Transcriptional repression of TRH promoter function by T3: analysis by in vivo gene transfer

We consider how an integrated in vivo model can be used to study the specif ic transcriptional effects of specific receptors in neuroendocrine systems. Our example is the role of thyroid receptor (TR) isoforms in mediating neg ative feedback effects of T-3 on TRH (thyrotropin releasing hormone) expres sion. The in vivo transfection method employed polyethylenimine (PEI) to in troduce genes directly into specific regions of the brains of mice, rats, a nd Xenopus tadpoles. In the mouse model, the technique has served to examin e TR effects on TRH transcription and on the pituitary-thyroid axis end poi nt: thyroid hormone secretion. When a TRH-luciferase construct is introduce d into the hypothalami of newborn mice TRH-luciferase transcription is regu lated physiologically, being significantly increased in hypothyroidism and decreased in T-3-treated animals. When various T-3-binding forms of TR beta or TR alpha are expressed in the hypothalamus, all TR beta isoforms give T -3-dependent regulation of TRH transcription, whereas TR alpha isoforms blo ck T-3-dependent transcription. Moreover, TR transcriptional effects are co rrelated with physiological consequences on circulating T-4. Thus, somatic gene transfer shows TR subtypes to have distinct, physiologically relevant effects on TRH transcription. The approach is an appealing alternative to g erminal transgenesis for studying specific neuroendocrine regulations at de fined developmental stages in different species.