Cai Liquan, Brown Donald D
Department of Embryology, Carnegie Institution of Washington, Baltimore, MD 21210-3399, USA.
Dev Biol. 2004 Feb 1;266(1):87-95. doi: 10.1016/j.ydbio.2003.10.005.
The thyroid gland synthesizes thyroxine (T4), which passes through the larval tadpole's circulatory system. The enzyme type II iodothyronine deiodinase (D2) converts thyroxine (T4) to the active hormone 3,5,3'-triiodothyronine (T3) in peripheral tissues. An early response to thyroid hormone (TH) in the Xenopus laevis tadpole is the stimulation of cell division in cells that line the brain ventricles, the lumen of the spinal cord, and the limb buds. These cells express constitutively high levels of D2 mRNA. Exogenous T4 induces early DNA synthesis in brain, spinal cord, and limb buds as efficiently as T3. The deiodinase inhibitor iopanoic acid blocks T4- but not T3-induced cell division. At metamorphic climax, both TH-induced cell division and D2 expression decrease in the brain. Then D2 expression appears in late-responding tissues including the anterior pituitary, the intestine, and the tail where cell division is reduced or absent. Therefore, constitutive expression of D2 occurs in the earliest target tissues of TH that will grow and differentiate, while TH-induced expression of D2 takes place in late-responding tissues that will remodel or die. This pattern of constitutive and induced D2 expression contributes to the timing of metamorphic changes in these tissues.
甲状腺合成甲状腺素（T4），其通过幼体蝌蚪的循环系统。II型碘甲状腺原氨酸脱碘酶（D2）在周围组织中将甲状腺素（T4）转化为活性激素3,5,3'-三碘甲状腺原氨酸（T3）。非洲爪蟾蝌蚪对甲状腺激素（TH）的早期反应是刺激脑室、脊髓腔和肢芽内衬细胞的细胞分裂。这些细胞组成性地高水平表达D2 mRNA。外源性T4与T3一样有效地诱导脑、脊髓和肢芽中的早期DNA合成。脱碘酶抑制剂碘番酸阻断T4诱导的细胞分裂，但不阻断T3诱导的细胞分裂。在变态高峰期，TH诱导的细胞分裂和D2表达在脑中均下降。然后D2表达出现在反应较晚的组织中，包括垂体前叶、肠道和尾部，这些组织中的细胞分裂减少或不存在。因此，D2的组成性表达发生在TH最早的将生长和分化的靶组织中，而TH诱导的D2表达发生在将重塑或死亡的反应较晚的组织中。这种组成性和诱导性D2表达模式有助于这些组织中变态变化的时间安排。
