Requirement for expression of WW domain containing transcription regulator 1 in bovine trophectoderm development.

WW domain-containing transcription regulator 1 (WWTR1) is one of the primary effectors in the Hippo pathway, which plays essential roles in cell differentiation into trophectoderm (TE) and inner cell mass cell lineages at the blastocyst stage. However, little is known about the roles of WWTR1 in preimplantation development. The present study aimed to explore the significance of WWTR1 expression in preimplantation development using an mRNA knockdown (KD) system in bovine embryos. We first quantitated WWTR1 expression at protein and mRNA levels from fertilization to blastocyst stage. WWTR1 proteins gradually shifted from extranuclear localization during the 16-cell stage to nuclear localization by morula stage. WWTR1 mRNA expression was also transiently upregulated at the 16-cell stage. WWTR1 KD efficiently repressed WWTR1 expression at protein and mRNA levels. The WWTR1 KD embryos developed to the blastocyst stage at rates equivalent to those of controls, but TE cell numbers were significantly decreased. Representative TE-expressed genes, including CDX2 and IFNT were also significantly decreased in WWTR1 KD blastocysts. These results provide the first demonstration that WWTR1 expression is responsible for normal TE cell development in preimplantation embryos.

Yes-associated protein 1 translocation through actin cytoskeleton organization in trophectoderm cells.

A mammalian embryo experiences the first cell segregation at the blastocyst stage, in which cells giving form to the embryo are sorted into two lineages; trophectoderm (TE) and inner cell mass (ICM). This first cell segregation process is governed by cell position-dependent Hippo signaling, which is a phosphorylation cascade determining whether Yes-associated protein 1 (YAP1), one of the key components of the Hippo signaling pathway, localizes within the nucleus or cytoplasm. YAP1 localization determines the transcriptional on/off switch of a key gene, Cdx2, required for TE differentiation. However, the control mechanisms involved in YAP1 nucleocytoplasmic shuttling post blastocyst formation remain unknown. This study focused on the mechanisms involved in YAP1 release from TE nuclei after blastocoel contraction in bovine blastocysts. The blastocysts contracted by blastocoel fluid aspiration showed that the YAP1 translocation from nucleus to cytoplasm in the TE cells was concomitant with the protruded actin cytoskeleton. This YAP1 release from TE nuclei in the contracted blastocysts was prevented by actin disruption and stabilization. In contrast, Y27632, which is a potent inhibitor of Rho-associated coiled-coil containing protein kinase 1/2 (ROCK) activity, was found to promote YAP1 nuclear localization in the TE cells of contracted blastocysts. Meanwhile, lambda protein phosphatase (LPP) treatment inducing protein dephosphorylation could not prevent YAP1 release from TE nuclei in the contracted blastocysts, indicating that YAP1 release from TE nuclei does not depend on the Hippo signaling pathway. These results suggested that blastocyst contraction causes YAP1 release from TE nuclei through actin cytoskeleton remodeling in a Hippo signaling-independent manner. Thus, the present study raised the possibility that YAP1 subcellular localization is controlled by actin cytoskeletal organization after the blastocyst formation. Our results demonstrate diverse regulatory mechanisms for YAP1 nucleocytoplasmic shuttling in TE cells.