LIGANDED THYROID HORMONE RECEPTOR INDUCES NUCLEOSOME REMOVAL AND HISTONE MODIFICATIONS TO ACTIVATE TRANSCRIPTION DURING INTESTINAL METAMORPHOSIS. We have previous shown that the heterodimers between TR and RXR (9-cis retinoic acid receptor) have dual functions for during frog development. In premetamorphic tadpoles, they repress gene expression in the absence of TH to prevent metamorphosis, thus ensuring a proper tadpole growth period. When TH is present either from endogenous synthesis during development or exogenous addition to the raring water of premetamorphic tadpoles, TR/RXR heterodimers activate TH-inducible genes to initiate metamorphosis. To investigate how they regulate gene expression in vivo, we analyzed histone modifications at TH target genes during intestinal metamorphosis in Xenopus tropicalis, 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. HISTONE H3K79 METHYLTRANSFERASE DOT1L IS DIRECTLY ACTIVATED BY THYROID HORMONE RECEPTOR DURING XENOPUS METAMORPHOSIS. To identify the direct target genes of TR during metamorphosis, we carried out a ChIP (chromatin immunoprecipitation)-on-chip analysis of the intestine by using a set of microarray chips covering a 8 kb region flanking each putative promoter of 17000 Xenopus tropicalis genes to look for genes bound by TR (unpublished observation). While the ChIP-on-chip data was very preliminary, it was of interest that one of the putative target genes thus identified corresponded to the Xenopus tropicalis homolog of the mammalian Dot1L gene, the only histone methyltransferase capable of methylating H3K79. We showed that Dot1L was directly regulated by TR via binding to a TH response element in the promoter region during metamorphosis in Xenopus tropicalis. We further showed that Dot1L expression in both the intestine and tail correlated with the transformation of the organs. As we have previously shown that methylation of histone H3 lysine (K) 79 in the promoter regions of TR target genes is increased upon activation of TH-inducible genes (above), our findings suggest that TR activates Dot1L, which in turn participates in metamorphosis through a positive feedback to enhance H3K79 methylation and gene activation by liganded TR. DIRECT ACTIVATION OF XENOPUS IODOTYROSINE DEIODINASE BY THYROID HORMONE RECEPTOR IN THE REMODELING INTESTINE DURING AMPHIBIAN METAMORPHOSIS. TH production in the thyroid involves incorporating inorganic iodide into thyroglobulin. The expression of iodotyrosine deiodinase (IYD, also known as iodotyrosine dehalogenase 1 (DEHAL1)) 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. Interestingly, our preliminary ChIP-on-chip study also discovered that Xenopus tropicalis IYD gene was bound by TR in the intestine. Furthermore, we showed that IYD was strongly upregulated 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 TR to a TH response element in the IYD promoter region. As 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 TR 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.
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