Koibuchi N, Fukuda H, Chin W W
Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA.
Endocrinology. 1999 Sep;140(9):3955-61. doi: 10.1210/endo.140.9.6997.
Thyroid hormone (TH) plays a critical role in normal cerebellar development. However, the molecular mechanisms of TH action in the developing cerebellum are not fully understood. This action could be exerted in part through brain-derived neurotropic factor (BDNF), as cerebellar BDNF messenger RNA (mRNA) expression is lower, and replacement of BDNF partially reverses the abnormal neurogenesis in the hypothyroid rat. The rat BDNF gene consists of four noncoding exons (exons I-IV), each of which is linked to a different promoter, and a protein-coding exon (exon V). To study promoter-specific regulation of the BDNF gene by TH, ribonuclease protection assay of each exon mRNA was performed using total developing rat cerebellar RNA. During cerebellar development, all exon mRNAs were detected, but with different expression patterns; among noncoding exon mRNAs, exon II mRNA was the most abundant. Daily TH replacement induced a 3-fold increase in exon II mRNA on postnatal day (P) 15. On P30, exon II mRNA was still much greater in the TH-replaced animal. Exon I mRNA was detected on P2 and P7. However, in contrast to exon II mRNA, TH treatment suppressed the expression of exon I mRNA on P2. Exon III and IV mRNAs were not detected on P2 and P7, but small amounts were observed starting on P15 in TH-replaced animals. They were not detected by P30 in hypothyroid animals. In contrast, in the cerebral cortex, although all exons are differentially regulated during development, the expression of each mRNA was not significantly altered by TH. These results indicate that TH regulates BDNF gene expression in a promoter-, developmental stage-, and brain region-specific manner, which may play an important role in region- and stage-specific regulation of brain development by TH.
甲状腺激素（TH）在正常小脑发育过程中发挥着关键作用。然而，TH在发育中小脑的作用分子机制尚未完全明确。这种作用可能部分通过脑源性神经营养因子（BDNF）来实现，因为小脑BDNF信使核糖核酸（mRNA）表达较低，而补充BDNF可部分逆转甲状腺功能减退大鼠的异常神经发生。大鼠BDNF基因由四个非编码外显子（外显子I-IV）和一个蛋白质编码外显子（外显子V）组成，每个非编码外显子都与一个不同的启动子相连。为了研究TH对BDNF基因启动子特异性的调控，利用发育中大鼠小脑总RNA对每个外显子mRNA进行核糖核酸酶保护分析。在小脑发育过程中，所有外显子mRNA均被检测到，但表达模式不同；在非编码外显子mRNA中，外显子II mRNA最为丰富。出生后第（P）15天，每日补充TH可使外显子II mRNA增加3倍。在P30时，补充TH的动物中外显子II mRNA仍远高于未补充组。外显子I mRNA在P2和P7时被检测到。然而，与外显子II mRNA不同的是，TH处理在P2时抑制了外显子I mRNA的表达。外显子III和IV mRNA在P2和P7时未被检测到，但在补充TH的动物中从P15开始可观察到少量表达。在甲状腺功能减退的动物中，到P30时仍未检测到。相比之下，在大脑皮层中，尽管所有外显子在发育过程中受到不同调控，但TH并未显著改变每个mRNA的表达。这些结果表明，TH以启动子、发育阶段和脑区特异性的方式调节BDNF基因表达，这可能在TH对脑发育的区域和阶段特异性调控中发挥重要作用。
