Differential regulation of two histidine ammonia-lyase genes during Xenopus development implicates distinct functions during thyroid hormone-induced formation of adult stem cells.
BACKGROUND
Organ-specific, adult stem cells are essential for organ-homeostasis and tissue repair and regeneration. The formation of such stem cells during vertebrate development remains to be investigated. Frog metamorphosis offers an excellent opportunity to study the formation of adult stem cells as this process involves essentially the transformations of all larval tissues/organs into the adult form. Of particular interest is the remodeling of the intestine. Early studies in Xenopus laevis have shown that this process involves complete degeneration of the larval epithelium and de novo formation of adult stem cells through dedifferentiation of some larval epithelial cells. A major advantage of this metamorphosis model is its total dependence on thyroid hormone (T3). In an effort to identify genes that are important for stem cell development, we have previously carried out tissue-specific microarray analysis of intestinal gene expression during Xenopus laevis metamorphosis.


RESULTS
We report the detailed characterization of one of the genes thus identified, the histidine ammonia-lyase (HAL) gene, which encodes an enzyme known as histidase or histidinase. We show that there are two duplicated HAL genes, HAL1 and HAL2, in both Xenopus laevis and Xenopus tropicalis, a highly related but diploid species. Interestingly, only HAL2 is highly upregulated by T3 and appears to be specifically expressed in the adult intestinal progenitor/stem cells while HAL1 is not expressed in the intestine during metamorphosis. Furthermore, when analyzed in whole animals, HAL1 appears to be expressed only during embryogenesis but not metamorphosis while the opposite appears to be true for HAL2.


CONCLUSIONS
Our results suggest that the duplicated HAL genes have distinct functions with HAL2 likely involved in the formation and/or proliferation of the adult stem cells during metamorphosis.
Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), 18 Library Dr., 20892 Bethesda, Maryland, USA.
Nga
N
Luu
Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), 18 Library Dr., 20892 Bethesda, Maryland, USA.
Luan
L
Wen
Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), 18 Library Dr., 20892 Bethesda, Maryland, USA.
Liezhen
L
Fu
Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), 18 Library Dr., 20892 Bethesda, Maryland, USA.|Present address: Division of Gene Structure and Function, Research Center for Genomic Medicine, Saitama Medical University, 1397-1 Yamane, 350-1241 Hidaka-shi, Saitama, Japan.
Kenta
K
Fujimoto
Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), 18 Library Dr., 20892 Bethesda, Maryland, USA.
Yun-Bo
YB
Shi
School of Basic Medical Sciences, Wuhan University, 430072 Wuhan, P.R. China.
Guihong
G
Sun