Thyroid endocrine disruption and external body morphology of Zebrafish

Transcriptomic and developmental effects of persistent organic pollutants in sentinel fishes collected near an arctic formerly used defense site

Alaska contains over 600 formerly used defense (FUD) sites, many of which serve as point sources of pollution. These sites are often co-located with rural communities that depend upon traditional subsistence foods, especially lipid-rich animals that bioaccumulate and biomagnify persistent organic pollutants (POPs). Many POPs are carcinogenic and endocrine-disrupting compounds that are associated with adverse health outcomes. Therefore, elevated exposure to POPs from point sources of pollution may contribute to disproportionate incidence of disease in arctic communities. We investigated PCB concentrations and the health implications of POP exposure in sentinel fishes collected near the Northeast Cape FUD site on Sivuqaq (St. Lawrence Island), Alaska. Sivuqaq residents are almost exclusively Yupik and rely on subsistence foods. At the request of the Sivuqaq community, we examined differential gene expression and developmental pathologies associated with exposure to POPs originating at the Northeast Cape FUD site. We found significantly higher levels of PCBs in Alaska blackfish (Dallia pectoralis) collected from contaminated sites downstream of the FUD site compared to fish collected from upstream reference sites. We compared transcriptomic profiles and histopathologies of these same blackfish. Blackfish from contaminated sites overexpressed genes involved in ribosomal and FoxO signaling pathways compared to blackfish from reference sites. Contaminated blackfish also had significantly fewer thyroid follicles and smaller pigmented macrophage aggregates. Conversely, we found that ninespine stickleback (Pungitius pungitius) from contaminated sites exhibited thyroid follicle hyperplasia. Despite our previous research reporting transcriptomic and endocrine differences in stickleback from contaminated vs. reference sites, we did not find significant differences in kidney or gonadal histomorphologies. Our results demonstrate that contaminants from the Northeast Cape FUD site are associated with altered gene expression and thyroid development in native fishes. These results are consistent with our prior work demonstrating disruption of the thyroid hormone axis in Sivuqaq residents.

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

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

Development of the integrated fish endocrine disruptor test-Part B: Implementation of thyroid-related endpoints

Given the vital role of thyroid hormones (THs) in vertebrate development, it is essential to identify chemicals that interfere with the TH system. Whereas, among nonmammalian laboratory animals, fish are the most frequently utilized test species in endocrine disruptor research, for example, in guidelines for the detection of effects on the sex hormone system, there is no test guideline (TG) using fish as models for thyroid-related effects; rather, amphibians are used. Therefore, the objective of the present project was to integrate thyroid-related endpoints for fish into a test protocol combining OECD TGs 229 (Fish Short-Term Reproduction Assay) and 234 (Fish Sexual Development Test). The resulting integrated Fish Endocrine Disruption Test (iFEDT) was designed as a comprehensive approach to covering sexual differentiation, early development, and reproduction and to identifying disruption not only of the sexual and/or reproductive system but also the TH system. Two 85-day exposure tests were performed using different well-studied endocrine disruptors: 6-propyl-2-thiouracil (PTU) and 17α-ethinylestradiol (EE2). Whereas the companion Part A of this study presents the findings on effects by PTU and EE2 on endpoints established in existing TGs, the present Part B discusses effects on novel thyroid-related endpoints such as TH levels, thyroid follicle histopathology, and eye development. 6-Propyl-2-thiouracil induced a massive proliferation of thyroid follicles in any life stage, and histopathological changes in the eyes proved to be highly sensitive for TH system disruption especially in younger life stages. For measurement of THs, further methodological development is required. 17-α-Ethinylestradiol demonstrated not only the well-known disruption of the hypothalamic-pituitary-gonadal axis, but also induced effects on thyroid follicles in adult zebrafish (Danio rerio) exposed to higher EE2 concentrations, suggesting crosstalk between endocrine axes. The novel iFEDT has thus proven capable of simultaneously capturing endocrine disruption of both the steroid and thyroid endocrine systems. Integr Environ Assess Manag 2024;20:830-845. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Biallelic variants in GTF3C5, a regulator of RNA polymerase III-mediated transcription, cause a multisystem developmental disorder

General transcription factor IIIC subunit 5 (GTF3C5) encodes transcription factor IIIC63 (TFIIIC63). It binds to DNA to recruit another transcription factor, TFIIIB, and RNA polymerase III (Pol III) to mediate the transcription of small noncoding RNAs, such as tRNAs. Here, we report four individuals from three families presenting with a multisystem developmental disorder phenotype with biallelic variants in GTF3C5. The overlapping features include growth retardation, developmental delay, intellectual disability, dental anomalies, cerebellar malformations, delayed bone age, skeletal anomalies, and facial dysmorphism. Using lymphoblastoid cell lines (LCLs) from two affected individuals, we observed a reduction in TFIIIC63 protein levels compared to control LCLs. Genome binding of TFIIIC63 protein is also reduced in LCL from one of the affected individuals. Additionally, approximately 40% of Pol III binding regions exhibited reduction in the level of Pol III occupancy in the mutant genome relative to the control, while approximately 54% of target regions showed comparable levels of Pol III occupancy between the two, indicating partial impairment of Pol III occupancy in the mutant genome. Yeasts with subject-specific variants showed temperature sensitivity and impaired growth, supporting the notion that the identified variants have deleterious effects. gtf3c5 mutant zebrafish showed developmental defects, including a smaller body, head, and eyes. Taken together, our data show that GTF3C5 plays an important role in embryonic development, and that biallelic variants in this gene cause a multisystem developmental disorder. Our study adds GTF3C5-related disorder to the growing list of genetic disorders associated with Pol III transcription machinery.

Differential gene expression and developmental pathologies associated with persistent organic pollutants in sentinel fish in Troutman Lake, Sivuqaq, Alaska

Persistent organic pollutants (POPs) are lipophilic compounds that bioaccumulate in animals and biomagnify within food webs. Many POPs are endocrine disrupting compounds that impact vertebrate development. POPs accumulate in the Arctic via global distillation and thereby impact high trophic level vertebrates as well as people who live a subsistence lifestyle. The Arctic also contains thousands of point sources of pollution, such as formerly used defense (FUD) sites. Sivuqaq (St. Lawrence Island), Alaska was used by the U.S. military during the Cold War and FUD sites on the island remain point sources of POP contamination. We examined the effects of POP exposure on ninespine stickleback (Pungitius pungitius) collected from Troutman Lake in the village of Gambell as a model for human exposure and disease. During the Cold War, Troutman Lake was used as a dump site by the U.S. military. We found that PCB concentrations in stickleback exceeded the U.S. Environmental Protection Agency's guideline for unlimited consumption despite these fish being low trophic level organisms. We examined effects at three levels of biological organization: gene expression, endocrinology, and histomorphology. We found that ninespine stickleback from Troutman Lake exhibited suppressed gonadal development compared to threespine stickleback (Gasterosteus aculeatus) studied elsewhere. Troutman Lake stickleback also displayed two distinct hepatic phenotypes, one with lipid accumulation and one with glycogen-type vacuolation. We compared the transcriptomic profiles of these liver phenotypes using RNA sequencing and found significant upregulation of genes involved in ribosomal and metabolic pathways in the lipid accumulation group. Additionally, stickleback displaying liver lipid accumulation had significantly fewer thyroid follicles than the vacuolated phenotype. Our study and previous work highlight health concerns for people and wildlife due to pollution hotspots in the Arctic, and the need for health-protective remediation.

Ciona spp. and ascidians as bioindicator organisms for evaluating effects of endocrine disrupting chemicals: A discussion paper

In context of testing, screening and monitoring of endocrine-disrupting (ED) type of environmental pollutants, tunicates could possibly represent a particularly interesting group of bioindicator organisms. These primitive chordates are already important model organisms within developmental and genomics research due to their central position in evolution and close relationship to vertebrates. The solitary ascidians, such as the genus Ciona spp. (vase tunicates), could possibly be extra feasible as ED bioindicators. They have a free-swimming, tadpole-like larval stage that develops extremely quickly (<20 h under favorable conditions), has a short life cycle (typically 2-3 months), are relatively easy to maintain in laboratory culture, have fully sequenced genomes, and transgenic embryos with 3D course data of the embryo ontogeny are available. In this article, we discuss possible roles of Ciona spp. (and other solitary ascidians) as ecotoxicological bioindicator organisms in general but perhaps especially for effect studies of contaminants with presumed endocrine disrupting modes of action.

Exposure to different temperature regimes at early life stages affects hatching, developmental morphology, larval growth, and muscle cellularity in rainbow trout, Oncorhynchus mykiss

In this study, the effects of temperature on hatching, yolk-sac absorption, larval metamorphosis, post-metamorphic growth, developmental morphology, and muscle cellularity were assessed in rainbow trout, during its early development (until 52 days post-hatching, dph). From the eyed-ova stage, embryos were exposed to either low (8 ± 1 °C, LT-8) or high (16 ± 1 °C, HT-16) temperatures until hatching. Following hatching, half of the sac-fry from LT-8 group were shifted to higher temperature (16 ± 1 °C, LHT-16), and half from HT-16 group were shifted to medium temperature (13 ± 1 °C, HMT-13), for larval rearing. Incubating the eyed-ova at 16 °C preceded the hatching by 6 days, synchronized hatching duration, and minimized hatchlings' size-variation. However, it yielded smaller and morphologically less developed individuals compared to those incubated continuously at 8 ± 1 °C. Post-hatch shifting of sac-fry to high and medium temperatures, respectively, from the initial low and high regimes differentially affected the length and weight of fish. The effect on length was immediate and temporary, but on weight, it appeared to be permanent. Red muscle hypertrophy was observed to be high in HT-16 and HMT-13 individuals (high-temperature incubated groups). White muscle hypertrophy was high in HT-16 and LHT-16 individuals (high post-hatch rearing temperature groups). The effect of early-life temperature regimes on developmental morphology was found to be strong at 22 dph (82.5%) and comparatively weak at 52 dph (65%). The post-hatch rearing temperature caused an immediate but temporary effect on fin development, mainly pectoral, caudal, and anal fin (seen only at 22 dph, not at 52 dph). Contrarily, incubation temperature affected fin position, in a delayed but persistent manner (subtle at 22 dph, but stronger at 52 dph). Overall, this study provides new insights on temperature-dependent changes in developmental morphology, muscle cellularity, and larval growth in rainbow trout and shows that incubation temperature affects ontogeny profoundly than post-hatch thermal regimes.

Environmentally relevant arsenic exposure affects morphological and molecular endpoints associated with reproduction in the Western mosquitofish, Gambusia affinis

Arsenic (As) exposure, even at low environmentally relevant levels, may cause detrimental health outcomes through developmental toxicity and by acting as an endocrine disrupting compound (EDC). Although several studies indicate that wildlife bioaccumulate As, few evaluate the health impact on fish species in their natural environment. In the U.S., As has a drinking water regulatory limit of 10 μg/L. In many parts of Arizona, surface water and groundwater have naturally elevated levels of As from geologic deposits and contamination is exacerbated by anthropogenic activity. In aquatic environments, the Western mosquitofish, Gambusia affinis, is a good bioindicator for EDC exposure because of the distinct androgen-related development of an intromittent organ, the gonapodium, in males. We evaluated morphological and reproductive outcomes in mosquitofish exposed to As. In a laboratory experiment, juvenile male mosquitofish were exposed to sodium arsenite (0 μg/L, 0.75 μg/L, 7.50 μg/L, and 75 μg/L) for 30 days, and in a field study, populations of adult male mosquitofish were collected in Arizona waterways with As levels above and below the World Health Organization's regulatory limit. In both studies, higher As exposure was significantly associated with altered hepatosomatic indices, altered fish morphology, shortened gonopodia, and lower gonopodia-somatic indices. In the field experiment, populations from surface water with higher As concentrations exhibited lower condition factors, lower gonadal-somatic indices, distinct gonopodia shapes, and altered estrogen receptor alpha and vitellogenin gene expression; androgen receptor expression was unchanged. Together, laboratory and field results suggest that As exposure at environmentally-relevant levels affects general growth and reproductive development in mosquitofish. Observed effects may further influence individual health, mobility, or reproductive function, and because G. affinis is a species known to tolerate and adapt to a wide range of environments, it serves as a local bioindicator species as well as a model organism for parallel field and laboratory studies.

Reproductive and thyroid endocrine axis cross-talk in rainbow trout (Oncorhynchus mykiss) alevins

The goal of this study was to characterize morphological and molecular effects in rainbow trout alevins after waterborne exposures to 17β-estradiol (E2; 0.0008 to 0.5 μg/L), triiodothyronine (T3; 0.52 to 65 μg/L), and various co-treatments for 21 to 23 days. Interestingly, there was no consistent evidence that E2 alone influenced growth, development or deformity rates, however, 65 μg/L T3 alone expedited development, and both 13 μg/L and 65 μg/L alone caused a unique opercular deformity not previously reported. In addition, some potentiation between E2 and T3 at lower concentrations suggests some cross-talk between these two hormonal pathways may also contribute to the development of this opercular deformity. Gene expression changes were observed, including induction of vtg in rainbow trout alevins at 0.02 μg/L concentration of E2, which is the lowest concentration reported to induce vtg in rainbow trout alevins. These data suggest low-level E2 does not negate abnormal growth and development caused by hyperthyroidism, and examining more time points is likely required to demonstrate a stronger response profile for individual hormones and endocrine axes cross-talk.

Developmental thyroid disruption causes long-term impacts on immune cell function and transcriptional responses to pathogen in a small fish model

Current evidence suggests thyroid hormones (THs) impact development of the immune system, but few studies have explored the connection between the thyroid and immune systems, especially in fish. This is important as some environmental contaminants disrupt TH homeostasis and may thus have negative impacts on the immune system. To determine the long-term consequences of early life stage (ELS) hypothyroidism on immune function, fathead minnows were exposed to the model thyroid hormone suppressant propylthiouracil (PTU) from < 1 to 30 days post hatch. Fish were transferred to clean water and raised to adulthood (5-7 months post hatch) at which time, several aspects of immune function were evaluated. Ex vivo assessment of immune cell function revealed significant decreases (1.2-fold) in the phagocytic cell activity of PTU-treated fish relative to the controls. Fish were also injected with Yersinia ruckeri to evaluate their in vivo immune responses across a suite of endpoints (i.e., transcriptomic analysis, leukocyte counts, spleen index, hematocrit, bacterial load and pathogen resistance). The transcriptomic response to infection was significantly different between control and PTU-treated fish, though no differences in bacterial load or pathogen resistance were noted. Overall, these results suggest that early life stage TH suppression causes long-term impacts on immune function at the molecular and cellular levels suggesting a key role for TH signaling in normal immune system development. This study lays the foundation for further exploration into thyroid-immune crosstalk in fish. This is noteworthy as disruption of the thyroid system during development, which can occur in response to chemicals present in the environment, may have lasting effects on immune function in adulthood.

High temperature stress response is not sexually dimorphic at the whole-body level and is dependent on androgens to induce sex reversal

The understanding of the molecular and endocrine mechanisms behind environmentally-induced sex reversal in fish is of great importance in the context of predicting the potential effects of climate change, especially increasing temperature. Here, we demonstrate the global effects of high temperature on genome-wide transcription in medaka (Oryzias latipes) during early development. Interestingly, data analysis did not show sexual dimorphic changes, demonstrating that thermal stress is not dependent on genotypic sex. Additionally, our results revealed significant changes in several pathways under high temperature, such as stress response from brain, steroid biosynthesis, epigenetic mechanisms, and thyroid hormone biosynthesis, among others. These microarray data raised the question of what the exact molecular and hormonal mechanisms of action are for female-to-male sex reversal under high temperatures in fish. Complementary gene expression analysis revealed that androgen-related genes increase in females (XX) experiencing high water temperature. To test the involvement of androgens in thermal-induced sex reversal, an androgen antagonist was used to treat XX medaka under a high-temperature setup. Data clearly demonstrated failure of female-to-male sex reversal when androgen action is inhibited, corroborating the importance of androgens in environmentally-induced sex reversal.

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.

Testing for thyroid hormone disruptors, a review of non-mammalian in vivo models

Thyroid hormones (THs) play critical roles in profound changes in many vertebrates, notably in mammalian neurodevelopment, although the precise molecular mechanisms of these fundamental biological processes are still being unravelled. Environmental and health concerns prompted the development of chemical safety testing and, in the context of endocrine disruption, identification of thyroid hormone axis disrupting chemicals (THADCs) remains particularly challenging. As various molecules are known to interfere with different levels of TH signalling, screening tests for THADCs may not rely solely on in vitro ligand/receptor binding to TH receptors. Therefore, alternatives to mammalian in vivo assays featuring TH-related endpoints that are more sensitive than circulatory THs and more rapid than thyroid histopathology are needed to fulfil the ambition of higher throughput screening of the myriad of environmental chemicals. After a detailed introduction of the context, we have listed current assays and parameters to assess thyroid disruption following a literature search of recent publications referring to non-mammalian models. Potential THADCs were mostly investigated in zebrafish and the frog Xenopus laevis, an amphibian model extensively used to study TH signalling.

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.

Developmental instability is associated with estrogenic endocrine disruption in the Chilean native fish species, Trichomycterus areolatus

Endocrine disrupting compounds (EDCs) are widespread contaminants that alter the normal functioning of the endocrine system. While they cause dysfunctions in essential biological processes, it is unclear whether EDCs also impact developmental stability. In the present study, we investigated the occurrence of estrogenic endocrine disrupting compounds in a small watershed of south-central Chile impacted by anthropogenic activities. Then, we assessed their relationship with internal levels of estrogenic active compounds and fluctuating asymmetry (FA), a proxy of developmental stability in organisms with bilateral symmetry, in a native fish species (Trichomycterus areolatus). Yeast estrogenic screen assays were performed to measure estrogenic activity in river sediments and in male fish tissues collected from 17 sites along the Chillán watershed, and geometric morphometrics used to estimate fluctuating asymmetry based on the shapes of 248 fish skulls. Estrogenic activity was detected both in sediments and male fish tissues at concentrations of up to 1005 ng and 83 ng 17β-estradiol equivalent/kg dw, respectively. No significant correlation was found between the two. However, fish tissue estrogenicity, water temperature and dissolved oxygen explained >80% of the FA population variation. By showing a significant relationship between estrogenic activity and FA of T. areolatus, our results indicate that developmental stability can be altered by estrogenic endocrine disruption, and that FA can be a useful indicator of sub-lethal stress in T. areolatus populations.

Morphometric signatures of exposure to endocrine disrupting chemicals in zebrafish eleutheroembryos

Understanding the mode of action of the different pollutants in human and wildlife health is a key step in environmental risk assessment. The aim of this study was to determine signatures that could link morphological phenotypes to the toxicity mechanisms of four Endocrine Disrupting Chemicals (EDCs): bisphenol A (BPA), perfluorooctanesulfonate potassium salt (PFOS), tributyltin chloride (TBT), and 17-ß-estradiol (E2). Zebrafish (Danio rerio) eleutheroembryos were exposed from 2 to 5 dpf to a wide range of BPA, PFOS, TBT and E2 concentrations. At the end of the exposures several morphometric features were assessed. Common and non-specific effects on larvae pigmentation or swim bladder area were observed after exposures to all compounds. BPA specifically induced yolk sac malabsorption syndrome and altered craniofacial parameters, whereas PFOS had specific effects on the notochord formation presenting higher rates of scoliosis and kyphosis. The main effect of E2 was an increase in the body length of the exposed eleutheroembryos. In the case of TBT, main alterations on the morphological traits were related to developmental delays. When integrating all morphometrical parameters, BPA showed the highest rates of malformations in terms of equilethality, followed by PFOS and, distantly, by TBT and E2. In the case of BPA and PFOS, we were able to relate our results with effects on the transcriptome and metabolome, previously reported. We propose that methodized morphometric analyses in zebrafish embryo model can be used as an inexpensive and easy screening tool to predict modes of action of a wide-range number of contaminants.

Larval exposure to environmentally relevant concentrations of triclosan impairs metamorphosis and reproductive fitness in zebrafish

Developmental exposure to endocrine disruptors can cause organizational changes resulting in latent and transgenerational disease. We exposed zebrafish to environmentally relevant concentrations of triclosan during the critical period of metamorphosis and somatic sex differentiation to determine effects on metamorphosis and reproduction. We use biological and morphological biomarkers to predict potential modes of action. Larval exposure to environmentally relevant concentrations of triclosan was sufficient to cause adverse effects in adults and their offspring. TCS exposure delays metamorphosis and impairs fecundity and fertility. Offspring from TCS-exposed fish show decreased survival and delayed maturation, but their reproductive capacity is not altered. Delays in metamorphosis in conjunction with morphological indicators suggest that toxicity may result from lowered thyroid hormones in parental fish. This work illustrates the importance of evaluating the latent effects of early exposure to environmental contaminants, and that further studies to evaluate the effects of triclosan on the thyroid axis are warranted.

Protective effect of mulberry crude extract against nonylphenol-induced thyroid disruption by inhibiting the activity of deiodinase in rats

Nonylphenol (NP) is an endocrine-disrupting chemical (EDC) that can lead to thyroid disruption. We explored NP-induced toxicity in the rat thyroid and evaluated the mitigating effects of mulberry crude extract (MCE) on NP toxicity. First, we aimed to evaluate NP-induced thyroid disruption by dosing Sprague-Dawley (SD) rats with NP (0, 30, 90, or 270 mg/kg body weight) daily for 28 days. Second, we aimed to determine whether MCE had a detoxifying effect on NP-induced thyroid disruption by dosing SD rats with NP (270 mg/kg body weight) or/and MCE (30, 60, or 120 mg/kg body weight) daily for 28 days. We found that NP significantly inhibited free triiodothyronin (FT₃) and free thyroxine (FT₄) activity in rat serum (P < 0.05), but MCE intervention significantly increased FT₃ and FT₄ serum levels (P < 0.05). It is possible that changes in hormonal composition might trigger the TRH-TSH-TH automatic feedback loop. The activity of the three iodothyronine deiodinases increased significantly after NP-dosing (P < 0.05), but only deiodinase3 (D3) was downregulated after MCE treatment (P < 0.05). Therefore, MCE might be an effective NP-detoxification agent against thyroid disruption because it regulates D3 activity.

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.

Ultrastructural Alterations in Thyrocytes of Zebrafish ( Danio rerio) after Exposure to Propylthiouracil and Perchlorate

Histopathology is a widely used approach to evaluate effects of endocrine-active chemicals in the thyroid. However, effects at an ultrastructural level have hardly been examined in fish thyroids. In the present study, zebrafish was exposed to sublethal concentrations of propylthiouracil (PTU; 0-50 mg/L) and perchlorate (PER; 0-5,000 µg/L) for 5 weeks in a modified early life-stage test. None of the treatments caused significant mortality (no observed effect concentrations for survival ≥50 mg/L [PTU] and ≥5,000 µg/L [PER]). PTU induced dose-dependent alterations in the rough endoplasmic reticulum (rER) in all exposure groups, whereas only the 2 highest PER exposure groups (500 and 5,000 µg/L) resulted in alterations of the rER. Both substances caused an increase in the numbers of lysosomes and mitochondria, with mitochondria displaying distorted cristae. Increased mitochondrial diameters were only observed in the PTU treatment. PER-exposed samples displayed an increase in apical microvilli. The highest PTU concentration (50 mg/L) showed first signs of cellular degeneration. Ultrastructural changes in zebrafish thyrocytes thus appear specific for different chemicals, most likely depending on their specific modes of action. Additional knowledge of subcellular changes in thyrocytes can help to better understand and interpret existing histological data in the future.

ZEBRAFISH AS AN IN VIVO MODEL FOR SUSTAINABLE CHEMICAL DESIGN

Heightened public awareness about the many thousands of chemicals in use and present as persistent contaminants in the environment has increased the demand for safer chemicals and more rigorous toxicity testing. There is a growing recognition that the use of traditional test models and empirical approaches is impractical for screening for toxicity the many thousands of chemicals in the environment and the hundreds of new chemistries introduced each year. These realities coupled with the green chemistry movement have prompted efforts to implement more predictive-based approaches to evaluate chemical toxicity early in product development. While used for many years in environmental toxicology and biomedicine, zebrafish use has accelerated more recently in genetic toxicology, high throughput screening (HTS), and behavioral testing. This review describes major advances in these testing methods that have positioned the zebrafish as a highly applicable model in chemical safety evaluations and sustainable chemistry efforts. Many toxic responses have been shown to be shared among fish and mammals owing to their generally well-conserved development, cellular networks, and organ systems. These shared responses have been observed for chemicals that impair endocrine functioning, development, and reproduction, as well as those that elicit cardiotoxicity and carcinogenicity, among other diseases. HTS technologies with zebrafish enable screening large chemical libraries for bioactivity that provide opportunities for testing early in product development. A compelling attribute of the zebrafish centers on being able to characterize toxicity mechanisms across multiple levels of biological organization from the genome to receptor interactions and cellular processes leading to phenotypic changes such as developmental malformations. Finally, there is a growing recognition of the links between human and wildlife health and the need for approaches that allow for assessment of real world multi-chemical exposures. The zebrafish is poised to be an important model in bridging these two conventionally separate areas of toxicology and characterizing the biological effects of chemical mixtures that could augment its role in sustainable chemistry.

Effects of thyroid endocrine manipulation on sex-related gene expression and population sex ratios in Zebrafish

Thyroid hormone reportedly induces masculinization of genetic females and goitrogen treatment delays testicular differentiation (ovary-to-testis transformation) in genetic males of Zebrafish. This study explored potential molecular mechanisms of these phenomena. Zebrafish were treated with thyroxine (T4, 2nM), goitrogen [methimazole (MZ), 0.15mM], MZ (0.15mM) and T4 (2nM) (rescue treatment), or reconstituted water (control) from 3 to 33days postfertilization (dpf) and maintained in control water until 45dpf. Whole fish were collected during early (25dpf) and late (45dpf) testicular differentiation for transcript abundance analysis of selected male (dmrt1, amh, ar) and female (cyp19a1a, esr1, esr2a, esr2b) sex-related genes by quantitative RT-PCR, and fold-changes relative to control values were determined. Additional fish were sampled at 45dpf for histological assessment of gonadal sex. The T4 and rescue treatments caused male-biased populations, and T4 alone induced precocious puberty in ∼50% of males. Male-biased sex ratios were accompanied by increased expression of amh and ar and reduced expression of cyp19a1a, esr1, esr2a, and esr2b at 25 and 45dpf and, unexpectedly, reduced expression of dmrt1 at 45dpf. Goitrogen exposure increased the proportion of individuals with ovaries (per previous studies interpreted as delay in testicular differentiation of genetic males), and at 25 and 45dpf reduced the expression of amh and ar and increased the expression of esr1 (only at 25dpf), esr2a, and esr2b. Notably, cyp19a1a transcript was reduced but via non-thyroidal pathways (not restored by rescue treatment). In conclusion, the masculinizing activity of T4 at the population level may be due to its ability to inhibit female and stimulate male sex-related genes in larvae, while the inability of MZ to induce cyp19a1a, which is necessary for ovarian differentiation, may explain why its "feminizing" activity on gonadal sex is not permanent.

A Rat α-Fetoprotein Binding Activity Prediction Model to Facilitate Assessment of the Endocrine Disruption Potential of Environmental Chemicals

Endocrine disruptors such as polychlorinated biphenyls (PCBs), diethylstilbestrol (DES) and dichlorodiphenyltrichloroethane (DDT) are agents that interfere with the endocrine system and cause adverse health effects. Huge public health concern about endocrine disruptors has arisen. One of the mechanisms of endocrine disruption is through binding of endocrine disruptors with the hormone receptors in the target cells. Entrance of endocrine disruptors into target cells is the precondition of endocrine disruption. The binding capability of a chemical with proteins in the blood affects its entrance into the target cells and, thus, is very informative for the assessment of potential endocrine disruption of chemicals. α-fetoprotein is one of the major serum proteins that binds to a variety of chemicals such as estrogens. To better facilitate assessment of endocrine disruption of environmental chemicals, we developed a model for α-fetoprotein binding activity prediction using the novel pattern recognition method (Decision Forest) and the molecular descriptors calculated from two-dimensional structures by Mold² software. The predictive capability of the model has been evaluated through internal validation using 125 training chemicals (average balanced accuracy of 69%) and external validations using 22 chemicals (balanced accuracy of 71%). Prediction confidence analysis revealed the model performed much better at high prediction confidence. Our results indicate that the model is useful (when predictions are in high confidence) in endocrine disruption risk assessment of environmental chemicals though improvement by increasing number of training chemicals is needed.