Blood Flow Forces in Shaping the Vascular System: A Focus on Endothelial Cell Behavior

Abstract

The endothelium is the cell monolayer that lines the interior of the blood vessels separating the vessel lumen where blood circulates, from the surrounding tissues. During embryonic development, endothelial cells (ECs) must ensure that a tight barrier function is maintained whilst dynamically adapting to the growing vascular tree that is being formed and remodeled. Blood circulation generates mechanical forces, such as shear stress and circumferential stretch that are directly acting on the endothelium. ECs actively respond to flow-derived mechanical cues by becoming polarized, migrating and changing neighbors, undergoing shape changes, proliferating or even leaving the tissue and changing identity. It is now accepted that coordinated changes at the single cell level drive fundamental processes governing vascular network morphogenesis such as angiogenic sprouting, network pruning, lumen formation, regulation of vessel caliber and stability or cell fate transitions. Here we summarize the cell biology and mechanics of ECs in response to flow-derived forces, discuss the latest advances made at the single cell level with particular emphasis on in vivo studies and highlight potential implications for vascular pathologies.

Keywords: Danio rerio (zebrafish); angiogenesis; cardiovascular; cilia; live imaging; low Reynolds number; mechanotransduction; stretch.

FIGURE 1

Blood flow-derived forces. Schematic representation of the mechanical forces experienced by endothelial cells due to blood circulation inside the vessels. The blood flow, measured by the volume flow rate Q, causes a shear stress σ_shear on the wall. The shear stress depends on the flow rate Q, the blood viscosity η and the vessel radius R. The relationship shown is valid in laminar flow, occurring at low Reynolds numbers. Independently of the flow, the hydrostatic pressure p causes a circumferential (hoop) stress σ_circ and an axial stress σ_axial. Contrary to shear stress, they increase linearly with the vessel radius. The circumferential stress has twice the magnitude of the axial stress. The equations hold for thin walls (D≪R). Furthermore, we assumed that no external forces are acting on the vessel, such that the pressure forces are fully counterbalanced by the stress in the walls (depending on the mechanical environment, the force-free condition may not always be fulfiled, especially in the axial direction).

FIGURE 2

Endothelial cell behaviors triggered by flow-derived mechanical cues during vascular network growth and development. (a) Arteries adjust their caliber in response to hemodynamic forces. Arteries can either reduce their diameter via coordinated cell shape changes (Sugden et al., 2017) or increase the diameter by cell proliferation and migration toward enlarging vessels (Udan et al., 2013; Poduri et al., 2017). (b) Blood pressure has been shown to play a central role in network growth. The location and growth of newly forming sprouts is set by local differences in blood pressure (high and low pressure, magenta and cyan, respectively, Ghaffari et al., 2015). In addition, intraluminal pressure causes inverse blebbing of the apical membrane that drives lumen growth in new sprouts – drawn after Gebala et al. (2016). Similarly, during vessel anastomosis the apical membranes of two adjacent tip cells are pushed toward each other in a blood pressure-dependent fashion – illustration after Lenard et al. (2013). (c) Vascular networks can be remodeled in response to shear stress. Cell polarization and migration against the flow direction can lead to retraction of poorly irrigated vessels – sketched after Franco et al. (2016) – while vessels exposed to vigorous flow are kept by shear stress induced translocation of YAP to the nucleus – depiction after Nakajima et al. (2017). (d) Flow-derived mechanical cues can lead to cell extrusion and the formation of new structures such as the atrioventricular canal (AVC) valve or trigger cell fate transitions as seen during hematopoietic stem cell formation (see Figure 3). During AVC valve formation oscillatory blood flow triggers expression of the transcription factor klf2a, which is thought to promote invasion of the cardiac jelly by endocardial cells thus initiating the formation of the valve leaflets. Diagram after Heckel et al. (2015) and Steed et al. (2016).

FIGURE 3

Hematopoietic stem cell emergence is blood flow dependent. (A) During definitive hematopoiesis, hematopoietic stem cell progenitors arise at the ventral side of the dorsal aorta (DA) via cell extrusion from the endothelium. (B) Potential hemogenic cells (blue) located at the ventral region of the DA are elongated along the anterior-posterior (AP) axis. (C) Cells are extruded from the endothelium by contraction of actomyosin that accumulated at the AP poles of the cell. (D) Concomitantly, the majority of endothelial cells move toward the ventral side of the DA (yellow arrows). At each heartbeat, the DA wall deforms asymmetrically: the deformation is highest (magenta) in the ventral region and lowest (blue) in the dorsal side. Drawings in (B,C) after Lancino et al. (2018).