Function And Transcriptional Regulation Of MicroRNAs During Erythropoiesis And Megakaryopoiesis

During embryonic development, mammalian hematopoiesis occurs in three waves. The first wave of hematopoietic cells contains primitive erythroid cells. The second and the third waves of hematopoietic cells are definitive hematopoiesis, the erythroid cells from which are called definitive erythroid cells. Primitive erythroid cells are much lager than definitive erythroid cells, and express distinct globin genes. In recent years, model systems have been developed to study hematopoiesis in vitro. Murine embryonic stem cells (ESCs) grown in suspension culture form cell aggregates called embryoid bodies (EBs), from which expression of CD41 marks the initiation of definitive hematopoiesis. But the marker of primitive hematopoiesis is largely unknown. In this study, we compared in vitro differentiation in vitro of wild-type from SCL deficient (SCL-/-) embryonic stem cells and observed that CD71+high cells disappeared from SCL-/-EBs. In addition, SCL knockdown in a erythrolekemia line, MEL, reduced the expression of CD71. Chromatin immunoprecipitation CHIP) experiments demonstrated that SCL was bound to the enhancer of the CD71 gene, implying that CD71 is a direct transcriptional target of SCL. Further study demonstrated that primitive erythroid cells were highly enriched in FACS purified CD71+high cells. As CD71+high cells mature in vitro, they undergo a Hbb-bhl to Hbb-y switch, a process that mimics in vivo maturation. Taken together, these results suggest that CD71+highh population from differentiated cells represent primitive erythroid blast from ESCs in vitro.Megakarycytes and erythroid are derived from a common progenitor called MEP (Megakarycyte-erythroid-progenitor), and its differentiation into these two lineages is strictly regulated. MiR-144 and miR-451 are co-transcribed microRNAs and are upregulated during definitive erythroid differentiation. MiR144/451 knockout mice displayed anemia. In this study, we found that miR-451 was highly enriched in primitive erythroid cells and definitive hematopoietic progenitors, and miR1 44/451 is a direct transcriptional target of SCL. Overexpression of miR144/451 in K562 cells or MEL cells promoted erythroid formation but blocked megakaryocytic differentiation. By contrast, miR-142 enhanced megakaryocytic differentiation but inhibited erythroid formation, suggesting a role of microRNAs in regulating megakarycyte-erythroid balance. CAP1 was further identified as a new direct target gene of miR-451. we demonstrated here that CAP1 was downregulated during erythroid maturation, and its knockdown led to increased expression of hemaglobin, thereby suggesting a negative role of CAP1 in red blood cell formation. But the molecular mechnism of CAP1 remains to be determined. Additional studies demonstrated that several transcription factors (such as FLI-1, ETS-1, ERG) in ETS family cooperated with GATA1 to regulate expression of megakaryocyte-specific chemokines, including PBP and CCL5. In addition, they promoted megakaryocytic specification by enhancing CD41+cell formation from K562 but inhibited expression of hemaglobin in erythroid. Taken together, these result demonstrated a strictly regulated balance between megakaryopoiesis and erythroid formation by microRNAs and transcription factors.