Understanding hematopoietic stem and progenitor cell diversity during ontogeny

PROJECT SUMMARY The goal of this project is to understand hematopoietic stem and progenitor cell diversity during ontogeny. Hematopoietic stem cells (HSCs) are self-renewing, multipotent stem cells that maintain the entire blood system through a process called hematopoiesis. During embryogenesis, HSCs emerge simultaneously with HSC-independent Progenitor Cells (HPCs) that have limited self-renewal capacity and a lesser differentiation potential than HSCs. Interestingly, HPCs have been shown to be necessary and sufficient to sustain embryonic hematopoiesis and are critical to hematopoietic health. Therefore, understanding how these progenitor populations are regulated differently from HSCs is critical to advancing protocols for generating HPCs and HSCs in vitro and for understanding aberrant hematopoiesis. Developmental hematopoiesis is layered, where overlapping waves produce blood cells in the embryo. HSCs and HPCs emerge from the hemogenic endothelium (HE) via an evolutionarily conserved process termed the endothelial-to- hematopoietic transition (EHT). Evidence in the field suggests that these HSCs and HPCs are emerging from distinct HE cells, indicating that HSC or HPC fates are determined early in hematopoietic development. One major difficulty in studying HSCs and HPCs is that they emerge in a spatially and temporally overlapping manner and express many of the same markers. Our lab developed a novel lineage tracing assay that temporarily distinguishes developmental HSCs and HPCs in vivo by assessing their distinct differentiation properties. Additionally, we performed scRNA-seq on newly emerging hematopoietic stem and progenitor cells (HSPCs) and defined distinct gene signatures for HSPC populations. Based on the collective findings, we hypothesize that the distinct differentiation by HSCs and HPCs is set up during their embryonic formation and is regulated by discrete factors. In Aim 1, we seek to understand the ontogeny and differentiation potential of a poorly defined HPC subset, Lymphoid Progenitors (LyPs), and in Aim 2, we seek to determine the impact of transcription factor Tcf15 in regulating HSPC specification and functionality. Knowledge gained from this proposal will identify distinct characteristics and molecular mechanisms that differentiate HSCs from HPC with implications for human hematopoietic and immune disorders and their treatments.