The use of transgenesis to explore the molecular basis of Xenopus laevis metamorphosis

Amphibian metamorphosis is controlled by thyroid hormone (TH). Thyroid hormone receptors are TH-dependent transcription factors. We have taken a molecular approach in Xenopus laevis to study the gene expression cascades since it is not amenable to traditional genetics. TH up- and down-regulated genes have been isolated, sequenced and identified. The focus of this thesis is to explore the transgenic method for the functional analysis of genes involved in Xenopus laevis metamorphosis.;One of the genes that is up-regulated by TH during X. laevis metamorphosis encodes a type III deiodinase (D3) which inactivates TH. The basal expression, the developmental profile, the induction by TH, and the cell and tissue localization of D3 gene expression suggest that this enzyme plays a role in modulating the tadpole's response to TH. To test this correlation, transgenic X. laevis tadpoles overexpressing D3 by a constitutive promoter were produced. The transgenic tadpoles are resistant to exogenous TH and inhibited in spontaneous metamorphosis. These results demonstrate that D3 can modulate the action of TH in vivo. Thus, D3 as well as dominant-negative thyroid hormone receptors are useful tools to study TH action at the molecular and cellular level.;There has been a long standing controversy over the roles of growth hormone (GH) and prolactin (PRL) in amphibian metamorphosis. PRL in particular has been widely considered to be the growth-promoting and the juvenile hormone of anuran tadpoles and to counteract the effects of TH. The roles of GH and PRL have been reinvestigated by using the transgenic method. Overexpression of GH or PRL in transgenic X. laevis does not prolong tadpole life demonstrating that neither GH nor PRL plays a biological role in amphibians that is equivalent to that of juvenile hormone in insect metamorphosis. Overexpression of GH, but not PRL, has dramatic growth-promoting effects in tadpoles and frogs. Overexpression of PRL produces tailed frogs by inhibiting specifically some but not all of the programs of tail resorption and by stimulating growth of fibroblasts in the tail.