AN IMMUNOCYTOCHEMICAL ANALYSIS OF THE EXPRESSION OF THYROID-HORMONE-RECEPTOR-ALPHA AND THYROID-HORMONE-RECEPTOR-BETA PROTEINS DURING NATURAL AND THYROID HORMONE-INDUCED METAMORPHOSIS IN XENOPUS

Amphibian metamorphosis is characterized by the upregulation of thyroi d hormone receptor (TR) mRNA in all tissues of tadpole during both the natural and thyroid hormone (TH)-induced development. The two TR gene s, termed alpha and beta, are members of a large multigene family of n uclear receptors related to the cellular homolog of the oncogene c-erb A. The phenomenon of upregulation is more marked for the beta than the alpha isoform. To determine whether or not the auto-induction of the transcripts is paralleled by that of TR proteins, non-crossreacting mo noclonal antibodies were prepared against Xenopus laevis TR alpha and beta (xTR alpha, beta) in order to analyze immunocytochemically their expression and localization. Three tadpole tissues that exemplify thre e major consequences of gene re-programing during natural and TH-induc ed metamorphosis were studied: (i) Liver that undergoes extensive func tional switching; (ii) small intestinal epithelium that exhibits subst antial cell death prior to major structural and biochemical modificati ons; and (iii) hind limb-bud as an example of de novo morphogenesis. i t was shown that xTR alpha protein is generally more abundant in these tissues, and its expression is developmentally and hormonally less re gulated, than is xTR beta. The auto-induction of xTR beta was particul arly intense at 5 days after administration of triiodo-thyronine (T-3) to both pre-metamorphic (stage 52) tadpoles and at the onset of natur al metamorphosis (stage 55). In the developing hind limb-bud al both s tages the upregulation of TR beta is topologically restricted, being p articularly intense in dense pockets of cells, presumably rich in chon drocytes. It was concluded that the distribution and expression of xTR alpha and beta proteins match partially, but not fully, those of thei r transcripts during natural and hormone-induced metamorphosis.