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The small GTPase Rho regulates the formation of actin stress fibers in adherent cells through activation of its effector proteins Rhokinase and mDia. We found in bovine aortic endothelial cells that inhibitions of Rho, Rho-kinase, and mDia (with C3, Y27632, and F1F2⌬1, respectively) suppressed stress fiber formation, but fibers appeared after 10% cyclic uniaxial stretch (1-Hz frequency). In contrast to the predominately perpendicular alignment of stress fibers to the stretch direction in normal cells, the stress fibers in cells with Rho pathway inhibition became oriented parallel to the stretch direction. In cells with normal Rho activity, the extent of perpendicular orientation of stress fibers depended on the magnitude of stretch. Expressing active RhoV14 plasmid in these cells enhanced the stretch-induced stress fiber orientation by an extent equivalent to an additional Ϸ3% stretch. This augmentation of the stretch-induced perpendicular orientation by RhoV14 was blocked by Y27632 and by F1F2⌬1. Thus, the activity of the Rho pathway plays a critical role in determining both the direction and extent of stretch-induced stress fiber orientation in bovine aortic endothelial cells. Our results demonstrate that the stretch-induced stress fiber orientation is a function of the interplay between Rho pathway activity and the magnitude of stretching.cytoskeletal dynamics ͉ endothelial cells ͉ mechanotransduction ͉ Rho-kinase T he tension generated by contraction of adherent cells against their underlying surface results in an internal stress field that depends on the organization of the cytoskeleton and the associated adhesive contacts (see ref. 1 for review). Intracellular forces have an important role in cellular functions such as migration, proliferation, apoptosis, differentiation, and gene expression (see refs. 2-4 for reviews). Actin stress fibers, which are formed in response to cell contraction (5), consist of bundles of actin microfilaments cross-linked by ␣-actinin, myosin, myosin light-chain, tropomyosin, and other proteins arranged in a manner similar to that in muscle sarcomeres (6). Stress fibers represent the main contractile apparatus in non-muscle cells (7) and are the primary structures associated with intracellular tension. Stress fibers terminate at focal adhesions, which attach the cell to the extracellular matrix (8). Isometric contraction of a cell would result in tension development in the stress fibers, which are anchored at their ends.The activation of the small GTPase Rho leads to stress fiber assembly (9) and cell contraction by means of myosin light chain phosphorylation (5), which is regulated by Rho-kinase, a downstream effector of Rho (10). mDia, another Rho effector, is also involved in stress fiber formation downstream of Rho activation (11), possibly by regulating actin polymerization and focal adhesion turnover through its association with profilin (12, 13) and src-tyrosine-kinase (14), respectively.Cyclic uniaxial stretch induces the orientation of stress fibers in endothelial cells ...

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A new parallel plate flow chamber that has a linear variation of shear stress, starting from a predetermined maximum value at the entrance and falling to zero at the exit, has been designed and tested. This is in contrast to the usual rectangular channel plan which produces a constant shear stress over the entire length. The new design is based on the theory of Hele-Shaw flow between parallel plates. To verify the efficacy of the flow channel, the effect of fluid shear stress on platelet adhesion to a fibrinogen-coated glass surface was tested. The percentage of attached platelets after 5 min of shear stress is shown to be a function of shear stress. With this new flow chamber, cell-cell interactions can be studied efficiently over a wide range of shear stress using a single run at constant discharge.

T he migration of vascular endothelial cells (ECs) plays an important role in angiogenesis and postangioplasty wound healing. Cell migration is a coordinated process consisting of adhesion at the leading edge and detachment at the rear (1, 2). The focal adhesions (FAs), cytoskeleton, and signaling pathways that mediate cell migration need to respond to diverse extracellular signals and translate them into precisely regulated intracellular responses. There have been many studies on EC migration in response to gradients of soluble chemicals (chemotaxis) and immobilized extracellular matrix (haptotaxis; refs. 3-6). However, the effect of mechanical environment on EC migration is not well understood.ECs are constantly subjected to shear stress, the tangential component of hemodynamic force caused by blood flow. It has been shown that shear stress induces EC monolayer remodeling, e.g., increase of stress fibers and alterations in gene expression (7,8). Shear stress can modulate EC migration in wounding area and vascular stent surface (9-12), but the kinetics and molecular mechanism of EC migration in response to shear stress remain to be determined.Integrins are transmembrane adhesion receptors that link the extracellular matrix to cytoskeletal proteins and signaling molecules at FAs (13-15). Integrin-matrix binding activates the signaling cascade at FAs to modulate cell migration (13,14). Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase that colocalizes with integrins at FAs. FAK mediates the FA dynamics and signaling in response to growth factors and integrin-ligand binding (16,17). Phosphorylation of FAK at Tyr-397 [p-FAK(Y397)] upon cell adhesion allows FAK to associate with Src, which triggers downstream signaling events such as phosphorylation of mitogen-activated kinases, p130 cas , and paxillin to mediate cell adhesion and migration (18)(19)(20)(21)(22)(23)(24). Recent studies show that FAK is required for mechanosensing and persistent migration of fibroblasts (25, 26). We and others have shown that shear stress induces a transient activation of FAK in EC monolayer (27)(28)(29). These previous studies focused on the analysis of the global activity of FAK by using traditional biochemical assays; the subcellular distribution and dynamics of FAK at FAs and the role of this spatial dynamics in cell migration in response to mechanical and chemical stimuli remain to be determined.Here, we defined the kinetics of shear stress-induced directional migration of ECs. By expressing green fluorescence protein (GFP)-tagged FAK, we demonstrated the molecular dynamics of FAK at FAs in migrating ECs in response to shear stress and serum. The results showed that p-FAK(Y397) was correlated with FAK dynamics at FAs. Our findings indicate that the spatial dynamics of signaling at FAs is critical in directional migration, and that mechanotaxis is an important mechanism controlling EC migration. Materials and MethodsCell Culture. Cell culture reagents were obtained from GIBCO͞ BRL. Bovine aortic ECs (BAECs) before passa...