Mechanical forces are powerful regulators of biology and disease. In the vasculature, the expression of particular cellular phenotypes appears to depend not only on a combination of intrinsic genetically programmed biology but also on local hemodynamic environmental factors induced by blood flow. In the spectrum of forces experienced by cardiovascular tissue, one major component is the wall shear stress or friction force exerted by the blood flow on the endothelium. Through dedicated receptors, the endothelial cells lining the surface of cardiovascular structures are able to sense the characteristics of this force and to transduce this mechanical signal into biochemical signals, altering in turn cellular function. Interestingly, while physiologic wall shear stress maintains homeostasis, wall shear stress abnormalities often correlate with disease states. In this context, the elucidation of the cause-and-effect relationships between cardiovascular biology and blood flow has the potential to advance the understanding of disease progression and to enable new diagnosis and treatments.
One challenge in the study of mechanobiology is the difficulty to replicate in the laboratory the native mechanical environment of cells and tissue. The wall shear stress environment of cardiovascular tissue and cells is often pulsatile, characterized by large temporal variations and abrupt changes in directionality. The MSCBL has invested in the design and development of a programmable device capable of replicating these characteristics. The device is based on the cone-and-plate principle and can subject cells or whole pieces of tissue to single-sided or double-sided wall shear stress. While the single-sided operation is appropriate for most vascular tissues for which only one surface is exposed to flow (e.g., arterial and venous walls), the double-sided operation enables the conditionning of tissue experiencing more complex flow environments (e.g., valve leaflets).
The aortic valve functions in a mechanically complex environment, experiencing transvalvular pressures and pulsatile and oscillatory shear stresses, as well as bending and axial stress. Although valves were originally thought to be passive pieces of tissue, recent evidence has suggested an intimate interplay between the hemodynamic environment and biological response of the valve. In particular, while normal stress levels contribute to the maintenance of valvular physiology and function, abnormal stresses are suspected to play a role in valvular disease. The MSCBL is conducting research to elucidate the cause-and-effect relationships between wall shear stress abnormalities and valvular disease.
CAVD is an active process leading to the formation of calcific lesions on the valve leaflets and presumably triggered by synergies between cardiovascular risk factors, molecular signaling networks and hemodynamic cues. However, the exact role played by flow abnormalities in the pathogenesis of CAVD are poorly understood. To fill this knowledge gap, the MSCBL has investigated the wall shear stress regulation of endothelial activation and tissue remodeling. Using the novel double-sided shear stress device, valve leaflets were subjected to different combinations of wall shear stress magnitude and frequency. Endothelial activation and paracrine signaling were investigated by measuring cell-adhesion molecule (ICAM-1, VCAM-1) and cytokine (BMP-4, TGF-beta1) expressions using standard biological assessment techniques (immunostaining, immunoblotting). This study confirmed the sensitivity of valve leaflets to both wall shear stress magnitude and frequency. Normal valve leaflets were able to sense wall shear stress abnormaltiies and to tranduce them into a pathological response within 48 hours. Among all the wall shear stress environments considered in the study, supra-physiologic levels and abnormal frequencies were found to be the most unfavorable to homeostasis and the most conducive to CAVD precursor events.
A potential source of valvular flow abnormalities is the bicuspid aortic valve (BAV), a heart valve anomaly consisting of two leaflets instead of the three present in the normal tricuspid aortic valve (TAV). The BAV is a major risk factor for calcification and the mechanical stresses generated by this abnormal valve anatomy could be responsible for promoting the calcification process. The MSCBL has investigated the role played by BAV flow on leaflet biology. The double-sided shear stress bioreactor was programmed to condition normal leaflet tissue to the native wall shear stress environments of TAV and BAV leaflets. Immunostaining, immunoblotting and zymography were performed to characterize endothelial activation, pro-inflammatory paracrine signaling, extracellular matrix remodeling and markers involved in valvular interstitial cell activation and osteogenesis. The results indicated that, while the wall shear stress experienced by the TAV and the non-coronary BAV leaflets essentially maintained valvular homeostasis, the wall shear stress normally present on the fused BAV leaflet promoted endothelial activation, paracrine signaling, catabolic enzyme secretion and activity, and bone matrix synthesis. These results demonstrate the key role played by BAV hemodynamic abnormalities in CAVD pathogenesis.
The ascending aorta (AA) downstream of a bicuspid aortic valve (BAV) is prone to asymmetric dilation and dissection. Interestingly, the site of dilation seems to correlate with the type of BAV leaflet fusion. For example, BAVs with left-right coronary cusp fusion promote dilation in the convexity of the AA wall. This apparent correlation suggests a role for hemodynamic stress abnormalities in the development of BAV aortopathy. As a first step toward the investigation of the potential causality between BAV aorta hemodynamics and asymmetric aortic dilation, our lab has quantified and compared the remodeling response of aortic wall tissue subjected ex vivo to the native WSS present in the convexity and concavity of TAV and BAV AAs. The results indicated the particular susceptibility of the WSS environment present on the convexity of the BAV AA to promote tissue remodeling via MMP-9 expression and activation.
The aortic valve achieves unidirectional blood flow between the left ventricle and the aorta. It normally consists of three leaflets that open during systole and close during diastole, under the pressure difference established between the ventricle and the aorta.
Calcific aortic valve disease is the most common aortic valve disorder. It affects 4% of adults over 65 years of age and consists of the formation of calcific lesions on the valve leaflets.
The bicuspid aortic valve is the most common congenital valvular defect and affects 2% of the population. While a normal aortic valve consists of three leaflets, the bicuspid aortic valve forms with only two, as a result of fusion between two adjacent leaflets.