Slow myosin ATP turnover in the super-relaxed state in tarantula muscle

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.

Myosin folding boosts solubility in cardiac muscle sarcomeres

The polymerization of myosin molecules into thick filaments in muscle sarcomeres is essential for cardiac contractility, with the attenuation of interactions between the heads of myosin molecules within the filaments being proposed to result in hypercontractility, as observed in hypertrophic cardiomyopathy (HCM). However, experimental evidence demonstrates that the structure of these giant macromolecular complexes is highly dynamic, with molecules exchanging between the filaments and a pool of soluble molecules on the minute timescale. Therefore, we sought to test the hypothesis that the enhancement of interactions between the heads of myosin molecules within thick filaments limits the mobility of myosin by taking advantage of mavacamten, a small molecule approved for the treatment of HCM. Myosin molecules were labeled in vivo with a green fluorescent protein (GFP) and imaged in intact hearts using multiphoton microscopy. Treatment of the intact hearts with mavacamten resulted in an unexpected > 5-fold enhancement in GFP-myosin mobility within the sarcomere. In vitro biochemical assays suggested that mavacamten enhanced the mobility of GFP-myosin by increasing the solubility of myosin molecules, through the stabilization of a compact/folded conformation of the molecules, once disassociated from the thick filaments. These findings provide alternative insight into the mechanisms by which molecules exchange into and out of thick filaments and have implications for how mavacamten may affect cardiac contractility.

Exploring the Connection Between Relaxed Myosin States and the Anrep Effect

The Anrep effect is an adaptive response that increases left ventricular contractility following an acute rise in afterload. Although the mechanistic origin remains undefined, recent findings suggest a two-phase activation of resting myosin for contraction, involving strain-sensitive and posttranslational phases. We propose that this mobilization represents a transition among the relaxed states of myosin-specifically, from the super-relaxed (SRX) to the disordered-relaxed (DRX)-with DRX myosin ready to participate in force generation. This hypothesis offers a unified explanation that connects myosin's SRX-DRX equilibrium and the Anrep effect as parts of a singular phenomenon. We underscore the significance of this equilibrium in modulating contractility, primarily studied in the context of hypertrophic cardiomyopathy, the most common inherited cardiomyopathy associated with diastolic dysfunction, hypercontractility, and left ventricular hypertrophy. As we posit that the cellular basis of the Anrep effect relies on a two-phased transition of myosin from the SRX to the contraction-ready DRX configuration, any dysregulation in this equilibrium may result in the pathological manifestation of the Anrep phenomenon. For instance, in hypertrophic cardiomyopathy, hypercontractility is linked to a considerable shift of myosin to the DRX state, implying a persistent activation of the Anrep effect. These valuable insights call for additional research to uncover a clinical Anrep fingerprint in pathological states. Here, we demonstrate through noninvasive echocardiographic pressure-volume measurements that this fingerprint is evident in 12 patients with hypertrophic obstructive cardiomyopathy before septal myocardial ablation. This unique signature is characterized by enhanced contractility, indicated by a leftward shift and steepening of the end-systolic pressure-volume relationship, and a prolonged systolic ejection time adjusted for heart rate, which reverses post-procedure. The clinical application of this concept has potential implications beyond hypertrophic cardiomyopathy, extending to other genetic cardiomyopathies and even noncongenital heart diseases with complex etiologies across a broad spectrum of left ventricular ejection fractions.

The biochemically defined super relaxed state of myosin-A paradox

The biochemical SRX (super-relaxed) state of myosin has been defined as a low ATPase activity state. This state can conserve energy when the myosin is not recruited for muscle contraction. The SRX state has been correlated with a structurally defined ordered (versus disordered) state of muscle thick filaments. The two states may be linked via a common interacting head motif (IHM) where the two heads of heavy meromyosin (HMM), or myosin, fold back onto each other and form additional contacts with S2 and the thick filament. Experimental observations of the SRX, IHM, and the ordered form of thick filaments, however, do not always agree, and result in a series of unresolved paradoxes. To address these paradoxes, we have reexamined the biochemical measurements of the SRX state for porcine cardiac HMM. In our hands, the commonly employed mantATP displacement assay was unable to quantify the population of the SRX state with all data fitting very well by a single exponential. We further show that mavacamten inhibits the basal ATPases of both porcine ventricle HMM and S1 (Ki, 0.32 and 1.76 μM respectively) while dATP activates HMM cooperatively without any evidence of an SRX state. A combination of our experimental observations and theories suggests that the displacement of mantATP in purified proteins is not a reliable assay to quantify the SRX population. This means that while the structurally defined IHM and ordered thick filaments clearly exist, great care must be employed when using the mantATP displacement assay.

Structure of the Drosophila melanogaster Flight Muscle Myosin Filament at 4.7 Å Resolution Reveals New Details of Non-Myosin Proteins

Striated muscle thick filaments are composed of myosin II and several non-myosin proteins which define the filament length and modify its function. Myosin II has a globular N-terminal motor domain comprising its catalytic and actin-binding activities and a long α-helical, coiled tail that forms the dense filament backbone. Myosin alone polymerizes into filaments of irregular length, but striated muscle thick filaments have defined lengths that, with thin filaments, define the sarcomere structure. The motor domain structure and function are well understood, but the myosin filament backbone is not. Here we report on the structure of the flight muscle thick filaments from Drosophila melanogaster at 4.7 Å resolution, which eliminates previous ambiguities in non-myosin densities. The full proximal S2 region is resolved, as are the connecting densities between the Ig domains of stretchin-klp. The proteins, flightin, and myofilin are resolved in sufficient detail to build an atomic model based on an AlphaFold prediction. Our results suggest a method by which flightin and myofilin cooperate to define the structure of the thick filament and explains a key myosin mutation that affects flightin incorporation. Drosophila is a genetic model organism for which our results can define strategies for functional testing.

Physical activity impacts resting skeletal muscle myosin conformation and lowers its ATP consumption

It has recently been established that myosin, the molecular motor protein, is able to exist in two conformations in relaxed skeletal muscle. These conformations are known as the super-relaxed (SRX) and disordered-relaxed (DRX) states and are finely balanced to optimize ATP consumption and skeletal muscle metabolism. Indeed, SRX myosins are thought to have a 5- to 10-fold reduction in ATP turnover compared with DRX myosins. Here, we investigated whether chronic physical activity in humans would be associated with changes in the proportions of SRX and DRX skeletal myosins. For that, we isolated muscle fibers from young men of various physical activity levels (sedentary, moderately physically active, endurance-trained, and strength-trained athletes) and ran a loaded Mant-ATP chase protocol. We observed that in moderately physically active individuals, the amount of myosin molecules in the SRX state in type II muscle fibers was significantly greater than in age-matched sedentary individuals. In parallel, we did not find any difference in the proportions of SRX and DRX myosins in myofibers between highly endurance- and strength-trained athletes. We did however observe changes in their ATP turnover time. Altogether, these results indicate that physical activity level and training type can influence the resting skeletal muscle myosin dynamics. Our findings also emphasize that environmental stimuli such as exercise have the potential to rewire the molecular metabolism of human skeletal muscle through myosin.

Cryo-EM structure of the folded-back state of human β-cardiac myosin

To save energy and precisely regulate cardiac contractility, cardiac muscle myosin heads are sequestered in an 'off' state that can be converted to an 'on' state when exertion is increased. The 'off' state is equated with a folded-back structure known as the interacting-heads motif (IHM), which is a regulatory feature of all class-2 muscle and non-muscle myosins. We report here the human β-cardiac myosin IHM structure determined by cryo-electron microscopy to 3.6 Å resolution, providing details of all the interfaces stabilizing the 'off' state. The structure shows that these interfaces are hot spots of hypertrophic cardiomyopathy mutations that are thought to cause hypercontractility by destabilizing the 'off' state. Importantly, the cardiac and smooth muscle myosin IHM structures dramatically differ, providing structural evidence for the divergent physiological regulation of these muscle types. The cardiac IHM structure will facilitate development of clinically useful new molecules that modulate IHM stability.

Cryo-EM structure of the folded-back state of human β-cardiac myosin

During normal levels of exertion, many cardiac muscle myosin heads are sequestered in an off-state even during systolic contraction to save energy and for precise regulation. They can be converted to an on-state when exertion is increased. Hypercontractility caused by hypertrophic cardiomyopathy (HCM) myosin mutations is often the result of shifting the equilibrium toward more heads in the on-state. The off-state is equated with a folded-back structure known as the interacting head motif (IHM), which is a regulatory feature of all muscle myosins and class-2 non-muscle myosins. We report here the human β-cardiac myosin IHM structure to 3.6 Å resolution. The structure shows that the interfaces are hot spots of HCM mutations and reveals details of the significant interactions. Importantly, the structures of cardiac and smooth muscle myosin IHMs are dramatically different. This challenges the concept that the IHM structure is conserved in all muscle types and opens new perspectives in the understanding of muscle physiology. The cardiac IHM structure has been the missing puzzle piece to fully understand the development of inherited cardiomyopathies. This work will pave the way for the development of new molecules able to stabilize or destabilize the IHM in a personalized medicine approach. *This manuscript was submitted to Nature Communications in August 2022 and dealt efficiently by the editors. All reviewers received this version of the manuscript before 9 208 August 2022. They also received coordinates and maps of our high resolution structure on the 18 208 August 2022. Due to slowness of at least one reviewer, this contribution was delayed for acceptance by Nature Communications and we are now depositing in bioRxiv the originally submitted version written in July 2022 for everyone to see. Indeed, two bioRxiv contributions at lower resolution but adding similar concepts on thick filament regulation were deposited this week in bioRxiv, one of the contributions having had access to our coordinates. We hope that our data at high resolution will be helpful for all readers that appreciate that high resolution information is required to build accurate atomic models and discuss implications for sarcomere regulation and the effects of cardiomyopathy mutations on heart muscle function.

Variants of the myosin interacting-heads motif

Under relaxing conditions, the two heads of myosin II interact with each other and with the proximal part (S2) of the myosin tail, establishing the interacting-heads motif (IHM), found in myosin molecules and thick filaments of muscle and nonmuscle cells. The IHM is normally thought of as a single, unique structure, but there are several variants. In the simplest ("canonical") IHM, occurring in most relaxed thick filaments and in heavy meromyosin, the interacting heads bend back and interact with S2, and the motif lies parallel to the filament surface. In one variant, occurring in insect indirect flight muscle, there is no S2-head interaction and the motif is perpendicular to the filament. In a second variant, found in smooth and nonmuscle single myosin molecules in their inhibited (10S) conformation, S2 is shifted ∼20 Å from the canonical form and the tail folds twice and wraps around the interacting heads. These molecule and filament IHM variants have important energetic and pathophysiological consequences. (1) The canonical motif, with S2-head interaction, correlates with the super-relaxed (SRX) state of myosin. The absence of S2-head interaction in insects may account for the lower stability of this IHM and apparent absence of SRX in indirect flight muscle, contributing to the quick initiation of flight in insects. (2) The ∼20 Å shift of S2 in 10S myosin molecules means that S2-head interactions are different from those in the canonical IHM. This variant therefore cannot be used to analyze the impact of myosin mutations on S2-head interactions that occur in filaments, as has been proposed. It can be used, instead, to analyze the structural impact of mutations in smooth and nonmuscle myosin.

Myosin Heavy Chain as a Novel Key Modulator of Striated Muscle Resting State

After years of intense research using structural, biological, and biochemical experimental procedures, it is clear that myosin molecules are essential for striated muscle contraction. However, this is just the tip of the iceberg of their function. Interestingly, it has been shown recently that these molecules (especially myosin heavy chains) are also crucial for cardiac and skeletal muscle resting state. In the present review, we first overview myosin heavy chain biochemical states and how they influence the consumption of ATP. We then detail how neighboring partner proteins including myosin light chains and myosin binding protein C intervene in such processes, modulating the ATP demand in health and disease. Finally, we present current experimental drugs targeting myosin ATP consumption and how they can treat muscle diseases.

Structure of the Flight Muscle Thick Filament from the Bumble Bee, Bombus ignitus, at 6 Å Resolution

Four insect orders have flight muscles that are both asynchronous and indirect; they are asynchronous in that the wingbeat frequency is decoupled from the frequency of nervous stimulation and indirect in that the muscles attach to the thoracic exoskeleton instead of directly to the wing. Flight muscle thick filaments from two orders, Hemiptera and Diptera, have been imaged at a subnanometer resolution, both of which revealed a myosin tail arrangement referred to as “curved molecular crystalline layers”. Here, we report a thick filament structure from the indirect flight muscles of a third insect order, Hymenoptera, the Asian bumble bee Bombus ignitus. The myosin tails are in general agreement with previous determinations from Lethocerus indicus and Drosophila melanogaster. The Skip 2 region has the same unusual structure as found in Lethocerus indicus thick filaments, an α-helix discontinuity is also seen at Skip 4, but the orientation of the Skip 1 region on the surface of the backbone is less angled with respect to the filament axis than in the other two species. The heads are disordered as in Drosophila, but we observe no non-myosin proteins on the backbone surface that might prohibit the ordering of myosin heads onto the thick filament backbone. There are strong structural similarities among the three species in their non-myosin proteins within the backbone that suggest how one previously unassigned density in Lethocerus might be assigned. Overall, the structure conforms to the previously observed pattern of high similarity in the myosin tail arrangement, but differences in the non-myosin proteins.

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.

Interacting-heads motif explains the X-ray diffraction pattern of relaxed vertebrate skeletal muscle

Electron microscopy (EM) shows that myosin heads in thick filaments isolated from striated muscles interact with each other and with the myosin tail under relaxing conditions. This "interacting-heads motif" (IHM) is highly conserved across the animal kingdom and is thought to be the basis of the super-relaxed state. However, a recent X-ray modeling study concludes, contrary to expectation, that the IHM is not present in relaxed intact muscle. We propose that this conclusion results from modeling with a thick filament 3D reconstruction in which the myosin heads have radially collapsed onto the thick filament backbone, not from absence of the IHM. Such radial collapse, by about 3-4 nm, is well established in EM studies of negatively stained myosin filaments, on which the reconstruction was based. We have tested this idea by carrying out similar X-ray modeling and determining the effect of the radial position of the heads on the goodness of fit to the X-ray pattern. We find that, when the IHM is modeled into a thick filament at a radius 3-4 nm greater than that modeled in the recent study, there is good agreement with the X-ray pattern. When the original (collapsed) radial position is used, the fit is poor, in agreement with that study. We show that modeling of the low-angle region of the X-ray pattern is relatively insensitive to the conformation of the myosin heads but very sensitive to their radial distance from the filament axis. We conclude that the IHM is sufficient to explain the X-ray diffraction pattern of intact muscle when placed at the appropriate radius.

Exploring the super-relaxed state of myosin in myofibrils from fast-twitch, slow-twitch, and cardiac muscle

Muscle myosin heads, in the absence of actin, have been shown to exist in two states, the relaxed (turnover ∼0.05 s-1) and super-relaxed states (SRX, 0.005 s-1) using a simple fluorescent ATP chase assay (Hooijman, P. et al (2011) Biophys. J.100, 1969-1976). Studies have normally used purified proteins, myosin filaments, or muscle fibers. Here we use muscle myofibrils, which retain most of the ancillary proteins and 3-D architecture of muscle and can be used with rapid mixing methods. Recording timescales from 0.1 to 1000 s provides a precise measure of the two populations of myosin heads present in relaxed myofibrils. We demonstrate that the population of SRX states is formed from rigor cross bridges within 0.2 s of relaxing with fluorescently labeled ATP, and the population of SRX states is relatively constant over the temperature range of 5 °C-30 °C. The SRX population is enhanced in the presence of mavacamten and reduced in the presence of deoxy-ATP. Compared with myofibrils from fast-twitch muscle, slow-twitch muscle, and cardiac muscles, myofibrils require a tenfold lower concentration of mavacamten to be effective, and mavacamten induced a larger increase in the population of the SRX state. Mavacamten is less effective, however, at stabilizing the SRX state at physiological temperatures than at 5 °C. These assays require small quantities of myofibrils, making them suitable for studies of model organism muscles, human biopsies, or human-derived iPSCs.

The mechanics of the heart: zooming in on hypertrophic cardiomyopathy and cMyBP-C

Hypertrophic cardiomyopathy (HCM), a disease characterized by cardiac muscle hypertrophy and hypercontractility, is the most frequently inherited disorder of the heart. HCM is mainly caused by variants in genes encoding proteins of the sarcomere, the basic contractile unit of cardiomyocytes. The most frequently mutated among them is MYBPC3, which encodes cardiac myosin-binding protein C (cMyBP-C), a key regulator of sarcomere contraction. In this review, we summarize clinical and genetic aspects of HCM and provide updated information on the function of the healthy and HCM sarcomere, as well as on emerging therapeutic options targeting sarcomere mechanical activity. Building on what is known about cMyBP-C activity, we examine different pathogenicity drivers by which MYBPC3 variants can cause disease, focussing on protein haploinsufficiency as a common pathomechanism also in nontruncating variants. Finally, we discuss recent evidence correlating altered cMyBP-C mechanical properties with HCM development.

Structural basis of the super- and hyper-relaxed states of myosin II

Super-relaxation is a state of muscle thick filaments in which ATP turnover by myosin is much slower than that of myosin II in solution. This inhibited state, in equilibrium with a faster (relaxed) state, is ubiquitous and thought to be fundamental to muscle function, acting as a mechanism for switching off energy-consuming myosin motors when they are not being used. The structural basis of super-relaxation is usually taken to be a motif formed by myosin in which the two heads interact with each other and with the proximal tail forming an interacting-heads motif, which switches the heads off. However, recent studies show that even isolated myosin heads can exhibit this slow rate. Here, we review the role of head interactions in creating the super-relaxed state and show how increased numbers of interactions in thick filaments underlie the high levels of super-relaxation found in intact muscle. We suggest how a third, even more inhibited, state of myosin (a hyper-relaxed state) seen in certain species results from additional interactions involving the heads. We speculate on the relationship between animal lifestyle and level of super-relaxation in different species and on the mechanism of formation of the super-relaxed state. We also review how super-relaxed thick filaments are activated and how the super-relaxed state is modulated in healthy and diseased muscles.

Two Classes of Myosin Inhibitors, Para-nitroblebbistatin and Mavacamten, Stabilize β-Cardiac Myosin in Different Structural and Functional States

In addition to a conventional relaxed state, a fraction of myosins in the cardiac muscle exists in a low-energy consuming super-relaxed (SRX) state, which is kept as a reserve pool that may be engaged under sustained increased cardiac demand. The conventional relaxed and the super-relaxed states are widely assumed to correspond to a structure where myosin heads are in an open configuration, free to interact with actin, and a closed configuration, inhibiting binding to actin, respectively. Disruption of the myosin SRX population is an emerging model in different heart diseases, such as hypertrophic cardiomyopathy, which results in excessive muscle contraction, and stabilizing them using myosin inhibitors is budding as an attractive therapeutic strategy. Here we examined the structure-function relationships of two myosin ATPase inhibitors, mavacamten and para-nitroblebbistatin, and found that binding of mavacamten at a site different than para-nitroblebbistatin populates myosin into the SRX state. Para-nitroblebbistatin, binding to a distal pocket to the myosin lever arm near the nucleotide-binding site, does not affect the usual myosin SRX state but instead appears to render myosin into a new, perhaps much more inhibited, 'ultra-relaxed' state. X-ray scattering-based rigid body modeling shows that both mavacamten and para-nitroblebbistatin induce novel conformations in human β-cardiac heavy meromyosin that diverge significantly from the hypothetical open and closed states, and furthermore, mavacamten treatment causes greater compaction than para-nitroblebbistatin. Taken together, we conclude that mavacamten and para-nitroblebbistatin stabilize myosin in different structural states, and such states may give rise to different functional energy-sparing states.

Dilated Cardiomyopathy Mutations and Phosphorylation disrupt the Active Orientation of Cardiac Troponin C

Cardiac troponin (cTn) is made up of three subunits, cTnC, cTnI, and cTnT. The regulatory N-terminal domain of cTnC (cNTnC) controls cardiac muscle contraction in a calcium-dependent manner. We show that calcium-saturated cNTnC can adopt two different orientations, with the "active" orientation consistent with the 2020 cryo-EM structure of the activated cardiac thin filament by Yamada et al. Using solution NMR ¹⁵N R₂ relaxation analysis, we demonstrate that the two domains of cTnC tumble independently (average R₂ 10 s^-1), being connected by a flexible linker. However, upon addition of cTnI_1-77, the complex tumbles as a rigid unit (R₂ 30 s^-1). cTnI phosphomimetic mutants S22D/S23D, S41D/S43D and dilated cardiomyopathy- (DCM-)associated mutations cTnI K35Q, cTnC D75Y, and cTnC G159D destabilize the active orientation of cNTnC, with intermediate ¹⁵N R₂ rates (R₂ 17-23 s^-1). The active orientation of cNTnC is stabilized by the flexible tails of cTnI, cTnI_1-37 and cTnI_135-209. Surprisingly, when cTnC is incorporated into complexes lacking these tails (cTnC-cTnI_38-134, cTnC-cTnT_223-288, or cTnC-cTnI_38-134-cTnT_223-288), the cNTnC domain is still immobilized, revealing a new interaction between cNTnC and the IT-arm that stabilizes a "dormant" orientation. We propose that the calcium sensitivity of the cardiac troponin complex is regulated by an equilibrium between active and dormant orientations, which can be shifted through post-translational modifications or DCM-associated mutations.

Relaxed tarantula skeletal muscle has two ATP energy-saving mechanisms

Myosin molecules in the relaxed thick filaments of striated muscle have a helical arrangement in which the heads of each molecule interact with each other, forming the interacting-heads motif (IHM). In relaxed mammalian skeletal muscle, this helical ordering occurs only at temperatures >20°C and is disrupted when temperature is decreased. Recent x-ray diffraction studies of live tarantula skeletal muscle have suggested that the two myosin heads of the IHM (blocked heads [BHs] and free heads [FHs]) have very different roles and dynamics during contraction. Here, we explore temperature-induced changes in the BHs and FHs in relaxed tarantula skeletal muscle. We find a change with decreasing temperature that is similar to that in mammals, while increasing temperature induces a different behavior in the heads. At 22.5°C, the BHs and FHs containing ADP.Pi are fully helically organized, but they become progressively disordered as temperature is lowered or raised. Our interpretation suggests that at low temperature, while the BHs remain ordered the FHs become disordered due to transition of the heads to a straight conformation containing Mg.ATP. Above 27.5°C, the nucleotide remains as ADP.Pi, but while BHs remain ordered, half of the FHs become progressively disordered, released semipermanently at a midway distance to the thin filaments while the remaining FHs are docked as swaying heads. We propose a thermosensing mechanism for tarantula skeletal muscle to explain these changes. Our results suggest that tarantula skeletal muscle thick filaments, in addition to having a superrelaxation-based ATP energy-saving mechanism in the range of 8.5-40°C, also exhibit energy saving at lower temperatures (<22.5°C), similar to the proposed refractory state in mammals.

Convergent evolutionary pathways toward energy saving in muscle?

Reconditi reviews research into the role temperature plays on motor disorders.

Cardiac myosin super relaxation (SRX): a perspective on fundamental biology, human disease and therapeutics

The fundamental basis of muscle contraction 'the sliding filament model' (Huxley and Niedergerke, 1954; Huxley and Hanson, 1954) and the 'swinging, tilting crossbridge-sliding filament mechanism' (Huxley, 1969; Huxley and Brown, 1967) nucleated a field of research that has unearthed the complex and fascinating role of myosin structure in the regulation of contraction. A recently discovered energy conserving state of myosin termed the super relaxed state (SRX) has been observed in filamentous myosins and is central to modulating force production and energy use within the sarcomere. Modulation of myosin function through SRX is a rapidly developing theme in therapeutic development for both cardiovascular disease and infectious disease. Some 70 years after the first discoveries concerning muscular function, modulation of myosin SRX may bring the first myosin targeted small molecule to the clinic, for treating hypertrophic cardiomyopathy (Olivotto et al., 2020). An often monogenic disease HCM afflicts 1 in 500 individuals, and can cause heart failure and sudden cardiac death. Even as we near therapeutic translation, there remain many questions about the governance of muscle function in human health and disease. With this review, we provide a broad overview of contemporary understanding of myosin SRX, and explore the complexities of targeting this myosin state in human disease.This article has an associated Future Leaders to Watch interview with the authors of the paper.

To lie or not to lie: Super-relaxing with myosins

Since the discovery of muscle in the 19th century, myosins as molecular motors have been extensively studied. However, in the last decade, a new functional super-relaxed (SRX) state of myosin has been discovered, which has a 10-fold slower ATP turnover rate than the already-known non-actin-bound, disordered relaxed (DRX) state. These two states are in dynamic equilibrium under resting muscle conditions and are thought to be significant contributors to adaptive thermogenesis in skeletal muscle and can act as a reserve pool that may be recruited when there is a sustained demand for increased cardiac muscle power. This report provides an evolutionary perspective of how striated muscle contraction is regulated by modulating this myosin DRX↔SRX state equilibrium. We further discuss this equilibrium with respect to different physiological and pathophysiological perturbations, including insults causing hypertrophic cardiomyopathy, and small-molecule effectors that modulate muscle contractility in diseased pathology.

Synthetic thick filaments: A new avenue for better understanding the myosin super-relaxed state in healthy, diseased, and mavacamten-treated cardiac systems

A hallmark feature of myosin-II is that it can spontaneously self-assemble into bipolar synthetic thick filaments (STFs) in low-ionic-strength buffers, thereby serving as a reconstituted in vitro model for muscle thick filaments. Although these STFs have been extensively used for structural characterization, their functional evaluation has been limited. In this report, we show that myosins in STFs mirror the more electrostatic and cooperative interactions that underlie the energy-sparing super-relaxed (SRX) state, which are not seen using shorter myosin subfragments, heavy meromyosin (HMM) and myosin subfragment 1 (S1). Using these STFs, we show several pathophysiological insults in hypertrophic cardiomyopathy, including the R403Q myosin mutation, phosphorylation of myosin light chains, and an increased ADP:ATP ratio, destabilize the SRX population. Furthermore, WT myosin containing STFs, but not S1, HMM, or STFs-containing R403Q myosin, recapitulated the ADP-induced destabilization of the SRX state. Studies involving a clinical-stage small-molecule inhibitor, mavacamten, showed that it is more effective in not only increasing myosin SRX population in STFs than in S1 or HMM but also in increasing myosin SRX population equally well in STFs made of healthy and disease-causing R403Q myosin. Importantly, we also found that pathophysiological perturbations such as elevated ADP concentration weakens mavacamten's ability to increase the myosin SRX population, suggesting that mavacamten-bound myosin heads are not permanently protected in the SRX state but can be recruited into action. These findings collectively emphasize that STFs serve as a valuable tool to provide novel insights into the myosin SRX state in healthy, diseased, and therapeutic conditions.

Structure of the shutdown state of myosin-2

Myosin-2 is essential for processes as diverse as cell division and muscle contraction. Dephosphorylation of its regulatory light chain (RLC) promotes an inactive, ‘shutdown’ state with the filament-forming tail folded onto the two heads¹, preventing filament formation and inactivating the motors². The mechanism by which this happens is obscure. Here we report a cryo-electron microscopy structure of shutdown smooth muscle myosin, with a resolution of 6 Å in the head region. A pseudo-atomic model, obtained by flexible fitting of crystal structures into the density and molecular dynamics simulations, describes interaction interfaces at the atomic level. The N-terminal extension of one RLC interacts with the tail and the other with the partner head, revealing how the RLCs stabilise the shutdown state in different ways and how their phosphorylation would allow myosin activation. Additional interactions between the three segments of the coiled coil, the motor domains and LCs stabilise the shutdown molecule. The structure of the lever in each head is competent to generate force upon activation. This shutdown structure is relevant to all myosin-2 isoforms and provides a framework for understanding their disease-causing mutations.

The stretch-shortening cycle effect is prominent in the inhibited force state

It has been suggested that residual force enhancement (RFE) contributes to the work enhancement observed in stretch-shortening cycles (SSC). Based on recent findings that RFE was preserved in the reduced force state, one may speculate that the SSC effect may be preserved in the reduced force state as well. The purpose of this study was to examine the magnitude of the SSC effect in inhibited skeletal muscle force states. Normal and inhibited force conditions were analyzed using skinned rabbit soleus fibres (N = 18). The inhibited force condition was achieved by adding 2,3-Butanedione monoxime into the activating solution. For both conditions, a SSC test and a pure shortening test were performed. In the SSC tests, fibres were activated at an average sarcomere length of 2.4 μm, and then stretched to 3.0 μm. Immediately after the end of the stretch, fibres were shortened to 2.4 μm. In the pure shortening tests, fibres were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work in the shortening phase of the SSC compared to the pure shortening condition was defined as the SSC effect index, and the magnitude of the SSC effect was compared between the normal and the inhibited force condition. The SSC effect was greater in the inhibited compared to the normal force condition (p < 0.001). We conclude that the SSC effect is at least in part preserved in the reduced force state.

Super-relaxed state of myosin in human skeletal muscle is fiber-type dependent

The myosin super-relaxed state (SRX) in skeletal muscle is hypothesized to play an important role in regulating muscle contractility and thermogenesis in humans but has only been examined in model organisms. Here we report the first human skeletal muscle SRX measurements, using quantitative epifluorescence microscopy of fluorescent 2'/3'-O-(N-methylanthraniloyl) ATP (mantATP) single-nucleotide turnover. Myosin heavy chain (MHC) isoform expression was determined using gel electrophoresis for each permeabilized vastus lateralis fiber, to allow for novel comparisons of SRX between fiber types. We find that the fraction of myosin in SRX is less in MHC IIA fibers than in MHC I and IIAX fibers (P = 0.008). ATP turnover of SRX is faster in MHC IIAX fibers compared with MHC I and IIA fibers (P = 0.001). We conclude that SRX biochemistry is measurable in human skeletal muscle, and our data indicate that SRX depends on fiber type as classified by MHC isoform. Extension from this preliminary work would provide further understanding regarding the role of SRX in human muscle physiology.

CaMKII activity contributes to homeometric autoregulation of the heart: A novel mechanism for the Anrep effect

KEY POINTS

The Anrep effect represents the alteration of left ventricular (LV) contractility to acutely enhanced afterload in a few seconds, thereby preserving stroke volume (SV) at constant preload. As a result of the missing preload stretch in our model, the Anrep effect differs from the slow force response and has a different mechanism. The Anrep effect demonstrated two different phases. First, the sudden increased afterload was momentary equilibrated by the enhanced LV contractility as a result of higher power strokes of strongly-bound myosin cross-bridges. Second, the slightly delayed recovery of SV is perhaps dependent on Ca²⁺ /calmodulin-dependent protein kinase II activation caused by oxidation and myofilament phosphorylation (cardiac myosin-binding protein-C, myosin light chain 2), maximizing the recruitment of available strongly-bound myosin cross-bridges. Short-lived oxidative stress might present a new facet of subcellular signalling with respect to cardiovascular regulation. Relevance for human physiology was demonstrated by echocardiography disclosing the Anrep effect in humans during handgrip exercise.

ABSTRACT

The present study investigated whether oxidative stress and Ca²⁺ /calmodulin-dependent protein kinase II (CaMKII) activity are involved in triggering the Anrep effect. LV pressure-volume (PV) analyses of isolated, preload controlled working hearts were performed at two afterload levels (60 and 100 mmHg) in C57BL/6N wild-type (WT) and CaMKII-double knockout mice (DKO^CaMKII ). In snap-frozen WT hearts, force-pCa relationship, H₂ O₂ generation, CaMKII oxidation and phosphorylation of myofilament and Ca²⁺ handling proteins were assessed. Acutely raised afterload showed significantly increased wall stress, H₂ O₂ generation and LV contractility in the PV diagram with an initial decrease and recovery of stroke volume, whereas end-diastolic pressure and volume, as well as heart rate, remained constant. Afterload induced increase in LV contractility was blunted in DKO^CaMKII -hearts. Force development of single WT cardiomyocytes was greater with elevated afterload at submaximal Ca²⁺ concentration and associated with increases in CaMKII oxidation and phosphorylation of cardiac-myosin binding protein-C, myosin light chain and Ca²⁺ handling proteins. CaMKII activity is involved in the regulation of the Anrep effect and associates with stimulation of oxidative stress, presumably starting a cascade of CaMKII oxidation with downstream phosphorylation of myofilament and Ca²⁺ handling proteins. These mechanisms improve LV inotropy and preserve stroke volume within few seconds.

The myosin interacting-heads motif present in live tarantula muscle explains tetanic and posttetanic phosphorylation mechanisms

Striated muscle contraction involves sliding of actin thin filaments along myosin thick filaments, controlled by calcium through thin filament activation. In relaxed muscle, the two heads of myosin interact with each other on the filament surface to form the interacting-heads motif (IHM). A key question is how both heads are released from the surface to approach actin and produce force. We used time-resolved synchrotron X-ray diffraction to study tarantula muscle before and after tetani. The patterns showed that the IHM is present in live relaxed muscle. Tetanic contraction produced only a very small backbone elongation, implying that mechanosensing-proposed in vertebrate muscle-is not of primary importance in tarantula. Rather, thick filament activation results from increases in myosin phosphorylation that release a fraction of heads to produce force, with the remainder staying in the ordered IHM configuration. After the tetanus, the released heads slowly recover toward the resting, helically ordered state. During this time the released heads remain close to actin and can quickly rebind, enhancing the force produced by posttetanic twitches, structurally explaining posttetanic potentiation. Taken together, these results suggest that, in addition to stretch activation in insects, two other mechanisms for thick filament activation have evolved to disrupt the interactions that establish the relaxed helices of IHMs: one in invertebrates, by either regulatory light-chain phosphorylation (as in arthropods) or Ca2+-binding (in mollusks, lacking phosphorylation), and another in vertebrates, by mechanosensing.

18O labeling on Ser45 but not on Ser35 supports the cooperative phosphorylation mechanism on tarantula thick filament activation

Thick filaments from some striated muscles are regulated by phosphorylation of myosin regulatory light chains (RLCs). A tarantula thick filament quasi-atomic model achieved by cryo-electron microscopy has advanced our understanding on how this regulation occurs. In native thick filaments, an asymmetric intramolecular interaction between the actin-binding region of one myosin head ("blocked") and the converter region of the other head ("free") switches both heads off, establishing the myosin interacting-heads motif (IHM). This structural finding, together with motility assays, sequence analysis, and mass spectrometry (MS) observations have suggested a cooperative phosphorylation activation (CPA) mechanism for thick filament activation. In the CPA mechanism, some myosin free heads are phosphorylated constitutively in Ser35 by protein kinase C (PKC) and -under Ca²⁺ control - others (free or blocked) heads temporally on Ser45 by myosin light chain kinase (MLCK), in a way that explains both force development and post-tetanic potentiation in tarantula striated muscle. We tested this model using MS to verify if Ca²⁺-activation phosphorylates de novo un-phosphorylated Ser35 heads. For this purpose, we standardized an approach based on ¹⁸O isotopic ATP labeling to accurately detect by MS-MS the RLC phosphorylation under Ca²⁺-activation. MS spectra showed de novo¹⁸O incorporation only on Ser45 but not on Ser35. As the constitutive Ser35 phosphorylation cannot be dephosphorylated, this result suggests that the number of RLCs on free heads with constitutively phosphorylated Ser35 does remain constant on Ca²⁺-activation supporting that the myosin has a basal activation and force modulation or potentiation is controlled by MLCK Ser45 phosphorylation.

The mesa trail and the interacting heads motif of myosin II

Myosin II molecules in the thick filaments of striated muscle form a structure in which the heads interact with each other and fold back onto the tail. This structure, the "interacting heads motif" (IHM), provides a mechanistic basis for the auto-inhibition of myosin in relaxed thick filaments. Similar IHM interactions occur in single myosin molecules of smooth and nonmuscle cells in the switched-off state. In addition to the interaction between the two heads, which inhibits their activity, the IHM also contains an interaction between the motor domain of one head and the initial part (subfragment 2, S2) of the tail. This is thought to be a crucial anchoring interaction that holds the IHM in place on the thick filament. S2 appears to cross the head at a specific location within a broader region of the motor domain known as the myosin mesa. Here, we show that the positive and negative charge distribution in this part of the mesa is complementary to the charge distribution on S2. We have designated this the "mesa trail" owing to its linear path across the mesa. We studied the structural sequence alignment, the location of charged residues on the surface of myosin head atomic models, and the distribution of surface charge potential along the mesa trail in different types of myosin II and in different species. The charge distribution in both the mesa trail and the adjacent S2 is relatively conserved. This suggests a common basis for IHM formation across different myosin IIs, dependent on attraction between complementary charged patches on S2 and the myosin head. Conservation from mammals to insects suggests that the mesa trail/S2 interaction plays a key role in the inhibitory function of the IHM.

β-Cardiac myosin hypertrophic cardiomyopathy mutations release sequestered heads and increase enzymatic activity

Hypertrophic cardiomyopathy (HCM) affects 1 in 500 people and leads to hyper-contractility of the heart. Nearly 40 percent of HCM-causing mutations are found in human β-cardiac myosin. Previous studies looking at the effect of HCM mutations on the force, velocity and ATPase activity of the catalytic domain of human β-cardiac myosin have not shown clear trends leading to hypercontractility at the molecular scale. Here we present functional data showing that four separate HCM mutations located at the myosin head-tail (R249Q, H251N) and head-head (D382Y, R719W) interfaces of a folded-back sequestered state referred to as the interacting heads motif (IHM) lead to a significant increase in the number of heads functionally accessible for interaction with actin. These results provide evidence that HCM mutations can modulate myosin activity by disrupting intramolecular interactions within the proposed sequestered state, which could lead to hypercontractility at the molecular level.

Hypertrophic cardiomyopathy (HCM) leads to hyper-contractility of the heart and is often caused by mutations in human β-cardiac myosin. Here authors show that four separate β-cardiac myosin mutations can modulate myosin activity by disrupting intramolecular interactions.

Advances in the Genetic Basis and Pathogenesis of Sarcomere Cardiomyopathies

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are common heart muscle disorders that are caused by pathogenic variants in sarcomere protein genes. HCM is characterized by unexplained cardiac hypertrophy (increased chamber wall thickness) that is accompanied by enhanced cardiac contractility and impaired relaxation. DCM is defined as increased ventricular chamber volume with contractile impairment. In this review, we discuss recent analyses that provide new insights into the molecular mechanisms that cause these conditions. HCM studies have uncovered the critical importance of conformational changes that occur during relaxation and enable energy conservation, which are frequently disturbed by HCM mutations. DCM studies have demonstrated the considerable prevalence of truncating variants in titin and have discerned that these variants reduce contractile function by impairing sarcomerogenesis. These new pathophysiologic mechanisms open exciting opportunities to identify new pharmacological targets and develop future cardioprotective strategies.

Phosphomimetic-mediated in vitro rescue of hypertrophic cardiomyopathy linked to R58Q mutation in myosin regulatory light chain

Myosin regulatory light chain (RLC) phosphorylation is important for cardiac muscle mechanics/function as well as for the Ca²⁺ -troponin/tropomyosin regulation of muscle contraction. This study focuses on the arginine to glutamine (R58Q) substitution in the human ventricular RLC (MYL2 gene), linked to malignant hypertrophic cardiomyopathy in humans and causing severe functional abnormalities in transgenic (Tg) R58Q mice, including inhibition of cardiac RLC phosphorylation. Using a phosphomimic recombinant RLC variant where Ser-15 at the phosphorylation site was substituted with aspartic acid (S15D) and placed in the background of R58Q, we aimed to assess whether we could rescue/mitigate R58Q-induced structural/functional abnormalities in vitro. We show rescue of several R58Q-exerted adverse phenotypes in S15D-R58Q-reconstituted porcine cardiac muscle preparations. A low level of maximal isometric force observed for R58Q- versus WT-reconstituted fibers was restored by S15D-R58Q. Significant beneficial effects were also observed on the V_max of actin-activated myosin ATPase activity in S15D-R58Q versus R58Q-reconstituted myosin, along with its binding to fluorescently labeled actin. We also report that R58Q promotes the OFF state of myosin, both in reconstituted porcine fibers and in Tg mouse papillary muscles, thereby stabilizing the super-relaxed state (SRX) of myosin, characterized by a very low ATP turnover rate. Experiments in S15D-R58Q-reconstituted porcine fibers showed a mild destabilization of the SRX state, suggesting an S15D-mediated shift in disordered-relaxed (DRX)↔SRX equilibrium toward the DRX state of myosin. Our study shows that S15D-phosphomimic can be used as a potential rescue strategy to abrogate/alleviate the RLC mutation-induced phenotypes and is a likely candidate for therapeutic intervention in HCM patients.

Hypertrophic cardiomyopathy disease results from disparate impairments of cardiac myosin function and auto-inhibition

Hypertrophic cardiomyopathies (HCM) result from distinct single-point mutations in sarcomeric proteins that lead to muscle hypercontractility. While different models account for a pathological increase in the power output, clear understanding of the molecular basis of dysfunction in HCM is the mandatory next step to improve current treatments. Here, we present an optimized quasi-atomic model of the sequestered state of cardiac myosin coupled to X-ray crystallography and in silico analysis of the mechanical compliance of the lever arm, allowing the systematic study of a large set of HCM mutations and the definition of different mutation classes based on their effects on lever arm compliance, sequestered state stability, and motor functions. The present work reconciles previous models and explains how distinct HCM mutations can have disparate effects on the motor mechano-chemical parameters and yet lead to the same disease. The framework presented here can guide future investigations aiming at finding HCM treatments.

Hypertrophic cardiomyopathy (HCM) is caused by point mutations in sarcomeric proteins. Here the authors develop an optimized model of the sequestered state of cardiac myosin and define the features affecting the lever arm compliance, allowing them to group mutations in classes and to elucidate the molecular mechanisms leading to cardiac dysfunction in HCM.

Age affects myosin relaxation states in skeletal muscle fibers of female but not male mice

The recent discovery that myosin has two distinct states in relaxed muscle–disordered relaxed (DRX) and super-relaxed (SRX)–provides another factor to consider in our fundamental understanding of the aging mechanism in skeletal muscle, since myosin is thought to be a potential contributor to dynapenia (age-associated loss of muscle strength independent of atrophy). The primary goal of this study was to determine the effects of age on DRX and SRX states and to examine their sex specificity. We have used quantitative fluorescence microscopy of the fluorescent nucleotide analog 2′/3′-O-(N-methylanthraniloyl) ATP (mantATP) to measure single-nucleotide turnover kinetics of myosin in skinned skeletal muscle fibers under relaxing conditions. We examined changes in DRX and SRX in response to the natural aging process by measuring the turnover of mantATP in skinned fibers isolated from psoas muscle of adult young (3–4 months old) and aged (26–28 months old) C57BL/6 female and male mice. Fluorescence decays were fitted to a multi-exponential decay function to determine both the time constants and mole fractions of fast and slow turnover populations, and significance was analyzed by a t-test. We found that in females, both the DRX and SRX lifetimes of myosin ATP turnover at steady state were shorter in aged muscle fibers compared to young muscle fibers (p ≤ 0.033). However, there was no significant difference in relaxation lifetime of either DRX (p = 0.202) or SRX (p = 0.804) between young and aged male mice. No significant effects were measured on the mole fractions (populations) of these states, as a function of sex or age (females, p = 0.100; males, p = 0.929). The effect of age on the order of myosin heads at rest and their ATPase function is sex specific, affecting only females. These findings provide new insight into the molecular factors and mechanisms that contribute to aging muscle dysfunction in a sex-specific manner.

Deciphering the super relaxed state of human β-cardiac myosin and the mode of action of mavacamten from myosin molecules to muscle fibers

Mutations in β-cardiac myosin, the predominant motor protein for human heart contraction, can alter power output and cause cardiomyopathy. However, measurements of the intrinsic force, velocity, and ATPase activity of myosin have not provided a consistent mechanism to link mutations to muscle pathology. An alternative model posits that mutations in myosin affect the stability of a sequestered, super relaxed state (SRX) of the protein with very slow ATP hydrolysis and thereby change the number of myosin heads accessible to actin. Here we show that purified human β-cardiac myosin exists partly in an SRX and may in part correspond to a folded-back conformation of myosin heads observed in muscle fibers around the thick filament backbone. Mutations that cause hypertrophic cardiomyopathy destabilize this state, while the small molecule mavacamten promotes it. These findings provide a biochemical and structural link between the genetics and physiology of cardiomyopathy with implications for therapeutic strategies.

Mavacamten stabilizes an autoinhibited state of two-headed cardiac myosin

We used transient biochemical and structural kinetics to elucidate the molecular mechanism of mavacamten, an allosteric cardiac myosin inhibitor and a prospective treatment for hypertrophic cardiomyopathy. We find that mavacamten stabilizes an autoinhibited state of two-headed cardiac myosin not found in the single-headed S1 myosin motor fragment. We determined this by measuring cardiac myosin actin-activated and actin-independent ATPase and single-ATP turnover kinetics. A two-headed myosin fragment exhibits distinct autoinhibited ATP turnover kinetics compared with a single-headed fragment. Mavacamten enhanced this autoinhibition. It also enhanced autoinhibition of ADP release. Furthermore, actin changes the structure of the autoinhibited state by forcing myosin lever-arm rotation. Mavacamten slows this rotation in two-headed myosin but does not prevent it. We conclude that cardiac myosin is regulated in solution by an interaction between its two heads and propose that mavacamten stabilizes this state.

Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals

Electron microscope studies have shown that the switched-off state of myosin II in muscle involves intramolecular interaction between the two heads of myosin and between one head and the tail. The interaction, seen in both myosin filaments and isolated molecules, inhibits activity by blocking actin-binding and ATPase sites on myosin. This interacting-heads motif is highly conserved, occurring in invertebrates and vertebrates, in striated, smooth, and nonmuscle myosin IIs, and in myosins regulated by both Ca²⁺ binding and regulatory light-chain phosphorylation. Our goal was to determine how early this motif arose by studying the structure of inhibited myosin II molecules from primitive animals and from earlier, unicellular species that predate animals. Myosin II from Cnidaria (sea anemones, jellyfish), the most primitive animals with muscles, and Porifera (sponges), the most primitive of all animals (lacking muscle tissue) showed the same interacting-heads structure as myosins from higher animals, confirming the early origin of the motif. The social amoeba Dictyostelium discoideum showed a similar, but modified, version of the motif, while the amoeba Acanthamoeba castellanii and fission yeast (Schizosaccharomyces pombe) showed no head-head interaction, consistent with the different sequences and regulatory mechanisms of these myosins compared with animal myosin IIs. Our results suggest that head-head/head-tail interactions have been conserved, with slight modifications, as a mechanism for regulating myosin II activity from the emergence of the first animals and before. The early origins of these interactions highlight their importance in generating the inhibited (relaxed) state of myosin in muscle and nonmuscle cells.

Regulation of Contraction by the Thick Filaments in Skeletal Muscle

Contraction of skeletal muscle cells is initiated by a well-known signaling pathway. An action potential in a motor nerve triggers an action potential in a muscle cell membrane, a transient increase of intracellular calcium concentration, binding of calcium to troponin in the actin-containing thin filaments, and a structural change in the thin filaments that allows myosin motors from the thick filaments to bind to actin and generate force. This calcium/thin filament mediated pathway provides the "START" signal for contraction, but it is argued that the functional response of the muscle cell, including the speed of its contraction and relaxation, adaptation to the external load, and the metabolic cost of contraction is largely determined by additional mechanisms. This review considers the role of the thick filaments in those mechanisms, and puts forward a paradigm for the control of contraction in skeletal muscle in which both the thick and thin filaments have a regulatory function. The OFF state of the thick filament is characterized by helical packing of most of the myosin head or motor domains on the thick filament surface in a conformation that makes them unavailable for actin binding or ATP hydrolysis, although a small fraction of the myosin heads are constitutively ON. The availability of the majority fraction of the myosin heads for contraction is controlled in part by the external load on the muscle, so that these heads only attach to actin and hydrolyze ATP when they are required. This phenomenon seems to be the major determinant of the well-known force-velocity relationship of muscle, and controls the metabolic cost of contraction. The regulatory state of the thick filament also seems to control the dynamics of both muscle activation and relaxation.

Coupling between myosin head conformation and the thick filament backbone structure

The recent high-resolution structure of the thick filament from Lethocerus asynchronous flight muscle shows aspects of thick filament structure never before revealed that may shed some light on how striated muscles function. The phenomenon of stretch activation underlies the function of asynchronous flight muscle. It is most highly developed in flight muscle, but is also observed in other striated muscles such as cardiac muscle. Although stretch activation is likely to be complex, involving more than a single structural aspect of striated muscle, the thick filament itself, would be a prime site for regulatory function because it must bear all of the tension produced by both its associated myosin motors and any externally applied force. Here we show the first structural evidence that the arrangement of myosin heads within the interacting heads motif is coupled to the structure of the thick filament backbone. We find that a change in helical angle of 0.16° disorders the blocked head preferentially within the Lethocerus interacting heads motif. This observation suggests a mechanism for how tension affects the dynamics of the myosin heads leading to a detailed hypothesis for stretch activation and shortening deactivation, in which the blocked head preferentially binds the thin filament followed by the free head when force production occurs.

Lessons from a tarantula: new insights into myosin interacting-heads motif evolution and its implications on disease

Tarantula's leg muscle thick filament is the ideal model for the study of the structure and function of skeletal muscle thick filaments. Its analysis has given rise to a series of structural and functional studies, leading, among other things, to the discovery of the myosin interacting-heads motif (IHM). Further electron microscopy (EM) studies have shown the presence of IHM in frozen-hydrated and negatively stained thick filaments of striated, cardiac, and smooth muscle of bilaterians, most showing the IHM parallel to the filament axis. EM studies on negatively stained heavy meromyosin of different species have shown the presence of IHM on sponges, animals that lack muscle, extending the presence of IHM to metazoans. The IHM evolved about 800 MY ago in the ancestor of Metazoa, and independently with functional differences in the lineage leading to the slime mold Dictyostelium discoideum (Mycetozoa). This motif conveys important functional advantages, such as Ca²⁺ regulation and ATP energy-saving mechanisms. Recent interest has focused on human IHM structure in order to understand the structural basis underlying various conditions and situations of scientific and medical interest: the hypertrophic and dilated cardiomyopathies, overfeeding control, aging and hormone deprival muscle weakness, drug design for schistosomiasis control, and conditioning exercise physiology for the training of power athletes.

Lessons from a tarantula: new insights into muscle thick filament and myosin interacting-heads motif structure and function

The tarantula skeletal muscle X-ray diffraction pattern suggested that the myosin heads were helically arranged on the thick filaments. Electron microscopy (EM) of negatively stained relaxed tarantula thick filaments revealed four helices of heads allowing a helical 3D reconstruction. Due to its low resolution (5.0 nm), the unambiguous interpretation of densities of both heads was not possible. A resolution increase up to 2.5 nm, achieved by cryo-EM of frozen-hydrated relaxed thick filaments and an iterative helical real space reconstruction, allowed the resolving of both heads. The two heads, "free" and "blocked", formed an asymmetric structure named the "interacting-heads motif" (IHM) which explained relaxation by self-inhibition of both heads ATPases. This finding made tarantula an exemplar system for thick filament structure and function studies. Heads were shown to be released and disordered by Ca²⁺-activation through myosin regulatory light chain phosphorylation, leading to EM, small angle X-ray diffraction and scattering, and spectroscopic and biochemical studies of the IHM structure and function. The results from these studies have consequent implications for understanding and explaining myosin super-relaxed state and thick filament activation and regulation. A cooperative phosphorylation mechanism for activation in tarantula skeletal muscle, involving swaying constitutively Ser35 mono-phosphorylated free heads, explains super-relaxation, force potentiation and post-tetanic potentiation through Ser45 mono-phosphorylated blocked heads. Based on this mechanism, we propose a swaying-swinging, tilting crossbridge-sliding filament for tarantula muscle contraction.

Hypertrophic cardiomyopathy and the myosin mesa: viewing an old disease in a new light

The sarcomere is an exquisitely designed apparatus that is capable of generating force, which in the case of the heart results in the pumping of blood throughout the body. At the molecular level, an ATP-dependent interaction of myosin with actin drives the contraction and force generation of the sarcomere. Over the past six decades, work on muscle has yielded tremendous insights into the workings of the sarcomeric system. We now stand on the cusp where the acquired knowledge of how the sarcomere contracts and how that contraction is regulated can be extended to an understanding of the molecular mechanisms of sarcomeric diseases, such as hypertrophic cardiomyopathy (HCM). In this review we present a picture that combines current knowledge of the myosin mesa, the sequestered state of myosin heads on the thick filament, known as the interacting-heads motif (IHM), their possible interaction with myosin binding protein C (MyBP-C) and how these interactions can be abrogated leading to hyper-contractility, a key clinical manifestation of HCM. We discuss the structural and functional basis of the IHM state of the myosin heads and identify HCM-causing mutations that can directly impact the equilibrium between the 'on state' of the myosin heads (the open state) and the IHM 'off state'. We also hypothesize a role of MyBP-C in helping to maintain myosin heads in the IHM state on the thick filament, allowing release in a graded manner upon adrenergic stimulation. By viewing clinical hyper-contractility as the result of the destabilization of the IHM state, our aim is to view an old disease in a new light.

Myosin II sequences for Lethocerus indicus

We present the genomic and expressed myosin II sequences from the giant waterbug, Lethocerus indicus. The intron rich gene appears relatively ancient and contains six regions of mutually exclusive exons that are alternatively spliced. Alternatively spliced regions may be involved in the asymmetric myosin dimer structure known as the interacting heads motif, as well as stabilizing the interacting heads motif within the thick filament. A lack of negative charge in the myosin S2 domain may explain why Lethocerus thick filaments display a perpendicular interacting heads motif, rather than one folded back to contact S2, as is seen in other thick filament types such as those from tarantula.

MYBPC3 mutations are associated with a reduced super-relaxed state in patients with hypertrophic cardiomyopathy

The “super-relaxed state” (SRX) of myosin represents a ‘reserve’ of motors in the heart. Myosin heads in the SRX are bound to the thick filament and have a very low ATPase rate. Changes in the SRX are likely to modulate cardiac contractility. We previously demonstrated that the SRX is significantly reduced in mouse cardiomyocytes lacking cardiac myosin binding protein–C (cMyBP-C). Here, we report the effect of mutations in the cMyBP-C gene (MYBPC3) using samples from human patients with hypertrophic cardiomyopathy (HCM). Left ventricular (LV) samples from 11 HCM patients were obtained following myectomy surgery to relieve LV outflow tract obstruction. HCM samples were genotyped as either MYBPC3 mutation positive (MYBPC3_mut) or negative (HCM_smn) and were compared to eight non-failing donor hearts. Compared to donors, only MYBPC3_mut samples display a significantly diminished SRX, characterised by a decrease in both the number of myosin heads in the SRX and the lifetime of ATP turnover. These changes were not observed in HCM_smn samples. There was a positive correlation (p < 0.01) between the expression of cMyBP-C and the proportion of myosin heads in the SRX state, suggesting cMyBP-C modulates and maintains the SRX. Phosphorylation of the myosin regulatory light chain in MYBPC3_mut samples was significantly decreased compared to the other groups, suggesting a potential mechanism to compensate for the diminished SRX. We conclude that by altering both contractility and sarcomeric energy requirements, a reduced SRX may be an important disease mechanism in patients with MYBPC3 mutations.

Effects of myosin variants on interacting-heads motif explain distinct hypertrophic and dilated cardiomyopathy phenotypes

Cardiac β-myosin variants cause hypertrophic (HCM) or dilated (DCM) cardiomyopathy by disrupting sarcomere contraction and relaxation. The locations of variants on isolated myosin head structures predict contractility effects but not the prominent relaxation and energetic deficits that characterize HCM. During relaxation, pairs of myosins form interacting-heads motif (IHM) structures that with other sarcomere proteins establish an energy-saving, super-relaxed (SRX) state. Using a human β-cardiac myosin IHM quasi-atomic model, we defined interactions sites between adjacent myosin heads and associated protein partners, and then analyzed rare variants from 6112 HCM and 1315 DCM patients and 33,370 ExAC controls. HCM variants, 72% that changed electrostatic charges, disproportionately altered IHM interaction residues (expected 23%; HCM 54%, p=2.6×10-19; DCM 26%, p=0.66; controls 20%, p=0.23). HCM variant locations predict impaired IHM formation and stability, and attenuation of the SRX state - accounting for altered contractility, reduced diastolic relaxation, and increased energy consumption, that fully characterizes HCM pathogenesis.

The myosin mesa and the basis of hypercontractility caused by hypertrophic cardiomyopathy mutations

Hypertrophic cardiomyopathy (HCM) is primarily caused by mutations in β-cardiac myosin and myosin-binding protein-C (MyBP-C). Changes in the contractile parameters of myosin measured so far do not explain the clinical hypercontractility caused by such mutations. We propose that hypercontractility is due to an increase in the number of myosin heads (S1) that are accessible for force production. In support of this hypothesis, we demonstrate myosin tail (S2)-dependent functional regulation of actin-activated human β-cardiac myosin ATPase. In addition, we show that both S2 and MyBP-C bind to S1 and that phosphorylation of either S1 or MyBP-C weakens these interactions. Importantly, the S1-S2 interaction is also weakened by four myosin HCM-causing mutations but not by two other mutations. To explain these experimental results, we propose a working structural model involving multiple interactions, including those with myosin's own S2 and MyBP-C, that hold myosin in a sequestered state.

Biophysical properties of human β-cardiac myosin with converter mutations that cause hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) affects 1 in 500 individuals and is an important cause of arrhythmias and heart failure. Clinically, HCM is characterized as causing hypercontractility, and therapies are aimed toward controlling the hyperactive physiology. Mutations in the β-cardiac myosin comprise ~40% of genetic mutations associated with HCM, and the converter domain of myosin is a hotspot for HCM-causing mutations; however, the underlying primary effects of these mutations on myosin's biomechanical function remain elusive. We hypothesize that these mutations affect the biomechanical properties of myosin, such as increasing its intrinsic force and/or its duty ratio and therefore the ensemble force of the sarcomere. Using recombinant human β-cardiac myosin, we characterize the molecular effects of three severe HCM-causing converter domain mutations: R719W, R723G, and G741R. Contrary to our hypothesis, the intrinsic forces of R719W and R723G mutant myosins are decreased compared to wild type and unchanged for G741R. Actin and regulated thin filament gliding velocities are ~15% faster for R719W and R723G myosins, whereas there is no change in velocity for G741R. Adenosine triphosphatase activities and the load-dependent velocity change profiles of all three mutant proteins are very similar to those of wild type. These results indicate that the net biomechanical properties of human β-cardiac myosin carrying these converter domain mutations are very similar to those of wild type or are even slightly hypocontractile, leading us to consider an alternative mechanism for the clinically observed hypercontractility. Future work includes how these mutations affect protein interactions within the sarcomere that increase the availability of myosin heads participating in force production.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.