Isolation of connective-tissue-specific genes involved in Xenopus intestinal remodeling: thyroid hormone up-regulates Tolloid/BMP-1 expression.

To clarify connective-tissue-specific genes involved in adult epithelial development during amphibian intestinal remodeling, we have isolated 16 cDNA clones derived from the anterior part of Xenopus laevis intestine cultured in vitro by using subtractive suppression hybridization. Among four genes identified, the expression of Xtld, a Xenopus homolog of Drosophila Tolloid closely related to bone morphogenic protein-1 (BMP-1), was most remarkably up-regulated during metamorphosis. To further explore the roles of Xtld in intestinal remodeling, we examined its developmental expression in the X. laevis intestine by in situ hybridization and northern blot analysis. Xtld mRNA first became detectable in the connective tissue just before the appearance of adult epithelial primordia. Subsequently, the level of Xtld mRNA reached a high in the connective tissue, concomitantly with adult epithelial development along the anteroposterior axis of the intestine. Thereafter, towards the completion of metamorphosis, the expression of Xtld mRNA was down-regulated. Thus, the expression profile of Xtld mRNA spatiotemporally correlates well with adult epithelial development in vivo. Furthermore, the present culture study has shown that thyroid hormone (TH) up-regulates the expression of Xtld mRNA organ-autonomously in the anterior part of the intestine, but not in its posterior part, and that TH up-regulation of Xtld expression is not mediated by the epithelium. These results suggest that TH directly up-regulates Xtld expression in the connective tissue along the anteroposterior axis, which in turn plays important roles in adult epithelial development during amphibian intestinal remodeling.