Polarization-independent regulation of the subcellular localization of Yes-associated protein 1 during preimplantation development

Cell polarization is a crucial developmental process that determines cell differentiation in mouse embryos. During this process, an extensively expressed transcriptional regulator, Yes-associated protein 1 (YAP1), is localized either to the cytoplasm or to the nucleus via HIPPO signaling. In mouse premorula embryos, YAP1 is present in the nuclei of all cells. Thereafter, YAP1 is distributed to the nuclei of outer cells or cytoplasm of inner cells, depending on the establishment of cell polarity and morula formation. However, the dynamics of YAP1 localization in other species, including ruminants, remain unclear. To gain an in-depth understanding of cell differentiation in mammalian embryos, we investigated YAP1 localization changes in bovine embryos. Unlike in mouse morulae, YAP1 displayed cytoplasmic localization in most cells, including the outer cells of bovine morulae, after the 32-cell stage. Next, we analyzed the relationship between cell polarity and nuclear localization of YAP1. Polarization of outer cells in the bovine morula began at the late 16-cell stage and was established by the late 32-cell stage, indicating that polarization preceded the nuclear localization of YAP1 in bovine embryos. To explore the regulation of YAP1 localization in bovine morula, we analyzed zona-free embryos and found that the presence of the zona pellucida significantly enhanced YAP1 cytoplasmic localization. Moreover, we observed ectopic expression of SRY-box transcription factor 2 in zona-free blastocysts, which indicated that cytoplasmic localization of YAP1 was associated with the suppression of pluripotency in the trophectoderm. These findings provide valuable insights into the molecular mechanisms underlying the first cell differentiation in mammalian embryos.