UCLA

Human embryonic stem cells (hESC) and induced pluripotent stem cells (hiPSC) are pluripotent stem cells (hPSCs) that have the capacity to differentiate into a panoply of various cell types and to serve as a model for human development. Nevertheless, it is unknown whether in vitro development of hPSCs mirrors in vivo human development. We performed comprehensive transcriptome profiling to compare hPSC-derived progeny with tissue-derived counterparts. While hESC and hiPSC make nearly identical progeny for the neural, hepatic and mesenchymal lineages, these hPSC-derived progeny maintained, regardless of the cell lineage, the expression of genes normally associated with early development, including LIN28A, LIN28B and DPPA4. These data and expression patterns in early human fetal tissue suggested the hPSC-derivatives were reflective of very early human development. The LIN28/let-7 pathway has been known to regulate development. We hypothesize that high Lin28 and low level let-7 activities may be inhibiting the developmental maturity in hPSC-derivatives. In addition, such a LIN28/let-7 expression pattern is also thought to contribute to tumor aggressiveness. We have established a human cell-based system to screen for small molecules that could modulate LIN28/let-7 activity. This screening could potentially unveil powerful small molecules that could modulate developmental maturity and therapeutically combat cancer. Previous reports and our own transcriptional profiling indicate that hPSCs are suitable for modeling early human development. Epithelial to mesenchymal transitions (EMTs) are thought to be essential to generate diversity of tissues during early fetal development, but these events are impossible to study at the molecular level in vivo in humans. The first EMT event in human development occurs just prior to generation of the primitive streak. Because hPSCs are thought to most closely resemble cells found in epiblast stage embryos prior to formation of the primitive streak, we sought to model this first human EMT in vitro with hPSCs. We have demonstrated the EMT progression in hPSCs with embryoid bodies. We also identified PTK7 as a marker of this EMT population, which was used to purify these cells for identification of novel markers of human germ layer development.