Regional structure-function relationships in mouse aortic valve tissue

Functional mechanical behavior of the murine pulmonary heart valve

Genetically modified mouse models provide a versatile and efficient platform to extend our understanding of the underlying disease processes and evaluate potential treatments for congenital heart valve diseases. However, applications have been limited to the gene and molecular levels due to the small size of murine heart valves, which prohibits the use of standard mechanical evaluation and in vivo imaging methods. We have developed an integrated imaging/computational mechanics approach to evaluate, for the first time, the functional mechanical behavior of the murine pulmonary heart valve (mPV). We utilized extant mPV high resolution µCT images of 1-year-old healthy C57BL/6J mice, with mPVs loaded to 0, 10, 20 or 30 mmHg then chemically fixed to preserve their shape. Individual mPV leaflets and annular boundaries were segmented and key geometric quantities of interest defined and quantified. The resulting observed inter-valve variations were small and consistent at each TVP level. This allowed us to develop a high fidelity NURBS-based geometric model. From the resultant individual mPV geometries, we developed a mPV shape-evolving geometric model (SEGM) that accurately represented mPV shape changes as a continuous function of transvalvular pressure. The SEGM was then integrated into an isogeometric finite element based inverse model that estimated the individual leaflet and regional mPV mechanical behaviors. We demonstrated that the mPV leaflet mechanical behaviors were highly anisotropic and nonlinear, with substantial leaflet and regional variations. We also observed the presence of strong axial mechanical coupling, suggesting the important role of the underlying collagen fiber architecture in the mPV. When compared to larger mammalian species, the mPV exhibited substantially different mechanical behaviors. Thus, while qualitatively similar, the mPV exhibited important functional differences that will need to accounted for in murine heart valve studies. The results of this novel study will allow detailed murine tissue and organ level investigations of semi-lunar heart valve diseases.

Engineering Heart Valve Interfaces Using Melt Electrowriting: Biomimetic Design Strategies from Multi-Modal Imaging

Interfaces within biological tissues not only connect different regions but also contribute to the overall functionality of the tissue. This is especially true in the case of the aortic heart valve. Here, melt electrowriting (MEW) is used to engineer complex, user-defined, interfaces for heart valve scaffolds. First, a multi-modal imaging investigation into the interfacial regions of the valve reveals differences in collagen orientation, density, and recruitment in previously unexplored regions including the commissure and inter-leaflet triangle. Overlapping, suturing, and continuous printing methods for interfacing MEW scaffolds are then investigated for their morphological, tensile, and flexural properties, demonstrating the superior performance of continuous interfaces. G-codes for MEW scaffolds with complex interfaces are designed and generated using a novel software and graphical user interface. Finally, a singular MEW scaffold for the interfacial region of the aortic heart valve is presented incorporating continuous interfaces, gradient porosities, variable layer numbers across regions, and tailored fiber orientations inspired by the collagen distribution and orientation from the multi-modal imaging study. The scaffold exhibits similar yield strain, hysteresis, and relaxation behavior to porcine heart valves. This work demonstrates the ability of a bioinspired approach for MEW scaffold design to address the functional complexity of biological tissues.

Pigmentation Affects Elastic Fiber Patterning and Biomechanical Behavior of the Murine Aortic Valve

The aortic valve (AoV) maintains unidirectional blood distribution from the left ventricle of the heart to the aorta for systemic circulation. The AoV leaflets rely on a precise extracellular matrix microarchitecture of collagen, elastin, and proteoglycans for appropriate biomechanical performance. We have previously demonstrated a relationship between the presence of pigment in the mouse AoV with elastic fiber patterning using multiphoton imaging. Here, we extended those findings using wholemount confocal microscopy revealing that elastic fibers were diminished in the AoV of hypopigmented mice (Kit^Wv and albino) and were disorganized in the AoV of K5-Edn3 transgenic hyperpigmented mice when compared to wild type C57BL/6J mice. We further used atomic force microscopy to measure stiffness differences in the wholemount AoV leaflets of mice with different levels of pigmentation. We show that AoV leaflets of K5-Edn3 had overall higher stiffness (4.42 ± 0.35 kPa) when compared to those from Kit^Wv (2.22 ± 0.21 kPa), albino (2.45 ± 0.16 kPa), and C57BL/6J (3.0 ± 0.16 kPa) mice. Despite the striking elastic fiber phenotype and noted stiffness differences, adult mutant mice were found to have no overt cardiac differences as measured by echocardiography. Our results indicate that pigmentation, but not melanocytes, is required for proper elastic fiber organization in the mouse AoV and dictates its biomechanical properties.

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.

A Method to Quantify Tensile Biaxial Properties of Mouse Aortic Valve Leaflets

Understanding aortic valve (AV) mechanics is crucial in elucidating both the mechanisms that drive the manifestation of valvular diseases as well as the development of treatment modalities that target these processes. Genetically modified mouse models have become the gold standard in assessing biological mechanistic influences of AV development and disease. However, very little is known about mouse aortic valve leaflet (MAVL) tensile properties due to their microscopic size (∼500 μm long and 45 μm thick) and the lack of proper mechanical testing modalities to assess uniaxial and biaxial tensile properties of the tissue. We developed a method in which the biaxial tensile properties of MAVL tissues can be assessed by adhering the tissues to a silicone rubber membrane utilizing dopamine as an adhesive. Applying equiaxial tensile loads on the tissue-membrane composite and tracking the engineering strains on the surface of the tissue resulted in the characteristic orthotropic response of AV tissues seen in human and porcine tissues. Our data suggest that the circumferential direction is stiffer than the radial direction (n = 6, P = 0.0006) in MAVL tissues. This method can be implemented in future studies involving longitudinal mechanical stimulation of genetically modified MAVL tissues bridging the gap between cellular biological mechanisms and valve mechanics in popular mouse models of valve disease.

Loss of flow responsive Tie1 results in Impaired Aortic valve remodeling

The mechanisms regulating endothelial cell response to hemodynamic forces required for heart valve development, especially valve remodeling, remain elusive. Tie1, an endothelial specific receptor tyrosine kinase, is up-regulated by oscillating shear stress and is required for lymphatic valve development. In this study, we demonstrate that valvular endothelial Tie1 is differentially expressed in a dynamic pattern predicted by disturbed flow during valve remodeling. Following valvular endocardial specific deletion of Tie1 in mice, we observed enlarged aortic valve leaflets, decreased valve stiffness and valvular insufficiency. Valve abnormalities were only detected in late gestation and early postnatal mutant animals and worsened with age. The mutant mice developed perturbed extracellular matrix (ECM) deposition and remodeling characterized by increased glycosaminoglycan and decreased collagen content, as well as increased valve interstitial cell expression of Sox9, a transcription factor essential for normal ECM maturation during heart valve development. This study provides the first evidence that Tie1 is involved in modulation of late valve remodeling and suggests that an important Tie1-Sox9 signaling axis exists through which disturbed flows are converted by endocardial cells to paracrine Sox9 signals to modulate normal matrix remodeling of the aortic valve.

Conditional deletion of RB1 in the Tie2 lineage leads to aortic valve regurgitation

Objective

Aortic valve disease is a complex process characterized by valve interstitial cell activation, disruption of the extracellular matrix culminating in valve mineralization occurring over many years. We explored the function of the retinoblastoma protein (pRb) in aortic valve disease, given its critical role in mesenchymal cell differentiation including bone development and mineralization.

Approach and results

We generated a mouse model of conditional pRb knockout (cKO) in the aortic valve regulated by Tie2-Cre-mediated excision of floxed RB1 alleles. Aged pRb cKO animals showed significantly more aortic valve regurgitation by echocardiography compared to pRb het control animals. The pRb cKO aortic valves had increased leaflet thickness without increased cellular proliferation. Histologic studies demonstrated intense α-SMA expression in pRb cKO leaflets associated with disorganized extracellular matrix and increased leaflet stiffness. The pRb cKO mice also showed increased circulating cytokine levels.

Conclusions

Our studies demonstrate that pRb loss in the Tie2-lineage that includes aortic valve interstitial cells is sufficient to cause age-dependent aortic valve dysfunction.

Dysregulation of hyaluronan homeostasis during aortic valve disease

Aortic valve disease (AVD) is one of the leading causes of cardiovascular mortality. Abnormal expression of hyaluronan (HA) and its synthesizing/degrading enzymes have been observed during latent AVD however, the mechanism of impaired HA homeostasis prior to and after the onset of AVD remains unexplored. Transforming growth factor beta (TGFβ) pathway defects and biomechanical dysfunction are hallmarks of AVD, however their association with altered HA regulation is understudied. Expression of HA homeostatic markers was evaluated in diseased human aortic valves and TGFβ1-cultured porcine aortic valve tissues using histology, immunohistochemistry and Western blotting. Further, porcine valve interstitial cell cultures were stretched (using Flexcell) and simultaneously treated with exogenous TGFβ1±inhibitors for activated Smad2/3 (SB431542) and ERK1/2 (U0126) pathways, and differential HA regulation was assessed using qRT-PCR. Pathological heavy chain HA together with abnormal regional expression of the enzymes HAS2, HYAL1, KIAA1199, TSG6 and IαI was demonstrated in calcified valve tissues identifying the collapse of HA homeostatic machinery during human AVD. Heightened TSG6 activity likely preceded the end-stage of disease, with the existence of a transitional, pre-calcific phase characterized by HA dysregulation. TGFβ1 elicited a fibrotic remodeling response in porcine aortic valves similar to human disease pathology, with increased collagen and HYAL to HAS ratio, and site-specific abnormalities in the expression of CD44 and RHAMM receptors. Further in these porcine valves, expression of HAS2 and HYAL1 was found to be differentially regulated by the Smad2/3 and ERK1/2 pathways, and CD44 expression was highly responsive to biomechanical strain. Leveraging the regulatory pathways that control both HA maintenance in normal valves and early postnatal dysregulation of HA homeostasis during disease may identify new mechanistic insight into AVD pathogenesis.

Proteomic Alterations Associated with Biomechanical Dysfunction are Early Processes in the Emilin1 Deficient Mouse Model of Aortic Valve Disease

Aortic valve (AV) disease involves stiffening of the AV cusp with progression characterized by inflammation, fibrosis, and calcification. Here, we examine the relationship between biomechanical valve function and proteomic changes before and after the development of AV pathology in the Emilin1-/- mouse model of latent AV disease. Biomechanical studies were performed to quantify tissue stiffness at the macro (micropipette) and micro (atomic force microscopy (AFM)) levels. Micropipette studies showed that the Emilin1-/- AV annulus and cusp regions demonstrated increased stiffness only after the onset of AV disease. AFM studies showed that the Emilin1-/- cusp stiffens before the onset of AV disease and worsens with the onset of disease. Proteomes from AV cusps were investigated to identify protein functions, pathways, and interaction network alterations that occur with age- and genotype-related valve stiffening. Protein alterations due to Emilin1 deficiency, including changes in pathways and functions, preceded biomechanical aberrations, resulting in marked depletion of extracellular matrix (ECM) proteins interacting with TGFB1, including latent transforming growth factor beta 3 (LTBP3), fibulin 5 (FBLN5), and cartilage intermediate layer protein 1 (CILP1). This study identifies proteomic dysregulation is associated with biomechanical dysfunction as early pathogenic processes in the Emilin1-/- model of AV disease.

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

In aortic valves, biaxial cyclic stretch is known to modulate cell differentiation, extracellular matrix (ECM) synthesis and organization. We designed a novel bioreactor that can apply independent and precise stretch along radial and circumferential directions in a tissue culture environment. While this bioreactor can be used for either native or engineered tissues, this study determined matrix remodeling and strain distribution of aortic cusps after culturing under biaxial stretch for 14 days. The contents of collagen and glycosaminoglycans were determined using standard biochemical assays and compared with fresh controls. Strain fields in static cusps were more uniform than those in stretched cusps, which indicated degradation of the ECM fibers. The glycosaminoglycan content was significantly elevated in the static control as compared to fresh or stretched cusps, but no difference was observed in collagen content among the groups. The strain profile of freshly isolated fibrosa vs. ventricularis and left, right, and noncoronary cusps were also determined by Digital Image Correlation technique. Distinct strain patterns were observed under stretch on fibrosa and ventricularis sides and among the three cusps. This work highlights the critical role of the anisotropic ECM structure for proper functions of native aortic valves and the beneficial effects of biaxial stretch for maintenance of the native ECM structure.

Cancer-Associated Fibroblasts Induce a Collagen Cross-link Switch in Tumor Stroma

Intratumoral collagen cross-links heighten stromal stiffness and stimulate tumor cell invasion, but it is unclear how collagen cross-linking is regulated in epithelial tumors. To address this question, we used Kras(LA1) mice, which develop lung adenocarcinomas from somatic activation of a Kras(G12D) allele. The lung tumors in Kras(LA1) mice were highly fibrotic and contained cancer-associated fibroblasts (CAF) that produced collagen and generated stiffness in collagen gels. In xenograft tumors generated by injection of wild-type mice with lung adenocarcinoma cells alone or in combination with CAFs, the total concentration of collagen cross-links was the same in tumors generated with or without CAFs, but coinjected tumors had higher hydroxylysine aldehyde-derived collagen cross-links (HLCC) and lower lysine-aldehyde-derived collagen cross-links (LCCs). Therefore, we postulated that an LCC-to-HLCC switch induced by CAFs promotes the migratory and invasive properties of lung adenocarcinoma cells. To test this hypothesis, we created coculture models in which CAFs are positioned interstitially or peripherally in tumor cell aggregates, mimicking distinct spatial orientations of CAFs in human lung cancer. In both contexts, CAFs enhanced the invasive properties of tumor cells in three-dimensional (3D) collagen gels. Tumor cell aggregates that attached to CAF networks on a Matrigel surface dissociated and migrated on the networks. Lysyl hydroxylase 2 (PLOD2/LH2), which drives HLCC formation, was expressed in CAFs, and LH2 depletion abrogated the ability of CAFs to promote tumor cell invasion and migration.

IMPLICATIONS

CAFs induce a collagen cross-link switch in tumor stroma to influence the invasive properties of tumor cells.

Development of a heart valve model surface for optimization of surface modifications

Current bioprosthetic valve replacements (BPVs) are susceptible to myriad complications, including calcification and thrombosis; however, recent research has explored surface modifications to encourage re-endothelialization of the tissue, preventing unwanted blood-tissue interactions. A bioprosthetic valve surface model (BVSM) was developed to facilitate rapid in vitro optimization of surface modification techniques for BPVs. The BVSM was manufactured by photopolymerization of PEGDA and collagen type I and subsequent addition of amine-rich peptide to provide reactive sites for surface modification. This BVSM mimics surface mechanical properties of bioprosthetic valve tissue, as measured by micropipette aspiration. The BVSM successfully mimics the latent toxic effects of glutaraldehyde fixation, as shown through MTT assay results. Amine content, assessed by XPS, was shown to be significantly lower in the BVSM than unfixed tissue. However, incubation of the surface with amine-reactive NHS-PEG-Cy5 revealed even coverage of the BVSM surface, suggesting that there exists sufficient surface reactive groups to anchor surface modifications, and that translation of the modification process to tissue will yield more complete modification of the BPV surface. These results indicate successful construction of a BVSM that mimics essential properties of bioprosthetic valve tissue and its usefulness for rapid in vitro optimization of surface modification methods for endothelialization.

STATEMENT OF SIGNIFICANCE

Current bioprosthetic valve replacements are susceptible to many complications, including calcification and thrombosis; however, recent research has explored surface modifications to encourage the integration of the replacement with the native tissue, which would prevent unwanted blood-tissue interactions. However, methods to analyze and optimize such modifications are limited by the complex surface topography, individual variability, and opacity of native tissue. Thus, we have developed a novel bioprosthetic valve tissue model (BVM) which mimics the important features of the bioprosthetic valve tissue and serves as a platform for rapid optimization and testing of surface modification strategies for tissue valves. Thus, the BVM will provide a needed platform to support rapid improvement of clinically available cardiovascular implants.

Heterogeneity of Mitral Leaflet Matrix Composition and Turnover Correlates with Regional Leaflet Strain

To determine how extracellular matrix and contractile valvular cells contribute to the heterogeneous motion and strain across the mitral valve (MV) during the cardiac cycle, regional MV material properties, matrix composition, matrix turnover, and cell phenotype were related to regional leaflet strain. Radiopaque markers were implanted into 14 sheep to delineate the septal (SEPT), lateral (LAT), and anterior and posterior commissural leaflets (ANT-C, POST-C). Videofluoroscopy imaging was used to calculate radial and circumferential strains. Mechanical properties were assessed using uniaxial tensile testing and micropipette aspiration. Matrix composition and cell phenotypes were immunohistochemically evaluated within each leaflet region [basal leaflet (BL), mid-leaflet (ML), and free edge]. SEPT-BL segments were stiffer and stronger than other valve tissues, while LAT segments demonstrated more extensibility and strain. Collagens I and III in SEPT were greater than in LAT, although LAT showed greater collagen turnover [matrix metalloprotease (MMP)-13, lysyl oxidase] and cell activation [smooth muscle alpha-actin (SMaA), and non-muscle myosin (NMM)]. MMP13, NMM, and SMaA were strongly correlated with each other, as well as with radial and circumferential strains in both SEPT and LAT. SMaA and MMP13 in POST-C ML was greater than ANT-C, corresponding to greater radial strains in POST-C. This work directly relates leaflet strain, material properties, and matrix turnover, and suggests a role for myofibroblasts in the heterogeneity of leaflet composition and strain. New approaches to MV repair techniques and ring design should preserve this normal coupling between leaflet composition and motion.

Cellular and Extracellular Matrix Basis for Heterogeneity in Mitral Annular Contraction

PURPOSE

Regional heterogeneity in mitral annular contraction, which is generally ascribed to the fibrous vs. muscular annular composition, ensures proper leaflet motion and timing of coaptation. It is unknown whether the fibroblast-like cells in the annulus modulate this heterogeneity, even though valvular interstitial cells (VICs) can be mechanically "activated."

METHODS

Fourteen sheep underwent implantation of radiopaque markers around the mitral annulus defining four segments: septal (SEPT), lateral (LAT), and anterior (ANT-C) and posterior (POST-C) commissures. Segmental annular contraction was calculated using biplane videofluoroscopy. Immunohistochemistry of annular cross sections assessed regional matrix content, matrix turnover, and cell phenotype. Micropipette aspiration measured the Young's modulus of the leaflets adjacent to the myocardial border.

RESULTS

Whereas SEPT contained more collagen I and III, LAT demonstrated more collagen and elastin turnover as shown by greater decorin, lysyl oxidase, and matrix metalloprotease (MMP)-13 and smooth muscle alpha-actin (SMaA). This greater matrix turnover paralleled greater annular contraction in LAT vs. SEPT (22.5% vs. 4.1%). Similarly, POST-C had more SMaA and MMP13 than ANT-C, consistent with greater annular contraction in POST-C (18.8% vs. 11.1%). Interestingly, POST-C had the greatest effective modulus, significantly higher than LAT.

CONCLUSIONS

These data suggest that matrix turnover by activated VICs relates to annular motion heterogeneity, maintains steady-state mechanical properties in the annulus, and could be a therapeutic target when annular motion is impaired. Conversely, alterations in this heterogeneous annular contraction, whether through disease or secondary to ring annuloplasty, could disrupt this normal pattern of cell-mediated matrix remodeling and further adversely impact mitral valve function.

Morphologic studies of cell endogenous repopulation in decellularized aortic and pulmonary homografts implanted in sheep

PURPOSES

The rationale of this study was to assess morphologically the effects of implantation of decellularized aortic and pulmonary homografts into growing sheep, with the objective to establish type and extent of cell repopulation and propensity to dystrophic calcification over a prolonged period of time.

METHODS

Pulmonary and aortic homografts were obtained from healthy euthanized juvenile sheep (35-45kg). Complete decellularization was accomplished in 0.5% sodium deoxycholate and 0.5% sodium dodecylsulfate for 24h. Twelve homografts from 11 animals were studied as follows: Gross, X-ray, histology, immunohistochemistry, morphometry, transmission electron microscopy and calcium content spectroscopy investigations were carried out.

RESULTS

Decellularization appeared complete in unimplanted homografts. The extracellular matrix was intact. Explanted homografts showed soft, pliable cusps without gross calcium deposits and tears; calcium content showed slight difference between aortic and pulmonary cusps (5.505±2.04 vs. 2.77±1.06mg/g dry weight, P=.04). Microscopic calcifications were observed in two aortic homografts on smooth muscle cells of repopulated homograft wall and on valvular interstitial cells, respectively. Inflammatory infiltrates were never seen. Cell repopulation occurred in homograft wall with actin smooth muscle and vimentin positive cells in media lamellar units (cell density per millimeter squared, 885.4±424.38 in native vs. 172.64±160.33 in implanted homograft, P<.01) as well as in cusps (cell density per millimeter squared, 495.96±63.92 in native vs. 184.66±140.74 in implanted homograft, P<.01). The percentage area of recellularization was 71.27±3.03 in the homograft wall and 22.16±3.06 in the cusps. Thickness of pulmonary explanted homograft wall and cusps was 900.68±321.52μm vs. 994.36±135.92μm and 204.75±66.64μm vs. 231.04±105.94, respectively (P=NS), whereas in aortic homograft wall and cusps it was 1358.604±423.79μm vs. 2065.32±431.46μm, P=.016, and 248.01±93.95μm vs. 390.30±104.81μm, P=.03, respectively. The endothelial lining was restored.

CONCLUSION

Endogenous cell repopulation in decellularized homografts occurs and persists following implantation, at both wall and cusp level, without evidence of immune reaction. Even in the long term, the cusps exhibit no structural deterioration and negligible calcification.

Cardiac valve cells and their microenvironment--insights from in vitro studies

During every heartbeat, cardiac valves open and close coordinately to control the unidirectional flow of blood. In this dynamically challenging environment, resident valve cells actively maintain homeostasis, but the signalling between cells and their microenvironment is complex. When homeostasis is disrupted and the valve opening obstructed, haemodynamic profiles can be altered and lead to impaired cardiac function. Currently, late stages of cardiac valve diseases are treated surgically, because no drug therapies exist to reverse or halt disease progression. Consequently, investigators have sought to understand the molecular and cellular mechanisms of valvular diseases using in vitro cell culture systems and biomaterial scaffolds that can mimic the extracellular microenvironment. In this Review, we describe how signals in the extracellular matrix regulate valve cell function. We propose that the cellular context is a critical factor when studying the molecular basis of valvular diseases in vitro, and one should consider how the surrounding matrix might influence cell signalling and functional outcomes in the valve. Investigators need to build a systems-level understanding of the complex signalling network involved in valve regulation, to facilitate drug target identification and promote in situ or ex vivo heart valve regeneration.

Asymmetric cell-matrix and biomechanical abnormalities in elastin insufficiency induced aortopathy

Aortopathy is characterized by vascular smooth muscle cell (VSMC) abnormalities and elastic fiber fragmentation. Elastin insufficient (Eln (+/-)) mice demonstrate latent aortopathy similar to human disease. We hypothesized that aortopathy manifests primarily in the aorto-pulmonary septal (APS) side of the thoracic aorta due to asymmetric cardiac neural crest (CNC) distribution. Anatomic (aortic root vs. ascending aorta) and molecular (APS vs. non-APS) regions of proximal aorta tissue were examined in adult and aged wild type (WT) and mutant (Eln (+/-)) mice. CNC, VSMCs, elastic fiber architecture, proteoglycan expression, morphometrics and biomechanical properties were examined using histology, 3D reconstruction, micropipette aspiration and in vivo magnetic resonance imaging (MRI). In the APS side of Eln (+/-) aorta, Sonic Hedgehog (SHH) is decreased while SM22 is increased. Elastic fiber architecture abnormalities are present in the Eln (+/-) aortic root and APS ascending aorta, and biglycan is increased in the aortic root while aggrecan is increased in the APS aorta. The Eln (+/-) ascending aorta is stiffer than the aortic root, the APS side is thicker and stiffer than the non-APS side, and significant differences in the individual aortic root sinuses are observed. Asymmetric structure-function abnormalities implicate regional CNC dysregulation in the development and progression of aortopathy.

Fibrocalcific aortic valve disease: opportunity to understand disease mechanisms using mouse models

Studies in vitro and in vivo continue to identify complex-regulated mechanisms leading to overt fibrocalcific aortic valve disease (FCAVD). Assessment of the functional impact of those processes requires careful studies of models of FCAVD in vivo. Although the genetic basis for FCAVD is unknown for most patients with FCAVD, several disease-associated genes have been identified in humans and mice. Some gene products which regulate valve development in utero also protect against fibrocalcific disease during postnatal aging. Valve calcification can occur via processes that resemble bone formation. But valve calcification can also occur by nonosteogenic mechanisms, such as formation of calcific apoptotic nodules. Anticalcific interventions might preferentially target either osteogenic or nonosteogenic calcification. Although FCAVD and atherosclerosis share several risk factors and mechanisms, there are fundamental differences between arteries and the aortic valve, with respect to disease mechanisms and responses to therapeutic interventions. Both innate and acquired immunity are likely to contribute to FCAVD. Angiogenesis is a feature of inflammation, but may also contribute independently to progression of FCAVD, possibly by actions of pericytes that are associated with new blood vessels. Several therapeutic interventions seem to be effective in attenuating the development of FCAVD in mice. Therapies which are effective early in the course of FCAVD, however, are not necessarily effective in established disease.

Age-related analysis of structural, biochemical and mechanical properties of the porcine mitral heart valve leaflets

The aim of this study was to assess structural and biochemical differences in the extracellular matrix of the fetal and adult porcine mitral heart valves in relation to their mechanical characteristics. Using tensile tests it was demonstrated that the material properties of porcine mitral heart valves progressively change with age. The collagen content of the adult heart valve, as estimated by hydroxyproline assay, increases three times as compared with fetal heart valves. Transmission electron microscopy demonstrated that the diameter of collagen fibrils increased in adult heart valves compared with fetal heart valves. The level of collagen cross-linking is lower in the fetal heart valve than the adult heart valve. The reported age differences in the material properties of fetal and adult porcine heart valves were associated with increases in collagen content, the diameter of collagen fibrils and the level of collagen cross-linking. These data lay a foundation for systematic elucidation of the structural determinants of material properties of heart valves during embryonic and postnatal valvulogenesis. They are also essential to define the desirable level of tissue maturation in heart valve tissue engineering.

Hesr2 knockout mice develop aortic valve disease with advancing age

OBJECTIVE

Acquired heart diseases, such as valve disease, are major causes of human morbidity and mortality. However, the pathological mechanisms underlying these diseases are largely unknown. Our aim is to identify the role of the hairy and enhancer of split-related (Hesr)-2 gene in the adult heart.

METHODS AND RESULTS

Echocardiography detected heart dysfunctions indicative of aortic valve anomalies, stenosis, and regurgitation, in ≈59% of >12-month-old Hesr2 knockout survivor mice. Morphological and histological analyses revealed thickened semilunar valves with increased fibrotic areas, indicating that sclerotic degeneration of valves is the main cause of aortic valve disease. The expression of osteogenic genes, such as osteopontin and sclerostin, were upregulated in the mutants, and the overexpression of sclerostin in endothelial cells resulted in thickened semilunar valves with increased fibrotic areas, similar to that seen in the Hesr2 knockout mice, suggesting that Hesr2 can regulate osteogenic gene expression in valves. Reduced left ventricular function, which may be caused by increased ventricular interstitial fibrosis, and enlarged myocardial cell size without ventricular wall thickening were found in both aortic valve stenosis/regurgitation-positive (33%) and aortic valve stenosis/regurgitation-negative (38%) subpopulations in 12-month-old survivor mice. Dilated left ventricular internal dimensions were specifically detected in the aortic valve stenosis/regurgitation-positive subpopulation, thus suggesting that the degeneration of cardiomyocytes is influenced by irregular hemodynamics.

CONCLUSIONS

These data revealed that survivor mice lacking the Hesr2 gene exhibit fibrosis in the aortic valve and ventricle in adulthood, thus suggesting that Hesr2 plays an important role in maintaining the homeostasis of the aortic valve and ventricle.

Multi-scale biomechanical remodeling in aging and genetic mutant murine mitral valve leaflets: insights into Marfan syndrome

Mitral valve degeneration is a key component of the pathophysiology of Marfan syndrome. The biomechanical consequences of aging and genetic mutation in mitral valves are poorly understood because of limited tools to study this in mouse models. Our aim was to determine the global biomechanical and local cell-matrix deformation relationships in the aging and Marfan related Fbn1 mutated murine mitral valve. To conduct this investigation, a novel stretching apparatus and gripping method was implemented to directly quantify both global tissue biomechanics and local cellular deformation and matrix fiber realignment in murine mitral valves. Excised mitral valve leaflets from wild-type and Fbn1 mutant mice from 2 weeks to 10 months in age were tested in circumferential orientation under continuous laser optical imaging. Mouse mitral valves stiffen with age, correlating with increases in collagen fraction and matrix fiber alignment. Fbn1 mutation resulted in significantly more compliant valves (modulus 1.34 ± 0.12 vs. 2.51 ± 0.31 MPa, respectively, P<.01) at 4 months, corresponding with an increase in proportion of GAGs and decrease in elastin fraction. Local cellular deformation and fiber alignment change linearly with global tissue stretch, and these slopes become more extreme with aging. In comparison, Fbn1 mutated valves have decoupled cellular deformation and fiber alignment with tissue stretch. Taken together, quantitative understanding of multi-scale murine planar tissue biomechanics is essential for establishing consequences of aging and genetic mutations. Decoupling of local cell-matrix deformation kinematics with global tissue stretch may be an important mechanism of normal and pathological biomechanical remodeling in valves.

A novel technique for quantifying mouse heart valve leaflet stiffness with atomic force microscopy

BACKGROUND AND AIM OF THE STUDY

The use of genetically altered small animal models is a powerful strategy for elucidating the mechanisms of heart valve disease. However, while the ability to manipulate genes in rodent models is well established, there remains a significant obstacle in determining the functional mechanical properties of the genetically mutated leaflets. Hence, a feasibility study was conducted using micromechanical analysis via atomic force microscopy (AFM) to determine the stiffness of mouse heart valve leaflets in the context of age and disease states.

METHODS

A novel AFM imaging technique for the quantification of heart valve leaflet stiffness was performed on cryosectioned tissues. Heart valve leaflet samples were obtained from wild-type mice (2 and 17 months old) and genetically altered mice (10-month-old Notch1 heterozygous and 20-month-old ApoE homozygous). Histology was performed on adjacent sections to determine the extracellular matrix characteristics of the scanned areas.

RESULTS

The 17-month-old wild-type, 10-month-old Notch1, and 20-month-old ApoE aortic valve leaflets were all significantly stiffer than leaflets from 2-month-old wild-type mice. Notch1 leaflets were significantly stiffer than all other leaflets examined, indicating that the Notch1 heterozygous mutation may alter leaflet stiffness, both earlier and to a greater degree than the homozygous ApoE mutation. However, these conclusions must be considered only preliminary due to the small sample size used in this proof-of-concept study.

CONCLUSION

It is believed that this technique can provide a powerful end-point analysis for determining the mechanical properties of heart valve leaflets from genetically altered mice. Further, the technique is complementary to standard histological processing, and does not require excess tissue for mechanical testing. In this proof-of-concept study, AFM was shown to be a powerful tool for investigators of heart valve disease who develop genetically altered animals for their studies.

Maladaptive matrix remodeling and regional biomechanical dysfunction in a mouse model of aortic valve disease

Aortic valve disease (AVD) occurs in 2.5% of the general population and often requires surgical intervention. Aortic valve malformation (AVM) underlies the majority of cases, suggesting a developmental etiology. Elastin haploinsufficiency results in complex cardiovascular problems, and 20-45% of patients have AVM and/or AVD. Elastin insufficient (Eln+/-) mice demonstrate AVM and latent AVD due to abnormalities in the valve annulus region. The objective of this study was to examine extracellular matrix (ECM) remodeling and biomechanical properties in regional aortic valve tissue and determine the impact of early AVM on late AVD in the Eln+/- mouse model. Aortic valve ECM composition and remodeling from juvenile, adult, and aged stages were evaluated in Eln+/- mice using histology, ELISA, immunohistochemistry and gelatin zymography. Aortic valve tissue biomechanical properties were determined using micropipette aspiration. Cartilage-like nodules were demonstrated within the valve annulus region at all stages identifying a developmental abnormality preceding AVD. Interestingly, maladaptive ECM remodeling was observed in early AVM without AVD and worsened with late AVD, as evidenced by increased MMP-2 and MMP-9 expression and activity, as well as abnormalities in ADAMTS-mediated versican processing. Cleaved versican was increased in the valve annulus region of aged Eln+/- mice, and this abnormality correlated temporally with adverse alterations in valve tissue biomechanical properties and the manifestation of AVD. These findings identify maladaptive ECM remodeling in functional AVM as an early disease process with a progressive natural history, similar to that seen in human AVD, emphasizing the importance of the annulus region in pathogenesis. Combining molecular and engineering approaches provides complementary mechanistic insights that may be informative in the search for new therapeutic targets and durable valve bioprostheses.

Quantification of embryonic atrioventricular valve biomechanics during morphogenesis

Tissue assembly in the developing embryo is a rapid and complex process. While much research has focused on genetic regulatory machinery, understanding tissue level changes such as biomechanical remodeling remains a challenging experimental enigma. In the particular case of embryonic atrioventricular valves, micro-scale, amorphous cushions rapidly remodel into fibrous leaflets while simultaneously interacting with a demanding mechanical environment. In this study we employ two microscale mechanical measurement systems in conjunction with finite element analysis to quantify valve stiffening during valvulogenesis. The pipette aspiration technique is compared to a uniaxial load deformation, and the analytic expression for a uniaxially loaded bar is used to estimate the nonlinear material parameters of the experimental data. Effective modulus and strain energy density are analyzed as potential metrics for comparing mechanical stiffness. Avian atrioventricular valves from globular Hamburger-Hamilton stages HH25-HH34 were tested via the pipette method, while the planar HH36 leaflets were tested using the deformable post technique. Strain energy density between HH25 and HH34 septal leaflets increased 4.6±1.8 fold (±SD). The strain energy density of the HH36 septal leaflet was four orders of magnitude greater than the HH34 pipette result. Our results establish morphological thresholds for employing the micropipette aspiration and deformable post techniques for measuring uniaxial mechanical properties of embryonic tissues. Quantitative biomechanical analysis is an important and underserved complement to molecular and genetic experimentation of embryonic morphogenesis.

Animal models of calcific aortic valve disease

Calcific aortic valve disease (CAVD), once thought to be a degenerative disease, is now recognized to be an active pathobiological process, with chronic inflammation emerging as a predominant, and possibly driving, factor. However, many details of the pathobiological mechanisms of CAVD remain to be described, and new approaches to treat CAVD need to be identified. Animal models are emerging as vital tools to this end, facilitated by the advent of new models and improved understanding of the utility of existing models. In this paper, we summarize and critically appraise current small and large animal models of CAVD, discuss the utility of animal models for priority CAVD research areas, and provide recommendations for future animal model studies of CAVD.

Cell–Matrix Interactions in the Pathobiology of Calcific Aortic Valve Disease

The hallmarks of calcific aortic valve disease (CAVD) are the significant changes that occur in the organization, composition, and mechanical properties of the extracellular matrix (ECM), ultimately resulting in stiffened stenotic leaflets that obstruct flow and compromise cardiac function. Increasing evidence suggests that ECM maladaptations are not simply a result of valve cell dysfunction; they also contribute to CAVD progression by altering cellular and molecular signaling. In this review, we summarize the ECM changes that occur in CAVD. We also discuss examples of how the ECM influences cellular processes by signaling through adhesion receptors (matricellular signaling), by regulating the presentation and availability of growth factors and cytokines to cells (matricrine signaling), and by transducing externally applied forces and resisting cell-generated tractional forces (mechanical signaling) to regulate a wide range of pathological processes, including differentiation, fibrosis, calcification, and angiogenesis. Finally, we suggest areas for future research that should lead to new insights into bidirectional cell–ECM interactions in the aortic valve, their contributions to homeostasis and pathobiology, and possible targets to slow or prevent the progression of CAVD.

Diet-induced aortic valve disease in mice haploinsufficient for the Notch pathway effector RBPJK/CSL

OBJECTIVE

Calcific aortic valve disease is similar to atherosclerosis in that both diseases result from chronic inflammation and endothelial dysfunction. Heterozygous NOTCH1 mutations have been associated to calcific aortic disease and a bicuspid aortic valve. We investigated whether mice with genetic inactivation of the Notch signaling pathway are prone to develop valve disease when exposed to a predisposing diet.

METHODS AND RESULTS

Using Doppler echocardiography, histology, immunohistochemistry, quantitative gene expression analysis, and cell culture assays, we examined the effect of a hypercholesterolemic diet supplemented with vitamin D on mice heterozygous for null mutations in the Notch1 receptor or the effector transcription factor gene RBPJk. After 16 weeks on the hyperlipidemic diet, calcific aortic disease was detected in heterozygous RBPJk mice. Analysis of valve leaflets revealed macrophage infiltration, enhanced collagen deposition, proosteogenic protein expression, and calcification. Heterozygous null Notch1 mice displayed milder histopathologic changes and did not develop any significant hemodynamic disturbance. Valvular disease correlated with reduced expression of the Notch target gene Hey1 in valves of RBPJk heterozygous mice fed the hyperlipidemic diet. Consistent with the in vivo data, Notch signaling inhibition in porcine valve interstitial cells led to downregulation of HEY1 transcription, activation of osteogenic markers, and increased calcified nodule formation.

CONCLUSIONS

We show that Notch signaling disruption via RBPJk heterozygous inactivation results in aortic valve disease. Notch1 heterozygous mice do not show functional impairment, suggesting that additional Notch receptors may be involved in aortic valve homeostasis and disease. Our data establish a genetic mouse model of calcific aortic valve disease and may help to identify a patient population with reduced valvular NOTCH signaling at risk for developing this disease.

Heart valve tissue engineering: quo vadis?

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and generally enhances survival and quality of life. However, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. A tissue engineered heart valve could, in principle, accommodate these requirements, especially somatic growth. This review provides a brief overview of the field of heart valve tissue engineering, with emphasis on recent studies and evolving concepts, especially those that establish design criteria and key hurdles that must be surmounted before clinical implementation.