Meis1 establishes the pre-hemogenic endothelial state prior to Runx1 expression [CITE-seq]

Hematopoietic stem and progenitor cells (HSPCs) originate from an endothelial-to-hematopoietic transition (EHT) during embryogenesis. Characterization of early hemogenic endothelial (HE) cells is required to understand what drives hemogenic specification and to accurately define cells capable of undergoing EHT. Using Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITE-seq), we defined the early subpopulation of pre-HE cells based on both surface markers and transcriptomes. We identified the transcription factor Meis1 as an essential regulator of hemogenic cell specification in the embryo prior to Runx1 expression. Meis1 is expressed at the earliest stages of EHT and distinguishes pre-HE cells primed towards the hemogenic trajectory from the arterial endothelial cells that continue towards a vascular fate. Endothelial-specific deletion of Meis1 impaired the formation of functional Runx1-expressing HE which significantly impeded the emergence of pre-HSPC via EHT. Our findings implicate Meis1 in a critical fate-determining step for establishing EHT potential in endothelial cells. CITE-Seq profiles of CD31+ cells FACS-isolated from dissected mouse AGM at embryonic day 10.5. Each replicate represents a pool of 2-5 embryos.造血干细胞和祖细胞（hematopoietic stem and progenitor cells, HSPCs）起源于胚胎发生过程中的内皮向造血转化（endothelial-to-hematopoietic transition, EHT）。亟需对早期生血内皮（hemogenic endothelial, HE）细胞进行特征解析，以阐明驱动生血特化的分子机制，并准确定义具备内皮向造血转化能力的细胞类群。本研究采用转录组与表位测序细胞索引技术（Cellular Indexing of Transcriptomes and Epitopes by Sequencing, CITE-seq），基于表面标志物与转录组特征，鉴定出前生血内皮（pre-HE）细胞的早期亚群。本研究鉴定出转录因子Meis1是胚胎中Runx1表达前调控生血细胞特化的必需因子。Meis1于内皮向造血转化的最早阶段即有表达，可将朝向生血轨迹定向的前生血内皮细胞与维持血管细胞命运的动脉内皮细胞区分开来。内皮特异性敲除Meis1会损害表达Runx1的功能性生血内皮细胞的形成，进而显著阻滞经内皮向造血转化产生造血干祖细胞前体（pre-HSPC）的过程。本研究结果表明，Meis1参与了内皮细胞获得内皮向造血转化潜能的关键命运决定步骤。本数据集包含从胚胎发育第10.5天的小鼠主动脉-性腺-中肾（aorta-gonad-mesonephros, AGM）区域解剖分离、经荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）得到的CD31阳性细胞的CITE-seq图谱。每个重复样本为2~5枚胚胎的混合样本。