A Rare Hematopoietic Stem Cell-Derived Megakaryocyte Progenitor Accumulates via Enhanced Survival and Contributes to Exacerbated Thrombopoiesis Upon Aging.

Distinct routes of cellular production from hematopoietic stem cells (HSCs) have defined our current view of hematopoiesis. Recently, we and others have challenged classical views of platelet generation, demonstrating that megakaryocyte progenitors (MkPs) and ultimately platelets can be specified via an alternate and additive route of HSC-direct specification specifically during aging. This "shortcut" pathway generates hyperactive platelets likely to contribute to age-related platelet-mediated morbidities. Here, we used single-cell RNA/CITEseq to demonstrate that these age-unique, noncanonical (nc)MkPs can be prospectively defined and experimentally isolated from wild-type mice. Surprisingly, this revealed that a rare population of ncMkPs also exists in young mice. Young and aged ncMkPs are functionally distinct from each other and from their canonical (c)MkP counterparts, with aged ncMkPs paradoxically and uniquely exhibiting enhanced survival and platelet generation capacity. We further demonstrate that aged HSCs generate significantly more ncMkPs than their younger counterparts, yet this is accomplished without strict clonal restriction. Together, these findings reveal significant phenotypic, functional, and aging-dependent heterogeneity among the MkP pool and uncover unique features of megakaryopoiesis throughout life, potentially offering cellular and molecular targets for the mitigation of age-related adverse thrombotic events.