NIDDK, Laboratory of Endocrinology and Receptor Biology, National Institutes of Health, Bethesda, MD 20892, USA.
Department of Cell, Developmental and Regenerative Biology, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA.
Endocrinology. 2023 Jan 9;164(3). doi: 10.1210/endocr/bqad006.
The function of a hormone receptor requires mechanisms to control precisely where, when, and at what level the receptor gene is expressed. An intriguing case concerns the selective induction of thyroid hormone receptor β2 (TRβ2), encoded by Thrb, in the pituitary and also in cone photoreceptors, in which it critically regulates expression of the opsin photopigments that mediate color vision. Here, we investigate the physiological significance of a candidate enhancer for induction of TRβ2 by mutagenesis of a conserved intron region in its natural context in the endogenous Thrb gene in mice. Mutation of e-box sites for bHLH (basic-helix-loop-helix) transcription factors preferentially impairs TRβ2 expression in cones whereas mutation of nearby sequences preferentially impairs expression in pituitary. A deletion encompassing all sites impairs expression in both tissues, indicating bifunctional activity. In cones, the e-box mutations disrupt chromatin acetylation, blunt the developmental induction of TRβ2, and ultimately impair cone opsin expression and sensitivity to longer wavelengths of light. These results demonstrate the necessity of studying an enhancer in its natural chromosomal context for defining biological relevance and reveal surprisingly critical nuances of level and timing of enhancer function. Our findings illustrate the influence of noncoding sequences over thyroid hormone functions.
激素受体的功能需要机制来精确控制受体基因表达的位置、时间和水平。一个有趣的例子涉及甲状腺激素受体 β2（TRβ2）的选择性诱导，该基因由 Thrb 编码，在垂体中以及在视锥细胞中都有表达，它在调节介导色觉的视蛋白光色素的表达中起着关键作用。在这里，我们通过对其天然的内源性 Thrb 基因中的保守内含子区域进行突变，研究了 TRβ2 诱导的候选增强子的生理意义。突变 bHLH（碱性螺旋-环-螺旋）转录因子的 e-box 位点优先损害视锥细胞中 TRβ2 的表达，而附近序列的突变则优先损害垂体中的表达。包含所有位点的缺失会损害两种组织中的表达，表明具有双功能活性。在视锥细胞中，e-box 突变破坏了染色质乙酰化，削弱了 TRβ2 的发育诱导，并最终损害了视蛋白的表达和对长波长光的敏感性。这些结果表明，在研究增强子时，有必要在其天然的染色体环境中研究其生物学相关性，并揭示增强子功能在水平和时间上令人惊讶的关键细微差别。我们的发现说明了非编码序列对甲状腺激素功能的影响。
