Characterization of the mechanistic linkages between iodothyronine deiodinase inhibition and impaired thyroid-mediated growth and development in Xenopus laevis using iopanoic acid

Type I Iodothyronine Deiodinase Functions in Thyroid Hormone Synthesis and Growth Regulation in Bivalve

Bivalve breeding programs account for over 50% of global marine aquaculture, and growth is one of the most important traits of them. With the purpose of investigating the key genes related to growth performance of Yesso scallop (Patinopecten yessoensis), an economically important bivalve, we conducted transcriptome comparison between fast- and slow-growing individuals, and identified 1,531 differentially expressed genes (DEGs), with 656 up-regulated in fast-growing scallops, categorizing into six molecular function terms, of which the thyroxine 5'-deiodinase activity term exhibited the lowest p values. This term contained only two genes, and they were both type I iodothyronine deiodinase (D1), the enzyme catalyzing triiodothyronine (T3) production in vertebrates. Meanwhile, T3 concentration was significantly higher in fast-growing scallops. To further understand D1 functions in bivalve growth, RNA interference (RNAi) was performed in the dwarf surf clam (Mulinia lateralis), a suitable bivalve for verifying the function of growth-related genes. Suppression of MlD1-1 and MlD1-2 expression was detected after RNAi, and in MlD1-1 RNAi group, dwarf surf clams exhibited significant reductions in growth, with shell length, shell width, shell height, and body weight decreasing by 9%, 9%, 9%, and 25%, respectively. Furthermore, the triiodothyronine (T3) and thyroxine (T4) levels of clams in MlD1-1 RNAi group were significantly decreased by 11% and increased by 5%, respectively, while no changes were observed in the MlD1-2 RNAi group. Our data imply that bivalve D1 could be a candidate gene for the breeding of fast-growing bivalve.

Testing the sensitivity of the medaka Transgenic Eleuthero-embryonic THYroid-Specific assay (TETHYS) to different mechanisms of action

There are many concerns about the impacts of Endocrine-Disrupting Chemicals on both wildlife and human populations. A plethora of chemicals have been shown to interfere with the Hypothalamic-Pituitary-Thyroid (HPT) axis in vertebrates. Disruption of the HPT axis is one of main endocrine criteria considered for the regulation of chemicals, along with the estrogen axis, androgen axis and steroidogenesis (EATS). In response to these concerns, the Organization for Economic Cooperation and Development (OECD) initiated the validation of test guidelines (TGs) covering the EATS modalities. Regarding thyroid activity and/or disruption assessment, three OECD TGs are validated, all of them using amphibians. To date, no OECD TGs based on fish are available for the detection of Thyroid Active Chemicals (TACs). To fill this gap, we developed a new test for the detection of TACs, the TETHYS assay (Transgenic Eleuthero-embryonic THYroid-Specific assay). This assay uses a medaka (Oryzias latipes) transgenic line Tg(tg:eGFP) expressing Green Fluorescent Protein in the thyroid follicles, under the control of the thyroglobulin promoter. This assay is performed at eleuthero-embryonic life-stages with an exposure length of 72 h. In the present study, the following reference chemicals with known thyroid hormone system mechanism of action have been tested: methimazole, sodium perchlorate, sodium tetrafluoroborate, diclofenac, iopanoic acid, sobetirome, NH-3 and 1-850. Except for the thyroid receptor antagonists, all chemicals tested were identified as thyroid active, modifying the total fluorescence and the size of the thyroid follicles. To investigate the test specificity, we tested three chemicals presumed to be inert on the HPT axis: cefuroxime, abamectin and 17α-ethinylestradiol. All were found to be inactive in the TETHYS assay. This promising New Approach Methodology can serve as a foundation for the development of a new OECD TG in the frame of regulatory assessment of chemicals for thyroid activity.

Gene biomarkers for the assessment of thyroid-disrupting activity in zebrafish embryos

Active ingredients of pesticides or biocides and industrial chemicals can negatively affect environmental organisms, potentially endangering populations and ecosystems. European legislation mandates that chemical manufacturers provide data for the environmental risk assessment of substances to obtain registration. Endocrine disruptors, substances that interfere with the hormone system, are not granted marketing authorization due to their adverse effects. Current methods for identifying disruptors targeting the thyroid hormone system are costly and require many amphibians. Consequently, alternative methods compliant with the 3R principle (replacement, reduction, refinement) are essential to prioritize risk assessment using reliable biomarkers at non-protected life stages. Our study focused on detecting robust biomarkers for thyroid-disrupting mechanisms of action (MoA) by analyzing molecular signatures in zebrafish embryos induced by deiodinase inhibitor iopanoic acid and thyroid peroxidase inhibitor methimazole. We exposed freshly fertilized zebrafish eggs to these compounds, measuring lethality, hatching rate, swim bladder size and transcriptomic responses. Both compounds significantly reduced swim bladder size, aligning with prior findings. Transcriptome analysis revealed specific molecular fingerprints consistent with the MoA under investigation. This analysis confirmed regulation directions seen in other studies involving thyroid disruptors and allowed us to identify genes like tg, scl2a11b, guca1d, cthrc1a, si:ch211-226h7.5, soul5, nnt2, cox6a2 and mep1a as biomarker genes for thyroid disrupting MoA in zebrafish embryos as per OECD test guideline 236. Future screening methods based on our findings will enable precise identification of thyroid-related activity in chemicals, promoting the development of environmentally safer substances. Additionally, these biomarkers could potentially be incorporated into legally mandated chronic toxicity tests in fish, potentially replacing amphibian tests for thyroid disruption screening in the future.

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.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Deiodinase inhibition impairs the formation of the three posterior swim bladder tissue layers during early embryonic development in zebrafish

Thyroid hormone system disruption (THSD) negatively affects multiple developmental processes and organs. In fish, inhibition of deiodinases, which are enzymes crucial for (in)activating thyroid hormones (THs), leads to impaired swim bladder inflation. Until now, the underlying mechanism has remained largely unknown. Therefore, the objective of this study was to identify the process during swim bladder development that is impacted by deiodinase inhibition. Zebrafish embryos were exposed to 6 mg/L iopanoic acid (IOP), a model deiodinase inhibitor, during 8 different exposure windows (0-60, 60-120, 24-48, 48-72, 72-96, 96-120, 72-120 and 0-120 h post fertilization (hpf)). Exposure windows were chosen based on the three stages of swim bladder development: budding (24-48 hpf), pre-inflation, i.e., the formation of the swim bladder tissue layers (48-72 hpf), and inflation phase (72-120 hpf). Exposures prior to 72 hpf, during either the budding or pre-inflation phase (or both), impaired swim bladder inflation, while exposure during the inflation phase did not. Based on our results, we hypothesize that DIO inhibition before 72 hpf leads to a local decrease in T3 levels in the developing swim bladder. Gene transcript analysis showed that these TH level alterations disturb both Wnt and hedgehog signaling, known to be essential for swim bladder formation, eventually resulting in impaired development of the swim bladder tissue layers. Improper development of the swim bladder impairs swim bladder inflation, leading to reduced swimming performance. This study demonstrates that deiodinase inhibition impacts processes underlying the formation of the swim bladder and not the inflation process, suggesting that these processes primarily rely on maternal rather than endogenously synthetized THs since TH measurements showed that THs were not endogenously synthetized during the sensitive period.

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.

Combining In Vitro and In Silico New Approach Methods to Investigate Type 3 Iodothyronine Deiodinase Chemical Inhibition Across Species

New approach methodologies (NAMs) are being developed to reduce and replace vertebrate animal testing in support of ecotoxicology and risk assessment. The US Environmental Protection Agency's Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) bioinformatic tool was used to evaluate amino acid sequence conservation of the type 3 iodothyronine deiodinase (DIO3) enzyme across species to demonstrate NAM applications for understanding effects of chemical interactions with a specific protein target. Existing literature was used to identify critical amino acids for thyroid hormone binding and interaction with a reducing cofactor. The SeqAPASS tool identifies whether known critical amino acids involved in ligand binding are exact, partial, or not matches across species compared with a template species based on molecular weight and side chain classification. This evaluation guided the design of variant proteins representing critical amino acid substitutions found in various species. Site-directed mutagenesis of the wild-type (WT) human DIO3 gene sequence was used to create six variant proteins expressed in cell culture, which were then tested in vitro for chemical inhibition. Significant differences in in vitro median inhibitory concentration results were observed among variants for potential competitive inhibitors. A molecular model representing the WT human DIO3 was constructed using Molecular Operating Environment (MOE) software and mutated in silico to create the six variants. The MOE Site Finder tool identified the proposed catalytic and cofactor sites and potential alternative binding sites. Virtual docking did not provide affinity scores with sufficient resolution to rank the potency of the chemical inhibitors. Chemical characteristics, function and location of substituted amino acids, and complexities of the protein target are important considerations in developing NAMs to evaluate chemical susceptibility across species. Environ Toxicol Chem 2023;42:1032-1048. © 2023 University of Wisconsin-Madison. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Amphibian toxicity testing for identification of thyroid disrupting chemicals

Both amphibian metamorphosis assay (AMA) and larval amphibian growth and development assay (LAGDA) can detect thyroid-mediated modality and adversity on the basis of morphological changes during the thyroid hormone-dependent metamorphosis. They are used for identification of thyroid hormone system disrupting chemicals (TDCs) for non-target organisms or the environment. The EU Guidance recommends that the AMA and the LAGDA should be used to address sufficient investigation of the thyroid-mediated modality and adversity, respectively. In the EU discussions over identification of TDCs, the necessity of using LAGDA as a follow-up of positive results of the AMA has been questioned because of the overlap between the endpoints and the exposure of both tests. This study analyzed similarities, differences, and sensitivity of these two assays in detection of TDCs. For agonists and most of antagonists of the hypothalamic-pituitary-thyroid (HPT) axis, both AMA and LAGDA can detect the thyroid-mediated modality and adversity. The LAGDA, as a follow-up of the positive results of the AMA, may not be needed because the results of AMA are considered enough for identification of TDCs. For chemicals like inhibitors of iodotyrosine deiodinase, the LAGDA is considered necessary for identification of TDCs because the thyroid-mediated adversity cannot be detected until Nieuwkoop and Faber (NF) stage 62. Incorporation of mechanistic endpoints into existing test guidelines and the use of Xenopus Eleutheroembryo Thyroid Assay (XETA), extended amphibian metamorphosis assay (EAMA) and adverse outcome pathways (AOPs) for testing and identification of TDCs are further discussed.