Aortic valve: anatomy and structure and the role of vasculature in the degenerative process

Pathological Mechanism and Treatment of Calcified Aortic Stenosis

Calcified aortic stenosis (AS) is one of the most common valvular heart diseases worldwide, characterized by progressive fibrocalcific remodeling and thickening of the leaflets, which ultimately leads to obstruction of blood flow. Its pathobiology is an active and complicated process, involving endothelial cell dysfunction, lipoprotein deposition and oxidation, chronic inflammation, phenotypic transformation of valve interstitial cells, neovascularization, and intravalvular hemorrhage. To date, no targeted drug has been proven to slow down or prevent disease progression. Aortic valve replacement is still the optimal treatment of AS. This article reviews the etiology, diagnosis, and management of calcified aortic stenosis and proposes novel potential therapeutic targets.

Neovascularization of native cardiac valves, and correlation with histopathologic, clinical, and radiologic features

Native cardiac valves in the setting of chronic injury undergo remodeling that includes fibrous thickening and dystrophic calcification, as well as neovascularization, that result in abnormal valve function. In order to characterize the presence of neovascularization in valves, a retrospective review of 1246 sequentially reviewed native cardiac valves of all types was performed, with correlation with other histopathologic features, and clinical and echocardiographic findings. Neovascularization was present in 55.5% of cases, with the greatest prevalence amongst aortic valves. While microvasculature (representing capillaries, venules, and/or lymphatics) was at least present in all cases of valves with neovascularization, arterial vessels were never identified in valves without also the finding of concomitant microvasculature present. Patients with neovascularization had a greater mean age and body mass index compared to those without, and the proportions of cases with significant coronary artery disease, dyslipidemia, diabetes mellitus, rheumatic fever, and malignancy were greater in the setting of valves with neovascularization compared to cases without. The rate of neovascularization increased with degree of valve thickening and/or calcification, and stenosis; in contrast, neovascularization was observed at a greater rate with decreasing degrees of regurgitation. The prevalence rates of hemosiderin-laden macrophages, osseous metaplasia, chondromatous metaplasia, smooth muscle, and chronic inflammation were greater in valves with neovascularization compared to valves without. Neovascularization within native cardiac valves is a frequent histopathologic alteration associated with chronic valve disease, likely representing a constituent of structural remodeling that mediates and reflects chronic injury.

Ischemia-Reperfusion Injury in Porcine Aortic Valvular Endothelial and Interstitial Cells

Ischemia-reperfusion injury (IRI) in the myocardium has been thoroughly researched, especially in acute coronary syndrome and heart transplantation. However, our understanding of IRI implications on cardiac valves is still developing. This knowledge gap becomes even more pronounced given the advent of partial heart transplantation, a procedure designed to implant isolated human heart valves in young patients. This study aims to investigate the effects of IRI on aortic valvular endothelial cells (VECs), valvular interstitial cells (VICs), and whole leaflet cultures (no separation of VECs and VICs). We employed two conditions: hypoxic cold storage reperfusion (HCSR) and normothermia (NT). Key markers, secreted protein acidic and cysteine rich (SPARC) (osteonectin), and inducible nitric oxide synthase (iNOS2) were evaluated. In the isolated cells under HCSR, VICs manifested a significant 15-fold elevation in SPARC expression compared to NT (p = 0.0016). Conversely, whole leaflet cultures exhibited a 1-fold increment in SPARC expression in NT over HCSR (p = 0.0011). iNOS2 expression in VECs presented a marginal rise in HCSR, whereas, in whole leaflet settings, there was a 1-fold ascent in NT compared to HCSR (p = 0.0003). Minor escalations in the adhesion molecules intercellular adhesion molecule (ICAM), vascular cell adhesion molecule (VCAM), E-selection, and P-selection were detected in HCSR for whole leaflet cultures, albeit without statistical significance. Additionally, under HCSR, VICs released a markedly higher quantity of IL-6 and IL-8, with respective p-values of 0.0033 and <0.0001. Interestingly, the IL-6 levels in VECs remained consistent across both HCSR and NT conditions. These insights lay the groundwork for understanding graft IRI following partial heart transplantation and hint at the interdependent dynamic of VECs and VICs in valvular tissue.

Hypoxia-inducible factor activation promotes osteogenic transition of valve interstitial cells and accelerates aortic valve calcification in a mice model of chronic kidney disease

Introduction

Valve calcification (VC) is a widespread complication in chronic kidney disease (CKD) patients. VC is an active process with the involvement of in situ osteogenic transition of valve interstitial cells (VICs). VC is accompanied by the activation of hypoxia inducible factor (HIF) pathway, but the role of HIF activation in the calcification process remains undiscovered.

Methods and result

Using in vitro and in vivo approaches we addressed the role of HIF activation in osteogenic transition of VICs and CKD-associated VC. Elevation of osteogenic (Runx2, Sox9) and HIF activation markers (HIF-1α and HIF-2α) and VC occurred in adenine-induced CKD mice. High phosphate (Pi) induced upregulation of osteogenic (Runx2, alkaline-phosphatase, Sox9, osteocalcin) and hypoxia markers (HIF-1α, HIF-2α, Glut-1), and calcification in VICs. Down-regulation of HIF-1α and HIF-2α inhibited, whereas further activation of HIF pathway by hypoxic exposure (1% O₂) or hypoxia mimetics [desferrioxamine, CoCl₂, Daprodustat (DPD)] promoted Pi-induced calcification of VICs. Pi augmented the formation of reactive oxygen species (ROS) and decreased viability of VICs, whose effects were further exacerbated by hypoxia. N-acetyl cysteine inhibited Pi-induced ROS production, cell death and calcification under both normoxic and hypoxic conditions. DPD treatment corrected anemia but promoted aortic VC in the CKD mice model.

Discussion

HIF activation plays a fundamental role in Pi-induced osteogenic transition of VICs and CKD-induced VC. The cellular mechanism involves stabilization of HIF-1α and HIF-2α, increased ROS production and cell death. Targeting the HIF pathways may thus be investigated as a therapeutic approach to attenuate aortic VC.

3D histopathology of stenotic aortic valve cusps using ex vivo microfocus computed tomography

Background

Calcific aortic stenosis (AS) is the most prevalent heart valve disease in developed countries. The aortic valve cusps progressively thicken and the valve does not open fully due to the presence of calcifications. In vivo imaging, usually used for diagnosis, does not allow the visualization of the microstructural changes associated with AS.

Methods

Ex vivo high-resolution microfocus computed tomography (microCT) was used to quantitatively describe the microstructure of calcified aortic valve cusps in full 3D. As case study in our work, this quantitative analysis was applied to normal-flow low-gradient severe AS (NF-LG-SAS), for which the medical prognostic is still highly debated in the current literature, and high-gradient severe AS (HG-SAS).

Results

The volume proportion of calcification, the size and number of calcified particles and their density composition was quantified. A new size-based classification considering small-sized particles that are not detected with in vivo imaging was defined for macro-, meso- and microscale calcifications. Volume and thickness of aortic valve cusps, including the complete thickness distribution, were also determined. Moreover, changes in the cusp soft tissues were also visualized with microCT and confirmed by scanning electron microscopy images of the same sample. NF-LG-SAS cusps contained lower relative amount of calcifications than HG-SAS. Moreover, the number and size of calcified objects and the volume and thickness of the cusps were also lower in NF-LG-SAS cusps than in HG-SAS.

Conclusions

The application of high-resolution ex vivo microCT to stenotic aortic valve cusps provided a quantitative description of the general structure of the cusps and of the calcifications present in the cusp soft tissues. This detailed description could help in the future to better understand the mechanisms of AS.

Atractylodin targets GLA to regulate D-mannose metabolism to inhibit osteogenic differentiation of human valve interstitial cells and ameliorate aortic valve calcification

Atractylodin (ATL) has been reported to exert anti-inflammatory effects. Osteogenic changes induced by inflammation in valve interstitial cells (VICs) play a key role in the development of calcified aortic valve disease (CAVD). This study aimed to investigate the anti-calcification effects of ATL on aortic valves. Human VICs (hVICs) were exposed to osteogenic induction medium (OM) containing ATL to investigate cell viability, osteogenic gene and protein expression, and anti-calcification effects. Gas chromatography-mass spectroscopy (GC-MS) metabolomics analysis was used to detect changes in the metabolites of hVICs stimulated with OM before and after ATL administration. The compound-reaction-enzyme-gene network was used to identify drug targets. Gene interference was used to verify the targets. ApoE-/- mice fed a high-fat (HF) diet were used to evaluate the inhibition of aortic valve calcification by ATL. Treatment with 20 μM ATL in OM prevented calcified nodule accumulation and decreases in the gene and protein expression levels of ALP, RUNX2, and IL-1β. Differential metabolite analysis showed that D-mannose was highly associated with the anti-calcification effect of ATL. The addition of D-mannose prevented calcified nodule accumulation and inhibited succinate-mediated HIF-1α activation and IL-1β production. The target of ATL was identified as GLA. Silencing of the GLA gene (si-GLA) reversed the anti-osteogenic differentiation of ATL. In vivo, ATL ameliorated aortic valve calcification by preventing decreases in GLA expression and the up-regulation of IL-1β expression synchronously. In conclusion, ATL is a potential drug for the treatment of CAVD by targeting GLA to regulate D-mannose metabolism, thereby inhibiting succinate-mediated HIF-1α activation and IL-1β production.

Proteolytic Degradation Is a Major Contributor to Bioprosthetic Heart Valve Failure

Background Whereas the risk factors for structural valve degeneration (SVD) of glutaraldehyde-treated bioprosthetic heart valves (BHVs) are well studied, those responsible for the failure of BHVs fixed with alternative next-generation chemicals remain largely unknown. This study aimed to investigate the reasons behind the development of SVD in ethylene glycol diglycidyl ether-treated BHVs. Methods and Results Ten ethylene glycol diglycidyl ether-treated BHVs excised because of SVD, and 5 calcified aortic valves (AVs) replaced with BHVs because of calcific AV disease were collected and their proteomic profile was deciphered. Then, BHVs and AVs were interrogated for immune cell infiltration, microbial contamination, distribution of matrix-degrading enzymes and their tissue inhibitors, lipid deposition, and calcification. In contrast with dysfunctional AVs, failing BHVs suffered from complement-driven neutrophil invasion, excessive proteolysis, unwanted coagulation, and lipid deposition. Neutrophil infiltration was triggered by an asymptomatic bacterial colonization of the prosthetic tissue. Neutrophil elastase, myeloblastin/proteinase 3, cathepsin G, and matrix metalloproteinases (MMPs; neutrophil-derived MMP-8 and plasma-derived MMP-9), were significantly overexpressed, while tissue inhibitors of metalloproteinases 1/2 were downregulated in the BHVs as compared with AVs, together indicative of unbalanced proteolysis in the failing BHVs. As opposed to other proteases, MMP-9 was mostly expressed in the disorganized prosthetic extracellular matrix, suggesting plasma-derived proteases as the primary culprit of SVD in ethylene glycol diglycidyl ether-treated BHVs. Hence, hemodynamic stress and progressive accumulation of proteases led to the extracellular matrix degeneration and dystrophic calcification, ultimately resulting in SVD. Conclusions Neutrophil- and plasma-derived proteases are responsible for the loss of BHV mechanical competence and need to be thwarted to prevent SVD.

Fibroblast growth factor 2 inhibits myofibroblastic activation of valvular interstitial cells

Heart valve disease is a growing problem worldwide. Though very common in older adults, the mechanisms behind the development of the disease aren't well understood, and at present the only therapeutic option is valve replacement. Valvular interstitial cells (VICs) may hold the answer. These cells can undergo pathological differentiation into contractile myofibroblasts or osteoblasts, leading to thickening and calcification of the valve tissue. Our study aimed to characterise the effect of fibroblast growth factor 2 (FGF-2) on the differentiation potential of VICs. We isolated VICs from diseased human valves and treated these cells with FGF-2 and TGF-β to elucidate effect of these growth factors on several myofibroblastic outcomes, in particular immunocytochemistry and gene expression. We used TGF-β as a positive control for myofibroblastic differentiation. We found that FGF-2 promotes a 'quiescent-type' morphology and inhibits the formation of α-smooth muscle actin positive myofibroblasts. FGF-2 reduced the calcification potential of VICs, with a marked reduction in the number of calcific nodules. FGF-2 interrupted the 'canonical' TGF-β signalling pathway, reducing the nuclear translocation of the SMAD2/3 complex. The panel of genes assayed revealed that FGF-2 promoted a quiescent-type pattern of gene expression, with significant downregulations in typical myofibroblast markers α smooth muscle actin, extracellular matrix proteins, and scleraxis. We did not see evidence of osteoblast differentiation: neither matrix-type calcification nor changes in osteoblast associated gene expression were observed. Our findings show that FGF-2 can reverse the myofibroblastic phenotype of VICs isolated from diseased valves and inhibit the calcification potential of these cells.

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.

Mechanisms and Drug Therapies of Bioprosthetic Heart Valve Calcification

Valve replacement is the main therapy for valvular heart disease, in which a diseased valve is replaced by mechanical heart valve (MHV) or bioprosthetic heart valve (BHV). Since the 2000s, BHV surpassed MHV as the leading option of prosthetic valve substitute because of its excellent hemocompatible and hemodynamic properties. However, BHV is apt to structural valve degeneration (SVD), resulting in limited durability. Calcification is the most frequent presentation and the core pathophysiological process of SVD. Understanding the basic mechanisms of BHV calcification is an essential prerequisite to address the limited-durability issues. In this narrative review, we provide a comprehensive summary about the mechanisms of BHV calcification on 1) composition and site of calcifications; 2) material-associated mechanisms; 3) host-associated mechanisms, including immune response and foreign body reaction, oxidative stress, metabolic disorder, and thrombosis. Strategies that target these mechanisms may be explored for novel drug therapy to prevent or delay BHV calcification.

Low-Gradient aortic stenosis; the diagnostic dilemma

Low-gradient (LG) aortic valve stenosis (AS) constitutes a significant subset among patients with severe aortic stenosis. This entity represents one of the most challenging heart conditions when it comes to diagnosis and management, mainly because of the discrepancy between the small aortic valve area (≤1.0 cm²) that is considered a severe AS, and low mean transvalvular pressure gradient (<40 mmHg), which is one of the criteria for nonsevere AS. LG AS is divided according to transvalvular aortic flow rate into normal-flow LG AS and low-flow LG (LFLG) AS; the latter category can be divided further according to left ventricular ejection fraction (LVEF) into classical LFLG AS if LVEF is depressed or paradoxical LFLG AS if LVEF is preserved. The primary diagnostic challenge in patients with LG AS is to confirm that AS is truly severe and not pseudosevere, which is assessed mainly by either dobutamine stress echocardiography or multidetector computed tomography. The management of symptomatic true severe LG AS is mainly by aortic valve replacement (AVR), whether surgical or transcatheter approach. Patients with LG severe AS have a generally worse prognosis and higher mortality compared with patients with high-gradient severe AS. Despite the survival benefit of AVR in patients with true severe LG AS, these patients have higher surgical risk post-AVR compared with high-gradient AS patients. Early recognition and correct diagnosis of a patient with LG AS is crucial to improve their mortality and morbidity.

Early Aberrant Angiogenesis Due to Elastic Fiber Fragmentation in Aortic Valve Disease

Elastic fiber fragmentation (EFF) is a hallmark of aortic valve disease (AVD), and neovascularization has been identified as a late finding related to inflammation. We sought to characterize the relationship between early EFF and aberrant angiogenesis. To examine disease progression, regional anatomy and pathology of aortic valve tissue were assessed using histochemistry, immunohistochemistry, and electron microscopy from early-onset (<40 yo) and late-onset (≥40 yo) non-syndromic AVD specimens. To assess the effects of EFF on early AVD processes, valve tissue from Williams and Marfan syndrome patients was also analyzed. Bicuspid aortic valve was more common in early-onset AVD, and cardiovascular comorbidities were more common in late-onset AVD. Early-onset AVD specimens demonstrated angiogenesis without inflammation or atherosclerosis. A distinct pattern of elastic fiber components surrounded early-onset AVD neovessels, including increased emilin-1 and decreased fibulin-5. Different types of EFF were present in Williams syndrome (WS) and Marfan syndrome (MFS) aortic valves; WS but not MFS aortic valves demonstrated angiogenesis. Aberrant angiogenesis occurs in early-onset AVD in the absence of inflammation, implicating EFF. Elucidation of underlying mechanisms may inform the development of new pharmacologic treatments.

Ultrastructural Pathology of Atherosclerosis, Calcific Aortic Valve Disease, and Bioprosthetic Heart Valve Degeneration: Commonalities and Differences

Atherosclerosis, calcific aortic valve disease (CAVD), and bioprosthetic heart valve degeneration (alternatively termed structural valve deterioration, SVD) represent three diseases affecting distinct components of the circulatory system and their substitutes, yet sharing multiple risk factors and commonly leading to the extraskeletal calcification. Whereas the histopathology of the mentioned disorders is well-described, their ultrastructural pathology is largely obscure due to the lack of appropriate investigation techniques. Employing an original method for sample preparation and the electron microscopy visualisation of calcified cardiovascular tissues, here we revisited the ultrastructural features of lipid retention, macrophage infiltration, intraplaque/intraleaflet haemorrhage, and calcification which are common or unique for the indicated types of cardiovascular disease. Atherosclerotic plaques were notable for the massive accumulation of lipids in the extracellular matrix (ECM), abundant macrophage content, and pronounced neovascularisation associated with blood leakage and calcium deposition. In contrast, CAVD and SVD generally did not require vasculo- or angiogenesis to occur, instead relying on fatigue-induced ECM degradation and the concurrent migration of immune cells. Unlike native tissues, bioprosthetic heart valves contained numerous specialised macrophages and were not capable of the regeneration that underscores ECM integrity as a pivotal factor for SVD prevention. While atherosclerosis, CAVD, and SVD show similar pathogenesis patterns, these disorders demonstrate considerable ultrastructural differences.