The function of the heart valve interstitial cells (VICs) is intimately connected to heart valve tissue remodeling and repair, as well as the onset and progression of valvular pathological processes. There is yet only very limited knowledge and extant models for the complex three-dimensional VIC internal stress-bearing structures, the associated cell-level biomechanical behaviors, and how they change under varying activation levels. Importantly, VICs are known to exist and function within the highly dynamic valve tissue environment, including very high physiological loading rates. Yet we have no knowledge on how these factors affect VIC function. To this end, we extended our previous VIC computational continuum mechanics model (Sakamoto, et al., 2016, "On Intrinsic Stress Fiber Contractile Forces in Semilunar Heart Valve Interstitial Cells Using a Continuum Mixture Model," J. Mech. Behav. Biomed. Mater., 54(244-258)). to incorporate realistic stress-fiber geometries, force-length relations (Hill model for active contraction), explicit α-smooth muscle actin (α-SMA) and F-actin expression levels, and strain rate. Novel micro-indentation measurements were then performed using cytochalasin D (CytoD), variable KCl molar concentrations, both alone and with transforming growth factor β1 (TGF-β1) (which emulates certain valvular pathological processes) to explore how α-SMA and F-actin expression levels influenced stress fiber responses under quasi-static and physiological loading rates. Simulation results indicated that both F-actin and α-SMA contributed substantially to stress fiber force generation, with the highest activation state (90 mM KCL + TGF-β1) inducing the largest α-SMA levels and associated force generation. Validation was performed by comparisons to traction force microscopy studies, which showed very good agreement. Interestingly, only in the highest activation state was strain rate sensitivity observed, which was captured successfully in the simulations. These unique findings demonstrated that only VICs with high levels of αSMA expression exhibited significant viscoelastic effects. Implications of this study include greater insight into the functional role of α-SMA and F-actin in VIC stress fiber function, and the potential for strain rate-dependent effects in pathological states where high levels of α-SMA occur, which appear to be unique to the valvular cellular in vivo microenvironment.

Event Abstract Back to Event An active contraction model of the valvular interstitial cell Michael S Sacks1 and Yusuke Sakamoto1 1 The University of Texas at Austin, Biomedical Engineering, United States 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 Keywords: modeling, heart valve, cell phenotype, Cell functionality Conference: 10th World Biomaterials Congress, Montréal, Canada, 17 May - 22 May, 2016. Presentation Type: General Session Oral Topic: Mechanobiology of cells on biomaterials Citation: Sacks M and Sakamoto Y (2016). An active contraction model of the valvular interstitial cell. Front. Bioeng. Biotechnol. Conference Abstract: 10th World Biomaterials Congress. doi: 10.3389/conf.FBIOE.2016.01.03024 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 28 Mar 2016; Published Online: 30 Mar 2016. Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Michael S Sacks Yusuke Sakamoto Google Michael S Sacks Yusuke Sakamoto Google Scholar Michael S Sacks Yusuke Sakamoto PubMed Michael S Sacks Yusuke Sakamoto Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Heart valve interstitial cells (VICs) play a critical role in the maintenance and pathophysiology of heart valve tissues. Normally quiescent in the adult, VICs can become activated in periods of growth and disease. When activated, VICs exhibit increased levels of cytokines and extracellular matrix (ECM) synthesis, and upregulated expression and strong contraction of α-smooth muscle actin (α-SMA) fibers. However, it remains unknown how expression and contraction of the α-SMA fibers, which vary among different VIC types, contribute to the overall VIC mechanical responses, including the nucleus and cytoskeleton contributions. In the present study, we developed a novel solid-mixture model for VIC biomechanical behavior that incorporated 1) the underlying cytoskeletal network, 2) the oriented α-SMA stress fibers with passive elastic and active contractile responses, 3) a finite deformable elastic nucleus. We implemented the model in a full 3D finite element simulation of a VIC based on known geometry. Moreover, we examined the respective mechanical responses of aortic and pulmonary VICs (AVICs and PVICs, respectively), which are known to have different levels of α-SMA expression levels and contractile behaviors. To calibrate the model, we simulated the combined mechanical responses of VICs in both micropipette aspiration (MA) and atomic force microscopy (AFM) experiments. These two states were chosen as the VICs were under significantly different mechanical loading conditions and activation states, with the α-SMA fibers inactivated in the MA studies while fully activated in the AFM studies. We also used the AFM to study the mechanical property of the nucleus. Our model predicted that the substantial differences found in stiffening of the AVIC compared to the PVICs was due to a 9 to 16 times stronger intrinsic AVIC α-SMA stress fiber contractile force. Model validation was done by simulating a traction force microscopy experiment to estimate the forces the VICs exert on the underlying substrate, and found good agreement with reported traction force microscopy results. Further, estimated nuclear stiffness for both AVICs and PVICs were similar and comparable to the literature, and were both unaffected by VIC activation level. These results suggest substantial functional differences between AVICs and PVICs at the subcellular level. Moreover, this first VIC computational biomechanical model is but a first step in developing a comprehensive, integrated view of the VIC pathophysiology and interactions with the valve ECM micro-environment based on simulation technologies.