Non-classical thyroid hormone action mediated by thyroid hormone receptor alpha and beta

甲状腺激素(TH)对正常发育、生长和新陈代谢是必不可少的。它们充当甲状腺激素受体(TRs)α和β的配体。受体位于靶基因的启动子上，与激素结合后诱导基因表达，被认为是其经典的作用方式。最近发现，TRs也可以独立于DNA结合发挥作用：它们可以激活一个细胞信号通路，即PI3K通路，然后该通路也可以诱导基因表达，但也可能介导TH的其他生理效应。这被描述为非经典的TH作用。非经典TH动作的生理学相关性目前尚不清楚。现在的挑战是确定TH的哪些效应是通过TRs的两种机制中的哪一种来调节的，经典的(DNA依赖的)或非经典的(DNA非依赖的)。因此，我们为TRs建立了小鼠模型，其中TRs被修改，使它们不能再与DNA结合，TRalphaGS和TRbetaGS小鼠。因此，由这些受体介导的TH的所有作用必须是DNA不依赖的。这些小鼠存活并具有生育能力，使我们能够在活体动物模型中研究TH的纯非经典、DNA无关的效应，并与TR野生型(WT)和TRKO(KO)小鼠进行比较：在TRWT小鼠中观察到的TH效应，在TRKO小鼠中观察到，但在TRGS小鼠中保存下来的TH效应必须依赖于TRS，但不依赖于DNA结合，构成非经典的TH作用。初步表型显示，TRalphaGS和TRbetaGS小鼠的表型与TRpha或TRbetaKO小鼠不同：TRalphaGS小鼠保持心率，而不是像TRalphaKO小鼠那样减慢；TH处理导致TRbetaGS小鼠肌肉中的慢纤维向快纤维转变而不是TRbetaKO小鼠；TH处理使TRbetaGS小鼠的血糖迅速降低，但在TRbetaKO小鼠中不是；TRbetaGS小鼠的体温高于TRbetaKO小鼠。这些初步数据有力地表明，非经典的TRR信号在体内是相关的，这挑战了当前的范式，即TH的主要效应是由基因决定的，这些基因是通过经典的tr作用于其启动子而诱导的。因此，本项目的目的是以缺乏经典TH作用的TRalphaGS和TRbetaGS小鼠为模型，研究非经典TH作用对TH整体效应的贡献。其具体目的是：1)确定通过TRbeta的非经典TH作用在哪里以及如何激活生热；2)确定通过TRbeta的非经典TH作用在胰岛β细胞功能中的作用；3)确定非经典TH作用通过TRpha在心肌肥大中的作用。这个项目将促进我们对TH作用的理解，特别是对由TRsα和β介导的非经典TH作用的生理相关性的理解。

Thyroid hormones (TH) are essential for normal development, growth, and metabolism. They act as ligands of the thyroid hormone receptors (TRs) alpha and beta. The TRs reside on the promoters of target genes and induce gene expression after hormone binding, which is seen as their classical mode of action. Recently, it was discovered that TRs can also act independent of DNA binding: they can activate a cellular signaling pathway, the PI3K pathway, which then also induces gene expression, but may also mediate other physiological effects of TH. This is characterized as non-classical TH action. The physiological relevance of non-classical TH action is currently unknown. The challenge is now to determine which effects of TH are mediated through which of the two mechanisms of the TRs, classical (DNA-dependent) or non-classical (DNA-independent). We therefore generated mouse models for the TRs, in which the TRs were modified so that they cannot bind to DNA anymore, the TRalphaGS and the TRbetaGS mice. All effects of TH mediated by these receptors must therefore be DNA-independent. These mice are viable and fertile and allow us to study purely non-classical, DNA-independent effects of TH in a live animal model in comparison to TR wild-type (WT) and TR knock-out (KO) mice: TH effects that are observed in TR WT mice, absent in TR KO mice, but preserved in TR GS mice must be dependent on TRs, but independent from DNA binding, constituting non-classical TH action. Preliminary phenotyping showed that the phenotype of TRalphaGS and TRbetaGS mice is not the same as that of TRalpha or TRbeta (KO) mice: Heart rate is maintained in TRalphaGS mice and not reduced as seen in TRalphaKO mice, TH treatment leads to a shift from slow to fast fibers in muscle from TRbetaGS mice but not from TRbetaKO mice, TH treatment rapidly decreases blood glucose in TRbetaGS mice, but not in TRbetaKO mice, and TRbetaGS mice have a higher body temperature than TRbetaKO mice.These preliminary data strongly suggest that non-classical TR signaling is relevant in vivo, which challenges the current paradigm that the main TH effects are determined by the genes that are induced via classical TR action on their promoters. Aim of this project, therefore, is to study the contribution of non-classical TH action to the overall effect of TH using the TRalphaGS and TRbetaGS mice with absent classical TH action as suitable models. The specific aims are: 1) To determine where and how non-classical TH action via TRbeta activates thermogenesis, 2) to determine the role of non-classical TH action via TRbeta in pancreatic beta-cell function, 3) to determine the role of non-classical TH action via TRalpha in cardiac hypertrophy. This project will advance our understanding of TH action and especially of the physiological relevance of non-classical TH action mediated by the TRs alpha and beta.