The molecular basis of the steep force-calcium relation in heart muscle

Mechanisms of myosin II force generation: insights from novel experimental techniques and approaches

Myosin II is a molecular motor that converts chemical energy derived from ATP hydrolysis into mechanical work. Myosin II isoforms are responsible for muscle contraction and a range of cell functions relying on the development of force and motion. When the motor attaches to actin, ATP is hydrolyzed and inorganic phosphate (Pi) and ADP are released from its active site. These reactions are coordinated with changes in the structure of myosin, promoting the so-called "power stroke" that causes the sliding of actin filaments. The general features of the myosin-actin interactions are well accepted, but there are critical issues that remain poorly understood, mostly due to technological limitations. In recent years, there has been a significant advance in structural, biochemical, and mechanical methods that have advanced the field considerably. New modeling approaches have also allowed researchers to understand actomyosin interactions at different levels of analysis. This paper reviews recent studies looking into the interaction between myosin II and actin filaments, which leads to power stroke and force generation. It reviews studies conducted with single myosin molecules, myosins working in filaments, muscle sarcomeres, myofibrils, and fibers. It also reviews the mathematical models that have been used to understand the mechanics of myosin II in approaches focusing on single molecules to ensembles. Finally, it includes brief sections on translational aspects, how changes in the myosin motor by mutations and/or posttranslational modifications may cause detrimental effects in diseases and aging, among other conditions, and how myosin II has become an emerging drug target.

Molecular determinants of skeletal muscle force loss in response to 5 days of dry immersion in human

BACKGROUND

Astronauts in Earth's orbit experience microgravity, resulting in a decline of skeletal muscle mass and function. On Earth, models simulating microgravity have shown that the extent of the loss in muscle force is greater than the loss in muscle mass. The reasons behind this disproportionate loss of muscle force are still poorly understood. In the present study, we hypothesize that alongside the loss in skeletal muscle mass, modifications in the expression profile of genes encoding critical determinants of resting membrane potential, excitation-contraction coupling and Ca2+ handling contribute to the decline in skeletal muscle force.

METHODS

Healthy male volunteers (n = 18) participated in a 5-day dry immersion (DI) study, an Earth-based model of simulated microgravity. Muscle force measurement and MRI analysis of the cross-sectional area of thigh muscles were performed before and after DI. Biopsies of the vastus lateralis skeletal muscle performed before and after DI were used for the determination Ca2+ properties of isolated muscle fibres, molecular and biochemical analyses.

RESULTS

The extent of the decline in force, measured as maximal voluntary contraction of knee extensors (-11.1%, P < 0.01) was higher than the decline in muscle mass (-2.5%, P < 0.01). The decline in muscle mass was molecularly supported by a significant repression of the anabolic IGF-1/Akt/mTOR pathway (-19.9% and -40.9% in 4E-BP1 and RPS6 phosphorylation, respectively), a transcriptional downregulation of the autophagy-lysosome pathway and a downregulation in the mRNA levels of myofibrillar protein slow isoforms. At the single fibre level, biochemical and tension-pCa curve analyses showed that the loss in force was independent of fibre type (-11% and -12.3% in slow and fast fibres, respectively) and Ca2+ activation properties. Finally, we showed a significant remodelling in the expression of critical players of resting membrane potential (aquaporin 4: -24.9%, ATP1A2: +50.4%), excitation-contraction coupling (CHRNA1: +75.1%, CACNA2D1: -23.5%, JPH2: -24.2%, TRDN: -15.6%, S100A1: +27.2%), and Ca2+ handling (ATP2A2: -32.5%, CASQ1: -15%, ORAI1: -36.2%, ATP2B1: -19.1%).

CONCLUSIONS

These findings provide evidence that a deregulation in the expression profile of critical molecular determinants of resting membrane potential, excitation-contraction coupling, and Ca2+ handling could be involved in the loss of muscle force induced by DI. They also provide the paradigm for the understanding of muscle force loss during prolonged bed rest periods as those encountered in intensive care unit.

Exosomal Preconditioning of Human iPSC-Derived Cardiomyocytes Beneficially Alters Cardiac Electrophysiology and Micro RNA Expression

Experimental evidence, both in vitro and in vivo, has indicated cardioprotective effects of extracellular vesicles (EVs) derived from various cell types, including induced pluripotent stem cell-derived cardiomyocytes. The biological effects of EV secretion, particularly in the context of ischemia and cardiac electrophysiology, remain to be fully explored. Therefore, the goal of this study was to unveil the effects of exosome (EXO)-mediated cell-cell signaling during hypoxia by employing a simulated preconditioning approach on human-induced pluripotent stem cell-derived cardiomyocytes (hIPSC-CMs). Electrophysiological activity of hIPSC-CMs was measured using a multielectrode array (MEA) system. A total of 16 h of hypoxic stress drastically increased the beat period. Moreover, hIPSC-CMs preconditioned with EXOs displayed significantly longer beat periods compared with non-treated cells after 16 h of hypoxia (+15.7%, p < 0.05). Furthermore, preconditioning with hypoxic EXOs resulted in faster excitation-contraction (EC) coupling compared with non-treated hIPSC-CMs after 16 h of hypoxia (-25.3%, p < 0.05). Additionally, microRNA (miR) sequencing and gene target prediction analysis of the non-treated and pre-conditioned hIPSC-CMs identified 10 differentially regulated miRs and 44 gene targets. These results shed light on the intricate involvement of miRs, emphasizing gene targets associated with cell survival, contraction, apoptosis, reactive oxygen species (ROS) regulation, and ion channel modulation. Overall, this study demonstrates that EXOs secreted by hIPSC-CM during hypoxia beneficially alter electrophysiological properties in recipient cells exposed to hypoxic stress, which could play a crucial role in the development of targeted interventions to improve outcomes in ischemic heart conditions.

Characterization of NEB pathogenic variants in patients reveals novel nemaline myopathy disease mechanisms and omecamtiv mecarbil force effects

Nebulin, a critical protein of the skeletal muscle thin filament, plays important roles in physiological processes such as regulating thin filament length (TFL), cross-bridge cycling, and myofibril alignment. Pathogenic variants in the nebulin gene (NEB) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking curative therapies. In this study, we examined a cohort of ten NEM2 patients, each with unique pathogenic variants, aiming to understand their impact on mRNA, protein, and functional levels. Results show that pathogenic truncation variants affect NEB mRNA stability and lead to nonsense-mediated decay of the mutated transcript. Moreover, a high incidence of cryptic splice site activation was found in patients with pathogenic splicing variants that are expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with either relatively normal or markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, or reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a pathogenic duplication variant in nebulin that resulted in a four-copy gain in the triplicate region of NEB and a much larger nebulin protein and longer TFL. Additionally, we investigated the effect of Omecamtiv mecarbil (OM), a small-molecule activator of cardiac myosin, on force production of type 1 muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87 to 318%, with the largest effects in patients with the lowest level of nebulin. In summary, this study indicates that post-transcriptional or post-translational mechanisms regulate nebulin expression. Moreover, we propose that the pathomechanism of NEM2 involves not only shortened but also elongated thin filaments, along with the disruption of actin-binding sites resulting from pathogenic splicing variants. Significantly, our findings highlight the potential of OM treatment to improve skeletal muscle function in NEM2 patients, especially those with large reductions in nebulin levels.

An In Silico Cardiomyocyte Reveals the Impact of Changes in CaMKII Signalling on Cardiomyocyte Contraction Kinetics in Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is characterised by asymmetric left ventricular hypertrophy, ventricular arrhythmias, and cardiomyocyte dysfunction that may cause sudden death. HCM is associated with mutations in sarcomeric proteins and is usually transmitted as an autosomal-dominant trait. The aim of this in silico study was to assess the mechanisms that underlie the altered electrophysiological activity, contractility, regulation of energy metabolism, and crossbridge cycling in HCM at the single-cell level. To investigate this, we developed a human ventricular cardiomyocyte model that incorporates electrophysiology, metabolism, and force generation. The model was validated by its ability to reproduce the experimentally observed kinetic properties of human HCM induced by (a) remodelling of several ion channels and Ca2+-handling proteins arising from altered Ca2+/calmodulin kinase II signalling pathways and (b) increased Ca2+ sensitivity of the myofilament proteins. Our simulation showed a decreased phosphocreatine-to-ATP ratio (-9%) suggesting a negative mismatch between energy expenditure and supply. Using a spatial myofilament half-sarcomere model, we also compared the fraction of detached, weakly bound, and strongly bound crossbridges in the control and HCM conditions. Our simulations showed that HCM has more crossbridges in force-producing states than in the control condition. In conclusion, our model reveals that impaired crossbridge kinetics is accompanied by a negative mismatch between the ATP supply and demand ratio. This suggests that improving this ratio may reduce the incidence of sudden death in HCM.

The cardiomyocyte origins of diastolic dysfunction: cellular components of myocardial "stiffness"

The impaired ability of the heart to relax and stretch to accommodate venous return is generally understood to represent a state of "diastolic dysfunction" and often described using the all-purpose noun "stiffness." Despite the now common qualitative usage of this term in fields of cardiac patho/physiology, the specific quantitative concept of stiffness as a molecular and biophysical entity with real practical interpretation in healthy and diseased hearts is sometimes obscure. The focus of this review is to characterize the concept of cardiomyocyte stiffness and to develop interpretation of "stiffness" attributes at the cellular and molecular levels. Here, we consider "stiffness"-related terminology interpretation and make links between cardiomyocyte stiffness and aspects of functional and structural cardiac performance. We discuss cross bridge-derived stiffness sources, considering the contributions of diastolic myofilament activation and impaired relaxation. This includes commentary relating to the role of cardiomyocyte Ca2+ flux and Ca2+ levels in diastole, the troponin-tropomyosin complex role as a Ca2+ effector in diastole, the myosin ADP dissociation rate as a modulator of cross bridge attachment and regulation of cross-bridge attachment by myosin binding protein C. We also discuss non-cross bridge-derived stiffness sources, including the titin sarcomeric spring protein, microtubule and intermediate filaments, and cytoskeletal extracellular matrix interactions. As the prevalence of conditions involving diastolic heart failure has escalated, a more sophisticated understanding of the molecular, cellular, and tissue determinants of cardiomyocyte stiffness offers potential to develop imaging and molecular intervention tools.

Effect of the N-terminal extension in myosin essential light chain A1 on the mechanism of actomyosin ATP hydrolysis

Myosin essential light chains A1 and A2 are identical isoforms except for an extension of ∼40 amino acids at the N terminus of A1 that binds F-actin. The extension has no bearing on the burst hydrolysis rate (M-ATP → M-ADP-Pi) as determined by chemical quench flow (100 μM isoenzyme). Whereas actomyosin-S1A2 steady state MgATPase (low ionic strength, 20 °C) is hyperbolically dependent on concentration: Vmax 7.6 s-1, Kapp 6.4 μM (F-actin) and Vmax 10.1 s-1, Kapp 5.5 μM (native thin filaments, pCa 4), the relationship for myosin-S1A1 is bimodal; an initial rise at low concentration followed by a decline to one-third the Vmax of S1A2, indicative of more than one rate-limiting step and A1-enforced flux through the slower actomyosin-limited hydrolysis pathway. In double-mixing stopped-flow with an indicator, Ca(II)-mediated activation of Pi dissociation (regulatedAM-ADP-Pi → regulatedAM-ADP + Pi) is attenuated by A1 attachment to thin filaments (pCa 4). The maximum accelerated rates of Pi dissociation are: 81 s-1 (S1A1, Kapp 8.9 μM) versus 129 s-1 (S1A2, Kapp 58 μM). To investigate apomyosin-S1-mediated activation, thin filaments (EGTA) are premixed with a given isomyosin-S1 and double-mixing is repeated with myosin-S1A1 in the first mix. Similar maximum rates of Pi dissociation are observed, 44.5 s-1 (S1A1) and 47.1 s-1 (S1A2), which are lower than for Ca(II) activation. Overall, these results biochemically demonstrate how the longer light chain A1 can contribute to slower contraction and higher force and the shorter version A2 to faster contraction and lower force, consistent with their distribution in different types of striated muscle.

Advancing clinical translation of cardiac biomechanics models: a comprehensive review, applications and future pathways

Cardiac mechanics models are developed to represent a high level of detail, including refined anatomies, accurate cell mechanics models, and platforms to link microscale physiology to whole-organ function. However, cardiac biomechanics models still have limited clinical translation. In this review, we provide a picture of cardiac mechanics models, focusing on their clinical translation. We review the main experimental and clinical data used in cardiac models, as well as the steps followed in the literature to generate anatomical meshes ready for simulations. We describe the main models in active and passive mechanics and the different lumped parameter models to represent the circulatory system. Lastly, we provide a summary of the state-of-the-art in terms of ventricular, atrial, and four-chamber cardiac biomechanics models. We discuss the steps that may facilitate clinical translation of the biomechanics models we describe. A well-established software to simulate cardiac biomechanics is lacking, with all available platforms involving different levels of documentation, learning curves, accessibility, and cost. Furthermore, there is no regulatory framework that clearly outlines the verification and validation requirements a model has to satisfy in order to be reliably used in applications. Finally, better integration with increasingly rich clinical and/or experimental datasets as well as machine learning techniques to reduce computational costs might increase model reliability at feasible resources. Cardiac biomechanics models provide excellent opportunities to be integrated into clinical workflows, but more refinement and careful validation against clinical data are needed to improve their credibility. In addition, in each context of use, model complexity must be balanced with the associated high computational cost of running these models.

Local Control Model of a Human Ventricular Myocyte: An Exploration of Frequency-Dependent Changes and Calcium Sparks

Calcium (Ca2+) sparks are the elementary events of excitation-contraction coupling, yet they are not explicitly represented in human ventricular myocyte models. A stochastic ventricular cardiomyocyte human model that adapts to intracellular Ca2+ ([Ca2+]i) dynamics, spark regulation, and frequency-dependent changes in the form of locally controlled Ca2+ release was developed. The 20,000 CRUs in this model are composed of 9 individual LCCs and 49 RyRs that function as couplons. The simulated action potential duration at 1 Hz steady-state pacing is ~0.280 s similar to human ventricular cell recordings. Rate-dependence experiments reveal that APD shortening mechanisms are largely contributed by the L-type calcium channel inactivation, RyR open fraction, and [Ca2+]myo concentrations. The dynamic slow-rapid-slow pacing protocol shows that RyR open probability during high pacing frequency (2.5 Hz) switches to an adapted "nonconducting" form of Ca2+-dependent transition state. The predicted force was also observed to be increased in high pacing, but the SR Ca2+ fractional release was lower due to the smaller difference between diastolic and systolic [Ca2+]SR. Restitution analysis through the S1S2 protocol and increased LCC Ca2+-dependent activation rate show that the duration of LCC opening helps modulate its effects on the APD restitution at different diastolic intervals. Ultimately, a longer duration of calcium sparks was observed in relation to the SR Ca2+ loading at high pacing rates. Overall, this study demonstrates the spontaneous Ca2+ release events and ion channel responses throughout various stimuli.

The Role of Ca2+ Sparks in Force Frequency Relationships in Guinea Pig Ventricular Myocytes

Calcium sparks are the elementary Ca2+ release events in excitation-contraction coupling that underlie the Ca2+ transient. The frequency-dependent contractile force generated by cardiac myocytes depends upon the characteristics of the Ca2+ transients. A stochastic computational local control model of a guinea pig ventricular cardiomyocyte was developed, to gain insight into mechanisms of force-frequency relationship (FFR). This required the creation of a new three-state RyR2 model that reproduced the adaptive behavior of RyR2, in which the RyR2 channels transition into a different state when exposed to prolonged elevated subspace [Ca2+]. The model simulations agree with previous experimental and modeling studies on interval-force relations. Unlike previous common pool models, this local control model displayed stable action potential trains at 7 Hz. The duration and the amplitude of the [Ca2+]myo transients increase in pacing rates consistent with the experiments. The [Ca2+]myo transient reaches its peak value at 4 Hz and decreases afterward, consistent with experimental force-frequency curves. The model predicts, in agreement with previous modeling studies of Jafri and co-workers, diastolic sarcoplasmic reticulum, [Ca2+]sr, and RyR2 adaptation increase with the increased stimulation frequency, producing rising, rather than falling, amplitude of the myoplasmic [Ca2+] transients. However, the local control model also suggests that the reduction of the L-type Ca2+ current, with an increase in pacing frequency due to Ca2+-dependent inactivation, also plays a role in the negative slope of the FFR. In the simulations, the peak Ca2+ transient in the FFR correlated with the highest numbers of SR Ca2+ sparks: the larger average amplitudes of those sparks, and the longer duration of the Ca2+ sparks.

Monoclonal Antibodies as Probes to Study Ligand-Induced Conformations of Troponin Subunits

Striated muscle contraction and relaxation is regulated by Ca2+ at the myofilament level via conformational modulations of the troponin complex. To understand the structure-function relationship of troponin in normal muscle and in myopathies, it is necessary to study the functional effects of troponin isoforms and mutations at the level of allosteric conformations of troponin subunits. Traditional methodologies assessing such conformational studies are laborious and require significant amounts of purified protein, while many current methodologies require non-physiological conditions or labeling of the protein, which may affect their physiological conformation and function. To address these issues, we developed a novel approach using site-specific monoclonal antibodies (mAb) as molecular probes to detect and monitor conformational changes of proteins. Here, we present examples for its application in studies of two subunits of troponin: the Ca2+-binding subunit, TnC, and the tropomyosin-binding/thin filament-anchoring subunit, TnT. Studies using a high-throughput microplate assay are compared with that using localized surface plasmon resonance (LSPR) to demonstrate the effectiveness of using mAb probes to assess ligand-induced conformations of troponin subunits in physiological conditions. The assays utilize relatively small amounts of protein and are free of protein modification, which may bias results. Detailed methodologies using various monoclonal antibodies (mAbs) are discussed with considerations for the optimization of assay conditions and the broader application in studies of other proteins as well as in screening of therapeutic reagents that bind a specific target site with conformational and functional effects.

Functional and Molecular Characterisation of Heart Failure Progression in Mice and the Role of Myosin Regulatory Light Chains in the Recovery of Cardiac Muscle Function

Heart failure (HF) as a result of myocardial infarction (MI) is a major cause of fatality worldwide. However, the cause of cardiac dysfunction succeeding MI has not been elucidated at a sarcomeric level. Thus, studying the alterations within the sarcomere is necessary to gain insights on the fundamental mechansims leading to HF and potentially uncover appropriate therapeutic targets. Since existing research portrays regulatory light chains (RLC) to be mediators of cardiac muscle contraction in both human and animal models, its role was further explored In this study, a detailed characterisation of the physiological changes (i.e., isometric force, calcium sensitivity and sarcomeric protein phosphorylation) was assessed in an MI mouse model, between 2D (2 days) and 28D post-MI, and the changes were related to the phosphorylation status of RLCs. MI mouse models were created via complete ligation of left anterior descending (LAD) coronary artery. Left ventricular (LV) papillary muscles were isolated and permeabilised for isometric force and Ca²⁺ sensitivity measurement, while the LV myocardium was used to assay sarcomeric proteins’ (RLC, troponin I (TnI) and myosin binding protein-C (MyBP-C)) phosphorylation levels and enzyme (myosin light chain kinase (MLCK), zipper interacting protein kinase (ZIPK) and myosin phosphatase target subunit 2 (MYPT2)) expression levels. Finally, the potential for improving the contractility of diseased cardiac papillary fibres via the enhancement of RLC phosphorylation levels was investigated by employing RLC exchange methods, in vitro. RLC phosphorylation and isometric force potentiation were enhanced in the compensatory phase and decreased in the decompensatory phase of HF failure progression, respectively. There was no significant time-lag between the changes in RLC phosphorylation and isometric force during HF progression, suggesting that changes in RLC phosphorylation immediately affect force generation. Additionally, the in vitro increase in RLC phosphorylation levels in 14D post-MI muscle segments (decompensatory stage) enhanced its force of isometric contraction, substantiating its potential in HF treatment. Longitudinal observation unveils potential mechanisms involving MyBP-C and key enzymes regulating RLC phosphorylation, such as MLCK and MYPT2 (subunit of MLCP), during HF progression. This study primarily demonstrates that RLC phosphorylation is a key sarcomeric protein modification modulating cardiac function. This substantiates the possibility of using RLCs and their associated enzymes to treat HF.

Computationally efficient model of myocardial electromechanics for multiscale simulations

A model of myocardial electromechanics is suggested. It combines modified and simplified versions of previously published models of cardiac electrophysiology, excitation-contraction coupling, and mechanics. The mechano-calcium and mechano-electrical feedbacks, including the strain-dependence of the propagation velocity of the action potential, are also accounted for. The model reproduces changes in the twitch amplitude and Ca2+-transients upon changes in muscle strain including the slow response. The model also reproduces the Bowditch effect and changes in the twitch amplitude and duration upon changes in the interstimulus interval, including accelerated relaxation at high stimulation frequency. Special efforts were taken to reduce the stiffness of the differential equations of the model. As a result, the equations can be integrated numerically with a relatively high time step making the model suitable for multiscale simulation of the human heart and allowing one to study the impact of myocardial mechanics on arrhythmias.

Omecamtiv Mecarbil: A Novel Mechanistic and Therapeutic Approach to Chronic Heart Failure Management

Heart failure (HF) is a major public health problem in the United States as well as worldwide. Chronic heart failure is a syndrome of reduced cardiac output resulting from impaired ventricular function, impaired filling, or a combination of both. Associated symptoms include dyspnea, fatigue, and decreased exercise tolerance. HF has a marked effect on morbidity and mortality, given limited therapeutic choices. The first line of therapeutic agents indicated in heart failure are beta-blockers. Other drugs and therapeutic modalities employed in HF treatment include angiotensin-receptor blockers (ARBs), sacubitril (neprilysin inhibitor) combination with the ARB, valsartan, small doses of aldosterone receptor antagonists (ARAs) in the setting of angiotensin-converting enzyme (ACE) inhibitors, and beta-blockers. Additionally, the sodium-glucose transporter-2 inhibitor, dapagliflozin in the setting of ACE inhibitors, ARBs, or sacubitril-valsartan plus beta-blocker have been employed. Other therapeutic modalities have included loop diuretics, digoxin, the hydralazine-isosorbide dinitrate combination, ivabradine, the inotropes, dobutamine, milrinone, and dopamine. Decreased cardiac contractility is central to the systolic HF. Therapeutic agents employed to increase cardiac contractility in HF are limited because of their mechanistic-related adverse effect profiles. Omecamtiv mecarbil (OM) is a first of its class cardiac myosin activator that increases the cardiac contractility by specifically binding to the catalytic S1 domain of cardiac myosin, to be employed in heart failure treatment. This agent has demonstrated benefit in reducing heart rate, peripheral vascular resistance, mean left arterial pressure, and left ventricular end-diastolic pressure in the animal models. Additionally, OM is known to improve systolic wall thickening, stroke volume (SV), and cardiac output (CO). OM increases systolic ejection time (SET), cardiac myocyte fractional shortening without significant increase of LV dP/dtmax, myocardial oxygen consumption, and myocyte intracellular calcium. The benefits of OM have been demonstrated through key trials, as (i) The Acute Treatment with Omecamtiv mecarbil to Increase Contractility in Acute Heart Failure (ATOMIC-AHF), and (ii) The Chronic Oral Study of Myosin Activation to Increase Contractility in Heart Failure (COSMIC-HF). The Global Approach to Lowering Adverse Cardiac Outcomes Through Improving Contractility in Heart Failure (GALACTIC-HF) trial is ongoing and can help provide further clinical data. OM provides a novel mechanism and therapeutic approach to managing patients with HF. Preclinical and clinical data suggest that OM capability can improve cardiac function, decrease ventricular wall stress, reverse ventricular remodeling, and promote sympathetic withdrawal.

In-silico human electro-mechanical ventricular modelling and simulation for drug-induced pro-arrhythmia and inotropic risk assessment

Human-based computational modelling and simulation are powerful tools to accelerate the mechanistic understanding of cardiac patho-physiology, and to develop and evaluate therapeutic interventions. The aim of this study is to calibrate and evaluate human ventricular electro-mechanical models for investigations on the effect of the electro-mechanical coupling and pharmacological action on human ventricular electrophysiology, calcium dynamics, and active contraction. The most recent models of human ventricular electrophysiology, excitation-contraction coupling, and active contraction were integrated, and the coupled models were calibrated using human experimental data. Simulations were then conducted using the coupled models to quantify the effects of electro-mechanical coupling and drug exposure on electrophysiology and force generation in virtual human ventricular cardiomyocytes and tissue. The resulting calibrated human electro-mechanical models yielded active tension, action potential, and calcium transient metrics that are in agreement with experiments for endocardial, epicardial, and mid-myocardial human samples. Simulation results correctly predicted the inotropic response of different multichannel action reference compounds and demonstrated that the electro-mechanical coupling improves the robustness of repolarisation under drug exposure compared to electrophysiology-only models. They also generated additional evidence to explain the partial mismatch between in-silico and in-vitro experiments on drug-induced electrophysiology changes. The human calibrated and evaluated modelling and simulation framework constructed in this study opens new avenues for future investigations into the complex interplay between the electrical and mechanical cardiac substrates, its modulation by pharmacological action, and its translation to tissue and organ models of cardiac patho-physiology.

Cardiac Disorders and Pathophysiology of Sarcomeric Proteins

The sarcomeric proteins represent the structural building blocks of heart muscle, which are essential for contraction and relaxation. During recent years, it has become evident that posttranslational modifications of sarcomeric proteins, in particular phosphorylation, tune cardiac pump function at rest and during exercise. This delicate, orchestrated interaction is also influenced by mutations, predominantly in sarcomeric proteins, which cause hypertrophic or dilated cardiomyopathy. In this review, we follow a bottom-up approach starting from a description of the basic components of cardiac muscle at the molecular level up to the various forms of cardiac disorders at the organ level. An overview is given of sarcomere changes in acquired and inherited forms of cardiac disease and the underlying disease mechanisms with particular reference to human tissue. A distinction will be made between the primary defect and maladaptive/adaptive secondary changes. Techniques used to unravel functional consequences of disease-induced protein changes are described, and an overview of current and future treatments targeted at sarcomeric proteins is given. The current evidence presented suggests that sarcomeres not only form the basis of cardiac muscle function but also represent a therapeutic target to combat cardiac disease.

Nonreentrant atrial tachycardia occurs independently of hypertrophic cardiomyopathy in RASopathy patients

Multifocal atrial tachycardia (MAT) has a well-known association with Costello syndrome, but is rarely described with related RAS/MAPK pathway disorders (RASopathies). We report 11 patients with RASopathies (Costello, Noonan, and Noonan syndrome with multiple lentigines [formerly LEOPARD syndrome]) and nonreentrant atrial tachycardias (MAT and ectopic atrial tachycardia) demonstrating overlap in cardiac arrhythmia phenotype. Similar overlap is seen in RASopathies with respect to skeletal, musculoskeletal and cutaneous abnormalities, dysmorphic facial features, and neurodevelopmental deficits. Nonreentrant atrial tachycardias may cause cardiac compromise if sinus rhythm is not restored expeditiously. Typical first-line supraventricular tachycardia anti-arrhythmics (propranolol and digoxin) were generally not effective in restoring or maintaining sinus rhythm in this cohort, while flecainide or amiodarone alone or in concert with propranolol were effective anti-arrhythmic agents for acute and chronic use. Atrial tachycardia resolved in all patients. However, a 4-month-old boy from the cohort was found asystolic (with concurrent cellulitis) and a second patient underwent cardiac transplant for heart failure complicated by recalcitrant atrial arrhythmia. While propranolol alone frequently failed to convert or maintain sinus rhythm, fleccainide or amiodarone, occasionally in combination with propranolol, was effective for RASopathy patient treatment for nonreentrant atrial arrhythmia. Our analysis shows that RASopathy patients may have nonreentrant atrial tachycardia with and without associated cardiac hypertrophy. While nonreentrant arrhythmia has been traditionally associated with Costello syndrome, this work provides an expanded view of RASopathy cardiac arrhythmia phenotype as we demonstrate mutant proteins throughout this signaling pathway can also give rise to ectopic and/or MAT.

Cardiac myosin activators for heart failure therapy: focus on omecamtiv mecarbil

Heart failure continues to be a major global health problem with a pronounced impact on morbidity and mortality and very limited drug treatment options especially with regard to inotropic therapy. Omecamtiv mecarbil is a first-in-class cardiac myosin activator, which increases the proportion of myosin heads that are tightly bound to actin and creates a force-producing state that is not associated with cytosolic calcium accumulation. Phase I and phase II studies have shown that it is safe and well tolerated. It produces dose-dependent increases in systolic ejection time (SET), stroke volume (SV), left ventricular ejection fraction (LVEF), and fractional shortening. In the ATOMIC-AHF trial, intravenous (IV) omecamtiv mecarbil did not improve dyspnoea overall but may have improved it in a high-dose group of acute heart failure patients. It did, however, increase SET, decrease left ventricular end-systolic diameter, and was well tolerated. The COSMIC-HF trial showed that a pharmacokinetic-based dose-titration strategy of oral omecamtiv mecarbil improved cardiac function and reduced ventricular diameters compared to placebo and had a similar safety profile. It also significantly reduced plasma N-terminal-pro B-type natriuretic peptide compared with placebo. The GALACTIC-HF trial is now underway and will compare omecamtiv mecarbil with placebo when added to current heart failure standard treatment in patients with chronic heart failure and reduced LVEF. It is expected to be completed in January 2021. The ongoing range of preclinical and clinical research on omecamtiv mecarbil will further elucidate its full range of pharmacological effects and its clinical usefulness in heart failure.

Functional significance of C-terminal mobile domain of cardiac troponin I

Ca²⁺-regulation of cardiac contractility is mediated through the troponin complex, which comprises three subunits: cTnC, cTnI, and cTnT. As intracellular [Ca²⁺] increases, cTnI reduces its binding interactions with actin to primarily interact with cTnC, thereby enabling contraction. A portion of this regulatory switching involves the mobile domain of cTnI (cTnI-MD), the role of which in muscle contractility is still elusive. To study the functional significance of cTnI-MD, we engineered two cTnI constructs in which the MD was truncated to various extents: cTnI(1-167) and cTnI(1-193). These truncations were exchanged for endogenous cTnI in skinned rat papillary muscle fibers, and their influence on Ca²⁺-activated contraction and cross-bridge cycling kinetics was assessed at short (1.9 μm) and long (2.2 μm) sarcomere lengths (SLs). Our results show that the cTnI(1-167) truncation diminished the SL-induced increase in Ca²⁺-sensitivity of contraction, but not the SL-dependent increase in maximal tension, suggesting an uncoupling between the thin and thick filament contributions to length dependent activation. Compared to cTnI(WT), both truncations displayed greater Ca²⁺-sensitivity and faster cross-bridge attachment rates at both SLs. Furthermore, cTnI(1-167) slowed MgADP release rate and enhanced cross-bridge binding. Our findings imply that cTnI-MD truncations affect the blocked-to closed-state transition(s) and destabilize the closed-state position of tropomyosin.

A simple model of cardiac muscle for multiscale simulation: Passive mechanics, crossbridge kinetics and calcium regulation

A simple model of cardiac muscle was designed for multiscale simulation of heart mechanics. Relaxed cardiac muscle was described as a transversally isotropic hyperelastic material. Active tension caused by actin-myosin crossbridges depends on the ensemble averaged strain of myosin heads bound to actin. Calcium activation was modeled by Ca²⁺ binding to troponin-C. To account for the dependence of troponin affinity for Ca²⁺ on myosin heads strongly bound to actin, the kinetics of troponin binding to Ca²⁺ in the overlap zone of the thin and thick filaments and outside it were separated. The changes in the length of these zones during muscle shortening or lengthening were accounted for explicitly. Simplified version of the model contains only 5 ordinary differential equations (ODE). Model parameters were estimated from a limited set of experiments with skeletal and cardiac muscle. Simulations have shown that model reproduces qualitatively a number of experimental observations: steady-state force-velocity and stiffness-velocity relations; mechanical responses to step changes in muscle length or load; steep Ca²⁺-tension relationship and its dependence on sarcomere length tension (the Frank-Starling mechanism); tension, shortening and Ca²⁺-transients in twitch isometric and isotonic contractions, tension development and redevelopment upon fast change in Ca²⁺ concentration or muscle release followed by re-stretch. We believe that the model can be effectively used for modeling contraction and relaxation of the heart.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Mechanical aberrations in hypetrophic cardiomyopathy: emerging concepts

Hypertrophic cardiomyopathy is the most common monogenic disorder in cardiology. Despite important advances in understanding disease pathogenesis, it is not clear how flaws in individual sarcomere components are responsible for the observed phenotype. The aim of this article is to provide a brief interpretative analysis of some currently proposed pathophysiological mechanisms of hypertrophic cardiomyopathy, with a special emphasis on alterations in the cardiac mechanical properties.

A Spatially Detailed Model of Isometric Contraction Based on Competitive Binding of Troponin I Explains Cooperative Interactions between Tropomyosin and Crossbridges

Biophysical models of cardiac tension development provide a succinct representation of our understanding of force generation in the heart. The link between protein kinetics and interactions that gives rise to high cooperativity is not yet fully explained from experiments or previous biophysical models. We propose a biophysical ODE-based representation of cross-bridge (XB), tropomyosin and troponin within a contractile regulatory unit (RU) to investigate the mechanisms behind cooperative activation, as well as the role of cooperativity in dynamic tension generation across different species. The model includes cooperative interactions between regulatory units (RU-RU), between crossbridges (XB-XB), as well more complex interactions between crossbridges and regulatory units (XB-RU interactions). For the steady-state force-calcium relationship, our framework predicts that: (1) XB-RU effects are key in shifting the half-activation value of the force-calcium relationship towards lower [Ca²⁺], but have only small effects on cooperativity. (2) XB-XB effects approximately double the duty ratio of myosin, but do not significantly affect cooperativity. (3) RU-RU effects derived from the long-range action of tropomyosin are a major factor in cooperative activation, with each additional unblocked RU increasing the rate of additional RU’s unblocking. (4) Myosin affinity for short (1–4 RU) unblocked stretches of actin of is very low, and the resulting suppression of force at low [Ca²⁺] is a major contributor in the biphasic force-calcium relationship. We also reproduce isometric tension development across mouse, rat and human at physiological temperature and pacing rate, and conclude that species differences require only changes in myosin affinity and troponin I/troponin C affinity. Furthermore, we show that the calcium dependence of the rate of tension redevelopment k_tr is explained by transient blocking of RU’s by a temporary decrease in XB-RU effects.

Force generation in cardiac muscle cells is driven by changes in calcium concentration. Relatively small changes in the calcium concentration over the course of a heart beat lead to the large changes in force required to fully contract and relax the heart. This is known as ‘cooperative activation’, and involves a complex interaction of several proteins involved in contraction. Current computer models which reproduce force generation often do not represent these processes explicitly, and stochastic approaches that do tend to require large amounts of computational power to solve, which limit the range of investigations in which they can be used. We have created an new computational model that captures the underlying physiological processes in more detail, and is more efficient than stochastic approaches, while still being able to run a large range of simulations. The model is able to explain the biological processes leading to the cooperative activation of muscle. In addition, the model reproduces how this cooperative activation translates to normal muscle function to generate force from changes in calcium across three different species.

Effect of levosimendan on the contractility of muscle fibers from nemaline myopathy patients with mutations in the nebulin gene

Background

Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. To date, no therapy exists that enhances the contractile strength of muscles of NM patients. Mutations in NEB, encoding the giant protein nebulin, are the most common cause of NM. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. We propose that the lower calcium-sensitivity of force generation in NEB-NM offers a therapeutic target. Levosimendan is a calcium sensitizer that is approved for use in humans and has been developed to target cardiac muscle fibers. It exerts its effect through binding to slow skeletal/cardiac troponin C. As slow skeletal/cardiac troponin C is also the dominant troponin C isoform in slow-twitch skeletal muscle fibers, we hypothesized that levosimendan improves slow-twitch muscle fiber strength at submaximal levels of activation in patients with NEB-NM.

Methods

To test whether levosimendan affects force production, permeabilized slow-twitch muscle fibers isolated from biopsies of NEB-NM patients and controls were exposed to levosimendan and the force response was measured.

Results

No effect of levosimendan on muscle fiber force in NEB-NM and control skeletal muscle fibers was found, both at a submaximal calcium level using incremental levosimendan concentrations, and at incremental calcium concentrations in the presence of levosimendan. In contrast, levosimendan did significantly increase the calcium-sensitivity of force in human single cardiomyocytes. Protein analysis confirmed that the slow skeletal/cardiac troponin C isoform was present in the skeletal muscle fibers tested.

Conclusions

These findings indicate that levosimendan does not improve the contractility in human skeletal muscle fibers, and do not provide rationale for using levosimendan as a therapeutic to restore muscle weakness in NEB-NM patients. We stress the importance of searching for compounds that improve the calcium-sensitivity of force generation of slow-twitch muscle fibers. Such compounds provide an appealing approach to restore muscle force in patients with NEB-NM, and also in patients with other neuromuscular disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0037-7) contains supplementary material, which is available to authorized users.

Myosin binding protein-C activates thin filaments and inhibits thick filaments in heart muscle cells

Myosin binding protein-C (MyBP-C) is a key regulatory protein in heart muscle, and mutations in the MYBPC3 gene are frequently associated with cardiomyopathy. However, the mechanism of action of MyBP-C remains poorly understood, and both activating and inhibitory effects of MyBP-C on contractility have been reported. To clarify the function of the regulatory N-terminal domains of MyBP-C, we determined their effects on the structure of thick (myosin-containing) and thin (actin-containing) filaments in intact sarcomeres of heart muscle. We used fluorescent probes on troponin C in the thin filaments and on myosin regulatory light chain in the thick filaments to monitor structural changes associated with activation of demembranated trabeculae from rat ventricle by the C1mC2 region of rat MyBP-C. C1mC2 induced larger structural changes in thin filaments than calcium activation, and these were still present when active force was blocked with blebbistatin, showing that C1mC2 directly activates the thin filaments. In contrast, structural changes in thick filaments induced by C1mC2 were smaller than those associated with calcium activation and were abolished or reversed by blebbistatin. Low concentrations of C1mC2 did not affect resting force but increased calcium sensitivity and reduced cooperativity of force and structural changes in both thin and thick filaments. These results show that the N-terminal region of MyBP-C stabilizes the ON state of thin filaments and the OFF state of thick filaments and lead to a novel hypothesis for the physiological role of MyBP-C in the regulation of cardiac contractility.

Pathomechanisms in heart failure: the contractile connection

Heart failure is a multi-factorial progressive disease in which eventually the contractile performance of the heart is insufficient to meet the demands of the body, even at rest. A distinction can be made on the basis of the cause of the disease in genetic and acquired heart failure and at the functional level between systolic and diastolic heart failure. Here the basic determinants of contractile function of myocardial cells will be reviewed and an attempt will be made to elucidate their role in the development of heart failure. The following topics are addressed: the tension generating capacity, passive tension, the rate of tension development, the rate of ATP utilisation, calcium sensitivity of tension development, phosphorylation of contractile proteins, length dependent activation and stretch activation. The reduction in contractile performance during systole can be attributed predominantly to a loss of cardiomyocytes (necrosis), myocyte disarray and a decrease in myofibrillar density all resulting in a reduction in the tension generating capacity and likely also to a mismatch between energy supply and demand of the myocardium. This leads to a decline in the ejection fraction of the heart. Diastolic dysfunction can be attributed to fibrosis and an increase in titin stiffness which result in an increase in stiffness of the ventricular wall and hampers the filling of the heart with blood during diastole. A large number of post translation modifications of regulatory sarcomeric proteins influence myocardial function by altering calcium sensitivity of tension development. It is still unclear whether in concert these influences are adaptive or maladaptive during the disease process.

A mechanistic model of Ca regulation of thin filaments in cardiac muscle

We present a model of Ca-regulated thin filaments in cardiac muscle where tropomyosin is treated as a continuous elastic chain confined in the closed position on the actin helix by electrostatic forces. The main distinction from previous works is that the intrinsic stress-free helical shape of the tropomyosin chain was taken into account explicitly. This results in the appearance of a new, to our knowledge, tension-like term in the energy functional and the equilibrium equation. The competitive binding of calcium and the mobile segment of troponin-I to troponin-C were described by a simple kinetic scheme. The values of dimensionless model parameters were estimated from published data. A stochastic Monte Carlo simulation of calcium curves has been performed and its results were compared to published data. The model explains the high cooperativity of calcium response of the regulated thin filaments even in the absence of myosin heads. The binding of myosin heads to actin increases the calcium sensitivity while not affecting its cooperativity significantly. When the presence of calcium-insensitive troponin-C was simulated in the model, both calcium sensitivity and cooperativity decreased. All these features were previously observed experimentally.

Structural dynamics of troponin during activation of skeletal muscle

Time-resolved changes in the conformation of troponin in the thin filaments of skeletal muscle were followed during activation in situ by photolysis of caged calcium using bifunctional fluorescent probes in the regulatory and the coiled-coil (IT arm) domains of troponin. Three sequential steps in the activation mechanism were identified. The fastest step (1,100 s(-1)) matches the rate of Ca(2+) binding to the regulatory domain but also dominates the motion of the IT arm. The second step (120 s(-1)) coincides with the azimuthal motion of tropomyosin around the thin filament. The third step (15 s(-1)) was shown by three independent approaches to track myosin head binding to the thin filament, but is absent in the regulatory head. The results lead to a four-state structural kinetic model that describes the molecular mechanism of muscle activation in the thin filament-myosin head complex under physiological conditions.

Clusters of bound Ca(2+) initiate contraction in fast skeletal muscle

Ca(2+)-binding to troponin C ultimately controls force in muscle leading to the expectation that the two curves, pCa/force and pCa/Ca(2+) binding, will coincide. Using an improved fluorescence apparatus to measure Ca(2+)-binding, we confirm a displacement between the position and shape of the pCa/Ca(2+)-binding and pCa/force curves. This displacement may be part of a mechanism that reduces the noise inherent in the control process. There must always be some Ca(2+)-binding events even at 10 or 100nM, well below threshold for muscle contraction. To minimize the response to such random binding events we suggest that clusters of adjacent Ca(2+)-binding sites must be filled before contraction is initiated. Clusters promote the reconfiguration of the thin filament to the "On" state; this simultaneously increases thin filaments' affinity for myosin heads and of troponin C for Ca(2+) producing the highly cooperative pCa/force curve. The cluster requirement displaces the Ca(2+)-binding from the force curve as observed. The thin filament conformational changes and the accompanying affinity increases introduce a discontinuity in the pCa/Ca(2+)-binding curve. The curve, therefore, is most appropriately fit by two separate Hill equations, a simple non-cooperative one (midpoint, pK1, n1∼1) for the foot and a second cooperative one (pK2, n2∼2.5) for the upper part. With this fit pK2 is larger than pK1 as our argument requires, in contrast to fitting to the sum of two Hill equations. It also expresses the idea that there may be three states of the thin filament.

Structural basis for the in situ Ca(2+) sensitization of cardiac troponin C by positive feedback from force-generating myosin cross-bridges

The in situ structural coupling between the cardiac troponin (cTn) Ca(2+)-sensitive regulatory switch (CRS) and strong myosin cross-bridges was investigated using Förster resonance energy transfer (FRET). The double cysteine mutant cTnC(T13C/N51C) was fluorescently labeled with the FRET pair 5-(iodoacetamidoethyl)aminonaphthelene-1-sulfonic acid (IAEDENS) and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (DDPM) and then incorporated into detergent skinned left ventricular papillary fiber bundles. Ca(2+) titrations of cTnC(T13C/N51C)AEDENS/DDPM-reconstituted fibers showed that the Ca(2+)-dependence of the opening of the N-domain of cTnC (N-cTnC) statistically matched the force-Ca(2+) relationship. N-cTnC opening still occurred steeply during Ca(2+) titrations in the presence of 1mM vanadate, but the maximal extent of ensemble-averaged N-cTnC opening and the Ca(2+)-sensitivity of the CRS were significantly reduced. At nanomolar, resting Ca(2+) levels, treatment with ADP·Mg in the absence of ATP caused a partial opening of N-cTnC. During subsequent Ca(2+) titrations in the presence of ADP·Mg and absence of ATP, further N-cTnC opening was stimulated as the CRS responded to Ca(2+) with increased Ca(2+)-sensitivity and reduced steepness. These findings supported our hypothesis here that strong cross-bridge interactions with the cardiac thin filament exert a Ca(2+)-sensitizing effect on the CRS by stabilizing the interaction between the exposed hydrophobic patch of N-cTnC and the switch region of cTnI.

Conserved Asp-137 is important for both structure and regulatory functions of cardiac α-tropomyosin (α-TM) in a novel transgenic mouse model expressing α-TM-D137L

α-Tropomyosin (α-TM) has a conserved, charged Asp-137 residue located in the hydrophobic core of its coiled-coil structure, which is unusual in that the residue is found at a position typically occupied by a hydrophobic residue. Asp-137 is thought to destabilize the coiled-coil and so impart structural flexibility to the molecule, which is believed to be crucial for its function in the heart. A previous in vitro study indicated that the conversion of Asp-137 to a more typical canonical Leu alters flexibility of TM and affects its in vitro regulatory functions. However, the physiological importance of the residue Asp-137 and altered TM flexibility is unknown. In this study, we further analyzed structural properties of the α-TM-D137L variant and addressed the physiological importance of TM flexibility in cardiac function in studies with a novel transgenic mouse model expressing α-TM-D137L in the heart. Our NMR spectroscopy data indicated that the presence of D137L introduced long range rearrangements in TM structure. Differential scanning calorimetry measurements demonstrated that α-TM-D137L has higher thermal stability compared with α-TM, which correlated with decreased flexibility. Hearts of transgenic mice expressing α-TM-D137L showed systolic and diastolic dysfunction with decreased myofilament Ca(2+) sensitivity and cardiomyocyte contractility without changes in intracellular Ca(2+) transients or post-translational modifications of major myofilament proteins. We conclude that conversion of the highly conserved Asp-137 to Leu results in loss of flexibility of TM that is important for its regulatory functions in mouse hearts. Thus, our results provide insight into the link between flexibility of TM and its function in ejecting hearts.

Integration of troponin I phosphorylation with cardiac regulatory networks

We focus here on the modulation of thin filament activity by cardiac troponin I phosphorylation as an integral and adaptive mechanism in cardiac homeostasis and as a mechanism vulnerable to maladaptive response to stress. We discuss a current concept of cardiac troponin I function in the A-band region of the sarcomere and potential signaling to cardiac troponin I in a network involving the ends of the thin filaments at the Z-disk and the M-band regions. The cardiac sarcomere represents a remarkable set of interacting proteins that functions not only as a molecular machine generating the heartbeat but also as a hub of signaling. We review how phosphorylation signaling to cardiac troponin I is integrated, with parallel signals controlling excitation-contraction coupling, hypertrophy, and metabolism.

The troponin I: inhibitory peptide uncouples force generation and the cooperativity of contractile activation in mammalian skeletal muscle

Hodges and his colleagues identified a 12 amino acid fragment of troponin I (TnI-ip) that inhibits Ca(2+)-activated force and reduces the effectiveness Ca(2+) as an activator. To understand the role of troponin C (TnC) in the extended cooperative interactions of thin filament activation, we compared the effect of TnI-ip with that of partial troponin TnC extraction. Both methods reduce maximal Ca(2+)-activated force and increase [Ca(2+)] required for activation. In contrast to TnC extraction, TnI-ip does not reduce the extended cooperative interactions between adjacent thin filament regulatory units as assessed by the slope of the pCa/force relationship. Additional evidence that TnI-ip does not interfere with extended cooperativity comes from studies that activate muscle by rigor crossbridges (RXBs). TnI-ip increases both the cooperativity of activation and the concentration of RXBs needed for maximal force. This shows that TnI-ip binding to TnC increases the stability of the relaxed state of the thin filament. TnI-ip, therefore, uncouples force generation from extended cooperativity in both Ca(2+) and RXB activated muscle contraction. Because maximum force can be reduced with no change-or even an increase-in cooperativity, force-generating crossbridges do not appear to be the primary activators of cooperativity between thin filament regulatory units of skeletal muscle.

Structural and kinetic effects of hypertrophic cardiomyopathy related mutations R146G/Q and R163W on the regulatory switching activity of rat cardiac troponin I

Mutations in cardiac troponin I (cTnI) that cause hypertrophic cardiomyopathy (HCM) have been reported to change the contractility of cardiac myofilaments, but the underlying molecular mechanism remains elusive. In this study, Förster resonance energy transfer (FRET) was used to investigate the specific structural and kinetic effects that HCM related rat cTnI mutations R146G/Q and R163W exert on Ca(2+) and myosin S1 dependent conformational transitions in rat cTn structure. Ca(2+)-induced changes in interactions between cTnC and cTnI were individually monitored in reconstituted thin filaments using steady state and time resolved FRET, and kinetics were determined using stopped flow. R146G/Q and R163W all changed the FRET distances between cTnC and cTnI in unique and various ways. However, kinetic rates of conformational transitions induced by Ca(2+)-dissociation were universally slowed when R146G/Q and R163W were present. Interestingly, the kinetic rates of changes in the inhibitory region of cTnI were always slower than that of the regulatory region, suggesting that the fly casting mechanism that normally underlies deactivation is preserved in spite of mutation. In situ rat myocardial fiber studies also revealed that FRET distance changes indicating mutation specific disruption of the cTnIIR-actin interaction were consistent with increased passive tension.

Enhanced Ca2+ binding of cardiac troponin reduces sarcomere length dependence of contractile activation independently of strong crossbridges

Calcium sensitivity of the force-pCa relationship depends strongly on sarcomere length (SL) in cardiac muscle and is considered to be the cellular basis of the Frank-Starling law of the heart. SL dependence may involve changes in myofilament lattice spacing and/or myosin crossbridge orientation to increase probability of binding to actin at longer SLs. We used the L48Q cardiac troponin C (cTnC) variant, which has enhanced Ca(2+) binding affinity, to test the hypotheses that the intrinsic properties of cTnC are important in determining 1) thin filament binding site availability and responsiveness to crossbridge activation and 2) SL dependence of force in cardiac muscle. Trabeculae containing L48Q cTnC-cTn lost SL dependence of the Ca(2+) sensitivity of force. This occurred despite maintaining the typical SL-dependent changes in maximal force (F(max)). Osmotic compression of preparations at SL 2.0 μm with 3% dextran increased F(max) but not pCa(50) in L48Q cTnC-cTn exchanged trabeculae, whereas wild-type (WT)-cTnC-cTn exchanged trabeculae exhibited increases in both F(max) and pCa(50). Furthermore, crossbridge inhibition with 2,3-butanedione monoxime at SL 2.3 μm decreased F(max) and pCa(50) in WT cTnC-cTn trabeculae to levels measured at SL 2.0 μm, whereas only F(max) was decreased with L48Q cTnC-cTn. Overall, these results suggest that L48Q cTnC confers reduced crossbridge dependence of thin filament activation in cardiac muscle and that changes in the Ca(2+) sensitivity of force in response to changes in SL are at least partially dependent on properties of thin filament troponin.

Regulating the contraction of insect flight muscle

The rapid movement of the wings in small insects is powered by the indirect flight muscles. These muscles are capable of contracting at up to 1,000 Hz because they are activated mechanically by stretching. The mechanism is so efficient that it is also used in larger insects like the waterbug, Lethocerus. The oscillatory activity of the muscles occurs a low concentration of Ca(2+), which stays constant as the muscles contract and relax. Activation by stretch requires particular isoforms of tropomyosin and the troponin complex on the thin filament. We compare the tropomyosin and troponin of Lethocerus and Drosophila with that of vertebrates. The characteristics of the flight muscle regulatory proteins suggest ways in which stretch-activation works. There is evidence for bridges between troponin on thin filaments and myosin crossbridges on the thick filaments. Recent X-ray fibre diffraction results suggest that a pull on the bridges activates the thin filament by shifting tropomyosin from a blocking position on actin. The troponin bridges are likely to contain extended sequences of tropomyosin or troponin I (TnI). Flight muscle has two isoforms of TnC with different Ca(2+)-binding properties: F1 TnC is needed for stretch-activation and F2 TnC for isometric contractions. In this review, we describe the structural changes in both isoforms on binding Ca(2+) and TnI, and discuss how the steric model of muscle regulation can apply to insect flight muscle.

Tropomyosin position on F-actin revealed by EM reconstruction and computational chemistry

Electron microscopy and fiber diffraction studies of reconstituted F-actin-tropomyosin filaments reveal the azimuthal position of end-to-end linked tropomyosin molecules on the surface of actin. However, the longitudinal z-position of tropomyosin along F-actin is still uncertain. Without this information, atomic models of F-actin-tropomyosin filaments, free of constraints imposed by troponin or other actin-binding proteins, cannot be formulated, and thus optimal interfacial contacts between actin and tropomyosin remain unknown. Here, a computational search assessing electrostatic interactions for multiple azimuthal locations, z-positions, and pseudo-rotations of tropomyosin on F-actin was performed. The information gleaned was used to localize tropomyosin on F-actin, yielding an atomic model characterized by protein-protein contacts that primarily involve clusters of basic amino acids on actin subdomains 1 and 3 juxtaposed against acidic residues on the successive quasi-repeating units of tropomyosin. A virtually identical model generated by docking F-actin and tropomyosin atomic structures into electron microscopy reconstructions of F-actin-tropomyosin validated the above solution. Here, the z-position of tropomyosin alongside F-actin was defined by matching the seven broad and narrow motifs that typify tropomyosin's twisting superhelical coiled-coil to the wide and tapering tropomyosin densities seen in surface views of F-actin-tropomyosin reconstructions. The functional implications of the F-actin-tropomyosin models determined in this work are discussed.

Expression of tropomyosin-κ induces dilated cardiomyopathy and depresses cardiac myofilament tension by mechanisms involving cross-bridge dependent activation and altered tropomyosin phosphorylation

Tropomyosin-kappa (TPM1-κ) is a newly discovered tropomyosin (TM) isoform that is exclusively expressed in the human heart and generated by an alternative splicing of the α-TM gene. We reported that TPM1-κ expression was increased in the hearts of patients with chronic dilated cardiomyopathy (DCM). To increase our understanding of the significance of this shift in isoform population, we generated transgenic (TG) mice expressing TPM1-κ in the cardiac compartment where TPM1-κ replaces 90% of the native TM. We previously showed that there was a significant inhibition of the ability of strongly bound cross-bridges to induce activation of TG myofilaments (Rajan et al., Circulation 121:410-418, 2010). Here, we compared the force-Ca(2+) relations in detergent extracted (skinned) fiber bundles isolated from non-transgenic (NTG) and TG-TPM1-κ hearts at two sarcomere lengths (SLs). Our data demonstrated a significant decrease in the Ca(2+) sensitivity of the myofilaments from TG-TPM1-κ hearts with no change in the maximum developed tension, length-dependent activation, and the ratio of ATPase rate to tension. There was also no difference in the affinity and cooperativity of Ca(2+)-binding to troponin in thin filaments reconstituted with either TPM1-κ or α-TM. We also compared protein phosphorylation in NTG and TG-TPM1-κ myofilaments. There was a decrease in the total phosphorylation of TPM1-κ compared to α-TM, but no significant change in other major sarcomeric proteins. Our results identify a novel mode of myofilament desensitization to Ca(2+) associated with a DCM linked switch in TM isoform population.

Effects of actin-myosin kinetics on the calcium sensitivity of regulated thin filaments

Activation of thin filaments in striated muscle occurs when tropomyosin exposes myosin binding sites on actin either through calcium-troponin (Ca-Tn) binding or by actin-myosin (A-M) strong binding. However, the extent to which these binding events contributes to thin filament activation remains unclear. Here we propose a simple analytical model in which strong A-M binding and Ca-Tn binding independently activates the rate of A-M weak-to-strong binding. The model predicts how the level of activation varies with pCa as well as A-M attachment, N·k(att), and detachment, k(det), kinetics. To test the model, we use an in vitro motility assay to measure the myosin-based sliding velocities of thin filaments at different pCa, N·k(att), and k(det) values. We observe that the combined effects of varying pCa, N·k(att), and k(det) are accurately fit by the analytical model. The model and supporting data imply that changes in attachment and detachment kinetics predictably affect the calcium sensitivity of striated muscle mechanics, providing a novel A-M kinetic-based interpretation for perturbations (e.g. disease-related mutations) that alter calcium sensitivity.