(K-414) Development of a Novel Bioreactor to Explore the Mechanobiology of the Endothelial to Mesenchymal Transition Pathway in the Aortic Valve

Introduction:: Bicuspid aortic valve disease (BAV) is the most common sexually dimorphic congenital heart deformation (CHD), affecting 1.5% of 46,XY men and 0.5% of 46,XX women. In this condition, the aortic valve is underdeveloped with only two cusps, which leads to altered blood flow dynamics and can promote the early onset of aortic valve disease. It is theorized that non-syndromic BAV cases are caused by mutations that impair a conserved phenotypic regulatory pathway called endothelial-mesenchymal transition (endMT), a mechanism of cellular plasticity that allows endothelial cells to lose their polarity and cell-cell adhesion, restructure their cytoskeleton and become invasive. EndMT has been observed to be critical for physiological heart septation and theorized to have a role in BAV. Microenvironmental factors such as substrate stiffness, extracellular matrix (ECM) composition, and disturbed flow are shown to activate endMT. Their respective influence to this pathway has been proven through various tissue engineering techniques, but they have not fully recapitulated the microenvironment of the endocardial cushions - embryonic tissues that mature into the aortic valve. In addition, sexual dimorphism in BAV continues to be understudied, and, to the best of our knowledge, there is no evidence of a model that aims to identify differences in endMT response pertinent to biological sex. To better analyze the effect of shear stress on endMT with respect to biological sex, our objective is to develop a novel system that integrates hydrogel scaffolds within a cone-plate bioreactor with the endocardial cushion microenvironment as a design constraint.

Materials and Methods::

We are integrating (1) hydrogel scaffolds, which mimic ECM composition and stiffness of the endocardial cushions using (2) a biocompatible recessed base that can expose (3) isolated male and female endothelial cells to uniform shear stress using a cone-plate system (CVS). Porcine aortic valve endothelial cells (PAVECs) were isolated from hearts and purified through flow assisted cell sorting (FACS) using an Alexa Fluor 488 igG1 primary antibody specific for porcine CD31, a standard endothelial biomarker. To prototype recessed bases, we used the Formlabs Form 3B stereolithography (SLA) 3D printer and Biomed Clear, a biocompatible resin. To inhibit the formation of turbulent flow in the CVS with the recessed base, we adjusted print orientation, support settings, and the UV post-cure protocol to optimize surface/edge resolution and prevent warping of the structure. We fabricated hydrogels out of 3.4 kDa polyethylene glycol diacrylate (PEGDA), hyaluronic acid (HA), and degradable PEG with a PQ peptide sequence (GGGPQGIAGQGK) (degPEGDA), while adjusting for composition and concentration to achieve a stiffness of < 5 kPa, a mechanical property that mimics the endocardial cushions. The hydrogels were synthesized through radical photopolymerization using white light, a biocompatible technique that uses 32.8mM N-Vinyl-2-Pyrrolidone (NVP), 10 μM Eosin Y and 1.5% v/v TEOA in HEPES buffer solution. The resulting hydrogels were allowed to swell for 24 hours in phosphate buffered saline (1xPBS) before being uniaxially compressed using a BOSE ELF 3200 mechanical tester to determine their elastic moduli between 0-20% strain using a MATLAB script.  

Results, Conclusions, and Discussions::

Results & Discussion

FACS from cell isolations of male and female porcine aortic valves highlighted the presence of two distinct populations that can be differentiated via CD31 expression. The identified cell lines were PAVECs (CD31+) and aortic valve interstitial cells (PAVICs) (CD31-). This data (Fig. 1) shows that the isolation technique yields a co-culture and requires purification to isolate PAVECs. Pertinent to prototyping the recessed base, adjusting the print orientation of the structure revealed that surface resolution and edge uniformity are dependent on printing angle. It was observed that low angle structure orientations (~20o) with respect to the printing platform results in surface ridges and edge imperfections, whereas higher printing angles (~60o) smoothen the top surface and improved edge consistency (Fig. 2). Printing angle was also observed to inversely affect the number of supports required while increasing the printing time. Biomed Clear resin without supports warped during UV-curing, while the incorporation of a full raft with supports eliminated the issue. The final design contained wells that allow gels to be crosslinked in-situ without leaking. Pertinent to scaffold design, we were able to synthesize various hydrogels composed of PEGDA, degPEGDA and a PEGDA-HA composite. It was observed that decreasing polymer concentrations decreased the compression moduli, and by doing so, we were able to meet the < 5kPa stiffness design criteria while maintaining all other crosslinking factors constant (Fig. 3). The hydrogels that met our design criteria were 2.5% and 3% PEGDA, 3.5% degPEGDA, and 0.25% HA/0.75% PEGDA composite. 

Conclusions

The significance of these results is that we were able to develop and implement a functional recessed base in the cone and plate system. This novel bioreactor will enable the scientific community to elucidate the effect of substrate composition and stiffness in the mechanobiology of endMT activation in conjunction with shear stress.

This work can be advanced by seeding the purified PAVECs onto the various < 5kPa hydrogels and optimizing for cell adhesion, cytotoxicity and sterility using live-dead staining and confocal imaging. Once successfully implemented, endMT activation can be analyzed using qRT-PCR while controlling for both hydrogel composition and shear stress.

Acknowledgements (Optional): : We would like to thank our funding sources, including the American Heart Association (20UFEL35260054) and the National Science
Foundation (2129122).

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