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

甲状腺激素（TH）是正常发育，生长和代谢所必需的。它们作为甲状腺激素受体（TR）α和β的配体。TRs位于靶基因的启动子上，并在激素结合后诱导基因表达，这被视为其经典的作用模式。最近，人们发现TR也可以独立于DNA结合发挥作用：它们可以激活细胞信号传导途径，PI3K途径，然后也诱导基因表达，但也可能介导TH的其他生理作用。这被表征为非经典TH作用。非经典TH作用的生理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TR的两种机制中的哪一种介导的，经典的（DNA依赖的）或非经典的（DNA不依赖的）。因此，我们产生了TRs的小鼠模型，其中TRs被修饰，使得它们不能再与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非依赖性的。这些小鼠是有活力的和可生育的，并且允许我们在活的动物模型中研究TH与TR野生型（WT）和TR敲除（KO）小鼠相比的纯粹非经典的、DNA非依赖性的作用：在TR WT小鼠中观察到的TH作用，在TR KO小鼠中不存在，但在TR GS小鼠中保留，必须依赖于TR，但独立于DNA结合，构成非经典的TH作用。初步表型分析显示，TRalpha GS和TRbetaGS小鼠的表型与TRalpha或TRbeta（KO）小鼠的表型不同：在TRalphaGS小鼠中维持心率，并且不像在TRalphaKO小鼠中所见的那样降低，TH处理导致TRbetaGS小鼠而不是TRbetaKO小鼠的肌肉中从慢纤维向快纤维的转变，TH处理快速降低TRbetaGS小鼠而不是TRbetaKO小鼠的血糖，这些初步数据强烈地表明非经典TR信号传导在体内是相关的，这挑战了目前的范式，即主要的TH效应是由通过经典TR作用于其启动子而诱导的基因决定的。因此，本项目的目的是使用缺乏经典TH作用的TRalpha GS和TRbetaGS小鼠作为合适的模型来研究非经典TH作用对TH的总体效应的贡献。具体目标是：1）确定通过TRbeta的非经典TH作用在何处以及如何激活产热，2）确定通过TRbeta的非经典TH作用在胰腺β细胞功能中的作用，3）确定通过TRalpha的非经典TH作用在心脏肥大中的作用。这个项目将促进我们对TH作用的理解，特别是对TRs α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.