Fluid shear stress combined with shear stress spatial gradients regulates vascular endothelial morphology

High shear stress (SS) causes local changes around arterial bifurcations, which are common sites for cerebral aneurysms. High SS and SS spatial gradient (SSG) are thought to play important roles in the pathology of cerebral aneurysms. However, whether SS and SSG independently affect the function and morphology of vascular endothelial cells (ECs) exposed to fluid flow remains unclear. This study evaluated the morphology of ECs exposed to various SS and SSG combinations. Confluent ECs were exposed to a SS of 2-60 Pa and a uniform SSG of 0, 5, 10, or 15 Pa mm^-1 for 24 h. Although ECs exposed to lower levels of SS/SSG were not oriented or elongated in the direction of flow, they began to exhibit orientation, elongation, and development of actin stress fibers under the conditions of SS with a SSG when the SS exceeded a threshold value depending on the magnitude of SSG. Using a simplified computational model, we found that the presence of a SSG affects the strain field in ECs, resulting in a morphological response. SS combined with a SSG can alter the localization of SS mechano-sensing proteins along the strain field as a result of shear flow. Our results suggest that the magnitude of the relationship between SS and SSG plays an important role in regulating morphological changes in ECs in response to fluid flow by regulating EC polarity.