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The Axolotl (Ambystoma mexicanum), a Neotenic Amphibian, Expresses Functional Thyroid Hormone Receptors

Centre National de la Recherche Scientifique, Unité Mixte de Recherche (UMR) 5665, Laboratoire de Biologie Moléculaire et Cellulaire de la Cellule, Ecole Normale Supérieure de Lyon (R.S., S.B., O.M., M.D., J.-M.V., V.L.), 69364, Lyon, France; Centre National de la Recherche Scientifique, UMR 5166, Evolution des Régulations Endocriniennes, Muséum National d’Histoire Naturelle (A.d.L., B.D.), 75321 Paris, France; and Institut National de la Santé et de la Recherche Médicale, Unité-476, Faculté de Médecine (M.M., A.M.), 13385 Marseille, France

Address all correspondence and requests for reprints to: Dr. Vincent Laudet, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5665, Laboratoire de Biologie Moléculaire et Cellulaire, Ecole Normale Supérieure de Lyon, 46 allée d’ Italie, 69364 Lyon Cedex 07, France. E-mail: vincent.laudet{at}ens-lyon.fr.

Neotenic amphibians such as the axolotl (Ambystoma mexicanum) are often unable to undergo metamorphosis under natural conditions. It is thought that neoteny represents a deviation from the standard course of amphibian ontogeny, affecting the thyroid axis at different levels from the central nervous system to peripheral organs. Thyroid hormone receptors (TRs) that bind the thyroid hormone (TH) T₃ have been described in axolotl. However, the full sequences of TR were needed to better characterize the TH response and to be able to assess their functional capacity at the molecular level. We report that each of the and ß axolotl TRs bind both DNA and TH, and they activate transcription in response to TH in a mammalian cell-based transient transfection assay. Moreover, both TRs are expressed in axolotl tissues. Interestingly, each TR gene generates alternatively spliced isoforms, harboring partial or total deletions of the ligand-binding domain, which are expressed in vivo. Further, we found that in the axolotl, TH regulates the expression of stromelysin 3 and collagenase 3, which are TH target genes in Xenopus. Taken together, these results suggest that axolotl TRs are functional and that the molecular basis of neoteny in the axolotl is not linked to a major defect in TH response in peripheral tissues.