Mechanism of NMDA receptor channel block by MK-801 and memantine.

The NMDA (N-methyl-D-aspartate) receptor transduces the binding of glutamate and glycine, coupling it to the opening of a calcium-permeable ion channel 1 . Owing to the lack of high-resolution structural studies of the NMDA receptor, the mechanism by which ion-channel blockers occlude ion permeation is not well understood. Here we show that removal of the amino-terminal domains from the GluN1-GluN2B NMDA receptor yields a functional receptor and crystals with good diffraction properties, allowing us to map the binding site of the NMDA receptor blocker, MK-801. This crystal structure, together with long-timescale molecular dynamics simulations, shows how MK-801 and memantine (a drug approved for the treatment of Alzheimer's disease) bind within the vestibule of the ion channel, promote closure of the ion channel gate and lodge between the M3-helix-bundle crossing and the M2-pore loops, physically blocking ion permeation.

Anesthetics affect peripheral venous pressure waveforms and the cross-talk with arterial pressure.

Analysis of peripheral venous pressure (PVP) waveforms is a novel method of monitoring intravascular volume. Two pediatric cohorts were studied to test the effect of anesthetic agents on the PVP waveform and cross-talk between peripheral veins and arteries: (1) dehydration setting in a pyloromyotomy using the infused anesthetic propofol and (2) hemorrhage setting during elective surgery for craniosynostosis with the inhaled anesthetic isoflurane. PVP waveforms were collected from 39 patients that received propofol and 9 that received isoflurane. A multiple analysis of variance test determined if anesthetics influence the PVP waveform. A prediction system was built using k-nearest neighbor (k-NN) to distinguish between: (1) PVP waveforms with and without propofol and (2) different minimum alveolar concentration (MAC) groups of isoflurane. 52 porcine, 5 propofol, and 7 isoflurane subjects were used to determine the cross-talk between veins and arteries at the heart and respiratory rate frequency during: (a) during and after bleeding with constant anesthesia, (b) before and after propofol, and (c) at each MAC value. PVP waveforms are influenced by anesthetics, determined by MANOVA: p value < 0.01, η2 = 0.478 for hypovolemic, and η2 = 0.388 for euvolemic conditions. The k-NN prediction models had 82% and 77% accuracy for detecting propofol and MAC, respectively. The cross-talk relationship at each stage was: (a) ρ = 0.95, (b) ρ = 0.96, and (c) could not be evaluated using this cohort. Future research should consider anesthetic agents when analyzing PVP waveforms developing future clinical monitoring technology that uses PVP.

Venous Physiology Predicts Dehydration in the Pediatric Population.

BACKGROUND: No standard dehydration monitor exists for children. This study attempts to determine the utility of Fast Fourier Transform (FFT) of a peripheral venous pressure (PVP) waveform to predict dehydration. MATERIALS AND METHODS: PVP waveforms were collected from 18 patients. Groups were defined as resuscitated (serum chloride ≥ 100 mmol/L) and hypovolemic (serum chloride < 100 mmol/L). Data were collected on emergency department admission and after a 20 cc/kg fluid bolus. The MATLAB (MathWorks) software analyzed nonoverlapping 10-s window signals; 2.4 Hz (144 bps) was the most demonstrative frequency to compare the PVP signal power (mmHg). RESULTS: Admission FFTs were compared between 10 (56%) resuscitated and 8 (44%) hypovolemic patients. The PVP signal power was higher in resuscitated patients (median 0.174 mmHg, IQR: 0.079-0.374 mmHg) than in hypovolemic patients (median 0.026 mmHg, IQR: 0.001-0.057 mmHg), (P < 0.001). Fourteen patients received a bolus regardless of laboratory values: 6 (43%) resuscitated and 8 (57%) hypovolemic. In resuscitated patients, the signal power did not change significantly after the fluid bolus (median 0.142 mmHg, IQR: 0.032-0.383 mmHg) (P = 0.019), whereas significantly increased signal power (median 0.0474 mmHg, IQR: 0.019-0.110 mmHg) was observed in the hypovolemic patients after a fluid bolus at 2.4 Hz (P < 0.001). The algorithm predicted dehydration for window-level analysis (sensitivity 97.95%, specificity 93.07%). The algorithm predicted dehydration for patient-level analysis (sensitivity 100%, specificity 100%). CONCLUSIONS: FFT of PVP waveforms can predict dehydration in hypertrophic pyloric stenosis. Further work is needed to determine the utility of PVP analysis to guide fluid resuscitation status in other pediatric populations.

Real-Time Transthoracic Vector Flow Imaging of the Heart in Pediatric Patients.

In children with congenital heart defects, Doppler ultrasound is the standard, bedside imaging modality. However, precise characterization of blood flow is challenging due to angle-dependent and one-dimensional velocity estimation. Contrast agent free Vector Flow Imaging is a new ultrasound technology that enables angle-independent visualization of the detailed flow field. Two piglets, one with normal cardiac anatomy and one with congenital heart disease comprised of valvular pulmonary stenosis, a dilated main pulmonary artery, and an incomplete atrioventricular canal defect, were imaged transthoracically and epicardially using a BK Ultrasound bk5000 with built-in vector flow imaging and a 5MHz linear probe. Subsequently, two children, one with normal cardiac anatomy and one with congenital heart disease comprised of aortic valve stenosis and coarctation of the aorta were imaged transthoracically. Transthoracic two-dimensional echocardiography and vector flow imaging were readily performed in both animals and were limited only by the geometry of the porcine thorax. In addition, transthoracic vector flow imaging was successfully performed in both children, and abnormal flow secondary to cardiac anomalies was visible. Adequate penetration was obtained to a depth of 6.5 cm. Our group has previously demonstrated for the first time that transthoracic vector flow imaging echocardiography is feasible and practicable in pediatric-sized patients, and this paper describes examples of these concepts and in-depth comparisons with traditional imaging modalities. This paper demonstrates that commercially available vector flow imaging technology can be utilized in pediatric cardiac applications as a bedside transthoracic imaging modality, providing advanced detail of blood flow patterns within the cardiac chambers, across valves, and in the great arteries.

New Mitral Valve Annuloplasty Concept: Optimizing Annular Dynamics and Force Distribution.

BACKGROUND: Temporal three-dimensional (3D) analysis of the mitral valve biomechanics has prompted a re-evaluation of surgical approaches and repair device designs to accommodate the natural dynamics of the valve. Such new designs strive to obtain lower annulus restraining forces, resulting in more natural leaflet and chordal stresses. A new annuloplasty system was evaluated using 3D motion and out-of-plane force analysis. It was hypothesized that this system would not impact the valve with adverse motion restrictions or high systolic annular forces compared to conventional flat rigid ring designs. METHODS: In an acute porcine set-up, six 80 kg pigs were monitored before and after implantation of the new annuloplasty system consisting of two half-rings with a saddle-shaped outline. Valvular 3D dynamic geometry was obtained using sonomicrometry before and after annuloplasty system implantation. Strain gauges mounted on the commissural segments provided the annular restraining force distribution perpendicular to the annular plane. RESULTS: The change in annular height to commissural width ratio from diastole to systole did not alter following implantation (p >0.05). Out-of-plane systolic restraining forces were 0.2 ± 0.1 N and 0.8 ± 0.3 N (mean ± SEM) in the posterior and anterior commissural segments, respectively, without any difference in-between (p >0.1). Forces in both commissural segments were significantly lowered compared to previous measurements with a flat and stiff mitral annuloplasty ring (p <0.01). Mitral annular septal-lateral distance, area, and circumference in the commissural segments were decreased after implantation (p <0.05). The cross-annular distance between the commissural segments and the lengths of the anterior and posterior annular segments did not change following implantation (p >0.05). CONCLUSIONS: The new annuloplasty system design maintained annular 3D dynamics and provided a minimized out-of-plane restraining force distribution compared to earlier studies on flat rigid rings. This may have important implications in the selection of annuloplasty devices in order to increase repair durability.

Optimizing peripheral venous pressure waveforms in an awake pediatric patient by decreasing signal interference.

The purpose of this technological notes paper is to describe our institution's experience collecting peripheral venous pressure (PVP) waveforms using a standard peripheral intravenous catheter in an awake pediatric patient. PVP waveforms were collected from patients with hypertrophic pyloric stenosis. PVP measurements were obtained prospectively at two time points during the hospitalization: admission to emergency department and after bolus in emergency department. Data was collected from thirty-two patients. Interference in the PVP waveforms data collection was associated with the following: patient or device motion, system set-up error, type of IV catheter, and peripheral intravenous catheter location. PVP waveforms can be collected in an awake pediatric patient and adjuncts to decrease signal interference can be used to optimize data collection.

Remodeling Mitral Annuloplasty Ring Concept with Preserved Dynamics of Annular Height.

BACKGROUND AND AIM OF THE STUDY: The configuration of the native annulus changes from nearly flat in the diastolic phase to saddle-shaped in the systolic phase. The present study was conducted to test a novel remodeling annuloplasty ring with built-in septal-lateral fixation and commissural axial flexibility so as to maintain the change in annular saddle shape. The study aim was to evaluate the in-vivo biomechanical performance of the novel annuloplasty ring, compared with the native valve and a semi-rigid and rigid annuloplasty ring. METHODS: All measurements were performed in vivo using a porcine model. A total of 28 pigs (bodyweight ca. 80 kg) were randomized to four groups: (i) with no ring; (ii) with a novel remodeling ring; (iii) with a semi-rigid ring (Physio I Ring, Edwards Lifesciences); and (iv) with a rigid ring (Classic Annuloplasty Ring, Edwards Lifesciences). Force measurements were performed using a dedicated transducer to determine remodeling capacity of the annuloplasty rings. Geometric parameters were measured by implanting sonomicrometry crystals along the mitral annulus. RESULTS: All ring groups significantly restricted the cyclic change of the mitral annulus compared with the 'no-ring' group. The change and maximum value of the annular height were maintained for the novel ring but were significantly decreased for the rigid and semi-rigid rings compared with the 'no-ring' group. Mitral annular force measurements confirmed that the overall remodeling capacity of the novel ring was comparable with the conventional ring groups, and significantly higher in the septal-lateral direction compared to the semi-rigid ring. CONCLUSIONS: In-vivo geometry and force measurements indicated that the intended design features of the new device were successfully provided. The novel ring concept with remodeling properties, combined with the advantages of a flexible annuloplasty ring, is unique. The maintenance of annular saddle shape and cyclic change in annular height may be an important step towards improved mitral valve repair.

Surgical Treatment of Functional Ischemic Mitral Regurgitation.

After myocardial infarction, functional ischemic mitral regurgitation (FIMR) is present in 21% of patients, and 3-13% have at least moderate FIMR. Currently, the 'gold-standard' treatment of FIMR is down-sized ring annuloplasty at the time of coronary artery bypass (CABG) surgery. However, this procedure has a failure rate of 20-30% in terms of recurrent FIMR after two to four years. In many ways, a cross-roads has been reached in terms of what constitutes optimal FIMR treatment: Is CABG combined with mitral valve ring annuloplasty better than CABG alone in moderate FIMR? Does mitral valve repair really produce better outcome than mitral valve replacement? And does adding an adjunct valvular repair or subvalvular left ventricular reverse remodeling procedure shift that balance? In order to shed further light on these questions and to help identify potential cornerstones in improving the 'gold standard' therapy, the present review addresses the current status and future perspectives of the surgical treatment of FIMR.

Papillary muscle force distribution after total tricuspid reconstruction using porcine extracellular matrix: in-vitro valve characterization.

BACKGROUND AND AIM OF THE STUDY: The use of extracellular matrix (ECM) from the porcine small intestinal submucosa has shown promising results in reconstructive heart surgery. The study aim was to compare native and ECM tricuspid valves with regards to tricuspid leaflet coaptation geometry and force development in the three papillary muscles, under normal ventricular pressures. METHODS: Six native porcine tricuspid valves and five ECM valves were examined in a static pressure right-heart simulator. Water was used in the ventricular chamber to induce adequate pressure changes (from 5 to 40 mmHg). Dedicated force transducers were used to measure force development in the three papillary muscles in parallel with ventricular pressure measurements. Relative leaflet area was defined as the percentage that one leaflet area comprised of the whole orifice area. The estimated peak leaflet pressure force was defined as leaflet area x peak ventricular pressure, and peak leaflet tethering force was calculated as the average of two adjacent papillary muscles forces. Valve competence and leaflet areas were documented using digital photography. RESULTS: The relative leaflet area of the anterior leaflet was significantly smaller (0.4 versus 0.5, p < 0.01) and that of the posterior leaflet was significantly larger (0.3 versus 0.2; p < 0.001) in ECM tube grafts compared to native valves. No difference was found between septal leaflet relative areas (p > 0.1). Accordingly, estimated peak leaflet pressure forces were greater in anterior leaflets and smaller in the posterior leaflets of native valves compared to ECM valves (p < 0.01). However, peak papillary muscle forces and peak leaflet tethering forces did not differ significantly between the valves. CONCLUSION: A competent and functional tricuspid valve can be constructed in vitro from ECM. In spite of different leaflet area distributions, the force distribution and tethering forces were not significantly different between the two valve types, indicating that a physiologically functioning tricuspid valve can be constructed from ECM.

Ross Confers More Favorable Left Ventricular Remodeling Compared With Mechanical Aortic Valve Replacement.

Background: Aortic valve disease results in left ventricular (LV) dilation and/or hypertrophy. Valve intervention may improve, but not normalize flow dynamics. We hypothesized that LV remodeling would be more favorable following the Ross procedure versus mechanical aortic valve replacement (mAVR). Methods: Patients who were 18 to 50 years of age and underwent Ross or mAVR from 2000 to 2016 at a single institution were retrospectively reviewed. Propensity score matching was performed and yielded 27 well-matched pairs. Demographics and echocardiographic variables of LV morphology and wall thickness were collected. Those with > mild residual valve disease were excluded. Primary endpoints included LV morphology. T test and Fisher exact test analysis were used for statistical comparison. Results: Average age at operation (Ross 35.3 ± 10.2 vs mAVR 37.3 ± 8.9 years) did not differ. Indication for operation was similar between groups. Preoperative echocardiographic variables did not differ. At average follow-up duration (Ross 7.9 ± 2.4 vs mAVR 7.3 ± 2.4 years), wall thickness was significantly smaller for Ross compared with mAVR (P = .00715). Only 4/27 (15%) of mAVR patients had normalized LV parameters compared with 16/27 (59%) of Ross patients (P = .000813). Residual hypertrophy was the most common long-term abnormality for mAVR. Conclusion: Following aortic valve replacement with the Ross procedure or mechanical aortic valve prosthesis, the Ross conferred more favorable LV remodeling compared with mAVR. Future directions include analyzing longer follow-up to determine if patterns persist and the impact on cardiac morbidity and mortality.

Surgical relocation of the papillary muscles in functional ischemic mitral regurgitation: what are the forces of the relocation stitches acting on the myocardium?

BACKGROUND AND AIM OF THE STUDY: In patients with chronic functional ischemic mitral regurgitation (FIMR), papillary muscle relocation has the potential to induce reverse left ventricular remodeling. However, in order to optimize function and durability, the forces imposed on the left ventricular myocardium by papillary muscle relocation should be assessed. METHODS: Eight pigs with FIMR were subjected to down-sized ring annuloplasty in combination with relocation of the anterior (5 mm) and posterior (15 mm) papillary muscles towards the respective trigone. Papillary muscle relocation was obtained by a 2-0 expanded polytetrafluoroethylene stitch fixed to the trigone, exteriorized through the myocardium overlying the papillary muscle, and fixed to an epicardial disc. Tension in these stitches was measured at a systolic blood pressure > 80 mmHg using a custom-made sliding caliper with a strain gauge mounted in line. This allowed assessment of the cyclic change from minimal diastolic to maximum systolic papillary muscle relocation stitch tension. RESULTS: Maximum cyclic change in the posterior papillary muscle (PPM) stitch tension was 1.1 N at 15 mm relocation. In comparison, the anterior papillary muscle (APM) tension was increased to a maximum of 1.4 N with only 5 mm relocation. Surprisingly, during each step of isolated PPM relocation, the APM stitch tension increased concomitantly, but in contrast APM relocation did not influence the magnitude of PPM stitch tension. There was no statistically significant difference between cyclic changes in APM and PPM stitch tension at any step of relocation. CONCLUSION: Papillary muscle relocation using stitches attached between epicardial discs and respective trigones induced a cyclic change in papillary muscle relocation stitch tension of 1.1-1.4 N. These values were in the range of normal tension in the mitral valve apparatus, and equivalent to only 19-24% of the total papillary muscle forces. Therefore, this technique does not appear to induce a non-physiologically high cyclic load on the mitral valve complex.

Mitral valve mechanics following posterior leaflet patch augmentation.

BACKGROUND AND AIM OF THE STUDY: Attention towards the optimization of mitral valve repair methods is increasing. Patch augmentation is one strategy used to treat functional ischemic mitral regurgitation (FIMR). The study aim was to investigate the force balance changes in specific chordae tendineae emanating from the posterior papillary muscle in a FIMR-simulated valve, following posterior leaflet patch augmentation. METHODS: Mitral valves were obtained from 12 pigs (body weight 80 kg). An in vitro test set-up simulating the left ventricle was used to hold the valves. The left ventricular pressure was regulated with water to simulate different static pressures during valve closure. A standardized oval pericardial patch (17 x 29 mm) was introduced into the posterior leaflet from mid P2 to the end of the P3 scallop. Dedicated miniature transducers were used to record the forces exerted on the chordae tendineae. Data were acquired before and after 12 mm posterior and 5 mm apical posterior papillary muscle displacement to simulate the effect from one of the main contributors of FIMR, before and after patch augmentation. RESULTS: The effect of displacing the posterior papillary muscle induced tethering on the intermediate chordae tendineae to the posterior leaflet, and resulted in a 39.8% force increase (p = 0.014). Posterior leaflet patch augmentation of the FIMR valve induced a 31.1% force decrease (p = 0.007). There was no difference in force between the healthy and the repaired valve simulations (p = 0.773). CONCLUSION: Posterior leaflet patch augmentation significantly reduced the forces exerted on the intermediate chordae tendineae from the posterior papillary muscle following FIMR simulation. As changes in chordal tension lead to a redistribution of the total stress exerted on the valve, patch augmentation may have an adverse long-term influence on mitral valve function and remodeling.

Principles of conduction and hydrophobic gating in K+ channels.

We present the first atomic-resolution observations of permeation and gating in a K(+) channel, based on molecular dynamics simulations of the Kv1.2 pore domain. Analysis of hundreds of simulated permeation events revealed a detailed conduction mechanism, resembling the Hodgkin-Keynes "knock-on" model, in which translocation of two selectivity filter-bound ions is driven by a third ion; formation of this knock-on intermediate is rate determining. In addition, at reverse or zero voltages, we observed pore closure by a novel "hydrophobic gating" mechanism: A dewetting transition of the hydrophobic pore cavity-fastest when K(+) was not bound in selectivity filter sites nearest the cavity-caused the open, conducting pore to collapse into a closed, nonconducting conformation. Such pore closure corroborates the idea that voltage sensors can act to prevent pore collapse into the intrinsically more stable, closed conformation, and it further suggests that molecular-scale dewetting facilitates a specific biological function: K(+) channel gating. Existing experimental data support our hypothesis that hydrophobic gating may be a fundamental principle underlying the gating of voltage-sensitive K(+) channels. We suggest that hydrophobic gating explains, in part, why diverse ion channels conserve hydrophobic pore cavities, and we speculate that modulation of cavity hydration could enable structural determination of both open and closed channels.

Gating and modulation of an inward-rectifier potassium channel.

Inward-rectifier potassium channels (Kirs) are lipid-gated ion channels that differ from other K+ channels in that they allow K+ ions to flow more easily into, rather than out of, the cell. Inward rectification is known to result from endogenous magnesium ions or polyamines (e.g., spermine) binding to Kirs, resulting in a block of outward potassium currents, but questions remain regarding the structural and dynamic basis of the rectification process and lipid-dependent channel activation. Here, we present the results of long-timescale molecular dynamics simulations starting from a crystal structure of phosphatidylinositol 4,5-bisphosphate (PIP2)-bound chicken Kir2.2 with a non-conducting pore. After introducing a mutation (G178R) that is known to increase the open probability of a homologous channel, we were able to observe transitions to a stably open, ion-conducting pore, during which key conformational changes occurred in the main activation gate and the cytoplasmic domain. PIP2 binding appeared to increase stability of the pore in its open and conducting state, as PIP2 removal resulted in pore closure, with a median closure time about half of that with PIP2 present. To investigate structural details of inward rectification, we simulated spermine binding to and unbinding from the open pore conformation at positive and negative voltages, respectively, and identified a spermine-binding site located near a previously hypothesized site between the pore cavity and the selectivity filter. We also studied the effects of long-range electrostatics on conduction and spermine binding by mutating charged residues in the cytoplasmic domain and found that a finely tuned charge density, arising from basic and acidic residues within the cytoplasmic domain, modulated conduction and rectification.

Functional dynamics and allosteric modulation of TRPA1.

The cation channel TRPA1 is a potentially important drug target, and characterization of TRPA1 functional dynamics might help guide structure-based drug design. Here, we present results from long-timescale molecular dynamics simulations of TRPA1 with an allosteric activator, allyl isothiocyanate (AITC), in which we observed spontaneous transitions from a closed, non-conducting channel conformation into an open, conducting conformation. Based on these transitions, we propose a gating mechanism in which movement of a regulatory TRP-like domain allosterically translates into pore opening in a manner reminiscent of pore opening in voltage-gated ion channels. In subsequent experiments, we found that mutations that disrupt packing of the S4-S5 linker-TRP-like domain and the S5 and S6 helices also affected channel activity. In simulations, we also observed A-967079, a known allosteric inhibitor, binding between helices S5 and S6, suggesting that A-967079 may suppress activity by stabilizing a non-conducting pore conformation-a finding consistent with our proposed gating mechanism.

Desensitization dynamics of the AMPA receptor.

To perform their physiological functions, amino methyl propionic acid receptors (AMPARs) cycle through active, resting, and desensitized states, and dysfunction in AMPAR activity is associated with various neurological disorders. Transitions among AMPAR functional states, however, are largely uncharacterized at atomic resolution and are difficult to examine experimentally. Here, we report long-timescale molecular dynamics simulations of dimerized AMPAR ligand-binding domains (LBDs), whose conformational changes are tightly coupled to changes in AMPAR functional states, in which we observed LBD dimer activation and deactivation upon ligand binding and unbinding at atomic resolution. Importantly, we observed the ligand-bound LBD dimer transition from the active conformation to several other conformations, which may correspond with distinct desensitized conformations. We also identified a linker region whose structural rearrangements heavily affected the transitions to and among these putative desensitized conformations, and confirmed, using electrophysiology experiments, the importance of the linker region in these functional transitions.

Design, fabrication, and evaluation of 3-D-printed cystotomy spoons as a retrieval method in dogs.

OBJECTIVE: Current cystotomy methods often implement the use of off-label devices, resulting in urocystolith extraction difficulty and potentially leading to postoperative complications and discomfort for the patient. The objective of this study was to create 3 novel 3-D printed cystotomy spoons that offer a dedicated solution for removing urocystoliths from a patient's urinary bladder. ANIMALS: Clinical use of the 3 novel 3-D printed cystotomy spoons were ultimately evaluated in 4 dogs and 1 cat that presented for urocystotlith removal at 3 different veterinary hospitals in northwest Arkansas. METHODS: The novel cystotomy spoons were designed using SolidWorks, 3-D printed with a Dental Surgical Guide resin, and underwent prototype testing that included chlorhexidine soaking, autoclave sterilization, 3-point bend testing, and Finite Element Analysis. The efficiency of the spoons was then evaluated through a limited proof-of-concept study utilizing a postoperative questionnaire for the participating clinicians. RESULTS: Practitioner feedback indicated positive experiences using 1 or more of the novel 3-D printed cystotomy spoons while performing a cystotomy surgery. However, successful use of the spoons was ultimately limited to dogs in the 23 to 34 kg weight range. CLINICAL RELEVANCE: Novel 3-D printed cystotomy spoons have the potential to mediate urocystolith extraction difficulty and reduce postoperative complications. Additionally, this research demonstrates how veterinarians might develop custom 3-D models and prints to meet patient-specific needs. As such, further development could impact the standard of healthcare and the veterinary industry by promoting the use of additive manufacturing in veterinary medicine.

Utilization of Engineering Advances for Detailed Biomechanical Characterization of the Mitral-Ventricular Relationship to Optimize Repair Strategies: A Comprehensive Review.

The geometrical details and biomechanical relationships of the mitral valve-left ventricular apparatus are very complex and have posed as an area of research interest for decades. These characteristics play a major role in identifying and perfecting the optimal approaches to treat diseases of this system when the restoration of biomechanical and mechano-biological conditions becomes the main target. Over the years, engineering approaches have helped to revolutionize the field in this regard. Furthermore, advanced modelling modalities have contributed greatly to the development of novel devices and less invasive strategies. This article provides an overview and narrative of the evolution of mitral valve therapy with special focus on two diseases frequently encountered by cardiac surgeons and interventional cardiologists: ischemic and degenerative mitral regurgitation.

A conserved protonation-dependent switch controls drug binding in the Abl kinase.

In many protein kinases, a characteristic conformational change (the "DFG flip") connects catalytically active and inactive conformations. Many kinase inhibitors--including the cancer drug imatinib--selectively target a specific DFG conformation, but the function and mechanism of the flip remain unclear. Using long molecular dynamics simulations of the Abl kinase, we visualized the DFG flip in atomic-level detail and formulated an energetic model predicting that protonation of the DFG aspartate controls the flip. Consistent with our model's predictions, we demonstrated experimentally that the kinetics of imatinib binding to Abl kinase have a pH dependence that disappears when the DFG aspartate is mutated. Our model suggests a possible explanation for the high degree of conservation of the DFG motif: that the flip, modulated by electrostatic changes inherent to the catalytic cycle, allows the kinase to access flexible conformations facilitating nucleotide binding and release.

Identification of two distinct inactive conformations of the beta2-adrenergic receptor reconciles structural and biochemical observations.

Fully understanding the mechanisms of signaling proteins such as G protein-coupled receptors (GPCRs) will require the characterization of their conformational states and the pathways connecting those states. The recent crystal structures of the beta(2)- and beta(1)-adrenergic receptors in a nominally inactive state constituted a major advance toward this goal, but also raised new questions. Although earlier biochemical observations had suggested that these receptors possessed a set of contacts between helices 3 and 6, known as the ionic lock, which was believed to form a molecular switch for receptor activation, the crystal structures lacked these contacts. The unexpectedly broken ionic lock has raised questions about the true conformation(s) of the inactive state and the role of the ionic lock in receptor activation and signaling. To address these questions, we performed microsecond-timescale molecular dynamics simulations of the beta(2)-adrenergic receptor (beta(2)AR) in multiple wild-type and mutant forms. In wild-type simulations, the ionic lock formed reproducibly, bringing the intracellular ends of helices 3 and 6 together to adopt a conformation similar to that found in inactive rhodopsin. Our results suggest that inactive beta(2)AR exists in equilibrium between conformations with the lock formed and the lock broken, whether or not the cocrystallized ligand is present. These findings, along with the formation of several secondary structural elements in the beta(2)AR loops during our simulations, may provide a more comprehensive picture of the inactive state of the beta-adrenergic receptors, reconciling the crystal structures with biochemical studies.