Inhibition of Pathological Differentiation of Valvular Interstitial Cells by C-Type Natriuretic Peptide

Intracellular proteomics and extracellular vesiculomics as a metric of disease recapitulation in 3D-bioprinted aortic valve arrays

In calcific aortic valve disease (CAVD), mechanosensitive valvular cells respond to fibrosis- and calcification-induced tissue stiffening, further driving pathophysiology. No pharmacotherapeutics are available to treat CAVD because of the paucity of (i) appropriate experimental models that recapitulate this complex environment and (ii) benchmarking novel engineered aortic valve (AV)-model performance. We established a biomaterial-based CAVD model mimicking the biomechanics of the human AV disease-prone fibrosa layer, three-dimensional (3D)-bioprinted into 96-well arrays. Liquid chromatography-tandem mass spectrometry analyses probed the cellular proteome and vesiculome to compare the 3D-bioprinted model versus traditional 2D monoculture, against human CAVD tissue. The 3D-bioprinted model highly recapitulated the CAVD cellular proteome (94% versus 70% of 2D proteins). Integration of cellular and vesicular datasets identified known and unknown proteins ubiquitous to AV calcification. This study explores how 2D versus 3D-bioengineered systems recapitulate unique aspects of human disease, positions multiomics as a technique for the evaluation of high throughput-based bioengineered model systems, and potentiates future drug discovery.

Single-cell RNA-sequencing analysis of aortic valve interstitial cells demonstrates the regulation of integrin signaling by nitric oxide

Calcific aortic valve disease (CAVD) is an increasingly prevalent condition among the elderly population that is associated with significant morbidity and mortality. Insufficient understanding of the underlying disease mechanisms has hindered the development of pharmacologic therapies for CAVD. Recently, we described nitric oxide (NO) mediated S-nitrosylation as a novel mechanism for preventing the calcific process. We demonstrated that NO donor or an S-nitrosylating agent, S-nitrosoglutathione (GSNO), inhibits spontaneous calcification in porcine aortic valve interstitial cells (pAVICs) and this was supported by single-cell RNA sequencing (scRNAseq) that demonstrated NO donor and GSNO inhibited myofibroblast activation of pAVICs. Here, we investigated novel signaling pathways that are critical for the calcification of pAVICs that are altered by NO and GSNO by performing an in-depth analysis of the scRNA-seq dataset. Transcriptomic analysis revealed 1,247 differentially expressed genes in pAVICs after NO donor or GSNO treatment compared to untreated cells. Pathway-based analysis of the differentially expressed genes revealed an overrepresentation of the integrin signaling pathway, along with the Rho GTPase, Wnt, TGF-β, and p53 signaling pathways. We demonstrate that ITGA8 and VCL, two of the identified genes from the integrin signaling pathway, which are known to regulate cell-extracellular matrix (ECM) communication and focal adhesion, were upregulated in both in vitro and in vivo calcific conditions. Reduced expression of these genes after treatment with NO donor suggests that NO inhibits calcification by targeting myofibroblast adhesion and ECM remodeling. In addition, withdrawal of NO donor after 3 days of exposure revealed that NO-mediated transcriptional and translational regulation is a transient event and requires continuous NO exposure to inhibit calcification. Overall, our data suggest that NO and S-nitrosylation regulate the integrin signaling pathway to maintain healthy cell-ECM interaction and prevent CAVD.

Molecular Mechanism of Induction of Bone Growth by the C-Type Natriuretic Peptide

The skeletal development process in the body occurs through sequential cellular and molecular processes called endochondral ossification. Endochondral ossification occurs in the growth plate where chondrocytes differentiate from resting, proliferative, hypertrophic to calcified zones. Natriuretic peptides (NPTs) are peptide hormones with multiple functions, including regulation of blood pressure, water-mineral balance, and many metabolic processes. NPTs secreted from the heart activate different tissues and organs, working in a paracrine or autocrine manner. One of the natriuretic peptides, C-type natriuretic peptide-, induces bone growth through several mechanisms. This review will summarize the knowledge, including the newest discoveries, of the mechanism of CNP activation in bone growth.

Development of a bi-layered cryogenic electrospun polylactic acid scaffold to study calcific aortic valve disease in a 3D co-culture model

Calcified aortic valve disease (CAVD) is the most prevalent valve disease in the elderly. Targeted pharmacological therapies are limited since the underlying mechanisms of CAVD are not well understood. Appropriate 3D in vitro models could potentially improve our knowledge of the disease. Here, we developed a 3D in vitro aortic heart valve model that resembles the morphology of the valvular extracellular matrix and mimics the mechanical and physiological behavior of the native aortic valve fibrosa and spongiosa. We employed cryogenic electrospinning to engineer a bi-layered cryogenic electrospun scaffold (BCES) with defined morphologies that allowed valvular endothelial cell (VEC) adherence and valvular interstitial cell (VIC) ingrowth into the scaffold. Using a self-designed cell culture insert allowed us to establish the valvular co-culture simultaneously by seeding VICs on one side and VECs on the other side of the electrospun scaffold. Proof-of-principle calcification studies were successfully performed using an established osteogenic culture protocol and the here designed 3D in vitro aortic heart valve model. STATEMENT OF SIGNIFICANCE: Three-dimensional (3D) electrospun scaffolds are widely used for soft tissue engineering since they mimic the morphology of the native extracellular matrix. Several studies have shown that cells behave more naturally on 3D materials than on the commonly used stiff two-dimensional (2D) cell culture substrates, which have no biological properties. As appropriate 3D models for the study of aortic valve diseases are limited, we developed a novel bi-layered 3D in vitro test system by using the versatile technique of cryogenic electrospinning in combination with the influence of different solvents to mimic the morphology, mechanical, and cellular distribution of a native aortic heart valve leaflet. This 3D in vitro model can be used to study valve biology and heart valve-impacting diseases such as calcification to elucidate therapeutic targets.

Dissecting Calcific Aortic Valve Disease-The Role, Etiology, and Drivers of Valvular Fibrosis

Calcific aortic valve disease (CAVD) is a highly prevalent and progressive disorder that ultimately causes gradual narrowing of the left ventricular outflow orifice with ensuing devastating hemodynamic effects on the heart. Calcific mineral accumulation is the hallmark pathology defining this process; however, fibrotic extracellular matrix (ECM) remodeling that leads to extensive deposition of fibrous connective tissue and distortion of the valvular microarchitecture similarly has major biomechanical and functional consequences for heart valve function. Significant advances have been made to unravel the complex mechanisms that govern these active, cell-mediated processes, yet the interplay between fibrosis and calcification and the individual contribution to progressive extracellular matrix stiffening require further clarification. Specifically, we discuss (1) the valvular biomechanics and layered ECM composition, (2) patterns in the cellular contribution, temporal onset, and risk factors for valvular fibrosis, (3) imaging valvular fibrosis, (4) biomechanical implications of valvular fibrosis, and (5) molecular mechanisms promoting fibrotic tissue remodeling and the possibility of reverse remodeling. This review explores our current understanding of the cellular and molecular drivers of fibrogenesis and the pathophysiological role of fibrosis in CAVD.

Inflammatory and Biomechanical Drivers of Endothelial-Interstitial Interactions in Calcific Aortic Valve Disease

Calcific aortic valve disease is dramatically increasing in global burden, yet no therapy exists outside of prosthetic replacement. The increasing proportion of younger and more active patients mandates alternative therapies. Studies suggest a window of opportunity for biologically based diagnostics and therapeutics to alleviate or delay calcific aortic valve disease progression. Advancement, however, has been hampered by limited understanding of the complex mechanisms driving calcific aortic valve disease initiation and progression towards clinically relevant interventions.

Recent Progress Toward Clinical Translation of Tissue-Engineered Heart Valves

Surgical replacement remains the primary option to treat the rapidly growing number of patients with severe valvular heart disease. Although current valve replacements-mechanical, bioprosthetic, and cryopreserved homograft valves-enhance survival and quality of life for many patients, the ideal prosthetic heart valve that is abundantly available, immunocompatible, and capable of growth, self-repair, and life-long performance has yet to be developed. These features are essential for pediatric patients with congenital defects, children and young adult patients with rheumatic fever, and active adult patients with valve disease. Heart valve tissue engineering promises to address these needs by providing living valve replacements that function similarly to their native counterparts. This is best evidenced by the long-term clinical success of decellularised pulmonary and aortic homografts, but the supply of homografts cannot meet the demand for replacement valves. A more abundant and consistent source of replacement valves may come from cellularised valves grown in vitro or acellular off-the-shelf biomaterial/tissue constructs that recellularise in situ, but neither tissue engineering approach has yet achieved long-term success in preclinical testing. Beyond the technical challenges, heart valve tissue engineering faces logistical, economic, and regulatory challenges. In this review, we summarise recent progress in heart valve tissue engineering, highlight important outcomes from preclinical and clinical testing, and discuss challenges and future directions toward clinical translation.

Nitric oxide prevents aortic valve calcification by S-nitrosylation of USP9X to activate NOTCH signaling

Calcific aortic valve disease (CAVD) is an increasingly prevalent condition, and endothelial dysfunction is implicated in its etiology. We previously identified nitric oxide (NO) as a calcification inhibitor by its activation of NOTCH1, which is genetically linked to human CAVD. Here, we show NO rescues calcification by an S-nitrosylation-mediated mechanism in porcine aortic valve interstitial cells and single-cell RNA-seq demonstrated NO regulates the NOTCH pathway. An unbiased proteomic approach to identify S-nitrosylated proteins in valve cells found enrichment of the ubiquitin-proteasome pathway and implicated S-nitrosylation of USP9X (ubiquitin specific peptidase 9, X-linked) in NOTCH regulation during calcification. Furthermore, S-nitrosylated USP9X was shown to deubiquitinate and stabilize MIB1 for NOTCH1 activation. Consistent with this, genetic deletion of Usp9x in mice demonstrated CAVD and human calcified aortic valves displayed reduced S-nitrosylation of USP9X. These results demonstrate a previously unidentified mechanism by which S-nitrosylation-dependent regulation of a ubiquitin-associated pathway prevents CAVD.

Effect of statin therapy on plasma C-type Natriuretic Peptides and Endothelin-1 in males with and without symptomatic coronary artery disease

C-type Natriuretic Peptide (CNP) and Endothelin-1 (ET-1) have reciprocal roles in maintaining vascular homeostasis and are acutely modulated by statins in human cultured endothelial cells. Whether these actions of statins in vitro are reflected in studies in vivo is unknown. In a prospective study of 66 subjects with or without post- acute coronary syndrome (ACS), plasma concentrations of bioactive CNP and bio-inactive aminoterminal proCNP (NTproCNP), ET-1, B-type Natriuretic Peptide (BNP) and high sensitivity C Reactive Protein (hsCRP) were measured together with lipids before and at intervals of 1, 2 and 7 days after commencing atorvastatin 40 mg/day - and for a further period of 6months in those with ACS. Plasma lipids fell significantly in all subjects but plasma CNP, NTproCNP and ET-1 were unchanged by atorvastatin. In ACS, baseline hsCRP, BNP and CNP but not NTproCNP or ET-1 were significantly raised compared to values in age-matched controls. The ratio of NTproCNP to CNP was significantly lower in ACS throughout the study and was unaffected by statin therapy. We conclude that conventional doses of atorvastatin do not affect plasma CNP products or ET-1. Elevated CNP after cardiac injury likely results from regulated changes in clearance, not enhanced production.

Aortic Valve Stenosis: From Basic Mechanisms to Novel Therapeutic Targets

Aortic valve stenosis is the most prevalent heart valve disease worldwide. Although interventional treatment options have rapidly improved in recent years, symptomatic aortic valve stenosis is still associated with high morbidity and mortality. Calcific aortic valve stenosis is characterized by a progressive fibro-calcific remodeling and thickening of the aortic valve cusps, which subsequently leads to valve obstruction. The underlying pathophysiology is complex and involves endothelial dysfunction, immune cell infiltration, myofibroblastic and osteoblastic differentiation, and, subsequently, calcification. To date, no pharmacotherapy has been established to prevent aortic valve calcification. However, novel promising therapeutic targets have been recently identified. This review summarizes the current knowledge of pathomechanisms involved in aortic valve calcification and points out novel treatment strategies.

MicroRNA-204 Deficiency in Human Aortic Valves Elevates Valvular Osteogenic Activity

Aortic valve interstitial cells (AVICs) play a major role in valvular calcification associated with calcific aortic valve disease (CAVD). Although AVICs from diseased valves display a pro-osteogenic phenotype, the underlying mechanism causing this remains unclear. MicroRNA-204 (miR-204) is a negative regulator of osteoblast differentiation. We sought to analyze miR-204 expression in diseased human aortic valves and determine the role of this miR in AVIC osteogenic activity associated with CAVD pathobiology. In situ hybridization and PCR analysis revealed miR-204 deficiency in diseased valves and in AVICs from diseased valves. MiR-204 mimic suppressed alkaline phosphatase (ALP) expression and calcium deposition in AVICs from diseased valves. MiR-204 antagomir enhanced ALP expression in AVICs from normal valves through induction of Runx2 and Osx, and expression of miR-204 antagomir in mouse aortic valves promoted calcium deposition through up-regulation of Runx2 and Osx. Further, miR-204 mimic suppressed the osteogenic responses to TGF-β1 in AVICs of normal valves. In conclusion, miR-204 deficiency contributes to the mechanism underlying elevated osteogenic activity in diseased aortic valves, and miR-204 is capable of reversing the pro-osteogenic phenotype of AVICs of diseased valves and suppressing AVIC osteogenic response to stimulation. Exogenous miR-204 may have therapeutic potential for inhibiting valvular calcification associated with CAVD progression.

MicroRNA-638 inhibits human aortic valve interstitial cell calcification by targeting Sp7

Calcific aortic valve disease (CAVD) is a complex heart valve disease involving a wide range of pathological changes. Emerging evidence indicates that osteogenic differentiation of human aortic valve interstitial cells (hAVICs) plays a key role in valve calcification. In this study, we aimed to investigate the function of miR-638 in hAVICs osteogenesis. Both miRNA microarray assay and qRT-PCR results demonstrating miR-638 was obviously up-regulated in calcific aortic valves compared with non-calcific valves. We also proved that miR-638 was significantly up-regulated during hAVICs osteogenic differentiation. Overexpression of miR-638 suppressed osteogenic differentiation of hAVICs in vitro, whereas down-regulation of miR-638 enhance the process. Target prediction analysis and dual-luciferase reporter assay confirmed that Sp7 transcription factor (Sp7) was a direct target of miR-638. Furthermore, knockdown of Sp7 inhibited osteogenic differentiation of hAVICs, which is similar to the results observed in up-regulation miR-638. Our data indicated that miR-638 plays an inhibitory role in hAVICs osteogenic differentiation, which may act by targeting Sp7. MiR-638 may be a potential therapeutic target for CAVD.

Discovery of O-glycans on atrial natriuretic peptide (ANP) that affect both its proteolytic degradation and potency at its cognate receptor

Atrial natriuretic peptide (ANP) is a peptide hormone that in response to atrial stretch is secreted from atrial myocytes into the circulation, where it stimulates vasodilatation and natriuresis. ANP is an important biomarker of heart failure where low plasma concentrations exclude cardiac dysfunction. ANP is a member of the natriuretic peptide (NP) family, which also includes the B-type natriuretic peptide (BNP) and the C-type natriuretic peptide. The proforms of these hormones undergo processing to mature peptides, and for proBNP, this process has previously been demonstrated to be regulated by O-glycosylation. It has been suggested that proANP also may undergo post-translational modifications. Here, we conducted a targeted O-glycoproteomics approach to characterize O-glycans on NPs and demonstrate that all NP members can carry O-glycans. We identified four O-glycosites in proANP in the porcine heart, and surprisingly, two of these were located on the mature bioactive ANP itself. We found that one of these glycans is located within a conserved sequence motif of the receptor-binding region, suggesting that O-glycans may serve a function beyond intracellular processing and maturation. We also identified an O-glycoform of proANP naturally occurring in human circulation. We demonstrated that site-specific O-glycosylation shields bioactive ANP from proteolytic degradation and modifies potency at its cognate receptor in vitro Furthermore, we showed that ANP O-glycosylation attenuates acute renal and cardiovascular ANP actions in vivo The discovery of novel glycosylated ANP proteoforms reported here significantly improves our understanding of cardiac endocrinology and provides important insight into the etiology of heart failure.

Development of calcific aortic valve disease: Do we know enough for new clinical trials?

Calcific aortic valve disease (CAVD), previously thought to represent a passive degeneration of the valvular extracellular matrix (VECM), is now regarded as an intricate multistage disorder with sequential yet intertangled and interacting underlying processes. Endothelial dysfunction and injury, initiated by disturbed blood flow and metabolic disorders, lead to the deposition of low-density lipoprotein cholesterol in the VECM further provoking macrophage infiltration, oxidative stress, and release of pro-inflammatory cytokines. Such changes in the valvular homeostasis induce differentiation of normally quiescent valvular interstitial cells (VICs) into synthetically active myofibroblasts producing excessive quantities of the VECM and proteins responsible for its remodeling. As a result of constantly ongoing degradation and re-deposition, VECM becomes disorganised and rigid, additionally potentiating myofibroblastic differentiation of VICs and worsening adaptation of the valve to the blood flow. Moreover, disrupted and excessively vascularised VECM is susceptible to the dystrophic calcification caused by calcium and phosphate precipitating on damaged collagen fibers and concurrently accompanied by osteogenic differentiation of VICs. Being combined, passive calcification and biomineralisation synergistically induce ossification of the aortic valve ultimately resulting in its mechanical incompetence requiring surgical replacement. Unfortunately, multiple attempts have failed to find an efficient conservative treatment of CAVD; however, therapeutic regimens and clinical settings have also been far from the optimal. In this review, we focused on interactions and transitions between aforementioned mechanisms demarcating ascending stages of CAVD, suggesting a predisposing condition (bicuspid aortic valve) and drug combination (lipid-lowering drugs combined with angiotensin II antagonists and cytokine inhibitors) for the further testing in both preclinical and clinical trials.

Differential expression patterns of Toll Like Receptors and Interleukin-37 between calcific aortic and mitral valve cusps in humans

BACKGROUND

Significant differences are mentioned in the progress of calcification between aortic and mitral valve. Evidence of inflammation in calcific aortic and mitral valve disease suggests that pathways of Toll Like Receptors (TLR) and Interleukin (IL)-37 expression may contribute to this process. We sought to investigate the role of TLR-mediated inflammatory response and IL-37 pathway expression on aortic and mitral valve calcification.

MATERIAL AND METHODS

One-hundred twenty stenotic valve cusps/leaflets (60 aortic, 60 mitral) were excised during surgery and were collected for histological, immunohistochemistry and morphometric analysis at our department. After total RNA isolation from a second part of valve cusps/leaflets, cDNA synthesis and quantitative reverse transcription polymerase chain reaction (qRT-PCR) protocols were performed and relative mRNA levels of target genes were assessed.

RESULTS

By histological analysis, the anti-inflammatory IL-37 levels were increased in mitral valve leaflets (MVL) compared to aortic valve cusps (AVCu) while all other biomarkers, including TLR, presented a reverse pattern with decreased levels as compared to AVCu. In terms of calcification biomarkers, only osteopontin differed between AVCu and MVL. mRNA analysis confirmed increased expression of IL-37 and decreased levels of TLR in MVL compared to AVCu.

CONCLUSIONS

Stenotic cusps of aortic valves express lower IL-37 and increased TLRs levels than stenotic mitral valve leaflets, suggesting a differential pro-calcification and pro-inflammatory profile between the two valves. This may explain the higher incidence of calcification of AVCu than MVL and offer therapeutic considerations.

Deficiency of Natriuretic Peptide Receptor 2 Promotes Bicuspid Aortic Valves, Aortic Valve Disease, Left Ventricular Dysfunction, and Ascending Aortic Dilatations in Mice

RATIONALE

Aortic valve disease is a cell-mediated process without effective pharmacotherapy. CNP (C-type natriuretic peptide) inhibits myofibrogenesis and osteogenesis of cultured valve interstitial cells and is downregulated in stenotic aortic valves. However, it is unknown whether CNP signaling regulates aortic valve health in vivo.

OBJECTIVE

The aim of this study is to determine whether a deficient CNP signaling axis in mice causes accelerated progression of aortic valve disease.

METHODS AND RESULTS

In cultured porcine valve interstitial cells, CNP inhibited pathological differentiation via the guanylate cyclase NPR2 (natriuretic peptide receptor 2) and not the G-protein-coupled clearance receptor NPR3 (natriuretic peptide receptor 3). We used Npr2^+/- and Npr2^+/-;Ldlr^-/- mice and wild-type littermate controls to examine the valvular effects of deficient CNP/NPR2 signaling in vivo, in the context of both moderate and advanced aortic valve disease. Myofibrogenesis in cultured Npr2^+/- fibroblasts was insensitive to CNP treatment, whereas aged Npr2^+/- and Npr2^+/-;Ldlr-/- mice developed cardiac dysfunction and ventricular fibrosis. Aortic valve function was significantly impaired in Npr2^+/- and Npr2^+/-;Ldlr^-/- mice versus wild-type littermates, with increased valve thickening, myofibrogenesis, osteogenesis, proteoglycan synthesis, collagen accumulation, and calcification. 9.4% of mice heterozygous for Npr2 had congenital bicuspid aortic valves, with worse aortic valve function, fibrosis, and calcification than those Npr2^+/- with typical tricuspid aortic valves or all wild-type littermate controls. Moreover, cGK (cGMP-dependent protein kinase) activity was downregulated in Npr2^+/- valves, and CNP triggered synthesis of cGMP and activation of cGK1 (cGMP-dependent protein kinase 1) in cultured porcine valve interstitial cells. Finally, aged Npr2^+/-;Ldlr^-/- mice developed dilatation of the ascending aortic, with greater aneurysmal progression in Npr2^+/- mice with bicuspid aortic valves than those with tricuspid valves.

CONCLUSIONS

Our data establish CNP/NPR2 signaling as a novel regulator of aortic valve development and disease and elucidate the therapeutic potential of targeting this pathway to arrest disease progression.

Mechanisms of Smooth Muscle Cell Differentiation Are Distinctly Altered in Thoracic Aortic Aneurysms Associated with Bicuspid or Tricuspid Aortic Valves

Cellular and molecular mechanisms of thoracic aortic aneurysm are not clear and therapeutic approaches are mostly absent. Thoracic aortic aneurysm is associated with defective differentiation of smooth muscle cells (SMC) of aortic wall. Bicuspid aortic valve (BAV) comparing to tricuspid aortic valve (TAV) significantly predisposes to a risk of thoracic aortic aneurysms. It has been suggested recently that BAV-associated aortopathies represent a separate pathology comparing to TAV-associated dilations. The only proven candidate gene that has been associated with BAV remains NOTCH1. In this study we tested the hypothesis that Notch-dependent and related TGF-β and BMP differentiation pathways are differently altered in aortic SMC of BAV- vs. TAV-associated aortic aneurysms. SMC were isolated from aortic tissues of the patients with BAV- or TAV-associated aortic aneurysms and from healthy donors used as controls. Gene expression was verified by qPCR and Western blotting. For TGF-β induced differentiation SMC were treated with the medium containing TGF-β1. To induce proosteogenic signaling we cultured SMC in the presence of specific osteogenic factors. Notch-dependent differentiation was induced via lentiviral transduction of SMC with activated Notch1 domain. MYOCD expression, a master gene of SMC differentiation, was down regulated in SMC of both BAV and TAV patients. Discriminant analysis of gene expression patterns included a set of contractile genes specific for SMC, Notch-related genes and proosteogenic genes and revealed that control cells form a separate cluster from both BAV and TAV group, while BAV- and TAV-derived SMC are partially distinct with some overlapping. In differentiation experiments TGF-β caused similar patterns of target gene expression for BAV- and TAV derived cells while the induction was higher in the diseased cells than in control ones. Osteogenic induction caused significant change in RUNX2 expression exclusively in BAV group. Notch activation induced significant ACTA2 expression also exclusively in BAV group. We show that Notch acts synergistically with proosteogenic factors to induce ACTA2 transcription and osteogenic differentiation. In conclusion we have found differences in responsiveness of SMC to Notch and to proosteogenic induction between BAV- and TAV-associated aortic aneurysms.

Altered MicroRNA Expression Is Responsible for the Pro-Osteogenic Phenotype of Interstitial Cells in Calcified Human Aortic Valves

BACKGROUND

The transition of aortic valve interstitial cells (AVICs) to myofibroblastic and osteoblast-like phenotypes plays a critical role in calcific aortic valve disease progression. Several microRNAs (miRs) are implicated in stem cell differentiation into osteoblast. We hypothesized that an epigenetic mechanism regulates valvular pro-osteogenic activity. This study examined miR profile in AVICs of calcified valves and identified miRs responsible for AVIC phenotypic transition.

METHODS AND RESULTS

AVICs were isolated from normal and diseased valves. The miR microarray analysis revealed 14 upregulated and 12 downregulated miRs in diseased AVICs. Increased miR-486 and decreased miR-204 levels were associated with higher levels of myofibroblastic biomarker α-smooth muscle actin and osteoblastic biomarkers runt-related transcription factor 2 (Runx2) and osterix (Osx). Cotransfection of miR-486 antagomir and miR-204 mimic in diseased AVICs reduced their ability to express Runx2 and Osx. The miR-486 mimic upregulated α-smooth muscle actin expression in normal AVICs through the protein kinase B pathway and moderately elevated Runx2 and Osx levels. Knockdown of α-smooth muscle actin attenuated Runx2 and Osx expression induced by miR-486. The miR-486 mimic and miR-204 antagomir synergistically promoted Runx2 and Osx expression and calcium deposition in normal AVICs and normal aortic valve tissue.

CONCLUSIONS

In AVICs of calcified valves, increased levels of miR-486 induce myofibroblastic transition to upregulate Runx2 and Osx expression and synergize with miR-204 deficiency to elevate cellular and valvular pro-osteogenic activity. These novel findings indicate that modulation of the epigenetic mechanism underlying valvular pro-osteogenic activity has therapeutic potential for prevention of calcific aortic valve disease progression.

Targeting vasoactive peptides for managing calcific aortic valve disease

Calcific aortic valve disease (CAVD) represents a spectrum of disease spanning from milder degrees of calcification of valve leaflets, i.e., aortic sclerosis, to severe calcification i.e., aortic stenosis (AS) with hemodynamic instability. The prevalence of CAVD is increasing rapidly due to the aging of the population, being up to 2.8% among patients over 75 years of age. Even without significant aortic valve stenosis, aortic sclerosis is associated with a 50% increased risk of myocardial infarction and death from cardiovascular causes. To date, there is no pharmacological treatment available to reverse or hinder the progression of CAVD. So far, the cholesterol-lowering therapies (statins) and renin-angiotensin system (RAS) blocking drugs have been the major pharmacological agents investigated for treatment of CAVD. Especially angiotensin receptor blockers (ARB)s and angiotensin convertase enzyme inhibitors (ACEI)s, have been under active investigation in clinical trials, but have proven to be unsuccessful in slowing the progression of CAVD. Several studies have suggested that other vasoactive hormones, including endothelin and apelin systems are also associated with development of AS. In the present review, we discuss the role of vasoactive factors in the pathogenesis of CAVD as novel pharmacological targets for the treatment of aortic valve calcification. Key messages Vasoactive factors are involved in the progression of calcific aortic valve disease. Endothelin and renin-angiotensin systems seem to be most prominent targets for therapeutic interventions in the view of valvular pathogenesis. Circulating vasoactive factors may provide targets for diagnostic tools of calcified aortic valve disease.

Integrated microRNA and messenger RNA analysis in aortic stenosis

Aortic valve stenosis (AS) is a major cause of morbidity and mortality, with no effective medical therapies. Investigation into the underlying biology of AS in humans is limited by difficulties in obtaining healthy valvular tissue for use as a control group. However, micro-ribonucleic acids (miRNAs) are stable in post-mortem tissue. We compared valve specimens from patients undergoing aortic valve replacement for AS to non-diseased cadaveric valves. We found 106 differentially expressed miRNAs (p < 0.05, adjusted for multiple comparisons) on microarray analysis, with highly correlated expression among up- and down-regulated miRNAs. Integrated miRNA/gene expression analysis validated the microarray results as a whole, while quantitative polymerase chain reaction confirmed downregulation of miR-122-5p, miR-625-5p, miR-30e-5p and upregulation of miR-21-5p and miR-221-3p. Pathway analysis of the integrated miRNA/mRNA network identified pathways predominantly involved in extracellular matrix function. A number of currently available therapies target products of upregulated genes in the integrated miRNA/mRNA network, with these genes being predominantly more peripheral members of the network. The identification of a group of tissue miRNA associated with AS may contribute to the development of new therapeutic approaches to AS. This study highlights the importance of systems biology-based approaches to complex diseases.

Biomechanical conditioning of tissue engineered heart valves: Too much of a good thing?

Surgical replacement of dysfunctional valves is the primary option for the treatment of valvular disease and congenital defects. Existing mechanical and bioprosthetic replacement valves are far from ideal, requiring concomitant anticoagulation therapy or having limited durability, thus necessitating further surgical intervention. Heart valve tissue engineering (HVTE) is a promising alternative to existing replacement options, with the potential to synthesize mechanically robust tissue capable of growth, repair, and remodeling. The clinical realization of a bioengineered valve relies on the appropriate combination of cells, biomaterials, and/or bioreactor conditioning. Biomechanical conditioning of valves in vitro promotes differentiation of progenitor cells to tissue-synthesizing myofibroblasts and prepares the construct to withstand the complex hemodynamic environment of the native valve. While this is a crucial step in most HVTE strategies, it also may contribute to fibrosis, the primary limitation of engineered valves, through sustained myofibrogenesis. In this review, we examine the progress of HVTE and the role of mechanical conditioning in the synthesis of mechanically robust tissue, and suggest approaches to achieve myofibroblast quiescence and prevent fibrosis.

Biophysical analysis of dystrophic and osteogenic models of valvular calcification

Calcific aortic valve disease (CAVD) is a significant cardiovascular disorder characterized by the formation of calcific nodules (CN) on the valve. In vitro assays studying the formation of these nodules were developed and have led to many significant mechanistic findings; however, the biophysical properties of CNs have not been clearly defined. A thorough analysis of dystrophic and osteogenic nodules utilizing scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and atomic force microscopy (AFM) was conducted to describe calcific nodule properties and provide a link between calcific nodule morphogenesis in vitro and in vivo. Unique nodule properties were observed for dystrophic and osteogenic nodules, highlighting the distinct mechanisms occurring in valvular calcification.

Directing valvular interstitial cell myofibroblast-like differentiation in a hybrid hydrogel platform

Three dimensional (3D) hydrogel platforms are powerful tools, providing controllable, physiologically relevant microenvironments that could aid in understanding how various environmental factors direct valvular interstitial cell (VIC) phenotype. Continuous activation of VICs and their transformation from quiescent fibroblast to activated myofibroblast phenotype is considered to be an initiating event in the onset of valve disease. However, the relative contribution VIC phenotypes is poorly understood since most 2D culture systems lead to spontaneous VIC myofibroblastic activation. Here, a hydrogel platform composed of photocrosslinkable versions of native valvular extracellular matrix components-methacrylated hyaluronic acid (HAMA) and methacrylated gelatin (GelMA)-is proposed as a 3D culture system to study VIC phenotypic changes. These results show that VIC myofibroblast-like differentiation occurs spontaneously in mechanically soft GelMA hydrogels. Conversely, differentiation of VICs encapsulated in HAMA-GelMA hybrid hydrogels, does not occur spontaneously and requires exogenous delivery of TGFβ1, indicating that hybrid hydrogels can be used to study cytokine-dependent transition of VICs. This study demonstrates that a hybrid hydrogel platform can be used to maintain a quiescent VIC phenotype and study the effect of environmental cues on VIC activation, which will aid in understanding pathobiology of valvular disease.

In vitro models of aortic valve calcification: solidifying a system

Calcific aortic valve disease (CAVD) affects 25% of people over 65, and the late-stage stenotic state can only be treated with total valve replacement, requiring 85,000 surgeries annually in the US alone (University of Maryland Medical Center, 2013, http://umm.edu/programs/services/heart-center-programs/cardiothoracic-surgery/valve-surgery/facts). As CAVD is an age-related disease, many of the affected patients are unable to undergo the open-chest surgery that is its only current cure. This challenge motivates the elucidation of the mechanisms involved in calcification, with the eventual goal of alternative preventative and therapeutic strategies. There is no sufficient animal model of CAVD, so we turn to potential in vitro models. In general, in vitro models have the advantages of shortened experiment time and better control over multiple variables compared to in vivo models. As with all models, the hypothesis being tested dictates the most important characteristics of the in vivo physiology to recapitulate. Here, we collate the relevant pieces of designing and evaluating aortic valve calcification so that investigators can more effectively draw significant conclusions from their results.

Isolation and culture of rat aortic valve interstitial cells

Objective:

Culturing aortic valve interstitial cells is a useful way to investigate the physiology and pathology of the aortic valve at the cellular level. The culture methods of the cells have been established in many species. However, the previous methods need some improvements.

Methods:

We evaluated various techniques with regard to the isolation of Sprague-Dawley (SD) rat aortic valve interstitial cells and established suitable conditions about the culture and passage of the primary cells. The specimens from the aortic valve were processed by tissue explant methods before seeding them onto the dishes.

Results:

The cells obtained emerged from the explants after 2 to 3 days and stained positive for a-SMA and vimentin protein. Moreover, transmission electron microscopy images showed that the cells had abundant mitochondria, prominent rough endoplasmic reticulum, and plentiful myofilaments.

Conclusion:

In the present study, we provided reliable and efficient methods for the isolation and culture of rat aortic valve interstitial cells that could serve for in vitro studies on aortic valve physiology and pathophysiology.

Differential Aortic and Mitral Valve Interstitial Cell Mineralization and the Induction of Mineralization by Lysophosphatidylcholine In Vitro

PURPOSE

Calcific aortic valve disease (CAVD) is a serious condition with vast uncertainty regarding the precise mechanism leading to valve calcification. This study was undertaken to examine the role of the lipid lysophosphatidylcholine (LPC) in a comparison of aortic and mitral valve cellular mineralization.

METHODS

The proportion of LPC in differentially calcified regions of diseased aortic valves was determined using thin layer chromatography (TLC). Next, porcine valvular interstitial cells (pVICs) from the aortic (paVICs) and mitral valve (pmVICs) were cultured with LPC (10-1 - 105 nM) and analyzed for cellular mineralization, alkaline phosphatase activity (ALPa), proliferation, and apoptosis.

RESULTS

TLC showed a higher percentage of LPC in calcified regions of tissue compared to non-calcified regions. In pVIC cultures, with the exception of 105 nM LPC, increasing concentrations of LPC led to an increase in phosphate mineralization. Increased levels of calcium content were exhibited at 104 nm LPC application compared to baseline controls. Compared to pmVIC cultures, paVIC cultures had greater total phosphate mineralization, ALPa, calcium content, and apoptosis, under both a baseline control and LPC-treated conditions.

CONCLUSIONS

This study showed that LPC has the capacity to promote pVIC calcification. Also, paVICs have a greater propensity for mineralization than pmVICs. LPC may be a key factor in the transition of the aortic valve from a healthy to diseased state. In addition, there are intrinsic differences that exist between VICs from different valves that may play a key role in heart valve pathology.

Cardiac valve cells and their microenvironment--insights from in vitro studies

During every heartbeat, cardiac valves open and close coordinately to control the unidirectional flow of blood. In this dynamically challenging environment, resident valve cells actively maintain homeostasis, but the signalling between cells and their microenvironment is complex. When homeostasis is disrupted and the valve opening obstructed, haemodynamic profiles can be altered and lead to impaired cardiac function. Currently, late stages of cardiac valve diseases are treated surgically, because no drug therapies exist to reverse or halt disease progression. Consequently, investigators have sought to understand the molecular and cellular mechanisms of valvular diseases using in vitro cell culture systems and biomaterial scaffolds that can mimic the extracellular microenvironment. In this Review, we describe how signals in the extracellular matrix regulate valve cell function. We propose that the cellular context is a critical factor when studying the molecular basis of valvular diseases in vitro, and one should consider how the surrounding matrix might influence cell signalling and functional outcomes in the valve. Investigators need to build a systems-level understanding of the complex signalling network involved in valve regulation, to facilitate drug target identification and promote in situ or ex vivo heart valve regeneration.

Osteopontin-CD44v6 interaction mediates calcium deposition via phospho-Akt in valve interstitial cells from patients with noncalcified aortic valve sclerosis

OBJECTIVE

The activation of valve interstitial cells (VICs) toward an osteogenic phenotype characterizes aortic valve sclerosis, the early asymptomatic phase of calcific aortic valve disease. Osteopontin is a phosphorylated acidic glycoprotein that accumulates within the aortic leaflets and labels VIC activation even in noncalcified asymptomatic patients. Despite this, osteopontin protects VICs against in vitro calcification. Here, we hypothesize that the specific interaction of osteopontin with CD44v6, and the related intracellular pathway, prevents calcium deposition in human-derived VICs from patients with aortic valve sclerosis.

APPROACH AND RESULTS

On informed consent, 23 patients and 4 controls were enrolled through the cardiac surgery and heart transplant programs. Human aortic valves and VICs were tested for osteogenic transdifferentiation, ex vivo and in vitro. Osteopontin-CD44 interaction was analyzed using proximity ligation assay and the signaling pathways investigated. A murine model based on angiotensin II infusion was used to mimic early pathological remodeling of the aortic valves. We report osteopontin-CD44 functional interaction as a hallmark of early stages of calcific aortic valve disease. We demonstrated that osteopontin-CD44 interaction mediates calcium deposition via phospho-Akt in VICs from patients with noncalcified aortic valve sclerosis. Finally, microdissection analysis of murine valves shows increased cusp thickness in angiotensin II-treated mice versus saline infused along with colocalization of osteopontin and CD44 as seen in human lesions.

CONCLUSIONS

Here, we unveil a specific protein-protein association and intracellular signaling mechanisms of osteopontin. Understanding the molecular mechanisms of early VIC activation and calcium deposition in asymptomatic stage of calcific aortic valve disease could open new prospective for diagnosis and therapeutic intervention.

Reversal of myofibroblastic activation by polyunsaturated fatty acids in valvular interstitial cells from aortic valves. Role of RhoA/G-actin/MRTF signalling

Valvular interstitial cells (VICs), the fibroblast-like cellular constituents of aortic heart valves, maintain structural integrity of valve tissue. Activation into contractile myofibroblasts occurs under pathological situations and under standard cell culture conditions of isolated VICs. Reversal of this phenotype switch would be of major importance in respect to fibrotic valve diseases. In this investigation, we found that exogenous polyunsaturated fatty acids (PUFAs) decreased contractility and expression of myofibroblastic markers like α-smooth muscle actin (αSMA) in cultured VICs from porcine aortic valves. The most active PUFAs, docosahexaenoic acid (DHA) and arachidonic acid (AA) reduced the level of active RhoA and increased the G/F-actin ratio. The G-actin-regulated nuclear translocation of myocardin-related transcription factors (MRTFs), co-activators of serum response factor, was also reduced by DHA and AA. The same effects were observed after blocking RhoA directly with C3 transferase. In addition, increased contractility after induction of actin polymerisation with jasplakinolide and concomitant expression of αSMA were ameliorated by active PUFAs. Furthermore, reduced αSMA expression under PUFA exposure was observed in valve tissue explants demonstrating physiological relevance. In conclusion, RhoA/G-actin/MRTF signalling is operative in VICs, and this pathway can be partially blocked by certain PUFAs whereby the activation into the myofibroblastic phenotype is reversed.

The living aortic valve: From molecules to function

The aortic valve lies in a unique hemodynamic environment, one characterized by a range of stresses (shear stress, bending forces, loading forces and strain) that vary in intensity and direction throughout the cardiac cycle. Yet, despite its changing environment, the aortic valve opens and closes over 100,000 times a day and, in the majority of human beings, will function normally over a lifespan of 70–90 years. Until relatively recently heart valves were considered passive structures that play no active role in the functioning of a valve, or in the maintenance of its integrity and durability. However, through clinical experience and basic research the aortic valve can now be characterized as a living, dynamic organ with the capacity to adapt to its complex mechanical and biomechanical environment through active and passive communication between its constituent parts. The clinical relevance of a living valve substitute in patients requiring aortic valve replacement has been confirmed. This highlights the importance of using tissue engineering to develop heart valve substitutes containing living cells which have the ability to assume the complex functioning of the native valve.

Mechanisms of calcification in aortic valve disease: role of mechanokinetics and mechanodynamics

The aortic valve is highly responsive to cyclical and continuous mechanical forces, at the macroscopic and cellular levels. In this report, we delineate mechanokinetics (effects of mechanical inputs on the cells) and mechanodynamics (effects of cells and pathologic processes on the mechanics) of the aortic valve, with a particular focus on how mechanical inputs synergize with the inflammatory cytokine and other biomolecular signaling to contribute to the process of aortic valve calcification.

Hemodynamic and cellular response feedback in calcific aortic valve disease

This review highlights aspects of calcific aortic valve disease that encompass the entire range of aortic valve disease progression from initial cellular changes to aortic valve sclerosis and stenosis, which can be initiated by changes in blood flow (hemodynamics) and pressure across the aortic valve. Appropriate hemodynamics is important for normal valve function and maintenance, but pathological blood velocities and pressure can have profound consequences at the macroscopic to microscopic scales. At the macroscopic scale, hemodynamic forces impart shear stresses on the surface of the valve leaflets and cause deformation of the leaflet tissue. As discussed in this review, these macroscale forces are transduced to the microscale, where they influence the functions of the valvular endothelial cells that line the leaflet surface and the valvular interstitial cells that populate the valve extracellular matrix. For example, pathological changes in blood flow-induced shear stress can cause dysfunction, impairing their homeostatic functions, and pathological stretching of valve tissue caused by elevated transvalvular pressure can activate valvular interstitial cells and latent paracrine signaling cytokines (eg, transforming growth factor-β1) to promote maladaptive tissue remodeling. Collectively, these coordinated and complex interactions adversely impact bulk valve tissue properties, feeding back to further deteriorate valve function and propagate valve cell pathological responses. Here, we review the role of hemodynamic forces in calcific aortic valve disease initiation and progression, with focus on cellular responses and how they feed back to exacerbate aortic valve dysfunction.

Molecular and cellular aspects of calcific aortic valve disease

Calcific aortic valve disease (CAVD) increasingly afflicts our aging population. One third of our elderly have echocardiographic or radiological evidence of calcific aortic valve sclerosis, an early and subclinical form of CAVD. Age, sex, tobacco use, hypercholesterolemia, hypertension, and type II diabetes mellitus all contribute to the risk of disease that has worldwide distribution. On progression to its most severe form, calcific aortic stenosis, CAVD becomes debilitating and devastating, and 2% of individuals >60 years are affected by calcific aortic stenosis to the extent that surgical intervention is required. No effective pharmacotherapies exist for treating those at risk for clinical progression. It is becoming increasingly apparent that a diverse spectrum of cellular and molecular mechanisms converge to regulate valvular calcium load; this is evidenced not only in histopathologic heterogeneity of CAVD, but also from the multiplicity of cell types that can participate in valve biomineralization. In this review, we highlight our current understanding of CAVD disease biology, emphasizing molecular and cellular aspects of its regulation. We end by pointing to important biological and clinical questions that must be answered to enable sophisticated disease staging and the development of new strategies to treat CAVD medically.

A 3D microfluidic platform incorporating methacrylated gelatin hydrogels to study physiological cardiovascular cell-cell interactions

The cardiovascular system is particularly well-suited to modelling with microfluidic technologies, and much progress has been made to create microfluidic devices that mimic the microvasculature. In contrast, microfluidic platforms that model larger blood vessels and heart valves are lacking, despite the clear potential benefits of improved physiological relevance and enhanced throughput over traditional cell culture technologies. To address this need, we developed a bilayer membrane microfluidic device to model the vascular/valvular three-dimensional environment. Key features of the platform include physiologically-relevant spatial arrangement of multiple cell types, fluid flow over an endothelial monolayer, a porous membrane that permits heterotypic cell interactions while maintaining cell compartmentalization, and a photopolymerizable gelatin methacrylate (gel-MA) hydrogel as a physiologically-relevant subendothelial 3D matrix. Processing guidelines were defined for successful in-channel polymerization of gel-MA hydrogels that were mechanically stable, had physiologically-relevant elastic moduli of 2-30 kPa, and supported over 80% primary cell viability for at least four days in culture. The platform was applied to investigate shear stress-regulated paracrine interactions between valvular endothelial cells and valvular interstitial cells. The presence of endothelial cells significantly suppressed interstitial cell pathological differentiation to α-smooth muscle actin-positive myofibroblasts, an effect that was enhanced when the endothelium was exposed to flow-induced shear stress. We expect this versatile organ-on-a-chip platform to have broad utility for mechanistic vascular and valvular biology studies and to be useful for drug screening in physiologically-relevant 3D cardiovascular microenvironments.

Side-specific endothelial-dependent regulation of aortic valve calcification: interplay of hemodynamics and nitric oxide signaling

Arterial endothelial cells maintain vascular homeostasis and vessel tone in part through the secretion of nitric oxide (NO). In this study, we determined how aortic valve endothelial cells (VEC) regulate aortic valve interstitial cell (VIC) phenotype and matrix calcification through NO. Using an anchored in vitro collagen hydrogel culture system, we demonstrate that three-dimensionally cultured porcine VIC do not calcify in osteogenic medium unless under mechanical stress. Co-culture with porcine VEC, however, significantly attenuated VIC calcification through inhibition of myofibroblastic activation, osteogenic differentiation, and calcium deposition. Incubation with the NO donor DETA-NO inhibited VIC osteogenic differentiation and matrix calcification, whereas incubation with the NO blocker l-NAME augmented calcification even in 3D VIC-VEC co-culture. Aortic VEC, but not VIC, expressed endothelial NO synthase (eNOS) in both porcine and human valves, which was reduced in osteogenic medium. eNOS expression was reduced in calcified human aortic valves in a side-specific manner. Porcine leaflets exposed to the soluble guanylyl cyclase inhibitor ODQ increased osteocalcin and α-smooth muscle actin expression. Finally, side-specific shear stress applied to porcine aortic valve leaflet endothelial surfaces increased cGMP production in VEC. Valve endothelial-derived NO is a natural inhibitor of the early phases of valve calcification and therefore may be an important regulator of valve homeostasis and pathology.

A time course investigation of the statin paradox among valvular interstitial cell phenotypes

Statin drugs are prescribed primarily for their ability to lower cholesterol, but may also exert beneficial side effects unrelated to cholesterol metabolism. Previous work has described a "statin paradox," where statin treatment decreased osteoblastic markers in valve myofibroblasts while increasing those same markers in preosteoblasts. However, valvular interstitial cells (VICs) themselves are a multipotent cell type, capable of differentiating into activated, myofibroblastic VICs (aVICs) and osteoblastic VICs (obVICs), motivating the question of whether the statin paradox can exist within an individual valve containing these phenotypically distinct VIC subpopulations. In the current study, a heterogeneous initial population of porcine VICs was differentiated into aVICs or obVICs and treated with simvastatin. Gene expression analysis was conducted daily over an 8-day time course to capture temporally dynamic changes in cell phenotype induced by statin treatment. These studies demonstrated that the two VIC populations, aVICs and obVICs, exhibited differential responses to statin treatment. Specifically, simvastatin increased the expression of osteoblastic markers in obVICs, but not in aVICs, while also suppressing the myofibroblastic phenotype in both aVICs and obVICs. These results indicate that the statin paradox can exist within the heterogeneous VIC population of an individual diseased valve and that statin efficacy in the context of calcific aortic valve disease (CAVD) may be dependent upon the cellular composition of the valve. These findings may have implications for clinical usage of statins, shedding light on how statin efficacy in CAVD may be dependent upon the disease stage or why some individuals exhibit better responsiveness to statin therapy.

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.

The elastic properties of valve interstitial cells undergoing pathological differentiation

Increasing evidence indicates that the progression of calcific aortic valve disease (CAVD) is influenced by the mechanical forces experienced by valvular interstitial cells (VICs) embedded within the valve matrix. The ability of VICs to sense and respond to tissue-level mechanical stimuli depends in part on cellular-level biomechanical properties, which may change with disease. In this study, we used micropipette aspiration to measure the instantaneous elastic modulus of normal VICs and of VICs induced to undergo pathological differentiation in vitro to osteoblast or myofibroblast lineages on compliant and stiff collagen gels, respectively. We found that VIC elastic modulus increased after subculturing on stiff tissue culture-treated polystyrene and with pathological differentiation on the collagen gels. Fibroblast, osteoblast, and myofibroblast VICs had distinct cellular-level elastic properties that were not fully explained by substrate stiffness, but were correlated with α-smooth muscle actin expression levels. C-type natriuretic peptide, a peptide expressed in aortic valves in vivo, prevented VIC stiffening in vitro, consistent with its ability to inhibit α-smooth muscle actin expression and VIC pathological differentiation. These data demonstrate that VIC phenotypic plasticity and mechanical adaptability are linked and regulated both biomechanically and biochemically, with the potential to influence the progression of CAVD.