Uncovering distinct stem-progenitor cell pathways for platelet replenishment

Every day 4 million of cells are turned over in the human body, about 90% of these cells are blood cells. The turn-over and therefore need for replenishment is particularly high for the short-lived myeloid lineages, including platelets. Hematopoietic stem cells (HSCs) ensure the constant replenishment of blood cells, passing through multiple stages of increasingly lineage-restricted progenitors. Although insults to the blood-forming system highlight the need for more rapid blood replenishment from HSCs, established models of hematopoiesis implicate only one, mandatory, differentiation pathway for each blood cell lineage. We previously identified a distinct HSC expressing von Willebrand factor (Vwf+ HSCs) that almost exclusively and stably replenishes platelets, unlike canonical Vwf- HSCs that replenish all the mature lineages. By tracking the clonal contribution of HSCs upon transplantation, we established that they utilize two cellularly and molecularly distinct differentiation pathways. Strikingly, single-cell RNA sequencing (scRNAseq) of the progenitor trajectories replenished by the two distinct HSC subsets revealed the absence of Flt3 RNA expression through the entire Vwf+ HSC progenitor pathway (platelet-restricted pathway). This finding allowed us to use Flt3Cretg/+ R26Tomato/+ mice for tracing the contribution of the two pathways both in steady-state and in response to stresses to the hematopoietic system. Importantly we were not only able to confirm the presence of the pathway in steady-state, but we also observed a selective activation of the platelet-restricted pathway upon progenitor depletion through cytotoxic treatments. These findings establish the existence of alternative HSC-progenitor pathways in platelet replenishment and provide a framework for enhancing platelet replenishment in settings where slow platelet recovery of platelets remains a significant challenge.