Modeling mixtures of thyroid gland function disruptors in a vertebrate alternative model, the zebrafish eleutheroembryo

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.

Mixture Concentration-Response Modeling Reveals Antagonistic Effects of Estradiol and Genistein in Combination on Brain Aromatase Gene (cyp19a1b) in Zebrafish

Comprehension of compound interactions in mixtures is of increasing interest to scientists, especially from a perspective of mixture risk assessment. However, most of conducted studies have been dedicated to the effects on gonads, while only few of them were. interested in the effects on the central nervous system which is a known target for estrogenic compounds. In the present study, the effects of estradiol (E2), a natural estrogen, and genistein (GEN), a phyto-estrogen, on the brain ER-regulated cyp19a1b gene in radial glial cells were investigated alone and in mixtures. For that, zebrafish-specific in vitro and in vivo bioassays were used. In U251-MG transactivation assays, E2 and GEN produced antagonistic effects at low mixture concentrations. In the cyp19a1b-GFP transgenic zebrafish, this antagonism was observed at all ratios and all concentrations of mixtures, confirming the in vitro effects. In the present study, we confirm (i) that our in vitro and in vivo biological models are valuable complementary tools to assess the estrogenic potency of chemicals both alone and in mixtures; (ii) the usefulness of the ray design approach combined with the concentration-addition modeling to highlight interactions between mixture components.

Assessment of synergistic thyroid disrupting effects of a mixture of EDCs in ovariectomized rats using factorial analysis and dose addition

Endocrine disrupting chemicals (EDCs) have been implicated in a broad spectrum of health problems related to reproduction, thyroid function, neurodevelopment, and metabolism. In many cases, EDCs in the environment are at extremely low concentrations which rarely induce health problems alone, however, a mixture of these EDCs may interact and induce potential additive and synergistic effects. Many mixture studies on EDCs were conducted in terms of high doses with the direct effect addition method, which didn't comply with the dose-response relationship of toxicants in the "S" or "U" shaped curves. In the present study, the thyroid disrupting effects of a mixture of three EDCs, propylthiouracil (PTU), polychlorinated biphenyls (PCBs), and ammonium perchlorate (AP), were measured in an ovariectomized rat model. Sixty female SD rats were ovariectomized bilaterally and randomly assigned to ovariectomization (OVX) control, PTU + PCBs, PTU + AP, PCBs + AP and PTU + PCBs + AP groups treated with doses at lowest observed adverse effect levels (LOAELs) or benchmark dose lower limits (BMDLs) obtained from our previous dose-response relationship studies. OVX control animals were treated with vehicle control while all other animals were treated with different combinations of EDCs by gavage for 8 days. The body weight change, serum total thyroxine (tT₄), triiodothyroxine (tT₃), the thyroid/body weight ratio, and thyroid histopathological endpoints were measured and analyzed using factorial analysis and dose addition. All EDC treated groups showed a marked change compared to vehicle control in serum tT₄, the thyroid/body weight ratio, and the thyroid epithelium/colloid ratio. Both factorial analysis and dose addition analysis showed a synergistic effect on thyroid function by PTU, PCBs and AP together, but the modes of interaction varied when either two were mixed at LOAELs. To conclude, a mixture of PTU, PCBs, and AP mainly acted synergistically on thyroid function and induced a significant health effect.

The fish embryo test (FET): origin, applications, and future

Originally designed as an alternative for the acute fish toxicity test according to, e.g., OECD TG 203, the fish embryo test (FET) with the zebrafish (Danio rerio) has been optimized, standardized, and validated during an OECD validation study and adopted as OECD TG 236 as a test to assess toxicity of embryonic forms of fish. Given its excellent correlation with the acute fish toxicity test and the fact that non-feeding developmental stages of fish are not categorized as protected stages according to the new European Directive 2010/63/EU on the protection of animals used for scientific purposes, the FET is ready for use not only for range-finding but also as a true alternative for the acute fish toxicity test, as required for a multitude of national and international regulations. If-for ethical reasons-not accepted as a full alternative, the FET represents at least a refinement in the sense of the 3Rs principle. Objections to the use of the FET have mainly been based on the putative lack of biotransformation capacity and the assumption that highly lipophilic and/or high molecular weight substances might not have access to the embryo due to the protective role of the chorion. With respect to bioactivation, the only substance identified so far as not being activated in the zebrafish embryo is allyl alcohol; all other biotransformation processes that have been studied in more detail so far were found to be present, albeit, in some cases, at lower levels than in adult fish. With respect to larger molecules, the extension of the test duration to 96 h (i.e., beyond hatch) has-at least for the substances tested so far-compensated for the reduced access to the embryo; however, more research is necessary to fully explore the applicability of the FET to substances with a molecular weight >3 kDa as well as substances with a neurotoxic mode of action. An extension of the endpoints to also cover sublethal endpoints makes the FET a powerful tool for the detection of teratogenicity, dioxin-like activity, genotoxicity and mutagenicity, neurotoxicity, as well as various forms of endocrine disruption.

In vivo zebrafish assays for analyzing drug toxicity

INTRODUCTION

Off-target effects represent one of the major concerns in the development of new pharmaceuticals, requiring large-scale animal toxicity testing. Faster, cheaper and more reliable assays based on zebrafish embryos (ZE) are being developed as major tools for assessing toxicity of chemicals during the drug-discovery process.

AREAS COVERED

This paper reviews techniques aimed to the analysis of in vivo sublethal toxic effects of drugs on major physiological functions, including the cardiovascular, nervous, neuromuscular, gastrointestinal and thyroid systems among others. Particular emphasis is placed on high-throughput screening techniques (HTS), including robotics, imaging technologies and image-analysis software.

EXPERT OPINION

The analysis of off-target effects of candidate drugs requires systemic analyses, as they often involve the complete organism rather than specific, tissue- or cell-specific targets. The unique physical and physiological characteristics of ZE make this system an essential tool for drug discovery and toxicity assessment. Different HTS methodologies applicable to ZE allow the screening of large numbers of different chemicals for many diverse and relevant toxic endpoints.

Involvement of purinergic system in inflammation and toxicity induced by copper in zebrafish larvae

The use of zebrafish (Danio rerio) is increasing as an intermediate preclinical model, to prioritize drug candidates for mammalian testing. As the immune system of the zebrafish is quite similar to that of mammals, models of inflammation are being developed for the screening of new drugs. The characterization of these models is crucial for studies that seek for mechanisms of action and specific pharmacological targets. It is well known that copper is a metal that induces damage and cell migration to hair cells of lateral line of zebrafish. Extracellular nucleotides/nucleosides, as ATP and adenosine (ADO), act as endogenous signaling molecules during tissue damage by exerting effects on inflammatory and immune responses. The present study aimed to characterize the inflammatory status, and to investigate the involvement of the purinergic system in copper-induced inflammation in zebrafish larvae. Fishes of 7 days post-fertilization were exposed to 10 μM of copper for a period of 24 h. The grade of oxidative stress, inflammatory status, copper uptake, the activity and the gene expression of the enzymes responsible for controlling the levels of nucleotides and adenosine were evaluated. Due to the copper accumulation in zebrafish larvae tissues, the damage and oxidative stress were exacerbated over time, resulting in an inflammatory process involving IL-1β, TNF-α, COX-2 and PGE2. Within the purinergic system, the mechanisms that control the ADO levels were the most involved, mainly the reactions performed by the isoenzyme ADA 2. In conclusion, our data shed new lights on the mechanisms related to copper-induced inflammation in zebrafish larvae.