Degeneration of Aortic Valves in a Bioreactor System with Pulsatile Flow

Abstract

Calcific aortic valve disease is the most common valvular heart disease in industrialized countries. Pulsatile pressure, sheer and bending stress promote initiation and progression of aortic valve degeneration. The aim of this work is to establish an ex vivo model to study the therein involved processes. Ovine aortic roots bearing aortic valve leaflets were cultivated in an elaborated bioreactor system with pulsatile flow, physiological temperature, and controlled pressure and pH values. Standard and pro-degenerative treatment were studied regarding the impact on morphology, calcification, and gene expression. In particular, differentiation, matrix remodeling, and degeneration were also compared to a static cultivation model. Bioreactor cultivation led to shrinking and thickening of the valve leaflets compared to native leaflets while gross morphology and the presence of valvular interstitial cells were preserved. Degenerative conditions induced considerable leaflet calcification. In comparison to static cultivation, collagen gene expression was stable under bioreactor cultivation, whereas expression of hypoxia-related markers was increased. Osteopontin gene expression was differentially altered compared to protein expression, indicating an enhanced protein turnover. The present ex vivo model is an adequate and effective system to analyze aortic valve degeneration under controlled physiological conditions without the need of additional growth factors.

Crosstalk of Diabetic Conditions with Static Versus Dynamic Flow Environment—Impact on Aortic Valve Remodeling

“…Discrepancies between this model and our models thus might not only be due to mechanical differences but also might also be ascribed to the lack of VEC in our models, since VEC are known to communicate with VIC [32] possibly through protein kinase C under HG [24]. Direct comparisons of static vs. dynamic environment under basal conditions have shown a slight trend towards lower gene expression levels of SPP1, contrary to our previous observations in a similar setting [28]. Here, lower glucose levels in our basal conditions compared to the aforementioned approach using HG medium as basal medium might be the reason for this difference.…”

“…A generally lower expression of TGFβ under static conditions might be the reason for the lack of susceptibility to diabetic conditions. Recent findings indicate that TGFβ expressions are generally higher in AV tissue cultivated under dynamic flow conditions than compared to a static cultivation environment, which is in line with our findings regarding direct comparison of basal conditions [28]. TGFβ is known to have an impact on VIC activation, i.e., ACTA2 expression [29].…”

“…Increased basal gene expression of ACTA2 in the static environment in direct comparison to the dynamic environment could be ascribed to enhanced stretch stress levels in this model, as it has been described before [26,27]. Differences in basal gene expression of ACTA2 compared to basal protein abundance might be indicative for a diverse turnover dependent on biomechanical influence, as we have already observed for other targets in a similar setting [28].…”

“…Thus, unchanged or even reduced TGFβ expression levels under static or dynamic flow conditions regarding diabetic treatment, respectively, might be the reason for reduced or unchanged ACTA2 expression levels. Increased TGFβ expression levels in a dynamic environment under basal conditions reflect recent findings with a similar approach [28] and might contribute to the thickening of AV tissue by affecting extracellular matrix expression [30].…”

“…In general, dynamic flow cultivation resulted in fibrosis and thus results in the thickening of the tissue in comparison to samples under static cultivation independent of diabetic conditions. We mainly ascribe this to a generally higher COL1A1 expression under dynamic flow cultivation compared to static cultivation, as we have observed this in a similar setting before [28]. Nevertheless, direct comparison of basal COL1A1 expression levels in the present work show a significantly reduced expression under dynamic flow cultivation albeit within a small range, which might be attributed to lower glucose levels in our basal NG conditions compared to the aforementioned approach using HG medium as the basal medium [28].…”