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 DuringXenopus tropicalisMetamorphosis

Background:There are two highly conserved thyroid hormone (triiodothyronine [T3]) receptor (TR) genes,TR alpha andTR beta, in all vertebrates, and the expression ofTR alpha but notTR beta 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 tropicalisundergoes 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 formedde novoand 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 TR alpha in the tadpole intestine by using knockout animals and RNA-seq analysis. Results:We observed that removing endogenous TR alpha 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 andTR alpha-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 notTR alpha-knockout tadpoles. Gene Ontology and biological pathway analyses revealed that surprisingly, these TR alpha-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 TR alpha-mediated T3 activation of the cell cycle program is involved in larval epithelial cell death and adult epithelial stem cell development during intestinal remodeling.