Unliganded thyroid hormone receptor is essential for Xenopus laevis eye development

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).

Hypothyroidism is a causal determinant of age-related cataract risk in European population: a Mendelian randomization study

Objective

To determine whether there is a causal relationship between thyroid dysfunction and the risk of age-related cataract (ARC) in the European population.

Design

A two-sample Mendelian randomization (MR) study.

Methods

Hypothyroidism, hyperthyroidism, free thyroxine (fT4), and thyrotropin (TSH) were selected as exposures. The single nucleotide polymorphisms (SNP) of hypothyroidism and hyperthyroidism were obtained from the genome-wide association studies (GWAS) of the IEU database, including 337,159 subjects. Data for fT4 and TSH (72,167 subjects) were extracted from the ThyroidOmics Consortium. ARC was used as the outcome. The SNPs associated with ARC were selected from a GWAS of 216,362 individuals in the FinnGen database. The main method used was the inverse variance-weighted method, together with four complementary methods. Sensitivity analyses were performed using Cochran's Q test, MR-PRESSO, MR-Egger regression and leave-one-out test. MR pleiotropy was used to test for pleiotropy. MR Steiger test was used to test for the directionality.

Results

Two-sample MR analysis revealed a positive association between genetically predicted hypothyroidism and risk of ARC (OR = 2.501, 95% CI: 1.325-4.720; P = 0.004). Hyperthyroidism, circulating fT4 and TSH levels did not have a significant causal effect on ARC (P > 0.05). The results were robust and reliable, and no horizontal pleiotropy was found after sensitivity analyses. In the MR Steiger test, we found no reverse causal effects of hypothyroidism on the ARC (P <0.001).

Conclusions

Our study provides strong evidence that hypothyroidism is a causal determinant of ARC risk.

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.

Essential and subtype-dependent function of thyroid hormone receptors during Xenopus metamorphosis

Thyroid hormone (T3) plays critical roles in organ metabolism and development in vertebrates. Anuran metamorphosis is perhaps the most dramatic and best studied developmental process controlled by T3. Many changes in different organs/tissues during anuran metamorphosis resemble the maturation/remodeling of the corresponding organs/tissues during mammalian postembryonic development. The plasma T3 level peaks during both anuran metamorphosis and mammalian postembryonic development. T3 exerts its developmental function through transcriptional regulation via T3 receptors (TRs). Studies on the metamorphosis of two highly related anurans, pseudo-tetraploid Xenopus laevis and diploid Xenopus tropicalis, have led to a dual function model for TRs during development. This has been supported by strong molecular and genetic evidence. Here we review some of the evidence with a focus on more recent gene knockout studies in Xenopus tropicalis. These studies have not only supported the model but also revealed novel and TR subtype-specific roles during Xenopus development, particularly a critical role of TRα in controlling developmental timing and rate.

Xenopus in revealing developmental toxicity and modeling human diseases

The Xenopus model offers many advantages for investigation of the molecular, cellular, and behavioral mechanisms underlying embryo development. Moreover, Xenopus oocytes and embryos have been extensively used to study developmental toxicity and human diseases in response to various environmental chemicals. This review first summarizes recent advances in using Xenopus as a vertebrate model to study distinct types of tissue/organ development following exposure to environmental toxicants, chemical reagents, and pharmaceutical drugs. Then, the successful use of Xenopus as a model for diseases, including fetal alcohol spectrum disorders, autism, epilepsy, and cardiovascular disease, is reviewed. The potential application of Xenopus in genetic and chemical screening to protect against embryo deficits induced by chemical toxicants and related diseases is also discussed.

Thyroid hormone receptor: A new player in epinephrine-induced larval metamorphosis of the hard-shelled mussel

Many marine invertebrate larvae undergo a dramatic morphological and physiological transition from a planktonic larva to a benthic juvenile. The mechanisms of this metamorphosis in bivalves are mainly unknown. The recent identification in bivalves of a thyroid hormone receptor (TR) gene raises the possibility that as occurs in vertebrate metamorphosis, TRs regulate this developmental process. An evolutionary study of TR receptors revealed they are ubiquitous in the molluscs. Knock-down of the TR gene in pediveliger larvae of the hard-shelled mussel, Mytilus coruscus (Mc), using electroporation of siRNA significantly (p < 0.01) reduced TR gene expression. TR gene knock-down decreased pediveliger larval metamorphosis by 54% and was associated with a significant (p < 0.01) reduction in viability compared to control larvae. The TR in the hard-shelled mussel appears to be an essential regulatory factor for the successful epinephrine-induced metamorphosis of the pediveliger larvae to post-larvae. It is hypothesised that the knock-down of TR by siRNA transfection affects the "competence" of pediveliger larvae for the metamorphic transition by reducing their ability to respond to the inducer. The involvement of TR in the epinephrine-induced metamorphosis of a mollusc, the hard-shelled mussel, suggests the role of TR in this process probably emerged early during evolution.

Thyroid hormone regulation of neural stem cell fate: From development to ageing

In the vertebrate brain, neural stem cells (NSCs) generate both neuronal and glial cells throughout life. However, their neuro‐ and gliogenic capacity changes as a function of the developmental context. Despite the growing body of evidence on the variety of intrinsic and extrinsic factors regulating NSC physiology, their precise cellular and molecular actions are not fully determined. Our review focuses on thyroid hormone (TH), a vital component for both development and adult brain function that regulates NSC biology at all stages. First, we review comparative data to analyse how TH modulates neuro‐ and gliogenesis during vertebrate brain development. Second, as the mammalian brain is the most studied, we highlight the molecular mechanisms underlying TH action in this context. Lastly, we explore how the interplay between TH signalling and cell metabolism governs both neurodevelopmental and adult neurogenesis. We conclude that, together, TH and cellular metabolism regulate optimal brain formation, maturation and function from early foetal life to adult in vertebrate species.

Effects of nitrate on development and thyroid hormone signaling pathway during Bufo gargarizans embryogenesis

Nitrate is known to disrupt the thyroid hormone, which is essential for the metamorphosis of amphibians. However, few studies are focused on the effects of nitrate on the maternal thyroid hormone in early amphibian embryos. We aimed to determine the impact of nitrate on maternal thyroid hormone signaling pathway in Bufo gargarizans embryos. B. gargarizans embryos were exposed to different concentrations of nitrate-nitrogen (NO3-N) for 7 days. High concentration of NO3-N (50, 100, and 200 mg/L) could induce embryonic malformation and influence the development of embryos. In addition, maternal T4 and components of the thyroid hormone (TH) signaling pathway were detected by ELISA and RNA-seq, respectively. The expression levels of mRNA related to thyroid hormone and oxidative stress were affected in the early developing embryos in all NO3-N treatment groups. However, the T4 levels and the spatial expression patterns of type II iodothyronine deiodinase (D2), type III iodothyronine deiodinase (D3), thyroid hormone receptor α (TRα), and thyroid hormone receptor β (TRβ) mRNA were not changed by nitrate. In conclusion, the results of our study highlight the crucial role of the maternal thyroid hormone signaling pathway in normal embryonic development, and the adverse effects of nitrate on the expression levels of mRNA related to thyroid hormone signaling pathway and oxidative stress in B. gargarizans embryos.

Photoreceptor Progenitors Depend Upon Coordination of gdf6a, thrβ, and tbx2b to Generate Precise Populations of Cone Photoreceptor Subtypes

Purpose

Replacing cone photoreceptors, the units of the retina necessary for daytime vision, depends upon the successful production of a full variety of new cones from, for example, stem cells. Using genetic experiments in a model organism with high cone diversity, zebrafish, we map the intersecting effects of cone development factors gdf6a, tbx2b, and thrβ.

Methods

We investigated these genes of interest by using genetic combinations of mutants, gene knockdown, and dominant negative gene expression, and then quantified cone subtype outcomes (which normally develop in tightly regulated ratios).

Results

Gdf6a mutants have reduced blue cones and, discovered here, reduced red cones. In combined gdf6a/tbx2b disruption, the loss of gdf6a in heterozygous tbx2b mutants reduced UV cones. Intriguingly, when we disrupted thrβ in gdf6a mutants by using a thrβ morpholino, their combined early disruption revealed a lamination phenotype. Disrupting thrβ activity via expression of a dominant negative thrβ (dnthrβ) at either early or late retinal development had differential outcomes on red cones (reduced abundance), versus UV and blue cones (increased abundance). By using dnthrβ in gdf6a mutants, we revealed that disrupting thrβ activity did not change gdf6a mutant cone phenotypes.

Conclusions

Gdf6a loss directly affects blue and red cones and indirectly affects UV cones by increasing sensitivity to additional disruption, such as reduced tbx2b, resulting in fewer UV cones. The effects of thrβ change through photoreceptor development, first promoting red cones and restricting UV cones, and later restricting UV and blue cones. The effects of gdf6a on UV, blue, and red cone development overlap with, but likely supersede, those of thrβ.

Effect of Thyroid Hormones on Neurons and Neurodevelopment

This review focuses on the current knowledge of the effects of thyroid hormones on central nervous system differentiation and development in animals and the human fetal brain. The outcomes of children with congenital hypothyroidism and of newborns with hypothyroid pregnant mothers are emphasized, focusing on how therapies could affect and especially improve the outcomes.

Opposite T3 Response of ACTG1-FOS Subnetwork Differentiate Tailfin Fate in Xenopus Tadpole and Post-hatching Axolotl

Amphibian post-embryonic development and Thyroid Hormones (TH) signaling are deeply and intimately connected. In anuran amphibians, TH induce the spectacular and complex process known as metamorphosis. In paedomorphic salamanders, at similar development time, raising levels of TH fail to induce proper metamorphosis, as many "larval" tissues (e.g., gills, tailfin) are maintained. Why does the same evolutionary conserved signaling pathway leads to alternative phenotypes? We used a combination of developmental endocrinology, functional genomics and network biology to compare the transcriptional response of tailfin to TH, in the post-hatching paedormorphic Axolotl salamander and Xenopus tadpoles. We also provide a technological framework that efficiently reduces large lists of regulated genes down to a few genes of interest, which is well-suited to dissect endocrine regulations. We first show that Axolotl tailfin undergoes a strong and robust TH-dependent transcriptional response at post embryonic transition, despite the lack of visible anatomical changes. We next show that Fos and Actg1, which structure a single and dense subnetwork of cellular sensors and regulators, display opposite regulation between the two species. We finally show that TH treatments and natural variations of TH levels follow similar transcriptional dynamics. We suggest that, at the molecular level, tailfin fate correlates with the alternative transcriptional states of an fos-actg1 sub-network, which also includes transcription factors and regulators of cell fate. We propose that this subnetwork is one of the molecular switches governing the initiation of distinct TH responses, with transcriptional programs conducting alternative tailfin fate (maintenance vs. resorption) 2 weeks post-hatching.

Evolutionary Conservation of Thyroid Hormone Receptor and Deiodinase Expression Dynamics in ovo in a Direct-Developing Frog, Eleutherodactylus coqui

Direct development is a reproductive mode in amphibians that has evolved independently from the ancestral biphasic life history in at least a dozen anuran lineages. Most direct-developing frogs, including the Puerto Rican coquí, Eleutherodactylus coqui, lack a free-living aquatic larva and instead hatch from terrestrial eggs as miniature adults. Their embryonic development includes the transient formation of many larval-specific features and the formation of adult-specific features that typically form postembryonically-during metamorphosis-in indirect-developing frogs. We found that pre-hatching developmental patterns of thyroid hormone receptors alpha (thra) and beta (thrb) and deiodinases type II (dio2) and type III (dio3) mRNAs in E. coqui limb and tail are conserved relative to those seen during metamorphosis in indirect-developing frogs. Additionally, thra, thrb, and dio2 mRNAs are expressed in the limb before formation of the embryonic thyroid gland. Liquid-chromatography mass-spectrometry revealed that maternally derived thyroid hormone is present throughout early embryogenesis, including stages of digit formation that occur prior to the increase in embryonically produced thyroid hormone. Eleutherodactylus coqui embryos take up much less 3,5,3'-triiodothyronine (T₃) from the environment compared with X. tropicalis tadpoles. However, E. coqui tissue explants mount robust and direct gene expression responses to exogenous T₃ similar to those seen in metamorphosing species. The presence of key components of the thyroid axis in the limb and the ability of limb tissue to respond to T₃ suggest that thyroid hormone-mediated limb development may begin prior to thyroid gland formation. Thyroid hormone-dependent limb development and tail resorption characteristic of metamorphosis in indirect-developing anurans are evolutionarily conserved, but they occur instead in ovo in E. coqui.

From axolotl to zebrafish: a comparative approach to the study of thyroid involvement in ocular development

While discussion of interactions between the thyroid gland and the eye mostly focus on thyroid eye disease, the involvement of the thyroid and its hormones on development of the eye before birth is also an important perspective that should not be forgotten. Experimental models involving amphibians and fish are valuable both because the developing larvae are not constrained within the adult but can be observed and manipulated as they grow autonomously and also because they metamorphose with substantial morphological changes driven by the hypothalamo-pituitary-thyroid axis occurring as they develop from eggs to adults. In this paper we will first discuss changes seen in the axolotl, a naturally occurring neotonous hypothyroid salamander and the Xenopus toad a widely used laboratory amphibian. Secondly we will document ocular changes in flatfish which exhibit remarkable anatomical alterations during their change from a pelagic to benthic lifestyle. Finally we will evaluate ocular changes in developing zebrafish when subjected to thyroid-disrupting chemicals. These experiments show the influence of the thyroid on ocular development when thyroid function is altered which may be of value in determining changes in human hypothyroid infants but are also important to note as these chemicals are widely used in plastics and also as flame retardants used to control wildfires. Run-off into water courses can damage both wildlife and humans consuming contaminated water and thyroid disruption may have significant effects on reproduction and development, although influences on ocular morphology have yet to be investigated.

Expression of the inactivating deiodinase, Deiodinase 3, in the pre-metamorphic tadpole retina

Thyroid hormone (TH) orchestrates amphibian metamorphosis. Thus, this developmental phase is often used to study TH-dependent responses in specific tissues. However, TH signaling appears early in development raising the question of the control of TH availability in specific cell types prior to metamorphosis. TH availability is under strict temporal and tissue-specific control by deiodinases. We examined the expression of the TH-inactivating enzyme, deiodinase type 3 (D3), during early retinal development. To this end we created a Xenopus laevis transgenic line expressing GFP from the Xenopus dio3 promoter region (pdio3) and followed pdio3-GFP expression in pre-metamorphic tadpoles. To validate retinal GFP expression in the transgenic line as a function of dio3 promoter activity, we used in situ hybridization to compare endogenous dio3 expression to reporter-driven GFP activity. Retinal expression of dio3 increased during pre-metamorphosis through stages NF41, 45 and 48. Both sets of results show dio3 to have cell-specific, dynamic expression in the pre-metamorphic retina. At stage NF48, dio3 expression co-localised with markers for photoreceptors, rods, Opsin-S cones and bipolar neurons. In contrast, in post-metamorphic juveniles dio3 expression was reduced and spatially confined to certain photoreceptors and amacrine cells. We compared dio3 expression at stages NF41 and NF48 with TH-dependent transcriptional responses using another transgenic reporter line: THbZIP-GFP and by analyzing the expression of T3-regulated genes in distinct TH availability contexts. At stage NF48, the majority of retinal cells expressing dio3 were negative for T3 signaling. Notably, most ganglion cells were virtually both dio3-free and T3-responsive. The results show that dio3 can reduce TH availability at the cellular scale. Further, a reduction in dio3 expression can trigger fine-tuned T3 action in cell-type specific maturation at the right time, as exemplified here in photoreceptor survival in the pre-metamorphic retina.

Effects of the herbicides linuron and S-metolachlor on Perez's frog embryos

Presence of pesticides in the environment and their possible effects on aquatic organisms are of great concern worldwide. The extensive use of herbicides in agricultural areas are one of the factors contributing to the known decline of amphibian populations. Thus, as non-target species, amphibians can be exposed in early life stages to herbicides in aquatic systems. In this context, this study aims to evaluate effects of increasing concentrations of two maize herbicides, linuron and S-metolachlor on embryos of the Perez' frog (Pelophylax perezi) during 192 h. Apical endpoints were determined for each herbicide: mortality, hatching rate, malformations and length. Frog embryos presented a LC₅₀ of 21 mg/l linuron and 37.5 mg/l S-metolachlor. Furthermore, sub-lethal concentrations of both herbicides affected normal embryonic development, delaying hatching, decreasing larvae length and causing several malformations. Length of larvae decreased with increasing concentrations of each herbicide, even at the lower concentrations tested. Malformations observed in larvae exposed to both herbicides were oedemas, spinal curvature and deformation, blistering and microphtalmia. Overall, these results highlight the need to assess adverse effects of xenobiotics to early life stages of amphibians regarding beside mortality the embryonic development, which could result in impairments at later stages. However, to unravel mechanisms involved in toxicity of these herbicides further studies regarding lower levels of biological organisation such as biochemical and genomic level should be performed.

Comparative approaches to understanding thyroid hormone regulation of neurogenesis

Thyroid hormone (TH) signalling, an evolutionary conserved pathway, is crucial for brain function and cognition throughout life, from early development to ageing. In humans, TH deficiency during pregnancy alters offspring brain development, increasing the risk of cognitive disorders. How TH regulates neurogenesis and subsequent behaviour and cognitive functions remains a major research challenge. Cellular and molecular mechanisms underlying TH signalling on proliferation, survival, determination, migration, differentiation and maturation have been studied in mammalian animal models for over a century. However, recent data show that THs also influence embryonic and adult neurogenesis throughout vertebrates (from mammals to teleosts). These latest observations raise the question of how TH availability is controlled during neurogenesis and particularly in specific neural stem cell populations. This review deals with the role of TH in regulating neurogenesis in the developing and the adult brain across different vertebrate species. Such evo-devo approaches can shed new light on (i) the evolution of the nervous system and (ii) the evolutionary control of neurogenesis by TH across animal phyla. We also discuss the role of thyroid disruptors on brain development in an evolutionary context.

Functional Characterization of Xenopus Thyroid Hormone Transporters mct8 and oatp1c1

Xenopus is an excellent model for studying thyroid hormone signaling as it undergoes thyroid hormone-dependent metamorphosis. Despite the fact that receptors and deiodinases have been described in Xenopus, membrane transporters for these hormones are yet to be characterized. We cloned Xenopus monocarboxylate transporter 8 (mct8) and organic anion-transporting polypeptide 1C1 (oatpc1c1), focusing on these two transporters given their importance for vertebrate brain development. Protein alignment and bootstrap analysis showed that Xenopus mct8 and oatp1c1 are closer to their mammalian orthologs than their teleost counterparts. We functionally characterized the two transporters using a radiolabeled hormones in vitro uptake assay in COS-1 cells. Xenopus mct8 was found to actively transport both T3 and T4 bidirectionally. As to the thyroid precursor molecules, diiodotyrosine (DIT) and monoiodotyrosine (MIT), both human and Xenopus mct8, showed active efflux, but no influx. Again similar to humans, Xenopus oatp1c1 transported T4 but not T3, MIT, or DIT. We used reverse transcription quantitative polymerase chain reaction and in situ hybridization to characterize the temporal and spatial expression of mct8 and oatp1c1 in Xenopus. Specific expression of the transporter was observed in the brain, with increasingly strong expression as development progressed. In conclusion, these results show that Xenopus thyroid hormone transporters are functional and display marked spatiotemporal expression patterns. These features make them interesting targets to elucidate their roles in determining thyroid hormone availability during embryonic development.

Thyroid Hormone Acts Locally to Increase Neurogenesis, Neuronal Differentiation, and Dendritic Arbor Elaboration in the Tadpole Visual System

Thyroid hormone (TH) regulates many cellular events underlying perinatal brain development in vertebrates. Whether and how TH regulates brain development when neural circuits are first forming is less clear. Furthermore, although the molecular mechanisms that impose spatiotemporal constraints on TH action in the brain have been described, the effects of local TH signaling are poorly understood. We determined the effects of manipulating TH signaling on development of the optic tectum in stage 46-49 Xenopus laevis tadpoles. Global TH treatment caused large-scale morphological effects in tadpoles, including changes in brain morphology and increased tectal cell proliferation. Either increasing or decreasing endogenous TH signaling in tectum, by combining targeted DIO3 knockdown and methimazole, led to corresponding changes in tectal cell proliferation. Local increases in TH, accomplished by injecting suspensions of tri-iodothyronine (T₃) in coconut oil into the midbrain ventricle or into the eye, selectively increased tectal or retinal cell proliferation, respectively. In vivo time-lapse imaging demonstrated that local TH first increased tectal progenitor cell proliferation, expanding the progenitor pool, and subsequently increased neuronal differentiation. Local T₃ also dramatically increased dendritic arbor growth in neurons that had already reached a growth plateau. The time-lapse data indicate that the same cells are differentially sensitive to T₃ at different time points. Finally, TH increased expression of genes pertaining to proliferation and neuronal differentiation. These experiments indicate that endogenous TH locally regulates neurogenesis at developmental stages relevant to circuit assembly by affecting cell proliferation and differentiation and by acting on neurons to increase dendritic arbor elaboration.

SIGNIFICANCE STATEMENT

Thyroid hormone (TH) is a critical regulator of perinatal brain development in vertebrates. Abnormal TH signaling in early pregnancy is associated with significant cognitive deficits in humans; however, it is difficult to probe the function of TH in early brain development in mammals because of the inaccessibility of the fetal brain in the uterine environment and the challenge of disambiguating maternal versus fetal contributions of TH. The external development of tadpoles allows manipulation and direct observation of the molecular and cellular mechanisms underlying TH's effects on brain development in ways not possible in mammals. We find that endogenous TH locally regulates neurogenesis at developmental stages relevant to circuit assembly by affecting neural progenitor cell proliferation and differentiation and by acting on neurons to enhance dendritic arbor elaboration.

Thyroid Hormone Receptor α Controls Developmental Timing and Regulates the Rate and Coordination of Tissue-Specific Metamorphosis in Xenopus tropicalis

Thyroid hormone (T3) receptors (TRs) mediate the effects of T3 on organ metabolism and animal development. There are two TR genes, TRα and TRβ, in all vertebrates. During animal development, TRα expression is activated earlier than zygotic T3 synthesis and secretion into the plasma, implicating a developmental role of TRα both in the presence and absence of T3. Using T3-dependent amphibian metamorphosis as a model, we previously proposed a dual-function model for TRs, in particular TRα, during development. That is, unliganded TR represses the expression of T3-inducible genes during premetamorphosis to ensure proper animal growth and prevent premature metamorphosis, whereas during metamorphosis, liganded TR activates target gene transcription to promote the transformation of the tadpole into a frog. To determine if TRα has such a dual function, we generated homozygous TRα-knockout animal lines. We show that, indeed, TRα knockout affects both premetamorphic animal development and metamorphosis. Surprisingly, we observed that TRα is not essential for amphibian metamorphosis, given that homozygous knockout animals complete metamorphosis within a similar time period after fertilization as their wild-type siblings. On the other hand, the timing of metamorphosis for different organs is altered by the knockout; limb metamorphosis occurs earlier, whereas intestinal metamorphosis is completed later than in wild-type siblings. Thus, our studies have demonstrated a critical role of endogenous TRα, not only in regulating both the timing and rate of metamorphosis, but also in coordinating temporal metamorphosis of different organs.

Frogs model man: In vivo thyroid hormone signaling during development

Thyroid hormone (TH) signaling comprises TH transport across cell membranes, metabolism by deiodinases, and molecular mechanisms of gene regulation. Proper TH signaling is essential for normal perinatal development, most notably for neurogenesis and fetal growth. Knowledge of perinatal TH endocrinology needs improvement to provide better treatments for premature infants and endocrine diseases during gestation and to counteract effects of endocrine disrupting chemicals. Studies in amphibians have provided major insights to understand in vivo mechanisms of TH signaling. The frog model boasts dramatic TH-dependent changes directly observable in free-living tadpoles with precise and easy experimental control of the TH response at developmental stages comparable to fetal stages in mammals. The hormones, their receptors, molecular mechanisms, and developmental roles of TH signaling are conserved to a high degree in humans and amphibians, such that with respect to developmental TH signaling "frogs are just little people that hop." The frog model is exceptionally illustrative of fundamental molecular mechanisms of in vivo TH action involving TH receptors, transcriptional cofactors, and chromatin remodeling. This review highlights the current need, recent successes, and future prospects using amphibians as a model to elucidate molecular mechanisms and functional roles of TH signaling during post-embryonic development.

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

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

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

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

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

Unliganded thyroid hormone receptor α controls developmental timing in Xenopus tropicalis

Thyroid hormone (T3) affects adult metabolism and postembryonic development in vertebrates. T3 functions mainly via binding to its receptors (TRs) to regulate gene expression. There are 2 TR genes, TRα and TRβ, with TRα more ubiquitously expressed. During development, TRα expression appears earlier than T3 synthesis and secretion into the plasma. This and the ability of TRs to regulate gene expression both in the presence and absence of T3 have indicated a role for unliganded TR during vertebrate development. On the other hand, it has been difficult to study the role of unliganded TR during development in mammals because of the difficulty to manipulate the uterus-enclosed, late-stage embryos. Here we use amphibian development as a model to address this question. We have designed transcriptional activator-like effector nucleases (TALENs) to mutate the TRα gene in Xenopus tropicalis. We show that knockdown of TRα enhances tadpole growth in premetamorphic tadpoles, in part because of increased growth hormone gene expression. More importantly, the knockdown also accelerates animal development, with the knockdown animals initiating metamorphosis at a younger age and with a smaller body size. On the other hand, such tadpoles are resistant to exogenous T3 treatment and have delayed natural metamorphosis. Thus, our studies not only have directly demonstrated a critical role of endogenous TRα in mediating the metamorphic effect of T3 but also revealed novel functions of unliganded TRα during postembryonic development, that is, regulating both tadpole growth rate and the timing of metamorphosis.

Thyroid hormone signaling during early neurogenesis and its significance as a vulnerable window for endocrine disruption

The essential roles of thyroid hormone (TH) in perinatal brain development have been known for decades. More recently, many of the molecular mechanisms underlying the multiple effects of TH on proliferation, differentiation, migration, synaptogenesis and myelination in the developing nervous system have been elucidated. At the same time data from both epidemiological studies and animal models have revealed that the influence of thyroid signaling on development of the nervous system, extends to all periods of life, from early embryogenesis to neurogenesis in the adult brain. This review focuses on recent insights into the actions of TH during early neurogenesis. A key concept is that, in contrast to the previous ideas that only the unliganded receptor was implicated in these early phases, a critical role of the ligand, T3, is increasingly recognized. These findings are considered in the light of increasing knowledge of cell specific control of T3 availability as a function of deiodinase activity and transporter expression. These requirements for TH in the early stages of neurogenesis take on new relevance given the increasing epidemiological data on adverse effects of TH lack in early pregnancy on children's neurodevelopmental outcome. These ideas lead logically into a discussion on how the actions of TH during the first phases of neurogenesis can be potentially disrupted by gestational iodine lack and/or chemical pollution. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

The Xenopus homologue of Down syndrome critical region protein 6 drives dorsoanterior gene expression and embryonic axis formation by antagonising polycomb group proteins

Mesoderm and embryonic axis formation in vertebrates is mediated by maternal and zygotic factors that activate the expression of target genes. Transcriptional derepression plays an important role in the regulation of expression in different contexts; however, its involvement and possible mechanism in mesoderm and embryonic axis formation are largely unknown. Here we demonstrate that XDSCR6, a Xenopus homologue of human Down syndrome critical region protein 6 (DSCR6, or RIPPLY3), regulates mesoderm and embryonic axis formation through derepression of polycomb group (PcG) proteins. Xdscr6 maternal mRNA is enriched in the endoderm of the early gastrula and potently triggers the formation of dorsal mesoderm and neural tissues in ectoderm explants; it also dorsalises ventral mesoderm during gastrulation and induces a secondary embryonic axis. A WRPW motif, which is present in all DSCR6 homologues, is necessary and sufficient for the dorsal mesoderm- and axis-inducing activity. Knockdown of Xdscr6 inhibits dorsal mesoderm gene expression and results in head deficiency. We further show that XDSCR6 physically interacts with PcG proteins through the WRPW motif, preventing the formation of PcG bodies and antagonising their repressor activity in embryonic axis formation. By chromatin immunoprecipitation, we demonstrate that XDSCR6 releases PcG proteins from chromatin and allows dorsal mesoderm gene transcription. Our studies suggest that XDSCR6 might function to sequester PcG proteins and identify a novel derepression mechanism implicated in embryonic induction and axis formation.

Metamorphosis in a frog that does not have a tadpole

The evolutionary removal of the tadpole from the frog life history is a very successful strategy, particularly in the tropics. These direct developers form limbs and a frog-like head early in embryogenesis, and they have reduced or lost tadpole-specific structures, like gills, a long, coiled intestine, and tadpole teeth and jaws. Despite the apparently continuous development to the frog morphology, the direct developer, Eleutherodactylus coqui, undergoes a cryptic metamorphosis requiring thyroid hormone. As in Xenopus laevis, there is a stimulation by corticotrophin-releasing factor (CRF) and an upregulation of thyroid hormone receptor β (thrb). In addition to changes in skin and muscle, thyroid hormone stimulates yolk utilization for froglet growth from a novel tissue, the nutritional endoderm. The activities of CRF and corticosterone (CORT) in metamorphosis may provide the basis for the multiple evolutionary origins of direct development in anuran amphibians. Potential roles for maternally supplied thyroid hormone and its receptor and for deiodinases in regulating tissue sensitivity to thyroid hormone should be the subjects of future investigations.

High-throughput sequencing will metamorphose the analysis of thyroid hormone receptor function during amphibian development

Amphibian metamorphosis is marked by dramatic thyroid hormone (T(3))-induced changes including de novo morphogenesis, tissue remodeling, and organ resorption through programmed cell death. These changes involve cascades of gene regulation initiated by thyroid hormone (TH). TH functions by regulating gene expression through TH receptors (TR). TR are DNA-binding transcription factors that belong to the steroid hormone receptor superfamily. In the absence of ligand, TR can repress gene expression by recruiting a corepressor complex, whereas liganded TR recruits a coactivator complex for gene activation. Earlier studies have led us to propose a dual function model for TR during development. In premetamorphic tadpoles, unliganded TR represses transcription involving corepressors. During metamorphosis, endogenous T(3) allows TR to activate gene expression. To fully understand the diversity of T(3) effects during metamorphosis, whole genome analysis of transcriptome and mechanism of TR action should be carried out. To this end, the new sequencing technologies have dramatically changed how fundamental questions in biology are being addressed and is now making the transition from technology development to being a standard for genomic and functional genomic analysis. This review focuses on the applications of high-throughput technologies to the field of amphibian metamorphosis.

Thyroid hormone signaling in the Xenopus laevis embryo is functional and susceptible to endocrine disruption

Thyroid hormone (TH) is essential for vertebrate brain development. Most research on TH and neuronal development focuses on late development, mainly the perinatal period in mammals. However, in human infants neuromotor development correlates best with maternal TH levels in the first trimester of pregnancy, suggesting that TH signaling could affect early brain development. Studying TH signaling in early embryogenesis in mammals is experimentally challenging. In contrast, free-living embryos, such as Xenopus laevis, permit physiological experimentation independent of maternal factors. We detailed key elements of TH signaling: ligands, receptors (TR), and deiodinases during early X. laevis development, before embryonic thyroid gland formation. Dynamic profiles for all components were found. Between developmental stages 37 and 41 (~48 h after hatching, coincident with a phase of continuing neurogenesis) significant increases in T(3) levels as well as in mRNA encoding deiodinases and TR occurred. Exposure of embryos at this developmental stage for 24 h to either a TH antagonist, NH-3, or to tetrabromobisphenol A, a flame retardant and known TH disruptor, differentially modulated the expression of a number of TH target genes implicated in neural stem cell function or neural differentiation. Moreover, 24-h exposure to either NH-3 or tetrabromobisphenol A diminished cell proliferation in the brain. Thus, these data show first, that TH signaling exerts regulatory roles in early X. laevis neurogenesis and second, that this period represents a potential window for endocrine disruption.

Regulation of thyroid hormone sensitivity by differential expression of the thyroid hormone receptor during Xenopus metamorphosis

During amphibian metamorphosis, a series of dynamic changes occur in a predetermined order. Hind limb morphogenesis begins in response to low levels of thyroid hormone (TH) in early prometamorphosis, but tail muscle cell death is delayed until climax, when TH levels are high. It takes about 20 days for tadpoles to grow from early prometamorphosis to climax. To study the molecular basis of the timing of tissue-specific transformations, we introduced thyroid hormone receptor (TR) expression constructs into tail muscle cells of Xenopus tadpoles. The TR-transfected tail muscle cells died upon exposure to a low level of thyroxine (T4). This cell death was suggested to be mediated by type 2 iodothyronine deiodinase (D2) that converts T4 to T3-the more active form of TH. D2 mRNA was induced in the TR-overexpressing cells by low levels of TH. D2 promoter contains a TH-response element (TRE) with a lower affinity for TR. These results show that the TR transfection confers the ability to respond to physiological concentrations of TH at early prometamorphosis to tail muscle cells through D2 activity and promotes TH signaling. We propose the positive feedback loop model to amplify the cell's ability to respond to low levels of T4.

Teratogenic effects of triphenyltin on embryos of amphibian (Xenopus tropicalis): a phenotypic comparison with the retinoid X and retinoic acid receptor ligands

Triphenyltin (TPT) has high binding affinity with the retinoid X receptor (RXR) in animals. The natural ligand of RXR, 9-cis-retinoic acid (RA), is known to induce featured malformations in vertebrate embryos by disrupting RA signal. Limited information is available on the TPT effects on amphibians. We exposed embryos of amphibian (Xenopus tropicalis) to TPT, 9-cis-RA, all-trans-RA (ligand of retinoic acid receptor, RAR), and LGD1069 (a selective ligand of RXR). The 72h LC50 of TPT was 5.25 μg Sn/L, and 72h EC50 was 0.96 μg Sn/L. TPT induced multiple malformations including enlarged proctodaeum and narrow fins. TPT at 5 μg Sn/L inhibited the differentiation of skins and muscles. The reduced brain, loss of external eyes and bent axis were observed in RXR and RAR ligands treatments. TPT and tributyltin (TBT) inhibited the mRNA expression of RXRα and increased that of TRβ. The phenotypes of malformations induced by TPT were similar to those by TBT and were much different from those by the RXR and RAR ligands. These results indicated that TPT was acute toxic and had high teratogenicity to amphibian embryos, and that TPT induced phenotypes of malformations. TPT and TBT might have a similar teratogenic mechanism, which seems not to be mainly mediated through RA signal.

Higher thyroid hormone receptor expression correlates with short larval periods in spadefoot toads and increases metamorphic rate

Spadefoot toad species display extreme variation in larval period duration, due in part to evolution of thyroid hormone (TH) physiology. Specifically, desert species with short larval periods have higher tail tissue content of TH and exhibit increased responsiveness to TH. To address the molecular basis of larval period differences, we examined TH receptor (TR) expression across species. Based on the dual function model for the role of TR in development, we hypothesized that desert spadefoot species with short larval periods would have (1) late onset of TR expression prior to the production of endogenous TH and (2) higher TR levels when endogenous TH becomes available. To test these hypotheses, we cloned fragments of TRα and TRβ genes from the desert spadefoot toads Scaphiopus couchii and Spea multiplicata and their non-desert relative Pelobates cultripes and measured their mRNA levels in tails using quantitative PCR in the absence (premetamorphosis) or presence (natural metamorphosis) of TH. All species express TRα and TRβ from the earliest stages measured (from just after hatching), but S. couchii, which has the shortest larval period, had more TRα throughout development compared to P. cultripes, which has the longest larval period. TRβ mRNA levels were similar across species. Exogenous T3 treatment induced faster TH-response gene expression kinetics in S. couchii compared to the other species, consistent with its higher TRα mRNA expression and indicative of a functional consequence of more TRα activity at the molecular level. To directly test whether higher TRα expression may contribute to shorter larval periods, we overexpressed TRα via plasmid injection into tail muscle cells of the model frog Xenopus laevis and found an increased rate of muscle cell death in response to TH. These results suggest that increased TRα expression evolved in S. couchii and contribute to its higher metamorphic rates.

Thyroid hormone deficiency disrupts rat eye neurodevelopment

Clinical and experimental studies have highlighted the role played by thyroid hormones (TH) in neural and neuro-sensorial development. However, knowledge on TH mechanisms on the developing visual system is still incomplete. To uncover TH actions on the eyes and vision we carried out a microscopical study on the role of TH in the developing retina and optic nerve, in a rat model of controlled TH deficiency (THD). Morphometric and stereological analyses of the retina and optic nerve showed a reduction in the volume of the eye (p<0.001) and optic nerve cross-sectional area (p<0.001), and thinning of the retinal layers (p<0.001). Glial development and myelination was significantly delayed in the THD optic nerves (p<0.001), as compared to controls. The data indicate that TH play an essential role in neuro-retinogenesis. Substitutive TH therapy in critical periods, should be considered in hypothyroidism-related eye disorders as well as neurodegenerative retinal processes.

Specific histone lysine 4 methylation patterns define TR-binding capacity and differentiate direct T3 responses

The diversity of thyroid hormone T(3) effects in vivo makes their molecular analysis particularly challenging. Indeed, the current model of the action of T(3) and its receptors on transcription does not reflect this diversity. Here, T(3)-dependent amphibian metamorphosis was exploited to investigate, in an in vivo developmental context, how T(3) directly regulates gene expression. Two, direct positively regulated T(3)-response genes encoding transcription factors were analyzed: thyroid hormone receptor β (TRβ) and TH/bZIP. Reverse transcription-real-time quantitative PCR analysis on Xenopus tropicalis tadpole brain and tail fin showed differences in expression levels in premetamorphic tadpoles (lower for TH/bZIP than for TRβ) and differences in induction after T(3) treatment (lower for TRβ than for TH/bZIP). To dissect the mechanisms underlying these differences, chromatin immunoprecipitation was used. T(3) differentially induced RNA polymerase II and histone tail acetylation as a function of transcriptional level. Gene-specific patterns of TR binding were found on the different T(3) -responsive elements (higher for TRβ than for TH/bZIP), correlated with gene-specific modifications of H3K4 methylation (higher for TRβ than for TH/bZIP). Moreover, tissue-specific modifications of H3K27 were found (lower in brain than in tail fin). This first in vivo analysis of the association of histone modifications and TR binding/gene activation during vertebrate development for any nuclear receptor indicate that chromatin context of thyroid-responsive elements loci controls the capacity to bind TR through variations in histone H3K4 methylation, and that the histone code, notably H3, contributes to the fine tuning of gene expression that underlies complex physiological T(3) responses.

Molecular and genetic studies suggest that thyroid hormone receptor is both necessary and sufficient to mediate the developmental effects of thyroid hormone

Thyroid hormone (TH) affects diverse biological processes and can exert its effects through both gene regulation via binding the nuclear TH receptors (TRs) and non-genomic actions via binding to cell surface and cytoplasmic proteins. The critical importance of TH in vertebrate development has long been established, ranging from the formation of human cretins to the blockage of frog metamorphosis due the TH deficiency. How TH affects vertebrate development has been difficult to study in mammals due to the complications associated with the uterus-enclosed mammalian embryos. Anuran metamorphosis offers a unique opportunity to address such an issue. Using Xenopus as a model, we and others have shown that the expression of TRs and their heterodimerization partners RXRs (9-cis retinoic acid receptors) correlates temporally with metamorphosis in different organs in two highly related species, Xenopuslaevis and Xenopus tropicalis. In vivo molecular studies have shown that TR and RXR are bound to the TH response elements (TREs) located in TH-inducible genes in developing tadpoles of both species. More importantly, transgenic studies in X. laevis have demonstrated that TR function is both necessary and sufficient for mediating the metamorphic effects of TH. Thus, the non-genomic effects of TH have little or no roles during metamorphosis and likely during vertebrate development in general.

Amphibian metamorphosis as a model for studying endocrine disruption on vertebrate development: effect of bisphenol A on thyroid hormone action

Thyroid hormone (TH) is essential for proper development in vertebrates. TH deficiency during gestation and early postnatal development produces severe neurological, skeletal, metabolism and growth abnormalities. It is therefore important to consider environmental chemicals that may interfere with TH signaling. Exposure to environmental contaminants that disrupt TH action may underlie the increasing incidence of human developmental disorders worldwide. One contaminant of concern is the xenoestrogen bisphenol A (BPA), a chemical widely used to manufacture polycarbonate plastics and epoxy resins. The difficulty in studying uterus-enclosed mammalian embryos has hampered the analysis on the direct effects of BPA during vertebrate development. As TH action at the cellular level is highly conserved across vertebrate species, amphibian metamorphosis serves as an important TH-dependent in vivo vertebrate model for studying potential contributions of BPA toward human developmental disorders. Using Xenopus laevis as a model, we and others have demonstrated the inhibitory effects of BPA exposure on metamorphosis. Genome-wide gene expression analysis revealed that surprisingly, BPA primarily targets the TH-signaling pathway essential for metamorphosis in Xenopus laevis. Given the importance of the genomic effects of TH during metamorphosis and the conservation in its regulation in higher vertebrates, these observations suggest that the effect of BPA in human embryogenesis is through the inhibition of the TH pathway and warrants further investigation. Our findings further argue for the critical need to use in vivo animal models coupled with systematic molecular analysis to determine the developmental effects of endocrine disrupting compounds.

Teratogenic effects of tetrabromobisphenol A on Xenopus tropicalis embryos

Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant and a known thyroid disruptor. We reported exposing Xenopus tropicalis embryos (NF10) to 0.01, 0.1 or 1 mg/L of TBBPA with or without 70 microg/L triiodothyronine (T(3)). Compared with the controls, 1 mg/L of TBBPA significantly reduced the body length of embryos after 24, 36 and 48 h of exposure. Embryos treated with TBBPA showed multiple malformations, including: abnormal eyes, skin hypopigmentation, enlarged proctodaeum, narrow fins and pericardial edemas. The effect of abnormal eyes manifested itself in the loss of pigmentation, reduction in size, or absence of external eyes. The degree of eye malformations was quantified with the index of eye malformations (IEM) with 0 being normal and 3 being severe. In the 1 mg/L TBBPA treatment groups, the incidence of total malformations (ITM) was 68-93%, and IEM was 0.8-0.9. T(3) showed no teratogenic effects on embryos, but it significantly enhanced TBBPA-induced teratogenic effects. In the T(3)+1 mg/L TBBPA treatment groups, ITM was 91-99%, and IEM was 1.8-1.9. Histological observations showed that the retinas were generally smaller, and the lenses were underdeveloped or even absent. These results indicate that TBBPA at relatively high concentration has teratogenic effects on X. tropicalis embryos. The results also suggest that thyroid hormone signaling might be involved in TBBPA induced-teratogenicity.

Fadrozole and finasteride exposures modulate sex steroid- and thyroid hormone-related gene expression in Silurana (Xenopus) tropicalis early larval development

Steroidogenic enzymes and their steroid products play critical roles during gonadal differentiation in amphibians; however their roles during embryogenesis remain unclear. The objective of this study was to investigate the expression and activity of aromatase (cyp19; estrogen synthase) and 5 beta-reductase (srd5 beta; 5 beta-dihydrotestosterone synthase) during amphibian embryogenesis. Expression and activity profiles of cyp19 and srd5 beta were first established during Silurana (Xenopus) tropicalis embryogenesis from Nieuwkoop-Faber (NF) stage 2 (2-cell stage; 1h post-fertilization) to NF stage 46 (beginning of feeding; 72 h post-fertilization). Exposures to fadrozole (an aromatase inhibitor; 0.5, 1.0 and 2.0 microM) and finasteride (a putative 5-reductase inhibitor; 25, 50 and 100 microM) were designed to assess the consequences of inhibiting these enzymes on gene expression in early amphibian larval development. Exposed embryos showed changes in both enzyme activities and sex steroid- and thyroid hormone-related gene expression. Fadrozole treatment inhibited cyp19 activity and increased androgen receptor and thyroid hormone receptor (alpha and beta) mRNAs. Finasteride treatment inhibited srd5 beta (activity and mRNA), decreased cyp19 mRNA and activity levels and increased estrogen receptor alpha mRNA. Both treatments altered the expression of deiodinases (thyroid hormone metabolizing enzymes). We conclude that cyp19 and srd5 beta are active in early embryogenesis and larval development in Silurana tropicalis and their inhibition affected transcription of genes associated with the thyroid and reproductive axes.

Expression and T3 regulation of thyroid hormone- and sex steroid-related genes during Silurana (Xenopus) tropicalis early development

In amphibians, thyroid hormones (THs) are the primary regulators of metamorphosis; however, their physiological role during embryogenesis remains unclear. First, we established complete developmental profiles for TH receptors (tr alpha and tr beta), deiodinases (dio; types 1, 2, 3), estrogen receptors (er alpha and er beta) and androgen receptor (ar) mRNA levels during embryogenesis and early larval stages in Silurana (Xenopus) tropicalis (from Nieuwkoop and Faber (NF) stage 2 until NF 46). Real-time RT-PCR analyses in whole embryos and larvae revealed that all transcripts except tr alpha were detected throughout development; tr alpha only appears after gastrulation. The first significant increase in the expression of tralpha and tr beta was observed before hatching, between NF 21 and NF 27 (2.5- and 11-fold, respectively). In order to test if these genes could be regulated by THs during early larval development, embryos were exposed to triiodothyronine (T3; 0.5, 5.0, 50 nM) from NF 27 to NF 46. T3 exposure caused a dose-dependent increase relative to control in the expression of tr alpha, tr beta, dio (types 2 and 3), ar, and 5 alpha-reductase type 1 in whole larvae. These results indicate that in S. tropicalis, tr and dio can be induced by T3 as early as NF 46, a response that had only been characterized later during frog metamorphosis. In addition, T3 also affected androgen-related gene expression, supporting our hypothesis that THs are involved in male development in frogs.

Effects of tributyltin (TBT) on Xenopus tropicalis embryos at environmentally relevant concentrations

Tributyltin (TBT) has been widely used as a biocide in antifouling paints and is a known endocrine disrupting chemical. In this paper, we exposed embryos of Xenopus tropicalis to 50-400ngL(-1) tributyltin chloride. TBT significantly decreased the survival rate, reduced the body length and retarded the development of embryos after 24, 36 and 48h of exposure. These effects of TBT were concentration- and time-dependent. Embryos treated with TBT showed multiple malformations. The most obvious alterations were abnormal eyes, enlarged proctodaeum, narrow fins, and skin hypopigmentation. Enlarged proctodaeum and narrow fins were mainly observed after 36 and 48h of exposure. The loss of eye pigmentation or the absence of external eyes occurred after 24 and 36h of exposure, while extended lenses or edemas of eyes were more commonly observed after 48h of exposure. Additional malformations included: small anterior region of heads, pericardial edemas, enlarged trunks, and bent tails. These results suggested that TBT is very toxic to X. tropicalis embryos at environmentally relevant concentrations.

Coordination of deiodinase and thyroid hormone receptor expression during the larval to juvenile transition in sea bream (Sparus aurata, Linnaeus)

To test the hypothesis that THs play an important role in the larval to juvenile transition in the marine teleost model, sea bream (Sparus auratus), key elements of the thyroid axis were analysed during development. Specific RT-PCR and Taqman quantitative RT-PCR were established and used to measure sea bream iodothyronine deiodinases and thyroid hormone receptor (TR) genes, respectively. Expression of deiodinases genes (D1 and D2) which encode enzymes producing T3, TRs and T4 levels start to increase at 20-30 days post-hatch (dph; beginning of metamorphosis), peak at about 45 dph (climax) and decline to early larval levels after 90-100 dph (end of metamorphosis) when fish are fully formed juveniles. The profile of these different TH elements during sea bream development is strikingly similar to that observed during the TH driven metamorphosis of flatfish and suggests that THs play an analogous role in the larval to juvenile transition in this species and probably also in other pelagic teleosts. However, the effect of T3 treatment on deiodinases and TR transcript abundance in sea bream is not as clear cut as in larval flatfish and tadpoles indicating divergence in the responsiveness of TH axis elements and highlighting the need for further studies of this axis during development of fish.

Dual functions of thyroid hormone receptors in vertebrate development: the roles of histone-modifying cofactor complexes

Thyroid hormone (TH) receptor (TR) plays critical roles in vertebrate development. Transcription studies have shown that TR activates or represses TH-inducible genes by recruiting coactivators or corepressors in the presence or absence of TH, respectively. However, the developmental roles of these TR cofactors remain largely unexplored. Frog metamorphosis is totally dependent on TH and mimics the postembryonic period in mammalian development during which TH levels are also high. We have previously proposed a dual function model for TR in the development of the anuran Xenopus laevis. That is, unliganded TR recruits corepressors to TH-inducible genes in premetamorphic tadpoles to repress these genes and prevent premature metamorphic changes and subsequently, when TH becomes available, liganded TR recruits coactivators to activate these same genes, leading to metamorphosis. Over the years, we and others have used molecular and genetic approaches to demonstrate the importance of the dual functions of TR in Xenopus laevis. In particular, unliganded TR has been shown to recruit histone deacetylase-containing corepressor complexes in premetamorphic tadpoles to control metamorphic timing. In contrast, metamorphosis requires TH-bound TR to recruit coactivator complexes containing histone acetyltransferases and methyltransferases to activate transcription. Furthermore, the concentrations of coactivators appear to regulate the rate of metamorphic progression. Studies in mammals also suggest that the dual function model for TR is conserved across vertebrates.