Regulation of adult intestinal epithelial stem cell development by thyroid hormone duringXenopus laevis metamorphosis

Comparisons of cell proliferation and cell death from tornaria larva to juvenile worm in the hemichordate Schizocardium californicum

Background

There are a wide range of developmental strategies in animal phyla, but most insights into adult body plan formation come from direct-developing species. For indirect-developing species, there are distinct larval and adult body plans that are linked together by metamorphosis. Some outstanding questions in the development of indirect-developing organisms include the extent to which larval tissue undergoes cell death during the process of metamorphosis and when and where the tissue that will give rise to the adult originates. How do the processes of cell division and cell death redesign the body plans of indirect developers? In this study, we present patterns of cell proliferation and cell death during larval body plan development, metamorphosis, and adult body plan formation, in the hemichordate Schizocardium californium (Cameron and Perez in Zootaxa 3569:79–88, 2012) to answer these questions.

Results

We identified distinct patterns of cell proliferation between larval and adult body plan formation of S. californicum. We found that some adult tissues proliferate during the late larval phase prior to the start of overt metamorphosis. In addition, using an irradiation and transcriptomic approach, we describe a genetic signature of proliferative cells that is shared across the life history states, as well as markers that are unique to larval or juvenile states. Finally, we observed that cell death is minimal in larval stages but begins with the onset of metamorphosis.

Conclusions

Cell proliferation during the development of S. californicum has distinct patterns in the formation of larval and adult body plans. However, cell death is very limited in larvae and begins during the onset of metamorphosis and into early juvenile development in specific domains. The populations of cells that proliferated and gave rise to the larvae and juveniles have a genetic signature that suggested a heterogeneous pool of proliferative progenitors, rather than a set-aside population of pluripotent cells. Taken together, we propose that the gradual morphological transformation of S. californicum is mirrored at the cellular level and may be more representative of the development strategies that characterize metamorphosis in many metazoan animals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-022-00198-1.

Amphibian Hormones, Calcium Physiology, Bone Weight, and Lung Use Call for a More Inclusive Approach to Understanding Ossification Sequence Evolution

Skeleton plays a huge role in understanding how vertebrate animals have diversified in phylogeny, ecology and behavior. Recent evo-devo research has used ossification sequences to compare skeletal development among major groups, to identify conserved and labile aspects of a sequence within a group, to derive ancestral and modal sequences, and to look for modularity based on embryonic origin and type of bone. However, questions remain about how to detect and order bone appearances, the adaptive significance of ossification sequences and their relationship to adult function, and the utility of categorizing bones by embryonic origin and type. Also, the singular focus on bone appearances and the omission of other tissues and behavioral, ecological and life history events limit the relevance of such analyses. Amphibians accentuate these concerns because of their highly specialized biphasic life histories and the exceptionally late timing, and high variability of their ossification sequences. Amphibians demonstrate a need for a whole-animal, whole-ontogeny approach that integrates the entire ossification process with physiology, behavior and ecology. I discuss evidence and hypotheses for how hormone mediation and calcium physiology might elicit non-adaptive variability in ossification sequence, and for adaptive strategies to partition larval habitats using bone to offset the buoyancy created by lung use. I also argue that understanding plasticity in ossification requires shifting focus away from embryonic development and adult function, and toward postembryonic mechanisms of regulating skeletal growth, especially ones that respond directly to midlife environments and behaviors.

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

Background

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

Results

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

Conclusions

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

EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate development

The gene ectopic viral integration site 1 (EVI) and its variant myelodysplastic syndrome 1 (MDS)/EVI encode zinc-finger proteins that have been recognized as important oncogenes in various types of cancer. In contrast to the established role of EVI and MDS/EVI in cancer development, their potential function during vertebrate postembryonic development, especially in organ-specific adult stem cells, is unclear. Amphibian metamorphosis is strikingly similar to postembryonic development around birth in mammals, with both processes taking place when plasma thyroid hormone (T3) levels are high. Using the T3-dependent metamorphosis in Xenopus tropicalis as a model, we show here that high levels of EVI and MDS/EVI are expressed in the intestine at the climax of metamorphosis and are induced by T3. By using the transcription activator-like effector nuclease gene editing technology, we have knocked out both EVI and MDS/EVI and have shown that EVI and MDS/EVI are not essential for embryogenesis and premetamorphosis in X. tropicalis On the other hand, knocking out EVI and MDS/EVI causes severe retardation in the growth and development of the tadpoles during metamorphosis and leads to tadpole lethality at the climax of metamorphosis. Furthermore, the homozygous-knockout animals have reduced adult intestinal epithelial stem cell proliferation at the end of metamorphosis (for the few that survive through metamorphosis) or during T3-induced metamorphosis. These findings reveal a novel role of EVI and/or MDS/EVI in regulating the formation and/or proliferation of adult intestinal adult stem cells during postembryonic development in vertebrates.-Okada, M., Shi, Y.-B. EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate 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.

Neuropeptide Y-Induced Orexigenic Action Is Attenuated by the Orexin Receptor Antagonist in Bullfrog Larvae

In bullfrog larvae at the pre- and pro-metamorphic stages, feeding behavior is regulated by appetite factors such as orexigenic peptides. In fact, food intake is enhanced by intracerebroventricular (ICV) administration of neuropeptide Y (NPY) and orexin A. Using goldfish, our previous study indicated that the orexigenic action of NPY is mediated by orexin A, suggesting the functional interaction between the two. However, there is little information about whether the action of orexin A mediates the orexigenic action of NPY in bullfrog larvae. Therefore, we examined the effect of the orexin receptor antagonist, SB334867 on the orexigenic action of NPY in larvae. The stimulatory effect of ICV injection of NPY at 10 pmol/g body weight (BW) on food intake was abolished by treatment with SB334867 at 60 pmol/g BW. These results suggest that, in bullfrog larvae, there is a neuronal relationship between the NPY and orexin systems, and that the orexigenic action of NPY is mediated by the orexin A-induced orexigenic action.

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.

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.

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.

Orexin A enhances food intake in bullfrog larvae

Orexin is a potent orexigenic peptide implicated in appetite regulation in rodents. However, except for teleost fish, the involvement of orexin in the regulation of feeding in non-mammalian vertebrates has not been well studied. Anuran amphibian larvae feed and grow during the pre- and prometamorphic stages. Therefore, orexigenic factors seem to play important roles in growing larvae. Indeed, our recent studies have demonstrated that neuropeptide Y and ghrelin exert orexigenic actions in bullfrog larvae during the prometamorphic stages. In this study, we examined the effect of intracerebroventricular (ICV) administration of synthetic orexin A on food intake in bullfrog larvae at the prometamorphic stages. Food intake was significantly increased by ICV administration of orexin A (at 6 pmol/g BW) during a 15-min observation period. The orexigenic action of orexin A at 6 pmol/g BW was blocked by treatment with an orexin receptor antagonist, SB334867, at 60 pmol/g BW. These results indicate that orexin A acts as an orexigenic factor in bullfrog larvae.

Ultrastructural and molecular changes in the developing small intestine of the toad Bufo regularis

The ontogenetic development of the small intestine of the toad Bufo regularis was investigated using twofold approaches, namely, ultrastructural and molecular. The former has been done using transmission electron microscope and utilizing the developmental stages 42, 50, 55, 60, 63, and 66. The most prominent ultrastructural changes were recorded at stage 60 and were more evident at stage 63. These included the appearance of apoptotic bodies/nuclei within the larval epithelium, the presence of macrophages, swollen mitochondria, distorted rough endoplasmic reticulum, chromatin condensation, and irregular nuclear envelop, and the presence of large vacuoles and lysosomes. The molecular investigation involved examining DNA content and fragmentation. The results showed that the DNA content decreased significantly during the metamorphic stages 60 and 63 compared with both larval (50 and 55) and postmetamorphic (66) stages. The metamorphic stages (60 and 63) displayed extensive DNA laddering compared with stages 50, 55, and 66. The percentage of DNA damage was 0.00%, 12.91%, 57.26%, 45.48%, and 4.43% for the developmental stages 50, 55, 60, 63, and 66, respectively. In conclusion, the recorded remodeling of the small intestine represents a model for clarifying the mechanism whereby cell death and proliferation are controlled.

Functional modifications associated with gastrointestinal tract organogenesis during metamorphosis in Atlantic halibut (Hippoglossus hippoglossus)

BACKGROUND

Flatfish metamorphosis is a hormone regulated post-embryonic developmental event that transforms a symmetric larva into an asymmetric juvenile. In altricial-gastric teleost fish, differentiation of the stomach takes place after the onset of first feeding, and during metamorphosis dramatic molecular and morphological modifications of the gastrointestinal (GI-) tract occur. Here we present the functional ontogeny of the developing GI-tract from an integrative perspective in the pleuronectiforme Atlantic halibut, and test the hypothesis that the multiple functions of the teleost stomach develop synchronously during metamorphosis.

RESULTS

Onset of gastric function was determined with several approaches (anatomical, biochemical, molecular and in vivo observations). In vivo pH analysis in the GI-tract lumen combined with quantitative PCR (qPCR) of α and β subunits of the gastric proton pump (H+/K+-ATPase) and pepsinogen A2 indicated that gastric proteolytic capacity is established during the climax of metamorphosis. Transcript abundance of ghrelin, a putative orexigenic signalling molecule produced in the developing stomach, correlated (p < 0.05) with the emergence of gastric proteolytic activity, suggesting that the stomach's role in appetite regulation occurs simultaneously with the establishment of proteolytic function. A 3D models series of the GI-tract development indicated a functional pyloric sphincter prior to first feeding. Observations of fed larvae in vivo confirmed that stomach reservoir function was established before metamorphosis, and was thus independent of this event. Mechanical breakdown of food and transportation of chyme through the GI-tract was observed in vivo and resulted from phasic and propagating contractions established well before metamorphosis. The number of contractions in the midgut decreased at metamorphic climax synchronously with establishment of the stomach's proteolytic capacity and its increased peristaltic activity. Putative osmoregulatory competence of the GI-tract, inferred by abundance of Na+/K+-ATPase α transcripts, was already established at the onset of exogenous feeding and was unmodified by metamorphosis.

CONCLUSIONS

The functional specialization of the GI-tract was not exclusive to metamorphosis, and its osmoregulatory capacity and reservoir function were established before first feeding. Nonetheless, acid production and the proteolytic capacity of the stomach coincided with metamorphic climax, and also marked the onset of the stomach's involvement in appetite regulation via ghrelin.

Stimulatory effect of ghrelin on food intake in bullfrog larvae

Ghrelin is a potent orexigenic peptide implicated in appetite regulation in rodents. However, except for teleost fish, the involvement of ghrelin in the regulation of feeding in non-mammalian vertebrates has not been well studied. Anuran amphibian larvae feed and grow during the pre- and prometamorphic stages, but, thereafter they stop feeding as the metamorphic climax approaches. Therefore, orexigenic factors seem to play important roles in growing larvae. In the present study, we examined the effect of intraperitoneal (IP) or intracerebroventricular (ICV) administration of synthetic bullfrog ghrelin (n-octanoylated 28-amino acid form) on food intake in larvae at the prometamorphic stages. Cumulative food intake was significantly increased by IP (8 and 16pmol/g body weight (BW)) or ICV (0.5 and 1pmol/g BW) administration of ghrelin during a 15-min observation period. The orexigenic action of ghrelin at 8pmol/g BW (IP) or at 0.5pmol/g BW (ICV) was blocked by treatment with a growth hormone secretagogue-receptor antagonist, [D-Lys(3)]GHRP-6 at 80pmol/g BW (IP) or at 5pmol/g BW (ICV). We then investigated the effect of feeding status on expression levels of the ghrelin transcript in the hypothalamus and gastrointestinal tract. Ghrelin mRNA levels in both were decreased 15 and 60min after feeding. These results indicate that ghrelin acts as an orexigenic factor in bullfrog larvae.

Effect of neuropeptide Y on food intake in bullfrog larvae

Neuropeptide Y (NPY) is a potent orexigenic neuropeptide implicated in appetite regulation in mammals. However, except for teleost fish such as the goldfish and zebrafish, the involvement of NPY in the regulation of feeding in non-mammalian vertebrates has not been well studied. Anuran amphibian larvae feed and grow during the pre- and pro-metamorphic stages, but, thereafter they stop feeding as the metamorphic climax approaches. Therefore, orexigenic factors seem to play important roles in pre- and pro-metamorphic larvae. We investigated the role of NPY in food intake using bullfrog larvae including pre- and pro-metamorphic stages, and examined the effect of feeding status on the expression level of the NPY transcript in the hypothalamus. NPY mRNA levels in hypothalamus specimens obtained from larvae that had been fasted for 3 days were higher than those in larvae that had been fed normally. We then investigated the effect of intracerebroventricular (ICV) administration of NPY on food intake in the larvae. Cumulative food intake was significantly increased by ICV administration of NPY (5 and 10 pmol/g body weight, BW) during a 15-min observation period. The NPY-induced orexigenic action (10 pmol/g BW) was blocked by treatment with a NPY Y1 receptor antagonist, BIBP-3226 (100 pmol/g BW). These results indicate that NPY acts as an orexigenic factor in bullfrog larvae.

Migration of Drosophila intestinal stem cells across organ boundaries

All components of the Drosophila intestinal tract, including the endodermal midgut and ectodermal hindgut/Malpighian tubules, maintain populations of dividing stem cells. In the midgut and hindgut, these stem cells originate from within larger populations of intestinal progenitors that proliferate during the larval stage and form the adult intestine during metamorphosis. The origin of stem cells found in the excretory Malpighian tubules ('renal stem cells') has not been established. In this paper, we investigate the migration patterns of intestinal progenitors that take place during metamorphosis. Our data demonstrate that a subset of adult midgut progenitors (AMPs) move posteriorly to form the adult ureters and, consecutively, the renal stem cells. Inhibiting cell migration by AMP-directed expression of a dominant-negative form of Rac1 protein results in the absence of stem cells in the Malpighian tubules. As the majority of the hindgut progenitor cells migrate posteriorly and differentiate into hindgut enterocytes, a group of the progenitor cells, unexpectedly, invades anteriorly into the midgut territory. Consequently, these progenitor cells differentiate into midgut enterocytes. The midgut determinant GATAe is required for the differentiation of midgut enterocytes derived from hindgut progenitors. Wingless signaling acts to balance the proportion of hindgut progenitors that differentiate as midgut versus hindgut enterocytes. Our findings indicate that a stable boundary between midgut and hindgut/Malpighian tubules is not established during early embryonic development; instead, pluripotent progenitor populations cross in between these organs in both directions, and are able to adopt the fate of the organ in which they come to reside.

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.

Genetic control of intestinal stem cell specification and development: a comparative view

Stem cells of the adult vertebrate intestine (ISCs) are responsible for the continuous replacement of intestinal cells, but also serve as site of origin of intestinal neoplasms. The interaction between multiple signaling pathways, including Wnt/Wg, Shh/Hh, BMP, and Notch, orchestrate mitosis, motility, and differentiation of ISCs. Many fundamental questions of how these pathways carry out their function remain unanswered. One approach to gain more insight is to look at the development of stem cells, to analyze the "programming" process which these cells undergo as they emerge from the large populations of embryonic progenitors. This review intends to summarize pertinent data on vertebrate intestinal stem cell biology, to then take a closer look at recent studies of intestinal stem cell development in Drosophila. Here, stem cell pools and their niche environment consist of relatively small numbers of cells, and questions concerning the pattern of cell division, niche-stem cell contacts, or differentiation can be addressed at the single cell level. Likewise, it is possible to analyze the emergence of stem cells during development more easily than in vertebrate systems: where in the embryo do stem cells arise, what structures in their environment do they interact with, and what signaling pathways are active sequentially as a result of these interactions. Given the high degree of conservation among genetic mechanisms controlling stem cell behavior in all animals, findings in Drosophila will provide answers that inform research in the vertebrate stem cell field.

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.

Spatiotemporal expression profile of no29/nucleophosmin3 in the intestine of Xenopus laevis during metamorphosis

A Xenopus laevis homolog of nucleophosmin/nucleoplasmin3 (NPM3), no29, has been previously identified as a thyroid hormone (TH)-response gene during TH-induced metamorphosis. X. laevis has another NPM3 homolog (npm3) in the pseudo-tetraploid genome, whereas X. tropicalis possesses one ortholog in the diploid genome. To assess the possible roles of these NPM3 homologs in amphibian metamorphosis, we have analyzed their expression profiles in X. laevis tadpoles. Levels of no29 and npm3 mRNA are rapidly up-regulated by exogenous TH in various organs of the premetamorphic tadpoles. Notably, in the small intestine, no29 and npm3 mRNA levels are transiently up-regulated during metamorphic climax, when progenitor/stem cells of the adult epithelium appear and actively proliferate. In situ hybridization analysis has revealed that the no29 transcript is specifically localized in adult epithelial progenitor/stem cells of the intestine during natural and TH-induced metamorphosis. Double-staining for in situ hybridization and immunohistochemistry has shown co-expression of no29 mRNA and no38 protein (an ortholog of NPM1), which is known to interact with NPM3 and to regulate cell proliferation in mammals. Thus, no29/npm3 might serve as a stem cell marker in the intestine during metamorphosis.

Spatio-temporal expression profile of stem cell-associated gene LGR5 in the intestine during thyroid hormone-dependent metamorphosis in Xenopus laevis

Background

The intestinal epithelium undergoes constant self-renewal throughout adult life across vertebrates. This is accomplished through the proliferation and subsequent differentiation of the adult stem cells. This self-renewal system is established in the so-called postembryonic developmental period in mammals when endogenous thyroid hormone (T3) levels are high.

Methodology/Principal Findings

The T3-dependent metamorphosis in anurans like Xenopus laevis resembles the mammalian postembryonic development and offers a unique opportunity to study how the adult stem cells are developed. The tadpole intestine is predominantly a monolayer of larval epithelial cells. During metamorphosis, the larval epithelial cells undergo apoptosis and, concurrently, adult epithelial stem/progenitor cells develop de novo, rapidly proliferate, and then differentiate to establish a trough-crest axis of the epithelial fold, resembling the crypt-villus axis in the adult mammalian intestine. The leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5) is a well-established stem cell marker in the adult mouse intestinal crypt. Here we have cloned and analyzed the spatiotemporal expression profile of LGR5 gene during frog metamorphosis. We show that the two duplicated LGR5 genes in Xenopus laevis and the LGR5 gene in Xenopus tropicalis are highly homologous to the LGR5 in other vertebrates. The expression of LGR5 is induced in the limb, tail, and intestine by T3 during metamorphosis. More importantly, LGR5 mRNA is localized to the developing adult epithelial stem cells of the intestine.

Conclusions/Significance

These results suggest that LGR5-expressing cells are the stem/progenitor cells of the adult intestine and that LGR5 plays a role in the development and/or maintenance of the adult intestinal stem cells during postembryonic development in vertebrates.

Changes in the distribution of corticotropin-releasing factor (CRF)-like immunoreactivity in the larval bullfrog brain and the involvement of CRF in the cessation of food intake during metamorphosis

In submammalian vertebrates, corticotropin-releasing factor (CRF) acts as an anorexigenic neuropeptide as well as a potent stimulator of corticotropin and thyrotropin release from the pituitary. As a step for demonstrating the involvement of CRF in the feeding regulation of anuran larvae, which are known to stop feeding toward the metamorphic climax, we studied firstly the changes in the distribution of CRF-like immunoreactivity (CRF-LI) in the brain of metamorphosing bullfrog larvae. Neuronal cell bodies showing CRF-LI were invariably present in the thalamic regions throughout larval development. Cells with CRF-LI were also found in the hypothalamus. The number of cells with CRF-LI in the hypothalamus, but not in the thalamus, showed a significant increase as metamorphosis progressed. Immunoreactive nerve fibers were observed mainly in the median eminence, and became abundant as metamorphosis proceeded. The number of cells showing CRF-LI in the hypothalamus as well as the density of immunoreactive fibers in the median eminence decreased at the end of metamorphosis. Secondly, we examined the effect of intracerebroventricular (ICV) injection of CRF on the food intake in the premetamorphic larvae. ICV injection of CRF at 10 pmol/g body weight (BW) induced a significant decrease of food intake during 15 min. The CRF-induced anorexigenic action was blocked by the treatment with a CRF receptor antagonist [alpha-helical CRF(9-41)] at 100 pmol/g BW. The results suggest the involvement of CRF in the accomplishment of metamorphosis through the pituitary and in the feeding restriction that occurs during the later stages of metamorphosis through the central nervous system.

Gating of the mitochondrial permeability transition pore by thyroid hormone

The calorigenic-thermogenic activity of thyroid hormone (T3) has long been ascribed to uncoupling of mitochondrial oxidative phosphorylation. However, the mode of action of T3 in promoting mitochondrial proton leak is still unresolved. Mitochondrial uncoupling by T3 is reported here to be transduced in vivo in rats and in cultured Jurkat cells by gating of the mitochondrial permeability transition pore (PTP). T3-induced PTP gating is shown here to be abrogated in inositol 1,4,5-trisphosphate (IP(3)) receptor 1 (IP(3)R1)(-/-) cells, indicating that the endoplasmic reticulum IP(3)R1 may serve as upstream target for the mitochondrial activity of T3. IP(3)R1 gating by T3 is due to its increased expression and truncation into channel-only peptides, resulting in IP(3)-independent Ca(2+) efflux. Increased cytosolic Ca(2+) results in activation of protein phosphatase 2B, dephosphorylation and depletion of mitochondrial Bcl2 (S70), and increase in mitochondrial free Bax leading to low-conductance PTP gating. The T3 transduction pathway integrates genomic and nongenomic activities of T3 in regulating mitochondrial energetics and may offer novel targets for thyromimetics designed to modulate energy expenditure.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

A model of transcriptional and morphological changes during thyroid hormone-induced metamorphosis of the axolotl

Anuran (frog) metamorphosis has long-served as a model of how thyroid hormones regulate post-embryonic development in vertebrates. However, comparatively little is known about urodele (salamander) metamorphosis. We conducted a detailed time-course study of induced metamorphosis in the Mexican axolotl (Ambystoma mexicanum) that probed metamorphic changes in morphology and gene expression in the skin. Using morphometrics, quantitative PCR, histology, and in situ hybridization we demonstrate that the development of transcriptional markers is fundamental to the resolution of early metamorphic events in axolotls. We then use linear and piecewise linear models to identify a sequence of morphological and transcriptional changes that define larval to adult remodeling events throughout metamorphosis. In addition, we show that transcriptional biomarkers are expressed in specific larval and adult cell populations of the skin and that temporal changes in these biomarkers correlate with tissue remodeling. We compare our results with other studies of natural and induced metamorphosis in urodeles and highlight what appear to be conserved features between urodele and anuran metamorphosis.

The xenoestrogen bisphenol A inhibits postembryonic vertebrate development by antagonizing gene regulation by thyroid hormone

Bisphenol A (BPA), a chemical widely used to manufacture plastics, is estrogenic and capable of disrupting sex differentiation. However, recent in vitro studies have shown that BPA can also antagonize T(3) activation of the T(3) receptor. The difficulty in studying uterus-enclosed mammalian embryos has hampered the analysis on the direct effects of BPA during vertebrate development. This study proposed to identify critical T(3) pathways that may be disrupted by BPA based on molecular analysis in vivo. Because amphibian metamorphosis requires T(3) and encompasses the postembryonic period in mammals when T(3) action is most critical, we used this unique model for studying the effect of BPA on T(3)-dependent vertebrate development at both the morphological and molecular levels. After 4 d of exposure, BPA inhibited T(3)-induced intestinal remodeling in premetamorphic Xenopus laevis tadpoles. Importantly, microarray analysis revealed that BPA antagonized the regulation of most T(3)-response genes, thereby explaining the inhibitory effect of BPA on metamorphosis. Surprisingly, most of the genes affected by BPA in the presence of T(3) were T(3)-response genes, suggesting that BPA predominantly affected T(3)-signaling pathways during metamorphosis. Our finding that this endocrine disruptor, well known for its estrogenic activity in vitro, functions to inhibit T(3) pathways to affect vertebrate development in vivo and thus not only provides a mechanism for the likely deleterious effects of BPA on human development but also demonstrates the importance of studying endocrine disruption in a developmental context in vivo.

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

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

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

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

Anterior regeneration in the hemichordate Ptychodera flava

Ptychodera flava is a hemichordate whose anterior structures regenerate reproducibly from posterior trunk pieces when amputated. We characterized the cellular processes of anterior regeneration with respect to programmed cell death and cell proliferation, after wound healing. We found scattered proliferating cells at day 2 of regeneration using a proliferating cell nuclear antigen antibody. On day 4, most proliferating cells were associated with the nerve tract under the epidermis, and on day 6, a small proboscis derived from proliferated cells was regenerated, and a mouth had broken though the epidermis. TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling) detected elevated levels of apoptosis in the endoderm that began furthest away from the region of wound healing, then moved anteriorly over 8 days. Posterior to anterior apoptosis is likely to remove digestive endoderm for later differentiation of pharyngeal endoderm. We hypothesize that P. flava regeneration is nerve dependent and that remodeling in the gut endoderm plays an important role in regeneration.

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

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