Growth, Development, and Intestinal Remodeling Occurs in the Absence of Thyroid Hormone Receptor α in Tadpoles of Xenopus tropicalis

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.

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.

Effects of hormones on intestinal stem cells

The maintenance of intestinal renewal and repair mainly depends on intestinal stem cells (ISCs), which can also contribute to the growth of intestinal tumours. Hormones, which are vital signalling agents in the body, have various effects on the growth and replacement of intestinal stem cells. This review summarises recent progress in the identification of hormones associated with intestinal stem cells. Several hormones, including thyroid hormone, glucagon-like peptide-2, androgens, insulin, leptin, growth hormone, corticotropin-releasing hormone and progastrin, promote the development of intestinal stem cells. However, somatostatin and melatonin are two hormones that prevent the proliferation of intestinal stem cells. Therefore, new therapeutic targets for the diagnosis and treatment of intestinal illnesses can be identified by examining the impact of hormones on intestinal stem cells.

Comparative Analysis of Transcriptome Profiles Reveals Distinct and Organ-Dependent Genomic and Nongenomic Actions of Thyroid Hormone in Xenopus tropicalis Tadpoles

Background: Thyroid hormone (triiodothyronine [T3]) is essential for development and organ metabolism in all vertebrates. T3 has both genomic and nongenomic effects on target cells. While much has been learnt on its genomic effects via T3 receptors (TRs) in vertebrate development, mostly through TR-knockout and TR-knockin studies, little is known about the effects of T3 on gene expression in animals in the absence of TR. We have been studying Xenopus metamorphosis as a model for mammalian postembryonic development, a period around birth when plasma T3 level peaks and many organs/tissues mature into their adult forms. We have recently generated TR double knockout (TRDKO) Xenopus tropicalis animals. This offers an opportunity to compare the effects of T3 on global gene expression in tadpole tissues in the presence or absence of TR. Methods: We analyzed the effects of T3 on gene expression in tadpole tail and intestine by using RNA-seq analysis on wild-type and TRDKO tadpoles with or without T3 treatment. Results: We observed that removing TRs reduced the number of genes regulated by T3 in both organs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that T3 affected distinct biological processes and pathways in wild-type and TRDKO tadpoles. Many GO terms and KEGG pathways that were enriched among genes regulated in wild-type tissues are likely involved in mediating the effects of T3 on metamorphosis, for example, those related to development, stem cells, apoptosis, and cell cycle/cell proliferation. However, such GO terms and pathways were not enriched among T3-regulated genes in TRDKO tadpoles. Instead, in TRDKO tadpoles, GO terms and pathways related to "metabolism" and "immune response" were highly enriched among T3-regulated genes. We further observed strong divergence in the TR-independent nongenomic effects of T3 in the intestine and tail. Conclusions: Our data suggest that T3 has distinct and organ-dependent effects on gene expression in developing tadpoles. The TR-mediated effects are consistent with the metamorphic changes, in agreement with the fact that TR is necessary and sufficient to mediate the effects of T3 on metamorphosis. T3 appears to have a major effect on metabolism and immune response via TR-independent nongenomic processes.

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.

Organ-specific effects on target binding due to knockout of thyroid hormone receptor α during Xenopus metamorphosis

Thyroid hormone (T3) is essential for normal development and metabolism, especially during postembryonic development, a period around birth in mammals when plasma T3 levels reach their peak. T3 functions through two T3 receptors, TRα and TRβ. However, little is known about the tissue-specific functions of TRs during postembryonic development because of maternal influence and difficulty in manipulation of mammalian models. We have studied Xenopus tropicalis metamorphosis as a model for human postembryonic development. By using TRα knockout (Xtr·thra^tmshi ) tadpoles, we have previously shown that TRα is important for T3-dependent intestinal remodeling and hindlimb development but not tail resorption during metamorphosis. Here, we have identified genes bound by TR in premetamorphic wild-type and Xtr·thra^tmshi tails with or without T3 treatment by using chromatin immunoprecipitation-sequencing and compared them with those in the intestine and hindlimb. Compared to other organs, the tail has much fewer genes bound by TR or affected by TRα knockout. Bioinformatic analyses revealed that among the genes bound by TR in wild-type but not Xtr·thra^tmshi organs, fewer gene ontology (GO) terms or biological pathways related to metamorphosis were enriched in the tail compared to those in the intestine and hindlimb. This difference likely underlies the drastic effects of TRα knockout on the metamorphosis of the intestine and hindlimb but not the tail. Thus, TRα has tissue-specific roles in regulating T3-dependent anuran metamorphosis by directly targeting the pathways and GO terms important for metamorphosis.

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.

Normal Table of Xenopus development: a new graphical resource

Normal tables of development are essential for studies of embryogenesis, serving as an important resource for model organisms, including the frog Xenopus laevis. Xenopus has long been used to study developmental and cell biology, and is an increasingly important model for human birth defects and disease, genomics, proteomics and toxicology. Scientists utilize Nieuwkoop and Faber's classic 'Normal Table of Xenopus laevis (Daudin)' and accompanying illustrations to enable experimental reproducibility and reuse the illustrations in new publications and teaching. However, it is no longer possible to obtain permission for these copyrighted illustrations. We present 133 new, high-quality illustrations of X. laevis development from fertilization to metamorphosis, with additional views that were not available in the original collection. All the images are available on Xenbase, the Xenopus knowledgebase (http://www.xenbase.org/entry/zahn.do), for download and reuse under an attributable, non-commercial creative commons license. Additionally, we have compiled a 'Landmarks Table' of key morphological features and marker gene expression that can be used to distinguish stages quickly and reliably (https://www.xenbase.org/entry/landmarks-table.do). This new open-access resource will facilitate Xenopus research and teaching in the decades to come.

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.

Thyroid hormone receptor α controls larval intestinal epithelial cell death by regulating the CDK1 pathway

Thyroid hormone (T3) regulates adult intestine development through T3 receptors (TRs). It is difficult to study TR function during postembryonic intestinal maturation in mammals due to maternal influence. We chose intestinal remodeling during Xenopus tropicalis metamorphosis as a model to study TR function in adult organ development. By using ChIP (chromatin immunoprecipitation)-Seq, we identified over 3000 TR-bound genes in the intestine of premetamorphic wild type or TRα (the major TR expressed during premetamorphosis)-knockout tadpoles. Surprisingly, cell cycle-related GO (gene ontology) terms and biological pathways were highly enriched among TR target genes even though the first major event during intestinal metamorphosis is larval epithelial cell death, and TRα knockout drastically reduced this enrichment. More importantly, treatment of tadpoles with cell cycle inhibitors blocked T3-induced intestinal remodeling, especially larval epithelial cell death, suggesting that TRα-dependent activation of cell cycle is important for T3-induced apoptosis during intestinal remodeling.

Tanizaki et al use ChIP-Seq to identify over 3000 Thyroid hormone (T3) receptor (TR)-bound genes in the intestine of premetamorphic wild type Xenopus tropicalis tadpoles and in TRα-knockouts. They show that treatment of tadpoles with cell cycle inhibitors blocked T3-induced intestinal remodeling, suggesting that TRα-dependent activation of the cell cycle is important for T3-induced apoptosis during intestinal remodelling.

Thyroid Hormone Receptor α Controls the Hind Limb Metamorphosis by Regulating Cell Proliferation and Wnt Signaling Pathways in Xenopus tropicalis

Thyroid hormone (T3) receptors (TRs) mediate T3 effects on vertebrate development. We have studied Xenopus tropicalis metamorphosis as a model for postembryonic human development and demonstrated that TRα knockout induces precocious hind limb development. To reveal the molecular pathways regulated by TRα during limb development, we performed chromatin immunoprecipitation- and RNA-sequencing on the hind limb of premetamorphic wild type and TRα knockout tadpoles, and identified over 700 TR-bound genes upregulated by T3 treatment in wild type but not TRα knockout tadpoles. Interestingly, most of these genes were expressed at higher levels in the hind limb of premetamorphic TRα knockout tadpoles than stage-matched wild-type tadpoles, suggesting their derepression upon TRα knockout. Bioinformatic analyses revealed that these genes were highly enriched with cell cycle and Wingless/Integrated (Wnt) signaling-related genes. Furthermore, cell cycle and Wnt signaling pathways were also highly enriched among genes bound by TR in wild type but not TRα knockout hind limb. These findings suggest that direct binding of TRα to target genes related to cell cycle and Wnt pathways is important for limb development: first preventing precocious hind limb formation by repressing these pathways as unliganded TR before metamorphosis and later promoting hind limb development during metamorphosis by mediating T3 activation of these pathways.

Proteomic Analysis of the Intestinal Resistance to Thyroid Hormone Mouse Model With Thyroid Hormone Receptor Alpha Mutations

Thyroid hormone is critical during the development of vertebrates and affects the function of many organs and tissues, especially the intestine. Triiodothyronine (T₃) is the active form and can bind to thyroid hormone nuclear receptors (TRs) to play a vital role in the development of vertebrates. The resistance to thyroid hormone α, as seen in patients, has been mimicked by the Thra^E403X mutation. To investigate the mechanisms underlying the effect of TRα1 on intestinal development, the present study employed proteomic analysis to identify differentially expressed proteins (DEPs) in the distal ileum between homozygous Thra^E403X/E403X and wild-type Thra^+/+ mice. A total of 1,189 DEPs were identified, including 603 upregulated and 586 downregulated proteins. Proteomic analysis revealed that the DEPs were highly enriched in the metabolic process, the developmental process, the transporter of the nutrients, and the intestinal immune system-related pathway. Of these DEPs, 20 proteins were validated by parallel reaction monitoring analysis. Our intestinal proteomic results provide promising candidates for future studies, as they suggest novel mechanisms by which TRα1 may influence intestinal development, such as the transport of intestinal nutrients and the establishment of innate and adaptive immune barriers of the intestine.

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.

Phenotypic Variation Through Ontogeny: Thyroid Axis Disruption During Larval Development in the Frog Pleurodema borellii

Studies of the effects of thyroid hormones on larval development in the frog Xenopus spp. have provided baseline information to identify developmental constraints and elucidate genetic and hormonal mechanisms driving development, growth, and life history transitions. However, this knowledge requires data based on other anurans to complete a comprehensive approach to the understanding of larval developmental diversity and phenotypic variation through ontogeny. Mesocosm experiments provide realistic data about environmental conditions and timing; this information is useful to describe anuran larval development and/or analyze endocrine disruption. In this study, mesocosm experiments of the larval development of the frog Pleurodema borellii were conducted to explore the consequences of thyroid axis disruption; the sensitivity of tadpoles to the methimazole (2.66 mg/l) and thyroxine (T4) (1.66 μg/l) was compared. These concentrations were selected based on previous studies in Pleurodema borellii. We test the effects of methimazole and thyroxine on development in early exposure (from beginning of larval development) and late exposure, 18 days after hatching, with doses administered every 48 h. Tadpoles were evaluated 31 days after hatching. Methimazole caused moderate hypertrophy of the thyroid gland, alteration in the growth rates, differentiation without inhibition of development, and an increase of developmental variability. Thyroxine produced slight atrophy of the thyroid gland, accelerated growth rates and differentiation, and minor developmental variability. In tadpoles at stages previous to metamorphose, skull development (differentiation of olfactory capsules, appearance of dermal bones, and cartilage remodeling) seemed to be unaltered by the disruptors. Moreover, similar abnormal morphogenesis converged in specimens under methimazole and thyroxine exposures. Abnormalities occurred in pelvic and pectoral girdles, and vent tube, and could have been originated at the time of differentiation of musculoskeletal tissues of girdles. Our results indicate that premetamorphic stages (Gosner Stages 25–35) are sensitive to minimal thyroid axis disruption, which produces changes in developmental rates; these stages would also be critical for appendicular musculoskeletal morphogenesis to achieve the optimal condition to start metamorphosis.

Xenopus laevis tadpoles exposed to metamifop: Changes in growth, behavioral endpoints, neurotransmitters, antioxidant system and thyroid development

Pesticides are a major cause of the reduction in the global amphibian population. In this study, the acute toxicity and chronic effects of metamifop on Xenopus laevis (X. laevis) tadpoles were investigated. The 96 h-LC₅₀ value of metamifop on X. laevis tadpoles was 0.634 mg/L, which indicated that metamifop was highly toxic to tadpoles. In the chronic toxicity study, tadpoles were exposed to 0.063 mg/L of metamifop. After 14, 21 and 35 d of exposure, metamifop significantly inhibited the body weight and neurotransmitter synthesis of tadpoles, caused abnormal behavior and interfered with fat metabolism. According to the results of antioxidant enzymes and malondialdehyde (MDA), tadpoles exposed to 0.063 mg/L metamifop suffered severe lipid oxidative damage. Compared with the control group, the thyroid hormone (TH) levels and related gene expression in tadpoles in the treatment group were affected, reflecting the endocrine interference effect of metamifop. The data of this study can enrich our knowledge of the effects of aryloxyphenoxy propionate pesticides on amphibians and highlight the role of metamifop and other pesticides play in global decline of amphibians.

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.

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.

Analysis of Thyroid Hormone Receptor α-Knockout Tadpoles Reveals That the Activation of Cell Cycle Program Is Involved in Thyroid Hormone-Induced Larval Epithelial Cell Death and Adult Intestinal Stem Cell Development During Xenopus tropicalis Metamorphosis

Background: There are two highly conserved thyroid hormone (triiodothyronine [T3]) receptor (TR) genes, TRα and TRβ, in all vertebrates, and the expression of TRα but not TRβ is activated earlier than T3 synthesis during development. In human, high levels of T3 are present during the several months around birth, and T3 deficiency during this period causes severe developmental abnormalities including skeletal and intestinal defects. It is, however, difficult to study this period in mammals as the embryos and neonates depend on maternal supply of nutrients for survival. However, Xenopus tropicalis undergoes a T3-dependent metamorphosis, which drastically changes essentially every organ in a tadpole. Of interest is intestinal remodeling, which involves near complete degeneration of the larval epithelium through apoptosis. Concurrently, adult intestinal stem cells are formed de novo and subsequently give rise to the self-renewing adult epithelial system, resembling intestinal maturation around birth in mammals. We have previously demonstrated that T3 signaling is essential for the formation of adult intestinal stem cells during metamorphosis. Methods: We studied the function of endogenous TRα in the tadpole intestine by using knockout animals and RNA-seq analysis. Results: We observed that removing endogenous TRα caused defects in intestinal remodeling, including drastically reduced larval epithelial cell death and adult intestinal stem cell proliferation. Using RNA-seq on intestinal RNA from premetamorphic wild-type and TRα-knockout tadpoles treated with or without T3 for one day, before any detectable T3-induced cell death and stem cell formation in the tadpole intestine, we identified more than 1500 genes, which were regulated by T3 treatment of the wild-type but not TRα-knockout tadpoles. Gene Ontology and biological pathway analyses revealed that surprisingly, these TRα-regulated genes were highly enriched with cell cycle-related genes, in addition to genes related to stem cells and apoptosis. Conclusions: Our findings suggest that TRα-mediated T3 activation of the cell cycle program is involved in larval epithelial cell death and adult epithelial stem cell development during intestinal remodeling.

Thyroid hormone directly activates mitochondrial fission process 1 (Mtfp1) gene transcription during adult intestinal stem cell development and proliferation in Xenopus tropicalis

Thyroid hormone (T3) regulates vertebrate development via T3 receptors (TRs). T3 level peaks during postembryonic development, a period around birth in mammals or metamorphosis in anurans. Anuran metamorphosis offers many advantages for studying T3 and TR function in vivo largely because of its total dependent on T3 and the dramatic changes affecting essentially all organs/tissues that can be easily manipulated. Earlier studies have shown that TRs are both necessary and sufficient for mediating the metamorphic effects of T3. Many candidate TR target genes have been identified during Xenopus tropicalis intestinal metamorphosis, a process that involves apoptotic degeneration of most of the larval epithelial cells and de novo development of adult epithelial stem cells. Among these putative TR target genes is mitochondrial fission process 1 (Mtfp1), a nuclear-encoded mitochondrial gene. Here, we report that Mtfp1gene expression peaks in the intestine during both natural and T3-induced metamorphosis when adult epithelial stem cell development and proliferation take place. Furthermore, we show that Mtfp1 contains a T3-response element within the first intron that is bound by TR to mediate T3-induced local histone H3K79 methylation and RNA polymerase recruitment in the intestine during metamorphosis. Additionally, we demonstrate that the Mtfp1 promoter can be activated by T3 in a reconstituted frog oocyte system in vivo and that this activation is dependent on the intronic TRE. These findings suggest that T3 activates Mtfp1 gene directly via the intronic TRE and that Mtfp1 in turn facilitate adult intestinal stem cell development/proliferation by affecting mitochondrial fission process.

Harmful Algal Bloom Toxicity in Lithobates catesbeiana Tadpoles

Harmful algal blooms (HAB) have become a major health concern worldwide, not just to humans that consume and recreate on contaminated waters, but also to the fauna that inhabit the environments surrounding affected areas. HABs contain heterotrophic bacteria, cyanobacterial lipopolysaccharide, and cyanobacterial toxins such as microcystins, that can cause severe toxicity in many aquatic species as well as bioaccumulation within various organs. Thus, the possibility of trophic transference of this toxin through the food chain has potentially important health implications for other organisms in the related food web. While some species have developed adaptions to attenuate the toxic effects of HAB toxins, there are still numerous species that remain vulnerable, including Lithobates catesbeiana (American bullfrog) tadpoles. In the current study we demonstrate that acute, short-term exposure of tadpoles to HAB toxins containing 1 µg/L (1 nmol/L) of total microcystins for only 7 days results in significant liver and intestinal toxicity within tadpoles. Exposed tadpoles had increased intestinal diameter, decreased intestinal fold heights, and a constant number of intestinal folds, indicating pathological intestinal distension, similar to what is seen in various disease processes, such as toxic megacolon. HAB-toxin-exposed tadpoles also demonstrated hepatocyte hypertrophy with increased hepatocyte binucleation consistent with carcinogenic and oxidative processes within the liver. Both livers and intestines of HAB-toxin-exposed tadpoles demonstrated significant increases in protein carbonylation consistent with oxidative stress and damage. These findings demonstrate that short-term exposure to HAB toxins, including microcystins, can have significant adverse effects in amphibian populations. This acute, short-term toxicity highlights the need to evaluate the influence HAB toxins may have on other vulnerable species within the food web and how those may ultimately also impact human health.

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.

Thyroid hormone receptor beta is critical for intestinal remodeling during Xenopus tropicalis metamorphosis

Background

Thyroid hormone (T3) is critical for development in all vertebrates. The mechanism underlying T3 effect has been difficult to study due to the uterus-enclosed nature of mammalian embryos. Anuran metamorphosis, which is dependent on T3 but independent of maternal influence, is an excellent model to study the roles of T3 and its receptors (TRs) during vertebrate development. We and others have reported various effects of TR knockout (TRα and TRβ) during Xenopus tropicalis development. However, these studies were largely focused on external morphology.

Results

We have generated TRβ knockout animals containing an out-frame-mutation of 5 base deletion by using the CRISPR/Cas9 system and observed that TRβ knockout does not affect premetamorphic tadpole development. We have found that the basal expression of direct T3-inducible genes is increased but their upregulation by T3 is reduced in the intestine of premetamorphic homozygous TRβ knockout animals, accompanied by reduced target binding by TR. More importantly, we have observed reduced adult stem cell proliferation and larval epithelial apoptosis in the intestine during T3-induced metamorphosis.

Conclusions

Our data suggest that TRβ plays a critical role in intestinal remodeling during metamorphosis.

Post-metamorphic carry-over effects of altered thyroid hormone level and developmental temperature: physiological plasticity and body condition at two life stages in Rana temporaria

Environmental stress induced by natural and anthropogenic processes including climate change may threaten the productivity of species and persistence of populations. Ectotherms can potentially cope with stressful conditions such as extremes in temperature by exhibiting physiological plasticity. Amphibian larvae experiencing stressful environments display altered thyroid hormone (TH) status with potential implications for physiological traits and acclimation capacity. We investigated how developmental temperature (T_dev) and altered TH levels (simulating proximate effects of environmental stress) influence the standard metabolic rate (SMR), body condition (BC), and thermal tolerance in metamorphic and post-metamorphic anuran larvae of the common frog (Rana temporaria) reared at five constant temperatures (14-28 °C). At metamorphosis, larvae that developed at higher temperatures had higher maximum thermal limits but narrower ranges in thermal tolerance. Mean CT_max was 37.63 °C ± 0.14 (low TH), 36.49 °C ± 0.31 (control), and 36.43 °C ± 0.68 (high TH) in larvae acclimated to different temperatures. Larvae were able to acclimate to higher T_dev by adjusting their thermal tolerance, but not their SMR, and this effect was not impaired by altered TH levels. BC was reduced by 80% (metamorphic) and by 85% (post-metamorphic) at highest T_dev. The effect of stressful larval conditions (i.e., different developmental temperatures and, to some extent, altered TH levels) on SMR and particularly on BC at the onset of metamorphosis was carried over to froglets at the end of metamorphic climax. This has far reaching consequences, since body condition at metamorphosis is known to determine metamorphic success and, thus, is indirectly linked to individual fitness in later life stages.

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.

Thyroid hormone activates Xenopus MBD3 gene via an intronic TRE in vivo

Thyroid hormone (T3) is important for adult organ function and vertebrate development. Amphibian metamorphosis is totally dependent on T3 and can be easily manipulated, thus offering a unique opportunity for studying how T3 controls vertebrate development. T3 controls frog metamorphosis through T3 receptor (TR)-mediated regulation of T3 response genes. To identify direct T3 response genes, we previously carried out a ChIP (chromatin immunoprecipitation)-on-chip analysis with a polyclonal anti-TR antibody on the tadpole intestine and identified many putative TR target genes. Among them is the methyl-CpG binding domain protein 3 (MBD3) gene, which has been implicated to play a role in epigenetic regulation of cellular processes as a subunit of the Mi-2/NuRD (Nucleosome Remodeling Deacetylase) complex. We show here that MBD3 is upregulated in the intestine and tail by T3 and its expression peaks at stage 62, the climax of metamorphosis. We further show that a putative TRE within the first intron of the MBD3 gene binds to TR/RXR in vitro and in vivo, and mediates T3 regulation of the MBD3 promoter in vivo.

Differences in responsiveness and sensitivity to exogenous disruptors of the thyroid gland in three anuran species

Anuran larval development comprises tissues/organs/systems that are: exclusively of larvae, able to be remodelled, and those of postmetamorphic stages. Also, the anuran larval development is characterized by inter-related parameters: time, size and shape forming part of growth and differentiation. The anuran metamorphosis starts when growth and differentiation achieve a threshold that differs among species since it is regulated by a number of external (environmental) and internal (hormonal) processes. Here we explore the consequences of exogenous disruptors on the thyroid gland (e.g., methimazole and thyroxine as T4) of three species by immersing premetamorphic tadpoles in predetermined concentrations of the disruptors for short periods (10 or 16 days). The species were Pleurodema borellii, Leptodactylus chaquensis, and Dermatonotus muelleri, which all breed in small temporary ponds during the summer, but differ in their ecomorphology. The experiments were conducted to evaluate the effects of these substances on larval development (based in Gosner larval stages), morphometric variation in body parameters (snout-vent and total length by larval stages), and thyroid gland histopathology at the end of the assays. In P. borelli and L. chaquensis, methimazole produces significant increment of size measurements (nonparametric Kruskal-Wallis, p < .05) during stages of digit differentiation and induced thyroid gland hypertrophy. In the three species, T4 exposure accelerated limb development and caused atrophy of thyroid gland. Prolonged T4 exposure in L. chaquensis and D. muelleri triggered metamorphic transformation in the gut and skull cartilages. Discussion about interspecific differences in responsiveness and sensitivity elucidates the importance of hormonal signals to morphological evolution.

Effects of nonylphenols on embryonic development and metamorphosis of Xenopus laevis: FETAX and amphibian metamorphosis toxicity test (OECD TG231)

Nonylphenols (NPs) are a group of endocrine-disrupting surfactants that mimic estrogen. To determine the developmental toxicity and thyroid-disrupting effect of NPs, the effects of exposure to nonylphenol (NP), 4-nonylphenol (4-NP), and nonylphenol ethoxylate (NP-12) were examined according to the frog embryo teratogenesis assay-Xenopus (FETAX) and Organization for Economic Co-operation and Development test guidelines 231 (TG231). In FETAX, the LC₅₀ values of NP, 4-NP, and NP-12 were 59.14 mg/L, 10.13 mg/L, and 14.60 mg/L, respectively. At 10.0 mg/L, NP, 4-NP, and NP-12 significantly decreased the total length of tadpoles, and NP and 4-NP increased gut malformation and bent tails. In surviving tadpoles, the EC₅₀ values for malformation of NP, 4-NP, and NP-12 were 4.66, 6.51, and 13.08 mg/L, respectively. The teratogenic indices of NP, 4-NP, and NP-12 were 12.69, 1.56, and 1.08, respectively, suggesting the teratogenic potential of NP and 4-NP. In a range-finder assay for TG231, the 96-h LC₅₀ values of NP, 4-NP, and NP-12 were 2.0, 2.0, and 10.57 mg/L, respectively. When NF stage 51 larvae were exposed for 21 days, larval growth was inhibited by NP, 4-NP, and NP-12 at 0.67, 0.07, and 0.37 mg/L, respectively. 4-NP at 0.07 mg/L accelerated the developmental stage and significantly increased hind limb length, while 0.67 mg/L 4-NP delayed the developmental stage and decreased hind limb length, suggesting a bimodal effect of 4-NP on metamorphosis. NP and NP-12 at test concentrations did not alter the larval stage, but NP-12 at 0.37 mg/L significantly decreased total length and tail length, suggesting growth inhibition in larvae. The total colloid area of thyroid follicles was significantly increased by 0.07 mg/L 4-NP but not by NP and NP-12, suggesting that 4-NP may interfere with thyroid function. Together, the developmental toxicity of NPs was in the following order: 4-NP, NP-12, and NP. 4-NP may alter metamorphosis driven by thyroid hormones in X. laevis.

Thyroid Hormone Receptor Alpha Is Required for Thyroid Hormone-Dependent Neural Cell Proliferation During Tadpole Metamorphosis

Thyroid hormone (T3) plays several key roles in development of the nervous system in vertebrates, controlling diverse processes such as neurogenesis, cell migration, apoptosis, differentiation, and maturation. In anuran amphibians, the hormone exerts its actions on the tadpole brain during metamorphosis, a developmental period dependent on T3. Thyroid hormone regulates gene transcription by binding to two nuclear receptors, TRα and TRβ. Our previous findings using pharmacological and other approaches supported that TRα plays a pivotal role in mediating T3 actions on neural cell proliferation in Xenopus tadpole brain. Here we used Xenopus tropicalis (X. tropicalis) tadpoles with an inactivating mutation in the gene that encodes TRα to investigate roles for TRα in mitosis and gene regulation in tadpole brain. Gross morphological analysis showed that mutant tadpoles had proportionally smaller brains, corrected for body size, compared with wildtype, both during prometamorphosis and at the completion of metamorphosis. This was reflected in a large reduction in phosphorylated histone 3 (pH3; a mitosis marker) immunoreactive (ir) nuclei in prometamorphic tadpole brain, when T3-dependent cell proliferation is maximal. Treatment of wild type premetamorphic tadpoles with T3 for 48 h induced gross morphological changes in the brain, and strongly increased pH3-ir, but had no effect in mutant tadpoles. Thyroid hormone induction of the direct TR target genes thrb, klf9, and thibz was dysregulated in mutant tadpoles. Analysis of gene expression by RNA sequencing in the brain of premetamorphic tadpoles treated with or without T3 for 16 h showed that the TRα accounts for 95% of the gene regulation responses to T3.

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.

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.

How thyroid hormones and their inhibitors affect cartilage growth and shape in the frog Xenopus laevis

Understanding how skeleton changes shape in ontogeny is fundamental to understanding how its shape diversifies in phylogeny. Amphibians pose a special case because their jaw and throat skeleton consists of cartilages that are dramatically reshaped midway through life to support new feeding and breathing styles. Although amphibian metamorphosis is commonly studied by immersing larvae in thyroid hormones (TH), how individual cartilages respond to TH is poorly understood. This study documents the effects of larval stage and TH type (T4 vs. T3), dose and deprivation on the size, shape and morphogenesis of the lower jaw and ceratohyal cartilages in the frog Xenopus laevis. It uses thyroid inhibitors to isolate the effects of each hormone at specific concentrations. It also deconstructs the TH responses into the effects on individual dimensions, and uses measures of percent change to eliminate the effects of body size and growth rate variation. As stage increases, T4 and T3 responses become increasingly similar to each other and to natural remodeling; the differences at low and intermediate stages result largely from abnormal responses to T3. Most notably, the beak-like lower jaw commonly observed at the lowest stage in other studies results largely from arrested growth of cartilage. TH responses are superimposed upon the growth typical for each stage so that cartilages can attain postmetamorphic shapes through dimensional changes that exceed those of natural metamorphosis. Using thyroid inhibitors alters the outcome of TH-induced remodeling, and T4 has almost the same capacity to induce metamorphic shape changes as T3. The results have implications for understanding how the starting shapes of larval elements affect morphogenesis, how chondrocytes behave to change cartilage shape, and how intracellular processing of TH might contribute to interspecific differences in shape change. Also, the data on animal mortality and which stages and doses most closely replicate natural remodeling have practical value for researchers who treat Xenopus tadpoles with TH.

Tail Resorption During Metamorphosis in Xenopus Tadpoles

Tail resorption in anuran tadpoles is one of the most physically and physiologically notable phenomena in developmental biology. A tail that is over twice as long as the tadpole trunk is absorbed within several days, while concurrently the tadpole's locomotive function is continuously managed during the transition of the driving force from the tail to hindlimbs. Elaborate regulation is necessary to accomplish this locomotive switch. Tadpole's hindlimbs must develop from the limb-bud size to the mature size and the nervous system must be arranged to control movement before the tail is degenerated. The order of the development and growth of hindlimbs and the regression of the tail are regulated by the increasing levels of thyroid hormones (THs), the intracellular metabolism of THs, the expression levels of TH receptors, the expression of several effector genes, and other factors that can modulate TH signaling. The tail degeneration that is induced by the TH surge occurs through two mechanisms, direct TH-responsive cell death (suicide) and cell death caused by the degradation of the extracellular matrix and a loss of cellular anchorage (murder). These pathways lead to the collapse of the notochord, the contraction of surviving slow muscles, and, ultimately, the loss of the tail. In this review, I focus on the differential TH sensitivity of the tail and hindlimbs and the mechanism of tail resorption during Xenopus metamorphosis.

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.

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 hormone levels and temperature during development alter thermal tolerance and energetics of Xenopus laevis larvae

Environmental variation induced by natural and anthropogenic processes including climate change may threaten species by causing environmental stress. Anuran larvae experiencing environmental stress may display altered thyroid hormone (TH) status with potential implications for physiological traits. Therefore, any capacity to adapt to environmental changes through plastic responses provides a key to determining species vulnerability to environmental variation. We investigated whether developmental temperature (T dev), altered TH levels and whether the interactive effect of both affect standard metabolic rate (SMR), body condition (BC), survival and thermal tolerance in larvae of the African clawed frog (Xenopus laevis) reared at five temperatures with experimentally altered TH levels. At metamorphosis, SMR, BC and survival were significantly affected by T dev, TH status and their interaction with the latter often intensified impacts. Larvae developing at warmer temperatures exhibited significantly higher SMRs and BC was reduced at warm T dev and high TH levels suggesting decreased ability to acclimate to variation in temperature. Accordingly, tadpoles that developed at warm temperatures had higher maximum thermal limits but more narrow thermal tolerance windows. High and low TH levels decreased and increased upper thermal limits, respectively. Thus, when experiencing both warmer temperatures and environmental stress, larvae may be less able to compensate for changes in T dev. Our results demonstrate that physiological traits in larvae of X. laevis are strongly affected by increased TH levels and warmer temperatures. Altered TH levels and increasing T dev due to global change may result in a reduced capacity for physiological plasticity. This has far reaching consequences since the energetic requirement at the onset of metamorphosis is known to determine metamorphic success and thus, is indirectly linked to individual fitness in later life stages.

The balance of two opposing factors Mad and Myc regulates cell fate during tissue remodeling

Cell proliferation and differentiation are two distinct yet coupled processes in development in diverse organisms. Understanding the molecular mechanisms that regulate this process is a central theme in developmental biology. The intestinal epithelium is a highly complex tissue that relies on the coordination of cell proliferation within the crypts and apoptosis mainly at the tip of the villi, preservation of epithelial function through differentiation, and homeostatic cell migration along the crypt-villus axis. Small populations of adult stem cells are responsible for the self-renewal of the epithelium throughout life. Surprisingly, much less is known about the mechanisms governing the remodeling of the intestine from the embryonic to adult form. Furthermore, it remains unknown how thyroid hormone (T3) affects stem cell development during this postembryonic process, which is around birth in mammals when T3 level increase rapidly in the plasma. Tissue remodeling during amphibian metamorphosis is very similar to the maturation of the mammalian organs around birth in mammals and is regulated by T3. In particular, many unique features of Xenopus intestinal remodeling during metamorphosis has enabled us and others to elucidate how adult stem cells are formed during postembryonic development in vertebrates. In this review, we will focus on recent findings on the role of Mad1/c-Myc in cell death and proliferation during intestinal metamorphosis and discuss how a Mad1-c-Myc balance controls intestinal epithelial cell fate during this T3-dependent process.

Dual function model revised by thyroid hormone receptor alpha knockout frogs

All vertebrates require thyroid hormone (TH) for normal growth and development. Plasma TH enters cells and alters gene expression via nuclear receptors TRα and TRβ. In-vitro studies showed that TRs function as repressors of TH-inducible genes in the absence of TH and as activators of those same genes in the presence of TH. A dual function model was proposed to harmonize these molecular TR actions with the dynamic expression of TRs and peak in production of TH experienced during development. Conclusive tests of the repression activity of TRs early in development as predicted by the model awaited gene knockout technology targeting TRα. At the molecular level, active repression of genes involved in metamorphosis by TRα in the absence of TH was confirmed in whole bodies and intestine from TRα knockout studies. As a consequence of this reduced repression in TRα knockout animals, initiation of limb morphogenesis occurs precociously. However, subsequent limb development is retarded during rising plasma TH levels due to reduced TR-dependent responsivity to TH. In contrast to the limbs, intestine remodeling is delayed by one to two developmental stages in TRα knockout animals, despite de-repressed levels of TH-induced genes during premetamorphosis. Surprisingly, in the absence of TRα, hind limbs do not require gene induction by TH signaling to complete morphological growth and development, which is contrary to prediction by the dual function model. Full evaluation of the dual function model for all organs awaits the production of TRα and TRβ double knockout frogs.

Thyroid Hormone Receptor α- and β-Knockout Xenopus tropicalis Tadpoles Reveal Subtype-Specific Roles During Development

Thyroid hormone (TH) binds TH receptor α (TRα) and β (TRβ) to induce amphibian metamorphosis. Whereas TH signaling has been well studied, functional differences between TRα and TRβ during this process have not been characterized. To understand how each TR contributes to metamorphosis, we generated TRα- and TRβ-knockout tadpoles of Xenopus tropicalis and examined developmental abnormalities, histology of the tail and intestine, and messenger RNA expression of genes encoding extracellular matrix-degrading enzymes. In TRβ-knockout tadpoles, tail regression was delayed significantly and a healthy notochord was observed even 5 days after the initiation of tail shortening (stage 62), whereas in the tails of wild-type and TRα-knockout tadpoles, the notochord disappeared after ∼1 day. The messenger RNA expression levels of genes encoding extracellular matrix-degrading enzymes (MMP2, MMP9TH, MMP13, MMP14, and FAPα) were obviously reduced in the tail tip of TRβ-knockout tadpoles, with the shortening tail. The reduction in olfactory nerve length and head narrowing by gill absorption were also affected. Hind limb growth and intestinal shortening were not compromised in TRβ-knockout tadpoles, whereas tail regression and olfactory nerve shortening appeared to proceed normally in TRα-knockout tadpoles, except for the precocious development of hind limbs. Our results demonstrated the distinct roles of TRα and TRβ in hind limb growth and tail regression, respectively.

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 Receptor in Amphibian Development

The amphibian Xenopus laevis has long been used as a model for studying vertebrate cell and developmental biology largely due to the easiness to manipulate this system in vivo and in vitro. While most of the developmental studies have been on Xenopus embryogenesis, considerable efforts have been made to understand its metamorphosis, a process mimicking postembryonic development in mammals when many organs mature into their adult forms in the presence of high levels of thyroid hormone (T3). Amphibian metamorphosis is totally dependent on T3 and offers a number of advantages for experimental analyses compared to the late stage, uterus-enclosed mammalian embryos. Earlier studies on metamorphosis in Xenopus laevis have revealed dual functions of T3 receptors (TR) during premetamorphic development and metamorphosis as well as important roles of TR-interacting corepressors and coactivators during these two periods, respectively. The development of gene-editing technologies that functions in amphibians in recent years has made possible for the first time to study function of endogenous TRs, especially in the highly related diploid anuran species Xenopus tropicalis. Here, we first review the current mechanistic understanding of the regulation of metamorphosis by T3 and TR, and then describe a detailed method to use TALEN to knock out TRα for studying its role in gene regulation by T3 in vivo and Xenopus development.

Dioxin Exposure Alters Molecular and Morphological Responses to Thyroid Hormone in Xenopus laevis Cultured Cells and Prometamorphic Tadpoles

Amphibian metamorphosis is driven by thyroid hormone (TH). We used prometamorphic tadpoles and a cell line of the African clawed frog (Xenopus laevis) to examine immediate effects of dioxin exposure on TH. Gene expression patterns suggest cross-talk between the thyroid hormone receptor (TR) and aryl hydrocarbon receptor (AHR) signaling pathways. In XLK-WG cells, expression of Cytochrome P450 1A6 (cyp1A6), an AHR target, was induced 1000-fold by 100 nM TCDD (2, 3, 7, 8 tetrachlorodibenzo-p-dioxin). Krüppel-Like Factor 9 (klf9), the first gene induced in a cascade of TH responses tied to metamorphosis, was upregulated over 5-fold by 50 nM triiodothyronine (T3) and 2-fold by dioxin. Co-exposure to T3 and TCDD boosted both responses, further inducing cyp1A6 by 75% and klf9 about 60%. Additional canonical targets of each receptor, including trβa and trβb (TR) and udpgt1a (AHR) responded similarly. Induction of TH targets by TCDD in XLK-WG cells predicts that exposure could speed metamorphosis. We tested this hypothesis in two remodeling events: tail resorption and hind limb growth. Resorption of ex vivo cultured tails was accelerated by 10 nM T3, while a modest increase in resorption by 100 nM TCDD lacked statistical significance. Hind limbs doubled in length over four days following 1 nM T3 treatment, but limb length was unaffected by 100 nM TCDD. TCDD co-exposure reduced the T3 effect by nearly 40%, despite TCDD induction of klf9 in whole tadpoles, alone or with T3. These results suggest that tissue-specific TCDD effects limit or reverse the increased metamorphosis rate predicted by klf9 induction.

Xenopus metamorphosis as a model to study thyroid hormone receptor function during vertebrate developmental transitions

A hormone-dependent developmental transition from aquatic to terrestrial existence occurs in all tetrapod vertebrates, such as birth, hatching, and metamorphosis. Thyroid hormones (TH) and their receptors (TRs) are key players in the tissue transformations comprising vertebrate developmental transitions. The African clawed frog, Xenopus, is a premier model for the role of TRs in developmental transitions because of the numerous and dramatic TH-dependent tissue transformations during metamorphosis and because of the endocrine, molecular, and genomic resources available. TRs are nuclear receptors that repress TH-response genes when plasma TH is minimal and that activate those same genes to induce tissue-specific gene regulation cascades when TH plasma levels increase. Tissue-specific TR expression levels help determine tissue sensitivity and responsivity to TH thereby regulating the initiation and rate of developmental change in TH-sensitive tissues which govern the tissue developmental asynchrony observed during metamorphosis. This review highlighting Xenopus presents the key experimental findings underpinning the roles TRs play in control of vertebrate developmental transitions.