PATJ and MPDZ are required for trophectoderm lineage specification in early mouse embryos

During the morula to blastocyst transformation, polarity establishment in outer cells is a prerequisite for trophectoderm lineage specification. This study reveals the roles of polarity proteins PATJ and MPDZ in trophectoderm lineage fate decision.In mouse preimplantation embryos, cell polarity plays a crucial role in the first lineage specification. PATJ and its homolog MPDZ are the main members of CRB-PALS1-PATJ (CRUMBS-Protein associated with Lin7 1-Pals-associated tight junction protein) apical polarity complex. They act as adaptor proteins connecting CRB-PALS1 and tight junction proteins, making them essential for cell polarization and stabilization of apical junctions. However, their roles in regulating trophectoderm differentiation and blastocyst development remain unclear. In this study, PATJ and/or MPDZ were downregulated by the microinjection of specific RNA interference constructs into zygotes. Downregulation of PATJ alone did not severely affect early embryonic development and trophectoderm lineage differentiation although it slowed down the blastocyst formation. Depletion of PATJ and MPDZ did not affect compaction and morula development but impaired blastocyst formation. Furthermore, the expression of trophectoderm-specific transcription factors and trophoblast differentiation was compromised in the absence of PATJ/MPDZ. These abnormalities might result from the breakdown of apical domain in the outer cells of the embryo. The loss of PATJ/MPDZ caused the breakdown of CRB and PAR polarity complexes as well as deficiencies in tight junctions and actin filaments. These defects led to ectopic activation of Hippo signaling in the outer cells of developing embryos, ultimately suppressing Cdx2 expression and trophectoderm differentiation. Altogether, PATJ and MPDZ are essential for trophectoderm lineage differentiation and normal blastocyst morphogenesis via the regulation of the establishment of apical domain, formation of tight junctions, phosphorylation and localization of YAP, and expression of trophectoderm-specific transcription factors.