Early expression of thyroid hormone receptor β and retinoid X receptor γ in the Xenopus embryo

Abstract

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor β ( XTRβ) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor γ (RXRγ), a potential dimerization partner for XTRβ, was also found to be expressed in the retina at early stages, and at later stages RXRγ was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTRβ and RXRγ, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTRβ and RXRγ in the developing Xenopus retina.