MAPK/ERK signaling blocks ectopic H3K9me3 heterochromatin formation to confer mesoderm and endoderm developmental competence.

During gastrulation, dynamic interplay among cell signaling pathways dictates cell fate decisions. While extensive studies have elucidated their critical roles in morphological regulation, how these signals orchestrate the epigenome to confer developmental competence remains unclear. In this study, we demonstrate that H3K9me3-marked facultative heterochromatin domains undergo global reorganization during differentiation of human pluripotent stem cells into mesoderm and endoderm, which arise through epithelial-mesenchymal transition (EMT), but not into ectoderm, which retains epithelial state. We identify the MAPK/ERK pathway, acting downstream of FGF signaling, as a key mediator of this reorganization. Specifically, the MAPK/ERK pathway prevents ectopic formation of H3K9me3 domains at EMT- and lineage-specific gene loci whose expression is necessary for mesoderm and endoderm differentiation. Collectively, our findings reveal a previously unrecognized role for MAPK/ERK signaling in reorganizing the H3K9me3 landscape to enable mesoderm and endoderm differentiation, bridging a critical gap in our knowledge of how cell signaling pathways shape the epigenetic landscape during development.