A correlation of thyroid hormone receptor gene expression with amphibian metamorphosis.

The expression of the thyroid hormone (TH) receptor genes alpha (TR alpha) and beta (TR beta) in Xenopus laevis begins after the embryo hatches. The TR alpha mRNA increases throughout the premetamorphosis stage of tadpole development, is maximal by prometamorphosis, and falls after climax of metamorphosis to a lower level in frogs. The TR beta mRNA is barely detectable during premetamorphosis. In synchrony with the onset of endogenous TH synthesis by the thyroid gland (prometamorphosis), the level of TR beta mRNA rises in parallel with endogenous TH, reaching a peak at the climax of metamorphosis (stage 61) and drops to approximately 10% of its peak level after metamorphosis. As suggested by this correlation, exogenous TH up-regulates TR beta mRNA as much as 20-fold during premetamorphosis, whereas TH up-regulates TR alpha mRNA by approximately 2-fold during the same period. Up-regulation of TR beta mRNA is the earliest response to exogenous TH by competent tadpoles yet detected.