Aortic valve sclerosis in mice deficient in endothelial nitric oxide synthase

Identification of congenital aortic valve malformations in juvenile natriuretic peptide receptor 2-deficient mice using high-frequency ultrasound

Mouse models of congenital aortic valve malformations are useful for studying disease pathobiology, but most models have incomplete penetrance [e.g., ∼2 to 77% prevalence of bicuspid aortic valves (BAVs) across multiple models]. For longitudinal studies of pathologies associated with BAVs and other congenital valve malformations, which manifest over months in mice, it is operationally inefficient, economically burdensome, and ethically challenging to enroll large numbers of mice in studies without first identifying those with valvular abnormalities. To address this need, we established and validated a novel in vivo high-frequency (30 MHz) ultrasound imaging protocol capable of detecting aortic valvular malformations in juvenile mice. Fifty natriuretic peptide receptor 2 heterozygous mice on a low-density lipoprotein receptor-deficient background (Npr2+/-;Ldlr-/-; 32 males and 18 females) were imaged at 4 and 8 wk of age. Fourteen percent of the Npr2+/-;Ldlr-/- mice exhibited features associated with aortic valve malformations, including 1) abnormal transaortic flow patterns on color Doppler (recirculation and regurgitation), 2) peak systolic flow velocities distal to the aortic valves reaching or surpassing ∼1,250 mm/s by pulsed-wave Doppler, and 3) putative fusion of cusps along commissures and abnormal movement elucidated by two-dimensional (2-D) imaging with ultrahigh temporal resolution. Valves with these features were confirmed by ex vivo gross anatomy and histological visualization to have thickened cusps, partial fusions, or Sievers type-0 bicuspid valves. This ultrasound imaging protocol will enable efficient, cost effective, and humane implementation of studies of congenital aortic valvular abnormalities and associated pathologies in a wide range of mouse models.NEW & NOTEWORTHY We developed a high-frequency ultrasound imaging protocol for diagnosing congenital aortic valve structural abnormalities in 4-wk-old mice. Our protocol defines specific criteria to distinguish mice with abnormal aortic valves from those with normal tricuspid valves using color Doppler, pulsed-wave Doppler, and two-dimensional (2-D) imaging with ultrahigh temporal resolution. This approach enables early identification of valvular abnormalities for efficient and ethical experimental design of longitudinal studies of congenital valve diseases and associated pathologies in mice.

Insulin and Insulin-Like Growth Factor 1 Signaling Preserves Sarcomere Integrity in the Adult Heart

Insulin and insulin-like growth factor 1 (IGF1) signaling is transduced by insulin receptor substrate 1 (IRS1) and IRS2. To elucidate physiological and redundant roles of insulin and IGF1 signaling in adult hearts, we generated mice with inducible cardiomyocyte-specific deletion of insulin and IGF1 receptors or IRS1 and IRS2. Both models developed dilated cardiomyopathy, and most mice died by 8 weeks post-gene deletion. Heart failure was characterized by cardiomyocyte loss and disarray, increased proapoptotic signaling, and increased autophagy. Suppression of autophagy by activating mTOR signaling did not prevent heart failure. Transcriptional profiling revealed reduced serum response factor (SRF) transcriptional activity and decreased mRNA levels of genes encoding sarcomere and gap junction proteins as early as 3 days post-gene deletion, in concert with ultrastructural evidence of sarcomere disruption and intercalated discs within 1 week after gene deletion. These data confirm conserved roles for constitutive insulin and IGF1 signaling in suppressing autophagic and apoptotic signaling in the adult heart. The present study also identifies an unexpected role for insulin and IGF1 signaling in regulating an SRF-mediated transcriptional program, which maintains expression of genes encoding proteins that support sarcomere integrity in the adult heart, reduction of which results in rapid development of heart failure.

Implication of Lipids in Calcified Aortic Valve Pathogenesis: Why Did Statins Fail?

Calcific Aortic Valve Disease (CAVD) is a fibrocalcific disease. Lipoproteins and oxidized phospholipids play a substantial role in CAVD; the level of Lp(a) has been shown to accelerate the progression of valve calcification. Indeed, oxidized phospholipids carried by Lp(a) into the aortic valve stimulate endothelial dysfunction and promote inflammation. Inflammation and growth factors actively promote the synthesis of the extracellular matrix (ECM) and trigger an osteogenic program. The accumulation of ECM proteins promotes lipid adhesion to valve tissue, which could initiate the osteogenic program in interstitial valve cells. Statin treatment has been shown to have the ability to diminish the death rate in subjects with atherosclerotic impediments by decreasing the serum LDL cholesterol levels. However, the use of HMG-CoA inhibitors (statins) as cholesterol-lowering therapy did not significantly reduce the progression or the severity of aortic valve calcification. However, new clinical trials targeting Lp(a) or PCSK9 are showing promising results in reducing the severity of aortic stenosis. In this review, we discuss the implication of lipids in aortic valve calcification and the current findings on the effect of lipid-lowering therapy in aortic stenosis.

Calcific aortic valve disease: from molecular and cellular mechanisms to medical therapy

Calcific aortic valve disease (CAVD) is a highly prevalent condition that comprises a disease continuum, ranging from microscopic changes to profound fibro-calcific leaflet remodelling, culminating in aortic stenosis, heart failure, and ultimately premature death. Traditional risk factors, such as hypercholesterolaemia and (systolic) hypertension, are shared among atherosclerotic cardiovascular disease and CAVD, yet the molecular and cellular mechanisms differ markedly. Statin-induced low-density lipoprotein cholesterol lowering, a remedy highly effective for secondary prevention of atherosclerotic cardiovascular disease, consistently failed to impact CAVD progression or to improve patient outcomes. However, recently completed phase II trials provide hope that pharmaceutical tactics directed at other targets implicated in CAVD pathogenesis offer an avenue to alter the course of the disease non-invasively. Herein, we delineate key players of CAVD pathobiology, outline mechanisms that entail compromised endothelial barrier function, and promote lipid homing, immune-cell infiltration, and deranged phospho-calcium metabolism that collectively perpetuate a pro-inflammatory/pro-osteogenic milieu in which valvular interstitial cells increasingly adopt myofibro-/osteoblast-like properties, thereby fostering fibro-calcific leaflet remodelling and eventually resulting in left ventricular outflow obstruction. We provide a glimpse into the most promising targets on the horizon, including lipoprotein(a), mineral-binding matrix Gla protein, soluble guanylate cyclase, dipeptidyl peptidase-4 as well as candidates involved in regulating phospho-calcium metabolism and valvular angiotensin II synthesis and ultimately discuss their potential for a future therapy of this insidious disease.

NOTCH Signaling in Aortic Valve Development and Calcific Aortic Valve Disease

NOTCH intercellular signaling mediates the communications between adjacent cells involved in multiple biological processes essential for tissue morphogenesis and homeostasis. The NOTCH1 mutations are the first identified human genetic variants that cause congenital bicuspid aortic valve (BAV) and calcific aortic valve disease (CAVD). Genetic variants affecting other genes in the NOTCH signaling pathway may also contribute to the development of BAV and the pathogenesis of CAVD. While CAVD occurs commonly in the elderly population with tri-leaflet aortic valve, patients with BAV have a high risk of developing CAVD at a young age. This observation indicates an important role of NOTCH signaling in the postnatal homeostasis of the aortic valve, in addition to its prenatal functions during aortic valve development. Over the last decade, animal studies, especially with the mouse models, have revealed detailed information in the developmental etiology of congenital aortic valve defects. In this review, we will discuss the molecular and cellular aspects of aortic valve development and examine the embryonic pathogenesis of BAV. We will focus our discussions on the NOTCH signaling during the endocardial-to-mesenchymal transformation (EMT) and the post-EMT remodeling of the aortic valve. We will further examine the involvement of the NOTCH mutations in the postnatal development of CAVD. We will emphasize the deleterious impact of the embryonic valve defects on the homeostatic mechanisms of the adult aortic valve for the purpose of identifying the potential therapeutic targets for disease intervention.

AMPK: Potential Therapeutic Target for Vascular Calcification

Vascular calcification (VC) is an urgent worldwide health issue with no available medical treatment. It is an active cell-driven process by osteogenic differentiation of vascular cells with complex mechanisms. The AMP-activated protein kinase (AMPK) serves as the master sensor of cellular energy status. Accumulating evidence reveals the vital role of AMPK in VC progression. AMPK is involved in VC in various ways, including inhibiting runt-related transcription factor 2 signaling pathways, triggering autophagy, attenuating endoplasmic reticulum stress and dynamic-related protein 1-mediated mitochondrial fission, and activating endothelial nitric oxide synthase. AMPK activators, like metformin, are associated with reduced calcification deposits in certain groups of patients, indicating that AMPK is a potential therapeutic target for VC.

Genetic and Developmental Contributors to Aortic Stenosis

Aortic stenosis (AS) remains one of the most common forms of valve disease, with significant impact on patient survival. The disease is characterized by left ventricular outflow obstruction and encompasses a series of stenotic lesions starting from the left ventricular outflow tract to the descending aorta. Obstructions may be subvalvar, valvar, or supravalvar and can be present at birth (congenital) or acquired later in life. Bicuspid aortic valve, whereby the aortic valve forms with two instead of three cusps, is the most common cause of AS in younger patients due to primary anatomic narrowing of the valve. In addition, the secondary onset of premature calcification, likely induced by altered hemodynamics, further obstructs left ventricular outflow in bicuspid aortic valve patients. In adults, degenerative AS involves progressive calcification of an anatomically normal, tricuspid aortic valve and is attributed to lifelong exposure to multifactoral risk factors and physiological wear-and-tear that negatively impacts valve structure-function relationships. AS continues to be the most frequent valvular disease that requires intervention, and aortic valve replacement is the standard treatment for patients with severe or symptomatic AS. While the positive impacts of surgical interventions are well documented, the financial burden, the potential need for repeated procedures, and operative risks are substantial. In addition, the clinical management of asymptomatic patients remains controversial. Therefore, there is a critical need to develop alternative approaches to prevent the progression of left ventricular outflow obstruction, especially in valvar lesions. This review summarizes our current understandings of AS cause; beginning with developmental origins of congenital valve disease, and leading into the multifactorial nature of AS in the adult population.

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.

Role of oxidative stress in calcific aortic valve disease and its therapeutic implications

Calcific aortic valve disease (CAVD) is the end result of active cellular processes that lead to the progressive fibrosis and calcification of aortic valve leaflets. In western populations, CAVD is a significant cause of cardiovascular morbidity and mortality, and in the absence of effective drugs, it will likely represent an increasing disease burden as populations age. As there are currently no pharmacological therapies available for preventing, treating, or slowing the development of CAVD, understanding the mechanisms underlying the initiation and progression of the disease is important for identifying novel therapeutic targets. Recent evidence has emerged of an important causative role for reactive oxygen species (ROS)-mediated oxidative stress in the pathophysiology of CAVD, inducing the differentiation of valve interstitial cells into myofibroblasts and then osteoblasts. In this review, we focus on the roles and sources of ROS driving CAVD and consider their potential as novel therapeutic targets for this debilitating condition.

Optical coherence tomography and multiphoton microscopy offer new options for the quantification of fibrotic aortic valve disease in ApoE-/- mice

Aortic valve sclerosis is characterized as the thickening of the aortic valve without obstruction of the left ventricular outflow. It has a prevalence of 30% in people over 65 years old. Aortic valve sclerosis represents a cardiovascular risk marker because it may progress to moderate or severe aortic valve stenosis. Thus, the early recognition and management of aortic valve sclerosis are of cardinal importance. We examined the aortic valve geometry and structure from healthy C57Bl6 wild type and age-matched hyperlipidemic ApoE-/- mice with aortic valve sclerosis using optical coherence tomography (OCT) and multiphoton microscopy (MPM) and compared results with histological analyses. Early fibrotic thickening, especially in the tip region of the native aortic valve leaflets from the ApoE-/- mice, was detectable in a precise spatial resolution using OCT. Evaluation of the second harmonic generation signal using MPM demonstrated that collagen content decreased in all aortic valve leaflet regions in the ApoE-/- mice. Lipid droplets and cholesterol crystals were detected using coherent anti-Stokes Raman scattering in the tissue from the ApoE-/- mice. Here, we demonstrated that OCT and MPM, which are fast and precise contactless imaging approaches, are suitable for defining early morphological and structural alterations of sclerotic murine aortic valves.

Evogliptin Suppresses Calcific Aortic Valve Disease by Attenuating Inflammation, Fibrosis, and Calcification

Calcific aortic valve disease (CAVD) accompanies inflammatory cell infiltration, fibrosis, and ultimately calcification of the valve leaflets. We previously demonstrated that dipeptidyl peptidase-4 (DPP-4) is responsible for the progression of aortic valvular calcification in CAVD animal models. As evogliptin, one of the DPP-4 inhibitors displays high specific accumulation in cardiac tissue, we here evaluated its therapeutic potency for attenuating valvular calcification in CAVD animal models. Evogliptin administration markedly reduced calcific deposition accompanied by a reduction in proinflammatory cytokine expression in endothelial nitric oxide synthase-deficient mice in vivo, and significantly ameliorated the mineralization of the primary human valvular interstitial cells (VICs), with a reduction in the mRNA expression of bone-associated and fibrosis-related genes in vitro. In addition, evogliptin ameliorated the rate of change in the transaortic peak velocity and mean pressure gradients in our rabbit model as assessed by echocardiography. Importantly, evogliptin administration in a rabbit model was found to suppress the effects of a high-cholesterol diet and of vitamin D2-driven fibrosis in association with a reduction in macrophage infiltration and calcific deposition in aortic valves. These results have indicated that evogliptin prohibits inflammatory cytokine expression, fibrosis, and calcification in a CAVD animal model, suggesting its potential as a selective therapeutic agent for the inhibition of valvular calcification during CAVD progression.

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.

Calcified Aortic Valve Disease in Patients With Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is a rare autosomal gene deficiency disease with increased low-density lipoprotein cholesterol, xanthoma, and premature coronary heart disease. Calcified aortic valve disease (CAVD) is prevalent in FH patients, resulting in adverse events and heavy health care burden. Aortic valve calcification is currently considered an active biological process, which shares several common risk factors with atherosclerosis, including aging, hypertension, dyslipidemia, and so on. Unfortunately, the pathogenesis and therapy of CAVD in FH are still controversial. There is no pharmacological intervention recommended to delay the development of CAVD in FH, and the only effective treatment for severe CAVD is aortic valve replacement. In this review, we summarize the detailed description of the pathophysiology, molecular mechanism, risk factors, and treatment of CAVD in FH patients.

Lack of Endothelial Nitric Oxide Synthase Accelerates Ectopic Calcification in Uremic Mice Fed an Adenine and High Phosphorus Diet

Ectopic calcification is a risk of cardiovascular disease in chronic kidney disease (CKD) patients, and impaired endothelial nitric oxide synthase (eNOS) is involved in the CKD complications. However, whether eNOS dysfunction is a cause of ectopic calcification in CKD remains to be elucidated. To address this issue, we investigated the role of eNOS in ectopic calcification in mice with renal injury caused by an adenine and high-phosphorus (Ade + HP) diet. DBA/2J mice, a calcification-sensitive strain, were fed Ade + HP for 3 weeks. Expression levels of eNOS-related genes were reduced significantly in their calcified aorta. C57BL/6J is a calcification-resistant strain, and wild-type mice showed mild calcified lesions in the aorta and kidney when given an Ade + HP diet for 4 weeks. In contrast, a lack of eNOS led to the development of severe aortic calcification accompanied by an increase in runt-related transcription factor 2, an osteochondrogenic marker. Increased renal calcium deposition and the tubular injury score were remarkable in mice lacking eNOS-fed Ade + HP. Exacerbation of ectopic calcification by a lack of eNOS is associated with increased oxidative stress markers such as nicotinamide adenine dinucleotide phosphate oxidases. In conclusion, eNOS is critically important in preventing ectopic calcification. Therefore, the maintenance of eNOS is useful to reduce cardiovascular disease events and to improve prognosis in CKD patients.

Aortic Valve Stenosis and Mitochondrial Dysfunctions: Clinical and Molecular Perspectives

Calcific aortic stenosis is a disorder that impacts the physiology of heart valves. Fibrocalcific events progress in conjunction with thickening of the valve leaflets. Over the years, these events promote stenosis and obstruction of blood flow. Known and common risk factors are congenital defects, aging and metabolic syndromes linked to high plasma levels of lipoproteins. Inflammation and oxidative stress are the main molecular mediators of the evolution of aortic stenosis in patients and these mediators regulate both the degradation and remodeling processes. Mitochondrial dysfunction and dysregulation of autophagy also contribute to the disease. A better understanding of these cellular impairments might help to develop new ways to treat patients since, at the moment, there is no effective medical treatment to diminish neither the advancement of valve stenosis nor the left ventricular function impairments, and the current approaches are surgical treatment or transcatheter aortic valve replacement with prosthesis.

l-Arginine prevents inflammatory and pro-calcific differentiation of interstitial aortic valve cells

BACKGROUND AND AIMS

Reduced bioavailability of nitric oxide (NO) has been implicated in the pathogenesis of calcific aortic stenosis. Herein, we investigated the effects of l-Arginine, the main precursor of NO, on the osteogenic differentiation of aortic interstitial valve cells (VICs).

METHODS

We isolated a clonal population of bovine VICs that expresses osteogenic markers and induces calcification of collagen matrix after stimulation with endotoxin (LPS 500 ng/mL). VICs were treated in vitro with different combinations of LPS ± l-Arginine (50 or 100 mM) and cell extracts were collected to perform proteomic (iTRAQ) and gene expression (RT-PCR) analysis.

RESULTS

l-Arginine prevents the over-expression of alkaline phosphatase (ALP, p < 0.001) and reduces matrix calcification (p < 0.05) in VICs treated with LPS. l-Arginine also reduces the over-expression of inflammatory molecules induced by LPS (TNF-alpha, IL-6 and IL-1beta, p < 0.001). The proteomic analysis allowed to identify 49 proteins with an altered expression profile after stimulation with LPS and significantly modified by l-Arginine. These include proteins involved in the redox homeostasis of the cells (i.e. Xanthine Oxidase, Catalase, Aldehyde Oxidase), remodeling of the extracellular matrix (i.e. ADAMTSL4, Basigin, COL3A1) and cellular signaling (i.e. Fibrillin-1, Legumain, S100A13). The RT-PCR analysis confirmed the modifications of Fibrillin-1, ADAMTSL4, Basigin and Xanthine Oxidase, whose expression levels increase after stimulation with LPS and are reduced by l-Arginine (p < 0.05).

CONCLUSIONS

l-Arginine prevents osteogenic differentiation of VICs and reduces matrix calcification. This effect is achieved through the modulation of proteins involved in the cellular redox system, remodeling of extracellular matrix and inflammatory activation of VICs.

Lysed Erythrocyte Membranes Promote Vascular Calcification

BACKGROUND

Intraplaque hemorrhage promotes atherosclerosis progression, and erythrocytes may contribute to this process. In this study we examined the effects of red blood cells on smooth muscle cell mineralization and vascular calcification and the possible mechanisms involved.

METHODS

Erythrocytes were isolated from human and murine whole blood. Intact and lysed erythrocytes and their membrane fraction or specific erythrocyte components were examined in vitro using diverse calcification assays, ex vivo by using the murine aortic ring calcification model, and in vivo after murine erythrocyte membrane injection into neointimal lesions of hypercholesterolemic apolipoprotein E-deficient mice. Vascular tissues (aortic valves, atherosclerotic carotid artery specimens, abdominal aortic aneurysms) were obtained from patients undergoing surgery.

RESULTS

The membrane fraction of lysed, but not intact human erythrocytes promoted mineralization of human arterial smooth muscle cells in culture, as shown by Alizarin red and van Kossa stain and increased alkaline phosphatase activity, and by increased expression of osteoblast-specific transcription factors (eg, runt-related transcription factor 2, osterix) and differentiation markers (eg, osteopontin, osteocalcin, and osterix). Erythrocyte membranes dose-dependently enhanced calcification in murine aortic rings, and extravasated CD235a-positive erythrocytes or Perl iron-positive signals colocalized with calcified areas or osteoblast-like cells in human vascular lesions. Mechanistically, the osteoinductive activity of lysed erythrocytes was localized to their membrane fraction, did not involve membrane lipids, heme, or iron, and was enhanced after removal of the nitric oxide (NO) scavenger hemoglobin. Lysed erythrocyte membranes enhanced calcification to a similar extent as the NO donor diethylenetriamine-NO, and their osteoinductive effects could be further augmented by arginase-1 inhibition (indirectly increasing NO bioavailability). However, the osteoinductive effects of erythrocyte membranes were reduced in human arterial smooth muscle cells treated with the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide or following inhibition of NO synthase or the NO receptor soluble guanylate cyclase. Erythrocytes isolated from endothelial NO synthase-deficient mice exhibited a reduced potency to promote calcification in the aortic ring assay and after injection into murine vascular lesions.

CONCLUSIONS

Our findings in cells, genetically modified mice, and human vascular specimens suggest that intraplaque hemorrhage with erythrocyte extravasation and lysis promotes osteoblastic differentiation of smooth muscle cells and vascular lesion calcification, and also support a role for erythrocyte-derived NO.

NFκB (Nuclear Factor κ-Light-Chain Enhancer of Activated B Cells) Activity Regulates Cell-Type-Specific and Context-Specific Susceptibility to Calcification in the Aortic Valve

OBJECTIVE

Although often studied independently, little is known about how aortic valve endothelial cells and valve interstitial cells interact collaborate to maintain tissue homeostasis or drive valve calcific pathogenesis. Inflammatory signaling is a recognized initiator of valve calcification, but the cell-type-specific downstream mechanisms have not been elucidated. In this study, we test how inflammatory signaling via NFκB (nuclear factor κ-light-chain enhancer of activated B cells) activity coordinates unique and shared mechanisms of valve endothelial cells and valve interstitial cells differentiation during calcific progression. Approach and Results: Activated NFκB was present throughout the calcific aortic valve disease (CAVD) process in both endothelial and interstitial cell populations in an established mouse model of hypercholesterolemia-induced CAVD and in human CAVD. NFκB activity induces endothelial to mesenchymal transformation in 3-dimensional cultured aortic valve endothelial cells and subsequent osteogenic calcification of transformed cells. Similarly, 3-dimensional cultured valve interstitial cells calcified via NFκB-mediated osteogenic differentiation. NFκB-mediated endothelial to mesenchymal transformation was directly demonstrated in vivo during CAVD via genetic lineage tracking. Genetic deletion of NFκB in either whole valves or valve endothelium only was sufficient to prevent valve-specific molecular and cellular mechanisms of CAVD in vivo despite the persistence of a CAVD inducing environment.

CONCLUSIONS

Our results identify NFκB signaling as an essential molecular regulator for both valve endothelial and interstitial participation in CAVD pathogenesis. Direct demonstration of valve endothelial cell endothelial to mesenchymal transformation transmigration in vivo during CAVD highlights a new cellular population for further investigation in CAVD morbidity. The efficacy of valve-specific NFκB modulation in inhibiting hypercholesterolemic CAVD suggests potential benefits of multicell type integrated investigation for biological therapeutic development and evaluation for CAVD.

Say NO to ROS: Their Roles in Embryonic Heart Development and Pathogenesis of Congenital Heart Defects in Maternal Diabetes

Congenital heart defects (CHDs) are the most prevalent and serious birth defect, occurring in 1% of all live births. Pregestational maternal diabetes is a known risk factor for the development of CHDs, elevating the risk in the child by more than four-fold. As the prevalence of diabetes rapidly rises among women of childbearing age, there is a need to investigate the mechanisms and potential preventative strategies for these defects. In experimental animal models of pregestational diabetes induced-CHDs, upwards of 50% of offspring display congenital malformations of the heart, including septal, valvular, and outflow tract defects. Specifically, the imbalance of nitric oxide (NO) and reactive oxygen species (ROS) signaling is a major driver of the development of CHDs in offspring of mice with pregestational diabetes. NO from endothelial nitric oxide synthase (eNOS) is crucial to cardiogenesis, regulating various cellular and molecular processes. In fact, deficiency in eNOS results in CHDs and coronary artery malformation. Embryonic hearts from diabetic dams exhibit eNOS uncoupling and oxidative stress. Maternal treatment with sapropterin, a cofactor of eNOS, and antioxidants such as N-acetylcysteine, vitamin E, and glutathione as well as maternal exercise have been shown to improve eNOS function, reduce oxidative stress, and lower the incidence CHDs in the offspring of mice with pregestational diabetes. This review summarizes recent data on pregestational diabetes-induced CHDs, and offers insights into the important roles of NO and ROS in embryonic heart development and pathogenesis of CHDs in maternal diabetes.

Endothelial progerin expression causes cardiovascular pathology through an impaired mechanoresponse

Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disorder characterized by accelerated cardiovascular disease with extensive fibrosis. It is caused by a mutation in LMNA leading to expression of truncated prelamin A (progerin) in the nucleus. To investigate the contribution of the endothelium to cardiovascular HGPS pathology, we generated an endothelium-specific HGPS mouse model with selective endothelial progerin expression. Transgenic mice develop interstitial myocardial and perivascular fibrosis and left ventricular hypertrophy associated with diastolic dysfunction and premature death. Endothelial cells show impaired shear stress response and reduced levels of endothelial nitric oxide synthase (eNOS) and NO. On the molecular level, progerin impairs nucleocytoskeletal coupling in endothelial cells through changes in mechanoresponsive components at the nuclear envelope, increased F-actin/G-actin ratios, and deregulation of mechanoresponsive myocardin-related transcription factor-A (MRTFA). MRTFA binds to the Nos3 promoter and reduces eNOS expression, thereby mediating a profibrotic paracrine response in fibroblasts. MRTFA inhibition rescues eNOS levels and ameliorates the profibrotic effect of endothelial cells in vitro. Although this murine model lacks the key anatomical feature of vascular smooth muscle cell loss seen in HGPS patients, our data show that progerin-induced impairment of mechanosignaling in endothelial cells contributes to excessive fibrosis and cardiovascular disease in HGPS patients.

Calcific aortic valve stenosis: hard disease in the heart: A biomolecular approach towards diagnosis and treatment

Calcific aortic valve stenosis (CAVS) is common in the ageing population and set to become an increasing economic and health burden. Once present, it inevitably progresses and has a poor prognosis in symptomatic patients. No medical therapies are proven to be effective in holding or reducing disease progression. Therefore, aortic valve replacement remains the only available treatment option. Improved knowledge of the mechanisms underlying disease progression has provided us with insights that CAVS is not a passive disease. Rather, CAVS is regulated by numerous mechanisms with a key role for calcification. Aortic valve calcification (AVC) is actively regulated involving cellular and humoral factors that may offer targets for diagnosis and intervention. The discovery that the vitamin K-dependent proteins are involved in the inhibition of AVC has boosted our mechanistic understanding of this process and has opened up novel avenues in disease exploration. This review discusses processes involved in CAVS progression, with an emphasis on recent insights into calcification, methods for imaging calcification activity, and potential therapeutic options.

Discovery of an Experimental Model of Unicuspid Aortic Valve

Background

The epithelial growth factor receptor family of tyrosine kinases modulates embryonic formation of semilunar valves. We hypothesized that mice heterozygous for a dominant loss‐of‐function mutation in epithelial growth factor receptor, which are Egfr^Vel/+ mice, would develop anomalous aortic valves, valve dysfunction, and valvular cardiomyopathy.

Methods and Results

Aortic valves from Egfr^Vel/+ mice and control mice were examined by light microscopy at 2.5 to 4 months of age. Additional Egfr^Vel/+ and control mice underwent echocardiography at 2.5, 4.5, 8, and 12 months of age, followed by histologic examination. In young mice, microscopy revealed anatomic anomalies in 79% of Egfr^Vel/+ aortic valves, which resembled human unicuspid aortic valves. Anomalies were not observed in control mice. At 12 months of age, histologic architecture was grossly distorted in Egfr^Vel/+ aortic valves. Echocardiography detected moderate or severe aortic regurgitation, or aortic stenosis was present in 38% of Egfr^Vel/+ mice at 2.5 months of age (N=24) and in 74% by 8 months of age. Left ventricular enlargement, hypertrophy, and reversion to a fetal myocardial gene expression program occurred in Egfr^Vel/+ mice with aortic valve dysfunction, but not in Egfr^Vel/+ mice with near‐normal aortic valve function. Myocardial fibrosis was minimal or absent in all groups.

Conclusions

A new mouse model uniquely recapitulates salient functional, structural, and histologic features of human unicuspid aortic valve disease, which are phenotypically distinct from other forms of congenital aortic valve disease. The new model may be useful for elucidating mechanisms by which congenitally anomalous aortic valves become critically dysfunctional.

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.

Dipeptidyl Peptidase-4 Induces Aortic Valve Calcification by Inhibiting Insulin-Like Growth Factor-1 Signaling in Valvular Interstitial Cells

Background:

Calcification of the aortic valve leads to increased leaflet stiffness and consequently to the development of calcific aortic valve disease. However, the underlying molecular and cellular mechanisms of calcification remain unclear. Here, we identified that dipeptidyl peptidase-4 (DPP-4, also known as CD26) increases valvular calcification and promotes calcific aortic valve disease progression.

Methods:

We obtained the aortic valve tissues from humans and murine models (wild-type and endothelial nitric oxide synthase-deficient-mice) and cultured the valvular interstitial cells (VICs) and valvular endothelial cells from the cusps. We induced osteogenic differentiation in the primary cultured VICs and examined the effects of the DPP-4 inhibitor on the osteogenic changes in vitro and aortic valve calcification in endothelial nitric oxide synthase-deficient-mice. We also induced calcific aortic stenosis in male New Zealand rabbits (weight, 2.5–3.0 kg) by a cholesterol-enriched diet+vitamin D2 (25 000 IU, daily). Echocardiography was performed to assess the aortic valve area and the maximal and mean transaortic pressure gradients at baseline and 3-week intervals thereafter. After 12 weeks, we harvested the heart and evaluated the aortic valve tissue using immunohistochemistry.

Results:

We found that nitric oxide depletion in human valvular endothelial cells activates NF-κB in human VICs. Consequently, the NF-κB promotes DPP-4 expression, which then induces the osteogenic differentiation of VICs by limiting autocrine insulin-like growth factor-1 signaling. The inhibition of DPP-4 enzymatic activity blocked the osteogenic changes in VICs in vitro and reduced the aortic valve calcification in vivo in a mouse model. Sitagliptin administration in a rabbit calcific aortic valve disease model led to significant improvements in the rate of change in aortic valve area, transaortic peak velocity, and maximal and mean pressure gradients over 12 weeks. Immunohistochemistry staining confirmed the therapeutic effect of Sitagliptin in terms of reducing the calcium deposits in the rabbit aortic valve cusps. In rabbits receiving Sitagliptin, the plasma insulin-like growth factor-1 levels were significantly increased, in line with DPP-4 inhibition.

Conclusions:

DPP-4-dependent insulin-like growth factor-1 inhibition in VICs contributes to aortic valve calcification, suggesting that DPP-4 could serve as a potential therapeutic target to inhibit calcific aortic valve disease progression.

Growth and maturation of heart valves leads to changes in endothelial cell distribution, impaired function, decreased metabolism and reduced cell proliferation

Risk factors of heart valve disease are well defined and prolonged exposure throughout life leads to degeneration and dysfunction in up to 33% of the population. While aortic valve replacement remains the most common need for cardiovascular surgery particularly in those aged over 65, the underlying mechanisms of progressive deterioration are unknown. In other cardiovascular systems, a decline in endothelial cell integrity and function play a major role in promoting pathological changes, and while similar mechanisms have been speculated in the valves, studies to support this are lacking. The goal of this study was to examine age-related changes in valve endothelial cell (VEC) distribution, morphology, function and transcriptomes during critical stages of valve development (embryonic), growth (postnatal (PN)), maintenance (young adult) and aging (aging adult). Using a combination of in vivo mouse, and in vitro porcine assays we show that VEC function including, nitric oxide bioavailability, metabolism, endothelial-to-mesenchymal potential, membrane self-repair and proliferation decline with age. In addition, density of VEC distribution along the endothelium decreases and this is associated with changes in morphology, decreased cell-cell interactions, and increased permeability. These changes are supported by RNA-seq analysis showing that focal adhesion-, cell cycle-, and oxidative phosphorylation-associated biological processes are negatively impacted by aging. Furthermore, by performing high-throughput analysis we are able to report the differential and common transcriptomes of VECs at each time point that can provide insights into the mechanisms underlying age-related dysfunction. These studies suggest that maturation of heart valves over time is a multifactorial process and this study has identified several key parameters that may contribute to impairment of the valve to maintain critical structure-function relationships; leading to degeneration and disease.

Shear-Sensitive Genes in Aortic Valve Endothelium

SIGNIFICANCE

Currently, calcific aortic valve disease (CAVD) is only treatable through surgical intervention because the specific mechanisms leading to the disease remain unclear. In this review, we explore the forces and structure of the valve, as well as the mechanosensors and downstream signaling in the valve endothelium known to contribute to inflammation and valve dysfunction.

RECENT ADVANCES

While the valvular structure enables adaptation to dynamic hemodynamic forces, these are impaired during CAVD, resulting in pathological systemic changes. Mechanosensing mechanisms-proteins, sugars, and membrane structures-at the surface of the valve endothelial cell relay mechanical signals to the nucleus. As a result, a large number of mechanosensitive genes are transcribed to alter cellular phenotype and, ultimately, induce inflammation and CAVD. Transforming growth factor-β signaling and Wnt/β-catenin have been widely studied in this context. Importantly, NADPH oxidase and reactive oxygen species/reactive nitrogen species signaling has increasingly been recognized to play a key role in the cellular response to mechanical stimuli. In addition, a number of valvular microRNAs are mechanosensitive and may regulate the progression of CAVD.

CRITICAL ISSUES

While numerous pathways have been described in the pathology of CAVD, no treatment options are available to avoid surgery for advanced stenosis and calcification of the aortic valve. More work must be focused on this issue to lead to successful therapies for the disease.

FUTURE DIRECTIONS

Ultimately, a more complete understanding of the mechanisms within the aortic valve endothelium will lead us to future therapies important for treatment of CAVD without the risks involved with valve replacement or repair. Antioxid. Redox Signal. 25, 401-414.

Pathological Investigation of Congenital Bicuspid Aortic Valve Stenosis, Compared with Atherosclerotic Tricuspid Aortic Valve Stenosis and Congenital Bicuspid Aortic Valve Regurgitation

Background

Congenital bicuspid aortic valve (CBAV) is the main cause of aortic stenosis (AS) in young adults. However, the histopathological features of AS in patients with CBAV have not been fully investigated.

Methods and Results

We examined specimens of aortic valve leaflets obtained from patients who had undergone aortic valve re/placement at our institution for severe AS with CBAV (n = 24, CBAV-AS group), severe AS with tricuspid aortic valve (n = 24, TAV-AS group), and severe aortic regurgitation (AR) with CBAV (n = 24, CBAV-AR group). We compared the histopathological features among the three groups. Pathological features were classified using semi-quantitative methods (graded on a scale 0 to 3) by experienced pathologists without knowledge of the patients’ backgrounds. The severity of inflammation, neovascularization, and calcium and cholesterol deposition did not differ between the CBAV-AS and TAV-AS groups, and these four parameters were less marked in the CBAV-AR group than in the CBAV-AS (all p<0.01). Meanwhile, the grade of valvular fibrosis was greater in the CBAV-AS group, compared with the TAV-AS and CBAV-AR groups (both p<0.01). In AS patients, thickness of fibrotic lesions was greater on the aortic side than on the ventricular side (both p<0.01). Meanwhile, thickness of fibrotic lesions was comparable between the aortic and ventricular sides in CBAV-AR patients (p = 0.35).

Conclusions

Valvular fibrosis, especially on the aortic side, was greater in patients with CBAV-AS than in those without, suggesting a difference in the pathogenesis of AS between CBAV and TAV.

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.

Embryonic Development of the Bicuspid Aortic Valve

Bicuspid aortic valve (BAV) is the most common congenital valvular heart defect with an overall frequency of 0.5%–1.2%. BAVs result from abnormal aortic cusp formation during valvulogenesis, whereby adjacent cusps fuse into a single large cusp resulting in two, instead of the normal three, aortic cusps. Individuals with BAV are at increased risk for ascending aortic disease, aortic stenosis and coarctation of the aorta. The frequent occurrence of BAV and its anatomically discrete but frequent co-existing diseases leads us to suspect a common cellular origin. Although autosomal-dominant transmission of BAV has been observed in a few pedigrees, notably involving the gene NOTCH1, no single-gene model clearly explains BAV inheritance, implying a complex genetic model involving interacting genes. Several sequencing studies in patients with BAV have identified rare and uncommon mutations in genes of cardiac embryogenesis. But the extensive cell-cell signaling and multiple cellular origins involved in cardiac embryogenesis preclude simplistic explanations of this disease. In this review, we examine the series of events from cellular and transcriptional embryogenesis of the heart, to development of the aortic valve.

Heart failure induces changes in acid-sensing ion channels in sensory neurons innervating skeletal muscle

Heart failure is associated with diminished exercise capacity, which is driven, in part, by alterations in exercise-induced autonomic reflexes triggered by skeletal muscle sensory neurons (afferents). These overactive reflexes may also contribute to the chronic state of sympathetic excitation, which is a major contributor to the morbidity and mortality of heart failure. Acid-sensing ion channels (ASICs) are highly expressed in muscle afferents where they sense metabolic changes associated with ischaemia and exercise, and contribute to the metabolic component of these reflexes. Therefore, we tested if ASICs within muscle afferents are altered in heart failure. We used whole-cell patch clamp to study the electrophysiological properties of acid-evoked currents in isolated, labelled muscle afferent neurons from control and heart failure (induced by myocardial infarction) mice. We found that the percentage of muscle afferents that displayed ASIC-like currents, the current amplitudes, and the pH dose-response relationships were not altered in mice with heart failure. On the other hand, the biophysical properties of ASIC-like currents were significantly different in a subpopulation of cells (40%) from heart failure mice. This population displayed diminished pH sensitivity, altered desensitization kinetics, and very fast recovery from desensitization. These unique properties define these channels within this subpopulation of muscle afferents as being heteromeric channels composed of ASIC2a and -3 subunits. Heart failure induced a shift in the subunit composition of ASICs within muscle afferents, which significantly altered their pH sensing characteristics. These results might, in part, contribute to the changes in exercise-mediated reflexes that are associated with heart failure.

Cardiovascular calcification: current controversies and novel concepts

Cardiovascular calcification is a commonly observed but incompletely understood mechanism of increased atherosclerotic plaque instability and accelerated aortic valve stenosis. Traditional histological staining and imaging techniques are nonspecific for the type of mineral present in calcified tissues, information that is critical for proper validation of in vitro and in vivo models. This review highlights current gaps in our understanding of the biophysical implications and the cellular mechanisms of valvular and vascular calcification and how they may differ between the two tissue types. We also address the hindrances of current cell culture systems, discussing novel platforms and important considerations for future studies of cardiovascular calcification.

Reciprocal interactions between mitral valve endothelial and interstitial cells reduce endothelial-to-mesenchymal transition and myofibroblastic activation

Thickening of mitral leaflets, endothelial-to-mesenchymal transition (EndMT), and activated myofibroblast-like interstitial cells have been observed in ischemic mitral valve regurgitation. We set out to determine if interactions between mitral valve endothelial cells (VEC) and interstitial cells (VIC) might affect these alterations. We used in vitro co-culture in Transwell™ inserts to test the hypothesis that VIC secrete factors that inhibit EndMT and conversely, that VEC secrete factors that mitigate the activation of VIC to a myofibroblast-like, activated phenotype. Primary cultures and clonal populations of ovine mitral VIC and VEC were used. Western blot, quantitative reverse transcriptase PCR (qPCR) and functional assays were used to assess changes in cell phenotype and behavior. VIC or conditioned media from VIC inhibited transforming growth factorβ (TGFβ)-induced EndMT in VEC, as indicated by reduced expression of EndMT markers α-smooth muscle actin (α-SMA), Slug, Snai1 and MMP-2 and maintained ability of VEC to mediate leukocyte adhesion, an important endothelial function. VEC or conditioned media from VEC reversed the spontaneous cell culture-induced change in VIC to an activated phenotype, as indicated by reduced expression of α-SMA and type I collagen, increased expression chondromodulin-1 (Chm1), and reduced contractile activity. These results demonstrate that mitral VEC and VIC secrete soluble factors that can reduce VIC activation and inhibit TGFβ-driven EndMT, respectively. These findings suggest that the endothelium of the mitral valve is critical for the maintenance of a quiescent VIC phenotype and that, in turn, VIC prevent EndMT. We speculate that disturbance of the ongoing reciprocal interactions between VEC and VICs in vivo may contribute to the thickened and fibrotic leaflets observed in ischemic mitral regurgitation, and in other types of valve disease.

Molecular biology of calcific aortic valve disease: towards new pharmacological therapies

Calcific aortic valve disease (CAVD) is a chronic process leading to fibrosis and mineralization of the aortic valve. Investigations in the last several years have emphasized that key underlying molecular processes are involved in the pathogenesis of CAVD. In this regard, the processing of lipids and their retention has been underlined as an important mechanism that triggers inflammation. In turn, inflammation promotes/enhances the mineralization of valve interstitial cells, the main cellular component of the aortic valve. On the other hand, transformation of valve interstitial cells into myofibroblasts and osteoblast-like cells is determined by several signaling pathways having reciprocal cross-talks. In addition, the mineralization of the aortic valve has been shown to rely on ectonucleotidase and purinergic signaling. In this review, the authors have highlighted key molecular underpinnings of CAVD that may have significant relevance for the development of novel pharmaceutical therapies.