Shear Stress Rapidly Alters the Physical Properties of Vascular Endothelial Cell Membranes by Decreasing Their Lipid Order and Increasing Their Fluidity

Abstract

Vascular endothelial cells (ECs) sense shear stress generated by flowing blood and alter their functions that play important roles in vascular homeostasis and pathophysiology. A unique feature of shear-stress-sensing is the rapid activation of many different types of membrane–bound molecules, including receptors, ion channels, and adhesion proteins, but the mechanisms remain unknown. One hypothesis is that shear stress might alter the physical properties of EC membranes, which lead to the activation of various membrane-bound molecules. To determine how shear stress influences the cell membrane, cultured human pulmonary artery ECs were exposed to shear stress in a flow-loading apparatus and examined for changes in membrane lipid order and fluidity by means of Laurdan two-photon imaging and FRAP measurements. Upon shear stress stimulation, the lipid order of EC membranes rapidly decreased in an intensity-dependent manner, and caveolar membrane domains changed form the liquid-ordered state to the liquid-disordered state. Notably, a similar decrease in lipid order occurred when artificial membranes of giant unilamellar vesicles composed of cholesterol and phospholipids were exposed to shear stress, suggesting that this is a physical phenomenon. Membrane fluidity increased over the entire EC membranes in response to shear stress. Addition of cholesterol to ECs abolished the effects of shear stress on the membrane lipid order and fluidity, and it markedly suppressed ATP release which is a well-known EC response to shear stress and is involved in shear stress Ca2 + signaling. These findings indicate that EC membranes, especially caveolar membrane domains, rapidly respond to shear stress by changing their physical properties, including their lipid phase order and fluidity, and that these changes may be linked to shear-stress-sensing and response mechanisms.KeywordsEndothelial cellshear stresscaveolaeplasma membranelipid ordermembrane fluidity