Calcific Aortic Valve Disease Is Associated with Layer-Specific Alterations in Collagen Architecture

Extracellular matrix in vascular homeostasis and disease

The extracellular matrix is an essential component and constitutes a dynamic microenvironment of the vessel wall with an indispensable role in vascular homeostasis and disease. From early development through to ageing, the vascular extracellular matrix undergoes various biochemical and biomechanical alterations in response to diverse environmental cues and exerts precise regulatory control over vessel remodelling. Advances in novel technologies that enable the comprehensive evaluation of extracellular matrix components and cell-matrix interactions have led to the emergence of therapeutic strategies that specifically target this fine-tuned network. In this Review, we explore various aspects of extracellular matrix biology in vascular development, disorders and ageing, emphasizing the effect of the extracellular matrix on disease initiation and progression. Additionally, we provide an overview of the potential therapeutic implications of targeting the extracellular matrix microenvironment in vascular diseases.

Collagen-mediated cardiovascular calcification

Cardiovascular calcification is a pathological process commonly observed in the elderly. Based on the location of the calcification, cardiovascular calcification can be classified into two main types: vascular calcification and valvular calcification. Collagen plays a critical role in the development of cardiovascular calcification lesions. The content and type of collagen are the result of a dynamic balance between synthesis and degradation. Unregulated processes can lead to adverse outcomes. During cardiovascular calcification, collagen not only serves as a scaffold for ectopic mineral deposition but also acts as a signal transduction pathway that mediates calcification by guiding the aggregation and nucleation of matrix vesicles and promoting the proliferation, migration and phenotypic changes of cells involved in the lesion. This review provides an overview of collagen subtypes in the cardiovascular system under physiological conditions and discusses their distribution. Additionally, we introduce pathological changes and mechanisms of collagen in blood vessels and heart valves. Then, the formation process and characteristic stages of cardiovascular calcification are described. Finally, we highlight the role of collagen in cardiovascular calcification, explore strategied for mediating calcification, and suggest potential directions for future research.

Aortic valve leaflet assessment to inform novel bioinspired materials: Understanding the impact of collagen fibres on the tissue's mechanical behaviour

Aortic stenosis is a prevalent disease that is treated with either mechanical or bioprosthetic valve replacement devices. However, these implants can experience problems with either functionality in the case of mechanical valves or long-term durability in the case of bioprosthetic valves. To enhance next generation prosthetic valves, such as biomimetic polymeric valves, an improved understanding of the native aortic valve leaflet structure and mechanical response is required to provide much needed benchmarks for future device development. This study aims to provide such information through imaging and mechanical testing of porcine aortic valve leaflet tissue. Using second harmonic generation imaging on cleared tissue it is shown that the fibre orientations are dependent on the leaflet type (left coronary, right coronary, non-coronary), while fibre crimp is not solely dependent on either of these factors. Uniaxial tensile testing of the leaflets and their layers showed that the ventricularis layer is stiffer than the fibrosa but the fibrosa dominates the mechanical response of the whole leaflet due to its higher thickness. Overall, this work provides a detailed assessment of the native porcine aortic valve leaflets' microstructure and mechanical response, delivering key information to aid the design and manufacture of future bioinspired valve implant devices.

Effect of Collagen Fiber Tortuosity Distribution on the Mechanical Response of Arterial Tissues

This study investigated the effect of collagen fiber tortuosity distribution on the biomechanical failure and prefailure properties of arterial wall tissue. An in-silico model of the arterial wall was developed using data obtained from combined multiphoton microscopy imaging and uni-axial tensile testing. Layer-dependent properties were prescribed for collagen, elastin, and ground substance. Collagen fibers were modeled as discrete anisotropic elements, while elastin and ground substance were modeled as homogeneous isotropic components. Our parametric analysis, using a finite element approach, revealed that different parameters of collagen fibers tortuosity distribution significantly influence both prefailure and failure biomechanical properties. Increased fiber tortuosity improved the tissue strength whereas the dispersion in the tortuosity distribution reduced it. This study provides novel insights into the structural-mechanical interdependencies in arterial walls, offering potential targets for clinical assessments and therapeutic interventions aimed at mitigating rupture risks.

Roles of Lysyl oxidases (LOX(L)) in pathologic calcification

Calcification of tissues involves the formation and deposition of calcium-containing crystals in the extracellular matrix (ECM). While this process is normal in bones, it becomes pathological when it occurs in cardiovascular and musculoskeletal soft tissues. Pathological calcification (PC) triggers detrimental pathways such as inflammation and oxidative stress, contributing to tissue damage and dysregulated tissue biomechanics, ultimately leading to severe complications and even death. The underlying mechanisms of PC remain elusive. Emerging evidence suggests a significant role of lysyl oxidases (LOX(L)) in PC. LOX(L) are a group of five enzymes involved in collagen cross-linking and ECM maturation. Beyond their classical role in bone mineralization, recent investigations propose new non-classical roles for LOX(L) that could be relevant in PC. In this review, we analyzed and summarized the functions of LOX(L) in cardiovascular and musculoskeletal PC, highlighting their deleterious roles in most studies. To date, specific inhibitors targeting LOX(L) isoforms are under development. New therapeutic tools targeting LOX(L) are warranted in PC and must avoid adverse effects on physiological bone mineralization.

Intracellular proteomics and extracellular vesiculomics as a metric of disease recapitulation in 3D-bioprinted aortic valve arrays

In calcific aortic valve disease (CAVD), mechanosensitive valvular cells respond to fibrosis- and calcification-induced tissue stiffening, further driving pathophysiology. No pharmacotherapeutics are available to treat CAVD because of the paucity of (i) appropriate experimental models that recapitulate this complex environment and (ii) benchmarking novel engineered aortic valve (AV)-model performance. We established a biomaterial-based CAVD model mimicking the biomechanics of the human AV disease-prone fibrosa layer, three-dimensional (3D)-bioprinted into 96-well arrays. Liquid chromatography-tandem mass spectrometry analyses probed the cellular proteome and vesiculome to compare the 3D-bioprinted model versus traditional 2D monoculture, against human CAVD tissue. The 3D-bioprinted model highly recapitulated the CAVD cellular proteome (94% versus 70% of 2D proteins). Integration of cellular and vesicular datasets identified known and unknown proteins ubiquitous to AV calcification. This study explores how 2D versus 3D-bioengineered systems recapitulate unique aspects of human disease, positions multiomics as a technique for the evaluation of high throughput-based bioengineered model systems, and potentiates future drug discovery.

The Role of Apoptosis and Oxidative Stress in a Cell Spheroid Model of Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD) is the most common heart valve disease among aging populations. There are two reported pathways of CAVD: osteogenic and dystrophic, the latter being more prevalent. Current two-dimensional (2D) in vitro CAVD models have shed light on the disease but lack three-dimensional (3D) cell-ECM interactions, and current 3D models require osteogenic media to induce calcification. The goal of this work is to develop a 3D dystrophic calcification model. We hypothesize that, as with 2D cell-based CAVD models, programmed cell death (apoptosis) is integral to calcification. We model the cell aggregation observed in CAVD by creating porcine valvular interstitial cell spheroids in agarose microwells. Upon culture in complete growth media (DMEM with serum), calcium nodules form in the spheroids within a few days. Inhibiting apoptosis with Z-VAD significantly reduced calcification, indicating that the calcification observed in this model is dystrophic rather than osteogenic. To determine the relative roles of oxidative stress and extracellular matrix (ECM) production in the induction of apoptosis and subsequent calcification, the media was supplemented with antioxidants with differing effects on ECM formation (ascorbic acid (AA), Trolox, or Methionine). All three antioxidants significantly reduced calcification as measured by Von Kossa staining, with the percentages of calcification per area of AA, Trolox, Methionine, and the non-antioxidant-treated control on day 7 equaling 0.17%, 2.5%, 6.0%, and 7.7%, respectively. As ZVAD and AA almost entirely inhibit calcification, apoptosis does not appear to be caused by a lack of diffusion of oxygen and metabolites within the small spheroids. Further, the observation that AA treatment reduces calcification significantly more than the other antioxidants indicates that the ECM stimulatory effect of AA plays a role inhibiting apoptosis and calcification in the spheroids. We conclude that, in this 3D in vitro model, both oxidative stress and ECM production play crucial roles in dystrophic calcification and may be viable therapeutic targets for preventing CAVD.

Lysyl oxidase-dependent extracellular matrix crosslinking modulates calcification in atherosclerosis and aortic valve disease

Extracellular matrix (ECM) is an active player in cardiovascular calcification (CVC), a major public health issue with an unmet need for effective therapies. Lysyl oxidase (LOX) conditions ECM biomechanical properties; thus, we hypothesized that LOX might impact on mineral deposition in calcific aortic valve disease (CAVD) and atherosclerosis. LOX was upregulated in calcified valves from two cohorts of CAVD patients. Strong LOX immunostaining was detected surrounding calcified foci in calcified human valves and atherosclerotic lesions colocalizing with RUNX2 on valvular interstitial cells (VICs) or vascular smooth muscle cells (VSMCs). Both LOX secretion and organized collagen deposition were enhanced in calcifying VICs exposed to osteogenic media. β-aminopropionitrile (BAPN), an inhibitor of LOX, attenuated collagen deposition and calcification. VICs seeded onto decellularized matrices from BAPN-treated VICs calcified less than cells cultured onto control scaffolds; instead, VICs exposed to conditioned media from cells over-expressing LOX or cultured onto LOX-crosslinked matrices calcified more. Atherosclerosis was induced in WT and transgenic mice that overexpress LOX in VSMC (TgLOXVSMC) by AAV-PCSK9D374Y injection and high-fat feeding. In atherosclerosis-challenged TgLOXVSMC mice both atherosclerosis burden and calcification assessed by near-infrared fluorescence (NIRF) imaging were higher than in WT mice. These animals also exhibited larger calcified areas in atherosclerotic lesions from aortic arches and brachiocephalic arteries. Moreover, LOX transgenesis exacerbated plaque inflammation, and increased VSMC cellularity, the rate of RUNX2-positive cells and both connective tissue content and collagen cross-linking. Our findings highlight the relevance of LOX in CVC and postulate this enzyme as a potential therapeutic target for CVC.

Bio-Inspired Fiber Reinforcement for Aortic Valves: Scaffold Production Process and Characterization

The application of tissue-engineered heart valves in the high-pressure circulatory system is still challenging. One possible solution is the development of biohybrid scaffolds with textile reinforcement to achieve improved mechanical properties. In this article, we present a manufacturing process of bio-inspired fiber reinforcement for an aortic valve scaffold. The reinforcement structure consists of polyvinylidene difluoride monofilament fibers that are biomimetically arranged by a novel winding process. The fibers were embedded and fixated into electrospun polycarbonate urethane on a cylindrical collector. The scaffold was characterized by biaxial tensile strength, bending stiffness, burst pressure and hemodynamically in a mock circulation system. The produced fiber-reinforced scaffold showed adequate acute mechanical and hemodynamic properties. The transvalvular pressure gradient was 3.02 ± 0.26 mmHg with an effective orifice area of 2.12 ± 0.22 cm2. The valves sustained aortic conditions, fulfilling the ISO-5840 standards. The fiber-reinforced scaffold failed in a circumferential direction at a stress of 461.64 ± 58.87 N/m and a strain of 49.43 ± 7.53%. These values were above the levels of tested native heart valve tissue. Overall, we demonstrated a novel manufacturing approach to develop a fiber-reinforced biomimetic scaffold for aortic heart valve tissue engineering. The characterization showed that this approach is promising for an in situ valve replacement.

Three-Dimensional Bioprinting of Ovine Aortic Valve Endothelial and Interstitial Cells for the Development of Multicellular Tissue Engineered Tissue Constructs

To investigate the pathogenic mechanisms of calcified aortic valve disease (CAVD), it is necessary to develop a new three-dimensional model that contains valvular interstitial cells (VIC) and valvular endothelial cells (VEC). For this purpose, ovine aortic valves were processed to isolate VIC and VEC that were dissolved in an alginate/gelatin hydrogel. A 3D-bioprinter (3D-Bioplotter^® Developer Series, EnvisionTec, Gladbeck, Germany) was used to print cell-laden tissue constructs containing VIC and VEC which were cultured for up to 21 days. The 3D-architecture, the composition of the culture medium, and the hydrogels were modified, and cell viability was assessed. The composition of the culture medium directly affected the cell viability of the multicellular tissue constructs. Co-culture of VIC and VEC with a mixture of 70% valvular interstitial cell and 30% valvular endothelial cell medium components reached the cell viability best tested with about 60% more living cells compared to pure valvular interstitial cell medium (p = 0.02). The tissue constructs retained comparable cell viability after 21 days (p = 0.90) with different 3D-architectures, including a "sandwich" and a "tube" design. Good long-term cell viability was confirmed even for thick multilayer multicellular tissue constructs. The 3D-bioprinting of multicellular tissue constructs with VEC and VIC is a successful new technique to design tissue constructs that mimic the structure of the native aortic valve for research applications of aortic valve pathologies.

Cellular-scale sex differences in extracellular matrix remodeling by valvular interstitial cells

Males acquire calcific aortic valve disease (CAVD) twice as often as females, yet stenotic valves from females display significantly higher levels of fibrosis compared to males with similar extent of disease. Fibrosis occurs as an imbalance between the production and degradation of the extracellular matrix (ECM), specifically type I collagen. This work characterizes ECM production and remodeling by male and female valvular interstitial cells (VICs) to better understand the fibrocalcific divergence between sexes evident in CAVD. Male and female VICs were assessed for gene and protein expression of myofibroblastic markers, ECM components, matrix metalloproteinases (MMPs), and tissue inhibitors of MMPs (TIMPs) via qRT-PCR and western blot. Overall metabolic activity was also measured. Activity assays for collagenase and gelatinase were performed to examine degradation behavior. Male VICs produced greater levels of myofibroblastic markers while female VICs showed greater metabolic activity and collagen production. In general, females displayed a greater level of MMP expression and production than males, but no sex differences were observed in TIMP production. Male VICs also displayed a greater level of collagenase and gelatinase activity than female VICs. This work displays sex differences in ECM remodeling by VICs that could be related to the sexual dimorphism in ECM structure seen in clinical CAVD.

Perspectives on pediatric congenital aortic valve stenosis: Extracellular matrix proteins, post translational modifications, and proteomic strategies

In heart valve biology, organization of the extracellular matrix structure is directly correlated to valve function. This is especially true in cases of pediatric congenital aortic valve stenosis (pCAVS), in which extracellular matrix (ECM) dysregulation is a hallmark of the disease, eventually leading to left ventricular hypertrophy and heart failure. Therapeutic strategies are limited, especially in pediatric cases in which mechanical and tissue engineered valve replacements may not be a suitable option. By identifying mechanisms of translational and post-translational dysregulation of ECM in CAVS, potential drug targets can be identified, and better bioengineered solutions can be developed. In this review, we summarize current knowledge regarding ECM proteins and their post translational modifications (PTMs) during aortic valve development and disease and contributing factors to ECM dysregulation in CAVS. Additionally, we aim to draw parallels between other fibrotic disease and contributions to ECM post-translational modifications. Finally, we explore the current treatment options in pediatrics and identify how the field of proteomics has advanced in recent years, highlighting novel characterization methods of ECM and PTMs that may be used to identify potential therapeutic strategies relevant to pCAVS.

HIF1A inhibitor PX-478 reduces pathological stretch-induced calcification and collagen turnover in aortic valve

HIF1A is significantly upregulated in calcified human aortic valves (AVs). Furthermore, HIF1A inhibitor PX-478 was shown to inhibit AV calcification under static and disturbed flow conditions. Since elevated stretch is one of the major mechanical stimuli for AV calcification, we investigated the effect of PX-478 on AV calcification and collagen turnover under a pathophysiological cyclic stretch (15%) condition. Porcine aortic valve (PAV) leaflets were cyclically (1 Hz) stretched at 15% for 24 days in osteogenic medium with or without PX-478. In addition, PAV leaflets were cyclically stretched at a physiological (10%) and 15% for 3 days in regular medium to assess its effect of on HIF1A mRNA expression. It was found that 100 μM (high concentration) PX-478 could significantly inhibit PAV calcification under 15% stretch, whereas 50 μM (moderate concentration) PX-478 showed a modest inhibitory effect on PAV calcification. Nonetheless, 50 μM PX-478 significantly reduced PAV collagen turnover under 15% stretch. Surprisingly, it was observed that cyclic stretch (15% vs. 10%) did not have any significant effect on HIF1A mRNA expression in PAV leaflets. These results suggest that HIF1A inhibitor PX-478 may impart its anti-calcific and anti-matrix remodeling effect in a stretch-independent manner.

The mechanistic pathways of oxidative stress in aortic stenosis and clinical implications

Despite the elucidation of the pathways behind the development of aortic stenosis (AS), there remains no effective medical treatment to slow or reverse its progress. Instead, the gold standard of care in severe or symptomatic AS is replacement of the aortic valve. Oxidative stress is implicated, both directly as well as indirectly, in lipid infiltration, inflammation and fibro-calcification, all of which are key processes underlying the pathophysiology of degenerative AS. This culminates in the breakdown of the extracellular matrix, differentiation of the valvular interstitial cells into an osteogenic phenotype, and finally, calcium deposition as well as thickening of the aortic valve. Oxidative stress is thus a promising and potential therapeutic target for the treatment of AS. Several studies focusing on the mitigation of oxidative stress in the context of AS have shown some success in animal and in vitro models, however similar benefits have yet to be seen in clinical trials. Statin therapy, once thought to be the key to the treatment of AS, has yielded disappointing results, however newer lipid lowering therapies may hold some promise. Other potential therapies, such as manipulation of microRNAs, blockade of the renin-angiotensin-aldosterone system and the use of dipeptidylpeptidase-4 inhibitors will also be reviewed.

Identification of Key Non-coding RNAs and Transcription Factors in Calcific Aortic Valve Disease

Background

Calcific aortic valve disease (CAVD) is one of the most frequently occurring valvular heart diseases among the aging population. Currently, there is no known pharmacological treatment available to delay or reverse CAVD progression. The regulation of gene expression could contribute to the initiation, progression, and treatment of CAVD. Non-coding RNAs (ncRNAs) and transcription factors play essential regulatory roles in gene expression in CAVD; thus, further research is urgently needed.

Materials and Methods

The gene-expression profiles of GSE51472 and GSE12644 were obtained from the Gene Expression Omnibus database, and differentially expressed genes (DEGs) were identified in each dataset. A protein-protein-interaction (PPI) network of DEGs was then constructed using the Search Tool for the Retrieval of Interacting Genes/Proteins database, and functional modules were analyzed with ClusterOne plugin in Cytoscape. Furthermore, Gene Ontology-functional annotation and Kyoto Encyclopedia of Genes and Genomes-pathway analysis were conducted for each functional module. Most crucially, ncRNAs and transcription factors acting on each functional module were separately identified using the RNAInter and TRRUST databases. The expression of predicted transcription factors and key genes was validated using GSE51472 and GSE12644. Furthermore, quantitative real-time PCR (qRT-PCR) experiments were performed to validate the differential expression of most promising candidates in human CAVD and control samples.

Results

Among 552 DEGs, 383 were upregulated and 169 were downregulated. In the PPI network, 15 functional modules involving 182 genes and proteins were identified. After hypergeometric testing, 45 ncRNAs and 33 transcription factors were obtained. Among the predicted transcription factors, CIITA, HIF1A, JUN, POU2F2, and STAT6 were differentially expressed in both the training and validation sets. In addition, we found that key genes, namely, CD2, CD86, CXCL8, FCGR3B, GZMB, ITGB2, LY86, MMP9, PPBP, and TYROBP were also differentially expressed in both the training and validation sets. Among the most promising candidates, differential expressions of ETS1, JUN, NFKB1, RELA, SP1, STAT1, ANCR, and LOC101927497 were identified via qRT-PCR experiments.

Conclusion

In this study, we identified functional modules with ncRNAs and transcription factors involved in CAVD pathogenesis. The current results suggest candidate molecules for further research on CAVD.

The role of neutrophil elastase in aortic valve calcification

Background

Calcific aortic valve disease (CAVD) is the most commonly valvular disease in the western countries initiated by inflammation and abnormal calcium deposition. Currently, there is no clinical drug for CAVD. Neutrophil elastase (NE) plays a causal role in inflammation and participates actively in cardiovascular diseases. However, the effect of NE on valve calcification remains unclear. So we next explore whether it is involved in valve calcification and the molecular mechanisms involved.

Methods

NE expression and activity in calcific aortic valve stenosis (CAVD) patients (n = 58) and healthy patients (n = 30) were measured by enzyme-linked immunosorbent assay (ELISA), western blot and immunohistochemistry (IHC). Porcine aortic valve interstitial cells (pVICs) were isolated and used in vitro expriments. The effects of NE on pVICs inflammation, apoptosis and calcification were detected by TUNEL assay, MTT assay, reverse transcription polymerase chain reaction (RT-PCR) and western blot. The effects of NE knockdown and NE activity inhibitor Alvelestat on pVICs inflammation, apoptosis and calcification under osteogenic medium induction were also detected by RT-PCR, western blot, alkaline phosphatase staining and alizarin red staining. Changes of Intracellular signaling pathways after NE treatment were measured by western blot. Apolipoprotein E⁻/⁻ (APOE⁻/⁻) mice were employed in this study to establish the important role of Alvelestat in valve calcification. HE was used to detected the thickness of valve. IHC was used to detected the NE and α-SMA expression in APOE⁻/⁻ mice. Echocardiography was employed to assess the heat function of APOE⁻/⁻ mice.

Results

The level and activity of NE were evaluated in patients with CAVD and calcified valve tissues. NE promoted inflammation, apoptosis and phenotype transition in pVICs in the presence or absence of osteogenic medium. Under osteogenic medium induction, NE silencing or NE inhibitor Alvelestat both suppressed the osteogenic differentiation of pVICs. Mechanically, NE played its role in promoting osteogenic differentiation of pVICs by activating the NF-κB and AKT signaling pathway. Alvelestat alleviated valve thickening and decreased the expression of NE and α-SMA in western diet-induced APOE⁻/⁻ mice. Alvelestat also reduced NE activity and partially improved the heart function of APOE⁻/⁻mice.

Conclusions

Collectively, NE is highly involved in the pathogenesis of valve calcification. Targeting NE such as Alvelestat may be a potential treatment for CAVD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03363-1.

The effect of the fibrocalcific pathological process on aortic valve stenosis in female patients: a finite element study

Calcific aortic valve disease (CAVD) is the most common heart valvular disease in the developed world. Most of the relevant research has been sex-blind, ignoring sex-related biological variables and thus under-appreciate sex differences. However, females present pronounced fibrosis for the same aortic stenosis (AS) severity compared with males, who exhibit more calcification. Herein, we present a computational model of fibrocalcific AV, aiming to investigate its effect on AS development. A parametric study was conducted to explore the influence of the total collagen fiber volume and its architecture on the aortic valve area (AVA). Towards that goal, computational models were generated for three females with stenotic AVs and different volumes of calcium. We have tested the influence of fibrosis on various parameters as fiber architecture, fibrosis location, and transvalvular pressure. We found that increased fiber volume with a low calcium volume could actively contribute to AS and reduce the AVA similarly to high calcium volume. Thus, the computed AVAs for our fibrocalcific models were 0.94 and 0.84 cm²and the clinical (Echo) AVAs were 0.82 and 0.8 cm². For the heavily calcified model, the computed AVA was 0.8 cm²and the clinical AVA was 0.73 cm². The proposed models demonstrated how collagen thickening influence the fibrocalcific-AS process in female patients. These models can assist in the clinical decision-making process and treatment development in valve therapy for female patients.

Differential proteome profile, biological pathways, and network relationships of osteogenic proteins in calcified human aortic valves

Calcific aortic valve disease (CAVD) is the most common heart valve disease requiring intervention. Most research on CAVD has focused on inflammation, ossification, and cellular phenotype transformation. To gain a broader picture into the wide range of cellular and molecular mechanisms involved in this disease, we compared the total protein profiles between calcified and non-calcified areas from 5 human valves resected during surgery. The 1413 positively identified proteins were filtered down to 248 proteins present in both calcified and non-calcified segments of at least 3 of the 5 valves, which were then analyzed using Ingenuity Pathway Analysis. Concurrently, the top 40 differentially abundant proteins were grouped according to their biological functions and shown in interactive networks. Finally, the abundance of selected osteogenic proteins (osteopontin, osteonectin, osteocalcin, osteoprotegerin, and RANK) was quantified using ELISA and/or immunohistochemistry. The top pathways identified were complement system, acute phase response signaling, metabolism, LXR/RXR and FXR/RXR activation, actin cytoskeleton, mineral binding, nucleic acid interaction, structural extracellular matrix (ECM), and angiogenesis. There was a greater abundance of osteopontin, osteonectin, osteocalcin, osteoprotegerin, and RANK in the calcified regions than the non-calcified ones. The osteogenic proteins also formed key connections between the biological signaling pathways in the network model. In conclusion, this proteomic analysis demonstrated the involvement of multiple signaling pathways in CAVD. The interconnectedness of these pathways provides new insights for the treatment of this disease.

Aortic valve cell microenvironment: Considerations for developing a valve-on-chip

Cardiac valves are sophisticated, dynamic structures residing in a complex mechanical and hemodynamic environment. Cardiac valve disease is an active and progressive disease resulting in severe socioeconomic burden, especially in the elderly. Valve disease also leads to a 50% increase in the possibility of associated cardiovascular events. Yet, valve replacement remains the standard of treatment with early detection, mitigation, and alternate therapeutic strategies still lacking. Effective study models are required to further elucidate disease mechanisms and diagnostic and therapeutic strategies. Organ-on-chip models offer a unique and powerful environment that incorporates the ease and reproducibility of in vitro systems along with the complexity and physiological recapitulation of the in vivo system. The key to developing effective valve-on-chip models is maintaining the cell and tissue-level microenvironment relevant to the study application. This review outlines the various components and factors that comprise and/or affect the cell microenvironment that ought to be considered while constructing a valve-on-chip model. This review also dives into the advancements made toward constructing valve-on-chip models with a specific focus on the aortic valve, that is, in vitro studies incorporating three-dimensional co-culture models that incorporate relevant extracellular matrices and mechanical and hemodynamic cues.

Comparison of Serotonin-Regulated Calcific Processes in Aortic and Mitral Valvular Interstitial Cells

Calcification is an important pathological process and a common complication of degenerative valvular heart diseases, with higher incidence in aortic versus mitral valves. Two phenotypes of valvular interstitial cells (VICs), activated VICs and osteoblastic VICs (obVICs), synergistically orchestrate this pathology. It has been demonstrated that serotonin is involved in early stages of myxomatous mitral degeneration, whereas the role of serotonin in calcific aortic valve disease is still unknown. To uncover the link between serotonin and osteogenesis in heart valves, osteogenesis of aortic and mitral VICs was induced in vitro. Actin polymerization and serotonin signaling were inhibited using cytochalasin D and serotonin inhibitors, respectively, to investigate the role of cell activation and serotonin signals in valvular cell osteogenesis. To evaluate calcification progress, calcium and collagen deposits along with the expression of protein markers, including the rate-limiting enzyme of serotonin synthesis [tryptophan hydroxylase 1 (TPH1)], were assessed. When exposed to osteogenic culture conditions and grown on soft surfaces, passage zero aortic VICs increased extracellular collagen deposits and obVIC phenotype markers. A more intense osteogenic process was observed in aortic VICs of higher passages, where cells were activated prior to osteogenic induction. For both, TPH1 expression was upregulated as osteogenesis advanced. However, these osteogenic changes were reversed upon serotonin inhibition. This discovery provides a better understanding of signaling pathways regulating VIC phenotype transformation and explains different manifestations of degenerative pathologies. In addition, the discovery of serotonin-based inhibition of valvular calcification will contribute to the development of potential novel therapies for calcific valvular diseases.

Engineering the aortic valve extracellular matrix through stages of development, aging, and disease

For such a thin tissue, the aortic valve possesses an exquisitely complex, multi-layered extracellular matrix (ECM), and disruptions to this structure constitute one of the earliest hallmarks of fibrocalcific aortic valve disease (CAVD). The native valve structure provides a challenging target for engineers to mimic, but the development of advanced, ECM-based scaffolds may enable mechanistic and therapeutic discoveries that are not feasible in other culture or in vivo platforms. This review first discusses the ECM changes that occur during heart valve development, normal aging, onset of early-stage disease, and progression to late-stage disease. We then provide an overview of the bottom-up tissue engineering strategies that have been used to mimic the valvular ECM, and opportunities for advancement in these areas.

Label-Free Multiphoton Microscopy for the Detection and Monitoring of Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD) is the most common valvular heart disease. CAVD results in a considerable socio-economic burden, especially considering the aging population in Europe and North America. The only treatment standard is surgical valve replacement as early diagnostic, mitigation, and drug strategies remain underdeveloped. Novel diagnostic techniques and biomarkers for early detection and monitoring of CAVD progression are thus a pressing need. Additionally, non-destructive tools are required for longitudinal in vitro and in vivo assessment of CAVD initiation and progression that can be translated into clinical practice in the future. Multiphoton microscopy (MPM) facilitates label-free and non-destructive imaging to obtain quantitative, optical biomarkers that have been shown to correlate with key events during CAVD progression. MPM can also be used to obtain spatiotemporal readouts of metabolic changes that occur in the cells. While cellular metabolism has been extensively explored for various cardiovascular disorders like atherosclerosis, hypertension, and heart failure, and has shown potential in elucidating key pathophysiological processes in heart valve diseases, it has yet to gain traction in the study of CAVD. Furthermore, MPM also provides structural, functional, and metabolic readouts that have the potential to correlate with key pathophysiological events in CAVD progression. This review outlines the applicability of MPM and its derived quantitative metrics for the detection and monitoring of early CAVD progression. The review will further focus on the MPM-detectable metabolic biomarkers that correlate with key biological events during valve pathogenesis and their potential role in assessing CAVD pathophysiology.

Dissecting Calcific Aortic Valve Disease-The Role, Etiology, and Drivers of Valvular Fibrosis

Calcific aortic valve disease (CAVD) is a highly prevalent and progressive disorder that ultimately causes gradual narrowing of the left ventricular outflow orifice with ensuing devastating hemodynamic effects on the heart. Calcific mineral accumulation is the hallmark pathology defining this process; however, fibrotic extracellular matrix (ECM) remodeling that leads to extensive deposition of fibrous connective tissue and distortion of the valvular microarchitecture similarly has major biomechanical and functional consequences for heart valve function. Significant advances have been made to unravel the complex mechanisms that govern these active, cell-mediated processes, yet the interplay between fibrosis and calcification and the individual contribution to progressive extracellular matrix stiffening require further clarification. Specifically, we discuss (1) the valvular biomechanics and layered ECM composition, (2) patterns in the cellular contribution, temporal onset, and risk factors for valvular fibrosis, (3) imaging valvular fibrosis, (4) biomechanical implications of valvular fibrosis, and (5) molecular mechanisms promoting fibrotic tissue remodeling and the possibility of reverse remodeling. This review explores our current understanding of the cellular and molecular drivers of fibrogenesis and the pathophysiological role of fibrosis in CAVD.

Collagen fiber regulation in human pediatric aortic valve development and disease

Congenital aortic valve stenosis (CAVS) affects up to 10% of the world population without medical therapies to treat the disease. New molecular targets are continually being sought that can halt CAVS progression. Collagen deregulation is a hallmark of CAVS yet remains mostly undefined. Here, histological studies were paired with high resolution accurate mass (HRAM) collagen-targeting proteomics to investigate collagen fiber production with collagen regulation associated with human AV development and pediatric end-stage CAVS (pCAVS). Histological studies identified collagen fiber realignment and unique regions of high-density collagen in pCAVS. Proteomic analysis reported specific collagen peptides are modified by hydroxylated prolines (HYP), a post-translational modification critical to stabilizing the collagen triple helix. Quantitative data analysis reported significant regulation of collagen HYP sites across patient categories. Non-collagen type ECM proteins identified (26 of the 44 total proteins) have direct interactions in collagen synthesis, regulation, or modification. Network analysis identified BAMBI (BMP and Activin Membrane Bound Inhibitor) as a potential upstream regulator of the collagen interactome. This is the first study to detail the collagen types and HYP modifications associated with human AV development and pCAVS. We anticipate that this study will inform new therapeutic avenues that inhibit valvular degradation in pCAVS and engineered options for valve replacement.

Reproducible In Vitro Tissue Culture Model to Study Basic Mechanisms of Calcific Aortic Valve Disease: Comparative Analysis to Valvular Interstitials Cells

The hallmarks of calcific aortic valve disease (CAVD), an active and regulated process involving the creation of calcium nodules, lipoprotein accumulation, and chronic inflammation, are the significant changes that occur in the composition, organization, and mechanical properties of the extracellular matrix (ECM) of the aortic valve (AV). Most research regarding CAVD is based on experiments using two-dimensional (2D) cell culture or artificially created three-dimensional (3D) environments of valvular interstitial cells (VICs). Because the valvular ECM has a powerful influence in regulating pathological events, we developed an in vitro AV tissue culture model, which is more closely able to mimic natural conditions to study cellular responses underlying CAVD. AV leaflets, isolated from the hearts of 6-8-month-old sheep, were fixed with needles on silicon rubber rings to achieve passive tension and treated in vitro under pro-degenerative and pro-calcifying conditions. The degeneration of AV leaflets progressed over time, commencing with the first visible calcified domains after 14 d and winding up with the distinct formation of calcium nodules, heightened stiffness, and clear disruption of the ECM after 56 d. Both the expression of pro-degenerative genes and the myofibroblastic differentiation of VICs were altered in AV leaflets compared to that in VIC cultures. In this study, we have established an easily applicable, reproducible, and cost-effective in vitro AV tissue culture model to study pathological mechanisms underlying CAVD. The valvular ECM and realistic VIC-VEC interactions mimic natural conditions more closely than VIC cultures or 3D environments. The application of various culture conditions enables the examination of different pathological mechanisms underlying CAVD and could lead to a better understanding of the molecular mechanisms that lead to VIC degeneration and AS. Our model provides a valuable tool to study the complex pathobiology of CAVD and can be used to identify potential therapeutic targets for slowing disease progression.

Enhancer-associated aortic valve stenosis risk locus 1p21.2 alters NFATC2 binding site and promotes fibrogenesis

Genome-wide association studies for calcific aortic valve stenosis (CAVS) previously reported strong signal for noncoding variants at 1p21.2. Previous study using Mendelian randomization suggested that the locus controls the expression of PALMD encoding Palmdelphin (PALMD). However, the molecular regulation at the locus and the impact of PALMD on the biology of the aortic valve is presently unknown. 3D genetic mapping and CRISPR activation identified rs6702619 as being located in a distant-acting enhancer, which controls the expression of PALMD. DNA-binding assay showed that the risk variant modified the DNA shape, which prevented the recruitment of NFATC2 and lowered the expression of PALMD. In co-expression network analysis, a module encompassing PALMD was enriched in actin-based process. Mass spectrometry and functional assessment showed that PALMD is a regulator of actin polymerization. In turn, lower level of PALMD promoted the activation of myocardin-related transcription factor and fibrosis, a key pathobiological process underpinning CAVS.

Extracellular Matrix in Calcific Aortic Valve Disease: Architecture, Dynamic and Perspectives

Calcific Aortic Valve Disease (CAVD) is the most common valvular heart disease in developed countries and in the ageing population. It is strongly correlated to median age, affecting up to 13% of the population over the age of 65. Pathophysiological analysis indicates CAVD as a result of an active and degenerative disease, starting with sclerosis and chronic inflammation and then leaflet calcification, which ultimately can account for aortic stenosis. Although CAVD has been firstly recognized as a passive event mostly resulting from a degenerative aging process, much evidences suggests that calcification arises from different active processes, involving both aortic valve-resident cells (valve endothelial cells, valve interstitial cells, mesenchymal stem cells, innate immunity cells) and circulating cells (circulating mesenchymal cells, immunity cells). Moreover, a role for the cell-derived "matrix vesicles" and extracellular matrix (ECM) components has also been recognized. The aim of this work is to review the cellular and molecular alterations occurring in aortic valve during CAVD pathogenesis, focusing on the role of ECM in the natural course of the disease.

Valve endothelial-interstitial interactions drive emergent complex calcific lesion formation in vitro

OBJECTIVE

Calcific aortic valve disease (CAVD) is an actively regulated degenerative disease process. Clinical lesions exhibit marked 3D complexity not represented in current in vitro systems. We here present a unique mechanically stressed 3D culture system that recapitulates valve interstitial cell (VIC) induced matrix calcification through myofibroblastic activation and osteoblastic differentiation. We test the hypothesis that valve endothelial (VEC) - interstitial collaborative interactions modulate the risk and complexity of calcific pathogenesis within mechanically stressed and pro-inflammatory environments.

APPROACH AND RESULTS

Porcine aortic valve endothelial and interstitial cells (VEC and VIC) were seeded in a mechanically constrained collagen hydrogels alone or in co-culture configurations. Raised 3D VIC-filled lesions formed within 7 days when cultured in osteogenic media (OGM), and surprisingly exacerbated by endothelial coculture. We identified a spatially coordinated pro-endochondral vs. pro-osteogenic signaling program within the lesion. VEC underwent Endothelial-to-Mesenchymal Transformation (EndMT) and populated the lesion center. The spatial complexity of molecular and cellular signatures of this 3D in vitro CAVD system were consistent with human diseased aortic valve histology. SNAI1 was highly expressed in the VEC and subendothelial direct VIC corroborates with human CAVD lesions. Spatial distribution of Sox9 vs. Runx2 expression within the developed lesions (Sox9 peri-lesion vs. Runx2 predominantly within lesions) mirrored their expression in heavily calcified human aortic valves. Finally, we demonstrate the applicability of this platform for screening potential pharmacologic therapies through blocking the canonical NFκB pathway via BAY 11-7082.

CONCLUSIONS

Our results establish that VEC actively induce VIC pathological remodeling and calcification via EndMT and paracrine signaling. This mechanically constrained culture platform enables the interrogation of accelerated cell-mediated matrix remodeling behavior underpinned by this cellular feedback circuit. The high fidelity of this complex 3D model system to human CAVD mechanisms supports its use to test mechanisms of intercellular communication in valves and their pharmacological control.

Label-free optical biomarkers detect early calcific aortic valve disease in a wild-type mouse model

Background

Calcific aortic valve disease (CAVD) pathophysiology is a complex, multistage process, usually diagnosed at advanced stages after significant anatomical and hemodynamic changes in the valve. Early detection of disease progression is thus pivotal in the development of prevention and mitigation strategies. In this study, we developed a diet-based, non-genetically modified mouse model for early CAVD progression, and explored the utility of two-photon excited fluorescence (TPEF) microscopy for early detection of CAVD progression. TPEF imaging provides label-free, non-invasive, quantitative metrics with the potential to correlate with multiple stages of CAVD pathophysiology including calcium deposition, collagen remodeling and osteogenic differentiation.

Methods

Twenty-week old C57BL/6J mice were fed either a control or pro-calcific diet for 16 weeks and monitored via echocardiography, histology, immunohistochemistry, and quantitative polarized light imaging. Additionally, TPEF imaging was used to quantify tissue autofluorescence (A) at 755 nm, 810 nm and 860 nm excitation, to calculate TPEF 755–860 ratio (A_860/525/(A_755/460 + A_860/525)) and TPEF Collagen-Calcium ratio (A_810/525/(A_810/460 + A_810/525)) in the murine valves. In a separate experiment, animals were fed the above diets till 28 weeks to assess for later-stage calcification.

Results

Pro-calcific mice showed evidence of lipid deposition at 4 weeks and calcification at 16 weeks at the valve commissures. The valves of pro-calcific mice also showed positive expression for markers of osteogenic differentiation, myofibroblast activation, proliferation, inflammatory cytokines and collagen remodeling. Pro-calcific mice exhibited lower TPEF autofluorescence ratios, at locations coincident with calcification, that correlated with increased collagen disorganization and positive expression of osteogenic markers. Additionally, locations with lower TPEF autofluorescence ratios at 4 and 16 weeks exhibited increased calcification at later 28-week timepoints.

Conclusions

This study suggests the potential of TPEF autofluorescence metrics to serve as a label-free tool for early detection and monitoring of CAVD pathophysiology.

Anisotropic Fiber-Reinforced Glycosaminoglycan Hydrogels for Heart Valve Tissue Engineering

This study investigates polymer fiber-reinforced protein-polysaccharide-based hydrogels for heart valve tissue engineering applications. Polycaprolactone and gelatin (3:1) blends were jet-spun to fabricate aligned fibers that possessed fiber diameters in the range found in the native heart valve. These fibers were embedded in methacrylated hydrogels made from gelatin, sodium hyaluronate, and chondroitin sulfate to create fiber-reinforced hydrogel composites (HCs). The fiber-reinforced gelatin glycosaminoglycan (GAG)-based HC possessed interconnected porous structures and porosity higher than fiber-only conditions. These fiber-reinforced HCs exhibited compressive modulus and biaxial mechanical behavior comparable to that of native porcine aortic valves. The fiber-reinforced HCs were able to swell higher and degraded less than the hydrogels. Elution studies revealed that less than 20% of incorporated gelatin methacrylate and GAGs were released over 2 weeks, with a steady-state release after the first day. When cultured with porcine valve interstitial cells (VICs), the fiber-reinforced composites were able to maintain higher cell viability compared with fiber-only samples. Quiescent VICs expressed alpha smooth muscle actin and calponin showing an activated phenotype, along with a few cells expressing the proliferation marker Ki67 and negative expression for RUNX2, an osteogenic marker. Our study demonstrated that compared with the hydrogels and fibers alone, combining both components can yield durable, reinforced composites that mimic heart valve mechanical behavior, while maintaining high cell viability and expressing positive activation as well as proliferation markers.

Identification of CD34+/PGDFRα+ Valve Interstitial Cells (VICs) in Human Aortic Valves: Association of Their Abundance, Morphology and Spatial Organization with Early Calcific Remodeling

Aortic valve interstitial cells (VICs) constitute a heterogeneous population involved in the maintenance of unique valvular architecture, ensuring proper hemodynamic function but also engaged in valve degeneration. Recently, cells similar to telocytes/interstitial Cajal-like cells described in various organs were found in heart valves. The aim of this study was to examine the density, distribution, and spatial organization of a VIC subset co-expressing CD34 and PDGFRα in normal aortic valves and to investigate if these cells are associated with the occurrence of early signs of valve calcific remodeling. We examined 28 human aortic valves obtained upon autopsy. General valve morphology and the early signs of degeneration were assessed histochemically. The studied VICs were identified by immunofluorescence (CD34, PDGFRα, vimentin), and their number in standardized parts and layers of the valves was evaluated. In order to show the complex three-dimensional structure of CD34+/PDGFRα+ VICs, whole-mount specimens were imaged by confocal microscopy, and subsequently rendered using the Imaris (Bitplane AG, Zürich, Switzerland) software. CD34+/PDGFRα+ VICs were found in all examined valves, showing significant differences in the number, distribution within valve tissue, spatial organization, and morphology (spherical/oval without projections; numerous short projections; long, branching, occasionally moniliform projections). Such a complex morphology was associated with the younger age of the subjects, and these VICs were more frequent in the spongiosa layer of the valve. Both the number and percentage of CD34+/PDGFRα+ VICs were inversely correlated with the age of the subjects. Valves with histochemical signs of early calcification contained a lower number of CD34+/PDGFRα+ cells. They were less numerous in proximal parts of the cusps, i.e., areas prone to calcification. The results suggest that normal aortic valves contain a subpopulation of CD34+/PDGFRα+ VICs, which might be involved in the maintenance of local microenvironment resisting to pathologic remodeling. Their reduced number in older age could limit the self-regenerative properties of the valve stroma.

Ovarian Cells Have Increased Proliferation in Response to Heparin-Binding Epidermal Growth Factor as Collagen Density Increases

It is well known that during ovarian cancer progression, the omentum transforms from a thin lacy organ to a thick tougher tissue. However, the mechanisms regulating this transformation and the implications of the altered microenvironment on ovarian cancer progression remain unclear. To address these questions, the global and local concentrations of collagen I were determined for normal and metastatic human omentum. Collagen I was increased 5.3-fold in omenta from ovarian cancer patients and localized to areas of activated fibroblasts rather than regions with a high density of cancer cells. Transforming growth factor beta 1 (TGFβ1) was detected in ascites from ovarian cancer patients (4 ng/mL), suggesting a potential role for TGFβ1 in the observed increase in collagen. Treatment with TGFβ1 induced fibroblast activation, proliferation, and collagen deposition in mouse omental explants and an in vitro model with human omental fibroblasts. Finally, the impact of increased collagen I on ovarian cancer cells was determined by examining proliferation on collagen I gels formulated to mimic normal and cancerous omenta. While collagen density alone had no impact on proliferation, a synergistic effect was observed with collagen density and heparin-binding epidermal growth factor treatment. These results suggest that TGFβ1 induces collagen deposition from the resident fibroblasts in the omentum and that this altered microenvironment impacts cancer cell response to growth factors found in ascites. Impact statement Using quantitative analysis of patient samples, in vitro models of the metastatic ovarian cancer microenvironment were designed with pathologically relevant collagen densities and growth factor concentrations. Studies in these models support a mechanism where transforming growth factor β1 in the ascites fluid induces omental fibroblast proliferation, activation, and deposition of collagen I, which then impacts tumor cell proliferation in response to additional ascites growth factors such as heparin-binding epidermal growth factor. This approach can be used to dissect mechanisms involved in microenvironmental modeling in multiple disease applications.

Integration of polarized spatial frequency domain imaging (pSFDI) with a biaxial mechanical testing system for quantification of load-dependent collagen architecture in soft collagenous tissues

Collagen fiber networks provide the structural strength of tissues, such as tendons, skin and arteries. Quantifying the fiber architecture in response to mechanical loads is essential towards a better understanding of the tissue-level mechanical behaviors, especially in assessing disease-driven functional changes. To enable novel investigations into these load-dependent fiber structures, a polarized spatial frequency domain imaging (pSFDI) device was developed and, for the first time, integrated with a biaxial mechanical testing system. The integrated instrument is capable of a wide-field quantification of the fiber orientation and the degree of optical anisotropy (DOA), representing the local degree of fiber alignment. The opto-mechanical instrument''s performance was assessed through uniaxial loading on tendon tissues with known collagen fiber microstructures. Our results revealed that the bulk fiber orientation angle of the tendon tissue changed minimally with loading (median ± 0.5*IQR of 52.7° ± 3.3° and 51.9° ± 3.3° under 0 and 3% longitudinal strains, respectively), whereas on a micro-scale, the fibers became better aligned with the direction of loading: the DOA (mean ± SD) increased from 0.149 ± 0.032 to 0.198 ± 0.056 under 0 and 3% longitudinal strains, respectively, p < 0.001. The integrated instrument was further applied to study two representative mitral valve anterior leaflet (MVAL) tissues subjected to various biaxial loads. The fiber orientations within these representative MVAL tissue specimens demonstrated noticeable heterogeneity, with the local fiber orientations dependent upon the sample, the spatial and transmural locations, and the applied loading. Our results also showed that fibers were generally better aligned under equibiaxial (DOA = 0.089 ± 0.036) and circumferentially-dominant loading (DOA = 0.086 ± 0.037) than under the radially-dominant loading (DOA = 0.077 ± 0.034), indicating circumferential predisposition. These novel findings exemplify a deeper understanding of the load-dependent collagen fiber microstructures obtained through the use of the integrated opto-mechanical instrument. STATEMENT OF SIGNIFICANCE: In this study, a novel quantitative opto-mechanical system was developed by combining a polarized Spatial Frequency Domain Imaging (pSFDI) device with a biaxial mechanical tester. The integrated system was used to quantify the load-dependent collagen fiber microstructures in representative tendon and mitral valve anterior leaflet (MVAL) tissues. Our results revealed that MVAL's fiber architectures exhibited load-dependent spatial and transmural heterogeneities, suggesting further microstructural complexity than previously reported in heart valve tissues. These novel findings were possible through the system's ability to, for the first time, capture the load-dependent collagen architecture in the mitral valve anterior leaflet tissue over a wide field of view (e.g., 10 × 10 mm for the MVAL tissue specimens). Such capabilities afford unique future opportunities to improve patient outcomes through concurrent mechanical and microstructural assessments of healthy and diseased tissues in conditions such as heart valve regurgitation and calcification.

Disease-inspired tissue engineering: Investigation of cardiovascular pathologies

Once focused exclusively on the creation of tissues to repair or replace diseased or damaged organs, the field of tissue engineering has undergone an important evolution in recent years. Namely, tissue engineering techniques are increasingly being applied to intentionally generate pathological conditions. Motivated in part by the wide gap between 2D cultures and animal models in the current disease modeling continuum, disease-inspired tissue-engineered platforms have numerous potential applications, and may serve to advance our understanding and clinical treatment of various diseases. This review will focus on recent progress toward generating tissue-engineered models of cardiovascular diseases, including cardiac hypertrophy, fibrosis, and ischemia reperfusion injury, atherosclerosis, and calcific aortic valve disease, with an emphasis on how these disease-inspired platforms can be used to decipher disease etiology. Each pathology is discussed in the context of generating both disease-specific cells as well as disease-specific extracellular environments, with an eye toward future opportunities to integrate different tools to yield more complex and physiologically relevant culture platforms. Ultimately, the development of effective disease treatments relies upon our ability to develop appropriate experimental models; as cardiovascular diseases are the leading cause of death worldwide, the insights yielded by improved in vitro disease modeling could have substantial ramifications for public health and clinical care.

Emerging Roles of Lysyl Oxidases in the Cardiovascular System: New Concepts and Therapeutic Challenges

Lysyl oxidases (LOX and LOX-likes (LOXLs) isoenzymes) belong to a family of copper-dependent enzymes classically involved in the covalent cross-linking of collagen and elastin, a pivotal process that ensures extracellular matrix (ECM) stability and provides the tensile and elastic characteristics of connective tissues. Besides this structural role, in the last years, novel biological properties have been attributed to these enzymes, which can critically influence cardiovascular function. LOX and LOXLs control cell proliferation, migration, adhesion, differentiation, oxidative stress, and transcriptional regulation and, thereby, their dysregulation has been linked to a myriad of cardiovascular pathologies. Lysyl oxidase could modulate virtually all stages of the atherosclerotic process, from endothelial dysfunction and plaque progression to calcification and rupture of advanced and complicated plaques, and contributes to vascular stiffness in hypertension. The alteration of LOX/LOXLs expression underlies the development of other vascular pathologies characterized by a destructive remodeling of the ECM, such as aneurysm and artery dissections, and contributes to the adverse myocardial remodeling and dysfunction in hypertension, myocardial infarction, and obesity. This review examines the most recent advances in the study of LOX and LOXLs biology and their pathophysiological role in cardiovascular diseases with special emphasis on their potential as therapeutic targets.

Heterogeneous multi-laminar tissue constructs as a platform to evaluate aortic valve matrix-dependent pathogenicity

Designing and constructing controlled in vitro cell culture platforms is imperative toward pinpointing factors that contribute to the development of calcific aortic valve disease. A 3D, laminar, filter paper-based cell culture system that was previously established as a method of analyzing valvular interstitial cell migration and protein expression was adapted here for studying the impact of specific extracellular matrix proteins on cellular viability and calcification proclivity. Hydrogels incorporating hyaluronan and collagen I, two prevalent valvular extracellular matrix proteins with altered pathological production, were designed with similar mechanics to parse out effects of the individual proteins on cell behavior. Laminar constructs containing varying combinations of discrete layers of collagen and hyaluronan were assembled to mimic native and pathological valve compositions. Proteinaceous and genetic expression patterns pertaining to cell viability and calcific potential were quantified via fluorescent imaging. A significant dose-dependency was observed, with increased collagen content associated with decreased viability and increased calcific phenotype. These results suggest that extracellular composition is influential in calcific aortic valve disease progression and will be key toward development of future tissue-engineered or pharmaceutical calcific aortic valve treatments. STATEMENT OF SIGNIFICANCE: Calcific aortic valve disease (CAVD), a widespread heart valve disorder, is characterized by fibrotic leaflet thickening and calcific nodule formation. This pathological remodeling is an active process mediated by the valvular interstitial cells (VICs). Currently, the only treatment available is surgical replacement of the valve - a procedure associated with significant long-term risk and morbidity. Development of effective alternate therapies is hindered by our poor understanding of CAVD etiology. Previous work has implicated the composition and mechanics of the extracellular matrix in the progression of CAVD. These individual factors and their magnitude of influence have not been extensively explored - particularly in 3D systems. Here, we have bridged this gap in understanding through the employment of a heterogeneous 3D filter-paper culture system.

Response of collagen matrices under pressure and hydraulic resistance in hydrogels

Extracellular matrices in animal tissue are hydrogels mostly made of collagen. In these matrices, collagen fibers are hierarchically assembled and cross-linked to form a porous and elastic material, through which migrating cells can move by either pushing through open matrix pores, or by actively digesting collagen fibers. The influence of matrix mechanical properties on cell behavior is well studied. Less attention has been focused on hydraulic properties of extracellular matrices, and how hydrodynamic flows in these porous hydrogels are influenced by matrix composition and architecture. Here we study the response of collagen hydrogels using rapid changes in the hydraulic pressure within a microfluidic device, and analyze the data using a poroelastic theory. Major poroelastic parameters can be obtained in a single experiment. Results show that depending on the density, porosity, and the degree of geometric confinement, moving micron-sized objects such as cells can experience substantially increased hydraulic resistance (by as much as 106 times) when compared to 2D environments. Therefore, in addition to properties such as mechanical stiffness, the fluidic environment of the cell is also likely to impact cell behavior.

Spatial Regulation of Valve Interstitial Cell Phenotypes within Three-Dimensional Micropatterned Hydrogels

Calcific aortic valve disease (CAVD) is the third leading cause of cardiovascular disease. CAVD exhibits progressive disruption of the normally highly organized and aligned extracellular matrix (ECM) structure within the valve leaflets simultaneously with myofibroblastic and/or osteogenic differentiation of indigenous endogenous valve interstitial cells (VIC). It is unclear how the alignment of VIC within their 3D microenvironment drives VIC phenotype or how alignment affects cellular responses to biochemical cues in physiological or pathological conditions. In this study, we implement a photolithographic technique to control the alignment and elongation of both normal and diseased human aortic VIC (HAVIC) within microengineered 3D hydrogels consisting of methacrylated hyaluronic acid and methacrylated gelatin. Stripe micropatterning created distinct alignment of HAVIC within a 3D culture system, which promoted spreading and enhanced their activation and osteogenic differentiation in pro-osteogenic conditions. HAVIC from a patient with CAVD exhibited greater susceptibility to myofibroblastic and osteogenic differentiation in culture. The roles of conjugated basic fibroblastic growth factor (bFGF) and RhoA/ROCK pathway in regulating HAVIC phenotypes were also investigated in the presence of aligned microtopography. The addition of bFGF was preventative to osteogenic differentiation for healthy HAVIC; however, it promoted osteogenic differentiation in diseased HAVIC. Inhibition of the ROCK pathway only decreased osteogenic differentiation for diseased HAVIC in the aligned formation. Collectively, these results improve our knowledge of the effects that VIC alignment has on VIC phenotypes and valve disease progression. The cell culture platform also enables a better understanding of the interplay between topography, biochemical cues, and VIC differentiation and provides information useful for directing differentiation as well as valve tissue regeneration.

Comparing the Role of Mechanical Forces in Vascular and Valvular Calcification Progression

Calcification is a prevalent disease in most fully developed countries and is predominantly observed in heart valves and nearby vasculature. Calcification of either tissue leads to deterioration and, ultimately, failure causing poor quality of life and decreased overall life expectancy in patients. In valves, calcification presents as Calcific Aortic Valve Disease (CAVD), in which the aortic valve becomes stenotic when calcific nodules form within the leaflets. The initiation and progression of these calcific nodules is strongly influenced by the varied mechanical forces on the valve. In turn, the addition of calcific nodules creates localized disturbances in the tissue biomechanics, which affects extracellular matrix (ECM) production and cellular activation. In vasculature, atherosclerosis is the most common occurrence of calcification. Atherosclerosis exhibits as calcific plaque formation that forms in juxtaposition to areas of low blood shear stresses. Research in these two manifestations of calcification remain separated, although many similarities persist. Both diseases show that the endothelial layer and its regulation of nitric oxide is crucial to calcification progression. Further, there are similarities between vascular smooth muscle cells and valvular interstitial cells in terms of their roles in ECM overproduction. This review summarizes valvular and vascular tissue in terms of their basic anatomy, their cellular and ECM components and mechanical forces. Calcification is then examined in both tissues in terms of disease prediction, progression, and treatment. Highlighting the similarities and differences between these areas will help target further research toward disease treatment.

Cell Sources for Tissue Engineering Strategies to Treat Calcific Valve Disease

Cardiovascular calcification is an independent risk factor and an established predictor of adverse cardiovascular events. Despite concomitant factors leading to atherosclerosis and heart valve disease (VHD), the latter has been identified as an independent pathological entity. Calcific aortic valve stenosis is the most common form of VDH resulting of either congenital malformations or senile “degeneration.” About 2% of the population over 65 years is affected by aortic valve stenosis which represents a major cause of morbidity and mortality in the elderly. A multifactorial, complex and active heterotopic bone-like formation process, including extracellular matrix remodeling, osteogenesis and angiogenesis, drives heart valve “degeneration” and calcification, finally causing left ventricle outflow obstruction. Surgical heart valve replacement is the current therapeutic option for those patients diagnosed with severe VHD representing more than 20% of all cardiac surgeries nowadays. Tissue Engineering of Heart Valves (TEHV) is emerging as a valuable alternative for definitive treatment of VHD and promises to overcome either the chronic oral anticoagulation or the time-dependent deterioration and reintervention of current mechanical or biological prosthesis, respectively. Among the plethora of approaches and stablished techniques for TEHV, utilization of different cell sources may confer of additional properties, desirable and not, which need to be considered before moving from the bench to the bedside. This review aims to provide a critical appraisal of current knowledge about calcific VHD and to discuss the pros and cons of the main cell sources tested in studies addressing in vitro TEHV.

The Genetic Regulation of Aortic Valve Development and Calcific Disease

Heart valves are dynamic, highly organized structures required for unidirectional blood flow through the heart. Over an average lifetime, the valve leaflets or cusps open and close over a billion times, however in over 5 million Americans, leaflet function fails due to biomechanical insufficiency in response to wear-and-tear or pathological stimulus. Calcific aortic valve disease (CAVD) is the most common valve pathology and leads to stiffening of the cusp and narrowing of the aortic orifice leading to stenosis and insufficiency. At the cellular level, CAVD is characterized by valve endothelial cell dysfunction and osteoblast-like differentiation of valve interstitial cells. These processes are associated with dysregulation of several molecular pathways important for valve development including Notch, Sox9, Tgfβ, Bmp, Wnt, as well as additional epigenetic regulators. In this review, we discuss the multifactorial mechanisms that contribute to CAVD pathogenesis and the potential of targeting these for the development of novel, alternative therapeutics beyond surgical intervention.

The Extracellular Matrix of Ovarian Cortical Inclusion Cysts Modulates Invasion of Fallopian Tube Epithelial Cells

A growing body of research supports the idea that the fallopian tube epithelium (FTE) is the precursor for most high-grade serous ovarian canacers (HGSOC) but that the ovary plays a critical role in tumor metastasis. Cortical inclusion cysts (CICs) in the ovarian cortex have been hypothesized to create a niche environment that plays a role in HGSOC progression. Through histological analysis of pathology samples from human ovaries, we determined that collagen I and III were elevated near CICs and that the collagen fibers in this dense region were oriented parallel to the cyst boundary. Using this information from human samples as design parameters, we engineered an in vitro model that recreates the size, shape, and extracellular matrix (ECM) properties of CICs. We found that FTE cells within our model underwent robust invasion that was responsive to stimulation with follicular fluid, while ovarian surface epithelial (OSE) cells, the native cells of the ovary, were not invasive. We provide experimental evidence to support a role of the extracellular matrix in modulating FTE cell invasion, as decreased collagen I concentration or the addition of collagen III to the matrix surrounding FTE cells increased FTE cell invasion. Taken together, we show that an in vitro model of CICs informed by the analysis of human tissue can act as an important tool for understanding FTE cell interactions with their environment.

Engineering Approaches to Study Cellular Decision Making

In their native environment, cells are immersed in a complex milieu of biochemical and biophysical cues. These cues may include growth factors, the extracellular matrix, cell-cell contacts, stiffness, and topography, and they are responsible for regulating cellular behaviors such as adhesion, proliferation, migration, apoptosis, and differentiation. The decision-making process used to convert these extracellular inputs into actions is highly complex and sensitive to changes both in the type of individual cue (e.g., growth factor dose/level, timing) and in how these individual cues are combined (e.g., homotypic/heterotypic combinations). In this review, we highlight recent advances in the development of engineering-based approaches to study the cellular decision-making process. Specifically, we discuss the use of biomaterial platforms that enable controlled and tailored delivery of individual and combined cues, as well as the application of computational modeling to analyses of the complex cellular decision-making networks.

Imaging the Cardiac Extracellular Matrix

Cardiovascular disease is the global leading cause of death. One route to address this problem is using biomedical imaging to measure the molecules and structures that surround cardiac cells. This cellular microenvironment, known as the cardiac extracellular matrix, changes in composition and organization during most cardiac diseases and in response to many cardiac treatments. Measuring these changes with biomedical imaging can aid in understanding, diagnosing, and treating heart disease. This chapter supports those efforts by reviewing representative methods for imaging the cardiac extracellular matrix. It first describes the major biological targets of ECM imaging, including the primary imaging target of fibrillar collagen. Then it discusses the imaging methods, describing their current capabilities and limitations. It categorizes the imaging methods into two main categories: organ-scale noninvasive methods and cellular-scale invasive methods. Noninvasive methods can be used on patients, but only a few are clinically available, and others require further development to be used in the clinic. Invasive methods are the most established and can measure a variety of properties, but they cannot be used on live patients. Finally, the chapter concludes with a perspective on future directions and applications of biomedical imaging technologies.

Label-free imaging of atherosclerotic plaques using third-harmonic generation microscopy

Multiphoton microscopy using laser sources in the mid-infrared range (MIR, 1,300 nm and 1,700 nm) was used to image atherosclerotic plaques from murine and human samples. Third harmonic generation (THG) from atherosclerotic plaques revealed morphological details of cellular and extracellular lipid deposits. Simultaneous nonlinear optical signals from the same laser source, including second harmonic generation and endogenous fluorescence, resulted in label-free images of various layers within the diseased vessel wall. The THG signal adds an endogenous contrast mechanism with a practical degree of specificity for atherosclerotic plaques that complements current nonlinear optical methods for the investigation of cardiovascular disease. Our use of whole-mount tissue and backward scattered epi-detection suggests THG could potentially be used in the future as a clinical tool.