Biomechanical control of hematopoiesis

During development, specific areas of the vascular system transiently express high levels of the master regulator of haematopoiesis Runx1, and produce hematopoietic progenitors from the hemogenic endothelium. Vascular hematopoietic activity has been recently shown to be the source of definitive hematopoietic progenitors and therefore of the adult haematopoietic system. While hemogenic activity appears in the blood vessels after initiation of the heart beat, it remains unknown whether the biomechanical forces imposed on the vascular wall at this developmental stage act as a determinant of hematopoietic potential. In this thesis, I explore the hypothesis that shear stress, the frictional force generated by intravascular fluid flow on the internal aspect of the vascular wall, promotes hematopoiesis. Using embryonic stem cells (ES) differentiated in vitro , I showed that fluid shear stress increases the expression of the prototypical marker of hemogenic endothelium Runx1, concomitantly upregulating other haematopoiesis related genes and augmenting hematopoietic colony-forming potential of cultured cells. Moreover, I found that shear stress increases hematopoietic colony forming potential and expression of hematopoietic markers in primary cultures of embryonic hemogenic regions and that abrogation of nitric oxide (NO), a mediator of shear stress-induced signalling, compromises hematopoietic potential in vitro and in vivo . I further validated in vivo these observations showing that shear stress is able to rescue the hematopoietic defect observed in Ncx1 -/- embryos that fail to initiate a heart-beat. Finally, I present preliminary evidence suggesting that this pro-hematopoietic effect of shear stress is mediated through a direct effect of biomechanical stimulation on the hemogenic endothelium and that biomechanical stimulation plays a role in the establishment of definitive hematopoietic precursors. Collectively, the results here reported demonstrate that shear stress stimulates embryonic hematopoiesis both in ES cell cultures and within murine embryos. Establishing a link between the initiation of vascular flow and embryonic perivascular hematopoietic development, these data define hematopoiesis as a shear stress dependent function and provide new perspectives for the manipulation and production of hematopoietic progenitors in vitro , a key process for the implementation of stem cell-based therapy of hematologic diseases.