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.

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.

In silico prediction of ARB resistance: A first step in creating personalized ARB therapy.

Angiotensin II type 1 receptor (AT1R) blockers (ARBs) are among the most prescribed drugs. However, ARB effectiveness varies widely, which may be due to non-synonymous single nucleotide polymorphisms (nsSNPs) within the AT1R gene. The AT1R coding sequence contains over 100 nsSNPs; therefore, this study embarked on determining which nsSNPs may abrogate the binding of selective ARBs. The crystal structure of olmesartan-bound human AT1R (PDB:4ZUD) served as a template to create an inactive apo-AT1R via molecular dynamics simulation (n = 3). All simulations resulted in a water accessible ligand-binding pocket that lacked sodium ions. The model remained inactive displaying little movement in the receptor core; however, helix 8 showed considerable flexibility. A single frame representing the average stable AT1R was used as a template to dock Olmesartan via AutoDock 4.2, MOE, and AutoDock Vina to obtain predicted binding poses and mean Boltzmann weighted average affinity. The docking results did not match the known pose and affinity of Olmesartan. Thus, an optimization protocol was initiated using AutoDock 4.2 that provided more accurate poses and affinity for Olmesartan (n = 6). Atomic models of 103 of the known human AT1R polymorphisms were constructed using the molecular dynamics equilibrated apo-AT1R. Each of the eight ARBs was then docked, using ARB-optimized parameters, to each polymorphic AT1R (n = 6). Although each nsSNP has a negligible effect on the global AT1R structure, most nsSNPs drastically alter a sub-set of ARBs affinity to the AT1R. Alterations within N298 -L314 strongly effected predicted ARB affinity, which aligns with early mutagenesis studies. The current study demonstrates the potential of utilizing in silico approaches towards personalized ARB therapy. The results presented here will guide further biochemical studies and refinement of the model to increase the accuracy of the prediction of ARB resistance in order to increase overall ARB effectiveness.

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 (GSK3ß) 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.

N-terminal extension in cardiac myosin-binding protein C regulates myofilament binding.

RATIONALE: Mutations in the gene encoding the sarcomeric protein cardiac myosin-binding protein C (cMyBP-C) are a leading cause of hypertrophic cardiomyopathy (HCM). Mouse models targeting cMyBP-C and use of recombinant proteins have been effective in studying its roles in contractile function and disease. Surprisingly, while the N-terminus of cMyBP-C is important to regulate myofilament binding and contains many HCM mutations, an incorrect sequence, lacking the N-terminal 8 amino acids has been used in many studies. OBJECTIVES: To determine the N-terminal cMyBP-C sequences in ventricles and investigate the roles of species-specific differences in cMyBP-C on myofilament binding. METHODS AND RESULTS: We determined cMyBP-C sequences in mouse and human by inspecting available sequence databases. N-terminal differences were confirmed using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Cosedimentation assays with actin or myosin were used to examine binding in mouse, human and chimeric fusion proteins of cMyBP-C. Time-resolved FRET (TR-FRET) with site-directed probes on cMyBP-C was employed to measure structural dynamics. LC-MS/MS supported the sequencing data that mouse cMyBP-C contains an eight-residue N-terminal extension (NTE) not found in human. Cosedimentation assays revealed that cardiac myosin binding was strongly influenced by the presence of the NTE, which reduced binding by 60%. 75% more human C0-C2 than mouse bound to myosin. Actin binding of mouse C0-C2 was not affected by the NTE. 50% more human C0-C2 than mouse bound to actin. TR-FRET indicates that the NTE did not significantly affect structural dynamics across domains C0 and C1. CONCLUSIONS: Our functional results are consistent with the idea that cardiac myosin binding of N-terminal cMyBP-C is reduced in the mouse protein due to the presence of the NTE, which is proposed to interfere with myosin regulatory light chain (RLC) binding. The NTE is a critical component of mouse cMyBP-C, and should be considered in extrapolation of studies to cMyBP-C and HCM mechanisms in human.

Prevention of skin carcinogenesis by the ß-blocker carvedilol.

The stress-related catecholamine hormones and the α- and ß-adrenergic receptors (α- and ß-AR) may affect carcinogenesis. The ß-AR GRK/ß-arrestin biased agonist carvedilol can induce ß-AR-mediated transactivation of the EGFR. The initial purpose of this study was to determine whether carvedilol, through activation of EGFR, can promote cancer. Carvedilol failed to promote anchorage-independent growth of JB6 P(+) cells, a skin cell model used to study tumor promotion. However, at nontoxic concentrations, carvedilol dose dependently inhibited EGF-induced malignant transformation of JB6 P(+) cells, suggesting that carvedilol has chemopreventive activity against skin cancer. Such effect was not observed for the ß-AR agonist isoproterenol and the ß-AR antagonist atenolol. Gene expression, receptor binding, and functional studies indicate that JB6 P(+) cells only express ß2-ARs. Carvedilol, but not atenolol, inhibited EGF-mediated activator protein-1 (AP-1) activation. A topical 7,12-dimethylbenz(α)anthracene (DMBA)-induced skin hyperplasia model in SENCAR mice was utilized to determine the in vivo cancer preventative activity of carvedilol. Both topical and oral carvedilol treatment inhibited DMBA-induced epidermal hyperplasia (P < 0.05) and reduced H-ras mutations; topical treatment being the most potent. However, in models of established cancer, carvedilol had modest to no inhibitory effect on tumor growth of human lung cancer A549 cells in vitro and in vivo. In conclusion, these results suggest that the cardiovascular drug carvedilol may be repurposed for skin cancer chemoprevention, but may not be an effective treatment of established tumors. More broadly, this study suggests that ß-ARs may serve as a novel target for cancer prevention.