Luu Nga, Fu Liezhen, Fujimoto Kenta, Shi Yun-Bo
Section on Molecular Morphogenesis, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland.
Endocrinology. 2017 Apr 1;158(4):1022-1033. doi: 10.1210/en.2016-1558.
Most vertebrate organs use adult stem cells to maintain homeostasis and ensure proper repair when damaged. How such organ-specific stem cells are formed during vertebrate development is largely unexplored. We have been using the thyroid hormone (T3)-dependent amphibian metamorphosis to address this issue. Early studies in Xenopus laevis have shown that intestinal remodeling involves complete degeneration of the larval epithelium and de novo formation of adult stem cells through dedifferentiation of some larval epithelial cells. We have further discovered that the histidine ammonia-lyase (HAL; also known as histidase or histidinase)-2 gene is strongly and specifically activated by T3 in the proliferating adult stem cells of the intestine during metamorphosis, implicating a role of histidine catabolism in the development of adult intestinal stem cells. To determine the mechanism by which T3 regulates the HAL2 gene, we have carried out bioinformatics analysis and discovered a putative T3 response element (TRE) in the HAL2 gene. Importantly, we show that this TRE is bound by T3 receptor (TR) in the intestine during metamorphosis. The TRE is capable of binding to the heterodimer of TR and 9-cis retinoic acid receptor (RXR) in vitro and mediate transcriptional activation by liganded TR/RXR in frog oocytes. More importantly, the HAL2 promoter containing the TRE can drive T3-dependent reporter gene expression to mimic endogenous HAL2 expression in transgenic animals. Our results suggest that the TRE mediates the induction of HAL2 gene by T3 in the developing adult intestinal stem cells during metamorphosis.
大多数脊椎动物器官利用成体干细胞来维持体内平衡，并在受损时确保适当的修复。在脊椎动物发育过程中，这些器官特异性干细胞是如何形成的，目前在很大程度上尚未得到探索。我们一直在利用甲状腺激素（T3）依赖的两栖类变态发育来解决这个问题。早期对非洲爪蟾的研究表明，肠道重塑涉及幼虫上皮的完全退化以及通过一些幼虫上皮细胞的去分化重新形成成体干细胞。我们进一步发现，组氨酸解氨酶（HAL；也称为组氨酸酶）-2基因在变态发育期间被T3强烈且特异性地激活于肠道增殖的成体干细胞中，这暗示组氨酸分解代谢在成体肠道干细胞发育中起作用。为了确定T3调节HAL2基因的机制，我们进行了生物信息学分析，并在HAL2基因中发现了一个假定的T3反应元件（TRE）。重要的是，我们表明在变态发育期间，这个TRE在肠道中与T3受体（TR）结合。该TRE在体外能够与TR和9-顺式视黄酸受体（RXR）的异二聚体结合，并在蛙卵母细胞中介导配体化的TR/RXR的转录激活。更重要的是，含有TRE的HAL2启动子能够驱动T3依赖的报告基因表达，以模拟转基因动物中内源性HAL2的表达。我们的结果表明，在变态发育期间，TRE介导T3对发育中的成体肠道干细胞中HAL2基因的诱导作用。
