Surviving Change: The Metabolic Journey of Hematopoietic Stem Cells

Hematopoietic stem cell (HSC) activation is accompanied by mitochondria activation and a shift in metabolic activity from glycolysis to OXPHOS, which provides energy and increases the production of ROS and other mitochondrial metabolites that can act both as signaling molecules and substrates/co-activators for epigenetic enzymes. Metabolically activated HSCs are poised to undergo lineage priming and produce different lineage-biased multipotent progenitors (MPP). However, activated HSCs must also return to quiescence to maintain the stem cell pool. In this context, autophagy plays an essential role by clearing activated mitochondria to allow OXPHOS-driven HSCs to efficiently revert to a mostly glycolysis-based quiescence metabolism. Without autophagy, HSCs display an overactive OXPHOS-driven metabolism that promotes myeloid-biased differentiation and loss of stemness, likely as a direct consequence of epigenetic reprogramming. At steady state, blood production reflects the differential generation by HSCs of a small number of myeloid-biased MPP2/3 and a large number of lymphoid-biased MPP4, which both contribute to myeloid output. In contrast, during blood regeneration, activated HSCs are induced to overproduce MPP2/3, and MPP4 are reprogrammed towards almost exclusive myeloid output in large part due to cytokine stimulations and the triggering of specific regulatory pathways. Altogether, the metabolic activation of HSCs and the remodeling of the MPP compartment represent emergency myelopoiesis pathways that are transiently activated during regeneration, and are continuously triggered in myeloid disease conditions.