Transforming growth factor-β regulates the growth of valve interstitial cells in vitro

Elastomeric Trilayer Substrates with Native-like Mechanical Properties for Heart Valve Leaflet Tissue Engineering

Heart valve leaflets have a complex trilayered structure with layer-specific orientations, anisotropic tensile properties, and elastomeric characteristics that are difficult to mimic collectively. Previously, trilayer leaflet substrates intended for heart valve tissue engineering were developed with nonelastomeric biomaterials that cannot deliver native-like mechanical properties. In this study, by electrospinning polycaprolactone (PCL) polymer and poly(l-lactide-co-ε-caprolactone) (PLCL) copolymer, we created elastomeric trilayer PCL/PLCL leaflet substrates with native-like tensile, flexural, and anisotropic properties and compared them with trilayer PCL leaflet substrates (as control) to find their effectiveness in heart valve leaflet tissue engineering. These substrates were seeded with porcine valvular interstitial cells (PVICs) and cultured for 1 month in static conditions to produce cell-cultured constructs. The PCL/PLCL substrates had lower crystallinity and hydrophobicity but higher anisotropy and flexibility than PCL leaflet substrates. These attributes contributed to more significant cell proliferation, infiltration, extracellular matrix production, and superior gene expression in the PCL/PLCL cell-cultured constructs than in the PCL cell-cultured constructs. Further, the PCL/PLCL constructs showed better resistance to calcification than PCL constructs. Trilayer PCL/PLCL leaflet substrates with native-like mechanical and flexural properties could significantly improve heart valve tissue engineering.

Tumor necrosis factor-α promotes and exacerbates calcification in heart valve myofibroblast populations

Pro-inflammatory cytokines play critical roles in regulating valvular interstitial cell (VIC) phenotypic changes that can cause heart valve fibrosis and calcification. Tumor necrosis factor alpha (TNF-α) is a cytokine known to influence VIC behavior and has been reported at high levels in calcified valves ex vivo. We sought to understand the specific effects of TNF-α on VIC phenotypes (eg, fibroblast, profibrotic activated myofibroblasts) and its link with heart valve disorders. We characterize human aortic valve tissue from patients with valve disorders and identify a high variability of fibrotic and calcific markers between tissues. These results motivated in vitro studies to explore the effects of TNF-α on defined VIC fibroblasts and profibrotic activated myofibroblasts, induced via FGF-2 and TGF-β1 treatment. Using 3D hydrogels to culture VICs, we measure the effect of TNF-α (0.1-10 ng/mL) on key markers of fibrosis (eg, αSMA, COL1A1) and calcification (eg, RUNX2, BMP2, and calcium deposits). We observe calcification in TNF-α-treated VIC activated myofibroblasts and identify the MAPK/ERK signaling cascade as a potential pathway for TNF-α mediated calcification. Conversely, VIC fibroblasts respond to TNF-α with decreased calcification. Treatment of VIC profibrotic activated myofibroblast populations with TNF-α leads to increased calcification. Our in vitro findings correlate with findings in diseased human valves and highlight the importance of understanding the effect of cytokines and signaling pathways on specific VIC phenotypes. Finally, we reveal MAPK/ERK as a potential pathway involved in VIC-mediated matrix calcification with TNF-α treatment, suggesting this pathway as a potential pharmaceutical target for aortic valve disease.

TGFβ2 and TGFβ3 mediate appropriate context-dependent phenotype of rat valvular interstitial cells

This study focused on characterizing the potential mechanism of valvular toxicity caused by TGFβ receptor inhibitors (TGFβRis) using rat valvular interstitial cells (VICs) to evaluate early biological responses to TGFβR inhibition. Three TGFβRis that achieved similar exposures in the rat were assessed. Two dual TGFβRI/-RII inhibitors caused valvulopathy, whereas a selective TGFβRI inhibitor did not, leading to a hypothesis that TGFβ receptor selectivity may influence the potency of valvular toxicity. The dual valvular toxic inhibitors had the most profound effect on altering VIC phenotype including altered morphology, migration, and extracellular matrix production. Reduction of TGFβ expression demonstrated that combined TGFβ2/β3 inhibition by small interfering RNA or neutralizing antibodies caused similar alterations as TGFβRis. Inhibition of TGFβ3 transcription was only associated with the dual TGFβRis, suggesting that TGFβRII inhibition impacts TGFβ3 transcriptional regulation, and that the potency of valvular toxicity may relate to alteration of TGFβ2/β3-mediated processes involved in maintaining proper balance of VIC phenotypes in the heart valve.

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TGFβ signaling blockade causes valvulopathy; VICs may be the cellular target

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VICs express TGFβ receptors, ligands, and pSMAD2/3, indicating autocrine regulation

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TGFβ2 and TGFβ3 maintain VIC phenotype; TGFβRis altered shape, migration, and ECM

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Maintaining TGFβ3 transcription may reduce the potency of toxicity

TGF-β Signaling Promotes Tissue Formation during Cardiac Valve Regeneration in Adult Zebrafish

Cardiac valve disease can lead to severe cardiac dysfunction and is thus a frequent cause of morbidity and mortality. Its main treatment is valve replacement, which is currently greatly limited by the poor recellularization and tissue formation potential of the implanted valves. As we still lack suitable animal models to identify modulators of these processes, here we used adult zebrafish and found that, upon valve decellularization, they initiate a rapid regenerative program that leads to the formation of new functional valves. After injury, endothelial and kidney marrow-derived cells undergo cell cycle re-entry and differentiate into new extracellular matrix-secreting valve cells. The TGF-β signaling pathway promotes the regenerative process by enhancing progenitor cell proliferation as well as valve cell differentiation. These findings reveal a key role for TGF-β signaling in cardiac valve regeneration and establish the zebrafish as a model to identify and test factors promoting cardiac valve recellularization and growth.

The effect of physiological stretch and the valvular endothelium on mitral valve proteomes

IMPACT STATEMENT

This work is important to the field of heart valve pathophysiology as it provides new insights into molecular markers of mechanically induced valvular degeneration as well as the protective role of the valvular endothelium. These discoveries reported here advance our current knowledge of the valvular endothelium and how its removal essentially takes valve leaflets into an environmental shock. In addition, it shows that static conditions represent a mild pathological state for valve leaflets, while 10% cyclic stretch provides valvular cell quiescence. These findings impact the field by informing disease stages and by providing potential new drug targets to reverse or slow down valvular change before it affects cardiac function.

FGF-2 inhibits contractile properties of valvular interstitial cell myofibroblasts encapsulated in 3D MMP-degradable hydrogels

Valvular interstitial cells (VICs) are responsible for the maintenance of the extracellular matrix in heart valve leaflets and, in response to injury, activate from a quiescent fibroblast to a wound healing myofibroblast phenotype. Under normal conditions, myofibroblast activation is transient, but the chronic presence of activated VICs can lead to valve diseases, such as fibrotic aortic valve stenosis, for which non-surgical treatments remain elusive. We monitored the porcine VIC response to exogenously delivered fibroblast growth factor 2 (FGF-2; 100 ng/ml), transforming growth factor beta 1 (TGF-β1; 5 ng/ml), or a combination of the two while cultured within 3D matrix metalloproteinase (MMP)-degradable 8-arm 40 kDa poly(ethylene glycol) hydrogels that mimic aspects of the aortic valve. Here, we aimed to investigate VIC myofibroblast activation and subsequent contraction or the reparative wound healing response. To this end, VIC morphology, proliferation, gene expression related to the myofibroblast phenotype [alpha smooth muscle actin (α-SMA) and connective tissue growth factor (CTGF)] and matrix remodeling [collagens (COL1A1 and COL3) and MMP1], and contraction assays were used to quantify the cell response. Treatment with FGF-2 resulted in increased cellular proliferation while reducing the myofibroblast phenotype, as seen by decreased expression of CTGF and α-SMA, and reduced contraction relative to untreated control, suggesting that FGF-2 encourages a reparative phenotype, even in the presence of TGF-β1. TGF-β1 treatment predictably led to an increased proportion of VICs exhibiting the myofibroblast phenotype, indicated by the presence of α-SMA, increased gene expression indicative of matrix remodeling, and bulk contraction of the hydrogels. Functional contraction assays and biomechanical analyses were performed on VIC encapsulated hydrogels and porcine aortic valve tissue explants to validate these findings.

Correlation between valvular interstitial cell morphology and phenotypes: A novel way to detect activation

Valvular interstitial cells (VICs) constitute the major cell population in heart valves. Quiescent fibroblastic VICs are seen in adult healthy valves. They become activated myofibroblastic VICs during development, in diseased valves and in vitro. 2D substrate stiffness within a 5-15 kPa range along with high passage numbers promote VIC activation in vitro. In this study, we characterize VIC quiescence and activation across a 1-21 kPa range of substrate stiffness and passages. We define a cell morphology characterization system for VICs as they transform. We hypothesize that VICs show distinct morphological characteristics in different activation states and the morphology distribution varies with substrate stiffness and passage number. Four VIC morphologies - tailed, spindle, rhomboid and triangle - account for the majority of VIC in this study. Using α-smooth muscle actin (α-SMA), non-muscle myosin heavy chain B (SMemb) and transforming growth factor β (TGF-β) as activation markers for validation, we developed a system where we categorize morphology distribution of VIC cultures, to be potentially used as a non-destructive detection method of activation state. We also show that this system can be used to force stiffness-induced deactivation. The reversibility in VIC activation has important implications in in vitro research and tissue engineering.

Dysregulation of hyaluronan homeostasis during aortic valve disease

Aortic valve disease (AVD) is one of the leading causes of cardiovascular mortality. Abnormal expression of hyaluronan (HA) and its synthesizing/degrading enzymes have been observed during latent AVD however, the mechanism of impaired HA homeostasis prior to and after the onset of AVD remains unexplored. Transforming growth factor beta (TGFβ) pathway defects and biomechanical dysfunction are hallmarks of AVD, however their association with altered HA regulation is understudied. Expression of HA homeostatic markers was evaluated in diseased human aortic valves and TGFβ1-cultured porcine aortic valve tissues using histology, immunohistochemistry and Western blotting. Further, porcine valve interstitial cell cultures were stretched (using Flexcell) and simultaneously treated with exogenous TGFβ1±inhibitors for activated Smad2/3 (SB431542) and ERK1/2 (U0126) pathways, and differential HA regulation was assessed using qRT-PCR. Pathological heavy chain HA together with abnormal regional expression of the enzymes HAS2, HYAL1, KIAA1199, TSG6 and IαI was demonstrated in calcified valve tissues identifying the collapse of HA homeostatic machinery during human AVD. Heightened TSG6 activity likely preceded the end-stage of disease, with the existence of a transitional, pre-calcific phase characterized by HA dysregulation. TGFβ1 elicited a fibrotic remodeling response in porcine aortic valves similar to human disease pathology, with increased collagen and HYAL to HAS ratio, and site-specific abnormalities in the expression of CD44 and RHAMM receptors. Further in these porcine valves, expression of HAS2 and HYAL1 was found to be differentially regulated by the Smad2/3 and ERK1/2 pathways, and CD44 expression was highly responsive to biomechanical strain. Leveraging the regulatory pathways that control both HA maintenance in normal valves and early postnatal dysregulation of HA homeostasis during disease may identify new mechanistic insight into AVD pathogenesis.

Fibroblast growth factor 9 (FGF9) regulation of cyclin D1 and cyclin-dependent kinase-4 in ovarian granulosa and theca cells of cattle

To determine the mechanism by which fibroblast growth factor 9 (FGF9) alters granulosa (GC) and theca (TC) cell proliferation, cell cycle proteins that regulate progression through G1 phase of the cell cycle, cyclin D1 (CCND1) and cyclin-dependent kinase-4 (CDK4; CCND1's catalytic partner), were evaluated. Ovaries were obtained from a local abattoir, GC were harvested from small (1-5 mm) and large (8-22 mm) follicles, and TC were harvested from large follicles. GC and TC were plated in medium containing 10% fetal calf serum followed by various treatments in serum-free medium. Treatment with 30 ng/mL of either FGF9 or IGF1 significantly increased GC numbers and when combined, synergized to further increase GC numbers by threefold. Abundance of CCND1 and CDK4 mRNA in TC and GC were quantified via real-time PCR. Alone and in combination with IGF1, FGF9 significantly increased CCND1 mRNA expression in both GC and TC. Western blotting revealed that CCND1 protein levels were increased by FGF9 in TC after 6 h and 12 h of treatment, but CDK4 protein was not affected. A mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway inhibitor, U0126, significantly reduced FGF9-induced CCND1 mRNA expression to basal levels. For the first time we show that CCND1 mRNA expression is increased by FGF9 in bovine TC and GC, and that FGF9 likely uses the MAPK pathway to induce CCND1 mRNA production in bovine TC.

Inflammation Drives Retraction, Stiffening, and Nodule Formation via Cytoskeletal Machinery in a Three-Dimensional Culture Model of Aortic Stenosis

In calcific aortic valve disease, the valve cusps undergo retraction, stiffening, and nodular calcification. The inflammatory cytokine, tumor necrosis factor (TNF)-α, contributes to valve disease progression; however, the mechanisms of its actions on cusp retraction and stiffening are unclear. We investigated effects of TNF-α on murine aortic valvular interstitial cells (VICs) within three-dimensional, free-floating, compliant, collagen hydrogels, simulating their natural substrate and biomechanics. TNF-α increased retraction (percentage of diameter), stiffness, and formation of macroscopic, nodular structures with calcification in the VIC-laden hydrogels. The effects of TNF-α were attenuated by blebbistatin inhibition of myosin II-mediated cytoskeletal contraction. Inhibition of actin polymerization with cytochalasin-D, but not inhibition of Rho kinase with Y27632, blocked TNF-α-induced retraction in three-dimensional VIC hydrogels, suggesting that actin stress fibers mediate TNF-α-induced effects. In the hydrogels, inhibitors of NF-κB blocked TNF-α-induced retraction, whereas simultaneous inhibition of c-Jun N-terminal kinase was required to block TNF-α-induced stiffness. TNF-α also significantly increased collagen deposition, as visualized by Masson's trichrome staining, and up-regulated mRNA expression of discoidin domain receptor tyrosine kinase 2, fibronectin, and α-smooth muscle actin. In human aortic valves, calcified cusps were stiffer and had more collagen deposition than noncalcified cusps. These findings suggest that inflammation, through stimulation of cytoskeletal contractile activity, may be responsible for valvular cusp retraction, stiffening, and formation of calcified nodules.

Heart valve health, disease, replacement, and repair: a 25-year cardiovascular pathology perspective

The past several decades have witnessed major advances in the understanding of the structure, function, and biology of native valves and the pathobiology and clinical management of valvular heart disease. These improvements have enabled earlier and more precise diagnosis, assessment of the proper timing of surgical and interventional procedures, improved prosthetic and biologic valve replacements and repairs, recognition of postoperative complications and their management, and the introduction of minimally invasive approaches that have enabled definitive and durable treatment for patients who were previously considered inoperable. This review summarizes the current state of our understanding of the mechanisms of heart valve health and disease arrived at through innovative research on the cell and molecular biology of valves, clinical and pathological features of the most frequent intrinsic structural diseases that affect the valves, and the status and pathological considerations in the technological advances in valvular surgery and interventions. The contributions of many cardiovascular pathologists and other scientists, engineers, and clinicians are emphasized, and potentially fruitful areas for research are highlighted.

Involvement of Immune Cell Network in Aortic Valve Stenosis: Communication between Valvular Interstitial Cells and Immune Cells

Aortic valve stenosis is a heart disease prevalent in the elderly characterized by valvular calcification, fibrosis, and inflammation, but its exact pathogenesis remains unclear. Previously, aortic valve stenosis was thought to be caused by chronic passive and degenerative changes associated with aging. However, recent studies have demonstrated that atherosclerotic processes and inflammation can induce valvular calcification and bone deposition, leading to valvular stenosis. In particular, the most abundant cell type in cardiac valves, valvular interstitial cells, can differentiate into myofibroblasts and osteoblast-like cells, leading to valvular calcification and stenosis. Differentiation of valvular interstitial cells can be trigged by inflammatory stimuli from several immune cell types, including macrophages, dendritic cells, T cells, B cells, and mast cells. This review indicates that crosstalk between immune cells and valvular interstitial cells plays an important role in the development of aortic valve stenosis.

Mitral valve disease--morphology and mechanisms

Mitral valve disease is a frequent cause of heart failure and death. Emerging evidence indicates that the mitral valve is not a passive structure, but--even in adult life--remains dynamic and accessible for treatment. This concept motivates efforts to reduce the clinical progression of mitral valve disease through early detection and modification of underlying mechanisms. Discoveries of genetic mutations causing mitral valve elongation and prolapse have revealed that growth factor signalling and cell migration pathways are regulated by structural molecules in ways that can be modified to limit progression from developmental defects to valve degeneration with clinical complications. Mitral valve enlargement can determine left ventricular outflow tract obstruction in hypertrophic cardiomyopathy, and might be stimulated by potentially modifiable biological valvular-ventricular interactions. Mitral valve plasticity also allows adaptive growth in response to ventricular remodelling. However, adverse cellular and mechanobiological processes create relative leaflet deficiency in the ischaemic setting, leading to mitral regurgitation with increased heart failure and mortality. Our approach, which bridges clinicians and basic scientists, enables the correlation of observed disease with cellular and molecular mechanisms, leading to the discovery of new opportunities for improving the natural history of mitral valve disease.

Role of cell-matrix interactions on VIC phenotype and tissue deposition in 3D PEG hydrogels

Valvular interstitial cells (VICs) respond to 3D matrix interactions in a complex manner, but understanding these effects on VIC function better is important for applications ranging from valve tissue engineering to studying valve disease. Here, we encapsulated VICs in poly(ethylene glycol) (PEG) hydrogels modified with three different adhesive ligands, derived from fibronectin (RGDS), elastin (VGVAPG) and collagen-1 (P15). By day 14, VICs became significantly more elongated in RGDS-containing gels compared to VGVAPG or P15. This difference in cell morphology appeared to correlate with global matrix metalloproteinase (MMP) activity, as VICs encapsulated in RGDS-functionalized hydrogels secreted higher levels of active MMP at day 2. VIC activation to a myofibroblast phenotype was also characterized by staining for α-smooth muscle actin (αSMA) at day 14. The percentage of αSMA⁺ VICs in the VGVAPG gels was the highest (56%) compared to RGDS (33%) or P15 (38%) gels. Matrix deposition and composition were also characterized at days 14 and 42 and found to depend on the initial hydrogel composition. All gel formulations had similar levels of collagen, elastin and chondroitin sulphate deposited as the porcine aortic valve. However, the composition of collagen deposited by VICs in VGVAPG-functionalized gels had a significantly higher collagen-X:collagen-1 ratio, which is associated with stenotic valves. Taken together, these data suggest that peptide-functionalized PEG hydrogels are a useful system for culturing VICs three-dimensionally and, with the ability to systematically alter biochemical and biophysical properties, this platform may prove useful in manipulating VIC function for valve regeneration. Copyright © 2013 John Wiley & Sons, Ltd.

TGF-β signalling and reactive oxygen species drive fibrosis and matrix remodelling in myxomatous mitral valves

AIMS

Myxomatous mitral valve disease (MMVD) is associated with leaflet thickening, fibrosis, matrix remodelling, and leaflet prolapse. Molecular mechanisms contributing to MMVD, however, remain poorly understood. We tested the hypothesis that increased transforming growth factor-β (TGF-β) signalling and reactive oxygen species (ROS) are major contributors to pro-fibrotic gene expression in human and mouse mitral valves.

METHODS AND RESULTS

Using qRT-PCR, we found that increased expression of TGF-β1 in mitral valves from humans with MMVD (n = 24) was associated with increased expression of connective tissue growth factor (CTGF) and matrix metalloproteinase 2 (MMP2). Increased levels of phospho-SMAD2/3 (western blotting) and expression of SMAD-specific E3 ubiquitin-protein ligases (SMURF) 1 and 2 (qRT-PCR) suggested that TGF-β1 signalling occurred through canonical signalling cascades. Oxidative stress (dihydroethidium staining) was increased in human MMVD tissue and associated with increases in NAD(P)H oxidase catalytic subunits (Nox) 2 and 4, occurring despite increases in superoxide dismutase 1 (SOD1). In mitral valves from SOD1-deficient mice, expression of CTGF, MMP2, Nox2, and Nox4 was significantly increased, suggesting that ROS can independently activate pro-fibrotic and matrix remodelling gene expression patterns. Furthermore, treatment of mouse mitral valve interstitial cells with cell permeable antioxidants attenuated TGF-β1-induced pro-fibrotic and matrix remodelling gene expression in vitro.

CONCLUSION

Activation of canonical TGF-β signalling is a major contributor to fibrosis and matrix remodelling in MMVD, and is amplified by increases in oxidative stress. Treatments aimed at reducing TGF-β activation and oxidative stress in early MMVD may slow progression of MMVD.

Wnt3a/β-catenin increases proliferation in heart valve interstitial cells

BACKGROUND

Valve interstitial cells (VICs), the most prevalent cells in the heart valve, mediate normal valve function and repair in valve injury and disease. The Wnt3a/β-catenin pathway, important for proliferation and endothelial-to-mesenchymal transition in endocardial cushion formation in valve development, is up-regulated in adult valves with calcific aortic stenosis. Therefore, we tested the hypothesis that Wnt3a/β-catenin signaling regulates proliferation in adult VICs.

METHODS

Porcine VICs were treated with 150 ng/ml of exogenous Wnt3a. To measure proliferation, cells were counted on day 4 posttreatment and stained for bromodeoxyuridine (BrdU) at 24 h posttreatment. β-Catenin small interfering RNA (siRNA) was used to knock down β-catenin expression. Apoptosis was measured with terminal deoxynucleotidyl transferase dUTP nick end labeling assay. To assess changes in β-catenin, cells were stained for β-catenin at days 1, 3, 6, and 9 posttreatment. Western blot for β-catenin was performed on whole cell, cytoplasmic, and nuclear extracts at day 4 posttreatment. To measure β-catenin-mediated transcription, TOPFLASH/FOPFLASH reporter assay was performed at 24 h posttreatment.

RESULTS

Wnt3a produced a significant increase in cell number at day 4 posttreatment and in the percentage of BrdU-positive nuclei at 24 h posttreatment. The increase in proliferation was abolished by β-catenin siRNA. Apoptosis was minimal in all conditions. Wnt3a produced progressively greater β-catenin staining as treatment length increased from 1 to 9 days. Wnt3a produced a significant increase in β-catenin protein in both whole cell and nuclear lysates after 4 days of treatment. Wnt3a significantly increased TOPFLASH/FOPFLASH reporter activity after 24 h of treatment.

CONCLUSION

Wnt3a/β-catenin signaling pathway is an important regulator of proliferation in adult VICs.

Sex-related differences in gene expression by porcine aortic valvular interstitial cells

While many large-scale risk factors for calcific aortic valve disease (CAVD) have been identified, the molecular etiology and subsequent pathogenesis of CAVD have yet to be fully understood. Specifically, it is unclear what biological phenomena underlie the significantly higher occurrence of CAVD in the male population. We hypothesized the existence of intrinsic, cellular-scale differences between male and female valvular interstitial cells (VICs) that contribute to male sex being a risk factor for CAVD. Differences in gene expression profiles between healthy male and female porcine VICs were investigated via microarray analysis. Mean expression values of each probe set in the male samples were compared to the female samples, and biological processes were analyzed for overrepresentation using Gene Ontology term enrichment analysis. There were 183 genes identified as significantly (fold change>2; P<0.05) different in male versus female aortic valve leaflets. Within this significant gene list there were 298 overrepresented biological processes, several of which are relevant to pathways identified in CAVD pathogenesis. In particular, pathway analysis indicated that cellular proliferation, apoptosis, migration, ossification, angiogenesis, inflammation, and extracellular matrix reorganization were all significantly represented in the data set. These gene expression findings also translated into functional differences in VIC behavior in the in vitro environment, as sex-related differences in proliferation and apoptosis were confirmed in VIC populations cultured in vitro. These data suggest that a sex-related propensity for CAVD exists on the cellular level in healthy subjects, a phenomenon that could have significant clinical implications. These findings also strongly support discontinuing the use of mixed-sex VIC cultures, thereby changing the current standard in the field.