Deiodinase knockdown during early zebrafish development affects growth, development, energy metabolism, motility and phototransduction

The generation gap in endocrine disruption: Can the integrated fish endocrine disruptor test (iFEDT) bridge the gap by assessing intergenerational effects of thyroid hormone system disruption?

Thyroid hormones (THs) act early in ontogenesis, even prior to the differentiation of thyrocytes. Maternal transfer of THs is therefore known to play an essential role in early development. Current OECD test guidelines for the assessment of TH system disruption (THSD) do not address inter- or transgenerational effects. The integrated fish endocrine disruptor test (iFEDT), a test combining parental and developmental exposure of filial fish, may fill this gap. We tested the ability of the iFEDT to detect intergenerational effects in zebrafish (Danio rerio): Parental fish were exposed to propylthiouracil (PTU), an inhibitor of TH synthesis, or not exposed. The offspring was submitted to a crossed experimental design to obtain four exposure scenarios: (1) no exposure at all, (2) parental exposure only, (3) embryonic exposure only, and (4) combined parental and embryonic exposure. Swim bladder inflation, visual motor response (VMR) and gene expression of the progeny were analysed. Parental, but not embryonic PTU exposure reduced the size of the swim bladder of 5 d old embryos, indicating the existence of intergenerational effects. The VMR test produced opposite responses in 4.5 d old embryos exposed to PTU vs. embryos derived from exposed parents. Embryonic exposure, but not parental exposure increased gene expression of thyroperoxidase, the target of PTU, most likely due to a compensatory mechanism. The gene expression of pde-6h (phosphodiesterase) was reduced by embryonic, but not parental exposure, suggesting downregulation of phototransduction pathways. Hence, adverse effects on swim bladder inflation appear more sensitive to parental than embryonic exposure and the iFEDT represents an improvement in the testing strategy for THSD.

Response characteristic and potential molecular mechanism of tail resorption in Bufo gargarizans after exposure to lead and copper, alone or combined

Tail resorption during amphibian metamorphosis is one of the most dramatic processes that is obligatorily dependent on thyroid hormone (TH). Heavy metals could result in thyroid gland damages and disturb TH homeostasis. Lead (Pb) and copper (Cu) often co-exist in natural aquatic ecosystems. However, there is still little information on how tail resorption responds to alone or combined exposure to Pb and Cu. Our study investigated the effects of Pb and Cu alone or combined exposure on the morphological parameters of the tail, histological changes of thyroid gland and tail, and gene expression programs involved in cell death of the tail in Bufo gargarizans tadpoles at the climax of metamorphosis. Results demonstrated that Pb, Cu and Pb-Cu mixture exposure resulted in a significantly longer tail compared with control. Damages to notochord, muscle, skin and spinal cord of the tail were found in Pb and Cu exposure groups. The colloid area, the height of follicular cells and number of phagocytic vesicles of thyroid gland in Pb-Cu mixture exposure groups were significantly reduced. In addition, the expression levels of TH, apoptosis, autophagy, degradation of cellular components and oxidative stress-related genes in the tail were significantly altered following Pb and Cu exposure. The present work revealed the relationship between environmental pollutants and tail resorption, providing scientific basis for amphibian protection.

Toxicogenomics of Five Cytostatics in Fathead Minnow (Pimephales promelas) Larvae

In this study, the toxicogenomic effects of five cytostatics (tamoxifen, methotrexate, capecitabine, cyclophosphamide, and ifosfamide) on fathead minnow (Pimephales promelas) larvae were evaluated. Post-fertilization eggs were exposed to increasing concentrations of the drugs for six days. The expression levels of two genetic biomarkers for toxicity and four thyroid hormone-related gene pathways were measured. Interestingly, the results showed that all concentrations of the five cytostatics affect the transcription levels of both toxicity biomarker genes. Additionally, the thyroid hormone-related genes had different expression levels than the control, with the most significant changes observed in those larvae exposed to cyclophosphamide and ifosfamide. While a previous study found no effects on fish morphology, this study suggests that the five cytostatics modify subtle molecular responses of P. promelas, highlighting the importance of assessing multibiological level endpoints throughout the lifecycle of animals to understand the full portrait of potential effects of cytostatics and other contaminants.

Bisphenol A alters retinal morphology, visually guided behavior, and thyroid hormone levels in zebrafish larvae

Bisphenols are industrial chemicals that are produced in large quantities and have been detected in all parts of the environment as well as in a multitude of different organisms including humans and fish. Several bisphenols, such as bisphenol A (BPA) and bisphenol F, have been shown to disrupt endocrine systems thereby affecting development and reproduction. While numerous studies investigated the effect of bisphenols on estrogen signaling, their impact on the thyroid hormone system (THS), which is vital for neurodevelopment including sensory development, has been explored to a lesser extent. The present work selected BPA as a representative for structurally similar bisphenols and assessed its impact on the THS as well as sensory development and function in zebrafish. To this end, zebrafish were exposed to BPA until up to 8 days post fertilization (dpf) and thyroid hormone levels, eye morphology, and sensory-mediated behaviors were analyzed. Zebrafish larvae exposed to BPA showed altered retinal layering, decreased motility across varying light conditions, and a loss of responsiveness to red light. Furthermore, whole-body levels of the thyroid hormones thyroxine (T4) and 3,5-diiodothyronine (3,5-T2) were significantly decreased in 5 dpf zebrafish. Taken together, BPA disrupted THS homeostasis and compromised visual development and function, which is pivotal for the survival of fish larvae. This work underlines the necessity for ongoing research on BPA and its numerous substitutes, particularly concerning their effects on the THS and neurodevelopment, to ensure a high level of protection for the environment and human health.

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.

Reversibility of Thyroid Hormone System-Disrupting Effects on Eye and Thyroid Follicle Development in Zebrafish (Danio rerio) Embryos

Early vertebrate development is partially regulated by thyroid hormones (THs). Environmental pollutants that interact with the TH system (TH system-disrupting chemicals [THSDCs]) can have massively disrupting effects on this essential phase. Eye development of fish is directly regulated by THs and can, therefore, be used as a thyroid-related endpoint in endocrine disruptor testing. To evaluate the effects of THSDC-induced eye malformations during early development, zebrafish (Danio rerio) embryos were exposed for 5 days postfertilization (dpf) to either propylthiouracil, a TH synthesis inhibitor, or tetrabromobisphenol A, which interacts with TH receptors. Subsequently, one half of the embryos were exposed further to the THSDCs until 8 dpf, while the other half of the embryos were raised in clean water for 3 days to check for reversibility of effects. Continued THSDC exposure altered eye size and pigmentation and induced changes in the cellular structure of the retina. This correlated with morphological alterations of thyroid follicles as revealed by use of a transgenic zebrafish line. Interestingly, effects were partly reversible after a recovery period as short as 3 days. Results are consistent with changes in TH levels measured in different tissues of the embryos, for example, in the eyes. The results show that eye development in zebrafish embryos is very sensitive to THSDC treatment but able to recover quickly from early exposure by effective repair mechanisms. Environ Toxicol Chem 2023;42:1276-1292. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Developmental exposure to triclosan and benzophenone-2 causes morphological alterations in zebrafish (Danio rerio) thyroid follicles and eyes

Thyroid hormones (THs) regulate a multitude of developmental and metabolic processes, which are responsible for vertebrate development, growth, and maintenance of homeostasis. THs also play a key role in neurogenesis of vertebrates and thus affect eye development, which is vital for foraging efficiency and for effective escape from predation. Currently, there are no validated test guidelines for the assessment of TH system-disrupting chemicals (THSDCs) in fish. Consequently, the present study was designed to demonstrate the suitability of novel thyroid-related endpoints in early life-stages of fish. Embryos of a transgenic zebrafish (Danio rerio) line expressing the reporter gene tg:mCherry in their thyrocytes were used to investigate the effects of the environmental THSDCs triclosan (TCS, antibacterial agent) and benzophenone-2 (BP-2, UV filter) on thyroid follicle and eye development. Both BP-2 and TCS caused thyroid follicle hyperplasia in transgenic zebrafish, thus confirming their role as THSDCs. The effect intensity on follicle size and fluorescence was comparable with a 1.7-fold increase for BP-2 and 1.6-fold for TCS. Alterations of the cellular structures of the retina indicate an impact of both substances on eye development, with a stronger impact of TCS. With respect to guideline development, results provide further evidence for the suitability of morphological changes in thyroid follicles and the eyes as novel endpoints for the sensitive assessment of THSD-related effects in fish.

Toxicity assessment of hexafluoropropylene oxide-dimer acid on morphology, heart physiology, and gene expression during zebrafish (Danio rerio) development

Hexafluoropropylene oxide-dimer acid (HFPO-DA) is one of the emerging replacements for the "forever" carcinogenic and toxic long-chain PFAS. HFPO-DA is a polymerization aid used for manufacturing fluoropolymers, whose global distribution and undetermined toxic properties are a concern regarding human and ecological health. To assess embryotoxic potential, zebrafish embryos were exposed to HFPO-DA at concentrations of 0.5-20,000 mg/L at 24-, 48-, and 72-h post-fertilization (hpf). Heart rate increased significantly in embryos exposed to 2 mg/L and 10 mg/L HFPO-DA across all time points. Spinal deformities and edema phenotypes were evident among embryos exposed to 1000-16,000 mg/L HFPO-DA at 72 hpf. A median lethal concentration (LC₅₀) was derived as 7651 mg/L at 72 hpf. Shallow RNA sequencing analysis of 9465 transcripts identified 38 consistently differentially expressed genes at 0.5 mg/L, 1 mg/L, 2 mg/L, and 10 mg/L HFPO-DA exposures. Notably, seven downregulated genes were associated with visual response, and seven upregulated genes were expressed in or regulated the cardiovascular system. This study identifies biological targets and molecular pathways affected during animal development by an emerging, potentially problematic, and ubiquitous industrial chemical.

Thyroid hormone disrupting potentials of benzisothiazolinone in embryo-larval zebrafish and rat pituitary GH3 cell line

Benzisothiazolinone (BIT), one of the most widely used antimicrobial agents in consumer products, has frequently been detected in the water environment. The present study was conducted to determine the adverse effects of BIT on the thyroid neuroendocrine system of zebrafish embryos/larvae. Rat pituitary (GH3) cell line was employed to support the underlying mechanism of thyroid hormone disrupting effects. Significant coagulation and hatching delay were observed in embryos exposed to 30 μg/L of BIT, which in turn remarkably decreased hatchability and larval survival. In BIT-exposed larvae, tshβ, tshr, and trh genes were significantly upregulated along with a decrease in thyroxine and triiodothyronine content, indicating that BIT decreased thyroid hormones and increased thyrotropin-releasing hormone and thyroid stimulating hormone secretion through a feedback circuit. The downregulation of trα and deio2 genes in the zebrafish larvae suggests the inhibition of thyroid hormone receptors and deiodination. Similar to the results in zebrafish, upregulation of tshβ and downregulation of trα, trβ, deio1, and deio2 genes were observed in GH3 cells. Our observations suggest that BIT can decrease the level of thyroid hormones by influencing central regulation, receptor binding, and deiodination.

Adverse effects of thyroid-hormone-disrupting chemicals 6-propyl-2-thiouracil and tetrabromobisphenol A on Japanese medaka (Oryzias latipes)

Thyroid-hormone-disrupting chemicals are increasingly attracting attention because of their potential harmful effects on animal health, including on fishes. Here, we investigated the effects of exposure to the thyroid-hormone-disrupting chemicals 6-propyl-2-thiouracil (PTU) and tetrabromobisphenol A (TBBPA) on swim bladder inflation, eye development, growth, swimming performance, and the expression of thyroid-related genes in Japanese medaka (Oryzias latipes). PTU exposure resulted in reductions in eye size, growth, and swim bladder inflation, and these effects led to poorer swimming performance. These phenotypic effects were accompanied by increased expression of the thyroid-stimulating hormone subunit beta (tshβ) paralog tshβ-like, but there were no significant changes in expression for tshβ, deiodinase 1 (dio1), deiodinase 2 (dio2), and thyroid hormone receptor alpha (trα) and beta (trβ). For PTU exposure, we identified the key event (swim bladder inflation reduction) and an adverse outcome (swimming performance reduction). No significant effects from TBBPA exposure were seen on swim bladder inflation, eye development, growth, or swimming performance. However, expression of tshβ-like and tshβ (significantly enhanced) and trα and trβ (significantly reduced) were affected by TBBPA exposure albeit not in dose-dependent manners. There were no effects of TBBPA on the expression of dio1 and dio2. We thus show that the two thyroid-hormone-disrupting chemicals PTU and TBBPA differ in their effect profiles with comparable effects on the studied phenotypes and thyroid-related gene expression to those reported in zebrafish.

Thyroid hormone disruption by bis-(2-ethylhexyl) phthalate (DEHP) and bis-(2-ethylhexyl) adipate (DEHA) in Japanese medaka Oryzias latipes

Pollution of water bodies with plasticizers is a serious environmental problem worldwide. In this study, we investigated the effects of plasticizers bis-(2-ethylhexyl) phthalate (DEHP) and bis-(2-ethylhexyl) adipate (DEHA) in Japanese medaka (Oryzias latipes). DEHP significantly increased the expression of all the genes tested: thyroid stimulating hormone beta subunit (tshβ-like), tshβ, deiodinase 1 (dio1), deiodinase 2 (dio2), and thyroid hormone receptor alpha (trα) and beta (trβ). However, DEHA only significantly increased tshβ at 7.4 µg/L but significantly decreased dio2 expression at 25.8, 111.1, and 412.6 4 µg/L, while other genes were not significantly affected. Both chemicals reduced eye size and total body length, but did not affect embryo development, hatching time and rate, and swimming performance. DEHA alone affected swim bladder inflation and not DEHP. This is the first report that not only DEHP but also DEHA disrupt thyroid hormone activity in fish. DEHP contamination (13.2 μg/L) was detected in tap water from Kobe, Japan; thus, tap water itself may disrupt thyroid hormone activity in Japanese medaka. Importantly, the effective concentration of DEHP for thyroid hormone-related gene expression and growth was close to or lower than DEHP concentrations reported in surface water elsewhere, indicating that DEHP contamination is a serious aquatic pollution.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

Zebrafish as an emerging tool for drug discovery and development for thyroid diseases

Zebrafish is a useful model for understanding human genetics and diseases and has evolved into a prominent scientific research model. The genetic structure of zebrafish is 70% identical to that of humans. Its small size, low cost, and transparent embryo make it a valuable tool in experimentation. Zebrafish and mammals possess the same molecular mechanism of thyroid organogenesis and development. Thus, thyroid hormone signaling, embryonic development, thyroid-related disorders, and novel genes involved in early thyroid development can all be studied using zebrafish as a model. Here in this review, we emphasize the evolving role of zebrafish as a possible tool for studying the thyroid gland in the context of physiology and pathology. The transcription factors nkx2.1a, pax2a, and hhex which contribute a pivotal role in the differentiation of thyroid primordium are discussed. Further, we have described the role of zebrafish as a model for thyroid cancer, evaluation of defects in thyroid hormone transport, thyroid hormone (TH) metabolism, and as a screening tool to study thyrotoxins. Hence, the present review highlights the role of zebrafish as a novel approach to understand thyroid development and organogenesis.

Visualizing and assessing the size-dependent oral uptake, tissue distribution, and detrimental effect of polystyrene microplastics in Eisenia fetida

Microplastics are widely distributed in the environment, their potential ecological risks on soil organism have attracted extensive attention, while the investigation of the size effect on its accumulation and toxicity in soil invertebrates are still lacking. In this study, we set out to explore the size-dependent effects of microplastics on soil invertebrates with different doses. Specifically, we investigated the effect of polystyrene (PS) microplastics on earthworm Eisenia fetida with three different sizes (70 nm, 1 μm and 10 μm) and exposure doses (0.5%, 5% and 10% w/w in food). Results showed that PS microplastics had no effects on the mortality of E. fetida, while an obvious growth inhibition with rising exposure concentrations was observed, especially under exposure of 70 nm plastic particles. Additionally, 70 nm PS microplastics induced more serious oxidative stress, energy depletion and histopathological damage on earthworms compared with larger sizes. The accumulation and distribution pattern of microplastics was size-dependent in earthworms after 3- and 7-day exposure as revealed by laser confocal microscopy. Notably, earthworms accumulated more micro-sized particles (MPs, 10 μm and 1 μm) but with less toxic responses, suggesting its weaker toxicity. The distribution pattern of MPs may explain the weak relation between accumulation and toxicity as they mainly distributed in epidermis of mid- and tail-section and the intestine of earthworm. In contrast, nano-sized particles (NPs, 70 nm) were more distributed in the head-section and subcutaneous tissue of the skin, which was in accordance with the obvious toxic responses found in earthworms exposing to NPs. Our study highlighted the importance of size in determining the accumulation, distribution and toxic effects of plastic particles towards soil invertebrates and advocates the necessity of ecological risk assessments of NPs.

Effects of tetramethyl bisphenol F on thyroid and growth hormone-related endocrine systems in zebrafish larvae

Trimethyl bisphenol F (TMBPF) has recently been used as a bisphenol A substitute in polymer coatings for metal cans containing beverages or food. This study investigated whether TMBPF disrupts the endocrine system associated with thyroid hormones and growth hormones employing zebrafish embryos and larvae. After 14 days of exposure, body weight was significantly reduced when zebrafish were exposed to a TMBPF concentration greater than 50 μg/L. The triiodothyronine levels were significantly increased, while growth hormone levels were significantly decreased in larvae exposed to 5 μg/L TMBPF. The transcription of genes associated with thyroid hormone production (trα, tpo, tg, and nis), deiodination (deio2), growth hormone production (gh1, ghrh, and ghra), and insulin-like growth factor (igf2a, igf2b, igf2r, igfbp1a, igfbp1b, igfbp2a, igfbp2b, and igfbp5a) was significantly upregulated, whereas the transcription of genes association with thyrotropin-releasing hormone (trh and trhr1) was significantly downregulated. These results suggest that hyperthyroidism, decrease in growth hormone, and regulation of genes involved in the hypothalamus-pituitary-thyroid and growth hormone/insulin-like growth factor might be responsible for the observed growth inhibition in larvae exposed to TMBPF. The bioaccumulation of TMBPF and its effects on the endocrine system after chronic exposure requires further investigation.

Patterning and Development of Photoreceptors in the Human Retina

Humans rely on visual cues to navigate the world around them. Vision begins with the detection of light by photoreceptor cells in the retina, a light-sensitive tissue located at the back of the eye. Photoreceptor types are defined by morphology, gene expression, light sensitivity, and function. Rod photoreceptors function in low-light vision and motion detection, and cone photoreceptors are responsible for high-acuity daytime and trichromatic color vision. In this review, we discuss the generation, development, and patterning of photoreceptors in the human retina. We describe our current understanding of how photoreceptors are patterned in concentric regions. We conclude with insights into mechanisms of photoreceptor differentiation drawn from studies of model organisms and human retinal organoids.

Fish toxicity testing for identification of thyroid disrupting chemicals

Identification of thyroid disrupting chemicals (TDCs), one of the most studied types of endocrine disruptors (EDs), is required according to EU regulations on industrial chemicals, pesticides, and biocides. Following that requirement, the use of fish as a unique non-mammalian model species for identification of EDs may be warranted. This study summarized and evaluated effects of TDCs on fish thyroid sensitive endpoints including thyroid hormones, thyroid related gene expression, immunostaining for thyroid follicles, eye size and pigmentation, swim bladder inflation as well as effects of TDCs on secondary sex characteristics, sex ratio, growth and reproduction. Changes in thyroid sensitive endpoints may reflect the balanced outcome of different processes of the thyroid cascade. Thyroid sensitive endpoints may also be altered by non-thyroid molecular or endocrine pathways as well as non-specific factors such as general toxicity, development, stress, nutrient, and the environmental factors like temperature and pH. Defining chemical specific effects on thyroid sensitive endpoints is important for identification of TDCs. Application of the AOP (adverse outcome pathway) concept could be helpful for defining critical events needed for testing and identification of TDCs in fish.

Deiodinases: How Nonmammalian Research Helped Shape Our Present View

Iodothyronine deiodinases are enzymes capable of activating and inactivating thyroid hormones (THs) and have an important role in regulating TH action in tissues throughout the body. Three types of deiodinases (D1, D2, and D3) were originally defined based on their biochemical characteristics. Cloning of the first complementary DNAs in the 1990s (Dio1 in rat and dio2 and dio3 in frog) allowed to confirm the existence of 3 distinct enzymes. Over the years, increasing genomic information revealed that deiodinases are present in all chordates, vertebrates, and nonvertebrates and that they can even be found in some mollusks and annelids, pointing to an ancient origin. Research in nonmammalian models has substantially broadened our understanding of deiodinases. In relation to their structure, we discovered for instance that biochemical properties such as inhibition by 6-propyl-2-thiouracil, stimulation by dithiothreitol, and temperature optimum are subject to variation. Data from fish, amphibians, and birds were key in shifting our view on the relative importance of activating and inactivating deiodination pathways and in showing the impact of D2 and D3 not only in local but also whole body T3 availability. They also led to the discovery of new local functions such as the acute reciprocal changes in D2 and D3 in hypothalamic tanycytes upon photostimulation, involved in seasonal rhythmicity. With the present possibilities for rapid and precise gene silencing in any species of interest, comparative research will certainly further contribute to a better understanding of the importance of deiodinases for adequate TH action, also in humans.

The effects of exposure to crude oil or PAHs on fish swim bladder development and function

The failure of the swim bladder to inflate during fish development is a common and sensitive response to exposure to petrochemicals. Here, we review potential mechanisms by which petrochemicals or their toxic components (polycyclic aromatic hydrocarbons; PAHs) may affect swim bladder inflation, particularly during early life stages. Surface films formed by oil can cause a physical barrier to primary inflation by air gulping, and are likely important during oil spills. The act of swimming to the surface for primary inflation can be arduous for some species, and may prevent inflation if this behavior is limited by toxic effects on vision or musculature. Some studies have noted altered gene expression in the swim bladder in response to PAHs, and Cytochrome P450 1A (CYP1A) can be induced in swim bladder or rete mirabile tissue, suggesting that PAHs can have direct effects on swim bladder development. Swim bladder inflation failure can also occur secondarily to the failure of other systems; cardiovascular impairment is the best elucidated of these mechanisms, but other mechanisms might include non-inflation as a sequela of disruption to thyroid signaling or cholesterol metabolism. Failed swim bladder inflation has the potential to lead to chronic sublethal effects that are as yet unstudied.

Cross-Taxa Distinctions in Mechanisms of Developmental Effects for Aquatic Species Exposed to Trifluralin

Standard ecological risk assessment practices often rely on larval and juvenile fish toxicity data as representative of the amphibian aquatic phase. Empirical evidence suggests that endpoints measured in fish early life stage tests are often sufficient to protect larval amphibians. However, the process of amphibian metamorphosis relies on endocrine cues that affect development and morphological restructuring and are not represented by these test endpoints. The present study compares developmental endpoints for zebrafish (Danio rerio) and the African clawed frog (Xenopus laevis), 2 standard test species, exposed to the herbicide trifluralin throughout the larval period. Danio rerio were more sensitive and demonstrated a reduction in growth measurements with increasing trifluralin exposure. Size of X. laevis at metamorphosis was not correlated with exposure concentration; however, time to metamorphosis was delayed relative to trifluralin concentration. Gene expression patterns indicate discrepancies in response by D. rerio and X. laevis, and dose-dependent metabolic activity suggests that trifluralin exposure perturbed biological pathways differently within the 2 species. Although many metabolites were correlated with exposure concentration in D. rerio, nontargeted hepatic metabolomics identified a subset of metabolites that exhibited a nonmonotonic response to trifluralin exposure in X. laevis. Linking taxonomic distinctions in cellular-level response with ecologically relevant endpoints will refine assumptions used in interspecies extrapolation of standard test effects and improve assessment of sublethal impacts on amphibian populations. Environ Toxicol Chem 2020;39:1797-1812. Published 2020. This article is a US government work and is in the public domain in the USA.

Toward an AOP Network-Based Tiered Testing Strategy for the Assessment of Thyroid Hormone Disruption

A growing number of environmental pollutants are known to adversely affect the thyroid hormone system, and major gaps have been identified in the tools available for the identification, and the hazard and risk assessment of these thyroid hormone disrupting chemicals. We provide an example of how the adverse outcome pathway (AOP) framework and associated data generation can address current testing challenges in the context of fish early life stage tests, and fish tests in general. We demonstrate how a suite of assays covering biological processes involved in the underlying toxicological pathways can be implemented in a tiered screening and testing approach for thyroid hormone disruption, using the levels of assessment of the OECD's Conceptual Framework for the Testing and Assessment of Endocrine Disrupting Chemicals as a guide.

Effect of Thyroperoxidase and Deiodinase Inhibition on Anterior Swim Bladder Inflation in the Zebrafish

A set of adverse outcome pathways (AOPs) linking inhibition of thyroperoxidase and deiodinase to impaired swim bladder inflation in fish has recently been developed. These AOPs help to establish links between these thyroid hormone (TH) disrupting molecular events and adverse outcomes relevant to aquatic ecological risk assessment. Until now, very little data on the effects of TH disruption on inflation of the anterior chamber (AC) of the swim bladder were available. The present study used zebrafish exposure experiments with three model compounds with distinct thyroperoxidase and deiodinase inhibition potencies (methimazole, iopanoic acid, and propylthiouracil) to evaluate this linkage. Exposure to all three chemicals decreased whole body triiodothyronine (T3) concentrations, either through inhibition of thyroxine (T4) synthesis or through inhibition of Dio mediated conversion of T4 to T3. A quantitative relationship between reduced T3 and reduced AC inflation was established, a critical key event relationship linking impaired swim bladder inflation to TH disruption. Reduced inflation of the AC was directly linked to reductions in swimming distance compared to controls as well as to chemical-exposed fish whose ACs inflated. Together the data provide compelling support for AOPs linking TH disruption to impaired AC inflation in fish.

ERGO: Breaking Down the Wall between Human Health and Environmental Testing of Endocrine Disrupters

ERGO (EndocRine Guideline Optimization) is the acronym of a European Union-funded research and innovation action, that aims to break down the wall between mammalian and non-mammalian vertebrate regulatory testing of endocrine disruptors (EDs), by identifying, developing and aligning thyroid-related biomarkers and endpoints (B/E) for the linkage of effects between vertebrate classes. To achieve this, an adverse outcome pathway (AOP) network covering various modes of thyroid hormone disruption (THD) in multiple vertebrate classes will be developed. The AOP development will be based on existing and new data from in vitro and in vivo experiments with fish, amphibians and mammals, using a battery of different THDs. This will provide the scientifically plausible and evidence-based foundation for the selection of B/E and assays in lower vertebrates, predictive of human health outcomes. These assays will be prioritized for validation at OECD (Organization for Economic Cooperation and Development) level. ERGO will re-think ED testing strategies from in silico methods to in vivo testing and develop, optimize and validate existing in vivo and early life-stage OECD guidelines, as well as new in vitro protocols for THD. This strategy will reduce requirements for animal testing by preventing duplication of testing in mammals and non-mammalian vertebrates and increase the screening capacity to enable more chemicals to be tested for ED properties.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

Transcriptomic analysis of microRNAs-mRNAs regulating innate immune response of zebrafish larvae against Vibrio parahaemolyticus infection

In recent years, microRNAs (miRNAs) have been shown to play important roles in immunity. Analyses of the functions of miRNAs and their targets are useful in understanding the regulation of the immune response. To understand the relationships between miRNAs and their targets during infection, we used zebrafish as an infection model in which to characterize the miRNA and mRNA transcriptomes of zebrafish larvae infected with Vibrio parahaemolyticus. We identified the differentially expressed miRNAs and mRNAs. Overall, 37 known zebrafish miRNAs were differentially expressed in the infection group and 107 predicted target genes of 26 miRNAs were differentially expressed in the mRNA transcriptome. These targets with specific Gene Ontology (GO) terms, such as peripheral nervous system neuron axonogenesis, organophosphate metabolic process, heme binding, protein binding, tetrapyrrole binding, protein dimerization activity, and aromatase activity, which regulate nerve conduction, energy metabolism, hematopoiesis, and protein synthesis. They were also associated with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways such as phototransduction, tryptophan metabolism, notch signaling, and purine metabolism. Our findings indicate that miRNAs regulate the innate immune response via complex networks, and zebrafish (Danio rerio, dre)-miR-205-3p, dre-miR-141-5p, dre-miR-200a-5p, dre-miR-92a-2-5p, dre-miR-192, and dre-miR-1788 may play important roles in the innate immune response by regulating target genes.

Leptin-a mediates transcription of genes that participate in central endocrine and phosphatidylinositol signaling pathways in 72-hour embryonic zebrafish (Danio rerio)

We analyzed microarray expression data to highlight biological pathways that respond to embryonic zebrafish Leptin-a (lepa) signaling. Microarray expression measures for 26,046 genes were evaluated from lepa morpholino oligonucleotide "knockdown", recombinant Leptin-a "rescue", and uninjected control zebrafish at 72-hours post fertilization. In addition to KEGG pathway enrichment for phosphatidylinositol signaling and neuroactive ligand-receptor interactions, Gene Ontology (GO) data from lepa rescue zebrafish include JAK/STAT cascade, sensory perception, nervous system processes, and synaptic signaling. In the zebrafish lepa rescue treatment, we found changes in the expression of homologous genes that align with mammalian leptin signaling cascades including AMPK (prkaa2), ACC (acacb), Ca²⁺/calmodulin-dependent kinase (camkk2), PI3K (pik3r1), Ser/Thr protein kinase B (akt3), neuropeptides (agrp2, cart1), mitogen-activated protein kinase (MAPK), and insulin receptor substrate (LOC794738, LOC100537326). Notch signaling pathway and ribosome biogenesis genes respond to knockdown of Leptin-a. Differentially expressed transcription factors in lepa knockdown zebrafish regulate neurogenesis, neural differentiation, and cell fate commitment. This study presents a role for zebrafish Leptin-a in influencing expression of genes that mediate phosphatidylinositol and central endocrine signaling.

Comparison of thyroid hormone disruption potentials by bisphenols A, S, F, and Z in embryo-larval zebrafish

Several structural analogues of bisphenol A (BPA), e.g., bisphenol F (BPF), bisphenol S (BPS), and bisphenol Z (BPZ), have been used as its substitutes in many applications and consequently detected in the environment, and human specimen such as urine and serum. While BPA has been frequently reported for thyroid hormone disruption in both experimental and epidemiological studies, less is known for the BPA analogues. In the present study, thyroid hormone disrupting effects of BPF, BPS and BPZ, were investigated, and compared with those of BPA, using embryo-larval zebrafish (Danio rerio). At 120 hpf, significant increases in T3 and/or T4 were observed in the larval fish following exposure to BPA, BPF, or BPS. Moreover, transcriptional changes of the genes related to thyroid development (hhex and tg), thyroid hormone transport (ttr) and metabolism (ugt1ab) were observed as well. Thyroid hormone (T4) disruption by BPF was observed even at the concentration (2.0 mg/L) lower than the effective concentration determined for BPA (>2.0 mg/L). Delayed hatching was observed by all tested bisphenols. Our results clearly show that these BPA analogues can disrupt thyroid function of the larval fish, and their thyroid hormone disruption potencies could be even greater than that of BPA. The concentrations which disrupt thyroid function of the larval fish were orders of magnitude higher than those occurring in the ambient environment. However, thyroid hormone disruption by longer term exposure and its consequences in the fish population, deserve further investigation.

Thyroid Hormone Disruptors Interfere with Molecular Pathways of Eye Development and Function in Zebrafish

The effects of thyroid hormone disrupting chemicals (THDCs) on eye development of zebrafish were investigated. We expected THDC exposure to cause transcriptional changes of vision-related genes, which find their phenotypic anchoring in eye malformations and dysfunction, as observed in our previous studies. Zebrafish were exposed from 0 to 5 days post fertilization (dpf) to either propylthiouracil (PTU), a thyroid hormone synthesis inhibitor, or tetrabromobisphenol-A (TBBPA), which interacts with thyroid hormone receptors. Full genome microarray analyses of RNA isolated from eye tissue revealed that the number of affected transcripts was substantially higher in PTU- than in TBBPA-treated larvae. However, multiple components of phototransduction (e.g., phosphodiesterase, opsins) were responsive to both THDC exposures. Yet, the response pattern for the gene ontology (GO)-class "sensory perception" differed between treatments, with over 90% down-regulation in PTU-exposed fish, compared to over 80% up-regulation in TBBPA-exposed fish. Additionally, the reversibility of effects after recovery in clean water for three days was investigated. Transcriptional patterns in the eyes were still altered and partly overlapped between 5 and 8 dpf, showing that no full recovery occurred within the time period investigated. However, pathways involved in repair mechanisms were significantly upregulated, which indicates activation of regeneration processes.

Disruption of deiodinase type 2 in zebrafish disturbs male and female reproduction

Thyroid hormones are crucial mediators of many aspects of vertebrate life, including reproduction. The key player is the biologically active 3,5,3'-triiodothyronine (T3), whose local bio-availability is strictly regulated by deiodinase enzymes. Deiodinase type 2 (Dio2) is present in many tissues and is the main enzyme for local T3 production. To unravel its role in different physiological processes, we generated a mutant zebrafish line, completely lacking Dio2 activity. Here we focus on the reproductive phenotype studied at the level of offspring production, gametogenesis, functioning of the hypothalamic-pituitary-gonadal axis and sex steroid production. Homozygous Dio2-deficient zebrafish were hypothyroid, displayed a delay in sexual maturity, and the duration of their reproductive period was substantially shortened. Fecundity and fertilization were also severely reduced. Gamete counts pointed to a delay in oogenesis at onset of sexual maturity and later on to an accumulation of oocytes in mutant ovaries due to inhibition of ovulation. Analysis of spermatogenesis showed a strongly decreased number of spermatogonia A at onset of sexual maturity. Investigation of the hypothalamic-pituitary-gonadal axis revealed that dysregulation was largely confined to the gonads with significant upregulation of igf3, and a strong decrease in sex steroid production concomitant with alterations in gene expression in steroidogenesis/steroid signaling pathways. Rescue of the phenotype by T3 supplementation starting at 4 weeks resulted in normalization of reproductive activity in both sexes. The combined results show that reproductive function in mutants is severely hampered in both sexes, thereby linking the loss of Dio2 activity and the resulting hypothyroidism to reproductive dysfunction.

Thyroid hormone signaling specifies cone subtypes in human retinal organoids

INTRODUCTION:

Cone photoreceptors in the human retina enable daytime, color, and high-acuity vision. The three subtypes of human cones are defined by the visual pigment that they express: blue-opsin (short wavelength; S), green-opsin (medium wavelength; M), or red-opsin (long wavelength; L). Mutations that affect opsin expression or function cause various forms of color blindness and retinal degeneration.

RATIONALE:

Our current understanding of the vertebrate eye has been derived primarily from the study of model organisms. We studied the human retina to understand the developmental mechanisms that generate the mosaic of mutually exclusive cone subtypes. Specification of human cones occurs in a two-step process. First, a decision occurs between S versus L/M cone fates. If the L/M fate is chosen, a subsequent choice is made between expression of L- or M-opsin. To determine the mechanism that controls the first decision between S and L/M cone fates, we studied human retinal organoids derived from stem cells.

RESULTS:

We found that human organoids and retinas have similar distributions, gene expression profiles, and morphologies of cone subtypes. During development, S cones are specified first, followed by L/M cones. This temporal switch from specification of S cones to generation of L/M cones is controlled by thyroid hormone (TH) signaling. In retinal organoids that lacked thyroid hormone receptor β (Thrβ), all cones developed into the S subtype. Thrβ binds with high affinity to triiodothyronine (T3), the more active form of TH, to regulate gene expression. We observed that addition of T3 early during development induced L/M fate in nearly all cones. Thus, TH signaling through Thrβ is necessary and sufficient to induce L/M cone fate and suppress S fate. TH exists largely in two states: thyroxine (T4), the most abundant circulating form of TH, and T3, which binds TH receptors with high affinity. We hypothesized that the retina itself could modulate TH levels to control subtype fates. We found that deiodinase 3 (DIO3), an enzyme that degrades both T3 and T4, was expressed early in organoid and retina development. Conversely, deiodinase 2 (DIO2), an enzyme that converts T4 to active T3, as well as TH carriers and transporters, were expressed later in development. Temporally dynamic expression of TH-degrading and -activating proteins supports a model in which the retina itself controls TH levels, ensuring low TH signaling early to specify S cones and high TH signaling later in development to produce L/M cones.

CONCLUSION:

Studies of model organisms and human epidemiology often generate hypotheses about human biology that cannot be studied in humans. Organoids provide a system to determine the mechanisms of human development, enabling direct testing of hypotheses in developing human tissue. Our studies identify temporal regulation of TH signaling as a mechanism that controls cone subtype specification in humans. Consistent with our findings, preterm human infants with low T3 and T4 have an increased incidence of color vision defects. Moreover, our identification of a mechanism that generates one cone subtype while suppressing the other, coupled with successful transplantation and incorporation of stem cell-derived photoreceptors in mice, suggests that the promise of therapies to treat human diseases such as color blindness, retinitis pigmentosa, and macular degeneration will be achieved in the near future. ■

The mechanisms underlying specification of neuronal subtypes within the human nervous system are largely unknown. The blue (S), green (M), and red (L) cones of the retina enable high-acuity daytime and color vision. To determine the mechanism that controls S versus L/M fates, we studied the differentiation of human retinal organoids. Organoids and retinas have similar distributions, expression profiles, and morphologies of cone subtypes. S cones are specified first, followed by L/M cones, and thyroid hormone signaling controls this temporal switch. Dynamic expression of thyroid hormone–degrading and –activating proteins within the retina ensures low signaling early to specify S cones and high signaling late to produce L/M cones. This work establishes organoids as a model for determining mechanisms of human development with promising utility for therapeutics and vision repair.

Gene transcription ontogeny of hypothalamic-pituitary-thyroid axis development in early-life stage fathead minnow and zebrafish

The hypothalamic-pituitary-thyroid (HPT) axis is known to play a crucial role in the development of teleost fish. However, knowledge of endogenous transcription profiles of thyroid-related genes in developing teleosts remains fragmented. We selected two model teleost species, the fathead minnow (Pimephales promelas) and the zebrafish (Danio rerio), to compare the gene transcription ontogeny of the HPT axis. Control organisms were sampled at several time points during embryonic and larval development until 33 days post-fertilization. Total RNA was extracted from pooled, whole fish, and thyroid-related mRNA expression was evaluated using quantitative polymerase chain reaction. Gene transcripts examined included: thyrotropin-releasing hormone receptor (trhr), thyroid-stimulating hormone receptor (tshr), sodium-iodide symporter (nis), thyroid peroxidase (tpo), thyroglobulin (tg), transthyretin (ttr), deiodinases 1, 2, 3a, and 3b (dio1, dio2, dio3a and 3b), and thyroid hormone receptors alpha and beta (thrα and β). A loess regression method was successful in identifying maxima and minima of transcriptional expression during early development of both species. Overall, we observed great similarities between the species, including maternal transfer, at least to some extent, of almost all transcripts (confirmed in unfertilized eggs), increasing expression of most transcripts during hatching and embryo-larval transition, and indications of a fully functional HPT axis in larvae. These data will aid in the development of hypotheses on the role of certain genes and pathways during development. Furthermore, this provides a background reference dataset for designing and interpreting targeted transcriptional expression studies both for fundamental research and for applications such as toxicology.

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

The adverse outcome pathway (AOP) framework can be used to help support the development of alternative testing strategies aimed at predicting adverse outcomes caused by triggering specific toxicity pathways. In this paper, we present a case-study demonstrating the selection of alternative in chemico assays targeting the molecular initiating events of established AOPs, and evaluate use of the resulting data to predict higher level biological endpoints. Based on two AOPs linking inhibition of the deiodinase (DIO) enzymes to impaired posterior swim bladder inflation in fish, we used in chemico enzyme inhibition assays to measure the molecular initiating events for an array of 51 chemicals. Zebrafish embryos were then exposed to 14 compounds with different measured inhibition potentials. Effects on posterior swim bladder inflation, predicted based on the information captured by the AOPs, were evaluated. By linking the two datasets and setting thresholds, we were able to demonstrate that the in chemico dataset can be used to predict biological effects on posterior chamber inflation, with only two outliers out of the 14 tested compounds. Our results show how information organized using the AOP framework can be employed to develop or select alternative assays, and successfully forecast downstream key events along the AOP. In general, such in chemico assays could serve as a first-tier high-throughput system to screen and prioritize chemicals for subsequent acute and chronic fish testing, potentially reducing the need for long-term and costly toxicity tests requiring large numbers of animals.

Regulation of Intracellular Triiodothyronine Is Essential for Optimal Macrophage Function

Innate immune cells, including macrophages, have recently been identified as target cells for thyroid hormone. We hypothesized that optimal intracellular concentrations of the active thyroid hormone triiodothyronine (T3) are essential for proinflammatory macrophage function. T3 is generated intracellularly by type 2 deiodinase (D2) and acts via the nuclear thyroid hormone receptor (TR). In zebrafish embryos, D2 knockdown increased mortality during pneumococcal meningitis. Primary murine D2 knockout macrophages exhibited impaired phagocytosis and partially reduced cytokine response to stimulation with bacterial endotoxin. These effects are presumably due to reduced intracellular T3 availability. Knockdown of the main TR in macrophages, TRα, impaired polarization into proinflammatory macrophages and amplified polarization into immunomodulatory macrophages. Intracellular T3 availability and action appear to play a crucial role in macrophage function. Our data suggest that low intracellular T3 action has an anti-inflammatory effect, possibly due to an effect on macrophage polarization mediated via the TRα. This study provides important insights into the link between the endocrine and innate immune system.

Transcriptomic and phenotypic profiling in developing zebrafish exposed to thyroid hormone receptor agonists

There continues to be a need to develop in vivo high-throughput screening (HTS) and computational methods to screen chemicals for interaction with the estrogen, androgen, and thyroid pathways and as complements to in vitro HTS assays. This study explored the utility of an embryonic zebrafish HTS approach to identify and classify endocrine bioactivity using phenotypically-anchored transcriptome profiling. Transcriptome analysis was conducted on zebrafish embryos exposed to 25 estrogen-, androgen-, or thyroid-active chemicals at concentrations that elicited adverse malformations or mortality at 120 h post-fertilization in 80% of animals exposed. Analysis of the top 1000 significant differentially expressed transcripts and developmental toxicity profiles across all treatments identified a unique transcriptional and phenotypic signature for thyroid hormone receptor agonists. This unique signature has the potential to be used as a tiered in vivo HTS and may aid in identifying chemicals that interact with the thyroid hormone receptor.

Multiple bioanalytical method to reveal developmental biological responses in zebrafish embryos exposed to triclocarban

Triclocarban (TCC) is a well-known antibacterial agent that is frequently detected in environmental, wildlife and human samples. The potential toxicological effects and action mechanism of TCC on vertebrate development has remained unclear. In the present study, we analyzed phenotypic alterations, thyroid hormone levels, thyroid hormone responsive genes, and proteomic profiles of zebrafish embryos after exposure to a series of concentrations of TCC from 6 h post-fertilization (hpf) to 120 hpf. The most nonlethal concentration (MNLC), lethal concentration 10% (LC₁₀) and lethal concentration 50% (LC₅₀) of TCC for exposures of 96 h were 133.3 μg/L, 147.5 μg/L and 215.8 μg/L, respectively. Our results showed that exposure to TCC decreased heart rate, delayed yolk absorption and swim bladder development at MNLC and LC₁₀. Exposure to MNLC of TCC inhibited thyroid hormone and altered expression of thyroid hormone responsive genes. Furthermore, exposure to 1/20 MNLC of TCC altered expression of proteins related to binding and metabolism, skeletal muscle development and function, as well as proteins involved in nervous system development and immune response, indicating TCC has potential health risks in wildlife and humans at low concentration level.

How zebrafish research has helped in understanding thyroid diseases

Next-generation sequencing technologies have revolutionized the identification of disease-causing genes, accelerating the discovery of new mutations and new candidate genes for thyroid diseases. To face this flow of novel genetic information, it is important to have suitable animal models to study the mechanisms regulating thyroid development and thyroid hormone availability and activity. Zebrafish ( Danio rerio), with its rapid external embryonic development, has been extensively used in developmental biology. To date, almost all of the components of the zebrafish thyroid axis have been characterized and are structurally and functionally comparable with those of higher vertebrates. The availability of transgenic fluorescent zebrafish lines allows the real-time analysis of thyroid organogenesis and its alterations. Transient morpholino-knockdown is a solution to silence the expression of a gene of interest and promptly obtain insights on its contribution during the development of the zebrafish thyroid axis. The recently available tools for targeted stable gene knockout have further increased the value of zebrafish to the study of thyroid disease. All of the reported zebrafish models can also be used to screen small compounds and to test new drugs and may allow the establishment of experimental proof of concept to plan subsequent clinical trials.

Mechanisms of Photoreceptor Patterning in Vertebrates and Invertebrates

Across the animal kingdom, visual systems have evolved to be uniquely suited to the environments and behavioral patterns of different species. Visual acuity and color perception depend on the distribution of photoreceptor (PR) subtypes within the retina. Retinal mosaics can be organized into three broad categories: stochastic/regionalized, regionalized, and ordered. We describe here the retinal mosaics of flies, zebrafish, chickens, mice, and humans, and the gene regulatory networks controlling proper PR specification in each. By drawing parallels in eye development between these divergent species, we identify a set of conserved organizing principles and transcriptional networks that govern PR subtype differentiation.

Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid

Inflation of the posterior and/or anterior swim bladder is a process previously demonstrated to be regulated by thyroid hormones. We investigated whether inhibition of deiodinases, which convert thyroxine (T4) to the more biologically active form, 3,5,3'-triiodothyronine (T3), would impact swim bladder inflation. Two experiments were conducted using a model deiodinase inhibitor, iopanoic acid (IOP). First, fathead minnow embryos were exposed to 0.6, 1.9, or 6.0 mg/L or control water until 6 d postfertilization (dpf), at which time posterior swim bladder inflation was assessed. To examine anterior swim bladder inflation, a second study was conducted with 6-dpf larvae exposed to the same IOP concentrations until 21 dpf. Fish from both studies were sampled for T4/T3 measurements and gene transcription analyses. Incidence and length of inflated posterior swim bladders were significantly reduced in the 6.0 mg/L treatment at 6 dpf. Incidence of inflation and length of anterior swim bladder were significantly reduced in all IOP treatments at 14 dpf, but inflation recovered by 18 dpf. Throughout the larval study, whole-body T4 concentrations increased and T3 concentrations decreased in all IOP treatments. Consistent with hypothesized compensatory responses, deiodinase-2 messenger ribonucleic acid (mRNA) was up-regulated in the larval study, and thyroperoxidase mRNA was down-regulated in all IOP treatments in both studies. These results support the hypothesized adverse outcome pathways linking inhibition of deiodinase activity to impaired swim bladder inflation. Environ Toxicol Chem 2017;36:2942-2952. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Duplication of Dio3 genes in teleost fish and their divergent expression in skin during flatfish metamorphosis

Deiodinase 3 (Dio3) plays an essential role during early development in vertebrates by controlling tissue thyroid hormone (TH) availability. The Atlantic halibut (Hippoglossus hippoglossus) possesses duplicate dio3 genes (dio3a and dio3b). Expression analysis indicates that dio3b levels change in abocular skin during metamorphosis and this suggests that this enzyme is associated with the divergent development of larval skin to the juvenile phenotype. In larvae exposed to MMI, a chemical that inhibits TH production, expression of dio3b in ocular skin is significantly up-regulated suggesting that THs normally modulate this genes expression during this developmental event. The molecular basis for divergent dio3a and dio3b expression and responsiveness to MMI treatment is explained by the multiple conserved TREs in the proximal promoter region of teleost dio3b and their absence from the promoter of dio3a. We propose that the divergent expression of dio3 in ocular and abocular skin during halibut metamorphosis contributes to the asymmetric pigment development in response to THs.

Implication of thyroid hormone signaling in neural crest cells migration: Evidence from thyroid hormone receptor beta knockdown and NH3 antagonist studies

Thyroid hormones (TH) have been mainly associated with post-embryonic development and adult homeostasis but few studies report direct experimental evidence for TH function at very early phases of embryogenesis. We assessed the outcome of altered TH signaling on early embryogenesis using the amphibian Xenopus as a model system. Precocious exposure to the TH antagonist NH-3 or impaired thyroid receptor beta function led to severe malformations related to neurocristopathies. These include pathologies with a broad spectrum of organ dysplasias arising from defects in embryonic neural crest cell (NCC) development. We identified a specific temporal window of sensitivity that encompasses the emergence of NCCs. Although the initial steps in NCC ontogenesis appeared unaffected, their migration properties were severely compromised both in vivo and in vitro. Our data describe a role for TH signaling in NCCs migration ability and suggest severe consequences of altered TH signaling during early phases of embryonic development.

Permanent Deiodinase Type 2 Deficiency Strongly Perturbs Zebrafish Development, Growth, and Fertility

Iodothyronine deiodinases are selenocysteine-containing enzymes that activate or inactivate thyroid hormones (THs). Deiodinase type 2 (Dio2) catalyzes the conversion of the prohormone T4 into the transcriptionally active T3 and is the predominant activating deiodinase in zebrafish. Using zinc finger nucleases, we generated two different dio2(-/-) mutant zebrafish lines to investigate the physiological function of this TH activator. The first line contains a deletion of 9 bp, resulting in an in-frame elimination of three conserved amino acids. The other line is characterized by an insertion of 4 bp, leading to the introduction of a premature stop-codon. Both lines completely lack Dio2 activity, resulting in a strong reduction of T3 abundancy in all tissues tested. Early development is clearly perturbed in these animals, as shown by a diverse set of morphometric parameters, defects in swim bladder inflation, and disturbed locomotor activity tested between 1 and 7 days after fertilization. Permanent Dio2 deficiency also provokes long-term effects because growth and especially fertility are severely hampered. Possible compensatory mechanisms were investigated in adult dio2(-/-) mutants, revealing a down-regulation of the inactivating deiodinase Dio3 and TH receptor transcript levels. As the first nonmammalian model with permanent Dio2 deficiency, these mutant zebrafish lines provide evidence that Dio2 is essential to assure normal development and to obtain a normal adult phenotype.

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.

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.

Deiodinase knockdown affects zebrafish eye development at the level of gene expression, morphology and function

Retinal development in vertebrates relies extensively on thyroid hormones. Their local availability is tightly controlled by several regulators, including deiodinases (Ds). Here we used morpholino technology to explore the roles of Ds during eye development in zebrafish. Transcriptome analysis at 3 days post fertilization (dpf) revealed a pronounced effect of knockdown of both T4-activating Ds (D1D2MO) or knockdown of T3-inactivating D3 (D3bMO) on phototransduction and retinoid recycling. This was accompanied by morphological defects (studied from 1 to 7 dpf) including reduced eye size, disturbed retinal lamination and strong reduction in rods and all four cone types. Defects were more prominent and persistent in D3-deficient fish. Finally, D3-deficient zebrafish larvae had disrupted visual function at 4 dpf and were less sensitive to a light stimulus at 5 dpf. These data demonstrate the importance of TH-activating and -inactivating Ds for correct zebrafish eye development, and point to D3b as a central player.

Thyroid disruption in zebrafish (Danio rerio) larvae: Different molecular response patterns lead to impaired eye development and visual functions

The vertebrate thyroid system is important for multiple developmental processes, including eye development. Thus, its environmentally induced disruption may impact important fitness-related parameters like visual capacities and behaviour. The present study investigated the relation between molecular effects of thyroid disruption and morphological and physiological changes of eye development in zebrafish (Danio rerio). Two test compounds representing different molecular modes of thyroid disruption were used: propylthiouracil (PTU), which is an enzyme-inhibitor of thyroid hormone synthesis, and tetrabromobisphenol A (TBBPA), which interacts with the thyroid hormone receptors. Both chemicals significantly altered transcript levels of thyroid system-related genes (TRα, TRβ, TPO, TSH, DIO1, DIO2 and DIO3) in a compound-specific way. Despite these different molecular response patterns, both treatments resulted in similar pathological alterations of the eyes such as reduced size, RPE cell diameter and pigmentation, which were concentration-dependent. The morphological changes translated into impaired visual performance of the larvae: the optokinetic response was significantly and concentration-dependently decreased in both treatments, together with a significant increase of light preference of PTU-treated larvae. In addition, swimming activity was impacted. This study provides first evidence that different modes of molecular action of the thyroid disruptors can be associated with uniform apical responses. Furthermore, this study is the first to show that pathological eye development, as it can be induced by exposure to thyroid disruptors, indeed translates into impaired visual capacities of zebrafish early life stages.

A high throughput passive dosing format for the Fish Embryo Acute Toxicity test

High throughput testing according to the Fish Embryo Acute Toxicity (FET) test (OECD Testing Guideline 236) is usually conducted in well plates. In the case of hydrophobic test substances, sorptive and evaporative losses often result in declining and poorly controlled exposure conditions. Therefore, our objective was to improve exposure conditions in FET tests by evaluating a passive dosing format using silicone O-rings in standard 24-well polystyrene plates. We exposed zebrafish embryos to a series of phenanthrene concentrations until 120h post fertilization (hpf), and obtained a linear dilution series. We report effect values for both mortality and sublethal morphological effects based on (1) measured exposure concentrations, (2) (lipid normalized) body residues and (3) chemical activity. The LC50 for 120hpf was 310μg/L, CBR50 (critical body residue) was 2.72mmol/kg fresh wt and La50 (lethal chemical activity) was 0.047. All values were within ranges expected for baseline toxicity. Impaired swim bladder inflation was the most pronounced morphological effect and swimming activity was reduced in all exposure concentrations. Further analysis showed that the effect on swimming activity was not attributed to impaired swim bladder inflation, but rather to baseline toxicity. We conclude that silicone O-rings (1) produce a linear dilution series of phenanthrene in the 120hpf FET test, (2) generate and maintain aqueous concentrations for reliable determination of effect concentrations, and allow for obtaining mechanistic toxicity information, and (3) cause no toxicity, demonstrating its potential as an extension of the FET test when testing hydrophobic chemicals.