Non-canonical thyroid hormone signaling mediates cardiometabolic effects in vivo

Thyroid hormone (TH) and TH receptors (TRs) α and β act by binding to TH response elements (TREs) in regulatory regions of target genes. This nuclear signaling is established as the canonical pathway for TH action. In addition, however, TRs can activate intra-cellular second messenger signaling pathways. Whether such non-canonical TR signaling is physiologically relevant in vivo is unknown. Here we generated knock-in mice with a mutation in the TR DNA-binding domain that abrogates binding to DNA and TREs and leads to a complete loss of canonical TH action. We show that several important physiological TH effects are preserved despite disrupted 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. Our findings provide first 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. Global H3K27ac ChIP-seq in livers of WT TR mice or mice with different TR mutations treated with T3 or control (PBS).