EXPRESSION PROFILING OF INTESTINAL TISSUES IMPLICATES TISSUE-SPECIFC GENES AND PATHWAYS ESSENTIAL FOR THYROID HORMONE-INDUCED ADULT STEM CELL DEVELOPMENT. We have shown earlier that the de novo formation of adult intestinal stem cells that requires TH signaling in both the larval epithelium (Ep) and non-epithelial tissues (non-Ep). To understand the underlying molecular mechanisms, it is critical to determine the genes and signaling pathways involved in this process. Thus, we have profiled the gene expression programs in the epithelial and non-epithelial tissues in X. laevis intestine to systematically determine changes underlying the cell autonomous and cell-cell interaction-dependent processes that are required for stem cell formation. We isolated the intestines of tadpoles from pre-metamorphosis (stage 56), metamorphic climax (stage 61/62, when larval epithelial cell death is near completion, cell proliferation is rapid and most of the cells are adult progenitor/stem cells), and end of metamorphosis (stage 66, when adult intestine is formed) and separated the Ep from the non-Ep of each intestine. High throughput gene expression profiling on the isolated tissues revealed that most of the genes regulated during intestinal metamorphosis were tissue-specific and that distinct signal transduction pathways were affected by T3 in the Ep and non-Ep, suggesting that their differential role in cell autonomous functions in stem cell development vs. in the stem cell niche formation. Our results further suggested the existence of potentially many novel stem cell associated genes, at least in the intestine during development TISSUE-SPECIFIC UPREGULATION OF MDS/EVI GENE TRANSCRIPTS IN THE INTESTINE BY THYROID HORMONE DURING XENOPUS METAMORPHOSIS. One of the genes that we discovered to be highly expressed in the epithelium at the climax of metamorphosis when most of the epithelial cells are adult progenitor/stem cells is the homolog of human ectopic viral integration site 1 (EVI). The EVI locus encodes EVI as well as its variant myelodysplastic syndrome 1 (MDS)/EVI generated via transcription from the upstream MDS promoter and alternative splicing. EVI and MDS/EVI have been implicated in a number of cancers including breast, leukemia, ovarian, and intestinal cancers. We showed that EVI and MDS/EVI transcripts were upregulated by TH in the epithelium but not the rest of the intestine in Xenopus laevis when adult stem cells are forming in the epithelium. Our results suggest that EVI and MDS/EVI are likely involved in the development and/or proliferation of newly forming adult intestinal epithelial cells. GENETIC ANALYSIS OF THE FUNCTION OF A THYROID HORMONE/AMINO ACID TRANSPORTER DURING MOUSE DEVELOPMENT. To regulate cellular processes, TH has to be actively transported into cells and this process is mediated by several different types of transporters. One of our previously identified TH-response genes in the intestine, LAT1, encodes the light chain of a heterodimeric system L type of TH transporter, which also transports several amino acids. Interestingly, LAT1 is highly upregulated at the climax of metamorphosis in the tadpole intestine, coinciding with the formation and rapid proliferation of the adult intestinal stem cells. We recently found out that LAT1 was also highly expressed in the mouse intestine during the neonatal period when the mouse intestine matured into the adult form, a process that appears also involves TH-dependent formation and/proliferation of the adult intestinal stem cells. Through a collaborative study, we generated a mouse line with the LAT1 gene floxed, which allows conditional knockout of the LAT1 upon expression of the Cre recombinase. While we are still in the process to investigate whether LAT1 affects adult intestinal stem cell development in mouse, we have recently shown through another collaboration that LAT1 is critical for antigen receptor-mediated the metabolic re-programming, a process essential for T cell differentiation.
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