The use of genetically modified mouse models for investigations of mechanisms of arterial stiffness

Arterial stiffness is an independent risk factor for cardiovascular morbidity and mortality. With age, the large arteries become stiffer, leading to hypertension, left ventricular hypertrophy, end organ damage, and cardiovascular disease. However, the underlying cellular and molecular mechanisms leading to increased arterial stiffness remain largely unknown. The scope of the work presented in this thesis encompasses the use of genetically modified mouse models to explore potential mechanisms of vascular stiffness. A novel, high-fidelity technique of estimating aortic stiffness by pulse wave velocity (PWV) in the mouse was developed and used to characterise the pressure-dependency of PWV in the mouse aorta. Transglutaminase 2 (TG2) is a multifunctional enzyme which was hypothesized to play a role in arterial stiffness due to its ability to crosslink various structural proteins in the extracellular matrix. Pressure-dependent PWV, vascular reactivity, aortic geometry and calcification, and cardiac function were investigated in TG2-/- and wild type (WT) mice. Findings suggest that TG2 plays a role in arterial stiffness at high blood pressures and left ventricular hypertrophy. The apolipoprotein E knockout (apoE-/-) mouse is a widely used model of human atherosclerosis. In vivo PWV measurements demonstrated increased stiffness in apoE-/- mice compared to control as early as 12 weeks of age. Endothelial dysfunction through an impaired relaxation response to acetylcholine was also observed. A study of the effects of early aging on vascular stiffness in the mouse was also performed. Between the ages of 12 and 36 weeks a significant increase in aortic stiffness was observed in WT mice. A slight but insignificant increase in PWV was observed in the apoE-/- mouse suggesting that the rate of change of PWV is strain dependent. The nitric oxide synthase inhibitor, L-NAME, was used to induce both increased TG2 activity and endothelial dysfunction, with the hypothesis that this would increase arterial stiffness. In TG2-/- mice and WT control, no differences in cardiovascular function were observed after L-NAME treatment. However, in 12 week old apoE-/- mice, L-NAME administration reduced PWV, suggesting the presence of compensatory mechanisms that account for the lack of nitric oxide bioavailability. This thesis provides new insights into potential mechanisms of increased vascular stiffness. More importantly, it highlights the utility of the genetically modified mouse as a tool for interrogating novel pathways to arterial stiffness, and a new method with which to do so. Use of this technique could lead to the identification of novel therapeutic targets for the treatment of vascular stiffness.