Tyler Nerton Thacher
Due to the pulsatile nature of blood flow, arteries are constantly exposed to dynamic mechanical forces; the pulsatility continuously stretches the vessel wall and the flow creates a frictional force on the interior surface. These stresses, referred to as cyclic circumferential stretch and shear stress, are known to determine arterial structure and morphology; modulation of which leads to the progression of vascular diseases such as hypertension and atherosclerosis. Yet the individual contributions of cyclic stretch and shear stress, with regards to vascular disease, have yet to be revealed. In this thesis I wish to identify the role of reduced cyclic stretch in the development of endothelial dysfunction and vascular remodeling, develop an experimental model for studying the autonomous effects of shear stress and cyclic stretch and how these two stimuli individually modulate markers of vascular disease in different regions of the vascular wall. I will begin by introducing the different structural and cellular components of the vascular wall and their individual functions. From here I will introduce how hemodynamic forces transmitted to the vascular wall due to the pulsatile nature of blood flow play an essential role maintaining arterial health and function. And as such, how deviations from a physiologic hemodynamic range can have catastrophic implications for the vasculature. Next I will introduce how certain hemodynamic conditions can stimulate cellular dysfunction and how this relates to initiation and progression of vascular disease. In the first paper, we set out to determine if reduction of cyclic stretch could be a factor which induces remodeling of the arterial wall. We found that reducing compliance caused a decrease in vascular smooth muscle function, as well as inducing switch in smooth muscle cell phenotype. Arteries exposed to a reduced cyclic stretch also exhibited increased matrix degradation and cellular proliferation than those allowed to stretch physiologically. These findings accent the importance of cyclic stretch in the maintenance of a differentiated and fully functional phenotype of vascular smooth muscle cells, as well as in the regulation of migratory properties, proliferation and matrix turnover in the vascular wall. In the second paper we investigated how reduction of cyclic stretch influences endothelial dysfunction and modulation of nitric oxide bioavailability. We observed that reduced compliance significantly decreases the activity of the enzyme responsible for producing nitric oxide (eNOS). Overall production of reactive oxygen species were also increased by reducing compliance, which we were able to attribute to stimulation of the superoxide generating NAD(P)H oxidase. We found that experimentally reduced compliance also caused a significant decrease in endothelial function, as assessed with bradykinin dependent vascular relaxation. The results from this study point out how reduced arterial compliance interrupts the eNOS activation pathway and increases vascular levels of oxidative stress, which together could explain the measured decreases on endothelial functionality. In the third article we used our experimental model to investigate how shear stress and cyclic stretch independently stimulate the vascular wall. We found that both oscillatory flow and reduced stretch are detrimental to endothelial function, whereas oscillatory flow alone, dominated total endogenous vascular wall superoxide anion production. Yet when superoxide anion production was analyzed in just the endothelial region we observed that it was modulated more significantly by reduced cyclic stretch than by oscillatory shear, emphasizing an important distinction between shear and stretch mediated effects to the vascular wall. Analysis of eNOS and nitro-tyrosine, the by-product of superoxide anion and nitric oxide, proved that they too are more significantly negatively modulated by oscillatory flow, than by reduced stretch. The findings from this study point out how shear and stretch stimulate regions of the vascular wall differently, affecting NO bioavailability and contributing to vascular disease. The goal of the fourth article is to further investigate how shear stress and cyclic stretch modulate markers of vascular remodelling in different regions of the arterial wall. We demonstrated that while total superoxide production, fibronectin expression and gelatinase activation are predominantly mediated by shear stress, their expression in the endothelial region is mediated by reduced cyclic stretch, which correlates well with results from total MMP-2 expression. By plotting intensity versus radius for these markers of vascular remodeling we are able to see that superoxide production and gelatinase activity follows trends indicating their expression is in part mediated by stress distributions through out the vascular wall, while fibronectin and p22-phox were much less or not at all. Most importantly these findings, when coupled with our results from tissue reactive studies, suggest that the arterial remodeling process triggered in the endothelial region due to reduced stretch causes the most significant changes in arterial smooth muscle function. Perturbed shear stress and reduced arterial compliance have both been implicated in the initiation and progression of vascular disease: this work provides a new perspective into how these stimuli are perceived through out the vascular wall. To conclude, this body of work has shown that cyclic stretching due to the pulsatile nature of blood flow is an essential stimulus regulating arterial remodeling and endothelial viability. We have performed experiments permitting the autonomous effects of cyclic stretch and shear stress to be studied. Yet more importantly, we have shown how cyclic stretch and shear stress stimulate the vascular wall in different regions and compared this data to arterial functionality studies. These results have indicated that although reduced cyclic stretch may not stimulate the total expression of certain markers of vascular disease as much as an OSC shear stress, it does so in specific regions of the vascular wall which can in fact have a more detrimental effect on arterial function. As reduced arterial cyclic stretching is associated with the aging process, this work gives insight into the progression of vascular disease over the course of a person's life time.
EPFL2009
