Vascular endothelial response to shear stress
A research study is underway to examine the detailed mechanical characterisation of soft vascular tissue. The project focuses on the study of the basic mechanisms and processes by which vascular endothelial cells physically respond to mechanical stimuli. This topic is of considerable importance from the point of view of understanding how arteries respond to the presence of implanted devices and ultimately to the design of devices that minimise adverse cellular reactions following implantation.
This is a collaborative research project with groups at the National University of Ireland Galway (led by Dr. Peter McHugh) and the University of Limerick (led by Dr. Tim McGloughlin)
The Research Team
Dr. Brendan McCormack Principal researcher School of Engineering, IT, Sligo.
Dr. Jeremy Bird Principal researcher School of Science, IT, Sligo.
Dr. Enda Gibney Post-Graduate Student School of Engineering, IT, Sligo.
Atherosclerosis is the accumulation of plaques, consisting predominantly of fat, inside the walls of arteries ref. 1. Most people in developed countries will have some advanced plaques, with considerable accumulation of extracellular lipid, by the time they reach early adulthood 2. These plaques can cause heart attacks, when they block up coronary arteries, or strokes, when they block arteries that supply blood to the brain.
For many years, these plaques were thought to build up on passive artery walls, but more recently, this view has been challenged. Many experiments have shown that the arterial wall plays an active role in the causation of atherosclerosis. This role is an extension of the role that the wall plays in inflammatory processes 1.
The formation of an atherosclerotic lesion is induced when the endothelial cells that line the arteries express chemokines and cell adhesion molecules (CAMs). The chemokines attract inflammatory white blood cells, especially monocytes and T-lymphocytes, which are captured by the CAMs and subsequently enter the intima, the inner layer of the artery 3. There, monocytes are transformed into active macrophages, which ingest low-density lipoproteins (LDL-cholesterol), and become foam cells filled with fat droplets. They form “fatty streaks”, which are the precursors of atherosclerotic plaques 1, 3, 4.
When these plaques expand into the artery they cause stenosis, a restriction of the blood channel that reduces blood delivery to tissues 1. Alternatively, when blood seeps through a fissure in the cap, T-lymphocytes in the core of the plaques induce foam cells to produce high levels of tissue factor, a potent clot inducer. This reacts with clotting precursors in the blood to form a thrombus. This may intrude into the lumen, to occlude the blood vessel completely 2.
Endothelial cells are important in vascular homeostasis through their response to both hormonal (wounding, infection) and hemodynamic (mechanical) stimuli. They have developed mechanisms to detect changes in these stimuli, and respond to these changes by secreting mediator proteins and molecules. These can alter the properties of the vascular wall, by regulating its permeability to plasma lipoproteins, attachment of leukocytes and release of many different molecular factors. In atherosclerosis and inflammation, induced CAMs interact with epitopes on the surface of monocytes in the blood plasma, to initiate the inflammatory response 5.
An important factor in the causation of atherosclerosis is the response of endothelial cells to the mechanical conditions created by blood flow and the cardiac cycle.
These can be broken down into three primary mechanical forces: pressure (caused by the hydrostatic forces of blood in the blood vessel), circumferential stretch or tension (caused by intercellular forces between endothelial cells as a result of the widening and narrowing of the arteries when blood is pumped through them), and wall shear stress (caused by friction between the moving blood and the stationary cells) 5.
Atherosclerotic lesions are generally found at curves, bifurcations, and the lateral walls of branching points in the arterial pathways 6, 7, 8, 9. Although blood flow in the mammalian arterial network is inherently complex and non-uniform, perturbations in blood flow are particularly large and show very similar patterns at these positions. Fluid mechanical analyses of these areas of altered blood flow patterns have shown that disturbed laminar shear stress is the common denominator in atherogenesis. This type of blood flow is characterized by recirculation vortices, in which the laminar flow detaches from the vessel wall at the start of the vortice and reattaches at its termination. Vortices give rise to oscillatory flow caused by flow reversal (backflow of the blood plasma) and to areas of stagnation. Oscillatory flow causes large temporal and spatial gradients of shear stress 5, 6, 9.
Similar blood flow patterns may also develop in areas of stenosis, at the anastomoses of surgical bypasses, and at the sites of angioplasty or stents, where the vessel wall has been disturbed. At these areas of low average shear stress, leukocytes may easily adhere to the vessel wall, without being swept away in the current 9.
Shear stress has also been shown to alter the surface expression of cell adhesion molecules. Laminar flow has been shown to increase the expression of Intercellular Cell Adhesion Molecule (ICAM)-1, but to reduce V(ascular)CAM-1 expression. Disturbed blood flow increases the expression of both E-selectin and VCAM-1, both of which are important in the capture of monocytes 10.
Gradients in shear stress and steady shear stress are therefore assumed to represent different biomechanical stimuli that activate distinct signaling pathways to regulate endothelial function. This is reinforced by in vitro evidence 8.
1. Libby P (2002) Atherosclerosis: the new view, Scientific American, 286(5), 28-37
2. Davies, MJ (1996) Stability and Instability: Two Faces of Coronary Atherosclerosis – The Paul Dudley White Lecture 1995, Circulation, 94, 2013-2020
3. Libby P (2001), Current Concepts of the Pathogenesis of the Acute Coronary Syndromes, Circulation, 104, 365
4. de Martin R; Hoeth M; Hofer-Warbinek R; Schmid JA (2000) The Transcription Factor NF-?B and the Regulation of Vascular Cell Function, Arteriosclerosis, Thrombosis, and Vascular Biology, 20, e83
5. Traub O; Berk BC (1998) Laminar Shear Stress- Mechanisms by Which Endothelial Cells Transduce an Atheroprotective Force, Arteriosclerosis, Thrombosis, and Vascular Biology, 18, 677-685
6. Nagel T; Resnick N; Dewey CF, Jr; Gimbrone MA, Jr (1999), Vascular Endothelial Cells Respond to Spatial Gradients in Fluid Shear Stress by Enhanced Activation of Transcription Factors, Arteriosclerosis, Thrombosis, and Vascular Biology, 19,1825-1834
7. Hsiai TK; Cho SK; Reddy S; Hama S; Navab M; Demer LL; Honda HM; Ho CM (2001), Pulsatile Flow Regulates Monocyte Adhesion to Oxidized Lipid-Induced Endothelial Cells, Arteriosclerosis, Thrombosis, and Vascular Biology, 21, 1770
8. Bao X; Lu C; Frangos JA (1999),Temporal Gradient in Shear But Not Steady Shear Stress Induces PDGF-A and MCP-1 Expression in Endothelial Cells-Role of NO, NF?B, and egr-1, Arteriosclerosis, Thrombosis, and Vascular Biology, 19, 996-1003
9. Skilbeck C; Westwood SM; Walker PG; David T; Nash GB (2001), Population of the Vessel Wall by Leukocytes Binding to P-Selectin in a Model of Disturbed Arterial Flow, Arteriosclerosis, Thrombosis, and Vascular Biology, 21, 1294
10. Chiu JJ; Wung BS; Shyy JYJ; Hsieh HJ; Wang DL (1997), Reactive Oxygen Species Are Involved in Shear Stress-Induced Intercellular Adhesion Molecule-1 Expression in Endothelial Cells, Arteriosclerosis, Thrombosis, and Vascular Biology, 17, 3570-3577
The approach will involve both macroscale and microscale analysis. At the macroscale (in projects carried out in the National University of Ireland Galway and the University of Limerick), the blood flow distributions in coronary arteries will be determined using both experimental testing and computational modelling techniques. In addition, the techniques will be used to determine stress distributions on the endothelial cells at the arterial walls.
The macroscale analysis will also be used to determine how the flow and stress conditions change in the presence of vascular devices and these modified conditions will also be applied in vitro.
At the microscale (carried out at IT Sligo), these flow and stress conditions will be used to determine the conditions to which endothelial cells will be subjected in vitro. In these experiments the response of the cells (deformation, growth, expression of cell adhesion molecules, etc.) will be observed and the response will be monitored as a function of variation in test conditions. The influence of anti-inflammatory drugs on the cell response will also be tested, in an effort to find a possible preventative drug for atherosclerosis.
A slightly more long-term goal within the project timeframe will be the development of models that will predict the in vitro behaviour. The successful generation of such models would represent a significant fundamental scientific and technological breakthrough.