Identification and Age-dependent Increase of Platelet Biased Human Hematopoietic Stem Cells

Hematopoietic stem cells (HSC) reconstitute multi-lineage human hematopoiesis after clinical bone marrow transplantation and are the cells-of-origin of hematological malignancies. Though HSC provide multi-lineage engraftment, individual murine HSCs are lineage-biased and contribute unequally to blood cell lineages. Now, by combining xenografting of molecularly barcoded adult human bone marrow (BM) HSCs and high-throughput single cell RNA sequencing we demonstrate that human individual BM HSCs are also functionally and transcriptionally lineage biased. Specifically, we identify platelet-biased and multi-lineage human HSCs. Quantitative comparison of transcriptomes from single HSCs from young, and aged, BM show that both the proportion of platelet-biased HSCs, and their level of transcriptional platelet priming, increases with age. Therefore, platelet-biased HSCs, as well as their increased prevalence and elevated transcriptional platelet priming during ageing, are conserved between human and murine hematopoiesis.

External signals regulate continuous transcriptional states in hematopoietic stem cells

Hematopoietic stem cells (HSCs) must ensure adequate blood cell production following distinct external stressors. A comprehensive understanding of in vivo heterogeneity and specificity of HSC responses to external stimuli is currently lacking. We performed single-cell RNA sequencing (scRNA-Seq) on functionally validated mouse HSCs and LSK (Lin-, c-Kit+, Sca1+) progenitors after in vivo pharmacological perturbation of niche signals interferon, granulocyte colony-stimulating factor (G-CSF), and prostaglandin. We identified six HSC states that are characterized by enrichment but not exclusive expression of marker genes. External signals induced rapid transitions between HSC states but transcriptional response varied both between external stimulants and within the HSC population for a given perturbation. In contrast to LSK progenitors, HSCs were characterized by a greater link between molecular signatures at baseline and in response to external stressors. Chromatin analysis of unperturbed HSCs and LSKs by scATAC-Seq suggested some HSC-specific, cell intrinsic predispositions to niche signals. We compiled a comprehensive resource of HSC- and LSK progenitor-specific chromatin and transcriptional features that represent determinants of signal receptiveness and regenerative potential during stress hematopoiesis.