Lin28调控小鼠胚胎干细胞内源性病毒表达和胚胎二细胞期合子基因组激活

Embryonic stem (ES) cells derive from inner cell mass of blastocyst of mouse early embryos and can differentiate to three germ layers and germ cells, but can not differentiate to trophectoderm and extraembryonic tissues, and therefore, they are pluripotent but not totipotent. Two-cell state cells are totipotent cells in an earlier stage of embryo development and have potential to differentiate to all cell types of embryonic and extraembryonic tissues. The two-cell state is characterized by activation of endogenous retrovirus (ERV) genes which often overlap with regulatory regions (such as enhances) of early development genes, thereby influence their expression and early embryo development. In cultured mouse ES cells, there is a rare and transient cell population with activated two-cell marker genes including ERVs. How are ERVs activated and inactivated during the 2-cell state, and how do these events influence the whole zygotic genome activation and early embryo development are important questions to be answered. We found the pluripotency factor Lin28 is sharply decreased at the entry of 2-cell state, and gets re-induced at the exit of two-cell state, and keeps expressed toward blastocyst state. Lose-of-function Lin28 ES cells, iPS cells, and gain-of-function let-7 ES cells show altered expression of ERVs. The proposal aims to study 1) the role of Lin28 in regulating ERVs; 2) the mechanism of the regulation through epigenetics; and 3) the role of Lin28 in regulating entry and exit of two-cell state embryo. This study will provide new knowledge about our understanding of zygotic genome activation during the two-cell state as well as transition from totipotency to pluripotency. Moreover,this study will shed light on our understanding of pluripotent stem cells and their application in regenerative medicine.

早期胚胎发育的二细胞期细胞具有分化成为三个胚层和胚外组织的能力，因此被称为全能干细胞。伴随二细胞期的一个特征是内源性病毒（ERV）基因的激活，这类基因可以影响周围发育基因的调控区域以及其表达。ERV如何在胚胎干细胞中被激活和关闭、其对合子基因组激活和二细胞期细胞命运决定和胚胎发育有什么作用等一系列重要生物学问题尚待解决。我们近期的研究表明多能性因子Lin28敲除的小鼠胚胎干细胞ERV基因的表达异常。基于此，我们提出科学假设，Lin28参与ERV和二细胞期发育调控。该项目拟在此基础上研究1）Lin28是否调控二细胞期ERV基因的激活和关闭；2）Lin28调控ERV的表观遗传学的分子机制；3）Lin28如何调控二细胞期细胞命运决定和胚胎的发育。研究的发现将为进一步理解早期胚胎发育的合子基因组激活、全能干细胞向多能干细胞的转变提供新知识，对干细胞在再生医学领域的应用具有重要意义。

哺乳动物胚胎发育是一个紧密调控、有序的过程，在小鼠胚胎发育到二细胞（2C）时期，合子基因组开始大量转录激活，并伴随着内源性病毒（Endogenous Retro Virus, ERV）的瞬时上调。此时的细胞被认为是具有全能性的细胞，具有形成胚胎的和胚胎外部组织的能力。胚胎干细胞（Embryonic Stem Cell, ESC）来自于胚胎早期发育至囊胚期的内细胞团，能够分化成为三个胚层以及生殖细胞，因此是具有多能性的细胞。现已有文章报道表观遗传因子、miRNA、LINE1等参与2C基因和ERV的表达调控，但ES-2C转变的稳态调控与维持的分子机制仍需要进一步探究。本研究首次揭示了定位于核仁中RNA结合蛋白LIN28在小鼠早期胚胎发育过程中的新功能，以及LIN28介导的核仁完整性在ES-2C状态转变和基因表达调控中的作用。这些研究将为进一步理解早期胚胎发育的合子基因组激活、二细胞期的全能干细胞向囊胚期的多能干细胞的转变提供新的视野，并对运用干细胞进行再生医学研究的探索和应用产生重要意义。