Understanding the role of thyroid hormone signaling during axolotl limb regeneration

组织再生潜力在物种和器官之间差异很大。然而，大多数生物，包括哺乳动物，在幼年阶段具有部分再生能力，随着动物的发育和组织的成熟而丧失。这种下降与甲状腺器官发生的开始和几个物种中全身甲状腺激素的存在有关。例如，青蛙在甲状腺激素诱导的变态过程中失去了肢体再生能力。相反，蝾螈（Ambystoma mexicanum）在其一生中维持低水平的甲状腺激素并显示出显著的组织再生能力，而甲状腺水平高得多的小鼠在所有三种中具有最低的组织再生能力。以前在美西螈中的研究表明，L-甲状腺素诱导的变态降低了再生率和保真度[1]，支持甲状腺信号传导、变态和再生能力之间不仅存在相关性而且存在因果关系的观点。虽然这些事实早已为人所知，但我们并不了解甲状腺途径发挥其抗再生作用的细胞和分子机制。 在这一建议中，我们希望解决以下基本问题。首先，甲状腺激素引起的不良再生反应-是由于甲状腺激素的全身效应还是其对特定细胞类型的直接影响，还是两者兼而有之？第二，甲状腺激素的变态和抗再生特性是协同作用的还是可以解耦的？第三，在关键细胞类型的分子水平上，变态意味着什么？在所有不同的可能性中，我们的中心假设是甲状腺治疗后的变态和不良再生行为彼此耦合，并且甲状腺激素治疗通过将成纤维细胞推向进一步分化的状态来发挥其抗再生和变态作用。我们根据文献和初步数据提供了令人信服的证据来支持这些假设，并概述了一系列实验，这些实验将使我们能够测试我们的假设并回答上述问题。

Tissue regenerative potential greatly varies across species and organs. Nevertheless, most organisms, including mammals, possess partial regenerative abilities in their juvenile stages, which is lost as the animal develops and tissue matures. This decline is correlated with the onset of thyroid gland organogenesis and the presence of systemic thyroid hormone in several species. For example, the frogs lose their limb regeneration ability during metamorphosis which is induced by thyroid hormones. On the contrary, axolotls (Ambystoma mexicanum) maintain a low level of the thyroid hormones and show remarkable tissue regeneration ability throughout their life whereas mouse with much higher thyroid level has the lowest tissue regeneration ability among all three. A previous study in axolotl suggests that L-thyroxine induced metamorphosis reduces regenerative rate and fidelity [1], supporting the idea that there is not only a correlative but also a causal relation between the thyroid signaling, metamorphosis and the regenerative ability. Although these facts have long been known, we do not understand the cellular and molecular mechanism by which the thyroid pathway exerts its anti-regenerative effects. In this proposal, we want to address the following fundamental questions. First, poor regenerative response caused by the thyroid hormone – is it due to a systemic effect of the thyroid hormone or its direct effect on a specific cell type or is it both? Second, are metamorphosis and anti-regenerative properties of the thyroid hormone synergistic or can they be decoupled? Third, what does metamorphosis mean at a molecular level in the key cell type? Among all different possibilities, our central hypotheses are metamorphosis and poor regenerative behavior upon the thyroid treatment are coupled with each other and that the thyroid hormone treatment exerts its anti-regenerative and metamorphic effect by pushing fibroblasts to a further differentiated state. We provide compelling evidences based on literature and our preliminary data to support these hypotheses and we outline a series of experiments that will allow us to test our hypotheses and answer the aforementioned questions.