基于“seesaw模型”建立的新的重编程体系的的表观遗传机制研究

The reprogramming factors that induce pluripotency have been identified primarily from embryonic stem cell (ESC)-enriched, pluripotency-associated factors. Recently, we have discovered that during mouse somatic cell reprogramming, pluripotency can be induced with lineage specifiers that are pluripotency rivals to suppress ESC identity, most of which are not enriched in ESCs. We found that OCT4 and SOX2, the core regulators of pluripotency, can be replaced by lineage specifiers that are involved in mesendodermal (ME) specification and in ectodermal (ECT) specification, respectively. OCT4 and its substitutes attenuated the elevated expression of a group of ECT genes whereas SOX2 and its substitutes curtailed a group of ME genes during reprogramming. Surprisingly, the two counteracting lineage specifiers can synergistically induce pluripotency in the absence of both OCT4 and SOX2. Our study suggests a “seesaw model," in which a balance that is established using pluripotency factors and/or counteracting lineage specifiers can facilitate reprogramming. To further study the epigenetic mechanism underlying the “seesaw model”, we will focus our research on the following two aspects: 1) To analyze the dynamics of histone modification and DNA methylation in the lineage-specifier induced reprogramming, and describe the epigenetic roadmap; 2) To further explore the roles of major epigenetic modulators in the lineage-specifier induced reprogramming, compare different reprogramming induction systems, and identify the major shared determinates in somatic reprogramming. This work should help better understand cell fate determination, improve the iPS induction efficiency, and develop novel methods for iPS induction.

成体细胞可以通过转入多潜能性相关基因完成重编程而被诱导成为“诱导的多潜能性干细胞（iPSC）”。我们实验室新的研究结果表明，最关键的两个多潜能性相关基因Oct4与Sox2可以分别被控制胚胎早期不同胚层分化的基因（如Gata3与Gmnn）所代替，基于这一新发现，我们提出了“seesaw模型”，从新的角度解释了传统的细胞命运决定的理论。基于“seesaw模型”，我们将从以下两方面对细胞重编程的表观遗传机制进行深入研究：1）分析我们新建立的重编程诱导体系过程中组蛋白及DNA修饰的动态变化，描述新体系中的表观遗传修饰改变的过程；2）研究调控表观遗传修饰的相关因子在新的重编程体系中的作用，描绘其新的表观遗传路径图谱，分析和比较不同重编程诱导体系中表观遗传调控的异同点，揭示新的重编程机制。我们的研究将为诱导iPSC提供新的思路和途径，从新的视角阐述细胞命运是如何决定和转变的。

成体细胞可以通过导入转录因子的方式获得多潜能性。这一技术被称为诱导多潜能干细胞技术（iPS），是近年来干细胞领域的里程碑。传统的iPS技术是通过向成体细胞中导入多潜能性相关基因而建立多潜能性。我们实验室此前的研究表明，谱系决定因子可以替代多潜能性基因，诱导成体细胞转变成iPS细胞。基于这一新发现，我们提出了“seesaw模型”，从新的角度解释了细胞命运决定的理论。此项目开展期间，基于“seesaw模型”，我们通过深入的分子机理研究首次发现谱系分化因子GATA的整个家族蛋白均可以被用于实现体细胞重编程。我们验证了之前的“跷跷板”模型，证实GATA家族的所有成员都能够抑制外胚层分化基因的表达。并且发现连接谱系分化因子和干性基因网络激活的中间桥梁因子Sall4。同时，我们发现除转录因子诱导重编程外，小分子化合物诱导不同细胞类型重编程的过程中也存在动态的“seesaw模型”，这一发现证明 “seesaw模型”是细胞命运转变的普适性模型。此外，我们还发现谱系重编程过程中也存在类似的“seesaw模型”。这些结果表明“seesaw模型”从新的视角阐述了细胞命运是如何决定和转变的，将为诱导iPSC和改变细胞命运提供新的思路和理论依据。