Endocrine Abstracts

Thyroid hormone (TH) and TH receptors (TRs) α and β are currently perceived as prototypical nuclear receptors, acting by binding to TH response elements (TREs) in regulatory regions of target genes. This nuclear signaling is long established as the canonical pathway for TH action. However, TRs can also act non-canonically, independent from DNA binding and outside the nucleus, by modulating second messenger signaling in vitro. Whether such non-canonical TR action is relevant in vivo has so far been unknown. The main reason is that the physiological role of TRs has been studied in vivo for more than fifteen years by comparing WT mice with TR knockout (TR KO) mice. Since canonical and non-canonical TR signaling are both present in WT and absent in TR KO mice, such a comparison could not distinguish between the two mechanisms. To solve this problem and separate both modes of TH/TR signaling, we abolished DNA-binding of TRs in TRαGS and TRβGS mice. We show that several important physiological TH effects are preserved despite disrupted TR DNA-binding, most notably heart rate, body temperature, blood glucose and triglyceride concentration, all of which were regulated by non-canonical TR signaling. Additionally, we confirm that TRE-binding defective TRβ leads to disruption of the hypothalamic-pituitary-thyroid axis with resistance to TH, while mutation of TRα causes a severe delay in skeletal development, showing TRE-mediated and tissue-specific canonical signaling. These results provide in vivo evidence that non-canonical TR signaling exerts important cardiometabolic effects, which are clearly separated from canonical actions. Consequently, these data challenge the current paradigm that TH action is exclusively mediated through regulation of gene transcription at the nuclear level.