Paracrine signals regulate human liver organoid maturation from iPSC

A self-organizing organoid model provides a new approach to study the mechanism of human liver organogenesis. Previous animal models documented that simultaneous paracrine signaling and cell-to-cell surface contact regulate hepatocyte differentiation. To dissect the relative contributions of the paracrine effects, we first established a liver organoid using human induced pluripotent stem cells (iPSC), mesenchymal stem cells (MSC), and human umbilical vein endothelial cells (HUVEC) as previously reported. Time-lapse imaging showed the iPSC-derived hepatic endoderm (HE-iPSC) self-assembled into three-dimensional organoids, resulting in hepatic gene induction. Progressive differentiation was demonstrated by hepatic protein production after in vivo organoid transplantation. To assess the paracrine contributions, we employed a transwell system in which HE-iPSC were separately co-cultured with MSC and/or HUVEC. Although the three-dimensional structure did not form, their soluble factors induced a hepatocyte-like phenotype in HE-iPSC, resulting in the expression of bile salt export pump. In conclusion, the mesoderm-derived paracrine signals promote hepatocyte maturation in liver organoids, but organoid self-organization requires cell-to-cell surface contact. Our in vitro model demonstrated a novel approach to identify developmental paracrine signals regulating the differentiation of human hepatocytes.