Nucleome programming is required for the foundation of totipotency in mammalian germline development [ChIP-Seq]

Germ cells are unique in engendering totipotency, yet the mechanisms underlying this capacity remain elusive. Here, we perform comprehensive and in-depth nucleome analysis of mouse germ-cell development in vitro, encompassing pluripotent precursors, primordial germ cells (PGCs) before and after epigenetic reprogramming, and spermatogonia/spermatogonial stem cells (SSCs). Although epigenetic reprogramming, including genome-wide DNA de-methylation, creates broadly open chromatin with abundant enhancer-like signatures, the augmented chromatin insulation safeguards transcriptional fidelity. These insulatory constraints are then erased en masse for spermatogonial development. Notably, despite distinguishing epigenetic programming, including global DNA re-methylation, the PGCs-to-spermatogonia/SSCs development entails further euchromatization. This accompanies substantial erasure of lamina-associated domains, generating spermatogonia/SSCs with a minimal peripheral attachment of chromatin except for pericentromeres—an architecture conserved in primates. Accordingly, faulty nucleome maturation, including persistent insulation and improper euchromatization, leads to impaired spermatogenic potential. Given that PGCs after epigenetic reprogramming serve as oogenic progenitors as well, our findings elucidate a principle for the nucleome programming that creates gametogenic progenitors in both sexes, defining a basis for nuclear totipotency. Nucleome profiling of various cell types along murine germ-cell development using ChIP-seq against histone modifications, transcription factors, and architectural proteins

生殖细胞 (Germ cells) 具备赋予细胞全能性 (totipotency) 的独特能力，但其支撑这一特性的分子机制仍未明确。本研究对体外培养的小鼠生殖细胞发育过程开展了全面且深入的核组 (nucleome) 分析，研究对象涵盖多能前体细胞 (pluripotent precursors)、表观重编程 (epigenetic reprogramming) 前后的原始生殖细胞 (primordial germ cells, PGCs)，以及精原细胞/精原干细胞 (spermatogonial stem cells, SSCs)。尽管表观重编程（包括全基因组DNA去甲基化 (DNA de-methylation)）可形成广泛开放且富集增强子样特征的染色质，但增强的染色质绝缘 (chromatin insulation) 作用保障了转录保真度 (transcriptional fidelity)。随后，这类绝缘约束被大规模清除，以推动精原细胞的发育进程。值得注意的是，尽管存在包含全基因组DNA重新甲基化在内的特异性表观编程，从PGCs到精原细胞/SSCs的发育过程仍伴随进一步的常染色质化 (euchromatization)。这一过程伴随着核纤层相关结构域 (lamina-associated domains) 的显著清除，使得精原细胞/SSCs的染色质仅在着丝粒旁区域 (pericentromeres) 保留极少的外周附着——这一染色质结构在灵长类动物中具有保守性。相应地，异常的核组成熟过程（如持续存在的染色质绝缘作用及异常的常染色质化）会导致生精潜能受损。鉴于表观重编程后的PGCs同时可作为卵原前体细胞，本研究的发现阐明了一套核组编程的核心原则：该过程可在两性个体中生成配子发生前体细胞，为细胞核全能性奠定了理论基础。本数据集通过针对组蛋白修饰、转录因子与结构蛋白的染色质免疫共沉淀测序 (ChIP-seq)，完成了小鼠生殖细胞发育全流程中各类细胞类型的核组谱分析。