胚胎干细胞（Embryonic stem cell，ESC）具有自我更新和分化的能力。Serum+lif培养条件下的小鼠ESC（mESC）与植入前的胚胎相似，被称为原始态mESC；而2i+lif（即ERK抑制剂和GSK3抑制剂)条件下的mESC被称为基态ESC。后者更接近早期囊胚细胞，具有更强的分化潜能，更有助于发育研究。我们前期研究发现，2i+lif条件下的mESC对BRD4依赖更少，且BRD4在两个系统下的全基因组分布不同。因此我们假设BRD4介导的基本转录装置的差异导致了两种不同的多能性状态。为了进一步阐明这一假说，解释2i+lif条件为什么能使mESC更接近早期胚胎细胞，本项目拟研究：1.BRD4在基因组上差异分布的机制，特别是与代谢改变表观修饰的关系；2. BRD4重新分布对转录，特别是转录延伸的影响；3. BRD4在mESC中的机制是否适用于人基态ESC。

Embryonic stem cells (ESCs) have the ability to self-renew without loosing their pluripotent characteristics and differentiate into multiple lineages upon treatment with specific signaling clues. Mouse ESCs cultured with serum+LIF are considered similar to the preimplantation embryo, and are therefore termed naïve. Yet, when cultured in medium with 2 inhibitors (2i, ERK inhibitor and GSK3 inhibitor) +LIF, they behave more similarly to the early blastocyst. This is termed ground state pluripotency. Notably, the recent derivation of human ESCs in naïve conditions is supposed to mimic the mouse ESC ground state. This is important because ESCs in ground state are thought to have improved differentiation potential and also represent a better model to understand development. We want to uncover the mechanisms underlying the different transcriptional network in mouse ESCs cultured in 2i naive conditions compared to serum+LIF, and also study whether these same mechanisms participate in the maintenance of naïve human ESCs. Our specific hypothesis is that changes in the basal transcriptional machinery mediated by repurposing of the bromodomain protein BRD4 are partly responsible for the observed differences in gene regulatory networks between the 2 systems. Our preliminary data showed that BRD4 was less necessary in mouse ESCs cultured in 2i+LIF compared to serum+LIF, and that this is caused by BRD4 redistribution along the genome. We want to explore the mechanisms involved in BRD4 redistribution, with emphasis on alterations in histone acetylation (BRD4 binds acetylated histones) mediated by changes in lipid metabolism (as a source of acetyl groups) in mouse ESCs cultured with 2i+LIF. We will also aim to understand the consequences of BRD4 redistribution in the reshaping of the transcriptional landscape, with particular emphasis on changes in transcriptional pause release by RNA polymerase II (a phenomenon triggered by BRD4) at key loci. Altogether, we expect to provide an explanation to why culture of mouse ESCs in 2i+LIF induces a more naïve state that mimics the early blastocyst. In addition, we will study whether these same mechanism apply to human naïve ESCs and if not whether manipulation of BRD4 can help produce human naïve ESCs with optimal characteristics.