On the 3D contractile properties of the aortic heart valve interstitial cell in health and disease

Date

2022-05-02

Authors

Khang, Alex, Ph. D.

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Abstract

Aortic valve interstitial cells (AVICs) are fibrobast-like cells that reside within all layers of the aortic valve (AV) and maintain extracellular matrix (ECM) turnover and remodeling. In disease, AVICs can undergo activation and take on a myofibroblastic phenotype characterized by increases in ECM deposition, remodeling, and cellular contractility brought about through expression of alpha-smooth muscle actin stress fibers (SFs). AVIC contractility via stress fibers is a physical indicator that reflects both AVIC activation level as well as biophysical state and is known to be correlated with crucial processes such as collagen deposition and ECM remodeling. My dissertation focuses on investigating the 3D contractile properties of AVICs within tissue-mimicking, 3D peptide-modified poly (ethlyene glycol) (PEG) hydrogels that crucially allow for direct visualization and assessment of AVIC behaviors. First, I used a flexure setup to quantify the contractile states of AVICs embedded within PEG gels, which showed similarity to our earlier native tissue work and demonstrated that the PEG gel environment reproduces many of the same functional characteristics as soft tissue. Then, I investigated the 3D contractile properties of AVICs in greater detail using 3D traction force microscopy and found that AVIC shape orientation and principal contractile direction were correlated. Further analysis showed that AVIC protrusions were the main drivers of AVIC contractile behaviors and that they deformed in a uniform, piston-like manner, indicative of highly-aligned underlying SFs. To gain deeper insight into SF architecture and contractile forces, I developed a 3D computational model of the contracting AVIC within a PEG hydrogel medium. First, the model predicted that AVICs stiffen the local material likely due to nascent ECM deposition. The local variations in hydrogel moduli were then incorporated with a mechanical model of the contracting AVIC which predicted that the greatest SF alignment and contractile force levels were localized at the AVIC protrusions, showing consistency with experimentation. Finally, I extended this approach to investigate intrinsic differences between AVICs extracted from human bicuspid AVs (BAVs) and structurally normal tricuspid AVs (NAVs) and found that AVICs from BAVs showed lower levels of activation as evidenced by lesser SF alignment and contractility. These findings suggest that intrinsic differences among the AVICs likely contribute to the increased rate of valve disease experienced by many BAV patients. In addition, this work highlights the importance of investigating cellular and sub-cellular differences among the BAV and NAV toward identifying targets for novel, non-surgical therapies.

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