Qiao, Y.; Xu, J.; Zheng, L.; Xiao, Z.; Huang, Z.; Liang, Z.; Zhou, X.; Ma, G.; Tong, G.; Esteban, M. A.; Hutchins, A. P.

Embryonic development is marked by the successive restriction of developmental potential and the specification of embryonic and extraembryonic lineages. Yet, how these lineage decisions are established, and how the epigenome is remodelled to promote and restrict cell fate transitions, remains poorly understood. Here, we demonstrate that SALL2 knockdown in primed human pluripotent stem cells (hPSCs) triggers a trophectoderm (TE)-like phenotype, characterized by palisade-like morphology and the up-regulation of TE-associated genes. Mechanistically, SALL2 physically interacts with the key TE driver TEAD4 and maintains bivalent, repressive chromatin (H3K4me3/H3K27me3) at TE-specific loci. Reduced SALL2 led to enhanced TEAD4 occupancy and disrupted H3K27me3 and increased active chromatin marks at TE genes. Importantly, depletion of TEAD4 abolished the TE-like phenotype induced by SALL2 knockdown, demonstrating that TEAD4 is required for the downstream effects of SALL2 loss. In support of this, blastoid-like aggregates can be generated from primed hPSCs with SALL2 knocked down. Together, our findings identify SALL2 as a key epigenetic barrier that restrains TE lineage commitment by limiting TEAD4-dependent activation of the trophoblast transcriptional program.