Hematopoiesis: Functional Insights by Cell Fate Conversion

Abstract: Blood cell formation, or hematopoiesis, is maintained by rare hematopoietic stem cells (HSCs) residing in the bone marrow (BM). HSCs may self-renew (the process whereby HSCs replicate to produce new HSCs) to preserve their own numbers, as well as initiate a highly coordinated hierarchical differentiation process that results in the production of mature effector blood cells. Aging is characterized by an overall loss of cellular and organ fitness, which has been suggested to result from alterations in tissue-resident somatic stem cell function. In the blood, aging presents with several functional alterations that can be traced to cell intrinsic alterations occurring in HSCs. HSC aging has been suggested to result from accumulation of DNA damage in genomic and/or mitochondrial DNA (mtDNA). We addressed the latter in article I using mice that rapidly accumulate mtDNA mutations and display several premature aging phenotypes. These “mutator mice” displayed several hematopoietic abnormalities associated with aging, including anemia and defects in lymphopoiesis. However, several hallmarks associated with HSC aging was lacking. In addition, the observed phenotypes appeared to result from alterations in progenitor cells rather than in HSCs. Thus, we concluded that mtDNA mutations are unlikely to be the main drivers of hematopoietic aging. In article II, we investigated the relevance of epigenetic alterations for hematopoietic aging. To this end, we generated induced pluripotent stem (iPS) cells from aged hematopoietic stem and progenitor cells (HSPCs), since cellular reprogramming coincides with an epigenetic reset of the somatic donor cells. We next redifferentiated the resulting iPS cells into blood in vivo and investigated the resulting hematopoiesis for age-related parameters. This revealed that the reformed blood system displayed a balanced lineage potential, with HSC numbers and function comparable to the young setting. Therefore, we concluded that a major component of hematopoietic aging involves reversible epigenetic alterations. While mtDNA mutations appeared to impact little on somatic stem cell function per se (article I), little is known about their impact on iPS cells function. To address this, we generated iPS cells from mutator HSPCs. Although mutator iPS cells were readily formed, these displayed severe differentiation defects (article III). We traced this to a failure of mutator iPS cells to utilize mitochondrial driven oxidative phosphorylation during differentiation. In article IV, we set out to identify individual novel regulators that may impact on early hematopoietic cell fate decisions. This led to the identification of the transcription factor Hepatic Leukemia Factor (Hlf). Enforced Hlf expression could direct early cell fate decisions of multipotent GMLPs by strongly favoring myelopoiesis over lymphopoiesis. This indicates that Hlf is a key determinant of lineage fate and an important component in the regulatory networks of multipotency.