Multiscale Structure and Function of the Aortic Valve Apparatus.

While studying the aortic valve in isolation has facilitated the development of life-saving procedures and technologies, the dynamic interplay of the aortic valve and its surrounding structures is vital to preserving their function across the wide range of conditions encountered in an active lifestyle. Our view is that these structures should be viewed as an integrated functional unit, herein referred to as the aortic valve apparatus (AVA). The coupling of the aortic valve and root, left ventricular outflow tract, and blood circulation is crucial for AVA's functions: unidirectional flow out of the left ventricle, coronary perfusion, reservoir function, and supporting left ventricular function. In this review, we explore the multiscale biological and physical phenomena that underly the simultaneous fulfilment of these functions. A brief overview of the tools used to investigate the AVA is included, such as: medical imaging modalities, experimental methods, and computational modelling, specifically fluid-structure interaction (FSI) simulations, is included. Some pathologies affecting the AVA are explored, and insights are provided on treatments and interventions that aim to maintain quality of life. The concepts explained in this paper support the idea of AVA being an integrated functional unit and help identify unanswered research questions. Incorporating phenomena through the molecular, micro, meso and whole tissue scales is crucial for understanding the sophisticated normal functions and diseases of the AVA.

Hypoxia-mediated regulation of the secretory properties of mitral valve interstitial cells.

The sophisticated function of the mitral valve depends to a large extent on its extracellular matrix (ECM) and specific cellular components. These are tightly regulated by a repertoire of mechanical stimuli and biological pathways. One potentially important stimulus is hypoxia. The purpose of this investigation is to determine the effect of hypoxia on the regulation of mitral valve interstitial cells (MVICs) with respect to the synthesis and secretion of extracellular matrix proteins. Hypoxia resulted in reduced production of total collagen and sulfated glycosaminoglycans (sGAG) in cultured porcine MVICs. Increased gene expression of matrix metalloproteinases-1 and -9 and their tissue inhibitors 1 and 2 was also observed after incubation under hypoxic conditions for up to 24 h. Hypoxia had no effect on MVIC viability, morphology, or phenotype. MVICs expressed hypoxia-inducible factor (HIF)-1α under hypoxia. Stimulating HIF-1α chemically caused a reduction in the amount of sGAG produced, similar to the effect observed under hypoxia. Human rheumatic valves had greater expression of HIF-1α compared with normal or myxomatous degenerated valves. In conclusion, hypoxia affects the production of certain ECM proteins and expression of matrix remodeling enzymes by MVICs. The effects of hypoxia appear to correlate with the induction of HIF-1α. This study highlights a potential role of hypoxia and HIF-1α in regulating the mitral valve, which could be important in health and disease.NEW & NOTEWORTHY This study demonstrates that hypoxia regulates extracellular matrix secretion and the remodeling potential of heart valve interstitial cells. Expression of hypoxia-induced factor-1α plays a role in these effects. These data highlight the potential role of hypoxia as a physiological mediator of the complex function of heart valve cells.

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 ß (TGFß1) 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 ß1 (TGFß1) (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 ß1 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.

Structural aortic wall abnormalities following the Nikaidoh operation, which could be reversible and include a healing process.

The Nikaidoh operation continues to be used for patients with transposition of the great arteries, ventricular septal defect and left ventricular outflow tract obstruction. We recently reported structural and functional changes in the aortic root during the follow-up of a patient who underwent the Nikaidoh operation. These changes necessitated re-operation. The pathophysiology of these changes and their potential for reversibility have not yet been studied. In this communication, we describe the extensive structural changes in the aortic wall of the same patient.

Valvulogenesis of a living, innervated pulmonary root induced by an acellular scaffold.

Heart valve disease is a major cause of mortality and morbidity worldwide with no effective medical therapy and no ideal valve substitute emulating the extremely sophisticated functions of a living heart valve. These functions influence survival and quality of life. This has stimulated extensive attempts at tissue engineering "living" heart valves. These attempts utilised combinations of allogeneic/ autologous cells and biological scaffolds with practical, regulatory, and ethical issues. In situ regeneration depends on scaffolds that attract, house and instruct cells and promote connective tissue formation. We describe a surgical, tissue-engineered, anatomically precise, novel off-the-shelf, acellular, synthetic scaffold inducing a rapid process of morphogenesis involving relevant cell types, extracellular matrix, regulatory elements including nerves and humoral components. This process relies on specific material characteristics, design and "morphodynamism".

Atypical Expression of Smooth Muscle Markers and Co-activators and Their Regulation in Rheumatic Aortic and Calcified Bicuspid Valves.

Objective: We have previously reported that human calcified aortic cusps have abundant expression of smooth muscle (SM) markers and co-activators. We hypothesised that cells in bicuspid aortic valve (BAV) cusps and those affected by rheumatic heart valve (RHV) disease may follow a similar phenotypic transition into smooth muscle cells, a process that could be regulated by transforming growth factors (TGFs). Aims: Cusps from eight patients with BAV and seven patients with RHV were analysed for early and late SM markers and regulators of SM gene expression by immunocytochemistry and compared to healthy aortic valves from 12 unused heart valve donors. The ability of TGFs to induce these markers in valve endothelial cells (VECs) on two substrates was assessed. Results: In total, 7 out of 8 BAVs and all the RHVs showed an increased and atypical expression of early and late SM markers α-SMA, calponin, SM22 and SM-myosin. The SM marker co-activators were aberrantly expressed in six of the BAV and six of the RHV, in a similar regional pattern to the expression of SM markers. Additionally, regions of VECs, and endothelial cells lining the vessels within the cusps were found to be positive for SM markers and co-activators in three BAV and six RHV. Both BAVs and RHVs were significantly thickened and HIF1α expression was prominent in four BAVs and one RHV. The ability of TGFßs to induce the expression of SM markers and myocardin was greater in VECs cultured on fibronectin than on gelatin. Fibronectin was shown to be upregulated in BAVs and RHVs, within the cusps as well as in the basement membrane. Conclusion: Bicuspid aortic valves and RHVs expressed increased numbers of SM marker-positive VICs and VECs. Concomittantly, these cells expressed MRTF-A and myocardin, key regulators of SM gene expression. TGFß1 was able to preferentially upregulate SM markers and myocardin in VECs on fibronectin, and fibronectin was found to be upregulated in BAVs and RHVs. These findings suggest a role of VEC as a source of cells that express SM cell markers in BAVs and RHVs. The similarity between SM marker expression in BAVs and RHVs with our previous study with cusps from patients with aortic stenosis suggests the existance of a common pathological pathway between these different pathologies.

Nine days extended release of adenosine from biocompatible MOFs under biologically relevant conditions.

Adenosine is a small molecule directly involved in maintaining homeostasis under pathological and stressful conditions. Due to its rapid metabolism, delivery vehicles capable of exhibiting extended release of adenosine are of paramount interest. Herein, we demonstrate a superior long-term (9 days) release profile of adenosine from biocompatible MOFs in a physiologically relevant environment. The key to the biocompatibility of MOFs is their stability under biologically relevant conditions. This study additionally highlights the interplay between the chemical stability of prototypal MOFs, assessed under physiological conditions, and their cytotoxicity profiles. Cytotoxicity of the prototypal Zn-based MOF (ZIF-8) and three Zr-based MOFs (UiO-66, UiO-66-NH2, and MOF-801) on six cell types was assessed. The cell types selected were valve interstitial cells (VICs), valve endothelial cells (VECs), adipose tissue-derived stem cells (ADSCs), and cell lines U937, THP1, and HeLa. Zr-based MOFs demonstrated a wide tolerance range in the cell culture cytotoxicity assays, demonstrating cell viability up to a very high dose of ∼1000 µg mL-1, as compared to ZIF-8 which showed notable cytotoxicity in as little as ∼100 µg mL-1 dose. This study demonstrates, for the first time, the utilization of biocompatible MOFs for adenosine delivery as well as establishes a direct link between structural instability in the cell culture medium and the observed cytotoxicity of the studied MOFs.

Profiling Genome-Wide DNA Methylation Patterns in Human Aortic and Mitral Valves.

Cardiac valves exhibit highly complex structures and specialized functions that include dynamic interactions between cells, extracellular matrix (ECM) and their hemodynamic environment. Valvular gene expression is tightly regulated by a variety of mechanisms including epigenetic factors such as histone modifications, RNA-based mechanisms and DNA methylation. To date, methylation fingerprints of non-diseased human aortic and mitral valves have not been studied. In this work we analyzed the differential methylation profiles of 12 non-diseased aortic and mitral valve tissue samples (in matched pairs). Analysis of methylation data [reduced representation bisulfite sequencing (RRBS)] of 16,101 promoters genome-wide revealed 584 differentially methylated (DM) promoters, of which 13 were reported in endothelial mesenchymal trans-differentiation (EMT), 37 in aortic and mitral valve disease and 7 in ECM remodeling. Both functional classification as well as network analysis showed that the genes associated with the DM promoters were enriched for WNT-, Cadherin-, Endothelin-, PDGF-, HIF-1 and VEGF- signaling implicated in valvular physiology and pathophysiology. Additional enrichment was detected for TGFB-, NOTCH- and Integrin- signaling involved in EMT as well as ECM remodeling. This data provides the first insight into differential regulation of human aortic and mitral valve tissue and identifies candidate genes linked to DM promoters. Our work will improve the understanding of valve biology, valve tissue engineering approaches and contributes to the identification of relevant drug targets.

Design and characterization of an in vivo injectable hydrogel with effervescently generated porosity for regenerative medicine applications.

Injectable hydrogels that polymerize directly in vivo hold significant promises in clinical settings to support the repair of damaged or failing tissues. Existing systems that allow cellular and tissue ingrowth after injection are limited because of deficient porosity and lack of oxygen and nutrient diffusion inside the hydrogels. Here is reported for the first time an in vivo injectable hydrogel in which the porosity does not pre-exist but is formed concomitantly with its in situ injection by a controlled effervescent reaction. The hydrogel tailorable crosslinking, through the reaction of polyethylene glycol with lysine dendrimers, allows the mixing and injection of precursor solutions from a dual-chamber syringe while entrapping effervescently generated CO2 bubbles to form highly interconnected porous networks. The resulting structures allow preserving modular mechanical properties (from 12.7 ± 0.9 to 29.9 ± 1.7 kPa) while being cytocompatible and conducive to swift cellular attachment, proliferation, in-depth infiltration and extracellular matrix deposition. Most importantly, the subcutaneously injected porous hydrogels are biocompatible, undergo tissue remodeling and support extensive neovascularisation, which is of significant advantage for the clinical repair of damaged tissues. Thus, the porosity and injectability of the described effervescent hydrogels, together with their biocompatibility and versatility of mechanical properties, open broad perspectives for various regenerative medicine or material applications, since effervescence could be combined with a variety of other systems of swift crosslinking. STATEMENT OF SIGNIFICANCE: A major challenge in hydrogel design is the synthesis of injectable formulations allowing easy handling and dispensing in the site of interest. However, the lack of adequate porosity inside hydrogels prevent cellular entry and, therefore, vascularization and tissue ingrowth, limiting the regenerative potential of a vast majority of injectable hydrogels. We describe here the development of an acellular hydrogel that can be injected directly in situ while allowing the simultaneous formation of porosity. Such hydrogel would facilitate handling through injection while providing a porous structure supporting vascularization and tissue ingrowth.

Severe degeneration of a sub-coronary pulmonary autograft in a young adult.

Background. The pulmonary autograft is currently the best valve substitute in terms of longevity and performance. However, there is no agreement about the optimal method of insertion (sub-coronary position or freestanding root). Objectives. We sought to examine the clinical status, detailed imaging and morphometric changes in an explanted pulmonary autograft 22 years after sub-coronary implantation. Methods. A 30-year-old female underwent pulmonary autograft replacement of a severely stenotic valve at the age of 7 years, after presenting to us with signs of moderate to severe heart failure. She underwent clinical examination, detailed imaging including echocardiographic and CT examination with computerised image analysis. The explanted valve was examined by morphometry. Results. Clinical examination showed signs of heart failure (NYHA III). Trans-thoracic and trans-oesophageal 2D echo showed severe malfunction of both the aortic and pulmonary valves associated with dilatation and hypertrophy of both the right and left ventricles. Surgical correction was performed by replacing both the pulmonary and aortic valves with Medtronic 27mm Freestyle valves. The pulmonary autograft showed degeneration of the trilamellar layering of the leaflets, loss and disorganisation of GAGs, increased collagen with fibrotic overgrowth, and markers of fibrosis, inflammation, and calcification. Post-operative imaging showed good correction of the haemodynamic lesions. Conclusion. The pulmonary autograft implanted into the sub-coronary position presented with adverse remodelling, which was detrimental to the functionality and longevity of the valve. Authorship. NL, AM, MN all contributed equally to this paper.

Biocompatibility and Application of Carbon Fibers in Heart Valve Tissue Engineering.

The success of tissue-engineered heart valves rely on a balance between polymer degradation, appropriate cell repopulation, and extracellular matrix (ECM) deposition, in order for the valves to continue their vital function. However, the process of remodeling is highly dynamic and species dependent. The carbon fibers have been well used in the construction industry for their high tensile strength and flexibility and, therefore, might be relevant to support tissue-engineered hearts valve during this transition in the mechanically demanding environment of the circulation. The aim of this study was to assess the suitability of the carbon fibers to be incorporated into tissue-engineered heart valves, with respect to optimizing their cellular interaction and mechanical flexibility during valve opening and closure. The morphology and surface oxidation of the carbon fibers were characterized by scanning electron microscopy (SEM). Their ability to interact with human adipose-derived stem cells (hADSCs) was assessed with respect to cell attachment and phenotypic changes. hADSCs attached and maintained their expression of stem cell markers with negligible differentiation to other lineages. Incorporation of the carbon fibers into a stand-alone tissue-engineered aortic root, comprised of jet-sprayed polycaprolactone aligned carbon fibers, had no negative effects on the opening and closure characteristics of the valve when simulated in a pulsatile bioreactor. In conclusion, the carbon fibers were found to be conducive to hADSC attachment and maintaining their phenotype. The carbon fibers were sufficiently flexible for full motion of valvular opening and closure. This study provides a proof-of-concept for the incorporation of the carbon fibers into tissue-engineered heart valves to continue their vital function during scaffold degradation.

Microstructure of the juvenile sheep aortic valve hinge region and the trilamellar sliding hypothesis.

Background: The aortic valve mechanism performs extremely sophisticated functions which depend on the microstructure of its component parts. The hinge mechanism of the aortic leaflets plays a crucial part in the overall function. However, the detailed microstructure and its relation to function has not been adequately studied. Methods: The aortic roots of juvenile sheep were fixed under physiologic pressure. Sections through all three sinuses were then performed to illustrate the microstructure of the hinge mechanism in different regions of the aortic root. Results: The hinge region in the different sinuses showed unique microstructure with a trilamellar topology with a dominant core consisting of glycosaminoglycans. The exact arrangement of the trilamellar structures varies around the aortic sinuses, which could have functional implications. These features allow the hinge to perform its complex functions through what we have described as "the trilamellar sliding hypothesis". Conclusion: The microstructure of the hinge mechanism is unique and enables it to perform it sophisticated functions.

Long-term adaptive versus maladaptive remodelling of the pulmonary autograft after the Ross operation.

OBJECTIVES: Following the Ross operation, the pulmonary autograft undergoes structural changes (remodelling). We sought to determine the extent, nature and possible determinants of long-term remodelling in the different components of the pulmonary autograft. METHODS: Ten pulmonary autografts and 12 normal control valves (6 pulmonary and 6 aortic) were examined by conventional histology, immunocytochemistry and electron microscopy. The structural changes were quantified by morphometry. RESULTS: The leaflets from free-standing root replacement valves demonstrated thickening to levels comparable to the normal aortic leaflets, largely due to the addition of a thin layer of 'neointima' formed of radial elastic fibres, collagen bundles and glycoaminoglycans, on the ventricular aspect of the leaflets. The leaflets of valves from sub-coronary implantation demonstrated a significantly thicker fibroelastic layer on the ventricularis and calcium deposition in the fibrosa. The media of the explanted valves showed increased number of lamellar units to levels comparable to normal aortic roots. Electron microscopy of valves inserted as free-standing roots showed increased organization into continuous layers. However, intralamellar components showed varying degrees of 'disorganization' in comparison to those in the normal aortic media. In addition, there was a marked increase in the number of vasa vasorum with thickened arteriolar wall in the outer media and adventitia. CONCLUSIONS: Following the Ross operation, in the very long term, all components of the autograft showed varying degrees of remodelling, which was judged to be largely adaptive. Defining the type, determinants and possible functional effects of remodelling could help in understanding and optimizing the results of the Ross operation.

Expression and function of mechanosensitive ion channels in human valve interstitial cells.

BACKGROUND: The ability of heart valve cells to respond to their mechanical environment represents a key mechanism by which the integrity and function of valve cusps is maintained. A number of different mechanotransduction pathways have been implicated in the response of valve cells to mechanical stimulation. In this study, we explore the expression pattern of several mechanosensitive ion channels (MSC) and their potential to mediate mechanosensitive responses of human valve interstitial cells (VIC). METHODS: MSC presence and function were probed using the patch clamp technique. Protein abundance of key MSC was evaluated by Western blotting in isolated fibroblastic VIC (VICFB) and in VIC differentiated towards myofibroblastic (VICMB) or osteoblastic (VICOB) phenotypes. Expression was compared in non-calcified and calcified human aortic valves. MSC contributions to stretch-induced collagen gene expression and to VIC migration were assessed by pharmacological inhibition of specific channels. RESULTS: Two MSC types were recorded in VICFB: potassium selective and cation non-selective channels. In keeping with functional data, the presence of both TREK-1 and Kir6.1 (potassium selective), as well as TRPM4, TRPV4 and TRPC6 (cationic non-selective) channels was confirmed in VIC at the protein level. Differentiation of VICFB into VICMB or VICOB phenotypes was associated with a lower expression of TREK-1 and Kir6.1, and a higher expression of TRPV4 and TRPC6. Differences in MSC expression were also seen in non-calcified vs calcified aortic valves where TREK-1, TRPM4 and TRPV4 expression were higher in calcified compared to control tissues. Cyclic stretch-induced expression of COL I mRNA in cultured VICFB was blocked by RN-9893, a selective inhibitor of TRPV4 channels while having no effect on the stretch-induced expression of COL III. VICFB migration was blocked with the non-specific MSC blocker streptomycin and by GSK417651A an inhibitor of TRPC6/3. CONCLUSION: Aortic VIC express a range of MSC that play a role in functional responses of these cells to mechanical stimulation. MSC expression levels differ in calcified and non-calcified valves in ways that are in part compatible with the change in expression seen between VIC phenotypes. These changes in MSC expression, and associated alterations in the ability of VIC to respond to their mechanical environment, may form novel targets for intervention during aortic valvulopathies.

Elevated p53 expression is associated with dysregulation of the ubiquitin-proteasome system in dilated cardiomyopathy.

AIMS: The molecular mechanisms that regulate cardiomyocyte apoptosis and their role in human heart failure (HF) are uncertain. Expression of the apoptosis regulator p53 is governed by minute double minute 2 (MDM2), an E3 enzyme that targets p53 for ubiquitination and proteasomal processing, and by the deubiquitinating enzyme, herpesvirus-associated ubiquitin-specific protease (HAUSP), which rescues p53 by removing ubiquitin chains from it. Here, we examined whether elevated expression of p53 was associated with dysregulation of ubiquitin-proteasome system (UPS) components and activation of downstream effectors of apoptosis in human dilated cardiomyopathy (DCM). METHODS AND RESULTS: Left ventricular myocardial samples were obtained from patients with DCM (n = 12) or from non-failing (donor) hearts (n = 17). Western blotting and immunohistochemistry revealed that DCM tissues contained elevated levels of p53 and its regulators MDM2 and HAUSP (all P < 0.01) compared with non-failing hearts. DCM tissues also contained elevated levels of polyubiquitinated proteins and possessed enhanced 20S-proteasome chymotrypsin-like activities (P < 0.04) as measured in vitro using a fluorogenic substrate. DCM tissues contained activated caspases-9 and -3 (P < 0.001) and reduced expression of the caspase substrate PARP-1 (P < 0.05). Western blotting and immunohistochemistry revealed that DCM tissues contained elevated expression levels of caspase-3-activated DNAse (CAD; P < 0.001), which is a key effector of DNA fragmentation in apoptosis and also contained elevated expression of a potent inhibitor of CAD (ICAD-S; P < 0.01). CONCLUSION: Expression of p53 in human DCM is associated with dysregulation of UPS components, which are known to regulate p53 stability. Elevated p53 expression and caspase activation in DCM was not associated with activation of both CAD and its inhibitor, ICAD-S. Our findings are consistent with the concept that apoptosis may be interrupted and therefore potentially reversible in human HF.

A novel role of the sympatho-adrenergic system in regulating valve calcification.

BACKGROUND: Aortic valve calcification is a progressive process resembling ossification. Recent evidence indicates that the sympathetic nervous system plays an important role in regulating bone deposition and resorption through the beta2-adrenergic receptors (beta2-ARs). The aim of this study is to determine the level and pattern of expression of beta2-ARs in human valve interstitial cells (ICs) and assess their influence on differentiation of the cells into an osteoblast-like phenotype. METHODS AND RESULTS: Immunohistochemical analysis demonstrated a high expression of beta2-ARs, beta1-ARs, beta3-AR,s and receptor activator of nuclear factor-kappaB (RANK) in calcified aortic valves. The expression of beta2-ARs and beta1-ARs mRNA was assessed by real-time TaqMan PCR in cultures of human aortic valve ICs. Human valve ICs treated with the selective beta2-AR agonist, salmeterol, in the presence of osteogenic medium showed a significant 5-fold decrease in the alkaline phosphatase (ALP) activity in comparison to cells treated with osteogenic medium only (P<0.05). Immunocytochemical staining of the valve ICs showed a concomitant reduction in osteocalcin expression. In addition, other beta2-AR agonists caused a reduction in the protein expression of bone markers including ALP, Cbfa-1, and periostin. Human valve ICs treated with norepinephrine, in the presence of osteogenic medium, did not show a significant reduction in the ALP activity. CONCLUSIONS: These findings suggest an important role of the beta2-ARs in regulating valve calcification and may identify potential therapeutic targets.

Effect of Side-Specific Valvular Shear Stress on the Content of Extracellular Matrix in Aortic Valves.

Responses of valve endothelial cells (VECs) to shear stresses are important for the regulation of valve durability. However, the effect of flow patterns subjected to VECs on the opposite surfaces of the valves on the production of extracellular matrix (ECM) has not yet been investigated. This study aims to investigate the response of side-specific flow patterns, in terms of ECM synthesis and/or degradation in porcine aortic valves. Aortic and ventricular sides of aortic valve leaflets were exposed to oscillatory and laminar flow generated by a Cone-and-Plate machine for 48 h. The amount of collagen, GAGs and elastin was quantified and compared to samples collected from the same leaflets without exposing to flow. The results demonstrated that flow is important to maintain the amount of GAGs and elastin in the valve, as compared to the effect of static conditions. Particularly, the laminar waveform plays a crucial role on the modulation of elastin in side-independent manner. Furthermore, the ability of oscillatory flow on the aortic surface to increase the amount of collagen and GAGs cannot be replicated by exposure of an identical flow pattern on the ventricular side of the valve. Side-specific responses to the particular patterns of flow are important to the regulation of ECM components. Such understanding is imperative to the creation of tissue-engineered heart valves that must be created from the "appropriate" cells that can replicate the functions of the native VECs to regulate the different constituents of ECM.

A Strategy to Enhance Secretion of Extracellular Matrix Components by Stem Cells: Relevance to Tissue Engineering.

The ability of cells to secrete extracellular matrix proteins is an important property in the repair, replacement, and regeneration of living tissue. Cells that populate tissue-engineered constructs need to be able to emulate these functions. The motifs, KTTKS or palmitoyl-KTTKS (peptide amphiphile), have been shown to stimulate production of collagen and fibronectin in differentiated cells. Molecular modeling was used to design different forms of active peptide motifs to enhance the efficacy of peptides to increase collagen and fibronectin production using terminals KTTKS/SKTTK/SKTTKS connected by various hydrophobic linkers, V4A3/V4A2/A4G3. Molecular dynamic simulations showed SKTTKS-V4A3-SKTTKS (P3), with palindromic (SKTTKS) motifs and SKTTK-V4A2-KTTKS (P5), maintained structural integrity and favorable surface electrostatic distributions that are required for functionality. In vitro studies showed that peptides, P3 and P5, showed low toxicity to human adipose-derived stem cells (hADSCs) and significantly increased the production of collagen and fibronectin in a concentration-dependent manner compared with the original active peptide motif. The 4-day treatment showed that stem cell markers of hADSCs remained stable with P3. The molecular design of novel peptides is a promising strategy for the development of intelligent biomaterials to guide stem cell function for tissue engineering applications.

Role of human valve interstitial cells in valve calcification and their response to atorvastatin.

BACKGROUND: Calcific aortic valve stenosis is a common disease in the elderly and is characterized by progressive calcification and fibrous thickening of the valve, but the cellular and molecular mechanisms are not fully understood. We hypothesized that human valve interstitial cells (ICs) are able to differentiate into osteoblast-like cells through the influence of defined mediators and that this process can be modulated pharmacologically. METHODS AND RESULTS: To test this hypothesis, we treated primary cultures of human aortic valve ICs with osteogenic media, bone morphogenic proteins ([BMPs] BMP-2, BMP-4, and BMP-7), and tissue growth factor-beta ([TGF-beta] TGF-beta1 and TGF-beta3) for 21 days. These mediators induced osteoblast differentiation of valve ICs by significantly increasing the activity and expression of alkaline phosphatase ([ALP] P<0.001). A cytokine protein array revealed that atorvastatin treatment (100 micromol/L) of human valve ICs caused a downregulation in levels of expression of BMP-2, BMP-6, TGF-beta1, and TGF-beta3 after 24 hours. In addition, human valve ICs treated with atorvastatin in the presence of osteogenic media showed a significant reduction in ALP activity in comparison to cells treated with osteogenic media only (P=<0.001). This was further confirmed with immunocytochemical staining of valve ICs, whereby atorvastatin markedly reduced the expression of ALP and osteocalcin induced by osteogenic media in comparison to untreated cells. CONCLUSIONS: These findings suggest that human valve ICs are capable of osteoblastic differentiation, by potential mediators which can be pharmacologically targeted by atorvastatin.

Characterization of structural and signaling molecules by human valve interstitial cells and comparison to human mesenchymal stem cells.

BACKGROUND AND AIM OF THE STUDY: Human mesenchymal stem cells (MSCs) are a potential cell source for the tissue engineering of biological structures, including cardiac valves. A comprehensive, phenotypic analysis of MSCs and, for the latter, their comparison with valve interstitial cells (ICs) is therefore essential. METHODS: Isolates of bone marrow-derived human MSCs and human cardiac valve ICs were extensively phenotyped for their expression of membrane proteins involved in adhesion and cell-cell communication, cytoskeletal components, extracellular matrix (ECM) proteins and gene expression of WNT/FZD/SFRP/DKK/LRP family members. RESULTS: MSCs and valve ICs (>80%) expressed fibroblast surface antigen, smooth muscle alpha-actin, vimentin and CD44; expression of MHC class I and II and calponin was inconsistent, and a small proportion expressed desmin and smooth muscle myosin. CD105 was weakly expressed by a low percentage of valve ICs (<10%) compared to MSCs (>90%). ECM components made by both cell types demonstrated similar levels and patterns of staining, although expression of elastin was not detected by both cell types. Adhesion molecule expression was highly variable among the MSC isolates and between the two cell types, with the predominant integrins being alphal, alpha3, alpha5, and beta1 by both cell types. PCR analysis of WNT/FZD/SFRP/LRP family members revealed a greater range of the WNT family of genes being expressed in MSCs compared to ICs. CONCLUSION: The study results provided an extensive fingerprint of valve ICs and of MSCs for the tissue engineering of biological structures and for the manipulation of their desired phenotype. MSCs represent a promising cell type for valve tissue engineering, and will require extensive phenotyping after differentiation.