转录延伸因子ELL3在胚胎干细胞染色质可塑性中的作用机制研究

Pluripotent stem cells, by virtue of their self-renewal and pluripotency properties hold significant promise for being used as an unlimited source for basic research on various developmental events and also for the therapeutic applications in genetic or degenerative diseases. Over the last few decades, researchers have made remarkable advances in understanding stem cell pluripotency and cellular reprogramming, which have led to the groundbreaking technology for generating induced pluripotent stem (iPS) cells. However, before iPS cell treatments can be put through clinical trials, one of the most important issues needed to be solved is how patient derived iPS cells or other pluripotent cells can be directly differentiated into specialized cell types consistently with high purity, high efficiency, and high safety. Chromatin plasticity is characteristic of embryonic stem cells and plays essential roles in their self-renewal and pluripotency. However, how chromatin plasticity is established and maintained is not well understood. Recently, we have identified ELL3 as the first transcription elongation factor possessing enhancer-specific binding activity. ELL3 is essential for stem cell specification through priming the future activation of many poised developmental genes by Super Elongation Complex (SEC). This work suggests a novel and critical function of enhancers regulated by ELL3 in chromatin plasticity control. Therefore, we plan to use ELL3 as an entry point to further uncover the enhancer-associated epigenetic network involved in establishing the pluripotent differentiation potentials of stem cells; 2) define the molecular mechanisms by which this network incorporates diverse developmental cues into transcription program to trigger the differentiation of stem cells in specific directions, while shut down other unfavorable cellular events. Our studies on ELL3-associated enhancer regulatory network and its roles in chromatin plasticity will advance our understanding on stem cell self-renewal and pluripotency.

干细胞因其自我更新及分化成不同类型细胞的潜能，已被广泛应用与发育相关的基础研究；同时可作为各种细胞和组织的来源，在再生医学领域具有非常广阔的应用前景。目前的研究表明：染色质可以作为主导因子调控胚胎干细胞自我更新及多向分化潜能。因此，研究染色质可塑性的形成及其调控机制具有重要意义。申请人的研究首次表明，在胚胎干细胞中，超级延伸复合物SEC亚基ELL3结合在处于暂停状态的增强子，调控了其相邻基因的启动子上暂停的Pol II水平及其在干细胞分化过程中的转录激活。ELL3的功能缺失影响了这些胚层特异性基因的激活，从而导致了胚胎干细胞向类胚体以及神经细胞分化的缺陷。本项目书拟通过研究ELL3相关的增强子调控网络，及其对启动子近段Pol II暂停和表观遗传学机器的作用，来探索胚胎干细胞染色质可塑性的形成及维持机制，以及它们对干细胞维持自我更新及多向分化潜能的重要作用。

干细胞因其自我更新及分化成不同类型细胞的潜能，已被广泛应用与发育相关的基础研究;同时可作为各种细胞和组织的来源，在再生医学领域具有非常广阔的应用前景。目前的研究表明:染色质可以作为主导因子调控胚胎干细胞自我更新及多向分化潜能。因此，研究染色质可塑性的形成及其调控机制具有重要意义。我们之前的研究首次表明，在胚胎干细胞中，超级延伸复合物SEC亚基ELL3结合在处于暂停状态的增强子，调控了其相邻基因的启动子上暂停的Pol II水平及其在干细胞分化过程中的SEC介导的转录激活。ELL3的功能缺失影响了这些胚层特异性基因的激活，从而导致了胚胎干细胞向类胚体以及神经细胞分化的缺陷。在本项目中，我们发现在胚胎干细胞中，Ell3可与Tet1互作，调控DNA去甲基化活性及组蛋白H3K27乙酰化水平，影响LINE-1的活性及表达。ELL3敲除可导致局部染色质结构重塑，影响相邻基因的表达，进而调控原始态胚胎干细胞向始发态分化进程。同时我们发现了SEC在调控启动子近端暂停的Pol II释放的相分离作用新机制，阐明了SEC在应激反应下聚集成相分离的液滴启动基因快速激活程序的分子机理。相关成果以通讯作者身份发表在Science Advances，Nucleic Acids Research，Mol Cell Biol等期刊上。