SPI1-KLF1/LYL1 Axis Regulates Lineage Commitment during Human Endothelial-to-Hematopoietic Transition

PU.1 ( SPI1) is a crucial transcription factor in hematopoiesis, yet its role in human endothelial-to-hematopoietic transition (EHT) remains unclear. Given that the complexity of in vivo hematopoiesis can mask subtle cellular transitions, we sought to investigate the function of SPI1 in both complex in vivo environments and more controlled in vitro models of EHT to elucidate its precise role. By comparing the human in vivo and in vitro EHT transcriptomes, we observed similarities in the transcriptional profiles of SPI1, suggesting its regulatory role in EHT. Knocking down SPI1 in in vitro-generated hemogenic endothelial cells (HECs) led to a decrease in the generation of hematopoietic progenitor cells (HPCs) and their differentiation potential. Through multi-omic analysis, we identified KLF1 and LYL1 - transcription factors specific to erythroid/myeloid and lymphoid cells, respectively - as downstream targets ofSPI1. Concomitant overexpression of KLF1 or LYL1 with SPI1 knockdown in in vitro-generated HECs partially rescues the reduction in HPC formation induced by SPI1 knockdown. Specifically, KLF1 overexpression restores myeloid lineage potential, while LYL1 overexpression re-establishes lymphoid lineage potential. We also observed a SPI1- LYL1 axis in the regulatory network in in vivo EHT. Taken together, our findings shed new light on the role of SPI1 in regulating lineage commitment during EHT, potentially contributing to the heterogeneity of hematopoietic stem cells (HSCs).