Allosteric modulation of cardiac myosin dynamics by omecamtiv mecarbil

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.

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.

Phospholamban inhibits the cardiac calcium pump by interrupting an allosteric activation pathway

Phospholamban (PLB) is a transmembrane micropeptide that regulates the sarcoplasmic reticulum Ca2+-ATPase (SERCA) in cardiac muscle, but the physical mechanism of this regulation remains poorly understood. PLB reduces the Ca2+ sensitivity of active SERCA, increasing the Ca2+ concentration required for pump cycling. However, PLB does not decrease Ca2+ binding to SERCA when ATP is absent, suggesting PLB does not inhibit SERCA Ca2+ affinity. The prevailing explanation for these seemingly conflicting results is that PLB slows transitions in the SERCA enzymatic cycle associated with Ca2+ binding, altering transport Ca2+ dependence without actually affecting the equilibrium binding affinity of the Ca2+-coordinating sites. Here, we consider another hypothesis, that measurements of Ca2+ binding in the absence of ATP overlook important allosteric effects of nucleotide binding that increase SERCA Ca2+ binding affinity. We speculated that PLB inhibits SERCA by reversing this allostery. To test this, we used a fluorescent SERCA biosensor to quantify the Ca2+ affinity of non-cycling SERCA in the presence and absence of a non-hydrolyzable ATP-analog, AMPPCP. Nucleotide activation increased SERCA Ca2+ affinity, and this effect was reversed by co-expression of PLB. Interestingly, PLB had no effect on Ca2+ affinity in the absence of nucleotide. These results reconcile the previous conflicting observations from ATPase assays versus Ca2+ binding assays. Moreover, structural analysis of SERCA revealed a novel allosteric pathway connecting the ATP- and Ca2+-binding sites. We propose this pathway is disrupted by PLB binding. Thus, PLB reduces the equilibrium Ca2+ affinity of SERCA by interrupting allosteric activation of the pump by ATP.

Exploring the Super-Relaxed State of Human Cardiac β-Myosin by Molecular Dynamics Simulations

Human β-cardiac myosin plays a critical role in generating the mechanical forces necessary for cardiac muscle contraction. This process relies on a delicate dynamic equilibrium between the disordered relaxed state (DRX) and the super-relaxed state (SRX) of myosin. Disruptions in this equilibrium due to mutations can lead to heart diseases. However, the structural characteristics of SRX and the molecular mechanisms underlying pathogenic mutations have remained elusive. To bridge this gap, we conducted molecular dynamics simulations and free energy calculations to explore the conformational changes in myosin. Our findings indicate that the size of the phosphate-binding pocket can serve as a valuable metric for characterizing the transition from the DRX to SRX state. Importantly, we established a global dynamic coupling network within the myosin motor head at the residue level, elucidating how the pathogenic mutation E483K impacts the equilibrium between SRX and DRX through allosteric effects. Our work illuminates molecular details of SRX and offers valuable insights into disease treatment through the regulation of SRX.

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.

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.

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.

Myosin modulators: emerging approaches for the treatment of cardiomyopathies and heart failure

Myosin modulators are a novel class of pharmaceutical agents that are being developed to treat patients with a range of cardiomyopathies. The therapeutic goal of these drugs is to target cardiac myosins directly to modulate contractility and cardiac power output to alleviate symptoms that lead to heart failure and arrhythmias, without altering calcium signaling. In this Review, we discuss two classes of drugs that have been developed to either activate (omecamtiv mecarbil) or inhibit (mavacamten) cardiac contractility by binding to β-cardiac myosin (MYH7). We discuss progress in understanding the mechanisms by which the drugs alter myosin mechanochemistry, and we provide an appraisal of the results from clinical trials of these drugs, with consideration for the importance of disease heterogeneity and genetic etiology for predicting treatment benefit.

Heart Failure Drug Modifies the Intrinsic Dynamics of the Pre-Power Stroke State of Cardiac Myosin

Omecamtiv mecarbil (OM), currently investigated for the treatment of heart failure, is the first example of a new class of drugs (cardiac myotropes) that can modify muscle contractility by directly targeting sarcomeric proteins. Using atomistic molecular dynamics simulations, we show that the binding of OM to the pre-power stroke state of cardiac myosin inhibits the functional motions of the protein and potentially affects Pi release from the nucleotide binding site. We also show that the changes in myosin ATPase activity induced by a set of OM analogues can be predicted from their relative affinity to the pre-power stroke state compared to the near rigor one, indicating that conformational selectivity plays an important role in determining the activity of these compounds.

Cholesterol binding to the sterol-sensing region of Niemann Pick C1 protein confines dynamics of its N-terminal domain

Lysosomal accumulation of cholesterol is a hallmark of Niemann Pick type C (NPC) disease caused by mutations primarily in the lysosomal membrane protein NPC1. NPC1 contains a transmembrane sterol-sensing domain (SSD), which is supposed to regulate protein activity upon cholesterol binding, but the mechanisms underlying this process are poorly understood. Using atomistic simulations, we show that in the absence of cholesterol in the SSD, the luminal domains of NPC1 are highly dynamic, resulting in the disengagement of the NTD from the rest of the protein. The disengaged NPC1 adopts a flexed conformation that approaches the lipid bilayer, and could represent a conformational state primed to receive a sterol molecule from the soluble lysosomal cholesterol carrier NPC2. The binding of cholesterol to the SSD of NPC1 allosterically suppresses the conformational dynamics of the luminal domains resulting in an upright NTD conformation. The presence of an additional 20% cholesterol in the membrane has negligible impact on this process. The additional presence of an NTD-bound cholesterol suppresses the flexing of the NTD. We propose that cholesterol acts as an allosteric effector, and the modulation of NTD dynamics by the SSD-bound cholesterol constitutes an allosteric feedback mechanism in NPC1 that controls cholesterol abundance in the lysosomal membrane.

Cholesterol is absorbed from LDL particles in esterified form, and is broken down to free cholesterol in the lysosomes of cells, from where cholesterol must be transported to other cellular compartments such as the plasma membrane. The Niemann Pick type C (NPC) diseases arise from deficient cholesterol transport and result from mutations in the cholesterol transport protein NPC1. Using computer simulations, we show that cholesterol, when bound to one part of NPC1, can control the structural transitions of an 8-nm distant, different part of NPC1 protein called the N-terminal domain (NTD). Such long-range control of protein conformations (allostery), controls a wide range of cellular functions mediated by proteins. Fundamental molecular insights into the function of the NPC1 protein can potentially lead to better pharmaceutical interventions for the NPC diseases.

Conformational distributions of isolated myosin motor domains encode their mechanochemical properties

Myosin motor domains perform an extraordinary diversity of biological functions despite sharing a common mechanochemical cycle. Motors are adapted to their function, in part, by tuning the thermodynamics and kinetics of steps in this cycle. However, it remains unclear how sequence encodes these differences, since biochemically distinct motors often have nearly indistinguishable crystal structures. We hypothesized that sequences produce distinct biochemical phenotypes by modulating the relative probabilities of an ensemble of conformations primed for different functional roles. To test this hypothesis, we modeled the distribution of conformations for 12 myosin motor domains by building Markov state models (MSMs) from an unprecedented two milliseconds of all-atom, explicit-solvent molecular dynamics simulations. Comparing motors reveals shifts in the balance between nucleotide-favorable and nucleotide-unfavorable P-loop conformations that predict experimentally measured duty ratios and ADP release rates better than sequence or individual structures. This result demonstrates the power of an ensemble perspective for interrogating sequence-function relationships.

Structure and functional analysis of the Legionella pneumophila chitinase ChiA reveals a novel mechanism of metal-dependent mucin degradation

Chitinases are important enzymes that contribute to the generation of carbon and nitrogen from chitin, a long chain polymer of N-acetylglucosamine that is abundant in insects, fungi, invertebrates and fish. Although mammals do not produce chitin, chitinases have been identified in bacteria that are key virulence factors in severe respiratory, gastrointestinal and urinary diseases. However, it is unclear how these enzymes are able to carry out this dual function. Legionella pneumophila is the causative agent of Legionnaires' disease, an often-fatal pneumonia and its chitinase ChiA is essential for the survival of L. pneumophila in the lung. Here we report the first atomic resolution insight into the pathogenic mechanism of a bacterial chitinase. We derive an experimental model of intact ChiA and show how its N-terminal region targets ChiA to the bacterial surface after its secretion. We provide the first evidence that L. pneumophila can bind mucins on its surface, but this is not dependent on ChiA. This demonstrates that additional peripheral mucin binding proteins are also expressed in L. pneumophila. We also show that the ChiA C-terminal chitinase domain has novel Zn2+-dependent peptidase activity against mammalian mucin-like proteins, namely MUC5AC and the C1-esterase inhibitor, and that ChiA promotes bacterial penetration of mucin gels. Our findings suggest that ChiA can facilitate passage of L. pneumophila through the alveolar mucosa, can modulate the host complement system and that ChiA may be a promising target for vaccine development.

Myocardial Contractility: Historical and Contemporary Considerations

The term myocardial contractility is thought to have originated more than 125 years ago and has remained and enigma ever since. Although the term is frequently used in textbooks, editorials and contemporary manuscripts its definition remains illusive often being conflated with cardiac performance or inotropy. The absence of a universally accepted definition has led to confusion, disagreement and misconceptions among physiologists, cardiologists and safety pharmacologists regarding its definition particularly in light of new discoveries regarding the load dependent kinetics of cardiac contraction and their translation to cardiac force-velocity and ventricular pressure-volume measurements. Importantly, the Starling interpretation of force development is length-dependent while contractility is length independent. Most historical definitions employ an operational approach and define cardiac contractility in terms of the hearts mechanical properties independent of loading conditions. Literally defined the term contract infers that something has become smaller, shrunk or shortened. The addition of the suffix “ility” implies the quality of this process. The discovery and clinical investigation of small molecules that bind to sarcomeric proteins independently altering force or velocity requires that a modern definition of the term myocardial contractility be developed if the term is to persist. This review reconsiders the historical and contemporary interpretations of the terms cardiac performance and inotropy and recommends a modern definition of myocardial contractility as the preload, afterload and length-independent intrinsic kinetically controlled, chemo-mechanical processes responsible for the development of force and velocity.

Targeted and Controlled Drug Delivery to a Rat Model of Heart Failure Through a Magnetic Nanocomposite

As a novel cardiac myosin activator, Omecamtive Mecarbil (OM) has shown promising results in the management of systolic heart failure in clinical examinations. However, the need for repeated administration along with dose-dependent side effects made its use elusive as a standard treatment for heart failure (HF). We hypothesized that improved cardiac function in systolic HF models would be achieved in lower doses by targeted delivery of OM to the heart. To test this hypothesis, a nanocomposite system was developed by composing chitosan and a magnetic core (Fe₃O₄), loaded with OM, and directed toward the rats' heart via a 0.3 T magnet. HF-induced rats were injected with saline, OM, and OM-loaded nanocomposite (n = 8 in each group) and compared with a group of healthy animals (saline injected, n = 8). Knowing the ejection fraction (EF) of healthy (93.68 ± 1.37%) and HF (71.7 ± 1.41%) rats, injection of nanocomposites was associated with improved EF (EF = 89.6 ± 1.40%). Due to increased heart targeting of nanocomposite (2.5 folds), improved cardiac function was seen with only 4% of the OM dose required for infusion, while injecting the same dose of OM without targeting was unable to stop HF progression (EF = 55.33 ± 3.16%) during 7 days. In conclusion, heart nanocomposites targeting improves the EF by up to 18% by only using 4% of the doses traditionally used in treating the HF.

New therapies for the treatment of heart failure: a summary of recent accomplishments

Despite continuous efforts to prevent cardiovascular diseases (CVDs), heart failure prevails as the number one cause of death in developed countries. To properly treat CVDs, scientists had to take a closer look at the factors that contribute to their pathogenesis and either modernize current pharmaceuticals or develop brand new treatments. Enhancement of current drugs, such as tolvaptan and omecamtiv mecarbil, sheds new light on already-known therapies. Tolvaptan, a vasopressin antagonist, could be adopted in heart failure therapy as it reduces pre- and afterload by decreasing systolic blood pressure and blood volume. Omecamtiv mecarbil, which is a myosin binding peptide, could aid cardiac contractility. The next generation vasodilators, serelaxin and ularitide, are based on naturally occurring peptides and they reduce peripheral vascular resistance and increase the cardiac index. In combination with their anti-inflammatory properties, they could turn out to be extremely potent drugs for heart failure treatment. Cardiotrophin has exceeded many researchers’ expectations, as evidence suggests that it could cause sarcomere hypertrophy without excessive proliferation of connective tissue. Rapid progress in gene therapy has caused it to finally be considered as one of the viable options for the treatment of CVDs. This novel therapeutic approach could restore stable heart function either by restoring depleted membrane proteins or by balancing the intracellular calcium concentration. Although it has been set back by problems concerning its long-term effects, it is still highly likely to succeed.

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

Omecamtiv mecarbil (OM) is a positive cardiac inotrope in phase-3 clinical trials for treatment of heart failure. Although initially described as a direct myosin activator, subsequent studies are at odds with this description and do not explain OM-mediated increases in cardiac performance. Here we show, via single-molecule, biophysical experiments on cardiac myosin, that OM suppresses myosin's working stroke and prolongs actomyosin attachment 5-fold, which explains inhibitory actions of the drug observed in vitro. OM also causes the actin-detachment rate to become independent of both applied load and ATP concentration. Surprisingly, increased myocardial force output in the presence of OM can be explained by cooperative thin-filament activation by OM-inhibited myosin molecules. Selective suppression of myosin is an unanticipated route to muscle activation that may guide future development of therapeutic drugs.