NR5A2 connects genome activation to the first lineage segregation in totipotent embryos
Zygotic genome activation (ZGA) marks the beginning of the embryonic program for a totipotent embryo, which gives rise to inner cell mass (ICM), where pluripotent epiblast arises, and extraembryonic trophectoderm. While much has been learned about pluripotency regulation, how ZGA is connected to the first lineage segregation in early embryos remains elusive. Here, we investigated the role of nuclear receptor transcription factors (NR TFs), whose motifs are highly enriched in accessible chromatin at the 2-cell (2C) to 8-cell (8C) stages after ZGA. We found NR5A2, an NR TF highly induced upon ZGA, is required for early development. Genome-wide chromatin binding in 2C and 8C embryos showed NR5A2 bound cis-regulatory elements which strongly enrich B1 elements where its motif is embedded. RNA-seq showed while the zygotic genome was largely activated at the 2C stage, 4-8C-specific gene activation was substantially impaired in Nr5a2 KD embryos. At the 8C stage, NR5A2 preferentially regulated its targets including a subset of early ICM genes. Consistently, NR5A2 was required for opening 8C-specific binding sites. Interestingly, while NR5A2's 2C-binding sites are largely closed in blastocyst, 8C-specific binding sites tend to stay open and are bound by master pluripotency TFs (NANOG, SOX2, and OCT4) in blastocyst or ESC. Further analyses showed NR5A2 regulates early ICM genes at the 8C stage, master pluripotency TFs regulate both early and late ICM genes at later stages. Taken together, these data identify NR5A2 as a key regulator in totipotent embryos that connects ZGA to the first lineage segregation in early mouse development. Overall design: By employing CUT&RUN, ATAC-seq, ChIP-seq and RNA-seq, we systematically examined the genome wide NR5A2 chromatin binding and function in mouse early embryos and  mESC cell lines.