A thiol-ene polymerization platform was used to synthesize peptide functionalized poly(ethylene glycol) (PEG) hydrogels, which were initially characterized and compared to theoretical predictions of Young’s modulus via a theoretical crosslinking density equation presented herein. After thorough characterization, this material system’s utility for answering specific biological hypotheses was demonstrated with the culture and observation of aortic valvular interstitial cells (VICs). Specifically, these materials were used to better understand the role of substrate elasticity and biochemical functionality on VIC α-smooth muscle (αSMA) expression and secretory properties (i.e., de novo ECM). The Young’s moduli of the hydrogels were varied from 28kPa (activating, 90% myofibroblasts) to 4kPa (non-activating, 15% myofibroblast) substrates, and the biochemical functionality was tailored by incorporating three small adhesive peptide sequences, RGDS, VGVAPG, and P15. To promote VIC adhesion, a basal [RGDS] of 0.8mM was used in all formulations, while the [VGVAPG] or [P15] were varied to be lower, equal, or higher than 0.8mM. The substrates with 1.2mM VGVAPG and all gels with P15 led to significantly higher αSMA expression for both stiff and soft substrates, as compared to 0.8mM RGDS alone. Importantly, all gel conditions were significantly lower than TCPS (~4–10 fold difference). The ECM produced significantly decreased as the total integrin binding peptide concentration increased, but was significantly higher than that expressed on TCPS. This easily tailored material system provides a useful culture platform to improve the fundamental understanding of VIC biology through isolating specific biological cues and observing VIC function.

The aortic heart valve opens and closes ≈3 billion times in a person's lifetime, making it arguably one of the most mechanically demanding environments in the body ( Figure 1A). The valve has 3 leaflets or cusps of thin tissue composed of collagen, elastin, and glycosaminoglycans 1 and is striated into 3 layers ( Figure 1B). Vascularization of the valve is not necessary because the cusps are thin enough for oxygen, nutrients, and waste to diffuse between the tissue and the surrounding blood.

The effects of valvular endothelial cell (VlvEC) paracrine signaling on VIC phenotype and nodule formation were tested using a co-culture platform with physiologically relevant matrix elasticities and diffusion distance. 100μm thin poly(ethylene glycol) (PEG) hydrogels of 3 to 27 kPa Young’s moduli were fabricated in transwell inserts. VICs were cultured on the gels, as VIC phenotype is known to change significantly within this range, while VlvECs lined the underside of the membrane. Co-culture with VlvECs significantly reduced VIC activation to the myofibroblast phenotype on all gels with the largest percent decrease on the 3 kPa gels (~70%), while stiffer gels resulted in approximately 20–30% decrease. Additionally, VlvECs significantly reduced αSMA protein expression (~2 fold lower) on both 3 and 27 kPa gels, as well as the number (~2 fold lower) of nodules formed on the 27kPa gels. Effects of VlvECs were prevented when nitric oxide (NO) release was inhibited with L-NAME, suggesting that VlvEC produced NO inhibits VIC activation. Withdrawal of L-NAME after 3, 5, and 7 days with restoration of VlvEC NO production for 2 additional days led to a partial reversal of VIC activation (~25% decrease). A potential mechanism by which VlvEC produced NO reduced VIC activation was studied by inhibiting initial and mid-stage cGMP pathway molecules. Inhibition of soluble guanylyl cyclase (sGC) with ODQ or protein kinase G (PKG) with RBrcGMP or stimulation of Rho kinase (ROCK) with LPA, abolished VlvEC effects on VIC activation. This work contributes substantially to the understanding of the valve endothelium’s role in preventing VIC functions associated with aortic valve stenosis initiation and progression.