THE THYROID-HORMONE RECEPTOR GENE (C-ERBA-ALPHA) IS EXPRESSED IN ADVANCE OF THYROID-GLAND MATURATION DURING THE EARLY EMBRYONIC-DEVELOPMENT OF XENOPUS-LAEVIS

The c-erbA proto-oncogene encodes the thyroid hormone receptor, a ligand-dependent transcription factor which plays an important role in vertebrate growth and development. To define the role of the thyroid hormone receptor in developmental processes, we have begun studying c-erbA gene expression during the ontogeny of Xenopus laevis, an organism in which thyroid hormone has well-documented effects on morphogenesis. Using polymerase chain reactions (PCR) as a sensitive assay of specific gene expression, we found that polyadenylated erbA-alpha RNA is present in Xenopus cells at early development stages, including the fertilized egg, blastula, gastrula, and neurula. By performing erbA-alpha-specific PCR on reverse-transcribed RNAs from high-density sucrose gradient fractions prepared from early-stage embryos, we have demonstrated that these erbA transcripts are recruited to polysomes. Therefore, erbA is expressed in Xenopus development prior to the appearance of the thyroid gland anlage in tailbud-stage embryos. This implies that erbA-alpha/thyroid hormone receptors may play ligand-independent roles during the early development of X. laevis. Quantitative PCR revealed a greater than 25-fold range in the steady-state levels of polyadenylated erbA-alpha RNA across early stages of development, as expressed relative to equimolar amounts of total embryonic RNA. Substantial increases in the levels of erbA-alpha RNA were noted at stages well after the onset of zygotic transcription at the mid-blastula transition, with accumulation of erbA-alpha transcripts reaching a relative maximum in advance of metamorphosis. We also show that erbA-alpha RNAs are expressed unequally across Xenopus neural tube embryos. This differential expression continues through later stages of development, including metamorphosis. This finding suggests that erbA-alpha/thyroid hormone receptors may play roles in tissue-specific processes across all of Xenopus development.