The earliest changes in gene expression in tadpole intestine induced by thyroid hormone

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.

Defining components of early thyroid hormone signalling through temperature-mediated activation of molecular memory in cultured Rana [lithobates] catesbeiana tadpole back skin

Thyroid hormones (THs) are essential signalling molecules for the postembryonic development of all vertebrates. THs are necessary for the metamorphosis from tadpole to froglet and exogenous TH administration precociously induces metamorphosis. In American bullfrog (Rana [Lithobates] catesbeiana) tadpoles, the TH-induced metamorphosis observed at a warm temperature (24 °C) is arrested at a cold temperature (4 °C) even in the presence of exogenous THs. However, when TH-exposed tadpoles are shifted from cold to warm temperatures (4 → 24 °C), they undergo TH-dependent metamorphosis at an accelerated rate even when the initial TH signal is no longer present. Thus, they possess a "molecular memory" of TH exposure that establishes the TH-induced response program at the cold temperature and prompts accelerated metamorphosis after a shift to a warmer temperature. The components of the molecular memory that allow the uncoupling of initiation from the execution of the metamorphic program are not understood. To investigate this, we used cultured tadpole back skin (C-Skin) in a repeated measures experiment under 24 °C only, 4 °C only, and 4 → 24 °C temperature shifted regimes and reverse transcription quantitative polymerase chain reaction (RT-qPCR) and RNA-sequencing (RNA-seq) analyses. RNA-seq identified 570, 44, and 890 transcripts, respectively, that were significantly changed by TH treatment. These included transcripts encoding transcription factors and proteins involved in mRNA structure and stability. Notably, transcripts associated with molecular memory do not overlap with those identified previously in cultured tail fin (C-fin) except for TH-induced basic leucine zipper-containing protein (thibz) suggesting that thibz may have a central role in molecular memory that works with tissue-specific factors to establish TH-induced gene expression programs.

Metamorphic gene regulation programs in Xenopus tropicalis tadpole brain

Amphibian metamorphosis is controlled by thyroid hormone (TH), which binds TH receptors (TRs) to regulate gene expression programs that underlie morphogenesis. Gene expression screens using tissues from premetamorphic tadpoles treated with TH identified some TH target genes, but few studies have analyzed genome-wide changes in gene regulation during spontaneous metamorphosis. We analyzed RNA sequencing data at four developmental stages from the beginning to the end of spontaneous metamorphosis, conducted on the neuroendocrine centers of Xenopus tropicalis tadpole brain. We also conducted chromatin immunoprecipitation sequencing (ChIP-seq) for TRs, and we compared gene expression changes during metamorphosis with those induced by exogenous TH. The mRNA levels of 26% of protein coding genes changed during metamorphosis; about half were upregulated and half downregulated. Twenty four percent of genes whose mRNA levels changed during metamorphosis had TR ChIP-seq peaks. Genes involved with neural cell differentiation, cell physiology, synaptogenesis and cell-cell signaling were upregulated, while genes involved with cell cycle, protein synthesis, and neural stem/progenitor cell homeostasis were downregulated. There is a shift from building neural structures early in the metamorphic process, to the differentiation and maturation of neural cells and neural signaling pathways characteristic of the adult frog brain. Only half of the genes modulated by treatment of premetamorphic tadpoles with TH for 16 h changed expression during metamorphosis; these represented 33% of the genes whose mRNA levels changed during metamorphosis. Taken together, our results provide a foundation for understanding the molecular basis for metamorphosis of tadpole brain, and they highlight potential caveats for interpreting gene regulation changes in premetamorphic tadpoles induced by exogenous TH.

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.

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.

Thyroid hormone signaling in the intestinal stem cells and their niche

Several studies emphasized the function of the thyroid hormones in stem cell biology. These hormones act through the nuclear hormone receptor TRs, which are T3-modulated transcription factors. Pioneer work on T3-dependent amphibian metamorphosis showed that the crosstalk between the epithelium and the underlying mesenchyme is absolutely required for intestinal maturation and stem cell emergence. With the recent advances of powerful animal models and 3D-organoid cultures, similar findings have now begun to be described in mammals, where the action of T3 and TRα1 control physiological and cancer-related stem cell biology. In this review, we have summarized recent findings on the multiple functions of T3 and TRα1 in intestinal epithelium stem cells, cancer stem cells and their niche. In particular, we have highlighted the regulation of metabolic functions directly linked to normal and/or cancer stem cell biology. These findings help explain other possible mechanisms by which TRα1 controls stem cell biology, beyond the more classical Wnt and Notch signaling pathways.

Essential roles of YAP-TEAD complex in adult stem cell development during thyroid hormone-induced intestinal remodeling of Xenopus laevis

During amphibian metamorphosis which is triggered by thyroid hormone (TH), the small intestine is extensively remodeled from the larval to adult form. In the Xenopus laevis intestine, some of the larval epithelial cells dedifferentiate into adult stem cells, which newly form the adult epithelium similar to the mammalian one. We have previously shown that TH-activated Shh, Wnt and Notch signaling pathways play important roles in adult epithelial development. Here we focus on the Hippo signaling pathway, which is known to interact with these pathways in the mammalian intestine. Our quantitative RT-PCR analysis indicates that the expression of genes involved in this pathway including YAP1, TAZ, TEAD1 and core kinases is differently regulated by TH in the metamorphosing intestine. Additionally, we show by in situ hybridization and immunohistochemistry that the transcriptional co-activator YAP1, a major effector of the Hippo signaling, is expressed in the adult stem cells and connective tissue cells surrounding them and that YAP1 protein is localized in either nucleus or cytoplasm of the stem cells. We further show that YAP1 binds its binding partner TEAD1 (transcription factor) in vivo and that their interaction is inhibited by verteporfin (VP). More importantly, by using VP in organ culture of the tadpole intestine, we experimentally demonstrate that the inhibition of YAP1-TEAD1 interaction decreases both TH-induced stem cells expressing LGR5 and nearby connective tissue cells in number and proliferation, leading to the failure of adult epithelial development. Our results indicate that YAP-TEAD complex is required for stem cell development during intestinal remodeling.

Thyroid Hormones Regulate Goblet Cell Differentiation and Fgf19-Fgfr4 Signaling

Hypothyroidism is a common pathological condition characterized by insufficient activity of the thyroid hormones (THs), thyroxine (T4), and 3,5,3'-triiodothyronine (T3), in the whole body or in specific tissues. Hypothyroidism is associated with inadequate development of the intestine as well as gastrointestinal diseases. We used a zebrafish model of hypothyroidism to identify and characterize TH-modulated genes and cellular pathways controlling intestine development. In the intestine of hypothyroid juveniles and adults, the number of mucus-secreting goblet cells was reduced, and this phenotype could be rescued by T3 treatment. Transcriptome profiling revealed dozens of differentially expressed genes in the intestine of hypothyroid adults compared to controls. Notably, the expression of genes encoding to Fgf19 and its receptor Fgfr4 was markedly increased in the intestine of hypothyroid adults, and treatment with T3 normalized it. Blocking fibroblast growth factor (FGF) signaling, using an inducible dominant-negative Fgfr transgenic line, rescued the number of goblet cells in hypothyroid adults. These results show that THs inhibit the Fgf19-Fgfr4 signaling pathway, which is associated with inhibition of goblet cell differentiation in hypothyroidism. Both the TH and Fgf19-Fgfr4 signaling pathways can be pharmaceutical targets for the treatment of TH-related gastrointestinal diseases.

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.

Developmental assays using invasive cane toads, Rhinella marina, reveal safety concerns of a common formulation of the rice herbicide, butachlor

Identifying the adverse impacts of pesticide exposure is essential to guide regulations that are protective of wildlife and human health. Within rice ecosystems, amphibians are valuable indicators because pesticide applications coincide with sensitive reproductive and developmental life stages. We conducted two experiments using wild cane toads (Rhinella marina) to test 1) whether environmentally relevant exposure to a commercial formulation of butachlor, an acetanilide herbicide used extensively in rice, affects amphibian development and 2) whether cane toad tadpoles are capable of acclimatizing to sub-lethal exposure. First, we exposed wild cane toads to 0.002, 0.02, or 0.2 mg/L of butachlor (Machete EC), during distinct development stages (as eggs and hatchlings, as tadpoles, or continuously) for 12 days. Next, we exposed a subset of animals from the first experiment to a second, lethal concentration and examined survivorship. We found that cane toads exposed to butachlor developed slower and weighed less than controls, and that development of the thyroid gland was affected: exposed individuals had smaller thyroid glands and thyrocyte cells, and more individual follicles. Analyses of the transcriptome revealed that butachlor exposure resulted in downregulation of transcripts related to metabolic processes, anatomic structure development, immune system function, and response to stress. Last, we observed evidence of acclimatization, where animals exposed to butachlor early in life performed better than naïve animals during a second exposure. Our findings indicate that the commercial formulation of butachlor, Machete EC, causes thyroid endocrine disruption in vertebrates, and suggest that exposure in lowland irrigated rice fields presents a concern for wildlife and human health. Furthermore, we establish that developmental assays with cane toads can be used to screen for adverse effects of pesticides in rice fields.

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.

How thyroid hormone regulates transformation of larval epithelial cells into adult stem cells in the amphibian intestine

In the amphibian intestine during metamorphosis, a small number of larval epithelial cells dedifferentiate into adult stem cells that newly form the adult epithelium analogous to the mammalian counterpart, while most of them undergo apoptosis. Because this larval-to-adult intestinal remodeling can be experimentally induced by thyroid hormone (TH) both in vivo and in vitro, TH response genes identified in the Xenopus intestine provide us valuable clues to investigating how adult stem cells and their niche are formed during postembryonic development. Their expression and functional analyses by using the culture and recent transgenic (Tg) techniques have shed light on key signaling pathways essential for intestinal stem cell development. The present review focuses on such recent findings and discusses the evolutionally conserved roles of TH in development or maintenance of the stem cells which are common to the terrestrial vertebrate intestines.

Thyroid Hormone-Induced Activation of Notch Signaling is Required for Adult Intestinal Stem Cell Development During Xenopus Laevis Metamorphosis

In Xenopus laevis intestine during metamorphosis, the larval epithelial cells are removed by apoptosis, and the adult epithelial stem (AE) cells appear concomitantly. They proliferate and differentiate to form the adult epithelium (Ep). Thyroid hormone (TH) is well established to trigger this remodeling by regulating the expression of various genes including Notch receptor. To study the role of Notch signaling, we have analyzed the expression of its components, including the ligands (DLL and Jag), receptor (Notch), and targets (Hairy), in the metamorphosing intestine by real‐time reverse transcription‐polymerase chain reaction and in situ hybridization or immunohistochemistry. We show that they are up‐regulated during both natural and TH‐induced metamorphosis in a tissue‐specific manner. Particularly, Hairy1 is specifically expressed in the AE cells. Moreover, up‐regulation of Hairy1 and Hairy2b by TH was prevented by treating tadpoles with a γ‐secretase inhibitor (GSI), which inhibits Notch signaling. More importantly, TH‐induced up‐regulation of LGR5, an adult intestinal stem cell marker, was suppressed by GSI treatment. Our results suggest that Notch signaling plays a role in stem cell development by regulating the expression of Hairy genes during intestinal remodeling. Furthermore, we show with organ culture experiments that prolonged exposure of tadpole intestine to TH plus GSI leads to hyperplasia of secretory cells and reduction of absorptive cells. Our findings here thus provide evidence for evolutionarily conserved role of Notch signaling in intestinal cell fate determination but more importantly reveal, for the first time, an important role of Notch pathway in the formation of adult intestinal stem cells during vertebrate development. Stem Cells2017;35:1028–1039

Orchestrating change: The thyroid hormones and GI-tract development in flatfish metamorphosis

Metamorphosis in flatfish (Pleuronectiformes) is a late post-embryonic developmental event that prepares the organism for the larval-to-juvenile transition. Thyroid hormones (THs) play a central role in flatfish metamorphosis and the basic elements that constitute the thyroid axis in vertebrates are all present at this stage. The advantage of using flatfish to study the larval-to-juvenile transition is the profound change in external morphology that accompanies metamorphosis making it easy to track progression to climax. This important lifecycle transition is underpinned by molecular, cellular, structural and functional modifications of organs and tissues that prepare larvae for a successful transition to the adult habitat and lifestyle. Understanding the role of THs in the maturation of organs and tissues with diverse functions during metamorphosis is a major challenge. The change in diet that accompanies the transition from a pelagic larvae to a benthic juvenile in flatfish is associated with structural and functional modifications in the gastrointestinal tract (GI-tract). The present review will focus on the maturation of the GI-tract during metamorphosis giving particular attention to organogenesis of the stomach a TH triggered event. Gene transcripts and biological processes that are associated with GI-tract maturation during Atlantic halibut metamorphosis are identified. Gene ontology analysis reveals core biological functions and putative TH-responsive genes that underpin TH-driven metamorphosis of the GI-tract in Atlantic halibut. Deciphering the specific role remains a challenge. Recent advances in characterizing the molecular, structural and functional modifications that accompany the appearance of a functional stomach in Atlantic halibut are considered and future research challenges identified.

Direct Activation of Amidohydrolase Domain-Containing 1 Gene by Thyroid Hormone Implicates a Role in the Formation of Adult Intestinal Stem Cells During Xenopus Metamorphosis

The T3-dependent anuran metamorphosis resembles postembryonic development in mammals, the period around birth when plasma T3 levels peak. In particular, the remodeling of the intestine during metamorphosis mimics neonatal intestinal maturation in mammals when the adult intestinal epithelial self-renewing system is established. We have been using intestinal metamorphosis to investigate how the organ-specific adult stem cells are formed during vertebrate development. Early studies in Xenopus laevis have shown that this process involves complete degeneration of the larval epithelium and de novo formation of adult stem cells. A tissue-specific microarray analysis of intestinal gene expression during Xenopus laevis metamorphosis has identified a number of candidate stem cell genes. Here we have carried out detailed analyses of one such gene, amidohydrolase domain containing 1 (AMDHD1) gene, which encodes an enzyme in the histidine catabolic pathway. We show that AMDHD1 is exclusively expressed in the proliferating adult epithelial stem cells during metamorphosis with little expression in other intestinal tissues. We further provide evidence that T3 activates AMDHD1 gene expression directly at the transcription level through T3 receptor binding to the AMDHD1 gene in the intestine. In addition, we have reported earlier that histidine ammonia-lyase gene, another gene in histidine catabolic pathway, is similarly regulated by T3 in the intestine. These results together suggest that histidine catabolism plays a critical role in the formation and/or proliferation of adult intestinal stem cells during metamorphosis.

A requirement for hedgehog signaling in thyroid hormone-induced postembryonic intestinal remodeling

Background

Intestinal remodeling during amphibian metamorphosis has long been studied as a model for the formation of the adult organs in vertebrates, especially the formation of adult organ-specific stem cells. Like all other processes during metamorphosis, this process is controlled by thyroid hormone (T3), which affects cell fate and behavior through transcriptional regulation of target genes by binding to T3 receptors (TRs). Earlier studies have shown that Sonic hedgehog (Shh) is induced by T3 in the developing adult stem cells and that the Shh receptor and other downstream components are present in the connective tissue and at lower levels in the muscles at the climax of intestinal remodeling. However, no in vivo studies have carried out to investigate whether Shh produced in the adult cells can regulate the connective tissue to promote intestinal maturation.

Results

We have addressed this issue by treating tadpoles with Shh inhibitor cyclopamine. We showed that cyclopamine but not the structurally related chemical tomatidine inhibited the expression of Shh response genes BMP4, Snai2, and Twist1. More importantly, we showed that cyclopamine reduced the cell proliferation of both the developing adult stem cells as well as cells in the other intestinal tissues at the climax of metamorphosis, leading to delayed/incomplete remodeling of the intestine at the end of metamorphosis. We further revealed that both Snai2 and Twist1 were strongly upregulated during metamorphosis in the intestine and their expression was restricted to the connective tissue.

Conclusions

Our results suggest that Shh indeed signals the connective tissue whereby it can increase adult stem cell proliferation and promote formation of the adult intestine.

The role of thyroid hormone signaling in the prevention of digestive system cancers

Thyroid hormones play a critical role in the growth and development of the alimentary tract in vertebrates. Their effects are mediated by nuclear receptors as well as the cell surface receptor integrin αVβ3. Systemic thyroid hormone levels are controlled via activation and deactivation by iodothyronine deiodinases in the liver and other tissues. Given that thyroid hormone signaling has been characterized as a major effector of digestive system growth and homeostasis, numerous investigations have examined its role in the occurrence and progression of cancers in various tissues of this organ system. The present review summarizes current findings regarding the effects of thyroid hormone signaling on cancers of the esophagus, stomach, liver, pancreas, and colon. Particular attention is given to the roles of different thyroid hormone receptor isoforms, the novel integrin αVβ3 receptor, and thyroid hormone-related nutrients as possible protective agents and therapeutic targets. Future investigations geared towards a better understanding of thyroid hormone signaling in digestive system cancers may provide preventive or therapeutic strategies to diminish risk, improve outcome and avert recurrence in afflicted individuals.

Thyroid hormone-induced cell-cell interactions are required for the development of adult intestinal stem cells

The mammalian intestine has long been used as a model to study organ-specific adult stem cells, which are essential for organ repair and tissue regeneration throughout adult life. The establishment of the intestinal epithelial cell self-renewing system takes place during perinatal development when the villus-crypt axis is established with the adult stem cells localized in the crypt. This developmental period is characterized by high levels of plasma thyroid hormone (T3) and T3 deficiency is known to impair intestinal development. Determining how T3 regulates adult stem cell development in the mammalian intestine can be difficult due to maternal influences. Intestinal remodeling during amphibian metamorphosis resembles perinatal intestinal maturation in mammals and its dependence on T3 is well established. A major advantage of the amphibian model is that it can easily be controlled by altering the availability of T3. The ability to manipulate and examine this relatively rapid and localized formation of adult stem cells has greatly assisted in the elucidation of molecular mechanisms regulating their formation and further revealed evidence that supports conservation in the underlying mechanisms of adult stem cell development in vertebrates. Furthermore, genetic studies in Xenopus laevis indicate that T3 actions in both the epithelium and the rest of the intestine, most likely the underlying connective tissue, are required for the formation of adult stem cells. Molecular analyses suggest that cell-cell interactions involving hedgehog and BMP pathways are critical for the establishment of the stem cell niche that is essential for the formation of the adult intestinal stem cells.

Establishment of intestinal stem cell niche during amphibian metamorphosis

In the amphibian intestine during metamorphosis, most of the larval epithelial cells undergo apoptosis, whereas a small number of them survive. These cells dedifferentiate into stem cells through interactions with the microenvironment referred to as "stem cell niche" and generate the adult epithelium analogous to the mammalian counterpart. Since all processes of the larval-to-adult intestinal remodeling can be experimentally induced by thyroid hormone (TH) both in vivo and in vitro, the amphibian intestine provides us a valuable opportunity to study how adult stem cells and their niche are formed during postembryonic development. To address this issue, a number of expression and functional analyses of TH response genes have been intensely performed in the Xenopus laevis over the past two decades, by using organ culture and transgenic techniques. We here review recent progress in this field, focusing on key signaling pathways involved in establishment of the stem cell niche and discuss their evolutionarily conserved roles in the vertebrate intestine.

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

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

Beyond synergy: corticosterone and thyroid hormone have numerous interaction effects on gene regulation in Xenopus tropicalis tadpoles

Hormones play critical roles in vertebrate development, and frog metamorphosis has been an excellent model system to study the developmental roles of thyroid hormone (TH) and glucocorticoids. Whereas TH regulates the initiation and rate of metamorphosis, the actions of corticosterone (CORT; the main glucocorticoid in frogs) are more complex. In the absence of TH during premetamorphosis, CORT inhibits development, but in the presence of TH during metamorphosis, CORT synergizes with TH to accelerate development. Synergy at the level of gene expression is known for three genes in frogs, but the nature and extent of TH and CORT cross talk is otherwise unknown. Therefore, to examine TH and CORT interactions, we performed microarray analysis on tails from Xenopus tropicalis tadpoles treated with CORT, TH, CORT+TH, or vehicle for 18 h. The expression of 5432 genes was significantly altered in response to either or both hormones. Using Venn diagrams and cluster analysis, we identified 16 main patterns of gene regulation due to up- or down-regulation by TH and/or CORT. Many genes were affected by only one of the hormones, and a large proportion of regulated genes (22%) required both hormones. We also identified patterns of additive or synergistic, inhibitory, subtractive, and annihilatory regulation. A total of 928 genes (17%) were regulated by novel interactions between the two hormones. These data expand our understanding of the hormonal cross talk underlying the gene regulation cascade directing tail resorption and suggest the possibility that CORT affects not only the timing but also the nature of TH-dependent tissue transformation.

Roles of Matrix Metalloproteinases and ECM Remodeling during Thyroid Hormone-Dependent Intestinal Metamorphosis in Xenopus laevis

Intestinal metamorphosis in anurans is an excellent model system for studying post-embryonic tissue remodeling and organ development in vertebrates. This process involves degeneration of the larval or tadpole form of its primary functional tissue, the simple tubular epithelium through apoptosis or programmed cell death. Concurrently, adult epithelial stem cells, whose origin remains to be determined, proliferate and differentiate to form a multiply folded, complex adult epithelium. The connective tissue and muscles also develop extensively during this period. Like all other changes during amphibian metamorphosis, intestinal remodeling is controlled by thyroid hormone (TH). Isolation and characterization of genes that are regulated by TH has implicated the involvement of matrix metalloproteinases (MMPs) in the remodeling of the extracellular matrix (ECM) during intestinal metamorphosis. Here we will review some studies, almost exclusively in Xenopus laevis, that support a role of MMPs, particularly stromelysin 3, and ECM remodeling in regulating cell fate and tissue morphogenesis.

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

BACKGROUND

Thyroid hormone (TH) receptor (TR) plays critical roles in vertebrate development. However, the in vivo mechanism of TR action remains poorly explored.

SCOPE OF REVIEW

Frog metamorphosis is controlled by TH and mimics the postembryonic period in mammals when high levels of TH are also required. We review here some of the findings on the developmental functions of TH and TR and the associated mechanisms obtained from this model system.

MAJOR CONCLUSION

A dual function model for TR in Anuran development was proposed over a decade ago. That is, unliganded TR recruits corepressors to TH response genes in premetamorphic tadpoles to repress these genes and prevent premature metamorphic changes. Subsequently, when TH becomes available, liganded TR recruits coactivators to activate these same genes, leading to metamorphic changes. Over the years, molecular and genetic approaches have provided strong support for this model. Specifically, it has been shown that unliganded TR recruits histone deacetylase containing corepressor complexes during larval stages to control metamorphic timing, while liganded TR recruits multiple histone modifying and chromatin remodeling coactivator complexes during metamorphosis. These complexes can alter chromatin structure via nucleosome position alterations or eviction and histone modifications to contribute to the recruitment of transcriptional machinery and gene activation.

GENERAL SIGNIFICANCE

The molecular mechanisms of TR action in vivo as revealed from studies on amphibian metamorphosis are very likely applicable to mammalian development as well. These findings provide a new perspective for understanding the diverse effects of TH in normal physiology and diseases caused by TH dysfunction. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone-induced sonic hedgehog signal up-regulates its own pathway in a paracrine manner in the Xenopus laevis intestine during metamorphosis

BACKGROUND

During Xenopus laevis metamorphosis, Sonic hedgehog (Shh) is directly induced by thyroid hormone (TH) at the transcription level as one of the earliest events in intestinal remodeling. However, the regulation of other components of this signaling pathway remains to be analyzed. Here, we analyzed the spatiotemporal expression of Patched (Ptc)-1, Smoothened (Smo), Gli1, Gli2, and Gli3 during natural and TH-induced intestinal remodeling.

RESULTS

We show that all of the genes examined are transiently up-regulated in the mesenchymal tissues during intestinal metamorphosis.

CONCLUSIONS

Interestingly, in the presence of protein synthesis inhibitors, Gli2 but not the others was induced by TH, suggesting that Gli2 is a direct TH response gene, while the others are likely indirect ones. Furthermore, we demonstrate by the organ culture experiment that overexpression of Shh enhances the expression of Ptc-1, Smo, and Glis even in the absence of TH, indicating that Shh regulates its own pathway components during intestinal remodeling.

Epithelial-connective tissue interactions induced by thyroid hormone receptor are essential for adult stem cell development in the Xenopus laevis intestine

In the amphibian intestine during metamorphosis, stem cells appear and generate the adult absorptive epithelium, analogous to the mammalian one, under the control of thyroid hormone (TH). We have previously shown that the adult stem cells originate from differentiated larval epithelial cells in the Xenopus laevis intestine. To clarify whether TH signaling in the epithelium alone is sufficient for inducing the stem cells, we have now performed tissue recombinant culture experiments using transgenic X. laevis tadpoles that express a dominant-positive TH receptor (dpTR) under a control of heat shock promoter. Wild-type (Wt) or dpTR transgenic (Tg) larval epithelium (Ep) was isolated from the tadpole intestine, recombined with homologous or heterologous nonepithelial tissues (non-Ep), and then cultivated in the absence of TH with daily heat shocks to induce transgenic dpTR expression. Adult epithelial progenitor cells expressing sonic hedgehog became detectable on day 5 in both the recombinant intestine of Tg Ep and Tg non-Ep (Tg/Tg) and that of Tg Ep and Wt non-Ep (Tg/Wt). However, in Tg/Wt intestine, they did not express other stem cell markers such as Musashi-1 and never generated the adult epithelium expressing a marker for absorptive epithelial cells. Our results indicate that, while it is unclear why some larval epithelial cells dedifferentiate into adult progenitor/stem cells, TR-mediated gene expression in the surrounding tissues other than the epithelium is required for them to develop into adult stem cells, suggesting the importance of TH-inducible epithelial-connective tissue interactions in establishment of the stem cell niche in the amphibian intestine.

Novel regulation of yolk utilization by thyroid hormone in embryos of the direct developing frog Eleutherodactylus coqui

Thyroid hormone (TH) is required for metamorphosis of the long, coiled tadpole gut into the short frog gut. Eleutherodactylus coqui, a direct developing frog, lacks a tadpole. Its embryonic gut is a miniature adult form with a mass of yolky cells, called nutritional endoderm, attached to the small intestine. We tested the TH requirement for gut development in E. coqui. Inhibition of TH synthesis with methimazole arrested gut development in its embryonic form. Embryos treated with methimazole failed to utilize the yolk in their nutritional endoderm, and survived for weeks without further development. Conversely, methimazole and 3,3',5-tri-iodo-l-thyronine, the active form of TH, stimulated gut development and utilization and disappearance of the nutritional endoderm. In Xenopus laevis, the receptor for TH, TRβ, is upregulated in response to TH. Similarly, EcTRβ, the E. coqui ortholog, was upregulated by TH in the gut. EcTRβ expression was high in the nutritional endoderm, suggesting a direct role for TH in yolk utilization by these cells. An initial step in the breakdown of yolk in X. laevis is acidification of the yolk platelet. E. coqui embryos in methimazole failed to acidify their yolk platelets, but acidification was stimulated by TH indicating its role in an early step of yolk utilization. In addition to a conserved TH role in gut development, a novel regulatory role for TH in yolk utilization has evolved in these direct developers.

Transdifferentiation of tadpole pancreatic acinar cells to duct cells mediated by Notch and stromelysin-3

The tadpole pancreas has differentiated acinar cells but an underdeveloped ductal system. At the climax of metamorphosis thyroid hormone (TH) induces the tadpole acinar cells to dedifferentiate to a progenitor state. After metamorphosis is complete the exocrine pancreas redifferentiates in the growing frog forming a typical vertebrate pancreas including a complex ductal system. A micro array analysis found that TH up regulates stromelysin 3 (ST3, matrix metalloproteinase 11) in the exocrine pancreas at metamorphic climax. Transgenic tadpoles were prepared with an elastase promoter driving either the ST3 gene or the constitutively active form of Notch (IC). Expression of the transgenes was controlled by the tetracycline system. A few days after either of these transgenes is activated by doxycycline the pancreatic acinar cells turn into duct-like cells. This transdetermination occurs without cell division since both acinar and ductal markers can be visualized transiently in the same cell. We propose that remodeling of the tadpole acinar cells is initiated when ST3 is up regulated by TH. Stromelysin-3 then cleaves and activates Notch.

Metamorphosis and the regenerative capacity of spinal cord axons in Xenopus laevis

Throughout the vertebrate subphylum, the regenerative potential of central nervous system axons is greatest in embryonic stages and declines as development progresses. For example, Xenopus laevis can functionally recover from complete transection of the spinal cord as a tadpole but is unable to do so after metamorphosing into a frog. Neurons of the reticular formation and raphe nucleus are among those that regenerate axons most reliably in tadpole and that lose this ability after metamorphosis. To identify molecular factors associated with the success and failure of spinal cord axon regeneration, we pharmacologically manipulated thyroid hormone (TH) levels using methimazole or triiodothyronine, to either keep tadpoles in a permanently larval state or induce precocious metamorphosis, respectively. Following complete spinal cord transection, serotonergic axons crossed the lesion site and tadpole swimming ability was restored when metamorphosis was inhibited, but these events failed to occur when metamorphosis was prematurely induced. Thus, the metamorphic events controlled by TH led directly to the loss of regenerative potential. Microarray analysis identified changes in hindbrain gene expression that accompanied regeneration-permissive and -inhibitory conditions, including many genes in the permissive condition that have been previously associated with axon outgrowth and neuroprotection. These data demonstrate that changes in gene expression occur within regenerating neurons in response to axotomy under regeneration-permissive conditions in which normal development has been suspended, and they identify candidate genes for future studies of how central nervous system axons can successfully regenerate in some vertebrates.

Apoptosis in amphibian organs during metamorphosis

During amphibian metamorphosis, the larval tissues/organs rapidly degenerate to adapt from the aquatic to the terrestrial life. At the cellular level, a large quantity of apoptosis occurs in a spatiotemporally-regulated fashion in different organs to ensure timely removal of larval organs/tissues and the development of adult ones for the survival of the individuals. Thus, amphibian metamorphosis provides us a good opportunity to understand the mechanisms regulating apoptosis. To investigate this process at the molecular level, a number of thyroid hormone (TH) response genes have been isolated from several organs of Xenopus laevis tadpoles and their expression and functional analyses are now in progress using modern molecular and genetic technologies. In this review, we will first summarize when and where apoptosis occurs in typical larva-specific and larval-to-adult remodeling amphibian organs to highlight that the timing of apoptosis is different in different tissues/organs, even though all are induced by the same circulating TH. Next, to discuss how TH spatiotemporally regulates the apoptosis, we will focus on apoptosis of the X. laevis small intestine, one of the best characterized remodeling organs. Functional studies of TH response genes using transgenic frogs and culture techniques have shown that apoptosis of larval epithelial cells can be induced by TH either cell-autonomously or indirectly through interactions with extracellular matrix (ECM) components of the underlying basal lamina. Here, we propose that multiple intra- and extracellular apoptotic pathways are coordinately controlled by TH to ensure massive but well-organized apoptosis, which is essential for the proper progression of amphibian metamorphosis.

Tissue-dependent induction of apoptosis by matrix metalloproteinase stromelysin-3 during amphibian metamorphosis

Matrix metalloproteinases (MMPs) are a superfamily of Zn(2+)-dependent proteases that are capable of cleaving the proteinaceous component of the extracellular matrix (ECM). The ECM is a critical medium for cell-cell interactions and can also directly signal cells through cell surface ECM receptors, such as integrins. In addition, many growth factors and signaling molecules are stored in the ECM. Thus, ECM remodeling and/or degradation by MMPs are expected to affect cell fate and behavior during many developmental and pathological processes. Numerous studies have shown that the expression of MMP mRNAs and proteins associates tightly with diverse developmental and pathological processes, such as tumor metastasis and mammary gland involution. In vivo evidence to support the roles of MMPs in these processes has been much harder to get. Here, we will review some of our studies on MMP11, or stromelysin-3, during the thyroid hormone-dependent amphibian metamorphosis, a process that resembles the so-called postembryonic development in mammals (from a few months before to several months after birth in humans when organ growth and maturation take place). Our investigations demonstrate that stromelysin-3 controls apoptosis in different tissues via at least two distinct mechanisms.

Identification of direct thyroid hormone response genes reveals the earliest gene regulation programs during frog metamorphosis

Thyroid hormone (T3) is essential for normal development and organ function throughout vertebrates. Its effects are mainly mediated through transcriptional regulation by T3 receptor (TR). The identification and characterization of the immediate early, direct target genes are thus of critical importance in understanding the molecular pathways induced by T3. Unfortunately, this has been hampered by the difficulty to study gene regulation by T3 in uterus-enclosed mammalian embryos. Here we used Xenopus metamorphosis as a model for vertebrate postembryonic development to identify direct T3 response genes in vivo. We took advantage of the ability to easily induce metamorphosis with physiological levels of T3 and to carry out microarray analysis in Xenopus laevis and genome-wide sequence analysis in Xenopus tropicalis. This allowed us to identify 188 up-regulated and 249 down-regulated genes by T3 in the absence of new protein synthesis in whole animals. We further provide evidence to show that these genes contain functional TREs that are bound by TR in tadpoles and that their promoters are regulated by TR in vivo. More importantly, gene ontology analysis showed that the direct up-regulated genes are enriched in categories important for transcriptional regulation and protein degradation-dependent signaling processes but not DNA replication. Our findings thus revealed the existence of interesting pathways induced by T3 at the earliest step of metamorphosis.

Differential regulation of cell type-specific apoptosis by stromelysin-3: a potential mechanism via the cleavage of the laminin receptor during tail resorption in Xenopus laevis

Matrix metalloproteinases (MMPs) have been extensively studied because of their functional attributes in development and diseases. However, relatively few in vivo functional studies have been reported on the roles of MMPs in postembryonic organ development. Amphibian metamorphosis is a unique model for studying MMP function during vertebrate development because of its dependence on thyroid hormone (T3) and the ability to easily manipulate this process with exogenous T3. The MMP stromelysin-3 (ST3) is induced by T3, and its expression correlates with cell death during metamorphosis. We have previously shown that ST3 is both necessary and sufficient for larval epithelial cell death in the remodeling intestine. To investigate the roles of ST3 in other organs and especially on different cell types, we have analyzed the effect of transgenic overexpression of ST3 in the tail of premetamorphic tadpoles. We report for the first time that ST3 expression, in the absence of T3, caused significant muscle cell death in the tail of premetamorphic transgenic tadpoles. On the other hand, only relatively low levels of epidermal cell death were induced by precocious ST3 expression in the tail, contrasting what takes place during natural and T3-induced metamorphosis when ST3 expression is high. This cell type-specific apoptotic response to ST3 in the tail suggests distinct mechanisms regulating cell death in different tissues. Furthermore, our analyses of laminin receptor, an in vivo substrate of ST3 in the intestine, suggest that laminin receptor cleavage may be an underlying mechanism for the cell type-specific effects of ST3.

Cell-cell interactions during remodeling of the intestine at metamorphosis in Xenopus laevis

Amphibian metamorphosis is accompanied by extensive intestinal remodeling. This process, mediated by thyroid hormone (TH) and its nuclear receptors, affects every cell type. Gut remodeling in Xenopus laevis involves epithelial and mesenchymal proliferation, smooth muscle thickening, neuronal aggregation, formation of intestinal folds, and shortening of its length by 75%. Transgenic tadpoles expressing a dominant negative TH receptor (TRDN) controlled by epithelial-, fibroblast-, and muscle-specific gene promoters were studied. TRDN expression in the epithelium caused abnormal development of virtually all cell types, with froglet guts displaying reduced intestinal folds, thin muscle and mesenchyme, absence of neurons, and reduced cell proliferation. TRDN expression in fibroblasts caused abnormal epithelia and mesenchyme development, and expression in muscle produced fewer enteric neurons and a reduced inter-muscular space. Gut shortening was inhibited only when TRDN was expressed in fibroblasts. Gut remodeling results from both cell-autonomous and cell-cell interactions.

Origin of the adult intestinal stem cells induced by thyroid hormone in Xenopus laevis

In the amphibian intestine during metamorphosis, de novo stem cells generate the adult epithelium analogous to the mammalian counterpart. Interestingly, to date the exact origin of these stem cells remains to be determined, making intestinal metamorphosis a unique model to study development of adult organ-specific stem cells. Here, to determine their origin, we made use of transgenic Xenopus tadpoles expressing green fluorescent protein (GFP) for recombinant organ cultures. The larval epithelium separated from the wild-type (Wt) or GFP transgenic (Tg) intestine before metamorphic climax was recombined with homologous and heterologous nonepithelial tissues and was cultivated in the presence of thyroid hormone, the causative agent of metamorphosis. In all kinds of recombinant intestine, adult progenitor cells expressing markers for intestinal stem cells such as sonic hedgehog became detectable and then differentiated into the adult epithelium expressing intestinal fatty acid binding-protein, a marker for absorptive cells. Notably, whenever the epithelium was derived from Tg intestine, both the adult progenitor/stem cells and their differentiated cells expressed GFP, whereas neither of them expressed GFP in the Wt-derived epithelium. Our results provide direct evidence that stem cells that generate the adult intestinal epithelium originate from the larval epithelium, through thyroid hormone-induced dedifferentiation.

Genome-wide identification of Xenopus matrix metalloproteinases: conservation and unique duplications in amphibians

Background

Matrix metalloproteinases (MMPs) are members of the superfamily of Zn²⁺ dependent extracellular or membrane-bound endopeptidases which have been implicated to play critical roles in vertebrate development and human pathogenesis. A number of MMP genes have been found to be upregulated in some or all organs during frog metamorphosis, suggesting that different MMPs may have different functions in various organs/tissues. The recent advances in EST (expressed sequence tag) sequencing and the completion of the genome of Xenopus (X.) tropicalis prompted us to systematically analyze the existence of MMPs in the Xenopus genome.

Results

We examined X. laevis and X. tropicalis ESTs and genomic sequences for MMPs and obtained likely homologs for 20 out of the 25 MMPs known in higher vertebrates. Four of the five missing MMPs, i.e. MMPs 8, 10, 12 and 27, were all encoded on human Chromosome 11 and the other missing MMP, MMP22 (a chicken MMP), was also absent in human genome. In addition, we identified several novel MMPs which appears to be derived from unique duplications over evolution, are present in the genomes of both Xenopus species.

Conclusion

We identified the homologs of most of the mammalian MMPs in Xenopus and discovered a number of novel MMPs. Our results suggest that MMP genes undergo dynamic changes over evolution. It will be of interest in the future to investigate whether MMP expression and functions during vertebrate development are conserved. The sequence information reported here should facilitate such an endeavor in the near future.

Novel functions of protein arginine methyltransferase 1 in thyroid hormone receptor-mediated transcription and in the regulation of metamorphic rate in Xenopus laevis

Protein arginine methyltransferase 1 (PRMT1) acts as a transcription coactivator for nuclear receptors through histone H4 R3 methylation. The in vivo function of PRMT1 is largely unknown. Here we investigated the role of PRMT1 in thyroid hormone (T3) receptor (TR)-mediated transcription in vivo during vertebrate development. By using intestinal remodeling during T3-dependent Xenopus laevis metamorphosis for in vivo molecular analysis, we first showed that PRMT1 expression was upregulated during metamorphosis when both TR and T3 were present. We then demonstrated a role for PRMT1 in TR-mediated transcription by showing that PRMT1 enhanced transcriptional activation by liganded TR in the frog oocyte transcription system and was recruited to the T3 response element (TRE) of the target promoter in the oocyte, as well as to endogenous TREs during frog metamorphosis. Surprisingly, we found that PRMT1 was only transiently recruited to the TREs in the target during metamorphosis and observed no PRMT1 recruitment to TREs at the climax of intestinal remodeling when both PRMT1 and T3 were at peak levels. Mechanistically, we showed that overexpression of PRMT1 enhanced TR binding to TREs both in the frog oocyte model system and during metamorphosis. More importantly, transgenic overexpression of PRMT1 enhanced gene activation in vivo and accelerated both natural and T3-induced metamorphosis. These results thus indicate that PRMT1 functions transiently as a coactivator in TR-mediated transcription by enhancing TR-TRE binding and further suggest that PRMT1 has tissue-specific roles in regulating the rate of metamorphosis.

Thyroid hormone-up-regulated hedgehog interacting protein is involved in larval-to-adult intestinal remodeling by regulating sonic hedgehog signaling pathway in Xenopus laevis

Sonic hedgehog (Shh) was previously shown to be involved in the larval-to-adult remodeling of the Xenopus laevis intestine. While Shh is transcriptionally regulated by thyroid hormone (TH), the posttranscriptional regulation of Shh signaling during intestinal remodeling is largely unknown. In the present study, we focused on a role of the pan-hedgehog inhibitor, hedgehog interacting protein (Hip), in the spatiotemporal regulation of Shh signaling. Using real-time reverse transcriptase-polymerase chain reaction and in situ hybridization, we show that Hip expression is transiently up-regulated during both natural and TH-induced metamorphosis and that Hip mRNA is localized in the connective tissue adjacent to the adult epithelial primordia expressing Shh. Interestingly, the expression of bone morphogenetic protein-4, a Shh target gene, is hardly detectable where Hip is strongly expressed. Finally, we demonstrate that Hip binds to the N-terminal fragment of processed Shh in vivo, suggesting that Hip suppresses Shh signaling through sequestering Shh.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Thyroid hormone regulation of stem cell development during intestinal remodeling

During amphibian metamorphosis the small intestine is remodeled from larval to adult form, analogous to the mammalian intestine. The larval epithelium mostly undergoes apoptosis, while a small number of stem cells appear, actively proliferate, and differentiate into the adult epithelium possessing a cell-renewal system. Because amphibian intestinal remodeling is completely controlled by thyroid hormone (T3) through T3 receptors (TRs), it serves as an excellent model for studying the molecular mechanism of the mammalian intestinal development. TRs bind T3 response elements in target genes and have dual functions by interacting with coactivators or corepressors in a T3-dependent manner. A number of T3 response genes have been isolated from the Xenopus laevis intestine. They include signaling molecules, matrix metalloproteinases, and transcription factors. Functional studies have been carried out on many such genes in vitro and in vivo by using transgenic and culture technologies. Here we will review recent findings from such studies with a special emphasis on the adult intestinal stem cells, and discuss the evolutionarily conserved roles of T3 in the epithelial cell-renewal in the vertebrate intestine.

Genes induced during the early developmental stages of the Cane Toad, Bufo (Chaunus) marinus

Metamorphosis, a critical stage in the development of toads and frogs, involves rapid levels of morphological change. In the current study, we have used microarray analysis to identify shifts in gene expression between tadpole and toadlet stages of the cane toad, Bufo (Chaunus) marinus. Here, we report on nine genes that show the greatest induction during metamorphosis; the gut-associated gastrokine and trefoil factor, blood components haemoglobins alpha/beta, apolipoprotein and serum albumin, a nasal gene olfactomedin, a lens gene gamma-crystallin, and a novel gene with low homology to frog harderin. We present both temporal and spatial expression patterns of these genes identified in developing and adult cane toads. This study extends our knowledge of the molecular basis of toad metamorphosis, and not only offers insights to the genes induced during the general remodelling that occurs but also reveals possible targets for control and manipulation of amphibian pest species, for example, the cane toad in Australia.