Quantifying Heart Valve Interstitial Cell Contractile State Using Highly Tunable Poly(Ethylene Glycol) Hydrogels

Abstract

Valve interstitial cells (VIC) are the primary cell type residing within heart valve tissues. In many valve pathologies, VICs become activated and will subsequently profoundly remodel the valve tissue extracellular matrix (ECM). A primary indicator of VIC activation is the upregulation of α--smooth muscle actin (αSMA) fibers, which in turn increase VIC contractility. Thus, contractile state reflects VIC activation and ECM biosynthesis levels. In general, cell contraction studies have largely utilized two--dimensional substrates,which are a vastly different mechanical environment than native tissues. To address this limitation, hydrogels have been a popular choice for studying cells in a three--dimensional environment due to their tunable properties and optical transparency, which allows for direct cell visualization. In the present study, we extended the use of hydrogels to study the active contractile behavior of VICs. Aortic VICs (AVIC) were encapsulated within poly(ethylene glycol) hydrogels and were subjected to flexural--deformation tests to assess the state of AVIC contraction. Using a finite element model of the bending experiment, we determined the effective shear modulus of the constructs and observed an increase in bending stiffness as a result of AVIC active contraction. Results indicated that AVIC contraction has a more pronounced effect on macroscale properties in softer gels (72 ± 21% increase in shear moduli within 2.5 kPa gels) and is dependent upon the availability of adhesion sites (0.5--1 mM CRGDS). We were able to image AVICs directly within the hydrogel and observed a time-dependent decrease in volume (time constant τ = 3.04 min) when the AVICs were induced into a hypertensive state. Our results indicate that AVIC contraction is regulated by the stiffness of the hydrogel, as part of the larger AVIC--biomaterial interactions. This finding suggests that AVIC contractile state can be profoundly modulated through their local micro environment. Thus, hydrogels can be used to study the mechanics of AVICs in a 3D micromechanical-emulating environment. Moving forward, this approach has the potential to be used towards delineating normal and diseased VIC biomechanical properties using highly tunable PEG biomaterials.