An immunocytochemical analysis of the expression of thyroid hormone receptor alpha and beta proteins during natural and thyroid hormone-induced metamorphosis in Xenopus

Amphibian metamorphosis is characterized by the upregulation of thyroid hormone receptor (TR) mRNA in all tissues of tadpole during both the natural and thyroid hormone (TH)-induced development. The two TR genes, termed α and β, are members of a large multigene family of nuclear receptors related to the cellular homolog of the oncogene c-erbA. The phenomenon of upregulation is more marked for the β than the α isoform. To determine whether or not the auto-induction of the transcripts is paralleled by that of TR proteins, non-cross-reacting monoclonal antibodies were prepared against Xenopus laevis TRα and β (xTRα, β) in order to analyze immunocytochemically their expression and localization. Three tadpole tissues that exemplify three major consequences of gene re-programing during natural and TH-induced metamorphosis were studied: (i) Liver that undergoes extensive functional switching; (ii) small intestinal epithelium that exhibits substantial cell death prior to major structural and biochemical modifications; and (iii) hind limb-bud as an example of de novo morphogenesis. It was shown that xTRα protein is generally more abundant in these tissues, and its expression is developmentally and hormonally less regulated, than is xTRβ. The auto-induction of xTRβ was particularly intense at 5 days after administration of triiodo-thyronine (T3) to both pre-metamorphic (stage 52) tadpoles and at the onset of natural metamorphosis (stage 55). In the developing hind limb-bud at both stages the upregulation of TRβ is topologically restricted, being particularly intense in dense pockets of cells, presumably rich in chondrocytes. It was concluded that the distribution and expression of xTRα and β proteins match partially, but not fully, those of their transcripts during natural and hormone-induced metamorphosis.