SUV39H2 CONTROLS TROPHOBLAST STEM CELL FATE

The placenta is constructed through the orchestration of trophoblast stem (TS) cell expansion and differentiation along a multi-lineage pathway. Dynamic regulation of histone H3K9 methylation is pivotal to cell differentiation for many cell lineages, but little is known about its involvement in trophoblast development. Among the twelve-known histone H3K9 methyltransferases, only SUV39H2 exhibited robust differential expression in stem versus differentiated rat TS cells. SUV39H2 transcript and protein expression were high in the stem state and rapidly declined as TS cells differentiated. Disruption of SUV39H2 expression in TS cells led to prominent phenotypic changes. Suv39h2-specific shRNA knockdown resulted in an arrest in TS cell proliferation and activation of trophoblast cell differentiation. These observations were reinforced by flow cytometry and transcript profiling. Histone H3K9 methylation status at specific loci exhibiting differentiation-dependent gene expression were regulated by SUV39H2 and also represented sites for SUV39H2 occupancy. Analyses of SUV39H2 on ex vivo rat blastocyst development supported its role in regulating TS cell expansion and differentiation. Finally, we identified SUV39H2 as a downstream target of CDX2, a master regulator of trophoblast lineage development. In summary, our findings indicate that SUV39H2 contributes to the maintenance of the TS cell stem state and restrains trophoblast cell differentiation and thus serves as a contributor to the epigenetic regulation of hemochorial placental development.胎盘通过滋养层干细胞（trophoblast stem cell, TS）的扩增及多谱系分化通路的协同调控得以构建。组蛋白H3K9甲基化（histone H3K9 methylation）的动态调控对诸多细胞谱系的细胞分化至关重要，但目前关于其在滋养层发育中的参与作用尚不明晰。在已发现的12种组蛋白H3K9甲基转移酶（histone H3K9 methyltransferase）中，仅SUV39H2在大鼠TS细胞的干细胞状态与分化状态间表现出显著的差异表达。SUV39H2的转录本与蛋白在干细胞状态下表达量较高，并随TS细胞分化迅速下调。干扰TS细胞中SUV39H2的表达会引发显著的表型改变：靶向Suv39h2的短发夹RNA（small hairpin RNA, shRNA）敲低可导致TS细胞增殖阻滞，并启动滋养层细胞分化程序。流式细胞术（flow cytometry）与转录组分析（transcript profiling）进一步验证了上述观察结果。在表现出分化依赖性基因表达特征的特定基因座处，组蛋白H3K9甲基化状态受SUV39H2调控，同时这些位点也是SUV39H2的结合靶点。对大鼠离体囊胚发育过程中SUV39H2的分析证实，其在调控TS细胞扩增与分化中发挥作用。最后，我们发现SUV39H2是尾型同源盒转录因子2（CDX2）的下游靶基因，而CDX2是滋养层谱系发育的核心调控因子。综上，本研究结果表明，SUV39H2有助于维持TS细胞的干细胞状态，并抑制滋养层细胞分化，因此其参与了血绒毛膜型胎盘发育的表观遗传调控。