Role of the cell cycle during human endothelial-to-haematopoietic transition

Haematopoietic stem cells (HSC) are responsible for the maintenance of blood homeostasis and are of fundamental importance for the treatment of a variety of life-threatening diseases. However, their scarce availability constitutes a major obstacle. Thus, understanding their embryonic specification is essential for the production of these cells in vitro from human pluripotent stem cells (hPSC), but also to develop new therapies. During development, HSCs first arise in the aorta-gonad-mesonephros (AGM) region of the embryo from a population of haemogenic endothelial cells lining the ventral portion of the dorsal aorta which undergo endothelial-to-haematopoietic transition (EHT). This process culminates with the generation of the first HSCs capable of multilineage differentiation and long-term engraftment. Little is known about the molecular mechanisms driving this process, especially in human where the AGM region is not easily accessible in vivo. In this study, I took advantage of hPSCs and single cell transcriptomics to draw an accurate picture of this developmental stage and uncover mechanisms by which the haemogenic endothelium generates early HSCs. Of particular interest, I show that most of the endothelial cells at this stage reside in a quiescent state, with a direct correlation between cell cycle entry and their ability to progress to the haematopoietic fate. Furthermore, I identify CDK4/6 and CDK1 as key regulators affecting this process. Ultimately, I propose here a direct link between the molecular machineries controlling cell cycle progression and cell fate decision, determining the capability of haemogenic endothelial cells to undertake the haematopoietic fate during EHT. These results will have a major impact on the improvement of protocols for the production of functional HSCs in vitro, but also on the advancement of new therapies based on the culture ex vivo of primary HSCs.My PhD was funded by the British Heart Foundation Oxbridge Centre of Regenerative Medicin