Critical Role of Coaptive Strain in Aortic Valve Leaflet Homeostasis: Use of a Novel Flow Culture Bioreactor to Explore Heart Valve Mechanobiology

Models for calcific aortic valve disease in vivo and in vitro

Calcific Aortic Valve Disease (CAVD) is prevalent among the elderly as the most common valvular heart disease. Currently, no pharmaceutical interventions can effectively reverse or prevent CAVD, making valve replacement the primary therapeutic recourse. Extensive research spanning decades has contributed to the establishment of animal and in vitro cell models, which facilitates a deeper understanding of the pathophysiological progression and underlying mechanisms of CAVD. In this review, we provide a comprehensive summary and analysis of the strengths and limitations associated with commonly employed models for the study of valve calcification. We specifically emphasize the advancements in three-dimensional culture technologies, which replicate the structural complexity of the valve. Furthermore, we delve into prospective recommendations for advancing in vivo and in vitro model studies of CAVD.

Challenges of aortic valve tissue culture - maintenance of viability and extracellular matrix in the pulsatile dynamic microphysiological system

BACKGROUND

Calcific aortic valve disease (CAVD) causes an increasing health burden in the 21st century due to aging population. The complex pathophysiology remains to be understood to develop novel prevention and treatment strategies. Microphysiological systems (MPSs), also known as organ-on-chip or lab-on-a-chip systems, proved promising in bridging in vitro and in vivo approaches by applying integer AV tissue and modelling biomechanical microenvironment. This study introduces a novel MPS comprising different micropumps in conjunction with a tissue-incubation-chamber (TIC) for long-term porcine and human AV incubation (pAV, hAV).

RESULTS

Tissue cultures in two different MPS setups were compared and validated by a bimodal viability analysis and extracellular matrix transformation assessment. The MPS-TIC conjunction proved applicable for incubation periods of 14-26 days. An increased metabolic rate was detected for pulsatile dynamic MPS culture compared to static condition indicated by increased LDH intensity. ECM changes such as an increase of collagen fibre content in line with tissue contraction and mass reduction, also observed in early CAVD, were detected in MPS-TIC culture, as well as an increase of collagen fibre content. Glycosaminoglycans remained stable, no significant alterations of α-SMA or CD31 epitopes and no accumulation of calciumhydroxyapatite were observed after 14 days of incubation.

CONCLUSIONS

The presented ex vivo MPS allows long-term AV tissue incubation and will be adopted for future investigation of CAVD pathophysiology, also implementing human tissues. The bimodal viability assessment and ECM analyses approve reliability of ex vivo CAVD investigation and comparability of parallel tissue segments with different treatment strategies regarding the AV (patho)physiology.

The Complex Relationship between Hypoxia Signaling, Mitochondrial Dysfunction and Inflammation in Calcific Aortic Valve Disease: Insights from the Molecular Mechanisms to Therapeutic Approaches

Calcific aortic valve stenosis (CAVS) is among the most common causes of cardiovascular mortality in an aging population worldwide. The pathomechanisms of CAVS are such a complex and multifactorial process that researchers are still making progress to understand its physiopathology as well as the complex players involved in CAVS pathogenesis. Currently, there is no successful and effective treatment to prevent or slow down the disease. Surgical and transcatheter valve replacement represents the only option available for treating CAVS. Insufficient oxygen availability (hypoxia) has a critical role in the pathogenesis of almost all CVDs. This process is orchestrated by the hallmark transcription factor, hypoxia-inducible factor 1 alpha subunit (HIF-1α), which plays a pivotal role in regulating various target hypoxic genes and metabolic adaptations. Recent studies have shown a great deal of interest in understanding the contribution of HIF-1α in the pathogenesis of CAVS. However, it is deeply intertwined with other major contributors, including sustained inflammation and mitochondrial impairments, which are attributed primarily to CAVS. The present review aims to cover the latest understanding of the complex interplay effect of hypoxia signaling pathways, mitochondrial dysfunction, and inflammation in CAVS. We propose further hypotheses and interconnections on the complexity of these impacts in a perspective of better understanding the pathophysiology. These interplays will be examined considering recent studies that shall help us better dissect the molecular mechanism to enable the design and development of potential future therapeutic approaches that can prevent or slow down CAVS processes.

Establishment of a resazurin-based aortic valve tissue viability assay for dynamic culture in a microphysiological system

BACKGROUND/AIM

Tissue pathogenesis of aortic valve (AV) stenosis is research focus in cardiac surgery. Model limitations of conventional 2D culture of human or porcine valvular interstitial/endothelial cells (VIC/VECs) isolated from aortic valve tissues but also limited ability of (small) animal models to reflect human (patho)physiological situation in AV position raise the need to establish an in vitro setup using AV tissues. Resulting aim is to approximate (patho)physiological conditions in a dynamic pulsatile Microphysiological System (MPS) to culture human and porcine AV tissue with preservation of tissue viability but also defined ECM composition.

MATERIALS/METHODS

A tissue incubation chamber (TIC) was designed to implement human or porcine tissues (3×5 mm2) in a dynamic pulsatile culture in conventional cell culture ambience in a MPS. Cell viability assays based on lactate dehydrogenase (LDH)-release or resazurin-conversion were tested for applicability in the system and applied for a culture period of 14 days with interval evaluation of tissue viability on every other day. Resazurin-assay setup was compared in static vs. dynamic culture using varying substance saturation settings (50-300μM), incubation times and tissue masses and was consequently adapted.

RESULTS

Sterile dynamic culture of human and porcine AV tissue segments was established at a pulsatile flow rate range of 0.9-13.4μl/s. Implementation of tissues was realized by stitching the material in a thermoplastic polyurethane (TPU)-ring and insertion in the TIC-MPS-system. Culture volume of 2 ml caused LDH dilution not detectable in standard membrane integrity assay setup. Therefore, detection of resazurin-conversion of viable tissue was investigated. Optimal incubation time for viability conversion was determined at two hours at a saturated concentration of 300μM resazurin. Measurement in static conditions was shown to offer comparable results as dynamic condition but allowing optimal handling and TIC sterilization protocols for long term culture. Preliminary results revealed favourable porcine AV tissue viability over a 14 day period confirmed via resazurin-assay comparing statically cultured tissue counterparts.

CONCLUSIONS

Human and porcine AV tissue can be dynamically cultured in a TIC-MPS with monitoring of tissue viability using an adapted resazurin-assay setup. Preliminary results reveal advantageous viability of porcine AV tissues after dynamic TIC-MPS culture compared to static control.

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.

Modeling Impaired Coaptation Effects on Mitral Leaflet Homeostasis Using a Flow-Culture Bioreactor

BACKGROUND

Mitral valve (MV) regurgitation constitutes an increasing burden of adult and pediatric cardiac disease tending to worsen over time. Whether altered mechanical forces on leaflets cause valve disease is unknown. Here we show that MV leaflet coaptive strain disruption alters expression of genes critical to leaflet homeostasis.

METHODS

We used a flow-culture bioreactor of rat MVs with flow-induced cyclic coaptation (cycling valve group; n = 4) or in a sustained open state (open valve group; n = 4). After 3 days of culture, leaflet RNA expression was profiled.

RESULTS

More than 48 genes exhibited markedly changed expression when coaptive leaflet strain was disrupted for 3 days (change >fourfold; p < 0.05; cycling vs open valves). Genes exhibiting highly altered expression included Angpt2, Vegf, Cd74, RT1-Da (HLA-DRA), and Igfbp3. Pathway analysis indicated the most significant signaling pathways regulating the expression changes were Hif1α and Tnfα when MV closure was disrupted.

CONCLUSIONS

Disruption of normal MV coaptive strain markedly alters the expression of leaflet genes, demonstrating that cyclic strain is critically important to leaflet homeostasis. We demonstrate a pattern of MV gene expression changes in which hypoxia signaling is prominently increased in response to disrupted strain cycles. Coaptive strain regulation of MV leaflet homeostasis implicates altered strain as a mechanism potentially initiating valve disease. Early repair may prevent progression of disease driven by altered coaptation.