Abstract
The limited number of hematopoietic stem cells (HSCs) in umbilical cord blood (UCB) units restricts their use for stem cell transplantation. Ex vivo treatment of UCB-CD34(+) cells with valproic acid (VPA) increases the number of transplantable HSCs. In this study, we demonstrate that HSC expansion is not merely a result of proliferation of the existing stem cells but, rather, a result of a rapid reprogramming of CD34(+)CD90(-) cells into CD34(+)CD90(+) cells, which is accompanied by limited numbers of cell divisions. Beyond this phenotypic switch, the treated cells acquire and retain a transcriptomic and mitochondrial profile, reminiscent of primary HSCs. Single and bulk RNA-seq revealed a signature highly enriched for transcripts characteristic of primary HSCs. The acquisition of this HSC signature is linked to mitochondrial remodeling accompanied by a reduced activity and enhanced glycolytic potential. These events act in concert with a modest upregulation of p53 activity to limit the levels of reactive oxygen species (ROS). Inhibition of either glycolysis or p53 activity impairs HSC expansion. This study indicates that a complex interplay of events is required for effective ex vivo expansion of UCB-HSCs.