Biophysical analysis of dystrophic and osteogenic models of valvular calcification

Focusing on the Native Matrix Proteins in Calcific Aortic Valve Stenosis

Calcific aortic valve stenosis (CAVS) is a widespread valvular heart disease affecting people in aging societies, primarily characterized by fibrosis, inflammation, and progressive calcification, leading to valve orifice stenosis. Understanding the factors associated with CAVS onset and progression is crucial to develop effective future pharmaceutical therapies. In CAVS, native extracellular matrix proteins modifications, play a significant role in calcification in vitro and in vivo. This work aimed to review the evidence on the alterations of structural native extracellular matrix proteins involved in calcification development during CAVS and highlight its link to deregulated biomechanical function.

Decellularized tissue-engineered heart valves calcification: what do animal and clinical studies tell us?

Cardiovascular diseases are the first cause of death worldwide. Among different heart malfunctions, heart valve failure due to calcification is still a challenging problem. While drug-dependent treatment for the early stage calcification could slow down its progression, heart valve replacement is inevitable in the late stages. Currently, heart valve replacements involve mainly two types of substitutes: mechanical and biological heart valves. Despite their significant advantages in restoring the cardiac function, both types of valves suffered from serious drawbacks in the long term. On the one hand, the mechanical one showed non-physiological hemodynamics and the need for the chronic anticoagulation therapy. On the other hand, the biological one showed stenosis and/or regurgitation due to calcification. Nowadays, new promising heart valve substitutes have emerged, known as decellularized tissue-engineered heart valves (dTEHV). Decellularized tissues of different types have been widely tested in bioprosthetic and tissue-engineered valves because of their superior biomechanics, biocompatibility, and biomimetic material composition. Such advantages allow successful cell attachment, growth and function leading finally to a living regenerative valvular tissue in vivo. Yet, there are no comprehensive studies that are covering the performance of dTEHV scaffolds in terms of their efficiency for the calcification problem. In this review article, we sought to answer the question of whether decellularized heart valves calcify or not. Also, which factors make them calcify and which ones lower and/or prevent their calcification. In addition, the review discussed the possible mechanisms for dTEHV calcification in comparison to the calcification in the native and bioprosthetic heart valves. For this purpose, we did a retrospective study for all the published work of decellularized heart valves. Only animal and clinical studies were included in this review. Those animal and clinical studies were further subcategorized into 4 categories for each depending on the effect of decellularization on calcification. Due to the complex nature of calcification in heart valves, other in vitro and in silico studies were not included. Finally, we compared the different results and summed up all the solid findings of whether decellularized heart valves calcify or not. Based on our review, the selection of the proper heart valve tissue sources (no immunological provoking residues), decellularization technique (no damaged exposed residues of the decellularized tissues, no remnants of dead cells, no remnants of decellularizing agents) and implantation techniques (avoiding suturing during the surgical implantation) could provide a perfect anticalcification potential even without in vitro cell seeding or additional scaffold treatment.

Celecoxib Is Associated With Dystrophic Calcification and Aortic Valve Stenosis

Calcific aortic valve disease is a progressive fibrocalcific process that can only be treated with valve replacement. Cadherin-11 has recently been identified as a potential therapeutic target for calcific aortic valve disease. The already approved drug celecoxib, a cyclooxygenase-2 inhibitor, binds cadherin-11, and was investigated as a therapeutic against calcific aortic valve disease. Unexpectedly, celecoxib treatment led to hallmarks of myofibroblast activation and calcific nodule formation in vitro. Retrospective electronic medical record analysis of celecoxib, ibuprofen, and naproxen revealed a unique association of celecoxib use and aortic stenosis.

Crystallinity of hydroxyapatite drives myofibroblastic activation and calcification in aortic valves

Calcific aortic valve disease (CAVD) is an inexorably degenerative pathology characterized by progressive calcific lesion formation on the valve leaflets. The interaction of valvular cells in advanced lesion environments is not well understood yet highly relevant as clinically detectable CAVD exhibits calcifications composed of non-stoichiometric hydroxyapatite (HA). In this study, Fourier transform infrared spectroscopic imaging was used to spatially analyze mineral properties as a function of disease progression. Crystallinity (size and perfection) increased with increased valve calcification. To study the relationship between crystallinity and cellular behavior in CAVD, valve cells were seeded into 3D mineral-rich collagen gels containing synthetic HA particles, which had varying crystallinities. Lower crystallinity HA drove myofibroblastic activation in both valve interstitial and endothelial cells, as well as osteoblastic differentiation in interstitial cells. Additionally, calcium accumulation within gels depended on crystallinity, and apoptosis was insufficient to explain differences in HA-driven cellular activity. The protective nature of endothelial cells against interstitial cell activation and calcium accumulation was completely inhibited in the presence of less crystalline HA particles. Elucidating valve cellular behavior post-calcification is of vital importance to better predict and treat clinical pathogenesis, and mineral-containing hydrogel models provide a unique 3D platform to evaluate valve cell responses to a later stage of valve disease.

STATEMENT OF SIGNIFICANCE

We implement a 3D in vitro platform with embedded hydroxyapatite (HA) nanoparticles to investigate the interaction between valve interstitial cells, valve endothelial cells, and a mineral-rich extracellular environment. HA nanoparticles were synthesized based on analysis of the mineral properties of calcific regions of diseased human aortic valves. Our findings indicate that crystallinity of HA drives activation and differentiation in interstitial and endothelial cells. We also show that a mineralized environment blocks endothelial protection against interstitial cell calcification. Our HA-containing hydrogel model provides a unique 3D platform to evaluate valve cell responses to a mineralized ECM. This study additionally lays the groundwork to capture the diversity of mineral properties in calcified valves, and link these properties to progression of the disease.

Correlative imaging reveals physiochemical heterogeneity of microcalcifications in human breast carcinomas

Microcalcifications (MCs) are routinely used to detect breast cancer in mammography. Little is known, however, about their materials properties and associated organic matrix, or their correlation to breast cancer prognosis. We combine histopathology, Raman microscopy, and electron microscopy to image MCs within snap-frozen human breast tissue and generate micron-scale resolution correlative maps of crystalline phase, trace metals, particle morphology, and organic matrix chemical signatures within high grade ductal carcinoma in situ (DCIS) and invasive cancer. We reveal the heterogeneity of mineral-matrix pairings, including punctate apatitic particles (<2 µm) with associated trace elements (e.g., F, Na, and unexpectedly Al) distributed within the necrotic cores of DCIS, and both apatite and spheroidal whitlockite particles in invasive cancer within a matrix containing spectroscopic signatures of collagen, non-collagen proteins, cholesterol, carotenoids, and DNA. Among the three DCIS samples, we identify key similarities in MC morphology and distribution, supporting a dystrophic mineralization pathway. This multimodal methodology lays the groundwork for establishing MC heterogeneity in the context of breast cancer biology, and could dramatically improve current prognostic models.

Living nano-micro fibrous woven fabric/hydrogel composite scaffolds for heart valve engineering

Regeneration and repair of injured or diseased heart valves remains a clinical challenge. Tissue engineering provides a promising treatment approach to facilitate living heart valve repair and regeneration. Three-dimensional (3D) biomimetic scaffolds that possess heterogeneous and anisotropic features that approximate those of native heart valve tissue are beneficial to the successful in vitro development of tissue engineered heart valves (TEHV). Here we report the development and characterization of a novel composite scaffold consisting of nano- and micro-scale fibrous woven fabrics and 3D hydrogels by using textile techniques combined with bioactive hydrogel formation. Embedded nano-micro fibrous scaffolds within hydrogel enhanced mechanical strength and physical structural anisotropy of the composite scaffold (similar to native aortic valve leaflets) and also reduced its compaction. We determined that the composite scaffolds supported the growth of human aortic valve interstitial cells (HAVIC), balanced the remodeling of heart valve ECM against shrinkage, and maintained better physiological fibroblastic phenotype in both normal and diseased HAVIC over single materials. These fabricated composite scaffolds enable the engineering of a living heart valve graft with improved anisotropic structure and tissue biomechanics important for maintaining valve cell phenotypes.

STATEMENT OF SIGNIFICANCE

Heart valve-related disease is an important clinical problem, with over 300,000 surgical repairs performed annually. Tissue engineering offers a promising strategy for heart valve repair and regeneration. In this study, we developed and tissue engineered living nano-micro fibrous woven fabric/hydrogel composite scaffolds by using textile technique combined with bioactive hydrogel formation. The novelty of our technique is that the composite scaffolds can mimic physical structure anisotropy and the mechanical strength of natural aortic valve leaflet. Moreover, the composite scaffolds prevented the matrix shrinkage, which is major problem that causes the failure of TEHV, and better maintained physiological fibroblastic phenotype in both normal and diseased HAVIC. This work marks the first report of a combination composite scaffold using 3D hydrogel enhanced by nano-micro fibrous woven fabric, and represents a promising tissue engineering strategy to treat heart valve injury.

Reproducible in vitro model for dystrophic calcification of cardiac valvular interstitial cells: insights into the mechanisms of calcific aortic valvular disease

Calcific aortic valvular disease (CAVD) is the most prevalent valvular pathology in the United States. Development of a pharmacologic agent to slow, halt, or reverse calcification has proven to be unsuccessful as still much remains unknown about the mechanisms of disease initiation. Although in vitro models of some features of CAVD exist, their utility is limited by the inconsistency of the size and time course of the calcified cell aggregates. In this study, we introduce and verify a highly reproducible in vitro method for studying dystrophic calcification of cardiac valvular interstitial cells, considered to be a key mechanism of clinical CAVD. By utilizing micro-contact printing, we were able to consistently reproduce cell aggregation, myofibroblastic markers, programmed cell death, and calcium accumulation within aggregates of 50-400 μm in diameter on substrates with moduli from 9.6 to 76.8 kPa. This method is highly repeatable, with 70% of aggregates staining positive for Alizarin Red S after one week in culture. Dense mineralized calcium-positive nanoparticles were found within the valvular interstitial cell aggregates as shown by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The area of micro-contact printed aggregates staining positive for caspase 3/7 activity increased from 5.9 ± 0.9% to 12.6 ± 4.5% over one week in culture. Z-VAD-FMK reduced aggregates staining positive for Alizarin Red S by 60%. The state of cell stress is hypothesized to play a role in the disease progression; traction force microscopy indicates high substrate stresses along the aggregate periphery which can be modulated by altering the size of the aggregates and the modulus of the substrate. Micro-contact printing is advantageous over the currently used in vitro model as it allows the independent study of how cytokines, substrate modulus, and pharmacologic agents affect calcification. This controlled method for aggregate creation has the potential to be used as an in vitro assay for the screening of promising therapeutics to mitigate CAVD.

Surface chemistry regulates valvular interstitial cell differentiation in vitro

The primary driver for valvular calcification is the differentiation of valvular interstitial cells (VICs) into a diseased phenotype. However, the factors leading to the onset of osteoblastic-like VICs (obVICs) and resulting calcification are not fully understood. This study isolates the effect of substrate surface chemistry on in vitro VIC differentiation and calcified tissue formation. Using ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold [CH3 (hydrophobic), OH (hydrophilic), COOH (COO(-), negative at physiological pH), and NH2 (NH3(+), positive at physiological pH)], we have demonstrated that surface chemistry modulates VIC phenotype and calcified tissue deposition independent of osteoblastic-inducing media additives. Over seven days VICs exhibited surface-dependent differences in cell proliferation (COO(-)=NH3(+)>OH>CH3), morphology, and osteoblastic potential. Both NH3(+)and CH3-terminated SAMs promoted calcified tissue formation while COO(-)-terminated SAMs showed no calcification. VICs on NH3(+)-SAMs exhibited the most osteoblastic phenotypic markers through robust nodule formation, up-regulated osteocalcin and α-smooth muscle actin expression, and adoption of a round/rhomboid morphology indicative of osteoblastic differentiation. With the slowest proliferation, VICs on CH3-SAMs promoted calcified aggregate formation through cell detachment and increased cell death indicative of dystrophic calcification. Furthermore, induction of calcified tissue deposition on NH3(+) and CH3-SAMs was distinctly different than that of media induced osteoblastic VICs. These results demonstrate that substrate surface chemistry alters VIC behavior and plays an important role in calcified tissue formation. In addition, we have identified two novel methods of calcified VIC induction in vitro. Further study of these environments may yield new models for in vitro testing of therapeutics for calcified valve stenosis, although additional studies need to be conducted to correlate results to in vivo models.

STATEMENT OF SIGNIFICANCE

Valvular interstitial cell (VIC) differentiation and aortic valve calcification is associated with increased risk of mortality and onset of other cardiovascular disorders. This research examines effects of in vitro substrate surface chemistry on VIC differentiation and has led to the identification of two materials-based initiation mechanisms of osteoblastic-like calcified tissue formation independent of soluble signaling methods. Such findings are important for their potential to study signaling cascades responsible for valvular heart disease initiation and progression as well providing in vitro disease models for drug development. We have also identified a VIC activating in vitro environment that does not exhibit confluence induced nodule formation with promise for the development of tissue regenerating scaffolds.

Revisiting cardiovascular calcification: A multifaceted disease requiring a multidisciplinary approach

The presence of cardiovascular calcification significantly predicts patients' morbidity and mortality. Calcific mineral deposition within the soft cardiovascular tissues disrupts the normal biomechanical function of these tissues, leading to complications such as heart failure, myocardial infarction, and stroke. The realization that calcification results from active cellular processes offers hope that therapeutic intervention may prevent or reverse the disease. To this point, however, no clinically viable therapies have emerged. This may be due to the lack of certainty that remains in the mechanisms by which mineral is deposited in cardiovascular tissues. Gaining new insight into this process requires a multidisciplinary approach. The pathological changes in cell phenotype that lead to the physicochemical deposition of mineral and the resultant effects on tissue biomechanics must all be considered when designing strategies to treat cardiovascular calcification. In this review, we overview the current cardiovascular calcification paradigm and discuss emerging techniques that are providing new insight into the mechanisms of ectopic calcification.