Nucleolar-based Dux repression is essential for 2-cell stage exit

Abstract: Upon fertilisation, the mammalian embryo must switch from dependence on maternal transcripts to transcribing its own genome, and in mice involves the transient upregulation of MERVL transposons and MERVL-driven genes at the 2-cell stage. The mechanisms and requirement for MERVL and 2-cell (2C) gene upregulation are poorly understood. Moreover, this MERVL-driven transcriptional program must be rapidly shut off to allow 2-cell exit and developmental progression. Here, we report that robust ribosomal RNA (rRNA) synthesis and nucleolar maturation are essential for exit from the 2C state. 2C-like cells and 2-cell embryos show similar immature nucleoli with altered structure and reduced rRNA output. We reveal that nucleolar disruption via blocking Pol I activity or preventing nucleolar phase separation enhances conversion to a 2C-like state in embryonic stem cells (ESCs) by detachment of the MERVL activator Dux from the nucleolar surface. In embryos, nucleolar disruption prevents proper Dux silencing and leads to 2-4 cell arrest. Our findings reveal an intriguing link between rRNA synthesis, nucleolar maturation and gene repression during early development.