Dev120238 1606..1615

Cell fate decisions are fundamental to the development of multicellular organisms. In mammals the first cell fate decision involves segregation of the pluripotent inner cell mass and the trophectoderm, a process regulated by cell polarity proteins, HIPPO signaling and lineagespecific transcription factors such as CDX2. However, the regulatory mechanisms that operate upstream to specify the trophectoderm lineage have not been established. Here we report that transcription factor AP-2γ (TFAP2C) functions asa novel upstream regulator ofCdx2 expressionandposition-dependentHIPPOsignaling inmice. Lossand gain-of-function studies and promoter analysis revealed that TFAP2C binding to an intronic enhancer is required for activation of Cdx2 expression during early development. During the 8-cell to morula transitionTFAP2Cpotentiates cell polarity tosuppressHIPPOsignaling in theoutsideblastomeres.TFAP2Cdepletion triggereddownregulation of PARD6B, loss of apical cell polarity, disorganization of F-actin, and activation of HIPPO signaling in the outside blastomeres. Rescue experiments usingPard6bmRNA restored cell polarity but only partially corrected position-dependent HIPPO signaling, suggesting that TFAP2C negatively regulates HIPPO signaling via multiple pathways. Several genes involved in regulation of the actin cytoskeleton (including Rock1, Rock2) were downregulated in TFAP2C-depleted embryos. Inhibition of ROCK1 and ROCK2 activity during the 8-cell to morula transition phenocopied TFAP2C knockdown, triggering a loss of position-dependent HIPPO signaling and decrease in Cdx2 expression. Altogether, these results demonstrate that TFAP2C facilitates trophectoderm lineage specification by functioning as a key regulator of Cdx2 transcription, cell polarity and position-dependent