Cell-specific and spatio-temporal expression of intestinal fatty acid-binding protein gene during amphibian metamorphosis.

Intestinal fatty acid-binding protein (IFABP) gene is known to be regulated during Xenopus metamorphosis. To determine the relationship between its regulation and cellular differentiation during metamorphosis, we have examined the distribution of IFABP mRNA in the Xenopus digestive tract by in situ hybridization techniques. Throughout all stages examined, transcripts of IFABP gene were observed exclusively in absorptive epithelial cells of the small intestine, and they decreased in amount towards the posterior intestine. Around stage 58, just before metamorphic climax, IFABP mRNA level began to decrease in larval absorptive cells that still remained intact morphologically. Thereafter, IFABP mRNA was no longer detected among larval cells. In turn, at stage 62, IFABP mRNA became detectable in some of the newly formed adult epithelium that had not yet developed a brush border, but not in the remaining larval cells. By the end of metamorphosis, IFABP mRNA became more abundant towards the crest of intestinal folds. These results suggest that IFABP gene expression is specific for absorptive epithelial cells of the small intestine and is regionally regulated along the intestinal anterior-posterior axis in both tadpoles and frogs and also along the trough-crest axis of frog intestinal folds. In addition, our present study directly shows that IFABP mRNA level decreases in larval absorptive cells but increases in adult ones during metamorphosis, preceding morphological changes of both types of cells. Therefore, the regulation of IFABP gene is an early event during both larval epithelial cell death and adult epithelial cell differentiation.