Phase Separation of SF3B1 Serves as a Critical Post-Transcriptional Regulator During Early Mouse Embryogenesis.

During early mammalian embryogenesis, totipotent zygotes and early blastomeres undergo extensive post-transcriptional regulation during the establishment of the first cell lineages; however, the functional contribution of alternative splicing to embryonic compaction and blastulation remains poorly understood. Here, we show that SF3B1, a core component of the spliceosome, is upregulated from the 4-cell stage and mediates highly dynamic splicing programs. Depletion of SF3B1 results in developmental arrest at the morula stage, accompanied by widespread transcriptomic dysregulation characterized by aberrant expression of transcription factors that impede pluripotency transition. Alternative splicing analysis further identifies that aberrantly spliced transcripts were significantly enriched in genes involved in cell cycle regulation, such as Cdk11b and Ccnb1. Importantly, we demonstrate that SF3B1 undergoes intrinsic, IDR-driven liquid-liquid phase separation both in vitro and in vivo, forming nuclear condensates in oocytes and early embryos, which is essential for successful development to the blastocyst stage. Together, our findings reveal that phase separation mediated SF3B1 splicing activity is a critical regulator of early mouse embryonic development.