Establishment of yeast reporter assay systems to detect ligands of thyroid hormone receptors α and β

Highly effective degradation of ibuprofen by alkaline metal-doped copper oxides via peroxymonosulfate activation: Mechanisms, degradation pathway and toxicity assessments

Redox ratios of Cu2+/Cu+ and adsorbed oxygen species (Oads) have shown great activity toward radical generation by activating peroxymonosulfate (PMS). Herein, different alkaline metal oxides (CaO, MgO and BaO) and various amounts of CaO are incorporated into CuO, which could tune the main active sites of redox ratios of Cu2+/Cu+ and Oads. The results show that CaO-CuO-5% exhibits the outstanding performance of PMS activation toward ibuprofen (IBF) degradation with excellent kinetics (k = 0.812 min-1). The X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculation show that the CaO-CuO-5% has the higher electron density with superior electron transfer ability and lower PMS adsorption energy. Based on radical scavengers and electron paramagnetic resonance spectrometer (EPR), a nonradical process is proposed to play the dominant role. The degradation pathway and the corresponding toxicity of degraded intermediates with residue PMS after reaction is evaluated by LC-MS/MS and bioassay experiments, indicating the lower antagonistic influence on human hormone receptors after advanced oxidation process. Mitigation of the Cu leaching with cyclic stability can be achieved. This study provides a facile method to optimize high-performance catalysts to activate PMS and offer practical environmental applications in the remediation of emerging contaminants.

A Novel Transgenic Model to Study Thyroid Axis Activity in Early Life Stage Medaka

Identifying endocrine disrupting chemicals in order to limit their usage is a priority and required according to the European Regulation. There are no Organization for Economic Co-operation and Development (OECD) test guidelines based on fish available for the detection of Thyroid axis Active Chemicals (TACs). This study aimed to fill this gap by developing an assay at eleuthero-embryonic life stages in a novel medaka (Oryzias latipes) transgenic line. This transgenic line expresses green fluorescent protein (GFP) in thyrocytes, under the control of the medaka thyroglobulin gene promoter. The fluorescence expressed in the thyrocytes is inversely proportional to the thyroid axis activity. When exposed for 72 h to activators (triiodothyronine (T3) and thyroxine (T4)) or inhibitors (6-N-propylthiouracil (PTU), Tetrabromobisphenol A (TBBPA)) of the thyroid axis, the thyrocytes can change their size and express lower or higher levels of fluorescence, respectively. This reflects the regulation of thyroglobulin by the negative feedback loop of the Hypothalamic-Pituitary-Thyroid axis. T3, T4, PTU, and TBBPA induced fluorescence changes with the lowest observable effect concentrations (LOECs) of 5 μg/L, 1 μg/L, 8 mg/L, and 5 mg/L, respectively. This promising tool could be used as a rapid screening assay and also to help decipher the mechanisms by which TACs can disrupt the thyroid axis in medaka.

Development of a yeast reporter gene assay to detect ligands of freshwater cladoceran Daphnia magna ultraspiracle, a homolog of vertebrate retinoid X receptors

Endocrine-disrupting chemicals (EDCs) often affect homeostatic regulation in living organisms by directly acting on nuclear receptors (NRs). Retinoid X receptors (RXRs), the most highly conserved members of the NR superfamily during evolution, function as partners to form heterodimers with other NRs, such as retinoic acid, thyroid hormone, and vitamin D3 receptors. RXRs also homodimerize and induce the expression of target genes upon binding with their natural ligand, 9-cis-retinoic acid (9cRA), and typical EDCs organotin compounds, such as tributyltin and triphenyltin. In the present study, we established a new yeast reporter gene assay (RGA) to detect the ligands of freshwater cladoceran Daphnia magna ultraspiracle (Dapma-USP), a homolog of vertebrate RXRs. D. magna has been used as a representative crustacean species for aquatic EDC assessments in the Organization for Economic Corporation and Development test guidelines. Dapma-USP was expressed along with the Drosophila melanogaster steroid receptor coactivator Taiman in yeast cells carrying the lacZ reporter plasmid. The RGA for detecting agonist activity of organotins and o-butylphenol was improved by use of mutant yeast strains lacking genes encoding cell wall mannoproteins and/or plasma membrane drug efflux pumps as hosts. We also showed that a number of other human RXR ligands, phenol and bisphenol A derivatives, and terpenoid compounds such as 9c-RA exhibited antagonist activity on Dapma-USP. Our newly established yeast-based RGA system is valuable as the first screening tool to detect ligand substances for Dapma-USP and for evaluating the evolutionary divergence of the ligand responses of RXR homologs between humans and D. magna.

In silico profiling of endocrine-disrupting potential of bisphenol analogues and their halogenated transformation products

Due to its endocrine-disrupting properties, bisphenol A (BPA) is being phased out from plastics, thermal paper and epoxy resins, and its replacements are being introduced into the market. Bisphenols are released into the environment, where they can undergo halogenation. Unlike BPA, the endocrine-disrupting potential of BPA analogues and their halogenated transformation products has not been extensively studied. The aim of this study was to evaluate the endocrine-disrupting potential of 18 BPA analogues and their halogenated derivatives by calculating affinities for 14 human nuclear receptors utilizing the Endocrine Disruptome and VirtualToxLab™ in silico tools. Our simulations identified AR, ERs, and GR as the most favorable targets of bisphenols and their derivatives. Several BPA analogues displayed a higher predicted potential for endocrine disruption than BPA. Our models highlighted BPZ and BPPH as the most hazardous in terms of predicted endocrine activities. Halogenation, in general, was predicted to increase the binding affinity of bisphenols for AR, ERβ, MR, GR, PPARγ, and TRβ. Notably, mono- or 2,2'-di-halogenated bisphenols exhibited the highest potential for endocrine disruption. In vitro corroboration of the obtained results should be the next milestone in evaluating the safety of BPA substitutes and their halogenated transformation products.

Detection of juvenile hormone agonists by a new reporter gene assay using yeast expressing Drosophila methoprene-tolerant

Juvenile hormones (JHs) are sesquiterpenoids that play important roles in the regulation of growth, metamorphosis, and reproduction in insects. Synthetic JH agonists (JHAs) have been used as insecticides and are categorized as a class of insect growth regulators (IGRs). Natural JHs and synthetic JHAs bind to the JH receptor methoprene‐tolerant (Met), which forms a functional JH‐receptor complex with steroid receptor coactivators, such as Drosophila melanogaster Taiman (Tai). The ligand‐bound Met–Tai complex induces the transcription of JH response genes by binding to specific DNA elements referred to as JH response elements (JHREs). In the present study, we established a reporter gene assay (RGA) for detecting natural JHs and synthetic JHAs in a yeast strain expressing D. melanogaster Met and Tai. The yeast RGA system detected various juvenoid ligands in a dose‐dependent manner. The rank order of the ligand potencies of the juvenoids examined in the yeast RGA linearly correlated with those of RGAs for Met–Tai established in mammalian and insect cells. Our new yeast RGA is rapid, easy to handle, cost‐effective, and valuable for screening novel JHAs.

In vitro and in vivo screening for environmentally friendly benzophenone-type UV filters with beneficial tyrosinase inhibition activity

Benzophenones (BPs) are a group of chemically similar organic compounds commonly used in formulations of sunscreen and other personal care products as UV filters to protect our skin against sunlight overexposure. Studies have shown that the occurrence of certain BPs (e.g., BP-3 and its metabolite BP-1) in multiple environmental matrices may increase the incidence of coral planulae bleaching and estrogenic effects on aquatic life. Currently, most BPs are not yet comprehensively screened in vitro and in vivo for their ecotoxicity under environmentally relevant concentrations. This study systematically assessed the in vitro and in vivo toxicity and activity of the 7 most commonly used BPs (BP-1, BP-2, BP-3, BP-4, BP-6, BP-7 and BP-8) to select BP alternatives with lower ecotoxicity and extra beneficial functions. BP-2 (LC₅₀ = 18.43 µM) was least toxic and BP-3 (LC₅₀ = 4.10 µM) and BP-8 (LC₅₀ =1.62 µM) were less and most toxic, respectively, in terms of 96-hr acute mortality of medaka larvae. BP-2 at environmentally relevant concentrations (5-50 nM) did not significantly alter locomotion and oxidative stress responses of medaka larvae from 24-hr to 7-day exposure, whereas BP-3 and BP-8 at 5 nM induced hypoactivity or changed fish swimming angles. Only BP-2 was able to inhibit in vitro mushroom tyrosinase activity, with EC₅₀ value 19.7 µM. Also, BP-2 could effectively suppress melanin formation and tyrosinase activity in zebrafish embryos. Among the 7 tested BPs, BP-2 was the least toxic and the most environmentally friendly UV filter with extra benefit for tyrosinase inhibition and could be a promising alternative to the use of toxic BPs.

Shining light on molecular communication

Molecules and combinations of molecules are the natural communication currency of microbes; microbes have evolved and been engineered to sense a variety of compounds, often with exquisite sensitivity. The availability of microbial biosensors, combined with the ability to genetically engineer biological circuits to process information, make microbes attractive bionanomachines for propagating information through molecular communication (MC) networks. However, MC networks built entirely of biological components suffer a number of limitations. They are extremely slow due to processing and propagation delays and must employ simple algorithms due to the still limited computational capabilities of biological circuits. In this work, we propose a hybrid bio-electronic framework which utilizes biological components for sensing but offloads processing and computation to traditional electronic systems and communication infrastructure. This is achieved by using tools from the burgeoning field of optogenetics to trigger biosensing through an optoelectronic interface, alleviating the need for computation and communication in the biological domain.

Construction of reporter gene assays using CWP and PDR mutant yeasts for enhanced detection of various sex steroids

Background

Sex steroid hormone receptors are classified into three classes of receptors: estrogen receptors (ER) α and β, androgen receptor (AR), and progesterone receptor (PR). They belong to the nuclear receptor superfamily and activate their downstream genes in a ligand-dependent manner. Since sex steroid hormones are involved in a wide variety of physiological processes and cancer development, synthetic chemical substances that exhibit sex steroid hormone activities have been applied as pharmaceuticals and consumed in large amounts worldwide. They are potentially hazardous contaminants as endocrine disruptors in the environment because they may induce inappropriate gene expression mediated by sex steroid hormone receptors in vivo.

Results

To develop simple reporter gene assays with enhanced sensitivity for the detection of sex steroid hormones, we newly established mutant yeast strains lacking the CWP and PDR genes encoding cell wall mannoproteins and plasma membrane drug efflux pumps, respectively, and expressing human ERα, ERβ, AR, and PR. Reporter gene assays with mutant yeast strains responded to endogenous and synthetic ligands more strongly than those with wild-type strains. Sex steroid hormone activities in some pharmaceutical oral tablets and human urine were also detectable in these yeast assays.

Conclusions

Yeast reporter gene assay systems for all six steroid hormone receptors, including previously established glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) assay yeasts, are now available. Environmental endocrine disrupters with steroid hormone activity will be qualitatively detectable by simple and easy procedures. The yeast-based reporter gene assay will be valuable as a primary screening tool to detect and evaluate steroid hormone activities in various test samples. Our assay system will strongly support the detection of agonists, antagonists, and inverse agonists of steroid hormone receptors in the field of novel drug discovery and assessments of environmental pollutants.

Environmental Pollutants Modulate Transcriptional Activity of Nuclear Receptors of Whales In Vitro

This study reports the transcriptional activity of fin (Balaenoptera physalus) and blue whale (Balaenoptera musculus) peroxisome proliferator-activated receptor γ (PPARG), glucocorticoid receptor (GR), and thyroid hormone receptor β (THRB), when exposed to 14 persistent organic pollutants (so-called "legacy" persistent organic pollutants (POPs)) and a synthetic mixture of POPs, using GAL4-UAS-based in vitro luciferase reporter gene assays. Polychlorinated biphenyls (PCBs) had both agonistic and antagonistic effects on PPARG and GR, and mainly antagonistic, except for PCB153, effects on THRB. 1,1,1-Trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and its metabolites had mainly antagonistic effects on all of the receptors, except for o,p'-DDT. Given that the ligand-binding domain (LBD) of PPARG is the same in killer whales, white whales, polar bears, and humans, and that GR-LBD is identical in killer whales and minke whales and that the LBD of THRB is the same in killer whales, white whales, and humans, it is likely that the results of this study are representative for these other species as well. It is important to note that several environmental pollutants modulated the transcriptional activity of tested nuclear receptors at environmentally relevant concentrations for whales.

Assessing the endocrine disrupting potentials and genotoxicity in environmental samples from Taiwanese rivers

Background

Surface waters receive a variety of organic pollutants via wastewater discharge, and sediment represents a sink for hydrophobic contaminants. In this study, we used in vitro yeast-based reporter gene assays and a Bacillus subtilis Rec-assay to examine the occurrence of endocrine disrupting activities and genotoxic potentials in samples collected from three Taiwanese rivers. Levels of 51 polycyclic aromatic hydrocarbons (PAHs) in muscles of fish captured from same rivers were also analyzed to assess in vivo pollution of PAHs.

Results

Antagonist activities for androgen receptor and retinoid X receptor (RXR) were detected in river water extracts at environmentally relevant concentrations., and sediment extracts exhibited RXR agonist, RXR antagonist, and genotoxic potentials concurrently. Σ16 PAHs in fish muscles ranged from 44.9–242.4 ng g^− 1 dry weight, representing 38 to 59% of the total 51 PAHs concentrations, and methylated PAHs of low molecular weight PAHs were often detected as well.

Conclusion

Taiwanese river sediment samples concomitantly exhibited RXR disrupting potentials and genotoxic activities, whereas RXR agonist and antagonist activities were simultaneously detected in several dry-season sediment extracts. PAH levels in fish muscles were categorized as minimally polluted by aromatic compounds, nonetheless, the presence of methylated PAHs in muscles samples may be of concern owing to the higher toxic potentials than their parent compounds.

New reporter gene assays for detecting natural and synthetic molting hormone agonists using yeasts expressing ecdysone receptors of various insects

Synthetic nonsteroidal ecdysone agonists, a class of insect growth regulators (IGRs), target the ecdysone receptor (EcR), which forms a heterodimer with ultraspiracle (USP) to transactivate ecdysone response genes. These compounds have high binding affinities to the EcR–USP complexes of certain insects and their toxicity is selective for certain taxonomic orders. In the present study, we developed reporter gene assay (RGA) systems to detect molting hormone (ecdysone) activity by introducing EcR–USP cDNA and a bacterial lacZ reporter gene into yeast. EcR and USP were derived from the insect species of three different taxonomic orders: Drosophila melanogaster (Diptera), Chilo suppressalis (Lepidoptera), and Leptinotarsa decemlineata (Coleoptera). Transcriptional coactivator taiman (Tai) cDNA cloned from D. melanogaster was also used in this RGA system. This yeast RGA system responded to various EcR ligands in a dose‐dependent and ecdysteroid‐specific manner. Furthermore, the insect order‐selective ligand activities of synthetic nonsteroidal ecdysone agonists were linearly related to their binding activities, which were measured against in vitro translated EcR–USP complexes. Our newly established yeast RGA is useful for screening new molting hormone agonists that work selectively on target insects.

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.

Detection of endocrine active substances in the aquatic environment in southern Taiwan using bioassays and LC-MS/MS

Endocrine active substances, including naturally occurring hormones and various synthetic chemicals have received much concern owing to their endocrine disrupting potencies. It is essential to monitor their environmental occurrence since these compounds may pose potential threats to biota and human health. In this study, yeast-based reporter assays were carried out to investigate the presence of (anti-)androgenic, (anti-)estrogenic, and (anti-)thyroid compounds in the aquatic environment in southern Taiwan. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was also used to measure the environmental concentrations of selected endocrine active substances for assessing potential ecological risks and characterizing contributions to the endocrine disrupting activities. Bioassay results showed that anti-androgenic (ND-7489 μg L(-1) flutamide equivalent), estrogenic (ND-347 ng L(-1) 17β-estradiol equivalent), and anti-thyroid activities were detected in the dissolved and particulate phases of river water samples, while anti-estrogenic activities (ND-10 μg L(-1) 4-hydroxytamoxifen equivalent) were less often found. LC-MS/MS analysis revealed that anti-androgenic and estrogenic contaminants, such as bisphenol A, triclosan, and estrone were frequently detected in Taiwanese rivers. In addition, their risk quotient values were often higher than 1, suggesting that they may pose an ecological risk to the aquatic biota. Further identification of unknown anti-androgenic and estrogenic contaminants in Taiwanese rivers may be necessary to protect Taiwan's aquatic environment.

Freeze-drying as suitable method to achieve ready-to-use yeast biosensors for androgenic and estrogenic compounds

Recombinant yeast assays (RYAs) have been proved to be a suitable tool for the fast screening of compounds with endocrine disrupting activities. However, ready-to-use versions more accessible to less equipped laboratories and field studies are scarce and far from optimal throughputs. Here, we have applied freeze-drying technology to optimize RYA for the fast assessment of environmental compounds with estrogenic and androgenic potencies. The effects of different cryoprotectants, initial optical density and long-term storage were evaluated. The study included detailed characterization of sensitivity, robustness and reproducibility of the new ready-to-use versions, as well as comparison with the standard assays. Freeze-dried RYAs showed similar dose-responses curves to their homolog standard assays, with Lowest Observed Effect Concentration (LOEC) and Median effective Concentration (EC50) of 1 nM and 7.5 nM for testosterone, and 0.05 nM and 0.5 nM for 17β-estradiol, respectively. Freeze-dried cells stored at 4 °C retained maximum sensitivity up to 2 months, while cells stored at -18 °C showed no decrease in sensitivity throughout the study (10 months). This ready-to-use RYA is easily accessible and may be potentially used for on-site applications.

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

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

Benzotriazole ultraviolet stabilizers show potent activities as human aryl hydrocarbon receptor ligands

Benzotriazole ultraviolet stabilizers (BUVSs) used in consumer products are raising concerns as new pollutants in the aquatic environment. We determined the agonistic activities of eight BUVSs and a chemically distinct UV absorber (4-methylbenzylidinecamphor) toward the human aryl hydrocarbon receptor (AhR) and thyroid hormone receptors alpha and beta. Although none of the BUVSs showed ligand activity against the thyroid hormone receptors, four of them (UV-P, UV-9, UV-326, and UV-090) showed significant AhR ligand activity. Their half-maximal effective concentrations (EC50) were 130 nM for UV-P, 460 nM for UV-9, and 5.1 μM for UV-090 (a value for UV-326 could not be determined). Of the numerous AhR ligands, it is well-known that those considered nontoxic are quickly metabolized by enzymes such as CYP1A1, which destroys their ability to function as ligands. Accordingly, we established a new yeast assay for simultaneous monitoring of both the strength of AhR ligand activity and ligand degradation by CYP1A1. We found the AhR ligand activities of the above four BUVSs to be stable in the presence of CYP1A1; therefore, they have the potential to accumulate and exert potent physiological effects in humans, analogous to polycyclic aromatic hydrocarbons and dioxins, which are known stable and toxic ligands.

Occurrence of xenobiotic ligands for retinoid X receptors and thyroid hormone receptors in the aquatic environment of Taiwan

Various synthetic compounds are frequently discharged into the environment via human activities. Among them, certain contaminants may disrupt normal physiological functions of wildlife and humans via interactions with nuclear receptors. To protect human health and the environment, it is important to detect environmental ligands for human nuclear receptors. In this study, yeast-based reporter gene assays were used to investigate the occurrence of xenobiotic ligands for retinoid X receptors (RXR) and thyroid hormone receptors (TR) in the aquatic environment of Taiwan. Experimental results revealed that RXR agonist/antagonist activity was detected in river water and sediment samples. In particular, high RXR agonist/antagonist activity was found in the samples collected near river mouths. Additionally, few samples also elicited significant TR antagonist activity. Our findings show that the aquatic environment of Taiwan was contaminated with RXR and TR ligands. Further study is necessary to identify these xenobiotic RXR and TR agonists and antagonists.

Development of yeast reporter assay for screening specific ligands of retinoic acid and retinoid X receptor subtypes

INTRODUCTION

Retinoic acids are essential for embryonic development, tissue organization, and homeostasis and act via retinoic acid receptors (RARs) that form heterodimers with retinoid X receptors (RXRs). Human RARs and RXRs include the three subtypes α, β, and γ, which have varying distributions and physiological functions among human tissues. Recent reports show that subtype-specific binding of several chemicals to RARs or RXRs may lead to endocrine disruption. To evaluate these ligand-like chemicals, convenient assay systems for each receptor subtype are required.

METHODS

We developed reporter assay yeasts to screen ligands for RXR subtype receptor homodimers. To screen RAR ligands, yeasts were engineered to express RAR subtypes with defective RXRα, which fails to bind to coactivators because of its shortened c-terminus.

RESULTS

These assay yeasts were validated using known RXR- and RAR-specific ligands and subtype-specific responses were clearly shown. Subtype-specific ligand activities of the suspected chemical RAR or RXR ligands o-t-butylphenol, triphenyltin chloride, tributyltin chloride, and 4-nonylphenol were determined.

DISCUSSION

The present assay yeasts may be valuable tools for subtype-specific assessments of unidentified environmental ligand chemicals and receptor-specific pharmaceuticals.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.