Mechanisms of thyroid hormone receptor action during development: lessons from amphibian studies

Intestinal remodeling during Xenopus metamorphosis as a model for studying thyroid hormone signaling and adult organogenesis

Intestinal development takes places in two phases, the initial formation of neonatal (mammals)/larval (anurans) intestine and its subsequent maturation into the adult form. This maturation occurs during postembryonic development when plasma thyroid hormone (T3) level peaks. In anurans such as the highly related Xenopus laevis and Xenopus tropicalis, the larval/tadpole intestine is drastically remodeled from a simple tubular structure to a complex, multi-folded adult organ during T3-dependent metamorphosis. This involved complete degeneration of larval epithelium via programmed cell death and de novo formation of adult epithelium, with concurrent maturation of the muscles and connective tissue. Here, we will summarize our current understanding of the underlying molecular mechanisms, with a focus on more recent genetic and genome-wide studies.

Overlapping action of T3 and T4 during Xenopus laevis development

Thyroid hormones are involved in many biological processes such as neurogenesis, metabolism, and development. However, compounds called endocrine disruptors can alter thyroid hormone signaling and induce unwanted effects on human and ecosystems health. Regulatory tests have been developed to detect these compounds but need to be significantly improved by proposing novel endpoints and key events. The Xenopus Eleutheroembryonic Thyroid Assay (XETA, OECD test guideline no. 248) is one such test. It is based on Xenopus laevis tadpoles, a particularly sensitive model system for studying the physiology and disruption of thyroid hormone signaling: amphibian metamorphosis is a spectacular (thus easy to monitor) life cycle transition governed by thyroid hormones. With a long-term objective of providing novel molecular markers under XETA settings, we propose first to describe the differential effects of thyroid hormones on gene expression, which, surprisingly, are not known. After thyroid hormones exposure (T3 or T4), whole tadpole RNAs were subjected to transcriptomic analysis. By using standard approaches coupled to system biology, we found similar effects of the two thyroid hormones. They impact the cell cycle and promote the expression of genes involves in cell proliferation. At the level of the whole tadpole, the immune system is also a prime target of thyroid hormone action.

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.

Cell cycle activation in thyroid hormone-induced apoptosis and stem cell development during Xenopus intestinal metamorphosis

Amphibian metamorphosis resembles mammalian postembryonic development, a period around birth when many organs mature into their adult forms and when plasma thyroid hormone (T3) concentration peaks. T3 plays a causative role for amphibian metamorphosis. This and its independence from maternal influence make metamorphosis of amphibians, particularly anurans such as pseudo-tetraploid Xenopus laevis and its highly related diploid species Xenopus tropicalis, an excellent model to investigate how T3 regulates adult organ development. Studies on intestinal remodeling, a process that involves degeneration of larval epithelium via apoptosis and de novo formation of adult stem cells followed by their proliferation and differentiation to form the adult epithelium, have revealed important molecular insights on T3 regulation of cell fate during development. Here, we review some evidence suggesting that T3-induced activation of cell cycle program is important for T3-induced larval epithelial cell death and de novo formation of adult intestinal stem cells.

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.

Steroid-receptor coactivator complexes in thyroid hormone-regulation of Xenopus metamorphosis

Anuran metamorphosis is perhaps the most drastic developmental change regulated by thyroid hormone (T3) in vertebrate. It mimics the postembryonic development in mammals when many organs/tissues mature into adult forms and plasma T3 level peaks. T3 functions by regulating target gene transcription through T3 receptors (TRs), which can recruit corepressor or coactivator complexes to target genes in the absence or presence of T3, respectively. By using molecular and genetic approaches, we and others have investigated the role of corepressor or coactivator complexes in TR function during the development of two highly related anuran species, the pseudo-tetraploid Xenopus laevis and diploid Xenopus tropicalis. Here we will review some of these studies that demonstrate a critical role of coactivator complexes, particularly those containing steroid receptor coactivator (SRC) 3, in regulating metamorphic rate and ensuring the completion of metamorphosis.

Stem cell development involves divergent thyroid hormone receptor subtype expression and epigenetic modifications in the amphibian intestine during metamorphosis

In the amphibian intestine during metamorphosis, most of the larval epithelial cells undergo apoptosis, while a small number of the epithelial cells dedifferentiate into stem cells (SCs). The SCs actively proliferate and then newly generate the adult epithelium analogous to the mammalian counterpart, which is continuously renewed from the SCs throughout adulthood. This larval-to-adult intestinal remodeling can be experimentally induced by thyroid hormone (TH) through interacting with the surrounding connective tissue that develops as the stem cell niche. Thus, the amphibian intestine provides us a valuable opportunity to study how the SCs and their niche are formed during development. To clarify the TH-induced and evolutionally conserved mechanism of SC development at the molecular level, numerous TH response genes have been identified in the Xenopus laevis intestine over the last three decades and extensively analyzed for their expression and function by using wild-type and transgenic Xenopus tadpoles. Interestingly, accumulating evidence indicates that thyroid hormone receptor (TR) epigenetically regulates the expression of TH response genes involved in the remodeling. In this review, we highlight recent progress in the understanding of SC development, focusing on epigenetic gene regulation by TH/TR signaling in the X. laevis intestine. We here propose that two subtypes of TRs, TRα and TRβ, play distinct roles in the intestinal SC development via different histone modifications in different cell types.

Simultaneous activation of genes encoding urea cycle enzymes and gluconeogenetic enzymes coincides with a corticosterone surge period before metamorphosis in Xenopus laevis

Amphibian tadpoles are postulated to excrete ammonia as nitrogen metabolites but to shift from ammonotelism to ureotelism during metamorphosis. However, it is unknown whether ureagenesis occurs or plays a functional role before metamorphosis. Here, the mRNA-expression levels of two urea cycle enzymes (carbamoyl phosphate synthetase I [CPSI] and ornithine transcarbamylase [OTC]) were measured beginning with stage-47 Xenopus tadpoles at 5 days post-fertilization (dpf), between the onset of feeding (stage 45, 4 dpf) and metamorphosis (stage 55, 32 dpf). CPSI and OTC expression levels increased significantly from stage 49 (12 dpf). Urea excretion was also detected at stage 47. A transient corticosterone surge peaking at stage 48 was previously reported, supporting the hypothesis that corticosterone can induce CPSI expression in tadpoles, as found in adult frogs and mammals. Stage-46 tadpoles were exposed to a synthetic glucocorticoid, dexamethasone (Dex, 10-500 nM) for 3 days. CPSI mRNA expression was significantly higher in tadpoles exposed to Dex than in tadpoles exposed to the vehicle control. Furthermore, glucocorticoid receptor mRNA expression increased during the pre-metamorphic period. In addition to CPSI and OTC mRNA upregulation, the expression levels of three gluconeogenic enzyme genes (glucose 6-phosphatase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase 1) increased with the onset of urea synthesis and excretion. These results suggest that simultaneous induction of the urea cycle and gluconeogenic enzymes coincided with a corticosterone surge occurring prior to metamorphosis. These metabolic changes preceding metamorphosis may be closely related to the onset of feeding and nutrient accumulation required for metamorphosis.

Crosstalk between Thyroid Hormone and Corticosteroid Signaling Targets Cell Proliferation in Xenopus tropicalis Tadpole Liver

Thyroid hormones (TH) and glucocorticoids (GC) are involved in numerous developmental and physiological processes. The effects of individual hormones are well documented, but little is known about the joint actions of the two hormones. To decipher the crosstalk between these two hormonal pathways, we conducted a transcriptional analysis of genes regulated by TH, GC, or both hormones together in liver of Xenopus tropicalis tadpoles using RNA-Seq. Among the differentially expressed genes (DE), 70.5% were regulated by TH only, 0.87% by GC only, and 15% by crosstalk between the two hormones. Gene ontology analysis of the crosstalk-regulated genes identified terms referring to DNA replication, DNA repair, and cell-cycle regulation. Biological network analysis identified groups of genes targeted by the hormonal crosstalk and corroborated the gene ontology analysis. Specifically, we found two groups of functionally linked genes (chains) mainly composed of crosstalk-regulated hubs (highly interactive genes), and a large subnetwork centred around the crosstalk-regulated genes psmb6 and cdc7. Most of the genes in the chains are involved in cell-cycle regulation, as are psmb6 and cdc7, which regulate the G2/M transition. Thus, the biological action of these two hormonal pathways acting together in the liver targets cell-cycle regulation.

Upregulation of proto-oncogene ski by thyroid hormone in the intestine and tail during Xenopus metamorphosis

Thyroid hormone (T3) is important for adult organ function and vertebrate development, particularly during the postembryonic period when many organs develop/mature into their adult forms. Amphibian metamorphosis is totally dependent on T3 and can be easily manipulated, thus offering a unique opportunity for studying how T3 controls postembryonic development in vertebrates. Numerous early studies have demonstrated that T3 affects frog metamorphosis through T3 receptor (TR)-mediated regulation of T3 response genes, where TR forms a heterodimer with RXR (9-cis retinoic acid receptor) and binds to T3 response elements (TREs) in T3 response genes to regulate their expression. We have previously identified many candidate direct T3 response genes in Xenopus tropicalis tadpole intestine. Among them is the proto-oncogene Ski, which encodes a nuclear protein with complex function in regulating cell fate. We show here that Ski is upregulated in the intestine and tail of premetamorphic tadpoles upon T3 treatment and its expression peaks at stage 62, the climax of metamorphosis. We have further discovered a putative TRE in the first exon that can bind to TR/RXR in vitro and mediate T3 regulation of the promoter in vivo. These data demonstrate that Ski is activated by T3 through TR binding to a TRE in the first exon during Xenopus tropicalis metamorphosis, implicating a role of Ski in regulating cell fate during metamorphosis.

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Effects of Separate and Combined Exposure of Cadmium and Lead on the Endochondral Ossification in Bufo gargarizans

Cadmium (Cd) and lead (Pb) are ubiquitous in aquatic environments and most studies have examined the potential effects of Cd or Pb alone on aquatic organisms. In the present study, chronic effects of Cd and Pb, alone and in combination, on Bufo gargarizans were investigated by exposing embryos to these contaminants throughout metamorphosis. Significant reductions in body mass and snout-to-vent length were observed in B. gargarizans at Gosner stage 42 (Gs 42) and Gs 46 exposed to a Cd/Pb mixture. Single and combined exposure with Cd and Pb induced histological alterations of the thyroid gland characterized by reduced colloid area and thickness of epithelial cells. There was a significant decrease in the maximum jump distance of froglets exposed to Cd alone and the Cd/Pb mixture, and the jumping capacity showed a positive correlation with hind limb length and tibia/fibula. Moreover, single metals and their mixture induced reduction of endochondral bone formation in B. gargarizans. Transcriptomic and real-time quantitative polymerase chain reaction results showed that genes involved in skeletal ossification (TRα, TRβ, Dio2, Dio3, MMP9, MMP13, Runx1, Runx2, and Runx3) were transcriptionally dysregulated by Cd and Pb exposure alone or in combination. Our results suggested that despite the low concentration tested, the Cd/Pb mixture induced more severe impacts on B. gargarizans. In addition, the Cd/Pb mixture might reduce chances of survival for B. gargarizans froglets by decreasing size at metamorphosis, impaired skeletal ossification, and reduction in jumping ability, which might result from dysregulation of genes involved in thyroid hormone action and endochondral ossification. The findings obtained could add a new dimension to understanding of the mechanisms underpinning skeletal ossification response to heavy metals in amphibians. Environ Toxicol Chem 2022;41:1228-1245. © 2022 SETAC.

Retinoid-X receptor agonists increase thyroid hormone competence in lower jaw remodeling of pre-metamorphic Xenopus laevis tadpoles

Thyroid hormone (TH) signaling plays critical roles during vertebrate development, including regulation of skeletal and cartilage growth. TH acts through its receptors (TRs), nuclear hormone receptors (NRs) that heterodimerize with Retinoid-X receptors (RXRs), to regulate gene expression. A defining difference between NR signaling during development compared to in adult tissues, is competence, the ability of the organism to respond to an endocrine signal. Amphibian metamorphosis, especially in Xenopus laevis, the African clawed frog, is a well-established in vivo model for studying the mechanisms of TH action during development. Previously, we’ve used one-week post-fertilization X. laevis tadpoles, which are only partially competent to TH, to show that in the tail, which is naturally refractive to exogenous T3 at this stage, RXR agonists increase TH competence, and that RXR antagonism inhibits the TH response. Here, we focused on the jaw that undergoes dramatic TH-mediated remodeling during metamorphosis in order to support new feeding and breathing styles. We used a battery of approaches in one-week-old tadpoles, including quantitative morphology, differential gene expression and whole mount cell proliferation assays, to show that both pharmacologic (bexarotene) and environmental (tributyltin) RXR agonists potentiated TH-induced responses but were inactive in the absence of TH; and the RXR antagonist UVI 3003 inhibited TH action. Bex and TBT significantly potentiated cellular proliferation and the TH induction of runx2, a transcription factor critical for developing cartilage and bone. Prominent targets of RXR-mediated TH potentiation were members of the matrix metalloprotease family, suggesting that RXR potentiation may emphasize pathways responsible for rapid changes during development.

Sperm associated antigen 7 is activated by T3 during Xenopus tropicalis metamorphosis via a thyroid hormone response element within the first intron

Thyroid hormone (T3) affects many diverse physiological processes such as metabolism, organogenesis, and growth. The two highly related frog species, diploid Xenopus tropicalis and pseudo tetraploid Xenopus laevis, have been used as models for analyzing the effects of T3 during vertebrate development. T3 regulates T3-inducible gene transcription through T3 receptor (TR)-binding to T3-response elements (TREs). We have previously identified sperm associated antigen 7 (spag7) as a candidate T3 target gene that is potentially involved in adult stem cell development and/or proliferation during intestinal metamorphosis. To investigate whether T3 regulates spag7 directly at the transcriptional level via TR, we first conducted qRT-PCR to analyze its expression during natural and T3-induced metamorphosis and found that spag7 was up-regulated during natural metamorphosis in the intestine, tail, brain and hindlimb, peaking at the climax of metamorphosis in all those organs, and upon T3 treatment of premetamorphic tadpoles. Next, we demonstrated that an intronic TRE in spag7, first identified through bioinformatic analysis, could bind to TR in vitro and in vivo during metamorphosis. A dual luciferase assay utilizing a reconstituted frog oocyte transcription system showed that the TRE could mediate promoter activation by liganded TR. These results indicate that spag7 expression is directly regulated by T3 through the TRE in the first intron during metamorphosis, implicating a role for spag7 early during T3-regulated tissue remodeling and resorption.

Thyroid hormone regulation of adult hippocampal neurogenesis: Putative molecular and cellular mechanisms

Adult hippocampal neurogenesis is sensitive to perturbations in thyroid hormone signaling, with evidence supporting a key role for thyroid hormone and thyroid hormone receptors (TRs) in the regulation of postmitotic progenitor survival and neuronal differentiation. In this book chapter we summarize the current understanding of the effects of thyroid hormone signaling on adult hippocampal progenitor development, and also critically address the role of TRs in regulation of distinct aspects of stage-specific hippocampal progenitor progression. We highlight actions of thyroid hormone on thyroid hormone responsive target genes, and the implications for hippocampal progenitor regulation. Given the influence of thyroid hormone on both mitochondrial and lipid metabolism, we discuss a putative role for regulation of metabolism in the effects of thyroid hormone on adult hippocampal neurogenesis. Finally, we highlight specific ideas that require detailed experimental investigation, and the need for future studies to obtain a deeper mechanistic insight into the influence of thyroid hormone and TRs in the developmental progression of adult hippocampal progenitors.

Increasing Hormonal Control of Skeletal Development: An Evolutionary Trend in Amphibians

The biphasic life history of amphibians includes metamorphosis, a complex developmental event that involves drastic changes in the morphology, physiology and biochemistry accompanying the transition from the larval to adult stage of development. Thyroid hormones (THs) are widely known to orchestrate this remodeling and, in particular, to mediate the development of the bony skeleton, which is a model system in evolutionary morphological studies of amphibians. Detailed experimental studies of the role of THs in the craniogenesis of diverse urodelan amphibians revealed that (i) these hormones affect both the timing and sequence of bone formation, (ii) TH involvement increases in parallel with the increase in divergence between larval and adult skull morphology, and (iii) among urodelans, TH-involvement in skull development changes from a minimum in basal salamanders (Hynobiidae) to the most pronounced in derived ones (Salamandridae and Plethodontidae). Given the increasing regulatory function of THs in urodelan evolution, we hypothesized a stronger involvement of THs in the control of skeletogenesis in anurans with their most complex and dramatic metamorphosis among all amphibians. Our experimental study of skeletal development in the hypo- and hyperthyroid yellow-bellied toad (Bombina variegata: Bombinatoridae) supports the greater involvement of THs in the mediation of all stages of anuran cranial and postcranial bones formation. Similar to urodelans, B. variegata displays enhancing TH involvement in the development of cranial bones that arise during larval ontogeny: while the hormonal impact on early larval ossifications is minimal, the skull bones forming during metamorphosis are strictly TH-inducible. However, in contrast to urodelans, all cranial bones, including the earliest to form, are TH-dependent in B. variegata; moreover, the development of all elements of the axial and limb skeleton is affected by THs. The more accentuated hormonal control of skeletogenesis in B. variegata demonstrates the advanced regulatory and inductive function of THs in the orchestration of anuran metamorphosis. Based on these findings, we discuss (i) changes in THs function in amphibian evolution and (ii) the role of THs in the evolution of life histories in amphibians.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Transcriptome and Methylome Analysis Reveal Complex Cross-Talks between Thyroid Hormone and Glucocorticoid Signaling at Xenopus Metamorphosis

Background: Most work in endocrinology focus on the action of a single hormone, and very little on the cross-talks between two hormones. Here we characterize the nature of interactions between thyroid hormone and glucocorticoid signaling during Xenopus tropicalis metamorphosis. Methods: We used functional genomics to derive genome wide profiles of methylated DNA and measured changes of gene expression after hormonal treatments of a highly responsive tissue, tailfin. Clustering classified the data into four types of biological responses, and biological networks were modeled by system biology. Results: We found that gene expression is mostly regulated by either T₃ or CORT, or their additive effect when they both regulate the same genes. A small but non-negligible fraction of genes (12%) displayed non-trivial regulations indicative of complex interactions between the signaling pathways. Strikingly, DNA methylation changes display the opposite and are dominated by cross-talks. Conclusion: Cross-talks between thyroid hormones and glucocorticoids are more complex than initially envisioned and are not limited to the simple addition of their individual effects, a statement that can be summarized with the pseudo-equation: TH ∙ GC > TH + GC. DNA methylation changes are highly dynamic and buffered from genome expression.

Sucralose Affects Thyroid Hormone Signaling in American Bullfrog [Rana (Lithobates) catesbeiana] Tadpoles

Nonnutritive sweeteners used in food and beverage products are widespread, persistent aquatic pollutants. Despite this, their impact on aquatic organisms, particularly vertebrates, is not well-studied. Recent findings in rodents suggest sucralose, a chlorinated disaccharide, alters thyroid hormone (TH) metabolism. Because amphibian tadpole metamorphosis is TH-dependent, we hypothesized sucralose may alter signaling for this postembryonic developmental process. The present study used the American bullfrog, Rana (Lithobates) catesbeiana, as a sensitive, environmentally relevant model for testing TH disruption in the absence and presence of thyroxine (T₄), a hormone that induces metamorphosis. Premetamorphic R. catesbeiana tadpoles were immersed in 1-, 15-, and 32-mg/L sucralose solutions ± 5 nM (3.9 µg/L) thyroxine (T₄) for 48 h. RNA transcripts encoding thyroid hormone receptors alpha and beta (thra and thrb) and TH-induced basic region leucine zipper protein (thibz) were analyzed in four tissues: back skin, liver, olfactory epithelium, and tail fin, using reverse transcription quantitative real-time PCR (RT-qPCR). We found that sucralose altered the expression of fundamental TH-response genes involved in anuran metamorphosis in a tissue- and TH-status dependent manner. As organochlorines induce xenobiotic metabolism, we isolated and characterized three novel R. catesbeiana gene transcripts involved in xenobiotic metabolism: pregnane X receptor (nr1i2), constitutive androstane receptor (nr1i3), and cytochrome p450 3a4 (cyp3a4). We analyzed their expression using RT-qPCR and found evidence of their modulation by sucralose. To our knowledge, these data are the first to show xenobiotic and thyroid-disrupting activities in amphibians and further investigations into cumulative effects of environmental sucralose exposure are warranted.

Life Without Thyroid Hormone Receptor

Thyroid hormone (T3) is critical not only for organ function and metabolism in the adult but also for animal development. This is particularly true during the neonatal period when T3 levels are high in mammals. Many processes during this postembryonic developmental period resemble those during amphibian metamorphosis. Anuran metamorphosis is perhaps the most dramatic developmental process controlled by T3 and affects essentially all organs/tissues, often in an organ autonomous manner. This offers a unique opportunity to study how T3 regulates vertebrate development. Earlier transgenic studies in the pseudo-tetraploid anuran Xenopus laevis revealed that T3 receptors (TRs) are necessary and sufficient for mediating the effects of T3 during metamorphosis. Recent gene knockout studies with gene-editing technologies in the highly related diploid anuran Xenopus tropicalis showed, surprisingly, that TRs are not required for most metamorphic transformations, although tadpoles lacking TRs are stalled at the climax of metamorphosis and eventually die. Analyses of the changes in different organs suggest that removal of TRs enables premature development of many adult tissues, likely due to de-repression of T3-inducible genes, while preventing the degeneration of tadpole-specific tissues, which is possibly responsible for the eventual lethality. Comparison with findings in TR knockout mice suggests both conservation and divergence in TR functions, with the latter likely due to the greatly reduced need, if any, to remove embryo/prenatal-specific tissues during mammalian postembryonic development.

polished rice mediates ecdysone-dependent control of Drosophila embryonic organogenesis

In many animals, progression of developmental stages is temporally controlled by steroid hormones. In Drosophila, the level of ecdysone titer oscillates and developmental stage transitions, such as larval molting and metamorphosis, are induced at each of ecdysone peaks. Ecdysone titer also peaks at the stage of mid-embryogenesis and the embryonic ecdysone is necessary for morphogenesis of several organs, although the regulatory mechanisms of embryonic organogenesis dependent on ecdysone signaling are still open questions. In this study, we find that absence or interruption of embryonic ecdysone signaling caused multiple defects in the tracheal system, including decrease in luminal protein deposition, uneven dilation of the dorsal trunk and loss of terminal branches. We also reveal that an ecdysone-inducible gene polished rice (pri) is essential for tip cell fate decision in dorsal branches. As over-expression of pri can restore the defects caused by disturbance of ecdysone biosynthesis, pri functions as one of the major mediators of embryonic ecdysone signal in tracheogenesis. These results demonstrate that ecdysone and its downstream target pri play essential roles in tracheal development by modulating cell fate decision.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

The development of adult intestinal stem cells: Insights from studies on thyroid hormone-dependent anuran metamorphosis

Vertebrates organ development often takes place in two phases: initial formation and subsequent maturation into the adult form. This is exemplified by the intestine. In mouse, the intestine at birth has villus, where most differentiated epithelial cells are located, but lacks any crypts, where adult intestinal stem cells reside. The crypt is formed during the first 3 weeks after birth when plasma thyroid hormone (T3) levels are high. Similarly, in anurans, the intestine undergoes drastic remodeling into the adult form during metamorphosis in a process completely dependent on T3. Studies on Xenopus metamorphosis have revealed important clues on the formation of the adult intestine during metamorphosis. Here we will review our current understanding on how T3 induces the degeneration of larval epithelium and de novo formation of adult intestinal stem cells. We will also discuss the mechanistic conservations in intestinal development between anurans and mammals.

Thyroid Hormone Receptor Is Essential for Larval Epithelial Apoptosis and Adult Epithelial Stem Cell Development but Not Adult Intestinal Morphogenesis during Xenopus tropicalis Metamorphosis

Vertebrate postembryonic development is regulated by thyroid hormone (T3). Of particular interest is anuran metamorphosis, which offers several unique advantages for studying the role of T3 and its two nuclear receptor genes, TRα and TRβ, during postembryonic development. We have recently generated TR double knockout (TRDKO) Xenopus tropicalis animals and reported that TR is essential for the completion of metamorphosis. Furthermore, TRDKO tadpoles are stalled at the climax of metamorphosis before eventual death. Here we show that TRDKO intestine lacked larval epithelial cell death and adult stem cell formation/proliferation during natural metamorphosis. Interestingly, TRDKO tadpole intestine had premature formation of adult-like epithelial folds and muscle development. In addition, T3 treatment of premetamorphic TRDKO tadpoles failed to induce any metamorphic changes in the intestine. Furthermore, RNA-seq analysis revealed that TRDKO altered the expression of many genes in biological pathways such as Wnt signaling and the cell cycle that likely underlay the inhibition of larval epithelial cell death and adult stem cell development caused by removing both TR genes. Our data suggest that liganded TR is required for larval epithelial cell degeneration and adult stem cell formation, whereas unliganded TR prevents precocious adult tissue morphogenesis such as smooth-muscle development and epithelial folding.

Effects of temperature on growth, development and the leptin signaling pathway of Bufo gargarizans

Climate change is one of the most important causes of the decline in amphibians. Changes in temperature have an important effect on the growth and development and energy metabolism of amphibians. The aim of this study is to unravel the effects of temperature on the leptin signaling pathway of Bufo gargarizans and its molecular mechanisms. Our results showed that high temperature accelerated the development rate of tadpoles, but reduced body size and mass, while low temperature deferred the development of tadpoles, but increased size and mass. Both high temperature and low temperature exposure caused pathological damage of the liver in B. gargarizans. The results of RT-qPCR revealed that the high temperature treatment significantly upregulated the transcript levels of genes related to thyroid hormone (DIO2 (D2), Thyroid Hormone Receptor-α (TRα)) and the leptin signaling pathway (Leptin Receptor (LepR), Janus kinase 1 (JAK1), Janus kinase 2 (JAK2), Tyrosine kinase 2 (TYK2), Signal Transducer And Activator Of Transcription 3 (STAT3), Signal Transducer And Activator Of Transcription 3.1 (STAT3.1), and Signal Transducer And Activator Of Transcription 6 (STAT6)), while there was a decrease of mRNA expression of these genes (TRα, Thyroid Hormone Receptor-Beta (TRβ), LepR, JAK1, and TYK2) in the liver of tadpoles exposed to high temperature compared with the intermediate temperature treatment. Therefore, our results suggested that temperature extremes might interfere with the thyroid and leptin signaling pathways and affect the growth and development of B. gargarizans. Furthermore, tissue injury of the liver could occur due to exposure to temperature extremes. This work promotes public awareness of environmental protection and species conservation needs, also provides valuable experimental data and a theoretical basis for the protection of amphibians.

Effects of sodium perchlorate and exogenous L-thyroxine on growth, development and leptin signaling pathway of Bufo gargarizans tadpoles during metamorphosis

Sodium perchlorate (NaClO₄) and exogenous L-thyroxine (T4), two kinds of endocrine-disrupting chemicals (EDCs), mainly affect the circulating thyroid hormones, which regulate the initiation and rate of metamorphosis in amphibian. The aim of this study is to evaluate the potential role of EDCs in regulating the development of tadpoles and leptin signaling pathway of liver during the metamorphosis of Bufo gargarizans. There was completely opposite result of average development stage of tadpoles and morphological parameters between the NaClO₄ and T4 exposure groups. Histological analysis revealed that NaClO₄ and T4 exposure both caused liver injury, such as the decreased size of hepatocytes, atrophy of nucleus, increased melanomacrophage centres and disappearance of hepatocyte membranes. In addition, the results of RT-qPCR revealed that NaClO₄ treatment significantly inhibited the transcript levels of genes related to thyroid hormone (D2, TRα and TRβ) and leptin signaling pathway (LepR, JAK1, JAK2, and TYK2), while there was an increase of mRNA expression of these genes in the liver of tadpoles administrated with T4 compared with control. This work lays an important foundation for assessing the risk of EDCs in relation to amphibian development during metamorphosis.

Remarkable metabolic reorganization and altered metabolic requirements in frog metamorphic climax

Background

Metamorphic climax is the crucial stage of amphibian metamorphosis responsible for the morphological and functional changes necessary for transition to a terrestrial habitat. This developmental period is sensitive to environmental changes and pollution. Understanding its metabolic basis and requirements is significant for ecological and toxicological research. Rana omeimontis tadpoles are a useful model for investigating this stage as their liver is involved in both metabolic regulation and fat storage.

Results

We used a combined approach of transcriptomics and metabolomics to study the metabolic reorganization during natural and T3-driven metamorphic climax in the liver and tail of Rana omeimontis tadpoles. The metabolic flux from the apoptotic tail replaced hepatic fat storage as metabolic fuel, resulting in increased hepatic amino acid and fat levels. In the liver, amino acid catabolism (transamination and urea cycle) was upregulated along with energy metabolism (TCA cycle and oxidative phosphorylation), while the carbohydrate and lipid catabolism (glycolysis, pentose phosphate pathway (PPP), and β-oxidation) decreased. The hepatic glycogen phosphorylation and gluconeogenesis were upregulated, and the carbohydrate flux was used for synthesis of glycan units (e.g., UDP-glucuronate). In the tail, glycolysis, β-oxidation, and transamination were all downregulated, accompanied by synchronous downregulation of energy production and consumption. Glycogenolysis was maintained in the tail, and the carbohydrate flux likely flowed into both PPP and the synthesis of glycan units (e.g., UDP-glucuronate and UDP-glucosamine). Fatty acid elongation and desaturation, as well as the synthesis of bioactive lipid (e.g., prostaglandins) were encouraged in the tail during metamorphic climax. Protein synthesis was downregulated in both the liver and tail. The significance of these metabolic adjustments and their potential regulation mechanism are discussed.

Conclusion

The energic strategy and anabolic requirements during metamorphic climax were revealed at the molecular level. Amino acid made an increased contribution to energy metabolism during metamorphic climax. Carbohydrate anabolism was essential for the body construction of the froglets. The tail was critical in anabolism including synthesizing bioactive metabolites. These findings increase our understanding of amphibian metamorphosis and provide background information for ecological, evolutionary, conservation, and developmental studies of amphibians.

Stem cell development involves divergent thyroid hormone receptor subtype expression and epigenetic modifications in the Xenopus metamorphosing intestine

In the intestine during metamorphosis of the frog Xenopus laevis, most of the larval epithelial cells are induced to undergo apoptosis by thyroid hormone (TH), and under continued TH action, the remaining epithelial cells dedifferentiate into stem cells (SCs), which then newly generate an adult epithelium analogous to the mammalian intestinal epithelium. Previously, we have shown that the precursors of the SCs that exist in the larval epithelium as differentiated absorptive cells specifically express receptor tyrosine kinase-like orphan receptor 2 (Ror2). By using Ror2 as a marker, we have immunohistochemically shown here that these SC precursors, but not the larval epithelial cells destined to die by apoptosis, express TH receptor α (TRα). Upon initiation of TH-dependent remodeling, TRα expression remains restricted to the SCs as well as proliferating adult epithelial primordia derived from them. As intestinal folds form, TRα expression becomes localized in the trough of the folds where the SCs reside. In contrast, TRβ expression is transiently up-regulated in the entire intestine concomitantly with the increase of endogenous TH levels and is most highly expressed in the developing adult epithelial primordia. Moreover, we have shown here that global histone H4 acetylation is enhanced in the SC precursors and adult primordia including the SCs, while tri-methylation of histone H3 lysine 27 is lacking in those cells during metamorphosis. Our results strongly suggest distinct roles of TRα and TRβ in the intestinal larval-to-adult remodeling, involving distinctive epigenetic modifications in the SC lineage.

Direct activation of tRNA methyltransferase-like 1 (Mettl1) gene by thyroid hormone receptor implicates a role in adult intestinal stem cell development and proliferation during Xenopus tropicalis metamorphosis

Background

Thyroid hormone (T3) plays an important role in vertebrate development. Compared to the postembryonic development of uterus-enclosed mammalian embryos, T3-dependent amphibian metamorphosis is advantageous for studying the function of T3 and T3 receptors (TRs) during vertebrate development. The effects of T3 on the metamorphosis of anurans such as Xenopus tropicalis is known to be mediated by TRs. Many putative TR target genes have been identified previously. Among them is the tRNA methyltransferase Mettl1.

Results

We studied the regulation of Mettl1 gene by T3 during intestinal metamorphosis, a process involves near complete degeneration of the larval epithelial cells via apoptosis and de novo formation of adult epithelial stem cells and their subsequent proliferation and differentiation. We observed that Mettl1 was activated by T3 in the intestine during both natural and T3-induced metamorphosis and that its mRNA level peaks at the climax of intestinal remodeling. We further showed that Mettl1 promoter could be activated by liganded TR via a T3 response element upstream of the transcription start site in vivo. More importantly, we found that TR binding to the TRE region correlated with the increase in the level of H3K79 methylation, a transcription activation histone mark, and the recruitment of RNA polymerase II by T3 during metamorphosis.

Conclusions

Our findings suggest that Mettl1 is activated by liganded TR directly at the transcriptional level via the TRE in the promoter region in the intestine during metamorphosis. Mettl1 in turn regulate target tRNAs to affect translation, thus facilitating stem cell formation and/or proliferation during intestinal remodeling.

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

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

Comprehensive RNA-Seq analysis of notochord-enriched genes induced during Xenopus tropicalis tail resorption

Anuran metamorphosis is perhaps the most dramatic developmental process regulated by thyroid hormone (TH). One of the unique processes that occur during metamorphosis is the complete resorption of the tail, including the notochord. Interestingly, recent gene knockout studies have shown that of the two known vertebrate TH receptors, TRα and TRβ, TRβ appears to be critical for notochord regression during tail resorption in Xenopus tropicalis. To determine the mechanisms underlying notochord regression, we carried out a comprehensive gene expression analysis in the notochord during metamorphosis by using RNA-Seq analyses of whole tail at stage 60 before any noticeable tail length reduction, whole tail at stage 63 when the tail length is reduced by about one half, and the rest of the tail at stage 63 after removing the notochord. This allowed us to identify many notochord-enriched, metamorphosis-induced genes at stage 63. Future studies on these genes should help to determine if they are regulated by TRβ and play any roles in notochord regression.

Organ-Specific Requirements for Thyroid Hormone Receptor Ensure Temporal Coordination of Tissue-Specific Transformations and Completion of Xenopus Metamorphosis

Background: Thyroid hormone (triiodothyronine [T3]) is essential for the development throughout vertebrates. Anuran metamorphosis mimics mammalian postembryonic development, a period around birth when plasma T3 level peaks and many organs/tissues mature into their adult forms. Compared with the uterus-enclosed mammalian embryos, tadpoles can be easily manipulated to study the roles of T3 and T3 receptors (TRs) in tissue remodeling and adult organ development. We and others have previously knocked out TRα or TRβ in the diploid anuran Xenopus tropicalis and reported distinct effects of the two receptor knockouts on metamorphosis. However, animals lacking either TRα or TRβ can complete metamorphosis and develop into reproductive adults. Methods: We have generated TRα and TRβ double knockout animals and carried out molecular and morphological analyses to determine if TR is required for Xenopus development. Results: We found that the TR double knockout tadpoles do not respond to T3, supporting the view that there are no other TR genes in X. tropicalis and that TR is essential for mediating the effects of T3 in vivo. Surprisingly, the double knockout tadpoles are able to initiate metamorphosis and accomplish many metamorphic changes, such as limb development. However, all double knockout tadpoles stall and eventually die at stage 61, the climax of metamorphosis, before tail resorption takes place. Analyses of the knockout tadpoles at stage 61 revealed various developmental abnormalities, including precocious ossification and extra vertebrae. Conclusions: Our data indicate that TRs are not required for the initiation of metamorphosis but is essential for the completion of metamorphosis. Furthermore, the differential effects of TR knockout on different organs/tissues suggest tissue-specific roles for TR to control temporal coordination and progression of metamorphosis in various organs.

A unique role of thyroid hormone receptor β in regulating notochord resorption during Xenopus metamorphosis

Tail resorption during anuran metamorphosis is perhaps the most dramatic tissue transformation that occurs during vertebrate development. Earlier studies in highly related anuran species Xenopus laevis and Xenopus tropicalis have shown that thyroid hormone (T3) receptor (TR) plays a necessary and sufficient role to mediate the causative effect of T3 on metamorphosis. Of the two known TR genes in vertebrates, TRα is highly expressed during both premetamorphosis and metamorphosis while TRβ expression is low in premetamorphic tadpoles but highly upregulated as a direct target gene of T3 during metamorphosis, suggesting potentially different functions during metamorphosis. Indeed, gene knockout studies have shown that knocking out TRα and TRβ has different effects on tadpole development. In particularly, homozygous TRβ knockout tadpoles become tailed frogs well after sibling wild type ones complete metamorphosis. Most noticeably, in TRβ-knockout tadpoles, an apparently normal notochord is present when the notochord in wild-type and TRα-knockout tadpoles disappears. Here, we have investigated how tail notochord resorption is regulated by TR. We show that TRβ is selectively very highly expressed in the notochord compared to TRα. We have also discovered differential regulation of several matrix metalloproteinases (MMPs), which are known to be upregulated by T3 and implicated to play a role in tissue resorption by degrading the extracellular matrix (ECM). In particular, MMP9-TH and MMP13 are extremely highly expressed in the notochord compared to the rest of the tail. In situ hybridization analyses show that these MMPs are expressed in the outer sheath cells and/or the connective tissue sheath surrounding the notochord. Our findings suggest that high levels of TRβ expression in the notochord specifically upregulate these MMPs, which in turn degrades the ECM, leading to the collapse of the notochord and its subsequent resorption during metamorphosis.

Staging and normal table of postembryonic development of the clownfish (Amphiprion ocellaris)

Background

The clownfish Amphiprion ocellaris is one of the rare coral reef fish species that can be reared in aquaria. With relatively short embryonic and larval development, it could be used as a model species to study the impact of global changes such as temperature rise or anthropogenic threats (eg, pollution) on the postembryonic development at molecular and endocrinological levels. Establishing a developmental table allows us to standardize sampling for the scientific community willing to conduct experiments on this species on different areas: ecology, evolution, and developmental biology.

Results

Here, we describe the postembryonic developmental stages for the clownfish A. ocellaris from hatching to juvenile stages (30 days posthatching). We quantitatively followed the postembryonic growth and described qualitative traits: head, paired and unpaired fins, notochord flexion, and pigmentation changes. The occurrence of these changes over time allowed us to define seven stages, for which we provide precise descriptions.

Conclusions

Our work gives an easy system to determine A. ocellaris postembryonic stages allowing, thus, to develop this species as a model species for coral reef fishes. In light of global warming, the access to the full postembryonic development stages of coral reef fish is important to determine stressors that can affect such processes.

Seven developmental stages have been identified to describe the larval development of the clownfish Amphiprion ocellaris.

Clownfish larvae undergo two distinct developmental growth phases that correspond to growth and metamorphosis.

A dichotomous key determination has been created to assist users in identifying the various developmental stages.

Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids

Thyroid hormone (TH) is the most important hormone in frog metamorphosis, a developmental process which will not occur in the absence of TH but can be induced precociously by exogenous TH. However, such treatments including in-vitro TH treatments often do not replicate the events of natural metamorphosis in many organs, including lung, brain, blood, intestine, pancreas, tail, and skin. A potential explanation for the discrepancy between natural and TH-induced metamorphosis is the involvement of glucocorticoids (GCs). GCs are not able to advance development by themselves but can modulate the rate of developmental progress induced by TH via increased tissue sensitivity to TH. Global gene expression analyses and endocrine experiments suggest that GCs may also have direct actions required for completion of metamorphosis independent of their effects on TH signaling. Here, we provide a new review and analysis of the requirement and necessity of TH signaling in light of recent insights from gene knockout frogs. We also examine the independent and interactive roles GCs play in regulating morphological and molecular metamorphic events dependent upon TH.

Involvement of epigenetic modifications in thyroid hormone-dependent formation of adult intestinal stem cells during amphibian metamorphosis

Amphibian metamorphosis has long been used as model to study postembryonic development in vertebrates, a period around birth in mammals when many organs/tissues mature into their adult forms and is characterized by peak levels of plasma thyroid hormone (T3). Of particular interest is the remodeling of the intestine during metamorphosis. In the highly-related anurans Xenopus laevis and Xenopus tropicalis, this remodeling process involves larval epithelial cell death and de novo formation of adult stem cells via dedifferentiation of some larval cells under the induction of T3, making it a valuable system to investigate how adult organ-specific stem cells are formed during vertebrate development. Here, we will review some studies by us and others on how T3 regulates the formation of the intestinal stem cells during metamorphosis. We will highlight the involvement of nucleosome removal and a positive feedback mechanism involving the histone methyltransferases in gene regulation by T3 receptor (TR) during this process.

Gene Expression Program Underlying Tail Resorption During Thyroid Hormone-Dependent Metamorphosis of the Ornamented Pygmy Frog Microhyla fissipes

Thyroid hormone (T3) is essential for vertebrate development, especially during the so-called postembryonic development, a period around birth in mammals when plasma T3 level peaks and many organs mature into their adult form. Compared to embryogenesis, postembryonic development is poorly studied in mammals largely because of the difficulty to manipulate the uterus-enclosed embryos and neonates. Amphibian metamorphosis is independent of maternal influence and can be easily manipulated for molecular and genetic studies, making it a valuable model to study postembryonic development in vertebrates. Studies on amphibian metamorphosis have been largely focused on the two highly related species Xenopus laevis and Xenopus tropicalis. However, adult X. laevis and X. tropicalis animals remain aquatic. This makes important to study metamorphosis in a species in which postmetamorphic frogs live on land. In this regard, the anuran Microhyla fissipes represents an alternative model for developmental and genetic studies. Here we have made use of the advances in sequencing technologies to investigate the gene expression profiles underlying the tail resorption program during metamorphosis in M. fissipes. We first used single molecule real-time sequencing to obtain 67, 939 expressed transcripts in M. fissipes. We next identified 4,555 differentially expressed transcripts during tail resorption by using Illumina sequencing on RNA samples from tails at different metamorphic stages. Bioinformatics analyses revealed that 11 up-regulated KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways and 88 Gene Ontology (GO) terms as well as 21 down-regulated KEGG pathways and 499 GO terms were associated with tail resorption. Our findings suggest that tail resorption in M. fissipes and X. laevis shares many programs. Future investigations on function and regulation of these genes and pathways should help to reveal the mechanisms governing amphibian tail resorption and adaptive evolution from aquatic to terrestrial life. Furthermore, analysis of the M. fissipes model, especially, on the changes in other organs associated with the transition from aquatic to terrestrial living, should help to reveal important mechanistic insights governing mammalian postembryonic developments.

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.

Functional Studies of Transcriptional Cofactors via Microinjection-Mediated Gene Editing in Xenopus

The anuran Xenopus laevis has been studied for decades as a model for vertebrate cell and developmental biology. More recently, the highly related species Xenopus tropicalis has offered the opportunity to carry out genetic studies due to its diploid genome as compared to the pseudo-tetraploid Xenopus laevis. Amphibians undergo a biphasic development: embryogenesis to produce a free-living tadpoles and subsequent metamorphosis to transform the tadpole to a frog. This second phase mimics the so-called postembryonic development in mammals when many organs/tissues mature into their adult form in the presence of high levels of plasma thyroid hormone (T3). The total dependence of amphibian metamorphosis on T3 offers a unique opportunity to study postembryonic development in vertebrates, especially with the recent development gene editing technologies that function in amphibians. Here, we first review the basic molecular understanding of the regulation of Xenopus metamorphosis by T3 and T3 receptors (TRs), and then describe a detailed method to use CRISPR to knock out the TR-coactivator SRC3 (steroid receptor coactivator 3), a histone acetyltransferase, in order to study its involvement in gene regulation by T3 in vivo and Xenopus development.

Thyroid hormones and androgens differentially regulate gene expression in testes and ovaries of sexually mature Silurana tropicalis

A series of ex vivo exposures using testicular and ovarian tissues of sexually mature Western clawed frogs (Silurana tropicalis) were designed to examine molecular mechanisms of thyroid hormone (TH) and androgen crosstalk sans hypophyseal feedback as well as investigate potential sex-specific differences. Tissues were exposed ex vivo to either triiodothyronine (T3), iopanoic acid (IOP), one co-treatment of IOP + 5α-dihydrotestosterone (5α-DHT), 5α-DHT, 5β-dihydrotestosterone (5β-DHT), or testosterone (T). Direct exposure to different androgens led to androgen specific increases in thyroid receptor and deiodinase transcripts in testes (trβ and dio1) but a decrease in expression in ovaries (trβ and dio3), suggesting that male and female frogs can be differently affected by androgenic compounds. Moreover, exposure to select androgens differentially increased estrogen-related transcription (estrogen receptor alpha (erα) and aromatase (cyp19)) and production (estradiol) in ovaries and testes indicating the activation of alternate metabolic pathways yielding estrogenic metabolites. Sex-steroid-related transcription (i.e., steroid 5α-reductase type 2 (srd5α2) and erα) and production (i.e., 5α-DHT) were also differentially regulated by THs. The presence and frequency of transcription factor binding sites in the putative promoter regions of TH- and sex steroid-related genes were also examined in S. tropicalis, rodent, and fish models using in silico analysis. In summary, this study provides an improved mechanistic understanding of TH- and androgen-mediated actions and reveals differential transcriptional effects as a function of sex in frogs.

RXR Ligands Modulate Thyroid Hormone Signaling Competence in Young Xenopus laevis Tadpoles

Appropriate thyroid hormone (TH) signaling through thyroid hormone receptors (TRs) is essential for vertebrate development. Amphibian metamorphosis is initiated and sustained through the action of TH on TRs, which are conserved across vertebrates. TRs heterodimerize with retinoid X receptors (RXRs) on thyroid hormone response elements (TREs) in the genome; however, in most cell line and adult animal studies, RXR ligands do not affect expression of TR target genes. We used a quantitative, precocious metamorphosis assay to interrogate the effects of the RXR agonist bexarotene (Bex) and the RXR antagonist UVI 3003 (UVI) on T3-induced resorption phenotypes in Xenopus laevis tadpoles 1 week postfertilization. Bex potentiated gill and tail resorption, and UVI abrogated T3 action. These results held in transgenic tadpoles bearing a TRE-driven luciferase reporter. Therefore, we used poly-A-primed RNA sequencing transcriptomic analysis to determine their effects on T3-induced gene expression. We also assayed the environmental pollutant tributyltin (TBT), which is an RXR agonist. We found that the proteases that carry out resorption were potentiated by Bex and TBT but were not significantly inhibited by UVI. However, several transcription factors from multiple families (sox4, fosl2, mxd1, mafb, nfib) were all inhibited by UVI and potentiated by Bex and TBT. All required T3 for induction. Time course analysis of gene expression showed that although the agonists could potentiate within 12 hours, the antagonist response lagged. These data indicate that the agonists and antagonist are not necessarily functioning through the same mechanism and suggest that RXR liganding may modulate TH competence in metamorphic signaling.

Identification and differential regulation of microRNAs during thyroid hormone-dependent metamorphosis in Microhyla fissipes

BACKGROUND

Anuran metamorphosis, which is obligatorily initiated and sustained by thyroid hormone (TH), is a dramatic example of extensive morphological, biochemical and cellular changes occurring during post-embryonic development. Thus, it provides an ideal model to understand the actions of the hormone and molecular mechanisms underlying these developmental and apoptotic processes. In addition to transcriptional factors, microRNAs (miRNAs) play key roles in diverse biological processes via post-transcriptional repression of mRNAs. However, the possible role of miRNAs in anuran metamorphosis is not well understood. Screening and identification of TH-responding miRNAs are required to reveal the integrated regulatory mechanisms of TH during metamorphosis. Given the specific role of TRs during M. fissipes metamorphosis and the characteristics of M. fissipes as an ideal model, Illumina sequencing technology was employed to get a full scope of miRNA in M. fissipes metamorphosis treated by T3.

RESULTS

Morphological and histological analysis revealed that 24 h T3 treatment M. fissipes tadpoles resembled that at the climax of natural metamorphosis. Thus, small RNA libraries were constructed from control and 24 h T3 treatment groups. A total of 164 conserved miRNAs and 36 predicted novel miRNAs were characterized. Furthermore, 5' first and ninth nucleotides of miRNAs were significantly enriched in U in our study. In all, 21 miRNAs were differentially expressed between the T3 and control groups (p < 0.01). A total of 10,206 unigenes were identified as target genes of these differentially expressed miRNAs. KEGG pathway analysis indicated that the most overrepresented miRNA target genes were enriched in the "PI3k-Akt signaling pathway". In addition, a network associated with the TH signaling pathway provides an opportunity to further understand the complex biological processes that occur in metamorphosis.

CONCLUSIONS

We identified a large number of miRNAs during M. fissipes metamorphosis, and 21 of them were differentially expressed in the two groups that represented two different metamorphic stages. These miRNAs may play important roles during metamorphosis. The study gives us clues for further studies of the mechanisms of anuran metamorphosis and provides a model to study the mechanism of TH-affected biological processes in humans.

Behavioral and molecular analyses of olfaction-mediated avoidance responses of Rana (Lithobates) catesbeiana tadpoles: Sensitivity to thyroid hormones, estrogen, and treated municipal wastewater effluent

Olfaction is critical for survival, facilitating predator avoidance and food location. The nature of the olfactory system changes during amphibian metamorphosis as the aquatic herbivorous tadpole transitions to a terrestrial, carnivorous frog. Metamorphosis is principally dependent on the action of thyroid hormones (THs), l-thyroxine (T₄) and 3,5,3'-triiodothyronine (T₃), yet little is known about their influence on olfaction during this phase of postembryonic development. We exposed Taylor Kollros stage I-XIII Rana (Lithobates) catesbeiana tadpoles to physiological concentrations of T₄, T₃, or 17-beta-estradiol (E₂) for 48h and evaluated a predator cue avoidance response. The avoidance response in T₃-exposed tadpoles was abolished while T₄- or E₂-exposed tadpoles were unaffected compared to control tadpoles. qPCR analyses on classic TH-response gene transcripts (thra, thrb, and thibz) in the olfactory epithelium demonstrated that, while both THs produced molecular responses, T₃ elicited greater responses than T₄. Municipal wastewater feed stock was spiked with a defined pharmaceutical and personal care product (PPCP) cocktail and treated with an anaerobic membrane bioreactor (AnMBR). Despite substantially reduced PPCP levels, exposure to this effluent abolished avoidance behavior relative to AnMBR effluent whose feed stock was spiked with vehicle. Thibz transcript levels increased upon exposure to either effluent indicating TH mimic activity. The present work is the first to demonstrate differential TH responsiveness of the frog tadpole olfactory system with both behavioral and molecular alterations. A systems-based analysis is warranted to further elucidate the mechanism of action on the olfactory epithelium and identify further molecular bioindicators linked to behavioral response disruption.

Functional analysis of thyroid hormone receptor beta in Xenopus tropicalis founders using CRISPR-Cas

Amphibians provide an ideal model to study the actions of thyroid hormone (TH) in animal development because TH signaling via two TH receptors, TRα and TRβ, is indispensable for amphibian metamorphosis. However, specific roles for the TRβ isoform in metamorphosis are poorly understood. To address this issue, we generated trβ-disrupted Xenopus tropicalis tadpoles using the CRISPR-Cas system. We first established a highly efficient and rapid workflow for gene disruption in the founder generation (F0) by injecting sgRNA and Cas9 ribonucleoprotein. Most embryos showed severe mutant phenotypes carrying high somatic mutation rates. Utilizing this founder analysis system, we examined the role of trβ in metamorphosis. trβ-disrupted pre-metamorphic tadpoles exhibited mixed responsiveness to exogenous TH. Specifically, gill resorption and activation of several TH-response genes, including trβ itself and two protease genes, were impaired. However, hind limb outgrowth and induction of the TH-response genes, klf9 and fra-2, were not affected by loss of trβ Surprisingly, trβ-disrupted tadpoles were able to undergo spontaneous metamorphosis normally, except for a slight delay in tail resorption. These results indicate TRβ is not required but contributes to the timing of resorptive events of metamorphosis.

Role of thyroid hormone in hepatic gene regulation, chromatin remodeling, and autophagy

Thyroid hormone (TH) actions on development and metabolism have been studied ever since the discovery of thyroxine almost a century ago. Initial studies focused on the physiological and biochemical actions of TH. Later, the cloning of the thyroid hormone receptor (THR) isoforms and the development of techniques enabled the study of TH regulation of complex cellular processes (such as gene transcription). Recently we found that TH activates secondary transcription factors such as FOXO1, to amplify gene transcription; and also is a potent inducer of autophagy that was critical for fatty acid β-oxidation in the liver. This review summarizes the recent advancements in our understanding of TH regulation of gene expression of metabolic genes (via co-regulators/transcription factors and epigenetic control) and autophagy in the liver. Our deeper understanding of TH action recently has led to the development of tissue- and THR isoform-specific TH mimetics that may be useful for the treatment of metabolic disorders.

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.

Direct Regulation of Histidine Ammonia-Lyase 2 Gene by Thyroid Hormone in the Developing Adult Intestinal Stem Cells

Most vertebrate organs use adult stem cells to maintain homeostasis and ensure proper repair when damaged. How such organ-specific stem cells are formed during vertebrate development is largely unexplored. We have been using the thyroid hormone (T3)-dependent amphibian metamorphosis to address this issue. Early studies in Xenopus laevis have shown that intestinal remodeling involves complete degeneration of the larval epithelium and de novo formation of adult stem cells through dedifferentiation of some larval epithelial cells. We have further discovered that the histidine ammonia-lyase (HAL; also known as histidase or histidinase)-2 gene is strongly and specifically activated by T3 in the proliferating adult stem cells of the intestine during metamorphosis, implicating a role of histidine catabolism in the development of adult intestinal stem cells. To determine the mechanism by which T3 regulates the HAL2 gene, we have carried out bioinformatics analysis and discovered a putative T3 response element (TRE) in the HAL2 gene. Importantly, we show that this TRE is bound by T3 receptor (TR) in the intestine during metamorphosis. The TRE is capable of binding to the heterodimer of TR and 9-cis retinoic acid receptor (RXR) in vitro and mediate transcriptional activation by liganded TR/RXR in frog oocytes. More importantly, the HAL2 promoter containing the TRE can drive T3-dependent reporter gene expression to mimic endogenous HAL2 expression in transgenic animals. Our results suggest that the TRE mediates the induction of HAL2 gene by T3 in the developing adult intestinal stem cells during metamorphosis.