Feedback on hypothalamic TRH transcription is dependent on thyroid hormone receptor N terminus

The β thyroid hormone receptor (TRβ), but not TRα1, plays a specific role in mediating T3-dependent repression of hypothalamic TRH transcription. To investigate the structural basis of isoform specificity, we compared the transcriptional regulation and DNA binding obtained with chimeric and N-terminally deleted TRs. Using in vivo transfection assays to follow hypothalamic TRH transcription in the mouse brain, we found that TRβ1 and chimeras with the TRβ1 N terminus did not affect either transcriptional activation or repression from the rat TRH promoter, whereas N-terminally deleted TRβ1 impaired T3-dependent repression. TRα1 or chimeras with the TRα1 N terminus reduced T3-independent transcriptional activation and blocked T3-dependent repression of transcription. Full deletion of the TRα1 N terminus restored ligand-independent activation of transcription. No TR isoform specificity was seen after transcription from a positive thyroid hormone response element. Gel mobility assays showed that all TRs tested bound specifically to the main negative thyroid hormone response element in the TRH promoter (site 4). Addition of neither steroid receptor coactivator 1 nor nuclear extracts from the hypothalamic paraventricular nuclei revealed any TR isoform specificity in binding to site 4. Thus N-terminal sequences specify TR T3-dependent repression of TRH transcription but not DNA recognition, emphasizing as yet unknown neuron-specific contributions to protein-promoter interactions in vivo.