Extracellular matrix remodeling and cell phenotypic changes in dysplastic and hemodynamically altered semilunar human cardiac valves

Excess Provisional Extracellular Matrix: A Common Factor in Bicuspid Aortic Valve Formation

A bicuspid aortic valve (BAV) is the most common cardiac malformation, found in 0.5% to 2% of the population. BAVs are present in approximately 50% of patients with severe aortic stenosis and are an independent risk factor for aortic aneurysms. Currently, there are no therapeutics to treat BAV, and the human mutations identified to date represent a relatively small number of BAV patients. However, the discovery of BAV in an increasing number of genetically modified mice is advancing our understanding of molecular pathways that contribute to BAV formation. In this study, we utilized the comparison of BAV phenotypic characteristics between murine models as a tool to advance our understanding of BAV formation. The collation of murine BAV data indicated that excess versican within the provisional extracellular matrix (P-ECM) is a common factor in BAV development. While the percentage of BAVs is low in many of the murine BAV models, the remaining mutant mice exhibit larger and more amorphous tricuspid AoVs, also with excess P-ECM compared to littermates. The identification of common molecular characteristics among murine BAV models may lead to BAV therapeutic targets and biomarkers of disease progression for this highly prevalent and heterogeneous cardiovascular malformation.

Degenerative aortic valve disease and diabetes: Implications for a link between proteoglycans and diabetic disorders in the aortic valve

Degenerative aortic valve disease in combination with diabetes is an increasing burden worldwide. There is growing evidence that particularly small leucine-rich proteoglycans are involved in the development of degenerative aortic valve disease. Nevertheless, the role of these molecules in this disease in the course of diabetes has not been elucidated in detail and previous studies remain controversial. Therefore, the aim of this study is to broaden the knowledge about small leucine-rich proteoglycans in degenerative aortic valve disease and the influence of diabetes and hyperglycaemia on aortic valves and valvular interstitial cells is examined. Analyses were performed using reverse-transcription polymerase chain reaction, Western blot, enzyme-linked immunosorbent assay, (immuno)histology and colorimetric assays. We could show that biglycan, but not decorin and lumican, is upregulated in degenerated human aortic valve cusps. Subgroup analysis reveals that upregulation of biglycan is stage-dependent. In vivo, loss of biglycan leads to stage-dependent calcification and also to migratory effects on interstitial cells within the extracellular matrix. In late stages of degenerative aortic valve disease, diabetes increases the expression of biglycan in aortic valves. In vitro, the combinations of hyperglycaemic with pro-degenerative conditions lead to an upregulation of biglycan. In conclusion, biglycan represents a potential link between degenerative aortic valve disease and diabetes.

Left-Ventricular Assist Device Impact on Aortic Valve Mechanics, Proteomics and Ultrastructure

BACKGROUND

Aortic regurgitation is a prevalent, detrimental complication of left ventricular assist devices (LVADs). The altered hemodynamics of LVADs results in aortic valves (AVs) having distinct mechanical stimulation. Our hypothesis was that the altered AV hemodynamics modulates the valve cells and matrix, resulting in changes in valvular mechanical properties that then can lead to regurgitation.

METHODS

AVs were collected from 16 LVAD and 6 non-LVAD patients at time of heart transplant. Standard demographic and preoperative data were collected and comparisons between the two groups were calculated using standard statistical methods. Samples were analyzed using biaxial mechanical tensile testing, mass spectrometry-based proteomics, and transmission electron microscopy to assess ultrastructure.

RESULTS

The maximum circumferential leaflet strain in LVAD patients was less than in non-LVAD patients (0.35 ± 0.10MPa versus 0.52 ± 0.18 MPa, p = 0.03) with a trend of reduced radial strain (p = 0.06) and a tendency for the radial strain to decrease with increasing LVAD duration (p = 0.063). Numerous proteins associated with actin and myosin, immune signaling and oxidative stress, and transforming growth factor β were increased in LVAD patients. Ultrastructural analysis showed a trend of increased fiber diameter in LVAD patients (46.2 ± 7.2 nm versus 45.1 ± 6.9 nm, p = 0.10), but no difference in fiber density.

CONCLUSIONS

AVs in LVAD patients showed decreased compliance and increased expression of numerous proteins related to valve activation and injury compared to non-LVAD patients. Further knowledge of AV changes leading to regurgitation in LVAD patients and the pathways by which they occur may provide an opportunity for interventions to prevent and/or reverse this detrimental complication.

Mapping Extracellular Matrix Proteins in Formalin-Fixed, Paraffin-Embedded Tissues by MALDI Imaging Mass Spectrometry

Collagens and elastin form the fundamental framework of all tissues and organs, and their expression and post-translational processing are tightly regulated in disease and health. Because of their unique structural composition and properties, it is a recognized challenge to access these protein structures within the complex tissue microenvironment to understand how localized changes modulate tissue health. We describe a new workflow using a combination of matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) with matrix metalloproteinase (MMP) enzymes to access and report on spatial localization of collagen and elastin sequences in formalin-fixed, paraffin-embedded (FFPE) tissues. The developed technology provides new access to collagens and elastin sequences localized to tissue features that were previously unattainable. This high-throughput technological advance should be applicable to any tissue regardless of disease type, tissue origin, or disease status and is thus relevant to all research: basic, translational, or clinical.

Heart valve health, disease, replacement, and repair: a 25-year cardiovascular pathology perspective

The past several decades have witnessed major advances in the understanding of the structure, function, and biology of native valves and the pathobiology and clinical management of valvular heart disease. These improvements have enabled earlier and more precise diagnosis, assessment of the proper timing of surgical and interventional procedures, improved prosthetic and biologic valve replacements and repairs, recognition of postoperative complications and their management, and the introduction of minimally invasive approaches that have enabled definitive and durable treatment for patients who were previously considered inoperable. This review summarizes the current state of our understanding of the mechanisms of heart valve health and disease arrived at through innovative research on the cell and molecular biology of valves, clinical and pathological features of the most frequent intrinsic structural diseases that affect the valves, and the status and pathological considerations in the technological advances in valvular surgery and interventions. The contributions of many cardiovascular pathologists and other scientists, engineers, and clinicians are emphasized, and potentially fruitful areas for research are highlighted.

Comparison of calcification potential of valvular interstitial cells isolated from individual aortic valve cusps

BACKGROUND

Calcific aortic valve disease (CAVD) is one of the most prevalent disorders among the elderly in developed countries. CAVD develops via cell-mediated processes, and clinical data show that CAVD initiates mostly in the noncoronary cusp of the aortic valve. Valvular interstitial cells (VICs) populate the inside of heart valves and are a heterogeneous cell population. The goal of this study is to elucidate the difference in calcification potential among VICs isolated from the left, right, and noncoronary cusps of porcine aortic valves.

METHODS AND RESULTS

VICs were isolated from each of the aortic valve cusps and cultured in calcifying medium for 14days to induce calcification. The samples were assessed for calcium deposits, nodule formation, and calcific markers using alizarin red and Von Kossa staining, alkaline phosphatase (ALP) staining, ALP enzyme activity assay, and Western blot. Extracellular matrix production and degradation were measured using collagen and glycosaminoglycan (GAG) assay and gelatin zymography. We observed that VICs isolated from the noncoronary cusp expressed greatest amount of the above calcific markers as compared to the coronary cusps. Also, collagen and GAG content was the greatest in noncoronary VICs. However, our zymography results showed significant difference only for active matrix metalloproteinase-2 expression between right and noncoronary VICs.

CONCLUSION

Our results suggest that VICs among the three cusps within aortic valve might be inherently different, where a subpopulation of VICs might be predisposed to calcification.

Integrating valve-inspired design features into poly(ethylene glycol) hydrogel scaffolds for heart valve tissue engineering

The development of advanced scaffolds that recapitulate the anisotropic mechanical behavior and biological functions of the extracellular matrix in leaflets would be transformative for heart valve tissue engineering. In this study, anisotropic mechanical properties were established in poly(ethylene glycol) (PEG) hydrogels by crosslinking stripes of 3.4 kDa PEG diacrylate (PEGDA) within 20 kDa PEGDA base hydrogels using a photolithographic patterning method. Varying the stripe width and spacing resulted in a tensile elastic modulus parallel to the stripes that was 4.1-6.8 times greater than that in the perpendicular direction, comparable to the degree of anisotropy between the circumferential and radial orientations in native valve leaflets. Biomimetic PEG-peptide hydrogels were prepared by tethering the cell-adhesive peptide RGDS and incorporating the collagenase-degradable peptide PQ (GGGPQG↓IWGQGK) into the polymer network. The specific amounts of RGDS and PEG-PQ within the resulting hydrogels influenced the elongation, de novo extracellular matrix deposition and hydrogel degradation behavior of encapsulated valvular interstitial cells (VICs). In addition, the morphology and activation of VICs grown atop PEG hydrogels could be modulated by controlling the concentration or micro-patterning profile of PEG-RGDS. These results are promising for the fabrication of PEG-based hydrogels using anatomically and biologically inspired scaffold design features for heart valve tissue engineering.

Myocardial extracellular matrix: an ever-changing and diverse entity

The cardiac extracellular matrix (ECM) is a complex architectural network consisting of structural and nonstructural proteins, creating strength and plasticity. The nonstructural compartment of the ECM houses a variety of proteins, which are vital for ECM plasticity, and can be divided into 3 major groups: glycoproteins, proteoglycans, and glycosaminoglycans. The common denominator for these groups is glycosylation, which refers to the decoration of proteins or lipids with sugars. This review will discuss the fundamental role of the matrix in cardiac development, homeostasis, and remodeling, from a glycobiology point of view. Glycoproteins (eg, thrombospondins, secreted protein acidic and rich in cysteine, tenascins), proteoglycans (eg, versican, syndecans, biglycan), and glycosaminoglycans (eg, hyaluronan, heparan sulfate) are upregulated on cardiac injury and regulate key processes in the remodeling myocardium such as inflammation, fibrosis, and angiogenesis. Albeit some parallels can be made regarding the processes these proteins are involved in, their specific functions are extremely diverse. In fact, under varying conditions, individual proteins can even have opposing functions, making spatiotemporal contribution of these proteins in the rearrangement of multifaceted ECM very hard to grasp. Alterations of protein characteristics by the addition of sugars may explain the immense, yet tightly regulated, variability of the remodeling cardiac matrix. Understanding the role of glycosylation in altering the ultimate function of glycoproteins, proteoglycans, and glycosaminoglycans in the myocardium may lead to the development of new biochemical structures or compounds with great therapeutic potential for patients with heart disease.

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

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