Niche-specific regulation of hematopoietic stem and progenitors by NR4A1

Abstract Hematopoietic stem and progenitor cells (HSPCs) have the potential to generate all cells in the blood. Although the differentiation of HSPCs depends on their interactions with the niche, the exact mechanisms that control lineage specification remain unclear. Adult HSPC development predominates in the bone marrow, but also occurs at extramedullary sites including the spleen and there favors myelopoiesis. To explore how the spleen regulates myeloid differentiation, we transplanted bone marrow-derived, myeloid-biased HSPC subsets, including multipotent progenitor populations MPP2s and MPP3s, and monitored their development. We discovered that MPP2s, not MPP3s, contributed ~2-fold more to the total cell number in the spleen than in the bone marrow. At this same time point, ~25% of the Lin−Sca1+cKit+ population in the spleen, but not the bone marrow, were MPP2-derived. These data indicate that for specific HSPC populations, the splenic environment promotes their differentiation and self-renewal. Since we recently defined a role for the nuclear receptor, NR4A1, in regulating myeloid progenitor differentiation, we wondered if NR4A1 also affected HSPCs. Using NR4A1GFP mice, we discovered that HSPCs expressed higher levels of NR4A1 in the spleen compared to the bone marrow. Almost all NR4A1+ HSPCs in the spleen co-expressed VLA-4, a known HSPC niche retention factor. The few NR4A1+ HSPCs (~10%) in the bone marrow also expressed VLA-4 suggesting that NR4A1 affects the location of HSPCs either within the local niche or via mobilization to distant sites like the spleen. Together, our data highlight the importance of the splenic microenvironment in HSPC maintenance, differentiation, and localization, in part by the influence of NR4A1.