Hematopoietic lineages diverge within the stem cell compartment

Abstract

Hematopoietic stem cells (HSCs) produce a highly diverse array of cell lineages. To assay hematopoietic differentiation with minimal experimental perturbation, non-invasive methods for heritable labeling (1-3) or barcoding (4-7) of HSCs in vivo have recently been developed and used to study lineage fate of HSCs in physiological conditions. However, the differentiation pathways leading from HSCs to mature cells remain controversial (8), with suggested models ranging from gradual lineage restriction in a branching cascade of progenitors to HSCs already making ultimate lineage decisions. Here we show, by iterating HSC fate-mapping, mitotic history tracking, single-cell RNA-sequencing and computational inference, that the major differentiation routes to megakaryocytes, erythro-myeloid cells and lymphocytes split within HSCs. We identify the hitherto elusive self-renewing source of physiological hematopoiesis as an HSC subpopulation co-expressing high levels of Sca-1 and CD201. Downstream, HSCs reduce Sca-1 expression and enter into either thrombopoiesis or erythro-myelopoiesis, or retain high Sca-1 levels and the ability to generate lymphocytes. Moreover, we show that a distinct population of CD48-/lo megakaryocyte progenitors links HSCs to megakaryocytes. This direct thrombopoiesis pathway is independent of the classical pathway of megakaryocyte differentiation via multipotent progenitors and becomes the dominant platelet production line upon enhanced thrombopoietin signaling. Our results define a hierarchy of self-renewal and lineage decisions within HSCs in native hematopoiesis. Methodologically, we provide a blueprint for mapping physiological differentiation pathways of stem cells and probing their regulation.

Longitudinal dynamics of clonal hematopoiesis identifies gene-specific fitness effects

“…3B) and to the rate of self-renewing divisions. Using the well-established rate = 1.3/year (once every 40 weeks, (Catlin et al, 2011)) we obtain an estimate on the number of HSPCs of ∼ 6500 (Barile et al, 2020;Morcos et al, 2020;Takahashi et al, 2021). Since we can only directly measure , it is possible that both a depletion of the pool of HSPCs at old age and heterogeneity in the self-renewal rate have enabled neutral events to reach high VAFs.…”

“…Both a study of somatic mutations in a 115-yr-old woman and a recent longitudinal study following the evolution of clones in an individual from age 103 to 110 (3 time points) found that likely benign clones achieved VAFs of up to 0.3 (van den Akker et al, 2020; Holstege et al, 2014)-suggesting a very small pool of stem cells. Another possible explanation is that the observed variants arose in cells with faster rates of self-renewing symmetric divisions λ , either due to natural selection of faster dividing HSPCs or because the observed variants in the LBC arose in different HSPC compartments, such as multipotent progenitor cells (Barile et al, 2020; Morcos et al, 2020; Takahashi et al, 2021). Since we can only directly measure θ , it is possible that both a depletion of the pool of HSPCs at old age and heterogeneity in the self-renewal rate λ have enabled neutral events to reach high VAFs.…”

Developmental cues license megakaryocyte priming in murine hematopoietic stem cells

“… 3 , 5 , 6 , 7 , 8 Functional studies have unveiled a role for CD41-expressing HSCs in mitigating the risk for thrombocytopenia after acute stress by forging a top-down emergency megakaryopoiesis bypass route. 9 , 10 , 11 , 12 However, when and how this poised HSC state is first established during ontogeny remains unexplored.…”

Clearance of genome-damaged cells from the hematopoietic system via p53 without contribution by the cGAS/STING axis

“…RH2 KO mice developed thrombocythemia (Figure 2A), while all other blood lineages are cytopenic. These seemingly contradictory findings are likely explained by the observation that thrombocytes can be generated by direct differentiation of HSCs into megakaryocytes without proliferation (Morcos, Li et al 2020), while maintenance of all other mature blood cells heavily relies on cell division of stem and progenitors cells. Interestingly, the thrombocythemia of RH2 hKO mice was reversed by loss of p53, in line with reported roles for p53 signaling in megakaryocyte endoreplication and thrombopoiesis (Apostolidis, Woulfe et al 2012, Yang, Liu et al 2020).…”