mixtures thereof is to analyze gene expression in organisms after a short time exposure to the chemical(s). Here we describe a protocol using Xenopus laevis embryos to detect endocrine disruption of the thyroid axis by analysis of gene expression and an alternative protocol for fluorescence read-out using a transgenic GFP-expressing Xenopus laevis line. Taken together, these methods allow detection of subtle changes in TH signaling by EDCs that either activate or inhibit TH signaling in vivo.

Effects of s-metolachlor on early life stages of marbled crayfish

The effects of s-metolachlor chronic exposure at concentrations of 1.1 μg/L (maximal real environmental concentration in the Czech Republic), 11 μg/L (environmental relevant concentration) and 110 μg/L on early life stages of marbled crayfish (Procambarus virginalis) was evaluated under laboratory conditions. All s-metolachlor exposures resulted in higher mortality, delay ontogenetic development with accompanied slower growth and excited behaviour (increase of total distance moved and walking speed). Significantly lower superoxide dismutase, catalase, glutathione S-transferase activity and reduced glutathione level was observed at two higher tested concentrations (11 and 110 μg/L) of s-metolachlor compared with the control. S-metolachlor in concentrations 110 μg/L showed alteration of the tubular system of hepatopancreas including focal disintegration of tubular epithelium and notable reduction in epithelial cells number, especially B-cells. In conclusion, potential risk associated with using of s-metolachlor in agriculture, due to effects on non-target aquatic organisms as documented on early life stages of marbled crayfish in this study, should be taken into account.

Enantioselective effects of chiral amide herbicides napropamide, acetochlor and propisochlor: The more efficient R-enantiomer and its environmental friendly

Amide herbicides, which are used extensively worldwide, are often chiral. Enantiomeric selectivity comes from the different effects of the enantiomers on target and non-target organisms. In this study, the enantiomers of three amide herbicides were purified by the semi-preparative column and were used to investigate the enantioselective effects on target Echinochloa crusgalli (lowland rice weeds), and non-target Microcystis aeruginosa, and the yeast transformed with the human TRβ plasmid organisms. The results showed that (i) the R-enantiomers of the three amide herbicides exhibited the strongest activity toward weed inhibition and the lowest toxicity toward non-target organisms; (ii) napropamide was better suited for controlling root growth, while acetochlor and propisochlor were better for leaves control; (iii) herbicides at certain low concentrations (0.01 mg L^-1 for acetochlor and propisochlor) could be utilized to promote plant growth. These findings encourage the use of R-amide herbicides instead of their racemates to increase the efficiency of weed control and reduce the risk to non-target organisms. On the other hand, the adverse effects are caused mostly by S-enantiomer, using R-enantiomer-enriched products may offer great environmental/ecological benefits.

Expression of the inactivating deiodinase, Deiodinase 3, in the pre-metamorphic tadpole retina

Thyroid hormone (TH) orchestrates amphibian metamorphosis. Thus, this developmental phase is often used to study TH-dependent responses in specific tissues. However, TH signaling appears early in development raising the question of the control of TH availability in specific cell types prior to metamorphosis. TH availability is under strict temporal and tissue-specific control by deiodinases. We examined the expression of the TH-inactivating enzyme, deiodinase type 3 (D3), during early retinal development. To this end we created a Xenopus laevis transgenic line expressing GFP from the Xenopus dio3 promoter region (pdio3) and followed pdio3-GFP expression in pre-metamorphic tadpoles. To validate retinal GFP expression in the transgenic line as a function of dio3 promoter activity, we used in situ hybridization to compare endogenous dio3 expression to reporter-driven GFP activity. Retinal expression of dio3 increased during pre-metamorphosis through stages NF41, 45 and 48. Both sets of results show dio3 to have cell-specific, dynamic expression in the pre-metamorphic retina. At stage NF48, dio3 expression co-localised with markers for photoreceptors, rods, Opsin-S cones and bipolar neurons. In contrast, in post-metamorphic juveniles dio3 expression was reduced and spatially confined to certain photoreceptors and amacrine cells. We compared dio3 expression at stages NF41 and NF48 with TH-dependent transcriptional responses using another transgenic reporter line: THbZIP-GFP and by analyzing the expression of T3-regulated genes in distinct TH availability contexts. At stage NF48, the majority of retinal cells expressing dio3 were negative for T3 signaling. Notably, most ganglion cells were virtually both dio3-free and T3-responsive. The results show that dio3 can reduce TH availability at the cellular scale. Further, a reduction in dio3 expression can trigger fine-tuned T3 action in cell-type specific maturation at the right time, as exemplified here in photoreceptor survival in the pre-metamorphic retina.

In Vivo Transfection of Naked DNA into Xenopus Tadpole Tail Muscle

In vivo gene transfer systems are important to study foreign gene expression and promoter regulation in an organism, with the benefit of exploring this in an integrated environment. Direct injection of plasmids encoding exogenous promoters and genes into muscle has numerous advantages: the protocol is easy, efficient, and shows time-persistent plasmid expression in transfected muscular cells. After injecting naked-DNA plasmids into tadpole tail muscle, transgene expression is strong, reproducible, and correlates with the amount of DNA injected. Moreover, expression is stable as long as the tadpoles remain, or are maintained, in premetamorphic stages. By directly expressing genes and regulated promoters in Xenopus tadpole muscle in vivo, one can exploit the powerful experimental advantages of gene transfer systems in an intact, physiologically normal animal.

Human amniotic fluid contaminants alter thyroid hormone signalling and early brain development in Xenopus embryos

Thyroid hormones are essential for normal brain development in vertebrates. In humans, abnormal maternal thyroid hormone levels during early pregnancy are associated with decreased offspring IQ and modified brain structure. As numerous environmental chemicals disrupt thyroid hormone signalling, we questioned whether exposure to ubiquitous chemicals affects thyroid hormone responses during early neurogenesis. We established a mixture of 15 common chemicals at concentrations reported in human amniotic fluid. An in vivo larval reporter (GFP) assay served to determine integrated thyroid hormone transcriptional responses. Dose-dependent effects of short-term (72 h) exposure to single chemicals and the mixture were found. qPCR on dissected brains showed significant changes in thyroid hormone-related genes including receptors, deiodinases and neural differentiation markers. Further, exposure to mixture also modified neural proliferation as well as neuron and oligodendrocyte size. Finally, exposed tadpoles showed behavioural responses with dose-dependent reductions in mobility. In conclusion, exposure to a mixture of ubiquitous chemicals at concentrations found in human amniotic fluid affect thyroid hormone-dependent transcription, gene expression, brain development and behaviour in early embryogenesis. As thyroid hormone signalling is strongly conserved across vertebrates the results suggest that ubiquitous chemical mixtures could be exerting adverse effects on foetal human brain development.

A screening assay for thyroid hormone signaling disruption based on thyroid hormone-response gene expression analysis in the frog Pelophylax nigromaculatus

Amphibian metamorphosis provides a wonderful model to study the thyroid hormone (TH) signaling disrupting activity of environmental chemicals, with Xenopus laevis as the most commonly used species. This study aimed to establish a rapid and sensitive screening assay based on TH-response gene expression analysis using Pelophylax nigromaculatus, a native frog species distributed widely in East Asia, especially in China. To achieve this, five candidate TH-response genes that were sensitive to T3 induction were chosen as molecular markers, and T3 induction was determined as 0.2 nmol/L T3 exposure for 48 hr. The developed assay can detect the agonistic activity of T3 with a lowest observed effective concentration of 0.001 nmol/L and EC50 at around 0.118-1.229 nmol/L, exhibiting comparable or higher sensitivity than previously reported assays. We further validated the efficiency of the developed assay by detecting the TH signaling disrupting activity of tetrabromobisphenol A (TBBPA), a known TH signaling disruptor. In accordance with previous reports, we found a weak TH agonistic activity for TBBPA in the absence of T3, whereas a TH antagonistic activity was found for TBBPA at higher concentrations in the presence of T3, showing that the P. nigromaculatus assay is effective for detecting TH signaling disrupting activity. Importantly, we observed non-monotonic dose-dependent disrupting activity of TBBPA in the presence of T3, which is difficult to detect with in vitro reporter gene assays. Overall, the developed P. nigromaculatus assay can be used to screen TH signaling disrupting activity of environmental chemicals with high sensitivity.

Towards a humanized PPARγ reporter system for in vivo screening of obesogens

Overeating and lack of exercise are major contributors to the current obesity epidemic, but environmental contaminants, or obesogens, are also considered to be potential actors. A common obesogen target is the Peroxisome Proliferator Activated Receptor Gamma (PPARγ). Screening for exogenous obesogens requires in vivo systems as many xenobiotics exert their effects through metabolites. We thus developed a humanized in vivo PPARγ reporter model, using Xenopus laevis larvae, a species possessing metabolic capacities comparable to mammals. A somatic transgenesis approach was used to co-express an expression vector for the human PPARγ protein simultaneously with one of a series of reporter vectors, each containing a PPARγ Response Element (PPRE)-eGFP sequence. Treatment of tadpoles with PPARγ agonists, antagonists or candidate obesogens, significantly modulated eGFP expression. Thus, the system provides a promising proof of principle for a sensitive and reliable humanized in vivo tool to screen both novel PPARγ drug ligands and potential endocrine disruptors or obesogens targeting this receptor.

In vitro effects of herbicides and insecticides on human breast cells

Numerous studies have indicated that the pesticides and herbicides used in agricultural processes in the United States and Europe may have detrimental effects upon human health. Many of these compounds have been indicated as potential endocrine and reproductive disruptors, although the studies have examined supraphysiological levels well above the US EPA safe levels for drinking water and have often examined these effects in "model" cell lines such as Chinese hamster ovary cells. We have now examined the cytotoxicity of more environmentally relevant concentrations of four herbicides, acetochlor, atrazine, cyanazine, and simazine, and two insecticides, chlorpyrifos and resmethrin, in three human breast cell lines. Interestingly, cytotoxicity was not observed in the estrogen-dependent MCF-7 mammary epithelial carcinoma cells; rather increases in cell viability were seen for some of the compounds at select concentrations. These results vary greatly from what was observed in the estrogen independent MDA-MB-231 breast cancer cells and the non-cancerous MCF-10A breast cells. This gives insight into how different tumors may respond to pesticide exposure and allows us to make more accurate conclusions about the potential cytotoxicity or, at times, stimulatory actions of these pesticides.

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.

C-fin: a cultured frog tadpole tail fin biopsy approach for detection of thyroid hormone-disrupting chemicals

There is a need for the development of a rapid method for identifying chemicals that disrupt thyroid hormone (TH) action while maintaining complex tissue structure and biological variation. Moreover, no assay to date allows a simultaneous screen of an individual's response to multiple chemicals. A cultured tail fin biopsy or C-fin assay was developed using Rana catesbeiana tadpoles. Multiple tail fin biopsies were taken per tadpole, cultured in serum-free medium, and then each biopsy was exposed to a different treatment condition. The effects of known disruptors of TH action were evaluated in the C-fin assay. Chemical exposure was performed +/- 10 nM 3,3',5-triiodothyronine and real-time quantitative polymerase chain reaction (QPCR) of two TH-responsive transcripts, TH receptor beta (TRbeta) and the Rana larval keratin type I (RLKI), was performed. Within 48 h of exposure to Triac (1-100 nM), roscovitine (0.6-60 microM), or genistein (1-100 microM), perturbations in TH signaling were detected. Tetrabromobisphenol A (TBBPA) (10-1,000 nM) showed no effect. Acetochlor (1-100 nM) elicited a modest effect on the TH-dependent induction of TRbeta transcript. These data reveal that a direct tissue effect may not be critical for TBBPA and acetochlor to disrupt TH action previously observed in intact tadpoles.

An innovative continuous flow system for monitoring heavy metal pollution in water using transgenic Xenopus laevis tadpoles

While numerous detection methods exist for environmental heavy metal monitoring, easy-to-use technologies combining rapidity with in vivo measurements are lacking. Multiwell systems exploiting transgenic tadpoles are ideal but require time-consuming placement of individuals in wells. We developed a real-time flow-through system, based on Fountain Flow cytometry, which measures in situ contaminant-induced fluorescence in transgenic amphibian larvae immersed in water samples. The system maintains the advantages of transgenic amphibians, but requires minimal human intervention. Portable and self-contained, it allows on-site measurements. Optimization exploited a transgenic Xenopus laevis bearing a chimeric gene with metal responsive elements fused to eGFP. The transgene was selectively induced by 1 microM Zn(2+). Using this tadpole we show the continuous flow method to be as rapid and sensitive as image analysis. Flow-through readings thus accelerate the overall process of data acquisition and render fluorescent monitoring of tadpoles suitable for on-site tracking of heavy metal pollution.

Transgenesis procedures in Xenopus

Stable integration of foreign DNA into the frog genome has been the purpose of several studies aimed at generating transgenic animals or producing mutations of endogenous genes. Inserting DNA into a host genome can be achieved in a number of ways. In Xenopus, different strategies have been developed which exhibit specific molecular and technical features. Although several of these technologies were also applied in various model organizms, the attributes of each method have rarely been experimentally compared. Investigators are thus confronted with a difficult choice to discriminate which method would be best suited for their applications. To gain better understanding, a transgenesis workshop was organized by the X-omics consortium. Three procedures were assessed side-by-side, and the results obtained are used to illustrate this review. In addition, a number of reagents and tools have been set up for the purpose of gene expression and functional gene analyses. This not only improves the status of Xenopus as a powerful model for developmental studies, but also renders it suitable for sophisticated genetic approaches. Twenty years after the first reported transgenic Xenopus, we review the state of the art of transgenic research, focusing on the new perspectives in performing genetic studies in this species.

An in vivo multiwell-based fluorescent screen for monitoring vertebrate thyroid hormone disruption

There is a pressing need for high throughput methods to assess potential effects of endocrine disrupting chemicals (EDCs). released into the environment. Currently our ability to identify effects in vitro exceeds that for in vivo monitoring. However, only in vivo analysis provides the full spectrum of physiological impacts exerted by a given chemical. With the aim of finding a physiological system compatible with automatic plate reading we tested the capacity of early embryonic stage Xenopus laevis tadpoles to monitor thyroid hormone (TH) disruption. Fluorescent transgenic X. laevis embryos bearing a TH/bZIP-eGFP construct, placed in 96 well plates, were used for a physiological-based screen for potential TH signaling disruptors. Using stage NF-45 embryos (time of thyroid gland formation) allowed rapid detection of chemical interference with both peripheral TR signaling and production of endogenous TH. Nanomolar concentrations of TH receptor agonists could be detected within 72 h. Moreover, when testing against a 5nM T3 challenge, the effects of inhibitors of TH production were revealed, including inhibitors of TH synthesis, (methimazole: 1 mM or sodium perchlorate: 3.56 microM), as well as antagonists acting at the receptor level (NH3: 2 microM) and a deiodinase inhibitor (iopanoic acid: 10 microM). Finally, we show that the thyroid disrupting activities of BPA (10 microM) and TBBPA (1 microM) can also be detected in this rapid screening protocol. Finally, this noninvasive technology using an automatic reading system shows low variability (around 5%) and permits detection of subtle changes in signaling by EDCs that either inhibit or activate TH signaling in vivo.

Identification of gene expression indicators for thyroid axis disruption in a Xenopus laevis metamorphosis screening assay. Part 1. Effects on the brain

Thyroid hormones (TH), thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)), play crucial roles in regulation of growth, development and metabolism in vertebrates and their actions are targets for endocrine disruptive agents. Perturbations in TH action can contribute to the development of disease states and the US Environmental Protection Agency is developing a high throughput screen using TH-dependent amphibian metamorphosis as an assay platform. Currently this methodology relies on external morphological endpoints and changes in central thyroid axis parameters. However, exposure-related changes in gene expression in TH-sensitive tissue types that occur over shorter time frames have the potential to augment this screen. This study aims to characterize and identify molecular markers in the tadpole brain. Using a combination of cDNA array analysis and real time quantitative polymerase chain reaction (QPCR), we examine the brain of tadpoles following 96 h of continuous exposure to T(3), T(4), methimazole, propylthiouracil, or perchlorate. This tissue was more sensitive to T(4) rather than T(3), even when differences in biological activity were taken into account. This implies that a simple conversion of T(4) to T(3) cannot fully account for T(4) effects on the brain and suggests distinctive mechanisms of action for the two THs. While the brain shows gene expression alterations for methimazole and propylthiouracil, the environmental contaminant, perchlorate, had the greatest effect on the levels of mRNAs encoding proteins important in neural development and function. Our data identify gene expression profiles that can serve as exposure indicators of these chemicals.

Microarray analysis identifies keratin loci as sensitive biomarkers for thyroid hormone disruption in the salamander Ambystoma mexicanum

Ambystomatid salamanders offer several advantages for endocrine disruption research, including genomic and bioinformatics resources, an accessible laboratory model (Ambystoma mexicanum), and natural lineages that are broadly distributed among North American habitats. We used microarray analysis to measure the relative abundance of transcripts isolated from A. mexicanum epidermis (skin) after exogenous application of thyroid hormone (TH). Only one gene had a >2-fold change in transcript abundance after 2 days of TH treatment. However, hundreds of genes showed significantly different transcript levels at days 12 and 28 in comparison to day 0. A list of 123 TH-responsive genes was identified using statistical, BLAST, and fold level criteria. Cluster analysis identified two groups of genes with similar transcription patterns: up-regulated versus down-regulated. Most notably, several keratins exhibited dramatic (1000 fold) increases or decreases in transcript abundance. Keratin gene expression changes coincided with morphological remodeling of epithelial tissues. This suggests that keratin loci can be developed as sensitive biomarkers to assay temporal disruptions of larval-to-adult gene expression programs. Our study has identified the first collection of loci that are regulated during TH-induced metamorphosis in a salamander, thus setting the stage for future investigations of TH disruption in the Mexican axolotl and other salamanders of the genus Ambystoma.

Evaluation of the effect of acetochlor on thyroid hormone receptor gene expression in the brain and behavior of Rana catesbeiana tadpoles

The thyroid hormones (THs) including 3,5,3'-triiodothyronine (T3), are important regulators of growth and development of the brain in vertebrates. Previous studies showed that acetochlor, a widely used herbicide, accelerates T3 -induced frog tadpole metamorphosis and elevates the T3 -dependent accumulation of the mRNAs encoding the TH receptors, TRalpha and beta, in the tail. Here we show that acetochlor affects the expression of these TR isoforms in the brain of Rana catesbeiana tadpoles. Premetamorphic tadpoles exposed to 10 nM acetochlor with and without 100 nM T(3) for 4 days showed substantial increases in TRalpha and TRbeta transcript levels and significant decreases in the TRalpha/TRbeta ratios in their brains. This change in TR ratios is recapitulated with 10 nM acetochlor in R. catesbeiana tadpole brains during prometamorphosis, a period in which THs are endogenously produced. Tail fin biopsies revealed an elevation in TRalpha and beta mRNA levels compared to control animals when exposed to 1 and 10 nM acetochlor for 6 days. When subsequently reared in clean water for 59 days, no alterations in metamorphic hallmarks (forelimb emergence, mouth development, tail regression) were noted compared to the controls. Since alterations in TR ratios/levels may impact brain development, we tested the escape behavior in premetamorphic tadpoles exposed to 10 nM acetochlor for 4 days. We did not detect any statistically significant differences that would indicate that acetochlor affects escape behavior. However, since the gene expression data suggest that brain function may be affected, additional studies examining different behaviors upon acetochlor exposure at environmentally-relevant concentrations are warranted.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

In vitro and in vivo analysis of the thyroid system-disrupting activities of brominated phenolic and phenol compounds in Xenopus laevis

We investigated the effects of the brominated phenolic and phenol compounds, some of which are brominated flame retardants, on the binding of (125)I-3,3',5-L-triiodothyronine ((125)I-T(3)) to purified Xenopus laevis transthyretin (xTTR) and to the ligand-binding domain of X. laevis thyroid hormone receptor beta (xTR LBD), on the induction of a T(3)-responsive reporter gene in a recombinant X. laevis cell line (XL58-TRE-Luc) and on T(3)-induced or spontaneous metamorphosis in X. laevis tadpoles. Of the brominated phenolic and phenol compounds tested, 3,3',5-tribromobisphenol A and 3,3'-dibromobisphenol A were the most potent competitors of (125)I-T(3) binding to xTTR and the xTR LBD, respectively. Structures with a bromine in either ortho positions with respect to the hydroxy group competed more efficiently with T(3) binding to xTTR and the xTR LBD. 3,3',5-Tribromobisphenol A and 3,3',5,5'-tetrabromobisphenol A, at 0.1-1.0 microM, exerted both T(3) agonist and antagonist activities in the T(3)-responsive reporter gene assay. Sera obtained from fetal bovine and bullfrog tadpoles weakened the T(3) agonist and antagonist activities of 3,3',5-tribromobisphenol A, but not the T(3) antagonist activity of o-t-butylphenol, for which xTTR has no significant affinity. The T(3) agonist and antagonist activities of 0.5 microM 3,3',5-tribromobisphenol A were confirmed in the in vivo, short-term gene expression assay in premetamorphic X. laevis tadpoles using endogenous, T(3)-responsive genes as molecular markers. Our results suggest that 3,3',5-tribromobisphenol A affects T(3) binding to xTTR and xTR and that it interferes with the intracellular T(3) signaling pathway.