Polycomb group protein Bmi1 promotes hematopoietic cell development from embryonic stem cells.

Bmi1 is a component of the Polycomb repressive complexes and essential for maintaining the pool of adult stem cells. Polycomb repressive complexes are key regulators for embryonic development by modifying chromatin architecture and maintaining gene repression. To assess the role of Bmi1 in pluripotent stem cells and on exit from pluripotency during differentiation, we studied forced Bmi1 expression in mouse embryonic stem cells (ESC). We found that ESC do not express detectable levels of Bmi1 RNA and protein and that forced Bmi1 expression had no obvious influence on ESC self-renewal. However, upon ESC differentiation, Bmi1 effectively enhanced development of hematopoietic cells. Global transcriptional profiling identified a large array of genes that were differentially regulated during ESC differentiation by Bmi1. Importantly, we found that Bmi1 induced a prominent up-regulation of Gata2, a zinc finger transcription factor, which is essential for primitive hematopoietic cell generation from mesoderm. In addition, Bmi1 caused sustained growth and a >100-fold expansion of ESC-derived hematopoietic stem/progenitor cells within 2-3 weeks of culture. The enhanced proliferative capacity was associated with reduced Ink4a/Arf expression in Bmi1-transduced cells. Taken together, our experiments demonstrate distinct activities of Bmi1 in ESC and ESC-derived hematopoietic progenitor cells. In addition, Bmi1 enhances the propensity of ESC in differentiating toward the hematopoietic lineage. Thus, Bmi1 could be a candidate gene for engineered adult stem cell derivation from ESC.