The Role of the Thyroid Axis in Fish

Immunotoxicity of thyroid hormone system disrupting compounds in fish

Endocrine disrupting compounds (EDCs) are among the most studied environmental pollutants in the field of (eco)toxicology, and different fish species are commonly used as model organisms, especially for studying reprotoxic effects. Despite the scientific and regulatory importance of EDCs, little attention has been given to their immunotoxic effects in fish. Basic knowledge and test systems for immune-related outcomes in fish are limited. For example, while the impact of estrogenic EDCs on the fish immune system has raised some attention in the last decade, thyroid hormone system disrupting compounds (THSDCs) and their impact on the fish immune system are less well studied. Thus, this literature review is aimed at describing the immunomodulatory roles of thyroid hormones (THs), as well as summarizing the existing research on the immunotoxicity of THSDCs in fish. A simplified potential adverse outcome pathway (AOP) was created, explaining the key events between THSD and lowered survival of fish experiencing pathogen infections along with chemical exposure. This AOP demonstrates that THSDCs can alter immune system functioning on a molecular, cellular, and organism level and, therefore, lead to reduced survival by lowering pathogen resistance of fish. However, available data were mainly limited to molecular analyses of immune-related biomarkers and included only few studies that conducted experiments demonstrating immunotoxic effects at organism level that can inform about population-relevant outcomes. Our putatively developed and simplified AOP can support the incorporation of immune-related endpoints in EDC testing guidelines and aid the development of risk assessments for THSDCs for human and environmental health.

Early life development and sex determination of brown trout affected by treated wastewater discharge

Artificial conditions limit the ability of laboratory studies to describe the complex effects of polluted environments on aquatic life. This study aimed to evaluate the impacts of treated wastewater discharge on the survival, growth, and sex ratio balance of the population of brown trout (Salmo trutta m. fario) in situ. Five floating incubators with 1000 eggs each were placed in the upstream reference and treated wastewater-affected sites in the Czech Republic for approximately three months. The hatched fish were grown in a natural environment for nearly one year. Water quality, including nutrients, temperature, pharmaceutical and personal care products, biological effects by bioassays and fish mortality, metabolic rate, and growth, were measured regularly. Up to 72 pharmaceutical and personal care products (7400-23000 ng/sampler) were detected in the passive samplers deployed downstream of the sewage treatment plant effluent. In vitro bioassays of the sampler extracts indicated elevated oestrogenic effects, transthyretin binding inhibition, and aryl hydrocarbon-mediated and androgenic potencies, showing endocrine-disrupting potential at the polluted site. The cumulated mortality of brown trout in the exposed group (9.67%) was significantly higher (p < 0.05) than in the control group (5.16%). In addition, the body size, growth, and metabolic rate of exposed fish were significantly lower (p < 0.05). The sex ratio of brown trout in the effluent-affected stretch was imbalanced, and sterile individuals were detected after several months of natural development in the stream. The observed effects of treated wastewater on the early developmental stages of aquatic wildlife could be connected to the development and readiness of adult individuals and, consequently, to the sustainability of freshwater ecosystems. Applying the hatching apparatus used in fishery practices, followed by comparing mortality, development, and sex with reference localities, seems to be a promising biomonitoring approach that can indicate hotspots for in-depth investigation and risk assessment.

Diversity and dynamics of fish ovaries: Insights into reproductive strategies, hormonal regulation, and ovarian development

Fish ovaries exhibit a remarkable diversity in shape, size, and organization, reflecting the myriad reproductive strategies employed by different species. This review delves into the intricate biology of fish ovaries, highlighting their structural diversity and the hormonal regulation that governs ovarian development and oocyte maturation. Key hormones include pituitary gonadotropins (GTHs) and maturation-inducing hormones (MIHs), which initiate oocyte growth and maturation. GTHs stimulate ovarian production of estradiol-17β and 17α,20β-DP, which induce oocyte maturation via MPF formation. Sex steroids like estrogens and progestogens, synthesized from cholesterol, play crucial roles. Other hormones, including growth hormone, prolactin, thyroid hormones, IGFs, ACTH, and melatonin, influence ovarian activity. The review also explores the varied reproductive strategies among fish, including oviparity and viviparity, and discusses how environmental factors like water temperature and photoperiod influence ovarian histology. Understanding the complex interplay between these factors is essential for advancing fisheries management, conservation, and aquaculture practices. Additionally, the evolutionary trajectory of fish ovaries underscores their adaptation to diverse ecological niches, contributing to the survival and reproductive success of fish species. The ovarian stroma provides structural support and houses various cell types, including dendritic cells (DCs), endocrine cells, and telocytes, contributing to follicle growth and hormone production, essential for reproductive success in fish. Fish ovaries are a crucial aspect of fish biology, with their structure and function intricately regulated by hormonal, environmental, and seasonal factors.

The interplay between thyroxine and cortisol in zebrafish: Effects on growth, physiology, and reproduction

In the current study, the role of thyroxine (T4) and cortisol (CO) on growth, reproduction, and whole-body hormone levels in male and female zebrafish was assayed. Fifty-seven days post-fertilization, zebrafish [132 females; initial weight: 0.3 ± 0.0 g and 132 males (initial weight: 0.2 ± 0.0 g)] were fed with diets including 10 mg T4 kg feed-1 (T4), 10 mg CO kg feed-1 (CO), 10 mg T4 kg feed-1 + 10 mg CO kg feed-1 (T4 + CO), and a control group (without hormones) for 28 days. At the end of the experiment, growth performance, feed utilization, whole-body hormone levels, gonadal development, and spawning performance were determined. The growth performance was improved following T4 administration in both males and females. Viscerosomatic and gonadosomatic indices in females were significantly enhanced in the T4 group compared with the other groups. Regarding oocyte developmental stages, a lower number of oocytes at final maturation were found in the CO group compared with the other treatments. Oocyte diameter was significantly increased in T4 compared with the other groups. Whole-body CO levels in both males and females were increased and decreased in CO and T4 treatments, respectively. Moreover, whole-body thyroid hormone levels in both sexes were higher in T4 group than the other groups. The hatching rate and survival rate at three days post-hatch were increased in the T4 group. Overall, the findings of this study underscore the stimulatory role of T4 in enhancing the physiological performance of zebrafish, leading to improved growth and increased reproductive output in both males and females.

Investigation of serum thyroid hormones, iodine and cobalt concentrations across common aquarium-housed elasmobranchs

Introduction

Thyroid disease is an important condition to understand in elasmobranchs, with goiters being predominant. To identify dysfunction, measuring serum thyroid hormone levels is a standard of practice for diagnosing disease in most species. Although these levels have been reported in elasmobranch literature, the testing methodology is varied and values are not clinically useful for most aquarium species. In a group of aquarium-housed elasmobranchs, thyroid hormone levels had been persistently low or not detectable in otherwise healthy animals as well as animals with thyroid disease. The concern for reliability of these results to diagnose thyroid disease, prompted a shift to serum iodine levels as a proxy to determine thyroid health.

Methods

This study assesses thyroid hormone and iodine levels as compared to thyroid disease stage in elasmobranchs with and without dietary supplementation, to determine the efficacy of using these serum values to guide clinical decisions.

Results

Serum thyroid hormone results were lower than the readable range of the standard curve in both sharks and rays; thus reported values are usually extrapolated. Including additional standards down to the limit of sensitivity improved detection, however increasing the sample volume tested was determined to be the most important factor for obtaining measurable results in low-value thyroid hormone samples. Serum iodine levels are reported in three groups of southern stingrays (Hypanus americanus). Other elasmobranch species maintained in aquaria with and without thyroid disease were used for biological comparisons. Non-goiter, diseased animals reliably had elevated levels (over baseline) of thyroid hormones and iodine; in goiter cases, hormones were not useful. Additionally, it was found that cobalt levels were also elevated in some disease states and correlated positively with serum iodine levels.

Conclusion

Current available thyroid testing may not provide clinically useful values unless methodology is adjusted, or disease is severe. Serum iodine may be a useful marker to investigate thyroid health. Further, while thyroid disease may be identifiable with thyroid hormones, it is not straightforward or substantial enough alone for diagnosis.

Maintaining the physiological stability during artificial spawning of Liza ramada in captivity

In captivity, the physiological condition of mature brood fish has a vital role for a successful artificial spawning. Therefore, the current study aimed to minimize endocrine and metabolic disruptions during routine handling, transportation, and acclimatization during artificial spawning in L. ramada. Here, we determined the impacts of transportation and handling, as well as the acclimation to different salinities on the levels of total thyroxine (T4), triiodothyronine (T3), cortisol, and glucose in the mature L. ramada females. The transportation procedures of cultured mature females of L. ramada without anesthesia induced physiological stress as reflected with a rapid elevation in serum cortisol and glucose concomitant with a decrease in T4 within 3 h. However, the anesthesia protocol and the gradual acclimatization to seawater (24 h) were successful in maintaining baseline concentrations of the measured hormones in mature L. ramada females. The recorded levels of thyroid hormones (T4 and T3) and cortisol proved that 40 mg l−1 of clove oil was superior to anesthetic tricaine methane sulfonate (MS-222). In parallel, clove oil as an anesthetic has a rapid induction time and longer recovery time compared to MS-222 in L. ramada anesthesia. So, the slow acclimation and clove oil anesthesia were crucial during the induction of spawning in L. ramada. Indeed, all injected females were physiologically stable and spawned within the appropriate time consistent with the histological observation of both ovary and liver. Together, these findings recommend that maintaining the physiological stability of broodstock is critical for the successful artificial spawning of mullet.

2-Ethylhexyl-4-methoxycinnamate on marine and coastal environments: A comprehensive review of its environmental significance and biological impact

Marine and coastal environments are constantly subjected to increasing pressures associated with population growth, industrialization development, pollution and higher demand feeding society's consumerism. Among these pressures, there has been an increasing concern towards UV filters occurrence in aquatic ecosystems due to a greater use of personal care products (PCPs). 2-ethylhexyl-4-methoxycinnamate (EHMC) is one of the most used UV filters in sunscreen formulations, yet few reports address its effects in biota. This literature review intends to collect the available information concerning the environmental presence of EHMC in marine and coastal ecosystems and their effects in biota. The EHMC effects have been reported for the taxonomic groups: Actinomycetes, Alphaproteobacteria, Bacilli, Cytophagia, Flavobacteriia, Gammaproteobacteria, Actinopterygii, Anthozoa, Bacillariophyceae, Bivalvia, Branchiopoda, Coccolithophyceae, Echinoidea, Gastropoda, Malacostraca, Annelida and Thecostraca. The reported literature evaluated endpoints mainly related to development, viability, mortality, estrogenicity, gene transcription disruptions, biochemical alterations and morphophysiological changes. Based on the available information, there is still a clear need for further investigations related to EHMC and its toxicological effects on marine and coastal organisms.

Combined effects of co-exposure to microcystin-LR and polystyrene microplastics on growth, brain pathology and thyroid hormone homeostasis in adult zebrafish

The concurrent presence of algal blooms and microplastics pollution in natural water bodies poses a novel threat. However, the joint effects of microcystin-LR (MCLR) in combination with polystyrene microplastics (PSMPs) on the thyroid endocrine system of adult fish remains unclear. In our study, male zebrafish (Danio rerio) were exposed to environmentally relevant concentrations of MCLR alone (0, 0.8, 4, 20 μg/L) and a mix of MCLR and PSMPs (100 μg/L) for 60 days. Alterations in brain histology, thyroid hormone (TH) levels, and the transcription levels of hypothalamic-pituitary-thyroid (HPT)-axis genes were used to assess the thyroid function. In the MCLR-only treatment groups, we observed mild brain tissue damage characterized by glial scarring and hyperemia. The presence of PSMPs exacerbated the brain damage cause by MCLR, resulting in more pronounced ventriculomegaly and hyperemia. No significant changes in whole-body thyroxine (T4) and triiodothyronine (T3) levels were observed in the MCLR-only groups, while a significant decrease was noted in the groups co-exposed to MCLR and PSMPs. Additionally, significant alterations in crh, tshβ, ttr, trα, and trβ expression levels in the combined exposure groups provided further confirmation that MCLR and PSMPs jointly cause thyroid endocrine disruption. Our findings suggest that the fish can trigger a compensatory mechanism to maintain thyroid hormone homeostasis in response to environmentally relevant concentrations of MCLR. However, the presence of PSMPs disrupts this self-regulatory equilibrium, thereby exacerbates the thyroid endocrine disruption cause by MCLR in zebrafish.

Hormonal symphony: The dynamic duo of IGF and EGF in gonadotropin-induced fish ovarian development and egg maturation

Fish oocyte maturation (FOM) is a critical biological process that occurs before ovulation and is influenced by gonadotropins, particularly luteinizing hormone (LH). The release of LH stimulates the ovarian follicle to produce a maturation-inducing hormone (MIH), specifically 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-DP), which initiates the formation of maturation-promoting factor (MPF) through the activation of cyclin B and cdc2 kinase. Insulin-like growth factor I (IGF-I) significantly regulates ovarian functions, including steroidogenesis, by activating its membrane receptors and the tyrosine kinase pathway. IGF-I influences oocyte maturation directly via the PI3 kinase pathway, independent of steroid hormones. Additionally, epidermal growth factor (EGF) promotes cell growth and differentiation by binding to its receptor (EGFR). It is implicated in mediating human chorionic gonadotropin (hCG)-induced DNA synthesis in ovarian follicles while suppressing apoptosis. The presence of EGF in follicle cells and oocytes, along with its higher expression in oocytes, suggests it may act as a paracrine signal regulating somatic cell activity. Recent studies indicate that the activin system in follicle cells could be a target for EGF activity. The EGFR signaling pathway enhances gonadotropin-induced steroidogenesis and governs the transition of oocyte maturation stages, essential for successful fertilization. This review synthesizes current research on the roles of gonadotropins, IGFs, and EGFs in fish oocyte maturation and ovarian steroid production.

Thyroid Endocrine Disrupting Potential of Fluoxetine in Zebrafish Larvae

Fluoxetine (FLX), a typical selective serotonin reuptake inhibitors, has been frequently detected in aquatic environment and wild fish. However, little is known about its effect on thyroid endocrine system. In the present study, zebrafish (Danio rerio) embryos were exposed to 1, 3, 10, and 30 μg/L of FLX for 6 days. Chemical analysis demonstrated that FLX and its metabolic product (nonfluoxetine, NFLX) were accumulated in zebrafish larvae. The exposure resulted in decreased thyroid hormones (THs) levels, indicating thyroid endocrine disruption. Moreover, thyroid-stimulating hormone (TSH) content was significantly inhibited in a concentration-dependent manner after exposure to FLX. Gene transcription in the hypothalamic-pituitary-thyroid (HPT) axis was further examined, and the results showed that the genes encoding corticotrophin-releasing hormone (crh) and thyrotropin-releasing hormone (trh) were significantly up-regulated as a compensatory mechanism for the decreased TH contents accompanied with decreased tshβ mRNA expression. In addition, genes involved in thyroid hormone synthesis (sodium/iodide symporter, nis, thyroglobulin, tg) and transport (transthyretin, ttr) were down-regulated after exposure to FLX in a concentration-dependent manner. The increased gene transcription of deiodinases (dio2) and uridinediphosphate-glucuronosyltransferase (ugt1ab) might be responsible for the decrease of TH contents. In addition, a significant inhibition in thyroid hormone receptors (trα and trβ) gene expression was observed upon treatment with FLX. All these results demonstrated that FLX could alter THs and TSH content as well as gene transcription in the HPT axis, exerting an endocrine disruption of the thyroid system in zebrafish larvae.

Ontogeny of adenohypophyseal cells, pituitary gland development, and structure in adults of Astyanax lacustris (Teleostei, Characiformes): an emerging Neotropical model fish species

Pituitary gland morphogenesis and the ontogeny of the adenohypophyseal (AH) cells of Astyanax lacustris are presented herein. This Characiformes species shows great ecological and commercial importance, and it has been increasingly used as animal model. For this study, A. lacustris specimens were collected from 0.5 to 120 days after hatching (dah) (adults). The entire animal or its head was appropriately fixed, and after histological processing, the sections were subjected to histochemical and immunohistochemical reactions, using homologous and heterologous antibodies. The first AH cells of A. lacustris were detected at 1 dah by the immunostaining of prolactin (PRL)-producing cells. The morphology of the gland presented changes in shape throughout the development, starting with elongation but more oval at the end. The neurohypophysis was differentiated at 3 dah, along with the identification of adrenocorticotropic hormone (ACTH), melanotropic hormone (MSH), thyroid-stimulating hormone (TSH), and follicle-stimulating hormone (FSH)-producing cells. Identification of the immunoreactive cells to anti-luteinizing hormone (LH), anti-somatolactin (SL), and anti-growth hormone (GH) antibodies occurred at 5 dah. At 20 dah, an increase in pituitary proportions and the presence of the pituitary stalk were observed. At 60 dah, the pituitary gland already had the same shape and distribution of AH cells seen in the adult. The ontogeny of adenohypophyseal cells in A. lacustris corroborates the heterogeneity in the appearance of these cell types in teleosts and suggests that these hormones actively participate during the post-hatching development of this species, even before the establishment of all endocrine axes. Our findings contribute to understanding the morphogenesis of the hypothalamic-pituitary axis in South American teleosts, providing essential data for the development of future studies related to pituitary gland morphophysiology under normal or experimental conditions.

Effects of L-NAME and air exposure on mitochondrial energetic markers, thyroid hormone receptor/regulator system and stress/ease-responsive receptor expression in the brain/gut axis of zebrafish

As a signal molecule, nitric oxide (NO) has several physiological actions in fish. However, the action of NO on the brain/gut axis, a classic inter-organal axis that bridges the gastrointestinal tract and the CNS, still requires more understanding. The short-term in vivo action of a NO inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), on mitochondrial energetic markers and the receptor expression of thyroid hormone (TH) and neuroendocrine hormones involved in stress/ease response was tested in the brain/gut axis of zebrafish exposed to either in non-stressed or air-exposed condition. L-NAME treatment decreased the NO content in brain and gut segments in non-stressed fish but rose upon L-NAME treatment in air-exposed fish that corresponded with the activation of inos, nnos, hif1a and hif1an transcript expressions. The brain/gut segments that showed spatial and differential sensitivity to L-NAME, modified the transcript expression patterns of stress (adra2da, adrb1, nr3c2)- and ease-responsive (htr2b, slc6a4a, mtnr1aa) hormone receptors. The expression pattern of the TH receptor/regulator system (thra, thrb, dio1, dio2, dio3) becomes more active in gut segments than brain segments upon L-NAME challenge in stressed zebrafish. The data provide evidence for a novel role of NO as an integrator of brain/gut axis segments in zebrafish, where the endogenously produced NO in mid-brain/posterior-gut axis aligns together upon air-exposure stress, providing a lead role to the posterior gut that activates and directs the neuroendocrine receptor expressions of stress/ease responsive genes. The data further invites studies exploring the therapeutic potential of L-NAME in this biomedical model to control the brain/gut axis segments.

Effect of TSH on aromatase expression of ovarian granulosa cells in obese mice

PURPOSE

Aromatase plays an important role in ovarian development, the normal progress of the menstrual cycle, and fertility status. Elevated aromatase activity is linked to obesity. There is a bidirectional relationship between obesity and thyroid function. Few studies have investigated the relationship between TSH and ovarian aromatase in obesity. Our aim was to investigate the effect of TSH on aromatase expression of ovarian granulosa cells in obese mice.

METHODS

Female mice pups were divided into an obesity group and a control group. Obese parameters and the time of pubertal onset were recorded. At the age of 5 weeks, blood and tissues were obtained. Serum aromatase and hormone concentrations were measured using ELISA. The granulosa cells were isolated and exposed to variable concentrations (0 μM, 1 μM, 10 μM, 100 μM) of TSH. The expression of CYP19A1 mRNA and protein were assessed via RT-qPCR and western blot.

RESULTS

In female mice, body weight, Lee's obesity index, and serum levels of E2, aromatase, and TSH were significantly higher in the obesity group compared to the control group, whereas the time of pubertal onset and serum T3 and T4 concentrations were significantly lower (all P < 0.001). In granulosa cells, the expression of CYP19A1 mRNA in the obesity group was lower than that in the control group at 1 μM and 100 μM concentrations of TSH (both P < 0.001). The expression of CYP19A1 protein in the obesity group was higher than that in the control group after TSH stimulation (P = 0.014, P < 0.001, and P = 0.004, respectively). With the increase of TSH concentrations, the expression of CYP19A1 mRNA and protein in the two groups significantly increased (all P < 0.001).

CONCLUSION

Early puberty and elevated serum aromatase and TSH levels were found in obese female mice. In the granulosa cells of obese mice, TSH directly regulates aromatase expression in a dose-dependent manner.

Advancing thyroid disease research: The role and potential of zebrafish model

Thyroid disorders significantly affect human metabolism, cardiovascular function, skeletal health, and reproductive systems, presenting a complex challenge due to their multifactorial nature. Understanding the underlying mechanisms and developing novel therapeutic approaches require appropriate models. Zebrafish, with their genetic tractability, short life cycle, and physiological relevance, have emerged as a valuable model for investigating thyroid diseases. This review provides a comprehensive analysis of the zebrafish thyroid gland's structure and function, explores its application in modeling thyroid pathologies such as hypothyroidism, hyperthyroidism, and thyroid cancer, and discusses current limitations and possible improvements. Furthermore, it outlines future directions for zebrafish-based research, focusing on enhancing the model's relevance to human thyroid disease and its potential to expedite the development of clinical therapies.

Thyroid and growth hormone endocrine disruption and mechanisms of homosalate and octisalate using wild-type, thrαa-/-, and dre-miR-499-/- zebrafish embryo/larvae

Homosalate (HS) and octisalate (OS), which are used in sunscreen for the purpose of blocking ultraviolet rays, are frequently detected in water environment. Although effects on estrogens and androgens have been reported, studies on thyroid and growth hormone endocrine disruption are limited. In the present study, larval mortality was compared in wild-type and two knockout fish (thyroid hormone receptor alpha a knockout (thrαa-/-) and dre-miR-499 knockout (dre-miR-499-/-)) after 96 h of exposure to HS and OS (0, 0.003, 0.03, 0.3, 3, 30 and 300 µg/L). To investigate the mechanisms of thyroid and growth hormone endocrine disruption, we measured the levels of triiodothyronine (T3), thyroxine (T4), thyroid stimulating hormone (TSH), growth hormone (GH), and insulin-like growth factor-1 (IGF-1), and the regulation of representative genes related to the hypothalamus-pituitary-thyroid (HPT) and GH/IGF axis in wild-type zebrafish exposed to target chemicals. The significantly lower larval survival rate of thrαa-/- and dre-miR-499-/- fish exposed to 300 μg/L of HS and OS suggest that thyroid hormone receptors and dre-miR-499 play a crucial role in the toxic effects of HS and OS. The finding of a significant increase in T3 and T4 in zebrafish larvae exposed to HS and OS supports a significant decrease in the crh gene. The reduction of GH and IGF-1 in fish exposed to HS and OS is well supported by the regulation of genes involved in the GH/IGF axis. Our observations suggest that exposure to HS and OS affects not only thyroid hormone receptors and their associated miRNAs, but also the feedback routes of HPT and GH/IGF axes, ultimately leading to growth reduction.

Gene expression in the hypothalamic-pituitary-thyroid axis in Seriola rivoliana early larvae development at different temperatures

We analyzed the expression of genes involved in the hypothalamic-pituitary-thyroid axis (HPT-axis) in the longfin yellowtail Seriola rivoliana early larva, including temperature effects (22, 26 and 28 °C) and days of development (day one, day two, and day six after hatching). We aimed to determine if egg and larval incubation at different temperatures could disrupt this critical endocrine axis, which, in an aquaculture context, it could provoke mortality during early metamorphosis. There was a significant interaction between temperature and developmental timing on the relative expression of thyrotropin releasing hormone (trh). Larvae at 22 °C was the longest and increased more trh expression than larvae at higher temperatures. Interestingly, thyrotropin stimulating hormone (tsh) was highly expressed after hatching. Subsequently, it was downregulated at any temperature at least until day four, suggesting a temporal inhibition of the HPT axis. Therefore, we suggest that tsh-binding (tshr) to follicles should have occurred from hatching, creating a further "cascade effect" of upregulation of larval thyroglobulin (tg) from day two in a temperature-dependent manner. Consequently, new thyroid hormones should have been produced after yolk sac absorption. The above may indicate a narrow window of larval survival, where the larval transition from endogenous to exogenous feeding would depend on the correct timing to synthesize tg. Temperature significantly affected the expressions of deiodinase 1 (dio1-downregulated) and deiodinase 2 (dio2-upregulated) after hatching. The expressions of thyroid receptors alpha (trα) and beta (trβ) remained constant after hatching without significant effects of temperature and days of development. Then, the differential expression on day six showed that all HPT-axis transcripts increased their expressions as larvae developed, which suggested a functional HPT. Finally, there was no evidence that any temperature would disrupt the endocrine's larval axis, which indicated that the longfin yellowtail has a wide temperature adaption. Nevertheless, based on tg upregulation, we suggest that larvae should be maintained around 25-26 °C after hatching for a better chance of survival and development.

Effects and phenotypic consequences of transient thyrotoxicosis and hypothyroidism at different stages of zebrafish Danio rerio (Teleostei; Cyprinidae) skeleton development

The effects and consequences of changes in thyroid hormones (THs) level are among the actively studied topics in teleost developmental and evolutionary biology. In most of the experimental models used, the altered hormonal status (either hypo- or hyperthyroidism) is a stable characteristic of the developing organism, and the observed phenotypic outcomes are the cumulative consequences of multiple TH-induced developmental changes. Meanwhile, the influence of the transient fluctuations of TH content on skeleton development has been much less studied. Here, we present experimental data on the developmental effects and phenotypic consequences of transient, pharmacologically induced thyrotoxicosis and hypothyroidism at different stages of ossified skeleton patterning in zebrafish. According to the results, the skeleton structures differed in TH sensitivity. Some showed a notable shift in the developmental timing and rate, while other demonstrated little or no response to changes in TH content. The developmental stages also differed in TH sensitivity. We identified a relatively short developmental period, during which changes in TH level significantly increased the developmental instability and plasticity, leading to phenotypic consequences comparable to those in fish with a persistent hypo- or hyperthyroidism. These findings allow this period to be considered as a critical developmental window.

Multi-omics profiling reveals the molecular mechanisms of H2O2-induced detrimental effects on Thamnaconus septentrionalis

BACKGROUND

Hydrogen peroxide (H2O2), a novel water treatment agent, can be used for disinfection, water quality adjustment, and disease prevention, while excessive H2O2 can injure farm animals, even leading to death. Hydrogen peroxide is a recommended disinfectant and bactericide for treating gill diseases and vibriosis in the greenfin horse-faced filefish Thamnaconus septentrionalis. However, its cumulative effect, toxic molecular mechanism and relevant signal transduction/metabolic networks in marine fishes are largely unknown.

RESULTS

We employed a multi-omics approach to investigate the detrimental effects of 50 mg/L H2O2 exposure (2 h/d) on filefish for 2 d, 4 d, and 6 d. Transcriptome sequencing showed that differentially expressed genes (DEGs) were mainly classified into functions such as signal transduction, nervous system, liver and bile acid metabolism, energy metabolism, cell adhesion and communication, inflammation and immune response. Metabolomic analysis found that the significantly changed metabolites (SCMs) were involved in phenylalanine metabolism, inflammatory mediator regulation, linoleic acid metabolism, and necroptosis. The main SCMs were cholic acid, carnitine C12:1, dimethylmalonic acid, glutamic acid, L-lactic acid, shikimic acid, 2-methylsuccinic acid, and others. Moreover, H2O2-induced oxidative stress also disturbs the balance of the gut microbiota, altering the microbial composition and affecting digestive processes.

CONCLUSIONS

Integrated multiomics analysis revealed that H2O2-induced detrimental impacts include mucosal damage, inflammatory and immune responses, altered energy metabolism, and gut microbiota disorders. These findings offer novel insights into the harmful effects and signal transduction/metabolic pathways triggered by H2O2 exposure in marine fishes.

Molecular characterization and expression analysis of thyroid hormone receptors in protogynous rice field eel, Monopterus albus

Thyroid hormones (THs) play important roles in growth, development, morphogenesis, reproduction, and so on. They are mainly meditated by binding to thyroid hormone receptors (TRs) in vertebrates. As important members of the nuclear receptor superfamily, TRs and their ligands are involved in many biological processes. To investigate the potential roles of TRs in the gonadal differentiation and sex change, we cloned and characterized the TRs genes in protogynous rice field eel (Monopterus albus). In this study, three types of TRs were obtained, which were TRαA, TRαB and TRβ, encoding preproproteins of 336-, 409- and 415-amino acids, respectively. Multiple alignments of the three putative TRs protein sequences showed they had a higher similarity. Tissue expression analysis showed that TRαA mainly expressed in the gonad, while TRαB and TRβ in the brain. During female-to-male sex reversal, the expression levels of all the three TRs showed a similar trend of increase followed by a decrease in the gonad. Intraperitoneal injection of triiodothyronine (T3) stimulated the expression of TRαA and TRαB, while it had no significant change on the expression of TRβ in the ovary. Gonadotropin-releasing hormone analogue (GnRHa) injection also significantly upregulated the expression levels of TRαA and TRαB after 6 h, while it had no significant effect on TRβ. These results demonstrated that TRs were involved in the gonadal differentiation and sex reversal, and TRα may play more important roles than TRβ in reproduction by the regulation of GnRHa in rice field eel.

Two pathways regulate insulin-like growth factor genes in the brain and liver of the tropical damselfish Chrysiptera cyanea: A possible role for melatonin in the actions of growth and thyroid hormones

External and internal factors are involved in controlling the growth of fishes. However, little is known about the mechanisms by which external factors trigger stimulus signals. This study explored the physiological roles of melatonin in the transcription of growth-related genes in the brain and liver of Chrysiptera cyanea, a tropical damselfish with long-day preference. In brain samples of this species collected at 4-h intervals, the transcript levels of arylalkylamine N-acetyltransferase2 (aanat2), the rate-limiting enzyme of melatonin synthesis, and growth hormone (gh) peaked at 20:00 and 00:00, respectively. Concomitantly, the transcript levels of insulin-like growth factors (igf1 and igf2) in the brain and liver were upregulated during the scotophase. Levels of iodothyronine deiodinases (dio2 and dio3), enzymes that convert thyroxine (T4) to triiodothyronine (T3) and reverse T3, respectively, increased in the brain (dio2 and dio3) and liver (dio2) during the photophase, whereas dio3 levels in the liver showed the opposite trend. Fish reared in melatonin-containing water exhibited significant increases in the transcription levels of gh and igf1 in the brain and igf1 in the liver, suggesting that growth in this fish is positively regulated by the GH/IGF pathway on a daily basis. Melatonin treatment also stimulated the transcript levels of dio2 and dio3 in the liver, but not in the brain. Fish consuming pellets containing T3, but not T4, showed significant increases in gh and igf1 in the brain and igf1 and igf2 in the liver, suggesting that the intercellular actions of the TH/IGF pathway have an impact on growth on a daily basis. In summary, IGF synthesis and action in the brain and liver undergo dual regulation by distinct hormone networks, which may also be affected by daily, seasonal, or nutritional factors.

The thyroid hormone system disrupting potential of resorcinol in fish

Environmental pollutants capable of interfering with the thyroid hormone (TH) system increasingly raise concern for both human and environmental health. Recently, resorcinol has received attention as a compound of concern due to its endocrine disrupting properties. It is a known inhibitor of thyroperoxidase (TPO), an enzyme required in TH synthesis, and therapeutic use of resorcinol exposure has led to hypothyroidism in humans. There is limited evidence concerning ecotoxicologically relevant effects of resorcinol in fish. A set of adverse outcome pathways (AOPs) has recently been developed linking thyroid hormone system disruption (THSD) to impaired swim bladder inflation and eye development in fish. In the present study, these AOPs were used to provide the background for testing potential THSD effects of resorcinol in zebrafish eleutheroembryos. We exposed zebrafish eleutheroembryos to resorcinol and assessed TH levels, swim bladder inflation and eye morphology. As a TPO inhibitor, resorcinol is expected to affect TH levels and eye morphology but not swim bladder inflation during embryonic development. Indeed, thyroxine (T4) levels were significantly decreased following resorcinol exposure. In contrast to our hypothesis, swim bladder inflation was impaired at 5 days post fertilization (dpf) and no effects on eye morphology were detected. Therefore, in vitro assays were performed to identify potential additional thyroid hormone system disruption-related mechanisms through which resorcinol may act. Two new mechanisms were identified: TH receptor (TR) antagonism and transthyretin (TTR) binding inhibition. Both of these mechanisms can plausibly be linked to impaired swim bladder inflation and could, therefore, explain the observed effect. Overall, our study contributes to the knowledge of the THSD potential of resorcinol both in vivo in the zebrafish model as well as in vitro.

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.

Optimization of Selenium Inclusion Level in the Larval Diet of Labeo rohita

The nutritional requirement of fish larvae remains a limiting factor in advanced aquaculture. Micronutrients are crucial for early development, but their dietary inclusion level in the larval feed of carps has not been standardized. The present study was executed to determine the optimum dietary inclusion level of organic and inorganic selenium in the larval feed of Rohu, Labeo rohita. A 35-day feeding trial in triplicate under semi-control conditions was conducted in 21 troughs divided into seven groups. Each trough (capacity 4.0 L) contained 200 larvae (average body weight 0.4 mg). The first group (control) was reared on nano-particulate basal diet (CP 50%), while three groups Se-Na(0.5), Se-Na(1), and Se-Na(1.5) were fed basal diet supplemented with graded levels (0.5-1.5 mg/kg diet) of inorganic form of Se, sodium selenite (Se-Na). The last three groups (Se-Met(0.5), Se-Met(1), and Se-Met(1.5)) were fed organic form of dietary Se, selenium methionine (Se-Met) at the same inclusion level as Se-Na. Results indicated the curvilinear relationship of dietary Se levels with body weight, activity of digestive enzymes (protease, amylase, lipases, and trypsin), and antioxidant enzymes (SOD, CAT, POD, and GSH-Px) activity, intestinal villi, width, and absorptive area. A positive correlation was observed with up to 0.5 and 1 mg/kg diet of Se-Na and Se-Met, respectively; however, above these levels, a negative impact was observed. The upregulation of growth hormone mediator (IGF-1) and downregulation of heat shock protein (HSP-70) also followed a similar trend in response to Se-Na and Se-Met inclusion. Based on the results, 1 mg/kg diet Se-Met could be considered the optimum level and is recommended for the early rearing of rohu larvae.

Adaptation in landlocked Atlantic salmon links genetics in wild and farmed salmon to smoltification

Increased knowledge of heritable traits in Atlantic Salmon (Salmo salar) is important to overcome bottlenecks in salmonid aquaculture. Atlantic salmonid populations, both landlocked and anadromous, represent an interesting model to gain insight into anadromy related traits, most notably, the probability to smoltify. While a previous study has identified several genomic regions diverging between anadromous and landlocked populations across the species range, the present study explores these data further with the aim to uncover if some of these genomic regions are linked to beneficial genetic traits associated with smoltification. In this study 17 of these loci were monitored in 669 anadromous salmon originating from 36 full-sibling families that had been reared under common garden conditions. The Smolt Index was calculated, using multiple visual markers, and provided a means of assessing smoltification stage. One SNP, located in Ssa04, showed a significant association with probability to smoltify, where individuals homozygous for the landlocked variant (LL) displayed a decrease in probability of smoltifying after one winter when compared with the homozygous for the anadromous variant (AA). This effect was independent of individual fish size. A separate common garden study comprising 200 individuals from either anadromous or landlocked strains showed that expression levels of ncor1, a thyroid mediator hormone located on the same chromosomal region (Ssa04), were significantly reduced in landlocked individuals post smoltification but remained constant in their anadromous counterparts. This study therefore suggests that while size is still the most important trigger for the induction of smoltification, there may also be an additional genetic component or trigger that has been 'lost' during the years deprived of SW transfer. In conclusion, the LL genotype identified here could potentially be used by the industry to delay smoltification and may also represent one of the first clues to the genetic regulation of smoltification in Atlantic salmon.

Co-exposure to microplastic and plastic additives causes development impairment in zebrafish embryos

Since the run off of microplastic and plastic additives into the aquatic environment through the disposal of plastic products, we investigated the adverse effects of co-exposure to microplastics and plastic additives on zebrafish embryonic development. To elucidate the combined effects between microplastic mixtures composed of microplastics and plastic additives in zebrafish embryonic development, polystyrene (PS), bisphenol S (BPS), and mono-(2-ethylhexyl) phthalate (MEHP) were chosen as a target contaminant. Based on non-toxic concentration of each contaminant in zebrafish embryos, microplastic mixtures which is consisted of binary and ternary mixed forms were prepared. A strong phenotypic toxicity to zebrafish embryos was observed in the mixtures composed with non-toxic concentration of each contaminant. In particular, the mixture combination with ≤ EC10 values for BPS and MEHP showed a with a strong synergistic effect. Based on phenotypic toxicity to zebrafish embryos, change of transcription levels for target genes related to cell damage and thyroid hormone synthesis were analyzed in the ternary mixtures with low concentrations that were observed non-toxicity. Compared with the control group, cell damage genes linked to the oxidative stress response and thyroid hormone transcription factors were remarkably down-regulated in the ternary mixture-exposed groups, whereas the transcriptional levels of cyp1a1 and p53 were significantly up-regulated in the ternary mixture-exposed groups (P < 0.05). These results demonstrate that even at low concentrations, exposure to microplastic mixtures can cause embryonic damage and developmental malformations in zebrafish, depending on the mixed concentration-combination. Consequently, our findings will provide data to examine the action mode of zebrafish developmental toxicity caused by microplastic mixtures exposure composed with microplastics and plastic additives.

Effects of thyroxin and luteinizing hormone releasing hormone on reproductive physiology of Rohu (Labeo rohita): Insights into spawning performance, oocyte maturation, steroidogenesis, and follicular development genes

As the global aquaculture industry grows, attention is increasingly turning towards assisted reproductive technologies. In this study, we examined the impact of D-Ala6, Pro9-Net-mGnRH (LHRHa: 0.4 mL/kg) and two doses (1 and 10 μg/kg fish) of thyroxin (T4) administered through a single injection on oocyte maturation, spawning performance, sex steroid hormone levels, as well as the expression of genes related to steroidogenesis and follicle development (ZP2, Cyp19a1a and SF-1) in Rohu (Labeo rohita). The study found that untreated female Rohu did not spawn, while those treated with LHRHa and thyroxin ovulated and spawned across a hormonal gradient. The highest spawning success was observed with a thyroxin dosage of 10 µg/kg (no significant change with a dose of 1 μg/kg), and female latency period decreased with increasing dosage. Additionally, females treated with thyroxin exhibited significantly higher fecundity than other experimental groups. Treatment with LHRHa and two doses of thyroxin significantly increased the gonadal somatic index compared to the control and sham groups. Hormonal treatment also led to increased fertilization success, hatching rate, and larval survival. At 12 h post-injection, females treated with thyroxin exhibited a significant decline in estradiol levels and expression of Zp2, Cyp19a1a, and SF-1 compared to other experimental groups. Levels of DHP significantly increased across the hormonal gradient. Histological analyses supported a steroidogenic shift, where oocyte maturation was accelerated by hormone administration, particularly with both doses of thyroxin. In conclusion, the findings suggest that thyroxin is a recommended treatment for assisted reproduction of Rohu due to its ability to induce spawning, increase fecundity and improve larval survival.

Iopanoic acid alters thyroid hormone-related gene expression, thyroid hormone levels, swim bladder inflation, and swimming performance in Japanese medaka

Disruption of the thyroid hormone system by synthetic chemicals is gaining attention owing to its potential negative effects on organisms. In this study, the effects of the dio-inhibitor iopanoic acid (IOP) on the levels of thyroid hormone and related gene expression, swim bladder inflation, and swimming performance were investigated in Japanese medaka. Iopanoic acid exposure suppressed thyroid-stimulating hormone β (tshβ), tshβ-like, iodotyronin deiodinase 1 (dio1), and dio2 expression, and increased T4 and T3 levels. In addition, IOP exposure inhibited swim bladder inflation, reducing swimming performance. Although adverse outcome pathways of thyroid hormone disruption have been developed using zebrafish, no adverse outcome pathways have been developed using Japanese medaka. This study confirmed that IOP inhibits dio expression (a molecular initiating event), affects T3 and T4 levels (a key event), and reduces swim bladder inflation (a key event) and swimming performance (an adverse outcome) in Japanese medaka.

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.

Identification of new endocrine disruptive transthyretin ligands in polluted waters using pull-down assay coupled to non-target mass spectrometry

Surface and treated wastewater are contaminated with highly complex mixtures of micropollutants, which may cause numerous adverse effects, often mediated by endocrine disruption. However, there is limited knowledge regarding some important modes of action, such as interference with thyroid hormone (TH) regulation, and the compounds driving these effects. This study describes an effective approach for the identification of compounds with the potential to bind to transthyretin (TTR; protein distributing TH to target tissues), based on their specific separation in a pull-down assay followed by non-target analysis (NTA). The method was optimized with known TTR ligands and applied to complex water samples. The specific separation of TTR ligands provided a substantial reduction of chromatographic features from the original samples. The applied NTA workflow resulted in the identification of 34 structures. Twelve compounds with available standards were quantified in the original extracts and their TH-displacement potency was confirmed. Eleven compounds were discovered as TTR binders for the first time and linear alkylbenzene sulfonates (LAS) were highlighted as contaminants of concern. Pull-down assay combined with NTA proved to be a well-functioning approach for the identification of unknown bioactive compounds in complex mixtures with great application potential across various biological targets and environmental compartments.

Thyroid neoplasia associated with nutritional deficiency in cultured jade perch in Hong Kong

Non-neoplastic thyroid hyperplasia is common in terrestrial animals, secondary to nutritional imbalances or other goitrogenic compounds. Thyroid hyperplasia is relatively common in teleost fish; however, malignant thyroid neoplasia is rarely reported. We diagnosed cases of thyroid neoplasia in a population of jade perch (Scortum barcoo). The 3,000 affected fish had grossly apparent, bilateral pharyngeal swellings. Histologic examination confirmed proliferative thyroid lesions ranging from hyperplasia to well-differentiated follicular cell carcinoma. In addition, the younger population of animals on the farm also had bacterial septicemia and mild Dactylogyrus sp. gill infections. Feed analysis revealed a severe deficiency of iodine and vitamin C in the homemade fish diet used on the farm. The concentrations of other minerals, such as zinc, were also on the lower end of the recommended requirements for freshwater fish. The farm was using surface water in its recirculating aquaculture system. We recommended a switch to a commercial aquaculture diet, as well as to use well water rather than surface water to avoid any contaminants, and to treat the younger fish with an antibiotic for bacterial septicemia. Our case provides evidence of progression from nutritional-associated thyroid hyperplasia to neoplasia in farmed teleost fish.

Disruption of Thyroid Hormone Receptor Thrab Leads to Female Infertility in Zebrafish

Thyroid hormones (THs) T4 and T3 are vital for development, growth, and metabolism. Thyroid dysfunction can also cause problems in fertility, suggesting involvement of THs in reproduction. In zebrafish, there exist 2 forms of TH receptor alpha gene (thraa and thrab). Disruption of these genes by CRISPR/Cas9 showed no reproductive irregularities in the thraa mutant; however, inactivation of the thrab gene resulted in female infertility. Although young female mutants (thrabm/m) showed normal ovarian development and folliculogenesis before sexual maturation, they failed to release eggs during oviposition after sexual maturation. This spawning failure was due to oviductal blockage at the genital papilla. The obstruction of the oviduct subsequently caused an accumulation of the eggs in the ovary, resulting in severe ovarian hypertrophy, abdominal distention, and disruption of folliculogenesis. Gene expression analysis showed expression of both TH receptors and estrogen receptors in the genital papilla, suggesting a direct TH action and potential interactions between thyroid and estrogen signaling pathways in controlling genital papilla development and function. In addition to their actions in the reproductive tracts, THs may also have direct effects in the ovary, as suggested by follicle atresia and cessation of folliculogenesis in the heterozygous mutant (thrab+/m), which was normal in all aspects of female reproduction in young and sexually mature fish but exhibited premature ovarian failure in aged females. In summary, this study provides substantial evidence for roles of THs in controlling the development and functions of both reproductive tract and ovary.

The ecological function of thyroid hormones

Thyroid hormones (TH) are central hormonal regulators, orchestrating gene expression and complex biological processes vital for growth and reproduction in variable environments by triggering specific developmental processes in response to external cues. TH serve distinct roles in different species: inducing metamorphosis in amphibians or teleost fishes, governing metabolic processes in mammals, and acting as effectors of seasonality. These multifaceted roles raise questions about the underlying mechanisms of TH action. Recent evidence suggests a shared ecological role of TH across vertebrates, potentially extending to a significant portion of bilaterian species. According to this model, TH ensure that ontogenetic transitions align with environmental conditions, particularly in terms of energy expenditure, helping animals to match their ontogenetic transition with available resources. This alignment spans post-embryonic developmental transitions common to all vertebrates and more subtle adjustments during seasonal changes. The underlying logic of TH function is to synchronize transitions with the environment. This review briefly outlines the fundamental mechanisms of thyroid signalling and shows various ways in which animals use this hormonal system in natural environments. Lastly, we propose a model linking TH signalling, environmental conditions, ontogenetic trajectory and metabolism. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Endocrine disruption in teleosts and amphibians is mediated by anthropogenic and natural environmental factors: implications for risk assessment

Environmental variation in the Anthropocene involves several factors that interfere with endocrine systems of wildlife and humans, presenting a planetary boundary of still unknown dimensions. Here, we focus on chemical compounds and other impacts of anthropogenic and natural origins that are adversely affecting reproduction and development. The main sink of these endocrine disruptors (EDs) is surface waters, where they mostly endanger aquatic vertebrates, like teleost fish and amphibians. For regulatory purposes, EDs are categorized into EATS modalities (oestrogenic, androgenic, thyroidal, steroidogenesis), only addressing endocrine systems being assessable by validated tests. However, there is evidence that non-EATS modalities-and even natural sources, such as decomposition products of plants or parasitic infections-can affect vertebrate endocrine systems. Recently, the disturbance of natural circadian light rhythms by artificial light at night (ALAN) has been identified as another ED. Reviewing the knowledge about EDs affecting teleosts and amphibians leads to implications for risk assessment. The generally accepted WHO-definition for EDs, which focuses exclusively on 'exogenous substances' and neglects parasitic infections or ALAN, seems to require some adaptation. Natural EDs have been involved in coevolutionary processes for ages without resulting in a general loss of biodiversity. Therefore, to address the 'One Health'-principle, future research and regulatory efforts should focus on minimizing anthropogenic factors for endocrine disruption. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

The effects of environmental changes on the endocrine regulation of feeding in fishes

Fishes are exposed to natural and anthropogenic changes in their environment, which can have major effects on their behaviour and their physiology, including feeding behaviour, food intake and digestive processes. These alterations are owing to the direct action of environmental physico-chemical parameters (i.e. temperature, pH, turbidity) on feeding physiology but can also be a consequence of variations in food availability. Food intake is ultimately regulated by feeding centres of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues such as the gastrointestinal tract. These endocrine signals stimulate or inhibit food intake, and interact with each other to maintain energy homeostasis. Changes in environmental conditions might change feeding habits and rates, thus affecting levels of energy stores, and the expression of endocrine appetite regulators. This review provides an overview of how environmental changes and food availability could affect feeding and these endocrine networks in fishes. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

Long-term dietary exposure to the non-steroidal anti-inflammatory drugs diclofenac and ibuprofen can affect the physiology of common carp (Cyprinus carpio) on multiple levels, even at "environmentally relevant" concentrations

The aim of the study was to evaluate the effects of emerging environmental contaminants, the non-steroidal anti-inflammatory drugs (NSAIDs) diclofenac (DCF) and ibuprofen (IBP), on physiological functions in juvenile common carp (Cyprinus carpio). Fish were exposed for 6 weeks, and for the first time, NSAIDs were administered through diet. Either substance was tested at two concentrations, 20 or 2000 μg/kg, resulting in four different treatments (DCF 20, DCF 2000, IBP 20, IBP 2000). The effects on haematological and biochemical profiles, the biomarkers of oxidative stress, and endocrine disruption were studied, and changes in RNA transcription were also monitored to obtain a comprehensive picture of toxicity. Fish exposure to high concentrations of NSAIDs (DCF 2000, IBP 2000) elicited numerous statistically significant changes (p < 0.05) in the endpoints investigated, with DCF being almost always more efficient than IBP. Compared to control fish, a decrease in total leukocyte count attributed to relative lymphopenia was observed. Plasma concentrations of total proteins, ammonia, and thyroxine, and enzyme activities of alanine aminotransferase (ALT), aspartate aminotransferase, and alkaline phosphatase (ALP) were significantly elevated in either group, as were the activities of certain hepatic antioxidant enzymes (superoxide dismutase, glutathione-S-transferase) in the DCF 2000 group. The transcriptomic profile of selected genes in the tissues of exposed fish was affected as well. Significant changes in plasma total proteins, ammonia, ALT, and ALP, as well as in the transcription of genes related to thyroid function and the antioxidant defense of the organism, were found even in fish exposed to the lower DCF concentration (DCF 20). As it was chosen to match DCF concentrations commonly detected in aquatic invertebrates (i.e., the potential feed source of fish), it can be considered "environmentally relevant". Future research is necessary to shed more light on the dietary NSAID toxicity to fish.

Structure-Related Thyroid Disrupting Effect of Perfluorooctanesulfonate-like Substances in Zebrafish Larvae

Chlorinated polyfluorooctane ether sulfonate (6:2 Cl-PFESA), hydrogenated polyfluorooctane ether sulfonate (6:2 H-PFESA), and chlorinated polyfluorooctanesulfonate (Cl-PFOS) share structural similarities with the regulated perfluorooctanesulfonate (PFOS), but their toxic potential is rarely known. Here, the thyroid disrupting potential of these four compounds in zebrafish larvae has been comparably investigated. PFOS, Cl-PFOS, and 6:2 Cl-PFESA were accumulated in the larvae at similar levels, approximately 1.3-1.6 times higher than 6:2 H-PFESA. Additionally, PFOS, Cl-PFOS, and 6:2 Cl-PFESA exhibited stronger disruption than 6:2 H-PFESA on genetic regulation, particularly concerning thyroid hormone (TH) activation and action and on TH homeostasis in both free and total forms of thyroxine (T4) and 3,5,3'-triiodothyronine (T3). These results indicate that chlorination or oxygen insertion does not substantially alter the thyrotoxicity of PFOS, but hydrogenation mitigates it. Molecular docking analysis and the luciferase reporter gene assay provided mechanistic perspectives that the PFOS-like substances could competitively replace THs to bind with TH plasma and membrane transporters, thereby disrupting TH transport and action, respectively. Moreover, they are also potent to disrupt TH synthesis and activation through Na+/K+-dependent transport of I- or competitive binding to the sites of deiodinases.

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

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

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.

Feeding frequency does not interact with BPA exposure to influence metabolism or behaviour in zebrafish (Danio rerio)

Resource limitation can constrain energy (ATP) production, and thereby affect locomotion and behaviour such as exploration of novel environments and boldness. Consequently, ecological processes such as dispersal and interactions within and between species may be influenced by food availability. Energy metabolism, and behaviour are regulated by endocrine signalling, and may therefore be impacted by endocrine disrupting compounds (EDCs) including bisphenol A (BPA) derived from plastic manufacture and pollution. It is important to determine the impacts of these novel environmental contexts to understand how human activity alters individual physiology and behaviour and thereby populations. Our aim was to determine whether BPA exposure interacts with feeding frequency to alter metabolism and behaviour. In a fully factorial experiment, we show that low feeding frequency reduced zebrafish (Danio rerio) mass, condition, resting metabolic rates, total distance moved and speed in a novel arena, as well as anxiety indicated by the number of times fish returned to a dark shelter. However, feeding frequency did not significantly affect maximal metabolic rates, aerobic scope, swimming performance, latency to leave a shelter, or metabolic enzyme activities (citrate synthase and lactate dehydrogenase). Natural or anthropogenic fluctuation in food resources can therefore impact energetics and movement of animals with repercussions for ecological processes such as dispersal. BPA exposure reduced LDH activity and body mass, but did not interact with feeding frequency. Hence, behaviour of adult fish is relatively insensitive to disruption by BPA. However, alteration of LDH activity by BPA could disrupt lactate metabolism and signalling and together with reduction in body mass could affect size-dependent reproductive output. BPA released by plastic manufacture and pollution can thereby impact conservation and management of natural resources.

Influence of stocking density and environmental factors on the expression of insulin-like growth factors in cage-reared butter catfish (Ompok bimaculatus, Bloch 1794) within a large reservoir ecosystem

In vertebrates, insulin-like growth like factors (IGFs) play an important role in growth and other physiological processes. The GH-IGF axis is considered a valuable tool to monitor fish growth performance. Herein, we report the molecular characterization of igf-1, igf-2, and β-actin transcripts and relative expression of igf-1 and igf-2 in the liver and muscle tissue of cage-reared butter catfish, Ompok bimaculatus, in response to different stocking densities (T1, 15 fingerlings m-3; T2, 25 fingerlings m-3; and T3, 35 fingerlings m-3) over 180 days of culture duration. The length of the partial amplified transcript sequence of Obigf-1, Obigf-2, and Obβ-actin was 325, 438, and 924 bp, respectively. Phylogenetically, Obigf-1 and Obigf-2 were closely clustered with catfishes, viz., Clarias magur, Bagarius yarrelli, and Silurus asotus. The expression of igf-1 was significantly downregulated in the liver at higher densities after 120 days as biomass in the cages increased, while igf-2 expression did not change with the stocking densities over the culture period. Cortisol concentration was significantly elevated in T3 groups post 150 days of the culture period and correlated negatively with the expression of igf-1 (p < 0.05) and igf-2 (p > 0.05). Environmental parameters, pH, TDS, hardness, conductivity, and alkalinity showed a significant positive correlation with hepatic IGF expression. Our study indicates that the liver-derived igf-1 plays a more important role in the regulation of growth in response to culture density in the species studied, and thus, igf-1 can be used effectively as a biomarker for growth. Furthermore, this study will help in planning a proper harvest schedule and optimize the culture practices of O. bimaculatus in an open water cage system.

Thyroid and reproduction in amphibians and reptiles

The relationship between the thyroid gland and reproduction in amphibians and reptiles has been studied for more than 100 years. Most studies suggest a positive involvement of thyroid hormones with some aspects of reproduction, but some studies support a negative role for thyroid hormones at certain life stages. Comprehensive studies of gene activation/suppression by thyroid hormones and their absence at various levels of the hypothalamo-pituitary-gonadal axis coupled with observations of adrenocorticoid activity, reproductive performance, and metabolic involvement are needed to understand this complex relationship.

Integrated transcriptomic and metabolomic analyses identify key factors in the vitellogenesis of juvenile Sichuan bream (Sinibrama taeniatus)

Vitellogenesis is the most important stage of ovarian maturation in fish, involving the synthesis and transport of essential yolk substances and their complex mechanisms and coordination process. The liver is the main tissue involved in the vitellogenesis of oviparous animals, but studies of vitellogenesis in fish rarely include the liver and ovary as a whole. The aim of this study was to explore the molecular mechanism and associated regulatory factors behind vitellogenesis in Sichuan bream (Sinibrama taeniatus). The different stages of oogenesis were first identified by successive histological observations. Then, ovary and liver tissues that developed to 115 days (stage II, previtellogenesis stage), 165 days (stage III, vitellogenesis stage) and 185 days (stage IV, late-vitellogenesis stage) were collected for transcriptomic and metabolomic analyses, and serum testosterone (T), 17β-estradiol (E2), vitellogenin (Vtg), triiodothyronine (T3), and thyroxine (T4) levels were measured at the corresponding stages. We found that energy redistribution during vitellogenesis is mainly regulated through glycolysis, fatty acid biosynthesis and the citrate cycle pathway. In the liver, energy metabolism was promoted by activating glucolipid metabolic pathways to provide sufficient ATP, but at the same time, the ovary tends to retain nutrients rather than decompose them to produce energy. In addition, we have identified several key factors involved in the metabolism of neutral lipids, polar lipids, amino acids and vitamins, which are involved in the assembly and transport of important yolk nutrients. The initiation of vitellogenesis was found to be associated with a surge in serum E2 levels, but the sustained increase in Vtg levels in the late stage may be due more to upregulation of the estrogen receptor. These results provide valuable information about the regulation of ovarian development in cultured fish.

Rapid hyperthyroidism-induced adaptation of salmonid fish in response to environmental pollution

The streams draining volcanic landscapes are often characterized by a complex series of factors that negatively affect hydrobionts and lead to declines in their populations. However, in a number of cases, a range of rapid adaptive changes ensure the resilience of hydrobiont populations. Here, we present both field and experimental data shedding light on the physiological basis of adaptation to heavy metal contamination in populations of Dolly Varden charr (Salvelinus malma) differing in duration of isolation in volcanic streams. The study reveals that isolated populations have a physiological phenotype that distinguishes them from populations inhabiting clean waters. They are characterized by a hyperthyroid status accompanied by an increased metabolic rate, elevated activity of antioxidant enzymes, decreased ionic conductivity of tissues and reduced stored energy reserves. Our experimental data reveal that hyperthyroidism is an adaptive characteristic enhancing the resistance to heavy metal contamination and shaping the evolution of these populations. The similarity of physiological, developmental and morphological changes in isolated populations suggests a common source and mechanisms underpinning this case of 'evolutionary rescue'. Thus, populations of S. malma trapped in volcanic streams represent a genuine case of rapid endocrine-driven adaptation to changing environmental stimuli.

Depleted uranium induces thyroid damage through activation of ER stress via the thrombospondin 1-PERK pathway

Depleted uranium (DU) can cause damage to the body, but its effects on the thyroid are unclear. The purpose of this study was to investigate the DU-induced thyroid damage and its potential mechanism in order to find new targets for detoxification after DU poisoning. A model of acute exposure to DU was constructed in rats. It was observed that DU accumulated in the thyroid, induced thyroid structure disorder and cell apoptosis, and decreased the serum T4 and FT4 levels. Gene screening showed that thrombospondin 1 (TSP-1) was a sensitive gene of DU, and the expression of TSP-1 decreased with the increase of DU exposure dose and time. TSP-1 knockout mice exposed to DU had more severe thyroid damage and lower serum FT4 and T4 levels than wild-type mice. Inhibiting the expression of TSP-1 in FRTL-5 cells aggravated DU-induced apoptosis, while exogenous TSP-1 protein alleviated the decreased viability in FRTL-5 cells caused by DU. It was suggested that DU may caused thyroid damage by down-regulating TSP-1. It was also found that DU increased the expressions of PERK, CHOP, and Caspase-3, and 4-Phenylbutyric (4-PBA) alleviated the DU-induced FRTL-5 cell viability decline and the decrease levels of rat serum FT4 and T4 caused by DU. After DU exposure, the PERK expression was further up-regulated in TSP-1 knockout mice, and the increased expression of PERK was alleviated in TSP-1 over-expressed cells, as well as the increased expression of CHOP and Caspase-3. Further verification showed that inhibition of PERK expression could reduce the DU-induced increased expression of CHOP and Caspase-3. These findings shed light on the mechanism that DU may activate ER stress via the TSP 1-PERK pathway, thereby leading to thyroid damage, and suggest that TSP-1 may be a potential therapeutic target for DU-induced thyroid damage.

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.

Tank color influences the response of tomato clownfish (Amphiprion frenatus) to an acute stress challenge

The trade of aquarium organisms is growing worldwide. This market depends on a continuous supply of healthy and colorful aquatic animals, but this sector has few initiatives. However, in the last decade, there has been a growing interest in researching captive breeding of these animals, aiming to develop a more sustainable aquarium hobby. Larviculture is an important phase in the cultivation process because the larvae are more sensitive to stress and variations in the bulk of variables, such as temperature, salinity, nutritional management, light intensity and spectrum, and environmental background colors. Because background color could be a promoter variable of proper welfare, we tested whether it affects the endocrine response of tomato clownfish Amphiprion frenatus larvae to an acute stress challenge. We show that background color influences the endocrine stress axis responsiveness in tomato clownfish. When fish were subjected to a standard acute stressor of 61 days after hatching, only fish adapted to white walls increased the whole-body cortisol levels. From the results presented herein, we recommend that white tanks be avoided for A. frenatus larviculture. Both, the less stress level and the good welfare condition of larvae reared in colored tanks may have robust, practical applications since almost all clownfish in the ornamental aquarium trade come from captive breeding.

The multi-level regulation of clownfish metamorphosis by thyroid hormones

Most marine organisms have a biphasic life cycle during which pelagic larvae transform into radically different juveniles. In vertebrates, the role of thyroid hormones (THs) in triggering this transition is well known, but how the morphological and physiological changes are integrated in a coherent way with the ecological transition remains poorly explored. To gain insight into this question, we performed an integrated analysis of metamorphosis of a marine teleost, the false clownfish (Amphiprion ocellaris). We show how THs coordinate a change in color vision as well as a major metabolic shift in energy production, highlighting how it orchestrates this transformation. By manipulating the activity of liver X regulator (LXR), a major regulator of metabolism, we also identify a tight link between metabolic changes and metamorphosis progression. Strikingly, we observed that these regulations are at play in the wild, explaining how hormones coordinate energy needs with available resources during the life cycle.

Determination of thyroid hormones in lake sturgeon (Acipenser fulvescens) tissues at different developmental stages using a sensitive liquid chromatography-mass spectrometry method

Thyroid hormones (TH) are known to play an important role in the growth and development of vertebrates. In fish species, TH regulates the larval-juvenile metamorphosis, and is crucial for development during early life stages. Monitoring the variations in TH levels at different life stages can provide insights into the regulation of metamorphosis and fish development. In this study, we developed an extremely sensitive method for the quantification of thyroxine (T4), triiodothyronine (T3), and reverse-triiodothyronine (rT3), in lake sturgeon (Acipenser fulvescens) tissues from eggs, free embryos, larvae, and juveniles. The target compounds were extracted by an enzymatic digestion method, followed by protein precipitation. Further cleanup was performed by liquid-liquid extraction (LLE) and solid phase extraction (SPE) using SampliQ OPT cartridges. The liquid-chromatography tandem mass spectrometry (LC-MS/MS) method used to quantify TH compounds showed remarkably high sensitivity with the limit of detection (LOD) and the limit of quantification (LOQ) ranging from < 1 pg/mL to 10 pg/mL and linearity in the range of 10-50,000 pg/mL. This method was validated for tissue samples across several early developmental stages and was checked for intra- and inter-day accuracy (78.3-111.2 %) and precision (0.1-4.9 %), matrix effect (75.4-134.1 %), and recovery (41.2-69.0 %). The method was successfully applied for the quantification and comparison of T4, T3 and rT3 in hatchery raised lake sturgeon samples collected at unique time points (i.e., days post fertilization dpf) including fertilized eggs (11 dpf), free embryos (14 dpf), larvae (22 dpf), juveniles (40 dpf) and older juveniles (74 dpf). With modifications, this method could be applied to other species important for agriculture or conservation.

Environmentally relevant concentrations of triclosan induce lethality and disrupt thyroid hormone activity in zebrafish (Danio rerio)

Triclosan is an antimicrobial agent that has been used in common household products and can be detected in water environment. In this study, therefore, I aimed at clarifying the effects of environmentally relevant concentrations of triclosan on the early life stage development in zebrafish. A lethal effect was observed: the lowest effect and the no effect concentrations were 70.6 and 48.4μg/L, respectively. These concentrations are very close to the environmentally detected residual concentrations. In 10.9, 19.8, 48.4, and 70.6μg/L of triclosan, the iodothyronine deiodinase 1 gene expression was found to be significantly increased when compared with that of the control group. These findings indicate that triclosan can potentially disrupt the thyroid hormone activity in zebrafish. The exposure to triclosan (at 149.2μg/L) was also found to inhibit the gene expression of insulin-like growth factor-1. My findings suggest that triclosan can exert a thyroid hormone-disrupting effect on fish.

Estrogenic and non-estrogenic effects of bisphenol A and its action mechanism in the zebrafish model: An overview of the past two decades of work

Bisphenol A (BPA) is the most simple and predominant component of the Bisphenol family. BPA is widely present in the environment and the human body as a result of its extensive usage in the plastic and epoxy resins of consumer goods like water bottles, food containers, and tableware. Since the 1930s, when BPA's estrogenic activity was first observed, and it was labeled as a "mimic hormone of E2", studies on the endocrine-disrupting effects of BPA then have been widely conducted. As a top vertebrate model for genetic and developmental studies, the zebrafish has caught tremendous attention in the past two decades. By using the zebrafish, the negative effects of BPA either through estrogenic signaling pathways or non-estrogenic signaling pathways were largely found. In this review, we tried to draw a full picture of the current state of knowledge on the estrogenic and non-estrogenic effects of BPA with their mechanisms of action through the zebrafish model of the past two decades, which may help to fully understand the endocrine-disrupting effects of BPA and its action mechanism, and give a direction for the future studies.