The present study used a molecular approach toward understanding the mechanism of hormone- and region-dependent remodeling of the small intestine during metamorphosis of Xenopus laevis. A protein spot was noticed on a two-dimensional polyacrylamide gel as a protein whose expression was metamorphic stage- and region-dependent. The protein was identified as the Xenopus homolog (Xcalbindin) of chick calbindin D28k. Xcalbindin expression in the intestine was restricted to absorptive cells in the posterior part, being detectable at stages 49-61, not detectable at stages 62-63, detectable again at stages 64-66, and finally becoming undetectable in the adult. During spontaneous metamorphosis, the level of Xcalbindin mRNA was significantly increased between stages 57 and 58, dramatically reduced at stage 59, and the mRNA was undetectable from stages 60-63, after which it was weakly re-expressed until the end of metamorphosis. Such up- and down-regulation of Xcalbindin mRNA was induced precociously by exogenous thyroid hormone. These results indicated that Xcalbindin is a specific marker of the differentiated absorptive cells of the intestine. Immunohistochemistry with specific antibodies against Xcalbindin demonstrated that precursor cells of adult intestinal epithelial cells expressed Xcalbindin. Considering these results, the origin of adult intestinal epithelial cells was discussed.

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Recent genome wide in silico analyses discovered a new family (type 2 or family H) of odorant receptors (ORs) in teleost fish and frogs. However, since there is no evidence of the expression of these novel OR genes in olfactory sensory neurons (OSN), it remains unknown if type 2 ORs (OR2) function as odorant receptors. In this study, we examined expression of OR2 genes in the frog Xenopus tropicalis. The overall gene expression pattern is highly complex and differs depending on the gene and developmental stage. RT-PCR analysis in larvae showed that all of the OR2η genes we identified were expressed in the peripheral olfactory system and some were detected in the brain and skin. Whole mount in situ hybridization of the larval olfactory cavity confirmed that at least two OR2η genes so far tested are expressed in the OSN. Because tadpoles are aquatic animals, OR2η genes are probably involved in aquatic olfaction. In adults, OR2η genes are expressed in the nose, brain, and testes to different degrees depending on the genes. OR2η expression in the olfactory system is restricted to the medium cavity, which participates in the detection of water-soluble odorants, suggesting that OR2ηs function as receptors for water-soluble odorants. Moreover, the fact that several OR2ηs are significantly expressed in non-olfactory organs suggests unknown roles in a range of biological processes other than putative odorant receptor functions.