Direct activation of folate receptor 4 by thyroid hormone suggests its role in the development of adult intestinal epithelium during Xenopus laevis metamorphosis

Amphibian metamorphosis is tightly regulated by thyroid hormone (TH). During this process, most larval epithelial cells in the Xenopus laevis intestine undergo apoptosis, whereas a small population dedifferentiates into adult epithelial stem cells. They subsequently proliferate and differentiate to form a trough-crest epithelial architecture similar to the mammalian crypt-villus axis. We have previously identified a number of TH-responsive genes likely involved in this intestinal remodeling. Here, we focus on one such gene, folate receptor 4 (folr4). We examined the spatiotemporal expression of folr4.L by using quantitative RT-PCR and in situ hybridization chain reaction (HCR) and found that folr4.L expression is highly upregulated during both natural and TH-induced metamorphosis. Interestingly, in the epithelium at the climax of metamorphosis, folr4.L is specifically expressed in the proliferating and/or differentiating adult epithelial cells located adjacent to proliferating adult stem cells, which express intestinal stem cell marker leucine-rich repeat-containing G protein-coupled 5 (lgr5). Moreover, we identified a TH response element (TRE) in the folr4.L promoter that binds to the heterodimer of TH receptor (TR) and 9-cis retinoic acid receptor (RXR) in vitro and mediates T3-dependent transcriptional activation in vivo. Phylogenetic analysis suggested that X. laevis Folr4.L may be more closely related to riboflavin binding protein (Rfbp) than mammalian FOLR4. These findings suggest that TH-induced Folr4.L might be involved in the development of adult intestinal epithelium.