Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UK.
Biophys J. 2012 Nov 21;103(10):2060-70. doi: 10.1016/j.bpj.2012.10.015. Epub 2012 Nov 20.
The interplay between epigenetic modification and chromatin compaction is implicated in the regulation of gene expression, and it comprises one of the most fascinating frontiers in cell biology. Although a complete picture is still lacking, it is generally accepted that the differentiation of embryonic stem (ES) cells is accompanied by a selective condensation into heterochromatin with concomitant gene silencing, leaving access only to lineage-specific genes in the euchromatin. ES cells have been reported to have less condensed chromatin, as they are capable of differentiating into any cell type. However, pluripotency itself-even prior to differentiation-is a split state comprising a naïve state and a state in which ES cells prime for differentiation. Here, we show that naïve ES cells decondense their chromatin in the course of downregulating the pluripotency marker Nanog before they initiate lineage commitment. We used fluorescence recovery after photobleaching, and histone modification analysis paired with a novel, to our knowledge, optical stretching method, to show that ES cells in the naïve state have a significantly stiffer nucleus that is coupled to a globally more condensed chromatin state. We link this biophysical phenotype to coinciding epigenetic differences, including histone methylation, and show a strong correlation of chromatin condensation and nuclear stiffness with the expression of Nanog. Besides having implications for transcriptional regulation and embryonic cell sorting and suggesting a putative mechanosensing mechanism, the physical differences point to a system-level regulatory role of chromatin in maintaining pluripotency in embryonic development.
表观遗传修饰和染色质紧缩的相互作用与基因表达的调控有关，它是细胞生物学最迷人的前沿领域之一。尽管目前还缺乏完整的画面，但人们普遍认为，胚胎干细胞（ES 细胞）的分化伴随着选择性浓缩成异染色质，伴随着基因沉默，只有染色质上的特定基因在常染色质中可被访问。已经报道 ES 细胞具有较少的浓缩染色质，因为它们能够分化为任何细胞类型。然而，多能性本身——甚至在分化之前——就是一种分裂状态，包括一个幼稚状态和一个 ES 细胞为分化做准备的状态。在这里，我们表明，在开始谱系承诺之前，幼稚的 ES 细胞在下调多能性标记 Nanog 的过程中去浓缩其染色质。我们使用光漂白后的荧光恢复，以及组蛋白修饰分析，并结合我们所知的一种新的光学拉伸方法，表明幼稚状态的 ES 细胞具有明显更硬的核，这与整体更浓缩的染色质状态相关。我们将这种生物物理表型与同时存在的表观遗传差异联系起来，包括组蛋白甲基化，并表明染色质浓缩和核硬度与 Nanog 的表达之间存在很强的相关性。除了对转录调控和胚胎细胞分选有影响，并暗示存在一种潜在的机械感觉机制外，这些物理差异表明染色质在维持胚胎发育中的多能性方面具有系统级的调节作用。
