Zygotic genome activation (ZGA) marks the beginning of the embryonic program for a totipotent embryo, which gives rise to the inner cell mass (ICM) where pluripotent epiblast arises, and extraembryonic trophectoderm. However, how ZGA is connected to the first lineage segregation in mammalian embryos remains elusive. Here, we investigated the role of nuclear receptor (NR) transcription factors (TFs), whose motifs are highly enriched and accessible from the 2-cell (2C) to 8-cell (8C) stages in mouse embryos. We found that NR5A2, an NR TF strongly induced upon ZGA, was required for this connection. Upon Nr5a2 knockdown or knockout, embryos developed beyond 2C normally with the zygotic genome largely activated. However, 4–8C-specific gene activation was substantially impaired and Nr5a2-deficient embryos subsequently arrested at the morula stage. Genome-wide chromatin binding analysis showed that NR5A2-bound cis-regulatory elements in both 2C and 8C embryos are strongly enriched for B1 elements where its binding motif is embedded. NR5A2 was not required for the global opening of its binding sites in 2C embryos but was essential to the opening of its 8C-specific binding sites. These 8C-specific, but not 2C-specific, binding sites are enriched near genes involved in blastocyst and stem cell regulation, and are often bound by master pluripotency TFs in blastocysts and embryonic stem cells (ESCs). Importantly, NR5A2 regulated key pluripotency genes Nanog and Pou5f1/Oct4, and primitive endoderm regulatory genes including Gata6 among many early ICM genes, as well as key trophectoderm regulatory genes including Tead4 and Gata3 at the 8C stage. By contrast, master pluripotency TFs NANOG, SOX2, and OCT4 targeted both early and late ICM genes in mouse ESCs. Taken together, these data identify NR5A2 as a key regulator in totipotent embryos that bridges ZGA to the first lineage segregation during mouse early development.

合子基因组激活（ZGA）标志着全能胚胎胚胎程序的开始，该程序产生内细胞团（ICM），其中多能外胚层和胚胎外滋养外胚层出现。然而，ZGA 如何与哺乳动物胚胎中的第一次谱系分离联系起来仍然难以捉摸。在这里，我们研究了核受体 (NR) 转录因子 (TF) 的作用，其基序高度富集，并且在小鼠胚胎的 2 细胞 (2C) 到 8 细胞 (8C) 阶段均可使用。我们发现这种连接需要 NR5A2（一种在 ZGA 上强烈诱导的 NR TF）。 Nr5a2敲低或敲除后，胚胎正常发育超过 2C，合子基因组大部分被激活。然而，4-8C 特异性基因激活严重受损， Nr5a2缺陷胚胎随后停滞在桑葚胚阶段。全基因组染色质结合分析表明，2C 和 8C 胚胎中 NR5A2 结合的顺式调节元件都高度富集 B1 元件，其中嵌入了其结合基序。 NR5A2 对于 2C 胚胎中其结合位点的整体开放不是必需的，但对于其 8C 特异性结合位点的开放至关重要。这些 8C 特异性而非 2C 特异性结合位点在参与囊胚和干细胞调节的基因附近富集，并且通常与囊胚和胚胎干细胞 (ESC) 中的主多能性 TF 结合。重要的是，NR5A2 调节关键的多能性基因Nanog和Pou5f1/Oct4以及许多早期 ICM 基因中的原始内胚层调节基因（包括Gata6） ，以及 8C 阶段的关键滋养外胚层调节基因（包括Tead4和Gata3） 。 相比之下，主多能转录因子 NANOG、SOX2 和 OCT4 靶向小鼠 ESC 中的早期和晚期 ICM 基因。综上所述，这些数据表明 NR5A2 是全能胚胎中的关键调节因子，在小鼠早期发育过程中将 ZGA 与第一次谱系分离联系起来。