Mitochondrial dynamics in hematopoietic stem cells

Hematopoietic stem cells (HSCs) take on the extraordinary role of sustaining life-long production of blood cells. Despite their indisputable therapeutic potential, HSC biology is poorly understood, and the field remains limited by the inability to maintain, expand, or generate HSCs in vitro. The aim of this study was to elucidate a particular gap in our understanding of the organellar cell biology of HSCs, specifically the role and function of the mitochondria. Several signaling pathways and biological processes converge onto the mitochondria, yet these organelles were found to be largely dispensable in HSCs on the basis of their predominantly glycolytic metabolism and reports of low mitochondrial content. Our studies show that MitoTracker Green (MTG), a frequently used fluorescent dye to measure mitochondrial mass in hematopoietic populations, is effluxed by HSCs resulting in their systematic and deceptive enrichment in the subset of cells with the lowest MTG fluorescence. Using dye-independent methods we discovered that HSCs have elevated mitochondrial content despite their reliance on glycolysis for ATP production. Moreover, mechanisms of mitochondrial quality control and clearance by autophagy appear to be comparatively lower in HSCs than in any other hematopoietic population we analyzed, suggesting HSCs maintain their mitochondria over time. To investigate the function of mitochondria in HSCs we generated mice with disruption of mitofusins (MFN) 1 and 2. These proteins are key mediators of mitochondrial fusion, a process that in coordination with mitochondrial fission regulates mitochondrial size, number, and function. Mice with deletion of Mfn1 and Mfn2 (DKO) die perinatally, are pale in appearance and their HSCs show complete loss of regenerative capacity. Several processes linked to dysfunctional mitochondrial fusion and known to be tightly regulated in HSCs are altered in these mutants, including mitochondrial morphology, mitochondrial mass, proliferation, and altered metabolism. Interestingly, one allele of Mfn1 is sufficient to rescue the hematopoietic function and lethality of DKO mice, while one allele of Mfn2 only rescues myeloid reconstitution. Taken together, our findings highlight the importance and complexity of mitochondrial function and dynamics in HSCs and have contributed to the recently increased appreciation of a vital role for mitochondria in HSCs.