THYROID HORMONE-DEPENDENT TRANSCRIPTIONAL REGULATION OF EXOGENOUS GENES TRANSFERRED INTO XENOPUS TADPOLE MUSCLE IN-VIVO

Metamorphosis in amphibians is marked by dramatic thyroid hormone-indu ced changes that include tail regression. To examine thyroid hormone e ffects on gene transcription during the early stages of tail resorptio n, we injected exogenous genes directly into the caudal skeletal muscl e of Xenopus tadpoles and followed their expression in vivo. Gene expr ession was both strong and reproducible, and it correlated with the am ount of DNA injected. Moreover, expression continued as long as the an imals were blocked in prometamorphosis by antithyroid drugs (for up to 4 months). Thyroid hormone-dependent effects on transcription were ex amined by using a palindromic thyroid hormone response element linked to a chloramphenicol acetyltransferase reporter gene. Reporter gene ex pressions were normalized for transfection efficiency by using a const itutively expressed luciferase construct. Physiological concentrations of 3,5,3' triiodo-L-thyronine (1 nM), applied for 120 hr, produced a 5-fold increase in transcription (P <0.05) from the thyroid hormone re sponse element but did not modify transcription from constitutive vira l promoters. This study thus demonstrates that by directly expressing genes in Xenopus tadpole muscle in vivo, one can exploit the powerful experimental advantages of gene transfer systems in an intact, physiol ogically normal animal.