TRαGS and TRβGS knock-in mice demonstrate physiological relevance of non-classical thyroid hormone action

Abstract

Introduction: Thyroid hormone (TH) and thyroid hormone receptors (TRs) can act either classically via binding to promoters of target genes or non-classically via activating the PI3K signaling pathway. Our aim is to determine which TH effects are mediated by which mechanism. We therefore generated knock-in mice for DNA binding domain mutations that abrogate promoter binding of the TRs (TRαGS and TRβGS). Thus, the classical nuclear action is lost, whereas the non-classical mechanism is preserved. Material & Methods: TRαGS and TRβGS mice were generated at the University of Duisburg-Essen using zinc finger nucleases. Results: Loss of genomic action of TR GS mutants was confirmed in vitro. In vivo, heart rate is the same in TRαGS mice as in wildtype (WT) mice (749 ± 10 vs. 740 ± 20, n.s.) and not reduced as seen in TRα knockout (KO) mice (694 ± 6, P< 0.0001). And TRβGS mice have a 1 °C higher core body temperature than TRβKO mice (37.7 °C vs. 36.6 °C). A subset of TH target genes is expressed at similar levels in WT and GS mice, but showed reduced levels in KO mice. This suggests that this induction is TR mediated, but independent from classical TR/promoter interaction. Conclusion: Despite loss of classical TR action, TRαGS and TRβGS knock-in mice differ phenotypically from knockout mice. Remarkably, heart rate is not reduced in TRαGS mice and TRβGS mice have a higher body temperature than TRβ-/- mice. These data show that non-classical TH/TR signaling is relevant in vivo. The TRαGS and TRβGS mice are suitable models to further study the contribution of non-classical TH action to the overall effect of TH.