3025 - Ebf1 as a Transcriptional Regulator of Bone Marrow Mesenchymal Stromal Cells

Abstract

Mesenchymal stem and progenitor cells (MSPC), a crucial component of BM niche orchestrate maintenance, trafficking and stage specific differentiation of hematopoietic stem and progenitor cells (HSPCs). Although, it is generally accepted that MSPCs are essential for hematopoietic homeostasis, little is known about the transcriptional networks governing MSPCs biology. Here, we identified Early B-cell factor 1 (Ebf1) as new transcription regulator of MSPCs activity. MSPCs isolated from Ebf1-/- mice show diminished capacity to form fibroblastic colonies (CFU-F) indicating reduced self-renewal. Cells expanded from these colonies display impaired in vitro differentiation towards osteoblasts, chondrocytes and adipocytes. Adoptive transfers of wild type HSPCs to Ebf1+/- recipient mice showed a decrease in the absolute numbers of HSPCs in primary recipients and reduced donor chimerism within the HSCP compartment in competitive secondary transplant experiments. Additionally, Prx1-Cre-mediated deletion of Ebf1 in MSPCs in vivo leads to reduced numbers of HSPCs and myeloid cells in the bone marrow. Single cell and bulk transcriptome analysis of MSPCs lacking Ebf1 revealed differences in the niche composition and decreased expression of lineage-instructive signals for myeloid cells. Interestingly, we also observed a reduced ability of HSPCs sorted from Prx1CreEbf1 fl/fl mice to form colonies in methylcellulose, suggesting not only impaired maintenance but also hindered function of these cells. HSPCs exposed to Ebf1-deficient niche exhibit reduced chromatin accessibility for AP-1, ETS, Runx and IRF transcription factors that are crucial for myeloid differentiation, which is consistent with decreased myeloid output observed in Prx1CreEbf1 fl/fl mice. These results support the hypothesis that defective niche can cause epigenetic reprograming of HSPCs. In conclusion, our study establishes Ebf1 as a novel regulator of MSPCs playing a crucial role in the maintenance and differentiation of HSPCs.