Identification of a thyroid hormone response element in the mouse Kruppel-like factor 9 gene to explain its postnatal expression in the brain.

Brain development is critically dependent on thyroid hormone (T(3)). Krüppel-like factor 9 (Klf9) is a T(3)-inducible gene in developing rat brain, and several lines of evidence support that KLF9 plays a key role in neuronal morphogenesis. Here we extend our findings to the mouse and demonstrate the presence of a functional T(3) response element (T(3)RE) in the 5' flanking region of the mouse Klf9 gene. Klf9 mRNA is strongly induced in the mouse hippocampus and cerebellum in a developmental stage- and T(3)-dependent manner. Computer analysis identified a near optimal direct repeat 4 (DR-4) T(3)RE 3.8 kb upstream of the Klf9 transcription start site, and EMSAs showed that T(3) receptor (TR)-retinoid X receptor heterodimers bound to the T(3)RE with high affinity. The T(3)RE acts as a strong positive response element in transfection assays using a minimal heterologous promoter. In the mouse neuroblastoma cell line N2a[TRbeta1], T(3) caused a dose-dependent up-regulation of Klf9 mRNA. Chromatin immunoprecipitation assays conducted with N2a[TRbeta1] cells showed that TRs associated with the Klf9 T(3)RE, and this association was promoted by T(3). Treatment of N2a[TRbeta1] cells with T(3) led to hyperacetylation of histones 3 and 4 at the T(3)RE site. Furthermore, TRs associated with the DR-4 T(3)RE in postnatal d 4 mouse brain, and histone 4 acetylation was greater at this site compared with other regions of the Klf9 gene. Our study identifies a functional DR-4 T(3)RE located in the mouse Klf9 gene to explain its regulation by T(3) during mammalian brain development.