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      Effects of Flow Patterns on the Localization and Expression of VE-Cadherin at Vascular Endothelial Cell Junctions: In vivo and in vitro Investigations

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          Atherosclerosis occurs preferentially at vascular curvature and branch sites where the vessel walls are exposed to fluctuating shear stress and have high endothelial permeability. Endothelial permeability is modulated by intercellular adhesion molecules such as VE-cadherin. This study was designed to elucidate the effects of different flow patterns on the localization and expression of VE-cadherin in endothelial cells (ECs) both in vivo and in vitro. VE-cadherin staining at EC borders was much stronger in the descending thoracic aorta and abdominal aorta, where the pulsatile flow has a strong net forward component than in the aortic arch and the poststenotic dilatation site beyond an experimental constriction, where the flow near the wall is complex and reciprocating with little net flow. With the use of flow chambers the effects of pulsatile flow (12 ± 4 dyn/cm<sup>2</sup> at 1 Hz) and reciprocating flow (0.5 ± 4 dyn/cm<sup>2</sup> at 1 Hz) on VE-cadherin organization in endothelial monolayers were studied in vitro. VE-cadherin staining was continuous along cell borders in static controls. Following 6 h of either pulsatile or reciprocating flow, the VE-cadherin staining at cell borders became intermittent. When the pulsatile flow was extended to 24, 48 or 72 h the staining around the cell borders became continuous again, but the staining was still intermittent when the reciprocating flow was similarly extended. Exposure to pulsatile or reciprocating flow for 6 and 24 h neither change the expression level of VE-cadherin nor its distribution between membrane and cytosol fractions as determined by Western blot and compared with static controls. These findings suggest that the cell junction remodeling induced by different flow patterns may result from a redistribution of VE-cadherin within the cell membrane. Both the in vivo and in vitro data indicate that pulsatile and reciprocating flow patterns have different effects on cell junction remodeling. The lack of junction reorganization in regions of reciprocating flow in vivo and in vitro may provide a mechanistic basis for the high permeability and the preferential localization of atherosclerosis in regions of the arterial stress with complex flow patterns and fluctuating shear stress.

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          Most cited references 29

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          Flow effects on prostacyclin production by cultured human endothelial cells.

          Endothelial cell functions, such as arachidonic acid metabolism, may be modulated by membrane stresses induced by blood flow. The production of prostacyclin by primary human endothelial cell cultures subjected to pulsatile and steady flow shear stress was measured. The onset of flow led to a sudden increase in prostacyclin production, which decreased to a steady rate within several minutes. The steady-state production rate of cells subjected to pulsatile shear stress was more than twice that of cells exposed to steady shear stress and 16 times greater than that of cells in stationary culture.
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            Differential Localization of VE- and N-Cadherins in Human Endothelial Cells: VE-Cadherin Competes with N-Cadherin for Junctional Localization

            The two major cadherins of endothelial cells are neural (N)-cadherin and vascular endothelial (VE)- cadherin. Despite similar level of protein expression only VE-cadherin is located at cell–cell contacts, whereas N-cadherin is distributed over the whole cell membrane. Cotransfection of VE-cadherin and N-cadherin in CHO cells resulted in the same distribution as that observed in endothelial cells indicating that the behavior of the two cadherins was not cell specific but related to their structural characteristics. Similar amounts of α- and β-catenins and plakoglobin were associated to VE- and N-cadherins, whereas p120 was higher in the VE-cadherin complex. The presence of VE-cadherin did not affect N-cadherin homotypic adhesive properties or its capacity to localize at junctions when cotransfectants were cocultured with cells transfected with N-cadherin only. To define the molecular domain responsible for the VE-cadherin–dominant activity we prepared a chimeric construct formed by VE-cadherin extracellular region linked to N-cadherin intracellular domain. The chimera lost the capacity to exclude N-cadherin from junctions indicating that the extracellular domain of VE-cadherin alone is not sufficient for the preferential localization of the molecule at the junctions. A truncated mutant of VE-cadherin retaining the full extracellular domain and a short cytoplasmic tail (Arg621–Pro702) lacking the catenin-binding region was able to exclude N-cadherin from junctions. This indicates that the Arg621–Pro702 sequence in the VE-cadherin cytoplasmic tail is required for N-cadherin exclusion from junctions. Competition between cadherins for their clustering at intercellular junctions in the same cell has never been described before. We speculate that, in the endothelium, VE- and N-cadherin play different roles; whereas VE-cadherin mostly promotes the homotypic interaction between endothelial cells, N-cadherin may be responsible for the anchorage of the endothelium to other surrounding cell types expressing N-cadherin such as vascular smooth muscle cells or pericytes.
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              Shear stress induces spatial reorganization of the endothelial cell cytoskeleton.

              The morphology of endothelial cells in vivo depends on the local hemodynamic forces. Cells are polygonal and randomly oriented in areas of low shear stress, but they are elongated and aligned in the direction of fluid flow in regions of high shear stress. Endothelial cells in vitro also have a polygonal shape, but the application of shear stress orients and elongates the cells in the direction of fluid flow. The corresponding spatial reorganization of the cytoskeleton in response to the applied hemodynamic forces is unknown. In this study, we determined the spatial reorganization of the cytoskeleton throughout the volume of cultured bovine aortic endothelial cells after the cells had been exposed to a physiological level of shear stress for 0, 1.5, 3, 6, 12, or 24 h. The response of the monolayer to shear stress was not monotonic; it had three distinct phases. The first phase occurred within 3 h. The cells elongated and had more stress fibers, thicker intercellular junctions, and more apical microfilaments. After 6 h of exposure, the monolayer entered the second phase, where the cells exhibited characteristics of motility. The cells lost their dense peripheral bands and had more of their microtubule organizing centers and nuclei located in the upstream region of the cell. The third phase began after 12 h of exposure and was characterized by elongated cells oriented in the direction of fluid flow. The stress fibers in these cells were thicker and longer, and the heights of the intercellular junctions and microfilaments were increased. These results suggest that endothelial cells initially respond to shear stress by enhancing their attachments to the substrate and neighboring cells. The cells then demonstrate characteristics of motility as they realign. The cells eventually thicken their intercellular junctions and increase the amount of apical microfilaments. The time course of rearrangement can be described as a constrained motility that produces a new cytoskeletal organization that alters how the forces produced by fluid flow act on the cell and how the forces are transmitted to the cell interior and substrate.

                Author and article information

                J Vasc Res
                Journal of Vascular Research
                S. Karger AG
                February 2005
                28 January 2005
                : 42
                : 1
                : 77-89
                Departments of Bioengineering and Medicine, and The Whitaker Institute of Biomedical Engineering, University of California, San Diego, La Jolla, Calif., USA
                83094 J Vasc Res 2005;42:77–89
                © 2005 S. Karger AG, Basel

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                Page count
                Figures: 7, References: 60, Pages: 13
                Research Paper


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