IntroductionAtherosclerosis is a progressive disorder initiated by biomechanical forces in areas of the vascular tree subjected to disturbed blood flow and a number of systemic factors, including hyperlipidemia, smoking, and diabetes (1, 2). Lipid uptake by the vascular wall leads over time to a gradual buildup of atherosclerotic plaques. Once formed, the plaque continues to expand, leading to a gradual restriction in lumen diameter and compromise of blood flow. Under certain conditions, plaque rupture can precipitate intravascular thrombosis, resulting in complete and sudden interruption of the arterial blood supply. The buildup, growth, and rupture of the plaque have all been associated with the presence of systemic and vascular wall inflammation (3-5). Despite strong data linking vessel wall inflammation to atherosclerosis progression, the mechanism(s) of inflammation-induced atherosclerotic disease progression remains obscure (4), while efforts to halt disease progression by antiinflammatory therapies have failed in clinical trial (3).Recent studies have documented a high incidence of endothelial-to-mesenchymal transition (EndMT) in a number of pathological conditions associated with inflammation, such as myocardial infarction (6), cerebral cavernous malformations (7), portal hypertension (8), pulmonary hypertension (9), and vascular graft failure (10, 11) among others. EndMT is characterized by a change in phenotype of normal endothelial cells (ECs) that assume the shape and properties of mesenchymal cells (fibroblasts, smooth muscle cells [SMCs]), including enhanced proliferation and migration; secretion of extracellular matrix proteins, such as fibronectin and collagen; and expression of various leukocyte adhesion molecules.TGF-β has been identified as the central player in driving EndMT progression (12, 13), but the processes leading to activation of its signaling remain poorly defined. We have previously reported that a reduction in endothelial FGF-mediated signaling induced by knockout of either the key intracellular signal mediator FGF receptor substrate 2 α (FRS2α) (10) or the primary FGF receptor in the endothelium (FGF receptor 1 [FGFR1]) (14) activates TGF-β signaling, leading to EndMT.The unusual feature of FGFR1 biology is that FGFR1's expression in ECs is affected by certain inflammatory stimuli, including IFN-γ, TNF-α, and IL-1β, that induce a profound reduction in its expression (10). Since these are precisely the inflammatory mediators found in atherosclerotic plaques, we set out to investigate the occurrence and role of EndMT in atherosclerosis.Studies in cell culture and mouse models demonstrated that both oscillatory fluid shear stress and soluble inflammatory mediators reduced FGFR1 expression and signaling and promoted TGF-β signaling and EndMT. In addition, analysis of human coronary arteries demonstrated a strong correlation among endothelial FGFR1 expression, increased TGF-β signaling, and the appearance of EndMT with the severity of atherosclerosis. Together, these findings point to...

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VE-cadherin plays a critical role in endothelial shear stress mechanotransduction by interacting with VEGFRs through their transmembrane domains.

SummaryForces acting on cells govern many important regulatory events during development, normal physiology, and disease processes. Integrin-mediated adhesions, which transmit forces between the extracellular matrix and the actin cytoskeleton, play a central role in transducing effects of forces to regulate cell functions. Recent work has led to major insights into the molecular mechanisms by which these adhesions respond to forces to control cellular signaling pathways. We briefly summarize effects of forces on organs, tissues, and cells; and then discuss recent advances toward understanding molecular mechanisms.

Vascular remodeling under conditions of growth or exercise, or during recovery from arterial restriction or blockage is essential for health, but mechanisms are poorly understood. It has been proposed that endothelial cells have a preferred level of fluid shear stress, or ‘set point’, that determines remodeling. We show that human umbilical vein endothelial cells respond optimally within a range of fluid shear stress that approximate physiological shear. Lymphatic endothelial cells, which experience much lower flow in vivo, show similar effects but at lower value of shear stress. VEGFR3 levels, a component of a junctional mechanosensory complex, mediate these differences. Experiments in mice and zebrafish demonstrate that changing levels of VEGFR3/Flt4 modulates aortic lumen diameter consistent with flow-dependent remodeling. These data provide direct evidence for a fluid shear stress set point, identify a mechanism for varying the set point, and demonstrate its relevance to vessel remodeling in vivo.DOI: http://dx.doi.org/10.7554/eLife.04645.001

Mutations of the endothelial BMP9/10 receptors Alk1 and endoglin are associated with vascular malformations in hereditary hemorrhagic telangiectasia (HHT). Baeyens et al. report that fluid flow potentiates BMP activation of Alk1 signaling to stabilize blood vessels. HHT lesions thus result from a defect in a synergistic mechanical/biochemical signaling pathway.

Cerebral cavernous malformations (CCMs) are vascular malformations located within the central nervous system often resulting in cerebral hemorrhage. Pharmacological treatment is needed, since current therapy is limited to neurosurgery. Familial CCM is caused by loss‐of‐function mutations in any of Ccm1, Ccm2, and Ccm3 genes. CCM cavernomas are lined by endothelial cells (ECs) undergoing endothelial‐to‐mesenchymal transition (EndMT). This switch in phenotype is due to the activation of the transforming growth factor beta/bone morphogenetic protein (TGFβ/BMP) signaling. However, the mechanism linking Ccm gene inactivation and TGFβ/BMP‐dependent EndMT remains undefined. Here, we report that Ccm1 ablation leads to the activation of a MEKK3‐MEK5‐ERK5‐MEF2 signaling axis that induces a strong increase in Kruppel‐like factor 4 (KLF4) in ECs in vivo. KLF4 transcriptional activity is responsible for the EndMT occurring in CCM1‐null ECs. KLF4 promotes TGFβ/BMP signaling through the production of BMP6. Importantly, in endothelial‐specific Ccm1 and Klf4 double knockout mice, we observe a strong reduction in the development of CCM and mouse mortality. Our data unveil KLF4 as a therapeutic target for CCM.

Atherosclerotic plaque localization correlates with regions of disturbed flow in which endothelial cells (ECs) align poorly, whereas sustained laminar flow correlates with cell alignment in the direction of flow and resistance to atherosclerosis. We now report that in hypercholesterolemic mice, deletion of syndecan 4 (S4 −/− ) drastically increased atherosclerotic plaque burden with the appearance of plaque in normally resistant locations. Strikingly, ECs from the thoracic aortas of S4 −/− mice were poorly aligned in the direction of the flow. Depletion of S4 in human umbilical vein endothelial cells (HUVECs) using shRNA also inhibited flow-induced alignment in vitro, which was rescued by re-expression of S4. This effect was highly specific, as flow activation of VEGF receptor 2 and NF-κB was normal. S4-depleted ECs aligned in cyclic stretch and even elongated under flow, although nondirectionally. EC alignment was previously found to have a causal role in modulating activation of inflammatory versus antiinflammatory pathways by flow. Consistent with these results, S4-depleted HUVECs in long-term laminar flow showed increased activation of proinflammatory NF-κB and decreased induction of antiinflammatory kruppel-like factor (KLF) 2 and KLF4. Thus, S4 plays a critical role in sensing flow direction to promote cell alignment and inhibit atherosclerosis. mechanotransduction | polarity | shear stress | atherosclerosis S yndecan 4 (S4) is a transmembrane heparan sulfate proteoglycan that serves as a coreceptor for extracellular matrix proteins and growth factors (1-3). S4−/− mice are viable and fertile (4, 5) but show defective wound healing consequent to impaired angiogenesis (6). They also have higher mortality after LPS injection (7) and exhibit defective muscle repair and myofiber organization as a result of inefficient differentiation and migration of muscle satellite cells (8). We and others have also demonstrated that S4 plays a critical role in the control of cell polarity, by controlling Rho GTPase activity (9-11), as well as in planar cell polarity (12). S4 has also been recently identified as a putative mechanosensor (13).Atherosclerosis is an inflammatory disease of large to midsized arteries that is the major cause of illness and death in developed nations and is rapidly increasing in developing nations (14,15). It is linked to a variety of risk factors including high LDL cholesterol level and triglycerides, diabetes, smoking, hypertension, sedentary lifestyle, and inflammatory mediators. However, atherosclerotic lesions occur selectively in regions of arteries that are subject to disturbances in fluid shear stress (FSS), the frictional force flowing blood exerts on the endothelium. Regions of arteries with lower flow magnitude, flow reversal, and other complex spatial/ temporal flow patterns are predisposed to atherosclerosis. Systemic risk factors appear to synergize with local biomechanical factors in the initiation and progression of atherosclerotic lesions (16).The importance of S4 in endothelial bi...