Cardiac myosin binding protein C phosphorylation affects cross-bridge cycle's elementary steps in a site-specific manner

MYBPC3-c.772G>A mutation results in haploinsufficiency and altered myosin cycling kinetics in a patient induced stem cell derived cardiomyocyte model of hypertrophic cardiomyopathy

Approximately 40% of hypertrophic cardiomyopathy (HCM) mutations are linked to the sarcomere protein cardiac myosin binding protein-C (cMyBP-C). These mutations are either classified as missense mutations or truncation mutations. One mutation whose nature has been inconsistently reported in the literature is the MYBPC3-c.772G > A mutation. Using patient-derived human induced pluripotent stem cells differentiated to cardiomyocytes (hiPSC-CMs), we have performed a mechanistic study of the structure-function relationship for this MYBPC3-c.772G > A mutation versus a mutation corrected, isogenic cell line. Our results confirm that this mutation leads to exon skipping and mRNA truncation that ultimately suggests ∼20% less cMyBP-C protein (i.e., haploinsufficiency). This, in turn, results in increased myosin recruitment and accelerated myofibril cycling kinetics. Our mechanistic studies suggest that faster ADP release from myosin is a primary cause of accelerated myofibril cross-bridge cycling due to this mutation. Additionally, the reduction in force generating heads expected from faster ADP release during isometric contractions is outweighed by a cMyBP-C phosphorylation mediated increase in myosin recruitment that leads to a net increase of myofibril force, primarily at submaximal calcium activations. These results match well with our previous report on contractile properties from myectomy samples of the patients from whom the hiPSC-CMs were generated, demonstrating that these cell lines are a good model to study this pathological mutation and extends our understanding of the mechanisms of altered contractile properties of this HCM MYBPC3-c.772G > A mutation.

N-Terminal Fragment of Cardiac Myosin Binding Protein C Modulates Cooperative Mechanisms of Thin Filament Activation in Atria and Ventricles

Cardiac myosin binding protein C (cMyBP-C) is one of the essential control components of the myosin cross-bridge cycle. The C-terminal part of cMyBP-C is located on the surface of the thick filament, and its N-terminal part interacts with actin, myosin, and tropomyosin, affecting both kinetics of the ATP hydrolysis cycle and lifetime of the cross-bridge, as well as calcium regulation of the actin-myosin interaction, thereby modulating contractile function of myocardium. The role of cMyBP-C in atrial contraction has not been practically studied. We examined effect of the N-terminal C0-C1-m-C2 (C0-C2) fragment of cMyBP-C on actin-myosin interaction using ventricular and atrial myosin in an in vitro motility assay. The C0-C2 fragment of cMyBP-C significantly reduced the maximum sliding velocity of thin filaments on both myosin isoforms and increased the calcium sensitivity of the actin-myosin interaction. The C0-C2 fragment had different effects on the kinetics of ATP and ADP exchange, increasing the affinity of ventricular myosin for ADP and decreasing the affinity of atrial myosin. The effect of the C0-C2 fragment on the activation of the thin filament depended on the myosin isoforms. Atrial myosin activates the thin filament less than ventricular myosin, and the C0-C2 fragment makes these differences in the myosin isoforms more pronounced.

Mechanical Evaluation of Frozen and Cryo-Sectioned Papillary Muscle Samples by Using Sinusoidal Analysis: Cross-bridge Kinetics and the Effect of Partial Ca2+ activation

The use of frozen and cryo-sectioned cardiac muscle preparations, introduced recently by (Feng & Jin, 2020), offers promising advantages of easy transport and exchange of muscle samples among collaborating laboratories. In this report, we examined integrity of such preparation by studying tension transients in response to sinusoidal length changes and following concomitant amplitude and phase shift in tension time courses at varying frequencies. We used sections with 70 μm thickness, isolated fiber preparations, and studied cross-bridge (CB) kinetics: we activated the preparations with saturating Ca2+, and varying concentrations of ATP and phosphate (Pi). Our experiments have demonstrated that this preparation has the normal active tension and elementary steps of the CB cycle. Furthermore, we investigated the effect of Ca2+ on the rate constants and found that the rate constant r 4 of the force generation step is proportionate to [Ca2+] when it is <5 μM. This observation suggests that the activation mechanism can be described by a simple second order reaction. As expected, we found that magnitude parameters including tension and stiffness are related to [Ca2+] by the Hill equation with cooperativity of 4-5, consistent to the fact that Ca2+ activation mechanisms involve cooperative multimolecular interactions. Our results are consistent with a long-held hypothesis that process C (phase 2 of step analysis) represents the CB detachment step, and process B (phase 3) represents the force generation step. In this report, we further found that constant H may also represent work performance step. Our experiments have demonstrated excellent CB kinetics with reduced noise and well-defined two exponentials, which are better than skinned fibers, and easier to handle and study than single myofibrils.

Right-ventricular dysfunction in HFpEF is linked to altered cardiomyocyte Ca2+ homeostasis and myofilament sensitivity

Aims

Heart failure with preserved ejection fraction (HFpEF) is frequently (30%) associated with right ventricular (RV) dysfunction, which increases morbidity and mortality in these patients. Yet cellular mechanisms of RV remodelling and RV dysfunction in HFpEF are not well understood. Here, we evaluated RV cardiomyocyte function in a rat model of metabolically induced HFpEF.

Methods and results

Heart failure with preserved ejection fraction‐prone animals (ZSF‐1 obese) and control rats (Wistar Kyoto) were fed a high‐caloric diet for 13 weeks. Haemodynamic characterization by echocardiography and invasive catheterization was performed at 22 and 23 weeks of age, respectively. After sacrifice, organ morphometry, RV histology, isolated RV cardiomyocyte function, and calcium (Ca²⁺) transients were assessed. ZSF‐1 obese rats showed a HFpEF phenotype with left ventricular (LV) hypertrophy, LV diastolic dysfunction (including increased LV end‐diastolic pressures and E/e′ ratio), and preserved LV ejection fraction. ZSF‐1 obese animals developed RV dilatation (50% increased end‐diastolic area) and mildly impaired RV ejection fraction (42%) with evidence of RV hypertrophy. In isolated RV cardiomyocytes from ZSF‐1 obese rats, cell shortening amplitude was preserved, but cytosolic Ca²⁺ transient amplitude was reduced. In addition, augmentation of cytosolic Ca²⁺ release with increased stimulation frequency was lost in ZSF‐1 obese rats. Myofilament sensitivity was increased, while contractile kinetics were largely unaffected in intact isolated RV cardiomyocytes from ZSF‐1 obese rats. Western blot analysis revealed significantly increased phosphorylation of cardiac myosin‐binding protein C (Ser282 cMyBP‐C) but no change in phosphorylation of troponin I (Ser23, 24 TnI) in RV myocardium from ZSF‐1 obese rats.

Conclusions

Right ventricular dysfunction in obese ZSF‐1 rats with HFpEF is associated with intrinsic RV cardiomyocyte remodelling including reduced cytosolic Ca²⁺ amplitudes, loss of frequency‐dependent augmentation of Ca²⁺ release, and increased myofilament Ca²⁺ sensitivity.

The structure of the native cardiac thin filament at systolic Ca2+ levels

Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.

Impact of heart rate on cross-bridge cycling kinetics in failing and nonfailing human myocardium

The force-frequency relationship (FFR) is an important regulatory mechanism that increases the force-generating capacity as well as the contraction and relaxation kinetics in human cardiac muscle as the heart rate increases. In human heart failure, the normally positive FFR often becomes flat, or even negative. The rate of cross-bridge cycling, which has been reported to affect cardiac output, could be potentially dysregulated and contribute to blunted or negative FFR in heart failure. We recently developed and herein use a novel method for measuring the rate of tension redevelopment. This method allows us to obtain an index of the rate of cross-bridge cycling in intact contracting cardiac trabeculae at physiological temperature and assess physiological properties of cardiac muscles while preserving posttranslational modifications representative of those that occur in vivo. We observed that trabeculae from failing human hearts indeed exhibit an impaired FFR and a reduced speed of relaxation kinetics. However, stimulation frequencies in the lower spectrum did not majorly affect cross-bridge cycling kinetics in nonfailing and failing trabeculae when assessed at maximal activation. Trabeculae from failing human hearts had slightly slower cross-bridge kinetics at 3 Hz as well as reduced capacity to generate force upon K⁺ contracture at this frequency. We conclude that cross-bridge kinetics at maximal activation in the prevailing in vivo heart rates are not majorly impacted by frequency and are not majorly impacted by disease.NEW & NOTEWORTHY In this study, we confirm that cardiac relaxation kinetics are impaired in filing human myocardium and that cross-bridge cycling rate at resting heart rates does not contribute to this impaired relaxation. At high heart rates, failing myocardium cross-bridge rates are slower than in nonfailing myocardium.

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.

Nebulin increases thin filament stiffness and force per cross-bridge in slow-twitch soleus muscle fibers

Nebulin stabilizes the thin filament and regulates force generation in skeletal muscle, but its precise role is not understood. Using conditional knockout mice, Kawai et al. demonstrate that nebulin functions to increase the force per cross-bridge in skinned slow-twitch soleus muscle fibers.

Nebulin (Neb) is associated with the thin filament in skeletal muscle cells, but its functions are not well understood. For this goal, we study skinned slow-twitch soleus muscle fibers from wild-type (Neb⁺) and conditional Neb knockout (Neb⁻) mice. We characterize cross-bridge (CB) kinetics and the elementary steps of the CB cycle by sinusoidal analysis during full Ca²⁺ activation and observe that Neb increases active tension 1.9-fold, active stiffness 2.7-fold, and rigor stiffness 3.0-fold. The ratio of stiffness during activation and rigor states is 62% in Neb⁺ fibers and 68% in Neb⁻ fibers. These are approximately proportionate to the number of strongly attached CBs during activation. Because the thin filament length is 15% shorter in Neb⁻ fibers than in Neb⁺ fibers, the increase in force per CB in the presence of Neb is ∼1.5 fold. The equilibrium constant of the CB detachment step (K₂), its rate (k₂), and the rate of the reverse force generation step (k₋₄) are larger in Neb⁺ fibers than in Neb⁻ fibers. The rates of the force generation step (k₄) and the reversal detachment step (k₋₂) change in the opposite direction. These effects can be explained by Le Chatelier’s principle: Increased CB strain promotes less force-generating state(s) and/or detached state(s). Further, when CB distributions among the six states are calculated, there is no significant difference in the number of strongly attached CBs between fibers with and without Neb. These results demonstrate that Neb increases force per CB. We also confirm that force is generated by isomerization of actomyosin (AM) from the AM.ADP.Pi state (ADP, adenosine diphophate; Pi, phosphate) to the AM*ADP.Pi state, where the same force is maintained after Pi release to result in the AM*ADP state. We propose that Neb changes the actin (and myosin) conformation for better ionic and hydrophobic/stereospecific AM interaction, and that the effect of Neb is similar to that of tropomyosin.

Development of apical hypertrophic cardiomyopathy with age in a transgenic mouse model carrying the cardiac actin E99K mutation

In both humans and mice, the Glu-99-Lys (E99K) mutation in the cardiac actin gene (ACTC) results in little understood apical hypertrophic cardiomyopathy (AHCM). To determine how cross-bridge kinetics change with AHCM development, we applied sinusoidal length perturbations to skinned papillary muscle fibres from 2- and 5-month old E99K transgenic (Tg) and non-transgenic (NTg) mice, and studied tension and its transients. These age groups were chosen because our preliminary studies indicated that AHCM develops with age. Fibres from 5-month old E99K mice showed significant decreases in tension, stiffness, the rate of the medium-speed exponential process and its magnitude compared to non-transgenic control. The nucleotide association constants increased with age, and they were significantly larger in E99K compared to NTg. However, there were no large differences in the rates of the cross-bridge detachment step, the rates of the force generation step, or the phosphate association constant. Our result on force/cross-bridge demonstrates that the decreased active tension of E99K fibres was caused by a decreased amount of force generated per each cross-bridge. The effects were generally less or insignificant at 2 months. A pCa-tension study showed increased Ca²⁺-sensitivity (pCa₅₀) with age in both the E99K and NTg sample groups, and pCa₅₀ was significantly larger (but only for 0.05-0.06 pCa units) in E99K than in NTg groups. A significant decrease in cooperativity (n_H) was observed only in 5-month old E99K mice. We conclude that the AHCM-causing ACTC E99K mutation is associated with progressive alterations in biomechanical parameters, with changes smaller at 2 months but larger at 5 months, correlating with the development of AHCM.

Myosin Rod Hypophosphorylation and CB Kinetics in Papillary Muscles from a TnC-A8V KI Mouse Model

The cardiac troponin C (TnC)-A8V mutation is associated with hypertrophic and restrictive cardiomyopathy (HCM and RCM) in human and mice. The residue affected lies in the N-helix, a region known to affect Ca²⁺-binding affinity to the N-terminal domain. Here we report on the functional effects of this mutation in skinned papillary muscle fibers from homozygous knock-in TnC-A8V mice. Muscle fibers from left ventricle were activated at 25°C under the ionic conditions of working cardiomyocytes. The pCa-tension relationship showed a 3× increase in Ca²⁺-sensitivity and a decrease (0.8×) in cooperativity (n_H) in mutant fibers. The elementary steps of the cross-bridge (CB) cycle were investigated by sinusoidal analysis. The ATP study revealed that there is no significant change in the affinity of ATP (K₁) for the myosin head. In TnC-A8V mutant fibers, the CB detachment rate (k₂) and its equilibrium constant (K₂) increased (1.5×). The phosphate study revealed that rate constant of the force-generation step (k₄) decreased (0.5×), reversal step (k_-4) increased (2×), and the phosphate-release step (1/K₅) increased (2×). Pro-Q Diamond staining of the skinned fibers samples revealed no significant changes in total phosphorylation of multiple sarcomeric proteins. Further investigation using liquid chromatography-tandem mass spectrometry revealed hypophosphorylation of the rod domain of myosin heavy chain in TnC-A8V mutant fibers compared to wild-type. Immunoblotting confirmed the results observed in the mass spectrometry analysis. The results suggest perturbed CB kinetics-possibly caused by changes in the α-myosin heavy chain phosphorylation profile-as a novel mechanism, to our knowledge, by which a mutation in TnC can have rippling effects in the myofilament and contribute to the pathogenesis of HCM/RCM.

Knockdown of fast skeletal myosin-binding protein C in zebrafish results in a severe skeletal myopathy

MyBPC: A muscle protein for all seasons.

Myosin-binding protein C (MyBPC) in the muscle sarcomere interacts with several contractile and structural proteins. Mutations in the cardiac isoform (MyBPC-3) in humans, or animal knockout, are associated with cardiomyopathy. Function of the fast skeletal isoform (MyBPC-2) in living muscles is less understood. This question was addressed using zebrafish models, combining gene expression data with functional analysis of contractility and small-angle x-ray diffraction measurements of filament structure. Fast skeletal MyBPC-2B, the major isoform, was knocked down by >50% using morpholino antisense nucleotides. These morphants exhibited a skeletal myopathy with elevated apoptosis and up-regulation of factors associated with muscle protein degradation. Morphant muscles had shorter sarcomeres with a broader length distribution, shorter actin filaments, and a wider interfilament spacing compared with controls, suggesting that fast skeletal MyBPC has a role in sarcomere assembly. Active force was reduced more than expected from the decrease in muscle size, suggesting that MyBPC-2 is required for optimal force generation at the cross-bridge level. The maximal shortening velocity was significantly increased in the MyBPC-2 morphants, but when related to the sarcomere length, the difference was smaller, reflecting that the decrease in MyBPC-2B content and the resulting myopathy were accompanied by only a minor influence on filament shortening kinetics. In the controls, equatorial patterns from small-angle x-ray scattering revealed that comparatively few cross-bridges are attached (as evaluated by the intensity ratio of the 11 and 10 equatorial reflections) during active contraction. X-ray scattering data from relaxed and contracting morphants were not significantly different from those in controls. However, the increase in the 11:10 intensity ratio in rigor was lower compared with that in controls, possibly reflecting effects of MyBPC on the cross-bridge interactions. In conclusion, lack of MyBPC-2 results in a severe skeletal myopathy with structural changes and muscle weakness.

Comparison of elementary steps of the cross-bridge cycle in rat papillary muscle fibers expressing α- and β-myosin heavy chain with sinusoidal analysis

In mammalian ventricles, two myosin heavy chain (MHC) isoforms have been identified. Small animals express α-MHC, whereas large animals express β-MHC, which contribute to a large difference in the heart rate. Sprague-Dawley rats possessing ~99% α-MHC were treated with propylthiouracil to result in 100% β-MHC. Papillary muscles were skinned, dissected into small fibers, and used for experiments. To understand the functional difference between α-MHC and β-MHC, skinned-fibers were activated under the intracellular ionic conditions: 5 mM MgATP, 1 mM Mg²⁺, 8 mM Pi, 200 mM ionic strength, pH 7.00 at 25 °C. Small amplitude sinusoidal length oscillations were applied in the frequency range 0.13-100 Hz (corresponding time domain: 1.6-1200 ms), and effects of Ca²⁺, Pi, and ATP were studied. The results show that Ca²⁺ sensitivity was slightly less (10-15%) in β-MHC than α-MHC containing fibers. Sinusoidal analysis at pCa 4.66 (full Ca²⁺ activation) demonstrated that, the apparent rate constants were 2-4× faster in α-MHC containing fibers. The ATP study demonstrated that, in β-MHC containing fibers, K ₁ (ATP association constant) was greater (1.7×), k ₂ and k _-2 (cross-bridge detachment and its reversal rate constants) were smaller (×0.6). The Pi study demonstrated that, in β-MHC containing fibers, k ₄ (rate constant of the force-generation step) and k _-4 were smaller (0.75× and 0.25×, respectively), resulting in greater K ₄ (3×). There were no differences in active tension, rigor stiffness, or K ₂ (equilibrium constant of the cross-bridge detachment step). Our study further demonstrated that there were no differences in parameters between fibers obtained from left and right ventricles, but with an exception in K ₅ (Pi association constant).

Dynamics of cross-bridge cycling, ATP hydrolysis, force generation, and deformation in cardiac muscle

Despite extensive study over the past six decades the coupling of chemical reaction and mechanical processes in muscle dynamics is not well understood. We lack a theoretical description of how chemical processes (metabolite binding, ATP hydrolysis) influence and are influenced by mechanical processes (deformation and force generation). To address this need, a mathematical model of the muscle cross-bridge (XB) cycle based on Huxley's sliding filament theory is developed that explicitly accounts for the chemical transformation events and the influence of strain on state transitions. The model is identified based on elastic and viscous moduli data from mouse and rat myocardial strips over a range of perturbation frequencies, and MgATP and inorganic phosphate (Pi) concentrations. Simulations of the identified model reproduce the observed effects of MgATP and MgADP on the rate of force development. Furthermore, simulations reveal that the rate of force re-development measured in slack-restretch experiments is not directly proportional to the rate of XB cycling. For these experiments, the model predicts that the observed increase in the rate of force generation with increased Pi concentration is due to inhibition of cycle turnover by Pi. Finally, the model captures the observed phenomena of force yielding suggesting that it is a result of rapid detachment of stretched attached myosin heads.

Molecular effects of the myosin activator omecamtiv mecarbil on contractile properties of skinned myocardium lacking cardiac myosin binding protein-C

Decreased expression of cardiac myosin binding protein-C (cMyBP-C) in the myocardium is thought to be a contributing factor to hypertrophic cardiomyopathy in humans, and the initial molecular defect is likely abnormal cross-bridge (XB) function which leads to impaired force generation, decreased contractile performance, and hypertrophy in vivo. The myosin activator omecamtiv mecarbil (OM) is a pharmacological drug that specifically targets the myosin XB and recent evidence suggests that OM induces a significant decrease in the in vivo motility velocity and an increase in the XB duty cycle. Thus, the molecular effects of OM maybe beneficial in improving contractile function in skinned myocardium lacking cMyBP-C because absence of cMyBP-C in the sarcomere accelerates XB kinetics and enhances XB turnover rate, which presumably reduces contractile efficiency. Therefore, parameters of XB function were measured in skinned myocardium lacking cMyBP-C prior to and following OM incubation. We measured ktr, the rate of force redevelopment as an index of XB transition from both the weakly- to strongly-bound state and from the strongly- to weakly-bound states and performed stretch activation experiments to measure the rates of XB detachment (krel) and XB recruitment (kdf) in detergent-skinned ventricular preparations isolated from hearts of wild-type (WT) and cMyBP-C knockout (KO) mice. Samples from donor human hearts were also used to assess the effects of OM in cardiac muscle expressing a slow β-myosin heavy chain (β-MHC). Incubation of skinned myocardium with OM produced large enhancements in steady-state force generation which were most pronounced at low levels of [Ca(2+)] activations, suggesting that OM cooperatively recruits additional XB's into force generating states. Despite a large increase in steady-state force generation following OM incubation, parallel accelerations in XB kinetics as measured by ktr were not observed, and there was a significant OM-induced decrease in krel which was more pronounced in the KO skinned myocardium compared to WT skinned myocardium (58% in WT vs. 76% in KO at pCa 6.1), such that baseline differences in krel between KO and WT skinned myocardium were no longer apparent following OM-incubation. A significant decrease in the kdf was also observed following OM incubation in all groups, which may be related to the increase in the number of cooperatively recruited XB's at low Ca(2+)-activations which slows the overall rate of force generation. Our results indicate that OM may be a useful pharmacological approach to normalize hypercontractile XB kinetics in myocardium with decreased cMyBP-C expression due to its molecular effects on XB behavior.