Activation of Rac1 by shear stress in endothelial cells mediates both cytoskeletal reorganization and effects on gene expression

Abstract

Hemodynamic shear stress is a fundamental determinant of vascular remodeling and atherogenesis. Changes in focal adhesions, cytoskeletal organization and gene expression are major responses of endothelial cells to shear stress. Here, we show that activation of the small GTPase Rac is essential for gene expression and for providing spatial information for shear stress-induced cell alignment. Fluorescence resonance energy transfer (FRET) localizes activated Rac1 in the direction of¯ow. This directional Rac1 activation is downstream of shear-induced new integrin binding to extracellular matrix. Additionally, Rac1 mediates ow-induced stimulation of nuclear factor kB (NF-kB) and the subsequent expression of intercellular cell adhesion molecule 1 (ICAM-1), an adhesion receptor involved in the recruitment of leukocytes to atherosclerotic plaque. These studies provide a unifying model linking three of the main responses to shear stress that mediate both normal adaptation to hemodynamic forces and in¯ammatory dysfunction of endothelial cells in atherosclerosis.

A model of localised Rac1 activation in endothelial cells due to fluid flow

“…Figure 3 gives the pattern of shear stress on the cell surface as a result of flow in a large artery. Under these conditions the maximum shear stress is 10 dynes cm −2 , which is similar to the wall shear stress calculated in in vitro flow chamber experiments (Wojciak-Stothard and Tzima et al, 2001Tzima et al, , 2002.…”

“…Using the derived parameters (see appendix) the time-course of Src activation is comparable to previously reported (Fleming et al, 2005;Jalali et al, 1998). The whole cell response for Rac, figure 9(d), is slightly less than reported (Tzima et al, 2002). However, the model predicts maximal Rac activation within 25 minutes, which is comparable to the observed response (Tzima et al, 2002).…”

“…Endothelial cells (ECs) respond to fluid flow by elongating in the direction of flow (Davies, 1995;Ballerman et al, 1998;Wojciak-Stothard and Ridley, 2003;Tzima et al, 2002;McCue et al, 2006). This response is believed to be crucial for the correct function of the endothelium.…”

“…In response to fluid flow, RhoA has been reported to contribute to cell shape change by transiently inducing cell contraction, subsequently allowing cells to elongate in the direction of flow (Wojciak-Stothard and Ridley, 2003). Rac activation is localised to the downstream edge in ECs responding to fluid flow (Tzima et al, 2002). In response to integrin activation, Src has been shown to activate a Rac GEF, Vav2 (Garrett et al, 2007), and p190RhoGAP (a GAP for Rho) (Arthur et al, 2000).…”

Ascorbic Acid Prevents Blood–Brain Barrier Disruption and Sensory Deficit Caused by Sustained Compression of Primary Somatosensory Cortex

“…In our model, despite no ischemic signs, cortical compression significantly reduced the size of the cortical microvessels. This may cause the alteration of the blood flow-associated hemodynamic forces such as shear stress and cyclic strain on the vascular endothelium, which have been evidenced to activate integrin and Rac1, to decrease the occludin tyrosine phosphorylation and increase the ZO-1 serine/threonine phosphorylation, and to modulate the expression, association, and distribution of occludin and ZO-1 (Colgan et al, 2007;Collins et al, 2006;Tzima et al, 2002). Moreover, increasing evidences have suggested that the BBB permeability is strictly regulated by the redistribution of tight junction proteins between the cell membrane and the cytoplasmic fraction of endothelial cells, and the underlying mechanism involves the phosphorylation of tyrosine, serine, or threonine in the tight junction proteins (Andras et al, 2007;Elias et al, 2009).…”