Human Organoids Reveal PTEN-driven Mesendoderm Specification via Retinoic Acid Signaling Suppression

During early embryogenesis, epiblast cells ingress through the primitive streak (PS) and commit to a mesendoderm (MES) fate, giving rise to both mesodermal and endodermal lineages. Despite advancements in human organoid technology for modeling pre-implantation blastocyst stage, robust systems that accurately recapitulate MES specification and PS formation during the post-implantation stage remain limited. Here, we show that human induced pluripotent stem cells can be used to generate three-dimensional MES organoids that faithfully mimic in vivo human MES specification, particularly from the anterior PS. We demonstrate that MES organoid formation is dependent on PTEN expression and that loss of PTEN severely impairs the formation of mesoderm and definitive endoderm derivatives arising from MES organoids. To elucidate the molecular mechanisms underlying these defects, we conducted an integrative multi-omics analysis, including transcriptome, chromatin accessibility profiling, metabolism, and phosphoproteomics, on wild-type and PTEN-/-MES cells. These analyses revealed that PTEN suppresses retinoic acid (RA) signaling during MES lineage commitment. Furthermore, we identified the RA-degrading enzyme CYP26A1 as a downstream effector of PTEN. Notably, while excessive RA induced by PTEN ablation is detrimental to MES cell generation, physiological levels of RA are necessary. Collectively, our human MES organoids offer a valuable model for dissecting early human development, and our findings identify PTEN as a key regulator of MES fate commitment through inhibition of RA signaling.