Identification and Characterization of Distinct Pathways for Platelet Replenishment in Steady-State Hematopoiesis

Rare multipotent hematopoietic stem cells (HSCs) safeguard the replenishment of millions of blood cells per second in humans, passing through multiple stages of increasingly lineage-restricted progenitors. The turn-over and therefore need for replenishment is particularly high for the short-lived myeloid lineages, including platelets. We previously identified a distinct subset of hematopoietic stem cells (HSCs) in mice, highly biased or even restricted towards exclusively replenishing platelets unlike the prototypical HSCs that produce all blood cell lineages (Carrelha J et al. Nature. 2018 Feb 1;554(7690):106-111).

Through single HSC transplantation studies, we recently established a non-hierarchical relationship between von Willebrand factor negative (Vwf^-) HSCs that replenish all blood cell lineages (Vwf^- multi-HSCs) and Vwf⁺ HSCs replenishing almost exclusively platelets (Vwf⁺ P-HSCs), and that they utilize cellularly, and molecularly distinct pathways for replenishment of distinct megakaryocyte-restricted progenitors (MkPs), multi-MkPs and P-MkPs, respectively (Carrelha J et al. Nat Immunol. 2024 Jun;25(6):1007-1019). In contrast, studies by Poscablo et al (Poscablo DM et al. Cell. 2024 Jun 6;187(12):3090-3107.e21), suggested that a platelet-restricted pathway is unique to old mice, raising the possibility that the platelet-restricted pathway in young mice is absent during steady-state and only activated upon transplantation. One of the most striking differences revealed by single-cell RNA sequencing (scRNAseq) of the progenitor trajectories replenished by transplanted single Vwf⁻ multi-HSCs and Vwf⁺ P-HSCs was the absence of Flt3 RNA expression through the entire Vwf⁺ P-HSC progenitor pathway, contrasting with all progenitors in the Vwf^- multi-HSC pathway having passed through a Flt3⁺ stage. We therefore investigated young Flt3Cre^tg/+ R26^Tomato/+ mice for the presence of the distinct MkPs replenished through the two pathways. Peripheral blood (PB) analysis of 12-week-old mice revealed that platelets were the only blood cell lineage in which a fraction (10%) of the cells were dTomato-negative, and likewise in the bone marrow (BM) 10% of MkPs were dTomato-negative in contrast to other lineage-restricted progenitors that were all >99% dTomato-positive. ScRNAseq analysis comparing steady-state dTomato-negative and dTomato-positive MkPs recapitulated the distinct gene expression signatures of Multi-MkPs and P-MkPs replenished by single cell transplanted Vwf^- Multi-HSCs and Vwf⁺ P-HSCs, respectively, including upregulation in P-MkPs of several genes (Clu, Ctse, Gda and Phactr), encoding proteins promoting the coagulation process. These findings establish that the platelet-restricted pathway initiated by P-HSCs is established and active already in young adult mice, although most platelets are replenished through the canonical multi-HSC pathway. In agreement with Poscablo et al, and the expansion of P-HSCs upon aging (Grover A et al. Nat Commun. 2016 Mar 24;7:11075), 2-year-old Flt3Cre^tg/+ R26^Tomato/+ mice showed a striking increase in dTomato-negative platelets in PB and MkPs in BM, both rising to 40%.

These findings establish a model to unravel the role of alternative HSC-progenitor pathways for platelet replenishment, the functionality of platelets they produce and how that is altered upon aging.