Tadpole competence and tissue-specific temporal regulation of amphibian metamorphosis: roles of thyroid hormone and its receptors.
Amphibian metamorphosis is a post-embryonic process that systematically transforms different tissues in a tadpole. Thyroid hormone plays a causative role in this complex process by inducing a cascade of gene regulation. While natural metamorphosis does not occur until endogenous thyroid hormone has been synthesized, tadpoles are competent to respond to exogenous thyroid hormone shortly after hatching. In addition, even though the metamorphic transitions of individual organs are all controlled by thyroid hormone, each occurs at distinct developmental stages. Recent molecular studies suggest that this competence of premetamorphic tadpoles to respond to the hormone and the developmental stage-dependent regulation of tissue-specific transformations are determined in part by the levels of thyroid hormone receptors and the concentrations of cellular free thyroid hormone. In addition, at least two genes, encoding a cytosolic thyroid hormone binding protein and a 5-deiodinase, respectively, are likely to be critical players in regulating cellular free thyroid hormone concentrations. This review discusses how all of these molecular components coordinate to induce amphibian metamorphosis in a correct spatial and temporal manner. These studies provide us with general clues as to how and why tissues become competent to respond to hormonal signals.
Shi
Y B
YB
Laboratory of Molecular Embryology, National Institute of Child Health and Human Development, National Institute of Health, Bethesda, MD 20892, USA.
Wong
J
J
Puzianowska-Kuznicka
M
M
Stolow
M A
MA
D000663:Amphibians / Q000254:growth & development*; D000818:Animals; D004625:Embryo, Nonmammalian / Q000502:physiology; D018507:Gene Expression Regulation, Developmental; D007453:Iodide Peroxidase / Q000096:biosynthesis; D008675:Metamorphosis, Biological; D009928:Organ Specificity; D018168:Receptors, Retinoic Acid / Q000096:biosynthesis; D011988:Receptors, Thyroid Hormone / Q000096:biosynthesis; D047488:Retinoid X Receptors; D013961:Thyroid Gland / Q000254:growth & development; D013963:Thyroid Hormones / Q000502:physiology; D014157:Transcription Factors / Q000096:biosynthesis; D014982:Xenopus laevis
D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D013963:Thyroid Hormones; D014157:Transcription Factors; D007453:Iodide Peroxidase