胚胎干细胞（embryonic stem cells,ESCs）来源于着床前囊胚的内细胞团,具有自我更新和发育全能性这两大特性，该特性决定了其具有巨大的研究价值和广泛的临床应用前景。近年来，由体细胞重编程为与ESCs特性相似的诱导性多能干细胞(induced pluripotent stem  cells,iPS cells）的建立更进一步拉进了干细胞和临床疾病治疗的距离。已有的大量研究表明，转录因子Oct4和Sox2对于维持ESCs以及iPS细胞的建立都起着重要的作用，它们蛋白水平的微小改变都可以影响到其下游靶基因的正常表达，进而导致ESCs分化。然而，至今仍不是很清楚Oct4和Sox2以及其下游靶基因在ESCs及其分化过程中是如何被精密调控的。

中科院上海生命科学院/上海交通大学医学院健康科学研究所干细胞生物重点实验室高芙蓉博士研究生在导师金颖研究员的指导下，以Oct4和Sox2的一个下游靶基因，成纤维生长因子4（fibroblast growth factor 4,FGF4），的增强子的一段寡核苷酸（含有Oct4和Sox2结合序列）为饵，通过亲和层析和蛋白质质谱分析发现了一个与Oct4和Sox2共同特异性调控FGF4表达的核蛋白，PARP1。PARP1是一个NAD+依赖的多聚ADP-核糖转移酶，参与多种重要的细胞功能的调控。她们的研究发现，PARP1可直接结合于FGF4的增强子上，在ESCs分化过程中，PARP1通过对Sox2进行多聚ADP-核糖修饰，使Sox2从FGF4增强子上解离并降解，从而正调控FGF4的表达。当PARP1缺失或处于低活性时，Sox2的多聚ADP-核糖修饰降低，使得Sox2与FGF4增强子的结合多于正常水平。同时，Sox2蛋白水平也增加，最终造成FGF4的表达下降。FGF4对于ESCs诱导的分化细胞的生长或存活是必需的，而加入外源的FGF4生长因子可以部分恢复因由PARP1-/-  ESCs来源的分化细胞的生长或存活的异常。该研究工作首次阐明蛋白翻译后修饰对于Sox2蛋白水平及其功能的重要调控作用，同时揭示了在ESCs分化过程中，PARP1对Sox2的蛋白水平和FGF4的转录的动态调控是必需的。这些发现有助于我们进一步阐明ESCs维持其自我更新和发育全能性的机制，并且对于研究体细胞重编程的分子机制有促进作用。

这项研究工作于2009年8月发表在《生物化学杂志》  (The Journal of Biological Chemistry)上，该研究工作得到了国家自然科学基金、上海市优秀学科带头人计划、上海市教育委员会重点学科建设项目和科学院创新项目等的支持。

推荐原始出处：

J. Biol. Chem., Vol. 284, Issue 33, 22263-22273, August 14, 2009

PARP1 Poly(ADP-ribosyl)ates Sox2 to Control Sox2 Protein Levels and FGF4 Expression during Embryonic Stem Cell Differentiation*

Furong Gao?, Sung Won Kwon||, Yingming Zhao||, and Ying Jin1

From the From the Key Laboratory of Stem Cell Biology, Institute of Health Sciences, Shanghai Institute of Biological Sciences, Chinese Academy of Sciences/Shanghai JiaoTong University School of Medicine, Shanghai 200025, China, ,  Shanghai Stem Cell Institute, Shanghai JiaoTong University School of Medicine, Shanghai 200025, China, , the ||Ben May Department for Cancer Research, University of Chicago, Chicago, Illinois 60637, and , ?Graduate School of Chinese Academy of Sciences, Beijing 100000, China

Transcription factors Oct4 and Sox2 are key players in maintaining the pluripotent state of embryonic stem cells (ESCs). Small changes in their levels disrupt normal expression of their target genes. However, it remains elusive how protein levels of Oct4 and Sox2 and expression of their target genes are precisely controlled in ESCs. Here we identify PARP1, a DNA-binding protein with an NAD+-dependent enzymatic activity, as a cofactor of Oct4 and Sox2 to regulate expression of their target gene FGF4. We demonstrate for the first time that PARP1 binds the FGF4 enhancer to positively regulate FGF4 expression. Our data show that PARP1 interacts with and poly(ADP-ribosyl)ates Sox2 directly, which may be a step required for dissociation and degradation of inhibitory Sox2 proteins from the FGF4 enhancer. When PARP1 activity is inhibited or absent, poly(ADP-ribosyl)ation of Sox2 decreases and association of Sox2 with FGF4 enhancers increases, accompanied by an elevated level of Sox2 proteins and reduced expression of FGF4. Significantly, specific knockdown of Sox2 expression by RNA interference can considerably abrogate the inhibitory effect of the poly(ADP- ribose) polymerase inhibitor on FGF4 expression. Interestingly, PARP1 deficiency does not affect undifferentiated ESCs but compromises cell survival and/or growth when ESCs are induced into differentiation. Addition of FGF4 can partially rescue the phenotypes caused by PARP1 deficiency during ESC differentiation. Taken together, this study uncovers new mechanisms through which Sox2 protein levels and FGF4 expression are dynamically regulated during ESC differentiation and adds a new member to the family of proteins regulating the properties of ESCs.