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.— Ishizuya‐Oka, A.,Hasebe, T., Buchholz, D. R., Kajita, M., Fu, L., Shi, Y.‐B. The origin of the adult intestinal stem cells induced by thyroid hormone in Xenopus laevis . FASEB J. 23, 2568–2575 (2009)

Adult organ-specific stem cells are essential for organ homeostasis and repair in adult vertebrates. The intestine is one of the best-studied organs in this regard. The intestinal epithelium undergoes constant self-renewal throughout adult life across vertebrates through the proliferation and subsequent differentiation of the adult stem cells. This self-renewal system is established late during development, around birth, in mammals when endogenous thyroid hormone (T3) levels are high. Amphibian metamorphosis resembles mammalian postembryonic development around birth and is totally dependent upon the presence of high levels of T3. During this process, the tadpole intestine, predominantly a monolayer of larval epithelial cells, undergoes drastic transformation. 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. We and others have studied the T3-dependent remodeling of the intestine in Xenopus laevis. Here we will highlight some of the recent findings on the origin of the adult intestinal stem cells. We will discuss observations suggesting that liganded T3 receptor (TR) regulates cell autonomous formation of adult intestinal progenitor cells and that T3 action in the connective tissue is important for the establishment of the stem cell niche. We will further review evidence suggesting similar T3-dependent formation of adult intestinal stem cells in other vertebrates.

In the amphibian intestine during metamorphosis, stem cells appear and generate the adult absorptive epithelium, analogous to the mammalian one, under the control of thyroid hormone (TH). We have previously shown that the adult stem cells originate from differentiated larval epithelial cells in the Xenopus laevis intestine. To clarify whether TH signaling in the epithelium alone is sufficient for inducing the stem cells, we have now performed tissue recombinant culture experiments using transgenic X. laevis tadpoles that express a dominant‐positive TH receptor (dpTR) under a control of heat shock promoter. Wild‐type (Wt) or dpTR transgenic (Tg) larval epithelium (Ep) was isolated from the tadpole intestine, recombined with homologous or heterologous nonepithelial tissues (non‐Ep), and then cultivated in the absence of TH with daily heat shocks to induce transgenic dpTR expression. Adult epithelial progenitor cells expressing sonic hedgehog became detectable on day 5 in both the recombinant intestine of Tg Ep and Tg non‐Ep (Tg/Tg) and that of Tg Ep and Wt non‐Ep (Tg/Wt). However, in Tg/Wt intestine, they did not express other stem cell markers such as Musashi‐1 and never generated the adult epithelium expressing a marker for absorptive epithelial cells. Our results indicate that, while it is unclear why some larval epithelial cells dedifferentiate into adult progenitor/stem cells, TR‐mediated gene expression in the surrounding tissues other than the epithelium is required for them to develop into adult stem cells, suggesting the importance of TH‐inducible epithelial‐connective tissue interactions in establishment of the stem cell niche in the amphibian intestine. STEM CELLS 2011;29:154–161

During amphibian larval‐to‐adult intestinal remodeling, progenitor cells of the adult epithelium actively proliferate and differentiate under the control of thyroid hormone (TH) to form the intestinal absorptive epithelium, which is analogous to the mammalian counterpart. We previously found that TH–up‐regulated expression of bone morphogenetic protein‐4 (BMP‐4) spatiotemporally correlates with adult epithelial development in the Xenopus laevis intestine. Here, we aimed to clarify the role of BMP‐4 in intestinal remodeling. Our reverse transcriptase‐polymerase chain reaction and in situ hybridization analyses indicated that mRNA of BMPR‐IA, a type I receptor of BMP‐4, is expressed in both the developing connective tissue and progenitor cells of the adult epithelium. More importantly, using organ culture and immunohistochemical procedures, we have shown that BMP‐4 not only represses cell proliferation of the connective tissue but promotes differentiation of the intestinal absorptive epithelium. In addition, we found that the connective tissue‐specific expression of BMP‐4 mRNA is up‐regulated by sonic hedgehog (Shh), whose epithelium‐specific expression is directly induced by TH. These results strongly suggest that the Shh/BMP‐4 signaling pathway plays key roles in the amphibian intestinal remodeling through epithelial–connective tissue interactions. Developmental Dynamics 235:3240–3249, 2006.

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.