Structural and functional effects of myosin-binding protein-C phosphorylation in heart muscle are not mimicked by serine-to-aspartate substitutions

Cardiac myosin binding protein-C phosphorylation as a function of multiple protein kinase and phosphatase activities

Phosphorylation of cardiac myosin binding protein-C (cMyBP-C) is a determinant of cardiac myofilament function. Although cMyBP-C phosphorylation by various protein kinases has been extensively studied, the influence of protein phosphatases on cMyBP-C's multiple phosphorylation sites has remained largely obscure. Here we provide a detailed biochemical characterization of cMyBP-C dephosphorylation by protein phosphatases 1 and 2 A (PP1 and PP2A), and develop an integrated kinetic model for cMyBP-C phosphorylation using data for both PP1, PP2A and various protein kinases known to phosphorylate cMyBP-C. We find strong site-specificity and a hierarchical mechanism for both phosphatases, proceeding in the opposite direction of sequential phosphorylation by potein kinase A. The model is consistent with published data from human patients and predicts complex non-linear cMyBP-C phosphorylation patterns that are validated experimentally. Our results suggest non-redundant roles for PP1 and PP2A under both physiological and heart failure conditions, and emphasize the importance of phosphatases for cMyBP-C regulation.

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.

Accessing isotopically labeled proteins containing genetically encoded phosphoserine for NMR with optimized expression conditions

Phosphoserine (pSer) sites are primarily located within disordered protein regions, making it difficult to experimentally ascertain their effects on protein structure and function. Therefore, the production of ¹⁵N- (and ¹³C)-labeled proteins with site-specifically encoded pSer for NMR studies is essential to uncover molecular mechanisms of protein regulation by phosphorylation. While genetic code expansion technologies for the translational installation of pSer in Escherichia coli are well established and offer a powerful strategy to produce site-specifically phosphorylated proteins, methodologies to adapt them to minimal or isotope-enriched media have not been described. This shortcoming exists because pSer genetic code expansion expression hosts require the genomic ΔserB mutation, which increases pSer bioavailability but also imposes serine auxotrophy, preventing growth in minimal media used for isotopic labeling of recombinant proteins. Here, by testing different media supplements, we restored normal BL21(DE3) ΔserB growth in labeling media but subsequently observed an increase of phosphatase activity and mis-incorporation not typically seen in standard rich media. After rounds of optimization and adaption of a high-density culture protocol, we were able to obtain ≥10 mg/L homogenously labeled, phosphorylated superfolder GFP. To demonstrate the utility of this method, we also produced the intrinsically disordered serine/arginine-rich region of the SARS-CoV-2 Nucleocapsid protein labeled with ¹⁵N and pSer at the key site S188 and observed the resulting peak shift due to phosphorylation by 2D and 3D heteronuclear single quantum correlation analyses. We propose this cost-effective methodology will pave the way for more routine access to pSer-enriched proteins for 2D and 3D NMR analyses.

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.

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.

Molecular noise filtering in the β-adrenergic signaling network by phospholamban pentamers

Phospholamban (PLN) is an important regulator of cardiac calcium handling due to its ability to inhibit the calcium ATPase SERCA. β-Adrenergic stimulation reverses SERCA inhibition via PLN phosphorylation and facilitates fast calcium reuptake. PLN also forms pentamers whose physiological significance has remained elusive. Using mathematical modeling combined with biochemical and cell biological experiments, we show that pentamers regulate both the dynamics and steady-state levels of monomer phosphorylation. Substrate competition by pentamers and a feed-forward loop involving inhibitor-1 can delay monomer phosphorylation by protein kinase A (PKA), whereas cooperative pentamer dephosphorylation enables bistable PLN steady-state phosphorylation. Simulations show that phosphorylation delay and bistability act as complementary filters that reduce the effect of random fluctuations in PKA activity, thereby ensuring consistent monomer phosphorylation and SERCA activity despite noisy upstream signals. Preliminary analyses suggest that the PLN mutation R14del could impair noise filtering, offering a new perspective on how this mutation causes cardiac arrhythmias.

I22: SAXS/WAXS beamline at Diamond Light Source - an overview of 10 years operation

Beamline I22 at Diamond Light Source is dedicated to the study of soft-matter systems from both biological and materials science. The beamline can operate in the range 3.7 keV to 22 keV for transmission SAXS and 14 keV to 20 keV for microfocus SAXS with beam sizes of 240 µm × 60 µm [full width half-maximum (FWHM) horizontal (H) × vertical (V)] at the sample for the main beamline, and approximately 10 µm × 10 µm for the dedicated microfocusing platform. There is a versatile sample platform for accommodating a range of facilities and user-developed sample environments. The high brilliance of the insertion device source on I22 allows structural investigation of materials under extreme environments (for example, fluid flow at high pressures and temperatures). I22 provides reliable access to millisecond data acquisition timescales, essential to understanding kinetic processes such as protein folding or structural evolution in polymers and colloids.

Novel insights into sarcomere regulatory systems control of cardiac thin filament activation

Our review focuses on sarcomere regulatory mechanisms with a discussion of cardiac-specific modifications to the three-state model of thin filament activation from a blocked to closed to open state. We discuss modulation of these thin filament transitions by Ca²⁺, by crossbridge interactions, and by thick filament–associated proteins, cardiac myosin–binding protein C (cMyBP-C), cardiac regulatory light chain (cRLC), and titin. Emerging evidence supports the idea that the cooperative activation of the thin filaments despite a single Ca²⁺ triggering regulatory site on troponin C (cTnC) cannot be considered in isolation of other functional domains of the sarcomere. We discuss long- and short-range interactions among these domains with the regulatory units of thin filaments, including proteins at the barbed end at the Z-disc and the pointed end near the M-band. Important to these discussions is the ever-increasing understanding of the role of cMyBP-C, cRLC, and titin filaments. Detailed knowledge of these control processes is critical to the understanding of mechanisms sustaining physiological cardiac state with varying hemodynamic load, to better defining genetic and acquired cardiac disorders, and to developing targets for therapies at the level of the sarcomeres.

cMyBP-C phosphorylation modulates the time-dependent slowing of unloaded shortening in murine skinned myocardium

In myocardium, phosphorylation of cardiac myosin-binding protein-C (cMyBP-C) is thought to modulate the cooperative activation of the thin filament by binding to myosin and/or actin, thereby regulating the probability of cross-bridge binding to actin. At low levels of Ca2+ activation, unloaded shortening velocity (Vo) in permeabilized cardiac muscle is comprised of an initial high-velocity phase and a subsequent low-velocity phase. The velocities in these phases scale with the level of activation, culminating in a single high-velocity phase (Vmax) at saturating Ca2+. To test the idea that cMyBP-C phosphorylation contributes to the activation dependence of Vo, we measured Vo before and following treatment with protein kinase A (PKA) in skinned trabecula isolated from mice expressing either wild-type cMyBP-C (tWT), nonphosphorylatable cMyBP-C (t3SA), or phosphomimetic cMyBP-C (t3SD). During maximal Ca2+ activation, Vmax was monophasic and not significantly different between the three groups. Although biphasic shortening was observed in all three groups at half-maximal activation under control conditions, the high- and low-velocity phases were faster in the t3SD myocardium compared with values obtained in either tWT or t3SA myocardium. Treatment with PKA significantly accelerated both the high- and low-velocity phases in tWT myocardium but had no effect on Vo in either the t3SD or t3SA myocardium. These results can be explained in terms of a model in which the level of cMyBP-C phosphorylation modulates the extent and rate of cooperative spread of myosin binding to actin.

High Throughput Screen Identifies Small Molecule Effectors That Modulate Thin Filament Activation in Cardiac Muscle

Current therapeutic interventions for both heart disease and heart failure are largely insufficient and associated with undesired side effects. Biomedical research has emphasized the role of sarcomeric protein function for the normal performance and energy efficiency of the heart, suggesting that directly targeting the contractile myofilaments themselves using small molecule effectors has therapeutic potential and will likely result in greater drug efficacy and selectivity. In this study, we developed a robust and highly reproducible fluorescence polarization-based high throughput screening (HTS) assay that directly targets the calcium-dependent interaction between cardiac troponin C (cTnC) and the switch region of cardiac troponin I (cTnI_SP), with the aim of identifying small molecule effectors of the cardiac thin filament activation pathway. We screened a commercially available small molecule library and identified several hit compounds with both inhibitory and activating effects. We used a range of biophysical and biochemical methods to characterize hit compounds and identified fingolimod, a sphingosin-1-phosphate receptor modulator, as a new troponin-based small molecule effector. Fingolimod decreased the ATPase activity and calcium sensitivity of demembranated cardiac muscle fibers in a dose-dependent manner, suggesting that the compound acts as a calcium desensitizer. We investigated fingolimod's mechanism of action using a combination of computational studies, biophysical methods, and synthetic chemistry, showing that fingolimod bound to cTnC repels cTnI_SP via mainly electrostatic repulsion of its positively charged tail. These results suggest that fingolimod is a potential new lead compound/scaffold for the development of troponin-directed heart failure therapeutics.

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.

Human cardiac myosin-binding protein C restricts actin structural dynamics in a cooperative and phosphorylation-sensitive manner

Cardiac myosin-binding protein C (cMyBP-C) is a thick filament-associated protein that influences actin-myosin interactions. cMyBP-C alters myofilament structure and contractile properties in a protein kinase A (PKA) phosphorylation-dependent manner. To determine the effects of cMyBP-C and its phosphorylation on the microsecond rotational dynamics of actin filaments, we attached a phosphorescent probe to F-actin at Cys-374 and performed transient phosphorescence anisotropy (TPA) experiments. Binding of cMyBP-C N-terminal domains (C0-C2) to labeled F-actin reduced rotational flexibility by 20-25°, indicated by increased final anisotropy of the TPA decay. The effects of C0-C2 on actin TPA were highly cooperative (n = ∼8), suggesting that the cMyBP-C N terminus impacts the rotational dynamics of actin spanning seven monomers (i.e. the length of tropomyosin). PKA-mediated phosphorylation of C0-C2 eliminated the cooperative effects on actin flexibility and modestly increased actin rotational rates. Effects of Ser to Asp phosphomimetic substitutions in the M-domain of C0-C2 on actin dynamics only partially recapitulated the phosphorylation effects. C0-C1 (lacking M-domain/C2) similarly exhibited reduced cooperativity, but not as reduced as by phosphorylated C0-C2. These results suggest an important regulatory role of the M-domain in cMyBP-C effects on actin structural dynamics. In contrast, phosphomimetic substitution of the glycogen synthase kinase (GSK3β) site in the Pro/Ala-rich linker of C0-C2 did not significantly affect the TPA results. We conclude that cMyBP-C binding and PKA-mediated phosphorylation can modulate actin dynamics. We propose that these N-terminal cMyBP-C-induced changes in actin dynamics help explain the functional effects of cMyBP-C phosphorylation on actin-myosin interactions.

Site-specific phosphorylation of myosin binding protein-C coordinates thin and thick filament activation in cardiac muscle

Phosphorylation of cardiac myosin binding protein-C (cMyBP-C) is a key regulator of myocardial contractility, and dephosphorylation of cMyBP-C is associated with heart failure. However, the molecular mechanisms underlying contractile regulation by cMyBP-C phosphorylation are poorly understood. We describe the kinase specificity of the multiple phosphorylation sites on cMyBP-C and show that they are interdependent and have distinct effects on the structure of the thin and thick filaments. The results lead to a model of regulation by cMyBP-C phosphorylation through altered affinity of cMyBP-C’s N terminus for thin and thick filaments, as well as their structures and associated regulatory states. Impairment of these mechanisms is likely to underlie the functional effects of mutations in filament proteins associated with cardiomyopathy.

The heart’s response to varying demands of the body is regulated by signaling pathways that activate protein kinases which phosphorylate sarcomeric proteins. Although phosphorylation of cardiac myosin binding protein-C (cMyBP-C) has been recognized as a key regulator of myocardial contractility, little is known about its mechanism of action. Here, we used protein kinase A (PKA) and Cε (PKCε), as well as ribosomal S6 kinase II (RSK2), which have different specificities for cMyBP-C’s multiple phosphorylation sites, to show that individual sites are not independent, and that phosphorylation of cMyBP-C is controlled by positive and negative regulatory coupling between those sites. PKA phosphorylation of cMyBP-C’s N terminus on 3 conserved serine residues is hierarchical and antagonizes phosphorylation by PKCε, and vice versa. In contrast, RSK2 phosphorylation of cMyBP-C accelerates PKA phosphorylation. We used cMyBP-C’s regulatory N-terminal domains in defined phosphorylation states for protein–protein interaction studies with isolated cardiac native thin filaments and the S2 domain of cardiac myosin to show that site-specific phosphorylation of this region of cMyBP-C controls its interaction with both the actin-containing thin and myosin-containing thick filaments. We also used fluorescence probes on the myosin-associated regulatory light chain in the thick filaments and on troponin C in the thin filaments to monitor structural changes in the myofilaments of intact heart muscle cells associated with activation of myocardial contraction by the N-terminal region of cMyBP-C in its different phosphorylation states. Our results suggest that cMyBP-C acts as a sarcomeric integrator of multiple signaling pathways that determines downstream physiological function.

Cardiac myosin binding protein-C phosphorylation regulates the super-relaxed state of myosin

Phosphorylation of cardiac myosin binding protein-C (cMyBP-C) accelerates cardiac contractility. However, the mechanisms by which cMyBP-C phosphorylation increases contractile kinetics have not been fully elucidated. In this study, we tested the hypothesis that phosphorylation of cMyBP-C releases myosin heads from the inhibited super-relaxed state (SRX), thereby determining the fraction of myosin available for contraction. Mice with various alanine (A) or aspartic acid (D) substitutions of the three main phosphorylatable serines of cMyBP-C (serines 273, 282, and 302) were used to address the association between cMyBP-C phosphorylation and SRX. Single-nucleotide turnover in skinned ventricular preparations demonstrated that phosphomimetic cMyBP-C destabilized SRX, whereas phospho-ablated cMyBP-C had a stabilizing effect on SRX. Strikingly, phosphorylation at serine 282 site was found to play a critical role in regulating the SRX. Treatment of WT preparations with protein kinase A (PKA) reduced the SRX, whereas, in nonphosphorylatable cMyBP-C preparations, PKA had no detectable effect. Mice with stable SRX exhibited reduced force production. Phosphomimetic cMyBP-C with reduced SRX exhibited increased rates of tension redevelopment and reduced binding to myosin. We also used recombinant myosin subfragment-2 to disrupt the endogenous interaction between cMyBP-C and myosin and observed a significant reduction in the population of SRX myosin. This peptide also increased force generation and rate of tension redevelopment in skinned fibers. Taken together, this study demonstrates that the phosphorylation-dependent interaction between cMyBP-C and myosin is a determinant of the fraction of myosin available for contraction. Furthermore, the binding between cMyBP-C and myosin may be targeted to improve contractile function.

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.