3164 – A CELLULAR TAXONOMY OF MURINE BONE MARROW STROMAL STEM AND PROGENITOR CELLS UPON DEVELOPMENT

Abstract

Mesenchymal stem cells (MSCs) in the bone marrow (BM) are thought to play pivotal roles in tissue (re)generation and provide a supportive microenvironment for hematopoietic stem cells (HSCs). MSCs have been well characterized in murine models and isolated using different combinations of markers. Yet, these markers define a broader and more heterogeneous population, of which MSCs are only a minority. Although several studies indicate that bona fide MSCs reside in the BM, further investigations are needed to define their identity and allow their prospective and effective isolation. Here, using a single cell RNA sequencing approach, we aimed to delineate the cellular heterogeneity of the MSC-enriched Sca-1+ CD51+ population throughout development. At the fetal stage (E17), the BMSC population appeared relatively homogeneous with subsets predicted to contain candidate MSCs based on marker expression and computational modeling. The frequency of putative MSC subsets declined upon aging. After birth, the BMSC population was more heterogeneous, and characterized by the emergence of clusters showing transcriptional wiring consistent with specialization towards differentiated (progenitor) subsets. This included the postnatal emergence of a Lepr+ population, expressing HSC regulatory genes and previously established as HSC niches. In the fetal BM, subsets of endothelial cells expressed HSC factors and may thus represent an initial supportive microenvironment for emerging hematopoiesis. Our results shed light on the cellular diversity of BMSCs from different developmental phases, providing a cellular taxonomy of this population at single cell resolution. The exploration of our dataset, and the comparison of transcriptional landscapes from different developmental stages, may provide a resource to identify and further validate factors with a predicted role in BM regeneration. Mesenchymal stem cells (MSCs) in the bone marrow (BM) are thought to play pivotal roles in tissue (re)generation and provide a supportive microenvironment for hematopoietic stem cells (HSCs). MSCs have been well characterized in murine models and isolated using different combinations of markers. Yet, these markers define a broader and more heterogeneous population, of which MSCs are only a minority. Although several studies indicate that bona fide MSCs reside in the BM, further investigations are needed to define their identity and allow their prospective and effective isolation. Here, using a single cell RNA sequencing approach, we aimed to delineate the cellular heterogeneity of the MSC-enriched Sca-1+ CD51+ population throughout development. At the fetal stage (E17), the BMSC population appeared relatively homogeneous with subsets predicted to contain candidate MSCs based on marker expression and computational modeling. The frequency of putative MSC subsets declined upon aging. After birth, the BMSC population was more heterogeneous, and characterized by the emergence of clusters showing transcriptional wiring consistent with specialization towards differentiated (progenitor) subsets. This included the postnatal emergence of a Lepr+ population, expressing HSC regulatory genes and previously established as HSC niches. In the fetal BM, subsets of endothelial cells expressed HSC factors and may thus represent an initial supportive microenvironment for emerging hematopoiesis. Our results shed light on the cellular diversity of BMSCs from different developmental phases, providing a cellular taxonomy of this population at single cell resolution. The exploration of our dataset, and the comparison of transcriptional landscapes from different developmental stages, may provide a resource to identify and further validate factors with a predicted role in BM regeneration.