胚胎干细胞（ESCs）和诱导多能干细胞（iPSCs）具有分化成为成人体内各种细胞类型的能力，特定病人来源的诱导多能干细胞（iPSCs）为治疗人类免疫缺失疾病和神经退行性疾病提供新的可行途径。此外，人类胚胎干细胞和诱导多能干细胞也成为人类疾病模型建立、药物研发以及临床治疗的重要工具。近年来，对胚胎干细胞的自我复制和细胞重编程分子机制的了解有了很大进步，但许多重要的生物学问题依然有待解决。 Sox2是维持胚胎干细胞和成体干细胞自我更新的核心因子，也是体细胞实现重编程的关键转录因子。尽管有研究显示Sox2在胚胎干细胞中被磷酸化，但Sox2的磷酸化对胚胎干细胞维持多能性和重编程过程的生物学意义有待进一步研究。在本文中，我们鉴定出Sox2蛋白新的磷酸化位点，同时证明Cdk2能磷酸化Sox2第39位和第253位丝氨酸残基。这两个位点的磷酸化对维持胚胎干细胞自我更新是非必需的，但对体细胞完成重编程建立多能性必不可少。 实验中，我们利用质谱分析鉴定Sox2蛋白8个新的磷酸化位点，包括2个酪氨酸残基和6个丝氨酸/苏氨酸残基。在体外，Cdk2直接结合Sox2，并且同步磷酸化Sox2的第39位和第253位丝氨酸残基。研究发现，Sox2在这两个位点的磷酸化对维持胚胎干细胞自我更新和正常的细胞周期进程是非必需的。然而，在三因子（OSK，Oct4、Sox2和Klf4）诱导下，Sox2的磷酸化能大大加速小鼠成纤维细胞的重编程过程，并影响诱导多能干细（iPSCs）生殖系传递能力。同时，Cdk2的活性调节对OSK三因子诱导体细胞完成重编程非常重要。 因此，本研究首次揭示细胞周期因子Cdk2对Sox2的磷酸化能增强干细胞多能性的建立，但并不影响胚胎干细胞多能性状态的维持。这也可能解释了为何包括p53, p21和Arf/Ink4在内的这些CDKs抑制因子是重编程过程的一大障碍。

Embryonic stem cells (ESCs) have the ability to differentiate into various cell types in the adult body. Particularly, patient-specific-induced pluripotent stem cells (iPSCs) offer even greater promise in treating human diseases due to the absence of immune rejection. Moreover, human ESCs and iPSCs have also become important tools for understanding human biology and disease mechanisms and for developing better and more effective drugs for many human diseases. Although much progress has been made in understanding the molecular mechanisms underlying ESC self-renewal and cellular reprogramming, many important biological questions remain to be answered. Sox2 is a key factor in maintaining selfrenewal of embryonic stem cells (ESCs) and adult stem cells as well as in reprogramming differentiated cells back into pluripotent or multipotent stem cells. Although previous studies have shown that Sox2 is phosphorylated in human ESCs, the biological significance of Sox2 phosphorylation in ESC maintenance and reprogramming has not been well understood. In this study, we have identified new phosphorylation sites on Sox2, and have further demonstrated that Cdk2-mediated Sox2 phosphorylation at S39 and S253 is required for establishing the pluripotent state during reprogramming but is dispensable for ESC maintenance. Mass-spectrometry analysis of purified Sox2 protein has identified new phosphorylation sites on two Tyrosine and six Serine/Threonine residues. Cdk2 physically interacts with Sox2 and phosphorylates Sox2 at S39 and S253 in vitro. Surprisingly, Sox2 phosphorylation at S39 and S253 is dispensable for ESC self-renewal and cell cycle progression. In addition, Sox2 phosphorylation enhances its ability to establish the pluripotent state during reprogramming by working with Oct4 and Klf4. Finally, Cdk2 can also modulate the ability of Oct4, Sox2 and Klf4 in reprogramming fibroblasts back into pluripotent stem cells. Therefore, this study has, for the first time, demonstrated that Sox2 phosphorylation by Cdk2 promotes the establishment, but not the maintenance, of the pluripotent state. It might also help explain why the inactivation of CDK inhibitors, such as p53, p21 and Arf/Ink4, promotes the induction of pluripotent stem cells.