Identification of genes mediating thyroid hormone action in the developing mouse cerebellum.

Despite the indispensable role thyroid hormone (TH) plays in brain development, only a small number of genes have been identified to be directly regulated by TH and its precise mechanism of action remains largely unknown, partly because most of the previous studies have been carried out at postnatal day 15 or later. In the present study, we screened for TH-responsive genes in the developing mouse cerebellum at postnatal day 4 when morphological alterations because of TH status are not apparent. Among the new candidate genes selected by comparing gene expression profiles of experimentally hypothyroid, hypothyroid with postnatal thyroxine replacement, and control animals using oligoDNA microarrays, six genes were confirmed by real-time PCR to be positively (orc1l, galr3, sort1, nlgn3, cdk5r2, and zfp367) regulated by TH. Among these, sort1, cdk5r2, and zfp367 were up-regulated already at 1 h after a single injection of thyroxine to the hypothyroid or control animal, suggesting them to be possible primary targets of the hormone. Cell proliferation and apoptosis examined by BrdU incorporation and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay revealed that hypothyroidism by itself did not enhance apoptosis at this stage, but rather increased cell survival, possibly through regulation of these newly identified genes.