Introduction: Valvular interstitial cells (VICs) play a critical role in the maintenance and pathophysiology of heart valve tissues. When activated, VICs engage in the tissue repair and remodeling, with the increased levels of cytokines and extracellular matrix (ECM) synthesis and strong contraction through the expression of α-smooth muscle actin (α-SMA) fibers. However, the abnormal mechanical loading conditions within the tissue cause the unregulated activities of the VICs, resulting in the heart valve disease. Thus, current research challenges aim at characterizing the mechanisms that activate the VIC contractility and the mechanical interactions of contractile VICs with the surrounding ECM. However, it remains unclear how active contraction of the α-SMA fibers contribute to the overall VIC mechanical responses as well as other mechanical constituents such as nucleus, cytoskeleton, and cytosolic fluid. The objective of this study is to investigate the roles of these different subcellular structures of the VICs, especially α-SMA stress fibers, to the VIC mechanical responses under different mechanical loading conditions and activation states.
Materials and Methods: We modeled the VIC as a continuum with two distinct domains: the cell nucleus and cytoplasm. The nucleus was modeled as an incompressible neo-Hookean material while the cytoplasm was modeled as a mixture of two solid phases: the basal, isotropic cytoskeleton phase and α-SMA stress fiber phase, which exhibit some orientations at each point described by an orientation density function. The α-SMA stress fibers also exhibit passive elastic and active contractile responses in the direction of the orientations. We developed VIC mechanical model, which integrated the data from two experiments: micropipette aspiration (MA) and atomic force microscopy (AFM) of the aortic VIC (AVIC) and pulmonary VIC (PVIC) that each exhibits different expression levels of the α-SMA and contraction strength. In the MA experiment, VICs are in inactivated states while in AFM experiment, VICs are in activated states, exhibiting higher level of α-SMA expression and contraction. Thus, using our model in conjunction with the experimental data, we investigated how the expression level and active contraction of the α-SMA fibers affect the effective mechanical responses of the VICs. We implemented our model on the finite element method and ran the simulations on FEBio software package using its plugin capability.
Results and Discussion: Using the experimental data, we investigated the nucleus, cytoskeleton, and stress fiber stiffness and contraction strength of the AVICs and PVICs. The contraction strength and nucleus stiffness are shown in Figure 1. There exists ~10-fold difference in the contractions strength between the AVICs and PVICs, implying that not only the expression level of the α-SMA fibers but also the contraction level increases from PVICs compared to AVICs. It is possible that the higher expression of the α-SMA fibers facilitates more efficient contraction. We also determined the nucleus stiffness of the AVICs and PVICs and found that no statistically significant differences (p=0.54) in the stiffness values. Thus, the nucleus stiffness is not influenced by the activation states of the VICs.
Conclusions: In this study, we developed the mechanical model of the VICs that is capable of capturing the mechanical responses of VICs under different activation states and loading conditions. It has been known that VICs exhibit significantly different stiffness values for the MA and AFM measurements. Our model explains this gap between these two measurements by attributing their differences to the active contraction strength of the α-SMA fibers. Thus, our model provides more holistic view of the VIC mechanics. Also, using our model with calibrated parameters, we can simulate and study the VICs within the native and engineered tisue environments, where the VICs undergo dynamic and rapid mechanical changes due to native ECM or biomaterial environment.
R01 HL-68816-01 and R01 HL119297