A study of extracellular matrix remodeling in aortic heart valves using a novel biaxial stretch bioreactor

Parametric optimization of culture chamber for cell mechanobiology research

Mechanical signals influence the morphology, function, differentiation, proliferation, and growth of cells. Due to the small size of cells, it is essential to analyze their mechanobiological responses with an in vitro mechanical loading device. Cells are cultured on an elastic silicone membrane substrate, and mechanical signals are transmitted to the cells by the substrate applying mechanical loads. However, large areas of non-uniform strain fields are generated on the elastic membrane, affecting the experiment's accuracy. In the study, finite-element analysis served as the basis of optimization, with uniform strain as the objective. The thickness of the basement membrane and loading constraints were parametrically adjusted. Through finite-element cycle iteration, the "M" profile basement membrane structure of the culture chamber was obtained to enhance the uniform strain field of the membrane. The optimized strain field of culture chamber was confirmed by three-dimensional digital image correlation (3D-DIC) technology. The results showed that the optimized chamber improved the strain uniformity factor. The uniform strain area proportion of the new chamber reached 90%, compared to approximately 70% of the current chambers. The new chamber further improved the uniformity and accuracy of the strain, demonstrating promising application prospects.

Design of a Mechanobioreactor to Apply Anisotropic, Biaxial Strain to Large Thin Biomaterials for Tissue Engineered Heart Valve Applications

Repair and replacement solutions for congenitally diseased heart valves capable of post-surgery growth and adaptation have remained elusive. Tissue engineered heart valves (TEHVs) offer a potential biological solution that addresses the drawbacks of existing valve replacements. Typically, TEHVs are made from thin, fibrous biomaterials that either become cell populated in vitro or in situ. Often, TEHV designs poorly mimic the anisotropic mechanical properties of healthy native valves leading to inadequate biomechanical function. Mechanical conditioning of engineered tissues with anisotropic strain application can induce extracellular matrix remodelling to alter the anisotropic mechanical properties of a construct, but implementation has been limited to small-scale set-ups. To address this limitation for TEHV applications, we designed and built a mechanobioreactor capable of modulating biaxial strain anisotropy applied to large, thin, biomaterial sheets in vitro. The bioreactor can independently control two orthogonal stretch axes to modulate applied strain anisotropy on biomaterial sheets from 13 × 13 mm² to 70 × 40 mm². A design of experiments was performed using experimentally validated finite element (FE) models and demonstrated that biaxial strain was applied uniformly over a larger percentage of the cell seeded area for larger sheets (13 × 13 mm²: 58% of sheet area vs. 52 × 31 mm²: 86% of sheet area). Furthermore, bioreactor prototypes demonstrated that over 70% of the cell seeding area remained uniformly strained under different prescribed protocols: equibiaxial amplitudes between 5 to 40%, cyclic frequencies between 0.1 to 2.5 Hz and anisotropic strain ratios between 0:1 (constrained uniaxial) to 2:1. Lastly, proof-of-concept experiments were conducted where we applied equibiaxial (ε_x = ε_y = 8.75%) and anisotropic (ε_x = 12.5%, ε_y = 5%) strain protocols to cell-seeded, electrospun scaffolds. Cell nuclei and F-actin aligned to the vector-sum strain direction of each prescribed protocol (nuclei alignment: equibiaxial: 43.2° ± 1.8°, anisotropic: 17.5° ± 1.7°; p < 0.001). The abilities of this bioreactor to prescribe different strain amplitude, frequency and strain anisotropy protocols to cell-seeded scaffolds will enable future studies into the effects of anisotropic loading protocols on mechanically conditioned TEHVs and other engineered planar connective tissues.

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.

Hyaluronic acid regulates heart valve interstitial cell contraction in fibrin-based scaffolds

Heart valve disease is associated with high morbidity and mortality worldwide resulting in hundreds of thousands of heart valve replacements each year. Tissue engineered heart valves (TEHVs) have the potential to overcome the major limitations of traditional replacement valves; however, leaflet retraction has led to the failure of TEHVs in preclinical studies. As native unmodified hyaluronic acid (HA) is known to promote healthy tissue development in native heart valves, we hypothesize that adding unmodified HA to fibrin-based scaffolds common to tissue engineering will reduce retraction by increasing cell-scaffold interactions and density of the scaffolds. Using a custom high-throughput culture system, we found that incorporating HA into millimeter-scale fibrin-based cell-populated scaffolds increases initial fiber diameter and cell-scaffold interactions, causing a cascade of mechanical, morphological, and cellular responses. These changes lead to higher levels of scaffold compaction and stiffness, increased cell alignment, and less bundling of fibrin fibers by the cells during culture. These effects significantly reduce scaffold retraction and total contractile force each by around 25%. These findings increase our understanding of how HA alters tissue remodeling and could inform the design of the next generation of tissue engineered heart valves to help reduce retraction. STATEMENT OF SIGNIFICANCE: Tissue engineered heart valves (TEHVs) have the potential to overcome the major limitations of traditional replacement valves; however, leaflet retraction induced by excessive myofibroblast activation has led to failure in preclinical studies. Developing valves are rich in hyaluronic acid (HA), which helps maintain a physiological environment for tissue remodeling without retraction. We hypothesized that adding unmodified HA to TEHVs would reduce retraction by increasing cell-scaffold interactions and density of the scaffolds. Using a high-throughput tissue culture platform, we demonstrate that HA incorporation into a fibrin-based scaffold can significantly reduce tissue retraction and total contractile force by increasing fiber bundling and altering cell-mediated matrix remodeling, therefore increasing gel density and stiffness. These finding increase our knowledge of native HA's effects within the extracellular matrix, and provide a new tool for TEHV design.

Lysyl oxidase interactions with transforming growth factor-β during angiogenesis are mediated by endothelin 1

Crosstalk between multiple components underlies the formation of mature vessels. Although the players involved in angiogenesis have been identified, mechanisms underlying the crosstalk between them are still unclear. Using the ex vivo aortic ring assay, we set out to dissect the interactions between two key angiogenic signaling pathways, vascular endothelial growth factor (VEGF) and transforming growth factor β (TGFβ), with members of the lysyl oxidase (LOX) family of matrix modifying enzymes. We find an interplay between VEGF, TGFβ, and the LOXs is essential for the formation of mature vascular smooth muscle cells (vSMC)-coated vessels. RNA sequencing analysis further identified an interaction with the endothelin-1 pathway. Our work implicates endothelin-1 downstream of TGFβ in vascular maturation and demonstrate the complexity of processes involved in generating vSMC-coated vessels.

Degeneration of Aortic Valves in a Bioreactor System with Pulsatile Flow

Calcific aortic valve disease is the most common valvular heart disease in industrialized countries. Pulsatile pressure, sheer and bending stress promote initiation and progression of aortic valve degeneration. The aim of this work is to establish an ex vivo model to study the therein involved processes. Ovine aortic roots bearing aortic valve leaflets were cultivated in an elaborated bioreactor system with pulsatile flow, physiological temperature, and controlled pressure and pH values. Standard and pro-degenerative treatment were studied regarding the impact on morphology, calcification, and gene expression. In particular, differentiation, matrix remodeling, and degeneration were also compared to a static cultivation model. Bioreactor cultivation led to shrinking and thickening of the valve leaflets compared to native leaflets while gross morphology and the presence of valvular interstitial cells were preserved. Degenerative conditions induced considerable leaflet calcification. In comparison to static cultivation, collagen gene expression was stable under bioreactor cultivation, whereas expression of hypoxia-related markers was increased. Osteopontin gene expression was differentially altered compared to protein expression, indicating an enhanced protein turnover. The present ex vivo model is an adequate and effective system to analyze aortic valve degeneration under controlled physiological conditions without the need of additional growth factors.