Clonal analysis of young and aged hematopoietic stem cells using cellular barcoding

Production of all blood cells during the entire lifespan is supported by a small population of hematopoietic stem cells (HSCs). Any adverse changes in this population caused by cellular stress, the acquisition of mutations or aging can lead to development of blood diseases, such as leukemia and bone marrow failure. In order to prevent or cure these conditions, it is necessary to understand how HSCs function normally. It is a matter of discussion how many HSCs are required for maintenance of blood production, whether all HSCs are similar, and how they change with aging. To answer these questions, it is important to measure the contribution of multiple HSCs simultaneously and in a quantitative manner. We have developed and validated a novel barcoding technique, which is based on DNA-tagging of hematopoietic cells with genome-integrating viral vectors. Using this technique, we labeled HSCs from young and aged mice and transplanted these cells into recipient mice. As each HSC was uniquely labeled with a barcode, its progeny could be followed in time. Our findings indicate that HSC clones are very heterogeneous with respect to their ability to contribute to the various types of mature cells. Aging decreased the number of mature cells produced per stem cell. However, both young and aged HSCs were preferentially located in certain skeletal sites, and were similarly affected by clinically relevant cytokine administration. In summary, we developed novel technology that enables studying the behavior of HSC in vivo and provides answers to multiple long-standing questions in HSC biology.