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

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. [ABSTRACT FROM AUTHOR]

Background: Thyroid hormone (triiodothyronine [T3]) is essential for the development throughout vertebrates. Anuran metamorphosis mimics mammalian postembryonic development, a period around birth when plasma T3 level peaks and many organs/tissues mature into their adult forms. Compared with the uterus-enclosed mammalian embryos, tadpoles can be easily manipulated to study the roles of T3 and T3 receptors (TRs) in tissue remodeling and adult organ development. We and others have previously knocked out TRα or TRβ in the diploid anuran Xenopus tropicalis and reported distinct effects of the two receptor knockouts on metamorphosis. However, animals lacking either TRα or TRβ can complete metamorphosis and develop into reproductive adults. Methods: We have generated TRα and TRβ double knockout animals and carried out molecular and morphological analyses to determine if TR is required for Xenopus development. Results: We found that the TR double knockout tadpoles do not respond to T3, supporting the view that there are no other TR genes in X. tropicalis and that TR is essential for mediating the effects of T3 in vivo. Surprisingly, the double knockout tadpoles are able to initiate metamorphosis and accomplish many metamorphic changes, such as limb development. However, all double knockout tadpoles stall and eventually die at stage 61, the climax of metamorphosis, before tail resorption takes place. Analyses of the knockout tadpoles at stage 61 revealed various developmental abnormalities, including precocious ossification and extra vertebrae. Conclusions: Our data indicate that TRs are not required for the initiation of metamorphosis but is essential for the completion of metamorphosis. Furthermore, the differential effects of TR knockout on different organs/tissues suggest tissue-specific roles for TR to control temporal coordination and progression of metamorphosis in various organs. [ABSTRACT FROM AUTHOR]

Background: The thyroid hormone triiodothyronine (T3) is critical for vertebrate development and affects the function of many adult tissues and organs. Its genomic effects are mediated by thyroid hormone nuclear receptors (TRs) present in all vertebrates. The discovery of patients with resistance to thyroid hormone (RTHβ) >50 years ago and subsequent identification of genetic mutations in only the THRB gene in these patients suggest that mutations in the THRA gene may have different pathological manifestations in humans. Indeed, the recent discovery of a number of human patients carrying heterozygous mutations in the THRA gene (RTHα) revealed a distinct phenotype that was not observed in RTH patients with THRB gene mutations (RTHβ). That is, RTHα patients have constipation, implicating intestinal defects caused by THRA gene mutations. Methods: To determine how TRα1 mutations affect the intestine, this study analyzed a mutant mouse expressing a strong dominantly negative TRα1 mutant (denoted TRα1PV; Thra1PV mice). This mutant mouse faithfully reproduces RTHα phenotypes observed in patients. Results: In adult Thra1PV/+ mice, constipation was observed just like in patients with TRα mutations. Importantly, significant intestinal defects were discovered, including shorter villi and increased differentiated cells in the crypt, accompanied by reduced stem-cell proliferation in the intestine. Conclusions: The findings suggest that further analysis of this mouse model should help to reveal the molecular and physiological defects in the intestine caused by TRα mutations and to determine the underlying mechanisms. [ABSTRACT FROM AUTHOR]