Dissertations / Theses on the topic 'Valvular interstitial cells'

"A quantitative understanding of the complex interactions between cells, soluble factors, and the biological and mechanical properties of biomaterials is required to guide cell remodeling towards regeneration of healthy tissue rather than fibrocontractive tissue. The goal of this thesis was to elucidate the interactions between the boundary stiffness of three-dimensional (3D) matrix and soluble factors on valvular interstitial cell (VIC) phenotype with a quantitative approach. The first part of the work presented in this thesis was to characterize the combined effects of boundary stiffness and transforming growth factor-β1 (TGF-β1) on cell-generated forces and collagen accumulation. We first generated a quantitative map of cell-generated tension in response to these factors by culturing VICs within micro-scale fibrin gels between compliant posts (0.15-1.05 nN/nm) in chemically-defined media with TGF-β1 (0-5 ng/mL). The VICs generated 100 to 3000 nN/cell after one week of culture, and multiple regression modeling demonstrated, for the first time, quantitative interaction (synergy) between these factors in a 3D culture system. We then isolated passive and active components of tension within the micro-tissues and found that cells cultured with high levels of stiffness and TGF-β1 expressed myofibroblast markers and generated substantial residual tension in the matrix yet, surprisingly, were not able to generate additional tension in response to membrane depolarization signifying a state of continual maximal contraction. In contrast, negligible residual tension was stored in the low stiffness and TGF-β1 groups indicating a lower potential for shrinkage upon release. We then studied if ECM could be generated under the low tension environment and found that TGF-β1, but not EGF, increased de novo collagen accumulation in both low and high tension environments roughly equally. Combined, these findings suggest that isometric cell force, passive retraction, and collagen production can be tuned by independently altering boundary stiffness and TGF-β1 concentration. In the second part, by using the quantitative information obtained from the first part, we investigated the effects of dynamic changes in stiffness on cell phenotype in a 3D protein matrix, quantitatively. Our novel method utilizing magnetic force to constrain the motion of one of two flexible posts between which VIC-populated micro-tissues were cultured effectively doubled the boundary stiffness and resulted in a significant increase in cell-generated forces. When the magnetic force was removed, the effective boundary stiffness was halved and the tissue tension dropped to 65-87% of the peak value. Surprisingly, following release the cell-generated forces continued to increase for the next two days rather than reducing down to the homeostatic tension level of the control group with identical (but constant) boundary stiffness. The rapid release of tension with the return to baseline boundary stiffness did not result in a decrease in number of cells with α-SMA positive stress fibers or an increase in apoptosis. When samples were entirely released from the boundaries and cultured free floating (where tension is minimal but cannot be measured), the proportion of apoptotic cells in middle region of the micro-tissues increased more than five-fold to 31%. Together, these data indicate that modest temporary changes in boundary stiffness can have lasting effects on myofibroblast activation and persistence in 3D matrices, and that a large decrease in the ability of the cells to generate tension is required to trigger de-differentiation and apoptosis. "

Calcific aortic valvular disease (CAVD) is the most common valvular pathology in the developed world. CAVD results in calcifications forming on the aortic valve leaflets, inhibiting proper closure and causing complications of stenosis and regurgitation. Although, the mechanisms behind the disease initiation are unknown, it is believed to be a cell-mediated phenomenon, and not the result of passive degradation of the valve as once believed due to the increased prevalence with age. Currently, there are no pharmaceutical options for the prevention or reversal of calcifications, the only treatment option is complete valve replacement, an imperfect solution. Hindering the development of potential therapeutics is that currently there are no adequate animal models which replicate the calcification and cell death seen in disease explanted valves. An in vitro model has been develop where valvular interstitial cells (VICs), the main cell type of the valve, are seeded at high density into tissue culture polystyrene dishes and cultured with TGF-β1. This results in VICs activating to the myofibroblast phenotype and forming cell aggregates. Due to currently unknown mechanisms, apoptosis occurs within the center of the aggregates and calcification ensues. Although simplistic, this model has been used to show that rate and frequency of aggregation is affected by cellular tension; conditions of high tension increase aggregation response, while conditions of low tension prevent aggregation and calcification from occurring. It is important to note; however, that despite its wide usage, the current model is limited as the aggregation and subsequent calcification are random occurrences and are not consistent across literature where same conditions for control samples are used. The motivation of the presented work is two-fold. First, high intracellular tension has been suggested as one of the mechanisms leading to disease in the valve. Despite the clear and important role of cell tension, VIC tension has never before been measured in a dynamic environment. The ways in which dynamic stimulation affects individual VIC tension is not known. In aim one, a method is developed to allow for long-term cyclic stretch of VICs with measurement of cell traction force. It was found that cyclic stretch decreased cell tension in cells with high prestress and increased cell tension for conditions of low prestress. Combined, these findings indicate a homeostatic cellular tension which is dependent upon the mechanical environment. In the second aim, a novel method for creating VIC aggregates is validated. Micro-contact printing, essentially “stampingâ€� of a protein in a defined pattern, is used to create circular aggregates on polyacrylamide gels. This method allows for the separation of the aggregation from the subsequent calcification, an improvement over the current in vitro model. The method is then used to explore the role of the distribution of tension in the initiation of disease

TRPM4 est un canal cationique non sélectif activé par le calcium intracellulaire impliqué dans le remodelage des fibroblastes cardiaques humains. Ces cellules partagent des caractéristiques communes avec les cellules interstitielles valvulaires (VIC) principales cellules qui composent les valves aortiques, notamment leur capacité de transdifférenciation phénotypique. La différenciation ostéogénique est une étape clé dans la calcification valvulaire menant au rétrécissement aortique calcifié. L’expression du canal TRPM4 est augmentée dans les valves aortiques issues de patients souffrant de rétrécissement aortique calcifié suggérant que le canal TRPM4 peut être impliqué dans les processus de calcification valvulaire. De plus, ce canal est impliqué dans le remodelage valvulaire aortique radio-induit dans un modèle murin. Cependant, à l’heure actuelle les acteurs cellulaires et les mécanismes TRPM4-dépendants impliqués dans ce remodelage demeurent inconnus. Le but de ce travail était de mettre en évidence une potentielle implication de TRPM4 dans la différenciation ostéogénique des VIC humaines (hVIC) et les mécanismes moléculaires impliqués.Ce travail a permis de mettre en évidence que le canal TRPM4 est préférentiellement exprimé à la membrane et une signature électrophysiologique du canal TRPM4 a été enregistrée. TRPM4 est impliqué dans la différenciation ostéogénique des hVIC aussi bien dans les cellules issues de valves calcifiées que dans les hVIC issues de valves non calcifiées. Cet effet passe par l’activation de diverses voies de signalisation, notamment une activation TRPM4-dépendante de la voie BMP2/SMAD1/5 et de la voie NFAT. De plus, nous avons pu observer une plus forte expression du canal TRPM4 dans les hVIC issues de valves calcifiées en comparaison des cellules issues de valves non calcifiées. De même, la culture des hVIC en milieu pro-calcifiant a conduit à une augmentation de l’expression du canal. Enfin, l’irradiation des hVIC à une dose de 8 Gy a entrainé une augmentation de la surface cellulaire associée à une sénescence cellulaire qui est TRPM4-dépendante.Dans l’ensemble, ce travail a permis de mettre en évidence le canal TRPM4 comme un nouvel acteur dans la différenciation des hVIC
TRPM4 is a non-selective cation channel activated by intracellular calcium involved in the remodeling of human cardiac fibroblasts. These cells share properties with the valvular interstitial cells (VIC), the main cells that compose the aortic valves, including their capacity for phenotypic transdifferentiation. Osteogenic differentiation is a key element in valve mineralization leading to calcified aortic stenosis. TRPM4 channel expression is increased in aortic valves from patients with calcified aortic stenosis suggesting that TRPM4 could be involved in the valve’s calcification process. Furthermore, this channel is involved in radiation-induced aortic valve remodeling in mouse. However, the cellular actors and TRPM4-dependant pathways involved in this remodeling remain unknown. The purpose of this study was to search for potential implication of TRPM4 in osteogenic differentiation of human VIC (hVIC) and underlying molecular mechanisms.This work demonstrated that the TRPM4 channel is preferentially expressed at the plasma membrane and a typical electrophysiological signature of TRPM4 was recorded on hVIC. TRPM4 is involved in the osteogenic differentiation of hVIC both in cells originating from calcified valves and in hVIC originating from non-calcified valves. This effect occured through the activation of various signaling pathways, including a TRPM4-dependent activation of the BMP2/SMAD1/5 pathway and the NFAT pathway. Moreover, we were able to observe a stronger expression of the TRPM4 channel in hVIC from calcified valves compared to cells from non-calcified valves. Likewise, the culture of hVIC in a pro-calcifying medium led to an increase in the expression of the channel. Finally, hVIC radiation at a dose of 8 Gy resulted in an increase in cell surface area associated to a cellular senescence which are both TRPM4-dependent.Overall, this work highlights the TRPM4 channel as a new player in the differentiation of hVIC

"During heart valve remodeling, and in many disease states, valvular interstitial cells (VICs) shift to an activated myofibroblast phenotype which is characterized by enhanced synthetic and contractile activity. Pronounced alpha smooth muscle actin (alpha-SMA)-containing stress fibers, the hallmark of activated myofibroblasts, are also observed when VICs are placed under tension due to altered mechanical loading in vivo or during in vitro culture on stiff substrates or under high mechanical loads and in the presence of transforming growth factor-beta 1 (TGF-beta 1). The work presented herein describes three distinct model systems for application of controlled mechanical environment to VICs cultured in vitro. The first system uses polyacrylamide (PA) gels of defined stiffness to evaluate the response of VICs over a large range of stiffness levels and TGF-beta 1 concentration. The second system controls the boundary stiffness of cell-populated gels using springs of defined stiffness. The third system cyclically stretches soft or stiff two-dimensional (2D) gels while cells are cultured on the gel surface as it is deformed. Through the use of these model systems, we have found that the level of 2D stiffness required to maintain the quiescent VIC phenotype is potentially too low for a material to both act as matrix to support cell growth in the non-activated state and also to withstand the mechanical loading that occurs during the cardiac cycle. Further, we found that increasing the boundary stiffness on a three-dimensional (3D) cell populated collagen gel resulted in increased cellular contractile forces, alpha-SMA expression, and collagen gel (material)stiffness. Finally, VIC morphology is significantly altered in response to stiffness and stretch. On soft 2D substrates, VICs cultured statically exhibit a small rounded morphology, significantly smaller than on stiff substrates. Following equibiaxial cyclic stretch, VICs spread to the extent of cells cultured on stiff substrates, but did not reorient in response to uniaxial stretch to the extent of cells stretched on stiff substrates. These studies provide critical information for characterizing how VICs respond to mechanical stimuli. Characterization of these responses is important for the development of tissue engineered heart valves and contributes to the understanding of the role of mechanical cues on valve pathology and disease onset and progression. While this work is focused on valvular interstitial cells, the culture conditions and methods for applying mechanical stimulation could be applied to numerous other adherent cell types providing information on the response to mechanical stimuli relevant for optimizing cell culture, engineered tissues or fundamental research of disease states."

Aortic valve stenosis (AVS) involves the transformation of valvular interstitial cells (VIC) into an osteoblastic phenotype. Such valvular disease is mostly associated with both thickening and calcification of the valve cusps, which is accompanied by inflammation and remodeling of the tissue. This process is mediated by the VIC that carry out an impressive array of functions throughout the calcification process. For this dissertation, I hypothesized that in AVS, VIC transform from a myofibroblast phenotype to osteoblast-like cells and that the canonical Wnt and TGFβ pathways and vitamin D3 interactively and collaboratively contribute to these phenomena. In order to test this hypothesis, I established an in vitro model of calcification by culturing human primary VIC in a pro-calcification conditioned medium. Calcified cells display several molecular characteristic features of human AVS, including increased levels of alkaline phosphatase and the formation of calcium nodules. These changes increased over time and peaked at 28 days of treatment. To define possible mechanisms of AVS, I first characterized human VIC in regards to the process of calcification. I showed for the first time in vitro that these VIC express bone specific markers, the characteristic of normal osteoblasts. To determine the factors involved in osteoblastic transformation in this model, I examined WNT3A and TGFβ, known to be involved in normal bone formation. Both calcified human aortic valve tissues and VIC express excess WNT3A and TGFβ1. Adding WNT3A and TGFβ1 to the VIC cultures increased the levels of cell mineralization. Further, the addition of DKK1, the WNT3A antagonist, decreased VIC calcification in vitro. By using various combinations of WNT3A, TGFB1 and DKK1, I made the novel observation that the suppression of DKK1 by TGFB1 allowed WNT3A to drive calcification in VIC in vitro. Finally, I examined the role vitamin D3 that is associated with vascular calcification in rats. Vitamin D3 can up-regulate VIC calcification in vitro, however its mechanism of action appears to be independent of the Wnt and TGFβ pathways. In conclusion, the canonical Wnt and TGFβ pathways function interactively through DKK1 to transform VIC to osteoblast-like cells and vitamin D3 promotes this process in an independent manner.
Medicine, Faculty of
Pathology and Laboratory Medicine, Department of
Graduate

During heart valve remodeling, and in many disease states, valvular interstitial cells (VICs) shift to an activated myofibroblast phenotype which is characterized by enhanced synthetic and contractile activity. Pronounced alpha smooth muscle actin (αSMA)-containing stress fibers, the hallmark of activated myofibroblasts, are also observed when VICs are placed under tension due to altered mechanical loading in vivo or during in vitro culture on stiff substrates or under high mechanical loads and in the presence of transforming growth factor-beta1 (TGF-β1). The work presented herein describes three distinct model systems for application of controlled mechanical environment to VICs cultured in vitro. The first system uses polyacrylamide (PA) gels of defined stiffness to evaluate the response of VICs over a large range of stiffness levels and TGF-β1 concentration. The second system controls the boundary stiffness of cell-populated gels using springs of defined stiffness. The third system cyclically stretches soft or stiff two-dimensional (2D) gels while cells are cultured on the gel surface as it is deformed. Through the use of these model systems, we have found that the level of 2D stiffness required to maintain the quiescent VIC phenotype is potentially too low for a material to both act as matrix to support cell growth in the non-activated state and also to withstand the mechanical loading that occurs during the cardiac cycle. Further, we found that increasing the boundary stiffness on a three-dimensional (3D) cell populated collagen gel resulted in increased cellular contractile forces, αSMA expression, and collagen gel (material) stiffness. Finally, VIC morphology is significantly altered in response to stiffness and stretch. On soft 2D substrates, VICs cultured statically exhibit a small rounded morphology, significantly smaller than on stiff substrates. Following equibiaxial cyclic stretch, VICs spread to the extent of cells cultured on stiff substrates, but did not reorient in response to uniaxial stretch to the extent of cells stretched on stiff substrates. These studies provide critical information for characterizing how VICs respond to mechanical stimuli. Characterization of these responses is important for the development of tissue engineered heart valves and contributes to the understanding of the role of mechanical cues on valve pathology and disease onset and progression. While this work is focused on valvular interstitial cells, the culture conditions and methods for applying mechanical stimulation could be applied to numerous other adherent cell types providing information on the response to mechanical stimuli relevant for optimizing cell culture, engineered tissues or fundamental research of disease states.

Aortic valve disease (AVD) is a large contributor to health costs in the United States affecting 2.8% of the population greater than 75 years old. With a growing elderly population due to medical advances, AVD will continue to rise in prevalence over time. Current treatments for AVD are insufficient due to a lack of preventative therapies and the bioprosthetic valves used for surgical replacement have major limitations. Tissue engineered heart valves (TEHVs) present an ideal solution to current AVD needs because of their biocompatibility, capability to integrate with the host’s tissue, and ability to utilize the natural repair mechanisms of the body. To achieve this goal, we designed synthetic environments with specific cell phenotypes and scaffold properties in order to direct cellular behavior and tissue growth in vitro. In this work cell subpopulations, mechanical stiffness of the substrate, and material surface charge were all studied to understand how the primary cells of the aortic valve, valvular interstitial cells (VICs), were affected by specific environmental cues. These studies were then translated from monolayer culture into a three-dimensional hydrogel system for the study of VICs in a more physically relevant cell culture system.

Differently loaded regions of the mitral valve contain distinct amounts and proportions of glycosaminoglycans (GAGs) and proteoglycans (PGs); these GAG/PG profiles are altered in abnormal loading conditions such as myxomatous degeneration. However, the role of mechanical stimulation on GAG and PG synthesis by valvular interstitial cells (VICs) is still unclear. This research analyzed first the PGs in differently loaded regions of mitral valve (leaflet and chordae) and then the effects of mechanical strains on GAG and PG synthesis by VICs using an in vitro 3-dimensional tissue-engineering model to develop a deeper understanding of valve mechanobiology. This original research investigated the specific PGs present in human mitral valves and found that the regions in compression (leaflets) are rich in versican and regions in tension (chordae) are rich in decorin and biglycan; these PGs were also detected in the engineered tissues seeded with VICs. Applying constraint increased the synthesis of decorin, biglycan and 4-sulfated GAGs. Constraint also increased versican secretion but reduced its retention within the engineered tissues. The application of constraint was found to be more influential than the directionality (biaxial vs. uniaxial) of strain. Constrained collagen gels containing leaflet cells retained more decorin and biglycan than did those containing chordal cells. The application of cyclic strains decreased the total GAG synthesis, increased the proportions of 4-sulfated GAGs, and reduced the proportions of hyaluronan. Synthesis of the PG versican was increased by leaflet cells and decreased by chordal cells in response to cyclic strain. Chordal cells were found to be more responsive to cyclic strains than leaflet cells, which has implications in the dramatic remodeling of myxomatous chordae tendineae. Synthesis of total GAGs, 4-sulfated GAGs and decorin was found to be strain dependent, whereas synthesis of versican and decorin was frequency dependent. In general, VICs within collagen gels synthesize GAG in proportions and amounts close to that of native valve tissue. This research is the first to show that strains can modulate GAG/PG synthesis by valve cells. These results provide insight into valve mechanobiology and pathology and have implications for understanding the remodeling process of many soft tissues.

The gold standard treatment for patients with AVD is surgical replacement of the aortic valve with either mechanical or fixed tissue prostheses. These implants have a limited lifespan and are associated with serious adverse events. Patient autologous tissue engineered heart valves (TEHVs) offer a solution. Vital to the development of a TEHV is determining a source of donor tissue(s) that most closely mimics the native valve tissue. In pursuit of determining an alternative cell source for patient autologous TEHVs we compared a number of phenotypic and genotypic characteristics of atrial fibroblasts, dermal fibroblasts and differentiated bone marrow-derived progenitor cells (BMCs) and made a comparison to valvular interstitial cells (VICS). We demonstrate that while VICs share some phenotypic similarities with fibroblasts and BMCs, they also possess unique characteristics and demonstrate differential mRNA expression of key regulatory pathways that may influence their phenotype.
October 2016

Calcific aortic valve disease (CAVD) occurs through multiple mutually non-exclusive mechanisms that are mediated by valvular interstitial cells (VICs). VICs undergo pathological differentiation during the progression of valve calcification; however the factors that regulate cellular differentiation are not well defined. Most commonly recognized are biochemical factors that induce pathological differentiation, but little is known regarding the biochemical factors that may suppress this process. Further, the contribution of matrix mechanics in valve pathology has been overlooked, despite increasing evidence of close relationships between changes in tissue mechanics, disease progression and the regulation of cellular response. In this thesis, the effect of matrix stiffness on the differentiation of and calcification by VICs in response to pro-calcific and anti-calcific biochemical factors was investigated. Matrix stiffness modulated the response of VICs to pro-calcific factors, leading to two distinct calcification processes. VICs cultured on the more compliant matrices underwent calcification via osteoblast differentiation, whereas those cultured on the stiffer matrices were prone to myofibroblast differentiation. The transition of fibroblastic VICs to myofibroblasts increased cellular contractility, which led to contraction-mediated, apoptosis-dependent calcification. In addition, C-type natriuretic peptide (CNP), a putative protective molecule against CAVD, was identified. CNP supressed myofibroblast and osteoblast differentiation of VICs, and thereby inhibited calcification in vitro. Matrix stiffness modulated the expression of CNP-regulated transcripts, with only a small number of CNP-regulated transcripts not being sensitive to matrix mechanics. These data demonstrate the combined effects of mechanical and biochemical cues in defining VIC phenotype and responses, with implications for the interpretation of in vitro models of VIC calcification and possibly disease devleopment. The findings from this thesis emphasize the necessity to consider both biochemical and mechanical factors in order to improve fundamental understanding of VIC biology.

The aortic valve maintains unidirectional blood flow between the left ventricle and the systemic circulation. When diseased, the valve is replaced either by a mechanical or a bioprosthetic heart valve, that carry issues such as thrombogenesis, long term structural failure, and calcification, necessitating the development of more structurally and biologically sufficient long-term replacements. Tissue engineering provides a possible avenue for development, combining cells, scaffolds, and biochemical factors to regenerate tissue. The overall goal of this dissertation was to create a foundation for the rational design of a tissue engineered aortic valve. The novel approach taken in this thesis research was to view each of the three leaflets as a laminate structure. The first three aims consider the leaflet as a laminate structure comprising of layers of collagen, elastin, and glycosaminoglycans (GAGs). In the first aim, the effect of GAGs on the tensile properties and stress relaxation in the leaflet was investigated, by removing GAGs through increasing amounts of hyaluronidase. A decrease in GAGs led to significantly higher elastic moduli, maximum stresses, and hysteresis in the leaflet. In the second aim, the 3D elastic fiber network of the leaflet was characterized using immunohistochemistry and scanning electron microscopy. This structure was found to have regionally varying thicknesses and patterns. In the third aim, a novel hydrogel-fiber composite design was proposed to match the anisotropy of the leaflet. This composite composed of aligned electrospun poly(ε-caprolactone) (PCL) within a poly(ethylene glycol) diacrylate (PEGDA) matrix. Surface modification and embedding of the PCL did not significantly alter the anisotropy or strength of the underlying PCL scaffold, providing the basis for an anisotropic, biocompatible scaffold. In the last aim, a novel co-culture model was designed using magnetic levitation as a layered structure of valvular endothelial cells and interstitial cells. This technique was used to create co-culture models within hours, while maintaining cell phenotype and function, and inducing extracellular matrix formation, as shown by immunohistochemical stains and their gene expression profiling. The overall result of this dissertation is a clearer understanding of the layered structure-function relationship of the aortic valve, and its application towards heart valve tissue engineering.