The myosin-binding protein C motif binds to F-actin in a phosphorylation-sensitive manner

The W792R HCM missense mutation in the C6 domain of cardiac myosin binding protein-C increases contractility in neonatal mouse myocardium

Missense mutations in cardiac myosin binding protein C (cMyBP-C) are known to cause hypertrophic cardiomyopathy (HCM). The W792R mutation in the C6 domain of cMyBP-C causes severe, early onset HCM in humans, yet its impact on the function of cMyBP-C and the mechanism through which it causes disease remain unknown. To fully characterize the effect of the W792R mutation on cardiac morphology and function in vivo, we generated a murine knock-in model. We crossed heterozygous W792RWR mice to produce homozygous mutant W792RRR, heterozygous W792RWR, and control W792RWW mice. W792RRR mice present with cardiac hypertrophy, myofibrillar disarray and fibrosis by postnatal day 10 (PND10), and do not survive past PND21. Full-length cMyBP-C is present at similar levels in W792RWW, W792RWR and W792RRR mice and is properly incorporated into the sarcomere. Heterozygous W792RWR mice displayed normal heart morphology and contractility. Permeabilized myocardium from PND10 W792RRR mice showed increased Ca2+ sensitivity, accelerated cross-bridge cycling kinetics, decreased cooperativity in the activation of force, and increased expression of hypertrophy-related genes. In silico modeling suggests that the W792R mutation destabilizes the fold of the C6 domain and increases torsion in the C5-C7 region, possibly impacting regulatory interactions of cMyBP-C with myosin and actin. Based on the data presented here, we propose a model in which mutant W792R cMyBP-C preferentially forms Ca2+ sensitizing interactions with actin, rather than inhibitory interactions with myosin. The W792R-cMyBP-C mouse model provides mechanistic insights into the pathology of this mutation and may provide a mechanism by which other central domain missense mutations in cMyBP-C may alter contractility, leading to HCM.

MYBPC3 D389V Variant Induces Hypercontractility in Cardiac Organoids

BACKGROUND

MYBPC3 , encoding cardiac myosin binding protein-C (cMyBP-C), is the most mutated gene known to cause hypertrophic cardiomyopathy (HCM). However, since little is known about the underlying etiology, additional in vitro studies are crucial to defining the underlying molecular mechanisms. Accordingly, this study aimed to investigate the molecular mechanisms underlying the pathogenesis of HCM associated with a polymorphic variant (D389V) in MYBPC3 by using human-induced pluripotent stem cell (hiPSC)-derived cardiac organoids (hCOs).

METHODS

The hiPSC-derived cardiomyocytes (hiPSC-CMs) and hCOs were generated from human subjects to define the molecular, cellular, and functional changes caused by the MYBPC3 D389V variant. This variant is associated with increased fractional shortening and is highly prevalent in South Asian descendants. Recombinant C0-C2, N'-region of cMyBP-C (wildtype and D389V), and myosin S2 proteins were also utilized to perform binding and motility assays in vitro .

RESULTS

Confocal and electron microscopic analyses of hCOs generated from noncarriers (NC) and carriers of the MYBPC3 D389V variant revealed the presence of highly organized sarcomeres. Furthermore, functional experiments showed hypercontractility with increased contraction velocity, faster calcium cycling, and faster contractile kinetics in hCOs expressing MYBPC3 D389V than NC hCOs. Interestingly, significantly increased cMyBP-C phosphorylation in MYBPC3 D389V hCOs was observed, but without changes in total protein levels, in addition to higher oxidative stress and lower mitochondrial membrane potential (ΔΨm). Next, spatial mapping revealed the presence of endothelial cells, fibroblasts, macrophages, immune cells, and cardiomyocytes in the hCOs. The hypercontractile function was significantly improved after treatment with the myosin inhibitor mavacamten (CAMZYOS®) in MYBPC3 D389V hCOs. Lastly, various in vitro binding assays revealed a significant loss of affinity in the presence of MYBPC3 D389V with myosin S2 region as a likely mechanism for hypercontraction.

CONCLUSIONS

Conceptually, we showed the feasibility of assessing the functional and molecular mechanisms of HCM using highly translatable hCOs through pragmatic experiments that led to determining the MYBPC3 D389V hypercontractile phenotype, which was rescued by administration of a myosin inhibitor. Novelty and Significance: What Is Known?: MYBPC3 mutations have been implicated in hypertrophic cardiomyopathy. D389V is a polymorphic variant of MYBPC3 predicted to be present in 53000 US South Asians owing to the founder effect. D389V carriers have shown evidence of hyperdynamic heart, and human-induced pluripotent stem cells (hiPSC)-derived cardiomyocytes with D389V show cellular hypertrophy and irregular calcium transients. The molecular mechanism by which the D389V variant develops pathological cardiac dysfunction remains to be conclusively determined.What New Information Does This Article Contribute ?: The authors leveraged a highly translational cardiac organoid model to explore the role of altered cardiac calcium handling and cardiac contractility as a common pathway leading to pathophysiological phenotypes in patients with early HCM. The MYBPC3 D389V -mediated pathological pathway is first studied here by comparing functional properties using three-dimensional cardiac organoids differentiated from hiPSC and determining the presence of hypercontraction. Our data demonstrate that faster sarcomere kinetics resulting from lower binding affinity between D389V-mutated cMyBP-C protein and myosin S2, as evidenced by in vitro studies, could cause hypercontractility which was rescued by administration of mavacamten (CAMZYOS®), a myosin inhibitor. In addition, hypercontractility causes secondary mitochondrial defects such as higher oxidative stress and lower mitochondrial membrane potential (ΔΨm), highlighting a possible early adaptive response to primary sarcomeric changes. Early treatment of MYBPC3 D389V carriers with mavacamten may prevent or reduce early HCM-related pathology. GRAPHICAL ABSTRACT: A graphical abstract is available for this article.

Cardiac myosin-binding protein C N-terminal interactions with myosin and actin filaments: Opposite effects of phosphorylation and M-domain mutations

N-terminal cardiac myosin-binding protein C (cMyBP-C) domains (C0-C2) bind to thick (myosin) and thin (actin) filaments to coordinate contraction and relaxation of the heart. These interactions are regulated by phosphorylation of the M-domain situated between domains C1 and C2. In cardiomyopathies and heart failure, phosphorylation of cMyBP-C is significantly altered. We aimed to investigate how cMyBP-C interacts with myosin and actin. We developed complementary, high-throughput, C0-C2 FRET-based binding assays for myosin and actin to characterize the effects due to 5 HCM-linked variants or functional mutations in unphosphorylated and phosphorylated C0-C2. The assays indicated that phosphorylation decreases binding to both myosin and actin, whereas the HCM mutations in M-domain generally increase binding. The effects of mutations were greatest in phosphorylated C0-C2, and some mutations had a larger effect on actin than myosin binding. Phosphorylation also altered the spatial relationship of the probes on C0-C2 and actin. The magnitude of these structural changes was dependent on C0-C2 probe location (C0, C1, or M-domain). We conclude that binding can differ between myosin and actin due to phosphorylation or mutations. Additionally, these variables can change the mode of binding, affecting which of the interactions in cMyBP-C N-terminal domains with myosin or actin take place. The opposite effects of phosphorylation and M-domain mutations is consistent with the idea that cMyBP-C phosphorylation is critical for normal cardiac function. The precision of these assays is indicative of their usefulness in high-throughput screening of drug libraries for targeting cMyBP-C as therapy.

Design Principles and Benefits of Spatially Explicit Models of Myofilament Function

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

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.

Drug discovery for heart failure targeting myosin-binding protein C

Cardiac MyBP-C (cMyBP-C) interacts with actin and myosin to fine-tune cardiac muscle contractility. Phosphorylation of cMyBP-C, which reduces the binding of cMyBP-C to actin and myosin, is often decreased in patients with heart failure (HF) and is cardioprotective in model systems of HF. Therefore, cMyBP-C is a potential target for HF drugs that mimic its phosphorylation and/or perturb its interactions with actin or myosin. We labeled actin with fluorescein-5-maleimide (FMAL) and the C0-C2 fragment of cMyBP-C (cC0-C2) with tetramethylrhodamine (TMR). We performed two complementary high-throughput screens (HTS) on an FDA-approved drug library, to discover small molecules that specifically bind to cMyBP-C and affect its interactions with actin or myosin, using fluorescence lifetime (FLT) detection. We first excited FMAL and detected its FLT, to measure changes in fluorescence resonance energy transfer (FRET) from FMAL (donor) to TMR (acceptor), indicating binding. Using the same samples, we then excited TMR directly, using a longer wavelength laser, to detect the effects of compounds on the environmentally sensitive FLT of TMR, to identify compounds that bind directly to cC0-C2. Secondary assays, performed on selected modulators with the most promising effects in the primary HTS assays, characterized the specificity of these compounds for phosphorylated versus unphosphorylated cC0-C2 and for cC0-C2 versus C1-C2 of fast skeletal muscle (fC1-C2). A subset of identified compounds modulated ATPase activity in cardiac and/or skeletal myofibrils. These assays establish the feasibility of the discovery of small-molecule modulators of the cMyBP-C-actin/myosin interaction, with the ultimate goal of developing therapies for HF.

Novel approaches to target fibroblast mechanotransduction in fibroproliferative diseases

The ability of cells to sense and respond to changes in mechanical environment is vital in conditions of organ injury when the architecture of normal tissues is disturbed or lost. Among the various cellular players that respond to injury, fibroblasts take center stage in re-establishing tissue integrity by secreting and organizing extracellular matrix into stabilizing scar tissue. Activation, activity, survival, and death of scar-forming fibroblasts are tightly controlled by mechanical environment and proper mechanotransduction ensures that fibroblast activities cease after completion of the tissue repair process. Conversely, dysregulated mechanotransduction often results in fibroblast over-activation or persistence beyond the state of normal repair. The resulting pathological accumulation of extracellular matrix is called fibrosis, a condition that has been associated with over 40% of all deaths in the industrialized countries. Consequently, elements in fibroblast mechanotransduction are scrutinized for their suitability as anti-fibrotic therapeutic targets. We review the current knowledge on mechanically relevant factors in the fibroblast extracellular environment, cell-matrix and cell-cell adhesion structures, stretch-activated membrane channels, stress-regulated cytoskeletal structures, and co-transcription factors. We critically discuss the targetability of these elements in therapeutic approaches and their progress in pre-clinical and/or clinical trials to treat organ fibrosis.

Cryo-electron tomography of intact cardiac muscle reveals myosin binding protein-C linking myosin and actin filaments

Myosin binding protein C (MyBP-C) is an accessory protein of the thick filament in vertebrate cardiac muscle arranged over 9 stripes of intervals of 430 Å in each half of the A-band in the region called the C-zone. Mutations in cardiac MyBP-C are a leading cause of hypertrophic cardiomyopathy the mechanism of which is unknown. It is a rod-shaped protein composed of 10 or 11 immunoglobulin- or fibronectin-like domains labelled C0 to C10 which binds to the thick filament via its C-terminal region. MyBP-C regulates contraction in a phosphorylation dependent fashion that may be through binding of its N-terminal domains with myosin or actin. Understanding the 3D organisation of MyBP-C in the sarcomere environment may provide new light on its function. We report here the fine structure of MyBP-C in relaxed rat cardiac muscle by cryo-electron tomography and subtomogram averaging of refrozen Tokuyasu cryosections. We find that on average MyBP-C connects via its distal end to actin across a disc perpendicular to the thick filament. The path of MyBP-C suggests that the central domains may interact with myosin heads. Surprisingly MyBP-C at Stripe 4 is different; it has weaker density than the other stripes which could result from a mainly axial or wavy path. Given that the same feature at Stripe 4 can also be found in several mammalian cardiac muscles and in some skeletal muscles, our finding may have broader implication and significance. In the D-zone, we show the first demonstration of myosin crowns arranged on a uniform 143 Å repeat.

An Update on MYBPC3 Gene Mutation in Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most prevalent genetically inherited cardiomyopathy that follows an autosomal dominant inheritance pattern. The majority of HCM cases can be attributed to mutation of the MYBPC3 gene, which encodes cMyBP-C, a crucial structural protein of the cardiac muscle. The manifestation of HCM's morphological, histological, and clinical symptoms is subject to the complex interplay of various determinants, including genetic mutation and environmental factors. Approximately half of MYBPC3 mutations give rise to truncated protein products, while the remaining mutations cause insertion/deletion, frameshift, or missense mutations of single amino acids. In addition, the onset of HCM may be attributed to disturbances in the protein and transcript quality control systems, namely, the ubiquitin-proteasome system and nonsense-mediated RNA dysfunctions. The aforementioned genetic modifications, which appear to be associated with unfavorable lifelong outcomes and are largely influenced by the type of mutation, exhibit a unique array of clinical manifestations ranging from asymptomatic to arrhythmic syncope and even sudden cardiac death. Although the current understanding of the MYBPC3 mutation does not comprehensively explain the varied phenotypic manifestations witnessed in patients with HCM, patients with pathogenic MYBPC3 mutations can exhibit an array of clinical manifestations ranging from asymptomatic to advanced heart failure and sudden cardiac death, leading to a higher rate of adverse clinical outcomes. This review focuses on MYBPC3 mutation and its characteristics as a prognostic determinant for disease onset and related clinical consequences in HCM.

cMyBP-C ablation in human engineered cardiac tissue causes progressive Ca2+-handling abnormalities

Truncation mutations in cardiac myosin binding protein C (cMyBP-C) are common causes of hypertrophic cardiomyopathy (HCM). Heterozygous carriers present with classical HCM, while homozygous carriers present with early onset HCM that rapidly progress to heart failure. We used CRISPR-Cas9 to introduce heterozygous (cMyBP-C+/-) and homozygous (cMyBP-C-/-) frame-shift mutations into MYBPC3 in human iPSCs. Cardiomyocytes derived from these isogenic lines were used to generate cardiac micropatterns and engineered cardiac tissue constructs (ECTs) that were characterized for contractile function, Ca2+-handling, and Ca2+-sensitivity. While heterozygous frame shifts did not alter cMyBP-C protein levels in 2-D cardiomyocytes, cMyBP-C+/- ECTs were haploinsufficient. cMyBP-C-/- cardiac micropatterns produced increased strain with normal Ca2+-handling. After 2 wk of culture in ECT, contractile function was similar between the three genotypes; however, Ca2+-release was slower in the setting of reduced or absent cMyBP-C. At 6 wk in ECT culture, the Ca2+-handling abnormalities became more pronounced in both cMyBP-C+/- and cMyBP-C-/- ECTs, and force production became severely depressed in cMyBP-C-/- ECTs. RNA-seq analysis revealed enrichment of differentially expressed hypertrophic, sarcomeric, Ca2+-handling, and metabolic genes in cMyBP-C+/- and cMyBP-C-/- ECTs. Our data suggest a progressive phenotype caused by cMyBP-C haploinsufficiency and ablation that initially is hypercontractile, but progresses to hypocontractility with impaired relaxation. The severity of the phenotype correlates with the amount of cMyBP-C present, with more severe earlier phenotypes observed in cMyBP-C-/- than cMyBP-C+/- ECTs. We propose that while the primary effect of cMyBP-C haploinsufficiency or ablation may relate to myosin crossbridge orientation, the observed contractile phenotype is Ca2+-mediated.

Muscle Mechanics and Thick Filament Activation: An Emerging Two-Way Interaction for the Vertebrate Striated Muscle Fine Regulation

Contraction in striated muscle is classically described as regulated by calcium-mediated structural changes in the actin-containing thin filaments, which release the binding sites for the interaction with myosin motors to produce force. In this view, myosin motors, arranged in the thick filaments, are basically always ready to interact with the thin filaments, which ultimately regulate the contraction. However, a new “dual-filament” activation paradigm is emerging, where both filaments must be activated to generate force. Growing evidence from the literature shows that the thick filament activation has a role on the striated muscle fine regulation, and its impairment is associated with severe pathologies. This review is focused on the proposed mechanical feedback that activates the inactive motors depending on the level of tension generated by the active ones, the so-called mechanosensing mechanism. Since the main muscle function is to generate mechanical work, the implications on muscle mechanics will be highlighted, showing: (i) how non-mechanical modulation of the thick filament activation influences the contraction, (ii) how the contraction influences the activation of the thick filament and (iii) how muscle, through the mechanical modulation of the thick filament activation, can regulate its own mechanics. This description highlights the crucial role of the emerging bi-directional feedback on muscle mechanical performance.

Synthetic growth hormone-releasing hormone agonist ameliorates the myocardial pathophysiology characteristic of heart failure with preserved ejection fraction

AIMS

To test the hypothesis that the activation of the growth hormone-releasing hormone (GHRH) receptor signalling pathway within the myocardium both prevents and reverses diastolic dysfunction and pathophysiologic features consistent with heart failure with preserved ejection fraction (HFpEF). Impaired myocardial relaxation, fibrosis, and ventricular stiffness, among other multi-organ morbidities, characterize the phenotype underlying the HFpEF syndrome. Despite the rapidly increasing prevalence of HFpEF, few effective therapies have emerged. Synthetic agonists of the GHRH receptors reduce myocardial fibrosis, cardiomyocyte hypertrophy, and improve performance in animal models of ischaemic cardiomyopathy, independently of the growth hormone axis.

METHODS AND RESULTS

CD1 mice received 4- or 8-week continuous infusion of angiotensin-II (Ang-II) to generate a phenotype with several features consistent with HFpEF. Mice were administered either vehicle or a potent synthetic agonist of GHRH, MR-356 for 4-weeks beginning concurrently or 4-weeks following the initiation of Ang-II infusion. Ang-II-treated animals exhibited diastolic dysfunction, ventricular hypertrophy, interstitial fibrosis, and normal ejection fraction. Cardiomyocytes isolated from these animals exhibited incomplete relaxation, depressed contractile responses, altered myofibrillar protein phosphorylation, and disturbed calcium handling mechanisms (ex vivo). MR-356 both prevented and reversed the development of the pathological phenotype in vivo and ex vivo. Activation of the GHRH receptors increased cAMP and cGMP in cardiomyocytes isolated from control animals but only cAMP in cardiac fibroblasts, suggesting that GHRH-A exert differential effects on cardiomyocytes and fibroblasts.

CONCLUSION

These findings indicate that the GHRH receptor signalling pathway(s) represents a new molecular target to counteract dysfunctional cardiomyocyte relaxation by targeting myofilament phosphorylation and fibrosis. Accordingly, activation of GHRH receptors with potent, synthetic GHRH agonists may provide a novel therapeutic approach to management of the myocardial alterations associated with the HFpEF syndrome.

Implications of S-glutathionylation of sarcomere proteins in cardiac disorders, therapies, and diagnosis

The discovery that cardiac sarcomere proteins are substrates for S-glutathionylation and that this post-translational modification correlates strongly with diastolic dysfunction led to new concepts regarding how levels of oxidative stress affect the heartbeat. Major sarcomere proteins for which there is evidence of S-glutathionylation include cardiac myosin binding protein C (cMyBP-C), actin, cardiac troponin I (cTnI) and titin. Our hypothesis is that these S-glutathionylated proteins are significant factors in acquired and familial disorders of the heart; and, when released into the serum, provide novel biomarkers. We consider the molecular mechanisms for these effects in the context of recent revelations of how these proteins control cardiac dynamics in close collaboration with Ca2+ fluxes. These revelations were made using powerful approaches and technologies that were focused on thin filaments, thick filaments, and titin filaments. Here we integrate their regulatory processes in the sarcomere as modulated mainly by neuro-humoral control of phosphorylation inasmuch evidence indicates that S-glutathionylation and protein phosphorylation, promoting increased dynamics and modifying the Frank-Starling relation, may be mutually exclusive. Earlier studies demonstrated that in addition to cTnI as a well-established biomarker for cardiac disorders, serum levels of cMyBP-C are also a biomarker for cardiac disorders. We describe recent studies approaching the question of whether serum levels of S-glutathionylated-cMyBP-C could be employed as an important clinical tool in patient stratification, early diagnosis in at risk patients before HFpEF, determination of progression, effectiveness of therapeutic approaches, and as a guide in developing future therapies.

Phosphorylation-dependent interactions of myosin-binding protein C and troponin coordinate the myofilament response to protein kinase A

PKA-mediated phosphorylation of sarcomeric proteins enhances heart muscle performance in response to β-adrenergic stimulation and is associated with accelerated relaxation and increased cardiac output for a given preload. At the cellular level, the latter translates to a greater dependence of Ca2+ sensitivity and maximum force on sarcomere length (SL), that is, enhanced length-dependent activation. However, the mechanisms by which PKA phosphorylation of the most notable sarcomeric PKA targets, troponin I (cTnI) and myosin-binding protein C (cMyBP-C), lead to these effects remain elusive. Here, we specifically altered the phosphorylation level of cTnI in heart muscle cells and characterized the structural and functional effects at different levels of background phosphorylation of cMyBP-C and with two different SLs. We found Ser22/23 bisphosphorylation of cTnI was indispensable for the enhancement of length-dependent activation by PKA, as was cMyBP-C phosphorylation. This high level of coordination between cTnI and cMyBP-C may suggest coupling between their regulatory mechanisms. Further evidence for this was provided by our finding that cardiac troponin (cTn) can directly interact with cMyBP-C in vitro, in a phosphorylation- and Ca2+-dependent manner. In addition, bisphosphorylation at Ser22/Ser23 increased Ca2+ sensitivity at long SL in the presence of endogenously phosphorylated cMyBP-C. When cMyBP-C was dephosphorylated, bisphosphorylation of cTnI increased Ca2+ sensitivity and decreased cooperativity at both SLs, which may translate to deleterious effects in physiological settings. Our results could have clinical relevance for disease pathways, where PKA phosphorylation of cTnI may be functionally uncoupled from cMyBP-C phosphorylation due to mutations or haploinsufficiency.

Cryo-Electron Microscopy Reveals Cardiac Myosin Binding Protein-C M-Domain Interactions with the Thin Filament

Cardiac myosin binding protein C (cMyBP-C) modulates cardiac contraction via direct interactions with cardiac thick (myosin) and thin (actin) filaments (cTFs). While its C-terminal domains (e.g. C8-C10) anchor cMyBP-C to the backbone of the thick filament, its N-terminal domains (NTDs) (e.g. C0, C1, M, and C2) bind to both myosin and actin to accomplish its dual roles of inhibiting thick filaments and activating cTFs. While the positions of C0, C1 and C2 on cTF have been reported, the binding site of the M-domain on the surface of the cTF is unknown. Here, we used cryo-EM to reveal that the M-domain interacts with actin via helix 3 of its ordered tri-helix bundle region, while the unstructured part of the M-domain does not maintain extensive interactions with actin. We combined the recently obtained structure of the cTF with the positions of all the four NTDs on its surface to propose a complete model of the NTD binding to the cTF. The model predicts that the interactions of the NTDs with the cTF depend on the activation state of the cTF. At the peak of systole, when bound to the extensively activated cTF, NTDs would inhibit actomyosin interactions. In contrast, at falling Ca2+ levels, NTDs would not compete with the myosin heads for binding to the cTF, but would rather promote formation of active cross-bridges at the adjacent regulatory units located at the opposite cTF strand. Our structural data provides a testable model of the cTF regulation by the cMyBP-C.

Transcriptome Profile Identifies Actin as an Essential Regulator of Cardiac Myosin Binding Protein C3 Hypertrophic Cardiomyopathy in a Zebrafish Model

Variants in cardiac myosin-binding protein C (cMyBP-C) are the leading cause of inherited hypertrophic cardiomyopathy (HCM), demonstrating the key role that cMyBP-C plays in the heart’s contractile machinery. To investigate the c-MYBPC3 HCM-related cardiac impairment, we generated a zebrafish mypbc3-knockout model. These knockout zebrafish displayed significant morphological heart alterations related to a significant decrease in ventricular and atrial diameters at systolic and diastolic states at the larval stages. Immunofluorescence staining revealed significant hyperplasia in the mutant’s total cardiac and ventricular cardiomyocytes. Although cardiac contractility was similar to the wild-type control, the ejection fraction was significantly increased in the mypbc3 mutants. At later stages of larval development, the mutants demonstrated an early cardiac phenotype of myocardium remodeling, concurrent cardiomyocyte hyperplasia, and increased ejection fraction as critical processes in HCM initiation to counteract the increased ventricular myocardial wall stress. The examination of zebrafish adults showed a thickened ventricular cardiac wall with reduced heart rate, swimming speed, and endurance ability in both the mypbc3 heterozygous and homozygous groups. Furthermore, heart transcriptome profiling showed a significant downregulation of the actin-filament-based process, indicating an impaired actin cytoskeleton organization as the main dysregulating factor associated with the early ventricular cardiac hypertrophy in the zebrafish mypbc3 HCM model.

Heterogeneous Distribution of Genetic Mutations in Myosin Binding Protein-C Paralogs

Myosin binding protein-C (MyBP-C) is a sarcomeric protein which regulates the force of contraction in striated muscles. Mutations in the MYBPC family of genes, including slow skeletal (MYBPC1), fast skeletal (MYBPC2) and cardiac (MYBPC3), can result in cardiac and skeletal myopathies. Nonetheless, their evolutionary pattern, pathogenicity and impact on MyBP-C protein structure remain to be elucidated. Therefore, the present study aimed to systematically assess the evolutionarily conserved and epigenetic patterns of MYBPC family mutations. Leveraging a machine learning (ML) approach, the Genome Aggregation Database (gnomAD) provided variants in MYBPC1, MYBPC2, and MYBPC3 genes. This was followed by an analysis with Ensembl’s variant effect predictor (VEP), resulting in the identification of 8,618, 3,871, and 3,071 variants in MYBPC1, MYBPC2, and MYBPC3, respectively. Missense variants comprised 61%–66% of total variants in which the third nucleotide positions in the codons were highly altered. Arginine was the most mutated amino acid, important because most disease-causing mutations in MyBP-C proteins are arginine in origin. Domains C5 and C6 of MyBP-C were found to be hotspots for most mutations in the MyBP-C family of proteins. A high percentage of truncated mutations in cMyBP-C cause cardiomyopathies. Arginine and glutamate were the top hits in fMyBP-C and cMyBP-C, respectively, and tryptophan and tyrosine were the most common among the three paralogs changing to premature stop codons and causing protein truncations at the carboxyl terminus. A heterogeneous epigenetic pattern was identified among the three MYBP-C paralogs. Overall, it was shown that databases using computational approaches can facilitate diagnosis and drug discovery to treat muscle disorders caused by MYBPC mutations.

Nanosurfer assay dissects β-cardiac myosin and cardiac myosin-binding protein C interactions

Cardiac myosin-binding protein C (cMyBP-C) modulates cardiac contractility through putative interactions with the myosin S2 tail and/or the thin filament. The relative contribution of these binding-partner interactions to cMyBP-C modulatory function remains unclear. Hence, we developed a "nanosurfer" assay as a model system to interrogate these cMyBP-C binding-partner interactions. Synthetic thick filaments were generated using recombinant human β-cardiac myosin subfragments (HMM or S1) attached to DNA nanotubes, with 14- or 28-nm spacing, corresponding to the 14.3-nm myosin spacing in native thick filaments. The nanosurfer assay consists of DNA nanotubes added to the in vitro motility assay so that myosins on the motility surface effectively deliver thin filaments to the DNA nanotubes, enhancing thin filament gliding probability on the DNA nanotubes. Thin filament velocities on nanotubes with either 14- or 28-nm myosin spacing were no different. We then characterized the effects of cMyBP-C on thin filament motility by alternating HMM and cMyBP-C N-terminal fragments (C0-C2 or C1-C2) on nanotubes every 14 nm. Both C0-C2 and C1-C2 reduced thin filament velocity four- to sixfold relative to HMM alone. Similar inhibition occurred using the myosin S1 construct, which lacks the myosin S2 region proposed to interact with cMyBP-C, suggesting that the cMyBP-C N terminus must interact with other myosin head domains and/or actin to slow thin filament velocity. Thin filament velocity was unaffected by the C0-C1f fragment, which lacks the majority of the M-domain, supporting the importance of this domain for inhibitory interaction(s). A C0-C2 fragment with phospho-mimetic replacement in the M-domain showed markedly less inhibition of thin filament velocity compared with its phospho-null counterpart, highlighting the modulatory role of M-domain phosphorylation on cMyBP-C function. Therefore, the nanosurfer assay provides a platform to precisely manipulate spatially dependent cMyBP-C binding-partner interactions, shedding light on the molecular regulation of β-cardiac myosin contractility.

Human cardiac myosin-binding protein C phosphorylation- and mutation-dependent structural dynamics monitored by time-resolved FRET

Cardiac myosin-binding protein C (cMyBP-C) is a thick filament-associated protein of the sarcomere and a potential therapeutic target for treating contractile dysfunction in heart failure. Mimicking the structural dynamics of phosphorylated cMyBP-C by small-molecule drug binding could lead to therapies that modulate cMyBP-C conformational states, and thereby function, to improve contractility. We have developed a human cMyBP-C biosensor capable of detecting intramolecular structural changes due to phosphorylation and mutation. Using site-directed mutagenesis and time-resolved fluorescence resonance energy transfer (TR-FRET), we substituted cysteines in cMyBP-C N-terminal domains C0 through C2 (C0-C2) for thiol-reactive fluorescent probe labeling to examine C0-C2 structure. We identified a cysteine pair that upon donor-acceptor labeling reports phosphorylation-sensitive structural changes between the C1 domain and the tri-helix bundle of the M-domain that links C1 to C2. Phosphorylation reduced FRET efficiency by ~18%, corresponding to a ~11% increase in the distance between probes and a ~30% increase in disorder between them. The magnitude and precision of phosphorylation-mediated TR-FRET changes, as quantified by the Z'-factor, demonstrate the assay's potential for structure-based high-throughput screening of compounds for cMyBP-C-targeted therapies to improve cardiac performance in heart failure. Additionally, by probing C1's spatial positioning relative to the tri-helix bundle, these findings provide new molecular insight into the structural dynamics of phosphoregulation as well as mutations in cMyBP-C. Biosensor sensitivity to disease-relevant mutations in C0-C2 was demonstrated by examination of the hypertrophic cardiomyopathy mutation R282W. The results presented here support a screening platform to identify small molecules that regulate N-terminal cMyBP-C conformational states.

Small Angle X-ray Diffraction as a Tool for Structural Characterization of Muscle Disease

Small angle X-ray fiber diffraction is the method of choice for obtaining molecular level structural information from striated muscle fibers under hydrated physiological conditions. For many decades this technique had been used primarily for investigating basic biophysical questions regarding muscle contraction and regulation and its use confined to a relatively small group of expert practitioners. Over the last 20 years, however, X-ray diffraction has emerged as an important tool for investigating the structural consequences of cardiac and skeletal myopathies. In this review we show how simple and straightforward measurements, accessible to non-experts, can be used to extract biophysical parameters that can help explain and characterize the physiology and pathology of a given experimental system. We provide a comprehensive guide to the range of the kinds of measurements that can be made and illustrate how they have been used to provide insights into the structural basis of pathology in a comprehensive review of the literature. We also show how these kinds of measurements can inform current controversies and indicate some future directions.

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.

Myofilament glycation in diabetes reduces contractility by inhibiting tropomyosin movement, is rescued by cMyBPC domains

Diabetes doubles the risk of developing heart failure (HF). As the prevalence of diabetes grows, so will HF unless the mechanisms connecting these diseases can be identified. Methylglyoxal (MG) is a glycolysis by-product that forms irreversible modifications on lysine and arginine, called glycation. We previously found that myofilament MG glycation causes sarcomere contractile dysfunction and is increased in patients with diabetes and HF. The aim of this study was to discover the molecular mechanisms by which MG glycation of myofilament proteins cause sarcomere dysfunction and to identify therapeutic avenues to compensate. In humans with type 2 diabetes without HF, we found increased glycation of sarcomeric actin compared to non-diabetics and it correlated with decreased calcium sensitivity. Depressed calcium sensitivity is pathogenic for HF, therefore myofilament glycation represents a promising therapeutic target to inhibit the development of HF in diabetics. To identify possible therapeutic targets, we further defined the molecular actions of myofilament glycation. Skinned myocytes exposed to 100 μM MG exhibited decreased calcium sensitivity, maximal calcium-activated force, and crossbridge kinetics. Replicating MG's functional affects using a computer simulation of sarcomere function predicted simultaneous decreases in tropomyosin's blocked-to-closed rate transition and crossbridge duty cycle were consistent with all experimental findings. Stopped-flow experiments and ATPase activity confirmed MG decreased the blocked-to-closed transition rate. Currently, no therapeutics target tropomyosin, so as proof-of-principal, we used a n-terminal peptide of myosin-binding protein C, previously shown to alter tropomyosin's position on actin. C0C2 completely rescued MG-induced calcium desensitization, suggesting a possible treatment for diabetic HF.

Proteomic analysis of the heart in normal aging mice

Aging induces pathological cardiovascular changes such as cardiac dysfunction and arteriosclerosis. With aging, heart cells, especially, become more susceptible to lethal damage. In this report, we tried to understand the precise mechanism of myocardial change resulting from aging by examining the heart proteome in aging mice using two-dimensional gel electrophoresis (2DE). The proteins were stained with fluorescence dyes (SYPRO Ruby and Pro-Q Diamond) and identified by subsequent MALDI-TOF-MS / MS. As a result, markedly altered levels of 14 proteins and 7 phosphoproteins were detected in the hearts of 3-, 7-, 11-, and 20-month-old mice. The functions of these identified proteins and phosphoproteins were energy metabolism, muscle contraction, glycolysis, and cytoskeletal support. Additionally, the results of Western blotting confirmed changes in the expression of FTH, CPNE5, and SUCLA2. These findings showed that aging modified the expression of proteins and phosphoproteins in the heart. We suggest that changes in the expression of these proteins are critical to the development of cardiac dysfunction resulting from aging. J. Med. Invest. 69 : 217-223, August, 2022.

Microscale thermophoresis suggests a new model of regulation of cardiac myosin function via interaction with cardiac myosin-binding protein C

The cardiac isoform of myosin-binding protein C (cMyBP-C) is a key regulatory protein found in cardiac myofilaments that can control the activation state of both the actin-containing thin and myosin-containing thick filaments. However, in contrast to thin filament–based mechanisms of regulation, the mechanism of myosin-based regulation by cMyBP-C has yet to be defined in detail. To clarify its function in this process, we used microscale thermophoresis to build an extensive interaction map between cMyBP-C and isolated fragments of β-cardiac myosin. We show here that the regulatory N-terminal domains (C0C2) of cMyBP-C interact with both the myosin head (myosin S1) and tail domains (myosin S2) with micromolar affinity via phosphorylation-independent and phosphorylation-dependent interactions of domain C1 and the cardiac-specific m-motif, respectively. Moreover, we show that the interaction sites with the highest affinity between cMyBP-C and myosin S1 are localized to its central domains, which bind myosin with submicromolar affinity. We identified two separate interaction regions in the central C2C4 and C5C7 segments that compete for the same binding site on myosin S1, suggesting that cMyBP-C can crosslink the two myosin heads of a single myosin molecule and thereby stabilize it in the folded OFF state. Phosphorylation of the cardiac-specific m-motif by protein kinase A had no effect on the binding of either the N-terminal or the central segments to the myosin head domain, suggesting this might therefore represent a constitutively bound state of myosin associated with cMyBP-C. Based on our results, we propose a new model of regulation of cardiac myosin function by cMyBP-C.

Assessment of the Contribution of a Thermodynamic and Mechanical Destabilization of Myosin-Binding Protein C Domain C2 to the Pathomechanism of Hypertrophic Cardiomyopathy-Causing Double Mutation MYBPC3Δ25bp/D389V

Mutations in the gene encoding cardiac myosin-binding protein-C (MyBPC), a thick filament assembly protein that stabilizes sarcomeric structure and regulates cardiac function, are a common cause for the development of hypertrophic cardiomyopathy. About 10% of carriers of the Δ25bp variant of MYBPC3, which is common in individuals from South Asia, are also carriers of the D389V variant on the same allele. Compared with noncarriers and those with MYBPC3^Δ25bp alone, indicators for the development of hypertrophic cardiomyopathy occur with increased frequency in MYBPC3^Δ25bp/D389V carriers. Residue D389 lies in the IgI-like C2 domain that is part of the N-terminal region of MyBPC. To probe the effects of mutation D389V on structure, thermostability, and protein–protein interactions, we produced and characterized wild-type and mutant constructs corresponding to the isolated 10 kDa C2 domain and a 52 kDa N-terminal fragment that includes subdomains C0 to C2. Our results show marked reductions in the melting temperatures of D389V mutant constructs. Interactions of construct C0–C2 D389V with the cardiac isoforms of myosin-2 and actin remain unchanged. Molecular dynamics simulations reveal changes in the stiffness and conformer dynamics of domain C2 caused by mutation D389V. Our results suggest a pathomechanism for the development of HCM based on the toxic buildup of misfolded protein in young MYBPC3^Δ25bp/D389V carriers that is supplanted and enhanced by C-zone haploinsufficiency at older ages.

Myosin motors that cannot bind actin leave their folded OFF state on activation of skeletal muscle

The myosin motors in resting skeletal muscle are folded back against their tails in the thick filament in a conformation that makes them unavailable for binding to actin. When muscles are activated, calcium binding to troponin leads to a rapid change in the structure of the actin-containing thin filaments that uncovers the myosin binding sites on actin. Almost as quickly, myosin motors leave the folded state and move away from the surface of the thick filament. To test whether motor unfolding is triggered by the availability of nearby actin binding sites, we measured changes in the x-ray reflections that report motor conformation when muscles are activated at longer sarcomere length, so that part of the thick filaments no longer overlaps with thin filaments. We found that the intensity of the M3 reflection from the axial repeat of the motors along the thick filaments declines almost linearly with increasing sarcomere length up to 2.8 µm, as expected if motors in the nonoverlap zone had left the folded state and become relatively disordered. In a recent article in JGP, Squire and Knupp challenged this interpretation of the data. We show here that their analysis is based on an incorrect assumption about how the interference subpeaks of the M3 reflection were reported in our previous paper. We extend previous models of mass distribution along the filaments to show that the sarcomere length dependence of the M3 reflection is consistent with <10% of no-overlap motors remaining in the folded conformation during active contraction, confirming our previous conclusion that unfolding of myosin motors on muscle activation is not due to the availability of local actin binding sites.

Cardiac myosin contraction and mechanotransduction in health and disease

Cardiac myosin is the molecular motor that powers heart contraction by converting chemical energy from ATP hydrolysis into mechanical force. The power output of the heart is tightly regulated to meet the physiological needs of the body. Recent multiscale studies spanning from molecules to tissues have revealed complex regulatory mechanisms that fine-tune cardiac contraction, in which myosin not only generates power output but also plays an active role in its regulation. Thus, myosin is both shaped by and actively involved in shaping its mechanical environment. Moreover, these studies have shown that cardiac myosin-generated tension affects physiological processes beyond muscle contraction. Here, we review these novel regulatory mechanisms, as well as the roles that myosin-based force generation and mechanotransduction play in development and disease. We describe how key intra- and intermolecular interactions contribute to the regulation of myosin-based contractility and the role of mechanical forces in tuning myosin function. We also discuss the emergence of cardiac myosin as a drug target for diseases including heart failure, leading to the discovery of therapeutics that directly tune myosin contractility. Finally, we highlight some of the outstanding questions that must be addressed to better understand myosin's functions and regulation, and we discuss prospects for translating these discoveries into precision medicine therapeutics targeting contractility and mechanotransduction.

Interaction of the C2 Ig-like Domain of Cardiac Myosin Binding Protein-C with F-actin

Cardiac muscle contraction depends on interactions between thick (myosin) and thin (actin) filaments (TFs). TFs are regulated by intracellular Ca2+ levels. Under activating conditions Ca2+ binds to the troponin complex and displaces tropomyosin from myosin binding sites on the TF surface to allow actomyosin interactions. Recent studies have shown that in addition to Ca2+, the first four N-terminal domains (NTDs) of cardiac myosin binding protein C (cMyBP-C) (e.g. C0, C1, M and C2), are potent modulators of the TF activity, but the mechanism of their collective action is poorly understood. Previously, we showed that C1 activates the TF at low Ca2+ and C0 stabilizes binding of C1 to the TF, but the ability of C2 to bind and/or affect the TF remains unknown. Here we obtained 7.5 Å resolution cryo-EM reconstruction of C2-decorated actin filaments to demonstrate that C2 binds to actin in a single structural mode that does not activate the TF unlike the polymorphic binding of C0 and C1 to actin. Comparison of amino acid sequences of C2 with either C0 or C1 shows low levels of identity between the residues involved in interactions with the TF but high levels of conservation for residues involved in Ig fold stabilization. This provides a structural basis for strikingly different interactions of structurally homologous C0, C1 and C2 with the TF. Our detailed analysis of the interaction of C2 with the actin filament provides crucial information required to model the collective action of cMyBP-C NTDs on the cardiac TF.

Amino terminus of cardiac myosin binding protein-C regulates cardiac contractility

Phosphorylation of cardiac myosin binding protein-C (cMyBP-C) regulates cardiac contraction through modulation of actomyosin interactions mediated by the protein's amino terminal (N')-region (C0-C2 domains, 358 amino acids). On the other hand, dephosphorylation of cMyBP-C during myocardial injury results in cleavage of the 271 amino acid C0-C1f region and subsequent contractile dysfunction. Yet, our current understanding of amino terminus region of cMyBP-C in the context of regulating thin and thick filament interactions is limited. A novel cardiac-specific transgenic mouse model expressing cMyBP-C, but lacking its C0-C1f region (cMyBP-C∆C0-C1f), displayed dilated cardiomyopathy, underscoring the importance of the N'-region in cMyBP-C. Further exploring the molecular basis for this cardiomyopathy, in vitro studies revealed increased interfilament lattice spacing and rate of tension redevelopment, as well as faster actin-filament sliding velocity within the C-zone of the transgenic sarcomere. Moreover, phosphorylation of the unablated phosphoregulatory sites was increased, likely contributing to normal sarcomere morphology and myoarchitecture. These results led us to hypothesize that restoration of the N'-region of cMyBP-C would return actomyosin interaction to its steady state. Accordingly, we administered recombinant C0-C2 (rC0-C2) to permeabilized cardiomyocytes from transgenic, cMyBP-C null, and human heart failure biopsies, and we found that normal regulation of actomyosin interaction and contractility was restored. Overall, these data provide a unique picture of selective perturbations of the cardiac sarcomere that either lead to injury or adaptation to injury in the myocardium.

Cardiac myosin-binding protein C interaction with actin is inhibited by compounds identified in a high-throughput fluorescence lifetime screen

Cardiac myosin-binding protein C (cMyBP-C) interacts with actin and myosin to modulate cardiac muscle contractility. These interactions are disfavored by cMyBP-C phosphorylation. Heart failure patients often display decreased cMyBP-C phosphorylation, and phosphorylation in model systems has been shown to be cardioprotective against heart failure. Therefore, cMyBP-C is a potential target for heart failure drugs that mimic phosphorylation or perturb its interactions with actin/myosin. Here we have used a novel fluorescence lifetime-based assay to identify small-molecule inhibitors of actin-cMyBP-C binding. Actin was labeled with a fluorescent dye (Alexa Fluor 568, AF568) near its cMyBP-C binding sites; when combined with the cMyBP-C N-terminal fragment, C0-C2, the fluorescence lifetime of AF568-actin decreases. Using this reduction in lifetime as a readout of actin binding, a high-throughput screen of a 1280-compound library identified three reproducible hit compounds (suramin, NF023, and aurintricarboxylic acid) that reduced C0-C2 binding to actin in the micromolar range. Binding of phosphorylated C0-C2 was also blocked by these compounds. That they specifically block binding was confirmed by an actin-C0-C2 time-resolved FRET (TR-FRET) binding assay. Isothermal titration calorimetry (ITC) and transient phosphorescence anisotropy (TPA) confirmed that these compounds bind to cMyBP-C, but not to actin. TPA results were also consistent with these compounds inhibiting C0-C2 binding to actin. We conclude that the actin-cMyBP-C fluorescence lifetime assay permits detection of pharmacologically active compounds that affect cMyBP-C-actin binding. We now have, for the first time, a validated high-throughput screen focused on cMyBP-C, a regulator of cardiac muscle contractility and known key factor in heart failure.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

Two groundbreaking papers published in 1954 laid out the theory of the mechanism of muscle contraction based on force-generating interactions between myofilaments in the sarcomere that cause filaments to slide past one another during muscle contraction. The succeeding decades of research in muscle physiology have revealed a unifying interest: to understand the multiscale processes-from atom to organ-that govern muscle function. Such an understanding would have profound consequences for a vast array of applications, from developing new biomimetic technologies to treating heart disease. However, connecting structural and functional properties that are relevant at one spatiotemporal scale to those that are relevant at other scales remains a great challenge. Through a lens of multiscale dynamics, we review in this article current and historical research in muscle physiology sparked by the sliding filament theory.

Cardiac myosin binding protein-C phosphorylation accelerates β-cardiac myosin detachment rate in mouse myocardium

Cardiac myosin binding protein-C (cMyBP-C) is a thick filament protein that influences sarcomere stiffness and modulates cardiac contraction-relaxation through its phosphorylation. Phosphorylation of cMyBP-C and ablation of cMyBP-C have been shown to increase the rate of MgADP release in the acto-myosin cross-bridge cycle in the intact sarcomere. The influence of cMyBP-C on Pi-dependent myosin kinetics has not yet been examined. We investigated the effect of cMyBP-C, and its phosphorylation, on myosin kinetics in demembranated papillary muscle strips bearing the β-cardiac myosin isoform from nontransgenic and homozygous transgenic mice lacking cMyBP-C. We used quick stretch and stochastic length-perturbation analysis to characterize rates of myosin detachment and force development over 0-12 mM Pi and at maximal (pCa 4.8) and near-half maximal (pCa 5.75) Ca2+ activation. Protein kinase A (PKA) treatment was applied to half the strips to probe the effect of cMyBP-C phosphorylation on Pi sensitivity of myosin kinetics. Increasing Pi increased myosin cross-bridge detachment rate similarly for muscles with and without cMyBP-C, although these rates were higher in muscle without cMyBP-C. Treating myocardial strips with PKA accelerated detachment rate when cMyBP-C was present over all Pi, but not when cMyBP-C was absent. The rate of force development increased with Pi in all muscles. However, Pi sensitivity of the rate force development was reduced when cMyBP-C was present versus absent, suggesting that cMyBP-C inhibits Pi-dependent reversal of the power stroke or stabilizes cross-bridge attachment to enhance the probability of completing the power stroke. These results support a functional role for cMyBP-C in slowing myosin detachment rate, possibly through a direct interaction with myosin or by altering strain-dependent myosin detachment via cMyBP-C-dependent stiffness of the thick filament and myofilament lattice. PKA treatment reduces the role for cMyBP-C to slow myosin detachment and thus effectively accelerates β-myosin detachment in the intact myofilament lattice.NEW & NOTEWORTHY Length perturbation analysis was used to demonstrate that β-cardiac myosin characteristic rates of detachment and recruitment in the intact myofilament lattice are accelerated by Pi, phosphorylation of cMyBP-C, and the absence of cMyBP-C. The results suggest that cMyBP-C normally slows myosin detachment, including Pi-dependent detachment, and that this inhibition is released with phosphorylation or absence of cMyBP-C.

Stress-dependent activation of myosin in the heart requires thin filament activation and thick filament mechanosensing

The efficiency of the heart as a pump depends on an autoregulatory mechanism, the Frank–Starling law of the heart, that potentiates the strength of contraction in response to an increase in ventricular filling. Disruption of this mechanism compromises the ability of the heart to pump blood, potentially leading to heart failure. We used fluorescent probes on myosin in heart muscle cells to investigate the molecular basis of the Frank–Starling mechanism. Our results show that the stronger contraction of heart muscle at longer lengths is due to a calcium-dependent interfilament signaling pathway that links stress sensing in the myosin-containing filaments with calcium activation of the actin-containing filaments. This pathway can potentially be targeted for treating heart failure.

Myosin-based regulation in the heart muscle modulates the number of myosin motors available for interaction with calcium-regulated thin filaments, but the signaling pathways mediating the stronger contraction triggered by stretch between heartbeats or by phosphorylation of the myosin regulatory light chain (RLC) remain unclear. Here, we used RLC probes in demembranated cardiac trabeculae to investigate the molecular structural basis of these regulatory pathways. We show that in relaxed trabeculae at near-physiological temperature and filament lattice spacing, the RLC-lobe orientations are consistent with a subset of myosin motors being folded onto the filament surface in the interacting-heads motif seen in isolated filaments. The folded conformation of myosin is disrupted by cooling relaxed trabeculae, similar to the effect induced by maximal calcium activation. Stretch or increased RLC phosphorylation in the physiological range have almost no effect on RLC conformation at a calcium concentration corresponding to that between beats. These results indicate that in near-physiological conditions, the folded myosin motors are not directly switched on by RLC phosphorylation or by the titin-based passive tension at longer sarcomere lengths in the absence of thin filament activation. However, at the higher calcium concentrations that activate the thin filaments, stretch produces a delayed activation of folded myosin motors and force increase that is potentiated by RLC phosphorylation. We conclude that the increased contractility of the heart induced by RLC phosphorylation and stretch can be explained by a calcium-dependent interfilament signaling pathway involving both thin filament sensitization and thick filament mechanosensing.

Contraction-relaxation coupling is unaltered by exercise training and infarction in isolated canine myocardium

The two main phases of the mammalian cardiac cycle are contraction and relaxation; however, whether there is a connection between them in humans is not well understood. Routine exercise has been shown to improve cardiac function, morphology, and molecular signatures. Likewise, the acute and chronic changes that occur in the heart in response to injury, disease, and stress are well characterized, albeit not fully understood. In this study, we investigated how exercise and myocardial injury affect contraction–relaxation coupling. We retrospectively analyzed the correlation between the maximal speed of contraction and the maximal speed of relaxation of canine myocardium after receiving surgically induced myocardial infarction, followed by either sedentary recovery or exercise training for 10–12 wk. We used isolated right ventricular trabeculae, which were electrically paced at different lengths, frequencies, and with increasing β-adrenoceptor stimulation. In all conditions, contraction and relaxation were linearly correlated, irrespective of injury or training history. Based on these results and the available literature, we posit that contraction–relaxation coupling is a fundamental myocardial property that resides in the structural arrangement of proteins at the level of the sarcomere and that this may be regulated by the actions of cardiac myosin binding protein C (cMyBP-C) on actin and myosin.

Complexity in genetic cardiomyopathies and new approaches for mechanism-based precision medicine

Genetic cardiomyopathies have been studied for decades, and it has become increasingly clear that these progressive diseases are more complex than originally thought. These complexities can be seen both in the molecular etiologies of these disorders and in the clinical phenotypes observed in patients. While these disorders can be caused by mutations in cardiac genes, including ones encoding sarcomeric proteins, the disease presentation varies depending on the patient mutation, where mutations even within the same gene can cause divergent phenotypes. Moreover, it is challenging to connect the mutation-induced molecular insult that drives the disease pathogenesis with the various compensatory and maladaptive pathways that are activated during the course of the subsequent progressive, pathogenic cardiac remodeling. These inherent complexities have frustrated our ability to understand and develop broadly effective treatments for these disorders. It has been proposed that it might be possible to improve patient outcomes by adopting a precision medicine approach. Here, we lay out a practical framework for such an approach, where patient subpopulations are binned based on common underlying biophysical mechanisms that drive the molecular disease pathogenesis, and we propose that this function-based approach will enable the development of targeted therapeutics that ameliorate these effects. We highlight several mutations to illustrate the need for mechanistic molecular experiments that span organizational and temporal scales, and we describe recent advances in the development of novel therapeutics based on functional targets. Finally, we describe many of the outstanding questions for the field and how fundamental mechanistic studies, informed by our more nuanced understanding of the clinical disorders, will play a central role in realizing the potential of precision medicine for genetic cardiomyopathies.

The N terminus of myosin-binding protein C extends toward actin filaments in intact cardiac muscle

Myosin and actin filaments are highly organized within muscle sarcomeres. Myosin-binding protein C (MyBP-C) is a flexible, rod-like protein located within the C-zone of the sarcomere. The C-terminal domain of MyBP-C is tethered to the myosin filament backbone, and the N-terminal domains are postulated to interact with actin and/or the myosin head to modulate filament sliding. To define where the N-terminal domains of MyBP-C are localized in the sarcomere of active and relaxed mouse myocardium, the relative positions of the N terminus of MyBP-C and actin were imaged in fixed muscle samples using super-resolution fluorescence microscopy. The resolution of the imaging was enhanced by particle averaging. The images demonstrate that the position of the N terminus of MyBP-C is biased toward the actin filaments in both active and relaxed muscle preparations. Comparison of the experimental images with images generated in silico, accounting for known binding partner interactions, suggests that the N-terminal domains of MyBP-C may bind to actin and possibly the myosin head but only when the myosin head is in the proximity of an actin filament. These physiologically relevant images help define the molecular mechanism by which the N-terminal domains of MyBP-C may search for, and capture, molecular binding partners to tune cardiac contractility.

Making waves: A proposed new role for myosin-binding protein C in regulating oscillatory contractions in vertebrate striated muscle

Myosin-binding protein C (MyBP-C) is a critical regulator of muscle performance that was first identified through its strong binding interactions with myosin, the force-generating protein of muscle. Almost simultaneously with its discovery, MyBP-C was soon found to bind to actin, the physiological catalyst for myosin's activity. However, the two observations posed an apparent paradox, in part because interactions of MyBP-C with myosin were on the thick filament, whereas MyBP-C interactions with actin were on the thin filament. Despite the intervening decades since these initial discoveries, it is only recently that the dual binding modes of MyBP-C are becoming reconciled in models that place MyBP-C at a central position between actin and myosin, where MyBP-C alternately stabilizes a newly discovered super-relaxed state (SRX) of myosin on thick filaments in resting muscle and then prolongs the "on" state of actin on thin filaments in active muscle. Recognition of these dual, alternating functions of MyBP-C reveals how it is central to the regulation of both muscle contraction and relaxation. The purpose of this Viewpoint is to briefly summarize the roles of MyBP-C in binding to myosin and actin and then to highlight a possible new role for MyBP-C in inducing and damping oscillatory waves of contraction and relaxation. Because the contractile waves bear similarity to cycles of contraction and relaxation in insect flight muscles, which evolved for fast, energetically efficient contraction, the ability of MyBP-C to damp so-called spontaneous oscillatory contractions (SPOCs) has broad implications for previously unrecognized regulatory mechanisms in vertebrate striated muscle. While the molecular mechanisms by which MyBP-C can function as a wave maker or a wave breaker are just beginning to be explored, it is likely that MyBP-C dual interactions with both myosin and actin will continue to be important for understanding the new functions of this enigmatic protein.

In the eye of the STORM: Tracking the myosin-binding protein C N terminus in heart muscle

Colson discusses a recent investigation of the localization of N-terminal myosin-binding protein C in cardiac muscle.

A high-throughput fluorescence lifetime-based assay to detect binding of myosin-binding protein C to F-actin

Binding properties of actin-binding proteins are typically evaluated by cosedimentation assays. However, this method is time-consuming, involves multiple steps, and has a limited throughput. These shortcomings preclude its use in screening for drugs that modulate actin-binding proteins relevant to human disease. To develop a simple, quantitative, and scalable F-actin-binding assay, we attached fluorescent probes to actin's Cys-374 and assessed changes in fluorescence lifetime upon binding to the N-terminal region (domains C0-C2) of human cardiac myosin-binding protein C (cMyBP-C). The lifetime of all five probes tested decreased upon incubation with cMyBP-C C0-C2, as measured by time-resolved fluorescence (TR-F), with IAEDANS being the most sensitive probe that yielded the smallest errors. The TR-F assay was compared with cosedimentation to evaluate in vitro changes in binding to actin and actin-tropomyosin arising from cMyBP-C mutations associated with hypertrophic cardiomyopathy (HCM) and tropomyosin binding. Lifetime changes of labeled actin with added C0-C2 were consistent with cosedimentation results. The HCM mutation L352P was confirmed to enhance actin binding, whereas PKA phosphorylation reduced binding. The HCM mutation R282W, predicted to disrupt a PKA recognition sequence, led to deficits in C0-C2 phosphorylation and altered binding. Lastly, C0-C2 binding was found to be enhanced by tropomyosin and binding capacity to be altered by mutations in a tropomyosin-binding region. These findings suggest that the TR-F assay is suitable for rapidly and accurately determining quantitative binding and for screening physiological conditions and compounds that affect cMyBP-C binding to F-actin for therapeutic discovery.

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.

Myosin Binding Protein-C Forms Amyloid-Like Aggregates In Vitro

This work investigated in vitro aggregation and amyloid properties of skeletal myosin binding protein-C (sMyBP-C) interacting in vivo with proteins of thick and thin filaments in the sarcomeric A-disc. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) found a rapid (5–10 min) formation of large (>2 μm) aggregates. sMyBP-C oligomers formed both at the initial 5–10 min and after 16 h of aggregation. Small angle X-ray scattering (SAXS) and DLS revealed sMyBP-C oligomers to consist of 7–10 monomers. TEM and atomic force microscopy (AFM) showed sMyBP-C to form amorphous aggregates (and, to a lesser degree, fibrillar structures) exhibiting no toxicity on cell culture. X-ray diffraction of sMyBP-C aggregates registered reflections attributed to a cross-β quaternary structure. Circular dichroism (CD) showed the formation of the amyloid-like structure to occur without changes in the sMyBP-C secondary structure. The obtained results indicating a high in vitro aggregability of sMyBP-C are, apparently, a consequence of structural features of the domain organization of proteins of this family. Formation of pathological amyloid or amyloid-like sMyBP-C aggregates in vivo is little probable due to amino-acid sequence low identity (<26%), alternating ordered/disordered regions in the protein molecule, and S–S bonds providing for general stability.

Biomarkers for Heart Failure Prognosis: Proteins, Genetic Scores and Non-coding RNAs

Heart failure (HF) is a complex disease in which cardiomyocyte injury leads to a cascade of inflammatory and fibrosis pathway activation, thereby causing decrease in cardiac function. As a result, several biomolecules are released which can be identified easily in circulating body fluids. The complex biological processes involved in the development and worsening of HF require an early treatment strategy to stop deterioration of cardiac function. Circulating biomarkers provide not only an ideal platform to detect subclinical changes, their clinical application also offers the opportunity to monitor disease treatment. Many of these biomarkers can be quantified with high sensitivity; allowing their clinical application to be evaluated beyond diagnostic purposes as potential tools for HF prognosis. Though the field of biomarkers is dominated by protein molecules, non-coding RNAs (microRNAs, long non-coding RNAs, and circular RNAs) are novel and promising biomarker candidates that encompass several ideal characteristics required in the biomarker field. The application of genetic biomarkers as genetic risk scores in disease prognosis, albeit in its infancy, holds promise to improve disease risk estimation. Despite the multitude of biomarkers that have been available and identified, the majority of novel biomarker candidates are not cardiac-specific, and instead may simply be a readout of systemic inflammation or other pathological processes. Thus, the true value of novel biomarker candidates in HF prognostication remains unclear. In this article, we discuss the current state of application of protein, genetic as well as non-coding RNA biomarkers in HF risk prognosis.

AAV9 gene transfer of cMyBPC N-terminal domains ameliorates cardiomyopathy in cMyBPC-deficient mice

Decreased cardiac myosin-binding protein C (cMyBPC) expression due to inheritable mutations is thought to contribute to the hypertrophic cardiomyopathy (HCM) phenotype, suggesting that increasing cMyBPC content is of therapeutic benefit. In vitro assays show that cMyBPC N-terminal domains (NTDs) contain structural elements necessary and sufficient to modulate actomyosin interactions, but it is unknown if they can regulate in vivo myocardial function. To test whether NTDs can recapitulate the effects of full-length (FL) cMyBPC in rescuing cardiac function in a cMyBPC-null mouse model of HCM, we assessed the efficacy of AAV9 gene transfer of a cMyBPC NTD that contained domains C0C2 and compared its therapeutic potential with AAV9-FL gene replacement. AAV9 vectors were administered systemically at neonatal day 1, when early-onset disease phenotypes begin to manifest. A comprehensive analysis of in vivo and in vitro function was performed following cMyBPC gene transfer. Our results show that a systemic injection of AAV9-C0C2 significantly improved cardiac function (e.g., 52.24 ± 1.69 ejection fraction in the C0C2-treated group compared with 40.07 ± 1.97 in the control cMyBPC^–/– group, P < 0.05) and reduced the histopathologic signs of cardiomyopathy. Furthermore, C0C2 significantly slowed and normalized the accelerated cross-bridge kinetics found in cMyBPC^–/– control myocardium, as evidenced by a 32.41% decrease in the rate of cross-bridge detachment (k_rel). Results indicate that C0C2 can rescue biomechanical defects of cMyBPC deficiency and that the NTD may be a target region for therapeutic myofilament kinetic manipulation.

Cardiac function improves following AAV9-mediated delivery of the C0C2 domains of cardiac myosin-binding protein C in a mouse model of hypertrophic cardiomyopathy.

Dilated cardiomyopathy mutations in thin-filament regulatory proteins reduce contractility, suppress systolic Ca2+, and activate NFAT and Akt signaling

Dilated cardiomyopathy (DCM) is clinically characterized by dilated ventricular cavities and reduced ejection fraction, leading to heart failure and increased thromboembolic risk. Mutations in thin-filament regulatory proteins can cause DCM and have been shown in vitro to reduce contractility and myofilament Ca²⁺-affinity. In this work we have studied the functional consequences of mutations in cardiac troponin T (R131W), cardiac troponin I (K36Q) and α-tropomyosin (E40K) using adenovirally transduced isolated guinea pig left ventricular cardiomyocytes. We find significantly reduced fractional shortening with reduced systolic Ca²⁺. Contraction and Ca²⁺ reuptake times were slowed, which contrast with some findings in murine models of myofilament Ca²⁺ desensitization. We also observe increased sarcoplasmic reticulum (SR) Ca²⁺ load and smaller fractional SR Ca²⁺ release. This corresponds to a reduction in SR Ca²⁺-ATPase activity and increase in sodium-calcium exchanger activity. We also observe dephosphorylation and nuclear translocation of the nuclear factor of activated T cells (NFAT), with concordant RAC-α-serine/threonine protein kinase (Akt) phosphorylation but no change to extracellular signal-regulated kinase activation in chronically paced cardiomyocytes expressing DCM mutations. These changes in Ca²⁺ handling and signaling are common to all three mutations, indicating an analogous pathway of disease pathogenesis in thin-filament sarcomeric DCM. Previous work has shown that changes to myofilament Ca²⁺ sensitivity caused by DCM mutations are qualitatively opposite from hypertrophic cardiomyopathy (HCM) mutations in the same genes. However, we find several common pathways such as increased relaxation times and NFAT activation that are also hallmarks of HCM. This suggests more complex intracellular signaling underpinning DCM, driven by the primary mutation.NEW & NOTEWORTHY Dilated cardiomyopathy (DCM) is a frequently occurring cardiac disorder with a degree of genetic inheritance. We have found that DCM mutations in proteins that regulate the contractile machinery cause alterations to contraction, calcium-handling, and some new signaling pathways that provide stimuli for disease development. We have used guinea pig cells that recapitulate human calcium-handling and introduced the mutations using adenovirus gene transduction to look at the initial triggers of disease before remodeling.

Genetic, clinical, molecular, and pathogenic aspects of the South Asian-specific polymorphic MYBPC3Δ25bp variant

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by ventricular enlargement, diastolic dysfunction, and increased risk for sudden cardiac death. Sarcomeric genetic defects are the predominant known cause of HCM. In particular, mutations in the myosin-binding protein C gene (MYBPC3) are associated with ~ 40% of all HCM cases in which a genetic basis has been established. A decade ago, our group reported a 25-base pair deletion in intron 32 of MYBPC3 (MYBPC3Δ25bp) that is uniquely prevalent in South Asians and is associated with autosomal dominant cardiomyopathy. Although our studies suggest that this deletion results in left ventricular dysfunction, cardiomyopathies, and heart failure, the precise mechanism by which this variant predisposes to heart disease remains unclear. Increasingly appreciated, however, is the contribution of secondary risk factors, additional mutations, and lifestyle choices in augmenting or modifying the HCM phenotype in MYBPC3Δ25bp carriers. Therefore, the goal of this review article is to summarize the current research dedicated to understanding the molecular pathophysiology of HCM in South Asians with the MYBPC3Δ25bp variant. An emphasis is to review the latest techniques currently applied to explore the MYBPC3Δ25bp pathogenesis and to provide a foundation for developing new diagnostic strategies and advances in therapeutics.

The hypertrophic cardiomyopathy mutations R403Q and R663H increase the number of myosin heads available to interact with actin

Hypertrophic cardiomyopathy (HCM) mutations in β-cardiac myosin and myosin binding protein-C (MyBP-C) lead to hypercontractility of the heart, an early hallmark of HCM. We show that hypercontractility caused by the HCM-causing mutation R663H cannot be explained by changes in fundamental myosin contractile parameters, much like the HCM-causing mutation R403Q. Using enzymatic assays with purified human β-cardiac myosin, we provide evidence that both mutations cause hypercontractility by increasing the number of functionally accessible myosin heads. We also demonstrate that the myosin mutation R403Q, but not R663H, ablates the binding of myosin with the C0-C7 fragment of MyBP-C. Furthermore, addition of C0-C7 decreases the wild-type myosin basal ATPase single turnover rate, while the mutants do not show a similar reduction. These data suggest that a primary mechanism of action for these mutations is to increase the number of myosin heads functionally available for interaction with actin, which could contribute to hypercontractility.

MyBP-C: one protein to govern them all

The heart is an extraordinarily versatile pump, finely tuned to respond to a multitude of demands. Given the heart pumps without rest for decades its efficiency is particularly relevant. Although many proteins in the heart are essential for viability, the non-essential components can attract numerous mutations which can cause disease, possibly through alterations in pumping efficiency. Of these, myosin binding protein C is strongly over-represented with ~ 40% of all known mutations in hypertrophic cardiomyopathy. Therefore, a complete understanding of its molecular function in the cardiac sarcomere is warranted. In this review, we revisit contemporary and classical literature to clarify both the current standing of this fast-moving field and frame future unresolved questions. To date, much effort has been directed at understanding MyBP-C function on either thick or thin filaments. Here we aim to focus questions on how MyBP-C functions at a molecular level in the context of both the thick and thin filaments together. A concept that emerges is MyBP-C acts to govern interactions on two levels; controlling myosin access to the thin filament by sequestration on the thick filament, and controlling the activation state and access of myosin to its binding sites on the thin filament. Such affects are achieved through directed interactions mediated by phosphorylation (of MyBP-C and other sarcomeric components) and calcium.

A Novel "Cut and Paste" Method for In Situ Replacement of cMyBP-C Reveals a New Role for cMyBP-C in the Regulation of Contractile Oscillations

RATIONALE

cMyBP-C (cardiac myosin-binding protein-C) is a critical regulator of heart contraction, but the mechanisms by which cMyBP-C affects actin and myosin are only partly understood. A primary obstacle is that cMyBP-C localization on thick filaments may be a key factor defining its interactions, but most in vitro studies cannot duplicate the unique spatial arrangement of cMyBP-C within the sarcomere.

OBJECTIVE

The goal of this study was to validate a novel hybrid genetic/protein engineering approach for rapid manipulation of cMyBP-C in sarcomeres in situ.

METHODS AND RESULTS

We designed a novel cut and paste approach for removal and replacement of cMyBP-C N'-terminal domains (C0-C7) in detergent-permeabilized cardiomyocytes from gene-edited Spy-C mice. Spy-C mice express a TEVp (tobacco etch virus protease) cleavage site and a SpyTag (st) between cMyBP-C domains C7 and C8. A cut is achieved using TEVp which cleaves cMyBP-C to create a soluble N'-terminal γC0C7 (endogenous [genetically encoded] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C) fragment and an insoluble C'-terminal SpyTag-C8-C10 fragment that remains associated with thick filaments. Paste of new recombinant (r)C0C7 domains is achieved by a covalent bond formed between SpyCatcher (-sc; encoded at the C'-termini of recombinant proteins) and SpyTag. Results show that loss of γC0C7 reduced myofilament Ca²⁺ sensitivity and increased cross-bridge cycling (k_tr) at submaximal [Ca²⁺]. Acute loss of γC0C7 also induced auto-oscillatory contractions at submaximal [Ca²⁺]. Ligation of rC0C7 (exogenous [recombinant] N'-terminal domains C0 to C7 of cardiac myosin binding protein-C)-sc returned pCa₅₀ and k_tr to control values and abolished oscillations, but phosphorylated (p)-rC0C7-sc did not completely rescue these effects.

CONCLUSIONS

We describe a robust new approach for acute removal and replacement of cMyBP-C in situ. The method revealed a novel role for cMyBP-C N'-terminal domains to damp sarcomere-driven contractile waves (so-called spontaneous oscillatory contractions). Because phosphorylated (p)-rC0C7-sc was less effective at damping contractile oscillations, results suggest that spontaneous oscillatory contractions may contribute to enhanced contractility in response to inotropic stimuli.

Altered C10 domain in cardiac myosin binding protein-C results in hypertrophic cardiomyopathy

AIMS

A 25-base pair deletion in the cardiac myosin binding protein-C (cMyBP-C) gene (MYBPC3), proposed to skip exon 33, modifies the C10 domain (cMyBP-CΔC10mut) and is associated with hypertrophic cardiomyopathy (HCM) and heart failure, affecting approximately 100 million South Asians. However, the molecular mechanisms underlying the pathogenicity of cMyBP-CΔC10mutin vivo are unknown. We hypothesized that expression of cMyBP-CΔC10mut exerts a poison polypeptide effect leading to improper assembly of cardiac sarcomeres and the development of HCM.

METHODS AND RESULTS

To determine whether expression of cMyBP-CΔC10mut is sufficient to cause HCM and contractile dysfunction in vivo, we generated transgenic (TG) mice having cardiac-specific protein expression of cMyBP-CΔC10mut at approximately half the level of endogenous cMyBP-C. At 12 weeks of age, significant hypertrophy was observed in TG mice expressing cMyBP-CΔC10mut (heart weight/body weight ratio: 4.43 ± 0.11 mg/g non-transgenic (NTG) vs. 5.34 ± 0.25 mg/g cMyBP-CΔC10mut, P < 0.05). Furthermore, haematoxylin and eosin, Masson's trichrome staining, as well as second-harmonic generation imaging revealed the presence of significant fibrosis and a greater relative nuclear area in cMyBP-CΔC10mut hearts compared with NTG controls. M-mode echocardiography analysis revealed hypercontractile hearts (EF: 53.4%±2.9% NTG vs. 66.4% ± 4.7% cMyBP-CΔC10mut; P < 0.05) and early diastolic dysfunction (E/E': 28.7 ± 3.7 NTG vs. 46.3 ± 8.4 cMyBP-CΔC10mut; P < 0.05), indicating the presence of an HCM phenotype. To assess whether these changes manifested at the myofilament level, contractile function of single skinned cardiomyocytes was measured. Preserved maximum force generation and increased Ca2+-sensitivity of force generation were observed in cardiomyocytes from cMyBP-CΔC10mut mice compared with NTG controls (EC50: 3.6 ± 0.02 µM NTG vs. 2.90 ± 0.01 µM cMyBP-CΔC10mut; P < 0.0001).

CONCLUSION

Expression of cMyBP-C protein with a modified C10 domain is sufficient to cause contractile dysfunction and HCM in vivo.