Promoter-specific regulation of the brain-derived neurotropic factor gene by thyroid hormone in the developing rat cerebellum

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-TV), 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 TV 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.