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

Human myxomatous mitral valve prolapse: role of bone morphogenetic protein 4 in valvular interstitial cell activation

Myxomatous mitral valve prolapse (MVP) is the most common cardiac valvular abnormality in industrialized countries and a leading cause of mitral valve surgery for isolated mitral regurgitation. The key role of valvular interstitial cells (VICs) during mitral valve development and homeostasis has been recently suggested, however little is known about the molecular pathways leading to MVP. We aim to characterize bone morphogenetic protein 4 (BMP4) as a cellular regulator of mitral VIC activation towards a pathologic synthetic phenotype and to analyze the cellular phenotypic changes and extracellular matrix (ECM) reorganization associated with the development of myxomatous MVP. Microarray analysis showed significant up regulation of BMP4-mediated signaling molecules in myxomatous MVP when compared to controls. Histological analysis and cellular characterization suggest that during myxomatous MVP development, healthy quiescent mitral VICs undergo a phenotypic activation via up regulation of BMP4-mediated pathway. In vitro hBMP4 treatment of isolated human mitral VICs mimics the cellular activation and ECM remodeling as seen in MVP tissues. The present study characterizes the cell biology of mitral VICs in physiological and pathological conditions and provides insights into the molecular and cellular mechanisms mediated by BMP4 during MVP. The ability to test and control the plasticity of VICs using different molecules may help in developing new diagnostic and therapeutic strategies for myxomatous MVP.

Epigenetic regulation of 5-lipoxygenase in the phenotypic plasticity of valvular interstitial cells associated with aortic valve stenosis

Valvular interstitial cells (VICs) are of mesenchymal origin and may differentiate into immune-like cells. This phenotypic plasticity is a key feature of aortic valve stenosis, but the role of epigenetic mechanisms has not previously been explored. Here we compared normal and calcified human aortic valve tissue. Calcified tissue exhibited decreased DNA-methylation in the promoter of the gene encoding the proinflammatory enzyme 5-lipoxygenase (5-LO), accompanied by increased 5-LO mRNA levels. Treatment of cultured VICs with the DNA methyltransferase inhibitor: 5-Aza-2'-deoxycytidine increased 5-LO mRNA levels and leukotriene production. These findings provide a first piece of evidence for epigenetic modifications of VICs in valvular heart disease.

Pathophysiologic mechanisms of calcific aortic stenosis

Calcific aortic stenosis (CAS) comprises the leading indication for valve replacement in the Western world. Until recently, progressive calcification was considered to be a passive process. Emerging evidence, however, suggests that degenerative aortic stenosis constitutes an active process involving stimulation of several pathophysiologic pathways such as inflammation and osteogenesis. In addition, CAS and atherosclerosis share common features regarding histopathology of lesions. These novel data raise a new perspective on the prevention and treatment of disease. The current article reviews the most important pathophysiologic mechanisms of senile aortic stenosis.

Mechanisms and clinical consequences of vascular calcification

Vascular calcification has severe clinical consequences and is considered an accurate predictor of future adverse cardiovascular events, including myocardial infarction and stroke. Previously vascular calcification was thought to be a passive process which involved the deposition of calcium and phosphate in arteries and cardiac valves. However, recent studies have shown that vascular calcification is a highly regulated, cell-mediated process similar to bone formation. In this article, we outline the current understanding of key mechanisms governing vascular calcification and highlight the clinical consequences. By understanding better the molecular pathways and genetic circuitry responsible for the pathological mineralization process novel drug targets may be identified and exploited to combat and reduce the detrimental effects of vascular calcification on human health.

Intracellular Ca(2+) accumulation is strain-dependent and correlates with apoptosis in aortic valve fibroblasts

Aortic valve (AV) disease is often characterized by the formation of calcific nodules within AV leaflets that alter functional biomechanics. In vitro, formation of these nodules is associated with osteogenic differentiation and/or increased contraction and apoptosis of AV interstitial cells (AVICs), leading to growth of calcium phosphate crystal structures. In several other cell types, increased intracellular Ca(2+) has been shown to be an important part in activation of osteogenic differentiability. However, elevated intracellular Ca(2+) is known to mediate cell contraction, and has also been shown to lead to apoptosis in many cell types. Therefore, a rise in intracellular Ca(2+) may precede cellular changes that lead to calcification, and fibroblasts similar to AVICs have been shown to exhibit increases in intracellular Ca(2+) in response to mechanical strain. In this study, we hypothesized that strain induces intracellular Ca(2+) accumulation through stretch-activated calcium channels. We were also interested in assessing possible correlations between intracellular Ca(2+) increases and apoptosis in AVICs. To test our hypothesis, cultured porcine AVICs were used to assess correlates between strain, intracellular Ca(2+), and apoptosis. Ca(2+) sensitive fluorescent dyes were utilized to measure real-time intracellular Ca(2+) changes in strained AVICs. Ca(2+) changes were then correlated with AVIC apoptosis using flow cytometric Annexin V apoptosis assays. These data indicate that strain-dependent accumulation of intracellular Ca(2+) is correlated with apoptosis in AVICs. We believe that these findings indicate early mechanotransductive events that may initiate AV calcification pathways.

Role of angiogenetic factors in cardiac valve homeostasis and disease

The aging of populations worldwide and the habitual consumption of food high in calories and cholesterol have led to recent increases in morbidity from calcific aortic valve disease. At the same time, rupture of the chordae tendineae cordis, which is a component of the mitral valve complex, is one of the major causes of mitral regurgitation. Surgery is the basis of treatment for these diseases, and little is known about their causes and mechanisms. A balance of angiogenetic and angioinhibitory factors is crucial for normal development and homeostasis of many organs. Although the heart is a vascular-rich organ, most of the cardiac valve complex is avascular like cartilage and tendons. Our studies have focused on the role of angiogenetic factors expressed in the cartilage and tendons in cardiac valve homeostasis. Recently, we found that chondromodulin-I, tenomodulin, and periostin play essential roles in degeneration and/or rupture of the cardiac valve complex by controlling angiogenesis and matrix metalloproteinase production. Here, we review the mechanistic insights provided by these studies and the proposed roles of angiogenetic factors in cardiac valve homeostasis and disease.

Molecular imaging insights into early inflammatory stages of arterial and aortic valve calcification

Traditional imaging modalities such as computed tomography, although perfectly adept at identifying and quantifying advanced calcification, cannot detect the early stages of this disorder and offer limited insight into the mechanisms of mineral dysregulation. This review presents optical molecular imaging as a promising tool that simultaneously detects pathobiological processes associated with inflammation and early stages of calcification in vivo at the (sub)cellular levels. Research into treatment of cardiovascular calcification is lacking, as shown by clinical trials that have failed to demonstrate the reduction of calcific aortic stenosis. Hence, the need to elucidate the pathways that contribute to cardiovascular calcification and to develop new therapeutic strategies to prevent or reverse calcification has driven investigations into the use of molecular imaging. This review discusses studies that have used molecular imaging methods to advance knowledge of cardiovascular calcification, focusing in particular on the inflammation-dependent mechanisms of arterial and aortic valve calcification.

Progenitor cells confer plasticity to cardiac valve endothelium

The endothelium covering the aortic, pulmonary, mitral, and tricuspid valves looks much like the endothelium throughout the vasculature, in terms of general morphology and expression of many endothelial markers. Closer examination, however, reveals important differences and hints of a unique phenotype that reflects the valvular endothelium's embryonic history, and potentially, its ability to maintain integrity and function over a life span of dynamic mechanical stress. A well-studied property that sets the cardiac valvular endothelium apart is the ability to transition from an endothelial to a mesenchymal phenotype-an event known as epithelial to mesenchymal transition (EMT). EMT is a critical step during embryonic valvulogenesis, it can occur in post-natal valves and has recently been implicated in the adaptive response of mitral valve leaflets exposed to a controlled in vivo setting designed to mimic the leaflet tethering that occurs in ischemic mitral regurgitation. In this review, we will discuss what is known about valvular endothelial cells, with a particular focus on post-natal, adult valves. We will put forth the idea that at subset of valvular endothelial cells are progenitor cells, which may serve to replenish valvular cells during normal cellular turnover and in response to injury and disease.

Inflammatory regulation of valvular remodeling: the good(?), the bad, and the ugly

Heart valve disease is unique in that it affects both the very young and very old, and does not discriminate by financial affluence, social stratus, or global location. Research over the past decade has transformed our understanding of heart valve cell biology, yet still more remains unclear regarding how these cells respond and adapt to their local microenvironment. Recent studies have identified inflammatory signaling at nearly every point in the life cycle of heart valves, yet its role at each stage is unclear. While the vast majority of evidence points to inflammation as mediating pathological valve remodeling and eventual destruction, some studies suggest inflammation may provide key signals guiding transient adaptive remodeling. Though the mechanisms are far from clear, inflammatory signaling may be a previously unrecognized ally in the quest for controlled rapid tissue remodeling, a key requirement for regenerative medicine approaches for heart valve disease. This paper summarizes the current state of knowledge regarding inflammatory mediation of heart valve remodeling and suggests key questions moving forward.

Molecular imaging insights into early inflammatory stages of arterial and aortic valve calcification

Traditional imaging modalities such as computed tomography, although perfectly adept at identifying and quantifying advanced calcification, cannot detect the early stages of this disorder and offer limited insight into the mechanisms of mineral dysregulation. This review presents optical molecular imaging as a promising tool that simultaneously detects pathobiological processes associated with inflammation and early stages of calcification in vivo at the (sub)cellular levels. Research into treatment of cardiovascular calcification is lacking, as shown by clinical trials that have failed to demonstrate the reduction of calcific aortic stenosis. Hence, the need to elucidate the pathways that contribute to cardiovascular calcification and to develop new therapeutic strategies to prevent or reverse calcification has driven investigations into the use of molecular imaging. This review discusses studies that have used molecular imaging methods to advance knowledge of cardiovascular calcification, focusing in particular on the inflammation-dependent mechanisms of arterial and aortic valve calcification.

Aortic valve disease and treatment: the need for naturally engineered solutions

The aortic valve regulates unidirectional flow of oxygenated blood to the myocardium and arterial system. The natural anatomical geometry and microstructural complexity ensures biomechanically and hemodynamically efficient function. The compliant cusps are populated with unique cell phenotypes that continually remodel tissue for long-term durability within an extremely demanding mechanical environment. Alteration from normal valve homeostasis arises from genetic and microenvironmental (mechanical) sources, which lead to congenital and/or premature structural degeneration. Aortic valve stenosis pathobiology shares some features of atherosclerosis, but its final calcification endpoint is distinct. Despite its broad and significant clinical significance, very little is known about the mechanisms of normal valve mechanobiology and mechanisms of disease. This is reflected in the paucity of predictive diagnostic tools, early stage interventional strategies, and stagnation in regenerative medicine innovation. Tissue engineering has unique potential for aortic valve disease therapy, but overcoming current design pitfalls will require even more multidisciplinary effort. This review summarizes the latest advancements in aortic valve research and highlights important future directions.

Differences in valvular and vascular cell responses to strain in osteogenic media

Calcification is the primary cause of failure of bioprosthetic and tissue-engineered vascular and valvular grafts. We used tissue-engineered collagen gels containing human aortic smooth muscle cells (HASMC) and human aortic valvular interstitial cells (HAVIC) as a model to investigate cell-mediated differences in early markers of calcification. The HASMCs and HAVICs were isolated from non-sclerotic human tissues. After 21 days of culture in either regular or osteogenic media with or without 10% cyclic strain at 1 Hz, the collagen gels were assessed for DNA content, collagen I, matrix metalloproteinase (MMP)-2 and glycosaminoglycan (GAG) content. The collagen gels containing HASMCs contained significantly greater amounts of collagen I and GAG compared to HAVICs. Although strain increased MMP-2 activity for both cell types, this trend was significant (p ≤ 0.05) only for HAVICs. Cultured gels were also assessed for osteogenic markers calcium content, alkaline phosphatase (ALP), and Runx2 and were present at greater amounts in gels containing HASMCs than HAVICs. Calcium content, Runx2 expression, and ALP activity were also modulated by mechanical strain. The results indicate that cell-mediated differences exist between the vascular and valvular calcification processes. Further investigation is necessary for improved understanding and to detect biomarkers for early detection or prevention of these diseases.

Viral and bacterial patterns induce TLR-mediated sustained inflammation and calcification in aortic valve interstitial cells

BACKGROUND

Aortic stenosis shares some ethiopathological features with atherosclerosis and increasing evidence links Toll-like receptors (TLRs) to atherogenesis.

METHODS

TLR-mediated inflammation and osteogenesis were investigated in human interstitial cells isolated from stenotic and non-stenotic aortic valves. TLR expression and signalling were evaluated by quantitative RT-PCR, flow cytometry, Western blot analysis, ELISA, and cytokine arrays. Osteogenesis was evaluated by measuring alkaline phosphatase activity.

RESULTS

Interstitial cells from control valves express most TLRs, being TLR4 the most abundant, whereas cells from stenotic valves express higher TLR4 and TLR2 and lower TLR5 and TLR9 transcript levels. When pro-inflammatory pathways were analyzed, we observed that TLR4, TLR2 and TLR3 ligands induced an early activation of NF-κB and p38 MAPK activation in cells from control and stenotic valves. Strikingly, when TLRs sensing viral patterns were studied, a sustained TLR3-mediated activation of NF-κB, a κB-independent induction of catalytically active cyclooxigenase (COX)-2 and ICAM-1 expression, and induction of expression of several chemokines were observed. TLR4, but not TLR2, engagement produced a similar but NF-κB-dependent effect. Moreover, TLR3 and TLR4 agonists induced alkaline phosphatase expression and activity.

CONCLUSIONS

Exposure of aortic valve interstitial cells to viral and Gram-negative bacteria molecular patterns induces distinct and long-term TLR-mediated pro-inflammatory and pro-osteogenic responses that might be relevant to the pathogenesis of degenerative aortic stenosis.

Differential proteoglycan and hyaluronan distribution in calcified aortic valves

BACKGROUND

While the prevalence of calcified aortic valve disease continues to rise and no pharmacological treatments exist, little is known regarding the pathogenesis of the disease. Proteoglycans and the glycosaminoglycan hyaluronan are involved in calcification in arteriosclerosis and their characterization in calcified aortic valves may lend insight into the pathogenesis of the disease.

METHODS

Fourteen calcified aortic valves removed during valve replacement surgery were immunohistochemically stained for the proteoglycans decorin, biglycan, and versican, as well as the glycosaminoglycan hyaluronan. Staining intensity was evaluated in the following regions of interest: center of calcified nodule, edge of nodule, tissue directly surrounding the nodule; center and tissue surrounding small "prenodules"; and fibrosa layer of normal regions of the leaflet distanced from the nodule.

RESULTS

Decorin, biglycan, and versican, as well as hyaluronan, were abundantly present immediately surrounding the calcified nodules, but minimally within the nodule itself. Expression of decorin and biglycan in and surrounding prenodules was greater than in the edge and center regions of mature nodules. The levels of expression of the proteoglycans and hyaluronan were highly correlated with one another in the different regions of the valve.

CONCLUSIONS

The three proteoglycans and hyaluronan demonstrated distinctive localization relative to nodules within calcified aortic valves, where they likely mediate lipid retention, cell proliferation, and extracellular matrix remodeling, and motivate further study. Comparisons between expression of these components in mature nodules and prenodules suggest distinct roles for these components in nodule progression, especially in the tissues surrounding the nodules.

Mitral valve endothelial cells with osteogenic differentiation potential

OBJECTIVE

Cardiac valvular endothelium is unique in its ability to undergo endothelial-to-mesenchymal transformation, a differentiation process that is essential for valve development and has been proposed as mechanism for replenishing the interstitial cells of mature valves. We hypothesized that the valvular endothelium contains endothelial cells that are direct precursors to osteoblastic valvular interstitial cells (VICs).

METHODS AND RESULTS

Clonal cell populations from ovine mitral valve leaflets were isolated by single cell plating. Mitral valvular endothelial and mesenchymal clones were tested for osteogenic, adipogenic, and chondrogenic differentiation, determined by the expression of lineage-specific markers. Mitral valvular endothelial clones showed a propensity for osteogenic, as well as chondrogenic differentiation that was comparable to a mitral valvular VIC clone and to bone marrow-derived mesenchymal stem cells. Osteogenic differentiation was not detected in nonvalvular endothelial cells. Regions of osteocalcin expression, a marker of osteoblastic differentiation, were detected along the endothelium of mitral valves that had been subjected in vivo to mechanical stretch.

CONCLUSIONS

Mitral valve leaflets contain endothelial cells with multilineage mesenchymal differentiation potential, including osteogenic differentiation. This unique feature suggests that postnatal mitral valvular endothelium harbors a reserve of progenitor cells that can contribute to osteogenic and chondrogenic VICs.

Differential expression of cartilage and bone-related proteins in pediatric and adult diseased aortic valves

Approximately 5 million people are affected with aortic valve disease (AoVD) in the United States. The most common treatment is aortic valve (AoV) replacement surgery, however, replacement valves are susceptible to failure, necessitating additional surgeries. The molecular mechanisms underlying disease progression and late AoV calcification are not well understood. Recent studies suggest that genes involved in bone and cartilage development play an active role in osteogenic-like calcification in human calcific AoVD (CAVD). In an effort to define the molecular pathways involved in AoVD progression and calcification, expression of markers of valve mesenchymal progenitors, chondrogenic precursors, and osteogenic differentiation was compared in pediatric non-calcified and adult calcified AoV specimens. Valvular interstitial cell (VIC) activation, extracellular matrix (ECM) disorganization, and markers of valve mesenchymal and skeletal chondrogenic progenitor cells were observed in both pediatric and adult AoVD. However, activated BMP signaling, increased expression of cartilage and bone-type collagens, and increased expression of the osteogenic marker Runx2 are observed in adult diseased AoVs. They are not observed in the majority of pediatric diseased valves, representing a marked distinction in the molecular profile between pediatric and adult diseased AoVs. The combined evidence suggests that an actively regulated osteochondrogenic disease process underlies the pathological changes affecting AoVD progression, ultimately resulting in stenotic AoVD. Both pediatric and adult diseased AoVs express protein markers of valve mesenchymal and chondrogenic progenitor cells while adult diseased AoVs also express proteins involved in osteogenic calcification. These findings provide specific molecular indicators of AoVD progression, which may lead to identification of early disease markers and the development of potential therapeutics.

Elevated cyclic stretch induces aortic valve calcification in a bone morphogenic protein-dependent manner

Calcified aortic valve (AV) cusps have increased expression of bone morphogenic proteins (BMPs) and transforming growth factor-beta1 (TGF-beta1). Elevated stretch loading on the AV is known to increase expression of matrix remodeling enzymes and pro-inflammatory proteins. Little, however, is known about the mechanism by which elevated stretch might induce AV calcification. We investigated the hypothesis that elevated stretch may cause valve calcification via a BMP-dependent mechanism. Porcine AV cusps were cultured in a stretch bioreactor, at 10% (physiological) or 15% (pathological) stretch and 70 beats per minute for 3, 7, and 14 days, in osteogenic media supplemented with or without high phosphate (3.8 mmol/L), TGF-beta1 (1 ng/ml), as well as the BMP inhibitor noggin (1, 10, and 100 ng/ml). Fresh cusps served as controls. Alizarin red and von Kossa staining demonstrated that 15% stretch elicited a stronger calcification response compared with 10% stretch in a fully osteogenic medium containing high phosphate and TGF-beta1. BMP-2, -4, and Runx2 expression was observed after 3 days on the fibrosa surface of the valve cusp and was stretch magnitude-dependent. Cellular apoptosis was highest at 15% stretch. Tissue calcium content and alkaline phosphatase activity were similarly stretch-dependent and were significantly reduced by noggin in a dose dependent manner. These results underline the potential role of BMPs in valve calcification due to altered stretch.

Effect of hydroxymethylglutaryl coenzyme-a reductase inhibitors on the long-term progression of rheumatic mitral valve disease

BACKGROUND

At present, no medical therapy is known to affect the progression of rheumatic mitral stenosis (MS). We sought to assess the effect of statin treatment on long-term progression of MS in a large population.

METHODS AND RESULTS

From our 20-year database, we identified all patients with rheumatic MS with > or =2 echocardiographies > or =1 year apart. Exclusion criteria were previous intervention on the mitral valve, more than moderate aortic regurgitation, or symptoms at first examination. The study sample included 315 patients (mean age, 61+/-12 years; 224 women); 35 patients (11.1%) were treated with statins, and 280 (88.9%) were not. Mean follow-up period was 6.1+/-4.0 years (range, 1 to 20). The rate of decrease in mitral valve area was significantly lower in the statin group compared with the untreated group (0.027+/-0.056 versus 0.067+/-0.082 cm(2)/y; P=0.005). The annualized change in mean transmitral gradient was lower in statin-treated patients (0.20+/-0.59 versus 0.58+/-0.96 mm Hg/y; P=0.023). The prevalence of fast MS progression (annual change in mitral valve area >0.08 cm(2)) was significantly lower in the statin group (P=0.008). An increase in systolic pulmonary artery pressure of >10 mm Hg was found in 17% of patients in the statin group versus 40% of untreated patients (P=0.045).

CONCLUSIONS

Our study shows a significantly slower progression of rheumatic MS in patients treated with statins. These findings could have an important impact in the early medical therapy of patients with rheumatic heart disease.

Common pathogenic features of atherosclerosis and calcific aortic stenosis: role of transforming growth factor-beta

Calcific aortic stenosis and atherosclerosis have been investigated separately in experimental in vitro and in vivo studies and in clinical studies. The similarities identified in both diseases suggest that similar pathogenic pathways are involved in both conditions. Most current therapeutic studies are focused on statins. The evidence suggests that statin effects on valves may, in large part, be independent of the lipid lowering effects of the drug. There are several molecules that play significant regulatory roles on the development and progression of valve sclerosis and calcification and on growth and complications of atherosclerotic plaques. The purpose of this review is to discuss the pathogenic features of the two conditions, highlight the important similarities, and then review the data that suggest that transforming growth factor-beta may play a key regulatory role in both diseases and that this is worthy of study as a potential therapeutic target for both conditions.

Regulation of valvular interstitial cell calcification by components of the extracellular matrix

Understanding the interactions between extracellular matrix (ECM) components and valvular interstitial cells (VICs) is relevant to both treating heart valve disease and designing heart valve tissue engineering scaffolds, yet the VIC-ECM relationship has not been well characterized. Thus, the aim of this study was to characterize VIC-ECM interactions, paying specific attention to whether ECM composition affected the in vitro calcification of VICs. Our results show that the number and size of calcific nodules formed in VIC cultures, as well as the expression of the mineralization markers alkaline phosphatase (ALP) and CBFa1, were highly dependent upon the composition of the culture surface. VICs cultured on certain ECM components, that is, collagen and fibronectin, were resistant to calcification, even upon treatment with mineralization-inducing growth factors. Meanwhile, cultures of VICs on fibrin, laminin, and heparin coatings had a high number of calcified nodules, although only VICs on fibrin expressed significantly elevated levels of ALP and CBFa1. Nodule composition analysis revealed the presence of multiple types of mineralization. Although apoptotic and necrotic cells were more concentrated in nodules, these nodules did contain a strong majority population of viable cells. Characterizing this ECM-dependence of VIC calcification will help us to identify appropriate biomaterial environments for heart valve tissue engineering as well as elucidate mechanisms of valvular disease.

Mechanisms of aortic valve calcification: the LDL-density-radius theory: a translation from cell signaling to physiology

Recent epidemiologic studies have revealed the risk factors associated for vascular atherosclerosis, including the male sex, smoking, hypertension, and elevated serum cholesterol, similar to the risk factors associated with the development of AV stenosis. An increasing number of models of experimental hypercholesterolemia demonstrate features of atherosclerosis in the AV, which are similar to the early stages of vascular atherosclerotic lesions. Experimental and clinical studies demonstrate that the hypercholesterolemic AV develops an atherosclerotic lesion which is proliferative and expresses high levels of osteoblast bone markers which mineralize over time to form bone. Calcification, the end-stage process of the disease, is necessary to understand as a prognostic indicator in the modification of this cellular process before it is too late. In summary, these findings suggest that medical therapies may have a potential role in patients in the early stages of this disease process to slow the progression to severe aortic stenosis and to delay the timing of the need for surgery. The translation of these experimental studies to clinical practice will be important to understand the potential for medical therapy for this disease process.

Notch1 represses osteogenic pathways in aortic valve cells

Calcific aortic stenosis is the third leading cause of adult heart disease and the most common form of acquired valvular disease in developed countries. However, the molecular pathways leading to calcification are poorly understood. We reported two families in which heterozygous mutations in NOTCH1 caused bicuspid aortic valve and severe aortic valve calcification. NOTCH1 is part of a highly conserved signaling pathway involved in cell fate decisions, cell differentiation, and cardiac valve formation. In this study, we examined the mechanism by which NOTCH1 represses aortic valve calcification. Heterozygous Notch1-null (Notch1(+/)(-)) mice had greater than fivefold more aortic valve calcification than age- and sex-matched wildtype littermates. Inhibition of Notch signaling in cultured sheep aortic valve interstitial cells (AVICs) also increased calcification more than fivefold and resulted in gene expression typical of osteoblasts. We found that Notch1 normally represses the gene encoding bone morphogenic protein 2 (Bmp2) in murine aortic valves in vivo and in aortic valve cells in vitro. siRNA-mediated knockdown of Bmp2 blocked the calcification induced by Notch inhibition in AVICs. These findings suggest that Notch1 signaling in aortic valve cells represses osteoblast-like calcification pathways mediated by Bmp2.

Statins block calcific nodule formation of valvular interstitial cells by inhibiting alpha-smooth muscle actin expression

OBJECTIVE

Calcific aortic stenosis, characterized by excessive fibrosis and deposition of bone-like calcified tissue, affects roughly 2% to 3% of the U.S. population over the age of 65. Recent studies have suggested that statins have a positive effect on the progression of aoritic stenosis, likely because of their ability to affect the resident cell population, known as valvular interstitial cells (VICs). VICs are fibroblastic cells that can differentiate to form activated myofibroblasts, displaying increased alpha smooth muscle actin (alphaSMA) expression, contractility, and collagen production.

METHODS AND RESULTS

In culture, VICs spontaneously form multicellular aggregates that subsequently develop into calcified nodules, providing an in vitro model for aortic stenosis. Using real-time microscopic tracking, we observed that confluent VIC monolayers spontaneously contract into rounded nodules, suggesting that myofibroblastic contractility is a critical step in the process of nodule formation. Overexpression of alphaSMA increased VIC calcific nodule formation and contractility, whereas knockdown of alphaSMA with siRNAs reduced these phenotypes, suggesting that the expression and contractile properties of alphaSMA are essential to the formation of nodules. Statin treatment of VICs reduced alphaSMA expression, inhibited contractility, and decreased nodule formation. When statins were used to treat preformed nodules, no decrease in the number of calcified nodules was observed, suggesting that statins may play more of a preventative role in aortic stenosis than a cure.

CONCLUSIONS

Our studies provide evidence of a causal relationship between VIC myofibroblastic activity and initial VIC calcific nodule formation. Furthermore, we demonstrate that pravastatin inhibition of calcific nodule formation is related to inhibition of myofibroblastic activity.

The in vivo diagnosis of early-stage aortic valve sclerosis using magnetic resonance imaging in a rabbit model

PURPOSE

To use magnetic resonance imaging (MRI) to identify and monitor early aortic valve sclerosis (AVS) induced by cholesterol feeding in rabbits. AVS is a highly prevalent disease process, affecting more than 25% of the population over age 65. A major obstacle to early stage medical management of AVS has been the lack of an objective noninvasive technique to identify its presence and monitor its progress.

MATERIALS AND METHODS

Retrospectively gated CINE fast spoiled gradient echo (fSPGR) images of aortic valve cusps were collected at 3-month intervals starting at 6 months using a 1.5 T MR scanner interfaced with a customized surface radiofrequency coil. At 16 months a subset of animals was sacrificed and excised cusps were examined with both high frequency ultrasound (US) and histopathological techniques to validate the MRI method.

RESULTS

MR and US analysis identified significant thickening of diseased AV cusps when compared to control (P < 0.05). Histopathological analysis confirmed the presence of human-like AVS in diseased rabbit valves.

CONCLUSION

Early AVS, exemplified by increased valve thickness, can be identified in vivo using high-resolution MRI.

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.

Identification and characterization of aortic valve mesenchymal progenitor cells with robust osteogenic calcification potential

Advanced valvular lesions often contain ectopic mesenchymal tissues, which may be elaborated by an unidentified multipotent progenitor subpopulation within the valve interstitium. The identity, frequency, and differentiation potential of the putative progenitor subpopulation are unknown. The objectives of this study were to determine whether valve interstitial cells (VICs) contain a subpopulation of multipotent mesenchymal progenitor cells, to measure the frequencies of the mesenchymal progenitors and osteoprogenitors, and to characterize the osteoprogenitor subpopulation because of its potential role in calcific aortic valve disease. The multilineage potential of freshly isolated and subcultured porcine aortic VICs was tested in vitro. Progenitor frequencies and self-renewal capacity were determined by limiting dilution and colony-forming unit assays. VICs were inducible to osteogenic, adipogenic, chondrogenic, and myofibrogenic lineages. Osteogenic differentiation was also observed in situ in sclerotic porcine leaflets. Primary VICs had strikingly high frequencies of mesenchymal progenitors (48.0 +/- 5.7%) and osteoprogenitors (44.1 +/- 12.0%). High frequencies were maintained for up to six population doublings, but decreased after nine population doublings to 28.2 +/- 9.9% and 5.8 +/- 1.3%, for mesenchymal progenitors and osteoprogenitors, respectively. We further identified the putative osteoprogenitor subpopulation as morphologically distinct cells that occur at high frequency, self-renew, and elaborate bone matrix from single cells. These findings demonstrate that the aortic valve is rich in a mesenchyma l progenitor cell population that has strong potential to contribute to valve calcification.

Efficacy of simvastatin treatment of valvular interstitial cells varies with the extracellular environment

OBJECTIVE

The lack of therapies that inhibit valvular calcification and the conflicting outcomes of clinical studies regarding the impact of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors on valve disease highlight the need for controlled investigations to characterize the interactions between HMG-CoA reductase inhibitors and valve tissue. Thus, we applied multiple in vitro disease stimuli to valvular interstitial cell (VIC) cultures and examined the impact of simvastatin treatment on VIC function.

METHODS AND RESULTS

VICs were cultured on 3 different substrates that supported various levels of nodule formation. Transforming growth factor (TGF)-beta1 was also applied as a disease stimulus to VICs on 2-D surfaces or encapsulated in 3-D collagen gels and combined with different temporal applications of simvastatin. Simvastatin inhibited calcific nodule formation in a dose-dependent manner on all materials, although the level of statin efficacy was highly substrate-dependent. Simvastatin treatment significantly altered nodule morphology, resulting in dramatic nodule dissipation over time, also in a substrate-dependent manner. These effects were mimicked in 3-D cultures, wherein simvastatin reversed TGF-beta1-induced contraction. Decreases in nodule formation were not achieved via the HMG-CoA reductase pathway, but were correlated with decreases in ROCK activity.

CONCLUSIONS

These studies represent a significant contribution to understanding how simvastatin may impact heart valve calcification.

Calcific aortic stenosis: lessons learned from experimental and clinical studies

Calcific aortic stenosis is the most common indication for surgical valve replacement in the United States. For years this disease has been described as a passive degenerative process during which serum calcium attaches to the valve surface and binds to the leaflet to form nodules. Therefore, surgical treatment of this disease has been the approach toward relieving outflow obstruction in these patients. Recent studies demonstrate an association between atherosclerosis and its risk factors for aortic valve disease. In 2008, there are increasing number of epidemiology and experimental studies to provide evidence that this disease process is not a passive phenomena. There is an active cellular process that develops within the valve leaflet and causes a regulated bone formation to develop. If the atherosclerotic hypothesis is important in the initiation of aortic stenosis, then treatments used in slowing the progression of atherosclerosis may be effective in patients with aortic valve disease. This review will discuss the pathogenesis and the potential for medical therapy in the management of patients with calcific aortic stenosis by examining the lessons provided from the experimental research.

Altered shear stress stimulates upregulation of endothelial VCAM-1 and ICAM-1 in a BMP-4- and TGF-beta1-dependent pathway

OBJECTIVE

Hemodynamics has been associated with aortic valve (AV) inflammation, but the underlying mechanisms are not well understood. Here we tested the hypothesis that altered shear stress conditions stimulate the expression of cytokines and adhesion molecules in AV leaflets via a bone morphogenic protein (BMP)- and transforming growth fact (TGF)-beta1-dependent pathway.

METHODS AND RESULTS

The ventricularis or aortic surface of porcine AV leaflets were exposed for 48 hours to unidirectional pulsatile and bidirectional oscillatory shear stresses ex vivo. Immunohistochemistry was performed to detect expressions of the 4 inflammatory markers VCAM-1, ICAM-1, BMP-4, and TGF-beta1. Exposure of the aortic surface to pulsatile shear stress (altered hemodynamics), but not oscillatory shear stress, increased expression of the inflammatory markers. In contrast, neither pulsatile nor oscillatory shear stress affected expression of the inflammatory markers on the ventricularis surface. The shear stress-dependent expression of VCAM-1, ICAM-1, and BMP-4, but not TGF-beta1, was significantly reduced by the BMP inhibitor noggin, whereas the TGF-beta1 inhibitor SB431542 blocked BMP-4 expression on the aortic surface exposed to pulsatile shear stress.

CONCLUSIONS

The results demonstrate that altered hemodynamics stimulates the expression of AV leaflet endothelial adhesion molecules in a TGF-beta1- and BMP-4-dependent manner, providing some potential directions for future drug-based therapies for AV diseases.

Shared gene expression profiles in developing heart valves and osteoblast progenitor cells

The atrioventricular (AV) valves of the heart develop from undifferentiated mesenchymal endocardial cushions, which later mature into stratified valves with diversified extracellular matrix (ECM). Because the mature valves express genes associated with osteogenesis and exhibit disease-associated calcification, we hypothesized the existence of shared regulatory pathways active in developing AV valves and in bone progenitor cells. To define gene regulatory programs of valvulogenesis relative to osteoblast progenitors, we undertook Affymetrix gene expression profiling analysis of murine embryonic day (E)12.5 AV endocardial cushions compared with E17.5 AV valves (mitral and tricuspid) and with preosteoblast MC3T3-E1 (subclone4) cells. Overall, MC3T3 cells were significantly more similar to E17.5 valves than to E12.5 cushions, supporting the hypothesis that valve maturation involves the expression of many genes also expressed in osteoblasts. Several transcription factors characteristic of mesenchymal and osteoblast precursor cells, including Twist1, are predominant in E12.5 cushion. Valve maturation is characterized by differential regulation of matrix metalloproteinases and their inhibitors as well as complex collagen gene expression. Among the most highly enriched genes during valvulogenesis were members of the small leucine-rich proteoglycan (SLRP) family including Asporin, a known negative regulator of osteoblast differentiation and mineralization. Together, these data support shared gene expression profiles of the developing valves and osteoblast bone precursor cells in normal valve development and homeostasis with potential functions in calcific valve disease.

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.

The emerging role of valve interstitial cell phenotypes in regulating heart valve pathobiology

The study of the cellular and molecular pathogenesis of heart valve disease is an emerging area of research made possible by the availability of cultures of valve interstitial cells (VICs) and valve endothelial cells (VECs) and by the design and use of in vitro and in vivo experimental systems that model elements of valve biological and pathobiological activity. VICs are the most common cells in the valve and are distinct from other mesenchymal cell types in other organs. We present a conceptual approach to the investigation of VICs by focusing on VIC phenotype-function relationships. Our review suggests that there are five identifiable phenotypes of VICs that define the current understanding of their cellular and molecular functions. These include embryonic progenitor endothelial/mesenchymal cells, quiescent VICs (qVICs), activated VICs (aVICs), progenitor VICs (pVICs), and osteoblastic VICs (obVICs). Although these may exhibit plasticity and may convert from one form to another, compartmentalizing VIC function into distinct phenotypes is useful in bringing clarity to our understanding of VIC pathobiology. We present a conceptual model that is useful in the design and interpretation of studies on the function of an important phenotype in disease, the activated VIC. We hope this review will inspire members of the investigative pathology community to consider valve pathobiology as an exciting new frontier exploring pathogenesis and discovering new therapeutic targets in cardiovascular diseases.

Insights Into Degenerative Aortic Valve Disease

Despite the dramatic decline of rheumatic heart disease over the past 5 decades, there has not been a concordant decline in the prevalence of valvular heart disease. Degenerative aortic valve disease (DAVD) has become the most common cause of valvular heart disease in the Western world, causing significant morbidity and mortality. No longer considered a benign consequence of aging, valve calcification is the result of an active process that, much like atherosclerotic vascular disease, is preceded by basement membrane disruption, inflammatory cell infiltration, and lipid deposition and is associated with diabetes, hypercholesterolemia, hypertension, and tobacco use. These realizations, in addition to pathological insights gained from emerging imaging modalities, have lead to the exploration of a variety of therapeutic interventions to delay or prevent the progression of DAVD. Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, angiotensin-converting enzyme, and matrix metalloproteinase have all been studied as potential disease modifiers. Moreover, tissue engineering, aided by emerging stem cell technology, holds immense potential for the treatment of valvular heart disease as adjuncts to surgical interventions. Here we review the epidemiology and pathophysiology of DAVD, in addition to highlighting emerging therapeutic interventions for this growing problem.