Evaluation of Histological and Molecular Endpoints for Enhanced Detection of Thyroid System Disruption in Xenopus laevis Tadpoles

Cross-taxa extrapolation: Is there a role for thyroid hormone conjugating liver enzymes during amphibian metamorphosis?

Chemical safety assessment includes evaluating the potential to disrupt the endocrine system in humans and wildlife. The thyroid hormone system shows high complexity which is conserved across vertebrates, allowing biological read-across between regulatory important taxa, namely mammals and amphibians. Potential thyroid disruption in aquatic vertebrates is typically investigated by activity assays (Amphibian Metamorphosis Assay (AMA), Xenopus Eleutheroembryo Thyroid Assay). Since neither assay is designed to provide detailed mechanistic information, mode of action analyses often rely on mammalian data, assuming overall cross-vertebrate conservation. This manuscript elaborates on the imperative that, despite overall conservation, the T-modality in metamorphosing amphibians needs to be understood in detail to justify biological read-across between mammals and amphibians. To this end, we revisit the AMA regarding amphibian developmental physiology, and the T-modality regarding mechanistic cross-vertebrate conservation. The importance of a mechanistic understanding for read-across is showcased based on the AMA's apparent insensitivity to at least one category of prototypical liver enzyme inducers. From a regulatory perspective, deeper mechanistic understanding is needed, not only to strengthen the scientific basis for designing testing strategies and interpreting study results, but also to allow the identification of data gaps and thus development of New Approach Methodologies (NAMs) to minimize vertebrate testing.

Methimazole disrupted skeletal ossification and muscle fiber transition in Bufo gargarizans larvae

Methimazole (MMI) is an emerging endocrine disrupting chemical (EDC) due to its increasing use in the treatment of thyrotoxicosis (hyperthyroidism), but its potential impact on amphibian development remains largely unexplored. In the present study, the effects of 8 mg/L MMI and 1 μg/L thyroxine (T4) exposure on skeletal ossification and muscle development in Bufo gargarizans tadpoles were comprehensively investigated by double skeletal staining, histological analysis and RNA sequencing. Our results indicated that MMI treatment down-regulated the expression levels of ossification-related genes (e.g., BMPs, MMPs, and Wnt9a) in cartilage, thereby delaying chondrocyte apoptosis and inhibiting hindlimb ossification. Muscle sarcomere was elongated in both the MMI and T4 treatment groups, which may lead to muscle weakness and consequently affect land motion. Additionally, we evaluated the expression levels of fast muscle-related genes (TNNI2 and TNNT3) and slow muscle-related genes (TNNI1 and TNNT1), revealing an opposite trend in the transition from fast to slow muscle after T4 and MMI exposures. In conclusion, these findings fill the data gap regarding MMI contamination in aquatic environments by revealing the negative effects of MMI on amphibian bone and muscle development. Future studies should address the toxicity of EDCs to wildlife and inform aquatic ecosystem conservation strategies.

Development of a Zebrafish Embryo-Based Test System for Thyroid Hormone System Disruption: 3Rs in Ecotoxicological Research

There is increasing concern regarding pollutants disrupting the vertebrate thyroid hormone (TH) system, which is crucial for development. Thus, identification of TH system-disrupting chemicals (THSDCs) is an important requirement in the Organisation for Economic Co-operation and Development (OECD) testing framework. The current OECD approach uses different model organisms for different endocrine modalities, leading to a high number of animal tests. Alternative models compatible with the 3Rs (replacement, reduction, refinement) principle are required. Zebrafish embryos, not protected by current European Union animal welfare legislation, represent a promising model. Studies show that zebrafish swim bladder inflation and eye development are affected by THSDCs, and the respective adverse outcome pathways (AOPs) have been established. The present study compared effects of four THSDCs with distinct molecular modes of action: Propylthiouracil (PTU), potassium perchlorate, iopanoic acid, and the TH triiodothyronine (T3) were tested with a protocol based on the OECD fish embryo toxicity test (FET). Effects were analyzed according to the AOP concept from molecular over morphological to behavioral levels: Analysis of thyroid- and eye-related gene expression revealed significant effects after PTU and T3 exposure. All substances caused changes in thyroid follicle morphology of a transgenic zebrafish line expressing fluorescence in thyrocytes. Impaired eye development and swimming activity were observed in all treatments, supporting the hypothesis that THSDCs cause adverse population-relevant changes. Findings thus confirm that the FET can be amended by TH system-related endpoints into an integrated protocol comprising molecular, morphological, and behavioral endpoints for environmental risk assessment of potential endocrine disruptors, which is compatible with the 3Rs principle. Environ Toxicol Chem 2024;00:1-18. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

How the Xenopus eleutheroembryonic thyroid assay compares to the amphibian metamorphosis assay for detecting thyroid active chemicals

The Xenopus Eleutheroembryonic Thyroid Assay (XETA) was recently published as an OECD Test Guideline for detecting chemicals acting on the thyroid axis. However, the OECD validation did not cover all mechanisms that can potentially be detected by the XETA. This study was therefore initiated to investigate and consolidate the applicability domain of the XETA regarding the following mechanisms: thyroid hormone receptor (THR) agonism, sodium-iodide symporter (NIS) inhibition, thyroperoxidase (TPO) inhibition, deiodinase (DIO) inhibition, glucocorticoid receptor (GR) agonism, and uridine 5'-diphospho-glucuronosyltransferase (UDPGT) induction. In total, 22 chemicals identified as thyroid-active or -inactive in Amphibian Metamorphosis Assays (AMAs) were tested using the XETA OECD Test Guideline. The comparison showed that both assays are highly concordant in identifying chemicals with mechanisms of action related to THR agonism, DIO inhibition, and GR agonism. They also consistently identified the UDPGT inducers as thyroid inactive. NIS inhibition, investigated using sodium perchlorate, was not detected in the XETA. TPO inhibition requires further mechanistic investigations as the reference chemicals tested resulted in opposing response directions in the XETA and AMA. This study contributes refining the applicability domain of the XETA, thereby helping to clarify the conditions where it can be used as an ethical alternative to the AMA.

Subacute toxicity and endocrine-disrupting effects of Fe2O3, ZnO, and CeO2 nanoparticles on amphibian metamorphosis

This study evaluated the potential toxic and endocrine-disrupting effects of sublethal concentrations of Fe2O3, CeO2 and ZnO nanoparticles (NPs) on amphibian metamorphosis. Tadpoles were exposed to several NPs concentrations, reaching a maximum of 1000 µg/L, for up to 21 days according to the amphibian metamorphosis assay (AMA). Some standard morphological parameters, such as developmental stage (DS), hind limb length (HLL), snout-to-vent length (SVL), wet body weight (WBW), and as well as post-exposure lethality were recorded in exposed organisms on days 7 and 21 of the bioassay. Furthermore, triiodothyronine (T3), thyroxine (T4) and malondialdehyde (MDA) levels and the activities of glutathione S-transferases (GST), glutathione reductase (GR), catalase (CAT), carboxylesterase (CaE), and acetylcholinesterase (AChE) were determined in exposed tadpoles as biomarkers. The results indicate that short-term exposure to Fe2O3 NPs leads to toxic effects, both exposure periods cause toxic effects and growth inhibition for ZnO NPs, while short-term exposure to CeO2 NPs results in toxic effects and long-term exposure causes endocrine-disrupting effects. The responses observed after exposure to the tested NPs during amphibian metamorphosis suggest that they may have ecotoxicological effects and their effects should be monitored through field studies.

Characterization of lesions of nutritional congenital goitre in cattle

There are few studies that classify and characterize the morphometric and immunohistochemical features of goitre in bovine thyroid glands (TGs). We investigated 39 bovine TGs (fetuses [9], stillbirths [18], neonates [12]) born to dams with low T4 hormone levels and no iodine supplementation and 10 (fetuses [3], stillbirths [3], neonates [4]) born to dams with normal T4 levels and supplemented with iodine. Body weight (BW), TG weight (TGW), TGW:BW ratio and histological lesions were determined. The TGs were classified histopathologically as normal gland (G0), mild goitre (G1), moderate goitre (G2) or severe goitre (G3). Various morphological and morphometric parameters were calculated from microscopic images using image analysis software. Immunohistochemistry was performed to detect proliferating cell nuclear antigen (PCNA). There were significant differences in the TGW:BW ratio among groups (P <0.05): 0.3 ± 0.1 in G0, 0.5 ± 0.3 in G1, 0.8 ± 0.3 in G2 and 1.3 ± 0.7 in G3. In G0, large homogeneous follicles with eosinophilic colloid were seen. In the groups with lesions (G1, G2 and G3), heterogeneity in follicle shape and size, height and area of thyroid follicular cells, height of thyroid follicular epithelium and PCNA immunolabelling were directly related to histopathological grade, with significant differences among groups (P <0.001), gradually increasing from G1 to G3 compared with G0. The TGW:BW ratio and histological grade were positively correlated (P = 0.008).

Effects of Chronic Roundup Exposure on Medaka Larvae

The use of glyphosate-based herbicides is increasing yearly to keep up with the growing demands of the agriculture world. Although glyphosate-based herbicides target the enzymatic pathway in plants, the effects on the endocrine systems of vertebrate organisms, mainly fish, are widely unknown. Many studies with glyphosate used high-exposure concentrations (mg/L), and the effect of environmentally relevant or lower concentrations has not been clearly understood. Therefore, the present study examined the effects of very low, environmentally relevant, and high concentrations of glyphosate exposure on embryo development and the thyroid system of Japanese medaka (Oryzias latipes). The Hd-rR medaka embryos were exposed to Roundup containing 0.05, 0.5, 5, 10, and 20 mg/L glyphosate (glyphosate acid equivalent) from the 8 h post-fertilization stage through the 14-day post-fertilization stage. Phenotypes observed include delayed hatching, increased developmental deformities, abnormal growth, and embryo mortality. The lowest concentration of glyphosate (0.05 mg/L) and the highest concentration (20 mg/L) induced similar phenotypes in embryos and fry. A significant decrease in mRNA levels for acetylcholinesterase (ache) and thyroid hormone receptor alpha (thrα) was found in the fry exposed to 0.05 mg/L and 20 mg/L glyphosate. The present results demonstrated that exposure to glyphosate formulation, at a concentration of 0.05 mg/L, can affect the early development of medaka larvae and the thyroid pathway, suggesting a link between thyroid functional changes and developmental alteration; they also showed that glyphosate can be toxic to fish at this concentration.

A biologically based computational model for the hypothalamic-pituitary-thyroid (HPT) axis in Xenopus laevis larvae

A biologically based computational model was developed to describe the hypothalamic-pituitary-thyroid (HPT) axis in developing Xenopus laevis larvae. The goal of this effort was to develop a tool that can be used to better understand mechanisms of thyroid hormone-mediated metamorphosis in X. laevis and predict organismal outcomes when those mechanisms are perturbed by chemical toxicants. In this report, we describe efforts to simulate the normal biology of control organisms. The structure of the model borrows from established models of HPT axis function in mammals. Additional features specific to X. laevis account for the effects of organism growth, growth of the thyroid gland, and developmental changes in regulation of thyroid stimulating hormone (TSH) by circulating thyroid hormones (THs). Calibration was achieved by simulating observed changes in stored and circulating levels of THs during a critical developmental window (Nieuwkoop and Faber stages 54-57) that encompasses widely used in vivo chemical testing protocols. The resulting model predicts that multiple homeostatic processes, operating in concert, can act to preserve circulating levels of THs despite profound impairments in TH synthesis. Represented in the model are several biochemical processes for which there are high-throughput in vitro chemical screening assays. By linking the HPT axis model to a toxicokinetic model of chemical uptake and distribution, it may be possible to use this in vitro effects information to predict chemical effects in X. laevis larvae resulting from defined chemical exposures.

Effects of the herbicide ametryn on development and thyroidogenesis of bullfrog tadpoles (Aquarana catesbeiana) under different temperatures

Thyroid hormones (TH) are essential for the metamorphosis of amphibians and their production can be influenced by environmental stressors, such as temperature fluctuations, and exposure to aquatic pollutants, such as herbicides. In the present study we evaluated the influence of different temperatures (25 and 32 °C) on the effects of the herbicide ametryn (AMT, 0 - control, 10, 50 and 200 ng.L^-1) for 16 days on thyroidogenesis of bullfrog tadpoles. Higher temperature and AMT exposure caused a delay in the development of tadpoles, despite no differences were noted in weight gain and total length of the animals. Levels of triiodothyronine (T3) and thyroxine (T4) were not altered neither by AMT nor by temperature, but the highest temperature caused a decrease in total area and number of follicles in the thyroid gland. Transcript levels of thyroid hormone receptors alpha and beta (TRα and TRβ) and iodothyronine deiodinase 3 (DIO3) were lower at 32 °C, which is consistent with developmental delay at the higher temperature. Tadpoles exposed to 200 ng.L^-1 of AMT at 25 °C also presented delayed development, which was consistent with lower TRα and DIO3 transcript levels. Lower levels of estradiol were noted in tadpoles exposed to AMT at the higher temperature, being also possibly related to a developmental delay. This study demonstrates that higher temperature and AMT exposure impair thyroidgenesis in bullfrog tadpoles, disrupting metamorphosis.

Effects of Separate and Combined Exposure of Cadmium and Lead on the Endochondral Ossification in Bufo gargarizans

Cadmium (Cd) and lead (Pb) are ubiquitous in aquatic environments and most studies have examined the potential effects of Cd or Pb alone on aquatic organisms. In the present study, chronic effects of Cd and Pb, alone and in combination, on Bufo gargarizans were investigated by exposing embryos to these contaminants throughout metamorphosis. Significant reductions in body mass and snout-to-vent length were observed in B. gargarizans at Gosner stage 42 (Gs 42) and Gs 46 exposed to a Cd/Pb mixture. Single and combined exposure with Cd and Pb induced histological alterations of the thyroid gland characterized by reduced colloid area and thickness of epithelial cells. There was a significant decrease in the maximum jump distance of froglets exposed to Cd alone and the Cd/Pb mixture, and the jumping capacity showed a positive correlation with hind limb length and tibia/fibula. Moreover, single metals and their mixture induced reduction of endochondral bone formation in B. gargarizans. Transcriptomic and real-time quantitative polymerase chain reaction results showed that genes involved in skeletal ossification (TRα, TRβ, Dio2, Dio3, MMP9, MMP13, Runx1, Runx2, and Runx3) were transcriptionally dysregulated by Cd and Pb exposure alone or in combination. Our results suggested that despite the low concentration tested, the Cd/Pb mixture induced more severe impacts on B. gargarizans. In addition, the Cd/Pb mixture might reduce chances of survival for B. gargarizans froglets by decreasing size at metamorphosis, impaired skeletal ossification, and reduction in jumping ability, which might result from dysregulation of genes involved in thyroid hormone action and endochondral ossification. The findings obtained could add a new dimension to understanding of the mechanisms underpinning skeletal ossification response to heavy metals in amphibians. Environ Toxicol Chem 2022;41:1228-1245. © 2022 SETAC.

Effects of sodium perchlorate and 6-propylthiouracil on metamorphosis and thyroid gland histopathology in the European common frog (Rana temporaria)

Several chemicals have been identified as thyroid hormone axis disrupting chemicals (THADCs) able to interfere with the thyroid hormone system during fetal life and early life stages, thereby impairing neurodevelopment in mammals and inducing development and growth disorders in fish and amphibians. However, identification of THADCs is particularly challenging, and thyroid modalities are currently only assessed in vivo by mammalian and amphibian tests. The aquatic African clawed frog (Xenopus laevis/tropicalis) is the model species of the amphibian test guidelines developed by the OECD and the United States Environmental Protection Agency, but as most European amphibians are semi-aquatic, concern has been raised whether the sensitivity of native European species is comparable to Xenopus. A shortened version of the OEDC test guideline 241 (Larval Amphibian Growth and Development Assay, LAGDA) was used to investigate the effects of two model THADCs on the metamorphosis and thyroid histopathology in the European common frog (Rana temporaria). R. temporaria eggs were collected on the field and exposed till metamorphic climax to sodium perchlorate (11.9-426.5 μg/L perchlorate concentrations) and 6-propylthiouracil (PTU: 1.23-47.7 mg/L). PTU severely delayed metamorphosis and affected several thyroid gland histopathological endpoints at slightly lower concentrations compared to Xenopus. As opposed to what was described in similar Xenopus studies, we observed no effect of perchlorate on the investigated endpoints. Interspecies differences may be linked to mechanisms of action.

Phenotypic Variation Through Ontogeny: Thyroid Axis Disruption During Larval Development in the Frog Pleurodema borellii

Studies of the effects of thyroid hormones on larval development in the frog Xenopus spp. have provided baseline information to identify developmental constraints and elucidate genetic and hormonal mechanisms driving development, growth, and life history transitions. However, this knowledge requires data based on other anurans to complete a comprehensive approach to the understanding of larval developmental diversity and phenotypic variation through ontogeny. Mesocosm experiments provide realistic data about environmental conditions and timing; this information is useful to describe anuran larval development and/or analyze endocrine disruption. In this study, mesocosm experiments of the larval development of the frog Pleurodema borellii were conducted to explore the consequences of thyroid axis disruption; the sensitivity of tadpoles to the methimazole (2.66 mg/l) and thyroxine (T4) (1.66 μg/l) was compared. These concentrations were selected based on previous studies in Pleurodema borellii. We test the effects of methimazole and thyroxine on development in early exposure (from beginning of larval development) and late exposure, 18 days after hatching, with doses administered every 48 h. Tadpoles were evaluated 31 days after hatching. Methimazole caused moderate hypertrophy of the thyroid gland, alteration in the growth rates, differentiation without inhibition of development, and an increase of developmental variability. Thyroxine produced slight atrophy of the thyroid gland, accelerated growth rates and differentiation, and minor developmental variability. In tadpoles at stages previous to metamorphose, skull development (differentiation of olfactory capsules, appearance of dermal bones, and cartilage remodeling) seemed to be unaltered by the disruptors. Moreover, similar abnormal morphogenesis converged in specimens under methimazole and thyroxine exposures. Abnormalities occurred in pelvic and pectoral girdles, and vent tube, and could have been originated at the time of differentiation of musculoskeletal tissues of girdles. Our results indicate that premetamorphic stages (Gosner Stages 25–35) are sensitive to minimal thyroid axis disruption, which produces changes in developmental rates; these stages would also be critical for appendicular musculoskeletal morphogenesis to achieve the optimal condition to start metamorphosis.

Thyroid Disrupting Chemicals in Mixture Perturb Thymocyte Differentiation in Xenopus laevis Tadpoles

Endocrine disrupting chemicals (EDCs) can perturb the hypothalamic-pituitary-thyroid axis affecting human and wildlife health. Thyroid hormones (TH) are crucial regulators of metabolism, growth, and differentiation. The perinatal stage is most reliant on TH, thus vulnerable to TH disrupting chemicals. Dysregulation of TH signaling during perinatal development can weaken T cell function in maturity, raising the question of whether TH disrupting chemicals can perturb thymocyte development. Using Xenopus laevis tadpoles as model, we determined TH disrupting effects and thymocyte alterations following exposure to a mixture of common waterborne TH disrupting chemicals at concentrations similar to those found in contaminated water. This mixture included naphthalene, ethylene glycol, ethoxylated nonylphenol, and octylphenol, which have documented TH disrupting activity. Besides hypertrophy-like pathology in the thyroid gland and delayed metamorphosis, exposure to the mixture antagonized TH receptor-induced transcription of the Krüppel-like factor 9 transcription factor and significantly raised thyroid-stimulating hormone gene expression in the brain, two genes that modulate thymocyte differentiation. Importantly, exposure to this mixture reduced the number of Xenopus immature cortical thymocyte-specific-antigen (CTX+) and mature CD8+ thymocytes, whereas co-exposure with exogenous TH (T3) abolished the effect. When each chemical of the mixture was individually tested, only ethylene glycol induced significant antagonist effects on brain, thymic gene expression, and CD8+ thymocytes. These results suggest that EDCs in mixture are more potent than each chemical alone to perturb thymocyte development through TH-dependent pathway, and provide a starting point to research TH influence on thymocyte development.

The Extended Amphibian Metamorphosis Assay: A Thyroid-Specific and Less Animal-Intensive Alternative to the Larval Amphibian Growth and Development Assay

The amphibian metamorphosis assay (AMA; US Environmental Protection Agency [USEPA] test guideline 890.1100 and Organisation for Economic Co-Operation and Development test guideline 231) has been used for more than a decade to assess the potential thyroid-mediated endocrine activity of chemicals. In 2013, in the context of the Endocrine Disruptor Screening Program of the USEPA, a Scientific Advisory Panel reviewed the results from 18 studies and recommended changes to the AMA test guideline, including a modification to a fixed-stage design rather than a fixed-time (i.e., 21-d) design. We describe an extended test design for the AMA (or EAMA) that includes thyroid histopathology and time to metamorphosis (Nieuwkoop-Faber [NF] stage 62), to address both the issues with the fixed-time design and the specific question of thyroid-mediated adversity in a shorter assay than the larval amphibian growth and development assay (LAGDA; Organisation for Economic Co-Operation and Development test guideline 241), using fewer animals and resources. A demonstration study was conducted with the EAMA (up to NF stage 58) using sodium perchlorate. Data analyses and interpretation of the fixed-stage design of the EAMA are more straightforward than the fixed-time design because the fixed-stage design avoids confounded morphometric measurements and thyroid histopathology caused by varying developmental stages at test termination. It also results in greater statistical power to detect metamorphic delays than the fixed-time design. By preferentially extending the AMA to NF stage 62, suitable data can be produced to evaluate thyroid-mediated adversity and preclude the need to perform a LAGDA for thyroid mode of action analysis. The LAGDA remains of further interest should investigations of longer term effects related to sexual development modulated though the hypothalamus-pituitary-gonadal axis be necessary. However, reproduction assessment or life cycle testing is currently not addressed in the LAGDA study design. This is better addressed by higher tier studies in fish, which should then include specific thyroid-related endpoints. Environ Toxicol Chem 2021;40:2135-2144. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Chronic Exposure to Two Gestagens Differentially Alters Morphology and Gene Expression in Silurana tropicalis

Gestagens are active ingredients in human and veterinary drugs with progestogenic activity. Two gestagens-progesterone (P4), and the synthetic P4 analogue, melengestrol acetate (MGA)-are approved for use in beef cattle agriculture in North America. Both P4 and MGA have been measured in surface water receiving runoff from animal agricultural operations. This project aimed to assess the morphometric and molecular consequences of chronic exposures to P4, MGA, and their mixture during Western clawed frog metamorphosis. Chronic exposure (from embryo to metamorphosis) to MGA (1.7 µg/L) or P4 + MGA (0.22 µg/L P4 + 1.5 µg/L MGA) caused a considerable dysregulation of metamorphic timing, as evidenced by an inhibition of growth, narrower head, and lack of forelimb emergence in all animals. Molecular analysis revealed that chronic exposure to the mixture induced an additive upregulation of neurosteroid-related (GABA_A receptor subunit α6 (gabra6) and steroid 5-alpha reductase 1 (srd5α1) gene expression in brain tissue. Chronic P4 exposure (0.26 µg/L P4) induced a significant upregulation of the expression hypothalamic-pituitary-gonadal (HPG)-related genes (ipgr, erα) in the gonadal mesonephros complex (GMC). Our data suggest that exposure to P4, MGA, and their mixture induces multiple endocrine responses and adverse effects in larval Western clawed frogs. This study helps to better our understanding of the consequences of chronic gestagen exposure and suggests that the implications and risk of high gestagen use in beef cattle feeding operations may extend to the aquatic environment.

Investigating Acute and Subchronic Effects of Neonicotinoids on Northwestern Salamander Larvae

This research investigated the adverse effects of neonicotinoids on the Northwestern salamander (Ambystoma gracile; NWS) after acute and subchronic exposures during early aquatic life stages via whole organism (i.e., growth, development) and molecular (i.e., gene expression) level endpoints. In a 96-h exposure, NWS larvae were exposed to four imidacloprid concentrations (250, 750, 2250, 6750 µg/L) and a water control treatment, and no effects on survival, body weight, snout-vent length (SVL), and total body length were observed. However, a significant 1.70- and 2.33-fold decrease in thyroid receptor β (TRβ) mRNA expression levels were detected in the larvae exposed to 750 and 2250 µg/L imidacloprid, respectively, compared with the larvae in the water control. In subsequent subchronic experiments, NWS larvae were exposed for 35 days to imidacloprid alone and an equal part mixture of neonicotinoids (imidacloprid, clothianidin, and thiamethoxam (ICT)) at three concentrations (10, 100 and 1000 µg total neonicotinoids/L) and a water control. In these experiments, there were no effects on larval survival, body weight, SVL, and total body length. However, advanced development of larvae in the 100 µg/L imidacloprid treatment was observed compared with the control after 35-day imidacloprid exposure, providing some evidence of disruption of the thyroid endocrine axis at an environmentally relevant concentration. Ultimately, there is a paucity of studies conducted examining the sensitivity of salamanders to pollutants; thus, this study reports novel findings that will contribute to understanding the sensitivity of a Caudate amphibian model to a common environmental pollutant.

Exposure Impacts of Environmentally Relevant Concentrations of a Glufosinate Ammonium Herbicide Formulation on Larval Development and Thyroid Histology of Xenopus laevis

Thyroid hormones play critical roles in body growth and development as well as reproduction. They also influence the activities of a wider variety of tissues and biological functions, such as osmoregulation, metabolism, and especially metamorphosis in organisms, such as frogs. These complex activities of thyroid hormones are prone to disruption by agricultural pesticides, often leading to modulation of growth and the reproductive system in particular. These substances include Glufosinate ammonium, Glyphosates, Imazapyr, Penoxsulam, and Diquat dibromide among other herbicides. In this study, the standardized Xenopus Metamorphosis Assay protocol was used to assess the potential thyroid-modulatory properties of the Glufosinate ammonium Basta formulation, at relevant environmental concentrations (0.05 mg/L, 0.15 mg/L, and 0.25 mg/L) for 21 days. The results showed that this formulation only reduced the hind-limb length among the morphological endpoints. Histological evaluation showed that the mean thyroid gland area and the mean thyroidal follicle epithelium height were significantly increased following 0.15 and 0.25 mg/L exposures. The present study confirmed that this Basta formulation interacts with the thyroid axis and therefore potentially pose health hazard to amphibian in particular and potentially metamorphic aquatic vertebrates. Furthermore, the result is a signal of inherent potential thyroid disrupting activities that must be further investigated and characterised in some of the aquatic herbicide formulations to safeguard the aquatic biodiversity.

Effect of ethylenethiourea on metamorphosis and ovary development: A comparative study of three larval frogs

Amphibian endocrine systems interact with each other during normal development. Interference with one of the endocrine systems may influence others. We studied the effect of a thyroid inhibitor (ethylenethiourea [ETU]) on metamorphosis and ovary development of three species, Sphaerotheca pashchima, Indosylvirana caesari, and Euphlyctis cyanophlyctis with different larval durations. We treated the tadpoles of these species with 50, 100, and 200 mg/L concentrations of ETU and studied their larval duration, size at metamorphosis, and ovary development. The results revealed that ETU affects metamorphosis, depending on the species and concentration. ETU delayed metamorphosis of E. cyanophlyctis tadpoles and did not affect metamorphosis in S. pashchima tadpoles. Lower concentrations of ETU stimulated metamorphosis in I. caesari tadpoles while high concentration delayed metamorphosis. In the tadpoles (E. cyanophlyctis) treated with higher concentrations of ETU, ovary development was advanced with an increased size of the diplotene oocytes. Oocyte size was smaller in the tadpoles (of I. caesari) treated with lower concentrations of ETU. These results demonstrated that the tadpoles of these species show different responses to the thyroid inhibitor, possibly due to the differences in the larval duration and sensitivity. Inhibition or acceleration of metamorphosis did not interfere in the ovary development of E. cyanophlyctis and I. caesari. These results will be useful in understanding the impact of endocrine disruptors on the interaction between thyroid and sex steroid hormones.

Long-term effects of in ovo exposure to an environmentally relevant dose of atrazine on the thyroid gland of Caiman latirostris

The increased incidence of human thyroid disorders, particularly in women, suggests that the exposure to endocrine-disrupting compounds (EDCs) together with sex-related factors could play a role in thyroid dysregulation. Since the herbicide atrazine (ATZ) is an environmental EDC suspected to behave as a thyroid disruptor, and Caiman latirostris is a crocodilian species highly sensitive to endocrine disruption that can be exposed to ATZ, this study aimed to describe the histoarchitecture and sexually dimorphic features of the thyroid gland of C. latirostris, and to determine the long-term effects of in ovo exposure to an environmentally relevant dose of ATZ (0.2 ppm) on its thyroid gland and growth. Control caimans showed no sexual dimorphisms. In contrast, ATZ-exposed caimans showed altered embryo growth but an unaltered temporal pattern of development and a sexually dimorphic response in the body condition index growth curves postnatally, which suggests a female-related increase in fat storage. Besides, both male and female exposed caimans showed increases in the size of the thyroid stromal compartment, content of interstitial collagen, and follicular hyperplasia, and decreases in the expression of androgen receptor in the follicular epithelium. ATZ-exposed females, but not males, also showed evidences of thyroid enlargement, colloid depletion, increased follicular epithelial height and increased presence of microfollicular structures. Our results demonstrate that prenatal exposure of caimans to ATZ causes thyroid disruption and that females were more vulnerable to ATZ than males. The effects were organizational and observed long after exposure ended. These findings alert on ATZ side-effects on the growth, metabolism, reproduction and development of non-target exposed organisms, particularly females.

Chronic effects of lead on metamorphosis, development of thyroid gland, and skeletal ossification in Bufo gargarizans

We examined the Pb²⁺ exposure on tadpoles of Bufo gargarizans from Gosner stage 26-42. Mortality, growth and development, time to metamorphosis, size, and skeletal ossification at metamorphic climax of Bufo gargarizans were examined. Also, histological characteristics of thyroid glands in tadpoles at Gosner stage 33, 38, and 42 as well as transcript levels of thyroid hormone-related genes in the hind-limb, tail, and liver of tadpoles at metamorphic climax were examined. Pb²⁺ exposure induced mortality in a concentration-dependent manner in Bufo gargarizans larvae. The significant increase in growth and development, percent metamorphosis, size at metamorphic climax, and skeletal ossification were observed at 50 μg Pb²⁺ L^-1; however, exposure to 1000 μg Pb²⁺ L^-1 resulted in the opposite effects in tadpoles. In addition, histological alterations of thyroid gland, such as follicular cell hyperplasia and colloid depletion could be found in 50-1000 μg Pb²⁺ L^-1 treatments. Furthermore, Pb²⁺ exposure at 1000 μg L^-1 resulted in significantly decreased transcript levels of Dio2, TRα and TRβ, and increased transcript levels of Dio3. In contrast, 50 μg Pb²⁺ L^-1 significantly upregulated the mRNA levels of Dio2, TRα, and TRβ, but it reduced the Dio3 expression. These results suggested that Pb²⁺ might disrupt TH homeostasis in tadpoles by histological alterations of thyroid gland and disturb the transcript levels of Dio2, Dio3, TRα, and TRβ, leading to altered growth and development, as well as percent metamorphosis and skeletal ossification. Further studies are needed to elucidate the underlying mechanisms of low-dose stimulation and high-dose inhibition effects.

Differences in responsiveness and sensitivity to exogenous disruptors of the thyroid gland in three anuran species

Anuran larval development comprises tissues/organs/systems that are: exclusively of larvae, able to be remodelled, and those of postmetamorphic stages. Also, the anuran larval development is characterized by inter-related parameters: time, size and shape forming part of growth and differentiation. The anuran metamorphosis starts when growth and differentiation achieve a threshold that differs among species since it is regulated by a number of external (environmental) and internal (hormonal) processes. Here we explore the consequences of exogenous disruptors on the thyroid gland (e.g., methimazole and thyroxine as T4) of three species by immersing premetamorphic tadpoles in predetermined concentrations of the disruptors for short periods (10 or 16 days). The species were Pleurodema borellii, Leptodactylus chaquensis, and Dermatonotus muelleri, which all breed in small temporary ponds during the summer, but differ in their ecomorphology. The experiments were conducted to evaluate the effects of these substances on larval development (based in Gosner larval stages), morphometric variation in body parameters (snout-vent and total length by larval stages), and thyroid gland histopathology at the end of the assays. In P. borelli and L. chaquensis, methimazole produces significant increment of size measurements (nonparametric Kruskal-Wallis, p < .05) during stages of digit differentiation and induced thyroid gland hypertrophy. In the three species, T4 exposure accelerated limb development and caused atrophy of thyroid gland. Prolonged T4 exposure in L. chaquensis and D. muelleri triggered metamorphic transformation in the gut and skull cartilages. Discussion about interspecific differences in responsiveness and sensitivity elucidates the importance of hormonal signals to morphological evolution.

Thyroid endocrine status and biochemical stress responses in adult male Wistar rats chronically exposed to pristine polystyrene nanoplastics

Toxicity evaluations of micro- or nano-sized plastics in rodent species commonly employed for toxicity analyses based on which risk assessment for humans could be performed are still largely lacking. Given this knowledge gap, the present work was aimed at determining the potential impact of chronic exposure to polystyrene nanoplastics (PS NPs) on the thyroid endocrine status and biochemical stress in a rat model. Young adult male rats were orally administered with PS NPs (1, 3, 6 and 10 mg kg^-1 day^-1) for five weeks. Thyroid hormones (THs) l-thyroxine (T4), l-triiodothyronine (T3), l-free triiodothyronine (FT3), and l-free thyroxine (FT4) as well as thyroid stimulating hormone (TSH) serum levels of normal rats and those exposed to PS NPs were compared. Serum levels of high-density lipoprotein (HDL), low-density lipoprotein (LDL), cholesterol, and creatinine, as well as glutamic oxaloacetic transaminase (GOT) and glutamic pyruvic transaminase (GPT) enzymes were also measured. Exposure to PS NPs suppressed the serum levels of T3 and circulating levels of THs, whereas TSH increased significantly. Though exposure to PS NPs did not affect the molar ratios of T3/T4, it induced a slight, but significant, increase in FT3/FT4. In addition, exposure to plastic nanoparticles showed signs of nephrotoxicity induction and kidney injury in exposed organisms as can be inferred from the significantly higher serum levels of creatinine in exposed groups. Our results provide clear evidence of an association between exposure to plastic NPs and thyroid endocrine disruption as well as metabolic deficit, and generate new leads for future research efforts.

Effects of nonylphenols on embryonic development and metamorphosis of Xenopus laevis: FETAX and amphibian metamorphosis toxicity test (OECD TG231)

Nonylphenols (NPs) are a group of endocrine-disrupting surfactants that mimic estrogen. To determine the developmental toxicity and thyroid-disrupting effect of NPs, the effects of exposure to nonylphenol (NP), 4-nonylphenol (4-NP), and nonylphenol ethoxylate (NP-12) were examined according to the frog embryo teratogenesis assay-Xenopus (FETAX) and Organization for Economic Co-operation and Development test guidelines 231 (TG231). In FETAX, the LC₅₀ values of NP, 4-NP, and NP-12 were 59.14 mg/L, 10.13 mg/L, and 14.60 mg/L, respectively. At 10.0 mg/L, NP, 4-NP, and NP-12 significantly decreased the total length of tadpoles, and NP and 4-NP increased gut malformation and bent tails. In surviving tadpoles, the EC₅₀ values for malformation of NP, 4-NP, and NP-12 were 4.66, 6.51, and 13.08 mg/L, respectively. The teratogenic indices of NP, 4-NP, and NP-12 were 12.69, 1.56, and 1.08, respectively, suggesting the teratogenic potential of NP and 4-NP. In a range-finder assay for TG231, the 96-h LC₅₀ values of NP, 4-NP, and NP-12 were 2.0, 2.0, and 10.57 mg/L, respectively. When NF stage 51 larvae were exposed for 21 days, larval growth was inhibited by NP, 4-NP, and NP-12 at 0.67, 0.07, and 0.37 mg/L, respectively. 4-NP at 0.07 mg/L accelerated the developmental stage and significantly increased hind limb length, while 0.67 mg/L 4-NP delayed the developmental stage and decreased hind limb length, suggesting a bimodal effect of 4-NP on metamorphosis. NP and NP-12 at test concentrations did not alter the larval stage, but NP-12 at 0.37 mg/L significantly decreased total length and tail length, suggesting growth inhibition in larvae. The total colloid area of thyroid follicles was significantly increased by 0.07 mg/L 4-NP but not by NP and NP-12, suggesting that 4-NP may interfere with thyroid function. Together, the developmental toxicity of NPs was in the following order: 4-NP, NP-12, and NP. 4-NP may alter metamorphosis driven by thyroid hormones in X. laevis.

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.

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.

Developmental expression profiles and thyroidal regulation of cytokines during metamorphosis in the amphibian Xenopus laevis

Early life-stages of amphibians rely on the innate immune system for defense against pathogens. While thyroid hormones (TH) are critical for metamorphosis and later development of the adaptive immune system, the role of TH in innate immune system development is less clear. An integral part of the innate immune response are pro-inflammatory cytokines - effector molecules that allow communication between components of the immune system. The objective of this study was to characterize the expression of key pro-inflammatory cytokines, tumor necrosis factor-α (TNFα), interleukin-1β (IL-1β) and interferon-γ (IFN-γ), throughout amphibian development and determine the impacts of thyroidal modulation on their expression. Xenopus laevis were sampled at various stages of development encompassing early embryogenesis to late prometamorphosis and cytokine expression was measured by real-time PCR. Expression of TNFα and IL-1β were transient over development, increasing with developmental stage, while IFN-γ remained relatively stable. Functionally athyroid, premetamorphic tadpoles were exposed to thyroxine (0.5 and 2 μg/L) or sodium perchlorate (125 and 500 μg/L) for seven days. Tadpoles demonstrated characteristic responses of advanced development with thyroxine exposure and delayed development (although to a lesser extent) and increased thyroid gland area and follicular cell height with sodium perchlorate exposure. Exposure to thyroxine for two days resulted in decreased expression of IL-1β in tadpole trunks. Sodium perchlorate had negligible effects on cytokine expression. Overall, these results demonstrate that cytokine transcript levels vary with stage of tadpole development but that their ontogenic regulation is not likely exclusively influenced by thyroid status. Understanding the direct and indirect effects of altered hormone status may provide insight into potential mechanisms of altered immune function during amphibian development.

Intrafollicular thyroid hormone staining in whole-mount zebrafish (Danio rerio) embryos for the detection of thyroid hormone synthesis disruption

Endocrine-disrupting chemicals are known to impact multiple hormonal axes of vertebrates, among which the thyroid system is crucial for multiple developmental and physiological processes. Thus, the present study focused on the semi-quantitative visualization of intrafollicular triiodothyronine (T3) and thyroxin (T4) in zebrafish embryos as a potential test system for the detection of disrupted thyroid hormone synthesis. To this end, an antibody-based fluorescence double-staining protocol for whole-mount zebrafish embryos and larvae was adapted to simultaneously detect intrafollicular T3 and T4. During normal development until 10 days post-fertilization (dpf), the number of thyroid follicles increased along the ventral aorta. Concentrations of T4 and T3, measured by fluorescence intensity, increased until 6 dpf, but decreased thereafter. Exposure of zebrafish embryos to propylthiouracil (PTU), a known inhibitor of TH synthesis, resulted in a significant decrease in the number of follicles that stained for T3, whereas a trend for increase in follicles that stained for T4 was observed. In contrast, fluorescence intensity for both thyroid hormones decreased significantly after exposure to PTU. Overall, the zebrafish embryo appears to be suitable for the simultaneous visualization and detection of changing intrafollicular TH contents during normal development and after PTU treatment.

The progestin norethisterone affects thyroid hormone-dependent metamorphosis of Xenopus laevis tadpoles at environmentally relevant concentrations

Previously, levonorgestrel (LNG) has been shown to be an endocrine disruptor of the amphibian thyroid system. In the present study, we investigated whether anti-thyroidal effects are a common property of progestins other than LNG. Premetamorphic Xenopus laevis tadpoles were exposed to norethisterone (NET) and dienogest DIE (each at 0.1-10nM) and LNG (10nM) until completion of metamorphosis. LNG and NET at all concentrations caused a significant developmental retardation whereas DIE did not impair time to metamorphosis. In LNG and 10nM NET exposed animals, tsh mRNA levels increased considerably later than the developmental delay occurred and thyroid histopathology showed no signs of TSH-hyperstimulation. Instead, thyroid glands from these treatments appeared inactive in producing thyroid hormones. Thyroidal transcript levels of dio2 and dio3 were increased by treatments with LNG and NET at 1nM and 10nM, whereas iyd mRNA was reduced by LNG and 10nM NET. Expression of slc5α5 was not changed by any treatment. Effects of DIE differed from those induced by LNG and NET. No developmental delay was measurable; however, tshβ and dio2 mRNAs were increased in pituitary glands of tadpoles exposed to 1.0nM and 10nM DIE. Thyroid histopathology displayed no abnormalities and thyroidal mRNA expression of the genes analyzed (slc5α5, iyd, dio2, dio3) was not changed by DIE. Overall, our results provide evidence that the anti-thyroidal effects already known from LNG are also present in another progestin, namely NET, even at environmentally relevant concentrations. In conclusion we suggest that progestins do not only pose an environmental risk in terms of their impact on reproductive success of aquatic vertebrates, but also with respect to their anti-thyroidal properties affecting amphibian metamorphosis.

Vulnerability of marsh frog Pelophylax ridibundus to the typical wastewater effluents ibuprofen, triclosan and estrone, detected by multi-biomarker approach

Pharmaceutical and personal care products (PPCPs) are the environmental pollutants of growing concern. The aim of this study was to indicate the effects of typical PPCPs on the marsh frog Pelophylax ridibundus. We treated male frogs with waterborne ibuprofen (IBU, 250ng·L^-1), triclosan (TCS, 500ng·L^-1), or estrone (E1, 100ng·L^-1) for 14days. Common vulnerability of the frogs was detected from dramatic decrease of Zn, total and metalated metallothionein (MT) concentrations, Zn/Cu ratio, the elevation of activity of glutathione-S-transferase, cathepsin D and DNA instability in the liver, the depletion of cholinesterase in the brain and cortisol in the blood plasma in all exposures. Nevertheless, lipofuscin concentration in the liver was always decreased. The groups were best distinguished by cytochrome P450 (CYP450) activity determined by ELISA. The exposure to IBU caused lesser damage, but elevated the levels of oxyradicals and glutathione (GSH and GSSG) and lysosomal membrane instability. Exposures to TCS and E1 provoked the endocrine disturbance (increased levels of vitellogenin and thyrotropin in blood plasma), decreased lactate dehydrogenase activity and increased level of pyruvate in the liver. TCS caused the increase of GSSG by 7.3 times and lactate levels. Only E1 lead to decrease of deiodinase activity in the liver, activation of CYP450 and caspase-3 and efflux of cathepsin D from lysosomes. Spectrophotometric and ELISA assays of MTs and CYP450 gave distinct results in E1-group. Broad disruption of the hormonal pathways caused by E1 could be of concern for the health status of frogs in their habitats.

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.

Development of yeast reporter assays for the enhanced detection of environmental ligands of thyroid hormone receptors α and β from Xenopus tropicalis

Thyroid hormones (THs) are involved in the regulation of metabolic homeostasis during the development and differentiation of vertebrates, particularly amphibian metamorphosis, which is entirely controlled by internal TH levels. Some artificial chemicals have been shown to exhibit TH-disrupting activities. In order to detect TH disruptors for amphibians, we herein developed a reporter assay using yeast strains expressing the thyroid hormone receptors (TRs) α and β together with the transcriptional coactivator SRC-1, all of which were derived from the frog Xenopus tropicalis (XT). These yeast strains responded to endogenous THs (T₂, T₃, and T₄) in a dose-dependent manner. They detected the TR ligand activities of some artificial chemicals suspected to exhibit TH-disrupting activities, as well as TR ligand activity in river water collected downstream of sewage plant discharges, which may have originated from human excrement. Moreover, the responses of XT TR strains to these endogenous and artificial ligands were stronger than those of yeast strains for human TRα and β assays, which had previously been established in our laboratory. These results indicate that the yeast reporter assay system for XT TRα and β is valuable for assessing TR ligand activities in environmental samples that may be particularly potent in amphibians.

Metabolic and immune impairments induced by the endocrine disruptors benzo[a]pyrene and triclosan in Xenopus tropicalis

Despite numerous studies suggesting that amphibians are highly sensitive to cumulative anthropogenic stresses, the role played by endocrine disruptors (EDs) in the decline of amphibian populations remains unclear. EDs have been extensively studied in adult amphibians for their capacity to disturb reproduction by interfering with the sexual hormone axis. Here, we studied the in vivo responses of Xenopus tropicalis males exposed to environmentally relevant concentrations of each ED, benzo[a]pyrene (BaP) and triclosan (TCS) alone (10 μg L(-1)) or a mixture of the two (10 μg L(-1) each) over a 24 h exposure period by following the modulation of the transcription of key genes involved in metabolic, sexual and immunity processes and the cellular changes in liver, spleen and testis. BaP, TCS and the mixture of the two all induced a marked metabolic disorder in the liver highlighted by insulin resistance-like and non-alcoholic fatty liver disease (NAFLD)-like phenotypes together with hepatotoxicity due to the impairment of lipid metabolism. For TCS and the mixture, these metabolic disorders were concomitant with modulation of innate immunity. These results confirmed that in addition to the reproductive effects induced by EDs in amphibians, metabolic disorders and immune system disruption should also be considered.

Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

In the present study, a hypothesized adverse outcome pathway linking inhibition of thyroid peroxidase (TPO) activity to impaired swim bladder inflation was investigated in two experiments in which fathead minnows (Pimephales promelas) were exposed to 2-mercaptobenzothiazole (MBT). Continuous exposure to 1mg MBT/L for up to 22 days had no effect on inflation of the posterior chamber of the swim bladder, which typically inflates around 6 days post fertilization (dpf), a period during which maternally-derived thyroid hormone is presumed to be present. In contrast, inflation of the anterior swim bladder, which occurs around 14dpf, was impacted. Specifically, at 14dpf, approximately 50% of fish exposed to 1mg MBT/L did not have an inflated anterior swim bladder. In fish exposed to MBT through 21 or 22dpf, the anterior swim bladder was able to inflate, but the ratio of the anterior/posterior chamber length was significantly reduced compared to controls. Both abundance of thyroid peroxidase mRNA and thyroid follicle histology suggest that fathead minnows mounted a compensatory response to the presumed inhibition of TPO activity by MBT. Time-course characterization showed that fish exposed to MBT for at least 4 days prior to normal anterior swim bladder inflation had significant reductions in anterior swim bladder size, relative to the posterior chamber, compared to controls. These results, along with similar results observed in zebrafish (see part II, this issue) are consistent with the hypothesis that thyroid hormone signaling plays a significant role in mediating anterior swim bladder inflation and development in cyprinids, and that role can be disrupted by exposure to thyroid hormone synthesis inhibitors. Nonetheless, possible thyroid-independent actions of MBT on anterior swim bladder inflation cannot be ruled out based on the present results. Overall, although anterior swim bladder inflation has not been directly linked to survival as posterior swim bladder inflation has, potential links to adverse ecological outcomes are plausible given involvement of the anterior chamber in sound production and detection.

Endocrine activities and cellular stress responses in the marsh frog Pelophylax ridibundus exposed to cobalt, zinc and their organic nanocomplexes

Metal-containing materials are extensively used in industry, personal care products and medicine, and their release in the environment causes concern for the potential impacts on aquatic organisms. We assessed endocrine disrupting potential of N-vinyl-2-pyrrolidone-based nanoparticles (Me-PSs) containing cobalt (Co(2+)) or zinc (Zn(2+)), using the marsh frog Pelophylax ridibundus as a model. Adult males were exposed for 14 days to waterborne Co(2+) (50μg/L), Zn(2+) (100μg/L) or corresponding concentrations of Co-PS, Zn-PS, or parental polymeric compound (PS). The indices of thyroid activity, vitellogenesis, cytochrome P450-dependent monooxygenases activity (EROD) and cytotoxicity markers were evaluated. Exposure to Co(2+) led to the elevation of serum thyrotropin (TSH) and hepatic deiodinase activities accompanied by the up-regulation of EROD activity. In contrast, the action of the polymer-containing substances (Co-PS, Zn-PS and PS) as well as free Zn(2+) caused a prominent decrease of EROD activity and a decrease in serum cortisol and TSH concentrations. Exposures to Zn(2+), Zn-PS and PS upregulated vitellogenesis in males. All exposures except Co(2+) caused neurotoxicity as indicated by the depletion of cholinesterase. These results demonstrate toxicity of Co- and Zn-containing Me-PSs and their parental compounds (Zn(2+) and PS) in frogs and indicate distinct mechanisms of Co(2+) action. Broad disruption of the hormonal pathways and reduced capacity for organic xenobiotic detoxification may have deleterious impacts on amphibian populations from habitats exposed to metallorganic pollution.

Effects of fluoride on metamorphosis, thyroid and skeletal development in Bufo gargarizans tadpoles

This study examined the effects of chronic fluoride exposure on metamorphosis, thyroid and skeletal development in tadpoles of Chinese Toad, Bufo gargarizans. The tadpoles were exposed to fluoride concentrations either at 0, 1, 5, 10, or at 50 mg L(-1) from Gosner stage 26 to Gosner stage 42. Body weight, total length and percentage of tadpoles reaching metamorphosis climax were recorded, and thyroid histological examinations were employed. In addition, mRNA expression of both deiodinase type 2 (D2) and deiodinase type 3 (D3) was analyzed by using RT-PCR and skeletal systems were investigated by using double-staining methodology at stage 42. Results showed that total length and body weight were unaffected by fluoride exposure at all concentrations while metamorphosis was strongly inhibited only by 50 mg L(-1) fluoride. Histomorphological measurements showed the percentage of colloid depletion in thyroid gland increased significantly, while the average diameter of follicles was significantly shorter at 50 mg L(-1) concentration. In addition, fluoride at 5 mg L(-1) can stimulate bone mineralization, while fluoride at 50 mg L(-1) can retard deposition of calcium. In conclusion, our study suggests that 50 mg L(-1) fluoride could damage follicular cells in thyroid gland and induce a sharp reduction in thyroid hormone probably through the up-regulation of D3 mRNA expression, and these influences on thyroid system may delay metamorphosis as well as ossification in bone tissue by inhibiting calcium deposition.

The antiestrogens tamoxifen and fulvestrant abolish estrogenic impacts of 17α-ethinylestradiol on male calling behavior of Xenopus laevis

Various synthetic chemicals released to the environment can interfere with the endocrine system of vertebrates. Many of these endocrine disrupting compounds (EDCs) exhibit estrogenic activity and can interfere with sexual development and reproductive physiology. More recently, also chemicals with different modes of action (MOAs), such as antiestrogenic, androgenic and antiandrogenic EDCs, have been shown to be present in the environment. However, to date EDC-research primarily focuses on exposure to EDCs with just one MOA, while studies examining the effects of simultaneous exposure to EDCs with different MOAs are rare, although they would reflect more real, natural exposure situations. In the present study the combined effects of estrogenic and antiestrogenic EDCs were assessed by analyzing the calling behavior of short-term exposed male Xenopus laevis. The estrogenic 17α-ethinylestradiol (EE2), and the antiestrogenic EDCs tamoxifen (TAM) and fulvestrant (ICI) were used as model substances. As previously demonstrated, sole EE2 exposure (10-10 M) resulted in significant alterations of the male calling behavior, including altered temporal and spectral parameters of the advertisement calls. Sole TAM (10-7 M, 10-8 M, 10-10 M) or ICI (10-7 M) exposure, on the other hand, did not affect any of the measured parameters. If frogs were co-exposed to EE2 (10-10 M) and TAM (10-7 M) the effects of EE2 on some parameters were abolished, but co-exposure to EE2 and ICI (10-7 M) neutralized all estrogenic effects. Thus, although EDCs with antiestrogenic MOA might not exhibit any effects per se, they can alter the estrogenic effects of EE2. Our observations demonstrate that there is need to further investigate the combined effects of EDCs with various, not only opposing, MOAs as this would reflect realistic wildlife situations.

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.

Thyroid disruption effects of environmental level perfluorooctane sulfonates (PFOS) in Xenopus laevis

Perfluorooctane sulfonate (PFOS), one of the emerging persistent organic pollutants (POPs), has caused growing international concern especially related to the potential disruption in the development and function of thyroid system. Xenopus laevis is an amphibian species widely used as a suitable amphibian model for thyroid disruption research. To study the thyroid disruption effects related to PFOS exposure at environmental low levels, X. laevis tadpoles were exposed to 0.1, 1, 10 and 100 μg/l PFOS in water respectively from stage 46/47 to stage 62. The results showed that the time to metamorphosis (presented by forelimb emergence, FLE) did not significantly change with PFOS exposure, but exhibited an increasing trend (except for 10 μg/l exposure). Partial colloid depletion was observed for PFOS exposure, but no significant histological abnormality was observed in treatment groups. In addition, PFOS exposure resulted in up-regulation of thyroid hormone-regulated genes-thyroid receptor beta A (TRβA), basic transcription element-binding protein (BTEB) and type II deiodinase (DI2) mRNA expression, presented as an inverted U-shaped dose response pattern. However, the mRNA expression of type III deiodinase (DI3) remained unaffected compared with the control. These results demonstrated that PFOS might disrupt the thyroid system in X. laevis tadpoles regarding FLE changes and regulation alternation of thyroid hormone-regulated genes. Our study has raised new concerns for possible thyroid disruption of PFOS in amphibians at environmental relevant levels.

Zebrafish eleutheroembryos provide a suitable vertebrate model for screening chemicals that impair thyroid hormone synthesis

Thyroxine-immunofluorescence quantitative disruption test (TIQDT) was designed to provide a simple, rapid, alternative bioassay for assessing the potential of chemical pollutants and drugs to disrupt thyroid gland function. This study demonstrated that zebrafish eleutheroembryos provided a suitable vertebrate model, not only for screening the potential thyroid disrupting effect of molecules, but also for estimating the potential hazards associated with exposure to chemicals directly impairing thyroxine (T4) synthesis. Amitrole, potassium perchlorate, potassium thiocyanate, methimazole (MMI), phloroglucinol, 6-propyl-2-thiouracil, ethylenethiourea, benzophenone-2, resorcinol, pyrazole, sulfamethoxazole, sodium bromide, mancozeb, and genistein were classified as thyroid gland function disruptors. Concordance between TIQDT on zebrafish and mammalian published data was very high and the physiological relevance of T4-intrafollicular content was clearly higher than regulation at the transcriptional level of tg or slc5a5. Moreover, concentration-response analysis provided information about the thyroid disrupting potency and hazard of selected positive compounds. Finally, the effect of perchlorate, but not MMI, was completely rescued by low-micromolar amounts of iodide. TIQDT performed on zebrafish eleutheroembryos is an alternative whole-organism screening assay that provides relevant information for environmental and human risk assessments.

Alterations along the Hypothalamic-Pituitary-Thyroid Axis of the Zebrafish (Danio rerio) after Exposure to Propylthiouracil

In the past, various approaches have been developed to detect adverse effects of pollutants on the thyroid of vertebrates, most of these with special emphasis on the South African clawed frog, Xenopus laevis. Although fish are primarily affected by thyroid-disrupting chemicals, studies into alterations of the thyroid of fish are scarce. Therefore, effects of the reference compound propylthiouracil on histopathology of the thyroid axis were analyzed in a modified early life-stage test with zebrafish (Danio rerio) exposed to propylthiouracil. The test substance induced dose-dependent alterations of thyroidal tissue concomitant with increases in the number of surrounding blood vessels. Despite this massive proliferation of the thyroid, zebrafish were not able to maintain thyroxin concentrations. The pituitary was affected displaying significant alterations in thyroid-stimulating hormone cell counts. Quantitative evaluation of pituitary surface areas revealed a dose-dependent increase of adenohypophyseal tissue. Distinct histopathological effects may contribute to a more easy identification and interpretation of alterations induced by thyroid-disrupting chemicals.

The synthetic gestagen levonorgestrel impairs metamorphosis in Xenopus laevis by disruption of the thyroid system

Synthetic gestagens, including levonorgestrel (LNG), are active compounds in contraceptives, and several studies report their occurrence in surface waters. However, information about endocrine-disrupting effects in nontarget organisms is scarce. The present study investigated effects of LNG exposure on thyroid hormone-dependent metamorphosis of Xenopus laevis. Premetamorphic X. laevis tadpoles at Nieuwkoop and Faber (NF) stage 48 were exposed in a flow-through culture system to four LNG concentrations (10(-11), 10(-10), 10(-9), and 10(-8)M) over the period of metamorphosis. At NF 58 and 66, tadpoles were examined sex specifically. Developmental time and organismal responses were recorded and correlated with molecular and histopathological endpoints. Exposure to 10(-8)M LNG caused an inhibition of metamorphosis resulting in developmental arrest at early climax stages as giant tadpoles or tailed frogs. In brain-pituitary tissue of NF 58 tadpoles, gene expression of thyroid-stimulating hormone (β-subunit; TSHβ), TH receptor β (TRβ), and deiodinase type 3 (D3) was not changed. Instead, prolactin (PRL) messenger RNA (mRNA) was significantly increased by 10(-9)M LNG in females and by 10(-8)M LNG in both sexes. In NF 66 tadpoles, mRNA levels of TSHβ mRNA were significantly increased in the 10(-9) and 10(-8)M LNG treatment groups indicating a hypothyroid state. No changes of TRβ, D3, and PRL gene expression were detected. Histopathological evaluation of thyroid gland sections revealed no typical sign of hypothyroidism but rather an inactivated appearance of the thyroid. In conclusion, our data demonstrate for the first time a completely new aspect of thyroid system disruption caused by synthetic gestagens in developing amphibians.

Thyroid disruption by Di-n-butyl phthalate (DBP) and mono-n-butyl phthalate (MBP) in Xenopus laevis

BACKGROUND

Di-n-butyl phthalate (DBP), a chemical widely used in many consumer products, is estrogenic and capable of producing seriously reproductive and developmental effects in laboratory animals. However, recent in vitro studies have shown that DBP and mono-n-butyl phthalate (MBP), the major metabolite of DBP, possessed thyroid hormone receptor (TR) antagonist activity. It is therefore important to consider DBP and MBP that may interfere with thyroid hormone system.

METHODOLOGY/PRINCIPAL FINDINGS

Nieuwkoop and Faber stage 51 Xenopus laevis were exposed to DBP and MBP (2, 10 or 15 mg/L) separately for 21 days. The two test chemicals decelerated spontaneous metamorphosis in X. laevis at concentrations of 10 and 15 mg/L. Moreover, MBP seemed to possess stronger activity. The effects of DBP and MBP on inducing changes of expression of selected thyroid hormone response genes: thyroid hormone receptor-beta (TRβ), retinoid X receptor gamma (RXRγ), alpha and beta subunits of thyroid-stimulating hormone (TSHα and TSHβ) were detected by qPCR at all concentrations of the compounds. Using mammalian two-hybrid assay in vitro, we found that DBP and MBP enhanced the interactions between co-repressor SMRT (silencing mediator for retinoid and thyroid hormone receptors) and TR in a dose-dependent manner, and MBP displayed more markedly. In addition, MBP at low concentrations (2 and 10 mg/L) caused aberrant methylation of TRβ in head tissue.

CONCLUSIONS

The current findings highlight potential disruption of thyroid signalling by DBP and MBP and provide data for human risk assessment.

Effects of metolachlor on transcription of thyroid system-related genes in juvenile and adult Japanese medaka (Oryzias latipes)

Metolachlor (MT) is one of the most important pesticides applied to corn and other crops for controlling broadleaf and grass weeds. However, the effects of MT on the thyroid system in fish remain to be elucidated. In the present experiment, transcription of genes related to the thyroid system, including thyrotropin releasing hormone (Trh), deiodinase 2 (Dio2), thyroid hormone receptor α (Thrα), and thyroid hormone receptor β (Thrβ), were induced by MT in a sex-, developmental stage-, and tissue- specific manner when medaka were exposed to various concentrations of MT for 14 days. The transcriptional levels of the genes were only significantly altered in both juvenile and adult female medaka in response to MT exposure. And the lowest concentrations able to significantly induce transcription of the selected genes were 10 and 100 μg/L in juvenile and adult female medaka, respectively. In adult female medaka, a significant up-regulation of these genes was detected only in the brain, with little or no effect in the liver. Furthermore, MT-induced (100 μg/L) transcription of thyroid system-related genes was enhanced significantly in male juvenile medaka in the presence of estrogen (E2) (50 and 100 ng/L). Moreover, the mRNA levels of Thrα and Thrβ in males increase with the combined treatments of 100 μg/L MT and 100 ng/L E2. Dio2 increased when exposed to 100 μg/L MT and 50 or 100 ng/L E2. The information obtained in the present study suggests that MT has the potential to influence several steps of the hypothalamus-pituitary-thyroid (HPT) axis homeostasis and to disrupt the thyroid system in medaka.

Predicting chemical impacts on vertebrate endocrine systems

Animals have evolved diverse protective mechanisms for responding to toxic chemicals of both natural and anthropogenic origin. From a governmental regulatory perspective, these protective responses complicate efforts to establish acceptable levels of chemical exposure. To explore this issue, we considered vertebrate endocrine systems as potential targets for environmental contaminants. Using the hypothalamic-pituitary-thyroid (HPT), hypothalamic-pituitary-gonad (HPG), and hypothalamic-pituitary-adrenal (HPA) axes as case examples, we identified features of these systems that allow them to accommodate and recover from chemical insults. In doing so, a distinction was made between effects on adults and those on developing organisms. This distinction was required because endocrine system disruption in early life stages may alter development of organs and organ systems, resulting in permanent changes in phenotypic expression later in life. Risk assessments of chemicals that impact highly regulated systems must consider the dynamics of these systems in relation to complex environmental exposures. A largely unanswered question is whether successful accommodation to a toxic insult exerts a fitness cost on individual animals, resulting in adverse consequences for populations. Mechanistically based mathematical models of endocrine systems provide a means for better understanding accommodation and recovery. In the short term, these models can be used to design experiments and interpret study findings. Over the long term, a set of validated models could be used to extrapolate limited in vitro and in vivo testing data to a broader range of untested chemicals, species, and exposure scenarios. With appropriate modification, Tier 2 assays developed in support of the U.S. Environmental Protection Agency's Endocrine Disruptor Screening Program could be used to assess the potential for accommodation and recovery and inform the development of mechanistically based models.

Effects of nonylphenol on early embryonic development, pigmentation and 3,5,3'-triiodothyronine-induced metamorphosis in Bombina orientalis (Amphibia: Anura)

Nonylphenol (NP) is an estrogenic endocrine disruptor in many aquatic species. In an effort to highlight the developmental toxicity of NP in amphibians, we examined the effects of NP on the embryonic survival, tadpole growth, melanophore development and metamorphosis of a native Korean amphibian species, Bombina orientalis (Anura). When treated to fertilized eggs, 1 μM NP significantly decreased embryonic survival at 48 h post fertilization (p.f.), suggesting that 1 μM NP can exert systemic toxicity in B. orientalis embryos. In the surviving embryos, there were no significant differences in malformation rates between NP-treated embryos and controls at 240 h p.f., suggesting no or low teratogenicity of NP in B. orientalis embryos. Below LC(50) NP significantly decreased body growth and development of melanophores at 0.1 μM, suggesting that NP far below the LC(50) targets multiple developmental events in tadpoles of this frog species. In metamorphosis assay using the premetamorphic tadpoles (corresponding to Nieuwkoop Faber stage 53 in Xenopus laevis) exogenous 3,5,3'-triiodothyronine (T3)-induced tail resorption was significantly decreased by 1 μM NP. However, NP (0.1 and 1 μM)-only treatment did not affected total body T3 and T4 levels, suggesting that NP at tested concentrations inhibits thyroid hormones action but not the synthesis of hormones during metamorphosis.