Spic a specific marker of ground state pluripotency that regulates one carbon metabolism

Understanding mechanisms of epigenetic regulation in embryonic stem cells (ESCs) is of fundamental importance for stem cell and developmental biology. Here we identify Spic, a member of the ETS family of transcription factors, as a specific marker of ground state pluripotency. We show that Spic is rapidly induced in ESCs cultured with GSK3-, MEK-inhibitors and LIF (2iL), and in response to MEK/ERK inhibition. ChIP-seq analysis demonstrated that Spic binds to enhancer elements that are associated with pluripotency genes. Interaction proteomics and genomic profiling confirmed that SPIC interacts with NANOG and stabilizes its binding to chromatin in 2iL-ESCs. Additional gain of function and loss of function experiments revealed that Spic controls genes involved in one carbon (1C) metabolism, Bhmt, Bhmt2, and Dmgdh, and the flux of SAM-to-SAH in 2iL-ESCs. By maintaining low levels of SAM, Spic controls the level of H3K4me3 and H3R17me2 histone methylation in ground state ESCs. Our data highlight the role of uncharacterized axillary transcription factors that link cellular metabolism to epigenetic regulation in ground state pluripotency.

解析胚胎干细胞（Embryonic Stem Cells, ESCs）中的表观遗传调控机制，对于干细胞生物学与发育生物学而言具有核心意义。本研究鉴定出ETS家族转录因子成员Spic，作为基底态多能性的特异性标志物。研究发现，在采用GSK3抑制剂、MEK抑制剂与白血病抑制因子（Leukemia Inhibitory Factor, LIF）构建的2iL培养体系中培养的ESCs，以及经MEK/ERK通路抑制处理的ESCs内，Spic的表达均会被快速诱导。染色质免疫共沉淀测序（Chromatin Immunoprecipitation sequencing, ChIP-seq）分析显示，Spic可结合与多能性基因相关的增强子元件。相互作用蛋白质组学与基因组谱分析证实，在2iL培养的ESCs中，SPIC可与NANOG相互作用，并稳定其与染色质的结合。后续的功能获得与功能缺失实验表明，在2iL培养的ESCs中，Spic可调控参与一碳（1C）代谢的Bhmt、Bhmt2与Dmgdh基因，以及S-腺苷甲硫氨酸（S-Adenosylmethionine, SAM）向S-腺苷同型半胱氨酸（S-Adenosylhomocysteine, SAH）的代谢通量。通过维持SAM的低水平状态，Spic可调控基底态ESCs中H3K4me3与H3R17me2两种组蛋白修饰的甲基化水平。本研究数据凸显了未被表征的辅助转录因子的功能：这类因子可将细胞代谢与基底态多能性中的表观遗传调控过程相联结。