Bicuspid aortic valve hemodynamics does not promote remodeling in porcine aortic wall concavity

AIM: To investigate the role of type-I left-right bicuspid aortic valve(LR-BAV) hemodynamic stresses in the remodeling of the thoracic ascending aorta(AA) concavity, in the absence of underlying genetic or structural defects.METHODS: Transient wall shear stress(WSS) profiles in the concavity of tricuspid aortic valve(TAV) and LR-BAV AAs were obtained computationally. Tissue specimens excised from the concavity of normal(nondilated) porcine AAs were subjected for 48 h to those stress environments using a shear stress bioreactor. Tissue remodeling was characterized in terms of matrix metalloproteinase(MMP) expression and activity via immunostaining and gelatin zymography.RESULTS: Immunostaining semi-quantification results indicated no significant difference in MMP-2 and MMP-9 expression between the tissue groups exposed to TAV and LR-BAV AA WSS(P = 0.80 and P = 0.19, respectively). Zymography densitometry revealed no difference in MMP-2 activity(total activity, active form and latent form) between the groups subjected to TAV AA and LR-BAV AA WSS(P = 0.08, P = 0.15 and P = 0.59, respectively).CONCLUSION: The hemodynamic stress environment present in the concavity of type-I LR-BAV AA does not cause any significant change in proteolytic enzyme expression and activity as compared to that present in the TAV AA.

Bone morphogenetic protein-4 and transforming growth factor-beta1 mechanisms in acute valvular response to supra-physiologic hemodynamic stresses

AIM:To explore ex vivo the role of bone morphogenetic protein-4(BMP-4) and transforming growth factorbeta1(TGF-β1) in acute valvular response to fluid shear stress(FSS) abnormalities.METHODS:Porcine valve leaflets were subjected ex vivo to physiologic FSS,supra-physiologic FSS magnitude at normal frequency and supra-physiologic FSS frequency at normal magnitude for 48 h in a double-sided cone-and-plate bioreactor filled with standard culture medium. The role of BMP-4 and TGF-β1 in the valvular response was investigated by promoting or inhibiting the downstream action of those cytokines via culture medium supplementation with BMP-4 or the BMP antagonist noggin,and TGF-β1 or the TGF-β1 inhibitor SB-431542,respectively. Fresh porcine leaflets were used as controls. Each experimental group consisted of six leaflet samples. Immunostaining and immunoblotting were performed to assess endothelial activation in terms of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 expressions,paracrine signaling in terms of BMP-4 and TGF-β1 expressions and extracellular matrix(ECM) remodeling in terms of cathepsin L,cathepsin S,metalloproteinases(MMP)-2 and MMP-9 expressions. Immunostained images were quantified by normalizing the intensities of positively stained regions by the number of cells in each image while immunoblots were quantified by densitometry. R E S U LT S :Regardless of the culture medium,physiologic FSS maintained valvular homeostasis. Tissue exposure to supra-physiologic FSS magnitude in standard medium stimulated paracrine signaling(TGF-β1:467% ± 22% vs 100% ± 6% in freshcontrols,BMP-4:258% ± 22% vs 100% ± 4% in fresh controls; P < 0.05) and ECM degradation(MMP-2:941% ± 90% vs 100% ± 19% in fresh controls,MMP-9:1219% ± 190% vs 100% ± 16% in fresh controls,cathepsin L:1187% ± 175% vs 100% ± 12% in fresh controls,cathepsin S:603% ± 88% vs 100% ± 13% in fresh controls; P < 0.05),while BMP-4 supplementation also promoted fibrosa activation and TGF-β1 inhibition reduced MMP-9 expression to the native tissue level(MMP-9:308% ± 153% with TGF-β1 inhibition vs 100% ± 16% in fresh control; P > 0.05). Supra-physiologic FSS frequency had no effect on endothelial activation and paracrine signaling regardless of the culture medium but TGF-β1 silencing attenuated FSS-induced ECM degradation via MMP-9 downregulation(MMP-9:302% ± 182% vs 100% ± 42% in fresh controls; P > 0.05).CONCLUSION:Valvular tissue is sensitive to FSS abnormalities. The TGF-β1 inhibitor SB-431542 is a potential candidate molecule for attenuating the effects of FSS abnormalities on valvular remodeling.

Effect of Bicuspid Aortic Valve Cusp Fusion on Aorta Wall Shear Stress: Preliminary Computational Assessment and Implication for Aortic Dilation

The bicuspid aortic valve (BAV) is a major congenital valvular abnormality and is associated with a high prevalence of aortic dilation, whose expression depends on the type of leaflet fusion. Although BAV hemodynamics is considered a potential pathogenic contributor, the impact of BAV fusion on ascending aorta (AA) wall shear stress (WSS) remains largely unknown. A fluid-structure interaction approach was implemented to predict the hemodynamics and WSS characteristics in realistic AA models subjected to the flow of a normal tricuspid aortic valve (TAV) and three BAV morphotypes (left-right coronary cusp fusion (LR), right-non coronary cusp fusion (RN) and non-left coronary cusp fusion (NL)). TAV flow conditions subjected the proximal and middle AA to a streamlined flow typical of flows in bends, while BAV flow conditions generated increased flow helicity. The LR-BAV orifice jet generated flow abnormalities primarily in the proximal AA, which were marked by a uniform WSS overload along the wall circumference and contrasted WSS directionalities on the wall convexity and concavity. Flow abnormalities generated by the RN-BAV and NL-BAV inlet flow conditions were more diffuse and consisted of WSS overloads in the convexity of the proximal and middle AA and contrasted WSS directionalities in the anterior and posterior wall regions. This study demonstrates the impact of the BAV morphotype on AA hemodynamics and provides quantitative evidence for the existence of WSS abnormalities in aortic wall regions prone to dilation.