Towards new therapies for calcific aortic valve disease

Abstract

Calcific aortic valve disease (CAVD) is characterized by progressive calcification of the aortic valve cusps. The end-stage (stenosis), can lead to heart failure and death. Approximately 2-3% of adults over 65 years of age are thought to suffer from valve stenosis, requiring aortic valve replacement. This amounts to a total number of approximately 300,000 aortic valve replacements worldwide and this number is thought to triple by 2050. Currently used prostheses for heart valve replacement therapies are sub-optimal, mostly because they lack living tissue, hence the ability to grow and repair and remodel in vivo. Therefore, the search for alternative treatment options is warranted. In the first part of this thesis, we aimed to improve currently used strategies for heart valve tissue engineering. These strategies have been successfully applied in laboratory settings and even in pre-clinical animal models, but protocols need to be adapted for translation from bench to bedside. For this purpose, we have investigated if animal serum in culture protocols can be replaced with human plasma/serum alternatives. Our initial results showed that human platelet-lysate (PL) was a suitable substitute: use of PL resulted in enhanced cell proliferation, collagen production and expression of matrix metalloproteinases (MMPs) when compared to fetal bovine serum (FBS). The results from these 2D cell studies suggested that the use of PL would results in reduced waiting time to obtain sufficient amounts of cells, while subsequent tissue culture would benefit from enhanced collagen production and matrix remodeling. Unfortunately, a follow-up of this study in 3D-tissue strips revealed that tissue produced in PL-containing medium was significantly weaker when compared to tissue cultured in FSB-containing medium. Therefore, we hypothesized that human serum (HS) may be a more suitable alternative for FBS, because it should contain less platelet-derived growth factors which may induce expression and activity of MMPs. Indeed, in a third study, we showed that the use of HS instead of PL increased tissue strength to a level comparable to FBS. From these studies, we concluded that matrix remodeling by MMPs and the length and arrangement of collagen fibers are important determinants for tissue strength. Moreover, we showed that sequential use of PL and HS is an optimal autologous culture protocol for heart valve tissue engineering, as PL enhances cell proliferation, while HS stimulates production of load-bearing tissue. In the second part, we focused on future targets for pharmacological treatment of CAVD, which might halt progression of the disease, thereby preventing the need for invasive surgery. Following a review on etiology and pathogenesis of CAVD, we identified Wnt-signaling as a potential new target in onset and progression of CAVD. We describe a new pathway in valve tissue in which BMP2 stimulates Wnt-signaling, leading to enhanced expression of osteogenic transcription factor Runx2, independent of Smad-signaling. In the final chapter, we compare microRNA expression patterns from healthy and diseased valve tissue, resulting in identification of microRNAs possibly involved in CAVD. Future studies will be performed to determine if these studies result in new targets for future pharmacological treatments.