Positive cardiac inotrope omecamtiv mecarbil activates muscle despite suppressing the myosin working stroke

The effect of omecamtiv mecarbil on actin-myosin interaction in the disused rat soleus muscle

Preventing muscle atrophy caused by disuse is a major concern in space, clinical, and rehabilitation medicine. This study aimed to attenuate the impact of disuse and support muscle function during hindlimb unloading using the β-myosin activator omecamtiv mecarbil (OM). We obtained soleus muscle myosin from rats in control, control with 10-day OM supplementation, hindlimb-unloaded, and hindlimb-unloaded with 10-day OM supplementation (prolonged treatment). To examine the direct effect of OM (direct treatment) on myosin from all groups, we added it to myosin in the flow cell at a concentration of 1 μM. Using an in vitro motility assay, we examined the sliding velocity of actin filaments and regulated thin filaments over soleus muscle myosin, the fraction of motile filaments, calcium sensitivity and Hill coefficient in the "pCa-velocity" and "pCa-fraction of motile filaments" relationships, relative force. Hindlimb unloading resulted in a slow-to-fast shift in the content of myosin heavy and light chains isoforms, an increased sliding velocity of actin filaments and regulated thin filaments over myosin. 10-day OM supplementation decreased the sliding velocity of actin filaments and regulated thin filaments over myosin slightly increasing calcium sensitivity in healthy rats and prevented the increase in the velocity caused by disuse without altering relative force, myosin isoform content. Direct treatment reduced the sliding velocity of actin filaments and regulated thin filaments over myosin while enhancing calcium sensitivity and relative force in all studied groups. Thus, both direct and prolonged OM treatment mitigated the effects of disuse on the functional characteristics of soleus muscle myosin.

How Omecamtiv Modulates Myosin Motion

Myosin VI is a unique reverse-directed motor protein in the myosin family. The D179Y mutation in Myosin VI is associated with deafness in mammals. This mutation destroys the processive motion of myosin and inhibits its functional activity due to an elevated phosphate release rate. The current work explores the way by which this mutation affects the phosphate release rate and changes the action of Myosin VI. Our study involves a wide range of approaches comprising free energy-based simulations, contact map analysis, binding energy investigation, structural inspection, renormalization simulation, multiple sequence alignment, and bioinformatics analysis. It is found that when the evolutionary conserved aspartic acid (D179) of Myosin VI is mutated to tyrosine (Y179), it leads to premature phosphate release from Myosin VI. Most importantly, the drug omecamtiv rescues the processivity of the mutant by slowing down the actin-independent phosphate release from Myosin VI. Thus, we also explore the molecular mechanism behind the premature phosphate release of the D179Y mutant of Myosin VI and the actin-independent slowing down of the phosphate release in the presence of omecamtiv. This phosphate release modulation is related to Myosin VI's processivity as found experimentally. Overall, our proposed model indicates that omecamtiv significantly alters the interaction between the P-loop of Myosin VI and the interfacial residues, which is the driving force behind the slowing down of the phosphate release of the D179Y mutant in the presence of omecamtiv. Finally, our study provides additional support to our proposal that the directionality of myosins is determined by the highest barrier along the cycle and not by any dynamical effect.

Mechanistic insights into effects of the cardiac myosin activator omecamtiv mecarbil from mechanokinetic modelling

Introduction

Small molecular compounds that affect the force, and motion-generating actin-myosin interaction in the heart have emerged as alternatives to treat or alleviate symptoms in severe debilitating conditions, such as cardiomyopathies and heart failure. Omecamtiv mecarbil (OM) is such a compound developed to enhance cardiac contraction. In addition to potential therapeutic use, its effects may help to elucidate myosin energy transduction mechanisms in health and disease and add insights into how the molecular properties govern contraction of large myosin ensembles in cardiac cells. Despite intense studies, the effects of OM are still incompletely understood.

Methods

Here we take an in silico approach to elucidate the issue. First, we modify a model, previously used in studies of skeletal muscle, with molecular parameter values for human ventricular β-myosin to make it useful for studies of both myosin mutations and drugs. Repeated tests lead to at a set of parameter values that allow faithful reproduction of range of functional variables of cardiac myocytes. We then apply the model to studies of OM.

Results and discussion

The results suggest that major effects of OM such as large reduction of the maximum velocity with more limited effects on maximum isometric force and slowed actin-activated ATPase can be accounted for by two key molecular effects. These encompass a reduced difference in binding free energy between the pre- and post-power-stroke states and greatly increased activation energy for the lever arm swing during the power-stroke. Better quantitative agreement, e.g., isometric force minimally changed from the control value by OM is achieved by additional changes in model parameter values previously suggested by studies of isolated proteins.

Omecamtiv Mecarbil in Systolic Heart Failure: Clinical Efficacy and Future Directions of a Novel Myosin-Activating Inotropic Agent

Heart failure with reduced ejection fraction (HFrEF) remains a global health challenge, associated with high morbidity, mortality, and rising healthcare costs. Despite advances in guideline-directed therapy, many patients, especially those with severely reduced ejection fraction, remain symptomatic. Traditional inotropes, including β-agonists and phosphodiesterase inhibitors, are limited in chronic HFrEF due to risks of arrhythmias, calcium overload, and increased myocardial oxygen demand. Omecamtiv mecarbil (OM) is a novel cardiac myosin activator that enhances systolic contraction by increasing the efficiency of actin-myosin interactions. It prolongs systolic ejection time and improves stroke volume without elevating intracellular calcium or energy consumption. Although theoretical concerns exist regarding impaired diastolic filling, clinical trials have not confirmed such adverse effects in most patients. This narrative review discusses OM's pharmacologic profile, clinical trial data, and its potential as an adjunct therapy in advanced HFrEF. While not yet guideline-recommended, OM may benefit patients who remain symptomatic despite optimal treatment.

Mavacamten inhibits myosin activity by stabilising the myosin interacting-heads motif and stalling motor force generation

Most sudden cardiac deaths in young people arise from hypertrophic cardiomyopathy, a genetic disease of the heart muscle, with many causative mutations found in the molecular motor beta-cardiac myosin that drives contraction. Therapeutic intervention has until recently been limited to symptomatic relief or invasive procedures. However, small molecule modulators of cardiac myosin are promising therapeutic options to target disease progression. Mavacamten is the first example to gain FDA approval but its molecular mode of action remains unclear, limiting our understanding of its functional effects in disease. To better understand this, we solved the cryoEM structures of beta-cardiac heavy meromyosin in three ADP.Pi-bound states, the primed motor domain in the presence and absence of mavacamten, and the sequestered autoinhibited interacting-heads motif (IHM) in complex with mavacamten, to 2.9 Å, 3.4 Å and 3.7 Å global resolution respectively. Together with quantitative crosslinking mass spectrometric analysis, these structures reveal how mavacamten inhibits myosin. Mavacamten stabilises ADP.Pi binding, stalling the motor domain in a primed state, reducing motor dynamics required for actin-binding cleft closure, and slowing progression through the force generation cycle. Within the two-headed myosin molecule, these effects are propagated and lead to stabilisation of the IHM, through increased contacts at the motor-motor interface. Critically, while mavacamten treatment can thus rescue cardiac muscle relaxation in diastole, it can also reduce contractile output in systole in the heart.

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.

Pharmacodynamics of single-dose omecamtiv mecarbil administered intravenously in clinically healthy cats

Background

Omecamtiv mecarbil (OM), a selective cardiac myosin activator, is gaining attention as a potential heart failure (HF) treatment because it can enhance cardiac contractility without adverse effects. Concerns were raised about arrhythmias with conventional HF treatments in cats. Despite OM's promise in veterinary medicine, no studies have confirmed its efficacy in cats.

Aim

This study aimed to investigate the pharmacodynamics of OM in cats.

Methods

Five clinically healthy cats were used. OM's efficacy was examined in three doses: 0.1, 0.25, and 1.0 mg/kg. To minimize the effect on heart rates, the cats were under general anesthesia, and baseline measurements were taken after the heart rate and blood pressure had stabilized. OM was administered as a single intravenous injection. Echocardiography was performed 15, 30, 60, 90, and 120 minutes after administration. Heart rate and blood pressure were measured before each echocardiographic examination.

Results

The heart rates decreased at all doses; significant reductions were seen at 0.25 and 1.0 mg/kg. All doses enhanced cardiac contractility, showing a dose-dependent effect. Blood pressure remained unchanged.

Conclusion

OM enhances cardiac contractility in cats, with 0.25 mg/kg being the optimal dose.

Danicamtiv reduces myosin's working stroke but enhances contraction by activating the thin filament

Heart failure is a leading cause of death worldwide, and even with current treatments, the 5-year transplant-free survival rate is only ~50-70%. As such, there is a need to develop new treatments for patients that improve survival and quality of life. Recently, there have been efforts to develop small molecules for heart failure that directly target components of the sarcomere, including cardiac myosin. One such molecule, danicamtiv, recently entered phase II clinical trials; however, its mechanism of action and direct effects on myosin's mechanics and kinetics are not well understood. Using optical trapping techniques, stopped flow transient kinetics, and in vitro reconstitution assays, we found that danicamtiv reduces the size of cardiac myosin's working stroke, and in contrast to studies in muscle fibers, we found that it does not affect actomyosin detachment kinetics at the level of individual crossbridges. We demonstrate that danicamtiv accelerates actomyosin association kinetics, leading to increased recruitment of myosin crossbridges and subsequent thin filament activation at physiologically-relevant calcium concentrations. Finally, we computationally model how the observed changes in mechanics and kinetics at the level of single crossbridges contribute to increased cardiac contraction and improved diastolic function compared to the related myotrope, omecamtiv mecarbil. Taken together, our results have important implications for the design of new sarcomeric-targeting compounds for heart failure.

From amoeboid myosin to unique targeted medicines for a genetic cardiac disease

The importance of fundamental basic research in the quest for much needed clinical treatments is a story that constantly must be retold. Funding of basic science in the USA by the National Institutes of Health and other agencies is provided under the assumption that fundamental research eventually will lead to improvements in healthcare worldwide. Understanding how basic research is connected to clinical developments is important, but just part of the story. Many basic science discoveries never see the light of day in a clinical setting because academic scientists are not interested in or do not have the inclination and/or support for entering the world of biotechnology. Even if the interest and inclination are there, often the unknowns about how to enter that world inhibit taking the initial step. Young investigators often ask me how I incorporated biotech opportunities into my otherwise purely academic research endeavors. Here I tell the story of the foundational basic science and early events of my career that led to forming the biotech companies responsible for the development of unique cardiac drugs, including mavacamten, a first in class human β-cardiac myosin inhibitor that is changing the lives of hypertrophic cardiomyopathy patients.

Myosin Isoform-Dependent Effect of Omecamtiv Mecarbil on the Regulation of Force Generation in Human Cardiac Muscle

Omecamtiv mecarbil (OM) is a small molecule that has been shown to improve the function of the slow human ventricular myosin (MyHC) motor through a complex perturbation of the thin/thick filament regulatory state of the sarcomere mediated by binding to myosin allosteric sites coupled to inorganic phosphate (Pi) release. Here, myofibrils from samples of human left ventricle (β-slow MyHC-7) and left atrium (α-fast MyHC-6) from healthy donors were used to study the differential effects of μmolar [OM] on isometric force in relaxing conditions (pCa 9.0) and at maximal (pCa 4.5) or half-maximal (pCa 5.75) calcium activation, both under control conditions (15 °C; equimolar DMSO; contaminant inorganic phosphate [Pi] ~170 μM) and in the presence of 5 mM [Pi]. The activation state and OM concentration within the contractile lattice were rapidly altered by fast solution switching, demonstrating that the effect of OM was rapid and fully reversible with dose-dependent and myosin isoform-dependent features. In MyHC-7 ventricular myofibrils, OM increased submaximal and maximal Ca2+-activated isometric force with a complex dose-dependent effect peaking (40% increase) at 0.5 μM, whereas in MyHC-6 atrial myofibrils, it had no effect or-at concentrations above 5 µM-decreased the maximum Ca2+-activated force. In both ventricular and atrial myofibrils, OM strongly depressed the kinetics of force development and relaxation up to 90% at 10 μM [OM] and reduced the inhibition of force by inorganic phosphate. Interestingly, in the ventricle, but not in the atrium, OM induced a large dose-dependent Ca2+-independent force development and an increase in basal ATPase that were abolished by the presence of millimolar inorganic phosphate, consistent with the hypothesis that the widely reported Ca2+-sensitising effect of OM may be coupled to a change in the state of the thick filaments that resembles the on-off regulation of thin filaments by Ca2+. The complexity of this scenario may help to understand the disappointing results of clinical trials testing OM as inotropic support in systolic heart failure compared with currently available inotropic drugs that alter the calcium signalling cascade.

Danicamtiv affected isometric force and cross-bridge kinetics similarly in skinned myocardial strips from male and female rats

Myotropes are pharmaceuticals that have recently been developed or are under investigation for the treatment of heart diseases. Myotropes have had varied success in clinical trials. Initial research into myotropes have widely focused on animal models of cardiac dysfunction in comparison with normal animal cardiac physiology-primarily using males. In this study we examined the effect of danicamtiv, which is one type of myotrope within the class of myosin activators, on contractile function in permeabilized (skinned) myocardial strips from male and female Sprague-Dawley rats. We found that danicamtiv increased steady-state isometric force production at sub-maximal calcium levels, leading to greater Ca2+-sensitivity of contraction for both sexes. Danicamtiv did not affect maximal Ca2+-activated force for either sex. Sinusoidal length-perturbation analysis was used to assess viscoelastic myocardial stiffness and cross-bridge cycling kinetics. Data from these measurements did not vary with sex, and the data suggest that danicamtiv slows cross-bridge cycling kinetics. These findings imply that danicamtiv increases force production via increasing cross-bridge contributions to activation of contraction, especially at sub-maximal Ca2+-activation. The inclusion of both sexes in animal models during the formative stages of drug development could be helpful for understanding the efficacy or limitation of a drug's therapeutic impact on cardiac function.

Dilated cardiomyopathy mutation in beta-cardiac myosin enhances actin activation of the power stroke and phosphate release

Inherited mutations in human beta-cardiac myosin (M2β) can lead to severe forms of heart failure. The E525K mutation in M2β is associated with dilated cardiomyopathy (DCM) and was found to stabilize the interacting heads motif (IHM) and autoinhibited super-relaxed (SRX) state in dimeric heavy meromyosin. However, in monomeric M2β subfragment 1 (S1) we found that E525K enhances (threefold) the maximum steady-state actin-activated ATPase activity (k cat) and decreases (eightfold) the actin concentration at which ATPase is one-half maximal (K ATPase). We also found a twofold to fourfold increase in the actin-activated power stroke and phosphate release rate constants at 30 μM actin, which overall enhanced the duty ratio threefold. Loaded motility assays revealed that the enhanced intrinsic motor activity translates to increased ensemble force in M2β S1. Glutamate 525, located near the actin binding region in the so-called activation loop, is highly conserved and predicted to form a salt bridge with another conserved residue (lysine 484) in the relay helix. Enhanced sampling molecular dynamics simulations predict that the charge reversal mutation disrupts the E525-K484 salt bridge, inducing conformations with a more flexible relay helix and a wide phosphate release tunnel. Our results highlight a highly conserved allosteric pathway associated with actin activation of the power stroke and phosphate release and suggest an important feature of the autoinhibited IHM is to prevent this region of myosin from interacting with actin. The ability of the E525K mutation to stabilize the IHM likely overrides the enhanced intrinsic motor properties, which may be key to triggering DCM pathogenesis.

Omecamtiv mecarbil and Mavacamten target the same myosin pocket despite opposite effects in heart contraction

Inherited cardiomyopathies are common cardiac diseases worldwide, leading in the late stage to heart failure and death. The most promising treatments against these diseases are small molecules directly modulating the force produced by β-cardiac myosin, the molecular motor driving heart contraction. Omecamtiv mecarbil and Mavacamten are two such molecules that completed phase 3 clinical trials, and the inhibitor Mavacamten is now approved by the FDA. In contrast to Mavacamten, Omecamtiv mecarbil acts as an activator of cardiac contractility. Here, we reveal by X-ray crystallography that both drugs target the same pocket and stabilize a pre-stroke structural state, with only few local differences. All-atom molecular dynamics simulations reveal how these molecules produce distinct effects in motor allostery thus impacting force production in opposite way. Altogether, our results provide the framework for rational drug development for the purpose of personalized medicine.

Roles of cMyBP-C phosphorylation on cardiac contractile dysfunction in db/db mice

Type 2 diabetes mellitus (T2DM) is a metabolic disease and comorbidity associated with several conditions, including cardiac dysfunction leading to heart failure with preserved ejection fraction (HFpEF), in turn resulting in T2DM-induced cardiomyopathy (T2DM-CM). However, the molecular mechanisms underlying the development of T2DM-CM are poorly understood. It is hypothesized that molecular alterations in myopathic genes induced by diabetes promote the development of HFpEF, whereas cardiac myosin inhibitors can rescue the resultant T2DM-mediated cardiomyopathy. To test this hypothesis, a Leptin receptor-deficient db/db homozygous (Lepr db/db) mouse model was used to define the pathogenesis of T2DM-CM. Echocardiographic studies at 4 and 6 months revealed that Lepr db/db hearts started developing cardiac dysfunction by four months, and left ventricular hypertrophy with diastolic dysfunction was evident at 6 months. RNA-seq data analysis, followed by functional enrichment, revealed the differential regulation of genes related to cardiac dysfunction in Lepr db/db heart tissues. Strikingly, the level of cardiac myosin binding protein-C phosphorylation was significantly increased in Lepr db/db mouse hearts. Finally, using isolated skinned papillary muscles and freshly isolated cardiomyocytes, CAMZYOS ® (mavacamten, MYK-461), a prescription heart medicine used for symptomatic obstructive hypertrophic cardiomyopathy treatment, was tested for its ability to rescue T2DM-CM. Compared with controls, MYK-461 significantly reduced force generation in papillary muscle fibers and cardiomyocyte contractility in the db/db group. This line of evidence shows that 1) T2DM-CM is associated with hyperphosphorylation of cardiac myosin binding protein-C and 2) MYK-461 significantly lessened disease progression in vitro, suggesting its promise as a treatment for HFpEF.

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.

Harnessing molecular mechanism for precision medicine in dilated cardiomyopathy caused by a mutation in troponin T

Familial dilated cardiomyopathy (DCM) is frequently caused by autosomal dominant point mutations in genes involved in diverse cellular processes, including sarcomeric contraction. While patient studies have defined the genetic landscape of DCM, genetics are not currently used in patient care, and patients receive similar treatments regardless of the underlying mutation. It has been suggested that a precision medicine approach based on the molecular mechanism of the underlying mutation could improve outcomes; however, realizing this approach has been challenging due to difficulties linking genotype and phenotype and then leveraging this information to identify therapeutic approaches. Here, we used multiscale experimental and computational approaches to test whether knowledge of molecular mechanism could be harnessed to connect genotype, phenotype, and drug response for a DCM mutation in troponin T, deletion of K210. Previously, we showed that at the molecular scale, the mutation reduces thin filament activation. Here, we used computational modeling of this molecular defect to predict that the mutant will reduce cellular and tissue contractility, and we validated this prediction in human cardiomyocytes and engineered heart tissues. We then used our knowledge of molecular mechanism to computationally model the effects of a small molecule that can activate the thin filament. We demonstrate experimentally that the modeling correctly predicts that the small molecule can partially rescue systolic dysfunction at the expense of diastolic function. Taken together, our results demonstrate how molecular mechanism can be harnessed to connect genotype and phenotype and inspire strategies to optimize mechanism-based therapeutics for DCM.

Omecamtiv Mecarbil in the treatment of heart failure: the past, the present, and the future

Heart failure, a prevailing global health issue, imposes a substantial burden on both healthcare systems and patients worldwide. With an escalating prevalence of heart failure, prolonged survival rates, and an aging demographic, an increasing number of individuals are progressing to more advanced phases of this incapacitating ailment. Against this backdrop, the quest for pharmacological agents capable of addressing the diverse subtypes of heart failure becomes a paramount pursuit. From this viewpoint, the present article focuses on Omecamtiv Mecarbil (OM), an emerging chemical compound said to exert inotropic effects without altering calcium homeostasis. For the first time, as a review, the present article uniquely started from the very basic pathophysiology of heart failure, its classification, and the strategies underpinning drug design, to on-going debates of OM's underlying mechanism of action and the latest large-scale clinical trials. Furthermore, we not only saw the advantages of OM, but also exhaustively summarized the concerns in sense of its effects. These of no doubt make the present article the most systemic and informative one among the existing literature. Overall, by offering new mechanistic insights and therapeutic possibilities, OM has carved a significant niche in the treatment of heart failure, making it a compelling subject of study.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Homologous mutations in human β, embryonic, and perinatal muscle myosins have divergent effects on molecular power generation

Mutations at a highly conserved homologous residue in three closely related muscle myosins cause three distinct diseases involving muscle defects: R671C in β-cardiac myosin causes hypertrophic cardiomyopathy, R672C and R672H in embryonic skeletal myosin cause Freeman-Sheldon syndrome, and R674Q in perinatal skeletal myosin causes trismus-pseudocamptodactyly syndrome. It is not known whether their effects at the molecular level are similar to one another or correlate with disease phenotype and severity. To this end, we investigated the effects of the homologous mutations on key factors of molecular power production using recombinantly expressed human β, embryonic, and perinatal myosin subfragment-1. We found large effects in the developmental myosins but minimal effects in β myosin, and magnitude of changes correlated partially with clinical severity. The mutations in the developmental myosins dramatically decreased the step size and load-sensitive actin-detachment rate of single molecules measured by optical tweezers, in addition to decreasing overall enzymatic (ATPase) cycle rate. In contrast, the only measured effect of R671C in β myosin was a larger step size. Our measurements of step size and bound times predicted velocities consistent with those measured in an in vitro motility assay. Finally, molecular dynamics simulations predicted that the arginine to cysteine mutation in embryonic, but not β, myosin may reduce pre-powerstroke lever arm priming and ADP pocket opening, providing a possible structural mechanism consistent with the experimental observations. This paper presents direct comparisons of homologous mutations in several different myosin isoforms, whose divergent functional effects are a testament to myosin's highly allosteric nature.

The structural OFF and ON states of myosin can be decoupled from the biochemical super- and disordered-relaxed states

There is a growing awareness that both thick-filament and classical thin-filament regulations play central roles in modulating muscle contraction. Myosin ATPase assays have demonstrated that under relaxed conditions, myosin may reside either in a high-energy-consuming disordered-relaxed (DRX) state available for binding actin to generate force or in an energy-sparing super-relaxed (SRX) state unavailable for actin binding. X-ray diffraction studies have shown that the majority of myosin heads are in a quasi-helically ordered OFF state in a resting muscle and that this helical ordering is lost when myosin heads are turned ON for contraction. It has been assumed that myosin heads in SRX and DRX states are equivalent to the OFF and ON states, respectively, and the terms have been used interchangeably. In this study, we use X-ray diffraction and ATP turnover assays to track the structural and biochemical transitions of myosin heads, respectively, induced with either omecamtiv mecarbil (OM) or piperine in relaxed porcine myocardium. We find that while OM and piperine induce dramatic shifts of myosin heads from the OFF to the ON state, there are no appreciable changes in the population of myosin heads in the SRX and DRX states in both unloaded and loaded preparations. Our results show that biochemically defined SRX and DRX can be decoupled from structurally defined OFF and ON states. In summary, while SRX/DRX and OFF/ON transitions can be correlated in some cases, these two phenomena are measured using different approaches, reflect different properties of the thick filament, and should be investigated and interpreted separately.

Roles of β-adrenoceptor Subtypes and Therapeutics in Human Cardiovascular Disease: Heart Failure, Tachyarrhythmias and Other Cardiovascular Disorders

β-Adrenoceptors (β-ARs) provide an important therapeutic target for the treatment of cardiovascular disease. Three β-ARs, β1-AR, β2-AR, β3-AR are localized to the human heart. Activation of β1-AR and β2-ARs increases heart rate, force of contraction (inotropy) and consequently cardiac output to meet physiological demand. However, in disease, chronic over-activation of β1-AR is responsible for the progression of disease (e.g. heart failure) mediated by pathological hypertrophy, adverse remodelling and premature cell death. Furthermore, activation of β1-AR is critical in the pathogenesis of cardiac arrhythmias while activation of β2-AR directly influences blood pressure haemostasis. There is an increasing awareness of the contribution of β2-AR in cardiovascular disease, particularly arrhythmia generation. All β-blockers used therapeutically to treat cardiovascular disease block β1-AR with variable blockade of β2-AR depending on relative affinity for β1-AR vs β2-AR. Since the introduction of β-blockers into clinical practice in 1965, β-blockers with different properties have been trialled, used and evaluated, leading to better understanding of their therapeutic effects and tolerability in various cardiovascular conditions. β-Blockers with the property of intrinsic sympathomimetic activity (ISA), i.e. β-blockers that also activate the receptor, were used in the past for post-treatment of myocardial infarction and had limited use in heart failure. The β-blocker carvedilol continues to intrigue due to numerous properties that differentiate it from other β-blockers and is used successfully in the treatment of heart failure. The discovery of β3-AR in human heart created interest in the role of β3-AR in heart failure but has not resulted in therapeutics at this stage.

Design Principles and Benefits of Spatially Explicit Models of Myofilament Function

Spatially explicit models of muscle contraction include fine-scale details about the spatial, kinetic, and/or mechanical properties of the biological processes being represented within the model network. Over the past 25 years, this has primarily consisted of a set of mathematical and computational algorithms representing myosin cross-bridge activity, Ca2+-activation of contraction, and ensemble force production within a half-sarcomere representation of the myofilament network. Herein we discuss basic design principles associated with creating spatially explicit models of myofilament function, as well as model assumptions underlying model development. A brief overview of computational approaches is introduced. Opportunities for new model directions that could investigate coupled regulatory pathways between the thick-filament and thin-filaments are also presented. Given the modular design and flexibility associated with spatially explicit models, we highlight some advantages of this approach compared to other model formulations.

Characterization of NEB mutations 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. Mutations in the nebulin gene ( NEB ) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking therapies targeting the underlying pathological mechanisms. In this study, we examined a cohort of ten NEM2 patients, each with unique mutations, aiming to understand their impact on mRNA, protein, and functional levels. Results show that truncation mutations 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 splicing mutations which is expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with relatively normal nebulin levels and others with markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, and a positive relation between the reduction in nebulin level and the reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a duplication mutation in nebulin that resulted in a 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 I muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87-318%, 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 splicing mutations. 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.

Discovery of a novel cardiac-specific myosin modulator using artificial intelligence-based virtual screening

Direct modulation of cardiac myosin function has emerged as a therapeutic target for both heart disease and heart failure. However, the development of myosin-based therapeutics has been hampered by the lack of targeted in vitro screening assays. In this study we use Artificial Intelligence-based virtual high throughput screening (vHTS) to identify novel small molecule effectors of human β-cardiac myosin. We test the top scoring compounds from vHTS in biochemical counter-screens and identify a novel chemical scaffold called 'F10' as a cardiac-specific low-micromolar myosin inhibitor. Biochemical and biophysical characterization in both isolated proteins and muscle fibers show that F10 stabilizes both the biochemical (i.e. super-relaxed state) and structural (i.e. interacting heads motif) OFF state of cardiac myosin, and reduces force and left ventricular pressure development in isolated myofilaments and Langendorff-perfused hearts, respectively. F10 is a tunable scaffold for the further development of a novel class of myosin modulators.

Omecamtiv mecarbil and Mavacamten target the same myosin pocket despite antagonistic effects in heart contraction

Inherited cardiomyopathies are amongst the most common cardiac diseases worldwide, leading in the late-stage to heart failure and death. The most promising treatments against these diseases are small-molecules directly modulating the force produced by β-cardiac myosin, the molecular motor driving heart contraction. Two of these molecules that produce antagonistic effects on cardiac contractility have completed clinical phase 3 trials: the activator Omecamtiv mecarbil and the inhibitor Mavacamten. In this work, we reveal by X-ray crystallography that both drugs target the same pocket and stabilize a pre-stroke structural state, with only few local differences. All atoms molecular dynamics simulations reveal how these molecules can have antagonistic impact on the allostery of the motor by comparing β-cardiac myosin in the apo form or bound to Omecamtiv mecarbil or Mavacamten. Altogether, our results provide the framework for rational drug development for the purpose of personalized medicine.

Dilated cardiomyopathy mutation in beta-cardiac myosin enhances actin activation of the power stroke and phosphate release

Inherited mutations in human beta-cardiac myosin (M2β) can lead to severe forms of heart failure. The E525K mutation in M2β is associated with dilated cardiomyopathy (DCM) and was found to stabilize the interacting heads motif (IHM) and autoinhibited super-relaxed (SRX) state in dimeric heavy meromyosin. However, in monomeric M2β subfragment 1 (S1) we found that E525K enhances (3-fold) the maximum steady-state actin-activated ATPase activity (kcat) and decreases (6-fold) the actin concentration at which ATPase is one-half maximal (KATPase). We also found a 3 to 4-fold increase in the actin-activated power stroke and phosphate release rate constants at 30 μM actin, which overall enhanced the duty ratio 3-fold. Loaded motility assays revealed that the enhanced intrinsic motor activity translates to increased ensemble force in M2β S1. Glutamate 525, located near the actin binding region in the so-called activation loop, is highly conserved and predicted to form a salt-bridge with another conserved residue (lysine 484) in the relay helix. Enhanced sampling molecular dynamics simulations predict that the charge reversal mutation disrupts the E525-K484 salt-bridge, inducing conformations with a more flexible relay helix and a wide phosphate release tunnel. Our results highlight a highly conserved allosteric pathway associated with actin activation of the power stroke and phosphate release and suggest an important feature of the autoinhibited IHM is to prevent this region of myosin from interacting with actin. The ability of the E525K mutation to stabilize the IHM likely overrides the enhanced intrinsic motor properties, which may be key to triggering DCM pathogenesis.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than that of slow anchoring myosin-1s found on endosomal membranes. We, therefore, propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

Danicamtiv Increases Myosin Recruitment and Alters Cross-Bridge Cycling in Cardiac Muscle

BACKGROUND

Modulating myosin function is a novel therapeutic approach in patients with cardiomyopathy. Danicamtiv is a novel myosin activator with promising preclinical data that is currently in clinical trials. While it is known that danicamtiv increases force and cardiomyocyte contractility without affecting calcium levels, detailed mechanistic studies regarding its mode of action are lacking.

METHODS

Permeabilized porcine cardiac tissue and myofibrils were used for X-ray diffraction and mechanical measurements. A mouse model of genetic dilated cardiomyopathy was used to evaluate the ability of danicamtiv to correct the contractile deficit.

RESULTS

Danicamtiv increased force and calcium sensitivity via increasing the number of myosins in the ON state and slowing cross-bridge turnover. Our detailed analysis showed that inhibition of ADP release results in decreased cross-bridge turnover with cross bridges staying attached longer and prolonging myofibril relaxation. Danicamtiv corrected decreased calcium sensitivity in demembranated tissue, abnormal twitch magnitude and kinetics in intact cardiac tissue, and reduced ejection fraction in the whole organ.

CONCLUSIONS

As demonstrated by the detailed studies of Danicamtiv, increasing myosin recruitment and altering cross-bridge cycling are 2 mechanisms to increase force and calcium sensitivity in cardiac muscle. Myosin activators such as Danicamtiv can treat the causative hypocontractile phenotype in genetic dilated cardiomyopathy.

Homologous mutations in β, embryonic, and perinatal muscle myosins have divergent effects on molecular power generation

Mutations at a highly conserved homologous residue in three closely related muscle myosins cause three distinct diseases involving muscle defects: R671C in β -cardiac myosin causes hypertrophic cardiomyopathy, R672C and R672H in embryonic skeletal myosin cause Freeman Sheldon syndrome, and R674Q in perinatal skeletal myosin causes trismus-pseudocamptodactyly syndrome. It is not known if their effects at the molecular level are similar to one another or correlate with disease phenotype and severity. To this end, we investigated the effects of the homologous mutations on key factors of molecular power production using recombinantly expressed human β , embryonic, and perinatal myosin subfragment-1. We found large effects in the developmental myosins, with the most dramatic in perinatal, but minimal effects in β myosin, and magnitude of changes correlated partially with clinical severity. The mutations in the developmental myosins dramatically decreased the step size and load-sensitive actin-detachment rate of single molecules measured by optical tweezers, in addition to decreasing ATPase cycle rate. In contrast, the only measured effect of R671C in β myosin was a larger step size. Our measurements of step size and bound times predicted velocities consistent with those measured in an in vitro motility assay. Finally, molecular dynamics simulations predicted that the arginine to cysteine mutation in embryonic, but not β , myosin may reduce pre-powerstroke lever arm priming and ADP pocket opening, providing a possible structural mechanism consistent with the experimental observations. This paper presents the first direct comparisons of homologous mutations in several different myosin isoforms, whose divergent functional effects are yet another testament to myosin's highly allosteric nature.

Estimation of crossbridge-state during cardiomyocyte beating using second harmonic generation

Estimation of dynamic change of crossbridge formation in living cardiomyocytes is expected to provide crucial information for elucidating cardiomyopathy mechanisms, efficacy of an intervention, and others. Here, we established an assay system to dynamically measure second harmonic generation (SHG) anisotropy derived from myosin filaments depended on their crossbridge status in pulsating cardiomyocytes. Experiments utilizing an inheritable mutation that induces excessive myosin-actin interactions revealed that the correlation between sarcomere length and SHG anisotropy represents crossbridge formation ratio during pulsation. Furthermore, the present method found that ultraviolet irradiation induced an increased population of attached crossbridges that lost the force-generating ability upon myocardial differentiation. Taking an advantage of infrared two-photon excitation in SHG microscopy, myocardial dysfunction could be intravitally evaluated in a Drosophila disease model. Thus, we successfully demonstrated the applicability and effectiveness of the present method to evaluate the actomyosin activity of a drug or genetic defect on cardiomyocytes. Because genomic inspection alone may not catch the risk of cardiomyopathy in some cases, our study demonstrated herein would be of help in the risk assessment of future heart failure.

Endocytic myosin-1 is a force-insensitive, power-generating motor

Myosins are required for clathrin-mediated endocytosis, but their precise molecular roles in this process are not known. This is, in part, because the biophysical properties of the relevant motors have not been investigated. Myosins have diverse mechanochemical activities, ranging from powerful contractility against mechanical loads to force-sensitive anchoring. To better understand the essential molecular contribution of myosin to endocytosis, we studied the in vitro force-dependent kinetics of the Saccharomyces cerevisiae endocytic type I myosin called Myo5, a motor whose role in clathrin-mediated endocytosis has been meticulously studied in vivo. We report that Myo5 is a low-duty-ratio motor that is activated ∼10-fold by phosphorylation, and that its working stroke and actin-detachment kinetics are relatively force-insensitive. Strikingly, the in vitro mechanochemistry of Myo5 is more like that of cardiac myosin than like that of slow anchoring myosin-1s found on endosomal membranes. We therefore propose that Myo5 generates power to augment actin assembly-based forces during endocytosis in cells.

Summary

Pedersen, Snoberger et al. measure the force-sensitivity of the yeast endocytic the myosin-1 called Myo5 and find that it is more likely to generate power than to serve as a force-sensitive anchor in cells. Implications for Myo5's role in clathrin-mediated endocytosis are discussed.

Phosphorylation Mimetic of Myosin Regulatory Light Chain Mitigates Cardiomyopathy-Induced Myofilament Impairment in Mouse Models of RCM and DCM

This study focuses on mimicking constitutive phosphorylation in the N-terminus of the myosin regulatory light chain (S15D-RLC) as a rescue strategy for mutation-induced cardiac dysfunction in transgenic (Tg) models of restrictive (RCM) and dilated (DCM) cardiomyopathy caused by mutations in essential (ELC, MYL3 gene) or regulatory (RLC, MYL2 gene) light chains of myosin. Phosphomimetic S15D-RLC was reconstituted in left ventricular papillary muscle (LVPM) fibers from two mouse models of cardiomyopathy, RCM-E143K ELC and DCM-D94A RLC, along with their corresponding Tg-ELC and Tg-RLC wild-type (WT) mice. The beneficial effects of S15D-RLC in rescuing cardiac function were manifested by the S15D-RLC-induced destabilization of the super-relaxed (SRX) state that was observed in both models of cardiomyopathy. S15D-RLC promoted a shift from the SRX state to the disordered relaxed (DRX) state, increasing the number of heads readily available to interact with actin and produce force. Additionally, S15D-RLC reconstituted with fibers demonstrated significantly higher maximal isometric force per cross-section of muscle compared with reconstitution with WT-RLC protein. The effects of the phosphomimetic S15D-RLC were compared with those observed for Omecamtiv Mecarbil (OM), a myosin activator shown to bind to the catalytic site of cardiac myosin and increase myocardial contractility. A similar SRX↔DRX equilibrium shift was observed in OM-treated fibers as in S15D-RLC-reconstituted preparations. Additionally, treatment with OM resulted in significantly higher maximal pCa 4 force per cross-section of muscle fibers in both cardiomyopathy models. Our results suggest that both treatments with S15D-RLC and OM may improve the function of myosin motors and cardiac muscle contraction in RCM-ELC and DCM-RLC mice.

Single-molecule mechanics and kinetics of cardiac myosin interacting with regulated thin filaments

The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and nonmuscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin-filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin-filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin-filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations at both saturating and physiologically relevant subsaturating calcium concentrations, thin-filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for the modeling of cardiac physiology and diseases.

Binding pocket dynamics along the recovery stroke of human β-cardiac myosin

The druggability of small-molecule binding sites can be significantly affected by protein motions and conformational changes. Ligand binding, protein dynamics and protein function have been shown to be closely interconnected in myosins. The breakthrough discovery of omecamtiv mecarbil (OM) has led to an increased interest in small molecules that can target myosin and modulate its function for therapeutic purposes (myosin modulators). In this work, we use a combination of computational methods, including steered molecular dynamics, umbrella sampling and binding pocket tracking tools, to follow the evolution of the OM binding site during the recovery stroke transition of human β-cardiac myosin. We found that steering two internal coordinates of the motor domain can recapture the main features of the transition and in particular the rearrangements of the binding site, which shows significant changes in size, shape and composition. Possible intermediate conformations were also identified, in remarkable agreement with experimental findings. The differences in the binding site properties observed along the transition can be exploited for the future development of conformation-selective myosin modulators.

Distinct effects of two hearing loss-associated mutations in the sarcomeric myosin MYH7b

For decades, sarcomeric myosin heavy chain proteins were assumed to be restricted to striated muscle where they function as molecular motors that contract muscle. However, MYH7b, an evolutionarily ancient member of this myosin family, has been detected in mammalian nonmuscle tissues, and mutations in MYH7b are linked to hereditary hearing loss in compound heterozygous patients. These mutations are the first associated with hearing loss rather than a muscle pathology, and because there are no homologous mutations in other myosin isoforms, their functional effects were unknown. We generated recombinant human MYH7b harboring the D515N or R1651Q hearing loss-associated mutation and studied their effects on motor activity and structural and assembly properties, respectively. The D515N mutation had no effect on steady-state actin-activated ATPase rate or load-dependent detachment kinetics but increased actin sliding velocity because of an increased displacement during the myosin working stroke. Furthermore, we found that the D515N mutation caused an increase in the proportion of myosin heads that occupy the disordered-relaxed state, meaning more myosin heads are available to interact with actin. Although we found no impact of the R1651Q mutation on myosin rod secondary structure or solubility, we observed a striking aggregation phenotype when this mutation was introduced into nonmuscle cells. Our results suggest that each mutation independently affects MYH7b function and structure. Together, these results provide the foundation for further study of a role for MYH7b outside the sarcomere.

Conformational changes linked to ADP release from human cardiac myosin bound to actin-tropomyosin

Following binding to the thin filament, β-cardiac myosin couples ATP-hydrolysis to conformational rearrangements in the myosin motor that drive myofilament sliding and cardiac ventricular contraction. However, key features of the cardiac-specific actin-myosin interaction remain uncertain, including the structural effect of ADP release from myosin, which is rate-limiting during force generation. In fact, ADP release slows under experimental load or in the intact heart due to the afterload, thereby adjusting cardiac muscle power output to meet physiological demands. To further elucidate the structural basis of this fundamental process, we used a combination of cryo-EM reconstruction methodologies to determine structures of the human cardiac actin-myosin-tropomyosin filament complex at better than 3.4 Å-resolution in the presence and in the absence of Mg2+·ADP. Focused refinements of the myosin motor head and its essential light chains in these reconstructions reveal that small changes in the nucleotide-binding site are coupled to significant rigid body movements of the myosin converter domain and a 16-degree lever arm swing. Our structures provide a mechanistic framework to understand the effect of ADP binding and release on human cardiac β-myosin, and offer insights into the force-sensing mechanism displayed by the cardiac myosin motor.

Functional assays reveal the pathogenic mechanism of a de novo tropomyosin variant identified in patient with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a leading cause of heart failure and a major indicator for heart transplant. Human genetic studies have identified over a thousand causal mutations for DCM in genes involved in a variety of cellular processes, including sarcomeric contraction. A substantial clinical challenge is determining the pathogenicity of novel variants in disease-associated genes. This challenge of connecting genotype and phenotype has frustrated attempts to develop effective, mechanism-based treatments for patients. Here, we identified a de novo mutation (T237S) in TPM1, the gene that encodes the thin filament protein tropomyosin, in a patient with DCM and conducted in vitro experiments to characterize the pathogenicity of this novel variant. We expressed recombinant mutant protein, reconstituted it into thin filaments, and examined the effects of the mutation on thin filament function. We show that the mutation reduces the calcium sensitivity of thin filament activation, as previously seen for known pathogenic mutations. Mechanistically, this shift is due to mutation-induced changes in tropomyosin positioning along the thin filament. We demonstrate that the thin filament activator omecamtiv mecarbil restores the calcium sensitivity of thin filaments regulated by the mutant tropomyosin, which lays the foundation for additional experiments to explore the therapeutic potential of this drug for patients harboring the T237S mutation. Taken together, our results suggest that the TPM1 T237S mutation is likely pathogenic and demonstrate how functional in vitro characterization of pathogenic protein variants in the lab might guide precision medicine in the clinic.

Danicamtiv increases myosin recruitment and alters the chemomechanical cross bridge cycle in cardiac muscle

Modulating myosin function is a novel therapeutic approach in patients with cardiomyopathy. Detailed mechanism of action of these agents can help predict potential unwanted affects and identify patient populations that can benefit most from them. Danicamtiv is a novel myosin activator with promising preclinical data that is currently in clinical trials. While it is known danicamtiv increases force and cardiomyocyte contractility without affecting calcium levels, detailed mechanistic studies regarding its mode of action are lacking. Using porcine cardiac tissue and myofibrils we demonstrate that Danicamtiv increases force and calcium sensitivity via increasing the number of myosin in the "on" state and slowing cross bridge turnover. Our detailed analysis shows that inhibition of ADP release results in decreased cross bridge turnover with cross bridges staying on longer and prolonging myofibril relaxation. Using a mouse model of genetic dilated cardiomyopathy, we demonstrated that Danicamtiv corrected calcium sensitivity in demembranated and abnormal twitch magnitude and kinetics in intact cardiac tissue.

Significance Statement

Directly augmenting sarcomere function has potential to overcome limitations of currently used inotropic agents to improve cardiac contractility. Myosin modulation is a novel mechanism for increased contraction in cardiomyopathies. Danicamtiv is a myosin activator that is currently under investigation for use in cardiomyopathy patients. Our study is the first detailed mechanism of how Danicamtiv increases force and alters kinetics of cardiac activation and relaxation. This new understanding of the mechanism of action of Danicamtiv can be used to help identify patients that could benefit most from this treatment.

dATP elevation induces myocardial metabolic remodeling to support improved cardiac function

Hallmark features of systolic heart failure are reduced contractility and impaired metabolic flexibility of the myocardium. Cardiomyocytes (CMs) with elevated deoxy ATP (dATP) via overexpression of ribonucleotide reductase (RNR) enzyme robustly improve contractility. However, the effect of dATP elevation on cardiac metabolism is unknown. Here, we developed proteolysis-resistant versions of RNR and demonstrate that elevation of dATP/ATP to ∼1% in CMs in a transgenic mouse (TgRRB) resulted in robust improvement of cardiac function. Pharmacological approaches showed that CMs with elevated dATP have greater basal respiratory rates by shifting myosin states to more active forms, independent of its isoform, in relaxed CMs. Targeted metabolomic profiling revealed a significant reprogramming towards oxidative phosphorylation in TgRRB-CMs. Higher cristae density and activity in the mitochondria of TgRRB-CMs improved respiratory capacity. Our results revealed a critical property of dATP to modulate myosin states to enhance contractility and induce metabolic flexibility to support improved function in CMs.

Single Molecule Mechanics and Kinetics of Cardiac Myosin Interacting with Regulated Thin Filaments

The cardiac cycle is a tightly regulated process wherein the heart generates force to pump blood to the body during systole and then relaxes during diastole. Disruption of this finely tuned cycle can lead to a range of diseases including cardiomyopathies and heart failure. Cardiac contraction is driven by the molecular motor myosin, which pulls regulated thin filaments in a calcium-dependent manner. In some muscle and non-muscle myosins, regulatory proteins on actin tune the kinetics, mechanics, and load dependence of the myosin working stroke; however, it is not well understood whether or how thin filament regulatory proteins tune the mechanics of the cardiac myosin motor. To address this critical gap in knowledge, we used single-molecule techniques to measure the kinetics and mechanics of the substeps of the cardiac myosin working stroke in the presence and absence of thin filament regulatory proteins. We found that regulatory proteins gate the calcium-dependent interactions between myosin and the thin filament. At physiologically relevant ATP concentrations, cardiac myosin's mechanics and unloaded kinetics are not affected by thin filament regulatory proteins. We also measured the load-dependent kinetics of cardiac myosin at physiologically relevant ATP concentrations using an isometric optical clamp, and we found that thin filament regulatory proteins do not affect either the identity or magnitude of myosin's primary load-dependent transition. Interestingly, at low ATP concentrations, thin filament regulatory proteins have a small effect on actomyosin dissociation kinetics, suggesting a mechanism beyond simple steric blocking. These results have important implications for both disease modeling and computational models of muscle contraction.

Significance Statement

Human heart contraction is powered by the molecular motor β-cardiac myosin, which pulls on thin filaments consisting of actin and the regulatory proteins troponin and tropomyosin. In some muscle and non-muscle systems, these regulatory proteins tune the kinetics, mechanics, and load dependence of the myosin working stroke. Despite having a central role in health and disease, it is not well understood whether the mechanics or kinetics of β-cardiac myosin are affected by regulatory proteins. We show that regulatory proteins do not affect the mechanics or load-dependent kinetics of the working stroke at physiologically relevant ATP concentrations; however, they can affect the kinetics at low ATP concentrations, suggesting a mechanism beyond simple steric blocking. This has important implications for modeling of cardiac physiology and diseases.

Comparative study of binding pocket structure and dynamics in cardiac and skeletal myosin

The development of small molecule myosin modulators has seen an increased effort in recent years due to their possible use in the treatment of cardiac and skeletal myopathies. Omecamtiv mecarbil (OM) is the first-in-class cardiac myotrope and the first to enter clinical trials. Its selectivity toward slow/beta-cardiac myosin lies at the heart of its function; however, little is known about the underlying reasons for selectivity to this isoform as opposed to other closely related ones such as fast-type skeletal myosins. In this work, we compared the structure and dynamics of the OM binding site in cardiac and in fasttype IIa skeletal myosin to identify possible reasons for OM selectivity. We found that the different shape, size, and composition of the binding pocket in skeletal myosin directly affects the binding mode and related affinity of OM, which is potentially a result of weaker interactions and less optimal molecular recognition. Moreover, we identified a side pocket adjacent to the OM binding site that shows increased accessibility in skeletal myosin compared with the cardiac isoform. These findings could pave the way to the development of skeletal-selective compounds that can target this region of the protein and potentially be used to treat congenital myopathies where muscle weakness is related to myosin loss of function.

Cardiac Sarcomere Signaling in Health and Disease

The cardiac sarcomere is a triumph of biological evolution wherein myriad contractile and regulatory proteins assemble into a quasi-crystalline lattice to serve as the central point upon which cardiac muscle contraction occurs. This review focuses on the many signaling components and mechanisms of regulation that impact cardiac sarcomere function. We highlight the roles of the thick and thin filament, both as necessary structural and regulatory building blocks of the sarcomere as well as targets of functionally impactful modifications. Currently, a new focus emerging in the field is inter-myofilament signaling, and we discuss here the important mediators of this mechanism, including myosin-binding protein C and titin. As the understanding of sarcomere signaling advances, so do the methods with which it is studied. This is reviewed here through discussion of recent live muscle systems in which the sarcomere can be studied under intact, physiologically relevant conditions.

Insights into the Mechanism of the Cardiac Drug Omecamtiv Mecarbil─A Computational Study

Omecamtiv mecarbil (OM) is a positive inotrope that is thought to bind directly to an allosteric site of the β-cardiac myosin. The drug is under investigation for the treatment of systolic heart failure. The drug is classified as a cardiac myosin modulator and has been observed to affect multiple vital steps of the cross-bridge cycle including the recovery stroke and the chemical step. We explored the free-energy surface of the recovery stroke of the human cardiac β-myosin in the presence of OM to determine its influence on this process. We also investigated the effects of OM on the recovery stroke in the presence of genetic cardiomyopathic mutations R712L, F764L, and P710R using metadynamics. We also utilized the method of transition path sampling to generate an unbiased ensemble of reactive trajectories for the ATP hydrolysis step in the presence of OM that were able to provide insight into the differences observed due to OM in the dynamics and mechanism of the decomposition of ATP to ADP and HPO₄^2-, a central part of the power generation in cardiac muscle. We studied chemistry in the presence of the same three mutations to further elucidate the effect of OM, and its use in the treatment of cardiac disease.

The force of the myosin motor sets cooperativity in thin filament activation of skeletal muscles

Contraction of striated muscle is regulated by a dual mechanism involving both thin, actin-containing filament and thick, myosin-containing filament. Thin filament is activated by Ca2+ binding to troponin, leading to tropomyosin displacement that exposes actin sites for interaction with myosin motors, extending from the neighbouring stress-activated thick filaments. Motor attachment to actin contributes to spreading activation along the thin filament, through a cooperative mechanism, still unclear, that determines the slope of the sigmoidal relation between isometric force and pCa (−log[Ca2+]), estimated by Hill coefficient nH. We use sarcomere-level mechanics in demembranated fibres of rabbit skeletal muscle activated by Ca2+ at different temperatures (12–35 °C) to show that nH depends on the motor force at constant number of attached motors. The definition of the role of motor force provides fundamental constraints for modelling the dynamics of thin filament activation and defining the action of small molecules as possible therapeutic tools.

Functional divergence of the sarcomeric myosin, MYH7b, supports species-specific biological roles

Myosin heavy chain 7b (MYH7b) is an evolutionarily ancient member of the sarcomeric myosin family, which typically supports striated muscle function. However, in mammals, alternative splicing prevents MYH7b protein production in cardiac and most skeletal muscles and limits expression to a subset of specialized muscles and certain nonmuscle environments. In contrast, MYH7b protein is abundant in python cardiac and skeletal muscles. Although the MYH7b expression pattern diverges in mammals versus reptiles, MYH7b shares high sequence identity across species. So, it remains unclear how mammalian MYH7b function may differ from that of other sarcomeric myosins and whether human and python MYH7b motor functions diverge as their expression patterns suggest. Thus, we generated recombinant human and python MYH7b protein and measured their motor properties to investigate any species-specific differences in activity. Our results reveal that despite having similar working strokes, the MYH7b isoforms have slower actin-activated ATPase cycles and actin sliding velocities than human cardiac β-MyHC. Furthermore, python MYH7b is tuned to have slower motor activity than human MYH7b because of slower kinetics of the chemomechanical cycle. We found that the MYH7b isoforms adopt a higher proportion of myosin heads in the ultraslow, super-relaxed state compared with human cardiac β-MyHC. These findings are supported by molecular dynamics simulations that predict MYH7b preferentially occupies myosin active site conformations similar to those observed in the structurally inactive state. Together, these results suggest that MYH7b is specialized for slow and energy-conserving motor activity and that differential tuning of MYH7b orthologs contributes to species-specific biological roles.

Insights into Muscle Contraction Derived from the Effects of Small-Molecular Actomyosin-Modulating Compounds

Bottom-up mechanokinetic models predict ensemble function of actin and myosin based on parameter values derived from studies using isolated proteins. To be generally useful, e.g., to analyze disease effects, such models must also be able to predict ensemble function when actomyosin interaction kinetics are modified differently from normal. Here, we test this capability for a model recently shown to predict several physiological phenomena along with the effects of the small molecular compound blebbistatin. We demonstrate that this model also qualitatively predicts effects of other well-characterized drugs as well as varied concentrations of MgATP. However, the effects of one compound, amrinone, are not well accounted for quantitatively. We therefore systematically varied key model parameters to address this issue, leading to the increased amplitude of the second sub-stroke of the power stroke from 1 nm to 2.2 nm, an unchanged first sub-stroke (5.3−5.5 nm), and an effective cross-bridge attachment rate that more than doubled. In addition to better accounting for the effects of amrinone, the modified model also accounts well for normal physiological ensemble function. Moreover, a Monte Carlo simulation-based version of the model was used to evaluate force−velocity data from small myosin ensembles. We discuss our findings in relation to key aspects of actin−myosin operation mechanisms causing a non-hyperbolic shape of the force−velocity relationship at high loads. We also discuss remaining limitations of the model, including uncertainty of whether the cross-bridge elasticity is linear or not, the capability to account for contractile properties of very small actomyosin ensembles (<20 myosin heads), and the mechanism for requirements of a higher cross-bridge attachment rate during shortening compared to during isometric contraction.

Hypertrophic cardiomyopathy: Mutations to mechanisms to therapies

Hypertrophic cardiomyopathy (HCM) affects more than 1 in 500 people in the general population with an extensive burden of morbidity in the form of arrhythmia, heart failure, and sudden death. More than 25 years since the discovery of the genetic underpinnings of HCM, the field has unveiled significant insights into the primary effects of these genetic mutations, especially for the myosin heavy chain gene, which is one of the most commonly mutated genes. Our group has studied the molecular effects of HCM mutations on human β-cardiac myosin heavy chain using state-of-the-art biochemical and biophysical tools for the past 10 years, combining insights from clinical genetics and structural analyses of cardiac myosin. The overarching hypothesis is that HCM-causing mutations in sarcomere proteins cause hypercontractility at the sarcomere level, and we have shown that an increase in the number of myosin molecules available for interaction with actin is a primary driver. Recently, two pharmaceutical companies have developed small molecule inhibitors of human cardiac myosin to counteract the molecular consequences of HCM pathogenesis. One of these inhibitors (mavacamten) has recently been approved by the FDA after completing a successful phase III trial in HCM patients, and the other (aficamten) is currently being evaluated in a phase III trial. Myosin inhibitors will be the first class of medication used to treat HCM that has both robust clinical trial evidence of efficacy and that targets the fundamental mechanism of HCM pathogenesis. The success of myosin inhibitors in HCM opens the door to finding other new drugs that target the sarcomere directly, as we learn more about the genetics and fundamental mechanisms of this disease.