Measurements of strain on single stress fibers in living endothelial cells induced by fluid shear stress (original) (raw)
Related papers
Vascular Health and Risk Management, 2008
Fluid shear stress is the mechanical force generated by the blood fl ow which is applied over the apical surface of endothelial cells in situ. The fi ndings of a recent study suggest that stress fi bers and its associated focal adhesions play roles in mechano-signal transduction mechanism. Stress fi bers are present along the apical and the basal portion of the endothelial cells. Endothelial cells respond to fl uid shear stress and change their morphological characteristics in both their cell shape and cytoskeletal organization. Atherosclerosis is a common disease of the arteries and it occurs in areas around the branching site of blood vessels where the cells are exposed to low fl uid shear stress. The organization of stress fi bers and focal adhesions are strongly infl uenced by shear stress, and therefore the generation of atherosclerotic lesions seem to be associated with the cytoskeletal components of endothelial cells. This review describes the possible role of the cytoskeleton as a mechano-transducer in endothelial cells in situ.
Circulation Research, 2000
It has been proposed previously that actin filaments and cell adhesion sites are involved in mechanosignal transduction. In this study, we present certain morphological evidence that supports this hypothesis. The 3D disposition of actin filaments and phosphotyrosine-containing proteins in endothelial cells in situ was analyzed by using confocal microscopy and image reconstruction techniques. Surgical coarctations were made in guinea pig aortas, and the same 3D studies were conducted on such areas 1 week later. Stress fibers (SFs) were present at both basal and apical regions of endothelial cells regardless of coarctation, and several phosphotyrosine-containing proteins were associated with SF ends. Apical SFs had one end attached to the apical cell membrane and the other attached to either the basal membrane or the lateral cell border. Within the coarctation area, the actin filament-containing and vinculin-containing structures became prominent, especially at the apical and the lateral regions. Substantially higher levels of anti-phosphotyrosine and anti-Src staining were detected in the constricted area, particularly at the cell-cell apposition, whereas the anti-focal adhesion kinase, anti-CT10-related kinase, anti-platelet endothelial cell adhesion molecule-l, anti-vinculin, and phalloidin staining intensities increased only slightly after coarctation. We propose that apical SFs directly transmit the mechanical force of flow from the cell apex to the lateral and/or basal SF anchoring sites and that the SF ends associated with signaling molecules are sites of signal transduction. Our results support the idea that the cell apposition area is the major fluid shear stress-dependent mechanosignal transduction site in endothelial cells.
Endothelial actin cytoskeleton remodeling during mechanostimulation with fluid shear stress
American Journal of Physiology-cell Physiology - AMER J PHYSIOL-CELL PHYSIOL, 2005
Fluid shear stress stimulation induces endothelial cells to elongate and align in the direction of applied flow. Using the complementary techniques of photoactivation of fluorescence and fluorescence recovery after photobleaching, we have characterized endothelial actin cytoskeleton dynamics during the alignment process in response to steady laminar fluid flow and have correlated these results to motility. Alignment requires 24 h of exposure to fluid flow, but the cells respond within minutes to flow and diminish their movement by 50%. Although movement slows, the actin filament turnover rate increases threefold and the percentage of total actin in the polymerized state decreases by 34%, accelerating actin filament remodeling in individual cells within a confluent endothelial monolayer subjected to flow to levels used by dispersed nonconfluent cells under static conditions for rapid movement. Temporally, the rapid decrease in filamentous actin shortly after flow stimulation is preceded by an increase in actin filament turnover, revealing that the earliest phase of the actin cytoskeletal response to shear stress is net cytoskeletal depolymerization. However, unlike static cells, in which cell motility correlates positively with the rate of filament turnover and negatively with the amount polymerized actin, the decoupling of enhanced motility from enhanced actin dynamics after shear stress stimulation supports the notion that actin remodeling under these conditions favors cytoskeletal remodeling for shape change over locomotion. Hours later, motility returned to pre-shear stress levels but actin remodeling remained highly dynamic in many cells after alignment, suggesting continual cell shape optimization. We conclude that shear stress initiates a cytoplasmic actin-remodeling response that is used for endothelial cell shape change instead of bulk cell translocation. atherosclerosis; cytoskeletal dynamics; endothelial cells; mechanotransduction ATHEROSCLEROTIC LESIONS DEVELOP at predictable locations in the arterial vascular network, forming primarily where vessels branch and bend while largely sparing nearby straight vessel segments (8). Vessel bending and branching disturbs normal blood flow patterns, generating complex local regions of flow separation and recirculation that alter the magnitude, direction, and frequency of forces applied to the vessel wall (23, 41). Endothelial cells, which line the vessel wall and regulate vascular homeostasis, are exquisitely sensitive to the forces produced by flowing blood (10, 11, 14). The blood flow profiles present at atherogenic regions cause endothelial injury
Shear-induced reorganization of endothelial cell cytoskeleton and adhesion complexes
Trends in cardiovascular medicine, 2004
Endothelial cells undergo profound morphologic changes in response to alterations in shear stresses that are imposed on them by blood flow, and these responses have important implications for physiologic and pathophysiologic function of blood vessels. Shear-induced changes in cell morphology represent a unique mode of cell motility: elongation of the cells in the direction of shear stress is achieved by a reorientation and assembly of F-actin stress fibers at the basal cell surface that ultimately protrudes the upstream and downstream limits of the plasma membrane. Shape change is also accommodated by dramatic reorganization of cell-substrate and cell-cell junctional complexes. Both of these structures are sites of mechanotransduction, which raises intriguing questions concerning how shear-induced regulation of endothelial cell physiology is integrated with cell morphologic responses.
Mapping Mechanical Strain of an Endogenous Cytoskeletal Network in Living Endothelial Cells
Biophysical Journal, 2003
A central aspect of cellular mechanochemical signaling is a change of cytoskeletal tension upon the imposition of exogenous forces. Here we report measurements of the spatiotemporal distribution of mechanical strain in the intermediate filament cytoskeleton of endothelial cells computed from the relative displacement of endogenous green fluorescent protein (GFP)-vimentin before and after onset of shear stress. Quantitative image analysis permitted computation of the principal values and orientations of Lagrangian strain from 3-D high-resolution fluorescence intensity distributions that described intermediate filament positions. Spatially localized peaks in intermediate filament strain were repositioned after onset of shear stress. The orientation of principal strain indicated that mechanical stretching was induced across cell boundaries. This novel approach for intracellular strain mapping using an endogenous reporter demonstrates force transfer from the lumenal surface throughout the cell.
A Model for Shear Stress Sensing and Transmission in Vascular Endothelial Cells
Biophysical Journal, 2003
Arterial endothelial cell (EC) responsiveness to flow is essential for normal vascular function and plays a role in the development of atherosclerosis. EC flow responses may involve sensing of the mechanical stimulus at the cell surface with subsequent transmission via cytoskeleton to intracellular transduction sites. We had previously modeled flow-induced deformation of EC-surface flow sensors represented as viscoelastic materials with standard linear solid behavior (Kelvin bodies). In the present article, we extend the analysis to arbitrary networks of viscoelastic structures connected in series and/or parallel. Application of the model to a system of two Kelvin bodies in parallel reveals that flow induces an instantaneous deformation followed by creeping to the asymptotic response. The force divides equally between the two bodies when they have identical viscoelastic properties. When one body is stiffer than the other, a larger fraction of the applied force is directed to the stiffer body. We have also probed the impact of steady and oscillatory flow on simple sensor-cytoskeleton-nucleus networks. The results demonstrated that, consistent with the experimentally observed temporal chronology of EC flow responses, the flow sensor attains its peak deformation faster than intracellular structures and the nucleus deforms more rapidly than cytoskeletal elements. The results have also revealed that a 1-Hz oscillatory flow induces significantly smaller deformations than steady flow. These results may provide insight into the mechanisms behind the experimental observations that a number of EC responses induced by steady flow are not induced by oscillatory flow.
Journal of Theoretical Biology, 2005
Interactions of cell adhesions, Rho GTPases and actin in the endothelial cells' response to external forces are complex and not fully understood, but a qualitative understanding of the mechanosensory response begins to emerge. Here, we formulate a mathematical model of the coupled dynamics of cell adhesions, small GTPases Rac and Rho and actin stress fibers guiding a directional reorganization of the actin cytoskeleton. The model is based on the assumptions that the interconnected cytoskeleton transfers the shear force to the adhesion sites, which in turn transduce the force into a chemical signal that activates integrins at the basal surface of the cell. Subsequently, activated and ligated integrins signal and transiently de-activate Rho, causing the disassembly of actin stress fibers and inhibiting the maturation of focal complexes into focal contacts. Focal complexes and ligated integrins activate Rac, which in turn enhances focal complex assembly. When Rho activity recovers, stress fibers re-assemble and promote the maturation of focal complexes into focal contacts. Merging stress fibers self-align, while the elevated level of Rac activity at the downstream edge of the cell is translated into an alignment of the cells and the newly forming stress fibers in the flow direction. Numerical solutions of the model equations predict transient changes in Rac and Rho that compare well with published experimental results. We report quantitative data on early alignment of the stress fibers and its dependence on cell shape that agrees with the model. r
Modeling shear stress sensing and transmission in vascular endothelial cells
Proceedings of the Second Joint 24th Annual Conference and the Annual Fall Meeting of the Biomedical Engineering Society] [Engineering in Medicine and Biology
Arterial endothelial cell (EC) responsiveness to flow is essential for normal vascular function and plays a role in the development of atherosclerosis. EC flow responses may involve sensing of the mechanical stimulus at the cell surface with subsequent transmission via cytoskeleton to intracellular transduction sites. We had previously modeled flow-induced deformation of EC-surface flow sensors represented as viscoelastic materials with standard linear solid behavior (Kelvin bodies). In the present article, we extend the analysis to arbitrary networks of viscoelastic structures connected in series and/or parallel. Application of the model to a system of two Kelvin bodies in parallel reveals that flow induces an instantaneous deformation followed by creeping to the asymptotic response. The force divides equally between the two bodies when they have identical viscoelastic properties. When one body is stiffer than the other, a larger fraction of the applied force is directed to the stiffer body. We have also probed the impact of steady and oscillatory flow on simple sensor-cytoskeleton-nucleus networks. The results demonstrated that, consistent with the experimentally observed temporal chronology of EC flow responses, the flow sensor attains its peak deformation faster than intracellular structures and the nucleus deforms more rapidly than cytoskeletal elements. The results have also revealed that a 1-Hz oscillatory flow induces significantly smaller deformations than steady flow. These results may provide insight into the mechanisms behind the experimental observations that a number of EC responses induced by steady flow are not induced by oscillatory flow.
Journal of Theoretical Biology, 2001
Fluid mechanical shear stress elicits humoral, metabolic, and structural responses in vascular endothelial cells (ECs); however, the mechanisms involved in shear stress sensing and transduction remain incompletely understood. Beyond being responsive to shear stress, ECs distinguish among and respond di!erently to di!erent types of shear stress. Recent observations suggest that endothelial shear stress sensing may occur through direct interaction of the #ow with cell-surface structures that act as primary #ow sensors. This paper presents a mathematical model for the shear stress-induced deformation of a #ow sensor on the EC surface. The sensor is modeled as a cytoskeleton-coupled viscoelastic structure exhibiting standard linear solid behavior. Since ECs respond di!erently to di!erent types of #ow, the deformation and resulting velocity of the sensor in response to steady, non-reversing pulsatile, and oscillatory #ow have been studied. Furthermore, the sensitivity of the results to changes in various model parameters including the magnitude of applied shear stress, the constants that characterize the viscoelastic behavior, and the pulsatile #ow frequency (f) has been investigated. The results have demonstrated that in response to a suddenly applied shear stress, the sensor exhibits a level of instantaneous deformation followed by gradual creeping to the long-term response. The peak deformation increases linearly with the magnitude of the applied shear stress and decreases for viscoelastic constants that correspond to sti!er sensors. While the sensor deformation depends on f for low f values, the deformation becomes f-independent above a critical threshold frequency. Finally, the peak sensor deformation is considerably larger for steady and non-reversing pulsatile #ow than for oscillatory #ow. If the extent of sensor deformation correlates with the intensity of #ow-mediated endothelial signaling, then our results suggest possible mechanisms by which ECs distinguish among steady, nonreversing pulsatile, and oscillatory shear stress.