Abstract
A gene responsive to thyroid hormone (TH) has been identified in the adult rat brain cerebral tissue. A cDNA probe differentially expressed in euthyroid, hypothyroid and hyperthyroid rat cerebral tissue, generated by reverse transcriptase-PCR differential display of mRNA, was used to screen the rat brain cDNA library. A 3.4 kb positive clone hybridized in Northern blots with a 3.8 kb mRNA that proved to be TH responsive (THR). The remaining coding sequence and a part of the 5' untranslated region of this cDNA were obtained by 5' rapid amplification of cDNA ends. The deduced amino acid sequence revealed that THR protein (THRP), a 68 kDa moiety, has 83% sequence similarity with c-Abl interactor protein (Abi-2), which is a substrate for tyrosine kinase activity of c-Abl. The extensive similarity between the two proteins suggests a potential role for THRP as a substrate for c-Abl. Northern analysis showed that the expression of THR mRNA in hyperthyroid rats is 6-fold that in euthyroid rats. There is also a 4-6-fold increase in the concentration of THRP, as analysed by Western analysis. Owing to the extensive similarity between rat THRP and human Abi-2, a polyclonal anti- (human Abi-2) antibody was successfully used for Western analysis of proteins from the rat tissues. The observed increase in both the mRNA and the protein did not decline after beta-adrenergic system blockade with propranolol, suggesting that the action of TH on the expression of this gene is not mediated through the beta-adrenergic system. Immunohistochemical studies revealed that neuronal cells were particularly rich in THRP. Both THR mRNA and THRP are rapidly induced in vivo after intravenous administration of thyroxine. Tissue distribution studies indicated that the cerebral tissue was particularly enriched with THR mRNA and 68 kDa THRP. A cDNA clone for a THR gene could provide a useful tool to study the molecular mechanisms of TH effects on cerebral tissue in adult animals.
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