Cell composition of the human pulmonary valve: a comparative study with the aortic valve--the VESALIO Project. Vitalitate Exornatum Succedaneum Aorticum labore Ingegnoso Obtinebitur

Strategies for Development of Synthetic Heart Valve Tissue Engineering Scaffolds

The current clinical solutions, including mechanical and bioprosthetic valves for valvular heart diseases, are plagued by coagulation, calcification, nondurability, and the inability to grow with patients. The tissue engineering approach attempts to resolve these shortcomings by producing heart valve scaffolds that may deliver patients a life-long solution. Heart valve scaffolds serve as a three-dimensional support structure made of biocompatible materials that provide adequate porosity for cell infiltration, and nutrient and waste transport, sponsor cell adhesion, proliferation, and differentiation, and allow for extracellular matrix production that together contributes to the generation of functional neotissue. The foundation of successful heart valve tissue engineering is replicating native heart valve architecture, mechanics, and cellular attributes through appropriate biomaterials and scaffold designs. This article reviews biomaterials, the fabrication of heart valve scaffolds, and their in-vitro and in-vivo evaluations applied for heart valve tissue engineering.

The effects of strain history on aortic valve interstitial cell activation in a 3D hydrogel environment

Aortic valves (AVs) undergo unique stretch histories that include high rates and magnitudes. While major differences in deformation patterns have been observed between normal and congenitally defective bicuspid aortic valves (BAVs), the relation to underlying mechanisms of rapid disease onset in BAV patients remains unknown. To evaluate how the variations in stretch history affect AV interstitial cell (AVIC) activation, high-throughput methods were developed to impart varied cyclical biaxial stretch histories into 3D poly(ethylene) glycol hydrogels seeded with AVICs for 48 h. Specifically, a physiologically mimicking stretch history was compared to two stretch histories with varied peak stretch and stretch rate. Post-conditioned AVICs were imaged for nuclear shape, alpha smooth muscle actin (αSMA) and vimentin (VMN) polymerization, and small mothers against decapentaplegic homologs 2 and 3 (SMAD 2/3) nuclear activity. The results indicated that bulk gel deformations were accurately transduced to the AVICs. Lower peak stretches lead to increased αSMA polymerization. In contrast, VMN polymerization was a function of stretch rate, with SMAD 2/3 nuclear localization and nuclear shape also trending toward stretch rate dependency. Lower than physiological levels of stretch rate led to higher SMAD 2/3 activity, higher VMN polymerization around the nucleus, and lower nuclear elongation. αSMA polymerization did not correlate with VMN polymerization, SMAD 2/3 activity, nor nuclear shape. These results suggest that a negative feedback loop may form between SMAD 2/3, VMN, and nuclear shape to maintain AVIC homeostatic nuclear deformations, which is dependent on stretch rate. These novel results suggest that AVIC mechanobiological responses are sensitive to stretch history and provide insight into the mechanisms of AV disease.

Carbon Nanotubes Substrates Alleviate Pro-Calcific Evolution in Porcine Valve Interstitial Cells

The quest for surfaces able to interface cells and modulate their functionality has raised, in recent years, the development of biomaterials endowed with nanocues capable of mimicking the natural extracellular matrix (ECM), especially for tissue regeneration purposes. In this context, carbon nanotubes (CNTs) are optimal candidates, showing dimensions and a morphology comparable to fibril ECM constituents. Moreover, when immobilized onto surfaces, they demonstrated outstanding cytocompatibility and ease of chemical modification with ad hoc functionalities. In this study, we interface porcine aortic valve interstitial cells (pVICs) to multi-walled carbon nanotube (MWNT) carpets, investigating the impact of surface nano-morphology on cell properties. The results obtained indicate that CNTs significantly affect cell behavior in terms of cell morphology, cytoskeleton organization, and mechanical properties. We discovered that CNT carpets appear to maintain interfaced pVICs in a sort of “quiescent state”, hampering cell activation into a myofibroblasts-like phenotype morphology, a cellular evolution prodromal to Calcific Aortic Valve Disease (CAVD) and characterized by valve interstitial tissue stiffening. We found that this phenomenon is linked to CNTs’ ability to alter cell tensional homeostasis, interacting with cell plasma membranes, stabilizing focal adhesions and enabling a better strain distribution within cells. Our discovery contributes to shedding new light on the ECM contribution in modulating cell behavior and will open the door to new criteria for designing nanostructured scaffolds to drive cell functionality for tissue engineering applications.

Role of p38 MAPK in Atherosclerosis and Aortic Valve Sclerosis

Atherosclerosis and aortic valve sclerosis are cardiovascular diseases with an increasing prevalence in western societies. Statins are widely applied in atherosclerosis therapy, whereas no pharmacological interventions are available for the treatment of aortic valve sclerosis. Therefore, valve replacement surgery to prevent acute heart failure is the only option for patients with severe aortic stenosis. Both atherosclerosis and aortic valve sclerosis are not simply the consequence of degenerative processes, but rather diseases driven by inflammatory processes in response to lipid-deposition in the blood vessel wall and the aortic valve, respectively. The p38 mitogen-activated protein kinase (MAPK) is involved in inflammatory signaling and activated in response to various intracellular and extracellular stimuli, including oxidative stress, cytokines, and growth factors, all of which are abundantly present in atherosclerotic and aortic valve sclerotic lesions. The responses generated by p38 MAPK signaling in different cell types present in the lesions are diverse and might support the progression of the diseases. This review summarizes experimental findings relating to p38 MAPK in atherosclerosis and aortic valve sclerosis and discusses potential functions of p38 MAPK in the diseases with the aim of clarifying its eligibility as a pharmacological target.

Sheep-Specific Immunohistochemical Panel for the Evaluation of Regenerative and Inflammatory Processes in Tissue-Engineered Heart Valves

The creation of living heart valve replacements via tissue engineering is actively being pursued by many research groups. Numerous strategies have been described, aimed either at culturing autologous living valves in a bioreactor (in vitro) or inducing endogenous regeneration by the host via resorbable scaffolds (in situ). Whereas a lot of effort is being invested in the optimization of heart valve scaffold parameters and culturing conditions, the pathophysiological in vivo remodeling processes to which tissue-engineered heart valves are subjected upon implantation have been largely under-investigated. This is partly due to the unavailability of suitable immunohistochemical tools specific to sheep, which serves as the gold standard animal model in translational research on heart valve replacements. Therefore, the goal of this study was to comprise and validate a comprehensive sheep-specific panel of antibodies for the immunohistochemical analysis of tissue-engineered heart valve explants. For the selection of our panel we took inspiration from previous histopathological studies describing the morphology, extracellular matrix composition and cellular composition of native human heart valves throughout development and adult stages. Moreover, we included a range of immunological markers, which are particularly relevant to assess the host inflammatory response evoked by the implanted heart valve. The markers specifically identifying extracellular matrix components and cell phenotypes were tested on formalin-fixed paraffin-embedded sections of native sheep aortic valves. Markers for inflammation and apoptosis were tested on ovine spleen and kidney tissues. Taken together, this panel of antibodies could serve as a tool to study the spatiotemporal expression of proteins in remodeling tissue-engineered heart valves after implantation in a sheep model, thereby contributing to our understanding of the in vivo processes which ultimately determine long-term success or failure of tissue-engineered heart valves.

Recellularization of decellularized heart valves: Progress toward the tissue-engineered heart valve

The tissue-engineered heart valve portends a new era in the field of valve replacement. Decellularized heart valves are of great interest as a scaffold for the tissue-engineered heart valve due to their naturally bioactive composition, clinical relevance as a stand-alone implant, and partial recellularization in vivo. However, a significant challenge remains in realizing the tissue-engineered heart valve: assuring consistent recellularization of the entire valve leaflets by phenotypically appropriate cells. Many creative strategies have pursued complete biological valve recellularization; however, identifying the optimal recellularization method, including in situ or in vitro recellularization and chemical and/or mechanical conditioning, has proven difficult. Furthermore, while many studies have focused on individual parameters for increasing valve interstitial recellularization, a general understanding of the interacting dynamics is likely necessary to achieve success. Therefore, the purpose of this review is to explore and compare the various processing strategies used for the decellularization and subsequent recellularization of tissue-engineered heart valves.

In Vitro Model of a Fibrosa Layer of a Heart Valve

The fibrosa layer of a cardiac aortic valve is composed mostly of a dense network of type I collagen fibers oriented in circumferential direction. This main layer bears the tensile load and responds to the high stress on a leaflet. The inner fibrosa layer is also the site of pathophysiologic changes that result in valvular dysfunction, including stenosis and regurgitation. In vitro studies of these changes are limited by the absence of a substrate that mimics the circumferentially oriented structure of the fibrosa layer. In heart valve tissue engineering, generation of this layer is challenging. This study aimed to develop an artificial fibrosa layer of a native aortic leaflet. A unique morphologically biomimicked, pliable, but standalone substrate with circumferentially oriented nanofibers was fabricated by electrospinning on a novel collector designed for this study. The substrate had low-bulk tensile stiffness and ultimate strength; thus, cultured valvular interstitial cells (VICs) showed a fibroblast phenotype that is generally observed in a healthy aortic leaflet. Furthermore, gene and protein expression and morphology of VICs in substrates were close to those in the fibrosa layer of a native aortic leaflet. This artificial fibrosa layer can be useful for in vitro studies of valvular dysfunctions.

Revisiting cardiovascular calcification: A multifaceted disease requiring a multidisciplinary approach

The presence of cardiovascular calcification significantly predicts patients' morbidity and mortality. Calcific mineral deposition within the soft cardiovascular tissues disrupts the normal biomechanical function of these tissues, leading to complications such as heart failure, myocardial infarction, and stroke. The realization that calcification results from active cellular processes offers hope that therapeutic intervention may prevent or reverse the disease. To this point, however, no clinically viable therapies have emerged. This may be due to the lack of certainty that remains in the mechanisms by which mineral is deposited in cardiovascular tissues. Gaining new insight into this process requires a multidisciplinary approach. The pathological changes in cell phenotype that lead to the physicochemical deposition of mineral and the resultant effects on tissue biomechanics must all be considered when designing strategies to treat cardiovascular calcification. In this review, we overview the current cardiovascular calcification paradigm and discuss emerging techniques that are providing new insight into the mechanisms of ectopic calcification.

Modulation of human valve interstitial cell phenotype and function using a fibroblast growth factor 2 formulation

Valve interstitial cells (VICs) are fibroblastic in nature however in culture it is widely accepted that they differentiate into a myofibroblastic phenotype. This study assessed a fibroblast culture media formulation for its ability to maintain the phenotype and function of VICs as in the intact healthy valve. Normal human VICs were cultured separately in standard DMEM and in fibroblast media consisting of FGF2 (10ng/ml), insulin (50ng/ml) and 2% FCS for at least a week. Cell morphology, aspect ratio, size, levels and distribution of protein expression, proliferation, cell cycle, contraction and migration were assessed. Some VICs and some valve endothelial cells expressed FGF2 in valve tissue and this expression was increased in calcified valves. VICs in DMEM exhibited large, spread cells whereas VICs in fibroblast media were smaller, elongated and spindly. Aspect ratio and size were both significantly higher in DMEM (p<0.01). The level of expression of α-SMA was significantly reduced in fibroblast media at day 2 after isolation (p<0.01) and the expression of α-SMA, SM22 and EDA-fibronectin was significantly reduced in fibroblast media at days 7 and 12 post-isolation (p<0.01). Expression of cytoskeletal proteins, bone marker proteins and extracellular matrix proteins was reduced in fibroblast media. Proliferation of VICs in fibroblast media was significantly reduced at weeks 1 (p<0.05) and 2 (p<0.01). Collagen gel contraction was significantly reduced in fibroblast media (p<0.05). VICs were found to have significantly fewer and smaller focal adhesions in fibroblast media (p<0.01) with significantly fewer supermature focal adhesions in fibroblast media (p<0.001). Ultrastructurally, VICs in fibroblast media resembled native VICs from intact valves. VICs in fibroblast media demonstrated a slower migratory ability after wounding at 72 hours (p<0.01). Treatment of human VICs with this fibroblast media formulation has the ability to maintain and to dedifferentiate the VICs back to a fibroblastic phenotype with phenotypic and functional characteristics ascribed to cells in the intact valve. This methodology is fundamental in the study of normal valve biology, pathology and in the field of tissue engineering.

Expression of smooth muscle cell markers and co-activators in calcified aortic valves

AIMS

Similar risk factors and mediators are involved in calcific aortic stenosis (CAS) and atherosclerosis. Since normal valves harbour a low percentage of smooth muscle cells (SMCs), we hypothesize that the SMC phenotype participates in the pathogenesis of CAS.

METHOD AND RESULTS

We analysed 12 normal and 22 calcified aortic valves for SMC markers and the expression of co-activators of SMC gene expression, myocardin and myocardin-related transcription factors (MRTF-A/B). Transforming growth factor β (TGFβ1) was used to upregulate SMC markers and co-activators in valve interstitial cells (VICs) and transmission electron microscopy (TEM) was used to detect the presence of SMC in atypical regions of the valve leaflets. Smooth muscle cell markers and co-activators, myocardin, MRTF-A, and MRTF-B, demonstrated an increased incidence and aberrant expression around calcified nodules in all 22 calcified valves as well as in surface and microvessel endothelial cells. Smooth muscle cell markers and MRTF-A were significantly increased in calcified valves. Transforming growth factor β1 (TGFβ1) (10 ng/mL) was able to significantly upregulate the expression of some SMC markers and MRTF-A in VICs. Transmission electron microscopy of the fibrosa layer of calcified valves demonstrated the presence of bundles of SMCs and smooth muscle-derived foam cells.

CONCLUSION

Smooth muscle cell markers and co-activators, myocardin and MRTFs, were aberrantly expressed in calcified valves. Transforming growth factor β1 was able to significantly upregulate SMC markers and MRTF-A in VICs. Transmission electron microscopy unequivocally identified the presence of SMCs in calcified regions of valve leaflets. These findings provide evidence that the SMC phenotype plays a role in the development of CAS.

Culture and characterisation of canine mitral valve interstitial and endothelial cells

Valve interstitial cells (VICs) have an important role in the aetiopathogenesis of myxomatous mitral valve disease (MMVD) in the dog. Furthermore, there is evidence that valve endothelial cells (VECs) also contribute to disease development. In addition to examining native valve tissue to understand MMVD, another strategy is to separately examine VIC and VEC biology under in vitro culture conditions. The aim of this study was to isolate and characterise canine mitral VICs and VECs from normal dog valves using a combination of morphology, immunohistochemistry and reverse transcription PCR (RT-PCR). Canine mitral VECs and VICs were isolated and cultured in vitro. The two cell populations exhibited different morphologies and growth patterns. VECs, but not VICs, expressed the endothelial markers, platelet endothelial cell adhesion molecule (PECAM-1 or CD31) and acetylated low density lipoprotein (Dil-Ac-LDL). Both VECs and VICs expressed vimentin and embryonic non-smooth muscle myosin heavy chain (SMemb), an activated mesenchymal cell marker. The myofibroblast marker, alpha smooth muscle actin (α-SMA), was detected at the mRNA level in both VEC and VIC cultures, but only at the protein level in VIC cultures. The morphological heterogeneity and expression of non-endothelial phenotypic markers in VEC cultures suggested that a mixture of cell types was present, which might be due to cell contamination and/or endothelial-mesenchymal transition (EndoMT). The use of a specific endothelial culture medium for primary VEC cultures enhanced the endothelial properties of the cells and reduced α-SMA and SMemb expression.

Engineering patient-specific valves using stem cells generated from skin biopsy specimens

BACKGROUND

Pediatric patients requiring valve replacement will likely require reoperations due to a progressive deterioration of valve durability and limited repair and growth potential. To address these concerns, we sought to generate a biologically active pulmonary valve using patient-specific valvular cells and decellularized human pulmonary valves.

METHODS

We generated induced pluripotent stem cells (iPSCs) by reprogramming skin fibroblast cells. We then differentiated iPSCs to mesenchymal stem cells (iPCSs-MSCs) using culture conditions that favored an epithelial-to-mesenchymal transition. Next, decellularized human pulmonary heart valves were seeded with iPCS-MSCs using a combination of static and dynamic culture conditions and cultured up to 30 days.

RESULTS

The iPSCs-MSCs displayed cluster of differentiation CD105 and CD90 expression exceeding 90% after four passages and could differentiate into osteocytes, chondrocytes, and adipocytes (n = 4). Consistent with an MSC phenotype, iPSCs-MSCs lacked expression of CD45 and CD34. Compared with bone marrow MSCs, iPSCs-MSC proliferated more readily by twofold but maintained a gene expression profile exceeding 80% identical to bone marrow MSCs. In repopulated pulmonary valves compared with decellularized pulmonary valves, immunohistochemistry demonstrated increased cellularity, α-smooth muscle actin expression, and increased presence of extracellular matrix components, such as proteoglycans and glycosaminoglycans, suggesting sustained cell function and maturation.

CONCLUSIONS

Our results demonstrate the feasibility of constructing a biologically active human pulmonary valve using a sustainable and proliferative cell source. The bioactive pulmonary valve is expected to have advantages over existing valvular replacements, which will require further validation.

A three-dimensional co-culture model of the aortic valve using magnetic levitation

The aortic valve consists of valvular interstitial cells (VICs) and endothelial cells (VECs). While these cells are understood to work synergistically to maintain leaflet structure and valvular function, few co-culture models of these cell types exist. In this study, aortic valve co-cultures (AVCCs) were assembled using magnetic levitation and cultured for 3 days. Immunohistochemistry and quantitative reverse-transcriptase polymerase chain reaction were used to assess the maintenance of cellular phenotype and function, and the formation of extracellular matrix. AVCCs stained positive for CD31 and α-smooth muscle actin (αSMA), demonstrating that the phenotype was maintained. Functional markers endothelial nitric oxide synthase (eNOS), von Willebrand factor (VWF) and prolyl-4-hydroxylase were present. Extracellular matrix components collagen type I, laminin and fibronectin also stained positive, with reduced gene expression of these proteins in three dimensions compared to two dimensions. Genes for collagen type I, lysyl oxidase and αSMA were expressed less in AVCCs than in 2-D cultures, indicating that VICs are quiescent. Co-localization of CD31 and αSMA in the AVCCs suggests that endothelial-mesenchymal transdifferentiation might be occurring. Differences in VWF and eNOS in VECs cultured in two and three dimensions also suggests that the AVCCs possibly have anti-thrombotic potential. Overall, a co-culture model of the aortic valve was designed, and serves as a basis for future experiments to understand heart valve biology.

Investigating the role of substrate stiffness in the persistence of valvular interstitial cell activation

During heart valve remodeling and in many disease states, valvular interstitial cells (VICs) shift to an activated myofibroblast phenotype characterized by enhanced synthetic and contractile activity. Pronounced alpha smooth muscle actin (αSMA)-positive stress fibers, the hallmark of activated myofibroblasts, are also observed in VICs cultured on stiff substrates especially in the presence of transforming growth factor-beta1 (TGF-β1), however, the detailed relationship between stiffness and VIC phenotype has not been explored. The goal of this study was to characterize VIC activation as a function of substrate stiffness over a wide range of stiffness levels including that of diseased valves (stiff), normal valves (compliant), and hydrogels for heart valve tissue engineering (very soft). VICs obtained from porcine aortic valves were cultured on stiff tissue culture plastic to activate them, then, cultured on collagen-coated polyacrylamide substrates of predefined stiffness in a high-throughput culture system to assess the persistence of activation. Metrics extracted from regression analysis demonstrate that relative to a compliant substrate, stiff substrates result in higher cell numbers, more pronounced expression of αSMA-positive stress fibers, and larger spread area which is in qualitative agreement with previous studies. Our data also indicate that VICs require a much lower substrate stiffness level to "deactivate" them than previously thought. The high sensitivity of VICs to substrate stiffness demonstrates the importance of the mechanical properties of materials used for valve repair or for engineering valve tissue.

Anisotropic strain transfer through the aortic valve and its relevance to the cellular mechanical environment

Aortic valve interstitial cells are responsible for maintaining the valve in response to their local mechanical environment. However, the complex organization of the extracellular matrix means cell strains cannot be directly derived from gross strains, and knowledge of tissue structure-function correlations is fundamental towards understanding mechanotransduction. This study investigates strain transfer through the valve, hypothesizing that organization of the valve matrix leads to non-homogenous local strains. Radial and circumferential samples were cut from aortic valve leaflets and subjected to quasi-static mechanical characterization. Further samples were imaged using confocal microscopy, to determine local strains in the matrix. Mechanical data demonstrated that the valve was significantly stronger and stiffer when loaded circumferentially, comparable with previous studies. Micromechanical studies demonstrated that strain transfer through the matrix is anisotropic and indirect, with local strains consistently smaller than applied strains in both orientations. Under radial loading, strains were transferred linearly to cells. However, under circumferential loading, strains were only one-third of applied values, with a less direct relationship between applied and local strains. This may result from matrix reorganization, and be important for preventing cellular damage during normal valve function. These findings should be taken into account when investigating interstitial cell behaviours, such as cell metabolism and mechanotransduction.

The effects of combined cyclic stretch and pressure on the aortic valve interstitial cell phenotype

Aortic valve interstitial cells (VIC) can exhibit phenotypic characteristics of fibroblasts, myofibroblasts, and smooth muscle cells. Others have proposed that valve cells become activated and exhibit myofibroblast or fibroblast characteristics during disease initiation and progression; however, the cues that modulate this phenotypic change remain unclear. We hypothesize that the mechanical forces experienced by the valve play a role in regulating the native phenotype of the valve and that altered mechanical forces result in an activated phenotype. Using a novel ex vivo cyclic stretch and pressure bioreactor, we subjected porcine aortic valve (AV) leaflets to combinations of normal and pathological stretch and pressure magnitudes. The myofibroblast markers α-SMA and Vimentin, along with the smooth muscle markers Calponin and Caldesmon, were analyzed using immunohistochemistry and immunoblotting. Tissue structure was analyzed using Movat's pentachrome staining. We report that pathological stretch and pressure inhibited the contractile and possibly myofibroblast phenotypes as indicated by downregulation of the proteins α-SMA, Vimentin, and Calponin. In particular, Calponin downregulation implies depolymerization of actin filaments and possible conversion to a more synthetic (non-contractile) phenotype. This agreed well with the increase in spongiosa and fibrosa thickness observed under elevated pressure and stretch that are typically indicative of increased matrix synthesis. Our study therefore demonstrates how cyclic stretch and pressure may possibly act together to modulate the AVIC phenotype.

Three-dimensional quantitative micromorphology of pre- and post-implanted engineered heart valve tissues

There is a significant gap in our knowledge of engineered heart valve tissue (EHVT) development regarding detailed three-dimensional (3D) tissue formation and remodeling from the point of in vitro culturing to full in vivo function. As a step toward understanding the complexities of EHVT formation and remodeling, a novel serial confocal microscopy technique was employed to obtain 3D microstructural information of pre-implant (PRI) and post-implant for 12 weeks (POI) EHVT fabricated from PGA:PLLA scaffolds and seeded with ovine bone-marrow-derived mesenchymal stem cells. Custom scaffold fiber tracking software was developed to quantify scaffold fiber architectural features such as length, tortuosity, and minimum scaffold fiber-fiber separation distance and scaffold fiber orientation was quantified utilizing a 3D fabric tensor. In addition, collagen and cellular density of ovine pulmonary valve leaflet tissue were also analyzed for baseline comparisons. Results indicated that in the unseeded state, scaffold fibers formed a continuous, oriented network. In the PRI state, the scaffold showed some fragmentation with a scaffold volume fraction of 7.79%. In the POI specimen, the scaffold became highly fragmented, forming a randomly distributed short fibrous network (volume fraction of 2.03%) within a contiguous, dense collagenous matrix. Both PGA and PLLA scaffold fibers were observed in the PRI and POI specimens. Collagen density remained similar in both PRI and POI specimens (74.2 and 71.5%, respectively), though the distributions in the transmural direction appeared slightly more homogenous in the POI specimen. Finally, to guide future 2D histological studies for large-scale studies (since acquisition of high-resolution volumetric data is not practical for all specimens), we investigated changes in relevant collagen and scaffold metrics (collagen density and scaffold fiber orientation) with varying section spacing. It was found that a sectioning spacing up to 25 μm (for scaffold morphology) and 50 μm (for collagen density) in both PRI and POI tissues did not result in loss of information fidelity, and that sectioning in the circumferential or radial direction provides the greatest preservation of information. This is the first known work to investigate EHVT microstructure over a large volume with high resolution and to investigate time evolving in vivo EHVT morphology. The important scaffold fiber structural changes observed provide morphological information crucial for guiding future structurally based constitutive modeling efforts focused on better understanding EHVT tissue formation and remodeling.

Extracellular matrix remodeling and cell phenotypic changes in dysplastic and hemodynamically altered semilunar human cardiac valves

INTRODUCTION

Congenital cardiac valve disease is common, affecting ∼1% of the population, with substantial morbidity and mortality, but suboptimal treatment options. Characterization of the specific matrix and valve cell phenotypic abnormalities in these valves could lend insight into disease pathogenesis and potentially pave the way for novel therapies.

METHODS

Thirty-five human aortic and pulmonic valves were categorized based on gross and microscopic assessment into control valves (n=21); dysplastic valves, all except one also displaying hemodynamic changes (HEMO/DYSP, n=6); and hemodynamically altered valves (HEMO, n=8). Immunohistochemistry was performed on valve sections and flow cytometry on valvular interstitial cells.

RESULTS

While both hemodynamically altered aortic and pulmonic valves demonstrated increased collagen turnover and cell activation, prolyl 4-hydroxylase and hyaluronan increased in hemodynamically altered aortic valves but decreased in hemodynamically altered pulmonic valves relative to control valves (P<.001). HEMO/DYSP aortic valves demonstrated decreased collagen and elastic fiber synthesis and turnover compared to both hemodynamically altered aortic valves and control aortic valves (each P<.006). Valvular interstitial cells from both hemodynamically altered and HEMO/DYSP pulmonic valves showed altered cell phenotype compared to control valves (each P<.032), especially increased non-muscle myosin. Furthermore, valvular interstitial cells from hemodynamically altered pulmonic valves and HEMO/DYSP aortic and pulmonic valves each demonstrated greater size and complexity compared to control valves (each P<.05).

CONCLUSIONS

Dysplastic semilunar valves displayed alterations in collagen and elastic fiber turnover that were distinct from valves similarly exposed to altered hemodynamics as well as to control valves. These results demonstrate that dysplastic valves are not simply valves with gross changes or loss of leaflet layers, but contain complex matrix and cell phenotype changes that, with future study, could potentially be targets for novel nonsurgical treatments.

The remodeling of cardiovascular bioprostheses under influence of stem cell homing signal pathways

Optimizing current heart valve replacement strategies by creating living prostheses is a necessity to alleviate complications with current bioprosthetic devices such as calcification and degeneration. Regenerative medicine, mostly in vitro tissue engineering, is the forerunner of this optimization search, yet here we show the functionality of an in vivo alternative making use of 2 homing axes for stem cells. In rats we studied the signaling pathways of stem cells on implanted bioprosthetic tissue (photooxidized bovine pericardium (POP)), by gene and protein expression analysis. We found that SDF-1alpha/CXCR4 and FN/VLA4 homing axes play a role. When we implanted vascular grafts impregnated with SDF-1alpha and/or FN as carotid artery interpositions, primitive cells were attracted from the circulation. Next, bioprosthetic heart valves, constructed from POP impregnated with SDF-1alpha and/or FN, were implanted in pulmonary position. As shown by CD90, CD34 and CD117 immunofluorescent staining they became completely recellularized after 5 months, had a normal function and biomechanical properties and specifically the combination of SDF-1alpha and FN had an optimal valve-cell phenotype.

The influence of heart valve leaflet matrix characteristics on the interaction between human mesenchymal stem cells and decellularized scaffolds

The potential for in vitro colonization of decellularized valves by human bone marrow mesenchymal stem cells (hBM-MSCs) towards the anisotropic layers ventricularis and fibrosa and in homo- vs. heterotypic cell-ECM interactions has never been investigated. hBM-MSCs were expanded and characterized by immunofluorescence and FACS analysis. Porcine and human pulmonary valve leaflets (p- and hPVLs, respectively) underwent decellularization with Triton X100-sodium cholate treatment (TRICOL), followed by nuclear fragment removal. hBM-MSCs (2x10(6) cells/cm(2)) were seeded onto fibrosa (FS) or ventricularis (VS) of decellularized PVLs, precoated with FBS and fibronectin, and statically cultured for 30 days. Bioengineered PVLs revealed no histopathological features but a reconstructed endothelium lining and the presence of fibroblasts, myofibroblasts and SMCs, as in the corresponding native leaflet. The two valve layers behaved differently as regards hBM-MSC repopulation potential, however, with a higher degree of 3D spreading and differentiation in VS than in FS samples, and with enhanced cell survival and colonization effects in the homotypic ventricularis matrix, suggesting that hBM-MSC phenotypic conversion is strongly influenced in vitro by the anisotropic valve microstructure and species-specific matching between extracellular matrix and donor cells. These findings are of particular relevance to in vivo future applications of valve tissue engineering.

Pro-osteogenic phenotype of human aortic valve interstitial cells is associated with higher levels of Toll-like receptors 2 and 4 and enhanced expression of bone morphogenetic protein 2

OBJECTIVES

Our aim was to determine whether aortic valve interstitial cells (AVICs) and pulmonary valve interstitial cells (PVICs) differ in expression of Toll-like receptor (TLR)2 and TLR4, response to TLR agonists, and osteogenic phenotypic changes.

BACKGROUND

Calcific stenosis occurs frequently in aortic valves but rarely in pulmonary valves. Studies have implicated AVICs in the inflammation associated with calcification and progression to stenosis. We previously reported that human AVICs express functional TLR2 and TLR4 and that stimulation of these receptors induces pro-osteogenic factor expression.

METHODS

Human aortic and pulmonary valve leaflets from the same heart were collected and interstitial cells isolated.

RESULTS

Aortic valves express more TLR2 and TLR4, in both tissue and isolated interstitial cells, than pulmonary valves. After stimulation with TLR2 and TLR4 agonists, AVICs express higher levels of pro-inflammatory and pro-osteogenic mediators (bone morphogenetic protein [BMP]-2, runt-related transcription factor 2) and greater osteogenic phenotypic changes (alkaline phosphatase [ALP] activity, calcified nodule formation) than PVICs. Silencing TLR2 and TLR4 in AVICs reduced BMP-2 expression and ALP activity to PVIC levels. ALP activity in AVICs induced by TLR2 and TLR4 agonists was abolished by BMP antagonism with Noggin and mimicked by stimulation with recombinant BMP-2. AVICs isolated from stenotic valves had greater expression of TLR2 and TLR4 and a greater BMP-2 response than AVICs from normal valves.

CONCLUSIONS

Greater expression of TLR2 and TLR4 and greater pro-inflammatory and pro-osteogenic responses to TLR2 and TLR4 agonists in AVICs than PVICs are associated with osteogenic phenotypic changes. These innate immune receptors may play a critical role in aortic valve calcification and stenosis.

Vena cava as autologous tissue for pulmonary valve substitute

In this study, we report on our first experience with the construction of a valve using autologous vena cava tissue for right ventricular outflow tract reconstruction. Simulating the clinical situation valves were built from tubular pieces of porcine inferior vena cava placed in a PTFE tube and investigated in a pulsatile flow simulator. Based on the given vena cava dimensions, conduits were constructed with diameters of 19 mm in bicuspid or tricuspid and 22 mm and 24 mm in bicuspid configuration. The lowest pressure gradients were observed in the 22 mm vena cava valves in bicuspid configuration (8.6+/-0.5 mmHg) compared to 24 mm valves (10.6+/-0.9 mmHg, P=0.0004) and 19 mm valves (13.4+/-1.5 mmHg, P=0.005). No differences could be found between 19 mm bicuspid and tricuspid valves. Concerning valve opening movements, a complete opening in the 19 mm and a nearly unhindered opening in 22 mm valves were registered. In 24 mm valves opening was incomplete. Leakage was increased in 19 mm bicuspid valves due to leaflet prolapse. In conclusion, construction of a valve mechanism from vena cava tissue is feasible. The in-vitro hemodynamic results are encouraging, animal experiments are ongoing to investigate the midterm function of these valves.

Tissue-engineered heart valve prostheses: ‘state of the heart’

In this article, we will review the current state of the art in heart valve tissue engineering. We provide an overview of mechanical and biological replacement options, outlining advantages and limitations of each option. Tissue engineering, as a field, is introduced, and specific aspects of valve tissue engineering are discussed (e.g., biomaterials, cells and bioreactors). Technological hurdles, which need to be overcome for advancement of the field, are also discussed.

Cofilin is a marker of myofibroblast differentiation in cells from porcine aortic cardiac valves

The formation of myofibroblasts in valve interstitial cell (VIC) populations contributes to fibrotic valvular disease. We examined myofibroblast differentiation in VICs from porcine aortic valves. In normal valves, cells immunostained for α-smooth muscle actin (α-SMA, a myofibroblast marker) were rare (0.69 ± 0.48%), but in sclerotic valves of animals fed an atherogenic diet, myofibroblasts were spatially clustered and abundant (31.2 ± 6.3%). In cultured VIC populations from normal valves, SMA-positive myofibroblasts were also spatially clustered, abundant (21% positive cells after 1 passage), and stained for collagen type I and vimentin but not desmin. For an analysis of stem cells, two-color flow cytometry of isolated cells stained with Hoechst 33342 demonstrated that 0.5% of VICs were side population cells; none stained for SMA. Upon culture, sorted side population cells generated ∼85% SMA-positive cells, indicating that some myofibroblasts originate from a rare population with stem cell characteristics. Plating cells on rigid collagen substrates enabled the formation of myofibroblasts after 5 days in culture, which was completely blocked by culture of cells on compliant collagen substrates. Exogenous tensile force also significantly increased SMA expression in VICs. Isotope-coded affinity tags and mass spectrometry were used to identify differentially expressed proteins in myofibroblast differentiation of VICs. Of the nine proteins that were identified, cofilin expression and phospho-cofilin were strongly increased by conditions favoring myofibroblast differentiation. Knockdown of cofilin with small-interfering RNA inhibited collagen gel contraction and reduced myofibroblast differentiation as assessed by the SMA incorporation into stress fibers. When compared with normal valves, diseased valves showed strong immunostaining for cofilin that colocalized with SMA in clustered cells. We conclude that in VICs, cofilin is a marker for myofibroblasts in vivo and in vitro that arise from a rare population of stem cells and require a rigid matrix for formation.

Expression of functional Toll-like receptors 2 and 4 in human aortic valve interstitial cells: potential roles in aortic valve inflammation and stenosis

Calcific aortic valve stenosis is the most common indication for surgical valve replacement. Inflammation appears to be one of the mechanisms involved in aortic valve calcification, and valve interstitial cells seem to contribute to that process. Although Toll-like receptors (TLRs) play an important role in the cellular inflammatory response, it is unknown whether human aortic valve interstitial cells (HAVICs) express functional TLRs. We examined the expression of TLR2 and TLR4 in human aortic valve leaflets and in isolated HAVICs and analyzed the response of cultured HAVICs to the TLR2 and TLR4 agonists peptidoglycan (PGN) and LPS. Abundant TLR2 and TLR4 proteins were found in human aortic valve leaflets and in isolated HAVICs, and both receptors were detected in the membrane and cytoplasm of cultured HAVICs. Stimulation by either PGN or LPS resulted in the activation of the NF-κB signaling pathway and the production of multiple proinflammatory mediators, including IL-6, IL-8, and ICAM-1. In addition, stimulation by either PGN or LPS upregulated the expression of bone morphogenetic protein-2 (BMP-2) and Runx2, factors associated with osteogenesis. This study demonstrates for the first time that HAVICs express TLR2 and TLR4 and that stimulation of HAVICs by PGN or LPS induces the expression of proinflammatory mediators and the upregulation of osteogenesis-associated factors. These results suggest that TLR2 and TLR4 may play a role in aortic valve inflammation and stenosis.

Differences in tissue-remodeling potential of aortic and pulmonary heart valve interstitial cells

Heart valve interstitial cells (VICs) appear to have a dynamic and reversible phenotype, an attribute speculated to be necessary for valve tissue remodeling during times of development and repair. Therefore, we hypothesized that the cytoskeletal (CSK) remodeling capability of the aortic and pulmonary VICs (AVICs and PVICs, respectively), which are dominated by smooth muscle alpha-actin, would exhibit unique contractile behaviors when seeded on collagen gels. Using a porcine cell source, we observed that VIC populations did not contract the gels at early time points (2 and 4 hours) as dermal fibroblasts did, but formed a central cluster of cells prior to contraction. After clustering, VICs appeared to radiate out from the center of the gels, whereas fibroblasts did not migrate but contracted the gels locally. VIC gels treated with transforming growth factor beta1 contracted the gels rapidly, revealing similar sensitivity to the cytokine. Moreover, we evaluated the initial mechanical state of the underlying CSK by comparing AVIC and PVIC stiffness with atomic force microscopy. Not only were AVICs significantly stiffer (p < 0.001) than the PVICs, but they also contracted the gels significantly more at 24 and 48 hours (p < 0.001). Taken together, these findings suggest that the AVICs are capable of inducing greater extra cellular matrix contraction, possibly manifesting in a more pronounced ability to remodel valvular tissues. Moreover, significant mechanobiological differences between AVICs and PVICs exist, and may have implications for understanding native valvular tissue remodeling. Elucidating these differences will also define important functional endpoints in the development of tissue engineering approaches for heart valve repair and replacement.

Molecular and functional characteristics of heart-valve interstitial cells

The cells that reside within valve cusps play an integral role in the durability and function of heart valves. There are principally two types of cells found in cusp tissue: the endothelial cells that cover the surface of the cusps and the interstitial cells (ICs) that form a network within the extracellular matrix (ECM) within the body of the cusp. Both cell types exhibit unique functions that are unlike those of other endothelial and ICs found throughout the body. The valve ICs express a complex pattern of cell-surface, cytoskeletal and muscle proteins. They are able to bind to, and communicate with, each other and the ECM. The endothelial cells on the outflow and inflow surfaces of the valve differ from one another. Their individual characteristics and functions reflect the fact that they are exposed to separate patterns of flow and pressure. In addition to providing a structural role in the valve, it is now known that the biological function of valve cells is important in maintaining the integrity of the cusps and the optimum function of the valve. In response to inappropriate stimuli, valve interstitial and endothelial cells may also participate in processes that lead to valve degeneration and calcification. Understanding the complex biology of valve interstitial and endothelial cells is an important requirement in elucidating the mechanisms that regulate valve function in health and disease, as well as setting a benchmark for the function of cells that may be used to tissue engineer a heart valve.

Heart valve tissue engineering: concepts, approaches, progress, and challenges

Potential applications of tissue engineering in regenerative medicine range from structural tissues to organs with complex function. This review focuses on the engineering of heart valve tissue, a goal which involves a unique combination of biological, engineering, and technological hurdles. We emphasize basic concepts, approaches and methods, progress made, and remaining challenges. To provide a framework for understanding the enabling scientific principles, we first examine the elements and features of normal heart valve functional structure, biomechanics, development, maturation, remodeling, and response to injury. Following a discussion of the fundamental principles of tissue engineering applicable to heart valves, we examine three approaches to achieving the goal of an engineered tissue heart valve: (1) cell seeding of biodegradable synthetic scaffolds, (2) cell seeding of processed tissue scaffolds, and (3) in-vivo repopulation by circulating endogenous cells of implanted substrates without prior in-vitro cell seeding. Lastly, we analyze challenges to the field and suggest future directions for both preclinical and translational (clinical) studies that will be needed to address key regulatory issues for safety and efficacy of the application of tissue engineering and regenerative approaches to heart valves. Although modest progress has been made toward the goal of a clinically useful tissue engineered heart valve, further success and ultimate human benefit will be dependent upon advances in biodegradable polymers and other scaffolds, cellular manipulation, strategies for rebuilding the extracellular matrix, and techniques to characterize and potentially non-invasively assess the speed and quality of tissue healing and remodeling.

Engineering complex tissues

This article summarizes the views expressed at the third session of the workshop "Tissue Engineering--The Next Generation," which was devoted to the engineering of complex tissue structures. Antonios Mikos described the engineering of complex oral and craniofacial tissues as a "guided interplay" between biomaterial scaffolds, growth factors, and local cell populations toward the restoration of the original architecture and function of complex tissues. Susan Herring, reviewing osteogenesis and vasculogenesis, explained that the vascular arrangement precedes and dictates the architecture of the new bone, and proposed that engineering of osseous tissues might benefit from preconstruction of an appropriate vasculature. Jennifer Elisseeff explored the formation of complex tissue structures based on the example of stratified cartilage engineered using stem cells and hydrogels. Helen Lu discussed engineering of tissue interfaces, a problem critical for biological fixation of tendons and ligaments, and the development of a new generation of fixation devices. Rita Kandel discussed the challenges related to the re-creation of the cartilage-bone interface, in the context of tissue engineered joint repair. Frederick Schoen emphasized, in the context of heart valve engineering, the need for including the requirements derived from "adult biology" of tissue remodeling and establishing reliable early predictors of success or failure of tissue engineered implants. Mehmet Toner presented a review of biopreservation techniques and stressed that a new breakthrough in this field may be necessary to meet all the needs of tissue engineering. David Mooney described systems providing temporal and spatial regulation of growth factor availability, which may find utility in virtually all tissue engineering and regeneration applications, including directed in vitro and in vivo vascularization of tissues. Anthony Atala offered a clinician's perspective for functional tissue regeneration, and discussed new biomaterials that can be used to develop new regenerative technologies.

Engineering of Biologically Active Living Heart Valve Leaflets Using Human Umbilical Cord–Derived Progenitor Cells

This study demonstrates the engineering of biologically active heart valve leaflets using prenatally available human umbilical cord-derived progenitor cells as the only cell source. Wharton's Jelly-derived cells and umbilical cord blood-derived endothelial progenitor cells were subsequently seeded on biodegradable scaffolds and cultured in a biomimetic system under biochemical or mechanical stimulation or both. Depending on the stimulation, leaflets showed mature layered tissue formation with functional endothelia and extracellular matrix production comparable with that of native tissues. This demonstrates the feasibility of heart valve leaflet fabrication from prenatal umbilical cord-derived progenitor cells as a further step in overcoming the lack of living autologous replacements with growth and regeneration potential for the repair of congenital malformation.

In vivo recellularization of plain decellularized xenografts with specific cell characterization in the systemic circulation: histological and immunohistochemical study

Decellularized biological scaffolds have been used for the tissue engineering of heart valves with good results in the pulmonary circulation. However, little information is available on the recellularization of plain decellularized biological scaffolds in the systemic circulation. The aim of this study was to determine whether plain decellularized xenografts (PDXs) can recellularize with specific cell characterization in the systemic circulation. The xenogenic aortic valved conduit grafts of rabbits were implanted in the abdominal aorta of dogs after decellularization. The grafts were explanted at 4, 12, or 24 weeks after implantation for histological, immunohistochemical examination, scanning electron microscope, and Western blot analysis. Although the valvular structures were completely lost after implantation, supravalvular conduits showed normal shapes and endothelialization. The PDXs were repopulated with basic vascular cell components in approximate natural proportions without immunological responses. The PDXs had been recellularized with specific cell characterization in the systemic circulation.

Activation of valvular interstitial cells is mediated by transforming growth factor-beta1 interactions with matrix molecules

Strategies for the tissue-engineering of living cardiac valve replacements are limited by a lack of appropriate scaffold materials that both permit cell viability and actively contribute to the growth of functional tissues. Components of the extracellular matrix can localize and modify growth factor signals, and by doing so impart instructional stimuli for direction of cell phenotype. Fibronectin, collagen I, and heparin were explored as affinity matrices for sequestering and presenting soluble signaling molecules to control differentiation of valvular interstitial cells (VICs) to myofibroblasts. VIC differentiation is commonly characterized by expression of stress fibers containing alpha smooth muscle actin (alpha-SMA), and transforming growth factor-beta1 (TGF-beta1) is a central mediator of this transition. Both fibronectin and heparin, which are known to possess TGF-beta1 binding interactions, were found to increase VIC alpha-SMA expression (120% and 258% of expression in controls), while VICs cultured on collagen I-modified substrates had diminished alpha-SMA expression (66% of control). Heparin treatment significantly stimulated VIC production of TGF-beta1 at all concentrations tested (50 to 400 mug/ml). Heparin-modified substrates were found to alter cell morphology through increased adsorption of serum proteins, specifically TGF-beta1. In sum, heparin produced alpha-SMA-positive myofibroblasts through both the de novo production of TGF-beta1, and its localization in the pericellular environment. The addition of heparin to fibronectin-modified substrates led to a synergistic increase in VIC alpha-SMA expression, produced by the reciprocal binding of fibronectin, heparin, cell-produced TGF-beta1. The characterization of molecules, both soluble and insoluble, that control VIC activation will be important for the development of tailored 3D culture environments for tissue-engineering applications.

Comparative study of cellular and extracellular matrix composition of native and tissue engineered heart valves

Tissue engineering of heart valves utilizes biodegradable or metabolizable scaffolds for remodeling by seeded autologous cells. The aim of this study was to determine and compare extracellular matrix (ECM) formations, cellular phenotypes and cell location of native and tissue engineered (TE) valve leaflets. Ovine carotid arteries, ovine and porcine hearts were obtained from slaughterhouses. Cells were isolated from carotid arteries and dissected ovine, porcine and TE leaflets. TE constructs were fabricated from decellularized porcine pulmonary valves, seeded ovine arterial cells and subsequent 16 days dynamic in vitro culture using a pulsatile bioreactor. Native and TE valves were studied by histology (hematoxylin-eosin, resorcin-fuchsin, Movat pentachrome), NIR femtosecond multiphoton laser scanning microscopy and scanning electron microscopy (SEM). Cells of native and TE tissues were identified and localized by immunohistochemistry. Arterial, valvular and re-isolated TE-construct cells were processed for immunocytochemistry and Western blotting. ECM analysis and SEM revealed characteristical and comparable structures in native and TE leaflets. Most cells in native leaflets stained strongly positive for vimentin. Cells positive to alpha-smooth muscle actin (alpha-SMA), myosin and calponin were only found at the ventricular (inflow) side of ovine aortic and porcine pulmonary valve leaflets. Cells from TE constructs had a strong expression of vimentin, alpha-SMA, myosin, calponin and h-caldesmon throughout the entire leaflet. Comparable ECM formation and endothelial cell lining of native and TE leaflets could be demonstrated. However, immunostaining revealed significant differences between valvular cell phenotypes of native and TE leaflets. These results may be essential for further cardiovascular tissue engineering efforts.

Heart Valve Tissue Engineering

Tissue-engineered heart valves have been proposed by physicians and scientists alike to be the ultimate solution for treating valvular heart disease. Rather than replacing a diseased or defective native valve with a mechanical or animal tissue-derived artificial valve, a tissue-engineered valve would be a living organ, able to respond to growth and physiological forces in the same way that the native aortic valve does. Two main approaches have been attempted over the past 10 to 15 years: regeneration and repopulation. Regeneration involves the implantation of a resorbable matrix that is expected to remodel in vivo and yield a functional valve composed of the cells and connective tissue proteins of the patient. Repopulation involves implanting a whole porcine aortic valve that has been previously cleaned of all pig cells, leaving an intact, mechanically sound connective tissue matrix. The cells of the patients are expected to repopulate and revitalize the acellular matrix, creating living tissue that already has the complex microstructure necessary for proper function and durability. Regrettably, neither of the 2 approaches has fared well in animal experiments, and the only clinical experience with tissue-engineered valves resulted in a number of early failures and patient death. This article reviews the technological details of the 2 main approaches, their rationale, their strengths and weaknesses, and the likely mechanisms for their failure. Alternative approaches to valvular tissue engineering, as well as the role of industry in shaping this field in the future, are also reviewed.

From Stem Cells to Viable Autologous Semilunar Heart Valve

BACKGROUND

An estimated 275,000 patients undergo heart valve replacement each year. However, existing solutions for valve replacement are complicated by the morbidity associated with lifelong anticoagulation of mechanical valves and the limited durability of bioprostheses. Recent advances in tissue engineering and our understanding of stem cell biology may provide a lifelong solution to these problems.

METHODS AND RESULTS

Mesenchymal stem cells were isolated from ovine bone marrow and characterized by their morphology and antigen expression through immunocytochemistry, flow cytometry, and capacity to differentiate into multiple cell lineages. A biodegradable scaffold was developed and characterized by its tensile strength and stiffness as a function of time in cell-conditioned medium. Autologous semilunar heart valves were then created in vitro using mesenchymal stem cells and the biodegradable scaffold and were implanted into the pulmonary position of sheep on cardiopulmonary bypass. The valves were evaluated by echocardiography at implantation and after 4 months in vivo. Valves were explanted at 4 and 8 months and examined by histology and immunohistochemistry. Valves displayed a maximum instantaneous gradient of 17.2+/-1.33 mm Hg, a mean gradient of 9.7+/-1.3 mm Hg, an effective orifice area of 1.35+/-0.17 cm2, and trivial or mild regurgitation at implantation. Gradients changed little over 4 months of follow-up. Histology showed disposition of extracellular matrix and distribution of cell phenotypes in the engineered valves reminiscent of that in native pulmonary valves.

CONCLUSIONS

Stem-cell tissue-engineered heart valves can be created from mesenchymal stem cells in combination with a biodegradable scaffold and function satisfactorily in vivo for periods of >4 months. Furthermore, such valves undergo extensive remodeling in vivo to resemble native heart valves.

Tissue engineering a functional aortic heart valve: an appraisal

Valvular heart disease is an important cause of morbidity and mortality, and currently available substitutes for failing hearts have serious limitations. A new promising alternative that may overcome these shortcomings is provided by the relatively new field of tissue engineering (TE). TE techniques involve the growth of autologous cells on a 3D matrix that can be a biodegradable polymer scaffold, or an acellular tissue matrix. These approaches provide the potential to create living matrix valve structures with an ability to grow, repair and remodel within the recipient. This article provides an appraisal of artificial heart valves and an overview of developments in TE that includes the current limitations and challenges for creating a fully functional valve. Biomaterials and stem cell technologies are now providing the potential for new avenues of research and if combined with advances in the rapid prototyping of biomaterials, the engineering of personalized, fully functional, and autologous tissue valve replacements, may become a clinical alternative.

Molecular and functional characterization of ovine cardiac valve-derived interstitial cells in primary isolates and cultures

At present the involvement of cardiac valve interstitial cells (VICs) in growth, repair, and tissue engineering is understudied. Therefore, this study aims at characterizing ovine VICs in order to provide a solid base for tissue engineering of heart valves. Ovine ICs of the four heart valves were isolated by the explant outgrowth method and expanded in vitro up to passage 5. Vimentin and collagen I gene expression from freshly isolated or cultured ICs was measured by reverse transcriptase-polymerase chain reaction. Immunocytochemical stainings of vimentin, alpha-smooth muscle actin (ASMA), smooth muscle myosin, and procollagen I were performed on aortic VICs. In addition, migration and extracellular matrix deposition were studied in vitro in aortic VICs. ICs show stable vimentin and collagen I expression in culture. Expression is approximately doubled in cultured ICs compared with fresh isolates. More than 95% of ICs in each passage stain for vimentin and procollagen I. Freshly isolated ICs are ASMA and myosin negative, but ICs in culture partially stain for these contractile markers. ICs have stable matrix production for up to five passages, associated with stable migration of the cells. We conclude that ovine valve interstitial cells undergo phenotypic modulation to activated myofibroblasts under culture conditions but retain stable matrix production.

Imaging of cardiovascular structures using near-infrared femtosecond multiphoton laser scanning microscopy

Multiphoton imaging represents a novel and very promising medical diagnostic technology for the high-resolution analysis of living biological tissues. We performed multiphoton imaging to analyzed structural features of extracellular matrix (ECM) components, e.g., collagen and elastin, of vital pulmonary and aortic heart valves. High-resolution autofluorescence images of collagenous and elastic fibers were demonstrated using multifluorophore, multiphoton excitation at two different wavelengths and optical sectioning, without the requirement of embedding, fixation, or staining. Collagenous structures were selectively imaged by detection of second harmonic generation (SHG). Additionally, routine histology and electron microscopy were integrated to verify the observed results. In comparison with pulmonary tissues, aortic heart valve specimens show very similar matrix formations. The quality of the resulting three-dimensional (3-D) images enabled the differentiation between collagenous and elastic fibers. These experimental results indicate that multiphoton imaging with near-infrared (NIR) femtosecond laser pulses may prove to be a useful tool for the nondestructive monitoring and characterization of cardiovascular structures.

Multiphoton autofluorescence imaging of intratissue elastic fibers

Multiphoton induced blue/green autofluorescence by near infrared femtosecond laser pulses has been used to selectively image intratissue elastic fibers in native and tissue engineered (TE) viable heart valves without any invasive tissue removal, embedding, fixation, and staining. Elastic fibers could be clearly distinguished from collagenous structures which emit ultraviolet/violet radiation when excited with intense ultrashort pulses due to second harmonic generation. Deep-tissue three-dimensional imaging of elastic fibers with submicron spatial resolution was performed by optical sectioning of heart valves using a multiphoton laser scanning microscope in connection with a tunable 80 MHz femtosecond laser source. The technology was used to diagnose extracellular matrix structures and cell resettlement of TE heart valves prior implantation. This novel non-invasive method opens the general possibility of high-resolution in situ imaging of elastic fibers, collagen structures and intracellular organelles in living intact tissues without staining.

The effects of cellular contraction on aortic valve leaflet flexural stiffness

The aortic valve (AV) leaflet contains a heterogeneous interstitial cell population composed predominantly of myofibroblasts, which contain both fibroblast and smooth muscle cell characteristics. The focus of the present study was to examine aortic valve interstitial cell (AVIC) contractile behavior within the intact leaflet tissue. Circumferential strips of porcine AV leaflets were mechanically tested under flexure, with the AVIC maintained in the normal, contracted, and contraction-inhibited states. Leaflets were flexed both with (WC) and against (AC) the natural leaflet curvature, both before and after the addition of 90 mM KCl to elicit cellular contraction. In addition, a natural basal tonus was also demonstrated by treating the leaflets with 10 microM thapsigargin to completely inhibit AVIC contraction. Results revealed a 48% increase in leaflet stiffness with AVIC contraction (from 703 to 1040 kPa, respectively) when bent in the AC direction (p=0.004), while the WC direction resulted only in 5% increase (from 491 to 516.5 kPa, respectively--not significant) in leaflet stiffness in the active state. Also, the loss of basal tonus of the AVIC population with thapsigargin treatment resulted in 76% (AC, p=0.001) and 54% (WC, p=0.036) decreases in leaflet stiffness at 5 mM KCl levels, while preventing contraction with the addition of 90 mM KCl as expected. We speculate that the observed layer dependent effects of AVIC contraction are primarily due to varying ECM mechanical properties in the ventricularis and fibrosa layers. Moreover, while we have demonstrated that AVIC contractile ability is a significant contributor to AV leaflet bending stiffness, it most likely serves a role in maintaining AV leaflet tissue homeostasis that has yet to be elucidated.

Valvular myofibroblast activation by transforming growth factor-beta: implications for pathological extracellular matrix remodeling in heart valve disease

The pathogenesis of cardiac valve disease correlates with the emergence of muscle-like fibroblasts (myofibroblasts). These cells display prominent stress fibers containing alpha-smooth muscle actin (alpha-SMA) and are believed to differentiate from valvular interstitial cells (VICs). However, the biological factors that initiate myofibroblast differentiation and activation in valves remain unidentified. We show that transforming growth factor-beta1 (TGF-beta1) mediates differentiation of VICs into active myofibroblasts in vitro in a dose-dependent manner, as determined by a significant increase in alpha-SMA and the dramatic augmentation of stress fiber formation and alignment. Additionally, TGF-beta1 and increased mechanical stress function synergistically to enhance contractility. In turn, contractile valve myofibroblasts exert tension on the extracellular matrix, resulting in a dramatic realignment of extracellular fibronectin fibrils. TGF-beta1 also inhibits valve myofibroblast proliferation without enhancing apoptosis. Our results are consistent with activation of a highly contractile myofibroblast phenotype by TGF-beta1 and are the first to connect valve myofibroblast contractility with pathological valve matrix remodeling. We suggest that the activation of contractile myofibroblasts by TGF-beta1 may be a significant first step in promoting alterations to the valve matrix architecture that are evident in valvular heart disease.

Smoothelin-positive cells in human and porcine semilunar valves

Our aim was to further characterize the interstitial cell phenotypes of normal porcine and human semilunar valves, information necessary for the design of bioengineered valves and for the understanding of valve disease processes such as aortic valve sclerosis. Existence of fibroblasts, myofibroblasts, and smooth muscle-like cells within semilunar heart valves has been established. However, the nature of the smooth muscle cell population has been controversial. We used immunochemical and western blotting methods to determine the status of smoothelin and smooth muscle alpha-actin in the valve. Our examination of valve interstitial cells confirmed the presence of terminally differentiated, contractile smooth muscle cells in situ. They were arranged in small bundles of 5-35 cells within the ventricularis or as individual cells scattered throughout the valvular layers in vivo, and were present in cells explanted from the valves in vitro. Colocalization of these proteins in semilunar heart valves was achieved with double-labeling experiments. Protein extraction, followed by coimmunoprecipitation, electrophoresis, and western blotting confirmed the immunochemical analysis and suggested that smooth muscle alpha-actin and smoothelin interact, as has been previously postulated. The presence of contractile smooth muscle within the valve may be an important factor in understanding valve pathology and in the design of tissue engineering efforts.

Cell characterization of porcine aortic valve and decellularized leaflets repopulated with aortic valve interstitial cells: the VESALIO Project (Vitalitate Exornatum Succedaneum Aorticum Labore Ingenioso Obtenibitur)

BACKGROUND

Heart valve bioprostheses for cardiac valve replacement are fabricated by xeno- or allograft tissues. Decellularization techniques and tissue engineering technologies applied to these tissues might contribute to the reduction in risk of calcification and immune response. Surprisingly, there are few data on the cell phenotypes obtained after cellularizing these naturally-derived biomaterials in comparison to those expressed in the intact valve.

METHODS

Aortic valve interstitial cells (VIC) were used to repopulate the corresponding valve leaflets after a novel decellularization procedure based on the use of ionic and nonionic detergents. VIC from leaflet microexplants at the third passage were utilized to repopulate the decellularized leaflets. Intact, decellularized and repopulated valve leaflets and cultured VIC were examined by immunocytochemical procedures with a panel of antibodies to smooth muscle and nonmuscle differentiation antigens. Intact and cellularized leaflets were also investigated with Western blotting and transmission electron microscopy, respectively.

RESULTS

Myofibroblasts and smooth muscle cells (SMC) were mostly localized to the ventricularis of the leaflet whereas fibroblasts were dispersed unevenly. Cultured VIC were comprised of myofibroblasts and fibroblasts with no evidence of endothelial cells and SMC. Two weeks after VIC seeding into decellularized leaflets, grafted cells were found penetrating the bioscaffold. The immunophenotypic and ultrastructural properties of the grafted cells indicated that a VIC heterogeneous mesenchymal cell population was present: fibroblasts, myofibroblasts, SMC, and endothelial cells.

CONCLUSIONS

VIC seeding on detergent-treated valve bioscaffolds has the cellular potential to reconstruct a viable aortic valve.

The cardiac valve interstitial cell

Cardiac valve interstitial cells (ICs) are a heterogeneous and dynamic population of specific cell types that have many unique characteristics. They are responsible for maintaining the extracellular scaffold that provides the mechanical characteristics vital for sustaining the unique dynamic behaviour of the valve. A number of cellular phenotypes can be distinguished: some are sparsely arranged throughout the valve leaflets, whilst others are arranged in thin bundles. These cells express molecular markers similar to those of skeletal, cardiac and smooth muscle cells (SMCs) and in particular, many ICs express smooth muscle (SM) alpha-actin, a marker of myofibroblasts. In this respect, these cells exhibit a profile unlike skin fibroblasts, which may allude to their role in valve function.

Cardiovascular tissue engineering: constructing living tissue cardiac valves and blood vessels using bone marrow, umbilical cord blood, and peripheral blood cells

Although atherosclerosis and valvular heart disease are among the leading causes of morbidity and mortality in developed nations, the substitute blood vessels and heart valves currently available all have significant limitations. During the past 10 years, a new field called tissue engineering has emerged, and several research groups are focusing their efforts on constructing living tissue replacement blood vessels and heart valves. In 2001 several exciting developments occurred with the use of progenitor and stem cells. This article introduces the essential concepts of cardiovascular tissue engineering, reviews achievements in the field, discusses the basic developmental biology of heart valves and blood vessels, and summarizes the 2001 research on progenitor and stem cells.

Oxygen diffusion and consumption of aortic valve cusps

To maintain tissue oxygenation, normal aortic valves contain a vascular bed where tissue thickness is greatest. Avascular "living" tissue-engineered heart valves have been proposed, yet little information exists regarding the magnitude of valve tissue metabolic activity or oxygen requirements. We therefore set out to measure the oxygen diffusivity (DO(2)) and oxygen consumption (VO(2)) of seven porcine aortic valve cusps in vitro at 37 degrees C using a chamber with a Clark oxygen sensor. Mean DO(2) and VO(2) were 1.06 x 10(-5) cm(2)/s and 3.05 x 10(-5) x ml O(2). ml tissue(-1) x s(-1), respectively. When modeled as a three-layered structure by using these values and a boundary condition of 100 mmHg at both surfaces, the average aortic cusp predicted a central mean PO(2) of 27 mmHg (range of 0-50 mmHg). The DO(2) value obtained was similar to that found for other vascular structures, but because our studies were carried out in vitro, the VO(2) measurements may be lower than that required by the functioning valves. These values provide an initial understanding of the oxygen supply possible from the cusp surfaces and the oxygen needs of the tissue.