Leaflet Region Research Articles

Evaluation of the Impact of Stress Distribution on Polyurethane Trileaflet Heart Valve Leaflets in the Open Configuration by Employing Numerical Simulation

It is costly and time-consuming to design and manufacture functional polyurethane heart valve prototypes, to evaluate and comprehend their hemodynamic behaviour. To enhance the rapid and effective design of replacement heart valves, to meet the minimum criteria of FDA and ISO regulations and specifications, and to reduce the length of required clinical testing, computational fluid dynamics (CFD) and finite element analysis (FEA) were used. The results revealed that when the flexibility of the stent was taken into consideration with a uniform leaflet thickness, stress concentration regions that were present close to the commissural attachment were greatly diminished. Furthermore, it was found that the stress on the leaflets was directly impacted by the effect of reducing the post height on both rigid and flexible stents. When varying the leaflet thickness was considered, the high-stress distribution close to the commissures appeared to reduce at thicker leaflet regions. However, thicker leaflets may result in a stiffer valve with a corresponding increase in pressure drop. It was concluded that a leaflet with predefined varying thickness may be a better option.

In Silico Analysis of the MitraClip in a Realistic Human Left Heart Model

Mitral valve regurgitation is a common heart valve disorder associated with significant morbidity and mortality. Transcatheter mitral valve repair using the MitraClip device has emerged as a safe and effective alternative for patients unsuitable for conventional surgery. However, the structural and hemodynamic implications of MitraClip implantation in the left ventricle have not been extensively explored. This study aimed to assess the structural and hemodynamic performance of the MitraClip device using a high-fidelity model of the human heart, specifically focusing on a healthy mitral valve geometry. The implantation of the MitraClip device was simulated using the finite element method for structural analysis and the lattice Boltzmann method for computational flow analysis. MitraClip implantation induced geometrical changes in the mitral valve, resulting in local maxima of principal stress in the valve leaflet regions constrained by the device. Hemodynamic assessment revealed slow-moving nested helical flow near the left ventricular wall and high flow velocities in the apex regions. Vorticity analysis indicated abnormal hemodynamic conditions induced by the double-orifice area configuration of the mitral valve after MitraClip implantation. By predicting possible adverse events and complications in a patient-specific manner, computational modeling supports evidence-based decision making and enhances the overall effectiveness and safety of transcatheter mitral valve repairs.

809 A CASE OF CASEOUS MITRAL ANNULUS CALCIFICATION

Abstract Caseous mitral annular calcification (CMAC) is a rare variant of degenerative mitral annular calcification (MAC) with an echocardiographic prevalence of 0.6% of all MACs and an overall prevalence of up to 0.07% in the general population. It primarily affects older patients with hypertension and chronic kidney disease. Due to the general benign prognosis, conservative management of this lesion is recommended in most cases. Herein we describe the case of a 78-year-old female patient admitted for sustained ventricular tachycardia treated with Amiodaron infusion followed by electrical cardioversion for hemodynamically instability. Her past medical history was remarkable for hypertension, HFpEF and chronic kidney disease. Trans-thoracic echocardiogram showed the presence of an echo-dense mass with central echo-lucency surrounded by calcifications at the posterior peri-annular and leaflet region of the mitral valve, not associated with valvular disfunction. To define the etiology of the arrythmia, a coronary angiography was performed and excluded significant coronary artery disease. Nevertheless, a round-shape hyperdense mass was observed in the left atrium near the atrio-ventricular groove (figure 1): during injection through coronary vessels the contrast was not visible in the lesion. In order to better define the nature of the mass, a Trans-esophageal Echocardiogram was performed and showed a “toothpaste-like” echo-dense mass with central echo-lucency surrounded by a calcified envelope at the posterior peri-annular region of the mitral valve suggestive for CMAC (figure 2); 3D zoom imaging clarified its relationship with P2 scallop of posterior mitral valve leaflet (figure 3). For the best tissue characterization and to definitely exclude other mass-like lesions, such as myxoma, papillary fibroelastoma, myocardial abscess, vegetations, lipomatous hypertrophy, Cardiac Magnetic Resonance was performed and showed low-signal intensity in both T1- and T2-weighted sequences, reflecting its calcium content. First-pass perfusion sequence reveals no contrast enhancement, whereas late gadolinium enhancement depicts a peripheral rim of enhancement confirming our suspect. In Conclusion, CMAC is a rare disorder requiring a multimodal imaging approach to differentiate it with other intra-cardiac masses. Its course is generally benign and whether not associated with valvular or hemodynamic significance, it could be considered as a bystander. Figure 1 Figure 2 Figure 3

Benchtop characterization of the tricuspid valve leaflet pre-strains

The pre-strains of biological soft tissues are important when relating their in vitro and in vivo mechanical behaviors. In this study, we present the first-of-its-kind experimental characterization of the tricuspid valve leaflet pre-strains. We use 3D photogrammetry and the reproducing kernel method to calculate the pre-strains within the central 10×10 mm region of the tricuspid valve leaflets from n=8 porcine hearts. In agreement with previous pre-strain studies for heart valve leaflets, our results show that all the three tricuspid valve leaflets shrink after being explanted from the ex vivo heart. These calculated strains are leaflet-specific and the septal leaflet experiences the most compressive changes. Furthermore, the strains observed after dissection of the central 10×10 mm region of the leaflet are smaller than when the valve is explanted, suggesting that our computed pre-strains are mainly due to the release of in situ annulus and chordae connections. The leaflets are then mounted on a biaxial testing device and preconditioned using force-controlled equibiaxial loading. We show that the employed preconditioning protocol does not 100% restore the leaflet pre-strains as removed during tissue dissection, and future studies are warranted to explore alternative preconditioning methods. Finally, we compare the calculated biomechanically oriented metrics considering five stress-free reference configurations. Interestingly, the radial tissue stretches and material anisotropies are significantly smaller compared to the post-preconditioning configuration. Extensions of this work can further explore the role of this unique leaflet-specific leaflet pre-strains on in vivo valve behavior via high-fidelity in-silico models. Statement of significanceThis study provides a first of its kind benchtop characterization of tricuspid valve leaflet pre-strains. We used 3D photogrammetry to reconstruct the central region of the tricuspid valve leaflets in three configurations. The associated configurational changes revealed compressive, leaflet-specific strains after dissection of the valve from its in situ environment. Interestingly, we found that biaxial preconditioning did not restore the measured pre-strains of the leaflets. Depending on the selection of the stress-free reference configuration, this led to substantial differences in the leaflet mechanics. Our findings and methodology are crucial when it comes to relating in vitro mechanical behaviors to valve function in vivo. Future studies can integrate our quantified pre-strains into in-silico simulations to get more realistic predictions about the valve function.

Importance of Non-Newtonian Modeling of Blood Flow for Calcified Aortic Valves: Relevance to Sub-Clinical Thrombosis

Objective: The Newtonian assumption for blood has often been applied for aortic valve studies where high shear rates can be seen on the ventricularis side. However, in the presence of calcifications this assumption is suspect due to large flow disturbances, especially on the fibrosa side. Regions of low wall shear stress (WSS) have been known to lead to thrombogenesis in the cardiovascular system, and thus accurate blood flow modeling is required to predict these locations. This study looked at the applicability of a Newtonian and non-Newtonian, Carreau, model for blood on a healthy and calcified aortic valve geometry. Methods: A reconstructed human aortic heart valve from an 82-year old patient at the early diastolic phase was commercially acquired (Valve - 012 - Heart Print catalog, Materialize Inc., Plymouth MI). Inlet velocity of 1 cm/s was prescribed and an 80-mmHg pressure condition for the outlet boundary condition. Fluent (ANSYS Inc, Canonsburg, PA) was used for solving the unsteady Navier-Stokes equations. Combinations of a healthy or calcified valve and a Newtonian or a non-Newtonian model were then simulated. Results: WSS showed little difference between Newtonian and non-Newtonian models in the healthy valve (Figure 1A and Figure 1B). However, there was an underestimation of WSS in the calcified case using the Newtonian model compared to non-Newtonian (Figure 1C and Figure 1D) in certain core leaflet regions. Conclusions: The Newtonian assumption can be used for modeling blood flow of a healthy aortic valve, but a non-Newtonian model must be used in cases of valve disease, such as severely calcified aortic valves. These irregularities affect determining WSS where a few dynes/cm2 mark the difference between regions being susceptible to potential thrombus or not. Figure 1. Results on the fibrosa side of the aortic valve A) WSS using a Newtonian model on a healthy valve. B) WSS using a non-Newtonian, Carreau, model on a healthy valve. C) WSS using a Newtonian model on calcified valve. D) WSS using a non-Newtonian, Carreau, model on a calcified valve

Optical coherence tomography and multiphoton microscopy offer new options for the quantification of fibrotic aortic valve disease in ApoE\u2212/\u2212 mice

Aortic valve sclerosis is characterized as the thickening of the aortic valve without obstruction of the left ventricular outflow. It has a prevalence of 30% in people over 65 years old. Aortic valve sclerosis represents a cardiovascular risk marker because it may progress to moderate or severe aortic valve stenosis. Thus, the early recognition and management of aortic valve sclerosis are of cardinal importance. We examined the aortic valve geometry and structure from healthy C57Bl6 wild type and age-matched hyperlipidemic ApoE−/− mice with aortic valve sclerosis using optical coherence tomography (OCT) and multiphoton microscopy (MPM) and compared results with histological analyses. Early fibrotic thickening, especially in the tip region of the native aortic valve leaflets from the ApoE−/− mice, was detectable in a precise spatial resolution using OCT. Evaluation of the second harmonic generation signal using MPM demonstrated that collagen content decreased in all aortic valve leaflet regions in the ApoE−/− mice. Lipid droplets and cholesterol crystals were detected using coherent anti-Stokes Raman scattering in the tissue from the ApoE−/− mice. Here, we demonstrated that OCT and MPM, which are fast and precise contactless imaging approaches, are suitable for defining early morphological and structural alterations of sclerotic murine aortic valves.

Analysis of Protein-Cardiolipin Interactions in the E. coli Inner Membrane

Integral membrane proteins are localised and functionally regulated by lipids present in the surrounding bilayer. Whilst bacteria such as E. coli have a relatively simple membrane when compared to eukaryotic cells, there is ample evidence that multiple proteins bind, and have their function regulated by, specific lipid interactions. There are a number of ways of identifying protein-lipid interactions in membrane proteins, including through molecular simulation. Here, we highlight the capabilities of this technique by running lipid-binding simulations of 42 different E. coli inner membrane proteins. Our data reveals a strong cross-membrane asymmetry in the nature of protein-lipid interactions, with a marked increase of anionic lipid binding to the inner leaflet regions of membrane proteins. This appears to be driven by an increase in basic residues, suggesting a structural basis for the positive inside rule. We have identified ca. 700 independent cardiolipin binding sites from our data, allowing us dissect the molecular basis of a prototypical cardiolipin headgroup binding site. Applying free energy calculations quantifies the relative contributions of each residue type to cardiolipin binding. These findings demonstrate the potential for large-scale simulations to reveal pertinent underlying biological information applying to a number of systems, and paves the way for future analyses of more complex protein-lipid interactions.

A molecular framework underlying the compound leaf pattern of Medicago truncatula.

Compound leaves show more complex patterns than simple leaves, and this is mainly because of a specific morphogenetic process (leaflet initiation and arrangement) that occurs during their development. How the relevant morphogenetic activity is established and modulated to form a proper pattern of leaflets is a central question. Here we show that the trifoliate leaf pattern of the model leguminous plant Medicago truncatula is controlled by the BEL1-like homeodomain protein PINNATE-LIKE PENTAFOLIATA1 (PINNA1). We identify PINNA1 as a determinacy factor during leaf morphogenesis that directly represses transcription of the LEAFY (LFY) orthologue SINGLE LEAFLET1 (SGL1), which encodes an indeterminacy factor key to the morphogenetic activity maintenance. PINNA1 functions alone in the terminal leaflet region and synergizes with another determinacy factor, the C2H2 zinc finger protein PALMATE-LIKE PENTAFOLIATA1 (PALM1), in the lateral leaflet regions to define the spatiotemporal expression of SGL1, leading to an elaborate control of morphogenetic activity. This study reveals a framework for trifoliate leaf-pattern formation and sheds light on mechanisms generating diverse leaf forms.

Lateral Leaflet Suppression 1 (LLS1), encoding the MtYUCCA1 protein, regulates lateral leaflet development in Medicago truncatula.

In species with compound leaves, the positions of leaflet primordiuminitiation are associated with local peaks of auxin accumulation. However, the role of auxin during the late developmental stages and outgrowth of compound leaves remains largely unknown. Using genome resequencing approaches, we identified insertion sites at four alleles of the LATERAL LEAFLET SUPPRESSION1 (LLS1) gene, encoding the auxin biosynthetic enzyme YUCCA1 in Medicago truncatula. Linkage analysis and complementation tests showed that the lls1 mutant phenotypes were caused by the Tnt1 insertions that disrupted the LLS1 gene. The transcripts of LLS1 can be detected in primordia at early stages of leaf initiation and later in the basal regions of leaflets, and finally in vein tissues at late leaf developmental stages. Vein numbers and auxin content are reduced in the lls1-1 mutant. Analysis of the lls1 sgl1 and lls1 palm1 double mutants revealed that SGL1 is epistatic to LLS1, and LLS1 works with PALM1 in an independent pathway to regulate the growth of lateral leaflets. Our work demonstrates that the YUCCA1/YUCCA4 subgroup plays very important roles in the outgrowth of lateral leaflets during compound leaf development of M.truncatula, in addition to leaf venation.

Abstract 17124: Aortic Valve Calcification as a Predictor of Post-TAVR Pacemaker Dependence

Introduction: Atrioventricular block requiring permanent pacemaker (PPM) implantation is a common complication of TAVR. Our group has previously reported that a significant number of patients receiving PPM immediately post-TAVR do not require right ventricular (RV) pacing at 30 days. The mechanism of AV block during TAVR is not fully understood, but is thought to partly be due to the mechanical stress of TAVR deployment and resultant tissue edema, resulting in possible injury to the nearby compact AV node. Aortic valve calcification (AVC) may worsen this condition and has been associated with an increased risk for post-TAVR PPM implantation. We performed a retrospective analysis to determine if AVC is predictive for long-term RV pacing in post-TAVR pacemaker patients at 30 days. Methods: Prospectively collected data of 262 consecutive patients who underwent TAVR with placement of a balloon-expandable valve at Rhode Island Hospital from March 2012 to October 2016 were analyzed. AVC data were derived from contrast-enhanced computed tomography and characterized by leaflet sector and region. Results: A total of 25 patients (11.1%) required post-TAVR PPM implantation. Seventeen patients did not require RV pacing at 30 days. Nine of these 17 patients had no RV pacing requirement within 10 days. Non-coronary leaflet (NCL) calcium volume was significantly higher in patients who were pacemaker dependent at 30 days ( p =0.01) and a calcium volume of >200mm 3 in the NCL was significantly associated with pacemaker dependence at 30 days (OR 51.9, 2.3-1170.2, p = 0.01). Pre-existing RBBB (OR 105.4, 4.52-2458.5, p=0.0002), bifascicular block (OR 12.50, 1.60-97.65, p=0.02), intra-procedural complete heart block (OR 12.83, 1.26-130.52, p =0.03), and QRS duration >120ms (OR 70.43, 3.23-1535.22, p=0.0002) were also significantly associated with pacemaker dependence at 30 days. Conclusions: Quantification of AVC by non-coronary leaflet calcium volume was found to be a novel predictor for RV pacing dependence at 30 days. The association of NCL calcification and pacemaker dependence may be related to the proximity of the conduction bundle to the non-coronary leaflet. Further studies are necessary to improve risk prediction for long-term RV pacing requirements following TAVR.

A New Method for the Anterior Mitral Leaflet Segmentation in Echocardiography Videos using the Virtual M-mode Space.

Rheumatic heart disease is responsible for the heart valve damage, caused by repeated episodes of rheumatic fever. The disease commonly inflames and scars the mitral valve of the heart, resulting in thicker, less mobile leaflets, with associated decrease in cardiac efficiency. It is important to measure and quantity the early manifestations of this disease, including variations of the thickness, shape and mobility of the leaflets. These manifestations are visible in an echocardiographic screening process. The first step towards the defined objective is to segment the anterior mitral leaflet throughout the cardiac cycle, enabling the future automatic quantification of mentioned clinical parameters. In this work, a new algorithm for the segmentation of the whole region of the anterior mitral leaflet in the virtual M-mode space is proposed. The algorithm requires a single initialization point on the posterior wall of the aorta, in the first frame of the video. A junction point is then computed, showing the location where the two leaflets connect. This junction point helps to automatically redefine the range of virtual M-mode images required to completely segment the region of the anterior mitral leaflet. The segmented anterior mitral leaflet region in the virtual M-mode space is transferred back to regular image space and its shape refined using localized active contours. Results suggest the suitability of the proposed algorithm for the segmentation of anterior mitral leaflet with a median Dice Similarity Coefficient of 0.63, and with median precision and recall of 58% and 73% respectively.

Signal dropout correction-based ultrasound segmentation for diastolic mitral valve modeling.

Three-dimensional ultrasound segmentation of mitral valve (MV) at diastole is helpful for duplicating geometry and pathology in a patient-specific dynamic phantom. The major challenge is the signal dropout at leaflet regions in transesophageal echocardiography image data. Conventional segmentation approaches suffer from missing sonographic data leading to inaccurate MV modeling at leaflet regions. This paper proposes a signal dropout correction-based ultrasound segmentation method for diastolic MV modeling. The proposed method combines signal dropout correction, image fusion, continuous max-flow segmentation, and active contour segmentation techniques. The signal dropout correction approach is developed to recover the missing segmentation information. Once the signal dropout regions of TEE image data are recovered, the MV model can be accurately duplicated. Compared with other methods in current literature, the proposed algorithm exhibits lower computational cost. The experimental results show that the proposed algorithm gives competitive results for diastolic MV modeling compared with conventional segmentation algorithms, evaluated in terms of accuracy and efficiency.

Cation Measurements and Gene Expression Analysis Suggest Tomato Leaf Marginal Necrosis is Caused by a Jasmonate Signal Induced by K<sup>+</sup> Starvation in the Tip Region of Leaflets

Cation Measurements and Gene Expression Analysis Suggest Tomato Leaf Marginal Necrosis is Caused by a Jasmonate Signal Induced by K<sup>+</sup> Starvation in the Tip Region of Leaflets

Invited Commentary

Leonardo da Vinci was a master at blending function of biologic systems into his graphic drawings. His illustrations of aortic flow currents most likely were rendered from the first models that recapitulated the dynamic function of any biologic system [1Gharib M. Kremers D. Koochesfahani M.M. Kemp M. Leonardo’s vision of flow visualization.Exp Fluids. 2002; 33: 219-223Crossref Scopus (52) Google Scholar]. Similarly, ever since echocardiography was developed and expanded in the 1970s and 1980s, cardiologists and surgeons have desired to develop structural models of heart valves. This article by Pouch and associates [2Pouch A.M. Jackson B.M. Lai E. et al.Modeling the myxomatous mitral valve with three-dimensional echocardiography.Ann Thorac Surg. 2016; 102: 703-711Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar] describes a complex imaging and reconstruction method that attempts to characterize both non-diseased and myxomatous mitral valves. This work from the Gorman Laboratory at the University of Pennsylvania is unique; it used three-dimensional echocardiographic reconstructions to study the material properties of non-diseased and myxomatous mitral valves. Moreover, they were able to make transforms that potentially could be printed three-dimensionally into actual solid models of different degenerative mitral lesions. Although the methodology described in the article is beyond the interest or scope of most practicing cardiac surgeons, the message from this work is important. Clearly, improvement in echocardiography has been an iterative process, albeit a rapid one. Future advancements assuredly will allow us to have dynamic models of mitral valves to be created instantaneously in the operating room. With image fusion techniques, the creation of onlay “blueprints” may be possible to guide more perfectly engineered mitral repairs. The complexity of mitral valve stress-strain (pressure versus deformation) relationships is so multifactorial that perfect modeling will be even more difficult than this article describes. The authors had to make assumptions. They were not able to negate the mechanical property alterations of velocity and anisotropy; this means that degenerative valve leaflets often have variable regional thicknesses with variable viscoelastic properties rather than uniform ones. Moreover, multiple areas of prolapse impose additional nonconstant mechanical properties. In the early 1990s, Kunzelman and associates [3Kunzelman K.S. Cochran R.P. Chuong C. Ring W.S. Verrier E.D. Eberhart R.D. Finite element analysis of the mitral valve.J Heart Valve Dis. 1993; 2: 326-340PubMed Google Scholar] first used finite element analysis to determine mitral valve stress strain relationships in an attempt to define what leaflet regions were at highest risk (strain) for failure. Their most recent in vitro modeling studies, using micro computed tomographic images, focuses on stress-strain interactions of normal mitral leaflets, papillary muscles, and chords throughout the cardiac cycle [4Toma M. Bloodworth IV, C.H. Einstein D.R. et al.High-resolution subject-specific mitral valve imaging and modeling: experimental and computational methods.Biomech Model Mechanobiol. 2016 Apr 19; ([Epub ahead of print])Crossref PubMed Scopus (24) Google Scholar]. Recently, Hossien’s group [5Hossien A. Nithiarasuc P. Cheriex E. Maessen J. Sardari Nia P. Ashraf S. A multi-dimensional dynamic quantification tool for the mitral valve.Interact Cardiovasc Thorac Surg. 2015; 21: 481-487Crossref PubMed Scopus (10) Google Scholar] in Maastricht modeled mitral valve dynamics from segmented transesophageal echocardiography data in healthy patients. Subsequently, they have been able to print solid models for teaching minimally invasive mitral repairs for patients with various types of degenerative disease (Personal Communication, Peyman Sardari Nia, MD, University of Maastricht, 2016). The present article brings us much closer to translating image data from degenerative mitral valves into useful models for teaching and aiding in providing tailored mitral repairs for individualized patients. Anyanwu and colleagues [6Anyanwu A. Itagaki S. Chikwee J. El-Eshmawi A. Adams D.H. A complexity scoring system for degenerative mitral valve repair.J Thorac Cardiovasc Surg. 2016; 151: 1661-1670Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar] suggested recently that mitral valve pathologic complexity be categorized with a view toward selecting repair potentials based on a surgeon’s experience and talents. Thus, the information gained from future echocardiographic modeling studies could help surgeons to categorize the pathologic complexities of degenerative valves and to determine the best technique for reattaining the near-perfect leaflet and chord stress-strain relationships. To this end, the authors now must be challenged to develop dynamic operating room models that can be translated contemporaneously. Mitral repair surgeons should stay abreast of this evolving and exciting imaging and modeling technology; it could help us save more mitral valves from the pathologists’ microscope. Modeling the Myxomatous Mitral Valve With Three-Dimensional EchocardiographyThe Annals of Thoracic SurgeryVol. 102Issue 3PreviewDegenerative mitral valve disease is associated with variable and complex defects in valve morphology. Three-dimensional echocardiography (3DE) has shown promise in aiding preoperative planning for patients with this disease but to date has not been as transformative as initially predicted. The clinical usefulness of 3DE has been limited by the laborious methods currently required to extract quantitative data from the images. Full-Text PDF

2015 4thTERMIS World CongressBoston, MassachusettsSeptember 8–11, 2015

2015 4^thTERMIS World CongressBoston, MassachusettsSeptember 8–11, 2015

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.

The N-Terminal Domain Acts as an Anchor during the Gating Cycle of MscL

This study utilizes a finite element (FE) model to probe the gating mechanism of the mechanosensitive channel of large conductance (MscL). When compared to molecular dynamics simulations, FE computations are generally more applicable due to their much longer simulation times as well as larger size scales. Herein we use an FE model of MscL, which has been developed as an initial template to provide not only information about the gating cycle of this channel but also to provide a structural framework for a mechanistic understanding of the gating of other MS channels at the continuum level. The FE model, unlike molecular dynamics simulations, utilized membrane tension corresponding to midpoint activation of MscL close to that seen in patch-clamp experiments (<12 mN/m). For MscL in the open state, our model indicates that the N-terminus and TM1 of each subunit become aligned to form a single helix. Previous mutagenic work suggests that the N-terminus of MscL, which connects the TM1 helix to the lipid bilayer at an angle of ∼95° in the closed state, is imperative for mechanosensation of the channel. In agreement with experimental data, deletion of the N-terminal domain from our model increases the tensional force required for the full opening of MscL. Moreover, we observed that during gating, the channel gate moves from the inner leaflet region towards the bilayer midplane as the membrane thins. Since we do not see this upward movement of the gate when the N-terminal is removed from our model this further implies that N-terminal region acts as an anchor in the gating process of MscL.Supported by the University International Postgraduate Award (UIPA) to N.B. and APP1079398 grant from NHMRC to B.M.

Modeling the electrostatic potential of asymmetric lipopolysaccharide membranes: the MEMPOT algorithm implemented in DelPhi.

Four chemotypes of the rough lipopolysaccharides (LPS) membrane from Pseudomonas aeruginosa were investigated by a combined approach of explicit water molecular dynamics (MD) simulations and Poisson-Boltzmann continuum electrostatics with the goal to deliver the distribution of the electrostatic potential across the membrane. For the purpose of this investigation, a new tool for modeling the electrostatic potential profile along the axis normal to the membrane, MEMbrane POTential (MEMPOT), was developed and implemented in DelPhi. Applying MEMPOT on the snapshots obtained by MD simulations, two observations were made: (a) the average electrostatic potential has a complex profile but is mostly positive inside the membrane due to the presence of Ca(2+) ions, which overcompensate for the negative potential created by lipid phosphate groups; and (b) correct modeling of the electrostatic potential profile across the membrane requires taking into account the water phase, while neglecting it (vacuum calculations) results in dramatic changes including a reversal of the sign of the potential inside the membrane. Furthermore, using DelPhi to assign different dielectric constants for different regions of the LPS membranes, it was investigated whether a single frame structure before MD simulations with appropriate dielectric constants for the lipid tails, inner, and the external leaflet regions, can deliver the same average electrostatic potential distribution as obtained from the MD-generated ensemble of structures. Indeed, this can be attained by using smaller dielectric constant for the tail and inner leaflet regions (mostly hydrophobic) than for the external leaflet region (hydrophilic) and the optimal dielectric constant values are chemotype-specific.

Measurement of Mitral Leaflet and Annular Geometry and Stress After Repair of Posterior Leaflet Prolapse: Virtual Repair Using a Patient-Specific Finite Element Simulation

Recurrent mitral regurgitation after mitral valve (MV) repair for degenerative disease occurs at a rate of 2.6% per year and reoperation rate progressively reaches 20% at 19.5 years. We believe that MV repair durability is related to initial postoperative leaflet and annular geometry with subsequent leaflet remodeling due to stress. We tested the hypothesis that MV leaflet and annular stress is increased after MV repair. Magnetic resonance imaging was performed before and intraoperative three-dimensional (3D) transesophageal echocardiography was performed before and after repair of posterior leaflet prolapse in a single patient. The repair consisted of triangular resection and annuloplasty band placement. Images of the heart were manually co-registered. The left ventricle and MV were contoured, surfaced, and a 3D finite element (FE) model was created. Elements of the posterior leaflet region were removed to model leaflet resection and virtual sutures were used to repair the leaflet defect and attach the annuloplasty ring. The principal findings of the current study are the following: (1) FE simulation of MV repair is able to accurately predict changes in MV geometry including changes in annular dimensions and leaflet coaptation; (2) average posterior leaflet stress is increased; and (3) average anterior leaflet and annular stress are reduced after triangular resection and mitral annuloplasty. We successfully conducted virtual mitral valve prolapse repair using FE modeling methods. Future studies will examine the effects of leaflet resection type as well as annuloplasty ring size and shape.

Expression of smooth muscle cell markers and co-activators in calcified aortic valves

Similar risk factors and mediators are involved in calcific aortic stenosis (CAS) and atherosclerosis. Since normal valves harbour a low percentage of smooth muscle cells (SMCs), we hypothesize that the SMC phenotype participates in the pathogenesis of CAS. We analysed 12 normal and 22 calcified aortic valves for SMC markers and the expression of co-activators of SMC gene expression, myocardin and myocardin-related transcription factors (MRTF-A/B). Transforming growth factor β (TGFβ1) was used to upregulate SMC markers and co-activators in valve interstitial cells (VICs) and transmission electron microscopy (TEM) was used to detect the presence of SMC in atypical regions of the valve leaflets. Smooth muscle cell markers and co-activators, myocardin, MRTF-A, and MRTF-B, demonstrated an increased incidence and aberrant expression around calcified nodules in all 22 calcified valves as well as in surface and microvessel endothelial cells. Smooth muscle cell markers and MRTF-A were significantly increased in calcified valves. Transforming growth factor β1 (TGFβ1) (10 ng/mL) was able to significantly upregulate the expression of some SMC markers and MRTF-A in VICs. Transmission electron microscopy of the fibrosa layer of calcified valves demonstrated the presence of bundles of SMCs and smooth muscle-derived foam cells. Smooth muscle cell markers and co-activators, myocardin and MRTFs, were aberrantly expressed in calcified valves. Transforming growth factor β1 was able to significantly upregulate SMC markers and MRTF-A in VICs. Transmission electron microscopy unequivocally identified the presence of SMCs in calcified regions of valve leaflets. These findings provide evidence that the SMC phenotype plays a role in the development of CAS.