Type I and type III collagen synthesis and composition in the valve matrix in aortic valve stenosis

A three-dimensional valve-on-chip microphysiological system implicates cell cycle progression, cholesterol metabolism and protein homeostasis in early calcific aortic valve disease progression

BACKGROUND

Calcific aortic valve disease (CAVD) is one of the most common forms of valvulopathy, with a 50 % elevated risk of a fatal cardiovascular event, and greater than 15,000 annual deaths in North America alone. The treatment standard is valve replacement as early diagnostic, mitigation, and drug strategies remain underdeveloped. The development of early diagnostic and therapeutic strategies requires the fabrication of effective in vitro valve mimetic models to elucidate early CAVD mechanisms.

METHODS

In this study, we developed a multilayered physiologically relevant 3D valve-on-chip (VOC) system that incorporated aortic valve mimetic extracellular matrix (ECM), porcine aortic valve interstitial cell (VIC) and endothelial cell (VEC) co-culture and dynamic mechanical stimuli. Collagen and glycosaminoglycan (GAG) based hydrogels were assembled in a bilayer to mimic healthy or diseased compositions of the native fibrosa and spongiosa. Multiphoton imaging and proteomic analysis of healthy and diseased VOCs were performed.

RESULTS

Collagen-based bilayered hydrogel maintained the phenotype of the VICs. Proteins related to cellular processes like cell cycle progression, cholesterol biosynthesis, and protein homeostasis were found to be significantly altered and correlated with changes in cell metabolism in diseased VOCs. This study suggested that diseased VOCs may represent an early, adaptive disease initiation stage, which was corroborated by human aortic valve proteomic assessment.

CONCLUSIONS

In this study, we developed a collagen-based bilayered hydrogel to mimic healthy or diseased compositions of the native fibrosa and spongiosa layers. When the gels were assembled in a VOC with VECs and VICs, the diseased VOCs revealed key insights about the CAVD initiation process.

STATEMENT OF SIGNIFICANCE

Calcific aortic valve disease (CAVD) elevates the risk of death due to cardiovascular pathophysiology by 50 %, however, prevention and mitigation strategies are lacking, clinically. Developing tools to assess early disease would significantly aid in the prevention of disease and in the development of therapeutics. Previously, studies have utilized collagen and glycosaminoglycan-based hydrogels for valve cell co-cultures, valve cell co-cultures in dynamic environments, and inorganic polymer-based multilayered hydrogels; however, these approaches have not been combined to make a physiologically relevant model for CAVD studies. We fabricated a bi-layered hydrogel that closely mimics the aortic valve and used it for valve cell co-culture in a dynamic platform to gain mechanistic insights into the CAVD initiation process using proteomic and multiphoton imaging assessment.

A synergistic strategy based on active hydroxymethyl amine compounds and fucoidan for bioprosthetic heart valves with enhancing anti-coagulation and anti-calcification properties

With an aging population, the patients with valvular heart disease (VHD) are growing worldwide, and valve replacement is a primary choice for these patients with severe valvular disease. Among them, bioprosthetic heart valves (BHVs), especially BHVs trough transcatheter aortic valve replacement, are widely accepted by patients on account of their good hemodynamics and biocompatibility. Commercial BHVs in clinic are prepared by glutaraldehyde cross-linked pericardial tissue with the risk of calcification and thrombotic complications. In the present study, a strategy combines improved hemocompatibility and anti-calcification properties for BHVs has been developed based on a novel non-glutaraldehyde BHV crosslinker hexakis(hydroxymethyl)melamine (HMM) and the anticoagulant fucoidan. Besides the similar mechanical properties and enhanced component stability compared to glutaraldehyde crosslinked PP (G-PP), the fucoidan modified HMM-crosslinked PPs (HMM-Fu-PPs) also exhibit significantly enhanced anticoagulation performance with a 72 % decrease in thrombus weight compared with G-PP in ex-vivo shunt assay, along with the superior biocompatibility, satisfactory anti-calcification properties confirmed by subcutaneous implantation. Owing to good comprehensive performance of these HMM-Fu-PPs, this simple and feasible strategy may offer a great potential for BHV fabrication in the future, and open a new avenue to explore more N-hydroxymethyl compound based crosslinker with excellent performance in the field of biomaterials.

Dynamic changes in mitral valve extracellular matrix, tissue mechanics and function in a mouse model of Marfan syndrome

OBJECTIVE

Mouse models of Marfan syndrome (MFS) with Fibrillin 1 (Fbn1) variant C1041G exhibit cardiovascular abnormalities, including myxomatous valve disease (MVD) and aortic aneurism, with structural extracellular matrix (ECM) dysregulation. In this study, we examine the structure-function-mechanics relations of the mitral valve related to specific transitions in ECM composition and organization in progressive MVD in MFS mice from Postnatal day (P)7 to 1 year-of-age.

APPROACH AND RESULTS

Mechanistic links between mechanical forces and biological changes in MVD progression were examined in Fbn1C1041G/+ MFS mice. By echocardiography, mitral valve dysfunction is prevalent at 2 months with a decrease in cardiac function at 6 months, followed by a preserved cardiac function at 12 months. Mitral valve (MV) regurgitation occurs in a subset of mice at 2-6 months, while progressive dilatation of the aorta occurs from 2 to 12 months. Mitral valve tissue mechanical assessments using a uniaxial Permeabilizable Fiber System demonstrate decreased stiffness of MFS MVs at all stages. Histological and microscopic analysis of ECM content, structure, and fiber orientation demonstrate that alterations in ECM mechanics, composition, and organization precede functional abnormalities in Fbn1C1041G/+MFS MVs. At 2 months, ECM abnormalities are detected with an increase in proteoglycans and decreased stiffness of the mitral valve. By 6-12 months, collagen fiber remodeling is increased with abnormal fiber organization in MFS mitral valve leaflets. At the same time, matrifibrocyte gene expression characteristic of collagen-rich connective tissue is increased, as detected by RNA in situ hybridization and qPCR. Together, these studies demonstrate early prevalence of proteoglycans at 2 months followed by upregulation of collagen structure and organization with age in MVs of MFS mice.

CONCLUSIONS

Altogether, our data indicate dynamic regulation of mitral valve structure, tissue mechanics, and function that reflect changes in ECM composition, organization, and gene expression in progressive MVD. Notably, increased collagen fiber organization and orientation, potentially dependent on increased matrifibrocyte cell activity, is apparent with altered mitral valve mechanics and function in aging MFS mice.

Association Between Use of Fluoroquinolones and Risk of Mitral or Aortic Valve Regurgitation: A Nationwide Cohort Study

Biological plausibility suggests that fluoroquinolones may lead to mitral valve regurgitation or aortic valve regurgitation (MR/AR) through a collagen degradation pathway. However, available real-world studies were limited and yielded inconsistent findings. We estimated the risk of MR/AR associated with fluoroquinolones compared with other antibiotics with similar indications in a population-based cohort study. We identified adult patients who initiated fluoroquinolones or comparison antibiotics from the nationwide Taiwanese claims database. Patients were followed for up to 60 days after cohort entry. Cox regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) of MR/AR comparing fluoroquinolones to comparison antibiotics after 1:1 propensity score (PS) matching. All analyses were conducted by type of fluoroquinolone (fluoroquinolones as a class, respiratory fluoroquinolones, and non-respiratory fluoroquinolones) and comparison antibiotic (amoxicillin/clavulanate or ampicillin/sulbactam, extended-spectrum cephalosporins). Among 6,649,284 eligible patients, the crude incidence rates of MR/AR ranged from 1.44 to 4.99 per 1,000 person-years across different types of fluoroquinolones and comparison antibiotics. However, fluoroquinolone use was not associated with an increased risk in each pairwise PS-matched comparison. HRs were 1.00 (95% CI, 0.89-1.11) for fluoroquinolones as a class, 0.96 (95% CI, 0.83-1.12) for respiratory fluoroquinolones, and 0.87 (95% CI, 0.75-1.01) for non-respiratory fluoroquinolones, compared with amoxicillin/clavulanate or ampicillin/sulbactam. Results were similar when fluoroquinolones were compared with extended-spectrum cephalosporins (HRs of 0.96, 95% CI, 0.82-1.12, HR, 1.05, 95% CI, 0.86-1.28, and HR, 0.88, 95% CI, 0.75-1.03, respectively). This large-scale cohort study did not find a higher risk of MR/AR with different types of fluoroquinolones in the adult population.

The Role of Apoptosis and Oxidative Stress in a Cell Spheroid Model of Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD) is the most common heart valve disease among aging populations. There are two reported pathways of CAVD: osteogenic and dystrophic, the latter being more prevalent. Current two-dimensional (2D) in vitro CAVD models have shed light on the disease but lack three-dimensional (3D) cell-ECM interactions, and current 3D models require osteogenic media to induce calcification. The goal of this work is to develop a 3D dystrophic calcification model. We hypothesize that, as with 2D cell-based CAVD models, programmed cell death (apoptosis) is integral to calcification. We model the cell aggregation observed in CAVD by creating porcine valvular interstitial cell spheroids in agarose microwells. Upon culture in complete growth media (DMEM with serum), calcium nodules form in the spheroids within a few days. Inhibiting apoptosis with Z-VAD significantly reduced calcification, indicating that the calcification observed in this model is dystrophic rather than osteogenic. To determine the relative roles of oxidative stress and extracellular matrix (ECM) production in the induction of apoptosis and subsequent calcification, the media was supplemented with antioxidants with differing effects on ECM formation (ascorbic acid (AA), Trolox, or Methionine). All three antioxidants significantly reduced calcification as measured by Von Kossa staining, with the percentages of calcification per area of AA, Trolox, Methionine, and the non-antioxidant-treated control on day 7 equaling 0.17%, 2.5%, 6.0%, and 7.7%, respectively. As ZVAD and AA almost entirely inhibit calcification, apoptosis does not appear to be caused by a lack of diffusion of oxygen and metabolites within the small spheroids. Further, the observation that AA treatment reduces calcification significantly more than the other antioxidants indicates that the ECM stimulatory effect of AA plays a role inhibiting apoptosis and calcification in the spheroids. We conclude that, in this 3D in vitro model, both oxidative stress and ECM production play crucial roles in dystrophic calcification and may be viable therapeutic targets for preventing CAVD.

Nuclear magnetic resonance spectroscopy to quantify major extracellular matrix components in fibro-calcific aortic valve disease

Fibro-calcific aortic valve disease (FCAVD) is a pathological condition marked by overt fibrous and calcific extracellular matrix (ECM) accumulation that leads to valvular dysfunction and left ventricular outflow obstruction. Costly valve implantation is the only approved therapy. Multiple pharmacological interventions are under clinical investigation, however, none has proven clinically beneficial. This failure of translational approaches indicates incomplete understanding of the underlying pathomechanisms and may result from a limited toolbox of scientific methods to assess the cornerstones of FCAVD: lipid deposition, fibrous and calcific ECM accumulation. In this study, we evaluated magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to both, qualitatively and quantitatively assess these key elements of FCAVD pathogenesis. NMR spectra showed collagen, elastin, triacylglycerols, and phospholipids in human control and FCAVD tissue samples (n = 5). Calcification, measured by the hydroxyapatite content, was detectable in FCAVD tissues and in valve interstitial cells under procalcifying media conditions. Hydroxyapatite was significantly higher in FCAVD tissues than in controls (p < 0.05) as measured by 31P MAS NMR. The relative collagen content was lower in FCAVD tissues vs. controls (p < 0.05). Overall, we demonstrate the versatility of NMR spectroscopy as a diagnostic tool in preclinical FCAVD assessment.

Sulfonated, oxidized pectin-based double crosslinked bioprosthetic valve leaflets for synergistically enhancing hemocompatibility and cytocompatibility and reducing calcification

Clinically frequently-used glutaraldehyde (GA)-crosslinked bioprosthetic valve leaflets (BVLs) are still curbed by acute thrombosis, malignant immunoreaction, calcification, and poor durability. In this study, an anticoagulant heparin-like biomacromolecule, sulfonated, oxidized pectin (SAP) with a dialdehyde structure was first obtained by modifying citrus pectin with sulfonation of 3-amino-1-propane sulfonic acid and then oxidating with periodate. Notably, a novel crosslinking approach was established by doubly crosslinking BVLs with SAP and the nature-derived crosslinking agent quercetin (Que), which play a synergistic role in both crosslinking and bioactivity. The double crosslinked BVLs also presented enhanced mechanical properties and enzymatic degradation resistance owing to the double crosslinking networks formed via CN bonds and hydrogen bonds, respectively, and good HUVEC-cytocompatibility. The in vitro and ex vivo assay manifested that the double-crosslinked BVLs had excellent anticoagulant and antithrombotic properties, owing to the introduction of SAP. The subcutaneous implantation also demonstrated that the obtained BVLs showed a reduced inflammatory response and great resistance to calcification, which is attributed to quercetin with multiple physiological activities and depletion of aldehyde groups by hydroxyl aldehyde reaction. With excellent stability, hemocompatibility, anti-inflammatory, anti-calcification, and pro-endothelialization properties, the obtained double-crosslinked BVLs, SAP + Que-PP, would have great potential to substitute the current clinical GA-crosslinked BVLs.

Integrated Analysis of LncRNA-Mediated ceRNA Network in Calcific Aortic Valve Disease

BACKGROUND

The high morbidity and mortality of calcific aortic valve disease (CAVD) represents an unmet clinical need to investigate the molecular mechanisms involved. Evidence suggests that long non-coding RNAs (lncRNAs) can act as competitive endogenous RNAs (ceRNAs) by binding to microRNAs and regulating target genes in cardiovascular diseases. Nevertheless, the role of lncRNAs related ceRNA regulation in CAVD remains unclear.

METHODS

RNAseq data of human diseased aortic valves were downloaded from GEO data sets (GSE153555, GSE199718), and differentially expressed lncRNAs (DElncRNAs), mRNAs (DEmRNAs) between CAVD and non-calcific aortic valve tissues with limma R package. Gene Ontology (GO) annotation, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Set Enrichment analysis (GSEA) were performed with clusterProfiler and gesaplot2 R package. The pivotal microRNAs were predicted by three databases intersection including TargetScan, MiRwalk, miRDB according to the genes related to the crucial pathways. ENCORI was used to predict targeted lncRNAs of hub microRNAs. We constructed lncRNA-miRNA-mRNA ceRNA network with Cytoscape software. The lncRNAs in ceRNA network were verified by RT-qPCR in human 30 calcific and 20 noncalcified aortic valve tissues.

RESULTS

In total, 1739 DEmRNAs and 266 DElncRNAs were identified in CAVD. GO, KEGG pathway, GSEA annotations suggested that most of these genes are enriched in extracellular matrix (ECM)-reporter interaction pathways. The ceRNA networks associated with ECM-reporter interaction are constructed and related lncRNAs including H19, SNHG3 and ZNF436-AS1 were significant upregulated in human calcific aortic valve tissues, which might be potential therapeutic targets for CAVD.

CONCLUSIONS

In this study, we proposed a novel lncRNA-miRNA-mRNA ceRNA network related to ECM-reporter interaction pathways, which potentially regulates CAVD progression.

The effect of the fibrocalcific pathological process on aortic valve stenosis in female patients: a finite element study

Calcific aortic valve disease (CAVD) is the most common heart valvular disease in the developed world. Most of the relevant research has been sex-blind, ignoring sex-related biological variables and thus under-appreciate sex differences. However, females present pronounced fibrosis for the same aortic stenosis (AS) severity compared with males, who exhibit more calcification. Herein, we present a computational model of fibrocalcific AV, aiming to investigate its effect on AS development. A parametric study was conducted to explore the influence of the total collagen fiber volume and its architecture on the aortic valve area (AVA). Towards that goal, computational models were generated for three females with stenotic AVs and different volumes of calcium. We have tested the influence of fibrosis on various parameters as fiber architecture, fibrosis location, and transvalvular pressure. We found that increased fiber volume with a low calcium volume could actively contribute to AS and reduce the AVA similarly to high calcium volume. Thus, the computed AVAs for our fibrocalcific models were 0.94 and 0.84 cm²and the clinical (Echo) AVAs were 0.82 and 0.8 cm². For the heavily calcified model, the computed AVA was 0.8 cm²and the clinical AVA was 0.73 cm². The proposed models demonstrated how collagen thickening influence the fibrocalcific-AS process in female patients. These models can assist in the clinical decision-making process and treatment development in valve therapy for female patients.

Aortic valve cell microenvironment: Considerations for developing a valve-on-chip

Cardiac valves are sophisticated, dynamic structures residing in a complex mechanical and hemodynamic environment. Cardiac valve disease is an active and progressive disease resulting in severe socioeconomic burden, especially in the elderly. Valve disease also leads to a 50% increase in the possibility of associated cardiovascular events. Yet, valve replacement remains the standard of treatment with early detection, mitigation, and alternate therapeutic strategies still lacking. Effective study models are required to further elucidate disease mechanisms and diagnostic and therapeutic strategies. Organ-on-chip models offer a unique and powerful environment that incorporates the ease and reproducibility of in vitro systems along with the complexity and physiological recapitulation of the in vivo system. The key to developing effective valve-on-chip models is maintaining the cell and tissue-level microenvironment relevant to the study application. This review outlines the various components and factors that comprise and/or affect the cell microenvironment that ought to be considered while constructing a valve-on-chip model. This review also dives into the advancements made toward constructing valve-on-chip models with a specific focus on the aortic valve, that is, in vitro studies incorporating three-dimensional co-culture models that incorporate relevant extracellular matrices and mechanical and hemodynamic cues.

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.

Collagen fiber regulation in human pediatric aortic valve development and disease

Congenital aortic valve stenosis (CAVS) affects up to 10% of the world population without medical therapies to treat the disease. New molecular targets are continually being sought that can halt CAVS progression. Collagen deregulation is a hallmark of CAVS yet remains mostly undefined. Here, histological studies were paired with high resolution accurate mass (HRAM) collagen-targeting proteomics to investigate collagen fiber production with collagen regulation associated with human AV development and pediatric end-stage CAVS (pCAVS). Histological studies identified collagen fiber realignment and unique regions of high-density collagen in pCAVS. Proteomic analysis reported specific collagen peptides are modified by hydroxylated prolines (HYP), a post-translational modification critical to stabilizing the collagen triple helix. Quantitative data analysis reported significant regulation of collagen HYP sites across patient categories. Non-collagen type ECM proteins identified (26 of the 44 total proteins) have direct interactions in collagen synthesis, regulation, or modification. Network analysis identified BAMBI (BMP and Activin Membrane Bound Inhibitor) as a potential upstream regulator of the collagen interactome. This is the first study to detail the collagen types and HYP modifications associated with human AV development and pCAVS. We anticipate that this study will inform new therapeutic avenues that inhibit valvular degradation in pCAVS and engineered options for valve replacement.

Extracellular Matrix in Calcific Aortic Valve Disease: Architecture, Dynamic and Perspectives

Calcific Aortic Valve Disease (CAVD) is the most common valvular heart disease in developed countries and in the ageing population. It is strongly correlated to median age, affecting up to 13% of the population over the age of 65. Pathophysiological analysis indicates CAVD as a result of an active and degenerative disease, starting with sclerosis and chronic inflammation and then leaflet calcification, which ultimately can account for aortic stenosis. Although CAVD has been firstly recognized as a passive event mostly resulting from a degenerative aging process, much evidences suggests that calcification arises from different active processes, involving both aortic valve-resident cells (valve endothelial cells, valve interstitial cells, mesenchymal stem cells, innate immunity cells) and circulating cells (circulating mesenchymal cells, immunity cells). Moreover, a role for the cell-derived "matrix vesicles" and extracellular matrix (ECM) components has also been recognized. The aim of this work is to review the cellular and molecular alterations occurring in aortic valve during CAVD pathogenesis, focusing on the role of ECM in the natural course of the disease.

Collagen networks within 3D PEG hydrogels support valvular interstitial cell matrix mineralization

Enzymatically degradable hydrogels were designed for the 3D culture of valvular interstitial cells (VICs), and through the incorporation of various functionalities, we aimed to investigate the role of the tissue microenvironment in promoting the osteogenic properties of VICs and matrix mineralization. Specifically, porcine VICs were encapsulated in a poly(ethylene glycol) hydrogel crosslinked with a matrix metalloproteinase (MMP)-degradable crosslinker (KCGPQG↓IWGQCK) and formed via a thiol-ene photoclick reaction in the presence or absence of collagen type I to promote matrix mineralization. VIC-laden hydrogels were treated with osteogenic medium for up to 15 days, and the osteogenic response was characterized by the expression of RUNX2 as an early marker of an osteoblast-like phenotype, osteocalcin (OCN) as a marker of a mature osteoblast-like phenotype, and vimentin (VIM) as a marker of the fibroblast phenotype. In addition, matrix mineralization was characterized histologically with Von Kossa stain for calcium phosphate. Osteogenic response was further characterized biochemically with calcium assays, and physically via optical density measurements. When the osteogenic medium was supplemented with calcium chloride, OCN expression was upregulated and mineralization was discernable at 12 days of culture. Finally, this platform was used to screen various drug therapeutics that were assessed for their efficacy in preventing mineralization using optical density as a higher throughput readout. Collectively, these results suggest that matrix composition has a key role in supporting mineralization deposition within diseased valve tissue.

Transforming growth factor-β1 promotes fibrosis but attenuates calcification of valvular tissue applied as a three-dimensional calcific aortic valve disease model

Calcific aortic valve disease (CAVD) is characterized by valvular fibrosis and calcification and driven by differentiating valvular interstitial cells (VICs). Expression data from patient biopsies suggest that transforming growth factor (TGF)-β1 is implicated in CAVD pathogenesis. However, CAVD models using isolated VICs failed to deliver clear evidence on the role of TGF-β1. Thus, employing cultures of aortic valve leaflets, we investigated effects of TGF-β1 in a tissue-based three-dimensional (3-D) CAVD model. We found that TGF-β1 induced phosphorylation of Mothers against decapentaplegic homolog (SMAD) 3 and expression of SMAD7, indicating effective downstream signal transduction in valvular tissue. Thus, TGF-β1 increased VIC contents of rough endoplasmic reticulum, Golgi, and secretory vesicles as well as tissue levels of RNA and protein. In addition, TGF-β1 raised expression of proliferation marker cyclin D1, attenuated VIC apoptosis, and upregulated VIC density. Moreover, TGF-β1 intensified myofibroblastic VIC differentiation as evidenced by increased α-smooth muscle actin and collagen type I along with diminished vimentin expression. In contrast, TGF-β1 attenuated phosphorylation of SMAD1/5/8 and upregulation of β-catenin while inhibiting osteoblastic VIC differentiation as revealed by downregulation of osteocalcin expression, alkaline phosphatase activity, and extracellular matrix incorporation of hydroxyapatite. Collectively, these effects resulted in blocking of valvular tissue calcification and associated disintegration of collagen fibers. Instead, TGF-β1 induced development of fibrosis. Overall, in a tissue-based 3-D CAVD model, TGF-β1 intensifies expressional and proliferative activation along with myofibroblastic differentiation of VICs, thus triggering dominant fibrosis. Simultaneously, by inhibiting SMAD1/5/8 activation and canonical Wnt/β-catenin signaling, TGF-β1 attenuates osteoblastic VIC differentiation, thus blocking valvular tissue calcification. These findings question a general phase-independent CAVD-promoting role of TGF-β1.NEW & NOTEWORTHY Employing aortic valve leaflets as a tissue-based three-dimensional disease model, our study investigates the role of transforming growth factor (TGF)-β1 in calcific aortic valve disease pathogenesis. We find that, by activating Mothers against decapentaplegic homolog 3, TGF-β1 intensifies expressional and proliferative activation along with myofibroblastic differentiation of valvular interstitial cells, thus triggering dominant fibrosis. Simultaneously, by inhibiting activation of Mothers against decapentaplegic homolog 1/5/8 and canonical Wnt/β-catenin signaling, TGF-β1 attenuates apoptosis and osteoblastic differentiation of valvular interstitial cells, thus blocking valvular tissue calcification. These findings question a general phase-independent calcific aortic valve disease-promoting role of TGF-β1.

Irbesartan ameliorates myocardial fibrosis in diabetic cardiomyopathy rats by inhibiting the TGFβ1/Smad2/3 pathway

Myocardial fibrosis (MF) is an important pathological change in diabetic cardiomyopathy. The aim of the present study was to investigate whether irbesartan serves a role in improving MF in a diabetic rat model. Fasting blood glucose (FBG), total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) levels were measured in rats using biochemical methods. Heart weight index (HWI), left ventricular weight index (LVWI), left ventricular systolic pressure (LVSP) and left ventricular end-diastolic pressure (LVEDP) were also measured, whilst type I collagen and hydroxyproline content in myocardial tissue was quantified. Western blotting was used to measure the expression of transforming growth factor β1 (TGFβ1), phosphorylated (p)-Smad2/3 and collagen type I α 1 chain (COL1A1) inmyocardial tissues or rat cardiac fibroblast (RCF) cells. Cell proliferation was measured using EdU staining. Procollagen type III N-terminal peptide (PIIINP) content, FBG, TC, TG and LDL-C levels were found to be significantly higher, whilst HDL-C levels were found to be significantly lower in rats in the diabetic group. Those in the diabetic group also exhibited significantly elevated HWI, LVWI, LVEDP, myocardial tissue type I collagen content and hydroxyproline content values, but significantly reduced LVSP. Changes in the aforementioned indicators were reversed after treatment with irbesartan, where the protein expression levels of TGFβ1 and p-Smad2/3 in myocardial tissue were also significantly reduced. In RCF cells, irbesartan significantly reversed high glucose-induced upregulation of TGFβ1 expression, Smad2/3 phosphorylation and COL1A1 expression, as well as reducing cell proliferation and rat type I PICP and PIIINP levels. Application of pirfenidone produced additive effects on reducing the expression levels of the proteins aforementioned when combined with irbesartan. Therefore, the present results demonstrated that irbesartan reduced the activity of the TGFβ1/Smad2/3 pathway and ameliorated diabetic MF by downregulating the expression of TGFβ1.

Integration of polarized spatial frequency domain imaging (pSFDI) with a biaxial mechanical testing system for quantification of load-dependent collagen architecture in soft collagenous tissues

Collagen fiber networks provide the structural strength of tissues, such as tendons, skin and arteries. Quantifying the fiber architecture in response to mechanical loads is essential towards a better understanding of the tissue-level mechanical behaviors, especially in assessing disease-driven functional changes. To enable novel investigations into these load-dependent fiber structures, a polarized spatial frequency domain imaging (pSFDI) device was developed and, for the first time, integrated with a biaxial mechanical testing system. The integrated instrument is capable of a wide-field quantification of the fiber orientation and the degree of optical anisotropy (DOA), representing the local degree of fiber alignment. The opto-mechanical instrument''s performance was assessed through uniaxial loading on tendon tissues with known collagen fiber microstructures. Our results revealed that the bulk fiber orientation angle of the tendon tissue changed minimally with loading (median ± 0.5*IQR of 52.7° ± 3.3° and 51.9° ± 3.3° under 0 and 3% longitudinal strains, respectively), whereas on a micro-scale, the fibers became better aligned with the direction of loading: the DOA (mean ± SD) increased from 0.149 ± 0.032 to 0.198 ± 0.056 under 0 and 3% longitudinal strains, respectively, p < 0.001. The integrated instrument was further applied to study two representative mitral valve anterior leaflet (MVAL) tissues subjected to various biaxial loads. The fiber orientations within these representative MVAL tissue specimens demonstrated noticeable heterogeneity, with the local fiber orientations dependent upon the sample, the spatial and transmural locations, and the applied loading. Our results also showed that fibers were generally better aligned under equibiaxial (DOA = 0.089 ± 0.036) and circumferentially-dominant loading (DOA = 0.086 ± 0.037) than under the radially-dominant loading (DOA = 0.077 ± 0.034), indicating circumferential predisposition. These novel findings exemplify a deeper understanding of the load-dependent collagen fiber microstructures obtained through the use of the integrated opto-mechanical instrument. STATEMENT OF SIGNIFICANCE: In this study, a novel quantitative opto-mechanical system was developed by combining a polarized Spatial Frequency Domain Imaging (pSFDI) device with a biaxial mechanical tester. The integrated system was used to quantify the load-dependent collagen fiber microstructures in representative tendon and mitral valve anterior leaflet (MVAL) tissues. Our results revealed that MVAL's fiber architectures exhibited load-dependent spatial and transmural heterogeneities, suggesting further microstructural complexity than previously reported in heart valve tissues. These novel findings were possible through the system's ability to, for the first time, capture the load-dependent collagen architecture in the mitral valve anterior leaflet tissue over a wide field of view (e.g., 10 × 10 mm for the MVAL tissue specimens). Such capabilities afford unique future opportunities to improve patient outcomes through concurrent mechanical and microstructural assessments of healthy and diseased tissues in conditions such as heart valve regurgitation and calcification.

Fiber Scaffold Patterning for Mending Hearts: 3D Organization Bringing the Next Step

Heart failure (HF) is a leading cause of death worldwide. The most common conditions that lead to HF are coronary artery disease, myocardial infarction, valve disorders, high blood pressure, and cardiomyopathy. Due to the limited regenerative capacity of the heart, the only curative therapy currently available is heart transplantation. Therefore, there is a great need for the development of novel regenerative strategies to repair the injured myocardium, replace damaged valves, and treat occluded coronary arteries. Recent advances in manufacturing technologies have resulted in the precise fabrication of 3D fiber scaffolds with high architectural control that can support and guide new tissue growth, opening exciting new avenues for repair of the human heart. This review discusses the recent advancements in the novel research field of fiber patterning manufacturing technologies for cardiac tissue engineering (cTE) and to what extent these technologies could meet the requirements of the highly organized and structured cardiac tissues. Additionally, future directions of these novel fiber patterning technologies, designs, and applicability to advance cTE are presented.

Heterogeneous multi-laminar tissue constructs as a platform to evaluate aortic valve matrix-dependent pathogenicity

Designing and constructing controlled in vitro cell culture platforms is imperative toward pinpointing factors that contribute to the development of calcific aortic valve disease. A 3D, laminar, filter paper-based cell culture system that was previously established as a method of analyzing valvular interstitial cell migration and protein expression was adapted here for studying the impact of specific extracellular matrix proteins on cellular viability and calcification proclivity. Hydrogels incorporating hyaluronan and collagen I, two prevalent valvular extracellular matrix proteins with altered pathological production, were designed with similar mechanics to parse out effects of the individual proteins on cell behavior. Laminar constructs containing varying combinations of discrete layers of collagen and hyaluronan were assembled to mimic native and pathological valve compositions. Proteinaceous and genetic expression patterns pertaining to cell viability and calcific potential were quantified via fluorescent imaging. A significant dose-dependency was observed, with increased collagen content associated with decreased viability and increased calcific phenotype. These results suggest that extracellular composition is influential in calcific aortic valve disease progression and will be key toward development of future tissue-engineered or pharmaceutical calcific aortic valve treatments. STATEMENT OF SIGNIFICANCE: Calcific aortic valve disease (CAVD), a widespread heart valve disorder, is characterized by fibrotic leaflet thickening and calcific nodule formation. This pathological remodeling is an active process mediated by the valvular interstitial cells (VICs). Currently, the only treatment available is surgical replacement of the valve - a procedure associated with significant long-term risk and morbidity. Development of effective alternate therapies is hindered by our poor understanding of CAVD etiology. Previous work has implicated the composition and mechanics of the extracellular matrix in the progression of CAVD. These individual factors and their magnitude of influence have not been extensively explored - particularly in 3D systems. Here, we have bridged this gap in understanding through the employment of a heterogeneous 3D filter-paper culture system.

Comparing the Role of Mechanical Forces in Vascular and Valvular Calcification Progression

Calcification is a prevalent disease in most fully developed countries and is predominantly observed in heart valves and nearby vasculature. Calcification of either tissue leads to deterioration and, ultimately, failure causing poor quality of life and decreased overall life expectancy in patients. In valves, calcification presents as Calcific Aortic Valve Disease (CAVD), in which the aortic valve becomes stenotic when calcific nodules form within the leaflets. The initiation and progression of these calcific nodules is strongly influenced by the varied mechanical forces on the valve. In turn, the addition of calcific nodules creates localized disturbances in the tissue biomechanics, which affects extracellular matrix (ECM) production and cellular activation. In vasculature, atherosclerosis is the most common occurrence of calcification. Atherosclerosis exhibits as calcific plaque formation that forms in juxtaposition to areas of low blood shear stresses. Research in these two manifestations of calcification remain separated, although many similarities persist. Both diseases show that the endothelial layer and its regulation of nitric oxide is crucial to calcification progression. Further, there are similarities between vascular smooth muscle cells and valvular interstitial cells in terms of their roles in ECM overproduction. This review summarizes valvular and vascular tissue in terms of their basic anatomy, their cellular and ECM components and mechanical forces. Calcification is then examined in both tissues in terms of disease prediction, progression, and treatment. Highlighting the similarities and differences between these areas will help target further research toward disease treatment.

Experimental Metabolic Syndrome Model Associated with Mechanical and Structural Degenerative Changes of the Aortic Valve

The purpose of this study was to test the hypothesis that an experimental high fat (HF) animal with metabolic syndrome results in structural degeneration of the aortic valve. Domestic pigs were divided (n = 12) and administered either a normal or HF diet. After 16-weeks, the HF diet group had increased weight (p ≤ 0.05), total cholesterol (p ≤ 0.05), and systolic and diastolic pressure (p ≤ 0.05). The aortic valve extracellular matrix showed loss of elastin fibers and increased collagen deposition in the HF diet group. Collagen was quantified with ELISA, which showed an increased concentration of collagen types 1 and 3 (p ≤ 0.05). In the HF diet group, the initial stages of microcalcification were observed. Uniaxial mechanical testing of aortic cusps revealed that the HF diet group expressed a decrease in ultimate tensile strength and elastic modulus compared to the control diet group (p ≤ 0.05). Western blot and immunohistochemistry indicated the presence of proteins: lipoprotein-associated phospholipase A2, osteopontin, and osteocalcin with an increased expression in the HF diet group. The current study demonstrates that experimental metabolic syndrome induced by a 16-week HF diet was associated with a statistically significant alteration to the physical architecture of the aortic valve.

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

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

Radical polymerization-crosslinking method for improving extracellular matrix stability in bioprosthetic heart valves with reduced potential for calcification and inflammatory response

In recent years, the number of heart valve replacements has multiplied with valve diseases because of aging populations and the surge in rheumatic heart disease in young people. Among them, bioprosthetic heart valves (BHVs) have become increasingly popular. Transcatheter aortic valve implantation (TAVI) valve as an emerging BHV has been increasingly applied to patients. However, the current commercially used BHVs treated with glutaraldehyde (Glut) still face the problem of durability. BHVs derived from Glut-treated xenogenetic tissues would undergo structural degeneration and calcification sometimes even as short as less than 10 years. This issue has already become a big challenge considering more and more young patients at the age of 50-60 s are receiving the BHV replacement. In our study, an approach that is totally different from the previous techniques named by us as the radical polymerization-crosslinking (RPC) method was developed to improve extracellular matrix stability, prevent calcification, and reduce inflammatory response in BHVs. The porcine pericardium (PP) tissue was decellularized, functionalized with methacryloyl groups, and subsequently crosslinked by radical polymerization. We found that high-density RPC treatment remarkably improved the stability of collagen and elastin of PP, enhanced its endothelialization potential, and provided reliable biomechanical performance as compared to Glut treatment. The in vivo rat model also confirmed the increased componential stability and the reduced inflammatory response of RPC-treated PP. Moreover, the RPC-treated PP showed better in vivo anticalcification potential than Glut-treated PP. STATEMENT OF SIGNIFICANCE: Bioprosthetic heart valves (BHVs) manufactured from glutaraldehyde (Glut)-treated xenogeneic tissues have been used to treat valve-related diseases for several decades. However, the durability of BHVs remains unresolved and becomes more pronounced particularly in younger patients. Although a number of new alternative methods for Glut crosslinking have been proposed, their overall performance is still far from ready to use in humans. In this study, radical polymerization was investigated for crosslinking the porcine pericardium (PP). This treatment was found to have advantages compared to Glut-treated PP in terms of stability, biocompatibility, and anticalcification potential with the hope of addressing the needs of more robust biomaterials for the fabrication of BHVs.

Cellular Mechanisms of Valvular Thickening in Early and Intermediate Calcific Aortic Valve Disease

Background:

Calcific aortic valve disease is common in an aging population. It is an ac-tive atheroinflammatory process that has an initial pathophysiology and similar risk factors as athero-sclerosis. However, the ultimate disease phenotypes are markedly different. While coronary heart dis-ease results in rupture-prone plaques, calcific aortic valve disease leads to heavily calcified and ossi-fied valves. Both are initiated by the retention of low-density lipoprotein particles in the subendotheli-al matrix leading to sterile inflammation. In calcific aortic valve disease, the process towards calcifica-tion and ossification is preceded by valvular thickening, which can cause the first clinical symptoms. This is attributable to the accumulation of lipids, inflammatory cells and subsequently disturbances in the valvular extracellular matrix. Fibrosis is also increased but the innermost extracellular matrix layer is simultaneously loosened. Ultimately, the pathological changes in the valve cause massive calcifica-tion and bone formation - the main reasons for the loss of valvular function and the subsequent myo-cardial pathology.

Conclusion:

Calcification may be irreversible, and no drug treatments have been found to be effec-tive, thus it is imperative to emphasize lifestyle prevention of the disease. Here we review the mecha-nisms underpinning the early stages of the disease.

Association of matrix metalloproteinase 3 and endogenous inhibitors with inflammatory markers in mitral valve disease and calcification

Calcific mitral valve stenosis (MVS) is a common disease characterized by extensive remodeling of the extracellular matrix via matrix metalloproteinases (MMPs). The mechanism of calcification due to extensive matrix remodeling remains unclear. In this study, we investigated the relationship between MMP-3, tissue inhibitors of metalloproteinases (TIMPs) as well as pro-inflammatory cytokines and the phenomenon of calcification in MVS. 212 patients having rheumatic mitral stenosis (RMS) and 155 healthy control subjects were recruited in the Cardiology Department of La Rabta Hospital University. Levels of MMP-3, TIMPs, IL-6 and TNF-α were measured by ELISA sandwich assay, hs-CRP was measured by immunoturbidimetry. Plasma levels of MMP-3, TIMP-1 and MMP-3/TIMP-2 ratio were lower only in RMS women in comparison to the control group. Calcification degree correlated positively with MMP-3 in women and men. In addition, calcification was correlated positively with MMP-3/TIMPs ratio in women patients. The inflammatory parameters were positively associated with extracellular matrix turnover biomarkers in men patients. In patients, the level of MMP-3 was increased in men and women with a calcification score ≥ 5. In addition, MMP-3 level predicted the occurrence of calcification. At ROC curves analysis, the cut-off MMP-3 level was in women was 9.21 ng/ml (sensitivity 51.1%, specificity 89.3%) and in men was 12.84 ng/ml (sensitivity 78.6%, specificity 77.8%). The high levels of MMP-3 and the biomarkers of inflammation contribute to valvular remodeling and calcification of the mitral valve.

Elevated Serotonin Interacts with Angiotensin-II to Result in Altered Valve Interstitial Cell Contractility and Remodeling

While the valvulopathic effects of serotonin (5HT) and angiotensin-II (Ang-II) individually are known, it was not clear how 5HT and Ang-II might interact, specifically in the context of the mechanobiological responses due to altered valve mechanics potentiated by these molecules. In this context, the hypothesis of this study was that increased serotonin levels would result in accelerated progression toward disease in the presence of angiotensin-II-induced hypertension. C57/BL6 J mice were divided into four groups and subcutaneously implanted with osmotic pumps containing: PBS (control), 5HT (2.5 ng/kg/min), Ang-II (400 ng/kg/min), and 5HT + Ang-II (combination). Blood pressure was monitored using the tail cuff method. Echocardiography was performed on the mice before surgery and every week thereafter to assess ejection fraction. After three weeks, the mice were sacrificed and their hearts excised, embedded and sectioned for analysis of the aortic valves via histology and immunohistochemistry. In separate experiments, porcine valve interstitial cells (VICs) were directly stimulated with 5HT (10^-7 M), Ang-II (100 nM) or both and assayed for cellular contractility, cytoskeletal organization and collagen remodeling. After three weeks, average systolic blood pressure was significantly increased in the 5HT, Ang-II and combination groups compared to control. Echocardiographic analysis demonstrated significantly reduced ejection fraction in Ang-II and the combination groups. H&E staining demonstrated thicker leaflets in the combination groups, suggesting a more aggressive remodeling process. Picrosirius red staining and image analysis suggested that the Ang-II and combination groups had the largest proportion of thicker collagen fibers. VIC orientation, cellular contractility and collagen gene expression was highest for the 5HT + Ang-II combination treatment compared to all other groups. Overall, our results suggest that 5HT and Ang-II interact to result in significantly detrimental alteration of function and remodeling in the valve.

Role of Noncanonical Wnt Signaling Pathway in Human Aortic Valve Calcification

OBJECTIVE

The mechanisms underlying the pathogenesis of aortic valve calcification remain unclear. With accumulating evidence demonstrating that valve calcification recapitulates bone development, the crucial roles of noncanonical Wnt ligands WNT5a, WNT5b, and WNT11 in osteogenesis make them critical targets in the study of aortic valve calcification.

APPROACH AND RESULTS

Using immunohistochemistry, real-time qPCR, Western blotting, and tissue culture, we examined the tissue distribution of WNT5a, WNT5b, and WNT11 in noncalcified and calcified aortic valves and their effects on human aortic valve interstitial cells (HAVICs). Only focal strong immunostaining for WNT5a was seen in and around areas of calcification. Abundant immunostaining for WNT5b and WNT11 was seen in inflammatory cells, fibrosis, and activated myofibroblasts in areas of calcified foci. There was significant correlation between WNT5b and WNT11 overall staining and presence of calcification, lipid score, fibrosis, and microvessels (P<0.05). Real-time qPCR and Western blotting revealed abundant expression of both Wnts in stenotic aortic valves, particularly in bicuspid valves. Incubation of HAVICs from noncalcified valves with the 3 noncanonical Wnts significantly increased cell apoptosis and calcification (P<0.05). Treatment of HAVICs with the mitogen-activated protein kinase-38β and GSK3β inhibitors significantly reduced their mineralization (P<0.01). Raman spectroscopy identified the inorganic phosphate deposits as hydroxyapatite and showed a significant increase in hydroxyapatite deposition in HAVICs in response to WNT5a and WNT11 (P<0.05). Similar crystallinity was seen in the deposits found in HAVICs treated with Wnts and in calcified human aortic valves.

CONCLUSIONS

These findings suggest a potential role for noncanonical Wnt signaling in the pathogenesis of aortic valve calcification.

Structural and Histochemical Alterations in the Aortic Valves of Elderly Patients: A Comparative Study of Aortic Stenosis, Aortic Regurgitation, and Normal Valves

The aim of this study was to reveal the pathogenesis of aortic stenosis (AS) and regurgitation (AR) by comparing differences in mechanical and biochemical alterations. We applied scanning acoustic microscopy (SAM) to measure the speed of sound (SOS) through valves to estimate the elasticity and monitor sensitivity to protease treatment, as the SOS is correlated with the stiffness of materials, which is reduced after digestion by proteases. The fibrosa of both the AS and AR groups were stiffer than the fibrosa of the normal group. The AR group displayed significantly stiffer fibrosa than the AS group, with the exception of calcified areas. The AS group showed significantly decreased SOS values following protease digestion, whereas the AR showed little reduction. The AS group presented type III collagen in the fibrosa and the ventricularis. In the AR group, both type I collagen and type III collagen coexisted in the fibrosa and the ventricularis. Upon immunostaining for advanced glycation end-products, the AS group showed sparse, weak staining, whereas the AR group presented a strong, band-like positive reaction in the fibrosa. In conclusion, tissue remodelling associated with damage and repair is associated with AS pathogenesis, whereas static chemical alterations with slow collagen turnover induce AR.

Identification of side- and shear-dependent microRNAs regulating porcine aortic valve pathogenesis

Aortic valve (AV) calcification is an inflammation driven process that occurs preferentially in the fibrosa. To explore the underlying mechanisms, we investigated if key microRNAs (miRNA) in the AV are differentially expressed due to disturbed blood flow (oscillatory shear (OS)) experienced by the fibrosa compared to the ventricularis. To identify the miRNAs involved, endothelial-enriched RNA was isolated from either side of healthy porcine AVs for microarray analysis. Validation using qPCR confirmed significantly higher expression of 7 miRNAs (miR-100, -130a, -181a/b, -199a-3p, -199a-5p, and -214) in the fibrosa versus the ventricularis. Upon bioinformatics analysis, miR-214 was selected for further investigation using porcine AV leaflets in an ex vivo shear system. Fibrosa and ventricularis sides were exposed to either oscillatory or unidirectional pulsatile shear for 2 days and 3 & 7 days in regular and osteogenic media, respectively. Higher expression of miR-214, increased thickness of the fibrosa, and calcification was observed when the fibrosa was exposed to OS compared to the ventricularis. Silencing of miR-214 by anti-miR-214 in whole AV leaflets with the fibrosa exposed to OS significantly increased the protein expression of TGFβ1 and moderately increased collagen content but did not affect AV calcification. Thus, miR-214 is identified as a side- and shear-dependent miRNA that regulates key mechanosensitive gene in AV such as TGFβ1.

Cells and extracellular matrix interplay in cardiac valve disease: because age matters

Cardiovascular aging is a physiological process affecting all components of the heart. Despite the interest and experimental effort lavished on aging of cardiac cells, increasing evidence is pointing at the pivotal role of extracellular matrix (ECM) in cardiac aging. Structural and molecular changes in ECM composition during aging are at the root of significant functional modifications at the level of cardiac valve apparatus. Indeed, calcification or myxomatous degeneration of cardiac valves and their functional impairment can all be explained in light of age-related ECM alterations and the reciprocal interplay between altered ECM and cellular elements populating the leaflet, namely valvular interstitial cells and valvular endothelial cells, is additionally affecting valve function with striking reflexes on the clinical scenario. The initial experimental findings on this argument are underlining the need for a more comprehensive understanding on the biological mechanisms underlying ECM aging and remodeling as potentially constituting a pharmacological therapeutic target or a basis to improve existing prosthetic devices and treatment options. Given the lack of systematic knowledge on this topic, this review will focus on the ECM changes that occur during aging and on their clinical translational relevance and implications in the bedside scenario.

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.

Fibrotic Aortic Valve Stenosis in Hypercholesterolemic/Hypertensive Mice

OBJECTIVE

Hypercholesterolemia and hypertension are associated with aortic valve stenosis (AVS) in humans. We have examined aortic valve function, structure, and gene expression in hypercholesterolemic/hypertensive mice.

APPROACH AND RESULTS

Control, hypertensive, hypercholesterolemic (Apoe(-/-)), and hypercholesterolemic/hypertensive mice were studied. Severe aortic stenosis (echocardiography) occurred only in hypercholesterolemic/hypertensive mice. There was minimal calcification of the aortic valve. Several structural changes were identified at the base of the valve. The intercusp raphe (or seam between leaflets) was longer in hypercholesterolemic/hypertensive mice than in other mice, and collagen fibers at the base of the leaflets were reoriented to form a mesh. In hypercholesterolemic/hypertensive mice, the cusps were asymmetrical, which may contribute to changes that produce AVS. RNA sequencing was used to identify molecular targets during the developmental phase of stenosis. Genes related to the structure of the valve were identified, which differentially expressed before fibrotic AVS developed. Both RNA and protein of a profibrotic molecule, plasminogen activator inhibitor 1, were increased greatly in hypercholesterolemic/hypertensive mice.

CONCLUSIONS

Hypercholesterolemic/hypertensive mice are the first model of fibrotic AVS. Hypercholesterolemic/hypertensive mice develop severe AVS in the absence of significant calcification, a feature that resembles AVS in children and some adults. Structural changes at the base of the valve leaflets include lengthening of the raphe, remodeling of collagen, and asymmetry of the leaflets. Genes were identified that may contribute to the development of fibrotic AVS.

Krox20 heterozygous mice: A model of aortic regurgitation associated with decreased expression of fibrillar collagen genes

BACKGROUND

The mechanism involved in the onset of aortic valve (AoV) disease remains unclear despite its poor prognosis and frequency. Recently, we reported that Krox20 (EGR2 in humans) is involved in AoV development and dysfunction.

AIM

Analyze Krox20 heterozygous mice (Krox20(+/-)) to discover whether incomplete expression of Krox20 can cause valvular diseases.

METHODS

Transcriptional levels of Col1a2/COL1A2 and Krox20/EGR2 in AoVs from Krox20(+/-) mice and human patients operated on for severe aortic regurgitation were evaluated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Human control valves were obtained from three transplanted patients without AoV disease. Twenty-one heterozygous Krox20(+/-) mice were compared with 35 controls at different ages. Three independent measurements of valve thickness were performed on magnified tissue sections using Image J software. In vivo valve structure and function were evaluated using the high-frequency Vevo(®) 2100 echocardiogram.

RESULTS

qRT-PCR analysis using AoVs from patients with severe aortic regurgitation showed a decrease in EGR2 expression associated with significant downregulation of COL1A2 expression (P<0.05). Similar results were observed in the AoVs of Krox20(+/-) mice. Anatomical examination revealed that incomplete invalidation of Krox20 caused significant thickening of the aortic leaflet compared with controls (145±22 vs. 75±24μm; P=0.01). Within the mutant group, this thickening worsened significantly over time (Krox20(+/-) mice aged>7 vs.<7months: 136±48 vs. 102±41μm; P<0.001). Moreover, the aortic leaflets of embryonic day 18.5 Krox20(+/-) embryos were significantly more thickened than those from controls, suggesting that this disease begins during embryonic development. Echo-Doppler analysis showed a significant increase in AoV dysfunction in heterozygous versus control mice (53% vs. 17%; P<0.001), suggesting a tight relationship between valve architecture and function. Morphometric analysis revealed that the most severe AoV dysfunction was always associated with the most thickened valves. Classic histological analysis revealed that mutant AoVs had extracellular matrix disorganization, with features of human myxomatous degeneration, including excess of proteoglycan deposition in spongiosa and reduction of collagen fibre in fibrosa, but no calcification.

CONCLUSION

Decreased expression of Krox20 in mice causes degeneration of the aortic leaflets and disorganization of the extracellular matrix, causing valvular dysfunction.

Manipulation of valve composition to elucidate the role of collagen in aortic valve calcification

Background

Extracellular matrix (ECM) disarray is found in calcific aortic valvular disease (CAVD), yet much remains to be learned about the role of individual ECM components in valvular interstitial cell (VIC) function and dysfunction. Previous clinical analyses have shown that calcification is associated with decreased collagen content, while previous in vitro work has suggested that the presence of collagen attenuates the responsiveness of VICs to pro-calcific stimuli. The current study uses whole leaflet cultures to examine the contributions of endogenous collagen in regulating the phenotype and calcification of VICs.

Methods

A “top-down” approach was used to characterize changes in VIC phenotype in response to collagen alterations in the native 3D environment. Collagen-deficient leaflets were created via enzymatic treatment and cultured statically for six days in vitro. After culture, leaflets were harvested for analysis of DNA, proliferation, apoptosis, ECM composition, calcification, and gene/protein expression.

Results

In general, disruption of collagen was associated with increased expression of disease markers by VICs in whole organ leaflet culture. Compared to intact control leaflets, collagen-deficient leaflets demonstrated increased VIC proliferation and apoptosis, increased expression of disease-related markers such as alpha-smooth muscle actin, alkaline phosphatase, and osteocalcin, and an increase in calcification as evidenced by positive von Kossa staining.

Conclusions

These results indicate that disruption of the endogenous collagen structure in aortic valves is sufficient to stimulate pathological consequences in valve leaflet cultures, thereby highlighting the importance of collagen and the valve extracellular matrix in general in maintaining homeostasis of the valve phenotype.

What can we learn about valvular heart disease from PET/CT?

Valvular heart disease is a major cause of morbidity and mortality, and with an aging population, its prevalence is increasing. Here, we review the evolving use of positron emission tomography/computed tomography in valvular heart disease, with particular focus on calcific aortic stenosis and infective endocarditis. In principle, the activity of any pathological process can be studied, as long as an appropriate radiotracer can be developed. We will review some of the early data using established tracers in the above and other conditions, providing discussion as to the future research and clinical roles of these techniques. Furthermore, we will discuss the potential impact of novel tracers that are currently under development or testing in preclinical models. It is hoped that such advanced imaging might improve the diagnosis, treatment and outlook for patients with valvular heart disease.

[Aortic stenosis and extracellular matrix remodeling]

Valvular heart diseases represent an important public health burden. With the decrease in the incidence of rheumatic heart disease, calcific aortic stenosis has now become the most common valvular disease in Western countries. Its prevalence increases with age, such that its affects about 4% of the elderly population and it is the most common motive for valve replacement. Several tissue abnormalities were observed in aortic valves from patients suffering from aortic stenosis: presence of large calcium deposits, inflammatory cells, lipids, and neocapillaries as well as extracellular matrix remodeling. The aortic valves show three characteristic layers: the fibrosa composed mainly of collagen bundles, the spongiosa which consists of a proteoglycan matrix, and the ventricularis which contains several elastic lamellae. The components of the extracellular matrix are synthesized by valvular mesenchymal cells. The turn-over of collagen and elastic fibers is low; the other macromolecules are more rapidly synthesized and hydrolysed. Serine proteases such as enzymes of the fibrinolytic system and matrix metalloproteinases play a role in the remodeling of the extracellular matrix. The hydrolysis of adhesive proteins, such as fibronectin, by plasmin triggers the apoptosis of valvular (myo)fibroblasts, a biological process named anoikis. Cellular events and extracellular matrix remodeling thus participate to the evolution of aortic valves towards aortic stenosis.

Tumor necrosis factor-α and receptor activator of nuclear factor-κB ligand augment human macrophage foam-cell destruction of extracellular matrix through protease-mediated processes

By secreting proteases such as cathepsins and matrix metalloproteinases (MMPs), macrophage foam cells may be a major cause of ruptured atherosclerotic plaques. The aims of the present study were to investigate in vitro role of human macrophage foam cells in degrading type I collagen, a major component of extracellular matrix (ECM) in plaques, and to establish whether the pro-inflammatory molecules, tumor necrosis factor (TNF)-alpha, and receptor activator of nuclear factor-κB ligand (RANK-L) increase this degradation. CD14+ monocytes isolated from peripheral blood were differentiated into macrophage foam cells and cultured on a type I collagen matrix in the presence of TNF-alpha and RANK-L. Matrix degradation was measured by the cathepsin K-generated C-terminal cross-linked telopeptide of type I collagen (CTX-I) and the MMP-generated carboxyterminal telopeptide of type I collagen (ICTP) in supernatants showing that macrophage foam cells secrete MMPs and cathepsin K, resulting in release of ICTP and CTX-I. Stimulation with TNF-alpha increased CTX-I and ICTP dose dependently, with ICTP levels increasing by 59% and CTX-I levels increasing by 43%. RANK-L enhanced the release of CTX-I and ICTP by 56% and 72%, respectively. This is, to our knowledge, the first data describing a simple in vitro system in which macrophage foam cells degradation of matrix proteins can be monitored. This degradation can be enhanced by cytokines since TNF-alpha and RANK-L significantly increased the matrix degradation. This in vitro system in part is a model system for the macrophage-mediated proteolytic degradation of the ECM, which is found in many diseases with an inflammatory component.

Fibrosis in endstage human heart failure: severe changes in collagen metabolism and MMP/TIMP profiles

OBJECTIVES

We studied fibrosis, collagen metabolism, MMPs/TIMPs and cytokine expression in various forms of human heart failure (HF) by quantitative immunofluorescent microscopy, Western blot, zymography, RT-PCR and in situ hybridization. In explanted human hearts with HF due to either dilated (DCM, n=6) or ischemic (ICM-BZ-borderzone, ICM-RZ-remote zone, n=7) or inflammatory (myocarditis, MYO, n=6) cardiomyopathy and 8 controls MMP2, 8, 9, 19, and TIMP1, 2, 3, 4 as well as procollagens I and III (PINP, PIIINP), mature collagen III (IIINTP) and the cross-linked collagen I degradation product (ICTP) were measured.

RESULTS

In comparison with controls, MMPs and TIMPs were significantly upregulated ranging (from highest to lowest) from ICM-BZ, DCM, ICM-RZ, MYO for all MMPs with the exception of MMP9 (highest in DCM), and for TIMPs from ICM-BZ, ICM-RZ, DCM and MYO. MMP2 and 9 were activated in all groups. The TIMP/MMP ratio was 1.3 for control, 1.9 in ICM-BZ (TIMP>MMP) and lowered to 1.0 in the other groups. Collagen I/collagen III ratio correlated significantly with the decrease in LVEDP. PINP was higher than ICTP in all groups. PIIINP elevation was present in DCM and ICM-RZ and IIINTP was up to 4-fold augmented in all groups. Fibrosin mRNA was upregulated in ICM-BZ, activin A in MYO but FGF1 and FGF2 remained unchanged. ANP mRNA was increased in all groups.

CONCLUSIONS

Although different degrees of severity of collagen metabolism, MMP/TIMP imbalance and cytokine expression in diverse forms of HF are present, the end product is collagen deposition. These findings suggest multiple mechanisms acting alone or in concert in fibrosis development in HF.

Animal models of calcific aortic valve disease

Calcific aortic valve disease (CAVD), once thought to be a degenerative disease, is now recognized to be an active pathobiological process, with chronic inflammation emerging as a predominant, and possibly driving, factor. However, many details of the pathobiological mechanisms of CAVD remain to be described, and new approaches to treat CAVD need to be identified. Animal models are emerging as vital tools to this end, facilitated by the advent of new models and improved understanding of the utility of existing models. In this paper, we summarize and critically appraise current small and large animal models of CAVD, discuss the utility of animal models for priority CAVD research areas, and provide recommendations for future animal model studies of CAVD.

Cell–Matrix Interactions in the Pathobiology of Calcific Aortic Valve Disease

The hallmarks of calcific aortic valve disease (CAVD) are the significant changes that occur in the organization, composition, and mechanical properties of the extracellular matrix (ECM), ultimately resulting in stiffened stenotic leaflets that obstruct flow and compromise cardiac function. Increasing evidence suggests that ECM maladaptations are not simply a result of valve cell dysfunction; they also contribute to CAVD progression by altering cellular and molecular signaling. In this review, we summarize the ECM changes that occur in CAVD. We also discuss examples of how the ECM influences cellular processes by signaling through adhesion receptors (matricellular signaling), by regulating the presentation and availability of growth factors and cytokines to cells (matricrine signaling), and by transducing externally applied forces and resisting cell-generated tractional forces (mechanical signaling) to regulate a wide range of pathological processes, including differentiation, fibrosis, calcification, and angiogenesis. Finally, we suggest areas for future research that should lead to new insights into bidirectional cell–ECM interactions in the aortic valve, their contributions to homeostasis and pathobiology, and possible targets to slow or prevent the progression of CAVD.

Vascular endothelial growth factor-secreting mast cells and myofibroblasts: a novel self-perpetuating angiogenic pathway in aortic valve stenosis

OBJECTIVE

To examine the proangiogenic potential of myofibroblasts and mast cells, 2 types of cells present in human aortic valves.

METHODS AND RESULTS

Aortic valve stenosis is an active atheroinflammatory disease, characterized by the accumulation of inflammatory cells and the neovascularization of the valves. A total of 85 stenotic valves and 20 control valves were obtained during valve replacement surgery. The results of immunohistochemistry analysis revealed stenotic aortic valves that contained 3 types of neovessels: small microvessels, medium microvessels, and organized arterioles. The distribution density of the neovessels was significantly higher in stenotic valves than in control valves (P<0.001) and correlated positively with valvular calcification gradus (r=0.26, P=0.02) and mast cell density (r=0.38, P<0.001). In the neovascularized areas of stenotic aortic valves, mast cells contained vascular endothelial growth factor and were degranulated, indicating their activation. The stimulation of cultured myofibroblasts derived from aortic valves with a mast cell-preconditioned medium, hypoxic culture conditions, or tobacco smoke all induced vascular endothelial growth factor secretion in the myofibroblasts. Finally, mast cell tryptase was able to degrade the antiangiogenic molecule endostatin in vitro.

CONCLUSIONS

Mast cells and myofibroblasts may accelerate the progression of aortic valve stenosis by altering the balance between angiogenic and antiangiogenic factors in the valves, thus promoting valvular neovascularization.

Human macrophage foam cells degrade atherosclerotic plaques through cathepsin K mediated processes

Background

Proteolytic degradation of Type I Collagen by proteases may play an important role in remodeling of atherosclerotic plaques, contributing to increased risk of plaque rupture.

The aim of the current study was to investigate whether human macrophage foam cells degrade the extracellular matrix (ECM) of atherosclerotic plaques by cathepsin K mediated processes.

Methods

We 1) cultured human macrophages on ECM and measured cathepsin K generated fragments of type I collagen (C-terminal fragments of Type I collagen (CTX-I) 2) investigated the presence of CTX-I in human coronary arteries and 3) finally investigated the clinical potential by measuring circulating CTX-I in women with and without radiographic evidence of aortic calcified atherosclerosis.

Results

Immune-histochemistry of early and advanced lesions of coronary arteries demonstrated co-localization of Cathepsin-K and CTX-I in areas of intimal hyperplasia and in shoulder regions of advanced plaques. Treatment of human monocytes with M-CSF or M-CSF+LDL generated macrophages and foam cells producing CTX-I when cultured on type I collagen enriched matrix. Circulating levels of CTX-I were not significantly different in women with aortic calcifications compared to those without.

Conclusions

Human macrophage foam cells degrade the atherosclerotic plaques though cathepsin K mediated processes, resulting in increase in levels of CTX-I. Serum CTX-I was not elevated in women with aortic calcification, likely due to the contribution of CTX-I from osteoclastic bone resorption which involves Cathepsin-K. The human macrophage model system may be used to identify important pathway leading to excessive proteolytic plaque remodeling and plaque rupture.