Aortic valve mechanobiology --- The effect of cyclic stretch

The aortic valve is a dynamic, elegant structure that functions in a complex mechanical environment interacting with this environment to drive critical cell-extracellular matrix interactions. Altered mechanical forces are believed to trigger changes in valve biology but the cellular and molecular events involved in these processes are not well characterized. The particular phenotypes of aortic valvular endothelial and interstitial cells appear to be dependent on a combination of intrinsic genetically programmed biology and the influence of the local mechanical factors. Studies indicate that pathological mechanical loading experienced by the valve cusps lead to degenerative disease, which is characterized among other things by inflammation, calcification, stenosis, regurgitation and ultimate valve failure.;The objective of the current thesis work was to understand the effects of cyclic stretch on valve mechanobiology with a focus on the key questions that are relevant in understanding degenerative aortic valve disease. An ex vivo bioreactor system was built, validated and utilized to subject valve cusp samples to controlled waveforms of cyclic stretch representing normal and pathological conditions. The following aspects of degenerative aortic valve disease were studied. 1) Extracellular matrix remodeling; 2) Aortic valve calcification; and 3) Serotonin-related valvular degeneration via stretchsensitive receptors in the aortic valve.;Normal physiological magnitude of cyclic stretch was observed to maintain normal extracellular matrix composition and synthesis in aortic valve cusps. Aortic valve remodeling was maximum at 15% stretch, but did not increase with further increases in cyclic stretch magnitude. At 20% stretch, cell apoptosis and proliferation were significantly higher than at the lower magnitudes of cyclic stretch. These results imply that the valve cells are quiescent at 10% stretch; activated, in a synthetic phenotype at 15% stretch, and in a proliferative phenotype at 20% stretch.;Specific aim 2 sought to elucidate the effects of cyclic stretch on aortic valve calcification. Aortic valve cells were observed to differentiate into an osteoblast-like phenotype resulting in rapid mineralization of the cusp tissue under elevated cyclic stretch (15%). These responses were demonstrated to be strongly BMP dependent, and were also positively correlated with cyclic stretch magnitude. BMP expression was primarily in the endothelial cells, leading to speculation that the endothelial cells are a key mechanotransducer in the valve cusp and effecting downstream signaling and activation of the valve interstitial cell. All stretch responses could be inhibited by the BMP antagonist noggin in a dose-dependent manner, further demonstrating the importance of this cytokine in valve calcification.;In specific aim 3, 5-HT dependent degenerative aortic valve disease was studied. This type of degenerative valvulopathy is observed to occur along with valve thickening, increased cellular proliferation and changes in valve geometry. 5-HT induced valve remodeling was shown to occur via a 5-HT 2A receptor-dependent mechanism, increasing collagen synthesis and crosslinking resulted due to the combined effects of 5-HT administration and cyclic stretch. These responses were more prominent at 15% stretch compared to 10% stretch implying that the 5-HT2A receptor subtype is mechanosensitive. Mechanical testing revealed that these changes in collagen biochemistry resulted in transition to the stiffer regime of the valve stress-strain curve at a lower level of strain. This can be mostly attributed to the increased collagen crosslinking as evidenced by increased expression of lysyl oxidase. In addition, increases in collagen synthesis appeared to occur independent of the increased cellular proliferation that was observed. Cell proliferation was in fact primarily influenced by stretch magnitude, but not 5-HT administration.;In conclusion, the importance of cyclic stretch in altering valve remodeling and degenerative disease has been demonstrated in this thesis work. It is hoped that this thesis will inspire further study into the specific mechanobiological mechanisms of different degenerative valvulopathies. This knowledge can then be used to develop treatments that target degenerative valve disease. In addition, the valve tissue responses to elevated cyclic stretch of collagen synthesis and crosslinking can be used as starting points for preconditioning and testing regimes for tissue engineered valve constructs.