Epigenetic regulation of thyroid hormone-induced adult intestinal stem cell development during anuran metamorphosis

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.

The ecological function of thyroid hormones

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

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.

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.

Identification of thyroid hormone response genes in the remodeling of dorsal muscle during Microhyla fissipes metamorphosis

INTRODUCTION

Extensive morphological, biochemical, and cellular changes occur during anuran metamorphosis, which is triggered by a single hormone, thyroid hormone (TH). The function of TH is mainly mediated through thyroid receptor (TR) by binding to the specific thyroid response elements (TREs) of direct response genes, in turn regulating the downstream genes in the cascade. The remodeling of dorsal skeletal muscle during anuran metamorphosis provides the perfect model to identify the immediate early and direct response genes that are important during apoptosis, proliferation, and differentiation of the muscle.

METHODS

In our current study, we performed Illumina sequencing combined with single-molecule real-time (SMRT) sequencing in the dorsal muscle of Microhyla fissipes after TH, cycloheximide (CHX), and TH_CHX treatment.

RESULTS AND DISCUSSION

We first identified 1,245 differentially expressed transcripts (DETs) after TH exposure, many of which were involved in DNA replication, protein processing in the endoplasmic reticulum, cell cycle, apoptosis, p53 signaling pathway, and protein digestion and absorption. In the comparison of the TH group vs. control group and TH_CHX group vs. CHX group overlapping gene, 39 upregulated and 6 downregulated genes were identified as the TH directly induced genes. Further analysis indicated that AGGTCAnnTnAGGTCA is the optimal target sequence of target genes for TR/RXR heterodimers in M. fissipes. Future investigations on the function and regulation of these genes and pathways should help to reveal the mechanisms governing amphibian dorsal muscle remodeling. These full-length and high-quality transcriptomes in this study also provide an important foundation for future studies in M. fissipes metamorphosis.

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.

Hormonal Regulation of Programmed Cell Death in Sea Urchin Metamorphosis

Programmed cell death (PCD) has been identified as a key process in the metamorphic transition of indirectly developing organisms such as frogs and insects. Many marine invertebrate species with indirect development and biphasic life cycles face the challenge of completing the metamorphic transition of the larval body into a juvenile when they settle into the benthic habitat. Some key characteristics stand out during this transition in comparison to frogs and insects: (1) the transition is often remarkably fast and (2) the larval body is largely abandoned and few structures transition into the juvenile stage. In sea urchins, a group with a drastic and fast metamorphosis, development and destruction of the larval body is regulated by endocrine signals. Here we provide a brief review of the basic regulatory mechanisms of PCD in animals. We then narrow our discussion to metamorphosis with a specific emphasis on sea urchins with indirect life histories and discuss the function of thyroid hormones and histamine in larval development, metamorphosis and settlement of the sea urchin Strongylocentrotus purpuratus. We were able to annotate the large majority of PCD related genes in the sea urchin S. purpuratus and ongoing studies on sea urchin metamorphosis will shed light on the regulatory architecture underlying this dramatic life history transition. While we find overwhelming evidence for hormonal regulation of PCD in animals, especially in the context of metamorphosis, the mechanisms in many marine invertebrate groups with indirect life histories requires more work. Hence, we propose that studies of PCD in animals requires functional studies in whole organisms rather than isolated cells. We predict that future work, targeting a broader array of organisms will not only help to reveal important new functions of PCD but provide a fundamentally new perspective on its use in a diversity of taxonomic, developmental, and ecological contexts.

A Role of Endogenous Histone Acetyltransferase Steroid Hormone Receptor Coactivator 3 in Thyroid Hormone Signaling During Xenopus Intestinal Metamorphosis

Background: Thyroid hormone (triiodothyronine [T3]) plays an important role in regulating vertebrate developmental, cellular, and metabolic processes via T3 receptor (TR). Liganded TR recruit coactivator complexes that include steroid receptor coactivators (SRC1, SRC2 or SRC3), which are histone acetyltransferases, to T3-responsive promoters. The functions of endogenous coactivators during T3-dependent mammalian adult organ development remain largely unclear, in part, due to the difficulty to access and manipulate late-stage embryos and neonates. We use Xenopus metamorphosis as a model for postembryonic development in vertebrates. This process is controlled by T3, involves drastic changes in every organ/tissue, and can be easily manipulated. We have previously found that SRC3 was upregulated in the intestine during amphibian metamorphosis. Methods: To determine the function of endogenous SRC3 during intestinal remodeling, we have generated Xenopus tropicalis animals lacking a functional SRC3 gene and analyzed the resulting phenotype. Results: Although removing SRC3 had no apparent effect on external development and animal gross morphology, the SRC3 (-/-) tadpoles displayed a reduction in the acetylation of histone H4 in the intestine compared with that in wild-type animals. Further, the expression of TR target genes was also reduced in SRC3 (-/-) tadpoles during intestinal remodeling. Importantly, SRC3 (-/-) tadpoles had inhibited/delayed intestinal remodeling during natural and T3-induced metamorphosis, including reduced adult intestinal stem cell proliferation and apoptosis of larval epithelial cells. Conclusion: Our results, thus, demonstrate that SRC3 is a critical component of the TR-signaling pathway in vivo during intestinal remodeling.

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.

Thyroid Hormone Receptor Alpha Mutations Lead to Epithelial Defects in the Adult Intestine in a Mouse Model of Resistance to Thyroid Hormone

BACKGROUND

The thyroid hormone triiodothyronine (T3) is critical for vertebrate development and affects the function of many adult tissues and organs. Its genomic effects are mediated by thyroid hormone nuclear receptors (TRs) present in all vertebrates. The discovery of patients with resistance to thyroid hormone (RTHβ) >50 years ago and subsequent identification of genetic mutations in only the THRB gene in these patients suggest that mutations in the THRA gene may have different pathological manifestations in humans. Indeed, the recent discovery of a number of human patients carrying heterozygous mutations in the THRA gene (RTHα) revealed a distinct phenotype that was not observed in RTH patients with THRB gene mutations (RTHβ). That is, RTHα patients have constipation, implicating intestinal defects caused by THRA gene mutations.

METHODS

To determine how TRα1 mutations affect the intestine, this study analyzed a mutant mouse expressing a strong dominantly negative TRα1 mutant (denoted TRα1PV; Thra1^PV mice). This mutant mouse faithfully reproduces RTHα phenotypes observed in patients.

RESULTS

In adult Thra1^PV/+ mice, constipation was observed just like in patients with TRα mutations. Importantly, significant intestinal defects were discovered, including shorter villi and increased differentiated cells in the crypt, accompanied by reduced stem-cell proliferation in the intestine.

CONCLUSIONS

The findings suggest that further analysis of this mouse model should help to reveal the molecular and physiological defects in the intestine caused by TRα mutations and to determine the underlying mechanisms.

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.

Genome-wide identification of thyroid hormone receptor targets in the remodeling intestine during Xenopus tropicalis metamorphosis

Thyroid hormone (T3) affects development and metabolism in vertebrates. We have been studying intestinal remodeling during T3-dependent Xenopus metamorphosis as a model for organ maturation and formation of adult organ-specific stem cells during vertebrate postembryonic development, a period characterized by high levels of plasma T3. T3 is believed to affect development by regulating target gene transcription through T3 receptors (TRs). While many T3 response genes have been identified in different animal species, few have been shown to be direct target genes in vivo, especially during development. Here we generated a set of genomic microarray chips covering about 8000 bp flanking the predicted transcription start sites in Xenopus tropicalis for genome wide identification of TR binding sites. By using the intestine of premetamorphic tadpoles treated with or without T3 and for chromatin immunoprecipitation assays with these chips, we determined the genome-wide binding of TR in the control and T3-treated tadpole intestine. We further validated TR binding in vivo and analyzed the regulation of selected genes. We thus identified 278 candidate direct TR target genes. We further provided evidence that these genes are regulated by T3 and likely involved in the T3-induced formation of adult intestinal stem cells during metamorphosis.

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

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

A balance of Mad and Myc expression dictates larval cell apoptosis and adult stem cell development during Xenopus intestinal metamorphosis

The Myc/Mad/Max network has long been shown to be an important factor in regulating cell proliferation, death and differentiation in diverse cell types. In general, Myc–Max heterodimers activate target gene expression to promote cell proliferation, although excess of c-Myc can also induce apoptosis. In contrast, Mad competes against Myc to form Mad–Max heterodimers that bind to the same target genes to repress their expression and promote differentiation. The role of the Myc/Mad/Max network during vertebrate development, especially, the so-called postembryonic development, a period around birth in mammals, is unclear. Using thyroid hormone (T3)-dependent Xenopus metamorphosis as a model, we show here that Mad1 is induced by T3 in the intestine during metamorphosis when larval epithelial cell death and adult epithelial stem cell development take place. More importantly, we demonstrate that Mad1 is expressed in the larval cells undergoing apoptosis, whereas c-Myc is expressed in the proliferating adult stem cells during intestinal metamorphosis, suggesting that Mad1 may have a role in cell death during development. By using transcription activator-like effector nuclease-mediated gene-editing technology, we have generated Mad1 knockout Xenopus animals. This has revealed that Mad1 is not essential for embryogenesis or metamorphosis. On the other hand, consistent with its spatiotemporal expression profile, Mad1 knockout leads to reduced larval epithelial apoptosis but surprisingly also results in increased adult stem cell proliferation. These findings not only reveal a novel role of Mad1 in regulating developmental cell death but also suggest that a balance of Mad and Myc controls cell fate determination during adult organ development.

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

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

Thyroid hormone regulation of adult intestinal stem cells: Implications on intestinal development and homeostasis

Organ-specific adult stem cells are essential for organ homeostasis, tissue repair and regeneration. The formation of such stem cells often takes place during postembryonic development, a period around birth in mammals when plasma thyroid hormone concentration is high. The life-long self-renewal of the intestinal epithelium has made mammalian intestine a valuable model to study the function and regulation and adult stem cells. On the other hand, much less is known about how the adult intestinal stem cells are formed during vertebrate development. Here, we will review some recent progresses on this subject, focusing mainly on the formation of the adult intestine during Xenopus metamorphosis. We will discuss the role of thyroid hormone signaling pathway in the process and potential molecular conservations between amphibians and mammals as well as the implications in organ homeostasis and human diseases.

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

The Sox transcriptional factors: Functions during intestinal development in vertebrates

The intestine has long been studied as a model for adult stem cells due to the life-long self-renewal of the intestinal epithelium through the proliferation of the adult intestinal stem cells. Recent evidence suggests that the formation of adult intestinal stem cells in mammals takes place during the thyroid hormone-dependent neonatal period, also known as postembryonic development, which resembles intestinal remodeling during frog metamorphosis. Studies on the metamorphosis in Xenopus laevis have revealed that many members of the Sox family, a large family of DNA binding transcription factors, are upregulated in the intestinal epithelium during the formation and/or proliferation of the intestinal stem cells. Similarly, a number of Sox genes have been implicated in intestinal development and pathogenesis in mammals. Futures studies are needed to determine the expression and potential involvement of this important gene family in the development of the adult intestinal stem cells. These include the analyses of the expression and regulation of these and other Sox genes during postembryonic development in mammals as well as functional investigations in both mammals and amphibians by using the recently developed gene knockout technologies.

Regulation of growth rate and developmental timing by Xenopus thyroid hormone receptor α

Thyroid hormone (TH) is critical for vertebrate postembryonic development, a period around birth in mammals when plasma TH levels are high. Interestingly, TH receptors (TRs), especially TRα, are expressed prior to the synthesis and secretion of zygotic TH, suggesting the existence of unliganded TR during development. However, the role of unliganded TR during mammalian development has been difficult to study, in part due to the relatively weak phenotype of TR knockout mice. Amphibian metamorphosis resembles postembryonic development in mammals and is controlled by TH via TRs. Like in mammals, TRα gene is highly activated and is the major TR expressed prior to the synthesis of endogenous TH. By using TALEN (transcriptional activator like effector nucleases)-mediated gene editing approach, we and others have now shown that unliganded TRα has two independent functions during Xenopus premetamorphosis, i.e. inhibiting growth rate and slowing development. Furthermore, molecular and transgenic studies have shown that unliganded TRα accomplishes these via the recruitment of histone deacetylase (HDAC)-containing corepressor complexes to repress the expression of TH-inducible genes.

Molecular and cytological analyses reveal distinct transformations of intestinal epithelial cells during Xenopus metamorphosis

Background

The thyroid hormone (T3)-induced formation of adult intestine during amphibian metamorphosis resembles the maturation of the mammalian intestine during postembryonic development, the period around birth when plasma T3 level peaks. This process involves de novo formation of adult intestinal stem cells as well as the removal of the larval epithelial cells through apoptosis. Earlier studies have revealed a number of cytological and molecular markers for the epithelial cells undergoing different changes during metamorphosis. However, the lack of established double labeling has made it difficult to ascertain the identities of the metamorphosing epithelial cells.

Results

Here, we carried out different double-staining with a number of cytological and molecular markers during T3-induced and natural metamorphosis in Xenopus laevis. Our studies demonstrated conclusively that the clusters of proliferating cells in the epithelium at the climax of metamorphosis are undifferentiated epithelial cells and express the well-known adult intestinal stem cell marker gene Lgr5. We further show that the adult stem cells and apoptotic larval epithelial cells are distinct epithelial cells during metamorphosis.

Conclusions

Our findings suggest that morphologically identical larval epithelial cells choose two alternative paths: programmed cell death or dedifferentiation to form adult stem cells, in response to T3 during metamorphosis with apoptosis occurring prior to the formation of the proliferating adult stem cell clusters (islets).

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.

Unliganded TRs regulate growth and developmental timing during early embryogenesis: evidence for a dual function mechanism of TR action

Recent genetic studies in the anuran Xenopus tropicalis reveal surprising new roles of thyroid hormone receptor in regulating growth and developmental timing in the absence of thyroid hormone.