Interactions between valvular cells: implications for heart valve tissue engineering

Thesis (Ph.D.)--Boston UniversityApproximately 1 in 1000 children are born with congenital cardiovascular defects yearly in the US, including many abnormalities in heart valves. Tissue engineered heart valves (TEHVs) offer a solution for replacement or repair of affected valves. However, its therapeutic application is limited, and in ovine models, no TEHV has performed satisfactorily in vivo for longer than twenty weeks, in part due to the absence of supporting data for selection of the appropriate cell type(s) to be incorporated into the construct. This partially owes to the lack of a full understanding of the cells that inhabit the valve, which includes valve interstitial cells (VICs) and valve endothelial cells (VECs), and on the molecular mechanism underlying their interactions that maintain valve homeostasis. During embryonic valve development, the vast majority of VICs are derived from VECs via endothelial to mesenchymal transformation (EMT). EMT in postnatal valves is rare but it has been implicated in diseased valves. Yet, relatively little is known about VECs and VICs in post-natal valves in terms of specialized features, and how VECs and VICs might influence each other. This lack of knowledge has made it difficult to determine what type of cells should be used to create a TEHV. In order to achieve the optimal construction of a tissue engineered heart valve we look to the native valve as our guide for proper valve structure and function. Examination of the native valve leaflets can contribute to our understanding of the proper cellular environment and how disruption of this environment affects the valves. Many common mitral valve pathologies including mitral valve prolapse are characterized by thickening of the valve spongiosa, the presence of activated myofibroblasts, and excessive remodeling of the extracellular matrix. By examining the cell-cell interactions in healthy native valves, and comparing this with observations from pathogenic valves, a greater understanding can be achieved and then applied to the field of TEHV. In this thesis we explored the cell dynamics of the heart valve as related to natural homeostasis, disease progression, and tissue engineering. Using an in vitro co-culture model we revealed a novel two-way communication between mitral valve endothelial and interstitial cells. We propose that this communication promotes a healthy valve phenotype and function by inhibiting EndMT and suppressing VIC activation. We made a similar observation in the aortic valve, where VEC-VIC communication may prevent the process of an EndMT mediated osteogenesis in the context of calcific aortic valve disease. We have also used the VEC-VIC co-culture model to identify possible candidate cell sources for a tissue engineered heart valve. And finally, we show that cells that populated a tissue engineered pulmonary valve leaflet, created using an acellular scaffold, are phenotypically and functionally similar to native valve cells. These studies contribute to an understanding of the dynamics of the cellular interactions between VECs and VICs, and provide a new framework for identifying and testing the functionality of appropriate cell sources for building a TEHV with the ability to grow with the child, maintain homeostasis, and prevent fibrosis and calcification