The biochemically defined super relaxed state of myosin-A paradox

Consecutive SSCs increase the SSC effect in skinned rat muscle fibres

Muscle function is essential for generating force and movement, with stretch-shortening cycles (SSCs) playing a fundamental role in the economy of everyday locomotion. Compared with pure shortening contractions, the SSC effect is characterised by increased force, work, and power output during the SSC shortening phase. Few studies have investigated whether SSC effects transfer across consecutive SSCs. Therefore, we investigated SSC effects over three consecutive SSCs in skinned rat muscle fibres by analysing the isometric force immediately before stretch onset (Fonset), the peak force at the end of stretching (Fpeak), and the minimum force at the end of shortening (Fmin), along with mechanical (WorkSSC) and shortening work (WorkSHO) at different activation levels (20%, 60%, and 100%). Each SSC was followed by an isometric hold phase, allowing force to return to a steady state. Results indicated an increase in both Fpeak (20.3%) and WorkSSC (60.9%) from SSC1 to SSC3 across all activation levels tested. At 20% and 60% activation, Fonset, Fmin, and WorkSHO increased (range: 4.5-28.5%) from SSC1 to SSC3. However, at 100% activation, Fonset and WorkSHO remained unchanged, while Fmin decreased (- 8.5%) from SSC1 to SSC3. These results suggest that the increase in SSC effects at submaximal activation may be primarily due to increased cross-bridge forces. The absence of increases in Fonset, Fmin, and WorkSHO at 100% activation suggests that increases in Fpeak and WorkSSC may not be attributed to increased cross-bridge force but could instead be caused by additional effects, possibly involving modulation of non-cross-bridge structures, likely titin, and their stiffness.

A combined experimental and computational analysis of mantATP turnover in skinned muscle fibers

Myosin is the primary motor protein in skeletal muscle, responsible for adenosine triphosphate (ATP) hydrolysis that drives muscle contraction. In addition to force production, resting myosin consumes ATP in futile cycles at two rates, the slower one being associated with the Super Relaxed State (SRX), in contrast to the less inhibited Disordered Relaxed State (DRX). The SRX is typically measured using the mantATP chasing technique, where the decay of a fluorescent ATP analogue is fitted using a multiexponential function. Recently, significant concerns have been raised regarding the use of this technique, particularly when applied to soluble myosin preparations. While skinned fibers offer the advantage of preserving the native thick filament structure and myosin cooperativity, limited diffusion and nonspecific mantATP binding pose challenges. In this study, we combine experimental data and in-silico modeling to dissect the contributions of different components in the mantATP chasing signal. We analyze control skinned fibers and fibers subjected to myosin extraction. Our analysis shows that the nonspecific component partially overlaps with the DRX timescale. In contrast, the slow component linked to myosin SRX nucleotide release is characterized by a time constant that significantly differs from those of the nonspecific signal and DRX, enabling its reliable estimation using this technique. Our findings indicate that evaluating nonspecific mantATP components is necessary to obtain a reliable estimation of both SRX and DRX. We validated our analysis by comparing populations and time constants obtained from chasing with mantATP to mantATPase rates in control conditions and upon piperine-induced SRX destabilization.

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

Introduction

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

Methods

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

Results and discussion

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

Dynamics of β-cardiac myosin between the super-relaxed and disordered-relaxed states

The super-relaxed (SRX) state of myosin ATPase activity is critical for striated muscle function, and its dysregulation is linked to cardiomyopathies. It is unclear whether the SRX state exchanges readily with the disordered-relaxed (DRX) state and whether the SRX state directly corresponds to the folded back interacting-heads motif. Using recombinant β-cardiac heavy meromyosin and subfragment 1, which cannot form the interacting-heads motif, we show that the SRX and DRX populations transition at a rate substantially faster than the ATP turnover rate, dependent on myosin head-tail interactions. Some mutations which cause hypertrophic or dilated cardiomyopathies alter the SRX-DRX equilibrium, but not all mutations. The cardiac myosin inhibitor mavacamten slows nucleotide release by an equal factor for both heavy meromyosin and subfragment 1, thus only indirectly influencing the occupancy time of the SRX state. These findings suggest that purified myosins undergo rapid switching between SRX and DRX states, refining our understanding of cardiomyopathy mechanisms.

Functional control of myosin motors in the cardiac cycle

Contraction of the heart is driven by cyclical interactions between myosin and actin filaments powered by ATP hydrolysis. The modular structure of heart muscle and the organ-level synchrony of the heartbeat ensure tight reciprocal coupling between this myosin ATPase cycle and the macroscopic cardiac cycle. The myosin motors respond to the cyclical activation of the actin and myosin filaments to drive the pressure changes that control the inflow and outflow valves of the heart chambers. Opening and closing of the valves in turn switches the myosin motors between roughly isometric and roughly isotonic contraction modes. Peak filament stress in the heart is much smaller than in fully activated skeletal muscle, although the myosin filaments in the two muscle types have the same number of myosin motors. Calculations indicate that only ~5% of the myosin motors in the heart are needed to generate peak systolic pressure, although many more motors are needed to drive ejection. Tight regulation of the number of active motors is essential for the efficient functioning of the healthy heart - this control is commonly disrupted by gene variants associated with inherited heart disease, and its restoration might be a useful end point in the development of novel therapies.

Dynamics of β-cardiac myosin between the super-relaxed and disordered-relaxed states

The super-relaxed (SRX) state of myosin ATPase activity is critical for striated muscle function, and its dysregulation is linked to cardiomyopathies. It is unclear whether the SRX state exchanges readily with the disordered-relaxed (DRX) state, and whether the SRX state directly corresponds to the folded back interacting-head motif (IHM). Using recombinant β-cardiac heavy meromyosin (HMM) and subfragment 1 (S1), which cannot form the IHM, we show that the SRX and DRX populations are in rapid equilibrium, dependent on myosin head-tail interactions. Some mutations which cause hypertrophic (HCM) or dilated (DCM) cardiomyopathies alter the SRX-DRX equilibrium, but not all mutations. The cardiac myosin inhibitor mavacamten slows nucleotide release by an equal factor for both HMM and S1, thus only indirectly influencing the occupancy time of the SRX state. These findings suggest that purified myosins undergo rapid switching between SRX and DRX states, refining our understanding of cardiomyopathy mechanisms.

Load-dependence of the activation of myosin filaments in heart muscle

Contraction of heart muscle requires activation of both the actin and myosin filaments. The mechanism of myosin filament activation is unknown, but the leading candidate hypothesis is direct mechano-sensing by the filaments. Here, we tested this hypothesis by activating intact trabeculae from rat heart by electrical stimulation under different loads and measuring myosin filament activation by X-ray diffraction. Unexpectedly, we found that the distinct structural changes in the myosin filament associated with activation had different dependences on the load. In early activation, all the structural changes indicated faster activation at higher load, as expected from the mechano-sensing hypothesis, but, at later times, the helical order of the myosin motors characteristic of the inactivated state was lost even at very low load. We conclude that mechano-sensing does operate in heart muscle, but it is supplemented by a previously undescribed mechanism that links myosin filament activation to actin filament activation. KEY POINTS: Myosin filament activation controls the strength and speed of contraction in heart muscle. Early activation of the myosin filament is determined by the filament load. At later times, myosin filament activation is controlled by a load independent pathway. This load independent pathway provides new targets and assays for drug development.

Mechanistic basis for rescuing hypertrophic cardiomyopathy with myosin regulatory light chain phosphorylation

We investigated the impact of the phosphomimetic (Ser15 → Asp15) myosin regulatory light chain (S15D-RLC) on the Super-Relaxed (SRX) state of myosin using previously characterized transgenic (Tg) S15D-D166V rescue mice, comparing them to the Hypertrophic Cardiomyopathy (HCM) Tg-D166V model and wild-type (WT) RLC mice. In the Tg-D166V model, we observed a disruption of the SRX state, resulting in a transition from SRX to DRX (Disordered Relaxed) state, which explains the hypercontractility of D166V-mutated myosin motors. The presence of the S15D moiety in Tg-S15D-D166V mice restored the SRX/DRX balance to levels comparable to Tg-WT, thus mitigating the hypercontractile behavior associated with the HCM-D166V mutation. Additionally, we investigated the impact of delivering the S15D-RLC molecule to the hearts of Tg-D166V mice via adeno-associated virus (AAV9) and compared their condition to AAV9-empty vector-injected or non-injected Tg-D166V animals. Tg-D166V mice injected with AAV9 S15D-RLC exhibited a significantly higher proportion of myosin heads in the SRX state compared to those injected with AAV9 empty vector or left non-injected. No significant effect was observed in Tg-WT hearts treated similarly. These findings suggest that AAV9-delivered phosphomimetic S15D-RLC modality mitigates the abnormal Tg-D166V phenotype without impacting the normal function of Tg-WT hearts. Global longitudinal strain analysis supported these observations, indicating that the S15D moiety can alleviate the HCM-D166V phenotype by restoring SRX stability and the SRX ↔ DRX equilibrium.

Reassessing the unifying hypothesis for hypercontractility caused by myosin mutations in hypertrophic cardiomyopathy

Significant advances in structural and biochemical research validate the 9-year-old hypothesis that cardiac hypercontractility seen in patients with hypertrophic cardiomyopathy is primarily caused by sarcomeric mutations that increase the number of myosin molecules available for actin interaction.

Dual effect of N-terminal deletion of cardiac myosin essential light chain in mitigating cardiomyopathy

We investigated the role of the N-terminus (residues 1-43) of the myosin essential light chain (N-ELC) in regulating cardiac function in hypertrophic (HCM-A57G) and restrictive (RCM-E143K) cardiomyopathy mice. Both models were cross-genotyped with N-ELC-truncated Δ43 mice, and the offspring were studied using echocardiography and muscle contractile mechanics. In A57G×Δ43 mice, Δ43 expression improved heart function and reduced hypertrophy and fibrosis. No improvements were seen in E143K×Δ43 compared to RCM-E143K mice. HCM-mutant pathology involved an impaired N-ELC tension sensor, disrupted N-ELC-actin interactions, an altered force-pCa relationship, and a destabilized myosin's super-relaxed state. Removal of the malfunctioning N-ELC sensor led to functional rescue in HCM-truncated mutant hearts. However, the RCM mutation could not be rescued by N-ELC deletion, likely due to its proximity to the myosin motor domain, affecting lever-arm rigidity and myosin function. This study provides insights into the role of N-ELC in the development and potential rescue of ELC-mutant cardiomyopathy.

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

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