Metamorphosis is inhibited in transgenic Xenopus laevis tadpoles that overexpress type III deiodinase

One of the genes that is up-regulated by thyroid hormone (TH) during Xenopu s laevis metamorphosis encodes a type LII deiodinase (D3) that inactivates TH. Transgenic X. laevis tadpoles overexpressing a GFP-D3 fusion protein we re produced. These transgenic tadpoles had high levels of deiodinase activi ty and were resistant to exogenous TH added 1 week after fertilization. The y developed normally throughout embryogenesis and premetamorphic stages but became retarded in their development late in prometamorphosis when endogen ous TH reaches its highest level. Gill and tail resorption were delayed and most of the animals arrested and died. One tadpole completed its metamorph osis without resorbing its tail. These results demonstrate that D3 can modu late the action of TR in vivo, and document the value of the new transgenic method for functional analysis of genes involved in metamorphosis.