Mechanical remodeling of the endothelial surface and actin cytoskeleton induced by fluid flow

Objective: The mechanism by which cultured endothelial cells respond to shear stress is controversial. The cell surface and cytoskeleton are in involved, but their roles are undefined. In this study, previously unknown changes in the surface detail and actin cytoskeleton of bovine aortic endothelial cells were identified. Methods: Actin filament content and filament number in resting and flow-oriented cells were determined by biochemical assays. The three-dimensional organization of the actin cytoskeleton in cells was defined in the confocal microscope and in the electron microscope after rapid-freezing, freeze-drying, and metal coating of detergent-permeabilized cells. Results: Endothelial cells have smooth apical membranes in situ. However, cultured cells exhibit surface microvilli which increase the apical surface area, exposing the ruffled surface to forces from fluid flow and potentially enhancing cell interactions with blood-borne white cells. Stereoscopic micrographs show that stress fibers are integrated into a complex distributed cytoplasmic structural actin network (DCSA). This lattice is formed by actin filaments that frequently cross and connect to each other, stress fibers, and microfilaments and microtubules. The cytoskeletons of cells cultured in static media lack apparent order when compared to cytoskeletons from cells which have been exposed to 24 hours of laminar flow. Conclusions: The DCSA physically connects the apical and basal cell membranes and fills the volume between nucleus and membrane, providing a mechanism for transmitting mechanical forces across cells and a signaling pathway from membrane to nucleus. Stress fibers increase the mechanical modulus of die DCSA, although this increase is probably-unnecessary to withstand the increase in shear stress caused by blood flow in vivo. This implies that actin rearrangements are not required for mechanical integrity, but serve an alternate function.