The motif of human cardiac myosin-binding protein C is required for its Ca2+-dependent interaction with calmodulin

Bringing into focus the central domains C3-C6 of myosin binding protein C

Myosin binding protein C (MyBPC) is a multi-domain protein with each region having a distinct functional role in muscle contraction. The central domains of MyBPC have often been overlooked due to their unclear roles. However, recent research shows promise in understanding their potential structural and regulatory functions. Understanding the central region of MyBPC is important because it may have specialized function that can be used as drug targets or for disease-specific therapies. In this review, we provide a brief overview of the evolution of our understanding of the central domains of MyBPC in regard to its domain structures, arrangement and dynamics, interaction partners, hypothesized functions, disease-causing mutations, and post-translational modifications. We highlight key research studies that have helped advance our understanding of the central region. Lastly, we discuss gaps in our current understanding and potential avenues to further research and discovery.

Mechanism of post-tetanic depression of slow muscle fibres

A brief tetanic stimulation has a very different effect on the subsequent isometric twitch force of fast and slow skeletal muscles. Fast muscle responds with an enhanced twitch force which doubles that of the pre-tetanic value, whereas slow muscle depresses the post-tetanic twitch by about 20%. Twitch potentiation of fast muscle has long been known to be due to myosin light chain 2 phosphorylation. It is proposed that post-tetanic twitch depression in slow muscle is due to the dephosphorylation of the slow isoform of the thick filament protein, myosin-binding protein-C, by Ca2+/calmodulin-activated phosphatase calcineurin, whilst its phosphorylation underlies the force enhancement due to β-adrenergic stimulation in slow and fast muscle.

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.

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.

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.

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.

Functional characterization of human myosin-binding protein C3 variants associated with hypertrophic cardiomyopathy reveals exon-specific cardiac phenotypes in zebrafish model

Myosin-binding protein C 3 (MYBPC3) variants are the most common cause of hypertrophic cardiomyopathy (HCM). HCM is a complex cardiac disorder due to its significant genetic and clinical heterogeneity. MYBPC3 variants genotype-phenotype associations remain poorly understood. We investigated the impact of two novel human MYBPC3 splice-site variants: V1: c.654+2_654+4dupTGG targeting exon 5 using morpholino MOe5i5; and V2: c.772+1G>A targeting exon 6 using MOe6i6; located within C1 domain of cMyBP-C protein, known to be critical in regulating sarcomere structure and contractility. Zebrafish MOe5i5 and MOe6i6 morphants recapitulated typical characteristics of human HCM with cardiac phenotypes of varying severity, including reduced cardiomyocyte count, thickened ventricular myocardial wall, a drastic reduction in heart rate, stroke volume, and cardiac output. Analysis of all cardiac morphological and functional parameters demonstrated that V2 cardiac phenotype was more severe than V1. Coinjection with synthetic human MYBPC3 messenger RNA (mRNA) partially rescued disparate cardiac phenotypes in each zebrafish morphant. While human MYBPC3 mRNA partially restored the decreased heart rate in V1 morphants and displayed increased percentages of ejection fraction, fractional shortening, and area change, it failed to revert the V1 ventricular myocardial thickness. These results suggest a possible V1 impact on cardiac contractility. In contrast, attempts to rescue V2 morphants only restored the ventricular myocardial wall hypertrophy phenotype but had no significant effect on impaired heart rate, suggesting a potential V2 impact on the cardiac structure. Our study provides evidence of an association between MYBPC3 exon-specific cardiac phenotypes in the zebrafish model providing important insights into how these genetic variants contribute to HCM disease.

Role of intrinsic disorder in muscle sarcomeres

The role and utility of intrinsically disordered regions (IDRs) is reviewed for two groups of sarcomeric proteins, such as members of tropomodulin/leiomodin (Tmod/Lmod) protein homology group and myosin binding protein C (MyBP-C). These two types of sarcomeric proteins represent very different but strongly interdependent functions, being responsible for maintaining structure and operation of the muscle sarcomere. The role of IDRs in the formation of complexes between thin filaments and Tmods/Lmods is discussed within the framework of current understanding of the thin filament length regulation. For MyBP-C, the function of IDRs is discussed in the context of MYBP-C-dependent sarcomere contraction and actomyosin activation.

Skeletal myosin binding protein-C: An increasingly important regulator of striated muscle physiology

The Myosin Binding Protein-C (MyBP-C) family is a group of sarcomeric proteins important for striated muscle structure and function. Comprising approximately 2% of the myofilament mass, MyBP-C has important roles in both contraction and relaxation. Three paralogs of MyBP-C are encoded by separate genes with distinct expression profiles in striated muscle. In mammals, cardiac MyBP-C is limited to the heart, and it is the most extensively studied owing to its involvement in cardiomyopathies. However, the roles of two skeletal paralogs, slow and fast, in muscle biology remain poorly characterized. Nonetheless, both have been recently implicated in the development of skeletal myopathies. This calls for a better understanding of their function in the pathophysiology of distal arthrogryposis. This review characterizes MyBP-C as a whole and points out knowledge gaps that still remain with respect to skeletal MyBP-C.

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

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

The Role of Cardiac Myosin Binding Protein C3 in Hypertrophic Cardiomyopathy-Progress and Novel Therapeutic Opportunities

Hypertrophic cardiomyopathy (HCM) is a common autosomal dominant genetic cardiovascular disorder marked by genetic and phenotypic heterogeneity. Mutations in the gene encodes the cardiac myosin-binding protein C, cMYBPC3 is amongst the various sarcomeric genes that are associated with HCM. These mutations produce mutated mRNAs and truncated cMyBP-C proteins. In this review, we will discuss the implications and molecular mechanisms involved in MYBPC3 different mutations. Further, we will highlight the novel targets that can be developed into potential therapeutics for the treatment of HMC. J. Cell. Physiol. 232: 1650-1659, 2017. © 2016 Wiley Periodicals, Inc.

A Highly Conserved Yet Flexible Linker Is Part of a Polymorphic Protein-Binding Domain in Myosin-Binding Protein C

The nuclear magnetic resonance (NMR) structure of the tri-helix bundle (THB) of the m-domain plus C2 (ΔmC2) of myosin-binding protein C (MyBP-C) has revealed a highly flexible seven-residue linker between the structured THB and C2. Bioinformatics shows significant patterns of conservation across the THB-linker sequence, with the linker containing a strictly conserved serine in all MyBP-C isoforms. Clinically linked mutations further support the functional significance of the THB-linker region. NMR, small-angle X-ray scattering, and binding studies show the THB-linker plus the first ten residues of C2 undergo dramatic changes when ΔmC2 binds Ca²⁺-calmodulin, with the linker and C2 N-terminal residues contributing significantly to the affinity. Modeling of all available experimental data indicates that the THB tertiary structure must be disrupted to form the complex. These results are discussed in the context of the THB-linker and the N-terminal residues of C2 forming a polymorphic binding domain that could accommodate multiple binding partners in the dynamic sarcomere.

Site-directed spectroscopy of cardiac myosin-binding protein C reveals effects of phosphorylation on protein structural dynamics

We have used the site-directed spectroscopies of time-resolved fluorescence resonance energy transfer (TR-FRET) and double electron-electron resonance (DEER), combined with complementary molecular dynamics (MD) simulations, to resolve the structure and dynamics of cardiac myosin-binding protein C (cMyBP-C), focusing on the N-terminal region. The results have implications for the role of this protein in myocardial contraction, with particular relevance to β-adrenergic signaling, heart failure, and hypertrophic cardiomyopathy. N-terminal cMyBP-C domains C0-C2 (C0C2) contain binding regions for potential interactions with both thick and thin filaments. Phosphorylation by PKA in the MyBP-C motif regulates these binding interactions. Our spectroscopic assays detect distances between pairs of site-directed probes on cMyBP-C. We engineered intramolecular pairs of labeling sites within cMyBP-C to measure, with high resolution, the distance and disorder in the protein's flexible regions using TR-FRET and DEER. Phosphorylation reduced the level of molecular disorder and the distribution of C0C2 intramolecular distances became more compact, with probes flanking either the motif between C1 and C2 or the Pro/Ala-rich linker (PAL) between C0 and C1. Further insight was obtained from microsecond MD simulations, which revealed a large structural change in the disordered motif region in which phosphorylation unmasks the surface of a series of residues on a stable α-helix within the motif with high potential as a protein-protein interaction site. These experimental and computational findings elucidate structural transitions in the flexible and dynamic portions of cMyBP-C, providing previously unidentified molecular insight into the modulatory role of this protein in cardiac muscle contractility.

Cardiac myosin binding protein-C: a structurally dynamic regulator of myocardial contractility

Cardiac myosin binding protein-C (cMyBP-C) is a modular protein anchored to the thick filament through interactions mediated by its C-terminal region. The N-terminal region of cMyBPC-C regulates myocardial contractility by modifying actin-myosin association. Phosphorylation of the N-terminal region diminishes cMyBP-C's capacity to regulate actin-myosin function. Despite a substantial body of literature, many issues remain unclear regarding the structural and functional roles of cMyBP-C. While no high-resolution structures of the intact protein exist, crystallographic and nuclear magnetic resonance (NMR) structures of isolated N-terminal domains provide important molecular details regarding cMyBP-C's role in controlling contractility. In this review, we summarize the emerging structural understanding of cMyBP-C with a particular emphasis placed on describing how its dynamic molecular interactions with both thin and thick filament proteins likely contribute to contractile regulation. Furthermore, we discuss the future directions and strategies by which we may improve the mechanistic understanding of its role in modulating cardiac muscle contraction.

Earning stripes: myosin binding protein-C interactions with actin

Myosin binding protein-C (MyBP-C) was first discovered as an impurity during the purification of myosin from skeletal muscle. However, soon after its discovery, MyBP-C was also shown to bind actin. While the unique functional implications for a protein that could cross-link thick and thin filaments together were immediately recognized, most early research nonetheless focused on interactions of MyBP-C with the thick filament. This was in part because interactions of MyBP-C with the thick filament could adequately explain most (but not all) effects of MyBP-C on actomyosin interactions and in part because the specificity of actin binding was uncertain. However, numerous recent studies have now established that MyBP-C can indeed bind to actin through multiple binding sites, some of which are highly specific. Many of these interactions involve critical regulatory domains of MyBP-C that are also reported to interact with myosin. Here we review current evidence supporting MyBP-C interactions with actin and discuss these findings in terms of their ability to account for the functional effects of MyBP-C. We conclude that the influence of MyBP-C on muscle contraction can be explained equally well by interactions with actin as by interactions with myosin. However, because data showing that MyBP-C binds to either myosin or actin has come almost exclusively from in vitro biochemical studies, the challenge for future studies is to define which binding partner(s) MyBP-C interacts with in vivo.

Cross-species mechanical fingerprinting of cardiac myosin binding protein-C

Cardiac myosin binding protein-C (cMyBP-C) is a member of the immunoglobulin (Ig) superfamily of proteins and consists of 8 Ig- and 3 fibronectin III (FNIII)-like domains along with a unique regulatory sequence referred to as the MyBP-C motif or M-domain. We previously used atomic force microscopy to investigate the mechanical properties of murine cMyBP-C expressed using a baculovirus/insect cell expression system. Here, we investigate whether the mechanical properties of cMyBP-C are conserved across species by using atomic force microscopy to manipulate recombinant human cMyBP-C and native cMyBP-C purified from bovine heart. Force versus extension data obtained in velocity-clamp experiments showed that the mechanical response of the human recombinant protein was remarkably similar to that of the bovine native cMyBP-C. Ig/Fn-like domain unfolding events occurred in a hierarchical fashion across a threefold range of forces starting at relatively low forces of ~50 pN and ending with the unfolding of the highest stability domains at ~180 pN. Force-extension traces were also frequently marked by the appearance of anomalous force drops suggestive of additional mechanical complexity such as structural coupling among domains. Both recombinant and native cMyBP-C exhibited a prominent segment ~100 nm-long that could be stretched by forces <50 pN before the unfolding of Ig- and FN-like domains. Combined with our previous observations of mouse cMyBP-C, these results establish that although the response of cMyBP-C to mechanical load displays a complex pattern, it is highly conserved across species.

A gain-of-function mutation in the M-domain of cardiac myosin-binding protein-C increases binding to actin

The M-domain is the major regulatory subunit of cardiac myosin-binding protein-C (cMyBP-C) that modulates actin and myosin interactions to influence muscle contraction. However, the precise mechanism(s) and the specific residues involved in mediating the functional effects of the M-domain are not fully understood. Positively charged residues adjacent to phosphorylation sites in the M-domain are thought to be critical for effects of cMyBP-C on cross-bridge interactions by mediating electrostatic binding with myosin S2 and/or actin. However, recent structural studies revealed that highly conserved sequences downstream of the phosphorylation sites form a compact tri-helix bundle. Here we used site-directed mutagenesis to probe the functional significance of charged residues adjacent to the phosphorylation sites and conserved residues within the tri-helix bundle. Results confirm that charged residues adjacent to phosphorylation sites and residues within the tri-helix bundle are important for mediating effects of the M-domain on contraction. In addition, four missense variants within the tri-helix bundle that are associated with human hypertrophic cardiomyopathy caused either loss-of-function or gain-of-function effects on force. Importantly, the effects of the gain-of-function variant, L348P, increased the affinity of the M-domain for actin. Together, results demonstrate that functional effects of the M-domain are not due solely to interactions with charged residues near phosphorylatable serines and provide the first demonstration that the tri-helix bundle contributes to the functional effects of the M-domain, most likely by binding to actin.

Molecular modeling of disease causing mutations in domain C1 of cMyBP-C

Cardiac myosin binding protein-C (cMyBP-C) is a multi-domain (C0-C10) protein that regulates heart muscle contraction through interaction with myosin, actin and other sarcomeric proteins. Several mutations of this protein cause familial hypertrophic cardiomyopathy (HCM). Domain C1 of cMyBP-C plays a central role in protein interactions with actin and myosin. Here, we studied structure-function relationship of three disease causing mutations, Arg177His, Ala216Thr and Glu258Lys of the domain C1 using computational biology techniques with its available X-ray crystal structure. The results suggest that each mutation could affect structural properties of the domain C1, and hence it's structural integrity through modifying intra-molecular arrangements in a distinct mode. The mutations also change surface charge distributions, which could impact the binding of C1 with other sarcomeric proteins thereby affecting contractile function. These structural consequences of the C1 mutants could be valuable to understand the molecular mechanisms for the disease.