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

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

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

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

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

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.

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.

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

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

Protein arginine methyltransferase 1 regulates cell proliferation and differentiation in adult mouse adult intestine

Background

Adult stem cells play an essential role in adult organ physiology and tissue repair and regeneration. While much has been learnt about the property and function of various adult stem cells, the mechanisms of their development remain poorly understood in mammals. Earlier studies suggest that the formation of adult mouse intestinal stem cells takes place during the first few weeks after birth, the postembryonic period when plasma thyroid hormone (T3) levels are high. Furthermore, deficiency in T3 signaling leads to defects in adult mouse intestine, including reduced cell proliferation in the intestinal crypts, where stem cells reside. Our earlier studies have shown that protein arginine methyltransferase 1 (PRMT1), a T3 receptor coactivator, is highly expressed during intestinal maturation in mouse.

Methods

We have analyzed the expression of PRMT1 by immunohistochemistry and studied the effect of tissue-specific knockout of PRMT1 in the intestinal epithelium.

Results

We show that PRMT1 is expressed highly in the proliferating transit amplifying cells and crypt base stem cells. By using a conditional knockout mouse line, we have demonstrated that the expression of PRMT1 in the intestinal epithelium is critical for the development of the adult mouse intestine. Specific removal of PRMT1 in the intestinal epithelium results in, surprisingly, more elongated adult intestinal crypts with increased cell proliferation. In addition, epithelial cell migration along the crypt-villus axis and cell death on the villus are also increased. Furthermore, there are increased Goblet cells and reduced Paneth cells in the crypt while the number of crypt base stem cells remains unchanged.

Conclusions

Our finding that PRMT1 knockout increases cell proliferation is surprising considering the role of PRMT1 in T3-signaling and the importance of T3 for intestinal development, and suggests that PRMT1 likely regulates pathways in addition to T3-signaling to affect intestinal development and/or homeostasis, thus affecting cell proliferating and epithelial turn over in the adult.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00627-z.

Murine intestinal stem cells are highly sensitive to modulation of the T3/TRα1-dependent pathway

The thyroid hormone T3 and its nuclear receptor TRα1 control gut development and homeostasis through the modulation of intestinal crypt cell proliferation. Despite increasing data, in-depth analysis on their specific action on intestinal stem cells is lacking. By using ex vivo 3D organoid cultures and molecular approaches, we observed early responses to T3 involving the T3-metabolizing enzyme Dio1 and the transporter Mct10, accompanied by a complex response of stem cell- and progenitor-enriched genes. Interestingly, specific TRα1 loss-of-function (inducible or constitutive) was responsible for low ex vivo organoid development and impaired stem cell activity. T3 treatment of animals in vivo not only confirmed the positive action of this hormone on crypt cell proliferation but also demonstrated its key action in modulating the number of stem cells, the expression of their specific markers and the commitment of progenitors into lineage-specific differentiation. In conclusion, T3 treatment or TRα1 modulation has a rapid and strong effect on intestinal stem cells, broadening our perspectives in the study of T3/TRα1-dependent signaling in these cells.

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

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

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

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

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

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

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

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

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.

Expression of hyaluronan synthases upregulated by thyroid hormone is involved in intestinal stem cell development during Xenopus laevis metamorphosis

During amphibian intestinal remodeling, thyroid hormone (TH) induces adult stem cells, which newly generate the absorptive epithelium analogous to the mammalian one. We have previously shown that hyaluronan (HA) is newly synthesized and plays an essential role in the development of the stem cells via its major receptor CD44 in the Xenopus laevis intestine. We here focused on HA synthase (HAS) and examined how the expression of HAS family genes is regulated during natural and TH-induced metamorphosis. Our quantitative RT-PCR analysis indicated that the mRNA expression of HAS2 and HAS3, but not that of HAS1 and HAS-rs, a unique Xenopus HAS-related sequence, is upregulated concomitantly with the development of adult epithelial primordia consisting of the stem/progenitor cells during the metamorphic climax. In addition, our in situ hybridization analysis indicated that the HAS3 mRNA is specifically expressed in the adult epithelial primordia, whereas HAS2 mRNA is expressed in both the adult epithelial primordia and nearby connective tissue cells during this period. Furthermore, by treating X. laevis tadpoles with 4-methylumbelliferone, a HA synthesis inhibitor, we have experimentally shown that inhibition of HA synthesis leads to suppression of TH-upregulated expression of leucine-rich repeat-containing G protein-coupled 5 (LGR5), an intestinal stem cell marker, CD44, HAS2, HAS3, and gelatinase A in vivo. These findings suggest that HA newly synthesized by HAS2 and/or HAS3 is required for intestinal stem cell development through a positive feedback loop and is involved in the formation of the stem cell niche during metamorphosis.

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.

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.

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

During development in all vertebrates, thyroid hormone receptors (TRs) are expressed before as well as during and after the peak in plasma thyroid hormone (TH) levels. Previously, we established a role for unliganded TRα in gene repression and developmental timing using tadpoles of TRα knockout (TRαKO) frogs. Here, we examined the role of liganded TRα on growth, development, and intestinal remodeling during natural and TH-induced metamorphosis. Disrupted TRα had little effect on growth during the larval period, but after metamorphosis, TRαKO juveniles grew more slowly than wild-type (WT) juveniles. TRαKO tadpoles developed faster throughout premetamorphosis when TH was low or absent, and despite their decreased responsivity to exogenous TH, TRαKO tadpoles not only were able to complete TH-dependent metamorphosis but also did so earlier than WT tadpoles. In contrast to external morphology, larval epithelial cell apoptosis and adult cell proliferation of intestinal remodeling were delayed in TRαKO tadpoles. Also, TRαKO intestines did not shrink in length to the full extent, and fewer intestinal folds into the lumen were present in TRαKO compared with WT juveniles. Such delayed remodeling occurred despite higher premetamorphic expression levels of TH target genes important for metamorphic progression-namely, TRβ, Klf9, and ST3. Furthermore, the decreased TH-dependent intestinal shrinkage was consistent with reduced TH response gene expression during natural and TH-induced metamorphosis. As in the TRα null mouse model, TRαKO frogs had statistically significant but surprisingly mild growth and development phenotypes with normal survival and fertility.

Frogs model man: In vivo thyroid hormone signaling during development

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

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.

The transcriptome of metamorphosing flatfish

BACKGROUND

Flatfish metamorphosis denotes the extraordinary transformation of a symmetric pelagic larva into an asymmetric benthic juvenile. Metamorphosis in vertebrates is driven by thyroid hormones (THs), but how they orchestrate the cellular, morphological and functional modifications associated with maturation to juvenile/adult states in flatfish is an enigma. Since THs act via thyroid receptors that are ligand activated transcription factors, we hypothesized that the maturation of tissues during metamorphosis should be preceded by significant modifications in the transcriptome. Targeting the unique metamorphosis of flatfish and taking advantage of the large size of Atlantic halibut (Hippoglossus hippoglossus) larvae, we determined the molecular basis of TH action using RNA sequencing.

RESULTS

De novo assembly of sequences for larval head, skin and gastrointestinal tract (GI-tract) yielded 90,676, 65,530 and 38,426 contigs, respectively. More than 57 % of the assembled sequences were successfully annotated using a multi-step Blast approach. A unique set of biological processes and candidate genes were identified specifically associated with changes in morphology and function of the head, skin and GI-tract. Transcriptome dynamics during metamorphosis were mapped with SOLiD sequencing of whole larvae and revealed greater than 8,000 differentially expressed (DE) genes significantly (p < 0.05) up- or down-regulated in comparison with the juvenile stage. Candidate transcripts quantified by SOLiD and qPCR analysis were significantly (r = 0.843; p < 0.05) correlated. The majority (98 %) of DE genes during metamorphosis were not TH-responsive. TH-responsive transcripts clustered into 6 groups based on their expression pattern during metamorphosis and the majority of the 145 DE TH-responsive genes were down-regulated.

CONCLUSIONS

A transcriptome resource has been generated for metamorphosing Atlantic halibut and over 8,000 DE transcripts per stage were identified. Unique sets of biological processes and candidate genes were associated with changes in the head, skin and GI-tract during metamorphosis. A small proportion of DE transcripts were TH-responsive, suggesting that they trigger gene networks, signalling cascades and transcription factors, leading to the overt changes in tissue occurring during metamorphosis.

Thyroid hormone activates Wnt/β-catenin signaling involved in adult epithelial development during intestinal remodeling in Xenopus laevis

During amphibian intestinal remodeling, thyroid hormone (TH) induces some larval epithelial cells to dedifferentiate into adult stem cells, which newly generate the absorptive epithelium analogous to the mammalian epithelium. To clarify molecular mechanisms underlying adult epithelial development, we here focus on TH response genes that are associated with the canonical Wnt pathway. Our quantitative reverse transcription plus polymerase chain reaction and immunohistochemical analyses indicate that all of the genes examined, including β-catenin, c-Myc and secreted frizzle-related protein 2 (SFRP2), are up-regulated in Xenopus laevis intestine during both natural and TH-induced metamorphosis. Moreover, immunoreactivity for nuclear β-catenin becomes detectable in adult stem cells from the start of their appearance and then increases in intensity in adult epithelial primordia derived from the stem cells, which actively proliferate and coexpress Wnt target genes c-Myc and LGR5. These expression profiles strongly suggest the involvement of the canonical Wnt pathway in the maintenance and/or proliferation of adult stem/progenitor cells. More importantly, by using organ cultures of the tadpole intestine, we have experimentally shown that the addition of exogenous SFRP2 protein to the culture medium promotes cell proliferation of the adult epithelial primordia, whereas inhibition of endogenous SFRP2 by its antibody suppresses their proliferation. The inhibition of SFRP2 suppresses larval epithelial changes in shape from simple columnar to stem-cell-like roundish cells, resulting in the failure of epithelial dedifferentiation. Thus, TH-up-regulated SFRP2 in the postembryonic intestine promotes adult stem cell development, possibly by acting as an agonist of both canonical and non-canonical Wnt signaling.

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.

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

Epigenetic modifications of histones are emerging as key factors in gene regulation by diverse transcription factors. Their roles during vertebrate development and pathogenesis are less clear. The causative effect of thyroid hormone (T3) on amphibian metamorphosis and the ability to manipulate this process for molecular and genetic studies have led to the demonstration that T3 receptor (TR) is necessary and sufficient for Xenopus metamorphosis, a process that resembles the postembryonic development (around birth) in mammals. Importantly, analyses during metamorphosis have provided some of the first in vivo evidence for the involvement of histone modifications in gene regulation by TR during vertebrate development. Furthermore, expression and functional studies suggest that various histone modifying epigenetic enzymes likely participate in multiple steps during the formation of adult intestinal stem cells during metamorphosis. The similarity between intestinal remodeling and the maturation of the mammalian intestine around birth when T3 levels are high suggests conserved roles for the epigenetic enzymes in mammalian adult intestinal stem cell development and/or proliferation.

Thyroid hormone-regulated Wnt5a/Ror2 signaling is essential for dedifferentiation of larval epithelial cells into adult stem cells in the Xenopus laevis intestine

Background and Aims

Amphibian intestinal remodeling, where thyroid hormone (T3) induces some larval epithelial cells to become adult stem cells analogous to the mammalian intestinal ones, serves as a unique model for studying how the adult stem cells are formed. To clarify its molecular mechanisms, we here investigated roles of non-canonical Wnt signaling in the larval-to-adult intestinal remodeling during Xenopus laevis metamorphosis.

Methods/Findings

Our quantitative RT-PCR (qRT-PCR) and immunohistochemical analyses indicated that the expressions of Wnt5a and its receptors, frizzled 2 (Fzd2) and receptor tyrosine kinase-like orphan receptor 2 (Ror2) are up-regulated by T3 and are spatiotemporally correlated with adult epithelial development in the X. laevis intestine. Notably, changes in morphology of larval absorptive epithelial cells expressing Ror2 coincide well with formation of the adult stem cells during metamorphosis. In addition, by using organ cultures of the tadpole intestine, we have experimentally shown that addition of exogenous Wnt5a protein to the culture medium causes morphological changes in the larval epithelium expressing Ror2 even in the absence of T3. In contrast, in the presence of T3 where the adult stem cells are formed in vitro, inhibition of endogenous Wnt5a by an anti-Wnt5a antibody suppressed the epithelial morphological changes, leading to the failure of stem cell formation.

Significance

Our findings strongly suggest that the adult stem cells originate from the larval absorptive cells expressing Ror2, which require Wnt5a/Ror2 signaling for their dedifferentiation accompanied by changes in cell morphology.

Thyroid hormones and their nuclear receptors: new players in intestinal epithelium stem cell biology?

Thyroid hormones participate in the development and homeostasis of several organs and tissues. It is well documented that they act via nuclear receptors, the TRs, which are transcription factors whose function is modulated by the hormone T3. Importantly, T3-induced physiological response within a cell depends on the specific TR expression and on the T3 bioavailability. However, in addition to this T3-dependent control of TR functionality, increasing data show that the action of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. By focusing on the intestinal epithelium of both amphibians and mammals we summarize here new data in support of a role for thyroid hormones and the TR nuclear receptors in stem cell biology. This new concept may be extended to other organs and have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer.

Postembryonic organogenesis of the digestive tube: why does it occur in worms and sea cucumbers but fail in humans?

We provide an integrative view of mechanisms that enable regeneration of the digestive tube in various animal models, including vertebrates, tunicates, echinoderms, insects, and flatworms. Two main strategies of regeneration of the endodermal luminal (mucosal) epithelium have evolved in metazoans. One of them involves proliferation of resident epithelial cells, while the other relies on recruitment of cells from extramucosal sources. In any of these two scenarios, either pluri-/multipotent stem cells or specialized differentiated cells can be used as the starting material. Posttraumatic visceral regeneration shares some common mechanisms with normal embryonic development as well as with organ homeostatic maintenance, but there are signaling pathways and/or cellular pools that are specific to the regenerative phenomena. Comparative analysis of the literature suggests that mammals share with spontaneously regenerating animals many of the regeneration-related adaptations and are able to efficiently repair components of their digestive tube at the level of individual tissues, but fail to do so at the whole-organ scale. We review what might cause this failure in the context of the current state of knowledge about various regenerative models.

Differential regulation of two histidine ammonia-lyase genes during Xenopus development implicates distinct functions during thyroid hormone-induced formation of adult stem cells

Background

Organ-specific, adult stem cells are essential for organ-homeostasis and tissue repair and regeneration. The formation of such stem cells during vertebrate development remains to be investigated. Frog metamorphosis offers an excellent opportunity to study the formation of adult stem cells as this process involves essentially the transformations of all larval tissues/organs into the adult form. Of particular interest is the remodeling of the intestine. 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 through dedifferentiation of some larval epithelial cells. A major advantage of this metamorphosis model is its total dependence on thyroid hormone (T3). In an effort to identify genes that are important for stem cell development, we have previously carried out tissue-specific microarray analysis of intestinal gene expression during Xenopus laevis metamorphosis.

Results

We report the detailed characterization of one of the genes thus identified, the histidine ammonia-lyase (HAL) gene, which encodes an enzyme known as histidase or histidinase. We show that there are two duplicated HAL genes, HAL1 and HAL2, in both Xenopus laevis and Xenopus tropicalis, a highly related but diploid species. Interestingly, only HAL2 is highly upregulated by T3 and appears to be specifically expressed in the adult intestinal progenitor/stem cells while HAL1 is not expressed in the intestine during metamorphosis. Furthermore, when analyzed in whole animals, HAL1 appears to be expressed only during embryogenesis but not metamorphosis while the opposite appears to be true for HAL2.

Conclusions

Our results suggest that the duplicated HAL genes have distinct functions with HAL2 likely involved in the formation and/or proliferation of the adult stem cells during metamorphosis.

Expression profiling of intestinal tissues implicates tissue-specific genes and pathways essential for thyroid hormone-induced adult stem cell development

The study of the epithelium during development in the vertebrate intestine touches upon many contemporary aspects of biology: to name a few, the formation of the adult stem cells (ASCs) essential for the life-long self-renewal and the balance of stem cell activity for renewal vs cancer development. Although extensive analyses have been carried out on the property and functions of the adult intestinal stem cells in mammals, little is known about their formation during development due to the difficulty of manipulating late-stage, uterus-enclosed embryos. The gastrointestinal tract of the amphibian Xenopus laevis is an excellent model system for the study of mammalian ASC formation, cell proliferation, and differentiation. During T3-dependent amphibian metamorphosis, the digestive tract is extensively remodeled from the larval to the adult form for the adaptation of the amphibian from its aquatic herbivorous lifestyle to that of a terrestrial carnivorous frog. This involves de novo formation of ASCs that requires T3 signaling in both the larval epithelium and nonepithelial tissues. To understand the underlying molecular mechanisms, we have characterized the gene expression profiles in the epithelium and nonepithelial tissues by using cDNA microarrays. Our results revealed that T3 induces distinct tissue-specific gene regulation programs associated with the remodeling of the intestine, particularly the formation of the ASCs, and further suggested the existence of potentially many novel stem cell-associated genes, at least in the intestine during development.

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.

Hormonal control of stem cell systems

Many organs respond to physiological challenges by changing tissue size or composition. Such changes may originate from tissue-specific stem cells and their supportive environment (niche). The endocrine system is a major effector and conveyor of physiological changes and as such could alter stem cell behavior in various ways. In this review, we examine how hormones affect stem cell biology in four different organs: the ovary, intestine, hematopoietic system, and mammary gland. Hormones control every stage of stem cell life, including establishment, expansion, maintenance, and differentiation. The effects can be cell autonomous or non-cell autonomous through the niche. Moreover, a single hormone can affect different stem cells in different ways or affect the same stem cell differently at various developmental times. The vast complexity and diversity of stem cell responses to hormonal cues allow hormones to coordinate the body's reaction to physiological challenges.

Tissue-specific upregulation of MDS/EVI gene transcripts in the intestine by thyroid hormone during Xenopus metamorphosis

Background

Intestinal remodeling during amphibian metamorphosis resembles the maturation of the adult intestine during mammalian postembryonic development when the adult epithelial self-renewing system is established under the influence of high concentrations of plasma thyroid hormone (T3). This process involves de novo formation and subsequent proliferation and differentiation of the adult stem cells.

Methodology/Principal Findings

The T3-dependence of the formation of adult intestinal stem cell during Xenopus laevis metamorphosis offers a unique opportunity to identify genes likely important for adult organ-specific stem cell development. We have cloned and characterized the ectopic viral integration site 1 (EVI) and its variant myelodysplastic syndrome 1 (MDS)/EVI generated via transcription from the upstream MDS promoter and alternative splicing. EVI and MDS/EVI have been implicated in a number of cancers including breast, leukemia, ovarian, and intestinal cancers. We show that EVI and MDS/EVI transcripts are upregulated by T3 in the epithelium but not the rest of the intestine in Xenopus laevis when adult stem cells are forming in the epithelium.

Conclusions/Significance

Our results suggest that EVI and MDS/EVI are likely involved in the development and/or proliferation of newly forming adult intestinal epithelial 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.

Thyroid hormone regulation of adult intestinal stem cell development: mechanisms and evolutionary conservations

The adult mammalian intestine has long been used as a model to study adult stem cell function and tissue renewal as the intestinal epithelium is constantly undergoing self-renewal throughout adult life. This is accomplished through the proliferation and subsequent differentiation of the adult stem cells located in the crypt. The development of this self-renewal system is, however, poorly understood. A number of studies suggest that the formation/maturation of the adult intestine is conserved in vertebrates and depends on endogenous thyroid hormone (T3). In amphibians such as Xenopus laevis, the process takes place during metamorphosis, which is totally dependent upon T3 and resembles postembryonic development in mammals when T3 levels are also high. During metamorphosis, the larval epithelial cells in the tadpole intestine undergo apoptosis and concurrently, adult epithelial stem/progenitor cells are formed de novo, which subsequently lead to the formation of a trough-crest axis of the epithelial fold in the frog, resembling the crypt-villus axis in the adult mammalian intestine. Here we will review some recent molecular and genetic studies that support the conservation of the development of the adult intestinal stem cells in vertebrates. We will discuss the mechanisms by which T3 regulates this process via its nuclear receptors.

Cytological and morphological analyses reveal distinct features of intestinal development during Xenopus tropicalis metamorphosis

Background

The formation and/or maturation of adult organs in vertebrates often takes place during postembryonic development, a period around birth in mammals when thyroid hormone (T3) levels are high. The T3-dependent anuran metamorphosis serves as a model to study postembryonic development. Studies on the remodeling of the intestine during Xenopus (X.) laevis metamorphosis have shown that the development of the adult intestine involves de novo formation of adult stem cells in a process controlled by T3. On the other hand, X. tropicalis, highly related to X. laevis, offers a number of advantages for studying developmental mechanisms, especially at genome-wide level, over X. laevis, largely due to its shorter life cycle and sequenced genome. To establish X. tropicalis intestinal metamorphosis as a model for adult organogenesis, we analyzed the morphological and cytological changes in X. tropicalis intestine during metamorphosis.

Methodology/Principal Findings

We observed that in X. tropicalis, the premetamorphic intestine was made of mainly a monolayer of larval epithelial cells surrounded by little connective tissue except in the single epithelial fold, the typhlosole. During metamorphosis, the larval epithelium degenerates and adult epithelium develops to form a multi-folded structure with elaborate connective tissue and muscles. Interestingly, typhlosole, which is likely critical for adult epithelial development, is present along the entire length of the small intestine in premetamorphic tadpoles, in contrast to X. laevis, where it is present only in the anterior 1/3. T3-treatment induces intestinal remodeling, including the shortening of the intestine and the typhlosole, just like in X. laevis.

Conclusions/Significance

Our observations indicate that the intestine undergoes similar metamorphic changes in X. laevis and X. tropicalis, making it possible to use the large amount of information available on X. laevis intestinal metamorphosis and the genome sequence information and genetic advantages of X. tropicalis to dissect the pathways governing adult intestinal development.

Direct activation of Xenopus iodotyrosine deiodinase by thyroid hormone receptor in the remodeling intestine during amphibian metamorphosis

Thyroid hormone (TH) plays critical roles during vertebrate postembryonic development. TH production in the thyroid involves incorporating inorganic iodide into thyroglobulin. The expression of iodotyrosine deiodinase (IYD; also known as iodotyrosine dehalogenase 1) in the thyroid gland ensures efficient recycling of iodine from the byproducts of TH biosynthesis: 3'-monoiodotyrosine and 3', 5'-diiodotyrosine. Interestingly, IYD is known to be expressed in other organs in adult mammals, suggesting iodine recycling outside the thyroid. On the other hand, the developmental role of iodine recycling has yet to be investigated. Here, using intestinal metamorphosis as a model, we discovered that the Xenopus tropicalis IYD gene is strongly up-regulated by TH during metamorphosis in the intestine but not the tail. We further demonstrated that this induction was one of the earliest events during intestinal metamorphosis, with IYD being activated directly through the binding of liganded TH receptors to a TH response element in the IYD promoter region. Because iodide is mainly taken up from the diet in the intestine and the tadpole stops feeding during metamorphosis when the intestine is being remodeled, our findings suggest that IYD transcription is activated by liganded TH receptors early during intestinal remodeling to ensure efficient iodine recycling at the climax of metamorphosis when highest levels of TH are needed for the proper transformations of different organs.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone activates protein arginine methyltransferase 1 expression by directly inducing c-Myc transcription during Xenopus intestinal stem cell development

Adult organ-specific stem cells are essential for organ homeostasis and tissue repair and regeneration. The formation of such stem cells during vertebrate development is poorly understood. Intestinal remodeling during thyroid hormone (T3)-dependent Xenopus metamorphosis resembles postembryonic intestinal maturation in mammals. During metamorphosis, the intestine is remodeled de novo via a yet unknown mechanism. Protein arginine methyltransferase 1 (PRMT1) is up-regulated in and required for adult intestinal stem cells during metamorphosis. PRMT1 up-regulation is the earliest known molecular event for the developing stem cells and is also conserved during zebrafish and mouse intestinal development. To analyze how PRMT1 is specifically up-regulated during the formation of the adult intestinal stem cells, we cloned the Xenopus PRMT1 promoter and characterized it in CaCo-2 cells, a human cell line with intestinal stem cell characteristics. Through a series deletion and mutational analyses, we showed that the stem cell-associated transcription factor c-Myc could bind to a conserved site in the first intron to activate the promoter. Furthermore, we demonstrated that during metamorphosis, both c-Myc and PRMT1 were highly up-regulated, specifically in the remodeling intestine but not the resorbing tail, and that c-Myc was induced by T3 prior to PRMT1 up-regulation. In addition, we showed that T3 directly activated the c-Myc gene during metamorphosis in the intestine via binding of the T3 receptor to the c-Myc promoter. These results suggest that T3 induces c-Myc transcription directly in the intestine, that c-Myc, in turn, activates PRMT1 expression, and that this is an important gene regulation cascade controlling intestinal stem cell development.

Liganded thyroid hormone receptor induces nucleosome removal and histone modifications to activate transcription during larval intestinal cell death and adult stem cell development

Thyroid hormone (T(3)) plays an important role in regulating multiple cellular and metabolic processes, including cell proliferation, cell death, and energy metabolism, in vertebrates. Dysregulation of T(3) signaling results in developmental abnormalities, metabolic defects, and even cancer. We used T(3)-dependent Xenopus metamorphosis as a model to study how T(3) regulates transcription during vertebrate development. T(3) exerts its metamorphic effects through T(3) receptors (TR). TR recruits, in a T(3)-dependent manner, cofactor complexes that can carry out chromatin remodeling/histone modifications. Whether and how histone modifications change upon gene regulation by TR during vertebrate development is largely unknown. Here we analyzed histone modifications at T(3) target genes during intestinal metamorphosis, a process that involves essentially total apoptotic degeneration of the simple larval epithelium and de novo development of the adult epithelial stem cells, followed by their proliferation and differentiation into the complex adult epithelium. We demonstrated for the first time in vivo during vertebrate development that TR induces the removal of core histones at the promoter region and the recruitment of RNA polymerase. Furthermore, a number of histone activation and repression marks have been defined based on correlations with mRNA levels in cell cultures. Most but not all correlate with gene expression induced by liganded TR during development, suggesting that tissue and developmental context influences the roles of histone modifications in gene regulation. Our findings provide important mechanistic insights on how chromatin remodeling affects developmental gene regulation in vivo.

Evolutionary insights into postembryonic development of adult intestinal stem cells

In the adult vertebrate intestine, multi-potent stem cells continuously generate all of the epithelial cells throughout the adulthood. While it has long been known that the frog intestine is formed via the development of adult intestinal stem cells during thyroid hormone (TH)-dependent metamorphosis, the basic structure of the adult intestine is formed by birth in mammals and it is unclear if the subsequent maturation of the intestine involves any changes in the intestinal stem cells. Two recent papers showing that B lymphocyte-induced maturation protein 1 (Blimp1) regulates postnatal epithelial stem cell reprogramming during mouse intestinal maturation support the model that adult intestinal stem cells are developed during postembryonic development in mammals, in a TH-dependent process similar to intestinal remodeling during amphibian metamorphosis. Since the formation of the adult intestine in both mammals and amphibians is closely associated with the adaptation from aquatic to terrestrial life during the peak of endogenous TH levels, the molecular mechanisms by which the adult stem cells are developed are likely evolutionally conserved.

Thyroid hormone-regulated expression of nuclear lamins correlates with dedifferentiation of intestinal epithelial cells during Xenopus laevis metamorphosis

In the Xenopus laevis intestine during metamorphosis, which is triggered by thyroid hormone (TH), the adult epithelium develops and replaces the larval one undergoing apoptosis. We have previously shown that progenitor/stem cells of the adult epithelium originate from some differentiated larval epithelial cells. To investigate molecular mechanisms underlying larval epithelial dedifferentiation into the adult progenitor/stem cells, we here focused on nuclear lamin A (LA) and lamin LIII (LIII), whose expression is generally known to be correlated with the state of cell differentiation. We analyzed the spatiotemporal expression of LA and LIII during X. laevis intestinal remodeling by reverse transcription PCR, Western blotting, and immunohistochemistry. At the onset of natural metamorphosis, when the adult epithelial progenitor cells appear as small islets, the expression of LA is down-regulated, but that of LIII is up-regulated only in the islets. Then, as the adult progenitor cells differentiate, the expression of LA is up-regulated, whereas that of LIII is down-regulated in the adult cells. As multiple intestinal folds form, adult epithelial cells positive for LIII become restricted only to the troughs of the folds. In addition, we have shown that TH up- or down-regulates the expression of these lamins in the premetamorphic intestine as during natural metamorphosis. These results indicate that TH-regulated expression of LA and LIII closely correlates with dedifferentiation of the epithelial cells in the X. laevis intestine, suggesting the involvement of the lamins in the process of dedifferentiation during amphibian metamorphosis.