PHYSIOLOGY OF THYROID HORMONE-DEPENDENT GENE EXPRESSION

描述（由申请人提供）：甲状腺激素（TH）是几乎所有人体组织正常生长、发育和功能所必需的。这项建议将研究TH如何控制骨骼肌分化，再生和功能。骨骼肌是公认的TH靶点。TH在很大程度上通过其对骨骼肌代谢的影响来控制静息能量消耗。TH对肌肉再生的作用的一个戏剧性的例子发生在营养不良（mdx）小鼠中，其中实验性甲状腺毒症加速了肌细胞的破坏，而当小鼠甲状腺功能减退时，这一过程被延缓。尽管甲状腺功能减退和甲状腺功能亢进患者存在肌肉功能障碍，但TH在骨骼肌中作用的机制仍知之甚少。TH作用的第一步是激素原T4的5'单脱碘，通过1型和2型硒代脱碘酶（D1和D2）形成3，5，3'三碘甲状腺原氨酸（T3）。形成的T3占T4的大部分作用。T3的作用需要其与核受体（TR 1和TR 2）结合。T3效应的终止和T4活化的阻止通过3型脱碘酶（D3）从碘甲腺原氨酸核中去除一个或两个内环碘来催化。T4激活的D2（但不是D1）和T4和T3失活的D3都在卫星细胞中表达，卫星细胞是肌肉干细胞的等价物。这些脱碘酶的存在允许调节细胞内卫星细胞T3浓度，以响应各种细胞信号，而不依赖于循环TH水平。一个引人注目的例子发生在实验性肌肉损伤后，它诱导Notch，Wnt/2-catenin，Sonic hedgehog，Tgf 2和Hif 11等的增加，所有这些都是已知激活Dio 3基因的。这导致损伤区域中D3的短暂早期增加，持续8-10天，并且与卫星细胞和成肌细胞前体细胞的扩增有关。FoxO 3介导的D2增加遵循增加的细胞内T4至T3转化。细胞内T3的增加促进替代受损肌细胞的成肌细胞前体库的分化。循环甲状腺激素浓度始终保持恒定。在维持正常循环T3浓度的Dio 2 null（D2 KO）小鼠中，损伤肌肉的修复显著延迟，并且T3依赖性MyoD 1及其下游靶点保持较低，表明细胞内D2介导的T3产生的增加是正常再生和分化所需的。本项目将利用遗传和生物化学技术评估D3和D2作用产生的肌肉细胞内T3动态变化的影响。我们将探讨这些变化如何促进骨骼肌分化和再生。我们还将确定脱碘酶活性的治疗操作是否可用于增强对创伤性或退行性肌肉损伤或老年人肌肉减少症等疾病的治疗。

DESCRIPTION (provided by applicant): Thyroid hormone (TH) is required for normal growth, development and function of nearly all human tissues. This proposal will examine how TH controls skeletal muscle differentiation, regeneration and function. Skeletal muscle is a well-recognized TH target. TH controls resting energy expenditure in large part through its effects on skeletal muscle metabolism. A dramatic example of the effect of TH on muscle regeneration occurs in the dystrophic (mdx) mouse in which experimental thyrotoxicosis accelerates myocyte destruction, while this process is retarded when mice are made hypothyroid. Despite this and the muscle dysfunction in hypothyroid and thyrotoxic patients, the mechanism(s) for the actions of TH in skeletal muscle are poorly understood. The first step in TH action is the 5' monodeiodination of the prohormone T4, to form 3,5,3' triiodothyronine (T3) by the types 1 and 2 selenodeiodinases (D1 and D2). The T3 formed accounts for most of the actions of T4. The effects of T3 require its binding to nuclear receptors (TR1 and TR2). Termination of the effects of T3 and prevention of T4 activation is catalyzed by removal of one or both inner ring iodines from the iodothyronine nucleus by the type 3 deiodinase (D3). Both the T4-activating D2 (but not D1) and the T4- and T3-inactivating D3 are expressed in the satellite cells which are the muscle stem cell equivalent. The presence of these deiodinases allows regulation of the intracellular satellite cell T3 concentration in response to various cellular cues independent of the circulating TH levels. A striking example occurs after experimental muscle injury which induces an increases in Notch, Wnt/2-catenin, Sonic hedgehog, Tgf2, and Hif11, among others, all of which are known to activate the Dio3 gene. This leads to a transient early increase of D3 in the injured region lasting 8-10 days and is associated with the expansion of the satellite and myoblast precursor cells. A FoxO3- mediated increase in D2 follows increasing intracellular T4 to T3 conversion. The increase in intracellular T3 facilitates differentiation of the myoblast precursor pool replacing the damaged myocytes. Circulating thyroid hormone concentrations remain constant throughout. In a Dio2 null (D2KO) mouse, which maintains a normal circulating T3 concentration, the repair of injured muscle is markedly delayed and the T3-dependent MyoD1 and its downstream targets remain low, indicating an increase in intracellular D2-mediated T3 production is required for normal regeneration and differentiation. This project will evaluate the effects of the dynamic changes in intracellular T3 in muscle produced by the actions of D3 and D2 using genetic and biochemical techniques. We will explore how these changes facilitate skeletal muscle differentiation and regeneration. We will also determine whether therapeutic manipulations of deiodinase activities could be used to enhance the treatment of conditions such as traumatic or degenerative muscle injury or the sarcopenia of the elderly.