Role of Hemodynamic Shear Stress Abnormalities in Calcific Aortic Valve Disease

The aortic valve (AV) ensures unidirectional flow between the left ventricle and the aorta by opening during cardiac systole and closing during diastole. As compared to the normal tricuspid aortic valve (TAV) anatomy, which consists of three leaflets, the bicuspid aortic valve (BAV) is characterized by the presence of two leaflets, and is the most prevalent congenital cardiac anomaly. Regardless of the valve morphology, a common cause of valvular failure is calcific aortic valve disease (CAVD), a condition characterized by increased thickness and stiffness on the valve leaflets. Historically, CAVD has been considered a passive degenerative disease but is now recognized as an active pathology involving inflammation, remodeling and ossification and presumably triggered by atherogenic risk factors and hemodynamic cues. While TAV and BAV calcification seem to share common biological pathways, the calcification of the BAV is more rapid and severe than the TAV. As evidenced by recent studies, the valve interacts closely with its surrounding hemodynamic environment to maintain valvular homeostasis by altering the biosynthetic behavior of valve cells. Physiologic fluid shear stress (FSS), the frictional fluid force acting on the leaflet endothelium, contributes to valvular homeostasis by regulating valvular protein expression, glycosaminoglycan and DNA synthesis and endothelial cell alignment. In contrast, FSS abnormalities have been shown to promote endothelial activation and leaflet inflammation. Supported by those observations, the central hypothesis of this thesis is that hemodynamic FSS abnormalities contribute to CAVD development by regulating AV inflammation, extracellular matrix remodeling and valvular osteogenesis. Therefore, the overall goal of this work is to characterize the role of FSS abnormalities on early progression of CAVD in both TAVs and BAVs. Specifically, this thesis will address the following research questions: 1) What are the mechanisms by which FSS alterations are transduced into valvular pathological responses? 2) Are BMP-4 and TGF-β1 potential target molecules for the pharmacological treatment of CAVD? and 3) What are the reasons for the early development and severity of BAV calcification? These questions will be addressed via three specific aims. 1) To elucidate the mechanisms of CAVD secondary to FSS magnitude & frequency abnormalities; 2) To investigate the role of BMP-4 and TGF-β1 in FSS-induced valvular endothelial activation and ECM remodeling; and 3) To elucidate the mechanisms of CAVD secondary to BAV hemodynamic abnormalities. The approach integrated the implementations of a novel ex vivo device to condition porcine AV leaflets under physiological and pathologically FSS environments, standard biological techniques to assess valvular biological response and pharmacological inhibition/promotion to elucidate the mechanisms of FSS signal transduction. As a first step, a FSS bioreactor was designed to replicate the native side-specific valvular FSS environment in the laboratory setting. The bioreactor was validated mechanically using fluid structure interaction modeling and biologically in terms of sterility and tissue viability. The programming of the bioreactor with both physiological and pathological waveforms permitted to investigate the role of FSS in valvular pathogenesis. Specific aim 1 sought to elucidate the modes of FSS mechanotransduction in AV tissue and the respective role of FSS magnitude and/or frequency abnormalities in CAVD. Porcine leaflets were subjected to 9 FSS environments defined by isolated/combined abnormalities in FSS magnitude and frequency. While cytokine expression was stimulated under elevated FSS magnitude at normal frequency, ECM degradation was stimulated under both elevated FSS magnitude at normal frequency and physiologic FSS magnitude at abnormal frequency. In contrast, combined FSS magnitude and frequency abnormalities essentially maintained valvular homeostasis. The results revealed the particular sensitivity of valvular tissue to FSS magnitude as opposed to FSS frequency. In specific aim 2, the goal was to determine the role of BMP-4 and TGF-β1 in FSS-induced valvular endothelial activation and ECM remodeling. The methodology involved the ex vivo exposure of porcine aortic valve leaflets to side-specific physiological/pathological FSS and the use of pharmacological promoters/inhibitors of BMP-4 or TGF-β1. The results revealed the important role played by TGF-β1 in valvular remodeling through MMP-9 regulation. Specific aim 3 investigated how BAV hemodynamic FSS abnormalities contribute to early progression of CAVD. Porcine leaflets were exposed ex vivo to the native TAV and type-1 BAV FSS and their acute pathological response was examined, including endothelial activation, pro-inflammatory paracrine signaling, VIC activation and osteogenesis. The results revealed the pathogenic potential of the native BAV hemodynamic environment and the particular vulnerability of the fused BAV leaflet to calcification. In conclusion, the contribution of FSS to the onset and early progression of CAVD has been demonstrated in this thesis work. The relationship between FSS magnitude/frequency and valvular responses can be used as input for prediction of CAVD progression. In addition, the findings from this dissertation can be used to further investigate the molecular signaling mechanisms in order to develop pharmacological treatments of CAVD.