Phosphatase PPP1CC Regulates the First Lineage Segregation by GAS5 in Mouse Preimplantation Embryos

The transcriptional effector of the Hippo signalling pathway, YAP, regulates the first lineage specification in mouse preimplantation embryos. However, how YAP undergoes dephosphorylation specifically in the trophectoderm (TE) but not in the inner cell mass (ICM) remains unresolved. Here, we discovered that the serine/threonine phosphatase PPP1CC exhibits uniform distribution prior to blastocyst formation but becomes specifically localised to the TE during the blastocyst stage. Through mediating YAP dephosphorylation in the outer cells of mouse morula, PPP1CC facilitates YAP nuclear translocation, thereby ultimately driving TE lineage specification. Importantly, the spatially restricted localisation of PPP1CC in TE is achieved via its interaction with the long non-coding RNA GAS5, which localises to the subcortical region throughout early mouse embryonic development. Knockdown of GAS5 phenocopies PPP1CC deficiency, causing developmental arrest at the morula stage accompanied by impaired YAP dephosphorylation in outer cells. Moreover, overexpression of GAS5 in one blastomere of the 2-cell stage biases its descendants predominantly towards the TE fate. In summary, our study identifies the GAS5-PPP1CC-YAP axis as a central regulator of first lineage specification during mouse preimplantation development, highlighting its critical role in reversible phosphorylation during early embryogenesis.